Water-absorbing resin composition

Abstract

To provide a water-absorbing resin composition a cross-linked polymer of which can be efficiently decomposed and treated by an oxidant while the required water absorption performance being satisfied at the time of use, thereby contributing to resource saving and reduction of environmental load.SOLUTION: There is provided a water-absorbing resin composition comprising a cross-linked polymer (A) having one or more monomers (A1) selected from the group consisting of a water-soluble unsaturated monocarboxylic acid (a1) and a salt thereof and an internal crosslinking agent (b) having a dimethylhydrazine skeleton as essential constituent units, wherein the content of the internal crosslinking agent (b) unit in the cross-linked polymer (A) is 0.005 mol% or more and 3.000 mol% or less and the decomposition index of the cross-linked polymer (A) represented by the following expression (1) is 90 or more. Decomposition index=(1-β / α)×100 (Expression 1) β: the net weight of the water-absorbing resin composition after decomposition. α: the net weight of the water-absorbing resin composition before decomposition.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a water-absorbing resin composition.

Background Art

[0002] As the usage amount of sanitary products increases, the problem of garbage disposal of used sanitary products is becoming a serious problem. Sanitary products, especially disposable diapers, have rapidly spread as essential products in an aging and low-birth-rate society, and their consumption is increasing rapidly.

[0003] Regarding the garbage disposal of used sanitary products, disposable diapers and the like are usually incinerated. However, since the proportion of moisture in the diapers is nearly 80%, a large amount of combustion energy is required for incineration. For this reason, a large load is imposed on the incinerator itself during the treatment, which results in shortening the life of the incinerator. In addition, incineration leads to air pollution and global warming, and also becomes a factor imposing a burden on the environment, so improvement is strongly desired. Also, in the field of nursing care, reducing the burden on nurses involved in the disposal of disposable diapers has become an issue.

[0004] In response to the above problems, studies are underway to reduce the environmental impact by recovering and reusing components from used sanitary products. Usually, sanitary products contain an absorber composed of pulp fibers, a water-absorbing resin composition, a non-woven fabric, a polyethylene sheet, etc. In order to reuse thermoplastic resins such as pulp fibers, non-woven fabrics, and plastic sheets as components, it is necessary to separate these pulp fibers, etc. from the water-absorbing resin composition. However, since the water-absorbing resin composition in the absorber of used sanitary products is in a gel state swollen by absorbing water, it is difficult to separate it as it is. Therefore, a technique has been proposed to decompose and solubilize the water-absorbing resin composition and separate the solubilized components of the pulp fibers and the water-absorbing resin composition. For example, there is a technique (Patent Documents 1 and 2) in which a sanitary product containing pulp fibers and a water-absorbing resin composition is treated with an aqueous solution containing ozone, and after decomposing and solubilizing the water-absorbing resin composition, the pulp fibers are recovered. In addition, as a technique for decomposing and solubilizing the water-absorbing resin composition, a technique using an oxidizing agent such as hydrogen peroxide as a decomposition method, and a method of irradiating electromagnetic waves (Patent Documents 3 to 6) are known. In addition, a method has been reported in which a crosslinked polymer compound is reacted with an oxidizing agent to selectively cleave only the crosslinking agent unit portion and convert it into a water-soluble polyacrylic acid (salt) (Patent Document 7).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0006] The recycling technology of the used sanitary products described above aims to recover pulp fibers and use them as recycled pulp, or to reuse thermoplastic resins or use them as solid fuels. On the other hand, the water-absorbing resin composition, which is the other member, is mostly decomposed and solubilized, discarded, or recycled as solid fuel or the like.

[0007] The cross-linked polymer compound described in Patent Document 7 can recover pulp fibers by solubilizing acrylic acid oligomers by cutting the cross-linking agent with an oxidizing agent. However, the molecular weight of the solubilized oligomers is small, and even if the solubilized product is discharged into the sewer, it is difficult to perform precipitation treatment, which may cause water pollution and lead to concerns about environmental impact. In addition, the cross-linked polymer compound described in Patent Document 7 has a problem of poor decomposition efficiency due to oxidizing agents because moisture absorption blocking occurs.

[0008] An object of the present invention is to provide a water-absorbing resin composition that can satisfy the necessary water absorption performance during use, can efficiently decompose a cross-linked polymer with an oxidizing agent, and can contribute to resource saving and reduction of environmental impact.

Means for Solving the Problems

[0009] The present invention is a water-absorbing resin composition containing a cross-linked polymer (A) having, as essential constituent units, one or more monomers (A1) selected from the group consisting of a water-soluble unsaturated monocarboxylic acid (a1) and its salts, and an internal cross-linking agent (b) represented by the following general formula (1), The content of the internal cross-linking agent (b) unit in the cross-linked polymer (A) is 0.005 mol% or more and 3.000 mol% or less, The water-absorbing resin composition has a decomposition index represented by the following formula (1) of 90 or more.

Chemical formula

Advantages of the Invention

[0010] According to the present invention, it is possible to provide a water-absorbing resin composition that can efficiently decompose a crosslinked polymer with an oxidizing agent while satisfying the necessary water absorption performance during use, and can contribute to resource saving and reduction of environmental load. In addition, by solubilizing the water-absorbing resin composition with an oxidizing agent, it becomes easier to reuse plastic materials such as pulp and non-woven fabric contained in sanitary products.

Embodiments for Carrying Out the Invention

[0011] <Water-absorbing resin composition> The water-absorbing resin composition of this embodiment is a water-absorbing resin composition containing a crosslinked polymer (A) having, as essential constituent units, one or more monomers (A1) selected from the group consisting of a water-soluble unsaturated monocarboxylic acid (a1) and its salts, and an internal crosslinking agent (b) represented by the following general formula (1), the content of the internal crosslinking agent (b) units in the crosslinked polymer (A) is 0.005 mol% or more and 3.000 mol% or less, the decomposition index represented by the following formula (1) of the water-absorbing resin composition is 90 or more.

Chemical

[0012] According to the water-absorbing resin composition of the present embodiment, it is possible to provide a water-absorbing resin composition that can efficiently decompose a crosslinked polymer with an oxidizing agent while satisfying the necessary water absorption performance during use, and can contribute to resource saving and reduction of environmental load.

[0013] [Crosslinked polymer (A)] [Monomer (A1)] The water-soluble unsaturated monocarboxylic acid (a1) can be used without particular limitation as long as it is a water-soluble unsaturated monocarboxylic acid. From the viewpoints of water absorption performance when formed into a crosslinked product and ease of availability, one or more selected from the group consisting of acrylic acid, methacrylic acid, and crotonic acid are preferable, and acrylic acid and methacrylic acid are more preferable.

[0014] Examples of the salt of the water-soluble unsaturated monocarboxylic acid (a1) include alkali metal (such as lithium, sodium, and potassium) salts, alkaline earth metal (such as magnesium and calcium) salts, and ammonium (NH 4 ) salts and the like. Among these salts, from the viewpoints of absorption performance and the like, alkali metal salts and ammonium salts are preferable, alkali metal salts are more preferable, and sodium salts are particularly preferable.

[0015] When the crosslinked polymer (A) contains a salt of the water-soluble unsaturated monocarboxylic acid (a1) as a structural unit, the ratio of the salt of the water-soluble unsaturated monocarboxylic acid (a1) [also referred to as the degree of neutralization of the water-soluble unsaturated monocarboxylic acid (a1)] to the total number of moles of the salt of the water-soluble unsaturated monocarboxylic acid (a1) and the water-soluble unsaturated monocarboxylic acid (a1) is 35 to 90 mol%, preferably 40 to 85 mol%, from the viewpoint of the decomposition index.

[0016] In this specification, water-soluble means that at least 100 g dissolves in 100 g of water at 25°C.

[0017] In addition to the monomer (A1), the crosslinked polymer (A) can have, as a structural unit, other monomers (A3) copolymerizable with the monomer (A1). The monomer (A3) is not particularly limited as long as it is a monomer copolymerizable with the monomer (A1), and water-soluble unsaturated dicarboxylic acids (a3) and their salts, other vinyl monomers (a5), etc. can be used.

[0018] The water-soluble unsaturated dicarboxylic acid (a3) can be used without particular limitation as long as it is a water-soluble unsaturated dicarboxylic acid. From the viewpoints of reactivity with the water-soluble unsaturated monocarboxylic acid (a1) and ease of availability, maleic acid, fumaric acid, methylene succinic acid, and citraconic acid are preferred, and methylene succinic acid is more preferred.

[0019] Examples of the salt of the water-soluble unsaturated dicarboxylic acid (a3) include alkali metal (such as lithium, sodium, and potassium) salts, alkaline earth metal (such as magnesium and calcium) salts, and ammonium (NH 4 ) salts, etc. Among these salts, from the viewpoints of absorption performance, etc., alkali metal salts and ammonium salts are preferred, more preferably alkali metal salts, and particularly preferably sodium salts.

[0020] When the crosslinked polymer (A) contains a salt of the water-soluble unsaturated dicarboxylic acid (a3) as a constituent unit, the ratio of the salt of the water-soluble unsaturated dicarboxylic acid (a3) to the total number of moles of the salt of the water-soluble unsaturated dicarboxylic acid (a3) and the water-soluble unsaturated dicarboxylic acid (a3) [also referred to as the degree of neutralization of the water-soluble unsaturated dicarboxylic acid (a3)] is 35 to 90 mol% from the viewpoint of the decomposition index, and preferably 40 to 85 mol%.

