Production method of recycled water-absorbing resin and production method of water-absorbing resin using recycled water-absorbing resin as part of material

Abstract

To provide a recycled water-absorbing resin which inhibits coloring.SOLUTION: A method for producing a recycled water-absorbing resin from a water-absorbing resin recovered from used absorbent articles or wastes occurring from a production process of the water-absorbing resin includes: a bleach step in which the water-absorbing resin is breached; and / or a white pigment adding step in which a white pigment is added to the water-absorbing resin.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a recycled water-absorbent resin, and a method for producing a water-absorbent resin using a recycled water-absorbent resin as part of a raw material. [Background technology]

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

[0003] Although absorbent articles are generally disposable, interest in recycling absorbent articles is growing due to growing environmental awareness. In particular, for used water-absorbent resins contained in absorbent articles, many techniques have been developed for recycling the used water-absorbent resins into water-absorbent resins or other useful substances (Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. WO2020 / 241025 [Patent Document 2] International Publication No. WO2020 / 213298 [Patent Document 3] International Publication No. WO2022 / 080342 Summary of the Invention [Problem to be solved by the invention]

[0005] Currently, various methods for recycling water absorbent resins from used absorbent articles, etc. have been developed. However, when the water absorbent resin recovered from used absorbent articles, etc. is recycled, the resulting recycled water absorbent resin may be colored and the color tone of the recycled water absorbent resin may deteriorate, and there is room for improvement in this regard.

[0006] In the above-mentioned circumstances, one aspect of the present invention is to provide a recycled water-absorbent resin in which coloring is suppressed. [Means for solving the problem]

[0007] In order to solve the above problems, one embodiment of the present invention includes the following configuration. [1] A method for producing a recycled water-absorbent resin from a water-absorbent resin recovered from used absorbent articles or waste generated in the process of producing a water-absorbent resin, the method comprising a bleaching step of bleaching the water-absorbent resin, and / or a white pigment adding step of adding a white pigment to the water-absorbent resin. [2] The method for producing a recycled water-absorbent resin according to [1], wherein in the bleaching step, the water-absorbent resin is bleached using an oxidizing bleach or a reducing bleach. [3] The method for producing the recycled water absorbent resin according to [1] or [2], comprising a washing step of washing the water absorbent resin, a dehydration step of dehydrating the water absorbent resin, and a drying step of drying the water absorbent resin, wherein the bleaching step is carried out after the dehydration step. [4] The method for producing a recycled water absorbent resin according to any one of [1] to [3], comprising a washing step of washing the water absorbent resin, a dehydration step of dehydrating the water absorbent resin, and a drying step of drying the water absorbent resin, wherein the white pigment addition step is carried out after the dehydration step. [5] In the bleaching step, the oxidizing bleach or reducing bleach is added in an amount of 0.01 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the water absorbent resin. [6] The method for producing a recycled water absorbent resin according to any one of [1] to [5], wherein in the white pigment addition step, the white pigment is added in an amount of 0.01 parts by mass or more and 100 parts by mass or less relative to 100 parts by mass of the water absorbent resin. [7] A method for producing a water absorbent resin, comprising using a recycled water absorbent resin obtained by the production method according to any one of [1] to [6] as a part of a raw material in a production process of the water absorbent resin using a monomer constituting the water absorbent resin as a raw material. [8] The method for producing a water absorbent resin according to [7], wherein the recycled water absorbent resin accounts for 1% by mass or more and 60% by mass or less of all raw water absorbent resin materials. [9] A method for producing a water-absorbent resin, comprising the steps of: (a) preparing a water-absorbent resin from a monomer constituting the water-absorbent resin; and (b) preparing a water-absorbent resin from the monomer;

[10] The method for producing a water absorbent resin according to [9], wherein the proportion of the solubilized polymer derived from the recycled water absorbent resin relative to the total amount of the water absorbent resin raw material is 1% by mass or more and 60% by mass or less. Effect of the Invention

[0008] According to one aspect of the present invention, it is possible to provide a recycled water absorbent resin in which coloring is suppressed. Furthermore, it is possible to provide a method for producing a water absorbent resin in which the recycled water absorbent resin or a solubilized polymer that is a decomposition product of the recycled water absorbent resin or a decomposition product of the recycled water absorbent resin is used as a part of a raw material of the water absorbent resin. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] <1. Manufacturing method of recycled water absorbent resin> [1-1] Technical Concept of the First Embodiment of the Present Invention Means for Solving the Problems While conducting research into recycled water absorbent resins obtained by recycling water absorbent resins contained in used absorbent articles and waste generated in the manufacturing process of water absorbent resins, the present inventors have found that the recycled water absorbent resins may become discolored and the color tone of the recycled water absorbent resins may deteriorate.

[0010] Since normal (unused) water absorbent resins are white, recycled water absorbent resins with deteriorated color (particularly yellowed) may remind users of the presence of human waste and give an unhygienic impression. Therefore, there is a problem that the above-mentioned colored water absorbent resins with deteriorated color are unsuitable for product applications, particularly for sanitary material applications, which are the main applications.

[0011] The present inventors presumed that the above-mentioned coloring was caused by the residual substances derived from body fluids such as human waste, and by thermal deterioration during the manufacturing process of the water-absorbent resin. Therefore, the present inventors attempted to remove as many substances derived from body fluids such as human waste as possible during the recycling process of the water-absorbent resin, and to suppress thermal deterioration during the manufacturing process by changing the conditions of the manufacturing process, etc. However, the effect of suppressing coloring by such attempts was not sufficient, and further improvement of coloring was necessary.

[0012] As a result of further intensive research into the above-mentioned problems, the present inventors have found that a recycled water absorbent resin in which coloring is sufficiently suppressed (in other words, color tone is improved) can be provided by further bleaching the used water absorbent resin and / or adding a white pigment to the used water absorbent resin in the recycling process of the used water absorbent resin, in other words, by carrying out a bleaching step and / or a white pigment adding step in the recycling process, and have completed the present invention. Such a recycled water absorbent resin in which coloring is suppressed is a recycled water absorbent resin with low environmental load, but is comparable to a normal (unused) water absorbent resin, and can be suitably used for product applications (for example, sanitary material applications).

[0013] [1-2] Manufacturing method of recycled water absorbent resin according to the first embodiment In a first embodiment of the present invention, there is provided a method for producing a recycled water absorbent resin from a water absorbent resin recovered from used absorbent articles or waste generated in the process of producing a water absorbent resin, the method including a bleaching step of bleaching the water absorbent resin and / or a white pigment adding step of adding a white pigment to the water absorbent resin. Hereinafter, the method for producing a recycled water absorbent resin according to the first embodiment of the present invention may be referred to as "this first production method".

[0014] According to the first manufacturing method, it is possible to provide a recycled water absorbent resin in which coloring is suppressed. The degree of "coloring" of the water absorbent resin (recycled water absorbent resin) can be measured or evaluated by the method described in the Examples.

[0015] [1-2-1] Absorbent articles In this specification, the term "absorbent article" refers to an article used for water absorption. An example of the "absorbent article" is an absorbent article comprising an absorbent body containing a water-absorbing resin and a fibrous material, a top sheet having liquid permeability, and a back sheet having liquid impermeability. The absorbent body is suitably manufactured by blending the water-absorbing resin with a fibrous material, or by sandwiching the water-absorbing resin between fibrous materials and molding the mixture into a film, a tube, a sheet, or other shape. Here, examples of the fibrous material include hydrophilic fibers such as crushed wood pulp, cotton linters, crosslinked cellulose fibers, rayon, cotton, wool, acetate, vinylon, and the like. Specific examples of the "absorbent article" include sanitary materials (sanitary products) such as paper diapers, sanitary napkins, adult incontinence products (incontinence pads), and pet sheets, but are not limited thereto.

[0016] The term "used absorbent article" in this first manufacturing method refers to an absorbent article (particularly, a sanitary material) that has absorbed any liquid (absorbed liquid), particularly bodily fluids such as urine and blood.

[0017] [1-2-2] Water-absorbing resin In this specification, the term "water-absorbent resin" refers to a water-swellable, water-insoluble polymer gelling agent, and generally means a conventional water-absorbent resin having a water absorption capacity of 10 to 1000 times its volume. More specifically, it is preferable that the water-absorbent resin before absorbing the liquid to be absorbed satisfies the physical property of CRC of 5 g / g or more as "water-swellability" specified in ERT441.2-02, and satisfies the physical property of Ext (water-soluble content) of 0 to 50 mass% as "water-swellability" specified in ERT470.2-02. Specific examples of water-absorbent resins include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, cellulose-based resins, (meth)acrylate crosslinked polymers, saponified crosslinked products of (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.

