JP6923997B2 - Manufacturing method of water-absorbent resin - Google Patents

Description

The present invention relates to a method for producing a water-absorbent resin and a water-absorbent resin. More specifically, the present invention relates to a method for producing a water-absorbent resin preferably used for sanitary materials, which has excellent water-absorbing properties, and a water-absorbent resin having specific performance.

Water-absorbent resins are widely used in sanitary materials such as disposable diapers and sanitary napkins, daily necessities such as pet sheets, and industrial materials such as water-stopping materials for cables. Many types of water-absorbent resins are known according to their uses. In sanitary materials such as disposable diapers and sanitary napkins, water-absorbent resins made of polymers of water-soluble ethylenically unsaturated monomers are mainly used. Water-absorbent resins used as sanitary materials are generally required to be highly safe because they are used in close proximity to the human body, and when they come into contact with body fluids such as urine and blood, they quickly release a large amount of body fluids. Water absorption characteristics that can be absorbed and retained are required. In particular, in recent years, sanitary materials have tended to be thinner due to demands for comfort when worn and convenience for carrying, and improvement in the amount of water absorption of the water-absorbent resin itself is required.

A water-absorbent resin composed of a polymer of a water-soluble ethylenically unsaturated monomer is generally considered to achieve a high water absorption amount by lowering the degree of cross-linking. In the production of a water-absorbent resin, a monomer is often polymerized using a persulfate as a polymerization initiator, which has advantages such as easy control of the polymerization reaction. However, when a persulfate is used, so-called self-crosslinking tends to proceed, which limits the reduction of the degree of cross-linking, and thus it is difficult to obtain a water-absorbent resin having a high water absorption amount.

In order to solve such a problem, Patent Document 1 describes that an azo compound is used as a polymerization initiator instead of a persulfate. However, when an azo compound is used as a polymerization initiator, the polymerization rate of the water-soluble ethylenically unsaturated monomer is insufficient, and the produced water-absorbent resin is unreacted with a monomer (hereinafter referred to as “unreacted monomer”). , These are referred to as "residual monomers"). Further, in the dehydration step or the drying step of removing water from the reaction system containing the produced water-absorbent resin by heating or the like, even if a part of the water-absorbent resin is decomposed, residual monomers may be generated. When such a water-absorbent resin containing a large amount of residual monomer is used as a sanitary material, it may cause a rash or rough skin on the user's skin by migrating to the outside after absorbing the liquid. In particular, the transfer of the residual monomer to the outside in the initial stage of use when the amount of liquid absorbed is small tends to be a problem.

Therefore, a method for suppressing the content of the residual monomer in the water-absorbent resin has been proposed. For example, in Patent Document 2, a radical polymerization initiator is added before or during drying of a slurry containing a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer by a reverse-phase suspension polymerization method. How to do it is described. Further, Patent Document 3 describes a method of adding a reducing substance such as sulfite after the polymerization of the water-soluble ethylenically unsaturated monomer.

Japanese Unexamined Patent Publication No. 2006-176570 JP-A-2002-105125 Japanese Unexamined Patent Publication No. 64-62317

However, in these conventional techniques, the properties required for the water-absorbent resin, that is, "high water absorption characteristics (high water retention capacity, high water absorption capacity under load), few residual monomers, and residual monomers at the initial stage of use" From the viewpoint of "having the characteristic that the amount of migration to the outside is small", it cannot be said that it is sufficient yet.

The subject of the present invention is a water-absorbent resin having high water-retaining ability, water-absorbing ability under a high load, extremely small amount of residual monomer, and a small amount of residual monomer transferred to the outside at the initial stage of use. It is an object of the present invention to provide a water-absorbent resin having a manufacturing method and specific performance, and a sanitary material in which the water-absorbent resin is used.

The present inventor has carried out diligent research to achieve the above-mentioned object. As a result, in the method of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound to produce a water-absorbent resin, an oil-soluble radical generator is added to the reaction system at a specific time of the dehydration step. It was found that by adding the resin, a water-absorbent resin having high water-retaining ability and high water-absorbing ability under load, a small content of residual monomer, and a small amount of residual monomer transferred to the outside at the initial stage of use can be obtained. , The present invention has been completed.

That is, the present invention provides the following method for producing a water-absorbent resin, a water-absorbent resin having specific performance, and a sanitary material using the water-absorbent resin.

[Item 1] A polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound to obtain a reaction system containing a water-absorbent resin precursor, and the above reaction in a petroleum hydrocarbon dispersion medium. and a dehydration step of removing water from the system, in the dehydration step, to the reaction system in the remaining water ratio is 5 to 75% calculated by the following equation (1), 2, 2'-azobis (4 - Methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis-isobutyronitrile, 1,1'-azobis (cyclohexane-1-carbonitrile), 2,2'-azobis (2,4-dimethyl) Valeronitrile), 2,2'-azobis (methyl isobutyrate) and 2,2'-azobis (2-methylbutyronitrile) are added at least one oil-soluble radical generator selected from the group. A method for producing a water-absorbent resin.

Amount of [Claim 2] The oil-soluble radical generator is from 0.0001 to 0.0020 mol per 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step, to claim 1 The method for producing a water-absorbent resin according to the above.

[Item 3] The water-soluble azo compound is 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin- At least one selected from the group consisting of 2-yl] propane] dihydrochloride and 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropion amidine] tetrahydrate. The method for producing a water-absorbent resin according to 1 or 2.

[Claim 4] wherein in the polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer by reversed phase suspension polymerization method, a manufacturing method of water absorbing resin according to any one of claim 1 3.

