Method for producing water-absorbent resin particles

Abstract

Disclosed is a method for producing water-absorbing resin particles that contain a cross-linked polymer, said method including a drying step for drying a cross-linked polymer, wherein the dry basis moisture content of the cross-linked polymer before the drying step is at least 100 mass%. The drying step is carried out such that the initial change rate of the dry basis moisture content, represented by following expression, is 6.0% / min or less.　The initial change rate velocity of the dry basis moisture content (% / min) = [{(the dry basis moisture content before the drying step － the dry basis moisture content 15 min after the drying step) / the dry basis moisture content before the drying step} × 100] / 15.

Description

Technical Field

[0001] The present invention relates to a method for producing water-absorbent resin particles.

Background Art

[0002] As a method for synthesizing a crosslinked polymer constituting water-absorbent resin particles, there is a polymerization method such as an aqueous solution polymerization method in which water is used during polymerization to obtain a massive water-containing gel (for example, Patent Document 1). Since the massive water-containing gel contains a large amount of water used during polymerization, it needs to be dried.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For example, when drying a water-containing gel using a drying device, the water-containing gel may adhere to the metal surface of the drying device and be difficult to peel off even after drying. In order to improve the peelability of the metal surface of the drying device, a metal processed with a resin such as a fluororesin may be used. However, a metal with a resin coating such as a fluororesin has problems such as a lower heat transfer rate, lower heat resistance, and easier deterioration compared to the original metal. Therefore, it is desirable that the crosslinked polymer after drying has high peelability even on a general metal surface without processing for improving peelability such as a resin coating.

[0005] An object of the present invention is to provide a method for producing water-absorbent resin particles excellent in peelability of a crosslinked polymer after drying from a metal surface.

Means for Solving the Problems

[0006] The present inventors have found that controlling the degree of drying, particularly in the initial stage of the drying process (from the start of the drying process for 15 minutes), is important for improving the peelability of the crosslinked polymer after the drying process when drying the crosslinked polymer.

[0007] The present invention provides a method for producing water-absorbent resin particles containing a crosslinked polymer, including a drying process for drying the crosslinked polymer, wherein the moisture content based on dry weight before the drying process of the crosslinked polymer is 100% by mass or more, and the drying process is carried out such that the initial change rate of the moisture content based on dry weight represented by the following formula is 6.0% / min or less. Initial change rate of moisture content based on dry weight (% / min) = [{(Moisture content based on dry weight before the drying process - Moisture content based on dry weight 15 minutes after the drying process) / Moisture content based on dry weight before the drying process}×100] / 15

[0008] Preferably, the drying process is carried out such that the initial change rate of the moisture content based on dry weight is 5.6% / min or less.

[0009] Preferably, the drying process is carried out such that the initial change rate of the moisture content based on dry weight is 3.0% / min or more.

[0010] Preferably, the method further includes crushing the crosslinked polymer before the drying process.

[0011] After the drying process, the method may further include pulverizing the crosslinked polymer.

Advantages of the Invention

[0012] The present invention can provide a method for producing water-absorbent resin particles excellent in peelability from a metal surface of the crosslinked polymer after drying.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments and can be variously modified and implemented within the scope of the gist.

[0014] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic". Similarly, "acrylate" and "methacrylate" are 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 step by step in this specification, the upper limit value or the lower limit value of a numerical range at a certain step can be arbitrarily combined with the upper limit value or the lower limit value of a numerical range at other steps. In the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples. "Water-soluble" means showing 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. The content of each component in the composition means the total amount of the plurality of substances corresponding to each component in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified. Regarding at least one (meth)acrylic acid compound selected from the group consisting of (meth)acrylic acid and its salts, the "content of (meth)acrylic acid compound" means the total amount of acrylic acid, acrylate, methacrylic acid and methacrylate. "Room temperature" means 25°C ± 2°C.

[0015] The method for producing water-absorbent resin particles containing a crosslinked polymer according to this embodiment includes a drying step of drying the crosslinked polymer.

[0016] [Polymerization] In the production method according to this embodiment, the crosslinked polymer can be obtained, for example, by polymerizing a monomer. The polymerization of the monomer can be carried out, for example, using an aqueous monomer solution containing the monomer.

[0017] The production method according to this embodiment is suitable for a polymerization method in which the crosslinked polymer obtained by polymerization is in the form of a hydrogel and needs to be dried, for example, the aqueous solution polymerization method. Hereinafter, the case of using the aqueous solution polymerization method will be described.

[0018] The monomer may contain ethylenically unsaturated monomers and may contain water-soluble ethylenically unsaturated monomers. Examples of the ethylenically unsaturated monomers include carboxylic acid-based monomers such as unsaturated carboxylic acids ((meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid, etc.) and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, etc.; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylamide, etc., and quaternized products thereof; sulfonic acid-based monomers such as vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methylpropane sulfonic acid, 2-(meth)acryloylethane sulfonic 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 salts thereof. The ethylenically unsaturated monomer may contain both (meth)acrylic acid and a salt of (meth)acrylic acid. Examples of salts of unsaturated carboxylic acids ((meth)acrylic acid, etc.) include alkali metal salts (sodium salt, potassium salt, etc.), ammonium salts, and the like.

