WO2020218164A1 - Water-absorbent resin particles and water-absorbent sheet - Google Patents

Abstract

These water-absorbent resin particles have a fluctuation in friction coefficient of 0.08 or more which is determined by a method including the following steps (4) and (5). (4) A tissue paper is placed on water-absorbent resin particles attached to an adhesive surface to cover all the water-absorbent resin particles. (5) The friction coefficient of the tissue paper surface is continuously determined along a straight line of 20 mm or longer by moving a probe having a piano wire as a contactor while pressing the probe against the surface opposite to the water-absorbent resin particles of the tissue paper.

Description

Water-absorbent resin particles and water-absorbent sheet

The present invention relates to water-absorbent resin particles and an absorbent sheet.

Absorbent articles for absorbing water-based liquids such as urine generally often have an absorber containing water-absorbent resin particles and hydrophilic fibers. In order to realize further thinning of the absorbent article, it is considered to reduce the proportion of relatively bulky hydrophilic fibers. For example, Patent Document 1 proposes a water-absorbing sheet structure having an absorbing layer in which the proportion of hydrophilic fibers such as pulp is extremely small.

Japanese Unexamined Patent Publication No. 2014-045914

If the ratio of the amount of hydrophilic fibers to the amount of water-absorbent resin particles in the absorbent layer is small, the permeation rate of the liquid by the absorbent article tends to decrease (the permeation time increases).

Therefore, one aspect of the present invention provides water-absorbent resin particles capable of increasing the permeation rate of a liquid by an absorbent article having an absorbent layer containing a high proportion of water-absorbent resin particles.

One aspect of the present invention is that the variation in the coefficient of friction is measured by a method including the following steps (1), (2), (3), (4), (5) and (6) in this order. The present invention relates to water-absorbent resin particles having a coefficient of 0.08 or more.
(1) An adhesive tape having a rectangular adhesive surface of 5 × 15 cm is placed on the horizontal plane of the workbench with the adhesive surface facing up.
(2) 0.02 g of water-absorbent resin particles per 1 cm ² are attached to the entire adhesive surface.
(3) A roller having a mass of 4.0 kg, a diameter of 10.5 cm, and a width of 6.0 cm is placed on the water-absorbent resin particles adhering to the adhesive surface, and then the roller is reciprocated once between the two short sides of the adhesive surface.
(4) Place the tissue paper on the water-absorbent resin particles adhering to the adhesive surface so that the entire water-absorbent resin particles are covered.
(5) By moving the probe having the piano wire as a contact while pressing it against the surface of the tissue paper on the opposite side of the water-absorbent resin particles, the friction coefficient of the surface of the tissue paper is continuously along a straight line of 20 mm or more. Measure.
(6) From the curve representing the relationship between the friction coefficient μ and the moving distance x of the probe, the fluctuation of the friction coefficient is calculated by the following formula. In these equations, MIU is the average coefficient of friction and MMD is the variation of the coefficient of friction.

Another aspect of the present invention relates to a water absorbing sheet provided with an absorbing layer containing the above water absorbing resin particles.

According to the present invention, the permeation rate of a liquid by an absorbent article having an absorbent layer containing a high proportion of water-absorbent resin particles can be increased without embossing.

It is sectional drawing which shows an example of the water absorption sheet. It is a top view which shows an example of the pattern of the adhesive formed on the core wrap sheet. It is sectional drawing which shows an example of an absorbent article. It is a top view which shows an example of a stirring blade (a flat plate blade which has a slit in a flat plate part). It is a schematic diagram which shows the method of measuring the water absorption under load. It is sectional drawing which shows an example of the simple water absorption sheet for measuring a friction coefficient.

Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

In this specification, "acrylic" and "methacryl" are collectively referred to as "(meth) acrylic". Similarly, "acrylate" and "methacrylate" are also referred to as "(meth) acrylate". "(Poly)" shall mean both with and without the "poly" prefix. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range of one step can be arbitrarily combined with the upper limit value or the lower limit value of the numerical range of another step. In the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples. “A or B” may include either A or B, or both. "Water-soluble" means that it exhibits a solubility in water of 5% by mass or more at 25 ° C. The materials exemplified in the present specification may be used alone or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances present in the composition when a plurality of substances corresponding to each component are present in the composition, unless otherwise specified.

Friction measured by a method including the following steps (1), (2), (3), (4), (5) and (6) using the water-absorbent resin particles according to the embodiment in this order. The variation of the coefficient is 0.08 or more.
(1) An adhesive tape having a rectangular adhesive surface of 5 × 15 cm is placed on the horizontal plane of the workbench with the adhesive surface facing up.
(2) 0.02 g of water-absorbent resin particles per 1 cm ² are attached to the entire adhesive surface.
(3) A roller having a mass of 4.0 kg, a diameter of 10.5 cm, and a width of 6.0 cm is placed on the water-absorbent resin particles adhering to the adhesive surface, and then the roller is reciprocated once between the two short sides of the adhesive surface.
(4) Place the tissue paper on the water-absorbent resin particles adhering to the adhesive surface so that the entire water-absorbent resin particles are covered.
(5) By moving the probe having the piano wire as a contact while pressing it against the surface of the tissue paper opposite to the water-absorbent resin particles, the friction coefficient of the surface of the tissue paper is set along a straight line of 20 mm or more. Measure continuously.
(6) From the curve representing the relationship between the friction coefficient μ and the moving distance x of the probe, the fluctuation of the friction coefficient is calculated by the following formula. In these equations, MIU is the average coefficient of friction and MMD is the variation of the coefficient of friction.

The reasonably large variation in the coefficient of friction measured by the above method corresponds to the fact that when a thin resin layer is formed of water-absorbent resin particles, the surface irregularities are relatively large. According to the findings of the present inventors, the water-absorbent resin particles forming the resin layer having a surface having large irregularities contribute to the improvement of the permeation rate of the absorbent article having the absorbent layer containing the water-absorbent resin particles in a high proportion. be able to. The present inventors presume that the surface area of the absorbing layer containing a high proportion of water-absorbent resin particles is increased, and as a result, the permeation rate is improved.

The roller that applies a load to the water-absorbent resin particles adhering to the adhesive surface is cylindrical and may be a stainless steel roller.

When the tissue paper placed on the water-absorbent resin particles in the above method is placed on the adhesive surface to which the water-absorbent resin particles are not attached, and the friction coefficient of the surface of the tissue paper is measured in the same state as above. The average friction coefficient MIU may be 0.21 ± 0.10, and the variation MMD of the friction coefficient may be 0.028 ± 0.02. The mass per unit area of the tissue paper may be 16 ± 2 g / m ² . The thickness of the tissue paper may be 0.12 ± 0.02 mm.

The probe may have, for example, 10 piano wires having a diameter of 0.5 mm, and these piano wires may be aligned in a certain direction. The load applied to the tissue paper by the probe with the piano wire as a contact is appropriately adjusted for proper measurement. This load may be, for example, 50 gf. The moving speed of the probe may be, for example, 10 mm / sec. The moving distance of the probe (the length of the portion where the friction coefficient is measured) may be 20 mm or more, and may be 30 mm or less. As the measuring device provided with the probe, for example, a friction tester manufactured by Kato Tech Co., Ltd., KES-SE-STP (trade name) can be used.

The fluctuation of the friction coefficient measured by the above method may be 0.09 or more or 0.10 or more from the viewpoint of the absorption performance of the absorbent article. From the viewpoint of the feel of the absorbent article, the fluctuation of the coefficient of friction may be 0.19 or less, or 0.18 or less. The average coefficient of friction measured by the above method may be 0.30 or more, 0.50 or less, or 0.40 or less.

