Method for producing crosslinked polymer particles and method for producing water-absorbent resin particles

Abstract

The method for producing cross-linked polymer particles according to the present invention comprises a step for obtaining a cross-linked polymer by polymerizing a monomer, a drying step for drying the cross-linked polymer, and a pulverizing step for pulverizing the cross-linked polymer after drying, wherein the difference between the surface temperature of the cross-linked polymer at the end of the drying step and the surface temperature of the cross-linked polymer at the start of the pulverizing step is more than 0°C and no more than 100°C.

Description

[Technical Field]

[0001] The present invention relates to a method for producing crosslinked polymer particles and a method for producing water-absorbent resin particles. [Background technology]

[0002] Conventionally, absorbent articles for absorbing liquids containing water as a main component (e.g., urine) have used absorbents containing water-absorbent resin particles (see, for example, Patent Document 1 below). The water-absorbent resin particles can be obtained, for example, by pulverizing a crosslinked polymer to obtain crosslinked polymer particles, and then subjecting the crosslinked polymer particles to additional crosslinking. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 06-345819 Summary of the Invention [Problem to be solved by the invention]

[0004] The water-absorbent resin particles in the absorbent are required to have not only a high water absorption capacity but also excellent liquid diffusibility. Liquid diffusibility generally tends to decrease as the amount of fine powder (fine powder content) in the water-absorbent resin particles increases. That is, when the fine powder in the water-absorbent resin particles swells, it tends to block the path through which the liquid diffuses, which is prone to so-called "gel blocking." When the water-absorbent resin particles undergo gel blocking, the liquid diffusibility in the absorbent deteriorates, and the inherent performance of the absorbent is not fully exhibited, resulting in an increased amount of liquid backflow. Therefore, it is required that the amount of fine powder in the crosslinked polymer particles used to obtain the water-absorbent resin particles is small.

[0005] An object of one aspect of the present invention is to provide a method for producing crosslinked polymer particles, which can produce crosslinked polymer particles with a small amount of fine powder.An object of another aspect of the present invention is to provide a method for producing water-absorbent resin particles using the crosslinked polymer particles. [Means for solving the problem]

[0006] One aspect of the present invention provides a method for producing crosslinked polymer particles, comprising: a step of obtaining a crosslinked polymer by polymerizing a monomer; a drying step of drying the crosslinked polymer; and a pulverizing step of pulverizing the crosslinked polymer after the drying step, wherein the temperature difference between the surface temperature of the crosslinked polymer at the end of the drying step and the surface temperature of the crosslinked polymer at the start of the pulverizing step is greater than 0°C and not greater than 100°C.

[0007] According to such a method for producing crosslinked polymer particles, crosslinked polymer particles with a small amount of fine powder can be obtained.

[0008] Another aspect of the present invention provides a method for producing water-absorbent resin particles, comprising a step of subjecting the crosslinked polymer particles obtained by the above-mentioned method for producing crosslinked polymer particles to additional crosslinking. [Effects of the Invention]

[0009] According to one aspect of the present invention, there is provided a method for producing crosslinked polymer particles, which can produce crosslinked polymer particles with a small amount of fine powder. According to another aspect of the present invention, there is provided a method for producing water-absorbent resin particles using the crosslinked polymer particles. DETAILED DESCRIPTION OF THE INVENTION

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments and can be practiced in various modified forms within the scope of the present invention.

[0011] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." "Acrylate" and "methacrylate" are similarly referred to as "(meth)acrylate." "Polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol." The same applies to other expressions containing "(poly)." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value 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 alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified. With respect to at least one (meth)acrylic acid compound selected from the group consisting of (meth)acrylic acid and its salts, the "content of the (meth)acrylic acid compound" refers to the total amount of acrylic acid, acrylic acid salts, methacrylic acid, and methacrylic acid salts. "Room temperature" refers to 25°C ± 2°C. The water content, median particle size, and particle size distribution may be those at room temperature.

[0012] The method for producing crosslinked polymer particles according to this embodiment comprises a polymerization step of obtaining a crosslinked polymer by polymerizing a monomer, a drying step of drying the crosslinked polymer, and a pulverization step of pulverizing the crosslinked polymer after the drying step, wherein the temperature difference ΔT (T1-T2) between the surface temperature T1 of the crosslinked polymer at the end of the drying step and the surface temperature T2 of the crosslinked polymer at the start of the pulverization step is greater than 0°C and not greater than 100°C.

[0013] The method for producing crosslinked polymer particles according to this embodiment makes it possible to obtain crosslinked polymer particles with a low amount of fine powder (the mass-based content of fine powder (particles greater than 0 μm and less than 250 μm)). This allows for the production of crosslinked polymer particles (crosslinked polymer particle groups) with a mass ratio (fine particle abundance ratio) of particles less than 250 μm in diameter to particles with a particle diameter of 250 μm or greater and less than 850 μm to be 50 mass% or less. The reason for this effect is unclear, but it is presumed that excessively large changes in the surface temperature of the crosslinked polymer between the end of the drying step and the start of the pulverization step tend to reduce the elasticity of the crosslinked polymer, leading to the crosslinked polymer being easily pulverized during the pulverization step. However, it is presumed that maintaining the surface temperature of the crosslinked polymer within the above-mentioned range between the end of the drying step and the start of the pulverization step tends to prevent the crosslinked polymer from being pulverized during the pulverization step, thereby reducing the amount of fine powder. However, the reason is not limited to this.

[0014] According to this embodiment, a method for adjusting the amount of fine powder in crosslinked polymer particles can be provided. The method for adjusting the amount of fine powder in crosslinked polymer particles according to this embodiment includes a step of obtaining a crosslinked polymer by polymerizing a monomer, a drying step of drying the crosslinked polymer, and a pulverization step of pulverizing the crosslinked polymer after the drying step, and adjusts the amount of fine powder in the crosslinked polymer particles based on the temperature difference between the surface temperature of the crosslinked polymer at the end of the drying step and the surface temperature of the crosslinked polymer at the start of the pulverization step.

[0015] The crosslinked polymer particles according to this embodiment are crosslinked polymer particles obtained by the method for producing crosslinked polymer particles according to this embodiment. The method for producing crosslinked polymer particles according to this embodiment is a method for producing crosslinked polymer particles that can obtain water-absorbent resin particles by applying additional crosslinking. According to the method for producing crosslinked polymer particles according to this embodiment, crosslinked polymer particles that can obtain water-absorbent resin particles having excellent liquid diffusibility in an absorbent body can be obtained. The method for producing water-absorbent resin particles according to this embodiment includes an additional crosslinking step of applying additional crosslinks to the crosslinked polymer particles obtained by the method for producing crosslinked polymer particles according to this embodiment.

