JP2015110801A - Water-absorbing resin, and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water-absorbing resin that has a proper particle diameter, has an excellent water-absorbing performance and a water-absorbing rate suitable for hygienic material, has less solubility in water, and is suitable for use in the hygienic material.SOLUTION: This invention relates to a method of producing a water-absorbing resin from a water-soluble ethylenically unsaturated monomer by reverse-phase suspension polymerization reaction of two or more steps, wherein, in the reverse-phase suspension polymerization reaction in at least one step, used is a radical polymerization initiator which is different from the reverse-phase suspension polymerization reaction in another step.

Description

  The present invention has an appropriate particle diameter as a characteristic suitable for a water-absorbing agent used for sanitary materials, has excellent water-absorbing performance (no-load water absorption and water absorption under load), and water absorption speed suitable for sanitary materials. The present invention relates to a method for producing a water-absorbent resin having a low water-soluble content.

  In recent years, water-absorbing resins have been widely used in various fields such as sanitary materials such as paper diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as water-stopping agents and anti-condensation agents. Yes. Among these fields, it is often used especially for sanitary materials such as paper diapers and sanitary products. Examples of such a water-absorbing resin include a hydrolyzate of starch-acrylonitrile graft copolymer, a neutralized product of starch-acrylic acid graft copolymer, a saponified product of vinyl acetate-acrylic acid ester copolymer, A partially neutralized acrylic acid is known.

  Usually, properties desired for a water-absorbing resin include a high water absorption, an excellent water absorption rate, a high gel strength after water absorption, and the like. In particular, the properties desired for the water-absorbent resin used in the absorbent material for hygiene materials include high water absorption, excellent water absorption rate, high gel strength after water absorption, water absorption under excellent load, Particle size, narrow particle size distribution, less reversal of the absorbed material to the outside of the absorber, excellent diffusibility of the absorbed material to the inside of the absorber, For example, there is little elution. Conventionally, a particularly high water absorption amount and an excellent water absorption rate have been regarded as important required characteristics for a water absorbent resin suitably used for absorbent articles used for sanitary materials.

  Absorbent articles typified by paper diapers have an absorbent body that absorbs liquids such as body fluids on a flexible liquid-permeable surface sheet (top sheet) disposed on the side in contact with the body and on the opposite side in contact with the body. It has a structure that is sandwiched between arranged liquid-impermeable back sheets (back sheets). The absorber is mainly composed of a water absorbent resin and hydrophilic fibers such as crushed pulp and acetate fibers.

  The absorption performance of the absorber is mainly affected by the water absorption performance and content of the water-absorbent resin, and the uniformity of resin dispersion in the absorber. Methods have been studied. However, even if the amount of the water-absorbing resin is increased or a water-absorbing resin with a high water absorption amount is used to increase the absorption performance of the absorber, the absorption performance of the absorber does not necessarily increase in some cases. was there.

  One of the causes is that the water-absorbing resin becomes a soft gel upon absorption of liquid, so-called “gel blocking phenomenon” occurs, the liquid diffusibility is greatly reduced, and the liquid permeation rate of the absorber is slowed down. Was considered. This “gel blocking phenomenon” means that when a dense absorbent body absorbs a liquid, the water absorbent resin existing near the surface layer absorbs the liquid, and the soft gel becomes denser near the surface layer. This is a phenomenon in which the penetration of the liquid into the absorber is hindered and the water absorbent resin inside cannot absorb the liquid efficiently.

  Therefore, when gel blocking occurs, even if the content of the water-absorbing resin in the absorbent body is increased, it is difficult to obtain an absorption performance commensurate with it. Further, if the crosslink density of the resin is lowered in order to increase the water absorption amount of the water absorbent resin, the gel becomes soft at the same time, so that the gel blocking phenomenon tends to occur.

  Further, in recent years, hygiene materials have been required to absorb even under the load of the wearer, and the absorption performance under the load of absorbent articles has been attracting attention. Functionality has come to be demanded. In order to increase the amount of water absorption under the load of the water absorbent resin, it is necessary to increase the crosslink density of the resin and strengthen the water absorption gel. However, when the crosslink density is increased to increase the strength of the water-absorbing gel, the swelling of the gel is suppressed, so that the amount of water absorption (without load) decreases.

  Therefore, as a method for simultaneously increasing the amount of water absorption under no load and the amount of water absorption under load, for example, a water-soluble polymerizable monomer is added to a hydrophobic organic substance that is inert to polymerization in the presence of a predetermined amount of a crosslinking agent and a water-soluble chain transfer agent. Method for producing superabsorbent polymer that undergoes reverse phase suspension polymerization in solvent (Patent Document 1), water-soluble ethylenically unsaturated monomer using water-soluble azo radical polymerization initiator in the presence of polyvalent glycidyl compound A method for producing water-absorbing resin particles (Patent Document 2) that reverse-phase suspension polymerizes is proposed. However, in terms of both the unloaded water absorption amount and the water absorption amount under load, a sufficiently satisfactory product has not been obtained.

  On the other hand, the water absorption rate of the water-absorbent resin has been considered to be preferable from the viewpoint of preventing liquid leakage in the sanitary material, but in recent years, if the water absorption rate of the resin is too high, the liquid in the absorbent article Therefore, it has been inferred that the above-described gel blocking phenomenon is more likely to be caused by forming a liquid pool at a place where the liquid is introduced. Therefore, it is preferable that the water-absorbent resin used for the sanitary material has an appropriate water absorption rate, more specifically, the rate at the initial stage of water absorption is slightly slow, and the speed from the water absorption period to the latter stage is rapid. It is more preferable that However, the above technique focuses on optimizing the amount of water absorption under no load and the amount of water absorption under load by the crosslinking density, and does not necessarily have a water absorption rate suitable for sanitary materials.

  Therefore, a method for obtaining a water absorption rate suitable for use of the absorbent by subjecting the water absorbent resin to a hydrophobic treatment (Patent Document 3), and a water absorption rate different from the water absorption rate of the water absorbent resin obtained by the polymerization reaction. A method of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a resin (Patent Document 4) has been proposed. However, although these methods improve the water absorption speed, they do not have the excellent water absorption performance (no-load water absorption amount and water absorption amount under load) which is the above-mentioned problem.

  Therefore, a water-absorbent resin that has all the requirements that are preferably used for sanitary materials, that is, has an appropriate particle size, and is suitable for hygienic materials with excellent water absorption performance (no-load water absorption and water absorption under load). The development of a water-absorbing resin having a water absorption rate and a low water-soluble content is awaited.

JP 2002-284805 A JP 2006-176570 A JP 2003-88551 A Japanese Patent Laid-Open No. 9-151224

  An object of the present invention is to provide a water absorbent resin having an appropriate particle size, excellent water absorption performance and water absorption speed suitable for sanitary materials, low water-soluble content, and suitable for sanitary material applications. is there. Furthermore, the objective of this invention is providing the absorber and absorbent article using the said water absorbing resin.

  As a result of intensive studies to solve the above-mentioned problems, the present inventor, in a method for producing a water-absorbent resin from a water-soluble ethylenically unsaturated monomer by two or more stages of reversed-phase suspension polymerization reaction, At least one stage of reversed-phase suspension polymerization reaction uses radical polymerization initiators different from those of the other-stage reversed-phase suspension polymerization reaction, so that (i) moderate particle size and (ii) excellent water absorption performance It has been found that a water-absorbing resin satisfying (iii) a water absorption rate suitable for sanitary materials and (iv) a small water-soluble content can be produced. The present invention has been completed as a result of further studies based on these findings.

That is, this invention includes the aspect shown below.
Item 1. A method for producing a water-absorbent resin from a water-soluble ethylenically unsaturated monomer by two or more stages of reversed-phase suspension polymerization reactions,
The water-soluble radical polymerization initiator different from the reverse phase suspension polymerization reaction in the other stage is used in the at least one stage reverse phase suspension polymerization reaction,
A method for producing a water-absorbent resin.
Item 2. Item 2. The method according to Item 1, wherein the water-soluble radical polymerization initiator is at least one selected from the group consisting of a persulfate, a redox initiator, and a water-soluble azo compound.
Item 3. In at least one stage of the reverse phase suspension polymerization reaction, a persulfate is used as the water soluble radical polymerization initiator, and in the other stage of the reverse phase suspension polymerization reaction, as the water soluble radical polymerization initiator, an azo compound and Item 3. The method according to Item 1 or 2, wherein at least one member selected from the group consisting of redox initiators is used.
Item 4. Any of claim | item 1-3 whose usage-amount of the water-soluble radical polymerization initiator in the reverse phase suspension polymerization reaction of each step | paragraph is 0.01-1 mass% with respect to the aqueous solution containing a water-soluble ethylenically unsaturated monomer. A method for producing the water-absorbent resin according to claim 1.
Item 5. Using an internal cross-linking agent in the first-stage and second-stage reversed-phase suspension polymerization reactions,
The molar ratio (C1) of the internal crosslinking agent used in the first stage to the water-soluble radical polymerization initiator used in the first stage, and the second stage relative to the water-soluble radical polymerization initiator used in the second stage. Item 5. The method for producing a water absorbent resin according to any one of Items 1 to 4, wherein the molar ratio (C2) of the internal crosslinking agent used satisfies the following relational expression (a).
0.8 <C2 / C1 <5.0 (a)
Item 6. Item 6. A water absorbent resin produced by the method according to any one of Items 1 to 5.
Item 7. A water absorbent resin having the following characteristics (a) to (d).
(A) The median particle size is 200 to 800 μm, and the uniformity of the particle size distribution is 1.0 to 2.2.
(B) Water absorption under no load is 35 g / g or more, and water absorption under load is 15 mL / g or more. (C) Initial water absorption rate is 0.35 mL / s or less, and medium-term water absorption rate is 0.20 mL / s. (D) The water-soluble content is 20% by mass or less. Item 8. An absorbent body comprising the water-absorbent resin according to Item 6 or 7 and hydrophilic fibers.
Item 9. Item 9. An absorbent article obtained by sandwiching the absorbent body according to Item 8 between a liquid-permeable sheet and a liquid-impermeable sheet.

