Method for producing water-absorbent resin particles

Abstract

Disclosed is a method for producing a water-absorbing resin particle, the method including coarsely pulverizing a water-containing gelatinous polymer formed from a mixture containing a monomer and water so as to form a coarsely pulverized product, and then coarsely pulverizing the coarsely pulverized product so as to form a coarsely pulverized product containing a plurality of structures having shapes that can pass through circular holes having diameters of 7 mm. The elapsed time t between the time at which the temperature of the mixture reaches a maximum value Tmax and the time at which coarse pulverization starts is 15 minutes or less, and the elapsed time is a period of time selected so that the amount of polymer particles obtained from the coarsely pulverized product that dissolve in a physiological saline solution at 25°C is less than said amount in a case where t is 30 minutes and T is X. X is a temperature within the range 70±3ºC. The temperature T of the water-containing gelatinous polymer at the point of time at which coarse pulverization starts falls within the range X±25ºC.

Description

[Technical Field]

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

[0002] Water-absorbent resin particles that can be used in absorbent articles such as sanitary products may be produced by a method that includes crushing or pulverizing a mass of hydrogel polymer formed by polymerization of a monomer (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-228604 Summary of the Invention [Problem to be solved by the invention]

[0004] By crushing the hydrogel polymer before drying so as to form a crushed product containing structures of as small a size as possible, it is possible to efficiently remove water from the hydrogel polymer. However, according to the findings of the present inventors, it has become clear that when a crushed product containing small-sized structures is formed, the soluble portion of the obtained water-absorbent resin particles, particularly in physiological saline, tends to increase.

[0005] Therefore, one aspect of the present disclosure relates to a method for reducing a soluble content of water-absorbent resin particles in physiological saline, in the case of producing water-absorbent resin particles by a method including crushing a hydrous gel-like polymer so as to form crushed material including small-sized structures. [Means for solving the problem]

[0006] One aspect of the present disclosure provides a method for producing water-absorbent resin particles containing polymer particles. The method includes: forming a mixture containing a monomer and water in which the monomer is dissolved in a reaction vessel; polymerizing the monomer in the mixture to form a hydrogel polymer in the reaction vessel, which is a gelled mixture containing a polymer of the monomer; crushing the hydrogel polymer in a mass form removed from the reaction vessel to form a crushed product; and forming polymer particles from the crushed product by a method including removing water and crushing. The hydrogel polymer in a mass form is crushed to form the crushed product containing a plurality of structures having a shape that can pass through a circular hole with a diameter of 7 mm. During the polymerization of the monomer, the temperature of the mixture is raised to a maximum temperature T max The time elapsed from when the temperature reaches t to when the crushing of the hydrogel polymer starts is t. The temperature of the hydrogel polymer at the time when the crushing of the hydrogel polymer starts is T. t is 15 minutes or less and is a time selected so that the soluble portion of the polymer particles obtained from the crushed product in physiological saline at 25°C is smaller than the value when t is 30 minutes and T is X. X is a temperature within the range of 70±3°C. T is a temperature within the range of X±25°C. [Effects of the Invention]

[0007] According to the method according to one aspect of the present disclosure, in the case of producing water-absorbent resin particles by a method including crushing a hydrous gel polymer so as to form a crushed material containing small-sized structures, it is possible to reduce the soluble portion of the water-absorbent resin particles in physiological saline. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present invention is not limited to the following embodiments.

[0009] As used herein, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also referred to as "(meth)acrylate." The same applies to other similar terms. "(Poly)" refers to both cases with and without the prefix "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" refers to 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. "Saline" refers to a 0.9% by mass aqueous solution of sodium chloride.

[0010] An example of a method for producing water-absorbent resin particles includes forming a mixture containing a monomer and water in which the monomer is dissolved in a reaction vessel, polymerizing the monomer in the mixture to form a hydrogel polymer in the reaction vessel, which is a gelled mixture containing a polymer of the monomer, crushing the hydrogel polymer in a mass form removed from the reaction vessel to form a crushed material, and forming polymer particles from the crushed material. The produced water-absorbent resin particles include polymer particles containing a polymer.

[0011] The mixture before the polymerization reaction can be an aqueous monomer solution containing water and a monomer dissolved in water. The concentration of the monomer in the mixture before the polymerization reaction (aqueous monomer solution) may be, for example, 20 to 50 mass %, 25 to 45 mass %, or 30 to 40 mass % based on the mass of the mixture.

[0012] The monomer is a compound that forms, by polymerization, a polymer that imparts water absorption properties to the polymer particles and the water-absorbent resin particles. The monomer may be an ethylenically unsaturated monomer. The polymer formed may be a crosslinked polymer.

