WO2005012369A1 - Process for producing water-absorbing resin - Google Patents

Abstract

A process for producing a water-absorbing resin which comprises the step of polymerizing a water-soluble ethylenic monomer in the presence of a crosslinking agent, characterized in that the polymerization of the monomer is conducted in the presence of lactic acid and/or a salt thereof; a water-absorbing resin obtained by the process; and a sanitary material employing the water-absorbing resin.

Description

 Method for producing water absorbent resin

Technical field

 The present invention relates to a method for producing a water absorbent resin. More specifically, the present invention relates to a method for producing a water-absorbent resin that can be suitably used in the field of sanitary materials such as disposable diapers and sanitary napkins. Light

 Background technology

 Conventionally, water-absorbent resins have been widely used in sanitary materials such as disposable diapers and sanitary napkins, and industrial materials such as waterproofing agents for cables.

 Examples of the water-absorbent resin include a hydrolyzate of starch-acrylonitrile graft copolymer, a neutralized product of starch-acrylic acid graft polymer, a saponified product of vinyl acetate-acrylate copolymer, and a polyacrylic acid portion. Neutralized substances and the like are known. In general, these water-absorbing resins have a high water absorption under a pressure higher than the atmospheric pressure (hereinafter referred to as water absorption under pressure), but have a high water absorption under the atmospheric pressure (hereinafter, referred to as “water absorption under pressure”). Water absorption under no pressure) is insufficient. Therefore, when these water-absorbing resins are used in adult diapers and the like, for which needs have been increasing in recent years, the water absorption under no pressure is low, and sufficient absorption performance cannot be exhibited.

 Examples of a method for producing a water-absorbing agent excellent in water absorption under pressure and water absorption under pressure include, for example, a water-absorbent resin having a lipoxyl group, an inorganic acid or an organic acid which can be dissolved in an aqueous liquid, and lipoxyl. A method of mixing a crosslinking agent capable of reacting with a group is known.

(For example, refer to Japanese Patent Application Laid-Open No. Hei 7-272825).

However, the water absorption under no pressure and the water absorption under pressure of the water absorbing agent obtained by such a method for producing a water absorbing agent are still insufficient from a practical point of view. Disclosure of the invention

 The present invention has been made in view of the prior art, and provides a method for producing a water-absorbent resin which has excellent water absorption under no pressure and water absorption under pressure, and can be suitably used for sanitary materials and the like. As an issue.

 That is, the present invention

 [1] A method for producing a water-absorbent resin having a step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a crosslinking agent, wherein the polymerization of the monomer is further carried out by lactic acid and / or A method for producing a water-absorbent resin, which is performed in the presence of a salt thereof,

 [2] The water-absorbent resin according to the above [1], wherein the amount of lactic acid and / or a salt thereof is 0.0001 to 0.1 mol per 1 mol of the water-soluble ethylenically unsaturated monomer. Production method,

 (3) The method for producing a water-absorbent resin according to (1) or (2), wherein the polymerization is performed by an aqueous solution polymerization method or a reverse phase suspension polymerization method,

 (4) a water-absorbent resin obtained by the method for producing a water-absorbent resin according to any one of the above (1) to (3), and

 (5) a sanitary material comprising the water-absorbent resin according to (4),

About.

 ADVANTAGE OF THE INVENTION According to this invention, the water absorption under pressure and the water absorption under pressure are both excellent, and the water-absorbing resin which can be suitably used for sanitary materials etc. can be manufactured efficiently. Brief Description of Drawings

FIG. 1 is a schematic explanatory view of a measuring device X used for measuring the amount of water absorption under pressure. In the figure, X is a measuring device, 1 is an electronic balance, 2 is a bottle, 3 is an air intake pipe, 4 is a conduit, 5 is a glass filter, 6 is a measuring unit, 7 is a computer, 8 is saline, and 9 is water absorption. , 60 indicates a cylinder, 61 indicates a nylon mesh, and 62 indicates a weight. BEST MODE FOR CARRYING OUT THE INVENTION

 The method for producing a water-absorbent resin of the present invention is a method for producing a water-absorbent resin having a step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a cross-linking agent. Is performed in the presence of lactic acid and / or a salt thereof. With such a configuration, according to the present invention, a water-absorbent resin excellent in both the water absorption under pressure and the water absorption under pressure can be efficiently produced. In the present specification, the term “water absorption” means both the water absorption under no pressure and the water absorption under pressure. When referring to the weight of the water-absorbent resin, it refers to the dry weight unless otherwise specified. The dry weight refers to the weight (g) of the water-absorbent resin after drying the resin at 105 ° C. for 2 hours.

