JPH0655838B2 - Surface treatment method for water absorbent resin - Google Patents

Description

TECHNICAL FIELD The present invention relates to a surface treatment method for a water-absorbent resin, and more specifically, to obtain a water-absorbent resin exhibiting an excellent initial water absorption speed and suction force. The present invention relates to a surface treatment method for a water-absorbent resin.

[Conventional technology]

Heretofore, attempts have been made to use a water-absorbent resin as an absorbent body for sanitary cotton, disposable diapers, or other sanitary materials that absorb body fluids. Examples of such a water absorbent resin include a hydrolyzate of a starch-acrylonitrile graft polymer (Japanese Patent Publication No. 49-43395) and a neutralized product of a starch-acrylic acid graft polymer (JP-A-51-125468).
No.), a saponified product of vinyl acetate-acrylic acid ester copolymer (JP-A No. 52-14689), a hydrolyzate of an acrylonitrile copolymer or an acrylamide copolymer (JP-B No. 53-15959), or these Crosslinked product and partially neutralized polyacrylic acid crosslinked product (JP-A-55-84)
No. 304) is known.

The desired properties of the water-absorbent resin include a high water-absorption capacity and an excellent water-absorption rate when in contact with an aqueous liquid, a high gel strength of a hydrous swelling gel, and an excellent property for sucking water from a substrate containing an aqueous liquid. The suction power etc. can be raised. However, the relationship between these properties does not necessarily show a positive correlation, and in particular, there is a contradictory relationship between the water absorption capacity and the water absorption rate, the gel strength and the suction force, and the higher the water absorption capacity, the lower these physical properties become. I will end up. Further, among those having a high water absorption capacity, there is a water absorbing liquid which forms a so-called "mamako" upon contact with an aqueous liquid, does not diffuse water throughout the water absorbent resin particles, and has an extremely low water absorption rate.

As a method for obtaining a water absorbent resin having a high water absorption capacity and a relatively good water absorption speed, a method of coating the surface of the water absorbent resin with a surfactant or a non-volatile hydrocarbon is known. However, with this method, although the dispersibility of the water-absorbent resin in the aqueous liquid is improved, a sufficient effect cannot be obtained in terms of improving the water-absorption rate and suction force of each water-absorbent resin particle.

Further, as a method for improving the above characteristics in a well-balanced manner,
A method of crosslinking the surface of the water absorbent resin with a crosslinking agent is also known. As such a cross-linking agent, polyhydric alcohols, polyhydric glycidyl ethers, haloepoxy compounds, polyhydric aldehydes, polyhydric amines, and polyhydric metal salts are used.

As a method for crosslinking the surface of the water absorbent resin using these crosslinking agents, a water absorbent resin powder and a crosslinking agent or a composition obtained by dissolving the crosslinking agent in a small amount of water and a hydrophilic organic solvent are directly mixed. However, a method of performing heat treatment if necessary (Japanese Patent Laid-Open No. 58-58
-180233, JP-A-59-189103, JP-A-61-16903), a method in which a water-absorbent resin is dispersed in a mixed solvent of water and a hydrophilic organic solvent and a cross-linking agent is added to react (JP-B-61-61). No. 48521), a method of reacting a resin with a cross-linking agent in the presence of water in an inert solvent (JP-B-6).
0-18690) and the like are known.

When the surface of the water-absorbent resin is treated and cross-linked, the important factors are uniform dispersion of the cross-linking agent on the surface of the water-absorbent resin powder and appropriate penetration into the vicinity of the surface, and the process is industrially advantageous. It is necessary to be. However, among these methods, in the method of directly mixing the water-absorbent resin powder and the crosslinking agent,
Although it depends on the type of the cross-linking agent used, the penetrability to the powder surface is generally small, and the cross-linking effect may be difficult to appear. On the other hand, a method of mixing a water-absorbent resin powder and a composition obtained by dissolving a crosslinking agent in a small amount of water and / or a hydrophilic organic solvent improves the permeability to the powder surface, Especially when water is used, if the amount of the hydrophilic organic solvent used is small, lumps are more likely to occur during mixing, and the finer the average particle size of the water-absorbent resin, the more difficult the uniform crosslinking of the particle surface becomes.

