CN113302228A - Water-absorbent resin particles and method for producing same - Google Patents

Abstract

The present invention provides a water-absorbent resin particle which can maintain a stable absorption rate and liquid permeability from immediately after the production of the water-absorbent resin particle until the use of an absorbent article, and can exert an excellent texture after the use in the absorbent article, and a method for producing the same. The present invention relates to: water-absorbent resin particles obtained by surface-crosslinking resin particles comprising a crosslinked polymer (A) containing a water-soluble vinyl monomer and a crosslinking agent as essential constituent units, the water-absorbent resin particles being in an amorphous crushed state and having an average normal sphericity of 0.800 to 0.900 and a variation width of the average normal sphericity after an impact resistance test of 0 to 0.015; and a process for producing water-absorbent resin particles, which comprises adding a gel blocking inhibitor prior to a drying step for obtaining a dried powder containing a crosslinked polymer (A), and drying the dried powder in the drying step by using an agitation dryer, wherein the weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (A) is 50% by weight or less based on the total weight of the dried powder.

Description

Water-absorbent resin particles and method for producing same

Technical Field

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

Background

Absorbent bodies made of hydrophilic fibers such as pulp and water-Absorbent resins (Super Absorbent polymers (hereinafter referred to as "SAP") mainly made of acrylic acid (salt) or the like have been widely used in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads. In recent years, from the viewpoint of improvement of QOL (quality of life), demands for these sanitary materials are shifting to lighter and thinner materials, and accordingly, it is desired to reduce the amount of hydrophilic fibers used. Therefore, the SAP is required to take on the role hitherto taken on by the hydrophilic fiber in the absorbent body.

For example, as an important function of the diaper, there is a reduction in leakage caused by high-speed absorption of urine. In the conventional absorbent, the absorption rate of urine is increased by the physical space existing between bulky hydrophilic fibers, but in the absorbent having a low amount of hydrophilic fibers and a high SAP ratio, the physical space is small and the absorption rate of urine is low because the absorbent has a filling structure between SAP particles. In addition, in the conventional absorbent body, the hydrophilic fiber has high urine diffusibility, and urine can be diffused throughout the absorbent body, whereas in the absorbent body having a high SAP ratio, the swollen gel may inhibit the diffusion of urine, and therefore the urine diffusibility in the absorbent body is significantly reduced. This decrease in diffusion property is a profound cause of diaper leakage, i.e., rewet of urine, by interacting with the decrease in absorption rate.

In order to solve the above problems, a technique for improving both liquid permeability and absorption rate of SAP has been studied. For example, the following methods are known: the surface of the SAP is crosslinked with an aqueous solution containing a specific organic crosslinking agent compound and a specific cation, and deformation of the swollen gel surface is suppressed, thereby effectively forming a gel gap (for example, see patent document 1). However, when only surface crosslinking is performed, the liquid permeability between swollen gels is not sufficiently satisfactory. As another method for improving the liquid permeability, the following methods are known: (1) a method of forming a physical space by adding an inorganic compound such as silica and talc; (2) a method of forming gel gaps by performing surface treatment using a hydrophobic polymer having a small surface free energy such as modified silicone to suppress fusion between swollen gels; and (3) a method of adding aluminum sulfate, aluminum lactate, or the like (see, for example, patent document 2, patent document 3, and patent document 4). However, in these methods, even if excellent characteristics are exhibited immediately after SAP is produced, there is a possibility that the liquid permeability or absorption rate of the water-absorbent resin particles may be lowered by collision, friction, or the like between particles or with the particles on the device wall surface during SAP transportation, manufacturing of absorbent articles such as SAP distribution, supply, and mixing, and further during transportation or actual use of the absorbent articles. Particularly, as the proportion of SAP in the absorbent body increases in recent years, the above problem becomes more remarkable.

Further, as the proportion of SAP in the absorbent body increases, there is not only the above-described problem of deterioration of physical properties due to contact between particles, but also gritty or uneven touch due to contact between particles of SAP particles during actual use of the absorbent article, and improvement of such deterioration of touch is also being sought.

Documents of the prior art

Patent document

Patent document 1: international publication No. 00/053664 pamphlet

Patent document 2: japanese laid-open patent publication No. 2012-161788

Patent document 3: japanese patent laid-open publication No. 2013-133399

Patent document 4: japanese patent laid-open publication No. 2014-512440

Disclosure of Invention

Problems to be solved by the invention

The present invention addresses the problem of providing water-absorbent resin particles that can maintain stable absorption rate and liquid permeability from immediately after the production of the water-absorbent resin particles until the use of an absorbent article, and that can exhibit excellent texture after the use in an absorbent article, and a method for producing the same.

Means for solving the problems

The present invention relates to water-absorbent resin particles having a structure in which resin particles are surface-crosslinked with a surface-crosslinking agent (d), the resin particles containing a crosslinked polymer (A) containing a water-soluble vinyl monomer (a1) and a crosslinking agent (b) as essential constituent units, wherein the water-absorbent resin particles have an amorphous crushed particle shape, an average normal Sphericity (SPHT) of 0.800 to 0.900, and a range of variation in average normal Sphericity (SPHT) after an impact resistance test of 0 to 0.015.

The present invention also relates to a method for producing water-absorbent resin particles, which comprises the steps of: a polymerization step of polymerizing a monomer composition containing a water-soluble vinyl monomer (a1) and a crosslinking agent (b) as essential constituent units to obtain a water-containing gel containing a crosslinked polymer (A); a gel pulverization step of kneading and chopping the aqueous gel to obtain aqueous gel particles containing (A); a drying step of drying the hydrogel particles to obtain a dried powder containing (A); a step of further pulverizing and/or classifying the dried powder to obtain resin particles containing (a); and a step of surface-crosslinking the surfaces of the resin particles with a surface-crosslinking agent (d), wherein a gel blocking preventive agent (c) is added before the drying step, and the drying step is performed by using an agitation dryer, so that the weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (a) obtained after drying is 50 wt% or less with respect to the total weight of the dried powder.

ADVANTAGEOUS EFFECTS OF INVENTION

The water-absorbent resin particles of the present invention have a specific average sphericity, maintain the particle shape even after the impact resistance test, and exhibit a stable absorption rate and liquid permeability. In addition, the water-absorbent resin particles obtained by the production method of the present invention can suppress the generation of coarse particles in the dried powder, and can improve the average sphericity of the water-absorbent resin particles, and thus the water-absorbent resin particles of the present invention can be suitably obtained. Therefore, an absorbent article to which the water-absorbent resin particles of the present invention and the water-absorbent resin particles obtained by the production method of the present invention are applied can exhibit stable absorption performance from immediately after the production of the water-absorbent resin particles until the use of the absorbent article, and has less gritty touch and excellent texture when the absorbent article is actually used.

Drawings

FIG. 1 is a sectional view schematically showing a filtration cylinder tube for measuring the flow rate of a gel.

FIG. 2 is a perspective view schematically showing a pressurizing shaft and a weight for measuring the liquid passing speed of the gel.

Detailed Description

The water-absorbent resin particles of the present invention have a structure in which resin particles containing a crosslinked polymer (a) containing a water-soluble vinyl monomer (a1) and a crosslinking agent (b) as essential constituent units are surface-crosslinked with a surface-crosslinking agent (d).

The water-soluble vinyl monomer (a1) in the present invention is not particularly limited, and known monomers such as vinyl monomers having at least 1 water-soluble substituent and an ethylenically unsaturated group (e.g., anionic vinyl monomers, nonionic vinyl monomers and cationic vinyl monomers) disclosed in paragraphs 0007 to 0023 of Japanese patent application laid-open No. 3648553, anionic vinyl monomers, nonionic vinyl monomers and cationic vinyl monomers disclosed in paragraphs 0009 to 0024 of Japanese patent application laid-open No. 2003-165883, and vinyl monomers having at least one selected from the group consisting of carboxyl groups, sulfo groups, phosphono groups, hydroxyl groups, carbamoyl groups, amino groups and ammonium groups disclosed in paragraphs 0041 to 0051 of Japanese patent application laid-open No. 2005-75982 can be used.

The water-soluble vinyl monomer (a1) is preferably an anionic vinyl monomer, and more preferably a vinyl monomer having a carboxylate group, sulfonate group, amino group, carbamoyl group, ammonium group, monoalkylammonium group, dialkylammonium group or trialkylammonium group, from the viewpoint of absorption performance or the like. Among these, vinyl monomers having a carboxylate group or a carbamoyl group are more preferable, and (meth) acrylic acid (salt) and (meth) acrylamide are further preferable, and (meth) acrylic acid (salt) is particularly preferable, and acrylic acid (salt) is most preferable.

The "carboxylate group" means "carboxyl group" or "carboxylate group", and the "sulfonate group" means "sulfo group" or "sulfonate group". The term (meth) acrylic acid (salt) means acrylic acid, acrylic acid salt, methacrylic acid or methacrylic acid salt, and the term (meth) acrylamide means acrylamide or methacrylamide. The salt includes an alkali metal (lithium, sodium, potassium, etc.) salt, an alkaline earth metal (magnesium, calcium, etc.) salt, or ammonium (NH)₄) Salts and the like. Among these salts, alkali metal salts and ammonium salts are preferable, alkali metal salts are more preferable, and sodium salts are particularly preferable, from the viewpoint of absorption characteristics and the like.

When an acid group-containing monomer such as acrylic acid or methacrylic acid is used as the water-soluble vinyl monomer (a1), a part of the acid group-containing monomer may be neutralized with a base. As the base to be neutralized, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, or an alkali metal carbonate such as sodium carbonate, sodium hydrogencarbonate or potassium carbonate can be generally used. The neutralization may be carried out before or during the polymerization of the acid group-containing monomer in the step of producing the water-absorbent resin, or the acid group-containing polymer may be neutralized in the state of a hydrogel containing the crosslinked polymer (a) described later.