[0016] In the polymerization step, a crosslinked polymer is obtained by polymerizing a monomer. In the polymerization step, a monomer composition containing a monomer may be polymerized to obtain a crosslinked polymer. The crosslinked polymer obtained in the polymerization step may be a crosslinked polymer gel.

[0017] The monomer composition may contain water, an organic solvent, etc. The monomer composition may be an aqueous monomer solution. Examples of a method for polymerizing the monomer composition include an aqueous solution polymerization method and a bulk polymerization method. Among these, the aqueous solution polymerization method is preferred from the viewpoints of easily obtaining good water absorption performance and easily increasing productivity. In the following, a case where the aqueous solution polymerization method is used as an example of the polymerization method will be described.

[0018] The monomer may include an ethylenically unsaturated monomer, and may include a water-soluble ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include carboxylic acid monomers such as unsaturated carboxylic acids, such as (meth)acrylic acid, maleic acid, maleic anhydride, and fumaric acid, and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers, such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof; and sulfonic acid monomers such as vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and salts thereof. The ethylenically unsaturated monomer may contain at least one (meth)acrylic acid compound selected from the group consisting of (meth)acrylic acid and its salts. The ethylenically unsaturated monomer may contain both (meth)acrylic acid and a salt of (meth)acrylic acid. Examples of salts of unsaturated carboxylic acids (e.g., (meth)acrylic acid) include alkali metal salts (e.g., sodium salts, potassium salts), and alkaline earth metal salts (e.g., calcium salts).

[0019] The acid group of an ethylenically unsaturated monomer having an acid group (e.g., (meth)acrylic acid) may be neutralized in advance with an alkaline neutralizing agent. Examples of alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. The alkaline neutralizing agent may be used in the form of an aqueous solution to simplify the neutralization procedure. Neutralization of the acid group may be carried out before, during, or after polymerization of the ethylenically unsaturated monomer as a raw material.

[0020] The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 10 to 100 mol%, 30 to 90 mol%, 40 to 85 mol%, or 50 to 80 mol%, from the viewpoints of easily obtaining good water absorption performance by increasing the osmotic pressure, enhancing safety, and suppressing problems caused by the presence of excess alkaline neutralizing agent. The "degree of neutralization" refers to the degree of neutralization of all acid groups in the ethylenically unsaturated monomer.

[0021] The content of the monomer (e.g., a (meth)acrylic acid compound) is preferably in the following ranges based on the total mass of the monomer composition. From the viewpoint of easily increasing productivity, the content of the monomer is preferably 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more. From the viewpoint of easily increasing water absorption performance, the content of the monomer is preferably 60% by mass or less, 55% by mass or less, 50% by mass or less, less than 50% by mass, 45% by mass or less, or 40% by mass or less. From these viewpoints, the content of the monomer is preferably 10 to 60% by mass. In the crosslinked polymer particles according to this embodiment, the content of the structural unit derived from the (meth)acrylic acid compound is preferably in the above-mentioned ranges for the content of the monomer based on the total mass of the crosslinked polymer particles.

[0022] The content of the (meth)acrylic acid compound is preferably within the following ranges based on the total amount of monomers and / or the total amount of ethylenically unsaturated monomers contained in the monomer composition. The content of the (meth)acrylic acid compound is preferably 50 mol% or more, 70 mol% or more, 90 mol% or more, 95 mol% or more, 97 mol% or more, or 99 mol% or more. The monomers and / or the ethylenically unsaturated monomers contained in the monomer composition may be substantially composed of (meth)acrylic acid compounds (an embodiment in which substantially 100 mol% of the monomers and / or the ethylenically unsaturated monomers contained in the monomer composition are (meth)acrylic acid compounds).

[0023] The monomer composition may contain a polymerization initiator. Polymerization of the monomers contained in the monomer composition may be initiated by adding a polymerization initiator to the monomer composition and, if necessary, heating, irradiating with light, or the like. Examples of the polymerization initiator include a photopolymerization initiator and a radical polymerization initiator, and a water-soluble radical polymerization initiator is preferred. From the viewpoint of easily improving water absorption performance, the polymerization initiator preferably contains at least one selected from the group consisting of an azo compound and a peroxide.

[0024] Azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane} dihydrochloride, and 2,2'-azobis[2-(N-benzylamidino)propane] dihydrochloride. acid salt, 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-(2-imidazolin-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], and the like. From the viewpoint of easily obtaining good water absorption performance, the azo compound preferably contains at least one selected from the group consisting of 2,2'-azobis(2-methylpropionamide) dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate.

[0025] Examples of the peroxide include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; and organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butylperoxyisobutyrate, and t-butylperoxypivalate. From the viewpoint of easily obtaining good water absorption performance and easily reducing unreacted monomers contained in the water absorbent resin particles, the peroxide preferably contains at least one selected from the group consisting of potassium persulfate, ammonium persulfate, and sodium persulfate.

[0026] The content of the polymerization initiator is preferably in the following range relative to 1 mole of the ethylenically unsaturated monomer (for example, a (meth)acrylic acid compound). From the viewpoint of easily improving water absorption performance and easily reducing the amount of unreacted monomer contained in the water absorbent resin particles, the content of the polymerization initiator is preferably 0.001 mmol or more, 0.005 mmol or more, 0.01 mmol or more, 0.05 mmol or more, 0.1 mmol or more, or 0.15 mmol or more. From the viewpoint of easily improving water absorption performance and easily avoiding a rapid polymerization reaction, the content of the polymerization initiator is preferably 5 mmol or less, 4 mmol or less, 2 mmol or less, 1 mmol or less, 0.9 mmol or less, 0.7 mmol or less, 0.5 mmol or less, 0.4 mmol or less, or 0.3 mmol or less. From these viewpoints, the content of the polymerization initiator is preferably 0.001 to 5 mmol.

[0027] The monomer composition may contain a reducing agent. Examples of the reducing agent include sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, L-ascorbic acid, etc. The polymerization initiator and the reducing agent may be used in combination.

[0028] The monomer composition may contain an oxidizing agent, such as hydrogen peroxide, sodium perborate, superphosphoric acid and its salts, or potassium permanganate.

[0029] The monomer composition may contain an internal crosslinking agent. By using the internal crosslinking agent, the resulting crosslinked polymer can have a crosslinked structure due to the internal crosslinking agent in addition to a self-crosslinked structure due to the polymerization reaction.

