Water-absorbent resin and method for producing the same

Abstract

An object is to provide a water-absorbent resin in which the properties such as water-absorbing property are maintained and the odor generated at the time of swelling is reduced. To achieve the aforementioned object, a water-absorbent resin after surface crosslinking in which the concentration of volatile components after standing for 15 minutes under the condition that the swelling ratio becomes 1.0 times is 3.5 ppm or less is provided.

Description

Technical Field

[0001] This invention relates to a water-absorbing resin that reduces the odor generated during swelling, and a method for manufacturing the same. Background Technology

[0002] Regarding the absorbent agents contained in sanitary materials (hygiene products, absorbent items) such as diapers and sanitary napkins, superabsorbent polymers (SAPs) are currently mostly used. These SAPs are generally made from surface-crosslinked absorbent resins.

[0003] Generally, people's perception of odor varies depending on their lifestyle habits and living environment. Sometimes, even a slight odor can evoke different reactions, ranging from indifference to significant discomfort. In recent years, with increased consumer awareness of health and safety, the field of sanitary products (hygiene items, absorbent products) has begun to pay attention to the issue of subtle odors, which was previously largely ignored.

[0004] It is known that in conventional sanitary materials, various trace impurities in the raw materials can cause odor during swelling, and a certain percentage of users have been found to find this odor unpleasant. The odor emitted by sanitary materials that have absorbed urine, etc., includes not only the odor of the urine itself, but also odors generated from the absorption of moisture in the urine or from contact with moisture. These odors are believed to originate from chemicals such as nonwoven fabrics, adhesives, glues, and absorbent resins.

[0005] Surface-crosslinked absorbent resins contain trace amounts of various impurities, such as unreacted substances from reactive raw materials, represented by residual monomers and residual crosslinking agents, and byproducts generated from the raw materials. Therefore, after surface-crosslinked absorbent resins absorb moisture (or urine) and swell, they will produce an odor originating from the absorbent resin, which can cause discomfort to some consumers.

[0006] To date, several methods have been developed to suppress the odor produced by surface-crosslinked absorbent resins during swelling.

[0007] For example, Patent Document 1 discloses a method for controlling the content of alcohol-based volatile substances or residual ethylene glycol within a specific range for a surface-crosslinked water-absorbing resin, thereby suppressing the occurrence of the aforementioned odor.

[0008] Patent document 2 discloses a method for adding sulfite or persulfate to a water-absorbing resin to react the residual monomer with the sulfite or persulfate, thereby reducing the residual monomer content in the water-absorbing resin.

[0009] Patent document 3 discloses a method for removing odor along with water by adding an anti-coagulation agent and water to a surface-crosslinked water-absorbing resin and then drying the water-absorbing resin.

[0010] Patent document 4 discloses a method for removing odor by adding an odor adsorbent composed of an aqueous solution containing cysteine ​​to a surface-crosslinked water-absorbing resin and then drying the water-absorbing resin.

[0011] Patent document 5 discloses a method for removing odor by using a post-crosslinking agent such as 2-oxazolinone to perform a post-crosslinking reaction on a water-absorbing resin.

[0012] Patent documents 6-7 disclose a method for removing odors originating from the dispersion medium of a water-absorbing resin obtained by reverse suspension polymerization by reducing the dispersion medium inside the resin particles.

[0013] (Existing technical literature)

[0014] Patent Document 1: WO2006 / 033477 (Japanese Patent Application Publication No. 2006-116535)

[0015] Patent Document 2: Japanese Patent Application Publication No. 2006-297373

[0016] Patent Document 3: WO2019 / 022389

[0017] Patent Document 4: Japanese Patent Application Publication No. 2009-515691

[0018] Patent Document 5: WO2006 / 042704

[0019] Patent Document 6: WO2012 / 108253

[0020] Patent Document 7: WO2009 / 025235 Summary of the Invention

[0021] (The problem the invention aims to solve)

[0022] In recent years, sanitary materials have been trending towards thinner designs. With this thinning, the content of the surface-crosslinked absorbent resin in these sanitary materials has increased, while the content of other components (such as pulp) has decreased. Here, pulp also possesses deodorizing properties by absorbing odors. Therefore, the decrease in pulp content with the thinning of sanitary materials leads to an increase in the amount of odor generated from the absorbent resin. Furthermore, for example, in the case of incontinence pads, the reduction in the amount of odor-absorbing pulp due to the thinning of the sanitary material makes the odor more easily noticeable to caregivers and users of sanitary products.

[0023] That is, in recent years, as the amount of odor emitted has increased with the thinning of sanitary materials, users have become more aware of the odor, and the aforementioned existing technologies have become insufficient to solve the odor problem that has emerged in recent years.

[0024] Furthermore, with the recent trend towards thinner sanitary materials, the required water absorption rate has also increased. This increased absorption rate is generally achieved by increasing the specific surface area of ​​the surface-crosslinked absorbent resin. However, if the specific surface area of ​​the surface-crosslinked absorbent resin increases, the odorous substances (volatile components from impurities contained in the absorbent resin) present within the surface-crosslinked absorbent resin will more easily volatilize to the outside, resulting in an increase in the amount of odor generated from the surface-crosslinked absorbent resin.

[0025] One aspect of the present invention is to provide a water-absorbing resin in which the water-absorbing properties are maintained and the odor generated during swelling is sufficiently reduced; and a method thereof.

[0026] (Technical means used to solve the problem)

[0027] The inventors conducted intensive research to solve the aforementioned problems and discovered that, for surface-crosslinked absorbent resins, if the concentration of volatile components during low-ratio swelling is kept below a certain value, the absorbent properties of the resin, such as its water absorption capacity, can be maintained while the odor generated during swelling is reduced. This invention also includes a method for manufacturing the same absorbent resin.

[0028] That is, one embodiment of the present invention includes the following aspects.

[0029] A water-absorbing resin, which is a surface-crosslinked water-absorbing resin, has a volatile component concentration of less than 3.5 ppm after standing for 15 minutes under a swelling ratio of 1.0.

[0030] The concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0 is defined as the total concentration of all substances detected by a photoionization detector (PID) with a 10.6 eV irradiation lamp after uniformly adding 10.0 g of physiological saline at 23.5 ± 0.5 °C to 10.0 g of absorbent resin in a 2 L sealable glass container at room temperature and pressure, and standing for 15 minutes in a sealed state. This concentration is expressed as a value based on the detection value converted from isobutylene calibration gas.

[0031] A method for manufacturing a water-absorbing resin includes: adding an aqueous liquid in droplet form to a surface-crosslinked water-absorbing resin to make the water content of the water-absorbing resin reach 7.5% by mass or more, and then drying the water-absorbing resin to which the aqueous liquid has been added in such a way that the decrease in water content reaches 7.5% by mass or more within 1 hour.

[0032] A method for manufacturing a water-absorbing resin includes: a step of contacting the water-absorbing resin with a supercritical solvent to remove volatile components from the water-absorbing resin, wherein...

[0033] The water-absorbing resin is mainly composed of polyacrylic acid (salt) based resin.

[0034] The absorbent resin has undergone internal cross-linking, and

[0035] The water-absorbing resin has undergone surface cross-linking.

[0036] [The effects of the invention]

[0037] According to one aspect of the present invention, a water-absorbing resin whose water-absorbing properties are maintained and whose odor generated during swelling is reduced, and a method thereof can be provided. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of an example of a supercritical extraction apparatus used in the method for manufacturing the absorbent resin according to the second embodiment of the present invention.

[0039] Figure 2 This is a schematic diagram of the sealed container used to determine the concentration of volatile components. Detailed Implementation

[0040] The embodiments of the present invention will now be described in detail. However, the present invention is not limited to these embodiments, and various modifications can be made within the scope described in this specification. Embodiments and examples obtained by appropriately combining the technical means disclosed in different embodiments and examples are also included within the technical scope of the present invention. In addition, unless otherwise specified in this specification, "A to B" expressing a numerical range means "A or more and B or less", and "ppm" means "mass ppm" or "weight ppm". In addition, "(meth)propylene" means "propylene and / or methpropylene", and "mass" and "weight" are considered synonyms. Furthermore, unless otherwise specified, the mass of the water-absorbing resin, etc., expresses a calculated solid content value.

[0041] [1] Definition of terminology

[0042] [1-1] Water-absorbing resin

[0043] In this specification, "water-absorbent resin" refers to a water-swellable, water-insoluble cross-linked polymer, which is generally in granular form. In addition, "water-swellable" means that the unpressurized absorption ratio (CRC) is 5 g / g or more as specified in NWSP 241.0.R2(15), and "water-insoluble" means that the soluble component content (Ext) is 50% by mass or less as specified in NWSP 270.0.R2(15).

[0044] The "hygroscopic resin" is preferably a hydrophilic crosslinked polymer obtained by crosslinking polymerization of unsaturated monomers with carboxyl groups. However, it does not need to be 100% by mass of the hydrophilic crosslinked polymer, and may contain additives, etc., provided that the desired properties such as CRC and Ext are met. The unsaturated monomers with carboxyl groups are preferably acrylic (salt) monomers.

[0045] In addition, in this specification, the term "water-absorbing resin" sometimes refers to "a polymer that has only undergone internal crosslinking, i.e., a polymer with substantially the same crosslinking density on both the interior and surface," or "a polymer that has undergone internal and surface crosslinking, i.e., a polymer with a relatively higher surface crosslinking density than its interior crosslinking density." In principle, there is no difference between "polymers that have only undergone internal crosslinking" and "polymers that have undergone internal and surface crosslinking," both described as "water-absorbing resins." However, when it is necessary to clearly distinguish whether or not surface crosslinking is present, "polymers that have only undergone internal crosslinking" are described as "water-absorbing resin before surface crosslinking" or "basic polymer" because they represent the state before surface crosslinking has been implemented. Conversely, "polymers that have undergone internal and surface crosslinking, i.e., a polymer with a relatively higher surface crosslinking density than its interior crosslinking density" are described as "water-absorbing resin after surface crosslinking" or "water-absorbing resin after surface crosslinking" because they represent the state after surface crosslinking has been implemented. Here, "before surface crosslinking" means "before the addition of a surface crosslinking agent," or "although a surface crosslinking agent has been added, the surface crosslinking reaction has not yet been initiated through heat treatment."

[0046] In addition, the term "water-absorbing resin" sometimes refers only to the resin component, but sometimes it also includes additives and other components other than resin.

[0047] [1-2] "NWSP"

[0048] "NWSP" is short for Non-Woven Standard Procedures-Edition 2015, a standardized evaluation method for nonwoven fabrics and their products jointly published by EDANA (European Disposables and Nonwovens Associations) and INDA (Association of the Nonwoven Fabrics Industry), and adopted uniformly in Europe and the United States. NWSP also includes standard testing methods for absorbent polymers. This specification uses the original NWSP (2015) as a benchmark to determine the physical properties of absorbent polymers.

[0049] In this specification, unless otherwise specifically stated, the methods for determining various properties of water-absorbing resins follow the methods described in the examples below.

[0050] [2] Water-absorbing resin

[0051] The water-absorbing resin of one embodiment of the present invention is a surface-crosslinked water-absorbing resin, and its volatile component concentration is less than 3.5 ppm after standing for 15 minutes under the condition that the swelling ratio is 1.0.

[0052] In their research to solve the aforementioned problem, the inventors discovered that the intensity of the odor emitted by the absorbent resin in sanitary materials varies depending on the location of the absorbent core. Specifically, when a diaper with an absorbent core composed of absorbent resin and hydrophilic fibers was laid out, odorless saline solution or artificial urine was injected into its center. After the liquid diffused and was absorbed, the odor was directly smelled. It was found that the odor near the center was weaker, while the odor in the area penetrated by the diffused liquid was stronger. Further detailed analysis of the absorbent core revealed that the absorbent resin near the center swelled considerably, while the swelling of the absorbent resin in the area penetrated by the diffused liquid was less than that near the center. Based on this result, it was found that the degree of odor varies with the swelling ratio of the absorbent resin; the lower the swelling ratio, the stronger the odor. It is believed that this difference in odor intensity is due to the fact that various impurities contained in the surface-crosslinked absorbent resin produce odorous substances (volatile components), and the amount of these odorous substances varies. As a method for determining the generation of these odorous substances (volatile components), the concentration of volatile components was measured. Unexpectedly, a correlation was found between the concentration of volatile components and the degree of odor (the higher the concentration of volatile components, the stronger the odor). It was also found that the lower the swelling ratio of the absorbent resin, the higher the concentration of volatile components generated. Furthermore, it was found that the concentration of volatile components was highest at a low swelling ratio of 1.0, and when the concentration of volatile components was controlled below a specified value, the odor generated after swelling when the absorbent containing this absorbent resin was significantly reduced in actual use in sanitary materials.

[0053] In previous odor measurements of absorbent resins during swelling, as disclosed in Patent Documents 1 and 6, the swelling ratios of 5 times or 7.5 times were determined arbitrarily and at random. However, the inventors unexpectedly and clearly discovered through research that the concentration of volatile components emitted from absorbent resins reaches its highest level under low swelling conditions, which are lower than previous measurement conditions. Regarding this low swelling condition, the correlation between low swelling conditions and odor levels in practical applications such as sanitary materials has not been studied until now. This invention is the first to discover that, in order to reduce the unpleasant odor of sanitary materials, it is most important to measure the concentration of volatile components emitted by absorbent resins under low swelling conditions, which have a high correlation with the unpleasant odor emitted by sanitary materials and the concentration of volatile components.

[0054] In this invention, "swelling ratio" refers to the ratio of the mass of aqueous liquid absorbed by the swollen absorbent resin to the mass of the absorbent resin before swelling. For example, a swelling ratio of 1.0 means that the mass of aqueous liquid absorbed by the swollen absorbent resin is 1.0 times the mass of the absorbent resin before swelling (the mass of the absorbed aqueous liquid is equal to the mass of the absorbent resin before swelling). Here, the absorbent resin before swelling refers to the absorbent resin that has not yet absorbed water, and its water content is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. In other words, it refers to an absorbent resin with a solid content of 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more.

[0055] The term "concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0" (hereinafter referred to as "concentration of volatile components at 1.0 times swelling") refers to the following: Under normal temperature and pressure, physiological saline is uniformly added to the absorbent resin in a sealable container to achieve a swelling ratio of 1.0, and the container is left to stand for 15 minutes in a sealed state. The concentration of each volatile component (gaseous substance, gaseous material) present in the sealed container is then detected by a photoionization detector (PID) equipped with a 10.6 eV irradiation lamp. Specifically, it is the value measured using the method described in the examples.

[0056] The volatile component concentration in this invention refers to the total concentration of all substances present in the sealed container that are detected by a photoionization detector (PID) with a 10.6 eV irradiation lamp, which is the detection value converted using isobutylene as a calibration gas.

[0057] Examples of volatile components detected by the photoionization detector include acetic acid, methyl acetate, acrylic acid, methyl acrylate, ethyl acrylate, acetaldehyde, acetone, toluene, ethanol, isopropanol, butanol, diethyl ether, ethyl mercaptan, furfural, heptane, hexane, isobutylene, ammonia, hydrogen sulfide, carbon disulfide, and nitrogen dioxide. Examples of substances not detected by the photoionization detector (PID) include water, oxygen, nitrogen, carbon dioxide, ozone, and hydrogen. In this specification, unless otherwise specified, "volatile components" refers to "substances detected by a photoionization detector (PID) equipped with a 10.6 eV illumination lamp."

[0058] In this specification, physiological saline is used as one embodiment, but pure water or artificial urine with specific components may also be used.

[0059] The concentration of volatile components at 1.0 times swelling is 3.5 ppm or less, more preferably 3.3 ppm or less, more preferably 3.0 ppm or less, more preferably 2.7 ppm or less, more preferably 2.5 ppm or less, more preferably 2.3 ppm or less, more preferably 1.9 ppm or less, more preferably 1.5 ppm or less, and more preferably 1.0 ppm or less.

[0060] If the concentration of volatile components at the 1.0 times swelling is below 3.5 ppm, then in the actual use of absorbents containing this water-absorbing resin in sanitary materials, the odor generated during swelling can be significantly reduced.

[0061] Regarding the water-absorbing resin of one embodiment of the present invention, the total concentration of each volatile component after standing for 15 minutes under conditions where the swelling ratios are 0.0, 0.5, 1.0, 2.5, 5.0, 10.0, and 20.0 times is preferably 9.5 ppm or less.

[0062] As mentioned above, the swelling ratio of the absorbent resin in sanitary materials varies depending on its placement within the absorbent body when absorbing urine, resulting in different concentrations of volatile components at each swelling ratio. Therefore, it is desirable not only to reduce the concentration of volatile components at the swelling ratio that results in the highest concentration (1.0 times the average), but also at other swelling ratios. The inventors have determined that, in order to control the concentration of volatile components generated from sanitary materials to be low, it is preferable to control the total concentration of volatile components at the seven swelling ratios to be below 9.5 ppm.

[0063] The term "the total concentration of each volatile component after standing for 15 minutes under swelling ratios of 0.0, 0.5, 1.0, 2.5, 5.0, 10.0, and 20.0" (hereinafter sometimes referred to as "the cumulative value of volatile components during swelling at each ratio") refers to the total concentration of volatile components at each swelling ratio after uniformly adding physiological saline to the absorbent resin in a sealable container with the swelling ratios of 0, 0.5, 1.0, 2.5, 5.0, 10.0, and 20.0, respectively, and then standing for 15 minutes in a sealed state. Specifically, this is the value measured by the measurement method described in the examples. The cumulative value of volatile components during swelling at each expansion ratio is preferably 9.5 ppm or less, more preferably 8.0 ppm or less, more preferably 7.5 ppm or less, more preferably 7.0 ppm or less, more preferably 6.5 ppm or less, more preferably 6.0 ppm or less, more preferably 5.0 ppm or less, and even more preferably 4.0 ppm or less, and even more preferably 3.5 ppm or less.

[0064] If the cumulative value of volatile components during swelling at each expansion ratio is below 9.5 ppm, then in the actual use of absorbents containing this water-absorbing resin in sanitary materials, the odor generated during swelling can be significantly reduced.

[0065] Regarding the water-absorbing resin of one embodiment of the present invention, the maximum value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling to 900 seconds after the water-absorbing resin is 0.5 ppm or less when the water-absorbing resin is swollen at a swelling ratio of 5.0 times.

[0066] The inventors noted that the unpleasant odor emitted from sanitary materials diminishes over time, and also noted that this phenomenon is related to the decrease in the concentration of volatile components emitted during the swelling of the absorbent resin over time, starting immediately after swelling. Furthermore, it was found that, in order to minimize the noticeability of unpleasant odors from sanitary materials, it is preferable to control the maximum concentration of volatile components emitted by the absorbent resin over time.

[0067] Here, the term "the maximum value of the volatile component concentration measured every 5 seconds during the period from the start of swelling to 900 seconds after the swelling of the superabsorbent resin, under the condition that the swelling ratio is 5.0 times" (hereinafter sometimes referred to as "the maximum volatile component concentration during swelling along the time axis") refers to the maximum value of the volatile component concentration measured every 5 seconds during the period from the start of swelling of the superabsorbent resin to 900 seconds after the swelling of the superabsorbent resin, in a sealed container, by uniformly adding physiological saline to the superabsorbent resin in such a way that the swelling ratio of the superabsorbent resin is 5.0 times, and in a sealed state, specifically the value measured by the measurement method described in the examples. The maximum volatile component concentration during swelling along the time axis is preferably 0.5 ppm or less, more preferably 0.4 ppm or less, more preferably 0.3 ppm or less, and more preferably 0.2 ppm or less.

[0068] If the maximum volatile component concentration during swelling along the time axis is below 0.5 ppm, then in the actual use of absorbents containing this water-absorbing resin in sanitary materials, the odor generated during swelling can be significantly reduced.

[0069] Regarding the water-absorbing resin of one embodiment of the present invention, the total value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling to 900 seconds after the water-absorbing resin is 50.0 ppm or less is preferably 50.0 ppm.

[0070] As mentioned above, the concentration of volatile components emitted by the absorbent resin in sanitary materials decreases over time. However, the inventors have discovered that the unpleasant odor emitted by sanitary materials depends not only on the concentration of volatile components at a given time, but also on the total amount emitted from the start of swelling. Therefore, it is preferable to keep this total amount low.

[0071] Here, the phrase "the total value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling to 900 seconds after the swelling of the superabsorbent resin, under the condition that the swelling ratio is 5.0 times" (hereinafter sometimes referred to as "the cumulative value of volatile components during swelling along the time axis") refers to the total value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling of the superabsorbent resin to 900 seconds after the swelling of the superabsorbent resin, in a sealed container, with physiological saline uniformly added to achieve a swelling ratio of 5.0 times, and in a sealed state, specifically the value measured by the measurement method described in the examples. The cumulative value of volatile components during swelling along the time axis is preferably 50.0 ppm or less, more preferably 45.0 ppm or less, more preferably 35.0 ppm or less, even more preferably 25.0 ppm or less, and particularly preferably 20.0 ppm or less.

[0072] If the cumulative value of volatile components during swelling along the time axis is below 50.0 ppm, then in the actual use of absorbents containing this water-absorbing resin in sanitary materials, the odor generated during swelling can be significantly reduced.

[0073] [2-1] Polyacrylic acid (salt) based water-absorbing resin

[0074] In one embodiment of the present invention, the water-absorbing resin preferably contains a polyacrylic acid (salt)-based water-absorbing resin as the main component, but is not limited thereto. In this specification, a polyacrylic acid (salt)-based water-absorbing resin refers to a hydrophilic crosslinked polymer formed by crosslinking polymerization of a monomer composition containing acrylic acid (salt) monomers. That is, a polyacrylic acid (salt)-based water-absorbing resin refers to a polymer having structural units derived from acrylic acid (salt) and having grafted components as arbitrary components.

[0075] In this specification, "acrylic acid (salt)" refers to acrylic acid and / or its salts, and "monomer composition containing acrylic acid (salt) monomers" refers to a monomer composition in which the content of acrylic acid (salt) is 50 mol% or more relative to all monomers except crosslinking agents.

[0076] In other words, polyacrylic acid (salt) based water-absorbing resin refers to a cross-linked polymer in which the content of structural units derived from acrylic acid (salt) is 50 mol% or more relative to all structural units constituting the polyacrylic acid (salt) based water-absorbing resin, and which has grafted components as any component.

[0077] More preferably, the polyacrylic acid (salt) based water-absorbing resin is a crosslinked polymer obtained by using acrylic acid (salt) as a raw material, preferably 50 mol% or more, more preferably 70 mol% or more, more preferably 90 mol% or more, and preferably 100 mol% or less, and especially preferably substantially 100 mol% of the monomer components other than the internal crosslinking agent participating in the polymerization reaction.

[0078] (monomer)

[0079] Monomers are raw material components (monomers) used to form absorbent resins (polymers), and include, for example, acrylic (salt) monomers, monomers other than acrylic (salt) monomers, and internal crosslinking agents. All monomers used in absorbent resins constitute a monomer composition. Examples of acrylic (salt) monomers include (meth)acrylic acid and its salts.

[0080] Among monomers permitted to be included in the monomer composition, excluding acrylic (salt) monomers, monomers having acid groups are preferred among monomers having unsaturated double bonds (vinyl unsaturated monomers). Specific examples of such monomers include maleic acid (anhydride), fumaric acid, butenoic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methylpropane sulfonic acid, 2-(meth)acryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, 2-hydroxyethyl methacryloyl phosphate, and / or their salts. One or more of these monomers may be used as needed.

[0081] Examples of salts include alkali metal salts, ammonium salts, and amine salts, with sodium salts, potassium salts, lithium salts, and ammonium salts being more preferred, and sodium salts being particularly preferred.

[0082] In addition, the monomer composition containing acrylic (salt) monomers is preferably neutralized in the range of 10 to 90 mol%, more preferably in the range of 40 to 80 mol%, and particularly preferably in the range of 60 to 75 mol%.

[0083] Therefore, monomer compositions containing acrylate monomers are preferably neutralized using a neutralizing solution containing alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, carbonates such as sodium carbonate and potassium carbonate, and monovalent alkaline compounds such as ammonia. Neutralization using a neutralizing solution containing sodium hydroxide is particularly preferred.

[0084] In addition to the monomers mentioned above, the monomer composition may also include hydrophilic or hydrophobic unsaturated monomers (hereinafter referred to as "other monomers") as needed. Examples of such other monomers include: unsaturated monomers containing thiol groups; phenolic unsaturated monomers containing hydroxyl groups; amide-containing unsaturated monomers such as N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; and amino-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide. The amount of other monomers used should not impair the physical properties of the obtained water-absorbing resin; specifically, it should be 50 mol% or less, more preferably 20 mol% or less, relative to the portion of the monomer composition excluding the internal crosslinking agent.

[0085] (Internal cross-linking agent)

[0086] Examples of internal crosslinking agents include: N,N'-methylenebis(meth)acrylamide, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, tri(hydroxymethyl)propane di(meth)acrylate, tri(hydroxymethyl)propane tri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethoxylated tri(hydroxymethyl)propane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipolypentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl methacrylate, etc. Considering reactivity and other factors, at least one internal crosslinking agent can be selected from these internal crosslinking agents.

[0087] In this invention, from the viewpoint of the water absorption performance of the water-absorbing resin, it is preferable to select an internal crosslinking agent having two or more polymerizable unsaturated groups, and more preferably an internal crosslinking agent having a (poly)alkylene glycol structure and having two or more polymerizable unsaturated groups. Examples of such polymerizable unsaturated groups include allyl and (meth)acrylate groups. (Meth)acrylate groups are preferred. Furthermore, regarding the internal crosslinking agent having a (poly)alkylene glycol structure and having two or more polymerizable unsaturated groups, polyethylene glycol di(meth)acrylate is an example. Here, the number of alkylene glycol units (hereinafter referred to as "n") is preferably 1 or more, more preferably 2 or more, further preferably 4 or more, particularly preferably 6 or more, and preferably 100 or less, more preferably 50 or less, further preferably 20 or less, and particularly preferably 10 or less.

[0088] Regarding the amount of the internal crosslinking agent, relative to the monomer composition excluding the internal crosslinking agent, it is preferably 0.0001 mol% or more, more preferably 0.001 mol% or more, and even more preferably 0.01 mol% or more, and preferably 10 mol% or less, more preferably 5 mol% or less, and even more preferably 1 mol% or less. By using an amount within this range, a water-absorbing resin with the desired water absorption properties can be obtained. On the other hand, if the amount is outside this range, the soluble water content may sometimes increase, or the absorption rate may sometimes decrease, as the gel strength decreases.

[0089] (Trace components)

[0090] In this invention, the monomer composition may sometimes contain trace amounts of polymerization inhibitors, Fe, propionic acid, acetic acid, acrylic acid dimer, and other impurities.

[0091] Regarding the polymerization inhibitors that may be contained in the monomer composition, examples include N-oxyl compounds, manganese compounds, and substituted phenolic compounds exemplified in International Publication No. 2008 / 096713, with substituted phenols, especially p-methoxyphenols, being preferred. The content of the polymerization inhibitor relative to the monomer composition is 5 to 200 ppm, preferably 5 to 160 ppm, further preferably 10 to 160 ppm, more preferably 10 to 100 ppm, further preferably 10 to 80 ppm, and most preferably 10 to 70 ppm.

[0092] The amount of iron (Fe) that may be contained in the monomer composition is preferably 2 ppm or less, more preferably 1.5 ppm or less, even more preferably 1.0 ppm or less, even more preferably 0.5 ppm or less, and particularly preferably 0.3 ppm or less. Here, from the perspective of the purification cost of alkali (especially caustic soda), the lower limit of the amount of Fe is 0.001 ppm, preferably 0.01 ppm.

[0093] Here, the amount of iron in the monomer composition can be quantified, for example, by the ICP emission spectroscopy method described in Japanese Industrial Standard JIS K1200-6. For reference on the specific quantification method, International Publication No. 2008 / 090961 can be cited.

[0094] The amount of propionic acid that may be contained in the monomer composition is preferably less than 500 ppm, more preferably less than 400 ppm, and even more preferably less than 300 ppm relative to the monomer composition.

[0095] The amount of acetic acid that may be contained in the monomer composition is less than 1% by mass relative to the monomer composition, preferably less than 5000 ppm, more preferably less than 3000 ppm, further preferably less than 2000 ppm, even more preferably less than 1000 ppm, and particularly preferably less than 500 ppm.

[0096] The acrylic dimer that may be contained in the monomer composition may be less than 1000 ppm relative to the monomer composition, preferably less than 500 ppm, more preferably less than 200 ppm, and especially preferably less than 100 ppm.