On the other hand, a method of dispersing a water-absorbent resin in a mixed solvent of water and a hydrophilic organic solvent and adding a cross-linking agent to react is advantageous in bringing the cross-linking agent into contact with the resin powder surface, but a large amount of hydrophilic The use of a water-soluble organic solvent causes fire hazard and deterioration of occupational health, and water and hydrophilic organic solvent are distilled off together during drying. As a result, water is removed to recover the hydrophilic solvent. It requires a process, is industrially disadvantageous, and is not a desirable method. Further, when a hydrophobic organic solvent such as cyclohexanone or n-heptane is used as the inert solvent, water is not uniformly dissolved in the organic solvent but exists as dispersed droplets, so that the distribution on the surface of the water absorbent resin is It becomes non-uniform and uniform surface cross-linking is not formed.

[Problems to be Solved by the Invention]

Thus, in order to improve various properties of the water-absorbent resin in a well-balanced manner, surface treatment such as forming a surface cross-linking layer on the water-absorbent resin has been attempted, but both have the above-mentioned problems and physical properties. So far, there has been no method that can satisfy both the industrial and the industrial level.

The present invention solves the above-mentioned problems of the prior art. Therefore, an object of the present invention is to provide a method of industrially advantageously uniformly and efficiently cross-linking the surface of the water-absorbent resin, and as a result, the water-absorption capacity and water-absorption rate when in contact with an aqueous liquid, the aqueous solution. An object of the present invention is to provide a well-balanced water-absorbent resin having various physical properties such as suction force of water from a base material containing a liquid.

[Means and Actions for Solving the Problems]

As a result of intensive studies to achieve the above-mentioned object, the present inventors have mixed a water-absorbent resin powder with a cross-linking agent or a mixed composition of the cross-linking agent and an organic agent, and then, under an atmosphere of a specific humidity. The present invention has been completed by finding that the surface of the water-absorbent resin can be treated uniformly and efficiently industrially advantageously by heating at a specific temperature to cause a crosslinking reaction.

That is, in the present invention, a water-absorbent resin powder having a carboxyl group is mixed with a cross-linking agent having two or more functional groups capable of reacting with the carboxyl group or a mixed composition of a cross-linking agent and an organic solvent, and the relative humidity is 20 to 40-250 in 100% atmosphere
The present invention relates to a surface treatment method for a water absorbent resin, which comprises heating at a temperature of ° C to cause a crosslinking reaction.

In the present invention, the water-absorbent resin to be surface-treated,
It is a conventionally known resin that absorbs a large amount of water and swells in water to form a hydrogel, and it is necessary to have a carboxyl group. For example, a hydrolyzate of a starch-acrylonitrile graft copolymer, a neutralized product of a starch-acrylic acid graft polymer, an acrylic acid ester-
Saponified product of vinyl acetate copolymer, hydrolyzed product of acrylonitrile polymer or acrylamide copolymer or crosslinked product thereof, crosslinked polyvinyl alcohol modified product containing carboxyl group, self-crosslinked polyacrylic acid (partial) neutralized product, poly Examples thereof include crosslinked products of partially neutralized acrylic acid and neutralized products of crosslinked isobutylene-maleic anhydride copolymer.

Such a water-absorbent resin is generally selected from unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic anhydride and neutralized products thereof.
It is obtained by polymerizing a monomer component essentially containing one or more species. Preferred are acrylic acid, methacrylic acid, and neutralized products thereof.