[0018] [1-2-3] Waste generated during the manufacturing process of water-absorbent resin In this specification, the term "waste generated during the manufacturing process of a water-absorbent resin" refers to a water-absorbent resin that is generated during the manufacturing process of a water-absorbent resin and is rejected from product use for some reason (for example, because its water-absorbing properties, color tone, etc. are not within the standard), and is usually discarded, and is therefore referred to as "waste." Examples of "waste generated during the manufacturing process of a water-absorbent resin" include, but are not limited to, fine powder, gel, and water-absorbent resin particles with excessively large or small particle diameters generated during the manufacturing process of a water-absorbent resin.

[0019] [1-2-4] Recycled water-absorbent resin In the present specification, the term "recycled water absorbent resin" refers to a water absorbent resin obtained by recycling a water absorbent resin containing a liquid to be absorbed, such as human waste, recovered from a used absorbent article and / or a water absorbent resin recovered from waste generated in the manufacturing process of the water absorbent resin, for an application requiring a water absorption function. In the present specification, such a water absorbent resin recovered from a used absorbent article or waste generated in the manufacturing process of the water absorbent resin, in other words, a water absorbent resin to be recycled, may be referred to as a "recovered water absorbent resin".

[0020] Usually, the recovered water absorbent resin does not have water absorption performance or may have very low water absorption performance, and may contain a large amount of impurities derived from body fluids or generated during the manufacturing process of the water absorbent resin. Therefore, the recovered water absorbent resin is not used for product applications such as absorbent articles for water absorption purposes, but is disposed of, thermally recycled as fuel, or composted. On the other hand, the "recycled water absorbent resin" is a water absorbent resin derived from the recovered water absorbent resin, but is obtained by recovering the liquid absorption performance of the recovered water absorbent resin so that it can be used again for water absorption purposes. In other words, the "recycled water absorbent resin" is intended to be a water absorbent resin made from the recovered water absorbent resin, has water absorption performance, and can be used for product applications such as absorbent articles.

[0021] [1-2-5] Cleaning process The first manufacturing method includes a washing step of washing the recovered water absorbent resin. The washing step according to the first manufacturing method can be said to be a step of removing impurities (e.g., body fluids such as human waste, impurities generated during the manufacturing process, etc.) present between particles of the swollen gel of the recovered water absorbent resin, on the particle surfaces, etc., by washing with a washing liquid (washing treatment). Specific washing methods for the recovered water absorbent resin include, for example, a method of adding a washing liquid to the recovered water absorbent resin, a method of immersing the recovered water absorbent resin in a washing liquid, a method of stirring the recovered water absorbent resin while immersing it in a washing liquid, etc., but are not limited thereto.

[0022] The washing liquid used in the washing step is not particularly limited as long as it is an aqueous liquid that can wash away impurities and does not alter the recovered water absorbent resin, but it is preferable that the recovered water absorbent resin can absorb the washing liquid and swell because it can remove more impurities, particularly more impurities inside the recovered water absorbent resin, when dehydrating the swollen gel of the recovered water absorbent resin in the dehydration step described below. Examples of such washing liquids include deionized water, tap water, distilled water, physiological saline, and seawater. These washing liquids may also contain additives such as surfactants and defoamers to the extent that they do not alter the recovered water absorbent resin.

[0023] The temperature of the cleaning liquid in the cleaning step is not particularly limited, and can be from room temperature to 100° C., or steam of the cleaning liquid can be used. When using a high-temperature cleaning liquid or steam of the cleaning liquid of 60° C. or higher, it is preferable because a sterilizing effect can be expected in addition to the cleaning effect.

[0024] The amount of the washing liquid used in the washing step (total amount used) is not particularly limited as long as the recovered water absorbent resin can be sufficiently washed, but for example, the amount is preferably 2,000 parts by mass or more, more preferably 10,000 parts by mass or more, and further preferably 100,000 parts by mass or more, relative to 100 parts by mass of the recovered water absorbent resin.

[0025] In the first production method, the washing step may be carried out at least once, but may also be carried out multiple times. For example, after the first washing step and the subsequent dehydration step, a second or subsequent washing step may be carried out.

[0026] [1-2-6] Dehydration process The first manufacturing method includes a dehydration step of dehydrating the recovered water absorbent resin, preferably the recovered water absorbent resin after the washing step. The dehydration step according to the first manufacturing method can be said to be a step of dehydrating (dehydrating) the recovered water absorbent resin (preferably a swollen gel of the recovered water absorbent resin) to discharge impurities present inside the water absorbent resin together with moisture to the outside of the system.

[0027] In the dehydration step according to the first manufacturing method, the method of dehydrating the recovered water absorbent resin is not particularly limited, and is not particularly limited as long as it is a method capable of reducing the moisture content of the swollen recovered water absorbent resin in a state of absorbing liquid (for example, the aqueous liquid used for washing). Specific dehydration methods include, for example, a method of contacting the swollen gel of the recovered water absorbent resin with various dehydrating liquids such as an aqueous solution containing a hydrophilic organic solvent, an aqueous solution with a low pH (for example, 3 or less), and an aqueous solution of a polyvalent metal salt (for example, calcium chloride), as well as a method of applying a voltage to the swollen gel using a pair of electrodes. Methods of contacting the swollen gel of the recovered water absorbent resin with the various dehydrating liquids include, but are not limited to, a method of adding the liquid to the recovered water absorbent resin gel (swollen gel) in a swollen state, a method of immersing the swollen gel in the liquid, and a method of stirring the swollen gel while immersing it in the liquid.

[0028] In the dehydration step, when the swollen gel of the recovered water absorbent resin is immersed in an aqueous solution containing a hydrophilic organic solvent, the concentration of the hydrophilic organic solvent in the aqueous solution in which the recovered water absorbent resin is immersed is preferably 35% by mass or more, more preferably 40% by mass or more, and even more preferably 45% by mass or more.

[0029] In this specification, the hydrophilic organic solvent refers to an organic solvent having a solubility of 20 g or more in 100 mL of water at 20° C. As the hydrophilic organic solvent, one having a solubility of 25 g or more in 100 mL of water at 20° C. is more preferable, and one having a solubility of 30 g or more in 100 mL of water at 20° C. is even more preferable.

[0030] Specific examples of such hydrophilic organic solvents include: (1) lower alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, tert-butyl alcohol, etc.; (2) ketones such as acetone, methyl ethyl ketone, etc.; (3) ethers such as tetrahydrofuran, dioxane, etc.; and (4) esters such as methyl acetate, methyl lactate, ethyl lactate, etc. These hydrophilic organic solvents may be used alone or in combination of two or more.

[0031] In the dehydration step, when the swollen gel of the water-absorbent resin is immersed in an aqueous solution with a low pH (for example, 3 or less), the pH of the aqueous solution is preferably 3 or less, and more preferably 0.5 to 2.5. As the aqueous solution, an aqueous solution containing an acid can be suitably used. As such an acid, either an inorganic acid or an organic acid can be used. In the case of an inorganic acid, for example, hydrochloric acid, sulfuric acid, etc. can be used. In the case of an organic acid, for example, tartaric acid, glycolic acid, malic acid, citric acid, succinic acid, acetic acid, etc. can be used. These acids may be used alone or in combination of two or more.

[0032] In the dehydration step, when the swollen gel of the recovered water absorbent resin is contacted with an aqueous solution of a polyvalent metal salt, examples of the polyvalent metal salt include salts of metals such as calcium, magnesium, aluminum, iron, cobalt, nickel, and copper with chlorine, carbonate, acetate, etc. These polyvalent metal salts may be used alone or in combination of two or more.

[0033] In the first production method, the dehydration step may be carried out at least once, but may also be carried out multiple times. In addition, the dehydration step is preferably carried out after the washing step.

[0034] [1-2-7] Bleaching process The first production method preferably includes a bleaching step of bleaching the recovered water absorbent resin.

[0035] In the bleaching step, the method of bleaching the recovered water absorbent resin is not particularly limited, but examples thereof include a method of directly adding a bleaching agent in the form of an aqueous solution or a slurry to the recovered water absorbent resin, a method of immersing the recovered water absorbent resin in an aqueous solution in which the bleaching agent is dissolved, and a method of adding the recovered water absorbent resin, the bleaching agent, and optionally water to a reaction vessel and stirring and mixing them with a stirring blade or the like. Among these, from the viewpoint of bleaching efficiency, it is preferable to bleach the recovered water absorbent resin in the bleaching step by adding a bleaching agent to the recovered water absorbent resin.

[0036] The bleaching agent that can be used in the bleaching step is not particularly limited, but examples of suitable deodorants include (a1) chlorine-based bleaching agents such as chlorine, hypochlorous acid (salt), and chlorine dioxide, or (a2) oxygen-based bleaching agents such as oxygen, hydrogen peroxide, ozone, peracetic acid (salt), percarbonic acid (salt), and perboric acid (salt), which show a bleaching effect by oxidatively decomposing and removing the substances that cause coloring; or reductive bleaching agents such as hydrosulfite, sodium borohydrite, and thiourea dioxide, which show a deodorizing effect by adding hydrogen to the double bond that is the binding site of the substances that cause coloring, thereby eliminating the double bond. These bleaching agents may be used alone or in combination of two or more. However, it is not preferable to use an oxidative bleaching agent and a reductive bleaching agent in combination, since the oxidative and reductive actions of each agent are offset. That is, in the bleaching step, it is preferable to bleach the recovered water absorbent resin using an oxidizing bleaching agent or a reducing bleaching agent (either one of them).