[0021] The vinyl monomer (a5) is not particularly limited, and known hydrophobic vinyl monomers (for example, the hydrophobic vinyl monomers disclosed in paragraphs 0028 to 0029 of Japanese Patent No. 3648553, the vinyl monomers disclosed in paragraph 0025 of JP-A-2003-165883, and the vinyl monomers disclosed in paragraph 0058 of JP-A-2005-75982) and the like can be used. Specifically, for example, the following vinyl monomers (i) to (iii) and the like can be used. (i) An aromatic ethylenic monomer having 8 to 30 carbon atoms Styrene such as styrene, α-methylstyrene, vinyltoluene, and hydroxystyrene; vinylnaphthalene; and halogen-substituted products of styrene such as dichlorostyrene. (ii) An aliphatic ethylenic monomer having 2 to 20 carbon atoms Alkenes (ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.); and alkadienes (butadiene, isoprene, etc.). (iii) An alicyclic ethylenic monomer having 5 to 15 carbon atoms Monoethylenically unsaturated monomers (pinene, limonene, indene, etc.); and polyethylenically vinyl monomers [cyclopentadiene, bicyclopentadiene, ethylidene norbornene, etc.].

[0022] The monomer (A3) may be used alone or in combination of two or more.

[0023] From the viewpoint of absorption performance and the like, the amount of substance of the monomer (A3) in the crosslinked polymer (A) is preferably 0 to 5 mol parts, more preferably 0 to 3 mol parts, particularly preferably 0 to 2 mol parts, and most preferably 0 to 1.5 mol parts, relative to 100 mol parts of the monomer (A1). From the viewpoint of absorption performance and the like, it is most preferable that the amount is 0 mol parts.

[0024] [Internal crosslinking agent (b)] The internal crosslinking agent (b) has a dimethylhydrazine skeleton (general formula: R 2 -CH 2 NHNHCH 2 -R 3 . R 2 , and R 3 are each independently an arbitrary atomic group). The dimethylhydrazine skeleton of the internal crosslinking agent (b) which is a constitutional unit of the crosslinked polymer (A) decomposes by reacting with an oxidizing agent other than oxygen. By this reaction, the dimethylhydrazine chain crosslinking the polymer chains is cleaved, and the three-dimensional crosslinked structure of the crosslinked polymer (A) is eliminated and converted into a linear polymer, thereby enabling it to be solubilized in water.

[0025] The two Rs in the general formula (1) 1 are each independently selected from hydrogen, an alkyl group, a hydroxy group, an amino group, a mercapto group, a substituted carbonyl group, and any alkyl group having one or more selected from a hydroxy group, an amino group, a mercapto group, and a substituted carbonyl group as substituents, and are not particularly limited as long as they are one or more selected. However, from the viewpoints of solubility in an aqueous solution, the absorption performance of the water-absorbing resin composition, and ease of availability, it is preferable that both are hydrogen.

[0026] The degradation product obtained by decomposing the crosslinked polymer (A) is preferably water-soluble from the viewpoint of sewage treatment. However, even if the degradation product is water-soluble, if its molecular weight is small, precipitation treatment in sewage treatment becomes difficult. The present inventors have found that by regulating the content of the internal crosslinking agent (b) units in the crosslinked polymer (A), the molecular weight of the degradation product can be controlled, and both solubilization of the degradation product in water and facilitation of precipitation treatment in sewage treatment can be achieved.

[0027] From the viewpoints of solubilizing the degradation product of the crosslinked polymer (A) in water and absorption performance, the content of the internal crosslinking agent (b) units in the crosslinked polymer (A) is 0.005 mol% or more, preferably 0.01 mol% or more, particularly preferably 0.05 mol% or more, and from the viewpoint of facilitating precipitation treatment in sewage treatment, it is 3.000 mol% or less.

[0028] In addition to the internal crosslinking agent (b), the crosslinked polymer (A) may also have, as constituent units as required, known crosslinking agents (for example, crosslinking agents having two or more ethylenically unsaturated groups disclosed in paragraphs 0031 to 0034 of Japanese Patent No. 3648553, crosslinking agents having at least one functional group capable of reacting with a water-soluble substituent and having at least one ethylenically unsaturated group, and crosslinking agents having at least two functional groups capable of reacting with a water-soluble substituent, crosslinking agents having two or more ethylenically unsaturated groups disclosed in paragraphs 0028 to 0031 of JP-A-2003-165883, crosslinking agents having an ethylenically unsaturated group and a reactive functional group and crosslinking agents having two or more reactive substituents, crosslinkable vinyl monomers disclosed in paragraph 0059 of JP-A-2005-75982, and crosslinkable vinyl monomers disclosed in paragraphs 0015 to 0016 of JP-A-2005-95759). By using the internal crosslinking agent (b) and an internal crosslinking agent (nb) other than the internal crosslinking agent (b) in combination, it is possible to improve the water absorption performance.

[0029] From the viewpoint of degradation performance, the amount of substance of the internal crosslinking agent (nb) other than the internal crosslinking agent (b) in the crosslinked polymer (A) is preferably 0 to 500 mol parts with respect to 100 mol parts of the internal crosslinking agent (b).

[0030] The decomposition index represented by the following formula (1) of the water-absorbing resin composition is 90 or more, preferably 95 or more, from the viewpoints of the load on the environment after decomposing the crosslinked polymer (A) and the ease of separation of the decomposition product (acrylic acid oligomer). When the decomposition index is less than 90, it cannot be said that the water-absorbing resin composition is sufficiently decomposed. For example, when flowing through a pipe, there is a risk of pipe blockage, and when flowing into the ocean, since it is difficult to decompose in nature, there is a concern that it will remain in the environment for a long time. Decomposition index = (1 - β / α) × 100 (Formula 1) β: Net weight of the water-absorbing resin composition after decomposition α: Net weight of the water-absorbing resin composition before decomposition

[0031] The decomposition index is determined by the following method carried out in an environment where the temperature of the measurement atmosphere is 25°C ± 2°C. (1) Weigh 0.600 g of the water-absorbing resin composition, then add it to 20 ml of a 5% aqueous sodium hypochlorite solution, stir at 25°C at a speed of 200 rpm using a Teflon (registered trademark) spatula (manufactured by Esco Corporation), and stop stirring after 30 minutes. (2) Then, transfer the entire amount of the stirred material to a tea bag (20 cm long, 10 cm wide) made of a nylon mesh with an aperture of 63 μm (JIS Z8801-1:2006). At this time, a spatula or the like may be used as necessary. (3) Then, put the tea bag into a centrifuge and centrifuge for 90 seconds at 150 G to remove excess moisture. (4) Transfer the entire amount of the content of the tea bag after centrifugal dehydration to a SUS-made vat (15 cm long, 25 cm wide, 5 cm high, vat weight B1). (5) Transfer the content of the tea bag in the SUS-made vat to a hot air dryer maintained at 150°C, let it stand for 3 hours, then take it out, cool it in a desiccator (manufactured by AS ONE Corporation, model number 3-6011-24) containing silica gel desiccant until it reaches room temperature, take it out after cooling, measure the weight (B2) of the SUS-made vat together, and calculate the net weight of the decomposed water-absorbing resin composition [β (= B2 - B1)]. (6) Similarly, 0.600 g of the water-absorbing resin composition is placed in a SUS-made vat (15 cm in length, 25 cm in width, 5 cm in height; vat weight W1), left standing in a down-draft dryer maintained at 150°C for 3 hours, taken out, cooled in a desiccator (model number 3-6011-24, manufactured by AS ONE Corporation) containing silica gel desiccant until it reaches room temperature, and the weight of the entire SUS-made vat (weight W2) is measured to obtain the net weight α (=W2 - W1) of the water-absorbing resin composition before decomposition. (7) Calculate the decomposition index according to the following (Formula 1). Decomposition index = (1 - β / α) × 100 (Formula 1) β: Net weight of the water-absorbing resin composition after decomposition α: Net weight of the water-absorbing resin composition before decomposition

[0032] The crosslinked polymer (A) contained in the water-absorbing resin composition preferably has a structure in which its surface is crosslinked by a surface crosslinking agent (c). By crosslinking the surface of the crosslinked polymer (A), the gel strength of the water-absorbing resin composition can be improved, and the desirable water retention amount and the absorption amount under load of the water-absorbing resin composition can be satisfied. In addition, blocking on the surface of the water-absorbing resin composition is suppressed, and uniform water absorption can be achieved. Therefore, even when performing decomposition treatment with an oxidizing agent, an improvement in decomposition efficiency can be expected.

[0033] The surface crosslinking agent (c) can be either an inorganic substance or an organic substance. As the surface crosslinking agent (c), known organic surface crosslinking agents (such as polyvalent glycidyl compounds, polyvalent amines, polyvalent aziridine compounds, and polyvalent isocyanate compounds described in JP-A-59-189103, polyhydric alcohols described in JP-A-58-180233 and JP-A-61-16903, silane coupling agents described in JP-A-61-211305 and JP-A-61-252212, alkylene carbonates described in JP-T-5-508425, polyvalent oxazoline compounds described in JP-A-11-240959, etc.) can be used. Among these surface crosslinking agents (c), from the viewpoints of economy and absorption characteristics, polyvalent glycidyl compounds, polyhydric alcohols, and polyvalent amines are preferred, more preferably polyvalent glycidyl compounds and polyhydric alcohols, particularly preferably polyvalent glycidyl compounds, and most preferably ethylene glycol diglycidyl ether. The surface crosslinking agent (c) may be used alone or in combination of two or more kinds.

[0034] The amount of the surface crosslinking agent (c) used (% by weight) is not particularly limited because it can be variously changed depending on the type of the surface crosslinking agent, the crosslinking conditions, the target performance, etc. However, from the viewpoint of absorption characteristics, etc., based on the weight of the crosslinked polymer (A), 0.001 to 3 is preferred, more preferably 0.005 to 2, and particularly preferably 0.01 to 1.5.

[0035] The water-absorbing resin composition contains the crosslinked polymer (A) and may contain other components such as residual solvents and residual crosslinking components as long as its performance is not impaired.