  According to the present invention, there is provided a water absorbent resin having an appropriate particle size, excellent water absorption performance and water absorption speed suitable for sanitary materials, and having a low water-soluble content and suitable for sanitary material applications. Can do. Such a water-absorbent resin of the present invention can be used as a material for an absorbent body and an absorbent article, and absorbs performance under any environment (that is, under no load or under load). Because of its excellent penetration rate and reversal, it is ideal for hygiene materials.

It is the schematic of the measuring apparatus used for the measurement of the water absorption amount under load. It is the schematic of the measuring apparatus used for the measurement of the initial water absorption rate of a water absorbing resin, and a medium term water absorption rate.

1. Production method of absorbent resin The production method of the absorbent resin of the present invention (hereinafter sometimes abbreviated as the production method of the present invention) is a water-soluble ethylenic unsaturation by a reverse phase suspension polymerization reaction of two or more stages. A method for producing a water-absorbent resin from a monomer, characterized in that a radical polymerization initiator different from the reverse-phase suspension polymerization reaction of the other stage is used in the reverse-phase suspension polymerization reaction of at least one stage. And
The polymerization method employed in the production method of the present invention is reverse phase suspension polymerization. Reverse phase suspension polymerization is a polymerization method in which radical polymerization is performed in monomer droplets while suspending a water-soluble monomer in an oily solvent.
Hereafter, the manufacturing method of this invention is explained in full detail.

Water-soluble ethylenically unsaturated monomer Examples of the water-soluble ethylenically unsaturated monomer used in the production method of the present invention include (meth) acrylic acid (in this specification, “acrylic”, and “ “Methacryl” is collectively referred to as “(meth) acryl”, “acrylate” and “methacrylate” are collectively referred to as “(meth) acrylate”, and the same applies hereinafter), 2- (meth) acrylamide-2- Such as methylpropanesulfonic acid and salts thereof, (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, etc. A nonionic monomer, and N, N-diethylaminoethyl (meth) acrylate, , Mention may be made of N- diethylaminopropyl (meth) acrylate, diethylamino (meth) amino group-containing unsaturated monomers such as acrylamide or its quaternary compound or the like. Among these water-soluble ethylenically unsaturated monomers, acrylic acid, methacrylic acid, and salts thereof, acrylamide, methacrylamide, and N, N-dimethylacrylamide are preferably used. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

  In the present invention, different water-soluble ethylenically unsaturated monomers may be used in the polymerization reaction in each stage, but the same water-soluble ethylenically unsaturated monomer is used in the polymerization reaction in each stage. It is preferable.

  When a monomer having an acid group such as (meth) acrylic acid or 2- (meth) acrylamide-2-methylpropanesulfonic acid is used as the water-soluble ethylenically unsaturated monomer, the acid group is You may neutralize beforehand with alkaline neutralizing agents, such as an alkali metal salt. Examples of such an alkaline neutralizer include alkali metal compounds such as sodium hydroxide and potassium hydroxide, and alkali metal compounds such as ammonium hydroxide. These alkaline neutralizing agents may be used in the form of an aqueous solution. Moreover, these alkaline neutralizers may be used alone or in combination of two or more.

  The degree of neutralization of all acid groups by the alkaline neutralizing agent increases the water absorption performance by increasing the osmotic pressure of the resulting water-absorbent resin, and the presence of excess alkaline neutralizing agent causes problems such as safety. From the viewpoint of avoiding this, it may be usually 0 to 100 mol%, preferably 30 to 90 mol%, more preferably 50 to 80 mol%.

  In reverse phase suspension polymerization, the water-soluble ethylenically unsaturated monomer is subjected to a radical polymerization reaction in a state dissolved in an aqueous solution. Hereinafter, a monomer solution subjected to a radical polymerization reaction, that is, an aqueous solution containing a water-soluble ethylenically unsaturated monomer may be referred to as a “monomer aqueous solution”. Here, in the monomer aqueous solution subjected to the suspension polymerization reaction in each stage, the concentration of the water-soluble ethylenically unsaturated monomer is not particularly limited, but is usually 20% by mass or more per monomer aqueous solution. What is necessary is just to set it as the range below a saturated density | concentration, Preferably it is 30-55 mass%, More preferably, it is 35-48 mass%. By setting it as such a density | concentration, high productivity can be maintained, avoiding a rapid reaction.

The oily solvent used in the polymerization reaction at each stage of the solvent is not particularly limited, but a hydrophobic hydrocarbon solvent is preferable. Examples of the hydrophobic hydrocarbon solvent include aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; Cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane, and other alicyclic hydrocarbons; benzene And aromatic hydrocarbons such as toluene and xylene. These hydrophobic hydrocarbon solvents may be used alone or in combination of two or more. Furthermore, in the present invention, different oily solvents may be used in the polymerization reaction of each stage, and the same oily solvent may be used in the polymerization reaction of each stage.

  Among these hydrophobic hydrocarbon solvents, from the viewpoint that the suspended state is good, particles having an excellent water absorption rate are easily obtained with a suitable particle size, are easily available industrially, and the quality is stable. N-heptane, cyclohexane and exol heptane (manufactured by ExxonMobil: mixed hydrocarbon solvent containing n-heptane, 2-methylhexane, 3-methylhexane and methylcyclohexane as main components) are preferably used.

In the polymerization reaction at each stage of the surfactant, it is desirable to add a surfactant in order to improve the dispersibility of the aqueous monomer solution and the droplet formability in an oily solvent. Examples of the surfactant include sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene glycerin fatty acid ester, sucrose fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene Alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropyl alkyl ether, polyethylene glycol fatty acid ester, alkyl glucoside, N -Alkyl gluconamides, polyoxyethylene fatty acid amides, and polyoxyethylene amides Nonionic surfactants such as kill amines, fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl ether sulfonates and Examples thereof include anionic surfactants such as salts thereof, polyoxyethylene alkylphenyl ether phosphoric acid and salts thereof, and polyoxyethylene alkyl ether phosphoric acid and salts thereof. These may be used alone or in combination of two or more. Furthermore, in the present invention, different surfactants may be used in the polymerization reaction of each stage, and the same surfactant may be used in the polymerization reaction of each stage.

  Among these surfactants, it is preferable to use at least one selected from the group consisting of polyglycerol fatty acid ester, sucrose fatty acid ester and sorbitan fatty acid ester from the viewpoint of dispersion stability of the monomer aqueous solution.

  The HLB value of the above-mentioned surfactant is not generally determined because the form of the primary particles obtained varies depending on the type of the surfactant. For example, if it is a sucrose fatty acid ester or a sorbitan fatty acid ester, the HLB value is 5 or less. If the thing of the range is a polyglyceryl fatty acid ester, what is necessary is just to use the thing of the range below HLB10.

  The addition amount of the surfactant is preferably 0.01 to 5 parts by mass and more preferably 0.05 to 3 parts by mass with respect to 100 parts by mass of the monomer aqueous solution. When the addition amount of the surfactant is less than 0.05 parts by mass, the dispersion stability of the aqueous monomer solution is lowered, which is not preferable. When the addition amount is more than 5 parts by mass, it is not economical because it is not economical. The usage form of the surfactant is not particularly limited, but a method of diluting or dissolving the surfactant in a small amount of an oily solvent in advance is preferable because the dispersion can be stabilized in a short time.

  In each stage of the polymerization reaction, a hydrophobic polymer dispersant may be further added in order to improve the dispersibility of the aqueous monomer solution in the oily solvent. As the hydrophobic polymer dispersant, it is preferable to select and use a solvent that dissolves or disperses in the solvent to be used. For example, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified Ethylene / propylene copolymer, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene , Oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose, anhydrous maleated polybutadiene, anhydrous maleated EPDM (ethylene / propylene / diene terpolymer) And the like. The molecular weight of these hydrophobic polymer dispersants is not particularly limited, but examples include those having a mass average molecular weight of 20,000 or less, preferably 10,000 or less, and more preferably 5,000 or less. These hydrophobic polymer dispersants may be used alone or in combination of two or more. Furthermore, in the present invention, different hydrophobic polymer dispersants may be used in the polymerization reaction of each stage, and the same hydrophobic polymer dispersant may be used in the polymerization reaction of each stage.

  Among these hydrophobic polymer-based dispersants, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer Polymer, maleic anhydride / ethylene / propylene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, and at least one selected from the group consisting of oxidized ethylene / propylene copolymer Is preferred.

