JPH1180248A - Manufacture of water absorbent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a water absorbent with high water absorbency under pressure and reduced 'de-absorption', (i.e., high water retaining property) by crosslinking a water absorbing resin in the surface vicinity followed by hydrophilization of the resin. SOLUTION: A water absorbing resin is swollen by absorbing a large amount of water, physiological saline solution, urine or the like to form a substantially water-insoluble hydrogel. The typical example of the resin is a hydrophilic polymer having a crosslinked structure obtained by polymerization of hydrophilic monomers mainly consisting of acrylic acid or salts thereof. The hydrophilic monomers mainly consisting of acrylic acid or salts thereof is preferably used in an aqueous solution state and polymerized by aqueous solution polymerization or reversed phase suspension polymerization. It is preferable that a crosslinking agent is directly added to and mixed with the water absorbing resin particles. The hydrophilization, for example, of a water absorbing resin prepared by reverse phase suspension polymerization is carried out by washing it with an organic solvent. In this case, the organic solvent is preferably heated and then brought into contact with the water absorbing resin.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a water absorbing agent. More specifically, the absorption capacity under pressure is high,
The present invention also relates to a method for producing a water-absorbing agent having a small amount of return.

[0002]

2. Description of the Related Art In recent years, water-absorbing agents using a water-absorbing resin have been widely used as one of the constituent materials of sanitary materials such as disposable diapers and sanitary napkins, so-called incontinence pads, for the purpose of absorbing body fluids. I have. In addition to sanitary materials, water-absorbent resins are widely used for the purpose of absorbing and retaining water, such as soil water retention agents and drip sheets for foods and the like.

Examples of the water-absorbing resin include a crosslinked product of a partially neutralized polyacrylic acid, a hydrolyzate of a starch-acrylonitrile graft polymer, a neutralized product of a starch-acrylic acid graft polymer, and vinyl acetate-acrylic acid. A saponified acid ester copolymer, a hydrolyzed product of an acrylonitrile copolymer or an acrylamide copolymer, a crosslinked product thereof, a crosslinked product of a cationic monomer, and the like are known.

[0004] Characteristics that should be provided to the above-mentioned water-absorbent resin include a high absorption capacity, an excellent absorption rate, liquid permeability, gel strength of a swollen gel, and an aqueous liquid when in contact with an aqueous liquid such as a body fluid. There is a demand for a suction amount for sucking water from the base material. However, the relationship between these properties does not always show a positive correlation. For example, the higher the absorption capacity, the lower the physical properties such as liquid permeability, gel strength, and absorption rate. Therefore, as a method for improving the water absorbing properties of such a water-absorbent resin in a well-balanced manner, a technique of cross-linking the vicinity of the surface of the water-absorbent resin is known, and various methods have been proposed so far. For example, as a crosslinking agent, a method using a polyhydric alcohol, a method using a polyvalent glycidyl compound, a polyvalent aziridine compound, a polyvalent amine compound, a polyvalent isocyanate compound, a method using glyoxal, a method using a polyvalent metal, a silane cup A method using a ring agent, a method using an epoxy compound and a hydroxy compound,
A method using an alkylene carbonate is known.

Although the balance of various physical properties of the water-absorbing resin is improved by these methods, it is still not enough. In particular, in recent years, a thin absorbent using a water-absorbent resin at a high concentration and a high scatter density, which is a recent trend, has a high absorption capacity under high load (for example, 50 g / cm ² ) (absorption capacity under pressure). And the amount of return is required to be small, and it is considerably improved by the above surface crosslinking method,
At present, it has not yet reached a sufficient physical property level.

[0006]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for producing a water-absorbing agent having a high absorption capacity under pressure and a small amount of return.

[0007]

In order to solve the above problems, the present invention has the following arrangement. (1) A method for producing a water-absorbing agent, comprising subjecting the surface of a water-absorbent resin to a cross-linking treatment and then subjecting the water-absorbent resin to a hydrophilic treatment. (2) The method for producing a water-absorbing agent according to the above (1), wherein the water-absorbing resin is obtained by reverse phase suspension polymerization.

