Water-absorbent resin and method for producing water-absorbent resin

Abstract

A water-absorbent resin which can be easily swelled and gelled by absorbing an aqueous liquid such as an anti-freezing solution even in a low-temperature environment, and which is characterized in that (a) the value of the Vortex water absorption time at 0 DEG C is 30 seconds or less; (b) the ratio of 0 DEG C Vortex water absorption time / 30 DEG C Vortex water absorption time is 5 or less; (c) the GEX value is 17 or more; and (d) the value of CRC is a value exceeding 25 g / g.

Description

TECHNICAL FIELD

[0001] The present application relates to a water-absorbent resin and a method for producing a water-absorbent resin. BACKGROUND

[0002] A water-absorbent resin (SAP / Super Absorbent Polymer) is a high molecular gelation agent having water-swellability and water-insolubility. The water-absorbent resin is used as a water-retaining agent for agriculture and horticulture, a water-stopping agent for industry, and the like, and is widely used in various fields as a material for an article utilizing the property of being able to absorb a liquid such as water.

[0003] The water-absorbent resin can be produced from a raw material selected from various monomers and hydrophilic polymers. As the water-absorbent resin, from the aspect of water-absorbing performance, a water-absorbent resin in which a polymer composed of an acid group-containing unsaturated monomer is the main component is produced the most in industry.

[0004] As a conventional water-absorbent resin obtained using an acid group-containing unsaturated monomer, for example, the following (A) to (C) can be listed.

[0005] (A) A water-absorbent resin formed of polyacrylate crosslinked material dried individuals, which are produced by performing a polymerization reaction on a monomer composition containing potassium acrylate, a divinyl-based compound, and a polymerization initiator with a monomer concentration of 55 to 80% by weight and vaporizing moisture by heating (Patent Document 1).

[0006] (B) A super absorbent resin produced by a production method including, in order, a stage of producing an unsaturated monomer by neutralizing an acid unsaturated monomer and a basic substance containing potassium base at a molar ratio within a range of 1:0.45 to 0.99, and a stage of polymerizing the unsaturated monomer (Patent Document 2).

[0007] (C) A super absorbent resin including: a base resin powder including a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acid group, at least a part of which is neutralized by a potassium salt; and a surface crosslinked layer formed on the base resin powder, including a second crosslinked polymer produced by further crosslinking the first crosslinked polymer using a surface crosslinking agent (Patent Document 3).

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 58-071907

[0011] Patent Document 2: Korean Patent No. 10-1274880

[0012] Patent Document 3: Korean Patent Publication No. 10-2020-0073044 SUMMARY

[0013] PROBLEMS TO BE SOLVED BY THE INVENTION

[0014] As the use of the water-absorbing resin, for example, the use as a solidifying agent for antifreeze liquid (leaked or discarded), sandbags, and a solidifying agent for leaked water such as nuclear power plant drainage, and the like can be exemplified. On the other hand, the existing water-absorbing resin has a problem that particularly in a low temperature environment, it is difficult to perform swelling gelation, and it is not possible to sufficiently exert the function as the solidifying agent or the like. However, a water-absorbing resin capable of solving the problem has not been provided so far.

[0015] The present application was completed in view of such a problem, and an object thereof is to provide a water-absorbing resin which is capable of easily performing swelling gelation by absorbing an aqueous liquid such as antifreeze liquid even in a low temperature environment, and capable of preventing the problem from occurring. Note that the "swelling gelation" means that the water-absorbing resin becomes a swelling gel by absorbing the aqueous liquid, and the flowability of the aqueous liquid is reduced.

[0016] SOLUTION TO THE PROBLEM

[0017] As a result of intensive studies by the present inventors and the like, it has been found that the existing water-absorbing resin has a property that the water-absorbing property changes depending on the surrounding temperature, and particularly in a low temperature environment, the property is reduced, and thus the problem occurs, and thus the present application has been conceived.

[0018] The water-absorbing resin of one embodiment of the present application relates to a water-absorbing resin which sufficiently satisfies the following (a) to (d) conditions.

[0019] (a) the value of the 0°C Vortex water absorption time is 30 seconds or less;

[0020] (b) the ratio of the value of the 0°C Vortex water absorption time to the value of the 30°C Vortex water absorption time (Vortex water absorption time ratio (0°C / 30°C)) is 5.0 or less;

[0021] (c) the GEX value is 17 or more;

[0022] (d) the value of the CRC is a value exceeding 25 g / g.

[0023] The production method of the water-absorbent resin of one embodiment of the present invention includes: a polymerization step of cross-linking and polymerizing a monomer composition containing an acid group-containing unsaturated monomer, and optionally containing a monomer other than the acid group-containing unsaturated monomer, to obtain a water-containing gel-like cross-linked polymer, wherein at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, the neutralized acid group-containing unsaturated monomer contains one or more kinds of salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts, the total content of the acid group-containing unsaturated monomer and the monomer other than the acid group-containing unsaturated monomer in the monomer composition is 30% by mass or more and less than 55% by mass with respect to the mass of the entire monomer composition, and the polymerization initiation temperature in the polymerization step is 60°C or higher.

[0024] Effects of Invention

[0025] The water-absorbent resin of one embodiment of the present invention exerts the following effect: even in a low-temperature environment, it can sufficiently swell and gel by absorbing an aqueous liquid such as an antifreeze. Furthermore, the production method of the water-absorbent resin of one embodiment of the present invention exerts the following effect: it can produce a water-absorbent resin that can sufficiently swell and gel by absorbing an aqueous liquid such as an antifreeze even in a low-temperature environment. DETAILED DESCRIPTION

[0026] Hereinafter, the embodiments of the present disclosure will be described in detail. However, the present invention is not limited to this, and various modifications can be made within the scope of the description, and embodiments obtained by appropriately combining the technical means respectively disclosed in different embodiments are also included in the technical scope of the present invention. Note that “~ acid (salt)” means “~ acid and / or a salt thereof”, and “(meth)acryloyl” means “acryloyl and / or methacryloyl”. Furthermore, unless otherwise specified, the mass of the water-absorbent resin indicates a value converted into a solid content.

[0027] [1] Definitions of terms

[0028] [1-1] Water-absorbent resin

[0029] The water-absorbent resin in this specification refers to a water-swellable and water-insoluble polymer gelation agent. Specifically, the water-absorbent resin refers to a polymer gelation agent having a CRC of a value exceeding 25 g / g as water-swellability, and an Ext of 50% by mass or less as water-insolubility. “CRC” refers to centrifuge retention capacity regulated in NWSP 241.0.R2 (15), and “Ext” refers to water-soluble content regulated in NWSP 241.0.R2 (15). Note that “NWSP” will be described later.

[0030] The water-absorbent resin of the present application can be a crosslinked polymer obtained by crosslinking polymerization of an acid group-containing unsaturated monomer, and preferably can be a crosslinked polymer obtained by crosslinking polymerization of an acid group-containing unsaturated monomer having a carboxyl group. The crosslinked polymer can be a hydrophilic crosslinked polymer. Furthermore, the water-absorbent resin is not limited to a form in which all of the water-absorbent resin is a crosslinked polymer, and can be a composition containing an additive or the like, as long as each of the above-mentioned properties (CRC, Ext) satisfies the above-mentioned numerical range.

[0031] The water-absorbent resin can be a resin subjected to surface crosslinking (also called post-crosslinking or secondary crosslinking), or can be a resin not subjected to surface crosslinking. Note that, in the present specification, a water-absorbent resin in which a prescribed surface crosslinking treatment is completed is also called a "water-absorbent resin subjected to surface crosslinking". Furthermore, a water-absorbent resin adjusted to a prescribed water content and particle diameter is also called a water-absorbent resin powder or a water-absorbent agent.

[0032] [1-2] Fine powder

[0033] "Fine powder" in the present specification means a water-absorbent resin in a particulate or powder form, and means a water-absorbent resin that passes through a sieve having a pore diameter of 150 μm when the entire water-absorbent resin of the present application is classified using the sieve. The fine powder preferably means a fine powder containing a water-absorbent resin in which a poly(meth)acrylic acid (salt)-based crosslinked polymer is a main component. The water-absorbent resin constituting the fine powder can be a water-absorbent resin subjected to surface crosslinking or a water-absorbent resin not subjected to surface crosslinking. Furthermore, the fine powder can also contain water and / or an additive described in the addition step of other additives described later. That is, the fine powder is not limited to a fine powder composed of 100% of a water-absorbent resin (a polymer having a water content of 0%), and can contain water and / or other trace components (for example, inorganic fine particles and the like) as a raw material of the water-absorbent resin.

[0034] [1-3] "Poly(meth)acrylic acid (salt)"

[0035] "Poly(meth)acrylic acid (salt)" in the present specification means polyacrylic acid and / or a salt thereof. The poly(meth)acrylic acid (salt) is a crosslinked polymer in which a structure derived from (meth)acrylic acid and / or a salt thereof (hereinafter, also called "(meth)acrylic acid (salt)") is contained as a main component in the form of a repeating unit, and a structure derived from an internal crosslinking agent is contained as an arbitrary component.

[0036] The "main component" means that the amount (content) of the (meth)acrylic acid (salt) is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, further preferably 90 to 100 mol%, and particularly preferably substantially 100 mol% relative to the entire monomers used in the polymerization. Note that the "main component" in the present specification means that even in the case where a substance other than the (meth)acrylic acid (salt) is used, the amount (content) of the (meth)acrylic acid (salt) is within the above range relative to the entire composition containing the substance.

[0037] Here, the poly(meth)acrylic acid salt contained in the water-absorbent resin is preferably a partially neutralized or completely neutralized poly(meth)acrylic acid salt, more preferably a monovalent salt, further preferably an alkali metal salt or an ammonium salt, and particularly preferably a salt selected from one or more of the group consisting of a potassium salt, a lithium salt, and an ammonium salt.

[0038] [1-4] Definitions of evaluation methods

[0039] The "NWSP" means "Non-Woven Standard Procedures - Edition 2015". The NWSP is a standard published by EDANA (European Disposables And Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) in the United States and Europe for the purpose of unifying the evaluation methods of nonwoven fabrics and products thereof, and includes a standard measurement method of a water-absorbent resin therein. In the present specification, the physical properties of the water-absorbent resin are measured in accordance with the "Non-Woven Standard Procedures - Edition 2015" unless otherwise specified. For the evaluation methods not described in the NWSP, the methods and conditions described in the examples are used for the measurement.

[0040] [1-4-1] "CRC" (NWSP 241.0.R2(15))

[0041] The "CRC" is an abbreviation of "Centrifuge Retention capacity", and means the absorption ratio (sometimes referred to as "water absorption ratio") of a water-absorbent resin in a 0.90 mass% sodium chloride aqueous solution for 30 minutes under no pressure (unit: g / g). As the measurement method of the "CRC", for example, the method described in the examples can be used.

[0042] [1-4-2] "PSD" (NWSP 220.0.R2(15))

[0043] "PSD" is short for Particle Size Distribution, which refers to the particle size distribution of a water-absorbing resin as determined by sieve grading. It should be noted that parameters representing "PSD" include, for example, the mass-average particle size (D50) and the logarithmic standard deviation (σζ) of the particle size distribution. As for the method of determining D50 and σζ, the method described in the examples can be used, specifically the same method described in columns 27-28 of U.S. Patent No. 7,638,570, "(3) Mass-average particle size (D50) and logarithmic standard deviation of particle size distribution".

[0044] [1-4-3] "Moisture content and solid content" (NWSP 230.0.R2(15))

[0045] "Moisture content" is the ratio of the drying weight loss of the water-absorbing resin to its initial weight (mass%). "Solid content" is the ratio of the dried weight to its initial weight (mass%). Moisture content and solid content can be determined according to the NWSP. For example, the methods described in the examples can be used for these determinations.

[0046] Furthermore, in this specification, the water content and solids content of the hydrogel refer to the water content and solids content of the hydrogel before drying, respectively. The water content and solids content of the hydrogel can be measured after the polymerization process described later and before the drying process described later. That is, the water content and solids content of the hydrogel can be measured using the hydrogel before gel pulverization or using the granular hydrogel after gel pulverization. The water content and solids content of the hydrogel can be determined according to NWSP. For example, the method described in the examples can be used as a method for measuring the water content and solids content of the hydrogel.

[0047] [1-4-4] "0°C Vortex water absorption time and 30°C Vortex water absorption time"

[0048] In this specification, "Vortex water absorption time" is an indicator of the water absorption rate of the superabsorbent resin, referring to the time (in seconds) required for 2g of superabsorbent resin to absorb 50ml of a 0.90% (w / w) sodium chloride aqueous solution to a specified state. "0°C Vortex water absorption time" refers to the Vortex water absorption time measured with the sodium chloride aqueous solution at 0°C. Furthermore, "30°C Vortex water absorption time" refers to the Vortex water absorption time measured with the sodium chloride aqueous solution at 30°C. The "0°C Vortex water absorption time" and "30°C Vortex water absorption time" can be measured, for example, by the methods described in the examples.

[0049] [1-4-5] "Ext" (NWSP 241.0.R2(15))

[0050] "Ext" is short for Extractables, which refers to the water-soluble components (the amount of water-soluble polymer in the superabsorbent resin) in the superabsorbent resin. As a method for determining the "Ext", the method described in the examples can be used, for example.

[0051] [1-4-6] "GEX"

[0052] The “GEX value” refers to the value defined by equation (1) or (2) below, where the unpressurized absorption ratio (CRC) is set as y (g / g) and the water-soluble content Ext is set as x (mass%). Generally, in the relationship between CRC and water-soluble content, a water-absorbing resin with high CRC and low water-soluble content is preferred, and CRC and water-soluble content have an inverse relationship. Therefore, in order to evaluate CRC and water-soluble content as a parameter, the GEX value is defined. The larger the GEX value, the higher the performance and the smaller the degradation of physical properties.

[0053] When x>1, the GEX value = (y+17) / ln(x)……(1)

[0054] Where ln(x) is the natural logarithm of x.

[0055] When x≤1, the GEX value = (y) / (x)……(2)

[0056] [1-4-7] Solid D50 of the particulate water-containing gel

[0057] In this specification, "solid-average particle size converted from solid components of the particulate hydrogel" (hereinafter, "SolidD50") refers to the solid-average particle size (D50) of the particulate hydrogel obtained by gel pulverization as described later. That is, the Solid D50 is equivalent to the solid-average particle size of the particles obtained when the particulate hydrogel is dried. As a method for determining the Solid D50, the method described in the examples can be used, for example.

[0058] [1-4-8] "Residual volatile component amount"

[0059] The term "residual volatile content" in this specification refers to the amount of volatile components derived from organic solvents or the like incorporated into the absorbent resin during the manufacturing process (e.g., polymerization). It should be noted that these volatile components are firmly contained within the absorbent resin. Therefore, even when subjected to treatments such as heating and / or depressurization to adjust the environment surrounding the absorbent resin to one where the volatile components would normally be released, they are hardly released to the outside of the absorbent resin. On the other hand, when the absorbent resin comes into contact with an aqueous liquid to form a swollen gel, a portion of the volatile components may be released from within the absorbent resin. The method described in the examples can be used, for example, to determine the "residual volatile content."

[0060] [1-5] Others

[0061] In this specification, the range "X~Y" means "above X and below Y". Furthermore, unless otherwise specified, the unit of mass "t (ton)" means "Metric ton", and "ppm" means "mass ppm" or "weight ppm". Additionally, "~acid (salt)" means "~acid and / or its salt", and "(meth)acryloyl" means "acryloyl and / or methacryloyl". For convenience, "liter" is sometimes written as "l" or "L". Furthermore, in the case of determining trace components, etc., anything below the detection limit is designated as ND (Non Detected).

[0062] [2] Embodiment 1: Water-absorbent resin

[0063] The water-absorbing resin of the present invention fully satisfies the following conditions (a) to (d).

[0064] (a) The water absorption time of Vortex at 0°C is less than 30 seconds;

[0065] (b) The ratio of the Vortex water absorption time at 0°C to the Vortex water absorption time at 30°C (Vortex water absorption time ratio (0°C / 30°C)) is less than 5;

[0066] (c) GEX value is 17 or higher;

[0067] (d) The CRC value exceeds 25g / g.

[0068] [2-1] Mechanism of the water-absorbent resin of the present invention

[0069] In one embodiment of the invention, sufficiently satisfying condition (a) means that the water-absorbing resin has a sufficiently fast absorption rate when absorbing an aqueous liquid at a temperature of 0°C. Here, "absorbing an aqueous liquid at a temperature of 0°C" is equivalent to setting the ambient temperature of the water-absorbing resin to 0°C. Therefore, the absorption rate under condition (a) is equivalent to the absorption rate of the water-absorbing resin in a low-temperature environment.

[0070] Furthermore, fully satisfying condition (b) means that even when the temperature of the aqueous liquid absorbed by the absorbent resin changes from 30°C to 0°C, the decrease in the absorption rate of the absorbent resin is sufficiently small. Here, changing the temperature of the aqueous liquid absorbed by the absorbent resin from 30°C to 0°C is equivalent to changing the temperature around the absorbent resin from 30°C to 0°C. Therefore, this situation is equivalent to changing the temperature of the environment in which the absorbent resin exists from 30°C to 0°C. It should be noted that in this specification, "low temperature" is not specifically limited but refers to a temperature below 10°C.

[0071] The water absorption properties of superabsorbent resins, such as their absorption rate, typically deteriorate at low temperatures. However, the superabsorbent resin of the present invention fully satisfies the conditions described in (a) and (b), therefore, even when the ambient temperature changes from, for example, room temperature to low temperature, the deterioration of the water absorption rate is minimal, and the water absorption rate at low temperatures is sufficiently fast. Therefore, the superabsorbent resin of the present invention exhibits excellent water absorption rate at low temperatures and displays a stable water absorption rate even when the ambient temperature changes.

[0072] In one embodiment of the invention, fully satisfying condition (c) means that the CRC value is high enough and the Ext value is low enough.

[0073] In one embodiment of the invention, fully satisfying condition (d) means that the CRC value is sufficiently high.

[0074] The water-absorbing resin of the present invention fully satisfies the conditions described in (c) and (d), thus enabling it to absorb a sufficient amount of aqueous liquid (water, antifreeze, leak water, etc.), and when it swells after absorbing water, the amount of water-soluble components that flow out is small. In other words, the water-absorbing resin of the present invention can absorb a sufficient amount of aqueous liquid and swell and gel, reducing the fluidity of the aqueous liquid and maintaining it in a substantially solid state. In particular, because the water-absorbing resin of the present invention fully satisfies the condition described in (d), it exhibits sufficient water absorption performance even at low temperatures. Therefore, the water-absorbing resin of the present invention can fully satisfy the necessary conditions described in (a) and (b).

[0075] As described above, the water-absorbing resin of the present invention fully satisfies the conditions described in (c) and (d), thus possessing excellent water absorption performance at room temperature. Furthermore, as it fully satisfies the conditions described in (a) and (b), this excellent water absorption performance does not deteriorate even in low-temperature environments. Therefore, the water-absorbing resin of the present invention exhibits the following effects: even in low-temperature environments, it can fully absorb aqueous liquids such as antifreeze, and can fully swell and gel by absorbing such aqueous liquids.

[0076] [2-2] Physical property values of the water-absorbent resin of the present invention

[0077] [2-2-1] 0°C Vortex water absorption time

[0078] From the viewpoint of improving the water absorption rate at low temperatures, the smaller the 0°C Vortex water absorption time of the water-absorbing resin of the present invention, the more preferred it is. This value is preferably 30 seconds or less, 20 seconds or less, and further preferably 15 seconds or less. Furthermore, this value is typically 1 second or more, and preferably 2 seconds or more.

[0079] [2-2-2] Vortex water absorption time ratio (0°C / 30°C)

[0080] From the viewpoint of achieving a stable water absorption rate unaffected by changes in ambient temperature, a smaller Vortex water absorption time ratio (0°C / 30°C) of the water-absorbing resin of the present invention is preferred. This value is 5.0 or less, preferably 4.0 or less. Furthermore, this value is typically 1.0 or more, preferably 2.0 or more.

