Water-absorbing agent composition and method for producing the same

Abstract

The present invention aims to provide a production method of a water-absorbing agent composition which can maintain the surface tension of an absorbed liquid even when a water-absorbing resin having a high specific surface area is surface-crosslinked, has high water-absorbing properties under pressure (e.g., absorbency against pressure (AAP), gap retention under pressure), and does not hinder an improvement in the absorption speed. The production method of the present invention is a production method of a water-absorbing agent composition which includes a surface-crosslinking step of a water-absorbing resin, the water-absorbing agent composition containing the water-absorbing resin as a main component, and the production method of the water-absorbing agent composition is characterized by: (1) making the specific surface area of the water-absorbing resin before surface-crosslinking 25 m 2 / kg or more; (2) using a specific peroxide and an organic surface-crosslinking agent; and (3) appropriately controlling the water vapor density in a heat treatment step.

Description

Technical Field

[0001] This invention relates to a water-absorbing agent composition with water-absorbing resin as the main component and a method for manufacturing the same. Background Technology

[0002] In sanitary materials such as disposable diapers, sanitary napkins, and so-called incontinence pads, absorbent resin compositions are widely used for absorbing bodily fluids. As such absorbent resins, cross-linked polyacrylic acid derivatives are known. Furthermore, in recent years, with the trend towards thinner sanitary materials, the aforementioned absorbent resin compositions not only require excellent absorbency characteristics such as high absorption rate under pressure, but also demand high absorption speed. Absorbent resins that increase the specific surface area by increasing the contact area between the absorbed liquid and the absorbent resin composition to accelerate absorption have been disclosed (Patent Documents 1-7). In addition, absorbent resins with a high specific surface area for absorbing blood have also been disclosed (Patent Document 8).

[0003] It is known that increasing the specific surface area will increase the absorption rate of the superabsorbent resin, but on the other hand, the water absorption characteristics under pressure (e.g., the absorption ratio under pressure (AAP)) will decrease due to the decrease in gel strength. Therefore, the absorption rate and the water absorption characteristics under pressure are in a trade-off relationship.

[0004] On the other hand, in order to improve the absorption rate under pressure (AAP), it is known that surface crosslinking agents that enable the reaction with the functional groups of the water-absorbing resin react near the surface of the water-absorbing resin to carry out surface crosslinking with a higher crosslinking density near the surface compared with the interior of the water-absorbing resin, and various surface crosslinking techniques have been proposed (e.g., Patent Document 9).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 97 / 03114

[0008] Patent Document 2: Japanese Patent Application Publication No. 10-057805

[0009] Patent Document 3: European Patent Application Publication No. 0872491

[0010] Patent Document 4: European Patent Application Publication No. 0937739

[0011] Patent Document 5: International Publication No. 99 / 03577

[0012] Patent Document 6: International Publication No. 2013 / 018571

[0013] Patent Document 7: International Publication No. 2016 / 111223

[0014] Patent Document 8: International Publication No. 02 / 085959

[0015] Patent Document 9: Japanese Patent No. 5128098

[0016] Patent Document 10: International Publication No. 2013 / 072268 Summary of the Invention

[0017] As mentioned above, various surface crosslinking techniques have been proposed to improve water absorption characteristics under pressure (e.g., pressure absorption ratio (AAP)).

[0018] On the other hand, when the inventors were studying the absorption behavior of water-absorbing agent compositions with increased specific surface area and water-absorbing resins, they found that even with increased specific surface area, the absorption rate in surface-crosslinked water-absorbing resins was not as fast as designed.

[0019] In order to increase the hindered absorption rate to the desired level as described above, further treatment to increase the specific surface area is required. However, excessive treatment to increase the specific surface area will lead to a decrease in physical properties such as the absorption ratio under pressure, which is a trade-off. As a result, the water-absorbing resin may not achieve the desired absorption characteristics, and the performance of the sanitary material may also be reduced.

[0020] On the other hand, methods to avoid excessively increasing the specific surface area by hydrophilizing the absorbent resin have also been considered. One known method is to incorporate a hydrophilic surfactant into the absorbent resin (e.g., Patent Document 10). However, the addition of this surfactant reduces the surface tension of the absorbed liquid, which can increase backflow in sanitary materials such as disposable diapers and incontinence pads.

[0021] In other words, the technical problem of the present invention is to provide a method for manufacturing a water-absorbing agent composition, which maintains the surface tension of the absorbed liquid even when the surface of a water-absorbing resin with a high specific surface area is cross-linked, and has high water absorption characteristics under pressure (e.g., absorption rate under pressure (AAP) and interstitial water retention rate under pressure) without hindering the increase of absorption rate. Furthermore, the technical problem of the present invention is to provide an absorbent composition that, despite having a high specific surface area, maintains the surface tension of the absorbed liquid and has high water absorption characteristics under pressure (e.g., absorption rate under pressure (AAP) and interstitial water retention rate under pressure).

[0022] The present invention, which can solve the above-mentioned technical problems, has the following structure.

[0023] That is, one aspect of the present invention is as follows.

[0024] [1] A method for manufacturing a water-absorbing composition, the method comprising a surface crosslinking step of a water-absorbing resin, wherein the water-absorbing composition is mainly composed of a surface-crosslinked water-absorbing resin, and the method for manufacturing the water-absorbing composition satisfies all of the following (1) to (3): (1) The specific surface area of ​​the water-absorbing resin before surface crosslinking is 25 m². 2 / kg or more; (2) The above surface crosslinking process is carried out in the presence of peroxide and organic surface crosslinking agent that can react with carboxyl groups; (3) The above surface crosslinking process has a heat treatment process, which is carried out at the same time or after the addition of the above organic surface crosslinking agent to the water-absorbing resin, in an environment with a water vapor density of more than 0.005 g / L.

[0025] [2] In the method for manufacturing the water-absorbing composition described in [1] above, it is preferable that the water vapor density is 0.6 g / L or less;

[0026] [3] In the method of manufacturing the water-absorbing composition described in [1] or [2] above, it is preferable that the shape of the water-absorbing resin before surface crosslinking is irregular and broken;

[0027] [4] In the method of manufacturing the absorbent composition described in any one of [1] to [3] above, it is preferable that the D50 (mass average particle size) of the absorbent resin before surface crosslinking is 250 μm or more and less than 550 μm, and the mass percentage of particles with a particle size of less than 150 μm contained in the absorbent resin before surface crosslinking is 3% by mass or less.

[0028] [5] In the method of manufacturing the water-absorbing composition described in any one of [1] to [4] above, it is preferable that the manufacturing method further includes a polymerization step of an unsaturated monomer aqueous solution, wherein the water-absorbing resin is obtained by foaming polymerization of an unsaturated monomer aqueous solution;

[0029] [6] In the method of manufacturing the absorbent composition according to any one of [1] to [5] above, it is preferable that the above-mentioned organic surface crosslinking agent is added to the above-mentioned absorbent resin in a solution state, and the concentration of the above-mentioned organic surface crosslinking agent in the surface crosslinking agent solution is 0.1% by mass or more and 60% by mass or less;

[0030] [7] In the method of manufacturing the water-absorbing composition described in any one of [1] to [6] above, it is preferable to add the above-mentioned organic surface crosslinking agent to the water-absorbing resin from two or more addition nozzles provided in the mixing device;

[0031] [8] In the method of manufacturing the absorbent composition described in any one of [1] to [7] above, it is preferable that the surface crosslinking process is carried out under a micro-decompression with a pressure difference of -10 kPa or more and 0 kPa or less relative to atmospheric pressure;

[0032] [9] In the method of manufacturing the absorbent composition described in any one of [1] to [8] above, it is preferred that the peroxide is a persulfate.

[0033] In addition, another aspect of the present invention is as follows.

[0034]

[10] A water-absorbing composition, with a surface-crosslinked water-absorbing resin as the main component, wherein the water-absorbing composition satisfies all of the following (1) to (2): (1) The specific surface area of ​​the above-mentioned water-absorbing composition is 25 m². 2 / kg or more; (2) For the above absorbent composition, the Washburn contact angle θ2 determined by 20% by mass of sodium chloride aqueous solution is 72° or less.

[0035]

[11] In the water-absorbing composition described in

[10] above, it is preferable that the shape of the surface-crosslinked water-absorbing resin is irregular and broken, the D50 (mass average particle size) is 250 μm or more and less than 550 μm, and the mass ratio of particles less than 150 μm is 3% by mass or less.

[0036]

[12] In the absorbent composition described in

[10] or

[11] above, it is preferred that the water content of the absorbent composition is more than 0% by mass and less than 5% by mass;

[0037]

[13] In any of the above-mentioned

[10] to

[12] water-absorbing agent compositions, it is preferable that the water-absorbing agent composition has an absorption ratio (AAP) of 20.0 g / g or more under pressure at a load of 4.83 kPa;

[0038]

[14] In any of the above-mentioned

[10] to

[13] water-absorbing compositions, it is preferable that the water-absorbing composition has a gap water retention rate of 9.0 g / g or more under pressure;

[0039]

[15] In any of the above-mentioned

[10] to

[14] water-absorbing compositions, it is preferable that the surface tension of the water-absorbing composition is 56 mN / m or more;

[0040]

[16] In any of the above-mentioned

[10] to

[15] water-absorbing agent compositions, it is preferable that the water absorption time Vortex of the above-mentioned water-absorbing agent composition is 50 seconds or less;

[0041]

[17] In any of the above-mentioned

[10] to

[16] water-absorbing compositions, it is preferred that the water-absorbing composition further comprises a peroxide decomposition product, wherein the concentration of the peroxide decomposition product on the surface of the water-absorbing composition is higher than the concentration inside the water-absorbing composition.

[0042]

[18] In any of the above-mentioned

[10] to

[17] water-absorbing composition, it is preferable that, when particles with a diameter of 300 μm or more are designated as particle group a and particles with a diameter of less than 300 μm are designated as particle group b after sieving by a JIS standard sieve with a mesh size of 300 μm, the amount of peroxide decomposition products present in particle group a after the above-mentioned water-absorbing composition is subjected to an impact test based on a paint oscillator is designated as C1 and the amount of peroxide decomposition products present in particle group b is designated as C2, and C2-C1 is greater than 0% by mass and less than 1% by mass;

[0043]

[19] In any of the absorbent compositions described in

[10] to

[18] above, it is preferable that the SFC (salt water flow induction) of the absorbent composition is 1 × 10⁻⁶. -7 cm 3 • above sec / g.

[0044] In addition, another aspect of the present invention is as follows.

[0045]

[20] An absorbent article comprising the absorbent composition described in any one of

[10] to

[19] above. Detailed Implementation

[0046] One aspect of the present invention is a method for manufacturing a water-absorbing composition, the method comprising a surface crosslinking step of a water-absorbing resin, wherein the water-absorbing composition uses a surface-crosslinked water-absorbing resin as the main component, and the method for manufacturing the water-absorbing composition satisfies all of the following (1) to (3).

[0047] (1) The specific surface area of ​​the water-absorbing resin before surface cross-linking is 25 m². 2 / kg or more.

[0048] (2) The above surface crosslinking process is carried out in the presence of peroxide and organic surface crosslinking agent that can react with carboxyl groups.

[0049] (3) The above-mentioned surface crosslinking process includes a heat treatment process, wherein the heat treatment process is carried out at the same time or after the addition of the above-mentioned organic surface crosslinking agent to the water-absorbing resin, in an environment with a water vapor density exceeding 0.005 g / L.

[0050] Furthermore, another aspect of the present invention is a water-absorbing composition, which is mainly composed of a surface-crosslinked water-absorbing resin, wherein the water-absorbing composition satisfies all of the following (1) to (2): (1) the specific surface area of ​​the above-mentioned water-absorbing composition is 25 m². 2 / kg or more; (2) For the above absorbent composition, the Washburn contact angle θ2 determined by 20% by mass of sodium chloride aqueous solution is 72° or less.

[0051] The inventors believe that, as described above, the reason why the absorption rate in water-absorbing agent compositions and water-absorbing resins with increased specific surface area does not increase sufficiently during surface crosslinking is that, in conventional surface crosslinking treatments of water-absorbing resins, the surface of the water-absorbing resin becomes slightly hydrophobic, and the affinity (liquid wettability) between the absorbed liquid and the water-absorbing resin decreases. Specifically, it is believed that the absorbed liquid has difficulty penetrating into the cavity formed on the surface of the water-absorbing resin with its high specific surface area, reducing the contact area between the absorbed liquid and the surface of the water-absorbing resin. This prevents the surface from being effectively utilized as an absorption surface in contact with the absorbed liquid, hindering the increase in absorption rate. In other words, it is believed that even if the specific surface area is increased by slightly sacrificing the absorption uptake ratio (AAP), which is a trade-off between absorption rate and interstitial water retention capacity, the desired absorption rate cannot be obtained.

[0052] The inventors discovered a method for precisely controlling the affinity between the surface of the absorbent resin and the liquid being absorbed by focusing on the composition of the absorbent composition and the treatment agent for surface crosslinking treatment of the absorbent resin, as well as the dew point and water vapor density in the heating treatment machine for the surface crosslinking reaction, thereby completing the present invention.

[0053] By employing the method described above, surface hydrophobicity can be suppressed even during surface cross-linking. In a water-absorbing resin and water-absorbing agent composition with increased specific surface area, a method can be provided to improve the affinity between the absorbed liquid and the surface of the water-absorbing resin, allowing the absorbed liquid to penetrate into the cavity of the water-absorbing resin. Thus, it is believed that the liquid easily penetrates into the particle cavity, resulting in an increased water absorption rate. Furthermore, it is believed that by increasing the affinity between the absorbed liquid and the surface of the water-absorbing resin, the amount of liquid retained in the gaps of the particle layer and the particle cavity, i.e., the interstitial water retention, is also increased, resulting in a water-absorbing resin with good absorption up to pressure (AAP) and interstitial water retention rate under pressure. Moreover, it is not necessary to excessively increase the specific surface area to improve absorption rate, etc., thus providing a water-absorbing agent composition that achieves a balance between increased specific surface area and a balanced absorption up to pressure (AAP) and interstitial water retention rate under pressure.

[0054] Furthermore, the inventors have focused on the contact angle as an indicator of the affinity between the surface of the absorbent resin and the absorbed liquid. Conventionally, to eliminate the influence of the characteristic that the absorbent resin absorbs the desired absorbed liquid (e.g., human urine, and physiological saline or artificial urine representing human urine), the contact angle of absorbent resin is measured using a high-speed camera. The absorbent resin is densely spread across a bed, the absorbed liquid is dropped into it, an image of the instant of contact is captured, and the image is analyzed to obtain the contact angle value. However, at the instant the droplet falls and contacts the absorbent resin bed, the influence of gravity on the droplet deformation cannot be eliminated, resulting in unstable contact angle data. On the other hand, by employing a modified Washburn method (permeability method), a more accurate contact angle measurement can be performed than with existing methods.

[0055] Furthermore, the inventors have discovered that by employing a modified Washburn method, the affinity between the surface of the absorbent resin and the absorbent liquid, which varies depending on the height of the specific surface area and the conditions of the surface crosslinking treatment, can be precisely grasped and controlled. As a result, by preparing an absorbent composition having the above-described structure, it is possible to obtain an absorbent composition that exhibits desired absorption up to pressure (AAP), interstitial water retention rate under pressure, and flow rate under pressure (SFC, GBP) while maintaining a fast absorption rate.

[0056] The following describes in detail the method for manufacturing the absorbent composition of the present invention. However, the scope of the present invention is not limited to these descriptions. For situations other than those described below, appropriate modifications and implementations may be made without prejudice to the spirit of the present invention. Specifically, the present invention is not limited to the following embodiments. Various modifications can be made within the scope shown in the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of the present invention.

[0057] [1] Definition of terminology

[0058] [1-1] Composition of water-absorbing resin and water-absorbing agent

[0059] In this specification, "water-absorbing resin" refers to a water-swellable and water-insoluble polymeric gelling agent, generally in powder form. In addition, "water-swellable" means that the CRC (absorption ratio without pressure) specified in NWSP 241.0.R2 (19) is 5 g / g or more, and "water-insoluble" means that the Ext (soluble content) specified in NWSP 270.0.R2 (19) is 50% by mass or less.

[0060] The aforementioned "water-absorbing resin" is preferably a hydrophilic crosslinked polymer formed by crosslinking and polymerizing unsaturated monomers with carboxyl groups, but it is not required that its total amount, i.e., 100% by mass, is a crosslinked polymer. It may also contain additives within the range that meets the aforementioned properties such as CRC and Ext.

[0061] In addition, the term "hygroscopic resin" sometimes refers to "a polymer that is cross-linked only internally, i.e., a polymer in which the cross-linking density of the interior and the surface are substantially the same" or "a polymer that is cross-linked internally and on the surface, i.e., a polymer in which the cross-linking density of the surface is higher than that of the interior."

[0062] In this specification, the terms "polymers with internal crosslinking only" and "polymers with internal and surface crosslinking" are essentially the same and are both described as "hygroscopic resins." However, when it is necessary to clearly distinguish whether or not surface crosslinking is involved, since the "polymers with internal crosslinking only" are applied before surface crosslinking, they are described as "hygroscopic resins before surface crosslinking," and since the "polymers with internal and surface crosslinking" are applied after surface crosslinking, they are described as "hygroscopic resins after surface crosslinking" or "hygroscopic resins with surface crosslinking." It should be noted that "before surface crosslinking" means "before the addition of a surface crosslinking agent" or "although after the addition of a surface crosslinking agent, but before the start of the heat-based crosslinking reaction."

[0063] In addition, the term "absorbent resin" sometimes refers only to the resin component, but it can also include additives and other components besides the resin.

[0064] In this specification, "water-absorbing composition" refers to a water-absorbing composition made by mixing the above-mentioned "water-absorbing resin" with any additives or other components other than the water-absorbing resin.

[0065] The aforementioned "water-absorbing agent composition" comprises a water-absorbing resin as a main component. The "main component" refers to the water-absorbing resin's mass percentage relative to the total water-absorbing agent composition, preferably 50% by mass or more, followed by 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, and less than 100% by mass. Furthermore, the aforementioned "water-absorbing agent composition" preferably includes trace amounts of water, or polyvalent metal salts, chelating agents, surfactants within a range capable of maintaining surface tension, surface-modified polymers, etc., as other components.

[0066] [1-2] Polyacrylic acid (salt) based water-absorbing resin

[0067] In this specification, "polyacrylic acid (salt) based water-absorbing resin" refers to a water-absorbing resin made from acrylic acid and / or its salts (hereinafter referred to as "acrylic acid (salt)"). That is, "polyacrylic acid (salt) based water-absorbing resin" is a polymer having structural units derived from acrylic acid (salt), and a polymer having grafted components as any component. Specifically, relative to the portion of the monomers participating in the polymerization reaction, excluding the internal crosslinking agent, the polyacrylic acid (salt) based water-absorbing resin is a polymer containing preferably 50 mol% or more, more preferably 70 mol% or more, further preferably 90 mol% or more, preferably 100 mol% or less, and more preferably substantially 100 mol% of acrylic acid (salt).

[0068] [1-3] "EDANA" and "NWSP"

[0069] "EDANA" is the abbreviation for the European Disposables and Nonwovens Association. Furthermore, "NWSP" is an abbreviation for Non-Woven Standard Procedure, representing the world standard test method for absorbent compositions or absorbent resins provided by EDANA. In this invention, unless otherwise specified, the physical properties of the absorbent compositions or absorbent resins are determined according to the original NWSP (2019 revised edition). It should be noted that, unless otherwise mentioned, the test methods in the following examples are followed in this specification.

[0070] [1-4]CRC (NWSP 241.0.R2(19))

[0071] "CRC" is short for Centrifuge Retention Capacity, which refers to the water absorption ratio of a water-absorbing agent composition or water-absorbing resin without pressure.

[0072] [1-5]Ext (NWSP 270.0.R2(19))

[0073] "Ext" is short for Extractables, referring to the amount of water-soluble components in a water-absorbing composition or resin. Specifically, it refers to the amount of polymer dissolved (in mass%) after adding 1.0 g of the water-absorbing composition or resin to 200 ml of a 0.9% (w / w) sodium chloride aqueous solution and stirring at 250 rpm for 1 hour or 16 hours. The amount of dissolved polymer is determined using pH titration. The stirring time is recorded when reporting the results.

[0074] [1-6]AAP (NWSP 242.0.R2(19))

[0075] "AAP" is short for Absorption Against Pressure, which refers to the water absorption ratio of a water-absorbing agent composition or water-absorbing resin under pressure.

[0076] [1-7]SFC

[0077] "SFC" is short for Saline Flow Conductivity, which refers to the flow permeability (unit: ×10) of a sodium chloride aqueous solution under pressure (load 2.07 kPa (0.3 psi)) of a water-absorbing agent composition or water-absorbing resin. -7 cm 3 ·sec / g).

[0078] [1-8] Particle size distribution

[0079] "Particle size distribution" refers to the particle size distribution of the absorbent composition or absorbent resin determined by sieving and grading. Specifically, using a set of trays with a diameter of 200 mm and sieve apertures of 710 μm, 600 μm, 300 μm, 150 μm, and 45 μm, 100.0 g of the absorbent composition or absorbent resin is graded in a vibrating sieve for 10 minutes, and the mass (unit: mass%) of the absorbent composition or absorbent resin remaining on each sieve and tray is measured.

[0080] [1-9] Specific surface area

[0081] "Specific surface area" refers to the surface area per unit mass of the water-absorbing agent composition or water-absorbing resin (unit: m²). 2 / kg).

[0082] [1-10] Interstitial water retention rate under pressure

[0083] "Interstitial water retention rate under pressure" refers to the mass of interstitial water relative to 1 g of the absorbent composition or absorbent resin under a load of 0.3 psi (2.07 kPa). Furthermore, the aforementioned "interstitial water" refers to the liquid retained between (between) the gel particles when 1 g of the absorbent composition or absorbent resin is swollen with a 0.69% by mass sodium chloride aqueous solution under a load of 0.3 psi (2.07 kPa) to form a gel particle layer. In other words, the interstitial water retention rate under pressure refers to the mass (in g / g) of the sodium chloride aqueous solution retained in the interstitial spaces of the absorbent composition or absorbent resin under a load of 2.07 kPa (0.3 psi) when the absorbent composition or absorbent resin is swollen with a 0.69% by mass sodium chloride aqueous solution.