[0097] In addition, other impurities that may be present in the monomer composition include protoanemonin, allyl acrylate, allyl alcohol, aldehydes (especially furfural), maleic acid, and benzoic acid. Regarding the content of these six other impurities in the monomer composition, it is preferable that at least one of these impurities is present in a content of 0-20 ppm, more preferably two or more of these impurities in a content of 0-20 ppm, further preferably three or more of these impurities in a content of 0-20 ppm, even more preferably four or more of these impurities in a content of 0-20 ppm, particularly preferably five or more of these impurities in a content of 0-20 ppm, and most preferably all six of these impurities in a content of 0-20 ppm. Regarding the content of each other impurity, it is preferably 0-10 ppm, more preferably 0-5 ppm, further preferably 0-3 ppm, particularly preferably 0-1 ppm, and most preferably ND (below the detection limit). That is, in the monomer composition, it is most preferably that the total content of all six other impurities is ND (below the detection limit). In addition, the total amount of other impurities (the total weight of the six other impurities relative to the monomer composition) is preferably 100 ppm or less, more preferably 0 to 20 ppm, and even more preferably 0 to 10 ppm.

[0098] The aforementioned trace components (and their derivatives) can sometimes cause changes in volatile components (odor) due to surface cross-linking processes, etc., as described later. Therefore, it is preferable to focus on reducing these trace components present in the monomer composition from the perspective of raw materials. In other words, by reducing the amount of the aforementioned trace components in the monomer composition, the possibility of these trace components causing changes in volatile components due to surface cross-linking processes, etc., will be reduced (i.e., odor from volatile components will be reduced), thereby alleviating the odor emitted from the surface-cross-linked water-absorbing resin.

[0099] (Surface crosslinking agent)

[0100] The water-absorbing resin of one embodiment of the present invention has undergone surface crosslinking. Regarding the surface crosslinking agent used, examples include those disclosed in U.S. Patent No. 7,183,456. Considering reactivity, at least one such surface crosslinking agent can be selected. Furthermore, from the viewpoints of operability of the surface crosslinking agent and the water-absorbing properties of the water-absorbing resin, it is preferable to select an organic surface crosslinking agent compound having two or more functional groups capable of reacting with carboxyl groups and forming covalent bonds.

[0101] Specific examples of surface crosslinking agents include: ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol, 1,2-hexanediol, and 1,3-hexanediol. Polyols such as 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 2,3-hexanediol, 2,4-hexanediol, glycerol, polyglycerol, diethanolamine, and triethanolamine; polyamines such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyallylamine, and polyethyleneimine; halogenated epoxides and condensates of polyamines and halogenated epoxides; 1,2-ethylidene biazolinones and other oxazolidinones; 1, 3-Dioxapentane-2-one (ethylene carbonate), 4-methyl-1,3-dioxapentane-2-one, 4,5-dimethyl-1,3-dioxapentane-2-one, 4,4-dimethyl-1,3-dioxapentane-2-one, 4-ethyl-1,3-dioxapentane-2-one, 4-hydroxymethyl-1,3-dioxapentane-2-one, 1,3-dioxahexane-2-one, 4-methyl-1,3-dioxahexane-2-one, 4,6-dioxapentane-2-one Compounds containing alkylene carbonates such as methyl-1,3-dioxane-2-one and 1,3-dioxane-2-one; polyglycidyl compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, di(glycerol) polyglycidyl ether, poly(glycerol) polyglycidyl ether, and glycidyl; oxabutane compounds; vinyl ether compounds; cyclic urea compounds; etc. These can be used individually or in combination of two or more.

[0102] [2-2] Volatile component reducing agent

[0103] The absorbent resin of one embodiment of the present invention may also contain a volatile component reducing agent. A volatile component reducing agent is a substance that has the function of preventing the volatilization of volatile components (at least one of the volatile components detected by the photoionization detector), for example, a substance that has the function of capturing volatile components to inhibit their volatilization. Regarding the mechanism of capturing volatile components to inhibit their volatilization, for example, the volatile components may be chemically or physically adsorbed. The volatile component reducing agent may contain at least one selected from reducing agents, surfactants, and inorganic acids (salts).

[0104] (reducing agent)

[0105] The reducing agent is not particularly limited and includes reducing agents with a carboxyl group, reducing agents with an amino group, phosphoric acid reducing agents, and sulfuric acid reducing agents. Examples of reducing agents with a carboxyl group include L-ascorbic acid, mercaptoacetic acid, mercaptopropionic acid, etc. Examples of reducing agents with an amino group include compounds containing hydrazide groups such as sebacic acid dihydrazide, adipic acid dihydrazide, succinic acid dihydrazide, malonic acid dihydrazide, etc.; amino acids such as L-cysteine ​​and cysteamine; aminooxy compounds such as hydroxylamine and hydroxylamine-O-sulfonic acid, aminooxyacetic acid, and their similar compounds, etc., which are compounds with the functional group shown in the following structural formula (1); etc. In addition, the amino acids, aminooxy compounds, aminooxyacetic acid, and compounds with the functional group shown in the above structural formula (1) may also be in the state of hydrochloride (hemihydrochloride) for stabilization.

[0106] H2N-O-···Formula (1)

[0107] Regarding compounds having the functional group shown in the structural formula (1), there is no particular limitation as long as they have the functional group shown in formula (1), but for example, compounds having the structures shown in the following chemical formulas (2) to (6) can be cited:

[0108] H2N-O—R···Formula (2)

[0109] (In formula (2), R is H, CH3, C2H5, C6H5CH2, or SO3H)

[0110]

[0111] (In formula (3), R is H, CH3, n-C3H7, iso-C3H7, n-C4H9, n-C6H) 13 nC 10 H 21 Or C6H5CH2. (R can be the same or different.)

[0112] H2N-O-CH2-CH2-COOH···Wu(4)

[0113]

[0114] (In formula (6), R1, R2, and R3 are H, C6H5, and C6H3C, respectively.) 12 CH3 or C2H5, R1, R2, and R3 may be the same or different.

[0115] Examples of phosphoric acid-based reducing agents include hypophosphorous acid, sodium hypophosphite, phosphorous acid, and sodium phosphite. Examples of sulfuric acid-based reducing agents include sodium sulfite and sodium bisulfite. Among the above-mentioned reducing agents, compounds containing hydrazide groups, such as amino groups, sebacic acid dihydrazide, adipic acid dihydrazide, succinic acid dihydrazide, and malonic acid dihydrazide, are more preferred, especially L-cysteine, cysteine, and aminooxyacetic acid (semi-hydrochloride). One or more of the above-mentioned reducing agents may be used as needed, but if they contain compounds that react with hydrazide groups (resins or compounds with active carbonyl groups such as ketone groups and / or aldehyde groups), the hydrazide groups will react and disappear. Therefore, the function of suppressing the volatilization of odorous substances (volatile components) will decrease, so it is preferable not to use compounds containing hydrazide groups and compounds that react with hydrazide groups in combination.

[0116] If the absorbent resin of one embodiment of the present invention contains a reducing agent, the content of the reducing agent is preferably 0.001 to 2.0% by mass, more preferably 0.005 to 1.5% by mass, and even more preferably 0.008 to 1.2% by mass, particularly preferably 0.01 to 1.0% by mass, relative to the total amount of the absorbent resin containing additives, etc.

[0117] (surfactant)

[0118] Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants.

[0119] Examples of anionic surfactants include: sodium salts of mixed fatty acids, sodium salts of semi-cured tallow fatty acids, sodium salts of stearate, potassium oleate, and potassium castor oil; alkyl sulfate salts such as sodium lauryl sulfate, sodium higher alcohol sulfate, sodium lauryl sulfate, and triethanolamine lauryl sulfate; alkylbenzene sulfonates such as sodium dodecylbenzene sulfonate; alkylnaphthalene sulfonates such as sodium alkylnaphthalene sulfonate; alkyl sulfosuccinates such as sodium dialkyl thiosuccinate and disodium polyoxyethylene alkyl thiosuccinate; and alkyl diphenyl ether disulfonates such as sodium dialkyl diphenyl ether disulfonate. Alkyl phosphates such as potassium phosphate; polyoxyethylene alkyl (or alkylallyl) sulfates such as sodium lauryl ether sulfate, sodium polyoxyethylene alkyl ether sulfate, triethanolamine polyoxyethylene alkyl ether sulfate, and sodium polyoxyethylene alkylphenyl ether sulfate; special reactive anionic surfactants; special carboxylic acid surfactants; sodium salts of β-naphthyl sulfonic acid formalin condensates and sodium salts of special aromatic sulfonic acid formalin condensates; special polycarboxylic acid polymeric surfactants; polyoxyethylene alkyl phosphates, etc.

[0120] Examples of nonionic surfactants include: polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene octadecenyl ether, polyoxyethylene higher alcohol ethers, and other polyoxyethylene alkyl ethers; polyoxyethylene nonylphenyl ether and other polyoxyethylene alkyl aryl ethers; polyoxyethylene derivatives; sorbitol monolaurate, sorbitol monopalmitate, sorbitol monostearate, sorbitol tristearate, sorbitol monooleate, sorbitol trioleate, sorbitol hemioleate, sorbitol distearate, and other sorbitol fatty acid esters; polyoxyethylene sorbitol monolaurate, polyoxyethylene sorbitol monolaurate, and poly... Polyoxyethylene sorbitol fatty acid esters, including polyoxyethylene sorbitol monopalmitate, polyoxyethylene sorbitol monostearate, polyoxyethylene sorbitol tristearate, polyoxyethylene sorbitol monooleate, and polyoxyethylene sorbitol trioleate; fatty acid polyoxyethylene sorbitol esters, including tetraoleic acid polyoxyethylene sorbitol ester; fatty acid glycerides, including glyceryl monostearate, glyceryl monooleate, and self-emulsifying glyceryl monostearate; fatty acid polyoxyethylene esters, including polyethylene glycol monolaurate, polyethylene glycol monostearate, polyethylene glycol distearate, and polyethylene glycol monooleate; polyoxyethylene alkylamines; polyoxyethylene hardened castor oil; and alkyl alkanolamines.

[0121] Examples of cationic and amphoteric surfactants include: alkylamine salts such as coconut alkylamine acetate and stearylamine acetate; quaternary ammonium salts such as lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, distearate dimethylammonium chloride, and alkylbenzyl dimethylammonium chloride; alkyl betaines such as lauryl betaine, stearyl betaine, and lauryl carboxymethyl hydroxyethyl imidazoline betaine; and amine oxides such as lauryl dimethylamine oxygen.

[0122] In addition, surfactants containing fluorine atoms are also available. Various types of surfactants containing fluorine atoms are used in one embodiment of the present invention. For example, substances that significantly enhance surface activity by replacing hydrogen atoms on the hydrophobic group of a general surfactant with fluorine atoms to form an alkyl group (perfluoroalkyl). Alternatively, the surfactant containing fluorine atoms may also be a surfactant that does not have a perfluoroalkyl group, but rather has an alkyl group in which a portion of the hydrogen atoms (e.g., one hydrogen atom) on the hydrophobic group is replaced with fluorine atoms. Furthermore, a composition comprising an alkane-based surfactant and a surfactant containing fluorine atoms may also be used. Regarding surfactants containing fluorine atoms, even if the same perfluorocarbon chain is used as the hydrophobic group, it is possible to change them into four types of surfactants—anionic, nonionic, cationic, and amphoteric—by altering the hydrophilic group. The carbon chain as the hydrophobic group can be linear or branched. Representative surfactants containing fluorine atoms include the following substances.

[0123] Fluoroalkyl (C2-C10) carboxylic acids, disodium N-perfluorooctylsulfonylglutamate, sodium 3-[fluoroalkyl (C6-C11)oxy]-1-alkyl (C3-C4) sulfonate, sodium 3-[ω-fluoroalkylyl (C6-C8)-N-ethylamino]-1-propylsulfonate, N-[3-(perfluorooctylsulfonamido)propyl]-N,N-dimethyl-N-carboxymethylammonium betaine, fluoroalkyl (C11-C20) carboxylic acids, perfluoroalkyl carboxylic acids (C7-C13), perfluorooctyl Diethanolamine perfluoroalkyl sulfonate, perfluoroalkyl (C4-C12) sulfonates (Li, K, Na), N-propyl-N-(2-hydroxyethyl)perfluorooctylsulfonamide, perfluoroalkyl (C6-C10)sulfonamidopropyltrimethylammonium salt, perfluoroalkyl (C6-C10)-N-ethylsulfonylglycine salt (K), di(N-perfluorooctylsulfonyl-N-ethylaminoethyl ester) phosphate, perfluoroalkyl (C6-C16) monoethyl phosphate, perfluoroalkyl quaternary ammonium iodide (trade name Fluorad FC-135; a cationic fluorinated surfactant manufactured by Sumitomo 3M Corporation), perfluoroalkyl alkoxylate (trade name Fluorad FC-171; a nonionic surfactant manufactured by Sumitomo 3M Corporation), perfluoroalkyl potassium sulfonate (trade names Fluorad FC-95 and FC-98; anionic surfactants manufactured by Sumitomo 3M Corporation). Here, the number written after the label "C" represents the number of carbon atoms. For example, C2 to C10 means "a carbon number of 2 or more but less than 10".

[0124] In one embodiment of the present invention, an organometallic surfactant can also be used. The organometallic surfactant used in one embodiment of the present invention is a surfactant whose molecular backbone and / or side chains have metals such as Si, Ti, Sn, Zr, Ge, etc., preferably a surfactant whose molecular backbone has Si, and more preferably a siloxane surfactant.

[0125] Representative organometallic surfactants include those described on page 34 of "New Handbook of Surfactants" by Yoshida, Kondo, Ōgaki, and Yamanaka, Engineering Publishing House, 1966. Regarding the metal contained in organometallic surfactants, Sn, Zr, Ge, etc., can be used instead of Si or Ti. The surfactant used in one embodiment of the present invention is not limited to the surfactants described above.

[0126] Among these surfactants, nonionic surfactants are preferred from a safety perspective, with polyoxyethylene alkyl ethers, polyoxyethylene alkyl aryl ethers, polyoxyethylene derivatives, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and fatty acid glycerides being more preferred, and sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and fatty acid glycerides being particularly preferred.

[0127] If the absorbent resin of one embodiment of the present invention contains a surfactant, the surfactant content is preferably 0.001 to 2.0% by mass, more preferably 0.005 to 1.5% by mass, and even more preferably 0.008 to 1.2% by mass, particularly preferably 0.01 to 1.0% by mass, relative to the total amount of the absorbent resin containing additives, etc.

[0128] (Inorganic acids (salts))

[0129] Inorganic acids (salts) are compounds containing inorganic acids and their salts, such as carbonates, phosphates, and sulfates. However, inorganic acids (salts) that act as reducing agents are excluded. Examples of carbonate compounds include sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate. Examples of phosphate compounds include disodium hydrogen phosphate, sodium dihydrogen phosphate, and trisodium phosphate.

[0130] In one embodiment of the present invention, if the absorbent resin contains an inorganic acid (salt), the content of the inorganic acid (salt) relative to the total amount of the absorbent resin containing additives, etc., is preferably 0.001 to 2.0% by mass, more preferably 0.005 to 1.5% by mass, further preferably 0.008 to 1.2% by mass, and particularly preferably 0.01 to 1.0% by mass. When the content of the inorganic acid (salt) is 0.005% by mass or more, odorous substances (volatile components) can be effectively removed. Furthermore, when the content of the inorganic acid (salt) is 1.5% by mass or less, the absorbent properties and other physical properties (including whiteness, AAP, etc.) of the obtained absorbent resin can be well maintained.

[0131] [2-3] Properties of water-absorbing resins

[0132] The absorbent resin of one embodiment of the present invention preferably has an unpressurized absorption ratio (CRC) of 23 g / g or more, more preferably 25 g / g or more, even more preferably 27 g / g or more, and particularly preferably 28 g / g or more. A higher upper limit for the CRC is preferred, but considering the balance with other physical properties, the CRC is preferably 50 g / g or less, more preferably 45 g / g, even more preferably 40 g / g or less, and particularly preferably 35 g / g or less.

[0133] Furthermore, the absorbency up to pressure (AAP) of the water-absorbing resin according to one embodiment of the present invention is preferably 15 g / g or more, more preferably 17 g / g or more, even more preferably 20 g / g or more, particularly preferably 23 g / g or more, and most preferably 24 g / g or more. While there is no particular limitation on the upper limit, from the viewpoint of balancing with other physical properties, AAP is preferably 50 g / g or less.

[0134] When the AAP content is above 15 g / g, the amount of liquid backflow (commonly known as "Re-Wet") when the absorbent is under pressure will not be excessive, making it suitable for absorbents in hygiene products such as diapers. Here, AAP can be controlled through particle size and surface cross-linking agents.

[0135] The saline conductivity (SFC) of the absorbent resin according to one embodiment of the present invention is preferably 1×10⁻⁶. -7 cm 3 • sec / g or higher, more preferably 10 × 10 -7 cm 3 •sec / g or higher, and more preferably 20×10 -7 cm 3 • sec / g or higher, especially preferably 30 × 10 -7 cm 3 • sec / g or higher. A higher upper limit for SFC is preferred, but there is no specific limitation.

[0136] The water absorption rate of the water-absorbing resin according to one embodiment of the present invention, based on the "Vortex method," is preferably 60 seconds or less, more preferably 45 seconds or less, further preferably 35 seconds or less, particularly preferably 33 seconds or less, and most preferably 30 seconds or less. The lower the lower limit, the more preferred, but there is no particular limitation.

[0137] In one embodiment of the present invention, the pressure-dependent absorption uptake ratio (PDAUP) of the absorbent resin is preferably 10 g / g or more, more preferably 12 g / g or more, and even more preferably 15 g / g or more. Higher upper limits are preferred, but there is no particular limitation.

[0138] The specific surface area of ​​the water-absorbing resin in one embodiment of the present invention is preferably 20 m². 2 / kg or more, preferably 25m 2 / kg or more, preferably 27m 2 / kg or more, and preferably 30m 2 / kg or more, and more preferably 32m 2 / kg or more. The specific surface area of ​​the water-absorbing resin is 20m². 2 At concentrations above a certain level (e.g., per kg), the water absorption properties can be maintained. In other words, it is possible to manufacture water-absorbing resins with significantly increased water absorption rates.

[0139] In one embodiment of the present invention, the solid content of the absorbent resin is preferably 80% by mass or more, more preferably 85% by mass or more, further preferably 90% by mass or more, particularly preferably 92% by mass or more, and most preferably 95% by mass or more. When the solid content is 80% by mass or more, the absorbent properties and other physical properties (including whiteness, AAP, etc.) of the obtained absorbent resin can be well maintained.

[0140] The absorbent resin of one embodiment of the present invention is preferably in the form of granules. The granular absorbent resin may be, for example, randomly broken (irregular), spherical, fibrous, rod-shaped, nearly spherical, or flat. Considering the requirement for good diffusion of liquid (urine) and minimal peeling from the pulp when used in hygiene products such as baby diapers, the absorbent resin is preferably in an irregular shape among the above-mentioned granular shapes.

[0141] [2-4] Uses of water-absorbing resins

[0142] Regarding the absorbent resin of one embodiment of the present invention, its absorbent properties and other physical properties are maintained, and the odor generated during swelling is also reduced, so it can be well used in absorbent articles such as diapers, incontinence pads, and medical pads.

[0143] This invention also includes absorbent articles containing the absorbent resin of this invention. An absorbent article according to one embodiment of this invention includes, for example, an absorbent body containing the absorbent resin. The absorbent body can be, for example, a composite containing the absorbent resin and hydrophilic fibers. If the absorbent body is a composite containing the absorbent resin and hydrophilic fibers, the content of the absorbent resin relative to the total mass of the absorbent body is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. In this way, the absorbent article can be made thinner, and even when the content of odor-absorbing hydrophilic fibers, etc., is reduced, the amount of odor generated is significantly reduced because the odor generated during the swelling of the absorbent resin of this invention is lessened.

[0144] A more specific example of an absorbent article according to an embodiment of the present invention may be an absorbent article comprising an absorbent body having: a liquid-permeable front pad disposed adjacent to the wearer’s body; a liquid-impermeable back pad disposed adjacent to the wearer’s clothing away from the wearer’s body; and the aforementioned absorbent resin disposed between the front pad and the back pad.

[0145] [3] Method for manufacturing water-absorbing resin

[0146] The method for manufacturing the water-absorbing resin according to one embodiment of the present invention is not particularly limited as long as it is a method capable of obtaining the aforementioned water-absorbing resin. The embodiments of the method for manufacturing the water-absorbing resin of the present invention will be described in detail below.

[0147] [3-1] First Embodiment

[0148] The method for manufacturing a water-absorbing resin according to the first embodiment of the present invention includes: adding an aqueous liquid in droplet form to a surface-crosslinked water-absorbing resin to make the water content of the water-absorbing resin reach 7.5% by mass or more, and then drying the water-absorbing resin to which the aqueous liquid has been added in such a way that the decrease in water content reaches 7.5% by mass or more within 1 hour.

[0149] The method for manufacturing the water-absorbing resin according to the first embodiment of the present invention preferably includes one or more of the steps described in i) to iii):

[0150] Step i) involves adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 Absorbent resin of / kg or more;

[0151] Step ii) involves adding an aqueous liquid in droplet form to the absorbent resin to make the water content of the absorbent resin reach more than 10% by mass.

[0152] Step iii): Add a volatile component reduction agent.

[0153] The method for manufacturing the water-absorbing resin according to the first embodiment of the present invention may also include the steps described in (A) and / or (B) below:

[0154] Step (A) involves adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 / kg or more of the surface-crosslinked water-absorbing resin;

[0155] Step (B) sequentially includes a polymerization process, a drying process for drying the hydrogel obtained in the polymerization process, and a surface crosslinking process, and adds a volatile component reducing agent after the polymerization process is completed.

[0156] Furthermore, the method for manufacturing the water-absorbing resin according to the first embodiment of the present invention preferably includes: step (A), adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 The surface-crosslinked water-absorbing resin described above / kg. Particularly preferred is the addition of step (A), where an aqueous liquid in droplet form is added to a surface area of ​​25 m². 2 / kg or more of the surface-crosslinked water-absorbing resin; step (B) sequentially includes a polymerization step, a drying step of drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and adding a volatile component reducing agent after the polymerization step is completed.

[0157] Particularly preferably, the method for manufacturing the water-absorbing resin according to the first embodiment of the present invention includes: adding an aqueous liquid in droplet form to a surface-crosslinked material having a specific surface area of ​​25 m². 2 A water-absorbing resin of at least / kg is used to bring the water content of the water-absorbing resin to at least 7.5% by mass, and then the water-absorbing resin to which the aqueous liquid has been added is dried such that the decrease in water content reaches at least 7.5% by mass within 1 hour.

[0158] For ease of explanation, the process of "adding an aqueous liquid in droplet form to a water-absorbing resin to make the water content of the water-absorbing resin reach 7.5% by mass or more" will be described as "aqueous liquid addition process", and the process of "drying the water-absorbing resin to which the aqueous liquid has been added in such a way that the decrease in water content reaches 7.5% by mass or more within 1 hour" will be described as "drying process after adding aqueous liquid".

[0159] In this first embodiment, the volatile component reducing agent has been described in the aforementioned "[Polyacrylic acid (salt) based water-absorbing resin]" section. Furthermore, regarding the process of adding the volatile component reducing agent, please refer to the description in the "[3-3] Third Embodiment" section below.

[0160] [3-1-1] Aqueous liquid addition process

[0161] The aqueous liquid addition process in this first embodiment is as follows: adding an aqueous liquid in droplet form to a surface-crosslinked water-absorbing resin (with a specific surface area of ​​25m²). 2 The process involves increasing the moisture content of the surface-crosslinked superabsorbent resin to 7.5% by mass or more (preferably with a water-absorbing resin content of 7.5% by mass or more). In other words, this process increases the moisture content of the surface-crosslinked superabsorbent resin.

[0162] In one embodiment of the present invention, the aqueous liquid is preferably water, and more preferably an aqueous solution containing the aforementioned volatile component reducing agent. The presence of the volatile component reducing agent in the aqueous liquid results in a water-absorbing resin with a lower concentration of volatile components. Additionally, the aqueous liquid may contain impurities such as organic components and conductive substances. However, these impurities may hinder the effectiveness of the present invention. Therefore, the less of these impurities (especially those that may hinder the effectiveness of the present invention) contained in the aqueous liquid, the better. However, the volatile component reducing agent is not considered one of the aforementioned impurities.

[0163] In one embodiment of the present invention, when the aqueous liquid contains organic components that may hinder the effectiveness of the present invention, the concentration of the organic components in the aqueous liquid is preferably 1000 ppm or less, more preferably 500 ppm or less, even more preferably 200 ppm or less, and particularly preferably 100 ppm or less. When the concentration of the organic components in the aqueous liquid is within this range, (i) the presence of the organic components will not hinder the effectiveness of the present invention. Furthermore, (ii) in the absorbent resin manufactured by the manufacturing method of the first embodiment of the present invention, there are fewer organic components originating from residual impurities, thus further reducing the occurrence of odor due to these organic components.

[0164] Here, organic components that may hinder the effectiveness of the present invention include, for example, aliphatic hydrocarbons (e.g., n-heptane, cyclohexane), aromatic hydrocarbons (e.g., benzene, toluene, dimethylbenzene), alcohols (e.g., ethanol, isopropanol), carboxylic acid copolymers, etc. The concentration of organic components in an aqueous liquid refers to the total amount of these organic components.

[0165] In one embodiment of the present invention, when the aqueous liquid contains a conductive substance, the conductive substance may remain in the absorbent resin manufactured by the manufacturing method of the first embodiment of the present invention, thereby reducing the osmotic pressure of the absorbent resin when absorbing urine, etc., which may cause a decrease in the absorbency of the absorbent resin. Furthermore, depending on the type of conductive substance contained in the aqueous liquid, the conductive substance may also be a cause of odor (i.e., may hinder the effectiveness of the present invention). Here, the amount of conductive substance in the aqueous liquid can be evaluated by the conductivity of the aqueous liquid. The higher the content of conductive substance (ions, etc.) in the aqueous liquid, the higher the conductivity of the aqueous liquid. Therefore, as long as it does not hinder the effectiveness of the present invention, it is not necessary to specify the conductivity of the aqueous liquid; 5 mS / cm or less is acceptable, preferably 2 mS / cm or less, more preferably 1 mS / cm or less, and particularly preferably 500 μS / cm or less. "The conductivity of the aqueous liquid is 5 mS / cm or less" means that the content of the conductive substance in the aqueous liquid is very small (small enough not to hinder the effectiveness of the present invention). Therefore, by controlling the conductivity to preferably below 5 mS / cm, the decrease in the water absorption performance of the absorbent resin and the generation of odor caused by residual conductive substances can be further reduced. Here, examples of conductive substances that may hinder the effectiveness of the present invention include magnesium ions, calcium ions, and aluminum ions.

[0166] Here, the conductivity of physiological saline (0.9% by mass saline) is approximately 15.7 mS / cm, the conductivity of 0.69% by mass saline used in the determination of saline conductivity (SFC) is approximately 12.5 mS / cm, the conductivity of tap water is 100–200 μS / cm, and the conductivity of pure water is approximately 1 μS / cm. The above conductivity values ​​are at a liquid temperature of 25°C.

[0167] As described above, the aqueous liquid is preferably water with very few impurities such as organic components and conductive substances, and is especially preferably an aqueous solution containing volatile component inhibitors with so few impurities as to not hinder the effect of the present invention.

[0168] In the aqueous liquid addition process, the aqueous liquid in droplet form is added to the absorbent resin to achieve a water content of 7.5% by mass or more, preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. By adding the aqueous liquid in droplet form, the aqueous liquid can be uniformly added to the absorbent resin. The absorbent resin can be stirred as needed during and / or after the addition of the aqueous liquid. By adding the aqueous liquid in droplet form to the absorbent resin to achieve a water content of 7.5% by mass or more, substances that cause odor (hereinafter referred to as "odor substances." Odor substances refer to odor-causing components) can be effectively removed in the subsequent "drying process after adding the aqueous liquid." Furthermore, in the aqueous liquid addition process, it is preferable to add the aqueous liquid in droplet form to the absorbent resin to achieve a water content of 45% by mass or less, more preferably 35% by mass or less. Here, the term "moisture content" in this invention refers to the percentage (mass %) of the mass of the aqueous liquid relative to the total mass of the absorbent resin obtained by combining the mass of the solid components and the mass of the aqueous liquid.

[0169] If an excessive amount of aqueous liquid is added during the aqueous liquid addition process, the swollen absorbent resin may adhere to each other and form lumps. Furthermore, in the subsequent drying process after adding the aqueous liquid, a considerable amount of aqueous liquid needs to be removed from the absorbent resin, meaning the absorbent resin remains in a swollen state for an extended period. Consequently, during the drying process after adding the aqueous liquid, lumps may form due to the swollen absorbent resin adhering to each other. If such lumps form, the aqueous liquid inside the lumps may not be sufficiently removed during the drying process. That is, if such lumps form, the absorbency and other physical properties of the resulting absorbent resin may decrease. Additionally, if the lumps are broken up to adjust to the desired particle size distribution described later, the surface cross-linking layer of the absorbent resin may be damaged, still leading to a decrease in the physical properties of the absorbent resin. Furthermore, if the aforementioned lumps occur, the load on the stirring and drying apparatus will increase when the absorbent resin to which the aqueous liquid has been added is stirred to dry, and may even fail to stir under certain conditions.