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

[0020] The degree of neutralization of the ethylenically unsaturated monomer by 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" is defined as the degree of neutralization with respect to all acid groups of the ethylenically unsaturated monomer.

[0021] The content of the monomer (for example, (meth)acrylic acid compound) may be 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 based on the total mass of the aqueous monomer solution. The content of the monomer may be 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, less than 45% by mass, or 40% by mass or less. From the viewpoint of easily adjusting the moisture content on a dry basis before the drying step described later to an appropriate range, the monomer content in the aqueous monomer solution is preferably 40% by mass or less.

[0022] In the polymerization reaction, from the viewpoints of suppressing side reactions such as self-crosslinking, adjusting the water-containing gel to a hardness that is easy to crush in the subsequent crushing step, and suppressing the deterioration of the solid content in the water-containing gel in the subsequent drying step, the water content of the aqueous monomer solution is preferably 60% by mass or more. The water content of the aqueous monomer solution can be determined by the following formula. Water content (%) of the aqueous monomer solution = 100 - Solid content ratio (%) of the aqueous monomer solution The solid content ratio of the aqueous monomer solution is the ratio of the compound that is converted into the solid content constituting the crosslinked polymer to the total amount of the aqueous monomer solution. Since the amounts of components other than the monomer contained in the aqueous monomer solution (for example, crosslinking agent, polymerization initiator) are minute compared to the amount of the monomer, the monomer content in the aqueous monomer solution may be regarded as the substantial solid content. The water content of the aqueous monomer solution may be, for example, 70% by mass or less, or 65% by mass or less.

[0023] (Meth)acrylic acid compound content may be 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 based on the total amount of monomers contained in the aqueous monomer solution and / or the total amount of ethylenically unsaturated monomers contained in the aqueous monomer solution. The monomers contained in the aqueous monomer solution may be in a form consisting essentially of (meth)acrylic acid compounds, i.e., a form where 100 mol% of the monomers contained in the aqueous monomer solution are (meth)acrylic acid compounds.

[0024] The aqueous monomer solution may contain a polymerization initiator. Polymerization of the monomers contained in the aqueous monomer solution may be initiated by adding a polymerization initiator to the aqueous monomer solution and, if necessary, performing heating, light irradiation, etc. Examples of the polymerization initiator include photoinitiators, radical polymerization initiators, etc., and water-soluble radical polymerization initiators are preferred. From the viewpoint of easily enhancing the water absorption performance, the polymerization initiator preferably contains at least one selected from the group consisting of azo compounds and peroxides.

[0025] As azo compounds, 2,2'-azobis[2-(N-phenylamidinopropane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidinopropane]} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidinopropane]} dihydrochloride, 2,2'-azobis[2-(N-benzylamidinopropane] dihydrochloride, 2,2'-azobis[2-(N-allylamidinopropane] dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidinopropane]} 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], etc. can be mentioned. From the viewpoint that good water absorption performance is easily obtained, 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.

[0026] 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-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate. From the viewpoints of easily obtaining good water absorption performance and easily reducing unreacted monomers contained in the water-absorbing resin particles, it is preferable to contain at least one selected from the group consisting of potassium persulfate, ammonium persulfate, and sodium persulfate.

[0027] From the viewpoints of easily enhancing water absorption performance and easily reducing the amount of unreacted monomers contained in the water-absorbing 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 per 1 mol of the ethylenically unsaturated monomer (for example, (meth)acrylic acid compound). From the viewpoints of easily enhancing 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.

[0028] The aqueous monomer solution may contain a reducing agent. Examples of the reducing agent include sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid. The polymerization initiator and the reducing agent may be used in combination.

[0029] The aqueous monomer solution may contain an oxidizing agent. Examples of the oxidizing agent include hydrogen peroxide, sodium perborate, phosphoric acid and its salts, and potassium permanganate.

[0030] The monomer aqueous solution may contain an internal crosslinking agent. By using the internal crosslinking agent, the resulting crosslinked polymer can have, as its internal crosslinking structure, in addition to the self-crosslinking structure by the polymerization reaction, a crosslinking structure by the internal crosslinking agent.

[0031] 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)acrylate 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 (such as maleic acid and fumaric acid); 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; di- or tri-(meth)acrylate esters obtained by reacting a polyepoxide with (meth)acrylic acid; di(meth)acrylate carbamyl esters obtained by reacting a polyisocyanate (such as tolylene diisocyanate and hexamethylene diisocyanate) with hydroxyethyl (meth)acrylate; allylated starch; allylated cellulose; diallyl phthalate; N,N',N''-triallyl isocyanurate; divinylbenzene; pentaerythritol; ethylenediamine; polyethyleneimine, and the like. From the viewpoints of easily enhancing the water absorption performance and excellent reactivity, 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, (poly)glycerin diglycidyl ether, polyethylene glycol diacrylate, (poly)propylene glycol diacrylate, trimethylolpropane triacrylate, glycerol triacrylate, and trimethylolpropane diacrylate.

[0032] 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 per 1 mol of the ethylenically unsaturated monomer (for example, (meth)acrylic acid compound). 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.