In [Claim 5] The polymerization step, and performing the reversed-phase suspension polymerization in two or more stages, the manufacturing method of the water-absorbent resin according to claim 4.

Item 6. The method for producing a water-absorbent resin according to any one of Items 1 to 5, wherein a cross-linking agent is added after the polymerization step to carry out a post-cross-linking reaction.

[Item 7] A water-absorbent resin containing a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, which satisfies all of the following (A) to (D).
(A) Saline water retention capacity is 38-65 g / g
(B) Physiological saline water absorption capacity under 4.14 kPa load is 18 ml / g or more (C) Residual monomer content is 150 ppm or less (D) External transfer amount of residual monomer at the initial stage of use is 30 ppm or less

[Item 8] The water-absorbent resin according to Item 7, wherein the medium particle size based on mass is 100 to 600 μm. Hereinafter, the term "medium particle size" means a mass-based medium particle size.

Item 9. Hygiene comprising a liquid permeable sheet, a liquid permeable sheet, and an absorber held between the sheets, wherein the absorber contains the water-absorbent resin according to Item 7 or 8. material.

According to the present invention, a water-absorbent resin having a high water-retaining ability, a water-absorbing ability under a high load, a small amount of residual monomer, and a small amount of residual monomer transferred to the outside at the initial stage of use is produced. A method is provided, which provides a water-absorbent resin having specific performance suitable for a sanitary material, as well as a sanitary material in which it is used.

It is a schematic diagram which shows the schematic structure of the apparatus for measuring the water absorption capacity of a physiological saline under a load of 4.14 kPa of a water-absorbent resin.

In the method for producing a water-absorbent resin according to the present invention, first, a water-soluble ethylenically unsaturated monomer is polymerized in the presence of a water-soluble azo compound to prepare a reaction system containing a gel-like substance of the water-absorbent resin. (Hereinafter referred to as "polymerization step"). The gel-like product of the water-absorbent resin prepared here is referred to as a "water-absorbent resin precursor" for convenience in order to distinguish it from the finally obtained water-absorbent resin.

As the water-soluble ethylenically unsaturated monomer used in the present invention, for example, (meth) acrylic acid (in the present specification, "acry" and "methacryl" are collectively referred to as "(meth) acrylic". (Similar) and salts thereof; 2- (meth) acrylamide-2-methylpropanesulfonic acid and salts thereof; (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol. Nonionic monomers such as (meth) acrylamide and polyethylene glycol mono (meth) acrylate; N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, diethylaminopropyl (meth) acrylamide and the like. Examples thereof include an unsaturated monomer containing an amino group and a quaternized product thereof. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

Of these, (meth) acrylic acid and salts thereof, (meth) acrylamide, and N, N-dimethylacrylamide are preferable as raw materials for producing the water-absorbent resin according to the present invention because they are easily available (meth). Acrylic acid and salts thereof are more preferred.

Further, the other water-soluble ethylenically unsaturated monomer can be added to acrylic acid and a salt thereof for copolymerization. In that case, the amount of acrylic acid and its salt used is preferably 70 to 100 mol% with respect to the total amount (mass) of the water-soluble ethylenically unsaturated monomer used in the polymerization step.

The water-soluble ethylenically unsaturated monomer is usually used as an aqueous solution from the viewpoint of easy removal of heat of reaction during polymerization. The concentration of the monomer in such an aqueous solution is usually preferably 20% by mass or more and preferably a saturation concentration or less, more preferably 25 to 70% by mass, still more preferably 30 to 55% by mass.

When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth) acrylic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, etc., the acid group is previously alkaline if necessary. Those neutralized with a Japanese agent may be used. Examples of such an alkaline neutralizer include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. These alkaline neutralizers are usually used as an aqueous solution in order to simplify the neutralization operation. The alkaline neutralizers may be used alone or in combination of two or more.

The degree of neutralization of the water-soluble ethylenically unsaturated monomer by the alkaline neutralizing agent enhances the water-absorbing property of the obtained water-absorbent resin and prevents excess alkaline neutralizing agent from being generated. It is preferably 10 to 90 mol%, more preferably 30 to 85 mol%, still more preferably 40 to 82 mol%, based on all the acid groups contained in the sex unsaturated monomer. , 50-80 mol% is even more preferred.

If necessary, a cross-linking agent may be added to the water-soluble ethylenically unsaturated monomer and subjected to a polymerization reaction. Examples of the cross-linking agent (referred to as an internal cross-linking agent) added to the water-soluble ethylenically unsaturated monomer before the polymerization reaction include (poly) ethylene glycol (in this specification, "polyethylene glycol" and "polyethylene glycol". "Ethylene glycol" is collectively referred to as "(poly) ethylene glycol". The same shall apply hereinafter.), (Poly) propylene glycol, 1,4-butanediol, trimethylolpropane, (poly) glycerin and other diols, triol and the like. Unsaturated polyesters obtained by reacting polyols with unsaturated acids such as (meth) acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N, N'-methylenebisacrylamide; polyepoxides and (meth) Di or tri (meth) acrylic acid esters obtained by reacting with acrylic acid; di (meth) obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth) acrylic acid. Acrylic acid carbamil esters; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N, N', N''-triallyl isocyanurate, divinylbenzene; (poly) ) Diglycidyl compounds such as ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, polyglycidyl compounds such as triglycidyl compound; Epihalohydrin compounds; isocyanate compounds such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate; 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl- Examples thereof include compounds having two or more reactive functional groups such as oxetan compounds such as 3-oxetan ethanol, 3-ethyl-3-oxetan ethanol, and 3-butyl-3-oxetan ethanol. Among these, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether and N, N'-methylenebisacrylamide are preferable from the viewpoint of cross-linking efficiency. These internal cross-linking agents may be used alone or in combination of two or more.