[0030] Examples of the internal crosslinking agent include compounds having two or more reactive functional groups (e.g., polymerizable unsaturated groups). Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols such as (poly)ethylene glycol, (poly)propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; unsaturated polyesters obtained by reacting the above polyols with unsaturated acids (maleic acid, fumaric acid, etc.); glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)glycerin polyglycidyl ether, and glycidyl (meth)acrylate; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; and compounds obtained by reacting polyepoxides with (meth)acrylic acid. Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters obtained; di(meth)acrylic acid carbamyl esters obtained by reacting a polyisocyanate (such as tolylene diisocyanate or hexamethylene diisocyanate) with hydroxyethyl (meth)acrylate; allylated starch; allylated cellulose; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; pentaerythritol; ethylenediamine; and polyethyleneimine. From the viewpoints of easily improving water absorption performance and excellent reactivity at low temperatures, the internal crosslinking agent preferably contains at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin triglycidyl ether, and (poly)glycerin diglycidyl ether.

[0031] The content of the internal crosslinking agent is preferably in the following ranges per mole of ethylenically unsaturated monomer (e.g., (meth)acrylic acid compound). From the viewpoint of easily obtaining good water absorption performance, the content of the internal crosslinking agent is preferably 0.001 mmol or more, 0.005 mmol or more, 0.01 mmol or more, 0.05 mmol or more, 0.07 mmol or more, 0.09 mmol or more, 0.1 mmol or more, 0.11 mmol or more, or 0.13 mmol or more. From the viewpoint of easily obtaining good water absorption performance, the content of the internal crosslinking agent is preferably 5 mmol or less, 4.5 mmol or less, 4 mmol or less, 3.5 mmol or less, 3 mmol or less, 2.5 mmol or less, 2 mmol or less, 1.5 mmol or less, 1 mmol or less, 0.9 mmol or less, 0.8 mmol or less, 0.7 mmol or less, 0.5 mmol or less, 0.4 mmol or less, or 0.3 mmol or less. From these viewpoints, the content of the internal crosslinking agent is preferably 0.001 to 5 mmol.

[0032] The monomer composition may contain additives such as chain transfer agents, thickeners, and inorganic fillers as components other than the above-mentioned components, as needed. Examples of chain transfer agents include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid, and acrolein. Examples of thickeners include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, and polyacrylamide. Examples of inorganic fillers include metal oxides, ceramics, and clay minerals.

[0033] Examples of the polymerization method for aqueous solution polymerization include a static polymerization method in which the monomer composition is polymerized without stirring (for example, in a static state), and an agitation polymerization method in which the monomer composition is polymerized while being stirred in a reaction vessel. In the static polymerization method, a single block-shaped gel is obtained at the completion of polymerization, which has approximately the same volume as the monomer composition present in the reaction vessel.

[0034] The polymerization may be carried out in a batchwise manner, semi-continuously, continuously, etc. For example, when the stationary polymerization method is carried out in a continuous manner, the polymerization reaction is carried out while continuously supplying the monomer composition to a continuous polymerization apparatus, and a gel can be obtained continuously.

[0035] The polymerization temperature varies depending on the polymerization initiator used, but is preferably 0 to 130°C or 10 to 110°C from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time to increase productivity, and facilitating the reaction by removing the heat of polymerization. The polymerization time is appropriately set depending on the type and amount of the polymerization initiator used, the reaction temperature, etc., but is preferably 1 to 200 minutes or 5 to 100 minutes.

[0036] In the drying step, the crosslinked polymer obtained in the polymerization step is dried to obtain a dried product. The crosslinked polymer dried in the drying step may be a crosslinked polymer that has been subjected to a treatment (for example, a crushing treatment described below) between the polymerization step and the drying step. In the drying step, a dried product can be obtained by removing liquid components (water, etc.) in the crosslinked polymer by heating and / or blowing air. The drying method may be natural drying, heat drying (for example, hot air drying), drying under reduced pressure, etc. From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the drying temperature is preferably within the temperature range described below as a preferred range for the surface temperature T1. The drying time may be 1 to 60 minutes, 5 to 50 minutes, 10 to 40 minutes, or 10 to 30 minutes.

[0037] The moisture content of the crosslinked polymer at the end of the drying step may be 10% by mass or less, 8% by mass or less, 5% by mass or less, or less than 5% by mass. The moisture content of the crosslinked polymer is defined as the ratio of the amount of moisture to the mass of the crosslinked polymer including moisture. The moisture content of the crosslinked polymer may be 0% by mass, more than 0% by mass, 0.1% by mass or more, or 0.5% by mass or more. The moisture content of the crosslinked polymer can be measured by the method described in the Examples below. "End of the drying step" refers to the point at which all drying treatments for the crosslinked polymer have been completed, and may be the point at which blowing of hot air onto the crosslinked polymer has been completed, the point at which the crosslinked polymer has been removed from the dryer, etc. The same applies hereinafter.

[0038] The surface temperature T1 of the crosslinked polymer at the end of the drying step (all drying steps) is preferably within the following range. From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the surface temperature T1 is preferably 250°C or less, 200°C or less, 190°C or less, 185°C or less, or 180°C or less. From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the surface temperature T1 is preferably 70°C or more, 80°C or more, 90°C or more, 100°C or more, 120°C or more, 140°C or more, 150°C or more, 160°C or more, 165°C or more, 170°C or more, 175°C or more, or 180°C or more. From these viewpoints, the surface temperature T1 is preferably 70 to 250°C or 150 to 200°C. The surface temperature T1 may be 175°C or less, 170°C or less, 165°C or less, or 160°C or less. The surface temperature T1 can be measured using a radiation thermometer.

[0039] The method for producing crosslinked polymer particles according to this embodiment may include multiple drying steps. The drying temperatures in the multiple drying steps may be the same or different. The drying means in the multiple drying steps may be the same or different. The method for producing crosslinked polymer particles according to this embodiment may include a first drying step for obtaining a crosslinked polymer having a moisture content of 30% by mass or less (for example, more than 10% by mass and 30% by mass or less), and a second drying step for obtaining a crosslinked polymer having a moisture content of 10% by mass or less.

[0040] The method for producing crosslinked polymer particles according to this embodiment may include a crushing step between multiple drying steps. In this case, by performing the crushing step after a certain amount of drying treatment to reduce the moisture content, it is possible to prevent excessive aggregation of the crosslinked polymer at the end of all drying treatments, and it is easy to confirm the effect of the temperature difference ΔT described above. Furthermore, since it is possible to make the crosslinked polymer uniform in size to a certain extent, it is possible to increase the specific surface area and dry it, thereby reducing the variation in moisture content between each particle of the crosslinked polymer (allowing for uniform drying). As a crusher for the crushing step, a roller mill, a hammer mill, etc. can be used.