[0033] The monomer aqueous solution may contain additives such as a chain transfer agent, a thickener, and an inorganic filler as components different from the above-mentioned components, if necessary. Examples of the chain transfer agent include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid, acrolein, and the like. Examples of the thickener include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, polyacrylamide, and the like. Examples of the inorganic filler include metal oxides, ceramics, viscous minerals, and the like.

[0034] Examples of the polymerization method for aqueous solution polymerization include a static polymerization method in which the monomer aqueous solution is polymerized in a state where it is not stirred (for example, in a static state); a stirring polymerization method in which the monomer aqueous solution is polymerized while being stirred in a reaction apparatus, and the like. In the static polymerization method, at the completion of polymerization, a single block-shaped gel that occupies substantially the same volume as the monomer aqueous solution present in the reaction vessel can be obtained.

[0035] The form of polymerization may be batchwise, semi-continuous, continuous, etc. For example, when the stationary polymerization method is carried out as continuous polymerization, the polymerization reaction is carried out while continuously supplying an aqueous monomer solution to a continuous polymerization apparatus, and a gel can be obtained continuously.

[0036] The polymerization temperature varies depending on the polymerization initiator used, but 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, it is preferably 0 to 130° C. or 10 to 110° C. 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.

[0037] After the hydrogel (hydrogel-like crosslinked polymer) is obtained by polymerization, it may be immediately subjected to the drying step described below, or may be subjected to the drying step after a certain period of time (e.g., 10 minutes to 1 hour). The hydrogel may be placed in an environment of 25°C or higher and lower than the drying temperature (e.g., 75°C or lower) until it is subjected to the drying step.

[0038] [Rough crushing] The crosslinked polymer is preferably crushed in advance before the drying step. That is, the production method according to the present embodiment preferably includes a step of crushing the crosslinked polymer before the drying step of the crosslinked polymer. By crushing the crosslinked polymer before the drying step, drying can be performed more efficiently. In the crushing step, for example, the lump-shaped hydrogel obtained by polymerization can be crushed.

[0039] For the coarse crushing, for example, a coarse crusher such as a kneader (pressure kneader, double-arm kneader, etc.), a meat chopper, a cutter mill, a farmer mill, etc. can be used. In the coarse crushing step, the lump-shaped hydrogel may be cut into pieces of, for example, about 5 cm square in advance, and the cut hydrogel may be subjected to the coarse crushing. When the polymerization step is carried out by stirring polymerization using a device such as a kneader, the polymerization step and the coarse crushing step may be carried out substantially simultaneously.

[0040] The crosslinked polymer after crushing (crushed gel, crushed polymer) may be in the form of particles or may have an elongated shape where the particles are connected. The size of the smallest side of the crushed gel may be, for example, about 0.1 to 15 mm, preferably about 1.0 to 10 mm. The size of the largest side of the crushed gel may be about 0.1 to 200 mm, preferably about 1.0 to 150 mm.

[0041] [Drying process] In the manufacturing method according to this embodiment, the drying process is carried out such that the initial change rate of the moisture content based on dry weight shown by the following formula is 6.0% / min or less. The moisture content based on dry weight before the drying process is 100% by mass or more. Initial change rate of moisture content based on dry weight (% / min)=[{(Moisture content based on dry weight before drying process - Moisture content based on dry weight 15 minutes after drying process) / Moisture content based on dry weight before drying process}×100] / 15

[0042] In this specification, drying means removing at least a part of the moisture contained in the crosslinked polymer by placing the crosslinked polymer in an environment of 80°C or higher. In this specification, the moisture content based on dry weight is the ratio (mass%) of the amount of moisture in the crosslinked polymer to the solid content (dry weight) in the crosslinked polymer. Therefore, when the moisture content based on dry weight is 100% by mass, the amount of moisture and the solid content contained in the crosslinked polymer are the same. Also, the initial change rate of the moisture content based on dry weight shown by the above formula means the average value per minute of the change rate of the moisture content based on dry weight in the 15 minutes from the start of the drying process.

[0043] According to the manufacturing method according to this embodiment, by controlling the degree of drying of the crosslinked polymer in the initial stage of the drying process (15 minutes from the start of the drying process), it is possible to prevent the crosslinked polymer from adhering to the metal surface of the drying device or the like after the drying process and improve the peelability. By the manufacturing method according to this embodiment, after the drying process is completed, for example, the crosslinked polymer can be peeled from the metal surface by natural dropping without applying force to the crosslinked polymer from the outside.

[0044] The crosslinked polymer is preferably dried by placing the crosslinked polymer on a metal surface. The metal surface may be, for example, a wire mesh or a metal plate having holes. Metals that have been resin-treated, such as fluororesin-treated, tend to have a lower heat transfer rate than untreated original metals, so in order to perform efficient drying, it is preferable that the metal surface of the drying device is not resin-treated. According to the manufacturing method of this embodiment, even if a metal that has not been resin-treated is used for at least a part of the metal surface of the drying device, the crosslinked polymer is excellent in peelability from the drying device.

[0045] In this specification, the dry weight moisture content before the drying step refers to the dry weight moisture content immediately before the crosslinked polymer is subjected to the drying step. For example, when the drying is performed using a drying device, the dry weight moisture content immediately before the crosslinked polymer is placed at a predetermined position in the drying device. When the production method includes a process of crushing the crosslinked polymer, the dry weight moisture content of the crosslinked polymer immediately after crushing may be the dry weight moisture content before the drying step.

