Method for recycling superabsorbent polymers, and method for manufacturing superabsorbent polymers.

Abstract

The present invention provides: a method for producing a recycled water absorbent resin which is suppressed in coloring and decrease of the water absorption performance; and a method for producing a water absorbent resin using the same. The present invention relates to: a method for producing a recycled water absorbent resin from used absorbent articles, the method comprising a urea removal step in which urea is removed so that the urea content in a recycled water absorbent resin is 2% by mass or less, and a regeneration step in which water absorption properties are regenerated; and the like.

Description

Technical Field

[0001] The present invention relates to a method for recycling a water-absorbing resin contained in a used absorbent article, and a method for producing a water-absorbing resin using the recycled water-absorbing resin or the like as a part of raw materials.

Background Art

[0002] In recent years, in absorbent articles such as paper diapers, sanitary napkins, and incontinence pads, from the viewpoint of body fluid absorption, a water-absorbing resin as a constituent material thereof has been widely used as a water absorbent. Examples of such water-absorbing resins include hydrolyzates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic ester copolymers, crosslinked products of partially neutralized acrylic acid polymers, and the like. Among them, from the viewpoint of water absorption performance, polyacrylic acid (salt)-based water-absorbing resins using acrylic acid and / or its salt as a monomer are the most industrially produced.

[0003] Although absorbent articles are generally disposable, due to the increasing environmental awareness, there is a growing interest in recycling sanitary materials (Patent Documents 1 and 2). Regarding the water-absorbing resin used in absorbent articles, technologies for recycling the water-absorbing resin into a water-absorbing resin or other useful substances have also been developed (Patent Documents 3 to 7). In addition, since used absorbent articles are attached with bacteria (general viable bacteria, staphylococci, Escherichia coli, etc.), technologies for sterilizing used absorbent articles are also known (Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document ]2

Patent Document 3

[0005] Currently, various methods are being developed for recycling superabsorbent polymers from used absorbent materials. However, the water absorption performance and color of superabsorbent polymers recycled using these technologies are inferior to those of unused superabsorbent polymers, and further improvements are needed. To improve this, it is possible to add inorganic alkali metal salts (sodium sulfite, sodium bisulfite) using conventional technology, but adding large amounts would lead to increased raw material costs.

[0006] The object of the present invention is to provide a method for producing recycled superabsorbent resin that does not increase raw material costs and reduces the decrease in water absorption performance and discoloration, and a method for producing superabsorbent resin using the same. [Means for solving the problem]

[0007] The inventors of this invention have found that conventional recycled superabsorbent polymers contain a large amount of urea as foreign matter and impurities. This is because, in conventional technology, the regeneration process was restricted in order to improve water absorption performance, and therefore these foreign matter and impurities could not be removed.

[0008] One embodiment of the present invention relates to a method for recycling superabsorbent resin contained in a used absorbent article, comprising: a urea removal step of removing urea from the superabsorbent resin contained in the used absorbent article so that the urea content in the recycled superabsorbent resin is 2% by mass or less; and a regeneration step of regenerating the water absorption properties of the superabsorbent resin after the urea removal step. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide a method for producing recycled superabsorbent resin in which the reduction in water absorption performance from unused products and discoloration derived from foreign matter and impurities are reduced. Furthermore, it is possible to provide a method for producing superabsorbent resin with excellent water absorption performance and excellent reduction in discoloration, using recycled superabsorbent resin and / or its decomposition products, which are water-soluble polymers, as part of the raw material for the superabsorbent resin. [Modes for carrying out the invention]

[0010] <First Embodiment> [1-1] Technical concept of the first embodiment In the absorbent article recycling methods disclosed in Patent Documents 1 to 3, since a large amount of bacteria originating from human waste adheres to used absorbent articles, sterilization is carried out by treatments such as high-temperature treatment, treatment with oxidizing agents, and immersion in a liquid with sterilizing properties. While these treatments do sterilize used absorbent articles, if a large amount of impurities originating from human waste remain, problems arise such as a decrease in the water absorption properties and discoloration of the recycled absorbent resin.

[0011] To address these problems, the inventors discovered that removing impurities derived from human waste from recycled superabsorbent polymers can reduce the decrease in water absorption performance and discoloration. In particular, reducing the residual amount of urea, which is abundant in human waste, to 2% by mass or less in recycled superabsorbent polymers is effective in reducing the decrease in water absorption performance and discoloration of the polymers.

[0012] [1-2] Method for manufacturing recycled superabsorbent resin according to one embodiment of the first embodiment 1 In one embodiment of the first embodiment, the method for producing recycled superabsorbent resin is a method for recycling superabsorbent resin containing absorbent liquids such as human waste that are present in used absorbent articles. The method includes a urea removal step of removing urea so that the urea content in the superabsorbent resin contained in the used absorbent article is 2% by mass or less, and a regeneration step of regenerating the performance of the superabsorbent resin after the urea removal step.

[0013] Furthermore, "absorbent resin after urea removal process" refers to the absorbent resin obtained after the urea removal process.

[0014] [1-2-1] Absorbent articles An "absorbent article" is an article used for water absorption purposes. More specifically, an "absorbent article" is an absorbent article comprising an absorbent body containing a water-absorbent resin and a fibrous material, a permeable surface sheet, and a liquid-impermeable back sheet. The absorbent body is preferably manufactured by blending the water-absorbent resin and the fibrous material, or by sandwiching the water-absorbent resin with the fibrous material and molding it into a film, tube, sheet, etc. Examples of the fibrous material include hydrophilic fibers such as crushed wood pulp, cotton linters, cross-linked cellulose fibers, rayon, cotton, wool, acetate, and vinylon.

[0015] In a method for manufacturing recycled superabsorbent resin according to one embodiment of the first embodiment, the "used absorbent article" particularly refers to used sanitary materials that have absorbed bodily fluids (absorbent liquids) such as urine and blood. Examples of such sanitary materials include disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), pet sheets, and other sanitary materials (sanitary products).

[0016] [1-2-2] Water-absorbing resin "Water-absorbing resin" refers to a water-swellable and water-insoluble polymer gelling agent, and although not particularly limited, it refers to a conventional water-absorbing resin having a water absorption ratio of 10 to 1000 times. More specifically, the water-absorbing resin before absorbing the liquid to be absorbed (hereinafter also referred to as "initial water-absorbing resin") preferably satisfies the physical property of CRC of 5 g / g or more defined by ERT441.2-02 as "water-swellable".

[0017] In the first embodiment, specific examples of the water-absorbing resin include, for example, polyacrylic acid (salt) resins, polysulfonic acid (salt) resins, maleic anhydride (salt) resins, polyacrylamide resins, polyvinyl alcohol resins, polyethylene oxide resins, polyaspartic acid (salt) resins, polyglutamic acid (salt) resins, polyalginic acid (salt) resins, starch resins, cellulose resins, (meth)acrylate cross-linked polymers, saponified cross-linked products of (meth)acrylate ester-vinyl acetate copolymers, starch-acrylate graft polymers and their cross-linked products, and the like.

[0018] [1-2-3] Recycled water-absorbing resin In the first to third embodiments, "recycled water-absorbing resin" is a water-absorbing resin containing a liquid to be absorbed such as urine contained in a used absorbent article, and is a resin that has been recycled for applications that require a water absorption function, which was previously considered to have no other use than to be discarded. That is, "recycled water-absorbing resin" is a water-absorbing resin taken out from a used absorbent article, and is usually a resin that has been discarded, thermally recycled as fuel, or composted, etc., and has been made reusable for water absorption purposes. In other words, "recycled water-absorbing resin" is a water-absorbing resin whose absorption performance has been restored and can be used for water absorption purposes. Examples of recycled water-absorbing resin include, but are not limited to, the following (i) to (vi).

[0019] (i) A water-absorbing resin in which the water-absorbing resin contained in a used absorbent article has been restored to its water absorption through the urea removal process and the regeneration process described below.

[0020] (ii) A water-absorbent resin produced by adding the recycled water-absorbent resin described in (i) above to a conventional water-absorbent resin production process using monomers that constitute the water-absorbent resin as raw materials.

[0021] (iii) A water-absorbent resin manufactured by adding a resin that has undergone only the urea removal step in (i) above (used water-absorbent resin after urea removal treatment) to a conventional water-absorbent resin manufacturing process that uses monomers constituting the water-absorbent resin as raw materials.

[0022] (iv) A water-absorbing resin produced by adding a product (water-soluble polymer) obtained by solubilizing water-absorbing resin contained in used absorbent articles, in the manufacturing process of a (conventional) water-absorbing resin that uses monomers constituting the water-absorbing resin as raw materials.

[0023] (v) In the manufacturing process of a (conventional) superabsorbent resin that uses monomers constituting the superabsorbent resin as raw materials, the superabsorbent resin contained in used absorbent articles is chemically treated to convert it into monomers (such as acrylic acid). Subsequently, these monomers are polymerized to form a water-soluble polymer, and the superabsorbent resin is manufactured using this water-soluble polymer as part of the raw materials.

[0024] (vi) In the manufacturing process of (conventional) superabsorbent resins that use monomers constituting the superabsorbent resin as raw materials, the superabsorbent resin contained in used absorbent articles is chemically treated to convert it into monomers (such as acrylic acid). Subsequently, the superabsorbent resin is manufactured using these monomers as part of the raw materials.

[0025] [1-2-4] Urea removal process In the first embodiment, the "urea removal step" is a step of removing urea, which causes a decrease in water absorption properties and discoloration, from the absorbent resin contained in the used absorbent article so that the urea content in the absorbent resin is 2% by mass or less. The "urea removal step" is not particularly limited as long as it is a method that can remove urea. Specific methods include, for example, hydrolysis of urea with enzymes, catalysts, etc., removal of urea under reduced pressure, washing with an aqueous solution, etc. (washing treatment), and dehydration of swollen absorbent resin (dehydration treatment). Among these treatment methods, washing treatment and dehydration treatment are preferred from the viewpoint of treatment cost and simplicity.

[0026] In these processing methods, while known methods such as using a large amount of washing water and increasing the number of washing and / or dewatering treatments are effective in reducing the urea content in the superabsorbent resin, it was difficult to achieve the urea content of 2% by mass or less, which is the objective of this application. The method of using washing and dewatering treatments in combination, as described in the examples of this application, is one of the particularly preferred methods because it has a large urea removal effect, and thereby achieves a urea content of 2% by mass or less.

[0027] There is no particular limit to the lower limit of urea content in superabsorbent polymers; however, a lower content tends to reduce the decrease in water absorption performance and discoloration. However, reducing the urea content to less than 0.1% by mass does not result in any further improvement and is undesirable because it increases the cost required for washing and / or dewatering treatments.

[0028] Furthermore, by performing the urea removal treatment described above, impurities derived from human waste other than urea can also be removed at the same time. Specific examples of such impurities include proteins, carbohydrates, and lipids derived from food residues contained in feces; general viable bacteria such as intestinal bacteria, E. coli, and lactic acid bacteria; components such as bile acids, mucin, indole, skatole, phenol, and p-cresol; and uric acid, hippuric acid, ammonia, and creatinine contained in urine.

