WO2025243778A1 - Water-absorbent resin mixture and method for producing water-absorbent resin using water-absorbent resin mixture as part of raw material - Google Patents

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

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 that uses the water-absorbent resin mixture as part of its raw material.

Super absorbent polymer (SAP) is a water-swellable, water-insoluble polymer gelling agent that is used in a variety of absorbent products, including disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), hygiene materials (sanitary products) such as pet sheets, soil water retention agents for agricultural and horticultural use, and industrial water-stopping agents.

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

In recent years, efforts have been made to recycle such used absorbent articles from the perspective of environmental protection and the effective use of resources. One method under consideration is to separate, recover, and reuse the components that make up the absorbent articles, particularly the pulp and water-absorbent resin, rather than relying on incineration.

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

JP 2019-85447 A Japanese Patent Application Laid-Open No. 2021-41310 Japanese Patent Application Publication No. 04-317785

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

According to the method described in Patent Document 1, the absorbent resin is removed from the pulp containing the remaining absorbent resin, which has been separated from used absorbent articles, and the absorbent resin is separated from the pulp. Furthermore, Cited Document 2 describes that foreign matter (pulp, etc.) can be separated from the recycled superabsorbent polymer by carrying out a foreign matter separation process in which foreign matter such as pulp is separated from the recycled superabsorbent polymer after the drying process.

However, when used absorbent articles are recycled, for example, as in the technologies described in Patent Documents 1 and 2 above, even after a process of separating 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") has been carried out, small amounts of materials other than the water-absorbent resin are still 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 reducing its water-absorbing properties and powder properties.

Furthermore, Cited Document 2 describes a method of using recycled water-absorbent resin as a raw material or semi-finished product for the production of ordinary water-absorbent resin. However, recycled water-absorbent resin that contains foreign matter and has reduced powder properties has different handling characteristics from the raw material or semi-finished product used to produce ordinary water-absorbent resin, making it extremely difficult to use. Therefore, there is a demand for recycled water-absorbent resin with minimal deterioration in water-absorption properties and powder properties.

The object of the present invention is to provide a technology that can produce a recycled water-absorbent resin mixture containing a water-absorbent resin and a small amount of material other than the water-absorbent resin, with little deterioration in water absorption properties and powder characteristics.

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) 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 materials other than the water-absorbent resin remaining on a JIS standard sieve with 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 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) subjecting the water-absorbent resin mixture to a solubilization technique for solubilizing the water-absorbent resin mixture in water, while suppressing solubilization of materials other than the water-absorbent resin in water;
b) removing the solubilized water-absorbent resin component obtained in a);
c) The remaining component obtained in b) is dried, and the content of materials other than the water-absorbent resin is determined.
[3] The content of the material other than the water absorbent resin remaining on the JIS standard sieve having an opening of 150 μm is calculated from the content P% by mass of the material other than the water absorbent resin in the water absorbent resin mixture obtained by the quantification method by the following formula:
Content (mass%) of material other than water absorbent resin remaining on JIS standard sieve having 150 μm opening=Content mass ratio of water absorbent resin mixture remaining on JIS standard sieve having 150 μm opening in water absorbent resin mixture×P mass%
The water-absorbent resin mixture according to [2], which is obtained by the following formula:
[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 one of [2] to [4], 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 any one of [1] to [5], wherein the water-absorbent resin mixture is in a powder form.
[7] The water-absorbent resin mixture according to any one of [1] to [6], 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 any one of [1] to [7], wherein the bulk density of the water-absorbent resin mixture is 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 mass% or less.
[10] The water-absorbent resin mixture according to any one of [1] to [9], wherein the mass average 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 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 a used absorbent article, 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, wherein the water-absorbent resin mixture according to any one of [1] to [13] is used 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 manufacturing method according to [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.

The following describes preferred embodiments of the present invention. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims. Furthermore, the embodiments described in this specification can be combined in any manner to create other embodiments.

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) 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 materials other than the water-absorbent resin remaining on a JIS standard sieve with 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.

According to the above-described embodiment, in a water-absorbent resin mixture or 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 that exhibits less deterioration (change) in water absorption properties (e.g., CRC) and powder properties (e.g., flow rate and bulk specific gravity) compared to the original water-absorbent resin, and in particular, less deterioration (change) in powder properties. Furthermore, according to the above-described embodiment, it is possible to obtain a recycled water-absorbent resin mixture that exhibits excellent water-absorption properties (e.g., Vortex (water absorption speed)).

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

[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.

(Water-absorbent resin mixture recovered from used absorbent articles)
In one embodiment of the present invention, the water-absorbent resin mixture includes that recovered from used absorbent articles. In this specification, "used absorbent articles" refers to used sanitary materials that have been used by consumers and have absorbed body fluids such as urine and blood. In other words, used absorbent articles contain 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.

Here, there are no particular restrictions on the state of use. For example, it can be one with solid waste (feces, etc.) attached, one with liquid waste (urine, menstrual blood, etc.) attached, or one with both attached, but in consideration of the cost and processing efficiency of the recycling process, it is preferable for it to be in a state where urine is the main component absorbed.

Furthermore, the used absorbent articles used in the present invention include those that are collected, recovered, and transported from facilities where users of absorbent articles live or stay, such as ordinary households, hospitals, and welfare facilities.

[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".

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 partially neutralized carboxyl groups. Specific examples of water-absorbent resins 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, etc.

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).

In this specification, the term "water-absorbent resin" is not limited to a composition in which the total amount (100% by mass) is solely the water-absorbent resin, but may also refer to a water-absorbent resin composition containing additives, etc.

The mass percentage of the water-absorbent resin 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 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. 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.

[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.

i) Pulp: wood pulp, semi-synthetic fibers such as rayon and acetate;
ii) Nonwoven fabric: nonwoven fabric using fibers such as rayon, polyester, polypropylene, polyethylene, etc.
iii) Resin films: 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 them, 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, more preferably to include pulp and/or nonwoven fabric, and even more preferably to include pulp. In particular, fibrous materials such as pulp and nonwoven fabric have a high bulk specific gravity and can be a factor that significantly reduces the flow characteristics 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.

Note that "materials other than water-absorbent resins" does not include materials that are present only inside water-absorbent resin particles, but rather materials that are present outside water-absorbent resin particles, or materials where at least a portion of a material other than water-absorbent resin is present outside water-absorbent resin particles. This is because materials other than water-absorbent resins that are present only inside water-absorbent resin particles have little adverse effect on the water absorption properties and powder characteristics of the water-absorbent resin mixture, which is the subject of this application.

Furthermore, materials other than the water-absorbent resin do not include additives added during the production of the water-absorbent resin. Examples of such additives include 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, and non-polymeric water-soluble compounds.

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 insoluble matter from the solubilized matter obtained by the solubilization treatment, adding nitric acid to the insoluble matter and dissolving it by heating, and then quantifying the inorganic metal atoms using ICP optical emission spectrometry. Therefore, the amount of foreign matter can be quantified with greater 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 Water-Absorbent Resin" described below.

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

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

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

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

(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.

(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, and the like.

(Non-polymer water-soluble compound)
Specific examples of the non-polymeric water-soluble compound include the compounds disclosed in the section "Non-polymeric water-soluble compound" of WO 2014/034667.

As a method for determining whether or not a material falls under the category of a material other than a water-absorbent resin, for example, in the method shown in the "Method for quantifying the content of materials other than a water-absorbent resin in a water-absorbent resin mixture" described below, the material that is not solubilized by a "solubilizer" that solubilizes only the water-absorbent resin and remains can be determined to be a "material other than a water-absorbent resin."

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

More specifically, the content of materials other than the water-absorbent resin is preferably determined by a method for quantifying materials other than the water-absorbent resin, including the following steps a) to c).

a) solubilizing the water-absorbent resin in the water-absorbent resin mixture using a water-absorbent resin solubilization technique that solubilizes the water-absorbent resin in water while suppressing the solubilization of materials other than the water-absorbent resin in water;
b) removing the solubilized water-absorbing resin obtained in a);
c) The remaining component obtained in b) is dried, and the content of materials other than the water-absorbent resin is determined. 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.

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. Specific details will be described in detail in the section below titled "Method for quantifying the content of materials other than the water-absorbent resin." In addition, when there are multiple solubilization methods, the content of materials other than the water-absorbent resin can be determined using any of the solubilization methods.

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) will be 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 that exhibits little reduction in water absorption properties and powder properties, and further, a water-absorbent resin mixture with improved water absorption properties (e.g., Vortex) can be obtained. Materials other than the water-absorbent resin degrade the water-absorbing properties and appearance of the recycled water-absorbent resin, so it is considered desirable to keep their content as low as possible. However, as mentioned above, from the standpoint of powder properties, a content of up to 4.0% by mass is acceptable (4.0% by mass is critical for powder properties), but surprisingly, when the content is above the lower limit, water-absorbing properties (for example, Vortex) are improved.