When the acid group-containing monomer is used, the degree of neutralization of the acid group is preferably 50 to 80 mol%. When the neutralization degree is less than 50 mol%, the resultant hydrogel polymer may have high adhesiveness, and the workability during production and use may be deteriorated. And the water-retention capacity of the resulting water-absorbent resin particles may decrease. On the other hand, if the neutralization degree exceeds 80%, the pH of the obtained resin increases, and there is a concern about safety to human skin.

As the structural unit of the crosslinked polymer (a), other vinyl monomer (a2) copolymerizable with the water-soluble vinyl monomer (a1) may be used as a structural unit. One kind of the other vinyl monomer (a2) may be used alone, or two or more kinds may be used in combination.

The other copolymerizable vinyl monomer (a2) is not particularly limited, and known hydrophobic vinyl monomers (e.g., the hydrophobic vinyl monomers disclosed in paragraphs 0028 to 0029 of Japanese patent No. 3648553, the hydrophobic vinyl monomers disclosed in paragraphs 0025 of Japanese patent application laid-open No. 2003-165883, and the hydrophobic vinyl monomers disclosed in paragraphs 0058 of Japanese patent application laid-open No. 2005-75982) and the like can be used, and specifically, the following vinyl monomers (i) to (iii) and the like can be used.

(i) An aromatic ethylenic monomer having 8 to 30 carbon atoms

And halogen-substituted styrene compounds such as styrene, α -methylstyrene, styrene such as vinyltoluene and hydroxystyrene, vinylnaphthalene, and dichlorostyrene.

(ii) Aliphatic ethylenic monomer having 2 to 20 carbon atoms

Olefins (ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.); and dienes (butadiene, isoprene, etc.), and the like.

(iii) Alicyclic ethylenic monomer having 5 to 15 carbon atoms

Monoethylenically unsaturated monomers (pinene, limonene, indene, and the like); and multiethylenic vinyl monomers [ cyclopentadiene, dicyclopentadiene, ethylidene norbornene, etc. ], and the like.

The content (% by mol) of the unit of the other vinyl monomer (a2) is preferably 0 to 5, more preferably 0 to 3, particularly preferably 0 to 2, and particularly preferably 0 to 1.5 based on the number by mol of the unit of the water-soluble vinyl monomer (a1) from the viewpoint of absorption performance and the like, and the content of the unit of the other vinyl monomer (a2) is most preferably 0% by mol from the viewpoint of absorption performance and the like.

The crosslinking agent (b) is not particularly limited, and known crosslinking agents (for example, a crosslinking agent having 2 or more ethylenically unsaturated groups disclosed in paragraphs 0031 to 0034 of Japanese patent No. 3648553, a crosslinking agent having at least 1 functional group reactive with a water-soluble substituent and having at least 1 ethylenically unsaturated group, and a crosslinking agent having at least 2 functional groups reactive with a water-soluble substituent, a crosslinking agent having 2 or more ethylenically unsaturated groups disclosed in paragraphs 0028 to 0031 of Japanese patent laid-open No. 2003-165883, a crosslinking agent having an ethylenically unsaturated group and a reactive functional group, and a crosslinking agent having 2 or more reactive substituents, a crosslinkable vinyl monomer disclosed in paragraph 0059 of Japanese patent laid-open No. 2005-75982, and a crosslinkable vinyl monomer disclosed in paragraphs 0015 to 0016 of Japanese patent laid-open No. 2005-95759) and the like can be used. Among these, from the viewpoint of absorption performance and the like, a crosslinking agent having 2 or more ethylenically unsaturated groups is preferable, poly (meth) allyl ether of a polyhydric alcohol having 2 to 40 carbon atoms, a (meth) acrylic acid ester of a polyhydric alcohol having 2 to 40 carbon atoms, and (meth) acrylamide of a polyhydric alcohol having 2 to 40 carbon atoms are more preferable, polyallyl ether of a polyhydric alcohol having 2 to 40 carbon atoms is particularly preferable, and pentaerythritol triallyl ether is most preferable. One crosslinking agent (b) may be used alone, or two or more crosslinking agents may be used in combination.

The content (% by mole) of the unit of the crosslinking agent (b) is preferably 0.001 to 5, more preferably 0.005 to 3, and particularly preferably 0.01 to 1 based on the number of moles of the unit of the water-soluble vinyl monomer (a1) (based on the total number of moles of (a1) to (a2) in the case of using another vinyl monomer (a 2)). When the content is in the range of 0.001 to 5, the absorption performance becomes better.

The method for producing water-absorbent resin particles of the present invention comprises the steps of: a polymerization step of polymerizing the monomer composition containing the water-soluble vinyl monomer (a1) and the crosslinking agent (b) as essential constituent units to obtain a water-containing gel containing the crosslinked polymer (A); a gel pulverization step of kneading and chopping the hydrogel to obtain hydrogel particles containing (A); a drying step of drying the hydrogel particles to obtain a dried powder containing (A); a step of further pulverizing and/or classifying the dried powder to obtain resin particles containing (a); and a step of surface-crosslinking the surface of the resin particle with a surface-crosslinking agent (d).

As the polymerization step, a hydrogel (a hydrogel-like material containing water in the crosslinked polymer) containing the crosslinked polymer (A) can be obtained by a known solution polymerization (adiabatic polymerization, film polymerization, spray polymerization, etc.; Japanese patent application laid-open No. 55-133413, etc.), a known suspension polymerization, or a known reversed-phase suspension polymerization (Japanese patent application laid-open No. 54-30710, Japanese patent application laid-open No. 56-26909, Japanese patent application laid-open No. 1-5808, etc.). The crosslinked polymer (A) may be 1 kind alone or a mixture of 2 or more kinds.

Among the polymerization methods, the solution polymerization method is preferred, and the aqueous solution polymerization method is particularly preferred because it is advantageous in terms of production cost because it is not necessary to use an organic solvent or the like, and the aqueous solution adiabatic polymerization method is most preferred because it is possible to obtain a water-absorbent resin having a large water retention amount and a small amount of water-soluble components, and it is not necessary to control the temperature during polymerization.

In the case of aqueous solution polymerization, a mixed solvent comprising water and an organic solvent may be used, and examples of the organic solvent include methanol, ethanol, acetone, methyl ethyl ketone, N-dimethylformamide, dimethyl sulfoxide, and a mixture of 2 or more thereof.

In the case of aqueous solution polymerization, the amount (wt%) of the organic solvent is preferably 40 or less, more preferably 30 or less, based on the weight of water.

The polymerization concentration, that is, the input concentration of the water-soluble vinyl monomer (a1) and the other vinyl monomer (a2) used as needed in the polymerization liquid is not particularly limited, but is preferably 10 to 55%, more preferably 20 to 45%, based on the weight of the polymerization liquid (that is, the total weight of the water-soluble vinyl monomer (a1) and the other vinyl monomer (a2), the solvent, the internal crosslinking agent (b), the polymerization catalyst, and the polymerization control agent, which will be described later). When the polymerization concentration is less than 10%, productivity may be lowered; when the polymerization concentration is more than 55%, side reactions such as self-crosslinking may occur, and the water-retention capacity of the water-absorbent resin particles to be obtained may be lowered.

When a catalyst is used for the polymerization, a conventionally known radical polymerization catalyst can be used, examples thereof include azo compounds [ azobisisobutyronitrile, azobiscyanovaleric acid, 2' -azobis (2-amidinopropane) hydrochloride, etc. ], inorganic peroxides (hydrogen peroxide, ammonium persulfate, potassium persulfate, sodium persulfate, etc.), organic peroxides [ benzoyl peroxide, di-t-butyl peroxide, cumene hydroperoxide, succinic acid peroxide, di (2-ethoxyethyl) peroxydicarbonate, etc. ], and redox catalysts (catalysts comprising a combination of a reducing agent such as alkali metal sulfite or bisulfite, ammonium sulfite, ammonium bisulfite, and ascorbic acid, and an oxidizing agent such as alkali metal persulfate, ammonium persulfate, hydrogen peroxide, and organic peroxide, etc.). These catalysts may be used alone, or two or more of these may be used in combination.

The amount (wt%) of the radical polymerization catalyst is preferably 0.0005 to 5, more preferably 0.001 to 2 based on the water-soluble vinyl monomer (a1) (based on the total weight of (a1) to (a2) in the case of using another vinyl monomer (a 2)).

In the polymerization, a polymerization control agent such as a chain transfer agent may be used as necessary, and specific examples thereof include sodium hypophosphite, sodium phosphite, alkyl mercaptan, alkyl halide, thiocarbonyl compound, and the like. These polymerization control agents may be used alone, or two or more of these may be used in combination.

The amount (wt%) of the polymerization control agent to be used is preferably 0.0005 to 5, more preferably 0.001 to 2 based on the water-soluble vinyl monomer (a1) (based on the total weight of (a1) to (a2) in the case of using another vinyl monomer (a 2)).

When the suspension polymerization method or the reversed-phase suspension polymerization method is employed as the polymerization method, the polymerization may be carried out in the presence of a conventionally known dispersant or surfactant as needed. In the case of the reversed-phase suspension polymerization method, the polymerization can be carried out using a conventionally known hydrocarbon solvent such as xylene, n-hexane, and n-heptane.

The polymerization initiation temperature may be suitably adjusted depending on the kind of the catalyst used, and is preferably 0 to 100 ℃, and more preferably 2 to 80 ℃.