[0029] [1-2-5] Washing process In the first embodiment, the washing process is a method of urea removal, and is a process of removing most of the human waste present between particles and on the surface of the swollen gel of the superabsorbent resin with an aqueous solution such as water or saline solution. Specific methods include, for example, applying an aqueous solution to the swollen superabsorbent resin gel, immersing the swollen gel in an aqueous solution, or stirring the swollen gel while immersing it in an aqueous solution.

[0030] The aqueous solution is not particularly limited as long as the amount of human waste components is reduced by the washing process compared to before treatment and the water-absorbing resin does not deteriorate, but it is preferable that the water-absorbing resin swells when it absorbs water. This is because the human waste components are more easily removed when dewatering after swelling. Examples of such aqueous solutions include deionized water, tap water, distilled water, physiological saline, and seawater, and these may contain other components.

[0031] For aqueous solutions, those with temperatures ranging from room temperature to 100°C can be used, and steam can also be used. When using high-temperature aqueous solutions or aqueous solutions with steam, not only cleaning effects but also sterilization effects can be expected.

[0032] The amount of aqueous solution used is preferably 2,000 parts by weight or more, more preferably 10,000 parts by weight or more, and even more preferably 100,000 parts by weight or more, per 100 parts by weight of the water-absorbing resin.

[0033] [1-2-6] Dehydration process In the first embodiment, the dehydration step is a method of urea removal, and is a step in which the swollen gel of the water-absorbent resin is contracted to discharge the sewage components present inside the water-absorbent resin out of the system.

[0034] The dehydration step in one embodiment of the present invention is not particularly limited as long as it is a process that can reduce the water content of the swollen water-absorbent resin. Specific methods include, for example, contacting the swollen gel of the water-absorbent resin with an aqueous solution containing a hydrophilic organic solvent; contacting the swollen gel with an aqueous solution with a low pH (e.g., 3 or less); contacting the swollen gel with an aqueous solution of a polyvalent metal salt such as calcium chloride; and applying a voltage to the swollen gel using a pair of electrodes. Specific examples of contacting the swollen gel of the water-absorbent resin with these liquids include pouring the liquid onto the swollen gel, immersing the swollen gel in the liquid, and stirring the swollen gel while it is immersed in the liquid.

[0035] When immersing a swollen gel of a water-absorbent resin in an aqueous solution containing a hydrophilic organic solvent, the concentration of the hydrophilic organic solvent in the aqueous solution to be immersed is preferably 35% by mass or more, more preferably 40% by mass or more, and even more preferably 45% by mass or more.

[0036] A hydrophilic organic solvent is an organic solvent whose solubility in 100 mL of water at 20°C is 20 g or more. More preferably, its solubility in 100 mL of water at 20°C is 25 g or more, and even more preferably, its solubility in 100 mL of water at 20°C is 30 g or more.

[0037] Examples of such hydrophilic organic solvents include (1) lower alcohols, such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and tert-butyl alcohol; (2) ketones, such as acetone and methyl ethyl ketone; (3) ethers, such as tetrahydrofuran and dioxane; and (4) esters, such as methyl acetate, methyl lactate, and ethyl lactate. These hydrophilic organic solvents may be used individually or in combination of two or more.

[0038] When immersing a swollen gel of a water-absorbent resin in a low pH aqueous solution (e.g., 3 or less), the pH of the aqueous solution is preferably 3 or less, and more preferably 0.5 to 2.5. Either inorganic or organic acids can be used as the acid in the aqueous solution. In the case of inorganic acids, hydrochloric acid, sulfuric acid, etc., can be used. In the case of organic acids, tartaric acid, glycolic acid, malic acid, citric acid, succinic acid, acetic acid, etc., can be used. These acids may be used individually or in combination of two or more.

[0039] When contacting with an aqueous solution of polyvalent metal salts, examples of polyvalent metal salts include calcium, magnesium, aluminum, iron, cobalt, nickel, and copper. These polyvalent metal salts may be used individually or in combination of two or more.

[0040] [1-2-7] Residual urea amount In the first embodiment, by performing a urea removal treatment, impurities derived from human waste contained in the superabsorbent resin can be reduced, thereby reducing the discoloration of the regenerated superabsorbent resin.

[0041] By keeping the residual amount of urea, which is a highly concentrated impurity derived from human waste, at 2% by mass or less, preferably 1.8% by mass or less, and more preferably 1.6% by mass or less, it is possible to prevent a decrease in the water absorption performance of the superabsorbent resin and reduce discoloration. The "residual amount of urea" refers to the urea content in the recycled superabsorbent resin.

[0042] There are no particular limitations on the method for quantifying the amount of urea remaining in the superabsorbent resin, as long as it is a method that can quantify urea. For example, one method is to disperse the superabsorbent resin in water, extract the urea contained in the superabsorbent resin into the aqueous layer, and then directly quantify the urea contained in the aqueous layer using liquid chromatography, gas chromatography, etc. Another method is to quantify urea using colorimetric methods such as the urease-indophenol method and the diacetyl-monoxime method.

[0043] [1-2-8] Regeneration process In the first embodiment, the regeneration process for recycled superabsorbent resin is a process of regenerating the water absorption properties of the superabsorbent resin after subjecting the superabsorbent resin contained in the used absorbent article to a urea removal treatment.

[0044] The regeneration process is not particularly limited as long as it restores the water absorption capacity of the superabsorbent resin. For example, known regeneration methods such as contacting a urine-absorbing superabsorbent resin with a polyvalent metal salt to dehydrate it, then treating it with an acidic solution, neutralizing it with an alkali metal salt, and then drying it can be used. (JP 2003-225645) A method in which a urine-absorbing superabsorbent resin is contacted with a solution containing a hydrophilic organic solvent to dehydrate it and then dried it. (JP 2003-326161) A method in which a urine-absorbing superabsorbent resin is contacted with a polyvalent metal salt to dehydrate it, then treated with an alkali metal salt aqueous solution for ion exchange and then dried it. (JP 2013-198862) A method in which a urine-absorbing superabsorbent resin is contacted with an acidic aqueous solution to dehydrate it, then neutralized with an alkali metal salt and dried it. (JP 2019-135046, etc.)

[0045] [1-2-9] Crushing (cutting) process In the first embodiment, the method for recycling water-absorbent resin includes a crushing (cutting) step in the urea removal step, or before or after the urea removal step, in which the used absorbent article is crushed (cut) to obtain crushed material (cut material).

[0046] The method for crushing (cutting) used absorbent articles is not particularly limited, and methods such as cutting with a cutter or other cutting tool can be used. By performing the crushing (cutting) process, the opportunity for the absorbent resin in the absorbent article to come into contact with washing water, dewatering liquid, etc., can be increased. This can improve the washing effect, dewatering effect, and speed. In addition, the above process makes it easier to separate the absorbent resin, pulp, nonwoven fabric, and other components from the used absorbent article.

[0047] [1-2-10] Separation process In the first embodiment, the method for recycling water-absorbent resins can carry out a separation step of separating water-absorbent resins from used absorbent articles in parallel with a urea removal step and / or a crushing (cutting) step. "Separating water-absorbent resins from used absorbent articles" means extracting the water-absorbent resins that have shrunk due to the discharge of the absorbed liquid from the gaps in the pulp, nonwoven fabric, etc. contained in the used absorbent articles and dispersing them in a processing liquid.

[0048] One method for separating the absorbent resin from used absorbent articles is to stir a mixture of the treatment liquid and the used absorbent articles. The absorbent resin dispersed in the treatment liquid can be recovered by conventional solid-liquid separation methods that separate soluble and insoluble substances, such as filtration or centrifugation.

[0049] [1-2-11] Sterilization / disinfection process In the above-mentioned method for recycling the superabsorbent resin, the sterilization and / or disinfection step for sterilizing and / or disinfecting the superabsorbent resin can be performed in parallel with or after the urea removal step and / or crushing (cutting) step.

[0050] One method for sterilizing and / or disinfecting superabsorbent polymers is to treat them with a disinfectant solution. The disinfectant solution is not particularly limited and any known solution can be used. Examples include aqueous sodium hypochlorite solution, aqueous chlorine dioxide solution, ozonated water, hydrogen peroxide solution, and electrolyzed water (acidic electrolyzed water). Sterilization and disinfection are preferably carried out in a tank with stirring blades, or by convection of the aqueous solution, to efficiently mix the superabsorbent polymer and the disinfectant.

[0051] [1-2-12] "EDANA" and "ERT" "EDANA" is an abbreviation for the European Disposables and Nonwovens Association. "ERT" is an abbreviation for the European standard (almost global standard) for the measurement of superabsorbent polymers (EDANA Recommended Test Methods). In one embodiment of the present invention, unless otherwise specified, the physical properties of the superabsorbent polymer are measured in accordance with the original ERT (revised in 2002 / publicly known document).

[0052] [1-2-13] Others In this specification, the range "X~Y" means "greater than or equal to X, and less than or equal to Y".

[0053] In this specification, unless otherwise specified, "ppm" means "ppm by weight".

[0054] In this specification, "~acid (salt)" means "~acid and / or its salt." "(meth)acrylic" means "acrylic and / or methacrylic."

[0055] In this specification, the unit of volume "liter" may be written as "l" or "L".

[0056] In this specification, "weight" and "mass," "weight%" and "mass%," and "parts by weight" and "parts by mass" are treated as synonyms.

[0057] [1-3] Method for recycling water-absorbent resin according to one embodiment of the first embodiment 2 The present invention further provides a method for recycling superabsorbent resin, comprising, as one step, a recycling method (a method for manufacturing recycled superabsorbent resin) according to one aspect of the first embodiment described in [1-2] above.

[0058] Specifically, a method for recycling superabsorbent resin according to one embodiment of the first embodiment includes a urea removal step of removing urea from the swollen gel of the superabsorbent resin contained in a used absorbent article so that the urea content in the recycled superabsorbent resin is 2% by mass or less, and a regeneration step of regenerating the water absorption of the superabsorbent resin after the urea removal step.

[0059] Furthermore, a preferred configuration includes a step of crushing (cutting) the used absorbent material simultaneously with or before / after the urea removal step.

[0060] Furthermore, the separation process for separating the water-absorbent resin from used absorbent articles can be carried out in parallel with the crushing (cutting) process after the urea removal process.

[0061] Furthermore, the sterilization and / or disinfection of the water-absorbing resin can be performed in parallel with, or before or after, the urea removal process.

[0062] According to a method for producing recycled superabsorbent resin according to one embodiment of the first embodiment, it is possible to produce recycled superabsorbent resin with less discoloration. Furthermore, the decrease in water absorption performance is also reduced.

