Manufacturing method for solubilized polymer

Abstract

[Problem] The present invention addresses the problem of providing a new manufacturing method for a solubilized polymer that is obtained by degrading a water-absorbing resin and that is capable of shortening the degradation time of the water-absorbing resin. [Solution] A manufacturing method for a solubilized polymer, said method involving preparing a water-absorbing resin (A) which serves as an object to be degraded and bringing the water-absorbing resin (A) into contact with a degrading agent, wherein: (i) the aforementioned contact is carried out in the presence of a water-absorbing resin (B) which is higher in degradability than the water-absorbing resin (A); or (ii) the water-absorbing resin (B) which is higher in degradability than the water-absorbing resin (A) is brought into contact with a degrading agent, a degradation product is prepared separately, and the aforementioned contact is carried out in the presence of the degradation product.

Description

Method for producing solubilizing polymers

[0001] The present invention relates to a method for producing a solubilized polymer.

[0002] Water-absorbent polymers used in various fields, including sanitary products such as sanitary napkins and disposable diapers, have a three-dimensional crosslinked structure of water-soluble polymers, so they absorb water and swell but do not dissolve. Therefore, for example, disposable diapers and the like that use these water-absorbent polymers are currently disposed of as waste by incineration or landfill after use, which is not desirable from the standpoint of environmental conservation.

[0003] To solve this problem, Patent Document 1 discloses a disposal method in which water-absorbing polymers are decomposed with hydrogen peroxide, but this method has problems in that it requires the use of a large amount of hydrogen peroxide and a long treatment time.

[0004] In order to solve this problem, Patent Document 2 proposes a decomposition method characterized by bringing a decomposition agent containing periodate as an oxidizing agent into contact with a water-absorbent polymer to decompose the water-absorbent polymer. Example 1 of the document shows an example in which sodium periodate is dissolved in distilled water, and an acrylate cross-linked polymer (water-absorbent polymer / granular: the same used in "Merrys" manufactured by Kao Corporation) is added to and mixed with this aqueous solution to decompose the water-absorbent polymer.

[0005] Japanese Patent Laid-Open No. 4-317784 Japanese Patent Laid-Open No. 2001-316519

[0006] The present inventors have found that the decomposition time of a water-absorbent resin cannot be shortened simply by using a decomposing agent such as an oxidizing agent as shown in the conventional examples.

[0007] Therefore, an object of the present invention is to provide a novel method for producing a solubilized polymer obtained by decomposing a water-absorbent resin, which method can shorten the decomposition time of the water-absorbent resin.

[0008] One aspect for achieving the above object is a method for producing a solubilized polymer, which comprises preparing a water-absorbent resin (A) that is a water-absorbent resin to be decomposed, and contacting the water-absorbent resin (A) with a decomposing agent, (i) comprising carrying out the contact in the presence of a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A), or (ii) comprising bringing a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A) into contact with a decomposing agent to separately prepare a decomposed product, and carrying out the contact in the presence of the decomposed product.

[0009] According to the present invention, it is possible to provide a novel method for producing a solubilized polymer obtained by decomposing a water-absorbent resin, which can shorten the decomposition time of the water-absorbent resin.

[0010] FIG. 1 is a diagram for schematically explaining the concepts of water-absorbing resin (A) and water-absorbing resin (B).

[0011] Preferred embodiments of the present invention will be described below. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims. Furthermore, the embodiments described in this specification can be arbitrarily combined to form other embodiments.

[0012] As used herein, the term "(meth)acrylic" encompasses both acrylic and methacrylic. Thus, for example, the term "(meth)acrylic acid" encompasses both acrylic acid and methacrylic acid. "Acid (salt)" means acid and / or its salt.

[0013] [1] Method for Producing Solubilized Polymer One aspect of the present invention is a method for producing a solubilized polymer, comprising preparing a water-absorbent resin (A) that is a water-absorbent resin to be decomposed, and contacting the water-absorbent resin (A) with a decomposing agent, (i) comprising carrying out the contact in the presence of a water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A), or (ii) comprising bringing a water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A) into contact with a decomposing agent to separately prepare a decomposed product, and carrying out the contact in the presence of the decomposed product.

[0014] (1-1) Water-absorbent resin First, the "water-absorbent resin" will be explained.

[0015] The water-absorbent resin is a water-swellable, water-insoluble polymer gelling agent, and refers to a conventional water-absorbent resin having a water absorption capacity of 10 to 1000 times, although it is not particularly limited thereto. More specifically, the water-absorbent resin before absorbing the liquid to be absorbed (hereinafter also referred to as "initial water-absorbent resin") can satisfy the physical property of "water-swellability" such that the CRC is 5 g / g or more. The definition of CRC will be described later. Unless otherwise specified, the water-absorbent resin disclosed in this specification refers to at least one of the water-absorbent resin (A) and the water-absorbent resin (B).

[0016] In this technical field, the water-absorbent resin may refer to individual particles, but in this specification, unless otherwise specified, it refers to an aggregate of particles.

[0017] The water-absorbent resin may be a polymer derived from a carboxyl group-containing unsaturated monomer. The water-absorbent resin may include a polymer having a partially neutralized carboxyl group. Specific examples of the water-absorbent resin include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, cellulose-based resins, (meth)acrylate crosslinked polymers, crosslinked products of saponified (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.

[0018] In this specification, the term "water-absorbent resin" is not limited to a case where the total amount (100% by mass) of the water-absorbent resin is the water-absorbent resin alone, but may also refer to a water-absorbent resin composition containing additives. Furthermore, in this specification, the term "water-absorbent resin" may include a fresh water-absorbent resin newly produced in a water-absorbent resin production process, a used water-absorbent resin contained in a used absorbent article, or a mixture thereof. Here, the term "used absorbent article" refers to a used sanitary material that has absorbed bodily fluids such as urine and blood. Examples of such sanitary material include disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), and pet sheets. The term "used water-absorbent resin" encompasses water-absorbent resins that have absorbed bodily fluids such as urine and blood in used absorbent articles, water-absorbent resins that have not absorbed bodily fluids such as urine and blood but were used in the manufacture of absorbent articles, and fine powder or non-standard water-absorbent resins generated during the manufacturing process of the water-absorbent resin. The used water-absorbent resin contained in a used absorbent article may become a hydrous gel in which water such as urine and water has been absorbed. Therefore, the water-absorbent resin may include a water-absorbent resin in a hydrogel state. In this way, the water-absorbent resin (A) and the water-absorbent resin (B) may each independently be a used water-absorbent resin or a fresh water-absorbent resin newly produced in a water-absorbent resin production process.

[0019] In this specification, the mass of the water-absorbent resin is a numerical value converted into a solid content unless otherwise specified. When the mass of the water-absorbent resin contained in a used absorbent article (for example, a used disposable diaper) is unknown, the mass may be calculated from the mass of the water-absorbent resin contained in an unused absorbent article (for example, an unused disposable diaper) and a general value of the water absorption capacity of the water-absorbent resin in the used absorbent article (for example, a used disposable diaper), or the mass of the water-absorbent resin may be measured by assuming that the content (solid content equivalent) of the water-absorbent resin relative to the total mass of the used absorbent article (used disposable diaper) is 3 mass% or more and 9 mass% or less.

[0020] In one embodiment of the present invention, the CRC of the water-absorbing resin (used as a raw material for the solubilizing polymer) is 20 g / g or more, 25 g / g or more, 27 g / g or more, or 30 g / g or more. In one embodiment of the present invention, the CRC of the water-absorbing resin (used as a raw material for the solubilizing polymer) is 70 g / g or less, 50 g / g or less, or 45 g / g or less. The definition of CRC is as described in the Examples section.

[0021] In one embodiment of the present invention, the AAP (0.3 psi) of the water-absorbing resin (used as a raw material for the solubilizing polymer) is 10 g / g or more, 15 g / g or more, 20 g / g or more, 22 g / g or more, 24 g / g or more, or 25 g / g or more. In one embodiment of the present invention, the AAP (0.3 psi) of the water-absorbing resin (used as a raw material for the solubilizing polymer) is 50 g / g or less, 45 g / g or less, 40 g / g or less, 35 g / g or less, or 30 g / g or less. The definition of AAP is as described in the Examples section.

[0022] (1-2) Solubilized Polymer Next, the target "solubilized polymer" will be explained.

[0023] A "solubilized polymer" is a polymer (water-soluble polymer) solubilized in water by decomposing a water-swellable (i.e., water-insoluble) water-absorbent resin. The water-solubilized polymer may exist in a dissolved state in water, or may exist as a solid and dissolve when placed in water. The solubilized polymer may be completely solubilized in water, or may be a partially solubilized polymer containing a partially unsolubilized polymer. If the polymer appears to be dissolved in water with no residual dissolved material visible in the water (if the interface between the water-absorbent resin and water is no longer visible in the water), it is considered that a solubilized polymer has been produced. Even if some undissolved swollen material remains, if the amount of water-absorbent resin is reduced compared to before the solubilization treatment, it is partially solubilized, and a solubilized polymer has been produced. Therefore, even if some undissolved swollen material remains in the solubilized polymer aqueous solution after solubilization (decomposition), it may be called a solubilized polymer aqueous solution (or dispersion). The solubilized polymer thus produced can be discarded, but it may also be reused as a raw material for producing a new water-absorbing resin.

[0024] In one embodiment of the present invention, the proportion of the water-absorbent resin that is solubilized is not particularly limited, and as described above, as long as the water-absorbent resin (water-absorbent resin (A)) is solubilized even partially, it is included in the category of "a method for producing a solubilized polymer", but is preferably 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more (upper limit 100% by mass) relative to the total water-absorbent resin. In addition, the proportion of the water-absorbent resin (A) that is solubilized is not particularly limited, but is preferably 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more (upper limit 100% by mass) relative to the water-absorbent resin (A).

[0025] The proportion of the water-absorbent resin that is solubilized can be determined, for example, by the following method.

[0026] <Method for determining the solubilized proportion of a water-absorbent resin> A solubilized polymer obtained by solubilizing a water-absorbent resin was filtered through a 100-mesh stainless steel wire netting whose mass had been measured in advance, and the insoluble matter gel remaining on the mesh was thoroughly washed with a large amount of deionized water. Thereby, water-soluble (solubilized) components contained in the solubilized polymer were washed away, and only the insoluble matter gel of the water-absorbent resin was left on the wire netting.

[0027] Next, the wire mesh on which the water-absorbent resin insoluble matter gel was placed was dried in a hot air circulating oven at 180°C for 3 hours to remove volatile components. The solubilized proportion of the water-absorbent resin was determined according to the following formula (a).