[0013] The ethylenically unsaturated monomer may include, for example, at least one compound selected from the group consisting of (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. When the ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The monomer may contain at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide; at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, and acrylamide; or at least one compound selected from the group consisting of acrylic acid and its salts, and methacrylic acid and its salts.

[0014] The mixture for polymerization may contain a monomer other than the ethylenically unsaturated monomer, and the proportion of the ethylenically unsaturated monomer (particularly, (meth)acrylic acid and its salts) may be 70 to 100 mol % based on the total amount of monomers in the mixture.

[0015] The mixture for polymerization may further contain a radical polymerization initiator. The radical polymerization initiator may include a persulfate, an azo compound, a peroxide, or a combination thereof. The amount of the radical polymerization initiator may be 0.01 to 15 mmol per 1 mol of the monomer.

[0016] Examples of persulfates include potassium persulfate, ammonium persulfate, and sodium persulfate.

[0017] Examples of azo compounds include 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[ 2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). In terms of forming large CRC polymer particles, the radical polymerization initiator may include at least one azo compound selected from 2,2′-azobis(2-amidinopropane) dihydrochloride, 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, and 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride.

[0018] Examples of peroxides include methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide.

[0019] The aqueous monomer solution may further comprise a chain transfer agent, such as hypophosphorous acid, phosphorous acid, or a combination thereof.

[0020] The aqueous monomer solution may contain an internal cross-linking agent. In this case, a hydrogel polymer containing a cross-linked polymer cross-linked by the internal cross-linking agent is formed. The amount of the internal cross-linking agent may be 0.002 to 0.04 millimoles per mole of the monomer.

[0021] The internal crosslinking agent may be a compound having two or more reactive functional groups (e.g., polymerizable unsaturated groups). The internal crosslinking agent may include a compound having a (meth)acrylic group, an allyl group, an epoxy group, or an amino group as a reactive functional group. Examples of compounds having a (meth)acrylic group include (poly)ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and N,N'-methylenebis(meth)acrylamide. Examples of compounds having an allyl group include triallylamine. Examples of compounds having an epoxy group include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin polyglycidyl ether, and epichlorohydrin. Examples of compounds having an amino group include triethylenetetramine, ethylenediamine, and hexamethylenediamine.

[0022] The reaction vessel is not particularly limited, and may be, for example, a vessel made of stainless steel, ceramics, synthetic resin, or steel. The reaction vessel may be placed on a belt conveyor. The depth (or thickness) of the mixture (aqueous monomer solution) formed in the reaction vessel may be, for example, 10 to 50 mm, 10 to 40 mm, or 15 to 35 mm. The maximum width in the horizontal direction of the mixture (aqueous monomer solution) formed in the reaction vessel may be, for example, 50 to 800 mm.

[0023] When the polymerization reaction starts in the mixture (aqueous monomer solution) in the reaction vessel, the temperature of the mixture rises and reaches a maximum value T maxThe temperature of the mixture before the temperature rise due to the polymerization reaction may be, for example, 0°C or higher and 50°C or lower, 40°C or lower, or 30°C or lower, or 10°C or higher and 50°C or lower, 40°C or lower, or 30°C or lower. T max may be, for example, 60°C or higher and 100°C or lower, 95°C or lower, or 90°C or lower; 70°C or higher and 100°C or lower, 95°C or lower, or 90°C or higher; or 75°C or higher and 100°C or lower, 95°C or lower, or 90°C or lower.

[0024] When the mixture temperature reaches its maximum value T max After the temperature reaches 100°C, the formed hydrogel polymer aggregates are removed from the reaction vessel and subjected to the subsequent crushing step. The hydrogel polymer may be maintained at a temperature equal to or higher than the temperature T described below in the reaction vessel and then removed from the reaction vessel. The hydrogel polymer removed from the reaction vessel may be divided, if necessary, to obtain a plurality of aggregates of a relatively small size. In this case, the aggregates are subjected to the crushing step. The minimum width of the aggregates may be, for example, about 5 to 70 mm.

[0025] By crushing the hydrogel polymer (or the aggregates), a crushed product containing structures smaller than a certain size is formed. More specifically, the hydrogel polymer is crushed to form a crushed product containing a plurality of structures having a shape that can pass through a circular hole with a diameter of 7 mm. Examples of crushing devices for crushing the hydrogel polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and farmer mills.