(Method for quantifying the content of materials other than water-absorbent resin (foreign matter content) in a water-absorbent resin mixture) (Invention A)
In this specification, "a method for quantifying the content of a material other than a water-absorbent resin in a water-absorbent resin mixture (a content of foreign matter)" is a method for quantifying the content of a non-solubilized component as a "material other than a water-absorbent resin (foreign matter)" by selectively solubilizing the water-absorbent resin contained in the water-absorbent resin mixture using a specific agent or the like. Hereinafter, the quantification method (Invention A) will be described in detail.

(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.

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

In recent years, efforts have been made to recycle such used absorbent articles from the perspective of environmental protection, etc. For example, sanitary materials contain raw materials such as pulp, nonwoven fabrics, and adhesives in addition to the aforementioned water-absorbent resin, and material recycling has been attempted in which these raw materials are separated from used sanitary materials and reused. Furthermore, a method has been developed in which the water-absorbent resin is solubilized (decomposed) and separated from other components, and then the solubilized polymer is added to the water-absorbent resin manufacturing process for reuse. One known technique for solubilizing (decomposing) water-absorbent resin is, for example, heating the water-absorbent resin in an aqueous solution of an oxidizing agent (Patent Document 3).

Furthermore, with the aim of quantifying the recovery efficiency of materials recovered from used absorbent articles and the amount of impurities in the resulting recycled products, Non-Patent Document 1 describes that a sedimentation separation method can be used to quantify the amount of pulp and water-absorbent resin contained in samples during the recycling process.

When recycling absorbent resin from used absorbent goods, a process is included to separate out materials other than the absorbent resin, such as the constituent materials of disposable diapers, such as pulp, nonwoven fabric, and plastic. However, even after this separation process, small amounts of foreign matter may still be present in the absorbent resin. The presence of these foreign matter reduces the absorbency and appearance of the recycled absorbent resin, so it is desirable for the foreign matter content to be as low as possible.

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

The present invention therefore also encompasses a quantification method for quantifying the content of materials other than water-absorbent resins with high measurement accuracy.

One aspect of the present invention is a method for quantifying materials other than the water-absorbent resin in a water-absorbent resin mixture, the method comprising: 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 step (2) of quantifying the materials other than the water-absorbent resin from the solubilized product, wherein step (1) uses a water-absorbent resin solubilization technique that suppresses the solubilization of materials other than the water-absorbent resin in water.

This configuration allows for a quantification method with little measurement error (deviation from the actual value) when quantifying materials other than the water-absorbent resin contained in a water-absorbent resin mixture. Furthermore, a simple quantification method can be provided when quantifying materials other than the water-absorbent resin contained in a water-absorbent resin mixture.

When recycling absorbent goods, the water-absorbent resin is usually separated from constituent materials originating from the absorbent goods other than the water-absorbent resin. Specific processes for recycling water-absorbent resin from used absorbent goods to obtain recycled water-absorbent resin include, for example, a cleaning and sterilization process for cleaning and sterilizing used absorbent goods; a crushing process for crushing the exterior materials of absorbent goods, 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 shrinking and dehydrating treatment (inactivation treatment); a cleaning and disinfection process for cleaning and/or disinfecting the water-absorbent resin; a water-absorbent resin regeneration process; and a drying process for drying the water-absorbent resin.

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

The water-absorbent resin obtained by recycling in this manner (hereinafter also referred to as recycled water-absorbent resin) is usually a mixture of water-absorbent resin and materials other than 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 has a very high water-absorbent resin content. In such recycled water-absorbent resin, materials (derived from absorbent articles) other than the water-absorbent resin (or 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-absorbency properties and discoloration. Therefore, recycled water-absorbent resin is required to have a low content of foreign matter, and therefore, accurate quantification of the foreign matter is required.

Non-Patent Document 1 proposes a method of determining the pulp amount 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 inventors discovered that a method that simply solubilizes the pulp to determine the pulp amount results in measurement errors from the actual content. The inventors hypothesized that such measurement errors are caused by the following two factors when quantifying the pulp amount after solubilization: i) unintentional dissolution of pulp by the solubilization treatment, and ii) the presence of water-absorbent resin that did not completely dissolve even after the solubilization treatment. Therefore, if the presence of water-absorbent resin that did not completely dissolve even after the solubilization treatment is not recognized, the quantified pulp amount may contain the water-absorbent resin, potentially resulting in measurement errors. In other words, it was discovered that the pulp content quantified when calculating the pulp recovery rate in Non-Patent Document 1 may include residual water-absorbent resin. As in Non-Patent Document 1, such residual water-absorbent resin is unlikely to become apparent in mixtures with a relatively high pulp content of 40% by mass or more, which are prepared for the purpose of pulp recovery. However, in the case of a low pulp content, such as recycled water-absorbent resin, the residual water-absorbent resin has been found to have a significant impact on measurement errors in the quantification of the amount of foreign matter, including pulp. Based on this knowledge, it has been discovered that measurement errors can be reduced in the quantification of materials other than the water-absorbent resin by using a solubilization technique that suppresses the solubilization of materials other than the water-absorbent resin in water, or preferably, promotes the solubilization of the water-absorbent resin in water.

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

A simple (qualitative) method for determining whether a water-absorbent resin or materials other than water-absorbent resins are contained in a water-absorbent resin mixture can be visual inspection or component analysis.

A visual method involves adding a certain amount of water to a water-absorbent resin mixture. Because water-absorbent resins swell significantly when they absorb water, it is possible to distinguish them from materials other than water-absorbent resins. Cobalt chloride may also be added to make visual identification easier.

Methods of component analysis that can be used include analytical techniques such as NMR and IR. In the case of absorbent resins 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 (a pulp-degrading enzyme) can be detected using liquid chromatography or the like. Furthermore, in the case of polyolefins, absorption and peaks specific to polyolefins can be detected using IR (infrared absorption spectroscopy) or solid-state NMR (nuclear magnetic resonance).

In one embodiment of the present invention, since the insolubilized matter (water-insoluble substances) in the solubilized matter is quantified after solubilization 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 that are solubilized by solubilization 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 to 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, and non-polymer water-soluble compounds. 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. While water-insoluble inorganic particles are likely added during the production of the water-absorbent resin, 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 material 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 atomic 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 salts, cationic polymers, chelating agents, inorganic reducing agents, α-hydroxycarboxylic acid compounds, water-insoluble inorganic particles, surfactants, and non-polymeric water-soluble compounds are as described above.

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

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.

In the quantification method of Non-Patent Document 1, the pulp content is at least approximately 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 high, at 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. For this reason, the method of the present invention is particularly effective in embodiments 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.

Therefore, in one preferred embodiment of the quantification method of the present invention, the target of the quantification method is a water-absorbent resin mixture having a high content of water-absorbent resin. Furthermore, in one preferred embodiment of the present invention, the target of the quantification method is a water-absorbent resin mixture that has undergone 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.

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

(Processing step for 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 carried out. 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 the sample for the separation described below.

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.

The above-mentioned 1. washing and dehydration treatment of the water-absorbent resin mixture and 2. separation of materials other than the water-absorbent resin by a sedimentation separation method are specifically 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 (temperature inside the oven: 90° C.) for 3 hours, and then cooled in a desiccator, and the mass (A1 (g)) of the filter paper is measured.
(2) Approximately 5 g of a sample (water-absorbent resin mixture) that has been dried in advance in a vacuum dryer (temperature inside the dryer: 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 a mixer to separate the water-absorbing resin from materials other than the water-absorbing 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 collected in a 300 ml beaker.
(6) The water-absorbent resin is separated into the 300 ml beaker and stirred using 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 using filter paper (different from that used in (1)). If the water-absorbent resin can be visually confirmed in the filtered materials other than the water-absorbent resin, remove it with tweezers or the like and transfer it to a 300 ml beaker, and repeat the operation of (6) until the water-absorbent resin can no longer be confirmed in the materials other than the water-absorbent resin.
(8) The water-absorbent resin in a 300 ml beaker is filtered using the filter paper whose mass has been measured in (1), dried in a reduced pressure dryer (temperature inside the dryer: 90°C) for 3 hours, and then allowed to cool in a desiccator, and the mass (A2 (g)) is measured.
(9) The mass ratio (X%) of the water-absorbent resin in the water-absorbent resin mixture is calculated using the following formula 1.

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. Furthermore, the rough separation can be a sedimentation separation method.

(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.

The solubilization process uses a water-absorbent resin solubilization method that suppresses the solubilization of materials other than the water-absorbent resin in water.

In many cases, recovered (used) absorbent articles contain not a single absorbent resin, but a mixture of absorbent resins (with different compositions). The decomposition of each absorbent resin often depends on its composition, and therefore, even if the same decomposition agent is used, the decomposition rate varies greatly depending on the type of absorbent resin.

For this reason, in 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 step (1), it is preferable to select an optimal 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.

(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).

The solubilization rate of materials other than the water-absorbent resin (for example, foreign matter such as pulp) (hereinafter also referred to as the "foreign matter solubilization rate") refers to the solubilization rate when materials other than the water-absorbent resin (for example, foreign matter such as pulp) are used for solubilization.

When measuring the foreign matter solubilization rate, materials other than the water-absorbent resin can be identified by considering the following in the order of (1) and (2).

