JP2020093244A - Water-absorbing resin particle - Google Patents

Abstract

To provide water-absorbing resin particles which can suppress leakage of a liquid.SOLUTION: There are provided water-absorbing resin particles in which when a speed index I of water absorption=(10 second value of non-pressure DW)×6 [mL/g] and a speed index II of water absorption=(3 minute value of non-pressure DW)×3 [mL/g], an index α of static suction calculated by the following expression: α=(I+II)/2 is 5.0 mL/g or more.SELECTED DRAWING: None

Description

The present invention relates to water absorbent resin particles.

Conventionally, an absorbent body containing water-absorbent resin particles has been used for an absorbent article for absorbing a liquid containing water as a main component such as urine. For example, the following Patent Document 1 describes a method for producing water-absorbent resin particles having a particle size that is suitable for use in absorbent articles such as diapers, and Patent Document 2 describes the effect of containing body fluid such as urine. As a conventional absorbent member, a method of using a hydrogel absorbent polymer having specific saline flow conductivity, performance under pressure and the like is disclosed.

JP-A-06-345819 Japanese Patent Publication No. 09-510889

In an absorbent article using a conventional absorbent body, the liquid to be absorbed is not sufficiently absorbed by the absorbent body, and a phenomenon in which excess liquid flows on the surface of the absorbent body (liquid running) easily occurs, resulting in absorption of the liquid. There is room for improvement in terms of leakage, which is leakage to the outside of the product.

An object of the present invention is to provide water-absorbent resin particles capable of suppressing liquid leakage of an absorber.

One aspect of the present invention is
Water absorption rate index I=(10 second value of unpressurized DW)×6 [mL/g],
When the water absorption rate index II=(3 minutes value of unpressurized DW)/3 [mL/g],
The following formula:
α=(I+II)/2
It relates to water-absorbent resin particles having a static suction index α of 5.0 mL/g or more.

The non-pressurized DW is the amount of the water-absorbent resin particles that have absorbed physiological saline until a predetermined time elapses after contact with physiological saline (concentration of 0.9 mass% saline) under no pressure. Is the water absorption rate. The unpressurized DW is represented by the absorption amount (mL) per 1 g of the water-absorbent resin particles before absorption of physiological saline. The 10-second value and the 3-minute value of the non-pressurized DW mean the absorption amounts 10 seconds and 3 minutes after the water-absorbent resin particles come into contact with the physiological saline, respectively. The water absorption rate indexes I and II correspond to values obtained by converting the 10-second value and the 3-minute value of the unpressurized DW into the water absorption rate per minute. The static suction index α corresponds to the average value of the water absorption rate indexes I and II, and a large static suction index α means that a relatively large water absorption rate is maintained for 10 seconds to 3 minutes. To do. According to the knowledge of the present inventor, the water-absorbent resin particles having a static suction index α of 5.0 mL/g or more can effectively suppress liquid leakage.

According to the present invention, it is possible to provide water-absorbent resin particles capable of suppressing liquid leakage.

It is sectional drawing which shows one Embodiment of an absorbent article. It is a schematic diagram which shows an example of the measuring method of pressureless DW. It is a schematic diagram showing a measuring device of water absorption under load of water-absorbent resin particles.

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

In the present specification, “acrylic” and “methacrylic” are collectively referred to as “(meth)acrylic”. Similarly, "acrylate" and "methacrylate" are also referred to as "(meth)acrylate". In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value of the numerical range of a certain stage can be arbitrarily combined with the upper limit value or the lower limit value of the numerical range of another stage. In the numerical range described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the examples. “A or B” may include either one of A and B, or may include both. “Water-soluble” means exhibiting a solubility of 5% by mass or more in water at 25° C. The materials exemplified in this specification may be used alone or in combination of two or more kinds. The content of each component in the composition means the total amount of the plurality of substances present in the composition, unless a plurality of substances corresponding to each component are present in the composition, unless otherwise specified.

When the following water absorption rate index coefficients I and II are obtained from the 10 second value and the 3 minute value of the non-pressurized DW of the water absorbent resin particles according to one embodiment,
Water absorption rate index I=(10-second value of unpressurized DW)×6 [mL/g]
Water absorption rate index II=(3 minutes value of unpressurized DW)/3 [mL/g]
The following formula:
α=(I+II)/2
The static suction index α calculated by is 5.0 mL/g or more. When the static suction index α is large, liquid leakage due to the water absorbent resin particles tends to be more effectively suppressed. Static suction index α is 6.0 mL/g or more, 7.0 mL/g or more, 9.0 mL/g or more, 10 mL/g or more, 11 mL/g or more, 12 mL/g or more, 13 mL/g or more, 14 mL/g Or more, or 15 mL/g or more. The upper limit of the static suction index α is not particularly limited, but is usually 25 mL/g or less.

The water absorption rate index I may be 2 mL/g or more and 30 mL/g or less. The water absorption rate index II may be 8 mL/g or more, and may be 30 mL/g or less. When the water absorption rate indexes I and II are within these numerical ranges, the static suction index α tends to be 5.0 mL/g.

The water-absorbent resin particles exhibiting a static suction index α of 5.0 mL/g or more can be obtained by, for example, attaching silica particles (for example, amorphous silica) to the polymer particles described later and increasing the surface area of the water-absorbent resin particles. By adjusting the cross-linking density in the surface layer of the water-absorbent resin particles, or a combination thereof.

The water retention capacity of the physiological saline of the water absorbent resin particles according to the present embodiment may be, for example, 20 to 60 g/g, 25 to 55 g/g, 30 to 50 g/g, or 32 to 42 g/g. The water retention capacity of the physiological saline is measured by the method described in Examples described later.

The water absorption amount of the physiological saline solution under the load of the water absorbent resin particles according to the present embodiment may be, for example, 10 to 40 mL/g, 15 to 35 mL/g, 20 to 30 mL/g, or 22 to 28 mL/g. . As the water absorption amount of the physiological saline under the load, the water absorption amount at the load of 4.14 kPa (25° C.) can be used. The amount of water absorption can be measured by the method described in Examples below.

Examples of the shape of the water absorbent resin particles according to the present embodiment include a substantially spherical shape, a crushed shape, and a granular shape. The median particle diameter of the water absorbent resin particles according to the present embodiment may be 250 to 850 μm, 300 to 700 μm, or 300 to 600 μm. The water-absorbent resin particles according to the present embodiment may have a desired particle size distribution at the time of being obtained by the production method described later, but the particle size distribution by performing an operation such as particle size adjustment using classification with a sieve. May be adjusted.

