Method for producing superabsorbent polymer particles

Abstract

The present invention relates to a method for producing water absorbent resin particles, the method comprising a step wherein a polymer is crosslinked by means of a surface crosslinking agent by having the polymer and the surface crosslinking agent react with each other in a mixture of a powder of polymer particles containing the polymer and a crosslinking agent solution containing the surface crosslinking agent and water. With respect to this method for producing water absorbent resin particles: the surface crosslinking agent contains an alkylene carbonate; the amount (mass) of the surface crosslinking agent used therefor is 0.00110 or less relative to a mass of 1 of the total solid content of the polymer particles; and the content (mass) of the alkylene carbonate is 0.009 to 0.027 relative to a mass of 1 of water in the crosslinking agent solution.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses a method for producing surface-crosslinked water-absorbent resin particles that can be used in absorbent articles such as sanitary products.

Prior Art Documents

Patent Documents

[0003] [[ID=2D]]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the production of water-absorbent resin particles containing polymer particles, although surface crosslinking of the polymer particles improves the absorption performance of the water-absorbent resin particles under pressure, the method by surface crosslinking is known to cause a decrease in the centrifugal retention capacity (CRC), which is the basic water absorption performance of the water-absorbent resin particles.

[0005] On the other hand, absorbent articles are required to have a high absorption amount and little backflow of the absorbed liquid. Generally, water-absorbent resin particles having excellent performance in these aspects are demanded.

[0006] One aspect of the present invention is to provide a method for producing water-absorbent resin particles that can generally exhibit suppression of a decrease in centrifugal retention capacity and improvement of absorption performance under pressure of the water-absorbent resin particles, that is, water-absorbent resin particles excellent in the sum of centrifugal retention capacity and absorption performance under pressure.

Means for Solving the Problems

[0007] One aspect of the present invention relates to a method for producing superabsorbent resin particles containing polymer particles. The method includes a step of crosslinking the polymer with the surface crosslinking agent by reacting the polymer with the surface crosslinking agent in a mixture of polymer particle powder containing the polymer and a crosslinking agent solution containing a surface crosslinking agent and water, wherein the surface crosslinking agent contains alkylene carbonate, the amount (mass) of the surface crosslinking agent used is 0.00110 or less per unit mass of the total solid content of the polymer particles, and the content (mass) of the alkylene carbonate per unit mass of water in the crosslinking agent solution is 0.009 or more and 0.027 or less. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a method for producing water-absorbent resin particles that can generally suppress the decrease in centrifugal retention capacity and improve the absorption performance of water-absorbent resin particles under pressure, that is, water-absorbent resin particles that are superior in the sum of centrifugal retention capacity and absorption performance under pressure. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view showing one embodiment of an absorbent article. [Figure 2] This is a schematic diagram showing a device for measuring water absorption under load. [Modes for carrying out the invention]

[0010] Several embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0011] In this specification, "(meth)acrylic" means both acrylic and methacrylic. Similarly, "acrylate" and "methacrylate" are written as "(meth)acrylate." The same applies to other similar terms. "(poly)" means both with and without the prefix "poly." In the numerical ranges described stepwise in this specification, the upper or lower limit of one step in the numerical range can be arbitrarily combined with the upper or lower limit of another step in the numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with the values ​​shown in the examples. "Water-soluble" means solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used individually or in combination of two or more. "Physiological saline" means a 0.9% by mass aqueous solution of sodium chloride. "Room temperature" means 25°C.

[0012] One embodiment of a method for producing superabsorbent polymer particles includes a step of crosslinking the polymer with a surface crosslinking agent by reacting the polymer with a surface crosslinking agent in a mixture of polymer particle powder containing the polymer and a crosslinking agent solution containing a surface crosslinking agent and water.

[0013] The polymer forming the polymer particles only needs to be capable of imparting water absorption to the polymer particles. The monomers constituting the polymer may be ethylenically unsaturated monomers. The polymer may also be a crosslinked polymer.

[0014] The ethylenically unsaturated monomers constituting the polymer may include, for example, at least one compound selected from the group consisting of (meth)acrylic acid and its salts, 2-(meth)acrylamide-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. If the ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The ethylenically unsaturated monomers constituting the polymer may include 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.

[0015] The polymer forming the polymer particles may have functional groups that react with the crosslinking agent. These functional groups may be, for example, carboxyl groups, amino groups, or combinations thereof. The carboxyl groups may be, for example, groups derived from acrylic acid, methacrylic acid, or salts thereof.

[0016] The polymer may contain monomer units derived from monomers other than ethylenically unsaturated monomers. The proportion of monomer units derived from ethylenically unsaturated monomers (particularly (meth)acrylic acid and its salts) in the polymer may be 70 to 100 mol% of the total amount of monomers.

[0017] Polymer particles can be obtained by polymerization methods selected from, for example, reverse-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization. Polymer particles containing crosslinked polymers (crosslinked polymer particles) can be obtained by self-crosslinking during polymerization, reaction with an internal crosslinking agent, or a combination thereof.