 "Water-soluble" in the water-soluble ethylenically unsaturated monomer used in the present invention refers to the property of dissolving the monomer in water, and is independent of the dissolution temperature if it is soluble in water. . The monomer is not particularly limited as long as it is a compound having a polymerizable ethylene group.

Specific examples of the monomer include (meth) acrylic acid [“(meth) acryl J” means “acryl” and “methacryl”. The same applies hereinafter), 2- (meth) acrylamide—2-methylpropanesulfonic acid or their metal salts; (meth) acrylamide, N, N-dimethylacrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol Nonionic monomers such as (meth) acrylamide; amino-containing unsaturated monomers such as getylaminoethyl (meth) acrylate and getylaminopropyl (meth) acrylate; and quaternized products thereof. These may be used alone or in combination of two or more. In addition, examples of the alkali metal in the alkali metal salt include lithium, sodium, and potassium, and among them, sodium and / or potassium are preferable from the viewpoint of excellent water absorption and economy. Preferred among the water-soluble ethylenically unsaturated monomers are acrylic acid or an alkali metal salt thereof, methacrylic acid or an alkali metal salt thereof, acrylamide, methacrylamide and the like because of industrial availability. N, N-dimethylacrylamide.

 Usually, the water-soluble ethylenically unsaturated monomer is preferably used as an aqueous solution. The concentration of the water-soluble ethylenically unsaturated monomer in an aqueous solution of a water-soluble ethylenically unsaturated monomer (hereinafter, referred to as a monomer aqueous solution) is preferably from 25% by weight to a saturated concentration. The water used is not particularly limited, and includes tap water, distilled water, ion-exchanged water, and the like.

When the water-soluble ethylenically unsaturated monomer used contains an acid group, the monomer aqueous solution may be used after neutralizing the acid group with an alkali metal. The degree of neutralization by the alkali metal is to increase the osmotic pressure of the resulting water-absorbent resin and increase the water absorption rate, while preventing the safety and other problems from being caused by the presence of excess metal. Therefore, it is preferably 10 to 100 mol% of the acid groups of the water-soluble ethylenically unsaturated monomer before neutralization. Examples of the alkali metal include lithium, sodium, potassium, and the like. Of these, sodium and Z or potassium are preferred. The neutralization of the acid group can be performed, for example, by dropping an aqueous solution of a compound containing an alkali metal such as sodium hydroxide, potassium hydroxide or the like into the monomer aqueous solution and mixing. The concentration of the alkali metal-containing compound in the aqueous solution is not particularly limited, but is usually about 20 to 50% by weight. As the monomer, some water-soluble monomers other than the water-soluble ethylenically unsaturated monomer may be used as long as the desired effects of the present invention are not inhibited. Such a water-soluble monomer can be used, for example, by mixing it with the monomer aqueous solution. The method for polymerizing the monomer is not particularly limited, and examples thereof include a reverse phase suspension polymerization method, an aqueous solution polymerization method, a bulk polymerization method, and a precipitation polymerization method. Among these, from the viewpoints of securing the water absorbing performance of the obtained water-absorbing resin and the ease of controlling the polymerization reaction, A liquid polymerization method or a reversed phase suspension polymerization method is preferable, and a reversed phase suspension polymerization method is particularly preferable. For details of these polymerization methods, see, for example, JP-A-56-131160.

 In the following, as a method for polymerizing the water-soluble ethylenically unsaturated monomer, a reverse phase suspension polymerization method will be described as an example, but the polymerization method is not limited thereto.

 In the reversed-phase suspension polymerization method, an aqueous monomer solution is dispersed in a hydrocarbon-based solvent containing a surfactant and / or a polymer protective colloid, and a water-soluble radical polymerization initiator is used in the presence of a crosslinking agent. The polymerization of the water-soluble ethylenically unsaturated monomer is carried out. Then, in the method for producing a water absorbent resin of the present invention, the polymerization of the monomer is carried out in the presence of lactic acid and / or a salt thereof. The reaction system may further include other components such as a compound containing a transition metal and a chain transfer agent.