The amount of the internal cross-linking agent usually used is 0.000001 to 0.01 with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step from the viewpoint of enhancing the water absorption characteristics of the obtained water-absorbent resin. It is preferably mol, more preferably 0.00001 to 0.005 mol.

The internal cross-linking agent may be added to the aqueous solution of the water-soluble ethylenically unsaturated monomer, or may be added in the polymerization step separately from the water-soluble ethylenically unsaturated monomer. Further, in order to control the water absorption characteristics of the water-absorbent resin, a chain transfer agent may be added, and examples of such a chain transfer agent include hypophosphates, thiols, thiol acids, and secondary. Examples include alcohols and amines.

Examples of the water-soluble azo compound used in the present invention include 1-[(1-cyano-1-methylethyl) azo] formamide and 2,2'-azobis [2- (N-phenylamidino) propane] two. Hydrochloride, 2,2'-azobis [2- [N- (4-chlorophenyl) amidino] propane] dihydrochloride, 2,2'-azobis [2- [N- (4-hydroxyphenyl) amidino] propane] Dihydrochloride, 2,2'-azobis [2- (N-benzylamidino) propane] dihydrochloride, 2,2'-azobis [2- (N-allylamidino) propane] dihydrochloride, 2,2' -Azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [N- (2-hydroxyethyl) amidino] propane] dihydrochloride, 2,2'-azobis [2- (5- (5-) 5- Methyl-2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2-( 4,5,6,7-Tetrahydro-1H-1,3-diazepine-2-yl) dihydrochloride, 2,2'-azobis [2- (5-hydroxy-3,4,5,6-tetrahydropyrimidine) -2-yl) Propane] dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane] dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane], 2,2'-azobis [2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide], 2, 2'-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, 2,2'-azobis [2-methyl-N- [1,1-bis (hydroxymethyl) ethyl] propionamide] , 2,2'-azobis [2-methyl-N- (2-hydroxyethyl) -propionamide], 2,2'-azobis (2-methylpropionamide) dihydrate, 4,4'-azobis-4 -Cyanovaleic acid, 2,2'-azobis [2- (hydroxymethyl) propionitrile], 2,2'-azobis [2- (2-imidazolin-2-yl) propane] disulfate, 2,2' -Azobis [N- (2-carboxyethyl) -2-methylpropion amidine] tetrahydrate and 2,2'-azobis [2-methyl-N- (2-hydroxyethyl) propionamide] and the like can be mentioned. Of these, from the viewpoint of water absorption characteristics, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2- Il] Propane] dihydrochloride and 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropion amidine] tetrahydrate are preferred.

The water-soluble azo compounds may be used alone or in combination of two or more. The water solubility in the production method according to the present invention means that it exhibits a solubility in water of 5% by mass or more at 25 ° C. These water-soluble azo compounds generate a radical species containing a nitrogen atom by a denitrification reaction, and the radical species is introduced into the terminal group of the polymer chain as a functional group containing an amino group, an imino group, a cyano group, or the like. Will be done.

From the viewpoint of reaction time, the amount of the water-soluble azo compound used is preferably 0.00005 to 0.01 mol with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. More preferably, it is 0.0001 to 0.008 mol.

The polymerization method in the polymerization step of the water-soluble ethylenically unsaturated monomer can be selected from typical polymerization methods such as an aqueous solution polymerization method, an emulsion polymerization method and a reverse phase suspension polymerization method. For example, in the case of the aqueous solution polymerization method, an aqueous solution of a water-soluble ethylenically unsaturated monomer, a water-soluble azo compound and, if necessary, an internal cross-linking agent are added to a reaction vessel and heated while stirring and mixing to polymerize. Can proceed. Further, in the case of the reverse phase suspension polymerization method, a surfactant and / or a polymer protective colloid is added into a petroleum-based hydrocarbon dispersion medium to dissolve it, and then a water-soluble ethylenically unsaturated monomer and water-soluble are added. The polymerization can be allowed to proceed by further adding an aqueous solution in which a sex azo compound and an internal cross-linking agent are mixed, if necessary, and heating with stirring. As the polymerization method, a reverse phase suspension polymerization method is preferable because the particle size of the water-absorbent resin obtained during the polymerization reaction can be controlled over a wide range.

In the reverse phase suspension polymerization method, a water-soluble ethylenically unsaturated monomer is further added to the water-absorbent resin precursor obtained by the reverse phase suspension polymerization, and the polymerization is carried out in two or more stages. You can also do it. In such a multi-stage reverse phase suspension polymerization method, the particle size of the obtained water-absorbent resin can be appropriately adjusted by aggregating the water-absorbent resin precursor obtained in the first-stage reverse-phase suspension polymerization. It becomes easier to obtain a desired particle size for a sanitary material such as a paper diaper.

Further, in the multi-stage reverse phase suspension polymerization method, the types of the water-soluble ethylenically unsaturated monomer, the water-soluble azo-based polymerization initiator used in the second and subsequent stages, the internal cross-linking agent used as necessary, and the water solubility. The amounts of the azo-based polymerization initiator and the internal cross-linking agent used as necessary for the water-soluble ethylenically unsaturated monomer are the same as described above.

Hereinafter, the case where the polymerization step is carried out by the reverse phase suspension polymerization method will be described in more detail.