[0063] <Second Embodiment> [2-1] Technical concept of the second embodiment In actual recycling facilities for used absorbent materials, various types and manufacturers of sanitary materials are recycled together. Therefore, the absorbent resins recovered from these facilities are a mixture of materials from various manufacturers and with different performance characteristics, making it difficult to obtain recycled absorbent resins with stable absorbency.

[0064] On the other hand, superabsorbent polymers for sanitary materials have a highly balanced combination of various water absorption properties, such as water absorption ratio, water absorption ratio under pressure, and liquid permeability, to meet various applications and required characteristics. Therefore, if the water absorption properties of recycled superabsorbent polymers are unstable, it becomes extremely difficult to adjust the water absorption properties of superabsorbent polymers for sanitary materials to the desired balance.

[0065] Therefore, the inventors considered that by mixing recycled superabsorbent resin, which has unstable water absorption properties, into part of the raw materials used when manufacturing new superabsorbent resin, it would be easier to adjust the balance of physical properties. Note that "manufacturing new superabsorbent resin" refers to the manufacturing of conventional (usual) superabsorbent resin using monomers that constitute superabsorbent resin as raw materials. In this case, conventional (usual) superabsorbent resin does not include resins that partially contain the aforementioned recycled superabsorbent resin, nor superabsorbent resins that use monomers produced by recycling as part of the raw materials.

[0066] Furthermore, by using recycled superabsorbent polymer that has undergone urea removal treatment to reduce the content of urine-derived components that cause a decrease in water absorption performance and discoloration, it was discovered that a superabsorbent polymer with even better physical property balance and reduced discoloration can be obtained, thus completing the second embodiment of the present invention.

[0067] [2-2] Method for producing a water-absorbent resin according to one aspect of the second embodiment 1 In one embodiment of the second embodiment, the method for producing a water-absorbent resin is a method in which recycled water-absorbent resin, which has been subjected to a urea removal treatment and then a regeneration treatment so that the urea content is 2% by mass or less, is used as part of the raw materials in the process of producing a new water-absorbent resin. The phrase "producing a new water-absorbent resin" means producing a conventional water-absorbent resin using monomers that constitute the water-absorbent resin as raw materials. The aforementioned process is one of the following steps: preparation of an aqueous monomer solution, polymerization, pulverization of a water-containing gel, drying, or surface crosslinking.

[0068] [2-2-1] Absorbent articles In the second embodiment, the definition of “absorbent article” is derived from the definition of “[1-2-1] absorbent article” in the first embodiment described above.

[0069] [2-2-2] Water-absorbing resin In the second embodiment, the definition of "water-absorbent resin" is the same as the definition of "[1-2-2] water-absorbent resin" in the first embodiment described above.

[0070] [2-2-3] Recycled superabsorbent resin In the second embodiment, the definition of "recycled superabsorbent polymer" is based on the definition of "[1-2-3] recycled superabsorbent polymer" in the first embodiment described above.

[0071] [2-2-4] Urea removal process In the second embodiment, the definition of the "urea removal step" is the same as the definition of the "[1-2-4] urea removal step" in the first embodiment described above.

[0072] [2-2-5] Washing process In the second embodiment, the definition of "cleaning step" is derived from the definition of "[1-2-5] cleaning step" in the first embodiment described above.

[0073] [2-2-6] Dehydration process In the second embodiment, the definition of the "dehydration process" is derived from the definition of the "[1-2-6] dehydration process" in the first embodiment described above.

[0074] [2-2-7] Residual urea amount In the second embodiment, the definition of "residual urea amount" is derived from the definition of "[1-2-7] residual urea amount" in the first embodiment described above.

[0075] [2-2-8] Regeneration process In the second embodiment, the definition of "regeneration process" is derived from the definition of "[1-2-8] regeneration process" in the first embodiment described above.

[0076] [2-2-9] Crushing (cutting) process In the second embodiment, the definition of the "crushing (cutting) process" is based on the definition of "[1-2-9] crushing (cutting) process" in the first embodiment described above.

[0077] [2-2-10] Separation process In the second embodiment, the definition of "separation step" is derived from the definition of "[1-2-10] separation step" in the first embodiment described above.

[0078] [2-2-11] Sterilization / disinfection process In the second embodiment, the definition of the "sterilization and disinfection process" is the same as the definition of the "[1-2-11] sterilization and disinfection process" in the first embodiment described above.

[0079] [2-2-12] Preparation of monomer aqueous solution This process involves preparing an aqueous solution containing acrylic acid (salt) as the main component (hereinafter referred to as "monomer aqueous solution"). While monomer slurry can also be used, provided that it does not degrade the water absorption performance of the resulting superabsorbent resin, this section will explain the monomer aqueous solution for convenience.

[0080] Furthermore, the term "main component" above refers to a quantity (content) of acrylic acid (salt) that is typically 50 mol% or more, preferably 70 mol% or more, and more preferably 90 mol% or more (upper limit is 100 mol%), relative to the total monomers (excluding internal crosslinking agents) used in the polymerization reaction of the water-absorbent resin.

[0081] (Acrylic acid) In one embodiment of the present invention, acrylic acid and / or its salt (hereinafter referred to as "acrylic acid (salt)") are used as monomers constituting the water-absorbent resin, from the viewpoint of the physical properties and productivity of the water-absorbent resin obtained.

[0082] The above-mentioned "acrylic acid" may be any known acrylic acid, and the polymerization inhibitor should preferably be present in an amount of 200 ppm or less, more preferably 10 to 160 ppm, and even more preferably 20 to 100 ppm, from the viewpoint of the polymerizability of the acrylic acid and the color tone of the water-absorbent resin. The polymerization inhibitor is preferably methoxyphenols, more preferably p-methoxyphenol. Furthermore, regarding impurities in the acrylic acid, the compound described in U.S. Patent Application Publication No. 2008 / 0161512 is also applicable to one embodiment of the present invention.

[0083] Furthermore, the "acrylate" mentioned above is obtained by neutralizing the acrylic acid with the basic composition described below. The acrylate may be a commercially available acrylate (for example, sodium acrylate) or one obtained by neutralization within a water-absorbent resin manufacturing plant.

[0084] (Basic composition) In one embodiment of the present invention, "basic composition" refers to a composition containing a basic compound, such as a commercially available aqueous sodium hydroxide solution.

[0085] Examples of the basic compounds mentioned above include alkali metal carbonates and / or bicarbonates, alkali metal hydroxides, ammonia, and organic amines. Among these, a strong basicity is desirable from the viewpoint of the physical properties of the resulting superabsorbent resin. Specifically, alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide are preferred, with sodium hydroxide being more preferred.

[0086] (neutralization) In one embodiment of the present invention, neutralization can be performed by selecting or using in combination either neutralization of acrylic acid (before polymerization) or neutralization of a hydrated gel-like crosslinked polymer obtained by crosslinking acrylic acid (after polymerization) (hereinafter referred to as "post-neutralization"). Furthermore, these neutralizations can be performed in a continuous or batch manner and are not particularly limited, but a continuous manner is preferred from the viewpoint of production efficiency, etc.

[0087] When the aforementioned recycled superabsorbent polymer is used as part of the raw materials in the production of superabsorbent polymer, the recycled superabsorbent polymer may contain basic compounds, which neutralize the acrylic acid before polymerization or the water-containing gel-like crosslinked polymer after polymerization. Therefore, as described below, the neutralization rate of the acrylic acid and the neutralization rate of the superabsorbent polymer as the final product are adjusted to a predetermined range, taking these basic compounds into account.

[0088] Furthermore, the conditions for the neutralization apparatus, neutralization temperature, residence time, etc., as described in International Publication No. 2009 / 123197, U.S. Patent Application Publication No. 2008 / 0194863, etc., also apply to this invention.

[0089] In one embodiment of the present invention, the neutralization rate is preferably 10 to 90 mol%, more preferably 40 to 85 mol%, even more preferably 50 to 80 mol%, and particularly preferably 60 to 75 mol%, relative to the acid groups of the monomer. If the neutralization rate is less than 10 mol%, the water absorption ratio may decrease significantly. On the other hand, if the neutralization rate exceeds 90 mol%, a water-absorbing resin with a high water absorption ratio under pressure may not be obtained. For example, a neutralization rate of 75 mol% means a mixture of 25 mol% acrylic acid and 75 mol% acrylate. This mixture may also be referred to as a partially neutralized acrylic acid product.

[0090] The above neutralization rates are the same even in the case of post-neutralization. Furthermore, the above neutralization rates also apply to the neutralization rate of the superabsorbent resin as the final product. The present invention is characterized by using the above-mentioned recycled superabsorbent resin as part of the raw materials for the production of superabsorbent resin. If the recycled superabsorbent resin contains the above-mentioned basic compound, the neutralization with acrylic acid (before polymerization) and / or post-neutralization with the water-containing gel-like crosslinked polymer are appropriately adjusted so that the neutralization rate of the superabsorbent resin as the final product, including this basic compound, falls within a predetermined range.

[0091] (Other monomers) In one embodiment of the present invention, "other monomers" refers to monomers other than the acrylic acid (salt) described above, and a water-absorbent resin can be produced by using other monomers in combination with acrylic acid (salt).

[0092] Other monomers mentioned above 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.

[0093] (Internal crosslinking agent) As an internal crosslinking agent used in one embodiment of the present invention, the compounds described in U.S. Patent No. 6,241,928 are also applicable to one embodiment of the present invention. One or more compounds are selected from these, taking reactivity into consideration.

[0094] Furthermore, from the viewpoint of the water absorption performance of the resulting superabsorbent resin, a compound having two or more polymerizable unsaturated groups is preferably used as the internal crosslinking agent. More preferably, a compound that is thermally decomposable at the following drying temperature, and even more preferably, a compound having two or more polymerizable unsaturated groups having (poly)alkylene glycol structural units, is used as the internal crosslinking agent.

[0095] Preferably, the polymerizable unsaturated group is an allyl group, a (meth)acrylate group, and more preferably a (meth)acrylate group. Furthermore, polyethylene glycol is preferred as the (poly)alkylene glycol structural unit, with n being preferably 1 to 100, more preferably 6 to 50.

[0096] The amount of the internal crosslinking agent used is preferably 0.0001 to 10 mol%, more preferably 0.001 to 1 mol%, relative to the total monomer. By using this amount within the above range, the desired water-absorbent resin can be obtained. If the amount used is too small, the gel strength tends to decrease and the water-soluble content tends to increase, and if the amount used is too large, the water absorption ratio tends to decrease, which is undesirable. The mol% relative to the total monomer refers to the percentage of moles of the internal crosslinking agent relative to the total number of moles of monomers contained in the monomer aqueous solution.

[0097] In one embodiment of the present invention, a method is preferably applied in which a predetermined amount of internal crosslinking agent is added to an aqueous monomer solution in advance, and the crosslinking reaction occurs simultaneously with polymerization. On the other hand, methods other than this method can also be employed, such as post-crosslinking by adding the internal crosslinking agent during and / or after polymerization; radical crosslinking using a radical polymerization initiator; or radiation crosslinking using active energy rays such as electron beams and ultraviolet rays. Furthermore, these methods can also be used in combination.