 The surfactant, polymer protective colloid, crosslinking agent, hydrocarbon-based solvent, water-soluble radical polymerization initiator, and other components used in the polymerization reaction are not particularly limited, and may be reversed phase. What is usually used may be used in the suspension polymerization method. Examples of the surfactant used in the present invention include nonionic surfactants such as sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, sorbitol fatty acid ester, and polyoxetylene alkylphenyl ether. Anionic surfactants such as fatty acid salts, alkyl benzene sulfonates, alkyl methyl taurates, polyoxyethylene alkyl phenyl ether sulfates, and polyoxetylene alkyl ether sulfonates. Among these, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are preferred.

Examples of the polymer protective colloid used in the present invention include, for example, ethyl cellulose, ethyl hydroxyethyl cellulose, polyethylene oxide, maleic anhydride polyethylene, maleic anhydride polybutadiene, maleic anhydride EPDM (ethyl acetate). Ren / propylene Z Genno Yuichi Polymer).

 The amount of the surfactant and the protective colloid is preferably 0.1 to 5 parts by weight, preferably 0.2 to 3 parts by weight, per 100 parts by weight of the aqueous solution of the water-soluble ethylenically unsaturated monomer. Parts are more preferred. When both are used, the mixing ratio of both is not particularly limited.

 Examples of the water-soluble radical polymerization initiator include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; benzoyl peroxide; Organic peroxides such as oxide; hydrogen peroxide; and azo compounds such as 2,2'-azobis (2-amidinopropane) dihydrochloride. Further, a water-soluble radical polymerization initiator and a sulfite or the like can be used in combination to be used as a redox polymerization initiator. Among them, potassium persulfate, ammonium persulfate, sodium persulfate, benzoyl peroxide and 2,2′-azobis (2-amidinopropane) are preferable from the viewpoint of easy access and good storage stability. Hydrochloride is preferred.

 The amount of the water-soluble radical polymerization initiator is usually 0.000 to 1 mol of the water-soluble ethylenically unsaturated monomer from the viewpoint of shortening the polymerization reaction time and preventing a rapid polymerization reaction. 5 to 0.01 mole is preferred.

 Examples of the hydrocarbon solvent include aliphatic hydrocarbons such as n-hexane, n-heptane, and lignin; alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane, and methylcyclohexane. Hydrogen; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these, n-hexane, n-heptane, and cyclohexane are preferable because they are easily available industrially, have stable quality, and are inexpensive.

The amount of the hydrocarbon solvent is usually 50 to 6 with respect to 100 parts by weight of the water-soluble ethylenically unsaturated monomer from the viewpoint of removing the heat of polymerization and making it easier to control the polymerization temperature. 100 parts by weight is preferable, and 100 to 550 parts by weight is more preferable. Examples of the crosslinking agent used in the present invention include di- or tri (meth) acrylic polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene daricol, polyoxypropylene glycol, and polyglycerin. Acid esters; unsaturated polyesters obtained by reacting the polyols with unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N, N, -methylenebisacrylamide; polyepoxides and Di- or tri (meth) acrylates obtained by reacting acrylic acid; polyisocyanates such as tolylene diisocyanate, hexamethylene diisocyanate and hydroxyethyl (meth) acrylate To obtain (Meth) acrylic acid rubamyl esters; arylated starch, arylated cellulose, diallyl phthalate, N, N ', N''-polymerizable unsaturated groups such as triallyl isocyanate, divinylbenzene, etc. Compound having two or more; (poly) ethylene glycol diglycidyl ether [(poly) means with or without the prefix of “poly”. same as below. Diglycidyl ether compounds such as (poly) propylene glycol diglycidyl ether; And compounds having two or more reactive functional groups, such as isocyanate compounds such as 1,4-tolylene diisocyanate and hexamethylene diisocyanate.

 The amount of the cross-linking agent is determined per 1 mol of the water-soluble ethylenically unsaturated monomer so that the obtained polymer is suppressed in water-soluble properties by moderate cross-linking and exhibits a sufficient water absorption. , 0.001 to 0.01 mole.

