JP2918807B2 - Water absorbing agent, method for producing the same, and absorbent article containing water absorbing agent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water absorbing agent suitably used for sanitary materials such as disposable diapers (disposable diapers) and sanitary napkins, so-called incontinence pads, a method for producing the same, and a water absorbing agent. The present invention relates to an absorbent article including:

[0002]

2. Description of the Related Art In recent years, sanitary materials such as disposable diapers and sanitary napkins, so-called incontinence pads, have been used as constituent materials thereof.
BACKGROUND ART Water-absorbing resins for the purpose of absorbing body fluids are widely used.

Examples of the above water-absorbing resin include cross-linked polyacrylic acid partially neutralized products (JP-A-55-84304,
JP-A-55-108407, JP-A-55-133413),
Hydrolyzate of starch-acrylonitrile graft polymer (JP-A-46-43995), neutralized product of starch-acrylic acid graft polymer (JP-A-51-125468), vinyl acetate-acrylate Saponified polymer (JP-A-52
-14689), a hydrolyzate of an acrylonitrile copolymer or an acrylamide copolymer (JP-A-53-15959) or a crosslinked product thereof, a cross-linked product of a cationic monomer (JP-A-58-154709) And JP-A-58-154710).

[0004] The properties that the above water-absorbent resin should have include excellent water absorption and water absorption speed when in contact with aqueous liquids such as body fluids, gel strength, and suction power for sucking water from a substrate containing the aqueous liquid. Is mentioned. And, conventionally, it has a plurality of physical properties in various specific ranges among these absorption characteristics, and exhibits excellent performance (water absorption characteristics) when used in sanitary materials such as disposable diapers and sanitary napkins. resin,
Alternatively, various absorbers and absorbent articles using the water absorbent resin have been proposed. For example, a water-absorbent resin (US Pat. No. 4,654,039) combining a specific gel volume, shear modulus, and extractable polymer content, a water-absorbent resin whose water absorption amount, water absorption speed, and gel strength are specified, Disposable diapers and sanitary napkins using resin (JP-A-60-185550, JP-A-60-185551, JP-A-60-185804), specific water absorption and water absorption rate, gel stability Disposable diaper using a water-absorbent resin having a water-absorbent resin (Japanese Patent Application Laid-Open No. 60-185805), a water-absorbent article provided with a water-absorbent resin having specified water absorption, suction power, and water-soluble component amount (Japanese Patent Application Laid-Open No. 63-185805). 21902), a water-absorbent sanitary article containing a water-absorbent resin having specified water absorption, water absorption under pressure, and gel breaking strength (JP-A-63-99861), water absorption and water absorption rate under pressure. Disposable diapers containing the specified water-absorbing resin (JP-A-2-34167), the amount of water absorption under pressure and the particle size Water-absorbing agent containing a specified amount of water-absorbing resin (European Patent No. 339,461), water-absorbing agent containing a specified amount of water-absorbing resin whose water absorption rate and water absorption under pressure in a short time are specified (European Patent No. 443,627) ), A water-absorbing composite material containing a specific amount or more of a water-absorbing resin having a specified deformation and a wicking index under load (European Patent No. 532,002) and the like are known.

On the other hand, in recent years, hygienic materials such as disposable diapers and sanitary napkins have become more sophisticated and thinner, and the amount of water-absorbing resin used per hygienic material, or mainly the water-absorbing resin and hydrophilic There is a tendency that the weight% (hereinafter, referred to as resin concentration) of the water-absorbent resin in the absorbent made of the conductive fiber increases. In other words, less hydrophilic fibers with a low bulk specific gravity,
By increasing the amount of the water-absorbing resin which is excellent in water-absorbing property and has a large bulk specific gravity, the ratio of the water-absorbing resin in the absorber is increased, thereby reducing the thickness of the sanitary material without reducing the water absorption.

[0006]

However, as a result of various studies conducted by the present inventors to increase the amount of water absorbed by the sanitary material, for example, to increase the resin concentration in the absorber, it was found that the resin concentration was higher than before. Using the absorber
In addition, it has been found that in order to prevent inconvenience such as leakage of an aqueous liquid from a sanitary material, it is not sufficient to simply control the above-described characteristics such as the amount of water absorption, water absorption speed, gel strength, and suction force. For example, in the case of a water-absorbent resin that has been particularly noticed in recent years and has only a very large amount of water absorption under pressure, when the resin concentration is increased, a phenomenon that the liquid diffusibility in the absorber is extremely reduced occurs. Have a point.

Further, the inventors of the present application have conducted various studies focusing on the water absorption characteristics of an absorber having a higher resin concentration than before, and as a result, have found that a mixture obtained by mixing a known water-absorbent resin with hydrophilic fibers is used as the absorber. When used as a resin, when the resin concentration is low, it exhibits a certain level of water absorption characteristics, but the resin concentration is 40% by weight
It has been found that when the temperature exceeds the above range, a phenomenon in which the liquid diffusivity rapidly decreases and the amount of water absorbed by the absorber decreases, and a phenomenon in which the water absorption capacity after a long period of time decreases and the return amount of the aqueous liquid increases, occur. That is, when a mixture of a known water-absorbent resin and hydrophilic fibers is used as an absorber, the above-described problems occur.

[0008] Accordingly, an object of the present invention is to solve the above-mentioned problems. For example, when used for sanitary materials and the like, the liquid diffusion is not affected by the resin concentration or the structure of the absorber, and is always extremely high. To provide a water-absorbing agent capable of exhibiting excellent performance (water-absorbing properties) such as a property and a stable water-absorbing amount over a long period of time, a method for producing the same, and an absorbent article containing the water-absorbing agent It is in.

[0009]

Means for Solving the Problems In order to achieve the above object, the inventors of the present invention have proposed a water absorbing agent and a method for producing the same, and
As a result of diligent studies on absorbent articles containing the water absorbing agent,
A hydrophilic unsaturated monomer containing acrylic acid as a main component, in which at least 50 mol% is neutralized in the presence of the crosslinking agent and the dispersant, using a crosslinking agent having a specific composition in a specific amount; To produce a water-absorbing agent precursor by aqueous solution polymerization, adjusting the water-absorbing agent precursor to a specific range of water content, and a specific range of particle size, and then performing heat treatment in the presence of a surface cross-linking agent. The water-absorbing agent obtained by the method has excellent performance (water-absorbing properties) not provided by the above-mentioned conventional water-absorbent resin, that is, even when the amount of the water-absorbent resin used is large or the resin concentration in the absorber is high, Almost independent of the structure of the absorber, etc., it has been found that it has extremely high liquid diffusivity and excellent performance such as maintaining a stable water absorption for a long time, and has completed the present invention. .

That is, in order to solve the above-mentioned problems, the water-absorbing agent according to claim 1 has a diffusion absorption capacity of 25 g / g or more after 60 minutes from the start of absorption and an amount of water-soluble component. Is more than 0 and 7% by weight or less.

[0011] In order to solve the above-mentioned problems, the absorbent article according to the second aspect of the present invention has a diffusion absorption capacity of 25 g / g or more after 60 minutes from the start of absorption and an amount of water-soluble component. It is characterized by having an absorbent containing a water-absorbing agent that is more than 0 and not more than 7% by weight.

According to a third aspect of the present invention, there is provided an absorbent article according to the second aspect, wherein the absorbent includes a hydrophilic fiber, and the hydrophilic property of the absorbent is reduced. The fiber is characterized in that the proportion of the fiber is more than 0 and not more than 60% by weight.

[0013] The method for producing a water-absorbing agent precursor according to the fourth aspect of the present invention is intended to solve the above-mentioned problems.
The above is a method for producing a water-absorbing agent precursor in which a hydrophilic unsaturated monomer mainly containing neutralized acrylic acid is polymerized in an aqueous solution in the presence of a crosslinking agent and a dispersing agent. An ester of an alcohol having 6 or less carbon atoms and a valency of 3 or more and an unsaturated carboxylic acid;
When all the hydroxyl groups of the alcohol having a valency of at least one form an ester bond with the unsaturated carboxylic acid, the molecular weight of the compound is 1
The ratio of the molecular weight to the molecular weight of this compound is 0.7
As described above, a cross-linking agent main component in a range of less than 1.3 and a high-boiling component having two or more alcohol skeletons in a molecule, and the weight ratio of the cross-linking agent main component to the high-boiling component is 75 /
It is characterized in that a crosslinking agent in the range of 25 to 99/1 is used in a range of 0.05 mol% or more and 0.5 mol% or less based on the hydrophilic unsaturated monomer.

According to a fifth aspect of the present invention, there is provided a method for producing a water-absorbing agent precursor according to the fourth aspect, wherein the number of carbon atoms is 6 or less and 3 or less. It is characterized in that the alcohol having a valency of 3 or more is trimethylolpropane and the unsaturated carboxylic acid is acrylic acid.