  The amount of addition in the case of using a hydrophobic polymer dispersant is preferably 5 parts by mass or less, more preferably 0.01 to 3 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer aqueous solution. 0.05 to 2 parts by mass is more preferable. Since many hydrophobic polymer-based dispersants are relatively expensive, an addition amount of 5 parts by mass or less is preferable from the viewpoint of production cost.

Water-Soluble Radical Polymerization Initiator In the production method of the present invention, a water-soluble radical polymerization initiator different from the reverse-phase suspension polymerization reaction in the other stage is used in at least one stage of the reverse-phase suspension polymerization reaction.

  The water-soluble radical polymerization initiator used in the present invention is not particularly limited as long as it is water-soluble and acts as a radical polymerization initiator for a water-soluble ethylenically unsaturated monomer. Persulfates such as potassium persulfate, ammonium persulfate, sodium persulfate; peroxides such as hydrogen peroxide; 1-[(1-cyano-1-methylethyl) azo] formamide, 2,2′-azobis [2 -(N-phenylamidino) propane] dihydrochloride, 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-methyl-2-imidazoline) -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-diazepin-2-yl) dihydrochloride, 2.2′-azobis [2- (5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl) Propane] dihydrochloride, 2,2′-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane] dihydrochloride, 2,2′-azobis (1-imino- 1-pyrrolidino-2-methylpropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane], , 2′-azobis [2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide], 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) diwater Salt, 4,4′-azobis-4-cyanovaleric acid, 2,2′-azobis [2- (hydroxymethyl) propionitrile], 2,2′-azobis [2- (2-imidazolin-2-yl) Propane] disulfate, 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate, 2,2′-azobis [2-methyl-N- (2-hydroxy) And water-soluble azo compounds such as ethyl) propionamide].

  In addition, as a water-soluble radical polymerization initiator, you may use the redox polymerization initiator which used the above-mentioned compound (oxidizer) and the reducing agent together. The reducing agent used for the redox polymerization initiator includes sulfites such as sodium sulfite, potassium sulfite, calcium sulfite, zinc sulfite and ammonium sulfite; sulfites such as sodium hydrogen sulfite, potassium hydrogen sulfite, calcium hydrogen sulfite and ammonium hydrogen sulfite Hydrogen salts; pyrosulfites such as sodium pyrosulfite, potassium pyrosulfite, ammonium pyrosulfite; sodium dithionite, potassium dithionite, ammonium dithionite, calcium dithionite, zinc dithionite, etc. Dithionites; trithionates such as potassium trithionate and sodium trithionate; tetrathionates such as potassium tetrathionate and sodium tetrathionate; sodium thiosulfate, potassium thiosulfate, ammonium thiosulfate, etc. Thiosulfate; sodium nitrite Nitrites such as potassium nitrite, calcium nitrite, zinc nitrite; ascorbic acids such as L-ascorbic acid, sodium L-ascorbate, D-ascorbic acid, sodium D-ascorbate; erythorbic acid, sodium erythorbate, etc. And erythorbic acid. Of these, sodium sulfite, L-ascorbic acid, and sodium L-ascorbate are preferably used because of their industrial availability and ease of control of the redox reaction. These reducing agents may be used individually by 1 type, and may be used in combination of 2 or more type.

Among the redox polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, 2,2′-azobis [2- (2-imidazolin-2-yl) propane], from the viewpoint that the effects of the present invention can be easily obtained. Dihydrochloride, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane] Hydrochloride, 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, and 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] The combined use of any oxidizing agent of tetrahydrate and any reducing agent of sodium sulfite or L-ascorbic acid is preferably used.

  In the production method of the present invention, water-soluble radical polymerization initiators different from those in the reverse-phase suspension polymerization reaction in the other stage may be used in at least one-stage reverse-phase suspension polymerization reaction. In the production method of the present invention, when a two-stage reversed-phase polymerization reaction is employed, different water-soluble radical polymerization initiators may be used in the first-stage and second-stage reversed-phase suspension polymerization reactions. Good. In addition, in the production method of the present invention, when three or more stages of reverse phase polymerization reactions are employed, water solutions different from each other in at least two stages of reverse phase suspension polymerization reactions among three or more stages of reverse phase polymerization reactions. As long as the reactive radical polymerization initiator is used, the same water-soluble radical polymerization initiator may be used in two or more stages. For example, when performing a three-stage reversed-phase suspension polymerization reaction, (1) using different water-soluble radical polymerization initiators in the first-stage and second-stage reversed-phase suspension polymerization reactions, the third stage A mode in which the same water-soluble radical polymerization initiator as in the first stage or the second stage is used in the reverse-phase suspension polymerization reaction of (2); (2) the same water-solubility in the first-stage and second-stage reverse-phase suspension polymerization reactions A mode in which a radical polymerization initiator is used and a water-soluble radical polymerization initiator different from the first and second stages is used in the third-stage reversed-phase suspension polymerization reaction; Any of the embodiments using different water-soluble radical polymerization initiators in the phase suspension polymerization reaction may be used. From the viewpoint that the water-absorbing resin has less water-soluble components and more effectively has an excellent water-absorbing capacity and water-absorbing speed, at least the first and second stages of the polymerization reaction are carried out. It is desirable to use different water-soluble radical polymerization initiators in the reaction.

The at least two water-soluble radical polymerization initiators used in the present invention are appropriately selected from the water-soluble radical polymerization initiators described above, while further reducing the water-soluble content of the water-absorbent resin. From the viewpoint of providing an excellent water absorption capacity and water absorption speed, the following water-soluble radical polymerization initiator (1) is used in at least one of the two or more stages of suspension polymerization reaction, and the following water-soluble is used in the other stage. It is desirable to use a radical polymerization initiator (2).
The water-soluble radical polymerization initiator (1) is at least one selected from the persulfates, and the water-soluble radical polymerization initiator (2) is selected from the water-soluble azo compound and the redox initiator. At least one;
Preferably, the water-soluble radical polymerization initiator (1) is at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate, and the water-soluble radical polymerization initiator (2) is 2,2 '-Azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- ( 2-hydroxyethyl) -2-imidazolin-2-yl] propane] dihydrochloride, 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, and 2,2′- At least one selected from azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate, or at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate Oxidant And a redox initiator comprising a combination of at least one reducing agent selected from sodium sulfite, L-ascorbic acid and salts thereof, and erythorbic acid and salts thereof;
More preferably, the water-soluble radical polymerization initiator (1) is at least one selected from potassium persulfate and ammonium persulfate, and the water-soluble radical polymerization initiator (2) is 2,2′-azobis [ 2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) ) -2-imidazolin-2-yl] propane] dihydrochloride, 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride and 2,2′-azobis [N- ( 2-carboxyethyl) -2-methylpropionamidine] tetrahydrate.

  The water-soluble radical polymerization initiators (1) and (2) exemplified above may be used in any one of the suspension polymerization reactions in two or more stages, but the water-soluble radical polymerization initiators (1) and (2) may be used. One of the polymerization initiators (1) and (2) is preferably used in the first-stage suspension polymerization reaction, and the other is preferably used in the second-stage suspension polymerization reaction. ) Is used in the first-stage suspension polymerization reaction, and the water-soluble radical polymerization initiator (2) is more preferably used in the second-stage suspension polymerization reaction.

  The water-soluble radical polymerization initiator is dissolved in an aqueous monomer solution and subjected to a suspension polymerization reaction. The amount of the water-soluble radical polymerization initiator used in the suspension polymerization reaction in each stage is appropriately set according to the type and amount of the water-soluble ethylenically unsaturated monomer to be used, reaction conditions, etc. Usually, it is 0.01 to 1 part by mass per 100 parts by mass of the water-soluble ethylenically unsaturated monomer to be added. When the amount is less than 0.01 parts by mass, the polymerization rate is low. When the amount is more than 1 part by mass, a rapid polymerization reaction occurs, which is not preferable.

Internal cross-linking agent In order to enhance the effect of the present invention, an internal cross-linking agent may be used in addition to the water-soluble radical polymerization initiator in the suspension polymerization reaction at each stage.

  As an internal crosslinking agent, for example, (poly) ethylene glycol [“(poly)” means with or without the prefix “poly”. The same shall apply hereinafter), polyols such as 1,4-butanediol, glycerin, trimethylolpropane, etc., and a poly- having two or more vinyl groups obtained by reacting polyols with unsaturated acids such as acrylic acid and methacrylic acid. Unsaturated esters, bisacrylamides such as N, N′-methylenebisacrylamide, (poly) ethylene glycol diglycidyl ether, (poly) ethylene glycol triglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin tri Examples thereof include polyglycidyl compounds containing two or more glycidyl groups such as glycidyl ether, (poly) propylene glycol polyglycidyl ether, and (poly) glycerol polyglycidyl ether. These may be used alone or in combination of two or more.

  The amount added when using the internal crosslinking agent is preferably 3 parts by mass or less, more preferably 0.0001 to 1 part by mass, and more preferably 0.001 to 100 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. 0.1 parts by mass is more preferable. If the amount added exceeds 3 parts by mass, crosslinking is excessive and the water absorption without load becomes too low, which is not preferable. The internal cross-linking agent is preferably added in advance to the monomer aqueous solution.