Examples of the lactic acid and / or its salt used in the present invention include lactic acid, silver lactate, magnesium lactate, calcium lactate, sodium lactate, potassium lactate and the like. Of the lactic acid or salts thereof, lactic acid is preferably used because it is industrially easily available and inexpensive. Lactic acid and z or a salt thereof are used in an amount of preferably 0.0001 to 0.1 mol, more preferably 0.0001, based on 1 mol of the water-soluble ethylenically unsaturated monomer. To 0.05 mol, more preferably 0.001 to 0.02 mol. When the amount of lactic acid and / or its salt is 0.0001 mol or more, the effect of adding lactic acid and / or Z or its salt is sufficiently obtained, while the amount of lactic acid and / or Z or its salt is reduced. When the amount is 0.1 mol or less, a resin excellent in both the water absorption under no pressure and the water absorption under pressure is preferably obtained.

 A hydrocarbon-based solvent, including the monomer aqueous solution, a surfactant and a polymer or a polymer protective colloid, a water-soluble radical polymerization initiator, and the like, lactic acid and / or a salt thereof, a crosslinking agent, and other components as required. The mixture is heated under stirring and reverse suspension polymerization is carried out in a water-in-oil system. The order of addition of each component can be appropriately adjusted and is not particularly limited. For example, a surfactant and / or a polymer protective colloid and a hydrocarbon solvent, a water-soluble radical polymerization initiator, and lactic acid It is preferable that the monomer aqueous solution is mixed with Z and / or Z or a salt thereof, a cross-linking agent, and if necessary, other components and the aqueous monomer solution in advance, and the resulting mixed solutions are mixed to initiate polymerization. You. Further, a multi-stage polymerization method in which a monomer aqueous solution is divided and added in plural times may be used. The polymerization of the water-soluble ethylenically unsaturated monomer is preferably performed by a multi-stage polymerization method from the viewpoint that the average particle size and the amount of water absorption of the obtained water-absorbent resin are easily controlled. When the polymerization is carried out by a multi-stage polymerization method, the preferred use amount range of each component is based on the total amount of each component used in each stage.

The reaction temperature of the polymerization reaction varies depending on the water-soluble radical polymerization initiator used, but the polymerization proceeds quickly, shortening the polymerization time, improving economic efficiency and easily removing the heat of polymerization. From the viewpoint of performing a smooth reaction, the temperature is preferably from 20 to 110 ° C, more preferably from 40 to 90, and still more preferably from 40 to 8 Ot :. The reaction time is usually preferably between 0.5 and 4 hours. The termination of the polymerization reaction can be confirmed, for example, by confirming that the temperature rise in the reaction system has stopped. to this Thus, the water-absorbing resin is usually obtained in a state of a hydrogel.

 After completion of the polymerization reaction, the obtained water-containing gel is dried using, for example, a reduced-pressure drier, a hot-air drier, or the like, to obtain the water-absorbent resin of the present invention.

 The shape of the obtained water-absorbing resin is not particularly limited, but from the viewpoint of improving the water-absorbing performance, the average particle size of the resin is preferably from 200 to 600 im. The average particle size can be measured, for example, by a method of sieving with a low tap sieve vibrator.

 In the present invention, the mechanism by which a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer in the presence of lactic acid and / or a salt thereof is excellent in both water absorption under pressure and water absorption under pressure Is not clear, but is guessed as follows. Normally, in order to obtain a water-absorbent resin having excellent water absorption under pressure, it is necessary to increase the amount of the crosslinking agent to increase the crosslinking density. The water absorption of the aqueous resin under no pressure is reduced. However, by performing the polymerization reaction in the presence of lactic acid and / or a salt thereof, the crosslinking density can be appropriately increased without increasing the amount of the cross-linking agent to be used. It is presumed that a water-absorbent resin having excellent water absorption under pressure can be obtained. The effect of lactic acid and / or Z or its salt on the microstructure of the water-absorbent resin is not yet known, and it is difficult to define the resin by its structure. The water absorption under pressure and the water absorption under pressure of the obtained water-absorbent resin are clearly superior to those of the conventional one.

 When a carboxyl group is present in the above water-absorbing resin, post-crosslinking treatment (surface cross-linking treatment) using a crosslinking agent containing two or more functional groups having reactivity with the lipoxyl group is performed using gel. It is preferable from the viewpoint of improving strength and water absorption under pressure. Any post-crosslinking agent may be used as long as it can react with the hydroxyl group in the water-absorbing resin. For example, the same cross-linking agent as described above can be used.