The water retention amount when the water-absorbent resin particles absorb the physiological saline (hereinafter, may be simply referred to as "water retention amount") may be 35 g / g or more. As a result, it is easy to secure a sufficient amount of water retention of the absorbent article even in a thin absorbent layer having a small amount of water-absorbent resin particles. From the same viewpoint, the water retention amount may be 38 g / g or more, or 40 g / g or more. The water retention amount may be 55 g / g or less, or 50 g / g or less. The water absorption amount of the water-absorbent resin particles is measured by the method described in Examples described later.

The amount of water absorption when the physiological saline is absorbed under a load of 2.07 kPa of the water-absorbent resin particles (hereinafter, may be simply referred to as "the amount of water absorption under load") may be 15 mL / g or more. When the amount of water absorption under load is large, high water absorption capacity can be maintained even under the load of the wearer of the absorbent article. From the same viewpoint, the amount of water absorption of the water-absorbent resin particles under load may be 18 mL / g or more, 20 mL / g or more, 25 mL / g or more, or 30 mL / g or more, 50 mL / g or less, or 45 mL / g. It may be less than or equal to g. The amount of water absorption under load is measured by the method described in Examples described later.

The shape of the water-absorbent resin particles may be, for example, substantially spherical, crushed, or granular. The water-absorbent resin particles according to the present embodiment may be in a form in which fine particles (primary particles) are aggregated (secondary particles) in addition to a form in which each is composed of a single particle. The medium particle size of the water-absorbent resin particles may be 250 to 850 μm, 300 to 700 μm, 350 to 650 μm, 400 to 600 μm, or 450 to 550 μm. The water-absorbent resin particles may have a desired particle size distribution at the time of being obtained by the production method described later, but the particle size distribution may be adjusted by performing an operation such as particle size adjustment using classification by a sieve. Good.

The water-absorbent resin particles can include, for example, a crosslinked polymer formed by polymerizing a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer has a monomer unit derived from an ethylenically unsaturated monomer.

The water-absorbent resin particles can be produced by a method including a step of polymerizing a monomer containing an ethylenically unsaturated monomer. Examples of the polymerization method include a reverse phase suspension polymerization method, an aqueous solution polymerization method, a bulk polymerization method, and a precipitation polymerization method. From the viewpoint of ensuring good water absorption characteristics of the obtained water-absorbent resin particles and facilitating control of the polymerization reaction, a reverse phase suspension polymerization method or an aqueous solution polymerization method may be applied. In the following, a reverse phase suspension polymerization method will be described as an example as a method for polymerizing an ethylenically unsaturated monomer.

The ethylenically unsaturated monomer may be water-soluble. Examples of water-soluble ethylenically unsaturated monomers include (meth) acrylic acid and its salts, 2- (meth) acrylamide-2-methylpropanesulfonic acid and its salts, (meth) acrylamide, N, N-dimethyl. (Meta) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) Examples thereof include acrylate and diethylaminopropyl (meth) acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more. Functional groups such as the carboxyl group and amino group of the above-mentioned monomers can function as functional groups capable of cross-linking in the surface cross-linking step described later.

From the viewpoint of industrial availability, the ethylenically unsaturated monomer is at least one selected from the group consisting of (meth) acrylic acid and salts thereof, acrylamide, methacrylamide, and N, N-dimethylacrylamide. It may contain a compound of the species. The ethylenically unsaturated monomer may contain (meth) acrylic acid and a salt thereof, and at least one compound selected from the group consisting of acrylamide. From the viewpoint of further enhancing the water absorption property, the ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof.

The ethylenically unsaturated monomer can be used in the polymerization reaction as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter, simply referred to as "monomer aqueous solution") is 20% by mass or more and the saturation concentration or less, 25 to 70% by mass, or 30. It may be up to 55% by mass. Examples of the water used in the aqueous solution include tap water, distilled water, ion-exchanged water and the like.

As the monomer for obtaining the water-absorbent resin particles, a monomer other than the above-mentioned ethylenically unsaturated monomer may be used. Such a monomer can be used, for example, by being mixed with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used is the total amount of the monomers (the total amount of the monomers for obtaining the water-absorbent resin particles. For example, the total amount of the monomers giving the structural unit of the crosslinked polymer. The same shall apply hereinafter). It may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol%. The ratio of (meth) acrylic acid and its salt may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% with respect to the total amount of the monomer. It may be. "Ratio of (meth) acrylic acid and its salt" means the ratio of the total amount of (meth) acrylic acid and its salt.

When the ethylenically unsaturated monomer has an acid group, the acid group may be neutralized with an alkaline neutralizer, and then the monomer solution may be used in the polymerization reaction. The degree of neutralization of an ethylenically unsaturated monomer by an alkaline neutralizing agent increases the osmotic pressure of the obtained water-absorbent resin particles and further enhances the water absorption characteristics (water absorption amount, etc.). It may be 10-100 mol%, 50-90 mol%, or 60-80 mol% of the acidic group in the body. Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, potassium hydroxide and potassium carbonate; ammonia and the like. The alkaline neutralizer may be used alone or in combination of two or more. The alkaline neutralizer may be used in the form of an aqueous solution to simplify the neutralization operation. Neutralization of the acid group of the ethylenically unsaturated monomer can be performed, for example, by adding an aqueous solution of sodium hydroxide, potassium hydroxide or the like to the above-mentioned monomer aqueous solution and mixing them.

In the reverse phase suspension polymerization method, the monomer aqueous solution is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and the ethylenically unsaturated monomer is polymerized using a radical polymerization initiator or the like. Can be done. As the radical polymerization initiator, a water-soluble radical polymerization initiator can be used.

Examples of the surfactant include nonionic surfactants and anionic surfactants. Nonionic surfactants include sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, and polyoxyethylene. Alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether, Examples thereof include polyethylene glycol fatty acid ester. Anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, and polyoxyethylene alkyl ether phosphates. , And the phosphate ester of polyoxyethylene alkyl allyl ether and the like. The surfactant may be used alone or in combination of two or more.

From the viewpoint that the W / O type reverse phase suspension is in a good state, water-absorbent resin particles having a suitable particle size can be easily obtained, and industrially available, the surfactant is a sorbitan fatty acid ester. It may contain at least one compound selected from the group consisting of polyglycerin fatty acid ester and sucrose fatty acid ester. From the viewpoint of easily improving the water absorption characteristics of the obtained water-absorbent resin particles, the surfactant may contain a sucrose fatty acid ester (for example, sucrose stearic acid ester).

The amount of the surfactant may be 0.05 to 10 parts by mass, 0.08 to 5 parts by mass, or 0.1 to 3 parts by mass with respect to 100 parts by mass of the monomer aqueous solution.

In reverse phase suspension polymerization, a polymer-based dispersant may be used in combination with the above-mentioned surfactant. Examples of the polymer dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, maleic anhydride-modified EPDM (ethylene / propylene / diene / terpolymer), and maleic anhydride. Modified polybutadiene, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, maleic anhydride / butadiene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer Examples thereof include coalescence, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose and the like. The polymer-based dispersant may be used alone or in combination of two or more. From the viewpoint of excellent dispersion stability of the monomer, the polymer-based dispersant includes maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, and maleic anhydride / ethylene copolymer. , Maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene / propylene copolymer It may be at least one selected from the group consisting of.

The amount of the polymer-based dispersant may be 0.05 to 10 parts by mass, 0.08 to 5 parts by mass, or 0.1 to 3 parts by mass with respect to 100 parts by mass of the monomer aqueous solution.