[0037] In the bleaching step, when the recovered water absorbent resin is bleached using a bleaching agent, the amount of the bleaching agent added is not particularly limited, but it is preferable to add 0.01 to 100 parts by mass of the bleaching agent relative to 100 parts by mass of the recovered water absorbent resin, preferably 0.05 to 50 parts by mass, and more preferably 0.1 to 10 parts by mass. By controlling the amount of the bleaching agent used within the above range, it is possible to sufficiently suppress coloration of the recovered water absorbent resin, and further suppress decomposition of the recovered water absorbent resin by the bleaching agent.

[0038] The temperature at which the bleaching step is carried out, in other words, the temperature of the mixture of the recovered water absorbent resin and the bleaching agent during the bleaching step, is not particularly limited, but from the viewpoint of exhibiting an efficient bleaching action, it is preferably 10°C or higher and 120°C or lower, more preferably 20°C or higher and 110°C or lower, and even more preferably 30°C or higher and 100°C or lower.

[0039] The pH at which the bleaching step is carried out, in other words, the pH of the mixture of the recovered water absorbent resin and the bleaching agent during the bleaching step, cannot be generally defined because the pH range in which the most bleaching effect is exhibited differs depending on the type of bleaching agent, but from the viewpoint of suppressing the decomposition of the recovered water absorbent resin under low or high pH conditions, it is preferable to carry out the bleaching step under conditions of pH 3 to 12 (in other words, to avoid bleaching treatment under conditions of pH less than 3 or pH more than 12). That is, in the bleaching step, it is preferable to adjust the pH of the mixture so that the pH is 3 to 12 (in other words, not to be less than pH 3 or not to be more than pH 12).

[0040] In the bleaching step, in addition to the above-mentioned bleaching agent, a bleaching activity enhancer having the effect of enhancing the effect of the bleaching agent may be added to the recovered water-absorbent resin. By further adding the bleaching activity enhancer, it is possible to provide a recovered water-absorbent resin with further suppressed coloration.

[0041] Specific examples of bleaching activity enhancers that can be used in the bleaching step include TAED (tetraacetylethylenediamine), NOBS (nonanoyloxybenzenesulfonic acid), DOBA (decanoyloxybenzoic acid), PAG (pentaacetylglucose), etc. These bleaching activity enhancers may be used alone or in combination of two or more.

[0042] In the present first manufacturing method, the timing of carrying out the bleaching step is not particularly limited and can be carried out at any timing, but it is preferable to carry out the bleaching step after the dehydration step (after the last dehydration step when the dehydration step is carried out multiple times) in order to prevent the bleaching agent from leaking out during the washing step and the dehydration step. In addition, from the viewpoint of improving the contact probability between the recovered water absorbent resin and the bleaching agent at the molecular level and carrying out bleaching more efficiently, it is preferable to carry out the bleaching step before the drying step. That is, in the present first manufacturing method, it is preferable to carry out the bleaching step between the dehydration step and the drying step.

[0043] [1-2-8] White pigment addition process The first manufacturing method preferably includes a white pigment adding step of adding a white pigment to the recovered water absorbent resin.

[0044] The white pigment that can be used in the white pigment addition step is not particularly limited as long as it can make the color tone of the water-absorbing resin appear whiter by scattering and reflecting light, but examples of suitable white pigments include zinc oxide, titanium oxide, barium sulfate, barium carbonate, precipitated calcium carbonate, chalk, gypsum, alumina white, clay, silica, talc, calcium silicate, white lead, zinc oxide lead, lithopone, zinc sulfide, basic lead sulfate, lead sulfate, antimony white, zirconium oxide, tin oxide, barium metaborate, and magnesium carbonate. These white pigments may be used alone or in combination of two or more.

[0045] Among the above white pigments, titanium oxide or silica is preferred because it has little adverse effect on the water absorption properties of the resulting recycled water absorbent resin and is relatively inexpensive.

[0046] In the white pigment addition step, the amount of the white pigment added is not particularly limited, but from the viewpoint of the balance between the color tone improvement effect and the cost aspect, it is preferable to add 0.01 parts by mass or more and 10 parts by mass or less of the white pigment relative to 100 parts by mass of the recovered water absorbent resin, it is preferable to add 0.05 parts by mass or more and 5 parts by mass or less, and it is further preferable to add 0.1 parts by mass or more and 1 part by mass or less.

[0047] In the white pigment addition step, examples of a method for adding a white pigment to a recovered water absorbent resin include a method for directly adding a white pigment to a recovered water absorbent resin in the form of an aqueous solution or a slurry, and a method for adding a recovered water absorbent resin, a white pigment, and optionally water to a reaction vessel and stirring and mixing with a stirring blade or the like, but are not limited thereto.

[0048] In this first manufacturing method, the timing of the white pigment addition step is not particularly limited and can be performed at any timing, but it is preferable to perform it after the dehydration step (after the last dehydration step if the dehydration step is performed multiple times) because this can prevent the white pigment from flowing out during the washing step and the dehydration step. When the white pigment addition step is performed after the dehydration step, it can be performed either before the drying step described below or after the drying step, but it is preferable to perform it after the drying step because this can suppress deterioration and scattering of the white pigment. That is, in this first manufacturing method, it is preferable to perform the white pigment addition step after the drying step. Moreover, the white pigment addition step may be performed simultaneously with the above-mentioned deodorant addition step.

[0049] From the viewpoint of providing a recycled water absorbent resin with reduced coloring, the first production method may include at least one of the above-mentioned bleaching step and the white pigment adding step, but from the viewpoint of providing a recycled water absorbent resin with further reduced coloring, it is preferable that the first production method includes both of the above-mentioned bleaching step and the white pigment adding step.

[0050] [1-2-9] Drying process The first manufacturing method includes a drying step of drying the recovered water absorbent resin. The drying step is a step of drying the recovered water absorbent resin in a moist state to a desired resin solid content. The resin solid content of the water absorbent resin is determined from the loss on drying of the water absorbent resin (the change in mass before and after heating 1 g of the water absorbent resin at 180°C for 3 hours), and is preferably 80 mass% or more, more preferably 85 mass% to 99 mass%, even more preferably 90 mass% to 98 mass%, and particularly preferably 92 mass% to 97 mass%.

[0051] In the drying step, the method for drying the recovered water absorbent resin is not particularly limited, but examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and high humidity drying using high temperature water vapor. Among these, from the viewpoint of drying efficiency, hot air drying is preferred as the drying method, and band drying in which hot air drying is performed on a ventilation belt is more preferred. In addition, in the drying step, only one of these drying methods may be performed, or two or more may be combined and performed.

[0052] When hot air drying is performed in the drying step, the drying temperature (hot air temperature) in the hot air drying is preferably 120°C or more and 250°C or less, more preferably 150°C or more and 200°C or less, from the viewpoint of the color tone and drying efficiency of the obtained recycled water absorbent resin. The drying conditions other than the above drying temperature, such as the wind speed of the hot air and the drying time, can be appropriately set according to the moisture content, total mass and target resin solid content of the recovered water absorbent resin to be dried. In addition, when band drying is performed, known drying conditions, for example, conditions described in International Publication Nos. 2006 / 100300, 2011 / 025012, 2011 / 025013, and 2011 / 111657, can be appropriately applied as drying conditions.

[0053] [1-2-10] Regeneration process The recovered water absorbent resin used in the first production method may have no water absorption performance or may have very low water absorption performance (particularly when the recovered water absorbent resin is a recovered water absorbent resin derived from used absorbent articles), but by subjecting the recovered water absorbent resin to a regeneration step, it is possible to impart (regenerate) sufficient water absorption performance (sufficient for product applications) to the recovered water absorbent resin.

[0054] The regeneration method of the recovered water absorbent resin in the regeneration step is not particularly limited as long as it restores the water absorption ability of the recovered water absorbent resin. For example, a method of contacting a polyvalent metal salt with a recovered water absorbent resin that has absorbed urine, dehydrating the resin, treating the resin with an acidic solution, neutralizing the resin with an alkali metal salt, and then drying the resin (JP Patent Publication No. 2003-225645); a method of contacting a recovered water absorbent resin that has absorbed urine with a solution containing a hydrophilic organic solvent, dehydrating the resin, and then drying the resin (JP Patent Publication No. 2003-326161); a method of contacting a polyvalent metal salt with a recovered water absorbent resin that has absorbed urine, dehydrating the resin, treating the resin with an aqueous alkali metal salt solution, subjecting the resin to ion exchange treatment, and then drying the resin (JP Patent Publication No. 2013-198862); a method of contacting a recovered water absorbent resin that has absorbed urine with an acidic aqueous solution, dehydrating the resin, neutralizing the resin with an alkali metal salt, and then drying the resin (JP Patent Publication No. 2019-135046, etc.); and other known regeneration methods can be used.