The water absorbent resin particles according to the present embodiment can include, for example, a crosslinked polymer formed by polymerization of 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. Among these, the reverse phase suspension polymerization method or the aqueous solution polymerization method is preferable from the viewpoints of ensuring good water absorbing properties of the water-absorbent resin particles to be obtained and easily controlling the polymerization reaction. In the following, the reverse phase suspension polymerization method will be described as an example of the method for polymerizing the ethylenically unsaturated monomer.

The ethylenically unsaturated monomer is preferably water-soluble, and examples thereof include (meth)acrylic acid and salts thereof, 2-(meth)acrylamido-2-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, N,N-diethylaminoethyl(meth)acrylate, N,N- Examples thereof include diethylaminopropyl (meth)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 kinds. A functional group such as a carboxyl group and an amino group of the above-mentioned monomer can function as a functional group capable of being crosslinked in the surface crosslinking step described later.

Among these, the ethylenically unsaturated monomer is selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide from the viewpoint of being industrially easily available. It is preferable to contain at least one compound selected, and it is more preferable to contain at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and acrylamide. From the viewpoint of further enhancing the water absorption property, the ethylenically unsaturated monomer preferably contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof.

Usually, the ethylenically unsaturated monomer is preferably used 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 preferably 20% by mass or more and the saturated concentration or less, and 25 to 70% by mass is preferable. More preferably, 30-55 mass% is still more preferable. Examples of 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 by being mixed with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used is preferably 70 to 100 mol% based on the total amount of the monomers. Above all, the proportion of (meth)acrylic acid and its salt is more preferably 70 to 100 mol% with respect to the total amount of the monomers.

When the ethylenically unsaturated monomer has an acid group, the aqueous monomer solution may be used after neutralizing the acid group with an alkaline neutralizing agent. The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent is determined from the viewpoint of increasing the osmotic pressure of the water-absorbent resin particles to be obtained and further improving the water absorption characteristics (water absorption rate, etc.). It is preferably 10 to 100 mol%, more preferably 50 to 90 mol%, and further preferably 60 to 80 mol% of the acidic groups in the monomer. Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. The alkaline neutralizing agents may be used alone or in combination of two or more kinds. The alkaline neutralizing agent may be used in the form of an aqueous solution in order to simplify the neutralizing operation. The acid group of the ethylenically unsaturated monomer can be neutralized by, for example, dropping an aqueous solution of sodium hydroxide, potassium hydroxide or the like into the above-mentioned aqueous monomer solution and mixing them.

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

Examples of the surfactant include nonionic surfactants and anionic surfactants. As the nonionic surfactant, sorbitan fatty acid ester and (poly)glycerin fatty acid ester (“(poly)” means both with and without the prefix “poly”. The same applies hereinafter. ), sucrose fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene castor Oils, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters and the like can be mentioned. Examples of anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, and polyoxyethylene alkyl ether phosphates. , And phosphoric acid ester of polyoxyethylene alkyl allyl ether. The surfactant may be used alone or in combination of two or more kinds.

The surfactant is a sorbitan fatty acid ester from the viewpoint that the W/O type reversed phase suspension is in a good state, water-absorbent resin particles having a suitable particle size are easily obtained, and industrially easily available. It is preferable to 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 properties of the water-absorbent resin particles obtained, the surfactant preferably contains sucrose fatty acid ester, and more preferably sucrose stearate.

The amount of the surfactant used is preferably 0.05 to 10 parts by mass with respect to 100 parts by mass of the aqueous monomer solution, from the viewpoint that the effect on the amount used can be sufficiently obtained and from the economical viewpoint. 0.08 to 5 parts by mass is more preferable, and 0.1 to 3 parts by mass is still more preferable.

In the reverse phase suspension polymerization, a polymeric dispersant may be used together with the above-mentioned surfactant. As the polymer dispersant, maleic anhydride modified polyethylene, maleic anhydride modified polypropylene, maleic anhydride modified ethylene/propylene copolymer, maleic anhydride modified EPDM (ethylene/propylene/diene/terpolymer), 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 coalesce, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymer, ethylene/acrylic acid copolymer, ethyl cellulose and ethyl hydroxyethyl cellulose. The polymeric dispersants may be used alone or in combination of two or more. As the polymer-based dispersant, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, maleic anhydride/ethylene copolymer is used, from the viewpoint of excellent dispersion stability of the monomer. Combined, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, polyethylene, polypropylene, ethylene/propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene/propylene copolymer At least one selected from the group consisting of coalescence is preferable.

The amount of the polymeric dispersant used is preferably 0.05 to 10 parts by mass with respect to 100 parts by mass of the aqueous monomer solution, from the viewpoint that the effect on the amount used can be sufficiently obtained, and from the economical viewpoint. , 0.08 to 5 parts by mass is more preferable, and 0.1 to 3 parts by mass is further preferable.

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

From the viewpoint of industrial availability and 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 media, for example, commercially available exol heptane (manufactured by Exxon Mobil: n-heptane and isomer hydrocarbons 75 to 85% content) is used. May be.

The amount of the hydrocarbon dispersion medium used is preferably 30 to 1000 parts by mass, and 40 to 500 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. More preferably, 50 to 300 parts by mass is even more preferable. When the amount of the hydrocarbon dispersion medium used is 30 parts by mass or more, control of the polymerization temperature tends to be easy. When the amount of the hydrocarbon dispersion medium used is 1000 parts by mass or less, the productivity of polymerization tends to be improved, which is economical.

The radical polymerization initiator is preferably water-soluble, and examples thereof include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t. -Butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, peroxides such as 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] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} Dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N- (2-hydroxyethyl)-propionamide], 4,4′-azobis(4-cyanovaleric acid), and other azo compounds. The radical polymerization initiator may be used alone or in combination of two or more kinds. As the radical polymerization initiator, potassium persulfate, ammonium persulfate, sodium persulfate, 2,2′-azobis(2-amidinopropane) dihydrochloride, 2,2′-azobis[2-(2-imidazoline-2- At least one selected from the group consisting of yl)propane] dihydrochloride and 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride Is preferred.