Examples of the petroleum hydrocarbon dispersion medium used in the reverse phase suspension polymerization method include n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, and 3-ethylpentane. An aliphatic hydrocarbon having 6 to 8 carbon atoms such as n-octane; cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans- Alicyclic hydrocarbons such as 1,3-dimethylcyclopentane; aromatic hydrocarbons such as benzene, toluene and xylene can be mentioned. These petroleum-based hydrocarbon dispersion media may be used alone or in combination of two or more. Among these petroleum-based hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are industrially easily available, have stable quality, and are inexpensive. Further, as an example of the mixture of the petroleum-based hydrocarbon dispersion medium, commercially available exol heptane (manufactured by ExxonMobil: containing 75 to 85% by mass of hydrocarbons of heptane and its isomer) may be used.

Since the amount of the petroleum-based hydrocarbon dispersion medium used is easy to control the polymerization temperature by removing the heat of polymerization, it is usually compared with 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for the first-stage polymerization. It is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass. The first-stage polymerization means a first-stage polymerization step in a single-stage polymerization step and a multi-stage polymerization.

Examples of the surfactant used in the reverse phase suspension polymerization method include nonionic surfactants such as sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, sorbitol fatty acid ester and polyoxyethylene alkylphenyl ether, and fatty acids. Examples thereof include anionic surfactants such as salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates and polyoxyethylene alkyl ether sulfonates. Of these, nonionic surfactants, particularly sorbitan fatty acid ester, polyglycerin fatty acid ester, or sucrose fatty acid ester are preferably used.

Examples of the polymer protective colloid include ethyl cellulose, ethyl hydroxyethyl cellulose, polyethylene oxide, polyethylene anhydride, polybutadiene anhydride, and EPDM anhydride (ethylene / propylene / diene / terpolymer). Can be mentioned.

From the viewpoint of stability of the reverse phase suspension polymerization method, the amount of the surfactant and the polymer protective colloid used should be 100 parts by mass of the aqueous solution of the water-soluble ethylenically unsaturated monomer used for the first-stage polymerization. On the other hand, it is preferably 0.1 to 5 parts by mass, and more preferably 0.2 to 3 parts by mass.

The reaction temperature in the polymerization step is preferably 20 to 110 ° C, more preferably 40 to 100 ° C. The polymerization reaction time is preferably 0.1 to 4 hours.

Next, water is removed from the reaction system in which the polymerization step is completed, that is, the reaction system containing the water-absorbent resin precursor (hereinafter, referred to as “dehydration step”). As a method of the dehydration step, a method of dispersing a reaction system containing a water-absorbent resin precursor in a petroleum-based hydrocarbon dispersion medium, applying energy such as heat from the outside, and removing water by co-boiling distillation is adopted, and vice versa. In the phase suspension polymerization method, a reaction system containing a water-absorbent resin precursor and a petroleum-based hydrocarbon dispersion medium can be obtained after the polymerization step, and the process can be directly shifted to the dehydration step, which is also preferable in terms of work efficiency.

As the petroleum-based hydrocarbon dispersion medium used in the dehydration step, the same petroleum-based hydrocarbon dispersion medium used in the above-mentioned reverse phase suspension polymerization can be used.

The oil-soluble radical generator added in the dehydration step may be added to the reaction system as it is, or may be separately dissolved in a petroleum-based hydrocarbon dispersion medium or the like and then added to the reaction system.

The timing of adding the oil-soluble radical generator is determined based on the residual water content of the reaction system calculated by the above formula (1). In the formula (1), "mass of water remaining in the reaction system" means the mass of the total amount of water existing in the reaction system at the time of calculating the residual water ratio, and is included in the reaction system before the start of the polymerization reaction. It is calculated by subtracting the mass of water removed from the reaction system by the time the residual water ratio is calculated from the total amount of water and water introduced as needed.

The upper limit of the residual water content when the oil-soluble radical generator is added is 75% or less, preferably 70% or less, more preferably 65% or less at any dehydration stage. Further, the lower limit of the residual water ratio when the oil-soluble radical generator is added is 5% or more, preferably 10% or more, and more preferably 15% or more. If an oil-soluble radical generator is added at a stage where the residual water content of the reaction system is higher than 75% or lower than 5%, the reaction of the oil-soluble radical generator with the water-absorbent resin precursor tends to be insufficient. Therefore, it becomes difficult to have both high water absorption characteristics and low residual monomer content.

The reaction temperature after the oil-soluble radical generator is added to the reaction system is preferably 40 to 120 ° C, more preferably 50 to 100 ° C. The reaction time with the water-absorbent resin precursor after the addition of the oil-soluble radical generator is usually 10 minutes to 3 hours.

The oil-soluble radical generators added to the reaction system in the dehydration step are 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis-isobutyronitrile, 1, 1'-azobis (cyclohexane-1-carbonitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (methyl isobutyrate), 2,2'-azobis (2-- Examples thereof include oil-soluble azo compounds (methylbutyronitrile). These oil-soluble radical generators may be used alone or in combination of two or more. The oil solubility in the production method according to the present invention means that the solubility in any of toluene, cyclohexane or ethanol at 25 ° C. is 5% by mass or more.

The amount of the oil-soluble radical generator added to the reaction system is preferably 0.0001 to 0.0020 mol, preferably 0.0001 to 0.0020 mol, based on 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. More preferably, 0.05 to 0.0015 mol.

In the production method of the present invention, it is preferable to carry out a cross-linking reaction after adding a cross-linking agent to the water-absorbent resin precursor obtained in the polymerization step. By carrying out a post-crosslinking reaction after the polymerization step, the water absorption characteristics can be further enhanced.