[0055] [1-2-11] Crushing (cutting) process The first manufacturing method may include a crushing (cutting) step of crushing (cutting) used absorbent articles into crushed (cut) materials.

[0056] In the crushing (cutting) step, the method of crushing (cutting) the used absorbent article is not particularly limited, and examples thereof include a method of cutting using a cutting tool such as a cutter. By carrying out the crushing (cutting) step, it is possible to increase the contact opportunity between the recovered water-absorbent resin in the absorbent article and the washing water, dehydration liquid, etc., thereby improving the washing effect, dehydration effect, and the speed thereof. It is also possible to facilitate separation of each component such as the water-absorbent resin, pulp, nonwoven fabric, etc. from the used absorbent article in the separation step described below.

[0057] In this first manufacturing method, the timing of carrying out the crushing (cutting) step is not particularly limited, but for example, it is preferable to carry out the crushing (cutting) step after the above-mentioned dehydration step, and it is more preferable to carry out the crushing (cutting) step between the above-mentioned dehydration step and the above-mentioned deodorant addition step.

[0058] [1-2-12] Separation process The first manufacturing method may include a separation step of separating the water-absorbent resin from a used absorbent article. "Separating the water-absorbent resin from a used absorbent article" refers to taking out the water-absorbent resin that has shrunk by discharging the absorbed liquid from gaps in the pulp, nonwoven fabric, etc. contained in the used absorbent article, and dispersing it in a treatment liquid.

[0059] As a method for separating the water-absorbing resin from the used absorbent article, for example, a method of stirring a mixture of the treatment liquid and the used absorbent article can be mentioned. The water-absorbing resin dispersed in the treatment liquid can be recovered by a conventional solid-liquid separation means for separating soluble and insoluble substances, for example, filtration, centrifugation, etc.

[0060] In the first manufacturing method, the timing of carrying out the separation step is not particularly limited, but for example, it is preferable to carry out the separation step after the dehydration step, and more preferably to carry out the separation step between the dehydration step and the deodorant addition step. Also, the crushing (cutting) step and the separation step can be carried out in parallel.

[0061] [1-2-13] Other processes In addition to the above-mentioned steps, the present first production method may include any steps (other steps) that can be carried out in a normal production process of a water absorbent resin, such as a classification step of classifying the obtained recycled water absorbent resin, a surface cross-linking step of surface cross-linking the obtained recycled water absorbent resin, a disinfection / sterilization step of disinfecting and / or sterilizing the recovered water absorbent resin, etc. As specific aspects of the other steps in the present first production method, for example, the aspects described in the classification step section, surface cross-linking step section, etc. of the second embodiment described later can be appropriately adopted.

[0062] [1-2-14] Colorability (yellowness index / YI value) of recycled water absorbent resin obtained by the first manufacturing method The YI value of the recycled water absorbent resin obtained by the first manufacturing method is preferably 20 or less, more preferably 18 or less, and even more preferably 16 or less. The lower limit of the YI value is not particularly limited and may be 0. The YI value of the water absorbent resin is an index showing the colorability of the water absorbent resin, and the YI value of the recycled water absorbent resin within the above range means that the coloration of the recycled water absorbent resin is further suppressed. The YI value of the water absorbent resin (recycled water absorbent resin) can be measured by the method described in the Examples.

[0063] [1-2-15] Water absorption properties of the water-absorbent resin obtained by the first manufacturing method The water absorption properties of the recycled water absorbent resin obtained by the first manufacturing method are preferably CRC of 25 g / g or more, more preferably 30 g / g or more, even more preferably 35 g / g or more, and even more preferably 40 g / g or more. Also, AAP (0.3 psi) is preferably 10 g / g or more, more preferably 15 g / g or more, more preferably 20 g / g or more, more preferably 22 g / g or more, even more preferably 24 g / g or more, and even more preferably 26 g / g or more. The CRC and / or AAP (0.3 psi) of the water absorbent resin (recycled water absorbent resin) being in the above range means that the water absorbent resin (recycled water absorbent resin) has excellent water absorption. The CRC and AAP (0.3 psi) of the water absorbent resin (recycled water absorbent resin) can be measured by the method described in the Examples.

[0064] [1-2-16] "EDANA" and "ERT" "EDANA" is an abbreviation for European Disposables and Nonwovens Associations. Also, "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard (almost a global standard) for measuring water-absorbent resins. In one embodiment of the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the original ERT (revised in 2002 / publicly known document).

[0065] [1-2-17] Other In this specification, unless otherwise specified, "ppm" means "ppm by mass."

[0066] In this specification, "acid (salt)" means "acid and / or its salt." "(Meth)acrylic" means "acrylic and / or methacrylic."

[0067] In this specification, the unit of volume "liter" may be written as "l" or "L".

[0068] <2. Manufacturing method of water-absorbent resin containing recycled water-absorbent resin as part of the raw material> [2-1] Technical Concept of the Second Embodiment In actual recycling sites for used absorbent articles, sanitary materials of various types, manufacturers, etc. are collectively recycled. Therefore, the water absorbent resins recovered therefrom are also a mixture of those from various manufacturers and with various performances, and it is difficult to obtain a recycled water absorbent resin having stable water absorption performance.

[0069] On the other hand, water absorbent resins for sanitary materials are highly balanced in various water absorption properties such as water absorption capacity, water absorption capacity under pressure, and liquid permeability in order to meet various applications and required characteristics. In other words, water absorbent resins for sanitary materials are required to have an excellent balance of various water absorption properties. However, as described above, recycled water absorbent resins often have unstable water absorption properties, and it is necessary to adjust the balance of water absorption properties in order to use recycled water absorbent resins for sanitary materials.

[0070] Therefore, the inventors thought that it would be easier to adjust the balance of physical properties by mixing the recycled water absorbent resin, which has unstable water absorption properties as described above, into part of the raw materials used when newly producing a water absorbent resin. Note that "newly producing a water absorbent resin" refers to the production of a conventional (normal) water absorbent resin using monomers that constitute the water absorbent resin as raw materials. In this case, the conventional (normal) water absorbent resin does not include resins that partially contain the above-mentioned recycled water absorbent resin, nor water absorbent resins that use monomers generated by recycling as part of the raw materials.

[0071] Then, it was found that by using the recycled water absorbent resin produced by the above-mentioned first production method as a recycled water absorbent resin which is a part of the raw material, a water absorbent resin containing the recycled water absorbent resin as a part of the raw material, which has an excellent balance of water absorption properties and is suppressed in coloration, was obtained, and the second embodiment of the present invention was completed.

[0072] [2-2] Method for producing water-absorbent resin according to the second embodiment In a second embodiment of the present invention, a method for producing a water-absorbent resin is provided, in which a recycled water-absorbent resin obtained by the first production method is used as a part of a raw material in a production process of the water-absorbent resin using a monomer constituting the water-absorbent resin as a raw material. Hereinafter, the "production method for an absorbent resin according to the second embodiment of the present invention" may be referred to as the "second production method".

[0073] According to the second production method, it is possible to provide a water-absorbing resin that has an excellent balance of water-absorbing properties, is suppressed in coloration, and contains a recycled water-absorbing resin as a part of the raw material.

[0074] In the present second manufacturing method, "manufacturing a water absorbent resin using a monomer constituting a water absorbent resin as a raw material" refers to a manufacturing method of a normal (non-recycled) water absorbent resin using a monomer constituting a water absorbent resin as a raw material, for example, a manufacturing method of a normal (non-recycled) water absorbent resin including a preparation step of a monomer aqueous solution, a polymerization step, a gel crushing step, a gel drying step, a classification step, and a surface crosslinking step. Therefore, one aspect of the present second manufacturing method can also be expressed as a manufacturing method of a water absorbent resin including a preparation step of a monomer aqueous solution, a polymerization step, a gel crushing step, a gel drying step, a classification step, and a surface crosslinking step, which includes a washing step of washing the recovered water absorbent resin, a dehydration step of dehydrating the recovered water absorbent resin, and a drying step of drying the recovered water absorbent resin, and a deodorant addition step of adding a deodorant to the recovered water absorbent resin is performed after the dehydration step, and a manufacturing method of a water absorbent resin using a recycled water absorbent resin obtained by a recycling method of a recovered water absorbent resin (i.e., a recycled water absorbent resin obtained by the present first manufacturing method) as a part of a raw material.

[0075] A specific embodiment of the second production method will be described in detail below, taking as an example a method including a step of preparing an aqueous monomer solution, a polymerization step, a gel crushing step, a gel drying step, a classification step, and a surface cross-linking step.