[0081] [2-2-3] 30°C Vortex water absorption time

[0082] From the viewpoint of improving the water absorption rate at 30°C, the smaller the Vortex water absorption time of the water-absorbing resin of the present invention at 30°C, the more preferred it is. This value is preferably 20 seconds or less, more preferably 10 seconds or less. Furthermore, this value is typically 0.5 seconds or more, preferably 1 second or more.

[0083] [2-2-4] GEX

[0084] The water-absorbing resin of the present invention preferably has a high GEX value because it can rapidly swell and gel by absorbing and retaining a sufficient amount of aqueous liquid. The value is 17 or higher, preferably 18 or higher. Furthermore, the value is typically 30 or lower, preferably 28 or lower.

[0085] [2-2-5] CRC

[0086] The CRC value of the superabsorbent resin of the present invention is preferably large because it can rapidly swell and gel by absorbing and retaining a sufficient amount of aqueous liquid. This value is greater than 25 g / g, preferably 30 g / g or more. Furthermore, the value is typically 50 g / g or less, preferably 45 g / g or less. The CRC of the superabsorbent resin of the present invention varies depending on various conditions such as monomer concentration, neutralization rate, polymerization conditions, gel pulverization conditions, drying conditions, and surface crosslinking conditions. Furthermore, by adjusting the amount of internal crosslinking agent used according to these conditions, the CRC of the superabsorbent resin of the present invention can be adjusted to a value greater than 25 g / g. Generally, the more internal crosslinking agent used, the lower the CRC of the resulting superabsorbent resin; conversely, the less internal crosslinking agent used, the higher the CRC of the resulting superabsorbent resin. Specifically, when the amount of internal crosslinking agent used is small, the distance between the crosslinking points of the crosslinked polymer constituting the resulting superabsorbent resin becomes longer, and the network structure of the crosslinked polymer can expand sufficiently when absorbing water molecules, thus increasing the CRC of the superabsorbent resin.

[0087] [2-2-6] Ext

[0088] The Ext of the water-absorbing resin of the present invention is generally 1% or more and 40% or less by mass, preferably 2% or more and 35% or less by mass, more preferably 3% or more and 30% or less by mass, further preferably 4% or more and 25% or less by mass, and particularly preferably 5% or more and 20% or less by mass.

[0089] [2-2-7] Mass average particle diameter

[0090] The lower limit of the mass-average particle size (D50) of the water-absorbing resin of the present invention is preferably 50 μm or more, more preferably 100 μm or more, further preferably 150 μm or more, and particularly preferably 200 μm or more. Furthermore, the upper limit of the mass-average particle size (D50) is preferably 600 μm or less, more preferably 550 μm or less, further preferably 500 μm or less, and particularly preferably 450 μm or less. The water-absorbing resin of the present invention can appropriately improve its water absorption performance by ensuring that the mass-average particle size (D50) is within the aforementioned range. As a method for determining the mass-average particle size (D50), the method described in the examples described later can be used, that is, the same method as described in columns 27-28 of U.S. Patent No. 7,638,570, "(3) Logarithmic standard deviation of mass-average particle size (D50) and particle size distribution".

[0091] [2-2-8] Particle size distribution

[0092] The proportion of particles with a particle size exceeding 850 μm in the absorbent resin of the present invention is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. The absorbent resin of the present invention contains preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more particles with a particle size of 150 to 850 μm. Ideally, the absorbent resin of the present invention contains particles with a particle size of 150 to 850 μm at 100% by mass. The logarithmic standard deviation (σζ) of the particle size distribution of the absorbent resin of the present invention is preferably 0.20 or more and 0.50 or less, more preferably 0.25 or more and 0.40 or less, and even more preferably 0.27 or more and 0.35 or less.

[0093] [2-3] Constituent elements of the water-absorbent resin

[0094] [2-3-1] Crosslinked polymer

[0095] The water-absorbing resin of the present invention typically comprises a crosslinked polymer of an unsaturated monomer containing an acid group, preferably comprising this crosslinked polymer as a main component. The crosslinked polymer is preferably a hydrophilic crosslinked polymer. The crosslinking polymerization used to obtain the crosslinked polymer can be carried out in the presence of a crosslinking agent (internal crosslinking agent) or through self-crosslinking of the monomer. The crosslinked polymer may contain monomers other than the unsaturated monomer containing an acid group. It should be noted that, unless otherwise specified, the term "monomer" in this specification includes the concept of a neutralizing salt.

[0096] <Unsaturated monomer containing acid group>

[0097] The "acid group" in the acid-containing unsaturated monomer is not particularly limited, and examples include carboxyl, sulfonyl, and phosphate groups, with carboxyl being preferred. Examples of the acid-containing unsaturated monomer include: (meth)acrylic acid, maleic acid (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)acryloylethanesulfonic acid, 2-(meth)acryloyloxypropanesulfonic acid, and 2-hydroxyethyl(meth)acryloyl phosphate. From the viewpoint of the water absorption performance of the water-absorbing resin of the present invention, (meth)acrylic acid, maleic acid (anhydride), itaconic acid, and cinnamic acid are preferred, and (meth)acrylic acid is more preferred.

[0098] In the water-absorbing resin of the present invention, at least a portion of the acid-containing unsaturated monomer may be a neutralized acid-containing unsaturated monomer. The term "neutralized acid-containing unsaturated monomer" refers to a neutralized salt formed by neutralizing the acid groups.

[0099] From the viewpoint of achieving a CRC of over 25 g / g for the water-absorbing resin of the present invention, it is preferable that the molar ratio (neutralization rate) of the neutralizing salt in the total molar number of the unsaturated monomer containing the acid group is a predetermined value or higher. From this viewpoint, the neutralization rate is preferably 40 mol% or higher, more preferably 50 mol% or higher, and even more preferably 60 mol% or higher.

[0100] On the other hand, the upper limit of the neutralization rate is only required to be below 99 mol%.

[0101] The neutralizing salt preferably comprises a salt with a monovalent cation, more preferably comprises one or more salts selected from alkali metal salts, ammonium salts and amine salts, and even more preferably comprises one or more salts selected from alkali metal salts and ammonium salts.

[0102] Here, the water absorption and retention of the superabsorbent resin of the present invention are achieved by the detachment of cations from the neutralized acid groups in the neutralized salt, and the coordination of water molecules (H2O) with the detached groups. Therefore, when the cations are easily detached, the water absorption and retention of the superabsorbent resin of the present invention are more efficient, and swelling and gelation are easier to achieve.

[0103] The detachment typically does not occur easily at low temperatures. Therefore, at low temperatures, the swelling and gelation state can easily differ depending on whether easily detachable cations are used or difficult-to-detach cations are used.

[0104] Therefore, by using an easily detachable cation as the cation, especially at low temperatures, the swelling and gelation of the superabsorbent resin of the present invention becomes easier. Thus, by including an easily detachable cation as the cation, the superabsorbent resin of the present invention can improve the water absorption rate at low temperatures.

[0105] Here, the potassium cation (K) + ) and ammonium cations (NH4) + All of these are easily detached cations due to their large ionic radius and weak bonding with the detached group. Furthermore, lithium cations (Li...) + The ionic radius of the cation is small, but its ionization tendency is high, thus conforming to the category of easily detached cations. For these reasons, it is particularly preferred that the neutralizing salt comprises one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts.

[0106] For the aforementioned reasons, when the neutralizing salt contains more salts containing easily detachable cations, the water-absorbing resin of the present invention can further improve the water absorption rate at low temperatures. From this viewpoint, the molar percentage of one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts relative to the total molar percentage of the neutralizing salt is preferably 50 mol% or more, more preferably 60 mol% or more, and even more preferably 70 mol% or more. Furthermore, the upper limit of the molar percentage of the one or more salts relative to the total molar percentage of the neutralizing salt can be 100 mol% or less, preferably 95 mol% or less, and more preferably 90 mol% or less.

[0107] The content of the acid-containing unsaturated monomer relative to the total amount of monomers (acid-containing unsaturated monomer + monomers other than acid-containing unsaturated monomer) that become the raw material of the water-absorbing resin is preferably 50 mol% to 100 mol, more preferably 70 mol% to 100 mol, even more preferably 90 mol% to 100 mol, and particularly preferably substantially 100 mol.

[0108] <Monomer other than unsaturated monomer containing acid group>

[0109] Other than the acid-containing unsaturated monomers mentioned above, any compound that can polymerize to form a water-absorbing resin is acceptable. Examples include: unsaturated monomers containing amide groups such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; unsaturated monomers containing amino groups such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide; unsaturated monomers containing mercapto groups; unsaturated monomers containing phenolic hydroxyl groups; and unsaturated monomers containing lactam groups such as N-vinylpyrrolidone.

[0110] <Internal crosslinking agent>

[0111] The crosslinked polymer may include an internal crosslinking agent as needed. There are no particular limitations on the internal crosslinking agent; known internal crosslinking agents may be used, including compounds having a total of two or more unsaturated bonds or reactive functional groups within a single molecule.

[0112] Examples of internal crosslinking agents include: N,N'-methylenebis(meth)acrylamide, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkylene, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl methacrylate, etc. Among these, considering reactivity, one or more can be used, with compounds having two or more polymerizable unsaturated groups being preferred.

[0113] The amount (content) of the internal crosslinking agent relative to the total amount of the acid-containing monomer is preferably 0.0001 mol% or more and 10 mol% or less, more preferably 0.001 mol% or more and 1 mol% or less, and even more preferably 0.01 mol% or more and 0.2 mol% or less.

[0114] The more internal crosslinking agent used, the lower the CRC of the resulting superabsorbent resin; conversely, the less internal crosslinking agent used, the higher the CRC. Specifically, with a low amount of internal crosslinking agent, the distance between the crosslinking points of the crosslinked polymer constituting the superabsorbent resin becomes longer. This allows the network structure of the crosslinked polymer to expand sufficiently when absorbing water molecules, resulting in a high CRC value for the superabsorbent resin. The amount (content) of the internal crosslinking agent in the superabsorbent resin of the present invention is as described above, thus its CRC is controlled to exceed 25 g / g.

[0115] [2-3-2] Surface crosslinking agent

[0116] The water-absorbing resin of the present invention can be a "surface-crosslinked water-absorbing resin". Therefore, the water-absorbing resin of the present invention can contain a surface crosslinking agent. As the surface crosslinking agent, a commonly used surface crosslinking agent can be used, which can react with multiple functional groups (preferably multiple carboxyl groups) of the water-absorbing resin. The surface crosslinking agent is preferably a surface crosslinking agent capable of forming covalent or ionic bonds with the functional groups, and is further preferably a surface crosslinking agent capable of forming covalent bonds with the functional groups.

[0117] Specific examples of the aforementioned surface crosslinking agents include: ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanediol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, ethylene-oxypropylene block copolymer, pentaerythritol, sorbitol, and other polyol compounds. Epoxy compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidyl, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether; polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine, as well as their inorganic or organic salts; 2,4-toluene. Diisocyanates, hexamethylene diisocyanates, and other polyisocyanate compounds; aziridine compounds such as polyaziridine; 1,2-ethylidene bisoxazoline, bisoxazoline, polyoxazoline, and other polyoxazoline compounds; urea, thiourea, guanidine, dicyandiamide, 2-oxazolidinone, and other carbonate derivatives; 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1, Alkyl carbonate esters such as 3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and 1,3-dioxepan-2-one; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepicochlorohydrin, and their polyamine adducts; oxacyclobutane compounds; silane coupling agents such as γ-epoxypropoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; and multi-metal compounds such as hydroxides, chlorides, sulfates, nitrates, or carbonates of zinc, calcium, magnesium, aluminum, iron, zirconium, etc. Two or more of these may be used in combination. Preferably, the surface crosslinking agent is selected from one or more of multi-metal ions, epoxy compounds, oxazoline compounds, and alkyl carbonate esters.

[0118] When the water-absorbing resin of the present invention contains a surface crosslinking agent, the content of the surface crosslinking agent relative to the total mass of the water-absorbing resin is preferably 0.005% by mass or more and 5% by mass or less, more preferably 0.01% by mass or more and 4% by mass or less, and even more preferably 0.02% by mass or more and 3% by mass or less.

[0119] [2-3-3] Fine powder

[0120] The absorbent resin of the present invention may contain the microparticles. The content of the microparticles relative to the total mass of the absorbent resin is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. Furthermore, the content of the microparticles is particularly preferably 0% by mass. As a method for controlling the content to a specific value (e.g., 15% by mass) or less, one example is removing the byproduct microparticles during manufacturing by performing a grading process described later.

[0121] By keeping the content below 15% by mass relative to the total mass of the absorbent resin, a decrease in the water absorption performance of the absorbent resin of the present invention can be prevented. It should be noted that the content refers to the proportion of absorbent resin that has passed through a 150μm mesh sieve.

[0122] [2-3-4] Residual volatile component

[0123] Preferably, in the absorbent resin of the present invention, the content of the residual volatile components is 0.4% by mass or less relative to the total mass of the absorbent resin. The absorbent resin of the present invention, by this configuration, has the following advantages: it reduces the unpleasant odor caused by odor during water absorption and the reduction in absorption performance due to hydrophobicity.

[0124] From the viewpoint that the aforementioned advantages are readily obtained, in the absorbent resin of the present invention, the content of the residual volatile components relative to the total mass of the absorbent resin is more preferably 0.2% by mass or less, and particularly preferably substantially free of the residual volatile components. Here, "substantially free of the residual volatile components" means that the amount of residual volatile components obtained by the determination method described later is "undetectable".

[0125] The residual volatile components originate from organic solvents used during polymerization and other processes in the manufacturing process described above. Therefore, a water-absorbing resin that substantially contains zero residual volatile components can be manufactured, for example, by performing the polymerization process described later using methods such as aqueous solution polymerization without the use of organic solvents, and by not using organic solvents in other processes.

[0126] Furthermore, even when organic solvents or the like are used in the manufacturing process, by using only a small amount, the content of the residual volatile components can be kept within the preferred range described above.

[0127] [2-3-5] Polyol

[0128] The absorbent resin of the present invention is preferably substantially free of polyols. Here, "substantially free of polyols" means that the polyol content of the absorbent resin of the present invention is "undetectable" in measurement results. Because the absorbent resin of the present invention is substantially free of said polyols, it has the advantage of reducing the unpleasant odor caused by odor during water absorption.

[0129] The polyols are typically used as crosslinking agents (internal crosslinking agents, surface crosslinking agents) in the manufacturing process of absorbent resins. Therefore, absorbent resins that substantially do not contain the polyols can be manufactured by methods that do not use polyols, for example, by using compounds other than polyols as crosslinking agents. Furthermore, when the absorbent resin of the present invention contains polyols, their content relative to the total mass of the absorbent resin is preferably 5% by mass or less, more preferably 3% by mass or less.

[0130] [2-3-6] [Other additive]

[0131] In addition to the constituent elements described above, the absorbent resin of the present invention may also contain other additives. These additives can be those commonly found in absorbent resins and are not particularly limited. Examples of such additives include: chelating agents, organic reducing agents, inorganic reducing agents, oxidizing agents, hydroxycarboxylic acid compounds, surfactants, compounds having phosphorus atoms, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, multi-metal salts, cationic polymers, and inorganic microparticles. The additives may be one or a mixture of two or more of the above-mentioned additives. The content of the additives is not particularly limited as long as it does not impair the function of the absorbent resin of the present invention; preferably, it is 5% by mass or less, more preferably 3% by mass or less, relative to the total mass of the absorbent resin.

[0132] <Polyvalent metal salt>

[0133] When the absorbent resin of the present invention contains a polyvalent metal salt as one of the other additives, aluminum, zirconium, etc. are preferably examples of the polyvalent metal salt. Furthermore, aluminum lactate and aluminum sulfate are preferred as usable multi-metal salts, and aluminum sulfate is more preferred.

[0134] As for the content (amount added) of the polyvalent metal salt, relative to 1g of the water-absorbing resin (water-absorbing resin powder), the metal cation is preferably less than 3.6 × 10⁻⁶. -5moles, more preferably less than 2.8 × 10⁻⁶. -5 Moles, preferably less than 2.0 × 10 -5 Mole. Here, water-absorbing resin (water-absorbing resin powder) refers to water-absorbing resin that does not contain the other additives mentioned above.

[0135] <Cationic polymer>

[0136] When the absorbent resin of the present invention contains a cationic polymer as one of the other additives, the cationic polymer preferably includes compounds exemplified in U.S. Patent No. 7,098,284. Among the compounds exemplified in U.S. Patent No. 7,098,284, an ethyleneamine polymer is preferred. The content (amount) of the cationic polymer is 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, relative to 100 parts by weight of the absorbent resin (absorbent resin powder). Here, absorbent resin (absorbent resin powder) refers to an absorbent resin that does not contain the other additives.

[0137] <Inorganic fine particle>

[0138] When the absorbent resin of the present invention contains inorganic microparticles as other additives, examples of such inorganic microparticles include: mineral products such as talc, kaolin, bleaching clay, hydrotalcite, bentonite, activated clay, barite, natural bitumen, strontium ore, ilmenite, and perlite; aluminum compounds such as aluminum sulfate tetradecahydrate (or its anhydrous form), potassium aluminum sulfate dodecahydrate, sodium aluminum sulfate dodecahydrate, ammonium aluminum sulfate dodecahydrate, aluminum chloride, polyaluminum chloride, and aluminum oxide; other multi-metal salts, multi-metal oxides, and multi-metal hydroxides such as calcium phosphate; hydrophilic amorphous silica; and oxide complexes such as silica-alumina-magnesium oxide complexes, silica-alumina complexes, and silica-magnesium oxide complexes. Two or more of these can be used together. The amount of such inorganic microparticles added is preferably less than 2.0 parts by weight, more preferably less than 1.5 parts by weight, and even more preferably less than 1.0 parts by weight, relative to 100 parts by weight of the absorbent resin (absorbent resin powder). Here, water-absorbing resin (water-absorbing resin powder) refers to water-absorbing resin that does not contain the other additives mentioned above.

[0139] [2-4] Production method

[0140] As a method for manufacturing the water-absorbing resin of the present invention, the following manufacturing method may be used, for example.

[0141] [3] Embodiment 2: Production method of water-absorbent resin

[0142] A method for manufacturing a water-absorbing resin according to one embodiment of the present invention (hereinafter referred to as "the manufacturing method of the present invention") includes: a polymerization step, wherein a monomer composition containing an acid-containing unsaturated monomer and optionally containing monomers other than the acid-containing unsaturated monomer is subjected to crosslinking polymerization to obtain a hydrogel-like crosslinked polymer, wherein at least a portion of the acid-containing unsaturated monomer is a neutralized acid-containing unsaturated monomer, the neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts, the total content of the acid-containing unsaturated monomer and monomers other than the acid-containing unsaturated monomer in the monomer composition is 30% by mass or more and less than 55% by mass relative to the total mass of the monomer composition, and the polymerization initiation temperature in the polymerization step is 60°C or more.

[0143] [3-1] Mechanism of the production method of the present invention

[0144] The manufacturing method of the present invention includes: a polymerization step, wherein a monomer composition containing an acid-containing unsaturated monomer and optionally containing monomers other than the acid-containing unsaturated monomer is subjected to crosslinking polymerization to obtain a hydrogel-like crosslinked polymer, wherein at least a portion of the acid-containing unsaturated monomer is a neutralized acid-containing unsaturated monomer, and the neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts.

[0145] Therefore, the absorbent resin manufactured by the manufacturing method of the present invention comprises a crosslinked polymer, wherein the monomer constituting the crosslinked polymer comprises potassium cations (K). + Lithium cations (Li) + ) or ammonium cation (NH4) + Neutralized acid groups.

[0146] Here, as described in column [2-3-1], it has been K + Li + and NH4 + Neutralized acid groups in water-absorbing resins due to K + Li + and NH4 + It is easy to detach, and therefore absorbs and retains water better, especially at low temperatures, and swells and gels more easily. Therefore, by the manufacturing method described above, it is possible to manufacture a water-absorbing resin that has a smaller value for the 0°C Vortex water absorption time and the Vortex water absorption time ratio (0°C / 30°C) and fully satisfies the conditions of (a) and (b).