(3) The method for producing a water-absorbing agent according to (1) or (2), wherein the hydrophilic treatment is a treatment with an organic solvent. (4) The method for producing a water-absorbing agent according to (3), wherein the organic solvent is heated and brought into contact with a water-absorbing resin to perform a hydrophilic treatment. (5) The cross-linking treatment according to any one of the above (1) to (4), wherein a cross-linking agent capable of reacting with a functional group of the water-absorbent resin is directly added to and mixed with the water-absorbent resin powder. A method for producing a water absorbing agent.

(6) The method according to any one of the above (1) to (5), wherein the hydrophilic treatment of the water-absorbent resin is performed before the cross-linking treatment near the surface of the water-absorbent resin.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is considered that a hydrophobic portion is present on the surface of the water-absorbent resin after the surface cross-linking treatment.
The cause may be due to the surface cross-linking treatment and unrelated to the surface cross-linking treatment. As the former, impurities and alteration occur on the surface of the water-absorbent resin due to the surface cross-linking treatment, and as the latter, the additives used in the polymerization (such as dispersants in the case of reverse phase suspension polymerization) remain. Alternatively, it may be derived from the properties of the resin itself. The present inventors believe that the hydrophobic portion hinders the effect of the surface crosslinking treatment, and after performing the surface crosslinking treatment, performing a hydrophilic treatment on the water-absorbing resin to hydrophilize the hydrophobic portion. did. This can enhance the effect of the surface crosslinking treatment (increase the absorption capacity of the water-absorbing agent under pressure and reduce the amount of return). In addition, since the hydrophilicity of the surface of the water-absorbing resin is increased, the water absorption speed is also increased.

In addition, the above effect can be further enhanced by performing the hydrophilic treatment of the water-absorbent resin before performing the surface crosslinking treatment. Hereinafter, the present invention will be described in detail. The water-absorbing resin that can be used in the present invention is one that absorbs and swells a large amount of water, physiological saline, urine and the like to form a substantially water-insoluble hydrogel. A typical example is a hydrophilic polymer having a crosslinked structure obtained by polymerizing a hydrophilic monomer containing acrylic acid or a salt thereof as a main component. Such materials include, for example, partially neutralized crosslinked polyacrylic acid polymers, crosslinked and partially neutralized starch-acrylic acid graft polymers, isobutylene-maleic acid copolymers, and vinyl acetate-acrylic acid copolymers. Saponified product, hydrolyzate of acrylamide (co) polymer,
It is disclosed in hydrolyzates of acrylonitrile polymers and the like. Among them, preferred are polyacrylate crosslinked polymers. As the polyacrylate crosslinked polymer, it is preferable that 50 to 95 mol% of the acid groups in the polymer are neutralized, and it is more preferable that 60 to 90 mol% is neutralized. Examples of the salt include an alkali metal salt, an ammonium salt, an amine salt and the like. This neutralization may be performed with a monomer before polymerization, or with a hydrogel polymer during or after polymerization.

The water-absorbing resin used in the present invention may be obtained by (co) polymerizing a monomer other than the acrylic acid or a salt thereof, which is preferably used as a main component of the monomer. Specific examples of other monomers include methacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, Anionic unsaturated monomers such as-(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, and the like. It can be. The amount of the monomer other than the acrylic acid used is usually 0 to 50 in all the monomers.
It is less than mol%, preferably 0 to 30 mol%, but is not limited thereto.

The cross-linking structure of the water-absorbent resin used in the present invention may be a self-cross-linking type using no cross-linking agent, or an internal cross-linking structure having two or more polymerizable unsaturated groups or two or more reactive groups. Examples thereof include those in which an agent is copolymerized or reacted, and those having a crosslinked structure in which an internal crosslinking agent is copolymerized or reacted are more preferable. Specific examples of these internal crosslinking agents include, for example, N, N′-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth)
Acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth)
Acrylate, glycerin tri (meth) acrylate,
Glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine,
Poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth) acrylate Can be done. Further, two or more of these internal crosslinking agents may be used. Above all, it is preferable to use a compound having two or more polymerizable unsaturated groups as an internal crosslinking agent in view of the water absorbing properties of the obtained water-absorbing resin. 0.005 to 3 mol%,
More preferably, it is 0.01 to 1.5 mol%.