[0041] The method for producing crosslinked polymer particles according to this embodiment may include a classification step of classifying the crosslinked polymer after the crushing step. In the classification step, particles having a particle diameter of 2.8 mm or more and less than 9.5 mm may be obtained.

[0042] In the pulverization step, the crosslinked polymer (dried product) is pulverized after the drying step to obtain a pulverized product. In the method for producing crosslinked polymer particles according to this embodiment, crosslinked polymer particles may be obtained as the pulverized product. The moisture content of the crosslinked polymer pulverized in the pulverization step may be in the range described above as the moisture content of the crosslinked polymer at the end of the drying step, and may be, for example, 10% by mass or less. The particle size of the crosslinked polymer pulverized in the pulverization step is, for example, 20 mm or less, 15 mm or less, 10 mm or less, 9.5 mm or less, or less than 9.5 mm.

[0043] In the pulverization step, the crosslinked polymer can be pulverized using a screen (mesh member, punched plate, etc.) having openings (through holes; mesh). This embodiment may be, for example, an embodiment in which the crosslinked polymer is pulverized while passing through the screen in the pulverization step. In this case, by passing the crosslinked polymer through the screen from one side to the other side, particles constituting the crosslinked polymer particles can be obtained on the other side of the screen. Furthermore, the particle size can be adjusted while pulverizing the crosslinked polymer, and particles having a diameter corresponding to the opening diameter of the screen openings can be obtained on the other side of the screen. By adjusting the opening diameter of the screen openings, the particle size and particle size distribution of the particles contained in the crosslinked polymer particles can be adjusted. The openings of the screen have an opening diameter larger than the diameter of at least a portion of the crosslinked polymer to be pulverized. The opening diameter (hole diameter) of the screen openings may be, for example, 0.08 to 10 mm, 0.50 to 2.0 mm, or 0.75 to 1.5 mm. The screen may be annular (e.g., circular), plate-shaped, or the like.

[0044] This embodiment may be, for example, an embodiment in which centrifugal force is applied to the crosslinked polymer in the pulverization step, thereby pulverizing the crosslinked polymer while passing it through a screen. In this case, the crosslinked polymer can be impact-pulverized by colliding it with a screen or another member (e.g., a rotating member described below) using centrifugal force. This embodiment may be, for example, an embodiment in which the screen is annular, and in the pulverization step, centrifugal force (centrifugal force directed from the inner periphery to the outer periphery) is applied to the crosslinked polymer on the inner periphery of the screen, thereby pulverizing the crosslinked polymer while passing it through the screen. In this case, by passing the crosslinked polymer through the screen from the inner periphery to the outer periphery of the screen, particles constituting the crosslinked polymer particles can be obtained on the outer periphery of the screen. Furthermore, particle size can be adjusted while pulverizing the crosslinked polymer by impact pulverization, and particles having a diameter corresponding to the opening diameter of the screen can be obtained on the outer periphery of the screen.

[0045] The pulverizer used in the pulverization step may have, for example, a sample stage to which the crosslinked polymer is supplied and an annular screen surrounding the sample stage. The sample stage may be rotatable, and centrifugal force may be applied to the crosslinked polymer by rotating the sample stage. For example, centrifugal force can be applied to the crosslinked polymer by rotating the sample stage around the central axis (axis perpendicular to the circumferential direction) of the annular screen.

[0046] In addition to the sample stage and the annular screen, the crusher may include a rotating member (e.g., a blade member) that can rotate along the inner wall of the annular screen. In this case, the crosslinked polymer can be easily impact-pulverized by causing the crosslinked polymer to collide with the rotating member using centrifugal force while rotating the rotating member. The rotating member may be disposed near the inner wall of the screen. In this case, shear force can be applied to the crosslinked polymer present between the rotating member and the inner wall of the screen, making it easy to pulverize the crosslinked polymer. The rotating member may be a member that extends along the central axis of the annular screen. The rotating member may be integral with the sample stage or may be separate from the sample stage. The rotating member may be rotatable together with the sample stage. A plurality of rotating members (e.g., six members) may be arranged at intervals around the outer periphery of the sample stage. The rotation speed of each of the sample stage and the rotating member may be, for example, 6,000 to 18,000 rpm.

[0047] As the pulverizer, a product name: ZM200 manufactured by Retsch; a product name: P-14 Rotor Speed ​​Mill (Pulverisette 14) manufactured by Fritsch Japan Co., Ltd., or the like can be used. In the pulverization step, a pulverizer in which the particles that pass through the screen are not further adjusted in particle size can be used. In the pulverization step, a pulverizer other than a roll mill (e.g., a multistage roll mill) can be used. In the pulverization step, a single pulverizer or multiple types of pulverizers can be used.

[0048] The surface temperature T2 of the crosslinked polymer at the start of the pulverization step is preferably within the following ranges. From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the surface temperature T2 is preferably 200°C or less, 180°C or less, 160°C or less, 150°C or less, 140°C or less, or 130°C or less. From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the surface temperature T2 is preferably 50°C or more, 55°C or more, 60°C or more, 80°C or more, more than 80°C, 90°C or more, more than 90°C, 100°C or more, more than 100°C, 110°C or more, 120°C or more, or 130°C or more. From these viewpoints, the surface temperature T2 is preferably 50 to 200°C, 100 to 160°C, or more than 100°C but not more than 160°C. The surface temperature T2 may be 120°C or less, 110°C or less, 100°C or less, 90°C or less, 80°C or less, or 60°C or less. The "start of the grinding process" refers to the time when the crosslinked polymer is supplied to the grinder, and may be the time when the crosslinked polymer starts to be supplied to the sample supply port of the grinder, etc. The surface temperature T2 can be measured using a radiation thermometer.

[0049] The temperature difference ΔT (T1-T2) between the surface temperature T1 of the crosslinked polymer at the end of the drying step and the surface temperature T2 of the crosslinked polymer at the start of the pulverization step is greater than 0°C and not greater than 100°C, from the viewpoint of obtaining crosslinked polymer particles with a small amount of fine powder. The surface temperature T1 is greater than the surface temperature T2 (T1>T2), and the temperature difference ΔT is a positive number.