[0098] (Other substances added to monomer aqueous solutions) In one embodiment of the present invention, from the viewpoint of improving the physical properties of the resulting water-absorbing resin, the following substances may be added when preparing the monomer aqueous solution.

[0099] Specifically, hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salt), and polyacrylic acid (salt) crosslinked products are added, preferably in amounts of 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 is 0% by mass); foaming agents such as carbonates and azo compounds, surfactants, chelating agents, chain transfer agents, etc. are added, preferably in amounts of 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less (lower limit is 0% by mass).

[0100] Furthermore, the above-mentioned substance may be added not only in the form of an aqueous monomer solution, but also in the form of an additive during polymerization, or these forms may be used in combination.

[0101] Furthermore, when a water-soluble resin or a water-absorbent resin is used as the hydrophilic polymer, a graft polymer or a water-absorbent resin composition (e.g., starch-acrylic acid polymer, PVA-acrylic acid polymer, etc.) can be obtained. These polymers and water-absorbent resin compositions are also within the scope of the present invention.

[0102] [2-2-13] Polymerization process This step involves polymerizing the acrylic acid (salt) monomer aqueous solution obtained in the above monomer aqueous solution preparation step to obtain a hydrated gel-like crosslinked polymer (hereinafter referred to as "hydrated gel").

[0103] (Polymerization initiator) The polymerization initiator used in one embodiment of the present invention is not particularly limited, as it is appropriately selected depending on the polymerization mode and other factors. Examples include pyrolysis-type polymerization initiators, photodegradation-type polymerization initiators, or redox-type polymerization initiators that use a reducing agent to promote the decomposition of these polymerization initiators. Specifically, one or more polymerization initiators disclosed in U.S. Patent No. 7,265,190 are used. From the viewpoint of ease of handling of the polymerization initiator and the physical properties of the water-absorbent resin, peroxides or azo compounds are preferably used, more preferably peroxides, and even more preferably persulfates.

[0104] The amount of polymerization initiator used is preferably 0.001 to 1 mol%, more preferably 0.001 to 0.5 mol%, relative to the monomer. The amount of reducing agent used is preferably 0.0001 to 0.02 mol%, relative to the monomer. The mol% relative to the monomer refers to the percentage of moles of the reducing agent relative to the total number of moles of monomer contained in the monomer aqueous solution.

[0105] Alternatively, the polymerization reaction may be carried out by irradiating with active energy rays such as radiation, electron beams, or ultraviolet rays instead of the polymerization initiator, or these active energy rays may be used in combination with the polymerization initiator.

[0106] (polymerization form) The polymerization mode applied to one embodiment of the present invention is not particularly limited, but from the viewpoint of water absorption characteristics and ease of polymerization control, spray droplet polymerization, aqueous solution polymerization, inverted phase suspension polymerization, more preferably aqueous solution polymerization, inverted phase suspension polymerization, and even more preferably aqueous solution polymerization are used. Among these, continuous aqueous solution polymerization is particularly preferred, and either continuous belt polymerization or continuous kneader polymerization can be applied.

[0107] Specific polymerization methods include continuous belt polymerization, disclosed in U.S. Patent No. 4,893,999, No. 6,241,928, and U.S. Patent Publication No. 2005 / 215734, and continuous kneader polymerization, disclosed in U.S. Patent No. 6,987,151, No. 6710,141, and others. By employing these continuous aqueous solution polymerization methods, the production efficiency of water-absorbent resins can be improved.

[0108] Furthermore, preferred forms of the above-mentioned continuous aqueous solution polymerization include "high-temperature initiation polymerization" and "high-concentration polymerization." "High-temperature initiation polymerization" refers to a form in which polymerization is initiated at a temperature of preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher (upper limit is the boiling point) of the monomer aqueous solution. "High-concentration polymerization" refers to a form in which polymerization is carried out at a monomer concentration of preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 45% by mass or higher (upper limit is the saturation concentration). These polymerization forms can also be used in combination.

[0109] Furthermore, in one embodiment of the present invention, polymerization can be carried out under an air atmosphere, but from the viewpoint of the color tone of the resulting water-absorbing resin, polymerization is preferably carried out under an inert gas atmosphere such as nitrogen or argon. In this case, for example, it is preferable to control the oxygen concentration to 1% by volume or less. It is also preferable to replace the dissolved oxygen in the monomer aqueous solution with an inert gas (for example, dissolved oxygen; less than 1 mg / l).

[0110] Furthermore, in one embodiment of the present invention, the polymerization method can also be foam polymerization, in which bubbles (especially the above-mentioned inert gas, etc.) are dispersed in an aqueous monomer solution for polymerization.

[0111] [2-2-14] Hydrated gel grinding process This step involves grinding the water-containing gel obtained in the polymerization step using, for example, a screw extruder such as a kneader or meat chopper, or a gel grinder such as a cutter mill, to obtain particulate water-containing gel (hereinafter referred to as "particulate water-containing gel"). Note that if the polymerization step is kneader polymerization, the polymerization step and water content The gel pulverization process is carried out simultaneously. However, in cases where particulate water-containing gel is obtained directly during the polymerization process, such as in gas-phase polymerization or reverse-phase suspension polymerization, the gel pulverization process may not be performed.

[0112] For gel grinding conditions and forms other than those mentioned above, the contents disclosed in International Publication No. 2011 / 126079 are preferably applied to the present invention.

[0113] [2-2-15] Drying process This step involves drying the particulate water-containing gel obtained in the polymerization step and / or water-containing gel pulverization step to a desired resin solid content to obtain a dried polymer. The resin solid content is determined from the loss on drying (the change in mass when 1 g of water-absorbing resin is heated at 180°C for 3 hours), and is preferably 80% by mass or more, more preferably 85-99% by mass, even more preferably 90-98% by mass, and particularly preferably 92-97% by mass.

[0114] The drying method for the above-mentioned particulate water-containing gel is not particularly limited, but examples include heating 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 steam. Among these, hot air drying is preferred from the viewpoint of drying efficiency, and band drying, which is performed by hot air drying on a ventilated belt, is more preferred.

[0115] The drying temperature (hot air temperature) in the above-mentioned hot air drying is preferably 120 to 250°C, more preferably 150 to 200°C, from the viewpoint of the color tone of the water-absorbing resin and drying efficiency. Drying conditions other than the above-mentioned drying temperature, such as the hot air velocity and drying time, may be set appropriately according to the water content, total weight, and target resin solid content of the particulate water-containing gel to be dried. When performing band drying, various conditions described in International Publication Nos. 2006 / 100300, 2011 / 025012, 2011 / 025013, 2011 / 111657, etc., may be appropriately applied as drying conditions.

[0116] By setting the drying temperature, drying time, etc., within the above range, the resulting water-absorbing resin can have its CRC (water absorption ratio), Ext (water-soluble content), and color tone within the desired range.

[0117] [2-2-16] Surface crosslinking process This process involves creating a portion with a higher crosslinking density in the surface layer (the portion of the water-absorbent resin powder from the surface down to several tens of micrometers) obtained through the above-described process, and consists of a mixing step, a heat treatment step, and a cooling step (optional).

[0118] In the surface crosslinking process, a surface-crosslinked water-absorbent resin (water-absorbent resin particles) is obtained by radical crosslinking on the surface of the water-absorbent resin powder, surface polymerization, crosslinking reaction with a surface crosslinking agent, etc.

[0119] (Surface crosslinking agent) The surface crosslinking agent used in one embodiment of the present invention is not particularly limited, but may include organic or inorganic surface crosslinking agents. Among these, organic surface crosslinking agents that react with carboxyl groups are preferred from the viewpoint of the physical properties of the water-absorbing resin and the handling of the surface crosslinking agent. For example, one or more surface crosslinking agents disclosed in U.S. Patent No. 7183456 are mentioned. More specifically, examples include polyhydric alcohol compounds, epoxy compounds, halo-epoxy compounds, polyhydric amine compounds or condensates thereof with halo-epoxy compounds, oxazoline compounds, oxazolidinone compounds, polyhydric metal salts, alkylene carbonate compounds, cyclic urea compounds, and the like.

[0120] The amount of surface crosslinking agent used (total amount if multiple are used) is preferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts by mass, per 100 parts by mass of water-absorbing resin powder. Furthermore, the surface crosslinking agent is preferably added as an aqueous solution, in which case the amount of water used is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, per 100 parts by mass of water-absorbing resin powder. If a hydrophilic organic solvent is used as needed, the amount used is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, per 100 parts by mass of water-absorbing resin powder.

[0122] (Mixing process) This step involves mixing the water-absorbent resin powder with the surface crosslinking agent. The method of mixing the surface crosslinking agent is not particularly limited, but for example, a surface crosslinking agent solution may be prepared in advance, and this solution may be mixed with the water-absorbent resin powder, preferably by spraying or dropping, and more preferably by spraying.

[0123] The apparatus for performing the mixing is not particularly limited, but preferably a high-speed stirring type mixer, and more preferably a high-speed stirring type continuous mixer.

[0124] (Heat treatment process) This process involves applying heat to the mixture discharged from the above mixing process to induce a crosslinking reaction on the surface of the water-absorbent resin powder.

[0125] The apparatus for carrying out the crosslinking reaction is not particularly limited, but a paddle dryer is preferred. The reaction temperature in the crosslinking reaction is set appropriately depending on the type of surface crosslinking agent used, but is preferably 50 to 300°C, more preferably 100 to 200°C.

[0126] (cooling process) This step is an optional step that is performed as needed after the heat treatment step described above. The cooling device is not particularly limited, but is preferably a device with the same specifications as the device used in the heat treatment step, and more preferably a paddle dryer. This is because it can be used as a cooling device by changing the heat transfer medium to a refrigerant. The water-absorbing resin particles obtained in the heat treatment step are forcibly cooled to 40 to 80°C, more preferably 50 to 70°C, as needed in this cooling step.

[0127] [2-3] Method for producing a water-absorbent resin according to one aspect of the second embodiment 2 One embodiment of the present invention further provides a method for producing a water-absorbent resin according to one aspect of the second embodiment described in [2-2] above.

[0128] Specifically, a method for producing a superabsorbent resin according to one embodiment of the second embodiment is a method in which recycled superabsorbent resin, which has been subjected to a urea removal treatment and then a regeneration treatment so that the urea content in the recycled superabsorbent resin is 2% by mass or less, is used as part of the raw materials in the process of producing the superabsorbent resin. The said process is one of the following steps: preparation of monomer aqueous solution, polymerization, pulverization of water-containing gel, drying, and surface crosslinking.

[0129] Methods for adding recycled superabsorbent polymer include adding the recycled superabsorbent polymer as is, adding the recycled superabsorbent polymer in a swollen gel state after being swollen with water, and adding the recycled superabsorbent polymer in a dispersed state in water.

