JP2022129644A - Manufacturing method of water-absorbing resin particle - Google Patents

Abstract

To provide a manufacturing method of a water-absorbing resin particle capable of enhancing an amount of physiological saline solution absorption under a load.SOLUTION: A manufacturing method of a water-absorbing resin particle includes: preparing a water-containing gel polymer; processing the water-containing gel polymer to obtain a cross-linking polymer particle in which a pure water absorption/CRC is within a range of 10.0 to 40.0 and a reduction rate of the pure water absorption/CRC is within a range of 5 to 45% with regard to a reference cross-linking polymer particle obtained by desiccating, at 180°C for 30 minutes, a particle similarly obtained without processing; and applying a surface cross-linking to the cross-linking polymer particle.SELECTED DRAWING: None

Description

The present invention relates to a method for producing water absorbent resin particles.

Water-absorbent resin particles are used in fields such as sanitary materials such as sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as water stop agents and anti-condensation agents. The water-absorbent resin particles can be obtained, for example, by obtaining crosslinked polymer particles having a structural unit derived from an ethylenically unsaturated monomer and then surface-crosslinking the crosslinked polymer particles (for example, Patent document 1).

JP-A-2004-359943

The water absorbent resin particles are required to have an excellent water absorption capacity. In particular, when water-absorbing resin particles are used in absorbent articles such as diapers, a load is applied to the water-absorbing resin particles when the absorbent article is worn on the user. It is required to improve the water absorption amount of the water absorbent resin particles in a loaded state.

An object of the present invention is to provide a method for producing water-absorbent resin particles capable of increasing the amount of water absorbed by physiological saline under load.

The method for producing water-absorbing resin particles of the present invention comprises preparing a water-containing gel polymer and treating the water-containing gel polymer so that the pure water absorption/CRC is 10.0 to 40.0. Within the range, and the reduction rate of pure water absorption/CRC with respect to the reference crosslinked polymer particles obtained by drying the particles similarly obtained without treatment at 180 ° C. for 30 minutes is in the range of 5 to 45% It involves obtaining a crosslinked polymer particle and subjecting the crosslinked polymer particle to surface cross-linking. CRC is an abbreviation for centrifugal retention volume.

The water content of the hydrogel polymer at the start of treatment may be 45 to 70% by mass.

The production method preferably includes crushing the hydrogel polymer before treatment.

INDUSTRIAL APPLICABILITY The present invention provides a method for producing water-absorbent resin particles capable of increasing the amount of water absorbed by physiological saline under load.

FIG. 2 is a schematic diagram showing an apparatus for measuring the amount of physiological saline water absorption under load.

Preferred embodiments of the present invention are described in detail below. However, the present invention is not limited to the following embodiments.

As used herein, "(meth)acrylic" means both acrylic and methacrylic. "Acrylate" and "methacrylate" are similarly written as "(meth)acrylate". The same is true for other similar terms. "(poly)" shall mean both with and without the "poly" prefix. In the numerical ranges described stepwise in this specification, the upper limit value or lower limit value of the numerical range in one step can be arbitrarily combined with the upper limit value or lower limit of the numerical range in another step. In the numerical ranges described herein, the upper or lower limits of the numerical ranges may be replaced with the values shown in the examples. “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 singly 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 unless otherwise specified when there are multiple substances corresponding to each component in the composition. "Physiological saline" refers to a 0.9% by mass sodium chloride aqueous solution.

In the method for producing water-absorbing resin particles according to the present embodiment, a water-containing gel-like polymer is prepared, and the water-containing gel-like polymer is treated so that the pure water absorption/CRC is 10.0 to 40.0. 0, and the reduction rate of pure water absorption/CRC with respect to the reference crosslinked polymer particles obtained by drying the particles similarly obtained without treatment at 180 ° C. for 30 minutes is in the range of 5 to 45%. and subjecting the crosslinked polymer particles to surface crosslinking.

[Water-containing gel-like polymer]
A water-containing gel-like polymer can be obtained by polymerizing a monomer containing an ethylenically unsaturated monomer. The water-containing gel-like polymer may be a polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer that contains water and becomes gel-like. The polymer has monomeric units derived from ethylenically unsaturated monomers.

Polymerization can be carried out, for example, by a polymerization method such as an aqueous solution polymerization method or a reverse phase suspension polymerization method. Polymerization of a monomer by an aqueous solution polymerization method will be described below.

It is preferred that the ethylenically unsaturated monomers are water soluble. Examples of ethylenically unsaturated monomers include carboxylic acid monomers such as (meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid and salts thereof; (meth)acrylamide, N,N-dimethyl ( Nonionic monomers such as meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol (meth)acrylamide, polyethylene glycol mono (meth)acrylate; N,N-diethylaminoethyl (meth)acrylate, N, N-diethylaminopropyl (meth)acrylate, amino group-containing unsaturated monomers such as diethylaminopropyl (meth)acrylamide and quaternized products thereof; vinylsulfonic acid, styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropane Examples include sulfonic acid-based monomers such as sulfonic acid, 2-(meth)acryloylethanesulfonic acid and salts thereof. Ethylenically unsaturated monomers may be used alone or in combination of two or more.

The ethylenically unsaturated monomer preferably contains at least one selected from the group consisting of (meth)acrylic acid and its salts, maleic acid, fumaric acid, (meth)acrylamide, and N,N-dimethylacrylamide. , (meth)acrylic acid and salts thereof. Also, (meth)acrylic acid and its salts may be copolymerized with other ethylenically unsaturated monomers. In this case, of the total amount of ethylenically unsaturated monomers, the (meth)acrylic acid and its salt are preferably used in an amount of 70 to 100 mol%, more preferably 80 to 100 mol%, and 90 to It is more preferable to use 100 mol %. The ethylenically unsaturated monomer preferably contains at least one of acrylic acid and its salt.

When the ethylenically unsaturated monomer has an acid group such as (meth)acrylic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, etc., the acid group may optionally be pretreated with an alkaline neutralizer. can be used after neutralization. Examples of such alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. These alkaline neutralizers may be used in the form of an aqueous solution in order to facilitate the neutralization operation. Alkaline neutralizers may be used alone or in combination of two or more. The acid groups may be neutralized before, during or after the polymerization of the raw ethylenically unsaturated monomer.

The degree of neutralization of the ethylenically unsaturated monomers by the alkaline neutralizer increases the osmotic pressure of the resulting water-absorbent resin particles, thereby enhancing the water absorption performance, and the safety resulting from the presence of excess alkaline neutralizer. From the viewpoint of avoiding problems such as Even more preferably ~80 mol%. Here, the degree of neutralization is the degree of neutralization with respect to all acid groups possessed by the ethylenically unsaturated monomer.

Ethylenically unsaturated monomers are usually preferably used in the form of 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") may be 20% by mass or more and the saturated concentration or less, and is 25 to 70% by mass. %, or 30 to 50 mass %.

The amount of the ethylenically unsaturated monomer used is based on the total amount of monomers (the total amount of monomers for obtaining water-absorbing resin particles. For example, the total amount of monomers that provide the structural units of the polymer. The same shall apply hereinafter.) 70 to 100 mol %, 80 to 100 mol %, 90 to 100 mol %, 95 to 100 mol %, or 100 mol %. Among them, the proportion 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 It may be 100 mol %. "Ratio of (meth)acrylic acid and its salt" means the ratio of the total amount of (meth)acrylic acid and its salt.