[0050] From the viewpoint of easily obtaining crosslinked polymer particles with a small amount of fine powder, the temperature difference ΔT is preferably less than 100° C., 95° C. or less, 90° C. or less, less than 90° C., 85° C. or less, 80° C. or less, less than 80° C., 75° C. or less, 70° C. or less, less than 70° C., 65° C. or less, 60° C. or less, less than 60° C., 55° C. or less, less than 55° C., 50° C. or less, less than 50° C., 45° C. or less, 40° C. or less, less than 40° C., 35° C. or less, or 30° C. or less. For example, the temperature difference ΔT is preferably greater than 0° C. and 90° C. or less, or greater than 0° C. and 85° C. or less. The temperature difference ΔT may be 1°C or more, 5°C or more, 10°C or more, 15°C or more, 20°C or more, 25°C or more, 30°C or more, 35°C or more, 40°C or more, 45°C or more, 50°C or more, 55°C or more, 60°C or more, 65°C or more, 70°C or more, 75°C or more, 80°C or more, 85°C or more, 90°C or more, or 95°C or more.

[0051] From the viewpoint of preventing a decrease in the elasticity of the crosslinked polymer, the time between the end of the drying step and the start of the pulverization step is preferably 10 minutes or less, 5 minutes or less, 3 minutes or less, less than 3 minutes, 2.5 minutes or less, 2 minutes or less, 1.5 minutes or less, 1 minute or less, 0.5 minutes or less, or 0.33 (1 / 3) minutes or less. The time between the end of the drying step and the start of the pulverization step may be more than 0 minutes, 0.1 minutes or more, 0.2 minutes or more, 0.3 minutes or more, 0.33 (1 / 3) minutes or more, 0.5 minutes or more, 1 minute or more, 1.5 minutes or more, 2 minutes or more, or 2.5 minutes or more.

[0052] The ambient temperature (the temperature of the atmosphere to which the crosslinked polymer is exposed) between the end of the drying step and the start of the pulverization step may be within the following ranges. The ambient temperature may be 150°C or less, 120°C or less, 100°C or less, 80°C or less, 50°C or less, less than 50°C, 40°C or less, 30°C or less, or 25°C or less. The ambient temperature may be 10°C or more, 15°C or more, 20°C or more, or 25°C or more. From these viewpoints, the ambient temperature may be 10 to 150°C. Between the end of the drying step and the start of the pulverization step, the crosslinked polymer may be left without being treated, or may be treated.

[0053] The method for producing crosslinked polymer particles according to this embodiment may include a crushing step between the polymerization step and the drying step. The crushing step is, for example, a step of crushing the crosslinked polymer obtained in the polymerization step to obtain a crushed product. As a crushing machine for the crushing step, for example, a kneader (a pressure kneader, a double-arm kneader, etc.), a meat chopper, a cutter mill, a farmer mill, etc. can be used. In the crushing step, the hydrogel of the crosslinked polymer may be crushed. In the crushing step, the crosslinked polymer (for example, the hydrogel of the crosslinked polymer) may be crushed using multiple devices. For example, the hydrogel of the crosslinked polymer may be cut and then finely divided using a meat chopper. The water content of the crosslinked polymer (for example, the hydrogel of the crosslinked polymer) crushed in the crushing step is, for example, more than 30% by mass.

[0054] According to the method for producing crosslinked polymer particles of this embodiment, by adjusting the temperature difference between the surface temperature of the crosslinked polymer at the end of the drying step and the surface temperature of the crosslinked polymer at the start of the pulverization step, crosslinked polymer particles having the following particle size distribution and / or median particle diameter can be obtained.

[0055] The proportion of particles having a particle diameter of less than 180 μm (more than 0 μm and less than 180 μm) in the crosslinked polymer particles according to this embodiment may be in the following ranges based on the total mass of the crosslinked polymer particles. The proportion of particles having a particle diameter of less than 180 μm may be 22% by mass or less, 21% by mass or less, 20% by mass or less, 19% by mass or less, 18% by mass or less, 17% by mass or less, or 16% by mass or less. The proportion of particles having a particle diameter of less than 180 μm may be 0% by mass or more, more than 0% by mass, 1% by mass or more, 3% by mass or more, 5% by mass or more, 8% by mass or more, 10% by mass or more, 12% by mass or more, or 15% by mass or more. From these perspectives, the proportion of particles having a particle diameter of less than 180 μm may be 0 to 22% by mass.

[0056] The proportion of particles having a particle diameter of less than 250 μm (more than 0 μm and less than 250 μm) in the crosslinked polymer particles according to this embodiment may be in the following ranges based on the total mass of the crosslinked polymer particles. The proportion of particles having a particle diameter of less than 250 μm may be 34% by mass or less, 33% by mass or less, 32% by mass or less, 31% by mass or less, 30% by mass or less, 29% by mass or less, 28% by mass or less, 27% by mass or less, or 26% by mass or less. The proportion of particles having a particle diameter of less than 250 μm may be 0% by mass or more, more than 0% by mass, 1% by mass or more, 3% by mass or more, 5% by mass or more, 8% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more. From these perspectives, the proportion of particles having a particle diameter of less than 250 μm may be 0 to 34% by mass.

[0057] The proportion of particles having a particle diameter of 250 μm or more and less than 850 μm in the crosslinked polymer particles according to this embodiment may be in the following ranges based on the total mass of the crosslinked polymer particles. The proportion of particles having a particle diameter of 250 μm or more and less than 850 μm may be 90% by mass or less, 85% by mass or less, 80% by mass or less, 75% by mass or less, 71% by mass or less, 70% by mass or less, 68% by mass or less, 67.5% by mass or less, 67% by mass or less, or 66% by mass or less. The proportion of particles having a particle diameter of 250 μm or more and less than 850 μm may be 65% by mass or more, 66% by mass or more, 66.4% by mass or more, 67% by mass or more, 67.5% by mass or more, 68% by mass or more, 69% by mass or more, or 70% by mass or more. From these perspectives, the proportion of particles having a particle diameter of 250 μm or more and less than 850 μm may be 65 to 90% by mass.

[0058] The proportion of particles having a particle diameter of 850 μm or more in the crosslinked polymer particles according to this embodiment may be in the following ranges based on the total mass of the crosslinked polymer particles. The proportion of particles having a particle diameter of 850 μm or more may be 10% by mass or less, less than 10% by mass, 8% by mass or less, 5% by mass or less, 4% by mass or less, or 3.6% by mass or less. The proportion of particles having a particle diameter of 850 μm or more may be 1% by mass or more, 1.2% by mass or more, 1.5% by mass or more, 1.8% by mass or more, 2% by mass or more, 2.5% by mass or more, 3% by mass or more, or 3.5% by mass or more. From these perspectives, the proportion of particles having a particle diameter of 850 μm or more may be 1 to 10% by mass.