[0084] By measuring the interstitial water retention rate under pressure, the liquid retention force between swollen gel particles can be determined, which can serve as an indicator to predict the amount of reverse osmosis when the absorbent is actually used in sanitary materials such as disposable diapers.

[0085] [1-11] Other

[0086] In this specification, “~acid (salt)” means “~acid and / or its salt”, and “(meth)acryloyl” means “acryloyl and / or methacryloyl”.

[0087] [2] Method for manufacturing water-absorbing agent composition

[0088] The absorbent composition of the present invention comprises an absorbent resin (preferably a polyacrylic acid (salt) based absorbent resin) and, where appropriate, an additive. Hereinafter, a method for manufacturing the above-described absorbent composition will be described in detail.

[0089] The method for manufacturing the water-absorbing agent composition includes a surface crosslinking step of the water-absorbing resin. Furthermore, the method for manufacturing the water-absorbing agent composition may include a monomer aqueous solution preparation step, a polymerization step, a gel pulverization step, a drying step, a pulverization step, a classification step, or an additive addition step. In addition to the above-mentioned steps, the manufacturing method of the present invention may further include, as needed, a cooling step, a rewetting step, a micronization granulation step, a conveying step, a storage step, a packaging step, and a preservation step.

[0090] The following is a description of each process.

[0091] [2-1] Preparation process of monomer aqueous solution

[0092] This process involves preparing an aqueous solution of monomers, preferably unsaturated monomers, more preferably unsaturated monomers having carboxyl groups, and even more preferably monomers containing acrylic acid (salt) as a main component, which are raw materials for becoming absorbent resins. The aqueous monomer solution preferably contains one or more polymerizable internal crosslinking agents. The term "main component" refers to the portion of the monomer supplied for the polymerization reaction, excluding the internal crosslinking agent, where the content of acrylic acid (salt) is 50 mol% or more (up to 100 mol%), preferably 70 mol% or more, more preferably 90 mol% or more, and preferably 100 mol% or less. It should be noted that a monomer slurry can also be used within a range that does not affect the absorbent properties of the absorbent composition obtained as the final product; however, for convenience, an aqueous monomer solution is described in this specification.

[0093] (Acrylic acid (salt))

[0094] In this invention, from the viewpoint of the physical properties of the absorbent composition or the absorbent resin, it is preferable to use known acrylic acid (salt) as a monomer (hereinafter sometimes referred to as "polymerizable monomer"). Known acrylic acid contains trace amounts of polymerization inhibitors, impurities, etc. As the polymerization inhibitor, methoxyphenols are preferred, and p-methoxyphenols are more preferred. From the viewpoint of the polymerizability of acrylic acid, the color of the absorbent composition or the absorbent resin, etc., the concentration of the polymerization inhibitor in the acrylic acid, on a mass basis, is preferably 10 ppm or more, more preferably 20 ppm or more, preferably 200 ppm or less, more preferably 160 ppm or less, and even more preferably 100 ppm or less. It should be noted that the preferred range of the concentration of the polymerization inhibitor in the acrylic acid can be set as a range defined by any combination selected from the above-mentioned upper and lower limits.

[0095] In addition to organic compounds such as acetic acid, propionic acid, and furfural, the compounds described in U.S. Patent Application Publication No. 2008 / 0161512 can be listed as impurities. Furthermore, as acrylates, salts obtained by neutralizing the aforementioned acrylic acid with the following basic compounds can be listed. The acrylates can be commercially available acrylates or salts obtained by neutralizing acrylic acid.

[0096] (Alkaline compounds)

[0097] In this invention, "alkaline compound" refers to a compound that exhibits alkalinity. Specifically, sodium hydroxide and the like are considered alkaline compounds. It should be noted that commercially available sodium hydroxide, at the ppm level (mass basis), contains heavy metals such as zinc, lead, and iron, and can strictly be described as a composition. In this invention, such compositions are also considered to be included within the scope of alkaline compounds.

[0098] Specific examples of the aforementioned alkaline compounds include alkali metal carbonates, bicarbonates, alkali metal hydroxides, ammonia, and organic amines. Among these, strongly alkaline compounds are selected from the viewpoint of the absorbent properties of the absorbent composition or the absorbent resin. Therefore, hydroxides of alkali metals such as sodium, potassium, and lithium are preferred, and sodium hydroxide is more preferred. It should be noted that, from an operability viewpoint, the alkaline compound is preferably prepared as an aqueous solution.

[0099] (Neutralization)

[0100] When using the salt obtained by neutralizing acrylic acid as the aforementioned acrylate, neutralization can be performed before, during, or after polymerization, or at multiple times or locations. Furthermore, from the viewpoint of production efficiency of the water-absorbing composition or the water-absorbing resin, continuous neutralization is preferred.

[0101] When acrylic acid (salt) is used in this invention, the neutralization rate 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, preferably 90 mol% or less, more preferably 85 mol% or less, further preferably 80 mol% or less, and particularly preferably 75 mol% or less. It should be noted that the preferred range of the neutralization rate of acrylic acid (salt) can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. By setting it to the range of the neutralization rate, it is easier to suppress the decrease in the water absorption performance of the water-absorbing agent composition or the water-absorbing resin.

[0102] It should be noted that the above-mentioned neutralization rate range applies to any neutralization occurring before, during, or after polymerization. Furthermore, it applies not only to the acid groups of the water-absorbing resin but also to the acid groups of the water-absorbing composition used in the final product.

[0103] (Other monomers)

[0104] In this invention, monomers other than acrylic acid (salt) (hereinafter referred to as "other monomers") may be used in combination with acrylic acid (salt) as needed. Specifically, examples of the aforementioned other monomers include: maleic acid, maleic anhydride, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluenesulfonic acid, vinyl toluenesulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, 2-(meth)acryloyl ethanesulfonic acid, 2-(meth)acryloyl propanesulfonic acid, 2-hydroxyethyl(meth)acryloyl phosphate, and other anionic unsaturated monomers and their salts; unsaturated monomers containing thiol groups; unsaturated monomers containing phenolic hydroxyl groups; unsaturated monomers containing amide groups such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; and unsaturated monomers containing amino groups such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide. Furthermore, the other monomers include water-soluble or hydrophobic unsaturated monomers. When using the other monomers, their usage is preferably 30 mol% or less (lower limit 0 mol%) relative to the monomers other than the internal crosslinking agent, more preferably 10 mol% or less, and even more preferably 5 mol% or less.

[0105] (Internal cross-linking agent)

[0106] In a preferred manufacturing method of the present invention, an internal crosslinking agent is used. Specifically, examples of internal crosslinking agents include: N,N'-methylenebis(meth)acrylamide, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated glycerol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol 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, and glycidyl methacrylate. These internal crosslinking agents can be used alone or in combination of two or more. Considering reactivity and other factors, one or more internal crosslinking agents can be selected. Furthermore, from the viewpoint of the water absorption properties of the water-absorbing composition or the water-absorbing resin, it is preferable to select an internal crosslinking agent having two or more polymerizable unsaturated groups; more preferably, an internal crosslinking agent exhibiting pyrolytic properties at the drying temperature described later; and even more preferably, an internal crosslinking agent having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structure.

[0107] Specifically, allyl and (meth)acrylate groups can be listed as polymerizable unsaturated groups. (Meth)acrylate groups are preferred. Furthermore, polyethylene glycol can be listed as the (poly)alkylene glycol structure, and polyethylene glycol, polyethylene glycol di(meth)acrylate, and (poly)propylene glycol di(meth)acrylate, etc., can be listed as internal crosslinking agents having a (poly)alkylene glycol structure. It should be noted that the number of alkylene glycol units (hereinafter sometimes expressed as "n") is preferably 1 or more, more preferably 6 or more, preferably 100 or less, more preferably 50 or less, further preferably 20 or less, and particularly preferably 10 or less. It should be noted that the preferred range of the number of alkylene glycol units can be set as a range defined by any combination selected from the above upper and lower limits. In the manufacturing method of the present invention, the number of alkylene glycol units is preferably, for example, 1 or more and 100 or less, more preferably 6 or more and 50 or less, further preferably 6 or more and 20 or less, and particularly preferably 6 or more and 10 or less. It should be noted that the number of alkylene glycol units is the average of the distributed number.

[0108] The amount of the internal crosslinking agent used relative to the monomers other than the internal crosslinking agent is preferably 0.0001 mol% or more, more preferably 0.001 mol% or more, even more preferably 0.01 mol% or more, preferably 10 mol% or less, more preferably 5 mol% or less, and even more preferably 1 mol% or less. It should be noted that the preferred range for the amount of the internal crosslinking agent can be defined as a range determined by any combination of the above-mentioned upper and lower limits. For example, the amount of the internal crosslinking agent used relative to the monomers other than the internal crosslinking agent is preferably 0.0001 mol% or more and 10 mol% or less, more preferably 0.001 mol% or more and 5 mol% or less, and even more preferably 0.01 mol% or more and 1 mol% or less. By setting the dosage within the range, the increase in the water-soluble component of the absorbent composition or absorbent resin and the decrease in the absorption ratio are suppressed, and it is easier to obtain an absorbent composition or absorbent resin with the desired absorbent properties (e.g., high absorption ratio under pressure, high interstitial water retention rate under pressure, and high liquid flow rate (SFC, GBP) under pressure).

[0109] The aforementioned internal crosslinking agent is preferably added in advance when preparing the monomer aqueous solution, in which case the polymerization reaction and the crosslinking reaction occur simultaneously. Alternatively, the polymerization reaction can be initiated without adding an internal crosslinking agent, and the crosslinking reaction can be carried out by adding the internal crosslinking agent during or after the polymerization reaction. Furthermore, these methods can be used in combination. Additionally, it can be configured as a self-crosslinking reaction without using an internal crosslinking agent.

[0110] (Substances added to the monomer aqueous solution)

[0111] In this invention, from the viewpoint of improving the physical properties of the water-absorbing composition or the water-absorbing resin, the following substances can be added to the monomer aqueous solution at any of the following points: during the preparation of the monomer aqueous solution, during the polymerization and crosslinking reactions, or after the polymerization and crosslinking reactions. Specifically, examples of such substances include: hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (hereinafter sometimes referred to as "PVA"), polyacrylic acid (salt), and crosslinks of polyacrylic acid (salt); carbonates; azo compounds; foaming agents that generate various bubbles; surfactants; chelating agents; chain transfer agents, and other compounds.

[0112] 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, preferably 0% by mass or more, and 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, preferably 0% by mass or more, and more preferably more than 0% by mass.

[0113] If a water-soluble resin or a water-absorbing resin is used as the aforementioned hydrophilic polymer, grafted polymers or water-absorbing resin compositions such as starch-acrylate (salt) copolymers and PVA-acrylate (salt) copolymers can be obtained. These grafted polymers or water-absorbing resin compositions are also included within the scope of polyacrylate (salt)-based water-absorbing resins of the present invention.

[0114] (Concentration of monomeric components)

[0115] According to the purpose, each of the above-mentioned substances and components (hereinafter referred to as "monomer components") is selected, and their respective amounts are specified in a manner that satisfies the above-mentioned range and mixed with each other to prepare a monomer aqueous solution. It should be noted that, in this invention, in addition to preparing the monomer into an aqueous solution, a mixed solution of water and a hydrophilic solvent can also be prepared, and such a form is also referred to as a monomer aqueous solution.

[0116] Furthermore, from the viewpoint of the physical properties of the absorbent composition or the absorbent resin, the total concentration of the monomer components is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. It should be noted that the preferred range of the total concentration of the monomer components described above can be set as a range defined by any combination selected from the above upper and lower limits. It should be noted that the concentration of the monomer components is calculated according to the following (Formula I).

[0117] The concentration (mass%) of the monomer component = {(mass of the monomer component) / (mass of the monomer aqueous solution)} × 100…… (Formula I)

[0118] In the above (Formula I), the "mass of monomer aqueous solution" does not include the mass of grafted components, water-absorbing resin, and hydrophobic organic solvents in reverse suspension polymerization.

[0119] [2-2] Polymerization process

[0120] This process involves polymerizing the monomer aqueous solution obtained in the above monomer aqueous solution preparation process, which contains a monomer with acrylic acid (salt) as the main component and one or more polymerizable internal crosslinking agents, to obtain a hydrogel.

[0121] (Polymerization initiator)

[0122] In this invention, it is preferable to use a polymerization initiator during polymerization. Examples of polymerization initiators include pyrolytic polymerization initiators, photodegradable polymerization initiators, or redox polymerization initiators that also incorporate a reducing agent that promotes the decomposition of these polymerization initiators. Specifically, examples of free radical polymerization initiators include sodium persulfate, potassium persulfate, ammonium persulfate, tert-butyl hydroperoxide, hydrogen peroxide, and 2,2'-azobis(2-amidinylpropane) dihydrochloride. Considering the polymerization method, more than one of these polymerization initiators can be selected. Furthermore, from the viewpoint of the operability of the polymerization initiator and the physical properties of the water-absorbing composition or water-absorbing resin, peroxides or azo compounds are preferred as polymerization initiators, peroxides are more preferred, and persulfates are even more preferred. In addition, when using an oxidizing free radical polymerization initiator, redox polymerization can be carried out using a reducing agent such as sodium sulfite, sodium bisulfite, ferrous sulfate, or L-ascorbic acid. It should be noted that when using peroxide as a polymerization initiator, it can be the same type as the peroxide used in the surface crosslinking process described later, or it can be a different type, preferably the same type (preferably both are persulfates, and more preferably sodium persulfate).

[0123] The amount of the polymerization initiator used relative to the monomers other than the internal crosslinking agent is preferably 0.001 mol% or more, more preferably 0.01 mol% or more, more preferably 1 mol% or less, more preferably 0.5 mol% or less, and even more preferably 0.1 mol% or less. Furthermore, the amount of the reducing agent used relative to the monomers other than the internal crosslinking agent is preferably 0.0001 mol% or more, more preferably 0.0005 mol% or more, more preferably 0.02 mol% or less, and more preferably 0.015 mol% or less. It should be noted that the preferred range for the amount of the polymerization initiator or the reducing agent used can be set to a range defined by any combination selected from the above upper and lower limits. By setting the amount used within the range, it is easier to obtain a water-absorbing composition or water-absorbing resin with the desired water-absorbing properties.

[0124] Furthermore, in this invention, the above-mentioned polymerization reaction can also be initiated by irradiation with active energy rays such as radiation, electron beams, or ultraviolet rays. Additionally, irradiation with active energy rays and the above-mentioned polymerization initiator can be used in combination.

[0125] (Aggregation method)

[0126] Examples of polymerization methods applicable to this invention include aqueous solution polymerization, reverse suspension polymerization, spray polymerization, droplet polymerization, bulk polymerization, and precipitation polymerization. From the viewpoint of ease of polymerization control and the water absorption properties of the superabsorbent composition or superabsorbent resin, aqueous solution polymerization or reverse suspension polymerization is preferred, aqueous solution polymerization is more preferred, and continuous aqueous solution polymerization is even more preferred. Reverse suspension polymerization is described in International Publication No. 2007 / 004529, International Publication No. 2012 / 023433, etc. Furthermore, examples of continuous aqueous solution polymerization include continuous belt polymerization described in U.S. Patent No. 4,893,999, U.S. Patent No. 6,906,159, U.S. Patent No. 7,091,253, U.S. Patent No. 7,741,400, U.S. Patent No. 8,519,212, and Japanese Patent Application Publication No. 2005-36,100, or continuous kneader polymerization described in U.S. Patent No. 6,987,151, etc.

[0127] Preferred methods for the aforementioned continuous aqueous solution polymerization include high-temperature initiation polymerization, high-concentration polymerization, and foaming polymerization. "High-temperature initiation polymerization" refers to a polymerization method in which the temperature of the monomer aqueous solution during polymerization initiation is preferably 30°C or higher, more preferably 35°C or higher, further preferably 40°C or higher, and particularly preferably 50°C or higher, with the upper limit temperature set at the boiling point of the monomer aqueous solution. "High-concentration polymerization" refers to a polymerization method in which the monomer concentration during polymerization initiation is preferably 30% by mass or higher, more preferably 35% by mass or higher, further preferably 40% by mass or higher, and particularly preferably 42% by mass or higher, with the upper limit concentration set at the saturation concentration of the monomer aqueous solution. Furthermore, "foaming polymerization" refers to a polymerization method in which the aforementioned monomer aqueous solution containing a foaming agent or bubbles is polymerized. It should be noted that these polymerization methods can be implemented individually or in combination of two or more.

[0128] The above-described foaming polymerization is one of the methods for increasing the specific surface area of ​​the present invention, and is one of the preferred solutions. Methods for dispersing bubbles in the foaming polymerization include: (I) a method of dispersing gas dissolved in the monomer aqueous solution as bubbles by decreasing solubility; (II) a method of dispersing gas as bubbles by introducing gas from the outside; (III) a method of foaming by adding a foaming agent to the monomer aqueous solution; etc. Furthermore, the above-described dispersion methods can be used in combination depending on the water absorption properties of the water-absorbing agent composition or the water-absorbing resin. It should be noted that the bubbles dispersed in the monomer aqueous solution are enclosed within the gel when the monomer aqueous solution gels as the polymerization reaction proceeds, and the resulting gel has a foamed shape; therefore, it is referred to as foaming polymerization in the present invention.

[0129] The gases dissolved in the monomer aqueous solution mentioned above (I) can be listed as oxygen used to stabilize the monomer, nitrogen, carbon dioxide, ozone, etc., which are inert gases, and their mixtures.

[0130] In the case of the method described above (II) where a gas is introduced from the outside and dispersed in the form of bubbles, the gas can specifically include: oxygen, air, nitrogen, carbon dioxide, ozone, and mixtures thereof. Among these, from the viewpoints of polymerizability and cost, inert gases such as nitrogen and carbon dioxide are preferred, and nitrogen is more preferred.

[0131] In the case of the method described in (III) above, where a foaming agent is added to an aqueous monomer solution to cause foaming, the foaming agent can specifically include: azo compounds, organic or inorganic carbonate solutions, dispersions, or powders with a particle size of 0.1 μm or more and 1000 μm or less, preferably carbonates or bicarbonates such as sodium carbonate, ammonium carbonate, and magnesium carbonate. In the aqueous monomer solution containing the aforementioned foaming agent or bubbles, a surfactant can be used to stably maintain the bubbles.

[0132] In the methods (I) to (III) disclosed as methods for dispersing bubbles in the aforementioned foaming polymerization, surfactants may be used in conjunction. Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, fluorinated surfactants, and organometallic surfactants. Specifically, surfactants described in International Publication No. 97 / 017397 and US Patent No. 6107358 may be cited.

[0133] The above-described polymerization methods can be carried out in an air environment, but from the viewpoint of the color of the water-absorbing agent composition or the water-absorbing resin, it is preferable to carry out the polymerization in an environment with inert gases such as nitrogen or argon, and more preferably in an environment with an oxygen concentration of 1% by volume or less. It should be noted that, regarding the dissolved oxygen in the monomer aqueous solution, it is also preferable to use an inert gas to fully replace it, and more preferably to ensure that the dissolved oxygen content is less than 1 mg / L beforehand.

[0134] The formation of foamed hydrogels, water-absorbing resins, and water-absorbing agent compositions through foaming polymerization results in faster water absorption rates for the water-absorbing agent compositions or resins. Furthermore, it facilitates the immobilization of the water-absorbing agent compositions into absorbent articles, making this process preferable. It should be noted that the foamed shape can be confirmed by observing the pores on the particle surface under an electron microscope. Furthermore, examples of pore size include pores with a diameter of 1 μm or more and 100 μm or less. The lower limit for the number of pores in each particle of water-absorbing agent composition or water-absorbing resin is preferably 1 or more, more preferably 10 or more. On the other hand, the upper limit is preferably 10,000 or less, more preferably 1,000 or less. The number of pores can be controlled by the aforementioned foaming polymerization. From the perspective of increasing the specific surface area of ​​the water-absorbing agent composition or water-absorbing resin, this foaming polymerization technique is preferred.

[0135] [2-3] Gel pulverization process

[0136] This step involves gel pulverizing the hydrogel obtained in the polymerization step described above to obtain particulate hydrogel (hereinafter referred to as "particulate hydrogel"). It should be noted that, to distinguish it from the "pulverization" in the pulverization step described below, the pulverization in this step is referred to as "gel pulverization." The aforementioned "gel pulverization" refers to using a gel pulverizer such as a kneader, meat grinder, or shredder to adjust the hydrogel to a specified size.

[0137] Regarding the implementation methods and operating conditions for gel pulverization, the contents described in Japanese Patent No. 5989913 or Japanese Patent No. 6067126 are preferably applied in this invention. It should be noted that when the polymerization method is kneading polymerization, the polymerization step and the gel pulverization step are performed simultaneously. Furthermore, when particulate hydrogels are obtained through polymerization steps such as reverse suspension polymerization, spray polymerization, or droplet polymerization, the gel pulverization step is considered to be performed simultaneously with the polymerization step. Moreover, the gel pulverization process of this invention can yield irregularly fragmented water-absorbing agent compositions and water-absorbing resins.

[0138] The particle size of the finely granulated hydrogel obtained by the gel pulverization process is preferably 0.05 mm or more and 10 mm or less. If the particle size of the hydrogel is too small, the physical properties of the resulting water-absorbing agent composition or water-absorbing resin may become lower. On the other hand, if the particle size of the hydrogel is too large, the drying of the hydrogel may become insufficient.

[0139] Furthermore, the D50 (mass-average particle size) of the above-mentioned particulate hydrogel is preferably 50 μm or more, more preferably 100 μm or more, even more preferably 140 μm or more, preferably 2000 μm or less, more preferably 1500 μm or less, and even more preferably 1000 μm or less. It should be noted that the preferred range of the D50 (mass-average particle size) of the above-mentioned particulate hydrogel can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. For example, controlling the D50 (mass-average particle size) of the above-mentioned particulate hydrogel within the above-mentioned preferred range using the method described in (B) below is one method for increasing the specific surface area of ​​the water-absorbing agent composition or the water-absorbing resin, and is one of the preferred solutions.