The gel pulverization step is a step of kneading and chopping the hydrogel containing the crosslinked polymer (a) obtained in the polymerization step to obtain hydrogel particles. The size (longest diameter) of the hydrogel particles after the gel pulverization step is preferably 50 μm to 10cm, more preferably 100 μm to 2cm, and particularly preferably 500 μm to 1 cm. When the size is in the range of 50 μm to 10cm, the drying property in the drying step becomes better.

The gel pulverization can be carried out by a known method, and kneading and chopping can be carried out by using a pulverizing apparatus (for example, a kneader, a universal mixer, a single-screw or twin-screw kneading extruder, a chopper, a meat chopper, and the like). From the viewpoint of controlling the water absorption time by the Vortex method (Vortex method), a pulverizing device (for example, a single-screw or twin-screw kneading extruder, a chopper, or the like) provided with a kneading and extruding mechanism is preferable.

The gel temperature in the gel pulverization step is preferably 70 to 120 ℃, and more preferably 80 to 110 ℃. When the gel temperature is lower than 70 to 120 ℃, not only a cooling step is required after the polymerization step and unnecessary energy is required, but also the adhesiveness of the hydrogel particles is improved and the hydrogel particles are liable to be insufficiently crushed; when the gel temperature is higher than this range, bumping of water may occur, and stable pulverization may not be possible.

As described above, the aqueous gel containing an acid group polymer obtained after polymerization may be mixed with an alkali before or during the gel pulverization step to neutralize the aqueous gel. The preferable ranges of the base and the degree of neutralization used for neutralizing the acid group-containing polymer are the same as those in the case of using the acid group-containing monomer.

In the method for producing water-absorbent resin particles of the present invention, the gel blocking inhibitor (c) is added before the drying step described later. The gel blocking inhibitor (c) is an additive for inhibiting blocking due to aggregation of hydrogel particles obtained in the gel pulverization step. By adding the gel blocking inhibitor (c), generation of coarse particles (particles having a particle diameter of 2.8mm or more are referred to as coarse particles) in the dried powder obtained after drying can be suppressed, the average sphericity of the water-absorbent resin particles can be improved, and the range of change in the average sphericity after the impact resistance test can be reduced.

The gel blocking preventive (c) includes a hydrophobic substance (c1) containing a hydrocarbon group, a hydrophobic substance (c2) as a silicone, and the like.

Examples of the hydrophobic substance (c1) containing a hydrocarbon group include polyolefin resins, polyolefin resin derivatives, polystyrene resins, polystyrene resin derivatives, waxes, long-chain fatty acid esters, long-chain fatty acids and salts thereof, long-chain fatty alcohols, quaternary ammonium salt surfactants, and mixtures of two or more of these.

Examples of the polyolefin resin include polymers { for example, polyethylene, polypropylene, polyisobutylene, poly (ethylene-isobutylene), isoprene and the like } having a weight average molecular weight of 1000 to 100 ten thousand, which are obtained by using an olefin having 2 to 4 carbon atoms { for example, ethylene, propylene, isobutylene, isoprene and the like } as an essential constituent monomer (the content of the olefin is at least 50% by weight based on the weight of the polyolefin resin).

Examples of the polyolefin resin derivative include polymers having a weight average molecular weight of 1000 to 100 ten thousand, in which a carboxyl group (-COOH) or a1, 3-oxo-2-oxapropylene (-COOCO-) is introduced into a polyolefin resin { for example, polyethylene thermally degraded products, polypropylene thermally degraded products, maleic acid-modified polyethylene, chlorinated polyethylene, maleic acid-modified polypropylene, ethylene-acrylic acid copolymers, ethylene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, maleated polybutadiene, ethylene-vinyl acetate copolymers, and maleates of ethylene-vinyl acetate copolymers }.

As the polystyrene resin, a polymer having a weight average molecular weight of 1000 to 100 ten thousand, or the like can be used.

Examples of the polystyrene resin derivative include polymers having a weight average molecular weight of 1000 to 100 ten thousand (for example, styrene-maleic anhydride copolymers, styrene-butadiene copolymers, styrene-isobutylene copolymers, etc.) containing styrene as an essential constituent monomer (the content of styrene is at least 50% by weight based on the weight of the polystyrene derivative).

Examples of the wax include waxes having a melting point of 50 to 200 ℃ { for example, paraffin wax, beeswax, carnauba wax, tallow, and the like }.

Examples of the long-chain fatty acid ester include esters of fatty acids having 8 to 25 carbon atoms and alcohols having 1 to 12 carbon atoms { for example, methyl laurate, ethyl laurate, methyl stearate, ethyl stearate, methyl oleate, ethyl oleate, monolaurin, monostearate monoglyceride stearate, distearoyl glyceride, monooleoyl glyceride, monolaurin laurate, monoester stearate pentaerythritol, monooleoyl pentaerythritol, monolaurate sorbitol, monostearate sorbitol, monooleoyl sorbitol, monoester palmitate sucrose, diester palmitate sucrose, triester palmitate sucrose stearate, monoester stearate sucrose, diester stearate sucrose stearate, triester stearate, tallow, etc. }.

Examples of the long-chain fatty acid and a salt thereof include fatty acids having 8 to 25 carbon atoms { for example, lauric acid, palmitic acid, stearic acid, oleic acid, behenic acid, and the like }. Examples of the salt include salts with calcium, magnesium, or aluminum (hereinafter, simply referred to as Ca, Mg, or Al) { for example, Ca palmitate, Al palmitate, Ca stearate, Mg stearate, and Al stearate }.

Examples of the long-chain aliphatic alcohol include aliphatic alcohols having 8 to 25 carbon atoms { for example, lauryl alcohol, palmityl alcohol, stearyl alcohol, oleyl alcohol, and the like }.

Examples of the quaternary ammonium salt surfactant include quaternary ammonium salts containing an aliphatic chain having 1 to 2 carbon atoms and 8 to 25 carbon atoms { for example, didecyldimethylammonium chloride, benzyldimethyldecylammonium chloride, benzyldimethyltetradecylammonium chloride, and dimethyldistearylammonium chloride }, with didecyldimethylammonium chloride and dimethyldistearylammonium chloride being preferred.

The mixture of 2 or more of these includes a mixture of a long-chain fatty acid ester and a long-chain fatty alcohol { for example, a mixture of sucrose diester stearate and stearyl alcohol }.

The hydrophobic substance (c2) as the polysiloxane includes polydimethylsiloxane, polyether-modified polysiloxane { polyoxyethylene-modified polysiloxane, poly (oxyethylene-oxypropylene) -modified polysiloxane, and the like }, carboxyl-modified polysiloxane, epoxy-modified polysiloxane, amino-modified polysiloxane, alkoxy-modified polysiloxane, and the like, and mixtures thereof.

The position of the organic group (modifying group) of the modified silicone { polyether-modified polysiloxane, carboxyl-modified polysiloxane, epoxy-modified polysiloxane, amino-modified polysiloxane, or the like } is not particularly limited, and may be any position of a side chain of the polysiloxane, both ends of the polysiloxane, one end of the polysiloxane, a side chain of the polysiloxane, and both ends. Among these, from the viewpoint of reducing coarse particles and the like, the side chain of polysiloxane and both the side chain and both ends of polysiloxane are preferable, and both the side chain and both ends of polysiloxane are more preferable.

As the organic group (modifying group) of the polyether-modified polysiloxane, a group containing a polyoxyethylene group or a poly (oxyethylene-oxypropylene) group, and the like are included. The content (number) of oxyethylene groups and/or oxypropylene groups contained in the polyether-modified polysiloxane is preferably 2 to 40, more preferably 5 to 30, particularly preferably 7 to 20, and most preferably 10 to 15 per 1 molecule of the polyether-modified polysiloxane. When the content is in the range of 2 to 40, the absorption characteristics become better. In addition, in the case of containing an oxyethylene group and an oxypropylene group, the content (% by weight) of the oxyethylene group is preferably 1 to 30, more preferably 3 to 25, and particularly preferably 5 to 20 based on the weight of the polysiloxane. When the content is in the range of 1 to 30, coarse particles can be further reduced.

Examples of the polyether-modified polysiloxane include the following products { modification site, type of alkylene oxide }, which are readily available from the market.

Manufactured by shin-Etsu chemical industries, Ltd

KF-945{ side chain, ethylene oxide and propylene oxide }, KF-6020{ side chain, ethylene oxide and propylene oxide }, X-22-6191{ side chain, ethylene oxide and propylene oxide }, X-22-4952{ side chain, ethylene oxide and propylene oxide }, X-22-4272{ side chain, ethylene oxide and propylene oxide }, X-22-6266{ side chain, ethylene oxide and propylene oxide }

Toray Dow Corning Co., Ltd

FZ-2110{ both ends, ethylene oxide and propylene oxide }, FZ-2122{ both ends, ethylene oxide and propylene oxide }, FZ-7006{ both ends, ethylene oxide and propylene oxide }, FZ-2166{ both ends, ethylene oxide and propylene oxide }, FZ-2164{ both ends, ethylene oxide and propylene oxide }, FZ-2154{ both ends, ethylene oxide and propylene oxide }, FZ-2203{ both ends, ethylene oxide and propylene oxide }, and FZ-2207{ both ends, ethylene oxide and propylene oxide }

Examples of the organic group (modifying group) of the carboxyl-modified polysiloxane include a carboxyl-containing group and the like, examples of the organic group (modifying group) of the epoxy-modified polysiloxane include an epoxy-containing group and the like, and examples of the organic group (modifying group) of the amino-modified polysiloxane include an amino-containing group (primary amino group, secondary amino group, tertiary amino group) and the like. The content (g/mol) of the organic group (modifying group) in the modified silicone is preferably 200 to 11000, more preferably 600 to 8000, and particularly preferably 1000 to 4000 in terms of carboxyl equivalent, epoxy equivalent, or amino equivalent. When the content is in the range of 200 to 11000, coarse particles can be further reduced. The carboxyl equivalent is determined in accordance with JIS C2101: 1999 "16. Total acid number test". In addition, the epoxy equivalent is in accordance with JIS K7236: 2001, was determined. In addition, amino equivalent is measured in accordance with JIS K2501: 2003, "8. potentiometric titration (alkali value/hydrochloric acid method)".