[0059] In the crosslinked polymer particles (crosslinked polymer particle group) according to this embodiment, the mass ratio (fine particle abundance ratio) of particles having a particle diameter of less than 250 μm to particles having a particle diameter of 250 μm or more and less than 850 μm is preferably 50 mass% or less, 48 ​​mass% or less, 46.5 mass% or less, 45 mass% or less, 40 mass% or less, or 38 mass% or less. This mass ratio can be measured by the method described in the Examples below.

[0060] The median particle diameter of the crosslinked polymer particles according to this embodiment is preferably in the following range. The median particle diameter may be 200 μm or more, 230 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, 330 μm or more, 350 μm or more, 360 μm or more, 370 μm or more, 380 μm or more, 390 μm or more, or 400 μm or more. The median particle diameter may be 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, 400 μm or less, 390 μm or less, 380 μm or less, 370 μm or less, or 360 μm or less. From these viewpoints, the median particle diameter may be 200 to 600 μm. The median particle diameter can be measured by the method described in the Examples below.

[0061] The crosslinked polymer particles according to this embodiment may have structural units derived from an ethylenically unsaturated monomer (e.g., a (meth)acrylic acid compound). The crosslinked polymer particles according to this embodiment may further contain other components such as a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts (e.g., pentasodium diethylenetriaminepentaacetate)), a flow improver (lubricant), etc. The other components may be located inside the crosslinked polymer, on the surface, or both.

[0062] The crosslinked polymer particles according to this embodiment may contain inorganic particles disposed on the surface of the crosslinked polymer. For example, by mixing the crosslinked polymer with the inorganic particles, the inorganic particles can be disposed on the surface of the crosslinked polymer. Examples of the inorganic particles include silica particles such as amorphous silica.

[0063] The water-absorbent resin particles according to this embodiment can be obtained by crosslinking the crosslinked polymer particles obtained in the pulverization step (crosslinking step). The crosslinking may be surface crosslinking of the crosslinked polymer particles. The crosslinking can be carried out, for example, by reacting a crosslinking agent (e.g., a surface crosslinking agent) with the crosslinked polymer particles. By carrying out crosslinking using a crosslinking agent, the crosslink density of the crosslinked polymer particles (e.g., the crosslink density near the surface of the crosslinked polymer particles) increases, which makes it easier to improve water absorption performance.

[0064] Examples of crosslinking agents include compounds containing two or more functional groups (reactive functional groups) that are reactive with functional groups derived from ethylenically unsaturated monomers. Examples of crosslinking agents include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanated compounds having two or more reactive functional groups, such as compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.); oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide.

[0065] The water-absorbent resin particles according to this embodiment may contain inorganic particles of a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts (e.g., pentasodium diethylenetriaminepentaacetate), etc.), a flowability improver (lubricant), etc., on their surfaces. For example, by mixing the cross-linked particles with inorganic particles, the inorganic particles can be arranged on the surfaces of the cross-linked particles. Examples of inorganic particles include silica particles such as amorphous silica.

[0066] The water-absorbent resin particles according to this embodiment can retain water and absorb body fluids such as urine, sweat, and blood (e.g., menstrual blood). The water-absorbent resin particles according to this embodiment can be used as a constituent component of an absorbent. This embodiment can be used in fields such as hygiene materials such as disposable diapers and sanitary products; agricultural and horticultural materials such as water retention agents and soil conditioners; and industrial materials such as waterproofing agents and anti-condensation agents.

[0067] The absorbent body according to the present embodiment contains the water-absorbent resin particles according to the present embodiment. The absorbent body according to the present embodiment may contain fibrous material, for example, a mixture containing water-absorbent resin particles and fibrous material. The absorbent body may have a configuration in which the water-absorbent resin particles and fibrous material are uniformly mixed, a configuration in which the water-absorbent resin particles are sandwiched between fibrous material formed in a sheet or layer shape, or other configurations.

[0068] Examples of fibrous materials include finely ground wood pulp, cotton, cotton linters, rayon, cellulosic fibers such as cellulose acetate, synthetic fibers such as polyamide, polyester, polyolefin, and mixtures of these fibers. Hydrophilic fibers can be used as the fibrous material.

[0069] To improve the shape retention of the absorbent body before and during use, the fibers may be bonded together by adding an adhesive binder to the fibrous material. Examples of adhesive binders include heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions.

[0070] Examples of heat-fusible synthetic fibers include full-melt binders such as polyethylene, polypropylene, and ethylene-propylene copolymers; and non-full-melt binders having a side-by-side or core-sheath structure of polypropylene and polyethylene. In the above-mentioned non-full-melt binders, only the polyethylene portion can be heat-fused.

[0071] Examples of hot melt adhesives include mixtures of base polymers such as ethylene-vinyl acetate copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, styrene-ethylene-propylene-styrene block copolymer, and amorphous polypropylene with tackifiers, plasticizers, antioxidants, and the like.

[0072] The adhesive emulsion may be, for example, 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.

[0073] The absorbent body according to the present embodiment may contain inorganic particles (e.g., amorphous silica), a deodorant, an antibacterial agent, a pigment, a dye, a fragrance, an adhesive, etc. When the water-absorbent resin particles contain inorganic particles, the absorbent body may contain inorganic particles in addition to the inorganic particles in the water-absorbent resin particles.

[0074] The absorbent body according to this embodiment may be, for example, in the form of a sheet, and the thickness of the absorbent body (for example, the thickness of a sheet-shaped absorbent body) may be 0.1 to 20 mm or 0.3 to 15 mm.

[0075] The content of the water-absorbent resin particles in the absorbent body may be 2 to 95 mass %, 10 to 80 mass %, or 20 to 60 mass % of the total of the water-absorbent resin particles and the fibrous material, from the viewpoint of easily obtaining sufficient absorption properties.

[0076] The content of water-absorbent resin particles in the absorbent body is set at 1 / m² from the viewpoint of obtaining sufficient absorption characteristics. 2 The content of the fibrous material in the absorbent body is preferably 100 to 1000 g, 150 to 800 g, or 200 to 700 g per m of the absorbent body from the viewpoint of easily obtaining sufficient absorption properties. 2 Preferably, the serving size is 50 to 800 g, 100 to 600 g, or 150 to 500 g.