[0147] Furthermore, in the manufacturing method of the present invention, the total content of the acid-containing unsaturated monomer and other monomers besides the acid-containing unsaturated monomer in the monomer composition, relative to the total mass of the monomer composition, is 30% by mass or more and less than 55% by mass. It should be noted that, here, the "total content of the acid-containing unsaturated monomer and other monomers besides the acid-containing unsaturated monomer" refers to the content (concentration) of all monomers in the monomer composition. Hereinafter, the "total content of the acid-containing unsaturated monomer and other monomers besides the acid-containing unsaturated monomer" will also be referred to as the "content of all monomers in the monomer composition".

[0148] If the total content of all monomers in the monomer composition exceeds 55% by mass, the content of water-soluble components in the resulting superabsorbent resin may increase, and the GEX value described later may not reach the desired range.

[0149] The manufacturing method of the present invention enables appropriate crosslinking polymerization by ensuring that the content of all monomers in the monomer composition is 30% by mass or more, thereby producing a water-absorbing resin with a desired amount of water-soluble components and a CRC exceeding 25 g / g. It can be considered that the water-absorbing resin contains a desired amount of water-soluble components through the aforementioned composition, and that the CRC is controlled to exceed 25 g / g.

[0150] Thus, the absorbent resin manufactured by the method of the present invention has a high CRC and a low content of water-soluble components, thereby controlling the GEX value to be 17 or higher. Therefore, the method of the present invention can manufacture an absorbent resin that fully satisfies the conditions described in (c) and (d).

[0151] As described above, the water-absorbing resin of the present invention can be manufactured using the manufacturing method of the present invention.

[0152] It should be noted that in the manufacturing method of the present invention, the polymerization initiation temperature in the polymerization step is 60°C or higher. Here, when preparing the "at least partially neutralized acid-containing unsaturated monomer" in the monomer composition supplied for the polymerization step, the temperature of the monomer composition typically reaches 60°C or higher due to the heat of neutralization. Therefore, generally, in order to control the polymerization initiation temperature at 60°C or higher, it is not necessary to further heat the monomer composition.

[0153] On the other hand, if the temperature of the monomer composition does not reach 60°C, it can be further heated to a temperature of 60°C or higher. Known heating methods can be used, and there are no particular limitations. Furthermore, the manufacturing method of the present invention, by setting the polymerization initiation temperature to 60°C or higher, enables polymerization to be carried out in a short time, thus achieving excellent productivity.

[0154] [3-2] Each step constituting the production method of the present invention

[0155] The manufacturing method of the present invention includes the polymerization step, and may also include, in addition to the polymerization step, the conventional steps of a method for manufacturing an absorbent resin. The steps that may constitute the manufacturing method of the present invention are described in detail below.

[0156] [3-2-1] Preparation step of monomer composition

[0157] The manufacturing method of the present invention may include a step of preparing a monomer composition comprising monomers that serve as raw materials for a water-absorbing resin. The monomer composition is typically an aqueous solution containing an unsaturated monomer with an acidic group as the main component and optionally containing monomers other than the unsaturated monomer with an acidic group. It should be noted that the monomer composition is preferably a homogeneous aqueous solution. Alternatively, as the monomer composition, a slurry of monomers (a dispersion exceeding the saturation concentration of the monomers) may also be used, provided it does not reduce the water-absorbing properties of the obtained water-absorbing resin.

[0158] <Unsaturated monomer containing acid group>

[0159] As an acid-containing unsaturated monomer, the acid-containing unsaturated monomers listed in the "<Acid-containing Unsaturated Monomers>" section of [2-3-1] above can be used, and the description in that section can be cited. The acid-containing unsaturated monomer includes one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts listed in that section, i.e., the neutralized acid-containing unsaturated monomer.

[0160] <Monomer other than unsaturated monomer containing acid group>

[0161] Monomers other than the acid-containing unsaturated monomers mentioned above may also be used, and reference can be made to the description in that column.

[0162] <Polymerization inhibitor>

[0163] From the viewpoint of polymerization stability, the monomer composition supplied for the polymerization process preferably contains a small amount of polymerization inhibitor. A preferred polymerization inhibitor is p-methoxyphenol. The amount of polymerization inhibitor contained in the monomer (particularly acrylic acid and its salts) is typically 1 ppm or more and 250 ppm or less, preferably 10 ppm or more and 160 ppm or less, more preferably 20 ppm or more and 80 ppm or less.

[0164] <Neutralizing agent>

[0165] As described above, in the manufacturing method of the present invention, at least a portion of the acid-containing unsaturated monomers in the monomer composition is neutralized, and the neutralized acid-containing unsaturated monomers comprise one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts. The manufacturing method of the present invention may include neutralizing at least a portion of the acid-containing unsaturated monomers using a neutralizing agent.

[0166] Here, the neutralizing agent used to neutralize at least a portion of the acid-containing unsaturated monomer is not particularly limited. The neutralizing agent may, for example, be an alkaline substance. Examples of alkaline substances such as inorganic salts like lithium hydroxide, potassium hydroxide, and ammonium carbonate can be listed as neutralizing agents for preparing one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts. Other examples of alkaline agents include inorganic salts such as sodium hydroxide and sodium carbonate, and amine-based organic compounds having amino and / or imino groups. Two or more neutralizing agents may be used in combination.

[0167] As methods for adjusting the neutralization rate of the acid-containing unsaturated monomer in the manufacturing method of the present invention, examples include: mixing the acid-containing unsaturated monomer and its neutralizing salt; adding a known neutralizing agent to the acid-containing unsaturated monomer; using a partially neutralizing salt of the acid-containing unsaturated monomer (i.e., a mixture of the acid-containing unsaturated monomer and its neutralizing salt) pre-adjusted to a predetermined neutralization rate; and so on. Furthermore, these methods can be combined.

[0168] The adjustment of the neutralization rate can be performed before the polymerization reaction of the acid-containing unsaturated monomer, during the polymerization reaction of the acid-containing unsaturated monomer, or on the hydrogel-like crosslinked polymer obtained after the polymerization reaction of the acid-containing unsaturated monomer. Furthermore, the neutralization rate can be adjusted at any stage—before, during, or after the polymerization reaction—or at multiple stages. It should be noted that in applications where unreacted substances from neutralizing agents such as alkaline substances may be problematic, it is preferable to adjust the neutralization rate before and / or during the polymerization reaction, and more preferably before the polymerization reaction.

[0169] <Internal crosslinking agent>

[0170] The monomer composition in the manufacturing method of the present invention may include an internal crosslinking agent. The internal crosslinking agent can be used to adjust the water absorption properties and / or gel strength of the obtained superabsorbent resin. The internal crosslinking agents that can be used in this process are those listed in the <Internal Crosslinking Agent> section of [2-3-1] above, and reference can be made to that section. In the manufacturing method of the superabsorbent resin, the use of an internal crosslinking agent is preferred. It should be noted that under polymerization conditions where the self-crosslinking reaction of the monomer is effective, the internal crosslinking agent may not be used.

[0171] <Other substance>

[0172] In the manufacturing method of the present invention, within the scope of achieving the objectives of the present invention, substances exemplified below (hereinafter referred to as "other substances") may also be added to the monomer composition.

[0173] Other substances include, for example: chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; foaming agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as ethylenediaminetetra(methylenephosphonic acid) or its metal salts, ethylenediaminetetraacetic acid or its metal salts, and diethylenetriaminepentaacetic acid or its metal salts; hydrophilic polymers such as polyacrylic acid (salts) and their cross-linked forms (e.g., micropowders containing water-absorbing resins), starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol. These other substances can be used alone or in combination of two or more.

[0174] There is no particular limitation on the amount of other substances used. The total content (total concentration) of other substances in the monomer composition is preferably 10% by mass or less, more preferably 0.001% by mass or more and 5% by mass or less, and particularly preferably 0.01% by mass or more and 1% by mass or less, relative to the total mass of the monomer composition other than the acid-containing monomer and the acid-containing monomer (hereinafter referred to as "total mass of monomers").

[0175] <Content of monomer in monomer composition>

[0176] In this process, from the viewpoint of controlling the GEX value to be 17 or higher and from the viewpoint of productivity, the content of all monomers in the monomer composition (= the total mass of the unsaturated monomers containing acid groups and monomers other than the unsaturated monomers containing acid groups / the mass of the monomer composition) is 30% by mass or more and less than 55% by mass, preferably 30% by mass or more and less than 53% by mass, more preferably 35% by mass or more and less than 52% by mass, and even more preferably 40% by mass or more and less than 50% by mass.

[0177] The lower the content of all monomers in the monomer composition, the less self-crosslinking occurs due to the entanglement of molecular chains, and the higher the CRC value of the resulting superabsorbent resin. From the viewpoint of achieving a CRC of more than 25 g / g for the superabsorbent resin of the present invention, the content of all monomers in the monomer composition is less than 55% by mass, preferably 53% by mass or less, more preferably 52% by mass or less, and even more preferably 50% by mass or less.

[0178] <Polymerization initiator>

[0179] The polymerization initiator used in the manufacturing method of the present invention is appropriately selected according to the polymerization mode, and therefore is not particularly limited. Examples of polymerization initiators include thermally decomposable polymerization initiators, photodecomposable polymerization initiators, or combinations thereof, or redox polymerization initiators that incorporate a reducing agent that promotes the decomposition of these polymerization initiators. Specifically, one or more of the polymerization initiators disclosed in U.S. Patent No. 7,265,190 may be used. It should be noted that, from the viewpoint of operability of the polymerization initiator and the physical properties of the water-absorbing resin, peroxides or azo compounds are preferred, peroxides are more preferred, and persulfates are even more preferred.

[0180] The amount of polymerization initiator used, relative to the total molar number of the acid-containing monomer and other monomers in the monomer composition (hereinafter referred to as the "total molar number of monomers"), is preferably 0.001 mol% or more and 1 mol% or less, more preferably 0.001 mol% or more and 0.5 mol% or less. Furthermore, when redox polymerization is required, the amount of the reducing agent used in conjunction with the oxidizing agent, relative to the total molar number of monomers, is preferably 0.0001 mol% or more and 0.02 mol% or less.

[0181] <Amount of dissolved oxygen>

[0182] It should be noted that, when the monomer composition is an aqueous solution, it is also preferable to reduce the dissolved oxygen in the monomer composition before polymerization by heating or by replacing it with an inert gas. For example, the dissolved oxygen is preferably reduced to below 5 ppm, more preferably to below 3 ppm, and particularly preferably to below 1 ppm.

[0183] Furthermore, when the monomer composition is an aqueous solution, bubbles (especially the inert gas, etc.) can be dispersed in the monomer composition. In this case, the polymerization in the polymerization process is a foaming polymerization.

[0184] [3-2-2] Polymerization step

[0185] The manufacturing method of the present invention includes a polymerization step. The polymerization step refers to the process of crosslinking the monomer composition to obtain a hydrogel-like crosslinked polymer (hereinafter sometimes referred to as "hydrogel"). It should be noted that, here, crosslinking the monomer composition means crosslinking the monomers contained in the monomer composition to obtain a hydrogel-like crosslinked polymer.

[0186] It should be noted that, in addition to the method of adding the aforementioned polymerization initiator to initiate the polymerization reaction, there are also methods such as irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays. Furthermore, it is also possible to add a polymerization initiator and then irradiate with active energy rays.

[0187] <Polymerization method>

[0188] Examples of polymerization methods in the polymerization process include droplet polymerization in the gas phase, aqueous solution polymerization, and reverse suspension polymerization, with aqueous solution polymerization being particularly preferred. That is, the polymerization process is particularly preferred in which the monomer composition is polymerized by aqueous solution polymerization to obtain the hydrogel-like crosslinked polymer. Among the aqueous solution polymerization methods, continuous aqueous solution polymerization is particularly preferred; examples include continuous belt polymerization and continuous kneading polymerization. Continuous aqueous solution polymerization improves the production efficiency of the superabsorbent resin. Furthermore, by using aqueous solution polymerization as the polymerization method, a superabsorbent resin that is substantially free of residual volatile components can be manufactured.

[0189] The polymerization initiation temperature is preferably 60°C or higher, more preferably 65°C or higher, even more preferably 70°C or higher, and particularly preferably 75°C or higher. It should be noted that when the monomer composition contains a solvent, the upper limit of the polymerization initiation temperature is the boiling point of the solvent.

[0190] By controlling the polymerization initiation temperature within the preferred range, the values ​​of (c) and (d) of the manufactured water-absorbing resin can be controlled within a more preferred range.

[0191] <Polymerization rate of water-containing gel-like crosslinked polymer>

[0192] From the viewpoint of preventing the agglomeration of the particulate hydrogel obtained in the subsequent hydrogel pulverization process during drying, and / or reducing residual monomers in the manufactured water-absorbing resin, the polymerization rate of the hydrogel-like crosslinked polymer obtained in the polymerization process is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and particularly preferably 99% by mass or more.

[0193] There is no particular upper limit to the polymerization rate of the hydrogel-like crosslinked polymer; ideally, it would be 100% by mass. However, high polymerization rates require long polymerization times and stringent polymerization conditions, which can sometimes lead to a decrease in productivity and / or physical properties. Therefore, an upper limit of 99.95% by mass, further to 99.9% by mass, and typically around 99.8% by mass is sufficient. The polymerization rate of the hydrogel-like crosslinked polymer is typically 98–99.99% by mass.

[0194] <Physical property of water-containing gel-like crosslinked polymer>

[0195] The solid content of the hydrogel-like crosslinked polymer relative to the total mass of the hydrogel-like crosslinked polymer is preferably 60% by mass or less, more preferably 57% by mass or less, and even more preferably 55% by mass or less. Furthermore, the solid content of the hydrogel-like crosslinked polymer relative to the total mass of the hydrogel-like crosslinked polymer is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more. By keeping the solid content within these preferred ranges, the amount of water-soluble components can be reduced.

[0196] [3-2-3] Water-containing gel pulverization step

[0197] The manufacturing method of the present invention preferably further includes: a hydrogel pulverization step, wherein the hydrogel-like crosslinked polymer is pulverized to obtain particulate hydrogel. Specifically, this step involves pulverizing the hydrogel-like crosslinked polymer obtained in the polymerization step during and / or after polymerization to obtain particulate hydrogel. To obtain water-absorbing resin particles (after surface crosslinking) of the target shape and properties in high yield, the particle size of the particulate hydrogel is adjusted to a preferred range described later. It should be noted that this step may be performed more than twice to obtain particulate hydrogels of the specified particle size.

[0198] Furthermore, if the target particle size of the hydrogel is obtained during the polymerization process, such as in reverse suspension polymerization or gas-phase polymerization, this step may be omitted. Additionally, if necessary, a shredding step can be performed after the polymerization process and before the gel pulverization process: the hydrogel-like crosslinked polymer can be cut or coarsely crushed to a size suitable for feeding into a gel pulverization device using a roller cutter, guillotine cutter, or the like. In particular, if the polymerization process is belt polymerization and a sheet-like or block-like hydrogel-like crosslinked polymer is obtained, the shredding step is preferred.

[0199] <Gel pulverization device>

[0200] In the manufacturing method of this invention, the type of gel pulverizing apparatus is not particularly limited as long as a particulate hydrogel of a specified particle size is obtained without impairing its water absorption properties. Examples of gel pulverizing apparatus include, for instance, gel pulverizers with multiple rotating agitators, such as intermittent or continuous twin-arm kneaders; screw extruders such as single-screw extruders, twin-screw extruders, and meat grinders; multi-screw mixers (kneaders) with two or more screws; and gel pulverizers such as shredders. It should be noted that when the polymerization step is kneading polymerization, the polymerization step and the hydrogel pulverizing step are performed simultaneously.

[0201] In one embodiment, a meat grinder or a multi-screw mixer with a twin-screw or larger screw extrusion mechanism is preferably used in the hydrogel pulverization process. In particular, when the hydrogel is obtained by aqueous solution polymerization, fine particles can be formed in the hydrogel pulverization process using a meat grinder or a multi-screw mixer with a twin-screw or larger screw extrusion mechanism. It should be noted that the pulverization conditions and methods for hydrogels, for example, disclosed in International Publication No. 2011 / 126079, are preferably applied to this invention.

[0202] <Gel fluidizing agent>

[0203] In the manufacturing method of the present invention, a gel flow agent can be added before and / or during the hydrogel pulverization step. This yields a particulate hydrogel containing the gel flow agent. By adding the gel flow agent, adhesion or bonding between the hydrogel particles can be prevented during the drying step described later, resulting in improved water absorption properties of the obtained absorbent resin. Furthermore, the load and amount of micropowder generated during the subsequent pulverization step after drying can be reduced. Moreover, when stirring drying is performed during the drying step, the effect becomes significant because the particle size of the obtained particulate dried polymer is close to the product particle size. From the viewpoint of ensuring that each particle of the obtained particulate hydrogel uniformly contains the gel flow agent, it is preferable to add the gel flow agent during the hydrogel pulverization step. It should be noted that, in cases where a hydrogel pulverization step is not required, such as in the case of reverse suspension polymerization during the polymerization step, it is also preferable to add the gel flow agent to the particulate hydrogel at least before the drying step.

[0204] The amount of gel flow agent added is appropriately set according to the water content of the hydrogel or particulate hydrogel and / or the type of gel flow agent. The amount added is preferably 0.001% to 0.5% by mass relative to the solid content of the hydrogel, more preferably 0.01% to 0.3% by mass, and even more preferably 0.02% to 0.2% by mass.

[0205] Examples of such gel flow agents include anionic, cationic, nonionic, and amphoteric surfactants, as well as their low-molecular-weight or high-molecular-weight surfactants, and high-molecular-weight lubricants.

[0206] (Surfactant)

[0207] Specifically, the surfactants used as gel flow agents include: (1) nonionic surfactants such as sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl glucoamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers; (2) octyl dimethylamine. Alkyl dimethylaminoacetic acid betaine, lauryl dimethylaminoacetic acid betaine, myristyl dimethylaminoacetic acid betaine, stearyl dimethylaminoacetic acid betaine, and other alkyl dimethylaminoacetic acid betaine; alkyl amamidopropyl betaine, coconut oil fatty acid amamidopropyl betaine, palm kernel oil fatty acid amamidopropyl betaine, and other alkyl amamidopropyl betaine; alkyl hydroxysulfonyl betaine, and other alkyl hydroxysulfonyl betaine; alkyl carboxymethyl hydroxyethyl imidazoline betaine, and other alkyl carboxymethyl hydroxyethyl imidazoline betaine, and other amphoteric surfactants; (3) alkyl aminodiacetic acid monosodium, lauryl aminodiacetic acid potassium, myristyl aminodiacetic acid sodium, and other alkyl aminodiacetic acid monoalkali metals, and other anionic surfactants; (4) long-chain alkyl dimethylaminoethyl quaternary ammonium salts, and other cationic surfactants. Two or more of these can be used together.

[0208] <High molecular lubricant>

[0209] In the manufacturing method of the present invention, within the scope of achieving the objectives of the present invention, the polymeric lubricants exemplified below can be added to the monomeric composition and / or hydrogel.

[0210] Specifically, examples of such polymeric lubricants include: maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose, and polyalkylene oxides such as polyethylene glycol. Their molecular weights (weight-average molecular weights) are preferably selected from 2 million to 2 million, more preferably from 4 million to 1 million. Two or more of these can be used in combination.

[0211] Furthermore, these polymeric lubricants and the aforementioned surfactants can also be used as gel flow agents. When surfactants and polymeric lubricants are used together, their total addition amount is appropriately set according to the polymerization method, the composition of the monomer composition, and the water content of the hydrogel. When added to the monomer composition, the total amount is set relative to the concentration of the monomer component; when added to the hydrogel, the total amount is set relative to its solids content; and when added to both, the total amount is set. These gel flow agents can be reused with the dispersing agents used in reverse suspension polymerization.