[0026] The crushed material can be an aggregate composed of multiple structures. The structures that make up the crushed material can be, for example, elongated structures, granular structures (particles), or a combination of these. The elongated structures may be curved, and if their maximum width is 7 mm or less, they can be said to have a shape that allows them to pass through a circular hole with a diameter of 7 mm. The granular structures (particles) may be irregular in shape, and may have a shape that allows them to pass through a circular hole with a diameter of 7 mm while changing direction.

[0027] The crushed material consisting of a plurality of elongated structures is formed, for example, by a crushing device (e.g., a meat chopper) having a discharge hole through which the crushed material is discharged. When the maximum width of the discharge hole is 8 mm or less, the crushed material contains elongated structures that can normally pass through a circular hole with a diameter of 7 mm. The crushed material consisting of granular structures is formed, for example, by a kneader.

[0028] During the polymerization of the monomers, the temperature of the mixture reaches a maximum value T max The time elapsed from the time when the temperature reaches t to the time when the hydrogel polymer starts to be crushed is t. The temperature of the hydrogel polymer at the time when the hydrogel polymer starts to be crushed is T.

[0029] Here, "the time when the hydrogel polymer starts to be crushed" refers to the time when the crushing device starts to apply stress to the hydrogel polymer formed in one reaction vessel, excluding the portion lost during removal from the reaction vessel. For example, when a crushing device (e.g., a meat chopper) is used that has a kneading chamber containing a spiral roll for kneading and extruding the hydrogel polymer, and a discharge section at the end of the kneading chamber with multiple discharge holes through which the crushed material is discharged, the time when the hydrogel polymer starts to be crushed is considered to be the time when the hydrogel polymer starts to be crushed when the roll is rotated. When a kneader having a kneading chamber equipped with blades is used, the time when the hydrogel polymer starts to be crushed when the hydrogel polymer starts to be crushed is considered to be the time when the hydrogel polymer starts to be crushed when the hydrogel polymer starts to be crushed. Not all of the hydrogel polymer formed in one reaction vessel needs to be crushed.

[0030] The elapsed time t is the time when the temperature of the mixture reaches T max The time point when the water-containing gel polymer reaches the predetermined value is set as t=0 [minutes], and the elapsed time t includes the time thereafter until the water-containing gel polymer is removed from the reaction vessel, and the time from the time the water-containing gel polymer is removed from the reaction vessel until the water-containing gel polymer starts to be fed into the crushing device. According to the findings of the present inventors, when the water-containing gel polymer is crushed so as to form a crushed product containing a plurality of structures having a shape that can pass through a circular hole with a diameter of 7 mm, if the elapsed time t is short, the amount of water-absorbent resin particles that dissolves in physiological saline tends to decrease.

[0031] The elapsed time t is determined within a range of 15 minutes or less based on the solubility of the polymer particles obtained from the crushed material in physiological saline at 25°C. More specifically, t is selected so that the solubility is smaller than the solubility of the polymer particles obtained from the crushed material in physiological saline at 25°C (reference solubility) when t is 30 minutes and T is X. X is a temperature selected arbitrarily within the range of 70±3°C, and may be, for example, 68°C.

[0032] The amount of polymer particles dissolved in saline at 25°C is calculated by substituting Wa and Wb obtained by a method including: stirring a dispersion consisting of 500 g of saline at 25°C and 2 g of polymer particles dispersed in the saline in a 500 mL beaker for 3 hours using a stirrer (cylindrical, 8 mm in diameter x 30 mm in length, without a ring) rotating at 600 rpm; filtering the dispersion through a 75 μm JIS standard sieve to recover the filtrate; placing 80 g of the obtained filtrate in a 100 mL beaker and drying it in a hot air dryer at 140°C for 15 hours; measuring the mass Wa of the solid component remaining in the 100 mL beaker; and measuring the mass Wb of the solid component remaining in the 100 mL beaker in a blank test using saline containing no polymer particles, into the following formula: Dissolved content (mass%) = [((Wa-Wb) / 80)×500 / 2]×100