The water absorbent resin particles obtained by the production method according to the present embodiment, for example, contain a polymer having a structural unit derived from an ethylenically unsaturated monomer, and the ethylenically unsaturated monomer is a (meth)acrylic It contains at least one compound selected from the group consisting of acids and salts thereof, and the ratio of (meth)acrylic acid and its salts is 70 to 100 mol% with respect to the total amount of monomers for obtaining water absorbent resin particles. It may be

The aqueous monomer solution may contain a polymerization initiator. Polymerization of the monomers contained in the aqueous monomer solution is initiated by adding a polymerization initiator to the aqueous monomer solution and, if necessary, heating or irradiating with light. Examples of the polymerization initiator include photopolymerization initiators and radical polymerization initiators, and among them, water-soluble radical polymerization initiators are preferably used. The polymerization initiator may be, for example, an azo compound, a peroxide, or the like.

Examples of azo compounds include 2,2′-azobis[2-(N-phenylamidino)propane]dihydrochloride and 2,2′-azobis{2-[N-(4-chlorophenyl)amidino]propane}. dihydrochloride, 2,2′-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2′-azobis[2-(N-benzylamidino)propane]dihydrochloride , 2,2′-azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2′-azobis(2-amidinopropane)dihydrochloride, 2,2′-azobis{2-[N- (2-hydroxyethyl)amidino]propane}dihydrochloride, 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2′-azobis[2 -(2-imidazolin-2-yl)propane]dihydrochloride, 2,2′-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane] Dihydrochloride, 2,2′-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane]dihydrochloride, 2,2′-azobis{2-[1 -(2-Hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2′-azobis(2-methylpropionamido)dihydrochloride, 2,2′-azobis[2-(2 -Imidazolin-2-yl)propane]disulfate dihydrate, 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, 2,2′-azobis Examples include azo compounds such as [2-methyl-N-(2-hydroxyethyl)propionamide]. From the viewpoint that it is easy to obtain water-absorbing resin particles having good water-absorbing performance, 2,2′-azobis(2-amidinopropane) dihydrochloride, 2,2′-azobis{2-[1-(2-hydroxy Ethyl)-2-imidazolin-2-yl]propane}dihydrochloride and 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate are preferred. These azo compounds may be used singly or in combination of two or more.

Examples of peroxides 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 organic peroxides such as -butyl peroxyacetate, t-butyl peroxy isobutyrate, t-butyl peroxy pivalate; and peroxides such as hydrogen peroxide. Among these peroxides, it is preferable to use potassium persulfate, ammonium persulfate, sodium persulfate, or hydrogen peroxide from the viewpoint of obtaining water-absorbing resin particles having good water-absorbing performance. More preferably, ammonium persulfate or sodium persulfate is used. These peroxides may be used individually by 1 type, and may use 2 or more types together.

A combination of a polymerization initiator and a reducing agent can also be used as a redox polymerization initiator. Reducing agents include, for example, sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

From the viewpoint of performance balance such as CRC and water absorption under load in the water-absorbing resin particles, the amount of the polymerization initiator is 0.05 to 1 mmol, 0.08 to 0.8 mmol, per 1 mol of the monomer. Or it may be 0.1 to 0.7 millimoles.

The aqueous monomer solution preferably contains an internal cross-linking agent. By containing an internal cross-linking agent, the obtained cross-linked polymer can have cross-linking due to the internal cross-linking agent in addition to self-crosslinking due to the polymerization reaction as its internal cross-linking structure.

Internal crosslinkers may include compounds having (meth)acrylic, allyl, epoxy, or amino groups. A compound having two or more of these reactive functional groups can be used as an internal cross-linking agent. Examples of compounds having (meth)acrylic groups include (poly)ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate (poly)propylene glycol di(meth)acrylate, glycerol tri(meth)acrylate, trimethylolpropane di(meth)acrylate and N,N'-methylenebis(meth)acrylamide. Examples of compounds having allyl groups include triallylamine. Examples of compounds having an epoxy group include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and epichlorohydrin. Examples of compounds with amino groups include triethylenetetramine, ethylenediamine, and hexamethylenediamine. The internal cross-linking agents may be used singly or in combination of two or more.

When using an internal cross-linking agent, the amount used is 0.02 to 1.0 mmol per 1 mol of the monomer, from the viewpoint of adjusting the performance balance of the water-absorbent resin particles such as CRC and water absorption under load. It may be from 0.05 to 0.8 millimoles, or from 0.1 to 0.6 millimoles.

The aqueous monomer solution may contain additives such as a chain transfer agent and a thickener, if necessary. Examples of chain transfer agents include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid and the like. Examples of thickeners include carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, polyacrylamide, and the like. These may be used individually by 1 type, and may use 2 or more types together. A solvent other than water, such as a water-soluble organic solvent, may be appropriately added to the aqueous monomer solution.

The polymerization method may be, for example, a stationary polymerization method in which the aqueous monomer solution is not stirred (for example, in a stationary state), or a stirring polymerization method in which the aqueous monomer solution is polymerized while being stirred in a reactor. you can It is preferable to obtain a water-containing gel-like polymer by aqueous solution static polymerization, which is a static polymerization method. In the stationary polymerization method, a single block-shaped water-containing gel-like polymer can be obtained that occupies substantially the same volume as the aqueous monomer solution existing in the reaction vessel when the polymerization is completed.

The mode of production of the hydrogel polymer may be batchwise, semi-continuous, continuous, or the like. For example, in the aqueous stationary continuous polymerization, the polymerization reaction is performed while the aqueous monomer solution is continuously supplied to a belt-conveyor-shaped continuous polymerization apparatus, and a continuous shape such as a belt-like hydrous gel can be obtained. .

The polymerization temperature varies depending on the polymerization initiator used, but is preferably 0 to 130° C., more preferably 10 to 110° C., from the viewpoint of increasing productivity by allowing the polymerization to proceed rapidly and shortening the polymerization time. The polymerization time is appropriately set according to the type or amount of the polymerization initiator used, the reaction temperature, etc., but is preferably 1 to 200 minutes, more preferably 5 to 100 minutes.

The resulting hydrous gel-like polymer may be pre-crushed prior to the treatment step (structural adjustment step) described below. Crush before the treatment step is preferable because the treatment step can be performed efficiently. Pulverization of the hydrogel polymer may be performed after the treatment step.

The crushed product obtained by crushing the hydrous gel polymer may be in the form of particles, or may have an elongated shape such as a series of particles. The minimum side size of the crushed product may be, for example, on the order of 0.1 to 15 mm, or on the order of 1.0 to 10 mm. The size of the largest side of the crushed material may be on the order of 0.1 to 200 mm, or on the order of 1.0 to 150 mm. As the crushing device, for example, kneaders (e.g., pressurized kneaders, double-arm kneaders, etc.), meat choppers, cutter mills, farmer mills, etc. can be used, and double-arm kneaders, meat choppers, and cutter mills are preferred. .

[Structure optimization process]
In the method for producing water-absorbing resin particles according to the present embodiment, by treating the hydrogel polymer, the pure water absorption amount/CRC is in the range of 10.0 to 40.0, and the pure water absorption amount/ Obtaining crosslinked polymer particles having a CRC reduction in the range of 5-45%. The pure water absorption/CRC reduction rate is the reduction rate based on the reference crosslinked polymer particles obtained by drying the particles similarly obtained without treatment at 180° C. for 30 minutes. Hereinafter, the treatment process is also referred to as a structure optimization process. The hydrous gel polymer to be treated may be, for example, a lump or granule.