[0028] Solubilized proportion of water absorbent resin (mass%)={1−[(total mass of wire mesh and insoluble matter in water absorbent resin after drying)−mass of wire mesh only] / (mass of water absorbent resin before solubilization)×100...Equation (a)

[0029] In this way, when carrying out the method for determining the solubilized ratio of a water absorbent resin, a substance that passes through the mesh can be considered to be a solubilized polymer present in a state dissolved in water, and a substance that passes through the mesh and falls to the bottom when an insoluble matter gel remaining on the mesh is thoroughly washed with a large amount of deionized water, that is, a substance that is confined in a solid form inside the gel and that was not dissolved in water at first but dissolves when it comes into contact with at least a part of the water by washing with a large amount of water, can also be considered to be a solubilized polymer present as a solid.

[0030] In one embodiment of the present invention, the weight average molecular weight of the solubilizing polymer is 10,000 to 1,000,000, 15,000 to 750,000, or 20,000 to 500,000. The weight average molecular weight can also be calculated from the elution time of the measurement sample using GPC, by creating a calibration curve (calibration curve) from the molecular weight and elution time using a standard polymer sample (standard polystyrene, etc.) with known molecular weight.

[0031] In one embodiment of the present invention, the solubilizing polymer may be in a form containing water, and in that case, the solids concentration of the aqueous solubilizing polymer solution may be 1% by mass or more and less than 100% by mass, 3% by mass or more and 98% by mass or less, or 5% by mass or more and 95% by mass or less.

[0032] (1-3) Decomposing Agent In one aspect of the present invention, the solubilized polymer can be produced by preparing a water-absorbent resin (A) that is a water-absorbent resin to be decomposed, and contacting the water-absorbent resin (A) with a decomposing agent.

[0033] An oxidizing agent may be used as the decomposing agent. Thus, in one embodiment of the present invention, the decomposing agent includes an oxidizing agent. Here, the oxidizing agent is a compound having oxidizing properties and generating radicals upon heating. Examples of the oxidizing agent include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, alkyl hydroperoxides, and peresters; perchlorates such as sodium perchlorate and potassium perchlorate; periodates such as sodium periodate and potassium periodate; percarbonates, perborates, and peracetic acid. The oxidizing agents may be used alone or in combination of two or more. Of these, hydrogen peroxide is preferably used.

[0034] A reducing agent may be used as the decomposing agent. The reducing agent is a compound having reducing properties that generates radicals when used in combination with the aforementioned oxidizing agent or the below-described compound that generates transition metal ions. Examples of reducing agents include sulfite (salt), hydrogen sulfite (salt), phosphorous acid (salt), hypophosphorous acid (salt), thiosulfate (salt), formic acid, oxalic acid, erythorbic acid, amines, ascorbic acid (salt) or derivatives thereof (e.g., L-ascorbic acid (salt), isoascorbic acid (salt), and alkyl esters of ascorbic acid), phosphate esters, and sulfate esters. The reducing agents may be used alone or in combination of two or more. Among these, sulfite (salt), hydrogen sulfite (salt), L-ascorbic acid (salt), and isoascorbic acid (salt) are preferably used.

[0035] A transition metal ion may be used as the decomposing agent. Examples of the transition metal ion include Cu. ２＋ , Ag ＋ , Fe ２＋ , Fe ３＋ , Al ３＋ , Ni ２＋ , Mn ２＋ and the like. The transition metal ions may be used alone or in combination of two or more. Examples of compounds that generate the transition metal ions include chlorides and hydrates thereof, such as ferrous chloride; organic acid salts and hydrates thereof, such as ferrous fumarate, ferrous oxalate, ferrous chloride, sodium ferrous citrate, ferrous gluconate, ferrous citrate, ferrous acetate; sulfates and hydrates thereof, such as ferrous sulfate; and the like.

[0036] An alkaline compound may be used as the decomposing agent. The alkaline compound is a compound that solubilizes the crosslinked portion of the water-absorbent resin by alkaline hydrolysis. Examples of such alkaline compounds include alkali metal hydroxides such as lithium hydroxide, potassium hydroxide, and sodium hydroxide; alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide; ammonia; and amine compounds such as aliphatic amines, alicyclic amines, and aromatic amines. The alkaline compounds may be used alone or in combination of two or more.

[0037] In one embodiment of the present invention, the decomposition agent comprises at least two selected from an oxidizing agent, a reducing agent, and a transition metal ion. In one embodiment of the present invention, the decomposition agent comprises an oxidizing agent, a reducing agent, and a transition metal ion. In one embodiment of the present invention, the decomposition agent comprises at least two selected from a peroxide, ascorbic acid (salt) or a derivative thereof, and a transition metal ion. In one embodiment of the present invention, the decomposition agent comprises a peroxide, ascorbic acid (salt) or a derivative thereof, and a transition metal ion.

[0038] (1-4) Evaluation of decomposition property of water-absorbent resin In one embodiment of the present invention, when a solubilized polymer is produced using a water-absorbent resin (A) as at least one of raw materials, the water-absorbent resin (A) is contacted with a decomposer in the presence of a water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A), or the water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A) is contacted with a decomposer to separately prepare a solubilized polymer of the water-absorbent resin (B) (also referred to as a “decomposition product” in this specification), and the water-absorbent resin (A) is contacted with a decomposer in the presence of the decomposition product.

[0039] The decomposition ability of a "water-absorbent resin" can be evaluated by decomposing the same amount of two or more types of water-absorbent resins under the same decomposition conditions and determining the length of the decomposition time.

[0040] When evaluating the decomposition properties of two types of water-absorbent resins, the water-absorbent resin with the shorter decomposition time can be used as the water-absorbent resin (B).

[0041] The case of evaluating the degradability of three or more types of water-absorbent resins will be described in several embodiments. In FIG. 1, n types of water-absorbent resins with different degradability are prepared. The n types of water-absorbent resins may differ in at least a portion of the types of raw materials and / or the amounts thereof. Among these, the water-absorbent resin (i) has the lowest degradability (longest decomposition time), and the water-absorbent resin (n) has the highest degradability (shortest decomposition time). In one embodiment of the production method of the present invention, as shown in FIG. 1, the water-absorbent resin (n) is the water-absorbent resin (B), and a combination of the water-absorbent resins (i) to (n-1) can be the water-absorbent resin (A).

[0042] Here, when evaluating the degradability of a "water-absorbent resin" which is a combination of such multiple water-absorbent resins, the "degradability of the water-absorbent resin" of the present invention evaluates the average degradability of an aggregate of particles constituting the water-absorbent resin, so it is preferable to uniformly mix the combined water-absorbent resins (in FIG. 1, it is particularly preferable to uniformly mix the water-absorbent resin (A) which is a combination of water-absorbent resins (i) to (n-1)). On the other hand, if the water-absorbent resin is already a uniform aggregate of particles, uniform mixing is not necessary. For example, as shown in [Production Example 1] in the Examples section, if the water-absorbent resin is produced in one batch, the obtained water-absorbent resin is already a uniform aggregate of particles, so uniform mixing is not necessary. The same applies to the water-absorbent resin produced in [Production Example 2] in the Examples section, since it is already a uniform aggregate of particles. On the other hand, in the case where water absorbent resins in which at least a part of the kinds of raw materials and / or the amounts of use thereof are different are produced in separate batches and a combination of these is used as a water absorbent resin, in order to evaluate the average degradability of an aggregate of particles, it is preferable to mix them to form a uniform aggregate.

[0043] In one embodiment of the manufacturing method of the present invention, although the water-absorbent resins (A) and (B) are not shown, as a modified example of FIG. 1, "a combination of a water-absorbent resin (n) and at least one water-absorbent resin having lower degradability than that (for example, a water-absorbent resin (n-1))" is the water-absorbent resin (B), and "other water-absorbent resins (in this embodiment, a combination of water-absorbent resins (n-2) to water-absorbent resins (i))" can also be considered as the water-absorbent resin (A). For example, when there are five types of water-absorbent resins with different degradability, "a combination of water-absorbent resins (iv) to water-absorbent resins (v)" can be considered as the water-absorbent resin (B), and "a combination of water-absorbent resins (i) to water-absorbent resins (iii)" can also be considered as the water-absorbent resin (A).

[0044] In one embodiment of the manufacturing method of the present invention, although the water absorbent resins (A) and (B) are not shown, as a modified example of FIG. 1 , by preparing a water absorbent resin (n+1) having even higher decomposability than the water absorbent resin (n), the water absorbent resin (n+1) can be made the water absorbent resin (B), and the “combination of the water absorbent resins (i) to (n)” can be made the water absorbent resin (A).

[0045] In this way, the decomposability of the water absorbent resin (A) and the water absorbent resin (B) can be determined by the length of the decomposition time when an aggregate of particles constituting the water absorbent resin (A) and an aggregate of particles constituting the water absorbent resin (B) are decomposed under the same decomposition conditions.

[0046] The production method according to one embodiment of the present invention may include a confirmation step of confirming the decomposition property of the water absorbent resin (A) (confirming the decomposition time). After the confirmation step, it is also preferable to prepare a water absorbent resin (B) that is a water absorbent resin having higher decomposition property than the water absorbent resin (A).

[0047] Examples of a method for preparing a water absorbent resin (B) (i.e., a highly degradable water absorbent resin) include preparing a water absorbent resin having a smaller amount of internal crosslinking agent as the water absorbent resin (B), preparing a water absorbent resin using an internal crosslinking agent species that is more easily degraded than the internal crosslinking agent species used in the water absorbent resin (A), synthesizing the water absorbent resin (B) by a polymerization method that is more easily degraded than the polymerization method used in the water absorbent resin (A), etc. Even if the amount of the internal crosslinking agent or the type of internal crosslinking agent in the water absorbent resin (A) is unknown, the water absorbent resin (B) can also be prepared by using an internal crosslinking agent species that is easily degraded, which will be described later, and making the amount used very small (by trying this several times as necessary).

[0048] An example of a polymerization method that is prone to decomposition is "aqueous solution polymerization of a pre-neutralized product," while an example of a polymerization method that is difficult to decompose is "reverse phase suspension polymerization" or "post-neutralization aqueous solution polymerization." The "aqueous solution polymerization of a pre-neutralized product" method is an aqueous solution polymerization method that uses a pre-neutralized monomer, and examples of the polymerization methods include those described in JP 2007-191708 A, JP 2004-155963 A, and WO 2013 / 073682. Examples of the "reverse phase suspension polymerization" method include those described in JP 03-227301 A, JP 11-005808 A, WO 2004 / 083284 A, WO 2009 / 025235 A, and WO 2013 / 018571. The "post-neutralization aqueous solution polymerization" method is a method in which unneutralized monomers are aqueous solution polymerized and then neutralized, and examples thereof include the polymerization methods described in JP-A-10-101735, JP-A-01-103606, JP-A-62-054751, JP-A-2002-527547, and JP-A-03-174414. "Pre-neutralization" and "post-neutralization" are also described in detail in "(2-1) Step of preparing aqueous monomer solution" below.