[0140] For the PSD (particle size distribution) of the aforementioned particulate hydrogel, the σζ (logarithmic standard deviation), which represents the narrowness of its particle size distribution, is preferably 0.2 or more, preferably 1.5 or less, more preferably 1.3 or less, and even more preferably 1.2 or less. The smaller the value of the above-mentioned σζ (logarithmic standard deviation of particle size distribution), the more uniform the particle size, which has the advantage of being able to dry more uniformly. However, in order to set the σζ (logarithmic standard deviation of particle size distribution) to be less than 0.2, special operations such as particle size control during polymerization before gel pulverization and classification of particulate hydrogels after gel pulverization are required. Therefore, from the point of view of productivity and cost, it is practically difficult to implement.

[0141] In this invention, it is ideal to control one or more of the following methods: (A) foaming polymerization of monomer aqueous solution, (B) pulverization and granulation of particulate hydrogel or its dried polymer, and (C) micron powder recycling, so that the specific surface area of ​​the water-absorbing resin is 25 m². 2 / kg or more.

[0142] As a foaming polymerization of the above-mentioned monomer aqueous solution, for example, by employing a foaming polymerization method, the specific surface area of ​​the water-absorbing resin can be increased to 25 m². 2 The foaming polymerization method, with a volume of / kg or more, includes: foaming polymerization in which a surfactant and a monomer aqueous solution coexist, i.e., the foaming polymerization method described in Japanese Patent No. 5647625 (specifically, for example, a method of generating bubbles in a monomer aqueous solution by reducing the solubility of dissolved gases in the monomer aqueous solution in the presence of a surfactant), foaming polymerization method in which gas is introduced from the outside into the monomer aqueous solution and dispersed in the form of bubbles for polymerization, and foaming polymerization method in which a foaming agent is added to the monomer aqueous solution to cause foaming, etc. Therefore, the water-absorbing resin is also preferably obtained by foaming polymerization of an unsaturated monomer aqueous solution.

[0143] Furthermore, as for granulation of the above-mentioned (B) particulate hydrogel or its dried polymer, for example, by using the gel pulverization method described in Japanese Patent No. 5989913, Japanese Patent No. 6067126, and International Publication No. 2016 / 204302 as the gel pulverization step, and further drying is performed, thereby increasing the specific surface area of ​​the water-absorbing resin to 25 m². 2 / kg or more. Furthermore, by appropriately controlling the die aperture, number of holes, die thickness, amount of warm water added, and screw shaft speed of a gel pulverizer such as a shredder, a water-absorbing resin with a desired specific surface area can also be obtained. It should be noted that the above-described granulation can be performed on the hydrogel during polymerization, or on the micronized product of the polymerized hydrogel while drying, or on the dried micronized product using water and / or organic or inorganic binders. Therefore, granules of the hydrogel containing the water-absorbing resin or its dried product are preferred.

[0144] Furthermore, as for the recycling of the aforementioned (C) micro-powder, for example, by recovering the micro-powder of the water-absorbing resin that has passed through a sieve with a mesh size of 150 μm during the polymerization process, gel pulverization process, and drying process, or by granulating and recycling the micro-powder, the specific surface area of ​​the water-absorbing resin can be increased to 25 m². 2 / kg or more. Therefore, the micronized recycled product containing the water-absorbing resin is also preferred. The methods (A) to (C) above can be carried out individually or in combination.

[0145] It should be noted that this increases the specific surface area of ​​the water-absorbing resin to 25m². 2 Methods using particles larger than / kg also contain a large amount of small particles. However, these methods often contain a large amount of small particles, particularly micro-powder that passes through a 150μm sieve. As a result, the resulting absorbent composition is prone to gelation blockage, and its liquid absorption and flow properties under pressure decrease, making it undesirable. Therefore, when adjusting the specific surface area using the micro-powder, methods (B) and / or (C) described above are preferred. In this invention, it is preferable to pay close attention to adjusting the particle size distribution and implement the adjustment method described later.

[0146] The methods for determining the D50 (mass-average particle size) and σζ (logarithmic standard deviation of particle size distribution) of the above-mentioned particulate hydrogels are performed by referring to the methods described in paragraphs 0257 to 0270 of International Publication No. 2016 / 111223.

[0147] [2-4] Drying process

[0148] This process involves drying the hydrogel and / or particulate hydrogel obtained in the above-described polymerization and / or gel pulverization processes to a desired resin solids content to obtain a dried polymer. The resin solids content of this dried polymer is determined based on the mass change of 1 g of absorbent resin heated at 180°C for 3 hours, and 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, preferably 99% by mass or less, more preferably 98% by mass or less, and further preferably 97% by mass or less. It should be noted that the preferred range of the resin solids content of the dried polymer can be defined as a range determined by any combination of the above-described upper and lower limits.

[0149] Specifically, drying methods for the aforementioned hydrogels and / or particulate hydrogels include: heating drying, hot air drying, vacuum drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying based on azeotropic dehydration with hydrophobic organic solvents, and high-humidity drying using high-temperature steam. From the viewpoint of drying efficiency, hot air drying is preferred, and belt drying, which involves hot air drying over a ventilation belt, is more preferred.

[0150] From the viewpoint of the color tone and drying efficiency of the absorbent composition or absorbent resin, the drying temperature in the above-mentioned hot air drying is preferably 100°C or higher, more preferably 150°C or higher, preferably 300°C or lower, and more preferably 200°C or lower. The preferred range of the above-mentioned drying temperature can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. It should be noted that the drying temperature in hot air drying is defined by the temperature of the hot air. In addition, regarding the drying conditions other than the above-mentioned drying temperature, such as the hot air velocity and drying time, they can be appropriately set according to the water content, total mass, and solid composition of the particulate hydrogel supplied for drying. When performing belt drying, the conditions described in International Publication Nos. 2006 / 100300, 2011 / 025012, 2011 / 025013, and 2011 / 111657 can be appropriately applied.

[0151] The drying time of the present invention is preferably 1 minute or more, more preferably 5 minutes or more, further preferably 10 minutes or more, preferably 10 hours or less, more preferably 3 hours or less, and even more preferably 1 hour or less. The preferred range of the above-mentioned drying time can be set to a range defined by any combination selected from the above-mentioned upper and lower limits. By setting the drying temperature and drying time within the range described above, the physical properties of the obtained water-absorbing composition can be set to a desired range. Furthermore, the physical properties of the water-absorbing resin as an intermediate product can also be set to a desired range. In addition, when drying is carried out by hot air drying, the wind speed of the hot air is preferably 0.5 m / s or more, preferably 3.0 m / s or less, and more preferably 2.0 m / s or less. It should be noted that for other drying conditions, appropriate settings can be made according to the water content and total mass of the particulate hydrogel being dried, as well as the target solid composition.

[0152] [2-5] Crushing process, grading process

[0153] This process involves pulverizing the dried polymer obtained from the aforementioned drying process and then adjusting its particle size to a desired range through a grading process. In this process, a water-absorbing resin before surface crosslinking is obtained. By undergoing a pulverizing process after drying, the water-absorbing resin becomes an irregularly broken piece.

[0154] Specifically, the pulverizers used in the above-mentioned pulverizing process can include high-speed rotary pulverizers such as roller mills, hammer mills, screw mills, and pin mills, vibratory mills, knuckle-type pulverizers, and cylindrical mixers. From the viewpoint of pulverizing efficiency, roller mills are preferred. Furthermore, multiple pulverizers can be used in combination.

[0155] As methods for particle size adjustment in the above-mentioned grading process, examples include sieve grading using a JIS standard sieve (JIS Z8801-1 (2000)) and air classifying. Among these, sieve grading is preferred from the viewpoint of grading efficiency. It should be noted that particle size adjustment of the water-absorbing agent composition or water-absorbing resin is not limited to the pulverizing process or grading process, but can also be implemented in the polymerization process, especially reverse suspension polymerization, droplet polymerization, or other processes such as granulation process or micron powder recovery process.

[0156] The proportion of particles with a particle size less than 150 μm in the water-absorbing resin before surface crosslinking after grading is preferably 3% by mass or less, more preferably less than 3% by mass, further preferably 2.5% by mass or less, and even more preferably 2% by mass or less. It should be noted that in continuous commercial production, it is sometimes very difficult to achieve a proportion of particles smaller than 150 μm of 0% by mass from the point of view of production efficiency. Therefore, the proportion of particles with a particle size less than 150 μm in the water-absorbing resin before surface crosslinking after grading is preferably more than 0% by mass, more preferably 0.1% by mass or more, further preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more. The preferred range for the proportion of particles smaller than 150 μm can be set as a range defined by any combination selected from the above upper and lower limits. Therefore, the proportion of particles with a particle size of less than 150 μm in the water-absorbing resin before surface crosslinking after grading can be, for example, more than 0% by mass and less than 3% by mass, more than 0% by mass and less than 3% by mass, more than 0.1% by mass and less than 2.5% by mass, more than 0.2% by mass and less than 2% by mass, or more than 0.3% by mass and less than 2% by mass. By keeping the proportion of particles with a particle size of less than 150 μm in the water-absorbing resin before surface crosslinking after grading within the above range, it is easier to further balance and control AAP (absorption rate under pressure) and SFC (salt flow induction), or the surface crosslinking agent can be easily and uniformly dispersed, thus improving the performance of the water-absorbing agent composition, which is therefore preferred.

[0157] Furthermore, the (ii) D50 (weight-average particle size) of the water-absorbing resin before surface crosslinking after grading is preferably 250 μm or more, more preferably 300 μm or more, even more preferably 330 μm or more, preferably less than 550 μm, more preferably less than 500 μm, and even more preferably less than 450 μm. The preferred range of the (ii) D50 (weight-average particle size) of the water-absorbing resin before surface crosslinking after grading can be set as a range defined by any combination selected from the above upper and lower limits. For example, the (ii) D50 (weight-average particle size) of the water-absorbing resin before surface crosslinking after grading is preferably 250 μm or more and less than 550 μm, more preferably 300 μm or more and less than 500 μm, and even more preferably 330 μm or more and less than 450 μm. By keeping the (ii) D50 (weight-average particle size) of the water-absorbing resin before surface crosslinking after grading within the above range, it is easier to further balance and control AAP (absorption rate under pressure) and SFC (salt flow induction), therefore this is preferable.

[0158] Furthermore, (iii) the particle size distribution of the absorbent resin before surface crosslinking is preferably within the range of (ii) above, with a D50 (mass-average particle size) and the proportion of particles smaller than 150 μm within the range of (i) above. It should be noted that in continuous commercial production, it is sometimes very difficult to achieve a proportion of particles smaller than 150 μm of 0% by mass from the perspective of production efficiency. Therefore, it is preferable to exceed 0% by mass, more preferably 0.1% by mass or more, further preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more. Specifically, the D50 (mass-average particle size) of the water-absorbing resin before surface crosslinking is 250 μm or more and less than 550 μm, 300 μm or more and less than 500 μm, or 330 μm or more and less than 450 μm, and the mass percentage of particles with a particle size of less than 150 μm contained in the above-mentioned water-absorbing resin is more than 0% by mass and less than 3% by mass, more than 0% by mass and less than 3% by mass, more than 0.1% by mass and less than 2.5% by mass, more than 0.2% by mass and less than 2% by mass, or more than 0.3% by mass and less than 2% by mass.

[0159] Furthermore, (iv) σζ (logarithmic standard deviation of particle size distribution) is preferably 0.20 or more, more preferably 0.25 or more, even more preferably 0.27 or more, preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. The preferred range of σζ (logarithmic standard deviation of particle size distribution) can be set as a range defined by any combination selected from the above upper and lower limits. The smaller the value of σζ (logarithmic standard deviation of particle size distribution), the more uniform the particle size, which has the advantage of reducing particle segregation. However, excessively reducing the σζ (logarithmic standard deviation of particle size distribution) requires repeated crushing and classification to remove coarse and fine particles, which may be disadvantageous from the point of view of productivity and cost.

[0160] The aforementioned particle size, i.e., (i) to (iv), applies not only to the water-absorbing resin before surface crosslinking but also to the water-absorbing resin and water-absorbing agent composition after surface crosslinking. Therefore, it is preferable to perform the surface crosslinking treatment, i.e., the surface crosslinking process, in a manner that maintains the particle size within the aforementioned range adjusted in the water-absorbing resin before surface crosslinking, and more preferably, to perform particle size adjustment by setting a granulation process after the surface crosslinking process. Furthermore, in this invention, (i) and (iv), (ii) and (iv), and (iii) and (iv) can be arbitrarily selected and combined, and in this case, the preferred ranges of each can be arbitrarily combined.

[0161] The specific surface area of ​​the above-mentioned water-absorbing resin before surface cross-linking is 25 m². 2 / kg or more. Even before surface crosslinking, the specific surface area of ​​the water-absorbing resin is 25m². 2When the water absorption rate is above / kg, according to the manufacturing method and water-absorbing composition of the present invention, a water-absorbing composition with high water absorption rate, high absorption ratio under pressure, and high interstitial water retention rate under pressure can also be obtained. A higher specific surface area of ​​the water-absorbing resin before surface crosslinking is preferred, with 26m² being the most desirable. 2 / kg or more, 27m 2 / kg or more, 28m 2 / kg or more, 29m 2 / kg or more, 30m 2 / kg or above, can be 36m 2 / kg or more. Furthermore, the upper limit of the specific surface area of ​​the water-absorbing resin before surface cross-linking is preferably 60 m². 2 / kg or less, 55m 2 / kg or less. From the viewpoint of improving water absorption rate, a higher specific surface area is more ideal. However, if the specific surface area is too high, excessive foaming polymerization is required in the polymerization process and excessively fine gel pulverization is required in the gel pulverization process, which may result in a decrease in AAP (Absorption Rate under Pressure) and SFC (Salt Flow Induction). On the other hand, if the specific surface area of ​​the water-absorbing resin is too small, it is difficult to obtain a water-absorbing resin with the desired water absorption rate, and therefore it is not preferred. It should be noted that the preferred range of the specific surface area of ​​the water-absorbing resin before surface crosslinking can be set as a range defined by any combination selected from the above upper and lower limits. For example, it can be 25m². 2 / kg or more and 60m 2 / kg or less, 26m 2 / kg or more and 60m 2 / kg or less, 27m 2 / kg or more and 60m 2 / kg or less, 28m 2 / kg or more and 60m 2 / kg or less, 29m 2 / kg or more and 60m 2 / kg or less, 30m 2 / kg or more and 55m 2 / kg or less, or 36m 2 / kg or more and 55m 2 / kg or less. It should be noted that the specific surface area of ​​the water-absorbing resin before surface cross-linking can be controlled by factors such as the pulverization conditions of the hydrogel.

[0162] The shape of the superabsorbent resin before surface crosslinking can be any of the following: spherical, granular, agglomerated, or irregularly broken. Considering the water absorption rate of the superabsorbent resin, an irregularly broken shape is preferred. Here, irregularly broken refers to particles with an irregular shape. In one embodiment of the present invention, the superabsorbent resin is preferably a pulverized product obtained by polymerization with an aqueous solution. On the other hand, without a pulverization process, spherical particles or granules of spherical particles obtained by spray-drop polymerization, such as reverse suspension polymerization or polymerization by spraying or adding polymeric monomers in a mist, are not irregularly broken. In one aspect of the present invention, the average roundness of the superabsorbent resin before surface crosslinking is preferably 0.83 or less, more preferably 0.80 or less, and even more preferably 0.75 or less.

[0163] [2-6] Surface crosslinking process

[0164] This process involves further setting a high-crosslinking-density portion on the surface layer of the absorbent resin obtained before surface crosslinking, as described in the preceding processes. It comprises a mixing process and a heat treatment process. In the surface crosslinking process, free radical crosslinking, surface polymerization, and crosslinking reactions with a surface crosslinking agent occur on the surface of the absorbent resin before the surface crosslinking process, resulting in a surface-crosslinked absorbent resin.

[0165] [2-6-1] Mixing process

[0166] This process involves mixing a surface crosslinking agent, preferably a solution containing the surface crosslinking agent (hereinafter referred to as "surface crosslinking agent solution"), with a water-absorbing resin before surface crosslinking in a mixing apparatus to obtain a humidified mixture.

[0167] (Organic surface crosslinking agent)

[0168] In this invention, an organic surface crosslinking agent capable of reacting with carboxyl groups is used during surface crosslinking. Specifically, examples of such organic surface crosslinking agents include: polyol compounds, amino alcohols, alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds. It is preferable to use at least one organic surface crosslinking agent selected from these. Furthermore, organic surface crosslinking agents capable of forming ester bonds with carboxyl groups are preferred. Examples of organic surface crosslinking agents that form ester bonds, preferably dehydrated ester bonds, with the functional group of a polyacrylic (salt)-based water-absorbing resin, such as with carboxyl groups, include organic surface crosslinking agents with hydroxyl groups within the molecule, such as polyol compounds or amino alcohol compounds; organic surface crosslinking agents that generate hydroxyl groups through ring-opening, such as alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds.

[0169] More specifically, the following organic surface crosslinking agents can be listed as examples: ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, 1-methyl-1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanemethanol, 1,2-cyclohexanediol... Alcohols, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylidene-oxypropylene block copolymer, pentaerythritol, mesoerythritol, D-sorbitol, sorbitol and other polyol compounds; ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidyl ether and other epoxy compounds; ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, polyamide polyamine Polyamine compounds and their inorganic or organic salts, such as aziridine salts; polyisocyanate compounds such as 2,4-toluene diisocyanate and hexamethylene diisocyanate; halogenated epoxy compounds such as epichlorohydrin, epibromopropane, and α-methylepoxychlorohydrin; polyoxazoline compounds such as 1,2-ethylidene bisoxazoline; oxazolidinone compounds such as N-acyloxazolidinone and 2-oxazolidinone; 1,3-dioxolane-2-one (ethylidene carbonate), 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl... Alkylene carbonate compounds such as 1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and 1,3-dioxane-heptane-2-one; cyclic urea compounds; oxetane compounds such as oxetane, 2-methyloxetane, 3-methyl-3-hydroxymethyloxetane, and 3-ethyl-3-hydroxymethyloxetane; and amino alcohol compounds such as ethanolamine. Furthermore, the aforementioned organic surface crosslinking agents can be used in combination with inorganic surface crosslinking agents such as hydroxides or chlorides of multivalent metals such as zinc, calcium, magnesium, aluminum, iron, and zirconium. These organic surface crosslinking agents can be used alone or in combination of two or more.

[0170] Among the aforementioned organic surface crosslinking agents, at least one organic surface crosslinking agent is preferably selected from the group consisting of polyol compounds, epoxy compounds, polyamine compounds and their salts, oxetane compounds, and alkylene carbonate compounds. More preferably, at least one organic surface crosslinking agent is selected from the group consisting of polyol compounds and alkylene carbonate compounds. The organic surface crosslinking agent is preferably selected from one or more of the group consisting of polyols with 3 or more and 8 carbon atoms (preferably 3 or more and 6 or less) and containing 2 or more and 3 or less hydroxyl groups in the molecule, epoxy compounds with 6 or more and 12 carbon atoms, alkylene carbonates with 3 or more and 5 carbon atoms, and oxetane compounds with 3 or more and 10 carbon atoms. More preferably, it is selected from one or more of the group consisting of polyols with 3 or more and 8 carbon atoms (preferably 3 or more and 6 or less) and containing 2 or more and 3 or less hydroxyl groups in the molecule, and alkylene carbonates with 3 or more and 5 or less carbon atoms. Furthermore, considering the reactivity of the aforementioned organic surface crosslinking agent and the heating temperature in the heat treatment process, one or more organic surface crosslinking agents (two can be used in combination) can be used. It should be noted that the organic surface crosslinking process can be performed more than twice to ensure its effectiveness. In this case, subsequent processes can use the same organic surface crosslinking agent as the first one, or different organic surface crosslinking agents can be used.

[0171] The amount of the above-mentioned organic surface crosslinking agent used is preferably 0.01 parts by mass or more, more preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 2 parts by mass or less, relative to 100 parts by mass of the water-absorbing resin before surface crosslinking. The amount of the organic surface crosslinking agent used is preferably 0.01 parts by mass or more and 10 parts by mass or less, more preferably 0.01 parts by mass or more and 5 parts by mass or less, and even more preferably 0.01 parts by mass or more and 2 parts by mass or less. By setting the amount of the organic surface crosslinking agent used within the above range, an optimal crosslinking structure can be formed on the surface layer of the water-absorbing resin before surface crosslinking, and it is easier to obtain a water-absorbing resin or water-absorbing agent composition with high physical properties. It should be noted that the amount used when using multiple organic surface crosslinking agents is their total amount.

[0172] The aforementioned organic surface crosslinking agent is preferably added to the aforementioned absorbent resin in solution form (as a surface crosslinking agent solution), and more preferably added to the absorbent resin before surface crosslinking in aqueous solution form. In this case, the amount of water used is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, further preferably 0.5 parts by mass or more, preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and further preferably 10 parts by mass or less, relative to 100 parts by mass of the absorbent resin before surface crosslinking. The preferred range for the amount of water used can be set to a range defined by any combination selected from the above upper and lower limits. By setting the amount of water used to the range described above, the treatability of the surface crosslinking agent solution is further improved, and the surface crosslinking agent can be easily and uniformly mixed into the absorbent resin before surface crosslinking.

[0173] Furthermore, the concentration of the organic surface crosslinking agent in the above-mentioned surface crosslinking agent solution is preferably 0.1% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, preferably 60% by mass or less, more preferably 50% by mass or less, even more preferably 45% by mass or less, and even more preferably 35% by mass or less. The preferred range of the concentration of the above-mentioned organic surface crosslinking agent can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. For example, the concentration of the organic surface crosslinking agent in the above-mentioned surface crosslinking agent solution is preferably 0.1% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, even more preferably 15% by mass or more and 45% by mass or less, and even more preferably 15% by mass or more and 35% by mass or less. By setting the concentration of the surface crosslinking agent within the above-mentioned range, an optimal crosslinking structure can be formed on the surface layer of the water-absorbing resin before surface crosslinking with a high specific surface area, thereby improving physical properties such as water absorption performance. It should be noted that the amount used when using multiple organic surface crosslinking agents is their total amount.

