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

Abstract

Provided is a water-absorbing resin which can absorb a water-based liquid such as an antifreeze to achieve a satisfactory level of swelling / gelation even under low-temperature environments. The water-absorbable resin has the following properties: (a) the value of Vortex at 0°C is 30 seconds or less; (b) the (Vortex at 0°C) / (Vortex at 30°C) ratio is 5 or less; (c) the GEX value is 17 or more; and (d) the value of CRC is greater than 25 g / g.

Description

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

[0001] The present invention relates to a water-absorbent resin and a method for producing the same.

[0002] A water-absorbent resin (SAP / Super Absorbent Polymer) is a water-swellable, water-insoluble polymer gelling agent. The water-absorbent resin is used as a material for products that utilize its ability to absorb liquids such as water, such as agricultural and horticultural water retention agents and industrial water-stopping agents, and is widely used in a variety of fields.

[0003] The water-absorbing resin can be produced from raw materials selected from a wide variety of monomers and hydrophilic polymers. In terms of water absorption performance, the water-absorbing resin mainly composed of a polymer composed of an acid group-containing unsaturated monomer is industrially produced most frequently.

[0004] Examples of conventional water-absorbent resins obtained by using an acid group-containing unsaturated monomer include the water-absorbent resins shown in (A) to (C) below.

[0005] (A) A water-absorbent resin consisting of a dried solid of a cross-linked polyacrylate, which is produced by heating a monomer composition containing potassium acrylate, a divinyl compound, and a polymerization initiator, and having a monomer concentration of 55 to 80% by weight, to carry out a polymerization reaction while evaporating water (Patent Document 1); (B) A superabsorbent resin produced by a production method which sequentially includes a step of neutralizing an acidic unsaturated monomer and a basic substance containing a potassium base in a molar ratio within the range of 1:0.45 to 0.99 to produce an unsaturated monomer, and a step of polymerizing the unsaturated monomer (Patent Document 2); (C) A superabsorbent resin comprising a base resin powder containing a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group, at least a portion of which is neutralized with a potassium salt; and a surface cross-linked layer formed on the base resin powder and containing a second cross-linked polymer obtained by further cross-linking the first cross-linked polymer with a surface cross-linking agent (Patent Document 3).

[0006] Japanese Patent Publication No. 58-071907 Korean Patent Publication No. 10-1274880 Korean Patent Publication No. 10-2020-0073044

[0007] Examples of uses of water-absorbent resins include solidification agents for (leaked or discarded) antifreeze, sandbags, and materials for solidification of leaked water such as nuclear power plant wastewater. On the other hand, conventional water-absorbent resins have a problem in that they are difficult to swell and gel, particularly in low-temperature environments, and cannot fully function as solidification agents, etc. However, no water-absorbent resin capable of solving the above problem has been provided to date.

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide a water-absorbent resin that can easily swell and gel by absorbing an aqueous liquid such as an antifreeze solution even in a low-temperature environment, thereby preventing the occurrence of the above problems. The "swelling and gelling" refers to the water-absorbent resin absorbing the aqueous liquid and becoming a swelled gel, thereby reducing the fluidity of the aqueous liquid.

[0009] As a result of intensive research, the present inventors have found that the water absorption performance of conventional water-absorbent resins changes depending on the ambient temperature, and that the performance deteriorates particularly in low-temperature environments, resulting in the above-mentioned problems, and have arrived at the present invention.

[0010] A water-absorbent resin according to one embodiment of the present invention relates to a water-absorbent resin that satisfies the following conditions (a) to (d):

[0011] (a) The 0°C vortex value is 30 seconds or less; (b) The ratio of the 0°C vortex value to the 30°C vortex value (vortex ratio (0°C / 30°C)) is 5.0 or less; (c) The GEX value is 17 or more; (d) The CRC value is more than 25 g / g.

[0012] A method for producing a water-absorbent resin according to one embodiment of the present invention is a method for producing a water-absorbent resin, comprising: a polymerization step of crosslinking-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 hydrogel-like crosslinked polymer, wherein at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer contains one or more 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 mass % or more and less than 55 mass % with respect to the mass of the entire monomer composition, and the polymerization initiation temperature in the polymerization step is 60° C. or more.

[0013] A water-absorbent resin according to one embodiment of the present invention has the effect of being able to sufficiently swell and gel when it absorbs an aqueous liquid such as an antifreeze solution even in a low-temperature environment. Also, a method for producing a water-absorbent resin according to one embodiment of the present invention has the effect of being able to produce a water-absorbent resin that can sufficiently swell and gel when it absorbs an aqueous liquid such as an antifreeze solution even in a low-temperature environment.

[0014] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to these, and various modifications are possible within the scope of the description. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. Note that "acid (salt)" means "acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic," respectively. Furthermore, the mass of the water-absorbent resin is expressed as a value converted into solid content, unless otherwise specified.

[0015] [1] Definition of Terms [1-1] Water-Absorbent Resin In this specification, the water-absorbent resin refers to a water-swellable, water-insoluble polymer gelling agent. Specifically, the water-absorbent resin refers to a polymer gelling agent whose water-swelling CRC exceeds 25 g / g and whose water-insolubility Ext is 50 mass% or less. "CRC" refers to the centrifuge retention capacity defined by NWSP241.0.R2(15), and "Ext" refers to the water-soluble content defined by NWSP241.0.R2(15). The term "NWSP" will be described later.

[0016] The water-absorbent resin of the present invention may be a crosslinked polymer obtained by crosslinking an acid group-containing unsaturated monomer, preferably a crosslinked polymer obtained by crosslinking an acid group-containing unsaturated monomer having a carboxyl group. The crosslinked polymer may be a hydrophilic crosslinked polymer. Furthermore, the water-absorbent resin is not limited to a form in which the entire amount is a crosslinked polymer, and may be a composition containing additives, etc., as long as the above-mentioned physical properties (CRC, Ext) satisfy the above-mentioned numerical ranges.

[0017] The water-absorbent resin may be surface-crosslinked (also known as post-crosslinked or secondary crosslinked), or may not be surface-crosslinked. In this specification, a water-absorbent resin for which a predetermined surface-crosslinking treatment has been completed may be separately referred to as a "surface-crosslinked water-absorbent resin." In addition, a water-absorbent resin adjusted to a predetermined moisture content and particle size may also be referred to as a water-absorbent resin powder or a water-absorbing agent.

[0018] [1-2] Fine Powder In this specification, the term "fine powder" refers to a particulate or powdery water-absorbent resin that passes through a sieve with a mesh size of 150 μm when the entire water-absorbent resin of the present invention is classified using the sieve. The fine powder preferably refers to a fine powder made of a water-absorbent resin whose main component is a poly(meth)acrylic acid (salt)-based crosslinked polymer. The water-absorbent resin constituting the fine powder may be a surface-crosslinked water-absorbent resin or a non-surface-crosslinked water-absorbent resin. The fine powder may also contain water and / or an additive described in the step of adding other additives described later. That is, the fine powder is not limited to a fine powder made of 100% water-absorbent resin (a polymer with a water content of 0%), but may also contain water and / or other minor components (e.g., inorganic fine particles) that are secondary raw materials for the water-absorbent resin.

[0019] [1-3] "Poly(meth)acrylic acid (salt)" In this specification, "poly(meth)acrylic acid (salt)" refers to poly(meth)acrylic acid and / or a salt thereof. Poly(meth)acrylic acid (salt) is a crosslinked polymer that contains, as a repeating unit, a structure derived from (meth)acrylic acid and / or a salt thereof (hereinafter also referred to as "(meth)acrylic acid (salt)") as the main component, and contains, as an optional component, a structure derived from an internal crosslinking agent.

[0020] The term "main component" means that the amount (content) of (meth)acrylic acid (salt) used 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 %, based on all the monomers used in the polymerization. Note that the term "main component" used in this specification also means that, even when a substance other than (meth)acrylic acid (salt) is used, the amount (content) of that substance used relative to the entire composition containing that substance is within the aforementioned range.

[0021] Here, the poly(meth)acrylic acid (salt) contained in the water absorbent resin is preferably a partially neutralized or fully neutralized poly(meth)acrylic acid salt, more preferably a monovalent salt, further preferably an alkali metal salt or an ammonium salt, particularly preferably one or more salts selected from the group consisting of a potassium salt, a lithium salt, and an ammonium salt.

[0022] [1-4] Definition of Evaluation Method The "NWSP" stands for "Non-Woven Standard Procedures - Edition 2015." The NWSP was jointly published by EDANA (European Disposables and Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to standardize evaluation methods for nonwoven fabrics and their products in the United States and Europe, and also includes a standard measurement method for water-absorbent resins. Unless otherwise specified, in this specification, the physical properties of the water-absorbent resin are measured in accordance with "Non-Woven Standard Procedures-Edition 2015". Regarding evaluation methods not described in NWSP, measurements are performed using the methods and conditions described in the examples.

[0023] [1-4-1] "CRC" (NWSP 241.0.R2(15)) "CRC" is an abbreviation for Centrifuge Retention Capacity, and means the absorption capacity (sometimes referred to as "absorption capacity") (unit: g / g) of a water-absorbent resin for a 0.90% by mass aqueous sodium chloride solution under no pressure for 30 minutes. As a measuring method for "CRC", for example, the method described in Examples can be adopted.

[0024] [1-4-2] "PSD" (NWSP 220.0.R2(15)) "PSD" is an abbreviation for Particle Size Distribution, and means the particle size distribution of a water-absorbent resin measured by sieve classification. In addition, as a parameter representing "PSD", for example, mass average particle diameter (D50) and logarithmic standard deviation of particle size distribution (σζ) can be mentioned. As a method for measuring the D50 and the σζ, for example, a method described in Examples, specifically, a method similar to the method described in "(3) Mass average particle diameter (D50) and logarithmic standard deviation of particle size distribution" in columns 27 to 28 of U.S. Pat. No. 7,638,570 can be adopted.

[0025] [1-4-3] "Moisture content and solid content" (NWSP 230.0.R2(15)) "Moisture content" means the ratio (mass%) of the loss on drying of the water absorbent resin to the mass before drying. "Solid content" is the ratio (mass%) of the mass after drying to the mass before drying. The moisture content and the solid content can be measured in accordance with NWSP. As a method for measuring these, for example, the methods described in the Examples can be adopted.

[0026] In this specification, the water content and solid content of the hydrogel refer to the water content and solid content of the hydrogel before drying, respectively. The water content and solid content of the hydrogel can be measured after the polymerization step described below and before the drying step described below. That is, the water content and solid content of the hydrogel may be measured on the hydrogel before gel crushing, or may be measured on the particulate hydrogel after gel crushing. The water content and solid content of the hydrogel may be measured in accordance with NWSP. The water content and solid content of the hydrogel can be measured, for example, as described in the Examples.

[0027] [1-4-4] "0°C Vortex and 30°C Vortex" In this specification, "vortex" is an index representing the water absorption rate of a water-absorbent resin, and means the time (unit: seconds) required for 2 g of water-absorbent resin to absorb 50 ml of a 0.90 mass % sodium chloride aqueous solution to a predetermined state. "0°C vortex" means the vortex measured when the temperature of the sodium chloride aqueous solution is set to 0°C. Furthermore, "30°C vortex" means the vortex measured when the temperature of the sodium chloride aqueous solution is set to 30°C. The "0°C vortex" and the "30°C vortex" can be measured, for example, by the method described in the Examples.

[0028] [1-4-5] "Ext" (NWSP241.0.R2(15)) "Ext" is an abbreviation for Extractables, and means the water-soluble content of the water-absorbent resin (the amount of water-soluble polymer in the water-absorbent resin). As a method for measuring the "Ext", for example, the method described in Examples can be adopted.

[0029] [1-4-6] "GEX" The "GEX value" is a value defined by the following formula (1) or (2) where the absorbency under no load (CRC) is y (g / g) and the water extractable content Ext is x (mass%). Generally, in the relationship between CRC and water extractable content, it is said that a water absorbent resin with a high CRC and a low water extractable content is preferable, but CRC and water extractable content are in a mutually contradictory relationship. Therefore, the GEX value is defined in order to evaluate the CRC and water extractable content as one parameter. The larger the GEX value, the higher the performance and the less the deterioration of physical properties.

[0030] If x>1, GEX value=(y+17) / ln(x) (1) where ln(x) is the natural logarithm of x.

[0031] When x≦1, the GEX value = (y) / (x) (2) [1-4-7] Mass average particle diameter (Solid D50) of particulate hydrogel in terms of solid content As used herein, the term "mass average particle diameter (Solid D50) of particulate hydrogel in terms of solid content" refers to the mass average particle diameter (D50) of particulate hydrogel obtained by gel pulverization, as described later. That is, the Solid D50 corresponds to the mass average particle diameter of particles obtained when the particulate hydrogel is dried. As a method for measuring the Solid D50, for example, the method described in the Examples can be adopted.

[0032] [1-4-8] "Amount of Residual Volatile Components" In this specification, the "amount of residual volatile components" refers to the amount of volatile components derived from organic solvents and the like used in the production process of a water-absorbent resin (e.g., a polymerization process, etc.) and incorporated into the water-absorbent resin. The volatile components are tightly sealed inside the water-absorbent resin. Therefore, even when the environment around the water-absorbent resin is adjusted to one in which the volatile components can generally be released by performing treatment such as heating and / or decompression, the volatile components are hardly released outside the water-absorbent resin. On the other hand, when the water-absorbent resin comes into contact with an aqueous liquid and forms a swollen gel, a part of the volatile components is released from inside the water-absorbent resin. As a method for measuring the "amount of residual volatile components," for example, the method described in the Examples can be adopted.

[0033] [1-5] Others In this specification, the range "X to Y" means "X or more, Y or less." Unless otherwise noted, the mass unit "t (ton)" means "Metric ton," and "ppm" means "ppm by mass" or "ppm by weight." Furthermore, "~acid (salt)" means "~acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic," respectively. For convenience, "liter" may be expressed as "l" or "L." Furthermore, when measuring trace components, amounts below the detection limit are expressed as N.D. (Non Detected).

[0034] [2] Embodiment 1: Water-absorbent resin The water-absorbent resin of the present invention satisfies the following conditions (a) to (d).

[0035] (a) The 0°C vortex value is 30 seconds or less; (b) The ratio of the 0°C vortex value to the 30°C vortex value (vortex ratio (0°C / 30°C)) is 5 or less; (c) The GEX value is 17 or more; (d) The CRC value is more than 25 g / g.

[0036] [2-1] Mechanism of the water-absorbent resin of the present invention In one embodiment of the present invention, satisfying the condition (a) above means that the water-absorbent resin has a sufficiently fast water-absorption rate when absorbing an aqueous liquid at a temperature of 0°C. Here, "absorbing an aqueous liquid at a temperature of 0°C" corresponds to setting the temperature around the water-absorbent resin to 0°C. Therefore, the water-absorption rate under the condition (a) above corresponds to the water-absorption rate of the water-absorbent resin in a low-temperature environment.

[0037] Furthermore, satisfying the condition (b) means that even when the temperature of the aqueous liquid absorbed by the water-absorbent resin is changed from 30°C to 0°C, the degree of decrease in the water absorption rate of the water-absorbent resin is sufficiently small. Here, the case where the temperature of the aqueous liquid absorbed by the water-absorbent resin is changed from 30°C to 0°C corresponds to the case where the temperature around the water-absorbent resin is changed from 30°C to 0°C. Therefore, the above case corresponds to the case where the temperature of the environment in which the water-absorbent resin exists is changed from 30°C to 0°C. In this specification, the term "low temperature" is not particularly limited, but means a temperature of 10°C or lower.

[0038] The water absorption performance, such as the water absorption rate, of a water-absorbent resin usually deteriorates in a low-temperature environment. However, the water-absorbent resin of the present invention satisfies the conditions (a) and (b) above, and therefore, even when the environmental temperature changes, for example, from room temperature to a low temperature, the degree of deterioration in the water absorption rate is small, and the water absorption rate in a low-temperature environment is sufficiently fast. Therefore, the water-absorbent resin of the present invention has an excellent water absorption rate in a low-temperature environment, and can exhibit a stable water absorption rate even when the environmental temperature changes.

[0039] In one embodiment of the present invention, satisfying the condition (c) above means that the CRC value is sufficiently high and Ext is sufficiently small.

[0040] In one embodiment of the present invention, satisfying the condition (d) above means that the CRC value is sufficiently high.

[0041] The water-absorbent resin of the present invention satisfies the conditions (c) and (d) above, and is therefore capable of absorbing a sufficient amount of aqueous liquid (water, antifreeze, leaked water, etc.), and when it absorbs water and becomes swollen, the amount of water-soluble matter that leaks out is small. In other words, the water-absorbent resin of the present invention absorbs a sufficient amount of aqueous liquid, swells and gels, reduces the fluidity of the aqueous liquid, and can maintain the aqueous liquid in a substantially solid state. In particular, the water-absorbent resin of the present invention satisfies the condition (d), and therefore can exhibit sufficient water-absorbing performance even in a low-temperature environment. Therefore, the water-absorbent resin of the present invention can satisfy the requirements (a) and (b) above.

[0042] As described above, the water-absorbent resin of the present invention satisfies the conditions (c) and (d) above, and therefore has excellent water-absorption performance at room temperature, and also satisfies the conditions (a) and (b) above, and therefore the excellent water-absorption performance does not deteriorate even in a low-temperature environment. Therefore, the water-absorbent resin of the present invention has the effect of being able to sufficiently absorb aqueous liquids such as antifreeze solutions even in a low-temperature environment, and being able to sufficiently swell and gel by absorbing the aqueous liquid.

[0043] [2-2] Physical Properties of the Water-Absorbent Resin of the Present Invention [2-2-1] 0°C Vortex The 0°C Vortex value of the water-absorbent resin of the present invention is preferably smaller from the viewpoint of increasing the water absorption rate in a low-temperature environment. The value is preferably 30 seconds or less, 20 seconds or less, or even 15 seconds or less. The value may usually be 1 second or more, and is preferably 2 seconds or more.

[0044] [2-2-2] Vortex Ratio (0°C / 30°C) The smaller the Vortex ratio (0°C / 30°C) of the water-absorbent resin of the present invention, the better, from the viewpoint of realizing a stable water absorption rate regardless of changes in environmental temperature. The value is 5.0 or less, and preferably 4.0 or less. Furthermore, the value may usually be 1.0 or more, and preferably 2.0 or more.

[0045] [2-2-3] 30°C Vortex The 30°C Vortex value of the water-absorbent resin of the present invention is preferably smaller from the viewpoint of increasing the water absorption rate in an environment of 30°C. The value is, for example, preferably 20 seconds or less, more preferably 10 seconds or less. Furthermore, the value may usually be 0.5 seconds or more, and is preferably 1 second or more.