[0036] Other examples of the other components include preservatives, fungicides, antibacterial agents, ultraviolet absorbers, antioxidants, colorants, fragrances, deodorants, fluidity improvers, inorganic powders, and organic fibrous substances. The amount is usually 5% by weight or less based on the weight of the water-absorbing resin composition.

[0037] From the perspective of water absorption performance, the water-absorbing resin composition preferably contains, as the other component, at least one typical element selected from the group consisting of iodine, tellurium, antimony, and bismuth. When the water-absorbing resin composition contains the typical element, the content of the typical element in the water-absorbing resin composition is preferably 0.0005 to 0.1% by weight, more preferably 0.001 to 0.05% by weight, from the perspective of water absorption performance.

[0038] There is no particular limitation on the shape of the water-absorbing resin composition, and examples thereof include irregular crushed shapes, flake shapes, pearl shapes, and rice grain shapes. Among these, an irregular crushed shape is preferable from the perspective of good entanglement with fibrous materials in applications such as disposable diapers and no fear of dropping off from fibrous materials.

[0039] The internal cross-linking agent (b) site of the water-absorbing resin composition is selectively decomposed by an oxidizing agent, and the decomposition product (acrylic acid oligomer) can be dissolved in water.

[0040] <Manufacturing method of water-absorbing resin composition> The manufacturing method of the water-absorbing resin composition of the present embodiment is a manufacturing method of the water-absorbing resin composition, and includes a polymerization step of obtaining a hydrogel containing the cross-linked polymer (A) and a drying step of drying the hydrogel.

[0041] 〔Polymerization step〕 The polymerization step is a step of polymerizing a monomer composition containing one or more monomers (A2) selected from the group consisting of the monomer (A1) and the monomer (a2) that becomes the water-soluble unsaturated monocarboxylic acid (a1) by hydrolysis, and the internal cross-linking agent (b) to obtain a hydrogel containing the cross-linked polymer (A).

[0042] The hydrolyzability of the monomer (a2) means the property of being hydrolyzed by the action of water and, if necessary, a catalyst (such as an acid or a base) to become water-soluble. The hydrolysis of the monomer (a2) may be carried out during polymerization, after polymerization, or both, but after polymerization is preferable from the perspective of the absorption performance of the resulting water-absorbing resin composition.

[0043] [Monomer (a2)] A monomer (a2) that becomes the water-soluble unsaturated monocarboxylic acid (a1) upon hydrolysis can be used together with or in place of the water-soluble unsaturated monocarboxylic acid (a1). The monomer (a2) is not particularly limited, and examples thereof include monomers having one hydrolyzable substituent that becomes a carboxy group upon hydrolysis. Examples of the hydrolyzable substituent include a group containing an acid anhydride (1,3-oxo-1-oxapropylene group [-COO-CO-]), a group containing an ester bond (alkyloxycarbonyl [-COOR], vinyloxycarbonyl, allyloxycarbonyl, propenyloxycarbonyl, etc.), and a cyano group. In addition, R contained in alkyloxycarbonyl is preferably an alkyl group having 1 to 3 carbon atoms (methyl, ethyl, and propyl), vinyl, allyl, or propenyl.

[0044] When producing the crosslinked polymer (A) having the monomer (A3) as a structural unit, the monomer composition contains the monomer (A3). From the viewpoint of absorption performance and the like, the amount of substance of the monomer (A3) in the monomer composition is preferably 0 to 5 mol parts, more preferably 0 to 3 mol parts, particularly preferably 0 to 2 mol parts, and most preferably 0 mol parts, based on 100 mol parts of the monomer (A1).

[0045] When producing the crosslinked polymer (A) having at least one selected from the group consisting of the water-soluble unsaturated dicarboxylic acid (a3) and its salts as a structural unit, a monomer (a4) that becomes the water-soluble unsaturated dicarboxylic acid (a3) upon hydrolysis can be used together with or in place of the water-soluble unsaturated dicarboxylic acid (a3).

[0046] The monomer (a4) is not particularly limited, and examples thereof include monomers having at least one hydrolyzable substituent.

[0047] The hydrolysis of the monomer (a4) may be carried out during polymerization, after polymerization, or both. From the viewpoint of the absorption performance of the resulting water-absorbing resin composition, post-polymerization is preferred.

[0048] Examples of the polymerization method of the monomer composition include known solution polymerization and known inverse phase suspension polymerization. Among the polymerization methods of the monomer composition, the preferred one is the solution polymerization method. Since it is not necessary to use an organic solvent or the like and it is advantageous in terms of production cost, the particularly preferred one is the aqueous solution polymerization method. A water-absorbing resin composition having a large water retention amount and a small amount of water-soluble components can be obtained, and the aqueous solution adiabatic polymerization method is most preferred because temperature control during polymerization is not required.

[0049] When carrying out aqueous solution polymerization, a mixed solvent containing water and an organic solvent can be used. Examples of the organic solvent include methanol, ethanol, acetone, methyl ethyl ketone, N,N-dimethylformamide, dimethyl sulfoxide, and mixtures of two or more of these.

[0050] When carrying out aqueous solution polymerization, the usage amount (weight%) of the organic solvent is preferably 40 or less, more preferably 30 or less, based on the weight of water.

[0051] When the polymerization method is the inverse phase suspension polymerization method, polymerization may be carried out in the presence of a conventionally known dispersant or surfactant, if necessary. Also, in the case of the inverse phase suspension polymerization method, polymerization can be carried out using a conventionally known hydrocarbon solvent such as xylene, normal hexane, and normal heptane.

[0052] The total weight percentage concentration of the monomer (A2) and the internal crosslinking agent (b) in the monomer composition is preferably 15 to 55% by weight based on the total weight of the polymerization solution at the start of polymerization. When the monomer (A3) is also used, the total weight percentage concentration of the monomer (A2), the monomer (A3), and the internal crosslinking agent (b) is preferably 15 to 55% by weight based on the total weight of the polymerization solution at the start of polymerization. If it is lower than this range, productivity may deteriorate, and if it is higher, sufficient gel strength may not be obtained.

[0053] The pH range of the polymerization solution during polymerization containing the monomer composition is preferably from 1 to 12, more preferably from 1 to 10, and even more preferably from 1 to 7. Within this range, surface crosslinking with the surface crosslinking agent described later proceeds efficiently, and the required water absorption performance can be easily obtained.

[0054] In the polymerization of the monomer composition, a known radical initiator can be used as necessary.

[0055] Examples of well-known radical initiators include azo compounds [such as azobisisobutyronitrile, azobiscyanovaleric acid, 2,2'-azobis(2-amidinopropane) hydrochloride, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], etc.], inorganic peroxides (such as hydrogen peroxide, ammonium persulfate, potassium persulfate, and sodium persulfate, etc.), organic peroxides [such as benzoyl peroxide, di-t-butyl peroxide, cumene hydroperoxide, succinic peroxide, and di(2-ethoxyethyl) peroxydicarbonate, etc.], redox catalysts (combinations of reducing agents such as sulfites or bisulfites of alkali metals, ammonium sulfite, ammonium bisulfite, and ascorbic acid, etc. and oxidizing agents such as persulfates of alkali metals, ammonium persulfate, hydrogen peroxide, and organic peroxides, etc.), photo radical generators [such as 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 1-hydroxycyclohexyl-phenylketone-hydroxyalkylphenone, α-aminoalkylphenone, etc.], etc. These radical initiators may be used alone or in combination of two or more thereof.

[0056] The amount of the radical initiator used is preferably 0.0005 to 5 mol parts, more preferably 0.001 to 2 mol parts, per 100 mol parts of the monomer (A1).

[0057] By polymerizing the monomer composition in the above polymerization step, the crosslinked polymer (A) can be obtained as a water-containing gel-like substance containing water (hereinafter also referred to as a water-containing gel).

[0058] 〔Gel cutting step〕 The method for producing the water-absorbing resin composition of the present embodiment may optionally have a gel cutting step of cutting the water-containing gel. The size (longest diameter) of the cut gel is preferably 50 μm to 10 cm, more preferably 100 μm to 2 cm, and particularly preferably 1 mm to 1 cm. Within this range, the drying property in the drying step becomes better.

[0059] The comminution of the aqueous gel can be carried out by a known method, and can be comminuted using a comminution device (for example, a Bepex mill, a rubber chopper, a Pharma mill, a mincing machine (meat chopper), an impact crusher, a roll crusher, etc.). Further, if necessary, an alkali can be mixed with the aqueous gel for neutralization.

[0060] 〔Neutralization step〕 The method for producing the water-absorbing resin composition of the present embodiment preferably has a neutralization step with an alkali from the viewpoint of decomposition efficiency. If not properly neutralized, when the water-absorbing resin composition absorbs water and changes into a gel state, agglomeration between gels occurs, and it is not preferable because the oxidizing agent hardly penetrates into the interior of the water-absorbing resin composition. The neutralization step with an alkali is preferably carried out simultaneously with or before the gel comminution step, and more preferably in the gel comminution step from the viewpoint of decomposition efficiency.

[0061] As the alkali, those known {Japanese Patent No. 3205168, etc.} can be used. Among these, from the viewpoint of water absorption performance, lithium hydroxide, sodium hydroxide, and potassium hydroxide are preferable, more preferably sodium hydroxide and potassium hydroxide, and particularly preferably sodium hydroxide.

[0062] The degree of neutralization of the acid groups of the crosslinked polymer (A) is 35 to 90 mol%, preferably 40 to 85 mol% from the viewpoint of decomposition efficiency. When the degree of neutralization of the acid groups of the crosslinked polymer (A) is within the range, it is easy to make the decomposition index within a predetermined range, which is preferable.

[0063] 〔Drying step〕 The method for producing the water-absorbing resin composition of the present embodiment has a drying step of drying the aqueous gel and distilling off the solvent (including water) in the aqueous gel.