The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of chain aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. As the hydrocarbon dispersion medium, a chain aliphatic hydrocarbon such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, n-octane; cyclohexane , Methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane and other alicyclic hydrocarbons; benzene, Examples include aromatic hydrocarbons such as toluene and xylene. The hydrocarbon dispersion medium may be used alone or in combination of two or more.

From the viewpoint of being industrially easily available and having stable quality, the hydrocarbon dispersion medium may contain at least one selected from the group consisting of n-heptane and cyclohexane. From the same viewpoint, as the mixture of the above-mentioned hydrocarbon dispersion medium, for example, a commercially available exol heptane (manufactured by ExxonMobil: containing 75 to 85% of n-heptane and isomeric hydrocarbons) may be used. Good.

The amount of the hydrocarbon dispersion medium is 30 to 1000 parts by mass, 40 to 500 parts by mass, or 50 parts by mass with respect to 100 parts by mass of the monomer aqueous solution from the viewpoint of appropriately removing the heat of polymerization and easily controlling the polymerization temperature. It may be up to 300 parts by mass. When the amount of the hydrocarbon dispersion medium is 30 parts by mass or more, the polymerization temperature tends to be easily controlled. When the amount of the hydrocarbon dispersion medium is 1000 parts by mass or less, the productivity of polymerization tends to be improved, which is economical.

The radical polymerization initiator may be water-soluble. Examples of water-soluble radical polymerization initiators are persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, and t-butyl cumylper. Peroxides such as oxides, t-butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, hydrogen peroxide; 2,2'-azobis (2-amidinopropane) dihydrochloride , 2,2'-azobis [2- (N-phenylamidino) propane] dihydrochloride, 2,2'-azobis [2- (N-allylamidino) propane] dihydrochloride, 2,2'-azobis [ 2- (2-Imidazolin-2-yl) propane] 2 hydrochloride, 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} 2 hydrochloride, 2,2'-azobis {2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2'-azobis [2-methyl-N- (2-hydroxy) Ethyl) -propionamide], azo compounds such as 4,4'-azobis (4-cyanovaleric acid) can be mentioned. The radical polymerization initiator may be used alone or in combination of two or more. The radical polymerization initiators are potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl). ) Propane] 2 hydrochloride and 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} 2 hydrochloride at least selected from the group. There may be.

The amount of the radical polymerization initiator may be 0.00005 to 0.01 mol per 1 mol of the ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.00005 mol or more, the polymerization reaction does not require a long time and is efficient. When the amount of the radical polymerization initiator is 0.01 mol or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

The exemplified radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

At the time of the polymerization reaction, the aqueous monomer solution may contain a chain transfer agent. Examples of the chain transfer agent include hypophosphates, thiols, thiolic acids, secondary alcohols, amines and the like.

In order to control the particle size of the water-absorbent resin particles, the monomer aqueous solution used for polymerization may contain a thickener. Examples of the thickener include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide and the like. If the stirring speed at the time of polymerization is the same, the higher the viscosity of the aqueous monomer solution, the larger the medium particle size of the obtained particles tends to be.

Cross-linking occurs by self-cross-linking during polymerization, but cross-linking may be performed by further using an internal cross-linking agent. When an internal cross-linking agent is used, it is easy to control the water absorption characteristics of the water-absorbent resin particles. The internal cross-linking agent is usually added to the reaction solution during the polymerization reaction. Examples of the internal cross-linking agent include di or tri (meth) acrylic acid esters of polyols such as ethylene glycol, propylene glycol, trimethylpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol and polyglycerin; Unsaturated polyesters obtained by reacting polyols with unsaturated acids (maleic acid, fumaric acid, etc.); bis (meth) acrylamides such as N, N'-methylenebis (meth) acrylamide; polyepoxides and (meth) Di or tri (meth) acrylic acid esters obtained by reacting with acrylic acid; di (meth) obtained by reacting polyisocyanate (tolylene diisocyanate, hexamethylene diisocyanate, etc.) with hydroxyethyl (meth) acrylate. ) Acrylic acid carbamil esters; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N, N', N "-triallyl isocyanurate, divinylbenzene; Poly such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. Glycidyl compounds; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; 2 reactive functional groups such as isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.) Examples thereof include compounds having more than one. The internal cross-linking agent may be used alone or in combination of two or more. The internal cross-linking agent may be a polyglycidyl compound or diglycidyl. It may be an ether compound. The internal cross-linking agent comprises at least one selected from the group consisting of (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether. It may be.

The amount of the internal cross-linking agent is 0 per mol of the ethylenically unsaturated monomer from the viewpoint that the water-soluble property is suppressed by appropriately cross-linking the obtained polymer and a sufficient amount of water absorption can be easily obtained. It may be mmol or more, 0.01 mmol or more, 0.015 mmol or more, 0.020 mmol or more, or 0.1 mol or less.

An aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, an internal cross-linking agent, etc., if necessary, and an oil containing a hydrocarbon-based dispersant, a surfactant, a polymer-based dispersant, etc., if necessary. Reversed phase suspension polymerization can be carried out in an aqueous system in oil by heating with stirring in a state where the phases are mixed.

When performing reverse phase suspension polymerization, a monomer aqueous solution containing an ethylenically unsaturated monomer is used as a hydrocarbon dispersion medium in the presence of a surfactant (and, if necessary, a polymer-based dispersant). Disperse in. At this time, before the start of the polymerization reaction, the timing of adding the surfactant, the polymer-based dispersant, etc. may be either before or after the addition of the monomer aqueous solution.

From the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water-absorbent resin, the monomer aqueous solution is dispersed in the hydrocarbon dispersion medium in which the polymer-based dispersant is dispersed, and then the surfactant is further dispersed. It may be allowed to carry out polymerization.

Reverse phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages. Reversed phase suspension polymerization may be carried out in two or three stages from the viewpoint of increasing productivity.

When reverse phase suspension polymerization is carried out in two or more stages, an ethylenically unsaturated monomer is added to the reaction mixture obtained in the first step polymerization reaction after the first step reverse phase suspension polymerization is carried out. It may be added and mixed, and the reverse phase suspension polymerization of the second and subsequent steps may be carried out in the same manner as in the first step. In the reverse phase suspension polymerization in each stage of the second and subsequent stages, in addition to the ethylenically unsaturated monomer, the above-mentioned radical polymerization initiator is used in the reverse phase suspension polymerization in each stage of the second and subsequent stages. Based on the amount of the ethylenically unsaturated monomer to be added, it may be added within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. In the reverse phase suspension polymerization in each stage after the second stage, an internal cross-linking agent may be used if necessary. When an internal cross-linking agent is used, it is added within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer based on the amount of the ethylenically unsaturated monomer provided in each stage, and the suspension is reversed. Muddy polymerization may be carried out.

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but by advancing the polymerization rapidly and shortening the polymerization time, the efficiency is improved and the heat of polymerization is easily removed to carry out the reaction smoothly. From the viewpoint, it may be 20 to 150 ° C. or 40 to 120 ° C. The reaction time is usually 0.5-4 hours. The completion of the polymerization reaction can be confirmed, for example, by stopping the temperature rise in the reaction system. As a result, the polymer of the ethylenically unsaturated monomer is usually obtained in the state of a hydrogel-like polymer.

After polymerization, cross-linking may be performed after polymerization by adding a cross-linking agent to the obtained hydrogel polymer and heating it. By performing cross-linking after polymerization, the degree of cross-linking of the hydrogel polymer can be increased, whereby the water-absorbing characteristics of the water-absorbent resin particles can be further improved.