According to a sixth aspect of the present invention, there is provided a method for producing a water-absorbing agent, wherein the water-absorbing agent precursor obtained in the fourth aspect has a water content of 10% or less and an average. The particle size is in the range of 200 μm to 600 μm and the particle size is 1
After adjusting the proportion of particles having a particle diameter of less than 06 μm to 10% by weight or less, the precursor of the water-absorbing agent is heat-treated in the presence of a surface crosslinking agent.

According to a seventh aspect of the present invention, there is provided a method of manufacturing a water absorbing agent according to the sixth aspect, wherein the surface crosslinking agent is capable of reacting with a carboxyl group. , Solubility parameter (SP value) is 12.5
(cal / cm ³ ) ^1/2 or more of the first surface cross-linking agent, and the solubility parameter is less than 12.5 (cal / cm ³ ) ^1/2 of the second surface cross-linking agent, and the treatment temperature of the heat treatment Is 160 ° C. or higher.

Hereinafter, the present invention will be described in detail. The diffusion absorption capacity in the present invention, the basis weight of the water-absorbent resin is high, and, taking into account the diffusion force of the aqueous liquid in a state where the resin particles are in close contact with each other by external force, to evaluate the water absorption of the water-absorbing agent Are new physical property values. The above-mentioned diffusion absorption capacity is calculated from a measured value after a predetermined time from the start of absorption, for example, 60 minutes after measurement under a predetermined condition. Diffuse absorption
The method of measuring the magnification will be described in detail.

(A) Diffusion Absorption Ratio First, a measuring device used for measuring the diffusion absorption ratio is described below.
A brief description is given below with reference to FIGS.

As shown in FIG. 1, the measuring device comprises a balance 1
And a container 2 having a predetermined capacity mounted on the balance 1, an outside air suction pipe 3, a conduit 4, a glass filter 6, and a measuring unit 5 mounted on the glass filter 6. I have. The container 2 has an opening 2a at the top and an opening 2b at the side surface thereof.
The outside air suction pipe 3 is fitted into the opening a, and the conduit 4 is attached to the opening 2b. The container 2 contains a predetermined amount of physiological saline 12. The lower end of the outside air suction pipe 3 is submerged in the physiological saline 12. The glass filter 6 is formed to have a diameter of 70 mm. And
The container 2 and the glass filter 6 are in communication with each other by a conduit 4. The glass filter 6 is fixed at a position slightly higher than the lower end of the outside air suction pipe 3.

As shown in FIG. 2, the measuring section 5 includes a filter paper 7, a sheet 8, a support cylinder 9, a wire mesh 10 attached to the bottom of the support cylinder 9, and a weight 11. doing. Then, the measuring unit 5 places the filter paper 7, the sheet 8, and the support cylinder 9 (that is, the wire mesh 10) on the glass filter 6 in this order, and also, inside the support cylinder 9, that is, the wire mesh 1.
The weight 11 is placed on the zero. Sheet 8
It is made of polyethylene terephthalate (PET), and is formed in a 0.1 mm-thick donut shape having an opening with a diameter of 18 mm at the center. The support cylinder 9 has an inner diameter of 60 mm. The wire mesh 10 is made of stainless steel, and is formed in 400 mesh (mesh size 38 μm). Then, a predetermined amount of the water-absorbing agent is evenly spread on the wire mesh 10. The weight of the weight 11 is adjusted so that a load of 20 g / cm ² can be uniformly applied to the wire netting 10, that is, the water absorbing agent.

The diffusion absorption capacity was measured using the measuring apparatus having the above-mentioned configuration. The measuring method will be described below.

First, predetermined preparation operations were performed, such as putting a predetermined amount of physiological saline 12 into the container 2 and fitting the outside air suction pipe 3 into the container 2. Next, the filter paper 7 was placed on the glass filter 6, and the sheet 8 was placed on the filter paper 7 such that the opening was located at the center of the glass filter 6. On the other hand, in parallel with these placing operations, 1.5 g of a water-absorbing agent (preferably 1.5 g of a water-absorbing agent whose particle diameter has been previously adjusted to 300 μm to 500 μm by an operation such as classification) inside the support cylinder 9, that is, on the wire mesh 10. ) Was evenly sprayed, and the weight 11 was placed on the water absorbing agent.

Next, on the sheet 8, a wire mesh 10, that is, the support cylinder 9 on which the water absorbing agent and the weight 11 are placed
Was placed such that the center part thereof coincided with the center part of the glass filter 6.

The weight W ₂ (g) of the physiological saline 12 absorbed by the water-absorbing agent for 20 minutes or 60 minutes after the support cylinder 9 is placed on the sheet 8 is measured by the balance 1.
It measured using. As shown in FIG. 3, after passing through the opening of the sheet 8, the physiological saline 12 was absorbed by the water-absorbing agent while diffusing almost uniformly in the lateral direction of the water-absorbing agent.

Then, based on the above weight W _{2, according} to the following equation, diffusion absorption capacity (g / g) = weight W ₂ (g) / weight of water absorbing agent (g), 20 minutes after the start of water absorption, or 60 The diffusion absorption capacity (g / g) after one minute was calculated.

The new property of the water-absorbing agent, that is, the water-absorbing resin, can be evaluated by the diffusion absorption capacity. That is, the diffusion absorption capacity determines how uniformly and quickly the water-absorbent resin can diffuse the aqueous liquid in the direction of the resin layer (hereinafter, referred to as the lateral direction). Can be evaluated. The lateral liquid diffusivity of the aqueous liquid is a particularly important factor in absorbing a large amount of the aqueous liquid. Then, from the results of the above evaluation, for example, the absorbent composed mainly of the water-absorbent resin and the hydrophilic fiber, in particular, the water-absorbent resin in the absorbent having a high weight percent (hereinafter referred to as resin concentration) of the water-absorbent resin is high. Water absorption behavior can also be easily predicted. The structure of the absorber will be described later.

In the above-mentioned prior application, there are many documents which evaluate the amount of water absorption under pressure. However, the conventional evaluation of the water absorption is performed only in a direction perpendicular to the resin layer direction (hereinafter, referred to as a vertical direction). For this reason, it is hardly evaluated how uniformly and quickly the aqueous liquid diffuses in the lateral direction. Therefore, from the results of the conventional evaluation described above, for example, in a disposable diaper or the like using an absorber having a high resin concentration, the water absorption behavior of the absorber cannot be accurately predicted.

The amount of the water-soluble component in the present invention means the re-diffusion property of the aqueous liquid after a long period of time or the retention of the aqueous liquid once absorbed for a long time in the case of the above-mentioned water-absorbent resin having excellent diffusion absorption capacity. It is a physical property value for evaluating the ability to perform.
The amount of the water-soluble component is determined by measurement under predetermined conditions. The method for measuring the amount of water-soluble components will be described in detail.

(B) Amount of Water-Soluble Component 0.500 g of a water-absorbing agent (or a water-absorbing agent precursor) was dispersed in 1000 ml of deionized water, stirred for 16 hours, and filtered with a filter paper. Next, 50 g of the obtained filtrate was placed in a 100 ml beaker, and 1 ml of a 0.1 N aqueous solution of sodium hydroxide, N / 200-
10.00 ml of an aqueous solution of methyl glycol chitosan, and
Four drops of an aqueous 0.1% toluidine blue solution were added.

Next, the above beaker solution was subjected to colloid titration using an N / 400-polyvinyl potassium sulfate aqueous solution,
The time point when the color of the solution changed from blue to reddish purple was determined as the end point of the titration, and the titer A (ml) was determined. Further, the same operation was performed using 50 g of deionized water instead of 50 g of the filtrate, and a blank titration was performed to determine a titer B (ml). Then, these titration amounts A and B and the neutralization ratio x of acrylic acid supplied to the water absorbing agent
(Mol%), the amount of water-soluble component (% by weight) = (B (ml) −A (ml)) × 0.01 × [72 · (100−x) + 94 ×] / 100 The component amount (% by weight) was calculated.

A water-absorbent resin having a water-soluble component content outside the range of the present invention, even if the diffusion absorption capacity is excellent, tends to increase the return amount of the aqueous liquid after a long time. On the other hand, the water-absorbing agent of the present invention, that is, the water-absorbing resin, satisfies both the above-mentioned two physical property values of the diffusion absorption capacity and the water-soluble component amount, and thus has excellent performance. .

The water absorbing agent of the present invention has a diffusion absorption capacity of 25 g / g.
As described above, the amount of the water-soluble component is 7% by weight or less. A water-absorbing agent having a diffusion absorption capacity of less than 25 g / g, that is, a water-absorbent resin, has a poor liquid diffusion property in a lateral direction in an absorber having a high resin concentration (high concentration), and has a small absorption capacity of the absorber. . The diffusion absorption capacity is more preferably 28 g / g or more, and 30 g / g or more.
/ g or more is more preferable, and 32 g / g or more is most preferable.
Even if the diffusion absorption capacity is 25 g / g or more, the water-absorbent resin having a water-soluble component content of more than 7% by weight has a large amount of aqueous liquid returned after a long time. The amount of the water-soluble component is more preferably 5% by weight or less, and further preferably 3% by weight or less. In the present invention, the diffusion absorption capacity is defined as a measured value 60 minutes after the start of absorption in the measurement under predetermined conditions.In addition to the above diffusion absorption capacity, further, 20 minutes after the start of absorption. The diffusion absorption capacity calculated from the measured value of
g / g or more of water-absorbent resin is preferable, and 20 g / g or more of water-absorbent resin is more preferable.