  In order to further enhance the effect of the present invention, the molar ratio (C1) of the internal crosslinking agent used in the first stage to the water-soluble radical polymerization initiator used in the first stage and the water solubility used in the second stage. It is preferable that the molar ratio (C2) of the internal crosslinking agent used in the second stage with respect to the polymerizable radical polymerization initiator satisfies the relational expression (a) of [0.8 <C2 / C1 <5.0] More preferably, the relational expression (a ′) of 1.0 <C2 / C1 <4.0] is satisfied, and the relational expression of [1.5 <C2 / C1 <3.0] (a ″) is satisfied. Is more preferable. As will be described later, when the reversed-phase suspension polymerization reaction is performed in two or more stages, the molar ratio of the internal crosslinking agent to the water-soluble radical polymerization initiator used in the third and subsequent stages is in the same range as C2. The molar ratio is applied.

The monomer aqueous solution used in each stage of additives other than the above may contain a chain transfer agent, a thickener, and the like as necessary. Examples of the chain transfer agent include compounds such as thiols, thiolic acids, secondary alcohols, hypophosphorous acid, and phosphorous acid. These may be used alone or in combination of two or more. Examples of the thickener include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, polyacrylic acid neutralized product, polyacrylamide and the like.

Number of stages of reversed-phase suspension polymerization reaction The production method of the present invention is carried out by multi-stage polymerization in which a reversed-phase suspension polymerization reaction is performed in two or more stages. In the present invention, the number of stages of the reverse-phase suspension polymerization reaction is not particularly limited, but from the viewpoint of obtaining a water-absorbing resin having a function particularly suitable for sanitary material applications, two to three stages are preferable, and two stages are Further preferred. In the present specification, the number of stages of the reverse phase suspension polymerization reaction means the number of times the reverse phase suspension polymerization reaction is performed.

First-stage suspension polymerization reaction The first-stage suspension polymerization reaction is a monomer aqueous solution containing a water-soluble radical polymerization initiator, a water-soluble ethylenically unsaturated monomer, and, if necessary, other additives. If necessary, an oily solvent containing other additives is prepared, and the monomer aqueous solution is suspended in the oily solvent.

  In the first stage suspension polymerization reaction, the mixing ratio of the oily solvent to the monomer aqueous solution is not particularly limited, but it is usually from the viewpoint of uniformly dispersing the monomer aqueous solution and facilitating control of the polymerization temperature. The oil-based solvent is preferably 40 to 600 parts by mass, more preferably 50 to 400 parts by mass, and still more preferably 60 to 200 parts by mass with respect to 100 parts by mass of the monomer aqueous solution.

  When the aqueous monomer solution is added and dispersed in the oily solvent, it is dispersed by agitation. However, the agitation conditions differ depending on the desired dispersed droplet diameter, and therefore cannot be determined in a general manner. The dispersed droplet diameter can be adjusted by the type of the stirring blade, the blade diameter, the rotational speed, and the like. Examples of the stirring blade include a propeller blade, a paddle blade, an anchor blade, a turbine blade, a fiddler blade, a ribbon blade, a full zone blade (manufactured by Shinko Pantech Co., Ltd.), and a max blend blade (manufactured by Sumitomo Heavy Industries, Ltd.). Supermix (produced by Satake Chemical Machinery Co., Ltd.) can be used.

Although the reaction temperature in the first stage suspension polymerization reaction varies depending on the type and amount of the polymerization initiator used, it cannot be determined unconditionally, but is preferably 20 to 100 ° C., more preferably 40 to 90 ° C. ° C. When the reaction temperature is lower than 20 ° C, the polymerization rate may be lowered, and when the reaction temperature is higher than 100 ° C, a rapid polymerization reaction occurs, which is not preferable.
The reaction time for the first-stage suspension polymerization reaction is appropriately set according to the type and amount of the polymerization initiator used, the reaction temperature, etc., preferably 5 to 200 minutes, more preferably 10 to 100. Minutes.

  The size of the polymer particles obtained by the first-stage suspension polymerization reaction is preferably a median particle size of 20 to 200 μm, preferably 30 to 200 μm, from the viewpoint that the finally obtained water absorbent resin has an appropriate particle size. 150 μm is more preferable, and 40 to 100 μm is more preferable. The median particle diameter of the first-stage polymer particles is a value measured for particles obtained by dehydration and drying after the completion of the first-stage polymerization. The median particle size is measured according to the method described in the examples.

Suspension polymerization reaction after the second stage The suspension polymerization reaction after the second stage is a single amount containing a water-soluble radical polymerization initiator, a water-soluble ethylenically unsaturated monomer, and other additives as required. The body aqueous solution is added to the polymerization reaction solution after completion of the previous suspension polymerization reaction. Moreover, you may add another additive to the polymerization reaction liquid after completion | finish of a previous suspension polymerization reaction other than monomer aqueous solution as needed.
By the suspension polymerization reaction after the second stage, the polymer particles obtained by the suspension polymerization reaction at the first stage can be aggregated to give the water absorbent resin a desired function suitable for sanitary material applications. Become.

  In the second and subsequent suspension polymerization reactions, it is desirable to reduce the action of the surfactant so that the added monomer aqueous solution does not form independent droplets. For example, it is preferable to carry out the suspension polymerization reaction in the second and subsequent stages after the surfactant and / or polymer dispersant are precipitated by cooling after the completion of the first stage polymerization.

  The amount of water-soluble ethylenically unsaturated monomer added after the second stage is not particularly limited, but is added at the first stage from the viewpoint of imparting an appropriate particle size to the finally obtained water-absorbent resin. 50 to 300 parts by mass, preferably 80 to 220 parts by mass, more preferably 100 to 200 parts by mass, and more preferably 120 to 180 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Further preferred.

  In the suspension polymerization reaction after the second stage, the stirring conditions, reaction temperature, and reaction time are the same as in the suspension polymerization reaction at the first stage.

Post-crosslinking reaction After completion of the multi-stage reversed-phase suspension polymerization reaction described above, the obtained water-absorbent resin further contains two or more functional groups having reactivity with the functional group derived from the water-soluble ethylenically unsaturated monomer. It is preferable to add a crosslinking agent (denoted as a post-crosslinking agent). By adding a post-crosslinking agent and reacting after polymerization, the cross-linking density of the surface layer of the water-absorbent resin particles is increased, and various performances such as water absorption under load, water absorption speed, gel strength, etc. can be improved. Particularly suitable performance is provided.

  The post-crosslinking agent used for the crosslinking reaction is not particularly limited as long as it can react with the functional group derived from the water-soluble ethylenically unsaturated monomer used for the polymerization. As post-crosslinking agents used, for example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin; (poly) Polyethylene such as ethylene glycol diglycidyl ether, (poly) ethylene glycol triglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether Glycidyl compounds; haloepoxy compounds such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; 2,4-tolylene diisocyanate, hexame Compounds having two or more reactive functional groups such as isocyanate compounds such as range isocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl- Examples thereof include oxetane compounds such as 3-oxetaneethanol, 3-ethyl-3-oxetaneethanol and 3-butyl-3-oxetaneethanol, oxazoline compounds such as 1,2-ethylenebisoxazoline, and carbonate compounds such as ethylene carbonate. These may be used alone or in combination of two or more.

  Among these, from the viewpoint of excellent reactivity with the functional group derived from the water-soluble ethylenically unsaturated monomer used for polymerization, (poly) ethylene glycol diglycidyl ether, (poly) ethylene glycol triglycidyl ether, Polyglycidyl compounds such as (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and (poly) glycerol polyglycidyl ether are preferred.

  The amount of the post-crosslinking agent added is 100 mass of the total amount of water-soluble ethylenically unsaturated monomers subjected to polymerization (that is, the dry weight of the polymer obtained by the multistage reverse phase suspension polymerization reaction described above). Part is preferably 0.005 to 5 parts by weight, more preferably 0.01 to 3 parts by weight, still more preferably 0.02 to 1 part by weight, and still more preferably 0.03 to 0.5 parts by weight. Especially preferably, it is 0.04-0.2 mass part.

  When the added amount of the post-crosslinking agent is less than 0.005 parts by mass, the water absorption capacity under load of the resulting water-absorbent resin, the water absorption rate, the gel strength, etc. cannot be improved, and the amount exceeds 5 parts by mass. This is not preferable because the water absorption is too low.

  When using a post-crosslinking agent, water or a hydrophilic organic solvent may be used as a solvent, if necessary. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, ethers such as diethyl ether, dioxane and tetrahydrofuran, N, N -Amides such as dimethylformamide, and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used alone or in combination of two or more. Of these, water, methyl alcohol, and ethyl alcohol are preferable, and water is more preferable, from the viewpoints of good dispersibility in the vicinity of the water-absorbent resin surface, low persistence, and safety when remaining.

  The timing for adding the post-crosslinking agent is not particularly limited as long as it is after completion of the final stage polymerization. The post-crosslinking reaction is preferably carried out in the presence of water in the range of 1 to 200 parts by weight, more preferably in the range of 5 to 100 parts by weight, with respect to 100 parts by weight of the water absorbent resin. Is more preferable, and a range of 20 to 50 parts by mass is even more preferable. Thus, by adjusting the water content when the post-crosslinking agent is added, post-crosslinking in the particle surface layer of the water-absorbent resin can be more suitably performed, and excellent water absorption performance can be exhibited.

  The reaction temperature in the post-crosslinking reaction is, for example, 50 to 250 ° C, preferably 60 to 180 ° C, 60 to 140 ° C, and more preferably 70 to 120 ° C. The reaction time in the post-crosslinking reaction is, for example, 5 to 600 minutes, preferably 20 to 500 minutes, more preferably 30 to 400 minutes, and more preferably 60 to 300 minutes.