[0077] The absorbent article according to the present embodiment includes the absorbent body according to the present embodiment. Other components of the absorbent article according to the present embodiment include a core wrap that maintains the shape of the absorbent body and prevents the components of the absorbent body from falling off or flowing; a liquid-permeable sheet that is disposed on the outermost side on the side where the liquid to be absorbed penetrates; and a liquid-impermeable sheet that is disposed on the outermost side on the side opposite the side where the liquid to be absorbed penetrates. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, parts for portable toilets, and animal waste disposal materials.

[0078] The absorbent body may be adhered to the top sheet. When the absorbent body is sandwiched or covered by a core wrap, it is preferable that at least the core wrap and the top sheet are adhered, and it is more preferable that the core wrap and the top sheet are adhered together and the core wrap and the absorbent body are adhered. Methods for adhering the absorbent body include a method in which a hot melt adhesive is applied to the top sheet in a striped or spiral pattern at predetermined intervals in the width direction, and a method in which a water-soluble binder such as starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, or other water-soluble polymer is used for adhesion. Furthermore, when the absorbent body contains heat-fusible synthetic fibers, a method of adhesion by heat-fusible bonding of the heat-fusible synthetic fibers may be used.

[0079] According to the present embodiment, it is possible to provide a method for absorbing liquid using the water-absorbent resin particles, absorbent body, or absorbent article according to the present embodiment. The method for absorbing liquid according to the present embodiment includes a step of bringing the water-absorbent resin particles, absorbent body, or absorbent article according to the present embodiment into contact with a liquid to be absorbed. According to the present embodiment, it is possible to provide applications of the water-absorbent resin particles, absorbent body, and absorbent article to liquid absorption.

[0080] According to the present embodiment, it is possible to provide a method for manufacturing an absorbent body using the above-mentioned water-absorbent resin particles. The method for manufacturing an absorbent body according to the present embodiment includes a water-absorbent resin particle manufacturing step for obtaining the above-mentioned water-absorbent resin particles. The method for manufacturing an absorbent body according to the present embodiment may include a step of mixing the water-absorbent resin particles and fibrous material after the water-absorbent resin particle manufacturing step. According to the present embodiment, it is possible to provide a method for manufacturing an absorbent article using the absorbent body obtained by the above-mentioned method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to the present embodiment includes an absorbent body manufacturing step for obtaining the absorbent body by the above-mentioned method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to the present embodiment may include a step of obtaining the absorbent article using the absorbent body and other constituent members of the absorbent article after the absorbent body manufacturing step, and in this step, the absorbent article is obtained, for example, by stacking the absorbent body and other constituent members of the absorbent article on each other. [Example]

[0081] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples. When the temperature during an experimental procedure is not specified below, the experimental procedure can be performed at room temperature.

[0082] <Preparation of Monomer Aqueous Solution> A 2-liter separable flask was charged with 340.0 g (4.72 mol) of acrylic acid. 291.7 g of ion-exchanged water was added to the acrylic acid in the separable flask with stirring. Next, 297.8 g of a 48% by mass aqueous solution of sodium hydroxide was added dropwise in an ice-water bath at approximately 3°C to prepare a partially neutralized solution of acrylic acid with a monomer concentration of 45% by mass.

[0083] <Preparation of cross-linked polymer> 888.30 g of the partially neutralized solution of acrylic acid, 144.39 g of ion-exchanged water, 0.412 g of polyethylene glycol diacrylate (n≒9) (internal crosslinking agent, NOF Corporation, Blenmer ADE-400A), and 16.15 g of a 2% by weight potassium persulfate aqueous solution were placed in a fluororesin-coated stainless steel tray (external dimensions: 297 mm x 232 mm x 50 mm height). A homogeneous mixture was formed inside the tray by stirring with two stir bars (8 mm diameter, 45 mm length, no rings). The top of the stainless steel tray was then covered with polyethylene film. The temperature of the mixture in the stainless steel tray was adjusted to 25°C, and the mixture was purged with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less. Next, while the mixture was stirred at 300 rpm, 3.39 g of a 0.5% by mass aqueous solution of L-ascorbic acid was added dropwise using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, syringe needle manufactured by Terumo Corporation).

[0084] The polymerization reaction started 2 minutes after the L-ascorbic acid aqueous solution was added dropwise. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled. 12 minutes after the end of the L-ascorbic acid aqueous solution addition, the installed thermometer indicated 75.8°C, after which the temperature began to drop. The stainless steel tray containing the hydrogel polymer (a hydrogel polymer containing water and a polymer) formed by the gelation of the reaction solution was immersed in a 75°C water bath, and the hydrogel polymer was allowed to age in that state for 20 minutes.

[0085] The entire amount of the aged hydrogel polymer was removed from the container and cut into pieces by making cuts at 5 cm intervals along the long side. The cut hydrogel was sequentially placed into a meat chopper (Kire Royal Co., Ltd., model number: 12VR-750SDX) and roughly crushed (fragmented) at room temperature. The diameter of the holes in the plate located at the outlet of the meat chopper was 6.4 mm. This fragmented particulate hydrogel was spread and placed on a wire mesh with openings of 0.8 cm x 0.8 cm, and then hot-air dried at 160°C for 30 minutes using a hot-air dryer (ADVANTEC, FV-320) to obtain a dried product. The moisture content of the dried product was 5% by mass.

[0086] The dried product was then cooled to room temperature and classified using JIS standard sieves with 9.5 mm and 2.8 mm openings. The dried product (particle size: 9.5 mm or more) remaining on the 9.5 mm JIS standard sieve was obtained. This dried product was placed in a polyethylene bag, and a 4.0 kg stainless steel roller (diameter: 10.5 cm, width: 6.0 cm) was run back and forth over the bag once to crush the dried product at room temperature, and then classified again using a sieve with the same openings. These operations were repeated until no dried product remained on the 9.5 mm JIS standard sieve. The product that passed through the 9.5 mm JIS standard sieve and remained on the 2.8 mm JIS standard sieve was obtained as crosslinked polymer (A).

[0087] <Crushing process> Example 1 40 g of crosslinked polymer (A) was uniformly placed in a fluororesin-coated stainless steel tray (external dimensions: 208 mm x 170 mm x 30 mm height) and then hot-air dried (heated) for 30 minutes at 160°C using a hot-air dryer (FV-320, manufactured by Advantec). At the end of drying, the surface temperature of the crosslinked polymer (A) (dried product) was 160°C, the moisture content was 3.9% by mass, and the particle size was 2.8 mm or more and less than 9.5 mm. After leaving the crosslinked polymer (A) at room temperature for 20 seconds, the surface temperature of the crosslinked polymer (A) (the surface temperature of the crosslinked polymer (A) when it was fed into the centrifugal grinder) was measured and found to be 130°C. The surface temperature was measured using a radiation thermometer (AD5611A, manufactured by A&D Co., Ltd.). Immediately thereafter, the crosslinked polymer (A) was fed to a centrifugal grinder (Retsch, ZM200, screen diameter: 1 mm, 6000 rpm) and ground to obtain irregularly shaped crosslinked polymer particles.