The absorbent article of the present invention is characterized in that it contains the above-mentioned water-absorbing agent having excellent properties as its absorbent. The absorbent may be hydrophilic if necessary in addition to the water-absorbing agent. It may contain fibers. When the absorber comprises, for example, a water-absorbing agent and hydrophilic fibers, examples of the structure of the absorber include a structure in which the water-absorbing agent and the hydrophilic fibers are uniformly mixed, and a layered hydrophilic fiber. A structure in which a water-absorbing agent is interposed, a layer in which the water-absorbing agent and hydrophilic fibers are uniformly mixed and formed into layers, and a layer in which the layered hydrophilic fibers are laminated, and a structure in which the water-absorbing agent and hydrophilic fibers are A configuration in which a layer is formed by mixing uniformly and a water-absorbing agent is sandwiched between the layer and the hydrophilic fibers formed in a layer. Note that the configuration of the absorber is not limited to the configuration described above.

Examples of the above hydrophilic fibers include cellulose fibers such as mechanical pulp, chemical pulp, semi-chemical pulp and dissolved pulp obtained from wood, and artificial cellulose fibers such as rayon and acetate. Among the fibers exemplified above, cellulose fibers are preferred. In addition, hydrophilic fibers, polyamide and polyester,
It may contain synthetic fibers such as polyolefin.
In addition, the hydrophilic fiber is not limited to the fiber exemplified above.

In order for the water-absorbing agent of the present invention to exhibit excellent properties, the ratio of the hydrophilic fibers in the absorbent should exceed 0,
It is preferably 60% by weight or less, 20% by weight or more, 40% by weight or less.
It is more preferable that the content be not more than weight%. Incidentally, the higher the resin concentration in the absorber, the more remarkable the properties of the water-absorbing agent of the present invention, but even if the absorbent contains hydrophilic fibers in any ratio, the water-absorbing agent of the present invention is , And its performance can be fully exhibited. In addition, the performance of the water absorbing agent is hardly affected by the structure of the absorber.

When the ratio of the hydrophilic fibers in the absorber is relatively small, the absorbers, that is, the hydrophilic fibers may be bonded to each other using an adhesive binder. By adhering the hydrophilic fibers to each other, the strength and shape retention of the absorbent before and during use of the water absorbing agent can be increased.

Examples of the adhesive binder include heat-fusible fibers such as polyolefin fibers such as polyethylene, polypropylene, ethylene-propylene copolymer and 1-butene-ethylene copolymer, and emulsions having adhesive properties. . These adhesive binders may be used alone or as a mixture of two or more. The weight ratio between the hydrophilic fiber and the adhesive binder is 50 / 50-99.
/ 1, preferably in the range of 70/30 to 95/5, and more preferably in the range of 80/20 to 95/5.

The water-absorbing agent according to the present invention uses a cross-linking agent having a certain composition in a certain amount, and in the presence of the cross-linking agent and the dispersing agent, an acrylic acid neutralized to 50% by mole or more. A water-absorbing agent precursor obtained by aqueous polymerization of a hydrophilic unsaturated monomer having as a main component a water-absorbing agent precursor in a specific range, and after adjusting the particle size in a specific range, the water-absorbing agent precursor is subjected to surface crosslinking. The heat treatment is carried out in the presence of an agent.

The main component of the hydrophilic unsaturated monomer used as a raw material in the present invention is acrylic acid and its neutralized product. In order to improve the water absorbing properties of the water absorbing agent, 50% by mole or more of acrylic acid, for example,
It is preferably neutralized with an alkali metal salt, ammonium salt, amine salt or the like. Furthermore, in order to further improve the water absorbing properties of the water absorbing agent, 65 mol% of acrylic acid
More preferably, about 80 mol% is neutralized.

The above-mentioned hydrophilic unsaturated monomer may contain an unsaturated monomer other than acrylic acid, if necessary. The unsaturated monomer other than acrylic acid is not particularly limited, but specifically, for example, methacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid, 2- (meth) acrylamide-2 Anionic unsaturated monomers such as -methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid and salts thereof; acrylamide, methacrylamide, N-ethyl (meth) acrylamide , Nn-propyl (meth) acrylamide, N
-Isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate , Vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine and other nonionic hydrophilic group-containing unsaturated monomers; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl Cationic unsaturated monomers such as (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide, and quaternary salts thereof are exemplified. When these other unsaturated monomers are used, they are preferably used in less than 50 mol% based on the total amount of acrylic acid used as a main component and its neutralized product.

In the present invention, the unsaturated carboxylic acid ester such as an acrylate having a plurality of functional groups (hereinafter, referred to as polyfunctional) having a carbon number of 6 or less and a trivalent or higher alcohol (hereinafter referred to as polyfunctional) is used as the crosslinking agent. (Hereinafter, referred to as a trivalent or higher alcohol) and an unsaturated carboxylic acid, or an ester exchange reaction between a trivalent or higher alcohol and an unsaturated carboxylic acid ester. However, an unsaturated carboxylic acid ester in which all of the hydroxyl groups of the above-mentioned trivalent or higher alcohol form an ester bond with the unsaturated carboxylic acid, that is, a desired unsaturated carboxylic acid ester can be obtained with high selectivity and high yield on an industrial scale. It is difficult at present to obtain at a low rate and at a low cost.
And, depending on the reaction conditions, usually, when the above-mentioned unsaturated carboxylic acid esters are synthesized industrially, in addition to the desired unsaturated carboxylic acid ester, a high boiling point having two or more alcohol skeletons in the molecule is generally used. Components are produced in large quantities.

Therefore, the present inventors have proposed that unsaturated carboxylic acid esters which are industrially available, that is, unsaturated carboxylic acid esters containing the high-boiling component, can be used as a crosslinking agent to form a hydrophilic unsaturated carboxylic acid ester. The polymer was subjected to aqueous polymerization to obtain a water-absorbent resin having excellent performance. As a result, when the aqueous solution polymerization is performed using the unsaturated carboxylic acid ester containing the high boiling component, the diffusion absorption capacity of the water absorbing agent is hardly improved, and the amount of the water-soluble component is hardly reduced. found. Therefore, further studies have been made in order to obtain a water-absorbing resin (that is, a water-absorbing agent) which is excellent in performance such as diffusion absorption capacity and liquid diffusivity and has a reduced amount of water-soluble components. As a result, in the presence of the dispersant, the amount of the water-soluble component is drastically reduced only when a cross-linking agent in which the content of the high-boiling component is controlled within a specific range is used in a specific amount. Further, it has been found that a water-absorbing agent excellent in performance such as diffusion absorption capacity can be obtained.

That is, the crosslinking agent used in the present invention comprises an ester of an alcohol having 6 or less carbon atoms and having a valency of 3 or more and an unsaturated carboxylic acid, and all the hydroxyl groups of the alcohol having a valency of 3 or more. Is a compound in which an ester bond is formed with the unsaturated carboxylic acid, and assuming that the molecular weight of the compound is 1, a crosslinking agent main component having a ratio of the molecular weight to the molecular weight of 0.7 or more and less than 1.3, A high-boiling component having two or more skeletons, and a weight ratio of the crosslinking agent main component to the high-boiling component is 75 /
A crosslinking agent that is in the range of 25-99 / 1.

The trihydric or higher valent alcohol that can be used in preparing the crosslinking agent of the present invention is not particularly limited, but may be glycerin, trimethylolethane, tetramethylolethane, trimethylolpropane, or tetrahydroxyl alcohol. Ethane, pentaerythritol and the like are preferred. Of these, trimethylolpropane is particularly preferred. Examples of the unsaturated carboxylic acid that can be used when preparing the crosslinking agent include the above-mentioned hydrophilic unsaturated monomer having a carboxyl group, and acrylic acid is particularly preferable.

As described above, the main component of the crosslinking agent is, among the polyfunctional unsaturated carboxylic esters containing the above trihydric or higher alcohol as an alcohol component and the above unsaturated carboxylic acid as an acid component, the trivalent unsaturated carboxylic acid esters. Compounds in which the ratio of the molecular weight to the molecular weight of this compound is 0.7 or more and less than 1.3, where 1 is the molecular weight of the compound when all the hydroxyl groups of the above alcohols form an ester bond with the unsaturated carboxylic acid. .