[0046] The moisture content on a dry basis before the drying step is 100% by mass or more, and may be 110% by mass or more, 120% by mass or more, 130% by mass or more, or 135% by mass or more. The moisture content on a dry basis before the drying step is preferably 233% by mass or less from the viewpoint of suppressing elution of monomers constituting the crosslinked polymer during the drying step and improving the peelability. The moisture content on a dry basis before the drying step may be 220% by mass or less, 210% by mass or less, 200% by mass or less, 190% by mass or less, 180% by mass or less, 170% by mass or less, or 160% by mass or less.

[0047] The dry-weight basis water content before the drying step can be adjusted, for example, by adjusting the water content of the aqueous monomer solution used in the polymerization of the crosslinked polymer. This is because the water content of the aqueous monomer solution affects the water content of the aggregated hydrogel obtained by polymerization and the dry-weight basis water content of the crosslinked polymer after crushing. The dry-weight basis water content before the drying step may be reduced by carrying out the polymerization step under a nitrogen stream, by distilling off the steam generated when the gel becomes hot due to the reaction heat during the polymerization step, and / or by subjecting the aggregated hydrogel obtained by polymerization to a crushing step under a nitrogen stream.

[0048] The drying temperature 15 minutes after the start of the drying process and 15 minutes after that is 80°C or higher, and may be 90°C or higher, 100°C or higher, 120°C or higher, 140°C or higher, 160°C or higher, 170°C or higher, 180°C or higher, 190°C or higher, or 195°C or higher. The drying temperature may be a temperature above the boiling point of water. The drying temperature may be 220°C or lower, 210°C or lower, or 200°C or lower. In this specification, the drying temperature may be the set temperature of the drying apparatus or the exposure atmosphere temperature of the hydrogel in the drying process. The drying process may be carried out at normal pressure or under reduced pressure. It is also possible that the environmental temperature temporarily drops below the predetermined drying temperature or below 80°C during the drying process.

[0049] It is preferable that the drying apparatus for drying the crosslinked polymer is set in advance to 80°C or higher, preferably the predetermined drying temperature, and the drying is started simultaneously when the crosslinked polymer is installed in the drying apparatus.

[0050] The cumulative time of the drying process may be set according to conditions such as the drying temperature so that the moisture content based on dry weight after drying is within an appropriate range. The time of the drying process is cumulative and may be, for example, 15 minutes or more, 20 minutes or more, 25 minutes or more, or 30 minutes or more, and may be 120 minutes or less, 90 minutes or less, or 60 minutes or less.

[0051] The drying of the crosslinked polymer can be carried out using a drying apparatus such as a hot air dryer, a vacuum dryer, a through-air belt dryer, a through-air band dryer, a rotary through-air dryer, a stirring dryer, a fluidized bed dryer, a vibrating fluidized bed dryer, or a vacuum dryer.

[0052] The initial change rate of the moisture content based on dry weight is 6.0% / min or less, and may be 5.8% / min or less, 5.6% / min or less, 5.3% / min or less, 5.0% / min or less, 4.5% / min or less, 4.2% / min or less, 4.0% / min or less, 3.8% / min or less, or 3.5% / min or less.

[0053] The initial change rate of the dry-basis moisture content may be 3.0% / min or more, preferably 3.5% / min or more. When the initial change rate is 3.5% / min or more, the peelability of the crosslinked polymer after drying can be further improved. The initial change rate may be 4.5% / min or more, or 5.0% / min or more.

[0054] The initial change rate of the dry-basis moisture content can be adjusted, for example, by adjusting the drying temperature, the amount of moisture volatilized from the crosslinked polymer and released outside the system, etc. within 15 minutes from the start of the drying process.

[0055] The dry-basis moisture content 15 minutes after the start of the drying process means the dry-basis moisture content 15 minutes after the start of the drying process. The dry-basis moisture content 15 minutes after the start of the drying process may be, for example, 10% by mass or more, and may also be 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, or 60% by mass or more. The dry-basis moisture content 15 minutes after the start of the drying process may be, for example, 80% by mass or less, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, or 30% by mass or less.

[0056] In the manufacturing method according to this embodiment, the mode of drying after 15 minutes from the start of the drying process is not particularly limited. For example, in order to perform more efficient drying, drying may be performed under stronger drying conditions than in the initial stage of the drying process. The drying temperature after 15 minutes from the start of the drying process may be higher or lower than that in the initial stage of the drying process.

[0057] In this specification, the moisture content based on the final dry weight refers to the moisture content based on the dry weight immediately after the drying process of the crosslinked polymer is completed. Immediately after the drying process is completed means, for example, when drying is performed using a drying device, it is immediately after the crosslinked polymer is taken out of the drying device. For example, when using a plurality of drying devices, it is immediately after being taken out of the last drying device used. When the crosslinked polymer is pulverized after the drying process, for example, the moisture content of the crosslinked polymer immediately before pulverization may be used as the moisture content based on the final dry weight. However, when moisture is added to the crosslinked polymer, such as by adding an aqueous additive solution to the crosslinked polymer, after the drying process and before the pulverization process, the moisture content of the crosslinked polymer before adding the moisture is used as the moisture content based on the final dry weight. The moisture content based on the final dry weight can be reduced, for example, by increasing the drying temperature, extending the drying time, etc.