[0046] [2-2-4] GEX The GEX value of the water-absorbent resin of the present invention is preferably large because the water-absorbent resin can absorb and retain a sufficient amount of aqueous liquid, thereby rapidly swelling and gelling. The value is 17 or more, preferably 18 or more. Furthermore, the value can usually be 30 or less, preferably 28 or less.

[0047] [2-2-5] CRC The CRC value of the water-absorbent resin of the present invention is also preferably high, since the water-absorbent resin can absorb and retain a sufficient amount of aqueous liquid, thereby rapidly swelling and gelling. The value is preferably greater than 25 g / g, and is preferably 30 g / g or greater. Furthermore, the value can typically be 50 g / g or less, and is preferably 45 g / g or less. The CRC of the water-absorbent resin of the present invention varies depending on various conditions, such as polymerization conditions (e.g., monomer concentration, neutralization rate), gel crushing conditions, drying conditions, and surface cross-linking conditions. Furthermore, by adjusting the amount of internal cross-linking agent used depending on the various conditions, the CRC of the water-absorbent resin of the present invention can be adjusted to a value greater than 25 g / g. Typically, the greater the amount of internal cross-linking agent used, the lower the CRC of the resulting water-absorbent resin, and the smaller the amount of internal cross-linking agent used, the higher the CRC of the resulting water-absorbent resin. In detail, when the amount of the internal cross-linking agent used is small, the distance between cross-linking points of the cross-linked polymer constituting the obtained water absorbent resin becomes long, and the network structure of the cross-linked polymer can be sufficiently expanded when absorbing water molecules, so that the CRC of the water absorbent resin becomes high.

[0048] [2-2-6] Ext The Ext of the water-absorbent resin of the present invention is usually 1% by mass or more and 40% by mass or less, preferably 2% by mass or more and 35% by mass or less, more preferably 3% by mass or more and 30% by mass or less, even more preferably 4% by mass or more and 25% by mass or less, and particularly preferably 5% by mass or more and 20% by mass or less.

[0049] [2-2-7] Mass Average Particle Diameter The lower limit of the mass average particle diameter (D50) of the water-absorbent resin of the present invention is preferably 50 μm or more, more preferably 100 μm or more, even more preferably 150 μm or more, and particularly preferably 200 μm or more. The upper limit of the mass average particle diameter (D50) is preferably 600 μm or less, more preferably 550 μm or less, even more preferably 500 μm or less, and particularly preferably 450 μm or less. The water-absorbent resin of the present invention can suitably improve its water absorption performance by having a mass average particle diameter (D50) within the above-mentioned range. The mass average particle diameter (D50) can be measured using the method described in the Examples below, i.e., a method similar to the method described in U.S. Pat. No. 7,638,570, columns 27-28, "(3) Mass Average Particle Diameter (D50) and Logarithmic Standard Deviation of Particle Size Distribution."

[0050] [2-2-8] Particle Size Distribution The water-absorbent resin of the present invention preferably has a proportion of particles having a particle diameter of more than 850 μm of 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. The water-absorbent resin of the present invention contains particles having a particle diameter of 150 to 850 μm in an amount of 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. The water-absorbent resin of the present invention ideally contains 100% by mass of particles having a particle diameter of 150 to 850 μm. The water-absorbent resin of the present invention has a logarithmic standard deviation (σζ) of the particle size distribution of preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and even more preferably 0.27 to 0.35.

[0051] [2-3] Components of Water-Absorbent Resin [2-3-1] Crosslinked Polymer The water-absorbent resin of the present invention typically contains a crosslinked polymer of an acid group-containing unsaturated monomer, and preferably contains the crosslinked polymer as a main component. The crosslinked polymer is preferably a hydrophilic crosslinked polymer. The crosslinking polymerization to obtain the crosslinked polymer may be crosslinking polymerization carried out in the presence of a crosslinking agent (internal crosslinking agent), or may be crosslinking polymerization carried out by self-crosslinking of a monomer. The crosslinked polymer may contain a monomer other than the acid group-containing unsaturated monomer as a monomer. In this specification, the term "monomer" is intended to include a neutralized salt unless otherwise specified.

[0052] <Acid Group-Containing Unsaturated Monomer> The "acid group" in the acid group-containing unsaturated monomer is not particularly limited, and examples thereof include a carboxy group, a sulfonic group, and a phosphate group, with a carboxy group being preferred. Examples of the acid group-containing unsaturated monomer include (meth)acrylic acid, maleic acid (anhydride), itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-hydroxyethyl (meth)acryloyl phosphate. From the viewpoint of the water absorption performance of the water-absorbent resin of the present invention, preferred are (meth)acrylic acid, maleic acid (anhydride), itaconic acid, and cinnamic acid, with a more preferred being (meth)acrylic acid.

[0053] In the water absorbent resin of the present invention, at least a part of the acid group-containing unsaturated monomer may be a neutralized acid group-containing unsaturated monomer. The "neutralized acid group-containing unsaturated monomer" means a neutralized salt obtained by neutralizing the acid group.

[0054] It is preferable that the ratio of the number of moles of the neutralized salt to the number of moles of all the acid group-containing unsaturated monomers (neutralization rate) is a predetermined value or more, from the viewpoint of making the CRC of the water absorbent resin of the present invention exceed 25 g / g. From this viewpoint, the neutralization rate is preferably 40 mol% or more, more preferably 50 mol% or more, and even more preferably 60 mol% or more.

[0055] On the other hand, the upper limit of the neutralization rate is 99 mol % or less.

[0056] The neutralized salt preferably includes a salt with a monovalent cation, more preferably includes one or more salts selected from alkali metal salts, ammonium salts, and amine salts, and further preferably includes one or more salts selected from alkali metal salts and ammonium salts.

[0057] Here, the water absorption and retention by the water absorbent resin of the present invention occurs when cations are released from the neutralized acid groups in the neutralized salt, and water molecules (H ２ Therefore, the easier it is for the cation to be released, the better the water absorption and retention of the water-absorbent resin of the present invention will be, and the easier it will be for swelling and gelation to occur.

[0058] The desorption generally does not occur easily at low temperatures, and therefore, at low temperatures, differences in the state of swelling and gelation tend to appear between cases where cations that are easily desorbed and cases where cations that are difficult to desorb are used.

[0059] Therefore, by using a cation that is easily released as the cation, the water absorbent resin of the present invention becomes more likely to swell and gel, particularly at low temperatures. Therefore, by including a cation that is easily released as the cation, the water absorbent resin of the present invention can improve its water absorption rate in a low-temperature environment.

[0060] Here, the potassium cation (K ＋ ) and ammonium cation (NH ４ ＋ ) are both cations that are easily released because they have a large ionic radius and the bond with the group after release is weak. ＋ ) has a small ionic radius but a high ionization tendency, and therefore corresponds to a cation that is easily desorbed. For these reasons, it is particularly preferable that the neutralized salt contains one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts.

[0061] For the reasons described above, when the neutralized salt contains a larger amount of salts containing cations that are easily released, the water-absorbing resin of the present invention can further improve its water absorption rate in a low-temperature environment. From this viewpoint, the ratio of the number of moles of one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts to the total number of moles of the neutralized salts 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 ratio of the number of moles of the one or more salts to the total number of moles of the neutralized salts can be 100 mol% or less, preferably 95 mol% or less, and more preferably 90 mol% or less.

[0062]

[0113] The content of the acid group-containing unsaturated monomer relative to the total amount of monomers serving as raw materials of the water absorbent resin (acid group-containing unsaturated monomer+monomers other than the acid group-containing unsaturated monomer) is preferably 50 mol% to 100 mol%, more preferably 70 mol% to 100 mol%, still more preferably 90 mol% to 100 mol%, and particularly preferably substantially 100 mol%.

[0063] <Monomers Other than Acid Group-Containing Unsaturated Monomers> The monomers other than the acid group-containing unsaturated monomers may be any compounds that can be polymerized to form a water-absorbent resin. Examples thereof include amide group-containing unsaturated monomers such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone.

[0064] <Internal Crosslinking Agent> The crosslinked polymer may contain an internal crosslinking agent, if necessary. The internal crosslinking agent is not particularly limited, and known agents can be used, including compounds having a total of two or more unsaturated bonds or reactive functional groups in one molecule.

[0065] Examples of the internal crosslinking agent include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, and glycidyl (meth)acrylate. Of these, one or more can be used in consideration of reactivity, and it is particularly preferred to use a compound having two or more polymerizable unsaturated groups.

[0066] The amount (content) of the internal crosslinking agent used 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, based on the total amount of the acid group-containing monomer.

[0067] The greater the amount of internal crosslinking agent used, the lower the CRC of the resulting water-absorbent resin, and the smaller the amount of internal crosslinking agent used, the higher the CRC of the resulting water-absorbent resin. In particular, when the amount of internal crosslinking agent used is small, the distance between crosslinking points of the crosslinked polymer constituting the resulting water-absorbent resin becomes longer, and the network structure of the crosslinked polymer can be sufficiently expanded when absorbing water molecules, so that the water-absorbent resin exhibits a high CRC. Since the amount (content) of the internal crosslinking agent used in the water-absorbent resin of the present invention is as described above, the CRC is controlled to a value exceeding 25 g / g.

[0068] [2-3-2] Surface Crosslinking Agent The water-absorbent resin of the present invention may be a surface-crosslinked water-absorbent resin. Therefore, the water-absorbent resin of the present invention may contain a surface crosslinking agent. As the surface crosslinking agent, a surface crosslinking agent that can be generally used may be adopted, and may be a surface crosslinking agent that can react with a plurality of functional groups (preferably a plurality of carboxy groups) of the water-absorbent resin. The surface crosslinking agent may preferably be a surface crosslinking agent that can form a covalent bond or an ionic bond with the functional group, and more preferably be a surface crosslinking agent that can form a covalent bond with the functional group.

[0069] Specific examples of the surface cross-linking agent include polyhydric alcohol compounds such as 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, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, and sorbitol; ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, and diglycerol polyglycidyl ether. epoxy compounds such as sorbitol, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidol, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether; polyvalent amine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine, and inorganic or organic salts thereof; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; aziridine compounds such as polyaziridine; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline, bisoxazoline, and polyoxazoline; carbonic acid derivatives such as urea, thiourea, guanidine, dicyandiamide, and 2-oxazolidinone;1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, 1,3-dioxopan-2-one alkylene carbonate compounds such as methyl acrylate; haloepoxy compounds such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and polyvalent amine adducts thereof; oxetane compounds; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; polyvalent metal compounds such as hydroxides, chlorides, sulfates, nitrates, or carbonates of zinc, calcium, magnesium, aluminum, iron, zirconium, and the like; and the like. Two or more of these may be used in combination. Among the surface crosslinking agents, one or more selected from polyvalent metal ions, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds are preferred.

[0070]

[0122] In the case where the water absorbent resin of the present invention contains a surface crosslinking agent, the content of the surface crosslinking agent 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 further preferably 0.02% by mass or more and 3% by mass or less, relative to the mass of the entire water absorbent resin.

[0071] [2-3-3] Fine Powder The water-absorbent resin of the present invention may contain the fine powder. The content of the fine powder is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to the mass of the entire water-absorbent resin. Furthermore, the content of the fine powder is particularly preferably 0% by mass. An example of a method for controlling the content to a specific value (for example, 15% by mass or less) includes a method of removing by-produced fine powder by carrying out a classification step described below during production.

[0072] When the content is 15% by mass or less with respect to the mass of the entire water-absorbent resin, it is possible to prevent a decrease in the water-absorbing performance of the water-absorbent resin of the present invention. Note that the content indicates the proportion of the water-absorbent resin that passes through a sieve with an opening of 150 μm.

[0073] [2-3-4] Residual Volatile Components In the water-absorbent resin of the present invention, the content of the residual volatile components is preferably 0.4 mass % or less relative to the mass of the entire water-absorbent resin. The water-absorbent resin of the present invention has the advantage that, due to this constitution, unpleasant odors during water absorption and a decrease in absorption performance due to hydrophobicity can be reduced.

[0074] From the viewpoint of making it easier to obtain the above-mentioned advantages, the water absorbent resin of the present invention more preferably has a content of the residual volatile components of 0.2 mass % or less relative to the mass of the entire water absorbent resin, and particularly preferably does not substantially contain the residual volatile components. Here, "substantially does not contain the residual volatile components" means that the amount of the residual volatile components obtained by the measurement method described below is "undetectable."

[0075] As described above, the residual volatile component is derived from an organic solvent used in polymerization during the production process, etc. Therefore, a water absorbent resin that does not substantially contain the residual volatile component can be produced, for example, by carrying out a polymerization step described later in a form that does not use an organic solvent, such as aqueous solution polymerization, and by adopting a method that does not use an organic solvent, etc. in other steps.

[0076] Furthermore, even when an organic solvent or the like is used in the production process, the content of the remaining volatile components can be kept within the above-mentioned preferred range by using a small amount of the organic solvent.

[0077] [2-3-5] Polyhydric Alcohol It is preferable that the water absorbent resin of the present invention is substantially free of polyhydric alcohol. Here, "substantially free of polyhydric alcohol" means that the measurement result of the polyhydric alcohol in the water absorbent resin of the present invention is "undetectable." Since the water absorbent resin of the present invention is substantially free of the polyhydric alcohol, it has the advantage of being able to reduce unpleasant odors during water absorption.

[0078] The polyhydric alcohol can generally be used as a crosslinking agent (internal crosslinking agent, surface crosslinking agent) in a method for producing a water absorbent resin. Therefore, the water absorbent resin that does not substantially contain the polyhydric alcohol can be produced by a method that does not use a polyhydric alcohol, for example, a method that uses a compound other than a polyhydric alcohol as a crosslinking agent. Furthermore, when the water absorbent resin of the present invention contains a polyhydric alcohol, the content thereof is preferably 5 mass % or less, more preferably 3 mass % or less, with respect to the mass of the entire water absorbent resin.

[0079] [2-3-6] Other Additives The water-absorbent resin of the present invention may contain other additives in addition to the above-described components. The additives may be additives that are generally contained in water-absorbent resins, and are not particularly limited. Examples of the additives include chelating agents, organic reducing agents, inorganic reducing agents, oxidizing agents, hydroxycarboxylic acid compounds, surfactants, phosphorus-containing compounds, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, polyvalent metal salts, cationic polymers, and inorganic fine particles. The additives may be one or a mixture of two or more of the additives described above. The content of the additives is not particularly limited as long as the content does not impair the functionality of the water-absorbent resin of the present invention, and is preferably 5% by mass or less, more preferably 3% by mass or less, based on the total mass of the water-absorbent resin.

[0080] <Polyvalent Metal Salt> When the water absorbent resin of the present invention contains a polyvalent metal salt as the other additive, preferred examples of the polyvalent metal of the polyvalent metal salt include aluminum, zirconium, etc. Preferred examples of the usable polyvalent metal salt include aluminum lactate and aluminum sulfate, and more preferred examples are aluminum sulfate.

[0081] The content (addition amount) of the polyvalent metal salt is preferably 3.6×10 metal cations per 1 g of the water-absorbent resin (water-absorbent resin powder). －５ less than 2.8 x 10 moles, more preferably less than 2.8 x 10 moles －５ mol, more preferably less than 2.0 × 10 －５Here, the water-absorbent resin (water-absorbent resin powder) means a water-absorbent resin that does not contain the other additives.

[0082] <Cationic Polymer> When the water-absorbent resin of the present invention contains a cationic polymer as the other additive, the cationic polymer is preferably a compound exemplified in U.S. Pat. No. 7,098,284. Among the compounds exemplified in U.S. Pat. No. 7,098,284, vinylamine polymer is preferred. The content (addition amount) of the cationic polymer is preferably less than 2.5 parts by mass, more preferably less than 2.0 parts by mass, and even more preferably less than 1.0 part by mass, relative to 100 parts by mass of the water-absorbent resin (water-absorbent resin powder). Here, the water-absorbent resin (water-absorbent resin powder) means a water-absorbent resin that does not contain the other additives.

[0083] <Inorganic fine particles> When the water absorbent resin of the present invention contains inorganic fine particles as the other additives, examples of the inorganic fine particles include minerals such as talc, kaolin, fuller's earth, hydrotalcite, bentonite, activated clay, barite, natural asphalt, strontium ore, ilmenite, and perlite; aluminum compounds such as aluminum sulfate 14-18 hydrate (or anhydrous aluminum sulfate), potassium aluminum sulfate 12-hydrate, sodium aluminum sulfate 12-hydrate, ammonium aluminum sulfate 12-hydrate, aluminum chloride, polyaluminum chloride, and aluminum oxide; other polyvalent metal salts such as calcium phosphate, polyvalent metal oxides, and polyvalent metal hydroxides; hydrophilic amorphous silicas; and oxide complexes such as silicon oxide-aluminum oxide-magnesium oxide complexes, silicon oxide-aluminum oxide complexes, and silicon oxide-magnesium oxide complexes. Two or more of these may be used in combination. The amount of the inorganic fine particles added is preferably less than 2.0 parts by mass, more preferably less than 1.5 parts by mass, and even more preferably less than 1.0 part by mass, relative to 100 parts by mass of the water-absorbent resin (water-absorbent resin powder). Here, the water-absorbent resin (water-absorbent resin powder) means a water-absorbent resin that does not contain the other additives.

[0084] [2-4] Production Method As a method for producing the water-absorbing resin of the present invention, for example, the following production method can be adopted.

[0085] [3] Embodiment 2: Manufacturing method of water-absorbent resin A manufacturing method of a water-absorbent resin according to one embodiment of the present invention (hereinafter referred to as "the manufacturing method of the present invention") is a manufacturing method of a water-absorbent resin, comprising: a polymerization step of crosslinking-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 hydrogel-like crosslinked polymer, wherein at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer contains one or more 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 mass % or more and less than 55 mass % with respect to the mass of the entire monomer composition, and the polymerization initiation temperature in the polymerization step is 60°C or more.

[0086] [3-1] Mechanism of the Production Method of the Present Invention The production method of the present invention comprises a polymerization step of crosslinking 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 hydrogel-like crosslinked polymer, wherein at least a portion of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer comprises one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts.

[0087] Therefore, the water-absorbent resin produced by the production method of the present invention contains a crosslinked polymer, and the monomer constituting the crosslinked polymer is a potassium cation (K ＋ ), lithium cation (Li ＋ ) or ammonium cation (NH ４ ＋ ) containing acid groups neutralized by

[0088] Here, as described in the above section [2-3-1], K ＋ , Li ＋ and N.H. ４ ＋The water-absorbing resin having an acid group neutralized by K ＋ , Li ＋ and N.H. ４ ＋ Therefore, the water-absorbent resin has a small 0°C vortex value and a small vortex ratio (0°C / 30°C), and satisfies the conditions (a) and (b).