The amount of the radical polymerization initiator used may be 0.00005 to 0.01 mol with respect to 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 used is 0.01 mol or less, it is easy to suppress a rapid polymerization reaction.

The above radical polymerization initiator can 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.

In the polymerization reaction, the aqueous monomer solution used for the polymerization may contain a chain transfer agent. Examples of the chain transfer agent include hypophosphites, thiols, thiolic acids, secondary alcohols, amines and the like.

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

Although crosslinking due to self-crosslinking occurs during the polymerization, crosslinking may be performed by further using an internal crosslinking agent. When the 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, trimethylolpropane, 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 carbamyl esters; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N,N′,N″-triallyl isocyanurate, and 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 Glycidyl compounds; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.) The internal cross-linking agent may be used alone or in combination of two or more. As the internal cross-linking agent, a polyglycidyl compound is preferable, and a diglycidyl ether compound. Is more preferable, and at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether is more preferable.

The amount of the internal cross-linking agent used is such that the water-soluble property is suppressed by the resulting polymer being appropriately cross-linked, and a sufficient amount of water absorption is easily obtained, based on 1 mol of the ethylenically unsaturated monomer, It may be 0 mmol or more, 0.02 mmol or more, 0.03 mmol or more, 0.04 mmol or more, or 0.05 mmol or more, or 0.15 mmol or less, or 0.10 mmol or less. Good. When the amount of the internal cross-linking agent is within these numerical ranges, it is easy to obtain water-absorbent resin particles having a suitable static suction index α for suppressing liquid leakage.

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

When carrying out reverse phase suspension polymerization, a monomer aqueous solution containing an ethylenically unsaturated monomer is added to a hydrocarbon dispersion medium in the presence of a surfactant (and, if necessary, a polymer dispersant). Disperse into. At this time, the surfactant, the polymeric dispersant, etc. may be added before or after the polymerization reaction is started, either before or after the addition of the aqueous monomer solution.

Among them, from the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water-absorbent resin, the surfactant is prepared by dispersing the aqueous monomer solution in the hydrocarbon dispersion medium in which the polymer dispersant is dispersed. It is preferable to carry out the polymerization after further dispersing.

The reverse phase suspension polymerization can be performed in one stage or in multiple stages of two or more stages. The reverse phase suspension polymerization is preferably carried out in 2 to 3 stages from the viewpoint of improving productivity.

When performing reverse phase suspension polymerization in multiple stages of two or more stages, after performing the first stage reverse phase suspension polymerization, the reaction mixture obtained in the first stage polymerization reaction is mixed with an ethylenically unsaturated monomer. The body may be added and mixed, and the reverse phase suspension polymerization of the second and subsequent stages may be carried out in the same manner as in the first stage. In the reverse phase suspension polymerization in each of the second and subsequent stages, in addition to the ethylenically unsaturated monomer, the above radical polymerization initiator is added in the reverse phase suspension polymerization in each of the second and subsequent stages. It is preferable to carry out reverse phase suspension polymerization by adding 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 to be added. In the reverse phase suspension polymerization in the second and subsequent stages, 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 to be supplied to each stage, and the reverse phase suspension is added. It is preferable to carry out turbid polymerization.

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but the polymerization is promoted rapidly and the polymerization time is shortened to improve economic efficiency, and the heat of polymerization is easily removed to smoothly carry out the reaction. From a viewpoint, 20-150 degreeC is preferable and 40-120 degreeC is more preferable. The reaction time is usually 0.5 to 4 hours. The completion of the polymerization reaction can be confirmed by, for example, stopping the temperature rise in the reaction system. Thereby, the polymer of the ethylenically unsaturated monomer is usually obtained in the state of a hydrogel polymer.

After the polymerization, crosslinking may be carried out after the polymerization by adding a crosslinking agent to the obtained hydrous gel polymer and heating it. By carrying out cross-linking after the polymerization, the degree of cross-linking of the hydrogel polymer can be increased, thereby further improving the water absorbing properties of the water absorbent resin particles.

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-methylepichlorohydrin, etc. 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 ethylene carbonate; bis[N , N-di(β-hydroxyethyl)]adipamide and the like. Among these, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether are preferable. .. 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, from the viewpoint that the resulting water-containing gel-like polymer exhibits suitable water absorption properties by being appropriately cross-linked, per 1 mol of the ethylenically unsaturated monomer, The amount is preferably 0 to 0.03 mol, more preferably 0 to 0.01 mol, and further preferably 0.00001 to 0.005 mol. When the amount of the cross-linking agent used for post-polymerization cross-linking is within the above range, water-absorbent resin particles having a large static suction index α can be easily obtained.

The post-polymerization crosslinking may be added at any time after the polymerization of the ethylenically unsaturated monomer used for the polymerization, and in the case of multi-stage polymerization, it is preferably added after the multi-stage polymerization. In consideration of heat generation during and after the polymerization, retention due to process delay, opening of the system at the time of adding the crosslinking agent, and fluctuation of water content due to addition of water accompanying addition of the crosslinking agent, the crosslinking agent for crosslinking after the polymerization is From the viewpoint of water content (described later), it is preferable to add in the range of [water content immediately after polymerization ±3%].

Subsequently, drying is performed to remove water from the obtained hydrous gel polymer. By drying, polymer particles containing a polymer of an ethylenically unsaturated monomer are obtained. As a drying method, for example, (a) the hydrogel polymer is dispersed in a hydrocarbon dispersion medium, and azeotropic distillation is performed by externally heating the mixture to reflux the hydrocarbon dispersion medium to remove water. The method, (b) the method of taking out the hydrous gel-like polymer by decantation and drying under reduced pressure, and (c) the method of separating the hydrous gel-like polymer by filtration and drying under reduced pressure are mentioned. Above all, it is preferable to use the method (a) because it is easy in the manufacturing process.

The particle diameter 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 coagulant into the system after the polymerization reaction or at the beginning of drying. By adding the aggregating agent, the particle diameter of the water-absorbent resin particles obtained can be increased. An inorganic coagulant can be used as the coagulant. Examples of the inorganic flocculant (for example, powdery 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 is preferably at least one selected from the group consisting of silica, aluminum oxide, talc and kaolin.