[0018] One example of a method for obtaining polymer particles by aqueous solution polymerization includes polymerizing an ethylenically unsaturated monomer in an aqueous monomer aqueous solution containing an ethylenically unsaturated monomer, a radical polymerization initiator, and water to form a bulk, water-containing gel polymer containing the polymer and water; roughly crushing the water-containing gel polymer to form a crushed product; drying the crushed product to obtain a dried product; and grinding the dried product to obtain polymer particles. The polymer particles powder after grinding may be classified by sieving or the like.

[0019] The radical polymerization initiator may include persulfates, azo compounds, peroxides, or combinations thereof. Using azo compounds as radical polymerization initiators tends to easily yield polymer particles exhibiting a high CRC (Critical Reduction Coefficient). Azo compounds may also be combined with peroxides.

[0020] Examples of azo compounds used as radical polymerization initiators 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-phenylamidinopropane] dihydrochloride, 2,2'-azobis[2-(N-allylamidinopropane] 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). From the perspective of forming polymer particles with a large CRC, 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.

[0021] Examples of persulfates used as radical polymerization initiators include potassium persulfate, ammonium persulfate, and sodium persulfate.

[0022] Examples of peroxides used as radical polymerization initiators 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 peroxy pivalate, and hydrogen peroxide.

[0023] When the amount of the radical polymerization initiator in the aqueous monomer solution is small, the CRC of the polymer particles tends to increase. From this perspective, the amount of the radical polymerization initiator may be 0.01 to 15 millimoles per 1 mole of the monomer units constituting the polymer in the polymer particles.

[0024] The aqueous monomer solution may further contain a chain transfer agent. The use of the chain transfer agent can also contribute to an increase in the CRC of the polymer particles. The chain transfer agent may contain, for example, hypophosphorous acid, phosphorous acid, or a combination thereof.

[0025] The aqueous monomer solution may contain an internal crosslinking agent. In that case, polymer particles containing a crosslinked polymer crosslinked by the internal crosslinking agent can be obtained. The internal crosslinking agent may be a compound having two or more reactive functional groups (for example, polymerizable unsaturated groups).

[0026] The internal crosslinking agent may contain a compound having a (meth)acrylic group, an allyl group, an epoxy group, or an amino group. When a compound having these reactive functional groups is used as the internal crosslinking agent, polymer particles showing a large CRC tend to be easily obtained. Examples of the compound having a (meth)acrylic group include (poly)ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and N,N'-methylenebis(meth)acrylamide. An example of the compound having an allyl group is triallylamine. Examples of the compound having an epoxy group include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and epichlorohydrin. Examples of the compound having an amino group include triethylenetetramine, ethylenediamine, and hexamethylenediamine.

[0027] When the amount of the internal crosslinking agent in the aqueous monomer solution is small, the CRC of the polymer particles tends to increase. From this perspective, the amount of the internal crosslinking agent may be 0.02 to 0.4 millimoles per 1 mole of the ethylenically unsaturated monomer.

[0028] The coarse material obtained by coarse grinding of a water-containing gel polymer may be particulate, or it may be elongated in shape with multiple particles linked together. The minimum width of the coarse material may be, for example, 0.1 to 15 mm, or 1.0 to 10 mm. The maximum width of the coarse material may be 0.1 to 200 mm, or 1.0 to 150 mm. Examples of equipment for coarse grinding include kneaders (e.g., pressurized kneaders, double-arm kneaders, etc.), meat choppers, cutter mills, and pharmacills.

[0029] Drying of the crushed material removes most of the water from it. The drying method may be a general method such as natural drying, heat drying, spray drying, freeze drying, or a combination thereof. The crushed material may be dried under atmospheric pressure or reduced pressure. The heating temperature for drying under atmospheric pressure may be 70-250°C or 80-200°C.

[0030] The water content of the dried product obtained by drying (or polymer particles before mixing with the crosslinking agent solution) may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less, and may be between 1% and 10% by mass. Here, the water content of the dried product refers to the ratio of water content in the polymer particles relative to the total mass of the dried product containing water. Typically, when a dried product containing water is heated at 200°C for 2 hours, the difference in mass of the dried product before and after heating can be considered as the water content in the dried product.

[0031] The method for grinding the dried material is not particularly limited. For example, the dried material can be ground using grinders such as centrifugal grinders, roller mills, stamp mills, jet mills, high-speed rotary grinders, and container-driven mills.

[0032] The polymer particle powder obtained by grinding may be classified. Classification means the operation of dividing a group of particles (powder) into two or more particle groups with different particle size distributions. A portion of the polymer particle powder after classification may be ground and classified again.

[0033] The classification method is not particularly limited, but may include, for example, screen classification or wind classification. Screen classification is a method of classifying particles on a screen into particles that pass through the mesh of the screen and particles that do not pass through the mesh by vibrating the screen. Screen classification can be performed using, for example, a vibrating screen, a rotary shifter, a cylindrical stirred screen, a blower shifter, or a rotary shaker. Wind classification is a method of classifying particles using the flow of air.

[0034] The median particle size of the polymer particles obtained by grinding and, if necessary, classification, before being mixed with the crosslinking agent solution may be, for example, 180 to 850 μm.

[0035] The centrifugal retention capacity (CRC) of the polymer particles before mixing with the crosslinking agent solution may be, for example, 40 to 65 g / g, preferably 40 to 62 g / g, and more preferably 40 to 58 g / g, from the viewpoint of increasing the strength of the polymer particles and ensuring that their shape is easily maintained in the manufacturing process after the polymerization step.