For example, when trimethylolpropane is used as the trivalent or higher alcohol and acrylic acid is used as the unsaturated carboxylic acid, the main components of the crosslinking agent are trimethylolpropane triacrylate, trimethylolpropane diacrylate, and trimethylolpropane diacrylate. Acrylate mono (β-acryloyloxypropionate) and the like. Further, for example, when glycerin is used as a trivalent or higher alcohol and acrylic acid is used as an unsaturated carboxylic acid, glycerin triacrylate, glycerin diacrylate and the like are mentioned as main components of a crosslinking agent. 3
When trimethylolethane is used as the alcohol having a valence of 3 or more and acrylic acid is used as the unsaturated carboxylic acid, the main components of the crosslinking agent include trimethylolethane triacrylate, trimethylolethane diacrylate, and the like. When tetramethylolethane is used as the trihydric or higher alcohol and acrylic acid is used as the unsaturated carboxylic acid, the main components of the crosslinking agent include tetramethylolethanetetraacrylate, tetramethylolethanetriacrylate and the like. When tetrahydroxyethane is used as the trihydric or higher alcohol and acrylic acid is used as the unsaturated carboxylic acid, the main components of the crosslinking agent include tetrahydroxyethanetetraacrylate, tetrahydroxyethanetriacrylate and the like. When pentaerythritol is used as the trihydric or higher alcohol and acrylic acid is used as the unsaturated carboxylic acid, the main components of the crosslinking agent include pentaerythritol tetraacrylate and pentaerythritol triacrylate.

The high-boiling components contained in the cross-linking agent are compounds obtained as by-products when synthesizing the main component of the cross-linking agent, although their structures are complicated. For example, when trimethylolpropane is used as a trihydric or higher alcohol and acrylic acid is used as an unsaturated carboxylic acid, the high boiling component is a condensate having two or more trimethylolpropane skeletons in the molecule, for example, trimethylolpropanetriol. Compounds formed by an addition reaction between acrylate and trimethylolpropane diacrylate, dimers of trimethylolpropane diacrylate, compounds formed by addition reaction between trimethylolpropane diacrylate and trimethylolpropane monoacrylate, and the like can be given.

The weight ratio of the main component of the crosslinking agent to the high-boiling component in the crosslinking agent must be in the range of 75/25 to 99/1, and in the range of 80/20 to 90/10. Is more preferred. When the weight ratio of the main component of the crosslinking agent to the high boiling component in the crosslinking agent is smaller than 75/25, that is, the ratio of the main component of the crosslinking agent to the total of the main component of the crosslinking agent and the high boiling component is less than 75% by weight ( In other words, when the proportion of the high-boiling component exceeds 25% by weight), even when the dispersant of the present invention is used, the performance such as the diffusion absorption capacity and the liquid diffusibility is excellent, and the amount of the water-soluble component is high. Because it is not possible to obtain a water absorbing agent with reduced
Not preferred. Further, the crosslinking agent in which the weight ratio of the crosslinking agent main component to the high boiling component is larger than 99/1, that is, the ratio of the crosslinking agent main component to the total of the crosslinking agent main component and the high boiling component is 99%
% By weight (that is, the proportion of high-boiling components is 1% by weight
(Less than) crosslinking agents are not preferred because they are difficult to obtain industrially easily. In addition, the ratio of the main component of the crosslinking agent is
Even when a cross-linking agent exceeding 99% by weight was used, no remarkable effect was observed as compared with the case where a cross-linking agent containing 75% to 99% by weight of the main component of the cross-linking agent was used.

The amount of the crosslinking agent used is in the range of 0.05 mol% or more and 0.5 mol% or less based on the amount of the hydrophilic unsaturated monomer.
The range is preferably 0.07 mol% or more and 0.2 mol% or less. When the amount of the crosslinking agent is less than 0.05 mol%, and
If it is more than 0.5 mol%, the diffusion and absorption capacity of the water-absorbing agent decreases, and the amount of water-soluble components increases, which is not preferable.
The amount of the cross-linking agent used is, in the case of a polyfunctional unsaturated carboxylic acid ester having a carbon number of 6 or less and a trivalent or higher alcohol as an alcohol component and an unsaturated carboxylic acid as an acid component, the trivalent or higher valent ester. A compound in which all the hydroxyl groups of the alcohol form an ester bond with the unsaturated carboxylic acid,
That is, it may be calculated based on the molecular weight of the main component of the crosslinking agent.

In the present invention, the dispersant used in the aqueous solution polymerization is preferably a compound which is compatible with the crosslinking agent and is uniformly dissolved in water, and generally, a compound which is industrially available is desirable. Examples of the dispersant include nonionics such as sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, polyoxyethylene acyl ether, and sucrose fatty acid ester. Anionic surfactants such as higher alcohol sulfates, alkyl naphthalene sulfonates, alkyl polyoxyethylene sulfates, and dialkyl sulfosuccinates; cationic surfactants such as alkyl quaternary ammonium salts and alkyl amine salts Surfactant; amphoteric surfactant such as alkyl betaine and lecithin; lipophilic polymer having carboxyl group, partially saponified polyvinyl alcohol, methylcellulose, carboxymethyl Le cellulose,
High molecular compounds such as hydroxyethylcellulose are exemplified. Among the dispersants exemplified above, water-soluble surfactants and water-dispersible surfactants are preferable, and nonionic surfactants having a hydrophilic-lipophilic balance (HLB) of 10 or more are most preferable.

The amount of the dispersant used is preferably in the range of 1% by weight to 100% by weight, more preferably in the range of 5% by weight to 50% by weight, based on the crosslinking agent. The amount of the dispersant used is
The amount is preferably 0.005 parts by weight or more and 0.5 parts by weight or less based on 100 parts by weight of the water-absorbent resin obtained from the hydrophilic unsaturated monomer.

The mixing method (addition method) of the dispersant during the aqueous solution polymerization is not particularly limited. For example, after the crosslinking agent and the dispersant are directly mixed, the mixed solution may be mixed with the hydrophilic unsaturated monomer or an aqueous solution thereof. Alternatively, after mixing a crosslinking agent and an aqueous solution or aqueous dispersion of a dispersant, the mixed solution may be mixed with a hydrophilic unsaturated monomer or an aqueous solution thereof. Alternatively, after mixing the crosslinking agent, the dispersant, and a part of the hydrophilic unsaturated monomer or an aqueous solution thereof, the mixed solution is mixed with the remaining portion of the hydrophilic unsaturated monomer or the aqueous solution thereof. You may. Alternatively, after mixing a crosslinking agent, an aqueous solution or aqueous dispersion of a dispersant, and a part of the hydrophilic unsaturated monomer or an aqueous solution thereof, the mixed solution is mixed with a hydrophilic unsaturated monomer or an aqueous solution thereof. May be mixed with the rest. After mixing the cross-linking agent, the dispersant, and a part of the hydrophilic unsaturated monomer or the aqueous solution thereof, the mixed solution, the hydrophilic unsaturated monomer or the aqueous solution thereof may be used. Is preferred. In addition, after mixing a crosslinking agent, a dispersant, and acrylic acid as a hydrophilic unsaturated monomer, the mixed solution is mixed with a residue of the hydrophilic unsaturated monomer or an aqueous solution thereof (that is, acrylic acid). Is more preferable.

When the hydrophilic unsaturated monomer is polymerized in an aqueous solution in the presence of a crosslinking agent and a dispersing agent, the concentration of the hydrophilic unsaturated monomer in the aqueous solution is 25% by weight or more, which is a saturated concentration. % Or less, more preferably 30% by weight or more and 45% by weight or less.

At the start of polymerization, potassium persulfate, ammonium persulfate, sodium persulfate, t-
Butyl hydroperoxide, hydrogen peroxide, 2,2 '
Radical polymerization initiators such as -azobis (2-amidinopropane) dihydrochloride, or active energy rays such as ultraviolet rays and electron beams can be used. When an oxidizing radical polymerization initiator is used, redox polymerization may be performed using a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid in combination. The amount of these polymerization initiators used is 0.001 mol%
It is preferably in the range of 2 mol%, and 0.01 mol% to 0.5 mol%
Is more preferable.

Further, the water-absorbing agent according to the present invention has a water content of the water-absorbing agent precursor obtained by the above-described aqueous solution polymerization.
After being adjusted by an operation such as classification so that the ratio of particles having a particle diameter of less than 10% and an average particle diameter within a range of 200 μm to 600 μm and less than 106 μm is 10% by weight or less,
It can be obtained by subjecting the water absorbing agent precursor to a heat treatment in the presence of a surface crosslinking agent. The water-absorbing agent precursor may be granulated in a predetermined shape, or may be in various shapes such as a spherical shape, a scale-like shape, an irregular crushed shape, and a granular shape. Further, the water-absorbing agent precursor may be a primary particle or a granulated primary particle. When the water content exceeds 10%, when the average particle size is out of the range of 200 μm to 600 μm, and when the ratio of particles having a particle size of less than 106 μm exceeds 10% by weight, the diffusion absorption capacity, etc. It is not preferable because a water-absorbing agent having excellent performance cannot be obtained.