In the structure optimization step, the pure water absorption amount/CRC of the obtained crosslinked polymer particles is in the range of 10.0 to 40.0, and the reduction rate of the pure water absorption amount/CRC is in the range of 5 to 45%. All you have to do is to make it happen. The crosslinked polymer particles obtained here are those in which the moisture content has been adjusted to the range of 0 to 15% by mass by passing through the structure optimization process, and the surface crosslinking described later is not performed. It is.

The water-containing gel-like polymer is subjected to a structural adjustment process, and the pure water absorption amount/CRC is in the range of 10.0 to 40.0, and the pure water absorption amount/CRC reduction rate is in the range of 5 to 45%. and subjecting the crosslinked polymer particles to surface cross-linking to increase the physiological saline water absorption under load of the obtained water-absorbing resin particles as compared with the standard crosslinked polymer particles. be able to. Although the mechanism by which such an effect is obtained is not clear, the present inventors presume as follows. In addition, the present invention is not limited to the following mechanism. By subjecting the water-containing gel polymer to an appropriate treatment step, the crosslinked structure and/or the main chain structure of the water-containing gel polymer is partly cut, and the three-dimensional structure of the water-containing gel polymer is optimized. It is believed that the subsequent surface cross-linking makes the network structure in the vicinity of the surface of the resulting water-absorbing resin particles more uniform, and improves the physiological saline water absorption under load.

The timing for performing the structure optimization step is before the water-containing gel-like polymer is dried. The water content of the water-containing gel-like polymer at the start of the structure optimization step is preferably 45 to 70% by mass, more preferably 50 to 65% by mass, and more preferably 50 to 60% by mass. More preferred. It is considered that the three-dimensional structure of the hydrous gel polymer is more likely to be optimized by optimizing the structure while the water content of the hydrous gel polymer is in an appropriate range.

The structure optimization step can be performed, for example, by heat-drying the hydrogel polymer. In addition to heat-drying, the hydrogel polymer is irradiated with microwaves and/or iron is applied to the hydrogel polymer. Addition of ions may be used in combination.

The water content of the polymer after the structural optimization step may be, for example, 0% by mass or more, 1% by mass or more, or 2% by mass or more, and is 15% by mass or less, 12% by mass or less, or 10% by mass or less. It's okay.

The temperature during heat drying may be, for example, 100° C. or higher, 120° C. or higher, 150° C. or higher, 180° C. or higher, or 200° C. or higher, and 250° C. or lower, 230° C. or lower, 220° C. or lower, or 210° C. or lower. good. The heating time may be adjusted depending on the size of the target hydrogel polymer, and may be, for example, 10 minutes or more, 20 minutes or more, 30 minutes or more, 1 hour or more, 5 hours or less, 4 hours or less, It may be 3 hours or less, or 2 hours or less. Heating is preferably carried out in an open system in order to remove at least part of the water content of the hydrogel polymer.

When microwave irradiation is used in addition to heat drying, it can be carried out, for example, by irradiating a coarsely crushed water-containing gel polymer with microwaves for about 5 minutes to 1 hour. Microwave irradiation can be performed on the polymer before heat drying, during heat drying, and/or after heat drying. Irradiation may be performed while stirring the polymer.

When iron ions are added in addition to drying by heating, the iron ions to be added may be divalent iron ions. Iron ions may be added in the form of salts. Salts may be, for example, iron(II) sulfate, iron(III) chloride, iron(III) nitrate. The concentration of iron ions to be added may be, for example, 0.1 to 100 ppm, preferably 0.5 to 50 ppm, relative to the hydrogel polymer. Iron ions may be added to the water-containing gelatinous polymer in the form of a liquid obtained by dissolving a salt in a solvent such as water. The temperature at which iron ions are added is not particularly limited, and may be, for example, 20°C or higher, 25°C or higher, 180°C or lower, 140°C or lower, 100°C or lower, 80°C or lower, 40°C or lower, or 30°C. may be: Addition of iron ions can be performed to the polymer before heat drying, during heat drying, and/or after heat drying.

When the iron ions are added to the polymer, the polymer may be granulated beforehand, or it may be granulated after the addition of the iron ions. After adding the iron ions, it is preferable to mix so that the iron ions are evenly distributed throughout the polymer.

[Crosslinked polymer particles]
Crosslinked polymer particles having a water content of 0 to 15% by mass can be obtained through the above-described structure optimization step. The pure water absorption/CRC of the crosslinked polymer particles obtained by the production method according to the present embodiment is in the range of 10.0 to 40.0. Pure water absorption and CRC are measured by the methods described in Examples below.

The pure water absorption/CRC of the crosslinked polymer particles is 10.5 or more, 11.0 or more, 12.0 or more, 13.0 or more, 13.5 or more, 14.0 or more, 14.5 or more, 15.5. 0 or more, 15.5 or more, 16.0 or more, 18.0 or more, 20.0 or more, 22.0 or more, 24.0 or more, 26.0 or more, 28.0 or more, 29.0 or more, 31. 0 or more, 32.0 or more, 33.0 or more, 34.0 or more, 35.0 or more, 36.0 or more, 37.0 or more, 38.0 or more, 39.0 or more, 40.0 or more, 41. It may be 0 or greater, 42.0 or greater, 43.0 or greater, or 44.0 or greater. The pure water absorption/CRC of the crosslinked polymer particles is 55.0 or less, 53.0 or less, 51.0 or less, 49.0 or less, 48.0 or less, 46.0 or less, 44.0 or less, 42.0 or less. 0 or less, 40.0 or less, 38.0 or less, 36.0 or less, 34.0 or less, 32.0 or less, 30.0 or less, 28.0 or less, 26.0 or less, 24.0 or less, 22. It may be 0 or less, 20.0 or less, 18.0 or less, or 16.0 or less.

The pure water absorption of the crosslinked polymer particles is, for example, 500 g/g or more, 600 g/g or more, 650 g/g or more, 800 g/g or more, 900 g/g or more, 1000 g/g or more, 1100 g/g or more, 1200 g/g or more. g or more, 1300 g/g or more, 1400 g/g or more, 1500 g/g or more, 1700 g/g or more, 1900 g/g or more, 2000 g/g or more, 2100 g/g or more, 2200 g/g or more, 2400 g/g or more, 2600, Or it may be 2800 g/g or more. The pure water absorption of the crosslinked polymer particles is 3500 g/g or less, 3400 g/g or less, 3300 g/g or less, 3100 or less, 3000 g/g or less, 2800 g/g or less, 2600 g/g or less, 2400 g/g or less, and 2200 g. /g or less, 2000 g/g or less, 1800 g/g or less, 1600 g/g or less, 1400 g/g or less, 1200 g/g or less, or 1000 g/g or less.

The CRC of the crosslinked polymer particles is, for example, 30 g/g or more, 35 g/g or more, 38 g/g or more, 40 g/g or more, 42 g/g or more, 44 g/g or more, 46 g/g or more, 48 g/g or more, It may be 50 g/g or greater, 51 g/g or greater, 53 g/g or greater, 55 g/g or greater, 57 g/g or greater, 59 g/g or greater, 61 g/g or greater, 63 g/g or greater, or 65 g/g or greater. The CRC of the crosslinked polymer particles is, for example, 75 g/g or less, 73 g/g or less, 71 g/g or less, 69 g/g or less, 67 g/g or less, 65 g/g or less, 63 g/g or less, 61 g/g or less, It may be 59 g/g or less, 57 g/g or less, 55 g/g or less, 53 g/g or less, 51 g/g or less, or 49 g/g or less.