[0033] The elapsed time t may be selected so that the dissolution fraction of the polymer particles obtained from the crushed material in saline at 25°C is 90% or less, 85% or less, or 80% or less of the reference dissolution fraction. The dissolution fraction of the polymer particles obtained from the crushed material in saline at 25°C may be 50% or more of the reference dissolution fraction. The elapsed time t may be 14 minutes or less, 13 minutes or less, 12 minutes or less, 11 minutes or less, 10 minutes or less, or less than 10 minutes. The elapsed time t may be 1 to 15 minutes, 2 to 15 minutes, 3 to 15 minutes, 4 to 15 minutes, 1 to 14 minutes, 2 to 14 minutes, 3 to 14 minutes, 4 to 14 minutes, 1 to 13 minutes, 2 to 13 minutes, 3 to 13 minutes, 4 to 13 minutes, 1 to 12 minutes, 2 to 12 minutes, 3 to 12 minutes, 4 to 12 minutes, 1 to 11 minutes, 2 to 11 minutes, 3 to 11 minutes, 4 to 11 minutes, 1 to 10 minutes, 2 to 10 minutes, 3 to 10 minutes, 4 to 10 minutes, 1 minute or more and less than 10 minutes, 2 minutes or more and less than 10 minutes, 3 minutes or more and less than 10 minutes, or 4 minutes or more and less than 10 minutes.

[0034] The temperature T of the hydrogel polymer at the time when crushing of the hydrogel polymer begins is within the range of X±25°C. When T is within the range of X±25°C, the elapsed time t at which the dissolved fraction becomes smaller than the reference dissolved fraction can be easily selected. T may be within the range of X±20°C, X±10°C, or X±5°C.

[0035] During the polymerization of the monomers, the temperature of the mixture reaches a maximum value T max From the time when the temperature of the mixture and the hydrogel polymer reaches T to the time when crushing of the hydrogel polymer is started, the temperature of the mixture and the hydrogel polymer may be constantly maintained at 40°C or higher, or T or higher. This makes it possible to obtain a more significant effect of reducing the dissolved content. In order to maintain the temperature at 40°C or higher, the hydrogel polymer may be heated as necessary.

[0036] Polymer particles can be obtained from the crushed material by a method including drying and pulverization. More specifically, the polymer particles can be obtained by a method including drying the crushed material to obtain a dried material and pulverizing the dried material.

[0037] Drying the crushed material removes most of the water from the crushed material. The moisture content of the dried material obtained by drying may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The moisture content of the dried material here refers to the proportion of water in the polymer particles based on the total mass of the dried material including water. Typically, when a dried material including water is heated at 200°C for 2 hours, the difference in mass of the dried material before and after heating can be considered to be the moisture content of the dried material. The drying method may be a common method such as natural drying, heat drying, air drying, freeze drying, or a combination of these. The crushed material may be dried under normal pressure or under reduced pressure. The heating temperature for drying under normal pressure may be 70 to 250°C, or 80 to 200°C.

[0038] The method for pulverizing the dried product is not particularly limited. For example, the dried product can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill.

[0039] The polymer particle powder obtained by pulverization may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups with different particle size distributions. A portion of the polymer particle powder after classification may be pulverized and classified again.

[0040] The classification method is not particularly limited, and may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not pass through by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing an air flow.

[0041] The polymer particles obtained through pulverization and, if necessary, classification may have a median particle diameter of, for example, 200 to 500 μm.The polymer particles before being mixed with a surface cross-linking agent solution described below may have a median particle diameter of, for example, 200 to 500 μm.

[0042] The polymer particles may be surface-crosslinked with a surface crosslinking agent. By surface crosslinking, the polymer near the surface of the polymer particles is crosslinked with the surface crosslinking agent. For example, the polymer particles can be surface-crosslinked by heating a mixture of powder of the polymer particles and a solution of the surface crosslinking agent. The above-mentioned dissolved fraction for selecting the elapsed time t is measured using the polymer particles before surface crosslinking.

[0043] The surface cross-linking agent solution may be a solution containing water and a surface cross-linking agent dissolved in water. The solvent contained in the surface cross-linking agent solution may be substantially water alone. The proportion of the solvent other than water may be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the surface cross-linking agent solution.

[0044] Examples of the surface cross-linking agent include alkylene carbonate compounds such as ethylene carbonate; polyol compounds 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, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin; Examples of surface crosslinking agents include haloepoxy compounds such as benzophenone, epibromohydrin, and α-methylepichlorohydrin; 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; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. These surface crosslinking agents may be used alone or in combination of two or more. The surface crosslinking agent may include an alkylene carbonate compound, a polyol compound, or a combination thereof. The ratio of the alkylene carbonate compound in the surface cross-linking agent may be 50 to 100 mass %, 60 to 100 mass %, 70 to 100 mass %, 80 to 100 mass %, or 90 to 100 mass % based on the total mass of the surface cross-linking agent.

[0045] From the viewpoint of the water absorption performance under pressure of the water-absorbing resin particles, the amount of the surface cross-linking agent may be 0.001 to 0.10 mol, 0.005 to 0.05, or 0.01 to 0.02 mol per 1 mol of the monomer unit constituting the polymer in the polymer particles.