(1) If the water-absorbent resin and materials other than the water-absorbent resin can be separated, for example, by rough separation in the processing step when calculating the mass content ratio of the water-absorbent resin, the separated materials other than the water-absorbent resin are used. Note that materials other than the water-absorbent resin (e.g., pulp) separated by this separation method may contain trace amounts 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, trace amounts of water-absorbent resin may be present.

(2) If the separation method in (1) does not allow for the collection of materials other than the water-absorbent resin in an amount sufficient to measure the foreign matter solubilization rate, the water-absorbent resin is solubilized and the materials other than the water-absorbent resin are collected.

The solubilization method is not particularly limited, but specific examples include a technique of solubilizing a water-absorbent resin by heat treatment in the presence of an oxidizing agent such as hydrogen peroxide or persulfate (e.g., JP 4-317785 A, JP 6-313008 A, JP 2003-321574 A), a technique of solubilizing a water-absorbent resin using ascorbic acid as a reducing agent under conditions of pH 4 or higher and pH 7.5 or lower (e.g., JP 05-247221 A), and a technique of solubilizing a water-absorbent resin using a reducing agent and transition metal ions (e.g., JP 2019- Examples of such techniques include a technique of solubilizing a water-absorbent resin by heating in the presence of an oxidizing water-soluble salt (e.g., WO 2021/042113), a technique of solubilizing a water-absorbent resin using ozone water (e.g., JP 2014-217835 A, JP 2017-100133 A), and a technique of solubilizing a water-absorbent resin by using an oxidizing agent in combination with a transition metal ion such as an iron ion or a copper ion (e.g., JP 11-172039 A). One form of the technique that can be used is a technique of solubilizing a water-absorbent resin by heat treatment in the presence of an oxidizing agent.

Note that this solubilization method may also solubilize materials other than the water-absorbent resin, and there is a possibility that a trace amount of water-absorbent resin (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) may be mixed in, but this does not pose a problem as long as a certain amount or more of materials other than the water-absorbent resin remain (the minimum amount necessary to measure the foreign matter solubilization rate).

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

(Method for solubilizing a water-absorbent resin to promote solubilization of the water-absorbent resin in water)
Examples of solubilization methods for water-absorbent resins that promote the solubilization of the water-absorbent resin in water (that can suppress the undissolved water-absorbent resin in water) include solubilization techniques that result in a solubilization rate of the water-absorbent resin in the 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 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 the water-absorbent resin and the decomposition agent), using a kneader with excellent mixing and stirring properties, and the like, in order to increase the decomposition rate of the water-absorbent resin.

When measuring the solubilization rate of a water-absorbent resin, a water-absorbent resin is used that has been separated from materials other than the water-absorbent resin by rough separation, for example, in the processing step used to calculate the mass content of the water-absorbent resin. Note that the water-absorbent resin separated by this separation method may contain trace amounts (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 materials other than the water-absorbent resin, but when measuring the solubilization rate of the water-absorbent resin, trace amounts of materials other than the water-absorbent resin may be present.

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

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 water-absorbent resin and pulp. Other examples include hypochlorous acid (salt) treatment, oxidation treatment with hydrogen peroxide, persulfuric acid, chlorine, or a water-soluble redox agent (e.g., JP 2013-150977 A), and solubilization of the water-absorbent resin using an acid or alkaline compound (e.g., JP 2020-49398 A). Other solubilization methods include thermal decomposition at high temperatures of 200°C or higher, and irradiation with radiation such as ultraviolet rays or electron beams.

In order to simultaneously suppress the solubilization of materials other than the water-absorbent resin in water, it is preferable to use a compound that generates an oxidizing agent, a reducing agent, or transition metal ions, or to use radiation, or a combination of these, as the treatment for solubilizing the water-absorbent resin in water.

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

An oxidizing agent is a compound that has oxidizing properties and generates radicals when heated. Radicals can also be generated by using an oxidizing agent in combination with a reducing agent and/or a compound that generates transition metal ions. Examples of oxidizing agents 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. Oxidizing agents may be used alone or in combination. Of these, hydrogen peroxide is preferred. In one embodiment, the amount of oxidizing agent may be 1 to 1,000 parts by weight, 10 to 500 parts by weight, 50 to 300 parts by weight, or 100 to 300 parts by weight, per 100 parts by weight of the water-absorbent resin mixture (solid content). This amount of oxidizing agent 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 oxidizing agent refers to the total amount when it is added in multiple portions.

A reducing agent is a compound with 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 its derivatives (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 to 300 parts by mass, 10 parts by mass to 200 parts by mass, 30 parts by mass to 200 parts by mass, or 60 parts by mass to 150 parts by mass, per 100 parts by mass of the water-absorbent resin mixture (solid content). This amount of reducing agent 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 reducing agent refers to the total amount when the reducing agent is added in multiple installments.

A transition metal ion may be used as the decomposing agent. When used in combination with the aforementioned oxidizing agent, the transition metal ion generates radicals through the Fenton reaction. When used in combination with the aforementioned reducing agent, radicals are also generated. Examples of transition metal ions include Cu ²⁺ , Ag ⁺ , Fe ²⁺ , Fe ³⁺ , Al ³⁺ , Ni ²⁺ , and Mn ²⁺ . The transition metal ions may be used alone or in combination of two or more. Of these, iron ions (Fe ²⁺ ) and copper ions (Cu ²⁺ ) are preferred, and iron ions (Fe ²⁺ ) are more preferred. 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 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.

An alkaline compound may be used as the decomposing agent. The alkaline compound is a compound that solubilizes the crosslinked portion of the water-absorbent resin by alkaline hydrolysis. Examples of such alkaline 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 alkaline compounds may be used alone or in combination. Of these, alkali metal hydroxides and alkaline earth metal hydroxides are preferred. In one embodiment, the content of the alkaline 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, per 100 parts by mass of the water-absorbent resin mixture (solid content).

In the solubilization treatment, it is preferable to add water to the water-absorbent resin mixture to promote the solubilization treatment. The water is not particularly limited as long as it is clean, and examples 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 to 9,900 parts by mass, 80 to 1,900 parts by mass, or 100 to 1,000 parts by mass, per 100 parts by mass of the water-absorbent resin mixture. Mixing water within this range allows the decomposing agent to be mixed uniformly, facilitating efficient decomposition.

Furthermore, the temperature at which the water-absorbent resin is decomposed is preferably relatively low, preferably between 20°C and 80°C, and more preferably between 25°C and 60°C, from the viewpoint of suppressing the solubilization of materials other than the water-absorbent resin in water.

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 work efficiency and decomposition efficiency.

From the perspective of improving decomposition efficiency, the solubilization treatment may be performed by adding the decomposition agent in multiple batches. "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 addition may be within 1 hour (lower limit: 0 minutes), within 30 minutes, within 20 minutes, within 10 minutes, or within 5 minutes after the completion of the previous addition.

The pH during the solubilization treatment is not particularly limited, but since this can suppress the decomposition of materials other than the water-absorbent resin, it is preferable to adjust the pH to a range of 5.0 to 10.0 (more preferably 5.5 to 9.5, and particularly preferably 6.0 to 9.0) before decomposition. The pH during the solubilization treatment can be adjusted as needed by adding a pH adjuster such as an acid or base. 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. Bases that can be used include, but are not limited to, sodium hydroxide, sodium carbonate, potassium hydroxide, and magnesium hydroxide.

The decomposition procedure yields a liquid solubilized product (a mixture containing the water-absorbent resin decomposition product).

1. Procedure for step (1)
In the step (1), the water-absorbent resin mixture is subjected to a solubilization treatment (decomposition treatment).

In one embodiment, step (1) comprises: (A) roughly separating the water-absorbent resin and materials other than the water-absorbent resin from the water-absorbent resin mixture; (B) examining solubilization conditions under which the solubilization rate of the roughly separated materials other than the water-absorbent resin is less than 30% by mass; (C) solubilizing the roughly separated water-absorbent resin using the solubilization conditions examined in (B) under which the solubilization rate is less than 30% by mass, and calculating the solubilization rate; (D) returning to (B) and examining the solubilization conditions again until optimized conditions are found under which the solubilization rate of the water-absorbent resin is 90% by mass or more, while maintaining the conditions under which the solubilization rate of the materials other than the water-absorbent resin is less than 30% by mass in (C); and (E) solubilizing the water-absorbent resin mixture using the solubilization method under the optimized conditions.

In one embodiment of the present invention, step (1) comprises: (A) roughly separating the water-absorbent resin from the water-absorbent resin mixture and materials other than the water-absorbent resin; (B) examining solubilization conditions under which the solubilization rate of the roughly separated water-absorbent resin is 90% by mass or more; (C) solubilizing the roughly separated materials other than the water-absorbent resin using the solubilization conditions examined in (B) under which the solubilization rate is 90% by mass or more, and calculating the solubilization rate; (D) returning to (B) and examining solubilization conditions again until optimized conditions are found under which the solubilization rate of the materials other than the water-absorbent resin is less than 30% by mass, while maintaining the conditions under which the solubilization rate of the water-absorbent resin is 90% by mass or more in (C); and (E) solubilizing the water-absorbent resin mixture using the solubilization method under the optimized conditions.