The carboxyl group-modified polysiloxane is preferably exemplified by the following commercial products { modification site, carboxyl group equivalent (g/mol) }, which can be easily obtained from the market.

Manufactured by shin-Etsu chemical industries, Ltd

X-22-3701E { side chain, 4000}, X-22-162C { both ends, 2300}, X-22-3710{ one end, 1450}

Toray Dow Corning Co., Ltd

BY 16-880{ side chain, 3500}, BY 16-750{ both ends, 750}, BY 16-840{ side chain, 3500}, SF8418{ side chain, 3500}

Examples of the epoxy-modified polysiloxane include the following products { modification site, epoxy equivalent }, which are readily available on the market.

Manufactured by shin-Etsu chemical industries, Ltd

X-22-343{ side chain, 525}, KF-101{ side chain, 350}, KF-1001{ side chain, 3500}, X-22-2000{ side chain, 620}, X-22-2046{ side chain, 600}, KF-102{ side chain, 3600}, X-22-4741{ side chain, 2500}, KF-1002{ side chain, 4300}, X-22-3000T { side chain, 250}, X-22-163{ both ends, 200}, KF-105{ both ends, 490}, X-22-163A { both ends, 1000}, X-22-163B { both ends, 1750}, X-22-163C { both ends, 2700}, X-22-169AS { both ends, 500}, X-22-169B { both ends, 1700}, X-22-173-DX { one end, DX } 4500, X-22-9002{ side chain and both termini, 5000}

Toray Dow Corning Co., Ltd

FZ-3720{ side chain, 1200}, BY 16-839{ side chain, 3700}, SF 8411{ side chain, 3200}, SF 8413{ side chain, 3800}, SF 8421{ side chain, 11000}, BY 16-876{ side chain, 2800}, FZ-3736{ side chain, 5000}, BY 16-855D { side chain, 180}, BY 16-8{ side chain, 3700}

The amino-modified silicone is preferably commercially available, for example, as the following products { modified site, amino equivalent }.

Manufactured by shin-Etsu chemical industries, Ltd

KF-865{ side chain, 5000}, KF-864{ side chain, 3800}, KF-859{ side chain, 6000}, KF-393{ side chain, 350}, KF-860{ side chain, 7600}, KF-880{ side chain, 1800}, KF-8004{ side chain, 1500}, KF-8002{ side chain, 1700}, KF-8005{ side chain, 11000}, KF-867{ side chain, 1700}, X-22-3820W { side chain, 55000}, KF-869{ side chain, 8800}, KF-861{ side chain, 2000}, X-22-3939A { side chain, 1500}, KF-877{ side chain, 5200}, PAM-E { both ends, 130}, KF-8010{ both ends, 430}, X-22-161A { both ends, 800}, X-22-161B }, KF-8010 } both ends, 1500-E { both ends }, KF-8010{ both ends }, KF-859{ both ends }, KF-8009 { both ends }, and KF-8000 }, wherein each of the end-8001 is a { both ends of the end of KF-8009 { side chain, and the end of the KF-8009 { side chain 2200, KF-8008{ two ends, 5700}, X-22-1660B-3{ two ends, 2200}, KF-857{ side chain, 2200}, KF-8001{ side chain, 1900}, KF-862{ side chain, 1900}, X-22-9192{ side chain, 6500}

Toray Dow Corning Co., Ltd

FZ-3707{ side chain, 1500}, FZ-3504{ side chain, 1000}, BY 16-205{ side chain, 4000}, FZ-3760{ side chain, 1500}, FZ-3705{ side chain, 4000}, BY 16-209{ side chain, 1800}, FZ-3710{ side chain, 1800}, SF 8417{ side chain, 1800}, BY 16-849{ side chain, 600}, BY 16-850{ side chain, 3300}, BY 16-879B { side chain, 8000}, BY 16-892{ side chain, 2000}, FZ-3501{ side chain, 3000}, FZ-3785{ side chain, 6000}, BY 16-872{ side chain, 1800}, BY 16-213{ side chain, 2700}, BY 16-203{ side chain, 1900}, BY 16-898{ side chain, 2900}, BY 16-890{ side chain, 1900}, BY 16-893 }, BY 16-213{ side chain, 2700}, and { side chain, 1800} 4000, FZ-3789 side chain, 1900, BY 16-871 both ends, 130, BY 16-853C both ends, 360, BY 16-853U both ends, 450

Examples of the mixture of these include a mixture of polydimethylsiloxane and carboxyl-modified polysiloxane, and a mixture of polyether-modified polysiloxane and amino-modified polysiloxane.

The viscosity (mPas, 25 ℃) of the hydrophobic substance (c2) as the polysiloxane is preferably 10 to 5000, more preferably 15 to 3000, and particularly preferably 20 to 1500. When the viscosity is in the range of 10 to 5000, the absorption characteristics become better. The viscosity is measured by a viscosity measuring method based on a cone and cone-plate rotational viscometer according to JIS Z8803-1991 "viscosity of liquid" 9 "{ for example, an E-type viscometer (manufactured by Toyobo industries, Ltd., RE80L, radius 7mm, angle 5.24X 10) adjusted at 25.0. + -. 0.5 ℃ is used^-2A conical cone of rad).

Among these gel blocking inhibitors (c), from the viewpoint of reducing coarse particles, long-chain fatty acid esters, long-chain fatty acid salts, long-chain fatty acid group alcohols, and hydrophobic substances as polysiloxanes are preferable, sucrose stearate, Mg stearate, stearyl alcohol, amino-modified polysiloxanes, and carboxyl-modified polysiloxanes are more preferable, and sucrose stearate monoester, sucrose stearate diester, Mg stearate, stearyl alcohol, and amino-modified polysiloxane are particularly preferable.

The step of adding the gel blocking inhibitor (c) is not particularly limited as long as it is performed before the drying step described later, and from the viewpoint of reducing coarse particles by the gel blocking inhibitor (c), a method of adding the gel blocking inhibitor (c) in the polymerization step or the gel pulverization step, more preferably in the gel pulverization step, and particularly preferably before or during the gel pulverization in the gel pulverization step is preferable.

The gel blocking inhibitor (c) is preferably added in an amount (% by weight) of 0.05 to 5.0, more preferably 0.08 to 1.0, and particularly preferably 0.1 to 0.5 based on the weight of the crosslinked polymer (A). If the amount of addition is less than 0.05, the effect of reducing coarse particles is insufficient, and the average sphericity tends to be reduced; if it exceeds 5.0, the effect of reducing coarse particles is not compatible with the amount of addition, which is uneconomical, and the absorption characteristics may be deteriorated.

The drying step is a step of drying the hydrogel particles obtained in the gel pulverization step to obtain a dried powder containing the crosslinked polymer (a). In this case, the solid content concentration of the gel before the drying step is preferably 10 to 55%, more preferably 25 to 45%. If the solid content concentration is less than 10 to 55%, the productivity is deteriorated, and if the solid content concentration is more than this range, the energy required for pulverization becomes too high, and the pulverizer may be damaged.

In the method for producing water-absorbent resin particles of the present invention, the drying step is performed using an agitation dryer. By using the agitation type dryer, aggregation of hydrogel particles during drying can be prevented, the amount of coarse particles in the dried powder obtained after drying can be suppressed, the average sphericity in the grinding step described later can be increased, and the range of variation in the average sphericity after the impact resistance test can be reduced.

In the present invention, the agitation type dryer is not limited as long as it can agitate the dried hydrogel particles, and may be in a form having an agitation means such as an agitation paddle, a rotary container, or an air flow. Specific examples of the stirring dryer include a tank type stirring dryer, a rotary dryer, a disk type dryer, a Nauta type dryer, a fluidized bed type dryer, and a pneumatic dryer. Among these, from the viewpoint of preventing aggregation of gel particles and convenience in handling, an agitation type dryer having an agitation means such as an agitation paddle or a rotary container is preferable.

The heating means of the dryer is not limited as long as it can apply heat necessary for drying, and examples thereof include heating means by convection heat transfer, conduction heat transfer, microwaves, infrared rays, and the like.

The drying temperature in the stirring dryer is preferably 100 to 230 ℃, and more preferably 120 to 200 ℃ from the viewpoints of drying efficiency and thermal deterioration of the crosslinked polymer. In the case of drying by introducing hot air, the temperature of the hot air may be increased, preferably 100 to 400 ℃, more preferably 200 to 400 ℃ in order to increase the drying rate, although the drying temperature depends on the water content of the gel particles to be dried.

When the solvent contains water, the water content (% by weight) of the dried powder of the crosslinked polymer (a) is preferably 0 to 20, more preferably 1 to 15, particularly preferably 2 to 13, and most preferably 3 to 12 based on the weight of the crosslinked polymer (a). When the water content is in the range of 0 to 20, the absorption performance becomes better.

When the solvent contains an organic solvent, the content (% by weight) of the organic solvent in the dried powder of the crosslinked polymer (a) is preferably 0 to 10, more preferably 0 to 5, particularly preferably 0 to 3, and most preferably 0 to 1 based on the weight of the crosslinked polymer (a). When the content is in the range of 0 to 10, the water-absorbent resin particles have better absorption performance.