[0089] In the production method of the present invention, the total content of the acid group-containing unsaturated monomer and monomers other than the acid group-containing unsaturated monomer in the monomer composition is 30% by mass or more and less than 55% by mass, based on the total mass of the monomer composition. Herein, "the total content of the acid group-containing unsaturated monomer and monomers other than the acid group-containing unsaturated monomer" means the content (concentration) of all monomers in the monomer composition. Hereinafter, "the total content of the acid group-containing unsaturated monomer and monomers other than the acid group-containing unsaturated monomer" will also be referred to as "the content of all monomers in the monomer composition."

[0090] When the content of all monomers in the monomer composition exceeds 55 mass %, the content of water-soluble matter in the obtained water absorbent resin may increase, and the GEX value described later may not fall within a desired range.

[0091] In the production method of the present invention, since the content of all monomers in the monomer composition is 30 mass% or more, the crosslinking polymerization reaction proceeds appropriately, and a water-absorbent resin having a desired content of water-soluble matter and a CRC of more than 25 g / g can be produced. It is considered that the water-absorbent resin contains a desired amount of water-soluble matter and has a CRC controlled to more than 25 g / g due to the above-mentioned configuration.

[0092] As described above, the water absorbent resin produced by the production method of the present invention has a high CRC and a low content of water-soluble matter, and therefore the GEX value is controlled to be not less than 17. Therefore, the production method of the present invention can produce a water absorbent resin that satisfies the conditions (c) and (d) above.

[0093] As described above, the production method of the present invention can produce the water-absorbing resin of the present invention.

[0094] In the production method of the present invention, the polymerization initiation temperature in the polymerization step is 60° C. or higher. When preparing the "at least partially neutralized acid group-containing unsaturated monomer" in the monomer composition to be subjected to the polymerization step, the temperature of the monomer composition generally reaches 60° C. or higher due to heat of neutralization. Therefore, it is usually not necessary to separately heat the monomer composition in order to control the polymerization initiation temperature to 60° C. or higher.

[0095] On the other hand, when the temperature of the monomer composition does not reach 60°C, the monomer composition may be heated separately to raise the temperature of the monomer composition to 60°C or higher. The heating method therefor may be any known heating method, and is not particularly limited. Furthermore, since the polymerization initiation temperature is 60°C or higher, the production method of the present invention exhibits the effect of excellent productivity since polymerization can be carried out in a short time.

[0096] [3-2] Each step constituting the production method of the present invention The production method of the present invention includes the polymerization step, and may include general steps constituting a production method of a water-absorbent resin other than the polymerization step. Each step that can constitute the production method of the present invention will be described in detail below.

[0097] [3-2-1] Step of Preparing Monomer Composition The production method of the present invention may include a step of preparing a monomer composition containing a monomer that is a raw material for a water-absorbent resin. The monomer composition is usually an aqueous solution containing an acid group-containing unsaturated monomer as a main component and optionally containing a monomer other than the acid group-containing unsaturated monomer. The monomer composition is preferably a homogeneous aqueous solution. On the other hand, as the monomer composition, a slurry liquid of a monomer (a dispersion liquid having a monomer concentration exceeding a saturation concentration) can also be used within a range in which the water absorption performance of the obtained water-absorbent resin is not reduced.

[0098] <Acid Group-Containing Unsaturated Monomer> As the acid group-containing unsaturated monomer, the acid group-containing unsaturated monomers listed in the section <Acid Group-Containing Unsaturated Monomer> in [2-3-1] above can be used, and the description in that section can be incorporated herein by reference. However, the acid group-containing unsaturated monomer includes a neutralized acid group-containing unsaturated monomer that is one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts of the acid group-containing unsaturated monomers listed above.

[0099] <Monomers other than acid group-containing unsaturated monomers> As the monomers other than the acid group-containing unsaturated monomers, monomers other than the acid group-containing unsaturated monomers listed in the above column can also be used, and the descriptions in that column can be used.

[0100] <Polymerization inhibitor> The monomer composition to be subjected to the polymerization step may preferably contain a small amount of polymerization inhibitor from the viewpoint of polymerization stability. A preferred polymerization inhibitor is p-methoxyphenol. The amount of polymerization inhibitor contained in the monomer (particularly acrylic acid and its salts) is usually 1 ppm or more and 250 ppm or less, preferably 10 ppm or more and 160 ppm or less, and more preferably 20 ppm or more and 80 ppm or less.

[0101] <Neutralizing Agent> As described above, the acid group-containing unsaturated monomer in the monomer composition in the production method of the present invention is at least partially neutralized, and the neutralized acid group-containing unsaturated monomer contains one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts. The production method of the present invention may include neutralizing at least a portion of the acid group-containing unsaturated monomer using a neutralizing agent.

[0102] The neutralizing agent used to neutralize at least a portion of the acid group-containing unsaturated monomer is not particularly limited. The neutralizing agent may be, for example, a basic substance. Examples of the neutralizing agent used to prepare one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts include basic substances such as inorganic salts, such as lithium hydroxide, potassium hydroxide, and ammonium carbonate. Other examples of the neutralizing agent include inorganic salts, such as sodium hydroxide and sodium carbonate, and basic substances, such as amine-based organic compounds having an amino group and / or an imino group. Two or more neutralizing agents may be used in combination.

[0103] In the production method of the present invention, the neutralization rate of the acid group-containing unsaturated monomer can be adjusted by mixing the acid group-containing unsaturated monomer with its neutralized salt; adding a known neutralizing agent to the acid group-containing unsaturated monomer; or using a partially neutralized salt of the acid group-containing unsaturated monomer (i.e., a mixture of the acid group-containing unsaturated monomer and its neutralized salt) that has been adjusted to a predetermined neutralization rate in advance. These methods may also be combined.

[0104] The neutralization rate may be adjusted before the start of the polymerization reaction of the acid group-containing unsaturated monomer, during the polymerization reaction of the acid group-containing unsaturated monomer, or on the hydrogel-like crosslinked polymer obtained after the completion of the polymerization reaction of the acid group-containing unsaturated monomer. The neutralization rate may be adjusted at any one of the following stages: before the start of the polymerization reaction, during the polymerization reaction, or after the completion of the polymerization reaction, or the neutralization rate may be adjusted at multiple stages. In applications where unreacted neutralizers such as basic substances may be a problem, the neutralization rate is preferably adjusted before the start of the polymerization reaction and / or during the polymerization reaction, more preferably before the start of the polymerization reaction.

[0105] <Internal Crosslinking Agent> The monomer composition in the production method of the present invention may contain an internal crosslinking agent. The internal crosslinking agent adjusts the water absorption performance and / or gel strength upon water absorption of the obtained water absorbent resin. As the internal crosslinking agent that can be used in this step, the internal crosslinking agents listed in the <Internal Crosslinking Agent> section of [2-3-1] above can be used, and the description in this section can be cited. In the production method of a water absorbent resin, an internal crosslinking agent is preferably used. Note that, under polymerization conditions in which the self-crosslinking reaction of the monomer is effective, the internal crosslinking agent does not need to be used.

[0106] <Other Substances> In the production method of the present invention, the following substances (hereinafter referred to as "other substances") may be added to the monomer composition within the scope in which the object of the present invention is achieved.

[0107] Examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; blowing agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as ethylenediaminetetra(methylenephosphinic acid) or a metal salt thereof, ethylenediaminetetraacetic acid or a metal salt thereof, and diethylenetriaminepentaacetic acid or a metal salt thereof; hydrophilic polymers such as polyacrylic acid (salts) and crosslinked products thereof (for example, fine powders made of water-absorbent resins), starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol. These other substances may be used alone or in combination of two or more.

[0108] The amount of the other substances used is not particularly limited. The total content (total concentration) of the 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, based on the total mass of the acid group-containing monomer and the monomers other than the acid group-containing monomer in the monomer composition (hereinafter referred to as "total mass of monomers").

[0109] <Content of Monomers in Monomer Composition> In this step, the content of all monomers in the monomer composition (=total mass of the acid group-containing unsaturated monomer and monomers other than the acid group-containing unsaturated monomer / mass of the monomer composition) is 30% by mass or more and less than 55% by mass, preferably 30% by mass or more and 53% by mass or less, more preferably 35% by mass or more and 52% by mass or less, and even more preferably 40% by mass or more and 50% by mass or less, from the viewpoint of controlling the GEX value to 17 or more and from the viewpoint of productivity.

[0110]

[0033] The lower the content of all monomers in the monomer composition, the less self-crosslinking resulting from entanglement of molecular chains occurs, and the higher the CRC of the obtained water-absorbent resin. From the viewpoint of making the CRC of the water-absorbent resin of the present invention exceed 25 g / g, the content of all monomers in the monomer composition is less than 55 mass%, preferably 53 mass% or less, more preferably 52 mass% or less, and even more preferably 50 mass% or less.

[0111] <Polymerization initiator> The polymerization initiator used in the production method of the present invention is not particularly limited, as it is appropriately selected depending on the polymerization form, etc. Examples of the polymerization initiator include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a combination of these, or a redox polymerization initiator used in combination with a reducing agent that promotes decomposition of the polymerization initiator. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 are used. Note that, from the viewpoint of the handleability of the polymerization initiator and the physical properties of the water-absorbent resin, preferably a peroxide or an azo compound, more preferably a peroxide, and even more preferably a persulfate is used.

[0112] The amount of the polymerization initiator used 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, based on the total moles of the acid group-containing monomer and monomers other than the acid group-containing monomer in the monomer composition (hereinafter referred to as "total moles of monomers"). When redox polymerization is performed as necessary, the amount of the reducing agent used in combination with the oxidizing agent is preferably 0.0001 mol % or more and 0.02 mol % or less, based on the total moles of the monomers.

[0113] <Dissolved Oxygen Amount> When the monomer composition is an aqueous solution, it is also preferable to reduce the dissolved oxygen in the monomer composition before polymerization by raising the temperature or replacing it with an inert gas. For example, the dissolved oxygen is reduced to preferably 5 ppm or less, more preferably 3 ppm or less, and particularly preferably 1 ppm or less.

[0114] When the monomer composition is an aqueous solution, bubbles (particularly the inert gas) can be dispersed in the monomer composition. In this case, the polymerization in the polymerization step is carried out as foam polymerization.

[0115] [3-2-2] Polymerization Step The production method of the present invention includes a polymerization step. The polymerization step is a step of crosslinking the monomer composition to obtain a hydrogel-like crosslinked polymer (hereinafter, may be referred to as "hydrogel"). Here, crosslinking the monomer composition means crosslinking the monomers contained in the monomer composition to obtain a hydrogel-like crosslinked polymer.

[0116] In addition to the method of adding a polymerization initiator and carrying out a polymerization reaction, there is also a method of irradiating with active energy rays such as radiation, electron beams, ultraviolet rays, etc. Furthermore, irradiation with active energy rays may be carried out in combination with adding a polymerization initiator.

[0117] <Polymerization form> Examples of the polymerization form in the polymerization step include droplet polymerization in a gas phase, aqueous solution polymerization, and reversed-phase suspension polymerization, and aqueous solution polymerization is particularly preferred. That is, it is particularly preferred that the polymerization step is a step in which the monomer composition is polymerized by aqueous solution polymerization to obtain the hydrogel-like crosslinked polymer. Among the aqueous solution polymerizations, continuous aqueous solution polymerization is particularly preferred, and examples thereof include continuous belt polymerization and continuous kneader polymerization. By adopting continuous aqueous solution polymerization as the polymerization form, the production efficiency of the water-absorbent resin is improved. Furthermore, by adopting aqueous solution polymerization as the polymerization form, it is possible to produce a water-absorbent resin that is substantially free of residual volatile components.

[0118] 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. When the monomer composition contains a solvent, the upper limit of the polymerization initiation temperature is the boiling point of the solvent.

[0119] By controlling the polymerization initiation temperature within the above-mentioned preferred range, the values ​​of (c) and (d) of the produced water-absorbent resin can be controlled within more preferred ranges.

[0120] <Polymerization rate of hydrogel-like crosslinked polymer> The polymerization rate of the hydrogel-like crosslinked polymer obtained in the polymerization step is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 98% by mass or more, and particularly preferably 99% by mass or more, from the viewpoint of preventing aggregation during drying of the particulate hydrogel obtained in the subsequent hydrogel crushing step and / or reducing residual monomers in the produced water absorbent resin.

[0121] The upper limit of the polymerization rate of the hydrogel-like crosslinked polymer is not particularly limited, and 100% by mass is ideal. However, a high polymerization rate requires a long polymerization time or strict polymerization conditions, which may result in a decrease in productivity and / or physical properties. Therefore, an upper limit of 99.95% by mass, further 99.9% by mass, and usually about 99.8% by mass is sufficient. The polymerization rate of the hydrogel-like crosslinked polymer is typically 98 to 99.99% by mass.

[0122] <Physical Properties of Hydrogel-Like Crosslinked Polymer> The solid content 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, based on the total mass of the hydrogel-like crosslinked polymer. Furthermore, the solid content 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, based on the total mass of the hydrogel-like crosslinked polymer. By keeping the solid content within these preferred ranges, the amount of water-soluble components can be reduced.

[0123] [3-2-3] Hydrogel Crushing Step The production method of the present invention preferably further comprises a hydrogel crushing step in which the hydrogel-like crosslinked polymer is crushed to obtain a particulate hydrogel. Specifically, this step is a step in which the hydrogel-like crosslinked polymer obtained in the polymerization step is crushed simultaneously with and / or after polymerization to obtain fine particles, thereby obtaining a particulate hydrogel. The particle size of the particulate hydrogel is adjusted to a preferred range described below so that (surface-crosslinked) water-absorbent resin particles having the desired shape and performance can be obtained in high yield. This step may be carried out two or more times to obtain a particulate hydrogel having a predetermined particle size.

[0124] Furthermore, when a particulate hydrogel having a desired particle size is obtained in the polymerization step, such as in reverse-phase suspension polymerization or gas-phase polymerization, this step does not need to be carried out. Furthermore, if necessary, after the polymerization step and before the gel-pulverization step, a shredding step may be carried out using a roller cutter, guillotine cutter, or the like, in which the hydrogel-like crosslinked polymer is cut or roughly crushed to a size that can be introduced into a gel-pulverizing device. In particular, when the polymerization step is belt polymerization and a sheet-like or block-like hydrogel-like crosslinked polymer is obtained, it is preferable to carry out the shredding step.

[0125] <Gel Crushing Apparatus> In the production method of the present invention, the type of gel crushing apparatus is not particularly limited as long as a particulate hydrogel of a predetermined particle size can be obtained without impairing the water absorption performance. Examples of gel crushing apparatus include gel crushers equipped with multiple rotating stirring blades, such as a batch or continuous double-arm kneader, screw extruders such as a single-screw extruder, a twin-screw extruder, or a meat chopper, double-screw kneaders (kneaders) with two or more screws, and gel crushers such as a cutter mill. Note that when the polymerization step is kneader polymerization, the polymerization step and the hydrogel crushing step are carried out simultaneously.

[0126] In one embodiment, a meat chopper or a double-screw kneader having two or more axes is preferably used in the hydrogel crushing step. In particular, when the hydrogel is obtained by aqueous solution polymerization, fine particles can be formed by using a meat chopper or a double-screw kneader having two or more axes in the hydrogel crushing step. Note that, for example, the contents disclosed in International Publication No. 2011 / 126079 are preferably applied to the present invention with regard to the hydrogel crushing conditions and form.

[0127] <Gel fluidizer> In the production method of the present invention, a gel fluidizer may be added before the hydrogel crushing step and / or during the hydrogel crushing step. This results in a particulate hydrogel containing a gel fluidizer. The addition of a gel fluidizer prevents adhesion or adhesion between hydrogel particles in the drying step described below, thereby improving the water absorption performance of the resulting water-absorbent resin. Furthermore, the load in the crushing step after drying described below is reduced, thereby reducing the amount of fine powder generated. Furthermore, when stirring and drying is performed in the drying step, the particle size of the resulting particulate dried polymer approaches the product particle size, making the above effect more pronounced. From the perspective of uniformly incorporating the gel fluidizer into each particle of the resulting particulate hydrogel, it is preferable to add the gel fluidizer during the hydrogel crushing step. Even when the hydrogel crushing step is not required, such as when reverse-phase suspension polymerization is performed in the polymerization step, it is preferable to add the gel fluidizer to the particulate hydrogel at least before the drying step.

[0128] The amount of the gel fluidizer to be added is appropriately determined depending on the water content of the hydrogel or particulate hydrogel and / or the type of gel fluidizer, and is preferably 0.001% by mass to 0.5% by mass, more preferably 0.01% by mass to 0.3% by mass, and even more preferably 0.02% by mass to 0.2% by mass, based on the solid content of the hydrogel.

[0129] Examples of the gel fluidizing agent include anionic, cationic, nonionic and amphoteric surfactants, as well as low-molecular-weight or high-molecular-weight surfactants thereof, polymeric lubricants, and the like.

[0130] (Surfactant) Specifically, surfactants used in gel fluidizers include: (1) sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin 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, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. (2) alkyldimethylaminoacetic acid betaines such as capryldimethylaminoacetic acid betaine, lauryldimethylaminoacetic acid betaine, myristyldimethylaminoacetic acid betaine, and stearyldimethylaminoacetic acid betaine; alkylamidopropyl betaines such as lauric acid amidopropyl betaine, coconut oil fatty acid amidopropyl betaine, and palm kernel oil fatty acid amidopropyl betaine; alkylhydroxysulfobetaines such as laurylhydroxysulfobetaine; and amphoteric surfactants such as alkylcarboxymethylhydroxyethylimidazolinium betaine and 2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine; (3) anionic surfactants such as monoalkali metal alkylaminodiacetates such as monosodium laurylaminodiacetate, potassium laurylaminodiacetate, and sodium myristylaminodiacetate; and (4) cationic surfactants such as long-chain alkyldimethylaminoethyl quaternary salts. Two or more of these may be used in combination.

[0131] <Polymer Lubricant> In the production method of the present invention, the polymer lubricants exemplified below can be added to the monomer composition and / or the hydrogel as long as the object of the present invention is achieved.

[0132] Specific examples of the polymer lubricant 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, ethylhydroxyethyl cellulose, and polyalkylene oxides such as polyethylene glycol. The molecular weight (weight average molecular weight) of these lubricants is preferably selected from the range of 2 to 2,000,000, more preferably 4 to 1,000,000. Two or more of these may be used in combination.

[0133] Furthermore, as a gel fluidizing agent, these polymeric lubricants may be used in combination with the surfactants. When a surfactant and a polymeric lubricant are used in combination, the total amount added is appropriately determined depending on the polymerization form, the composition of the monomer composition, and the water content of the hydrogel. When added to the monomer composition, the concentration is set relative to the monomer component; when added to the hydrogel, the concentration is set relative to the solids content; and when added to both, the total is set as the above. These gel fluidizing agents may overlap with the dispersing aid used in performing reversed-phase suspension polymerization.

[0134] The total amount of surfactant and polymer lubricant added is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, and is preferably 0.05% by mass or more, particularly preferably 0.1% by mass or more.