[0064] As the drying method in the drying step, microwave drying, thin film drying method using a drum dryer, etc., (heating) vacuum drying method, freeze drying method, drying method using infrared rays, decantation, filtration, etc. can be applied.

[0065] The drying temperature in the drying step is 100 to 450 °C, preferably 150 to 400 °C. When the drying temperature is high, the drying time is shortened, resulting in improved productivity. However, when the drying temperature is high, there is a risk that the water retention capacity, the absorption amount under load, and the liquid permeability of the water-absorbing resin composition may decrease due to thermal degradation, and there is also a risk that the color tone of the water-absorbing resin composition may deteriorate. If the drying temperature is less than 100 °C, sufficient drying cannot be achieved, and the productivity of the water-absorbing resin composition decreases.

[0066] In the drying step, from the viewpoint of suppressing changes in the soluble content while suppressing changes in color and taste after long-term storage in a high-temperature and high-humidity environment, the drying time is preferably within 60 minutes, more preferably within 40 minutes. Also, generally, the drying time is preferably 10 minutes or more. If the drying time is short, undried matter may be generated, causing clogging in the subsequent pulverization step.

[0067] 〔Pulverization step〕 The method for producing the water-absorbing resin composition of the present embodiment may include a pulverization step of pulverizing the water-absorbing resin composition obtained in the drying step to obtain a particulate water-absorbing resin composition containing the crosslinked polymer (A).

[0068] In the pulverization step, there are no particular limitations on the method of pulverizing the water-absorbing resin composition obtained in the drying step, and pulverization apparatuses (for example, hammer mills, impact mills, roll mills, and jet air classifying mills) can be used. The pulverized water-absorbing resin composition can be adjusted in particle size by sieving or the like if necessary.

[0069] 〔Surface crosslinking step〕 The method for producing the water-absorbing resin composition of the present embodiment may include a surface crosslinking step of crosslinking the surface of the crosslinked polymer (A) with a surface crosslinking agent after the polymerization step of obtaining the hydrogel or after the pulverization step.

[0070] The water-absorbing resin composition obtained through the surface crosslinking process has a structure in which the surface of the crosslinked polymer (A) is crosslinked by the surface crosslinking agent (c). By crosslinking the surface of the crosslinked polymer (A), the gel strength of the water-absorbing resin composition can be improved, and the desired water retention capacity and absorption amount under load of the water-absorbing resin composition can be satisfied. In addition, blocking on the surface of the water-absorbing resin composition is suppressed, and uniform water absorption can be achieved, so an improvement in decomposition efficiency can also be expected when performing decomposition treatment with an oxidizing agent.

[0071] The usage amount (weight %) of the surface crosslinking agent (c) can vary variously depending on the type of the surface crosslinking agent, the crosslinking conditions, the target performance, etc., and thus is not particularly limited. However, from the viewpoint of absorption characteristics, etc., based on the weight of the crosslinked polymer (A), 0.001 to 3 is preferable, more preferably 0.005 to 2, and particularly preferably 0.01 to 1.5.

[0072] The surface crosslinking of the crosslinked polymer (A) can be carried out by mixing the crosslinked polymer (A) and the surface crosslinking agent (c) and heating them. As the mixing method of the crosslinked polymer (A) and the surface crosslinking agent (c), methods of uniformly mixing the crosslinked polymer (A) and the surface crosslinking agent (c) using mixing devices such as a cylindrical mixer, a screw mixer, a screw extruder, a turbulizer, a Nauta mixer, a double-arm kneader, a fluidized mixer, a V-type mixer, a mincing mixer, a ribbon mixer, a fluidized mixer, an air current mixer, a rotating disk mixer, a conical blender, and a roll mixer can be mentioned. At this time, the surface crosslinking agent (c) may be diluted with water and / or any solvent and used.

[0073] The temperature when mixing the crosslinked polymer (A) and the surface crosslinking agent (c) is not particularly limited, but 10 to 150°C is preferable, more preferably 20 to 100°C, and particularly preferably 25 to 80°C.

[0074] After mixing the crosslinked polymer (A) and the surface crosslinking agent (c), heat treatment is usually carried out. From the viewpoint of the breakage resistance of the water-absorbing resin composition, the heating temperature is preferably 100 to 180°C, more preferably 110 to 175°C, and particularly preferably 120 to 170°C. If the heating is at 180°C or lower, indirect heating using steam is possible, which is advantageous in terms of equipment. If the heating temperature is less than 100°C, the absorption performance may deteriorate. Also, the heating time can be appropriately set according to the heating temperature, but from the viewpoint of absorption performance, it is preferably 5 to 60 minutes, more preferably 10 to 40 minutes. It is also possible to further surface crosslink the water-absorbing resin composition obtained by surface crosslinking using the same or different surface crosslinking agents as the surface crosslinking agent used initially.

[0075] The method for producing the water-absorbing resin composition may, if necessary, screen the water-absorbing resin composition to adjust the particle size. The average particle diameter of the obtained particles is preferably 100 to 600 μm, more preferably 200 to 500 μm. It is preferable that the content of fine particles is small, and the content of particles of 100 μm or less is preferably 3% by weight or less, and more preferably the content of particles of 150 μm or less is 3% by weight or less.

[0076] The water-absorbing resin composition constitutes an absorber used in sanitary products together with pulp fibers. Examples of sanitary products include disposable diapers, sanitary napkins, incontinence products, pet sheets, cat litter, etc. The manufacturing method of sanitary products and the like is the same as those known (described in JP-A-2003-225565, JP-A-2006-131767, JP-A-2005-097569, etc.).

[0077] The water retention amount (g / g) of the physiological saline with respect to 0.9% by weight physiological saline of the water-absorbing resin composition is 10 to 60, preferably 20 to 55, from the viewpoint of the absorption performance of sanitary products. The water retention amount of 0.9% by weight physiological saline is measured by the method described in the examples.

[0078] The absorption amount under load (g / g) of the water-absorbing resin composition in 0.9 wt% physiological saline is 10 to 27, more preferably 12 or more. If it is less than 10, leakage is likely to occur during repeated use, which is not preferable. Also, from the viewpoints of performance balance with other physical properties and productivity, the upper limit value is preferably 26 or less. The absorption amount under load can be appropriately adjusted by the types and amounts of the internal crosslinking agent (b) and the surface crosslinking agent. Therefore, for example, when it is necessary to increase the absorption amount under load, it can be easily achieved by increasing the amounts of the internal crosslinking agent (b) and the surface crosslinking agent. The absorption amount under load (g / g) of the water-absorbing resin composition can be measured by the method described in the examples.

[0079] <Method for treating water-absorbing resin composition> The method for treating the water-absorbing resin composition of the present embodiment includes a decomposition step of decomposing the water-absorbing resin composition with an oxidizing agent. In the crosslinked polymer (A) contained in the water-absorbing resin composition, a part of the polymer is decomposed by the oxidizing agent. The water-absorbing resin composition decomposed in the decomposition step may be a water-absorbing resin composition contained in a sanitary product.

[0080] 〔Decomposition step〕 The oxidizing agent used in the decomposition step is not particularly limited as long as the crosslinked polymer (A) can be decomposed with the oxidizing agent and solubilized in water.

[0081] As the oxidizing agent used for decomposing the water-absorbing resin composition, for example, it is preferably at least one selected from the group consisting of hydrogen peroxide, ozone, hypochlorite, chlorite, chlorate, perchlorate, percarbonate, perborate, percarboxylic acid, and nitroxyl radical compounds. In the production method of the present invention, oxygen in the air can also act as an oxidizing agent, but it is preferable to use the above-exemplified oxidizing agents other than oxygen, and more preferably to use one or more oxidizing agents selected from the above group.

[0082] The addition amount (ppm) of the oxidizing agent is 100 to 50,000, more preferably 500 to 30,000, based on the weight of the water-absorbing resin composition. If it is 100 or less, efficient decomposition does not proceed and the resin composition remains. If it is 50,000 or more, it is not preferable because it cuts the chemical bonds of the main chain other than the bonds of the internal crosslinking agent (b), hindering precipitation removal with the flocculant.

[0083] As hydrogen peroxide, it is preferably dissolved in water to form an aqueous hydrogen peroxide solution, but it may also be a solution of hydrogen peroxide water in an organic solvent. As the aqueous hydrogen peroxide solution, a 30 wt% concentration one is easily available and the liquid volume is small, which is preferable.

[0084] Hypochlorites, chlorites, chlorates, perchlorates, percarbonates, perborates, etc. are preferably alkali metal salts or alkaline earth metal salts, more preferably sodium salts, potassium salts, calcium salts, and particularly preferably sodium hypochlorite salt (NaClO). These salts can also be dissolved in a solvent such as water and used. Furthermore, when treating the crosslinked polymer (A) obtained by taking it out from used diapers or contained in used diapers, hypochlorites, chlorites, chlorates, perchlorates, etc. can be used to perform solubilization treatment while performing sterilization or disinfection. Therefore, for example, when reusing the pulp contained in used diapers, there is no need to perform sterilization or disinfection again, or the amount of the disinfectant used can be reduced, which is economical.

[0085] The decomposition treatment can be carried out while heating. The temperature (°C) when the decomposition treatment is carried out while heating is preferably 25 to 85, more preferably 30 to 80, and even more preferably 35 to 75. It is preferable within this range because the decomposition of the water-absorbing resin composition by the oxidizing agent proceeds efficiently. Also, the decomposition step may be carried out under stirring or under standing (without stirring).

[0086] When the water-absorbing resin composition is contained in a sanitary product, after separating the water-absorbing resin composition from the sanitary product, the water-absorbing resin composition may be decomposed, or from the viewpoint of treatment efficiency, the water-absorbing resin composition contained in the used sanitary product may be decomposed without separating it from the sanitary product. When decomposing the water-absorbing resin composition contained in the used sanitary product without separating it from the sanitary product, the decomposition treatment may be performed, for example, while washing the sanitary product, or may be performed after washing the sanitary product.