[0049] In one embodiment of the present invention, the water-absorbent resin (B) is obtained by aqueous solution polymerization of a pre-neutralized product. More specifically, for example, it is obtained by polymerizing partially or fully neutralized acrylic acid (salt). In one embodiment of the present invention, the water-absorbent resin (A) is obtained by reversed-phase suspension polymerization. More specifically, for example, the water-absorbent resin (A) is obtained by dispersing an aqueous solution containing acrylic acid (salt) in an organic solvent and polymerizing it. In one embodiment of the present invention, the water-absorbent resin (A) is obtained by post-neutralization aqueous solution polymerization. More specifically, for example, the water-absorbent resin (A) is obtained by neutralizing after acrylic acid polymerization.

[0050] In one embodiment of the present invention, the ratio of the decomposition time (minutes) of the water-absorbent resin (A) to the decomposition time (minutes) of the water-absorbent resin (B) is 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, or 1.9 or more. In one embodiment of the present invention, the ratio of the decomposition time (minutes) of the water-absorbent resin (A) to the decomposition time (minutes) of the water-absorbent resin (B) is not particularly limited because the use of a water-absorbent resin with higher decomposability can more efficiently achieve the intended effects of the present invention. To give an example, the ratio may be, for example, 100 or less, 50 or less, 30 or less, or 10 or less, although a ratio exceeding these values ​​may also be used. The conditions for measuring the "decomposition time" referred to here may be as follows. When a fresh water-absorbent resin or a used water-absorbent resin that has been dried (maintained in equilibrium with atmospheric humidity) is used as the water-absorbent resin, 73.3 parts by mass of deionized water is added to 10.0 parts by mass of the water-absorbent resin, and the mixture is left at room temperature for 12 hours or more (particularly 12 hours) to prepare a gel-like water-absorbent resin. When using a used gel-type water-absorbent resin, it is dried (maintaining equilibrium with atmospheric humidity) to form a water-absorbent resin, and 73.3 parts by mass of deionized water is added to 10.0 parts by mass of the water-absorbent resin. The mixture is left at room temperature for 12 hours or more (particularly 12 hours) to produce a gel-type water-absorbent resin. 100 parts by mass of the gel-type water-absorbent resin and 0.67 parts by mass of a 30% by mass aqueous hydrogen peroxide solution as a decomposing agent are added to a reaction vessel, and the mixture is stirred for 100 revolutions or more over 2 minutes (particularly 100 revolutions over 2 minutes) using a stirring means such as a spatula to obtain a mixture. The reaction vessel containing the mixture is placed in a thermostatic chamber at 80 ° C. The time of placement is considered the start of decomposition, and the decomposition time is determined according to "(c) Decomposition Time" in the Examples section. In this way, regardless of whether the water-absorbent resin is in a dry state or a hydrated (gel) state, the above-specified amount of water and the above-specified amount of the decomposing agent are added to 10.0 parts by mass (solid content equivalent) of the water-absorbent resin to obtain a mixture, and the reaction vessel containing the mixture is placed in a thermostatic chamber at 80°C to determine the decomposition time. Note that the conditions for measuring the "decomposition time" may be scaled up by multiplying each by one. Note that the decomposition conditions when producing the solubilized polymer in one embodiment of the present invention may be the same as or different from the above-specified conditions.

[0051] (1-5) Internal Crosslinking Agent Examples of the internal crosslinking agent used in the production of the water-absorbing resin include an internal crosslinking agent having two or more polymerizable unsaturated groups (such as an allyl group or a (meth)acrylate group), or an internal crosslinking agent that is thermally decomposable at a drying temperature. Specific examples of the internal crosslinking agent used in the production of the water-absorbing resin include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol, and the like. Examples of the internal crosslinking agent include methyl tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate. The number of alkylene glycol units (hereinafter sometimes referred to as n) may be 1 to 100, 6 to 50, 6 to 20, or 6 to 10. Among the above, examples of the internal crosslinking agent that are easily decomposed include (poly)ethylene glycol di(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, and polyethylene glycol. In one embodiment of the present invention, the ethylene glycol unit (n) of the (poly)ethylene glycol di(meth)acrylate may be, for example, 1 to 100, 6 to 50, 6 to 20, or 6 to 10.

[0052] The amount of the internal crosslinking agent used in the production of the water absorbent resin may be 0.0001 mol % or more and 10 mol % or less, 0.001 mol % or more and 5 mol % or less, or 0.01 mol % or more and 1 mol % or less, relative to the total amount of the monomers (excluding the internal crosslinking agent). As described above, the amount of the internal crosslinking agent used in the water absorbent resin (B) is preferably set to be less than the amount of the internal crosslinking agent used in the water absorbent resin (A).

[0053] (1-6) Embodiments of Production of Solubilized Polymer One aspect of the present invention is a method for producing a solubilized polymer, comprising preparing a water-absorbent resin (A) that is a water-absorbent resin to be decomposed, and contacting the water-absorbent resin (A) with a decomposing agent, (i) comprising carrying out the contact in the presence of a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A), or (ii) comprising bringing a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A) into contact with a decomposing agent to separately prepare a decomposed product, and carrying out the contact in the presence of the decomposed product.

[0054] The water-absorbent resin (A) is defined as "a water-absorbent resin to be decomposed," but (i) when the contact is carried out in the presence of the water-absorbent resin (B), the objects to be decomposed include not only the water-absorbent resin (A) but also the water-absorbent resin (B), and a solubilized polymer is produced by decomposing these. In other words, the objects to be decomposed include not only the water-absorbent resin (A) but also the water-absorbent resin (B). Similarly, (ii) when the contact is carried out in the presence of a decomposition product and the decomposition product contains a portion of unsolubilized polymer, the objects to be decomposed include not only the water-absorbent resin (A) but also the unsolubilized polymer, and a solubilized polymer is produced by decomposing these. In other words, the objects to be decomposed may include not only the water-absorbent resin (A) but also the unsolubilized polymer derived from the decomposition product.

[0055] The mechanism by which the decomposition time of the water-absorbent resin is shortened in the production method of one embodiment of the present invention is presumed to be as follows. However, the technical scope of the present invention is not limited by such a mechanism. That is, it is thought that during the decomposition of the water-absorbent resin (A), the decomposition product of the water-absorbent resin (B) present functions like a plasticizer, thereby suppressing aggregation of the water-absorbent resin (A) and increasing the surface area of ​​the water-absorbent resin (A), thereby improving the frequency of contact of the decomposing agent with the water-absorbent resin (A) and accelerating the decomposition of the water-absorbent resin (A).

[0056] Various methods can be used as a method for producing a solubilized polymer by contacting a water-absorbent resin (A) with a decomposing agent, as long as it is based on the premise that (i) the contact is carried out in the presence of a water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A), or (ii) a decomposition product is separately produced by contacting a water-absorbent resin (B) that is a water-absorbent resin having a higher decomposability than the water-absorbent resin (A) with a decomposing agent, and the contact is carried out in the presence of the decomposition product. For example, a technique for heat treatment in the presence of hydrogen peroxide as a decomposing agent (e.g., JP-A-04-317785), a technique for using ascorbic acid as a reducing agent as a decomposing agent under conditions of pH 4 to pH 7.5 (e.g., JP-A-05-247221), a technique for decomposing using a reducing agent and transition metal ions as a decomposing agent (e.g., JP-A-2019-131789), a technique for decomposing by heating in the presence of an oxidative aqueous salt as a decomposing agent (e.g., WO2021 / 042113), a technique for decomposing using ozone water as a decomposing agent (e.g., JP-A-2017-100133), a technique for decomposing using an oxidizing agent and transition metal ions such as iron ions or copper ions in combination as a decomposing agent (e.g., JP-A-11-172039), and a technique for decomposing using an alkaline compound as a decomposing agent (e.g., JP-A-2020-49398). Furthermore, for example, shear force can be applied to the water-absorbent resin before decomposing the water-absorbent resin and / or during the period from the start of the decomposition to the end of the decomposition to reduce the particle size of the water-absorbent resin. By reducing the particle size of the water-absorbent resin to be (being) contacted with the decomposing agent in advance, compared with the particle size of the prepared water-absorbent resin, the surface area of ​​the water-absorbent resin can be increased, and the time required to obtain a solubilized polymer can be shortened. The particle size reduction of the water-absorbent resin may be performed on either a water-absorbent resin in a dry state (maintaining equilibrium with atmospheric humidity) or a water-absorbent resin in a hydrated state.The crushing device used when applying shear force to the water absorbent resin to reduce the particle size is not particularly limited, and when the water absorbent resin is in a dry state, a jaw crusher, a cone crusher, an impact crusher, a roll crusher, an autogenous crusher, a stamp mill, a stone-type crusher, a crusher, a ring mill, a roller mill, a jet mill, a pin mill, a vibration mill, a planetary mill, a bead mill, an attritor, a hammer mill, a cutter mill, etc. can be used. When the water absorbent resin is in a gel state in a hydrous state, a gel crusher equipped with a plurality of rotating stirring blades, such as a batch or continuous double-arm kneader, a single-screw extruder, a twin-screw extruder, a meat chopper, a disperser, a homomixer, a colloid mill, a roll mill, a high-pressure injection type disperser, a rotary mill, a vibration mill, a planetary mill, an attritor, a bead mill, etc. can be used.

[0057] The apparatus for producing a solubilized polymer used in the present invention is not particularly limited as long as it is an apparatus capable of decomposing the polymer while maintaining the desired decomposition conditions constant, and examples thereof include a tank-type decomposition apparatus, a decomposition apparatus equipped with a stirrer, and a multi-shaft decomposition apparatus. From the viewpoint of reducing the frequency of equipment maintenance and maintaining performance, the inner wall surface of the decomposition apparatus is preferably made of a corrosion-resistant (acid-resistant, alkali-resistant) material. Examples of such corrosion-resistant materials include stainless steel, Hastelloy steel, titanium steel, glass-lined steel, resin-lined steel, and metal spray-coated steel. Examples of stainless steel include austenitic stainless steels such as SUS304, SUS304L, SUS304LN, SUS312L, SUS316, SUS316L, SUS316N, SUS316LN, SUS317, SUS317N, and SUS317LN; ferritic stainless steels such as SUS430, SUS430F, SUS434, and SUS444; and martensitic stainless steels such as SUS410, SUS410F2, SUS410J1, SUS410S, and SUS431. Examples of resin-lined steel include those lined with rubber, fluororesins (such as polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylenepropylene copolymer, ethylene tetrafluoroethylene copolymer, and polyvinylidene fluoride), and thermosetting resins (such as epoxy resin, bisphenol polyester, and vinyl ester). Examples of metal-sprayed steel include those coated with metals or ceramics by flame spraying, high-velocity flame spraying, arc spraying, plasma spraying, and wire explosion spraying. The lining or coating may be applied to the entire equipment, or only to areas particularly susceptible to corrosion (such as the gas-liquid interface). Other corrosion prevention measures besides the use of corrosion-resistant materials include periodic water washing of the equipment, sufficient stirring or standing before heating when adding acids or alkalis to ensure a uniform pH level, and removing oxygen from the solution (such as inert gas replacement, degassing, or the use of oxygen scavengers).