[0174] Furthermore, the hydrophilic organic solvent can be used in combination with the water described above to prepare the surface crosslinking agent solution as needed. In this case, the amount of hydrophilic organic solvent used is preferably 5 parts by mass or less (minimum 0 parts by mass) relative to 100 parts by mass of the absorbent resin before surface crosslinking, more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less. Specifically, examples of the hydrophilic organic solvent include: lower alcohols such as methanol; ketones such as acetone; ethers such as dioxane; amides such as N,N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; and polyols such as ethylene glycol. However, while these hydrophilic organic solvents function as mixing aids to uniformly disperse the surface crosslinking agent on the surface of the absorbent resin, from a commercial point of view, they lead to increased costs. Therefore, even when used, it is preferable to limit the amount used to the minimum possible.

[0175] In addition, various additives added in “[2-8] Additives and their addition process” below may be added to the above surface crosslinking agent solution in a range of 5 parts by mass or less relative to 100 parts by mass of the water-absorbing resin before surface crosslinking, or added separately in the mixing process.

[0176] (Combined use of peroxides and organic surface crosslinking agents)

[0177] In this invention, peroxides are mixed in a step following the polymerization step described above. The mixed peroxides, together with the aforementioned organic surface crosslinking agent, undergo a surface crosslinking reaction with the water-absorbing resin. The type of peroxide is not particularly limited, but preferably includes: hydrogen peroxide; persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; inorganic peroxides such as permanganate and perchlorate; and organic peroxides such as cumene hydrogen peroxide, tert-butyl hydrogen peroxide, di-tert-butyl peroxide, benzoyl peroxide, and lauroyl peroxide. One or more of these can be used. Among these, inorganic peroxides are preferred from the perspective of high reactivity and excellent safety, and persulfates are particularly preferred. For example, in Japan, ammonium persulfate is used as a food additive.

[0178] The amount of peroxide used (the amount of peroxide present in the surface crosslinking process, preferably the amount of peroxide added to the absorbent resin before surface crosslinking in the surface crosslinking process) relative to 100 parts by mass of the absorbent resin before surface crosslinking is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, particularly preferably 0.05 parts by mass or more, preferably 3 parts by mass or less, preferably 2 parts by mass or less, more preferably 1 part by mass or less, and particularly preferably 0.5 parts by mass or less. The preferred range of the amount of peroxide used can be set as a range defined by any combination selected from the above upper and lower limits. For example, the amount of peroxide used relative to 100 parts by mass of the absorbent resin before surface crosslinking is preferably 0.001 parts by mass or more and 3 parts by mass or less, preferably 0.01 parts by mass or more and 2 parts by mass or less, more preferably 0.02 parts by mass or more and 1 part by mass or less, and particularly preferably 0.05 parts by mass or more and 0.5 parts by mass or less. By using the above-mentioned peroxide in an amount of 0.001 parts by mass or more, the desired effects (improved liquid permeability and controlled affinity between the surface of the absorbent resin and the absorbed liquid) can be easily obtained. Furthermore, by using the above-mentioned peroxide in an amount of 3 parts by mass or less, the degradation of the absorbent resin and the decrease in physical properties such as the increase of water-soluble components can be suppressed.

[0179] The mass ratio of organic surface crosslinking agent to peroxide used in the surface crosslinking process is preferably 1:0.005 to 1:1, more preferably 1:0.01 to 1:0.8, even more preferably 1:0.03 to 1:0.5, even more preferably 1:0.05 to 1:0.5, and particularly preferably 1:0.05 to 1:0.4. When the amount of peroxide relative to the organic surface crosslinking agent is within the above range, the deterioration of the absorbent composition and the decrease in physical properties such as the increase of water-soluble components can be suppressed. In addition, the possibility of the absorbent composition turning brown or yellow can be suppressed, making it suitable for sanitary materials with a high absorbent content.

[0180] Water can be used when mixing the peroxide and the water-absorbing resin. The amount of water used is preferably 0.5 parts by mass or more, more preferably 1.0 parts by mass or more, more preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, relative to 100 parts by mass of the water-absorbing resin. The preferred range for the amount of water used can be set to a range defined by any combination selected from the above-mentioned upper and lower limits. For example, the amount of water used is preferably 0.5 parts by mass or more and 10 parts by mass or less, more preferably 1.0 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the water-absorbing resin. By setting it to the lower limit or above, it is easy to uniformly mix it into the water-absorbing resin. Furthermore, when it is below the upper limit, the formation of agglomerates and other decreases in mixability can be suppressed. In addition, the peroxide can coexist in the above-mentioned surface crosslinking agent solution.

[0181] The timing of adding peroxides is not particularly limited, as long as the peroxide and the organic surface crosslinking agent can coexist during the surface crosslinking process. This can be done in any step after the polymerization step. "After the polymerization step" refers to the process including the polymerization step. As described in the polymerization process description, some peroxides, such as sodium persulfate and potassium persulfate, are used as polymerization initiators. Furthermore, peroxides can also function as surface crosslinking agents.

[0182] Therefore, if the peroxide added in the polymerization step, which remains in the reaction system but is not used in the polymerization reaction, is located near the surface of the water-absorbing resin, the peroxide will undergo a crosslinking reaction together with the organic surface crosslinking agent in the surface crosslinking step. Thus, the peroxide can be added only in the polymerization step. In the case where the peroxide is added only in the polymerization step, it is preferable that the aforementioned preferred amount of peroxide remains in the surface crosslinking step.

[0183] Furthermore, peroxides can be added at any time after the polymerization process, such as after the polymerization process and before the drying process, during the drying process, after the drying process, during and before the pulverization process, during and before the grading process, during and before the granulation process, during other conveying processes, intermediate storage processes (hoppers, etc.), and after the surface crosslinking process. A preferred approach is to add the peroxide during the surface crosslinking process. When adding the peroxide outside the aforementioned polymerization process, it is preferable that the preferred amount of peroxide remains at the surface crosslinking process. If the peroxide and the organic surface crosslinking agent can coexist near the surface of the absorbent resin during the surface crosslinking process, the peroxide and the organic surface crosslinking agent together induce a crosslinking reaction on the particle surface, thus increasing the gel strength. Therefore, the liquid diffusivity and flowability of the particulate absorbent composition can be improved, and the affinity between the absorbent resin surface and the absorbed liquid can be increased.

[0184] In the surface crosslinking process, the presence of water facilitates the mixing of peroxide and water-absorbing resin, as well as the mixing of organic surface crosslinking agent and water-absorbing resin. As a result, the surface crosslinking reaction is reliably carried out through the combined use of peroxide and organic surface crosslinking agent, which is therefore preferred. The amount of water used is as described above.

[0185] Thus, as long as the peroxide and the organic surface crosslinking agent can coexist in the surface crosslinking process, the effects of the present invention can be obtained. However, in the manufacturing method of the present invention, it is particularly preferred to add an aqueous solution containing the peroxide and the organic surface crosslinking agent to the water-absorbing resin before surface crosslinking (before the heat treatment process) in the surface crosslinking process.

[0186] In this case, an aqueous solution containing peroxide and an organic surface crosslinking agent is added to the absorbent resin before surface crosslinking, thus both the peroxide and the organic surface crosslinking agent function as surface crosslinking agents simultaneously. It should be noted that "surface crosslinking agent" refers to a substance capable of surface crosslinking. As mentioned above, if the peroxide and the organic surface crosslinking agent coexist in the surface crosslinking process, the affinity between the absorbent resin surface and the absorbed liquid can be improved. Therefore, the timing of adding the peroxide and the organic surface crosslinking agent does not need to be simultaneous. By adding an aqueous solution containing both peroxide and the organic surface crosslinking agent to the absorbent resin during the surface crosslinking process, a particularly high level of effectiveness can be achieved.

[0187] Typically, in the surface crosslinking process, the absorbent resin and the surface crosslinking agent are mixed and then heated. However, if one of the peroxide or the organic surface crosslinking agent is added first, the first added component will penetrate deeper into the absorbent resin before the other is added, which may result in a decrease in the crosslinking ratio near the surface of the obtained absorbent resin.

[0188] On the other hand, it is believed that when added in the form of an aqueous solution containing peroxide and organic surface crosslinking agent, a crosslinking reaction of the two components occurs near the surface, thus forming a stronger surface crosslinking layer than before, and improving gel strength.

[0189] In one embodiment, the manufacturing method of the present invention is carried out by adding an aqueous solution of peroxide after the polymerization step and before the surface crosslinking step, preferably after the drying step, and adding an aqueous solution of an organic surface crosslinking agent in the surface crosslinking step. In this case, the crosslinking of the peroxide also occurs inside the particles, so the absorbency ratio under pressure (AAP) is slightly lower compared to the method of adding an aqueous solution containing peroxide and organic surface crosslinking agent to the absorbent resin in the surface crosslinking step.

[0190] From this perspective, it can be argued that peroxide-based crosslinking near the surface of the absorbent resin is more effective. It should be noted that "after the polymerization process" refers to including the polymerization process, while "before the surface crosslinking process" refers to excluding the surface crosslinking process.

[0191] It should be noted that the moisture content of the mixture obtained by adding an aqueous solution of peroxide after the drying process is preferably 0.5% by mass or more and less than 10% by mass. When the moisture content is 0.5% by mass or more, it is easy to mix uniformly. Furthermore, when the moisture content is less than 10% by mass, the formation of clumps is suppressed, and the mixability and workability are improved.

[0192] Here, a heating step may be included after the drying step, after adding the aqueous solution of the peroxide, and before the surface crosslinking step. The heating temperature is not particularly specified, but is preferably 40°C or higher, more preferably 50°C or higher, preferably 200°C or lower, and more preferably 150°C or lower. The preferred range of the heating temperature can be set to a range defined by any combination of the above-mentioned upper and lower limits. According to the above configuration, undesirable agglomerates in the mixture obtained by adding the aqueous solution of the peroxide can be easily broken up, and operability can be improved.

[0193] The surface crosslinking reaction is preferably performed by mixing the water-absorbing resin before surface crosslinking as described above with an organic surface crosslinking agent, peroxide, and water, followed by heat treatment. The heat treatment temperature also depends on the surface crosslinking agent used, and is preferably 130°C or higher, more preferably 150°C or higher, even more preferably 180°C or higher, and preferably 300°C or lower, more preferably 250°C or lower, even more preferably 230°C or lower. The preferred range of the heat treatment temperature can be set as a range defined by any combination selected from the above upper and lower limits. The heat treatment temperature is preferably 130°C or higher and 300°C or lower, more preferably 150°C or higher and 250°C or lower, even more preferably 180°C or higher and 230°C or lower. When the heat treatment temperature is less than 130°C, the absorption ratio under pressure, liquid permeability, and other absorption characteristics may not be sufficiently improved. When the heat treatment temperature exceeds 300°C, the water-absorbing resin may deteriorate, and various properties may decrease, which should be noted.

[0194] It should be noted that the heat treatment temperature described above is determined by the temperature of the heat medium, but in cases where the temperature of the heat medium cannot be determined, such as with microwaves, it is determined by the material temperature. The heat treatment time is preferably 1 minute or more, more preferably 5 minutes or more, preferably 2 hours or less, and more preferably 1 hour or less. The preferred range of the heat treatment time can be set as a range determined by any combination selected from the above upper and lower limits.

[0195] Furthermore, to achieve a more uniform mixing of the absorbent resin and the surface crosslinking agent, non-crosslinking water-soluble inorganic bases (preferably alkali metal salts, ammonium salts, alkali metal hydroxides, and ammonia or its hydroxides), non-reducing alkali metal salt pH buffers (preferably bicarbonates, dihydrogen phosphates, hydrogen phosphates, etc.), surfactants, and inorganic particles (fumed silica, colloidal silica, and colloidal alumina, etc.) can be coexisted during the mixing process. The amount used depends on the type and particle size of the absorbent resin, and is preferably 0.005 parts by weight or more, more preferably 0.05 parts by weight or more, more preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, relative to 100 parts by weight of the absorbent resin. The preferred range of the above-mentioned amounts can be defined as a range determined by any combination selected from the above upper and lower limits.

[0196] There are no particular limitations on the mixing method of the water-absorbing resin and the surface crosslinking agent. Examples include: impregnating the water-absorbing resin in a hydrophilic organic solvent and mixing it with a surface crosslinking agent dissolved in water and / or the hydrophilic organic solvent as needed; spraying the water-absorbing resin directly in a mist or adding the surface crosslinking agent dissolved in water and / or the hydrophilic organic solvent directly to the water-absorbing resin and mixing it.

[0197] (Mixed methods, mixed conditions)

[0198] As a method for mixing the aforementioned water-absorbing resin prior to surface crosslinking with the aforementioned surface crosslinking agent solution (which, depending on the case, is a surface crosslinking agent solution containing peroxide), the surface crosslinking agent solution is prepared in advance. The water-absorbing resin prior to surface crosslinking is preferably added to the solution by means of mist spraying, dripping, or straight rod flow, and then mixed. More preferably, the method of mist spraying and mixing is mentioned.

[0199] It should be noted that, in the addition using a straight-flowing rod, the inner diameter of the addition nozzle is preferably designed such that the flow velocity of the surface crosslinking agent solution within the nozzle is preferably between 1 m / s and 20 m / s, more preferably between 1 m / s and 10 m / s, and even more preferably between 1 m / s and 5 m / s. If the flow velocity within the nozzle is 1 m / s or more, the shape is less likely to become droplet-like, and intermittent addition can be suppressed, thus it is preferred from the viewpoint of mixing. On the other hand, if the flow velocity within the nozzle is 20 m / s or less, it can suppress the situation where some of the surface crosslinking agent solution is discharged in a liquid state without being mixed with the absorbent resin due to the dispersion of the surface crosslinking agent solution added into the mixing device, or the situation where some of the surface crosslinking agent solution is dispersed in the mixing device in the opposite direction to the travel direction of the absorbent resin, leading to increased adhesion and blockage in the mixing device, thus it is preferred.

[0200] To ensure a more uniform mixing of the superabsorbent resin before surface crosslinking with the surface crosslinking agent solution, it is preferable to add the surface crosslinking agent solution from multiple points in the mixing apparatus. When adding from multiple points, the number of adding nozzles is preferably 2 or more, more preferably 5 or more, further preferably 10 or more, and particularly preferably 20 or more. The upper limit for the number of adding nozzles is 100 or less. While a higher number of adding nozzles results in a more uniform mixing of the superabsorbent resin and the surface crosslinking agent solution, it also increases the probability of nozzle blockage during liquid delivery, making it difficult to identify the nozzle where the blockage occurred.

[0201] As the mixing apparatus for performing the above-mentioned mixing, it is preferable to have a mixing apparatus that has the torque required to uniformly and reliably mix the water-absorbing resin and the surface crosslinking agent solution before surface crosslinking. The mixing apparatus is preferably a high-speed stirring mixer, more preferably a high-speed stirring continuous mixer. It should be noted that the rotational speed of the high-speed stirring mixer is preferably 100 rpm or more, more preferably 300 rpm or more, preferably 10,000 rpm or less, and more preferably 2,000 rpm or less. The preferred range of the rotational speed can be set to a range defined by any combination selected from the above-mentioned upper and lower limits.

[0202] From the viewpoint of miscibility with the surface crosslinking agent solution and the cohesiveness of the humidified mixture, the temperature of the absorbent resin supplied to this process before surface crosslinking is preferably 35°C or higher, preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 60°C or lower. Furthermore, the mixing time is preferably 1 second or higher, more preferably 5 seconds or higher, preferably 1 hour or lower, and more preferably 10 minutes or lower. The preferred ranges for the temperature of the absorbent resin before surface crosslinking and the mixing time can be set as a range defined by any combination of the above-mentioned upper and lower limits.

[0203] [2-6-2] Heat treatment process

[0204] (Heat treatment apparatus)

[0205] This step involves applying heat to the humidified mixture obtained from the above mixing step to induce a cross-linking reaction on the surface of the absorbent resin before surface cross-linking. The heat treatment is performed simultaneously with or after the addition of the aforementioned organic surface cross-linking agent to the absorbent resin. Preferably, the heat treatment is performed after the addition of the aforementioned organic surface cross-linking agent to the absorbent resin; more preferably, the heat treatment is performed after the addition of the organic surface cross-linking agent and the peroxide to the absorbent resin. The heat treatment of the humidified mixture can be performed by heating the humidified mixture in a static state or by heating it in a flowing state using a dynamic force such as stirring. For the purpose of uniformly heating the humidified mixture as a whole, heating under stirring is preferred. Specifically, examples of heat treatment apparatus for performing the above heat treatment include paddle dryers, multi-finned dryers, and tower dryers.

[0206] When using the aforementioned heat treatment apparatus, especially a continuous conductive heat transfer type heat treatment apparatus, the heat transfer area (heating area) is not particularly limited as long as it can control the powder temperature of the mixture within the following temperature range. The preferred area relative to the supply quantity of the mixture being processed is 5 m³. 2 / (t / hr) or more, preferably 10m 2 / (t / hr) or more, preferably 100m 2 / (t / hr) or less, preferably 50m 2 / (t / hr) or less. The preferred range of the heat transfer area can be set to a range defined by any combination selected from the above upper and lower limits. By setting the heat transfer area to the above range, it is easy to control the powder temperature of the mixture and the inner wall temperature of the heat treatment apparatus within the following temperature range, and stable operation can be achieved, therefore it is preferred.

[0207] The heat treatment apparatus used in this invention is preferably a device equipped with a gas supply mechanism and / or a gas exhaust mechanism (hereinafter sometimes referred to as a "gas supply / exhaust mechanism") in a dryer or heating furnace. The gas supply / exhaust mechanism controls the ambient water vapor density within the heat treatment apparatus. In this case, it is preferable to not only provide an intake port / exhaust port, but also to use a blower or the like to adjust the amount and pressure of the gas flowing within the heat treatment apparatus. Furthermore, the intake port / exhaust port is not limited to one location; considering the size of the heat treatment apparatus used and the ambient water vapor density, multiple locations can be provided.

[0208] Furthermore, the heating method, stirring method, gas supply method, and exhaust method in the heat treatment apparatus can also be used as a combination of multiple heat treatment apparatuses of the same or different forms.

[0209] (Water vapor density inside the heat treatment device)

[0210] The lower limit of the water vapor density inside the heat treatment apparatus used in this heat treatment process is greater than 0.005 g / L, preferably greater than 0.05 g / L, more preferably 0.07 g / L or more, further preferably 0.10 g / L or more, even more preferably 0.15 g / L or more, and particularly preferably 0.20 g / L or more. When the water vapor density is less than or equal to 0.005 g / L, a large amount of water evaporates from the absorbent resin, and the evaporation rate is also fast, so it is impossible to obtain an absorbent resin with excellent affinity for the desired absorbent liquid. Furthermore, the upper limit of the water vapor density inside the heat treatment apparatus used in this heat treatment process is preferably less than or equal to 0.60 g / L, more preferably less than or equal to 0.55 g / L, and even more preferably less than or equal to 0.50 g / L. The preferred range of the above water vapor density can be set as a range defined by any combination selected from the above upper and lower limits. The water vapor density inside the heat treatment apparatus used in this heat treatment process is preferably greater than 0.005 g / L and less than 0.60 g / L, more preferably greater than 0.05 g / L and less than 0.60 g / L, even more preferably greater than 0.07 g / L and less than 0.60 g / L, even more preferably greater than 0.10 g / L and less than 0.60 g / L, even more preferably greater than 0.15 g / L and less than 0.55 g / L, and particularly preferably greater than 0.20 g / L and less than 0.50 g / L. By keeping the water vapor density below the above-mentioned upper limit, the water vapor density can be easily controlled without placing an excessive burden on the production equipment. It should be noted that detailed information such as the water vapor density measurement conditions is described in the embodiments.

[0211] In this invention, the water vapor density in this process refers to the average water vapor density of the gas in the upper space of the mixture within the heat treatment apparatus, and is preferably measured vertically above the mixture being heated by the heating section within the heat treatment apparatus.

[0212] In this invention, water vapor density refers to the value obtained by dividing the weight of water contained in the gas by the amount of non-condensable gas, and has the dimension of "weight / volume". Specifically, condensable components (water) are captured from the sampled gas by cooling, solvent absorption, etc., and measured. The amount of residual non-condensable gas is measured using a gas meter or the like. Water vapor density is calculated by dividing the weight of the captured water by the volume of the non-condensable gas converted to standard conditions at 0°C and 1 atmosphere. It should be noted that in this invention, "non-condensable gas" refers to a substance that is gaseous under standard conditions at 0°C and 1 atmosphere. Preferably, the non-condensable gas used is air or nitrogen, or a mixture of air and nitrogen.

[0213] (Operating conditions)

[0214] To achieve the effects of this invention, a key feature is that the water vapor density inside the heat treatment apparatus used in the heat treatment process is controlled to exceed 0.005 g / L. The operating conditions in the heat treatment process will be described in detail below.

[0215] In this invention, in order to control the water vapor density within the heat treatment apparatus within a desired range, a specified gas is introduced through the intake port of the heat treatment apparatus, or gas present in the upper space of the mixture within the heat treatment apparatus is discharged through the exhaust port. The flow rate of the intake and exhaust gas is not particularly limited, but is at least greater than 0.1 (Nm³). 3 / hr), preferably 10000 (Nm 3 / hr or less, preferably 5000 (Nm 3 Below 3000 (Nm³ / hr), further preferably 3000 (Nm³ / hr) 3 / hr or less. Furthermore, the ratio relative to the amount of the mixture being processed is preferably 3000 (Nm³). 3 / t) or less, more preferably 1000 (Nm 3 / t) and below. It should be noted that "Nm 3 "" refers to the volume of gas converted to standard conditions (0°C, 1 atmosphere).

[0216] Furthermore, the gas introduced into the heat treatment apparatus is not particularly limited as long as the water vapor density can be controlled within the aforementioned range; methods using steam, dry air, nitrogen, helium, argon, or dry air are examples. Additionally, to control the water vapor density, methods utilizing water vapor generated from the water contained in the absorbent resin during the heating process in this step are also examples.