The amount of the post-crosslinking agent to be used depends on the amount of water absorption under pressure Although it varies depending on the type of the crosslinking agent, it is usually preferably 0.005 to 7 parts by weight, more preferably 0.005 to 5 parts by weight, and still more preferably 100 parts by weight of the water-soluble ethylenically unsaturated monomer. 0.01 to 2 parts by weight, particularly preferably 0.01 to 1 part by weight. When the amount of the post-crosslinking agent is 0.005 parts by weight or more based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the crosslinking density near the surface of the water-absorbent resin can be sufficiently increased, On the other hand, if the amount is 7 parts by weight or less, the amount of the crosslinking agent used is appropriate.

 The post-crosslinking time of the water-absorbent resin by the post-crosslinking agent may be any time after the completion of the polymerization reaction of the water-soluble ethylenically unsaturated monomer, and is not particularly limited. From the viewpoint of easy control of the water absorption, it is preferable to perform post-crosslinking when the water content of the water-absorbent resin is 5 to 45% by weight. The water content (% by weight) of the water-absorbent resin was determined by measuring the weight a (g) of the water-containing gel after the polymerization reaction and before drying, and the weight b (g) after drying at 105 at 2 hours. The following formula: a (g) one b (g)

Water content (% by weight) = X 100

 b (g).

 The post-crosslinking treatment of the water-absorbent resin with the post-crosslinking agent is carried out, for example, by directly adding the post-crosslinking agent after the polymerization reaction of the water-soluble ethylenically unsaturated monomer to powder or an aqueous solution of about 0.5 to 50% by weight It can be carried out by adding to the water-absorbent resin in the form and reacting them at 50 to 150 ° for about 0.5 to 8 hours.

 After completion of the post-crosslinking reaction, water and a hydrocarbon solvent are distilled off by a known method, whereby a dried product of the post-crosslinked water-absorbing resin can be obtained.

 The water-absorbing resin of the present invention can be used by being mixed with, for example, a gel stabilizer, a metal chelating agent, silica and the like. In addition, if desired, the resin may be used according to a known method.

0 Alternatively, granulation or molding may be performed before use.

 The water-absorbing resin obtained as described above, particularly the post-crosslinked water-absorbing resin, is a water-absorbing resin having excellent water absorption under no pressure and water absorption under pressure. Such a water-absorbing resin is included in the present invention. Further, by using such a water-absorbing resin, for example, hygienic materials such as disposable diapers and sanitary napkins, and industrial materials such as a waterproofing agent for cables, etc., which are very excellent in water absorbing ability, can be obtained. The water-absorbent resin of the present invention is suitably used, among others, for producing sanitary materials. The present invention also provides such a sanitary material.

 The sanitary material and the like can be produced using the water-absorbent resin of the present invention according to a known method (for example, US Pat. No. 5,147,443, Japanese Patent Application Laid-Open No. 5-200006). No. 8). The amount of the resin to be used is not particularly limited, and may be an amount capable of obtaining a desired effect according to the application. Example

 Next, the present invention will be described in more detail based on examples, but the present invention is not limited to only these examples. In the following, each resin was prepared by a reverse phase suspension polymerization method. Example 1

 In a 100-OmL 5-neck cylindrical round bottom flask equipped with a stirrer, reflux condenser, dropping funnel, thermometer and nitrogen gas inlet tube, n-heptane 340 g, sucrose fatty acid ester (Mitsubishi 0.92 g of S-370) was added, and the mixture was dispersed, heated and dissolved, and then cooled to 55 ° C.

 Separately, to a 50 OmL Erlenmeyer flask, 92 g (1.02 mol) of an 80% by weight aqueous acrylic acid solution was added. While cooling from the outside, 102.2 g (0.76 mol) of a 30% by weight aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of acrylic acid. 50.2 g of water and water-soluble radical polymerization initiator persulfuric acid

1 Add potassium llg (0.4 lmmo 1), crosslinker ethylene glycol diglycidyl ether 8.3 mg (0.047 mmo 1), lactic acid 0.46 g (0.5 lmmo 1) and polymerize in the first stage An aqueous monomer solution was prepared.

 This monomer aqueous solution was added and dispersed in the above-mentioned five-necked cylindrical round-bottom flask under stirring with stirring, and the system was sufficiently purged with nitrogen, and then heated, and the bath temperature was maintained at 70 ° C. Then, after the first-stage polymerization reaction was performed for 1 hour, one polymerization slurry solution was cooled to room temperature.