Drying or multistage suspension polymerization reaction, and after a post-crosslinking reaction as necessary, the resulting water-absorbent resin is sentenced to a drying step. In the present invention, the drying step may be performed under normal pressure or reduced pressure, and may be performed under an air stream such as nitrogen in order to increase drying efficiency. When a drying process is a normal pressure, 70-250 degreeC is preferable, as for drying temperature, 80-180 degreeC is more preferable, 80-140 degreeC is further more preferable, 90-130 degreeC is still more preferable. In the case of reduced pressure, the drying temperature is preferably 60 to 100 ° C, more preferably 70 to 90 ° C.

  The moisture content after drying of the water absorbent resin thus obtained is less than 20% by mass, preferably 1 to 15% by mass, more preferably 3 to 10% by mass from the viewpoint of imparting fluidity. In order to improve fluidity, amorphous silica powder may be added. The moisture content is measured according to the method described in the examples.

2. Water-absorbing resin Moreover, this invention provides the water-absorbing resin obtained by the said manufacturing method. The water-absorbent resin obtained by the above production method has the characteristics of having an appropriate particle size, excellent water-absorbing performance, a water-absorbing speed suitable for sanitary materials, and a low water-soluble content.

  The reason why the water-absorbent resin obtained by the above production method can have such properties has not been sufficiently elucidated, and although a limited interpretation of the present invention is not desired, it is presumed as follows. In the conventional method, an azo initiator, a redox initiator, or the like is used to improve the amount of water absorption under the load of the water absorbent resin, but the water absorption rate of the obtained water absorbent resin is not suitable. On the other hand, persulfuric acid-based initiators are considered to be excellent in water absorption speed and water-soluble content, but the water absorption amount under load of the resulting water-absorbent resin tended to be low. Surprisingly, as a result of using these initiators in different polymerization reactions in a multi-stage reverse phase suspension polymerization reaction in which a water-absorbent resin having a particle size and particle size distribution suitable for sanitary materials is obtained, It is considered that a water-absorbing resin having advantageous water-absorbing performance derived from each initiator was obtained.

  The median particle diameter of the water-absorbent resin of the present invention is, for example, 200 to 800 μm, but from the viewpoint of being suitably used for sanitary material applications, 250 to 600 μm is more preferable, 300 to 550 μm is still more preferable, and 350 to More preferably, it is 500 μm. The medium particle diameter of the water-absorbent resin is 200 μm or more from the viewpoint of improving the absorption performance of the absorber by avoiding the use of fine powder that is poor in fluidity as a powder and easily generates a gel blocking phenomenon upon water absorption. It is preferable that the median particle diameter of the water-absorbent resin is 800 μm or less from the viewpoint of improving the tactile sensation by reducing the jerky feel of the absorbent body. The median particle size is measured according to the method described in the examples below.

  The uniformity of the particle size distribution of the water absorbent resin of the present invention is, for example, 1.0 to 2.2, preferably 1.0 to 2.0, more preferably 1.2 to 1.8. . When using a water-absorbent resin as an absorbent body, if there are many large particles, the absorbent body after compression becomes partially hard, which is not preferable. Furthermore, if there are many small particles, it is not preferable because the particles easily move in the thin absorber and the uniformity is impaired. Therefore, it is preferable that the water absorbent resin used for the absorber has a narrow particle size distribution, in other words, a smaller uniformity of the particle size distribution. Since the water-absorbent resin of the present invention satisfying the above range has a small uniformity of particle size distribution, it can be suitably used for sanitary materials. The uniformity of the particle size distribution is measured according to the method described in Examples described later.

  The physiological saline water absorption of the water-absorbent resin of the present invention is, for example, 35 g / g, preferably 40 to 85 g / g, more preferably 45 to 75 g / g, and particularly preferably 50 to 70 g / g. . By satisfying such a range of numerical values, the gel can be kept strong to prevent gel blocking, and excessive crosslinking can be avoided to increase the absorption capacity. The physiological saline water absorption is measured according to the method described in the examples described later.

  The physiological saline water absorption amount under the load of the water absorbent resin of the present invention is, for example, 15 mL / g or more, preferably 17 mL / g or more, more preferably 20 mL / g or more, and further preferably 21 mL / g. g or more, particularly preferably 23 mL / g or more. When these values are higher, the liquid can be absorbed more even when used under load. The water-absorbent resin of the present invention that satisfies the numerical values in the above range can maintain the performance when used as an absorber. The physiological saline water absorption amount under the load of the water absorbent resin is measured according to the method described in the examples described later.

  The initial water absorption rate of the water-absorbent resin of the present invention is, for example, 0.35 mL / s or less, preferably from the viewpoint of suppressing the occurrence of gel blocking phenomenon at the initial stage of liquid permeation and promoting liquid diffusion in the absorbent body. Is 0.05 to 0.30 mL / s, more preferably 0.10 to 0.25 mL / s. 0.05 mL / s or more is more preferable from the viewpoint of ensuring a dry feeling on the skin in the initial stage of liquid permeation while allowing the liquid to diffuse. In the present specification, the initial water absorption speed is the amount of water absorbed (mL) per second in the water absorption time of 0 to 30 seconds. The initial water absorption rate of the water-absorbent resin is measured according to the method described in Examples described later.

  The mid-term water absorption rate of the water-absorbent resin of the present invention is, for example, 0.20 mL / s or more, but the liquid diffused in the initial stage of liquid permeation is quickly absorbed to the extent that no leakage occurs from the absorbent body, and enters the skin. From the viewpoint of ensuring the dry feeling, it is preferably 0.25 to 0.60 mL / s, and more preferably 0.30 to 0.50 mL / s. In the present specification, the medium-term water absorption rate is the amount of water absorbed (mL) per second in the water absorption time of 60 to 120 seconds. The medium-term water absorption rate of the water-absorbent resin is measured according to the method described in Examples described later.

  The water absorption rate of the water absorbent resin of the present invention is more preferably 20 to 70 seconds from the viewpoint of increasing the liquid penetration rate and increasing the dry feeling on the skin when the water absorbent resin is used in the absorbent body. Is 25 to 60 seconds, more preferably 30 to 50 seconds. The water absorption rate of the water absorbent resin can be measured by the method described in Examples described later.

  The water-soluble content of the water-absorbent resin of the present invention is, for example, 20% by mass or less, preferably 18% by mass or less, and more preferably 15% by mass or less. When the water-soluble content is higher than 20% by mass, the water-soluble content eluted from the gel after water absorption may touch the skin of the wearer of the absorbent article and cause discomfort such as sliminess and skin irritation. The water-soluble content is measured according to the method described in Examples described later.

Furthermore, the present invention also provides a water-absorbing resin that can satisfy all of the following performances (a) to (d). About the suitable range of the following (i)-(d) performance, it is the same as that of the above.
(A) The median particle size is 200 to 800 μm, and the uniformity of the particle size distribution is 1.0 to 2.2.
(B) Water absorption under no load is 35 g / g or more, and water absorption under load is 15 mL / g or more. (C) Initial water absorption rate is 0.35 mL / s or less, and medium-term water absorption rate is 0.20 mL / s. The water-absorbing resin satisfying the performances (i) to (d) above can be obtained by, for example, the above production method.

  The water-absorbent resin is used as a constituent material of the absorber. As a preferable embodiment of the absorbent body, one containing the water-absorbent resin and hydrophilic fibers can be given. Examples of the structure of the absorber include a mixing structure in which water-absorbing resin particles and hydrophilic fibers are uniformly blended, a sandwich structure in which water-absorbing resin particles are held between layered hydrophilic fibers, and a water-absorbing resin. Although the structure etc. which wrapped particle | grains and the hydrophilic fiber with the tissue are mentioned, this invention is not limited to this illustration. The absorbent body may contain synthetic fibers as a reinforcing material.

  The content of the water absorbent resin in the absorber is preferably 10 to 90% by mass, more preferably 15 to 80% by mass, still more preferably 20 to 70% by mass, and particularly preferably 30 to 65% by mass. %. When the content of the water-absorbent resin is less than 10% by mass, the absorption capacity is decreased, which tends to increase liquid leakage and reversion. Moreover, when content of a water absorbing resin exceeds 90 mass%, there exists a tendency for an absorber to become high cost or the touch of an absorber becomes hard.

  Examples of the hydrophilic fiber include cellulose fibers such as cotton-like pulp, mechanical pulp, chemical pulp, and semi-chemical pulp obtained from wood, and artificial cellulose fibers such as rayon and acetate, but the present invention is only such examples. It is not limited to. The hydrophilic fiber may contain a fiber made of a synthetic resin such as polyamide, polyester, or polyolefin.

  The said absorber can be used as a constituent material of an absorbent article. As an example of the structure of the absorbent article, the absorber is interposed between a liquid permeable sheet (top sheet) through which aqueous liquid can pass and a liquid impermeable sheet (back sheet) through which aqueous liquid cannot pass. The structure held in Specific examples of the absorbent article include sanitary materials such as disposable diapers, sanitary products, incontinence pads, and pet sheets; industrial materials such as water-stopping agents and anti-condensation agents. When the absorbent article is a sanitary material, the liquid-permeable sheet is usually disposed on the side that contacts the body, and the liquid-impermeable sheet is disposed on the side that does not contact the body.