[0087] Separation of the water-absorbing resin composition from the sanitary product can be performed by a known method. As a known method for separating the water-absorbing resin composition from a sanitary product, there can be exemplified a separation method (Japanese Patent Application Laid-Open No. 2002-273731) that utilizes the difference in specific gravity and sedimentation rate between pulp and the water-absorbing resin composition by putting pulverized diapers into a tank filled with water.

[0088] When decomposing the water-absorbing resin composition separated from the sanitary product, the treatment method of the water-absorbing resin composition of the present embodiment preferably includes a pulp recovery step of recovering pulp fibers from the sanitary product from which the water-absorbing resin composition has been separated. Regarding the content of the water-absorbing resin composition remaining in the pulp fibers separated and recovered from the sanitary product, it is 20% or less, more preferably 15% or less, and still more preferably 5% or less with respect to the weight of the absorbent resin composition contained in the sanitary product before use. When it is within this range, it can be reused as recycled pulp without deteriorating the handling and physical properties of the pulp fibers. Note that the water-absorbing resin composition remaining in the recovered pulp fibers means only the resin component excluding the absorbed water, and its content can be determined by drying to remove the water.

[0089] The water-absorbing resin composition to be subjected to the decomposition step may hold water. By reacting with an oxidizing agent, specific bonds in the crosslinked polymer (A) are selectively cleaved, so that decomposition products (such as acrylic acid oligomers) of the water-absorbing resin composition can be solubilized in the water held by the water-absorbing resin composition. Then, the decomposition products generated in the decomposition step will dissolve or be suspended in the water held by the water-absorbing resin composition before decomposition.

[0090] From the viewpoint of facilitating the sedimentation treatment in sewage treatment, the number average molecular weight (number average molecular weight in terms of PEO) of the decomposition product is preferably 500,000 or more, more preferably 550,000 or more. From the viewpoint of solubilizing the decomposition product of the crosslinked polymer (A) in water, it is preferably 1,500,000 or less, more preferably 1,200,000 or less, and particularly preferably 1,000,000 or less. When the number average molecular weight of the decomposition product is within this range, the decomposition product is soluble in water, and when the decomposition product is reacted with a flocculant, strong coarse flocs can be formed. Since the formed flocs are difficult to be broken and redispersed, the stability and treatment speed of the flocculation treatment can be significantly improved, making the sewage treatment easier. The number average molecular weight can be measured by the following measurement method.

[0091] [Measurement method of number average molecular weight] The number average molecular weight of the decomposition product of the crosslinked polymer (A) can be measured using the following apparatus and conditions. [1] Apparatus: Gel Permeation Chromatograph "HLC-8120GPC", manufactured by Tosoh Corporation [2] Columns: "TSKgel G6000PWxl" and "TSKgel G3000PWxl" [both manufactured by Tosoh Corporation] are connected in series. [3] Eluent: A solution prepared by dissolving 0.5 wt% sodium acetate in methanol / water = 30 / 70 (volume ratio). [4] Standard substance: Polyethylene glycol [5] Injection conditions: Sample concentration 0.25 wt%, column temperature 40 °C

[0092] In the decomposition step, it is preferable to add a flocculant to an aqueous solution of the decomposition product obtained by solubilizing the water-absorbing resin composition by decomposition treatment, perform flocculation treatment to form flocs of the decomposition product, and then separate the flocs with a solid-liquid separator.

[0093] Further, the method for treating the water-absorbing resin composition of the present embodiment may include a pulp separation step of separating pulp fibers derived from the sanitary product from an aqueous solution of the decomposition product obtained by solubilizing the water-absorbing resin composition by decomposition treatment after the decomposition step.

[0094] For the separation of pulp fibers in the pulp separation step, the flocculant may be added to the aqueous solution obtained in the decomposition step for flocculation treatment, and then the pulp fibers and insoluble components derived from the water-absorbing resin composition (such as decomposition residues of the crosslinked polymer (A) and water-insoluble additives) may be separated at once with a solid-liquid separator, or the pulp fibers may be separated from the aqueous solution obtained in the decomposition step using a net, or the pulp fibers may be separated from the aqueous solution obtained in the decomposition step with a solid-liquid separator.

[0095] As the flocculant, known flocculants for water treatment can be used. Examples include inorganic flocculants such as aluminum sulfate, polyaluminum chloride (PAC), aluminum chloride, ferric chloride, and polyferric sulfate (polyiron), and nonionic, anionic, cationic, and amphoteric organic polymer flocculants. From the viewpoint of flocculation efficiency, a combination of an inorganic flocculant and an anionic or amphoteric organic polymer flocculant or a cationic organic polymer flocculant is preferable.

[0096] When the water-absorbing resin composition contained in the used sanitary product is subjected to decomposition treatment without separating it from the sanitary product, the water-absorbing resin composition is solubilized in water by the decomposition, which facilitates the separation of the pulp fibers derived from the sanitary product and can increase the recovery rate of the pulp. Further, when the sanitary product contains a thermoplastic resin such as a non-woven fabric or a film, the residual amount of the water-absorbing resin composition can be reduced by this decomposition treatment when these are recovered and reused as resources, and the utilization value can be increased.

[0097] That is, when the sanitary product contains pulp fibers, the treatment method of the water-absorbing resin composition preferably includes a pulp separation step of separating the pulp fibers after the decomposition step.

[0098] The content of the water-absorbing resin composition remaining in the pulp fibers recovered in the pulp separation step is 20% or less, more preferably 15% or less, and even more preferably 5% or less, based on the weight of the absorbent resin composition contained in the sanitary product before use. Within this range, it can be reused as recycled pulp without deteriorating the handling or physical properties of the pulp fibers.

[0099] Note that the water-absorbing resin composition remaining in the recovered pulp fibers means only the resin component excluding the absorbed water, and its content can be determined by drying to remove the water.

[0100] The treatment method of the water-absorbing resin composition may further have a crushing step. The crushing step may be a step of crushing the absorber contained in the sanitary product and containing the water-absorbing resin composition, a step of crushing the sanitary product, or a step of crushing the composition obtained in the decomposition step, and there is no limitation on the order of the crushing step, the decomposition step, and the pulp separation step.

[0101] Preferable treatment methods including the crushing step include a treatment method of a water-absorbent resin composition further including a step of crushing the sanitary product before the decomposition step, a treatment method of a water-absorbent resin composition including a step of crushing the composition obtained in the decomposition step between the decomposition step and the pulp separation step, and a treatment method of a water-absorbent resin composition having a step of crushing the sanitary product simultaneously with the decomposition treatment in the decomposition treatment step.

[0102] The crushing in the crushing step can be performed using a known pulverizer or crusher, such as a disposer-type crusher (the sanitary product is thrown against the wall by a high-speed rotating turntable and crushed by a fixed or variable hammer attached to the peripheral part of the turntable and a fixed blade on the wall, etc.) used for garbage crushers, a cutter mill, a single-shaft crusher, a double-shaft crusher, a coaxial crusher, a hammer crusher, and a ball mill. Among them, since the materials of sanitary products often include plastic sheets, non-woven fabrics, and stretchable materials, a disposer-type crusher and a cutter mill that cut with blades while rotating at high speed are particularly suitable.

[0103] When crushing the sanitary product, the crushed product obtained in the crushing step may be an aqueous suspension. Methods for obtaining the crushed product as an aqueous suspension include a method of adding water to swell the sanitary product and then crushing it, a method of adding water while crushing, and a method of adding water after crushing. Among them, from the viewpoint of reducing the load on the crusher, a method of adding water to swell the sanitary product and then crushing it is preferable.

[0104] The preferable range of the size of the crushed product of the sanitary product after crushing depends on the method of separating the water-absorbent resin composition from the sanitary product, but it is preferably crushed so that it is 300 mm or less, more preferably 100 mm or less in the longitudinal direction. If the length of a single piece is 300 mm or less, it is considered that the treatment efficiency with the decomposing agent is improved, the treatment time can be shortened, and the separation of the members after crushing becomes easier. The size of the crushed product can be appropriately adjusted according to the type of the above-mentioned pulverizer or crusher and treatment conditions.

[0105] Note that the hygiene product to be shredded may be shredded as it is, shredded after separating the absorbent body from the hygiene product, or shredded after separating the pulp fibers and the water-absorbing resin composition from the hygiene product.

[0106] In the crushing step, when crushing the composition obtained by the solubilization treatment, it can be carried out by crushing the above composition using the above-known crusher or pulverizer.

[0107] In the decomposition step, when carrying out the step of crushing the hygiene product simultaneously with the decomposition treatment, it can be carried out by putting the hygiene product and the above treatment agent into the above-known crusher or pulverizer and crushing them.

[0108] When the hygiene product to be treated contains dirt such as urine and feces, such as used disposable diapers, it is preferably sterilized in advance from a hygienic point of view. The sterilization treatment may be carried out at any time after the separation step, between these steps, or during a plurality of these steps in the decomposition step, separation step, and separation step, and sterilization and disinfection treatment may be carried out on the insoluble matter derived from the pulp fibers and the water-absorbing resin composition separated and recovered in the separation step.

[0109] Examples of the sterilization method include a method by heat treatment at 100°C or higher, a method by ultraviolet irradiation, a method by a bactericide, a method using an aqueous solution in which ozone is dissolved as described in JP-A-2016-000881, a method by hypochlorous acid (salt) as described in JP-A-2013-150977, and a method of using a gas of an alkylating agent such as ethylene oxide or formaldehyde in the presence of ozone.