[0036] In this specification, CRC is a value measured under conditions of 25±2°C and 50±10% humidity. The method for measuring CRC is as follows: a nonwoven fabric bag having a rectangular main surface of 60 mm × 85 mm and containing polymer particles (or superabsorbent resin particles) of mass Mc (g) is immersed in physiological saline at 25±2°C for 30 minutes to swell the polymer particles and form a gel; the gel in the nonwoven fabric bag is dehydrated by applying a centrifugal force of 250 G for 3 minutes using a centrifuge; the mass Ma (g) of the gel and nonwoven fabric bag after dehydration is measured; and the nonwoven fabric bag without polymer particles is immersed in physiological saline for 30 minutes and then dehydrated by applying a centrifugal force of 250 G for 3 minutes using a centrifuge, and the mass Mb (g) of the nonwoven fabric bag after dehydration is measured; and the following formula: CRC[g / g] = {(Ma-Mb)-Mc} / Mc This includes calculating the CRC. Mc is 0.2 ± 0.002 g. While the nonwoven fabric bag containing the polymer particles is immersed in physiological saline, the physiological saline may be stirred as necessary to ensure that the polymer particles absorb water to their limit. The method for measuring CRC will be described in more detail in the examples below.

[0037] By reacting the polymer with a surface crosslinking agent in a mixture of polymer particle powder and a crosslinking agent solution, the polymer near the surface of the polymer particles is primarily crosslinked by the surface crosslinking agent. It is preferable to apply the crosslinking agent solution as evenly as possible to the entire surface of the polymer particles. This makes it easier to obtain water-absorbing resin particles with uniform surface crosslinking near the surface.

[0038] The crosslinking agent solution may be a solution containing water and a surface crosslinking agent dissolved in water. The surface crosslinking agent contains alkylene carbonate. The alkylene carbonate may be, for example, at least one compound selected from the group consisting of 1,3-dioxolan-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one (propylene carbonate), 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one (glycerin carbonate), 1,3-dioxan-2-one (trimethylene carbonate), 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxepan-2-one, and may also be ethylene carbonate.

[0039] The amount (concentration) of alkylene carbonate in the crosslinking agent solution may be, for example, 0.1% by mass or more, 0.5% by mass or more, or 0.7% by mass or more, based on the total amount of the crosslinking agent solution, and may be 5.0% by mass or less, 3.0% by mass or less, 2.0% by mass or less, 1.6% by mass or less, or 1.4% by mass or less.

[0040] The surface crosslinking agent contained in the crosslinking agent solution may consist substantially of alkylene carbonate, or it may contain other surface crosslinking agents (other surface crosslinking agents). The ratio of alkylene carbonate in the surface crosslinking agent may be 50-100% by mass, 60-100% by mass, 70-100% by mass, 80-100% by mass, or 90-100% by mass, based on the total mass of the surface crosslinking agent.

[0041] Other examples of surface crosslinking agents include 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, epibromhydr Examples include phosphorus and halo-epoxy compounds such as 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 other surface crosslinking agents may be used individually or in combination of two or more.

[0042] The amount of surface crosslinking agent used (total amount used) is 0.00110 or less per unit mass of the total solid content of the polymer particles, and may be 0.00108 or less, 0.00106 or less, 0.00100 or less, 0.00090 or less, 0.00080 or less, 0.00070 or less, 0.00060 or less, or 0.00055 or less. From the viewpoint of achieving a better sum of centrifugal holding capacity and absorption performance under pressure, the mass ratio of the content of surface crosslinking agent in the crosslinking agent solution to the total solid content of the polymer particles may be 0.00030 or more, 0.00035 or more, 0.00040 or more, 0.00045 or more, or 0.00050 or more. From a similar viewpoint, the mass ratio of the content of the surface crosslinking agent in the crosslinking agent solution to the total mass of solids of the polymer particles may be, for example, 0.00030 to 0.00110, 0.00035 to 0.00110, 0.00040 to 0.00110, 0.00045 to 0.00110, or 0.00050 to 0.00110. In this specification, "total mass of solids of polymer particles" means the total mass of polymer particles minus the mass of water contained in the polymer particles.

[0043] The amount of alkylene carbonate (by mass) per unit mass of water in the crosslinking agent solution is between 0.009 and 0.027. The lower limit may be, for example, 0.011 or more, 0.013 or more, or 0.015 or more. The upper limit may be, for example, 0.025 or less, 0.023 or less, 0.021 or less, 0.019 or less, or 0.017 or less. The amount of alkylene carbonate per unit mass of water in the crosslinking agent solution is as follows: 0.011 to 0.027, 0.013 to 0.027, 0.015 to 0.027, 0.009 to 0.025, 0.011 to 0.025, 0.013 to 0.025, 0.015 to 0.025, 0.009 to 0.023, 0.011 to 0.023, 0.013 to 0.023, 0.0 It may be 15 or more and 0.023 or less, 0.009 or more and 0.021 or less, 0.011 or more and 0.021 or less, 0.013 or more and 0.021 or less, 0.015 or more and 0.021 or less, 0.009 or more and 0.019 or less, 0.011 or more and 0.019 or less, 0.013 or more and 0.019 or less, 0.015 or more and 0.019 or less, 0.009 or more and 0.017 or less, 0.011 or more and 0.017 or less, 0.013 or more and 0.017 or less, or 0.015 or more and 0.017 or less.