2. Procedure for step (2)
In the step (2), the amount of materials other than the water-absorbent resin is determined from the solubilized product.

The amount of materials other than the water-absorbent resin can be determined, 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. Conventional solid-liquid separation methods, such as filtration using a mesh filter or the like, or centrifugation, can be used for the separation. A specific method for quantifying the amount of materials other than the water-absorbent resin includes removing the liquid from the solubilized material and weighing the residue. More specifically, the amount of materials other than the water-absorbent resin can be determined by suction-filtering the solubilized material using filter paper, drying the filtrate, and measuring the mass of the dried material. Specifically, the amount of materials other than the water-absorbent resin can be determined by the method described in the Examples below. In one embodiment, 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.

The mass proportion (%) 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 quantifying 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, the solids mass. Specifically, it is the solids mass calculated by the following method.

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

[Recycling method]
The present invention also provides a method for recycling a water absorbent resin, comprising separating materials other than a water absorbent resin from a water absorbent resin mixture until a content of materials (foreign matter) other than a water absorbent resin in a water absorbent resin mixture, determined by the above-mentioned quantification method, 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.

Specifically, if the content of materials other than the water-absorbent resin in the mixture exceeds 10% by mass as determined by the above-mentioned quantification, the materials other than the water-absorbent resin are separated.

Methods for separating materials other than water-absorbent resins include, for example, centrifugal separation using a cyclone separator or the like to take advantage of the difference in specific gravity in the gas, sorting by size through sieving, and separation using a drum screen separator.

In this way, a water-absorbent resin mixture with a reduced content of foreign matter is obtained, which improves the water absorption properties of the recycled water-absorbent resin and prevents deterioration of the appearance (e.g., discoloration).

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

Invention A includes the following aspects and configurations:

1. A step (1) of solubilizing 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;
A step (2) of quantifying materials other than the water-absorbent resin from the solubilized product;
A method for quantifying a material other than a water-absorbent resin in a water-absorbent resin mixture, comprising:
The quantification method, wherein the solubilization treatment uses a water-absorbent resin solubilization technique that suppresses solubilization of materials other than the water-absorbent resin in water.

2. The quantification method described in 1., wherein the mass proportion of the water-absorbent resin in the water-absorbent resin mixture is 70 mass% or more.

3. The quantification method described in 1. or 2., wherein the material other than the water-absorbent resin is a constituent material derived from an absorbent article.

4. The quantification method described in any one of 1. to 3., wherein the material other than the water-absorbent resin includes at least one material selected from the group consisting of pulp, nonwoven fabric, and resin film.

5. The quantification method described in any one of 1. to 4., wherein the water-absorbent resin mixture is recovered from used absorbent articles.

6. The quantification method described in 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% by mass or more.

7. The quantification method described in any one of 1. to 6., wherein a solubilization method is used that results in a solubilization rate of less than 30% by mass of materials other than the water-absorbent resin.

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 the content of materials other than the water-absorbent resin in the water-absorbent resin mixture, as determined by the quantification method described in any one of 1. to 7., is 10 mass% or less, thereby obtaining a water-absorbent resin.

The above is a description of Invention A.

(Method for quantifying the content of materials (foreign matter) other than water absorbent resin remaining on a JIS standard sieve with a mesh size of 150 μm)
[0062] In this specification, "a method for quantifying content of materials (foreign matter) other than a water-absorbent resin remaining on a JIS standard sieve with a mesh size of 150 µm" means mass % of materials other than a water-absorbent resin that cannot pass through a JIS standard sieve with a mesh size of 150 µm, among all materials other than a water-absorbent resin contained in a water-absorbent resin mixture, relative to the water-absorbent resin mixture.

The 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 that materials other than the water-absorbent resin that have a size that allows them to remain on a JIS standard sieve with a mesh size of 150 μm have a particularly strong effect on the powder properties. Even if the content of materials other than the water-absorbent resin is approximately the same, if the content of materials other than the water-absorbent resin that remain on a JIS standard sieve with a mesh size of 150 μm exceeds 1.0 mass%, the powder properties will be significantly reduced. In other words, as described above, from the standpoint of powder properties, it is acceptable for the content of materials other than the water-absorbent resin that remain on a JIS standard sieve with a mesh size of 150 μm to be up to 1.0 mass% (there is a critical point in the powder properties at 1.0 mass%).

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 adverse effect on the properties of the water-absorbent resin mixture, particularly powder properties such as flow rate. 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 can be obtained that exhibits little deterioration in water absorption properties and powder properties.

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 standpoint 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% by mass or more and less than 1.0% by mass, 0.005% by mass or more and 1% by mass or less, 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.

The "content of the material other than the water-absorbent resin remaining on the JIS standard sieve having an opening of 150 μm" is preferably calculated from the content P% by mass of the material other than the water-absorbent resin in the water-absorbent resin mixture by the following formula 2:
Content (mass%) of material other than water absorbent resin remaining on JIS standard sieve having 150 μm mesh size=Content mass ratio of water absorbent resin mixture remaining on JIS standard sieve having 150 μm mesh size in water absorbent resin mixture×P mass% Equation 2
is calculated by

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

A method for obtaining a water-absorbent resin mixture in which the content of materials other than the water-absorbent resin (foreign matter) is adjusted to fall within the range described in the present invention is generally to use an appropriate combination of known foreign matter separation methods on a water-absorbent resin mixture containing a large amount of foreign matter, which has been recovered from used absorbent articles by a known method.

Publicly known methods for separating foreign matter include, for example, a "wet separation" method in which the water-absorbent resin is separated from the foreign matter in a solution, and a "dry separation" method in which a dry water-absorbent resin mixture is separated into the water-absorbent resin and the foreign matter.

Examples of "wet separation" methods include: i) a method in which the difference in size or length between the water-absorbent resin and foreign matter in the solution is utilized to separate them using a screen or sieve (e.g., JP 2024-088438); ii) a method in which the difference in specific gravity between the water-absorbent resin and foreign matter is utilized to separate them using a separation device such as a liquid cyclone (e.g., JP 2019-85447); and iii) a method in which the difference in sedimentation speed between the water-absorbent resin and foreign matter in the solution is utilized, or the difference in the ease with which they rise or settle when bubbled with air or the like (e.g., JP 2024-94574).

In particular, if you want 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), the shape of the liquid cyclone, the number of processing stages and processing speed (in case ii), or conditions such as the amount of bubbling air and the stirring speed of the solution (in case iii).

Examples of "dry separation" methods include: i) a method in which the difference in size or length between the water-absorbent resin and the foreign matter is used to separate them using a screen or sieve; ii) a method in which the difference in specific gravity between the water-absorbent resin and the foreign matter is used to separate them using a separation device such as a cyclone (e.g., Patent Publication Nos. 2022-015986 and 2007-203170); iii) a method in which a mixture of water-absorbent resin and foreign matter is transported by air and the difference in the ease with which the water-absorbent resin and the foreign matter settle is used to separate them (e.g., Patent Publication No. 2023-100208); and iv) a method in which the mixture of water-absorbent resin and foreign matter is moved, for example, on rollers, and the air in the gaps is sucked out to recover and separate the low-specific-gravity components (e.g., Patent Publication No. 2001-336077).

In particular, if you want to remove large foreign objects that have 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 cyclone processing speed and cyclone shape (in case ii), appropriately adjusting conditions such as the amount of air and transport distance when transporting by air (in case iii), or adjusting the amount and speed of air suction (in case iv).

[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, a water-absorbent resin mixture with little deterioration in water absorption properties and powder characteristics can be obtained. Furthermore, by controlling the flow rate of the water-absorbent resin mixture within the above range, a water-absorbent resin mixture with improved fluidity can be obtained. 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.

(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.

(moisture content)
In one embodiment of the present invention, the water 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 water 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 water 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 water 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.

(Mass average particle diameter (D50))
In one embodiment of the present invention, the mass median 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 median 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.

(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.

(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, and particularly preferably 50 seconds or less. The lower limit is not particularly limited, but is preferably 1 second or more, more preferably 5 seconds or more.

By keeping Vortex within the above range, it becomes possible to absorb a specified amount of liquid in a short period of time. When used in the absorbent body of absorbent articles such as disposable diapers, the user's skin feels wet for less time, causing less discomfort and reducing leakage.

(shape)
In one embodiment of the present invention, the water-absorbent resin mixture is preferably in the form of powder.

[1-5] Raw materials for water-absorbent resin mixtures (constituent materials derived from absorbent articles)
In one embodiment of the present invention, materials other than the water-absorbent resin constituting the water-absorbent resin mixture may 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 comprising 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 the fibrous material, or by sandwiching the water-absorbent resin between the fibrous materials and molding the blend into a film, a cylinder, a sheet, or the like. Examples of the fibrous material include hydrophilic fibers such as pulverized wood pulp, cotton linters, crosslinked cellulose fibers, rayon, cotton, wool, acetate, and vinylon.