The CRC, pure water absorption, and pure water absorption/CRC of the crosslinked polymer particles are obtained by further pulverizing and classifying the crosslinked polymer particles obtained through the structure optimization process, and having a particle diameter of 180 to 850 μm. It is preferable to measure with the

The median particle size of the crosslinked polymer particles may be, for example, 200-500 μm, or 300-500 μm.

The crosslinked polymer particles have a pure water absorption/CRC reduction rate in the range of 5 to 45%. The reduction rate was obtained by drying particles obtained by producing a hydrogel polymer in the same manner as the water-absorbing resin particles according to the present embodiment at 180° C. for 30 minutes, except that the structure optimization step was not performed. It is calculated based on the reference crosslinked polymer particles. More specifically, a water-containing gel polymer is produced in the same manner as the water-absorbing resin particles according to the present embodiment except that the structure optimization step is not performed, and after crushing the water-containing gel polymer, the structure is Without performing the optimization step, it is dried at 180 ° C. for 30 minutes using a hot air dryer (for example, model: FV-320 manufactured by ADVANTEC), further pulverized and classified to obtain particles with a particle size of 180 to 850 μm. The particles taken out are used as reference crosslinked polymer particles.

Pure water absorption/CRC reduction rate is 6% or more, 8% or more, 10% or more, 12% or more, 14% or more, 16% or more, 18% or more, 20% or more, 25% or more, 30% or more. , 35% or more, or 40% or more; It may be 20% or less, 18% or less, or 16% or less.

The CRC of the reference crosslinked polymer particles may be, for example, 40 g/g or greater, 45 g/g or greater, 50 g/g or greater, or 55 g/g or greater, and may be 60 g/g or less, or 58 g/g or less. . The pure water absorption of the reference crosslinked polymer particles is, for example, 800 g/g or more, 850 g/g or more, 900 g/g or more, 1000 g/g or more, 1500 g/g or more, 2000 g/g or more, 2500 g/g or more, or 3000 g/g or more, 4000 g/g or less, 3500 g/g or less, 3000 g/g or less, 2500 g/g or less, 2000 g/g or less, 1500 g/g or less, 1200 g/g or less, or 1100 g/g or less It's okay.

Pure water absorption/CRC of the reference crosslinked polymer particles may be, for example, 15 or more, 17 or more, 20 or more, 25 or more, 30 or more, 35 or more, 40 or more, 45 or more, 50 or more, 55 or more, It may be 65 or less, 63 or less, 61 or less, 59 or less, 55 or less, 50 or less, 45 or less, 40 or less, 35 or less, 30 or less, 25 or less, 20 or less.

The crosslinked polymer particles obtained through the structure optimization step are subjected to surface crosslinking. Prior to surface cross-linking, the cross-linked polymer particles may be ground and/or classified.

For pulverization, for example, roller mills (roll mills), stamp mills, jet mills, high-speed rotary pulverizers (ultracentrifugal pulverizers, hammer mills, pin mills, rotor mills, rotor beater mills, etc.), container-driven mills (rotary mills, vibration mills, planetary mills, etc.) can be used. The grinder may have an opening on the exit side, such as a perforated plate, screen, grid, etc., to control the maximum particle size of the ground particles. The shape of the opening may be polygonal, circular, etc., and the maximum diameter of the opening may be 0.1-5 mm, 0.3-3.0 mm, or 0.5-1.5 mm.

As the classification method, a known classification method can be used, and for example, screen classification or wind classification may be used. Screen classification is a method of classifying particles on a screen into particles that pass through the mesh of the screen and particles that do not pass through the mesh of the screen by vibrating the screen. Screen classification can be performed using, for example, a vibrating screen, a rotary shifter, a cylindrical stirring screen, a blower shifter, or a low-tap shaker. Pneumatic classification is a method of classifying particles using air flow.

[Surface cross-linking]
The method for producing water-absorbent resin particles according to the present embodiment includes a step of surface-crosslinking crosslinked polymer particles. That is, the water-absorbent resin particles obtained by the production method according to the present embodiment are surface-crosslinked. Surface cross-linking can be performed, for example, by adding a cross-linking agent for surface cross-linking (surface cross-linking agent) to the cross-linked polymer particles and reacting them. The addition of the surface cross-linking agent can be performed, for example, after pulverizing the crosslinked polymer particles, and may be performed before or after classification. By subjecting the crosslinked polymer particles to the surface cross-linking treatment, the crosslink density in the vicinity of the surface of the crosslinked polymer particles is increased, so that the water absorbing performance of the obtained water absorbent resin particles can be enhanced.

The addition of the surface cross-linking agent to the crosslinked polymer particles can be carried out, for example, by adding a surface cross-linking agent solution or spraying the surface cross-linking agent solution. From the viewpoint of uniformly dispersing the surface cross-linking agent on the particle surface, it is preferable to dissolve the surface cross-linking agent in a solvent such as water and/or alcohol and add it as a surface cross-linking agent solution. In addition, the surface cross-linking step may be performed once or divided into two or more times.

The surface cross-linking agent may contain, for example, two or more functional groups (reactive functional groups) having reactivity with functional groups derived from ethylenically unsaturated monomers. Examples of surface cross-linking agents include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; Polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerol diglycidyl ether, (poly) glycerol triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether; epichlorohydrin, epi haloepoxy compounds such as bromhydrin and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol; Oxetane compounds such as 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol;1 oxazoline compounds such as ,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these, (poly)ethylene glycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly) ) Polyglycidyl compounds such as glycerol polyglycidyl ether and/or polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, polyoxyethylene glycol, and polyoxypropylene glycol are preferred, and polyglycidyl compounds are more preferred. These surface cross-linking agents may be used alone or in combination of two or more. For example, polyglycidyl compounds and polyols may be used in combination.

From the viewpoint of moderately increasing the crosslink density in the vicinity of the surface of the water-absorbent resin particles, the amount of the surface cross-linking agent to be added is preferably 0.5 to 100 mol of the total amount of the ethylenically unsaturated monomers used in the polymerization. 0001 to 4.0 mol, more preferably 0.001 to 2.0 mol.

The surface cross-linking step is preferably carried out in the presence of 1 to 200 parts by mass of water per 100 parts by mass of the ethylenically unsaturated monomer. The water content can be adjusted by appropriately using water and/or a water-soluble organic solvent such as alcohol. By adjusting the water content during the surface cross-linking step, cross-linking in the vicinity of the particle surface of the water-absorbing resin particles can be performed more favorably.

The treatment temperature of the surface cross-linking agent is appropriately set according to the surface cross-linking agent to be used, and may be 20 to 250°C. The treatment time with the surface cross-linking agent may be 1-200 minutes or 5-100 minutes. Surface cross-linking may be performed only once, or may be performed at multiple timings.

[Water absorbent resin particles]
The water-absorbent resin particles obtained by the production method according to the present embodiment include a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, such as diethylenetriaminepentaacetic acid pentasodium salt), and improved fluidity. Additional ingredients such as agents (lubricants) may also be included. The additional component may be placed inside, on the surface of, or both of the water-absorbing resin particles.

The water absorbent resin particles may further contain a plurality of inorganic particles arranged on the surface of the particles. The production method according to this embodiment may further include a step of adhering inorganic particles to the surfaces of the particles.