The post-crosslinking reaction can usually be carried out at any time after the polymerization step, but it is preferably carried out after the oil-soluble radical generator is added and reacted in the dehydration step. Examples of the cross-linking agent (post-cross-linking agent) used in the post-crosslinking reaction include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin; (poly) ethylene glycol diglycidyl ether, (poly). Polyglycidyl compounds such as glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether, trimethylolpropane triglycidyl ether; epichlorohydrin, epibromhydrin, α -Haloepoxy compounds such as methyl epichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol , 3-Methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, 3-butyl-3-oxetane ethanol and other oxetane compounds; 1,2-ethylenebisoxazoline and other oxazoline compounds; Examples thereof include hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide. Among these post-crosslinking agents, (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether, etc. Polyglycidyl compound is preferably used. These post-crosslinking agents may be used alone or in combination of two or more.

The amount of the post-crosslinking agent used varies depending on the type of the post-crosslinking agent, but is usually 0.00001 to 0.01 mol with respect to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomer used in the polymerization step. It is preferably 0.00005 to 0.005 mol, more preferably 0.0001 to 0.002 mol, and even more preferably 0.0001 to 0.002 mol.

The reaction between the water-absorbent resin precursor and the post-crosslinking agent is preferably carried out in the presence of water. Therefore, when the post-crosslinking agent is added, it is preferable that an appropriate amount of water remains in the reaction system, and the post-crosslinking agent is preferably added to the reaction system as an aqueous solution. The amount of water in the reaction system when the post-crosslinking agent is added to the reaction system (when the post-crosslinking agent is added as an aqueous solution, the amount of water derived from the aqueous solution is included) is usually used in the polymerization step. It is preferably 5 to 300 parts by mass, more preferably 10 to 100 parts by mass, and more preferably 10 to 50 parts by mass with respect to 100 parts by mass of the total amount of the water-soluble ethylenically unsaturated monomer. More preferred. The amount of water in the reaction system means the total amount of water remaining in the reaction system during dehydration and water added to the reaction system as needed for post-crosslinking.

In this way, after reacting the oil-soluble radical generator and preferably the post-crosslinking agent with the water-absorbent resin precursor, a drying treatment for removing water, a petroleum-based hydrocarbon dispersion medium, etc. is performed. Water-absorbent resin can be obtained. Examples of the drying treatment include a method in which energy such as heat is applied from the outside and distillation or hot air blowing is used. The drying treatment may be performed under normal pressure or reduced pressure. , In order to improve the drying efficiency, it may be carried out under an air flow such as nitrogen, or these methods may be used in combination. When the drying treatment is under normal pressure, the drying temperature is preferably 70 to 250 ° C, more preferably 80 to 180 ° C, and even more preferably 80 to 140 ° C. The drying temperature when the drying treatment is reduced pressure is preferably 60 to 100 ° C, more preferably 70 to 90 ° C.

The water-absorbent resin according to the present invention obtained by the above example includes sanitary materials such as disposable diapers and sanitary products, daily necessities such as pet sheets, water-retaining materials, agricultural and horticultural materials such as soil conditioners, and water-stopping for electric power / communication cables. It can be used in various fields such as materials and industrial materials such as dew condensation prevention materials, but it has the following characteristics (A) to (D) and is particularly safe for the human body, especially the skin, that is, It is particularly suitable as a sanitary material because it can reduce the effect on the skin (rough skin).
(A) Saline water retention capacity is 38-65 g / g
(B) Physiological saline water absorption capacity under 4.14 kPa load is 18 ml / g or more (C) Residual monomer content is 150 ppm or less (D) External transfer amount of residual monomer at the initial stage of use is 30 ppm or less

In the use of the water-absorbent resin according to the present invention as a sanitary material, the physiological saline water retention capacity is preferably 40 to 55 g / g from the viewpoint of increasing the absorption capacity and reducing the amount of liquid reversion. ..

Further, the physiological saline water absorption capacity under a load of 4.14 kPa is preferably 20 ml / g or more, preferably 21 ml, from the viewpoint of reducing the amount of liquid reversion when pressure is applied to the sanitary material after absorption. More preferably, it is / g or more.

The residual monomer content is preferably 100 ppm or less, more preferably 80 ppm or less, from the viewpoint of the effect on the skin (reduction of rough skin).

The water-absorbent resin according to the present invention has an excellent property that the amount of residual monomer transferred to the outside is remarkably small when the swelling ratio of the water-absorbent resin is low as in the initial stage of use. When used, the effect on the skin (rough skin) can be significantly reduced. The amount of residual monomer transferred to the outside at the initial stage of use is preferably 20 ppm or less, more preferably 15 ppm or less.

The particle size is preferably 100 to 600 μm, more preferably 200 to 550 μm, and even more preferably 250 to 500 μm from the viewpoint of handleability as a powder. , 300-450 μm is even more preferable.

The sanitary material using the water-absorbent resin of the present invention preferably has a form in which an absorber containing the water-absorbent resin is sandwiched between a liquid-permeable sheet and a liquid-impermeable sheet. The liquid permeable sheet used here is, for example, a non-woven fabric or a porous sheet made of a polyethylene resin, a polypropylene resin, a polyester resin, a polyamide resin, or the like. On the other hand, the liquid impermeable sheet is, for example, a film made of a synthetic resin such as a polyethylene resin, a polypropylene resin and a polyvinyl chloride resin, or a film made of a composite material of the synthetic resin and a non-woven fabric.