[0044] The crosslinking agent solution may contain substantially only a surface crosslinking agent and water, or it may further contain components other than the surface crosslinking agent and water. For example, the crosslinking agent solution may further contain a monohydric alcohol. When the crosslinking agent solution contains a monohydric alcohol, the absorption rate of the surface crosslinking agent to the polymer particles is suppressed, making it easier for the surface crosslinking agent to come into uniform contact with the entire particle and also with the entire surface of each particle. As a result, when the crosslinking agent solution contains a monohydric alcohol, the absorption performance of the water-absorbing resin particles (e.g., absorption performance under pressure) is further improved. Examples of monohydric alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. The monohydric alcohol may be 2-propanol. If the crosslinking agent solution contains a monohydric alcohol, the monohydric alcohol content (by mass) may be, for example, 0.05000 or less, 0.03000 or less, or 0.01000 or less, or 0.00100 or more, 0.00300 or more, or 0.00500 or more, relative to 1 unit of the total mass of the polymer particles.

[0045] The water content (by mass) in the crosslinking agent solution may be 0.02000 or more, 0.03000 or more, 0.04000 or more, 0.05000 or more, 0.06000 or more, 0.07000 or more, 0.08000 or more, or 0.09000 or more, per unit mass of the total solid content of the polymer particles. The water content (by mass) in the crosslinking agent solution may be 0.20000 or less, 0.15000 or less, 0.12000 or less, 0.10000 or less, 0.08000 or less, 0.06000 or less, or 0.04000 or less, per unit mass of the total solid content of the polymer particles.

[0046] In the crosslinking process described above, by adjusting the amount of surface crosslinking agent used and the ratio of the mass of alkylene carbonate to the mass of water in the crosslinking agent solution within the above range, it is possible to produce superabsorbent polymer particles with excellent sum of centrifugal retention capacity and absorption performance under pressure. The mechanism by which such effects are achieved is not particularly limited, but the following mechanism is considered possible. By adjusting the amount of surface crosslinking agent used and the ratio of the mass of alkylene carbonate to the mass of water in the crosslinking agent solution within the above range, the thickness of the surface crosslinking layer containing the surface-crosslinked polymer particles in the polymer particles is controlled to an appropriate range. It is presumed that this is how the above-mentioned effects are achieved.

[0047] The method for mixing the crosslinking agent solution with polymer particles may involve dropping the crosslinking agent solution onto the polymer particles or spraying it. From the viewpoint of ensuring even adhesion across the entire surface, spraying the crosslinking agent solution onto the polymer particles is preferable.

[0048] The polymer and surface crosslinking agent contained in the polymer particles can be reacted by heating. The heating temperature and heating time for surface crosslinking are adjusted to ensure that the crosslinking reaction proceeds appropriately, taking into consideration the type of surface crosslinking agent, etc. For example, the heating temperature for surface crosslinking may be 80°C or higher, 100°C or higher, 120°C or higher, 150°C or higher, or even exceed 180°C, or even 190°C or higher. The heating temperature for surface crosslinking may be 250°C or lower. The heating time for surface crosslinking may be, for example, 5 to 90 minutes.

[0049] Surface-crosslinked polymer particles may be further dried or classified as needed. Inorganic particles may be attached to the surface of the polymer particles. Examples of inorganic particles include silica particles such as amorphous silica. Surface-crosslinked polymer particles should not be further pulverized. If surface-crosslinked polymer particles are not further pulverized, it becomes easier to obtain superabsorbent resin particles with uniform surface crosslinking near the surface (superabsorbent resin particles that do not have fracture surfaces that are not surface-crosslinked).

[0050] According to the method of this embodiment, it is possible to easily obtain water-absorbent polymer particles (surface-crosslinked polymer particles) in which the sum of the amount of physiological saline absorbed (AUL 0.6 psi) under a 4.14 kPa load (CRC + AUL 0.6 psi) of CRC and the absorption performance of the water-absorbent polymer particles under pressure is large. Furthermore, as an embodiment of the present invention, a method is provided in which the polymer is crosslinked by a surface crosslinking agent by reacting the polymer with a surface crosslinking agent in a mixture of a polymer particle powder containing the polymer and a crosslinking agent solution containing a surface crosslinking agent and water, wherein the surface crosslinking agent contains alkylene carbonate, the amount (mass) of the surface crosslinking agent used is 0.00110 or less per 1 unit of the total mass of the solid content of the polymer particles, and the amount (mass) of the alkylene carbonate used is 0.009 or more and 0.027 or less per 1 unit of the mass of water in the crosslinking agent solution, thereby suppressing the decrease in the centrifugal holding capacity of the water-absorbent polymer particles, improving the amount of absorption under pressure, and consequently improving the sum of the centrifugal holding capacity and the absorption performance under pressure.