The content and moisture of the organic solvent are measured by an infrared moisture meter [ for example, JE400 manufactured by katt corporation, ltd.: 120. + -. 5 ℃ for 30 minutes, ambient humidity before heating of 50. + -. 10% RH, lamp specification of 100V, 40W ] and the weight loss of the sample after heating.

After drying, some other components such as residual solvent and residual crosslinking component may be contained within a range not to impair the performance.

The proportion of the weight of particles having a particle diameter of 2.8mm or more in the dried powder of the crosslinked polymer (a) obtained after drying to the total weight of the dried powder is 50% by weight or less, preferably 45% by weight or less, more preferably 40% by weight or less, particularly preferably 35% by weight or less, and most preferably 30% by weight or less. When the weight ratio of the particles having a particle diameter of 2.8mm or more is more than 50% by weight, the average normal sphericity of the water-absorbent resin particles is low, and the range of variation in the average normal sphericity after the impact resistance test may be increased. On the other hand, the lower limit value, the lower the limit value, the lower the limit value is, although not particularly limited, the lower the limit value is, from the viewpoint of productivity of the water-absorbent resin particles, the lower the limit value is, and the lower the limit value is.

The weight ratio of particles having a particle diameter of 2.8mm or more to the total weight of the dry powder was measured by the method described in Perry's Chemical Engineers' Handbook 6 th edition (Myglo-Hill book Co., 1984, page 21) using a Ro-Tap type test vibratory screening machine and a standard sieve (JIS Z8801-1: 2006). That is, the JIS standard sieves were combined in the order of 4.0mm, 2.8mm, 1.4mm and tray from above. About 50g of the test particles were put on the uppermost stage sieve and vibrated for 5 minutes by means of a Ro-Tap test vibratory screening machine. The weight of the measured particles on each sieve and tray was weighed, and the total weight was set to 100% by weight, and the weight fraction of the particles on each sieve was determined. The total weight fraction of particles of 4.0mm or more and 2.8mm or more is defined as the weight ratio of particles having a particle diameter of 2.8mm or more.

The resin particles containing the crosslinked polymer (A) are adjusted in particle size and particle size distribution by pulverization and/or classification. The pulverization method is not particularly limited, and a known pulverization device (for example, a hammer mill, an impact mill, a roll mill, a jet mill, or the like) can be used. Among these, a roll mill is preferable from the viewpoint of controlling the particle size distribution. In addition, in order to control the particle size distribution, the oversize product may be pulverized again after classification. When the oversize product is pulverized again, the same or different types of pulverizers may be used.

In the method of classification, in order to control the particle size distribution of the pulverized resin particles, a plurality of sieves having specific meshes or a single sieve may be used for classification. The classifying device is not particularly limited, and known methods such as a vibrating screen, an in-plane moving screen, a movable mesh screen, a forced stirring screen, and a sonic screen are used, and a vibrating screen and an in-plane moving screen are preferably used. In order to control the particle size distribution of the resin particles, it is preferable to remove part or all of the particles (oversize material) remaining on the screen having the specific mesh and the particles (undersize material) having passed through the screen having the specific mesh.

The weight average particle diameter (μm) of the resin particles containing the crosslinked polymer (A) obtained by grinding and/or classification is preferably 200 to 450, more preferably 200 to 400, and particularly preferably 200 to 370. When the average sphericity is more than 200 to 450, the average sphericity of the water-absorbent resin particles tends to decrease, and the range of variation in average sphericity after the impact resistance test tends to increase. On the other hand, if the amount is less than this range, the flowability of the particles is deteriorated, and the amount of the particles added is likely to be deviated during the production of the diaper.

The weight average particle size was measured by a method described in the Peltier Chemical Engineers manual (Perry's Chemical Engineers' Handbook) 6 th edition (Mgelo-Hill book, 1984, page 21) using a Ro-Tap type test vibratory sieve and a standard sieve (JIS Z8801-1: 2006). That is, the JIS standard sieves were combined in the order of 1000. mu.m, 850. mu.m, 710. mu.m, 500. mu.m, 425. mu.m, 355. mu.m, 250. mu.m, 150. mu.m, 125. mu.m, 75. mu.m, and 45 μm from above and a tray. About 50g of the test particles were put on the uppermost stage sieve and vibrated for 5 minutes by a Ro-Tap type test vibratory screening machine. The weight of the particles measured on each sieve and tray was weighed, and the total weight was taken as 100% by weight, the weight fraction of the particles on each sieve was obtained, and the obtained value was plotted on a logarithmic probability paper (the horizontal axis represents the mesh (particle diameter) of the sieve, and the vertical axis represents the weight fraction), and then each point was drawn to obtain the particle diameter corresponding to the weight fraction of 50% by weight, and this was taken as the weight average particle diameter.

The water-absorbent resin particles of the present invention have a structure in which resin particles containing a crosslinked polymer (A) are surface-crosslinked with a surface-crosslinking agent (d). By having a structure in which the surface is crosslinked by the surface crosslinking agent, the absorption characteristics (liquid passing speed, absorption amount under load, and the like) can be improved.

In the production process of the present invention, as the surface-crosslinking agent (d) in the step of subjecting the resin particles containing the crosslinked polymer (A) to surface-crosslinking treatment with the surface-crosslinking agent, there can be used known surface-crosslinking agents { e.g., polyglycidyl compounds, polyhydric alcohols, polyamines, polyazepines, polyisocyanates, silane coupling agents, polyvalent metals, and the like } of Japanese patent application laid-open Nos. 59-189103, 58-180233, 61-169903, 61-211305, 61-252212, 51-136588, 61-257235, and the like. Among these surface cross-linking agents, from the viewpoint of economy and absorption characteristics, a polyglycidyl compound, a polyhydric alcohol and a polyamine are preferable, a polyglycidyl compound and a polyhydric alcohol are more preferable, a polyglycidyl compound is particularly preferable, and ethylene glycol diglycidyl ether is most preferable.

The amount (% by weight) of the surface-crosslinking agent to be used is not particularly limited since it may be variously changed depending on the kind of the surface-crosslinking agent, the crosslinking condition, the target performance, and the like, and is preferably 0.001 to 3, more preferably 0.005 to 2, and particularly preferably 0.01 to 1 based on the weight of the resin particles containing the crosslinked polymer (a) from the viewpoint of absorption characteristics and the like.

As the method for surface crosslinking with a surface crosslinking agent, a method known as { for example, Japanese patent No. 3648553, Japanese patent application laid-open Nos. 2003-165883, 2005-75982, and 2005-95759 } can be applied.

After the step of surface crosslinking with the surface crosslinking agent, the particle size can be adjusted by further screening.

The water-absorbent resin particles of the present invention may contain inorganic fine particles and/or polyvalent metal salt as necessary. Therefore, the production method of the present invention may include a step of surface-treating the resin particles containing the crosslinked polymer (a) with the inorganic fine particles and/or the polyvalent metal salt. By containing the inorganic fine particles and/or the polyvalent metal salt, the liquid permeability and blocking resistance of the water-absorbent resin particles are improved.

Examples of the inorganic fine particles include silica, alumina, zirconia, titania, zinc oxide, talc, and the like. In addition, as the polyvalent metal salt, a salt of at least one metal selected from the group consisting of magnesium, calcium, zirconium, aluminum and titanium with the above-mentioned inorganic acid or organic acid may be mentioned. Among these, silica, alumina, aluminum sulfate, sodium aluminum sulfate, and aluminum lactate are preferable. These may be used alone or in combination of two or more.

The amount (wt%) of the inorganic fine particles or polyvalent metal salt used is preferably 0.01 to 2.0, more preferably 0.05 to 1.0 based on the weight of the resin particles containing the crosslinked polymer (a) from the viewpoint of absorption performance (particularly liquid permeability and blocking resistance).

In the case of surface treatment with inorganic fine particles and/or polyvalent metal salt, the step of mixing with the inorganic fine particles and/or polyvalent metal salt may be performed in any of the processes before, after, and simultaneously with the step of surface crosslinking with the surface crosslinking agent.

In the production method of the present invention, the particle size can be further adjusted after the step of surface treatment with inorganic fine particles and/or polyvalent metal salt.

The water-absorbent resin particles of the present invention may contain other additives { for example, a known preservative (e.g., jp 2003-225565 a, jp 2006-131767 a, etc.), a fungicide, an antibacterial agent, an antioxidant, an ultraviolet absorber, a coloring agent, a fragrance, a deodorant, an organic fiber, etc. }. When these additives are contained, the content (% by weight) of the additives is preferably 0.001 to 10, more preferably 0.01 to 5, particularly preferably 0.05 to 1, and most preferably 0.1 to 0.5 based on the weight of the resin particles containing the crosslinked polymer (A).

The water-absorbent resin particles of the present invention have an amorphous crushed shape. It is preferable that the particle shape is in an amorphous crushed state, because not only water-absorbent resin particles having a high water absorption rate (particularly, a short water absorption time by the vortex method) can be obtained, but also the particles are favorably entangled with fibrous materials and are not likely to be separated from the fibrous materials in the use of disposable diapers or the like. The particles may be aggregates (or granules) of them or may be porous structures of them, as long as they are in an amorphous crushed state. Therefore, for example, it is preferable that the shape is not a regular sphere produced by the reversed-phase suspension polymerization method. The preferred average sphericity of the water-absorbent resin particles is as follows.