[0212] The total amount of surfactant and polymeric lubricant added is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, more preferably 0.05% by mass or more, and particularly preferably 0.1% by mass or more.

[0213] <Hydrogen peroxide>

[0214] In the manufacturing method of the present invention, hydrogen peroxide may be added in a step prior to the drying step. By adding hydrogen peroxide in a step prior to the drying step and heating in the drying step or a subsequent step, the cross-linking structure in the water-absorbing resin is cleaved by hydrogen peroxide when heat is applied after the drying step, which can efficiently increase the absorption ratio (CRC) and obtain a water-absorbing resin with a CRC exceeding 25 g / g. More specifically, it is preferable to add hydrogen peroxide to the monomer composition before the polymerization step and / or to the hydrogel in the gel breaking step.

[0215] The amount of hydrogen peroxide added relative to the monomer composition and / or hydrogel is appropriately set considering the selected polymerization conditions and the amount of hydrogen peroxide remaining before drying (ppm). For example, if the improvement of the physical properties of the resulting water-absorbing resin, especially the absorption ratio (CRC), is taken into account, the total mass (%) of the solid component of the monomer composition or hydrogel crosslinked polymer is preferably 50 ppm (0.005 wt%) or more, more preferably 50 ppm (0.005 wt%) to 10000 ppm (1.0 wt%), and even more preferably 100 ppm (0.01 wt%) to 5000 ppm (0.5 wt%).

[0216] There are no particular limitations on the method of adding hydrogen peroxide. However, for ease of addition, it is preferable to add it in the form of an aqueous solution containing dissolved hydrogen peroxide. The concentration of the aqueous solution is not particularly limited, but is typically around 1–40% by mass. Furthermore, the aqueous hydrogen peroxide solution may contain a small amount of a hydrophilic solvent, such as methanol, ethanol, n-propanol, or isopropanol.

[0217] Furthermore, when hydrogen peroxide is added, after the drying process, the hydrogel-like crosslinked polymer and / or the dried polymer are preferably heated to a maximum temperature exceeding 160°C. This heating breaks down the crosslinking structure in the absorbent resin with hydrogen peroxide, resulting in a particulate absorbent with a high absorption ratio (CRC). Hereinafter, the maximum temperature is preferably, in the order of exceeding 160°C and below 250°C, above 170°C and below 220°C, and above 180°C and below 200°C. Regarding the above-described method of adding hydrogen peroxide and the heating after the drying process, the conditions described in WO22 / 163849 can be used.

[0218] <Solid content rate of water-containing gel>

[0219] The solid content of the hydrogel supplied for the hydrogel pulverization process (hereinafter referred to as gel solid content) is preferably 25% by mass or more. From the viewpoints of preventing the hydrogel particles from agglomerating after pulverization, the energy required for pulverization, drying efficiency, and absorption performance, the gel solid content is more preferably 25% to 75% by mass, more preferably 30% to 70% by mass, even more preferably 35% to 65% by mass, and particularly preferably 40% to 60% by mass.

[0220] <Moisture content of particulate water-containing gel>

[0221] The water content of the particulate hydrogel (hereinafter referred to as gel water content) is determined by the measurement method described in the examples below. From the viewpoint of the flowability of the particulate hydrogel during the drying process described later, the gel water content is preferably 25% by mass or more, more preferably 30% by mass or more, further preferably 35% by mass or more, particularly preferably 40% by mass or more, and extremely preferably 43% by mass or more. Excessive high-concentration polymerization may reduce the physical properties of the water-absorbing resin; from the viewpoint of drying efficiency and absorption performance, the gel water content is preferably 75% by mass or less, more preferably 60% by mass or less, and particularly preferably 55% by mass or less.

[0222] <Particle diameter of particulate water-containing gel>

[0223] From the viewpoint of controlling the particle size of the obtained hydroabsorbent resin, the mass-average particle size of the particulate hydrogel obtained by the gel pulverization process (particulate hydrogel before drying) is preferably less than 1 mm in terms of solid content. The mass-average particle size in terms of solid content is preferably 10 μm to 1000 μm, more preferably 20 μm to 800 μm, further preferably 40 μm to 500 μm, particularly preferably 50 μm to 300 μm, and most preferably 60 μm to 200 μm. It should be noted that the mass-average particle size (Solid D50) of the particulate hydrogel in terms of solid content can be measured by the method described in the examples.

[0224] It should be noted that solid content conversion refers to converting the measured particle size and other physical properties of an aqueous gel-like cross-linked polymer into the corresponding physical properties of the water-absorbing resin solids in the aqueous gel-like cross-linked polymer. For example, if the aqueous gel-like cross-linked polymer has a water content of 50% (50% solid content), then the conversion is to the measured physical property value of the aqueous gel-like cross-linked polymer multiplied by 2.

[0225] (Gel pulverization using meat grinder)

[0226] In one embodiment, the gel grinding process preferably uses a meat grinder (screw extruder) as shown in Figure 1 of WO2013 / 002387 for gel grinding. This meat grinder has a die (also called a "template" or "perforated plate") at the extrusion outlet. The dimensions of the meat grinder can be appropriately set according to the desired production volume. Furthermore, the thickness of the perforated plate, the pore size of the perforated plate, the porosity of the perforated plate, the screw shaft speed, and the feeding rate of the hydrogel-like crosslinked polymer can be appropriately set according to the dimensions of the apparatus, without particular limitation.

[0227] Furthermore, in the grinding process using a meat grinder, it is preferable to pass the hydrogel through the meat grinder more than twice, and more preferably more than three times. In this invention, the gel pulverizer (GGE(1)) used for gel grinding of the hydrogel is preferably 10 to 500 J / g, more preferably 15 to 400 J / g, further preferably 20 to 300 J / g, even more preferably 45 to 250 J / g, and particularly preferably 25 to 200 J / g.

[0228] When the gel pulverizing energy (GGE(1)) is less than 10 J / g, it is sometimes impossible to pulverize the hydrogel-like crosslinked polymer into particulate hydrogels with the desired particle size. Conversely, when the gel pulverizing energy (GGE(1)) exceeds 500 J / g, the load on the pulverizing device increases, and it may be damaged during continuous operation. In addition, in this case, applying excessive shear / compression force to the hydrogel-like crosslinked polymer can sometimes lead to an increase in the amount of water-soluble components generated and a decrease in physical properties.

[0229] The gel pulverization energy (GGE(2)) is preferably 5-300 J / g, more preferably 6-280 J / g, 8-260 J / g, 9-250 J / g, or 10-240 J / g.

[0230] When the gel pulverization energy (GGE(2)) is less than 5 J / g, it is sometimes impossible to pulverize the hydrogel-like crosslinked polymer into particulate hydrogels with the desired particle size. Conversely, when the gel pulverization energy (GGE(2)) exceeds 300 J / g, applying excessive shear / compression forces to the hydrogel-like crosslinked polymer can sometimes lead to a decrease in physical properties, such as an increase in the amount of water-soluble components generated.

[0231] It should be noted that, in the case of multiple pulverizations, the gel pulverization energy (GGE(1)) or (GGE(2)) of the entire multiple pulverizations can be calculated by adding the gel pulverization energy (GGE(1)) or (GGE(2)) of each pulverization.

[0232] Gel pulverization energy refers to GGE (1) or GGE (2) as described in International Publication No. 2016 / 204302, which refers to the mechanical energy required per unit weight (unit weight of the hydrogel-like crosslinked polymer) of the gel pulverizing apparatus when pulverizing a hydrogel-like crosslinked polymer. The gel pulverization energy (GGE (1)) is defined as including the energy during the idling of the gel pulverizer, and the gel pulverization energy (GGE (2)) is defined as subtracting the energy during the idling of the gel pulverizer. In the case of multiple gel pulverizations, the gel pulverization energy assigned in each pulverization is summed to calculate the overall gel pulverization energy of the multiple pulverizations.

[0233] As a gel pulverizer using a meat grinder (screw extruder), the conditions described in WO2016 / 204302 can be applied to this invention.

[0234] (Gel pulverization using multi-screw type kneader of two or more screws)

[0235] In one embodiment, the hydrogel pulverization process preferably uses a multi-screw mixer as shown in Figures 1 and 2 of WO2022 / 065365. As shown in Figures 1 and 2, in the hydrogel pulverization process after the polymerization process, a gel pulverization device is used, having an inlet, a body with multiple rotating shafts, and an outlet. Each rotating shaft has a pulverizing unit. In this gel pulverization device, the hydrogel-like crosslinked polymer continuously fed into the body through the inlet is pulverized by the pulverizing units of each rotating shaft, and continuously removed as particulate hydrogel-like crosslinked polymer from the outlet. That is, the hydrogel-like crosslinked polymer fed into the body through the inlet is pulverized by the pulverizing units of each rotating shaft while moving from the inlet to the outlet. It should be noted that in this invention, the body refers to the machine body portion equipped with multiple rotating shafts and pulverizing units, and is also referred to as a barrel, tank, shell, etc. Preferably, the hydrogel-like crosslinked polymer fed into the body from the inlet is kept at a temperature above 50°C and moves from the inlet to the outlet while being crushed by the crushing units of each rotating shaft inside the multi-screw mixer.

[0236] The multi-screw mixer, as long as it is continuous, can be vertical (where the hydrogel travels in the up-down direction), horizontal, or lateral (where the hydrogel travels in the left-right or lateral direction). Furthermore, in both vertical and horizontal gel pulverizing devices, an inclination of 0° to 90° relative to the horizontal direction is possible. For example, in the case of the horizontal continuous pulverizing unit shown in Figure 1 of WO2022 / 065365, an appropriate inclination can be provided as needed, which can be downward or upward from the inlet towards the outlet (i.e., relative to the hydrogel's travel direction). Typically, the inclination angle is 0° to 10°, preferably 0° to 1°, and particularly preferably 0°.

[0237] From the viewpoint of continuous and stable gel pulverization, the multi-screw mixer preferably has a heating unit and / or a heat preservation unit. There are no particular limitations on the heating unit and / or heat preservation unit; however, from the viewpoint of preventing the adhesion and aggregation of hydrogels and particulate hydrogels, a heating unit that performs direct heat transfer and / or indirect heat transfer is preferred. Direct heat transfer is achieved through convection, and indirect heat transfer is achieved through heat conduction from the heating surface (the contact surface with the hydrogel, the heat source portion) of the gel pulverizing device heated by the heat medium. More preferably, the heating unit is a ventilated heating type for direct heat transfer and an external wall heating type for indirect heat transfer.

[0238] From the viewpoint of continuous and stable gel pulverization, the multi-screw mixer preferably has a heating unit and / or insulation unit on the outer surface of its main body, more preferably a heating unit. Examples of such insulation units include covering a portion or the entire outer surface of the main body with insulating material (preferably 50% or more, more preferably 80% or more, and particularly preferably the entire surface). Furthermore, examples of heating units include electric heat tracing pipes, steam heat tracing pipes, and jackets heated by a heat medium. By equipping the multi-screw mixer with such heating and / or insulation units, the gel pulverization process can be performed in a more favorable temperature range. Furthermore, the deterioration of gel pulverization quality caused by seasonal and diurnal temperature variations can be avoided. Moreover, when starting the gel pulverization device, it can be smoothly guided to stable operation.

[0239] The type of pulverizing unit in each rotating shaft is not particularly limited as long as the effects of the present invention can be achieved. For example, various shapes of disks can be listed as objects that have a shearing effect on hydrogels. Disks are sometimes referred to as chips, paddles, elements, kneading devices, rotors, etc. The shape of the disk is not particularly limited, and can be appropriately selected from circular, elliptical, or roughly triangular shapes. Different shapes of disks can also be used in combination, and their arrangement can be appropriately adjusted according to the particle size of the target particulate hydrogel and the energy required for pulverization. In addition, as pulverizing units, arms, blades, scrapers, cut disks (CDs), etc., can also be used together.

[0240] The distance (gap) between the disc and the body (barrel) sometimes varies depending on the position. When the shortest distance between the outer periphery of the disc and the inner wall of the body (barrel) is defined as the minimum gap C, the minimum gap C relative to the maximum diameter D of the disc is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less. It should be noted that "D" is an abbreviation for Diameter, and when using multiple discs with different diameters, it refers to the diameter of the largest disc. As long as it is below the above upper limit, the shear force between the barrel and the disc during gel pulverization will be stronger, and the gel pulverization efficiency will be good. In addition, the minimum gap C relative to the maximum diameter D of the disc is preferably 0.2% or more, more preferably 0.5% or more, and even more preferably 1% or more. As long as it is above the above lower limit, it can prevent the disc from contacting the inner wall of the body (barrel) and prevent the ingress of metal foreign matter caused by wear. In a preferred embodiment of the present invention, the minimum gap C relative to the maximum diameter D of the disc is 0.2% to 20%.

[0241] The rotational speeds of the multiple rotating shafts in the multi-screw mixer can be uniform or non-uniform, depending on the device, but are preferably in the range of 1 rpm to 1000 rpm, more preferably in the range of 3 rpm to 500 rpm, and even more preferably in the range of 5 rpm to 300 rpm. Furthermore, when the rotational speeds of the rotating shafts are different, the ratio of the rotational speed of one rotating shaft to the rotational speed of another rotating shaft is typically in the range of 1 to 10, preferably in the range of 1 to 2.

[0242] Furthermore, the rotation directions of the multiple rotating shafts can be either unidirectional (rotating in the same direction) or antidirectional (rotating in opposite directions). Unidirectional shafts offer self-cleaning properties, while antidirectional shafts offer strong shearing forces. The rotation direction of each shaft is appropriately selected based on its combination with the arrangement (disc pattern) of the aforementioned pulverizing units.

[0243] The multi-screw mixer preferably has the function of supplying water and / or steam to the interior of the main body. Therefore, according to one embodiment of the invention, water and / or steam are supplied to the interior of the main body during the gel pulverization process. As a unit for supplying water and / or steam, the gel pulverization device may have multiple inlets. The location of the water and / or steam inlets is not limited, but it is preferably located on the inlet side containing the hydrogel. In addition, water and steam may also be supplied from different inlets.

[0244] When adding water vapor, a gas can be mixed with the water vapor and added as a mixed gas. The gas is not particularly limited and can be, for example, air, dry air, nitrogen, etc. The pressure of the added water vapor is not particularly limited, but is preferably 0.2 to 0.8 MPa. The temperature of the water and / or water vapor (including the mixed gas) is not particularly limited, but is preferably 50°C or higher, more preferably 60°C or higher, further preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of preventing excessive heating and drying of the hydrogel, the temperature of the water and / or water vapor (including the mixed gas) is preferably below 200°C, more preferably below 170°C, further preferably below 150°C, even more preferably below 120°C, and particularly preferably below 100°C. Preferably, the temperature of the water and / or water vapor supplied to the interior of the compound is 50 to 120°C. The temperature of the hydrogel and particulate hydrogel in the multi-screw mixer can also be adjusted according to the temperature and amount of water and / or water vapor (including the mixed gas) used. In this case, water vapor and / or mixed gases act as a direct heat transfer medium, heating or maintaining the temperature of the hydrogel and particulate hydrogel inside the bulk at a specified temperature. It should be noted that additives such as gel flow agents, crosslinking agents, oxidizing agents, reducing agents, and polymerization initiators (described later) can be incorporated into the added water and / or water vapor (including mixed gases).

[0245] The amount of water and / or water vapor supplied is preferably 0.1% to 50% by mass, more preferably 0.5% to 40% by mass, and even more preferably 1% to 30% by mass, relative to the mass of the hydrogel in terms of solid content.

[0246] More preferably, before the hydrogel is fed into the multi-screw mixer, the temperature of the interior (inner surface) of the body is preferably heated to 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and even more preferably 80°C or higher. This reduces the adhesion of the hydrogel to the inner surface of the body, and further improves the water absorption properties of the resulting absorbent resin. That is, when performing gel pulverization using the multi-screw mixer, it is preferable that the inner surface of the body is heated to the aforementioned temperature or higher before the hydrogel is fed into the body and at the start of gel pulverization. From the viewpoint of maintaining the temperature above 50°C during the gel pulverization process, it is preferable to maintain the temperature of the interior (inner surface) of the body within the aforementioned range during the gel pulverization process.

[0247] The temperature for continuously pulverizing the hydrogel-like crosslinked polymer is preferably above 50°C, more preferably above 60°C, even more preferably above 70°C, and even more preferably above 80°C.

[0248] There is no particular upper limit to the temperature for continuous pulverization of hydrogel-like crosslinked polymers, but from the viewpoint of preventing excessive heating and drying of the hydrogel, it is preferably below 200°C, more preferably below 170°C, further preferably below 150°C, even more preferably below 130°C, and particularly preferably below 110°C.

[0249] Figures 1 and 2 of WO2022 / 065365 show an example of a multi-screw mixer (gel pulverizer) that can be preferably used in the present invention. Figure 1 is a partial sectional side view of the gel pulverizer, and Figure 2 is an enlarged view of the gel pulverizer (view of the central part of the body from above). Hereinafter, the basic configuration and method of use of the gel pulverizer are the same as those of the gel pulverizer described in Figures 1 and 2.

[0250] The rotating shaft of the gel pulverizing apparatus has multiple discs. The discs may have the same or different shapes, but are preferably different. The combination of discs can be appropriately modified, for example, by referring to patent documents (Japanese Patent Application Publication No. 2005-35212), depending on the physical properties of the hydrogel and the desired size of the pulverized gel.

[0251] Examples of gel pulverizing devices (multi-screw mixers) with such basic configurations include, for instance, multi-screw mixers (kneaders) with twin or more screws. Specifically, twin-screw, three-screw, four-screw, or eight-screw mixers can be cited. From a production efficiency perspective, continuous operation is preferred for this gel pulverizing device. Specifically, examples of gel pulverizing devices include: CKH type continuous mixers (Honda Iron Works Co., Ltd.), twin-screw extruders TEX (Nippon Steel Works Co., Ltd.), twin-screw extruders TEXαIII (Nippon Steel Works Co., Ltd.), CONTINUOUS KNEADER (Dalton Co., Ltd.), KRC mixing reactors (KRC HYBRID REACTER, Kurimoto Iron Works Co., Ltd.), KRC kneaders (KURIMOTO-READCOCONTINUOUS KNEADER, Kurimoto Iron Works Co., Ltd.), KEX extruders (KEX EXTRUDER, Kurimoto Iron Works Co., Ltd.), and KEXD extruders (KEXD...). The equipment includes EXTRUDER (Kurimoto Iron Works Co., Ltd.), KNEADER-RUDER twin-arm kneader (Moriyama Co., Ltd.), TEX-SSG twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-CS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-SX twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-DS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-A twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-B twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-BS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), and the WDR series four-screw and eight-screw compounding extruders (Technovel Co., Ltd.). Therefore, in a preferred embodiment of the present invention, the gel pulverizing device is a continuous multi-screw compounding machine.

[0252] When using a multi-screw mixer for pulverization, the upper limit of the gel pulverization energy (GGE(2)) for pulverizing hydrogels is preferably 150 [J / g] or less, more preferably 130 [J / g] or less, and even more preferably 120 [J / g] or less. Furthermore, the lower limit is preferably 20 [J / g] or more, more preferably 40 [J / g] or more, and even more preferably 50 [J / g] or more. For example, in this invention, the gel pulverization energy (GGE(1)) for pulverizing hydrogels is 20 to 150 [J / g], preferably 40 to 130 [J / g], and even more preferably 50 to 120 [J / g]. Furthermore, the GGE(2) is 20 to 150 [J / g], preferably 40 to 130 [J / g], and even more preferably 50 to 120 [J / g]. By controlling GGE(1) and GGE(2) within the specified range, gel pulverization can be performed while applying appropriate shear / compression force to the hydrogel. The calculation methods for GGE(1) and GGE(2) are the same as those for calculating gel pulverization energy when using a meat grinder.