[0046] The heating temperature and heating time for surface cross-linking are adjusted so that the cross-linking reaction proceeds appropriately, taking into consideration the type of surface cross-linking agent, etc. For example, the heating temperature for surface cross-linking may be 80 to 200° C. The heating time for surface cross-linking may be, for example, 5 to 90 minutes.

[0047] The surface-crosslinked polymer particles may be further dried or classified as necessary. The polymer particles may be used as water-absorbent resin particles as they are, or, for example, inorganic particles may be attached to the surface of the polymer particles. That is, the water-absorbent resin particles may contain polymer particles and inorganic particles attached to the surface of the polymer particles. Examples of inorganic particles include silica particles such as amorphous silica.

[0048] The produced water-absorbent resin particles are used to form an absorbent body that constitutes an absorbent article such as a diaper, for example. [Example]

[0049] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0050] 1. Production of water-absorbent resin particles (polymer particles) Example 1 93.43 g (1.30 mol) of acrylic acid was placed in a separable flask with an internal volume of 2 L. 79.96 g of ion-exchanged water was added to the acrylic acid in the separable flask with stirring. Next, 81.77 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare a partially neutralized solution of acrylic acid with a monomer concentration of 45% by mass.

[0051] 255.16 g of the prepared partially neutralized acrylic acid solution, 39.49 g of ion-exchanged water, 2.37 g of a 5% by weight aqueous solution of polyethylene glycol diacrylate (approximately 9 oxyethylene groups) as an internal crosslinker (NOF Corporation, Blenmar ADE-400A), and 4.64 g (0.34 mmol) of a 2% by weight aqueous solution of potassium persulfate were placed in a fluororesin-coated stainless steel tray (opening interior dimensions: 175 mm x 130 mm, bottom interior dimensions: 155 x 110 mm, height: 30 mm). A stirrer (8 mm diameter, 45 mm length, no ring) was placed in the center of the stainless steel tray, and a homogeneous liquid mixture was formed by stirring with the stirrer. A thermometer was placed in the center of the stainless steel tray to measure the temperature of the mixture. The opening of the stainless steel tray was then covered with polyethylene film. After adjusting the temperature of the mixture to 25°C, nitrogen gas was bubbled into the mixture through a tube inserted therein to replace the atmosphere with nitrogen until the dissolved oxygen level reached 0.1 ppm or less. Next, while the mixture was being stirred at 300 rpm, 0.97 g of an aqueous solution of L-ascorbic acid with a concentration of 0.5% by mass was added dropwise to the mixture toward the center of a stainless steel tray using a syringe (a 3 mL disposable syringe manufactured by Henke Sas Wolf GmbH, with a syringe needle manufactured by Terumo Corporation).

[0052] The polymerization reaction started immediately after the L-ascorbic acid aqueous solution was added dropwise. One minute after the addition of the L-ascorbic acid aqueous solution was completed, the nitrogen substitution tube was removed from the reaction system and stirring was stopped. As the polymerization reaction progressed, the viscosity of the mixture (reaction liquid) increased, and then the mixture gelled. 12 minutes after the addition of the L-ascorbic acid aqueous solution was completed, the thermometer measuring the temperature of the mixture reached a maximum value of 84°C.

[0053] Within 30 seconds after the thermometer reached its maximum value, it was confirmed that the thermometer indicated a temperature at least 0.1°C lower than the maximum value. The gelled mixture, a mass of hydrogel polymer, was then removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was roughly crushed by quickly introducing it into a kneading chamber equipped with rollers of a meat chopper (Kire Royal Co., Ltd., 12VR-750SDX) while the rollers were rotating.

[0054] The time elapsed from when the temperature of the polymerization mixture reached its maximum value until the hydrogel polymer began to be fed into the meat chopper was 4 minutes. The temperature of the hydrogel polymer immediately before being fed into the meat chopper was 73°C. Crushed material containing elongated structures of the hydrogel polymer was discharged from multiple circular discharge holes in a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the discharge holes was 6.4 mm. Crushing using the meat chopper was continued for 3 minutes, 3 minutes after the hydrogel polymer was fed. The resulting crushed material was an aggregate formed of multiple elongated structures 4 to 6 mm wide.

[0055] The obtained coarsely crushed material was spread on a wire mesh with 0.8 cm × 0.8 cm openings and dried with hot air at 180°C for 30 minutes to obtain a dried product. The dried product was pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved using a sieve with 850 μm openings and a sieve with 180 μm openings by shaking for 1 minute. The polymer particles that passed through the 850 μm opening sieve but remained on the 180 μm opening sieve were collected.