[0130] When adding recycled superabsorbent polymer in the "preparation step of monomer aqueous solution," the recycled superabsorbent polymer may be mixed with the monomer aqueous solution, or it may be mixed with water beforehand and then mixed with other raw materials. From the viewpoint of uniform mixing of raw materials, it is preferable to mix the recycled superabsorbent polymer with the monomer aqueous solution.

[0131] When adding recycled superabsorbent polymer in the polymerization process, it may be added before or after the start of polymerization. From the viewpoint of uniformity of the polymer components, it is preferable to add the recycled superabsorbent polymer before the start of polymerization.

[0132] Recycled superabsorbent polymer water content When adding the recycled superabsorbent resin during the gel pulverization process, it may be added before pulverization or during pulverization. It may also be added in portions. From the viewpoint of uniformity of the gel components, it is preferable to add the recycled superabsorbent resin before gel pulverization.

[0133] When adding recycled superabsorbent polymer in the "drying process," the water-containing gel and the recycled superabsorbent polymer may be mixed before drying, or they may be dried without mixing. Mixing the recycled superabsorbent polymer powder and the water-containing gel before drying allows the water from the water-containing gel to transfer to the recycled superabsorbent polymer powder. Therefore, this is preferable because it can be expected to shorten the drying speed and reduce the formation of undried gel.

[0134] When adding recycled superabsorbent polymer in the "surface crosslinking process," the dried superabsorbent polymer and the recycled superabsorbent polymer may be mixed beforehand, or they may be processed without mixing. From the viewpoint of uniform mixing between the surface treatment liquid and the superabsorbent polymer, it is preferable to use powdered recycled superabsorbent polymer and handle everything in powder form.

[0135] The proportion of recycled superabsorbent polymer to all superabsorbent polymer raw materials is 1 to 60% by mass, preferably 1 to 50% by mass, more preferably 1 to 40% by mass, and even more preferably 1 to 30% by mass. The term "all superabsorbent polymer raw materials" refers to all raw materials used in the manufacturing process of the superabsorbent polymer. Examples of such raw materials include acrylic acid (salt), basic composition, other monomers, internal crosslinking agents, substances added to monomer aqueous solutions (such as starch), polymerization initiators, and surface crosslinking agents. The term "manufacturing process of the superabsorbent polymer" refers to the monomer aqueous solution preparation step, polymerization step, water-containing gel pulverization step, drying step, and surface crosslinking step described in [2-2] above. The proportion is the percentage of the mass of the solid content of the recycled superabsorbent polymer to the total mass of the solid content of all superabsorbent polymer raw materials and the solid content of the recycled superabsorbent polymer.

[0136] According to a method for producing a water-absorbent resin according to one embodiment of the second embodiment, it is possible to produce a water-absorbent resin with less reduction in water absorption performance and less discoloration due to the mixed recycled water-absorbent resin.

[0137] <Third Embodiment> [3-1] Technical concept of the third embodiment In the second embodiment, the inventors stated that by mixing recycled superabsorbent polymer, which has undergone regeneration treatment after urea removal treatment, into a portion of the raw materials used when manufacturing a new superabsorbent polymer, a superabsorbent polymer with even better physical property balance and reduced discoloration can be obtained. The phrase "manufacturing a new superabsorbent polymer" refers to manufacturing a conventional superabsorbent polymer using monomers that constitute the superabsorbent polymer as raw materials.

[0138] However, when recycled superabsorbent polymers are used as part of the raw materials in the manufacture of superabsorbent polymers, the recycled polymers absorb moisture from the raw materials, increasing the viscosity of the mixed material. This can lead to problems such as reduced handling properties and non-uniformity of the mixed material. Therefore, the inventors have found that by solubilizing the recycled superabsorbent polymers to create a water-soluble polymer, the viscosity of the mixed material can be significantly reduced, thereby resolving the problems of reduced handling properties and non-uniformity of the mixed material.

[0139] Furthermore, this solubilization treatment can be applied not only to recycled superabsorbent polymers, but also to recycled superabsorbent polymer intermediate products after urea removal treatment, or to used superabsorbent polymers after urea removal treatment. Specifically, recycled superabsorbent polymers that have been regenerated after urea removal treatment to a urea content of 2% by mass or less; recycled superabsorbent polymer intermediate products after urea removal treatment; or used superabsorbent polymers after urea removal treatment were used as part of the raw materials when newly manufacturing superabsorbent polymers. The inventors found that this resulted in a superabsorbent polymer with better physical property balance and reduced discoloration, and thus completed a third embodiment of the present invention.

[0140] The "recycled superabsorbent resin intermediate treatment product after urea removal treatment" and the "used superabsorbent resin after urea removal treatment" will be described later.

[0141] [3-2] Method for manufacturing recycled superabsorbent resin according to one aspect of the third embodiment 1 In one embodiment of the third embodiment, the method for producing a water-absorbent resin includes a solubilization step in which a recycled water-absorbent resin that has been regenerated after a urea removal treatment so that the urea content is 2% by mass or less; an intermediate product of recycled water-absorbent resin after urea removal treatment; or a used water-absorbent resin after urea removal treatment is solubilized to obtain a water-soluble polymer. The method for producing a water-absorbent resin further includes using the water-soluble polymer as part of the raw materials in any of the steps of producing the water-absorbent resin: the monomer aqueous solution preparation step, the polymerization step, the water-containing gel pulverization step, the drying step, and the surface crosslinking step.

[0142] [3-2-1] Absorbent articles In the third embodiment, the definition of “absorbent article” is derived from the definition of “[1-2-1] absorbent article” in the first embodiment described above.

[0143] [3-2-2] Water-absorbing resin In the third embodiment, the definition of "absorbent polymer" is based on the definition of "[1-2-2] absorbent polymer" in the first embodiment described above.

[0144] [3-2-3] Recycled superabsorbent resin In the third embodiment, the definition of "recycled superabsorbent polymer" is derived from the definition of "[1-2-3] recycled superabsorbent polymer" in the first embodiment described above.

[0145] [3-2-4] Recycled superabsorbent polymer intermediate treatment product after urea removal treatment In the third embodiment, "recycled superabsorbent resin intermediate treatment product after urea removal treatment" refers to a product obtained at an intermediate stage of the regeneration process carried out to produce the "recycled superabsorbent resin" described above.

[0146] Specifically, for example, when using the method described in Japanese Patent Publication No. 2003-225645 as a recycling method, the "recycled superabsorbent polymer intermediate treatment product after urea removal treatment" is one of the following i) to iii). i) A water-containing gel obtained by treating the superabsorbent polymer in used absorbent articles with an acidic solution after urea removal treatment (dehydration by contact with a polyvalent metal salt) (neutralization and drying treatments have not been performed). ii) After the urea removal treatment, the material is treated with an acidic solution and then dried (neutralization treatment is not performed). iii) After the urea removal treatment, the material is treated with an acidic solution and then neutralized (without drying).

[0147] The method described in Japanese Patent Publication No. 2003-225645 involves contacting a water-absorbing resin that has absorbed urine with a polyvalent metal salt to dehydrate it, then treating it with an acidic solution, neutralizing it with an alkali metal salt, and finally drying it.

[0148] [3-2-5] Used superabsorbent resin after urea removal treatment In the third embodiment, "used superabsorbent resin after urea removal treatment" refers to superabsorbent resin recovered from used absorbent articles (superabsorbent resin contained in used absorbent articles) that has undergone urea removal treatment (urea removal process).

[0149] [3-2-6] Urea removal process In the third embodiment, the definition of the "urea removal step" is the same as the definition of the "[1-2-4] urea removal step" in the first embodiment described above.

[0150] [3-2-7] Washing process In the third embodiment, the definition of "cleaning step" is the same as the definition of "[1-2-5] cleaning step" in the first embodiment described above.

[0151] [3-2-8] Dehydration process In the third embodiment, the definition of the "dehydration process" is the same as the definition of the "[1-2-6] dehydration process" in the first embodiment described above.

[0152] [3-2-9] Residual urea amount In the third embodiment, the definition of "residual urea amount" is derived from the definition of "[1-2-7] residual urea amount" in the first embodiment described above.

[0153] [3-2-10] Regeneration process In the third embodiment, the definition of “regeneration process” is derived from the definition of “[1-2-8] regeneration process” in the first embodiment described above.

[0154] [3-2-11] Solubilization process In the third embodiment, the solubilization step is a process of solubilizing the superabsorbent resin and / or recycled superabsorbent resin contained in the used absorbent article in water to obtain a water-soluble polymer. The water-soluble polymer obtained in the solubilization step may be entirely solubilized, or it may be a partially solubilized product containing polymer that is not partially solubilized.

[0155] There are no particular limitations on the method for solubilizing superabsorbent polymers, as long as it can decompose the polymer and make it solubilized in water. Examples include: a method of decomposing and solubilizing superabsorbent polymers with a reducing agent and transition metal ions (Japanese Patent Publication No. 2019-131789); a method of solubilizing superabsorbent polymers using ascorbic acid as a reducing agent under pH 4 to 7.5 conditions (Japanese Patent No. 3146053); a method of solubilizing superabsorbent polymers by heat treatment in the presence of an oxidizing agent (Japanese Patent No. 3091251); a method of solubilizing superabsorbent polymers using chlorine dioxide under acidic conditions (Japanese Patent Publication No. 2019-108639); a method of solubilizing superabsorbent polymers using hydrogen peroxide (Japanese Patent Publication No. 2019-108640); and a method of solubilizing superabsorbent polymers with ozone-containing water (Japanese Patent Publication No. 2019-085343, etc.).

[0156] The resulting water-soluble polymer may be used directly in an aqueous solution, or it may be dried into a solid before use.

[0157] From the viewpoint of handling properties and miscibility with other raw materials, a lower molecular weight is preferable for water-soluble polymers. However, from the viewpoint of balancing the water absorption properties of the water-absorbent resin and reducing soluble content, a higher molecular weight is preferable. A weight-average molecular weight of 1 to 1 million is preferred, more preferably 200,000 to 800,000, and even more preferably 300,000 to 600,000, considering the balance between these two factors.

[0158] [3-2-12] Preparation of monomer aqueous solution In the third embodiment, the definition of "preparation step of monomer aqueous solution" is based on the definition of "[2-2-12] preparation step of monomer aqueous solution" in the second embodiment described above.

[0159] [3-2-13] Polymerization process In the third embodiment, the definition of "polymerization process" is derived from the definition of "[2-2-13] polymerization process" in the second embodiment described above.

[0160] [3-2-14] Hydrated gel grinding process In the third embodiment, the definition of the "hydrogenated gel grinding step" is based on the definition of the "[2-2-14] hydrogenated gel grinding step" in the second embodiment described above.

[0161] [3-2-15] Drying process In the third embodiment, the definition of "drying process" is derived from the definition of "[2-2-15] drying process" in the second embodiment described above.