Examples of the cross-linking agent for performing post-polymerization cross-linking include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; Compounds having two or more epoxy groups such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether; epichlorohydrin, epibromhydrin, α-methylepicrolhydrin, etc. Haloepoxy compounds; compounds having two or more isocyanate groups such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylenecarbonate; bis [N , N-di (β-hydroxyethyl)] hydroxyalkylamide compounds such as adipamide can be mentioned. Cross-linking agents for post-polymerization cross-linking are (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether. It may be a polyglycidyl compound such as. These cross-linking agents may be used alone or in combination of two or more.

The amount of the cross-linking agent used for post-polymerization cross-linking is 1 mol of the water-soluble ethylenically unsaturated monomer from the viewpoint of appropriately cross-linking the obtained hydrogel-like polymer to exhibit suitable water absorption characteristics. It may be 0 to 0.03 mol, 0 to 0.01 mol, or 0.00001 to 0.005 mol.

The cross-linking agent for post-polymerization cross-linking is added to the reaction solution after the polymerization reaction of the ethylenically unsaturated monomer. In the case of multi-stage polymerization, a cross-linking agent for post-polymerization cross-linking may be added after the multi-stage polymerization. Considering the fluctuation of water content due to heat generation during and after polymerization, retention due to process delay, opening of the system when adding a cross-linking agent, and addition of water due to addition of a cross-linking agent, the cross-linking agent for post-polymerization cross-linking is From the viewpoint of water content (described later), it may be added in the region of [water content immediately after polymerization ± 3% by mass].

Subsequently, water is removed from the obtained hydrogel polymer. Drying to remove water gives polymer particles containing a polymer of ethylenically unsaturated monomers. Examples of the drying method include (a) a method of removing water by co-boiling distillation in a state where the hydrogel polymer is dispersed in a hydrocarbon dispersion medium, and (b) taking out the hydrogel polymer by decantation and reducing the pressure. Examples thereof include a method of drying, and (c) a method of filtering the hydrogel polymer by a filter and drying under reduced pressure.

The particle size of the water-absorbent resin particles can be adjusted by adjusting the rotation speed of the stirrer during the polymerization reaction, or by adding a flocculant into the system after the polymerization reaction or in the early stage of drying. By adding a flocculant, the particle size of the obtained water-absorbent resin particles can be increased. As the flocculant, an inorganic flocculant can be used. Examples of the inorganic flocculant (for example, powdered inorganic flocculant) include silica, zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, hydrotalcite and the like. From the viewpoint of excellent aggregating effect, the aggregating agent may be at least one selected from the group consisting of silica, aluminum oxide, talc and kaolin.

In reverse phase suspension polymerization, a coagulant is previously dispersed in a hydrocarbon dispersion medium of the same type as that used in the polymerization or water, and then this is placed in a hydrocarbon dispersion medium containing a hydrogel polymer under stirring. May be mixed with.

The amount of the flocculant is 0.001 to 1 part by mass, 0.005 to 0.5 part by mass, or 0.01 to 0.2 with respect to 100 parts by mass of the ethylenically unsaturated monomer used for the polymerization. It may be a mass part. When the amount of the flocculant is within these ranges, it is easy to obtain water-absorbent resin particles having a desired particle size distribution.

The polymerization reaction can be carried out using various stirrers having stirring blades. As the stirring blade, a flat plate blade, a lattice blade, a paddle blade, a propeller blade, an anchor blade, a turbine blade, a Faudler blade, a ribbon blade, a full zone blade, a max blend blade and the like can be used. The flat plate blade has a shaft (stirring shaft) and a flat plate portion (stirring portion) arranged around the shaft. Further, the flat plate portion may have a slit or the like. When a flat plate blade is used as the stirring blade, the uniformity of cross-linking of the polymer in the formed polymer particles tends to be high. The water-absorbent resin particles containing the polymer particles having high cross-linking uniformity tend to easily form a resin layer showing a moderately large variation in friction coefficient MMD.

In the production of the water-absorbent resin particles, the surface portion of the hydrogel polymer may be crosslinked (surface crosslinked) using a crosslinking agent in any of the drying steps and subsequent steps. By performing surface cross-linking, it is easy to control the water absorption characteristics of the water-absorbent resin particles. The water content of the surface-crosslinked hydrogel polymer may be 5 to 50% by mass, 10 to 40% by mass, or 15 to 35% by mass. The water content (mass%) of the water-containing gel polymer is calculated by the following formula.
Moisture content = [Ww / (Ww + Ws)] x 100
Ww: Necessary when mixing a flocculant, a surface cross-linking agent, etc. to the amount obtained by subtracting the amount of water discharged to the outside of the system by the drying step from the amount of water contained in the monomer aqueous solution before polymerization in the entire polymerization step. The water content of the hydrogel polymer calculated by adding the water content used according to.

Ws: The amount of solids calculated from the amount of materials such as ethylenically unsaturated monomers, cross-linking agents, and initiators that make up the hydrogel polymer.

Examples of the cross-linking agent (surface cross-linking agent) for performing surface cross-linking include compounds having two or more reactive functional groups. Examples of cross-linking agents include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol diglycidyl. Polyglycidyl compounds such as ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, trimethylpropan triglycidyl ether (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether; epichlorohydrin , Epibrom hydrin, α-methyl epichlorohydrin and other haloepoxy compounds; 2,4-tolylene diisocyanate, hexamethylene diisocyanate and other isocyanate compounds; 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol; 1,2-ethylenebisoxazoline and the like. Oxazoline compounds; carbonate compounds such as ethylene carbonate; hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide can be mentioned. The surface cross-linking agent may be used alone or in combination of two or more. The surface cross-linking agent may be a polyglycidyl compound, and may be (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and It may contain at least one selected from the group consisting of polyglycerol polyglycidyl ether.

The amount of the surface cross-linking agent is 0.00001 to 0.02 mol, 0.00005 to 0.01 mol, or 0.0001 to 0.005 with respect to 1 mol of the ethylenically unsaturated monomer used for the polymerization. It may be a mole. When the amount of the surface cross-linking agent is 0.00001 mol or more, the cross-linking density on the surface portion of the water-absorbent resin particles is sufficiently increased, and the gel strength of the water-absorbent resin particles can be easily increased. When the amount of the surface cross-linking agent is 0.02 mol or less, it is easy to increase the water absorption amount of the water-absorbent resin particles.

After surface cross-linking, water and a hydrocarbon dispersion medium are distilled off to obtain polymer particles which are dried products of surface-cross-linked water-absorbent resin particles.

The water-absorbent resin particles according to the present embodiment may be composed of only polymer particles, but various additional particles selected from, for example, a gel stabilizer, a metal chelating agent, a fluidity improver (lubricant), and the like. Ingredients can be further included. Additional components may be placed inside the polymer particles, on the surface of the polymer particles, or both. The additional component may be a fluidity improver (lubricant). The fluidity improver may contain inorganic particles. Examples of the inorganic particles include silica particles such as amorphous silica.

The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer particles. For example, by mixing the polymer particles and the inorganic particles, the inorganic particles can be arranged on the surface of the polymer particles. The inorganic particles may be silica particles such as amorphous silica. When the water-absorbent resin particles include inorganic particles arranged on the surface of the polymer particles, the ratio of the amount of the inorganic particles to the mass of the polymer particles is 0.2% by mass or more, 0.5% by mass or more, 1 It may be 0.0% by mass or more, 1.5% by mass or more, 5.0% by mass or less, or 3.5% by mass or less. The inorganic particles here usually have a minute size as compared with the size of the polymer particles. For example, the average particle size of the inorganic particles may be 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle size here can be a value measured by a dynamic light scattering method or a laser diffraction / scattering method.