In the polymerization, starch, cellulose,
A derivative of starch / cellulose, a hydrophilic polymer such as a crosslinked product of polyvinyl alcohol, polyacrylic acid (salt), or polyacrylic acid (salt), or a chain transfer agent such as hypophosphorous acid (salt) may be added. . When polymerizing the above-described monomer containing acrylic acid or a salt thereof as a main component to obtain the water-absorbing resin used in the present invention, bulk polymerization or precipitation polymerization can be performed, From the viewpoint of easy control of polymerization and polymerization, it is preferable to carry out aqueous solution polymerization or reversed-phase suspension polymerization using a monomer as an aqueous solution.

The dispersing agent used in the case of performing the inverse suspension polymerization includes sorbitan fatty acid ester, sucrose fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkyl. Nonionic surfactants such as phenyl ether and polyoxyethylene acyl ester; anionic surfactants such as higher alcohol sulfates, alkyl naphthalene sulfonates, alkyl polyoxyethylene sulfates, and dialkyl sulfosuccinates; Cationic surfactants such as quaternary ammonium salts and alkylamine salts; amphoteric surfactants such as alkyl betaine and lecithin;
Polymer surfactants such as polymers having a lipophilic carboxyl group or ethylene oxide chain; cellulose derivatives such as cellulose ethers and cellulose esters; Among them, the use of a nonionic surfactant having an HLB value of 3 or more or a dispersant such as a cellulose derivative is more preferable because the effect of the present invention is more remarkable. These dispersants are used in an amount of 0.0
It is preferably 1 to 10% by weight, more preferably 0.5 to 5%.
% By weight.

The hydrophobic organic solvent used in the inverse suspension polymerization is not particularly limited as long as it is inert to polymerization and other reactions. For example, aliphatic hydrocarbons such as n-pentane, n-heptane, n-hexane, n-octane and the like; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane and decalin; chlorobenzene, bromobenzene, Halogenated hydrocarbons such as carbon chloride and 1,2-dichloroethane; and aromatic hydrocarbons such as benzene, toluene and xylene. Particularly preferred are n-hexane, cyclohexane, chlorobenzene, toluene and xylene. The amount of the hydrophobic organic solvent to be used is preferably 0.4 to 20 parts by weight, more preferably 0.7 to 10 parts by weight, based on 1 part by weight of the aqueous monomer solution.

In conducting the polymerization, radical polymerization of potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, 2,2′-azobis (2-amidinopropane) dihydrochloride, etc. Initiators, active energy rays such as ultraviolet rays and electron beams, and the like can be used. When an oxidizing radical polymerization initiator is used, redox polymerization may be performed by 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 usually 0.001 to 2 mol%, preferably 0.01 to 0.5 mol%.

The shape of the water-absorbent resin obtained by the above-mentioned polymerization can be any of various shapes such as irregularly crushed, spherical, fibrous, rod-like, substantially spherical, and flat. In the present invention, the water-absorbent resin thus obtained is subjected to a hydrophilic treatment as required, and then a cross-linking treatment is performed in the vicinity of the surface. For the surface cross-linking treatment, a cross-linking agent capable of reacting with a functional group of the water-absorbent resin, for example, an acidic group may be used, and a known cross-linking agent used for the purpose is usually exemplified. When the functional group of the water absorbent resin is a carboxyl group, for example, 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-pentadiol, 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, polyoxypropylene, oxyethyleneoxypropylene block Polyhydric alcohol compounds such as polymers, pentaerythritol and sorbitol; ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol Polyglycol compounds such as diglycidyl ether and glycidol; polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyallylamine and polyethyleneimine; 2,4-tolylenediisocyanate;
Polyvalent isocyanate compounds such as hexamethylene diisocyanate; polyvalent oxazoline compounds such as 1,2-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-one, 4-hydroxymethyl-1,3-dioxolan-2-one,
1,3-dioxan-2-one, 4-methyl-1,3-
Alkylene carbonate compounds such as dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; epichlorohydrin, epibromohydrin, α-methyl Haloepoxy compounds such as epichlorohydrin; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; zinc, calcium,
One or more compounds selected from hydroxides such as magnesium, aluminum, iron and zirconium and polyvalent metal compounds such as chlorides; Preferably, it contains at least one selected from a polyhydric alcohol compound, a polyvalent amine compound, a polyvalent epoxy compound, and an alkylene carbonate compound. These surface cross-linking agents may be used alone or in combination of two or more.