[0056] Example 2 A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum value of 81°C 12 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Within 30 seconds after the thermometer reached its maximum value, it was confirmed that the thermometer indicated a temperature 0.1°C lower than the maximum value. The stainless steel tray containing the hydrogel polymer was then immersed in a water bath at 80°C. The hydrogel polymer was allowed to stand in the water bath for 6 minutes, then removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was roughly crushed using a meat chopper in the same manner as in Example 1. Ten minutes elapsed from when the temperature of the polymerization mixture reached its maximum value until the hydrogel polymer began to be fed into the meat chopper. The temperature of the hydrogel polymer immediately before being fed into the kneading chamber of the meat chopper was 68°C. The obtained coarsely ground product was dried, pulverized and classified in the same manner as in Example 1 to obtain polymer particles.

[0057] Example 3 A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum value of 83°C 12 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Within 30 seconds of the thermometer reaching its maximum value, it was confirmed that the thermometer indicated a temperature 0.1°C lower than the maximum value. The stainless steel tray containing the hydrogel polymer was then immersed in a water bath at 80°C. After allowing the hydrogel polymer to stand in the water bath for 11 minutes, it was removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was roughly crushed using a meat chopper in the same manner as in Example 1. The time from when the temperature of the polymerization mixture reached its maximum value to when the hydrogel polymer began to be introduced into the kneading chamber of the meat chopper was 15 minutes. The temperature of the hydrogel polymer immediately before being introduced into the kneading chamber of the meat chopper was 70°C. The obtained coarsely ground product was dried, pulverized and classified in the same manner as in Example 1 to obtain polymer particles.

[0058] Example 4 A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum of 80°C 13 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Within 30 seconds after the thermometer reached its maximum reading, the thermometer was confirmed to indicate a temperature 0.1°C lower than the maximum reading. The hydrogel polymer was then removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was placed in a polyethylene bag with a zipper (inside zipper size: 280 mm x 200 mm, thickness: 0.04 mm) while minimizing air contamination. The polyethylene bag containing the hydrogel polymer was immersed in an ice bath at −20°C containing 78 parts by mass of ice and 22 parts by mass of NaCl. After allowing the hydrogel polymer to stand in the ice bath for 3 minutes, it was removed from the polyethylene bag and roughly crushed using a meat chopper in the same manner as in Example 1. The temperature of the ice bath was −10.8°C after the hydrogel polymer had stood for 3 minutes. The time from when the temperature of the polymerization mixture reached its maximum to when the hydrogel polymer began to be introduced into the kneading chamber of the meat chopper was 4 minutes. The temperature of the hydrogel polymer was 49°C just before it was introduced into the kneading chamber of the meat chopper. The obtained coarsely ground product was dried, pulverized and classified in the same manner as in Example 1 to obtain polymer particles.

[0059] (Comparative Example 1) A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum of 80°C 12 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Within 30 seconds of the thermometer reaching its maximum value, it was confirmed that the thermometer indicated a temperature 0.1°C lower than the maximum value. The stainless steel tray containing the hydrogel polymer was then immersed in a water bath at 80°C. After allowing the hydrogel polymer to stand in the water bath for 26 minutes, it was removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was roughly crushed using a meat chopper in the same manner as in Example 1. The time from when the temperature of the polymerization mixture reached its maximum value to when the hydrogel polymer began to be introduced into the kneading chamber of the meat chopper was 30 minutes. The temperature of the hydrogel polymer immediately before being introduced into the kneading chamber of the meat chopper was 68°C. The obtained coarsely ground product was dried, pulverized and classified in the same manner as in Example 1 to obtain polymer particles.

[0060] (Comparative Example 2) A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum value of 84°C 12 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Within 30 seconds of the thermometer reaching its maximum reading, the thermometer was confirmed to indicate a temperature 0.1°C lower than the maximum reading. The stainless steel tray containing the hydrogel polymer was then immersed in a water bath at 80°C. After allowing the hydrogel polymer to stand in the water bath for 11 minutes, it was removed from the stainless steel tray and immediately cut into pieces approximately 6 cm wide. The cut hydrogel polymer was placed in a polyethylene bag with a zipper (inside zipper size: 280 mm x 200 mm, thickness: 0.04 mm) while minimizing air inclusion. The polyethylene bag containing the hydrogel polymer was immersed in an ice bath at −20°C containing 78 parts by mass of ice and 22 parts by mass of NaCl. After allowing the hydrogel polymer to stand in the ice bath for 3 minutes, it was removed from the polyethylene bag and roughly crushed using a meat chopper in the same manner as in Example 1. The temperature of the ice bath was −12.5°C after the hydrogel polymer had stood for 3 minutes. The time from when the temperature of the polymerization mixture reached its maximum to when the hydrogel polymer started to be fed into the kneading chamber of the meat chopper was 15 minutes. The temperature of the hydrogel polymer just before being fed into the kneading chamber of the meat chopper was 20°C. The obtained coarsely ground product was dried, pulverized and classified in the same manner as in Example 1 to obtain polymer particles.