[0170] By controlling the moisture content of the water-absorbing resin to which the water-absorbing liquid has been added within the aforementioned range during the water-absorbing liquid addition process, the swelling state of the water-absorbing resin can be quickly relieved during the subsequent drying process after the water-absorbing liquid addition. Therefore, the formation of lumps can be effectively prevented, thereby maintaining the water absorption and other physical properties of the obtained water-absorbing resin.

[0171] The amount of aqueous liquid added in droplet form during the aqueous liquid addition process can be easily set based on the moisture content of the absorbent resin to which the aqueous liquid has been added, i.e., the target moisture content. For example, to achieve a moisture content of 7.5% by mass for the absorbent resin after the addition of the aqueous liquid, 7.5 parts by mass of aqueous liquid should be added relative to 92.5 parts by mass of the absorbent resin (solid component) before the addition of the aqueous liquid.

[0172] In the aqueous liquid addition process, the temperature (powder temperature) of the absorbent resin before adding the aqueous liquid is preferably controlled at 90°C to 160°C, more preferably at 90°C to 140°C. Furthermore, the temperature (powder temperature) of the absorbent resin after adding the aqueous liquid is preferably controlled at 60°C to 150°C, more preferably at 70°C to 140°C. Additionally, the temperature (powder temperature) of the absorbent resin to which the aqueous liquid has been added is preferably controlled to 80°C to 160°C within 30 minutes, more preferably to 90°C to 160°C. That is, preferably, the absorbent resin to which the aqueous liquid has been added is subjected to a drying process after adding the aqueous liquid within 30 minutes.

[0173] Furthermore, in the aqueous liquid addition process, the temperature of the aqueous liquid before addition is preferably controlled between 5°C and 90°C, more preferably between 10°C and 70°C. Here, it is preferable to add the aqueous liquid to the water-absorbing resin in a shorter time.

[0174] By controlling the temperature of the absorbent resin before adding the aqueous liquid (powder temperature), the temperature of the absorbent resin after adding the aqueous liquid (powder temperature), and the temperature of the added aqueous liquid in the manner described above, the added aqueous liquid can quickly penetrate into the interior of the absorbent resin particles. As a result, the affinity between the aqueous liquid and the odorous substance (volatile component) is improved. Therefore, in the subsequent drying process after the addition of the aqueous liquid, the odorous substance (volatile component) can be effectively removed along with the aqueous liquid while maintaining the water absorption properties and other physical properties of the absorbent resin unchanged.

[0175] [3-1-2] Drying process after adding aqueous liquid

[0176] The drying process after adding the aqueous liquid in this embodiment is a process of drying the water-absorbing resin to which the aqueous liquid has been added, such that the decrease in water content reaches 7.5% by mass or more within 1 hour. That is, this process is a process of reducing the water content of the surface-crosslinked water-absorbing resin by 7.5% by mass or more within 1 hour.

[0177] Alternatively, the aqueous liquid addition step and the subsequent drying step can be designed as continuous steps using devices such as a stirring and drying apparatus, or they can be designed as individual steps. That is, the method for manufacturing the water-absorbing resin according to the first embodiment of the present invention can be a continuous process where the aqueous liquid addition step and the subsequent drying step are performed as continuous steps, or it can be a batch process where the aqueous liquid addition step and the subsequent drying step are performed as individual steps. Considering production efficiency, a continuous process is preferred for manufacturing the water-absorbing resin.

[0178] In the drying process after adding the aqueous liquid, the absorbent resin to which the aqueous liquid has been added is dried such that the decrease in moisture content reaches 7.5% by mass or more, preferably 10.0% by mass or more, more preferably 15.0% by mass or more, and particularly preferably 20.0% by mass or more within one hour. Furthermore, if the moisture content reaches 27.5% by mass or more after adding the aqueous liquid in the aforementioned aqueous liquid addition process, it is more preferable to dry the absorbent resin to which the aqueous liquid has been added so that the moisture content reaches 20.0% by mass or less within one hour. In this way, odorous substances (volatile components) contained in the absorbent resin can be effectively removed along with the aqueous liquid. Moreover, the greater the decrease in moisture content, the greater the amount of odorous substances (volatile components) removed along with the aqueous liquid, thus further reducing the generation of odors attributable to these odorous substances (volatile components). Here, the phrase "within 1 hour" in this invention means that the elapsed time from the moment the aqueous liquid is added to the absorbent resin is within 1 hour.

[0179] In the drying process after adding the aqueous liquid, if the water-absorbing resin cannot be dried such that the decrease in water content is less than 7.5% by mass within one hour, or if the water content reaches 27.5% by mass or more after adding the aqueous liquid in the aqueous liquid addition process, and if the water-absorbing resin with added aqueous liquid cannot be dried such that its water content is less than 20.0% by mass within one hour, then the water-absorbing resin will remain in a swollen state for a long time. Therefore, it is possible for swollen water-absorbing resin to adhere to each other and form lumps. If such lumps form, it may be impossible to sufficiently remove the aqueous liquid inside the lumps during the drying process after adding the aqueous liquid. That is, if such lumps form, the water absorption properties and other physical properties of the resulting water-absorbing resin may decrease. Furthermore, if the lumps are broken up to achieve the desired particle size distribution described later, the surface cross-linking layer of the absorbent resin may be damaged, still resulting in a decrease in the physical properties of the absorbent resin. Moreover, if lumps are formed, the load on the stirring-drying apparatus will increase when the absorbent resin, to which the aqueous liquid has been added, is stirred for drying, and may even fail to stir under certain conditions.

[0180] The time from the completion of the aqueous liquid addition step to the start of the drying step following the addition of the aqueous liquid is preferably shorter, more preferably within 30 minutes. Most preferably, the aqueous liquid addition step and the drying step following the addition of the aqueous liquid are performed continuously. This shortens the time the water-absorbing resin remains in a swollen state.

[0181] In the drying process after the addition of the aqueous liquid, it is preferable to dry the water-absorbing resin to which the aqueous liquid has been added under stirring and / or airflow conditions. This efficiently reduces the moisture content of the water-absorbing resin. Known devices can be used for the stirring and drying apparatus used to perform the stirring, and for the airflow generating apparatus used to generate the airflow.

[0182] In the drying process after the addition of the aqueous liquid, it is preferable to dry the water-absorbing resin to which the aqueous liquid has been added at a reduced pressure of 0.0 kPa to 10.0 kPa, more preferably at a reduced pressure of 0.1 kPa to 5.0 kPa. This efficiently reduces the moisture content of the water-absorbing resin. For example, a method for controlling the reduced pressure within the above range can be implemented by using a dryer to perform the drying process after the addition of the aqueous liquid, and using an exhaust fan and / or a vacuum pump to control the pressure inside the dryer within the above range. By controlling the reduced pressure within the above range, the dispersion of the water-absorbing resin and its dust caused by the airflow during depressurization can be suppressed. Furthermore, it is not preferable to excessively increase the reduced pressure, as this would complicate the drying process and require large-scale equipment.

[0183] In the drying process after the addition of the aqueous liquid, the device temperature for reducing the moisture content of the water-absorbing resin is preferably 60°C to 160°C, more preferably 80°C to 160°C, and even more preferably 100°C to 150°C. The device temperature for reducing the moisture content of the water-absorbing resin refers to, for example, the inner wall temperature of a dryer used in the drying process after the addition of the aqueous liquid; or, for example, the temperature of an airflow used in the drying process. By using a temperature within the above-preferred range as the device temperature for reducing the moisture content of the water-absorbing resin, the drying time can be shortened, resulting in improved productivity of the water-absorbing resin. Furthermore, the deterioration of the water-absorbing resin at high temperatures can be suppressed, resulting in the effective removal of odorous substances (volatile components) from the water-absorbing resin along with the aqueous liquid while maintaining the water absorption properties and other physical properties of the obtained water-absorbing resin. Here, the device temperature for reducing the moisture content of the water-absorbing resin will sometimes be referred to as the "drying temperature."

[0184] In the drying process after the addition of the aqueous liquid, the absorbent resin to which the aqueous liquid has been added is dried such that the decrease in water content reaches 7.5% by mass or more within a drying time of preferably 5 minutes to 1 hour, and more preferably 10 minutes to 50 minutes. By completing the drying within the specified time, the drying time can be shortened, the deterioration of the absorbent resin caused by drying at high temperatures can be suppressed, and damage to the absorbent resin caused by prolonged stirring or collision with airflow can also be suppressed. In addition, by drying for a drying time of 5 minutes or more, odorous substances (volatile components) can be effectively removed.

[0185] As described above, the drying temperature and drying time can be appropriately set to achieve the desired reduction in moisture content. However, if drying is carried out at a low temperature for a long time, the cross-linked layer of the water-absorbing resin may be mechanically damaged and destroyed within the dryer, resulting in a decrease in the physical properties of the water-absorbing resin. On the other hand, if drying is carried out at a high temperature for a short time, although the aforementioned damage to the surface cross-linked layer can be suppressed, the high temperature may cause the water-absorbing resin to deteriorate. Therefore, it is preferable that the drying temperature and drying time simultaneously satisfy the above range.

[0186] [3-1-3] Water-absorbing resin after surface crosslinking

[0187] Regarding the water-absorbing resin provided for the aqueous liquid addition process, after surface cross-linking, it is preferred to have a specific surface area of ​​25 m². 2 / kg or more, preferably with a specific surface area of ​​27m² 2 / kg or above, and preferably with a specific surface area of ​​30m². 2 / kg or more, and then 32m 2 / kg or more, especially 35m 2 / kg or more. Even after performing the aqueous liquid addition process and the subsequent drying process, the specific surface area of ​​the surface-crosslinked water-absorbing resin remains almost unchanged. Therefore, the specific surface area of ​​the water-absorbing resin after performing the aqueous liquid addition process and the subsequent drying process is still 25m². 2 The absorbency rate is above / kg, so the water absorption performance and other physical properties are maintained. That is, even if the water-absorbing resin with a high water absorption rate is subjected to the above-described water-based liquid addition process and the drying process after the water-based liquid addition, the water absorption rate of the water-absorbing resin can still be maintained.

[0188] The unpressurized absorption ratio (CRC) of the surface-crosslinked water-absorbing resin supplied to the aqueous liquid addition process is preferably 23 g / g or more, more preferably 25 g / g or more, even more preferably 27 g / g or more, particularly preferably 28 g / g or more, and the higher the upper limit, the more preferred. However, from the viewpoint of balancing with other physical properties, it is preferably 50 g / g or less, more preferably 40 g / g or less, and even more preferably 35 g / g or less.

[0189] The pressure absorption ratio (AAP) of the surface-crosslinked water-absorbing resin supplied to the aqueous liquid addition process is preferably 15 g / g or more, more preferably 17 g / g or more, even more preferably 20 g / g or more, particularly preferably 23 g / g or more, and the higher the upper limit, the more preferred. However, from the viewpoint of balancing with other physical properties, it is preferably 50 g / g or less, more preferably 40 g / g or less, and even more preferably 30 g / g or less.

[0190] The water absorption rate of the surface-crosslinked water-absorbing resin supplied to the aqueous liquid addition process, based on the Vortex method, is preferably 35 seconds or less, more preferably 33 seconds or less, and even more preferably 30 seconds or less. The lower the lower limit, the more preferred, but there is no particular limitation.

[0191] The saline conductivity (SFC) of the surface-crosslinked absorbent resin supplied to the aqueous liquid addition process is preferably 10 × 10⁻⁶. -7 cm 3 • sec / g or higher, more preferably 20 × 10 -7 cm 3 •sec / g or higher, and more preferably 30×10 - 7 cm 3 • sec / g or higher. A higher upper limit is preferred, but there is no specific limitation.

[0192] The residual monomer content of the surface-crosslinked water-absorbing resin supplied to the aqueous liquid addition process is preferably 1000 ppm or less, more preferably 700 ppm or less, and even more preferably 500 ppm or less. The lower the lower limit, the more preferred, but there is no particular limitation.

[0193] By controlling the various physical properties of the surface-crosslinked water-absorbing resin supplied to the aqueous liquid addition process within the above-mentioned preferred range, a water-absorbing resin with various physical properties within a good range after the aqueous liquid addition process and the subsequent drying process can be obtained.

[0194] [3-1-4] Method for manufacturing water-absorbing resin after surface crosslinking

[0195] Regarding the method for manufacturing the surface-crosslinked absorbent resin supplied to the aqueous liquid addition step, known methods can be employed. Examples of such manufacturing methods include those shown below. However, it should be noted that the method for manufacturing the surface-crosslinked absorbent resin does not need to include all of the steps shown below; at least the polymerization step, drying step, and surface crosslinking step are sufficient. Furthermore, the method for manufacturing the absorbent resin according to the first embodiment of the present invention can include each of the steps shown below, but it is not necessary to include all of these steps.

[0196] [3-1-4-1] Preparation process of monomer aqueous solution

[0197] This step is the preparation of an aqueous solution of a monomer composition (hereinafter sometimes referred to as "monomer aqueous solution"), wherein the monomer composition contains monomers including the aforementioned acrylic (salt) monomers and at least one of the aforementioned internal crosslinking agents. Although the monomer slurry form can also be used, this specification uses an aqueous solution as an example for ease of understanding.

[0198] (monomer)

[0199] The monomers used in this process have been explained in the previous section on [Polyacrylic Acid (Salt) Based Water Absorbent Resins], and will not be repeated here.

[0200] (Neutralization using alkaline compounds)

[0201] In one embodiment of the present invention, the acrylic acid is preferably partially neutralized with an alkaline compound. That is, in one embodiment of the present invention, a water-absorbing resin in which the acid groups of polyacrylic acid have been partially neutralized is preferred.

[0202] Examples of such basic compounds include: alkali metal carbonates and bicarbonates; alkali metal hydroxides; ammonia; and organic amines. From the viewpoint of the absorbency of the absorbent resin, a strongly basic compound is preferred. From an operational perspective, this basic compound is preferably in aqueous solution form.

[0203] The neutralization can be performed before, during, or after polymerization, or it can be performed at multiple times or in multiple stages. Furthermore, from the viewpoint of increasing the production efficiency of the water-absorbing resin, continuous neutralization is preferred.

[0204] In this invention, when acrylic acid (salt) is used, the neutralization rate of acrylic acid (salt) relative to the acid groups of the monomer is preferably 10 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, particularly preferably 60 mol% or more, and preferably 90 mol% or less, more preferably 85 mol% or less, further preferably 80 mol% or less, and particularly preferably 75 mol% or less. By using a neutralization rate within this range, the decrease in the water absorption performance of the water-absorbing resin can be suppressed. Here, the above-mentioned requirement for the neutralization rate also applies to neutralization performed at any stage before, during, or after polymerization. Furthermore, the above-mentioned requirement for the neutralization rate also applies to water-absorbing resins.

[0205] Here, the neutralization rate of polyacrylic acid (salt) resin refers to the ratio of the number of moles of partially neutralized acid groups in the polyacrylic acid (salt) resin to the total number of moles of acid groups in the polyacrylic acid (salt) resin.

[0206] (Internal cross-linking agent)

[0207] The internal crosslinking agent used in this process and its dosage have been explained in the previous section on [Polyacrylic Acid (Salt) Based Water Absorbent Resin], and will not be repeated here.

[0208] In one embodiment of the present invention, the timing of adding the internal crosslinking agent should be such that uniform crosslinking of the polymer can be achieved. Examples include adding the internal crosslinking agent to the monomer aqueous solution before polymerization, or to the aqueous gel polymer during or after polymerization. Preferably, the method of adding a predetermined amount of internal crosslinking agent to the monomer aqueous solution beforehand is preferred.

[0209] (Substances added to the monomer aqueous solution)

[0210] In one embodiment of the present invention, when preparing the monomer aqueous solution, the following substance may be added to the monomer aqueous solution, the solution in the reaction, or the solution after the reaction at any stage during the polymerization reaction and the crosslinking reaction, or at any stage after the polymerization reaction and the crosslinking reaction, in order to improve the physical properties of the water-absorbing resin.

[0211] Examples of such substances include: starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (PVA), polyacrylic acid (salt), polyacrylic acid (salt) crosslinks, and other hydrophilic polymers; carbonates, azo compounds, various foaming agents that generate bubbles, surfactants, chelating agents, chain transfer agents, and other compounds.

[0212] The amount of the hydrophilic polymer added relative to the monomer aqueous solution is preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, particularly preferably 5% by mass or less, and preferably 0% by mass or more, even more preferably more than 0% by mass. Furthermore, the amount of the compound added relative to the monomer aqueous solution is preferably 5% by mass or less, more preferably 1% by mass or less, even more preferably 0.5% by mass or less, and preferably 0% by mass or more, even more preferably more than 0% by mass.

[0213] When a water-soluble resin or a water-absorbing resin is used as the hydrophilic polymer, a grafted polymer or a water-absorbing resin can be obtained, such as a starch-acrylate (salt) copolymer, a PVA-acrylate (salt) copolymer, etc. These grafted polymers or water-absorbing resins are also included in the category of polyacrylate (salt) based water-absorbing resins.

[0214] (Concentration of monomer composition)

[0215] According to the purpose, various monomers, internal crosslinking agents, other substances and components (hereinafter referred to as "monomer components") are selected, and their amounts are specified in a manner that satisfies the aforementioned range. They are then mixed together to prepare a mixture of monomer components (monomer composition). This mixture is then added to water to obtain an aqueous solution of the monomer composition (also called a monomer aqueous solution). Here, in the first embodiment of the present invention, in addition to preparing the monomer into an aqueous solution, a mixed solution of the monomer, water and a hydrophilic solvent can also be prepared.

[0216] Furthermore, from the viewpoint of the physical properties of water-absorbing resins, the total concentration of the monomer composition is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, and preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. This monomer composition concentration can be calculated using the following formula (A);

[0217] Monomer composition concentration (mass %) = [(mass of monomer composition) / (mass of monomer aqueous solution)] × 100 Equation (A)

[0218] Here, in formula (A), "mass of monomer aqueous solution" does not include the mass of grafted components, the mass of water-absorbing resin, or the mass of hydrophobic organic solvent used in reverse suspension polymerization.

[0219] [3-1-4-2] Polymerization process

[0220] This process involves polymerizing an aqueous monomer solution to obtain a hydrogel-like polymer (hydrogel-like crosslinked polymer). Preferably, the process involves polymerizing an aqueous monomer solution containing a monomer primarily composed of acrylic acid (salt) and at least one polymerizable internal crosslinking agent, obtained from the preparation process of the aforementioned aqueous monomer solution, to obtain a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel").

[0221] (Polymerization initiator)

[0222] Regarding the polymerization initiator used in one embodiment of the present invention, one or more polymerization initiators commonly used in the manufacture of water-absorbing resins can be selected, depending on the type of polymerizable monomer and polymerization conditions. Examples of polymerization initiators include thermally decomposable initiators and photodecomposable initiators.

[0223] Examples of thermally decomposable initiators include: persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, tert-butyl peroxide, and methyl ethyl ketone peroxide; azo compounds such as azonitrile compounds, azomididine compounds, cyclic azomididine compounds, azoamide compounds, alkyl azo compounds, 2,2'-azobis(2-amidinylpropane) dihydrochloride, and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride; and so on.

[0224] Examples of photodegradation initiators include benzoin derivatives, benzoyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds.

[0225] Among these, persulfates are preferred considering cost and the ability to suppress the formation of residual monomers. Furthermore, a reducing agent that promotes the decomposition of the persulfate or peroxide, or other oxidizing polymerization initiator, can be used to combine the oxidizing polymerization initiator and the reducing agent to obtain a redox system initiator. Examples of reducing agents include: sodium sulfite, sodium bisulfite, and other (bi)sulfites; reducing metals such as L-ascorbic acid and ferrous salts; and amines.

[0226] The amount of the polymerization initiator relative to the monomer composition excluding the internal crosslinking agent is preferably 0.001 mol% or more, more preferably 0.010 mol% or more, and preferably 1.000 mol% or less, more preferably 0.500 mol% or less, and even more preferably 0.100 mol% or less. Furthermore, the amount of the reducing agent relative to the monomer excluding the internal crosslinking agent is preferably 0.0001 mol% or more, more preferably 0.0005 mol% or more, and preferably 0.0200 mol% or less, more preferably 0.0150 mol% or less. By using amounts within this range, a water-absorbing resin with the desired water-absorbing properties can be obtained.

[0227] In another embodiment of the present invention, the polymerization reaction can also be initiated by irradiation with active energy rays such as radiation, electron beams, or ultraviolet light. Furthermore, irradiation with active energy rays can be used in conjunction with the polymerization initiator.

[0228] (Aggregation Form)

[0229] Examples of polymerization methods applicable to one embodiment of the present invention include aqueous solution polymerization, reverse suspension polymerization, spray polymerization, droplet polymerization, mass polymerization, and precipitation polymerization. Among these, from the viewpoint of ease of control of polymerization and the water absorption properties of the water-absorbing resin, aqueous solution polymerization or reverse suspension polymerization is preferred, and aqueous solution polymerization is more preferred. Aqueous solution polymerization is disclosed in Japanese Patent Application Publication No. 4-255701, etc. Reverse suspension polymerization is disclosed in International Publication Nos. 2007 / 004529 and 2012 / 023433, etc.

[0230] Preferred forms of aqueous solution polymerization include high-temperature initiation polymerization, high-concentration polymerization, and foaming polymerization. "High-temperature initiation polymerization" refers to a polymerization form where the temperature of the monomer aqueous solution at the start of polymerization is preferably 35°C or higher, more preferably 40°C or higher, even more preferably 45°C or higher, particularly preferably 50°C or higher, and preferably below the boiling point of the monomer aqueous solution. "High-concentration polymerization" refers to a polymerization form where the monomer concentration at the start of polymerization is preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, particularly preferably 45% by mass or higher, and preferably below the saturation concentration of the monomer aqueous solution. "Foaming polymerization" refers to a polymerization form in which the monomer aqueous solution containing a foaming agent or bubbles is polymerized. These polymerization forms can be carried out individually or in combination of two or more. Furthermore, the polymerization of the aqueous solution can be either batch or continuous, but from the viewpoint of production efficiency, continuous polymerization is preferred.

[0231] Furthermore, examples of continuous aqueous solution polymerization include continuous belt polymerization disclosed in U.S. Patent Nos. 4,893,999, 6,906,159, 7,091,253, 7,741,400, 8,519,212, and Japanese Patent Application Publication No. 2005-36,100, as well as kneader-based continuous polymerization disclosed in U.S. Patent No. 6,987,151.

[0232] Methods for dispersing bubbles used in the foaming polymerization include: reducing the solubility of the gas dissolved in the monomer aqueous solution to disperse the gas as bubbles; introducing gas from the outside to disperse the gas as bubbles; and adding a foaming agent to the monomer aqueous solution to cause foaming. Furthermore, these dispersion methods can be appropriately combined and implemented according to the target properties of the water-absorbing resin.

[0233] If the gas is introduced from the outside, examples of such gas include oxygen, air, nitrogen, carbon dioxide, ozone, and mixtures thereof. From the viewpoints of polymerizability and cost, inert gases such as nitrogen and carbon dioxide are preferred, and nitrogen is even more preferred.

[0234] Examples of suitable foaming agents include azo compounds, organic or inorganic carbonate solutions, dispersions, and powders with particle sizes ranging from 0.1 μm to 1000.0 μm. Among these, inorganic carbonates are preferred, specifically sodium carbonate, ammonium carbonate, magnesium carbonate, and bicarbonates.

[0235] By gel pulverizing the foamed hydrogel obtained through foaming polymerization, it becomes easier to dry. Furthermore, by polymerizing into a foamed water-absorbing resin, the water absorption rate of the resin can be increased. The pores on the surface of the water-absorbing resin, for example, pores with a diameter of 1 μm or more and 100 μm or less, can be examined using an electron microscope to determine whether it is foamed. The average number of pores per particle of water-absorbing resin is preferably 1 or more, more preferably 10 or more, and preferably 10,000 or less, more preferably 1,000 or less. The number of pores can be controlled through the foaming polymerization process.

[0236] [3-1-4-3] Gel pulverization process

[0237] This step is a process of pulverizing the hydrogel during and / or after the polymerization process. Specifically, the hydrogel can be pulverized during or after the polymerization process. That is, this step is a process of gel pulverizing the hydrogel to obtain a granular hydrogel crosslinked polymer (hereinafter referred to as "granular hydrogel"). To distinguish it from the "pulverization" in the pulverization process described later, this step is referred to as "gel pulverization". In addition, the object of gel pulverization is not only the hydrogel obtained in the polymerization process, but also, unless otherwise specified, the object includes the granulated gel obtained after reuse as described later. Unless otherwise specified, the same applies to gels in other processes.

[0238] The gel pulverization refers to the process of adjusting hydrogels to a specified size using a kneader, meat grinder, spiral extruder, or gel pulverizer.

[0239] When pulverizing hydrogels, it is preferable to add warm water to the gel pulverizer. Adding warm water results in granular hydrogels with low adhesion and good aeration, making them easier to dry, and is therefore preferred. The temperature of the warm water is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, and most preferably 100°C or lower.

[0240] Regarding the implementation method and operating conditions of gel pulverization, if the aqueous solution polymerization is used, the formulation method described in the literature disclosing continuous aqueous solution polymerization can be employed. Furthermore, the content described in International Publication No. 2011 / 126079 is also well applicable to the first embodiment of the present invention. Here, if the polymerization mode is kneader polymerization, this is equivalent to performing the polymerization step and the gel pulverization step simultaneously. Additionally, in the first embodiment of the present invention, the gel pulverization step yields a randomly fragmented, water-absorbing resin.

[0241] Furthermore, in the method for manufacturing the water-absorbing resin according to one embodiment of the present invention, the micron powder reuse step more preferably includes: a granulation step, in which the removed micron powder is mixed with an aqueous liquid to obtain a granulated gel; and a granulated gel addition step, in which the granulated gel is added to the hydrogel in at least one step and / or between steps during the process from the end of the gel pulverization step to the completion of the drying step. In addition, in the gel pulverization step of the first embodiment of the present invention, it is more preferable to appropriately control the gel pulverization energy. Regarding the granular hydrogel obtained by gel pulverization using the specified gel pulverization energy described later, even when the mixture of the granular hydrogel and the granulated gel is dried using a ventilated belt dryer in a lamination manner, the mixture is less prone to dense lamination. Therefore, compared to the case of using granular hydrogel obtained by gel pulverization under normal conditions, drying can be completed in a very short time. Furthermore, the granular hydrogel is readily compatible with the granulated gel described later, thus facilitating uniform drying. Furthermore, even from the perspective of the physical properties of the obtained superabsorbent resin, its water absorption rate is highly rated. This can be evaluated, for example, by the FSR described in International Publication No. 2009 / 016055 and the Vortex described in JIS K 7224 (1996) "Test method for water absorption rate of superabsorbent resins".

[0242] Here, in one embodiment of the present invention, "gel grinding energy" refers to the mechanical energy required by the gel grinding device relative to each unit mass, i.e., the unit mass of the hydrogel, when gel grinding a hydrogel, but excluding the energy for heating and cooling the outer casing and the energy of the introduced water and steam. Here, based on the English spelling "Gel Grinding Energy", "gel grinding energy" is abbreviated as "GGE".

[0243] If the gel pulverizer is driven by three-phase alternating current, then GGE can be calculated using the following formula (I).

[0244] GGE [J / g] = {√3 × voltage × current × power factor × motor efficiency} / {mass of hydrous gel fed into the gel pulverizer per second} ... Equation (I)

[0245] The aforementioned "power factor" and "motor efficiency" are inherent values ​​of the device, varying depending on the operating conditions of the gel pulverizer, and range from 0 to 1. These values ​​can be obtained from the device manufacturer. Furthermore, if the gel pulverizer is driven by single-phase AC power, GGE can be calculated by simply replacing "√3" with "1" in the above formula (I). Here, the unit of voltage is "[V]", the unit of current is "[A]", and the unit for the mass of the hydrous gel is "[g / s]".

[0246] Regarding the "power factor" and "motor efficiency" in the GGE, the values ​​are taken during gel pulverization. Since the current value during no-load operation is sometimes low, the power factor and motor efficiency during no-load operation are appropriately defined using equation (I). The "mass of hydrogel fed into the gel pulverizer per second" [g / s] in equation (I) is, for example, a value converted from the unit of the supply of hydrogel continuously supplied via a metering feeder to "[g / s]". However, as will be discussed later, the hydrogel may sometimes also contain granulated gel obtained through reuse.