The amount of the surface cross-linking agent used in the present invention varies depending on the type of the cross-linking agent to be used and the purpose thereof, but is usually based on 100 parts by weight of the solid content of the water-absorbent resin of the present invention. 0.001 to 10 parts by weight, preferably 0.0
The amount is in the range of 1 to 5 parts by weight, and if the amount is within this range, a water-absorbing agent having excellent absorption capacity under pressure can be obtained. When the amount of the cross-linking agent exceeds 10 parts by weight, it is not only uneconomical, but also tends to be excessive in achieving an appropriate surface cross-linking effect, and the absorption capacity may be excessively lowered. Conversely, if the amount is less than 0.001 part by weight, the effect of improving the absorption characteristics under pressure may not be obtained.

In the present invention, it is preferable to directly add and mix a crosslinking agent to the water absorbent resin powder. This is because if the mixing is performed in a solvent, the crosslinking agent may not be uniformly added to the water absorbent resin. Therefore, it is preferable to dry the water-absorbent resin into powder before performing the surface crosslinking treatment.
The device used when mixing the water-absorbing resin and the surface cross-linking agent in the present invention may be an ordinary device, for example,
Cylindrical mixer, screw mixer, screw extruder, turbulizer, Nauter mixer, V mixer, ribbon mixer, double arm kneader, flow mixer,
An air flow type mixer, a rotating disk type mixer, a roll mixer, a tumbling type mixer and the like can be mentioned, and the mixing speed may be either high or low.

In the present invention, water, steam, or an aqueous liquid comprising water and a hydrophilic organic solvent may be added before or after mixing the water-absorbent resin with the surface cross-linking agent and before the cross-linking reaction is carried out. Good. If the cross-linking agent reacts with a water-absorbent resin by a covalent bond, such as a polyhydric alcohol, a polyhydric epoxy compound, or an alkylene carbonate compound, the post-addition of water, water vapor, or an aqueous liquid greatly improves the absorption characteristics under pressure. Is preferred. Examples of the hydrophilic organic solvent used in this case include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, n-butyl alcohol, iso-butyl alcohol and t-butyl alcohol; acetone and the like. Ketones; ethers such as dioxane and tetrahydrofuran; amides such as N, N-dimethylformamide; and sulfoxides such as dimethylsulfoxide. The amount of water used in this case varies depending on the type and particle size of the water-absorbent resin, but is usually 10 parts by weight or less, preferably 1 to 100 parts by weight based on the solid content of the water-absorbent resin. The range is 5 parts by weight. Similarly, the amount of the hydrophilic organic solvent used is generally 10 parts by weight or less, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the solid content of the water-absorbent resin.

In the present invention, after mixing the water-absorbing resin and the surface cross-linking agent, if necessary, a heating treatment is further performed depending on the type of the cross-linking agent to cross-link the vicinity of the surface. When the heat treatment is performed in the present invention, the treatment temperature is preferably about 60 ° C. or more and 230 ° C. or less. When the heat treatment temperature is lower than 60 ° C., the heat treatment takes a long time to cause a decrease in productivity. In addition, uniform crosslinking is not achieved, and a resin having high water absorption under pressure which is the object of the present invention is obtained. May not be possible. On the other hand, when the temperature exceeds 230 ° C., the water-absorbing resin partially deteriorates, and the water absorption capacity may decrease. Depending on the type of crosslinking agent used,
The heat treatment temperature is more preferably in the range of 80 to 200 ° C, and still more preferably in the range of 120 to 180 ° C.

The heat treatment can be carried out using a usual dryer or heating furnace, for example, a groove-type mixing dryer, a rotary dryer, a desk dryer, a fluidized-bed dryer, a gas-flow dryer, and an infrared dryer. Etc. can be exemplified. In the present invention, after performing such a surface cross-linking treatment, the water-absorbing resin is subjected to a hydrophilic treatment. The hydrophilization treatment after the surface cross-linking treatment will be described below, but the same applies to the hydrophilization treatment that can be performed as needed before the surface cross-linking treatment unless otherwise specified.