[0061] (Reference example 1) A hydrogel polymer mass was formed in the stainless steel tray by polymerization in the mixture in the same manner as in Example 1. The temperature of the mixture reached a maximum of 80°C 13 minutes after the completion of the dropwise addition of the L-ascorbic acid aqueous solution. Thereafter, polymer particles were obtained by the same procedure as in Example 1, except that the plate attached to the end of the kneading chamber of the meat chopper was changed to one having a circular discharge hole with a diameter of 9.6 mm. The crushed material formed in this process was an aggregate formed of multiple elongated structures with a width of 8 mm. The time from when the temperature of the polymerization reaction mixture reached its maximum value to when the hydrogel polymer began to be introduced into the kneading chamber of the meat chopper was 4 minutes. The temperature of the hydrogel polymer immediately before being introduced into the kneading chamber of the meat chopper was 68°C.

[0062] (Reference example 2) A hydrogel polymer mass was formed in a stainless steel tray by polymerization in the mixture in the same manner as in Example 1. 13 minutes after the end of the dropwise addition of the L-ascorbic acid aqueous solution, the temperature of the mixture reached a maximum of 83°C. The temperature of the hydrogel polymer immediately before being charged into the kneading chamber of the meat chopper was 68°C. Thereafter, polymer particles were obtained by the same procedure as in Example 3, except that the plate attached to the end of the kneading chamber of the meat chopper was changed to one having a circular discharge hole with a diameter of 9.6 mm. The time from when the temperature of the polymerization reaction mixture reached its maximum value to when the hydrogel polymer began to be fed into the kneading chamber of the meat chopper was 15 minutes. The temperature of the hydrogel polymer immediately before being fed into the kneading chamber of the meat chopper was 68°C.

[0063] (Reference example 3) A hydrogel polymer mass was formed in a stainless steel tray by polymerization in the mixture in the same manner as in Example 1. 13 minutes after the end of the dropwise addition of the L-ascorbic acid aqueous solution, the temperature of the mixture reached a maximum value of 82°C. The temperature of the hydrogel polymer immediately before being introduced into the kneading chamber of the meat chopper was 69°C. Thereafter, polymer particles were obtained by the same procedure as in Comparative Example 1, except that the plate attached to the end of the kneading chamber of the meat chopper was changed to one having a circular discharge hole with a diameter of 9.6 mm. The time from when the temperature of the polymerization reaction mixture reached its maximum value to when the hydrogel polymer began to be fed into the kneading chamber of the meat chopper was 30 minutes. The temperature of the hydrogel polymer immediately before being fed into the meat chopper was 69°C.

[0064] 2. Evaluation of water-absorbent resin particles 2-1.CRC The CRC of the water-absorbent resin particles was measured in an environment of a temperature of 25°C ± 2°C and a humidity of 50% ± 10% by the following procedure in accordance with the EDANA method (NWSP 241.0.R2(15), pages 769 to 778). A nonwoven fabric (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) measuring 60 mm x 170 mm was folded in half lengthwise to reduce the size to 60 mm x 85 mm. A 60 mm x 85 mm nonwoven fabric bag was produced by heat-sealing the nonwoven fabric together on both sides extending in the lengthwise direction. The width of the sealed portion was 5 mm. 0.2 g of precisely weighed water-absorbent resin particles was placed in the nonwoven fabric bag. Next, the remaining side extending in the short direction of the nonwoven fabric bag was heat-sealed to close the nonwoven fabric bag. Multiple nonwoven bags were floated in 1000 g of saline in a stainless steel tray (240 mm × 320 mm × 45 mm) without overlapping each other, so that the entire nonwoven bag was completely wetted. One minute after placing the nonwoven bags in the saline, the nonwoven bags were immersed in the saline using a spatula. After the nonwoven fabric bag was immersed in the saline solution for 29 minutes, i.e., 30 minutes after the nonwoven fabric bag was placed in the saline solution, the nonwoven fabric bag was removed from the saline solution. The removed nonwoven fabric bag and the gel formed therein were dehydrated using a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). The dehydration time was 3 minutes after the centrifugal force in the centrifuge reached 250 G. After dehydration, the total mass Ma (g) of the gel and nonwoven fabric bag was weighed. A nonwoven fabric bag not containing water-absorbent resin particles was treated in the same manner as above, and then the mass Mb (g) of the nonwoven fabric bag was measured. The CRC was calculated using the following formula. Mc is the precisely weighed value of 0.2 g of the water-absorbent resin particles used in the measurement. CRC[g / g] = {(Ma-Mb)-Mc} / Mc