The shape of the water absorbent resin particles obtained by the production method according to the present embodiment may be, for example, a crushed shape or a shape formed by aggregation of crushed particles. The median particle diameter of the water absorbent resin particles may be 250 μm or more, 280 μm or more, 300 μm or more, 320 μm or more, or 340 μm or more, and may be 850 μm or less, 800 μm or less, 750 μm or less, 700 μm or less, 650 μm or less, 600 μm or less, or 550 μm. 500 μm or less, 450 μm or less, 420 μm or less, 400 μm or less, or 380 μm or less.

The CRC of the water absorbent resin particles is, for example, 30 g/g or more, 32 g/g or more, 34 g/g or more, 36 g/g or more, 38 g/g or more, 40 g/g or more, 42 g/g or more, 44 g/g or more, may be 46 g/g or greater, 48 g/g or greater, or 50 g/g or greater; 60 g/g or less; 58 g/g or less; 56 g/g or less; It may be 48 g/g or less, 46 g/g or less, 44 g/g or less, or 42 g/g or less. In general, there is a trade-off relationship that the higher the CRC of the water-absorbing resin particles, the lower the water absorption under load. It has excellent water absorption and can also maintain a relatively high CRC.

The physiological saline water absorption amount of the water absorbent resin particles under load is, for example, 15 mL/g or more, 17 mL/g or more, 19 mL/g or more, 21 mL/g or more, 23 mL/g or more, 25 mL/g or more, 27 mL/g. or more, or may be 29 mL/g or more, 35 mL/g or less, 33 mL/g or less, 31 mL/g or less, 29 mL/g or less, 27 mL/g or less, 25 mL/g or less, 23 mL/g or less, 21 mL/g or less, or 19 mL/g or less.

The water-absorbent resin particles obtained by the production method according to the present embodiment are based on the water-absorbent resin particles obtained by surface-crosslinking the reference crosslinked polymer particles under the same conditions, and the physiological saline water absorption amount under load is is, for example, 7% or more, 8% or more, 10% or more, 12% or more, 14% or more, 16% or more, 18% or more, 20% or more, or 22% or more, 30% 28% or less, 26% or less, 24% or less, 22% or less, or 20% or less.

The water-absorbing resin particles obtained by the production method according to the present embodiment have excellent water-absorbing properties. It can be used in fields such as industrial materials such as agents.

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

(Production example 1)
[Polymerization step]
93.48 g (1.30 mol) of acrylic acid was placed in a separable flask having an inner volume of 2 L. 78.32 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Then, 81.56 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare a partially neutralized sodium acrylic acid solution having a monomer concentration of 45% by mass in which 75% by mass of acrylic acid was neutralized. did.

253.36 g of the prepared sodium acrylic acid partially neutralized solution, 46.82 g of ion-exchanged water, and 0.09 g of polyethylene glycol diacrylate as an internal cross-linking agent (Tokyo Chemical Industry Co., Ltd., polyethylene glycol diacrylate N≈9) was placed in a circular aluminum vat (opening and bottom diameter: 150 mm, height: 60 mm) coated with fluororesin. A uniform liquid mixture (aqueous monomer solution) was formed by placing one stirring bar (8 mm in diameter, 30 mm in length, without ring) at the center of the circular vat and stirring.

A thermometer for measuring the temperature of the mixture was placed in the aluminum vat at a distance of 3 cm from the center. After that, the opening of the aluminum vat was covered with a polyethylene film. After adjusting the temperature of the mixture to 25° C., the reaction system was replaced with nitrogen by bubbling nitrogen gas through a tube inserted into the mixture until the dissolved oxygen content became 0.1 ppm or less. Next, while stirring the mixture at 500 rpm, a tube for nitrogen replacement was removed from the reaction system, and 0.15 g of a 20% by mass sodium persulfate aqueous solution and 10% by mass of 2,2′-azobis(2-amidinopropane) dichloride were added. 0.64 g of hydrochloride (V-50) aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the reaction solution. One minute after the addition, using a syringe (3 mL disposable syringe manufactured by HENKE SASS WOLF, injection needle manufactured by Terumo Corporation), 0.50% by mass of L-ascorbic acid aqueous solution 0.26 g and 0.35% by mass of excess 1.33 g of an aqueous hydrogen oxide solution was added dropwise to the mixture in the aluminum vat.

A polymerization reaction started immediately after the aqueous hydrogen peroxide solution was added dropwise. Stirring was stopped 1 minute after the end of dropping the aqueous hydrogen peroxide solution. After the viscosity of the reaction solution increased with the progress of the polymerization reaction, the mixture gelled. A thermometer for measuring the temperature of the mixture indicated a maximum value of 68° C. 7 minutes after the completion of dropping the aqueous hydrogen peroxide solution. After that, the aluminum vat containing the hydrogel polymer formed by the gelation of the reaction solution was immersed in a water bath at 75° C., and the hydrogel polymer was aged in that state for 20 minutes.

[Crushing process]
The hydrous gel polymer was taken out from the aluminum vat and immediately cut to a width of about 5 cm. The cut water-containing gel-like polymer was coarsely pulverized with a meat chopper (12VR-750SDX manufactured by Kiren Royal Co., Ltd.). Crude material (A) containing elongated structures of hydrous gelatinous polymer was discharged from a plurality of circular discharge holes of a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the outlet hole was 6.4 mm. Crude crushing by the meat chopper was continued for 3 minutes after the hydrogel polymer was added. The granules obtained were aggregates formed by a plurality of elongated structures with a width of 4-6 mm. The moisture content of the granulate ranged from 45-70%.

(Example 1)
[Structure optimization process]
100 g of the coarsely crushed product (A) was rounded into a sphere with a diameter of 7.5 cm, placed on a stainless steel sieve with an inner diameter of 7.5 cm, a height of 2 cm, and an opening of 0.3 mm. 320), a structure optimization step was performed by hot air drying at 200° C. for 1 hour to obtain a dried product.

[Pulverization process]
The dried product was lightly crushed into about 2 cm sides by hand and pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved by shaking for 1 minute using a sieve with an opening of 850 μm and a sieve of 180 μm. By classification, crosslinked polymer particles were obtained as a fraction that passed through the 850 μm sieve and did not pass through the 180 μm sieve.

[Surface cross-linking step]
A surface cross-linking agent solution was prepared by mixing 0.015 g of ethylene carbonate, 0.600 g of 2-propanol, and 0.891 g of deionized water. 15 g of the crosslinked polymer particles were weighed into a cylindrical separable flask with an inner diameter of 11 cm and equipped with a fluororesin anchor-type stirring blade. Next, while stirring at 500 rpm, the above surface cross-linking agent solution was taken with a 3 ml syringe, set a fine atomizer (Oral) [manufactured by Yoshikawa Kasei Co., Ltd.] at the tip of the syringe, and sprayed into the separable flask for 100 seconds. A mixture was obtained by stirring. The mixture was heated at 200° C. for 40 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 µm. By classification, water absorbent resin particles (1) passed through a sieve with an opening of 850 μm were obtained.

(Example 2)
100 g of the crushed product (A) is tightly packed in a stainless steel sieve with a diameter of 7.5 cm, a height of 2 cm, and an opening of 0.3 mm. was subjected to hot air drying. After that, it is further crushed by hand into 1.5 cm sides, spread on a wire mesh with an inner diameter of 20 cm, a height of 4.5 cm, and an opening of 0.3 mm, and placed in a hot air dryer (manufactured by ADVANTEC, model: FV-320). was further subjected to hot air drying at 200° C. for 30 minutes to perform a structure optimization step to obtain a dried product. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (2).