[0217] It should be noted that the pressure within the aforementioned heat treatment apparatus is preferably slightly reduced. Specifically, the pressure difference (gauge pressure) relative to atmospheric pressure is preferably -10 kPa or more and 0 kPa or less, more preferably -5 kPa or more and 0 kPa or less, and even more preferably -2 kPa or more and 0 kPa or less. Adjusting the pressure difference (gauge pressure) is one method to control the water vapor density within the heat treatment apparatus within the desired range. By maintaining the pressure within the heat treatment apparatus within the aforementioned range, the water vapor density can be easily controlled to exceed 0.005 g / L.

[0218] In this heat treatment process, it is preferable to heat the mixture described above. By controlling the temperature of the heat medium in the heat treatment apparatus, the ambient temperature or dew point inside the heat treatment apparatus, the water vapor density, the inner wall temperature, the heat transfer area, and the residence time, the powder temperature is controlled to a desired temperature range. The powder temperature is preferably 150°C or higher, more preferably 160°C or higher, even more preferably 170°C or higher, preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 230°C or lower. The preferred range of the powder temperature can be defined as any combination selected from the above upper and lower limits. When the powder temperature is less than 150°C, the covalent bonds used to form the surface crosslinking layer sometimes become insufficient. On the other hand, when the powder temperature exceeds 250°C, the water-absorbing resin deteriorates due to heating, and the desired water-absorbing resin cannot be obtained, which is therefore undesirable.

[0219] The powder temperature of the above mixture refers to the highest temperature in the reaction process. In the continuous process, it is evaluated by the temperature of the water-absorbing resin (the reactant after heating the mixture of water-absorbing resin and surface crosslinking agent) that has just been discharged from the heat treatment device.

[0220] There is no particular limitation on the residence time (heat treatment time) in the heat treatment apparatus, but it is preferably 5 minutes or more and 90 minutes or less, and more preferably 10 minutes or more and 60 minutes or less.

[0221] Regarding the control of the aforementioned water vapor density, considering factors such as the heat transfer from the inner wall of the heat treatment apparatus, the heat transfer from the absorbent resin, and the amount of water vapor generated by the absorbent resin, the gas supply, discharge, and temperature can be appropriately controlled. Specifically, methods such as installing a measuring device in the heat treatment apparatus and adjusting by introducing the aforementioned gas as needed, or adjusting by changing the gas discharge rate and pressure, can be cited. In this invention, multiple control methods can be appropriately combined.

[0222] It should be noted that the water vapor density varies with the location of the heating element and the duration of the treatment. Ideally, it should be controlled within a constant range within the heat treatment apparatus. "Constant range" means that, based on the total heat treatment time, the water vapor density is preferably within 50% or more, more preferably 70% or more, and even more preferably 80% or more of the above-mentioned ranges, with the variation preferably within 0.017 g / L, more preferably within 0.009 g / L, even more preferably within 0.007 g / L, and particularly preferably within 0.006 g / L.

[0223] Furthermore, as long as the airflow rate is within the above range, the water vapor density at an appropriate measuring point within the gas discharge mechanism of the heat treatment apparatus can also be set to the water vapor density inside the heat treatment apparatus as specified in this invention.

[0224] (Ambient dew point and ambient temperature inside the heat treatment unit)

[0225] In this invention, the ambient dew point and ambient temperature in this process refer to the average dew point and average temperature of the gas in the upper space of the mixture within the heat treatment apparatus, and are preferably measured vertically above the mixture being heated by the heating section within the heat treatment apparatus.

[0226] Preferably, the lower limit of the ambient dew point inside the heat treatment apparatus used in this heat treatment process is controlled to be above 0°C, 10°C, 20°C, 30°C, 40°C, or 45°C, and the upper limit is controlled to be below 100°C. If the ambient dew point is too low, a large amount of water evaporates from the absorbent resin at a fast rate, which may prevent the obtaining of an absorbent resin with excellent affinity for the desired absorbent liquid.

[0227] Furthermore, the ambient temperature inside the heat treatment apparatus used in this heat treatment process is preferably controlled to be above 100°C and below 300°C, more preferably above 100°C and below 250°C, and even more preferably above 100°C and below 230°C. When the ambient temperature is below 100°C, the moisture evaporated from the absorbent resin condenses inside the heating apparatus, causing the absorbent resin to adhere and hindering stable continuous production, potentially leading to a decrease in productivity and physical properties.

[0228] [2-7] Cooling process

[0229] This step is an arbitrary step added as needed after the heat treatment step in the above-mentioned surface crosslinking process. This step involves forcibly cooling the surface-crosslinked water-absorbing resin after the above-mentioned heat treatment step to a specified temperature, so as to quickly end the surface crosslinking reaction.

[0230] The cooling of the surface-crosslinked absorbent resin described above can be performed under static conditions or under flowing conditions using dynamic methods such as stirring. Considering the need for uniform cooling of the entire absorbent resin, cooling under stirring is preferred. From this perspective, cooling devices for performing this cooling include paddle dryers, multi-finned dryers, and tower dryers. It should be noted that these cooling devices can also adopt the same specifications as the heat treatment equipment used in the heat treatment process. This is because by changing the heat medium of the heat treatment equipment to a cold medium, it can be used as a cooling device.

[0231] The cooling temperature in this process can be appropriately set based on the heating temperature in the heat treatment process, the water-absorbing agent composition, or the water-absorbing properties of the surface-crosslinked water-absorbing resin. Specifically, the temperature of the surface-crosslinked water-absorbing resin is preferably below 150°C, more preferably below 100°C, further preferably below 90°C, particularly preferably below 80°C, preferably above 20°C, and more preferably above 30°C. The preferred temperature range of the surface-crosslinked water-absorbing resin can be set as a range defined by any combination of the above-mentioned upper and lower limits.

[0232] The shape of the surface-crosslinked superabsorbent resin can be any of the following: spherical, granular, agglomerated, or irregularly fragmented. Considering the water absorption rate of the superabsorbent resin, an irregularly fragmented shape is preferred. Furthermore, if the superabsorbent resin is crushed after surface crosslinking, the surface crosslinking effect is reduced; therefore, the shape of the superabsorbent resin before and after surface crosslinking is preferably irregularly fragmented. Irregularly fragmented superabsorbent resin can be obtained by pulverizing hydrogels or dry polymers. In one aspect of the present invention, the average roundness of the particulate superabsorbent composition is preferably 0.83 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. In the case of agglomerated particles, the average roundness of the primary particles is within the above range.

[0233] The mass proportion of particles smaller than 150 μm, (ii) D50 (mass-average particle size), (iii) D50 (mass-average particle size) and particles smaller than 150 μm, (iv) σζ (logarithmic standard deviation of particle size distribution), and combinations thereof and preferred ranges in the surface-crosslinked water-absorbing resin are as described above.

[0234] [2-8] Additives and their addition processes

[0235] In this invention, additives can be added to any one or more of the following: the water-absorbing resin before surface crosslinking, the water-absorbing resin during surface crosslinking, and the water-absorbing resin after surface crosslinking. In other words, the water-absorbing agent composition may contain additives in addition to the water-absorbing resin. These additives include liquid permeability enhancers or similar agents, other additives, etc., and one or more of these can be used, or a combination of two or more.

[0236] [2-8-1] Additives

[0237] (Liquid permeability improver or its components)

[0238] As a fluid permeability enhancer used in this invention, an additive that has the function of improving the saline flow induction (hereinafter referred to as "SFC") and gel bed permeability under load or no load (hereinafter referred to as "GBP") of the water-absorbing agent composition or water-absorbing resin can be used, for example, 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.

[0239] These additives may also be used for other purposes besides improving liquid permeability, such as acting as anti-caking agents under hygroscopic conditions, flow control agents for powders, deodorizers, fragrances, and binders for water-absorbing resins. It should be noted that when added for other functional purposes, they are referred to as conjugates. The amount of the aforementioned liquid permeability improvers or conjugates added is appropriately determined based on the selected compound. It should be noted that not only when using these additives alone, but also when using two or more in combination, the appropriate range of their respective addition amounts can be appropriately selected within the ranges described below.

[0240] The above-mentioned "GBP" is an abbreviation for Gel Bed Permeability, which is the permeability of a 0.9% by mass sodium chloride aqueous solution relative to the absorbent composition or absorbent resin under load or free swelling. It is a value determined according to the GBP test method described in International Publication No. 2005 / 016393.

[0241] (Polyvalent metal salts)

[0242] When using a polyvalent metal salt, the polyvalent metal cation of the polyvalent metal salt is preferably divalent or higher, more preferably trivalent or higher, and most preferably tetravalent or lower. Furthermore, aluminum, zirconium, etc., can be listed as usable polyvalent metals. Therefore, aluminum lactate, zirconium lactate, aluminum sulfate, zirconium sulfate, etc., can be listed as polyvalent inorganic salts usable in this process. Among these, from the viewpoint of improving the effect of SFC, aluminum lactate or aluminum sulfate is more preferred, and aluminum sulfate is even more preferred. The amount of the above-mentioned polyvalent metal salt added is preferably 0 moles or more and less than 3.6 × 10⁻⁶ moles per 1 g of absorbent resin. -5 moles, more preferably 0 moles or more and less than 1.4 × 10⁻⁶ moles. -5 The mole, more preferably 0 moles or more and less than 1.0 × 10⁻⁶. -5 Moore.

[0243] (Catonic polymer)

[0244] When using cationic polymers, the substances described in U.S. Patent No. 7,098,284 can be listed as cationic polymers. Among these, ethyleneamine polymers are more preferred from the viewpoint of improving the effects of SFC and GBP. Furthermore, the mass-average molecular weight of the cationic polymer is preferably 5,000 or more and 1,000,000 or less.

[0245] Relative to 100 parts by weight of the water-absorbing resin, the amount of the above-mentioned cationic polymer added is preferably 0 parts by weight or more, more preferably more than 0 parts by weight, more preferably less than 2.5 parts by weight, more preferably less than 2.0 parts by weight, and even more preferably less than 1.0 parts by weight.

[0246] (Inorganic particles)

[0247] When using inorganic microparticles, substances described in U.S. Patent No. 7,638,570 can be listed as inorganic microparticles. Among these, from the viewpoint of improving the effects of SFC and GBP, silica (amorphous fumed silica, colloidal silica, etc.) is preferred.

[0248] When the primary particle size is less than 20 nm, the inorganic microparticles are added in a manner that is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, more preferably less than 1.2 parts by mass, more preferably less than 1.0 parts by mass, and even more preferably less than 0.5 parts by mass, relative to 100 parts by mass of the water-absorbing resin. Furthermore, when the primary particle size is 20 nm or more, the inorganic microparticles are added in a manner that is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, more preferably less than 2.0 parts by mass, more preferably less than 1.5 parts by mass, and even more preferably less than 1.0 parts by mass, relative to 100 parts by mass of the water-absorbing resin.

[0249] (Other additives)

[0250] Other additives include, specifically: chelating agents, inorganic reducing agents, aromatic substances, organic reducing agents, hydroxycarboxylic acid compounds, surfactants, compounds containing phosphorus atoms, oxidizing agents, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, etc. One or more of these other additives may be used. Among them, chelating agents are preferred, and amino polycarboxylic acids or amino polyphosphoric acids are more preferred. Specifically, examples of chelating agents include those described in 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, Japanese Patent Application Publication No. 2005-097519, 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.

[0251] Other additives, especially chelating agents, are preferably added or contained in the range of 0.001% by mass or more and 1% by mass relative to the monomer or water-absorbing resin.

[0252] [2-8-2] Additive addition process

[0253] The additives described above may be added before, during, or in the process of at least one of the steps selected from the preparation of the monomer aqueous solution, polymerization step, gel pulverization step, drying step, pulverization step, classification step, and surface crosslinking step. Preferably, they are added before, after, or in the process of any step following the polymerization step.

[0254] When the above-mentioned additive is added to the absorbent resin, if the additive is a liquid or a solution of an aqueous medium such as water, it is preferable to spray the liquid or solution onto the absorbent resin in a mist form and apply sufficient torque to ensure that the absorbent resin and the additive are mixed uniformly and reliably. On the other hand, if the above-mentioned additive is in the form of a powder or other solid, it can be dry-mixed with the absorbent resin, or an aqueous liquid such as water can be used as a binder.

[0255] Specifically, the apparatus used in the above-mentioned mixing process can be categorized as follows: stirring mixer, cylindrical mixer, double-walled conical mixer, V-type mixer, belt mixer, screw mixer, flow-type rotary mixer, air-flow mixer, double-arm kneader, internal mixer, pulverizing kneader, rotary mixer, screw extruder, etc. When using a stirring mixer, its rotational speed is preferably 5 rpm or more, more preferably 10 rpm or more, preferably 10,000 rpm or less, and more preferably 2,000 rpm or less.

[0256] [2-9] Other processes

[0257] In addition to the above-mentioned processes, this invention may include granulation, sizing, micron powder removal, micron powder recovery, micron powder reuse, iron removal, etc., as needed. Furthermore, it may also include conveying, storage, bundling, and preservation processes.

[0258] [3] Properties of the water-absorbing agent composition

[0259] The absorbent composition obtained through the above process is a final product ready for shipment. The absorbent composition of the present invention is an absorbent composition with absorbent resin as the main component, and the absorbent composition satisfies all of the following (1) to (2).

[0260] (1) The specific surface area of ​​the above-mentioned absorbent composition is 25 m². 2 / kg or more.

[0261] (2) For the above absorbent composition, the Washburn contact angle θ2, measured with a 20% by mass aqueous solution of sodium chloride, is less than 72°.

[0262] [3-1] Specific surface area

[0263] By setting the specific surface area of ​​the water-absorbing agent composition of the present invention to 25m² 2 A specific surface area of ​​26 m² or higher yields even better water absorption rate and interstitial water retention. A higher specific surface area of ​​the water-absorbing agent composition is preferred, with 26 m² being the most desirable. 2 / kg or more, 27m 2 / kg or more, 28m 2 / kg or more, 29m 2 / kg or more, 30m 2 / kg or above, can be 36m 2 / kg or more. Furthermore, the specific surface area of ​​the absorbent composition of the present invention is preferably 60 m². 2 / kg or less, 55m 2 / kg or less. The preferred range of the specific surface area of ​​the above-mentioned superabsorbent composition can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. From the viewpoint of accelerating water absorption and improving interstitial water retention, a higher specific surface area is more ideal. However, if the specific surface area is too high, excessive foaming polymerization is required in the polymerization process and excessively fine gel pulverization is required in the gel pulverization process, which may result in a decrease in AAP (absorption rate under pressure) and SFC (salt flow induction). On the other hand, if the specific surface area of ​​the superabsorbent composition is too small, it is difficult to obtain a superabsorbent composition with the desired water absorption rate and interstitial water retention, and therefore it is not preferred. The specific surface area of ​​the superabsorbent composition can be, for example, 25m². 2 / kg or more and 60m 2 / kg or less, 26m 2 / kg or more and 60m 2 / kg or less, 27m 2 / kg or more and 60m 2 / kg or less, 28m 2 / kg or more and 60m 2 / kg or less, 29m 2 / kg or more and 60m 2 / kg or less, 30m 2 / kg or more and 55m 2 / kg or less, or 36m 2 / kg or more and 55m 2 / kg or less.

[0264] It should be noted that, in this specification, "specific surface area" refers to the surface area per unit mass of the water-absorbing agent composition or water-absorbing resin (unit: m²). 2 / kg, is determined by analyzing the three-dimensional image data of the absorbent composition or absorbent resin obtained using the microfocus X-ray CT system (Shimadzu Corporation: inspeXio SMX-100CT) described later using high-speed three-dimensional analysis software (Ratoc Systems: TRI / 3D-VOL-FCS64).

[0265] [3-2] Contact angles θ1 and θ2 according to the Washburn method

[0266] The Washburn contact angle θ1 (unit: °) of the absorbent composition of the present invention is preferably 65° or less, 63° or less, 60° or less, 58° or less, 56° or less, 55° or less, 54° or less, 53° or less, 52° or less, and 51° or less, and is also preferably 20° or more, 22° or more, 25° or more, 28° or more, and 30° or more. The preferred range of the Washburn contact angle θ1 described above can be defined as a range determined by any combination of the above upper and lower limits. The Washburn contact angle θ1 (unit: °) of the absorbent composition of the present invention can be, for example, 20° or more and 65° or less, 22° or more and 63° or less, 25° or more and 60° or less, 28° or more and 58° or less, 30° or more and 56° or less, 30° or more and 55° or less, 30° or more and 54° or less, 30° or more and 53° or less, 30° or more and 52° or less, or 30° or more and 51° or less.

[0267] The Washburn contact angle θ2 (unit: °) of the absorbent composition of the present invention is preferably 72° or less, 70° or less, 68° or less, 67° or less, 65° or less, 63° or less, 61° or less, or 60° or less, and preferably 25° or more, 27° or more, 30° or more, 33° or more, 35° or more, 40° or more, 45° or more, or 50° or more. The preferred range of the Washburn contact angle θ2 can be defined as a range determined by any combination of the above upper and lower limits. If the Washburn contact angle θ2 (unit: °) of the absorbent composition of the present invention exceeds 72°, it is difficult to improve the water absorption characteristics under pressure (e.g., AAP, interstitial water retention rate under pressure) while maintaining the advantage of high specific surface area, i.e., fast water absorption rate. The Washburn contact angle θ2 (unit: °) of the absorbent composition of the present invention is preferably, for example, 25° or more and 72° or less, 27° or more and 70° or less, 30° or more and 68° or less, 33° or more and 67° or less, 35° or more and 65° or less, 40° or more and 63° or less, 45° or more and 61° or less, or 50° or more and 60° or less.

[0268] By controlling the Washburn contact angle θ2 to below 72°, and preferably controlling the Washburn contact angle θ1 to below 65°, a water-absorbing agent composition with excellent affinity between the water-absorbing resin surface and the absorbed liquid, as well as excellent interstitial water retention, can be obtained. Furthermore, considering the excellent affinity between the water-absorbing resin surface and the absorbed liquid, smaller Washburn contact angles θ1 and θ2 are preferred. However, to achieve Washburn contact angles of less than 20° for θ1 and less than 25° for θ2, excessive additional treatment and excessive addition of hydrophilic surfactants are required, which is not preferred from an economic point of view.

[0269] The Washburn contact angle is calculated by measuring the time and wetting load of the liquid as it rises from the bottom of a tube filled with powder, using a capillary constant calculated with a contact angle of 0° (hexane, methanol, etc.). It can be determined by analyzing the permeation velocity data obtained using a high-performance surface tensiometer (Kyowa Interface Science Co., Ltd.: DyneMaster series DY-500) described later, using the accompanying analytical software DYNALYZER.

[0270] [3-3] Other physical properties of the absorbent composition

[0271] The absorbent composition of the present invention preferably has at least one of the following physical properties (a) to (k) within a preferred range.

[0272] (a) D50 (mass-average particle size), (b) mass proportion of particles with a diameter less than 150 μm, (c) σζ (logarithmic standard deviation of particle size distribution), (d) CRC (absorption ratio without pressure), (e) AAP (absorption ratio under pressure), (f) interstitial water retention rate under pressure, (g) water content, (h) SFC (flow induction of brine), (i) Vortex (water absorption rate), (j) surface tension, (k) residual peroxide.

[0273] Furthermore, the absorbent composition of the present invention may also combine any two or more of the preferred ranges of the above-described properties (a) to (k). Preferably, it is a combination of at least the preferred ranges of (e) AAP (absorption rate under pressure) and (g) moisture content; more preferably, it is a combination of the preferred ranges of (f) interstitial water retention rate under pressure; or, preferably, it is a combination of the preferred ranges of (e) AAP (absorption rate under pressure) and (f) interstitial water retention rate under pressure; further preferably, it is a combination of the preferred ranges of (i) Vortex (absorption rate); and even more preferably, it may also include the preferred ranges of (a) to (c). Most preferably, it includes all the preferred ranges of (a) to (k). It should be noted that the specific preferred ranges of the above-described properties (a) to (k) are the ranges described in the following items.

[0274] (a) D50 (mass-average particle size), (b) mass proportion of particles with a particle size less than 150 μm, (c) σζ (logarithmic standard deviation of particle size distribution)

[0275] The D50 (mass-average particle size), the mass ratio of particles with a particle size of less than 150 μm, and σζ (logarithmic standard deviation of particle size distribution) of the water-absorbing agent composition of the present invention are set to be the same as the range of the water-absorbing resin after grading and before surface crosslinking.

[0276] By setting the (a) D50 (mass-average particle size) of the above-mentioned absorbent composition within the aforementioned range, the preferred absorption properties, AAP (absorption rate under pressure) and SFC (saline flow induction), can be further balanced and well controlled. If the D50 (mass-average particle size) is too small, the gel bulk density may become too high, or the preferred absorption properties, AAP and SFC, may become too low. On the other hand, if the D50 (mass-average particle size) is too large, the particle roughness of the absorbent composition becomes noticeable, sometimes resulting in a poorer skin feel and wearing comfort when used in absorbent products such as disposable diapers and sanitary napkins.

[0277] Furthermore, by setting the mass proportion of particles with a particle size of less than 150 μm in (b) within the aforementioned range, it is easier to further balance and control AAP (absorption rate under pressure) and SFC (salt flow induction). (b) If the mass proportion of particles with a particle size of less than 150 μm is too high, not only is there a possibility that the AAP (absorption rate under pressure), which is the preferred absorption characteristic, may become too low, but there is also a possibility that the working environment may deteriorate due to dust dispersion at the site where the desiccant composition is processed, and that the processability may become difficult due to the accumulation of particles in the device, which is therefore not preferred.

[0278] Furthermore, it is preferred that (a) the D50 (mass-average particle size) of the absorbent composition is 250 μm or more and less than 550 μm, and (b) the mass percentage of particles with a particle size less than 150 μm is 3% by mass or less. More preferably, the absorbent composition satisfies both the above-mentioned range of D50 (mass-average particle size) and the above-mentioned range of the mass percentage of particles with a particle size less than 150 μm. By satisfying both of these conditions, the above-mentioned effects can be obtained synergistically. It should be noted that the D50 (mass-average particle size) of the absorbent composition and the mass percentage of particles with a particle size less than 150 μm are determined by the method described in the examples.

