Method for producing water-absorbent resin particles

Abstract

The present invention discloses a method for producing water-absorbing resin particles containing polymer particles. This method includes: a step for forming a hydrous gel-like polymer through a polymerization reaction in a reaction solution; and a step for forming a powder of polymer particles by pulverizing a dried product containing the polymer. When five measurement points are arranged in series on a straight line perpendicular to an extension direction X of a side wall surface, among the highest temperatures at which the temperature of the reaction solution reaches during the polymerization reaction, measured respectively at the five measurement points, at a point of time when the maximum value Tmax is indicated at any one of the five measurement points, the standard deviation of the temperatures of the reaction solution, measured respectively at the five measurement points, is 12-30, and Tmax is 80-100°C.

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 including forming a mass of hydrogel polymer by a polymerization reaction of monomers in a reaction liquid contained in a reaction vessel, and pulverizing a solid dried product containing the polymer obtained by drying the hydrogel polymer (e.g., Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-228604 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-160866 Summary of the Invention [Problem to be solved by the invention]

[0004] The powder obtained by pulverizing a dried polymer-containing material contains fine powder, but in general, fine powder that passes through a sieve with an opening of 180 μm is often difficult to use as a product as it is. Therefore, in order to improve the production yield, it is desirable that the proportion of fine powder in the powder obtained by pulverization is as small as possible.

[0005] One aspect of the present disclosure relates to a method for reducing the proportion of fine powder in powder formed by pulverization when producing water-absorbent resin particles by a method including pulverizing a dried material containing a polymer. [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 according to one aspect of the present disclosure includes the steps of forming a hydrous gel polymer containing water and a polymer by a polymerization reaction in a reaction liquid containing a monomer and water contained in a reaction vessel, removing water from the hydrous gel polymer to obtain a dried product containing a polymer, and pulverizing the dried product to form a powder of polymer particles.

[0007] The reaction vessel has two sidewalls extending in a certain direction and facing each other, and a bottom surface forming a recess together with the two sidewalls. When five measurement points are arranged in series on a line in the liquid surface of the reaction liquid perpendicular to the extending direction of the two sidewalls, the maximum temperature T among the maximum temperatures that the temperature of the reaction liquid reaches during the polymerization reaction at each of the five measurement points is calculated. max At the time when T is indicated at any of the five measurement points, the standard deviation of the temperature of the reaction solution at each of the five measurement points is 12 to 30, and T max The temperature of the reaction liquid at each of the five measurement points is measured on the bottom surface at a position directly below the measurement point. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, in the case of producing water-absorbent resin particles by a method including pulverizing a dried material containing a polymer, it is possible to reduce the proportion of fine powder in powder formed by pulverization. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a schematic top view showing an example of measurement points placed on the liquid surface of a reaction liquid in a process of forming a hydrogel polymer by a polymerization reaction in the reaction liquid. [Figure 2]FIG. 2 is a schematic cross-sectional view showing an example of a measurement point placed on the liquid surface of a reaction liquid in a process of forming a hydrogel polymer by a polymerization reaction in the reaction liquid. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention is not limited to the following examples.

[0011] In this specification, "(meth)acrylic" means both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(Poly)" means 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 a numerical range in another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more.

[0012] One embodiment of a method for producing water-absorbent resin particles includes the steps of: forming a hydrous gel polymer containing water and a polymer by a polymerization reaction in a reaction liquid containing a monomer and water; forming a dried product containing a polymer by removing water from the hydrous gel polymer; and pulverizing the dried product to form a powder of water-absorbent resin particles.

[0013] The polymerization reaction is carried out in a reaction liquid contained in a reaction vessel having a bottom surface and two side wall surfaces extending from the end of the bottom surface under conditions such that the temperature of the reaction liquid measured at five measurement points arranged on the liquid surface of the reaction liquid varies to a certain extent.

