JP2995276B2 - Manufacturing method of water absorbent resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a water-absorbing resin. More specifically, the present invention relates to a method for producing a water-absorbent resin having a reduced residual monomer content.

[0002]

2. Description of the Related Art Conventionally, a hydrated polymer obtained by polymerizing an acrylic acid salt and / or a monomer containing acrylic acid as a main component and a crosslinkable monomer in an aqueous solvent and, if necessary, neutralizing the polymer is heated. As a method for reducing the residual monomer content in the solid water-absorbing resin obtained by drying, a method of adding and mixing a reducing substance after polymerization (Japanese Patent Laid-Open No. 1-62317), an aqueous solution of the reducing substance in water-absorbing resin powder (JP-A-4-106108), a method of adding and mixing a peroxide and / or an azo compound after polymerization (JP-A-1-103644),
A method in which an aqueous solution of a radical polymerization initiator is sprayed on a water-absorbent resin powder and heated (JP-A-64-24808) has been proposed.

[0003]

However, in these methods, although the residual monomer is reduced, a side reaction such as cleavage of a main chain polymer or cleavage of a crosslinking point due to excessive radicals generated during heating and drying is caused. There is. As a result, there arises a problem that the absorption performance is decreased due to the decrease in the molecular weight and the amount of the water-soluble component is increased. Furthermore, since compounds such as reducing substances, peroxides, and azo compounds, which are unnecessary components in principle, are used as components that exhibit a water absorbing function, these compounds or their decomposition products are contained in products. Remains, and depending on the type of compound used, safety may be an issue.

[0004]

Means for Solving the Problems In view of the above problems, the present inventors have developed a method for reducing the residual monomer content in a water-absorbing resin without using an unnecessary compound as a water-absorbing resin component. As a result of intensive studies, the present invention has been reached.

That is, the present invention provides a dimer acid content of 0.1.
5% by weight or less of a non-neutralized acrylic acid as a main component, and a water-containing polymer obtained by polymerizing a polymerizable monomer component containing a crosslinkable polymerizable monomer in an aqueous solvent is mixed with an alkaline substance. A method for producing a water-absorbent resin having a reduced residual monomer content, which comprises partially neutralizing so as to have a degree of 50 to 90 mol% and drying by heating to produce a solid water-absorbent resin. .

In the present invention, the content of dimer acid in acrylic acid is 0.5% or less, preferably 0.1% by weight.
35% or less, more preferably 0.3% or less. When the dimer acid content exceeds 0.5% by weight, the residual monomer content in the water-absorbent resin is extremely increased, which is problematic in terms of safety. In particular, when the water-absorbent resin is used directly or indirectly in contact with the skin, for example, when used as an absorbent for sanitary goods (sanitary napkins, children's and adult paper diapers, etc.), the residual water-absorbent resin remains in the water-absorbent resin. It is important to reduce the monomer content. Here, the dimer acid content is a value measured by a method using liquid chromatography described later.

In the present invention, the dimer acid is a dimer formed by the addition reaction of two molecules of acrylic acid represented by the following formula. 2CH ₂ ＣＨCHCOOH → CH ₂ ＣＨCHCOOCH ₂ CH ₂ COOH This dimer acid is produced during the production of an acrylic acid monomer, but most of it is usually removed by a distillation operation or the like. However, dimer acid is gradually generated during storage of acrylic acid, and may reach a dimer acid content of several weight% order depending on storage conditions and temperature (especially in summer). Since dimer acid has a polymerizable double bond, it is copolymerized with monomers such as acrylic acid by polymerization and is incorporated into the main chain polymer, which is a particularly serious problem at the stage after polymerization. Are few and overlooked.
The trouble is that the dimer acid once incorporated into the main chain polymer is decomposed by the heat of heating and drying the water-containing polymer to produce acrylic acid again, which remains as a residual monomer in the dried product. is there. Furthermore, the generation of acrylic acid by decomposition of dimer acid is also caused when the surface of the resin obtained by heating and drying is crosslinked by heating with a crosslinking agent having at least two groups capable of reacting with a functional group in the polymer. Occur.