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 from chemical fibers such as polypropylene or polyester; a water-absorbing body containing a water-absorbing material such as a water-absorbing resin or pulp; a waterproof material such as a resin film such as polyethylene film, or paper or nonwoven fabric (resin-treated paper or resin-treated nonwoven fabric) that has been treated with resin, such as polyethylene-laminated paper or polyethylene-laminated nonwoven fabric; and an adhesive (binder) that bonds these components together. Note that "absorbent article" encompasses both unused and used absorbent articles.

[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.

The recycling method is not particularly limited as long as it satisfies the above-mentioned purpose, but for example, it involves processing used absorbent articles through any of the processes of washing, dehydration, regeneration, crushing, separation, sterilization/disinfection, etc. Specific examples include the methods described in JP 2013-198862 A and JP 2019-135046 A. Furthermore, water-absorbent resins and water-absorbent resin mixtures that have been recycled in this way and can be reused for water absorption purposes are sometimes referred to as "recycled water-absorbent resins."

[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.

On the other hand, water-absorbent resins for sanitary materials have a highly balanced range of absorbency properties, such as absorbency capacity, absorbency under pressure, and liquid permeability, to meet a variety of applications and required characteristics. In other words, water-absorbent resins for sanitary materials are required to have an excellent balance of various absorbency properties. However, as mentioned above, recycled water-absorbent resins often have unstable absorbency properties, and in order to use recycled water-absorbent resins for sanitary material applications, it is necessary to adjust the balance of absorbency properties.

The inventors therefore considered that by mixing recycled water-absorbent resin, which has unstable water-absorbing properties as described above, into the raw materials used when newly producing water-absorbent resin, it would be easier to adjust the balance of physical properties. Note that "producing a new water-absorbent resin" refers to the production of conventional (normal) water-absorbent resin using the monomers that make up the water-absorbent resin as raw materials.

The above-mentioned "production of conventional (normal) water-absorbent resin using the monomers that make up the water-absorbent resin as raw materials" refers to a method for producing a normal (non-recycled) water-absorbent resin using the monomers that make up the water-absorbent resin as raw materials, for example, a method for producing a normal (non-recycled) water-absorbent resin that includes the steps of preparing an aqueous monomer solution, polymerization, gel crushing, drying, classification, and surface cross-linking.

The phrase "using the water-absorbent resin mixture as part of the raw materials" means that the water-absorbent resin mixture is added in at least one of the steps of preparing the aqueous monomer solution, the polymerization step, the gel-crushing step, the drying step, the classification step, and the surface-crosslinking step. In other words, the present invention also encompasses a method for producing a water-absorbent resin, in which the water-absorbent resin mixture is used as part of the raw materials in the process for producing the water-absorbent resin from the monomers that make up the water-absorbent resin.

Below, one embodiment of the present invention will be described using as an example a method for producing a water-absorbent resin, which includes a monomer aqueous solution preparation step, a polymerization step, a gel crushing step, a drying step, a classification step, and a surface cross-linking step.

[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.

Furthermore, the term "main component" means that the amount (content) of acrylic acid (salt) used is 50 mol% or more out of 100 mol% of the total amount of monomers used in the polymerization reaction of the water-absorbent resin (in other words, the monomers contained in the aqueous monomer solution, excluding the internal crosslinking agent). The content of acrylic acid (salt) out of 100 mol% of the total amount of monomers is preferably 70 mol% or more, and more preferably 90 mol% or more (upper limit 100 mol%).

(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 of 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.

Furthermore, "acrylate" refers to acrylic acid neutralized with a basic composition. The acrylate may be a commercially available acrylate (e.g., sodium acrylate), or it may be one obtained by neutralization within a water-absorbent resin manufacturing plant.

(Basic Composition)
In this specification, "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, strongly basic compounds, for example, hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, are preferred, and sodium hydroxide is more preferred.

(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 neutralization is preferred from the viewpoint of production efficiency, etc. Regarding the conditions for the neutralization, such as the apparatus, neutralization temperature, and residence time, the conditions described in International Publication No. 2009/123197 and U.S. Patent Application Publication No. 2008/0194863, etc., also apply to the present invention.

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, a water-absorbent resin with a sufficient water absorption capacity can be provided, and by setting the neutralization rate to 90 mol% or less, a water-absorbent resin with a higher water absorption capacity under pressure can be provided.

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% acrylic acid and 75 mol% acrylic acid salt. Such a mixture is sometimes 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.

Furthermore, in this manufacturing method, the recycled water-absorbent resin is used as part of the raw material in the production of the water-absorbent resin. When such 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 may contain basic compounds. These basic compounds may neutralize the acrylic acid before polymerization or the hydrogel-like cross-linked polymer after polymerization. Therefore, taking into account neutralization by the basic compounds derived from this 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.

200 g of physiological saline (0.9% by mass sodium chloride aqueous solution) is measured out and placed in a 250 ml plastic container with a lid. 1.00 g of the water-absorbent resin mixture is added to the aqueous solution and stirred for 16 hours to extract the soluble components in the water-absorbent resin. This extract is filtered through a 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 resulting filtrate is measured out and used as the measurement solution.

First, titrate only the saline solution with 0.1N NaOH aqueous solution to a pH of 10, then titrate with 0.1N HCl aqueous solution to a pH of 2.7 and measure the blank titer ([bNaOH] ml, [bHCl] ml).

Next, perform the same titration procedure on the test solution to determine the titration volume ([NaOH] ml, [HCl] ml). The neutralization rate can be calculated according to Equation 3 below.

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

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

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.

(Internal crosslinking agent)
The aqueous monomer solution preferably contains an internal crosslinking agent. Examples of the internal crosslinking agent 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.

Furthermore, from the viewpoint of the water absorption performance of the resulting water-absorbent resin, the internal crosslinking agent is preferably a compound having two or more polymerizable unsaturated groups, more preferably a compound that is thermally decomposable at the drying temperature described below, and even more preferably a compound having two or more polymerizable unsaturated groups that have a (poly)alkylene glycol structural unit.

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

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 this range, a water-absorbent resin with 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 resulting water-absorbent resin tends to decrease and the water-soluble content tends to increase excessively. If the amount of the internal crosslinking agent used is more than 10 mol%, the water absorption capacity of the resulting water-absorbent resin tends to decrease, which is undesirable. 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.

In one embodiment of the present invention, a method is preferably used in which a predetermined amount (within the above-mentioned range) of internal crosslinking agent is added to an aqueous monomer solution in advance, and the crosslinking reaction is initiated simultaneously with polymerization. However, other methods that can be used include a post-crosslinking method in which an internal crosslinking agent is added during and/or after polymerization; a radical crosslinking method using a radical polymerization initiator; and a radiation crosslinking method using active energy rays such as electron beams or ultraviolet rays. These methods can also be used in combination.

(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.

Furthermore, 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 of the compounds described in U.S. Patent Application Publication No. 2008/0161512 can also be applied to the method for producing a water-absorbent resin that uses the water-absorbent resin mixture of the present invention as part of a raw material.

(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.

If 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.

Furthermore, if 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.

Furthermore, the above-mentioned other substances may be added to the reaction system in the polymerization step described below. In that 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 be within the above-mentioned range.

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

[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").

(Polymerization initiator)
In the polymerization step, it is preferable to carry out a 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.

The amount of polymerization initiator used in the polymerization step is preferably 0.001 mol % or more and 1 mol % or less, and 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). Furthermore, 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.

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

(polymerization form)
The polymerization mode 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, spray-droplet polymerization, aqueous solution polymerization, and reversed-phase suspension polymerization are preferred, and aqueous solution polymerization and reversed-phase suspension polymerization are more preferred, and aqueous solution polymerization is even more preferred. Among these, continuous aqueous solution polymerization is particularly preferred. Modes such as continuous belt polymerization and continuous kneader polymerization can also be applied. Furthermore, foam polymerization can also be applied, in which polymerization is carried out by dispersing bubbles (particularly inert gases or the like) in an aqueous monomer solution.

Specific polymerization methods include continuous belt polymerization, which is disclosed in U.S. Patent Nos. 4,893,999, 6,241,928, and U.S. Patent Application Publication No. 2005/215734, and continuous kneader polymerization, which is disclosed in U.S. Patent Nos. 6,987,151 and 6,710,141. By employing these continuous aqueous solution polymerization methods, the production efficiency of water-absorbent resins can be improved.

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). "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.

In the polymerization step, polymerization can be carried out in a desired atmosphere, for example, in an air atmosphere. However, from the viewpoint of the color tone of the resulting water-absorbent resin, it is preferable to carry out polymerization in 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. It is also preferable to replace the dissolved oxygen in the aqueous monomer solution with an inert gas (for example, so that the dissolved oxygen is less than 1 mg/L).

In one embodiment of the present invention, the polymerization can also be carried out as foam polymerization, in which bubbles (especially the inert gases mentioned above) are dispersed in an aqueous monomer solution.

[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").

In the polymerization step, if kneader polymerization is used as the polymerization method, the polymerization step and gel crushing step are carried out simultaneously. Furthermore, in cases where a particulate hydrogel is obtained directly during the polymerization process, such as gas-phase polymerization or reverse-phase suspension polymerization, the gel crushing step may not be carried out.

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

[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.