Further, the absorber used here is preferably a composite of a water-absorbent resin and hydrophilic fibers. This composite has, for example, a mixing structure in which a water-absorbent resin and a hydrophilic fiber are uniformly blended, a sandwich structure in which a water-absorbent resin is held between layered hydrophilic fibers, and a mixture of the water-absorbent resin and the hydrophilic fiber. Is preferably wrapped in a wrapping sheet having liquid permeability such as tissue paper. The hydrophilic fibers used in the composite include, for example, cotton-like pulp obtained from wood, mechanical pulp, cellulose fibers such as chemical pulp and semi-chemical pulp, artificial cellulose fibers such as rayon and acetate, and the like. The hydrophilic fiber may contain synthetic fibers such as polyamide resin fiber, polyester resin fiber and polyolefin resin fiber.

Hereinafter, the present invention will be described in detail based on Examples and Comparative Examples, but the present invention is not limited to these Examples and the like.

Example 1
<Polymerization process>
A 2 L five-necked cylindrical round-bottom flask equipped with a stirrer, a reflux condenser, a dropping funnel, a thermometer, and a nitrogen gas introduction tube is charged with 300 g of n-heptane, and a sucrose fatty acid having an HLB of 3.0. 0.74 g of an ester (trade name "S-370" of Mitsubishi Chemical Corporation) was added, and the temperature was raised to 80 ° C. to dissolve the mixture while stirring, and then the mixture was cooled to 55 ° C.

Separately, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 147.6 g (0.77 mol) of a 20.9 mass% sodium hydroxide aqueous solution was cooled therein. Was added dropwise to neutralize the acrylic acid. In addition to this neutralizing solution, 0.055 g (0.0002 mol) of 2,2'-azobis (2-amidinopropane) dihydrochloride which is a water-soluble azo compound and ethylene glycol diglycidyl ether 0 which is an internal cross-linking agent are added. 1.012 g (0.00007 mol) was added to prepare a monomer aqueous solution for the first-stage polymerization.

In addition, 128.8 g (1.44 mol) of an 80.5 mass% acrylic acid aqueous solution was placed in another 500 mL Erlenmeyer flask, and 160.6 g (1. 08 mol) was added dropwise to neutralize 75 mol% of acrylic acid. Furthermore, 0.077 g (0.0003 mol) of 2,2'-azobis (2-amidinopropane) dihydrochloride which is a water-soluble azo compound and 0.008 g (0.03 mol) of ethylene glycol diglycidyl ether which is an internal cross-linking agent. 0005 mol) was added to prepare a monomer aqueous solution for the second stage polymerization.

In the five-necked cylindrical round-bottom flask, the entire amount of the monomer aqueous solution for first-stage polymerization obtained above was added and dispersed under stirring, and the inside of the system was sufficiently replaced with nitrogen, and then the flask was placed. The mixture was stirred for 1 hour while being immersed in a water bath at 70 ° C. Then, after cooling the obtained polymerization slurry liquid to 26 ° C., the whole amount of the monomer aqueous solution for the second stage polymerization was added under stirring. After sufficiently replacing the inside of the system with nitrogen, the flask was stirred for 2 hours while being immersed in a water bath at 70 ° C. to obtain a reaction system containing a water-absorbent resin precursor.

<Dehydration process>
Next, the five-necked cylindrical round-bottom flask is heated in an oil bath at 120 ° C. to azeotrope the water of the reaction system and n-heptane, and 177 g of water is discharged to the outside of the system while refluxing n-heptane. Extracted (residual water rate: 61%). Here, as an oil-soluble radical generator, a solution prepared by dissolving 0.575 g (0.0035 mol) of 2,2'-azobis-isobutyronitrile in 28 g of n-heptane was added, and the mixture was held at 80 ° C. for 20 minutes. .. Subsequently, water and n-heptane were azeotroped, and 79 g of water was extracted out of the system while circulating n-heptane, and then a 2% aqueous solution of ethylene glycol diglycidyl ether as a post-crosslinking agent. 42 g (0.0005 mol) was added and kept at 80 ° C. for 2 hours. After that, n-heptane and water were distilled off to obtain 229.0 g of a water-absorbent resin as an aggregate of spherical particles.

Example 2
In Example 1, the same operation as in Example 1 was carried out except that the oil-soluble radical generator was changed to 0.806 g (0.0035 mol) of 2,2'-azobis (methyl isobutyrate). As an agglomerate, 228.9 g of a water-absorbent resin was obtained.

Example 3
In Example 2, the amount of dehydration when the oil-soluble radical generator was added was changed from 177 g to 214 g (residual water ratio: 44%), and was removed from the system from after the addition of the oil-soluble radical generator to before the addition of the post-crosslinking agent. The same operation as in Example 2 was carried out except that the amount of water was changed to 42 g, and 228.4 g of a water-absorbent resin was obtained as an aggregate of spherical particles.

Example 4
In Example 2, the amount of dehydration when the oil-soluble radical generator was added was changed from 177 g to 240 g (residual water ratio: 32%), and was removed from the system from after the addition of the oil-soluble radical generator to before the addition of the post-crosslinking agent. The same operation as in Example 2 was carried out except that the amount of water was changed to 16 g, and 228.1 g of a water-absorbent resin was obtained as an aggregate of spherical particles.

Example 5
In Example 1, the same operation as in Example 1 was carried out except that the oil-soluble radical generator was changed to 0.855 g (0.0035 mol) of 1,1'-azobis (cyclohexane-1-carbonitrile). 228.2 g of a water-absorbent resin was obtained as an agglomerate of spherical particles.

Example 6
In Example 1, the same operation as in Example 1 was performed except that the oil-soluble radical generator was changed to 0.497 g (0.002 mol) of 2,2'-azobis (2,4-dimethylvaleronitrile). , 228.5 g of water-absorbent resin was obtained as an agglomerate of spherical particles.