As the surface cross-linking agent in the present invention, a compound capable of reacting with a carboxyl group or a known surface cross-linking agent which is usually used for this purpose is suitable. Examples of the surface cross-linking agent include ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl. -1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4
-Diol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, poly Dihydric or higher alcohol compounds such as oxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol and sorbitol; ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, poly Epoxy such as glycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether and glycidol Polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine and polyethyleneimine; polyvalent isocyanate compounds such as 2,4-tolylenediisocyanate and hexamethylenediisocyanate; Polyoxazole compounds such as -ethylenebisoxazoline; 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-
On, 4-hydroxymethyl-1,3-dioxolane-
2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-
Alkylene carbonate compounds such as 1,3-dioxan-2-one and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; γ-gly Silane coupling agents such as sildoxypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane; and polyvalent metal compounds such as hydroxides or chlorides such as zinc, calcium, magnesium, aluminum, iron and zirconium. . These surface cross-linking agents may be used alone or in combination of two or more.

In particular, when the surface cross-linking agent is a combination of the first surface cross-linking agent and the second surface cross-linking agent having different solubility parameters (SP values), a water-absorbing agent having even more excellent diffusion absorption capacity is obtained. be able to. The above-mentioned solubility parameter is a value generally used as a factor indicating the polarity of a compound. In the present invention, the above-mentioned solubility parameter is compared with the Polymer Handbook, 3rd edition (published by WILEY INTERSCIENCE), pages 527 to 5
Solvent solubility parameter δ (cal / c) described on page 39
m ³ ) A value of ^1/2 shall be applied. Further, regarding the solubility parameter of the solvent not described in the above page, the Sma described in page 524 of the polymer handbook is used.
The value derived by substituting Hoy's cohesive energy constant described on page 525 into the formula of II is applied.

The first surface cross-linking agent is preferably a compound capable of reacting with a carboxyl group and having a solubility parameter of 12.5 (cal / cm ³ ) ^1/2 or more, more preferably 13.0 (cal / cm ³ ) ^1/2 or more. Compounds are more preferred. Examples of the first surface cross-linking agent include ethylene glycol, propylene glycol, glycerin, pentaerythritol, sorbitol, ethylene carbonate (1,3-dioxolan-2-one),
Propylene carbonate (4-methyl-1,3-dioxolan-2-one) and the like, but are not limited to these compounds. These first surface cross-linking agents may be used alone or in combination of two or more.

The second surface crosslinking agent is preferably a compound capable of reacting with a carboxyl group and having a solubility parameter of less than 12.5 (cal / cm ³ ) ^1/2 , and 9.5 (cal / cm ³ ) ^{1/2 to} 12.0 (cal / c
Compounds in the range m ³ ) ^1/2 are more preferred. As the second surface cross-linking agent, for example, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol,
1,3-butanediol, 1,4-butanediol,
1,5-pentanediol, 2,4-pentanediol, 1,6-hexanediol, 2,5-hexanediol, trimethylolpropane, diethanolamine,
Triethanolamine, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylenediamine, diethylenetriamine, triethyleneamine Ethylenetetramine, 2,4-tolylenediisocyanate, hexamethylenediisocyanate, 4,5-dimethyl-1,3-dioxolan-2-one, epichlorohydrin, epibromohydrin, and the like, but are not limited to these compounds. It is not something to be done. These second surface cross-linking agents may be used alone or in combination of two or more.

In the present invention, as the surface cross-linking agent, one or more compounds selected from the group of the first surface cross-linking agent and one or two types selected from the group of the second surface cross-linking agent It is particularly preferable to use a mixture of the above compounds.

The amount of the surface cross-linking agent used depends on the compound to be used and the combination thereof, but the solid content of the water-absorbing agent precursor is not limited.
The amount is preferably in the range of 0.001 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, per 100 parts by weight. When the first surface cross-linking agent and the second surface cross-linking agent are used as a mixture, the amount of the first surface cross-linking agent used is from 0.01 part by weight to 100 parts by weight of the solid content of the water-absorbing agent precursor. 5 parts by weight, the amount of the second surface crosslinking agent used is preferably in the range of 0.001 part by weight to 1 part by weight, and the amount of the first surface crosslinking agent used is 0.1 part by weight.
More preferably, the amount of the second surface cross-linking agent used is in the range of 0.005 to 0.5 parts by weight. By using the above surface cross-linking agent, the cross-linking density in the vicinity of the surface of the water-absorbing agent precursor, that is, the water-absorbing resin, can be made higher than the inside. When the use amount of the surface cross-linking agent exceeds 10 parts by weight, not only is it uneconomical, but also the amount of the surface cross-linking agent becomes excessive in forming an optimum cross-linked structure in the water-absorbing agent. The amount of the surface crosslinking agent used is 0.0
If the amount is less than 01 parts by weight, it is difficult to obtain the effect of improving the performance of the water-absorbing agent, such as the diffusion absorption capacity, and therefore it is not preferable. When mixing the water absorbing agent precursor and the surface crosslinking agent, it is preferable to use water as a solvent.
The amount of water used depends on the type and particle size of the water absorbing agent precursor, the water content, etc., but based on 100 parts by weight of the solid content of the water absorbing agent precursor.
It is preferably more than 0 and not more than 20 parts by weight, more preferably in the range of 0.5 to 10 parts by weight.

When mixing the water absorbing agent precursor and the surface crosslinking agent, a hydrophilic organic solvent may be used as a solvent, if necessary. Examples of the above hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol; ketones such as acetone; dioxane And ethers such as tetrahydrofuran; N, N-
Examples include amides such as dimethylformamide; sulfoxides such as dimethyl sulfoxide. The amount of the hydrophilic organic solvent used depends on the type and particle size of the water-absorbing agent precursor, the water content, etc., based on 100 parts by weight of the solid content of the water-absorbing agent precursor.
It is preferably at most 20 parts by weight, more preferably within the range of 0.1 to 10 parts by weight.

When mixing the water absorbing agent precursor and the surface crosslinking agent, for example, the water absorbing agent precursor may be dispersed in the above-mentioned hydrophilic organic solvent, and then the surface crosslinking agent may be mixed. However, the mixing method is not particularly limited. Among various mixing methods, a method in which a surface cross-linking agent dissolved in water and / or a hydrophilic organic solvent, if necessary, is directly sprayed or dropped onto the water absorbing agent precursor to be mixed. In the case of mixing using water, fine powder insoluble in water, a surfactant or the like may be allowed to coexist.

The mixing device used for mixing the water absorbing agent precursor and the surface cross-linking agent preferably has a large mixing force in order to mix both uniformly and reliably.
Examples of the mixing device include a cylindrical mixer, a double-walled conical mixer, a V-shaped mixer, a ribbon-type mixer, a screw-type mixer, a fluid-type furnace rotary desk-type mixer,
An air-flow type mixer, a double-arm type kneader, an internal mixer, a pulverization type kneader, a rotary mixer, a screw type extruder and the like are preferable.

After mixing the water-absorbing agent precursor and the surface cross-linking agent, heat treatment is performed to crosslink the vicinity of the surface of the water-absorbing agent precursor. The treatment temperature of the heat treatment depends on the surface cross-linking agent to be used, but is preferably 80 ° C or more and 250 ° C or less, more preferably 120 ° C or more and 230 ° C or less. When the first surface crosslinking agent and the second surface crosslinking agent are used in combination, 1
60 ° C. or higher is preferred. When the treatment temperature is lower than 80 ° C (less than 160 ° C when the first and second surface cross-linking agents are used), a uniform crosslinked structure is not formed, and therefore, the water absorption having excellent performance such as diffusion absorption capacity. Can not get the agent,
Not preferred. In addition, since the heat treatment takes time, the productivity is reduced. If the treatment temperature exceeds 250 ° C., the precursor of the water-absorbing agent is deteriorated, so that the performance of the water-absorbing agent is undesirably reduced.

The above heat treatment can be carried out using a usual dryer or heating furnace. Examples of the dryer include a groove-type mixing dryer, a rotary dryer, a desk dryer, a fluidized-bed dryer, a flash dryer, and an infrared dryer.

The water absorbing agent obtained by the above production method is
The diffusion absorption capacity after 60 minutes from the start of absorption is 25 g / g or more, and the amount of water-soluble component is more than 0 and 7% by weight or less. For this reason, the above-mentioned water-absorbing agent has excellent water-absorbing properties as described above. It is not clear why the water-absorbing agent according to the present invention exhibits very excellent properties such as diffusion absorption capacity, but during the aqueous solution polymerization, a specific amount of a cross-linking agent in which the content of high boiling components is controlled within a specific range is used. And, by using a dispersant, the high-boiling component is uniformly dispersed in the reaction system, and the original cross-linking ability of the main component of the cross-linking agent, which has been hindered by the high-boiling component, is maximized. It is presumed that the cross-linking reaction by the boiling component is also efficiently performed, and thereby the amount of the water-soluble component is drastically reduced.

For this reason, the above-mentioned water-absorbing agent has excellent water-absorbing properties as described above. Therefore, the water-absorbing agent, for example, when used in absorbent materials such as disposable diapers and sanitary napkins, so-called incontinence pads and the like, when the amount of water-absorbing resin used is large, or when the resin concentration in the absorbent material Even in the case of high water absorption, the diffusion absorption capacity is high, the amount of water-soluble components is small, and the return amount of the aqueous liquid after a long time has passed is very low. And excellent performance (water absorption characteristics) such as maintaining the water absorption amount over a long period of time.