[0014] Fig. 1 is a schematic top view showing an example of a measurement point placed in the liquid surface of a reaction liquid containing a monomer and water in a process of forming a hydrogel polymer by a polymerization reaction in the reaction liquid, and Fig. 2 is a cross-sectional view of Fig. 1. Reaction vessel 10 shown in Figs. 1 and 2, which contains reaction liquid 1, has two opposing sidewalls 11 and 12 extending along the same constant direction X, two opposing sidewalls 13 and 14 extending along a direction perpendicular to the direction X of extension of sidewalls 11 and 12, and a bottom surface 15 which, together with these sidewalls, forms a recess for containing reaction liquid 1. Sidewalls 11, 12, 13, and 14 may be perpendicular to bottom surface 15 or may be inclined.

[0015] Five measurement points P1, P2, P3, P4, and P5 are arranged in series in this order on a line 2 on the liquid surface 1S of the reaction liquid 1, perpendicular to the extension direction X of the two opposing side wall surfaces 11 and 12. The five measurement points consist of two measurement points P1 and P5 located 1 cm from each of the two side wall surfaces 11 and 12, and three measurement points P2, P3, and P4 located at positions dividing the space between the two measurement points P1 and P5 into four equal parts. The distance W0 between the side wall surfaces 11 and 12 and the measurement points P1 and P2 is 1 cm. The distance W12 between the measurement points P1 and P2, the distance W23 between the measurement points P2 and P3, the distance W34 between the measurement points P3 and P4, and the distance W45 between the measurement points P4 and P5 are all the same. Measurement point P3 is located in the center between the two side wall surfaces 11 and 12.

[0016] During the polymerization reaction, temperatures T1, T2, T3, T4, and T5 of the reaction liquid at five measurement points P1, P2, P3, P4, and P5, respectively, are measured on the bottom surface 15 at positions P10, P20, P30, P40, and P50 directly below the respective measurement points. If inclined side wall surfaces 11 and 12 are located directly below measurement points P1 and P5 at both ends, temperatures T1 and T5 are measured on the side wall surfaces 11 and 12 directly below measurement points P1 and P5 at both ends. The positions on the bottom surface 15 at which the temperature of the reaction liquid 1 is measured can be within 0.5 cm of the bottom surface 15 (or side wall surfaces 11 and 12) in the depth direction of the reaction liquid 1.

[0017] The temperatures T1, T2, T3, T4, and T5 of the reaction liquid 1 shown at the five measurement points P1, P2, P3, P4, and P5, respectively, rise as the polymerization reaction progresses, reach a maximum temperature, and then drop. The temperatures T1, T2, T3, T4, and T5 of the reaction liquid 1 at each measurement point are the maximum temperature T max At the time when the temperature T1, T2, T3, T4, and T5 of the reaction liquid shown at each of the five measurement points P1, P2, P3, P4, and P5 is indicated, the standard deviation (hereinafter sometimes referred to as "temperature deviation") of the temperature T1, T2, T3, T4, and T5 of the reaction liquid shown at each of the five measurement points P1, P2, P3, P4, and P5 is 12 to 30. For example, if the maximum temperature T3 of the reaction liquid 1 at measurement point P3 reaches during the polymerization reaction is the maximum temperature T1, T2, T3, T4, and T5 of the maximum temperatures T1, T2, T3, T4, and T5 of the reaction liquid 1 at each measurement point during the polymerization reaction, the standard deviation of the temperature T3 of the reaction liquid 1 at measurement point P3 is 12 to 30. max When the temperature T3 of the reaction solution 1 at the measurement point P3 is T max The temperature deviation calculated from temperatures T1, T2, T3, T4 and T5 at the time when the temperature reaches T1 is 12 to 30°C.

[0018] If the temperature deviation of the reaction liquid is relatively large, such as 12 to 30, the uniformity of the polymerization reaction decreases, and the dried product containing the polymer tends to become soft, which is thought to result in suppressing the generation of fine powder during pulverization. From the viewpoint of further reducing the proportion of fine powder in the polymer particle powder, the temperature deviation of the reaction liquid may be 12 or more and 25 or less, 23 or less, or 13 or more and 30 or less, 25 or less, 23 or less, or 20 or less.