[0076] In addition, with regard to the definitions and specific aspects of the "absorbent article", "water-absorbent resin", and "recycled water-absorbent resin" according to the second embodiment of the present invention, as well as the first manufacturing method ("cleaning process", "dehydration process", "deodorant addition process", "drying process", "antibacterial agent addition process", "regeneration process", "crushing (cutting) process", and "separation process"), the descriptions in the above-mentioned first embodiment are appropriately cited.

[0077] [2-2-1] Preparation of monomer aqueous solution The second production method preferably includes a monomer aqueous solution preparation step of preparing an aqueous solution containing acrylic acid (salt) as a main component (hereinafter referred to as "monomer aqueous solution"). In the second production method, a monomer slurry liquid can be used in addition to the monomer aqueous solution within a range in which the water absorption performance of the obtained water absorbent resin is not deteriorated, but for the sake of convenience, the monomer aqueous solution will be described in this section.

[0078] The above-mentioned "main component" means that the amount (content) of acrylic acid (salt) used is 50 mol% or more in the total amount (100 mol%) of monomers (in other words, monomers contained in the aqueous monomer solution, excluding the internal crosslinking agent) to be subjected to the polymerization reaction of the water absorbent resin. The content of acrylic acid (salt) in the total amount (100 mol%) of monomers is preferably 70 mol% or more, and more preferably 90 mol% or more (upper limit: 100 mol%).

[0079] (acrylic acid) In the second production method, from the viewpoint of the physical properties and productivity of the resulting water absorbent resin, acrylic acid and / or its salt (hereinafter referred to as "acrylic acid (salt)") is used as a monomer serving as a raw material for the water absorbent resin. As such "acrylic acid", known acrylic acid can be used.

[0080] The term "acrylate" refers to acrylic acid neutralized with a basic composition. The acrylate may be a commercially available acrylate (e.g., sodium acrylate) or may be one obtained by neutralization in a water absorbent resin manufacturing plant.

[0081] (Basic Composition) In the present specification, the term "basic composition" refers to a composition containing a basic compound. More specifically, the basic compound may be an alkali metal carbonate and / or hydrogen carbonate, an alkali metal hydroxide, ammonia, an organic amine, or the like. Among these, from the viewpoint of the physical properties of the obtained water absorbent resin, a strongly basic basic compound, for example, an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide, or lithium hydroxide, is preferred, and sodium hydroxide is more preferred.

[0082] (neutralization) In the second production method, it is preferable to neutralize at least a part of the acrylic acid. As the neutralization in the second production method, either neutralization of the acrylic acid (before polymerization) or neutralization of the hydrogel crosslinked polymer obtained by crosslinking and polymerizing the acrylic acid (after polymerization) (hereinafter referred to as "post-neutralization") can be selected or used in combination. In addition, the mode of these neutralizations is not particularly limited, and may be continuous or batchwise, but continuous is preferable from the viewpoint of production efficiency, etc.

[0083] Regarding the conditions such as the neutralization apparatus, neutralization temperature, and residence time, the conditions described in International Publication No. 2009 / 123197 and US Patent Application Publication No. 2008 / 0194863 are also applied to the present invention.

[0084] The neutralization rate of the monomer (monomer having an acid group including acrylic acid) in the second production method is preferably 10 mol% to 90 mol%, more preferably 40 mol% to 85 mol%, even more preferably 50 mol% to 80 mol%, and particularly preferably 60 mol% to 75 mol% in the total amount of the acid groups of the monomer (100 mol%). By setting the neutralization rate to 10 mol% or more, a water absorbent resin having a sufficient water absorption capacity can be provided, and by setting the neutralization rate to 90 mol% or less, a water absorbent resin having a higher water absorption capacity under pressure can be provided. The neutralization rate of the aqueous monomer solution will be further explained by taking as an example a case where the aqueous monomer solution contains only acrylic acid as a monomer component. In this case, the neutralization rate of the monomer is 75 mol% means that the monomer component contained in the aqueous monomer solution is a mixture of 25 mol% acrylic acid and 75 mol% acrylic acid salt. Such a mixture may be called a partially neutralized product of acrylic acid.

[0085] The above neutralization rate is the same in the case of post-neutralization. The above neutralization rate is also applied to the neutralization rate of the water-absorbent resin as a final product. In the second manufacturing method, the recycled water-absorbent resin is used as a part of the raw material in the manufacture of the water-absorbent resin. When such a recycled water-absorbent resin is used as a part of the raw material in the manufacture of the water-absorbent resin, the recycled water-absorbent resin may contain a basic compound. And, this basic compound may neutralize the acrylic acid before polymerization or the hydrogel-like crosslinked polymer after polymerization. Therefore, taking into account the neutralization by the basic compound derived from this recycled water-absorbent resin, the neutralization (post-neutralization) rate of the monomer in the aqueous monomer solution, the hydrogel-like polymer, and the water-absorbent resin as a final product is adjusted to a predetermined range.

[0086] (Other monomers) The aqueous monomer solution may contain a monomer (other monomer) other than the acrylic acid (salt). In the second production method, such other monomer can be used in combination with the acrylic acid (salt) to produce a water-absorbing resin.

[0087] The other monomer may be a water-soluble or hydrophobic unsaturated monomer. Specifically, an anionic unsaturated monomer such as (anhydrous) maleic acid, fumaric acid, crotonic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, or 2-hydroxyethyl (meth)acryloyl phosphate and / or a salt thereof may be suitably used as the other monomer. Moreover, one type of these other monomers may be used alone, or two or more types may be used in combination.

[0088] (Internal crosslinking agent) The aqueous monomer solution preferably contains an internal crosslinking agent. As the internal crosslinking agent that can be contained in the aqueous monomer solution, for example, the compounds described in U.S. Patent No. 6,241,928 can be mentioned. Among these, one or more compounds can be used as the internal crosslinking agent in consideration of reactivity.

[0089] In addition, from the viewpoint of the water absorption performance of the obtained water absorbent resin, the internal crosslinking agent is preferably a compound having two or more polymerizable unsaturated groups, more preferably a compound having thermal decomposition properties at the drying temperature described below, and further preferably a compound having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structural unit.

[0090] The polymerizable unsaturated group is preferably, for example, an allyl group or a (meth)acrylate group, more preferably a (meth)acrylate group. The (poly)alkylene glycol structural unit is preferably polyethylene glycol, and the number n is preferably 1 or more and 100 or less, more preferably 6 or more and 50 or less.

[0091] The amount of the internal crosslinking agent used (content in the aqueous monomer solution) is preferably 0.0001 mol% or more and 10 mol% or less, more preferably 0.001 mol% or more and 1 mol% or less, based on the total amount of the monomer. By setting the amount of the internal crosslinking agent used within the above range, a water-absorbing resin having a desired water-absorbing performance can be obtained. If the amount of the internal crosslinking agent used is less than 0.0001 mol%, the gel strength of the obtained water-absorbing resin tends to decrease and the water-soluble content tends to increase excessively, and if the amount of the internal crosslinking agent used is more than 10 mol%, the water absorption capacity of the obtained water-absorbing resin tends to decrease, which is not preferable. The mol% relative to the total monomer is the percentage of the number of moles of the internal crosslinking agent relative to the total number of moles of the monomer contained in the aqueous monomer solution.

[0092] In the second production method, a method is preferably applied in which a predetermined amount (within the above range) of an internal crosslinking agent is added to the monomer aqueous solution in advance, and the crosslinking reaction is started simultaneously with polymerization. On the other hand, in addition to this method, a method of post-crosslinking by adding an internal crosslinking agent during and / or after polymerization; a method of radical crosslinking using a radical polymerization initiator; a method of radiation crosslinking using active energy rays such as electron beams and ultraviolet rays, etc. can also be adopted. These methods can also be used in combination.

[0093] (Polymerization inhibitor) From the viewpoint of the polymerizability of acrylic acid and the color tone of the water-absorbing resin, the aqueous monomer solution preferably contains a polymerization inhibitor in an amount of 200 ppm or less, more preferably 10 ppm or more and 160 ppm or less, and even more preferably 20 ppm or more and 100 ppm or less. The polymerization inhibitor is not particularly limited, but is preferably a methoxyphenol, more preferably p-methoxyphenol.

[0094] The aqueous monomer solution according to the second production method may contain impurities derived from each component in the aqueous monomer solution. For example, as for impurities derived from acrylic acid, the description of the compounds described in U.S. Patent Application Publication No. 2008 / 0161512 can be applied to the second embodiment of the present invention.

[0095] (Other substances) The aqueous monomer solution may contain substances (other substances) other than the above-mentioned components from the viewpoint of improving the physical properties of the resulting water-absorbing resin.

[0096] Examples of other substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salts), and crosslinked polyacrylic acid (salts); and other additives such as foaming agents such as carbonates and azo compounds, surfactants, chelating agents, and chain transfer agents.