In addition, a deodorant, a fragrance,
Various inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers,
Fertilizers, oxidizing agents, reducing agents, water, salts and the like may be added, thereby imparting various functions to the water absorbing agent.

[0070]

As described above, the water-absorbing agent having the above-mentioned structure is hardly affected by the structure of the absorber even when the amount of the water-absorbing resin used is large or the resin concentration in the absorber is high. Excellent absorption capacity, small amount of water-soluble component, and low return of aqueous liquid after a long time, very high liquid diffusivity, and excellent retention such as stable water absorption for a long time Performance (water absorption properties) can be shown. Thereby, an absorbent article exhibiting the above-described excellent performance can be provided. The water-absorbing agent can be particularly suitably used, for example, in disposable diapers and sanitary napkins, which are desired to be highly functional and thin, for hygiene materials such as incontinence pads. Further, according to the above-mentioned method, a water-absorbing agent exhibiting the above-described excellent performance can be produced.

[0071]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but the present invention is not limited thereto. In addition, various properties of the water absorbing agent were measured by the following methods.

(C) Water absorption capacity 0.2 g of a water-absorbing agent (or a water-absorbing agent precursor) is uniformly placed in a non-woven tea bag type bag (40 mm × 150 mm), and is placed in a 0.9% by weight aqueous sodium chloride solution (physiological saline). Immersion.
After 60 minutes, the tea bag-type bag was pulled out and drained for a certain period of time, and then the weight W ₁ (g) of the tea bag-type bag was measured. The same operation was performed without using a water absorbing agent, and the weight W ₀ (g) at that time was measured. Then, from these weights W ₁ · W ₀ , the water absorption capacity (g) is calculated according to the following equation: water absorption capacity (g / g) = (weight W ₁ (g) −weight W ₀ (g)) / weight of the water absorbing agent (g). / g) was calculated.

(D) Water absorption capacity under pressure First, a measuring apparatus used for measuring the water absorption capacity under pressure will be briefly described below with reference to FIG.

As shown in FIG. 4, the measuring device includes a burette 21, a measuring table 22, a conduit 23, a glass filter 24 mounted on the measuring table 22, and a glass filter 24 mounted on the glass filter 24. And a measuring unit 25. The burette 21 is formed in a substantially T-shaped tubular shape having a main pipe part 21a and a side pipe part 21b. The main pipe portion 21a
An opening 21c is provided at the top and an opening 21d is provided at the bottom. A plug 26 is fitted into the opening 21c, and a conduit 23 is attached to the opening 21d. The side pipe portion 21b is provided so as to protrude from the center of the side portion of the main pipe portion 21a, and has a distal end opening upward. The measuring table 22 has an upper surface with a diameter of 70 mm.
Is formed. The buret 21 and the measuring table 22 are communicated with each other by a conduit 23, and the buret 21, the measuring table 22, and the conduit 23, that is, the communicating portion contain a predetermined amount of artificial urine 20. Note that a scale is provided on the main pipe portion 21a of the burette 21. In the burette 21, the position of the liquid surface 20a in the side tube portion 21b is lower than the position of the liquid surface 20b in the main tube portion 21a by a predetermined height.

The measuring section 25 has a filter paper 27, a support cylinder 28, a nonwoven fabric 29 attached to the bottom of the support cylinder 28, and a weight 30. And the measuring unit 25
Is a filter paper 27 and a support cylinder 28 on a glass filter 24.
(That is, the nonwoven fabric 29) is placed in this order,
The weight 30 is placed inside the support cylinder 28, that is, on the nonwoven fabric 29. The glass filter 24 has a diameter of 70 mm
Is formed. The support cylinder 28 has an inner diameter of 55 mm. The measuring table 22 is set such that the position of the filter paper 27 is equal to the position of the liquid surface 20a in the side tube section 21b when the measuring section 25 is placed.
Then, a predetermined amount of the water-absorbing agent is evenly spread on the nonwoven fabric 29. The weight of the weight 30 is adjusted so that a load of 20 g / cm ² can be uniformly applied to the nonwoven fabric 29, that is, the water absorbing agent.

The water absorption capacity under pressure was measured using the measuring device having the above-mentioned configuration. The measuring method will be described below.

First, a predetermined amount of artificial urine (composition: urea 1.9% by weight, NaCl 0.8% by weight, CaCl ₂ 0.1% by weight, M
gSO ₄ 0.1% by weight) 20 was prepared in advance and the artificial urine 20 was prepared.
Was placed in the communication section described above. Further, predetermined preparation operations such as plugging a plug 26 into the opening 21c of the burette 21 and setting the measurement table 22 at a predetermined height were performed. Next, the filter paper 27 was placed on the glass filter 24 installed at the center of the measuring table 22. On the other hand, in parallel with this placing operation, the water absorbing agent is placed inside the support cylinder 28, that is, on the nonwoven fabric 29.
20 g was evenly dispersed, and a weight 30 was placed on the water absorbing agent.

Next, a non-woven fabric 29, that is, the support cylinder 28 on which a water-absorbing agent and a weight 30 are placed is placed on a filter paper 27.
Was placed such that the center part thereof coincided with the center part of the glass filter 24.

The volume V ₁ (ml) of the artificial urine 20 absorbed by the water-absorbing agent for 30 minutes was measured by reading the scale of the burette 21 from the time when the support cylinder 28 was placed on the filter paper 27. Then, from the above volume V ₁ , water absorption under pressure 30 minutes after the start of water absorption according to the following formula: water absorption capacity under pressure (ml / g) = volume V ₁ (ml) / weight of water absorbing agent (g) Magnification (ml
/ g) was calculated.

(E) Diffusion Absorption Ratio of Absorbent Article First, a measuring apparatus used for measuring the diffusion absorption ratio of an absorbent article will be briefly described with reference to FIGS. 5 and 6. For convenience of description, the configuration having the same function as the measuring device used for measuring the diffusion absorption capacity,
The same reference numerals are added, and the description is omitted.

As shown in FIG. 5, the measuring device is a balance 1
, Container 2, outside air suction pipe 3, conduit 4, diameter 1
It is composed of a glass filter 6 having a size of 20 mm and a measuring unit 15 mounted on the glass filter 6. As shown in FIG. 6, the measuring unit 15 has the filter paper 7, the sheet 8, the support square tube 19, and the weight 11.
The wire mesh is not provided.

The measuring section 15 has a filter paper 7, a sheet 8, and a support square tube 19 placed in this order on the glass filter 6, and a weight 11 placed inside the support square tube 19. The sheet 8 is made of polyethylene terephthalate, and is formed in a rectangular shape with a thickness of 0.1 mm having a rectangular opening of 12.5 mm × 100 mm at the center. Support square tube 19
Has an inner dimension of 100 mm x 100 mm. Then, an absorbent article having a predetermined size is placed inside the support rectangular tube 19. Other configurations of the measuring device are the same as those of the measuring device used for measuring the diffusion absorption capacity.

The diffusion absorption capacity of the absorbent article was measured by using the measuring apparatus having the above-mentioned structure. The measuring method will be described below.

First, an absorbent article was formed in a size of 100 mm × 100 mm. Further, a predetermined preparation operation was performed. next,
The filter paper 7 was placed on the glass filter 6, and the sheet 8 was placed on the filter paper 7 such that the opening was located at the center of the glass filter 6. Next, the supporting square tube 19 was placed on the sheet 8 such that the center portion thereof coincided with the center portion of the glass filter 6.

Thereafter, the absorbent article is placed inside the support rectangular tube 19, that is, on the sheet 8, and the weight 1 is placed on the absorbent article.
1 was placed. In addition, the mounting operation of the absorbent article and the weight 11 was performed quickly.

The weight W ₃ (g) of the physiological saline 12 absorbed by the absorbent article for 60 minutes was measured using the balance 1 from the time when the absorbent article was placed on the sheet 8. As shown in FIG. 7, after passing through the opening of the sheet 8, the physiological saline solution 12 was absorbed by the absorbent article while diffusing almost uniformly in the lateral direction in the absorbent article.

Then, from the above weight W ₃ , 60 minutes from the start of water absorption according to the following equation: diffusion absorption capacity (g / g) of absorbent article = weight W ₃ (g) / weight of absorbent article (g) Thereafter, the diffusion absorption capacity (g / g) of the absorbent article was calculated.

The crosslinking agents used in Examples and Comparative Examples were prepared by the following preparation methods.

[Preparation Method 1] In a 1 L four-necked flask equipped with a stirrer, a water separation tube equipped with a cooling tube, a thermometer, and an air blow tube, 134 g of trimethylolpropane, 238 g of acrylic acid, and 170 parts of toluene
g, p-toluenesulfonic acid 24 g and hydroquinone
Add 0.6 g and blow the air into the flask
Heated to 30 ° C. Then, the reaction was carried out for a predetermined time while distilling off the reaction product water out of the system by azeotropic dehydration with toluene.