[0019] The maximum temperature T among the temperatures T1, T2, T3, T4, and T5 of the reaction liquid 1 reached during the polymerization reaction max is 80 to 100°C. max By setting the temperature to 80°C or higher, the polymerization reaction can be easily carried out until the entire reaction solution 1 is gelled. When the temperature of the reaction solution starts to rise due to the start of the polymerization reaction or when the addition of the cleavage accelerator solution described later is completed, any one of the temperatures T1, T2, T3, T4, and T5 can be set to T maxThe time required to reach this may be, for example, 1 to 20 minutes.

[0020] The width of the liquid surface 1S of the reaction liquid 1 in the direction in which the five measurement points P1, P2, P3, P4, and P5 are arranged (the direction of the line 2) may be 10 to 500 cm, 10 to 300 cm, 10 to 100 cm, or 10 to 80 cm.

[0021] In the case of the reaction vessel 10 shown in Figure 1, the width of the side wall surfaces 11 and 12 is shorter than the width of the side wall surfaces 13 and 14. Five measurement points for determining the temperature deviation may be arranged on a straight line perpendicular to the side wall surfaces 13 and 14. When the reaction vessel has two pairs of opposing side wall surfaces as in the embodiment of Figure 1, the temperature deviation of the reaction liquid measured at the five measurement points arranged between one or both of the two pairs of side wall surfaces can be 12 to 30°C.

[0022] The polymerization reaction may be allowed to proceed while the reaction vessel is moved along the extending direction of the side wall surfaces 11, 12. In this case, at any position in the moving direction of the reaction vessel, the temperature deviation of the reaction liquid at five measurement points arranged in series on a straight line perpendicular to the moving direction of the reaction vessel may be 12 to 30°.

[0023] When the polymerization reaction is initiated by adding a cleavage accelerator liquid containing a cleavage accelerator for initiating the polymerization reaction at a low temperature to the reaction liquid 1 as described below, the line 2 on which the measurement points P1, P2, P3, P4, and P5 are arranged may be a line passing through the position on the liquid surface 1S where the cleavage accelerator liquid is added or in the vicinity thereof. For example, the line 2 may be a line passing through an area within 1 cm from the position on the liquid surface 1S where the cleavage accelerator liquid is added.

[0024] The cleavage accelerator liquid may be poured toward one position within the liquid surface 1S, or may be poured simultaneously into two or more positions within the liquid surface 1S. If the number of positions into which the cleavage accelerator liquid is poured simultaneously is small, the temperature deviation tends to increase, and T maxThe temperature tends to increase. A long reaction vessel or a reaction vessel mounted on an endless belt may be used, and the cleavage accelerator liquid may be added two or more times sequentially while the reaction vessel is moved to change the addition position on the liquid surface of the reaction solution. In this case, for each addition of the cleavage accelerator liquid, five measurement points are placed on a line passing through or near the position where the cleavage accelerator liquid was added, and the polymerization reaction conditions are adjusted so that the temperature deviation of the reaction solution measured at each of the five measurement points is maintained within a range of 12 to 30°C.

[0025] The entire amount of the cleavage accelerator liquid is added over a period of, for example, 5 to 120 seconds to the reaction solution 1. If it takes a long time to add the cleavage accelerator liquid to the reaction solution 1, the temperature deviation of the reaction solution tends to increase.

[0026] The temperature of reaction solution 1 before the addition of the cleavage accelerator solution is T max The temperature is adjusted to 80 to 100° C., and may be, for example, 0 to 40° C. or 10 to 30° C. The amount of dissolved oxygen in the reaction liquid 1 may be, for example, 0.1 ppm or less.

[0027] During the polymerization reaction, the reaction solution 1 may be stirred. If the stirring intensity is low, the standard deviation of the temperature of the reaction solution measured at five measurement points tends to increase. For example, when the reaction solution 1 is stirred using stirrers 3 placed on the bottom surface 15 of the reaction vessel 10, if the rotation speed and / or number of the stirrers 3 is low, the temperature deviation of the reaction solution 1 tends to increase. The rotation speed and number of the stirrers can be adjusted so that the temperature deviation is 12 to 30 rpm depending on the type of stirrer, the amount of the reaction solution 1, etc. For example, the number of stirrers installed in one reaction vessel may be 1 to 3, and the rotation speed of the stirrers may be 50 to 300 rpm.