In order to obtain the water absorbent resin used in the present invention, other monomers may be used in combination with the unsaturated carboxylic acid, if necessary. Specific examples of other monomers include 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, and styrenesulfone. Anionic monomers such as acids and salts thereof; (meth) acrylamide, N-substituted (meth) acrylamide, 2-
Nonionic hydrophilic group-containing monomers such as hydroxylethyl (meth) acrylate and 2-hydroxypropyl (meth) acrylate; diethylaminoethyl (meth) acrylate, diethylaminopropyl (meth) acrylate,
Examples thereof include cationic monomers such as dimethylaminopropyl (meth) acrylamide and their graded products.
Preferably, it consists of 2- (meth) acryloylethanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid and salts thereof, dimethylaminoethyl (meth) acrylate and its quaternary compound, and (meth) acrylamide. One or two or more selected from the group. The amount of the carboxyl groups contained in the water absorbent resin is not particularly limited, but it is preferable that the carboxyl groups are present in an amount of 0.01 equivalent or more per 100 g of the water absorbent resin.

Further, the water-absorbent resin is preferably one obtained by copolymerizing a very small amount of a cross-linking agent having two or more polymerizable unsaturated groups or reactive functional groups, rather than a self-crosslinking type resin which does not use a cross-linking agent.

Examples of these crosslinking agents include N, N'-methylenebis (meth) acrylamide, N-methylol (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate. , Glycerin tri (meth) acrylate, glycerin (meth) acrylate, (meth)
Acrylic acid polyvalent metal salt, trimethylolpropane tri (meth) acrylate, triallylamine, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, glycidyl (meth) acrylate, ethylene glycol diglycidyl ether, glycerine tri (di) Examples thereof include glycidyl ether and polyethylene glycol diglycidyl ether. Moreover, you may use these crosslinking agents in mixture of 2 or more types. The amount of these cross-linking agents to be used is generally about 0.001 to 0.5 mol% with respect to the monomer component consisting of the unsaturated carboxylic acid and other monomer used as necessary.

As the polymerization method for obtaining the water absorbent resin, various methods such as aqueous solution polymerization, reverse phase suspension polymerization, precipitation polymerization, and bulk polymerization can be adopted. Is preferred.

The monomer concentration in the aqueous solution of the monomer component when synthesizing the water absorbent resin by the aqueous solution polymerization or the reverse phase suspension polymerization method can be selected over a wide range, but is generally 25% by weight or more, and more preferable. It is preferably 30% by weight or more to a saturated concentration.

As the polymerization initiator used in the polymerization, persulfates such as potassium persulfate, sodium persulfate and ammonium persulfate which are commonly used water-soluble radical polymerization initiators; hydrogen peroxide, t-butylhydrogen are used. Hydroperoxy such as peroxide and cumene hydroperoxide; azo compounds such as 2,2′-azobis-2-amidinopropane dihydrochloride. These polymerization initiators can be used as a mixture of two or more kinds, and further, a redox type initiator obtained by combining with a reducing agent such as sulfite, L-ascorbic acid and ferric salt can also be used. Wear.

In the present invention, the powder obtained by drying the water absorbent resin thus obtained is mixed with a crosslinking agent or a mixed composition of the crosslinking agent and an organic agent to obtain a relative humidity of 20 to 100%.
It is achieved by cross-linking the surface of the powder at a temperature of 40 to 250 ° C.