The hydrophilization treatment is a treatment for hydrophilizing the surface of the water-absorbent resin, and means that the wettability of the surface is improved before and after the treatment. To determine whether the wettability has been improved, for example, 0.3 g of the water-absorbing resin is evenly applied in a layered manner to a circular container having an inner diameter of 15 mm, and a drop of about 0.015 g of blue-colored deionized water is applied from above. Degree of penetration of deionized water from the resin surface layer when added with a micro syringe, or maximum diameter of absorbed resin area (maximum absorption diameter) (mm)
Can be determined by measuring. The permeation of the deionized water from the resin surface layer is performed more quickly after the treatment, and those in which the maximum diameter of the absorbed resin area is wider after the treatment has improved wettability, that is, the hydrophilic treatment is performed. It can be determined that this has been done.

The specific method of the hydrophilic treatment differs depending on the cause of forming the hydrophobic portion on the surface of the water-absorbing resin. For example, when a water-absorbent resin is produced by reverse-phase suspension polymerization, the surfactant used as a dispersant during polymerization is a cause of hydrophobicity, and therefore, it is preferable to wash the surfactant, using an organic solvent. Washing is more preferable.

The organic solvent is not particularly limited as long as it can wash the dispersant, but in order to enhance the washing effect, it is preferable to use one that does not swell the water-absorbing resin. The preferred swelling ratio of the water-absorbing resin during the treatment is less than 2 times. As the organic solvent, not only a hydrophilic organic solvent but also a hydrophobic organic solvent can be used.
As the hydrophilic organic solvent, specifically, for example, methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,
Lower alcohols such as isobutyl alcohol and t-butyl alcohol; ketones such as acetone; ethers such as dioxane and tetrahydrofuran; amides such as N, N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; Examples of the organic solvent include aliphatic hydrocarbons such as n-pentane, n-heptane, n-hexane and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane and decalin; chlorobenzene, bromo Halogenated hydrocarbons such as benzene, carbon tetrachloride, and 1,2-dichloroethane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among them, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-hexane and cyclohexane are preferably used.

In the case where the water-absorbing resin is subjected to hydrophilic treatment with an organic solvent, it is preferred that the absorption ratio under pressure and the return amount can be further improved by bringing the organic solvent into contact with the water-absorbing resin in a heated state. The heating temperature is preferably equal to or lower than the boiling point of the solvent, and is generally about 40 to 120 ° C. depending on the type of the organic solvent used. The hydrophilic treatment is preferably performed in a state where the water-absorbent resin is dried. When performing the hydrophilization treatment in a state of being wetted with an organic solvent or water, when cross-linking treatment is performed in the vicinity of the surface, conversely, the water absorption rate may be reduced or the water absorption magnification may be sharply reduced, so care must be taken. . Therefore, the hydrophilic treatment after the surface crosslinking treatment is preferably performed after the water-absorbent resin is dried. However,
When heating is performed in the surface cross-linking treatment, the water-absorbing resin after the surface cross-linking treatment is in a dry state, and thus the hydrophilic treatment can be directly performed without performing drying again. When performing the hydrophilic treatment before the surface crosslinking treatment,
It is preferable that the water-absorbing resin obtained by the reverse phase suspension polymerization or the aqueous solution polymerization is filtered and dried, and the hydrophilic treatment is performed in a state where the polymerization solvent and water are removed.

The water-absorbing agent obtained by the present invention is used in a water-absorbing article such as a disposable diaper or a sanitary napkin obtained by compounding a water-absorbing resin with, for example, a fibrous material.
When used under high concentration conditions of 50% by weight or more (ratio to the total of fibrous materials such as water-absorbent resin and pulp), it can exhibit sufficient absorption capacity even when a higher load is applied. Further thinning of the article is possible. Further, the water-absorbing agent of the present invention can be applied to various uses such as an absorbent article for body fluids such as a wound protecting material and a wound healing material, a drip absorbing material for food, a freshness retaining material, a water stopping material and a soil water retaining material. It is preferably used.