[0135] <Hydrogen Peroxide> In the production method of the present invention, hydrogen peroxide may be added in a step before the drying step. By adding hydrogen peroxide in a step before the drying step and heating in the drying step or a subsequent step, when heat is applied in the drying step or a subsequent step, the crosslinked structure in the water absorbent resin is cleaved by the hydrogen peroxide, thereby efficiently increasing the water absorbency capacity (CRC), and a water absorbent resin having a CRC of more than 25 g / g can be obtained. More specifically, it is preferable to add hydrogen peroxide to the monomer composition before the polymerization step and / or to the hydrogel during the gel-crushing step.

[0136] The amount of hydrogen peroxide added to the monomer composition and / or hydrogel is appropriately set in consideration of the selected polymerization conditions and the amount of hydrogen peroxide (ppm) remaining before drying. For example, in consideration of the improvement in the physical properties of the obtained water-absorbent resin, particularly the absorbency (CRC), the total amount is preferably 50 ppm (0.005 mass%) or more, more preferably 50 ppm (0.005 mass%) to 10,000 ppm (1.0 mass%), and still more preferably 100 ppm (0.01 mass%) to 5,000 ppm (0.5 mass%), relative to the solids mass (%) of the monomer composition or the hydrogel-like crosslinked polymer.

[0137] The method for adding hydrogen peroxide is not particularly limited. It is preferable to add hydrogen peroxide as an aqueous solution, since this method facilitates the addition. The concentration of the aqueous solution is not particularly limited, and is usually about 1 to 40% by mass. The aqueous hydrogen peroxide solution may also contain a small amount of a hydrophilic solvent, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, or isopropyl alcohol.

[0138] Furthermore, when hydrogen peroxide is added, the hydrogel cross-linked polymer and / or the dried polymer is preferably heated in the drying step and thereafter so that the maximum temperature reached exceeds 160°C. By such heating, the cross-linked structure in the water-absorbent resin is cleaved by the hydrogen peroxide, and a particulate water-absorbing agent with a high water absorption capacity (CRC) can be obtained. The maximum temperature reached is, in order of preference, more than 160°C and not more than 250°C, 170°C or more and 220°C or less, and 180°C or more and 200°C or less. Regarding the method of adding hydrogen peroxide and the heating in the drying step and thereafter, the conditions described in WO22 / 163849 may be adopted.

[0139] <Solid content of hydrogel> The solid content of the hydrogel subjected to the hydrogel crushing step (hereinafter referred to as gel solid content) is preferably 25% by mass or more. From the viewpoints of preventing aggregation of hydrogel particles after hydrogel crushing, the energy required for crushing, drying efficiency, and absorption performance, the gel solid content is more preferably 25% by mass to 75% by mass, even more preferably 30% by mass to 70% by mass, even more preferably 35% by mass to 65% by mass, and particularly preferably 40% by mass to 60% by mass.

[0140] <Moisture content of particulate hydrogel> The moisture content of the particulate hydrogel (hereinafter referred to as gel moisture content) is determined by the measurement method described in the Examples below. From the viewpoint of the fluidity of the particulate hydrogel in the drying step described below, the gel moisture content is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 35% by mass or more, particularly preferably 40% by mass or more, and extremely preferably 43% by mass or more. Excessively high-concentration polymerization may deteriorate the physical properties of the water-absorbent resin, and from the viewpoint of drying efficiency and absorption performance, the gel moisture content is preferably 75% by mass or less, more preferably 60% by mass or less, and particularly preferably 55% by mass or less.

[0141] <Particle diameter of particulate hydrogel> From the viewpoint of controlling the particle diameter of the obtained water-absorbent resin, the mass average particle diameter of the particulate hydrogel obtained through the gel crushing step (particulate hydrogel before drying) is preferably 1 mm or less in terms of solid content. The mass average particle diameter in terms of solid content is preferably 10 μm to 1000 μm, more preferably 20 μm to 800 μm, even more preferably 40 μm to 500 μm, particularly preferably 50 μm to 300 μm, and most preferably 60 μm to 200 μm. The mass average particle diameter (Solid D50) of the particulate hydrogel in terms of solid content can be measured by the method described in the Examples.

[0142] The solid content conversion refers to physical properties obtained by measuring various physical properties such as particle size of a hydrogel-like crosslinked polymer and then converting them into physical properties per solid content of the water-absorbent resin in the hydrogel-like crosslinked polymer. For example, in the case of a hydrogel-like crosslinked polymer with a water content of 50% (solid content of 50%), the conversion is performed by multiplying the measured physical property values ​​of the hydrogel-like crosslinked polymer by two.

[0143] (Gel Crushing Using a Meat Chopper) In one embodiment, in the hydrogel crushing step, gel crushing is preferably performed using a meat chopper (screw extruder) as shown in FIG. 1 of WO 2013 / 002387. The meat chopper is equipped with a die (also referred to as a "die plate" or "perforated plate") at the extrusion port. The size of the meat chopper may be appropriately set according to the desired production amount. In addition, the thickness of the perforated plate, the hole diameter of the perforated plate, the perforation rate of the perforated plate, the screw shaft rotation speed, the introduction rate of the hydrogel-like cross-linked polymer, etc. may be appropriately set according to the size of the apparatus and are not particularly limited.

[0144] In the case of crushing with a meat chopper, it is preferable to pass the hydrogel through the meat chopper preferably two or more times, more preferably three or more times. In the present invention, the (GGE (1)) for gel crushing the hydrogel is preferably 10 to 500 J / g, more preferably 15 to 400 J / g, even more preferably 20 to 300 J / g, still more preferably 45 to 250 J / g, and particularly preferably 25 to 200 J / g.

[0145] If the gel crushing energy (GGE(1)) is less than 10 J / g, the hydrogel crosslinked polymer may not be crushed into a particulate hydrogel having the desired particle size. Conversely, if the gel crushing energy (GGE(1)) exceeds 500 J / g, the load on the crushing device increases, and there is a risk of the device breaking down during continuous operation. In addition, in this case, excessive shear and compression forces may be applied to the hydrogel crosslinked polymer, which may result in a decrease in physical properties such as an increase in the amount of water-soluble matter produced.

[0146] The gel crushing energy (GGE(2)) is preferably 5 to 300 J / g, more preferably 6 to 280 J / g, 8 to 260 J / g, 9 to 250 J / g, or 10 to 240 J / g.

[0147] If the gel crushing energy (GGE(2)) is less than 5 J / g, the hydrogel crosslinked polymer may not be crushed into a particulate hydrogel having the desired particle size. Conversely, if the gel crushing energy (GGE(2)) is more than 300 J / g, excessive shear and compression forces may be applied to the hydrogel crosslinked polymer, resulting in a decrease in physical properties such as an increase in the amount of water-soluble components produced.

[0148] In addition, when grinding is performed multiple times, the gel grinding energy (GGE(1)) or (GGE(2))) for each grinding is added up, and the gel grinding energy (GGE(1)) or (GGE(2))) for the entire multiple grindings can be calculated.

[0149] Gel grinding energy is GGE(1) or GGE(2) described in International Publication No. 2016 / 204302, and refers to the mechanical energy per unit weight (unit weight of hydrogel-like cross-linked polymer) required by the gel grinding device when grinding a hydrogel-like cross-linked polymer. The gel grinding energy (GGE(1)) is defined to include the energy during idle operation of the gel grinder, and the gel grinding energy (GGE(2)) is defined by subtracting the energy during idle operation of the gel grinder. When gel grinding is performed multiple times, the gel grinding energy applied in each grinding is summed to calculate the gel grinding energy for the entire multiple grinding operations.

[0150] For gel pulverization using a meat chopper (screw extruder), the conditions described in WO2016 / 204302 can be applied to the present invention.

[0151] (Gel Crushing Using a Double-Shaft Kneader with Two or More Shafts) In one embodiment, the hydrogel crushing step preferably involves gel crushing using a double-shaft kneader as shown in FIGS. 1 and 2 of WO 2022 / 065365. As shown in FIGS. 1 and 2, a gel crushing device having an inlet, a main body incorporating multiple rotating shafts, and an outlet is used in the hydrogel crushing step after the polymerization step. Each rotating shaft has a crushing means. In this gel crushing device, the hydrogel-like crosslinked polymer continuously introduced into the main body from the inlet is crushed by the crushing means of each rotating shaft, and is continuously removed from the outlet as particulate hydrogel-like crosslinked polymer. That is, the hydrogel-like crosslinked polymer introduced into the main body from the inlet is crushed by the crushing means of each rotating shaft as it moves from the inlet to the outlet. In this invention, the "main body" refers to the body portion in which multiple rotating shafts and crushing means are installed, and is also referred to as a barrel, trough, casing, etc. Preferably, the hydrogel crosslinked polymer introduced into the main body through the inlet is pulverized by the pulverizing means of each rotating shaft inside the double-shaft kneader while moving from the inlet to the discharge outlet while being maintained at 50°C or higher.

[0152] As long as the double-shaft kneader is continuous, it may be vertical (the direction of travel of the hydrogel is vertical) or horizontal or horizontal (the direction of travel of the hydrogel is horizontal or horizontal). In addition, vertical and horizontal gel grinding devices may have an inclination of 0° to 90° with respect to the horizontal direction. For example, in the case of the horizontal continuous grinding device shown in FIG. 1 of WO2022 / 065365, an appropriate inclination is provided as needed, and the inclination may be downward or upward from the inlet toward the outlet (i.e., with respect to the direction of travel of the hydrogel). Typically, the inclination angle is 0° to 10°, preferably 0° to 1°, and particularly preferably 0°.

[0153] From the viewpoint of continuously and stably crushing the gel, the double-shaft kneader preferably has a heating means and / or a heat-retaining means. The heating means and / or the heat-retaining means are not particularly limited, but from the viewpoint of preventing adhesion and aggregation of the hydrous gel and particulate hydrous gel, a heating means using direct heat transfer by convection and / or indirect heat transfer by heat conduction from the heating surface (contact surface with the hydrous gel, heat source portion) of the gel crusher heated by a heat medium is preferred. More preferred heating means are a ventilation heating type for direct heat transfer and an outer wall heating type for indirect heat transfer.

[0154] From the viewpoint of continuously and stably crushing the gel, the double-shaft kneader preferably includes a heating means and / or a heat-retaining means, more preferably a heating means, on the outer surface of the main body. Examples of the heat-retaining means include covering a portion or the entire outer surface of the main body (preferably 50% or more of the outer surface of the main body, more preferably 80% or more, and particularly preferably the entire surface) with a heat insulating material. Examples of the heating means include an electric trace, a steam trace, and a jacket heated by a heat medium. By including the heating means and / or the heat-retaining means in the double-shaft kneader, the gel crushing process can be carried out in a more preferable temperature range. Furthermore, deterioration of the gel crushing quality due to temperature differences such as the season and day and night can be avoided. Furthermore, the gel crushing device can be smoothly guided to stable operation when starting up.

[0155] As long as the effects of the present invention can be obtained, the type of crushing means possessed by each rotating shaft is not particularly limited. For example, disks of various shapes can be used as means having a shearing effect on the hydrogel. Disks may be called chips, paddles, elements, kneading, rotors, etc. The shape of the disk is not particularly limited and can be appropriately selected from circular, elliptical, approximately triangular, etc. Disks of different shapes can also be used in combination, and their arrangement is appropriately adjusted from the perspective of the particle size of the target particulate hydrogel and the energy required for crushing. Furthermore, arms, vanes, blades, cut disks (CDs), etc. may be used in combination as crushing means.

[0156] The distance (clearance) between the disk and the main body (barrel) may vary depending on the location. When the minimum clearance C is the shortest distance between the outer periphery of the disk and the inner wall of the main body (barrel), the minimum clearance C is preferably 20% or less of the maximum diameter D of the disk, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less. Note that "D" stands for diameter, and when multiple disks of different diameters are used, it refers to the diameter of the largest disk. If the minimum clearance C is less than the above upper limit, the shear force between the barrel and the disk during gel crushing is strong, improving gel crushing efficiency. Furthermore, the minimum clearance C is preferably 0.2% or more of the maximum diameter D of the disk, more preferably 0.5% or more, and even more preferably 1% or more. If the minimum clearance C is greater than the above lower limit, contact between the disk and the inner wall of the main body (barrel) is prevented, and the intrusion of metal foreign matter due to wear is prevented. In a preferred embodiment of the present invention, the minimum clearance C is 0.2 to 20% of the maximum diameter D of the disk.

[0157] The rotation speeds of the multiple rotating shafts of the double-shaft kneader may be uniform or non-uniform and may be set appropriately depending on the device, but are preferably in the range of 1 rpm to 1000 rpm, more preferably 3 rpm to 500 rpm, and even more preferably 5 rpm to 300 rpm. When the rotation speeds of the rotating shafts are different, the ratio of the rotation speed of one rotating shaft to the rotation speed of the other rotating shaft is usually in the range of 1 to 10, and preferably in the range of 1 to 2.

[0158] The rotation direction of the multiple rotating shafts may be a synchronous type, in which each rotating shaft rotates in the same direction, or a counter-rotating type, in which each rotating shaft rotates in the opposite direction. A synchronous type device is expected to have self-cleaning properties, while a counter-rotating type device is expected to have strong shearing forces. The rotation direction of each rotating shaft is appropriately selected in combination with the arrangement of the grinding means (disk pattern) described above.

[0159] The double-shaft kneader preferably has a function of supplying water and / or steam into the main body. Therefore, according to one embodiment of the present invention, water and / or steam is supplied into the main body in the gel crushing step. The gel crushing device may be provided with a plurality of inlets as a means for supplying water and / or steam. The installation positions of the water and / or steam inlets are not important, but they are preferably installed on the side of the inlet for the hydrogel. Furthermore, water and steam may be supplied from different inlets.

[0160] When adding water vapor, a gas may be mixed with the water vapor and added as a mixed gas. The gas is not particularly limited and may be, for example, air, dry air, nitrogen, etc. The pressure of the added water vapor is not particularly limited and 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 and is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of preventing excessive temperature rise and drying of the hydrogel, the temperature of the water and / or water vapor (including the mixed gas) is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 120°C or lower, and particularly preferably 100°C or lower. In a preferred embodiment, the temperature of the water and / or water vapor supplied to the inside of the main body is 50 to 120°C. The temperatures of the hydrogel and particulate hydrogel in the double-shaft kneader can also be adjusted by the temperature and amount of water and / or steam (including mixed gas) used. In this case, the steam and / or mixed gas acts as a heat medium for direct heat transfer, and the hydrogel and particulate hydrogel inside the main body are heated or kept at a predetermined temperature. Note that additives such as a gel fluidizing agent, a crosslinking agent, an oxidizing agent, a reducing agent, and a polymerization initiator, which will be described later, may be blended into the water and / or steam (including mixed gas) to be added.

[0161] The amount of water and / or water vapor supplied is preferably 0.1% by mass to 50% by mass, more preferably 0.5% by mass to 40% by mass, and even more preferably 1% by mass to 30% by mass, based on the mass of the hydrogel in terms of solid content.

[0162] More preferably, before the hydrogel is introduced into the double-shaft kneader, the temperature inside (inner surface) of the main body is 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 adhesion of the hydrogel to the inner surface of the main body, and further improves the water absorption performance of the obtained water-absorbent resin. That is, in gel crushing using the double-shaft kneader, it is preferable that the inner surface of the main body is heated to the above-mentioned temperature or higher before introducing the hydrogel and at the start of gel crushing. From the viewpoint of maintaining the temperature in the gel crushing step at 50°C or higher, it is preferable that the temperature inside (inner surface) of the main body is maintained within the above range in the gel crushing step.

[0163] The temperature at which the hydrogel crosslinked polymer is continuously pulverized is preferably 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.

[0164] The upper limit of the temperature at which the hydrogel crosslinked polymer is continuously pulverized is not particularly limited, but from the viewpoint of preventing excessive temperature rise and drying of the hydrogel, it is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferably 110°C or lower.

[0165] Figures 1 and 2 of WO2022 / 065365 show an example of a double-shaft kneader (gel crusher) suitable for use in the present invention. Figure 1 is a partially cutaway side view of this gel crusher, and Figure 2 is an enlarged view of this gel crusher (a view of the center of the main body as seen from above). Hereinafter, the basic configuration and method of use of this gel crusher are the same as those of the gel crusher shown in Figures 1 and 2.

[0166] The rotation shaft of the gel crusher has multiple disks. The disks may have the same or different shapes, but preferably have different shapes. The combination of disks is appropriately changed depending on the physical properties of the hydrogel, the size of the crushed gel to be obtained, etc., with reference to, for example, Patent Document (JP 2005-35212 A).

[0167] An example of a gel grinding device (double-shaft kneader) having such a basic configuration is a double-shaft kneader (kneader) having two or more shafts. Specific examples include double-shaft, triple-shaft, quadruple-shaft, and eight-shaft kneaders. A continuous type gel grinding device is preferably used from the viewpoint of production efficiency. Specifically, examples of gel crushing devices include a CKH-type continuous kneader (Honda Iron Works Co., Ltd.), a twin-screw extruder TEX (The Japan Steel Works, Ltd.), a twin-screw extruder TEXαIII (The Japan Steel Works, Ltd.), a continuous kneader (CONTINUOUS KNEADER, Dalton Co., Ltd.), a KRC hybrid reactor (KRC HYBRID REACTER, Kurimoto Iron Works Co., Ltd.), a KRC kneader (KURIMOTO-READCO CONTINUOUS KNEADER, Kurimoto Iron Works Co., Ltd.), a KEX extruder (KEX EXTRUDER, Kurimoto Iron Works Co., Ltd.), and a KEXD extruder (KEXD Examples of such a kneader include the twin-screw extruder TEX-SSG (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-CS (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-SX (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-DS (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-A (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-B (Toshiba Machine Co., Ltd.), the twin-screw extruder TEX-BS (Toshiba Machine Co., Ltd.), the four-screw and eight-screw extruders WDR series (Technovel Corporation), and the like. Therefore, in a preferred embodiment of the present invention, the gel crusher is a continuous double-screw kneader.

[0168] In the pulverization using a double-shaft kneader, the gel crushing energy (GGE (2)) for gel crushing the hydrogel is preferably 150 [J / g] or less as an upper limit, more preferably 130 [J / g] or less, and even more preferably 120 [J / g] or less. 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 the present invention, the gel crushing energy (GGE (1)) for gel crushing the hydrogel is 20 to 150 [J / g], preferably 40 to 130 [J / g], and more preferably 50 to 120 [J / g]. The GGE (2) is 20 to 150 [J / g], preferably 40 to 130 [J / g], and more preferably 50 to 120 [J / g]. By controlling the GGE (1) and CGE (2) within the above ranges, the gel can be crushed while applying appropriate shear and compression forces to the hydrogel. The calculation method for the GGE (1) and GGE (2) is the same as the calculation method for the gel crushing energy described in the gel crushing with a meat chopper.