The cross-linking agent in this case is not particularly limited as long as it has two or more functional groups capable of reacting with the carboxyl group of the water-absorbent resin, but is preferably a hydrophilic compound, more preferably a water-soluble compound. For example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, glycerin, polyglycerin, propylene glycol, diethanolamine, triethanolamine, polyoxypropyl, oxyethyleneoxypropylene block copolymer, pentaerythritol, sorbitol, etc. Polyhydric alcohols; ethylene glycol diglycidyl ether,
Polyvalent polyglycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, sosorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, etc. Glycidyl ethers; 2,2-bishydroxymethylbutanol-tris [3- (1-aziridinyl) propionate], 1,
Polyvalent aziridine compounds such as 6-hexamethylenediethyleneurea and diphenylmethane-bis-4,4'-N, N'-diethyleneurea; haloepoxy compounds such as epichlorohydrin and α-methylchlorohydrin; glutaraldehyde, glyoxal and the like Polyvalent aldehydes; polyamines such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine; 2,
Examples include polyvalent isocyanates such as 4-toluylene diisocyanate and hexamethylene diisocyanate; polyvalent metal salts such as aluminum chloride, magnesium chloride, calcium chloride, aluminum sulfate, magnesium sulfate and calcium sulfate. Particularly preferred are polyhydric alcohols, polyhydric glycidyl ethers, polyhydric amines, and polyhydric metal salts. The amount of these cross-linking agents used depends on the type, but is generally 0.00
5-5% by weight is suitable. If this amount is less than 0.005% by weight, the surface treatment effect will not be exhibited, and if it exceeds 5% by weight, the water absorption capacity may decrease.

When these cross-linking agents are mixed with the water-absorbent resin powder, it is preferable to use a composition in which the cross-linking agent is mixed with an organic solvent because the cross-linking agent can be uniformly distributed on the surface of the resin powder. The organic solvent that can be used in this case is not particularly limited as long as it dissolves the cross-linking agent and does not affect the performance of the water-absorbent resin, but when a hydrophilic or water-soluble cross-linking agent is used as the cross-linking agent, A hydrophilic organic solvent is preferable. Examples thereof include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, fio-propyl alcohol, n-butyl alcohol, iso-butyl alcohol and t-butyl alcohol, and ketones such as acetone and methyl ethyl ketone. And ethers such as dioxane and tetrahydrofuran.

When these organic solvents are used, the amount thereof generally depends on the kind of the solvent, but is generally less than 50% by weight, preferably 0.1 to 30% by weight, more preferably 1 to 1% by weight of the water-absorbent resin.
It is 0% by weight. If it is 50% by weight or more, an excessive amount of the organic solvent remains after mixing with the water-absorbent resin powder, and a process for distilling off the organic solvent is required, which is not industrially or economically advantageous.

In the present invention, as a method of mixing a cross-linking agent or a mixed composition of the cross-linking agent and an organic solvent with the water-absorbent resin powder, it is common to spray or drop / mix these treatment liquids on the water-absorbent resin. Is. As a mixer used for mixing, a mixer having a large mixing force is preferable for uniform mixing, but a normal mixer or a kneader can be used. For example, a cylinder type mixer, a double cone type mixer, a V type mixer, a ribbon type mixer, a screw type mixer, a fluidized type mixer, a rotary disc type mixer, an air flow type mixer, a double arm type mixer. Examples include a kneader, an internal mixer, a muller type kneader, a roll mixer, a screw type extruder, and the like.

The water-absorbent resin powder and the cross-linking agent after being mixed by such a procedure are 40 to 250 in an atmosphere of relative humidity 20 to 100%.
By heating at a temperature of ℃, the cross-linking reaction occurs efficiently. When the relative humidity is less than 20%, it is difficult for the carboxyl groups on the surface of the resin powder and the crosslinking agent to come into contact with each other, and the crosslinking reaction does not proceed sufficiently. The higher the relative humidity, the more the crosslinking agent penetrates into the inside of the resin powder, and the crosslinking reaction inside the powder proceeds, so that the water absorbing agent obtained tends to have a lower water absorption capacity. Therefore,
Keeping the penetration of the cross-linking agent near the surface of the powder in an optimum state,
The relative humidity is preferably 30 to 80% in order to make the surface of the powder necessary and sufficiently crosslinked. If the humidity during heating is less than 40 ° C., the crosslinking reaction does not proceed sufficiently and unreacted crosslinking agent remains. On the contrary, if the temperature exceeds 250 ° C, the water absorbent resin deteriorates. 60 to 200 in order to achieve a sufficient cross-linking reaction without damaging the water-absorbent resin
It is preferred to heat at ° C.