In the reverse phase suspension polymerization, as a method of adding a flocculant, after preliminarily dispersing the flocculant in a hydrocarbon dispersion medium or water of the same kind as that used in the polymerization, under stirring, the hydrogel polymer The method of mixing in the hydrocarbon dispersion medium containing is preferable.

The amount of the aggregating agent added is preferably 0.001 to 1 part by mass, and 0.005 to 0.5 part by mass, relative to 100 parts by mass of the ethylenically unsaturated monomer used for the polymerization. Is more preferable, and 0.01 to 0.2 parts by mass is further preferable. When the addition amount of the aggregating agent is within the above range, it is easy to obtain water-absorbent resin particles having a target particle size distribution.

In the production of the water-absorbent resin particles, it is preferable that the surface portion of the hydrogel polymer is crosslinked (surface crosslinked) with a crosslinking agent in the drying step or any of the subsequent steps. By carrying out surface cross-linking, it is easy to control the water absorption characteristics of the water absorbent resin particles. The surface cross-linking is preferably carried out at a timing when the hydrogel polymer has a specific water content. The time of surface cross-linking is preferably a time when the water content of the hydrogel polymer is 5 to 50% by mass, more preferably 10 to 40% by mass, and further preferably 15 to 35% by mass. The water content (mass %) of the hydrogel polymer is calculated by the following formula.
Moisture content=[Ww/(Ww+Ws)]×100
Ww: Required when mixing the coagulant, surface cross-linking agent, etc. to the amount obtained by subtracting the amount of water discharged to the outside of the system from the drying process from the amount of water contained in the aqueous monomer solution before the polymerization in the entire polymerization process The water content of the hydrogel polymer including the water content used according to the above.
Ws: Solid content calculated from the charged amounts of materials such as an ethylenically unsaturated monomer, a cross-linking agent, and an initiator that compose 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. As the cross-linking agent, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol and polyglycerin; (poly)ethylene glycol diglycidyl ether, Polyglycidyl compounds such as (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether; epichlorohydrin, epi Haloepoxy compounds such as bromhydrin and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, Oxetane compounds such as 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol and 3-butyl-3-oxetane ethanol; Oxazolines such as 1,2-ethylenebisoxazoline Compounds: carbonate compounds such as ethylene carbonate; hydroxyalkyl amide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The cross-linking agent may be used alone or in combination of two or more kinds. The cross-linking agent is preferably a polyglycidyl compound, such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglyceride. At least one selected from the group consisting of glycidyl ether is more preferable.

The amount of the surface cross-linking agent used is, from the viewpoint that the resulting hydrogel polymer exhibits suitable water-absorbing properties by being appropriately cross-linked, usually the total amount of the ethylenically unsaturated monomer used for the polymerization. 0.00001-0.02 mol is preferable with respect to 1 mol, 0.00005-0.01 mol is more preferable, 0.0001-0.005 mol is still more preferable. When the amount of the surface-crosslinking agent used is 0.00001 mol or more, the crosslink density in the surface portion of the water-absorbent resin particles is sufficiently increased, and the gel strength of the water-absorbent resin particles is easily increased. When the amount of the surface cross-linking agent used is 0.02 mol or less, the water retention capacity of the water absorbent resin particles can be easily increased. When the amount of the surface cross-linking agent is 0.0003 mol or less, or 0.00025 mol or less, based on 1 mol of the total amount of the ethylenically unsaturated monomer used for polymerization, the static suction coefficient α is 5. It is particularly easy to obtain water-absorbent resin particles of 0 mL/g or more.

After the surface cross-linking, water and the hydrocarbon dispersion medium are distilled off by a known method to obtain polymer particles which are surface cross-linked dry products.

One embodiment of the method for producing the water absorbent resin particles may further include a step of evaluating the leak property of the obtained water absorbent resin particles by the method according to the above-described embodiment. For example, water-absorbent resin particles whose measured value of the diffusion distance D described later is smaller than a reference value (for example, 14 cm) may be selected. This makes it possible to more stably manufacture the water absorbent resin particles capable of suppressing the liquid leakage of the absorbent article.

The water-absorbent resin particles according to the present embodiment may be composed only of polymer particles, but for example, a gel stabilizer, a metal chelating agent (ethylenediamine tetraacetic acid and its salt, diethylenetriamine pentaacetic acid and its salt, such as diethylenetriamine). 5 sodium acetate, etc.), and various additional components selected from fluidity improvers (lubricants) and the like. The additional components may be located within the polymer particles, on the surface of the polymer particles, or both. As the additional component, a fluidity improver (lubricant) is preferable, and among them, inorganic particles are more preferable. Examples of the inorganic particles include silica particles such as amorphous silica.

The water absorbent resin particles may include a plurality of inorganic particles arranged on the surface of the polymer particles. For example, the inorganic particles can be arranged on the surface of the polymer particles by mixing the polymer particles and the inorganic 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 inorganic particles to the mass of the polymer particles is 0.2% by mass or more, 0.5% by mass or more, 1.0 It may be at least mass%, or at least 1.5 mass%, may be at most 5.0 mass%, or may be at most 3.5 mass%. 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 diameter here can be a value measured by a dynamic light scattering method or a laser diffraction/scattering method.

The absorber according to one embodiment contains the water absorbent resin particles according to the present embodiment. The absorbent body according to the present embodiment can contain a fibrous material, and is, for example, a mixture containing water-absorbent resin particles and a fibrous material. The structure of the absorbent body may be, for example, a structure in which the water-absorbent resin particles and the fibrous material are uniformly mixed, and the water-absorbent resin particles are sandwiched between the fibrous materials formed into a sheet or layer. It may be a configuration or another configuration.

Examples of the fibrous material include finely pulverized wood pulp; cotton; cotton linters; rayon; cellulosic fibers such as cellulose acetate; synthetic fibers such as polyamide, polyester and polyolefin; and a mixture of these fibers. The fibrous material may be used alone or in combination of two or more kinds. Hydrophilic fibers can be used as the fibrous material.

The mass ratio of the water absorbent resin particles in the absorber may be 2 to 100 mass%, 10 to 80 mass% or 20 to 60 mass% with respect to the total of the water absorbent resin particles and the fibrous material.

The fibers may be bonded to each other by adding an adhesive binder to the fibrous material in order to improve shape retention before and during use of the absorbent body. Examples of the adhesive binder include heat-fusible synthetic fibers, hot melt adhesives and adhesive emulsions. The adhesive binder may be used alone or in combination of two or more kinds.