[0169] (Gel Temperature) In gel pulverization using a double-screw kneader, from the viewpoint of continuously pulverizing the hydrogel-like crosslinked polymer, the temperature T1 of the hydrogel-like crosslinked polymer fed into the inlet of the gel pulverization device in the gel pulverization step is preferably 50°C or higher. Hereinafter, the temperature T1 will also be referred to as the "gel temperature T1 at the inlet" or simply as the "gel temperature T1." This gel temperature T1 is preferably measured using a thermometer installed at the inlet. From the viewpoint of preventing adhesion of the gel-pulverized hydrogel to the device, this gel temperature T1 is preferably 60°C or higher, and from the viewpoint of further improving the water absorption performance of the water-absorbent 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. The gel temperature T1 can be adjusted within a desired range by keeping the hydrogel-like cross-linked polymer warm after its temperature has risen due to the heat of polymerization when it is fed into the gel crusher, or by heating the obtained hydrogel-like cross-linked polymer.

[0170] From the viewpoint of preventing aggregation of the crushed hydrogels, the temperature T2 of the particulate hydrogel-like crosslinked polymer discharged from the gel crusher 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, the temperature T2 is also referred to as the "gel temperature T2 at the outlet" or simply "gel temperature T2." Preferably, the temperature T2 is set within the relevant temperature range, and the temperature T1 is set within the aforementioned temperature range. This gel temperature T2 is preferably measured using a thermometer installed at the outlet. Note that the gel temperature T2 can be adjusted within a desired range by appropriately adjusting the set temperature of the heating means and / or heat-retaining means of the gel crusher, and further, the residence time of the hydrogel-like crosslinked polymer inside the gel crusher.

[0171] In gel pulverization using a double-screw kneader, it is preferable to add the above-mentioned gel fluidizer. The gel fluidizer may be added in one go or in two or more divided doses, and the addition position may be any position in the kneader. When the gel fluidizer is added in multiple divided doses, it is preferable to add it at different positions.

[0172] [3-2-4] Drying Step The production method of the present invention may include a drying step. The drying step is a step of drying a particulate hydrogel (preferably containing a gel fluidizer) to a desired moisture content to obtain a dried polymer, preferably a particulate dried polymer. The particulate hydrogel subjected to the drying step is not limited to one obtained via a gel crushing step, but may also be obtained by, for example, reverse phase suspension polymerization. Furthermore, the (particulate) dried polymer obtained in the drying step may include a granule formed by physical or chemical adhesion of a plurality of particles (hereinafter referred to as a dried granule).

[0173] <Temperature of particulate hydrogel> From the viewpoint of preventing the particulate hydrogel from becoming large due to adhesion between the particulate hydrogel and the fine granulated material, the temperature T2 of the particulate hydrogel subjected to the drying step is controlled to preferably 50° C. or higher, more preferably 60° C. or higher, even more preferably 70° C. or higher, particularly preferably 80° C. or higher, and most preferably 90° C. or higher. Furthermore, from the viewpoint of preventing discoloration and / or performance degradation of the material to be dried, this temperature T2 is preferably 130° C. or lower, more preferably 110° C. or lower, and even more preferably 105° C. or lower. Typically, this temperature T2 is measured by a contact thermometer at the center of the material layer (particulate hydrogel and / or dried material) (for example, at a position about 5 cm deep when the material thickness is 10 cm).

[0174] The drying method in the drying step is not particularly limited, and stationary drying, stirring drying, fluidized bed drying, etc. are appropriately used. Also, various drying methods can be used, such as heat drying, hot air drying, reduced pressure drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration drying with a hydrophobic organic solvent, and high-humidity drying using high-temperature steam. From the viewpoint of drying efficiency, heat drying or hot air drying is preferred, and stirring-type heat drying and / or fluidized bed drying, in which the material to be dried is dried while being moved, are more preferred.

[0175] <Drying Apparatus> The drying apparatus used in the drying step is not particularly limited, and one or more of a heat transfer conduction dryer, a radiant heat transfer dryer, a hot air heat transfer dryer, a dielectric heating dryer, etc. may be appropriately selected. It may be a batch type or a continuous type. It may also be a direct heating type or an indirect heating type. Examples of the drying apparatus include heat transfer dryers such as a ventilation band type, a ventilation circuit type, a ventilation vertical type, a parallel flow band type, a ventilation tunnel type, a ventilation stirring type, a ventilation rotary type, a fluidized bed type, and an air flow type.

[0176] In the case of a band type or the like in which drying is performed in a stationary state, a large lump or block-like dried product is obtained, and when this is pulverized and processed into a particulate product with a particle size (for example, 850 μm to 150 μm), a large amount of fine powder may be generated. On the other hand, if an agitator dryer is used that is equipped with a fluidizing means for fluidizing the material to be dried in the dryer (for example, a stirring blade provided in the dryer and / or rotation of the dryer itself) and one or more heating means, a particulate dried product (hereinafter sometimes referred to as a particulate dried polymer) can be obtained, which is preferable, and a continuous agitator dryer is more preferable.

[0177] The heating means in the drying step is not particularly limited. As the heating means, from the viewpoint of drying efficiency and reducing thermal damage to the water absorbent resin, a heating means using direct heat transfer by convection heat transfer and / or indirect heat transfer by heat conduction from the heating surface (surface in contact with the material to be dried) of a dryer heated by a heat medium is preferred. More preferred heating means in the drying step are a ventilation heating type for direct heat transfer, and an outer wall heating type or a tubular heating type for indirect heat transfer.

[0178] The stirring method and form are not particularly limited, as long as the particulate hydrogel and finely powdered granulated material in the drying apparatus are fluidized by stirring means such as a stirring blade or a rotating cylinder. In this specification, a dryer in which stirring is performed by rotating a cylindrical container containing the material to be dried, that is, a dryer in which the stirring means is a rotating cylinder, is referred to as a rotary dryer. In the production method according to the present invention, for example, rotary dryers include rotary dryers, rotary kilns, and tube dryers, and continuous stirring dryers that are not classified as rotary dryers include single- or twin-shaft disk dryers and single- or twin-shaft paddle dryers. Specifically, examples of continuous agitation dryers that are not rotary dryers include Solid Air (manufactured by Hosokawa Micron Corporation), CD dryer (manufactured by Kurimoto Iron Works, Ltd.), and paddle dryer (manufactured by Nara Machinery Works, Ltd.), and examples of rotary dryers that can be used include steam tube dryer (manufactured by Kurimoto Iron Works, Ltd.), steam tube dryer (manufactured by Ube Machinery Co., Ltd.), steam tube dryer (manufactured by Tsukishima Kikai Co., Ltd.), steam tube dryer (manufactured by Mitsui Engineering & Shipbuilding Co., Ltd.), rotary kiln (manufactured by Kurimoto Iron Works, Ltd.), and rotary dryer (manufactured by Okawara Manufacturing Co., Ltd.).

[0179] From the viewpoint of reducing mechanical damage to the material to be dried, a preferred drying apparatus is a rotary dryer (a dryer using a rotating container (further, a cylindrical container, particularly a horizontal cylindrical container such as a rotary drum) that accommodates and rotates the material to be dried). From the viewpoint of reducing thermal and mechanical damage, a rotary dryer having one or more heating means selected from a ventilated heating type for direct heat transfer, and an outer wall heating type and a tubular heating type for indirect heat transfer is more preferred. Furthermore, when a rotary dryer uses only a ventilated heating type, as in a rotary kiln, problems such as scattering of the dried material due to ventilation and generation of large amounts of waste gas may occur. Therefore, for indirect heat transfer, a rotary dryer using one or more heating means selected from an outer wall heating type and a tubular heating type is preferred. Furthermore, a tubular heating type is more preferred because the use of multiple heating tubes increases the heat transfer area inside the dryer, enabling efficient drying. An example of such a rotary dryer is a rotary dryer with a heating tube.

[0180] For example, this rotary dryer with heating tubes includes a rotating container (particularly a cylindrical container, more specifically a horizontal cylindrical container) that accommodates and rotates the material to be dried, and multiple heating tubes located inside the rotating container, extending axially, and rotating together with the rotating container. In this rotary dryer, the material to be dried flows within the container primarily due to the rotation of the rotating container and the action of the multiple heating tubes that rotate together with the rotating container, resulting in minimal mechanical and thermal damage. This prevents the generation of fine powder and deterioration of physical properties during the drying process. Furthermore, this dryer dries by indirect heat transfer from the heating tubes, which has the advantages of eliminating the scattering that occurs in hot air drying (e.g., ventilated band dryers and ventilated heated rotary kilns) and eliminating the need for large amounts of waste gas treatment. Furthermore, this dryer allows for a larger heat transfer area within the dryer by increasing the number of heating tubes. The larger heat transfer area enables shorter drying times and reduces residence time within the device, further reducing thermal damage.

[0181] Preferably, the rotary dryer is provided with a heating means or a heat-retaining means on the outer circumferential surface of the rotary vessel. In this dryer, the mixture of the particulate hydrogel and the finely powdered granulated material contained therein is heated by the rotation of the rotary vessel through contact with the plurality of heating tubes or by heat conduction from the heating tubes. The inner surface of the rotary vessel is also heated by radiant heat from the plurality of heating tubes, and if necessary, the mixture is further heated on the inner wall by the heating means or the heat-retaining means located on the outer circumferential surface of the rotary vessel, thereby shortening the drying time by heat conduction from the inner wall.

[0182] Preferably, the rotary dryer is provided with an addition means inside the rotary container for adding an additive to the contents contained in the rotary container. An example of this addition means is a spray device. With a rotary dryer having an addition means, it is possible to add a surface additive as an additive to a mixture of particulate hydrogel and fine powder granulated material contained in the rotary container, mix them, and then heat-treat and dry them by contact with multiple heating tubes or by heat conduction. In this case, the surface cross-linking step and drying step described below are performed in one step, thereby improving production efficiency.

[0183] As long as the object of the present invention is not impaired, the rotary dryer may be provided with other fluidizing means for fluidizing the contents in addition to agitating the contents by rotating the rotary container. Examples of such other fluidizing means include a scraper or agitating blades installed on the inner surface of the rotary container.

[0184] As long as the object of the present invention is achieved, the number of dryers that can be used in the drying process may be one or two or more. A combination of dryers with different specifications may also be used. For example, the rotary dryer described above may be combined with another agitator dryer that is not classified as such or a band dryer that is classified as a material transport type. While it is preferable to include at least one agitator dryer, the type and number of dryers to be combined are not limited.

[0185] Preferably, a dryer equipped with a function for introducing gas therein (preferably, a means for introducing and discharging gas) is used in the drying step. Examples of the means for introducing and discharging gas include a gas inlet and an outlet. The gas acts as a carrier gas and promotes drying by discharging water vapor and the like generated during drying to the outside of the device. Furthermore, when heated gas is used, the gas also acts as a heat medium, further promoting drying. Preferably, nitrogen, water vapor, and a mixture of these with air are used. When a mixed gas containing water vapor (hereinafter also referred to as a high-humidity mixed gas) is used, the inside of the device becomes a low-oxygen state, preventing oxidation and / or deterioration during drying. As a result, improved performance and low coloration of the water-absorbent resin can be achieved. Furthermore, this is preferable because it makes it possible to prevent aggregation and agglomeration of the particulate hydrogel and fine-powder granulated material during drying.

[0186] In the drying step, a dryer having a function of maintaining its interior at pressurized, normal, or reduced pressure may be used. When a pressurized state is maintained, the pressure is adjusted, for example, by increasing the amount of carrier gas introduced into the dryer. Preferably, the degree of pressurization relative to atmospheric pressure is a slight pressure of more than 0 kPa and not more than 0.01 kPa. When a reduced pressure is maintained, the degree of pressurization is adjusted, for example, by changing the amount of suction of exhaust gases (carrier gas introduced, water vapor generated during drying, etc.) from the dryer.

[0187] The degree of reduced pressure relative to atmospheric pressure is preferably more than 0 kPa and not more than 5 kPa, more preferably more than 0 kPa and not more than 2 kPa, and even more preferably a slight reduced pressure of 0.01 kPa to 0.5 kPa. By setting the degree of reduced pressure within the above range, water vapor and the like generated during drying can be efficiently removed without excessively removing heat from the inside of the dryer, thereby shortening the drying time. In addition, aggregation of the particulate hydrogel and fine powder granulated product during the drying step can be prevented.

[0188] Note that the "pressure level relative to atmospheric pressure" and the "depressurization level relative to atmospheric pressure" refer to the pressure difference from atmospheric pressure and are expressed as the absolute value of the difference from atmospheric pressure. For example, if the atmospheric pressure is standard atmospheric pressure (101.3 kPa) and the depressurization level relative to atmospheric pressure is 10 kPa, the actual air pressure is 91.3 kPa.

[0189] In the drying step, the gas may be introduced into the dryer from one or more locations to adjust the atmospheric dew point inside the dryer. It is preferable to appropriately adjust the atmospheric dew point mainly depending on the moisture content of the particulate hydrogel to be introduced into the dryer. The atmospheric dew point is measured when the air is exhausted from the dryer, and is preferably 60°C or higher, more preferably 65°C or higher, and even more preferably 70°C or higher. The upper limit is not particularly limited, but is preferably 100°C or lower. By setting the dew point within the above range, drying is promoted, and the moisture content of the resulting (particulate) dried polymer is adjusted to a predetermined range, thereby improving water absorption performance.

[0190] Drying conditions are appropriately selected depending on the type of dryer or the moisture content of the particulate hydrous gel, 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 the above range, the drying time is moderately long, which is economical. Furthermore, when the drying temperature is within the above range, deterioration of the physical properties and significant coloration of the obtained water-absorbent resin are unlikely to occur, which is preferable. Furthermore, from the viewpoint of making the CRC of the obtained water-absorbent resin exceed 25 g / g, it is preferable that the drying temperature conditions be the temperature conditions described in WO22 / 163849.

[0191] The drying time is preferably 1 minute to 10 hours, more preferably 5 minutes to 2 hours, even more preferably 10 minutes to 120 minutes, and particularly preferably 20 minutes to 60 minutes. When the drying time is within the above range, there is no need to excessively increase the drying temperature, and deterioration of the physical properties and / or significant coloring of the water-absorbent resin is unlikely to occur, which is preferable. Furthermore, when the drying time is within the above range, it is economical because it prevents the drying device from becoming too large and avoids a decrease in the processing capacity.

[0192] In the manufacturing method according to the present invention, when the drying apparatus is a rotary dryer, its Froude number (Fr = ω2 * r / g) is appropriately set from the viewpoint of drying efficiency, depending on the size of the apparatus and the drying throughput (amount dried per hour). The Froude number Fr is preferably in the range of 0.001 to 1, more preferably 0.005 to 0.5, even more preferably 0.01 to 0.3, and particularly preferably 0.02 to 0.2. The Froude number Fr is the ratio of the centrifugal acceleration ω2 * r acting on the material to be dried while being stirred in a rotating container to the gravitational acceleration g. (ω is the angular velocity of the rotor: rad / sec, and r is the representative radius of the rotor: m.)

[0193] In the case of an agitation dryer having an agitation means for agitating the material to be dried with a rotating shaft equipped with agitating blades, the rotation speed of the rotating shaft is set appropriately depending on the device, but it is preferable to set the rotation speed so that the Froude number Fr falls within the above range. Examples of the agitating blades include arms, blades, paddles, and cut disks (CDs). The specific rotation speed is usually in the range of 1 rpm to 10,000 rpm, more preferably 5 rpm to 500 rpm, and even more preferably 10 rpm to 300 rpm.

[0194] The peripheral speed (V) of the stirring blade defined by the following formula (3) is appropriately set depending on the apparatus, but is usually 0.15 m / s or more and 25 m / s or less.

[0195] Circumferential speed (V) (m / s) = 2πr × n / 60 (3) In equation (3), V is the peripheral speed of the impeller (unit: m / s), r is the diameter of the impeller (unit: m), and n is the rotation speed of the impeller per unit time (unit: rpm).

[0196] In the case of a rotary mixer in which the container containing the material to be dried itself rotates, the rotation speed of the container is appropriately set depending on the size of the device, the drying throughput (amount dried per hour), etc. The rotation 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. The maximum peripheral 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.

[0197] In addition, the filling rate of the rotary dryer (effective volume of the rotary container (m ３ ) to the filling volume (m ３ From the viewpoint of heat treatment efficiency, the filling rate is preferably in the range of 5% to 95%, more preferably 6% to 50%, and even more preferably 10% to 40%.

[0198] Furthermore, when using a rotary dryer, it is preferable to increase the heat transfer area relative to the internal volume of the rotary container in order to efficiently transfer heat to the contents of the rotary container. This heat transfer area relative to the internal volume is determined by the effective volume (m ３ ) to the heat transfer area (m ２ The larger this ratio, the more efficient the heat transfer and the faster the temperature rise rate of the contents. As a result, the drying time is shortened, which reduces thermal and mechanical damage and improves productivity. This ratio is set appropriately depending on the specifications, shape, and shape of the contents of the dryer, but is preferably set to 10 m －１ More than 12m, preferably －１ , more preferably 15 m －１That's all. The effective volume refers to the internal volume of the rotating vessel in which the contents are accommodated, and the heat transfer area refers to the area of ​​the heating surface that can add heat to the contents accommodated in the rotating vessel. Specifically, the heat transfer area is the sum of the area of ​​the outer peripheral surfaces of the multiple heating tubes and the area of ​​the inner peripheral surface of the rotating vessel.

[0199] (Additives) Examples of additives used in the drying step include the above-mentioned gel fluidizing agents (surfactants, polymer lubricants, etc.) as well as the below-mentioned surface cross-linking agents (post-cross-linking agents). By adding a surface cross-linking agent before drying or during drying, adhesion during stirring and drying can be reduced, and further, a general surface cross-linking effect after drying (for example, improvement in water absorption capacity under pressure) is also exhibited, so that an advantage can be obtained that the surface cross-linking step after the drying step can also be omitted. From the viewpoint of making the CRC of the obtained water absorbent resin exceed 25 g / g, it is preferable to add the above-mentioned hydrogen peroxide before or during drying.

[0200] [3-2-5] Surface Cross-Linking Step The production method of the present invention may include a surface cross-linking step. The surface cross-linking step is a step of performing a cross-linking reaction in the surface layer of particles made of a water-absorbent resin (a region approximately several tens of μm deep from the particle surface). Typically, the surface cross-linking step includes a surface cross-linking agent addition step of adding a surface cross-linking agent that reacts with functional groups (particularly carboxy groups) of the water-absorbent resin to a particulate hydrogel or a dried polymer, and a heat treatment step of heat-treating the particulate hydrogel or dried polymer containing this surface cross-linking agent, and preferably includes a cooling step after the heat treatment step. If necessary, after the drying step and before the surface cross-linking step, a sizing step as described later in [3-2-7] is performed to obtain a particulate dried polymer having a preferred particle size. For example, when static drying is performed in the drying step, the resulting dried polymer contains relatively large aggregates, so it is preferable to perform a sizing step before the surface cross-linking step.