[0058] The moisture content based on the final dry weight may be 15% by mass or less, 10% by mass or less, 8% by mass or less, 6% by mass or less, or 5% by mass or less. From the viewpoint of reducing the adhesion of the crosslinked polymer to the pulverizer and reducing the load on the pulverizer to suppress mechanical failure when the pulverization process of the crosslinked polymer is performed after the drying process, the moisture content based on the final dry weight is preferably 15% by mass or less. The moisture content based on the final dry weight may be, for example, 1% by mass or more, 2% by mass or more, 3% by mass or more, 3.5% by mass or more, or 4% by mass or more.

[0059] [Peeling] After the drying process, the dried crosslinked polymer can be taken out of the drying device. When the crosslinked polymer is placed on a metal surface during the drying process, the crosslinked polymer is peeled off from the metal surface. According to the manufacturing method according to this embodiment, the crosslinked polymer can be peeled off from the metal surface of the drying device, for example, by natural dropping, without applying an external force.

[0060] [Pulverization] The crosslinked polymer (crushed and dried polymer) after the coarse crushing and drying processes is preferably further pulverized. That is, the manufacturing method according to this embodiment preferably includes a pulverization process of the crosslinked polymer after the drying process. By pulverization, particulate crosslinked polymers (polymer particles) having a smaller particle diameter can be obtained.

[0061] For pulverization, for example, pulverizers such as roller mills, stamp mills, jet mills, high-speed rotary pulverizers (hammer mills, pin mills, rotor beater mills, etc.), and container-driven mills (rotary mills, vibration mills, planetary mills, etc.) can be used. Preferably, a high-speed rotary pulverizer is used. The pulverizer may have an opening such as a perforated plate, screen, grid, etc. on the outlet side to control the maximum particle size of the pulverized particles. The shape of the opening may be polygonal, circular, etc., and the maximum diameter of the opening may be 0.1 to 5 mm, 0.3 to 3.0 mm, or 0.5 to 1.5 mm.

[0062] [Classification, particle size adjustment] The particulate crosslinked polymer obtained by pulverization may be further classified. The production method according to this embodiment may include a step of classifying the crosslinked polymer after pulverization. Classification refers to an operation of dividing a certain particle group into two or more particle groups with different particle size distributions according to the particle size. Also, multiple classification steps may be performed, such as pulverizing the particles after classification again and repeating the pulverization step and the classification step, or a classification step may be performed after the surface crosslinking step described later. Classification of the particles can be performed by methods such as screen classification and air classification. The particles may be granulated as necessary. The crosslinked polymers of each particle size obtained by classification may be mixed again as necessary to adjust the particle size so as to have a desired particle size distribution.

[0063] [Surface crosslinking] The production method of the water-absorbing resin particles according to this embodiment may include a step of performing surface crosslinking of the polymer particles. Surface crosslinking can be performed, for example, by adding a crosslinking agent (surface crosslinking agent) for performing surface crosslinking to the polymer particles and reacting them.

[0064] The surface crosslinking 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 the surface crosslinking agent 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; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 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] [Water-absorbing resin particles] The water-absorbing resin particles obtained by the production method according to this embodiment contain the above-described particulate crosslinked polymer (polymer particles). The water-absorbing resin particles may consist only of the polymer particles, and may further contain additional components such as a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, such as sodium diethylenetriaminepentaacetate), and a fluidity improver (lubricant). The additional components can be disposed inside the polymer particles, on the surface thereof, or both.

[0066] The water-absorbing resin particles may contain a plurality of inorganic particles disposed on the surface of the polymer particles. The production method according to the present embodiment may further include a step of attaching inorganic particles to the surface of the polymer particles.

[0067] The shape of the water-absorbing 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-absorbing resin particles may be 130 to 800 μm, 200 to 850 μm, 250 to 700 μm, 300 to 600 μm, or 300 to 450 μm.

[0068] The water-absorbing resin particles obtained by the production method according to the present embodiment are excellent in water absorption and can be used, for example, in the fields of sanitary materials such as paper diapers and sanitary products, agro-horticultural materials such as water retention agents and soil improvers, and industrial materials such as water stop agents and dew condensation preventers.

Examples

[0069] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples only.

[0070] <Production Example 1> [Preparation of Monomer Aqueous Solution] 339.39 g (4.71 mol) of acrylic acid was placed in a 2 L separable flask. 292.30 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Then, a partially neutralized solution of acrylic acid with a monomer concentration of 45.0% by mass (neutralization rate: 75.8 mol%) was prepared by dropping 297.45 g of a 48% by mass aqueous sodium hydroxide solution under an ice water bath at about 3°C.