[0029]

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. In the present invention, various properties of the water absorbing agent were measured by the following methods. (A) Absorption capacity 0.2 g of a water-absorbing agent was uniformly placed in a nonwoven fabric bag (60 mm × 60 mm) and immersed in a 0.9% by weight aqueous sodium chloride solution (physiological saline). After 60 minutes, the bag is pulled up and drained at 250 G for 3 minutes using a centrifuge, and then the bag weight W1 (g)
Was measured. The same operation was performed without using a water absorbing agent, and the weight W0 (g) at that time was measured. From these weights W1 and W0, the absorption capacity (g / g) is calculated according to the following equation: absorption capacity (g / g) = (weight W1 (g) -weight W0 (g)) / weight of water absorbing agent (g / g). ) Was calculated. (B) Absorption Capacity Under Pressure First, a measuring apparatus used for measuring the absorption capacity under pressure will be briefly described below with reference to FIG.

As shown in FIG. 1, the measuring device is 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. In the container 2, a predetermined amount of artificial urine 11 (composition: 0.2% by weight of sodium sulfate, 0.2% by weight of potassium chloride, 0.05% of magnesium chloride hexahydrate)
% Aqueous solution of 0.025% by weight of calcium chloride dihydrate, 0.085% by weight of ammonium dihydrogen phosphate and 0.015% by weight of diammonium hydrogen phosphate). The lower end of the outside air suction pipe 3 is submerged in the artificial urine 11. The above glass filter 6 is formed with a diameter of 70 mm. The container 2 and the glass filter 6 are in communication with each other by the conduit 4. The upper part of the glass filter 6 is fixed at a position slightly higher than the lower end of the outside air suction pipe 3.

The measuring section 5 includes a filter paper 7 and a support cylinder 8.
And a wire mesh 9 attached to the bottom of the support cylinder 8, and a weight 10. Then, the measuring unit 5 places the filter paper 7 and the support cylinder 8 (that is, the wire mesh 9) on the glass filter 6 in this order, and places the weight 10 inside the support cylinder 8, that is, on the wire mesh 9. It has been. Support cylinder 8
Is formed with an inner diameter of 60 mm. The wire mesh 9 is made of stainless steel and formed in a 400 mesh (mesh size 38 μm). Then, a predetermined amount of the water-absorbing agent is evenly spread on the wire mesh 9. The weight of the weight 10 is adjusted so that a load of 50 g / cm ² can be uniformly applied to the wire mesh 9, that is, the water absorbing agent.

The absorption capacity under pressure was measured using the measuring apparatus having the above-mentioned configuration. The measuring method will be described below. First, predetermined preparation operations such as putting a predetermined amount of artificial urine 11 into the container 2 and fitting the outside air suction pipe 3 into the container 2 were performed. Next, the filter paper 7 was placed on the glass filter 6. On the other hand, in parallel with these placing operations, 0.9 g of the water-absorbing agent was evenly sprayed on the inside of the support cylinder, that is, on the wire mesh 9, and the weight 10 was placed on the water-absorbing agent.

Next, on the filter paper 7, the wire cylinder 9, that is, the support cylinder 8 on which the water absorbing agent and the weight 10 were placed was placed. Then, the weight W2 (g) of the artificial urine 11 absorbed by the water-absorbing agent was measured using the balance 1 for 60 minutes after the support cylinder 8 was placed on the filter paper 7. Then, from the above weight W2, according to the following formula, absorption capacity under pressure (g / g) = weight W2 (g) / weight of water absorbing agent (g), absorption capacity under pressure 60 minutes after the start of absorption ( g /
g) was calculated. (C) Water absorption rate Spray 1.0 g of a water absorbing agent into a petri dish, and place the artificial urine there.
20 g were added. The time T1 until the resin completely absorbed the liquid and the liquid on the gel surface disappeared was measured, and the water absorption rate was calculated according to the following equation.

Water absorption speed (g / g / sec) = 20 ÷ 1.0 ÷ T1 (d) Return amount 50 parts by weight of a water absorbing agent and 50 parts by weight of wood pulp are mixed with a mixer.
And dry-mixed. The resulting mixture is then
Wire formed into 0 mesh (mesh size 38μm)
Air-making on a screen using a batch-type air-making machine
To form a web of 100 mm x 100 mm
did. In addition, this web is pressured 2kg / cm ^TwoPress for 5 seconds
By doing, the basis weight is about 360 g / m^TwoWas obtained. This
Of the absorbent was 3.6 g.