 In a separate 50 OmL Erlenmeyer flask, 11.1 g (1.32mo1) of an 80% by weight aqueous solution of acrylic acid was added, and while cooling, 132.2g of a 30% by weight aqueous solution of sodium hydroxide (0.92mo1) was added. ) To neutralize 75 mol% of acrylic acid, then add 27.4 g of water, 0.14 g of potassium persulfate (0.52 mm o 1), 0.059 g of lactic acid (0.66 mmo) 1) was added to prepare a monomer aqueous solution for the second-stage polymerization, which was cooled in an ice-water bath.

 After the entire amount of the aqueous monomer solution for the second stage polymerization was added to the polymerization slurry solution, the inside of the system was sufficiently replaced with nitrogen again, and the temperature was raised.The bath temperature was maintained at 7 Ot: The polymerization reaction of the eyes was performed for 2 hours. After the completion of the polymerization, the mixture was heated in an oil bath at 120 ° C., and only 260 g of water was removed from the system by azeotropic distillation to obtain a gel. The water content of the obtained gel was 30% by weight.

 8.15 g of a 2% by weight aqueous solution of ethylene dalicol diglycidyl ether was added to the obtained gel, and water and n-heptane were removed by distillation and dried to obtain 213.5 g of a water-absorbent resin. . The average particle size of the resin was 356.

 Here, the average particle size of the water-absorbent resin is the integrated weight obtained by sequentially integrating the weight of the water-absorbent resin remaining on each sieve when the water-absorbent resin is classified by a sieve, This corresponds to the sieve opening when it reaches 50% by weight of the total weight.

 Specifically, 100 g of a water-absorbent resin was weighed, and this was weighed into seven standard sieves (850 m, 500 urn, 300 m, 250 m from the top) corresponding to JIS-Z 8801-1982.

2 τη, 180 rn, 1 06 ^ m, 75 m open sieves are stacked one on top of the other.) Put the water-absorbent resin into the top sieve and use a low-tap sieve vibrator. After sieving by vibrating for 1 minute, weighing is carried out, and based on the result, the cumulative weight becomes 50% by weight. The particle diameter is calculated by the following formula: Average particle diameter (; X (D—B) + B.

 In the formula, A is the value obtained when the integrated value is obtained when the integrated weight is less than 50% by weight and the integrated value at the time closest to 50% by weight is calculated. The integrated value (g), and B is the opening m) of the sieve at the time when the integrated value was obtained. C is the value obtained by sequentially calculating the weight from the larger particle size and obtaining the integrated value at the time when the integrated weight is 50% by weight or more and closest to 50% by weight. g), and D is the sieve opening (β m) at the time when the integrated value was obtained. Example 2

 In Example 1, the amount of lactic acid in the aqueous monomer solution for the first-stage polymerization was 0.460 g (5.10 mmol), and the amount of lactic acid in the aqueous monomer solution for the second-stage polymerization was 20.1 g of a water-absorbent resin was obtained in the same manner as in Example 1 except that the weight was changed to 0.590 g (6.58 mmo 1). The average particle size of the resin was 372 m. Example 3

In Example 1, the amount of lactic acid in the aqueous monomer solution for the first stage polymerization was 0.230 g (2.55 mmo 1), and the amount of lactic acid in the aqueous monomer solution for the second stage polymerization was Of water-absorbent resin 2 in the same manner as in Example 1 except that the amount of water-absorbent resin 2 was changed to 0.295 g (3.29 mmo 1). 18.2 g were obtained. The average particle size of the resin was 360 m. Comparative Example 1

 212.1 g of a water-absorbent resin was obtained in the same manner as in Example 1 except that lactic acid was not used. The average particle size of the resin was 345 m. Comparative Example 2

 340 g of n-heptane, sucrose fatty acid ester (produced by Mitsubishi Chemical Corporation) in a 100-OmL five-neck cylindrical round bottom flask equipped with a stirrer, reflux condenser, dropping funnel, thermometer and nitrogen gas inlet tube Trade name: S-370) 0.92 g was added, and the mixture was dispersed, heated and dissolved, and then cooled to 55 ° C.