[0247] In one embodiment of the present invention, the gel pulverization energy (GGE) for gel pulverization is preferably 100 J / g or less, more preferably 80 J / g or less, even more preferably 60 J / g or less, and more preferably 20 J / g or more, even more preferably 25 J / g or more, and even more preferably 30 J / g or more. By controlling the gel pulverization energy within the above range, gel pulverization can be performed while applying appropriate shear and compressive forces to the hydrogel.

[0248] Here, if a screw extruder or multiple screw extruders are used after the gel is polymerized in the kneader, that is, if multiple devices are used for gel pulverization, the total energy consumed by each device is the gel pulverization energy (GGE).

[0249] Furthermore, by controlling the gel pulverization energy as described above, even better results can be obtained by adding warm water at the aforementioned temperature in combination. Alternatively, gel pulverization based on the aforementioned gel pulverization energy can be performed after performing conventional gel pulverization.

[0250] From the viewpoint of ease of drying and the resulting water-absorbing resin properties, the particle size of the finely granulated hydrogel obtained through the gel pulverization process is preferably in the range of 0.1 mm to 10.0 mm. Furthermore, the mass-average particle size (D50) of the granular hydrogel is 0.1 mm to 5.0 mm, more preferably 0.1 mm to 2.0 mm. When the mass-average particle size (D50) of the granular hydrogel is within the above range, drying can be sufficiently achieved. In the first embodiment of the present invention, the mass-average particle size of the hydrogel provided to the drying process is preferably within the above range, and more preferably both the particle size and the mass-average particle size satisfy the above range.

[0251] Regarding the particle size of the granular hydrogel, the logarithmic standard deviation (σζ) of its particle size distribution is preferably 0.2 to 1.5, more preferably 0.2 to 1.3, and even more preferably 0.2 to 1.2. The logarithmic standard deviation (σζ) of the particle size distribution reflects the narrowness of the particle size distribution; the smaller the value, the more uniform the particle size, thus providing the advantage of uniform drying. However, to achieve a logarithmic standard deviation (σζ) of less than 0.2, special operations are required, such as particle size control during polymerization before gel pulverization, or grading of the granular hydrogel after pulverization. However, from a production and cost perspective, this is practically difficult to achieve.

[0252] Here, in order to increase the specific surface area of ​​the water-absorbing resin described later, the gel pulverization method described in International Publication No. 2011 / 126079 is preferred. Alternatively, this gel pulverization method can be used in combination with the aforementioned foaming polymerization.

[0253] In addition, in order to achieve uniform and efficient drying, the water content of the granular hydrogel is preferably 30% by mass or more, more preferably 45% by mass or more, and preferably 70% by mass or less, more preferably 55% by mass or less.

[0254] [3-1-4-4] Drying process

[0255] This process involves drying the pulverized hydrogel. Specifically, this process includes drying the granular hydrogel to achieve a desired solid content, thereby obtaining a dried polymer; or, if a granulating gel is added to the granular hydrogel, drying both the granulating gel and the granular hydrogel to achieve a desired solid content, thereby obtaining a dried polymer. The value of this solid content, i.e., the value obtained by subtracting the water content from 100% by mass of the gel, is preferably 80% by mass or more, more preferably 85% by mass or more, more preferably 90% by mass or more, particularly preferably 92% by mass or more, and preferably 99% by mass or less, more preferably 98% by mass or less, and particularly preferably 97% by mass or less. When the solid content of the dried polymer is within the above range, pulverization, grading, and surface crosslinking can be performed efficiently. Here, "drying complete" in this specification refers to a state where the solid content has reached 80% by mass. In this process, the dried polymer may sometimes be in a clumpy state, and the water content of the clumpy may differ between the top and bottom, and between the center and the edges. In this case, dry polymer can be appropriately obtained from different locations, broken up as needed, and then the moisture content can be measured and the average value taken.

[0256] In this specification, dried polymers that do not meet the above-specified solid content are sometimes referred to as "undried material". The "material to be dried" or "particulate hydrogel" in the drying process sometimes includes both particulate hydrogels and granulated gels. Furthermore, the drying process in the first embodiment of the present invention is particularly effective for cases containing both particulate hydrogels and granulated gels. Similarly, in other processes, hydrogels and their processed products may include granulated gels and their processed products.

[0257] Drying methods in the drying process include, for example, heating drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, azeotropic dehydration drying with hydrophobic organic solvents, high-humidity drying using high-temperature steam, and stirring drying. From the viewpoint of drying efficiency, stirring drying and hot air drying are preferred. For stirring drying, paddle dryers or rotary drum dryers are preferred. For hot air drying, batch-type air-purifying dryers or air-purifying belt dryers are preferred for hot air drying on their air-purifying conveyor belts. Using air-purifying belt dryers prevents mechanical damage and friction to the dried polymer and particulate hydrogels during the drying process, thus avoiding the formation of fine powders, and achieves efficient drying.

[0258] From the perspective of drying efficiency, the drying temperature, i.e., the hot air temperature, in one embodiment of the present invention is preferably 120°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher, and preferably 250°C or lower, more preferably 230°C or lower, and even more preferably 200°C or lower. Furthermore, the drying time is selected to be 10 minutes to 120 minutes, more preferably 20 minutes to 90 minutes, and even more preferably 30 minutes to 60 minutes. When the drying temperature and drying time are within this range, the physical properties of the obtained water-absorbing resin are within the expected range. Regarding other drying conditions, they can be appropriately set according to the water content, total mass, and target solid content of the granular hydrogel and granulated gel to be dried. When performing belt drying, the conditions described in International Publication No. 2006 / 100300, International Publication No. 2011 / 025012, International Publication No. 2011 / 025013, and International Publication No. 2011 / 111657 can be used as appropriate.

[0259] [3-1-4-5] Crushing process, grading process

[0260] The pulverizing process is the process of pulverizing the dried polymer, and the grading process is the process of removing microparticles from the pulverized polymer. Specifically, the dried polymer obtained through the drying process is pulverized in the pulverizing process and adjusted to a particle size within a desired range in the grading process to obtain a cross-linked polymer. By performing the pulverizing process after drying, granular cross-linked polymers (hereinafter also referred to as cross-linked polymers) can be obtained.

[0261] Examples of pulverizers used in the pulverizing process include high-speed rotary pulverizers such as roller mills, hammer mills, spiral mills, and pin mills, vibratory mills, toggle mills, and cylindrical agitators. From the viewpoint of pulverizing efficiency, roller mills are preferred. Furthermore, multiple pulverizers can be used in combination.

[0262] Examples of particle size adjustment methods in the grading process include sieve grading using a JIS standard sieve (JIS Z8801-1(2000)) or air classifying. From the viewpoint of grading efficiency, sieve grading is preferred. However, from the viewpoint of ease of pulverization, the grading process can also be performed additionally before the pulverizing process.

[0263] Regarding the particle size distribution of the crosslinked polymer, the mass-average particle size (D50) of the crosslinked polymer is preferably 300 μm or more and 600 μm or less, and the proportion of particles smaller than 150 μm is 5% by mass or less. More preferably, the upper limit of the mass-average particle size (D50) is 500 μm, further preferably 450 μm, and especially preferably 400 μm. In addition, the proportion of particles smaller than 150 μm is more preferably 4% by mass or less, further preferably 3% by mass or less, and especially preferably 2% by mass or less. Furthermore, the logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 or more, more preferably 0.25 or more, further preferably 0.27 or more, and preferably 0.50 or less, more preferably 0.45 or less, further preferably 0.43 or less, especially preferably 0.40 or less, and most preferably 0.35 or less. The logarithmic standard deviation (σζ) of the particle size distribution expresses the narrowness of the particle size distribution. The smaller the value, the more uniform the particle size, which means less particle distribution bias. Preferably, the mass-average particle size (D50) and the proportion of particles smaller than 150 μm both satisfy the above range. More preferably, the mass-average particle size (D50), the proportion of particles smaller than 150 μm, and the logarithmic standard deviation all satisfy the above range. Appropriate combinations of the above ranges can be made.

[0264] Here, the mass-average particle size (D50) and the logarithmic standard deviation (σζ) can be determined according to the determination method described in "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" in U.S. Patent No. 7,638,570.

[0265] The aforementioned particle size requirements also apply to the base material absorbent resin after the pulverization and grading processes. Therefore, if surface crosslinking is performed, it is preferable to perform surface crosslinking treatment in a manner that maintains the particle size within the aforementioned range obtained by adjusting the crosslinked polymer; more preferably, a granulation process is performed after the surface crosslinking process to adjust the particle size. Therefore, the absorbent resin of one embodiment of the present invention preferably satisfies the following: the mass-average particle size (D50) and the proportion of particles smaller than 150 μm are within the aforementioned range. More preferably, the mass-average particle size (D50), the proportion of particles smaller than 150 μm, and the logarithmic standard deviation (σζ) of the particle size distribution are within the aforementioned range. Furthermore, it is preferable that the mass-average particle size (D50) of the absorbent resin of one embodiment of the present invention is 300–600 μm, the proportion of particles smaller than 150 μm is 5% by mass or less, and the logarithmic standard deviation (σζ) of its particle size distribution is 0.20–0.50.

[0266] [3-1-4-6] Surface crosslinking process

[0267] This process involves further forming a higher crosslinking density portion on the surface layer of the crosslinked polymer obtained through the aforementioned processes, as needed. This process includes mixing, heat treatment, and cooling. In this surface crosslinking process, free radical crosslinking, surface polymerization, and crosslinking reactions with a surface crosslinking agent occur on the surface of the crosslinked polymer, thereby obtaining a base water-absorbing resin.

[0268] The maximum temperature reached by the crosslinked polymer during the surface crosslinking process (powder temperature), which is also the maximum temperature reached by the crosslinked polymer during the heat treatment process (powder temperature), is preferably above 180°C, and more preferably above 190°C.

[0269] [3-1-4-6-1] Mixing process

[0270] This process involves mixing a solution containing a surface crosslinking agent (hereinafter referred to as "surface crosslinking agent solution") with a crosslinking polymer in a mixing device to obtain a base water-absorbing resin.

[0271] (Surface crosslinking agent)

[0272] In the first embodiment of the present invention, a surface crosslinking agent is used during surface crosslinking. This surface crosslinking agent has been described in the aforementioned section on "[Polyacrylic (salt) based water-absorbing resin]".

[0273] The amount of the surface crosslinking agent, or the total amount of multiple surface crosslinking agents used, is preferably 0.01 to 10.00 parts by weight relative to 100 parts by weight of the crosslinked polymer, more preferably 0.01 to 5.00 parts by weight, and even more preferably 0.01 to 2.00 parts by weight. When the amount of surface crosslinking agent is within this range, an optimal crosslinking structure can be formed on the surface layer of the crosslinked polymer, thereby obtaining a highly absorbent resin with excellent physical properties.

[0274] The surface crosslinking agent is preferably added to the crosslinking polymer in the form of an aqueous solution. In this case, the amount of water is preferably 0.1 to 20.0 parts by weight, more preferably 0.3 to 15.0 parts by weight, and even more preferably 0.5 to 10 parts by weight, relative to 100 parts by weight of the crosslinking polymer. Within this range, the operability of the surface crosslinking agent solution is improved, and the surface crosslinking agent is uniformly mixed into the crosslinking polymer.

[0275] Additionally, if necessary, a hydrophilic organic solvent can be used in conjunction with the water to form the surface crosslinking agent solution. In this case, the amount of hydrophilic organic solvent used is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less, relative to 100 parts by mass of the crosslinking polymer. Examples of such hydrophilic organic solvents include: lower alcohols such as methanol; ketones such as acetone; ethers such as dioxane; amides such as N,N-dimethylformamide; sulfones such as dimethyl sulfoxide; and polyols such as ethylene glycol. However, when using these hydrophilic organic solvents, it is preferable to limit the amount used to as low as possible.

[0276] Alternatively, the various additives described in the “[3-1-4-7] Additives and their addition process” section can be added to the surface crosslinking agent solution in a range of less than 5 parts by weight, or added separately through a mixing process.

[0277] (Mixing methods, mixing conditions)

[0278] Regarding the mixing of the crosslinking polymer and the surface crosslinking agent solution, the following methods can be selected: prepare the surface crosslinking agent solution in advance, preferably spray or drip the solution onto the crosslinking polymer, more preferably spray the solution, thereby mixing.

[0279] The mixing apparatus used for the mixing preferably has the torque required to uniformly and reliably mix the crosslinked polymer and the surface crosslinking agent. This mixing apparatus is preferably a high-speed stirring mixer, more preferably a high-speed stirring continuous mixer. The rotational speed of the high-speed stirring mixer is preferably 100 rpm or more, more preferably 300 rpm or more, and preferably 10,000 rpm or less, more preferably 2,000 rpm or less.

[0280] From the viewpoint of miscibility with the surface crosslinking agent solution and cohesiveness of the humidified mixture, the temperature of the crosslinking polymer supplied to this process is 35°C to 80°C, more preferably 35°C to 70°C, and even more preferably 35°C to 60°C. Furthermore, the mixing time is preferably 1 second or more, more preferably 5 seconds or more, and preferably 1 hour or less, more preferably 10 minutes or less.

[0281] [3-1-4-6-2] Heat treatment process

[0282] This step involves heating the base water-absorbing resin obtained in the mixing step to induce a cross-linking reaction on the surface of the cross-linked polymer. Regarding the heat treatment of the base water-absorbing resin, it can be heated in a static state or heated in a flowing state using dynamic forces such as stirring. However, from the perspective of ensuring uniform heating of the humidified mixture as a whole, heating under stirring is preferred. From the above perspective, examples of heat treatment apparatus for performing this heat treatment include paddle dryers, multi-blade agitators, and tower dryers.

[0283] From the perspective of the type and amount of surface crosslinking agent and the water absorption performance of the water-absorbing resin, the heating temperature in this process is preferably 150℃~250℃, more preferably 170℃~250℃, even more preferably 170℃~230℃, and even more preferably 180℃~230℃. Furthermore, the heating time is preferably at least 5 minutes, more preferably at least 7 minutes. By controlling the heating temperature and heating time within the above ranges, the water absorption performance of the obtained water-absorbing resin can be improved, and therefore this is preferable.

[0284] [3-1-4-6-3] Cooling process

[0285] This step is an optional step that is added as needed after the heat treatment step and / or the drying step. This step involves forcibly cooling the high-temperature water-absorbing resin after the heat treatment step to a specified temperature to rapidly stop the surface crosslinking reaction.

[0286] Regarding the cooling of the absorbent resin, cooling can be performed in a static state or in a flowing state using dynamic methods such as stirring. However, from the perspective of achieving uniform cooling of the absorbent resin as a whole, cooling under stirring is preferred. From this viewpoint, examples of cooling devices for performing this cooling include paddle dryers, multi-blade agitators, and tower dryers. These cooling devices can be of the same specifications as the heat treatment equipment used in the heat treatment process. By replacing the heat transfer medium in the heat treatment equipment with a refrigerant, it can be used as a cooling device.

[0287] The cooling temperature in this process can be appropriately set according to the heating temperature in the heat treatment process and the water absorption performance of the water-absorbing resin, preferably 40℃~100℃, more preferably 50℃~90℃, and even more preferably 50℃~70℃ or below.

[0288] [3-1-4-7] Additives and their addition process

[0289] [3-1-4-7-1] Surface Modifier

[0290] Surface modifiers are additives added to modify the surface of absorbent resin particles. Specifically, examples include permeability improvers, anti-caking agents under hygroscopic conditions, powder flow control agents, and binders for absorbent resins. Especially from the viewpoint of improving permeability, at least one compound selected from polyvalent metal salts, cationic polymers, and inorganic microparticles can be used, and two or more can be used in combination as needed. The amount of surface modifier added can be appropriately set according to the selected compound. To achieve the purpose of modifying the surface of absorbent resin particles, the surface modifier addition step is preferably performed after the polymerization step, more preferably after the drying step, and even more preferably after the surface crosslinking step. Furthermore, the surface modifier can be added in any one or more steps.

[0291] (Polyvalent metal salts)

[0292] If a polyvalent metal salt is used, the polyvalent metal cation of the salt is preferably divalent or higher, more preferably divalent or higher, and preferably tetravalent or lower, and even more preferably trivalent or tetravalent. Examples of usable polyvalent metals include aluminum and zirconium. Therefore, examples of polyvalent metal salts usable in this process include aluminum lactate, zirconium lactate, aluminum sulfate, and zirconium sulfate. From the viewpoint of improving the surface conductivity (SFC) of brine, aluminum lactate or aluminum sulfate is more preferred, and aluminum sulfate is even more preferred.

[0293] The amount of the polyvalent metal salt added relative to 1g of the absorbent resin is preferably 0 mol or more, and more preferably less than 5.0 × 10⁻⁶. －5 Moles, more preferably less than 4.0 × 10－5 Moles, and preferably less than 3.0 × 10 －5 Moore.

[0294] In addition, the solution containing the polyvalent metal may also contain monovalent metal compounds such as sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, and sodium lactate, as modifiers to adjust the permeability of the polyvalent metal into the absorbent resin.

[0295] (Catonic polymer)

[0296] If a cationic polymer is used, the substance described in U.S. Patent No. 7,098,284 can be cited as such a cationic polymer. Among these, a vinylamine polymer is more preferred from the viewpoint of improving permeability. Furthermore, the mass-average molecular weight of the cationic polymer is preferably 5,000 or more and 1,000,000 or less.

[0297] The amount of the cationic polymer added is preferably 0 parts by weight or more relative to 100 parts by weight of the water-absorbing resin, and more preferably less than 5.0 parts by weight, more preferably less than 4.0 parts by weight, and even more preferably less than 3.0 parts by weight.

[0298] (Inorganic particles)

[0299] If inorganic particles are used, the substances described in U.S. Patent No. 7,638,570 can be cited as examples of inorganic particles. Among these, silica is preferred from the viewpoint of improving permeability.

[0300] The primary particle size of the inorganic microparticles is preferably less than 100 nm, more preferably less than 80 nm, and even more preferably 50 nm. Furthermore, the inorganic microparticles can be in the form of powder or a suspension. The amount added relative to 100 parts by weight of the absorbent resin is preferably 0 parts by weight or more, and preferably less than 5.0 parts by weight, more preferably less than 4.0 parts by weight, and even more preferably less than 3.0 parts by weight. Here, if the inorganic microparticles are added to the absorbent resin in the form of a suspension, the amount of inorganic microparticles added is calculated based on the amount of solid component of the inorganic microparticles in the suspension.

[0301] [3-1-4-7-2] Other additives

[0302] Other additives include chelating agents, hydroxycarboxylic acid compounds, compounds containing phosphorus atoms, oxidizing agents, organic powders such as metal soaps, deodorants, antibacterial agents, pulp and thermoplastic fibers, aromatic substances such as terpene aromatic compounds and phenolic aromatic compounds, and one or more of these can be used. Chelating agents are preferred, and amino polycarboxylic acids or amino polyphosphates are more preferred. Specific examples include Japanese Patent Application Publication No. 11-060975, International Publication No. 2007 / 004529, International Publication No. 2011 / 126079, International Publication No. 2012 / 023433, Japanese Patent Application Publication No. 2009-509722, and Japanese Patent Application Publication No. 2005- Chelating agents described in Japanese Patent Application Publication No. 097519, Japanese Patent Application Publication No. 2011-074401, Japanese Patent Application Publication No. 2013-076073, Japanese Patent Application Publication No. 2013-213083, Japanese Patent Application Publication No. 59-105448, Japanese Patent Application Publication No. 60-158861, Japanese Patent Application Publication No. 11-241030, and Japanese Patent Application Publication No. 2-41155.

[0303] The amount or content of other additives (preferably chelating agents) is preferably in the range of 0.001% by mass to 1.000% by mass relative to the monomer or water-absorbing resin.

[0304] The additive may be added before, during, or in the course of at least one of the aforementioned steps: monomer aqueous solution preparation, polymerization, gel pulverization, drying, pulverization, classification, and surface crosslinking. Preferably, it is added before, during, or in the course of any step following the polymerization step.

[0305] [3-1-4-7-3] Additive Addition Process

[0306] If the additive is added to the absorbent resin and the additive is in the form of a liquid or a solution containing an aqueous medium such as water, it is preferable to spray the liquid or solution onto the absorbent resin and apply sufficient torque to uniformly and thoroughly mix the absorbent resin and the additive. On the other hand, if the additive is a solid such as a powder, it can be dry-mixed with the absorbent resin, and an aqueous liquid such as water can be used as a binder.

[0307] Specifically, examples of apparatus used for the mixing include stirred mixers, cylindrical mixers, double-walled conical mixers, V-shaped mixers, bowtie mixers, spiral mixers, flow-type swirl mixers, airflow mixers, double-arm kneaders, internal mixers, pulverizing kneaders, rotary mixers, and spiral extruders. When using a stirred mixer, its rotational speed is preferably 5 rpm or more, more preferably 10 rpm or more, and preferably 10,000 rpm or less, more preferably 2,000 rpm or less.

[0308] [3-1-4-8] Granulation process

[0309] In one embodiment of the present invention, in addition to the aforementioned steps, a granulation step may be performed as needed. The granulation step refers to: performing an aqueous liquid addition step and a drying step on the surface-crosslinked absorbent resin obtained from the surface crosslinking step, and then adjusting its particle size to a desired range to obtain a water-absorbing resin, i.e., a water-absorbing agent, in a factory-ready state as the final product. However, if there is no pulverization or grading step before the surface crosslinking step, the subsequent operations after the surface crosslinking step are performed as pulverization and grading steps. The particle size adjustment method in the granulation step can be the same as that used in the grading step. Furthermore, if the absorbent resin agglomerates during the surface crosslinking step and the surface modifier addition step, it can also be broken down, for example, by light pulverization. Additionally, the particle size distribution after particle size adjustment can be appropriately designed according to the application, preferably to the same degree as that used in the grading step. Therefore, grading can be performed using sieves or the like to satisfy the desired mass-average particle size (D50), the proportion of that mass-average particle size (D50), and the logarithmic standard deviation.

[0310] [3-1-4-9] Micro powder reuse process

[0311] This process involves recycling the micropowder removed in the grading process to the stage before the drying process is completed. Specifically, it involves recycling the micropowder obtained in the manufacturing process of the superabsorbent resin to the preparation process, preferably to the stage before the drying process, thereby manufacturing superabsorbent resin. The recycled micropowder is preferably the micropowder removed in the grading process, and more preferably the micropowder removed in the grading process and the granulation process, etc. Here, the micropowder does not necessarily have to be recycled to a superabsorbent resin manufacturing process that is exactly the same as the superabsorbent resin manufacturing process that produced the micropowder; it can also be recycled to other superabsorbent resin manufacturing processes whose differences do not affect the main idea of ​​the first embodiment of the present invention. For example, the micropowder generated on one production line can be recycled to an adjacent production line, or the polymerization conditions can be changed during the process between the micropowder removal treatment and the micropowder recycling treatment on the same production line.

[0312] [3-1-4-9-1] Granulation process

[0313] This process involves mixing the removed micropowder with an aqueous solution to obtain a granulated gel. A granulated gel is defined as a gel in which, under an optical microscope, individual particles aggregate, condense, or fuse into larger granular gels, and preferably possesses strength that will not be damaged by grading or handling operations.

[0314] (Micro powder)

[0315] In the first embodiment of the present invention, all the micropowder obtained during the manufacturing process of the water-absorbing resin is considered, preferably the micropowder removed in the grading process, more preferably the micropowder removed in both the grading and granulation processes, and an aqueous liquid is added to this micropowder for granulation. The mixing ratio (mass ratio) of the micropowder removed in the grading process to the micropowder removed in the granulation process is preferably 99:1 to 50:50, more preferably 98:2 to 60:40, and even more preferably 95:5 to 70:30. Since the micropowder removed in the granulation process has already undergone the surface crosslinking process, and sometimes not only the surface crosslinking process but also the surface modifier addition process described in the "Surface Modifier" section above, it is advantageous to reduce the cohesiveness of the granulated gel by using the micropowder removed in the granulation process at a predetermined ratio. In addition, in the first embodiment of the present invention, for example, the micropowder obtained from the removal of filter bags or the like in each manufacturing process can be used for granulation. Furthermore, micropowder obtained from removal in different processes and micropowder obtained from removal in other manufacturing processes (other manufacturing apparatus) can be used in combination. Additionally, the micropowder and the hydrogel dried together can be of the same composition or different compositions, but preferably, the micropowder used is derived from the hydrogel dried together and has the same composition as the hydrogel.

[0316] The size of the microparticles supplied for granulation is preferably smaller than the final product size of the water-absorbing resin. For example, the mass-average particle size (D50) of the microparticles, as defined by JIS standard sieve grading, is preferably 150 μm or less, more preferably 106 μm or less. The lower limit of the mass-average particle size (D50) of the microparticles is preferably 38 μm, more preferably 45 μm. Although this process targets microparticles, aggregates exceeding the final product size can also be appropriately pulverized and supplied as microparticles for granulation. Preferably, the content of particles with a particle size lower than 150 μm as defined by JIS standard sieve grading is preferably 50% to 100% by mass, more preferably 70% to 100% by mass, and even more preferably 90% to 100% by mass. In addition, from the perspective of granulation strength, the shape of the microparticles is more preferably random than that obtained by aqueous solution polymerization, compared to the spherical shape obtained by reverse suspension polymerization. In addition, as mentioned above, the micro powder can be micro powder removed after the surface crosslinking process usually performed in the manufacture of water-absorbing resin, micro powder removed before the surface crosslinking process, or a mixture thereof.

[0317] A granulated gel can be obtained by adding an aqueous solution to the micropowder, preferably to a mixture formed by mixing the micropowder in a predetermined ratio. Regarding the granulated gel, micropowders of various particle sizes obtained from the aforementioned single or multiple processes are used. If, during the granulation process, the mixing of the micropowder with the aqueous solution produces a large gel-like substance exceeding the aforementioned range, it is preferable to remove this large gel-like substance using a grading method such as a sieve. Alternatively, the removed large gel-like substance can be dried and pulverized for reuse as needed.

[0318] The temperature of the micronized powder when mixed with the aqueous solution is preferably 40°C to 120°C, more preferably 50°C to 100°C, and even more preferably 60°C to 90°C. Increasing the temperature of the micronized powder improves its miscibility with the aqueous solution, making it easier to obtain the desired granulation gel. However, if the temperature of the micronized powder is raised too high, the heating cost will increase. The temperature of the micronized powder can be appropriately adjusted as needed by methods such as: heating from the outside using hot air; maintaining the temperature after heating in the drying process; or cooling by blowing room temperature air. Preferably, the micronized powder is heated or maintained in a container equipped with a heating means such as a heated steam pipe.

[0319] (Aqueous liquid)

[0320] Regarding the aqueous solution used to mix with the micronized powder, specific examples include aqueous solutions containing the following components: water, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, and other lower alcohols; ketones such as acetone; ethers such as dioxane and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfones such as dimethyl sulfoxide. From the perspective of physical properties and granulation strength, the water content of the aqueous solution is preferably in the range of 90% to 100% by mass, more preferably in the range of 99% to 100% by mass, and particularly preferably composed solely of water. Furthermore, without affecting the effects of the first embodiment of the present invention, the aqueous solution may also contain small amounts of other additives such as crosslinking agents, chelating agents, surfactants, polymerization initiators, oxidants, reducing agents, and hydrophilic polymers. One or more additives may be added, regardless of their type when two or more are added. For example, by using an aqueous solution containing the polymerization initiator and reducing agent described in the polymerization process, the occurrence of residual monomers in the granulated gel and the hydrogel can be reduced. A preferred polymerization initiator is persulfate, and a preferred reducing agent is (bi)sulfite. For example, by using an aqueous solution containing an oxidizing agent, the decrease in physical properties such as absorption rate during the drying of the granulated gel can sometimes be suppressed. A preferred oxidizing agent is at least one selected from chlorite, hypochlorite, and peroxide, more preferably hydrogen peroxide. For example, by using an aqueous solution containing a surfactant, the granulated gel can contain a surfactant, thereby effectively suppressing the aggregation of the granulated gel. Furthermore, by using an aqueous solution containing a crosslinking agent and / or a hydrophilic polymer, the aggregation strength of the granulated gel can be improved, thereby suppressing re-micronization in subsequent processes. The crosslinking agent is selected from the aforementioned internal crosslinking agents and surface crosslinking agents, and the hydrophilic polymer is selected from the aforementioned hydrophilic polymers added to the monomer aqueous solution.

[0321] Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants.

[0322] Furthermore, if the micronized powder contains the aforementioned crosslinking agents, chelating agents, surfactants, polymerization initiators, oxidants, reducing agents, etc., then it is not necessary to specifically add additives to the aqueous solution, or only the missing additives may be added to the aqueous solution. The micronized powder is particularly preferably containing chelating agents, surfactants, oxidants, reducing agents, etc., as described in the "Additive Addition Process" section.