[0058] In one embodiment of the present invention, (i) the water-absorbent resin (A) is contacted with the decomposing agent in the presence of the water-absorbent resin (B).

[0059] In one embodiment of the present invention, the abundance ratio of the water-absorbent resin (B) is 10 parts by mass or more, 20 parts by mass or more, 30 parts by mass or more, 40 parts by mass or more, 45 parts by mass or more, 50 parts by mass or more, 60 parts by mass or more, 70 parts by mass or more, 80 parts by mass or more, or 90 parts by mass or more, relative to 100 parts by mass of the water-absorbent resin (A). In one embodiment of the present invention, the abundance ratio of the water-absorbent resin (B) is 1000 parts by mass or less, 750 parts by mass or less, 500 parts by mass or less, 400 parts by mass or less, 300 parts by mass or less, 200 parts by mass or less, 150 parts by mass or less, 120 parts by mass or less, 95 parts by mass or less, 85 parts by mass or less, 75 parts by mass or less, 65 parts by mass or less, 55 parts by mass or less, 45 parts by mass or less, 35 parts by mass or less, 25 parts by mass or less, or 20 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin (A).

[0060] In one embodiment of the present invention, the mass ratio of the water-absorbent resin (A) to the water-absorbent resin (B) is 85:15 to 15:85, 85:15 to 55:45, or 85:15 to 60:40. In one embodiment of the present invention, the mass ratio of the water-absorbent resin (A) to the water-absorbent resin (B) is 80:20 to 30:70, 70:30 to 40:60, or 70:30 to 50:50.

[0061] In one embodiment of the present invention, the contact between the water-absorbent resin (A) and the decomposing agent may be carried out in the presence of water. The decomposing agent may be in a form dissolved or dispersed in water. Therefore, by using the water, the contact between the water-absorbent resin (A) and the decomposing agent may be carried out in the presence of water. Of course, water may be added for the contact between the water-absorbent resin (A) and the decomposing agent. Examples of water include tap water, industrial water, ion-exchanged water, and pure water.

[0062] In one embodiment of the present invention, the mixing ratio of water to the water-absorbent resin (A) is not particularly limited, but the amount of water mixed is 50 parts by mass or more and 9900 parts by mass or less, 75 parts by mass or more and 3000 parts by mass or less, or 100 parts by mass or more and 2000 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin (A).

[0063]

[0122] In one embodiment of the present invention, the amount of the decomposing agent (when two or more kinds are used in combination, the total amount) used is, in terms of active ingredient, 0.001 part by mass or more and 60 parts by mass or less, 0.003 part by mass or more and 50 parts by mass or less, or 0.004 part by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the water absorbent resin (A).

[0064]

[0122] In one embodiment of the present invention, when a transition metal ion is used as at least a part of the decomposing agent, the amount of a compound which generates the transition metal ion to be used (the total amount when two or more kinds are used in combination) is, in terms of active ingredient, 0.005 parts by mass or more and 50 parts by mass or less, 0.01 parts by mass or more and 30 parts by mass or less, or 0.02 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the water absorbent resin (A).

[0065]

[0122] In one embodiment of the present invention, when ascorbic acid (salt) or a derivative thereof is used as at least a part of the decomposition agent, the amount of the ascorbic acid (salt) or a derivative thereof to be used (the total amount when two or more kinds are used in combination) is 0.1 part by mass or more and 50 parts by mass or less, 0.2 part by mass or more and 30 parts by mass or less, or 0.2 part by mass or more and 20 parts by mass or less, converted into an active ingredient, relative to 100 parts by mass of the water absorbent resin (A).

[0066]

[0122] In one embodiment of the present invention, when a peroxide is used as at least a part of a decomposing agent, the amount of the peroxide used (the total amount when two or more kinds are used in combination) is, in terms of active ingredient, 0.1 part by mass or more and 50 parts by mass or less, 0.2 part by mass or more and 30 parts by mass or less, or 0.3 part by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the water absorbent resin (A).

[0067] In one embodiment of the present invention, the pH of the mixture to be subjected to the decomposition reaction, which contains the water-absorbent resin (A) and the decomposing agent, is adjusted to a range of 6.5 to 8.5, 7.0 to 8.5, or 7.5 to 8.5. The pH of the mixture can be adjusted, as necessary, by adding a pH adjuster such as an acid or a base. Examples of acids that can be used include inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid and citric acid. Examples of bases that can be used include sodium hydroxide, sodium carbonate, potassium hydroxide, and magnesium hydroxide. The pH can be measured using a pH meter (a LAQUA act D-71 pH meter manufactured by Horiba, Ltd.). The amount of the pH adjuster is not particularly limited, as long as it is an amount that can adjust the pH to the desired level.

[0068] In one embodiment of the present invention, when obtaining a mixture containing a water-absorbent resin (A), a decomposing agent, and a water-absorbent resin (B) (here, in the embodiment (ii), "water-absorbent resin (B)" can be read as "separately prepared decomposition product"), i.e., a mixture to be subjected to a decomposition reaction, at least two of these, preferably all of them, may be mixed using a continuous mixer in order to mix them as uniformly as possible. In one embodiment of the present invention, the continuous mixer is a device that performs mixing by continuously supplying a mixture containing a water-absorbent resin (A), a decomposing agent, and a water-absorbent resin (B), and mixing the mixture while the mixture passes through a stirring section inside the continuous mixer and is discharged as a stirred mixture. When a continuous mixer is used, the mixture to be subjected to the decomposition reaction can be mixed more uniformly. From the viewpoint of performing more uniform mixing, it is preferable that the continuous mixer has a large mixing power. Such a continuous mixer is not particularly limited as long as it is capable of mixing in the above-mentioned manner. Examples of continuous mixers include cylindrical mixers, double-walled conical mixers, high-speed stirring mixers such as Turbulizer (registered trademark), V-shaped mixers, ribbon mixers, screw mixers, double-arm kneaders, grinding kneaders, rotary mixers, airflow mixers, continuous Lödige mixers, and continuous Prussian mixers.

[0069] In one embodiment of the present invention, in a continuous mixer, a mixture containing a water absorbent resin (A), a decomposing agent, and a water absorbent resin (B) is supplied through a feeder, and stirred in a stirring unit provided inside the continuous mixer, and the mixed mixture is discharged through a discharge port of the continuous mixer, and these processes are continuously performed.

[0070] In one embodiment of the present invention, the mixing time of a mixture containing a water absorbent resin (A), a decomposing agent, and a water absorbent resin (B) is, for example, from 5 minutes to 120 minutes, from 10 minutes to 60 minutes, or from 15 minutes to 45 minutes, although it varies depending on the scale, when the time when the mixture is supplied to a feeder of a continuous mixer is taken as the starting point and the time when all the stirred mixture is discharged from a discharge port of the continuous mixer is taken as the end point.

[0071] In one embodiment of the present invention, the concentration (mass) of all water-absorbent resins in a mixture containing a water-absorbent resin (A), a decomposing agent, and a water-absorbent resin (B) is 5% by mass or more, 7% by mass or more, 10% by mass or more, 30% by mass or more, 50% by mass or more, or 60% by mass or more, when the mass of the entire mixture is 100% by mass. When the concentration of all water-absorbent resins in the mixture is 5% by mass or more, the yield of solubilized polymer obtained from the mixture can be suitably improved. In one embodiment of the present invention, the concentration (mass) of all water-absorbent resins in a mixture containing a water-absorbent resin (A), a decomposing agent, and a water-absorbent resin (B) is 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 55% by mass or less, or 50% by mass or less, when the mass of the entire mixture is 100% by mass. When the concentration of all water-absorbent resins in the mixture is 90% by mass or less, a sufficient amount of decomposing agent can be contained in the mixture. As a result, the decomposition efficiency can be suitably improved.

[0072] As described above, the mixture is mixed using a continuous mixer, and the mixture discharged from the continuous mixer (which is usually in the form of a moistened mixture, a gel mixture, a slurry mixture, or the like) may be supplied to an apparatus for producing a solubilized polymer.

[0073] The decomposition of the water-absorbent resin (A) can be accelerated by heating.

[0074] In one embodiment of the present invention, the temperature condition when producing a solubilized polymer by contacting the water-absorbent resin (A) with a decomposing agent is, for example, 10° C. or higher and 130° C. or lower, 20° C. or higher and 120° C. or lower, 30° C. or higher and 110° C. or lower, 50° C. or higher and 100° C. or lower, or 70° C. or higher and 100° C. or lower. By setting the temperature of the mixture containing the water-absorbent resin (A) and the decomposing agent within such a range, the time required to obtain a solubilized polymer can be shortened.

[0075] In one embodiment of the present invention, in order to reduce the amount of remaining peroxide (e.g., hydrogen peroxide), an additive such as sodium carbonate may be added and the mixture may be stirred, for example, at a temperature of 10° C. to 90° C. for 10 minutes to 10 hours. The amount of remaining hydrogen peroxide can be measured by the method described in the Examples.

[0076] In one embodiment of the present invention, (ii) the contact of the water-absorbent resin (A) with the decomposer involves contacting the water-absorbent resin (B) with the decomposer to separately prepare a decomposition product (i.e., a solubilized polymer of the water-absorbent resin (B)), and the contact is carried out in the presence of the decomposition product. The decomposer used to decompose the water-absorbent resin (B) may be the same as or different from the decomposer used to produce the solubilized polymer. In one embodiment of the present invention, the abundance ratio of the decomposition product is 10 parts by mass or more, 20 parts by mass or more, 30 parts by mass or more, 40 parts by mass or more, 45 parts by mass or more, 50 parts by mass or more, 60 parts by mass or more, 70 parts by mass or more, 80 parts by mass or more, or 90 parts by mass or more, relative to 100 parts by mass of the water-absorbent resin (A). In one embodiment of the present invention, the abundance ratio of the decomposition product is 500 parts by mass or less, 400 parts by mass or less, 300 parts by mass or less, 200 parts by mass or less, 150 parts by mass or less, 120 parts by mass or less, 95 parts by mass or less, 85 parts by mass or less, 75 parts by mass or less, 65 parts by mass or less, 55 parts by mass or less, 45 parts by mass or less, 35 parts by mass or less, 25 parts by mass or less, or 20 parts by mass or less, relative to 100 parts by mass of the water absorbent resin (A). Except for the above, in this embodiment, the explanation of the embodiment (i) in which the contact between the water absorbent resin (A) and the decomposition agent is performed in the presence of the water absorbent resin (B) can be similarly applied.