[0253] (Gel temperature)

[0254] When using a multi-screw mixer for gel pulverization, from the viewpoint of continuously pulverizing hydrogel-like crosslinked polymers, the temperature T1 of the hydrogel-like crosslinked polymer fed into the inlet of the gel pulverizing device during the gel pulverization process is preferably 50°C or higher. Hereinafter, this temperature T1 will also be referred to as "gel temperature T1 at the inlet" or simply "gel temperature T1". This gel temperature T1 is preferably measured using a thermometer installed at the inlet. From the viewpoint of preventing the hydrogel-like polymer from adhering to the device after pulverization, this gel temperature T1 is preferably 60°C or higher; from the viewpoint of further improving the water absorption performance of the water-absorbing resin particles, it is more preferably 70°C or higher, and even more preferably 80°C or higher. From the viewpoint of preventing excessive drying, the gel temperature T1 is preferably 130°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 90°C or lower. For the same reason, the gel temperature during pulverization is preferably 130°C or lower. It should be noted that for hydrogel-like crosslinked polymers fed into the gel pulverizing device, the gel temperature T1 can be adjusted to the desired range by keeping the hydrogel-like crosslinked polymer, whose temperature rises due to the heat of polymerization, or by heating the obtained hydrogel-like crosslinked polymer.

[0255] From the viewpoint of preventing the hydrous gels from agglomerating after gel pulverization, the temperature T2 of the particulate hydrous gel crosslinked polymer discharged from the gel pulverizer is preferably 60°C to 140°C, more preferably 70°C to 130°C, even more preferably 80°C to 120°C, particularly preferably 80°C to 115°C, and most preferably 100°C to 115°C. Hereinafter, this temperature T2 will also be referred to as "gel temperature T2 at the discharge port" or simply "gel temperature T2". It is preferable that temperature T2 is set within the above-mentioned temperature range, and temperature T1 is also within the above temperature range. This gel temperature T2 is preferably measured by a thermometer installed at the discharge port. It should be noted that the gel temperature T2 can be adjusted to the desired range by appropriately adjusting the set temperature of the heating unit and / or the heat preservation unit of the gel pulverizer, and further adjusting the residence time of the hydrous gel crosslinked polymer inside the gel pulverizer.

[0256] When using a multi-screw mixer for gel pulverization, it is preferable to add the aforementioned gel flow agent. The gel flow agent can be added all at once or in multiple additions (two or more times), and can be added at any location on the mixer. When adding the gel flow agent in multiple additions, it is preferable to add it at different locations.

[0257] [3-2-4] Drying step

[0258] The manufacturing method of the present invention may include a drying step. The drying step refers to the process of drying a particulate hydrogel (preferably containing a gel flow agent) to a desired moisture content to obtain a dried polymer, preferably a particulate dried polymer. It should be noted that the particulate hydrogel supplied for the drying step is not limited to being obtained via a gel pulverization process; for example, it can also be obtained through reverse suspension polymerization. Furthermore, the (particulate) dried polymer obtained in the drying step may contain granules (hereinafter, dried granules) formed by the physical or chemical attachment of multiple particles.

[0259] <Temperature of particulate water-containing gel>

[0260] From the viewpoint of preventing the formation of large particles due to the adhesion between the particulate hydrogel and the micronized granules, the temperature T2 of the particulate hydrogel supplied in the drying process is preferably controlled to be 50°C or higher, more preferably 60°C or higher, further preferably 70°C or higher, particularly preferably 80°C or higher, and most preferably 90°C or higher. Furthermore, from the viewpoint of preventing coloration and / or performance degradation of the dried material, this temperature T2 is preferably 130°C or lower, more preferably 110°C or lower, and further preferably 105°C or lower. Typically, this temperature T2 is measured at the center of the material layer (particulate hydrogel and / or dried material) (e.g., approximately 5 cm in the case of a material thickness of 10 cm) using a contact thermometer.

[0261] There are no particular limitations on the drying method used in the aforementioned drying process; static drying, stirred drying, fluidized bed drying, etc., can be appropriately used. In addition, various drying methods can be employed, such as heating drying, hot air drying, reduced pressure drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration drying with hydrophobic organic solvents, and high-humidity drying using high-temperature steam. From the viewpoint of drying efficiency, heating drying or hot air drying is preferred, and stirred heating drying and / or fluidized bed drying, which involves moving the material being dried while drying, is even more preferred.

[0262] <Drying device>

[0263] As a drying device used in the drying process, there are no particular limitations; one or more of the following can be appropriately selected: heat transfer dryer, radiation heat transfer dryer, hot air heat transfer dryer, dielectric heating dryer, etc. It can be intermittent or continuous. Furthermore, it can be direct heating or indirect heating. Examples of heat transfer dryers include ventilated belt dryers, ventilated loop dryers, ventilated vertical dryers, horizontal flow belt dryers, ventilated tunnel dryers, ventilated stirred dryers, ventilated rotary dryers, fluidized bed dryers, and airflow dryers.

[0264] When using belt dryers or similar methods with static drying, large lumps or agglomerates of dried material are obtained. Therefore, when these are pulverized and processed into particulate products with a particle size (e.g., 850 μm or more and 150 μm or less), a large amount of fine powder is sometimes generated. On the other hand, if a stirred dryer equipped with a flow unit that allows the material to be dried to flow in the dryer (e.g., the stirring blades equipped in the dryer and / or the rotation of the dryer itself) and one or more heating units are used, particulate dried material (hereinafter sometimes referred to as particulate dried polymer) can be obtained. Therefore, a continuous stirred dryer is preferred, and a continuous stirred dryer is even more preferred.

[0265] The heating unit in the drying process is not particularly limited. From the viewpoint of drying efficiency and reducing thermal damage to the absorbent resin, the heating unit is preferably a direct heat transfer and / or indirect heat transfer unit. Direct heat transfer is achieved through convection, and indirect heat transfer is achieved through heat conduction from the heating surface of the dryer (the contact surface with the object being dried) heated by the heat-transfer medium. More preferably, the heating unit in the drying process is a ventilated heating type for direct heat transfer and an external wall heating type or a tubular heating type for indirect heat transfer.

[0266] The stirring method and manner are not particularly limited, as long as the particulate hydrogel and micro-powder granules within the drying apparatus flow through a stirring unit such as a stirring blade or a rotating cylinder. In this specification, a dryer that uses the rotation of a cylindrical container holding the material to be dried for stirring, i.e., a dryer whose stirring unit is based on a rotating cylinder, is called a rotary dryer. In the manufacturing method of this invention, examples of rotary dryers include, for example, rotary drum dryers, rotary kilns, and tubular dryers; examples of continuous stirring dryers that are not rotary dryers include single-screw or twin-screw disc dryers and single-screw or twin-screw paddle dryers. Specifically, as non-rotary continuous stirring dryers, Solid Air (manufactured by Hosokawa Micron Co., Ltd.), CD dryers (manufactured by Kurimoto Iron Works Co., Ltd.), and paddle dryers (manufactured by Nara Machinery Co., Ltd.) can be used; as rotary dryers, steam tube dryers (manufactured by Kurimoto Iron Works Co., Ltd.), steam tube dryers (manufactured by Ube Industries, Ltd.), steam tube dryers (manufactured by Tsukishima Machinery Co., Ltd.), steam tube dryers (manufactured by Mitsui Shipbuilding Co., Ltd.), rotary kilns (manufactured by Kurimoto Iron Works Co., Ltd.), and drum dryers (manufactured by Okawahara Manufacturing Co., Ltd.) can be used.

[0267] From the viewpoint of reducing mechanical damage to the dried material, a preferred drying device is a rotary dryer (a dryer based on a rotating container (further, a cylindrical container, particularly a horizontal cylindrical container such as a rotating drum) that holds and rotates the dried material). From the viewpoint of reducing thermal and mechanical damage, a rotary dryer having one or more heating units selected from direct heat transfer (air-heated type) and indirect heat transfer (external wall heating type and tubular heating type) is more preferred. Furthermore, in rotary dryers, such as rotary kilns that are heated only by air-heated methods, problems such as the scattering of dried material due to airflow and the generation of large amounts of waste gas may sometimes occur. Therefore, as a rotary dryer, it is preferable to have one or more heating units selected from indirect heat transfer (external wall heating type and tubular heating type). Moreover, tubular heating, by using multiple heating tubes, can increase the heat transfer area inside the dryer, thus enabling efficient drying, and is therefore more preferred. As such a rotary dryer, a rotary dryer with heating tubes can be cited as an example.

[0268] For example, this rotary dryer with heating tubes includes: a rotating container (particularly a cylindrical container, and more particularly a horizontal cylindrical container) that internally houses and rotates the material to be dried, and multiple heating tubes located inside the rotating container, extending along its axial direction, and rotating together with the rotating container. In this rotary dryer, the material to be dried flows within the container mainly through the rotation of the rotating container and the action of the multiple heating tubes rotating together with the rotating container, thus minimizing mechanical and thermal damage. This prevents the generation of fine powder and deterioration of physical properties during the drying process. Furthermore, in this dryer, drying is carried out through indirect heat transfer originating from the heating tubes, thus eliminating the scattering and large-scale waste gas treatment required by hot air drying (e.g., ventilated belt dryers and ventilated heated rotary kilns). Moreover, by increasing the number of heating tubes, the heat transfer area inside the dryer can be increased. Due to the large heat transfer area, drying can be carried out in a short time, and the residence time inside the device is also shortened, further reducing thermal damage.

[0269] Preferably, the rotary dryer has a heating unit or a heat-insulating unit on the outer circumferential surface of its rotating container. In this dryer, the mixture of particulate hydrogel and micro-powder granules contained inside is heated by the rotation of the rotating container through contact with multiple heating tubes or by heat conduction from the heating tubes. The inner surface of the rotating container is also heated by radiant heat from the multiple heating tubes, but if necessary, the mixture is further heated through the heating unit or heat-insulating unit located on the outer circumferential surface of the rotating container, thus shortening the drying time through heat conduction from the inner wall.

[0270] Preferably, the rotary dryer includes an additive unit inside the rotating container for adding additives to the contents contained within the rotating container. An example of this additive unit is a spray device. According to the rotary dryer with the additive unit, a surface additive can also be added as an additive to the mixture of particulate hydrogel and micronized granules contained in the rotating container, and then mixed, followed by heating through contact with or heat conduction with multiple heating tubes and drying. In this case, since the surface crosslinking process and the drying process described later are performed in one step, manufacturing efficiency is improved.

[0271] As long as it does not impede the purpose of the present invention, the rotary dryer may include, in addition to agitation of the contents by rotation of the rotating container, other flow units that allow the contents to flow. Examples of other flow units include a top plate and stirring blades disposed on the inner surface of the rotating container.

[0272] As long as the objectives of this invention can be achieved, the number of dryers used in the drying process can be as few as one or more than two. Multiple dryers of different specifications can also be used in combination. For example, the rotary dryer can be combined with other stirred dryers that are not rotary dryers or belt dryers classified as material transfer dryers. Preferably, at least one stirred dryer is included, but the type and number of dryers combined are not limited.

[0273] Preferably, a dryer with the function of introducing gas into its interior (preferably a unit for introducing and discharging gas) is used in the drying process. Examples of units for introducing and discharging gas include a gas inlet and an outlet. The gas acts as a carrier gas, promoting drying by discharging water vapor and the like generated during drying outside the apparatus. Furthermore, when heated gas is used, the gas also acts as a heat medium, further promoting drying. Preferably, nitrogen, water vapor, and mixtures thereof with air are used. When using a mixture containing water vapor (hereinafter also referred to as a high-humidity mixture), the apparatus becomes a low-oxygen state, preventing oxidation and / or deterioration during drying. As a result, improved performance and reduced coloring of the water-absorbing resin can be achieved. Furthermore, this is preferred because it prevents the agglomeration / bulking of particulate hydrogels and micro-powder granules during drying.

[0274] In the drying process, a dryer capable of operating under pressure, atmospheric pressure, or reduced pressure can be used. When operating under pressure, this can be adjusted, for example, by increasing the amount of carrier gas introduced into the dryer. Preferably, the pressure relative to atmospheric pressure is a slight increase, exceeding 0 kPa but less than 0.01 kPa. Furthermore, when operating under reduced pressure, the pressure can be adjusted, for example, by changing the amount of exhaust gas (introduced carrier gas, water vapor generated during drying, etc.) drawn from the dryer.

[0275] The pressure reduction relative to atmospheric pressure is preferably greater than 0 kPa and less than 5 kPa, more preferably greater than 0 kPa and less than 2 kPa, and even more preferably a slight pressure reduction of more than 0.01 kPa and less than 0.5 kPa. By setting the pressure reduction within this range, excessive heat is not drawn from the inside of the dryer, and water vapor and other substances generated during drying can be effectively removed, thus shortening the drying time. Furthermore, it prevents the agglomeration of particulate hydrogels and micro-powder granules during the drying process.

[0276] It should be noted that "pressure increase relative to atmospheric pressure" and "pressure decrease relative to atmospheric pressure" refer to the pressure difference with atmospheric pressure, expressed as the absolute value of the difference. For example, when the atmospheric pressure is standard atmospheric pressure (101.3 kPa) and the pressure decrease relative to atmospheric pressure is 10 kPa, the actual air pressure is 91.3 kPa.

[0277] In the drying process, the dew point of the atmosphere inside the dryer can be adjusted by introducing the aforementioned gas from one or more points. Preferably, the dew point is appropriately adjusted mainly based on the water content of the particulate hydrogel fed into the dryer. The dew point is measured when exhausting from the dryer, and is preferably 60°C or higher, more preferably 65°C or higher, and even more preferably 70°C or higher. There is no particular upper limit, but it is preferably 100°C or lower. By setting the dew point within the above range, drying can be promoted, and the water content of the resulting (particulate) dried polymer can be adjusted to a predetermined range, thereby improving its water absorption properties.

[0278] The drying conditions are appropriately selected based on the type of dryer or the moisture content of the particulate hydrogel, etc. The drying temperature (material temperature or heat medium temperature) is preferably 100°C or higher and 300°C or lower, more preferably 150°C or higher and 250°C or lower, even more preferably 160°C or higher and 220°C or lower, and particularly preferably 170°C or higher and 200°C or lower. When the drying temperature is within this range, the drying time is of an appropriate length, and therefore economical. Furthermore, when the drying temperature is within this range, it is less likely to cause deterioration of the physical properties of the obtained water-absorbing resin and significant discoloration, which is therefore preferable. Moreover, from the viewpoint of obtaining a CRC of more than 25 g / g for the obtained water-absorbing resin, it is preferable to set the drying temperature conditions as described in WO22 / 163849.

[0279] Furthermore, the drying time is preferably 1 minute or more and 10 hours or less, more preferably 5 minutes or more and 2 hours or less, even more preferably 10 minutes or more and 120 minutes or less, and particularly preferably 20 minutes or more and 60 minutes or less. When the drying time is within this range, there is no need to excessively increase the drying temperature, and the water-absorbing resin is less prone to physical property deterioration and / or significant discoloration, thus it is preferred. Furthermore, when the drying time is within this range, it is possible to prevent the drying equipment from becoming excessively large, and also to avoid a reduction in throughput, thus it is economical.

[0280] In the manufacturing method of the present invention, from the viewpoint of drying efficiency, when the drying device is a rotary dryer, its Froude number (Fr = ω²*r / g) is appropriately set according to the device size and drying capacity (drying amount per unit time). The Froude number Fr is preferably in the range of 0.001 or more and 1 or less, more preferably in the range of 0.005 or more and 0.5 or less, even more preferably in the range of 0.01 or more and 0.3 or less, and particularly preferably in the range of 0.02 or more and 0.2 or less. The Froude number Fr refers to the ratio of the centrifugal acceleration ω²*r acting on the material to be dried and stirred in the rotating container to the gravitational acceleration g. (ω is the angular velocity of the rotating body: rad / sec, r is the representative radius of the rotating body: m).

[0281] Furthermore, in the case of a stirring dryer having a stirring unit that stirs the material to be dried by a rotating shaft equipped with stirring blades, the rotational speed of the rotating shaft is appropriately set according to the device, but it is preferable to set the speed such that the Froude number Fr is within the range described above. Examples of stirring blades include arms, blades, impellers, and cutting discs (CDs). Specifically, the rotational speed is typically 1 rpm or more and 10,000 rpm or less, more preferably 5 rpm or more and 500 rpm or less, and even more preferably 10 rpm or more and 300 rpm or less.

[0282] In addition, the circumferential speed (V) of the stirring blade, as defined by the following formula (3), is set appropriately according to the device, and is usually above 0.15 m / s and below 25 m / s.

[0283] Circular velocity (V) (m / s) = 2πr × n / 60……(3)

[0284] In Equation (3), V is the circumferential speed of the stirring blade (unit: m / s), r is the diameter of the stirring blade (unit: m), and n is the rotational speed of the stirring blade per unit time (unit: rpm).

[0285] In the case of a rotary mixer in which the container holding the material to be dried rotates, the rotational speed of the container is appropriately set according to the size of the device and the drying capacity (the amount of material dried per unit time). The rotational speed of the container is preferably 1 rpm or more and 250 rpm or less, more preferably 1 rpm or more and 100 rpm or less, and even more preferably 2 rpm or more and 50 rpm or less. Furthermore, the maximum circumferential speed is not particularly limited, but is preferably 0.05 m / s or more and 10 m / s or less, more preferably 0.1 m / s or more and 8 m / s or less, and even more preferably 0.15 m / s or more and 5 m / s or less.

[0286] In addition, the fill ratio (fill volume of the container (m³)) in the rotary dryer 3) relative to the effective volume (m) of the rotating container 3 The ratio of the two components should be appropriately selected. From the viewpoint of heat treatment efficiency, the filling rate is preferably 5% or more and 95% or less, more preferably 6% or more and 50% or less, and even more preferably in the range of 10% or more and 40% or less.

[0287] Furthermore, when using a rotary dryer, in order to efficiently transfer heat to the contents of the rotating container, it is preferable to increase the heat transfer area relative to its internal volume. The heat transfer area relative to this internal volume is defined as the heat transfer area (m²). 2 ) relative to the effective volume (m) of the rotating container 3 The ratio of heat transfer area to effective volume is used. A higher ratio results in higher heat transfer efficiency and a faster temperature rise of the contents. This leads to shorter drying time, reduced thermal and mechanical damage, and increased productivity. This ratio is appropriately set based on the dryer's specifications, shape, and the shape of the contents, and is preferably 10m². -1 The above, preferably 12m -1 More preferably 15m -1 The above should be noted. It is important to understand that the effective volume refers to the internal volume of the rotating container holding the contents, and the heat transfer area refers to the area of ​​the heating surface capable of imparting heat to the contents contained in the rotating container. Specifically, the heat transfer area is the sum of the areas of the outer circumferential surfaces of the multiple heating tubes and the area of ​​the inner circumferential surface of the rotating container.

[0288] (Additive)

[0289] Examples of additives used in the drying process, besides the aforementioned gel flow agents (surfactants, polymeric lubricants, etc.), include surface crosslinking agents (post-crosslinking agents) described later. By adding a surface crosslinking agent before drying and then during drying, adhesion during agitated drying can be reduced, and the usual post-drying surface crosslinking effect (e.g., increased water absorption ratio under pressure) is also exhibited. Therefore, the advantage of being able to omit the post-drying surface crosslinking step can be obtained. From the viewpoint of obtaining a CRC of more than 25 g / g for the resulting water-absorbing resin, it is preferable to add the hydrogen peroxide before or during drying.

[0290] [3-2-5] Surface crosslinking step

[0291] The manufacturing method of the present invention may include a surface crosslinking step. The surface crosslinking step refers to a step in which a crosslinking reaction is carried out on the surface layer (a region from the particle surface to a depth of approximately tens of μm) of particles composed of a water-absorbing resin. Typically, the surface crosslinking step includes: a surface crosslinking agent addition step, in which a surface crosslinking agent that reacts with the functional groups (especially carboxyl groups) of the water-absorbing resin is added to the particulate hydrogel or dry polymer; and a heat treatment step, in which the particulate hydrogel or dry polymer containing the surface crosslinking agent is heated, preferably including a cooling step after the heat treatment step. It should be noted that, if necessary, a granulation step described later [3-2-7] is performed after the drying step and before the surface crosslinking step to produce a particulate dry polymer with a preferred particle size. For example, if static drying is performed during the drying step, since the resulting dry polymer contains large agglomerates, it is preferable to perform the granulation step before the surface crosslinking step.