As a device for treating the powder of the water-absorbent resin under such conditions, a known dryer or a heating furnace equipped with a gas supply device containing water vapor for producing the specified atmosphere is used. For example, a conduction heat transfer type, a radiant heat transfer type, a hot air heat transfer type, a dielectric heating type dryer or a heating furnace provided with a gas supply device is suitable. Specifically, a belt type, a groove type stirring type, a rotary type, a disc type, which is equipped with a mixed gas of water vapor and air and / or an inert gas or a water vapor supply device,
Examples thereof include a kneading type, fluidized bed type, air flow type, infrared type, electron beam type dryer or heating furnace. However, in terms of simplicity, a hot air heat transfer type dryer or a heating furnace is preferable.

〔The invention's effect〕

According to the method of the present invention, since water is not present when mixing the water-absorbent resin powder and the cross-linking agent, even when the water-absorbent resin powder has a small average particle size of, for example, 250 μm or less Can be mixed uniformly without generating large aggregates. During the crosslinking reaction, the water-absorbent resin powder and the crosslinking agent react efficiently and sufficiently, so that the risk of the crosslinking agent remaining unreacted is extremely small. Further, the amount of organic solvent used is small, which is industrially and economically advantageous. The water-absorbing agent after the surface treatment of the powder of the water-absorbent resin is excellent in water absorption capacity and water absorption rate, and also has a large suction force from the base material when brought into contact with the base material containing the aqueous liquid. It is a thing.

Therefore, the water-absorbing agent after being surface-treated by the method of the present invention is used for applications such as paper diapers, water-absorbing agents for sanitary materials such as sanitary cotton, dew condensation preventives for building materials, agricultural and horticultural water-retaining agents or desiccants. It can be preferably used.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited to these Examples.

The performance of the water absorbent resin was measured by the method described below.

(1) Water absorption ratio A non-woven tea bag bag (40 m) containing 0.2 g of water absorbent resin
m × 150 mm), dipped in 0.9 wt% sodium chloride aqueous solution for 3 minutes and 30 minutes, pull up this tea bag type bag and drain it for a certain period of time, then measure its weight, The water absorption ratio was calculated.

(2) Water absorption rate Artificial urine (containing 1.9% by weight of urea, 0.8% by weight of sodium chloride, 0.1% by weight of calcium chloride and 0.1% by weight of magnesium sulfate) 1.0 ml of water absorbing resin was added to 20 ml of artificial urine, and all the water absorbing resin was artificial. The water absorption speed was defined as the time taken for the swollen gel to lose its fluidity after absorbing urine.

(3) Suction power Artificial urine 2 on tissue paper (55 mm x 75 mm)
0 ml was added to prepare a base material containing artificial urine, and 1.0 g of the water absorbent resin was placed on the base material. After 10 minutes, the swollen gel was sampled and the weight thereof was measured to obtain the force of sucking the liquid from the tissue paper. In addition, the presence or absence of Mamako of the water absorbent resin added at the same time was observed.

Reference Example 1 Sodium acrylate 1410 g, acrylic acid 361 g
And trimethylolpropane triacrylate 1.18g
Was dissolved in 3290 g of ion-exchanged water, and static polymerization was performed at 55 to 80 ° C. in a nitrogen atmosphere using 6.76 g of ammonium persulfate and 0.25 g of sodium bisulfite to obtain a gel-like hydropolymer. The gel-like water-containing polymer was dried with a hot air dryer at 180 ° C., crushed with a hammer crusher, and sieved with a 28-mesh wire net to obtain a 28-mesh pass product (powder A). The average particle size was 300 μm.