Examples of the heat-fusible synthetic fibers include polyethylene-, polypropylene-, ethylene-propylene copolymer, and other fully-fused binders; side-by-side polypropylene and polyethylene, and non-fully-fused binders having a core-sheath structure. In the above non-total melting type binder, only the polyethylene portion can be heat-sealed.

Examples of hot melt adhesives include ethylene-vinyl acetate copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, and styrene-ethylene-propylene-styrene block copolymer. And a mixture of a base polymer such as amorphous polypropylene and a tackifier, a plasticizer, an antioxidant and the like.

Examples of the adhesive emulsion include a polymer of at least one monomer selected from the group consisting of methyl methacrylate, styrene, acrylonitrile, 2-ethylhexyl acrylate, butyl acrylate, butadiene, ethylene, and vinyl acetate.

The absorber according to this embodiment may contain an inorganic powder (for example, amorphous silica), a deodorant, an antibacterial agent, a fragrance, or the like. When the water-absorbent resin particles contain inorganic particles, the absorber may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles.

The shape of the absorber according to the present embodiment is not particularly limited, and may be, for example, a sheet shape. The thickness of the absorber (for example, the thickness of the sheet-like absorber) may be, for example, 0.1 to 20 mm and 0.3 to 15 mm.

The absorbent article according to the present embodiment includes the absorbent body according to the present embodiment. The absorbent article according to the present embodiment is a core wrap that retains the shape of an absorbent body; a liquid permeable sheet that is arranged at the outermost side of a side into which a liquid to be absorbed enters; a side into which a liquid to be absorbed enters. A liquid impermeable sheet or the like arranged on the outermost side on the opposite side can be used. Examples of absorbent articles include diapers (eg, paper diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, simple toilet parts, animal excrement disposal materials, etc. ..

FIG. 1 is a sectional view showing an example of an absorbent article. The absorbent article 100 shown in FIG. 1 includes an absorber 10, core wraps 20a and 20b, a liquid permeable sheet 30, and a liquid impermeable sheet 40. In the absorbent article 100, the liquid impermeable sheet 40, the core wrap 20b, the absorber 10, the core wrap 20a, and the liquid permeable sheet 30 are laminated in this order. In FIG. 1, there is a portion where there is a gap between the members, but the members may be in close contact with each other without the gap.

The absorber 10 includes the water-absorbent resin particles 10a according to the present embodiment and a fiber layer 10b containing a fibrous material. The water absorbent resin particles 10a are dispersed in the fiber layer 10b.

The core wrap 20a is arranged on one surface side of the absorbent body 10 (the upper side of the absorbent body 10 in FIG. 1) while being in contact with the absorbent body 10. The core wrap 20b is arranged on the other surface side of the absorbent body 10 (below the absorbent body 10 in FIG. 1) while being in contact with the absorbent body 10. The absorber 10 is arranged between the core wrap 20a and the core wrap 20b. Examples of the core wraps 20a and 20b include tissues and non-woven fabrics. The core wrap 20a and the core wrap 20b have, for example, a main surface having the same size as the absorber 10.

The liquid permeable sheet 30 is arranged at the outermost side on the side where the liquid to be absorbed enters. The liquid permeable sheet 30 is arranged on the core wrap 20a while being in contact with the core wrap 20a. Examples of the liquid permeable sheet 30 include a nonwoven fabric made of a synthetic resin such as polyethylene, polypropylene, polyester and polyamide, and a porous sheet. The liquid impermeable sheet 40 is arranged on the outermost side of the absorbent article 100 on the side opposite to the liquid permeable sheet 30. The liquid impermeable sheet 40 is arranged below the core wrap 20b in a state of being in contact with the core wrap 20b. Examples of the liquid impermeable sheet 40 include a sheet made of a synthetic resin such as polyethylene, polypropylene and polyvinyl chloride, a sheet made of a composite material of these synthetic resins and a non-woven fabric, and the like. The liquid permeable sheet 30 and the liquid impermeable sheet 40 have, for example, a main surface wider than the main surface of the absorber 10, and the outer edge portions of the liquid permeable sheet 30 and the liquid impermeable sheet 40 are It extends around the absorber 10 and the core wraps 20a, 20b.

The size relationship among the absorber 10, the core wraps 20a and 20b, the liquid permeable sheet 30, and the liquid impermeable sheet 40 is not particularly limited, and is appropriately adjusted according to the application of the absorbent article and the like. Further, the method of retaining the shape of the absorbent body 10 using the core wraps 20a and 20b is not particularly limited, and the absorbent body may be wrapped with a plurality of core wraps as shown in FIG. 1, and the absorbent body may be wrapped with one core wrap. But it's okay.

According to the present embodiment, it is possible to provide a liquid absorbing method using the water absorbent resin particles, the absorber or the absorbent article according to the present embodiment. The liquid absorbing method according to the present embodiment includes a step of bringing a liquid to be absorbed into contact with the water absorbent resin particles, the absorber or the absorbent article according to the present embodiment.

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

1. Preparation Example 1 of Water-Absorbent Resin Particles
First-stage polymerization reaction A reflux condenser, a dropping funnel, a nitrogen gas introducing pipe, and a stirrer equipped with a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages having an inner diameter of 11 cm and an internal volume of 2 L. A round bottom cylindrical separable flask was prepared. Into this flask were placed 293 g of n-heptane and 0.736 g of a maleic anhydride-modified ethylene/propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) as a polymeric dispersant. The reaction liquid in the flask was heated to 80° C. with stirring to dissolve the polymer dispersant in n-heptane. Then, the reaction solution was cooled to 50°C.

A beaker having an inner volume of 300 mL was charged with 92.0 g (1.03 mol) of an acrylic acid aqueous solution having a concentration of 80.5% by mass. While cooling the beaker from the outside, 147.7 g of a sodium hydroxide aqueous solution having a concentration of 20.9 mass% was added dropwise to the acrylic acid aqueous solution, thereby neutralizing 75 mol% of acrylic acid. Then, in an aqueous solution of acrylic acid, 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HECAW-15F) as a thickener, and 2,2′-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator. Dissolve 0.092 g (0.339 mmol) and 0.018 g (0.068 mmol) of potassium persulfate and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent. A monomer aqueous solution was prepared.