[0110] As one of the preferred embodiments of the method for treating the water-absorbing resin composition, a disposer wastewater treatment system can be mentioned. Disposer wastewater treatment is a system in which food waste is crushed by a disposer attached to the sink drain in the kitchen and discharged into the sewer or septic tank together with the wastewater from the water supply. It is a wastewater treatment system that reduces waste and is excellent in terms of hygiene and convenience, and has been widely popularized especially in apartment buildings and the like. Recently, efforts have been made to apply a similar system to used sanitary products (for example, Japanese Patent Application Laid-Open No. 2020-054936, etc.). In order to deploy the wastewater treatment system to sanitary products, it is important to reduce the water-swellability of the water-absorbing resin composition and prevent poor drainage and pipe blockage due to deposition and adhesion in the drain pipe. The treatment method of the present invention is preferable because it can solubilize the water-absorbing resin and thus can solve such problems.

[0111] By the method for treating the water-absorbing resin composition, the insoluble components derived from the pulp fibers and the water-absorbing resin composition are recovered together or separately. In the separation step, the insoluble components derived from the separated and recovered pulp fibers and water-absorbing resin can be recycled and used as solid fuel or recycled pulp. Among them, the insoluble components derived from the pulp fibers and the water-absorbing resin composition obtained by the method for treating the water-absorbing resin composition have the characteristic of low moisture content, so not only is the combustion efficiency excellent during incineration treatment, but it can also be preferably used for recycling as solid fuel or the like. In addition, the pulp fibers recovered alone among the insoluble components contain few components derived from the water-absorbing resin composition and are also useful for recycling as recycled pulp.

[0112] Further, the method for treating the water-absorbing resin composition may further include a step of drying the insoluble components derived from the pulp fibers and the water-absorbing resin composition separated and recovered in the separation step. Drying can be performed by a known method.

Examples

[0113] Hereinafter, the present invention will be further described by way of examples and comparative examples, but the present invention is not limited thereto. Hereinafter, unless otherwise specified, "parts" means parts by weight and "%" means % by weight.

[0114] <Evaluation Method> Each evaluation item was measured by the following method in a room at 25 ± 2°C and a humidity of 50 ± 10%. The temperature of the physiological saline used was adjusted to 25°C ± 2°C in advance before use.

[0115] 〔Measurement Method of Water Retention Capacity with Respect to 0.9% by Weight Physiological Saline〕 1.00 g of the measurement sample was placed in a tea bag (20 cm long, 10 cm wide) made of a nylon mesh with an opening size of 63 μm (JIS Z8801-1:2006), immersed in 1,000 ml of physiological saline (salt concentration 0.9%) without stirring for 1 hour, then pulled out and hung for 15 minutes to drain the water. Thereafter, the entire tea bag was placed in a centrifuge and centrifuged at 150 G for 90 seconds to remove the excess physiological saline, and the weight (h1) including the tea bag was measured. The water retention capacity was determined from the following formula. The temperature of the physiological saline and the measurement atmosphere used was 25°C ± 2°C. Water retention capacity (g / g) = (h1) - (h2) Note that (h2) is the weight of the tea bag measured by the same operation as above when there is no measurement sample.

[0116] 〔Measurement Method of Absorption Amount under Load with Respect to 0.9% by Weight Physiological Saline〕 In a cylindrical plastic tube (inner diameter: 25 mm, height: 34 mm) with a nylon mesh having an opening of 63 μm (JIS Z8801-1:2006) attached to the bottom surface, 0.16 g of a measurement sample sieved in the range of 250 to 500 μm using a standard sieve was weighed. After making the cylindrical plastic tube vertical and arranging the measurement sample on the nylon mesh so that it had a substantially uniform thickness, a weight (weight: 210.6 g, outer diameter: 24.5 mm) was placed on this measurement sample. After weighing the total weight (M1) of this entire cylindrical plastic tube, the cylindrical plastic tube containing the measurement sample and the weight was vertically placed in a petri dish (diameter: 12 cm) containing 60 ml of physiological saline (salt concentration 0.9%) with the nylon mesh side facing down and immersed for 60 minutes. After 60 minutes, the cylindrical plastic tube was pulled out of the petri dish, tilted obliquely to collect the water adhering to the bottom at one place and let it drip as water droplets to remove the excess water. Then, the total weight (M2) of the cylindrical plastic tube containing the measurement sample and the weight was weighed, and the absorption amount under load was determined from the following formula. The temperature of the physiological saline and the measurement atmosphere used was 25°C ± 2°C. Absorption amount under load (g / g) = {(M2) - (M1)} / 0.16

[0117] 〔Decomposition index of the water-absorbing resin composition〕 Weighed 0.600 g of the water-absorbing resin composition into a 50-ml conical beaker, then added it to 20 ml of a 5% aqueous sodium hypochlorite solution, and stirred at 25°C at a speed of 200 rpm using a Teflon (registered trademark) spatula (manufactured by Esco Corporation). After 30 minutes, the stirring was stopped. Then, the entire amount was transferred to a tea bag (20 cm long, 10 cm wide) made of a nylon mesh with an opening size of 63 μm (JIS Z8801-1:2006). At this time, a spatula or the like may be used as needed. Then, the tea bag was placed in a centrifuge and centrifugally dehydrated at 150 G for 90 seconds to remove excess moisture. The entire content of the tea bag was transferred to a SUS-made vat (15 cm long, 25 cm wide, 5 cm high; vat weight B1). After standing in a hot air dryer adjusted to 150°C for 3 hours, it was taken out, cooled in a desiccator (model number 3-6011-24, manufactured by AS ONE Corporation) containing silica gel desiccant until it reached room temperature, taken out after cooling, and the weight of the SUS-made vat was measured (weight B2) to obtain the pure content weight β (= B2 - B1) of the water-absorbing resin composition after decomposition. Furthermore, the decomposition index was calculated according to the following (Formula 1). Similarly, 0.600 g of the water-absorbing resin composition was placed in a SUS-made vat (15 cm long, 25 cm wide, 5 cm high; vat weight W1). After standing in a hot air dryer adjusted to 150°C for 3 hours, it was taken out, cooled in a desiccator (model number 3-6011-24, manufactured by AS ONE Corporation) containing silica gel desiccant until it reached room temperature, taken out after cooling, and the weight of the SUS-made vat was measured (weight W2) to obtain the pure content weight α (= W2 - W1) of the water-absorbing resin composition before decomposition. Furthermore, the decomposition index was calculated according to the following (Formula 1). The temperature of the measurement atmosphere was 25°C ± 2°C. Decomposition index = (1 - β / α) × 100 (Formula 1) β: Pure content weight of the water-absorbing resin composition after decomposition α: Pure content weight of the water-absorbing resin composition before decomposition

[0118] In this specification, the pure component mass of the water-absorbing resin composition before decomposition refers to the weight excluding the amount of water contained in the water-absorbing resin composition, and the amount of water was obtained by the following method for measuring the water content. The pure component mass of the water-absorbing resin composition after decomposition refers to the weight of the water-absorbing resin composition remaining after decomposing the water-absorbing resin composition with an oxidizing agent, washing it with water 50 times the weight before decomposition, and drying it at 150 °C for 3 hours.

[0119] 〔Method for Measuring Water Content of Water-Absorbing Resin Composition〕 Using an infrared moisture meter [(manufactured by KETT Co., Ltd., JE400, etc.: 120 ± 5 °C, 30 minutes, ambient humidity before heating 50 ± 10% RH, lamp specification 100V, 40W)], 2.00 g of the water-absorbing resin composition was heated, and the ratio of the weight loss of the water-absorbing resin composition before drying to the weight was taken as the water content (%).

[0120] 〔Visual Confirmation of Remaining Water-Absorbing Resin Composition〕 Weighed 0.600 g of the water-absorbing resin composition into a 50 ml conical beaker, then added it to 20 ml of a 5% aqueous sodium hypochlorite solution, stirred it at 25 °C at a speed of 200 rpm using a Teflon (registered trademark) spatula (manufactured by ESK Co., Ltd.), and stopped stirring after 30 minutes. Five minutes after stopping the stirring, the beaker was visually observed from the side to confirm the coloring state of the water-absorbing resin composition, and the state of the water-absorbing resin composition after decomposition was evaluated according to the following criteria. The less coloring of the water-absorbing resin composition means that more of it was solubilized in water and efficient decomposition occurred. 〇: There is no or very little part where the coloring of the water-absorbing resin composition is confirmed. △: There is a part where the coloring of the water-absorbing resin composition is confirmed. ×: There are many parts where the coloring of the water-absorbing resin composition is confirmed.