Reference Example 2 Polymerization was carried out in the same manner as in Reference Example 1, pulverized, and then sieved with a 60-mesh wire net to obtain a 60-mesh passing product (powder B). The average particle size was 125 μm.

Reference Example 3 Cyclohexane 2.5 was placed in a five-necked separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas introducing tube and a dropping funnel, and 20 g of sorbitan monostearate as a dispersant was added and dissolved. , Nitrogen gas was blown in to drive out the dissolved oxygen. Separately, in a flask, 282 g of sodium acrylate, 72.1 g of acrylic acid and 0.04 g of N, N'-methylenebisacrylamide were dissolved in 658 g of deionized water, and then 0.5 g of potassium persulfate was added and dissolved, and then nitrogen gas was blown. The oxygen dissolved in the aqueous solution was expelled.

Then, the aqueous monomer solution in the flask was added to the separable flask and dispersed by stirring at 180 rpm. Then, the bath temperature was raised to 60 ° C. to start the polymerization reaction, and then the temperature was maintained for 2 hours to complete the polymerization. After completion of the polymerization, most of the water was distilled off by azeotropic dehydration, cyclohexane was removed by filtration, and the obtained solid was dried under reduced pressure at 80 ° C. to obtain a water-absorbent resin powder (powder C). . All of these passed through a 100-mesh wire net and had an average particle size of 80 μm.

Reference Example 4 Sodium salt of 2-sulfoethyl methacrylate 30.2
g, sodium acrylate 4.2 g, acrylic acid 1.1 g and N, N'-methylenebisacrylamide 0.005 g were dissolved in deionized water 65.9 g, and then nitrogen gas was blown in to drive out dissolved oxygen. While maintaining this monomer aqueous solution at 30 ° C., 0.05 g of potassium persulfate and 0.003 of sodium bisulfite were added, and static polymerization was performed to obtain a gel-like hydropolymer. The hydrogel polymer was dried with a hot air dryer at 180 ° C., pulverized with a hammer type powder machine, and sieved with a 100 mesh wire net to obtain a polymer (powder D) having a size passing through 100 mesh. It was The average particle size was 70 μm.

Example 1 100 parts of the powder A obtained in Reference Example 1 was mixed with a mixed composition of 0.1 part of ethylene glycol diglycidyl ether and 1 part of methanol. The powder A after mixing did not generate a large lump at all, and when the powder A was passed through 28 mesh, all the powder A passed. The obtained mixture was heated at 80 ° C. for 1 hour under stirring in a double-arm type kneader adjusted to an atmosphere of relative humidity 55% to obtain a water absorbing agent (1). The result 1 is shown.

Example 2 100 parts of the powder A obtained in Reference Example 1 was mixed with 0.5 part of epichlorohydrin. 28 after mixing powder A
When passing through the mesh, all the powder A passed. The resulting mixture was heat treated for 2 hours at 70 ° C. in a fluidized bed dryer adjusted to an atmosphere of relative humidity 65% to obtain a water absorbing agent (2).
Got

The results are shown in Table 1.

Example 3 100 parts of the powder B obtained in Reference Example 2 was mixed with a mixed composition of 0.4 parts of diethylene glycol and 2 parts of ethanol. When the powder B after mixing was passed through 60 mesh, all the powder B passed. The resulting mixture was heat-treated at 180 ° C. for 1 hour under stirring in a double-arm type kneader adjusted to an atmosphere of relative humidity of 30% to obtain a water absorbing agent (3).
The results are shown in Table 1.

Example 4 100 parts of the powder C obtained in Reference Example 3 was mixed with a mixed composition of 0.1 part of glycerol polyglycidyl ether and 7 parts of acetone. When the powder B after mixing was passed through 100 mesh, all the powders C were passed. The resulting mixture was heat-treated for 90 minutes at 90 ° C. in a fluidized bed dryer adjusted to an atmosphere of 70% relative humidity to obtain a water absorbing agent (4). The results are shown in Table 1.