FIG. 1 is a cross-sectional view showing an example of a water absorption sheet. The water absorbing sheet 50 shown in FIG. 1 has an absorbing layer 10 and two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b are arranged on both sides of the absorption layer 10. In other words, the absorbent layer 10 is arranged inside the core wrap sheets 20a and 20b. The absorbent layer 10 is held in shape by being sandwiched between the two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b may be two sheets, one folded sheet, or one bag body.

The water absorbing sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the absorbing layer 10. FIG. 2 is a plan view showing an example of an adhesive pattern formed on the core wrap sheet. The adhesive 21 shown in FIG. 2 forms a pattern composed of a plurality of linear portions arranged at intervals on the core wrap sheet 20a. However, the pattern of the adhesive 21 is not limited to this. An adhesive layer may be interposed between the core wrap sheets 20a and 20b on both sides and the absorption layer 10. The adhesive 21 is not particularly limited, and may be, for example, a hot melt adhesive.

The absorption layer 10 has the water-absorbent resin particles 10a according to the above-described embodiment and the fiber layer 10b containing a fibrous material. The absorption layer 10 does not have to have the fiber layer 10b. The content of the water-absorbent resin particles in the absorption layer may be 70 to 100% by mass, 80 to 100% by mass, or 90 to 100% by mass based on the mass of the absorption layer 10.

The thickness of the absorption layer 10 is not particularly limited, but may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less in a dry state, and is 0.1 mm or more or 0.3 mm or less. It may be the above. The mass per unit area of the absorption layer 10 may be 1000 g / m ² or less, 800 g / m ² or less, 600 g / m ² or less, or 100 g / m ² or more.

The fibrous material constituting the fiber layer 10b can be, for example, a cellulosic fiber, a synthetic fiber, or a combination thereof. Examples of cellulosic fibers include crushed wood pulp, cotton, cotton linters, rayon and cellulosic acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may be hydrophilic fibers (for example, pulp). The average fiber length of the fibrous material is usually 0.1 to 10 mm and may be 0.5 to 5 mm.

The absorption layer 10 (or fiber layer 10b) may further contain an inorganic powder (for example, amorphous silica), a deodorant, an antibacterial agent, a fragrance, and the like. When the water-absorbent resin particles 10a contain inorganic particles, the absorption layer 10 may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles 10a.

The core wrap sheets 20a and 20b may be, for example, a non-woven fabric. The two core wrap sheets 20a and 20b can be the same or different non-woven fabrics. The non-woven fabric may be a non-woven fabric composed of short fibers (that is, staples) (short-fiber non-woven fabric) or a non-woven fabric composed of long fibers (that is, filaments) (long-fiber non-woven fabric). Staples are not limited to this, but generally may have a fiber length of several hundred mm or less.

The core wrap sheets 20a and 20b are laminated including a thermal bond non-woven fabric, an air-through non-woven fabric, a resin bond non-woven fabric, a spunbond non-woven fabric, a melt blow non-woven fabric, an air-laid non-woven fabric, a spunlace non-woven fabric, a point bond non-woven fabric, or two or more kinds of non-woven fabrics selected from these. It can be a body.

The non-woven fabric used as the core wrap sheets 20a and 20b can be a non-woven fabric formed of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamides such as nylon, and Examples thereof include fibers containing a synthetic resin selected from rayon. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the non-woven fabric may be polyolefin fibers, polyester fibers or a combination thereof. The core wrap sheets 20a and 20b may be tissue paper.

The water-absorbent sheet 50 is sandwiched between, for example, the water-absorbent resin particles 10a or a mixture containing the water-absorbent resin particles 10a and the fibrous material and the core wrap sheets 20a and 20b, and the formed structure is heated as necessary. It can be obtained by the method of pressurizing. If necessary, the adhesive 21 is arranged between the core wrap sheets 20a and 20b and the water-absorbent resin particles 10a or a mixture containing the same.

The water absorption sheet 50 is used, for example, for producing various absorbent articles. Examples of absorbent articles include diapers (eg paper diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, toilet components, and animal waste treatment materials. Can be mentioned.

FIG. 3 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in FIG. 3 includes a water absorbing sheet 50, a liquid permeable sheet 30, and a liquid impermeable sheet 40. In other words, the water absorbing sheet 50 is sandwiched between the liquid permeable sheet 30 and the liquid impermeable sheet 40.

The liquid permeable sheet 30 is arranged at the position of the outermost layer on the side where the liquid to be absorbed enters. The liquid permeable sheet 30 is arranged on the outside of the core wrap sheet 20b in contact with the core wrap sheet 20b. The liquid permeable sheet 40 is arranged at the position of the outermost layer on the side opposite to the liquid permeable sheet 30 in the absorbent article 100. The liquid impermeable sheet 40 is arranged on the outside of the core wrap sheet 20a in contact with the core wrap sheet 20a. The liquid permeable sheet 30 and the liquid permeable sheet 40 have a main surface wider than the main surface of the water absorbing sheet 50, and the outer edges of the liquid permeable sheet 30 and the liquid permeable sheet 40 are an absorbent layer. It extends around 10 and the core wrap sheets 20a and 20b. However, the magnitude relationship between the absorbent layer 10, the core wrap sheets 20a and 20b, the liquid permeable sheet 30, and the liquid permeable sheet 40 is not particularly limited, and is appropriately adjusted according to the use of the absorbent article and the like. ..

The liquid permeable sheet 30 may be a non-woven fabric. The non-woven fabric used as the liquid permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From this point of view, the liquid permeable sheet 30 is obtained by the pulp and paper test method No. A non-woven fabric having a hydrophilicity of 5 to 200 measured according to the measuring method of 68 (2000) may be used. The hydrophilicity of the non-woven fabric may be 10 to 150. Pulp and paper test method No. For details of 68, for example, WO2011 / 086843 can be referred to.

The non-woven fabric having hydrophilicity may be formed of fibers showing appropriate hydrophilicity such as rayon fiber, or obtained by hydrophilizing a hydrophobic chemical fiber such as polyolefin fiber or polyester fiber. It may be formed of rayon fibers. As a method for obtaining a non-woven fabric containing hydrophobic chemical fibers that have been hydrophobized, for example, a method for obtaining a non-woven fabric by a spunbond method using a mixture of hydrophobic chemical fibers and a hydrophilic agent, hydrophobic chemistry. Examples thereof include a method of accommodating a hydrophilic agent when producing a spunbonded nonwoven fabric from fibers, and a method of impregnating a spunbonded nonwoven fabric obtained by using a hydrophobic chemical fiber with a hydrophilic agent. Examples of the hydrophilizing agent include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acids. Nonionic surfactants such as esters, silicone-based surfactants such as polyoxyalkylene-modified silicones, and stain-releasing agents made of polyester-based, polyamide-based, acrylic-based, and urethane-based resins are used.

The amount of texture (mass per unit area) of the non-woven fabric used as the liquid permeable sheet 30 is from the viewpoint of imparting good liquid permeability, flexibility, strength and cushioning property to the absorbent article, and the liquid of the absorbent article. From the viewpoint of increasing the permeation rate, it may be 5 to 200 g / m ² , 8 to 150 g / m ² , or 10 to 100 g / m ² . The thickness of the liquid permeable sheet 30 may be 20 to 1400 μm, 50 to 1200 μm, or 80 to 1000 μm.