  Examples of the liquid permeable sheet include a nonwoven fabric made of polyethylene, polypropylene, polyester, polyamide, and the like, and a porous synthetic resin sheet. Examples of the liquid-impermeable sheet include films made of polyethylene, polypropylene, polyester, polyamide, etc., films made of composite materials of these synthetic resins and nonwoven fabrics, etc., but the present invention is limited only to such examples. It is not something.

  Since the sizes of the liquid permeable sheet and the liquid impermeable sheet differ depending on the application of the absorbent article, they cannot be determined in general. Therefore, it is preferable to appropriately adjust the size according to the application.

  EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited only to these examples.

  The median particle size of the water-absorbent resins obtained in each of the examples and comparative examples, the uniformity of the median particle size, the moisture content, the amount of saline water absorbed (under load and under no load), the initial water absorption rate, and the medium-term water absorption The speed, the water absorption speed of physiological saline, and the water-soluble content were evaluated by the following methods.

(A) Median particle diameter 50 g of a water-absorbent resin is passed through a JIS standard sieve with a mesh opening size of 250 μm. When an amount larger than mass% remained on the sieve, the median particle diameter was measured using the sieve combination (B).
(A) JIS standard sieve from above, sieve with 425 μm openings, sieve with 250 μm openings, sieve with 180 μm openings, sieves with 150 μm openings, sieves with 106 μm openings, sieves with 75 μm openings, 45 μm openings Combined in order of sieve and saucer.
(B) JIS standard sieve from above, sieve with an opening of 850 μm, sieve with an opening of 600 μm, sieve with an opening of 500 μm, sieve with an opening of 425 μm, sieve with an opening of 300 μm, sieve with an opening of 250 μm, sieve with an opening of 150 μm Combined in order of sieve and saucer.
About 50 g of the water-absorbing resin was put on the combined uppermost sieve and classified by shaking for 20 minutes using a low-tap shaker.
After classification, the mass of the water-absorbent resin remaining on each sieve is calculated as a percentage by mass with respect to the total amount, and the mass of the water-absorbent resin remaining on the sieve opening and the sieve is calculated by integrating in order from the larger particle size. The relationship between percentage and integrated value was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50 mass% was defined as the median particle diameter.

(B) Uniformity of particle size distribution In the measurement of the middle particle size of the middle particles, the particle size (X1) corresponding to an integrated mass percentage of 15.9% by mass and the corresponding particle size of 84.1% by mass (X2) was determined, and the uniformity was determined by the following formula.
Uniformity = X1 / X2
That is, when the particle size distribution is narrow, the uniformity is generally close to 1, and when the particle size distribution is wide, the uniformity is greater.

(C) Water absorption amount without load of physiological saline 500 g of 0.9 mass% sodium chloride aqueous solution is put into a 500 mL beaker containing a rotor having a length of 3 cm, and the water absorbent resin is added while stirring the liquid with a magnetic stirrer. 0 g is precisely weighed (Wa) and added so as not to become mamaco, and then stirred at a stirring speed of 600 rpm for 1 hour.
The gel was filtered with a JIS standard sieve having an aperture of 20 μm and a diameter of 106 μm, and excess water of the gel remaining on the sieve was roughly drained with a fluororesin plate, and the sieve was tilted and allowed to stand for 30 minutes for further draining. The weight (Wb) of the gel remaining on the sieve was measured, and the unloaded water absorption amount of physiological saline was calculated from the following formula.
Water absorption amount without load of physiological saline (g / g) = Wb ÷ Wa

(D) Water absorption amount under load The water absorption amount under load of the water-absorbent resin was measured using a measuring apparatus Y schematically shown in FIG. The measuring apparatus Y shown in FIG. 1 includes a burette unit 7, a conduit 8, a measuring table 9, and a measuring unit 10 placed on the measuring table 9. The burette part 7 has a rubber stopper 74 connected to the upper part of the burette 70, an air introduction pipe 71 and a cock 72 connected to the lower part, and the air introduction pipe 71 has a cock 73 at the tip. A conduit 8 is attached between the buret unit 7 and the measuring table 9, and the inner diameter of the conduit 8 is 6 mm. At the center of the measuring table 9, there is a hole with a diameter of 2 mm, and a conduit 8 is connected. The measuring unit 10 includes a cylinder 100 (made of Plexiglas), a nylon mesh 101 bonded to the bottom of the cylinder 40, and a weight 102. The inner diameter of the cylinder 100 is 20 mm. The opening of the nylon mesh 101 is 75 μm (200 mesh). And at the time of measurement, the water-absorbing resin 11 is uniformly distributed on the nylon mesh 101. The weight 102 has a diameter of 19 mm and a mass of 119.6 g. This weight is placed on the water absorbent resin 11 so that a load of 4.14 kPa can be applied to the water absorbent resin 11.
Next, the measurement procedure will be described. The measurement is performed in a room at 25 ° C. First, the cock 72 and the cock 73 of the burette part 7 are closed, 0.9 mass% saline adjusted to 25 ° C. is poured from the upper part of the burette 70, the stopper is plugged on the upper part of the burette, and then the cock of the burette part 7 is placed. 72, open the cock 73. Next, the height of the measuring table 9 is adjusted so that the surface of the 0.9 mass% saline solution coming out from the conduit port at the center of the measuring table 9 and the upper surface of the measuring table 9 have the same height. .
Separately, 0.10 g of water-absorbing resin 11 particles are uniformly spread on the nylon mesh 101 of the cylinder 100, and the weight 102 is placed on the water-absorbing resin 11 to prepare the measuring unit 10. Next, the measurement unit 10 is placed so that the center portion thereof coincides with the conduit port at the center portion of the measurement table 9.
From the point of time when the water absorbent resin 11 starts to absorb water, the amount of decrease in the 0.9 mass% saline solution in the burette 100 (that is, the amount of 0.9 mass% saline solution absorbed by the water absorbent resin 11) Wc (mL) is read. . The amount of water absorption under the load of the water absorbent resin 11 after 60 minutes from the start of water absorption was determined by the following equation.
Water absorption under load (mL / g) = Wc / 0.10

(E) Initial water absorption rate and medium-term water absorption rate of the water-absorbent resin The initial water absorption rate and medium-term water absorption rate of the water-absorbent resin were measured using the measuring apparatus shown in FIG.
The measuring device was composed of a burette part 1 and a conduit 2, a measuring table 3, a nonwoven fabric 4, a gantry 6, and a clamp 7. The burette unit 1 has a rubber stopper 14 connected to the upper part of the buret 10 whose scale is written in units of 0.1 mL, an air introduction pipe 11 and a cock 12 connected to the lower part, and a cock 13 connected to the lower end of the buret 10. Had. The burette part 1 was fixed with a clamp 7. A conduit 2 was attached between the burette unit 1 and the measuring table 3, and the inner diameter of the conduit 2 was 6 mm. A hole with a diameter of 2 mm was formed at the center of the measuring table 3 and the conduit 2 was connected. The measuring table 3 was supported at an appropriate height by the gantry 6.
The measurement of the initial water absorption rate and the medium-term water absorption rate using such a measuring apparatus X was performed according to the following procedure. The measurement was performed in a room at a temperature of 25 ° C. and a humidity of 45 to 75%. First, the cock 12 and the cock 13 of the burette part 1 are closed, 0.9 mass% saline adjusted to 25 ° C. is poured from the upper part of the burette 10, the stopper at the upper part of the burette is plugged with the rubber stopper 14, 12. The cock 13 was opened. Next, the inside of the conduit 2 is filled with 0.9% by mass saline solution while removing bubbles, and the surface of the 0.9% by mass saline solution coming out from the conduit port at the center of the measuring table 3 and the upper surface of the measuring table 3 The height of the measuring table 3 was adjusted so that the heights of and were the same.
Next, a nonwoven fabric 4 (hydrophilic rayon spunlace having a basis weight of 25 g / m 2) cut to 30 × 30 mm was laid on the conduit port at the center of the measuring table 3, and the nonwoven fabric was allowed to absorb water until equilibrium was reached. In the state where the non-woven fabric was absorbing water, bubbles were generated from the air inlet tube 11 to the burette 10, but it was determined that the bubble generation stopped within a few minutes, and it was determined that equilibrium was reached. After equilibration, the scale of the burette 10 was read and the zero point was confirmed.
Separately, 0.10 g of the water-absorbent resin 5 was accurately measured and charged at once into the center of the nonwoven fabric 4. The amount of decrease in the 0.9 mass% saline solution in the burette 10 (that is, the amount of 0.9 mass% saline solution absorbed by the particles of the water absorbent resin 5) is sequentially read, and 30 seconds from the introduction of the water absorbent resin 5. The subsequent weight loss Wc (g) of 0.9% by mass saline was recorded as the initial water absorption per 0.10 g of the water absorbent resin. In the same manner, a weight loss Wd (g) of 0.9% by mass saline solution in 60 to 120 seconds from the introduction of the water absorbent resin 5 was recorded as a medium-term water absorption amount per 0.10 g of the water absorbent resin. . The measurement was carried out five times for one type of water-absorbent resin, and an average value of three points excluding the lowest value and the highest value was used.
The quotient obtained by converting the amount Wc (mL) of 0.9% by mass salt water absorbed by the water-absorbent resin 5 after 30 seconds from the addition to the amount of water absorbed per gram of the water-absorbent resin, and further dividing by 30 (seconds). Was the initial water absorption rate (mL / s) of the water-absorbent resin. That is, the initial water absorption rate (mL / s) = Wc ÷ (0.10 × 30).
Further, the amount Wd (mL) of 0.9% by mass salt water absorbed by the water-absorbent resin 5 between 60 and 120 seconds after the addition is converted into the amount of water absorbed per 1 g of the water-absorbent resin, and further 60 (seconds). The quotient obtained by dividing was taken as the medium water absorption rate (mL / s) of the water absorbent resin. That is, medium-term water absorption rate (mL / s) = Wd ÷ (0.10 × 60).