Comparative Example 1
In Example 1, the same operation as in Example 1 was carried out except that the oil-soluble radical generator was not added during the dehydration step, and 228.3 g of a water-absorbent resin was obtained as an agglomerate of spherical particles.

Comparative Example 2
In Example 1, 0.949 g (0.0035) of 2,2'-azobis (2-amidinopropane) dihydrochloride, which is a water-soluble azo compound, is used instead of the oil-soluble radical generator added during the dehydration step. The same operation as in Example 1 was carried out except that an aqueous solution prepared by dissolving mol) in 8 g of water was added to obtain 229.5 g of a water-absorbent resin as an agglomerate of spherical particles.

Comparative Example 3
In Example 2, the same operation as in Example 2 was carried out except that the amount of dehydration when the oil-soluble radical generator was added was changed from 177 g to 90 g (residual water ratio: 101%), and as an agglomerate of spherical particles. 228.8 g of water-absorbent resin was obtained.

Comparative Example 4
In Example 2, instead of the water-soluble azo compound 2,2'-azobis (2-amidinopropane) dihydrochloride added to the monomer aqueous solution for the first-stage polymerization, a water-soluble peroxide is used. 2,2'-Azobis (2-amidino), which is a water-soluble azo compound, is added to the monomer aqueous solution for the second-stage polymerization using 0.054 g (0.0002 mol) of potassium persulfate. Propane) Use 0.076 g (0.0003 mol) of potassium persulfate, which is a water-soluble peroxide, instead of dihydrochloride, and remove it from the system after the addition of the oil-soluble radical generator and before the addition of the post-crosslinking agent. The same operation as in Example 2 was carried out except that the amount of water obtained was changed to 87 g, and 228.4 g of a water-absorbent resin was obtained as an agglomerate of spherical particles.

Evaluation Method For the water-absorbent resins obtained in each Example and Comparative Example, the saline water-retaining ability, the saline water-absorbing ability under a load of 4.14 kPa, the residual monomer content, the amount of residual monomer transferred to the outside at the initial stage of use, and The medium particle size was measured by the following method. The results are shown in Table 1.

(Saline water retention capacity)
Put 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline) in a 500 mL beaker, and add 2.0 g of water-absorbent resin to this while stirring at 600 r / min so as not to generate lumps. , Distributed. Stirring was continued for 30 minutes to swell the water-absorbent resin, and then all the contents of the beaker were poured into a cotton bag (Membroad No. 60, width 100 mm x length 200 mm). Then, this cotton bag whose upper part is tied with a rubber band is dehydrated for 1 minute using a dehydrator (product number "H-122" manufactured by Japan Centrifuge Co., Ltd.) set to have a centrifugal force of 167 G, and after dehydration. The mass Wa (g) of the cotton bag containing the swollen gel was measured. Further, as a comparative control, the same operation was carried out without adding the water-absorbent resin to the physiological saline, and the mass Wb (g) of the wet cotton bag was measured. Then, the physiological saline water retention capacity was calculated from the following formula.
Physiological salt water retention capacity (g / g) = [Wa-Wb] (g) / mass of water-absorbent resin (g)

(Saline water absorption capacity under 4.14 kPa load)
The measurement was performed using the measuring device 100 outlined in FIG. In the figure, the measuring device 100 includes a burette unit 1, a conduit 2, a measuring table 3, and a measuring unit 4 placed on the measuring table 3. The burette unit 1 has a burette 10. The upper part of the burette 10 can be closed with a rubber stopper 14, and the air introduction pipe 11 and the cock 12 are connected to the lower part. The air introduction pipe 11 has a cock 13 at its tip. The conduit 2 has an inner diameter of 6 mm, and connects the cock 12 of the burette portion 1 and the measuring table 3. The measuring table 3 is prepared so as to be movable in the vertical direction, and a hole (conduit port) having a diameter of 2 mm is provided in the center thereof, and one end of the conduit 2 is connected to the hole. The measuring unit 4 has a cylinder 40 made of plexiglass, a polyamide mesh 41 adhered to one opening of the cylinder 40, and a weight 42 movable in the vertical direction in the cylinder 40. The cylinder 40 is placed on the measuring table 3 via the mesh 41, and has an inner diameter of 20 mm. The opening of the polyamide mesh 41 is 75 μm (200 mesh). The weight 42 has a diameter of 19 mm and a mass of 119.6 g, and a load of 4.14 kPa can be applied to the water-absorbent resin 5 uniformly arranged on the polyamide mesh 41 as described later.

The physiological saline water absorption capacity under a load of 4.14 kPa was measured by this measuring device 100 in a room at 25 ° C. As a specific measurement procedure, first, the cock 12 and the cock 13 of the burette portion 1 were closed, and physiological saline adjusted to 25 ° C. was put into the burette 10 from the upper part. Next, after closing the upper part of the burette 10 with the rubber stopper 14, the cock 12 and the cock 13 are opened, and the water surface of the physiological saline solution that comes out from the conduit port of the measuring table 3 through the conduit 2 and the upper surface of the measuring table 3 are contacted. The height of the measuring table 3 was adjusted so as to have the same height. On the other hand, in the measuring unit 4, 0.10 g of the water-absorbent resin 5 is uniformly arranged on the polyamide mesh 41 in the cylinder 40, the weight 42 is arranged on the water-absorbent resin 5, and the cylinder 40 is placed in the center thereof. Was installed so as to coincide with the conduit port at the center of the measuring table 3.