[0323] When granulating micronized powder by mixing it with an aqueous solution, it is preferable to use a preheated aqueous solution. Using a heated aqueous solution allows for uniform granulation of the micronized powder in a short time, thereby improving productivity. The temperature of the aqueous solution is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, particularly preferably 70°C or higher, and preferably below the boiling point of the aqueous solution, more preferably below 100°C. The boiling point of the aqueous solution can be adjusted by adding salts and solvents, and by applying pressure conditions such as depressurization and pressurization. Alternatively, the aforementioned temperature can be substantially achieved by simultaneously adding water vapor and the aqueous solution at room temperature.

[0324] The amount of aqueous solution added is preferably less than 100 parts by weight relative to 100 parts by weight of the micronized powder (in its original form), more preferably less than 80 parts by weight, even more preferably less than 50 parts by weight, and preferably more than 10 parts by weight, more preferably more than 15 parts by weight, and even more preferably more than 20 parts by weight. Adding less than 100 parts by weight of the aqueous solution helps to reduce the burden during drying. On the other hand, adding more than 10 parts by weight of the aqueous solution results in sufficient granulation strength and uniform mixing of the micronized powder, preventing damage to the granules.

[0325] (Mixing device)

[0326] In the first embodiment of the present invention, the mixing apparatus used to mix the aqueous liquid and the micro powder is not particularly limited. For example, if a container-type mixer is used, a mechanical stirring mixer is preferred. Specific examples include turbulent mixers (manufactured by Hosokawa Micron Co., Ltd.), Loedige mixers (manufactured by Loedige Corporation), and mortar mixers (manufactured by West Japan Testing Machine Co., Ltd.). Furthermore, both batch mixers and continuous mixers can be used for mixing.

[0327] In the first embodiment of the present invention, it is preferable to mix the heated aqueous liquid and the heated micropowder in the aforementioned mixing apparatus. In the first embodiment of the present invention, it is more preferable to heat the mixing apparatus in addition to heating the aqueous liquid and the micropowder; specifically, it is preferable to heat the walls of the mixing apparatus and / or stirring means such as stirring blades. By mixing the mixture, with the aqueous liquid and micropowder all heated to a predetermined temperature, the formation of the large gel-like substance can be more effectively suppressed, and granulated gels with the desired particle size can be obtained more easily. In the first embodiment of the present invention, such an effect can be obtained even without heating the micropowder, aqueous liquid, and mixing apparatus; however, it is preferable to heat at least one of them, more preferably two, and even more preferably all of them to a predetermined temperature, thereby obtaining even better results.

[0328] During mixing, the heating temperature within the mixing device, preferably the heating temperature of the inner wall of the mixing device and / or the heating temperature of the stirring means, is preferably 50°C to 120°C, more preferably 55°C to 100°C, further preferably 60°C to 90°C, particularly preferably 65°C to 90°C, and most preferably 70°C to 90°C. By heating the mixing device, preferably by heating the inner wall or the stirring means, and more preferably by heating both, the micro-powder can be uniformly granulated in a short time, thereby improving productivity. The temperature within the mixing device can be appropriately adjusted, for example, by supplying heated gas or by conducting electricity or heat.

[0329] In the first embodiment of the present invention, if the micropowder is mixed with an aqueous liquid, granulation is preferably performed by high-speed mixing. By performing high-speed mixing, the formation of the large gel-like material can be suppressed, thus eliminating the need for the large mixing force required due to the appearance of the large gel-like material, and avoiding the following problems: the gel-like agglomerates become a mixed state, causing the main chain to break or entangle, resulting in the deterioration of the water-absorbing resin.

[0330] The high-speed mixing refers to a short time from the moment the raw material powder comes into contact with the aqueous liquid to the formation of a granulated gel within the mixing device. That is, the time from when the raw material is added to the mixing device to when the granulated gel is removed is short. The mixing time is preferably 3 minutes or less, more preferably 1 minute or less, and more preferably 1 second or more, more preferably 5 seconds or more. A mixing time of 5 seconds or more ensures that the aqueous liquid and powder are uniformly mixed, preventing the formation of a large, integrated gel. Furthermore, a mixing time of 3 minutes or less prevents an increase in the water-soluble content of the resulting absorbent resin and a decrease in the absorption rate under pressure, thus preventing a decline in the performance of the absorbent resin.

[0331] Therefore, as a means to achieve high-speed mixing, it is preferable to add the raw materials into the mixing device in a short time. When adding the aqueous liquid slowly by means of spraying or other methods, if the addition time of any one or both raw materials is prolonged, the mixing time will also be prolonged, which may cause the micropowder to form large agglomerates, or the water-absorbing resin may deteriorate due to prolonged mixing. The micropowder and the aqueous liquid can be added to the mixing device simultaneously, or one can be added at different times, followed by the other. Therefore, in the case of simultaneous addition, the time from the start to the end of the addition of both raw materials is preferably 60 seconds or less, more preferably 30 seconds or less, and even more preferably 10 seconds or less. In addition, in the case of different addition times, the time from the start to the end of the addition of the later raw material is preferably 60 seconds or less, more preferably 30 seconds or less, and even more preferably 10 seconds or less.

[0332] Furthermore, to achieve high-speed mixing, a high-speed impeller mixer is preferred. In this case, the impeller speed is preferably 100 rpm or more, more preferably 200 rpm or more, even more preferably 300 rpm or more, and preferably 5000 rpm or less, more preferably 4000 rpm or less, even more preferably 3000 rpm or less. The direction of the impeller's rotation axis is not limited; it can be either vertical (plumb line direction) or horizontal.

[0333] [3-1-4-9-2] Granulation and gelation process

[0334] This process involves adding a granulating gel to the hydrogel during at least one of the polymerization and drying processes, and / or between these processes, before the end of the drying process. Specifically, it is preferable to add the granulating gel to the hydrogel during at least one of the following stages: during the polymerization process; after the polymerization process and before the gel pulverization process; during the gel pulverization process; after the gel pulverization process and before the drying process; and during the drying process. Here, since a hydrogel is also obtained during the polymerization process, the granulating gel can be added during that polymerization process. In addition, polymers with a solid content of less than 80% by mass are generally considered hydrogels during the drying process. That is, a hydrogel may still be present during the middle stage of the drying process, so the granulating gel can be added during that drying process. Preferably, the granulating gel is added to the hydrogel after the gel pulverization process and before the drying process, or during the drying process; more preferably, the granulating gel is added to the hydrogel after the gel pulverization process and before the drying process. Adding a granulating gel to a pulverized hydrogel in this way results in a smaller particle size difference between the two, making them easier to mix and reducing the likelihood of uneven drying. Especially when the pulverization energy is controlled, the hydrogel can be granulated, further suppressing uneven drying. On the other hand, adding a granulating gel before or during the pulverization process may increase the load on the pulverizer, leading to unstable pulverization or uncontrollable gel particle size. Here, "before the process" and "after the process" include all processes before or after the current process.

[0335] In the granulation gel addition process, the solid content of the granulation gel is more than 50% by mass and less than 90% by mass.

[0336] (Solid content)

[0337] In this invention, regarding the step of adding the granulated gel to the hydrogel, it is preferable to appropriately control the solid content of both the granulated gel and the hydrogel. That is, if the solid content of both the granulated gel and the hydrogel is too low, incomplete drying may occur locally, or aggregates may easily form. Conversely, if the solid content is too high, there is a tendency for an increase in residual monomer content. In this invention, the solid content of the granulated gel and / or the hydrogel is preferably within an appropriate range. The solid content of the hydrogel is preferably 30% to 70% by mass, more preferably 45% to 55% by mass, and even more preferably 45% to 50% by mass. The solid content of the granulated gel is preferably 50% to 90% by mass, more preferably 55% to 85% by mass, and even more preferably 60% to 80% by mass. Preferably, the solid content of the granulated gel during the addition step is within the above range, more preferably, the solid content of the granulated gel is within the above range, and even more preferably, the temperatures of the granulated gel and the hydrogel are within the above range.

[0338] In the first embodiment of the present invention, the ratio between the granulating gel and the hydrogel can be appropriately determined based on the amount of separated microparticles and the amount of solid components in the granulating gel. From the viewpoint of the physical properties of the absorbent resin, the amount of granulating gel added is typically 10 to 50 parts by mass relative to 100 parts by mass of the hydrogel (in its original form), preferably 15 to 40 parts by mass, and more preferably 20 to 30 parts by mass. According to the method of the first embodiment of the present invention, even if the proportion of granulating gel is 10 parts by mass or more, uneven drying can be suppressed. However, if the proportion of granulating gel is too high, the final quality and physical properties of the absorbent resin as the final product may be significantly affected by the reused microparticles, i.e., the granulating gel.

[0339] The hydrogel containing the added granulating gel is processed in the drying step. The drying conditions for the mixed gel are the same as those in the drying step, so their description is omitted. Furthermore, the pulverizing and grading steps performed after the drying step are the same as those in the pulverizing and grading steps. The surface crosslinking and granulation steps can be performed as needed to obtain the water-absorbing resin as the product. Additionally, the micropowder obtained in the grading step can also be processed in the recycling step.

[0340] [3-2] Second Embodiment

[0341] The method for manufacturing the water-absorbing resin according to the second embodiment of the present invention includes a step of contacting the water-absorbing resin with a supercritical solvent to remove impurities from the water-absorbing resin (hereinafter, in this specification, the step of contacting the water-absorbing resin with a supercritical solvent to remove impurities from the water-absorbing resin is sometimes referred to as the "impurity removal step"), wherein the water-absorbing resin is mainly composed of polyacrylic acid (salt) resin, and the water-absorbing resin has undergone internal crosslinking and surface crosslinking.

[0342] Here, the impurities are not particularly limited, but are preferably organic compounds with two or more carbon atoms. Examples include low molecular weight compounds such as: unreacted substances derived from reactive raw materials, such as residual acrylic (salt) monomers and residual crosslinking products; impurities contained in the raw materials; byproducts derived from the raw materials; and so on.

[0343] [3-2-1] Impurity Removal Process

[0344] Regarding the method for contacting the base superabsorbent resin with the supercritical solvent in the impurity removal process, there are no particular limitations as long as the base superabsorbent resin can be brought into contact with the supercritical solvent. For example, a method can be given where the base superabsorbent resin is loaded into an extraction tank that serves as a fixed bed or a fluidized bed, and the base superabsorbent resin is brought into contact with the supercritical solvent within the extraction tank. Furthermore, the contact between the base superabsorbent resin and the supercritical solvent can be performed continuously or in batches. Moreover, if the superabsorbent resin production process is continuous, the contact process can be performed continuously by arranging multiple extraction tanks in parallel.

[0345] The compounds used as supercritical solvents are not particularly limited, but for the purpose of energy consumption reduction and device miniaturization, compounds that achieve supercriticality at the lowest possible temperature and pressure are preferred. Regarding the selected compound, the temperature at which the compound achieves supercriticality is preferably below 150°C, more preferably below 120°C, and even more preferably below 100°C. Furthermore, regarding the selected compound, the pressure at which the compound achieves supercriticality is preferably below 100 MPa, more preferably below 50 MPa, and even more preferably below 30 MPa.

[0346] Examples of substances selected as supercritical solvents based on the aforementioned temperature and pressure ranges include ethylene, carbon dioxide, ethane, nitrous oxide, propylene, dichlorofluoromethane, propane, dichlorofluoromethane, and ammonia. Among these, supercritical carbon dioxide is preferred as a supercritical solvent due to its operability and ease of achieving the conditions for a supercritical state.

[0347] The following is an example of a method for contacting a base water-absorbing resin with a supercritical solvent, using supercritical carbon dioxide as a supercritical solvent and employing a method that... Figure 1 The impurity removal process implemented by the supercritical extraction apparatus with the structure shown will be described.

[0348] like Figure 1 As shown, the supercritical extraction device consists of a carbon dioxide tank 1, a pressure regulating valve 2, a high-pressure liquid delivery pump 3, a cooling device 4, a pressure-resistant extraction tank 5, a pressure reducing valve 6, and a flow meter 7. Additionally, Figure 1 The middle arrow indicates the flow direction of carbon dioxide gas and supercritical carbon dioxide.

[0349] First, the superabsorbent resin base material is loaded into the pressure-resistant extraction tank 5 and set. Next, gaseous carbon dioxide (carbon dioxide gas) is pumped from the carbon dioxide tank 1 into the high-pressure liquid delivery pump 3. At this time, the temperature and pressure of the carbon dioxide in the high-pressure liquid delivery pump 3 are adjusted using the pressure regulating valve 2 and the cooling device 4, thereby making the carbon dioxide supercritical. Through this operation, supercritical carbon dioxide is prepared inside the high-pressure liquid delivery pump 3. Then, the supercritical carbon dioxide is injected into the pressure-resistant extraction tank 5 using the high-pressure liquid delivery pump 3, thereby bringing the superabsorbent resin base material into contact with the supercritical carbon dioxide. At this time, the temperature and pressure inside the pressure-resistant extraction tank 5 are controlled to maintain the supercritical carbon dioxide in a supercritical state. Then, the pressure is reduced by the pressure reducing valve 6 after the supercritical carbon dioxide has come into contact with the superabsorbent resin in the pressure-resistant extraction tank 5, thereby changing the state of the carbon dioxide gas and discharging it from the pressure-resistant extraction tank 5 into the outside atmosphere.

[0350] In one embodiment of the present invention, a supercritical solvent refers to a substance that, upon reaching a supercritical state exceeding its critical temperature and critical pressure, possesses a density similar to that of a liquid and a low viscosity similar to that of a gas, thus becoming an intermediate state between a liquid and a gas. Table 1 shows the temperatures and pressures at which typical compounds can reach a supercritical state. Carbon dioxide, in particular, is preferred, with a critical temperature of approximately 31°C (31.1°C) and a critical pressure of approximately 7.4 MPa (7.38 MPa).

[0351] [Table 1]

[0352] (Table 1)

[0353]

[0354] In one embodiment of the present invention, preferably, the compound constituting the supercritical solvent is stored in a gaseous state beforehand, and the supercritical solvent is prepared by controlling the temperature and pressure of the gas to be above the critical temperature and critical pressure range just before the impurity removal process is to be carried out. Then, the prepared supercritical solvent is brought into contact with the water-absorbing resin.

[0355] The supercritical solvent is an intermediate state between the liquid and the gas described above, and the molecules of the compounds constituting it are small, thus allowing the supercritical solvent to penetrate the tiny crevices present within the absorbent resin. Furthermore, the supercritical solvent has the same polarity as typical organic solvents such as hexane and toluene.

[0356] Here, regarding the impurities in the base water-absorbing resin, namely unreacted substances originating from reactive raw materials such as residual monomers and residual crosslinking products, impurities contained in the raw materials, and by-products originating from the raw materials, it is known that they are usually composed of or contain a large number of low molecular weight organic compounds.

[0357] Furthermore, it is known that the low molecular weight organic compound has the same polarity as the typical organic solvent. That is, the low molecular weight organic compound and the supercritical solvent have the same degree of polarity, and therefore have a high affinity between them.

[0358] Therefore, in the impurity removal process of one embodiment of the present invention, through contact between the supercritical solvent and the base water-absorbing resin, the supercritical solvent enters the tiny pores inside the base water-absorbing resin, thereby enabling the extraction of the low molecular weight organic compounds constituting the impurities present inside the base water-absorbing resin from the water-absorbing resin. As a result, in the impurity removal process of the second embodiment of the present invention, the impurities themselves can be removed from the base water-absorbing resin.

[0359] Furthermore, the base water-absorbing resin is insoluble in the supercritical solvent. Therefore, the base water-absorbing resin will not deteriorate due to the supercritical solvent. In addition, in the second embodiment of the present invention, the supercritical solvent evaporates from the base water-absorbing resin in gaseous form after the impurity removal process. Therefore, no compounds constituting the supercritical solvent remain in the treated water-absorbing resin. Therefore, the water-absorbing properties, such as water absorption, of the water-absorbing resin will not decrease due to residual compounds from the supercritical solvent in the treated water-absorbing resin.

[0360] Therefore, the impurity removal process of one embodiment of the present invention has the following effect: while maintaining the water absorption properties of the water-absorbing resin, it can remove the impurities contained in the water-absorbing resin.

[0361] In the impurity removal process of one embodiment of the present invention, the contact time between the base water-absorbing resin and the supercritical solvent is preferably 1 second to 1000 minutes, more preferably 1 minute to 900 minutes, and even more preferably 2 minutes to 800 minutes. By controlling the contact time within the above range, impurities contained in the base water-absorbing resin can be effectively dissolved in the supercritical solvent, resulting in the effective removal of the impurities from the base water-absorbing resin.

[0362] In the impurity removal process of one embodiment of the present invention, the volume of the supercritical solvent in contact with the base water-absorbing resin is preferably 0.1 mL to 1000 L per 1 g of the water-absorbing resin, more preferably 1 mL to 500 L, and even more preferably 10 mL to 250 L. By controlling the volume of the supercritical solvent in contact with each 1 g of the base water-absorbing resin within the above range, impurities contained in the base water-absorbing resin can be effectively dissolved in the supercritical solvent, resulting in the effective removal of the impurities from the base water-absorbing resin. Here, "each 1 g of water-absorbing resin" refers to "each 1 g of water-absorbing resin added to the extraction tank".

[0363] In the impurity removal process of one embodiment of the present invention, from the viewpoint of being able to effectively remove the impurities from the base water-absorbing resin, the compound constituting the supercritical solvent preferably remains in a supercritical state during the process of contacting the base water-absorbing resin with the supercritical solvent.

[0364] From the above perspective, in the impurity removal process of one embodiment of the present invention, the temperature at which the base water-absorbing resin comes into contact with the supercritical solvent is preferably 30°C or higher, more preferably 35°C or higher, and even more preferably 40°C or higher. Furthermore, in the impurity removal process of one embodiment of the present invention, the pressure at which the base water-absorbing resin comes into contact with the supercritical solvent is preferably 4.2 MPa or higher, more preferably 7.0 MPa or higher, and even more preferably 10.0 MPa or higher.

[0365] In the impurity removal process of one embodiment of the present invention, if the temperature and pressure at which the base water-absorbing resin comes into contact with the supercritical solvent are excessively high, some structures and / or internal structures of the water-absorbing resin may change, leading to a decrease in the water absorption performance and other physical properties of the water-absorbing resin. Furthermore, from a cost perspective, this equates to excessive industrial energy consumption, and is therefore uneconomical and undesirable.

[0366] From the viewpoint of avoiding a decrease in the physical properties of the absorbent resin and an increase in cost, in the impurity removal process of the second embodiment of the present invention, the temperature at which the base material absorbent resin comes into contact with the supercritical solvent is preferably 200°C or lower, more preferably 150°C or lower. Furthermore, in the impurity removal process of the second embodiment of the present invention, the pressure at which the base material absorbent resin comes into contact with the supercritical solvent is preferably 200 MPa or lower, more preferably 150 MPa or lower.

[0367] Furthermore, considering the impurities to be removed from the superabsorbent resin and the affinity between the supercritical solvent and the superabsorbent resin, water and / or an organic solvent can be added to the supercritical solvent to change its polarity. Examples of such organic solvents include low molecular weight organic solvents, with ethanol, methanol, isopropanol, and acetone being preferred. The water and / or organic solvent can also be provided to the impurity removal process individually or as a mixed solution together with the supercritical solvent. Alternatively, a small amount of water can be added to the superabsorbent resin beforehand to induce a swollen state, and then, in the impurity removal process, the mixture of the supercritical solvent and the low molecular weight organic solvent is brought into contact with the swollen superabsorbent resin.

[0368] [3-2-2] Properties of the absorbent resin before treatment

[0369] In one embodiment of the present invention, the water-absorbing resin (hereinafter also referred to as "water-absorbing resin before treatment") supplied to the impurity removal process is not particularly limited as long as it is mainly composed of polyacrylic acid (salt) resin and has undergone internal cross-linking and surface cross-linking.

[0370] Here, the following method for determining the physical properties of the absorbent resin before treatment is also applicable to the determination of the physical properties of the absorbent resin obtained by the "method for treating absorbent resin" according to an embodiment of the present invention (hereinafter also referred to as "treated absorbent resin"), the determination of the physical properties of the absorbent resin manufactured by the "method for manufacturing absorbent resin" according to an embodiment of the present invention, and the determination of the physical properties of the "absorbent resin" according to an embodiment of the present invention.

[0371] In one embodiment of the present invention, the absorbent resin before treatment preferably comprises 90% by mass or more, more preferably 93% by mass or more, and even more preferably 95% by mass or more of particulate absorbent resin with a particle size of 45 μm or more and 850 μm or less.

[0372] In one embodiment of the present invention, by adjusting the particle size of the particulate superabsorbent resin, which constitutes 90% or more of the superabsorbent resin before treatment, to the aforementioned range, when the treated superabsorbent resin is used as an absorbent in hygiene products such as diapers, the roughness and discomfort caused to the wearer by large-particle superabsorbent resin can be reduced. Furthermore, by adjusting the particle size of the particulate superabsorbent resin, which constitutes 90% or more of the superabsorbent resin before treatment, to the aforementioned range, dust problems caused by the dispersion of small-particle superabsorbent resin can be suppressed when the superabsorbent resin is handled in a hygiene product manufacturing plant. Moreover, by adjusting the particle size of the particulate superabsorbent resin, which constitutes 90% or more of the superabsorbent resin before treatment, to the aforementioned range, it is also expected that the internal gap size of the superabsorbent resin before treatment can be controlled to a size that allows the supercritical solvent to easily penetrate. When the size of the slit is controlled to allow the supercritical solvent to easily enter, the impurities can be effectively removed by the supercritical solvent that has entered the slit.

[0373] Regarding the method for adjusting the particle size of the particulate superabsorbent resin, which accounts for more than 90% by mass in the superabsorbent resin before the treatment, to the above-mentioned range, a known method for adjusting the particle size of superabsorbent resin can be used.

[0374] The proportion of particulate superabsorbent resin with a particle size of 45 μm or more and 850 μm or less in the superabsorbent resin before treatment can be determined according to the determination method described in "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" in US Patent No. 7,638,570.

[0375] Specifically, under room temperature (20-25℃) and humidity of 50%RH, 10.0g of absorbent resin was fed to a JIS standard sieve (THEIIDA TESTING SIEVE; diameter 8cm) consisting of sieves with mesh sizes of 850μm, 710μm, 600μm, 500μm, 300μm, 150μm, and 45μm, and a receiving dish. The resin was then classified for 5 minutes using a vibrating classifier (IIDA SIEVE SHAKER, TYPE: ES-65, SER. No. 0501). Afterward, the mass of absorbent resin remaining on each sieve was measured. Based on the measurement results, the ratio of the mass of absorbent resin remaining on each sieve to the mass of absorbent resin before classification (10.0g), i.e., the residual percentage R, was calculated. This yielded the proportion of particulate absorbent resin with a particle size of 45μm or larger and 850μm or smaller in the total absorbent resin before treatment.

[0376] In one embodiment of the present invention, the unpressurized absorption ratio (CRC) of the absorbent resin before treatment is preferably 20 g / g or more, more preferably 22 g / g or more, and even more preferably 25 g / g or more, with the higher the upper limit being more preferred. However, from the viewpoint of balancing with other physical properties, it is preferably 50 g / g or less, more preferably 48 g / g or less, and even more preferably 45 g / g or less.

[0377] In one embodiment of the present invention, the absorbent polymer under pressure (AAP) of the pretreated absorbent resin is preferably 5 g / g or more, more preferably 7 g / g or more, and even more preferably 10 g / g or more, with a higher upper limit being more preferred. However, from the viewpoint of balancing with other physical properties, it is preferably 40 g / g or less, more preferably 38 g / g or less, and even more preferably 35 g / g or less.

[0378] In one embodiment of the present invention, the water content of the absorbent resin before treatment is preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 15% by mass or less.

[0379] In one embodiment of the present invention, the water absorption rate of the absorbent resin before treatment, based on the Vortex method, is preferably 10 seconds or more, more preferably 15 seconds or more, and even more preferably 20 seconds or more, with the higher the upper limit being more preferred. However, from the viewpoint of balancing with other physical properties, it is preferably 100 seconds or less, more preferably 90 seconds or less, and even more preferably 80 seconds or less.

[0380] In one embodiment of the present invention, the saline conductivity (SFC) of the absorbent resin before treatment is preferably 1×10⁻⁶. -7 cm 3 • sec / g or higher, more preferably 2×10 -7 cm 3•sec / g or higher, and more preferably 5×10 -7 cm 3 • sec / g or higher, with higher upper limits being preferred. However, from the perspective of balancing with other physical properties, 200 × 10⁻⁶ is preferred. -7 cm 3 • sec / g or less, more preferably 150 × 10 -7 cm 3 • sec / g or less, and more preferably 100 × 10 -7 cm 3 • sec / g or less.

[0381] If the above-mentioned physical property values ​​of the absorbent resin before treatment in one embodiment of the present invention meet the above-mentioned range, then the absorbent resin obtained by the absorbent resin treatment method of one embodiment of the present invention can also have excellent physical properties. Therefore, from this point of view, the above-mentioned physical property values ​​of the absorbent resin before treatment preferably meet the above-mentioned range.

[0382] [3-2-3] Method for manufacturing water-absorbing resin before treatment

[0383] Regarding the method for manufacturing the pre-treatment absorbent resin in one embodiment of the present invention, a known method can be used. For example, the same method as that described in the first embodiment above, which is the same method used to manufacture the surface-crosslinked absorbent resin supplied to the aqueous liquid addition step, can be cited as an example. Furthermore, the method for manufacturing the absorbent resin in the second embodiment of the present invention can include each step described in the first embodiment, but does not necessarily need to include all of these steps.

[0384] [3-2-4] Physical properties of the treated absorbent resin

[0385] In one embodiment of the present invention, regarding the amount of impurities contained in the absorbent resin after the impurity treatment process, based on 100% of the total mass of the absorbent resin, the amount of impurities is preferably 2% or less by mass, more preferably 1% or less by mass, and even more preferably 0.5% or less by mass.

[0386] Regarding the method for treating the base water-absorbing resin according to an embodiment of the present invention, in the impurity removal process, based on the impurity content of 100% by mass before treatment, it is preferable to reduce the impurity content by 30% by mass or more, more preferably by 50% by mass or more.

[0387] The impurities mentioned may be common impurities found in water-absorbing resins that have undergone internal and surface cross-linking. Examples of such impurities include unreacted substances originating from reactive raw materials, such as residual monomers and residual cross-linking agents, impurities contained in the raw materials, and byproducts generated incidentally from the raw materials. In addition, there may be unidentified substances among the impurities and byproducts that cannot be identified by various analytical methods, but in this case, they can be identified by observing changes in peak intensity using methods such as chromatography.

[0388] [3-3] Third Embodiment

[0389] The method for manufacturing the water-absorbing resin according to the third embodiment of the present invention is a method for manufacturing the water-absorbing resin after surface crosslinking, comprising: a polymerization step, a drying step of drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and a step of adding a volatile component reducing agent after the polymerization step is completed.

[0390] [3-3-1] Process of adding volatile component reducing agent

[0391] In the method for manufacturing the water-absorbing resin according to the third embodiment of the present invention, it is sufficient to include a step of adding a volatile component reducing agent after the polymerization step is completed. However, it is more preferable to include the step of adding a volatile component reducing agent after the drying step of drying the hydrogel obtained in the polymerization step is completed. Furthermore, it is more preferable to include the step of adding a volatile component reducing agent after the surface crosslinking step is completed.

[0392] The volatile component reducing agent has been described in the aforementioned "[Polyacrylic acid (salt) based water-absorbing resin]" section. The volatile component reducing agent may contain at least one selected from reducing agents, surfactants, and inorganic acids (salts), more preferably containing a reducing agent, and even more preferably containing a reducing agent with an amino group, and even more preferably containing amino acids (hydrochloride), aminooxy compounds (hydrochloride), aminooxyacetic acids (hydrochloride), and compounds (hydrochloride) having the functional group shown in the aforementioned structural formula (1), and compounds containing hydrazide groups, especially preferably containing amino acids (hydrochloride), aminooxy compounds (hydrochloride), aminooxyacetic acids (hydrochloride), and compounds (hydrochloride) having the functional group shown in the aforementioned structural formula (1).

[0393] When the volatile component reducing agent is added to the water-absorbing resin, the method of addition is not particularly limited. For example, an aqueous solution prepared by dissolving the volatile component reducing agent in an aqueous medium such as water, or a dispersion prepared by suspending the volatile component reducing agent in an aqueous medium, can be prepared, and then the solution or dispersion can be added to the water-absorbing resin to mix with the resin. Alternatively, if the volatile component reducing agent is a solid such as a powder, it can be dry-mixed with the water-absorbing resin, and an aqueous liquid such as water can be used as a binder. Preferably, the volatile component reducing agent is added to the water-absorbing resin in the form of an aqueous solution.