[0121] 〔Method for Measuring Number-Average Molecular Weight of Decomposition Product〕 In the same manner as the measurement of the decomposition index of the water-absorbing resin composition, 0.600 g of the water-absorbing resin composition was weighed into a 50-ml conical beaker, and then added to 20 ml of an aqueous 5% sodium hypochlorite solution. At 25°C, it was stirred at a speed of 200 rpm using a Teflon (registered trademark) spatula (manufactured by Esco Corporation), and the stirring was stopped after 30 minutes. Next, the entire amount was transferred to a tea bag (20 cm in length, 10 cm in width) made of a nylon mesh with an aperture of 63 μm (JIS Z8801-1:2006). At this time, a part of the solution in which the decomposition products filtered by the tea bag were dissolved was collected as a sample for molecular weight measurement. The number average molecular weight of the decomposition products was measured under the following conditions. 〔Measurement conditions for number average molecular weight〕 [1] Apparatus: Gel permeation chromatograph "HLC-8120GPC", manufactured by Tosoh Corporation [2] Columns: "TSKgel G6000PWxl" and "TSKgel G3000PWxl" [both manufactured by Tosoh Corporation] were connected in series. [3] Eluent: A solution prepared by dissolving 0.5 wt% of sodium acetate in methanol / water = 30 / 70 (volume ratio). [4] Standard substance: Polyethylene glycol [5] Injection conditions: Sample concentration 0.25 wt%, column temperature 40°C

[0122] <Evaluation of decomposition of water-absorbing resin composition> 〔Example 1〕 157 parts (2.18 mol parts) of acrylic acid, 0.3 part (0.0021 mol part) of internal cross-linking agent (b-1) (N,N-diacryloylhydrazine), and 344.65 parts of deionized water were maintained at 3 °C while being stirred and mixed. Nitrogen was introduced into this mixture to reduce the dissolved oxygen content to 1 ppm or less, and then 0.63 part of 1% hydrogen peroxide aqueous solution, 1.1774 parts of 2% ascorbic acid aqueous solution, and 2.355 parts of 2% 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] aqueous solution were added and mixed to initiate polymerization. After the temperature of the mixture reached 90 °C, polymerization was carried out at 90 ± 2 °C for about 5 hours to obtain a hydrogel (1). Next, 128.42 parts of 48.5% sodium hydroxide aqueous solution (1.58 mol parts as sodium hydroxide) were added and mixed while cutting 502.27 parts of this hydrogel (1) into small pieces with a mincer to obtain hydrogel (1) particles. Further, the hydrogel (1) particles were dried with a through-air band dryer {150 °C, wind speed 2 m / s}, then pulverized with a juicer mixer, sieved, and adjusted to a particle size range of 710 to 150 μm in aperture to obtain dry particles (DP-1). While vigorously stirring 100 parts of this dry particles (DP-1), 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) were added while spraying and mixed, and left standing at 150 °C for 30 minutes for surface cross-linking to obtain a water-absorbing resin composition (P-1). The water content rate of the water-absorbing resin composition (P-1) was 3.4%. By the above method, the water retention capacity, absorption amount under load, and decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0123] [Example 2] In Example 1, a water-absorbing resin composition (P-2) was obtained in the same manner as in Example 1, except that the adjustment of the particle size range with an aperture of 710 to 150 μm was changed to the adjustment of the particle size range with an aperture of 1.2 mm to 150 μm. By the above method, the water retention capacity, absorption amount under load, and decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0124] [Example 3] In Example 1, except that the addition of 128.42 parts of a 48.5% sodium hydroxide aqueous solution was changed to the addition of 62.4 parts (0.76 mol part as sodium hydroxide) of a 48.5% sodium hydroxide aqueous solution, and the addition of 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) was changed to 4.00 parts, a water-absorbing resin composition (P-3) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0125] [Example 4] In Example 1, except that the addition of 128.42 parts of a 48.5% sodium hydroxide aqueous solution was changed to the addition of 142.69 parts (1.73 mol part as sodium hydroxide) of a 48.5% sodium hydroxide aqueous solution, and the addition of 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) was changed to 6.00 parts, a water-absorbing resin composition (P-4) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0126] [Example 5] In Example 1, except that 0.3 part (0.0021 mol part) of the internal crosslinking agent (b-1) (N,N-diacryloylhydrazine) was changed to 0.72 part (0.050 mol part), and the addition of 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) was changed to 2.50 parts, a water-absorbing resin composition (P-5) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0127] [Example 6] In Example 1, except that 0.3 part (0.0021 mol part) of the internal crosslinking agent (b-1) (N,N-diacryloylhydrazine) was changed to 6.5 parts (0.45 mol part), and the addition of 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) was changed to 1.00 part, a water-absorbing resin composition (P-6) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0128] 〔Comparative Example 1〕 157 parts (2.18 mol parts) of acrylic acid, 0.55 part (0.0021 mol part) of internal cross-linking agent (b-2) {pentaerythritol triallyl ether}, and 344.65 parts of deionized water were maintained at 3 °C while being stirred and mixed. Nitrogen was introduced into this mixture to reduce the dissolved oxygen content to 1 ppm or less, and then 0.63 part of 1% hydrogen peroxide aqueous solution, 1.1774 parts of 2% ascorbic acid aqueous solution, and 2.355 parts of 2% 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] aqueous solution were added and mixed to initiate polymerization. After the temperature of the mixture reached 90 °C, polymerization was carried out at 90 ± 2 °C for about 5 hours to obtain a hydrogel (H1). Next, 128.42 parts of 48.5% sodium hydroxide aqueous solution (1.73 mol parts as sodium hydroxide) were added and mixed while cutting 502.27 parts of this hydrogel (H1) with a mincer to obtain hydrogel (H1) particles. Further, after drying the hydrogel (H1) particles in a vent-type band dryer {150 °C, wind speed 2 m / s}, they were pulverized with a juicer mixer, sieved, and adjusted to a particle size range of 710 - 150 μm to obtain dried particles (DR-1). While rapidly stirring 100 parts of this dried particle (DR-1), 5.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) were added while spray-spraying and mixed, and left standing at 150 °C for 30 minutes for surface cross-linking to obtain a water-absorbing resin composition (R-1). By the above method, the water retention capacity, absorption amount under load, and decomposition index were measured, and visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1. Regarding the number average molecular weight of the decomposition products, since no peak of acrylic acid oligomer could be confirmed in the obtained chromatograph, "-" was described.

[0129] [Comparative Example 2] 157 parts (2.18 mol parts) of acrylic acid, 0.3 part (0.0021 mol part) of internal crosslinking agent (b-1) (N,N-diacryloylhydrazine), and 344.65 parts of deionized water were maintained at 3°C while being stirred and mixed. After introducing nitrogen into this mixture to reduce the dissolved oxygen content to 1 ppm or less, 0.63 part of 1% hydrogen peroxide aqueous solution, 1.1774 parts of 2% ascorbic acid aqueous solution, and 2.355 parts of 2% 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] aqueous solution were added and mixed to initiate polymerization. After the temperature of the mixture reached 90°C, polymerization was carried out at 90 ± 2°C for about 5 hours to obtain a hydrogel (H2). Next, while chopping 502.27 parts of this hydrogel (H2) with a mincer, 128.42 parts of 48.5% sodium hydroxide aqueous solution (1.73 mol parts as sodium hydroxide) were added and mixed to obtain hydrogel (H2) particles. Further, after drying the hydrogel (H2) particles with a through-air band dryer {150°C, wind speed 2 m / s}, they were pulverized with a jet mixer, sieved, and adjusted to a particle size range of 710 - 150 μm to obtain dry particles (DR-2). While vigorously stirring 100 parts of these dry particles (DR-2), 2.00 parts of a 2% water / methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water / methanol = 70 / 30) were added while spray-spraying and mixed, and left standing at 150°C for 30 minutes for surface crosslinking to obtain a water-absorbing resin composition (R-2). By the above method, the water retention capacity, absorption amount under load, and decomposition index were measured, and visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0130] [Comparative Example 3] In Example 1, a water-absorbing resin composition (R-3) was obtained in the same manner as in Example 1, except that 128.42 parts of 48.5% sodium hydroxide aqueous solution was changed to 178.36 parts (2.16 mol parts as sodium hydroxide). By the above method, the water retention capacity, absorption amount under load, and decomposition index were measured, and visual confirmation of the water-absorbing resin composition remaining after decomposition and the number average molecular weight of the decomposition products were measured, and the results are shown in Table 1.

[0131] [Comparative Example 4] In Example 1, an aqueous sodium hydroxide solution (48.5%) of 128.42 parts was changed to 53.9 parts (0.65 mol part as sodium hydroxide), and an aqueous sodium hydroxide solution (48.5%) of 128.42 parts was changed to 53.9 parts (0.65 mol part as sodium hydroxide), and a water-absorbing resin composition (R-4) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the water-absorbing resin composition remaining after decomposition was visually confirmed, and the number average molecular weight of the decomposition product was measured, and the results are shown in Table 1.

[0132] [Comparative Example 5] In Example 1, an aqueous sodium hydroxide solution (48.5%) of 128.42 parts was changed to 170.8 parts (2.07 mol part as sodium hydroxide), and an aqueous sodium hydroxide solution (48.5%) of 128.42 parts was changed to 170.8 parts (2.07 mol part as sodium hydroxide), and a water-absorbing resin composition (R-5) was obtained in the same manner as in Example 1. By the above method, the water retention amount, the absorption amount under load, and the decomposition index were measured, and the water-absorbing resin composition remaining after decomposition was visually confirmed, and the number average molecular weight of the decomposition product was measured, and the results are shown in Table 1.

[0133] <Evaluation of the water-absorbing resin composition remaining in the pulp> [Example 7] The water-absorbing resin composition (P-1) was uniformly spread by hand on a non-woven fabric (N-1) {non-woven fabric basis weight: 25 g / m 2} which is a diffusible member (V) so as to have a basis weight of 200 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd., and water was uniformly sprayed from above so as to be 17.5 g / m 2 to obtain an absorber (S-1'). This absorber (S-1') was cut into a rectangle of 10 cm × 15 cm, and the absorber (S-1') was sandwiched between a water-permeable sheet (N-2) {basis weight: 15.5 g / m 2 , Filter Paper No. 2 manufactured by Advantec Co., Ltd.} which is a diffusible member (V) of the same size as the absorber (S-1') to obtain an absorber (S-1). Further, a polyethylene sheet (polyethylene film UB-1 manufactured by Tamapoly Co., Ltd.) was used as the back sheet on the back surface, and a non-woven fabric (N-1) (non-woven fabric basis weight: 25 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd.) was arranged on the outermost surface to prepare an absorbent article (Q-1) [the water-absorbing resin composition (P-1) contained in the water-absorbing article (Q-1) is 3 g].