[0028] The polymerization reaction time is adjusted so that the polymerization reaction proceeds sufficiently to form a hydrogel polymer. After the temperature of the reaction solution begins to decrease, the reaction vessel 10 may be kept at 50 to 80°C to complete the polymerization reaction.

[0029] The reaction vessel 10 is not particularly limited, and may be made of, for example, stainless steel, ceramics, synthetic resin, or steel. The reaction vessel 10 may be placed on a belt conveyor. The depth of the reaction solution 1 in the reaction vessel 10 may be, for example, 10 to 50 mm, 10 to 40 mm, or 15 to 35 mm.

[0030] Before the polymerization reaction, the reaction liquid 1 can be an aqueous monomer solution containing water and a monomer dissolved in water. As the polymerization reaction proceeds, the reaction liquid 1 thickens and subsequently gels to form a hydrogel polymer. Generally, the reaction liquid 1 loses fluidity as the polymerization reaction proceeds, but in this specification, for convenience, the mixture after the reaction liquid 1 has lost fluidity may also be referred to as the reaction liquid.

[0031] The concentration of the monomer in the reaction liquid 1 may be, for example, 20 to 50 mass %, or 20 to 40 mass %, based on the mass of the reaction liquid 1.

[0032] 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 polymer may be a crosslinked polymer. The monomer may be an ethylenically unsaturated monomer.

[0033] 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.

[0034] The reaction liquid 1 may contain a monomer other than the ethylenically unsaturated monomer. The proportion of the ethylenically unsaturated monomer (particularly, (meth)acrylic acid and its salts) may be 70 to 100 mol%, 80 to 100 mol%, or 90 to 100 mol% relative to the total amount of monomers in the reaction liquid. The proportion of (meth)acrylic acid and its salts in the ethylenically unsaturated monomers may be 70 to 100 mol%, 80 to 100 mol%, or 90 to 100 mol%.

[0035] Reaction liquid 1 may further contain an initiator (particularly, a radical polymerization initiator). The initiator may include a persulfate, an azo compound, an organic peroxide, or a combination thereof. The amount of the initiator may be 0.01 to 15 millimoles per mole of the monomer. When two or more initiators are used, the amount of each initiator may be 0.01 to 15 millimoles per mole of the monomer.

[0036] Examples of persulfates include potassium persulfate, ammonium persulfate, and sodium persulfate.

[0037] Examples of azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, Hydrochloride, 2,2'-azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-( 2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide]. From the viewpoint of forming polymer particles exhibiting a large CRC (water absorption capacity without load), the radical polymerization initiator may contain 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.

[0038] Examples of organic peroxides include 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.

[0039] The reaction solution 1 may contain an initiator and a cleavage accelerator. The cleavage accelerator is a compound that lowers the temperature at which the polymerization reaction is initiated by the initiator. When the polymerization reaction is initiated at a low temperature, water-absorbent resin particles having better water absorption performance are more likely to be obtained. The cleavage accelerator may be, for example, a reducing agent, an oxidizing agent, or a combination thereof. A part or all of the cleavage accelerator may be added later to the reaction solution containing the monomer, the initiator, and water, thereby initiating the polymerization reaction. The amount of the cleavage accelerator may be, for example, 0.01 to 1.0 mol per 1 mol of the initiator.

[0040] The reducing agent used as the cleavage promoter may be, for example, at least one compound selected from the group consisting of sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid. The oxidizing agent used as the cleavage promoter may be, for example, at least one compound selected from the group consisting of hydrogen peroxide, sodium perborate, superphosphoric acid, superphosphates, and potassium permanganate.