[0051] The CRC (unit: g / g) + AUL 0.6psi (unit: mL / g) value shown by the water-absorbent resin particles may be, for example, 56 or higher, 58 or higher, 60 or higher, 62 or higher, and 75 or lower, 73 or lower, or 71 or lower. In practical terms, the CRC (unit: g / g) + AUL 0.6psi (unit: mL / g) value shown by the water-absorbent resin particles is preferably in the range of 60 to 75.

[0052] The CRC of the water-absorbent polymer particles may be, for example, 38 g / g or more, 39 g / g or more, 40 g / g or more, 41 g / g or more, or 42 g / g or more, and may be 50 g / g or less, or 45 g / g or less. The method for measuring CRC will be described in detail in the examples below. The CRC of the water-absorbent polymer particles may also be measured for the fraction that has passed through a sieve with a mesh size of 850 μm from the powder of surface-crosslinked polymer particles.

[0053] The AUL 0.6 psi of the water-absorbent resin particles may be, for example, 20 mL / g or more, or 25 mL / g or more, and may be 45 mL / g or less, 35 mL / g or less, or 30 mL / g or less. The method for measuring AUL 0.6 psi will be described in detail in the examples below.

[0054] The manufactured superabsorbent resin particles are used, for example, to form absorbent bodies that make up absorbent articles such as diapers. Figure 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Figure 1 comprises a sheet-like absorbent body 10, core wraps 20a and 20b, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40. In the absorbent article 100, the liquid-impermeable sheet 40, core wrap 20b, absorbent body 10, core wrap 20a, and liquid-permeable sheet 30 are laminated in this order. The absorbent body 10 has superabsorbent resin particles 10a manufactured by the method according to the above embodiment and a fiber layer 10b containing fibrous material. The superabsorbent resin particles 10a are dispersed within the fiber layer 10b. [Examples]

[0055] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[0056] (Manufacturing Example 1) [Preparation of partially neutralized acrylic acid solution] 365.78 (5.08 mol) of acrylic acid was placed in a round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L, equipped with a stirrer. While stirring the acrylic acid, 306.48 g of deionized water was added to the separable flask, and then 319.15 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare 991.41 g of a partially neutralized sodium solution (neutralization rate 75 mol%) of acrylic acid with a monomer concentration of 45% by mass.

[0057] [Polymerization process] 991.41 g of the partially neutralized sodium acrylate solution with a monomer concentration of 45% by mass prepared as described above was mixed with 189.30 g of deionized water and 0.63 g of polyethylene glycol diacrylate (n≒9) (Tokyo Chemical Industries, Ltd., polyethylene glycol diacrylate N=9) to obtain a reaction solution (monomer aqueous solution). Next, this reaction solution was supplied to a 3L stainless steel double-arm kneader with a jacket and two sigma-type blades, equipped with a thermometer and a lid that can be opened and closed, and a nitrogen blowing tube. The reaction solution was then subjected to nitrogen gas purging for 75 minutes while maintaining the temperature at 25°C. Next, while stirring the reaction mixture at 40 rpm, 1.14 g of a 20.00% by mass sodium persulfate aqueous solution and 5.00 g of a 10.00% by mass (2,2'-azobis(2-amidinopropane) dihydrochloride (V-50) aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) were added to the reaction mixture. One minute after the addition, 2.00 g of a 0.50% by mass L-ascorbic acid aqueous solution and 10.39 g of a 0.35% by mass hydrogen peroxide aqueous solution were added to the reaction mixture. Three minutes after the addition of the L-ascorbic acid aqueous solution and hydrogen peroxide aqueous solution, the temperature of the reaction mixture began to rise, and polymerization started. After 14 minutes, the maximum temperature during polymerization reached 37.9°C. Thereafter, stirring was continued while raising the jacket temperature to above 60°C, and the hydrated gel polymer was removed 60 minutes after the start of polymerization.

[0058] [Drying and grinding process] The obtained water-containing gel-like polymer was spread on a wire mesh with a mesh size of 0.8 cm × 0.8 cm and dried by hot air drying at 180°C for 30 minutes to obtain a dried product. The dried product was pulverized using a centrifugal mill (Retsch ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved using a sieve with a mesh size of 850 μm and a sieve with a mesh size of 180 μm by shaking for 1 minute. By classification, the cross-linked polymer particles (A) that passed through the 850 μm sieve but not the 180 μm sieve were recovered. At this time, the water content of the cross-linked polymer particles (A) was 5.0% by mass.

[0059] (Example 1) [Surface crosslinking process] 15 g of crosslinked polymer particles (A) were weighed into a round-bottom cylindrical separable flask with an inner diameter of 11 cm, equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 500 rpm, 0.015 g of ethylene carbonate (hereinafter also referred to as "EC"), 0.600 g of 2-propanol (hereinafter also referred to as "IPA"), and 0.547 g of deionized water were mixed. The entire amount of the resulting crosslinking agent solution was taken with a syringe, a fine atomizer (Oral) [manufactured by Yoshikawa Kasei Co., Ltd.] was attached to the tip of the syringe, and the solution was sprayed into the separable flask. The mixture was stirred for 100 seconds to obtain the mixture. This mixture was heated at 200°C for 40 minutes. After cooling to room temperature, the mixture was classified using a sieve with a mesh size of 850 μm. Classification yielded superabsorbent resin particles that passed through the 850 μm sieve (superabsorbent resin particles of Example 1). The CRC and AUL 0.6psi of the obtained superabsorbent resin particles were measured.