[0008]

[0009]

In the present invention, a polymerizable monomer component containing acrylic acid as a main component and a crosslinkable polymerizable monomer is used, and after polymerization in an aqueous solvent, the water-containing polymer is partially neutralized with an alkaline substance. Examples of the alkaline substance used for the neutralization include alkali metal compounds (such as sodium hydroxide and potassium hydroxide), alkali metal carbonates (such as sodium carbonate and sodium bicarbonate), ammonia, and amine compounds (such as methylamine and trimethylamine). Alkylamines; alkanolamines such as triethanolamine and diethanolamine) and two or more of these;
The degree of neutralization is usually 50 to 90 mol%, preferably 60 to 85 mol%.
Mol%. When the degree of neutralization is less than 50 mol%, the resulting hydropolymer has high tackiness, and it is difficult to heat-dry the water-absorbent resin with good workability. If it exceeds 90 mol%, the pH of the resulting water-absorbent resin becomes alkaline, which is not preferable from the viewpoint of safety on human skin.

In the present invention, in order to improve the strength of the water-absorbing gel,
The crosslinkable polymerizable monomer is used. Examples of the crosslinkable polymerizable monomer include the following (1) and (2). (1) a compound having at least two polymerizable double bonds; and (2) a functional group having at least one polymerizable double bond and reactive with a monomer (for example, a carboxyl group of acrylic acid). Having at least one of

Examples of the compound of the above (1) include the following. Bis (meth) acrylamide: N, N'-alkylene (C
₁ -C ₆₎ bis (meth) acrylamides example N, N'-methylene bisacrylamide. Di- or polyester of polyols and unsaturated mono- or polycarboxylic acids: di- or tri- (meth) acrylic acid of polyols [ethylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, etc.] Ester: unsaturated polyester [obtained by reacting the above polyols with an unsaturated acid such as maleic acid] and diester
-Or tri- (meth) acrylate [obtained by the reaction of polyepoxide with (meth) acrylic acid]
Such. Carbamyl ester: polyisocyanate [tolylene diisocyanate, hexamethylene diisocyanate,
4,4'-diphenylmethane diisocyanate and NCO group-containing prepolymer (obtained by reacting the above polyisocyanate with an active hydrogen atom-containing compound) and the like, and carbamyl ester obtained by the reaction of hydroxyethyl (meth) acrylate . Di or polyvinyl compounds: divinyl benzene, divinyl toluene, divinyl xylene, divinyl ether,
Divinyl ketone, trivinyl benzene, etc. Di- or poly- (meth) allyl ethers of polyols: di- or poly- (meth) allyl ethers of polyols [alkylene glycols, glycerin, polyalkylene glycols, polyalkylene polyols, carbohydrates, etc.] such as polyethylene glycol diallyl ether and Allylated starch, allylated cellulose. Di- or poly-allyl esters of polycarboxylic acids: diallyl phthalate, diallyl adipate and the like. Esters of unsaturated mono- or poly-carboxylic acids with mono (meth) allyl ethers of polyols: such as (meth) acrylates of polyethylene glycol monoallyl ether. Allyloxyalkanes: tetraallyloxyethane, neopentyl glycol triallyl ether and the like.

Examples of the compound (2) include groups reactive with (meth) acrylic acid and / or other copolymerizable monomers such as carboxyl groups and groups reactive with carboxylic anhydride groups (hydroxyl groups). Group, an epoxy group, a cationic group, etc.).
Specifically, non-ionic group-containing unsaturated compounds such as hydroxyl group-containing unsaturated compounds [N-methylol (meth) acrylamide and the like] and epoxy group-containing unsaturated compounds [glycidyl (meth) acrylate and the like] and cationic group-containing unsaturated compounds Saturated compounds such as quaternary ammonium base-containing unsaturated compounds [N, N, N-trimethyl-N- (meth) acryloyloxyethyltrimethylammonium chloride, N, N, N-triethyl-N- (meth) acryloyloxy Ethylammonium chloride], and 3
And quaternary amino group-containing unsaturated compounds [dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, etc.].