[0041] When two or more cleavage accelerators are used, they may be added separately to the reaction solution. For example, the polymerization reaction may be initiated by sequentially adding a cleavage accelerator solution containing a reducing agent (e.g., L-ascorbic acid) and a cleavage accelerator solution containing an oxidizing agent (e.g., hydrogen peroxide) to a reaction solution containing a monomer, an initiator, and water. Alternatively, the polymerization reaction may be initiated by adding a cleavage accelerator solution containing a reducing agent (e.g., L-ascorbic acid) to a reaction solution containing a monomer, an initiator, and water. The cleavage accelerator solution may be an aqueous solution containing the cleavage accelerator and water.

[0042] Reaction solution 1 may further contain a chain transfer agent. The chain transfer agent may include, for example, hypophosphorous acid, phosphorous acid, or a combination thereof.

[0043] The reaction solution 1 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.

[0044] 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.

[0045] The hydrogel polymer mass formed by the polymerization reaction is removed from the reaction vessel 10. Prior to the step of removing water from the hydrogel polymer to obtain a dried polymer-containing material, the hydrogel polymer may be crushed to form a crushed material containing structures of a certain size. By forming a crushed material, water can be efficiently removed. The structures constituting the crushed material may be, for example, elongated structures, granular structures (particles), or a combination thereof. The crushed material may contain multiple structures having a shape that can pass through a circular hole with a diameter of 10 mm or 7 mm. The elongated structures may be curved, and as long as their maximum width is 10 mm or less, they can be said to have a shape that can pass through a circular hole with a diameter of 10 mm. The granular structures (particles) may be irregular in shape or may have a shape that can pass through a circular hole with a diameter of 10 mm while changing direction. Examples of crushing devices for crushing the hydrogel polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and Pharmamills.

[0046] Drying the hydrogel polymer or its crushed product removes most of the water from the hydrogel polymer. The moisture content of the dried polymer-containing product obtained by drying may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less, or may be 0% by mass or more. The moisture content of the dried product here refers to the proportion of water in the polymer particles based on the total mass of the dried product containing water. Typically, when the dried product containing water to be measured is heated at 200°C for 2 hours, the difference in mass of the dried product before and after heating can be considered to be the moisture content of the dried product used for measurement. 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 product 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.

[0047] The method for pulverizing the dried polymer-containing material is not particularly limited. For example, the dried polymer 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.

[0048] 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.

[0049] 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.

[0050] 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. The particle size distribution may be adjusted by mixing two or more powders obtained by classification and having different median particle diameters.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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]

[0058] The present invention is not limited to the following examples.

[0059] 1. Crosslinked polymer particles Example 1 Preparation of aqueous monomer solution A 2-L separable flask was charged with 325.9 g (4.52 mol) of acrylic acid. 280.4 g of ion-exchanged water was added to the acrylic acid in the separable flask with stirring. Next, 283.2 g of a 48% by mass aqueous solution of sodium hydroxide was added dropwise in an ice-water bath at approximately 3°C to prepare an aqueous monomer solution containing partially neutralized acrylic acid as a monomer and having a monomer concentration of 45% by mass.

[0060] polymerization reaction 888.10 g of the aqueous monomer solution, 150.04 g of ion-exchanged water, 0.930 g of polyethylene glycol diacrylate (n = approximately 9, internal crosslinking agent, NOF Corporation, Blenmer ADE-400A), and 6.47 g of a 5% by mass aqueous solution of 2,2'-azobis(2-amidinopropane) dihydrochloride (V-50, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were sequentially placed into a 2 L beaker with handles, and the mixture in the beaker was stirred.

[0061] The reaction vessel used was a 5 cm high stainless steel tray coated with fluororesin, with a rectangular bottom measuring 25 cm x 19 cm, four side walls surrounding the bottom, and a rectangular opening measuring 28 cm x 22 cm. The mixture from the beaker was placed in the reaction vessel, and the mixture was stirred with two stir bars (8 mm diameter, 45 mm length, no rings) to form a homogeneous reaction solution. The top of the reaction vessel was then sealed with polyethylene film.