[0292] [3-2-5-1] Surface crosslinking agent addition step

[0293] The surface crosslinking agent addition step is a step of adding a surface crosslinking agent to particulate hydrogels and / or particulate dried polymers. The surface crosslinking agent is added to the particulate hydrogels before or during drying, or to the particulate dried polymers after drying or granulation. The particulate hydrogels and / or particulate dried polymers may contain micronized granules.

[0294] When the particulate hydrogel obtained in the gel pulverization process is fed into the surface crosslinking process during the drying process, the water content of the particulate hydrogel with added surface crosslinking agent is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 45% by mass or less, and even more preferably 20% by mass or more and 40% by mass or less. By adding the surface crosslinking agent within the above range, excessive aggregation and adhesion during drying are reduced. Furthermore, the effect of increased absorption ratio under pressure obtained by performing the surface crosslinking process after the drying process also occurs when surface crosslinking is performed simultaneously with drying, which is therefore preferred.

[0295] When surface crosslinking is performed simultaneously with drying, the surface crosslinking process and granulation process following the drying process can be omitted, thus significantly simplifying the manufacturing process. Furthermore, in the separately required surface crosslinking and conveying processes, there is virtually no generation of micropowder or destruction of the surface crosslinking structure due to process damage to the water-absorbing resin, which is therefore preferable.

[0296] Furthermore, when a surface crosslinking process is performed separately after the drying process, the solid content of the (particulate) dried polymer when the surface crosslinking agent is added is preferably 80% by mass or more, more preferably 85% by mass or more and 99.8% by mass or less, even more preferably 90% by mass or more and 99.7% by mass or less, even more preferably 92% by mass or more and 99.5% by mass or less, particularly preferably 96% by mass or more and 99.5% by mass or less, and extremely preferably 98% by mass or more and 99.5% by mass or less.

[0297] The temperature of the water-absorbing resin supplied for the surface crosslinking process after the drying process, i.e., the temperature of the (particulate) dried polymer when the surface crosslinking agent is added, is preferably 40 to 120°C, more preferably 60°C or higher and 100°C or lower. Furthermore, when surface crosslinking is performed simultaneously with drying, the temperature of the water-absorbing resin during the drying process, i.e., the temperature of the particulate hydrogel when the surface crosslinking agent is added, is preferably in the range of 70°C or higher and 150°C or lower, more preferably 80°C or higher and 130°C or lower. This temperature is measured during the drying process in the same manner as the gel temperature described above.

[0298] (Surface crosslinking agent)

[0299] The surface crosslinking agent that can be used in the surface crosslinking process can be any of the surface crosslinking agents listed in the [2-3-2] section above, and the description in that section can be referenced.

[0300] (Surface crosslinking agent solution)

[0301] The amount of the surface crosslinking agent added, calculated based on the solid content, is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, relative to the particulate hydrogel or particulate dry polymer. Furthermore, as a lower limit, it is preferably 0.001% by mass.

[0302] As for the method of adding the surface crosslinking agent, from the perspective of ease of addition, it is preferable to add it in the form of a solution dissolved in water or an organic solvent. The concentration of the surface crosslinking agent solution is preferably 1% by mass or more, more preferably 2% by mass or more. The total amount of solvent selected from water and organic solvent, in terms of solid content, is preferably 0% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 8% by mass or less, and even more preferably 0.5% by mass or more and 5% by mass or less. When water and organic solvent are used together as the solvent for the surface crosslinking agent solution, water is preferably the main component of the solvent.

[0303] [3-2-5-2] Heat treatment step

[0304] The heat treatment process constituting the surface crosslinking process is a process of heating a particulate hydrogel or particulate dry polymer containing a surface crosslinking agent to obtain a surface-crosslinked dry polymer.

[0305] <Surface crosslinking temperature>

[0306] By heating a particulate hydrogel or particulate dry polymer containing a surface crosslinking agent to above 100°C, a surface-crosslinked dry polymer can be obtained. This heating temperature (surface crosslinking temperature) is appropriately selected depending on the type of surface crosslinking agent added. From the viewpoint of heat treatment efficiency, the surface crosslinking temperature is preferably, for example, above 100°C and below 250°C, more preferably above 120°C and below 230°C, and even more preferably above 150°C and below 210°C. By appropriately selecting the above-mentioned surface crosslinking temperature, as well as the type and amount of the surface crosslinking agent used, the water-absorbing resin manufactured by the manufacturing method of the present invention can be made to have a CRC exceeding 25 g / g.

[0307] <Heating time>

[0308] The heating time at the surface crosslinking temperature is appropriately set according to the moisture content of the particulate hydrogel or particulate dry polymer, the type of surface crosslinking agent, etc. As a temporary indicator, it is sufficient to heat until the moisture content of the resulting water-absorbing resin powder is less than 10% by mass, and the heating time is in the range of 10 minutes to 120 minutes, preferably in the range of 30 minutes to 90 minutes.

[0309] <Heating method>

[0310] The heating device used in the heat treatment process is not particularly limited as long as the objective of the present invention can be achieved. From the viewpoint of minimizing uneven heating, it is preferable to use a device with a stirring mechanism (hereinafter sometimes referred to as a stirring-type indirect heating device) that achieves heat transfer through solid-solid contact. From the viewpoint of obtaining a surface-crosslinked, particulate, dried polymer after heat treatment, it is preferable to use the aforementioned stirred dryer (preferably a rotary dryer) as the heating device for the heat treatment process in the drying step. For example, it is preferable to use the aforementioned rotary dryer with heating tubes, adding a surface crosslinking agent as an additive to the particulate hydrogel and heating it. Thus, the drying process and the surface crosslinking process are performed simultaneously, obtaining a particulate, dried polymer with a specified moisture content (solid content) and surface crosslinking in one step.

[0311] [3-2-6] Cooling step

[0312] The manufacturing method of the present invention may include a cooling step. The cooling step refers to a step of forcibly cooling the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer to a desired temperature after the aforementioned drying step or heat treatment step and before the subsequent granulation step. In the aforementioned dryer, where the surface crosslinking step and the drying step are performed in one step, the surface crosslinking treatment is appropriately performed in a rotating container, and the solid content or moisture content of the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer is adjusted to a desired range. Then, the cooling step is performed before supplying it to the granulation step.

[0313] Specifically, relative to the temperature t°C of the (particulate) dried polymer after the drying process and / or the surface-crosslinked (particulate) dried polymer after the surface crosslinking process, it is forcibly cooled to preferably below (t-20)°C, more preferably below (t-30)°C, and even more preferably below (t-40)°C. For example, when the temperature t of the (particulate) dried polymer and / or the surface-crosslinked (particulate) dried polymer is 150°C or higher and 250°C or lower, before being supplied to the granulation process, the (particulate) dried polymer and / or the surface-crosslinked (particulate) dried polymer is forcibly cooled to preferably above 50°C and below 130°C, more preferably above 60°C and below 100°C, and even more preferably above 65°C and below 90°C. By cooling to this temperature range, the operability and grading accuracy during crushing in the granulation process are improved, and the physical properties of the manufactured water-absorbing resin are improved.

[0314] <Cooling method>

[0315] In the cooling process, there are no particular limitations on the method of cooling (particulate) dried polymers and / or surface-crosslinked (particulate) dried polymers. A continuous cooler with ventilated or conductive heat transfer cooling units is preferred.

[0316] [3-2-7] Granulation step

[0317] The manufacturing method of the present invention may include a granulation process. The granulation process refers to the process of adjusting the particle size of the dried polymer or the surface-crosslinked (particulate) dried polymer. By performing the granulation process after the surface crosslinking process, a water-absorbing resin powder with a particle size or particle size distribution controlled at a high level can be obtained.

[0318] Preferably, the granulation process includes a pulverizing step and / or a grading step. More preferably, the granulation process includes a crushing step and / or a grading step. Preferably, the granulation process obtains the water-absorbing resin powder with controlled particle size and particle size distribution through only a pulverizing step or a crushing step, and ideally, the granulation process obtains the water-absorbing resin powder with controlled particle size and particle size distribution through only a crushing step.

[0319] The pulverization step is, for example, as follows: The blocky dried polymer or the strongly agglomerated granular dried polymer obtained during the drying or heat treatment process is pulverized using a pulverizer to adjust the particle size. It should be noted that this pulverization step differs from the gel pulverization step in that the dried polymer being pulverized has undergone a drying process.

[0320] Examples of pulverizers used in the pulverizing process include high-speed rotary pulverizers such as roller mills, hammer mills, screw mills, and needle mills; vibrating mills; toggle pulverizers; and cylindrical mixers.

[0321] The crushing step involves breaking down (particle-like) dried polymer, which has been loosened and agglomerated through a drying or heat treatment process, using a crusher to adjust the particle size. In this crushing step, the particle size can be adjusted using a weaker grinding process than the pulverizing step described above.

[0322] The crusher used in the crushing step is preferably a crusher that causes minimal mechanical damage to (particulate) dry polymers or surface-crosslinked (particulate) dry polymers. Specifically, examples include roller granulators (Matsubo Co., Ltd.), granulators (Kurimoto Iron Works Co., Ltd.), and RoundelMill (Tokuju Works Co., Ltd.).

[0323] The grading step involves removing coarse particles and fine powders from (particulate) dried polymers, surface-crosslinked (particulate) dried polymers, or their pulverized or crushed forms using a classifier. The classifier used in this step is a vibrating or shaking sieve classifier that utilizes a screen.

[0324] The size of the coarse particles and micro powder removed in the grading step of the granulation process is appropriately set according to the particle size of the water-absorbing resin or water-absorbing agent as the final product. Preferably, the particle size of the coarse particles is 2000 μm or more, more preferably 850 μm or more. As described above, the micro powder is water-absorbing resin that has passed through a sieve with a mesh size of 150 μm.

[0325] (Particle diameter of dried polymer or dried polymer subjected to surface crosslinking)

[0326] From the viewpoint of reducing micronized powder, the mass-average particle size (D50) of the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer supplied for the granulation process is preferably 200 μm or more, more preferably 300 μm or more, further preferably 400 μm or more, and particularly preferably 500 μm or more. From the viewpoint of improving the efficiency of the crushing step, the mass-average particle size is preferably 2000 μm or less, further preferably 1500 μm or less, and 1000 μm or less. This mass-average particle size can be determined, for example, by the same method as described in columns 27-28 of U.S. Patent No. 7638570, "(3) Mass-average particle size (D50) and logarithmic standard deviation of particle size distribution".

[0327] (Particle diameter of water-absorbent resin powder)

[0328] From the viewpoint of water absorption performance, the mass-average particle size (D50) of the water-absorbing resin powder obtained through the granulation process is preferably 200 μm or more, more preferably 200 μm or more and 600 μm or less, further preferably 250 μm or more and 550 μm or less, and particularly preferably 300 μm or more and 500 μm or less. This mass-average particle size can be determined, for example, by the same method described in columns 27-28 of U.S. Patent No. 7,638,570, “(3) Mass-average particle size (D50) and logarithmic standard deviation of particle size distribution”.

[0329] From the viewpoint of producing high-quality water-absorbing resin (water absorbent), the main component of the water-absorbing resin powder is preferably particles with a particle size of 150 μm or more and 850 μm or less. The proportion of particles with a particle size of 150 μm or more and 850 μm or less contained in the water-absorbing resin powder is preferably 90% by mass or more and 100% by mass, more preferably 95% by mass or more and 100% by mass or less, further preferably 97% by mass or more and 100% by mass or less, and particularly preferably 99% by mass or more and 100% by mass or less. The proportion of particles with a particle size less than 150 μm and a particle size greater than 850 μm contained in the water-absorbing resin powder is preferably 10% by mass or less, more preferably 5% by mass or less, further preferably 3% by mass or less, and particularly preferably 1% by mass.

[0330] [3-2-8] Step of adding other additives

[0331] The manufacturing method of the present invention may include a step of adding other additives. The step of adding other additives refers to any step performed to impart various additional functions to the water-absorbing resin (water-absorbing agent) and to improve its water absorption performance; it is a step of adding the following other additives to the water-absorbing resin powder obtained through a surface crosslinking step. From the viewpoint of additive effect, it is preferable that the following other additives are present on the surface of each particle of the water-absorbing resin powder. Therefore, the step of adding other additives is preferably performed simultaneously with or separately from the surface crosslinking step, and more preferably after the surface crosslinking step.

[0332] Other additives can be listed in column [2-3-6], and the information in that column can be cited.

[0333] [3-2-9] Other steps

[0334] In addition to the steps described above, the manufacturing method of the present invention may further include, as needed, a rewetting step, a crushing step, a grading step, a granulation step, a conveying step, a storage step, a packaging step, and a preservation step.

[0335] Furthermore, the present invention may include the inventions shown below [1] to

[11] .

[0336] [1] A water-absorbing resin that fully satisfies the following conditions (a) to (d).

[0337] (a) The water absorption time of Vortex at 0°C is less than 30 seconds;

[0338] (b) The ratio of the Vortex water absorption time at 0°C to the Vortex water absorption time at 30°C (Vortex water absorption time ratio (0°C / 30°C)) is 5 or less;

[0339] (c) GEX value is 17 or higher;

[0340] (d) The CRC value exceeds 25g / g.

[0341] [2] The water-absorbing resin according to [1] contains a crosslinked polymer of an acid-containing unsaturated monomer, at least a portion of which is a neutralized acid-containing unsaturated monomer, and the neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts.

[0342] [3] The water-absorbing resin according to [1] or [2], wherein the molar ratio of the number of one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts to the total molar ratio of the neutralized acid-containing unsaturated monomer is 50 mol% or more and 100 mol% or less. [4] The water-absorbing resin according to any one of [1] to [3], wherein the mass-average particle size is 50 μm or more and 600 μm or less.

[0343] [5] The water-absorbing resin according to any one of [1] to [4], wherein the content of residual volatile components is less than 0.4% by mass relative to the total mass of the water-absorbing resin.

[0344] [6] The water-absorbing resin according to any one of [1] to [5], wherein the content of micro powder is 15% by mass or less relative to the total mass of the water-absorbing resin, and the micro powder is the water-absorbing resin that has passed through the sieve after the total mass of the water-absorbing resin has been classified using a sieve with a mesh size of 150 μm.

[0345] [7] A method for manufacturing a water-absorbing resin, comprising: a polymerization step, wherein a monomer composition containing an acid-containing unsaturated monomer and a monomer other than the acid-containing unsaturated monomer is subjected to crosslinking polymerization to obtain a hydrogel-like crosslinked polymer, wherein at least a portion of the acid-containing unsaturated monomer is a neutralized acid-containing unsaturated monomer, the neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts, wherein the total content of the acid-containing unsaturated monomer and the monomer other than the acid-containing unsaturated monomer in the monomer composition is 30% by mass or more and less than 55% by mass relative to the total mass of the monomer composition, and wherein the polymerization initiation temperature in the polymerization step is 60°C or more.

[0346] [8] In the method for manufacturing the water-absorbing resin according to [7], the solid content of the water-containing gel crosslinked polymer is 60% by mass or less relative to the total mass of the water-containing gel crosslinked polymer.

[0347] [9] The method for manufacturing the water-absorbing resin according to [7] or [8] further includes: a hydrogel pulverization step, wherein the hydrogel-like crosslinked polymer is pulverized to obtain particulate hydrogel.

[0348]

[10] The method for manufacturing a water-absorbing resin according to any one of [9], wherein the hydrogel pulverization step is a step of pulverizing the hydrogel-like crosslinked polymer in such a way that the mass-average particle size calculated from the solid components of the particulate hydrogel is less than 1 mm.

[0349]

[11] The method for manufacturing a water-absorbing resin according to any one of [7] to

[10] , wherein the polymerization step is a step of obtaining the hydrogel-like crosslinked polymer by polymerizing the monomer composition by aqueous solution polymerization.

[0350] This invention is not limited to the embodiments described above. Various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of this invention.

[0351] Examples

[0352] The present invention will be described in more detail with reference to the embodiments and comparative examples shown below, but the present invention is not limited thereto. Embodiments obtained by appropriately combining the technical means disclosed in the various embodiments are also included within the scope of the present invention.

[0353] It should be noted that the term "water-absorbing resin" as used below refers to granular dried material that has undergone a drying process, granular dried material that has undergone surface cross-linking, or water-absorbing resin powder, as well as water-absorbing resin powder that has undergone surface cross-linking. "Hydrogel" refers to hydrogel-like cross-linked polymer or particulate hydrogel that has not undergone a drying process.

[0354] In addition, unless otherwise specified, the electrical appliances used in the experimental examples (including the instruments used to measure the properties of the water-absorbing resins) all used a 60Hz, 200V or 100V power supply. Furthermore, unless otherwise specified, the properties of the water-absorbing resins and hydrogels described below were measured at room temperature (20℃~25℃) and a relative humidity of 50%RH±10%.

[0355] For convenience, "liter" is sometimes written as "l" or "L".

[0356] (Evaluation method)

[0357] The physical properties of the absorbent resins and hydrogels obtained in Examples 1-6 and Comparative Examples 1-4 were determined / evaluated using the methods shown below.

[0358] (A) "CRC"

[0359] The CRC of the absorbent resin was determined according to NWSP 241.0.R2 (15). Specifically, 0.2 g of absorbent resin was placed in a nonwoven bag and immersed in a large excess of 0.9% sodium chloride aqueous solution for 30 minutes to allow it to swell freely. Then, the water was removed by centrifuging (250G) for 3 minutes, and the mass after water removal was measured. The absorbency ratio of the absorbent resin (unit: g / g) was calculated.

[0360] (B) "PSD"

[0361] The mass-average particle size (D50) and the logarithmic standard deviation of the particle size distribution of the superabsorbent resin were determined by the same method as described in columns 27-28, “(3) Mass-average particle size (D50) and logarithmic standard deviation of particle size distribution”, of U.S. Patent No. 7,638,570. Here, “micron content” refers to the mass percentage (mass%) of the superabsorbent resin that passed through a 150 μm mesh sieve relative to the total mass of superabsorbent resin.

[0362] (C) "Moisture content and solid content"

[0363] The moisture content of the absorbent resin was determined according to NWSP. Specifically, the moisture content of the absorbent resin was determined by changing the mass of the sample to about 1.0 g, the drying temperature to 180 °C, and the drying time to 24 hours, as described in the method for determining moisture content in NWSP 230.0.R2 (15). Specifically, first, about 1.0 g of absorbent resin was accurately weighed (this mass is set as M (g)). The M (g) of absorbent resin was placed into an aluminum cup with a bottom diameter of 50 mm, and then the total mass W1 (g) of the sample placed in the cup was accurately weighed. Next, the cup was placed in an oven at an ambient temperature of 180 °C. After 24 hours, the cup was removed from the oven, and the total mass W2 (g) of the cup was accurately weighed. Using W1 (g) and W2 (g), the moisture content (100-α) (mass%) of the absorbent resin was calculated according to the following formula (4). It should be noted that α represents the solid content (mass%) of the water-absorbing resin.

[0364] The water content of the superabsorbent resin (100-α) (mass%) = {(W1-W2) / M}×100……(4)

[0365] The water content and solids content of the hydrogel were also determined according to NWSP. Specifically, the mass of the sample was changed to approximately 2.0 g, and the same method as the method for determining the water content of the superabsorbent resin described above was used. That is, the precise mass M' (g) of the sample (hydrogel), the total mass W3 (g) of the sample placed in the aluminum cup, and the total mass W4 (g) of the cup after being removed from the oven were accurately weighed. Then, using W3 (g) and W4 (g), the water content (100-α') (mass%) of the hydrogel was calculated according to the following formula (5) in the same manner as the method for determining the water content of the superabsorbent resin described above. It should be noted that α' is the solids content (mass%) of the hydrogel. Furthermore, the water content and solids content of the hydrogel determined here are used as the water content and solids content of the hydrogel for the calculation of other parameters.