The first-stage monomer aqueous solution was added to the above reaction liquid in the separable flask, and the reaction liquid was stirred for 10 minutes. Next, a surfactant solution containing 6.62 g of n-heptane and 0.736 g of sucrose stearate (HLB:3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) was added to the reaction solution and stirred. The inside of the system was sufficiently replaced with nitrogen while stirring the reaction liquid with the blade rotating at 550 rpm. Then, the polymerization reaction was allowed to proceed for 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 polymer was obtained.

Second-stage polymerization reaction 128.8 g (1.43 mol) of an acrylic acid aqueous 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 an aqueous solution of sodium hydroxide having a concentration of 27 mass% was added dropwise to the aqueous solution of acrylic acid to thereby neutralize 75 mol% of acrylic acid. Subsequently, 0.129 g (0.475 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.026 g (0.095 mmol) of potassium persulfate were dissolved in an acrylic acid aqueous solution, A second stage monomer aqueous solution was prepared.

The first-stage polymerized slurry liquid in the separable flask was cooled to 25° C. while stirring with the rotation speed of the stirring blade set to 1000 rpm. The total amount of the second-stage monomer aqueous solution was added thereto, and then the system was replaced with nitrogen for 30 minutes. Then, the polymerization reaction was allowed to proceed for 60 minutes while heating the separable flask in a water bath at 70°C. As a cross-linking agent for post-polymerization cross-linking, 0.580 g (0.067 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to obtain a hydrogel polymer.

0.265 g of an aqueous solution of diethylenetriamine pentaacetic acid 5-sodium acetate having a concentration of 45% by mass was added to the reaction solution containing the hydrous gel polymer under stirring. Then, the flask was immersed in an oil bath set at 125° C., and 238.5 g of water was extracted out of 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 cross-linking reaction by the surface cross-linking agent was allowed to proceed at 83° C. for 2 hours. It was

From the reaction solution after the surface crosslinking reaction, n-heptane was distilled off by heating at 125° C. to obtain polymer particles (dry product). The obtained polymer particles were passed through a sieve with a mesh opening of 850 μm. Thereafter, 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Tokseal NP-S) with respect to the mass of the polymer particles is mixed with the polymer particles, and the water-absorbent resin particles 232 containing the amorphous silica are mixed. 0.1 g was obtained. The median particle diameter of the particles was 396 μm.

Example 2
Water-absorbent resin particles containing amorphous silica in the same procedure as in Example 1 except that the amount of amorphous silica was changed to 2.0% by mass with respect to the mass of polymer particles (dry product). 236.3 g was obtained. The median particle diameter of the water absorbent resin particles was 393 μm.

Example 3
The water-soluble radical polymerization initiator in the first stage polymerization reaction was changed to 0.0736 g (0.272 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. , The water-soluble radical polymerization initiator in the second-stage polymerization reaction was changed to 0.090 g (0.334 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. Polymer particles (dry product) were obtained by the same procedure as in Example 1 except for the above. The amount of water extracted from the reaction solution containing the hydrogel polymer by azeotropic distillation was 247.9 g.
0.5% by mass of amorphous silica (Oriental Silicas Corporation, Tokusil NP-S) was mixed with the polymer particles to obtain 231.0 g of water-absorbent resin particles. The median particle diameter of the water absorbent resin particles was 355 μm.

Example 4
The water-soluble radical polymerization initiator in the first stage polymerization reaction was changed to 0.0736 g (0.272 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. , The water-soluble radical polymerization initiator in the second-stage polymerization reaction was changed to 0.090 g (0.334 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. Polymer particles (dry product) were obtained by the same procedure as in Example 1 except for the above. The amount of water extracted from the reaction solution containing the hydrogel polymer by azeotropic distillation was 239.7 g.
0.5% by mass of amorphous silica (Oriental Silicas Corporation, Tokusil NP-S) was mixed with the polymer particles to obtain 229.2 g of water-absorbent resin particles. The median particle diameter of the water absorbent resin particles was 377 μm.

Comparative Example 1
The water-soluble radical polymerization initiator in the first stage polymerization reaction was changed to 0.0736 g (0.272 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. , The water-soluble radical polymerization initiator in the second-stage polymerization reaction was changed to 0.090 g (0.334 mmol) of potassium persulfate without using 2,2′-azobis(2-amidinopropane) dihydrochloride. That 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether was used as the internal cross-linking agent, no cross-linking agent for post-polymerization cross-linking was added, and the water content after the second-stage polymerization Example 1 was repeated except that 256.1 g of water was extracted out of the system by azeotropic distillation in the gel polymer and 0.1% by mass of amorphous silica was mixed with the polymer particles. 230.8 g of water-absorbent resin particles were obtained in the same manner as in. The median particle diameter of the water absorbent resin particles was 349 μm.

Comparative example 2
The amount of ethylene glycol diglycidyl ether as the internal cross-linking agent in the first-stage polymerization reaction was changed to 0.0046 g (0.026 mmol), and ethylene was used as the internal cross-linking agent in the preparation of the second-stage aqueous liquid. 0.0116 g (0.067 mmol) of glycol diglycidyl ether was used, no cross-linking agent for post-polymerization cross-linking was added, and azeotropic distillation was carried out in the hydrogel polymer after the second-stage polymerization. Except that 219.2 g of water was extracted out of the system by distillation and the amount of the aqueous ethylene glycol diglycidyl ether solution having a concentration of 2% by mass as the surface crosslinking agent was changed to 6.62 g (0.761 mmol). By the same procedure as in Example 1, 226.6 g of water-absorbent resin particles containing amorphous silica was obtained. The median particle diameter of the water absorbent resin particles was 356 μm.

Comparative Example 3
The amount of ethylene glycol diglycidyl ether as an internal cross-linking agent in the first stage polymerization reaction was changed to 0.0046 g (0.026 mmol), and the amount of ethylene glycol diglycidyl ether was changed as an internal cross-linking agent in the preparation of the second-stage aqueous liquid. 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether was used, no cross-linking agent for post-polymerization cross-linking was added, and in the hydrogel polymer after the second stage polymerization, 220.6 g of water-absorbent resin particles containing amorphous silica were obtained by the same procedure as in Example 1 except that 234.2 g of water was extracted from the system by boiling distillation. The median particle diameter of the water absorbent resin particles was 355 μm.