[0097] When the aqueous monomer solution contains a hydrophilic polymer as an additional substance, the content of the hydrophilic polymer is preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less (the lower limit is 0% by mass) relative to the total amount (100% by mass) of the aqueous monomer solution. When the aqueous monomer solution contains other additives as additional substances, the content of the other additives is preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less (the lower limit is 0% by mass) relative to the total amount (100% by mass) of the aqueous monomer solution.

[0098] The above-mentioned other substances may be added to the reaction system in the polymerization step described below. In this case, it is preferable that the total amount of the other substances contained in the aqueous monomer solution and the other substances added to the reaction system in the polymerization step is within the above-mentioned range.

[0099] 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., a starch-acrylic acid polymer, a PVA-acrylic acid polymer, etc.) is obtained. These polymers and water-absorbent resin compositions also fall within the scope of one embodiment of the present invention.

[0100] [2-2-2] Polymerization process The second manufacturing method preferably includes a polymerization step of polymerizing the monomers (particularly, acrylic acid (salt)-based monomers) in the aqueous monomer solution obtained in the aqueous monomer solution preparation step to obtain a hydrous gel-like crosslinked polymer (hereinafter referred to as "hydrous gel").

[0101] (Polymerization initiator) In the polymerization step, it is preferable to carry out a polymerization reaction of the monomer using a polymerization initiator. The polymerization initiator used in the polymerization step is appropriately selected depending on the polymerization form and the like, and is not particularly limited. For example, a thermal decomposition type polymerization initiator, a photodecomposition type polymerization initiator, or a redox type polymerization initiator used in combination with a reducing agent that promotes the decomposition of these polymerization initiators can be mentioned. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 can be suitably used. In addition, from the viewpoint of handling and the physical properties of the obtained water-absorbing resin, the polymerization initiator is preferably a peroxide or an azo compound, more preferably a peroxide, and even more preferably a persulfate.

[0102] The amount of the polymerization initiator used in the polymerization step is preferably 0.001 mol% or more and 1 mol% or less, more preferably 0.001 mol% or more and 0.5 mol% or less, based on the total amount of the monomers to be polymerized (monomers contained in the aqueous monomer solution). When a reducing agent is used in combination (i.e., when a redox polymerization initiator is used), the amount of the reducing agent used is preferably 0.0001 mol% or more and 0.02 mol% or less, based on the total amount of the monomers. The mol% relative to the monomer refers to the percentage of the number of moles of the polymerization initiator or reducing agent relative to the total number of moles of the monomers contained in the aqueous monomer solution.

[0103] In addition, instead of the above polymerization initiator, the polymerization reaction may be carried out by irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays, or the polymerization reaction may be carried out by using these active energy rays in combination with the polymerization initiator.

[0104] (polymerization form) The polymerization form in the polymerization step is not particularly limited, but from the viewpoint of the water absorption properties of the obtained water absorbent resin and the ease of polymerization control, etc., preferably, spray droplet polymerization, aqueous solution polymerization, reversed phase suspension polymerization, more preferably aqueous solution polymerization, reversed phase suspension polymerization, and even more preferably aqueous solution polymerization are mentioned. Among them, continuous aqueous solution polymerization is particularly preferable. Forms such as continuous belt polymerization and continuous kneader polymerization can also be applied. In addition, a foaming polymerization form in which bubbles (particularly inert gas, etc.) are dispersed in a monomer aqueous solution to carry out polymerization can also be applied.

[0105] As specific polymerization forms, continuous belt polymerization is disclosed in U.S. Patent Nos. 4,893,999, 6,241,928, U.S. Patent Application Publication No. 2005 / 215734, etc., and continuous kneader polymerization is disclosed in U.S. Patent Nos. 6,987,151, 6,710,141, etc. By adopting these continuous aqueous solution polymerizations, the production efficiency of the water absorbent resin can be improved.

[0106] In addition, preferred forms of the continuous aqueous solution polymerization include "high-temperature initiated polymerization" and "high-concentration polymerization." "High-temperature initiated polymerization" refers to a form in which polymerization is initiated at a temperature of preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher (upper limit is the boiling point). "High-concentration polymerization" refers to a form in which polymerization is performed 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 saturated concentration). These polymerization forms can also be used in combination.

[0107] In the polymerization step, the polymerization can be carried out under a desired atmosphere, for example, under an air atmosphere, but from the viewpoint of the color tone of the obtained water absorbent resin, it is preferable to carry out the polymerization under an inert gas atmosphere such as nitrogen or argon. In this case, it is preferable to control the oxygen concentration to 1 volume % or less. It is also preferable to replace the dissolved oxygen in the aqueous monomer solution with an inert gas (for example, to make the dissolved oxygen less than 1 mg / l).

[0108] In one embodiment of the present invention, the polymerization may be carried out as foaming polymerization in which bubbles (particularly the above-mentioned inert gases or the like) are dispersed in an aqueous monomer solution.

[0109] [2-2-3] Gel crushing process The second manufacturing method includes a gel crushing step in which the hydrogel obtained in the polymerization step is crushed by a screw extruder such as a kneader or a meat chopper, or a gel crusher such as a cutter mill, to obtain a particulate hydrogel (hereinafter referred to as a "particulate hydrogel"). When kneader polymerization is used as the polymerization form in the polymerization step, the polymerization step and the gel crushing step are carried out simultaneously. When a particulate hydrogel is directly obtained in the polymerization process, such as in gas phase polymerization or reversed phase suspension polymerization, the gel crushing step may not be carried out.

[0110] Regarding gel crushing conditions and forms other than those described above, the contents disclosed in WO 2011 / 126079 are preferably applied to the present invention.

[0111] [2-2-4] Gel drying process The second manufacturing method preferably includes a gel drying step in which the particulate hydrogel obtained in the polymerization step and / or gel crushing step is dried to a desired resin solid content to obtain a dried polymer. The resin solid content of the dried polymer is determined from the loss on drying (the change in mass when 1 g of the dried polymer is heated at 180° C. for 3 hours), and is preferably 80% by mass or more, more preferably 85% by mass to 99% by mass, even more preferably 90% by mass to 98% by mass, and particularly preferably 92% by mass to 97% by mass.

[0112] The method for drying the particulate hydrogel is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, high humidity drying using high temperature water vapor, etc. Among these, from the viewpoint of drying efficiency, hot air drying is preferred, and band drying in which hot air drying is performed on a ventilation belt is more preferred.

[0113] The drying temperature (hot air temperature) in the hot air drying is preferably 120°C or higher and 250°C or lower, more preferably 150°C or higher and 200°C or lower, from the viewpoint of the color tone and drying efficiency of the obtained water-absorbent resin. The drying conditions other than the drying temperature, such as the hot air speed and drying time, can be appropriately set according to the water content and total mass of the particulate hydrogel to be dried and the intended resin solid content. In addition, when band drying is performed, the conditions described in International Publication Nos. 2006 / 100300, 2011 / 025012, 2011 / 025013, 2011 / 111657, etc. are appropriately applied as drying conditions.

[0114] [2-2-5] Classification process The second production method preferably includes a classification step of classifying the dried polymer obtained in the drying step to obtain a particulate dried polymer having a desired particle size.

[0115] In the classification step, the method for classifying the dried polymer includes sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), air flow classification, etc. Among them, from the viewpoint of classification efficiency, sieve classification is preferably selected.

[0116] The particle size of the particulate dry polymer obtained in the classification step is not particularly limited, but for example, the mass average particle size (D50) is preferably 200 μm or more and 500 μm or less, more preferably 250 μm or more and 450 μm or less, and even more preferably 300 μm or more and 400 μm or less. Also, the ratio of particles smaller than 150 μm is preferably less than 5 mass%, more preferably 4 mass% or less, and even more preferably 3 mass% or less.

[0117] In the classification step, the dried polymer may be appropriately pulverized to obtain a particulate dried polymer of a desired particle size. The pulverization method is not particularly limited, and a high-speed rotary pulverizer such as a roll mill, a hammer mill, a screw mill, or a pin mill, a vibration mill, a knuckle type pulverizer, or a cylindrical mixer may be used.

[0118] [2-2-6] Surface crosslinking process The second production method preferably includes a surface cross-linking step, which is a step of providing a portion with a higher cross-linking density on the surface layer (a portion several tens of μm from the surface of the particulate dry polymer) of the particulate dry polymer obtained through the above-mentioned classification step. The surface cross-linking step is composed of a mixing step, a heat treatment step, and a cooling step (optional).

[0119] In the surface cross-linking step, a surface-cross-linked water-absorbent resin is obtained by radical cross-linking on the surface of the particulate dry polymer, surface polymerization, a cross-linking reaction with a surface cross-linking agent, etc. Therefore, the particulate dry polymer to be subjected to the surface cross-linking step can also be said to be a water-absorbent resin precursor.