[0062] The width of the reaction solution surface was 26 cm in the longitudinal direction of the reaction vessel, and the depth of the reaction solution was 1.7 cm. Five measurement points were set on the reaction solution surface along a line perpendicular to the longitudinal direction of the reaction vessel: two points P1 and P5, located 1 cm from the two opposing side walls in the longitudinal direction of the reaction vessel, and three points P2, P3, and P4, dividing the space between these two points into four equal parts. At each of measurement points P1, P2, P3, P4, and P5, a thermometer (CT-320WP, Custom Co., Ltd.) was inserted vertically into the reaction solution so that it reached the bottom of the reaction vessel. The five measurement points were aligned linearly along the longitudinal direction of the reaction vessel, at the center of the shorter side of the vessel. Two stir bars were installed between measurement points P1 and P2 and between measurement points P4 and P5.

[0063] The temperature of the reaction solution in the reaction vessel was adjusted to 25°C, and the dissolved oxygen content in the reaction solution was adjusted to 0.1 ppm or less by nitrogen substitution. Next, 3.40 g of a 0.5% by mass L-ascorbic acid aqueous solution was added to the reaction solution using a syringe (10 mL disposable syringe, manufactured by Terumo Corporation, syringe needle, manufactured by Terumo Corporation) and the reaction solution was thoroughly stirred. While the reaction solution was stirred at 100 rpm, 3.70 g of a 0.35% by mass hydrogen peroxide aqueous solution was added dropwise over 4 seconds using a syringe (10 mL disposable syringe, manufactured by Terumo Corporation, syringe needle, manufactured by Terumo Corporation) toward the central measurement point P3.

[0064] The polymerization reaction began simultaneously with the addition of the hydrogen peroxide solution. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled, forming a hydrogel polymer containing water and crosslinked polymers. The temperatures indicated by the five thermometers each reached a maximum temperature and then decreased as the polymerization reaction progressed. Three minutes after the completion of the hydrogen peroxide solution addition, the thermometer at measurement point P3 recorded 99°C, the highest temperature among the five thermometers measured during the polymerization reaction. The temperatures indicated by the five thermometers at the time when the thermometer at measurement point P3 reached a maximum temperature of 99°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated. Six minutes after the completion of the hydrogen peroxide solution addition, the reaction vessel containing the hydrogel polymer was immersed in a 75°C water bath, and the hydrogel polymer was kept in this state for 20 minutes to complete the polymerization reaction.

[0065] Crushing and drying The hydrogel polymer mass removed from the reaction vessel was cut into 5 cm width pieces. The cut hydrogel polymer was sequentially placed into a meat chopper (Kire Royal Co., Ltd., model number: 12VR-750SDX) and roughly crushed at room temperature. The diameter of the circular discharge holes in the plate located at the outlet of the meat chopper was 6.4 mm. The roughly crushed hydrogel polymer was spread on a wire mesh with 0.8 cm x 0.8 cm openings and dried by heating at 180°C for 30 minutes using a hot air dryer (ADVANTEC, FV-320) to obtain a dried product containing a crosslinked polymer.

[0066] crushing The dried product was pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter: 1 mm, 6000 rpm) to obtain a powder of polymer particles.

[0067] Example 2 A powder of polymer particles was obtained using the same procedure as in Example 1, except that the rotation speed of the stirrer during the dropwise addition of the 0.35% by mass aqueous hydrogen peroxide solution was changed to 300 rpm, and 3.70 g of the 0.35% by mass aqueous hydrogen peroxide solution was added dropwise over 63 seconds. Three minutes after the end of the dropwise addition of the aqueous hydrogen peroxide solution, the thermometer at measurement point P2 indicated a maximum temperature of 80°C, which was the highest of the maximum temperatures indicated by each of the five thermometers during the polymerization reaction. The temperatures indicated by the five thermometers at the time when the thermometer at measurement point P2 indicated a maximum temperature of 80°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated.

[0068] Example 3 630.55 g of an aqueous monomer solution prepared in the same manner as in Example 1, 68.30 g of ion-exchanged water, 0.660 g of polyethylene glycol diacrylate (n = approximately 9, internal crosslinking agent, NOF Corporation, Blenmer ADE-400A), 1.44 g of a 2% by mass aqueous potassium persulfate solution, and 4.02 g of a 5% by mass aqueous 2,2'-azobis(2-amidinopropane) dihydrochloride solution (V-50, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were sequentially placed into a 1 L beaker with handles, and the mixture in the beaker was stirred.