Among these crosslinking monomers, preferred are the crosslinking monomers of (1), and more preferred are bis (meth) acrylamides, di (meth) acrylamides and diamines of polyols and unsaturated monocarboxylic acids. -Or poly-esters and allyloxyalkanes, particularly preferred are N, N'-methylenebisacrylamide, ethylene glycol diacrylate, trimethylolpropane triacrylate, tetraallyloxyethane and neopentyl glycol triallyl ether.

The amount of the crosslinkable monomer is usually 0.001 to 10% by weight, preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight based on the total weight of the monomer and the crosslinkable monomer. It is. When the amount of the crosslinkable monomer is less than 0.001% by weight, the resulting water-absorbent resin has a low gel strength after water absorption and becomes a sol. On the other hand, if it exceeds 10% by weight, on the contrary, the strength of the gel becomes too high and the absorption performance is reduced.

Other polymerizable monomers which can be used together with acrylic acid and the crosslinkable polymerizable monomer as needed as the polymerizable monomer component include, for example, unsaturated mono- or polycarboxylic acid. [Methacrylic acid, crotonic acid, sorbic acid, maleic acid, itaconic acid, maleic anhydride, etc.], monomers containing sulfonic acid groups [vinyl sulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, etc. Aliphatic or aromatic vinyl sulfonic acid; sulfoethyl (meth) acrylate,
(Meth) acrylsulfonic acid such as sulfopropyl (meth) acrylate, (meth) acrylamidesulfonic acid such as 2-acrylamido-2-methylpropanesulfonic acid, etc., and a polymerizable monomer having a phosphoric acid group [2 -Hydroxyethyl (meth) acryloyl phosphate, phenyl-2-acryloyloxyethyl phosphate, etc.], alkyl (C ₁ -C ₈ ) esters of unsaturated carboxylic acids [methyl methacrylate, etc.], (meth) acrylamide and its derivatives, acetic acid Vinyl and the like. The amount of the other polymerizable monomers in the total polymerizable monomers is
It is usually 0 to 30%, preferably 0 to 20% on a weight basis. When the amount of the other polymerizable monomer exceeds 30% by weight, the absorption performance of the obtained water-absorbent resin is reduced.

If necessary, a polysaccharide can be used together with acrylic acid and, if necessary, other polymerizable monomers. Examples of polysaccharides include starch (raw starch such as wheat starch; modified starch such as oxidized starch, alkyl etherified starch, and oxyalkylated starch), cellulose [cellulose obtained from wood, leaves, stems, seed hairs, and the like; Alkyl etherified cellulose,
Organic acid esterified cellulose, oxidized cellulose, processed cellulose such as hydroxyalkyl etherified cellulose, etc.]. The amount of the polysaccharide is usually 0 to 30% by weight, preferably 0 to 20% by weight based on the total amount of the polymerizable monomer.
% By weight. When the amount of the polysaccharide exceeds 30% by weight, the absorption performance decreases.

In the present invention, the polymerization is carried out in an aqueous solvent. If necessary, a water-soluble organic solvent may be used in combination.
For example, lower alcohols (such as methanol and ethanol), ketones (such as acetone and methyl ethyl ketone), dimethyl sulfoxide, and dimethylformamide are exemplified. The amount of these water-soluble organic solvents is usually 30% by weight or less, preferably 20% by weight, based on the total amount of the solvent.
It is as follows.

The polymerization method in the present invention may be a conventionally known method, for example, an aqueous solution polymerization method using a radical polymerization catalyst, an inverse suspension polymerization method and the like. Further, a polymerization method of irradiating radiation, an electron beam, ultraviolet light, or the like can be employed.