[0077] [2] Method for producing a water-absorbent resin, and method for producing a water-absorbent resin using a solubilized polymer obtained by decomposing a water-absorbent resin as part of the raw material At least a part of the raw material for the solubilized polymer, which is the object of the production method of one aspect of the present invention, is a water-absorbent resin (A), which is the "water-absorbent resin" to be decomposed. The solubilized polymer obtained by contacting the water-absorbent resin (A) with a decomposing agent may be discarded as is, but from the perspective of promoting environmental conservation, it may also be used as part of the raw material to produce a "water-absorbent resin". Therefore, below, an embodiment of the method for producing a water-absorbent resin and an embodiment of the method for producing a water-absorbent resin using a solubilized polymer as part of the raw material will be collectively described.

[0078] First, the method for producing a water-absorbent resin using a solubilized polymer as a part of the raw material is a production method in which the solubilized polymer obtained by decomposing the water-absorbent resin (A) is used as a raw material in at least one of the steps of preparing a monomer aqueous solution, polymerization, crushing a hydrogel, drying, and surface cross-linking. Also, the solubilized polymer may be cross-linked with a cross-linking agent (such as an internal cross-linking agent or a surface cross-linking agent) to form a cross-linked polymer, and then mixed with a newly produced water-absorbent resin in at least one of the steps. Furthermore, the solubilized polymer may be cross-linked with a cross-linking agent to form a cross-linked polymer, and then the cross-linked polymer may be directly manufactured into a product by the same process as conventional (including steps such as drying, crushing, classification, surface treatment, cooling, and sizing).

[0079] (2-1) Step of Preparing Aqueous Monomer Solution This step is a step of preparing an aqueous solution containing acrylic acid (salt) as a main component (hereinafter referred to as "aqueous monomer solution"). The "main component" means that the amount (content) of acrylic acid (salt) used is usually 50 mol % or more, preferably 70 mol % or more, and more preferably 90 mol % or more (upper limit 100 mol %) based on the total amount of monomers (excluding the internal crosslinking agent) used in the polymerization reaction of the water absorbent resin.

[0080] (Acrylic Acid) In one embodiment of the present invention, acrylic acid and / or a salt thereof (hereinafter referred to as "acrylic acid (salt)") is used as a monomer constituting a water absorbent resin. The "acrylic acid salt" is obtained by neutralizing the acrylic acid with the following basic composition, and the acrylic acid salt may be a commercially available acrylic acid salt (e.g., sodium acrylate), or may be one obtained by neutralization in a production plant for a water absorbent resin.

[0081] (Basic Composition) In one embodiment of the present invention, the term "basic composition" refers to a composition containing a basic compound, such as a commercially available aqueous sodium hydroxide solution. Specific examples of the basic compound include alkali metal salts such as alkali metal carbonates and / or hydrogen carbonates, and alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. Other basic compounds include ammonia and organic amines.

[0082] (Neutralization) In one embodiment of the present invention, neutralization can be performed by either neutralizing acrylic acid (before polymerization) (also referred to as pre-neutralization) or neutralizing a hydrogel-like crosslinked polymer obtained by crosslinking and polymerizing acrylic acid (after polymerization) (hereinafter referred to as "post-neutralization"). When a solubilized polymer is used as at least a part of the raw material to produce a water-absorbent resin, if the solubilized polymer contains a basic compound, this basic compound neutralizes the acrylic acid before polymerization or the hydrogel-like crosslinked polymer after polymerization. Therefore, the neutralization rate of the water-absorbent resin as a final product may be adjusted to a predetermined range by taking this basic compound into consideration. Note that, with regard to conditions such as the neutralization apparatus, neutralization temperature, and residence time, the conditions described in International Publication No. 2009 / 123197 and U.S. Patent Application Publication No. 2008 / 0194863 may also be applied to the present invention. In one embodiment of the present invention, neutralization may be performed by adding a basic compound to an aqueous monomer solution in multiple stages.

[0083] In one embodiment of the present invention, the neutralization ratio is 10 mol% to 90 mol%, 40 mol% to 85 mol%, 50 mol% to 80 mol%, or 60 mol% to 75 mol% of the acid groups of the monomer. A neutralization ratio of 75 mol% means 25 mol% acrylic acid and 75 mol% acrylic acid salt. This is also sometimes referred to as a partially neutralized product of acrylic acid. The same neutralization ratio applies to post-neutralization.

[0084] (Other Monomers) In one embodiment of the present invention, the term "other monomers" refers to monomers other than the acrylic acid (salt), and the water-absorbent resin can be produced by using other monomers in combination with acrylic acid (salt). Examples of the other monomers include water-soluble or hydrophobic unsaturated monomers. Specifically, the compounds described in U.S. Patent Application Publication No. 2005 / 0215734 (excluding acrylic acid) are also applicable to one embodiment of the present invention.

[0085] (Internal Crosslinking Agent) The explanation given in (1-5) above can be applied to the type and amount of the internal crosslinking agent used in one embodiment of the present invention. In one embodiment of the present invention, a method in which a predetermined amount of internal crosslinking agent is added in advance to an aqueous monomer solution and a crosslinking reaction is carried out simultaneously with polymerization is preferably applied. On the other hand, in addition to this method, a method in which an internal crosslinking agent is added before and / or after polymerization to perform post-crosslinking, a radical crosslinking method using a radical polymerization initiator, a radiation crosslinking method using active energy rays such as electron beams or ultraviolet rays, and the like can also be employed. These methods can also be used in combination.

[0086] (Other substances added to aqueous monomer solution) In one embodiment of the present invention, the following substances may be added during preparation of the aqueous monomer solution.

[0087] Specifically, any one of hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salt), and crosslinked polyacrylic acid (salt) may be added in an amount of preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less (lower limit: 0% by mass); any one of foaming agents such as carbonates, azo compounds, and bubbles, surfactants, chelating agents, and chain transfer agents may be added in an amount of preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less (lower limit: 0% by mass, more than 0% by mass, 0.005% by mass or more, or 0.01% by mass or more). Furthermore, the above substances may be added not only to the aqueous monomer solution but also during the polymerization, or these forms may be used in combination.

[0088] (2-2) Polymerization Step This step is a step of polymerizing the acrylic acid (salt)-based monomer aqueous solution obtained in the step of preparing the aqueous monomer solution to obtain a hydrogel-like crosslinked polymer (which may also be referred to as a "hydrogel").

[0089] (Polymerization Initiator) The polymerization initiator used in one embodiment of the present invention is not particularly limited and can be appropriately selected depending on the polymerization form, etc. Examples include thermally decomposable polymerization initiators, photodecomposable polymerization initiators, and redox-based polymerization initiators that are used in combination with a reducing agent that promotes the decomposition of these polymerization initiators. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 are used. From the viewpoints of the handleability of the polymerization initiator and the physical properties of the water-absorbing resin, peroxides or azo compounds are preferably used, more preferably peroxides, and even more preferably persulfates. The amount of the polymerization initiator used is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5 mol%, based on the monomer. The amount of the reducing agent used is preferably 0.0001 mol% to 0.02 mol%, based on the monomer. Instead of using the polymerization initiator, the polymerization reaction may be carried out by irradiating with active energy rays such as radiation, electron beams, and ultraviolet rays, or these active energy rays and the polymerization initiator may be used in combination.

[0090] (Polymerization form) The polymerization form applicable to one embodiment of the present invention is not particularly limited, but examples thereof include spray-droplet polymerization, aqueous solution polymerization, and reversed-phase suspension polymerization. Specific polymerization forms include continuous belt polymerization as disclosed in U.S. Pat. Nos. 4,893,999, 6,241,928, and U.S. Patent Application Publication No. 2005 / 215734, and continuous kneader polymerization as disclosed in U.S. Pat. Nos. 6,987,151 and 6,710,141.

[0091] (2-3) Hydrogel Crushing Step This step is a step in which the hydrogel obtained in the polymerization step is crushed using a gel crusher such as a screw extruder (e.g., a kneader or a meat chopper) or a cutter mill to obtain a particulate hydrogel (hereinafter also referred to as "particulate hydrogel"). When the polymerization step is kneader polymerization, the polymerization step and the hydrogel crushing step are carried out simultaneously. In addition, when a particulate hydrogel is obtained directly during the polymerization process, such as in gas phase polymerization or reverse phase suspension polymerization, the hydrogel crushing step may not be carried out. Gel crushing conditions and forms other than those described above may be the same as those disclosed in WO 2011 / 126079.

[0092] (2-4) Drying Step This step is a step of obtaining a dried polymer by drying the particulate hydrogel obtained in the polymerization step and / or hydrogel crushing step to a desired resin solid content. The resin solid content is determined from the loss on drying (the change in mass when 1 g of the water-absorbent resin is heated at 180°C for 3 hours), and is, for example, 80% by mass or more, 85% by mass to 99% by mass, 90% by mass to 98% by mass, or 92% by mass to 97% by mass.

[0093] The method for drying the particulate hydrogel is not particularly limited, but examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and high-humidity drying using high-temperature water vapor.

[0094] The drying temperature (hot air temperature) in the hot air drying is 120°C or higher and 250°C or lower, or 150°C or higher and 200°C or lower. The air drying time is, for example, 5 minutes or higher and 120 minutes or lower. Drying conditions other than the drying temperature, such as the hot air speed and drying time, may be appropriately set depending on the water content and total mass of the particulate hydrogel to be dried and the target resin solid content. When band drying is performed, the conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. may be appropriately applied.

[0095] (2-5) Surface Cross-Linking Step

[0123] This step is a step of providing a portion with a higher cross-linking density on the surface layer (portion several tens of µm from the surface of the water absorbent resin precursor) of the water absorbent resin precursor obtained through the above-mentioned steps, and can be composed of a mixing step, a heat treatment step, and a cooling step.

[0096] In the surface cross-linking step, a surface-cross-linked water-absorbent resin is obtained by radical cross-linking, surface polymerization, cross-linking reaction with a surface cross-linking agent, or the like on the surface of the water-absorbent resin precursor.

[0097] (Surface cross-linking agent) The surface cross-linking agent used in one embodiment of the present invention is not particularly limited, and may be an organic or inorganic surface cross-linking agent.For example, an organic surface cross-linking agent that reacts with a carboxyl group may be used, and one or more surface cross-linking agents disclosed in U.S. Patent No. 7,183,456 may be used.More specifically, polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyamine compounds or their condensates with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, cyclic urea compounds, etc. may be used.

[0098]

[0044] The amount of the surface crosslinking agent used (when a plurality of agents are used, the total amount used) is, for example, 0.01 part by mass or more and 10 parts by mass or less, or 0.01 part by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin precursor. The surface crosslinking agent may be added as an aqueous solution, and in this case, the amount of water used is, for example, 0.1 part by mass or more and 20 parts by mass or less, or 0.5 part by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin precursor. Furthermore, when a hydrophilic organic solvent is used as necessary, the amount used is, for example, 10 parts by mass or less, more preferably 5 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin precursor.