[0134] The absorbent article (Q-1) was placed in a 5 L polyethylene beaker, and 150 g of ion-exchanged water at 25°C was further added to allow the absorbent article (Q-1) to absorb water completely. Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water at 25°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, a methyl red solution (water / ethanol = 10 / 90) was used as the coloring solution of the water-absorbing resin composition, the water-absorbing resin composition remaining in the pulp was visually confirmed, and the results evaluated according to the following visual evaluation criteria are shown in Table 2. The less coloring of the water-absorbing resin composition, the less water-absorbing resin composition remains in the pulp, which means that the pulp is easy to reuse. <Visual evaluation criteria for the water-absorbing resin composition remaining in the pulp> 〇: There is no part where the coloring of the water-absorbing resin composition is confirmed, or it is extremely slight. △: There is a part where the coloring of the water-absorbing resin composition is confirmed. ×: There are many parts where the coloring of the water-absorbing resin composition is confirmed.

[0135] [Example 8] The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 30°C, and 150 g of ion-exchanged water temperature-controlled to 30°C was allowed to be fully absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water temperature-controlled to 30°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a hot air dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0136] 〔Example 9〕 The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 35°C, and 150 g of ion-exchanged water temperature-controlled to 35°C was allowed to be fully absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water temperature-controlled to 35°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a hot air dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0137] 〔Example 10〕 The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 50°C, and 150 g of ion-exchanged water thermostatically controlled at 50°C was allowed to be fully absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water thermostatically controlled at 50°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0138] [Example 11] The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 80°C, and 150 g of ion-exchanged water thermostatically controlled at 80°C was allowed to be fully absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water thermostatically controlled at 80°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes using a three-one motor. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0139] [Example 12] In Example 9, the same operations as in Example 9 were carried out except that 0.05 g of a 1% aqueous solution of sodium hypochlorite was changed to 15 g, and the water-absorbing resin composition remaining on the pulp was visually confirmed.

[0140] [Example 13] In Example 9, the same operations as in Example 9 were carried out except that 0.05 g of a 1% aqueous solution of sodium hypochlorite was changed to 0.3 g, and the water-absorbing resin composition remaining on the pulp was visually confirmed.

[0141] [Example 14] The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 50°C, and 150 g of ion-exchanged water thermostatically controlled at 50°C was allowed to be fully absorbed by the absorbent article (Q-1). Next, 3 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water thermostatically controlled at 50°C were added, and after standing for 30 minutes, the whole amount was recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, taken out, and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining on the pulp was visually confirmed in the same manner as in Example 7.

[0142] [Example 15] In Example 9, the same operations as in Example 9 were carried out except that 0.05 g of a 1% aqueous solution of sodium hypochlorite was changed to 0.1 g of a 3% aqueous solution of hydrogen peroxide, and the water-absorbing resin composition remaining on the pulp was visually confirmed.

[0143] [Example 16] The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 20°C, and 150 g of ion-exchanged water thermostatically controlled at 20°C was allowed to be completely absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water thermostatically controlled at 20°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and nonwoven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Next, after removing the nonwoven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0144] 〔Example 17〕 The absorbent article (Q-1) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 85°C, and 150 g of ion-exchanged water thermostatically controlled at 85°C was allowed to be completely absorbed by the absorbent article (Q-1). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water thermostatically controlled at 85°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and nonwoven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Next, after removing the nonwoven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread out in a SUS vat (20 cm in length × 30 cm in width × 5 cm in height), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0145] 〔Example 18〕 The water-absorbing resin composition (P-6) had a basis weight of 200 g / m 2It was evenly scattered by hand on the nonwoven fabric (N-1) {basis weight of nonwoven fabric: 25 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd.}, and water was evenly sprayed from above at 17.5 g / m 2 to obtain an absorber (S-2'). This absorber (S-2') was cut into a rectangle of 10 cm × 15 cm, and the absorber (S-2') was sandwiched between a water-permeable sheet (N-2) {basis weight 15.5 g / m 2 , Filter Paper No. 2 manufactured by Advantec Co., Ltd.} which is a diffusive member (V) of the same size as the absorber (S-2') to obtain an absorber (S-2). Further, a polyethylene sheet (polyethylene film UB-1 manufactured by Tamapoly Co., Ltd.) was used as the back sheet on the back surface, and the nonwoven fabric (N-1) (basis weight of nonwoven fabric: 25 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd.) was placed on the outermost surface to prepare an absorbent article (Q-2) [The water-absorbing resin composition (P-6) contained in the water-absorbing article was 3 g].

[0146] The absorbent article (Q-2) was placed in a 5 L polyethylene beaker installed on a mantle heater set at 25°C, and further 150 g of ion-exchanged water at 25°C was allowed to be fully absorbed by the absorbent article (Q-2). Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water at 25°C were added, and using a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.), it was stirred at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and nonwoven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the nonwoven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread in a SUS vat (vertical 20 cm × horizontal 30 cm × height 5 cm), left standing in a down-draft dryer set at 130°C for 6 hours to be sufficiently dried, and then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0147] 〔Comparative Example 6〕 The water-absorbing resin composition (R-2) had a basis weight of 200 g / m 2It was uniformly scattered by hand on a non-woven fabric (N-1) {basis weight of non-woven fabric: 25 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd.}, and water was uniformly sprayed from above so as to be 17.5 g / m 2 to obtain an absorber (RS-1'). This absorber (RS-1') was cut into a rectangle of 10 cm × 15 cm, and the absorber (RS-1') was sandwiched between a water-permeable sheet (N-2) {basis weight 15.5 g / m 2 , Filter Paper No. 2 manufactured by Advantec Co., Ltd.} to obtain an absorber (RS-1). Further, a polyethylene sheet (polyethylene film UB-1 manufactured by Tamapoly Co., Ltd.) was used as the back sheet on the back surface, and a non-woven fabric (N-1) (basis weight of non-woven fabric: 25 g / m 2 , 2.2T 44-SMK manufactured by Toyobo Co., Ltd.) was arranged on the outermost surface to prepare an absorbent article (Q-3) [The water-absorbing resin composition (R-1) contained in the water-absorbing article was 3 g].

[0148] The absorbent article (Q-3) was placed in a 5 L polyethylene beaker installed in a mantle heater set at 25°C, and 150 g of ion-exchanged water at 25°C was further absorbed by the absorbent article (Q-3) in its entirety. Next, 0.05 g of a 1% aqueous solution of sodium hypochlorite and 1 L of ion water at 25°C were added, and a homogenizer (product name: Excel Auto Homogenizer, manufactured by Nippon Seiki Co., Ltd.) was used to stir at 1000 rpm for 5 minutes. The members of the absorbent article such as pulp and non-woven fabric adhering to the homogenizer were completely recovered into the disposable cup using ion water. Then, after removing the non-woven fabric floating on the water surface, it was filtered through a 400-mesh SUS wire mesh and washed with ion water. Further, the pulp remaining on the wire mesh was spread in a SUS vat (length 20 cm × width 30 cm × height 5 cm), left standing in a hot air dryer set at 130°C for 6 hours to be sufficiently dried, and then taken out and allowed to cool to room temperature. Next, the water-absorbing resin composition remaining in the pulp was visually confirmed in the same manner as in Example 7.

[0149] The evaluation results related to each example and comparative example are shown in Tables 1 and 2. In addition, since the amount of the decomposition product related to Comparative Example 1 was small, the number average molecular weight could not be measured.

[0150]

Table 1

[0151]

Table 2

[0152] From the results shown in Table 1, it can be seen that the water-absorbing resin compositions according to Examples 1 to 6 have a larger decomposition index than the water-absorbing resin compositions according to Comparative Examples 1 to 5, less water-absorbing resin composition remaining after decomposition, and a larger number-average molecular weight of the decomposition products dissolved in water. The fact that the number-average molecular weight of the decomposition products dissolved in water of the water-absorbing resin compositions according to Examples 1 to 6 is larger than that of the decomposition products dissolved in water of the water-absorbing resin compositions according to Comparative Examples 1 to 5 means that it is easier to separate pulp fibers, non-woven fabrics, polyethylene, etc., which are members of sanitary products, from the acrylic acid oligomer. Also, in the treatment of sanitary products containing the water-absorbing resin composition, from the results shown in Table 2, it can be seen that in Examples 7 to 18 using the water-absorbing resin composition of the present invention, there is almost no water-absorbing resin composition remaining in the pulp, and it is easier to reuse the pulp compared to the comparative examples.

Claims

1. A water-absorbing resin composition containing a crosslinked polymer (A) having, as essential constituent units, one or more monomers (A1) selected from the group consisting of a water-soluble unsaturated monocarboxylic acid (a1) and salts thereof, and an internal crosslinking agent (b) represented by the following general formula (1), wherein the content of the internal crosslinking agent (b) units in the crosslinked polymer (A) is 0.005 mol% or more and 3.000 mol% or less, and the water-absorbing resin composition has a decomposition index represented by the following formula (1) of 90 or more. 【Chemical 1】 (In general formula (1), R １ is each independently one or more selected from hydrogen, an alkyl group, a hydroxy group, an amino group, a mercapto group, a substituted carbonyl group, and any alkyl group having one or more selected from a hydroxy group, an amino group, a mercapto group, and a substituted carbonyl group as substituents.) Decomposition index = (1 - β / α) × 100 (Formula 1) β: The pure content weight of the water-absorbing resin composition after decomposition α: The pure content weight of the water-absorbing resin composition before decomposition

2. An absorber containing the water-absorbing resin composition according to Claim 1.

3. A sanitary product containing the absorber according to Claim 2.

4. A sanitary product according to Claim 3, containing pulp fibers.

5. A method for treating a water-absorbing resin composition, comprising a decomposition step of decomposing the water-absorbing resin composition according to Claim 1 with an oxidizing agent.

6. The method for treating a water-absorbing resin composition according to Claim 5, wherein the water-absorbing resin composition is contained in a sanitary product.

7. The method for treating a water-absorbing resin composition according to Claim 5 or 6, wherein the sanitary product contains pulp fibers, and comprising a pulp separation step of separating the pulp fibers derived from the sanitary product after the decomposition step.

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