[0279] (d) CRC (Absorption Ratio without Pressure)

[0280] The absorbent composition of the present invention preferably has a CRC (absorbency ratio without pressure) of 25 g / g or more, preferably 40 g / g or less, more preferably 38 g / g or less, further preferably 35 g / g or less, and particularly preferably 33 g / g or less. If the above-mentioned CRC (absorbency ratio without pressure) is too low, the absorbency ratio of the absorbent composition will decrease, and it may not be suitable for use as an absorbent in absorbent articles such as disposable diapers and sanitary napkins. On the other hand, if the above-mentioned CRC (absorbency ratio without pressure) is too high, the gel strength may become weak.

[0281] (e) AAP (Absorption Ratio under Pressure)

[0282] The absorbent polymer (AAP) of the absorbent composition of the present invention is preferably 20.0 g / g or more, more preferably 23.0 g / g or more, even more preferably 25.5 g / g or more, more preferably 26.0 g / g or more, even more preferably 26.5 g / g or more, preferably 32.0 g / g or less, more preferably 31.0 g / g or less, even more preferably 30.0 g / g or less, and even more preferably 29.0 g / g or less. The preferred range of the AAP can be defined as a range determined by any combination selected from the above upper and lower limits. The absorbent composition of the present invention preferably has an AAP (absorbency ratio under pressure) of 20.0 g / g or more and 32.0 g / g or less, more preferably 23.0 g / g or more and 31.0 g / g or less, more preferably 25.5 g / g or more and 30.0 g / g or less, more preferably 26.0 g / g or more and 29.0 g / g or less, and even more preferably 26.5 g / g or more and 29.0 g / g or less. By setting the above-mentioned AAP (absorbency ratio under pressure) within the above range, the absorbent composition can resist the load and absorb the absorbed liquid even when pressure is applied to the absorbent body, thus becoming an absorbent resin or absorbent composition suitable for use as an absorbent body in absorbent articles such as disposable diapers and sanitary napkins.

[0283] (f) Interstitial water retention rate under pressure

[0284] The interstitial water retention rate of the absorbent composition of the present invention under pressure is preferably 9.0 g / g or more, 9.3 g / g or more, 9.5 g / g or more, 10.0 g / g or more, 11.0 g / g or more, and 12.0 g / g or more. There is no particular limitation on the upper limit, but it is preferably 19.0 g / g or less, and can be 18.0 g / g or less, or 17.0 g / g or less. The preferred range of the interstitial water retention rate under pressure can be defined as a range determined by any combination of the above upper and lower limits. When the interstitial water retention rate under pressure is less than 9.0 g / g, when manufacturing the absorbent body of absorbent articles such as disposable diapers, there is a possibility that urine and blood, etc., cannot be fully absorbed under pressure, resulting in leakage. Therefore, it is not suitable as the absorbent body for absorbent articles such as disposable diapers. The interstitial water retention rate of the absorbent composition of the present invention under pressure is, for example, 9.0 g / g or more and 19.0 g / g or less, 9.3 g / g or more and 19.0 g / g or less, 9.5 g / g or more and 19.0 g / g or less, 10.0 g / g or more and 19.0 g / g or less, 11.0 g / g or more and 18.0 g / g or less, and 12.0 g / g or more and 17.0 g / g or less.

[0285] (g) Moisture content

[0286] The water content of the absorbent composition of the present invention is preferably 5% by mass or less, and more preferably 4% by mass or less, 3% by mass or less, and 2% by mass or less. Furthermore, it is preferable to have a water content of more than 0% by mass. If the water content exceeds 5% by mass, the surface of the absorbent composition becomes sticky, the flowability of the powder decreases, and the workability deteriorates, which is therefore undesirable.

[0287] (h) SFC (Saline Flow Inducibility)

[0288] From the perspective of improving the interstitial water retention rate under pressure, the SFC (salt water flow induction) of the absorbent composition of the present invention is preferably 1×10⁻⁶. -7 cm 3 • sec / g or higher, exceeding 3×10 -7 cm 3 ·sec / g, 5×10 -7 cm 3 ·sec / g or higher, 10×10 -7 cm 3 ·sec / g or higher, 20×10 -7 cm 3 ·sec / g or higher, 25×10 -7 cm 3 ·sec / g or higher, 30×10-7 cm 3 • sec / g or higher. The SFC (salt water flow induction) of the absorbent composition of the present invention is preferably 120 × 10⁻⁶. -7 cm 3 · sec / g or less, 110×10 -7 cm 3 · sec / g or less, 100×10 -7 cm 3 · sec / g or less, 90×10 -7 cm 3 · sec / g or less, 80×10 -7 cm 3 · sec / g or less, 70×10 -7 cm 3 · sec / g or less, 60×10 -7 cm 3 · sec / g or less, 50×10 -7 cm 3 · sec / g or less, less than 40 × 10 -7 cm 3 • sec / g. The preferred range of the above-mentioned SFC (salt water flow induction) can be set as a range defined by any combination selected from the above upper and lower limits. The SFC (salt water flow induction) of the absorbent composition of the present invention is, for example, 1 × 10⁻⁶. -7 cm 3 • sec / g or higher and 120 × 10 -7 cm 3 · sec / g or less, or more than 3×10 -7 cm 3 ·sec / g and 110×10 -7 cm 3 · sec / g or less, 5×10 -7 cm 3 • sec / g or higher and 100×10 -7 cm 3 · sec / g or less, 10×10 -7 cm 3 • sec / g or higher and 90 × 10 -7 cm 3 · sec / g or less, 20×10 -7 cm 3 • sec / g or higher and 80×10 -7 cm 3 · sec / g or less, 25×10 -7 cm 3 • sec / g or higher and 70 × 10 -7 cm3 · sec / g or less, 25×10 - 7 cm 3 • sec / g or higher and 60×10 -7 cm 3 · sec / g or less, 25×10 -7 cm 3 • sec / g or higher and 50 × 10 -7 cm 3 · sec / g or less, 30×10 -7 cm 3 • sec / g or higher and less than 40 × 10 -7 cm 3 •sec / g. By setting the SFC to the lower limit or above, even when the absorbent composition is used at a high concentration in thin sanitary materials, gel adhesion can be suppressed, and the decrease in the absorbency properties of the sanitary materials can be suppressed. When the SFC is below the upper limit, the decrease in CRC, which is a compromise, can be suppressed.

[0289] (i) Vortex (water absorption rate)

[0290] The upper limit of the Vortex (absorption rate) of the absorbent composition of the present invention is preferably 50 seconds or less, 48 ​​seconds or less, 46 seconds or less, 44 seconds or less, 42 seconds or less, 40 seconds or less, 38 seconds or less, and 36 seconds or less, sequentially. Furthermore, the lower limit of the Vortex (absorption rate) of the absorbent composition of the present invention is preferably more than 10 seconds, more preferably more than 15 seconds. The preferred range of the above-mentioned Vortex (absorption rate) can be defined as a range determined by any combination selected from the above-mentioned upper and lower limits. For example, the Vortex (absorption rate) of the absorbent composition of the present invention is more than 10 seconds and less than 50 seconds, more than 10 seconds and less than 48 seconds, more than 10 seconds and less than 46 seconds, more than 10 seconds and less than 44 seconds, more than 10 seconds and less than 42 seconds, more than 10 seconds and less than 40 seconds, more than 15 seconds and less than 38 seconds, and more than 15 seconds and less than 36 seconds. When the Vortex (absorption rate) is slow (e.g., exceeding 50 seconds), the resulting absorbent composition absorbs bodily fluids such as urine and blood at a slower rate, potentially leading to leakage. Therefore, this absorbent composition is unsuitable as an absorbent for disposable diapers or similar absorbent products. It should be noted that the Vortex (absorption rate) can be controlled through foaming polymerization, gel pulverization, or the manufacturing method of this invention.

[0291] (j) Surface tension

[0292] The surface tension of the absorbent composition of the present invention is preferably 56 mN / m or more, more preferably 58 mN / m or more, even more preferably 60 mN / m or more, and particularly preferably 65 mN / m or more. The upper limit is not particularly limited, but from the viewpoint of balancing with other physical properties, it is preferably 75 mN / m or less. When the surface tension is 56 mN / m or more, the amount of backflow of liquid when pressure is applied to the absorbent does not become excessive, thus making it suitable as an absorbent composition for use in absorbent articles such as disposable diapers. The preferred range of surface tension described above can be defined as a range determined by any combination of the above upper and lower limits.

[0293] (k) Peroxide residue

[0294] The absorbent composition of the present invention preferably has peroxide decomposition products on its surface, and more preferably, the concentration on the surface of the absorbent composition is higher than the concentration inside the absorbent composition. According to the manufacturing method of the absorbent composition of the present invention, the residual components of peroxide (peroxide decomposition products) are more distributed on the particle surface than inside the particles of the absorbent composition. More specifically, according to the manufacturing method of one embodiment of the absorbent composition of the present invention, the peroxide is coexisted with an organic surface crosslinking agent in the surface crosslinking process. Therefore, compared with the case where the peroxide is added only as a polymerization initiator during polymerization, the concentration of peroxide decomposition products on the surface of the absorbent composition is higher than the concentration inside the absorbent composition. With such a configuration, the effects of the present invention are more easily obtained, and therefore preferred. It should be noted that the peroxide is decomposed, for example, by heat treatment during surface crosslinking, becoming decomposition products. In the case of sodium persulfate, the peroxide decomposition product is sodium sulfate; in the case of potassium persulfate, the peroxide decomposition product is potassium sulfate.

[0295] The residual amount of peroxide after decomposition on the particle surface of the absorbent composition of the present invention is preferably more than 0% by mass, more than 0.005% by mass, more than 0.010% by mass, more than 0.015% by mass, and more than 0.020% by mass, and preferably less than 1.0% by mass, less than 0.7% by mass, less than 0.35% by mass, and less than 0.1% by mass. The preferred range of the above-mentioned residual amount of peroxide can be set as a range defined by any combination selected from the above-mentioned upper and lower limits. For example, the above-mentioned residual amount of peroxide is more than 0% by mass and less than 1.0% by mass, more than 0.005% by mass and less than 0.7% by mass, more than 0.010% by mass and less than 0.35% by mass, more than 0.015% by mass and less than 0.1% by mass, and more than 0.020% by mass and less than 0.1% by mass. When the residual peroxide content is less than 0% by mass, i.e., when the residual peroxide content on the particle surface is equal to or less than the residual peroxide content inside the particle, the peroxide cannot effectively act on the particle surface of the absorbent composition, and therefore the desired effect (controlling the affinity between the absorbent resin surface and the absorbed liquid) cannot be obtained. On the other hand, when the residual peroxide content exceeds 1.0% by mass, it will cause deterioration of the absorbent composition, resulting in an increase in water-soluble components and a decrease in physical properties, which is not preferable from this perspective.

[0296] It should be noted that the above-mentioned residual peroxide content was determined by the following methods (a) to (e).

[0297] (a) The water-absorbing composition of the present invention is subjected to a prescribed impact test.

[0298] (b) Using a JIS standard sieve with a mesh size of 300 μm, the superabsorbent composition subjected to the above impact test was sieved into particle group a with a particle size of 300 μm or larger and particle group b with a particle size of less than 300 μm.

[0299] (c) Let the content of peroxide decomposition products present in the above particle group a be C1.

[0300] (d) Let the content of peroxide decomposition products present in the above particle group b be C2. At this time,

[0301] (e) The above-mentioned residual peroxide content is calculated by “C2-C1”.

[0302] Through the above impact resistance test, most of the shaved surface portion of the absorbent composition became particle group b. Therefore, it can be said that the larger the C2-C1 ratio, the more peroxides are present on the surface of the absorbent composition.

[0303] Furthermore, details of the impact test described in (a) above are set forth in the embodiments.

[0304] [3-4] Relationship between water-absorbing resin and water-absorbing agent composition

[0305] Relative to the total amount of the absorbent composition, the amount of absorbent resin contained in the absorbent composition of the present invention is preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more. It should be noted that, when including the various additives described above, the amount of absorbent resin contained in the absorbent composition is not 100% by mass.

[0306] Furthermore, the shape of the absorbent composition of the present invention can be any of the following: spherical, granular, agglomerated, irregularly broken, etc., but considering the water absorption rate, an irregularly broken shape is preferred.

[0307] [4] Uses of water-absorbing agent compositions

[0308] The absorbent composition of the present invention is preferably used as an absorbent or absorbent layer (hereinafter collectively referred to as "absorbent") in absorbent articles such as disposable diapers and sanitary napkins, and more preferably as an absorbent in absorbent articles in which a large amount of absorbent is used per absorbent article.

[0309] The aforementioned absorbent refers to an absorbent formed by molding a particulate absorbent composition into a sheet, fiber, or tubular shape, preferably molded into a sheet to form an absorbent layer. In addition to the absorbent composition of the present invention, absorbent materials such as pulp fibers, adhesives, or nonwoven fabrics may also be used during molding. In this case, the amount of absorbent composition in the absorbent (hereinafter referred to as "core concentration") is preferably 50% by mass or more, more preferably 60% by mass or more, further preferably 70% by mass or more, particularly preferably 80% by mass or more, and preferably 100% by mass or less. By setting the core concentration within the above range, when the above absorbent is used in absorbent articles, even if the absorbent composition absorbs urine and gels, a suitable space can be formed between the gel particles.

[0310] [5] Absorbent items

[0311] The absorbent article of the present invention includes the absorbent body described above, and typically has a liquid-permeable surface sheet and a liquid-impermeable back sheet. Examples of absorbent articles include disposable diapers and sanitary napkins.

[0312] In the case of absorbent articles such as disposable diapers, the disposable diaper is manufactured by sandwiching an absorbent containing the absorbent composition of the present invention between a liquid-permeable top sheet located on the side in contact with the skin when worn and a liquid-impermeable bottom sheet located on the outside when worn. It should be noted that the disposable diaper is also provided with components known to those skilled in the art, such as adhesive tape for securing the disposable diaper after wearing.

[0313] The absorbent article of the present invention, when the absorbent body absorbs liquid and the absorbent composition swells and gels, creates suitable spaces between the gel particles, thus preventing moisture retention and reducing the stuffiness of disposable diapers. Furthermore, when the absorbent composition contains fragrances or deodorants, these components are suitably volatilized through these spaces. Therefore, utilizing any one or both of the above effects, an absorbent article comfortable for the wearer and their caregiver can be provided. It should be noted that, in addition to the aforementioned disposable diapers and sanitary napkins, the absorbent composition of the present invention can also be suitably used for applications such as pet urine absorbents and urine gels for portable toilets.

[0314] Example

[0315] The present invention is described below with reference to specific embodiments. However, the present invention is not limited to the embodiments described below, and may be implemented by appropriate modifications within the scope of the preceding and following description, all of which are included within the technical scope of the present invention. Furthermore, in the present invention, unless otherwise specifically mentioned, the methods for measuring the above-mentioned physical properties are based on the methods described in the embodiments.

[0316] The methods for determining each physical property are described below. It should be noted that, for example, when the object of the test is not a water-absorbing composition, "water-absorbing composition" in the following description can be replaced with "particulate hydrogel", "water-absorbing resin before surface crosslinking", or "water-absorbing resin after surface crosslinking".

[0317] [D50 (mass-average particle size), mass proportion of particles smaller than 150 μm, σζ (log-standard deviation of particle size distribution)]

[0318] The D50 (mass-average particle size) and the mass ratio of particles with a particle size of less than 150 μm, as well as σζ (logarithmic standard deviation of particle size distribution) of the absorbent composition of the present invention, were determined according to the determination method described in U.S. Patent No. 7,638,570.

[0319] [CRC (Rate of Absorption at No Pressure)]

[0320] The CRC (Cycle of Absorbent Composition without Pressure) of the absorbent composition of the present invention was determined according to the EDANA method (NWSP241.0.R2(19)). Specifically, 0.2 g of the absorbent composition was placed in a nonwoven bag and immersed in a large excess of 0.9% sodium chloride aqueous solution for 30 minutes to allow the absorbent composition to swell freely. Then, after dehydration using a centrifuge (250G), the Cycle of Absorbent Composition without Pressure (CRC) (unit: g / g) was determined.

[0321] [AAP (Absorption Ratio under Pressure)]

[0322] The AAP (Absorption Rate under Pressure) of the absorbent composition of the present invention was determined according to the EDANA method (NWSP242.0.R2(19)). Specifically, 0.9 g of the absorbent composition was subjected to a large excess of 0.9% by mass sodium chloride aqueous solution at 4.83 kPa (49 g / cm³). 2 After swelling for 1 hour under a load of 0.7 psi, the absorption rate (AAP) under pressure was measured (unit: g / g).

[0323] [SFC (Saline Flow Inducibility)]

[0324] The SFC (salt flow induction) of the absorbent composition of the present invention was determined according to the method described in U.S. Patent No. 5,669,894.

[0325] [Washburn contact angles θ1 and θ2]

[0326] Regarding the Washburn contact angles θ1 and θ2 of the absorbent composition of the present invention, the rate at which the absorbent composition draws liquid from the sample through capillary force was measured using a surface tensiometer (DY-500, Kyowa Interface Science Co., Ltd.) and the measurement software DYNALYZER (Ver. 2.2.5.0), and calculated based on the Lucas-Washburn formula (Equation 1 below). It should be noted that θ2 was obtained by changing the filter paper (θ1) to a stainless steel sieve (θ2) with a mesh size of 400 mesh (36 μm), the same as the sieve used in the AAP measurement, in order to eliminate the adverse effects of the absorbent composition on liquid absorption. The measurement principle, test procedure, etc., are the same.

[0327] (Measurement principle)

[0328] The capillary phenomenon of the absorbent structure containing the water-absorbing composition in this invention depends on the contact angle θ of the liquid wetting the particles or particulate matter. The contact angle θ is calculated according to the following formulas (1) and (2). When the object of measurement is a powder, a penetration rate test using two liquids is generally performed to determine the contact angle θ.

[0329] In this instruction manual, firstly, a permeation rate test is conducted using n-hexane (contact angle θ is infinitely close to zero), and "c" is calculated according to the following (Equation 2). Next, a permeation rate test is conducted using a 20% by mass sodium chloride aqueous solution with the liquid temperature adjusted to 15°C. The "c" calculated using the above-mentioned n-hexane and the measurement result are substituted into the following (Equation 1) to calculate the contact angle θ with the 20% by mass sodium chloride aqueous solution.

[0330] [Formula 1]

[0331]

[0332] in,

[0333] m: Mass absorbed (g)

[0334] t: Measurement time (seconds)

[0335] As parameters for a 20% by mass sodium chloride aqueous solution at 15℃,

[0336] η: Absolute viscosity of the liquid (mPa·s) = 1.768

[0337] ρ: Density of the liquid (g / cm³) 3 =1.150,

[0338] σ: Surface tension of the liquid (mN / m) = 79.99.

[0339] It should be noted that "m" in the above (Equation 1) 2 " / t" refers to the rate at which a 20% (by mass) sodium chloride aqueous solution is drawn into the desiccant composition by capillary force, measured in m between 10 and 20 seconds after the start of the measurement. 2 The difference is used to calculate the permeation rate.

[0340] [Formula 2]

[0341]

[0342] in,

[0343] m: Mass absorbed (g)

[0344] t: Measurement time (seconds)

[0345] As a parameter for n-hexane at 20°C,

[0346] η: Absolute viscosity of the liquid (mPa·s) = 0.320

[0347] ρ: Density of the liquid (g / cm³) 3 =0.66,

[0348] σ: Surface tension of the liquid (mN / m) = 18.4

[0349] cosθ: The contact angle of the liquid = 1 (equivalent to θ = 0° for n-hexane).

[0350] It should be noted that "m" in the above (Equation 2) 2 " / t" refers to the rate at which n-hexane is drawn into the desiccant composition by capillary force, measured between 10 and 20 seconds after the start of the measurement. 2 The difference is used to calculate the permeation rate.

[0351] (Experimental procedure: θ1)

[0352] Next, the test procedure is shown: θ1. It should be noted that the following operations are performed in each measurement. Furthermore, all consumables with their model numbers are listed in the "Surface Tensiometer DyneMaster Series Options" web catalog on the Kyowa Interface Science Co., Ltd. homepage. Unless otherwise specified, the test procedure follows the video manual on the CD accompanying the DyneMaster DY-500.

[0353] First, to determine the constant "c", the following steps are performed. Prepare homogeneous n-hexane (FUJIFILM WakoPure Chemical, premium reagent grade) and heat it to a specific temperature (20°C). Next, assemble the analytical column using round powder analysis filter paper (model 7221), a Teflon column (model 7279), and a Teflon column connector (model 8077). At this point, without the desiccant composition, use the included compressor to compress the filter paper, filling the gap between the connector and the filter paper, ensuring the connector surface is flush with the bottom surface of the filter paper.

[0354] Next, weigh 3.0 g of the absorbent composition and place it into a Teflon (registered trademark) column. Then, without using the included compressor, gently tap the side of the column 10 times with your fingers to smooth the surface of the absorbent composition. Then, install the column holder (model 7175) on the upper part of the column and suspend it from the top of the measuring device using a small hook (model 6947).

[0355] Next, prepare the test solution. First, thoroughly rinse the accompanying culture dish (model 1196) with pure water to remove sebum, surfactants, and other substances that could affect surface tension. Then, rinse it three times with n-hexane to fully displace the surface. Next, inject n-hexane, heated to 20°C, into the line shown in the diagram into the culture dish, and place the culture dish on the worktable of the apparatus.

[0356] Next, the measurement conditions are set using the measurement software in the following order.

[0357] 1) Select “Powder Contact Angle Measurement” from “Selection of Measurement Category”.

[0358] 2) "Measurement Settings" > "Measurement" > Select "Powder Penetration Rate Measurement" from "Measurement".

[0359] 3) In “Measurement Settings” > “Measurement” > “Basic Settings”, enter the liquid contact sensitivity of 0.03g, the measurement end time of 60 seconds, and the sampling interval of 0.1 seconds. Select “Full Automatic” in the action method, uncheck the effective range, and enter 10.0% for both the start range and end range in the calculation range.