[0099] (Mixing step) This step is a step of mixing the water-absorbent resin precursor and the surface crosslinking agent. The method of mixing the surface crosslinking agent is not particularly limited, but includes a method of preparing a surface crosslinking agent solution in advance and spraying or dropping the liquid onto the water-absorbent resin precursor to mix them. The device for performing the mixing is not particularly limited, but includes, for example, a high-speed stirring type mixer, more preferably a high-speed stirring type continuous mixer.

[0100] (Heat Treatment Step) This step is a step of applying heat to the resultant product discharged from the mixing step to cause a crosslinking reaction on the surface of the water-absorbent resin precursor. The device for performing the crosslinking reaction is not particularly limited, but for example, a paddle dryer can be mentioned. The reaction temperature in the crosslinking reaction is appropriately set depending on the type of surface crosslinking agent used, and is, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 200°C or lower.

[0101] (Cooling Step) The cooling device is preferably one having the same specifications as the device used in the heat treatment step.

[0102] (Form of addition of solubilizing polymer) Methods for adding the solubilizing polymer include adding it as an aqueous solution, adding it as a powder after drying it, and adding it as an aqueous dispersion (slurry) with some undissolved material remaining.

[0103] When the solubilized polymer is added in the "step of preparing an aqueous monomer solution," the solubilized polymer (or its aqueous solution, or its aqueous dispersion) may be mixed with the aqueous monomer solution, or the solubilized polymer (or its aqueous solution, or its aqueous dispersion) may be mixed with water in advance, and then the monomer (or its aqueous solution) may be mixed. From the viewpoint of uniform mixing of the raw materials, it is preferable to mix the solubilized polymer (or its aqueous solution, or its aqueous dispersion) with the aqueous monomer solution.

[0104] When the solubilizing polymer is added in the "polymerization step", the solubilizing polymer (or its aqueous solution or aqueous dispersion) may be added before or after the start of polymerization. From the viewpoint of uniformity of the polymer component, it is preferable to add the solubilizing polymer before the start of polymerization.

[0105] When the solubilizing polymer is added in the "hydrogel crushing step," the solubilizing polymer (or its aqueous solution or aqueous dispersion) may be added before gel crushing, or may be added during gel crushing. Furthermore, it may be added in portions. From the viewpoint of uniformity of the gel component, it is preferable to add the solubilizing polymer (or its aqueous solution or aqueous dispersion) before gel crushing.

[0106] When the solubilizing polymer is added in the "drying step", the particulate hydrogel and the solubilizing polymer (or its aqueous solution or aqueous dispersion) may be mixed before drying, or may be dried without being mixed.

[0107]

[0063] When a solubilizing polymer is added in the "surface cross-linking step", a water-absorbent resin precursor dried in advance and a solubilizing polymer (or an aqueous solution or dispersion thereof) may be mixed, or the treatment may be performed without mixing. From the viewpoint of uniform mixing of a surface cross-linking agent solution and a water-absorbent resin precursor, it is preferable to use a powdery solubilizing polymer and handle all of them in a powdery state.

[0108] The proportion of the solubilized polymer in the entire water absorbent resin raw material is 1 mass % or more and 60 mass % or less, preferably 1 mass % or more and 50 mass % or less, more preferably 1 mass % or more and 40 mass % or less, and further preferably 1 mass % or more and 30 mass % or less.

[0109] In all of the above-mentioned methods of addition, it is also possible to crosslink at least a part of the solubilizing polymer and use it as a crosslinked polymer.

[0110] (2-6) Water absorption properties of water absorbent resins produced using solubilized polymers as part of the raw materials The water absorption properties of water absorbent resins produced using solubilized polymers as part of the raw materials may be CRC of 20 g / g or more, 25 g / g or more, 27 g / g or more, or 30 g / g or more. The AAP (0.3 psi) may be 10 g / g or more, 15 g / g or more, 20 g / g or more, 22 g / g or more, 24 g / g or more, or 25 g / g or more. The measurement methods for CRC and AAP will be described later. Furthermore, for example, the above-mentioned examples may be applied to the upper limits of CRC and AAP.

[0111] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0112] The present invention encompasses the following aspects and configurations.

[0113] 1. A method for producing a solubilized polymer, comprising preparing a water-absorbent resin (A) that is a water-absorbent resin to be decomposed, and contacting the water-absorbent resin (A) with a decomposing agent, (i) comprising carrying out the contact in the presence of a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A), or (ii) a method for producing a solubilized polymer, comprising bringing a water-absorbent resin (B) that is a water-absorbent resin that is more decomposable than the water-absorbent resin (A) into contact with a decomposing agent to separately prepare a decomposed product, and carrying out the contact in the presence of the decomposed product.

[0114] 2. The method according to 1., wherein the mass ratio of the water-absorbent resin (A) to the water-absorbent resin (B) is 85:15 to 15:85.

[0115] 3. The method for producing a solubilized polymer according to 1. or 2., wherein at least one of the decomposition agents includes an oxidizing agent.

[0116] 4. The method according to 3., wherein the oxidizing agent comprises one or more selected from peroxides.

[0117] 5. The method for producing a solubilized polymer according to any one of 1. to 4., wherein the ratio of the decomposition time (minutes) of the water absorbent resin (A) to the decomposition time (minutes) of the water absorbent resin (B) is 1.5 or more.

[0118] 6. The method according to any one of 1. to 5., wherein at least one of the contacting steps is carried out using a continuous mixer.

[0119] 7. The method according to any one of 1. to 6., wherein a concentration of all of the water absorbent resins in the mixture obtained by the contact is 5% by mass or more and 60% by mass or less, when the mass of the entire mixture is 100% by mass.

[0120] 8. The method according to any one of 1. to 7., further comprising reducing the particle size of at least one of the water absorbent resin (A) and the water absorbent resin (B) after preparing the water absorbent resin (A) and before the contacting.

[0121] 9. A method for producing a water-absorbent resin, comprising obtaining a solubilized polymer by the method according to any one of 1. to 8., and using the solubilized polymer as at least a part of a raw material for the water-absorbent resin.

[0122] 10. The method for producing a water-absorbent resin according to 9., wherein the mass of the solubilized polymer used is 1 mass % or more and 60 mass % or less with respect to the mass of the entire raw material.

[0123] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples. Examples obtained by appropriately combining the technical means disclosed in each example are also included within the scope of the present invention. Unless otherwise noted, measurements were taken at room temperature (20°C or higher and 25°C or lower) and at a relative humidity of 50±5% RH. For convenience, "liter" may be expressed as "l" or "L," and "mass%" may be expressed as "wt%."

[0124] (Evaluation Method) (a) CRC "CRC" of a water-absorbent resin is an abbreviation for "Centrifuge Retention Capacity" and indicates the absorption capacity (unit: g / g) of a water-absorbent resin for a 0.90 mass % aqueous sodium chloride solution under no pressure for 30 minutes.

[0125] 0.200 g of water-absorbent resin was uniformly placed in a nonwoven fabric bag (85 mm x 60 mm) (manufactured by Nankoku Pulp Industries Co., Ltd., trade name: Heatlon Paper, model: GSP-22) and heat-sealed, and then immersed in a large excess (usually about 500 ml) of 0.90% by mass sodium chloride aqueous solution at room temperature. After 30 minutes, the bag was pulled out and drained for 3 minutes using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122) at a centrifugal force (250 G) according to EDANA ABSORBENCY II 441.1-99, and the mass W1 (g) of the bag was measured. The same operation was performed without using the water-absorbent resin, and the mass W0 (g) at that time was measured. Then, from these W1 and W0, the CRC (centrifuge retention capacity) was calculated according to the following formula (1): "EDANA" is an abbreviation for European Disposables and Nonwovens Associations.

[0126] (b) AAP0.3 "AAP" in the water-absorbent resin is an abbreviation for "Absorption Against Pressure" and indicates the absorption capacity against pressure (unit: g / g) for a 0.90% by mass aqueous sodium chloride solution.

[0127] AAP0.3 was measured in accordance with NWSP 242.0.R2(15) except that the pressure condition was changed from 0.7 psi to 0.3 psi. Specifically, a large excess of a 0.9 mass % aqueous solution of sodium chloride was used, and 0.9 g of the water-absorbing agent composition was subjected to pressure of 2.07 kPa (21 g / cm) for 1 hour. ２ After swelling under a pressure of 0.3 psi (0.05 psi), the absorbency against pressure (AAP) was measured. "NWSP" stands for "Non-Woven Standard Procedures - Edition 2015."

[0128] (c) Decomposition Time After adding and mixing a decomposing agent (aqueous solution) to a water absorbent resin (a gel-like simulated used water absorbent resin), the reaction vessel containing the mixture was placed at a predetermined temperature (particularly, 80 to 100°C), and the time when the vessel was placed was determined as the start of decomposition. Thereafter, the state of the mixture was visually confirmed every 10 minutes, and the time when it was confirmed that the water absorbent resin (a gel-like simulated used water absorbent resin) had disappeared and was completely solubilized [that is, the presence of an interface between the water absorbent resin (a gel-like simulated used water absorbent resin) and water could no longer be visually confirmed] was determined as the end of decomposition.

[0129] (d) Solids Concentration of Solubilized Polymer Aqueous Solution A solubilized polymer aqueous solution with a mass Wa (g) was placed on an aluminum dish with a mass Wb (g) and dried in a hot air circulating oven at 180°C for 3 hours, and then the total mass Wc (g) of the solubilized polymer and the aluminum dish was measured. The solids concentration of the solubilized polymer aqueous solution was calculated according to the following formula (2): Solids Concentration of Solubilized Polymer Aqueous Solution (% by mass) = {(mass Wc - mass Wb) / Wa} x 100 (2)

[0130] (e) Mass Average Molecular Weight, Number Average Molecular Weight, and Molecular Weight Distribution of Solubilizing Polymer In the present invention, the molecular weight of the solubilizing polymer was measured according to the following procedure.

[0131] (Preparation of Measurement Sample) First, the solubilized polymer was dissolved in the following solvent to prepare a solution with a concentration of 0.1% by mass. Then, the obtained solubilized polymer solution was passed through a filter (GL Sciences: GL Chromatodisc, aqueous 25A, pore size 0.2 μm) to prepare a measurement sample.

[0132] Solvent: aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 or higher, pH 6.38 or lower). GPC measurement was carried out using the measurement sample under the following measurement conditions.