In a method using a radical polymerization catalyst (for example, an aqueous solution polymerization method or an inverse suspension polymerization method), examples of the polymerization catalyst include an azo compound [azobiscyanovaleric acid,
2'-azobis (2-amidinopropane) hydrochloride, etc.], inorganic peroxides [hydrogen peroxide, ammonium persulfate, potassium persulfate, sodium persulfate, etc.], organic peroxides [benzoyl peroxide, cumene hydroperoxide] , Succinic peroxide, etc.) and redox catalysts [reducing agents such as alkali metal sulfites or bisulfites, ammonium sulfite, ammonium bisulfite, ascorbic acid and alkali metal persulfates, ammonium persulfate, peroxides etc. Consisting of a combination of oxidizing agents] and two or more of these.

The amount of the polymerization catalyst may be the same as usual, for example, usually 0.0001 to 5% by weight, preferably 0.0005 to 1% by weight based on the total weight of all polymerizable monomers and crosslinkable monomers.
It is. Other polymerization conditions (polymerization concentration, polymerization initiation temperature,
The polymerization time, the aging temperature, etc.) may be the conventionally known conditions. For example, the polymerization concentration is usually 10 to 80% by weight, preferably 15 to 70% by weight, and more preferably 20 to 50% by weight.

In the present invention, the solvent in the water-containing polymer is removed by heating and drying. Heat drying temperature is usually 110-2
30 ° C, preferably 120-200 ° C, more preferably 120-180
° C. When the drying temperature is lower than 110 ° C., it is unlikely that the dimer acid is decomposed by the heat generated during the heat drying to produce acrylic acid again, but the drying takes too much time and is inefficient. On the other hand, if the drying temperature exceeds 230 ° C., the resin deteriorates or undergoes thermal decomposition, and the absorption performance is reduced. Furthermore, the rapid decomposition of the dimer acid increases the residual monomer content even if the drying time is reduced. The temperature during drying does not necessarily need to be kept constant. The first half of drying, in which the water content of the water-containing polymer is high, is treated at a high temperature (for example, 180 to 200 ° C.), and the second half of drying, in which the water content is low, is relatively low. Processing at lower temperatures (eg, 130-150 ° C.) is also possible.

The drying time can be arbitrarily selected depending on the solvent content in the water-containing polymer, the drying temperature and the drying method, and is not particularly limited, but is usually 1 to 120 minutes, preferably 3 to 60 minutes, more preferably 3 to 60 minutes. ~ 45 minutes. When drying at a high temperature, the shorter the drying time, the better.

The method of drying may be a known method, for example, a method in which a water-containing polymer is finely crushed if necessary, laminated on a perforated plate, a wire mesh, a flat plate, a belt, etc., and dried with hot air, a rotary kiln or a fluidizer. A method of flowing a material to be dried by a drier or the like and drying with hot air, a method of heating and drying by contacting a hydropolymer with a thin plate on the surface of a hot plate or a heat roller, a method of heating and drying under reduced pressure, infrared rays or microwaves. Drying method and the like are mentioned. In order to shorten the drying time, two or more of these methods may be combined. A preferred drying method for practicing the present invention is a method of laminating a water-containing polymer on a perforated plate, a wire mesh, a flat plate, a belt, or the like, and drying with hot air, and a method of fluidizing a material to be dried by a rotary kiln or a fluidized drier. Hot air drying method such as hot air drying method. The solid water-absorbent resin obtained by heating and drying does not necessarily have to be in a completely dry state, and has a water content that does not hinder handling as a solid (for example,
(Moisture content: 0.5 to 10% by weight).