[0060] (Comparative Example 1) The superabsorbent resin particles of Comparative Example 1 were obtained in the same manner as in Example 1, except that the amount of deionized water was changed to 0.096 g.

[0061] (Comparative Example 2) The water-absorbing resin particles of Comparative Example 2 were obtained in the same manner as in Example 1, except that the amount of deionized water was changed to 0.150 g.

[0062] (Example 2) The superabsorbent resin particles of Example 2 were obtained in the same manner as in Example 1, except that the amount of deionized water was changed to 0.891 g.

[0063] (Example 3) The superabsorbent resin particles of Example 3 were obtained in the same manner as in Example 1, except that the amount of deionized water was changed to 1,500 g.

[0064] (Comparative Example 3) The superabsorbent resin particles of Comparative Example 3 were obtained in the same manner as in Example 1, except that the amount of deionized water was changed to 4,500 g.

[0065] (Example 4) The superabsorbent polymer particles of Example 4 were obtained in the same manner as in Example 1, except that the amount of 2-propanol was changed to 0.075 g and the amount of deionized water to 0.891 g.

[0066] (Example 5) Absorbent polymer particles of Example 5 were obtained in the same manner as in Example 1, except that the amount of ethylene carbonate was changed to 0.0075 g and the amount of deionized water was changed to 0.510 g.

[0067] (Comparative Example 4) The superabsorbent resin particles of Comparative Example 4 were obtained in the same manner as in Example 1, except that the amount of ethylene carbonate was changed to 0.0075 g and the amount of deionized water was changed to 0.891 g.

[0068] (Example 6) Absorbent polymer particles of Example 6 were obtained in the same manner as in Example 1, except that the amount of ethylene carbonate was changed to 0.0113 g and the amount of deionized water was changed to 0.891 g.

[0069] (Example 7) Absorbent resin particles of Example 7 were obtained in the same manner as in Example 1, except that a crosslinking agent solution obtained by mixing 0.015 g of ethylene carbonate and 0.547 g of deionized water was used.

[0070] (Example 8) Absorbent polymer particles of Example 8 were obtained in the same manner as in Example 1, except that a crosslinking agent solution was used, which was obtained by mixing 0.0075 g of ethylene carbonate, 0.600 g of 2-propanol, 0.0075 g of propylene glycol (hereinafter also referred to as "PG"), and 0.510 g of deionized water.

[0071] (Manufacturing example 2) [Polymerization process] 93.48 g (1.30 mol) of acrylic acid was placed in a 2 L separable flask. 78.32 g of deionized water was added to the acrylic acid in the separable flask while stirring. Then, 81.56 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare a sodium-partially neutralized acrylic acid solution with a monomer concentration of 45% by mass, in which 75 mol% of the acrylic acid was neutralized.

[0072] 253.36 g of a partially neutralized sodium solution of acrylic acid, 46.73 g of deionized water, and 0.17 g of polyethylene glycol diacrylate (Tokyo Chemical Industries, polyethylene glycol diacrylate N≒9) as an internal crosslinking agent were placed in a circular aluminum tray coated with fluororesin (diameter of opening and bottom: 150 mm, height: 60 mm). A stirring bar (diameter 8 mm, length 30 mm, no ring) was placed in the center of the circular tray, and the mixture was stirred to form a homogeneous liquid mixture (monomer aqueous solution). A thermometer was placed 3 cm away from the center of the aluminum tray to measure the temperature of the mixture. The opening of the aluminum tray was then covered with polyethylene film. After adjusting the temperature of the mixture to 25°C, the reaction system was purged with nitrogen gas by bubbling nitrogen gas through a tube inserted into the mixture until the dissolved oxygen content was 0.1 ppm or less. Next, while stirring the mixture at 500 rpm, the nitrogen purging tube was removed from the reaction system, and 0.15 g of a 20% by mass sodium persulfate aqueous solution and 0.64 g of a 10% by mass (2,2'-azobis(2-amidinopropane) dihydrochloride (V-50) aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) were added to the reaction solution. One minute after the addition, 0.26 g of a 0.50% by mass L-ascorbic acid aqueous solution and 1.33 g of a 0.35% by mass hydrogen peroxide aqueous solution were added dropwise to the mixture in the aluminum tray using a syringe (3 mL disposable syringe manufactured by Henke Sass Wolf, with a needle manufactured by Terumo Corporation).

[0073] Immediately after the addition of the hydrogen peroxide solution dropwise, the polymerization reaction began. Stirring was stopped one minute after the completion of the addition of the hydrogen peroxide solution. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the mixture gelled. Six minutes after the completion of the addition of the hydrogen peroxide solution, the thermometer measuring the temperature of the mixture showed a maximum value of 75°C. Subsequently, an aluminum tray containing the hydrated gel polymer, which was formed by the gelling of the reaction solution and contained water and polymer, was immersed in a 75°C water bath, and the hydrated gel polymer was allowed to mature in that state for 20 minutes.

