Method for producing polymer solution and method for producing water-absorbing resin particles

Abstract

This method for producing an acidified polymer solution comprises: a step for bringing a water-absorbing resin having an acidic group into contact with a base component to obtain an uncrosslinked polymer solution containing an uncrosslinked polymer formed through cleavage of a crosslinked structure of the water-absorbing resin; and a step for bringing the uncrosslinked polymer solution into contact with a cation exchange resin to obtain an acidified polymer solution containing an acidified polymer that has a degree of neutralization lower than the degree of neutralization of the uncrosslinked polymer.

Description

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

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

[0002]

[0003] Generally, disposable sanitary products are constructed by enclosing pulp fibers and a water-absorbent resin between a water-impermeable cover sheet and a water-permeable nonwoven fabric, and the water-absorbent resin absorbs water and swells to absorb bodily waste. After use, such sanitary products are disposed of by incineration or landfilling, but recently, research has been conducted into the recovery and reuse of components from used sanitary products. For example, Patent Document 1 discloses a water-absorbent resin decomposition technology that focuses on cleavage of crosslinks within the water-absorbent resin, and a technology for recovering the water-soluble recycled polymer obtained by decomposing the water-absorbent resin as an aggregate by crosslinking with multivalent metal ions.

[0003] Japanese Patent Application Laid-Open No. 2020-49398

[0004] When the crosslinked structure of a water-absorbent resin is cleaved under alkaline conditions, it is necessary to adjust the degree of neutralization of the polymer before regenerating the water-absorbent resin, from the viewpoints of adjusting the performance of the water-absorbent resin and improving skin compatibility. In this case, the present inventors noticed that when an acidic polymer solution is produced by adjusting the degree of neutralization of the polymer by adding an acid, the water absorption performance of the regenerated water-absorbent resin obtained using the acidic polymer solution tends to be low, and further found that the main reason for this is that the acidic polymer solution contains a salt generated during neutralization with an acid.

[0005] Therefore, one aspect of the present invention relates to a method for producing an acidified polymer solution that can be used to produce water-absorbent resin particles having excellent water-absorbing properties.

[0006] One aspect of the present disclosure includes, for example, the following [1] to [5]. [1] A method for producing an acidified polymer solution, comprising: contacting a water-absorbent resin having acidic groups with a basic component to obtain an uncrosslinked polymer solution containing an uncrosslinked polymer formed by cleavage of a crosslinked structure of the water-absorbent resin; and contacting the uncrosslinked polymer solution with a cation exchange resin to obtain an acidified polymer solution containing an acidified polymer having a degree of neutralization lower than that of the uncrosslinked polymer. [2] The production method according to [1], wherein the step of obtaining the acidified polymer solution comprises mixing the uncrosslinked polymer solution after contact with the cation exchange resin with the uncrosslinked polymer solution that has not been contacted with the cation exchange resin. [3] The production method according to [2], wherein the mixing step comprises setting a target degree of neutralization of the acidified polymer, and adjusting a mixing ratio of the uncrosslinked polymer solution after contact with the cation exchange resin and the uncrosslinked polymer solution that has not been contacted with the cation exchange resin based on the target value. [4] The method according to [2] or [3], wherein a mass of the cation exchange resin to be brought into contact with the uncrosslinked polymer solution is 1.0X (g) to 1.5X (g), based on a mass X (unit: g) of the cation exchange resin necessary to bring the degree of neutralization of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol%. [5] A method for producing water-absorbent resin particles, comprising a step of crosslinking the acidified polymer contained in the acidified polymer solution obtained by the method according to any one of [1] to [4], to obtain a water-absorbent resin.

[0007] According to one aspect of the present invention, there is provided a method for producing an acidified polymer solution that can be used to produce water-absorbent resin particles having excellent water-absorbing properties.

[0008] The present invention relates to an absorbent article having an absorbent core, a nonwoven fabric bag, a water-absorbent resin, and a water-absorbent resin.

[0009] The present invention is not limited to the following examples.

[0010] In this specification, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with a value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. "Physiological saline" refers to a 0.9% by mass aqueous sodium chloride solution. "Standard sieve" refers to a test sieve (metal mesh sieve) specified in JIS Z 8801-1:2019. Unless otherwise specified, the examples and comparative examples were carried out in an environment of 1 atmosphere, normal temperature and humidity, and the various parameters disclosed in this specification were also values ​​measured in the same environment, with the temperature of the various samples being normal temperature. "1 atmosphere" is 101,325 Pa, "normal temperature" is 25°C, and "normal humidity" is 50% RH.

[0011] The method for producing an acidified polymer solution of the present invention includes a step of contacting a water-absorbent resin having an acidic group with a basic component to obtain an uncrosslinked polymer solution containing an uncrosslinked polymer formed by cleavage of a crosslinked structure of the water-absorbent resin (uncrosslinked polymer solution preparation step), and a step of contacting the uncrosslinked polymer solution with a cation exchange resin to obtain an acidified polymer solution containing an acidified polymer having a neutralization degree lower than that of the uncrosslinked polymer (acidified polymer solution preparation step).

[0012] <Water-absorbent resin> The water-absorbent resin whose crosslinked structure is cleaved may be a recycled water-absorbent resin. The recycled water-absorbent resin may be a used water-absorbent resin (i.e., a water-absorbent resin that has become gelatinous due to absorption of liquid), an unused water-absorbent resin (e.g., waste water-absorbent resin generated during the production process of a water-absorbent resin), or a mixture of both.

[0013] Examples of absorbent articles containing recycled water-absorbent resins include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste treatment materials. The absorbent articles may be used. The recycled water-absorbent resin recovered from used absorbent articles may form a gel due to absorption of liquid during use.

[0014] The recycled water-absorbing resin contains water and a water-absorbing resin. The recycled water-absorbing resin may contain fibrous materials such as cellulosic fibers and synthetic fibers.

[0015] The water-absorbent resin has an acidic group. The water-absorbent resin having an acidic group contains, for example, a polymer (crosslinked polymer) of an ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include (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. The ethylenically unsaturated 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.

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

[0017] The water-absorbing resin contains, for example, a crosslinked polymer having a poly(meth)acrylic acid structure.As such crosslinked polymer, for example, the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid to obtain a polymer, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond.

[0018] The shape of the recycled water absorbent resin is not particularly limited, and if it is a used gel, it may be in the form of an irregular lump, and if it is an unused one (generally a dry powder), it may be in the form of an irregular crushed shape, flakes, granules, etc.

[0019] <Uncrosslinked Polymer Solution Preparation Step> In the uncrosslinked polymer solution preparation step, a water-absorbent resin is contacted with a base component to obtain an uncrosslinked polymer solution containing an uncrosslinked polymer formed by cleavage of the crosslinked structure of the water-absorbent resin. The uncrosslinked polymer solution may be a mixture of an uncrosslinked polymer (a crosslinked polymer whose crosslinked structure has been cleaved) and a base component. The uncrosslinked polymer solution may or may not contain a water-absorbent resin (a crosslinked polymer whose crosslinked structure has not been cleaved). The water-absorbent resin contained in the uncrosslinked polymer solution is typically a water-absorbent resin whose crosslinked structure was not cleaved in the uncrosslinked polymer solution preparation step due to reasons such as a lack of a base component. When the crosslinked polymer contained in the water-absorbent resin is a crosslinked polymer having a poly(meth)acrylic acid structure, the uncrosslinked polymer may be a polymer having a poly(meth)acrylic acid structure, and the uncrosslinked polymer solution may be a mixture of a polymer having a poly(meth)acrylic acid structure and a base component. In this step, by contacting a water absorbent resin having an acidic group with a basic component, the crosslinked structure of the water absorbent resin is cleaved and the acidic group contained in the water absorbent resin is neutralized with the basic component. The degree of neutralization of the uncrosslinked polymer obtained through this step is typically about 100 mol%.

[0020] The base component may be an inorganic base component, for example, at least one inorganic base component selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or may be a monovalent inorganic base component from the viewpoint of suppressing aggregation of the polymer after cleavage of the crosslinked structure, for example, at least one monovalent inorganic base component selected from the group consisting of sodium hydroxide, ammonia, and potassium hydroxide.

[0021] The content of the base component in the uncrosslinked polymer solution may be 0.003 mol / kg or more, 0.01 mol / kg or more, 0.1 mol / kg or more, 0.2 mol / kg or more, 0.4 mol / kg or more, 0.5 mol / kg or more, 0.6 mol / kg or more, 0.7 mol / kg or more, or 0.8 mol / kg or more, or may be 2 mol / kg or less, 1.5 mol / kg or less, 1 mol / kg or less, 0.9 mol / kg or less, or 0.8 mol / kg or less, relative to the total amount of the uncrosslinked polymer solution. The content of the base component in the uncrosslinked polymer solution may be 0.003 mol / kg to 2 mol / kg, 0.01 mol / kg to 1.5 mol / kg, 0.1 mol / kg to 1 mol / kg, 0.2 mol / kg to 0.9 mol / kg, or 0.5 mol / kg to 0.8 mol / kg, relative to the total amount of the uncrosslinked polymer solution.

[0022] The pH of the uncrosslinked polymer solution may be 7.0 or more, 7.5 or more, 8.0 or more, 8.5 or more, 9.0 or more, 9.5 or more, or 10.0 or more, and may be 14.0 or less, 13.5 or less, 13.0 or less, 12.5 or less, 12.0 or less, 11.5 or less, or 11.0 or less. The pH of the uncrosslinked polymer solution may be, for example, 7.0 to 14.0, 7.5 to 13.5, 8.0 to 13.0, 8.5 to 12.5, 9.0 to 12.0, 9.5 to 11.5, or 10.0 to 11.0.

[0023] The uncrosslinked polymer solution preparation step may include an operation of adjusting the temperature of the uncrosslinked polymer solution, from the viewpoint that the weight-average molecular weight of the acidified polymer obtained in the acidified polymer solution preparation step described below is likely to increase and that the water-absorbing performance of the water-absorbent resin particles produced from the acidified polymer is more excellent. For example, the uncrosslinked polymer solution preparation step may include an operation of adjusting the uncrosslinked polymer solution to 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, or 90°C or higher; it may include an operation of adjusting the uncrosslinked polymer solution to 150°C or lower, 120°C or lower, 110°C or lower, or 100°C or lower; or it may include an operation of adjusting the uncrosslinked polymer solution to 50°C to 150°C, 60°C to 120°C, or 70°C to 100°C.

[0024] <Acidified Polymer Solution Preparation Step> In the acidified polymer solution preparation step, the uncrosslinked polymer solution is contacted with a cation exchange resin to obtain an acidified polymer solution containing an acidified polymer. The degree of neutralization of the acidified polymer is lower than that of the uncrosslinked polymer. Furthermore, by contacting the uncrosslinked polymer solution with the cation exchange resin, the degree of neutralization of the acidified polymer contained in the obtained acidified polymer solution can be adjusted while substantially preventing salt generation.

[0025] The degree of neutralization of an acidified polymer refers to the ratio of neutralized acidic groups (acidic groups that have formed salts) to the total amount of acidic groups (including neutralized acidic groups) in the acidified polymer. For example, when the acidified polymer is a partially neutralized product of polyacrylic acid, the degree of neutralization of the acidified polymer refers to the ratio of carboxylate to the total amount of carboxyl groups and carboxylate contained in the partially neutralized product of polyacrylic acid. By contacting an uncrosslinked polymer solution with a cation exchange resin, substantially no salt is produced, and therefore the purity of the resulting acidified polymer (i.e., the water-absorbent resin particles that can be produced) is increased, and water-absorbent resin particles with excellent water absorption performance can be obtained.

[0026] By contacting the uncrosslinked polymer solution with a cation exchange resin, metal cations dissolved in the uncrosslinked polymer solution as cations are adsorbed and removed. Examples of metal cations adsorbed by the cation exchange resin include sodium ions, potassium ions, chromium ions, iron ions, nickel ions, calcium ions, cobalt ions, copper ions, magnesium ions, and aluminum ions.

[0027] A cation exchange resin is a resin having an ion exchange group in its molecular structure. Examples of the ion exchange group include a strongly acidic sulfonic acid group and a weakly acidic carboxyl group. The ion exchange group of the cation exchange resin may be a sulfonic acid group, which can undergo an ion exchange reaction with a salt that may be contained in the uncrosslinked polymer solution (typically, a salt generated by contacting the water-absorbent resin with a basic component in the uncrosslinked polymer solution preparation step) (can decompose the salt), thereby improving the water absorption performance of the water-absorbent resin particles. In other words, the cation exchange resin may be a strongly acidic cation exchange resin, which can undergo an ion exchange reaction with a salt that may be contained in the uncrosslinked polymer solution, thereby improving the water absorption performance of the water-absorbent resin particles. Specific examples of the cation exchange resin include Diaion SK1BH, Diaion SK104H, Amberlite HPR1200, DOWEX650C, and DOWEX50WX8.

[0028] The degree of neutralization of the acidified polymer contained in the acidified polymer solution may be 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, or 75 mol% or more from the viewpoint of improving the water absorption performance of the water-absorbent resin by increasing the proportion of neutralized acidic groups, and may be 90 mol% or less, 85 mol% or less, 80 mol% or less, or 75 mol% or less from the viewpoint of improving skin compatibility. From these viewpoints, the degree of neutralization of the acidified polymer contained in the acidified polymer solution may be 40 mol% to 90 mol%, 60 mol% to 85 mol%, or 70 mol% to 80 mol%. When the degree of neutralization of the acidified polymer contained in the acidified polymer solution is used as a target value when determining the mixing ratio, the target value may be within the above range. The neutralization degree of the acidified polymer can be calculated from the following formula. Neutralization degree (mol%) = neutralized acidic groups / total amount of acidic groups (including neutralized acidic groups) in the acidified polymer

[0029] The acidified polymer solution may be obtained by contacting the entire uncrosslinked polymer solution obtained in the uncrosslinked polymer solution preparation step with a cation exchange resin. The acidified polymer solution may be obtained by contacting a portion of the uncrosslinked polymer solution obtained in the uncrosslinked polymer solution preparation step with a cation exchange resin to obtain the uncrosslinked polymer solution after contact with the cation exchange resin (hereinafter also referred to as a neutralization degree-adjusted solution), and then mixing the remaining portion of the uncrosslinked polymer solution (the uncrosslinked polymer solution not contacted with the cation exchange resin; hereinafter also referred to as an unadjusted solution) with the neutralization degree-adjusted solution. Alternatively, the acidified polymer solution may be obtained by contacting a certain lot of uncrosslinked polymer solution with a cation exchange resin to obtain a neutralization degree-adjusted solution, and then mixing the neutralization degree-adjusted solution obtained from the certain lot with the unadjusted solution from another lot. That is, the acidified polymer solution preparation step may include a step of mixing the neutralization degree-adjusted solution and the unadjusted solution.

[0030] When the entire uncrosslinked polymer solution is brought into contact with the cation exchange resin to obtain an acidified polymer solution, the degree of neutralization of the acidified polymer can be adjusted by appropriately adjusting the amount of the cation exchange resin used.

[0031] When the method for producing an acidified polymer solution includes a step of mixing a neutralization degree-adjusted solution and an unadjusted solution, the target neutralization degree of the acidified polymer can be achieved with little error. In other words, if an excess amount of cation exchange resin is used, the neutralization degree of the acidified polymer contained in the neutralization degree-adjusted solution will be approximately 0 mol%, while the neutralization degree of the uncrosslinked polymer contained in the unadjusted solution will be approximately 100 mol%. Therefore, an acidified polymer solution containing an acidified polymer having a target neutralization degree can be obtained with high precision by simply adjusting the mixing ratio of the neutralization degree-adjusted solution and the unadjusted solution, without adjusting the amount of cation exchange resin used. In addition, in this case, compared with the case where an acidified polymer solution is obtained by bringing the whole of the uncrosslinked polymer solution into contact with a cation exchange resin, the amount of the uncrosslinked polymer solution that comes into contact with the cation exchange resin can be reduced, and therefore the amount of the uncrosslinked polymer adsorbed on the surface of the cation exchange resin (i.e., the amount of loss of the uncrosslinked polymer taken up by the ion exchange resin) is reduced, and the amount of the obtained acidified polymer is increased, and therefore the amount of water absorbent resin particles that can be produced is also increased.

[0032] The mixing of the neutralization degree-adjusted liquid and the unadjusted liquid may involve setting a target value for the neutralization degree of the acidified polymer contained in the final acidified polymer solution, and adjusting the mixing ratio of the neutralization degree-adjusted liquid (the uncrosslinked polymer solution after contact with the cation exchange resin) and the unadjusted liquid (the uncrosslinked polymer solution that has not been contacted with the cation exchange resin) based on the target value.

[0033] Specifically, the mixing ratio of the neutralization degree-adjusted liquid and the unadjusted liquid can be adjusted as follows: That is, for example, when the target value for the neutralization degree of the acidified polymer contained in the acidified polymer solution is set to 75% and the neutralization degree-adjusted liquid and the unadjusted liquid are mixed, for example, an unadjusted liquid in which the neutralization degree of the acidified polymer is 100 mol % and a neutralization degree-adjusted liquid in which the neutralization degree of the acidified polymer is 0 mol % are prepared, and 75 parts by mass of the unadjusted liquid and 25 parts by mass of the neutralization degree-adjusted liquid are mixed to obtain an acidified polymer solution with a neutralization degree of 75%.

[0034] The higher the degree of neutralization of the acidified polymer contained in the neutralization degree-adjusted solution, the more likely the viscosity of the neutralization degree-adjusted solution will be, and the more likely the amount of acidified polymer adsorbed to the surface of the cation exchange resin will be. From this perspective, the degree of neutralization of the acidified polymer may be 5 mol% or less, 3 mol% or less, 1 mol% or less, or 0 mol%. From the same perspective, the degree of neutralization of the acidified polymer contained in the neutralization degree-adjusted solution may be 0 mol% to 5 mol%, 0 mol% to 3 mol%, or 0 mol% to 1 mol%. If the degree of neutralization of the acidified polymer is 0 mol% to 1 mol%, it is easy to adjust the degree of neutralization of the acidified polymer solution obtained by mixing with the unadjusted solution so that there is little error from the target value.

[0035] When the amount of the monomer unit constituting the uncrosslinked polymer contained in the uncrosslinked polymer solution is A (unit: mol), the valence of the acidic group of the monomer unit constituting the uncrosslinked polymer contained in the uncrosslinked polymer solution is B (unit: eq / mol), the amount of the cation exchange resin is X (unit: g), the ion exchange capacity of the cation exchange resin is C (unit: eq / L), and the density of the cation exchange resin is D (unit: g / L), the amount X of the cation exchange resin necessary to bring the degree of neutralization of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol% by contact with the cation exchange resin is theoretically represented by the following formula (1). When preparing a neutralization degree adjusting solution, the amount X of cation exchange resin required to bring the neutralization degree of the uncrosslinked polymer contained in the uncrosslinked polymer solution to be contacted with the cation exchange resin to 0 mol % by contact with the cation exchange resin can be theoretically expressed by the following formula (1) in the same manner as above, by replacing A and B above with the amount of monomer units constituting the uncrosslinked polymer contained in the uncrosslinked polymer solution to be contacted with the cation exchange resin and the valence of the acidic group of the monomer units constituting the uncrosslinked polymer contained in the uncrosslinked polymer solution to be contacted with the cation exchange resin, respectively. X=(A×B×D) / C (1)

[0036] When the mass of the cation exchange resin necessary to adjust the degree of neutralization of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol % is X (unit: g), the mass of the cation exchange resin to be contacted with the uncrosslinked polymer solution may be 1.0X (g) to 1.5X (g), or from the viewpoint of easily and reliably adjusting the degree of neutralization of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol %, it may be more than 1.0X (g), 1.1X (g) or more, or 1.2X (g) or more, or from the viewpoint of reducing the amount of uncrosslinked polymer adsorbed on the surface of the cation exchange resin, it may be 1.4X (g) or less, 1.3X (g) or less, or 1.2X (g) or less.

[0037] After obtaining the acidified polymer solution, solid-liquid separation may be performed to remove impurities from the acidified polymer solution. For example, the acidified polymer solution may be subjected to filtration or ultrafiltration. Before filtering the acidified polymer solution, the pH of the acidified polymer solution may be adjusted to near neutral.

[0038] <Crosslinking Step> A step of crosslinking the acidified polymer contained in the acidified polymer solution obtained through the above-mentioned acidified polymer solution preparation step to obtain a water-absorbent resin (regenerating the water-absorbent resin) may be carried out.

[0039] After obtaining the acidified polymer solution and before carrying out the crosslinking step, the acidified polymer solution may be diluted, heated, or have a salt (a salt containing a monovalent cation) added thereto in order to reduce the viscosity of the acidified polymer solution. Furthermore, after obtaining the acidified polymer solution and before crosslinking the acidified polymer, the acidified polymer solution may be subjected to a sterilization treatment.

[0040] In the crosslinking step, for example, a solution containing a crosslinker capable of forming a covalent bond with a functional group of the acidified polymer is prepared, and the acidified polymer is crosslinked via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the acidified polymer solution may be gelled. In this case, the entire acidified polymer solution loses fluidity, and a gel containing the crosslinked polymer and water is formed. When crosslinking progresses to the extent that a gel is formed, it is particularly easy to obtain water-absorbent resin particles having high water absorption capacity and a small amount of dissolved matter.

[0041] Examples of functional groups possessed by the acidified polymer include carboxyl groups. When the acidified polymer has carboxyl groups, the acidified polymer is crosslinked by reaction of the carboxyl groups with a crosslinking agent and / or reaction between the carboxyl groups themselves. The covalent bond may be at least one selected from the group consisting of an ester bond, a thioester bond, an amide bond, an ether bond, and a carbon-carbon bond. The acidified polymer may be crosslinked, for example, via at least one group selected from the group consisting of a carboxylic acid ester group, a thioester group, an amide group, an acid anhydride group, an oxyalkylene group, and an oxyarylene group.

[0042] Examples of the crosslinking agent include aliphatic polyhydric alcohols such as (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, and pentaerythritol; glycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; bisacrylamide compounds such as N,N'-methylenebis(meth)acrylamide; allylated starch; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0043] The amount of crosslinking agent may be 0.0001 parts by weight or more, 0.001 parts by weight or more, or 0.005 parts by weight or more, and may be 15 parts by weight or less, 10 parts by weight or less, or 5 parts by weight or less, per 100 parts by weight of acidified polymer. The amount of crosslinking agent may be 0.0001 parts by weight to 15 parts by weight, 0.001 parts by weight to 10 parts by weight, or 0.005 parts by weight to 5 parts by weight, per 100 parts by weight of acidified polymer.

[0044] The reaction temperature when crosslinking the acidified polymer is appropriately set depending on the type, amount, etc. of the crosslinking agent used, but may be 50° C. or higher, 80° C. or higher, or 100° C. or higher, and may be 220° C. or lower, 200° C. or lower, or 180° C. or lower. The reaction temperature when crosslinking the acidified polymer may be 50° C. to 220° C., 80° C. to 200° C., or 100° C. to 180° C.

[0045] The reaction time for crosslinking the acidified polymer is appropriately set depending on the type and amount of the crosslinking agent used, the reaction temperature, etc., but may be 1 to 200 minutes, or 5 to 150 minutes.

[0046] After obtaining a crosslinked polymer through the crosslinking step, a surface crosslinking step of surface crosslinking the crosslinked polymer may be carried out. The surface crosslinking can be carried out, for example, by adding a crosslinking agent for surface crosslinking (surface crosslinking agent) to the crosslinked polymer and allowing it to react.

[0047] The surface cross-linking agent may be a compound having two or more reactive functional groups that are reactive with the cross-linked polymer. The surface cross-linking agent may be the same as or different from the cross-linking agent in the cross-linking step.

[0048] The reactive functional groups of the surface cross-linking agent may be carbonate groups, alcoholic hydroxyl groups, epoxy groups, halogeno groups in haloepoxy compounds, isocyanate groups, oxetanyl groups, oxazoline groups, or combinations thereof. Carbonate groups are considered two reactive functional groups because they can react with two other molecules.

[0049] Examples of surface cross-linking agents having a carbonate group include alkylene carbonates (ethylene carbonate, etc.). Examples of surface cross-linking agents having an alcoholic hydroxyl group include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin, and hydroxyalkylamide compounds (bis[N,N-di(β-hydroxyethyl)]adipamide, etc.). Examples of surface cross-linking agents having two or more epoxy groups include (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. Examples of haloepoxy compounds having an epoxy group and a halogeno group include epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of surface crosslinking agents having an isocyanate group include 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Examples of surface crosslinking agents having an oxetanyl group include 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.

[0050] The amount of the surface crosslinking agent may be 0.001 parts by mass or more, 0.005 parts by mass or more, or 0.01 parts by mass or more, and may be 5 parts by mass or less, 3 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the crosslinked polymer. The amount of the crosslinking agent may be 0.001 parts by mass to 5 parts by mass, 0.005 parts by mass to 3 parts by mass, or 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of the crosslinked polymer.

[0051] A regenerated water-absorbent resin containing a crosslinked polymer is formed by a method including removing water from a crosslinked polymer obtained through an optional surface crosslinking step. When the reaction liquid to be subjected to the crosslinking step is an aqueous solution, the water-absorbent resin particles are formed by a method including a drying step of removing water from a block of crosslinked polymer formed by gelling the reaction liquid itself, drying the crosslinked polymer to form a dried product, and a pulverizing step of pulverizing the dried product.

[0052] The method for drying the crosslinked polymer may be a common method such as a squeezing method such as centrifugation, dehydration using an organic solvent, natural drying, heat drying, air drying, freeze drying, or a combination thereof. The heating temperature for drying may be 80°C to 220°C, 90°C to 200°C, or 100°C to 180°C from the viewpoint of efficiently removing water.

[0053] The moisture content of the dried product may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The moisture content of the dried product means the proportion of moisture in the dried product based on the total amount of the dried product. When a crosslinked polymer containing water is heated at 200°C for 2 hours, the difference in mass of the crosslinked polymer before and after heating can be considered to be the moisture content of the dried product.

[0054] Before drying the crosslinked polymer, the crosslinked polymer may be crushed to form a crushed product containing structures of a certain size. By forming a crushed product, water can be efficiently removed. The structures constituting the crushed product can be, for example, elongated structures, granular structures (particles), or a combination thereof. The crushed product may include a plurality of 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 and 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 crosslinked polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and farm mills.

[0055] The dried product is pulverized to form water-absorbent resin particles. The pulverization method is not particularly limited. For example, the dried product can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill.

[0056] The water-absorbent resin particles obtained by pulverization may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the water-absorbent resin particles after classification may be pulverized and classified again.

[0057] 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.

[0058] The median particle size of the crosslinked polymer powder obtained through pulverization and, if necessary, classification may be, for example, 200 μm to 500 μm, or 300 μm to 500 μm. The particle size distribution may be adjusted by mixing two or more powders obtained by classification and having different median particle sizes.

[0059] The centrifuge retention capacity (CRC) of the water-absorbent resin particles may be 36 g / g or more, 38 g / g or more, 40 g / g or more, 42 g / g or more, 44 g / g or more, or 46 g / g or more. The larger the CRC of the water-absorbent resin particles, the better the water retention capacity, and if the CRC is 36 g / g or more, it can be said that the water-absorbent resin particles have sufficient water absorption performance. The upper limit of the CRC of the water-absorbent resin particles is not particularly limited, but may be, for example, 70 g / g or less, 60 g / g or less, or 50 g / g or less. The CRC of the water-absorbent resin particles may be 36 g / g to 70 g / g, 38 g / g to 60 g / g, or 40 g / g to 50 g / g. The CRC of the water-absorbent resin particles is a value measured by the method described in the Examples below.

[0060] The dissolved content of the water-absorbent resin particles may be 25% by mass or less, 24% by mass or less, 23% by mass or less, 22% by mass or less, 21% by mass or less, 20% by mass or less, or 19% by mass or less. The smaller the dissolved content of the water-absorbent resin particles, the more the dissolution of the polymer can be suppressed. If the dissolved content is 25% by mass or less, the dissolution of the polymer can be sufficiently suppressed, and the water absorption performance of the water-absorbent resin particles is easily improved. The lower limit of the dissolved content of the water-absorbent resin particles is not particularly limited, but may be, for example, 1% by mass or more, 2% by mass or more, or 3% by mass or more. The dissolved content of the water-absorbent resin particles may be 1% by mass to 25% by mass, 2% by mass to 24% by mass, or 3% by mass to 23% by mass. The dissolved content of the water-absorbent resin particles is a value measured by the method described in the Examples below.

[0061] <Absorbent article> Fig. 1 is a cross-sectional view showing an example of an absorbent article having an absorbent body containing water-absorbent resin particles. The absorbent article 100 shown in Fig. 1 comprises a water-absorbent sheet 50 having a film-like absorbent body 10, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40.

[0062] The water-absorbent sheet 50 includes an absorbent body 10 containing a powder of water-absorbent resin particles 1, and two core wrap sheets 20a and 20b. The absorbent body 10 is disposed inside the core wrap sheets 20a and 20b. The absorbent body 10 maintains its shape by being sandwiched between the two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b may consist of two sheets, a single folded sheet, or a single bag. A sheet member that does not have any other components on the outside of the core wrap sheets 20a and 20b that wrap the absorbent body 10 is sometimes referred to as a water-absorbent sheet.

[0063] The absorbent body 10 is a component that mainly contains a powder of water-absorbent resin particles 1 and is retained to have a certain shape. The absorbent body 10 may contain fibrous material 3 in addition to the powder of water-absorbent resin particles 1, or may not contain fibrous material 3. The content of the water-absorbent resin particles 1 in the absorbent body 10 may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less, based on the mass of the absorbent body 10.

[0064] The thickness of the absorbent body 10 may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less, or may be 0.1 mm or more, or 0.3 mm or more. The thickness of the absorbent body 10 may be 0.1 mm or more and 20 mm or less. The mass per unit area of ​​the absorbent body 10 is 1000 g / m ２ Below, 800g / m ２ or less than 600 g / m ２ or less, and ２ It may be more than that.

[0065] The fibrous material 3 can be, for example, a cellulosic fiber, a synthetic fiber, or a combination thereof. Examples of cellulosic fibers include comminuted wood pulp, cotton, cotton linters, rayon, and cellulose acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may also be a hydrophilic fiber (e.g., pulp).

[0066] The absorbent 10 may further contain inorganic powder (e.g., amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the water-absorbent resin particles 1 contain inorganic particles, the absorbent 10 may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles 1.

[0067] The water-absorbent sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the absorbent body 10. An adhesive layer may be interposed between the core wrap sheets 20a, 20b on both sides and the absorbent body 10. The adhesive 21 is not particularly limited, and may be, for example, a hot-melt adhesive.

[0068] The core wrap sheets 20a, 20b may be, for example, nonwoven fabrics. The two core wrap sheets 20a, 20b may be the same or different nonwoven fabrics. The nonwoven fabric may be a nonwoven fabric made of short fibers (i.e., staple) (short fiber nonwoven fabric) or a nonwoven fabric made of long fibers (i.e., filaments) (long fiber nonwoven fabric). The staple may have a fiber length of, but is not limited to, typically several hundred millimeters or less.

[0069] The core wrap sheets 20a, 20b may be a thermal bonded nonwoven fabric, an air-through nonwoven fabric, a resin bonded nonwoven fabric, a spunbonded nonwoven fabric, a meltblown nonwoven fabric, an airlaid nonwoven fabric, a spunlace nonwoven fabric, a point bonded nonwoven fabric, or a laminate containing two or more types of nonwoven fabric selected from these.

[0070] The nonwoven fabric used as the core wrap sheets 20a, 20b can be made of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include fibers containing synthetic resins selected from polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamides such as nylon, and rayon. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the nonwoven fabric may be polyolefin fibers, polyester fibers, or a combination thereof. The core wrap sheets 20a, 20b may also be tissue paper.

[0071] The water-absorbent sheet 50 may be used to manufacture various other absorbent articles. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste disposal materials. The absorbent bodies that make up these absorbent articles often move or deform due to the movements of the user of the absorbent article, etc.

[0072] The liquid-permeable sheet 30 is positioned as the outermost layer on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is positioned outside the core wrap sheet 20b while in contact with the core wrap sheet 20b. The liquid-impermeable sheet 40 is positioned as the outermost layer on the opposite side of the absorbent article 100 from the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is positioned outside the core wrap sheet 20a while in contact with the core wrap sheet 20a. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have main surfaces that are wider than the main surface of the water-absorbent sheet 50, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wrap sheets 20a, 20b. However, the size relationships among the absorbent body 10, the core wrap sheets 20a, 20b, the liquid-permeable sheet 30, and the liquid-impermeable sheet 40 are not particularly limited and may be appropriately adjusted depending on the intended use of the absorbent article, etc.

[0073] The liquid-permeable sheet 30 may be a nonwoven fabric. The nonwoven fabric used as the liquid-permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From that viewpoint, the liquid-permeable sheet 30 may be a nonwoven fabric having a hydrophilicity of 5 to 200 as measured in accordance with the Pulp and Paper Testing Method No. 68 (2000) of the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may also be 10 to 150. For details of Pulp and Paper Testing Method No. 68, see, for example, WO2011 / 086843.

[0074] Hydrophilic nonwoven fabrics may be made from fibers with moderate hydrophilicity, such as rayon, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin and polyester fibers. Methods for obtaining nonwoven fabrics containing hydrophilized hydrophobic chemical fibers include spunbonding a mixture of hydrophobic chemical fibers and a hydrophilizing agent, adding a hydrophilizing agent to the hydrophobic chemical fibers, and impregnating a spunbond nonwoven fabric obtained from hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0075] The basis weight (mass per unit area) of the nonwoven fabric used as the liquid-permeable sheet 30 is set to 5 g / m from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and from the viewpoint of increasing the liquid permeation rate of the absorbent article. ２ ~200g / m ２ , 8 g / m ２ ~150g / m ２ , or 10 g / m ２ ~100g / m ２ The thickness of the liquid-permeable sheet 30 may be 20 μm to 1400 μm, 50 μm to 1200 μm, or 80 μm to 1000 μm.

[0076] The liquid-impermeable sheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out through the liquid-impermeable sheet 40. The liquid-impermeable sheet 40 may be a resin sheet or a nonwoven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The nonwoven fabric may be a spunbond / meltblown / spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. The liquid-impermeable sheet 40 may be a composite sheet of a resin sheet and a nonwoven fabric (e.g., a spunbond nonwoven fabric or a spunlace nonwoven fabric). The liquid-impermeable sheet 40 may be breathable to reduce stuffiness during wear and to alleviate discomfort to the wearer. For example, a low-density polyethylene (LDPE) resin sheet can be used as the breathable liquid-impermeable sheet 40.

[0077] In order to ensure flexibility and not impair the wearing comfort of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 is 10 g / m ２ ~50g / m ２ may be.

[0078] The present invention will be described in more detail below with reference to examples.

[0079] <Evaluation Method> The centrifuge retention capacity (CRC) and the dissolved content were measured according to the following evaluation method. The measurement results are shown in Table 1.

[0080] [Centrifuge Retention Capacity (CRC)] The centrifuge retention capacity (CRC) was measured by the following procedure with reference to the EDANA method (NWSP 241.0.R2(19)).

[0081] Two pieces of nonwoven fabric measuring 60 mm x 85 mm were heat-sealed together at 5 mm-wide edges along each of three sides to produce a nonwoven fabric bag measuring 60 mm x 85 mm. Approximately 0.2 g of precisely weighed water-absorbent resin particles was placed in the nonwoven fabric bag. The nonwoven fabrics were then heat-sealed together at the open edge of one side to close the nonwoven fabric bag shown in FIG. 2 .

[0082] A plurality of nonwoven fabric bags containing water-absorbent resin particles were floated on 500 g of physiological saline contained in a stainless steel tray (240 mm × 320 mm × 45 mm) without overlapping each other, thereby completely wetting the entire nonwoven fabric bag. One minute after floating the nonwoven fabric bags on the physiological saline, the entire nonwoven fabric bag was immersed in the physiological saline using a spatula.

[0083] After 30 minutes had passed since the nonwoven fabric bag began to float in the saline solution, i.e., the total time of 1 minute of floating time and 29 minutes of immersion time, the nonwoven fabric bag was removed from the saline solution. The removed nonwoven fabric bag was dehydrated for 3 minutes at a centrifugal force of 250 G using a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). After dehydration, the mass Wc [g] of the nonwoven fabric bag including the mass of the gel was weighed. The same operation was also performed on a nonwoven fabric bag that did not contain the water-absorbent resin particles to be measured, and the mass B [g] of the nonwoven fabric bag after dehydration was measured. The CRC [g / g] was calculated according to the following formula. Sc [g] is the precisely weighed value of 0.2 g of the mass of the water-absorbent resin particles to be measured. CRC [g / g] = {(Wc - B) - Sc} / Sc

[0084] [Dissolved Content] 500 g of saline in a 500 mL beaker was stirred with a stirrer (cylindrical, 8 mm diameter x 30 mm length, no ring) rotating at 600 rpm. The temperature of the saline was 25 ° C. 2.000 g of water-absorbent resin particles were added thereto, and the dispersion containing the water-absorbent resin particles was stirred for 3 hours. The dispersion was filtered through a standard sieve with a mesh size of 75 μm, and the filtrate was recovered. 80 g of the obtained filtrate was weighed into a weighed 100 mL beaker that had been previously weighed at 140 ° C. The filtrate in the beaker was heated in a 140 ° C. air-blowing dryer (FV-320, manufactured by ADVANTEC) for 15 hours to remove moisture, and the mass Wa (g) of the remaining solid component was measured. A blank test was performed using the same procedure as above, without adding water-absorbent resin particles to the saline, and the mass Wb (g) of the solid component remaining in the beaker was measured. The dissolved content was calculated according to the following formula. Dissolved content [mass%] = {((Wa-Wb) / 80) x 500 / 2} x 100

[0085] [Example 1] A commercially available disposable diaper (Merrys Pants, Suhada Sarasa Air Through, L size, manufactured by Kao Corporation) was prepared, and a water-absorbent resin containing a crosslinked polymer containing acrylic acid and an acrylate salt as a monomer unit (the valence of the acidic group of the monomer unit constituting the crosslinked polymer is 1) was collected from inside the disposable diaper.

[0086] As shown in Figure 3, a reflux condenser, a nitrogen gas inlet pipe, and a stirrer (a 4L round-bottom cylindrical separable flask equipped with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 100 mm, manufactured by DURAN Co., Ltd.) were prepared. 150 g of water-absorbent resin collected from disposable diapers was weighed into the separable flask, and 1350 g of ion-exchanged water was added and allowed to stand for 5 minutes to swell the water-absorbent resin. Next, 1500 g of a 0.5 mol / kg aqueous sodium hydroxide solution was added to the separable flask, and the internal temperature was maintained at 80 ° C. for 24 hours. While stirring with a stirring blade at 160 rpm, the crosslinked structure of the crosslinked polymer of the water-absorbent resin particles was cleaved, and 3000 g of an uncrosslinked polymer solution was obtained.

[0087] Next, 750 g of the uncrosslinked polymer solution was placed in a 5 L polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 100 mm). While stirring, 750 g of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), and a solid polymer was precipitated to obtain a polymer dispersion. After the ethanol was added dropwise, stirring was stopped. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a 75 μm mesh for 30 minutes to drain, thereby obtaining a purified uncrosslinked polymer. The moisture content and residual sodium hydroxide content of the purified uncrosslinked polymer were measured using the following methods.

[0088] (Moisture content) 2.0 g of the purified uncrosslinked polymer was placed in an aluminum wheel case (No. 8) previously adjusted to a constant weight (Wf (g)), and the total weight (Wg (g)) of the purified uncrosslinked polymer and the aluminum wheel case was precisely weighed. The purified uncrosslinked polymer was dried together with the aluminum wheel case in a hot air dryer (manufactured by ADVANTEC) with the internal temperature set to 105°C for 2 hours, and then allowed to cool in a desiccator, and the mass after drying (Wh (g)) was measured. The moisture content of the purified uncrosslinked polymer was calculated from the following formula: Moisture content [mass%] = 100 × [(Wg - Wf) - (Wh - Wf)] ÷ (Wg - Wf)

[0089] (Method for measuring residual sodium hydroxide rate) 0.139 g of purified uncrosslinked polymer (moisture content 64% by mass) and 49.861 g of ion-exchanged water were weighed into a 100 mL glass beaker and mixed for 60 minutes while stirring with a magnetic stir bar (8 mmφ x 30 mm without ring) to prepare a 0.1% by mass uncrosslinked polymer solution (50 g). Measurement of residual sodium hydroxide was performed using an automatic titrator (COM-1600) manufactured by Hiranuma Sangyo Co., Ltd. While stirring the prepared uncrosslinked polymer solution, 0.0125 mL of 0.1 M hydrochloric acid was added dropwise every 2 seconds until the pH reached 10. Subsequently, 0.025 mL of the same hydrochloric acid was added dropwise every 2 seconds to the same solution until the pH reached 2.7. Separately, 50 g of ion-exchanged water (Blank) was prepared, and 0.025 mL of 0.1 M sodium hydroxide was added dropwise every 2 seconds until the pH reached 10. Subsequently, 0.025 mL of 0.1 M hydrochloric acid was added dropwise every 2 seconds to the same solution until the pH reached 2.7. The amount of hydrochloric acid added until the pH of the prepared uncrosslinked polymer solution reached 10 (A (mL)), the amount of hydrochloric acid added until the pH of the prepared uncrosslinked polymer solution reached 10 from 10 to 2.7: B (mL), the amount of hydrochloric acid added until the pH of the Blank reached 10 to 2.7: B1 (mL), the molecular weight of sodium hydroxide: C (40 g / mol) and the molecular weight of sodium acrylate: D (94 g / mol) were used to calculate the residual rate of sodium hydroxide relative to the pure uncrosslinked polymer content not containing water, based on the following formula. Residual rate of sodium hydroxide [mass %]=100×A×C / {D×(B−B1)}

[0090] 50 g of purified uncrosslinked polymer (moisture content 64%) was weighed into a 500 mL polybeaker and stirred at 300 rpm for 2 hours using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 50 mm) to obtain a 10% diluted and purified uncrosslinked polymer solution. 18.7 g of cation exchange resin (DOWEX 650C strongly acidic ion exchange resin No. 7 (H type), manufactured by DuPont, ion exchange capacity: 2 eq / L, density: 781.9 g / L) was added to the 10% diluted and purified uncrosslinked polymer solution (180 g), and stirred for 1 hour using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four-blade inclined paddle blade with a blade diameter of 50 mm). After stirring, the cation exchange resin was filtered off by pressure filtration to obtain an acidified polymer solution. The amount of cation exchange resin added was determined based on the uncrosslinked polymer concentration in the uncrosslinked polymer solution and the residual rate of sodium hydroxide in the uncrosslinked polymer, assuming that the degree of neutralization of the uncrosslinked polymer was 100 mol%, by calculating the value that would result in a degree of neutralization of the acidified polymer of 75 mol%. The degree of neutralization of the acidified polymer contained in the obtained acidified polymer solution was 75 mol%. The degree of neutralization of the acidified polymer was measured by the following method.

[0091] (Method for measuring the degree of neutralization of acidified polymer) The moisture content of the acidified polymer contained in the obtained acidified polymer solution was measured using the method described above. 0.7692 g of acidified polymer solution containing this acidified polymer (moisture content 93.5%) was weighed out and 49.2308 g of ion-exchanged water was placed in a 100 mL glass beaker and stirred for 60 minutes with a magnetic stir bar (8 mmφ x 30 mm without ring) to prepare a 0.1 mass% acidified polymer solution (50 g). The degree of neutralization was measured using an automatic titrator (COM-1600) manufactured by Hiranuma Sangyo Co., Ltd. While stirring the prepared acidified polymer solution, 0.025 mL of 0.1 M sodium hydroxide was added dropwise every 2 seconds until the pH reached 10. Subsequently, 0.025 mL of 0.1 M hydrochloric acid was added dropwise to the same solution every 2 seconds until the pH reached 2.7. Separately, the same procedure was carried out on 50 g of ion-exchanged water (Blank). The degree of neutralization of the acidified polymer was calculated based on the following formula: A (mL) of sodium hydroxide added until the pH of the prepared acidified polymer solution reached 10, B (mL) of hydrochloric acid added until the pH of the prepared acidified polymer solution reached 2.7 from 10 to 2.7, A1 (mL) of sodium hydroxide added until the pH of the Blank reached 10, and B1 (mL) of hydrochloric acid added until the pH of the Blank reached 2.7 from 10 to 2.7. Neutralization degree of the acidified polymer [mol %] = 100 × {(B - B1) - (A - A1)} / (B - B1).

[0092] To the acidified polymer solution (100 g) containing this acidified polymer (water content 93.5%), 0.325 g of a crosslinker (1% by mass aqueous solution of ethylene glycol diglycidyl ether) was added and stirred for 30 minutes. After stirring, the acidified polymer was internally crosslinked by drying in a hot air dryer at 115 ° C for 3 hours to obtain a water-absorbent resin. The water-absorbent resin was pulverized with a centrifugal crusher (1 mm screen, 6000 rpm), and then pulverized using an 850 μm standard sieve and an 180 μm standard sieve. The water-absorbent resin (particle size 180-850 μm) that passed through the 850 μm standard sieve but not the 180 μm standard sieve was collected. The collected water-absorbent resin (4.55 g) was placed in a separable flask, and 0.2275 g of a crosslinker (1% by mass aqueous solution of ethylene glycol diglycidyl ether) was sprayed and added, followed by stirring for 100 seconds. After stirring, the mixture was dried in a hot air dryer at 120°C for 2 hours to perform surface crosslinking. After drying, the mixture was passed through an 850 µm standard sieve, and the water-absorbent resin that fell below the 850 µm sieve was collected as water-absorbent resin particles. The CRC and soluble content of the collected water-absorbent resin particles were measured. The measurement results are shown in Table 1.

[0093] Example 2 After obtaining 100 g of a purified uncrosslinked polymer (moisture content 64%) in the same manner as in Example 1, a portion (25 g) of the purified uncrosslinked polymer was placed in a 300 mL plastic beaker. 65 g of ion-exchanged water was added to the 300 mL plastic beaker, and the mixture was stirred at 300 rpm for 2 hours using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 50 mm), to obtain a purified uncrosslinked polymer solution diluted to 10%. A cation exchange resin (DOWEX 650C strongly acidic ion exchange resin No. 7 (H type), manufactured by DuPont, ion exchange capacity: 2 eq / L, density: 781.9 g / L) was added to a 300 mL polybeaker (1.2 times the amount required to adjust the neutralization degree of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol%), and the mixture was stirred for 1 hour with a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four-blade inclined paddle with a blade diameter of 50 mm). After stirring, the cation exchange resin was filtered off by pressure filtration to obtain a neutralization degree-adjusted solution. The amount of cation exchange resin added was determined based on the assumption that the neutralization degree of the uncrosslinked polymer was 100 mol%, and the value at which the neutralization degree of the acidified polymer was 0 mol% was determined based on the uncrosslinked polymer concentration of the uncrosslinked polymer solution and the residual rate of sodium hydroxide in the uncrosslinked polymer, and the excess amount (1.2 times the amount in this example) was determined. The degree of neutralization of the acidified polymer contained in the obtained neutralization degree adjusted solution was measured by the above-mentioned method and found to be 0 mol %.

[0094] 25 parts by mass of the neutralization degree adjusting solution and the remaining 75 parts by mass of the uncrosslinked polymer solution were mixed for 30 minutes to obtain an acidified polymer solution. The degree of neutralization of the acidified polymer contained in the acidified polymer solution was measured by the above-mentioned method and was found to be 75%. Thereafter, the water-absorbent resin particles were recovered in the same manner as in Example 1, and the CRC and soluble content of the recovered water-absorbent resin particles were measured. The measurement results are shown in Table 1.

[0095] Comparative Example 1 50 g of purified uncrosslinked polymer (moisture content 64%) obtained in the same manner as in Example 1 was weighed into a 500 mL plastic beaker and stirred at 300 rpm for 2 hours using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 50 mm) to obtain an uncrosslinked polymer solution diluted and purified to 10%. Assuming that the degree of neutralization of the uncrosslinked polymer is 100 mol%, the amount of hydrochloric acid (1.74 g) required to neutralize the uncrosslinked polymer contained in the diluted and purified uncrosslinked polymer solution to 75 mol% was calculated based on the uncrosslinked polymer concentration of the uncrosslinked polymer solution and the residual rate of sodium hydroxide in the uncrosslinked polymer, and this amount of hydrochloric acid was added to the diluted and purified uncrosslinked polymer solution. The mixture was stirred for 1 hour using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four inclined paddles with a blade diameter of 50 mm) to obtain an acidified polymer solution. The degree of neutralization of the acidified polymer contained in the acidified polymer solution was measured by the method described above, and was found to be 75 mol%. Thereafter, the water-absorbent resin particles were recovered using the same procedure as in Example 1, and the CRC and soluble content of the recovered water-absorbent resin particles were measured. The measurement results are shown in Table 1.

[0096] (Reference Example 1) A water-absorbent resin was collected from the disposable diaper (Merry's Pants, Smooth Air Through, L size, manufactured by Kao Corporation) used in the Examples. The CRC and soluble content of the collected water-absorbent resin were measured. The measurement results are shown in Table 1.

[0097]

[0098] Examples 1 and 2 are simulations of the case where a cation exchange resin is used to prepare an acidified polymer solution and used water-absorbent resin contained in used disposable diapers is regenerated. On the other hand, Comparative Example 1 is a simulation of the case where an acid is used to prepare an acidified polymer solution and used water-absorbent resin contained in used disposable diapers is regenerated. The water-absorbent resin particles of Examples 1 and 2 have a larger CRC and a smaller dissolved content than the water-absorbent resin particles of Comparative Example 1, and therefore can be said to have excellent water absorption performance.

[0099] 1...water-absorbent resin particles, 10...absorbent body, 20a, 20b...core wrap sheet, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 50...water-absorbent sheet, 100...absorbent article, 110...round-bottom cylindrical separable flask, 120...four-inclined paddle blade, 130...nitrogen gas inlet pipe, 140...reflux condenser, 150...oxygen concentration meter, 160...gas outlet pipe, 170...stirring motor, 180...shaft holder, 185...stirring shaft, 190, 195...three-way cock, 200...treatment tank, 250...heat seal portion, 300...nonwoven fabric bag.

Claims

1. A method for producing an acidified polymer solution, comprising: a step of contacting a water-absorbent resin having acidic groups with a basic component to obtain an uncrosslinked polymer solution containing an uncrosslinked polymer formed by cleavage of a crosslinked structure of the water-absorbent resin; and a step of contacting the uncrosslinked polymer solution with a cation exchange resin to obtain an acidified polymer solution containing an acidified polymer having a degree of neutralization lower than that of the uncrosslinked polymer.

2. The manufacturing method of claim 1, wherein the step of obtaining the acidified polymer solution comprises a step of mixing the uncrosslinked polymer solution after contact with the cation exchange resin with the uncrosslinked polymer solution that has not been contacted with the cation exchange resin.

3. The manufacturing method according to claim 2, wherein the mixing step includes setting a target value for the degree of neutralization of the acidified polymer, and adjusting a mixing ratio of the uncrosslinked polymer solution after contact with the cation exchange resin and the uncrosslinked polymer solution not contacted with the cation exchange resin based on the target value.

4. The manufacturing method according to any one of claims 1 to 3, wherein the mass of the cation exchange resin to be brought into contact with the uncrosslinked polymer solution is 1.0X (g) to 1.5X (g), where X (unit: g) is the mass of the cation exchange resin required to bring the degree of neutralization of the uncrosslinked polymer contained in the uncrosslinked polymer solution to 0 mol%.

5. A method for producing water-absorbent resin particles, comprising a step of crosslinking the acidified polymer contained in the acidified polymer solution obtained by the production method according to any one of claims 1 to 3 to obtain a water-absorbent resin.

Images