 Separately, a 50 OmL Erlenmeyer flask was charged with 92 g (1.02 mol) of an 80% by weight aqueous solution of acrylic acid. While cooling from the outside, 102.2 g (0.76 mol) of a 30% by weight aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of acrylic acid. In addition, 50.2 g of water, 0.1 llg (0.4 lmmo 1) of potassium persulfate as a water-soluble radical polymerization initiator, and 8.3 mg (0.047 mmo 1) of ethylene glycol diglycidyl ether as a cross-linking agent were added. This was added to prepare a monomer aqueous solution for the first-stage polymerization.

 This monomer aqueous solution was added and dispersed in the above-mentioned five-necked cylindrical round-bottom flask under stirring with stirring, and the system was sufficiently purged with nitrogen, and then heated, and the bath temperature was maintained at 70 ° C. Then, after performing the first-stage polymerization reaction for 1 hour, one polymerization slurry solution was cooled to room temperature.

 In a separate 50 OmL Erlenmeyer flask, 11.1 g (1.32mo1) of an 80% by weight aqueous solution of acrylic acid was added, and while cooling, 132.2g of a 30% by weight aqueous solution of sodium hydroxide (0.92mo1) was added. ) To neutralize 75 mol% of acrylic acid, and add 27.4 g of water and 0.14 g (0.52 mmol) of potassium persulfate to obtain monomer for second-stage polymerization. An aqueous solution was prepared and cooled in an ice-water bath.

Four After the whole amount of the monomer aqueous solution for the second stage polymerization was added to the polymerization slurry solution, the inside of the system was sufficiently replaced with nitrogen again, and the temperature was raised. The polymerization reaction of the eyes was performed for 2 hours. After completion of the polymerization, the mixture was heated in an oil bath at 120 ° C., and only 260 g of water was removed from the system by azeotropic distillation to obtain a gel. The water content of the obtained gel was 30% by weight.

 A solution of 0.105 g (1.17 mmo 1) of lactic acid in 8.158% of a 2% by weight aqueous solution of ethylene dalicol diglycidyl ether was added to the obtained gel, and water and n-heptane were distilled off. And dried to obtain 213.5 g of a water absorbent resin. The average particle size of the resin was 356 m. Test example

 The water-absorbent resins obtained in the above Examples and Comparative Examples were evaluated by the following methods. The measurement of the water absorption of the water-absorbing resin was performed at room temperature.

 (1) Water absorption under no pressure of water absorbent resin

 The water absorption of the water-absorbent resin under no pressure was determined as the water absorption of physiological saline by the resin under atmospheric pressure, and the water-absorbent resins obtained in the above Examples and Comparative Examples were compared.

2.0 g of the water-absorbent resin was weighed out into a cotton bag (membrane No. 60, width 100 mm x length 200 mm), and placed in a 50 OmL beaker. 500 g of physiological saline was poured into the cotton bag at a time, and the saline was dispersed so as not to cause water absorbent resin mamako. The upper part of the cotton bag was tied with a rubber band and left for 1 hour to sufficiently swell the water-absorbent resin. The cotton bag is dehydrated for 1 minute using a dehydrator (manufactured by Domestic Centrifuge Co., Ltd., product number: H-122) set to have a centrifugal force of 167G. The weight Wa (g) was measured. The same operation was performed without adding the water-absorbent resin, the wet weight of the cotton bag when wet Wb (g) was measured, and the water absorption under no pressure was calculated from the following equation. Wa (g) -Wb (g)

Water absorption under no pressure (g / g) =

 Weight of water absorbent resin (g)

(2) Water absorption under pressure of water-absorbent resin

 The water absorption under pressure of the water-absorbent resin was determined as the water absorption of physiological saline by the resin under a pressure of 2.07 kPa, and the water-absorbent resins obtained in the above Examples and Comparative Examples were compared. .

 The amount of water absorption under pressure of the water-absorbent resin was measured using a measuring device X shown in FIG. The measuring device X shown in FIG. 1 is composed of an electronic balance 1, a bottle 2 placed on the electronic balance 1, an air suction pipe 3, a conduit 4, a glass filter 1 and a glass filter 1. 5 and a measuring unit 6 placed on it.

 The electronic balance 1 is connected to a computer 7 so that the weight change can be recorded in seconds or minutes. The bottle 2 holds a physiological saline solution 8 therein, and has an air suction pipe 3 inserted into an opening at the top, while a conduit 4 is attached to the body. The lower end of the air suction pipe 3 is immersed in a physiological saline solution 8. The diameter of the glass filter 5 is 25 mm. As the glass filter 5, a glass filter No. 1 (pore size: 100 to 160 τα) of the Mutual Physical and Chemical Glass Laboratory was used.