[0088] Example 2 Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the drying temperature of the hot air dryer was changed to 180°C and the standing time at room temperature was changed to 30 seconds. The surface temperature of the crosslinked polymer (A) at the end of drying was 180°C, and the surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 150°C.

[0089] Example 3 Crosslinked polymer particles were obtained by the same procedure as in Example 1, except that the time left at room temperature was changed to 30 seconds. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 110°C.

[0090] Example 4 Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the drying temperature of the hot air dryer was changed to 180°C and the standing time at room temperature was changed to 1 minute. The surface temperature of the crosslinked polymer (A) at the end of drying was 180°C, and the surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 110°C.

[0091] Example 5 Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the time for leaving at room temperature was changed to 1 minute. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 90°C.

[0092] Example 6 Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the standing time at room temperature was changed to 2 minutes and 30 seconds. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 60°C.

[0093] (Comparative Example 1) Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the standing time at room temperature was changed to 3 minutes. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 50°C.

[0094] (Comparative Example 2) Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the time for leaving at room temperature was changed to 4 minutes. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 40°C.

[0095] (Comparative Example 3) Crosslinked polymer particles were obtained in the same manner as in Example 1, except that the time left at room temperature was changed to 15 minutes. The surface temperature of the crosslinked polymer (A) when fed to the centrifugal grinder was 24°C.

[0096] <Moisture content> The moisture content was measured using the following method. 5.00 g of the object to be measured was placed in a fluororesin-coated stainless steel tray (constant weight: W1 [g], external dimensions: 208 mm × 170 mm × 30 mm height), and the total mass of the stainless steel tray and the object to be measured, W2 [g], was precisely weighed. Next, the object to be measured in the stainless steel tray was dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) with the internal temperature set to 200°C. After the object to be measured (powder) was allowed to cool in a desiccator, the total mass of the stainless steel tray and the object to be measured, W3 [g], was obtained as the dry mass. The moisture content [mass%] of the object to be measured was then calculated using the following formula: Moisture content = [{(W2-W1)-(W3-W1)} / (W2-W1)]×100

[0097] <Particle size distribution and fine particle abundance ratio> Using a sieve shaker (manufactured by Iida Seisakusho Co., Ltd., Patent No. 531413), the particle size distribution of 30 g of crushed crosslinked polymer particles was measured using JIS standard sieves with mesh sizes of 850 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 180 μm, and a receiving pan. In measuring the particle size distribution, the mass proportions (mass percentages) of particle size ranges of "more than 0 μm but less than 180 μm," "180 μm or more but less than 250 μm," "250 μm or more but less than 300 μm," "300 μm or more but less than 425 μm," "425 μm or more but less than 500 μm," "500 μm or more but less than 850 μm," and "850 μm or more" were calculated. Based on the calculation results, the mass proportion of the particle size range "less than 250 μm" was calculated as the sum of the mass proportions of "more than 0 μm and less than 180 μm" and "180 μm or more and less than 250 μm." Furthermore, the mass proportion of the particle size range "250 μm or more and less than 850 μm" was calculated as the sum of the mass proportions of the particle size ranges "250 μm or more and less than 300 μm," "300 μm or more and less than 425 μm," "425 μm or more and less than 500 μm," and "500 μm or more and less than 850 μm." The mass proportion of particles less than 250 μm in diameter to particles with a particle size of "250 μm or more and less than 850 μm" was then calculated (fine particle abundance ratio [mass %] = [mass proportion of particles less than 250 μm in diameter] / [mass proportion of particles with a particle size of 250 μm or more and less than 850 μm] × 100). The results are shown in Table 1. When the ratio of fine particles is 50 mass % or less, it can be determined that the amount of fine powder is small.

[0098] <Median particle size> Regarding the particle size distribution described above, the mass of particles remaining on the sieve was integrated in descending order of particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50% by mass was obtained as the median particle size. The results are shown in Table 1.

[0099] [Table 1]

Claims

1. polymerizing the monomers to obtain a crosslinked polymer; a drying step of drying the crosslinked polymer; a pulverization step of pulverizing the crosslinked polymer after the drying step, the surface temperature of the crosslinked polymer at the end of the drying step is 90°C or higher, A method for producing crosslinked polymer particles, wherein the temperature difference between the surface temperature of the crosslinked polymer at the end of the drying step and the surface temperature of the crosslinked polymer at the start of the pulverization step is greater than 0°C and not greater than 100°C.

2. A process for obtaining a crosslinked polymer by polymerizing a monomer; a drying step of drying the crosslinked polymer; a pulverization step of pulverizing the crosslinked polymer after the drying step, the surface temperature of the crosslinked polymer at the start of the pulverization step is 90°C or higher; A method for producing crosslinked polymer particles, wherein the temperature difference between the surface temperature of the crosslinked polymer at the end of the drying step and the surface temperature of the crosslinked polymer at the start of the pulverization step is greater than 0°C and not greater than 100°C.

3. The method for producing crosslinked polymer particles according to claim 1 or 2, wherein the temperature difference is greater than 0°C and not more than 90°C.

4. The method for producing crosslinked polymer particles according to any one of claims 1 to 3, wherein the temperature difference is greater than 0°C and not more than 85°C.

5. The method for producing crosslinked polymer particles according to any one of claims 1 to 4, wherein the surface temperature of the crosslinked polymer is 150 to 200°C at the end of the drying step.

6. The method for producing crosslinked polymer particles according to any one of claims 1 to 5, wherein the surface temperature of the crosslinked polymer at the start of the pulverization step is greater than 100°C and not greater than 160°C.

7. The method for producing crosslinked polymer particles according to any one of claims 1 to 6, wherein the time between the end of the drying step and the start of the pulverizing step is less than 3 minutes.

8. The method for producing crosslinked polymer particles according to any one of claims 1 to 7, wherein the monomer comprises an ethylenically unsaturated monomer.

9. The method for producing crosslinked polymer particles according to claim 8, wherein the ethylenically unsaturated monomer comprises at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof.

10. A method for producing water-absorbent resin particles, comprising a step of subjecting the crosslinked polymer particles obtained by the method for producing crosslinked polymer particles according to any one of claims 1 to 9 to additional crosslinking.

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