[0120] (Surface cross-linking agent) In the surface crosslinking step, it is preferable to crosslink the surface of the particulate dried polymer using a surface crosslinking agent. The surface crosslinking agent is not particularly limited, but may be an organic or inorganic surface crosslinking agent. Among them, from the viewpoint of the physical properties of the water-absorbent resin, the handling property of the surface crosslinking agent, etc., an organic surface crosslinking agent that reacts with a carboxyl group is preferable. Specifically, as the surface crosslinking agent, one or more compounds disclosed in U.S. Patent No. 7,183,456 can be used. More specifically, as the surface crosslinking agent, polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, cyclic urea compounds, etc. may be used. One of these surface crosslinking agents may be used alone, or two or more may be used in combination. In addition, it is preferable to use these surface crosslinking agents in the state of an aqueous solution (i.e., in the state of a surface crosslinking liquid).

[0121] The amount of the surface crosslinking agent used in the surface crosslinking step (the total amount used when a plurality of agents are used) is preferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts by mass, relative to 100 parts by mass of the particulate dry polymer. The surface crosslinking agent is preferably added in the form of an aqueous solution, and in this case, the amount of water used is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, relative to 100 parts by mass of the particulate dry polymer. Furthermore, a hydrophilic organic solvent may be used in combination as necessary, and in that case, the amount of the hydrophilic organic solvent used is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, relative to 100 parts by mass of the particulate dry polymer. Each additive added in the "re-wetting step" described later can also be mixed into the aqueous solution of the surface crosslinking agent within the range of 5 parts by mass or less.

[0122] (Mixing process) The surface crosslinking step includes a mixing step of mixing the particulate dry polymer with the above-mentioned surface crosslinking agent. The method of mixing the surface crosslinking agent in the mixing step is not particularly limited, but for example, a method of preparing a surface crosslinking agent solution in advance and mixing the solution with the particulate dry polymer by preferably spraying or dropping, more preferably spraying, can be mentioned.

[0123] The device for mixing the particulate dry polymer and the surface crosslinking agent is not particularly limited, but is preferably a high-speed stirring type mixer, more preferably a high-speed stirring type continuous mixer.

[0124] (Heat treatment process) The surface cross-linking step includes a heat treatment step in which the mixture discharged from the mixing step is heated to cause a cross-linking reaction on the surface of the particulate dry polymer. The heat treatment step can also be called a cross-linking reaction step.

[0125] The apparatus for carrying out the heat treatment is not particularly limited, but preferably includes a paddle dryer. The heating temperature in the heat treatment, in other words, the reaction temperature in the crosslinking reaction, is appropriately set according to the type of the surface crosslinking agent used, but is preferably 50°C or more and 300°C or less, more preferably 100°C or more and 200°C or less.

[0126] (cooling process) The surface cross-linking step preferably includes a cooling step of cooling the surface-cross-linked particulate dry polymer after the heat treatment.

[0127] In the cooling step, 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 medium to a refrigerant. In addition, the surface crosslinked particulate dry polymer obtained in the heat treatment step is forcibly cooled, if necessary, to preferably 40°C or more and 80°C or less, more preferably 50°C or more and 70°C or less, in the cooling step.

[0128] [2-2-7] Use of recycled water-absorbent resin In the second manufacturing method, the recycled water absorbent resin obtained in the first manufacturing method is used as a part of the raw material of the obtained water absorbent resin. In the second manufacturing method, "using the recycled water absorbent resin as a part of the raw material of the obtained water absorbent resin" means that the recycled water absorbent resin is added to the water absorbent resin raw material (for example, the monomer aqueous solution, the hydrogel, the dried polymer, and / or the particulate dried polymer) in any of the above-mentioned steps, specifically, in any of the steps of preparing the monomer aqueous solution, the polymerization step, the gel crushing step, the gel drying step, the classification step, and the surface crosslinking step.

[0129] The method of adding the recycled water absorbent resin to the water absorbent resin raw material is not particularly limited, and examples thereof include a method of adding the recycled water absorbent resin as it is, a method of adding the recycled water absorbent resin in a state of a swollen gel obtained by swelling the recycled water absorbent resin with water, and a method of adding the recycled water absorbent resin in a state of being dispersed in water.

[0130] More specifically, when the recycled water absorbent resin is added in the "preparation step of the aqueous monomer solution", the recycled water absorbent resin may be added to the aqueous monomer solution, or the recycled water absorbent resin may be added to water in advance and then mixed with other raw materials such as monomers. From the viewpoint of uniform mixing of the raw materials, it is preferable to add the recycled water absorbent resin to the aqueous monomer solution.

[0131] When the recycled water absorbent resin is added in the "polymerization step", the recycled water absorbent resin may be added before the start of polymerization, or may be added after the start of polymerization. From the viewpoint of uniformity of the polymer component, it is preferable to add the recycled water absorbent resin before the start of polymerization.

[0132] When the recycled water absorbent resin is added in the "gel crushing step", the recycled water absorbent resin may be added before crushing, or may be added during crushing. It may also be added in portions. From the viewpoint of uniformity of the gel component, it is preferable to add the recycled water absorbent resin before gel crushing.

[0133] When the recycled water absorbent resin is added in the "gel drying step", the hydrous gel and the recycled water absorbent resin may be mixed before drying, or after drying. When the recycled water absorbent resin powder and the hydrous gel are mixed before drying, the moisture of the hydrous gel is transferred to the recycled water absorbent resin powder. Therefore, it is preferable because it is expected to have effects such as shortening the drying speed and reducing the generation of undried gel.

[0134] When the recycled water absorbent resin is added in the "classification step", the dried polymer and the recycled water absorbent resin may be mixed before or after classification. From the viewpoint of uniformity of particle size, it is preferable to add the recycled water absorbent resin before classification.

[0135] When the recycled water absorbent resin is added in the "surface cross-linking step", the particulate dried polymer and the recycled water absorbent resin may be mixed in advance, or may be treated without mixing. From the viewpoint of uniform mixing of the surface cross-linking liquid and the water absorbent resin, it is preferable to use the particulate recycled water absorbent resin and subject all the surface cross-linked materials (i.e., the particulate dried polymer and the particulate recycled water absorbent resin) to surface cross-linking in the form of particulates.

[0136] In the second manufacturing method, the recycled water absorbent resin may be added at any of the above timings. The entire amount of the recycled water absorbent resin may be added at one time at any timing, or may be added in portions at multiple timings. The recycled water absorbent resin may be added in different forms depending on the timing of addition.

[0137] In the second manufacturing method, the ratio of the recycled water absorbent resin to all the raw materials for the water absorbent resin is not particularly limited, but is preferably 1% by mass or more and 60% by mass or less, more preferably 3% by mass or more and 50% by mass or less, and even more preferably 5% by mass or more and 40% by mass or less. In other words, in the second manufacturing method, it is preferable to add the recycled water absorbent resin so that the ratio of the recycled water absorbent resin to all the raw materials for the water absorbent resin is within the above range. The above-mentioned "all raw materials for the water absorbent resin" refers to all the raw materials used in the manufacturing process of the water absorbent resin (i.e., the total amount of the recycled water absorbent resin to be added and the water absorbent resin prepared from the aqueous monomer solution). More specifically, the raw materials include the recycled water absorbent resin, and the non-recycled water absorbent resin and its raw materials (e.g., acrylic acid (salt); basic composition; other monomers; internal crosslinking agents; other substances (starch, etc.); polymerization initiators; and surface crosslinking agents). The above-mentioned ratio is the ratio of the solid content of the recycled water absorbent resin to the solid content of all the raw materials for the water absorbent resin.

[0138] [2-2-8] Colorability (yellowness index / YI value) of water absorbent resin obtained by the second production method The YI value of the water absorbent resin obtained by the second manufacturing method is preferably 20 or less, more preferably 16 or less, and even more preferably 12 or less. The lower limit of the YI value is not particularly limited and may be 0. The YI value of the water absorbent resin is an index showing the colorability of the water absorbent resin, and the YI value of the water absorbent resin within the above range means that the coloring of the water absorbent resin is further suppressed. The YI value of the water absorbent resin can be measured by the method described in the Examples.

[0139] [2-2-9] Water absorption properties of the water-absorbent resin obtained by the second manufacturing method The water-absorbing properties of the water-absorbing resin obtained by the second manufacturing method are preferably CRC of 25 g / g or more, more preferably 30 g / g or more, even more preferably 35 g / g or more, and even more preferably 40 g / g or more. Also, AAP (0.3 psi) is preferably 10 g / g or more, more preferably 15 g / g or more, more preferably 20 g / g or more, more preferably 22 g / g or more, even more preferably 24 g / g or more, and even more preferably 26 g / g or more. The CRC and / or AAP (0.3 psi) of the water-absorbing resin in the above range means that the water-absorbing resin has excellent water absorption. The CRC and AAP (0.3 psi) of the water-absorbing resin can be measured by the method described in the Examples.

[0140] <3. Manufacturing method of water-absorbent resin containing solubilized polymer derived from recycled water-absorbent resin as part of the raw material> [3-1] Technical concept of the third embodiment The inventors have stated that in the second embodiment, by mixing the recycled water absorbent resin obtained by the present first production method with a part of a raw material used in newly producing a water absorbent resin, a water absorbent resin further excellent in balance of water absorption properties and suppression of coloration can be obtained.