[0366] The water content of the hydrogel (100-α') (mass%) = {(W3-W4) / M'}×100……(5)

[0367] (D) "0°C Vortex water absorption time and 30°C Vortex water absorption time"

[0368] To prepare a test solution, add 0.02 parts by mass of edible blue No. 1 (Brilliant Blue) to 1000 parts by mass of a 0.90% saline aqueous solution. Measure 50 ml of the test solution into a 100 ml beaker. Adjust the temperature of the test solution in the beaker to 0°C. While stirring at 600 rpm using a cylindrical stir bar (40 mm long and 8 mm wide), add 2.00 g of superabsorbent resin to the test solution. Then, according to the standards described in JISK 7224-1996 "Explanation of the Test Method for Water Absorption Rate of Superabsorbent Resin," the time from the moment the superabsorbent resin is added to the test solution is taken as the starting point, and the time from the moment the superabsorbent resin absorbs the saline solution and covers the tip of the stir bar is taken as the ending point. The time from this starting point to this ending point is defined as the "0°C Vortex water absorption time" (unit: seconds). It should be noted that if the time from this starting point to this ending point is less than 1.0 second, it should be marked to one decimal place.

[0369] In addition, a test solution with the liquid temperature adjusted to 30°C was used, and the "Vortex water absorption time at 30°C" was measured using the same method as the "Vortex water absorption time at 0°C". Then, using the "Vortex water absorption time at 0°C" and the "Vortex water absorption time at 30°C", the Vortex water absorption time ratio (0°C / 30°C) was calculated as (Vortex water absorption time at 0°C) / (Vortex water absorption time at 30°C). It should be noted that the measurements of the Vortex water absorption time at 0°C and 30°C were conducted at room temperature (20°C–25°C) and a relative humidity of 50%RH ± 10%.

[0370] (E) "Ext"

[0371] Add 1.0 g of water-absorbing resin to 200 ml of 0.9% sodium chloride aqueous solution, stir at 500 rpm for 16 hours, and then determine the amount of substance dissolved in the aqueous solution by pH titration. The measured value is taken as Ext (water-soluble component) (unit: mass%).

[0372] Furthermore, 2.0 g of hydrogel was used instead of 1.0 g of absorbent resin. Otherwise, the amount of substance dissolved in the aqueous solution was determined using the same method as for determining the Ext of the absorbent resin. Next, based on the water content of the hydrogel determined by method (C), the amount of substance was converted to solid content, thereby calculating the Ext (unit: mass%) in the hydrogel.

[0373] (F) "GEX"

[0374] Using the CRC and Ext (water-soluble component) of the water-absorbing resin determined by the method described above, the GEX value of the water-absorbing resin is calculated based on the following formulas (1) and (2). It should be noted that in the following formulas (1) and (2), CRC is set as y (g / g) and Ext is set as x (mass %).

[0375] When x>1, the GEX value = (y+17) / ln(x)……(1)

[0376] Where ln(x) is the natural logarithm of x.

[0377] When x≤1, the GEX value = (y) / (x)……(2)

[0378] (G) Particle diameter of particulate hydrogel

[0379] <Distribution of particle diameter, mass average particle diameter (Gel D50) of particulate hydrogel>

[0380] The particle size distribution and mass-average particle size (Gel D50) of particulate hydrogels were determined using the following method.

[0381] 20 g of particulate hydrogel (solid content α, mass%) at 20–25 °C was added to 1000 g of a 20% (mass) sodium chloride aqueous solution (hereinafter referred to as "surfactant aqueous solution") containing 0.08% (mass) sodium lauryl sulfate (surfactant) to prepare a dispersion. The dispersion was then stirred at 300 rpm for 16 hours using a stir bar with a length of 50 mm and a diameter of 7 mm. The container used was a cylindrical polypropylene container (21 cm high, 8 cm in diameter, with an internal volume of approximately 1.14 L).

[0382] After the stirring is completed, the dispersion is added from the top of a stacked sieve consisting of eight JIS standard sieves stacked on a rotating disc. All sieves in the stacked sieve have a diameter of 21 cm, and the mesh sizes from the top sieve are 8 mm, 4 mm, 2 mm, 1 mm, 0.60 mm, 0.30 mm, 0.15 mm, and 0.075 mm, respectively. Next, 100 g of an additional surfactant aqueous solution is used to wash out all the remaining particulate hydrogel from the container onto the stacked sieve. Then, the particulate hydrogel is graded as follows: while the stacked sieve is rotated by hand (speed: 20 rpm), 6000 g of an additional surfactant aqueous solution is injected from a height of 30 cm above the stacked sieve using a spray (72 holes, flow rate: 6.0 L / min). This operation is to ensure that the injection range (50 cm) is within the specified range. 2 The process involves injecting the gel four times, ensuring no part of the entire upper surface of the stacked sieve is missed. After dehydrating the particulate hydrogel on the first section of the stacked sieve for approximately 2 minutes, it is weighed. The same procedure is performed on the second and subsequent sections of the stacked sieve, weighing the particulate hydrogel remaining on each sieve after dehydration. It should be noted that the sieves constituting the stacked sieve are appropriately modified according to the particle size of the particulate hydrogel. For example, if the particulate hydrogel has a small (fine) particle size and clogging occurs in a sieve with a mesh size of 0.15 mm and / or 0.075 mm, the sieve with a mesh size of 0.15 mm and / or 0.075 mm is replaced with a larger diameter JIS standard sieve (30 cm diameter, 0.15 mm mesh and / or 30 cm diameter, 0.075 mm mesh) for grading.

[0383] Based on the mass of the particulate hydrogel remaining on each sieve, the proportion (mass%) of the particulate hydrogel relative to the total mass of the particulate hydrogel is calculated using the following equation (6). The mesh size of each sieve used in the grading is converted based on the following equation (7). The converted mesh size and the mass percentage of the particulate hydrogel remaining on each sieve are plotted on logarithmic probability paper. The value of the mesh size corresponding to 50% of the mass is read as the mass-average particle size (Gel D50) of the particulate hydrogel.

[0384] X(%)=(w / W)×100……(6)

[0385] R (mm) = (20 / W) 1 / 3 ×r……(7)

[0386] In equations (6) and (7), X is the mass % (%) of the particulate hydrogel remaining on each sieve after grading and dehydration; w is the mass (g) of the particulate hydrogel remaining on each sieve after grading and dehydration; W is the total mass (g) of the particulate hydrogel remaining on each sieve after grading and dewatering; R is the mesh size (mm) of the sieve when converted to particulate hydrogel before swelling; and r is the mesh size (mm) of the sieve used in grading.

[0387] <Solid content-converted mass average particle diameter (Solid D50) of particulate hydrogel>

[0388] Based on the solid content (α) of the particulate hydrogel and the mass-average particle size (Gel D50) of the particulate hydrogel determined by the method described in (C) above, the mass-average particle size converted from the solid content (the mass-average particle size of the particulate hydrogel after drying, Solid D50) is calculated according to the following formula (8).

[0389] Solid D50=Gel D50×(α / 100) 1 / 3 ... (8)

[0390] In the formula (8), Gel D50 is the mass-average particle size (μm) of the particulate hydrogel particles; α is the solid content (mass%) of the particulate hydrogel; and Solid D50 is the mass-average particle size (μm) of the particulate hydrogel converted to dry matter.

[0391] (H) "Amount of residual volatile component"

[0392] The amount of volatile components originating from organic solvents, etc., that are embedded inside the absorbent resin particles in the gas volatilized when a mixture of dimethylformamide (DMF) and 25% by mass phosphoric acid aqueous solution with absorbent resin particles is heated at 110°C is quantified. The measured value is converted into the value per 1g of absorbent resin as the residual volatile component amount. The specific steps are as follows.

[0393] (H-1) Preparation of calibration curve

[0394] Prepare heptane, dimethylformamide (DMF), and a 25% (w / w) aqueous solution of phosphoric acid in a sealable glass container. This is to suppress errors caused by volatilization during the assay. Cool them before use if necessary.

[0395] First, accurately weigh 0.15 g of heptane into a 200 mL volumetric flask, and add DMF to make a total of 200 mL, which is used as standard solution (1). Next, accurately measure 10 mL of standard solution (1) into a 20 mL volumetric flask using a pipette, and add DMF to make a total of 20 mL, diluting standard solution 1 by 2 times, which is used as standard solution (2).

[0396] Similarly, standard solution (3) is obtained by diluting standard solution (2) by 2 times. Furthermore, standard solution (4) is obtained by diluting standard solution (3) by 2 times. And, similarly, standard solution (5) is obtained by diluting standard solution (4) by 2 times.

[0397] Accurately measure 4 mL of standard solution (1) into a 20 mL vial and add it. Then, accurately measure 5 mL of 25% (w / w) phosphoric acid aqueous solution and add it. Quickly seal the vial using a rubber septum and an aluminum cap, and shake the vial for 1 minute to mix. Perform the same operation on standard solutions (2) to (5) to prepare the solution for the standard curve.

[0398] While stirring the vial, the mixture was heated at 110°C for 2 hours. Then, 1 mL of the gas phase fraction was injected into the gas chromatograph to obtain chromatograms of the solutions used to prepare each calibration curve. Calibration curves were constructed using the precisely weighed mass of heptane used in preparing the calibration curve solutions and the peak areas of the chromatograms.

[0399] (H-2) Measurement of amount of residual volatile component

[0400] Prepare DMF and a 25% (w / w) aqueous phosphoric acid solution. Accurately weigh 0.10 g of the absorbent resin obtained in the examples and comparative examples into 20 mL vials. Add 4 mL of DMF and 5 mL of the 25% (w / w) aqueous phosphoric acid solution to the vial. Quickly seal the vial using a rubber septum and an aluminum cap, and shake the vial for 1 minute to mix. While mixing by shaking, heat the vial at 110°C for 2 hours, then inject 1 mL of the gas phase into a gas chromatograph to obtain a chromatogram.

[0401] Based on the peak areas of the obtained chromatograms and the previously prepared standard curve, the amount of volatile components contained in the absorbent resin (accurate weighing value of 0.10 g) was calculated. The calculated value was then converted to a value per 1 g of absorbent resin, and this converted value was taken as the residual volatile component amount (mass %).

[0402] The following are the conditions for the gas chromatograph.

[0403] Device: GC-2010 Plus (manufactured by Shimadzu Corporation).

[0404] Filler: 25% squalane, Shimalite (NAW) (101) 80-100 mesh.

[0405] Column: 3.2mmφ×2.1m.

[0406] Column temperature: 80℃.

[0407] Inlet temperature: 180℃.

[0408] Detector temperature: 180℃.

[0409] Detector: FID.

[0410] Carrier gas: N2.

[0411] [Example 1]

[0412] (Preparation step of monomer composition)

[0413] The following materials were added to a 2-liter polypropylene container and mixed to prepare a monomer composition (a1):

[0414] Acrylic acid 372.3 g (5.17 mol); 10% by mass aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent 54.05 g (0.2 mol% relative to monomer); 0.1% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid (DTPA·3Na) 25.80 g (0.0011 mol% relative to monomer); 48.5% by mass aqueous solution of potassium hydroxide 131.5 g; deionized water (ion-exchanged water) 298.3 g.

[0415] Next, the monomer composition (a1) was cooled while being stirred. When the temperature (liquid temperature) of the monomer composition (a1) reached 50°C, 304.9 g of a 48.5% by mass potassium hydroxide aqueous solution, adjusted to 40°C, was added and mixed to prepare monomer composition (a2). At this point, the temperature of the monomer composition (a2) rose to 92.5°C due to the heat of neutralization.

[0416] (Polymerization step)

[0417] Next, 13.1 g of a 5.0% by mass potassium persulfate aqueous solution (0.047 mol% relative to the monomer) was added to the stirred monomer composition (a2) to obtain monomer composition (a'2). At this time, the temperature of monomer composition (a2) was 90°C. Then, monomer composition (a'2) (monomer concentration 43% by mass, acrylic acid neutralization rate 73 mol%) was immediately poured into a stainless steel tank container (bottom 340×340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open atmosphere. It should be noted that the time from the addition of potassium hydroxide aqueous solution to monomer composition (a1) until the monomer composition (a'2) was poured into the tank container was set to 1 minute. The tank container was preheated to a surface temperature of 50°C using a heating plate (NEO HOTPLATE HI-1000 / Iinouchi Seieido Co., Ltd.), and heating was continued until the polymerization was completed. The time from the observation of the temperature rise of the monomer composition (a'2) (from the start of polymerization) until the maximum temperature (above 100°C) is 10 seconds. Through the above polymerization reaction, a hydrogel-like crosslinked polymer (b1) is obtained.

[0418] (Pulverization step of hydrogel)

[0419] The obtained hydrogel-like crosslinked polymer (b1) was fed into a screw extruder for gel pulverization. The screw extruder used was a meat grinder with a die (extrusion nozzle) having a diameter of 100 mm, an orifice diameter of 6.4 mm, 83 orifices, an opening ratio of 34.0%, and a thickness of 10 mm; the screw outer diameter was 86 mm, and the shell inner diameter was 88 mm. Simultaneously with the supply of the hydrogel-like crosslinked polymer (b1), warm water at 80°C, steam, and a 10% (w / w) aqueous solution of lauryl dimethylaminoacetic acid betaine were supplied, and gel pulverization (first gel pulverization) was performed to obtain particulate hydrogel (c3). It should be noted that, relative to the solid composition of the hydrogel-like crosslinked polymer (b1), the supply was in the form of 1% (w / w) warm water at 80°C, 1% (w / w) steam, and 0.08% (w / w) lauryl dimethylaminoacetic acid betaine as an active ingredient. Next, warm water and steam were supplied in the same manner to further gel-crush the particulate hydrogel (c3) obtained in the first gel crushing (second gel crushing) to obtain particulate hydrogel (c2). The mass-average particle size (hereinafter referred to as "Solid D50") of the obtained particulate hydrogel (c2) was 239 μm.

[0420] Next, warm water and steam were supplied in the same manner, and the particulate hydrogel (c2) obtained in the second gel pulverization was further pulverized (third gel pulverization) to obtain particulate hydrogel (c1). The Solid D50 of the obtained particulate hydrogel (c1) was 150 μm.

[0421] (Drying step of particulate hydrogel)

[0422] The obtained particulate hydrogel (c1) was dried using a hot air dryer. The dryer was equipped with a cage (30cm × 20cm bottom) made of a metal mesh with a mesh size of 1.2mm. 500g of particulate hydrogel (c1) was spread roughly evenly on the bottom surface of the cage, and hot air at 190°C was blown from below for 30 minutes to obtain the dried product (A'1).

[0423] (Pulverization and classification step of dried product)

[0424] The cooled and dried material (A'1) was fed into a roller mill for pulverization and classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm. The component that passed through the 850 μm sieve but did not pass through the 150 μm sieve was collected to obtain the water-absorbing resin (A1).

[0425] [Example 2]

[0426] The particulate hydrogel (c2) obtained by pulverizing the second gel in Example 1 (the pulverization step of the hydrogel) was dried in the same manner as in Example 1 (the drying step of the particulate hydrogel). Then, the particulate hydrogel (c2) was pulverized and graded in the same manner as in Example 1 (the pulverization and grading step of the dried material) to obtain the water-absorbing resin (A2). That is, the particulate hydrogel (c2) was fed to the drying step of the particulate hydrogel without performing the third gel pulverization, and the water-absorbing resin (A2) was obtained by the same method as in Example 1.

[0427] [Example 3]

[0428] For the absorbent resin (A1) obtained in Example 1, 2 parts by weight of hydrophilic silica (REOLOSIL (trademark registered) QS-20, manufactured by Tokuyama Co., Ltd.) were mixed. The mixing was performed by placing 30g of the absorbent resin (A1) and the hydrophilic silica together in a 225ml mayonnaise jar (inner diameter 52mm, height 110mm), and vibrating at 750 CPM for 1 minute using a paint shaker (manufactured by Toyo Seiki Co., Ltd.). As a result, absorbent resin (A3) was obtained.

[0429] [Example 4]

[0430] The absorbent resin (A2) obtained in Example 2 was mixed with hydrophilic silica (REOLOSIL (registered trademark) QS-20, manufactured by Tokuyama Co., Ltd.) in the same manner as in Example 3 to obtain absorbent resin (A4).

[0431] [Example 5]

[0432] (Preparation step of monomer composition)

[0433] In Example 1 (Preparation Steps of the Monomer Composition), except for the following aspects, the monomer composition was prepared by the same method as in Example 1 to obtain monomer composition (a3):

[0434] The amount of 10% by mass of the added polyethylene glycol diacrylate aqueous solution (molecular weight 523) was changed to 24.32 g (0.09 mol% relative to the monomer); the amount of deionized water (ion-exchanged water) was changed to 328.1 g.

[0435] (Polymerization step)

[0436] In the polymerization step of Example 1, monomer composition (a3) ​​was used instead of monomer composition (a2), and otherwise polymerization was carried out by the same method to obtain hydrogel-like crosslinked polymer (b2).

[0437] (Pulverization step of hydrogel)

[0438] In Example 1 (the hydrogel pulverization process), a hydrogel-like crosslinked polymer (b2) was used instead of a hydrogel-like crosslinked polymer (b1). Otherwise, the same method was used for (first gel pulverization), (second gel pulverization), and (third gel pulverization) to obtain particulate hydrogels (c1'). Furthermore, warm water and steam were supplied to further pulverize the particulate hydrogels (c1') obtained in the third gel pulverization (fourth gel pulverization) to obtain particulate hydrogels (c4). The solid D50 of the obtained particulate hydrogels (c4) was 120 μm.

[0439] (Drying step of particulate hydrogel)

[0440] The obtained particulate hydrogel (c4) was dried in the same manner as in Example 1 (drying process of particulate hydrogel) to obtain the dried product (A'5).

[0441] (Pulverization and classification step of dried product)

[0442] The dried material (A'5) obtained was pulverized and graded in the same manner as in Example 1 (pulverization and grading process of dried material) to obtain water-absorbing resin (A5).

[0443] (Addition step)

[0444] The obtained absorbent resin (A5) was mixed with hydrophilic silica (REOLOSIL (registered trademark) QS-20, manufactured by Tokuyama Co., Ltd.) in the same manner as in Example 3 to obtain absorbent resin (A6).

[0445] [Example 6]

[0446] (Preparation step of monomer composition)

[0447] The monomer composition (a2) was obtained in the same manner as in Example 1 (preparation process of monomer composition).

[0448] (Polymerization step)

[0449] A hydrogel-like crosslinked polymer (b1) was obtained in the same manner as in Example 1 (polymerization process).

[0450] (Pulverization step of hydrogel)

[0451] As a gel pulverizing device, a twin-screw mixer with a main body (barrel) having two rotating shafts rotating in the same direction is used to pulverize the hydrogel-like crosslinked polymer (b1) to obtain particulate hydrogel (c5). Each rotating shaft is equipped with a circular plate-shaped disk that mainly serves as a pulverizing unit. The barrel has a jacketed structure and a gas inlet that passes through the jacket and introduces water vapor into the main body.

[0452] First, a 105°C heat medium is circulated inside the jacket to maintain the internal temperature of the main body (barrel) at 105°C. Then, the rotation speed is set to 50 rpm, and the hydrogel (b1) is fed into the inlet of the twin-screw mixer at a feed rate of 0.25 kg / min for gel pulverization to obtain particulate hydrogel (c5). At this time, while supplying the hydrogel-like crosslinked polymer (b1), water at 90°C is supplied, followed by a 10% by mass aqueous solution of lauryl dimethylaminoacetic acid betaine as a gel flow agent, and then water vapor at 0.6 MPa is supplied from the gas inlet. The amount of water supplied at 90°C is 11% by mass relative to the solid content of the hydrogel-like crosslinked polymer (b1). The amount of water vapor supplied at 0.6 MPa is 24% by mass relative to the solid content of the hydrogel-like crosslinked polymer (b1). The amount of lauryl dimethylaminoacetic acid betaine, as an active ingredient, is 0.08% by mass relative to the solid content of the hydrogel-like crosslinked polymer (b1). The diameter D of the disc used for gel pulverization was 50 mm, and the minimum gap between the barrel and the disc was 1 mm (2% of the disc diameter D). The solid D50 of the resulting particulate hydrogel (c5) was 147 μm.