(F) Saline water absorption rate of water-absorbing resin This test was conducted in a room adjusted to 25 ° C ± 1 ° C. In a 100 mL beaker, 50 ± 0.1 g of physiological saline is weighed, a magnetic stirrer bar (without 8 mmφ × 30 mm ring) is added, the beaker is immersed in a constant temperature water bath, and the liquid temperature is 25 ± 0. Adjusted to 2 ° C. Next, after placing a beaker on a magnetic stirrer and generating a vortex in physiological saline at a rotation speed of 600 r / min, 2.0 ± 0.002 g of a water absorbent resin is quickly added to the beaker, Using a stopwatch, the time (seconds) from the addition of the water-absorbent resin to the point at which the vortex on the liquid surface converges was measured and taken as the physiological saline water-absorption rate of the water-absorbent resin.

(G) Water-soluble content of water-absorbent resin particles 500 g of physiological saline was weighed into a 500 mL beaker, and a magnetic stirrer bar (8 mmφ × 30 mm) was added and placed on the magnetic stirrer. Subsequently, the magnetic stirrer bar was adjusted to rotate at 600 rpm, and then 2.0 g of the water-absorbent resin particles were quickly poured and dispersed between the center of the vortex in the beaker and the side of the beaker and stirred for 3 hours. The water-absorbent resin particle-dispersed water after stirring for 3 hours was filtered with a JIS standard sieve (aperture 75 μm), and the obtained filtrate was further suction filtered using a Kiriyama funnel (filter paper No. 6).
80 ± 0.01 g of the filtrate obtained in a 100 mL beaker previously dried at 140 ° C. and cooled to room temperature was weighed and constant in a hot air dryer (ADVANTEC) with the internal temperature set at 140 ° C. And the mass We (g) of the solid content of the filtrate was measured.
On the other hand, it carried out similarly to the said operation, without using a water-absorbent resin particle | grain, measured blank mass Wf (g), and computed the water-soluble part from following Formula.
Water-soluble content (mass%) = [(We−Wf) × (500 ÷ 80)] ÷ 2 × 100

(H) Moisture content Approximately 2.5 g of the water-absorbent resin is precisely weighed in an aluminum cup (Wg), dried for 2 hours using a hot air dryer at 105 ° C., and then the mass (Wh) of the dried water-absorbent resin is measured. Thus, the moisture content was calculated from the following formula. (The tare mass before and after drying the aluminum cup was constant.)
Moisture content (mass%) = (Wg−Wh) ÷ Wg × 100

Example 1
As a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer, a round bottom cylindrical separable flask with an inner diameter of 100 mm equipped with a stirring blade having two inclined paddle blades with a blade diameter of 50 mm in two stages was prepared. 500 mL of n-heptane was taken in this flask, 0.92 g of sucrose stearate ester (manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370), maleic anhydride-modified ethylene / propylene copolymer (Mitsui Chemical Co., Ltd.) Manufactured, high wax 1105A) 0.92 g was added, the temperature was raised to 80 ° C. to dissolve the surfactant, and then cooled to 50 ° C.
On the other hand, 92 g of 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 154.3 g of 20.0% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside to neutralize 75 mol%. After performing, it was stirred at room temperature and completely dissolved. 0.11 g of ammonium persulfate and 9.2 mg of N, N′-methylenebisacrylamide were added and dissolved to prepare a first aqueous monomer solution.
The rotation speed of the stirrer was set to 450 rpm, the monomer aqueous solution was added to the separable flask, and the system was replaced with nitrogen, maintained at 35 ° C. for 30 minutes, and then immersed in a water bath at 70 ° C. The first stage post-polymerization slurry was obtained by warming and polymerizing. (Note that this post-polymerization slurry was obtained by azeotropically distilling water and n-heptane using a 120 ° C. oil bath, extracting only water out of the system, and then evaporating n-heptane and drying. The median particle diameter of the spherical primary particles was 80 μm.)
On the other hand, 110.4 g of 80.5 mass% acrylic acid aqueous solution was put into another 500 mL Erlenmeyer flask, and 149.9 g of 24.7 mass% sodium hydroxide aqueous solution was dripped while cooling from the outside, and 75 mol%. After neutralization, 0.13 g of 2,2′-azobis (2-amidinopropane) dihydrochloride and 33.1 mg of N, N′-methylenebisacrylamide were added and dissolved. A monomer aqueous solution was prepared.
After changing the stirring rotation speed of the post-polymerization slurry to 1000 rpm, cooling to 23 ° C., adding the second-stage monomer aqueous solution into the system, holding for 30 minutes while replacing with nitrogen, and again The second stage post-polymerization slurry was obtained by immersing the flask in a 70 ° C. water bath and raising the temperature to carry out the polymerization.
Next, the temperature was raised using an oil bath at 120 ° C., and 256.1 g of water was withdrawn from the system while refluxing n-heptane by azeotropically distilling water and n-heptane. Form of secondary particles in which spherical primary particles are aggregated by adding 8.10 g of a 2% by weight aqueous solution of diglycidyl ether and maintaining at 80 ° C. for 2 hours, and then evaporating n-heptane and drying. 213.8 g of a water absorbent resin having The obtained water-absorbent resin had a median particle diameter of 400 μm and a moisture content of 6% by mass. Table 1 shows the measurement results of each performance.

Example 2
In Example 1, instead of ammonium persulfate added to the first stage monomer, 0.11 g of potassium persulfate was added, the temperature of the slurry after polymerization was changed to 24 ° C., and water and n- Example 1 except that the water extracted from the system azeotropically with heptane was changed to 258.1 g.
The same operation as in Example 1 was performed to obtain 216.1 g of a water absorbent resin having a secondary particle form in which spherical primary particles were aggregated. The obtained water-absorbent resin had a median particle size of 360 μm and a moisture content of 7% by mass. Table 1 shows the measurement results of each performance.

Example 3
In Example 2, instead of 2,2′-azobis (2-amidinopropane) dihydrochloride added to the second stage monomer, 0.03 g of potassium persulfate and 0.002 g of L-ascorbic acid were added. Except for the addition, the same operation as in Example 2 was performed to obtain 215.8 g of a water absorbent resin having a form of secondary particles in which spherical primary particles were aggregated. The obtained water-absorbent resin had a median particle size of 440 μm and a moisture content of 7% by mass. Table 1 shows the measurement results of each performance.

Example 4
As a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer, a round bottom cylindrical separable flask with an inner diameter of 100 mm equipped with a stirring blade having two inclined paddle blades with a blade diameter of 50 mm in two stages was prepared. 500 mL of n-heptane was taken in this flask, 0.92 g of sucrose stearate ester (manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370), maleic anhydride-modified ethylene / propylene copolymer (Mitsui Chemical Co., Ltd.) Manufactured, high wax 1105A) 0.92 g was added, the temperature was raised to 80 ° C. to dissolve the surfactant, and then cooled to 50 ° C.
On the other hand, 92 g of 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 154.3 g of 20.0% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside to neutralize 75 mol%. After performing, it was stirred at room temperature and completely dissolved. 0.11 g of ammonium persulfate and 18.4 mg of ethylene glycol diglycidyl ether were added and dissolved to prepare a first aqueous monomer solution.
The rotation speed of the stirrer was set to 500 rpm, the monomer aqueous solution was added to the separable flask, and the system was replaced with nitrogen, maintained at 35 ° C. for 30 minutes, and then immersed in a 70 ° C. water bath to rise. The first stage post-polymerization slurry was obtained by warming and polymerizing. (Note that this post-polymerization slurry was obtained by azeotropically distilling water and n-heptane using a 120 ° C. oil bath, extracting only water out of the system, and then evaporating n-heptane and drying. The median particle size of the spherical primary particles was 60 μm.)
On the other hand, 128.8 g of 80.5% by mass acrylic acid aqueous solution was placed in another 500 mL Erlenmeyer flask, and 174.9 g of 24.7% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside. After neutralization, 0.12 g of 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride and 51.5 mg of ethylene glycol diglycidyl ether were added and dissolved. A second-stage monomer aqueous solution was prepared.
After changing the stirring rotation speed of the post-polymerization slurry to 1000 rpm, cooling to 21 ° C., adding the second stage monomer aqueous solution into the system, holding for 30 minutes while replacing with nitrogen, and again The second stage post-polymerization slurry was obtained by immersing the flask in a 70 ° C. water bath and raising the temperature to carry out the polymerization.
Next, the temperature was raised using an oil bath at 120 ° C., and 259.2 g of water was extracted out of the system while refluxing n-heptane by azeotropically distilling water and n-heptane. Form of secondary particles in which spherical primary particles are aggregated by adding 8.83 g of a 2% by weight aqueous solution of diglycidyl ether and maintaining at 80 ° C. for 2 hours, and then evaporating n-heptane and drying. 233.1 g of a water absorbent resin having The median particle diameter of the obtained water-absorbent resin was 490 μm, and the water content was 6% by mass. Table 1 shows the measurement results of each performance.