After adding 40 g of physiological saline to the above absorbent material, and one minute later, ten kitchen towels folded into four pieces are placed on the top sheet, and a load of 10 kg is applied for one minute to pass through the top sheet. The amount of return to the kitchen towel was examined. Reference Example 1 800 g of cyclohexane was placed in a 2000 ml four-neck separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube and a dropping funnel, and 3.0 g of sorbitan monostearate was added and dissolved as a dispersant. Then, nitrogen gas was blown in to expel dissolved oxygen.

Separately, in a flask, 141 g of sodium acrylate, 36 g of acrylic acid, 0.478 g of polyethylene glycol diacrylate (n = 8), and 413 g of ion-exchanged water
A monomer aqueous solution was prepared, and nitrogen gas was blown in to expel dissolved oxygen dissolved in the aqueous solution. Next, 10% of sodium persulfate was added to the aqueous monomer solution in this flask.
After adding 1.0 g of the aqueous solution, the whole amount was added to the separable flask, and dispersed by stirring at 230 rpm. Thereafter, the bath temperature was raised to 60 ° C. to start the polymerization reaction, and the temperature was maintained at this temperature for 2 hours to complete the polymerization. After completion of the polymerization, most of the water was distilled off by azeotropic dehydration to obtain a cyclohexane suspension of the polymer, and a resin having a water content of 20% was obtained by filtration. Further, the resultant was dried under reduced pressure at 80 ° C. to obtain a water absorbent resin (1) having a water content of 5%. The average particle size of the water absorbent resin (1) is 11
It was 0 μm. [Example 1] A surface cross-linking agent composed of 1.0 part by weight of propylene glycol, 0.05 part by weight of ethylene glycol diglycidyl ether, 3 parts by weight of water, and 1 part by weight of isopropyl alcohol per 100 parts by weight of the water absorbent resin (1). Was mixed. The above mixture was heated at 185 ° C. for 50 minutes. Then, 10 g of the surface-crosslinked resin having a water content of 2% obtained is
The mixture was added to 500 ml of heated methanol, stirred for 1 hour, filtered and dried to obtain a hydrophilic treatment to obtain the water absorbing agent (1) of the present invention. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the water absorbing agent (1). [Example 2] 10 g of the surface-crosslinked resin having a water content of 2% obtained in Example 1 was added to 500 ml of cyclohexane heated to 70 ° C, stirred for 1 hour, filtered and dried to perform a hydrophilic treatment. Then, a water absorbing agent (2) of the present invention was obtained. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the water absorbing agent (2). [Example 3] 10 g of the surface-crosslinked resin having a water content of 2% obtained in Example 1 was added to 500 ml of methanol at 20 ° C, stirred for 1 hour, and then subjected to a hydrophilization treatment by filtration, whereby the present invention was carried out. Water absorbing agent (3) was obtained. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate and return amount of the water absorbing agent (3).
It described in. [Comparative Example 1] A comparative water-absorbing agent (1) was obtained from 10 g of the surface-crosslinked resin having a water content of 2% obtained in Example 1 without performing a hydrophilic treatment. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the comparative water absorbing agent (1). REFERENCE EXAMPLE 2 A water-absorbent resin (2) having a water content of 6% was obtained in the same manner as in Reference Example 1, except that sucrose fatty acid ester (HLB = 6) was used instead of sorbitan monostearate. The average particle size of the water absorbent resin (2) is 160 μm
Met. Example 4 0.1 part by weight of ethylene glycol diglycidyl ether and 100 parts by weight of water were added to 100 parts by weight of the water absorbent resin (2).
Parts by weight and a surface crosslinking agent consisting of 1 part by weight of isopropyl alcohol were mixed. The above mixture was heated at 150 ° C. for 60 minutes. Thereafter, 10 g of the surface-crosslinked resin having a water content of 1% obtained was added to 500 ml of methanol heated to 60 ° C.
After stirring for 1 hour, the solution was subjected to a hydrophilic treatment by filtration and drying to obtain a water absorbing agent (4) of the present invention. This water absorbing agent (4)
Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the sample. Comparative Example 2 Comparative water-absorbing agent (2) was obtained without subjecting 10 g of the surface-crosslinked resin having a water content of 1% obtained in Example 4 to hydrophilic treatment. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the comparative water absorbing agent (2). Reference Example 3 The same operation as in Reference Example 1 was carried out except that ethyl cellulose was used instead of sorbitan monostearate to obtain a water-absorbent resin (3) having a water content of 4%. The average particle size of the water absorbent resin (3) was 250 μm. Example 5 0.1 part by weight of ethylene glycol diglycidyl ether and 100 parts by weight of water
Parts by weight and a surface crosslinking agent consisting of 1 part by weight of isopropyl alcohol were mixed. The above mixture was heated at 150 ° C. for 60 minutes. Thereafter, 10 g of the surface-crosslinked resin having a water content of 1% obtained was added to 500 ml of cyclohexane heated to 70 ° C., stirred for 1 hour, filtered and dried to perform a hydrophilic treatment, and the water absorbing agent of the present invention (5 ) Got. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the water absorbing agent (5). [Comparative Example 3] A comparative water-absorbing agent (3) was obtained without subjecting 10 g of the surface-crosslinked resin having a water content of 1% obtained in Example 5 to hydrophilic treatment. Table 1 shows the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the comparative water absorbing agent (3). [Comparative Example 4] In Example 1, a hydrophilic treatment was performed without performing surface cross-linking to obtain a comparative water absorbing agent (4). Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of this comparative water absorbing agent (4). [Example 6] 100 g of the water absorbent resin (1) obtained in Reference Example 1
Was added to 1000 ml of methanol heated to 60 ° C., and the mixture was stirred for 1 hour, followed by filtration and drying to perform a hydrophilic treatment, thereby obtaining a water-absorbent resin (1 ′).