In the present invention, the cross-linking agent having at least two groups capable of reacting with the functional groups in the polymer is obtained by heating and drying and, if necessary, pulverizing and adjusting the particle size to obtain a solid water-absorbing resin. Thus, a water-absorbent resin having higher gel strength and improved absorption characteristics can be produced by cross-linking by heating in a usual manner. Examples of the crosslinking agent used for the heating surface crosslinking include polyglycidyl compounds such as ethylene glycol diglycidyl ether, glycerin-1,3-diglycidyl ether, glycerin triglycidyl ether, and polyethylene glycol diglycidyl ether; glycerin, ethylene glycol And polyol compounds such as polyethylene glycol; and polyamine compounds such as ethylene diamine and polyamide polyamine epichlorohydrin resin. Among these, preferred crosslinking agents are polyglycidyl compounds, polyol compounds and polyamine compounds.

The residual monomer content of the water-absorbing resin thus obtained is usually 500 ppm or less, and the residual monomer content becomes 400 ppm or less by selecting more preferable conditions.

[0027]

The present invention will be further described with reference to the following examples and comparative examples, but the present invention is not limited to these examples. The dimer acid content, residual monomer content, absorption amount and water-soluble component amount were measured by the following methods. Hereinafter, unless otherwise specified,% indicates% by weight.

Dimer acid content: 1.00 g of raw acrylic acid is placed in a 1-liter volumetric flask, and distilled water is added to make 1 liter, which is used as a measurement sample. The sample is injected into a high-performance liquid chromatograph [Hi-SepLC, manufactured by Shimadzu Corporation] to determine the peak area of dimer acid. Separately, a calibration curve (a graph showing the relationship between the amount of the dimer acid and the peak area) is created from a dimer acid aqueous solution having a known concentration, and the dimer acid content in the monomer is determined from the calibration curve. Measurement conditions Column: SCR-101H, 0.3
MX7.9 mul Developing solvent: 0.015% H ₃ PO ₃ purified aqueous solution Flow rate: 0.5 ml / min Sample injection volume: 100 μl Detector: UV detector, wavelength 195 nm Residual monomer content: water absorbent resin composition in 300 ml beaker Then, add 1.0 g of the product, add 249.0 g of 0.9% saline, and stir for 3 hours. The filtrate obtained by filtering the gel after extraction with filter paper is used as a measurement sample. The sample solution is injected into the high performance liquid chromatography to determine the peak area of the remaining monomer. Separately, a calibration curve (a graph showing the relationship between the monomer amount and the peak area) is prepared from an aqueous solution of an acrylic acid monomer having a known concentration, and the residual monomer content is determined from the calibration curve. The measurement conditions were the same as for the dimer acid content. Absorption: 1.0 g of the water-absorbent resin composition was placed in a 250-mesh nylon net tea bag, immersed in 0.9% saline for 1 hour to absorb water, drained for 15 minutes, and the weight gain was measured. Was taken as the absorption capacity. Amount of water-soluble component: The amount of water-soluble component after 3 hours of extraction was measured by the method described in JP-A-62-54751.

Example 1 A reaction vessel equipped with a thermometer and a nitrogen gas inlet tube was charged with 100 g of acrylic acid having a dimer acid content of 0.18%, 0.2 g of N, N'-methylenebisacrylamide and 300 g of deionized water, and stirred. The temperature of the contents was kept at 10 ° C. After introducing nitrogen gas into the contents to reduce the dissolved oxygen content to 1 ppm or less, 0.005 g of hydrogen peroxide, 0.0025 g of ascorbic acid, and 0.1 g of 2,2'-azobis (amidinopropane) hydrochloride are added for polymerization. Was started. While kneading the water-containing polymer obtained by polymerization for about 5 hours with an extruder equipped with a perforated plate, 116 g of a 35% aqueous sodium hydroxide solution was added to neutralize 73 mol% of acrylic acid, and at the same time, 1 cm ³ or less. Into small pieces. The neutralized water-containing polymer was laminated on a wire net to a thickness of about 2 cm and dried with hot air at 140 to 150 ° C for 45 minutes. The obtained dried product was pulverized to a particle size of 20 to 100 mesh to obtain a water-absorbent resin composition (1) of the present invention. Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin. 1 and 2 show liquid chromatographic charts showing the dimer acid content and the residual monomer content, respectively.