After completion of the reaction, the reaction solution was transferred to a separating funnel,
500 g of 10 wt% NaOH water 300 g of 5 wt% NaOH water
Then, the unreacted acrylic acid was neutralized and separated, and then washed several times with 500 g of water until the washings became neutral. Thereafter, 0.06 g of hydroquinone monomethyl ether was added to the organic layer,
The reaction product was obtained by distilling off toluene under reduced pressure.

The reaction product was analyzed by gas chromatography (GC) and gel permeation chromatography (GPC). As a result, trimethylolpropane triacrylate, trimethylolpropane diacrylate, In addition, the weight ratio of trimethylolpropane diacrylate mono (β-acryloyloxypropionate) to a compound having two or more trimethylolpropane skeletons corresponding to high boiling components was 84/16. That is, by the above reaction and operation, a crosslinking agent A having a weight ratio of the crosslinking agent main component to the high boiling component of 84/16 was obtained.

[Preparation method 2] The same reaction and operation as in Preparation method 1 were carried out except that the reaction time was longer than the reaction time in Preparation method 1, and the weight ratio of the main component of the crosslinking agent to the high boiling component was 78. / 22 was obtained.

[Preparation Method 3] The same reaction and operation as in Preparation Method 1 were carried out except that the reaction time was shorter than the reaction time in Preparation Method 1, and the weight ratio of the main component of the crosslinking agent to the high boiling component was 89. / 11 was obtained.

[Preparation Method 4] The same reaction and operation as in Preparation Method 2 were carried out, except that the reaction time was further extended than the reaction time in Preparation Method 2,
A comparative crosslinking agent having a weight ratio of the crosslinking agent main component to the high boiling component of 72/28 was obtained.

Next, an aqueous solution polymerization of a hydrophilic unsaturated monomer was carried out using the cross-linking agents obtained by the above Preparation Methods 1 to 4.

Example 1 4.8 g of a crosslinking agent A and 1.0 g of polyoxyethylene sorbitan monostearate as a dispersant were dissolved in 414 g of acrylic acid as a hydrophilic unsaturated monomer.
37 of sodium acrylate as hydrophilic unsaturated monomer
4382 g of a weight% aqueous solution and 551 g of ion-exchanged water were added to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a stainless steel double-armed kneader with a capacity of 10 L having two sigma-type blades and a capacity of 10 L, and the system was maintained while maintaining the reaction solution at 30 ° C. The atmosphere was replaced with nitrogen gas. Subsequently, the reaction solution was stirred while being kept at 30 ° C., and 2.40 g of sodium persulfate and 0.12 g of L-ascorbic acid were added to initiate polymerization. Then, 40 minutes after the start of the polymerization, the hydrogel polymer was taken out.

The obtained hydrogel polymer was subdivided into finely divided products, and the finely divided products were spread on a 50-mesh wire net and dried with hot air at 150 ° C. for 90 minutes. Next, the dried product is pulverized using a vibration mill, and further classified by a 20-mesh wire net to obtain a water content of 4% and an average particle size of 350%.
μm and the proportion of particles having a particle size of less than 106 μm is 3
% By weight of an amorphous crushed water-absorbing resin, that is, a water-absorbing agent precursor. The water absorption capacity and the amount of water-soluble components (hereinafter simply referred to as results) of the obtained water-absorbing agent precursor are also shown in Table 1.

Example 2 The same reaction and operation as in Example 1 were carried out except that 4.8 g of crosslinking agent B was used in place of crosslinking agent A in Example 1, and the water content was 5% and the average particle size was 5%. An irregularly crushed water-absorbing resin having a diameter of 300 μm and a particle size of less than 106 μm and having a ratio of particles of 6% by weight, that is, a water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Example 3 The same reaction and operation as in Example 1 were carried out except that 4.8 g of cross-linking agent C was used instead of cross-linking agent A in Example 1, and the water content was 8% and the average particle size was 8%. An amorphous crushed water-absorbing resin having a diameter of 480 μm and a particle size of less than 106 μm in which the proportion of particles was 0.5% by weight, that is, a water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

[Example 4] The amount of the crosslinking agent A used in Example 1 was changed from 4.8 g to 6.8 g.
The same reaction and operation as in Example 1 were carried out, except that the water content was 5%, the average particle size was 380 μm, and
An amorphous crushed water-absorbent resin having a particle size of less than 106 μm and a ratio of particles of 2% by weight, that is, a water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Example 5 The amount of the crosslinking agent A used in Example 1 was changed from 4.8 g to 13.6 g.
The same reaction and operation as in Example 1 were carried out, except that the water content was 4%, the average particle size was 400 μm, and
An amorphous crushed water-absorbing resin having a particle size of less than 106 μm and a ratio of particles of 1% by weight, that is, a water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Example 6 In 29 g of acrylic acid as a hydrophilic unsaturated monomer, 6.8 g of a crosslinking agent A and 2.3 g of a partially saponified polyvinyl alcohol as a dispersant were dissolved. 4382 37% by weight aqueous solution of sodium acrylate as a saturated monomer
g, 385 g of acrylic acid as a hydrophilic unsaturated monomer, and 283 g of ion-exchanged water to prepare a reaction solution. Thereafter, the same reaction and operation as in Example 1 were performed, and the water content was 8%, the average particle size was 520 μm, and the particle size was 106 μm.
An amorphous crushed water-absorbing resin having a particle ratio of less than 0% by weight, that is, a water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Comparative Example 1 Comparative crosslinking agent 4.8 was used in place of crosslinking agent A in Example 1.
The same reaction and operation as in Example 1 were carried out except that g was used, and the water content was 5%, the average particle size was 340 μm, and
An amorphous crushed comparative water-absorbing resin having a particle size of less than 106 μm and a proportion of particles of 5% by weight, that is, a comparative water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Comparative Example 2 Example 1 was repeated except that polyoxyethylene sorbitan monostearate was not used as the dispersant in Example 1.
The same reaction and operation as described above were carried out, and the amorphous water-absorbing comparative resin having a water content of 4%, an average particle diameter of 370 μm, and a particle diameter of less than 106 μm of 3% by weight, that is, Thus, a comparative water absorbing agent precursor was obtained. The obtained results are shown in Table 1.

Comparative Example 3 The amount of the cross-linking agent A used in Example 1 was changed from 4.8 g to 2.0 g.
The same reaction and operation as in Example 1 were carried out, except that the water content was 5%, the average particle size was 390 μm, and
An amorphous crushed comparative water-absorbing resin having a particle size of less than 106 μm and a ratio of particles of 2% by weight, ie, a comparative water-absorbing agent precursor was obtained. The obtained results are shown in Table 1.

[0106]

[Table 1]

Example 7 100 parts by weight of the water-absorbing agent precursor obtained in Example 1 was mixed with glycerin (SP value: δ = 16.5 (cal / c
m ³ ) ^1/2 ) 0.5 part by weight and ethylene glycol diglycidyl ether (SP value: δ = 10.2
(cal / cm ³ ) ^1/2 ) A surface cross-linking agent consisting of 0.1 part by weight, 3 parts by weight of water, and 1 part by weight of isopropyl alcohol was mixed. The obtained mixture was heated at 200 ° C. for 40 minutes to obtain a water absorbing agent. The water absorption capacity, the amount of water-soluble components, the diffusion absorption capacity, and the water absorption capacity under pressure (hereinafter, simply referred to as the result) of the obtained water absorbing agent are also shown in Table 2.

[Examples 8 and 9] Except that 100 parts by weight of the water-absorbing agent precursor obtained in Examples 4 and 5 were used in order, the same heat treatment as in Example 7 was performed. A water absorbing agent was obtained. The results obtained are shown in Table 2.

[Comparative Examples 4 and 5] Comparative water-absorbing agent precursor 100 obtained in Comparative Examples 1 and 3
The same heat treatment as in Example 7 was performed, except that the parts by weight were used in order, to obtain the corresponding comparative water-absorbing agents. The results obtained are shown in Table 2.

Comparative Example 6 A partially neutralized and crosslinked starch-acrylic acid graft polymer (trade name: Sunwet IM3900P; manufactured by Hoechst Celanese Corporation) was used as a comparative water absorbing agent. The results obtained are shown in Table 2.

Comparative Example 7 A partially neutralized and crosslinked acrylic acid polymer (trade name: DiaWet US2-45Z; manufactured by Mitsubishi Yuka Co., Ltd.) was used as a comparative water absorbing agent. The results obtained are shown in Table 2.

Comparative Example 8 A partially neutralized and crosslinked acrylic acid polymer (trade name: AquaKeep SA-60; manufactured by Sumitomo Seika Co., Ltd.) was used as a comparative water absorbing agent. The results obtained are shown in Table 2.

Comparative Example 9 A polymer water-absorbing agent was taken out from a paper diaper called Mamipoco (trade name) manufactured by Unicharm Co., Ltd. and used as a comparative water-absorbing agent. The results obtained are shown in Table 2.