The apparent density (g/ml) of the water-absorbent resin particles is preferably 0.54 to 0.70, more preferably 0.56 to 0.68, and particularly preferably 0.58 to 0.66. When the apparent density is in the range of 0.54 to 0.70, the absorption performance is further improved. The apparent density was measured in accordance with JIS K7365: 1999 at 25 ℃.

The average spherical particle Size (SPHT) of the water-absorbent resin particles is 0.800 to 0.900, preferably 0.800 to 0.870, more preferably 0.800 to 0.850, and most preferably 0.800 to 0.840. When the average Sphericity (SPHT) is less than 0.800, not only the range of variation in average Sphericity (SPHT) after the impact resistance test described later is increased, but also gritty or uneven touch due to contact between particles of the SAP particles is increased, and the feel of the absorbent body is deteriorated. When the average positive Sphericity (SPHT) is greater than 0.900, the absorbent body comes off from the fibrous material. The average normal sphericity of the water-absorbent resin particles can be controlled by reducing the coarse particles of the dried powder obtained after drying, as described above.

As a method for measuring the average positive sphericity, a method of deriving the average positive sphericity of a measurement sample by image analysis, for example, measurement using a Camsizer (registered trademark) image analysis system (manufactured by Retsch Technology GmbH) can be used. That is, 30.0g of the measurement sample was allowed to freely fall little by little, and the falling measurement sample was continuously imaged by a CCD camera. The images taken are analyzed, from which the average sphericity of the test sample is deduced. The arithmetic average of the average normal sphericity of the particles derived from the number N of analysis points being 3 is defined as the average normal sphericity of the present invention.

The range of variation in average positive Sphericity (SPHT) after the impact resistance test is 0 to 0.015, preferably 0 to 0.0130, and more preferably 0 to 0.010. If the amount exceeds 0 to 0.015, the physical properties of the water-absorbent resin particles after the impact resistance test may be deteriorated, and the liquid permeability and the absorption rate may be easily changed. The range of variation in average normal sphericity after the impact resistance test of the water-absorbent resin particles can be controlled by reducing coarse particles of the dried powder obtained after drying, as described above.

< method of measuring average spherical Positive Density (SPHT) after impact resistance test >

The average Sphericity (SPHT) after the impact test of the present invention is measured as follows. 30g of water-absorbent resin particles were put into a 3L separable round flask (manufactured by ASONE), a nylon net (JIS Z8801-1: 2000) having a mesh opening of 63 μm with a 6mm hole at the center was laid on the upper part of the separable flask, and a four-neck separable lid (main tube TS29/42, side tube TS24/40, 24/40, 15/35 manufactured by ASONE) was further provided thereon. Next, a stainless steel tube (6 mm in outside diameter and 4mm in inside diameter) was inserted through the nylon net in the main tube TS29/42 of the four-necked separable cap so that the tip thereof was positioned at 45mm from the bottom surface of the separable flask. The other end of the stainless steel pipe is provided with a polyurethane pipe (with the length of 1500mm and the inner diameter of 8.5mm) and is connected with an air pipeline capable of realizing the pressure of more than 0.3 MPa. Then, the air line was opened at a pressure of 0.2MPa, and air blowing was carried out for 3 minutes to remove the water-absorbent resin particles. Using the water-absorbent resin particles, the average positive Sphericity (SPHT) after the impact resistance test was measured using a Camsizer (registered trademark) image analysis system (manufactured by Retsch Technology GmbH) in the same manner as described above. The range of change in average Sphericity (SPHT) after the impact resistance test was determined by the following equation.

(range of change in average sphericity after impact resistance test) (average sphericity after impact resistance test) - (average sphericity before impact resistance test)

The water-absorbent resin particles of the present invention preferably have a weight average particle diameter (μm) of 200 to 450, more preferably 200 to 400, and particularly preferably 200 to 370. When the weight average particle diameter is larger than the range of 200 to 450, the average normal sphericity of the water-absorbent resin particles tends to be low, and the range of variation in the average normal sphericity after the impact resistance test may be large. On the other hand, if the weight average particle diameter is less than this range, the flowability of the particles is deteriorated, and the amount of the particles added is likely to be deviated during the production of the diaper.

In the water-absorbent resin particles of the present invention, the weight ratio of particles having a particle diameter of 500 μm or more to the total water-absorbent resin particles is preferably 5% by weight or less, more preferably 3% by weight or less. If the amount is more than 5% by weight, the average sphericity of the water-absorbent resin particles tends to decrease, the range of variation in average sphericity after the impact resistance test increases, gritty or uneven touch due to contact between particles of the SAP particles increases, and the feel of the absorbent body may deteriorate.

In the water-absorbent resin particles of the present invention, the weight ratio of the particles having a particle diameter of less than 150 μm to the whole water-absorbent resin particles is preferably 3% by weight or less, and more preferably 1% by weight or less. If the weight ratio is more than 3% by weight, the gel permeation rate may be lowered.

In the water-absorbent resin particles of the present invention, the water retention capacity of physiological saline (0.9 wt% saline solution, the same applies hereinafter) is preferably 25 to 45g/g, and more preferably 30 to 40 g/g. When the water retention amount is in the range of 25 to 45g/g, the absorbent article can exhibit a sufficient absorption amount and can achieve a liquid flow rate.

The water-absorbent resin particles of the present invention have a gel permeation rate based on physiological saline of preferably 10 ml/min or more, more preferably 40 ml/min or more, and particularly preferably 70 ml/min or more. If the liquid permeation rate is less than 10 ml/min, the permeation rate into the absorbent body is reduced, and leakage may occur. The higher the upper limit value is, the more preferable is it, there is no particular limitation, and from the viewpoint of the water retention amount, it is preferably 1000 ml/min or less, more preferably 500ml or less, and particularly preferably 100ml or less.

The water-absorbent resin particles of the present invention have an absorption rate by the vortex method of preferably 45 seconds or less, more preferably 40 seconds or less, and particularly preferably 35 seconds or less. If it exceeds 45 seconds, leakage of the absorbent body is likely to occur. The lower limit value is preferably lower, and is not particularly limited, but is preferably 10 seconds or more, and more preferably 15 seconds or more, from the viewpoint of achieving both the average spherical uniformity and the like.

The water-absorbent resin particles of the present invention have a water content (% by weight) of preferably 0 to 20, more preferably 1 to 15, particularly preferably 2 to 13, and most preferably 3 to 12. When the water content is in the range of 0 to 20, the absorption performance is improved, and the range of variation of the average normal sphericity after the impact resistance test may be reduced.

The water-absorbent resin particles of the present invention constitute an absorber included in an absorbent article. The absorbent material may be the water-absorbent resin particles alone or may be the absorbent material together with other materials. As other materials, fibrous materials and the like can be cited. The structure, production method, and the like of the absorber in the case of using together with a fibrous material are the same as those of known techniques (japanese patent application laid-open nos. 2003-225565, 2006-131767, 2005-097569, and the like).

When the water-absorbent resin particles of the present invention are used together with a fibrous material to form an absorbent body, the weight ratio of the water-absorbent resin particles to the fibers (weight of the water-absorbent resin particles/weight of the fibers) is preferably 60/40 to 90/10, and more preferably 75/25 to 85/15.

As the absorbent article, the present invention can be suitably used not only as a sanitary product such as a disposable diaper and a sanitary napkin, but also as an article used in various applications such as an absorbent or a retention agent for various aqueous liquids, and a gelling agent, which will be described later. The method for producing the absorbent article and the like are the same as the known techniques (the methods described in japanese patent laid-open nos. 2003-225565, 2006-131767, 2005-097569 and the like).

Examples

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. Unless otherwise specified, parts means parts by weight and% means% by weight. The water retention capacity of the water-absorbent resin particles in physiological saline, the gel flow rate of the physiological saline, and the absorption rate by the vortex method were measured by the following methods.

< method for measuring Water Retention amount of physiological saline >

A measuring sample (1.00 g) was added to a tea bag (length 20cm, width 10cm) made of a nylon net having a mesh opening of 63 μm (JIS Z8801-1: 2006), and the bag was immersed in 1,000ml of physiological saline without stirring for 1 hour, and then suspended for 15 minutes to remove water. Thereafter, the tea bag was put into a centrifugal separator together with the tea bag, and the centrifugal separator was centrifuged at 150G for 90 seconds to remove the remaining physiological saline, and the weight including the tea bag was measured (h1), and the water retention was determined by the following equation. The temperature of the physiological saline used and the temperature of the measurement atmosphere were 25 ℃. + -. 2 ℃. The weight of the tea bag after the centrifugal dehydration was measured in the same manner as described above except that the measurement sample was not used, and this was designated as (h 2).

The water retention capacity (g/g) of physiological saline is (h1) - (h2)

< method for measuring gel permeation speed Using physiological saline >

The measurement was performed by the following procedure using the instrument shown in fig. 1 and 2.

The measurement sample 0.32g was immersed in 150ml of physiological saline 1 for 30 minutes to prepare swollen gel particles 2. Then, a filtration cylinder tube having a metal mesh 6 (mesh 106 μm, JIS Z8801-1: 2006) and a freely openable/closable cock 7 (inner diameter of liquid passing portion 5mm) at the bottom of a vertically standing cylinder 3{ diameter (inner diameter) 25.4mm, length 40cm, and scale marks 4 and 5 provided at positions 60ml and 40ml from the bottom part, respectively }, the prepared swollen gel particles 2 were transferred into the above-mentioned filtration cylinder tube together with physiological saline with the cock 7 closed, and then a circular wire mesh 8 (mesh opening 150 μm, diameter 25mm) was placed on the swollen gel particles 2 by a pressing shaft 9 (weight 22g, length 47cm) perpendicularly bonded to the wire mesh surface so that the wire mesh and the swollen gel particles were in contact with each other, and a weight 10(88.5g) was further placed on the pressing shaft 9 and allowed to stand for 1 minute. Then, the cock 7 was opened, and the time (T1; second) required for the liquid level in the filtration cylinder tube to reach from 60ml mark 4 to 40ml mark 5 was measured, and the gel permeation rate (ml/min) was determined by the following equation.