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

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 resulting water-absorbent resin, drying efficiency, etc. Drying conditions other than the drying temperature, such as the hot air speed and drying time, can be set appropriately depending on the water content and total mass of the particulate hydrogel to be dried and the desired resin solid content. When band drying is performed, the various conditions described in WO 2006/100300, WO 2011/025012, WO 2011/025013, WO 2011/111657, etc., are appropriately applied as drying conditions.

[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.

In the classification process, methods for classifying the dried polymer include sieve classification using JIS standard sieves (JIS Z8801-1 (2000)) and airflow classification. Among these, sieve classification is preferably selected from the standpoint of classification efficiency.

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 or more and 500 μm or less, more preferably 250 μm or more and 450 μm or less, and even more preferably 300 μm or more and 400 μm or less. Furthermore, 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.

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

[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).

In this surface cross-linking step, a surface-cross-linked water-absorbent resin is obtained through radical cross-linking near 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 subjected to the surface cross-linking step can also be said to be a water-absorbent resin precursor.

(Surface cross-linking agent)
In the surface cross-linking step, it is preferable to cross-link the surface of the particulate dried polymer using a surface cross-linking agent. The surface cross-linking agent is not particularly limited, but examples thereof include organic or inorganic surface cross-linking agents. Among these, organic surface cross-linking agents that react with carboxyl groups are preferred from the viewpoints of the physical properties of the water-absorbent resin and the handleability of the surface cross-linking agent. Specifically, one or more compounds disclosed in U.S. Patent No. 7,183,456 can be used as the surface cross-linking agent. More specifically, examples of the surface cross-linking 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 cross-linking agents may be used alone, or two or more may be used in combination. It is also preferable to use these surface cross-linking agents in the form of an aqueous solution (i.e., in the form of a surface cross-linking agent solution).

The amount of surface cross-linking agent used in the surface cross-linking 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, per 100 parts by mass of the particulate dry polymer. The surface cross-linking agent is preferably added in the form of an aqueous solution, in which case the amount of water used is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the particulate dry polymer. Furthermore, a hydrophilic organic solvent may be used in combination, if necessary. In that 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, per 100 parts by mass of the particulate dry polymer.

(Mixing process)
The surface cross-linking step includes a mixing step of mixing the particulate dry polymer and the surface cross-linking agent. A method for mixing the surface cross-linking agent in the mixing step is not particularly limited, but for example, a method in which a surface cross-linking agent solution is prepared in advance and the solution is preferably sprayed or dropped onto, and more preferably sprayed onto, the particulate dry polymer to mix them may be mentioned.

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

(Heat treatment process)
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 called a cross-linking reaction step.

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

(cooling process)
The surface cross-linking step preferably includes a cooling step of cooling the surface-cross-linked particulate dried polymer after the heat treatment.

The cooling device used 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 dry polymer obtained in the heat treatment step is forcibly cooled as needed in the cooling step, preferably to a temperature of 40°C or higher and 80°C or lower, more preferably 50°C or higher and 70°C or lower.

[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.

The method of adding the water-absorbent resin mixture according to the present invention to the water-absorbent resin raw material is not particularly limited, but examples include adding the water-absorbent resin mixture as is, adding the water-absorbent resin mixture in the form of a swollen gel obtained by swelling the water-absorbent resin mixture with water, and adding the water-absorbent resin mixture in the form of a dispersion in water.

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 the 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. From the viewpoint of uniform mixing of the raw materials, it is preferable to add the water-absorbent resin mixture to the aqueous monomer solution.

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

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

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 or after drying. If the water-absorbent resin mixture powder and the hydrous gel are mixed before drying, the moisture in the hydrous gel will migrate to the water-absorbent resin mixture. This is therefore preferable, as it is expected to have effects such as a shorter drying rate and a reduced production of undried gel.

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 they may be mixed after classification. From the perspective of particle size uniformity, it is preferable to add the water-absorbent resin mixture before classification.

When the water-absorbent resin mixture is added in the "surface-crosslinking step," the particulate dry polymer and the water-absorbent resin mixture may be mixed in advance, or the treatment may be carried out without mixing. From the viewpoint of uniform mixing of the surface-crosslinking agent solution and the water-absorbent resin, it is preferable to use a particulate water-absorbent resin mixture and subject all of the objects to be surface-crosslinked (i.e., the particulate dry polymer and the particulate water-absorbent resin mixture) to surface-crosslinking in particulate form.

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 timings. Furthermore, the entire amount of the water-absorbent resin mixture to be added may be added all at once at any timing, or may be added in portions at multiple timings. Furthermore, the water-absorbent resin mixture may be added in different forms depending on the timing of addition.

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 the aqueous monomer solution).

More specifically, the raw materials include water-absorbent resin mixtures, as well as non-water-absorbent resin mixtures and their raw materials (for example, acrylic acid (salts); basic compositions; other monomers; internal cross-linking agents; other substances (starch, etc.); polymerization initiators; and surface cross-linking agents). The above ratio is the ratio of the solid content of the water-absorbent resin mixture to the solid content of all the water-absorbent resin raw materials.

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

[Measurement and evaluation method]
(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 left to stand in a constant temperature and humidity chamber for 6 hours or more, and its mass was measured (a1: for sample measurement, b1: for blank measurement).

A 250 ml plastic container with a lid containing a 35 mm long stir bar and 100 g of deionized water was placed in a thermostatic bath at 40°C, and then the water-absorbent resin solubilizer (also simply referred to as solubilizer) 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 solubilizer).

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 stirrer stopped turning shortly after due to swelling of the water-absorbent resin, but if the reaction was continued, part of the water-absorbent resin was solubilized, allowing the stirrer to rotate. After a predetermined time, the solubilization reaction was terminated (see each Example, Comparative Example, Reference Example, and Comparative Reference Example for the number of solubilizer additions, addition timing, and solubilization time), and a solubilized liquid was obtained.

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. After washing, the filter paper was dried for 3 hours in an oven adjusted to 105°C. After drying, the filter paper was left to stand in a constant temperature and humidity chamber for at least 6 hours, 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, and left to stand in a constant temperature and humidity chamber for at least 6 hours, and its mass was measured (b2).

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

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

(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.

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

The content (mass%) of materials (foreign matter) other than the water-absorbent resin remaining on the JIS standard sieve with a mesh size of 150 μm was calculated according to the following formula 5.

The content (mass%) of materials other than water-absorbent resin remaining on a 150 μm JIS standard sieve = (W2 × P) / (W1 + W2) ... Equation 5

(c) Mass average particle diameter (D50)
The mass median particle size (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 US Patent Application Publication No. 2006/204755.

(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.

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

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

(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.

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

Flow rate (g/sec) = (mass of absorbent resin or absorbent resin mixture (g)) / (time required to complete fall (sec)) ... Equation 7

(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.

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

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

(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.

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

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

(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.

First, 0.02 parts by mass of Food Blue No. 1 (CAS No. 3844-45-9; a type of food additive) was added to 1,000 parts by mass of a 0.9% by mass aqueous solution of sodium chloride to color the solution, and the liquid temperature was adjusted to 30°C. This was used as the test liquid. Next, 50 mL of the test liquid was measured and placed in a 100 mL beaker, and a cylindrical stirrer measuring 40 mm in length and 8 mm in diameter was placed in the beaker, and stirring was initiated at 600 rpm. Subsequently, 2.0 g of a water-absorbent resin or a water-absorbent resin mixture was added to the test liquid while stirring, and the Vortex (water absorption rate) (unit: seconds) was measured.

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 Polymers." Specifically, the start point was the time when the water-absorbent resin or water-absorbent resin mixture was added to the test liquid, and the end point was the time when the added water-absorbent resin or water-absorbent resin mixture absorbed the test liquid and gelled, and the gel covered the cylindrical stirrer, and the interval between these points was taken as the vortex (water absorption rate) (unit: seconds).

(i) AAP 0.3
"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.

Specifically, 0.9 g of a water-absorbent resin or a water-absorbent resin mixture was swelled for 1 hour using a large excess of 0.9 mass % sodium chloride aqueous solution under a pressure of 2.07 kPa (21 g/cm ² , 0.3 psi), and then the AAP (absorbency against pressure) (unit: g/g) was measured.

(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).

A 250 ml plastic container with a lid containing a 35 mm long stir bar and 100 g of deionized water was placed in a thermostatic bath at 40°C, and then the solubilizer 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 solubilizer).

Next, the water-absorbent resin was weighed and added to the aqueous solubilizing agent solution to solubilize 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 of the container were suction-filtered using qualitative filter paper (No. 5A) whose mass had been measured in advance, and the container and any residue remaining on the filter paper were 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, and then the dried filter paper was left to stand in a constant temperature and humidity chamber for at least 6 hours, and its 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 at least 6 hours, and its mass was measured (b4).

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

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

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

(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).

A 250 ml plastic container with a lid containing a 35 mm long stir bar and 100 g of deionized water was placed in a thermostatic bath at 40°C, and then the solubilizer 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 solubilizer).