(Comparative example 1)
80 g of the crushed product (A) is wrapped in aluminum foil cut into 25 cm × 30 cm (antibacterial cooking foil 25 cm × 8 m, Toyo Aluminum Echo Products Co., Ltd.) to a size of 8 cm × 14 cm, and dried in a hot air dryer (manufactured by ADVANTEC, model: FV). -320) and subjected to hot air drying at 200° C. for 1 hour. The obtained processed coarsely crushed material is spread on a wire mesh having a size of 23 cm × 23 cm and an opening of 0.8 cm × 0.8 cm, and a hot air dryer (manufactured by ADVANTEC, model: FV-320) is used to Further, a dried product was obtained by subjecting it to hot air drying at 180° C. for 30 minutes. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (3).

(Comparative example 2)
A dried product was obtained in the same manner as in Example 1, except that 100 g of the crushed product (A) was spread out on a wire mesh having a size of 23 cm×23 cm and an opening of 0.3 mm without being rolled. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (4).

(Comparative Example 3)
A dried product was obtained in the same manner as in Example 1, except that the drying time at 200° C. was changed to 2 hours. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (5).

(Reference example 1)
Place 100 g of the crushed material (A) on a wire mesh with a size of 23 cm × 23 cm and a mesh size of 0.8 cm × 0.8 cm so that the crushed material (A) does not overlap, and hot air dryer (manufactured by ADVANTEC, model : FV-320), and subjected to hot air drying at 180°C for 30 minutes to obtain a dried product. The pulverization step was carried out in the same manner as in Example 1 to obtain reference crosslinked polymer particles. The reference crosslinked polymer particles were subjected to the surface cross-linking step in the same manner as in Example 1 to obtain water absorbent resin particles (6).

(Production example 2)
[Polymerization step]
93.48 g (1.30 mol) of acrylic acid was placed in a separable flask having an inner volume of 2 L. 78.32 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Then, 81.56 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare a partially neutralized sodium acrylic acid solution having a monomer concentration of 45% by mass in which 75% by mass of acrylic acid was neutralized. did.

253.36 g of the prepared sodium acrylic acid partially neutralized solution, 46.73 g of ion-exchanged water, and 0.17 g of polyethylene glycol diacrylate as an internal cross-linking agent (Tokyo Chemical Industry Co., Ltd., polyethylene glycol diacrylate N≈9) was placed in a circular aluminum vat (opening and bottom diameter: 150 mm, height: 60 mm) coated with fluororesin. A uniform liquid mixture (aqueous monomer solution) was formed by placing one stirring bar (8 mm in diameter, 30 mm in length, without ring) at the center of the circular vat and stirring.

A thermometer for measuring the temperature of the mixture was placed in the aluminum vat at a distance of 3 cm from the center. After that, the opening of the aluminum vat was covered with a polyethylene film. After adjusting the temperature of the mixture to 25° C., the reaction system was replaced with nitrogen by bubbling nitrogen gas through a tube inserted into the mixture until the dissolved oxygen content became 0.1 ppm or less. Next, while the mixture was stirred at 500 rpm, a tube for nitrogen replacement was removed from the reaction system, and 0.15 g of a 20% by mass sodium persulfate aqueous solution and 10% by mass of (2,2'-azobis(2-amidinopropane) 0.64 g of dihydrochloride (V-50) aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the reaction solution.One minute after the addition, a syringe (3 mL disposable syringe manufactured by HENKE SASS WOLF, Terumo Corporation) was added. Using an injection needle), 0.26 g of 0.50% by mass L-ascorbic acid aqueous solution and 1.33 g of 0.35% by mass hydrogen peroxide aqueous solution were dropped into the mixture in the aluminum vat.

A polymerization reaction started immediately after the aqueous hydrogen peroxide solution was added dropwise. Stirring was stopped 1 minute after the end of dropping the aqueous hydrogen peroxide solution. After the viscosity of the reaction solution increased with the progress of the polymerization reaction, the mixture gelled. A thermometer for measuring the temperature of the mixture indicated a maximum value of 77° C. 6 minutes after the completion of dropping the aqueous hydrogen peroxide solution. After that, the aluminum vat containing the hydrogel polymer formed by the gelation of the reaction solution was immersed in a water bath at 75° C., and the hydrogel polymer was aged in that state for 20 minutes.

[Crushing process]
The hydrous gel polymer was taken out from the aluminum vat and immediately cut to a width of about 5 cm. The cut water-containing gel-like polymer was coarsely pulverized with a meat chopper (12VR-750SDX manufactured by Kiren Royal Co., Ltd.). Crude material (B) containing elongated structures of hydrous gel-like polymer was discharged from a plurality of circular discharge holes of a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the outlet hole was 6.4 mm. Crude crushing by the meat chopper was continued for 3 minutes after the hydrogel polymer was added. The granules obtained were aggregates formed by a plurality of elongated structures with a width of 4-6 mm. The moisture content of the granulate ranged from 45-70%.

(Example 3)
[Structure optimization process]
An aqueous solution was prepared by mixing and dissolving 0.038 g of iron sulfate heptahydrate and 1000 g of deionized water. Furthermore, 49 g of deionized water was further mixed with 1 g of the obtained aqueous solution to prepare an aqueous iron sulfate solution. To 100 g of the crushed material (B), the entire amount of the iron sulfate aqueous solution was added, and the mixture was stirred with a spatula to obtain a mixture. The obtained mixture was spread on a wire mesh with a size of 23 cm × 23 cm and an opening of 0.8 cm × 0.8 cm, and dried at 180 ° C. using a hot air dryer (manufactured by ADVANTEC, model: FV-320). A dried product was obtained by subjecting it to hot air drying for 30 minutes. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (7).

(Example 4)
An aqueous solution was prepared by mixing and dissolving 0.038 g of iron sulfate heptahydrate and 1000 g of deionized water. Furthermore, 40 g of deionized water was further mixed with 10 g of the obtained aqueous solution to prepare an aqueous iron sulfate solution. A dried product was obtained in the same manner as in Example 3, except that the iron sulfate aqueous solution added to the coarsely ground product (B) was changed to this iron sulfate aqueous solution. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (8).

(Example 5)
A dried material was obtained in the same manner as in Comparative Example 1 except that 80 g of the coarsely ground material (B) was used and the drying time at 200° C. was changed to 2 hours. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (9).

(Reference example 2)
Place 100 g of the crushed material (B) on a wire mesh with a size of 23 cm × 23 cm and an opening of 0.8 cm × 0.8 cm so that the crushed material (B) does not overlap, and dry it with a hot air dryer (manufactured by ADVANTEC, model: FV-320) was used for hot air drying at 180° C. for 30 minutes to obtain a dried product. The pulverization step was carried out in the same manner as in Example 1 to obtain reference crosslinked polymer particles. The reference crosslinked polymer particles were subjected to the surface cross-linking step in the same manner as in Example 1 to obtain water absorbent resin particles (10).

(Production example 3)
[Polymerization step]
93.48 g (1.30 mol) of acrylic acid was placed in a separable flask having an inner volume of 2 L. 78.32 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Then, 81.56 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare a partially neutralized sodium acrylic acid solution having a monomer concentration of 45% by mass in which 75% by mass of acrylic acid was neutralized. did.

253.36 g of the prepared sodium acrylic acid partially neutralized solution, 46.77 g of ion-exchanged water, and 0.14 g of polyethylene glycol diacrylate as an internal cross-linking agent (Tokyo Chemical Industry Co., Ltd., polyethylene glycol diacrylate N≈9) was placed in a circular aluminum vat (opening and bottom diameter: 150 mm, height: 60 mm) coated with fluororesin. A uniform liquid mixture (aqueous monomer solution) was formed by placing one stirring bar (8 mm in diameter, 30 mm in length, without ring) at the center of the circular vat and stirring.