[0453] (Drying step of particulate hydrogel)

[0454] The obtained particulate hydrogel (c5) was dried in the same manner as in Example 1 (drying process of particulate hydrogel) to obtain the dried product (A'7).

[0455] (Pulverization and classification step of dried product)

[0456] The dried material (A'7) obtained was pulverized and graded in the same manner as in Example 1 (pulverization and grading process of dried material) to obtain water-absorbing resin (A7).

[0457] (Step of adding silica to water-absorbent resin)

[0458] The resulting absorbent resin (A7) was mixed with hydrophilic silica in the same manner as in Example 5 (addition step) to obtain absorbent resin (A8).

[0459] [Example 7]

[0460] (Preparation step of monomer composition)

[0461] The following materials were added to and mixed in a 2-liter polypropylene container to prepare a monomer composition (a4):

[0462] Acrylic acid 440.1 g (6.11 mol); 10% by mass aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent 31.94 g (0.1 mol% relative to monomer); 0.1% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid (DTPA·3Na) 25.80 g (0.0009 mol% relative to monomer); deionized water (ion-exchanged water) 462.8 g.

[0463] Next, the monomer composition (a4) was heated to 45°C using a thermostatic bath. After heating, 328.8 g of a 30% by mass ammonia solution at 25°C was added, and the monomer composition (a5) was prepared by mixing. At this point, the temperature of the monomer composition (a5) rose to 91.0°C due to the heat of neutralization.

[0464] (Polymerization step)

[0465] Next, 15.5 g of a 5.0% by mass sodium persulfate aqueous solution (0.053 mol% relative to the monomer) was added to the stirred monomer composition (a5) to obtain monomer composition (a'5). At this time, the temperature of monomer composition (a5) was 90.5°C. Then, monomer composition (a'5) (monomer concentration 43% by mass, acrylic acid neutralization rate 73 mol%) was immediately poured into a stainless steel tank container (bottom 340×340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open atmosphere. It should be noted that the time from the addition of potassium hydroxide aqueous solution to monomer composition (a4) until the monomer composition (a'5) was poured into the tank container was set to 1 minute. The tank container was preheated to a surface temperature of 50°C using a heating plate (NEO HOTPLATE HI-1000 / Iinouchi Seieido Co., Ltd.), and heating was continued until the polymerization was completed. The time from the observation of the temperature rise of the monomer composition (a'5) (from the start of polymerization) until the maximum temperature (above 100°C) is 10 seconds. Through the above polymerization reaction, a hydrogel-like crosslinked polymer (b3) is obtained.

[0466] (Pulverization step of hydrogel)

[0467] The obtained hydrogel-like crosslinked polymer (b3) was fed into a screw extruder for gel pulverization. The screw extruder used was a meat grinder with a die head having a diameter of 100 mm, an orifice diameter of 6.4 mm, 83 orifices, an opening ratio of 34.0%, and a thickness of 10 mm at the front end (extrusion nozzle); the screw outer diameter was 86 mm; and the shell inner diameter was 88 mm. Simultaneously with the supply of the hydrogel-like crosslinked polymer (b3), warm water at 80°C, steam, and a 10% (w / w) aqueous solution of lauryl dimethylaminoacetic acid betaine were supplied, and gel pulverization (first gel pulverization) was performed to obtain particulate hydrogel (c5). It should be noted that, relative to the solid composition of the hydrogel-like crosslinked polymer (b3), the supply was in the form of 1% (w / w) of warm water at 80°C, 1% (w / w) of steam, and 0.08% (w / w) of lauryl dimethylaminoacetic acid betaine as an active ingredient. Next, warm water and steam were supplied to further gel-crush the particulate hydrogel (c5) obtained in the first gel crushing (second gel crushing) to obtain particulate hydrogel (c6). The mass-average particle size (Gel D50) of the obtained particulate hydrogel (c6) was 120 μm.

[0468] (Drying step of particulate hydrogel)

[0469] The obtained particulate hydrogel (c6) was dried using a hot air dryer. The dryer was equipped with a cage (30cm × 20cm bottom) made of a metal mesh with a mesh size of 1.2mm. 250g of particulate hydrogel (c1) was spread roughly evenly on the bottom surface of the cage, and hot air at 100°C was blown from below for 50 minutes to obtain the dried product (A'8).

[0470] (Pulverization and classification step of dried product)

[0471] The dried material (A'8) obtained was pulverized and graded in the same manner as in Example 1 (pulverization and grading process of dried material) to obtain water-absorbing resin (A9).

[0472] (Step of adding silica to water-absorbent resin)

[0473] The resulting absorbent resin (A9) was mixed with hydrophilic silica in the same manner as in Example 5 (addition step) to obtain absorbent resin (A10).

[0474] [Comparative Example 1]

[0475] (Preparation step of monomer composition)

[0476] The following materials are added to a 2-liter polypropylene container and mixed to prepare a monomer composition (ca1):

[0477] Acrylic acid 422.0 g (5.86 mol); 10% by mass aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent 61.26 g (0.2 mol% relative to monomer); 0.1% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid (DTPA·3Na) 25.80 g (0.00096 mol% relative to monomer); 48.5% by mass aqueous solution of sodium hydroxide 135.2 g; deionized water (ion-exchanged water) 323.5 g.

[0478] Next, the monomer composition (Ca1) was cooled while being stirred. When the temperature (liquid temperature) of the monomer composition (Ca1) reached 48°C, 217.3 g of a 48.5% by mass sodium hydroxide aqueous solution, adjusted to 40°C, was added and mixed to prepare the monomer composition (Ca2). At this point, the temperature of the monomer composition (Ca2) rose to 91.5°C due to the heat of neutralization.

[0479] (Polymerization step)

[0480] Next, 14.9 g of a 5.0% by mass sodium persulfate aqueous solution (0.053 mol% relative to the monomer) was added to the stirred monomer composition (ca2) to obtain monomer composition (ca'2). At this time, the temperature of the monomer composition (ca2) was 90°C. Then, the monomer composition (ca'2) (monomer concentration 43% by mass, acrylic acid neutralization rate 73 mol%) was immediately poured into a stainless steel tank-type container (bottom 340×340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open atmosphere. It should be noted that the time from the addition of 48.5% by mass sodium hydroxide aqueous solution to the monomer composition (ca2) until the monomer composition (ca'2) was poured into the tank-type container was set to 1 minute. The tank-type container was preheated to a surface temperature of 50°C using a heating plate (NEO HOTPLATE HI-1000 / Iinouchi Seieido Co., Ltd.), and heating was continued until the polymerization was completed. The time from the observation of the temperature rise of the monomer composition (ca'2) from the start of polymerization to the reaching of the maximum temperature (above 100°C) is 10 seconds. Through the above polymerization reaction, a hydrogel-like crosslinked polymer (cb1) is obtained.

[0481] (Pulverization step of hydrogel)

[0482] The obtained hydrogel-like crosslinked polymer (CB1) was fed into a screw extruder for gel pulverization. The screw extruder used was a die with a diameter of 100 mm, an orifice diameter of 6.4 mm, 83 orifices, an opening ratio of 34.0%, and a thickness of 10 mm at the front end (extrusion nozzle); the screw outer diameter was 86 mm, and the shell inner diameter was 88 mm. Simultaneously with the supply of the hydrogel-like crosslinked polymer (CB1), warm water at 80°C, steam, and a 10% (w / w) aqueous solution of lauryl dimethylaminoacetic acid betaine were supplied, and gel pulverization (first gel pulverization) was performed to obtain particulate hydrogel (CC2). It should be noted that, relative to the solid composition of the hydrogel (CB1), the supply was based on 1% (w / w) of warm water at 80°C, 1% (w / w) of steam, and 0.08% (w / w) of lauryl dimethylaminoacetic acid betaine as an active ingredient. Next, warm water and steam were supplied to further gel-crush the particulate hydrogel (cc2) obtained in the first gel crushing (second gel crushing) to obtain particulate hydrogel (cc1). The Solid D50 of the obtained particulate hydrogel (cc1) was 140 μm.

[0483] (Drying step of particulate hydrogel)

[0484] The obtained particulate hydrogel (cc1) was dried using a hot air dryer. The dryer was equipped with a cage (30cm × 20cm bottom) made of a metal mesh with a mesh size of 1.2mm. 500g of particulate hydrogel (cc1) was spread roughly evenly on the bottom surface of the cage, and hot air at 190°C was blown from below for 30 minutes to obtain the dried product (cA'1).

[0485] (Pulverization and classification step of dried product)

[0486] The cooled and dried material (cA'1) was fed into a roller mill for pulverization and classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm. The component that passed through the 850 μm sieve but did not pass through the 150 μm sieve was collected to obtain the water-absorbing resin (cA1).

[0487] [Comparative Example 2]

[0488] The absorbent resin (cA1) obtained in Comparative Example 1 was mixed with hydrophilic silica (REOLOSIL (registered trademark) QS-20, manufactured by Tokuyama Co., Ltd.) in the same manner as in Example 3 to obtain absorbent resin (cA2).

[0489] [Comparative Example 3]

[0490] (Preparation step of monomer composition)

[0491] A hydrogel-like crosslinked polymer (cb1) was obtained in the same manner as in Comparative Example 1 (polymerization process of monomer composition).

[0492] (Pulverization step of hydrogel)

[0493] In Example 6 (the process of pulverizing the hydrogel), a hydrogel-like crosslinking polymer (cb1) was used instead of the hydrogel-like crosslinking polymer (b1). Otherwise, the gel was pulverized using the same method to obtain particulate hydrogels (cc2). The solid D50 of the obtained particulate hydrogels (cc2) was 158 μm.

[0494] (Drying step of particulate hydrogel)

[0495] The obtained particulate hydrogel (cc2) was dried in the same manner as in Comparative Example 1 (drying process of particulate hydrogel) to obtain the dried product (cA'3).

[0496] (Pulverization and classification step of dried product)

[0497] The dried product (cA'3) obtained was pulverized and classified in the same manner as in Comparative Example 1 (pulverization and classification process of dried product) to obtain water-absorbing resin (cA3).

[0498] [Comparative Example 4]

[0499] A water-absorbing resin (cA4) was obtained according to Example 1 of Japanese Patent Publication No. 02-14925. Specifically, firstly, 72.1 g of acrylic acid was added to 22.2 g of deionized water. Then, 49.5 g of potassium hydroxide with a purity of 85% as a neutralizing agent and 0.01 g of N,N-methylenebisacrylamide as a divinyl group compound (0.0065 mol% relative to the monomer) were added sequentially. Through the above operations, an aqueous solution of potassium acrylate with a monomer concentration of 70% by mass (neutralization degree 75%) was prepared.

[0500] The potassium acrylate aqueous solution was kept at 70°C. 2.9 g of an 18% aqueous solution of ammonium persulfate (0.5% by mass relative to the total mass of potassium acrylate, free acrylic acid, and N,N-methylenebisacrylamide, hereinafter the same) and 1.7 g of a 30.6% aqueous solution of sodium bisulfite (0.5% by mass) were then mixed into the solution. The resulting mixture was then poured into a tank container, allowing it to spread into a layer approximately 10 mm thick. The polymerization reaction began after approximately 30 seconds and was completed within approximately 1 minute. The maximum temperature during this period was 120°C.

[0501] This yields a strip-shaped dried solid of potassium polyacrylate crosslinked polymer with a moisture content of 11%. This strip-shaped dried solid is equivalent to a hydrogel-like crosslinked polymer. The strip-shaped dried solid of potassium polyacrylate crosslinked polymer is then pulverized using a pulverizer, thereby producing a pulverized resin (cA4).

[0502] [Comparative Example 5]

[0503] (Preparation step of monomer composition)

[0504] The monomer composition (a2) was obtained in the same manner as in Example 1 (preparation process of monomer composition).

[0505] (Polymerization step)

[0506] A hydrogel-like crosslinked polymer (b1) was obtained in the same manner as in Example 1 (polymerization process).

[0507] (Pulverization step of hydrogel)

[0508] The die orifice diameter of the screw extruder was changed to 11.0 mm. Otherwise, the first gel pulverization was performed in the same manner as in Example 1 to obtain particulate hydrogel (cc3). The Solid D50 of the obtained particulate hydrogel (cc3) was 1568 μm.

[0509] (Drying step of particulate hydrogel)

[0510] The obtained particulate hydrogel (cc3) was dried in the same manner as in Example 1 (drying process of particulate hydrogel) to obtain the dried product (cA'5).

[0511] (Pulverization and classification step of dried product)

[0512] The dried material (cA'5) obtained was pulverized and graded in the same manner as in Example 1 (pulverization and grading process of dried material) to obtain water-absorbing resin (cA5).

[0513] (Step of adding silica to water-absorbent resin)

[0514] The resulting absorbent resin (cA5) was mixed with hydrophilic silica in the same manner as in Example 5 (addition step) to obtain absorbent resin (cA6).

[0515] [Result A: Properties of hydrogel and water-absorbent resin]

[0516] The properties of the hydrogels, particulate hydrogels, and water-absorbing resins prepared in Examples 1-7 and Comparative Examples 1-5 are shown in Tables 1-3 below. It should be noted that "hydrogel" in Table 1 below refers to the hydrogel obtained through the polymerization process and in its state before being pulverized in the hydrogel pulverization process, i.e., the hydrogel-like crosslinked polymer. Furthermore, "particulate hydrogel" in Table 2 below refers to the hydrogel in its pulverized state during the hydrogel pulverization process.

[0517] [Table 1]

[0518]

[0519] [Table 2]

[0520]

[0521] [Table 3]

[0522]

[0523] As shown in Table 3, the absorbent resins A1-A4, A6, A8, and A10 manufactured in Examples 1-7 fully satisfy the necessary conditions (a)-(d) and conform to the absorbent resins of the present invention. Furthermore, the manufacturing methods of the absorbent resins in Examples 1-7 conform to the manufacturing method of the present invention. Therefore, the absorbent resin of the present invention can be manufactured using the manufacturing method of the present invention.

[0524] [Examples 8 to 14, Comparative Examples 6 to 9: Freeze-thaw cycle time]

[0525] The curing time of antifreeze was measured using the water-absorbing resins A1-A4, A6, A8, and A10 manufactured in Examples 1-7, and the water-absorbing resins cA1-cA3 and cA6 manufactured in Comparative Examples 1-3 and 5, respectively. Specifically, the curing time of the antifreeze was measured by adding 1 g of water-absorbing resin to 10 ml of a 37% ethylene glycol aqueous solution in a 20 ml threaded tube (27 mm outer diameter).

[0526] Specifically, the curing time was determined using the following method. First, 2 parts by mass of Aurora (trademark registered) long-lasting antifreeze coolant (92% ethylene glycol concentration, manufactured by Tokyo Fine Chemical Co., Ltd.) and 3 parts by mass of deionized water were mixed to obtain a 37% by mass ethylene glycol aqueous solution. 10g of the obtained 37% by mass ethylene glycol aqueous solution was added to a threaded tube (27mm in diameter, 55mm in height, manufactured by Maruemu Co., Ltd., No. 5) and placed on a horizontal stand. 1g of absorbent resin was added to the threaded tube, and the 37% by mass ethylene glycol aqueous solution was allowed to swell under static conditions. The curing time was then measured until the liquid stopped seeping out when the threaded tube was tilted at 90°. The temperature of the 37% by mass ethylene glycol aqueous solution during the measurement was 0℃±1℃ and 30℃±1℃ (corresponding to "0℃" and "30℃" in Table 4 below, respectively).

[0527] [Result B]

[0528] The curing times of the antifreeze measured in Examples 8-14 and Comparative Examples 6-9 are shown in Table 4 below.

[0529] [Table 4]

[0530]

[0531] As shown in Table 4, it was confirmed that the water-absorbing resins A1 to A4, A6, A8 and A10 (Examples 8 to 14) conforming to the present invention are superior to the water-absorbing resins cA1 to cA3 and cA6 (Comparative Examples 6 to 9) conforming to the present invention in terms of curing time, especially curing time at low temperature.

[0532] As can be seen from the above, the absorbent resin of the present invention is particularly capable of sufficient swelling and gelation due to absorption by aqueous liquids such as antifreeze, even at low temperatures. Furthermore, as described above, the manufacturing method of the present invention can produce the absorbent resin of the present invention. Therefore, it can be said that this method can produce an absorbent resin that can sufficiently swell and gel due to absorption by aqueous liquids such as antifreeze, even at low temperatures.

[0533] Industrial applicability

[0534] The water-absorbing resin of one embodiment of the present invention and the method for manufacturing the water-absorbing resin of one embodiment of the present invention can be used to manufacture water-absorbing articles that can fully swell and gel due to absorption of aqueous liquids such as antifreeze even at low temperatures.

Claims

1. A water-absorbing resin that fully satisfies the following conditions (a) to (d): (a) The water absorption time of Vortex at 0°C is less than 30 seconds; (b) The ratio of the Vortex water absorption time at 0°C to the Vortex water absorption time at 30°C, i.e., the Vortex water absorption time ratio at 0°C / 30°C is 5.0 or less; (c) GEX value is 17 or higher; (d) The CRC value exceeds 25g / g.

2. The absorbent resin according to claim 1, wherein it comprises a cross-linked polymer of unsaturated monomers containing acid groups. At least a portion of the acid-containing unsaturated monomers are neutralized acid-containing unsaturated monomers, and, The neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts.

3. The water-absorbing resin according to claim 2, wherein, The molar percentage of one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts is 50 mol% or more and 100 mol% or less relative to the total molar percentage of the neutralized acid-containing unsaturated monomer.

4. The water-absorbing resin according to any one of claims 1 to 3, wherein, The mass-average particle size is greater than 50 μm and less than 600 μm.

5. The water-absorbing resin according to any one of claims 1 to 4, wherein, The content of residual volatile components is less than 0.4% by mass relative to the total mass of the absorbent resin.

6. The water-absorbing resin according to any one of claims 1 to 5, wherein, The content of the micronized powder is less than 15% by mass relative to the total mass of the absorbent resin. The micro powder is the water-absorbing resin that has passed through the sieve after being classified as a whole using a sieve with a mesh size of 150μm.

7. A method for manufacturing a water-absorbing resin, comprising: The polymerization process involves crosslinking a monomer composition containing an unsaturated monomer with an acidic group, or any monomer other than an unsaturated monomer with an acidic group, to obtain a hydrogel-like crosslinked polymer. At least a portion of the acid-containing unsaturated monomers are neutralized acid-containing unsaturated monomers. The neutralized acid-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts. Relative to the total mass of the monomer composition, the total content of the acid-containing unsaturated monomer and monomers other than the acid-containing unsaturated monomer in the monomer composition is 30% by mass or more and less than 55% by mass, and... The polymerization initiation temperature in the polymerization process is above 60°C.

8. The method for manufacturing the water-absorbing resin according to claim 7, wherein, The solid content of the hydrogel-like crosslinked polymer is less than 60% by mass relative to the total mass of the hydrogel-like crosslinked polymer.

9. The method for manufacturing the water-absorbing resin according to claim 7 or 8, wherein, The process further includes a hydrogel pulverization step, in which the hydrogel-like crosslinked polymer is pulverized to obtain particulate hydrogel.

10. The method for manufacturing the water-absorbing resin according to claim 9, wherein, The hydrogel pulverization process is a process of pulverizing the hydrogel-like crosslinked polymer in such a way that the mass-average particle size, calculated from the solid components of the particulate hydrogel, is less than 1 mm.

11. The method for manufacturing the water-absorbing resin according to any one of claims 7 to 10, wherein, The polymerization process is a process of polymerizing the monomer composition through aqueous solution polymerization to obtain the hydrogel-like crosslinked polymer.

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