The liquid impermeable sheet 40 prevents the liquid absorbed by the absorbing layer 10 from leaking to the outside from the liquid impermeable sheet 40 side. The liquid impermeable sheet 40 may be a resin sheet or a non-woven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The non-woven fabric may be a spunbond / meltblow / spunbond (SMS) non-woven fabric in which a water-resistant melt-blow non-woven fabric is sandwiched between high-strength spunbond non-woven fabrics. The liquid impermeable sheet 40 may be a composite sheet of a resin sheet and a non-woven fabric (for example, a spunbonded non-woven fabric or a spunlaced non-woven fabric). The liquid impermeable sheet 40 may have breathability from the viewpoint that stuffiness at the time of wearing is reduced and discomfort given to the wearer can be reduced. As the liquid impermeable sheet 40 having breathability, for example, a sheet of low density polyethylene (LDPE) resin can be used.

From the viewpoint of ensuring flexibility so as not to impair the wearing feeling of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 may be 10 to 50 g / m ² .

The absorbent article 100 can be manufactured, for example, by a method including arranging the water absorbing sheet 50 between the liquid permeable sheet 30 and the liquid impermeable sheet 40. A laminate in which the liquid permeable sheet 40, the water absorbing sheet 50, and the liquid permeable sheet 30 are laminated in this order is pressurized as necessary. Alternatively, the liquid permeable sheet 30, the core wrap sheet 20b, the water-absorbent resin particles 10a, or the mixture containing the water-absorbent resin particles 10a and the fibrous material, and the core wrap sheet 20a and the liquid impermeable sheet 40 are used. The absorbent article 100 can also be obtained by arranging in this order and pressurizing the formed structure while heating if necessary.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

1. 1. Preparation of water-absorbent resin particles Example 1
<First stage polymerization reaction>
A round-bottomed cylindrical separable flask having an inner diameter of 11 cm and an internal volume of 2 L equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction pipe, and a stirrer was prepared. A stirring blade 200 whose outline is shown in FIG. 4 was attached to the stirring machine. The stirring blade 200 includes a shaft 200a and a flat plate portion 200b. The flat plate portion 200b is welded to the shaft 200a and has a curved tip. The flat plate portion 200b is formed with four slits S extending along the axial direction of the shaft 200a. The four slits S are arranged in the width direction of the flat plate portion 200b, the width of the two inner slits S is 1 cm, and the width of the two outer slits S is 0.5 cm. The length of the flat plate portion 200b is about 10 cm, and the width of the flat plate portion 200b is about 6 cm.

In the prepared separable flask, 293 g of n-heptane and 0.736 g of a dispersant (maleic anhydride-modified ethylene / propylene copolymer, manufactured by Mitsui Chemicals, Inc., high wax 1105A) were mixed. The dispersant was dissolved in n-heptane by heating the mixture in the separable flask to 80 ° C. while stirring with a stirrer. The formed reaction solution was cooled to 50 ° C.

92.0 g (1.03 mol) of an acrylic acid aqueous solution having a concentration of 80.5 mass% was placed in a beaker having an internal volume of 300 mL. While cooling from the outside, 147.7 g of an aqueous sodium hydroxide solution having a concentration of 20.9% by mass was added dropwise to the aqueous acrylic acid solution, thereby neutralizing 75 mol% of acrylic acid. 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Co., Ltd., HECAW-15F) as a thickener and 2,2'-azobis (2-amidinopropane) 2 as a water-soluble radical polymerization initiator in the neutralized acrylic acid aqueous solution. Dissolve these by adding 0.092 g (0.339 mmol) of hydrochloride, 0.0162 g (0.068 mmol) of sodium persulfate, and 0.0046 g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent. The first-stage monomer aqueous solution was prepared.

The first-stage monomer aqueous solution was added to the reaction solution in the separable flask described above, and the reaction solution was stirred for 10 minutes. Next, a surfactant solution containing 6.62 g of n-heptane and 0.736 g of sucrose stearic acid ester (HLB: 3, Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370) was added to the reaction solution, and the mixture was stirred. The inside of the system was sufficiently replaced with nitrogen while stirring the reaction solution at a blade rotation speed of 425 rpm. Then, the polymerization reaction was allowed to proceed over 60 minutes while heating the separable flask in a water bath at 70 ° C. By this polymerization reaction, a first-stage polymerization slurry liquid containing a hydrogel-like polymer was obtained.

<Second stage polymerization reaction>
128.8 g (1.44 mol) of an aqueous acrylic acid solution having a concentration of 80.5 mass% was placed in a beaker having an internal volume of 500 mL. While cooling the beaker from the outside, 159.0 g of a sodium hydroxide aqueous solution having a concentration of 27% by mass was added dropwise to the acrylic acid aqueous solution, thereby neutralizing 75 mol% of acrylic acid. Next, in an aqueous acrylic acid solution, 0.129 g (0.475 mmol) of 2,2'-azobis (2-amidinopropane) dihydrochloride salt and 0.0226 g (0.095 mmol) of sodium persulfate as a water-soluble radical polymerization initiator. ) And 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent were added to dissolve them to prepare a second-stage monomer aqueous solution.

The first-stage polymerized slurry liquid in the separable flask was cooled to 22 ° C. while stirring at a stirring blade rotation speed of 650 rpm. The whole amount of the second-stage monomer aqueous solution was added thereto, and then the inside of the system was replaced with nitrogen over 30 minutes. Then, while heating the separable flask in a water bath at 70 ° C., the second-stage polymerization reaction was allowed to proceed over 60 minutes.

0.589 g of a 45% by mass diethylenetriamine-5 sodium acetate aqueous solution was added to the reaction solution after the second stage polymerization reaction under stirring. Then, a separable flask was immersed in an oil bath at 125 ° C., and 229.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water. Then, 4.42 g (0.507 mmol) of an ethylene glycol diglycidyl ether aqueous solution having a concentration of 2% by mass was added to the reaction solution as a surface cross-linking agent, and the reaction solution was held at 83 ° C. for 2 hours to obtain a surface cross-linking agent. The cross-linking reaction was allowed to proceed.

From the reaction solution after the cross-linking reaction with the surface cross-linking agent, n-heptane was distilled off by heating at 125 ° C. to obtain a dried product of polymer particles. The obtained polymer particles were passed through a sieve having an opening of 850 μm. Then, 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxile NP-S) with respect to the mass of the polymer particles was mixed with the polymer particles, and the water-absorbent resin particles 214 containing the amorphous silica 9.9 g was obtained. The medium particle size of the water-absorbent resin particles was 470 μm.

Example 2
The radical polymerization initiator used in the preparation of the aqueous solution in the first stage was changed to 0.0648 g (0.272 mmol) of sodium persulfate, and the amount of ethylene glycol diglycidyl ether added as an internal cross-linking agent was 0. It was changed to 010 g (0.057 mmol), the rotation speed of the stirrer at the time of nitrogen substitution was changed to 350 rpm in the preparation of the first stage polymerized slurry liquid, and the radical used in the preparation of the second stage aqueous liquid liquid. The same as in Example 1 except that the polymerization initiator was changed to 0.0907 g (0.381 mmol) of sodium persulfate and the amount of water extracted to the outside of the system by azeotropic distillation was changed to 271.0 g. 215.8 g of water-absorbent resin particles were obtained. The medium particle size of the water-absorbent resin particles was 470 μm.

Example 3
The radical polymerization initiator used in the preparation of the aqueous solution in the first stage was changed to 0.0648 g (0.272 mmol) of sodium persulfate, and the amount of ethylene glycol diglycidyl ether added as an internal cross-linking agent was 0. The change to 010 g (0.057 mmol), the radical polymerization initiator used in the second stage aqueous solution preparation was changed to 0.0907 g (0.381 mmol) of sodium persulfate, and the second polymerization. In the reaction, the polymerization slurry solution of the first stage was cooled to 25 ° C., the entire amount of the monomer aqueous solution of the second stage was added, and the amount of water extracted to the outside of the system by co-boiling distillation was 256. 230.2 g of water-absorbent resin particles were obtained in the same manner as in Example 1 except that the amount was changed to 5.5 g. The medium particle size of the water-absorbent resin particles was 358 μm.