[0074] [Crushing process] The gelled mixture, a hydrated gel-like polymer, was removed from the aluminum tray and immediately cut into pieces approximately 5 cm wide. The cut hydrated gel-like polymer was coarsely crushed using a meat chopper (Kirei Royal Co., Ltd., 12VR-750SDX). Coarsely crushed material containing elongated structures of the hydrated gel-like polymer was discharged from multiple circular discharge holes on a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the discharge holes was 6.4 mm. Coarse crushing by the meat chopper was continued for 3 minutes from the time the hydrated gel-like polymer was introduced. The resulting coarsely crushed material was an aggregate formed from multiple elongated structures with a width of 4-6 mm.

[0075] [Drying and grinding process] The obtained coarse material was spread on a wire mesh with a mesh size of 0.8 cm × 0.8 cm and dried by hot air drying at 180°C for 30 minutes to obtain a dried product. The dried product was pulverized using a centrifugal mill (Retsch ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved using a sieve with a mesh size of 850 μm and a sieve with a mesh size of 180 μm by shaking for 1 minute. By classification, the cross-linked polymer particles (B) were recovered as the fraction that passed through the 850 μm sieve but not through the 180 μm sieve. At this time, the moisture content of the cross-linked polymer particles (B) was 5.0 mass%.

[0076] (Example 9) The water-absorbing resin particles of Example 9 were obtained in the same manner as in Example 2, except that cross-linked polymer particles (B) were used.

[0077] <Centrifugal retention capacity (CRC)> The centrifugal retention capacity (CRC) was measured using the following procedure, referencing the EDANA method (NWSP 241.0.R2(15), pages 769-778). The measurements were performed under conditions of 25°C ± 2°C and 50% ± 10% humidity.

[0078] A 60mm x 170mm nonwoven fabric (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papilia Co., Ltd.) was folded in half lengthwise to adjust its size to 60mm x 85mm. A 60mm x 85mm nonwoven fabric bag was created by heat-sealing the two ends of the nonwoven fabric together along the lengthwise direction (a 5mm wide sealed section was formed along both ends). 0.2g of superabsorbent resin particles were weighed and placed inside the nonwoven fabric bag. The bag was then closed by heat-sealing the remaining end along the short side.

[0079] The nonwoven fabric bags were floated in a stainless steel tray (240mm x 320mm x 45mm) containing 1000g of physiological saline solution at a liquid temperature of 25±2℃, ensuring that the nonwoven fabric bags were not folded over, thereby completely wetting the entire nonwoven fabric bags. One minute after placing the nonwoven fabric bags in the physiological saline solution, the bags were immersed in the solution using a spatula to obtain nonwoven fabric bags containing gel.

[0080] After 30 minutes (1 minute floating and 29 minutes immersion) of the nonwoven fabric bag in physiological saline, the bag was removed from the saline solution. The bag was then placed in a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reached 250G, the nonwoven fabric bag was dewatered for 3 minutes. After dewatering, the mass Ma [g] of the nonwoven fabric bag, including the mass of the gel, was weighed. The same procedure as described above was performed on the nonwoven fabric bag without containing the superabsorbent resin particles, and the mass Mb [g] of the dewatered nonwoven fabric bag was measured. CRC [g / g] was calculated based on the following formula. Mc [g] is the accurate weighing value of 0.2g of superabsorbent resin particles used in the measurement. CRC[g / g] = {(Ma-Mb)-Mc} / Mc

[0081] [Measurement of saline absorption under a 4.14 kPa load (AUL 0.6 psi)] The absorption performance of the water-absorbent resin particles under pressure was evaluated by the amount of saline solution absorbed under a 4.14 kPa load. The amount of saline solution absorbed under a 4.14 kPa load was measured using a measuring device schematically shown in Figure 2. The measuring device comprises a burette section 1, a clamp 3, a conduit 5, a stand 11, a measuring platform 13, and a measuring unit 4 placed on the measuring platform 13. The burette section 1 has a burette tube 21 with markings, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the lower end of the burette tube 21, and an air inlet tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette section 1 is fixed by the clamp 3. The flat measuring platform 13 has a through hole 13a with a diameter of 2 mm formed in its center and is supported by a stand 11 with adjustable height. The through-hole 13a of the measuring table 13 and the stopcock 22 of the burette section 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0082] The measuring unit 4 comprises a plexiglass cylinder 31, a polyamide mesh 32 bonded to one opening of the cylinder 31, and a weight 33 that is movable vertically within the cylinder 31. The cylinder 31 is placed on the measuring stand 13 via the polyamide mesh 32. The inner diameter of the cylinder 31 is 20 mm. The mesh opening of the polyamide mesh 32 is 75 μm (200 mesh). The weight 33 has a diameter of 19 mm and a mass of 119.6 g, and can apply a load of 4.14 kPa (0.6 psi) to water-absorbing resin particles 10a uniformly arranged on the polyamide mesh 32, as described later.