[0394] Regarding the apparatus used for the mixing, examples include stirring mixers, cylindrical mixers, double-walled conical mixers, V-shaped mixers, bowtie mixers, spiral mixers, flow-type swirl mixers, airflow mixers, double-arm kneaders, internal mixers, pulverizing kneaders, rotary mixers, and spiral extruders. When using a stirring mixer, its rotational speed is not particularly limited, but it is preferably 5 rpm or more, more preferably 10 rpm or more, and preferably 10,000 rpm or less, more preferably 2,000 rpm or less.

[0395] The volatile component reducing agent may contain at least one selected from reducing agents, surfactants, and inorganic acids (salts). The reducing agent is added to the absorbent resin in a manner that reaches a specified content relative to the total amount of the absorbent resin containing additives, etc. Similarly, the surfactant is added to the absorbent resin in a manner that reaches a specified content relative to the total amount of the absorbent resin containing additives, etc. Likewise, the inorganic acid (salt) is added to the absorbent resin in a manner that reaches a specified content relative to the total amount of the absorbent resin containing additives, etc. The specified contents of the reducing agent, surfactant, and inorganic acid (salt) mentioned above are the same as the contents required for "reducing agent," "surfactant," and "inorganic acid (salt)" as described above.

[0396] In one embodiment of the present invention, if the volatile component reducing agent is prepared into a solution or dispersion using an aqueous medium and the solution or dispersion is added to the water-absorbing resin, or if an aqueous liquid such as water is used as an adhesive, the addition treatment of the solution or dispersion, the aqueous liquid used as an adhesive, and the drying treatment after the addition of the aqueous liquid are preferably performed in the same manner as the "aqueous liquid addition process" (the process of adding an aqueous liquid to the surface-crosslinked water-absorbing resin as described in column [3-1-1] above) and the "drying process after adding an aqueous liquid" (the process of drying the water-absorbing resin to which the aqueous liquid has been added as described in column [3-1-2] above) described in the first embodiment above. More specifically, the following method can be used: Preferably, an aqueous solution containing a volatile component reducing agent is added as an aqueous liquid using the "aqueous liquid addition process" described in the first embodiment (described in column [3-1-1]), more preferably an aqueous solution containing a reducing agent having an amino group is added, and drying is performed using the "drying process after adding the aqueous liquid" described in the first embodiment (described in column [3-1-2]). A more preferred embodiment is that the aqueous solution containing the reducing agent having an amino group is added in droplet form to a surface area of ​​25 m². 2 When the water-absorbing resin with a surface cross-linking content of more than 27.5% is obtained, the water-absorbing resin is dried such that its water content reaches less than 20% by weight within 1 hour.

[0397] Furthermore, if a volatile component reducing agent is added after the surface crosslinking process is completed, the physical properties of the surface-crosslinked absorbent resin supplied in this addition process are preferably the same as those described in the first embodiment for the "surface-crosslinked absorbent resin".

[0398] [3-3-2] Method for manufacturing water-absorbing resin before treatment

[0399] In one embodiment of the present invention, the method for manufacturing the water-absorbing resin for the volatile component reducing agent addition step can be a known method.

[0400] If the manufacturing method of one embodiment of the present invention includes a step of adding a volatile component reducing agent after the polymerization step (polymerization step) of polymerizing a monomer composition containing a (meth)acrylate (salt) monomer to obtain a hydrogel-like crosslinked material, then the manufacturing method of the water-absorbing resin before treatment can include at least a manufacturing method that includes the polymerization step within the scope of the manufacturing method of the first embodiment described above.

[0401] Furthermore, if the manufacturing method of one embodiment of the present invention includes a step of adding a volatile component reducing agent after the drying step of drying the hydrogel obtained in the polymerization step is completed, then the manufacturing method of the water-absorbing resin before treatment can be: a manufacturing method that includes at least the polymerization step and the drying step of drying the hydrogel obtained in the polymerization step, which are within the scope of the manufacturing method of the first embodiment described above.

[0402] Furthermore, if the manufacturing method of one embodiment of the present invention includes a step of adding a volatile component reducing agent after the completion of the surface crosslinking step, then the manufacturing method of the water-absorbing resin before treatment can include at least the polymerization step, the drying step of drying the hydrogel obtained in the polymerization step, and the manufacturing method of the surface crosslinking step, which are all within the scope of the manufacturing method of the first embodiment described above.

[0403] Furthermore, the method for manufacturing the water-absorbing resin according to the third embodiment of the present invention may further include: a polymerization step, a drying step of drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and a step of adding a volatile component reducing agent (preferably a reducing agent having an amino group). It may also include each of the steps described in the manufacturing method of the first embodiment, but it is not necessary to include all of these steps.

[0404] Furthermore, regarding the manufacturing method of one embodiment of the present invention, the following manufacturing method can be cited: it includes the step of adding an aqueous liquid to a surface-crosslinked water-absorbing resin as described in [3-1-1] above (aqueous liquid addition step), and the step of drying the water-absorbing resin to which the aqueous liquid has been added as described in [3-1-2] above (drying step after adding aqueous liquid), and includes a step of adding an aqueous solution containing a volatile component reducing agent (especially an amino-containing reducing agent) instead of the aqueous liquid used in the aqueous liquid addition step.

[0405] In conventional technology, when using sanitary articles (absorbent articles) containing absorbent resin, the odor from substances volatilized from the surface-crosslinked absorbent resin (volatile components of the surface-crosslinked absorbent resin) mixes with the odor emitted from urine absorbed by the absorbent resin (urine odor), thus forming an unpleasant odor. Therefore, some users of absorbent articles find this odor offensive. However, this embodiment suppresses the odor from the volatile components of the surface-crosslinked absorbent resin, thereby suppressing the aforementioned unpleasant odor. In particular, for absorbent resins that have undergone surface crosslinking at temperatures above 150°C (such as polyols and alkylene carbonate compounds as surface crosslinking agents), it is necessary to suppress the aforementioned unpleasant odor, and this embodiment effectively suppresses this unpleasant odor.

[0406] One embodiment of the present invention can be the following scheme.

[0407] [1] A surface-crosslinked water-absorbing resin, wherein the concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0 is below 3.5 ppm, wherein,

[0408] The concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0 is defined as the total concentration of all substances detected by a photoionization detector (PID) with a 10.6 eV irradiation lamp after uniformly adding 10.0 g of physiological saline at 23.5 ± 0.5 °C to 10.0 g of absorbent resin in a 2 L sealable glass container at room temperature and pressure, and standing for 15 minutes in a sealed state. This concentration is expressed as a value based on the detection value converted from isobutylene calibration gas.

[0409] [2] According to the water-absorbing resin described in [1], the total concentration of each volatile component after standing for 15 minutes under the condition that the swelling ratios are 0.0 times, 0.5 times, 1.0 times, 2.5 times, 5.0 times, 10.0 times and 20.0 times is 9.5 ppm.

[0410] [3] The water-absorbing resin according to [1] or [2], wherein the maximum value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling to 900 seconds after the water-absorbing resin is 0.4 ppm or less when the water-absorbing resin is swollen at a swelling ratio of 5.0.

[0411] [4] The water-absorbing resin according to any one of [1] to [3], wherein the total value of the concentration of volatile components measured every 5 seconds during the period from the start of swelling to 900 seconds after the water-absorbing resin is 50.0 ppm or less when the water-absorbing resin is swollen under the condition that the swelling ratio is 5.0 times.

[0412] [5] The absorbent resin according to any one of [1] to [4] has an unpressurized absorption ratio (CRC) of 23 g / g or more and an apressurized absorption ratio (AAP) of 15 g / g or more.

[0413] [6] The water-absorbing resin according to any one of [1] to [5] has a mass-average particle size (D50) of 300 to 600 μm, the proportion of particles smaller than 150 μm in the water-absorbing resin is 5% by mass or less, and the logarithmic standard deviation (σζ) of the particle size distribution of the water-absorbing resin is 0.20 to 0.50.

[0414] [7] The water-absorbing resin according to any one of [1] to [6] contains a volatile component reducing agent.

[0415] [8] The water-absorbing resin according to any one of [1] to [7] has a specific surface area of ​​25 m². 2 / kg or more.

[0416] [9] An absorbent article comprising any one of [1] to [8] of the absorbent resin.

[0417]

[10] The absorbent article according to [9], wherein the absorbent article comprises an absorbent as a composite, the composite comprising the absorbent resin and hydrophilic fibers, wherein the content of the absorbent resin is 60% by mass or more relative to the total mass of the absorbent.

[0418]

[11] A method for manufacturing the absorbent resin described in any one of [1] to [8], characterized in that:

[0419] In order including

[0420] The polymerization process includes a polymerization step of polymerizing a monomer composition containing acrylic (salt) monomers to obtain a hydrogel-like crosslinked material, a drying step of drying the hydrogel-like crosslinked material obtained in the polymerization step, and a surface crosslinking step, and after the polymerization step is completed, an addition step of a reducing agent having an amino group is included.

[0421]

[12] The method for manufacturing the water-absorbing resin according to

[11] includes, after the surface crosslinking process is completed, an amino-containing reducing agent is added.

[0422]

[13] The method for manufacturing the water-absorbing resin according to

[11] or

[12] includes the step of adding the amino-containing reducing agent in the form of an aqueous solution.

[0423]

[14] The method for manufacturing a water-absorbing resin according to any one of

[11] to

[13] , wherein the amino-containing reducing agent comprises a compound containing an acylhydrazine group.

[0424]

[15] A method for manufacturing a water-absorbing resin includes: adding an aqueous liquid in droplet form to a surface-crosslinked water-absorbing resin to make the water content of the water-absorbing resin reach 7.5% by mass or more, and then drying the water-absorbing resin to which the aqueous liquid has been added in such a way that the decrease in water content reaches 7.5% by mass or more within 1 hour.

[0425]

[16] The method for manufacturing the water-absorbing resin according to

[15] includes the following steps (A) and / or (B):

[0426] Step (A) involves adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 / kg or more of the surface-crosslinked water-absorbing resin;

[0427] Step (B) sequentially includes a polymerization process, a drying process for drying the hydrogel obtained in the polymerization process, and a surface crosslinking process, and adds a volatile component reducing agent after the polymerization process is completed.

[0428]

[17] The method for manufacturing the water-absorbing resin according to

[16] includes: step (A), adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 The surface-crosslinked water-absorbing resin of / kg or above

[0429] and,

[0430] When the water content of the water-absorbing resin reaches 27.5% by mass or more due to the addition of an aqueous liquid in droplet form, the water-absorbing resin with the added aqueous liquid is dried such that its water content reaches 20% by mass or less within 1 hour.

[0431]

[18] The method for manufacturing the water-absorbing resin according to

[16] or

[17] , wherein the manufacturing method includes: step (B), which sequentially includes a polymerization step, a drying step for drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and adding a volatile component reducing agent after the polymerization step is completed.

[0432]

[19] A method for manufacturing a water-absorbing resin includes a step of contacting the water-absorbing resin with a supercritical solvent to remove volatile components from the water-absorbing resin, wherein,

[0433] The water-absorbing resin is mainly composed of polyacrylic acid (salt) resin, and the water-absorbing resin has undergone internal cross-linking and surface cross-linking.

[0434]

[20] The method for manufacturing the water-absorbing resin according to

[11] includes, after the drying process of the hydrogel obtained by the polymerization process, an amino-containing reducing agent addition process is included.

[0435] This invention is not limited to the embodiments described above. Various modifications can be made within the scope shown in the specification. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of this invention.

[0436] [Example]

[0437] The present invention will be described in more detail below through examples and comparative examples, but the interpretation of the present invention is not limited to these examples, and the embodiments obtained by appropriately combining the technical means disclosed in the various embodiments are also included in the scope of the present invention.

[0438] <Determination / Evaluation of Physical Properties of Water-Absorbent Resins>

[0439] The following describes the method for determining absorbent resins (including absorbent agents). It should be noted that absorbent resins that have been stored for a long period, or absorbent resins (including absorbent agents) removed from absorbent materials such as absorbent bodies and diapers, may absorb moisture, resulting in a moisture content exceeding 10% by mass (or a solid content of less than 90% by mass). Even in such cases, the concentration of volatile components of the absorbent resin is determined under conditions where the moisture content exceeds 10% by mass. As for CRC, AAP, SFC, absorption rate, PDAUP, mass-average particle size, and specific surface area, these can also be determined under conditions where the moisture content exceeds 10% by mass, but it is preferable to adjust the moisture content of the absorbent resin to below 10% by mass before measurement. A method for adjusting the moisture content of the absorbent resin to below 10% by mass can be cited as drying at 80°C and under reduced pressure (below 10.0 kPa) for 24 hours.

[0440] The determination and evaluation of the physical properties of the absorbent resins (including absorbent agents) obtained in Examples 1 to 12 and Comparative Examples 1 to 14 described below were carried out according to the following methods.

[0441] [Determination of volatile component concentration]

[0442] The concentration of volatile components in the absorbent resin was determined using a MiniRAE Lite portable VOC monitor PGM-7300 manufactured by RAE Systems. A 10.6 eV ultraviolet lamp was used in the PID detector of the device. The device was calibrated using 100 ppm isobutylene calibration gas, thereby determining the volatile component concentration based on isobutylene.

[0443] The "volatile component concentration at 1.0 times swelling", "cumulative value of volatile components during swelling at each ratio", "maximum volatile component concentration during swelling along the time axis", and "cumulative value of volatile components during swelling along the time axis" of the water-absorbing resin were determined according to the following method.

[0444] (a) "Concentration of volatile components at 1.0 times swelling"

[0445] Open as Figure 2The glass bottle 8 (a short storage bottle, "2L Mame-kun"; manufactured by Ishizuka Glass Co., Ltd.) with a capacity of 2L and a polyethylene cap 9 having a small cap 10 with a diameter of 2.2cm, is shown. Next, 10.0g of the absorbent resin obtained in the examples and comparative examples described later was evenly distributed inside the glass bottle 8. After distributing the absorbent resin, 10.0g of physiological saline at a temperature of 23.5℃±0.5℃ was evenly injected into the glass bottle 8 using a 10.0mL syringe (manufactured by NIPRO Co., Ltd.). After injecting the physiological saline into the glass bottle 8, the cap 9 was quickly closed to seal the glass bottle 8, and then it was placed indoors at a room temperature of 24℃.

[0446] After sealing the glass bottle 8 for 15 minutes, the small cap 10 was opened, and the probe of the VOC monitor PGM-7300 was inserted. When inserting the probe, the tip was positioned 1-2 cm from the bottom of the glass bottle 8. The volatile component concentration was measured for 1 minute, and the maximum volatile component concentration displayed on the monitor during this 1-minute period was taken as the "volatile component concentration at 1.0 times the swelling value".

[0447] (b) "Total concentration of each volatile component"

[0448] The following values ​​for "volatile component concentration at 0x swelling", "volatile component concentration at 0.5x swelling", "volatile component concentration at 1.0x swelling", "volatile component concentration at 2.5x swelling", "volatile component concentration at 5.0x swelling", "volatile component concentration at 10.0x swelling", and "volatile component concentration at 20.0x swelling" are added together to calculate the "cumulative value of volatile components at each swelling ratio".

[0449] The determinations of “volatile component concentration at 0 times swelling”, “volatile component concentration at 0.5 times swelling”, “volatile component concentration at 2.5 times swelling”, “volatile component concentration at 5.0 times swelling”, “volatile component concentration at 10.0 times swelling”, and “volatile component concentration at 20.0 times swelling” were performed in the following manner.

[0450] (b-1) "Concentration of volatile components at 0 times swelling"

[0451] Open as Figure 2The glass bottle 8 (a short storage bottle, "2L Mame-kun"; manufactured by Ishizuka Glass Co., Ltd.) shown has a large polyethylene cap 9 and a small cap 10 with a diameter of 2.2 cm, and a capacity of 2L. Next, 10.0g of the absorbent resin obtained in the examples and comparative examples described later was evenly distributed inside the glass bottle 8. After distributing the absorbent resin, the large cap 9 was quickly closed to seal the glass bottle 8, and then it was placed indoors at room temperature (24°C) for 15 minutes.

[0452] Next, the small cap 10 was opened, and the probe of the volatile component monitor PGM-7300 was inserted. When inserting the probe, the tip was positioned 1-2 cm from the bottom of the glass bottle 8. The VOC monitor was used to measure the concentration for 1 minute, and the maximum value of the volatile component concentration displayed on the monitor during this 1-minute period was taken as the "volatile component concentration at 0 times swelling".

[0453] (b-2) "Concentration of volatile components at 0.5 times swelling"

[0454] 10.0g of physiological saline at 23.5℃±0.5℃ was replaced with 5.0g of physiological saline at 23.5℃±0.5℃, and was uniformly injected into the glass bottle 8 using a 10.0mL syringe (manufactured by NIPRO Corporation). Otherwise, the determination of "concentration of volatile components at 1.0 times swelling" was performed in the same manner as described in (a) above.

[0455] (b-3) "Concentration of volatile components at 2.5 times swelling"

[0456] 10.0g of physiological saline at 23.5℃±0.5℃ was replaced with 25g of physiological saline at 23.5℃±0.5℃, and the saline was uniformly injected into the glass bottle 8 using a 50mL glass beaker. Otherwise, the determination was performed in the same manner as the determination of "concentration of volatile components at 1.0 times swelling" described in (a) above.

[0457] (b-4) "Concentration of volatile components at 5.0 times swelling"

[0458] 10.0g of physiological saline at 23.5℃±0.5℃ was replaced with 50g of physiological saline at 23.5℃±0.5℃, and the saline was uniformly injected into the glass bottle 8 using a 100mL glass beaker. Otherwise, the determination was performed in the same manner as the determination of "concentration of volatile components at 1.0 times swelling" described in (a) above.

[0459] (b-5) "Concentration of volatile components at 10.0 times swelling"

[0460] 10.0g of physiological saline at 23.5℃±0.5℃ was replaced with 100g of physiological saline at 23.5℃±0.5℃, and was evenly injected into the glass bottle 8 using a 200mL glass beaker. Otherwise, the determination was performed in the same manner as the determination of "concentration of volatile components at 1.0 times swelling" described in (a) above.

[0461] (b-6) "Concentration of volatile components at 20.0 times swelling"

[0462] 10.0g of physiological saline at 23.5℃±0.5℃ was replaced with 200g of physiological saline at 23.5℃±0.5℃, and the saline was uniformly poured into the glass bottle 8 using a 200mL glass beaker. Otherwise, the determination was performed in the same manner as the determination of "concentration of volatile components at 1.0 times swelling" described in (a) above.

[0463] (c) "Maximum volatile component concentration during swelling along the time axis" and (d) "Cumulative value of volatile components during swelling along the time axis".

[0464] Open as Figure 2 The glass bottle 8 (a short storage bottle, "2L Mame-kun"; manufactured by Ishizuka Glass Co., Ltd.) with a capacity of 2L and a polyethylene large cap 9 having a small cap 10 with a diameter of 2.2cm, is shown. Next, 10.0g of the absorbent resin obtained in the examples and comparative examples described later was evenly distributed inside the glass bottle 8. After distributing the absorbent resin, 50.0g of physiological saline at a temperature of 23.5℃±0.5℃ was evenly injected into the glass bottle 8 using a 100mL beaker. After injecting the physiological saline into the glass bottle 8, the large cap 9 was quickly closed to seal the glass bottle 8, and the small cap 10 was opened to insert the probe of the volatile component monitor PGM-7300. Here, when inserting the probe, the tip of the probe was positioned 1-2cm away from the bottom of the glass bottle 8. The values ​​displayed on the volatile component monitor were recorded every 5 seconds from the time the physiological saline was injected. The measurement was conducted continuously until 900 seconds had elapsed since the injection of physiological saline, recording a total of 180 volatile component concentrations. The highest volatile component concentration among these 180 was taken as the "maximum volatile component concentration during swelling along the time axis." Furthermore, the sum of the 180 volatile component concentrations was taken as the "cumulative value of volatile components during swelling along the time axis." The operating environment temperature was 24°C.

[0465] Here, in this determination method, even without using absorbent resin and performing the same operation as described above, it is still possible to detect volatile components. In this case, the concentration of volatile components detected inside the glass bottle 8 before it was filled with absorbent resin is subtracted to make a correction.

[0466] [Absorption Ratio (CRC) without Pressure]

[0467] The CRC of the absorbent resin was determined according to NWSP 241.0.R2(15). Specifically, 0.2 g of absorbent resin was placed in a nonwoven bag and then immersed in an excess of 0.9% by mass sodium chloride aqueous solution for 30 minutes to allow the absorbent resin to swell freely. After that, it was dehydrated by centrifuging (250G) for 3 minutes, and then its absorbency ratio (CRC) under no pressure was determined (unit: g / g).

[0468] [Absorption Rate under Pressure (AAP)]

[0469] The AAP of the superabsorbent resin was determined according to NWSP 242.0.R2(15). However, in this invention, the pressure conditions were changed to 4.83 kPa (49 g / cm³). 2 The test was conducted at 0.7 psi. Specifically, 0.9 g of the absorbent resin was subjected to a test at 4.83 kPa (49 g / cm³). 2 The solution was swollen in an excess of 0.9% sodium chloride aqueous solution for 1 hour under a pressure of 0.7 psi, and then its AAP (absorption rate under pressure) (unit: g / g) was measured. That is, in this specification, the AAP (absorption rate under pressure) is the measured value under a pressure of 4.83 kPa.

[0470] [Salt-based flow conversion (SFC)]

[0471] Saline conductivity (SFC) of absorbent resin (unit: ×10) -7 cm 3· The sec / g was determined according to the method described in U.S. Patent No. 5,669,894.

[0472] Specifically, 0.900g of absorbent resin is uniformly placed into a container, then the absorbent resin is impregnated in artificial urine, and the absorbent resin is swollen under pressure of 2.07kPa. The artificial urine is prepared by mixing 0.25g of calcium chloride dihydrate, 2.0g of potassium chloride, 0.50g of magnesium chloride hexahydrate, 2.0g of sodium sulfate, 0.85g of ammonium dihydrogen phosphate, 0.15g of diammonium hydrogen phosphate, and 994.25g of pure water.

[0473] After 60 minutes of pressurization, the height (cm) of the swollen absorbent resin gel layer was recorded. Next, the gel layer was pressurized to 2.07 kPa, and 0.69% (w / w) saline solution was flowed through it under this pressure. The room temperature was adjusted to 20–25°C. Using a scale and a computer, the amount of saline solution flowing through the gel layer was recorded every 20 seconds, and the flow rate Fs(T) was measured. The flow rate Fs(T) was calculated by dividing the mass (g) of saline solution added every 20 seconds by the flow time (s). The moment when the hydrostatic pressure of the saline solution became constant, thus stabilizing the flow rate, was defined as Ts. Using data measured over a 10-minute period starting from Ts, the flow rate Fs(T=0) was calculated. In other words, Fs(T=0) was calculated by plotting Fs(T) relative to time and based on the results obtained using the least squares method. Fs (T = 0) is the initial flow rate (g / s) of the saline solution flowing through the gel layer. Then, the saline solution conductivity (SFC) is calculated according to the following equation (7).

[0474] SFC={Fs(T=0)×L0} / (ρ×A×ΔP) Equation (7)

[0475] In equation (7), L0 is the height of the gel layer (cm), and ρ is the density of the saline solution (g / cm³). 3 ), where A is the cross-sectional area A (cm²) of the gel layer. 2 ), where ΔP is the hydrostatic pressure (dyne / cm) on the gel layer. 2 ).

[0476] [Water absorption rate based on the Vortex method]

[0477] The water absorption rate (in seconds) of the superabsorbent resin based on the Vortex method was determined in accordance with JIS K 7224 (1996) and following the procedure below.

[0478] Specifically, 0.02 parts by weight of the food additive Instant Blue No. 1 (CAS No. 3844-45-9) were first added to 1000 parts by weight of physiological saline for staining, and the liquid temperature was adjusted to 30℃. This solution was used as the test solution.

[0479] Next, 50 mL of the test solution was measured and placed into a 100 mL beaker. A cylindrical stir bar (stirring disc) with a length of 40 mm and a diameter of 8 mm was then placed into the beaker, and stirring was started at 600 rpm. Then, 2.0 g of water-absorbing resin was added to the stirred test solution, and the time it took for the cylindrical stir bar to be coated with the gelled test solution was measured, thus obtaining the water absorption rate based on the Vortex method. Hereinafter, the water absorption rate based on the Vortex method will be abbreviated as "Vortex method water absorption rate".

[0480] [Permeability-dependent absorption uptake ratio under pressure (PDAUP)]

[0481] The PDAUP of the absorbent resin was determined according to NWSP 243.0.R2(15). Specifically, 5.00 g of the absorbent resin was subjected to a pressure of 4.83 kPa (49 g / cm³). 2 After swelling in a 0.9% sodium chloride aqueous solution for 1 hour under a load of 0.7 psi, its water absorption ratio (PDAUP) (unit: g / g) was determined.

[0482] [Medium-mean particle size and logarithmic standard deviation]

[0483] The mass-average particle size (D50; in μm) of the superabsorbent resin and the logarithmic standard deviation (σζ) of the particle size distribution of the superabsorbent resin particles were determined according to the determination method described in U.S. Patent No. 7,638,570, "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution".

[0484] [Percentage of particles with a diameter of 150μm]

[0485] 10g of superabsorbent resin was classified using a JIS standard sieve (JIS Z8801-1(2000)) with a mesh size of 150μm or an equivalent sieve. Classification was performed for 5 minutes using a vibratory classifier (IIDA SIEVE SHAKER; ES-65; SER. No. 0501). After classification, the percentage of particles with a diameter less than 150μm was calculated using the following formula [mass %].

[0486] The percentage of particles with a diameter less than 150 μm [mass %] = {(mass of particles sieved through a 150 μm mesh [g]) / (mass of the superabsorbent resin [g])} × 100

[0487] [D50 (mass-average particle size) of granular hydrogels]

[0488] On the other hand, the mass-average particle size of the particulate hydrogel crosslinked polymer was determined using the following method.

[0489] Specifically, 20g of pulverized granular hydrogel (solid content α% by mass) at 20–25°C was added to 500g of a 20% by mass sodium chloride aqueous solution (hereinafter referred to as "EMAL aqueous solution") containing 0.08% by mass EMAL 20C (surfactant; manufactured by Kao Corporation) to prepare a dispersion. Then, the mixture was stirred at 300 rpm for 1 hour using a stir bar with a length of 50mm and a diameter of 7mm (a cylindrical polypropylene container with a height of 21cm and a diameter of 8cm, with a capacity of approximately 1.14L, was used).

[0490] After stirring, the dispersion was injected into the center of each JIS standard sieve (21cm in diameter; mesh sizes: 8mm, 4mm, 2mm, 1mm, 0.60mm, 0.30mm, 0.15mm, 0.075mm) stacked on a turntable. After rinsing all the particulate hydrogel onto the sieves with 100g of EMAL aqueous solution, the sieves were rotated by hand (20rpm) while a shower head (72 holes, 6.0L / min) was used to spray the solution from a height of 30cm above the sieves over a 50cm area. 2 6000g of EMAL aqueous solution was poured over the entire sieve to classify the granular hydrogel. The granular hydrogel on the first sieve after classification was drained for approximately 2 minutes and then weighed. The same process was repeated for the second and subsequent sieves. After draining, the residual granular hydrogel on each sieve was weighed.

[0491] The mass ratio X (in mass%) of the residual particulate hydrogel on each sieve relative to the total mass of the particulate hydrogel was calculated using Equation (8). For particulate hydrogels with a residual solid content of α% by mass on the sieve, the mesh size R(α) (in mm) of the corresponding sieve used for the particulate hydrogel was calculated according to Equation (9). The X and R(α) of the residual particulate hydrogel on each sieve were plotted on logarithmic probability paper to create a graph (particle size distribution) showing the relationship between the cumulative weight ratio of X and R(α). The particle size at which the residual percentage is equivalent to 50% by mass was read from this graph and used as the mass-average particle size (D50) of the particulate hydrogel.

[0492] X = (w / W) × 100 Equation (8)

[0493] R(α)=(20 / W) 1 / 3 ×r Equation (9)

[0494] Where X, w, W, R(α) and r refer to the following values.

[0495] X: Mass % of granular hydrogel remaining on each sieve after grading and draining (unit: mass %)

[0496] w: The individual mass (in g) of the granular hydrogel remaining on each sieve after grading and draining.

[0497] W: Total mass of granular hydrogel remaining on each sieve after grading and draining (unit: g)

[0498] R(α): The corresponding mesh size of the granular hydrogel after classification, converted to solid content α (mass%) (unit: mm; conversion value).

[0499] r: JIS standard sieve mesh size (unit: mm; measured value) obtained by grading granular hydrogels swollen in a 20% sodium chloride aqueous solution containing 0.08% by mass EMAL 20C (surfactant; manufactured by Kao Corporation).