[0162] [3-2-16] Surface crosslinking process In the third embodiment, the definition of "surface crosslinking process" is based on the definition of "[2-2-16] surface crosslinking process" in the second embodiment described above.

[0163] [3-3] Method for producing a water-absorbent resin according to one aspect of the third embodiment 2 One embodiment of the present invention further provides a method for producing a water-absorbent resin according to one aspect of the second embodiment described in [3-2] above.

[0164] Specifically, in one embodiment of the third embodiment, the method for producing a water-absorbent resin includes a solubilization step to obtain a water-soluble polymer by performing a solubilization treatment on a recycled water-absorbent resin that has been regenerated after urea removal treatment; an intermediate processed recycled water-absorbent resin after urea removal treatment; or a used water-absorbent resin after urea removal treatment, so that the urea content is 2% by mass or less. In the method for producing the water-absorbent resin, the water-soluble polymer is used as part of the raw materials in any of the steps of preparing an aqueous monomer solution, polymerization, water-containing gel pulverization, drying, or surface crosslinking when producing the water-absorbent resin.

[0165] Methods for adding the solubilized water-soluble polymer in the above step include adding it as an aqueous solution, adding it as a powder after drying, and adding it as a slurry with partially undissolved material remaining. If the solubilized water-soluble polymer contains a basic compound, adding the polymer may change the neutralization rate of the monomer and the neutralization rate of the water-absorbing resin in the final product. Therefore, the amount of basic compound contained in the solubilized water-soluble polymer is taken into consideration, and the neutralization rate of the monomer and the water-absorbing resin in the final product are adjusted to fall within a predetermined range.

[0166] When adding a solubilized water-soluble polymer in the "preparation step of monomer aqueous solution," the solubilized water-soluble polymer may be mixed with the monomer aqueous solution, or the solubilized water-soluble polymer may be mixed with water beforehand and then mixed with other raw materials. From the viewpoint of uniform mixing of raw materials, it is preferable to mix the solubilized water-soluble polymer with the monomer aqueous solution.

[0167] When adding a solubilized water-soluble polymer in the "polymerization step," the solubilized water-soluble polymer may be added before the start of polymerization or after the start of polymerization. From the viewpoint of uniformity of the polymer components, it is preferable to add the solubilized water-soluble polymer before the start of polymerization.

[0168] Solubilized water-soluble polymers water contentWhen adding during the gel pulverization process, the solubilized water-soluble polymer may be added before pulverization or during pulverization. It may also be added in portions. From the viewpoint of uniformity of the gel components, it is preferable to add the solubilized water-soluble polymer before gel pulverization.

[0169] When adding a solubilized water-soluble polymer during the "drying process," the water-containing gel and the solubilized water-soluble polymer may be mixed before drying, or they may be dried without mixing.

[0170] When adding a solubilized water-soluble polymer in the "surface crosslinking step," the water-absorbent resin that has been dried beforehand may be mixed with the solubilized water-soluble polymer, or the process may be carried out without mixing. From the viewpoint of uniform mixing between the surface treatment liquid and the water-absorbent resin, it is preferable to use a powdered solubilized water-soluble polymer and handle everything in powder form.

[0171] The proportion of the solubilized water-soluble polymer relative to all water-absorbent resin raw materials is 1 to 60% by mass, preferably 1 to 50% by mass, more preferably 1 to 40% by mass, and even more preferably 1 to 30% by mass. "All water-absorbent resin raw materials" are as described above. The above proportion is the percentage of the mass of the solid content of the water-soluble polymer relative to the total mass of the solid content of all water-absorbent resin raw materials and the solid content of the water-soluble polymer.

[0172] According to a method for producing a water-absorbent resin according to one embodiment of the third embodiment, it is possible to produce a water-absorbent resin with less reduction in water absorption performance and less discoloration due to the added solubilized water-soluble polymer.

[0173] This invention encompasses the following inventions.

[0174] One embodiment of the present invention relates to a method for recycling superabsorbent resin contained in a used absorbent article, comprising: a urea removal step of removing urea from the superabsorbent resin contained in the used absorbent article so that the urea content in the recycled superabsorbent resin is 2% by mass or less; and a regeneration step of regenerating the water absorption properties of the superabsorbent resin after the urea removal step.

[0175] A second embodiment of the present invention relates to a method for producing a water-absorbent resin, wherein the recycled water-absorbent resin, which has been subjected to a urea removal treatment and then a regeneration treatment so that the urea content is 2% by mass or less, is used as part of the raw materials in the process of producing a (conventional) water-absorbent resin using monomers constituting the water-absorbent resin as raw materials. The process is one of the following steps: preparation of an aqueous monomer solution, polymerization, pulverization of a water-containing gel, drying, and surface crosslinking.

[0176] A third embodiment of the present invention relates to a method for producing a water-soluble polymer of a water-absorbing resin, wherein a recycled water-absorbing resin that has been regenerated after urea removal so that the urea content is 2% by mass or less; an intermediate processed recycled water-absorbing resin after urea removal; or a used water-absorbing resin after urea removal is subjected to a decomposition treatment to solubilize it in water. Furthermore, the invention relates to a method for producing a water-soluble polymer of a water-absorbing resin in which this water-soluble polymer is used as part of the raw materials in a process for producing a (conventional) water-absorbing resin using monomers constituting the water-absorbing resin as raw materials. The process is one of the following steps: preparation of an aqueous monomer solution, polymerization, pulverization of a water-containing gel, drying, and surface crosslinking.

[0177] Furthermore, the present invention includes the following inventions.

[0178] [1] A method for recycling absorbent resin contained in used absorbent articles, A urea removal step is performed to remove urea from the absorbent resin contained in the used absorbent article so that the urea content in the recycled absorbent resin is 2% by mass or less. A regeneration step to restore the water absorption properties of the water-absorbing resin after the urea removal step, A method for recycling superabsorbent polymers, comprising the following characteristics.

[0179] [2] The method for recycling an absorbent resin according to [1], wherein the urea removal step includes a washing step of washing the swollen gel of the absorbent resin with an aqueous solution, and / or a dehydration step of dehydrating the absorbed liquid from the swollen gel.

[0180] [3] Recycled superabsorbent resin derived from used absorbent articles, manufactured by the recycling method described in [1] or [2] above.

[0181] [4] A method for producing an absorbent resin, comprising adding recycled absorbent resin derived from used absorbent articles as described in [3] above, in the process of producing an absorbent resin using monomers constituting the absorbent resin as raw materials.

[0182] [5] The method for producing an absorbent resin according to [4], wherein the recycled absorbent resin derived from used absorbent articles accounts for 1 to 60% by mass of all absorbent resin raw materials.

[0183] [6] A method for recycling absorbent resin according to [1] or [2], further comprising a solubilization step of solubilizing the recycled absorbent resin derived from the used absorbent article after the regeneration step.

[0184] [7] A water-soluble polymer derived from recycled superabsorbent resin, manufactured by the recycling method described in [6] above.

[0185] [8] A method for producing an absorbent resin, wherein in the process of producing an absorbent resin using monomers constituting the absorbent resin as raw materials, a water-soluble polymer derived from the recycled absorbent resin described in [7] above is used as part of the raw materials.

[0186] [9] A method for recycling absorbent resin contained in used absorbent articles, A urea removal step is performed to remove urea from the absorbent resin contained in used absorbent articles so that the urea content in the absorbent resin contained in the used absorbent articles is 2% by mass or less. A method for recycling absorbent resins, comprising a solubilization step of solubilizing the absorbent resin contained in the used absorbent article after the urea removal step.

[0187]

[10] A water-soluble polymer derived from used absorbent articles, manufactured by the recycling method described in [9] above.

[0188]

[11] A method for producing an absorbent resin, wherein in the process of producing an absorbent resin using monomers constituting the absorbent resin as raw materials, a water-soluble polymer derived from used absorbent articles as described in

[10] above is used as part of the raw materials.

[0189]

[12] An absorbent article comprising the recycled superabsorbent resin described in [3] above.

[0190]

[13] An absorbent article comprising a water-absorbent resin manufactured by the manufacturing method described in any of [4], [5], [8], or

[11] above.

[0191] The present invention is not limited to the embodiments described above, 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. [Examples]

[0192] The present invention will be described more specifically with reference to the following examples and comparative examples, but the present invention is not limited to these, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention.

[0193] (a) Urea content 200.0 g of 0.9% by mass saline solution was weighed into a 250 ml lidded plastic container. 1.00 g of superabsorbent resin or water absorbent was added to the aqueous solution and stirred with a stirrer for 16 hours to extract the urea contained in the resin. This extract was filtered through a filter (0.45 μm), and the amount of urea contained in the superabsorbent resin was quantified by analyzing the resulting filtrate using liquid chromatography (HPLC).

[0194] (b) CRC "CRC" is an abbreviation for Centrifuge Retention Capacity, and it indicates the absorption ratio of a water-absorbent polymer in 0.90 mass% saline solution under no pressure for 30 minutes.

[0195] 0.200 g of superabsorbent polymer was uniformly placed in a nonwoven fabric bag (85 mm x 60 mm) made of Nankoku Pulp Industry Co., Ltd., product name: Heatron Paper, model: GSP-22, and heat-sealed. Then, the bag was 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 removed and drained for 3 minutes using a centrifuge (Kokusan Co., Ltd., centrifuge: model H-122) at a centrifugal force (250 G) as described in edana ABSORBENCY II 441.1-99, and the mass W1 (g) of the bag was measured. The same procedure was also performed without the superabsorbent polymer, and the mass W0 (g) was measured. From these W1 and W0 values, the centrifuge holding capacity (CRC) (g / g) was calculated according to formula 1 below. CRC(g / g)=(W1(g)-W0(g)) / (mass of water absorbent resin(g) ) -1··(1)

[0196] A stainless steel 400-mesh wire mesh was fused to the bottom of a plastic support cylinder with an inner diameter of 25 mm. Next, 0.16 g of water-absorbing resin was uniformly scattered on the mesh under room temperature (20-25°C) and humidity of 50 RH. A piston and load were then placed on top in that order, and the mass W2 (g) of this measuring device was measured. The piston was adjusted to uniformly apply a load of 2.07 kPa (0.3 psi) to the water-absorbing resin, had an outer diameter slightly smaller than 25 mm, did not create a gap with the support cylinder, and did not hinder vertical movement.

[0197] A 90mm diameter glass filter (manufactured by Sogo Rikagaku Glass Manufacturing Co., Ltd., pore diameter: 100-120μm) was placed inside a 150mm diameter Petri dish, and 0.90% by mass saline solution (20-25℃) was added to the same level as the top surface of the glass filter. A 90mm diameter filter paper (ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No.2), thickness 0.26mm, retained particle size 5μm) was placed on top, ensuring that the entire surface was wet, and any excess liquid was removed. The above measuring apparatus was placed on the filter paper and the liquid was absorbed under load. After 1 hour, the measuring apparatus was lifted, and its mass W3 (g) was measured. Then, AUL0.3 was calculated from W2 and W3 according to formula 2 below. AUL0.3 = (W3(g) - W2(g)) / (mass of absorbent resin) ... (2) (d) Dry color tone The coloration of the superabsorbent polymer after the drying process was observed visually. Superabsorbent polymers without coloration appear white, while those with coloration due to impurities such as urea appear yellow. The closer to white, the better.