[0201] [3-2-5-1] Surface Crosslinking Agent Addition Step The surface crosslinking agent addition step is a step of adding a surface crosslinking agent to the particulate hydrogel and / or the particulate dry polymer. The surface crosslinking agent is added to the particulate hydrogel before or during drying, or to the particulate dry polymer after drying or sizing. The particulate hydrogel and / or the particulate dry polymer may contain a fine powder granulated material.

[0202] When the particulate hydrogel obtained in the gel crushing step is subjected to the surface cross-linking step during drying, the water content of the particulate hydrogel at the time of adding the surface cross-linking agent is preferably in the range of 10% by mass to 50% by mass, more preferably 15% by mass to 45% by mass, and even more preferably 20% by mass to 40% by mass. By adding the surface cross-linking agent in such a range, excessive aggregation and adhesion during drying are reduced. Furthermore, the effect of improving the absorbency under pressure obtained by carrying out the surface cross-linking step after the drying step is also exhibited when surface cross-linking is carried out simultaneously with drying, which is preferable.

[0203] In the case where surface crosslinking is carried out simultaneously with drying in this way, the surface crosslinking step and the sizing step after the drying step can be omitted, and therefore the production step can be significantly simplified. Furthermore, it is preferable because the generation of fine powder and the destruction of the surface crosslinked structure resulting from process damage to the water absorbent resin in the separately required surface crosslinking step and the transportation step do not substantially occur.

[0204]

[0122] In addition, in the case where a surface crosslinking step is separately carried out after the drying step, the solid content of the (particulate) dry polymer at the time of addition of the surface crosslinking agent 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, still 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.

[0205] The temperature of the water absorbent resin to be subjected to the surface cross-linking step after the drying step, i.e., the temperature of the (particulate) dried polymer at the time of adding the surface cross-linking agent, is preferably 40 to 120°C, more preferably 60°C or higher and 100°C or lower. In addition, when surface cross-linking is performed simultaneously with drying, the temperature of the water absorbent resin during drying, i.e., the temperature of the particulate hydrogel at the time of adding the surface cross-linking agent, 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 in the same manner as the gel temperature described above in the drying step.

[0206] (Surface Crosslinking Agent) As the surface crosslinking agent that can be used in the surface crosslinking step, the surface crosslinking agents listed in the above section [2-3-2] can be used, and the description in that section can be cited.

[0207] (Surface Crosslinking Agent Solution) The amount of the surface crosslinking agent added is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, calculated as a solid content, relative to the particulate hydrogel or the particulate dry polymer. The lower limit is preferably 0.001% by mass.

[0208] The surface cross-linking agent is preferably added as a solution dissolved in water or an organic solvent for ease of addition. The concentration of this surface cross-linking agent solution is preferably 1% by mass or more, more preferably 2% by mass or more. The total amount of the solvent selected from water and an organic solvent 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, in terms of solid content, with respect to the particulate hydrogel or the particulate dry polymer. When water and an organic solvent are used in combination as the solvent for the surface cross-linking agent solution, it is preferable that water is the main component of the solvent.

[0209] [3-2-5-2] Heat Treatment Step The heat treatment step constituting the surface cross-linking step is a step of heat-treating a particulate hydrogel or a particulate dry polymer containing a surface cross-linking agent to obtain a surface-cross-linked dry polymer.

[0210] <Surface cross-linking temperature> A surface-cross-linked dried polymer is obtained by heating a particulate hydrogel or a particulate dry polymer containing a surface cross-linking agent to 100°C or higher. This heating temperature (surface cross-linking temperature) is appropriately selected depending on the type of surface cross-linking agent to be added. For example, from the viewpoint of heat treatment efficiency, the surface cross-linking temperature is preferably 100°C or higher and 250°C or lower, more preferably 120°C or higher and 230°C or lower, and even more preferably 150°C or higher and 210°C or lower. By appropriately selecting the above surface cross-linking temperature and the type and amount of the above-mentioned surface cross-linking agent, the water absorbent resin produced by the production method of the present invention can be made to have a CRC of more than 25 g / g.

[0211] <Heating Time> The time for the heat treatment at the surface crosslinking temperature is appropriately set depending on the moisture content of the particulate hydrogel or the particulate dried polymer, the type of the surface crosslinking agent, etc. As a rough guide, it is sufficient to heat until the moisture content of the obtained water absorbent resin powder becomes 10 mass % or less, and the time for the heat treatment is in the range of 10 minutes or more and 120 minutes or less, preferably in the range of 30 minutes or more and 90 minutes or less.

[0212] <Heating Method> The heating device used in the heat treatment step is not particularly limited as long as the object of the present invention is achieved. As the heating device, a device having a stirring mechanism in a conductive heat transfer system by solid-solid contact (hereinafter, sometimes referred to as a stirring-type indirect heating type) is preferably used from the viewpoint of reducing the occurrence of uneven heating. Since a surface-crosslinked particulate dried polymer is obtained after the heat treatment, the stirring dryer (preferably a rotary dryer) described above in the drying step is preferably also used as the heating device in the heat treatment step. For example, it is preferable to add a surface crosslinking agent as an additive to the particulate hydrogel using the rotary dryer with heating tube described above and heat the resulting mixture. In this way, the drying step and the surface crosslinking step are carried out simultaneously, and a particulate dried polymer adjusted to a predetermined water content (solid content) and surface-crosslinked is obtained in one step.

[0213] [3-2-6] Cooling Step The production method of the present invention may include a cooling step. The cooling step is a step of forcibly cooling the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer to a desired temperature after the drying step or the heat treatment step described above and before the sizing step described below. When the surface crosslinking step and the drying step are performed in a single step in the dryer described above, the surface crosslinking treatment is properly performed in the rotary container, and the solid content or water content of the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer is adjusted to a desired range. Thereafter, a cooling step is performed before the product is subjected to the sizing step.

[0214] Specifically, the (particulate) dried polymer after the drying step and / or the surface-crosslinked (particulate) dried polymer after the surface-crosslinking step is forcibly cooled to preferably (t-20)°C or less, more preferably (t-30)°C or less, and even more preferably (t-40)°C or less, relative to the temperature t°C of the (particulate) dried polymer after the drying step and / or the surface-crosslinked (particulate) dried polymer. 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, the (particulate) dried polymer and / or the surface-crosslinked (particulate) dried polymer is forcibly cooled to preferably 50°C or higher and 130°C or lower, more preferably 60°C or higher and 100°C or lower, and even more preferably 65°C or higher and 90°C or lower, before being subjected to the sizing step. By cooling to this temperature range, the workability and classification accuracy during crushing in the sizing step are improved, and the physical properties of the produced water-absorbent resin are improved.

[0215] <Cooling Method> In the cooling step, the method for cooling the (particulate) dried polymer and / or the surface-crosslinked (particulate) dried polymer is not particularly limited. Preferably, a continuous cooler having a ventilation heat transfer type or conductive heat transfer type cooling means is used.

[0216]

[0111] [3-2-7] Particle Size Sizing Step The production method of the present invention may include a particle size sizing step. The particle size sizing step is a step of adjusting the particle size of a dried polymer or a surface-crosslinked (particulate) dried polymer. By carrying out the particle size sizing step after the surface crosslinking step, a water absorbent resin powder in which the particle size or particle size distribution is controlled at a high level can be obtained.

[0217]

[0044] Preferably, the sizing step includes a pulverization step and / or a classification step. More preferably, the sizing step includes a crushing step and / or a classification step. A sizing step in which a water absorbent resin powder having a controlled particle size and particle size distribution is obtained only by a pulverization step or a crushing step is preferred, and a sizing step in which a water absorbent resin powder having a controlled particle size and particle size distribution is obtained only by a crushing step is ideal.

[0218] The pulverization step is a step in which a bulk dried polymer or a strongly aggregated particulate dried polymer obtained when static drying is performed in the drying step or the heat treatment step is pulverized in a pulverizer to adjust the particle size. Note that this pulverization step differs from the gel pulverization step described above in that the dried polymer to be pulverized has undergone a drying step.

[0219] Examples of the pulverizer used in the pulverization step include high-speed rotary pulverizers such as roll mills, hammer mills, screw mills and pin mills, vibration mills, knuckle-type pulverizers and cylindrical mixers.

[0220] The disintegration step is a step in which the particle size is adjusted by disintegrating the loosely aggregated (particulate) dried polymer obtained through the drying step or the heat treatment step with a disintegrator. In the disintegration step, the particle size can be adjusted by less intense pulverization than in the above-mentioned pulverization step.

[0221] The crusher used in the crushing step is preferably one that causes little mechanical damage to the (particulate) dried polymer or the surface-crosslinked (particulate) dried polymer, and specific examples thereof include a roll granulator (Matsubo Corporation), a granulator (Kurimoto, Ltd.), and a randell mill (Tokuju Kogyosho Co., Ltd.).

[0222] The classification step is a process of removing coarse particles and fine powder from the (particulate) dried polymer, the surface-crosslinked (particulate) dried polymer, or a pulverized or crushed product thereof using a classifier. As the classifier used in the classification step, a vibrating or oscillating sieve classifier using a sieve mesh is used.

[0223] The sizes of the coarse particles and fine powder removed in the classification step of the sizing process are appropriately set depending on the particle diameter of the water-absorbent resin or water-absorbing agent as the final product. Preferably, the particle diameter of the coarse particles is 2000 μm or more, more preferably 850 μm or more. As described above, the fine powder is a water-absorbent resin that passes through a sieve with an opening of 150 μm.

[0224] (Particle diameter of dried polymer or surface-crosslinked dried polymer) From the viewpoint of reducing fine powder, the mass average particle diameter (D50) of the (particulate) dried polymer or surface-crosslinked (particulate) dried polymer subjected to the sizing step is preferably 200 μm or more, more preferably 300 μm or more, even more preferably 400 μm or more, and particularly preferably 500 μm or more. From the viewpoint of improving the efficiency of the disintegration step, the mass average particle diameter is preferably 2000 μm or less, further preferably 1500 μm or less, or even 1000 μm or less. The mass average particle diameter can be measured, for example, by a method similar to the method described in columns 27-28 of U.S. Pat. No. 7,638,570, "(3) Mass average particle diameter (D50) and logarithmic standard deviation of particle size distribution."

[0225]

[0044] (Particle diameter of water absorbent resin powder) From the viewpoint of water absorption performance, the mass median particle diameter (D50) of the water absorbent resin powder obtained through a sizing step is preferably 200 µm or more, more preferably 200 µm or more and 600 µm or less, even more preferably 250 µm or more and 550 µm or less, and particularly preferably 300 µm or more and 500 µm or less. The mass median particle diameter can be measured, for example, by a method similar to the method described in "(3) Mass median particle diameter (D50) and logarithmic standard deviation of particle size distribution" in columns 27 to 28 of U.S. Pat. No. 7,638,570.

[0226] From the viewpoint of improving the quality of the produced water-absorbent resin (water-absorbing agent), it is preferable that the main component of the water-absorbent resin powder is particles having a particle diameter of 150 μm or more and 850 μm or less. The proportion of particles having a particle diameter of 150 μm or more and 850 μm or less contained in the water-absorbent 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, even more 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 proportions of particles having a particle diameter of less than 150 μm and more than 850 μm contained in the water-absorbent resin powder are both preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and particularly preferably 1% by mass.

[0227] [3-2-8] Step of Adding Other Additives The production method of the present invention may include a step of adding other additives. The step of adding other additives is an optional step carried out for the purpose of imparting various additional functions to a water absorbent resin (water absorbing agent) and improving water absorption performance, and is a step of adding the following other additives to a water absorbent resin powder obtained through a surface cross-linking step. From the viewpoint of the addition effect, it is preferable that the following other additives are present on the surface of each particle of the water absorbent resin powder. Therefore, the step of adding other additives is preferably carried out simultaneously with or separately from the surface cross-linking step, and more preferably carried out after the surface cross-linking step.

[0228] Other additives include those listed in the section [2-3-6], and the descriptions in that section can be used by reference.

[0229] [3-2-9] Other Steps In addition to the steps described above, the production method of the present invention may further include a rewetting step, a pulverizing step, a classification step, a granulating step, a transporting step, a storing step, a packaging step, a preservation step, and the like, as necessary.

[0230] The present invention may also include the following inventions [1] to

[11] .

[0231] [1] A water-absorbent resin that satisfies the following conditions (a) to (d): (a) the 0°C vortex value is 30 seconds or less; (b) the ratio of the 0°C vortex value to the 30°C vortex value (vortex ratio (0°C / 30°C)) is 5 or less; (c) the GEX value is 17 or more; (d) the CRC value is more than 25 g / g.

[0232] [2] The water absorbent resin according to [1], which comprises a crosslinked polymer of an acid group-containing unsaturated monomer, at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer contains one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts.

[0233] [3] The water-absorbent resin according to [1] or [2], wherein the ratio of the number of moles of the one or more salts selected from the group consisting of potassium salts, lithium salts, and ammonium salts to the number of moles of the whole of the neutralized acid group-containing unsaturated monomers is 50 mol % or more and 100 mol % or less. [4] The water-absorbent resin according to any one of [1] to [3], wherein the mass average particle diameter is 50 μm or more and 600 μm or less.

[0234] [5] The water-absorbent resin according to any one of [1] to [4], wherein the content of the residual volatile component is 0.4 mass % or less relative to the mass of the entire water-absorbent resin.

[0235] [6] The water absorbent resin according to any one of [1] to [5], wherein a content of fine powder is 15 mass % or less with respect to a mass of the entire water absorbent resin, and the fine powder is a water absorbent resin that passes through a sieve having an opening of 150 μm when the entire water absorbent resin is classified using the sieve.

[0236] [7] A method for producing a water-absorbent resin, comprising: a polymerization step of crosslinking 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 hydrogel-like crosslinked polymer, wherein at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer contains one or more 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 mass % or more and less than 55 mass % with respect to the mass of the entire monomer composition, and the polymerization initiation temperature in the polymerization step is 60°C or more.

[0237] [8] The method for producing a water-absorbent resin according to [7], wherein the solid content of the hydrogel crosslinked polymer is 60 mass % or less with respect to the total mass of the hydrogel crosslinked polymer.

[0238] [9] The method for producing a water-absorbent resin according to [7] or [8], further comprising a hydrogel crushing step of crushing the hydrogel crosslinked polymer to obtain a particulate hydrogel.

[0239]

[10] The method for producing a water-absorbent resin according to any one of [9], wherein the hydrogel crushing step is a step of crushing the hydrogel-like crosslinked polymer so that the mass average particle diameter of the particulate hydrogel converted into solid content becomes 1 mm or less.

[0240]

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

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

[0241] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0242] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention.

[0243] In addition, the term "water-absorbent resin" described below means a granular dried material that has undergone a drying step, a surface-crosslinked granular dried material or water-absorbent resin powder, and a surface-crosslinked water-absorbent resin powder, and the term "hydrogel" means a hydrogel-like crosslinked polymer or a particulate hydrogel that has not undergone a drying step.

[0244] In addition, unless otherwise noted, a power supply of 200 V or 100 V at 60 Hz was used for the electrical equipment (including equipment for measuring the physical properties of the water-absorbent resin) used in the experimental examples. In addition, unless otherwise noted, the physical properties of the water-absorbent resin and hydrous gel described below were measured under conditions of room temperature (20°C to 25°C) and a relative humidity of 50% RH ± 10%.

[0245] For convenience, "liter" is sometimes written as "l" or "L".

[0246] (Evaluation Methods) The physical properties of the water-absorbent resins and hydrogels obtained in Examples 1 to 6 and Comparative Examples 1 to 4 were measured and evaluated by the methods described below.

[0247] (A) "CRC" The CRC of the water-absorbent resin was measured in accordance with NWSP 241.0.R2 (15). Specifically, 0.2 g of the water-absorbent resin was placed in a nonwoven bag, and then immersed in a large excess of a 0.9% by mass aqueous solution of sodium chloride for 30 minutes to allow free swelling. Thereafter, the bag was centrifuged (250 G) for 3 minutes to drain the water, and the mass was measured, and the water absorption capacity (unit: g / g) of the water-absorbent resin was calculated.

[0248] (B) "PSD" The mass median particle diameter (D50) and the logarithmic standard deviation (σζ) of particle size distribution of a water absorbent resin were measured by the same method as the method described in "(3) Mass median particle diameter (D50) and logarithmic standard deviation of particle size distribution" in columns 27 to 28 of U.S. Patent No. 7,638,570. Here, the "fine powder content" refers to the content (mass%) of a water absorbent resin that passes through a sieve with an opening of 150 μm relative to the mass of the entire water absorbent resin.

[0249] (C) "Moisture content and solid content" The moisture content of the water-absorbent resin was measured in accordance with NWSP. Specifically, among the moisture content measurement methods described in NWSP 230.0. R2 (15), the moisture content of the water-absorbent resin was measured by a method in which the mass of the sample was changed to about 1.0 g, the drying temperature was changed to 180°C, and the drying time was changed to 24 hours. Specifically, first, about 1.0 g of the water-absorbent resin was accurately weighed (this mass was defined as M (g)). After the water-absorbent resin of M (g) was placed in an aluminum cup having a bottom diameter of 50 mm, the total mass W of the sample in the cup was calculated. １ The cup was then placed in an oven set at an atmospheric temperature of 180° C. After 24 hours, the cup was removed from the oven and the total mass W of the cup was measured. ２ (g) was accurately weighed. １ (g) and W ２ Using the (g), the water content of the water-absorbent resin (100-α) (mass%) was calculated according to the following formula (4). α is the solid content (mass%) of the water-absorbent resin. Water content of the water-absorbent resin (100-α) (mass%) = {(W １ -W ２) / M}×100 (4) The water content and solid content of the hydrogel were also measured in accordance with NWSP. Specifically, the same method as the method for measuring the water content of the water-absorbent resin described above was used, except that the mass of the sample was changed to about 2.0 g. That is, the exact mass M' (g) of the sample (hydrogel), the total mass W of the sample in the aluminum cup, ３ (g) and the total mass W of the cup after removal from the oven ４ (g) was accurately weighed. ３ (g) and W ４ Using the sample (g), the water content (100-α') (mass%) of the hydrogel was calculated according to the following formula (5), in the same manner as in the method for measuring the water content of the water-absorbent resin described above. α' is the solid content (mass%) of the hydrogel. The water content and solid content of the hydrogel measured here were used as the water content and solid content of the hydrogel to calculate other parameters. Water content of hydrogel (100-α') (mass%) = {(W ３ -W ４ ) / M'} × 100 (5) (D) "0 ° C Vortex and 30 ° C Vortex" 0.02 parts by mass of Food Blue No. 1 (Brilliant Blue) was added to 1,000 parts by mass of a 0.90% by mass sodium chloride aqueous solution to prepare a test solution. 50 ml of the test solution was measured into a 100 ml beaker. The test solution in the beaker was adjusted to a liquid temperature of 0 ° C, and 2.00 g of water-absorbent resin was added to the test solution while stirring at 600 rpm using a cylindrical stirrer with a length of 40 mm and a diameter of 8 mm. Then, in accordance with the standards described in JIS K 7224-1996 "Explanation of the Water Absorption Rate Test Method for Super Absorbent Polymers," the starting point was the time when the water-absorbent resin was added to the test solution, and the end point was the time when the water-absorbent resin absorbed saline and covered the stirrer tip. The time from the start point to the end point was expressed as "0°C Vortex" (unit: seconds). If the time from the start point to the end point is less than 1.0 second, it is expressed to one decimal place.