[0071] [Preparation of Crushed Gel] (Polymerization Step) 889.28 g of the above acrylic acid partially neutralized solution, 143.40 g of ion-exchanged water, 0.412 g of polyethylene glycol diacrylate (n≈9) (manufactured by NOF Corporation, Blemmer ADE-400A) as an internal cross-linking agent, and 16.14 g of an aqueous potassium persulfate solution with a concentration of 2% by mass were placed in an 18-8 stainless steel vat (outer dimensions: 297 mm×232 mm×height 50 mm) coated with a fluororesin, and stirred with two stirrers (diameter 8 mm, length 45 mm) to form a uniform mixture in the stainless steel vat. Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25°C, a nitrogen introduction tube made of fluororesin with an inner diameter of 3 mm (flow rate 200 ml / min) was inserted, and the mixture in the vat was purged with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less. Next, while stirring the mixture at 300 rpm, 3.39 g of a 0.5% by mass aqueous L-ascorbic acid solution was dropped using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, injection needle manufactured by Terumo Corporation).

[0072] After dropping the aqueous L-ascorbic acid solution, the polymerization reaction started 3 minutes later. As the polymerization reaction proceeded, the viscosity of the reaction solution increased, and then the reaction solution gelled. At the 14th minute after the dropping of the aqueous L-ascorbic acid solution was completed, the installed thermometer showed 86.0°C, and then the temperature began to decrease.

[0073] The stainless steel vat containing the hydrogel (hydrogel-like crosslinked polymer) formed by the gelation of the reaction solution was immersed in a water bath at 75°C, and the hydrogel was heated in that state for 20 minutes to complete the polymerization reaction sufficiently.

[0074] (Crushing process) The total amount of the hydrogel after the polymerization process was taken out of the container, and cuts were made at 5 cm intervals along the long side for cutting. The cut hydrogels were sequentially put into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd. for crushing (subdivision) to obtain crushed gels. The diameter of the holes in the plate located at the outlet of the meat chopper was 6.4 mm. The moisture content of the crushed gels at this time (moisture content A based on the wet weight before the drying process) was 58.2% by mass.

[0075] <Production Example 2> [Preparation of Monomer Aqueous Solution] 340.60 g (4.73 mol) of acrylic acid was placed in a 2 L separable flask. While stirring the acrylic acid in the separable flask, 292.87 g of ion-exchanged water was added. Next, 295.98 g of a 48 mass% aqueous sodium hydroxide solution was added dropwise under an ice water bath at about 3°C to prepare a partially neutralized solution of acrylic acid with a monomer concentration of 45.0 mass% (neutralization rate: 75.1 mol%).

[0076] [Preparation of Crushed Gel] (Polymerization Step) 888.10 g of the above acrylic acid partially neutralized solution, 144.55 g of ion-exchanged water, 0.413 g of polyethylene glycol diacrylate (n≈9) (NOF Corporation, Blemmer ADE-400A) as an internal crosslinking agent, and 16.17 g of a 2 mass% aqueous potassium persulfate solution were placed in an 18-8 stainless steel bath (outer dimensions: 297 mm×232 mm×height 50 mm) coated with a fluororesin, and stirred with two stirrers (diameter 8 mm, length 45 mm) to form a uniform mixture in the stainless steel bath. Then, the upper part of the stainless steel bath was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel bath to 25°C, a nitrogen introduction tube made of fluororesin with an inner diameter of 3 mm (flow rate: 200 ml / min) was inserted, and the mixture in the bath was purged with nitrogen to adjust the dissolved oxygen amount to 0.1 ppm or less. Next, while stirring the mixture at 300 rpm, 3.40 g of a 0.5 mass% aqueous L-ascorbic acid solution was added dropwise using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, injection needle manufactured by Terumo Corporation).

[0077] After dropping the aqueous L-ascorbic acid solution, the polymerization reaction started 1 minute later. As the polymerization reaction proceeded, the viscosity of the reaction solution increased, and then the reaction solution gelled. At the 10-minute point after the dropping of the aqueous L-ascorbic acid solution was completed, the thermometer installed showed 94.6 °C, and then the temperature began to decrease. The subsequent steps were carried out in the same manner as in Production Example 1. Incidentally, the water content based on the wet weight of the crushed gel obtained after the crushing step (water content A based on the wet weight before the drying step) was 58.1% by mass.

[0078] <Production Example 3> The polymerization step was carried out in the same manner as in Production Example 2. After dropping the aqueous L-ascorbic acid solution, the polymerization reaction started 1 minute later. As the polymerization reaction proceeded, the viscosity of the reaction solution increased, and then the reaction solution gelled. At the 11-minute point after the dropping of the aqueous L-ascorbic acid solution was completed, the thermometer installed showed 72.6 °C, and then the temperature began to decrease. The subsequent steps were also carried out in the same manner as in Production Example 2. The water content based on the wet weight of the crushed gel obtained after the crushing step (water content A based on the wet weight before the drying step) was 58.6% by mass.