[0198] [Manufacturing Example 1] Method for producing water-absorbent resin (1) 550 parts of a 38% by mass aqueous solution of sodium acrylate (neutralization rate 71 mol%) were dissolved in 0.44 parts of polyethylene glycol diacrylate (molecular weight 523) to prepare the reaction solution. Next, this reaction solution was degassed under a nitrogen gas atmosphere for 30 minutes.

[0199] Next, the reaction solution was supplied to a jacketed stainless steel double-arm kneader with two sigma-shaped blades and an openable / closable lid, and the system was purged with nitrogen gas while maintaining the reaction solution at 30°C. Subsequently, 0.24 parts of ammonium persulfate and 0.012 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20-95°C, and the hydrated gel-like polymer was removed 60 minutes after the start of polymerization. The obtained hydrated gel-like polymer was dried with hot air at 150°C for 100 minutes. Next, the dried material was pulverized using a vibratory mill, and an amorphous crushed water-absorbing resin precursor with an average particle size of 400 μm was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve.

[0200] 100 parts by mass of the obtained superabsorbent resin precursor was mixed with a surface crosslinking agent composition solution consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The resulting mixture was heat-treated at 210°C for 40 minutes to obtain a white superabsorbent resin (1). The average particle size of the superabsorbent resin (1) was 400 μm, the water-soluble component content was 9%, the CRC was 41.7 (g / g), and the AUL 0.3 was 31.7 (g / g).

[0201] <Manufacturing of recycled water-absorbing resin> [Example 1] 20.0 g of superabsorbent resin (1) was placed in a beaker, and 600.0 g of artificial urine (urea 1.9% by mass, sodium chloride 0.80% by mass, magnesium chloride hexahydrate 0.10% by mass, calcium chloride dihydrate 0.10% by mass, deionized water 97.1% by mass) was added thereto. After standing for 24 hours, simulated used superabsorbent resin was prepared.

[0202] 1800.0g of deionized water was added to a beaker and stirred for 15 minutes to perform a water washing operation. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1956.5g of the filtrate was returned to the beaker. 1900.0g of acetone was added, and the mixture was stirred for 15 minutes to perform a dehydration operation. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 104.2g of the filtrate was dried in a 180°C oven for 3 hours. The dried material was then pulverized using a vibratory mill, and an amorphous, fragmented recycled superabsorbent polymer precursor (1) was obtained by passing it through an 850μm sieve and remaining on a 106μm sieve.

[0203] 100 parts by mass of the obtained recycled superabsorbent polymer precursor (1) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a white recycled superabsorbent polymer (1). The urea content of the recycled superabsorbent polymer (1) was 1.5% by mass, the CRC was 40.0 (g / g), and the AUL 0.3 was 30.2 (g / g).

[0204] [Example 2] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0205] 1800.0g of deionized water was added to a beaker and stirred for 15 minutes to perform a water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1956.3g of the filtrate was returned to the beaker. 1800.0g of fresh deionized water was added, and stirred for 15 minutes to perform a second water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 2764.8g of the filtrate was returned to the beaker. 2800.0g of acetone was added, and stirred for 15 minutes to perform a dehydration. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 88.1g of the filtrate was dried in an oven at 180°C for 3 hours. The dried material was then crushed using a vibratory mill, and an amorphous, crushed recycled superabsorbent polymer precursor (2) was obtained by passing it through an 850μm sieve and remaining on a 106μm sieve.

[0206] 100 parts by mass of the obtained recycled superabsorbent polymer precursor (2) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a white recycled superabsorbent polymer (2). The urea content of the recycled superabsorbent polymer (2) was 0.6% by mass, the CRC was 41.1 (g / g), and the AUL 0.3 was 30.7 (g / g).

[0207] [Example 3] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0208] 1800.0g of deionized water was added to a beaker and stirred for 15 minutes to perform a water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1948.3g of the filtrate was returned to the beaker. 1500.0g of 2.7% by mass citric acid aqueous solution was added and stirred for 15 minutes to perform a dehydration. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 372.8g of the filtrate was returned to the beaker. 1800.0g of fresh deionized water was added and stirred for 15 minutes to perform a water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 606.0g of the filtrate was returned to the beaker. Next, while stirring the filtrate with a spatula, 64.2g of 10% sodium hydroxide solution was added dropwise to neutralize it. After standing for 15 minutes, it was dried in an oven at 180°C for 3 hours. Next, the dried material was pulverized using a vibratory mill, and then an irregularly shaped, crushed recycled superabsorbent polymer precursor (3) was obtained, which passed through a sieve with an 850 μm mesh size and remained on a sieve with a 106 μm mesh size.

[0209] 100 parts by mass of the obtained recycled superabsorbent polymer precursor (3) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a white recycled superabsorbent polymer (3). The urea content of the recycled superabsorbent polymer (3) was 1.7% by mass, the CRC was 39.3 (g / g), and the AUL 0.3 was 30.1 (g / g).

[0210] [Comparative Example 1] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0211] The simulated used superabsorbent polymer was dried in an oven at 180°C for 3 hours. Next, the dried material was crushed using a vibratory mill, and an amorphous, fragmented comparative recycled superabsorbent polymer precursor (C1) was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve.

[0212] 100 parts by mass of the obtained comparative recycled superabsorbent polymer precursor (C1) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a yellow-colored comparative recycled superabsorbent polymer (C1). The urea content of the comparative recycled superabsorbent polymer (C1) was 29.0% by mass, the CRC was 24.3 (g / g), and the AUL 0.3 was 17.6 (g / g).

[0213] [Comparative Example 2] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0214] 1800.0 g of deionized water was added to a beaker and stirred for 15 minutes to perform a water washing procedure. Subsequently, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1934.5 g of the filtrate was dried in an oven at 180°C for 3 hours. The dried material was then pulverized using a vibratory mill, and an amorphous, fragmented comparative recycled superabsorbent polymer precursor (C2) was obtained by passing it through an 850 μm sieve and remaining on a 106 μm sieve.

[0215] 100 parts by mass of the obtained comparative recycled superabsorbent polymer precursor (C2) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a yellow-colored comparative recycled superabsorbent polymer (C2). The urea content of the comparative recycled superabsorbent polymer (C2) was 24.9% by mass, the CRC was 27.8 (g / g), and the AUL 0.3 was 23.8 (g / g). [Comparative Example 3] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0216] 1800.0g of deionized water was added to a beaker and stirred for 15 minutes to perform a water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1944.1g of the filtrate was returned to the beaker. A fresh 1800.0g of deionized water was added, and stirred for 15 minutes to perform a second water wash. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 2830.3g of the filtrate was dried in an oven at 180°C for 3 hours. The dried material was then crushed using a vibratory mill, and an amorphous, fragmented comparative recycled superabsorbent polymer precursor (C3) was obtained by passing it through an 850μm sieve and remaining on a 106μm sieve.

[0217] 100 parts by mass of the obtained comparative recycled superabsorbent polymer precursor (C3) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a yellow-colored comparative recycled superabsorbent polymer (C3). The urea content of the comparative recycled superabsorbent polymer (C3) was 22.3% by mass, the CRC was 32.4 (g / g), and the AUL 0.3 was 24.6 (g / g).

[0218] [Comparative Example 4] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0219] 1000.0 g of acetone was added to a beaker and stirred for 15 minutes to perform a dehydration procedure. Subsequently, the contents of the beaker were filtered through a 100-mesh stainless steel sieve, and 102.2 g of the filtrate was dried in an oven at 180°C for 3 hours. The dried material was then pulverized using a vibratory mill, and an amorphous, fragmented comparative recycled superabsorbent polymer precursor (C4) was obtained by passing it through an 850 μm sieve and remaining on a 106 μm sieve.

[0220] 100 parts by mass of the obtained comparative recycled superabsorbent polymer precursor (C4) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a comparative recycled superabsorbent polymer (C4) that was pale yellow in color. The urea content of the comparative recycled superabsorbent polymer (C4) was 3.6% by mass, the CRC was 36.6 (g / g), and the AUL 0.3 was 28.9 (g / g).

[0221] [Comparative Example 5] 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0222] 1500.0g of a 2.7% by mass citric acid aqueous solution was added to a beaker and stirred for 15 minutes to perform dehydration. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 333.4g of the filtrate was returned to the beaker. 2000.0g of deionized water was added, and stirred for 15 minutes to perform a water washing operation. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 536.5g of the filtrate was placed in a 2000mL container. Next, while stirring the filtrate with a spatula, 64.2g of a 10% sodium hydroxide solution was added dropwise to perform neutralization. After standing for 15 minutes, it was dried in an oven at 180°C for 3 hours. Then, the dried material was crushed using a vibratory mill, and an amorphous, crushed comparative recycled superabsorbent polymer precursor (C5) was obtained, which passed through an 850μm sieve and remained on a 106μm sieve.

[0223] 100 parts by mass of the obtained comparative recycled superabsorbent polymer precursor (C5) was mixed with a surface crosslinking agent composition solution (ethylene glycol diglycidyl ether / propylene glycol / water = 0.04 parts by mass / 0.9 parts by mass / 3 parts by mass) and heat-treated at 210°C for 40 minutes. This yielded a yellow-colored comparative recycled superabsorbent polymer (C5). The urea content of the comparative recycled superabsorbent polymer (C5) was 3.0% by mass, the CRC was 35.7 (g / g), and the AUL 0.3 was 30.1 (g / g).

[0224] Table 1 shows the evaluation results for urea content, CRC, AUL 0.3, and dried color tone of the superabsorbent resins obtained in the manufacturing example, example, and comparative example.

[0225] [Table 1] From the results in Table 1, the superabsorbent resins in the examples with a urea content of 2% by mass or less showed little decrease in water absorption performance (CRC, AUL 0.3) compared to the superabsorbent resin in Production Example 1, and no discoloration of the dried material was observed. On the other hand, the superabsorbent resins in the comparative examples with a urea content exceeding 2% by mass showed a decrease in water absorption performance and discoloration of the dried material.

[0226] Based on the above results, the recycled superabsorbent resin according to one embodiment of the present invention is an excellent recycled superabsorbent resin with less reduction in water absorption performance and discoloration compared to unused superabsorbent resin. Fat It can be said that it is possible.

[0227] [Example 4] (Manufacturing of superabsorbent resins using recycled superabsorbent resins as raw materials) A reaction solution was prepared by dissolving 0.42 parts of polyethylene glycol diacrylate (molecular weight 523) in 539.6 parts of a 36.8% by mass aqueous solution of sodium acrylate (neutralization rate 71 mol%).