The amount of decrease in the saline solution in the burette 10 60 minutes after the water-absorbent resin 5 starts absorbing the saline solution from the conduit 2 (that is, the amount of the saline solution absorbed by the water-absorbent resin 5) Wc (ml) ) Was read, and the physiological saline water absorption capacity of the water-absorbent resin 5 under a load of 4.14 kPa was calculated by the following formula.

4. Physiological saline water absorption capacity under a load of 14 kPa (ml / g) = Wc (ml) / mass of water-absorbent resin (g)

(Residual monomer content)
500 g of physiological saline was placed in a 500 mL beaker, 2.0 g of a water-absorbent resin was added thereto, and the mixture was stirred for 60 minutes. The contents of the beaker were passed through a JIS standard sieve having a mesh size of 75 μm, and then filtered through a filter paper (No. 3 manufactured by ADVANTEC) to separate the absorbed gel-like resin and physiological saline as a filtrate. .. Then, the amount of residual monomer dissolved in the obtained filtrate was measured by high performance liquid chromatography. The residual monomer to be measured here is acrylic acid and its alkali metal salt. This measured value was converted into a value per mass of the water-absorbent resin and used as the residual monomer content (ppm).

(Amount of residual monomer transferred to the outside at the initial stage of use)
In a vacuum suction filtration device (Albac Kiko, Portable Aspirator MDA-015) having a filter holder having an inner diameter of a liquid passing portion of 35 mm, a filter paper (ADVANTEC, No. 3) is placed on the filter holder portion and placed on the filter paper. 0.5 g of the water-absorbent resin was uniformly arranged. With the suction pressure adjusting valve closed (that is, the maximum decompression degree), 50 g of physiological saline was poured from the upper part of the filter paper at one time. Then, the amount of residual monomer dissolved in the obtained filtrate was measured by high performance liquid chromatography. This measured value was converted into a value per mass of the water-absorbent resin and used as the amount of residual monomer transferred to the outside (ppm) at the initial stage of use.

(Medium particle size)
0.25 g of amorphous silica (Degussa Japan Co., Ltd., Sipernat 200) was mixed with 50 g of the water-absorbent resin as a lubricant. This is passed through a sieve having a mesh size of 250 μm of a JIS standard sieve, and when the amount remaining on the sieve is 50% by mass or more, the combination of JIS standard sieves (A) shown below (combination A). In the case of less than 50% by mass, the medium particle size was measured using the combination of JIS standard sieves (combination B) shown below (B).

(A) From top to bottom, a sieve with an opening of 710 μm, a sieve with an opening of 600 μm, a sieve with an opening of 500 μm, a sieve with an opening of 400 μm, a sieve with an opening of 300 μm, a sieve with an opening of 250 μm, and a sieve with an opening of 150 μm. And the order of the saucer.

(B) From top to bottom, a sieve with a mesh size of 400 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 180 μm, a sieve with a mesh size of 150 μm, a sieve with a mesh size of 106 μm, a sieve with a mesh size of 75 μm, and a sieve with a mesh size of 45 μm. And the order of the saucer.

The water-absorbent resin was placed on the best combined sieve and shaken for 10 minutes using a low-tap shaker to classify. After classification, the mass of the water-absorbent resin remaining on each sieve is calculated as a mass percentage with respect to the total amount, and by integrating in order from the one with the largest particle size, the mesh size of the sieve and the mass of the water-absorbent resin remaining on the sieve are calculated. The relationship with the integrated value of the percentage was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to the cumulative mass percentage of 50% by mass was defined as the medium particle size.

According to Table 1, the water-absorbent resins obtained in the examples all have high physiological saline water-retaining ability, physiological saline water-absorbing ability under a high 4.14 kPa load, and the residual monomer content is extremely low. Moreover, it can be seen that the amount of residual monomer transferred to the outside at the initial stage of use is small.

100 Measuring device 1 Burette part 10 Burette 11 Air introduction pipe 12 Cock 13 Cock 14 Rubber stopper 2 Conduit 3 Measuring table 4 Measuring part 40 Cylindrical 41 Nylon mesh 42 Weight 5 Water-absorbent resin

Claims (6)

A polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound to obtain a reaction system containing a water-absorbent resin precursor, and water from the reaction system in a petroleum-based hydrocarbon dispersion medium. In the dehydration step, when the residual water ratio calculated by the following formula (1) is 5 to 75%, the reaction system is subjected to 2,2'-azobis (4-methoxy-2, 2). 4-Dimethylvaleronitrile), 2,2'-azobis-isobutyronitrile, 1,1'-azobis (cyclohexane-1-carbonitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), It is characterized by adding at least one oil-soluble radical generator selected from the group consisting of 2,2'-azobis (methyl isobutyrate) and 2,2'-azobis (2-methylbutyronitrile). A method for producing a water-absorbent resin.

The water absorption according to claim 1, wherein the amount of the oil-soluble radical generator added is 0.0001 to 0.0020 mol with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. A method for producing a sex resin. The water-soluble azo compound is 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl]. Claim 1 or 2 which is at least one selected from the group consisting of [propane] dihydrochloride and 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate. The method for producing a water-absorbent resin according to. The method for producing a water-absorbent resin according to any one of claims 1 to 3, wherein the water-soluble ethylenically unsaturated monomer is polymerized by a reverse phase suspension polymerization method in the polymerization step. The method for producing a water-absorbent resin according to claim 4, wherein in the polymerization step, the reverse phase suspension polymerization method is carried out in multiple stages of two or more stages. The method for producing a water-absorbent resin according to any one of claims 1 to 5, wherein a cross-linking agent is added after the polymerization step to carry out a post-cross-linking reaction.

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