[0500] [Specific surface area]

[0501] A microfocused X-ray CT system (inspeXioSMX-100CT manufactured by Shimadzu Corporation) was used to acquire 3D image data of the superabsorbent polymer (SAP), and the 3D image data was analyzed using high-speed 3D analysis software (TRI / 3D-VOL-FCS64 manufactured by Ratoc System Engineering Co., Ltd.) to determine the specific surface area of ​​the superabsorbent polymer.

[0502] The 3D image data obtained using the inspeXio SMX-100CT was acquired as follows: 1g of absorbent resin was filled into a small glass vial with a carcass diameter of 1cm and a total height of 4cm to prepare a sample. Then, double-sided tape was attached to the bottom of the vial to fix it to the material stage of the inspeXio SMX-100CT. Under this fixed condition, measurements were taken under the following conditions to obtain the 3D image data.

[0503] X-ray tube voltage (kV): 50

[0504] X-ray tube current (μA): 40

[0505] Size in inches: 4.0

[0506] X-ray filter: None

[0507] SDD (mm): 500

[0508] SRD (mm): 40

[0509] Z(mm): 108

[0510] X(mm): 0

[0511] Y(mm): 0

[0512] CT Mode 1: CBCT

[0513] CT Mode 2: Conventional Scan

[0514] Scanning angle: Full scan

[0515] Number of observations: 1200

[0516] Average: 5

[0517] Number of multidimensional rotations: None

[0518] Proofread by: YZ

[0519] Layer thickness (mm): 0.008

[0520] Layer spacing (mm): 0.010

[0521] Scaling factor: 50

[0522] BHC data: None

[0523] High-precision mode: Available

[0524] FOV XY (mm): 5.0

[0525] FOV Z (mm): 4.0

[0526] 3D pixel size (mm / voxel): 0.010

[0527] In addition, the image analysis using TRI / 3D-VOL-FCS64 is performed according to the following steps (1) to (6).

[0528] (1) Set the L value to 37580 and perform extraction processing on all particles (hygroscopic resin particles) in the measurement area; (2) Perform processing to exclude particles with a particle size of 10 voxels or less that are considered noise; (3) Perform extraction processing on the individual air bubbles inside each particle; (4) Perform synthesis processing on particles that were originally in a single state but can be regarded as multiple states, or perform separation processing on particles that were originally in a multiple state but can be regarded as a single state; (5) Perform processing to exclude particles at the edges; (6) Set the unit to voxel and calculate the total surface area, apparent total volume, and total volume of individual air bubbles for all particles in the measurement area. Here, apparent total volume refers to the total volume of all particles calculated assuming that there are no individual air bubbles inside the particles.

[0529] Using the values ​​obtained from the image analysis, the specific surface area of ​​the absorbent resin was calculated according to the following formula (10). Regarding the "true density of the absorbent resin" in formula (10), it is fixed at 1.7 kg / m³ in this invention. 3 The calculations are performed. The true density is determined according to the method disclosed in Japanese Patent No. 6093751. Alternatively, if the true density is unclear, the dry density of the absorbent resin pulverized to a particle size of less than 45 μm is measured and taken as the true density.

[0530] Specific surface area (m²) 2 / kg) = Total surface area (m²) 2 ) / {(apparent total volume (m 3 ) - Total volume of individual bubbles (m³) 3 True density of the absorbent resin (kg / m³) 3 Formula (10).

[0531] [Moisture content, solid content]

[0532] The moisture content (in mass%) of the absorbent resin was determined according to NWSP 230.0.R2(15). However, in this invention, the determination conditions specified in NWSP 230.0.R2(15) were changed, that is, the amount of absorbent resin as material was changed to 1.0 g and the drying temperature was changed to 180°C.

[0533] The solid content (unit: mass%) of the water-absorbing resin is calculated using the moisture content value measured as described above, by the following formula (11).

[0534] Solid content (mass%) = 100 - moisture content (mass%) Equation (11).

[0535] [Odor Assessment]

[0536] (Production of the absorber)

[0537] Two pieces of nonwoven fabric (LFPWTF hot-melt paper manufactured by Daio Paper Co., Ltd.) cut to 10cm x 16cm were overlapped, and their three edges were heat-sealed together using a heat-sealing machine, thus creating a nonwoven bag with only one opening. Next, 10 parts by weight of absorbent resin were filled into the nonwoven bag, and then the remaining edge was heat-sealed to seal it in a way that the absorbent resin would not leak out, thus obtaining an absorbent material for evaluation.

[0538] (Odor evaluation of absorbers)

[0539] The odor assessment of the absorber is carried out in the following steps (1) to (4).

[0540] (1) First, in such Figure 2 The absorbent was placed in a 2L glass bottle 8 (a short storage bottle, "2L Mame-kun"; manufactured by Ishizuka Glass Co., Ltd.) with a polyethylene cap 9 and a small cap 10 of 2.2cm in diameter; (2) 50g of physiological saline at a temperature of 23.5±0.5℃ was injected; (3) the cap 9 was quickly closed to seal the glass bottle 8, and it was placed in a room at a room temperature of 24℃; (4) after 10 minutes of sealing the glass bottle 8, the cap 9 was opened, and 10 adult evaluators smelled the odor in the air above the absorbent to evaluate the odor. Therefore, only one glass bottle 8 was prepared for each evaluator.

[0541] Regarding the evaluation method, if there is no odor, it is rated as 0 points; if a strong, unpleasant odor is detected, it is rated as 5 points. That is, the evaluation method is divided into 6 stages according to the degree of discomfort experienced by the subject. The lower the odor score, the less unpleasant the odor.

[0542] <Judgment Criteria>

[0543] 0: Same as the surrounding air

[0544] 1. There is a slight odor, but I can't quite put my finger on what it is (it's not unpleasant, and I don't mind it).

[0545] 2: There is a foul odor, but it is not unpleasant.

[0546] 3: It smells bad and is unpleasant.

[0547] 4: I strongly smell a foul odor, but it's not unpleasant.

[0548] 5: I have a strong feeling of foul odor, which is unpleasant.

[0549] Each evaluator conducted an odor assessment based on the aforementioned criteria, and then calculated the average value (rounding off the decimal places). This average value was used as the odor assessment value for the swollen gel formed by the swelling of absorbent resin. The reason for setting the rating of "strongly smelly odor, but not unpleasant" lower than "smells bad and is unpleasant" is that the unpleasantness of the odor varies from person to person. Therefore, when absorbent resin is used in sanitary materials, products with "strongly smelly odor" pose a higher risk of adverse effects.

[0550] [acrylic acid]

[0551] In the acrylic acid used in the examples, the amount of p-methoxyphenol was 70 ppm, and the amounts of protoanemonin and aldehyde were both "ND" (less than 1 ppm). In addition, the amount of acetic acid in the acrylic acid was 1470 ppm, the amount of propionic acid was 270 ppm, and the amount of acrylic acid dimer was 90 ppm.

[0552] [Manufacturing Example 1]

[0553] 1.8 g of polyethylene glycol acrylate (with an average ethoxylated molar number of 9) was dissolved in 2000 g of a sodium acrylate aqueous solution with a neutralization rate of 75 mol% (monomer concentration of 39% by mass, Fe content of 0.28 ppm), and this was used as reaction solution (1). The resulting reaction solution (1) was injected into a stainless steel cylinder-shaped container with dimensions of 320 mm (length) × 220 mm (width) × 50 mm (height) and equipped with a magnetic stirrer. At this time, the depth of the reaction solution was 23 mm. The top of the cylinder-shaped container was sealed with a polyethylene film having a nitrogen inlet, an outlet, and a polymerization initiator inlet. Then, the cylinder-shaped container was immersed in a water bath at a temperature of 25°C, and nitrogen gas was introduced into the reaction solution while maintaining the temperature of the reaction solution at 25°C, thereby removing dissolved oxygen from the reaction solution. Next, nitrogen gas was introduced into the space above the reaction solution in the cylinder-shaped container and the gas was continuously vented. That is, the atmosphere of the space was replaced with nitrogen gas.

[0554] Subsequently, 10.5 g of a 10% by mass aqueous solution of sodium persulfate and 1.4 g of a 1% by mass aqueous solution of L-ascorbic acid were added to the aforementioned jar-shaped container as polymerization initiators and thoroughly mixed using a magnetic stirrer. Two minutes after the addition of the polymerization initiators, the polymerization reaction began. The polymerization temperature was then controlled by intermittently immersing the jar-shaped container in a water bath at 12°C until the bottom of the container was 10 mm above the water surface. Fifteen minutes after the start of the polymerization reaction, the polymerization temperature reached 85°C (peak temperature). To cure the resulting hydrogel-like polymer, the jar-shaped container was then immersed in a water bath at 60°C until the bottom of the container was 10 mm above the water surface and maintained for 20 minutes. The obtained hydrogel polymer was then gel-crushed using a meat grinder (model HL-G22SN; manufactured by REMACOM Co., Ltd.) equipped with 11 mm diameter and 18 pores to obtain granular hydrogel crosslinked polymer (1). The mass-average particle size of the granular hydrogel crosslinked polymer (1) was 2500 μm.

[0555] The granular hydrogel crosslinked polymer (1) was spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 180°C for 30 minutes using a batch air-drying dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). The dried material was then pulverized using a roller mill, and the pulverized material was graded using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded granular crosslinked polymer powder (a) with a particle size of 150 μm to 710 μm and a randomly fragmented state, and micro-powdered crosslinked polymer powder (b) with a particle size of less than 150 μm.

[0556] By repeatedly performing the above operations, 500g of crosslinked polymer powder (b) was obtained. 300g of this crosslinked polymer powder (b) was placed in a 5L type mortar mixer (manufactured by West Japan Test Machine Manufacturing Co., Ltd.) kept warm in a water bath at 80°C. Then, while the stirring blades of the mortar mixer were rotated at high speed at 60Hz and 100V, 450g of water, adjusted to 80°C for granulation, was added to the mortar mixer in one go. Within 10 seconds after the water was added, the crosslinked polymer powder (b) and water were mixed to form granules. After 10 minutes from the time the water was added, the granules were removed, and granulated gel with a particle size of 3-10mm was obtained. Then, 600g of the obtained granulated gel was slightly mixed with 600g of granular hydrogel crosslinked polymer (1) obtained by repeatedly performing the above operations. The resulting mixture was then spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 180°C for 30 minutes using a batch air-drying dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). Next, the dried material was pulverized using a roller mill, and then classified using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded a crosslinked polymer powder (c) with a particle size of 150 μm to 710 μm and a randomly fragmented shape. The physical properties of this crosslinked polymer powder (c) and the crosslinked polymer powder (a) are shown in Table 2.

[0557] [Manufacturing Example 2]

[0558] An aqueous solution (1) was prepared by mixing 400 parts by weight of acrylic acid, 185 parts by weight of 48% sodium hydroxide aqueous solution, 2.3 parts by weight of polyethylene glycol diacrylate (with an average molar addition of 9 ethoxy groups), 1.3 parts by weight of 2% diethylenetriamine 5-acetic acid 3-sodium aqueous solution, 5 parts by weight of 10% polyoxyethylene octadecenyl ether (manufactured by Kao Corporation) aqueous solution, and 368 parts by weight of deionized water. Here, the deionized water was preheated to 40°C.

[0559] Next, while stirring the aqueous solution (1), 185 parts by mass of a 48% sodium hydroxide aqueous solution were injected into the aqueous solution (1) over approximately 30 seconds under open atmospheric conditions, and the mixture was prepared. Thus, a monomeric aqueous solution (1) was prepared. Here, due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomeric aqueous solution (1) rose to approximately 84°C.

[0560] Subsequently, when the temperature of the monomer aqueous solution (1) reached 83°C, 13 parts by mass of 5% sodium persulfate aqueous solution as a polymerization initiator were added and stirred for about 5 seconds, which was used as the reaction solution (2).

[0561] Next, the reaction solution (2) was poured into a stainless steel cylinder-shaped container (340×340mm at the bottom, 25mm in height, with Teflon (registered trademark) lining the inside) under open air. Here, the cylinder-shaped container was preheated with a heating plate to a surface temperature of 40°C.

[0562] After the reaction solution (2) was poured into the cylinder-shaped container, the polymerization reaction began within 1 minute. Due to this polymerization reaction, the reaction solution (2) generated water vapor and expanded upwards and in all directions, thus continuing the polymerization reaction. It then contracted to a size slightly larger than the bottom of the cylinder-shaped container. The polymerization reaction ended within approximately 1 minute. Through this polymerization reaction, a hydrogel-like crosslinked polymer (2) was obtained.

[0563] Next, the hydrogel-like crosslinked polymer (2) was cut into appropriate sizes and then gel-crushed using a meat grinder (model HL-G22SN; manufactured by REMACOM Co., Ltd.) equipped with 8mm diameter and 33 pores to obtain granular hydrogel-like crosslinked polymer (2). The mass-average particle size of the granular hydrogel-like crosslinked polymer (2) was 700μm.

[0564] The granular hydrogel crosslinked polymer (2) was spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 180°C for 30 minutes using a batch air-dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). The dried material was then pulverized using a roller mill, and the pulverized material was graded using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded a randomly fragmented (granular) crosslinked polymer powder (d) with a particle size of 150 μm to 710 μm. The properties of this crosslinked polymer powder (d) are shown in Table 2.

[0565] [Manufacturing Example 3]

[0566] An aqueous solution (2) was prepared by mixing 400 parts by mass of acrylic acid, 185 parts by mass of 48% sodium hydroxide aqueous solution, 2.5 parts by mass of polyethylene glycol diacrylate (with an average molar addition of 9 ethoxy groups), 1.3 parts by mass of 2% diethylenetriamine 5-acetic acid 3-sodium aqueous solution, and 373 parts by mass of deionized water in a 2L polypropylene container. Here, the deionized water was preheated to 40°C.

[0567] Next, while stirring the aqueous solution (2), 185 parts by mass of a 48% sodium hydroxide aqueous solution were injected into the aqueous solution (2) over approximately 30 seconds under open atmospheric conditions, and the mixture was prepared. Thus, a monomeric aqueous solution (2) was prepared. Here, due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomeric aqueous solution (2) rose to approximately 84°C.

[0568] Then, when the temperature of the monomer aqueous solution (2) reached 83°C, 13 parts by mass of 5% sodium persulfate aqueous solution as a polymerization initiator were added and stirred for about 5 seconds, which was used as the reaction solution (3).

[0569] Next, the reaction solution (3) was poured into a stainless steel cylinder-shaped container (340×340mm at the bottom, 25mm in height, with Teflon (registered trademark) lining the inside) under open air. Here, the cylinder-shaped container was preheated with a heating plate to a surface temperature of 40°C.

[0570] After the reaction solution (3) was poured into the cylinder-shaped container, the polymerization reaction began within 1 minute. Due to this polymerization reaction, the reaction solution (3) generated water vapor and expanded upwards and in all directions, thus continuing the polymerization reaction. It then contracted to a size slightly larger than the bottom of the cylinder-shaped container. The polymerization reaction ended within approximately 1 minute. Through this polymerization reaction, a hydrogel-like crosslinked polymer (3) was obtained.

[0571] Next, the hydrogel-like crosslinked polymer (3) was cut into appropriate sizes and then gel-crushed using a meat grinder (model HL-G22SN; manufactured by REMACOM Co., Ltd.) equipped with 6mm diameter and 52 pores to obtain granular hydrogel-like crosslinked polymer (3). The mass-average particle size of the granular hydrogel-like crosslinked polymer (3) was 400μm.

[0572] The granular hydrogel crosslinked polymer (3) was spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 180°C for 30 minutes using a batch air-drying dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). The dried material was then pulverized using a roller mill, and the pulverized material was graded using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded a randomly fragmented (granular) crosslinked polymer powder (e) with a particle size of 150 μm to 710 μm. The properties of this crosslinked polymer powder (e) are shown in Table 2.

[0573] [Manufacturing Example 4]

[0574] An aqueous solution (3) was prepared by mixing 291.0 parts by weight of acrylic acid, 0.43 parts by weight of polyethylene glycol diacrylate (with an average molar addition of ethoxy groups of 9), 3.6 parts by weight of an acrylic acid solution prepared by dissolving 1.0% by weight of IRGACURE (registered trademark) 184 in acrylic acid, 0.61 parts by weight of 0.45% by weight of sodium diethylenetriamine 5-acetate, and 255 parts by weight of deionized water. Here, the deionized water was preheated to 50°C.

[0575] Next, while stirring the aqueous solution (3), 247 parts by mass of a 48% sodium hydroxide aqueous solution were injected into the aqueous solution (3) over approximately 30 seconds under open atmospheric conditions, and the mixture was prepared. Thus, a monomeric aqueous solution (4) was prepared. Here, due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomeric aqueous solution (4) rose to approximately 100°C.

[0576] Subsequently, when the temperature of the monomer aqueous solution (4) reached 98°C, 1.8 parts by mass of 3% sodium persulfate aqueous solution as a polymerization initiator were added and stirred for about 1 second, which was used as the reaction solution (4).

[0577] Next, the reaction solution (4) was poured into a stainless steel cylinder-shaped container (340×340mm bottom, 25mm height, with Teflon (registered trademark) lining the inside) under open air. Additionally, ultraviolet light was applied while the reaction solution (4) was being poured into the stainless steel cylinder-shaped container.

[0578] After the reaction solution (4) was poured into the cylinder-shaped container, the polymerization reaction began within 1 minute. After 3 minutes, the ultraviolet irradiation was stopped, and a hydrogel-like crosslinked polymer (4) was obtained.

[0579] Next, the hydrogel-like crosslinked polymer (4) was cut into appropriate sizes and then gel-crushed using a meat grinder (model HL-G22SN; manufactured by REMACOM Co., Ltd.) equipped with 7.5 mm diameter and 38 pores to obtain granular hydrogel-like crosslinked polymer (4). The mass-average particle size of the granular hydrogel-like crosslinked polymer (4) was 1000 μm.

[0580] The particulate hydrogel crosslinked polymer (4) was spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 180°C for 30 minutes using a batch air-drying dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). The dried material was then pulverized using a roller mill, and the pulverized material was graded using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded a randomly fragmented (particulate) crosslinked polymer powder (f) with a particle size of 150 μm to 710 μm. The properties of this crosslinked polymer powder (f) are shown in Table 2.

[0581] [Manufacturing Example 5]

[0582] A reactor was constructed by fitting a lid onto a 10L double-arm stainless steel kneader equipped with two Sigma-shaped stirring blades and a housing. 425.2 parts by mass of acrylic acid, 4499.5 parts by mass of a 37% sodium acrylate aqueous solution, 538.5 parts by mass of deionized water, and 4.3 parts by mass of polyethylene glycol diacrylate (with an average ethoxylated molar number of 9) were added to the reactor to prepare reaction solution (5). The reaction solution (5) was then degassed under a nitrogen atmosphere for 30 minutes.

[0583] Next, while stirring the reaction solution (5), 28.3 parts by mass of 10% sodium persulfate aqueous solution and 1.5 parts by mass of 1% L-ascorbic acid aqueous solution were added. Polymerization began approximately 1 minute after the addition. After 17 minutes of polymerization, the temperature of the reaction solution (5) reached the peak polymerization temperature of 86°C. After 60 minutes of polymerization, the hydrogel-like crosslinked polymer (5) was removed from the reactor. Here, the obtained hydrogel-like crosslinked polymer (5) was a finely granulated hydrogel-like crosslinked polymer. The mass-average particle size of this granular hydrogel-like crosslinked polymer (5) was 1500 μm.

[0584] The granular hydrogel crosslinked polymer (5) was spread on a 50-mesh (300 μm) metal mesh and dried with hot air at 170°C for 65 minutes using a batch air-drying dryer (model 71-S6; manufactured by Satake Chemical Machinery Co., Ltd.). The dried product was then pulverized using a roller mill, and the pulverized product was graded using metal meshes with mesh sizes of 710 μm and 150 μm. This yielded randomly fragmented (granular) crosslinked polymer powder (g) with particle sizes ranging from 150 μm to 710 μm. The physical properties of this crosslinked polymer powder (g) are shown in Table 2.

[0585] [Table 2]

[0586] (Table 2)

[0587]

[0588] [Comparative Example 1]

[0589] 100 parts by weight of the obtained crosslinked polymer powder (c) were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent consisting of 0.3 parts by weight of ethylene carbonate, 0.7 parts by weight of propylene glycol, and 3 parts by weight of deionized water. The resulting mixture was heated for 40 minutes using a mixer at a heat transfer medium temperature of 210°C and then broken down to pass through a JIS standard sieve with a mesh size of 710 μm, thereby obtaining a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (1). The physical properties of this base water-absorbing resin (1) are shown in Table 3.

[0590] [Comparative Example 2]

[0591] 100 parts by weight of the crosslinked polymer powder (c) obtained in Manufacturing Example 1 were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent composed of 1.0 parts by weight of triethylene glycol and 3 parts by weight of deionized water using a sprayer. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (2), was obtained. The physical properties of this base water-absorbing resin (2) are shown in Table 3.

[0592] [Comparative Example 3]

[0593] 100 parts by weight of the crosslinked polymer powder (c) obtained in Manufacturing Example 1 were sprayed with a 6.0 part by weight aqueous solution of a surface crosslinking agent consisting of 1.0 part by weight of ethylene carbonate, 1.0 part by weight of propylene carbonate, and 4 parts by weight of deionized water using a sprayer. All other procedures were performed in the same manner as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (3), was obtained. The physical properties of this base water-absorbing resin (3) are shown in Table 3.

[0594] [Comparative Example 4]

[0595] 100 parts by weight of the crosslinked polymer powder (d) obtained in Manufacturing Example 2 were sprayed with an aqueous solution (3.3 parts by weight) of a surface crosslinking agent consisting of 0.8 parts by weight of ethylene glycol and 2.5 parts by weight of deionized water using a sprayer. All other procedures were performed in the same manner as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (4), was obtained. The physical properties of this base water-absorbing resin (4) are shown in Table 3.

[0596] [Comparative Example 5]

[0597] 100 parts by weight of the crosslinked polymer powder (d) obtained in Manufacturing Example 2 were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent consisting of 0.8 parts by weight of propylene glycol, 0.8 parts by weight of 1,6-hexanediol, and 2.4 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (5), was obtained. The physical properties of this base water-absorbing resin (5) are shown in Table 3.

[0598] [Comparative Example 6]

[0599] 100 parts by weight of the crosslinked polymer powder (d) obtained in Manufacturing Example 2 were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent composed of 0.5 parts by weight of triethylene glycol, 0.5 parts by weight of propylene glycol, and 3.0 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (6), was obtained. The physical properties of this base water-absorbing resin (6) are shown in Table 3.

[0600] [Comparative Example 7]

[0601] 100 parts by weight of the obtained crosslinked polymer powder (e) were sprayed with an aqueous solution (3.8 parts by weight) of a surface crosslinking agent consisting of 0.4 parts by weight of 1,4-butanediol, 0.6 parts by weight of propylene glycol, and 2.8 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (7), was obtained. The physical properties of this base water-absorbing resin (7) are shown in Table 3.

[0602] [Comparative Example 8]

[0603] 100 parts by weight of the obtained crosslinked polymer powder (e) were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent consisting of 0.3 parts by weight of triethylene glycol, 0.3 parts by weight of 1,6-hexanediol, and 3.4 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (8), was obtained. The physical properties of this base water-absorbing resin (8) are shown in Table 3.

[0604] [Comparative Example 9]

[0605] 100 parts by weight of the obtained crosslinked polymer powder (e) were sprayed with an aqueous solution (4.0 parts by weight) of a surface crosslinking agent consisting of 0.4 parts by weight of ethylene carbonate, 0.7 parts by weight of 1,6-hexanediol, and 2.9 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (9), was obtained. The physical properties of this base water-absorbing resin (9) are shown in Table 3.

[0606] [Comparative Example 10]

[0607] 100 parts by weight of the obtained crosslinked polymer powder (a) were sprayed with an aqueous solution (5.0 parts by weight) of a surface crosslinking agent consisting of 0.4 parts by weight of ethylene carbonate, 0.7 parts by weight of 1,6-hexanediol, and 3.9 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (10), was obtained. The physical properties of this base water-absorbing resin (10) are shown in Table 3.

[0608] [Comparative Example 11]

[0609] 100 parts by weight of the obtained crosslinked polymer powder (f) were sprayed with an aqueous solution (5.03 parts by weight) of a surface crosslinking agent consisting of 0.03 parts by weight of ethylene glycol diglycidyl ether, 1.50 parts by weight of propylene glycol, and 3.50 parts by weight of deionized water. The resulting mixture was heated for 45 minutes in a mixer at a heat transfer medium temperature of 100°C and then broken down to pass through a JIS standard sieve with a mesh size of 710 μm, thereby obtaining the surface-crosslinked water-absorbing resin, namely the base water-absorbing resin (11). The physical properties of the base water-absorbing resin (11) are shown in Table 3.

[0610] [Comparative Example 12]

[0611] 100 parts by weight of the obtained crosslinked polymer powder (g) were sprayed with an aqueous solution (3.8 parts by weight) of a surface crosslinking agent consisting of 0.3 parts by weight of 1,4-butanediol, 0.5 parts by weight of propylene glycol, and 3.0 parts by weight of deionized water. The procedure was otherwise the same as in Comparative Example 1. Thus, a surface-crosslinked water-absorbing resin, namely, a base water-absorbing resin (12), was obtained. The physical properties of this base water-absorbing resin (12) are shown in Table 3.

[0612] [Table 3]

[0613]

[0614] [Example 1]

[0615] The base material, water-absorbing resin (1), was heated to 150°C and then fed into an indirect-heating twin-shaft dryer (model CD-80; manufactured by Kurimoto Iron Works Co., Ltd.) with an internal temperature set at 150°C at a feed rate of 3.0 kg / hr. Simultaneously, deionized water was uniformly added from the raw material inlet using a sprayer at a feed rate of 2.0 kg / hr. The rotation speed of the stirring blades of the indirect-heating twin-shaft dryer was set to 20 rpm, and the baffle at the dryer outlet was adjusted to ensure that the amount of powder (water-absorbing resin and deionized water) retained inside the dryer was 2.5 kg, thus continuously performing stirring and drying. The residence time of the powder inside the dryer, i.e., the drying time, was 50 minutes. By continuously drying and discharging the powder, water-absorbing resin (1) was obtained. The physical properties of this water-absorbing resin (1) are shown in Table 4.

[0616] [Example 2]

[0617] The base water-absorbing resin (1) was replaced with a base water-absorbing resin (2), and the temperature inside the dryer was changed to 60°C. The base water-absorbing resin (2) was fed at a rate of 2.5 kg / hr, and tap water was added at a rate of 0.28 kg / hr. The residence time of the base water-absorbing resin (2) inside the dryer, i.e., the drying time, was adjusted to 60 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, water-absorbing resin (2) was obtained. The physical properties of the water-absorbing resin (2) are shown in Table 4.

[0618] [Example 3]

[0619] The base material, water-absorbing resin (3), was heated to 150°C and then fed into an indirect-heating twin-shaft dryer (model CD-80; manufactured by Kurimoto Iron Works Co., Ltd.) with an internal temperature set at 85°C at a feed rate of 3.00 kg / hr. Simultaneously, a 3.18% by mass sodium sesquicarbonate aqueous solution was uniformly added from the raw material inlet using a sprayer at a feed rate of 0.66 kg / hr. Here, the rotation speed of the stirring blades of the indirect-heating twin-shaft dryer was set to 20 rpm, and the baffle at the dryer outlet was adjusted to maintain the amount of powder (water-absorbing resin and 3.18% by mass sodium sesquicarbonate aqueous solution) retained inside the dryer at 2.5 kg, thereby continuously performing stirring and drying. The residence time of the powder inside the dryer, i.e., the drying time, was 50 minutes. By continuously drying and discharging the powder, water-absorbing resin (3) was obtained. The physical properties of the absorbent resin (3) are shown in Table 4.

[0620] [Example 4]

[0621] The base water-absorbing resin (3) was replaced with a base water-absorbing resin (4), and 0.4% by mass of L-cysteine ​​aqueous solution was added at a feed rate of 0.61 kg / hr instead of sodium hemicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (4) was obtained. The physical properties of this water-absorbing resin (4) are shown in Table 4.

[0622] [Example 5]

[0623] The base water-absorbing resin (5) was used instead of the base water-absorbing resin (3), and 0.09% by mass of fatty acid glycerides (manufactured by Kao Corporation; trade name: EXCEL 122V) aqueous solution was added at a feed rate of 1.36 kg / hr instead of the addition of sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (5) was obtained. The physical properties of this water-absorbing resin (5) are shown in Table 4.

[0624] [Example 6]

[0625] The base water-absorbing resin (6) was used instead of the base water-absorbing resin (3), and 1.00% by mass sodium dihydrogen phosphate aqueous solution was added at a feed rate of 0.61 kg / hr instead of sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (6) was obtained. The physical properties of this water-absorbing resin (6) are shown in Table 4.