[0069] A 3-cm-tall, fluororesin-coated stainless steel tray was used as the reaction vessel. The tray had a rectangular bottom measuring 16 cm x 20 cm, four sidewalls surrounding the bottom, and a rectangular opening measuring 19 cm x 23 cm. The mixture from the beaker was placed in the reaction vessel, and the mixture was stirred with two stir bars (8 mm diameter, 30 mm length, no rings) to form a homogeneous reaction solution. The top of the reaction vessel was then sealed with polyethylene film.

[0070] The width of the reaction solution surface was 18 cm along the short side of the reaction vessel, and the depth of the reaction solution was 1.6 cm. Five measurement points were set on the reaction solution surface along a line perpendicular to the longitudinal direction of the reaction vessel: two points P1 and P5, located 1 cm from the two opposing side walls along the short side of the reaction vessel, and three points P2, P3, and P4, dividing the space between these two points into four equal parts. At each of measurement points P1, P2, P3, P4, and P5, a thermometer (CT-320WP, Custom Co., Ltd.) was inserted vertically into the reaction solution so that it reached the bottom of the reaction vessel. The five measurement points were aligned linearly along the short side of the reaction vessel, at the center of the short side of the reaction vessel. Two stir bars were installed between measurement points P1 and P2 and between measurement points P4 and P5.

[0071] The temperature of the reaction solution in the reaction vessel was adjusted to 25°C, and the dissolved oxygen content in the reaction solution was adjusted to 0.1 ppm or less by nitrogen substitution. Next, while stirring at 100 rpm, 2.41 g of a 0.5% by mass L-ascorbic acid aqueous solution and 2.62 g of a 0.35% by mass hydrogen peroxide aqueous solution were sequentially added dropwise over 3 seconds using a syringe (10 mL disposable syringe, manufactured by Terumo Corporation, with a syringe needle manufactured by Terumo Corporation) toward the central measurement point P3.

[0072] The polymerization reaction began simultaneously with the addition of the hydrogen peroxide solution. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled, forming a hydrogel polymer containing water and crosslinked polymers. Five minutes after the completion of the addition of the hydrogen peroxide solution, the temperature at measurement point P3 reached 95°C, the highest temperature among the five thermometers measured during the polymerization reaction. The temperatures measured by the five thermometers at the time when the thermometer at measurement point P3 reached a maximum temperature of 95°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated. Eight minutes after the completion of the addition of the hydrogen peroxide solution, the reaction vessel containing the hydrogel polymer was immersed in a 75°C water bath, and the hydrogel polymer was kept in this state for 20 minutes to complete the polymerization reaction.

[0073] The formed hydrogel polymer was crushed, dried and pulverized in the same manner as in Example 1 to obtain a powder of polymer particles.

[0074] (Comparative Example 1) Polymer particles were obtained in the same manner as in Example 1, except that the rotation speed of the stirrer during the dropwise addition of the 0.35% by mass aqueous hydrogen peroxide solution was changed to 300 rpm. Three minutes after the end of the dropwise addition of the aqueous hydrogen peroxide solution, the thermometer at measurement point P3 showed a maximum temperature of 98°C, the highest of the maximum temperatures shown by each of the five thermometers during the polymerization reaction. The temperatures shown by the five thermometers at the time when the thermometer at measurement point P3 showed a maximum temperature of 98°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated.

[0075] (Comparative Example 2) Polymer particles were obtained in the same manner as in Example 1, except that the rotation speed of the stirrer during the dropwise addition of the 0.35% by mass aqueous hydrogen peroxide solution was changed to 500 rpm. Two minutes after the end of the dropwise addition of the aqueous hydrogen peroxide solution, the thermometer at measurement point P4 showed a maximum temperature of 94°C, the highest of the maximum temperatures shown by each of the five thermometers during the polymerization reaction. The temperatures shown by the five thermometers at the time when the thermometer at measurement point P4 showed a maximum temperature of 94°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated.