Next, materials other than the water-absorbent resin were weighed and added to the solubilizing agent 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 of the container were suction filtered using qualitative filter paper (No. 5A) whose mass had been measured in advance, and the container and any residue remaining on the filter paper were 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, and then the dried filter paper was left to stand in a constant temperature and humidity chamber for at least 6 hours, and its mass was measured (a6). 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 at least 6 hours, and its mass was measured (b6).

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

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

Foreign matter solubilization rate (mass%) = [1 - content of materials other than absorbent resin after solubilization (g) / amount of materials other than absorbent resin (g)] x 100 ... Equation 13

[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′).

Next, the aqueous monomer solution (a') was cooled while 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 the 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, forming a transparent, homogeneous solution.

Next, 12.1 g of a 4.0% by 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-shaped container (bottom 340 x 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-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 began 70 seconds after the aqueous monomer solution (a) was poured into the bat-shaped container.

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 cross-linked polymer (hereinafter referred to as "hydrogel") (S1) was removed. This series of operations was carried out in an open-to-air system. The hydrogel (S1) obtained in 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 screen and dried with hot air at 190°C for 60 minutes. It was then crushed using a vibration mill and passed through a sieve with 850 μm openings. The water-absorbent resin precursor (S1) was obtained in an irregularly crushed form with an average particle diameter of 350 μm, remaining on a 150 μm sieve.

100 parts by mass of the obtained water-absorbent resin precursor (S1) was uniformly mixed with a surface-crosslinking 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 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 (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.

[Production 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. The mixture was then placed in a vacuum dryer (internal temperature: 90°C) and dried for 3 hours to obtain a dried product. The dried product was then classified using a JIS standard sieve with 2 mm openings, and only the pulp was extracted from the mixture (pulp extracted from disposable diapers).

The disposable diaper pulp obtained by the above procedure was crushed using a crusher (Orient Crusher Co., Ltd.; vertical crusher VM27-S) until the entire amount could pass through a 1.5 mm outlet screen. The crushed pulp was then classified using a JIS standard sieve with a mesh size of 150 μm, yielding pulp that passed through the JIS standard sieve with a mesh size of 150 μm (model (A) of materials other than water-absorbent resin) and pulp that remained on the JIS standard sieve with a mesh size of 150 μm (model (B) of materials other than water-absorbent resin).

[Example 1]
99.9% by mass of the water-absorbent resin (S1) obtained in Production Example 1 and 0.10% by 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).

The obtained water-absorbent resin mixture (1) had a content of materials other than the water-absorbent resin of 0.10% 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 0.00% by mass, the mass average particle diameter (D50) was 345 μm, the moisture content was 4.8% by mass, the flow rate was 10.4 g/sec, the bulk specific gravity was 0.65 g/ml, and the CRC was 36.0 g/g. The measurement results are shown in Table 1.

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

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

A 250 ml plastic container with a lid containing a 35 mm long stir bar and 100 g of deionized water was placed in a 40°C thermostatic bath, and then water-absorbent resin solubilizer (28.50 g of 30% by 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.

Next, 9.99 g of the weighed water-absorbent resin mixture (1) was added to initiate the solubilization reaction of the water-absorbent resin. One hour after the addition of the water-absorbent resin mixture (1) (initiation 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 to complete the solubilization reaction (total of 3 hours), and a solubilized liquid was obtained.

Under the solubilization conditions of Example 1, the solubilization rate of the water-absorbent resin was determined to be 99.9% by mass. Furthermore, a similar solubilization procedure was performed using only 0.010 g of material model (A) other than the water-absorbent resin, and the solubilization rate of the pulp (foreign matter solubilization rate) was determined to be 6.0% by mass.

Next, under the above solubilization conditions, the content of materials other than the water-absorbent resin was determined.

[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.

[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.

[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.

[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.

[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.

[Example 5]
99.9% by mass of the water-absorbent resin (S1) obtained in Production Example 1 and 0.10% by 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% by 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% by mass. The measurement results of the physical properties of the water-absorbent resin mixture (5) are shown in Table 1.

[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 materials other than the water-absorbent resin in the water-absorbent resin mixture (6) was 0.50 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 0.47 mass%. The measurement results of the physical properties of the water-absorbent resin mixture (6) are shown in Table 1.

[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.

[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.

[Example 8]
A water-absorbent resin (S1) obtained by carrying out the same operation as in Production Example 1 multiple times and pulp removed from diapers obtained by carrying out 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 paper diaper".

Next, the following operations were carried out on the simulated used disposable diapers as a water-absorbent resin recycling process. The simulated used disposable diapers and five times the amount of ethanol were added to a reaction vessel equipped with a stirring blade, and the mixture was stirred for 10 minutes, causing the water-absorbent resin to settle to the bottom of the vessel while discharging the deionized water it had absorbed. After stirring was stopped, the pulp floating on the top layer of the vessel was removed by decantation. Deionized water and ethanol were again added to the remaining gel-like water-absorbent resin, and the mixture was stirred, after which the pulp floating on the top layer of the vessel was removed by decantation.

The contents of the container were then filtered through a 100-mesh stainless steel wire mesh and dried in an oven at 160°C for 2 hours. The dried material was then pulverized using a vibration mill, and a particulate recycled water-absorbent resin mixture (8-1) was obtained that passed through a sieve with 850 μm openings.

This recycled water-absorbent resin mixture (8-1) was operated using a cyclone centrifuge (dome diameter 600 mm) under conditions of an air volume of 13 m ³ /min and a supply rate of recycled water-absorbent resin mixture of 1 kg/min, to produce a recycled water-absorbent resin (8-2) with a reduced pulp content.

Next, the recycled water-absorbent resin mixture (8-2) was separated using a JIS standard sieve with a mesh size of 150 μm into a recycled water-absorbent resin mixture (8-2a) that remained on the sieve and a recycled water-absorbent resin mixture (8-2b) that passed through the sieve.

The recycled water-absorbent resin mixture (8-2a) was again run under the same conditions using a cyclone centrifuge to obtain a recycled water-absorbent resin (8-2a') with a reduced content of "pulp remaining on a JIS standard sieve with a mesh size of 150 μm."

Next, the recycled water-absorbent resin mixture (8-2b) that had passed through a 150 μm standard sieve was thoroughly mixed with recycled water-absorbent resin (8-2a') in which the "pulp remaining on the 150 μm JIS standard sieve" had been reduced, to produce the recycled water-absorbent resin mixture (8-3).

The obtained recycled water absorbent resin mixture (8-3) was measured according to the procedures described in [Measurement and Evaluation Methods]. The content of materials other than the water absorbent resin in 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 for the physical properties of the recycled water absorbent resin mixture (8-3) are shown in Table 1.

In addition, the content 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 it was added, and the solubilization time were the same as in Example 1.

(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.

Furthermore, it can be seen that Example 8, which is a water-absorbent resin mixture recycled from simulated used disposable diapers, also has excellent powder properties and water-absorbing properties.

In addition, in Examples 2, 3, 4, and 4-2, it is believed that by mixing a 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 150 μm openings aggregated and remained on the sieve with 150 μm openings.

(Reference Example of Invention A (Quantitative Method))
Invention A (quantitative method) will be specifically described below with reference to a reference example and a comparative reference example. Note that the measurement methods for physical properties and the like in Invention A are the same as those in the above [Measurement and evaluation methods].

[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').

Next, the aqueous monomer solution (b') was cooled while stirring. When the liquid temperature reached 40.0°C, 100.8 g of 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare 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, forming a transparent, homogeneous solution.

Next, 12.1 g of a 4.0% by 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 x 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 began 60 seconds after the aqueous monomer solution (b) was poured into the bat-shaped container.

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 cross-linked polymer (hereinafter referred to as "hydrogel") (S3) was removed. This series of operations was carried out in an open-to-air system. The hydrogel (S3) obtained in 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 screen and dried with hot air at 190°C for 60 minutes. It was then crushed using a vibration mill and passed through a sieve with openings of 850 μm, remaining on a 150 μm sieve to obtain an irregularly crushed water-absorbent resin precursor (S3) with an average particle diameter of 350 μm.

100 parts by mass of the obtained water-absorbent resin precursor (S3) was uniformly mixed with a surface-crosslinking 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 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 water-absorbent resin (S3). The CRC of water-absorbent resin (S3) was 31.3 (g/g), and the AAP0.3 was 30.4 (g/g).

[Reference example 1-1]
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, 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.

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 ferrous sulfate heptahydrate were added. Stirring was continued for another 2 hours to complete the solubilization reaction (3 hours in total), yielding solubilized liquid (1-A). The solubilization process up to this point is referred to as Solubilization Process 1.

The amount of undissolved water-absorbent resin in the solubilized solution (1-A) was measured using the following method.

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 foreign matter content measurement section, and the residue remaining on the filter paper was washed with a total of 100 g of deionized water. After washing, the filter paper was dried for 3 hours in an oven adjusted to 105°C. After drying, the filter paper was left to stand in a constant temperature and humidity chamber for at least 6 hours, 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 at least 6 hours, and the mass was measured.

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

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

Solubilization was carried out using the above-mentioned solubilization method (solubilization treatment 1) as follows.

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 initiate the solubilization reaction of the water-absorbent resin.