[0114]

[Table 2]

Example 10 75 parts by weight of the water-absorbing agent obtained in Example 7 and 25 parts by weight of ground wood pulp as hydrophilic fibers were dry-mixed using a mixer. The obtained mixture was formed into a web having a size of 100 mm × 100 mm, and the web was pressed at a pressure of 2 kg / cm ² to form a web.
By pressing for minutes, an absorbent article comprising an absorbent having a basis weight of about 0.035 g / cm ² was obtained. The diffusion absorption capacity (hereinafter, simply referred to as a result) of the obtained absorbent article is also shown in Table 3.

[Examples 11 and 12] An absorber was used in the same manner as in Example 10 except that the water absorbing agents obtained in Examples 8 and 9 were used in place of the water absorbing agent in Example 10. Was prepared, and corresponding absorbent articles were obtained. The results obtained are shown in Table 3.

Comparative Examples 10 to 15 In the same manner as in Example 10 except that the comparative water absorbing agents obtained in Comparative Examples 4 to 9 were used in place of the water absorbing agent in Example 10, respectively. Absorbers were prepared, and corresponding comparative absorbent articles were obtained. Table 3 shows the obtained results.
It is described in accordance with.

[0118]

[Table 3]

Example 13 75 parts by weight of the water-absorbing agent obtained in Example 7 and 25 parts by weight of ground wood pulp as hydrophilic fibers were dry-mixed using a mixer. Next, the resulting mixture was air-formed on a wire screen formed into a 400 mesh (mesh size: 38 μm) using a batch-type air-forming device to form a web having a size of 120 mm × 400 mm. . The web was pressed at a pressure of 2 kg / cm ² for 5 seconds to obtain an absorbent having a basis weight of about 0.047 g / cm ² .

Subsequently, a back sheet made of liquid-impermeable polypropylene and having a so-called leg gather,
The above-described absorber and a top sheet made of liquid-permeable polypropylene are adhered to each other in this order using a double-sided tape, and two so-called tape fasteners are attached to the adhered material to thereby form an absorbent article (ie, , A disposable diaper). The weight of this absorbent article was 46 g.

The above-mentioned absorbent article was mounted on a so-called Kewpie doll (55 cm long, 5 kg in weight), the doll was placed in a prone position, and a tube was inserted between the absorbent article and the doll to urinate the human body. At the position corresponding to the position to be performed, apply 50 ml of physiological saline at a time every 20 minutes.
50 ml was injected. After the doll was left at 37 ° C. for 16 hours, the absorbent article was removed.

At the center of the water-absorbing portion of the removed absorbent article, ten so-called kitchen towels were placed one on top of the other, and a load of 10 kg was applied to these kitchen towels for one minute. And the amount (g) of physiological saline returned from the absorbent article was measured. The obtained results are shown in Table 4.

[Examples 14 and 15] The same operation as in Example 13 was performed except that the water absorbing agents obtained in Examples 8 and 9 were used in place of the water absorbing agent in Example 13. And the corresponding absorbent articles were obtained. And the amount (g) of physiological saline returned from these absorbent articles was measured. The obtained results are shown in Table 4.

Comparative Examples 16 and 17 The same operation as in Example 13 was carried out except that the comparative water-absorbing agents obtained in Comparative Examples 4 and 5 were used in place of the water-absorbing agent in Example 13. Was performed to obtain a corresponding absorbent article for comparison. Then, the return amount (g) of the physiological saline from these comparative absorbent articles was measured. The obtained results are shown in Table 4.

[0125]

[Table 4]

As is clear from the results shown in Tables 1 to 4, the water absorbing agent of the present invention uses an absorbent having a high diffusion absorption capacity, a small amount of water-soluble components and a high resin concentration. The absorbent article obtained by the above method has a small amount of aqueous liquid returned after a long time, and exhibits excellent performance (water absorption properties) such as extremely high liquid diffusivity and a stable water absorption over a long period of time. You can see that.

[0127]

The above water-absorbing agent has a high diffusion absorption capacity even when the amount of the water-absorbing resin used is large, or even when the resin concentration in the absorber is high, regardless of the structure of the absorber. Excellent performance (water absorption properties) such as low water-soluble components, little return of aqueous liquid after a long time, very high liquid diffusivity, and stable water absorption over a long period of time Can be shown. Thereby, there is an effect that an absorbent article exhibiting the above-described excellent performance can be provided. The water-absorbing agent can be particularly suitably used, for example, in disposable diapers and sanitary napkins, which are desired to be highly functional and thin, for hygiene materials such as incontinence pads.

Further, according to the method for producing a water-absorbing agent of the present invention, the diffusion and absorption capacity is high, the amount of water-soluble components is small, and the return amount of the aqueous liquid after a long time has passed is very low. It is possible to produce a water-absorbing agent exhibiting excellent performance (water-absorbing properties) such as diffusibility and maintaining a stable amount of water absorption over a long period of time.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a measuring device used for measuring a diffusion absorption capacity, which is one of the properties exhibited by a water absorbing agent according to the present invention.

FIG. 2 is a sectional view of a main part of the measuring device.

FIG. 3 is an explanatory diagram illustrating a diffusion direction of a physiological saline in the measuring device.

FIG. 4 is a schematic cross-sectional view of a measuring device used for measuring a water absorption capacity under pressure, which is one of the properties exhibited by the water absorbing agent in the present invention.

FIG. 5 is a schematic cross-sectional view of a measuring device used for measuring a diffusion absorption capacity, which is one of the properties exhibited by the absorbent article according to the present invention.

6 is a sectional view of a main part of the measuring device of FIG.

FIG. 7 is an explanatory diagram illustrating a diffusion direction of a physiological saline in the measuring device of FIG. 5;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 balance 2 container 3 outside air suction pipe 4 conduit 5 measuring section 6 glass filter 7 filter paper 8 sheet 9 support cylinder 10 wire mesh 11 weight 12 physiological saline 15 measuring section 19 supporting square cylinder 20 artificial urine 21 burette 22 measuring table 23 conduit 24 glass Filter 25 Measuring unit 27 Filter paper 28 Support cylinder 29 Non-woven fabric 30 Weight

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kinya Nagasago 992, Nishioki, Okihama-shi, Aboshi-ku, Himeji-shi, Hyogo Nippon Shokubai Polymer Research Laboratory Co., Ltd. (1) References of Polymer Research Laboratory of Nippon Shokubai Co., Ltd. (56) References JP-A-2-12103 (JP, A) JP-A-6-14319 (JP, A) JP-A-7-88171 (JP, A) Kaihei 8-57311 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01J 20/26 C08F 8/00

Claims (7)

(57) [Claims] 1. A diffusion absorption capacity of 25 g after 60 minutes from the start of absorption.
/ g or more, and the water-soluble component amount is more than 0 and 7% by weight or less. 2. A diffusion absorption capacity of 25 g after 60 minutes from the start of absorption.
An absorbent article characterized by having an absorbent containing a water-absorbing agent having a water-soluble component amount of not less than 0 / g and not more than 0 and not more than 7% by weight. 3. The absorbent article according to claim 2, wherein the absorbent contains hydrophilic fibers, and the proportion of the hydrophilic fibers in the absorbent is more than 0 and 60% by weight or less. 4. A method for producing a water-absorbing agent precursor, wherein an aqueous solution of a hydrophilic unsaturated monomer containing acrylic acid as a main component, of which at least 50 mol% is neutralized, is polymerized in the presence of a crosslinking agent and a dispersing agent. The cross-linking agent comprises an ester of an alcohol having 6 or less carbon atoms and having a valency of 3 or more and an unsaturated carboxylic acid, and all the hydroxyl groups of the alcohol having a valency of 3 or more are the unsaturated carboxylic acids. Assuming that the molecular weight of the compound when an ester bond is formed with the compound 1, the crosslinking agent has a ratio of the molecular weight to the molecular weight of the compound of 0.7 or more and less than 1.3, and two or more alcohol skeletons in the molecule. A crosslinking agent comprising a high boiling component and having a weight ratio of the crosslinking agent main component to the high boiling component in the range of 75/25 to 99/1, with respect to the hydrophilic unsaturated monomer For use in the range of 0.05 mol% or more and 0.5 mol% or less A method for producing a water-absorbing agent precursor. 5. The water absorbing agent precursor according to claim 4, wherein the alcohol having 6 or less carbon atoms and having 3 or more valences is trimethylolpropane and the unsaturated carboxylic acid is acrylic acid. Production method. 6. The water-absorbing agent precursor obtained in claim 4,
Moisture content is 10% or less and average particle size is 200μm ~ 600μ
m, and the proportion of particles having a particle size of less than 106 μm is adjusted to be 10% by weight or less, and then the water absorbing agent precursor is heat-treated in the presence of a surface crosslinking agent. Method for producing a water absorbing agent. 7. The solubility parameter (SP value) of the surface crosslinking agent capable of reacting with a carboxyl group is 12.5 (cal / cm ³ ).
The first surface cross-linking agent of ^1/2 or more, and the solubility parameter is composed of a second surface cross-linking agent of less than 12.5 (cal / cm ³ ) ^1/2 , and the heat treatment temperature is 160 ° C. or more. The method for producing a water-absorbing agent according to claim 6, characterized in that: