Water-absorbent resin mixture and method for producing water-absorbent resin using water-absorbent resin mixture as part of raw material

Abstract

Provided is technology whereby it is possible to obtain a recycled water-absorbent resin mixture that contains a water-absorbent resin and a small amount of materials other than the water-absorbent resin, and that exhibits little deterioration in water absorption properties and powder characteristics. This water-absorbent resin mixture contains a water-absorbent resin and a material other than the water-absorbent resin and satisfies i) and ii): i) the content of materials other than the water-absorbent resin is 0.01-4.0 mass% with respect to the water-absorbent resin mixture; ii) the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with openings of 150 μm is 0-1.0 mass% with respect to the water-absorbent resin mixture.

Description

Water-absorbent resin mixture and method for producing water-absorbent resin using water-absorbent resin mixture as part of raw material

[0001] The present invention relates to a water-absorbent resin mixture, a method for recycling the same, and a method for producing a water-absorbent resin using the water-absorbent resin mixture as part of a raw material.

[0002] Super absorbent polymers (SAPs) are water-swellable, water-insoluble polymer gelling agents, and are used in a variety of absorbent articles, including sanitary materials (hygienic products) such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), and pet sheets, as well as soil water retention agents for agricultural and horticultural use and industrial water-stopping agents.

[0003] Among these, the main applications of water-absorbent resins are sanitary materials such as disposable diapers, sanitary napkins, etc. After being used for a short period of time (at most about one day), these sanitary materials are discarded in large quantities as used absorbent articles and incinerated.

[0004] In recent years, attempts have been made to recycle such used absorbent articles from the viewpoints of environmental protection, effective utilization of resources, etc. As one measure, a method has been considered in which components of the absorbent article, particularly pulp and water-absorbent resin, are separated, recovered, and reused, without relying on incineration.

[0005] For example, Patent Document 1 describes a method for recovering pulp from used absorbent articles by subjecting an inactivated aqueous solution containing pulp and a water-absorbent resin separated from the used absorbent articles to solid-liquid separation and then treating the solution by a specific method. Also, Patent Document 2 describes a method for producing a recycled superabsorbent polymer by inactivating a used superabsorbent polymer derived from used sanitary products with an acidic solution and then subjecting the polymer to a specific treatment.

[0006] Japanese Patent Application Laid-Open No. 2019-85447 Japanese Patent Application Laid-Open No. 2021-41310 Japanese Patent Application Laid-Open No. 04-317785

[0007] National Environmental Research Association Journal, Vol. 36, No. 1 (2011), P51-58

[0008] According to the method described in Patent Document 1, the water-absorbent resin is removed from the pulp containing the remaining water-absorbent resin, which has been separated from a used absorbent article, and the water-absorbent resin is separated from the pulp. Also, Patent Document 2 describes that foreign matter (pulp, etc.) can be separated from the recycled superabsorbent polymer by performing a foreign matter separation step of separating foreign matter such as pulp from the recycled superabsorbent polymer after the drying step.

[0009] However, for example, when used absorbent articles are recycled as in the techniques described in the above-mentioned Patent Documents 1 and 2, even after a separation step of the water-absorbent resin from materials other than the water-absorbent resin (pulp, nonwoven fabric, resin film, rubber, adhesive, etc.) (sometimes referred to as "foreign matter"), a small amount of materials other than the water-absorbent resin is contained in the separated water-absorbent resin. The presence of these materials other than the water-absorbent resin not only deteriorates the appearance of the recycled water-absorbent resin, but also poses the problem of deteriorating the water-absorbing properties and powder properties.

[0010] Furthermore, although Patent Document 2 describes a method of using a recycled water absorbent resin as a raw material or semi-finished product for producing a normal water absorbent resin, the recycled water absorbent resin containing foreign matter and having reduced powder properties has a problem of being very difficult to use because its handleability differs from that of the raw material or semi-finished product for producing a normal water absorbent resin. Therefore, there is a demand for a recycled water absorbent resin with little deterioration in water absorption properties and powder properties.

[0011] Therefore, an object of the present invention is to provide a technique by which a recycled water absorbent resin mixture containing a water absorbent resin and a small amount of a material other than the water absorbent resin can be obtained, which is less likely to deteriorate in water absorption properties and powder characteristics.

[0012] In order to solve the above problems, one aspect of the present invention includes the following configuration. [1] A water-absorbent resin mixture containing a water-absorbent resin and a material other than the water-absorbent resin, which satisfies the following i) and ii): i) a content of the material other than the water-absorbent resin is 0.01% by mass or more and 4.0% by mass or less with respect to the water-absorbent resin mixture; ii) a content of the material other than the water-absorbent resin remaining on a JIS standard sieve having an opening of 150 μm is 0% by mass or more and 1.0% by mass or less with respect to the water-absorbent resin mixture. [2] The water-absorbent resin mixture according to [1], wherein the content of a material other than the water-absorbent resin is determined by a method for quantifying a material other than the water-absorbent resin, the method including the following steps a) to c): a) subjecting the water-absorbent resin mixture to a solubilization technique for the water-absorbent resin mixture, which solubilizes a water-absorbent resin component in water and suppresses solubilization of materials other than the water-absorbent resin in water; b) removing the solubilized water-absorbent resin component obtained in a); c) drying the remaining component obtained in b), and determining the content of a material other than the water-absorbent resin. [3] The content of the material other than the water-absorbent resin remaining on the JIS standard sieve having a mesh size of 150 μm is determined from the content rate P mass % of the material other than the water-absorbent resin in the water-absorbent resin mixture determined by the quantification method by the following formula: Content (mass %) of the material other than the water-absorbent resin remaining on the JIS standard sieve having a mesh size of 150 μm=Content mass ratio of the water-absorbent resin mixture remaining on the JIS standard sieve having a mesh size of 150 μm×P mass %. [4] The water-absorbent resin mixture according to either [2] or [3], wherein a solubilization method is used such that a solubilization rate of the water-absorbent resin in the water-absorbent resin mixture is 90 mass % or more. [5] The water-absorbent resin mixture according to any of [2] to [4], wherein a solubilization method is used such that a solubilization rate of the material other than the water-absorbent resin is less than 30 mass %. [6] The water-absorbent resin mixture according to any of [1] to [5], wherein the water-absorbent resin mixture is in the form of a powder. [7] The water-absorbent resin mixture according to any one of [1] to [6], wherein the water-absorbent resin mixture has a flow rate of 7.0 g / sec or more. [8] The water-absorbent resin mixture according to any one of [1] to [7], wherein the water-absorbent resin mixture has a bulk specific gravity of 0.55 g / ml or more.[9] The water-absorbent resin mixture according to any one of [1] to [8], wherein the water-absorbent resin mixture has a moisture content of 20% by mass or less.

[10] The water-absorbent resin mixture according to any one of [1] to [9], wherein the water-absorbent resin mixture has a mass-average particle diameter (D50) of 200 μm or more and 600 μm or less.

[11] The water-absorbent resin mixture according to any one of [1] to

[10] , wherein the material other than the water-absorbent resin is a constituent material derived from an absorbent article.

[12] The water-absorbent resin mixture according to any one of [1] to

[11] , wherein the material other than the water-absorbent resin comprises at least one selected from the group consisting of pulp, nonwoven fabric, and resin film.

[13] The water-absorbent resin mixture according to any one of [1] to

[12] , wherein the water-absorbent resin mixture is recovered from used absorbent articles.

[14] A method for recycling a water-absorbent resin contained in used absorbent articles, wherein the recycled water-absorbent resin comprises the water-absorbent resin mixture according to any one of [1] to

[13] .

[15] A method for producing a water-absorbent resin, comprising using the water-absorbent resin mixture according to any one of [1] to

[13] as a part of a raw material in a process for producing a water-absorbent resin using a monomer constituting the water-absorbent resin as a raw material.

[16] The production method according to

[15] , wherein a proportion of the water-absorbent resin mixture to all raw materials for the water-absorbent resin is 1% by mass or more and 60% by mass or less.

[0013] Preferred embodiments of the present invention will be described below. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims. Furthermore, the embodiments described in this specification can be arbitrarily combined to form other embodiments.

[0014] One embodiment of the present invention is a water-absorbent resin mixture containing a water-absorbent resin and a material other than the water-absorbent resin, wherein the water-absorbent resin mixture satisfies the following i) and ii): i) a content of the material other than the water-absorbent resin is 0.01% by mass or more and 4.0% by mass or less with respect to the water-absorbent resin mixture; ii) a content of the material other than the water-absorbent resin remaining on a JIS standard sieve having an opening of 150 μm is 0% by mass or more and 1.0% by mass or less with respect to the water-absorbent resin mixture.

[0015] According to the above-described embodiment, in a water-absorbent resin mixture or a recycled water-absorbent resin containing a water-absorbent resin and a material other than the water-absorbent resin, by controlling the amount of the material other than the water-absorbent resin contained therein and the size of the material other than the water-absorbent resin within a certain range, it is possible to obtain a recycled water-absorbent resin mixture in which the deterioration (change) of water absorption properties (e.g., CRC) and powder properties (e.g., flow rate and bulk specific gravity) is small compared to the original water-absorbent resin, and in particular the deterioration (change) of powder properties is small. Furthermore, according to the above-described embodiment, it is possible to obtain a recycled water-absorbent resin mixture excellent in water-absorption properties (e.g., Vortex (water absorption speed)).

[0016] As used herein, the term "(meth)acrylic" encompasses both acrylic and methacrylic. Thus, for example, the term "(meth)acrylic acid" encompasses both acrylic acid and methacrylic acid. As used herein, "acid (salt)" means "acid and / or its salt."

[0017] [1] Water-absorbent resin mixture [1-1] Water-absorbent resin mixture In this specification, the term "water-absorbent resin mixture" refers to a mixture of a water-absorbent resin and a material other than the water-absorbent resin. Examples of the water-absorbent resin mixture include a mixture recovered from an unused absorbent article and a mixture recovered from a used absorbent article. In this embodiment, the water-absorbent resin mixture is preferably a mixture recovered from a used absorbent article, for which the quantitative determination of materials other than the water-absorbent resin is more necessary.

[0018] (Water-absorbent resin mixture recovered from used absorbent articles) In one embodiment of the present invention, the water-absorbent resin mixture includes one recovered from used absorbent articles. In this specification, "used absorbent article" refers to a used sanitary material that has been used by a consumer and has absorbed body fluids such as excrement, urine, blood, etc. In other words, a used absorbent article contains a water-absorbent resin that has swollen with body fluids. Examples of sanitary materials include sanitary materials (hygienic products) such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), and pet sheets.

[0019] Here, the state of use is not particularly limited. For example, the state may be one in which solid waste (such as feces) is attached, one in which liquid waste (such as urine or menstrual blood) is attached, or one in which both are attached, but in consideration of the cost and processing efficiency of the recycling process, it is preferable that the state be one in which urine is absorbed as the main component.

[0020] In addition, the used absorbent articles used in the present invention include, for example, those discarded from ordinary households, hospitals, welfare facilities, and other facilities where users of absorbent articles live or stay, which are collected, recovered, and transported.

[0021] [1-2] Water-absorbent resin In this specification, the term "water-absorbent resin" refers to a water-swellable, water-insoluble polymer gelling agent, and is not particularly limited, but refers to a commonly used water-absorbent resin having a water absorption capacity of 10 to 1000 times. More specifically, it is preferable that the water-absorbent resin before absorbing the liquid to be absorbed satisfies the physical property of CRC of 5 g / g or more as defined in ERT441.2-02 as "water-swellability", and the physical property of Ext (water-soluble content) of 0 mass% or more and 50 mass% or less as defined in ERT470.2-02 as "water-insolubility".

[0022] The water-absorbent resin may be a polymer derived from a carboxyl group-containing unsaturated monomer. The water-absorbent resin may include a polymer having a partially neutralized carboxyl group. Specific examples of the water-absorbent resin include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, cellulose-based resins, (meth)acrylate crosslinked polymers, crosslinked saponified (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.

[0023] In one embodiment of the present invention, the water-absorbent resin is a polymer derived from a carboxyl group-containing unsaturated monomer, and the water-absorbent resin may include a polymer having a partially neutralized carboxyl group. Furthermore, in one embodiment of the present invention, the water-absorbent resin is a polyacrylic acid (salt)-based resin. Furthermore, the polyacrylic acid (salt)-based resin may be a crosslinked structure of a polymer derived from a carboxyl group-containing unsaturated monomer (a polymer of a carboxyl group-containing unsaturated monomer and an internal crosslinking agent (e.g., a compound described in U.S. Patent No. 6,241,928)). Furthermore, the polyacrylic acid (salt)-based resin may be surface-crosslinked with a surface crosslinking agent (e.g., a surface crosslinking agent disclosed in U.S. Patent No. 7,183,456).

[0024] In this specification, the "water-absorbent resin" is not limited to an embodiment in which the total amount (100 mass %) is the water-absorbent resin alone, but may be a water-absorbent resin composition containing additives and the like.

[0025] The mass percentage of the water-absorbent resin contained in the water-absorbent resin mixture is preferably 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more. The mass percentage of the water-absorbent resin contained in the water-absorbent resin mixture to be solubilized may be 99.99% by mass or less, 99.9% by mass or less, 99.5% by mass or less, or 99% by mass or less. The mass percentage of the water-absorbent resin contained in the water-absorbent resin mixture to be solubilized may be 70% by mass or more and 99.99% by mass or less, 75% by mass or more and 99.9% by mass or less, 80% by mass or more and 99.5% by mass or less, or 85% by mass or more and 99% by mass or less. Here, the content of the water-absorbent resin in the water-absorbent resin mixture can be determined by a quantification method using a water-absorbent resin solubilization technique described below that suppresses the solubilization of materials other than the water-absorbent resin in water.

[0026] [1-3] Materials other than water-absorbent resin In this specification, "materials other than water-absorbent resin" refers to constituent components other than the water-absorbent resin in the water-absorbent resin mixture. Specifically, the constituent components other than the water-absorbent resin are preferably foreign matter other than the water-absorbent resin composition including additives contained in the manufacturing process of the water-absorbent resin. For example, in the case of a water-absorbent resin mixture obtained in the process of recycling from an absorbent article, constituent components other than the water-absorbent resin that constitutes the absorbent article, such as pulp (preferably fibrous pulp), nonwoven fabric (and / or fibrous materials derived therefrom; in this specification, nonwoven fabric refers to nonwoven fabric and / or fibrous materials derived therefrom), resin films, rubber, adhesives, etc., correspond to foreign matter. Specifically, the following can be mentioned as foreign matter.

[0027] i) Pulp: wood pulp, semi-synthetic fibers such as rayon and acetate, ii) Nonwoven fabric: nonwoven fabric using fibers such as rayon, polyester, polypropylene, and polyethylene, iii) Resin film: polyethylene and polypropylene films, iv) Rubber: polyurethane, natural rubber, synthetic rubber, etc., v) Others: hot melt adhesives, pressure sensitive adhesives, etc. for joining components together Among these, materials other than the water-absorbent resin are contained in a high amount in the absorbent article, are easily mixed into the water-absorbent resin mixture, and are easily solubilized by solubilization treatment, so it is preferable to include at least one selected from the group consisting of pulp, nonwoven fabric, and resin film, it is more preferable to include pulp and / or nonwoven fabric, and it is even more preferable to include pulp. In particular, fibrous materials such as pulp and nonwoven fabric have a high bulk density and can be a factor that significantly reduces the flow properties of the water-absorbent resin, so it is more preferable to include pulp and / or nonwoven fabric, as this makes it easier to achieve the effects of the present invention.

[0028] Incidentally, the material other than the water-absorbent resin does not include a material that exists only inside the water-absorbent resin particle, but corresponds to a material that exists outside the water-absorbent resin particle, or a material in which at least a part of the material other than the water-absorbent resin exists outside the water-absorbent resin particle, even if the material corresponds to a material other than the water-absorbent resin. This is because a material other than the water-absorbent resin that exists only inside the water-absorbent resin particle has little adverse effect on the water absorption properties and powder characteristics of the water-absorbent resin mixture, which is the subject of the present application.

[0029] Furthermore, the materials other than the water-absorbent resin do not include additives added during the production of the water-absorbent resin, such as water-soluble polyvalent metal cation-containing compounds, polyvalent metal salts, cationic polymers, chelating agents, inorganic reducing agents, α-hydroxycarboxylic acid compounds, water-insoluble inorganic particles, surfactants, non-polymeric water-soluble compounds, etc.

[0030] In particular, the water-insoluble inorganic particles are likely to be added during the production of the water-absorbent resin, and the absence of water-insoluble inorganic particles allows for more accurate quantification of the amount of foreign matter derived from absorbent articles such as pulp. The amount of water-insoluble inorganic particles can be quantified, for example, by separating the insolubilized matter from the solubilized matter obtained by the solubilization treatment, adding nitric acid to the insolubilized matter and dissolving it by heating, and then quantifying the inorganic metal atoms by ICP emission spectrometry. Therefore, the amount of foreign matter can be quantified with higher accuracy by subtracting the quantified amount of water-insoluble inorganic particles from the content determined by the "method for quantifying the content of materials other than the water-absorbent resin" described below.

[0031] (Polyvalent Metal Salt and / or Cationic Polymer) Specific examples of the polyvalent metal salt and / or cationic polymer include the compounds disclosed in “[7] Polyvalent Metal Salt and / or Cationic Polymer” of WO 2011 / 040530.

[0032] (Chelating Agent) Specific examples of the chelating agent include the compounds disclosed in “[2] Chelating Agents” of WO 2011 / 040530.

[0033] (Inorganic Reducing Agent) Specific examples of the inorganic reducing agent include the compounds disclosed in the section “(3) Inorganic Reducing Agents” of WO 2011 / 040530.

[0034] (α-Hydroxycarboxylic Acid Compound) Specific examples of the α-hydroxycarboxylic acid compound include the compounds disclosed in “[6] α-Hydroxycarboxylic Acid Compound” of WO 2011 / 040530.

[0035] (Water-insoluble inorganic particles) Specific examples of water-insoluble inorganic particles include polymetallic compounds such as hydrotalcite, silicon dioxide (silica), aluminum hydroxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, metal phosphates (e.g., calcium phosphates such as tricalcium phosphate, barium phosphate, aluminum phosphate), metal borates (e.g., titanium borate, aluminum borate, iron borate, magnesium borate, manganese borate, calcium borate), silicic acid or salts thereof, clay, diatomaceous earth, zeolite, bentonite, kaolin, activated clay, and the like.

[0036] (Surfactant) Specific examples of the surfactant include surfactants disclosed in WO 97 / 017397 and U.S. Pat. No. 6,107,358, i.e., nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants, etc.

[0037] (Non-polymerized water-soluble compound) Specific examples of the non-polymerized water-soluble compound include the compounds disclosed in the "Non-polymerized water-soluble compound" section of WO 2014 / 034667.

[0038] As a method for determining whether or not a material corresponds to a material other than a water absorbent resin, for example, in the method shown in "Method for quantifying content of material other than water absorbent resin in water absorbent resin mixture" described later, a material that is not solubilized by a "solubilizer" that solubilizes only a water absorbent resin and remains can be determined as a "material other than a water absorbent resin".

[0039] (Content of materials other than water-absorbent resin) (Content of foreign matter) In this specification, "content of materials other than water-absorbent resin (content of foreign matter)" means mass % of all "materials other than water-absorbent resin" contained in a water-absorbent resin mixture. When the content of the materials other than water-absorbent resin is not known, such as in a used absorbent article, the content of the materials other than water-absorbent resin can be determined by a quantification method for materials other than water-absorbent resin in a water-absorbent resin mixture, in which a solubilization treatment is performed to solubilize the water-absorbent resin in water to obtain a solubilized product, and the amount of the materials other than water-absorbent resin is quantified from the solubilized product, and the solubilization treatment uses a solubilization technique for the water-absorbent resin that suppresses the solubilization of materials other than the water-absorbent resin in water.

[0040] More specifically, the content of the material other than the water-absorbing resin is preferably determined by a method for quantifying the material other than the water-absorbing resin, which includes the following steps a) to c).

[0041] a) solubilizing the water-absorbent resin in water using a water-absorbent resin solubilization technique for solubilizing the water-absorbent resin in the water-absorbent resin mixture while suppressing the solubilization of materials other than the water-absorbent resin in water; b) removing the solubilized water-absorbent resin obtained in a); c) drying the remaining component obtained in b), and determining the content of materials other than the water-absorbent resin. More specifically, the content can be determined in accordance with the "method for quantifying the content of materials other than the water-absorbent resin" (hereinafter referred to as "Invention A") described below.

[0042] In this case, it is preferable to use a solubilization method that results in a solubilization rate of the water-absorbent resin in the water-absorbent resin mixture of 90% by mass or more. Furthermore, it is preferable to use a solubilization method that results in a solubilization rate of materials other than the water-absorbent resin of less than 30% by mass. Specifically, this will be described in detail in the section "Method for quantifying the content of materials other than the water-absorbent resin" described later. In addition, when there are multiple solubilization methods, the content of materials other than the water-absorbent resin can be determined by any of the solubilization methods.

[0043] In one embodiment of the present invention, the content of the material other than the water-absorbent resin is 0.01% by mass or more and 4.0% by mass or less, preferably 0.05% by mass or more and 3.5% by mass or less, more preferably 0.1% by mass or more and 3.0% by mass or less, and may also be 0.15% by mass or more and 4.0% by mass or less, 0.15% by mass or more and 3.5% by mass or less, 0.2% by mass or more and 3.0% by mass or less, or 0.3% by mass or more and 2.0% by mass or less. If the content of the material other than the water-absorbent resin exceeds 4.0% by mass, the powder properties (particularly, fluidity and bulk specific gravity) are significantly reduced. On the other hand, by controlling the content of the material other than the water-absorbent resin within the above range, a water-absorbent resin mixture can be obtained which exhibits little reduction in water absorption properties and powder properties, and further, a water-absorbent resin mixture can be obtained which exhibits improved water absorption properties (e.g., Vortex). Materials other than the water-absorbent resin deteriorate the water absorption properties and appearance of the recycled water-absorbent resin, and therefore it is considered desirable that the content thereof be as small as possible. However, as described above, from the viewpoint of powder properties, a content of up to 4.0 mass % is permissible (4.0 mass % is critical for powder properties), while surprisingly, when the content is equal to or more than the lower limit, the water absorption properties (for example, Vortex) are improved.

[0044]

[0033] (Method for quantifying content (foreign matter content) of material other than water absorbent resin in water absorbent resin mixture) (Invention A) In this specification, "method for quantifying content (foreign matter content) of material other than water absorbent resin in water absorbent resin mixture" is a method for selectively solubilizing a water absorbent resin contained in a water absorbent resin mixture in water by using a specific agent or the like, and quantifying the content of a component that has not been solubilized as "material other than water absorbent resin (foreign matter)". Hereinafter, the quantification method (Invention A) will be described in detail.

[0045] (Invention A) The present specification includes, as one aspect, Invention A relating to a method for quantifying the content (foreign matter content) of a material other than a water-absorbent resin in a water-absorbent resin mixture. The quantification method can be used when determining the content of a material other than the water-absorbent resin of the present invention. Furthermore, the description of the quantification method is incorporated by reference in relation to the content of a material other than the water-absorbent resin of the present invention.

[0046] As described above, after being used for a short period of time (at most about one day), sanitary materials are discarded in large quantities as used absorbent articles and incinerated.

[0047] In recent years, attempts have been made to recycle such used absorbent articles from the viewpoint of environmental protection, etc. For example, sanitary materials contain raw materials such as pulp, nonwoven fabrics, adhesives, etc. in addition to the above-mentioned water-absorbent resin, and material recycling has been attempted in which these raw materials are separated from used sanitary materials and reused. Also, a method has been developed in which the water-absorbent resin is solubilized (decomposed) to separate it from other components, and then the solubilized polymer is added to a production process for the water-absorbent resin for reuse. As a technique for solubilizing (decomposing) the water-absorbent resin, for example, a technique in which the water-absorbent resin is heated in an aqueous solution of an oxidizing agent is known (Patent Document 3).

[0048] Furthermore, for the purpose of quantifying the recovery efficiency of materials recovered from used absorbent articles and the amount of impurities in the obtained recycled products, Non-Patent Document 1 describes that a sedimentation separation method can be used as a method for quantifying the amount of pulp and water-absorbent resin contained in a sample during a recycling process.

[0049] When recycling a water-absorbent resin from used absorbent articles, a process of separating foreign matter such as materials other than the water-absorbent resin, for example, in the case of disposable diapers, constituent materials such as pulp, nonwoven fabric, plastic, etc. is included, but even after such a separation process, a small amount of foreign matter may still be contained in the water-absorbent resin. The presence of these foreign matters deteriorates the water-absorbing properties and appearance of the recycled water-absorbent resin, so it is desirable that the content of foreign matter is as small as possible.

[0050] In order to increase the purity of the water-absorbent resin in the recycled water-absorbent resin, it is necessary to detect with high accuracy the amount of materials other than the water-absorbent resin contained in the sample during the recycling process.

[0051] Therefore, the present invention also includes a method for quantifying the content of materials other than the water-absorbent resin with high measurement accuracy.

[0052] One aspect of the present invention is a method for quantifying a material other than a water-absorbent resin in a water-absorbent resin mixture, the method comprising: a step (1) of performing a solubilization treatment on a water-absorbent resin mixture containing a water-absorbent resin and a material other than the water-absorbent resin to solubilize the water-absorbent resin in water to obtain a solubilized product; and a step (2) of quantifying the material other than the water-absorbent resin from the solubilized product, wherein in the step (1), a water-absorbent resin solubilization technique that suppresses solubilization of the material other than the water-absorbent resin in water is used.

[0053] With such a configuration, when quantifying a material other than the water-absorbent resin contained in a water-absorbent resin mixture, a quantitative determination method with less measurement error (deviation from an actual value) can be provided. Also, when quantifying a material other than the water-absorbent resin contained in a water-absorbent resin mixture, a simple quantitative determination method can be provided.

[0054] In recycling absorbent articles, the water-absorbent resin is usually separated from constituent materials derived from the absorbent article other than the water-absorbent resin. Specific examples of processes for recycling the water-absorbent resin from used absorbent articles to obtain recycled water-absorbent resin include a cleaning and sterilization process for cleaning and sterilizing used absorbent articles; a crushing process for crushing exterior materials and the like of absorbent articles, specifically a process for breaking nonwoven fabrics or plastic sheets such as top sheets and back sheets that constitute molded products such as sanitary materials, and releasing the water-absorbent resin fixed inside; a process for separating materials other than the water-absorbent resin; an inactivation and dehydration treatment process for subjecting the water-absorbent resin to a shrinkage and dehydration treatment (inactivation treatment); a cleaning and disinfection process for cleaning and / or disinfecting the water-absorbent resin; a regeneration process for the water-absorbent resin; and a drying process for drying the water-absorbent resin.

[0055] In addition, the separated and recovered water-absorbent resin is chemically decomposed, and the resulting solubilized polymer is added to the manufacturing process of the water-absorbent resin for reuse.

[0056] The water-absorbent resin obtained by recycling in this manner (hereinafter also referred to as recycled water-absorbent resin) is usually a mixture of the water-absorbent resin and materials other than the water-absorbent resin. This mixture has undergone a separation process in which foreign matter such as pulp, nonwoven fabric, and resin sheets is removed from the absorbent article, and therefore the content of the water-absorbent resin is very high. In such recycled water-absorbent resin, materials (originating from the absorbent article) other than the water-absorbent resin (or the water-absorbent resin containing additives added during the manufacturing process of the water-absorbent resin) are foreign matter, and their inclusion can cause a decrease in water-absorption properties and discoloration. Therefore, recycled water-absorbent resins are required to have a low content of foreign matter, and therefore, accurate quantification of the foreign matter is required.

[0057] Non-Patent Document 1 proposes a method for determining the pulp content from the residue after solubilizing and removing the water-absorbent resin from a water-absorbent resin / pulp mixture. While it is desirable for such a pulp quantification method to have as little measurement error as possible, the present inventors have discovered that a method that simply solubilizes the pulp to determine the pulp content results in measurement errors relative to the actual content. The inventors hypothesized that such measurement errors are due to the following reasons: i) pulp unintentionally dissolved by the solubilization treatment, and ii) the presence of water-absorbent resin that is not completely dissolved by the solubilization treatment, when quantifying the pulp content after the solubilization treatment. Therefore, it was thought that if the presence of water-absorbent resin that is not completely dissolved by the solubilization treatment is not recognized, the quantified pulp content may contain the water-absorbent resin, resulting in measurement errors. In other words, it was found that the pulp content quantified when calculating the pulp recovery rate in Non-Patent Document 1 may include residual water-absorbent resin. It has been found that such residual water absorbent resin is unlikely to become apparent in a mixture having a relatively high pulp content of 40% by mass or more for the purpose of pulp recovery, as in Non-Patent Document 1, but in the case of a low pulp content such as recycled water absorbent resin, the residual water absorbent resin significantly affects measurement errors in the quantification of the amount of foreign matter such as pulp. Based on the above knowledge, it has been found that in the quantification of materials other than the water absorbent resin, measurement errors can be reduced by using a solubilization technique that suppresses the solubilization of materials other than the water absorbent resin in water, preferably, that promotes the solubilization of the water absorbent resin in water.

[0058] Therefore, one preferred embodiment of the present invention is a method for quantifying a content of a material other than a water-absorbent resin in a water-absorbent resin mixture, the method comprising: a step (1) of performing a solubilization treatment on a water-absorbent resin mixture containing a water-absorbent resin and a material other than a water-absorbent resin, the material being at least one kind selected from the group consisting of pulp, a nonwoven fabric, and a resin film, to solubilize the water-absorbent resin in water to obtain a solubilized product; and a step (2) of quantifying the amount of the material other than the water-absorbent resin from the solubilized product, wherein in the step (1), a water-absorbent resin solubilization technique that suppresses solubilization of the material other than the water-absorbent resin in water is used.

[0059] As a method for simply (qualitatively) confirming whether or not the water-absorbent resin or materials other than the water-absorbent resin are contained in the water-absorbent resin mixture, a visual method, a method by component analysis, or the like can be used.

[0060] A visual method may include adding a certain amount of water to a water-absorbent resin mixture. Since the water-absorbent resin absorbs water and swells significantly, it is possible to distinguish it from materials other than the water-absorbent resin. Cobalt chloride may also be added to facilitate visual discrimination.

[0061] As a method for component analysis, analytical means such as NMR and IR can be used. In the case of a water-absorbent resin whose main component is polyacrylic acid, nonwoven fabrics and resin films whose main component is polyolefin, and pulp whose main component is cellulose can be distinguished by analysis. Specifically, for example, in the case of pulp, "glucose" produced by hydrolyzing pulp by adding cellulase (pulp-degrading enzyme) can be detected by liquid chromatography or the like. Furthermore, in the case of polyolefin, absorption and peaks specific to polyolefin can be detected by IR (infrared absorption spectroscopy) or solid-state NMR (nuclear magnetic resonance).

[0062] In one embodiment of the present invention, since the insolubilized matter (water-insoluble substances) in the solubilized matter is quantified after the solubilization treatment of the water-absorbent resin, it is preferable that the materials other than the water-absorbent resin to be quantified are water-insoluble. Here, water-insoluble refers to a material that dissolves less than 1 g in 100 g of water at 25°C. The water-absorbent resin (composition) may contain additives added during the production of the water-absorbent resin. Among such additives, those solubilized by the solubilization treatment are not subject to quantification. In one embodiment, the materials other than the water-absorbent resin to be quantified do not include additives added during the production of the water-absorbent resin. Examples of additives of the water-absorbent resin that are not subject to quantification (compounds that are not subject to quantification as foreign matter) include, for example, water-soluble polyvalent metal cation-containing compounds, polyvalent metal salts, cationic polymers, chelating agents, inorganic reducing agents, α-hydroxycarboxylic acid compounds, water-insoluble inorganic particles, surfactants, non-polymer water-soluble compounds, etc. In one embodiment, the materials other than the water-absorbent resin to be quantified may be organic. In one embodiment, the materials to be quantified other than the water-absorbent resin do not contain water-insoluble inorganic particles. Water-insoluble inorganic particles are likely to be added during the production of the water-absorbent resin, but the absence of water-insoluble inorganic particles allows for more accurate quantification of the amount of foreign matter derived from absorbent articles, such as pulp. The amount of water-insoluble inorganic particles can be quantified, for example, by separating the insolubilized matter from the solubilized matter obtained by the solubilization treatment, adding nitric acid to the insolubilized matter, dissolving it by heating, and then quantifying the inorganic metal atoms using ICP emission spectrometry. Therefore, in one embodiment of the present invention, the amount of foreign matter can be quantified with higher accuracy by subtracting the quantified amount of water-insoluble inorganic particles from the insolubilized matter obtained by the solubilization treatment. The polyvalent metal salt, cationic polymer, chelating agent, inorganic reducing agent, α-hydroxycarboxylic acid compound, water-insoluble inorganic particle, surfactant, and non-polymer water-soluble compound are as described above.

[0063] The water-absorbent resin mixture may be subjected to a homogeneous mixing treatment as described below before being used as a sample for solubilization treatment.

[0064] The mass percentage of the water-absorbent resin in the water-absorbent resin mixture to be solubilized is preferably 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more. The mass percentage of the water-absorbent resin in the water-absorbent resin mixture to be solubilized may be 99.99% by mass or less, 99.9% by mass or less, 99.5% by mass or less, or 99% by mass or less. The mass percentage of the water-absorbent resin in the water-absorbent resin mixture to be solubilized may be 70% by mass or more and 99.99% by mass or less, 75% by mass or more and 99.9% by mass or less, 80% by mass or more and 99.5% by mass or less, or 85% by mass or more and 99% by mass or less.

[0065] In the quantification method of Non-Patent Document 1, the pulp content is at least about 40%. Therefore, even if the water-absorbent resin is mixed into the pulp after separation, the impact on the accuracy of the quantification of the pulp content is relatively small. However, for example, when the mass content of the water-absorbent resin in the water-absorbent resin mixture is as high as 70 mass% or more, the influence of the water-absorbent resin contamination is likely to be large in the quantification of materials other than the water-absorbent resin, which is likely to lead to measurement errors. Therefore, the method of the present invention is particularly effective in an embodiment in which the mass content of the water-absorbent resin in the water-absorbent resin mixture to be solubilized is 70 mass% or more, 75 mass% or more, 80 mass% or more, 85 mass% or more, or 90 mass% or more.

[0066] Therefore, in a preferred embodiment of the quantification method of the present invention, a water-absorbent resin mixture having a high content ratio of a water-absorbent resin is the target of the quantification method. Also, in a preferred embodiment of the present invention, a water-absorbent resin mixture after a step of separating foreign matter (preferably at least one selected from the group consisting of pulp, nonwoven fabric, and resin film) from a used absorbent article is the target of the quantification method.

[0067] The content mass ratio of the water-absorbent resin in the water-absorbent resin mixture referred to here is, for example, the mass ratio of the amount of the water-absorbent resin after the water-absorbent resin mixture has been treated in the following steps to the amount of the mixture before the treatment. Therefore, the content mass ratio of the water-absorbent resin in the water-absorbent resin mixture referred to here is an approximate value and may not be an accurate value.

[0068] (Processing steps when calculating the content mass ratio of water-absorbent resin in water-absorbent resin mixture) 1. Washing and dehydration treatment of water-absorbent resin mixture When the water-absorbent resin has absorbed urine or the like and is swollen, a regeneration treatment is performed. Specifically, 200 parts by mass of acetone is added to 100 parts by mass of the water-absorbent resin mixture (in a swollen state), and the mixture is stirred at 20°C for 30 minutes to shrink and dehydrate the water-absorbent resin. The discharged liquid is filtered, and the remaining gel is rinsed with 100 parts by mass of physiological saline, thereby washing the water-absorbent resin mixture. 200 parts by mass of acetone is added again to the washed water-absorbent resin mixture, and the mixture is stirred at 20°C for 30 minutes to shrink and dehydrate the water-absorbent resin again. The discharged liquid is filtered, and the remaining gel is dried in a reduced pressure dryer (chamber temperature 90°C) for 3 hours. The sample after this treatment is used as a sample for the following separation.

[0069] 2. Separation of materials other than water-absorbent resin by sedimentation method Materials other than water-absorbent resin are separated by sedimentation method, and the mass of the water-absorbent resin collected by filtration is measured.

[0070] Specifically, the 1. washing and dehydration treatment of the water-absorbent resin mixture and the 2. separation of materials other than the water-absorbent resin by the sedimentation separation method are the following methods: (1) Filter paper (for example, ADVANTEC Toyo Co., Ltd., product name: qualitative filter paper No. 5A, thickness 0.22 mm, retention particle size 7 μm) is dried in a vacuum dryer (chamber temperature 90° C.) for 3 hours, and after cooling in a desiccator, the mass of the filter paper (A1 (g)) is measured. (2) Approximately 5 g of a sample (water-absorbent resin mixture) that has been previously dried in a vacuum dryer (chamber temperature 90° C.) for 3 hours is placed in a 500 ml beaker, and the sample mass (B (g)) is accurately measured. (3) 400 mL of distilled water and 5 mL of a 4% cobalt (II) chloride hexahydrate solution are added to a 500 ml beaker, and the mixture is stirred for 10 minutes to color the water-absorbent resin. (4) The mixture is stirred with a stirrer or mixer to separate the water-absorbent resin from materials other than the water-absorbent resin. (5) The entire amount is placed in a 500 ml separatory funnel, shaken, and then allowed to settle and separate, and the settled water-absorbent resin is separated into a 300 ml beaker. (6) The 300 ml beaker into which the water-absorbent resin has been separated is stirred with a stirrer, and the supernatant liquid containing materials other than the water-absorbent resin is returned to the separatory funnel. (7) The materials other than the water-absorbent resin remaining in the separatory funnel are filtered out with filter paper (different from that in (1)). If the water-absorbent resin can be visually confirmed among the filtered materials other than the water-absorbent resin, it is removed with tweezers or the like and transferred to a 300 ml beaker, and the operation of (6) is repeated until the water-absorbent resin can no longer be confirmed among the materials other than the water-absorbent resin. (8) The water-absorbent resin in the 300 ml beaker is filtered using the filter paper whose mass was measured in (1), dried in a reduced pressure dryer (temperature inside the dryer: 90°C) for 3 hours, allowed to cool in a desiccator, and then the mass (A2 (g)) is measured. (9) The content mass ratio (X%) of the water-absorbent resin in the water-absorbent resin mixture is calculated using the following formula 1.

[0071] X(%)=(A2-A1) / B×100 Formula 1 In addition, in a treatment step when calculating the content mass ratio of the water absorbent resin in a water absorbent resin mixture, separating the water absorbent resin from a material other than the water absorbent resin from the water absorbent resin mixture is hereinafter also referred to as rough separation. In addition, the rough separation can be a sedimentation separation method.

[0072] (I) Solubilization Treatment In the step (1) of obtaining the solubilized product, the water-absorbent resin mixture is subjected to a solubilization treatment (decomposition treatment). The solubilization treatment refers to a treatment of solubilizing the water-absorbent resin in water.

[0073] In the solubilization treatment, a method for solubilizing the water-absorbing resin that suppresses the solubilization of materials other than the water-absorbing resin in water is used.

[0074] In many cases, collected (used) absorbent articles contain not a single water-absorbent resin but a mixture of various water-absorbent resins (having different compositions). The decomposition of each water-absorbent resin often depends on its composition, and therefore, even if the same decomposing agent is used, the decomposition rate varies greatly depending on the type of water-absorbent resin.

[0075] For this reason, in the step (1), it is preferable to use a solubilization method that suppresses the solubilization of materials other than the water-absorbent resin in water and increases the solubilization rate of the water-absorbent resin. In other words, in the step (1), it is preferable to select an optimum solubilization treatment method (or control the solubilization conditions) that suppresses the solubilization of materials other than the water-absorbent resin in water and preferably promotes the solubilization of the water-absorbent resin in water.

[0076] (II) Solubilization method of water-absorbent resin capable of suppressing the solubilization of materials other than the water-absorbent resin in water Examples of solubilization methods of water-absorbent resin capable of suppressing the solubilization of materials other than the water-absorbent resin in water include solubilization methods in which the solubilization rate of materials other than the water-absorbent resin (e.g., foreign matter such as pulp) is less than 30% by mass, less than 25% by mass, less than 20% by mass, less than 15% by mass, or less than 10% by mass. The method for adjusting the solubilization rate to within the above range is not particularly limited, but specific examples include not using a strong alkaline compound and performing the solubilization at a relatively low temperature (e.g., 60 ° C or less). Here, in one specific embodiment, it is desirable not to use any of lithium hydroxide, potassium hydroxide, sodium hydroxide, magnesium hydroxide, sodium carbonate, and potassium carbonate in the solubilization treatment. Furthermore, in order to adjust the solubilization rate to within the above range, it is preferable not to perform hypochlorite treatment (it is preferable not to use hypochlorite in the solubilization treatment).

[0077] The solubilization rate of a material other than the water-absorbent resin (for example, a foreign matter such as pulp) (hereinafter also referred to as "foreign matter solubilization rate") refers to the solubilization rate when a material other than the water-absorbent resin (for example, a foreign matter such as pulp) is used to solubilize the material.

[0078] Materials other than the water-absorbent resin when measuring the foreign matter solubilization rate can be identified by examining them in the order of (1) and (2) below.

[0079] (1) If the water-absorbent resin and a material other than the water-absorbent resin can be separated, for example, by rough separation in the processing step when calculating the mass ratio of the water-absorbent resin, the separated material other than the water-absorbent resin is used. Note that the material other than the water-absorbent resin separated by this separation method (e.g., pulp) may be contaminated with a trace amount of water-absorbent resin (e.g., 10% by mass or less (lower limit: 0% by mass), 5% by mass or less, 1% by mass or less, or 0.1% by mass or less), but in measuring the foreign matter solubilization rate, a trace amount of water-absorbent resin may be present.

[0080] (2) When the separation method of (1) cannot collect enough materials other than the water-absorbent resin to measure the foreign matter solubilization rate, the water-absorbent resin is solubilized and the materials other than the water-absorbent resin are collected.

[0081] The solubilization method is not particularly limited, and specifically, there are known techniques for solubilizing a water-absorbent resin by heat treatment in the presence of an oxidizing agent such as hydrogen peroxide or a persulfate (for example, JP-A-4-317785, JP-A-6-313008, JP-A-2003-321574), a technique for solubilizing a water-absorbent resin by using ascorbic acid as a reducing agent under a condition of pH 4 or more and pH 7.5 or less (for example, JP-A-05-247221), a technique for solubilizing a water-absorbent resin by using a reducing agent and a transition metal ion (for example, JP-A-2019- No. 131789), a technique of solubilizing a water-absorbent resin by heating in the presence of an oxidative water-soluble salt (for example, WO2021 / 042113), a technique of solubilizing a water-absorbent resin by using ozone water (for example, JP2014-217835A, JP2017-100133A), a technique of solubilizing a water-absorbent resin by using an oxidizing agent and a transition metal ion such as an iron ion or a copper ion in combination (for example, JP11-172039A), and the like can be mentioned, and as one form, a technique of solubilizing a water-absorbent resin by heat treatment in the presence of an oxidizing agent may be used.

[0082] Incidentally, according to the solubilization method, there is a possibility that materials other than the water-absorbent resin may also be solubilized, and a small amount (for example, 10 mass % or less (lower limit: 0 mass %), 5 mass % or less, 1 mass % or less, or 0.1 mass % or less) of the water-absorbent resin may be mixed in, but there is no problem as long as a certain amount or more of materials other than the water-absorbent resin remain (the minimum amount necessary for measuring the foreign matter solubilization rate).

[0083] The foreign matter solubilization rate can be determined by the method described in the Examples.

[0084] (Method for solubilizing a water-absorbent resin that promotes the solubilization of the water-absorbent resin in water) Examples of a method for solubilizing a water-absorbent resin that promotes the solubilization of the water-absorbent resin in water (that can suppress the water-absorbent resin from remaining undissolved in water) include solubilization techniques that result in a solubilization rate of the water-absorbent resin in a water-absorbent resin mixture of 90% by mass or more (upper limit 100% by mass), 95% by mass or more, 98% by mass or more, 99% by mass or more, 99.5% by mass or more, or 99.9% by mass or more. The method for adjusting the solubilization rate to fall within the above range is not particularly limited, but specific examples thereof include increasing the amount of decomposition agent used; extending the decomposition time; increasing the decomposition temperature; reducing the viscosity of the decomposition system (aqueous solution containing a water-absorbent resin and a decomposition agent); using a kneader with excellent mixing and stirring properties; and the like.

[0085] The water-absorbent resin used when measuring the solubilization rate of a water-absorbent resin is a water-absorbent resin that has been separated from a material other than the water-absorbent resin by, for example, rough separation in the processing step when calculating the mass proportion of the water-absorbent resin contained. Note that the water-absorbent resin separated by this separation method may be contaminated with a trace amount (for example, 10% by mass or less (lower limit: 0% by mass), 5% by mass or less, 1% by mass or less, or 0.1% by mass or less) of a material other than the water-absorbent resin, but in measuring the solubilization rate of the water-absorbent resin, a trace amount of a material other than the water-absorbent resin may be present.

[0086] The solubilization rate of the water-absorbent resin refers to the solubilization rate when solubilized using the water-absorbent resin. The solubilization rate of the water-absorbent resin can be determined by the method described in the examples.

[0087] The method for solubilizing the water-absorbent resin in water is not particularly limited, and known methods can be used. For example, the above-mentioned solubilization method can be applied to a mixture of a water-absorbent resin and pulp. Furthermore, there are hypochlorous acid (salt) treatment, a technique of oxidation treatment with hydrogen peroxide, persulfuric acid, chlorine, or a water-soluble redox agent (for example, JP 2013-150977 A), a technique of solubilizing the water-absorbent resin using an acid or alkali compound (for example, JP 2020-49398 A), and other solubilization methods, such as a method of thermal decomposition at a high temperature of 200 ° C. or higher, and a method of irradiating with radiation such as ultraviolet rays or electron beams.

[0088]

[0123] As the treatment for solubilizing a water-absorbent resin in water, from the viewpoint of simultaneously suppressing solubilization of materials other than the water-absorbent resin in water, a form using an oxidizing agent, a reducing agent, and a compound that generates transition metal ions, a form using radiation, or a combination thereof is preferred.

[0089]

[0113] Furthermore, among the above-mentioned methods for solubilizing a water absorbent resin, from the viewpoint of measurement accuracy of the content of a material other than a water absorbent resin, it is preferable to use a solubilization method which can almost completely solubilize a water absorbent resin contained in a water absorbent resin mixture and can suppress solubilization of materials other than a water absorbent resin, and it is most preferable to use a method of adding an oxidizing agent, a reducing agent, or a transition metal compound, specifically, a "method for quantifying the content of a material other than a water absorbent resin (content of foreign matter)" described in Examples.

[0090] The oxidizing agent is a compound having oxidizing properties and generates radicals upon heating. Radicals can also be generated by using the oxidizing agent in combination with a reducing agent and / or a compound that generates transition metal ions. Examples of the oxidizing agent include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, alkyl hydroperoxides, and peresters; perchlorates such as sodium perchlorate and potassium perchlorate; periodates such as sodium periodate and potassium periodate; percarbonates, perborates, and peracetic acid. The oxidizing agent may be used alone or in combination of two or more. Of these, hydrogen peroxide is preferably used. In one embodiment, the amount of the oxidizing agent may be 1 part by mass to 1,000 parts by mass, 10 parts by mass to 500 parts by mass, 50 parts by mass to 300 parts by mass, or 100 parts by mass to 300 parts by mass, relative to 100 parts by mass of the water-absorbent resin mixture (solid content). When the amount of the oxidizing agent is within the above range, the solubilization rate of the water-absorbent resin can be improved, and the solubilization of materials other than the water-absorbent resin can be easily suppressed. Here, the amount of the oxidizing agent is the total amount when the oxidizing agent is added in multiple portions.

[0091] The reducing agent is a compound having reducing properties that generates radicals when used in combination with the aforementioned oxidizing agent or the below-mentioned compound that generates transition metal ions. Examples of reducing agents include sulfurous acid (salts), hydrogen sulfite (salts), phosphorous acid (salts), hypophosphorous acid (salts), thiosulfuric acid (salts), formic acid, oxalic acid, erythorbic acid, amines, ascorbic acid (salts) or derivatives thereof (e.g., L-ascorbic acid (salts), isoascorbic acid (salts), and alkyl esters of ascorbic acid), phosphate esters, and sulfate esters. The reducing agents may be used alone or in combination of two or more. Of these, sulfurous acid (salts), hydrogen sulfite (salts), L-ascorbic acid (salts), and isoascorbic acid (salts) are preferably used. In one embodiment, the amount of the reducing agent may be 1 part by mass or more and 300 parts by mass or less, 10 parts by mass or more and 200 parts by mass or less, 30 parts by mass or more and 200 parts by mass or less, or 60 parts by mass or more and 150 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin mixture (solid content). This amount of the reducing agent can improve the solubilization rate of the water-absorbent resin, and also easily suppress the solubilization of materials other than the water-absorbent resin. Here, the amount of the reducing agent refers to the total amount when the reducing agent is added in multiple portions.

[0092] A transition metal ion may be used as the decomposing agent. The transition metal ion generates radicals by the Fenton reaction when used in combination with the aforementioned oxidizing agent. Furthermore, the transition metal ion generates radicals when used in combination with the aforementioned reducing agent. Examples of the transition metal ion include Cu. ２＋ , Ag ＋ , Fe ２＋ , Fe ３＋ , Al ３＋ , Ni ２＋ , Mn ２＋ The transition metal ions may be used alone or in combination of two or more. Among these, iron ions (Fe ２＋ ), copper ions (Cu ２＋ ) is preferred, and iron ions (Fe ２＋) is more preferable. Examples of the compound that generates the transition metal ions include chlorides and hydrates thereof, such as ferrous chloride; organic acid salts and hydrates thereof, such as ferrous fumarate, ferrous oxalate, ferrous chloride, sodium ferrous citrate, ferrous gluconate, ferrous citrate, and ferrous acetate; and sulfates and hydrates thereof, such as ferrous sulfate and ferrous sulfate heptahydrate. These compounds may be used alone or in combination of two or more. In one embodiment, the amount of the compound that generates the transition metal ions may be 0.1 to 50 parts by mass, 0.5 to 30 parts by mass, 1 to 20 parts by mass, or 6 to 20 parts by mass, relative to 100 parts by mass of the water-absorbent resin mixture (solid content). The amount of the transition metal ions mentioned above can improve the solubilization rate of the water-absorbent resin and easily suppress the solubilization of materials other than the water-absorbent resin. Here, the amount of the compound that generates the transition metal ions refers to the total amount when the compound is added in multiple installments.

[0093] An alkali compound may be used as the decomposing agent. The alkali compound is a compound that solubilizes the crosslinked portion of the water-absorbent resin by alkaline hydrolysis. Examples of such alkali compounds include alkali metal hydroxides such as lithium hydroxide, potassium hydroxide, and sodium hydroxide; alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide; ammonia; and amine compounds such as aliphatic amines, alicyclic amines, and aromatic amines. The alkali compounds may be used alone or in combination of two or more. Among these, alkali metal hydroxides and alkaline earth metal hydroxides are preferred. In one embodiment, the content of the alkali compound as the decomposing agent may be 30 parts by mass or less (lower limit: 0 parts by mass), 20 parts by mass or less, 10 parts by mass or less, 5 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the water-absorbent resin mixture (solid content).

[0094] In the solubilization treatment, it is preferable to add water to the water-absorbent resin mixture in order to promote the solubilization treatment. Here, the water is not particularly limited as long as it is clean, and examples thereof include tap water, industrial water, ion-exchanged water, and pure water. The mixing ratio of water to the water-absorbent resin mixture is not particularly limited. The amount of water mixed is, for example, 66 parts by mass or more and 9900 parts by mass or less, 80 parts by mass or more and 1900 parts by mass or less, or 100 parts by mass or more and 1000 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin mixture. By mixing water within the above range, the decomposing agent is mixed uniformly, and decomposition is likely to be carried out efficiently.

[0095] Further, the temperature for decomposing the water-absorbent resin is preferably relatively low, from the viewpoint of suppressing solubilization of materials other than the water-absorbent resin in water, and is preferably 20°C or higher and 80°C or lower, and more preferably 25°C or higher and 60°C or lower.

[0096] The solubilization time may be, for example, from 5 minutes to less than 12 hours, from 10 minutes to 10 hours, from 20 minutes to 8 hours, from 30 minutes to 6 hours, from 1 hour to 4 hours, or from 2.5 hours to 4 hours, taking into consideration the working efficiency and the decomposition efficiency.

[0097] From the viewpoint of improving the decomposition efficiency, the solubilization treatment may be performed by adding the decomposing agent in multiple batches. The term "multiple batches" means two or more batches, and may be two to five batches, two to three batches, or even just two batches. Furthermore, when adding multiple batches, each batch may be added within 1 hour (lower limit: 0 minutes), 30 minutes, 20 minutes, 10 minutes, or 5 minutes after the completion of the previous batch.

[0098] The pH in the solubilization treatment is not particularly limited, but since decomposition of materials other than the water-absorbent resin can be suppressed, it is preferable to adjust the pH to a range of 5.0 to 10.0 (more preferably 5.5 to 9.5, particularly preferably 6.0 to 9.0). The pH in the solubilization treatment can be adjusted as needed by adding a pH adjuster such as an acid or a base. Examples of acids that can be used include, but are not limited to, inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid and citric acid. Examples of bases that can be used include, but are not limited to, sodium hydroxide, sodium carbonate, potassium hydroxide, and magnesium hydroxide.

[0099] By the decomposition operation, a liquid solubilized product (a mixture containing a decomposition product of the water-absorbent resin) is obtained.

[0100] [1. Procedure of Step (1)] In the step (1), the water-absorbent resin mixture is subjected to a solubilization treatment (decomposition treatment).

[0101] In one embodiment, the step (1) comprises: (A) roughly separating a water absorbent resin and a material other than the water absorbent resin from the water absorbent resin mixture; (B) examining solubilization conditions such that the solubilization rate of the roughly separated material other than the water absorbent resin becomes less than 30% by mass; (C) solubilizing the roughly separated water absorbent resin using the solubilization conditions examined in (B) such that the solubilization rate becomes less than 30% by mass, and calculating the solubilization rate; (D) in (C), while maintaining the conditions such that the solubilization rate of the material other than the water absorbent resin becomes less than 30% by mass, examining the solubilization conditions again by returning to (B) until an optimized condition such that the solubilization rate of the water absorbent resin becomes 90% by mass or more is found; and (E) solubilizing the water absorbent resin mixture using the solubilization technique of the optimized condition.

[0102] Moreover, in one embodiment of the present invention, the step (1) comprises: (A) roughly separating a water absorbent resin and a material other than the water absorbent resin from the water absorbent resin mixture; (B) examining solubilization conditions such that the solubilization rate of the roughly separated water absorbent resin becomes 90% by mass or more; (C) solubilizing the material other than the roughly separated water absorbent resin using the solubilization conditions examined in (B) such that the solubilization rate becomes 90% by mass or more, and calculating the solubilization rate; (D) in (C), while maintaining the conditions such that the solubilization rate of the water absorbent resin becomes 90% by mass or more, examining the solubilization conditions again by returning to (B) until an optimized condition is found such that the solubilization rate of the material other than the water absorbent resin becomes less than 30% by mass; and (E) solubilizing the water absorbent resin mixture using the solubilization technique of the optimized condition.

[0103] 2. Procedure of Step (2) In the step (2), materials other than the water-absorbent resin are quantitatively determined from the solubilized product.

[0104] The quantification of materials other than the water-absorbent resin can be carried out, for example, by separating soluble and insoluble substances from the solubilized material obtained in step (1), removing the soluble substances, and quantifying the insoluble substances. A conventional solid-liquid separation method, such as filtration using a mesh filter or the like, or centrifugation, can be used for the separation. A specific method for quantifying materials other than the water-absorbent resin includes a method of removing liquid from the solubilized material and weighing the residue. More specifically, the quantification of materials other than the water-absorbent resin can be carried out by filtering the solubilized material by suction using filter paper, drying the filtrate, and measuring the mass of the dried material. Specifically, the quantification of materials other than the water-absorbent resin can be carried out by the method described in the following examples. In one embodiment, the steps include: b) removing the solubilized water-absorbent resin from the solubilized material; and c) drying the remaining component obtained in step b) to determine the content of materials other than the water-absorbent resin.

[0105] The content mass ratio (%) of the material other than the water-absorbent resin in the water-absorbent resin mixture can be calculated from the content (g) of the material other than the water-absorbent resin determined by the quantification of the material other than the water-absorbent resin and the sample mass (g) of the water-absorbent resin mixture. The sample mass of the water-absorbent resin mixture is, for example, a solid content mass. Specifically, it is a solid content mass calculated by the following method.

[0106] Solid content mass of water-absorbent resin mixture: A water-absorbent resin mixture is dried in a reduced-pressure dryer (degree of vacuum of 300 torr or less, temperature inside the dryer of 90°C) for 3 hours, and then allowed to cool in a desiccator containing a desiccant, and then measured.

[0107] [Recycling method] The present invention also provides a recycling method for a water absorbent resin, comprising separating a material other than a water absorbent resin from a water absorbent resin mixture until a content of a material (foreign matter) other than a water absorbent resin, determined by the above-mentioned quantification method, in a water absorbent resin mixture becomes 10% by mass or less (may be 5% by mass or less, 1% by mass or less, or 0.1% by mass or less), to obtain a water absorbent resin.

[0108] Specifically, when the content of materials other than the water-absorbing resin in the mixture exceeds 10 mass % as determined by the above-mentioned quantitative analysis, the materials other than the water-absorbing resin are separated.

[0109] Examples of methods for separating materials other than the water-absorbent resin include a method utilizing a difference in specific gravity in gas by performing centrifugation using a cyclone separator or the like, a method of sorting by size by sieving, and a method of separating using a drum screen separator.

[0110] In this way, a water-absorbent resin mixture having a reduced content of foreign matter can be obtained, and the water-absorbing properties of the recycled water-absorbent resin can be improved, and deterioration in appearance (for example, coloration) can be suppressed.

[0111] Although the embodiments of the present invention (Invention A) have been described in detail, it is clear that this is for illustrative and exemplary purposes only and is not limiting, and the scope of the present invention should be interpreted by the appended claims.

[0112] Invention A includes the following aspects and configurations.

[0113] 1. A method for quantifying materials other than the water-absorbent resin in a water-absorbent resin mixture, the method comprising: a step (1) of performing a solubilization treatment on a water-absorbent resin mixture containing a water-absorbent resin and a material other than the water-absorbent resin, to solubilize the water-absorbent resin in water and obtain a solubilized product; and a step (2) of quantifying the materials other than the water-absorbent resin from the solubilized product, wherein in the solubilization treatment, a water-absorbent resin solubilization technique that suppresses solubilization of the materials other than the water-absorbent resin in water is used.

[0114] 2. The method for quantifying the amount of the water-absorbent resin according to 1., wherein the content by mass of the water-absorbent resin in the water-absorbent resin mixture is 70% by mass or more.

[0115] 3. The method for quantifying the amount of water according to 1. or 2., wherein the material other than the water-absorbent resin is a constituent material derived from an absorbent article.

[0116] 4. The method for quantifying water according to any one of 1. to 3., wherein the material other than the water-absorbent resin includes at least one selected from the group consisting of pulp, nonwoven fabric, and resin film.

[0117] 5. The method for quantifying the amount of water absorbent resin according to any one of 1. to 4., wherein the water-absorbent resin mixture is recovered from a used absorbent article.

[0118] 6. The quantification method according to any one of 1. to 5., wherein a solubilization technique is used such that the solubilization rate of the water-absorbent resin in the water-absorbent resin mixture is 90 mass % or more.

[0119] 7. The quantification method according to any one of 1. to 6., wherein a solubilization method is used such that the solubilization rate of the material other than the water-absorbent resin is less than 30 mass %.

[0120] 8. A method for recycling a water-absorbent resin, comprising separating materials other than the water-absorbent resin from a water-absorbent resin mixture until a content of materials other than the water-absorbent resin in the water-absorbent resin mixture, determined by the quantification method according to any one of 1. to 7., becomes 10 mass % or less, to obtain a water-absorbent resin.

[0121] The above is the explanation of Invention A.

[0122] (Method for quantifying content of material (foreign matter) other than water absorbent resin remaining on JIS standard sieve with mesh size of 150 μm) In this specification, "method for quantifying content of material (foreign matter) other than water absorbent resin remaining on JIS standard sieve with mesh size of 150 μm" means % by mass of material other than water absorbent resin which cannot pass through JIS standard sieve with mesh size of 150 μm, among all materials other than water absorbent resin contained in a water absorbent resin mixture, relative to the water absorbent resin mixture.

[0123] The present inventors have found that the size of materials other than the water-absorbent resin affects the powder properties of a water-absorbent resin mixture, and in particular, materials other than the water-absorbent resin having a size that allows them to remain on a JIS standard sieve with a mesh size of 150 μm have a strong effect on the powder properties. Even if the content of materials other than the water-absorbent resin is approximately the same, when the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm exceeds 1.0 mass%, the powder properties are significantly reduced. In other words, as described above, from the viewpoint of powder properties, the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm is permissible up to 1.0 mass% (1.0 mass% is critical for powder properties).

[0124]

[0033] The higher the "content of materials (foreign matter) other than the water-absorbent resin remaining on the JIS standard sieve with a mesh size of 150 μm," the greater the tendency for adverse effects on the properties of the water-absorbent resin mixture, particularly powder properties such as flow rate, to increase. Therefore, in one embodiment of the present invention, the content of materials other than the water-absorbent resin remaining on the JIS standard sieve with a mesh size of 150 μm is 0% by mass or more and 1.0% by mass or less, preferably 0% by mass or more and less than 1.0% by mass, more preferably 0% by mass or more and 0.9% by mass or less, and even more preferably 0% by mass or more and 0.8% by mass or less, relative to the water-absorbent resin mixture. By controlling the content of materials other than the water-absorbent resin remaining on the JIS standard sieve with a mesh size of 150 μm within the above range, a water-absorbent resin mixture with little deterioration in water absorption properties and powder properties can be obtained.

[0125] The lower limit of the "content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm" is 0% by mass, but from the viewpoint of improving water absorption properties, particularly Vortex (water absorption speed), it is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and even more preferably more than 0.01% by mass. The "content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm" may be 0% by mass or more and 1.0% by mass or less, 0.005% by mass or more and less than 1.0% by mass, 0.005% by mass or more and 0.01% by mass or more and 0.9% by mass or less, or more than 0.01% by mass and 0.8% by mass or less.

[0126] "The content of material other than water absorbent resin remaining on a JIS standard sieve having a mesh size of 150 μm" is preferably determined from a content rate P of material other than water absorbent resin in a water absorbent resin mixture by the following formula 2: Content (mass %) of material other than water absorbent resin remaining on a JIS standard sieve having a mesh size of 150 μm=Content mass ratio of water absorbent resin mixture remaining on a JIS standard sieve having a mesh size of 150 μm in the water absorbent resin mixture×P mass % ... formula 2

[0127] The content P (mass %) of the material other than the water-absorbent resin can be determined by a method for quantifying the content of the material other than the water-absorbent resin. Specifically, it can be determined by a method described in Examples below.

[0128]

[0113] As a method for obtaining a water absorbent resin mixture adjusted so that the content of materials (foreign matter) other than a water absorbent resin falls within the range described in the present invention, the water absorbent resin mixture is generally obtained by appropriately combining known foreign matter separation methods with respect to a water absorbent resin mixture recovered from a used absorbent article by a known method and containing a large amount of foreign matter.

[0129]

[0113] Examples of known foreign matter separation methods include a "wet separation" method in which a water absorbent resin and foreign matter are separated in a solution, and a "dry separation" method in which a water absorbent resin mixture in a dry state is separated into a water absorbent resin and foreign matter.

[0130] Examples of "wet separation" methods include: i) a method of separating the water-absorbing resin and foreign matter in a solution using a screen or sieve by utilizing the difference in size and length (JP 2024-088438, etc.); ii) a method of separating the water-absorbing resin and foreign matter using a separation device such as a liquid cyclone by utilizing the difference in specific gravity (JP 2019-85447, etc.); and iii) a method of separating the water-absorbing resin and foreign matter by utilizing the difference in sedimentation rate in the solution, or the difference in ease of rising and settling when bubbling with air or the like (JP 2024-94574, etc.).

[0131] In particular, when it is desired to remove large foreign matter that has a significant adverse effect on powder properties, this can be achieved by appropriately adjusting the size and shape of the screen or sieve (in the case of i), by appropriately adjusting the shape of the liquid cyclone, the number of treatment stages and the treatment speed (in the case of ii), or by appropriately adjusting conditions such as the amount of air to be bubbling and the stirring speed of the solution (in the case of iii).

[0132] Examples of "dry separation" methods include: i) a method of separating the water-absorbent resin and foreign matter using a screen or sieve by taking advantage of the difference in size and length; ii) a method of separating the water-absorbent resin and foreign matter using a separation device such as a cyclone by taking advantage of the difference in specific gravity (JP 2022-015986, JP 2007-203170, etc.); and iii) a method of air-transporting a mixture of water-absorbent resin and foreign matter and separating the water-absorbent resin and foreign matter by taking advantage of the difference in ease of settling (JP 2023-100208, etc.); iv) a method of collecting and separating low-specific-gravity components by sucking air from the space while moving a mixture of water-absorbent resin and foreign matter, for example, on a roller (JP 2001-336077, etc.).

[0133] In particular, when it is desired to remove large foreign matter that has a significant adverse effect on powder properties, this can be achieved by appropriately adjusting the size and shape of the screen or sieve (in case i), appropriately adjusting the processing speed and shape of the cyclone (in case ii), appropriately adjusting conditions such as the amount of air and the transport distance during pneumatic transport (in case iii), or adjusting the amount and speed of air suction (in case iv).

[0134] [1-4] Physical Properties of Water-Absorbent Resin Mixture (Flow Rate) In one embodiment of the present invention, the flow rate of the water-absorbent resin mixture is preferably 7.0 g / sec or more, more preferably 7.5 g / sec or more, even more preferably 8.0 g / sec or more, and particularly preferably 8.5 g / sec or more. By controlling the flow rate of the water-absorbent resin mixture within the above range, it is possible to obtain a water-absorbent resin mixture with little deterioration in water absorption properties and powder characteristics. Furthermore, by controlling the flow rate of the water-absorbent resin mixture within the above range, it is possible to obtain a water-absorbent resin mixture with improved fluidity. The flow rate of the water-absorbent resin mixture may be 7.0 g / sec or more and 20 g / sec or less, 7.5 g / sec or more and 20 g / sec or less, 8.0 g / sec or more and 20 g / sec or less, or 8.5 g / sec or more and 15 g / sec or less.

[0135] (Bulk Specific Gravity) In one embodiment of the present invention, the bulk specific gravity of the water-absorbent resin mixture is preferably 0.55 g / ml or more, more preferably 0.57 g / ml or more, and even more preferably 0.59 g / ml or more. By controlling the bulk specific gravity of the water-absorbent resin mixture within the above range, it is possible to obtain a water-absorbent resin mixture with little deterioration in water absorption properties and powder characteristics. Furthermore, by controlling the bulk specific gravity of the water-absorbent resin mixture within the above range, transportation costs are advantageous. The bulk specific gravity of the water-absorbent resin mixture may be 0.55 g / ml or more and 0.80 g / ml or less, 0.57 g / ml or more and 0.75 g / ml or less, or 0.59 g / ml or more and 0.70 g / ml or less.

[0136] (Moisture Content) In one embodiment of the present invention, the moisture content of the water-absorbent resin mixture is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably 8% by mass or less. By controlling the moisture content of the water-absorbent resin mixture within the above range, it is possible to obtain a water-absorbent resin mixture with little deterioration in water absorption properties and powder characteristics. The lower limit of the moisture content of the water-absorbent resin mixture is, for example, 1% by mass or more, 2% by mass or more, or 3% by mass or more. The moisture content of the water-absorbent resin mixture may be 1% by mass or more and 20% by mass or less, 2% by mass or more and 15% by mass or less, 2% by mass or more and 10% by mass or less, or 3% by mass or more and 8% by mass or less.

[0137] (Mass average particle diameter (D50)) In one embodiment of the present invention, the mass average particle diameter (D50) of the water-absorbent resin mixture is preferably 200 μm or more and 600 μm or less, more preferably 250 μm or more and 550 μm or less, and further preferably 300 μm or more and 500 μm or less. By controlling the mass average particle diameter (D50) of the water-absorbent resin mixture within the above range, it is possible to obtain a water-absorbent resin mixture with little deterioration in water absorption properties and powder characteristics.

[0138] (CRC) In one embodiment of the present invention, the CRC of the water-absorbent resin mixture is 20 g / g or more and 70 g / g or less, 25 g / g or more and 60 g / g or less, 27 g / g or more and 50 g / g or less, 30 g / g or more and 45 g / g or less, 35 g / g or more and 45 g / g or less, 35.5 g / g or more and 45 g / g or less, or 36.0 g / g or more and 40 g / g or less. When the CRC of the water-absorbent resin mixture is in this range, the absorption amount is excellent and the balance with other physical properties is also excellent.

[0139] (Vortex (water absorption rate)) In one embodiment of the present invention, the vortex of the water-absorbent resin mixture is preferably 60 seconds or less, more preferably 53 seconds or less, even more preferably 52 seconds or less, particularly preferably 50 seconds or less. The lower limit value is not particularly limited, but is preferably 1 second or more, more preferably 5 seconds or more.

[0140] By setting the Vortex within the above range, it becomes possible to absorb a predetermined amount of liquid in a short time. When used in the absorbent body of an absorbent article such as a disposable diaper, the user's skin feels wet for a shorter period of time, which reduces discomfort and leakage.

[0141] (Shape) In one embodiment of the present invention, the shape of the water-absorbent resin mixture is preferably powder.

[0142] [1-5] Raw Materials for the Water-Absorbent Resin Mixture (Constituent Materials Derived from Absorbent Articles) In one embodiment of the present invention, materials other than the water-absorbent resin that constitute the water-absorbent resin mixture can include constituent materials derived from absorbent articles. In this specification, the term "absorbent article" refers to an article used for absorbing water. Examples include disposable diapers, sanitary napkins, incontinence pads, panty liners, and pet sheets, which are used to absorb liquids excreted by humans or animals. More specifically, the term "absorbent article" refers to an absorbent article that includes an absorbent core containing a water-absorbent resin and a fibrous material, a liquid-permeable top sheet, and a liquid-impermeable back sheet. The absorbent core is preferably produced by blending the water-absorbent resin with a fibrous material, or by sandwiching the water-absorbent resin between fibrous materials and molding the blend into a film, a cylinder, a sheet, or the like. Examples of fibrous materials include hydrophilic fibers such as pulverized wood pulp, cotton linters, crosslinked cellulose fibers, rayon, cotton, wool, acetate, and vinylon.

[0143] More specifically, an example of the configuration of a disposable diaper, which is one type of absorbent article, includes a surface material such as a nonwoven fabric made of chemical fibers such as polypropylene or polyester; a water absorbent body containing a water-absorbing material such as a water-absorbing resin or pulp; a waterproof material such as a resin film such as a polyethylene film, or paper or nonwoven fabric that has been treated with resin, such as polyethylene-laminated paper or polyethylene-laminated nonwoven fabric (resin-treated paper or resin-treated nonwoven fabric); and an adhesive (binder) that bonds these components together. Note that "absorbent article" includes both unused and used absorbent articles.

[0144] [2] Method for recycling water-absorbent resin containing water-absorbent resin mixture In one embodiment of the present invention, the water-absorbent resin mixture is separated and recovered from used absorbent articles (which may partially include unused absorbent articles), and recycled so that it can be reused for water absorption. That is, the present invention also encompasses a method for recycling a water-absorbent resin contained in used absorbent articles, wherein the recycled water-absorbent resin contains the above-mentioned water-absorbent resin mixture.

[0145] The recycling method is not particularly limited as long as it satisfies the above-mentioned purpose, and for example, a method in which used absorbent articles are treated through any of steps such as washing, dehydration, regeneration, crushing, separation, sterilization / disinfection, etc. Specific examples thereof include the methods described in JP-A-2013-198862 and JP-A-2019-135046. Furthermore, a water-absorbent resin or a water-absorbent resin mixture that has become reusable for the purpose of absorbing water through such a recycling treatment may be referred to as a "recycled water-absorbent resin."

[0146] [3] Manufacturing method of water-absorbent resin using a water-absorbent resin mixture as part of a raw material At actual recycling sites of used absorbent articles, sanitary materials of various types and manufacturers are recycled together. Therefore, the water-absorbent resin recovered therefrom is a mixture of products from various manufacturers and products having water-absorbing properties, and it is difficult to obtain a recycled water-absorbent resin having stable water-absorbing properties.

[0147] On the other hand, water absorbent resins for sanitary materials are those in which the balance of various water absorption properties such as water absorption capacity, water absorption capacity under pressure, and liquid permeability has been highly adjusted in order to meet various applications and required properties. In other words, water absorbent resins for sanitary materials are required to have an excellent balance of various water absorption properties. However, as mentioned above, recycled water absorbent resins often have unstable water absorption properties, and in order to use recycled water absorbent resins for sanitary material applications, it is necessary to adjust the balance of water absorption properties.

[0148] Therefore, the present inventors thought that it would be easy to adjust the balance of physical properties by mixing a recycled water-absorbent resin having unstable water-absorbing properties as described above into a part of the raw materials used when newly producing a water-absorbent resin. Note that "newly producing a water-absorbent resin" refers to the production of a conventional (normal) water-absorbent resin using a monomer constituting the water-absorbent resin as a raw material.

[0149] The above-mentioned "production of a conventional (normal) water-absorbing resin using a monomer constituting a water-absorbing resin as a raw material" means a production method of a normal (non-recycled) water-absorbing resin using a monomer constituting a water-absorbing resin as a raw material, for example, a production method of a normal (non-recycled) water-absorbing resin including a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a classification step, and a surface cross-linking step.

[0150] And, the above-mentioned "using a water-absorbent resin mixture as a part of a raw material" means that a water-absorbent resin mixture is added in at least one step of a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a classification step and a surface-crosslinking step. That is, the present invention also encompasses a method for producing a water-absorbent resin, in which the above-mentioned water-absorbent resin mixture is used as a part of a raw material in a production process of a water-absorbent resin using a monomer constituting the water-absorbent resin as a raw material.

[0151] Hereinafter, one embodiment of the present invention will be described by taking as an example a method for producing a water-absorbing resin, which includes a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a classification step, and a surface-crosslinking step.

[0152] [3-1] Step of Preparing Aqueous Monomer Solution In one aspect of the present invention, it is preferable to include a step of preparing an aqueous monomer solution, in which an aqueous solution containing acrylic acid (salt) as a main component (hereinafter referred to as "aqueous monomer solution"). In this production method, a slurry liquid of a monomer can also be used in addition to the aqueous monomer solution within a range in which the water absorption performance of the obtained water absorbent resin is not reduced, but for convenience, the aqueous monomer solution will be described in this section.

[0153] Further, the "main component" means that the amount (content) of acrylic acid (salt) used is 50 mol% or more in 100 mol% of the total amount of monomers to be subjected to a polymerization reaction of a water absorbent resin (in other words, monomers contained in an aqueous monomer solution, excluding an internal crosslinking agent). The content of acrylic acid (salt) in 100 mol% of the total amount of monomers is preferably 70 mol% or more, more preferably 90 mol% or more (upper limit: 100 mol%).

[0154] (Acrylic Acid) In one aspect of the present invention, acrylic acid and / or a salt thereof (hereinafter referred to as "acrylic acid (salt)") is used as a monomer serving as a raw material for a water absorbent resin, from the viewpoint of the physical properties and productivity of the resulting water absorbent resin. As such "acrylic acid", known acrylic acids can be used.

[0155] Further, the term "acrylate" means a salt obtained by neutralizing acrylic acid with a basic composition. The acrylate may be a commercially available acrylate (e.g., sodium acrylate), or may be a salt obtained by neutralization in a production plant for a water-absorbent resin.

[0156] (Basic composition) In the present specification, the term "basic composition" means a composition containing a basic compound. More specific examples of the basic compound include carbonates and / or hydrogencarbonates of alkali metals, hydroxides of alkali metals, ammonia, organic amines, etc. Among these, from the viewpoint of the physical properties of the obtained water absorbent resin, a strongly basic basic compound, for example, a hydroxide of an alkali metal such as sodium hydroxide, potassium hydroxide, or lithium hydroxide, is preferred, and sodium hydroxide is more preferred.

[0157] (Neutralization) In one embodiment of the present invention, it is preferable to neutralize at least a portion of the acrylic acid. Neutralization in this production method can be performed by either neutralizing the acrylic acid (before polymerization) or neutralizing the hydrogel-like crosslinked polymer obtained by crosslinking and polymerizing acrylic acid (after polymerization) (hereinafter referred to as "post-neutralization"). The neutralization method is not particularly limited, and may be continuous or batchwise, although continuous is preferred from the viewpoint of production efficiency, etc. Regarding the conditions for the neutralization apparatus, neutralization temperature, residence time, etc., the conditions described in International Publication No. 2009 / 123197, U.S. Patent Application Publication No. 2008 / 0194863, etc., also apply to the present invention.

[0158] In one aspect of the present invention, the neutralization rate of the monomer (a monomer having an acid group including acrylic acid) is preferably 10 mol% or more and 90 mol% or less, more preferably 40 mol% or more and 85 mol% or less, even more preferably 50 mol% or more and 80 mol% or less, and particularly preferably 60 mol% or more and 75 mol% or less, based on 100 mol% of the total amount of acid groups in the monomer. By setting the neutralization rate to 10 mol% or more, it is possible to provide a water-absorbent resin having a sufficient water absorption capacity, and by setting the neutralization rate to 90 mol% or less, it is possible to provide a water-absorbent resin having a higher water absorption capacity under pressure.

[0159] The neutralization rate of the aqueous monomer solution will be further explained using an example in which the aqueous monomer solution contains only acrylic acid as a monomer component. In this case, a neutralization rate of 75 mol% of the monomer means that the monomer component contained in the aqueous monomer solution is a mixture of 25 mol% of acrylic acid and 75 mol% of an acrylic acid salt. Such a mixture may be referred to as a partially neutralized product of acrylic acid. The neutralization rate is the same in the case of post-neutralization. The neutralization rate also applies to the neutralization rate of the water-absorbent resin as a final product.

[0160] In this production method, the recycled water-absorbent resin is used as part of the raw material in the production of the water-absorbent resin. When such a recycled water-absorbent resin is used as part of the raw material in the production of the water-absorbent resin, there is a possibility that the recycled water-absorbent resin contains a basic compound. This basic compound may neutralize the acrylic acid before polymerization or the hydrogel-like cross-linked polymer after polymerization. Therefore, taking into account the neutralization by the basic compound derived from the recycled water-absorbent resin, the neutralization (post-neutralization) rates of the monomer in the aqueous monomer solution, the hydrogel-like polymer, and the water-absorbent resin as the final product are adjusted to fall within a predetermined range. The neutralization rate is measured as follows.

[0161] 200 g of physiological saline (0.9 mass % sodium chloride aqueous solution) is measured into a 250 ml plastic container with a lid, 1.00 g of a water-absorbent resin mixture is added to the aqueous solution, and the mixture is stirred for 16 hours to extract the soluble matter in the water-absorbent resin. This extract is filtered using one sheet of filter paper (ADVANTEC Toyo Co., Ltd., product name: Qualitative Filter Paper No. 2, thickness 0.26 mm, retention particle size 5 μm), and 50.0 g of the obtained filtrate is measured and used as a measurement solution.

[0162] First, physiological saline alone is titrated to pH 10 with 0.1 N NaOH aqueous solution, and then titrated to pH 2.7 with 0.1 N HCl aqueous solution, and the blank titer ([bNaOH] ml, [bHCl] ml) is measured.

[0163] Next, the same titration procedure is carried out on the test solution to determine the titer ([NaOH] ml, [HCl] ml). The neutralization rate can be calculated according to the following formula 3.

[0164] Neutralization rate (mol%) = [1 - ([NaOH] - [bNaOH]) / ([HCl] - [bHCl])] x 100 Equation 3

[0165] (Other Monomers) The aqueous monomer solution may contain a monomer (other monomer) other than the acrylic acid (salt). In the present production method, such other monomers can be used in combination with the acrylic acid (salt) to produce a water-absorbent resin.

[0166] Examples of the other monomer include water-soluble or hydrophobic unsaturated monomers. Specifically, anionic unsaturated monomers and / or salts thereof, such as (anhydrous) maleic acid, fumaric acid, crotonic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-hydroxyethyl (meth)acryloyl phosphate, can be suitably used as the other monomer. These other monomers may be used singly or in combination of two or more.

[0167] (Internal Crosslinking Agent) The aqueous monomer solution preferably contains an internal crosslinking agent. Examples of internal crosslinking agents that can be contained in the aqueous monomer solution include the compounds described in U.S. Patent No. 6,241,928. One or more of these compounds can be used as the internal crosslinking agent in consideration of reactivity.

[0168]

[0133] Moreover, from the viewpoint of water absorption performance of the obtained water absorbent resin and the like, as the internal crosslinking agent, a compound having two or more polymerizable unsaturated groups is preferable, a compound having thermal decomposition properties at the drying temperature described below is more preferable, and a compound having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structural unit is further preferable.

[0169] The polymerizable unsaturated group is preferably, for example, an allyl group or a (meth)acrylate group, more preferably a (meth)acrylate group. The (poly)alkylene glycol structural unit is preferably polyethylene glycol, and the number n (the number of alkylene glycol structural units) is preferably 1 or more and 100 or less, more preferably 6 or more and 50 or less.

[0170] The amount of the internal crosslinking agent used (content in the aqueous monomer solution) is preferably 0.0001 mol% or more and 10 mol% or less, more preferably 0.001 mol% or more and 1 mol% or less, relative to the total amount of monomers. By setting the amount of the internal crosslinking agent used within the above range, a water-absorbent resin having the desired water absorption performance can be obtained. Note that, if the amount of the internal crosslinking agent used is less than 0.0001 mol%, the gel strength of the obtained water-absorbent resin tends to decrease and the water-soluble content tends to increase excessively. On the other hand, if the amount of the internal crosslinking agent used is more than 10 mol%, the water absorption capacity of the obtained water-absorbent resin tends to decrease, which is not preferable. Note that the mol% relative to the total amount of monomers refers to the percentage of the number of moles of the internal crosslinking agent relative to the total number of moles of monomers contained in the aqueous monomer solution.

[0171] In one embodiment of the present invention, a method is preferably applied in which a predetermined amount (within the above-mentioned range) of an internal cross-linking agent is added to an aqueous monomer solution in advance, and the cross-linking reaction is initiated simultaneously with polymerization. On the other hand, in addition to this method, a method of post-crosslinking by adding an internal cross-linking agent during and / or after polymerization, a radical cross-linking method using a radical polymerization initiator, a radiation cross-linking method using active energy rays such as electron beams or ultraviolet rays, etc. can also be used. These methods can also be used in combination.

[0172] (Polymerization inhibitor) From the viewpoint of the polymerizability of acrylic acid and the color tone of the water absorbent resin, the aqueous monomer solution preferably contains a polymerization inhibitor in an amount of 200 ppm or less, more preferably 10 ppm or more and 160 ppm or less, and even more preferably 20 ppm or more and 100 ppm or less. The polymerization inhibitor is not particularly limited, but is preferably a methoxyphenol, more preferably p-methoxyphenol.

[0173]

[0033] In addition, the aqueous monomer solution may contain impurities derived from each component in the aqueous monomer solution. For example, with regard to impurities derived from acrylic acid, the description relating to compounds described in U.S. Patent Application Publication No. 2008 / 0161512 can be applied to a method for producing a water absorbent resin using the water absorbent resin mixture of the present invention as a part of a raw material.

[0174] (Other Substances) The aqueous monomer solution may contain substances (other substances) other than the above-mentioned components from the viewpoint of improving the physical properties of the resulting water-absorbent resin. Examples of the other substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salt), and crosslinked polyacrylic acid (salt); and other additives such as foaming agents such as carbonates and azo compounds, surfactants, chelating agents, and chain transfer agents.

[0175] When the aqueous monomer solution contains a hydrophilic polymer as another substance, the content of the hydrophilic polymer is preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less (the lower limit is 0% by mass), relative to the total amount (100% by mass) of the aqueous monomer solution.

[0176] Furthermore, when the aqueous monomer solution contains other additives as other substances, the content of the other additives is preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less (the lower limit is 0% by mass), relative to the total amount (100% by mass) of the aqueous monomer solution.

[0177] The other substances may be added to the reaction system in the polymerization step described below. In this case, it is preferable that the total amount of the other substances contained in the aqueous monomer solution and the other substances added to the reaction system in the polymerization step is within the above-mentioned range.

[0178] When a water-soluble resin or a water-absorbent resin is used as the hydrophilic polymer, a graft polymer or a water-absorbent resin composition (for example, a starch-acrylic acid polymer, a PVA-acrylic acid polymer, etc.) is obtained. These polymers and water-absorbent resin compositions also fall within the scope of one embodiment of the present invention.

[0179] [3-2] Polymerization Step In one aspect of the present invention, it is preferable to include a polymerization step of polymerizing the monomers (particularly, acrylic acid (salt)-based monomers) in the aqueous monomer solution obtained in the aqueous monomer solution preparation step to obtain a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel").

[0180] (Polymerization initiator) In the polymerization step, it is preferable to carry out the polymerization reaction of the monomers using a polymerization initiator. The polymerization initiator used in the polymerization step is appropriately selected depending on the polymerization form, etc., and is not particularly limited. For example, a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a redox-based polymerization initiator used in combination with a reducing agent that promotes the decomposition of these polymerization initiators can be mentioned. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 can be suitably used. Note that, from the viewpoints of handleability and the physical properties of the obtained water-absorbent resin, the polymerization initiator is preferably a peroxide or an azo compound, more preferably a peroxide, and even more preferably a persulfate.

[0181] The amount of polymerization initiator used in the polymerization step is preferably 0.001 mol % or more and 1 mol % or less, more preferably 0.001 mol % or more and 0.5 mol % or less, based on the total amount of monomers to be polymerized (monomers contained in the aqueous monomer solution). When a reducing agent is used in combination (i.e., when a redox-based polymerization initiator is used), the amount of reducing agent used is preferably 0.0001 mol % or more and 0.02 mol % or less, based on the total amount of monomers. The "mol %" relative to the monomer refers to the percentage of the number of moles of the polymerization initiator or reducing agent relative to the total number of moles of monomers contained in the aqueous monomer solution.

[0182] Instead of using the polymerization initiator, the polymerization reaction may be carried out by irradiating with active energy rays such as radiation, electron beams, and ultraviolet rays, or by using these active energy rays in combination with the polymerization initiator.

[0183] (Polymerization form) The polymerization form in the polymerization step is not particularly limited, but from the viewpoints of the water absorption properties of the obtained water-absorbent resin and ease of polymerization control, etc., preferably, spray-droplet polymerization, aqueous solution polymerization, and reversed-phase suspension polymerization are mentioned, more preferably, aqueous solution polymerization, reversed-phase suspension polymerization, and even more preferably, aqueous solution polymerization. Among these, continuous aqueous solution polymerization is particularly preferred. Forms such as continuous belt polymerization and continuous kneader polymerization can also be applied. Furthermore, a foaming polymerization form in which polymerization is carried out by dispersing bubbles (particularly, an inert gas or the like) in an aqueous monomer solution can also be applied.

[0184] As specific polymerization modes, continuous belt polymerization is disclosed in U.S. Pat. Nos. 4,893,999, 6,241,928, U.S. Patent Application Publication No. 2005 / 215734, etc., and continuous kneader polymerization is disclosed in U.S. Pat. Nos. 6,987,151, 6,710,141, etc. By employing these continuous aqueous solution polymerizations, it is possible to improve the production efficiency of a water absorbent resin.

[0185] Furthermore, preferred forms of the continuous aqueous solution polymerization include "high-temperature initiated polymerization" and "high-concentration polymerization." "High-temperature initiated polymerization" refers to a form in which polymerization is initiated at a temperature of preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher (the upper limit is the boiling point) of the aqueous monomer solution to be polymerized. "High-concentration polymerization" refers to a form in which polymerization is carried out at a monomer concentration of preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 45% by mass or higher (the upper limit is the saturated concentration). These polymerization forms can also be used in combination.

[0186] In the polymerization step, polymerization can be carried out under a desired atmosphere, for example, under an air atmosphere. However, from the viewpoint of the color tone of the obtained water-absorbent resin, polymerization is preferably carried out under an inert gas atmosphere such as nitrogen or argon. In this case, it is preferable to control the oxygen concentration to 1% by volume or less. Furthermore, it is preferable to replace dissolved oxygen in the aqueous monomer solution with an inert gas (for example, so that the dissolved oxygen is less than 1 mg / L).

[0187] In one embodiment of the present invention, the polymerization may be carried out as foam polymerization, in which bubbles (particularly the inert gases) are dispersed in an aqueous monomer solution.

[0188] [3-3] Gel Crushing Step In one embodiment of the present invention, the hydrogel obtained in the polymerization step is crushed using a screw extruder such as a kneader or a meat chopper, or a gel crusher such as a cutter mill to obtain a particulate hydrogel (hereinafter referred to as "particulate hydrogel").

[0189] In the polymerization step, when kneader polymerization is adopted as the polymerization form, the polymerization step and the gel crushing step are carried out simultaneously. In addition, when a particulate hydrogel is directly obtained in the polymerization process, such as in gas phase polymerization or reverse phase suspension polymerization, the gel crushing step may not be carried out.

[0190] Regarding gel crushing conditions and forms other than those described above, the contents disclosed in WO 2011 / 126079 are preferably applied to the present invention.

[0191] [3-4] Drying Step In one embodiment of the present invention, the particulate hydrogel obtained in the polymerization step and / or gel crushing step is preferably dried to a desired resin solid content to obtain a dried polymer. The resin solid content of the dried polymer is determined from the loss on drying (the change in mass when 1 g of the dried polymer is heated at 180 ° C. for 3 hours), and is preferably 80% by mass or more, more preferably 85% by mass to 99% by mass, even more preferably 90% by mass to 98% by mass, and particularly preferably 92% by mass to 97% by mass.

[0192] The method for drying the particulate hydrogel is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, high-humidity drying using high-temperature water vapor, etc. Among these, from the viewpoint of drying efficiency, hot air drying is preferred, and band drying in which hot air drying is performed on a ventilated belt is more preferred.

[0193] The drying temperature (hot air temperature) in the hot air drying is preferably 120°C or higher and 250°C or lower, more preferably 150°C or higher and 200°C or lower, from the viewpoints of the color tone of the obtained water-absorbent resin, drying efficiency, etc. Drying conditions other than the drying temperature, such as the hot air speed and drying time, can be appropriately set depending on the water content and total mass of the particulate hydrogel to be dried and the target resin solid content. When band drying is performed, the conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. are appropriately applied as drying conditions.

[0194] [3-5] Classification Step In one embodiment of the present invention, it is preferable to include a classification step in which the dried polymer obtained in the drying step is classified to obtain a particulate dried polymer having a desired particle size.

[0195] In the classification step, methods for classifying the dried polymer include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), air flow classification, etc. Among these, sieve classification is preferably selected from the viewpoint of classification efficiency.

[0196] The particle size of the particulate dried polymer obtained in the classification step is not particularly limited, but for example, the mass average particle diameter (D50) is preferably 200 μm to 500 μm, more preferably 250 μm to 450 μm, and even more preferably 300 μm to 400 μm. The proportion of particles smaller than 150 μm is preferably less than 5% by mass, more preferably 4% by mass or less, and even more preferably 3% by mass or less.

[0197] In the classification step, the dried polymer may be appropriately pulverized to obtain a particulate dried polymer of a desired particle size. The pulverization method is not particularly limited, and a high-speed rotary pulverizer such as a roll mill, a hammer mill, a screw mill, or a pin mill, a vibration mill, a knuckle-type pulverizer, or a cylindrical mixer may be used.

[0198] [3-6] Surface Cross-Linking Step In one aspect of the present invention, it is preferable to include a surface cross-linking step, which is a step of providing a portion with a higher cross-linking density on the surface layer (a portion several tens of μm from the surface of the particulate dry polymer) of the particulate dry polymer obtained through the classification step. The surface cross-linking step is composed of a mixing step, a heat treatment step, and a cooling step (optional).

[0199] In the surface cross-linking step, a surface-cross-linked water absorbent resin is obtained by radical cross-linking in the vicinity of the surface of the particulate dry polymer, surface polymerization, a cross-linking reaction with a surface cross-linking agent, etc. Therefore, the particulate dry polymer to be subjected to the surface cross-linking step can also be said to be a water absorbent resin precursor.

[0200] (Surface Crosslinking Agent) In the surface crosslinking step, it is preferable to crosslink the surface of the particulate dried polymer using a surface crosslinking agent. The surface crosslinking agent is not particularly limited, but examples thereof include organic or inorganic surface crosslinking agents. Among these, organic surface crosslinking agents that react with carboxyl groups are preferred from the viewpoints of the physical properties of the water-absorbent resin, the handleability of the surface crosslinking agent, and the like. Specifically, one or more compounds disclosed in U.S. Patent No. 7,183,456 can be used as the surface crosslinking agent. More specifically, examples of the surface crosslinking agent include polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyamine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, and cyclic urea compounds. One of these surface crosslinking agents may be used alone, or two or more may be used in combination. Furthermore, it is preferable to use these surface crosslinking agents in the form of an aqueous solution (i.e., in the form of a surface crosslinking agent solution).

[0201] The amount of surface crosslinking agent used in the surface crosslinking step (the total amount used when multiple agents are used) is preferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts by mass, relative to 100 parts by mass of the particulate dry polymer. The surface crosslinking agent is preferably added in the form of an aqueous solution, and in this case, the amount of water used is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, relative to 100 parts by mass of the particulate dry polymer. Furthermore, if necessary, a hydrophilic organic solvent may be used in combination, and in this case, the amount of the hydrophilic organic solvent used is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, relative to 100 parts by mass of the particulate dry polymer.

[0202] (Mixing step) The surface cross-linking step includes a mixing step of mixing the particulate dry polymer and the surface cross-linking agent. A method of mixing the surface cross-linking agent in the mixing step is not particularly limited, but for example, a method of preparing a surface cross-linking agent solution in advance and mixing the solution with the particulate dry polymer by preferably spraying or dropping, more preferably spraying, can be mentioned.

[0203] The device for mixing the particulate dry polymer and the surface cross-linking agent is not particularly limited, but is preferably a high-speed stirring mixer, more preferably a high-speed stirring continuous mixer.

[0204] (Heat Treatment Step) The surface cross-linking step includes a heat treatment step in which the mixture discharged from the mixing step is heated to cause a cross-linking reaction on the surface of the particulate dry polymer. The heat treatment step can also be said to be a cross-linking reaction step.

[0205] The apparatus for performing the heat treatment is not particularly limited, but preferably includes a paddle dryer. The heating temperature in the heat treatment, in other words, the reaction temperature in the crosslinking reaction, is appropriately set depending on the type of surface crosslinking agent used, but is preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 200°C or lower.

[0206] (Cooling Step) The surface cross-linking step preferably includes a cooling step of cooling the surface-cross-linked particulate dried polymer after the heat treatment.

[0207] The cooling device in the cooling step is not particularly limited, but is preferably a device with the same specifications as the device used in the heat treatment step, and more preferably a paddle dryer. This is because it can be used as a cooling device by changing the heat medium to a refrigerant. The surface-crosslinked particulate dried polymer obtained in the heat treatment step is forcibly cooled, if necessary, in the cooling step to preferably 40°C or higher and 80°C or lower, more preferably 50°C or higher and 70°C or lower.

[0208] [4] Use of Water-Absorbent Resin Mixture In one embodiment of the present invention, the water-absorbent resin mixture is used as a part of the raw material when producing a water-absorbent resin. The phrase "using the water-absorbent resin mixture as a part of the raw material when producing a water-absorbent resin" means that the water-absorbent resin mixture is added to a water-absorbent resin raw material (for example, a monomer aqueous solution, a hydrogel, a dried polymer, and / or a particulate dried polymer) in any of the above-mentioned steps, specifically, in any of the steps of preparing a monomer aqueous solution, the polymerization step, the gel crushing step, the drying step, the classification step, and the surface cross-linking step.

[0209]

[0123] The method of adding the water-absorbent resin mixture according to the present invention to a water-absorbent resin raw material is not particularly limited, but examples thereof include a method of adding the water-absorbent resin mixture as it is, a method of adding the water-absorbent resin mixture in a state of a swollen gel obtained by swelling the water-absorbent resin mixture with water, a method of adding the water-absorbent resin mixture in a state of being dispersed in water, and the like.

[0210] More specifically, when the water absorbent resin mixture is added in the "step of preparing an aqueous monomer solution", the water absorbent resin mixture may be added to an aqueous monomer solution, or the water absorbent resin mixture may be added to water in advance and then mixed with other raw materials such as monomers. Note that, from the viewpoint of uniform mixing of raw materials, it is preferable to add the water absorbent resin mixture to an aqueous monomer solution.

[0211] When the water-absorbent resin mixture is added in the "polymerization step", the water-absorbent resin mixture may be added before the start of polymerization, or may be added after the start of polymerization. From the viewpoint of uniformity of the polymer component, it is preferable to add the water-absorbent resin mixture before the start of polymerization.

[0212] When the water-absorbent resin mixture is added in the "gel crushing step", the water-absorbent resin mixture may be added before crushing, or may be added during crushing. Furthermore, the water-absorbent resin mixture may be added in divided portions. From the viewpoint of uniformity of the gel component, it is preferable to add the water-absorbent resin mixture before gel crushing.

[0213] When the water-absorbent resin mixture is added in the "drying step", the hydrous gel and the water-absorbent resin mixture may be mixed before drying, or may be mixed after drying. When the powder of the water-absorbent resin mixture and the hydrous gel are mixed before drying, the moisture in the hydrous gel is transferred to the water-absorbent resin mixture. Therefore, this is preferable because effects such as a reduction in the drying speed and a reduction in the generation of undried gel can be expected.

[0214] When the water-absorbent resin mixture is added in the "classification step", the dried polymer and the water-absorbent resin mixture may be mixed before classification, or may be mixed after classification. From the viewpoint of uniformity of particle size, it is preferable to add the water-absorbent resin mixture before classification.

[0215] When the water absorbent resin mixture is added in the "surface cross-linking step", the particulate dry polymer and the water absorbent resin mixture may be mixed in advance, or the treatment may be performed without mixing. From the viewpoint of uniform mixing of the surface cross-linking agent solution and the water absorbent resin, it is preferable to use the particulate water absorbent resin mixture and subject all the objects to be surface cross-linked (i.e., the particulate dry polymer and the particulate water absorbent resin mixture) to surface cross-linking in the form of particles.

[0216] In the method for producing a water-absorbent resin according to the present invention, the water-absorbent resin mixture may be added at any of the above-mentioned timings. Also, the entire amount of the water-absorbent resin mixture to be added may be added at once at any timing, or may be added in portions at multiple timings. Also, the water-absorbent resin mixture may be added in different forms depending on the timing of addition.

[0217] In one aspect of the present invention, the proportion of the water-absorbent resin mixture relative to all the water-absorbent resin raw materials is not particularly limited, but is preferably 1% by mass or more and 60% by mass or less, more preferably 3% by mass or more and 50% by mass or less, and even more preferably 5% by mass or more and 40% by mass or less. In other words, in the production method of the present invention, it is preferable to add the water-absorbent resin mixture so that the proportion of the water-absorbent resin mixture relative to all the water-absorbent resin raw materials falls within the above-mentioned range. Note that the "all the water-absorbent resin raw materials" refers to all the raw materials used in the production process of the water-absorbent resin (i.e., the total amount of the water-absorbent resin mixture to be added and the water-absorbent resin prepared from an aqueous monomer solution).

[0218] More specifically, the raw materials include a water-absorbent resin mixture, and a non-water-absorbent resin mixture and raw materials thereof (for example, acrylic acid (salt); basic composition; other monomers; internal cross-linking agents; other substances (starch, etc.); polymerization initiators; and surface cross-linking agents). The ratio is the ratio of the solid content of the water-absorbent resin mixture to the solid content of all the raw materials of the water-absorbent resin.

[0219] The effects of the present invention will be explained using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention. In the following examples, unless otherwise specified, operations were performed under conditions of room temperature (20°C or higher and 25°C or lower) and relative humidity 45±5% RH. Furthermore, unless otherwise specified, "%" and "parts" mean "% by mass" and "parts by mass," respectively.

[0220] [Measurement and Evaluation Methods] (a) Content of Materials Other Than Water-Absorbent Resin (Content of Foreign Matter) Unused filter paper (ADVANTEC Toyo Co., Ltd., product name: Qualitative Filter Paper No. 5A, thickness 0.22 mm, retention particle size 7 μm) was dried for 3 hours in an oven adjusted to 105° C., and then allowed to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured (a1: for sample measurement, b1: for blank measurement).

[0221] A 250 ml capacity plastic container with a lid containing a stirring bar having a length of 35 mm and 100 g of deionized water was adjusted to a temperature in a thermostatic bath at 40°C, and then a water absorbent resin solubilizer (also simply referred to as a solubilizer) was added and dissolved while stirring at 500 rpm (for the type and amount of the solubilizer, see each of the Examples, Comparative Examples, Reference Examples, and Comparative Reference Examples).

[0222] Next, 10.0 g of the weighed water-absorbent resin mixture was added to initiate the solubilization reaction of the water-absorbent resin. After the addition of the water-absorbent resin mixture, the stirring bar stopped rotating after a while due to swelling of the water-absorbent resin, but if the reaction was continued, a part of the water-absorbent resin was solubilized and the stirring bar became rotatable. After a predetermined time, the solubilization reaction was terminated (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the number of times the solubilizer was added, the timing of addition, and the solubilization time), and a solubilized liquid was obtained.

[0223] Next, the contents of the container were suction filtered using qualitative filter paper (No. 5A) whose mass had been measured in advance, and the container was washed with a total of 100 g of deionized water, and any residue remaining on the filter paper was washed. The washed filter paper was dried for 3 hours in an oven adjusted to 105°C. The dried filter paper was left to stand in a constant temperature and humidity chamber for 6 hours or more, and its mass was measured (a2). A blank filter paper was also washed with 100 g of deionized water, dried for 3 hours in an oven adjusted to 105°C, left to stand in a constant temperature and humidity chamber for 6 hours or more, and its mass was measured (b2).

[0224] The content of materials other than the water-absorbent resin (foreign matter content) was calculated according to the following formula 4.

[0225] Content of materials other than water-absorbent resin (foreign matter content) (mass%) = [(a2-a1)-(b2-b1)] / 10.0×100 ... Formula 4

[0226] (b) Content of Material (Foreign Matter) Other Than Water Absorbent Resin Remained on JIS Standard Sieve Having a Mesh Size of 150 μm A water absorbent resin mixture was classified using a JIS standard sieve having a mesh size of 150 μm, and a fraction (mass W1 (g)) that passed through the JIS standard sieve and a fraction (mass W2 (g)) remaining on the JIS standard sieve were measured.

[0227] 10.00 g of the fraction remaining on the JIS standard sieve was weighed, and in accordance with the method described in the above (a) Content of material other than water absorbent resin (content of foreign matter), a content rate P (%) of material other than water absorbent resin contained in the fraction remaining on the JIS standard sieve was determined.

[0228] According to the following formula 5, the content (mass%) of materials (foreign matter) other than the water-absorbent resin remaining on a JIS standard sieve with an opening of 150 μm was determined.

[0229] Content (mass%) of materials other than the water-absorbent resin remaining on a JIS standard sieve with an opening of 150 μm=(W2×P) / (W1+W2) Equation 5

[0230] (c) Mass Median Particle Diameter (D50) The mass median particle diameter (D50) of the water-absorbent resin or the water-absorbent resin mixture was measured using a JIS standard sieve in accordance with the measurement method described in U.S. Patent Application Publication No. 2006 / 204755.

[0231] (d) Moisture Content A water-absorbent resin or a water-absorbent resin mixture W3 (g) was placed on an aluminum dish having a mass W4 (g), and dried for 3 hours in a hot air circulating oven at 180° C. Thereafter, a total mass W5 (g) of the water-absorbent resin or the water-absorbent resin mixture and the aluminum dish was measured.

[0232] The moisture content of the water-absorbent resin or the water-absorbent resin mixture was calculated according to the following formula 6.

[0233] Moisture content (mass%) = [1-(W5-W4) / W3]×100... Formula 6

[0234] (e) Flow Rate A stopper at the bottom of a funnel with an orifice damper (manufactured by Coesfield, orifice inner diameter 10 mm, angle 20°, height 145 mm) was closed, and 100 g of a weighed water-absorbent resin or a water-absorbent resin mixture was placed in the funnel. A timer was started at the same time as the stopper was opened, and the time until all of the water-absorbent resin or the water-absorbent resin mixture in the funnel fell was measured.

[0235] The flow rate was calculated according to the following formula 7.

[0236] Flow rate (g / sec) = (mass (g) of water-absorbent resin or water-absorbent resin mixture) / (time required to finish falling (sec)) ... Equation 7

[0237] (f) Bulk specific gravity: Using the same funnel with damper as that for the flow rate, it was measured in accordance with JIS K 3362:2008. 100.0 g of a water-absorbent resin or a water-absorbent resin mixture that had been thoroughly mixed to eliminate bias due to particle size was placed in a funnel with the damper closed, and then the damper was quickly opened, and the water-absorbent resin or the water-absorbent resin mixture was dropped into a receiver having an internal volume of 100 ml. The weight (unit: g) (referred to as mass W6) of the receiver was measured in advance.

[0238] Next, the water absorbent resin or the water absorbent resin mixture that had risen up from the receiver was scraped off with a glass rod, and then the weight (unit: g) of the receiver containing the water absorbent resin or the water absorbent resin mixture (referred to as mass W7) was accurately measured to the nearest 0.1 g, and the bulk specific gravity was calculated according to the following formula 8.

[0239] Bulk specific gravity (g / mL) = (W7 - W6) / 100 ... Equation 8

[0240] (g) CRC "CRC" of a water-absorbent resin or a water-absorbent resin mixture is an abbreviation for "Centrifuge Retention Capacity", and indicates the absorption capacity of a water-absorbent resin or a water-absorbent resin mixture for a 0.90 mass % aqueous sodium chloride solution (saline solution) under no pressure for 30 minutes.

[0241] Specifically, 0.200 g of a water-absorbent resin or a water-absorbent resin mixture was uniformly placed in a bag (85 mm x 60 mm) made of nonwoven fabric (manufactured by Nankoku Pulp Industries Co., Ltd., trade name: Heatlon Paper, model: GSP-22), and heat-sealed, and then immersed in a large excess (usually about 500 ml) of a 0.90% by mass aqueous sodium chloride solution at room temperature. After 30 minutes, the bag was pulled out and drained for 3 minutes using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122 / 250G), and the mass W8 (g) of the bag was measured. In addition, the same operation was performed without using a water-absorbent resin or a water-absorbent resin mixture, and the mass W9 (g) at that time was measured. Then, from these W8 and W9, the centrifuge retention capacity (CRC) (g / g) was calculated according to the following formula 9.

[0242] CRC (g / g) = {(W8 - W9) / (mass of water-absorbent resin or water-absorbent resin mixture)} - 1 ... Equation 9

[0243] (h) Vortex (Water Absorption Rate) The vortex (water absorption rate) of the water-absorbent resin or the water-absorbent resin mixture was measured by the following procedure.

[0244] First, 0.02 parts by mass of Food Blue No. 1 (CAS No. 3844-45-9; a type of food additive) was added to 1000 parts by mass of a 0.9% by mass aqueous solution of sodium chloride to color it, and the liquid temperature was adjusted to 30 ° C. This was used as a test liquid. Next, 50 mL of the test liquid was weighed into a 100 mL beaker, a cylindrical stirrer having a length of 40 mm and a diameter of 8 mm was placed in the beaker, and stirring was started at 600 rpm. Subsequently, 2.0 g of a water-absorbent resin or a water-absorbent resin mixture was added to the test liquid during stirring, and the Vortex (water absorption rate) (unit: seconds) was measured.

[0245] The start and end points of the water absorption rate were determined in accordance with the description in JIS K 7224 (1996) "Explanation of Test Method for Water Absorption Rate of Super Absorbent Resins." Specifically, the start point was the time when the water-absorbent resin or the water-absorbent resin mixture was added to the test liquid, and the end point was the time when the added water-absorbent resin or the water-absorbent resin mixture absorbed the test liquid and gelled, and the gel covered the cylindrical stirrer, and the interval between these points was defined as a vortex (water absorption rate) (unit: seconds).

[0246] (i) AAP0.3 "AAP" of a water-absorbent resin or a water-absorbent resin mixture is an abbreviation for Absorption Against Pressure, and indicates the water absorption capacity under pressure for 0.90 mass% saline solution. AAP0.3 was measured in accordance with NWSP242.0.R2(15) except that the pressure condition was changed from 0.7 psi to 0.3 pai.

[0247] Specifically, a large excess of a 0.9% by mass aqueous solution of sodium chloride is used, and 0.9 g of a water-absorbing resin or a water-absorbing resin mixture is subjected to 2.07 kPa (21 g / cm) suction for 1 hour. ２ After swelling under a pressure of 0.3 psi, the AAP (absorbency against pressure) (unit: g / g) was measured.

[0248] (j) Solubilization Rate of Water-Absorbent Resin Unused filter paper (ADVANTEC Toyo Co., Ltd., product name: qualitative filter paper No. 5A, thickness 0.22 mm, retention particle diameter 7 μm) was dried for 3 hours in an oven adjusted to 105° C., and then allowed to stand in a constant temperature and humidity chamber for 6 hours or more, and its mass was measured (a3: for sample measurement, b3: for blank measurement).

[0249] A 250 ml plastic container with a lid containing a 35 mm long stirrer and 100 g of deionized water was placed in a thermostatic bath at 40°C, and then the solubilizing agent was added and dissolved while stirring at 500 rpm (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the type and amount of solubilizing agent).

[0250] Next, the water-absorbent resin was weighed and then added to the solubilizing agent aqueous solution to solubilize the water-absorbent resin (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the number of times the solubilizing agent was added, the timing of addition, and the solubilization time). After the solubilization reaction was completed, the contents in the container were suction-filtered using a qualitative filter paper (No. 5A) whose mass had been measured in advance, and the container and residue remaining on the filter paper were washed with a total of 100 g of deionized water. The washed filter paper was then dried for 3 hours in an oven adjusted to 105 ° C, and the dried filter paper was left to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured (a4). Blank filter paper was also washed with 100 g of deionized water, dried for 3 hours in an oven adjusted to 105 ° C, and then left to stand in a constant humidity chamber for 6 hours or more, and the mass was measured (b4).

[0251] The water-absorbent resin content (g) after the solubilization treatment was calculated according to the following formula 10.

[0252] Water-absorbent resin content after solubilization treatment (g)=[(a4-a3)-(b4-b3)] Equation 10 The solubilization rate of the water-absorbent resin was calculated by the following Equation 11.

[0253] Solubilization rate (mass%) of water-absorbent resin=[1−content of water-absorbent resin after solubilization treatment (g) / amount of water-absorbent resin (g)]×100 Equation 11

[0254] (k) Foreign Matter Solubilization Rate Unused filter paper (ADVANTEC Toyo Co., Ltd., product name: Qualitative Filter Paper No. 5A, thickness 0.22 mm, retention particle size 7 μm) was dried for 3 hours in an oven adjusted to 105° C., and then allowed to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured (a5: for sample measurement, b5: for blank measurement).

[0255] A 250 ml plastic container with a lid containing a 35 mm long stirrer and 100 g of deionized water was placed in a thermostatic bath at 40°C, and then the solubilizing agent was added and dissolved while stirring at 500 rpm (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the type and amount of solubilizing agent).

[0256] Next, materials other than the water-absorbent resin were weighed and then added to the solubilizing agent aqueous solution to solubilize the materials other than the water-absorbent resin (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the number of solubilizing agent additions, addition timing, and solubilization time). After the solubilization reaction was completed, the contents in the container were suction filtered using a qualitative filter paper (No. 5A) whose mass had been measured in advance, and the container and residue remaining on the filter paper were washed with a total of 100 g of deionized water. The washed filter paper was dried in an oven adjusted to 105 ° C for 3 hours, and then the dried filter paper was left to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured (a6). Blank filter paper was also washed with 100 g of deionized water, dried in an oven adjusted to 105 ° C for 3 hours, and then left to stand in a constant humidity chamber for 6 hours or more, and the mass was measured (b6).

[0257] The content (g) of materials other than the water-absorbent resin after the solubilization treatment was calculated according to the following formula 12.

[0258] Content of materials other than water-absorbent resin after solubilization treatment (g)=[(a6-a5)-(b6-b5)] Equation 12 The foreign matter solubilization rate was calculated by the following Equation 13.

[0259] Foreign matter solubilization rate (mass%)=[1−content (g) of material other than water-absorbent resin after solubilization treatment / amount (g) of material other than water-absorbent resin]×100 Equation 13

[0260] [Production Example 1] A reaction vessel was charged with 244.9 g of acrylic acid, 0.71 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid·trisodium (DTPA·3Na), 103.7 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 389.0 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (a').

[0261] Next, the aqueous monomer solution (a') was cooled with stirring. When the liquid temperature reached 40.0°C, 100.9 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (a). At this time, the temperature of the aqueous monomer solution (a) rose to 77.9°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved and became a transparent, homogeneous solution.

[0262] Next, 12.1 g of a 4.0 mass % aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (a), and the mixture was immediately poured into a stainless steel bat-type container (bottom 340 × 340 mm, height 25 mm, inner surface Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (a) into the bat-type container was 55 seconds, and the bat-type container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction started 70 seconds after the aqueous monomer solution (a) was poured into the bat-type container.

[0263] The polymerization reaction proceeded by expanding and foaming in all directions while generating water vapor, and then shrunk to a size slightly larger than the tub-shaped container. Three minutes after the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (S1) was removed. This series of operations was carried out in an open-air system. The hydrogel (S1) obtained by the polymerization reaction was cut into strips and crushed using a meat chopper with a die diameter of 7.5 mm. The hydrogel was then spread on a 50-mesh wire net and dried with hot air at 190°C for 60 minutes. The resulting product was then crushed using a vibration mill and passed through a sieve with an opening of 850 μm to obtain an irregularly crushed water-absorbent resin precursor (S1) with an average particle size of 350 μm, remaining on a sieve with an opening of 150 μm.

[0264] 100 parts by mass of the obtained water-absorbent resin precursor (S1) was mixed uniformly with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water, and the mixture was heated at 100 ° C. for 45 minutes. After cooling, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (S1). The obtained water-absorbent resin (S1) had a mass average particle diameter (D50) of 352 μm, a water content of 4.5% by mass, a flow rate of 10.5 g / sec, a bulk specific gravity of 0.65 g / ml, and a CRC of 36.0 g / g.

[0265] [Manufacturing Example 2] A commercially available disposable diaper (manufactured by Elleair Co., Ltd.; (GOO.N Plus), purchase date: January 14, 2022) was disassembled, and a mixture of the constituent water-absorbent resin and pulp was extracted. Thereafter, the mixture was placed in a reduced pressure dryer (temperature inside: 90°C) and dried for 3 hours to obtain a dried product. Thereafter, the dried product was classified using a JIS standard sieve with a mesh size of 2 mm, and only the pulp was extracted from the mixture (pulp extracted from disposable diapers).

[0266] The pulp removed from the disposable diapers obtained by the above operation was continuously pulverized using a pulverizer (manufactured by Orient Crusher Co., Ltd.; vertical pulverizer VM27-S) until the entire amount could pass through a 1.5 mm outlet-side screen. Subsequently, the pulverized pulp was classified using a JIS standard sieve with a mesh size of 150 μm, to obtain pulp that passed through the JIS standard sieve with a mesh size of 150 μm (model (A) of a material other than a water-absorbent resin) and pulp that remained on the JIS standard sieve with a mesh size of 150 μm (model (B) of a material other than a water-absorbent resin).

[0267] [Example 1] 99.9 mass% of the water absorbent resin (S1) obtained in Production Example 1 and 0.10 mass% of a material model (A) other than the water absorbent resin obtained in Production Example 2 were mixed well to be uniform, thereby obtaining a water absorbent resin mixture (1) (powder form).

[0268] The obtained water-absorbent resin mixture (1) had a content of materials other than the water-absorbent resin of 0.10% by mass, an amount of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm of 0.00% by mass, a mass average particle diameter (D50) of 345 μm, a moisture content of 4.8% by mass, a flow rate of 10.4 g / sec, a bulk specific gravity of 0.65 g / ml, and a CRC of 36.0 g / g. The measurement results are shown in Table 1.

[0269] In Table 1, the total content (mass%) of foreign matter (materials other than water absorbent resin) is the content of materials (pulp) other than the water absorbent resin determined by the measurement method described in the above (a) Content of materials other than water absorbent resin (foreign matter content).

[0270] In the examples and comparative examples, the solubilization rate of the water-absorbent resin and the solubilization rate of foreign matter were determined for the solubilized liquid obtained by the following procedure.

[0271] A 250 ml plastic container with a lid containing a stirring bar having a length of 35 mm and 100 g of deionized water was adjusted to a temperature in a thermostatic bath at 40°C, and then a water absorbent resin solubilizer (28.50 g of 30 mass % hydrogen peroxide, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate) was added and dissolved while stirring at 500 rpm.

[0272] Next, 9.99 g of the weighed water-absorbent resin mixture (1) was added to start the solubilization reaction of the water-absorbent resin. One hour after the addition of the water-absorbent resin mixture (1) (start of solubilization), 28.50 g of 30 mass% hydrogen peroxide, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were additionally added. Stirring was continued for another 2 hours to complete the solubilization reaction (total 3 hours), and a solubilized liquid was obtained.

[0273] The solubilization rate of the water-absorbent resin was determined to be 99.9% by mass under the solubilization conditions in Example 1. In addition, the same solubilization operation was performed using only 0.010 g of the material model (A) other than the water-absorbent resin, and the solubilization rate of the pulp (solubilization rate of foreign matter) was determined to be 6.0% by mass.

[0274] Next, under the above solubilization conditions, the contents of materials other than the water-absorbent resin were determined.

[0275] [Example 2] A water-absorbent resin mixture (2) was obtained by the same operation as in Example 1, except that 99.5 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 0.50 mass% of the material model (A) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (2) was 0.52 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.010 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (2) are shown in Table 1.

[0276] [Example 3] A water-absorbent resin mixture (3) was obtained by the same operation as in Example 1, except that 99.0 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 1.0 mass% of the material model (A) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (3) was 0.99 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.013 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (3) are shown in Table 1.

[0277] [Example 4] A water-absorbent resin mixture (4) was obtained by the same operation as in Example 1, except that 97.0 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 3.0 mass% of the material model (A) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (4) was 3.04 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.034 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (4) are shown in Table 1.

[0278] [Example 4-2] A water-absorbent resin mixture (4-2) was obtained by the same operation as in Example 1, except that 96.1 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 3.9 mass% of the material model (A) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (4-2) was 3.91 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.039 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (4-2) are shown in Table 1.

[0279] [Comparative Example 1] A water-absorbent resin mixture (C1) was obtained by the same operation as in Example 1, except that 95.0 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 5.0 mass% of the material model (A) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (C1) was 4.95 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.033 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (C1) are shown in Table 1.

[0280] [Example 5] 99.9 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 0.10 mass% of the material model (B) other than the water-absorbent resin obtained in Production Example 2 were mixed well to be uniform, to obtain a water-absorbent resin mixture (5). The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (5) was 0.11 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.10 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (5) are shown in Table 1.

[0281] [Example 6] A water-absorbent resin mixture (6) was obtained by the same operation as in Example 1, except that 99.5 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 0.50 mass% of the material model (B) other than the water-absorbent resin obtained in Production Example 2 were used. The content of the material other than the water-absorbent resin in the water-absorbent resin mixture (6) was 0.50 mass%, and the content of the material other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.47 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (6) are shown in Table 1.

[0282] [Example 7] A water-absorbent resin mixture (7) was obtained by the same operation as in Example 1, except that 99.0 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 1.0 mass% of the material model (B) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (7) was 0.98 mass%, and the content of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.97 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (7) are shown in Table 1.

[0283] [Comparative Example 2] A water-absorbent resin mixture (C2) was obtained by the same operation as in Example 1, except that 98.0 mass% of the water-absorbent resin (S1) obtained in Production Example 1 and 2.0 mass% of the material model (B) other than the water-absorbent resin obtained in Production Example 2 were used. The content of materials other than the water-absorbent resin in the water-absorbent resin mixture (C2) was 2.04 mass%, and the amount of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 2.03 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (C2) are shown in Table 1.

[0284] [Example 8] A water-absorbent resin (S1) obtained by performing the same operation as in Production Example 1 multiple times and pulp removed from diapers obtained by performing the same operation as in Production Example 2 multiple times were added in a mass ratio of 1:1 and thoroughly mixed to become uniform. Then, 20 times the amount of deionized water (substitute for human urine) was added to the mixture to prepare a "simulated used disposable diaper".

[0285] Next, the following operation was carried out on the simulated used disposable diapers as a recycling treatment of the water-absorbent resin. The simulated used disposable diapers and five times the amount of ethanol were added to a reaction vessel equipped with a stirring blade, and then the mixture was stirred for 10 minutes, whereupon the water-absorbent resin settled to the bottom of the vessel while discharging the absorbed deionized water. After stirring was stopped, the pulp floating in the upper layer of the vessel was removed by decantation. Deionized water and ethanol were again added to the remaining gel-like substance of the water-absorbent resin, and the mixture was stirred, after which the pulp floating in the upper layer of the vessel was removed by decantation.

[0286] Next, the content of the container was filtered through a 100-mesh stainless steel wire mesh, and then dried for 2 hours in an oven at 160° C. Next, the dried product was pulverized using a vibration mill, and a particulate recycled water absorbent resin mixture (8-1) was obtained which passed through a sieve with an opening of 850 μm.

[0287] This recycled water-absorbent resin mixture (8-1) was centrifuged with a cyclone centrifugal separator (dome diameter 600 mm) at a volume of 13 m ３ / min, and the supply rate of the recycled water-absorbent resin mixture was 1 kg / min, to produce a recycled water-absorbent resin (8-2) having a reduced pulp content.

[0288] Next, the recycled water-absorbent resin mixture (8-2) was separated into a recycled water-absorbent resin mixture (8-2a) remaining on the sieve and a recycled water-absorbent resin mixture (8-2b) that passed through the sieve using a JIS standard sieve with an opening of 150 μm.

[0289] The recycled water absorbent resin mixture (8-2a) was again operated under the same conditions using a cyclone centrifuge, thereby obtaining a recycled water absorbent resin (8-2a') having a reduced content of "pulp remaining on a JIS standard sieve with an opening of 150 μm".

[0290] Subsequently, the recycled water absorbent resin mixture (8-2b) that had passed through a 150 μm standard sieve and the recycled water absorbent resin (8-2a′) in which “pulp remaining on a JIS standard sieve having an opening of 150 μm” was reduced were thoroughly mixed to prepare a recycled water absorbent resin mixture (8-3).

[0291] The obtained recycled water-absorbent resin mixture (8-3) was measured according to the procedure described in [Measurement and evaluation method], and the content of materials other than the water-absorbent resin of the recycled water-absorbent resin mixture (8-3) was 1.18% by mass, the amount of materials other than the water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm was 0.49% by mass, the mass average particle diameter was 325 μm, the moisture content was 6.1% by mass, the flow rate was 10.1 g / sec, the bulk specific gravity was 0.61 g / ml, the CRC was 35.5 g / g, and the Vortex was 48.3 seconds. The measurement results of the physical properties of the recycled water-absorbent resin mixture (8-3) are shown in Table 1.

[0292] The contents of materials other than the water-absorbent resin in the recycled water-absorbent resin mixture (8-3), the composition of the solubilizing agent, the number of times of addition, and the solubilizing time were the same as those in Example 1.

[0293]

[0294] (Summary) From the results in Table 1, it can be seen that the water-absorbent resin mixtures containing the water-absorbent resins of Examples 1 to 7 and material models other than the water-absorbent resin have excellent powder properties (flow rate, bulk specific gravity) and water absorption properties (CRC, Vortex). On the other hand, it can be seen that Comparative Examples 1 and 2 show a particularly large decrease in powder properties compared to Production Example 1, which does not contain any materials other than the water-absorbent resin. Incidentally, with regard to Vortex, it can be seen that the Vortex value is smaller when materials other than the water-absorbent resin are included, compared to Production Example 1, which does not contain any materials other than the water-absorbent resin, i.e., the water absorption rate is improved. However, since the CRC, which indicates the long-term water absorption capacity, is much greater for the water-absorbent resin than for materials other than the water-absorbent resin, there is a tendency for the CRC to decrease as the content ratio of materials other than the water-absorbent resin increases. Furthermore, since the powder properties also rapidly deteriorate as the content ratio of materials other than the water-absorbent resin increases, it can be said that a water-absorbent resin mixture in which the content ratio of materials other than the water-absorbent resin of the present application is specified is preferable from the viewpoint of balancing powder properties and water absorption properties.

[0295] Furthermore, it is also apparent that Example 8, which is a water-absorbent resin mixture recycled from simulated used paper diapers, has excellent powder characteristics and water-absorbing properties.

[0296] In addition, in Examples 2, 3, 4 and 4-2, it is considered that by mixing the material model (A) other than the water-absorbent resin obtained in Production Example 2 with the water-absorbent resin, a small amount of pulp that passed through the JIS standard sieve with an opening of 150 μm aggregated and remained on the sieve with an opening of 150 μm.

[0297] (Reference Examples Related to Invention A (Quantitative Method)) Invention A (Quantitative Method) will be specifically described below in Reference Examples and Comparative Reference Examples. Note that the methods for measuring physical properties, etc. in Invention A are the same as those described in [Measurement and Evaluation Methods] above.

[0298] [Production Example 3] A reaction vessel was charged with 244.8 g of acrylic acid, 1.42 g (0.080 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid·trisodium (DTPA·3Na), 103.7 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 389.9 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (b').

[0299] Next, the aqueous monomer solution (b') was cooled with stirring. When the liquid temperature reached 40.0°C, 100.8 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (b). At this time, the temperature of the aqueous monomer solution (b) rose to 78.2°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved and became a transparent, homogeneous solution.

[0300] Next, 12.1 g of a 4.0 mass % aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (b), and the mixture was immediately poured into a stainless steel bat-shaped container (bottom 340 × 340 mm, height 25 mm, inner surface Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (b) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction started 60 seconds after the aqueous monomer solution (b) was poured into the bat-shaped container.

[0301] The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then shrunk to a size slightly larger than the tub-shaped container. Three minutes after the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (S3) was removed. This series of operations was carried out in an open-air system. The hydrogel (S3) obtained by the polymerization reaction was cut into strips and crushed using a meat chopper with a die diameter of 7.5 mm. The hydrogel (S3) was then spread on a 50-mesh wire net and dried with hot air at 190°C for 60 minutes. The resulting product was then crushed using a vibration mill and passed through a sieve with an opening of 850 μm to obtain an irregularly crushed water-absorbent resin precursor (S3) with an average particle size of 350 μm, remaining on a sieve with an opening of 150 μm.

[0302] A surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water-absorbent resin precursor (S3), and the mixture was heat-treated at 100 ° C. for 45 minutes. The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (S3). The CRC of the water-absorbent resin (S3) was 31.3 (g / g), and the AAP0.3 was 30.4 (g / g).

[0303] [Reference Example 1-1] A reaction vessel containing a stirring bar and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40°C, and then 28.50 g of 30 mass% hydrogen peroxide solution, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added and dissolved while stirring. Subsequently, 9.000 g of the water absorbent resin (S3) obtained in Production Example 3 was added to start the solubilization reaction of the water absorbent resin.

[0304] One hour after the start of solubilization, 28.50 g of 30% by mass hydrogen peroxide, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours, and the solubilization reaction was completed (total 3 hours), yielding solubilized solution (1-A). The solubilization treatment up to this point is referred to as solubilization treatment 1.

[0305] The amount of undissolved water-absorbent resin in the solubilized liquid (1-A) was measured by the following method.

[0306] The solubilized solution (1-A) was suction filtered using qualitative filter paper (No. 5A), the mass of which had been measured in the same manner as described in the column for measuring foreign matter content, and the residue remaining on the filter paper was washed with a total of 100 g of deionized water. The washed filter paper was dried for 3 hours in an oven adjusted to 105°C. The dried filter paper was left to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured. A blank filter paper was also washed with 100 g of deionized water, dried for 3 hours in an oven adjusted to 105°C, and left to stand in a constant temperature and humidity chamber for 6 hours or more, and the mass was measured.

[0307] The solubilization rate of the water-absorbent resin under the above solubilization conditions was determined to be 100.0% by mass.

[0308] Next, a similar solubilization operation (solubilization treatment 1) was performed using only 1.000 g of pulp obtained by decomposing a commercially available disposable diaper, and the solubilization rate of the pulp (foreign matter solubilization rate) was determined to be 3.490 mass%.

[0309] Solubilization was carried out using the solubilization method (solubilization treatment 1) as follows.

[0310] 9.000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 1.000 g of pulp obtained by decomposing commercially available disposable diapers (the same pulp used to determine the pulp solubilization rate) were thoroughly mixed to obtain a water-absorbent resin mixture (1-1) containing 10.00% by mass of pulp. A 250 ml plastic container with a lid containing a stirrer and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40 ° C., and then 28.50 g of 30% by mass hydrogen peroxide solution, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added and dissolved while stirring at 500 rpm. Subsequently, 10.00 g of the water-absorbent resin mixture (1-1) was added to start the solubilization reaction of the water-absorbent resin.

[0311] One hour after the start of solubilization, 28.50 g of 30% by mass hydrogen peroxide, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours, and the solubilization reaction was completed (3 hours in total), yielding solubilized solution (1-1). The amount of foreign matter in the resulting solubilized solution (1-1) was measured according to the foreign matter content measurement method, and the foreign matter content was found to be 9.651% by mass.

[0312] The error in the measurement results for the pulp charge ratio (10.00 mass%) was (9.651-10.00) / 10.00×100=-3.490 mass%.

[0313] [Reference Example 1-2] 9.9000 g of a water-absorbent resin and 0.1000 g of pulp were subjected to the same solubilization treatment 1 as in Reference Example 1-1, and the solubilization rate of the water-absorbent resin and the solubilization rate of the pulp were determined. The solubilization rate of the water-absorbent resin was 99.99% by mass, and the solubilization rate of the pulp was 4.661% by mass.

[0314] Next, the same solubilization treatment as in Reference Example 1-1 was carried out except that a water-absorbent resin mixture (1-2) in which the pulp content was changed to 1.000 mass% was used, and the foreign matter content was determined. The results are shown in Table 2.

[0315] [Reference Example 1-3] 9.9900 g of a water-absorbent resin and 0.01000 g of pulp were subjected to the same solubilization treatment 1 as in Reference Example 1-1, and the solubilization rate of the water-absorbent resin and the solubilization rate of the pulp were determined. The solubilization rate of the water-absorbent resin was 99.99% by mass, and the solubilization rate of the pulp was 6.033% by mass.

[0316] Next, the same solubilization treatment as in Reference Example 1-1 was carried out except that a water-absorbent resin mixture (1-3) in which the pulp content was changed to 0.1000 mass% was used, and the foreign matter content was determined. The results are shown in Table 2.

[0317] [Reference Example 2-1] The same operation (solubilization treatment 2) as in the solubilization treatment 1 of Reference Example 1-1 was carried out except that the total time of the solubilization reaction was shortened to 2 hours, and the solubilization rate of the water-absorbent resin and the solubilization rate of the pulp were determined. The solubilization rate of the water-absorbent resin was 99.91 mass%, and the solubilization rate of the pulp was 4.628 mass%.

[0318] Next, the water-absorbent resin mixture (1-1) was subjected to solubilization treatment 2, and the content of foreign matter was determined. The results are shown in Table 2.

[0319] [Reference Example 2-2] 9.9000 g of a water-absorbent resin and 0.1000 g of pulp were subjected to solubilization treatment 2 to determine the solubilization rate of the water-absorbent resin and the solubilization rate of the pulp. The solubilization rate of the water-absorbent resin was 99.93 mass %, and the solubilization rate of the pulp was 3.801 mass %.

[0320] Next, the water-absorbent resin mixture (1-2) was subjected to solubilization treatment 2, and the content of foreign matter was determined. The results are shown in Table 2.

[0321] [Reference Example 2-3] 9.9900 g of a water-absorbent resin and 0.01000 g of pulp were subjected to solubilization treatment 2 to determine the solubilization rate of the water-absorbent resin and the solubilization rate of the pulp. The solubilization rate of the water-absorbent resin was 99.97 mass %, and the solubilization rate of the pulp was 4.923 mass %.

[0322] Next, the water-absorbent resin mixture (1-3) was subjected to solubilization treatment 2, and the content of foreign matter was determined. The results are shown in Table 2.

[0323] [Reference Example 3-1] A reaction vessel containing a stirring bar and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40°C, and then 14.25 g of 30 mass% hydrogen peroxide solution, 2.500 g of L-ascorbic acid, and 0.2500 g of iron sulfate heptahydrate were added and dissolved while stirring. Subsequently, 9.000 g of the water absorbent resin (S3) obtained in Production Example 3 was added to start a solubilization reaction of the water absorbent resin.

[0324] One hour after the start of solubilization, 14.25 g of 30% by mass hydrogen peroxide, 2.500 g of L-ascorbic acid, and 0.2500 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours, and the solubilization reaction was completed (a total of 3 hours), yielding solubilized solution (3-A). The solubilization treatment up to this point is referred to as solubilization treatment 3.

[0325] The solubilization rate of the water-absorbent resin was determined from the solubilized liquid (3-A) and was found to be 99.44% by mass.

[0326] Next, the same solubilization procedure was carried out using only 1.000 g of pulp obtained by decomposing a commercially available disposable diaper, and the solubilization rate of the pulp was determined to be 3.111% by mass.

[0327] Solubilization was carried out using the above-mentioned solubilization method (solubilization treatment 3) as follows.

[0328] 9.000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 1.000 g of pulp obtained by decomposing commercially available disposable diapers (the same pulp used to determine the pulp solubilization rate) were thoroughly mixed to obtain a water-absorbent resin mixture (3-1) containing 10.00% by mass of pulp. A 250 ml plastic container with a lid containing a stirrer and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40 ° C., and then 14.25 g of 30% by mass hydrogen peroxide, 2.500 g of L-ascorbic acid, and 0.2500 g of iron sulfate heptahydrate were added and dissolved while stirring at 500 rpm. Subsequently, 10.00 g of the water-absorbent resin mixture (3-1) was added to start the solubilization reaction of the water-absorbent resin.

[0329] One hour after the start of solubilization, 14.25 g of 30% by mass hydrogen peroxide, 2.500 g of L-ascorbic acid, and 0.2500 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours, and the solubilization reaction was completed (total 3 hours), yielding solubilized solution (3-1). The amount of foreign matter in the resulting solubilized solution (3-1) was measured according to the foreign matter content measurement method, and the foreign matter content was found to be 10.19% by mass.

[0330] The error in the measurement results for the pulp feed ratio (10.00 mass%) was (10.19-10.00) / 10.00 x 100 = 1.930 mass%.

[0331] [Reference Example 4-1] 9.000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 1.000 g of a polyolefin nonwoven fabric obtained by decomposing a commercially available disposable diaper were each subjected to solubilization treatment 1, and the solubilization rate of the water-absorbent resin and the solubilization rate of the nonwoven fabric (solubilization rate of foreign matter) were determined. The solubilization rate of the water-absorbent resin was 99.99% by mass, and the solubilization rate of the nonwoven fabric was 1.233% by mass.

[0332] Solubilization was carried out using the solubilization method (solubilization treatment 1) as follows.

[0333] 9.000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 1.000 g of a polyolefin nonwoven fabric obtained by decomposing a commercially available paper diaper (the same as the nonwoven fabric for which the nonwoven fabric solubilization rate was determined) were thoroughly mixed to obtain a water-absorbent resin mixture (4-1) containing 10.00 mass% of nonwoven fabric. A reaction vessel containing a stirrer and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40 ° C., and then 28.50 g of 30 mass% hydrogen peroxide solution, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added and dissolved while stirring. Subsequently, 10.00 g of the water-absorbent resin mixture (4-1) was added to start the solubilization reaction of the water-absorbent resin.

[0334] One hour after the start of solubilization, 28.50 g of 30% by mass hydrogen peroxide, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours, and the solubilization reaction was completed (total 3 hours), yielding solubilized solution (4-1). The amount of foreign matter in the resulting solubilized solution (4-1) was measured according to the foreign matter content measurement method, and the foreign matter content was found to be 9.886% by mass.

[0335] The error in the measurement results for the nonwoven fabric feed ratio (10.00 mass%) was (9.886-10.00) / 10.00×100=-1.140 mass%.

[0336] [Reference Example 4-2] 9.9000 g of a water-absorbent resin and 0.1000 g of a nonwoven fabric were subjected to solubilization treatment 1, and the solubilization rate of the water-absorbent resin and the solubilization rate of the nonwoven fabric were determined. The solubilization rate of the water-absorbent resin was 99.99 mass%, and the solubilization rate of the nonwoven fabric was 1.359 mass%.

[0337] Next, the foreign matter content was determined in the same manner as in Reference Example 4-1, except that a water-absorbent resin mixture (4-2) in which the content of the nonwoven fabric was changed to 1.000 mass% was used. The results are shown in Table 2.

[0338] [Reference Example 4-3] 9.9900 g of a water-absorbent resin and 0.01000 g of pulp were subjected to solubilization treatment 1, and the solubilization rate of the water-absorbent resin and the solubilization rate of the nonwoven fabric were determined. The solubilization rate of the water-absorbent resin was 99.99 mass%, and the solubilization rate of the nonwoven fabric was 1.582 mass%.

[0339] Next, the foreign matter content was determined in the same manner as in Reference Example 4-1, except that a water-absorbent resin mixture (4-3) in which the content of the nonwoven fabric was changed to 0.1000 mass% was used. The results are shown in Table 2.

[0340] [Comparative Reference Example 1] 19.80 g of the water-absorbent resin (S3) obtained in Production Example 3 and 0.2000 g of pulp obtained by decomposing commercially available disposable diapers were thoroughly mixed to obtain a comparative water-absorbent resin mixture (1) containing 1.000 mass% of pulp. 200.0 g of deionized water was added to a reaction vessel containing the entire amount of the comparative water-absorbent resin mixture (1) and a stirrer, and the water-absorbent resin was allowed to swell for a while. 66.50 g of calcium chloride dihydrate was added thereto, and the absorbed water-absorbent resin began to release water and shrink. Stirring was then continued for 30 minutes to separate the water-absorbent resin from the pulp.

[0341] The entire liquid was placed in a separatory funnel and shaken, after which the water-absorbent resin was allowed to settle and separate. The settled water-absorbent resin was extracted from the bottom of the separatory funnel into a container. The solution extracted from the bottom was further stirred with a stirrer to further separate the pulp mixed in the water-absorbent resin, and then the supernatant liquid containing the pulp was returned to the separatory funnel by decantation.

[0342] The pulp remaining in the separatory funnel was suction-filtered using qualitative filter paper (No. 5A) in the same manner as in measuring the "foreign matter content." The container was washed with an appropriate amount of deionized water, and the residue remaining on the filter paper was washed away. The container was then dried and the mass was measured to determine the foreign matter content, which was 0.5321% by mass. The error in the measurement result relative to the pulp charge ratio (1.000% by mass) was (0.5321 - 1.000) / 1.000 x 100 = -46.79% by mass.

[0343] [Comparative Reference Example 2] A reaction vessel containing a stirring bar and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40°C, and then 7.150 g of 30 mass% hydrogen peroxide water, 1.250 g of L-ascorbic acid, 0.1250 g of iron sulfate heptahydrate, and 3.000 g of 48 mass% sodium hydroxide aqueous solution were added and dissolved while stirring. Subsequently, 9.900 g of the water absorbent resin (S3) obtained in Production Example 3 was added to start the solubilization reaction of the water absorbent resin.

[0344] One hour after the start of solubilization, 7.150 g of 30% by mass hydrogen peroxide, 1.250 g of L-ascorbic acid, and 0.1250 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours to complete the solubilization reaction (3 hours in total), yielding solubilized solution (1-C). The solubilization treatment up to this point is referred to as solubilization treatment 4.

[0345] The solubilization rate of the water-absorbent resin was determined from the solubilized liquid (1-C) and was found to be 99.99% by mass.

[0346] Next, a similar solubilization procedure (solubilization treatment 4) was carried out using only 0.1000 g of pulp obtained by decomposing a commercially available disposable diaper, and the solubilization rate of the pulp was determined to be 32.31% by mass.

[0347] 9.900 g of the water-absorbent resin (S3) obtained in Production Example 3 and 0.1000 g of pulp obtained by decomposing commercially available paper diapers (the same pulp used to determine the pulp solubilization rate) were thoroughly mixed to obtain a comparative water-absorbent resin mixture (2) containing 1.000 mass% pulp. A 250 ml plastic container with a lid containing a stirrer and 100.0 g of deionized water was immersed in a thermostatic bath adjusted to 40 ° C., and then 7.150 g of 30 mass% hydrogen peroxide solution, 1.250 g of L-ascorbic acid, 0.1250 g of iron sulfate heptahydrate, and 3.000 g of 48 mass% sodium hydroxide aqueous solution were added and dissolved while stirring at 500 rpm.

[0348] Subsequently, 10.00 g of comparative water-absorbent resin mixture (2) was added to initiate the solubilization reaction of the water-absorbent resin. One hour after the start of solubilization, 7.150 g of 30% by mass hydrogen peroxide solution, 1.250 g of L-ascorbic acid, and 0.1250 g of iron sulfate heptahydrate were added. Stirring was continued for another 2 hours to complete the solubilization reaction (3 hours in total), and comparative solubilized liquid (2) was obtained. The pH of comparative solubilized liquid (2) was 11.2. The amount of foreign matter in the obtained comparative solubilized liquid (2) was measured according to the foreign matter content measurement method, and the foreign matter content was 0.6868% by mass. The error in the measurement result for the pulp feed ratio (1.000% by mass) was (0.6868-1.000) / 1.000 x 100 = -31.32% by mass.

[0349] [Comparative Reference Example 3] 5.000 g of sodium hypochlorite solution (chlorine content 5% or more, manufactured by Kishida Chemical Co., Ltd.) and 95.00 g of deionized water were added to a reaction vessel containing a stirrer, and the mixture was stirred at 500 rpm for 5 minutes. The pH of the solution was measured and found to be 10.1. Furthermore, 0.9000 g of the water-absorbent resin (S3) obtained in Production Example 3 was added, and the mixture was stirred at 500 rpm for 5 minutes. The reaction vessel was then immersed in a thermostatic bath adjusted to 80 ° C. to initiate the solubilization reaction of the water-absorbent resin.

[0350] The solubilization reaction was continued for 6 hours, and the solubilized solution (1-D) was obtained. The solubilization treatment up to this point was designated as solubilization treatment 5.

[0351] The solubilization rate of the water-absorbent resin was determined from the solubilized liquid (1-D) and was found to be 94.21% by mass.

[0352] Next, a similar solubilization procedure (solubilization treatment 5) was carried out using only 0.1000 g of pulp obtained by decomposing a commercially available disposable diaper, and the solubilization rate of the pulp was determined to be 35.14% by mass.

[0353] 0.9000 g of the water absorbent resin (S3) obtained in Production Example 3 and 0.1000 g of pulp obtained by decomposing a commercially available disposable diaper (the same as the pulp for which the pulp solubilization rate was determined) were thoroughly mixed to obtain a comparative water absorbent resin mixture (3) containing 10.00 mass % of pulp.

[0354]

[0223] To a reaction vessel containing a stirrer placed therein, 5.000 g of a sodium hypochlorite solution (chlorine content of 5% or more, manufactured by Kishida Chemical Co., Ltd.) and 95.00 g of deionized water were added and stirred for 5 minutes at 500 rpm, thereafter 1.000 g of a comparative water absorbent resin mixture (3) was added and stirred for 5 minutes at 500 rpm, and then the reaction vessel was immersed in a thermostatic bath whose temperature was adjusted to 80°C, to start a solubilization reaction of a water absorbent resin.

[0355] Stirring was continued for 6 hours to complete the solubilization reaction, yielding Comparative Solubilized Solution (3). The amount of foreign matter in the resulting Comparative Solubilized Solution (3) was measured according to the foreign matter content measurement method, and the foreign matter content was found to be 12.69% by mass. The error in the measurement result relative to the pulp charge ratio (10.00% by mass) was (12.69-10.00) / 10.00 x 100 = 26.89% by mass.

[0356]

[0357] Reference Examples 1 to 4, in which the solubilization rate of the water-absorbent resin was increased while the solubilization rate of foreign matter such as pulp and nonwoven fabric was kept low, were able to quantify the foreign matter content with high measurement accuracy while keeping the measurement error rate low. Furthermore, it can be seen that in this case, the higher the solubilization rate of the water-absorbent resin was (Reference Example 2 → Reference Example 1), the lower the measurement error rate could be kept even when the foreign matter content rate was low.

[0358] On the other hand, Comparative Reference Example 1 in which the foreign matter content was quantified by the "sedimentation separation method" utilizing the difference in sedimentation velocity between the water-absorbent resin and the foreign matter had a large measurement error of -46.79 mass%. In this method, under conditions in which the water-absorbent resin is present in a larger amount than the foreign matter, it is thought that the amount of foreign matter measured is lower than the set amount because a small amount of foreign matter becomes physically entangled with the water-absorbent resin.

[0359] In Comparative Reference Example 2, in which treatment with a solubilizing agent was performed under alkaline conditions, the solubilization rate of the water-absorbent resin was very high, but the solubilization rate of the pulp also increased, resulting in a large error in the measurement of the foreign matter content. Furthermore, in Comparative Reference Example 3, in which sodium hypochlorite was used as the solubilizing agent, the solubilization rate of the pulp increased, resulting in a large error in the measurement of the foreign matter content.

[0360] The above is a reference example relating to Invention A (quantitation method).

[0361] This application is based on Japanese Patent Application No. 2024-081547 filed on May 20, 2024 and Japanese Patent Application No. 2024-081551 filed on May 20, 2024, the disclosures of which are incorporated herein by reference in their entirety.

Claims

1. A water-absorbent resin mixture containing a water-absorbent resin and a material other than the water-absorbent resin, which satisfies the following i) and ii): i) the content of the material other than the water-absorbent resin is 0.01% by mass or more and 4.0% by mass or less with respect to the water-absorbent resin mixture; ii) the content of the material other than the water-absorbent resin remaining on a JIS standard sieve having an opening of 150 μm is 0% by mass or more and 1.0% by mass or less with respect to the water-absorbent resin mixture.

2. A water-absorbent resin mixture according to claim 1, wherein the content of a material other than a water-absorbent resin is determined by a method for quantifying a material other than a water-absorbent resin, the method including the following steps a) to c): a) solubilizing the water-absorbent resin in water using a water-absorbent resin solubilization method for the water-absorbent resin mixture, which solubilizes the water-absorbent resin in water and suppresses solubilization of materials other than the water-absorbent resin in water; b) removing the solubilized water-absorbent resin obtained in a); c) drying the remaining component obtained in b), and determining the content of a material other than a water-absorbent resin.

3. A water absorbent resin mixture according to claim 2, wherein a content of a material other than a water absorbent resin remaining on said JIS standard sieve having a mesh size of 150 μm is determined from a content rate P mass % of said material other than a water absorbent resin in a water absorbent resin mixture determined by said quantification method by the following formula: Content (mass %) of material other than a water absorbent resin remaining on the JIS standard sieve having a mesh size of 150 μm=Content mass ratio of the water absorbent resin mixture remaining on the JIS standard sieve having a mesh size of 150 μm in the water absorbent resin mixture×P mass %.

4. The water-absorbent resin mixture according to claim 2, wherein a solubilization method is used that allows the solubilization rate of the water-absorbent resin in the water-absorbent resin mixture to be 90 mass % or more.

5. The water-absorbent resin mixture according to claim 2, wherein a solubilization method is used such that the solubilization rate of materials other than the water-absorbent resin is less than 30 mass %.

6. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture is in the form of powder.

7. The water-absorbent resin mixture according to claim 1 or 2, wherein the flow rate of the water-absorbent resin mixture is 7.0 g / sec or more.

8. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture has a bulk density of 0.55 g / ml or more.

9. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture has a moisture content of 20% by mass or less.

10. The water-absorbent resin mixture according to claim 1 or 2, wherein the mass median particle diameter (D50) of the water-absorbent resin mixture is 200 μm or more and 600 μm or less.

11. The water-absorbent resin mixture according to claim 1 or 2, wherein the material other than the water-absorbent resin is a constituent material derived from an absorbent article.

12. The water-absorbent resin mixture according to claim 1 or 2, wherein the material other than the water-absorbent resin comprises at least one selected from the group consisting of pulp, nonwoven fabric, and resin film.

13. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture is recovered from used absorbent articles.

14. A method for recycling absorbent resin contained in used absorbent articles, wherein the recycled absorbent resin comprises the absorbent resin mixture according to claim 1 or 2.

15. A method for producing a water-absorbent resin, in which the water-absorbent resin mixture according to claim 1 or 2 is used as part of the raw material in the process of producing a water-absorbent resin from the monomers constituting the water-absorbent resin.

16. The manufacturing method according to claim 15, wherein the proportion of the water-absorbent resin mixture relative to all the water-absorbent resin raw materials is 1% by mass or more and 60% by mass or less.

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