[0250] The "30°C Vortex" was measured using the same method as the "0°C Vortex", except that the test liquid used was adjusted to a liquid temperature of 30°C. Next, the "0°C Vortex" and "30°C Vortex" were used to calculate the vortex ratio (0°C / 30°C) = (0°C Vortex) / (30°C Vortex). The measurements of the 0°C Vortex and 30°C Vortex were carried out at room temperature (20°C to 25°C) and a relative humidity of 50% RH ± 10%.

[0251] (E) "Ext" 1.0 g of a water-absorbent resin was added to 200 ml of a 0.9 mass % aqueous sodium chloride solution, and the mixture was stirred at 500 rpm for 16 hours. Thereafter, the amount of a substance dissolved in the aqueous solution was measured by pH titration, and the measured value was taken as Ext (water-soluble content) (unit: mass %).

[0252] In addition, the amount of the substance dissolved in the aqueous solution was measured by the same method as the above-mentioned method for measuring the Ext of the water-absorbent resin, except that 2.0 g of the hydrogel was used instead of 1.0 g of the water-absorbent resin. Subsequently, the amount of the substance was converted into a solid content based on the water content of the hydrogel measured by the method (C) above, to calculate the Ext (unit: mass%) in the hydrogel.

[0253] (F) "GEX" Using the CRC of the water-absorbent resin and Ext (water-soluble content) of the water-absorbent resin measured by the above-mentioned method, the GEX value of the water-absorbent resin was calculated based on the following formulas (1) and (2). In the following formulas (1) and (2), CRC is defined as y (g / g) and Ext is defined as x (mass%). When x > 1, GEX value = (y + 17) / ln(x) (1) Here, ln(x) is the natural logarithm of x. When x ≦ 1, GEX value = (y) / (x) (2) (G) Particle size of particulate hydrous gel <Particle size distribution and mass average particle size (Gel D50) of particulate hydrous gel> The particle size distribution and mass average particle size (Gel D50) of the particulate hydrous gel were measured by the following method.

[0254] 20 g of particulate hydrogel (solid content α% by mass) at a temperature of 20-25°C was added to 1,000 g of a 20% by mass aqueous solution of sodium chloride containing 0.08% by mass of polyoxyethylene sodium lauryl sulfate (surfactant) (hereinafter referred to as "aqueous surfactant solution") to prepare a dispersion. The dispersion was then stirred at 300 rpm for 16 hours using a stirrer tip measuring 50 mm in length and 7 mm in diameter. The container used was a cylindrical polypropylene container (height 21 cm, diameter 8 cm, internal volume approximately 1.14 L).

[0255] After the stirring was completed, the dispersion was poured from the top of the stacked sieve into the center of a stacked sieve consisting of eight JIS standard sieves placed on a turntable. The sieves constituting the stacked sieve all had a diameter of 21 cm, and the sieve openings were 8 mm, 4 mm, 2 mm, 1 mm, 0.60 mm, 0.30 mm, 0.15 mm, and 0.075 mm, in order from the top sieve. Subsequently, using 100 g of another surfactant aqueous solution, all of the particulate hydrogel remaining in the container was washed out onto the stacked sieve. Thereafter, while rotating the stacked sieve by hand (speed: 20 rpm), another 6000 g of surfactant aqueous solution was poured using a shower (72 holes, liquid volume: 6.0 [L / min]) from a height of 30 cm above the stacked sieve, and the particulate hydrogel was classified. This operation was carried out within a water pouring range (50 cm ２ ) was poured evenly four times so that it was distributed over the entire upper surface of the laminated sieve. The particulate hydrogel on the first sieve of the classified laminated sieve was drained for about 2 minutes and then weighed. The second and subsequent sieves of the laminated sieve were also classified in the same manner, and after draining, the particulate hydrogel remaining on each sieve was weighed. The sieves constituting the laminated sieve were appropriately changed depending on the particle diameter of the particulate gel. For example, when the particle diameter of the particulate hydrogel is small (fine) and clogging occurs in a sieve with an opening of 0.15 mm and / or 0.075 mm, the sieve with an opening of 0.15 mm and / or 0.075 mm is replaced with a JIS standard sieve with a larger diameter (a sieve with a diameter of 30 cm and an opening of 0.15 mm and / or a sieve with a diameter of 30 cm and an opening of 0.075 mm), and classification is performed.

[0256] The ratio (mass%) of the particulate hydrogel to the total mass of the particulate hydrogel was calculated from the mass of the particulate hydrogel remaining on each sieve using the following formula (6). The mesh size of each sieve used for classification was converted based on the following formula (7). The converted mesh size and the mass% of the particulate hydrogel remaining on each sieve were plotted on logarithmic probability paper. The mesh size value corresponding to 50% by mass was read and used as the mass average particle size (Gel D50) of the particulate hydrogel.

[0257] X (%) = (w / W) x 100... (6) R (mm) = (20 / W) １／３ ×r (7) In the formulas (6) and (7), X: mass % (%) of the particulate hydrogel remaining on each sieve after classification and draining; w: mass (g) of each of the particulate hydrogels remaining on each sieve after classification and draining; W: total mass (g) of the particulate hydrogel remaining on each sieve after classification and draining; R: sieve opening (mm) when converted to particulate hydrogel before swelling; and r: sieve opening (mm) used for classification.

[0258] <Solid content-equivalent mass average particle diameter of particulate hydrogel (Solid D50)> From the solid content (α) of the particulate hydrogel measured by the method (C) above and the mass average particle diameter (Gel D50) of the particulate hydrogel, the solid content-equivalent mass average particle diameter (mass average particle diameter of the particulate hydrogel after drying, Solid D50) was calculated according to the following formula (8): Solid D50 = Gel D50 × (α / 100) １／３ (8) In the formula (8), Gel D50: mass average particle diameter (μm) of the particulate hydrogel particles, α: solid content (% by mass) of the particulate hydrogel, and Solid D50: mass average particle diameter (μm) of the particulate hydrogel converted into a dry product.

[0259] (H) "Amount of remaining volatile components" When a mixed liquid of dimethylformamide (DMF), a 25 mass % aqueous phosphoric acid solution, and water absorbent resin particles was heated at 110°C, the amount of volatile components derived from an organic solvent or the like taken into the inside of the water absorbent resin particles in the vaporized gas was quantified, and the measured value was converted into a value per 1 g of water absorbent resin, which was taken as the amount of remaining volatile components. A specific procedure is shown below.

[0260] (H-1) Preparation of a calibration curve Heptane, dimethylformamide (DMF), and a 25% by mass aqueous solution of phosphoric acid were prepared in a stoppered glass container. These were cooled as necessary to reduce errors due to evaporation during measurement.

[0261] First, 0.15 g of heptane was precisely weighed into a 200 mL volumetric flask, and DMF was added to bring the total volume to 200 mL, which was designated as standard solution (1). Next, 10 mL of standard solution (1) was precisely measured using a volumetric pipette into a 20 mL volumetric flask, and DMF was added to bring the total volume to 20 mL. Standard solution 1 was diluted two-fold, which was designated as standard solution (2).

[0262] Similarly, standard solution (2) was diluted 2-fold to obtain standard solution (3). Similarly, standard solution (3) was diluted 2-fold to obtain standard solution (4). Similarly, standard solution (4) was diluted 2-fold to obtain standard solution (5).

[0263] 4 mL of standard solution (1) was accurately measured and added to a 20 mL vial, and 5 mL of 25% by mass aqueous phosphoric acid solution was then accurately measured and added. The vial was quickly sealed with a rubber septum and an aluminum cap, and the vial was shaken and mixed for 1 minute. The same procedure was performed on standard solutions (2) to (5), and solutions for creating a calibration curve were prepared.

[0264] The vials were heated at 110°C for 2 hours while being shaken and stirred, and then 1 mL of the gas phase was injected into a gas chromatograph to obtain a chromatogram for each calibration curve preparation solution. A calibration curve was prepared using the mass of heptane precisely weighed when preparing the calibration curve preparation solutions and the peak area of ​​the chromatogram.

[0265] (H-2) Measurement of the amount of remaining volatile components DMF and a 25% by mass aqueous phosphoric acid solution were prepared. 0.10 g of the water-absorbent resin obtained in each of the Examples and Comparative Examples was precisely weighed into a 20 mL vial. 4 mL of DMF and 5 mL of a 25% by mass aqueous phosphoric acid solution were added to the vial. The vial was quickly sealed using a septum rubber and an aluminum cap, and the vial was shaken and mixed for 1 minute. After the vial was heated at 110 ° C. for 2 hours while being shaken and mixed, 1 mL of the gas phase was injected into a gas chromatograph to obtain a chromatogram.

[0266] The amount of volatile components contained in the water-absorbent resin (precisely weighed value of 0.10 g) was calculated from the peak area of ​​the obtained chromatogram and the calibration curve prepared above. The calculated value was converted into a value per 1 g of the water-absorbent resin, and the amount of residual volatile components (% by mass) was determined.

[0267] The gas chromatograph conditions are as follows:

[0268] Apparatus: GC-2010 Plus (Shimadzu Corporation) Packing: Squalane 25% Shimalite (NAW) (101) 80-100 mesh Column: 3.2 mmφ×2.1 m Column temperature: 80°C Inlet temperature: 180°C Detector temperature: 180°C Detector: FID Carrier gas: N2 Example 1 (Preparation process of monomer composition) The following materials were placed in a 2-liter polypropylene container and mixed to prepare a monomer composition (a1): 372.3 g (5.17 mol) of acrylic acid; 54.05 g (0.2 mol% relative to the monomer) of a 10% by mass aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent; 25.80 g of a 0.1% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid (DTPA.3Na) (0.0011 mol % relative to the monomer); 131.5 g of a 48.5% by mass aqueous solution of potassium hydroxide; 298.3 g of deionized water (ion-exchanged water).

[0269] Next, the monomer composition (a1) was cooled with stirring. When the temperature (liquid temperature) of the monomer composition (a1) reached 50°C, 304.9 g of a 48.5% by mass aqueous potassium hydroxide solution adjusted to 40°C was added and mixed to produce a monomer composition (a2). At this time, the temperature of the monomer composition (a2) rose to 92.5°C due to heat of neutralization.

[0270] (Polymerization Step) Next, 13.1 g of a 5.0 mass % aqueous potassium persulfate solution (0.047 mol % relative to the monomer) was added to the stirred monomer composition (a2) to obtain a monomer composition (a'2). At this time, the temperature of the monomer composition (a2) was 90°C. Thereafter, the monomer composition (a'2) (monomer concentration 43 mass %, neutralization rate of acrylic acid 73 mol %) was immediately poured into a stainless steel bat-type container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open-to-air system. The time from adding the aqueous potassium hydroxide solution to the monomer composition (a1) to pouring the monomer composition (a'2) into the bat-type container was 1 minute. The bat-type container was preheated to a surface temperature of 50°C using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.), and heating was continued until the end of polymerization. The time from when the temperature of the monomer composition (a'2) began to rise (from the start of polymerization) until it reached the maximum temperature (over 100°C) was 10 seconds. A hydrogel crosslinked polymer (b1) was obtained by the above polymerization reaction.

[0271] (Hydrogel Crushing Step) The obtained hydrogel-like crosslinked polymer (b1) was put into a screw extruder and subjected to gel pulverization. As the screw extruder, a meat chopper having a die with a diameter of 100 mm, a hole diameter of 6.4 mm, 83 holes, an opening rate of 34.0%, and a thickness of 10 mm at the tip (extrusion port), an outer diameter of the screw shaft of 86 mm, and an inner diameter of the casing of 88 mm was used. Gel pulverization (first gel pulverization) was carried out while simultaneously supplying 80 ° C. hot water, steam, and a 10 mass% concentration aqueous solution of lauryl dimethyl amino acetic acid betaine with the hydrogel-like crosslinked polymer (b1), to obtain a particulate hydrogel (c3). Note that 80 ° C. hot water was 1 mass%, water vapor was 1 mass%, and lauryl dimethyl amino acetic acid betaine was supplied so that the active ingredient was 0.08 mass% relative to the solid content of the hydrogel-like crosslinked polymer (b1). Subsequently, the particulate hydrogel (c3) obtained by the first gel crushing was further crushed (second gel crushing) while supplying hot water and steam in the same manner to obtain a particulate hydrogel (c2). The obtained particulate hydrogel (c2) had a mass average particle diameter (solid D50) of 239 μm.

[0272] Subsequently, the particulate hydrogel (c2) obtained by the second gel crushing was further crushed (third gel crushing) while supplying hot water and steam in the same manner to obtain a particulate hydrogel (c1). The obtained particulate hydrogel (c1) had a Solid D50 of 150 μm.

[0273] (Step of drying particulate hydrogel) The obtained particulate hydrogel (c1) was dried using a hot air dryer. This dryer was equipped with a basket (bottom size 30 cm × 20 cm) made of a wire mesh with a mesh size of 1.2 mm. 500 g of the particulate hydrogel (c1) was spread almost uniformly on the bottom surface of the basket, and hot air at 190 ° C. was blown from below for 30 minutes to obtain a dried product (A'1).

[0274] (Pulverization and classification process of dried product) The cooled dried product (A'1) was supplied to a roll mill and pulverized, and then 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 a water-absorbent resin (A1).

[0275] [Example 2] The particulate hydrogel (c2) obtained by the second gel pulverization in the (hydrogel pulverization step) of Example 1 was dried in the same manner as in the (particulate hydrogel drying step) of Example 1. Thereafter, the particulate hydrogel (c2) was pulverized and classified in the same manner as in the (dried product pulverization and classification step) of Example 1 to obtain a water-absorbent resin (A2). That is, the water-absorbent resin (A2) was obtained by the same method as in Example 1, except that the particulate hydrogel (c2) was subjected to the particulate hydrogel drying step without performing the third gel pulverization.

[0276] Example 3 The water absorbent resin (A1) obtained in Example 1 was mixed with 2 parts by mass of hydrophilic silica (Reolosil (registered trademark) QS-20, manufactured by Tokuyama Corporation). The mixing was carried out by placing 30 g of the water absorbent resin (A1) together with the hydrophilic silica in a 225 ml mayonnaise bottle (inner diameter 52 mm, height 110 mm), and shaking the bottle at a vibration speed of 750 CPM for 1 minute using a paint shaker (manufactured by Toyo Seiki Seisaku-sho, Ltd.). As a result, a water absorbent resin (A3) was obtained.

[0277] [Example 4] The water absorbent resin (A2) obtained in Example 2 was mixed with hydrophilic silica (Reolosil (registered trademark) QS-20, manufactured by Tokuyama Corporation) in the same manner as in Example 3, thereby obtaining a water absorbent resin (A4).

[0278] [Example 5] (Preparation step of monomer composition) A monomer composition was prepared in the same manner as in Example 1 (Preparation step of monomer composition), to obtain a monomer composition (a3), except for the following points: - The amount of 10 mass % aqueous polyethylene glycol diacrylate solution (molecular weight 523) to be added was changed to 24.32 g (0.09 mol % relative to the monomer); - The amount of deionized water (ion-exchanged water) to be used was changed to 328.1 g.

[0279] (Polymerization step) Polymerization was carried out in the same manner as in (Polymerization step) of Example 1, except that the monomer composition (a3) ​​was used instead of the monomer composition (a2), to obtain a hydrogel-like crosslinked polymer (b2).

[0280] (Hydrogel Crushing Step) In the (Hydrogel Crushing Step) of Example 1, (First Gel Crushing), (Second Gel Crushing), and (Third Gel Crushing) were carried out by the same method except that the hydrogel-like crosslinked polymer (b2) was used instead of the hydrogel-like crosslinked polymer (b1), to obtain a particulate hydrogel (c1'). Furthermore, while similarly supplying hot water and steam, the particulate hydrogel (c1') obtained by the third gel crushing was further gel-crushed (Fourth Gel Crushing) to obtain a particulate hydrogel (c4). The obtained particulate hydrogel (c4) had a Solid D50 of 120 μm.

[0281] (Step of drying particulate hydrogel) The obtained particulate hydrogel (c4) was dried in the same manner as in (step of drying particulate hydrogel) of Example 1 to obtain a dried product (A'5).

[0282] (Step of pulverizing and classifying dried product) The obtained dried product (A'5) was pulverized and classified in the same manner as in (Step of pulverizing and classifying dried product) of Example 1 to obtain a water-absorbent resin (A5).

[0283] (Addition Step) The obtained water absorbent resin (A5) was mixed with hydrophilic silica (Reolosil (registered trademark) QS-20, manufactured by Tokuyama Corporation) in the same manner as in Example 3 to obtain a water absorbent resin (A6).

[0284] Example 6 (Preparation of Monomer Composition) A monomer composition (a2) was obtained in the same manner as in Example 1 (Preparation of Monomer Composition).

[0285] (Polymerization Step) A hydrogel crosslinked polymer (b1) was obtained in the same manner as in (Polymerization Step) of Example 1.

[0286] (Hydrogel Crushing Step) A twin-screw kneader equipped with a main body (barrel) incorporating two rotating shafts rotating in the same direction was used as a gel crushing device to crush the hydrogel-like crosslinked polymer (b1) and obtain a particulate hydrogel (c5). Each rotating shaft was provided with a circular disk, which was the main crushing means. The barrel had a jacket structure, and a gas inlet penetrating the jacket was provided to introduce water vapor into the main body.

[0287] First, a heat medium at 105 ° C. was circulated inside the jacket, and the temperature inside the main body (barrel) was maintained at 105 ° C. Then, the rotation speed was set to 50 rpm, and the hydrogel (b1) was fed into the inlet of the twin-screw kneader at a feed rate of 0.25 kg / min to perform gel pulverization, thereby obtaining a particulate hydrogel (c5). At that time, water at 90 ° C. was fed simultaneously with the hydrogel-like crosslinked polymer (b1), and further, a 10 mass% concentration aqueous solution of lauryl dimethyl amino acetic acid betaine as a gel fluidizing agent, and further, water vapor at 0.6 MPa was fed from the gas inlet. The amount of water fed at 90 ° C. was 11 mass% relative to the solid content of the hydrogel-like crosslinked polymer (b1). The amount of water fed at 0.6 MPa was 24 mass% relative to the solid content of the hydrogel-like crosslinked polymer (b1). The amount of lauryldimethylaminoacetate betaine added as an active ingredient was 0.08% by mass based on the solid content of the hydrogel crosslinked polymer (b1). The diameter D of the disk used for gel crushing was 50 mm, and the minimum clearance between the barrel and the disk was 1 mm (2% of the disk diameter D). The obtained particulate hydrogel (c5) had a Solid D50 of 147 μm.