The gel permeation speed (ml/min) based on physiological saline is 20ml multiplied by 60/(T1T2)

The temperature of the physiological saline used and the measurement atmosphere was measured at 25 ℃. + -. 2 ℃, and T2 was the time measured by the same procedure as described above in the case where no measurement sample was present.

< absorption Rate by Eddy Current method >

50g of physiological saline was added to a 100ml beaker, and temperature adjustment was performed at 25. + -. 2 ℃. Next, a stirrer (30 mm in length, 8mm in diameter at the center and 7mm in diameter at the end) was placed in the center of the beaker, and the physiological saline was stirred at 600 rpm. 2.000g of the measurement sample was put into the vicinity of the wall surface of the beaker. The measurement sample to be used is adjusted by using a sample splitter or the like so as to be sampled in a representative particle diameter state. The absorption time is measured as the time (seconds) from the time when the measurement sample is put into the liquid mixture, and the time (seconds) until the liquid surface of the liquid mixture of the measurement sample and the physiological saline is flattened (the time point when the diffuse reflection light from the liquid surface disappears). The test was carried out at 25. + -. 3 ℃ and 60. + -. 5 RH%.

< measurement of average Spherical Positive (SPHT) after impact resistance test >

The measurement was performed by the above-mentioned measurement method.

< Change in gel permeation Rate >

The rate of change of the gel permeation rate was determined by the following equation. In the following formula, the same samples as those used in the measurement of the average Sphericity (SPHT) after the impact resistance test were used for the gel permeation rate after the impact resistance test with respect to the gel permeation rate by physiological saline.

(rate of change in gel permeation rate [% ]) { (rate of permeation of gel of water-absorbent resin particles [ ml/min ]) - (rate of permeation of gel after impact resistance test [ ml/min ]) }/(rate of permeation of gel of water-absorbent resin particles [ ml/min ]) × 100

< absorption Rate Change Rate by Eddy Current method >

The rate of change of the absorption rate by the eddy current method was obtained by the following equation. In the following formula, the absorption rate by the vortex method after the impact resistance test was measured using the same sample as that used for the measurement of the average normal Sphericity (SPHT) after the impact resistance test.

(rate of change of absorption rate by vortex method [% ]) { (absorption rate by vortex method [ sec ] of water-absorbent resin particles) - (absorption rate by vortex method [ sec ] after impact resistance test/(absorption rate by vortex method [ sec ] of water-absorbent resin particles) × 100

< method for evaluating foreign body sensation >

Evaluation of the foreign body sensation by the tactile sensation was performed by 10 monitoring persons. Specifically, 100g of water-absorbent resin particles were put into a polyethylene bag (140X 100X 0.04mm) with a zipper, and the sample was gently kneaded in one direction (for example, clockwise) in a previously marked region using the index finger and/or middle finger, and the foreign substance sensation thereof was evaluated according to the following evaluation criteria.

Less than 2 people feel gravel

More than 2 persons and less than 4 persons feel a gritty feeling

More than 4 persons and less than 6 persons feel a gritty feeling

More than 6 people feel gravel

< example 1>

Water-soluble vinyl monomer (a1-1) { acrylic acid, made by mitsubishi chemical corporation, purity 100% }155 parts (2.15 parts by mol), crosslinking agent (b1) { pentaerythritol triallyl ether, made by Daiso corporation }0.6225 parts (0.0024 parts by mol), and deionized water 340.27 parts were kept at 3 ℃ with stirring and mixing. After nitrogen was flowed into the mixture to reduce the dissolved oxygen amount to 1ppm or less, 0.62 parts of a 1% hydrogen peroxide solution, 1.1625 parts of a 2% ascorbic acid aqueous solution, and 2.325 parts of a 2% 2, 2' -azobis [ 2-methyl-N- (2-hydroxyethyl) -propionamide ] aqueous solution were added and mixed to initiate polymerization. After the temperature of the mixture reached 90 ℃ and was polymerized at 90. + -. 2 ℃ for about 5 hours, thereby obtaining a crosslinked-polymerized aqueous gel (1).

Then, 51.13 parts of 48.5% sodium hydroxide aqueous solution was added and mixed to 200 parts of the hydrogel (1) while kneading and chopping the hydrogel with a chopper (12 VR-400K, made by ROYAL corporation), and the hydrogel was chopped 3 times with a chopper (12 VR-400K, made by ROYAL corporation) to obtain a chopped gel. Subsequently, 0.076 part of (c-1) { sucrose stearate } was added and mixed, and the mixture was further chopped 1 time by a chopper (12 VR-400K, ROYAL Co.) to obtain aqueous gel particles. Subsequently, the total amount of the hydrogel particles was charged into a kneader (desktop kneader PNV-1, manufactured by Kabushiki Kaisha) equipped with a Sigma-type rotary blade and a heat-retaining jacket, and dried at a rotation speed of 40rpm and a jacket temperature of 180 ℃ for 60 minutes to obtain a dried powder (1) containing a crosslinked polymer. The weight ratio of the particles having a particle diameter of 2.8mm or more in the dry powder (1) to the total weight of the dry powder (1) was 41 wt%. Next, the dried powder (1) was pulverized with a roll mill (RM-10 type roll mill, Seiko Co., Ltd.) with a gap of 0.35mm, and then sieved to have a mesh opening in the particle size range of 710 to 150 μm, thereby obtaining resin particles (A-1) comprising a crosslinked polymer. The weight average particle diameter of the crushed and classified resin particles (A-1) containing the crosslinked polymer was 420. mu.m.

Next, to 30 parts of the resin particles (a-1), a mixed solution obtained by mixing 0.081 parts of 15% ethylene glycol diglycidyl ether as a surface crosslinking agent, 0.90 parts of a 50% propylene glycol aqueous solution as a solvent, and 0.20 parts of Klebosol (registered trademark) 30CAL25 (merck colloidal silica) as inorganic fine particles was added, and after uniform mixing, the mixture was heated at 130 ℃ for 30 minutes to obtain surface-crosslinked resin particles. Subsequently, the resultant was passed through a sieve having a mesh opening of 850 μm, whereby water-absorbent resin particles (P-1) were obtained.

< example 2>

Water-absorbent resin particles (P-2) were obtained in the same manner as in example 1, except that in example 1, the rotation speed of the kneader was changed from 40rpm to 30 rpm. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (2) in example 2 to the total weight of the dry powder (2) in example 2 was 49 wt%, and the weight average particle diameter of the resin particles (a-2) containing the crosslinked polymer after the pulverization classification was 436 μm.

< example 3>

Water-absorbent resin particles (P-3) were obtained in the same manner as in example 1, except that in example 1, the rotation speed of the kneader was changed from 40rpm to 50 rpm. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (3) in example 3 to the total weight of the dry powder (3) in example 3 was 35% by weight, and the weight average particle diameter of the resin particles (a-3) containing the crosslinked polymer after the pulverization classification was 400 μm.

< example 4>

Water-absorbent resin particles (P-4) were obtained in the same manner as in example 1, except that 0.076 part of the gel blocking inhibitor (c-1) { sucrose stearate } was changed to 0.152 part in example 1. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (4) in example 4 to the total weight of the dry powder (4) in example 4 was 33 wt%, and the weight average particle diameter of the resin particles (a-4) containing the crosslinked polymer after the pulverization classification was 393 μm.

< example 5>

A water-absorbent resin particle (P-5) was obtained in the same manner as in example 1 except that 0.076 part of the anti-blocking agent (c-1) { sucrose stearate } was changed to 0.152 part, and 0.113 part of the anti-blocking agent (c-2) { Naroasty (registered trademark) CL-20, anionic surfactant (nonylphenol EOA (2 mol EO added)) } produced industrially by Sanyo chemical synthesis was used in combination with the above-described example 1. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (5) in example 5 to the total weight of the dry powder (5) in example 5 was 27% by weight, and the weight average particle diameter of the resin particles (a-5) containing a crosslinked polymer after pulverization and classification was 385 μm.

< example 6>

Water-absorbent resin particles (P-6) were obtained in the same manner as in example 5, except that in example 5, the particle diameter of the mesh openings 710 to 150 μm of the sieve after the dry powder had been pulverized was changed to a particle diameter range of 500 to 150. mu.m. The weight average particle diameter of the crushed and classified resin particles (A-6) containing a crosslinked polymer was 365. mu.m.

< example 7>

Water-absorbent resin particles (P-7) were obtained in the same manner as in example 5, except that in example 5, the particle diameter of the sieved mesh 710 to 150 μm after the dry powder had been pulverized was changed to a particle diameter range of 300 to 150. mu.m. The weight-average particle diameter of the crushed and classified resin particles (A-7) containing a crosslinked polymer was 202 μm.

< example 8>

Water-absorbent resin particles (P-8) were obtained in the same manner as in example 5, except that in example 5, the rotation speed of the kneader was set to 40rpm, and the particle diameter of the mesh openings 710 to 150 μm of the sieve after pulverizing the dried powder was changed to a particle diameter range of 500 to 150 μm. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (8) in example 8 to the total weight of the dry powder (8) in example 8 was 22% by weight, and the weight average particle diameter of the resin particles (a-8) containing a crosslinked polymer after pulverization and classification was 340 μm.