Comparative Example 4
The amount of ethylene glycol diglycidyl ether as the internal cross-linking agent in the first-stage polymerization reaction was changed to 0.0368 g (0.211 mmol). In the second-stage aqueous liquid preparation, ethylene was used as the internal cross-linking agent. 0.0515 g (0.296 mmol) of glycol diglycidyl ether was used, no cross-linking agent for post-polymerization cross-linking was added, and azeotropic distillation was carried out in the hydrogel polymer after the second-stage polymerization. 222.2 g of water-absorbent resin particles containing amorphous silica were obtained by the same procedure as in Example 1 except that 286.9 g of water was extracted from the system by distillation. The median particle diameter of the particles was 396 μm.

2. Evaluation 2-1. Saline Retention Capacity A cotton bag (Membroad No. 60, width 100 mm x length 200 mm) in which 2.0 g of water-absorbent resin particles was weighed was placed in a 500 mL beaker. Pour 0.9 g of a 0.9% by mass aqueous sodium chloride solution (physiological saline) into a cotton bag containing water-absorbent resin particles at one time so that it will not stick, and tie the upper part of the cotton bag with a rubber band and let it stand for 30 minutes. The water-absorbent resin particles were swollen with. The cotton bag after 30 minutes has been 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 a cotton bag containing the swollen gel after dehydration. The mass Wa(g) of was measured. The same operation was performed without adding the water-absorbent resin particles, the empty mass Wb (g) of the cotton bag when wet was measured, and the water retention capacity of the physiological saline was calculated from the following formula.
Saline retention capacity (g/g)=[Wa-Wb]/2.0

<Water absorption amount of water-absorbent resin particles under load>
The water absorption amount (room temperature, 25° C.±2° C.) of the physiological saline solution under load (under pressure) of the water absorbent resin particles was measured using the measuring device Y shown in FIG. The measuring device Y includes a burette section 61, a conduit 62, a measuring table 63, and a measuring section 64 placed on the measuring table 63. The buret part 61 has a buret 61a extending in the vertical direction, a rubber stopper 61b arranged at the upper end of the buret 61a, a cock 61c arranged at the lower end of the buret 61a, and one end near the cock 61c extending into the buret 61a. It has an air introducing pipe 61d and a cock 61e arranged on the other end side of the air introducing pipe 61d. The conduit 62 is attached between the burette portion 61 and the measuring table 63. The inner diameter of the conduit 62 is 6 mm. A hole having a diameter of 2 mm is formed in the center of the measuring table 63, and the conduit 62 is connected to the hole. The measuring unit 64 has a cylinder 64a (made of acrylic resin (plexiglass)), a nylon mesh 64b adhered to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The opening of the nylon mesh 64b is 75 μm (200 mesh). Then, at the time of measurement, the water-absorbent resin particles 65 to be measured are evenly spread on the nylon mesh 64b. The weight 64c has a diameter of 19 mm, and the weight 64c has a mass of 120 g. The weight 64c is placed on the water absorbent resin particles 65, and a load of 4.14 kPa can be applied to the water absorbent resin particles 65.

After putting 0.100 g of the water-absorbent resin particles 65 into the cylinder 64a of the measuring device Y, the weight 64c was placed and the measurement was started. Since the same volume of air as the physiological saline solution absorbed by the water-absorbent resin particles 65 is rapidly and smoothly supplied to the inside of the buret 61a from the air introduction pipe, the water level of the physiological saline solution inside the buret 61a is reduced. Is the amount of physiological saline absorbed by the water-absorbent resin particles 65. The scale of the buret 61a is engraved from 0 mL to 0.5 mL in a downward direction from the top, and the burette 61a of the buret 61a before the start of water absorption and the burette 61a 60 minutes after the start of water absorption are used as the water level of the physiological saline. The scale Vb was read and the amount of water absorption under load was calculated from the following formula. The results are shown in Table 1.
Water absorption under load [mL/g]=(Vb-Va)/0.1

<Medium particle size>
50 g of the water absorbent resin particles were used for measuring the medium particle size.

From the JIS standard sieve from the top, a sieve having an opening of 850 μm, a sieve having an opening of 500 μm, a sieve having an opening of 425 μm, a sieve having an opening of 300 μm, a sieve having an opening of 250 μm, a sieve having an opening of 180 μm, a sieve having an opening of 150 μm, and Combined in the order of the saucers.

The water-absorbent resin particles were put into the combined uppermost sieve and shaken for 20 minutes using a low-tap shaker for classification. After the 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 to obtain a particle size distribution. With respect to this particle size distribution, the relationship between the mesh opening of the sieve and the integrated value of the mass percentage of the water-absorbent resin particles remaining on the sieve was plotted on a logarithmic probability paper by sequentially accumulating on the sieve in descending order of particle size. 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.

2-2. Unpressurized DW
The non-pressurized DW of the particles of the water absorbent resin was measured using the measuring device shown in FIG. The measurement was carried out 5 times for one type of water-absorbent resin particle, and the average value of the measured values of 3 points excluding the minimum value and the maximum value was obtained.
The measuring device includes a buret unit 1, a conduit 5, a measuring table 13, a nylon mesh sheet 15, a gantry 11, and a clamp 3. The burette part 1 includes a burette tube 21 with graduations, a rubber stopper 23 that tightly plugs the upper opening of the burette tube 21, a cock 22 connected to the lower end of the burette tube 21, and a lower portion of the burette tube 21. And an air introducing pipe 25 and a cock 24 which are connected to each other. The bullet part 1 is fixed by a clamp 3. The flat plate-shaped measuring table 13 has a through hole 13a having a diameter of 2 mm formed in the center thereof, and is supported by a pedestal 11 whose height is variable. The through hole 13 a of the measuring table 13 and the cock 22 of the buret part 1 are connected by the conduit 5. The inner diameter of the conduit 5 is 6 mm.