Example 5 100 parts of the powder D obtained in Reference Example 4 was mixed with a mixed composition of 0.3 part of ethylene glycol diglycidyl ether and 9 parts of isopropyl alcohol. When the powder D after mixing was passed through 100 mesh, all the powder D passed. The obtained mixture was stirred at a temperature of 80 ° C. in a paddle-type dryer adjusted to an atmosphere of relative humidity of 45% to 1
Heat treatment was performed for 20 minutes to obtain a water absorbing agent (5). The results are shown in Table 1.

Example 6 In the same manner as in Example 5, after heat-treating the mixture obtained by mixing the powder D and the crosslinking agent at 85 ° C. for 15 minutes in a fluidized bed dryer adjusted to an atmosphere of 60% relative humidity, Further, a water absorbing agent (6) was obtained by heat treatment at a temperature of 180 ° C. for 10 minutes in an atmosphere having a relative humidity of 9%. The results are shown in Table 1.

Example 7 In Example 4, the atmosphere during heat treatment was set to a relative humidity of 9
The same operation as in Example 4 was repeated except that the content was changed to 0% to obtain a water absorbing agent (7). The results are shown in Table 1.

Comparative columns 1 to 4 Powders A to D obtained in Reference Examples 1 to 4 were used as comparative water absorbing agents (1 ') to (4'), and each performance was tested. The results are shown in Table 1.

Comparative Example 5 A comparative water absorbing agent (5 ′) was obtained by repeating the same operation as in Example 1 except that the atmosphere during the heat treatment was changed to 9% relative humidity. The results are shown in Table 1.

Comparative Example 6 Same as Example 3 except that the composition of the mixed composition in Example 3 was 0.1 part of ethylene glycol diglycidyl ether, 8 parts of water and 2 parts of methanol, and the atmosphere during heat treatment was 2% relative humidity. The above procedure was repeated to obtain a comparative water absorbing agent (6 '). The results are shown in Table 1. Incidentally, when the powder A after being mixed with the mixed composition containing the cross-linking agent was passed through 28 mesh, there was a lump that did not pass through.

As is clear from the results shown in the table, the water-absorbing agent obtained by surface-treating the powder of the water-absorbent resin according to the method of the present invention does not become muddy during water absorption, has a large water absorption ratio, and has a high water absorption rate. In addition, the suction force is much larger than the conventional one. Moreover, since no water is used at the time of mixing the powder and the crosslinking agent, no lump is formed. Furthermore, as shown in the examples, no hydrophilic organic solvent is used at the time of mixing, or even if it is used, the amount used is small, economical,
It turns out that it is very advantageous industrially.

Front page continuation (72) Inventor Tadao Shimomura 5-8 Nishiomitabicho, Suita City, Osaka Prefecture Yasuhide Nomura, Examiner, Central Research Laboratory, Nippon Shokubai Chemical Co., Ltd. (56) Reference JP-A-58-180233 ) JP-A-59-189103 (JP, A)

Claims (3)

[Claims] 1. A water-absorbent resin powder having a carboxyl group is mixed with a cross-linking agent having two or more functional groups capable of reacting with the carboxyl group or a mixed composition of the cross-linking agent and an organic solvent to obtain a relative humidity of 20. 40 to 250 in 100% atmosphere
A surface treatment method for a water-absorbent resin, which comprises heating at a temperature of ° C to cause a crosslinking reaction. 2. A cross-linking agent having two or more functional groups capable of reacting with a carboxyl group is used in an amount of 0.005 to 5 parts by weight and an organic solvent in an amount of 0.1 to 50 parts by weight, based on 100 parts by weight of the water-absorbent resin powder. The surface treatment method of the water absorbent resin according to claim 1. 3. The surface treatment method for a water absorbent resin according to claim 1, wherein the average particle diameter of the water absorbent resin powder is 250 μm or less.