[0079] <Example 1> (Drying step) 60 g of the crushed gel obtained in Production Example 1 was uniformly placed within a range of 15 cm in diameter from the center of a JIS sieve (20 cm in diameter) with an eye opening of 1.7 mm. Thereafter, a metal petri dish (167 g) with a diameter of 15 cm was placed on the crushed gel, and further, a 250 g weight was placed on the petri dish to apply a uniform load over the entire surface of the crushed gel, thereby pressing the crushed gel against the JIS sieve for 10 seconds. Thereafter, the metal petri dish and the weight were removed, and the upper surface opening of the JIS sieve on which the crushed gel was placed was covered with aluminum foil (manufactured by Toyo Aluminum Eco Products Co., Ltd., cooking foil). The JIS sieve covered with aluminum foil was placed in a hot air dryer (FV-320, manufactured by ADVANTEC) preset to 200°C, and a drying process was performed for 30 minutes to obtain a crushed and dried polymer. After the drying process, after removing the JIS sieve on which the crushed and dried polymer was placed from the dryer, a peel rate test was immediately carried out. Thereafter, the moisture content based on the wet weight (final moisture content based on the wet weight C) of the crushed and dried polymer was measured, and the moisture content based on the dry weight (final moisture content based on the dry weight c) was calculated. The final moisture content based on the wet weight C was 5.1% by mass.

[0080] Separately, after covering the upper surface opening of the JIS sieve with aluminum foil in the same manner as above, the crushed gel was taken out 15 minutes after the start of the drying process at 200°C. The moisture content based on the wet weight (moisture content based on the wet weight B after 15 minutes of the drying process) of the taken-out crushed gel was measured, and the moisture content based on the dry weight (moisture content based on the dry weight b after 15 minutes of the drying process) was calculated. The moisture content based on the wet weight B after 15 minutes of the drying process was 41.0% by mass.

[0081] <Example 2> (Drying process) After covering the upper surface opening of the JIS sieve with aluminum foil, a drying process was carried out in the same manner as in Example 1 except that 24 vent holes with a diameter of 3 mm were made in the aluminum foil at intervals of 2 to 3 cm. The moisture content based on the wet weight (final moisture content based on the wet weight C) of the crushed and dried polymer obtained after the drying process was 4.7% by mass.

[0082] Separately, in the same manner as above, the upper surface opening of the JIS sieve was covered with aluminum foil, and 24 vent holes with a diameter of 3 mm were made at intervals of 2 to 3 cm in the aluminum foil. Then, 15 minutes after the start of the drying process at 200 °C, the crushed gel was taken out. When the moisture content based on the wet weight of the taken-out crushed gel (moisture content based on the wet weight B after 15 minutes of the drying process) was measured, it was 35.6 mass%.

[0083] <Example 3> (Drying process) Using the crushed gel obtained in Production Example 2, in the same manner as in Example 1, the crushed gel was pressed against the JIS sieve with a metal petri dish and a weight for 10 seconds. The metal petri dish and the weight were removed, and the JIS sieve on which the crushed gel was placed was put into a hot air dryer (manufactured by ADVANTEC, FV-320) previously set at 180 °C. After 15 minutes, the JIS sieve on which the crushed gel was placed was taken out from the hot air dryer and immediately transferred to another hot air dryer (manufactured by ADVANTEC, FV-320) previously set at 200 °C, and the drying process was further carried out for 15 minutes to obtain a crushed and dried polymer. After the drying process of 30 minutes in total, the peel rate test was immediately carried out. Then, when the moisture content based on the wet weight of the crushed and dried polymer (final moisture content based on the wet weight C) was measured, it was 3.7 mass%.

[0084] Separately, when the moisture content based on the wet weight of the crushed gel taken out 15 minutes after the start of the drying process at 180 °C (moisture content based on the wet weight B after 15 minutes of the drying process) was measured, it was 19.7 mass%.

[0085] <Example 4> The drying process temperature for 30 minutes was changed to 180 °C, and the drying process was carried out in the same manner as in Example 1 except that the upper surface opening of the JIS sieve was not covered with aluminum foil. The moisture content based on the wet weight of the obtained crushed and dried polymer (final moisture content based on the wet weight C) was 5.1 mass%. Separately, when the moisture content based on the wet weight of the crushed gel taken out 15 minutes after the start of the drying process at 180 °C (moisture content based on the wet weight B after 15 minutes of the drying process) was measured, it was 19.5 mass%.

[0086] <Example 5> Using the crushed gel obtained after the crushing step in Production Example 3, the drying step was carried out in the same manner as in Example 3 except that the drying step temperature for 15 minutes from the start of the drying step was changed to 160°C. The moisture content based on wet weight (final moisture content based on wet weight C) of the obtained crushed and dried polymer was 3.6% by mass. Separately, the moisture content based on wet weight (moisture content based on wet weight B after 15 minutes of the drying step) of the crushed gel taken out 15 minutes after the start of the drying step at the same 160°C as above was measured and found to be 23.0% by mass.

[0087] <Comparative Example 1> The drying step was carried out in the same manner as in Example 1 except that the top of the JIS sieve was not covered with aluminum foil. The moisture content based on wet weight (final moisture content based on wet weight C) of the obtained crushed and dried polymer was 2.7% by mass. Separately, the moisture content based on wet weight (moisture content based on wet weight B after 15 minutes of the drying step) of the crushed gel taken out 15 minutes after the start of the drying step at the same 200°C as above was measured and found to be 8.3% by mass.