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 ferrous 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.

The error in the measurement results for the pulp feed ratio (10.00 mass%) was (9.651 - 10.00) / 10.00 x 100 = -3.490 mass%.

[Reference example 1-2]
The solubilization treatment 1 was carried out in the same manner as in Reference Example 1-1 for 9.9000 g of a water-absorbent resin and 0.1000 g of pulp, 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.

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% by mass was used, and the foreign matter content was determined. The results are shown in Table 2.

[Reference example 1-3]
The solubilization rate of the water-absorbent resin and the solubilization rate of the pulp were determined by performing the same solubilization treatment 1 as in Reference Example 1-1 on 9.9900 g of a water-absorbent resin and 0.01000 g of pulp. 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.

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% by mass was used, and the foreign matter content was determined. The results are shown in Table 2.

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

Next, solubilization treatment 2 was performed on the water-absorbent resin mixture (1-1), and the foreign matter content was determined. The results are shown in Table 2.

[Reference example 2-2]
Solubilization treatment 2 was performed on 9.9000 g of a water-absorbent resin and 0.1000 g of pulp, 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 99.93 mass%, and the solubilization rate of the pulp was 3.801 mass%.

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

[Reference example 2-3]
Solubilization treatment 2 was performed on 9.9900 g of a water-absorbent resin and 0.01000 g of pulp, 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 99.97 mass%, and the solubilization rate of the pulp was 4.923 mass%.

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

[Reference example 3-1]
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 14.25 g of 30 mass% hydrogen peroxide water, 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 the solubilization reaction of the water absorbent resin.

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 ferrous sulfate heptahydrate were added. Stirring was continued for another 2 hours to complete the solubilization reaction (3 hours in total), yielding solubilized liquid (3-A). The solubilization process up to this point is referred to as solubilization process 3.

The solubilization rate of the water-absorbent resin was determined from the solubilized solution (3-A) to be 99.44% by mass.

Next, a similar solubilization procedure was performed using only 1.000 g of pulp obtained by decomposing commercially available disposable diapers, and the solubilization rate of the pulp was determined to be 3.111% by mass.

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

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 initiate the solubilization reaction of the water-absorbent resin.

One hour after the start of solubilization, an additional 14.25 g of 30% by mass hydrogen peroxide, 2.500 g of L-ascorbic acid, and 0.2500 g of ferrous 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 (3-1). 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.

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%.

[Reference example 4-1]
[0110] 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.

Solubilization was carried out using the above-mentioned solubilization method (solubilization treatment 1) as follows.

9.000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 1.000 g of a polyolefin nonwoven fabric (the same as the nonwoven fabric for which the nonwoven fabric solubilization rate was determined) obtained by decomposing commercially available disposable diapers were thoroughly mixed to obtain a water-absorbent resin mixture (4-1) containing 10.00 mass% 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 water, 5.000 g of L-ascorbic acid, and 0.5000 g of iron sulfate heptahydrate were added with stirring and dissolved. Subsequently, 10.00 g of the water-absorbent resin mixture (4-1) was added to initiate the solubilization reaction of the water-absorbent resin.

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 ferrous 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 (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.

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

[Reference example 4-2]
Solubilization treatment 1 was performed on 9.9000 g of a water-absorbent resin and 0.1000 g of a nonwoven fabric, 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%.

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 nonwoven fabric content was changed to 1.000% by mass was used. The results are shown in Table 2.

[Reference example 4-3]
Solubilization treatment 1 was performed on 9.9900 g of a water-absorbent resin and 0.01000 g of pulp, 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%.

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 nonwoven fabric content was changed to 0.1000% by mass was used. The results are shown in Table 2.

[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% 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 and the pulp.

The entire liquid was placed in a separatory funnel and shaken, after which the absorbent resin was allowed to settle and separate. The settled 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 contained in the absorbent resin, and the supernatant liquid containing the pulp was then decanted back into the separatory funnel.

The pulp remaining in the separatory funnel was suction filtered using qualitative filter paper (No. 5A) using the same method as for 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. After drying, the material was weighed and the foreign matter content was determined to be 0.5321% by mass. The error in the measurement result for the pulp feed ratio (1.000% by mass) was (0.5321 - 1.000) / 1.000 x 100 = -46.79% by mass.

[Comparative Reference Example 2]
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 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.

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

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.

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

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 disposable 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 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 at 500 rpm.

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, 1.250 g of L-ascorbic acid, and 0.1250 g of ferrous sulfate heptahydrate were added. Stirring was continued for another two hours, and the solubilization reaction was completed (a total of three hours), yielding comparative solubilized liquid (2). 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 found to be 0.6868% by mass. The error in the measurement result for a pulp feed ratio (1.000% by mass) was (0.6868 - 1.000) / 1.000 x 100 = -31.32% by mass.

[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.

Stirring was continued for 6 hours, completing the solubilization reaction and obtaining solubilized solution (1-D). The solubilization process up to this point is referred to as solubilization process 5.

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

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

0.9000 g of the water-absorbent resin (S3) obtained in Production Example 3 and 0.1000 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 comparative water-absorbent resin mixture (3) containing 10.00% by mass of pulp.

5,000 g of sodium hypochlorite solution (chlorine content of 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 stirred at 500 rpm for 5 minutes, after which 1,000 g of comparative water-absorbent resin mixture (3) was added and stirred at 500 rpm for 5 minutes, and the reaction vessel was then immersed in a thermostatic bath adjusted to 80°C to initiate the solubilization reaction of the water-absorbent resin.

Stirring was continued for 6 hours, completing the solubilization reaction and obtaining comparative solubilized liquid (3). The amount of foreign matter in the resulting comparative solubilized liquid (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 feed ratio (10.00% by mass) was (12.69 - 10.00) / 10.00 x 100 = 26.89% by mass.

Reference Examples 1 to 4, which increased the solubilization rate of the water-absorbent resin while keeping the solubilization rate of foreign matter such as pulp and nonwoven fabric low, enabled the quantification of foreign matter content with high measurement accuracy and low measurement error rate. It can also be seen that the higher the solubilization rate of the water-absorbent resin (Reference Example 2 → Reference Example 1), the lower the measurement error rate can be kept even when the foreign matter content rate is low.

On the other hand, Comparative Reference Example 1, in which the foreign matter content was quantified using the "sedimentation separation method," which utilizes the difference in sedimentation speed between the absorbent resin and foreign matter, had a large measurement error of -46.79% by mass. With this method, when there is more absorbent resin than foreign matter, a small amount of foreign matter becomes physically entangled with the absorbent resin, which is thought to result in the amount of foreign matter being measured as lower than the set amount.

Furthermore, 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 was also high, 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 was also high, resulting in a large error in the measurement of the foreign matter content.

The above is a reference example regarding Invention A (quantitative method).

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

Claims (16)

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 materials other than the water-absorbent resin remaining on a JIS standard sieve with 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. The water-absorbent resin mixture according to claim 1, wherein the content of the material other than the water-absorbent resin is determined by a method for quantifying a material other than the water-absorbent resin, the method comprising the following steps a) to c):
a) solubilizing the water-absorbent resin in the water-absorbent resin mixture using a water-absorbent resin solubilization technique that solubilizes the water-absorbent resin in water while suppressing the solubilization of materials other than the water-absorbent resin in water;
b) removing the solubilized water-absorbing resin obtained in a);
c) The remaining component obtained in b) is dried, and the content of materials other than the water-absorbent resin is determined. The content of the material other than the water absorbent resin remaining on the JIS standard sieve having an opening of 150 μm is calculated from the content P% by mass of the material other than the water absorbent resin in the water absorbent resin mixture obtained by the quantification method using the following formula:
Content (mass%) of material other than water absorbent resin remaining on JIS standard sieve having 150 μm opening=Content mass ratio of water absorbent resin mixture remaining on JIS standard sieve having 150 μm opening in water absorbent resin mixture×P mass%
The water-absorbing resin mixture according to claim 2, which is determined by the following formula: The water-absorbent resin mixture according to claim 2, wherein a solubilization method is used such that the solubilization rate of the water-absorbent resin in the water-absorbent resin mixture is 90% by mass or more. 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% by mass. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture is in powder form. 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. The water-absorbent resin mixture according to claim 1 or 2, wherein the bulk specific gravity of the water-absorbent resin mixture is 0.55 g/ml or more. 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. The water-absorbent resin mixture according to claim 1 or 2, wherein the mass average particle diameter (D50) of the water-absorbent resin mixture is 200 μm or more and 600 μm or less. 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. The water-absorbent resin mixture according to claim 1 or 2, 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. The water-absorbent resin mixture according to claim 1 or 2, wherein the water-absorbent resin mixture is recovered from used absorbent articles. A method for recycling absorbent resin contained in used absorbent articles, wherein the recycled absorbent resin comprises the absorbent resin mixture described in claim 1 or 2. 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 materials in the process for producing the water-absorbent resin from the monomers that constitute the water-absorbent resin. The manufacturing method described in claim 15, wherein the proportion of the water-absorbent resin mixture relative to all water-absorbent resin raw materials is 1% by mass or more and 60% by mass or less.