A thermometer for measuring the temperature of the mixture was placed in the aluminum vat at a distance of 3 cm from the center. After that, the opening of the aluminum vat was covered with a polyethylene film. After adjusting the temperature of the mixture to 25° C., the reaction system was replaced with nitrogen by bubbling nitrogen gas through a tube inserted into the mixture until the dissolved oxygen content became 0.1 ppm or less. Next, while the mixture was stirred at 500 rpm, a tube for nitrogen replacement was removed from the reaction system, and 0.15 g of a 20% by mass sodium persulfate aqueous solution and 10% by mass of (2,2'-azobis(2-amidinopropane) 0.64 g of dihydrochloride (V-50) aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the reaction solution.One minute after the addition, a syringe (3 mL disposable syringe manufactured by HENKE SASS WOLF, Terumo Corporation) was added. Using an injection needle), 0.26 g of a 0.50% by mass L-ascorbic acid aqueous solution and 1.33 g of a 0.35% by mass aqueous hydrogen peroxide solution were dropped into the mixture in the aluminum vat.

A polymerization reaction started immediately after the aqueous hydrogen peroxide solution was added dropwise. Stirring was stopped 1 minute after the end of dropping the aqueous hydrogen peroxide solution. After the viscosity of the reaction solution increased with the progress of the polymerization reaction, the mixture gelled. A thermometer for measuring the temperature of the mixture indicated a maximum value of 66° C. 7 minutes after the completion of dropping the aqueous hydrogen peroxide solution. After that, the aluminum vat containing the hydrogel polymer formed by the gelation of the reaction solution was immersed in a water bath at 75° C., and the hydrogel polymer was aged in that state for 20 minutes.

[Crushing process]
The hydrous gel polymer was taken out from the aluminum vat and immediately cut to a width of about 5 cm. The cut water-containing gel-like polymer was coarsely pulverized with a meat chopper (12VR-750SDX manufactured by Kiren Royal Co., Ltd.). Crushed material (C) containing elongated structures of hydrous gel-like polymer was discharged from a plurality of circular discharge holes of a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the outlet hole was 6.4 mm. Crude crushing by the meat chopper was continued for 3 minutes after the hydrogel polymer was added. The granules obtained were aggregates formed by a plurality of elongated structures with a width of 4-6 mm. The moisture content of the granulate ranged from 45-70%.

(Reference example 3)
Place 100 g of the crushed material (C) on a wire mesh with a size of 23 cm × 23 cm and an opening of 0.8 cm × 0.8 cm so that the crushed material (C) does not overlap, and hot air dryer (manufactured by ADVANTEC, model : FV-320), and subjected to hot air drying at 180°C for 30 minutes to obtain a dried product. The steps after pulverization were carried out in the same manner as in Example 1 to obtain water absorbent resin particles (11).

The crosslinked polymer particles or water-absorbent resin particles obtained in Examples, Comparative Examples, and Reference Examples were evaluated by the following methods for pure water absorption, CRC, pure water absorption/CRC, median particle size, under load Physiological saline water absorption (AUL) and water content were measured. The results are shown in the text or in Tables 1 and 2.

<Pure water absorption>
The amount of pure water absorption was measured in an environment of 25±2° C. and 50±10% humidity according to the following procedure. 1000 g of ion-exchanged water was weighed into a 1000 mL beaker. Next, 0.1 g of the crosslinked polymer particles were dispersed in physiological saline while stirring at 600 rpm using a magnetic stirrer bar (8 mmφ×30 mm length, without ring) so as not to generate lumps. By allowing the particles to swell sufficiently by leaving them in the stirred state for 60 minutes, a dispersion containing a swollen gel was obtained. Subsequently, after measuring the total mass Wa (g) of two 75 μm standard sieves, the above dispersion was divided into two and passed through the two standard sieves. Excess moisture was removed by leaving the sieve tilted at an angle of about 30 degrees with respect to the horizontal for 30 minutes. The total weight Wb (g) of the sieve on which the swollen gel remained was measured, and the pure water absorption amount Wd (g/g) was determined by the following formula. Wc (g) is a value obtained by accurately weighing 0.1 g of the crosslinked polymer particles used for measurement.
Pure water absorption Wd (g/g) = (Wb-Wa-Wc)/Wc

<Centrifuge retention capacity (CRC)>
The centrifuge retention capacity (CRC) was measured by the following procedure with reference to the EDANA method (NWSP 241.0.R2(15), pages 769-778). The measurement was performed under an environment of temperature 25°C ± 2°C and humidity 50% ± 10%.

A 60 mm×170 mm non-woven fabric (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) was folded in half in the longitudinal direction to adjust the size to 60 mm×85 mm. A 60 mm×85 mm non-woven fabric bag was produced by heat-sealing the non-woven fabrics together on both sides extending in the longitudinal direction (crimped portions with a width of 5 mm were formed on both sides along the longitudinal direction). 0.2 g of particles (water-absorbent resin particles or crosslinked polymer particles) were accurately weighed and contained in the interior of the non-woven fabric bag. After that, the non-woven fabric bag was closed by crimping the remaining side extending in the transverse direction by heat sealing.

The entire non-woven bag was completely wetted by floating it on 1000 g of physiological saline contained in a stainless steel vat (240 mm×320 mm×45 mm) without folding the non-woven bag. One minute after the non-woven fabric bag was put into the physiological saline, the non-woven fabric bag was immersed in the physiological saline with a spatula to obtain a non-woven fabric bag containing the gel.

Thirty minutes after putting the non-woven fabric bag into the physiological saline (total of 1 minute of floating time and 29 minutes of immersion time), the non-woven fabric bag was taken out from the physiological saline. Then, the non-woven fabric bag was placed in a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reached 250 G, the non-woven fabric bag was dehydrated for 3 minutes. After dehydration, the mass Ma (g) of the nonwoven bag containing the mass of the gel was weighed. The nonwoven bag was subjected to the same operation as described above without accommodating the particles, and the mass Mb (g) of the nonwoven bag after dehydration was measured. CRC (g/g) was calculated based on the following formula. Mc(g) is a value obtained by accurately weighing 0.2 g of the water-absorbent resin particles or crosslinked polymer particles used in the measurement.
CRC (g/g) = [(Ma-Mb)-Mc]/Mc

<Pure water absorption amount/CRC>
The value of pure water absorption/CRC was calculated by dividing the pure water absorption Wd (g/g) by CRC (g/g).

<Pure water absorption amount/CRC reduction rate>
The reduction rate of pure water absorption/CRC with respect to reference crosslinked polymer particles (crosslinked polymer particles obtained by drying at 180° C. for 30 minutes without performing the structure optimization step) was calculated by the following formula.
Pure water absorption/CRC reduction rate (%) = {[(pure water absorption of reference crosslinked polymer particles/CRC) - (pure water absorption of measurement target/CRC)]/(reference crosslinked polymer particles Pure water absorption/CRC)}×100
Specifically, in Examples 1 and 2 and Comparative Examples 1 to 3, the crosslinked polymer particles of Reference Example 1 were used as reference crosslinked polymer particles, and in Examples 3 to 5, the crosslinked polymer particles of Reference Example 2 were used. Particles were calculated as reference crosslinked polymer particles.