[0088] <Comparative Example 2> The drying step was carried out in the same manner as in Example 3 except that the order of the 180°C hot air dryer and the 200°C hot air dryer was switched, and after drying for 15 minutes with the 200°C hot air dryer, it was quickly transferred to the 180°C dryer. The moisture content based on wet weight (final moisture content based on wet weight C) of the obtained crushed and dried polymer was 3.7% by mass. Separately, the moisture content based on wet weight (moisture content based on wet weight B after 15 minutes of the drying step) of the crushed gel taken out 15 minutes after the start of the drying step at the same 200°C as above was measured and found to be 8.4% by mass.

[0089] [Measurement of moisture content based on wet weight] The moisture content-based water content rates A before the drying process of the measurement sample, B after 15 minutes of the drying process, and C after the final moisture content were measured by the following method. For the measurement of the moisture content-based water content rate A before the drying process, 20.0 g of the crushed gel obtained in the crushing process was sampled and used as the measurement sample. For the measurement of the moisture content-based water content rate B after 15 minutes of the drying process, 20.0 g of the crushed gel taken out 15 minutes after the start of the drying process was sampled and used as the measurement sample. For the measurement of the final moisture content-based water content rate C, 20.0 g of the crushed and dried polymer obtained immediately after the entire drying process (160 to 200 °C, 30 minutes) was sampled and used as the measurement sample.

[0090] The above measurement sample was placed in a stainless steel vat (outer dimensions: 185 mm × 140 mm × height 30 mm) coated with a fluororesin and pre-weighed to a constant weight (W1 (g)), and the total mass W2 (g) of the stainless steel vat and the measurement sample was accurately weighed. The accurately weighed measurement sample was dried in a hot air dryer (manufactured by ADVANTEC, model: FV-320) with the internal temperature set to 200 °C for 2 hours. After the dried measurement sample was allowed to cool in a desiccator, the total mass W3 (g) of the stainless steel vat and the measurement sample was accurately weighed. The moisture content-based water content rate of the measurement sample was calculated from the following formula. Moisture content-based water content rate (mass%) = [((W2 - W1) - (W3 - W1)) / (W2 - W1)] × 100

[0091] [Calculation of dry matter-based water content rate] Using the values of the moisture content-based water content rate A before the drying process, B after 15 minutes of the drying process, and C after the final moisture content obtained by the above method, the dry matter-based water content rate a before the drying process, b after 15 minutes of the drying process, and c after the final dry matter were calculated from the following formula, respectively. Dry matter-based water content rate (mass%) = [moisture content-based water content rate / (100 - moisture content-based water content rate)] × 100

[0092] [Initial change rate of dry matter-based water content rate] The initial change rate of the dry matter-based water content rate of the water-containing gel was calculated from the following formula using the calculated dry matter-based water content rate a before the drying process and b after 15 minutes of the drying process. Initial change rate of dry-basis moisture content (% / min) = [({Initial dry-basis moisture content a before drying process - Dry-basis moisture content b 15 minutes after drying process 1} / Initial dry-basis moisture content a before drying process) × 100] / 15 (min)

[0093] [Peeling rate test] After the drying process, the JIS sieve with the crushed and dried polymer on it was taken out from the hot air dryer while keeping the mesh surface of the JIS sieve parallel to the ground without applying vibration. Within 5 seconds after the removal (during which time, without applying vibration to the JIS sieve and while keeping the mesh surface of the JIS sieve parallel to the ground), the JIS sieve was turned 180° up and down in 3 seconds, and the crushed and dried polymer that fell from the JIS sieve was collected in a vat. The weight W4 (g) of the fallen crushed and dried polymer and the weight W5 (g) of the crushed and dried polymer remaining attached to the JIS sieve were measured. The peeling rate was calculated by the following formula. The results are shown in Table 1. Peeling rate (mass %) = [W4 / (W4 + W5)] × 100

[0094]

Table 1

[0095] In the production method of the examples where the initial change rate of the dry-basis moisture content was 6.0% / min or less, the crosslinked polymer after the excellent drying process showed peelability. Particularly in Examples 3 and 5, even when the drying conditions after 15 minutes from the start of the drying process were made stronger, the crosslinked polymer after the drying process showed high peelability. On the other hand, in the comparative examples where the initial change rate of the dry-basis moisture content exceeded 6.0% / min, it was shown that the peeling rate was low even when the drying conditions after 15 minutes from the start of the drying process were made milder.

Claims

1. A method for producing water-absorbent resin particles containing a crosslinked polymer, comprising: a drying step of drying the crosslinked polymer, wherein the water content based on dry weight before the drying step of the crosslinked polymer is 100% by mass or more, and the drying step is carried out such that the initial change rate of the water content based on dry weight represented by the following formula is 6.0% / min or less. Initial change rate of water content based on dry weight (% / min) = [({(Water content based on dry weight before drying step - Water content based on dry weight 15 minutes after drying step) / Water content based on dry weight before drying step} × 100)] / 15

2. The method according to claim 1, wherein the drying step is carried out such that the initial change rate of the water content based on dry weight is 5.6% / min or less.

3. The method according to claim 1 or 2, wherein the drying step is carried out such that the initial change rate of the water content based on dry weight is 3.0% / min or more.

4. The method according to any one of claims 1 to 3, further comprising crushing the crosslinked polymer before the drying step.

5. The method according to any one of claims 1 to 4, further comprising pulverizing the crosslinked polymer after the drying step.

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