Examples 2 to 4 The water-absorbent resin composition (2) of the present invention was prepared in the same manner as in Example 1 except that the dimer acid content in the monomer was changed to the amount shown in Table 1. To (4) were obtained. Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin.

Example 5 The neutralized water-containing polymer obtained in the same manner as in Example 1 was stretched in the form of a thin film on the surface of a drum dryer heated to 190 ° C. and dried for 5 minutes. The obtained dried product was pulverized to a particle size of 20 to 100 mesh to obtain a water-absorbent resin composition (5) of the present invention. Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin.

Example 6 A 100% resin composition obtained in the same manner as in Example 1 was coated on the particle surface with 10% of ethylene glycol diglycidyl ether.
3 g of the aqueous solution was uniformly spray-added and heat-treated at 140 ° C. for 20 minutes to obtain a surface-crosslinked water-absorbent resin composition (6). Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin.

Reference Example 1 A 1-liter stainless steel reaction vessel was charged with 78 g of sodium acrylate and 22 g of acrylic acid. The dimer acid content in this monomer was 0.18%. Further, 0.2 g of N, N'-methylenebisacrylamide and 186 g of deionized water were charged, and the temperature of the contents was maintained at 10 ° C while stirring and mixing. After injecting nitrogen into the contents to reduce the dissolved oxygen content to 1 ppm or less, hydrogen peroxide 0.00
5 g, 0.0025 g of ascorbic acid and 2,2′-
Azobis (amidinopropane) hydrochloride
2 g was added to initiate the polymerization. The hydrated polymer obtained by polymerization for about 5 hours was crushed into small pieces of 1 cm ³ or less using a gel crusher (manufactured by Horai Tekko Co., Ltd.), and then laminated on a wire mesh to a thickness of about 2 cm. Hot air drying was performed at ~ 150 ° C for 45 minutes. The obtained dried product was pulverized to a particle size of 20 to 100 mesh to obtain a water-absorbent resin composition (7).
Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin.

Reference Example 2 In a 200 ml flask, 32.7 g of acrylic acid and 39.3 g of deionized water were charged, and 28.0 g of a 48% aqueous sodium hydroxide solution was gradually added with stirring while cooling to 20 to 30 ° C. After neutralizing 74 mol% of acrylic acid by dropping, the dimer acid content was checked and found to be 0.18%. After dissolving 0.033 g of N, N'-methylenebisacrylamide in the aqueous monomer solution and then adding 0.1 g of potassium persulfate and dissolving at room temperature, the nitrogen solution is introduced by flowing nitrogen gas. The amount of oxygen was set to 1 ppm or less. 400 g of n-hexane was charged into a 1-liter flask equipped with a reflux condenser, and 5 g of sorbitan monostearate was dissolved. Then, nitrogen gas was introduced to reduce the amount of dissolved oxygen in the solution to 1 ppm or less. Then, the temperature was maintained at about 60 ° C. by a warm bath, and the above-mentioned aqueous solution of a polymerizable monomer containing potassium persulfate was added dropwise with stirring to carry out polymerization for about 3 hours, followed by aging for further 2 hours. Thereafter, the water-containing polymer was filtered off from the polymerization dispersion, and n-hexane was removed with a centrifugal dehydrator to obtain a pearl-shaped water-containing polymer. The hydrated polymer is heated at 140 to 150 ° C. for 40 minutes.
Water-absorbent resin composition (8)
I got Table 1 shows the results of measurement of the residual monomer content, absorption amount, and water-soluble component amount of this resin.

Comparative Example 1 A comparative water-absorbent resin composition (a) was obtained in the same manner as in Example 1 except that a monomer having a dimer acid content of 0.77% was used. Table 1 also shows the results of measuring the performance of the resin. 1 and 2 show liquid chromatographic charts showing the dimer acid content and the residual monomer content.