[0065] 2-2.Soluble content The dissolved polymer particle content was measured under an environment of 25°C ± 2°C and 50% ± 10% humidity. 500 g of saline solution in a 500 mL beaker was stirred using a stirrer (cylindrical, 8 mm diameter x 30 mm length, no ring) rotating at 600 rpm. The saline solution was at 25°C. 2,000 g of water-absorbent resin particles were added, and the dispersion containing the polymer particles was stirred for 3 hours. The dispersion was filtered through a 75 μm standard sieve, and the filtrate was collected. 80 g of the resulting filtrate was weighed and placed in a weighed 100 mL beaker that had been pre-weighed at 140°C. The filtrate in the beaker was heated in a hot air dryer (Advantec, FV-320) at 140°C for 15 hours to remove moisture, and the mass of the remaining solid component, Wa (g), was measured. A blank test was performed using the same procedure as above, but without adding polymer particles to the saline solution, and the mass of the remaining solid component, Wb (g), was measured. The dissolved content was calculated using the following formula. Dissolved content (mass%) = [((Wa-Wb) / 80)×500 / 2]×100

[0066] 3.Results The evaluation results are shown in Table 1. When the width of the elongated structures constituting the crushed material is 7 mm or less as in Examples 1 to 3 and Comparative Example 1, a tendency for the soluble content to decrease as t becomes shorter for similar values ​​of T. On the other hand, when the width of the elongated structures constituting the crushed material exceeds 7 mm as in Reference Examples 1 to 3, no particular correlation was observed between t and the soluble content.

[0067] Taking Comparative Example 1, in which t was 30 minutes and T was 68°C as a standard, polymer particles with a dissolved content lower than the standard dissolved content can be obtained by selecting t as a time of 15 minutes or less, so long as T is within the range of 68±25°C as in Examples 1 to 4. In Comparative Example 2, in which T was 20°C, the dissolved content increased compared to the standard even when t was 15 minutes.

[0068] [Table 1]

Claims

1. A method for producing water-absorbent resin particles containing polymer particles, the method comprising: forming a mixture in a reaction vessel comprising a monomer and water having the monomer dissolved therein; polymerizing the monomer in the mixture to form a hydrogel polymer in the reaction vessel, the hydrogel polymer being the gelled mixture containing a polymer of the monomer; crushing the hydrogel polymer mass removed from the reaction vessel to form a crushed product; forming polymer particles from the crushed material by a process including drying and grinding; Including, The hydrogel polymer in a lump form is crushed to form the crushed material, which is an aggregate formed of a plurality of structures having a shape that can pass through a circular hole with a diameter of 7 mm and a maximum width of 4 mm or more, During polymerization of the monomers, the temperature of the mixture reaches a maximum value T ｍａｘ is the elapsed time from when the temperature reaches t to when the hydrogel polymer starts to be crushed, the temperature of the hydrogel polymer at the time when the hydrogel polymer starts to be crushed is T; t is 15 minutes or less, and is a time selected so that the solubility of the polymer particles obtained from the crushed product in physiological saline at 25°C is smaller than the value when t is 30 minutes and T is X, and X is a temperature within the range of 70±3°C; T is a temperature within the range of X ± 25°C; method.

2. During polymerization of the monomers, the temperature of the mixture reaches a maximum value T ｍａｘ 2. The method according to claim 1, wherein the temperature of the mixture and the hydrogel polymer is maintained at 40° C. or higher from the time when the temperature reaches 40° C. to the time when the hydrogel polymer is started to be crushed.

3. 3. The method according to claim 1 or 2, wherein t is a time selected so that the solubility of the polymer particles obtained from the crushed product in physiological saline at 25°C is 90% or less of the value when t is 30 minutes and T is X.

4. 4. The method according to claim 1, wherein t is 12 minutes or less.

5. The method further comprises dividing the hydrogel polymer mass removed from the reaction vessel into a plurality of masses; the plurality of agglomerates are crushed to obtain the crushed material; The method according to any one of claims 1 to 4.

6. The method according to any one of claims 1 to 5, wherein the monomer comprises at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof.

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