[0228] Next, the above reaction solution and 10.5 g of recycled superabsorbent polymer (1) obtained in Example 1 were added to a jacketed stainless steel double-arm kneader with two sigma-shaped blades and an openable / closable lid. The system was then purged with nitrogen gas for 30 minutes while maintaining the reaction solution at 30°C. Subsequently, 0.24 parts of ammonium persulfate and 0.012 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20-95°C, and the hydrated gel-like polymer was removed 60 minutes after the start of polymerization. The obtained hydrated gel-like polymer was dried with hot air at 150°C for 100 minutes. Next, the dried material was pulverized using a vibratory mill, and an amorphous crushed superabsorbent polymer precursor (4) was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve. The obtained superabsorbent polymer precursor (4) was colorless and exhibited a white color.

[0229] [Comparative Example 6] (Manufacturing of superabsorbent resins using recycled superabsorbent resins as raw materials) A reaction solution was prepared by dissolving 0.42 parts of polyethylene glycol diacrylate (molecular weight 523) in 539.6 parts of a 36.8% by mass aqueous solution of sodium acrylate (neutralization rate 71 mol%).

[0230] Next, the above reaction solution and 10.5 g of the comparative recycled superabsorbent polymer (C1) obtained in Comparative Example 1 were added to a jacketed stainless steel double-arm kneader with two sigma-shaped blades and an openable lid. The system was then purged with nitrogen gas for 30 minutes while maintaining the reaction solution at 30°C. Subsequently, 0.24 parts of ammonium persulfate and 0.012 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20-95°C, and the hydrated gel-like polymer was removed 60 minutes after the start of polymerization. The obtained hydrated gel-like polymer was dried with hot air at 150°C for 100 minutes. Next, the dried material was pulverized using a vibratory mill, and an amorphous crushed comparative superabsorbent polymer precursor (C6) was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve. The obtained comparative superabsorbent polymer precursor (C6) was pale yellow in color.

[0231] [Manufacturing Example 2] (Production of water-soluble polymers by solubilizing used superabsorbent polymers) 20.0g of superabsorbent resin (1) was placed in a beaker, 600.0g of artificial urine was added, and the mixture was left to stand for 24 hours to create simulated used superabsorbent resin.

[0232] 1800.0g of deionized water was added to the beaker and stirred for 15 minutes to perform a water washing operation. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and 1942.5g of the filtrate was returned to the beaker. 1900.0g of acetone was added to the filtrate and stirred for 15 minutes to perform a dehydration operation. Next, the contents of the beaker were filtered through a 100-mesh stainless steel mesh, and the amount of urea contained in the filtrate was measured to be 1.2% by mass. 106.4g of this filtrate was returned to the beaker. 100g of water was added to the beaker and heated to 80°C while stirring, and 1.43g of 30% by mass hydrogen peroxide aqueous solution, 0.050g of iron(II) sulfate heptahydrate, 1.00g of L-ascorbic acid, and 3.00g of sulfuric acid were added. This initiated the decomposition of the simulated used superabsorbent polymer. After 90 minutes, the aqueous solution was filtered through a 100-mesh wire mesh to obtain an aqueous solution of water-soluble polymer (1) in which the used superabsorbent polymer had been solubilized. The weight-average molecular weight of water-soluble polymer (1) was 108,000.

[0233] After thoroughly washing the insoluble material remaining on the filtered wire mesh with deionized water, the wire mesh and the insoluble material were dried in a 180°C oven for 2 hours. The decomposition rate of the water-absorbing resin was then calculated according to the following formula and was found to be 99.7%.

[0234] Decomposition rate of water-absorbent polymer (%) = [1 - {(mass of wire mesh + insoluble matter after drying) - (mass of wire mesh)} / (mass of water-absorbent polymer before decomposition)] × 100.

[0235] An aqueous solution of the water-soluble polymer (1) was concentrated using an evaporator to obtain a concentrated water-soluble polymer solution (1) with a solid content concentration of 21.1% by mass.

[0236] [Example 5] (Manufacturing of superabsorbent resin using water-soluble polymers obtained by solubilizing used superabsorbent resin as raw material) A reaction solution was prepared by dissolving 0.42 parts of polyethylene glycol diacrylate (molecular weight 523) in 522.5 parts of a 38% by mass aqueous solution of sodium acrylate (neutralization rate 71 mol%).

[0237] Next, the above reaction solution and 50.0 g of the water-soluble polymer concentrate (1) obtained in Production Example 2 were added to a jacketed stainless steel double-arm kneader with two sigma-shaped blades and an openable lid. The system was then purged with nitrogen gas for 30 minutes while maintaining the reaction solution at 30°C. Subsequently, 0.24 parts of ammonium persulfate and 0.012 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20-95°C, and the hydrated gel-like polymer was removed 60 minutes after the start of polymerization. The obtained hydrated gel-like polymer was dried with hot air at 150°C for 100 minutes. Next, the dried material was pulverized using a vibratory mill, and an amorphous crushed superabsorbent resin precursor (5) was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve. The obtained superabsorbent resin precursor (5) was colorless and exhibited a white color.

[0238] [Manufacturing Example 3] (Production of comparative water-soluble polymers by solubilizing used superabsorbent polymers) 20.0g of superabsorbent resin (1) was placed in a beaker, and 600.0g of artificial urine was added to it. After leaving it for 24 hours, simulated used superabsorbent resin was prepared. The amount of urea contained in this simulated used superabsorbent resin was measured to be 28.8% by mass.

[0239] Next, 400.0 g of water was added to the beaker and the mixture was heated to 80°C while stirring. Then, 1.43 g of 30% by mass hydrogen peroxide aqueous solution, 0.050 g of iron(II) sulfate heptahydrate, 1.00 g of L-ascorbic acid, and 3.00 g of sulfuric acid were added. This initiated the decomposition of the simulated used superabsorbent polymer. After 90 minutes, the aqueous solution was filtered through a 100-mesh wire mesh to obtain an aqueous solution of comparative water-soluble polymer (C1) in which the used superabsorbent polymer had been solubilized. The weight-average molecular weight of the decomposed superabsorbent polymer (comparative water-soluble polymer (C1)) was 101,000.

[0240] After thoroughly washing the insoluble material remaining on the filtered wire mesh with deionized water, the wire mesh and the insoluble material were dried in a 180°C oven for 2 hours, and the decomposition rate was determined to be 99.4%. An aqueous solution of the comparative water-soluble polymer (C1) was concentrated using an evaporator to obtain a comparative water-soluble polymer concentrate (C1) with a solid content concentration of 21.2% by mass.

[0241] [Comparative Example 7] (Manufacturing of superabsorbent resins using decomposed used superabsorbent resins as raw materials) A reaction solution was prepared by dissolving 0.42 parts of polyethylene glycol diacrylate (molecular weight 523) in 522.5 parts of a 38% by mass aqueous solution of sodium acrylate (neutralization rate 71 mol%).

[0242] Next, the above reaction solution and 50.0 g of the comparative water-soluble polymer concentrate (C1) obtained in Production Example 3 were added to a jacketed stainless steel double-arm kneader with two sigma-shaped blades and an openable / closable lid. The system was then purged with nitrogen gas for 30 minutes while maintaining the reaction solution at 30°C. Subsequently, 0.24 parts of ammonium persulfate and 0.012 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20-95°C, and the hydrated gel-like polymer was removed 60 minutes after the start of polymerization. The obtained hydrated gel-like polymer was dried with hot air at 150°C for 100 minutes. Next, the dried material was pulverized using a vibratory mill, and an amorphous crushed comparative superabsorbent polymer precursor (C7) was obtained, which passed through an 850 μm sieve and remained on a 106 μm sieve. The obtained comparative superabsorbent polymer precursor (C7) was pale yellow in color.

[0243] Table 2 shows the evaluation results of the dried color tone of the water-absorbing resins obtained in the examples and comparative examples.

[0244] [Table 2] As shown in Table 2, the superabsorbent resins of Examples 4 and 5, which used recycled superabsorbent resins and water-soluble polymers derived from used absorbent articles as part of the raw materials, with a urea content of 2% by mass or less, did not show any discoloration in the dried product. On the other hand, the superabsorbent resins of Comparative Examples 6 and 7, which used recycled superabsorbent resins and water-soluble polymers derived from used absorbent articles as part of the raw materials, with a urea content exceeding 2% by mass, showed discoloration in the dried product.

[0245] Based on the above results, the method for producing a water-absorbent resin using recycled water-absorbent resin and a water-soluble polymer derived from used absorbent articles as part of the raw materials according to one embodiment of the present invention can be said to be an excellent method for producing a water-absorbent resin with minimal discoloration.

[0246] The present invention is not limited to the embodiments described above, 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.

Claims

1. A method for recycling absorbent resin contained in used absorbent articles, A urea removal step is performed to remove urea from the absorbent resin contained in the used absorbent article so that the urea content in the recycled absorbent resin is 2% by mass or less. A regeneration step to restore the water absorption properties of the water-absorbing resin after the urea removal step, A method for recycling superabsorbent polymers, comprising the following characteristics.

2. The urea removal step includes a washing step of washing the swollen gel of the water-absorbent resin with an aqueous solution, and / or a dehydration step of dehydrating the absorbent liquid from the swollen gel. A method for recycling a water-absorbent resin according to claim 1.

3. A recycled superabsorbent resin derived from a used absorbent article that has absorbed urine, A recycled superabsorbent polymer with a urea content of 2% by mass or less.

4. A method for producing an absorbent resin, comprising adding the recycled absorbent resin described in claim 3 to the process of producing an absorbent resin using monomers constituting the absorbent resin as raw materials.

5. The method for producing an absorbent resin according to claim 4, wherein the recycled absorbent resin accounts for 1 to 60% by mass of all absorbent resin raw materials.

6. A method for recycling an absorbent resin according to claim 1, further comprising a solubilization step of solubilizing the recycled absorbent resin derived from the used absorbent article after the regeneration step.

7. A water-soluble polymer derived from recycled superabsorbent resin as described in Claim 3, A water-soluble polymer having a urea content of 2% by mass or less in the recycled superabsorbent resin.

8. A method for producing a water-absorbent resin, comprising using the water-soluble polymer described in claim 7 as part of the raw materials in a process for producing a water-absorbent resin using monomers constituting the water-absorbent resin as raw materials.

9. A method for recycling absorbent resin contained in used absorbent articles, A urea removal step is performed to remove urea from the absorbent resin contained in used absorbent articles so that the urea content in the absorbent resin contained in the used absorbent articles is 2% by mass or less. A method for recycling absorbent resins, comprising a solubilization step of solubilizing the absorbent resin contained in the used absorbent article after the urea removal step.

10. An absorbent article comprising the recycled water-absorbent resin described in claim 3.

11. An absorbent article comprising an absorbent resin that uses the recycled absorbent resin described in claim 3 or the water-soluble polymer described in claim 7 as part of its raw materials.

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