< example 9>

In example 6, surface-crosslinked resin particles were obtained in the same manner as in example 6 except that 0.081 parts of 15% ethylene glycol diglycidyl ether as a surface crosslinking agent (relative to the crushed and classified resin particles (a-6) containing a crosslinked polymer) was changed to 0.200 parts. Then, 0.045 part of silica (AEROSIL (registered trademark, hereinafter the same)) as inorganic fine particles was added thereto while stirring at a high speed (a high-speed stirring paddle mixer (registered trademark, hereinafter the same) manufactured by Hosokawa Micron Co., Ltd., rotational speed 2000rpm), and the mixture was heated at 80 ℃ for 30 minutes and then passed through a sieve having a mesh opening of 850. mu.m, thereby obtaining water-absorbent resin particles (P-9).

< example 10>

In example 6, surface-crosslinked resin particles were obtained in the same manner as in example 6 except that 0.081 parts (relative to the crushed and classified resin particles (a-6)) of 15% ethylene glycol diglycidyl ether as a surface crosslinking agent was changed to 0.200 parts, and a mixed solution obtained by mixing 0.51 parts of a 50% propylene glycol aqueous solution as a solvent and 0.225 parts of sodium aluminum sulfate hexadecahydrate (fuji film, wako pure chemical industries, inc.) as a polyvalent metal salt was added to perform surface crosslinking. While stirring at high speed (high speed stirring paddle mixer manufactured by Hosokawa Micron Co., Ltd., rotational speed 2000rpm), 0.045 part of silica (AEROSIL 200) as inorganic fine particles was added, and after heating at 80 ℃ for 30 minutes, the resultant was passed through a sieve having mesh openings of 850. mu.m, thereby obtaining water-absorbent resin particles (P-10).

< comparative example 1>

Water-absorbent resin particles (R-1) for comparison were obtained in the same manner as in example 1, except that in example 1, the rotation speed of the kneader was changed from 40rpm to 20 rpm. The weight ratio of particles having a particle diameter of 2.8mm or more in the dry powder (ratio 1) in comparative example 1 to the total weight of the dry powder (ratio 1) in comparative example 1 was 73 wt%, and the weight average particle diameter of the resin particles (B-1) containing the crosslinked polymer after pulverization and classification was 430 μm.

< comparative example 2>

In example 1, a water-absorbent resin particle (R-2) for comparison was obtained by following the same operation as in example 1 except that in example 1, the water-absorbent resin particle was further chopped 1 time by a chopper without adding a gel blocking inhibitor, the rotation speed of the kneader was changed to 40rpm and the drying time in the kneader was changed to 2 hours in the case of 1 hour. The weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (ratio 2) in comparative example 2 to the total weight of the dried powder (ratio 2) in comparative example 2 was 95% by weight, and the weight average particle diameter of the resin particles (B-2) containing the crosslinked polymer after the pulverization classification was 385 μm.

< comparative example 3>

In example 1, comparative water-absorbent resin particles (R-3) were obtained by following the same procedure as in example 1 except that the particles were further chopped 1 time by a chopper without adding a gel blocking inhibitor and drying by a kneader was changed to drying by an air-through type dryer (manufactured by TABAI ESPEC, drying conditions: hot air temperature 150 ℃, air speed 2m/s, 60 minutes). In the drying, the total amount of hydrogel particles was uniformly spread on an SUS disk (20 cm square in width and 5cm in depth). The weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (ratio 3) in comparative example 3 to the total weight of the dried powder (ratio 3) in comparative example 3 was 98% by weight, and the weight average particle diameter of the resin particles (B-3) containing a crosslinked polymer after pulverization and classification was 411 μm.

< comparative example 4>

Water-absorbent resin particles (R-4) for comparison were obtained in the same manner as in example 1, except that in example 1, the drying by the kneader was changed to drying by an air-through type dryer (manufactured by TABAI ESPEC, drying conditions: hot air temperature 150 ℃, air speed 2m/s, 60 minutes). In the drying, the total amount of hydrogel particles was uniformly spread on an SUS disk (20 cm square in width and 5cm in depth). The weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (ratio 4) in comparative example 4 to the total weight of the dried powder (ratio 4) in comparative example 4 was 97% by weight, and the weight average particle diameter of the resin particles (B-4) containing a crosslinked polymer after pulverization classification was 415 μm.

The evaluation results of the water retention capacities, weight-average particle diameters, average sphericities after impact resistance tests, gel passing speeds, gel passing speed changes, absorption speeds by the vortex method, absorption speed changes by the vortex method, and foreign matter feelings of the water-absorbent resin particles (P-1) to (P-10) obtained in examples 1 to 10 and the water-absorbent resin particles (R-1) to (R-4) for comparison obtained in comparative examples 1 to 4 are shown in Table 1.

As shown in Table 1, it was found that the water-absorbent resin particles of the present invention (examples 1 to 10) had an average normal sphericity in a specific high range as compared with the water-absorbent resin particles for comparison (comparative examples 1 to 4), and the water-absorbent resin particles had a small change width of the average normal sphericity after the impact resistance test, and had excellent results of the evaluation of the foreign matter sensation. It is also found that the change rate of the gel flow and the change rate of the absorption rate by the vortex method are reduced by making the change width of the average normal sphericity within a specific range and smaller. That is, it can be said that the water-absorbent resin particles of the present invention can exhibit stable absorption performance even in the presence of a physical load from the outside. Examples 6 to 10 are examples in which the weight average particle size was reduced so that the average normal sphericity and the increase width of the average normal sphericity were within a specific range, and it was found from the results of evaluation of the rate of change in absorption performance (liquid passing rate and absorption rate) and the foreign matter sensation that these characteristics were highly compatible.

On the other hand, as is clear from the results in table 1, in order to control the average normal sphericity and the range of variation in average normal sphericity after the impact resistance test to specific ranges, it is effective to control the weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder obtained after drying to specific ranges in the production process of the water-absorbent resin particles.

Industrial applicability

The water-absorbent resin particles of the present invention are suitable for use in an absorbent material comprising water-absorbent resin particles and a fibrous material, and are useful in absorbent articles { disposable diapers, sanitary napkins, blood-retention agents for medical use, and the like } comprising the absorbent material. In addition, the water-soluble polymer can be used in various applications such as a pet urine absorbent, a portable toilet urine gelling agent, a fresh-keeping agent for fruits and vegetables, a water drop absorbent for meat, fish and shellfish, a cold-keeping agent, a disposable warm patch, a gelling agent for batteries, a water-retaining agent for plants and soil, a dew condensation preventing agent, a water-stopping agent, a sealing material, and artificial snow.

Description of the symbols

1 physiological saline

2 aqueous gel particles

3 cylinders

4 graduation mark at the position 60ml from the bottom

5 Scale lines at a position 40ml from the bottom

6 Metal mesh

7 cock

8 round metal net

9 pressurized shaft

10 weight

Claims (11)

1. A water-absorbent resin particle having a structure in which a resin particle is surface-crosslinked with a surface-crosslinking agent (d), the resin particle comprising a crosslinked polymer (A) containing a water-soluble vinyl monomer (a1) and a crosslinking agent (b) as essential constituent units, wherein the water-absorbent resin particle has an amorphous crushed particle shape, an average normal Sphericity (SPHT) of 0.800 to 0.900, and a range of variation in average normal Sphericity (SPHT) after an impact resistance test of 0 to 0.015.

2. The water-absorbent resin particles according to claim 1, wherein the weight-average particle diameter is 200 μm to 450 μm.

3. The water-absorbent resin particles according to claim 2, wherein the weight-average particle diameter is 200 μm to 370 μm.

4. Water-absorbent resin particles according to any one of claims 1 to 3, which comprise inorganic fine particles and/or polyvalent metal salt.

5. Water-absorbent resin particles according to any one of claims 1 to 4, wherein the gel permeation rate based on 0.9% by weight of physiological saline is 10 ml/min or more.

6. Water-absorbent resin particles according to any one of claims 1 to 5, wherein the absorption rate by the vortex method is 45 seconds or less.

7. The water-absorbent resin particles according to any one of claims 1 to 6, wherein the water retention capacity of 0.9 wt.% physiological saline is 25g/g to 45 g/g.

8. A method for producing water-absorbent resin particles, comprising the steps of: a polymerization step of polymerizing a monomer composition containing a water-soluble vinyl monomer (a1) and a crosslinking agent (b) as essential constituent units to obtain a water-containing gel containing a crosslinked polymer (A); a gel pulverization step of kneading and chopping the aqueous gel to obtain aqueous gel particles containing (A); a drying step of drying the hydrogel particles to obtain a dried powder containing (A); a step of further pulverizing and/or classifying the dried powder to obtain resin particles containing (a); and a step of surface-crosslinking the surfaces of the resin particles with a surface-crosslinking agent (d), wherein a gel blocking preventive agent (c) is added before the drying step, and the drying step is performed using an agitation dryer, so that the weight ratio of particles having a particle diameter of 2.8mm or more in the dried powder (a) obtained after drying is 50 wt% or less with respect to the total weight of the dried powder.

9. The process for producing water-absorbent resin particles according to claim 8 wherein the anti-gelling agent (c) is a hydrophobic substance (c1) containing a hydrocarbon group and/or a hydrophobic substance (c2) which is a polysiloxane.

10. The process for producing water-absorbent resin particles according to claim 8 or 9, wherein the amount of the anti-blocking adhesive (c) added is from 0.05 to 5.0% by weight based on the weight of the crosslinked polymer (A).

11. The method for producing water-absorbent resin particles according to any one of claims 8 to 10, wherein the solid content concentration of the hydrogel particles before the drying step is 10 to 55% by weight.

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