Comparative Example 1
As a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer, a round bottom cylindrical separable flask with an inner diameter of 100 mm equipped with a stirring blade having two inclined paddle blades with a blade diameter of 50 mm in two stages was prepared. 500 mL of n-heptane was taken in this flask, 0.92 g of sucrose stearate ester (manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370), maleic anhydride-modified ethylene / propylene copolymer (Mitsui Chemical Co., Ltd.) Manufactured, high wax 1105A) 0.92 g was added, the temperature was raised to 80 ° C. to dissolve the surfactant, and then cooled to 50 ° C.
On the other hand, 92 g of 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 154.3 g of 20.0% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside to neutralize 75 mol%. After performing, it was stirred at room temperature and completely dissolved. 0.11 g of potassium persulfate and 9.2 mg of N, N′-methylenebisacrylamide were added and dissolved to prepare a first aqueous monomer solution.
The rotation speed of the stirrer was set to 450 rpm, the monomer aqueous solution was added to the separable flask, and the system was replaced with nitrogen, maintained at 35 ° C. for 30 minutes, and then immersed in a water bath at 70 ° C. The first stage post-polymerization slurry was obtained by warming and polymerizing. (Note that this post-polymerization slurry was obtained by azeotropically distilling water and n-heptane using a 120 ° C. oil bath, extracting only water out of the system, and then evaporating n-heptane and drying. The median particle diameter of the spherical primary particles was 80 μm.)
On the other hand, 110.4 g of 80.5 mass% acrylic acid aqueous solution was put into another 500 mL Erlenmeyer flask, and 149.9 g of 24.7 mass% sodium hydroxide aqueous solution was dripped while cooling from the outside, and 75 mol%. Then, 0.13 g of potassium persulfate and 11.0 mg of N, N′-methylenebisacrylamide were added and dissolved to prepare a second stage monomer aqueous solution.
After changing the stirring rotation speed of the post-polymerization slurry to 1000 rpm, cooling to 23 ° C., adding the second-stage monomer aqueous solution into the system, holding for 30 minutes while replacing with nitrogen, and again The second stage post-polymerization slurry was obtained by immersing the flask in a 70 ° C. water bath and raising the temperature to carry out the polymerization.
Next, the temperature was raised using an oil bath at 120 ° C., and 259.1 g of water was withdrawn from the system while refluxing n-heptane by azeotropically distilling water and n-heptane. Form of secondary particles in which spherical primary particles are aggregated by adding 8.10 g of a 2% by weight aqueous solution of diglycidyl ether and maintaining at 80 ° C. for 2 hours, and then evaporating n-heptane and drying. 213.5 g of a water absorbent resin having The obtained water-absorbent resin had a median particle size of 360 μm and a moisture content of 6% by mass. Table 1 shows the measurement results of each performance.

Comparative Example 2
In Comparative Example 1, instead of potassium persulfate added to the first stage monomer, 0.11 g of 2,2′-azobis (2-amidinopropane) dihydrochloride was added, and N, N′— The amount of methylenebisacrylamide was changed to 18.4 mg, and in place of potassium persulfate added to the second stage monomer, 0.13 g of 2,2′-azobis (2-amidinopropane) dihydrochloride was added. Was added, and the amount of N, N′-methylenebisacrylamide was changed to 33.1 mg, and the same operation as in Comparative Example 1 was performed, and water absorption having a form of secondary particles in which spherical primary particles were aggregated 215.0 g of functional resin was obtained. The median particle diameter of the obtained water-absorbent resin was 390 μm, and the moisture content was 7% by mass. Table 1 shows the measurement results of each performance.

Comparative Example 3
In Comparative Example 1, 0.014 g of sodium hypophosphite monohydrate was added to the first-stage monomer, the amount of N, N′-methylenebisacrylamide was changed to 18.4 mg, and Comparative Example 1 except that 0.017 g of sodium hypophosphite monohydrate was added to the second stage monomer and the amount of N, N′-methylenebisacrylamide was changed to 22.1 mg. The operation was performed to obtain 216.1 g of a water-absorbent resin having a form of secondary particles in which spherical primary particles were aggregated. The obtained water-absorbent resin had a median particle size of 420 μm and a moisture content of 8% by mass. Table 1 shows the measurement results of each performance.

  From Table 1, the water-absorbent resin of Example 1-4 has an appropriate particle size, excellent water absorption performance (no load water absorption and water absorption under load), and water absorption speed suitable for sanitary materials. It can be seen that the water-soluble content is small. In contrast, in the water-absorbent resin of Comparative Example 1-3 in which the water-soluble radical polymerization initiator used in the first and second stages is the same, it is possible to satisfy the characteristics required for the hygiene material absorber. There wasn't. Specifically, the water-absorbing resin of Comparative Example 1 is inferior in water-absorbing capacity under load. Therefore, when the water-absorbing resin is used as an absorbent material for sanitary materials, liquid leakage occurs under load (when mounted). There is a fear. Further, in the water-absorbing resins of Comparative Examples 2 and 3, the initial water absorption rate is almost the same as that of the example, so that the gel blocking phenomenon is suppressed. However, since the medium-term water absorption rate is inferior, the occurrence of liquid leakage and skin dryness is ensured. There is a problem in terms of. Furthermore, since the water-absorbent resins of Comparative Examples 2 and 3 have high values of water-soluble components, they are likely to cause discomfort and skin irritation when used as an absorbent material for sanitary materials. Moreover, since the water absorption speed | rate is comparatively inferior in any comparative example, it is thought that skin dry feeling is inferior.

  The present invention provides a water-absorbent resin having excellent water absorption performance (no-load water absorption and water absorption under load) and a water absorption speed suitable for sanitary materials. Therefore, such a water-absorbent resin is suitably used for absorbent materials and absorbent articles for hygiene materials.

The addition amount of the surfactant is preferably 0.01 to 5 parts by mass and more preferably 0.05 to 3 parts by mass with respect to 100 parts by mass of the monomer aqueous solution. When the addition amount of the surfactant is less than 0.01 parts by mass, the dispersion stability of the aqueous monomer solution is lowered, and this is not preferable. When the addition amount is more than 5 parts by mass, it is not economical because it is not economical. The usage form of the surfactant is not particularly limited, but a method of diluting or dissolving the surfactant in a small amount of an oily solvent in advance is preferable because the dispersion can be stabilized in a short time.

The physiological saline water absorption amount under no load of the water absorbent resin of the present invention is, for example, 35 g / g or more , preferably 40 to 85 g / g, more preferably 45 to 75 g / g, and particularly preferably 50. -70 g / g. By satisfying such a range of numerical values, the gel can be kept strong to prevent gel blocking, and excessive crosslinking can be avoided to increase the absorption capacity. The physiological saline water absorption is measured according to the method described in the examples described later.

Claims (9)

A method for producing a water-absorbent resin from a water-soluble ethylenically unsaturated monomer by two or more stages of reversed-phase suspension polymerization reactions,
The water-soluble radical polymerization initiator different from the reverse phase suspension polymerization reaction in the other stage is used in the at least one stage reverse phase suspension polymerization reaction,
A method for producing a water-absorbent resin.   The method according to claim 1, wherein at least one selected from the group consisting of a persulfate, a redox initiator and a water-soluble azo compound is used as the water-soluble radical polymerization initiator.   In at least one stage of the reverse phase suspension polymerization reaction, a persulfate is used as the water soluble radical polymerization initiator, and in the other stage of the reverse phase suspension polymerization reaction, as the water soluble radical polymerization initiator, an azo compound and The method according to claim 1 or 2, wherein at least one selected from the group consisting of redox initiators is used.   The usage-amount of the water-soluble radical polymerization initiator in the reverse phase suspension polymerization reaction of each step | paragraph is 0.01-1 mass% per aqueous solution containing a water-soluble ethylenically unsaturated monomer. The manufacturing method of the water absorbing resin in any one. Using an internal cross-linking agent in the first-stage and second-stage reversed-phase suspension polymerization reactions,
The molar ratio (C1) of the internal crosslinking agent used in the first stage to the water-soluble radical polymerization initiator used in the first stage, and the second stage relative to the water-soluble radical polymerization initiator used in the second stage. The manufacturing method of the water absorbing resin in any one of Claims 1-4 with which the molar ratio (C2) of the internal crosslinking agent used satisfy | fills the following relational expression (a).
0.8 <C2 / C1 <5.0 (a)   A water-absorbent resin produced by the method according to claim 1. A water absorbent resin having the following characteristics (a) to (d).
(A) The median particle size is 200 to 800 μm, and the uniformity of the particle size distribution is 1.0 to 2.2.
(B) Water absorption under no load is 35 g / g or more, and water absorption under load is 15 mL / g or more. (C) Initial water absorption rate is 0.35 mL / s or less, and medium-term water absorption rate is 0.20 mL / s. (D) Water-soluble content is 20% by mass or less   The absorber containing the water absorbing resin of Claim 6 or 7, and a hydrophilic fiber.   An absorbent article comprising the absorbent body according to claim 8 sandwiched between a liquid-permeable sheet and a liquid-impermeable sheet.

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