[0288] (Step of drying particulate hydrogel) The obtained particulate hydrogel (c5) was dried in the same manner as in (step of drying particulate hydrogel) of Example 1 to obtain a dried product (A'7).

[0289] (Step of pulverizing and classifying dried product) The obtained dried product (A'7) was pulverized and classified in the same manner as in (Step of pulverizing and classifying dried product) of Example 1 to obtain a water-absorbent resin (A7).

[0290] (Step of adding silica to water-absorbent resin) The obtained water-absorbent resin (A7) was mixed with hydrophilic silica in the same manner as in (the adding step) of Example 5, to obtain a water-absorbent resin (A8).

[0291] [Example 7] (Process for preparing monomer composition) The following materials were placed in a 2-liter polypropylene container and mixed to prepare a monomer composition (a4): 440.1 g (6.11 mol) of acrylic acid; 31.94 g (0.1 mol % relative to the monomer) of a 10 mass % aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent; 25.80 g (0.0009 mol % relative to the monomer) of a 0.1 mass % aqueous solution of diethylenetriaminepentaacetic acid trisodium (DTPA 3Na); 462.8 g of deionized water (ion-exchanged water).

[0292] Next, the temperature of the monomer composition (a4) was adjusted to 45° C. in a thermostatic bath. After the temperature adjustment, 328.8 g of a 30 mass % aqueous ammonia solution at 25° C. was added and mixed to prepare a monomer composition (a5). At this time, the temperature of the monomer composition (a5) rose to 91.0° C. due to heat of neutralization.

[0293] (Polymerization Step) Next, 15.5 g of a 5.0 mass % aqueous solution of sodium persulfate (0.053 mol% relative to the monomer) was added to the stirred monomer composition (a5) to obtain a monomer composition (a'5). At this time, the temperature of the monomer composition (a5) was 90.5°C. Thereafter, the monomer composition (a'5) (monomer concentration 43 mass%, neutralization rate of acrylic acid 73 mol%) was immediately poured into a stainless steel bat-type container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coated) in an open-to-air system. Note that the time from adding the aqueous potassium hydroxide solution to the monomer composition (a4) to pouring the monomer composition (a'5) into the bat-type container was 1 minute. The vat-shaped container was preheated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) to a surface temperature of 50°C, and heating was continued until the end of polymerization. The time from when the temperature of the monomer composition (a'5) began to rise (from the start of polymerization) until it reached its maximum temperature (over 100°C) was 10 seconds. A hydrogel-like crosslinked polymer (b3) was obtained by the above polymerization reaction.

[0294] (Hydrogel Crushing Step) The obtained hydrogel-like crosslinked polymer (b3) was put into a screw extruder and subjected to gel pulverization. As the screw extruder, a meat chopper having a die with a diameter of 100 mm, a hole diameter of 6.4 mm, 83 holes, an opening rate of 34.0%, and a thickness of 10 mm at the tip (extrusion port), an outer diameter of the screw shaft of 86 mm, and an inner diameter of the casing of 88 mm was used. Gel pulverization (first gel pulverization) was carried out while simultaneously supplying 80 ° C. hot water, steam, and a 10 mass% concentration aqueous solution of lauryl dimethyl amino acetic acid betaine with the hydrogel-like crosslinked polymer (b3), to obtain a particulate hydrogel (c5). Note that 80 ° C. hot water was 1 mass%, steam was 1 mass%, and lauryl dimethyl amino acetic acid betaine was supplied so that the active ingredient was 0.08 mass% relative to the solid content of the hydrogel-like crosslinked polymer (b3). Subsequently, the particulate hydrogel (c5) obtained by the first gel crushing was further crushed (second gel crushing) while supplying hot water and steam in the same manner to obtain a particulate hydrogel (c6). The mass average particle diameter (Gel D50) of the obtained particulate hydrogel (c6) was 120 μm.

[0295] (Step of drying particulate hydrogel) The obtained particulate hydrogel (c6) was dried using a hot air dryer. This dryer was equipped with a basket (bottom size 30 cm × 20 cm) made of a wire mesh with a mesh size of 1.2 mm. 250 g of the particulate hydrogel (c1) was spread almost uniformly on the bottom of the basket, and hot air at 100°C was blown from below for 50 minutes to obtain a dried product (A'8).

[0296] (Step of pulverizing and classifying dried product) The obtained dried product (A'8) was pulverized and classified in the same manner as in (Step of pulverizing and classifying dried product) of Example 1 to obtain a water-absorbent resin (A9).

[0297] (Step of adding silica to water-absorbent resin) The obtained water-absorbent resin (A9) was mixed with hydrophilic silica in the same manner as in (the adding step) of Example 5, to obtain a water-absorbent resin (A10).

[0298] [Comparative Example 1] (Process for preparing monomer composition) The following materials were charged into a 2-liter polypropylene container and mixed to prepare a monomer composition (ca1): 422.0 g (5.86 mol) of acrylic acid; 61.26 g (0.2 mol% relative to the monomer) of a 10% by mass aqueous solution of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent; 25.80 g (0.00096 mol% relative to the monomer) of a 0.1% by mass aqueous solution of diethylenetriaminepentaacetic acid trisodium (DTPA 3Na); 135.2 g of a 48.5% by mass aqueous solution of sodium hydroxide; 323.5 g of deionized water (ion-exchanged water).

[0299] Next, the monomer composition (ca1) was cooled while stirring. When the temperature (liquid temperature) of the monomer composition (ca1) reached 48°C, 217.3 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to produce a monomer composition (ca2). At this time, the temperature of the monomer composition (ca2) rose to 91.5°C due to the heat of neutralization.

[0300] (Polymerization step) Next, 14.9 g of a 5.0 mass% aqueous solution of sodium persulfate (0.053 mol% relative to the monomer) was added to the stirred monomer composition (ca2) to obtain a monomer composition (ca'2). At this time, the temperature of the monomer composition (ca2) was 90 ° C. Thereafter, the monomer composition (ca'2) (monomer concentration 43 mass%, neutralization rate of acrylic acid 73 mol%) was immediately poured into a stainless steel bat-type container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open-to-air system. Note that the time from adding 48.5 mass% aqueous sodium hydroxide solution to the monomer composition (ca2) to pouring the monomer composition (ca'2) into the bat-type container was 1 minute. The vat-shaped container was preheated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) to a surface temperature of 50°C, and heating was continued until the end of polymerization. The time from when the temperature of the monomer composition (ca'2) began to rise (from the start of polymerization) until it reached its maximum temperature (over 100°C) was 10 seconds. A hydrogel-like crosslinked polymer (cb1) was obtained by the above polymerization reaction.

[0301] (Hydrogel Crushing Step) The obtained hydrogel-like crosslinked polymer (cb1) was put into a screw extruder and subjected to gel pulverization. As the screw extruder, a meat chopper having a die with a diameter of 100 mm, a hole diameter of 6.4 mm, 83 holes, an opening rate of 34.0%, and a thickness of 10 mm at the tip (extrusion port), an outer diameter of the screw shaft of 86 mm, and an inner diameter of the casing of 88 mm was used. Gel pulverization (first gel pulverization) was performed while simultaneously supplying 80 ° C. hot water, steam, and a 10 mass% concentration aqueous solution of lauryl dimethyl amino acetic acid betaine with the hydrogel-like crosslinked polymer (cb1), to obtain a particulate hydrogel (cc2). Note that 80 ° C. hot water was supplied at 1 mass%, water vapor was supplied at 1 mass%, and lauryl dimethyl amino acetic acid betaine was supplied at 0.08 mass% as an active ingredient relative to the solid content of the hydrogel (cb1). Subsequently, the particulate hydrogel (cc2) obtained by the first gel crushing was further crushed (second gel crushing) while supplying hot water and steam in the same manner to obtain a particulate hydrogel (cc1). The obtained particulate hydrogel (cc1) had a Solid D50 of 140 μm.

[0302] (Step of drying particulate hydrogel) The obtained particulate hydrogel (cc1) was dried using a hot air dryer. This dryer was equipped with a basket (bottom size 30 cm × 20 cm) made of a wire mesh with a mesh size of 1.2 mm. 500 g of the particulate hydrogel (cc1) was spread almost uniformly on the bottom surface of the basket, and hot air at 190 ° C. was blown from below for 30 minutes to obtain a dried product (cA'1).

[0303] (Pulverization and classification process of dried product) The cooled dried product (cA'1) was supplied to a roll mill and pulverized, and then 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 a water-absorbent resin (cA1).

[0304] [Comparative Example 2] The water absorbent resin (cA1) obtained in Comparative Example 1 was mixed with hydrophilic silica (Reolosil (registered trademark) QS-20, manufactured by Tokuyama Corporation) in the same manner as in Example 3, to obtain a water absorbent resin (cA2).

[0305] Comparative Example 3 (Preparation step of monomer composition) A hydrogel crosslinked polymer (cb1) was obtained in the same manner as in Comparative Example 1 (Polymerization step of monomer composition).

[0306] (Hydrogel Crushing Step) Gel crushing was carried out in the same manner as in Example 6 (Hydrogel Crushing Step), except that the hydrogel-like crosslinked polymer (cb1) was used instead of the hydrogel-like crosslinked polymer (b1), to obtain a particulate hydrogel (cc2). The obtained particulate hydrogel (cc2) had a Solid D50 of 158 μm.

[0307] (Step of drying particulate hydrogel) The obtained particulate hydrogel (cc2) was dried in the same manner as in (step of drying particulate hydrogel) of Comparative Example 1 to obtain a dried product (cA'3).

[0308] (Pulverization and Classification of Dried Product) The obtained dried product (cA'3) was pulverized and classified in the same manner as in (Pulverization and Classification of Dried Product) of Comparative Example 1 to obtain a water-absorbent resin (cA3).

[0309] [Comparative Example 4] A water-absorbent resin (cA4) was obtained according to Example 1 of JP-B 02-14925. Specifically, first, 72.1 g of acrylic acid was added to 22.2 g of deionized water. Further, 49.5 g of potassium hydroxide with a purity of 85% as a neutralizing agent and 0.01 g (0.0065 mol% relative to the monomer) of N,N-methylenebisacrylamide as a divinyl compound were sequentially added thereto. By the above operations, an aqueous potassium acrylate solution (degree of neutralization 75%) with a monomer concentration of 70 mass% was prepared.

[0310] The aqueous potassium acrylate 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; the same applies hereinafter) and 1.7 g (0.5% by mass) of a 30.6% aqueous solution of sodium hydrogen sulfite were mixed therein. The resulting mixture was then poured into a vat-shaped container and spread into a layer about 10 mm thick. After about 30 seconds, a polymerization reaction started and was completed in about 1 minute. The maximum temperature during this period was about 120°C.

[0311] Thus, a strip-shaped dry solid of a crosslinked potassium polyacrylate having a moisture content of 11% was obtained. The strip-shaped dry solid corresponds to a hydrous gel-like crosslinked polymer. The strip-shaped dry solid of the crosslinked potassium polyacrylate was pulverized using a pulverizer to obtain a water-absorbent resin (cA4).

[0312] Comparative Example 5 (Step of preparing a monomer composition) A monomer composition (a2) was obtained in the same manner as in (Step of preparing a monomer composition) of Example 1.

[0313] (Polymerization Step) A hydrogel crosslinked polymer (b1) was obtained in the same manner as in (Polymerization Step) of Example 1.

[0314] (Hydrogel Crushing Step) A particulate hydrogel (cc3) was obtained by carrying out the first gel crushing in the same manner as in Example 1, except that the die hole diameter of the screw extruder was changed to 11.0 mm. The obtained particulate hydrogel (cc3) had a Solid D50 of 1568 μm.

[0315] (Step of drying particulate hydrogel) The obtained particulate hydrogel (cc3) was dried in the same manner as in (step of drying particulate hydrogel) of Example 1 to obtain a dried product (cA'5).

[0316] (Step of pulverizing and classifying dried product) The obtained dried product (cA'5) was pulverized and classified in the same manner as in (Step of pulverizing and classifying dried product) of Example 1 to obtain a water-absorbent resin (cA5).

[0317] (Step of adding silica to water-absorbent resin) The obtained water-absorbent resin (cA5) was mixed with hydrophilic silica in the same manner as in (the adding step) of Example 5, to obtain a water-absorbent resin (cA6).

[0318] [Result A: Properties of Hydrogel and Water-Absorbent Resin] The properties of the hydrogels, particulate hydrogels, and water-absorbent resins produced in Examples 1 to 7 and Comparative Examples 1 to 5 are shown in Tables 1 to 3 below. Note that "hydrogel" in Table 1 below means the hydrogel obtained by the polymerization step and in a state before being pulverized in the hydrogel pulverization step, i.e., the hydrogel-like crosslinked polymer. Furthermore, "particulate hydrogel" in Table 2 below means the hydrogel in a state pulverized in the hydrogel pulverization step.

[0319] As shown in Table 3, the water-absorbent resins A1 to A4, A6, A8, and A10 produced in Examples 1 to 7 satisfy the requirements (a) to (d) above, and correspond to the water-absorbent resin of the present invention. Furthermore, the production methods for the water-absorbent resins in Examples 1 to 7 correspond to the production method of the present invention. Therefore, the water-absorbent resin of the present invention can be produced by the production method of the present invention.

[0320] [Examples 8 to 14, Comparative Examples 6 to 9: Solidification time of antifreeze solution] Measurement of the solidification time of the antifreeze solution was carried out for each of the water absorbent resins A1 to A4, A6, A8, and A10 produced in Examples 1 to 7, and the water absorbent resins cA1 to cA3 and cA6 produced in Comparative Examples 1 to 3, and 5. In detail, the solidification time of the antifreeze solution was measured as the solidification time after adding 1 g of the water absorbent resin to 10 ml of a 37 mass % aqueous ethylene glycol solution in a 20 ml screw tube (outer diameter 27 mm).

[0321] Specifically, the solidification time was measured using the following method. First, 2 parts by mass of Aurora (registered trademark) Long Life Coolant (ethylene glycol concentration 92%, 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. 10 g of the obtained 37% by mass ethylene glycol aqueous solution was placed in a screw tube (outer diameter 27 mm, height 55 mm, manufactured by Maruemu Co., Ltd., No. 5) and placed on a horizontal stand. 1 g of water-absorbent resin was added to the screw tube, and the 37% by mass ethylene glycol aqueous solution was allowed to swell while standing. After tilting the screw tube 90 degrees, the solidification time was measured as the time until the liquid stopped seeping out. The temperatures of the 37% by mass ethylene glycol aqueous solution during measurement were 0°C ± 1°C and 30°C ± 1°C (corresponding to "0°C" and "30°C" in Table 4 below, respectively).

[0322] [Result B] The solidification times of the antifreeze solutions measured in Examples 8 to 14 and Comparative Examples 6 to 9 are shown in Table 4 below.

[0323] As shown in Table 4, it was confirmed that the water-absorbent resins A1 to A4, A6, A8 and A10 (Examples 8 to 14) corresponding to the water-absorbent resin of the present invention are superior in solidification time, particularly in solidification time at low temperatures, to the water-absorbent resins cA1 to cA3 and cA6 (Comparative Examples 6 to 9) not corresponding to the water-absorbent resin of the present invention.

[0324] From the above, it can be said that the water-absorbent resin of the present invention can sufficiently swell and gel upon absorption of an aqueous liquid such as an antifreeze solution, even in a low-temperature environment. Furthermore, as described above, the production method of the present invention can produce the water-absorbent resin of the present invention. Therefore, it can be said that the method can produce a water-absorbent resin that can sufficiently swell and gel upon absorption of an aqueous liquid such as an antifreeze solution, even in a low-temperature environment.

[0325] The water-absorbent resin according to one embodiment of the present invention and the method for producing the water-absorbent resin according to one embodiment of the present invention can be utilized for producing a water-absorbent article that can sufficiently perform swelling and gelling due to absorption of an aqueous liquid such as an antifreeze solution even in a low-temperature environment.

Claims

1. A water-absorbent resin that satisfies the following conditions (a) to (d): (a) the 0°C vortex value is 30 seconds or less; (b) the ratio of the 0°C vortex value to the 30°C vortex value (vortex ratio (0°C / 30°C)) is 5.0 or less; (c) the GEX value is 17 or more; (d) the CRC value is more than 25 g / g.

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

3. The water absorbent resin according to claim 2, wherein the ratio of the number of moles of one or more salts selected from the group consisting of potassium salts, lithium salts and ammonium salts to the number of moles of the entire neutralized acid group-containing unsaturated monomers is 50 mol % or more and 100 mol % or less.

4. The water-absorbent resin according to any one of claims 1 to 3, having a mass-average particle size of 50 µm or more and 600 µm or less.

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

6. The water absorbent resin according to any one of claims 1 to 5, wherein the content of fine powder is 15 mass% or less with respect to the mass of the entire water absorbent resin, and the fine powder is water absorbent resin that passes through a sieve having an opening of 150 μm when the entire water absorbent resin is classified using the sieve.

7. A method for producing a water-absorbent resin, comprising a polymerization step of crosslinking and polymerizing a monomer composition containing an acid group-containing unsaturated monomer and optionally a monomer other than the acid group-containing unsaturated monomer to obtain a hydrous gel-like crosslinked polymer, wherein at least a part of the acid group-containing unsaturated monomer is a neutralized acid group-containing unsaturated monomer, and the neutralized acid group-containing unsaturated monomer contains one or more 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 mass% or more and less than 55 mass% based on the mass of the entire monomer composition, and the polymerization initiation temperature in the polymerization step is 60°C or more.

8. The method for producing a water-absorbent resin according to claim 7, wherein the solid content in the hydrogel-like crosslinked polymer is 60 mass % or less based on the total mass of the hydrogel-like crosslinked polymer.

9. The method for producing a water-absorbent resin according to claim 7 or 8, further comprising a hydrogel crushing step of crushing the hydrogel-like crosslinked polymer to obtain a particulate hydrogel.

10. The method for producing a water-absorbent resin according to claim 9, wherein the hydrous gel crushing step is a step of crushing the hydrous gel-like crosslinked polymer so that the particulate hydrous gel has a mass average particle diameter of 1 mm or less in terms of solid content.

11. The method for producing a water-absorbent resin according to any one of claims 7 to 10, wherein the polymerization step is a step of polymerizing the monomer composition by aqueous solution polymerization to obtain the hydrous gel-like cross-linked polymer.