The measurement was performed in an environment of a temperature of 25° C. and a humidity of 60±10%. First, the cock 22 and the cock 24 of the buret part 1 were closed, and 0.9 mass% saline solution 50 adjusted to 25° C. was put into the buret tube 21 through the opening in the upper part of the buret tube 21. The saline solution concentration of 0.9 mass% is a concentration based on the mass of the saline solution. After sealing the opening of the burette tube 21 with the rubber stopper 23, the cock 22 and the cock 24 were opened. The inside of the conduit 5 was filled with 0.9 mass% saline solution 50 to prevent air bubbles from entering. The height of the measuring table 13 was adjusted such that the height of the water surface of the 0.9 mass% saline solution that reached the through hole 13a was the same as the height of the upper surface of the measuring table 13. After the adjustment, the height of the water surface of the 0.9 mass% saline solution 50 in the burette pipe 21 was read on the scale of the burette pipe 21, and the position was set as the zero point (reading value at 0 second).

A nylon mesh sheet 15 (100 mm×100 mm, 250 mesh, thickness of about 50 μm) was laid in the vicinity of the through hole 13 a on the measuring table 13, and a cylinder having an inner diameter of 30 mm and a height of 20 mm was placed at the center thereof. To this cylinder, 1.00 g of water-absorbent resin particles 10a was uniformly sprayed. After that, the cylinder was carefully removed to obtain a sample in which the water-absorbent resin particles 10a were circularly dispersed in the central portion of the nylon mesh sheet 15. Then, the nylon mesh sheet 15 on which the water-absorbent resin particles 10a were placed was moved quickly so that the center of the nylon mesh sheet 15 was at the position of the through-hole 13a, and the water-absorbent resin particles 10a were quickly dissipated to start the measurement. . Water absorption was started (0 second) at the time when bubbles were first introduced into the burette pipe 21 from the air introduction pipe 25.

The decrease amount of the 0.9 mass% saline solution 50 in the buret tube 21 (that is, the amount of 0.9 mass% saline solution absorbed by the water absorbent resin particles 10a) is sequentially read in units of 0.1 mL to obtain the water absorbent resin particles. The weight loss Wc (g) of the 0.9 mass% saline solution 50 was read 10 seconds, 1 minute, 3 minutes, 5 minutes, and 10 minutes after the start of water absorption of 10a. From Wc, a 10-second value, a 1-minute value, a 3-minute value, a 5-minute value and a 10-minute value of the non-pressurized DW were calculated by the following formula. The non-pressurized DW is the amount of water absorption per 1.00 g of the water absorbent resin particles 10a.
Unpressurized DW value (mL/g)=Wc/1.00

2-3. Leakability of water-absorbent resin particles (1) Preparation of artificial urine Sodium chloride, calcium chloride and magnesium sulfate were dissolved in ion-exchanged water at the following concentrations. A small amount of Blue No. 1 was added to the resulting solution to obtain blue colored artificial urine. The obtained artificial urine was used as a test solution for leak evaluation. The following concentrations are based on the total mass of artificial urine.
Artificial urine composition NaCl: 0.780 mass%
CaCl ₂ : 0.022 mass%
MgSO _4: 0.038 wt%
Blue No. 1: 0.002 mass%

(2) Leakage test The liquid leakability of the water absorbent resin particles was evaluated by the following procedures i), ii), iii), iv) and v).
i) A strip-shaped adhesive tape (Piaoran tape manufactured by Diatex Co., Ltd.) having a length of 15 cm and a width of 5 cm is placed on a laboratory table so that the adhesive surface faces upward, and the water-absorbent resin particles 3 are placed on the adhesive surface. 0.0 g was evenly sprayed. A stainless steel roller (mass 4.0 kg, diameter 10.5 cm, width 6.0 cm) was placed on top of the dispersed water-absorbent resin particles, and the roller was reciprocated 3 times between both ends in the longitudinal direction of the adhesive tape. It was Thereby, a water absorbing layer made of water absorbing resin particles was formed on the adhesive surface of the adhesive tape.
ii) The adhesive tape was stood vertically and lifted to remove excess water absorbent resin particles from the water absorbent layer. Again, the roller was placed on the water absorbing layer and reciprocated between the both ends in the longitudinal direction of the adhesive tape three times.
iii) In a room at a temperature of 25±2° C., an acrylic resin plate having a rectangular flat main surface with a length of 30 cm and a width of 55 cm is parallel to the horizontal plane, and the main surface and the horizontal plane form 30 degrees. I fixed it like an eggplant. On the main surface of the fixed acrylic plate, an adhesive tape on which a water absorbing layer was formed was pasted in such a direction that the water absorbing layer was exposed and its longitudinal direction was perpendicular to the width direction of the acrylic resin plate.
iv) From a height of about 1 cm from the surface at a position of about 1 cm from the upper end of the water-absorbing layer, 0.25 mL of the test solution having a liquid temperature of 25° C. was used for 1 with a micropipette (Pipeman Neo P1000N manufactured by MS Equipment Co., Ltd.) All injected within seconds.
v) 30 seconds after the injection of the test liquid was started, the maximum value of the moving distance of the test liquid injected into the water absorbing layer was read and recorded as the diffusion distance D. The diffusion distance D is the distance connecting the dropping point (injection point) and the longest arrival point on the main surface with a straight line perpendicular to the short side of the acrylic resin plate.

2-3. Results Evaluation results are shown in Table 1. It was confirmed that the water-absorbent resin particles of Examples 1 to 4 having the static suction index α of the predetermined value or more exhibited a small diffusion coefficient and were capable of suppressing liquid leakage. The diffusion distance “14*” indicates that the test liquid leaked from the lower part of the water absorption layer.

DESCRIPTION OF SYMBOLS 1... Burette part, 3... Clamp, 5... Conduit, 10... Absorber, 10a... Water absorbing resin particle, 10b... Fiber layer, 11... Stand, 13... Measuring stand, 13a... Through hole, 15... Nylon mesh sheet, 20a, 20b... Core wrap, 21... Burette tube, 22... Cock, 23... Rubber stopper, 24... Cock, 25... Air introduction tube, 50... 0.9 mass% saline solution, 30... Liquid permeable sheet, 40... Liquid Impermeable sheet, 100... Absorbent article.

Claims (3)

Water absorption rate index I=(10 second value of unpressurized DW)×6 [mL/g],
When the water absorption rate index II=(3 minutes value of unpressurized DW)/3 [mL/g],
The following formula:
α=(I+II)/2
The water-absorbent resin particles having a static suction index α of 5.0 mL/g or more calculated by. The water absorbent resin particles according to claim 1, wherein the water absorption rate index I is 2 mL/g or more. The water absorbent resin particles according to claim 1 or 2, wherein the water absorption rate index II is 8 mL/g or more.

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