[0133] (GPC Measurement) The measurement sample was subjected to GPC measurement using a Viscotec TDAmax manufactured by Malvern Instruments. The measurement device was equipped with a size exclusion chromatograph, a refractive index detector, a light scattering detector, and a capillary viscometer. The measurement device and measurement conditions were as follows:

[0134] Pump, autosampler: Viscotec GPCmax (manufactured by Malvern Instruments) Guard column: OHpak SB-G (manufactured by Showa Denko K.K.) Column: Two OHpak SB-806MHQ (manufactured by Showa Denko K.K.) connected in series Detector: Viscotec TDAmax (manufactured by Malvern Instruments) Solvent: Aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 or higher, pH 6.38 or lower) Flow rate: 0.5 ml / min Injection volume: 100 μl.

[0135] The water used in the GPC measurement was pure water from which impurities had been thoroughly removed. The GPC measurement was performed under conditions where the detector baseline was stable, particularly under conditions where there were no noise peaks in the light scattering detector.

[0136] The measuring device was calibrated using polyoxyethylene glycol (mass average molecular weight (Mw): 21,966, molecular weight distribution (Mw / Mn): 1.0, differential refractive index (dn / dc): 0.132, solvent refractive index: 1.33) as a standard sample.

[0137] The differential refractive index (dn / dc) of the solubilized polymer to be measured was set to 0.12, and the refractive index of the solvent was set to 1.33. Data collection and analysis of the refractive index, light scattering intensity, and viscosity were performed using software (Viscotek OmniSEC 4.7.0 (registered trademark)).

[0138] Based on the refractive index (RI), light scattering intensity (angle: 7°) (LALS) obtained in the above measurements, and data obtained from the viscometer (DP), the weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of the solubilized polymer were calculated.

[0139] (f) Amount of Residual Hydrogen Peroxide The amount of hydrogen peroxide remaining in the solubilized polymer aqueous solution was calculated by coloring the remaining hydrogen peroxide with titanium sulfate, a color reagent, and measuring the absorbance according to the following procedure. Specifically, 0.50 g of the solubilized polymer aqueous solution, 8.5 g of industrially purified water, and 1.0 g of a 10% by mass calcium chloride aqueous solution were added and mixed to precipitate the decomposition product, which was then filtered through a 0.2 μm chromatographic disk. 0.30 g of 1 mol / L sulfuric acid (2N) was added to the filtrate and mixed, followed by color development with 0.10 g of 30% by mass titanium(IV) sulfate. Absorbance was measured using a ratio beam spectrophotometer U-5100 (manufactured by Hitachi High-Technologies Corporation) with the attached 10 mm square cell, and industrially purified water was used as the standard. Absorbance at a wavelength of 410 nm was measured using the spectrophotometer. On the other hand, a calibration curve of hydrogen peroxide amount-absorbance was prepared by similarly reacting a calibration curve sample with known hydrogen peroxide concentrations with titanium (IV) sulfate, and the "amount of hydrogen peroxide remaining in the aqueous solubilized polymer solution" was calculated from the calibration curve.

[0140] [Production Example 1] A reaction vessel was charged with 244.8 g of acrylic acid, 1.42 g (0.080 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid·trisodium (DTPA·3Na), 103.7 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 388.9 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (a').

[0141] Next, the aqueous monomer solution (a') was cooled with stirring. When the liquid temperature reached 40.0°C, 100.8 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (a). At this time, the temperature of the aqueous monomer solution (a) rose to 78.2°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0142] Next, 12.1 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (a), and the mixture was immediately poured into a stainless steel bat-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open-to-air system. The time from the start of the second-stage neutralization to pouring the aqueous monomer solution (a) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction began 60 seconds after the aqueous monomer solution (a) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating water vapor, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (1) was removed. This series of operations was performed in an open-to-air system.

[0143] The hydrogel (1) obtained by the polymerization reaction was cut into strips and crushed using a meat chopper with a die diameter of 7.5 mm. The hydrogel (1) was then spread on a 50-mesh wire net and dried with hot air at 190°C for 60 minutes.

[0144] Next, the mixture was pulverized using a vibration mill and further classified using JIS standard sieves with openings of 850 μm and 150 μm, thereby obtaining an irregularly pulverized water-absorbent resin precursor (1).

[0145] A surface crosslinking agent solution containing 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water absorbent resin precursor (1), and the mixture was subjected to heat treatment at 100° C. for 45 minutes.

[0146] Thereafter, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (A). The obtained water-absorbent resin (A) had a CRC of 31.3 g / g and an AAP0.3 of 30.4 g / g.

[0147] [Production Example 2] A reaction vessel was charged with 244.9 g of acrylic acid, 0.71 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid·trisodium (DTPA·3Na), 103.7 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 389.0 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (b').

[0148] Next, the aqueous monomer solution (b') was cooled with stirring. When the liquid temperature reached 40.0°C, 100.9 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (b). At this time, the temperature of the aqueous monomer solution (b) rose to 78.0°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0149] Next, 12.1 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (b), and the mixture was immediately poured into a stainless steel bat-shaped container in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (b) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction began 60 seconds after the aqueous monomer solution (b) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. The hydrogel (2) was removed 3 minutes after the start of the polymerization reaction. This series of operations was performed in an open-to-air system.

[0150] The hydrogel (2) obtained by the polymerization reaction was cut into strips and crushed using a meat chopper with a die diameter of 7.5 mm. The hydrogel (2) was then spread on a 50-mesh wire net and dried with hot air at 190°C for 60 minutes.

[0151] Next, the mixture was pulverized using a vibration mill and further classified using JIS standard sieves with openings of 850 μm and 150 μm, thereby obtaining an irregularly pulverized water-absorbent resin precursor (2).

[0152] A surface crosslinking agent solution containing 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water absorbent resin precursor (2), and the mixture was subjected to heat treatment at 100°C for 45 minutes.

[0153] Thereafter, the mixture was cooled and passed through a JIS standard sieve with an opening of 710 μm to obtain a water-absorbent resin (B). The obtained water-absorbent resin (B) had a CRC of 35.9 g / g and an AAP0.3 of 28.9 g / g.

[0154] [Reference Example 1] 73.3 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (S1).

[0155] To a reaction vessel were added the total amount of the simulated used water absorbent resin (S1), 14.9 parts by mass of a 15 mass % aqueous solution of sodium carbonate as a pH adjuster (pH of the mixture was adjusted to 8.0 or more and 8.5 or less), and 0.67 parts by mass of a 30 mass % aqueous solution of hydrogen peroxide as a decomposing agent, and the mixture was stirred and mixed with a spatula at 100 revolutions over 2 minutes, to obtain a mixture.

[0156] Subsequently, the reaction vessel containing the mixture was placed in a thermostatic bath at 80° C. to initiate the decomposition reaction. 60 minutes after the start of the reaction, it was confirmed that the gel-like substance had been completely solubilized, and an aqueous solution of solubilized polymer (S1) was obtained.

[0157] Next, 8.1 parts by mass of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (S1), and the mixture was stirred at 80° C. for 30 minutes and then cooled to room temperature. This operation removed hydrogen peroxide remaining in the aqueous solution of solubilized polymer (S1).

[0158] The resulting aqueous solubilized polymer solution (S1) had a solids concentration of 9.24% by mass, and the weight average molecular weight of the solubilized polymer (S1) was 248,000.

[0159] [Comparative Example 1] 73.3 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (C1).

[0160]

[0222] To a reaction vessel were added the whole amount of the simulated used water absorbent resin (C1), 14.9 parts by mass of a 15 mass% aqueous solution of sodium carbonate as a pH adjuster (pH of the mixture was adjusted to 8.0 or more and 8.5 or less), and 0.67 parts by mass of a 30 mass% aqueous solution of hydrogen peroxide as a decomposing agent, and the mixture was stirred and mixed with a spatula at 100 revolutions over 2 minutes, to obtain a mixture.

[0161] Subsequently, the reaction vessel containing the mixture was placed in a thermostatic bath at 80° C. to initiate the decomposition reaction. 120 minutes after the start of the reaction, it was confirmed that the gel-like substance had been completely solubilized, and an aqueous solution of solubilized polymer (C1) was obtained.

[0162] Next, 8.1 parts by mass of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (C1), and the mixture was stirred at 80° C. for 30 minutes and then cooled to room temperature. This operation removed hydrogen peroxide remaining in the aqueous solution of solubilized polymer (C1).

[0163] The resulting aqueous solution of solubilized polymer (C1) had a solids concentration of 9.02% by mass, and the mass average molecular weight of solubilized polymer (C1) was 304,000.

[0164] [Example 1] 146.7 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1 and 10.0 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (1).

[0165] To a reaction vessel were added the total amount of the simulated used water absorbent resin (1), 29.9 parts by mass of a 15% by mass aqueous solution of sodium carbonate as a pH adjuster (pH of the mixture was adjusted to 8.0 or more and 8.5 or less), and 1.33 parts by mass of a 30% by mass aqueous solution of hydrogen peroxide as a decomposing agent, and the mixture was stirred and mixed with a spatula at 100 revolutions over 2 minutes, to obtain a mixture.

[0166] Subsequently, the reaction vessel containing the mixture was placed in a thermostatic bath at 80° C. to initiate the decomposition reaction. 80 minutes after the start of the reaction, it was confirmed that the gel-like substance had been completely solubilized, and an aqueous solution of solubilized polymer (1) was obtained.

[0167] Next, 16.1 parts by mass of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (1), and the mixture was stirred at 80° C. for 30 minutes and then cooled to room temperature. This operation removed hydrogen peroxide remaining in the aqueous solution of solubilized polymer (1).

[0168] The resulting aqueous solution of solubilized polymer (1) had a solids concentration of 9.24% by mass, and the mass average molecular weight of solubilized polymer (1) was 299,000.

[0169] [Example 2] 104.9 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1 and 4.30 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (2).

[0170] To a reaction vessel were added the total amount of the simulated used water absorbent resin (2), 21.4 parts by mass of a 15% by mass aqueous solution of sodium carbonate as a pH adjuster (pH of the mixture was adjusted to 8.0 or more and 8.5 or less), and 0.95 parts by mass of a 30% by mass aqueous solution of hydrogen peroxide as a decomposing agent, and the mixture was stirred and mixed with a spatula at 100 revolutions over 2 minutes, to obtain a mixture.

[0171] Subsequently, the reaction vessel containing the mixture was placed in a thermostatic bath at 80° C. to initiate the decomposition reaction. 80 minutes after the start of the reaction, it was confirmed that the gel-like substance had been completely solubilized, and an aqueous solution of solubilized polymer (2) was obtained.

[0172] Next, 11.5 parts by mass of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (2), and the mixture was stirred at 80° C. for 30 minutes and then cooled to room temperature. This operation removed hydrogen peroxide remaining in the aqueous solution of solubilized polymer (2).

[0173] The resulting aqueous solution of solubilized polymer (2) had a solids concentration of 9.11% by mass, and the mass average molecular weight of solubilized polymer (2) was 321,000.