Comparative Example 1
The stirring blade was changed to one with a 4-blade inclined paddle blade with a blade diameter of 5 cm in two stages, and the radical polymerization initiator used in the preparation of the aqueous liquid in the first stage was 2,2'-azobis (2-amidino). Changed to 0.092 g (0.339 mmol) of propane) dihydrochloride and 0.018 g (0.068 mmol) of potassium persulfate, and rotation of the stirrer at the time of nitrogen substitution in the preparation of the polymerization slurry solution in the first stage. The number was changed to 550 rpm, and the radical polymerization initiator used in the second stage aqueous solution preparation was 0.129 g (0.475 mmol) of 2,2'-azobis (2-amidinopropane) dihydrochloride. After changing to 0.026 g (0.095 mmol) of potassium persulfate and preparing the aqueous solution in the second stage, the inside of the separable flask system was cooled to 25 ° C., and the stirrer rotation speed at that time was changed to 1000 rpm. 228.6 g of water-absorbent resin particles were obtained in the same manner as in Example 1 except that the amount of water extracted to the outside of the system by the radical polymerization was changed to 215.8 g. The medium particle size of the water-absorbent resin particles was 339 μm.

Comparative Example 2
The stirring blade was changed to one with a 4-piece inclined paddle blade with a blade diameter of 5 cm in two stages, and the radical polymerization initiator used in the preparation of the aqueous liquid in the first stage was 2,2'-azobis (2-amidino). Propane) dihydrochloride 0.110 g (0.407 mmol) was changed, no cross-linking agent was added for internal cross-linking, and the thickener used was changed to 1.38 g of hydroxylethyl cellulose. In the preparation of the first-stage polymerization slurry liquid, the rotation speed of the stirrer at the time of nitrogen substitution was changed to 400 rpm, and after the first-stage polymerization reaction, co-boiling without performing the second-stage polymerization reaction. Except that the amount of water extracted to the outside of the system by distillation was changed to 109 g, and the amount of ethylene glycol diglycidyl ether aqueous solution having a concentration of 2% by mass as a surface cross-linking agent was changed to 9.20 g (1.056 mmol). In the same manner as in Example 1, 90.0 g of water-absorbent resin particles were obtained. The medium particle size of the water-absorbent resin particles was 395 μm.

2. Evaluation 2-1. Water retention amount The water retention amount (room temperature, 25 ° C ± 2 ° C) of the physiological saline of the water-absorbent resin particles was measured by the following procedure. A cotton bag (Membroad No. 60, width 100 mm x length 200 mm) weighing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. Pour 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline) into a cotton bag containing water-absorbent resin particles at a time so that mamaco cannot be formed, tie the upper part of the cotton bag with a rubber ring, and let it stand for 30 minutes. The water-absorbent resin particles were swollen with. After 30 minutes, the cotton bag is dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to have a centrifugal force of 167 G, and the cotton bag containing the swelling gel after dehydration. The mass Wa (g) of was measured. The same operation was performed without adding the water-absorbent resin particles, and the empty mass Wb (g) of the cotton bag when wet was measured. The amount of water retained was calculated from the following formula.
Water retention (g / g) = [Wa-Wb] /2.0

2-2. Amount of water absorption under load The amount of water absorption of water-absorbent resin particles with respect to physiological saline under a load of 2.07 kPa was measured with the measuring device Y shown in FIG. 5 under an environment of a temperature of 25 ° C. ± 2 ° C. and a humidity of 50 ± 10%. Measured using. The measuring device Y is composed of a burette unit 71, a conduit 72, a measuring table 73, and a measuring unit 74 placed on the measuring table 73. The burette portion 71 has a burette 71a extending in the vertical direction, a rubber stopper 71b arranged at the upper end of the burette 71a, a cock 71c arranged at the lower end of the burette 71a, and one end extending into the burette 71a in the vicinity of the cock 71c. It has an air introduction pipe 71d and a cock 71e arranged on the other end side of the air introduction pipe 71d. The conduit 72 is attached between the burette portion 71 and the measuring table 73. The inner diameter of the conduit 72 is 6 mm. A hole having a diameter of 2 mm is formed in the central portion of the measuring table 73, and the conduit 72 is connected to the hole. The measuring unit 74 has a cylinder 74a (made of acrylic resin (plexiglass)), a nylon mesh 74b adhered to the bottom of the cylinder 74a, and a weight 74c. The inner diameter of the cylinder 74a is 20 mm. The opening of the nylon mesh 74b is 75 μm (200 mesh). Then, at the time of measurement, the water-absorbent resin particles 75 to be measured are uniformly sprinkled on the nylon mesh 74b. The diameter of the weight 74c is 19 mm, and the mass of the weight 74c is 59.8 g. The weight 74c is placed on the water-absorbent resin particles 75, and a load of 2.07 kPa can be applied to the water-absorbent resin particles 75.

After 0.100 g of the water-absorbent resin particles 75 were placed in the cylinder 74a of the measuring device Y, the weight 74c was placed and the measurement was started. Since the same volume of air as the physiological saline absorbed by the water-absorbent resin particles 75 is quickly and smoothly supplied to the inside of the burette 71a from the air introduction pipe, the water level of the physiological saline inside the burette 71a is reduced. However, the amount of physiological saline absorbed by the water-absorbent resin particles 75 is obtained. The scale of the burette 71a is engraved from top to bottom in 0 mL to 0.5 mL increments. As the water level of the physiological saline, the scale Va of the burette 71a before the start of water absorption and the scale Vb of the burette 71a 60 minutes after the start of water absorption are read, and the amount of water absorption under load (under a load of 2.07 kPa) is read from the following formula. The amount of water absorbed with respect to physiological saline) was calculated.
Water absorption under load [mL / g] = (Vb-Va) /0.1

2-3. Friction coefficient A simple water absorption sheet for measuring the friction coefficient was prepared. FIG. 6 is a cross-sectional view showing the produced simple water absorption sheet. Place the adhesive tape 25 (Biolan tape (trade name) manufactured by Diatex Co., Ltd.) having a rectangular adhesive surface 25S of 5 x 15 cm on the horizontal surface of the workbench with the adhesive surface 25S facing up. It was. 1.5 g of water-absorbent resin particles were uniformly sprayed over the entire adhesive surface 25S. A 4.0 kg roller (stainless steel, diameter 10.5 cm, width 6.0 cm) is placed on the end of the resin layer 10A of the water-absorbent resin particles adhered to the adhesive surface 25 by spraying along the short side, and the roller is adhered. One round trip was made between the two short sides of the surface 25S. Next, a tissue paper 27 (mass per unit area: 16 g / m ² ) covering the entire resin layer 10A was placed on the resin layer 10A of the water-absorbent resin particles to prepare a sample for evaluating the friction coefficient. The evaluation sample was prepared so that the standard deviation of the mass of the water-absorbent resin particles sprayed on each region (5 × 3.75 cm) when divided into four equal parts in the longitudinal direction was 0.05 or less. did.