[0076] (Comparative Example 3) The polymerization reaction was carried out using the same procedure as in Example 1, except that the method of dripping the 0.35% by mass aqueous hydrogen peroxide solution was changed to a method in which five syringes (10 mL disposable syringes manufactured by Terumo Corporation, syringe needles manufactured by Terumo Corporation) were used to drip 0.74 g of the 0.35% by mass aqueous hydrogen peroxide solution simultaneously over one second toward the vicinity of each of the five measurement points. Three minutes after the end of dripping of the hydrogen peroxide solution, the thermometer at measurement point P5 indicated a maximum temperature of 63°C, the highest of the five maximum temperatures indicated by each of the five thermometers during the polymerization reaction. The temperatures indicated by the five thermometers at the time when the thermometer at measurement point P5 indicated a maximum temperature of 63°C were recorded, and the standard deviation (temperature deviation) of these five temperatures was calculated. Because a portion of the reaction solution did not gel and remained liquid, a block of hydrogel polymer could not be obtained.

[0077] 2. Particle size ratio of 180 to 850 μm JIS standard sieves with mesh sizes of 850 μm, 500 μm, 425 μm, 300 μm, 250 μm, 180 μm, and 106 μm were stacked on a tray, in this order from top to bottom. 10 g of crosslinked polymer particle powder was placed on the top 850 μm mesh sieve, and the powder was classified using a continuous, fully automatic ultrasonic vibration sieving measuring device (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.). After classification, the mass of the polymer particles remaining on each sieve was measured, and the particle size ratio [%] of 180 to 850 μm was calculated using the following formula: Particle size ratio [%] of 180 to 850 μm = (total mass [g] of crosslinked polymer particles remaining on sieves with openings of 500 μm, 425 μm, 300 μm, 250 μm, and 180 μm) / (mass [g] of crosslinked polymer particles used in measurement) × 100

[0078] Table 1 shows the particle size distribution and the particle size ratio calculated from it. In the table, "850 μm on" means the ratio of the crosslinked polymer particles remaining on the 850 μm mesh to the mass of the crosslinked polymer particles used in the measurement. This also applies to "500 μm on," etc. "Pass" means the ratio of the crosslinked polymer particles that passed through the 106 μm mesh sieve to the mass of the crosslinked polymer particles used in the measurement.

[0079] [Table 1] [Explanation of symbols]

[0080] 1...reaction liquid, 10...reaction vessel, 1S...liquid surface, 11, 12, 13, 14...side wall surface, 15...bottom surface, P1, P2, P3, P4, P5...measurement points, P10, P20, P30, P40, P50...positions where the temperature of the reaction liquid is measured, X...extension direction of the side wall surface.

Claims

1. A method for producing water-absorbent resin particles containing polymer particles, the method comprising: forming a hydrogel polymer containing water and a polymer by a polymerization reaction in a reaction solution containing a monomer and water contained in a reaction vessel; removing water from the hydrous gel polymer to form a dried product containing the polymer; pulverizing the dried product to form a powder of polymer particles; Including, the monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, the reaction vessel has two side wall surfaces extending in a certain direction and facing each other, and a bottom surface forming a recess together with the two side wall surfaces; When five measurement points are arranged in series on a straight line in the liquid surface of the reaction liquid perpendicular to the extending direction of the two side wall surfaces, the maximum temperature T among the maximum temperatures that the temperature of the reaction liquid reaches during the polymerization reaction at each of the five measurement points is ｍａｘ At the time when T is indicated at any of the five measurement points, the standard deviation of the temperature of the reaction solution at each of the five measurement points is 12 to 30, and T ｍａｘ is 80 to 100°C, The five measurement points are composed of two measurement points at positions 1 cm from each of the two side wall surfaces and three measurement points at positions dividing the space between the two measurement points into four equal parts, The temperature of the reaction liquid at each of the five measurement points is measured on the bottom surface at a position directly below each measurement point.

2. 2. The method according to claim 1, wherein the width of the liquid surface of the reaction liquid in the direction in which the five measurement points are arranged is 10 to 500 cm.

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