[0174] [Example 3] 86.5 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1 and 1.80 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (3).

[0175] To a reaction vessel, the total amount of the simulated used water absorbent resin (3), 17.6 parts by mass of a 15% by mass aqueous solution of sodium carbonate as a pH adjuster (pH of the mixture was adjusted to 8.0 or more and 8.5 or less), and 0.79 parts by mass of a 30% by mass aqueous solution of hydrogen peroxide as a decomposing agent were added, and the mixture was stirred and mixed with a spatula at 100 revolutions for 2 minutes, to obtain a mixture.

[0176] The reaction vessel containing the mixture was then placed in a thermostatic chamber at 80° C. to initiate the decomposition reaction. 100 minutes after the start of the reaction, it was confirmed that the gel-like substance had been completely solubilized, yielding an aqueous solution of solubilized polymer (3).

[0177] Next, 9.5 parts by mass of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (3), and the mixture was stirred at 80° C. for 30 minutes and then cooled to room temperature. This operation removed hydrogen peroxide remaining in the aqueous solution of solubilized polymer (3).

[0178] The resulting aqueous solution of solubilized polymer (3) had a solids concentration of 9.24% by mass, and the mass average molecular weight of solubilized polymer (3) was 286,000.

[0179] [Example 4] 73.3 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (4-1). 73.3 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (4-2).

[0180] 1500 g of the simulated used water absorbent resin (4-1), 1500 g of the simulated used water absorbent resin (4-2), 540 g of a 15 mass % aqueous sodium carbonate solution as a pH adjuster, and 24.0 g of a 30 mass % aqueous hydrogen peroxide solution as a decomposing agent were subjected to continuous mixing treatment for 20 minutes using a continuous high-speed stirring mixer (small-sized Super Turbulizer manufactured by Hosokawa Micron Corporation ("Turbulizer" is a registered trademark of Hosokawa Micron Corporation)), to obtain a humidified material (4).

[0181] Subsequently, the obtained humidified material (4) was placed in a reaction vessel equipped with a paddle stirring blade, and immersed in a warm water bath at 80° C., and the decomposition reaction was initiated by stirring with the stirring blade at 60 rpm. 50 minutes after the start of the reaction, it was visually confirmed that the simulated used water absorbent resin had been completely solubilized, and an aqueous solution of solubilized polymer (4) was obtained.

[0182] Next, 292 g of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (4), and the mixture was stirred at 80° C. for 2 hours and then cooled to room temperature. By this operation, the hydrogen peroxide remaining in the aqueous solution of solubilized polymer (4) was removed to 23 ppm.

[0183] The resulting aqueous solution of solubilized polymer (4) was removed from the reaction vessel, and the reaction vessel was inspected to find no undissolved residue. The aqueous solution of solubilized polymer (4) had a solids concentration of 9.55% by mass. The mass-average molecular weight of solubilized polymer (4) was 278,000.

[0184] [Example 5] 73.3 parts by mass of deionized water was added to 10.0 parts by mass of the water absorbent resin (A) obtained in Production Example 1, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (5-1). 13.2 parts by mass of deionized water was added to 1.80 parts by mass of the water absorbent resin (B) obtained in Production Example 2, and the mixture was left to stand at room temperature for 12 hours or more to prepare a gel-like simulated used water absorbent resin (5-2).

[0185] 1500 g of the simulated used water absorbent resin (5-1), 279 g of the simulated used water absorbent resin (5-2), 320 g of a 15 mass % aqueous sodium carbonate solution as a pH adjuster, and 14.2 g of a 30 mass % aqueous hydrogen peroxide solution as a decomposing agent were subjected to continuous mixing treatment for 20 minutes using a continuous high-speed stirring mixer (small-sized Super Turbulizer manufactured by Hosokawa Micron Corporation), to obtain a humidified material (5).

[0186] Subsequently, the obtained humidified material (5) was placed in a reaction vessel equipped with a paddle stirring blade, and immersed in a warm water bath at 80° C., and the decomposition reaction was initiated by stirring with the stirring blade at 60 rpm. 70 minutes after the start of the reaction, it was visually confirmed that all of the simulated used water absorbent resin had been solubilized, and an aqueous solution of solubilized polymer (5) was obtained.

[0187] Next, 173 g of a 15% by mass aqueous solution of sodium carbonate was added to the aqueous solution of solubilized polymer (5), and the mixture was stirred at 80° C. for 2 hours and then cooled to room temperature. By this operation, hydrogen peroxide remaining in the aqueous solution of solubilized polymer (5) was removed to 20 ppm.

[0188] The resulting aqueous solution of solubilized polymer (5) was removed from the reaction vessel, and the reaction vessel was inspected, but no undissolved solids were found. The aqueous solution of solubilized polymer (5) had a solids concentration of 9.52% by mass. The mass-average molecular weight of solubilized polymer (5) was 311,000.

[0189] The results of Reference Example 1, Comparative Example 1, and Examples 1 to 5 are shown in Table 1.

[0190]

[0191] (Summary) In Comparative Example 1, in which the water-absorbent resin (A) having poor degradability (long decomposition time) was decomposed alone, 120 minutes was required for decomposition of the water-absorbent resin. On the other hand, in Examples 1 to 3, in which the water-absorbent resin (B) having better degradability than the water-absorbent resin (A) (shorter decomposition time) was used in combination, a shorter decomposition time was observed than in Comparative Example 1.

[0192] Since the decomposition property of a water-absorbent resin is essentially determined by decomposition conditions such as the amount of decomposer used and the decomposition temperature, the decomposition time in Examples 1 to 3 should be 120 minutes, the same as the decomposition time of the water-absorbent resin (A) with a long decomposition time (the time required for 10 parts by mass of the water-absorbent resin (A) to disappear and be completely solubilized), or longer due to the additional presence of the water-absorbent resin (B). Therefore, the shortened decomposition time observed in Examples 1 to 3, assuming that the amount of decomposer relative to the total amount of the water-absorbent resin (A) and the water-absorbent resin (B) was the same, can be said to be an unexpected result. Although the cause is unknown, it is thought to be the effect of some kind of interaction due to the combined use of the water-absorbent resin (B). Perhaps, during the decomposition of the water-absorbent resin (A), the presence of the decomposition product of the water-absorbent resin (B) functions like a plasticizer, thereby suppressing the aggregation of the water-absorbent resin (A) and increasing its surface area. This is thought to improve the frequency of contact of the decomposer with the water-absorbent resin (A), thereby accelerating the decomposition of the water-absorbent resin (A).

[0193] On the other hand, in Examples 4 and 5, when the raw materials, i.e., the water absorbent resin (A), the water absorbent resin (B), the pH adjuster, and the decomposer, were charged into the reaction vessel in Examples 1 and 3, respectively, all of the raw materials were continuously mixed using a continuous high-speed stirring mixer. By this continuous mixing process, the time required for solubilization was shortened from 80 minutes in Example 1 to 50 minutes in Example 4, and from 100 minutes in Example 3 to 70 minutes in Example 5. From these facts, it is considered that homogenization of the raw materials by the continuous mixer resulted in improved decomposability.

[0194] Example 6 A reaction vessel was charged with 220.4 g of acrylic acid, 0.64 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 324.7 g of the solubilized polymer aqueous solution (1) obtained in Example 1, 1.83 g of a 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid trisodium (DTPA 3Na), 93.4 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 25.3 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (c').

[0195] Next, the aqueous monomer solution (c') was cooled with stirring. When the liquid temperature reached 40.0°C, 90.8 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (c). At this time, the temperature of the aqueous monomer solution (c) rose to 78.2°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0196] Next, 10.9 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (c), and the mixture was immediately poured into a stainless steel bat-shaped container in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (c) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction began 65 seconds after the aqueous monomer solution (c) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating water vapor, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel (6) was removed. This series of operations was performed in an open-to-air system.

[0197] The hydrogel (6) obtained by the polymerization reaction was cut into strips and crushed using a meat chopper with a die diameter of 7.5 mm. The hydrogel (6) was then spread on a 50-mesh wire netting and dried with hot air at 190°C for 60 minutes.

[0198] Next, the mixture was pulverized using a vibration mill and further classified using JIS standard sieves with openings of 850 μm and 150 μm, to obtain an irregularly pulverized water-absorbent resin precursor (6).

[0199] A surface crosslinking agent solution containing 0.040 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water absorbent resin precursor (6), and the mixture was subjected to heat treatment at 100° C. for 45 minutes.

[0200] Thereafter, the mixture was cooled and passed through a JIS standard sieve with an opening of 710 μm to obtain a water-absorbent resin (6). The obtained water-absorbent resin (6) had a CRC of 36.4 g / g and an AAP0.3 of 27.6 g / g.

[0201]

[0202] This application is based on Japanese Patent Application No. 2023-124764 filed on July 31, 2023 and Japanese Patent Application No. 2023-124937 filed on July 31, 2023, the disclosures of which are incorporated herein by reference in their entirety.

Claims

1. A method for producing a solubilized polymer, comprising preparing a water absorbent resin (A) to be decomposed, and contacting the water absorbent resin (A) with a decomposing agent, (i) comprising carrying out the contact in the presence of a water absorbent resin (B) which is a water absorbent resin having a higher decomposability than the water absorbent resin (A), or (ii) comprising bringing a water absorbent resin (B) which is a water absorbent resin having a higher decomposability than the water absorbent resin (A) into contact with a decomposing agent to separately prepare a decomposition product, and carrying out the contact in the presence of the decomposition product.

2. The method according to claim 1, wherein the mass ratio of the water-absorbent resin (A) to the water-absorbent resin (B) is 85:15 to 15:

85.

3. The method of claim 1, wherein at least one of the decomposition agents comprises an oxidizing agent.

4. The method of claim 3, wherein the oxidizing agent comprises one or more selected from peroxides.

5. The method according to claim 1, wherein the ratio of the decomposition time (min) of said water-absorbent resin (A) to the decomposition time (min) of said water-absorbent resin (B) is 1.5 or more.

6. The method of claim 1, wherein at least one of said contacting steps comprises using a continuous mixer.

7. The method according to claim 1, wherein the concentration of all the water-absorbent resins in the mixture obtained by the contact is 5% by mass or more and 60% by mass or less, when the mass of the entire mixture is 100% by mass.

8. The method according to claim 1, further comprising reducing the particle size of at least one of the water-absorbing resin (A) and the water-absorbing resin (B) after preparing the water-absorbing resin (A) and before the contacting.

9. A method for producing a water-absorbent resin, comprising obtaining a solubilized polymer by the method according to any one of claims 1 to 8 and using the solubilized polymer as at least a part of a raw material for the water-absorbent resin.

10. The method for producing a water-absorbent resin according to claim 9, wherein the mass of the solubilizing polymer used is 1 mass % or more and 60 mass % or less based on the mass of the entire raw material.