In an environment with a temperature of 25 ± 2 ° C. and a humidity of 50 ± 10%, a probe of a friction tester (KES-SE-STP (trade name) manufactured by Kato Tech Co., Ltd.) was used as a resin layer 10A of water-absorbent resin particles of tissue paper. The friction coefficient of the surface was continuously measured over a predetermined operating distance by reciprocating in a straight line while pressing against the surface 27S on the opposite side. The measurement conditions are as follows. The operating distance is the distance the probe has moved on the surface of the tissue paper. From this measurement, a curve showing the relationship between the friction coefficient μ and the moving distance x of the sensor was obtained.
・ Contact: Piano wire ・ Sens: H
・ Movement speed: 10 mm / sec ・ Load: 50 gf
・ Operating distance: 26.67 mm

From the obtained curve, the average friction coefficient (MIU) and the fluctuation of the friction coefficient (MMD) were calculated by the following formulas. In these equations, μ is the coefficient of friction at each measurement point, and x is the distance traveled by the sensor to each measurement point. The same measurement was performed 5 times, and the average value of 3 values excluding the maximum value and the minimum value among the obtained measured values was obtained. The average value is shown in Table 1. When the tissue paper used in the test was placed directly on the adhesive surface and the friction coefficient on the surface of the tissue paper was measured in the same state as above, MIU was 0.21 and MMD was 0.028.

2-4. Preparation of Water Absorption Sheet for Evaluation of Permeation Rate of Water Absorption Sheet An air-through non-woven fabric having a basis weight of 30 g / m ² unwound from a roll was cut into a size of 41 cm × 14 cm. 12 g of water-absorbent resin particles were uniformly dispersed on the outer surface of the roll of the air-through non-woven fabric using an air flow type mixing device (Padformer manufactured by Otec Co., Ltd.). 0.5 g of hot melt adhesive was applied along 12 straight lines at 1 mm intervals on the surface of a spunbonded non-woven fabric having a size of 41 cm × 14 cm and having a grain size of 13 g / m ² by a hot melt coating machine. The pattern of application was a spiral stripe. These were overlapped so that the ends of the spunbonded non-woven fabric and the air-through non-woven fabric were aligned with the surface of the spunbonded non-woven fabric to which the hot melt adhesive was attached and the surface of the air-through non-woven fabric to which the water-absorbent resin particles were sprayed on the inside. Next, the whole is sandwiched between release papers and pressed at a pressure of 0.1 MPa while heating at 110 ° C. using a laminating machine to obtain a spunbonded non-woven fabric, a hot melt adhesive, an absorbent layer composed of water-absorbent resin particles, and an absorbent layer. An evaluation water-absorbing sheet having the same structure as that of FIG. 1 was obtained in which the air-through nonwoven fabric was arranged in this order.

2-5. Penetration rate In an environment with a temperature of 25 ± 2 ° C. and a humidity of 50 ± 10%, the evaluation absorbent sheet was placed on the horizontal surface of the workbench with the air-through non-woven fabric facing upward. A liquid container having a capacity of 100 mL and a cylinder for liquid injection having an inner diameter of 3 cm extending from the bottom of the liquid container were prepared, and the tip of the input port was in contact with the central portion of the absorbent article for evaluation. It was held in a state. 50 mL of artificial urine at 25 ± 1 ° C. was poured into the liquid storage portion of the cylinder at once, and the artificial urine was absorbed by the water absorption sheet from the tip of the inlet. Using a stopwatch, the time from the time when the artificial urine was added until the artificial urine completely disappeared from the cylinder was measured. This time was defined as the first permeation rate (seconds). The same operation was performed twice more at intervals of 30 minutes, and the second and third permeation times (seconds) were measured. Artificial urine was prepared by mixing the following components.
-Ion-exchanged water: 5919.6 g
・ NaCl: 60.0 g
・ CaCl2 ・ H2O: 1.8g
-MgCl2.6H2O: 3.6g
・ Edible blue No. 1 (for coloring): 0.15 g
-Triton X-100 (1%): 15.0 g

2-6. Medium particle size 50 g of water-absorbent resin particles were used for measuring the medium particle size. The measurement was performed in an environment with a temperature of 25 ± 2 ° C. and a humidity of 50 ± 10%. From the top, JIS standard sieves have a mesh size of 850 μm, a mesh size of 500 μm, a mesh size of 425 μm, a mesh size of 300 μm, a mesh size of 250 μm, a mesh size of 180 μm, a mesh size of 150 μm, and a sieve. Combined in the order of the saucer. Water-absorbent resin particles were placed in the uppermost sieve and classified according to JIS Z 8815 (1994) using a low-tap shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage with respect to the total amount, and the particle size distribution was obtained. By integrating the mass percentages of the water-absorbent resin particles remaining on the sieve in order from the larger particle size with respect to this particle size distribution, the opening of the sieve and the integrated value of the mass percentages of the water-absorbent resin particles remaining on the sieve are integrated. The relationship with is plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to the cumulative mass percentage of 50% by mass was defined as the medium particle size.

Table 1 shows the evaluation results. Fluctuations in the coefficient of friction Absorbent articles with water-absorbent sheets made with water-absorbent resin particles with a specific MMD showed improved permeation rates.

10 ... Absorbent layer, 10a ... Water-absorbent resin particles, 10b ... Fiber layer, 20a ... Core wrap sheet, 20b ... Core wrap sheet, 21 ... Adhesive, 30 ... Liquid permeable sheet, 40 ... Liquid impermeable sheet, 50 ... water absorption sheet, 71 ... burette part, 71a ... burette, 71b ... rubber stopper, 71c ... cock, 71d ... air introduction pipe, 71e ... cock, 72 ... conduit, 73 ... measuring table, 74 ... measuring part, 74a ... cylinder, 74b ... Nylon mesh, 74c ... Weight, 75 ... Water-absorbent resin particles, 100 ... Absorbent article, 200 ... Stirring blade, 200a ... Shaft, 200b ... Flat plate, S ... Slit.

Claims (6)

1. The variation of the coefficient of friction is 0.08 or more, which is measured by a method including the following steps (1), (2), (3), (4), (5) and (6) in this order. Water-absorbent resin particles.
   (1) An adhesive tape having a rectangular adhesive surface of 5 × 15 cm is placed on the horizontal plane of the workbench with the adhesive surface facing up.
   (2) 0.02 g of water-absorbent resin particles per 1 cm ² are attached to the entire adhesive surface.
   (3) A roller having a mass of 4.0 kg, a diameter of 10.5 cm, and a width of 6.0 cm is placed on the water-absorbent resin particles adhering to the adhesive surface, and then the roller is reciprocated once between the two short sides of the adhesive surface.
   (4) Place the tissue paper on the water-absorbent resin particles adhering to the adhesive surface so that the entire water-absorbent resin particles are covered.
   (5) By moving the probe having the piano wire as a contact while pressing it against the surface of the tissue paper opposite to the water-absorbent resin particles, the friction coefficient of the surface of the tissue paper is set along a straight line of 20 mm or more. Measure continuously.
   (6) From the curve representing the relationship between the friction coefficient μ and the moving distance x of the probe, the fluctuation of the friction coefficient is calculated by the following formula. In these equations, MIU is the average coefficient of friction and MMD is the variation of the coefficient of friction.
   

   
2. The water-absorbent resin particle according to claim 1, wherein the water-absorbent resin particle has a water retention amount of 35 g / g or more when it absorbs physiological saline.
3. The water-absorbent resin particle according to claim 1, wherein the water-absorbent resin particle has a water absorption amount of 15 mL / g or more when it absorbs physiological saline under a load of 2.07 kPa.
4. A water-absorbing sheet comprising an absorbing layer containing the water-absorbing resin particles according to any one of claims 1 to 3.
5. The water-absorbent sheet according to claim 4, wherein the content of the water-absorbent resin particles is 70 to 100% by mass based on the mass of the absorption layer.
6. The water absorbing sheet according to claim 4 or 5, wherein the water absorbing sheet further includes a core wrap sheet, and the absorbing layer is arranged inside the core wrap sheet.

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