<Moisture content>
2.0 g of the sample is placed in an aluminum foil case (No. 8) that has been adjusted to a constant weight (W1 (g)) in advance, the opening of the aluminum foil case is lightly closed, and the total mass W2 (g) of the aluminum foil case containing the sample is precisely calculated. weighed. The aluminum foil case containing the sample was dried for 2 hours with a hot air dryer (manufactured by ADVANTEC, model: FV-320) whose internal temperature was set at 200°C. After drying, the aluminum foil case containing the sample was allowed to cool to room temperature in a desiccator. The total mass W3 (g) of the aluminum foil case containing the sample after standing to cool was measured. The moisture content of the sample was calculated from the following formula. The moisture contents of the dried products obtained in Examples, Comparative Examples and Reference Examples were all within the range of 0 to 15% by mass.
Moisture content [mass%] = {[(W2-W1)-(W3-W1)]/(W2-W1)}×100

<Measurement of AUL (physiological saline water absorption under 4.14 kPa load)>
Physiological saline water absorption under a load of 4.14 kPa (water absorption under load) was measured using a measuring apparatus outlined in FIG. The measuring device comprises a burette part 1 , a clamp 3 , a conduit 5 , a pedestal 11 , a measuring table 13 and a measuring part 4 placed on the measuring table 13 . The burette part 1 includes a burette tube 21 with a scale, a rubber stopper 23 sealing an upper opening of the burette tube 21, a cock 22 connected to the tip of the lower part of the burette tube 21, and a lower part of the burette tube 21. It has an air introduction pipe 25 and a cock 24 connected to the . The burette part 1 is fixed with a clamp 3 . A flat plate-shaped measuring stand 13 has a through hole 13a with a diameter of 2 mm formed in its central portion, and is supported by a pedestal 11 whose height is variable. Through hole 13 a of measuring table 13 and cock 22 of burette portion 1 are connected by conduit 5 . The inner diameter of conduit 5 is 6 mm.

The measurement unit 4 has a Plexiglas cylinder 31 , a polyamide mesh 32 adhered to one opening of the cylinder 31 , and a weight 33 vertically movable within the cylinder 31 . Cylinder 31 is placed on measuring table 13 via polyamide mesh 32 . The inner diameter of the cylinder 31 is 20 mm. The opening of the polyamide mesh 32 is 75 μm (200 meshes). The weight 33 has a diameter of 19 mm and a mass of 119.6 g, and can apply a load of 4.14 kPa (0.6 psi) to the water absorbent resin particles 10a uniformly arranged on the polyamide mesh 32 as described later. can.

The physiological saline water absorption under a load of 4.14 kPa was measured in a room at 25° C. using the measuring apparatus shown in FIG. First, the cocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass physiological saline adjusted to 25° C. was introduced into the burette tube 21 through the upper opening of the burette tube 21 . Next, after sealing the upper opening of the burette tube 21 with a rubber stopper 23, the cocks 22 and 24 were opened. The interior of the conduit 5 was filled with a 0.9 mass % saline solution 50 so as not to introduce air bubbles. The height of the measuring table 13 was adjusted so that the height of the water surface of the 0.9 mass % saline solution reaching the inside of the through-hole 13 a was the same as the height of the upper surface of the measuring table 13 . At this time, it was confirmed that the same volume of air as the 0.9 mass % saline solution 50 sucked out from the through-hole 13a was rapidly supplied from the air introduction pipe 25 into the burette. After the adjustment, the height of the water surface of the physiological saline solution 50 in the burette tube 21 was read from the scale of the burette tube 21, and the position was taken as the zero point (read value at 0 seconds).

In the measurement unit 4, 0.10 g of water-absorbing resin particles 10a are uniformly arranged on a polyamide mesh 32 in a cylinder 31, a weight 33 is arranged on the water-absorbing resin particles 10a, and the cylinder 31 is set so that its center is It was installed so as to match the conduit port in the center of the measuring table 13 . Decreased amount of physiological saline in burette tube 21 60 minutes after water-absorbing resin particles 10a began to absorb physiological saline from conduit 5 (that is, amount of physiological saline absorbed by water-absorbing resin particles 10a) We (ml) was read, and the physiological saline water absorption amount of the water absorbent resin particles 10a under a load of 4.14 kPa was calculated according to the following formula. Table 1 shows the results.
Physiological saline water absorption under 4.14 kPa load (ml/g) = We (ml)/mass of water-absorbing resin particles (g)

In Examples 1 and 2, the pure water absorption amount/CRC in the obtained crosslinked polymer particles was in the range of 10 to 40, and the reduction rate of the pure water absorption amount/CRC based on Reference Example 1 was It was 5 to 45%, and it was confirmed that the structure optimization process was properly performed. In Production Example 1, the water absorbent resin particles obtained in Examples achieved an AUL increase rate of more than 6% based on Reference Example 1 in which the structure optimization step was not performed. On the other hand, even if the heat drying process was performed, the pure water absorption amount / CRC of the crosslinked polymer particles exceeded 40 in Comparative Examples 1 and 2, and the decrease in the pure water absorption amount / CRC of the crosslinked polymer particles. The water-absorbing resin particles of Comparative Example 3, which had a ratio exceeding 45% with respect to the standard crosslinked polymer particles, showed an increase in AUL of 6% or less.

In Examples 3 to 5, the pure water absorption amount/CRC in the obtained crosslinked polymer particles was in the range of 10 to 40, and the reduction rate of the pure water absorption amount/CRC based on Reference Example 2 was 5 to 5. It was 45%, and it was confirmed that the structure optimization process was performed appropriately. Also in Production Example 2, similarly to Production Example 1, the water-absorbing resin particles obtained in Examples achieved an AUL increase rate of more than 6% based on Reference Example 2 in which the structure optimization step was not performed. . Further, in Reference Example 3 in which the structure optimization step was not performed, by adjusting the charged internal cross-linking agent, the CRC was reduced in the same manner as in Example 5 at the stage of the crosslinked polymer particles before performing surface cross-linking. A higher one was obtained, but the value of pure water absorption/CRC could not be decreased, and the AUL in the resulting water-absorbing resin particles was significantly lower than in Example 5 after the same surface cross-linking step. rice field. Therefore, although it is possible to improve the CRC of the crosslinked polymer particles by adjusting the polymerization conditions, etc., it is necessary to control the pure water absorption amount/CRC value and improve the AUL of the water-absorbent resin particles by optimizing the structure. The process was shown to be effective.

DESCRIPTION OF SYMBOLS 1... Burette part 3... Clamp 4... Measuring part 5... Conduit 10a... Water-absorbent resin particles 11... Stand 13... Measuring stand 13a... Through hole 21... Burette tube 22... Cock 23... Rubber plug, 24... cock, 25... air introduction tube, 31... cylinder, 32... polyamide mesh, 33... weight, 50... physiological saline.

Claims (3)

preparing a hydrous gel-like polymer;
By treating the water-containing gel polymer, the pure water absorption amount / CRC is in the range of 10.0 to 40.0, and the particles obtained in the same manner without treatment are dried at 180 ° C. for 30 minutes. obtaining crosslinked polymer particles having a pure water absorption/CRC reduction rate of 5 to 45% with respect to the reference crosslinked polymer particles obtained by
A method for producing water-absorbent resin particles, comprising surface-crosslinking the crosslinked polymer particles. 2. The method according to claim 1, wherein the hydrogel polymer has a water content of 45 to 70% by mass at the start of the treatment. 3. The method of claim 1 or 2, comprising granulating the hydrogel polymer prior to the treatment.

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