 Bottle 2 and glass filter 5 are connected to each other by conduit 4. The glass filter 5 is fixed at a position slightly higher than the lower end of the air suction pipe 3. The measuring section 6 has a cylinder 60, a nylon mesh 61 adhered to the bottom of the cylinder 60, and a weight 62 having a diameter of 19 mm and a weight of 59.8 g. The inner diameter of the cylinder 60 is 20 mm. The nylon mesh 61 is formed in a 200 mesh (mesh size: 75 m). Then, a predetermined amount of the water-absorbing resin 9 is evenly spread on the nylon mesh 61. Weight 62

6 It is placed on the water-absorbent resin 9 so that a load of 2.07 kPa can be uniformly applied to the water-absorbent resin 9.

 In the measuring device X having such a configuration, first, a predetermined amount of the physiological saline 8 and the air suction pipe 3 are put into the bottle 2 to prepare for the measurement. Next, 0.10 g of the water-absorbent resin 9 is evenly spread on the nylon mesh 61 of the cylinder 60, and the weight 62 is placed on the water-absorbent resin 9. The measuring unit 6 is placed on the glass filter 5 so that the center of the measuring unit 6 is aligned with the center of the glass filter 5.

 On the other hand, when the computer 7 connected to the electronic balance 1 is started and the water absorption is started, the reduced weight of the physiological saline 8 in the bottle 2 (the physiological saline absorbed by the water-absorbing resin 9) continues from the time when the water absorption is started. The weight of water 8) Wc (g) was recorded on the computer 7 in seconds based on the value obtained from the electronic balance 1. The water absorption under pressure of the water-absorbent resin 9 after a lapse of 60 minutes from the start of water absorption was determined by dividing the weight Wc (g) after the lapse of 60 minutes by the weight of the water-absorbent resin 9 (0.10 g).

 Table 1 summarizes the evaluation results obtained by the above methods (1) and (2). table 1

表 1に示された結果から、 実施例 1〜 3で得られた吸水性樹脂は、 いずれも、 加圧下吸水量については比較例 1〜2と同レベルであるものの、 無加圧下吸水量 は多く、 無加圧下吸水量および加圧下吸水量の両方に優れたものであることがわ かる。 比較例 1の吸水性樹脂は重合反応を乳酸の非存在下に行って得られたもの であり、 一方、 比較例 2の吸水性樹脂は比較例 1と同様にして重合反応を行い、 反応終了後に乳酸で処理して得られたものであるが、 無加圧下吸水量については、 実施例の吸水性樹脂に比し明らかに劣っていることがわかる。 産業上の利用可能性

From the results shown in Table 1, the water-absorbing resins obtained in Examples 1 to 3 all have the same level of water absorption under pressure as Comparative Examples 1 and 2, but water absorption under no pressure is It is clear that both water absorption under pressure and water absorption under pressure are excellent. Call The water-absorbent resin of Comparative Example 1 was obtained by performing a polymerization reaction in the absence of lactic acid, while the water-absorbent resin of Comparative Example 2 performed a polymerization reaction in the same manner as in Comparative Example 1, and the reaction was terminated. It was obtained by subsequent treatment with lactic acid, and it can be seen that the water absorption under no pressure is clearly inferior to the water-absorbent resin of the example. Industrial applicability

 The present invention provides a method for producing a water-absorbent resin that can be suitably used in the field of sanitary materials such as disposable diapers and sanitary napkins.

8

Claims

The scope of the claims 1. A method for producing a water-absorbent resin having a step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a crosslinking agent, wherein the polymerization of the monomer is further carried out by lactic acid and / or A method for producing a water-absorbent resin, which is performed in the presence of a salt. 2. The production of the water-absorbent resin according to claim 1, wherein the amount of lactic acid and Z or a salt thereof is 0.0001 to 0.1 mol per 1 mol of the water-soluble ethylenically unsaturated monomer. 3. The method for producing a water-absorbent resin according to claim 1, wherein the polymerization is performed by an aqueous solution polymerization method or a reverse phase suspension polymerization method. 4. A water-absorbent resin obtained by the method for producing a water-absorbent resin according to any one of claims 1 to 3. 5. A sanitary material comprising the water absorbent resin according to claim 4. 9