The same operation as in Example 1 was performed using the water-absorbing resin (1 ′) subjected to the above-mentioned hydrophilic treatment to obtain a water-absorbing agent (6) which was further subjected to a hydrophilic treatment after surface crosslinking. Table 1 shows the results of the absorption capacity, absorption capacity under pressure, water absorption rate, and return amount of the water absorbing agent (6). Further, Examples 1 to
Regarding the water absorbing agents (1) to (6), before and after the hydrophilization treatment, the degree of penetration and the maximum absorption diameter from the resin surface layer were measured by the above-described method, and the change in wettability was observed.
The results are shown in Table 2.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

According to the present invention, since the hydrophilic treatment is performed after the surface cross-linking treatment, the effect of the surface cross-linking treatment (the absorption capacity of the water-absorbing agent under pressure and the return amount can be reduced) can be enhanced. In addition, since the hydrophilicity of the surface of the water-absorbing resin is increased, the water absorption speed is also increased. Therefore, according to the present invention, it is possible to obtain a water-absorbing agent having a high absorption capacity under pressure, a small amount of return, and a high water absorption rate.

The water-absorbing agent obtained by the present invention is used in a water-absorbing article such as a disposable diaper or a sanitary napkin obtained by compounding a water-absorbing resin with, for example, a fibrous material.
When used under high concentration conditions of 50% by weight or more (ratio to the total of fibrous materials such as water-absorbent resin and pulp), it can exhibit sufficient absorption capacity even when a higher load is applied. Further thinning of the article is possible.

[Brief description of the drawings]

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

[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 support cylinder 9 wire mesh 10 weight 11 artificial urine

Claims (6)

[Claims] 1. A method for producing a water-absorbing agent, comprising subjecting a surface of a water-absorbent resin to a cross-linking treatment and then subjecting the water-absorbent resin to a hydrophilic treatment. 2. The method for producing a water-absorbing agent according to claim 1, wherein the water-absorbing resin is obtained by reversed-phase suspension polymerization. 3. The method for producing a water absorbing agent according to claim 1, wherein the hydrophilic treatment is a treatment with an organic solvent. 4. The method for producing a water-absorbing agent according to claim 3, wherein the hydrophilic treatment is performed by bringing the organic solvent into contact with a water-absorbing resin in a heated state. 5. The water absorption according to claim 1, wherein in the crosslinking treatment, a crosslinking agent capable of reacting with a functional group of the water absorbing resin is directly added to and mixed with the water absorbing resin powder. Method of manufacturing the agent. 6. The hydrophilic treatment of the water-absorbent resin before the cross-linking treatment near the surface of the water-absorbent resin.
The production method according to any one of the above.