[0360] 4) In “Measurement Settings” > “Measurement” > “Measurement Component Settings”, enter 5.00 mm for the powder column radius.

[0361] 5) In “Measurement Settings” > “Measurement” > “Sample Settings”, enter the values ​​of surface tension, density, viscosity of liquid sample and density of powder sample.

[0362] 6) In the lower part of “Test Settings” > “Test” > “Sample Settings”, select “Weight” from “Porosity”, “Weight”, and “Filling Height”, and enter the mass of the absorbent composition to be filled.

[0363] 7) In “Measurement Settings” > “Measurement” > “Table Action Settings”, enter 0.200mm / s for both the table rising speed and the table falling speed, and enter 2.0mm for the falling distance after measurement.

[0364] 8) As needed, set the vertical and horizontal axes of the charts you want to observe during and immediately after the measurement, as well as the output parameters, from “Measurement Settings” > “Display” > “Chart Axis Settings” and “Output Settings”.

[0365] It should be noted that after the measurement is completed, a CSV file containing all the information required for the calculations using the above (Formula 1) and above (Formula 2) will be obtained.

[0366] After setting the measurement conditions, the operating software raises the device's worktable until it is fully close to the column joint, then begins the measurement. The measurement automatically ends after the set measurement end time. The square of the mass (m) of the liquid collected between 10 and 20 seconds after the start of the measurement is calculated. 2 The change in (m) is assumed to be linear, and a regression line is defined. The slope of the regression line is taken as "m". 2 Calculate the value of " / t". Repeat the above operation three times, changing the sample each time. Substitute the average of the three measured values ​​into the above (Equation 2) to calculate the constant "c".

[0367] Next, as the test solution, a 20% (w / w) sodium chloride aqueous solution was prepared, with the temperature adjusted to 15°C. Hexane was replaced with the test solution. Otherwise, the same procedures as described above were performed to determine "m". 2The average value of " / t". It should be noted that the test solution is not a 0.9% sodium chloride aqueous solution (physiological saline), and the temperature is also 36°C lower than body temperature. This is to inhibit the absorption of the absorbent composition.

[0368] Based on the constant "c" obtained from the above operation and the "m" of a 20% (w / w) sodium chloride aqueous solution... 2 / t”, and calculate the Washburn contact angle θ using the above (Formula 1). It should be noted that the contact angle θ obtained through the above operation is set as “θ1”.

[0369] (Experimental procedure: θ2)

[0370] In the above experimental step: θ1, the filter paper for powder determination (model 7221) was changed to a sieve used for AAP determination cylinders. Additionally, "m" 2 / t” is set as the slope of the regression line from 0 to 10 seconds after the start of the measurement. Otherwise, the same operation as the above test procedure: θ1 is performed. It should be noted that in the measurement of contact angle θ2, in order to eliminate the influence of the absorbent composition on the test liquid, the following absorption mass correction is performed.

[0371] The above-mentioned absorption mass correction uses the difference between the "measured value of the absorbed mass" of the 20% sodium chloride aqueous solution between 0 and 10 seconds after the start of the measurement and the "absorbed mass" of the 20% sodium chloride aqueous solution between 0 and 10 seconds after the start of the measurement as the absorption correction absorbed mass.

[0372] The above-mentioned "absorption quality" was determined by the following method.

[0373] First, in the method for determining the FSR (water absorption rate) described in International Publication No. 2009 / 016055, paragraphs 0196-0197, which is cited by reference, the absorbent was changed to 2g of a 20% by mass sodium chloride aqueous solution at 15°C, and the FSR (water absorption rate) of the absorbent composition was determined. Hereinafter, the absorbent was changed to 2g of a 20% by mass sodium chloride aqueous solution at 15°C, and the determined FSR (water absorption rate) of the absorbent composition is set as FSR(20). It should be noted that the temperature and concentration of the sodium chloride aqueous solution here are matched to those used in the determination of the contact angle.

[0374] Next, the mesh of the contact angle measuring column is set to a height of 0 mm, and a 1 mm scale is attached to the outer surface of the column along the height direction. 3.0 g of the absorbent composition is placed into the column, and then the side is tapped 10 times. The height of the absorbent composition is then read from the scale, and the mass of the absorbent composition is divided by the height of the absorbent composition to calculate the mass of the absorbent composition per 1 mm.

[0375] In each measurement, the absorption state of the 20% sodium chloride aqueous solution was captured as a dynamic image. The difference (mm) in liquid height at 10 seconds and 20 seconds after the start of the measurement was read from the scale was multiplied by the mass of the absorbent composition of 1 mm mentioned above, and the value obtained was set as the mass of the absorbent composition involved in absorption (a).

[0376] Next, based on the mass of the absorbent composition involved in absorption (a) and the FSR (20) mentioned above, the absorbed mass per 0.1 seconds is calculated, and the cumulative mass from 0 seconds to 10 seconds is set as the "absorbed mass". Specifically,

[0377] The mass absorbed (x) in 0.1 seconds is a × (1 / 100) × FSR (20) × 0.1 seconds.

[0378] The absorbed mass (y) in 0.2 seconds is x + a × (2 / 100) × FSR (20) × 0.1 seconds.

[0379] The absorbed mass (z) at 0.3 seconds is y + a × (3 / 100) × FSR (20) × 0.1 seconds. Repeat this process to calculate the cumulative absorbed mass from 0 seconds to 10 seconds.

[0380] Based on the constant "c" obtained in the above operation and the m of a 20% (w / w) sodium chloride aqueous solution after absorption mass correction... 2 / t, calculate the Washburn contact angle θ using the above (Equation 1). It should be noted that the contact angle θ obtained through the above operation is set as "θ2".

[0381] [Interstitial water retention rate under pressure]

[0382] The interstitial water retention rate of the absorbent composition of the present invention under pressure was determined according to the determination method described in paragraphs 0225 to 0251 of International Publication No. 2016 / 111223, which is referenced by reference.

[0383] Specific surface area

[0384] The specific surface area of ​​the absorbent composition of the present invention was determined according to the determination method described in International Publication No. 2021 / 162072.

[0385] Specifically, the determination was made by analyzing three-dimensional image data of the absorbent composition or absorbent resin obtained using a microfocus X-ray CT system (Shimadzu SMX-100CT) with high-speed three-dimensional analysis software (Ratoc Systems: TRI / 3D-VOL-FCS64). The X-ray CT system measurement conditions and the calculation of specific surface area (using the analysis software) are as described in International Publication No. 2021 / 162072, paragraphs 0182-0184 (US Application Publication No. 2023 / 076935, paragraphs 0301-0327), which are cited by reference.

[0386] [Moisture content] (WSP230.3 (10))

[0387] "Moisture content" refers to the amount of water in the absorbent composition or absorbent resin, which is determined by the drying loss of 1 g of absorbent composition or absorbent resin after drying at 105°C for 3 hours. WSP230.3(10) describes "Moisture Content", but it is essentially the same content.

[0388] [Vortex (water absorption rate)]

[0389] The Vortex (water absorption rate) of the absorbent composition of the present invention was determined according to JIS K 7224 (1996) following these steps: First, 0.02 parts by weight of Edible Blue No. 1 (CAS No. 3844-45-9) as a food additive was added to 1000 parts by weight of physiological saline for coloring, and the liquid temperature was adjusted to 30°C. This was used as the test solution. Next, 50 mL of the above test solution was measured into a 100 mL beaker, and a cylindrical stir bar with a length of 40 mm and a diameter of 8 mm was placed in the beaker, and stirring was started at 600 rpm. Next, 2.0 g of absorbent resin was added to the above-stirred test solution, and the time until the stir bar was covered by the test solution was measured as the water absorption rate based on the Vortex method.

[0390] [Surface tension]

[0391] The surface tension of the absorbent composition of the present invention is determined by the following method.

[0392] First, add 40 ml of a 0.9% sodium chloride aqueous solution, adjusted to 23°C to 24°C, to a thoroughly cleaned 50 ml beaker. Use a surface tension meter (KRUSS K11 automatic surface tension meter) to measure the surface tension of the 0.9% sodium chloride aqueous solution. During this measurement, the surface tension value must be within the range of 72 mN / m to 74 mN / m.

[0393] Next, after adjusting the surface tension measurement to 23°C to 24°C, a thoroughly cleaned cylindrical stir bar (25 mm long, 7 mm in cross-sectional diameter) and 0.5 g of desiccant composition were added to a beaker containing 40 ml of 0.9% sodium chloride aqueous solution. The mixture was stirred at 350 rpm for 3 minutes. After 3 minutes, stirring was stopped, and the mixture was allowed to stand for 2 minutes. After the desiccant composition had absorbed water and settled, the same procedure was repeated to measure the surface tension of the supernatant. It should be noted that this measurement used the plate method with platinum plates. The plates were thoroughly cleaned with deionized water and heated with a gas torch before each measurement.

[0394] [Residual Peroxide Content]

[0395] The amount of peroxide decomposition products present on the particle surface of the absorbent composition can be determined by the following method.

[0396] (Pretreatment 1: Impact test)

[0397] 30g of the absorbent composition and 10g of 6mm diameter glass beads (precision fractionation filling sodium-calcium glass beads) were placed in a glass container (Mayonnaise 225, Yamamura Glass Co., Ltd., Japan) with a diameter of 6cm and a height of 11cm. The container was then sealed with a resin inner and outer cap (TOP Co., Ltd., Product Code: 604-003, Product Name: Inner and Outer Caps Included in the Mayonnaise Bottle PP Cap Set). The absorbent composition was then pulverized by shaking a coating shaker (Toyo Seiki Co., Ltd., Test Disperser: Product No. 488) at 800 times / minute. Furthermore, the shaking time was adjusted for each absorbent composition tested to satisfy the following formula (3).

[0398] 0.2 < (mass of particle swarm a) / (total mass of particle swarm a and particle swarm b) < 0.3 ... Equation (3)

[0399] It should be noted that details of the aforementioned paint shaker are described in Japanese Patent Application Publication No. 9-235378. After the shaker was used, glass beads were removed using a JIS standard sieve with a mesh size of 2mm.

[0400] (Pretreatment 2: Screening)

[0401] The pulverized absorbent composition obtained in the impact test of pretreatment 1 above was sieved into particle group a (300 μm or larger) and particle group b (smaller than 300 μm) using a JIS standard sieve. Most of the surface portion of the particles that was removed by the pulverization process passed through the 300 μm JIS standard sieve.

[0402] (Quantitative analysis of peroxides)

[0403] The persulfate added as a peroxide in the embodiments of this application decomposes during the manufacturing process of the absorbent composition (mainly the drying process and the heat treatment process), and the residual components (peroxide decomposition products) exist in the absorbent composition in the form of sulfate. For example, in the case of sodium persulfate, the residual component is sodium sulfate. The quantification of sodium sulfate is described in detail below, but the quantification of other substances is also determined according to this method.

[0404] The following are examples of ion chromatography used for the quantitative determination of sodium sulfate.

[0405] Ion chromatography system: Dionex Integrion RFIC system.

[0406] Chromatographic columns: Dionex IonPac AS-18, Dionex IonPac AG-18.

[0407] Sodium sulfate, the analyte, is dissolved in a 0.1% (w / w) formaldehyde aqueous solution to prepare a standard aqueous solution of arbitrary concentration. The standard aqueous solution is analyzed using the ion chromatography method described above, and a standard curve is constructed based on the relationship between the peak area of ​​the obtained chromatogram and the concentration of sodium sulfate. It should be noted that since the peaks detected by ion chromatography originate from sulfate ions generated by the ionization of sodium sulfate, the mass conversion is performed using sulfate ions as the total amount of sodium sulfate.

[0408] Next, 0.1 g each of the absorbent compositions of particle group a and particle group b obtained in pretreatment 2 were placed into cylindrical polypropylene containers with a capacity of approximately 250 mL. 100 g of 0.1% (w / w) formaldehyde aqueous solution was then added, and the mixture was stirred at 500 rpm for 1 hour using a cylindrical stir bar with a length of 25 mm. After stirring, the supernatant of the absorbent composition without swelling was collected and filtered through a sample pretreatment filter (GL chromatographic plate, 25A, 0.2 μm, Cat. No. 5040-28502) to obtain extract a and extract b of the analyte.

[0409] For extracts a and b, quantitative analysis based on the above-mentioned ion chromatography was performed, and the content of residual sodium sulfate C1 and C2 in the desiccant composition of particle group a and particle group b was determined according to the pre-prepared standard curve.

[0410] It should be noted that, unless otherwise stated, the raw material compounds, reaction reagents, and solvents used in the examples and comparative examples are commercially available products (e.g., commercially available products sold by Nippon Shokubai Co., Ltd.). Furthermore, the type of standard aqueous solution is changed according to the aforementioned residual components. For example, in the case of potassium persulfate, since the residual component is potassium sulfate, a potassium sulfate aqueous solution is used as the standard aqueous solution.

[0411] It should be noted that the content of peroxide remaining in the absorbent compositions of particle groups a and b is the value after correction for solid components by the following (Formula 4).

[0412] Moisture content corrected content (mass%) = Content (mass%) / (Amount of water absorbent composition added 0.1g × Amount of solid component (mass%)) ... (Equation 4)

[0413] The amount of solid component in (Equation 4) is calculated based on (Equation 5).

[0414] Solid content (mass%) = 100 - Moisture content (mass%) ... (Equation 5)

[0415] It should be noted that the moisture content of the absorbent composition was determined as described above.

[0416] [Water vapor density in the heat treatment apparatus]

[0417] In the manufacturing method of the present invention, the water vapor density in the heat treatment apparatus is determined by the following method. That is, the measurement is performed in the environment vertically above the contents (water-absorbing resin) heated by the heating section of the heat treatment apparatus, by capturing the gas located within 5 cm, preferably within 3 cm, and more preferably within 1 cm of the powder surface of the contents.

[0418] Methods for capturing the aforementioned gases include accumulating them in a cylindrical container of appropriate capacity, and using a pump to draw them in and condense or absorb the organic surface crosslinking agent and water vapor (condensable components) along the way. From the viewpoint of measurement accuracy, the latter method is preferred, and an apparatus having the following configuration is preferred.

[0419] As a gas collection device, a device having the following components is preferred: a sampling line, which is a rigid tube, preferably made of SUS, with an inner diameter of 1 mm or more and 10 mm or less, and is both heat-resistant and chemical-resistant; a gas switching section, preferably a heat-resistant 60,000-valve; a collection section for condensing or absorbing condensable components; a flow measurement section for measuring the flow rate of non-condensable gas passing through the collection section; and a suction pump connected downstream of the flow measurement section.

[0420] Furthermore, it is preferable that the portion upstream of the aforementioned collection section is kept at a temperature equal to or higher than the temperature of the gas being collected at the inlet of the sampling pipeline. It should be noted that the temperature during insulation is preferably 100°C or higher, more preferably 100°C or higher and 150°C or lower. In addition, in cases of blockage due to dust, it is preferable to install a filter, a cyclone dust collection device, or the like in the sampling pipeline.

[0421] The flow rate of the captured gas is preferably 10 seconds or less, more preferably 5 seconds or less, and even more preferably 3 seconds or less, obtained by dividing the volume from the inlet of the sampling line to the inlet of the flow measurement unit by the flow rate of the non-condensable gas. It should be noted that the water content in the captured condensate (solution of the condensing component) can be determined by Karl Fischer titration or the like.

[0422] <Manufacturing of Water-Absorbent Resins>

[0423] [Manufacturing Example 1]

[0424] (Preparation process of monomer aqueous solution)

[0425] A monomer aqueous solution (S1') was prepared by adding 421.7 parts by weight of acrylic acid, 140.4 parts by weight of 48% sodium hydroxide aqueous solution, 2.3 parts by weight of polyethylene glycol diacrylate (PEGDA, n:9), 1.3 parts by weight of 2.0% trisodium diethylenetriaminepentaacetate aqueous solution, 4.4 parts by weight of 1.0% polyoxyethylene (20) sorbitan monostearate (manufactured by Kao Corporation) aqueous solution, and 390.3 parts by weight of deionized water to a 2L polypropylene container and mixing them. It should be noted that the deionized water was preheated to 40°C.

[0426] (Polymerization process)

[0427] Next, while stirring the above monomer aqueous solution (S1'), the mixture was cooled. When the liquid temperature reached 39°C, 211.9 parts by mass of a 48% sodium hydroxide aqueous solution was added to the monomer aqueous solution (S1') over approximately 20 seconds under open atmospheric conditions and mixed to prepare the monomer aqueous solution (S1). At this time, the temperature of the monomer aqueous solution (S1) rose to approximately 81°C due to the heat of neutralization and heat of solution generated during the mixing process.

[0428] Next, in the stirred monomer aqueous solution (S1), nitrogen gas was introduced into the monomer aqueous solution (S1) for 10 seconds using a Kinoshita-type glass ball filter (manufactured by Kinoshita Rika Kogyo Co., Ltd.: Filter Particle No. 4) at a pressure of 0.1 MPa and a flow rate of 0.1 L / min. Then, 17.6 parts by mass of 4.0% sodium persulfate aqueous solution were added, and the mixture was stirred further for about 5 seconds. The solution was then poured into a stainless steel tank-shaped container (bottom surface 340×340 mm, height 25 mm, inner surface: Teflon (registered trademark) coating) under open atmospheric conditions. It should be noted that the time from the start of the second stage of neutralization to the injection of the monomer aqueous solution (S1) into the tank-shaped container was set at 55 seconds. The tank-shaped container was heated to a surface temperature of 40°C using a heating plate (manufactured by Inouchi Seieido Co., Ltd.: NEO HOTPLATE HI-1000). After the monomer aqueous solution (S1) was injected into the tank-shaped container, the polymerization reaction began after 59 seconds. In the polymerization reaction, the material expands and foams upwards in all directions while generating water vapor, and then shrinks to a size slightly larger than the bottom of the trough-shaped container. The polymerization reaction (expansion and shrinkage) ends within approximately 1 minute. Three minutes after the start of the polymerization reaction, the hydrogel (S1) is removed.

[0429] (Gel pulverization process)

[0430] Next, the hydrogel (S1) was cut into appropriate sizes and fed into a shredder for gel pulverization to obtain particulate hydrogel (S1). The shredder has a porous plate at its front end with a diameter of 100 mm, a pore size of 6.4 mm, 83 pores, an opening ratio of 34%, and a thickness of 10 mm. The screw shaft has an outer diameter of 86 mm, a rotational speed of 130 rpm, and an inner diameter of 88 mm. The resulting particulate hydrogel (S1) has a mass-average particle size of 360 μm.

[0431] (Drying process)

[0432] Next, the above-mentioned particulate hydrogel (S1) was spread out and placed on a metal mesh with a mesh size of 300 μm, and then placed in a hot air dryer. Then, the particulate hydrogel (S1) was dried by passing hot air at 190°C for 30 minutes to obtain a dried polymer (S1).

[0433] (Grinding process, grading process)

[0434] Next, the dried polymer (S1) was fed into a roller mill (manufactured by Inokuchi Giken Co., Ltd.: WML type roller mill) for pulverization, and then classified using two JIS standard sieves with mesh sizes of 710 μm and 150 μm, thereby obtaining an irregularly broken, uncrosslinked pre-crosslinked water-absorbing resin (S1). The obtained pre-crosslinked water-absorbing resin (S1) had a CRC of 33.0 g / g and a specific surface area of ​​41 m². 2 / kg, with a D50 of 390μm and a particle size less than 150μm, the mass ratio of particles is 2.6% by mass. The physical properties of the water-absorbing resin (S1) before surface crosslinking are also recorded in Tables 1-1 and 2-1.

[0435] (Surface crosslinking process)

[0436] Next, relative to 100 parts by weight of the water-absorbing resin (S1) before surface crosslinking, a surface crosslinking agent solution containing 0.18 parts by weight of 1,6-hexanediol, 0.4 parts by weight of triethylene glycol, 0.01 parts by weight of 10.0% polyoxyethylene (20) sorbitan monostearate aqueous solution and 3.0 parts by weight of deionized water was added from two locations using a high-speed continuous mixer and mixed uniformly to obtain a mixture (S1).

[0437] Next, in a heat treatment apparatus (heat medium temperature 205°C) where the ambient temperature was adjusted to 180°C and the water vapor density was adjusted to 0.42 g / L by adjusting the gauge pressure inside the heat treatment apparatus, the above mixture (S1) was heated while being stirred for 30 minutes. Then, it was crushed until it passed through a JIS standard sieve with a mesh size of 850 μm, thereby obtaining the surface-crosslinked water-absorbing resin (S1).

[0438] [Comparative Example 1]

[0439] The water-absorbing resin (S1) before surface crosslinking in Manufacturing Example 1 was set as the water-absorbing composition (C1) obtained in Comparative Example 1. The conditions in Comparative Example 1 are shown in Tables 1-2, and the physical properties of the water-absorbing composition (C1) are shown in Tables 1-3.

[0440] [Comparative Example 2]

[0441] The surface-crosslinked absorbent resin (S1) from Manufacturing Example 1 was used as the surface-crosslinked absorbent composition (C2) obtained in Comparative Example 2. The conditions in Comparative Example 2 are shown in Tables 1-2, and the physical properties of the absorbent composition (C2) are shown in Tables 1-3.

[0442] [Example 1]

[0443] In Comparative Example 2, relative to 100 parts by weight of the surface-crosslinked absorbent resin (S1) before surface crosslinking, the surface crosslinking agent solution was changed to a mixed solution containing 0.18 parts by weight of 1,6-hexanediol, 0.4 parts by weight of triethylene glycol, 0.01 parts by weight of 10.0% by weight of polyoxyethylene (20) sorbitan monostearate aqueous solution, 0.2 parts by weight of sodium persulfate, and 3.0 parts by weight of deionized water. Otherwise, the same operation as in Comparative Example 2 was performed to obtain the surface-crosslinked absorbent resin (1). It should be noted that the surface-crosslinked absorbent resin (1) is referred to as the absorbent composition (1). The conditions in Example 1 are shown in Tables 1-2, and the physical properties of the absorbent composition (1) are shown in Tables 1-3.

[0444] [Example 2]

[0445] In Example 1, the gauge pressure inside the heat treatment apparatus was adjusted to adjust the water vapor density of the heat treatment apparatus to 0.13 g / L. Otherwise, the same operation as in Example 1 was performed to obtain the surface-crosslinked water-absorbing resin (2). It should be noted that the surface-crosslinked water-absorbing resin (2) is referred to as the water-absorbing agent composition (2). The conditions in Example 2 are shown in Tables 1-2, and the physical properties of the water-absorbing agent composition (2) are shown in Tables 1-3.

[0446] [Comparative Example 3]

[0447] In Comparative Example 2, the gauge pressure inside the heat treatment apparatus was adjusted to bring the water vapor density of the heat treatment apparatus to 0.13 g / L. Otherwise, the same procedures as in Comparative Example 2 were performed to obtain the surface-crosslinked water-absorbing resin (C3). It should be noted that the surface-crosslinked water-absorbing resin (C3) is designated as the water-absorbing agent composition (C3). The conditions in Comparative Example 3 are shown in Tables 1-2, and the physical properties of the water-absorbing agent composition (C3) are shown in Tables 1-3.

[0448]

[0449] 1) 150↓: Particle density smaller than 150μm

[0450]

[0451] *) Abbreviation for organic surface crosslinking agent

[0452] HD: 1,6-Hexanediol

[0453] TEG: Triethylene Glycol

[0454]

[0455] 1) Unit of SFC: ×10 -7 cm3 ·sec / g

[0456] 2) 150↓: Particle density smaller than 150μm

[0457] (Summarize)

[0458] Based on the results of the absorbent compositions of Example 1 and Comparative Example 2 under the same conditions except for the addition of peroxide in the surface crosslinking process, the absorbent composition of Example 1, with a contact angle θ2 of 72° or less, had a lower Vortex and higher AAP and interstitial water retention rate under pressure compared to the absorbent composition of Comparative Example 2. Similarly, based on the results of the absorbent compositions of Example 2 and Comparative Example 3 under the same conditions except for the addition of peroxide in the surface crosslinking process, the absorbent composition of Example 2, with a contact angle θ2 of 72° or less, had a lower Vortex and higher AAP and interstitial water retention rate under pressure compared to the absorbent composition of Comparative Example 3. Furthermore, based on the value of peroxide residue on the surface of the absorbent composition, it can be seen that the peroxide added in the surface treatment process remains on the particle surface and plays a role. It should be noted that no surface treatment was performed in Comparative Example 1, therefore the values ​​of AAP and interstitial water retention rate under pressure were very small, and therefore were not measured.

[0459] [Comparative Example 4]

[0460] In the surface crosslinking process of Manufacturing Example 1, relative to 100 parts by weight of the absorbent resin (S1) before surface crosslinking, the surface crosslinking agent solution was changed to a solution containing 0.4 parts by weight of 1,4-butanediol, 0.6 parts by weight of propylene glycol, 0.01 parts by weight of 10.0% by weight of polyoxyethylene (20) sorbitan monostearate aqueous solution, 0.1 parts by weight of sodium persulfate, and 2.7 parts by weight of deionized water. Furthermore, the mixture (S1) was heated while being stirred for 30 minutes using a heat treatment apparatus (heat medium temperature 200°C) with the ambient temperature adjusted to 180°C and the water vapor density adjusted to 0.05 g / L by adjusting the gauge pressure in the heat treatment apparatus. Otherwise, the same operation as in Manufacturing Example 1 was performed to prepare the surface-crosslinked absorbent composition (C4). The conditions in Comparative Example 4 are shown in Table 2-2, and the physical properties of the absorbent composition (C4) are shown in Table 2-3.

[0461] [Comparative Example 5]

[0462] In Example 1, the gauge pressure in the heat treatment apparatus was adjusted to adjust the water vapor density to 0.05 g / L. Otherwise, the same procedures as in Example 1 were performed to obtain the surface-crosslinked water-absorbing resin (C5). It should be noted that the surface-crosslinked water-absorbing resin (C5) is designated as the water-absorbing composition (C5). The conditions in Comparative Example 5 are shown in Table 2-2, and the physical properties of the water-absorbing composition (C5) are shown in Table 2-3.

[0463] [Example 3]

[0464] In Comparative Example 4, the gauge pressure inside the heat treatment apparatus was adjusted to adjust the water vapor density of the heat treatment apparatus to 0.42 g / L. Otherwise, the same operation as in Comparative Example 4 was performed to obtain the surface-crosslinked water-absorbing resin (3). It should be noted that the surface-crosslinked water-absorbing resin (3) is referred to as the water-absorbing agent composition (3). The conditions in Example 3 are shown in Table 2-2, and the physical properties of the water-absorbing agent composition (3) are shown in Table 2-3.

[0465] [Example 4]

[0466] In Example 1, the gauge pressure inside the heat treatment apparatus was adjusted to adjust the water vapor density of the heat treatment apparatus to 0.51 g / L. Otherwise, the same operation as in Example 1 was performed to obtain the surface-crosslinked water-absorbing resin (4). It should be noted that the surface-crosslinked water-absorbing resin (4) is referred to as the water-absorbing agent composition (4). The conditions in Example 4 are shown in Table 2-2, and the physical properties of the water-absorbing agent composition (4) are shown in Table 2-3.

[0467]

[0468]

[0469] *) Abbreviation for organic surface crosslinking agent

[0470] HD: 1,6-Hexanediol

[0471] TEG: Triethylene Glycol

[0472] BD: 1,4-Butanediol

[0473] PG: Propylene Glycol

[0474]

[0475] 1) Unit of SFC: ×10 -7 cm 3 ·sec / g

[0476] 2) 150↓: Particle density smaller than 150μm

[0477] (Summarize)

[0478] Based on the results of the absorbent compositions of Examples 1, 2, 4 and 5 under the same conditions except for the water vapor density in the surface crosslinking process, as well as Examples 3 and 4, the absorbent compositions of the examples with a contact angle θ2 of 72° or less have low Vortex, high AAP, and high interstitial water retention rate under pressure.

[0479] [Manufacturing Example 2]

[0480] According to Reference Example 1 described in Japanese Patent No. 4926474, a water-absorbing resin (S2) before surface crosslinking and a water-absorbing resin (S2) after the surface crosslinking process were obtained.

[0481] (Preparation process of monomer aqueous solution)

[0482] 15.0 parts by weight of polyethylene glycol diacrylate (average molecular weight: 523) were added to 5500 parts by weight of sodium acrylate aqueous solution with a neutralization rate of 75 mol% (monomer concentration: 38 mol%) and mixed to prepare monomer aqueous solution (S2).

[0483] (Polymerization process, gel pulverization process)

[0484] After degassing the monomer aqueous solution (S2) under nitrogen for 30 minutes, the monomer aqueous solution (S2) was added to a reactor formed by a jacketed stainless steel double-arm kneader with a volume of 10L and two SIGMA-type blades and a cover. While maintaining the temperature of the monomer aqueous solution (S2) at 30°C, nitrogen was purged into the reactor.

[0485] Next, while stirring the monomer aqueous solution (S2) in the reactor, 2.46 parts by mass of sodium persulfate and 0.10 parts by mass of L-ascorbic acid were added, and polymerization began after about 1 minute. The polymerization reaction continued as is at 30°C to 90°C. 60 minutes after the start of the polymerization reaction, the particulate hydrogel (S2) from the reactor contents was removed. The particulate hydrogel (S2) was subdivided into particles of approximately 5 mm in size.

[0486] (Drying process)

[0487] The above-mentioned particulate hydrogel (S2) was placed on a metal mesh with a mesh size of 300 μm and then placed in a hot air dryer. The particulate hydrogel (S2) was then dried by passing hot air at 150°C for 90 minutes to obtain a dried polymer (S2). It should be noted that the dried polymer (S2) contained no undried material.

[0488] (Grinding process, grading process)

[0489] Next, the dried polymer (S2) was fed into a roller mill (WML type roller mill, manufactured by Inokuchi Giken Co., Ltd.) for pulverization. Then, it was classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm. Through this grading process, an irregularly shaped, fragmented pre-crosslinked water-absorbing resin (S2) with a particle size greater than 150 μm and less than 850 μm was obtained. The water-absorbing resin (S2) had a CRC of 33 g / g and a specific surface area of ​​22 m². 2 / kg, with a D50 (weight-average particle size) of 410 μm and a mass ratio of particles smaller than 150 μm of 2.2% by mass. The physical properties of the water-absorbing resin (S2) before surface crosslinking are also described in Table 3-1.

[0490] (Surface crosslinking process)

[0491] Next, relative to 100 parts by weight of the water-absorbing resin (S2) before surface crosslinking, a surface crosslinking agent solution containing 0.18 parts by weight of 1,6-hexanediol, 0.4 parts by weight of triethylene glycol, 0.2 parts by weight of sodium persulfate, and 3.0 parts by weight of deionized water was added from two locations using a high-speed continuous mixer and mixed uniformly to obtain a mixture (S2).

[0492] Next, in a heat treatment apparatus (heat medium temperature 205°C) where the ambient temperature was adjusted to 180°C and the water vapor density was adjusted to 0.42 g / L by adjusting the gauge pressure inside the heat treatment apparatus, the above mixture (S2) was heated while being stirred for 30 minutes. Then, it was crushed until it passed through a JIS standard sieve with a mesh size of 850 μm, thereby obtaining the surface-crosslinked water-absorbing resin (S2).

[0493] [Comparative Example 6]

[0494] The surface-crosslinked absorbent resin (S2) from Manufacturing Example 2 was used as the absorbent composition (C6) obtained in Comparative Example 6. The conditions of Comparative Example 6 are shown in Table 3-2, and the physical properties of the absorbent composition (C6) are shown in Table 3-3.

[0495] [Manufacturing Example 3]

[0496] In Manufacturing Example 1, the amount of polyethylene glycol diacrylate was changed to 2.5 parts by weight, and the pore size of the perforated plate of the shredder was changed to 9.5 mm. Otherwise, the same operations as in Manufacturing Example 1 were performed to obtain an irregularly fragmented, pre-crosslinked, water-absorbing resin (S3). It should be noted that the mass-average particle size of the particulate hydrogel (S3) obtained during manufacturing was 700 μm, the pre-crosslinked, irregularly fragmented water-absorbing resin (S3) had a CRC of 33 g / g, and a specific surface area of ​​27 m². 2 / kg, D50 (mass average particle size) is 390μm, and the mass ratio of particles smaller than 150μm is 2.5 by mass.

[0497] Next, relative to 100 parts by weight of the water-absorbing resin (S3) before surface crosslinking, a surface crosslinking agent solution containing 0.18 parts by weight of 1,6-hexanediol, 0.4 parts by weight of triethylene glycol, 0.2 parts by weight of sodium persulfate, and 3.0 parts by weight of deionized water was added from two locations using a high-speed continuous mixer and mixed uniformly to obtain a mixture (S3).

[0498] Next, in a heat treatment apparatus (heat medium temperature 205°C) where the ambient temperature was adjusted to 180°C and the water vapor density was adjusted to 0.42 g / L by adjusting the gauge pressure inside the heat treatment apparatus, the above mixture (S3) was heated while being stirred for 30 minutes. Then, it was crushed until it passed through a JIS standard sieve with a mesh size of 850 μm, thereby obtaining the surface-crosslinked water-absorbing resin (S3).

[0499] [Example 5]

[0500] The surface-crosslinked absorbent resin (S3) from Manufacturing Example 3 was used as the absorbent composition (5) obtained in Example 5. The conditions in Example 5 are shown in Table 3-2, and the physical properties of the absorbent composition (5) are shown in Table 3-3.

[0501] [Example 6]

[0502] In Manufacturing Example 3, relative to 100 parts by mass of the absorbent resin (S3) before surface crosslinking, the surface crosslinking agent solution was changed to a mixed solution containing 0.7 parts by mass of propylene glycol, 0.4 parts by mass of ethylene carbonate, 0.2 parts by mass of sodium persulfate, and 2.9 parts by mass of deionized water. Otherwise, the same operation as in Manufacturing Example 3 was performed to obtain the surface-crosslinked absorbent resin (6). It should be noted that the surface-crosslinked absorbent resin (6) is referred to as the absorbent composition (6). The conditions in Example 6 are shown in Table 3-2, and the physical properties of the absorbent composition (6) are shown in Table 3-3.

[0503] [Comparative Example 7]

[0504] In Example 6, compared to 100 parts by mass of the pre-crosslinked water-absorbing resin (S3), the surface crosslinking agent solution was changed to a mixed solution containing 0.7 parts by mass of propylene glycol, 0.4 parts by mass of ethylene carbonate, and 2.9 parts by mass of deionized water. Otherwise, the same operation as in Example 6 was performed to obtain the surface-crosslinked water-absorbing resin (C7). It should be noted that the surface-crosslinked water-absorbing resin (C7) is designated as the water-absorbing composition (C7). The conditions of Comparative Example 7 are shown in Table 3-2, and the physical properties of the water-absorbing composition (C7) are shown in Table 3-3.

[0505] [Example 7]

[0506] In Manufacturing Example 3, compared to 100 parts by mass of the absorbent resin (S3) before surface crosslinking, the surface crosslinking agent solution was changed to a mixed solution containing 1.1 parts by mass of ethylene carbonate, 0.2 parts by mass of sodium persulfate, and 2.9 parts by mass of deionized water. Otherwise, the same operation as in Manufacturing Example 3 was performed to obtain the absorbent resin (7) after surface crosslinking. It should be noted that the absorbent resin (7) after surface crosslinking is referred to as the absorbent composition (7). The conditions in Example 7 are shown in Table 3-2, and the physical properties of the absorbent composition (7) are shown in Table 3-3.

[0507] [Example 8]

[0508] In Manufacturing Example 3, compared to 100 parts by mass of the absorbent resin (S3) before surface crosslinking, the surface crosslinking agent solution was changed to a mixed solution containing 0.8 parts by mass of glycerol, 0.2 parts by mass of sodium persulfate, and 3.0 parts by mass of deionized water. Otherwise, the same operation as in Manufacturing Example 3 was performed to obtain the absorbent resin (8) after surface crosslinking. It should be noted that the absorbent resin (8) after surface crosslinking is referred to as the absorbent composition (8). The conditions in Example 8 are shown in Table 3-2, and the physical properties of the absorbent composition (8) are shown in Table 3-3.

[0509] [Example 9]

[0510] In Example 8, the temperature of the heat medium in the heat treatment apparatus was changed to 200°C, and the same operation as in Example 8 was performed to obtain the surface-crosslinked water-absorbing resin (9). It should be noted that the surface-crosslinked water-absorbing resin (9) is referred to as the water-absorbing agent composition (9). The conditions in Example 9 are shown in Table 3-2, and the physical properties of the water-absorbing agent composition (6) are shown in Table 3-3.

[0511] [Comparative Example 8]

[0512] In Example 8, compared to 100 parts by mass of the absorbent resin (S3) before surface crosslinking, the surface crosslinking agent solution was changed to a mixed solution containing 0.8 parts by mass of glycerol and 3.0 parts by mass of deionized water, and the heat medium temperature of the heat treatment apparatus was changed to 200°C. Otherwise, the same operation as in Example 8 was performed to obtain the absorbent resin (C8) after surface crosslinking. It should be noted that the absorbent resin (C8) after surface crosslinking is referred to as the absorbent composition (C8). The conditions of Comparative Example 8 are shown in Table 3-2, and the physical properties of the absorbent composition (C8) are shown in Table 3-3.

[0513]

[0514] 1) 150↓: Particle density smaller than 150μm

[0515]

[0516] *) Abbreviation for organic surface crosslinking agent

[0517] PG: Propylene Glycol

[0518] HD: 1,6-Hexanediol

[0519] TEG: Triethylene Glycol

[0520] EC: Ethylene carbonate

[0521] Gly: Glycerin

[0522]

[0523] 1) Unit of SFC: ×10 -7 cm 3 ·sec / g

[0524] 2) 150↓: Particle density smaller than 150μm

[0525] (Summarize)

[0526] Based on the results of the superabsorbent compositions of Comparative Example 6 and Example 5 under the same conditions except for the difference in the specific surface area of ​​the superabsorbent resin, compared with the superabsorbent composition of Comparative Example 6, the superabsorbent composition of Example 5, which has a contact angle θ2 of 72° or less, has a lower Vortex and higher AAP and interstitial water retention rate under pressure.

[0527] Furthermore, based on the results of the absorbent compositions of Examples 6, 7, 9, and 8 under the same conditions except for the addition of peroxide in the surface crosslinking process, the absorbent compositions of the examples with a contact angle θ2 of 72° or less exhibited lower Vortex, higher AAP, and higher interstitial water retention rate under pressure compared to the absorbent compositions of the comparative examples. Moreover, based on the value of the amount of peroxide residue on the surface of the absorbent composition, it can be seen that the peroxide added in the surface treatment process remains on the particle surface and exerts its effect. Furthermore, in the absorbent compositions of Examples 7 and 8, which used different types of surface crosslinking agents, Vortex was also low, and AAP and interstitial water retention rate under pressure were also high.

[0528] The Washburn contact angle θ2 of the absorbent compositions (1) to (9) is 72° or less. It is believed that the preparation method of this application can improve the affinity between the absorbed liquid and the surface of the absorbent composition, and that by making it easier for the liquid to penetrate into the cavity formed on the surface of the absorbent resin, the interstitial water retention rate under pressure can be improved. Therefore, the amount of backflow in absorbent articles such as disposable diapers can be reduced.

[0529] Furthermore, based on the value of the residual peroxide on the surface of the absorbent composition, it can be seen that the peroxide added in the surface treatment process remains on the particle surface and plays a role.

[0530] Industrial availability

[0531] The absorbent composition of the present invention can be suitably used, for example, in disposable diapers. Furthermore, the absorbent composition of the present invention can also be suitably used in various applications besides disposable diapers, such as absorbent articles (sanitary napkins, incontinence pads, etc.), soil moisture retainers for agriculture and horticulture, and water-stopping agents for industrial use.

[0532] This application is based on Japanese Patent Application No. 2023-193270, filed on November 13, 2023, the disclosure of which is incorporated herein by reference in its entirety.

Claims

1. A method for manufacturing a water-absorbing composition, the method comprising a surface crosslinking step of a water-absorbing resin, wherein the water-absorbing composition is mainly composed of a surface-crosslinked water-absorbing resin, and the method for manufacturing the water-absorbing composition satisfies all of the following (1) to (3): (1) The specific surface area of the water-absorbent resin before surface crosslinking is 25 m2 / g or more 2 / kg or more; (2) The surface crosslinking process is carried out in the presence of peroxide and an organic surface crosslinking agent that can react with carboxyl groups; (3) The surface crosslinking process includes a heat treatment process, wherein the heat treatment process is carried out at or after the addition of the organic surface crosslinking agent to the water-absorbing resin in an environment with a water vapor density exceeding 0.005 g / L.

2. The manufacturing method according to claim 1, wherein, The water vapor density is below 0.60 g / L.

3. The manufacturing method according to claim 1, wherein, The water-absorbing resin before surface cross-linking is irregularly broken in shape.

4. The manufacturing method according to claim 1, wherein, The D50 (weight-average particle size) of the water-absorbing resin before surface crosslinking is greater than 250 μm and less than 550 μm, and the mass percentage of particles with a particle size less than 150 μm contained in the water-absorbing resin before surface crosslinking is less than 3% by mass.

5. The manufacturing method according to claim 1, wherein, The manufacturing method further includes a polymerization step of the monomer aqueous solution. The water-absorbing resin is obtained by foaming polymerization of a monomer aqueous solution.

6. The manufacturing method according to claim 1, wherein, The organic surface crosslinking agent is added to the water-absorbing resin in solution form, wherein the concentration of the organic surface crosslinking agent in the solution is 0.1% by mass or more and 60% by mass or less.

7. The manufacturing method according to claim 1, wherein, The organic surface crosslinking agent is added to the absorbent resin through two or more nozzles located in the mixing device.

8. The manufacturing method according to claim 1, wherein, The surface crosslinking process is carried out under a micro-pressure reduction with a pressure difference of more than -10 kPa and less than 0 kPa relative to atmospheric pressure.

9. The manufacturing method according to claim 1, wherein, The peroxide is a persulfate.

10. A water-absorbing composition having a surface-crosslinked water-absorbing resin as the main component, wherein the water-absorbing composition satisfies all of the following (1) to (2): (1) The specific surface area of ​​the absorbent composition is 25 m². 2 / kg or more; (2) For the absorbent composition, the Washburn contact angle θ2, determined by a 20% by mass aqueous solution of sodium chloride, is less than 72°.

11. The absorbent composition according to claim 10, wherein, The surface-crosslinked water-absorbing resin has an irregular, broken shape, with D50 (i.e., a mass-average particle size of 250 μm or more and less than 550 μm) and a mass percentage of particles smaller than 150 μm of less than 3% by mass.

12. The absorbent composition according to claim 10, wherein, The absorbent composition has an absorption rate (AAP) of 20.0 g / g or more under pressure of 4.83 kPa.

13. The absorbent composition according to claim 10, wherein, The water-absorbing agent composition has a gap water retention rate of 9.0 g / g or more under pressure.

14. The absorbent composition according to claim 10, wherein, The surface tension of the absorbent composition is above 56 mN / m.

15. The absorbent composition according to claim 10, wherein, The absorbent composition has an absorption time (Vortex) of less than 50 seconds.

16. The absorbent composition according to claim 10, wherein, The absorbent composition also contains peroxide decomposition products. The concentration of this peroxide decomposition product on the surface of the absorbent composition is higher than that inside the absorbent composition.

17. The absorbent composition according to claim 10, wherein, When particles with a diameter of 300 μm or larger are classified as particle group a and particles with a diameter of less than 300 μm are classified as particle group b, and the amount of peroxide decomposition products present in particle group a after the water absorbent composition is subjected to an impact test based on a coating oscillator is defined as C1 and the amount of peroxide decomposition products present in particle group b is defined as C2, C2-C1 exceeds 0% by mass and is less than 1% by mass.

18. The absorbent composition according to claim 10, wherein, The absorbent composition has an SFC (salt water) flow induction rate of 1×10⁻⁶. -7 cm 3 • above sec / g.

19. An absorbent article comprising an absorbent composition according to any one of claims 10 to 18.

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