[0626] [Example 7]

[0627] The base water-absorbing resin (3) was replaced with a base water-absorbing resin (7), and 0.67% by mass of adipic acid dihydrazide aqueous solution was added at a feed rate of 0.45 kg / hr instead of sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (7) was obtained. The physical properties of this water-absorbing resin (7) are shown in Table 4.

[0628] [Example 8]

[0629] 30 parts by weight of the base water-absorbing resin (8) and 3 parts by weight of adipic dihydrazide were placed into a 225 ml wide-mouth bottle and mixed by vibration (vibration for 3 minutes at room temperature) using a paint mixer (manufactured by Toyo Seiki Co., Ltd.) to obtain the water-absorbing resin (8). The physical properties of the water-absorbing resin (8) are shown in Table 4.

[0630] [Example 9]

[0631] 50 parts by weight of the base water-absorbing resin (9) were loaded into a dispensing stainless steel tubing (manufactured by GL SCIENCES Co., Ltd.; Cat. no. 6010-15023), and supercritical carbon dioxide was injected for 24 hours at a flow rate of 7.0 g / min under conditions of 83.5°C and 21.0 MPa. Thus, water-absorbing resin (9) was obtained. The physical properties of this water-absorbing resin (9) are shown in Table 4.

[0632] [Example 10]

[0633] The base absorbent resin (3) was replaced with a base absorbent resin (10), and 0.50% by mass of a sodium polyoxyethylene alkyl thiosuccinate-based anionic surfactant (manufactured by Sanyo Chemical Co., Ltd.; trade name: BEAULIGHT ESS) aqueous solution was added at a feed rate of 0.24 kg / hr instead of the sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, absorbent resin (10) was obtained. The properties of this absorbent resin (10) are shown in Table 4.

[0634] [Example 11]

[0635] The base absorbent resin (3) was replaced with a base absorbent resin (11), and 0.33% by mass of an aqueous solution of glyceryl monooleate (manufactured by Kao Corporation; trade name: RHEODOL MO-60) was added at a feed rate of 0.37 kg / hr instead of an aqueous solution of sodium sesquicarbonate. Except for these changes, the same operation as in Example 3 was performed. Thus, absorbent resin (11) was obtained. The physical properties of this absorbent resin (11) are shown in Table 4.

[0636] [Example 12]

[0637] The base water-absorbing resin (3) was replaced with a base water-absorbing resin (12), and a 1.00% by mass sodium triphosphate aqueous solution was added at a feed rate of 1.07 kg / hr instead of a sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (12) was obtained. The properties of this water-absorbing resin (12) are shown in Table 4.

[0638] [Example 13]

[0639] The base absorbent resin (1) was replaced with a base absorbent resin (3), and the base absorbent resin (3) was fed at a rate of 2.5 kg / hr. Deionized water was also added at a rate of 1.67 kg / hr, thereby adjusting the residence time of the base absorbent resin (3) inside the dryer, i.e., the drying time, to 60 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (13) was obtained. The physical properties of the absorbent resin (13) are shown in Table 4.

[0640] [Example 14]

[0641] The base absorbent resin (1) was replaced with a base absorbent resin (4), and the temperature inside the dryer was changed to 90°C. The base absorbent resin (4) was fed at a rate of 3.0 kg / hr, and deionized water was added at a rate of 0.75 kg / hr. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (14) was obtained. The properties of this absorbent resin (14) are shown in Table 4.

[0642] [Example 15]

[0643] The base absorbent resin (1) was replaced with a base absorbent resin (5), and the temperature inside the dryer was changed to 120°C. The base absorbent resin (5) was fed at a rate of 2.5 kg / hr, and deionized water was added at a rate of 1.07 kg / hr. The residence time of the base absorbent resin (5) inside the dryer, i.e., the drying time, was adjusted to 60 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (15) was obtained. The physical properties of the absorbent resin (15) are shown in Table 4.

[0644] [Example 16]

[0645] The base absorbent resin (1) was replaced with a base absorbent resin (6), and the temperature inside the dryer was changed to 120°C. The base absorbent resin (6) was fed at a rate of 2.5 kg / hr, and deionized water was added at a rate of 1.67 kg / hr. The residence time of the base absorbent resin (6) inside the dryer, i.e., the drying time, was adjusted to 60 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (16) was obtained. The physical properties of the absorbent resin (16) are shown in Table 4.

[0646] [Example 17]

[0647] The base absorbent resin (1) was replaced with a base absorbent resin (7), and the base absorbent resin (7) was fed at a rate of 3.75 kg / hr. Deionized water was also added at a rate of 1.6 kg / hr, thereby adjusting the residence time of the base absorbent resin (7) inside the dryer, i.e., the drying time, to 40 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (17) was obtained. The physical properties of the absorbent resin (17) are shown in Table 4.

[0648] [Example 18]

[0649] The base absorbent resin (1) was replaced with a base absorbent resin (8), and the temperature inside the dryer was changed to 90°C. The base absorbent resin (8) was fed at a rate of 5.0 kg / hr, and deionized water was added at a rate of 0.55 kg / hr. The residence time of the base absorbent resin (8) inside the dryer, i.e., the drying time, was adjusted to 30 minutes. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (18) was obtained. The physical properties of the absorbent resin (18) are shown in Table 4.

[0650] [Example 19]

[0651] The base absorbent resin (1) was replaced with a base absorbent resin (9), and the temperature inside the dryer was changed to 100°C. The base absorbent resin (9) was fed at a rate of 3.0 kg / hr, and deionized water was added at a rate of 0.75 kg / hr. Except for these changes, the same operation as in Example 1 was performed. Thus, absorbent resin (19) was obtained. The physical properties of this absorbent resin (19) are shown in Table 4.

[0652] [Example 20]

[0653] The base water-absorbing resin (2) was used instead of the base water-absorbing resin (3), and 1.5% by mass of aminooxyacetic acid hemihydrochloride aqueous solution was added at a feed rate of 0.30 kg / hr instead of the addition of sodium sesquicarbonate aqueous solution. Except for these changes, the same operation as in Example 3 was performed. Thus, water-absorbing resin (20) was obtained. The properties of this water-absorbing resin (20) are shown in Table 4.

[0654] [Comparative Example 13]

[0655] 100 parts by weight of the base absorbent resin (4) were sprayed with an aqueous liquid containing 0.03 parts by weight of a copolymer consisting of 73 mol% methacrylic acid and 27 mol% methoxy polyethylene glycol methacrylate, and 6 parts by weight of deionized water. Here, the ethylene glycol addition number n of the methoxy polyethylene glycol methacrylate was 25, and the mass-average molecular weight (Mw) of the copolymer was 20,000. The resulting mixture was placed in a mixer with a heat transfer medium temperature of 98°C, the pressure was reduced to 700 mmH2O, and the mixture was stirred for 60 minutes. Thus, a comparative absorbent resin (13) was obtained. Various conditions are recorded in Table 4, and the physical properties of the comparative absorbent resin (13) are shown in Table 4.

[0656] [Comparative Example 14]

[0657] According to Example 4 of WO2012 / 108253, a comparative water-absorbing resin (14) was obtained. The specific preparation method is as follows.

[0658] (Preparation of the first aqueous solution)

[0659] 92 g of an 80.5% by mass aqueous solution of acrylic acid was weighed and placed into a 500 ml Erlenmeyer flask. While cooling from the outside, 156.2 g of a 23.7% by mass aqueous solution of sodium hydroxide was added dropwise to achieve a 90 mol% neutralization. The solution was then stirred at room temperature to ensure complete dissolution. 0.11 g of potassium persulfate and 9.2 mg of ethylene glycol diglycidyl ether were then added to dissolve the monomer, thus preparing the first aqueous solution of the monomer.

[0660] (Preparation of the second aqueous solution)

[0661] 128.8 g of an 80.5% by mass aqueous solution of acrylic acid was weighed and placed into a 500 ml Erlenmeyer flask. Then, while cooling from the outside, 150.2 g of a 23.0% by mass aqueous solution of sodium hydroxide was added dropwise to achieve a 60 mol% neutralization. Next, 0.15 g of potassium persulfate and 12.9 mg of N,N'-methylenebisacrylamide were added to dissolve the monomer, thus preparing a second aqueous solution. The temperature of this second aqueous solution was maintained at approximately 23 °C.

[0662] (Process 1)

[0663] A dispensable cylindrical flask with an inner diameter of 100 mm and a reflux condenser, equipped with a reflux cooler, a dropping funnel, a nitrogen inlet tube, and a stirrer (with a double-mounted, 50 mm diameter, four-bladed, inclined impeller), was prepared. 500 ml of n-heptane was added to the flask, along with 0.92 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Ltd.'s commercial product "High Wax 1105A"). The mixture was then heated to 80°C to dissolve, and subsequently cooled to 60°C.

[0664] The stirrer was set to 300 rpm, and the first aqueous solution was added all at once to a dispensable flask using a funnel. The flask's internal temperature was then adjusted to 40°C and stirred for 10 minutes to disperse the solution. Next, using a funnel, 0.92 g of sucrose stearate (Ryoto sucrose ester S-370, manufactured by Mitsubishi Chemical FOODS Co., Ltd.) as a surfactant was added to the dispensable flask by heating and dissolving it in 8.5 g of n-heptane. The stirring speed was then increased to 500 rpm to further disperse the first aqueous solution.

[0665] Next, the stirrer was set to 450 rpm, and the contents of the dispensable flask were purged with nitrogen while maintaining the temperature at 40°C for 30 minutes. Afterward, the flask was immersed in a 70°C water bath for heating, and polymerization yielded a slurry of spherical primary particles. An azeotropic reaction of water and n-heptane was performed in a 120°C oil bath to remove only water from a portion of the slurry system. The n-heptane was then evaporated for drying, resulting in spherical primary particles with a median particle size of 80 μm.

[0666] (Process 2)

[0667] The stirring speed of the slurry obtained in step 1 was changed to 1000 rpm, and the mixture was cooled to 23°C. Then, a second aqueous solution was added to the slurry. Next, the flask was purged with nitrogen and kept at this temperature for 30 minutes. Then, the flask was immersed in a 70°C water bath again to raise the temperature, thereby carrying out polymerization. As a result, a slurry containing secondary particles formed by the aggregation of primary particles was obtained.

[0668] (Post-crosslinking process)

[0669] Following step 2, the flask was heated in a 120°C oil bath to induce an azeotropic reaction between water and n-heptane, thereby removing 251.7 g of water from the system while the n-heptane was refluxed. Next, 8.83 g of a 2% aqueous solution of ethylene glycol diglycidyl ether as a post-crosslinking agent was added to the contents of the flask, and the mixture was kept at 80°C for 2 hours. Afterward, the n-heptane was evaporated for drying. Thus, 230.9 g of a comparative water-absorbing resin (4) composed of two particles with completed surface crosslinking was obtained.

[0670] [Table 4]

[0671]

[0672] [summary]

[0673] As can be clearly seen from Table 4, the concentration of volatile components at 1.0 times swelling of the water-absorbing resins 1 to 20 obtained in Examples 1 to 20 is all below 3.5 ppm, and the odor evaluation is all 0 to 2, that is, all of them have no unpleasant odor at all or almost no noticeable odor.

[0674] As can be clearly seen from Tables 3 and 4, the base water-absorbing resins 1 to 12 obtained in Comparative Examples 1 to 12, and the comparative water-absorbing resins (13) and (14), all had a volatile component concentration of 3.9 ppm or higher when 1.0 times swollen, and the odor rating of all were 3 or higher, meaning that an unpleasant odor was clearly noticeable in all of them.

[0675] A comparison between Examples 1-20 and Comparative Examples 1-14 clearly shows that if the concentration of volatile components at 1.0 times swelling of the water-absorbing resin is 3.5 ppm or less, a water-absorbing resin that sufficiently reduces odor during swelling can be achieved. Furthermore, it is also evident that the various physical properties of water-absorbing resins 1-20 are maintained, meeting the recent demands for absorbency and other requirements in sanitary materials.

[0676] [Industry availability]

[0677] The absorbent resin of one embodiment of the present invention can be used in the manufacture of: an absorbent resin whose absorbent properties are maintained and whose odor generated during swelling is reduced, and sanitary materials containing the absorbent resin.

[0678] <Explanation of Figure Markers>

[0679] 1. Carbon dioxide cylinder

[0680] 2. Pressure regulating valve

[0681] 3. High-pressure liquid delivery pump

[0682] 4. Cooling device

[0683] 5 Pressure-resistant extraction tank

[0684] 6. Pressure reducing valve

[0685] 7 Flowmeter

[0686] 8 glass bottles

[0687] 9. Polyethylene cap

[0688] 10. Polyethylene cap

Claims

1. A method for manufacturing a water-absorbing resin, characterized in that, In order, they include: The polymerization process involves polymerizing a monomer composition comprising acrylic monomers and / or acrylate monomers to obtain a hydrogel-like crosslinked material, a drying process involving drying the hydrogel-like crosslinked material obtained in the polymerization process, and a surface crosslinking process. and, After the surface crosslinking process is completed, a process of adding a reducing agent with amino groups is included. The reducing agents having amino groups include compounds containing hydrazide groups. The water-absorbing resin is a water-absorbing resin that has undergone surface cross-linking. The volatile component concentration of the water-absorbing resin after standing for 15 minutes at a swelling ratio of 1.0 is below 3.5 ppm (i.e., the volatile component concentration at 1.0 swelling ratio). The concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0 is defined as the total concentration of all substances detected by a photoionization detector (PID) with a 10.6 eV irradiation lamp after uniformly adding 10.0 g of physiological saline at 23.5 ± 0.5 °C to 10.0 g of absorbent resin in a 2 L sealable glass container at room temperature and pressure, and standing for 15 minutes in a sealed state. This concentration is expressed as a value based on the detection value converted from isobutylene calibration gas.

2. The method for manufacturing the water-absorbing resin according to claim 1, It includes the step of adding the reducing agent having an amino group in the form of an aqueous solution.

3. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The compound containing the hydrazide group is selected from one or more of sebacate dihydrazide, adipic acid dihydrazide, succinate dihydrazide, and malonate dihydrazide.

4. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The reducing agent having an amino group comprises one or more selected from amino acids or their hydrochlorides, aminooxy compounds or their hydrochlorides, aminooxyacetic acids or their hydrochlorides, and compounds or their hydrochlorides having the functional group shown in the following structural formula (1). H2N-O-···Formula (1).

5. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The reducing agent containing an amino group comprises one or more selected from L-cysteine, cysteine, and aminooxyacetic acid or their semi-hydrochlorides.

6. The method for manufacturing the water-absorbing resin according to claim 1, It includes the step of adding the reducing agent having an amino group in the form of a dispersion.

7. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The amount of reducing agent is 0.001 to 2.0% by mass relative to the total amount of the absorbent resin.

8. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The amount of reducing agent is 0.005 to 1.5% by mass relative to the total amount of the absorbent resin.

9. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The amount of reducing agent is 0.008 to 1.2% by mass relative to the total amount of the absorbent resin.

10. The method for manufacturing the water-absorbing resin according to claim 1, wherein, The amount of reducing agent is 0.01 to 1.0% by mass relative to the total amount of the absorbent resin.

11. A method for manufacturing a water-absorbing resin, comprising: An aqueous liquid in droplet form is added to a surface-crosslinked water-absorbing resin to bring the resin's moisture content to 7.5% by mass or more. The resin with the added liquid is then dried such that its moisture content decreases to 7.5% by mass or more within one hour. The aqueous liquid comprises at least one of the following: a reducing agent having a carboxyl group, a reducing agent having an amino group, a phosphoric acid reducing agent, a sulfuric acid reducing agent, and a surfactant.

12. The method for manufacturing the water-absorbing resin according to claim 11, wherein, A water-based liquid in droplet form is added to a surface-crosslinked water-absorbing resin to make the water content of the water-absorbing resin reach more than 10.0% by mass.

13. The method for manufacturing the water-absorbing resin according to claim 11, wherein, A water-based liquid in droplet form is added to a surface-crosslinked water-absorbing resin to make the water content of the water-absorbing resin reach more than 15.0% by mass.

14. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The aqueous liquid is an aqueous solution containing a volatile degrading agent.

15. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The aqueous liquid is a dispersion containing a volatile reducing agent.

16. The method for manufacturing the water-absorbing resin according to claim 14 or 15, wherein, The volatile component reducing agent is a reducing agent with amino groups.

17. The method for manufacturing the water-absorbing resin according to claim 11, wherein, When the aqueous liquid contains organic components selected from aliphatic hydrocarbons, aromatic hydrocarbons, alcohols and carboxylic acid copolymers, the concentration of the organic components in the aqueous liquid is less than 1000 ppm.

18. The method for manufacturing the water-absorbing resin according to claim 11, wherein, When the aqueous liquid contains organic components selected from aliphatic hydrocarbons, aromatic hydrocarbons, alcohols and carboxylic acid copolymers, the concentration of the organic components in the aqueous liquid is less than 200 ppm.

19. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The conductivity of the aqueous liquid at a liquid temperature of 25°C is less than 5 mS / cm.

20. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The conductivity of the aqueous liquid at a liquid temperature of 25°C is less than 1 mS / cm.

21. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The temperature of the absorbent resin before adding the water-based liquid is controlled between 90℃ and 160℃. The temperature of the water-absorbing resin after adding water-based liquid binder is controlled between 60℃ and 150℃.

22. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The temperature of the absorbent resin before adding the water-based liquid is controlled between 90°C and 140°C. The temperature of the water-absorbing resin after adding water-based liquid binder is controlled between 70℃ and 140℃.

23. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The temperature of the absorbent resin to which the aqueous liquid has been added is controlled to 80℃~160℃ within 30 minutes.

24. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The temperature of the absorbent resin to which the aqueous liquid has been added is controlled to 90℃~160℃ within 30 minutes.

25. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The temperature of the aqueous liquid added beforehand is controlled between 5°C and 90°C.

26. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried such that the decrease in water content reaches more than 10.0% by mass within 1 hour.

27. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried such that the decrease in water content reaches more than 15.0% by mass within 1 hour.

28. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried such that the decrease in water content reaches more than 20.0% by mass within 1 hour.

29. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried under stirring and / or airflow conditions.

30. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried under reduced pressure of 0.0 kPa to 10.0 kPa.

31. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the aqueous liquid has been added is dried for a drying time of 5 minutes or more.

32. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The water-absorbing resin to which the water-based liquid has been added is dried within 30 minutes after the addition of the water-based liquid.

33. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The specific surface area of ​​the surface-crosslinked water-absorbing resin is 27 m². 2 / kg or more.

34. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The specific surface area of ​​the surface-crosslinked water-absorbing resin is 30 m². 2 / kg or more.

35. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The specific surface area of ​​the surface-crosslinked water-absorbing resin is 32 m². 2 / kg or more.

36. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The specific surface area of ​​the surface-crosslinked water-absorbing resin is 35 m². 2 / kg or more.

37. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The cross-linked water-absorbing resin has an unpressurized absorption ratio (CRC) of 23 g / g or higher.

38. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The cross-linked water-absorbing resin has an unpressurized absorption ratio (CRC) of 25 g / g or higher.

39. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The surface-crosslinked water-absorbing resin has an unpressurized absorption ratio (CRC) of 28 g / g or higher.

40. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The absorbency ratio (AAP) of the surface-crosslinked water-absorbing resin under a pressure of 4.83 kPa is 20 g / g or more.

41. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The absorbency ratio (AAP) of the surface-crosslinked water-absorbing resin under a pressure of 4.83 kPa is above 23 g / g.

42. The method for manufacturing the water-absorbing resin according to claim 11, wherein, The absorbency ratio (AAP) of the surface-crosslinked water-absorbing resin under 4.83 kPa pressure is 25 g / g or more.

43. The method for manufacturing the water-absorbing resin according to claim 11, comprising the steps (A) and / or (B): Step (A): Add the aqueous liquid in droplet form to a surface area of ​​25 m². 2 / kg or more of the surface-crosslinked water-absorbing resin; Step (B) sequentially includes a polymerization step, a drying step of drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and after the surface crosslinking step is completed, a volatile component reducing agent is added.

44. The method for manufacturing the water-absorbing resin according to claim 43, wherein, The manufacturing method includes: step (A), adding an aqueous liquid in droplet form to a surface area of ​​25 m². 2 / kg or more of the surface-crosslinked water-absorbing resin, and, When the water content of the water-absorbing resin reaches 27.5% by mass or more due to the addition of an aqueous liquid in droplet form, the water-absorbing resin with the added aqueous liquid is dried such that its water content reaches 20% by mass or less within 1 hour.

45. The method for manufacturing the water-absorbing resin according to claim 43 or 44, wherein, The manufacturing method includes step (B), which sequentially comprises a polymerization step, a drying step of drying the hydrogel obtained in the polymerization step, and a surface crosslinking step, and after the surface crosslinking step is completed, an additive to reduce volatile components is added.

46. ​​A water-absorbing resin, which is a water-absorbing resin manufactured by the method for manufacturing the water-absorbing resin according to any one of claims 1 to 45. The water-absorbing resin is a water-absorbing resin that has undergone surface cross-linking. The surface cross-linking layer of the water-absorbing resin contains at least one of the following: a reducing agent containing an acylhydrazine group, a reducing agent containing a carboxyl group, a phosphoric acid-based reducing agent, and a sulfuric acid-based reducing agent. The volatile component concentration of the water-absorbing resin after standing for 15 minutes at a swelling ratio of 1.0 is below 3.5 ppm (i.e., the volatile component concentration at 1.0 swelling ratio). The concentration of volatile components after standing for 15 minutes under a swelling ratio of 1.0 is defined as the total concentration of all substances detected by a photoionization detector (PID) with a 10.6 eV irradiation lamp after uniformly adding 10.0 g of physiological saline at 23.5 ± 0.5 °C to 10.0 g of absorbent resin in a 2 L sealable glass container at room temperature and pressure, and standing for 15 minutes in a sealed state. This concentration is expressed as a value based on the detection value converted from isobutylene calibration gas.

47. The water-absorbing resin according to claim 46, wherein, The water-absorbing resin has a water content of less than 20% by mass before swelling.

48. The water-absorbing resin according to claim 46, wherein, The water content of the absorbent resin before swelling is less than 15% by mass.

49. The water-absorbing resin according to claim 46, wherein, The water-absorbing resin has a water content of less than 10% by mass before swelling.

50. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 3.3 ppm.

51. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 3.0 ppm.

52. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 2.7 ppm.

53. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 2.5 ppm.

54. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 2.3 ppm.

55. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0-fold swelling is below 1.9 ppm.

56. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0 times swelling is below 1.5 ppm.

57. The water-absorbing resin according to claim 46, wherein, The concentration of volatile components at the 1.0-fold swelling is below 1.0 ppm.

58. The water-absorbing resin according to claim 46, wherein, The total concentration of each volatile component after standing for 15 minutes under swelling ratios of 0.0, 0.5, 1.0, 2.5, 5.0, 10.0, and 20.0, i.e., the cumulative value of volatile components during swelling at each ratio, is less than 9.5 ppm.

59. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion rate is below 8.0 ppm.

60. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion ratio is below 7.5 ppm.

61. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion ratio is below 7.0 ppm.

62. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion ratio is below 6.5 ppm.

63. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion rate is below 6.0 ppm.

64. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion rate is below 5.0 ppm.

65. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion rate is below 4.0 ppm.

66. The water-absorbing resin according to claim 58, wherein, The cumulative value of volatile components during swelling at each expansion rate is below 3.5 ppm.

67. The water-absorbing resin according to claim 46, wherein, The water-absorbing resin was swollen at a swelling ratio of 5.0, and the maximum concentration of volatile components measured every 5 seconds from the start of swelling until 900 seconds had elapsed was 0.5 ppm or less, which is the maximum concentration of volatile components during swelling along the time axis.

68. The water-absorbing resin according to claim 67, wherein, The maximum volatile component concentration during swelling along the time axis is below 0.4 ppm.

69. The water-absorbing resin according to claim 67, wherein, The maximum volatile component concentration during swelling along the time axis is below 0.3 ppm.

70. The water-absorbing resin according to claim 67, wherein, The maximum volatile component concentration during swelling along the time axis is below 0.2 ppm.

71. The water-absorbing resin according to claim 46, wherein, The water-absorbing resin was swollen at a swelling ratio of 5.0, and the total concentration of volatile components measured every 5 seconds from the start of swelling until 900 seconds had elapsed, i.e., the cumulative value of volatile components during swelling along the time axis, was less than 50.0 ppm.

72. The water-absorbing resin according to claim 71, wherein, The cumulative value of volatile components during swelling along the time axis is below 45.0 ppm.

73. The water-absorbing resin according to claim 71, wherein, The cumulative value of volatile components during swelling along the time axis is below 35.0 ppm.

74. The water-absorbing resin according to claim 71, wherein, The cumulative value of volatile components during swelling along the time axis is below 25.0 ppm.

75. The water-absorbing resin according to claim 71, wherein, The cumulative value of volatile components during swelling along the time axis is below 20.0 ppm.

76. The water-absorbing resin according to claim 46, wherein, The water-absorbing resin is a polyacrylic acid-based water-absorbing resin and / or a polyacrylate-based water-absorbing resin, and is a crosslinked polymer comprising more than 50 mol% of acrylic acid-derived structural units and / or acrylate-derived structural units relative to all structural units constituting the polyacrylic acid-based water-absorbing resin and / or polyacrylate-based water-absorbing resin.

77. The water-absorbing resin according to claim 46, wherein, The absorbent resin has an unpressurized absorption ratio (CRC) of 23 g / g or more, and an absorbent absorption ratio (AAP) of 15 g / g or more under 4.83 kPa pressure.

78. The absorbent resin according to claim 46, wherein the unpressurized absorption ratio (CRC) is 25 g / g or more.

79. The absorbent resin according to claim 46, wherein the unpressurized absorption ratio (CRC) is 27 g / g or more.

80. The absorbent resin according to claim 46, wherein the unpressurized absorption ratio (CRC) is 28 g / g or more.

81. The absorbent resin according to claim 46, wherein the absorption ratio (AAP) under pressure of 4.83 kPa is 17 g / g or more.

82. The absorbent resin according to claim 46, wherein its absorption ratio (AAP) under pressure of 4.83 kPa is 20 g / g or more.

83. The absorbent resin according to claim 46, wherein its absorption ratio (AAP) under pressure of 4.83 kPa is 23 g / g or more.

84. The absorbent resin according to claim 46, wherein its absorption ratio (AAP) under pressure of 4.83 kPa is 24 g / g or more.

85. The water-absorbing resin according to claim 46, Its saline conductivity (SFC) is 10 × 10⁻⁶. -7 cm 3 • sec / g or more.

86. The water-absorbing resin according to claim 46, Its saline conductivity (SFC) is 20 × 10⁻⁶. -7 cm 3 • sec / g or more.

87. The water-absorbing resin according to claim 46, wherein, Its saline conductivity (SFC) is 30 × 10⁻⁶. -7 cm 3 • sec / g or more.

88. The water-absorbing resin according to claim 46, Its water absorption rate, based on the "Vortex method," is less than 60 seconds.

89. The water-absorbing resin according to claim 46, Its water absorption rate, based on the "Vortex method," is less than 45 seconds.

90. The water-absorbing resin according to claim 46, Its water absorption rate, based on the "Vortex method," is less than 35 seconds.

91. The water-absorbing resin according to claim 46, Its water absorption rate, based on the "Vortex method," is less than 30 seconds.

92. The water-absorbing resin according to claim 46, Its pressure-dependent absorption uptake ratio (PDAUP) is over 10 g / g.

93. The water-absorbing resin according to claim 46, Its pressure-dependent absorption uptake ratio (PDAUP) is above 12 g / g.

94. The water-absorbing resin according to claim 46, Its pressure-dependent absorption uptake ratio (PDAUP) is above 15 g / g.

95. The water-absorbing resin according to claim 46, wherein, The mass-average particle size (D50) of this water-absorbing resin is 300–600 μm. The proportion of particles smaller than 150 μm in this water-absorbing resin is less than 5% by mass. The logarithmic standard deviation (σζ) of the particle size distribution of this water-absorbing resin is 0.20–0.

50.

96. The water-absorbing resin according to claim 46, wherein the specific surface area is 25 m². 2 / kg or more.

97. The water-absorbing resin according to claim 46, Its specific surface area is 27m². 2 / kg or more.

98. The water-absorbing resin according to claim 46, Its specific surface area is 30m². 2 / kg or more.

99. The water-absorbing resin according to claim 46, Its specific surface area is 32m². 2 / kg or more.

100. The water-absorbing resin according to claim 46, It has an irregular, granular shape.

101. An absorbent article comprising the absorbent resin according to any one of claims 46 to 100.

102. The absorbent article according to claim 101, wherein, The absorbent article comprises an absorbent body as a composite, the composite comprising the absorbent resin and hydrophilic fibers. The content of the water-absorbing resin is 60% or more by mass relative to the total mass of the absorbent.

Images