Comparative Example 2 A comparative water-absorbent resin composition was prepared in the same manner as in Comparative Example 1 except that an aqueous solution in which 0.8 g of sodium bisulfite was dissolved in 10 g of water was added to the neutralized water-containing polymer. (B) was obtained. Table 1 also shows the results of measuring the performance of the resin.

COMPARATIVE EXAMPLE 3 In Example 1, a monomer having a dimer acid content of 0.93% was used.
Example 1 except that an aqueous solution of 0 g dissolved in 10 g of water was added.
In the same manner as in the above to obtain a comparative water-absorbent resin composition (C). Table 1 also shows the results of measuring the performance of the resin.

Comparative Example 4 A comparative water-absorbent resin composition (d) was obtained in the same manner as in Example 7, except that a monomer having a dimer acid content of 0.93% was used. Table 1 also shows the results of measuring the performance of the resin.

Comparative Example 5 A comparative water-absorbent resin composition (e) was obtained in the same manner as in Example 8, except that a monomer having a dimer acid content of 0.86% was used. Table 1 also shows the results of measuring the performance of the resin.

[0040]

[Table 1]

[0041]

According to the present invention, a water absorbing resin having the following effects can be produced. As is clear from the results in Table 1, a water-absorbent resin having a low residual monomer content can be obtained. Unlike the conventional residual monomer reduction method, a side reaction such as cleavage of a main chain polymer or cleavage of a crosslinking point does not occur at the time of heating and drying, so that a water-absorbent resin having high absorption performance can be obtained. A water-absorbing resin having a small amount of water-soluble components can be obtained. A highly safe water-absorbing resin can be obtained.

Because of the above effects, the water-absorbent resin of the present invention obtained by the production method of the present invention can be used as an absorbent material,
Use in contact with the human body such as sanitary materials (diaper diapers for children and adults, sanitary napkins, incontinence pads, etc.); Use in contact with foods such as freshness preserving material for fruits and vegetables, drip absorbing material for meat and seafood, and cold insulator Useful for various uses such as soil contact with crops and seeds such as soil water retention agent and seed coating agent; and use by humans such as prevention of dew condensation on interior building materials.

[Brief description of the drawings]

FIG. 1 is a liquid chromatography chart showing the dimer acid content in a monomer (starting acrylic acid) in Example 1 and Comparative Example 1.

FIG. 2 is a liquid chromatography chart showing the residual acrylic acid monomer content in products in Example 1 and Comparative Example 1.

────────────────────────────────────────────────── ─── Continuing from the front page Judge of the Joint Panel Ryuichi Ide Judge Norihiro Sano Judge Masaritsu Seki (56) References JP-A-6-56931 (JP, A) JP-A-3-31306 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C08F 20/06 C08F 6/00

Claims (4)

(57) [Claims] 1. A polymerizable monomer component having a dimer acid content of 0.5% by weight or less and containing unneutralized acrylic acid as a main component and containing a crosslinkable polymerizable monomer in an aqueous solvent. The obtained water-containing polymer is partially neutralized with an alkaline substance so as to have a degree of neutralization of 50 to 90 mol%, and dried by heating to produce a solid water-absorbing resin. A method for producing a reduced water absorbent resin. 2. The hydrated polymer is dried in an amount of 180 to 2 in the first half of drying.
Heat and dry with hot air at a temperature of 00 ° C.
The method according to claim 1, wherein the drying is performed by heating with hot air at a temperature of 150C. 3. The process according to claim 1, wherein the surface of the resin obtained by heating and drying is crosslinked by heating with a crosslinking agent having at least two groups capable of reacting with a functional group in the polymer. 4. The process according to claim 1, wherein the residual monomer content in the obtained water-absorbent resin is from 10 to 500 ppm.