[0083] The measurement of the saline absorption capacity under a 4.14 kPa load using the measuring device shown in Figure 2 was performed in a room at 25°C. First, the stopcocks 22 and 24 of the burette section 1 were closed, and 0.9 mass% saline solution adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. Next, the top opening of the burette tube 21 was sealed with the rubber stopper 23, and then the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9 mass% saline solution 50 to prevent air bubbles from entering. The height of the measuring platform 13 was adjusted so that the water level of the 0.9 mass% saline solution that reached the through-hole 13a was the same as the height of the top surface of the measuring platform 13. At this time, it was confirmed that the same volume of air as the 0.9 mass% saline solution 50 drawn out from the through-hole 13a was quickly supplied into the burette through the air introduction pipe 25. After adjustment, the level of the physiological saline solution 50 in the burette tube 21 was read using the scale on the burette tube 21, and that position was set as the zero point (reading at 0 seconds).

[0084] In the measurement unit 4, 0.10 g of superabsorbent resin particles 10a were uniformly arranged on the polyamide mesh 32 inside the cylinder 31, a weight 33 was placed on the superabsorbent resin particles 10a, and the cylinder 31 was positioned so that its center coincided with the conduit opening in the center of the measurement stand 13. The amount of saline solution in the burette tube 21 (i.e., the amount of saline solution absorbed by the superabsorbent resin particles 10a) Wc (ml) was read 60 minutes after the superabsorbent resin particles 10a began to absorb saline solution from the conduit 5, and the saline solution absorption capacity of the superabsorbent resin particles 10a under a load of 4.14 kPa was calculated using the following formula. The results are shown in Table 1. 4.14 kPa load: Physiological saline absorption capacity (ml / g) = Wc (ml) / Mass of absorbent resin particles (g)

[0085] <Moisture content> Place 2.0 g of cross-linked polymer particles into an aluminum foil case (No. 8) that has been pre-weighted to a constant weight (W1(g)), lightly close the opening of the aluminum foil case, and accurately weigh the total mass W2(g) of the aluminum foil case containing the sample. Dry the aluminum foil case containing the sample described above in a hot air dryer (ADVANTEC, model: FV-320) with the internal temperature set to 200°C for 2 hours. Allow the aluminum foil case containing the sample to cool to room temperature in a desiccator. Measure the total mass W3(g) of the aluminum foil case containing the sample after cooling. Calculate the water content of the sample using the following formula. Moisture content [mass%]=[{(W2-W1)-(W3-W1)} / (W2-W1)]×100

[0086] [Table 1]

[0087] The results are shown in Table 1. In Table 1, "Surface crosslinking agent / (Solid content of polymer particles) mass ratio" indicates the amount (mass) of surface crosslinking agent used in the crosslinking agent solution relative to the total mass of solid content of polymer particles (1), and "EC / water mass ratio" indicates the ratio of the mass of ethylene carbonate to the mass of water in the crosslinking agent solution. In each example, it was confirmed that surface-crosslinked particles (water-absorbing resin particles) could be obtained that could generally exhibit suppression of CRC decrease and improvement of AUL 0.6 psi, that is, the sum of centrifugal holding capacity and absorption performance under pressure was improved. [Explanation of Symbols]

[0088] 1...Burette section, 3...Clamp, 4...Measurement section, 5...Conduit, 10...Absorbent material, 10a...Water-absorbent resin particles, 10b...Fiber layer, 11...Stand, 13...Measurement platform, 13a...Through hole, 20a, 20b...Core wrap, 21...Burette tube, 22...Stopcock, 23...Rubber stopper, 24...Stopcock, 25...Air inlet tube, 30...Liquid permeable sheet, 31...Cylinder, 32...Polyamide mesh, 33...Weight, 40...Liquid impermeable sheet, 50...Physiological saline solution, 100...Absorbent article.

Claims

1. A method for producing water-absorbing resin particles containing polymer particles, wherein the method is The process includes a step of crosslinking the polymer with the surface crosslinking agent by reacting the polymer with the surface crosslinking agent in a mixture of polymer particle powder containing the polymer and a crosslinking agent solution containing a surface crosslinking agent and water. The aforementioned crosslinking agent solution contains a monohydric alcohol, The surface crosslinking agent comprises alkylene carbonate, The amount (mass) of the surface crosslinking agent used is 0.00045 or more and 0.00110 or less with respect to 1 mass of the total solid content of the polymer particles. A method wherein the amount (mass) of alkylene carbonate in the crosslinking agent solution is 0.009 or more and 0.027 or less per unit mass of water.

2. The method according to claim 1, wherein the amount (mass) of the surface crosslinking agent used is 0.00050 or more and 0.00110 or less with respect to 1 mass of the total amount of solids of the polymer particles.

3. The method according to claim 1 or 2, wherein the alkylene carbonate is ethylene carbonate.

4. A method for suppressing the decrease in centrifugal retention capacity of water-absorbent resin particles containing polymer particles and for improving the absorption amount of the water-absorbent resin particles under pressure, wherein the method is The method includes crosslinking the polymer with the surface crosslinking agent by reacting the polymer with the surface crosslinking agent in a mixture of polymer particle powder containing the polymer and a crosslinking agent solution containing a surface crosslinking agent and water. The surface crosslinking agent comprises alkylene carbonate, The amount (mass) of the surface crosslinking agent used is 0.00045 or more and 0.00110 or less with respect to 1 mass of the total solid content of the polymer particles. A method wherein the amount (mass) of alkylene carbonate in the crosslinking agent solution is 0.009 or more and 0.027 or less per unit mass of water.

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