[0141] On the other hand, the present inventors have found that when a recycled water absorbent resin is used as a part of the raw materials used in producing a water absorbent resin, the recycled water absorbent resin absorbs moisture in the raw materials and increases the viscosity of the mixed material, resulting in problems such as a decrease in the handleability of the mixed material and non-uniformity of the mixed material.

[0142] As a result of intensive research to solve the above problems, the present inventors have found that by subjecting a recycled water absorbent resin to a solubilization treatment to form a solubilized polymer, and using the solubilized polymer as a part of the raw material used when producing a water absorbent resin, the viscosity of the mixed material can be significantly reduced, and the handleability and uniformity of the mixed material can be improved; in other words, while using a recycled water absorbent resin as a part of the raw material, a water absorbent resin having a better balance of water absorption properties and suppressed coloration can be obtained, and have completed a third embodiment of the present invention.

[0143] [3-2] Method for producing water-absorbent resin according to the third embodiment In a third embodiment of the present invention, a method for producing a water-absorbent resin is provided, in which a solubilized polymer derived from a recycled water-absorbent resin obtained by the first production method is used as a part of a raw material in a production process of the water-absorbent resin using a monomer constituting the water-absorbent resin as a raw material. Hereinafter, the method for producing a water-absorbent resin according to the third embodiment of the present invention may be referred to as "the third production method".

[0144] According to the third production method, it is possible to provide a water-absorbing resin which has a better balance of water-absorbing properties, is suppressed in coloration, and uses recycled water-absorbing resin as part of the raw material.

[0145] In the third manufacturing method, the term "manufacturing process of water-absorbent resin using monomers constituting water-absorbent resin as raw materials" refers to a manufacturing process (method) of a normal (non-recycled) water-absorbent resin using monomers constituting water-absorbent resin as raw materials, for example, a manufacturing process (method) of a normal (non-recycled) water-absorbent resin including a monomer aqueous solution preparation process, a polymerization process, a gel crushing process, a gel drying process, a classification process, and a surface cross-linking process. The term "solubilized polymer derived from recycled water-absorbent resin" in the third manufacturing method refers to a solubilized polymer obtained by subjecting recycled water-absorbent resin to a solubilization treatment. In other words, the third manufacturing method can be said to include a solubilization process of solubilizing recycled water-absorbent resin to obtain a solubilized polymer. Therefore, one aspect of the present third production method can also be expressed as a production method for a water absorbent resin, which includes a step of preparing an aqueous monomer solution, a polymerization step, a gel crushing step, a gel drying step, a classification step and a surface cross-linking step, and further includes a washing step of washing the recovered water absorbent resin, a dehydration step of dehydrating the recovered water absorbent resin, and a drying step of drying the recovered water absorbent resin, in which a deodorant adding step of adding a deodorant to the recovered water absorbent resin is carried out after the dehydration step, and which includes a solubilization step of solubilizing a recycled water absorbent resin obtained by a recycling method for recovered water absorbent resin (i.e., a recycled water absorbent resin obtained by the present first production method), and which uses a solubilized polymer obtained in the solubilization step as a part of a raw material.

[0146] Hereinafter, a specific embodiment of the third production method will be described in detail, taking as an example a method including a solubilization step of solubilizing a recycled water absorbent resin, a step of preparing an aqueous monomer solution, a polymerization step, a gel crushing step, a gel drying step, a classification step, and a surface crosslinking step.

[0147] In addition, with regard to the definitions and specific aspects of the "absorbent article", "water-absorbent resin", and "recycled water-absorbent resin" according to the third embodiment of the present invention, as well as the first manufacturing method ("cleaning process", "dehydration process", "deodorant addition process", "drying process", "antibacterial agent addition process", "regeneration process", "crushing (cutting) process", and "separation process"), the descriptions in the first embodiment described above are appropriately cited.

[0148] [3-2-1] Solubilization process The third manufacturing method includes a solubilization step of solubilizing the recycled water absorbent resin (hereinafter simply referred to as "recycled water absorbent resin") obtained by the first manufacturing method. The solubilization step can also be said to be a step of solubilizing the recycled water absorbent resin in water to obtain a solubilized polymer. The solubilized polymer obtained in the solubilization step may be entirely solubilized in water, or may be a partially solubilized product containing a polymer that is not partially solubilized.

[0149] The method for solubilizing the recycled water absorbent resin is not particularly limited as long as it is a method that can decompose the water absorbent resin and solubilize it in water. For example, a method of decomposing and solubilizing the water absorbent resin with a reducing agent and a transition metal ion (for example, a method described in JP 2019-131789 A); a method of solubilizing the water absorbent resin using ascorbic acid as a reducing agent under conditions of pH 4 to 7.5 (for example, a method described in JP 3146053 A); a method of solubilizing the water absorbent resin by heat treatment in the presence of an oxidizing agent (for example, a method described in JP 3091251 A); a method of solubilizing the water absorbent resin using chlorine dioxide under acidic conditions (for example, a method described in JP 2019-108639 A); a method of solubilizing the water absorbent resin using hydrogen peroxide (for example, a method described in JP 2019-108640 A); a method of solubilizing the water absorbent resin with ozone water containing ozone (for example, a method described in JP 2019-085343 A, etc.), etc. can be mentioned.

[0150] The solubilized polymer obtained in the solubilization step may be used as it is in the form of an aqueous solution as a part of the water-absorbent resin raw material, or may be dried to form a solid and then used as a part of the water-absorbent resin raw material.

[0151] The molecular weight of the solubilized polymer obtained in the solubilization step is preferably lower from the viewpoints of handling of the polymer aqueous solution and mixability with other raw materials, but is preferably higher from the viewpoints of the balance of water absorption properties of the water absorbent resin, reduction of soluble content, etc. Therefore, from the viewpoint of obtaining a solubilized polymer that has a good balance between mixability with other raw materials and water absorption properties and has a reduced soluble content, the mass average molecular weight of the solubilized polymer obtained in the solubilization step is preferably from 10,000 to 1,000,000, more preferably from 20,000 to 800,000, and even more preferably from 30,000 to 600,000.

[0152] [3-2-2] Other processes Regarding specific aspects of the production steps of a water absorbent resin that can be included in the present third production method, i.e., the step of preparing an aqueous monomer solution, the polymerization step, the gel crushing step, the gel drying step, the classification step and the surface cross-linking step, the description of the above-mentioned second embodiment is appropriately cited.

[0153] [3-2-3] Use of solubilized polymers derived from recycled water-absorbent resins In the third manufacturing method, the solubilized polymer derived from the recycled water absorbent resin obtained in the first manufacturing method is used as a part of the raw material of the obtained water absorbent resin. In the third manufacturing method, "the solubilized polymer derived from the recycled water absorbent resin is used as a part of the raw material of the obtained water absorbent resin" means that the solubilized polymer derived from the recycled water absorbent resin is added to the water absorbent resin raw material (e.g., the monomer aqueous solution, the hydrogel, the dried polymer, and / or the particulate dried polymer) in any of the manufacturing steps of the water absorbent resin that may be included in the third manufacturing method, specifically, in any of the steps of preparing the monomer aqueous solution, the polymerization step, the gel crushing step, the gel drying step, the classification step, and the surface crosslinking step.

[0154] The method of adding the solubilized polymer derived from the recycled water absorbent resin in the above-mentioned process includes, but is not limited to, a method of adding it as an aqueous solution, a method of adding it as a powder after drying once, and a method of adding it in a slurry state with some undissolved matter remaining.

[0155] In addition, the solubilized polymer derived from the recycled water absorbent resin may contain a basic compound derived from the recycled water absorbent resin as a raw material. When the solubilized polymer derived from the recycled water absorbent resin contains a basic compound, the neutralization rate of the monomer and / or the water absorbent resin as a final product may change by adding the polymer to the water absorbent resin raw material. Therefore, when the solubilized polymer derived from the recycled water absorbent resin is added to the water absorbent resin raw material, it is preferable to adjust the neutralization rate of the monomer and the neutralization rate of the water absorbent resin as a final product to fall within a predetermined range, taking into account the amount of the basic compound contained in the polymer.

[0156] More specifically, when the solubilized polymer derived from the recycled water absorbent resin is added in the "preparation step of the aqueous monomer solution", the solubilized polymer derived from the recycled water absorbent resin may be mixed into the aqueous monomer solution, or the solubilized polymer derived from the recycled water absorbent resin may be mixed in advance with water and then mixed with other raw materials. From the viewpoint of uniform mixing of the raw materials, it is preferable to mix the solubilized polymer derived from the recycled water absorbent resin into the aqueous monomer solution.

[0157] When the solubilizing polymer derived from the recycled water absorbent resin is added in the "polymerization step", the solubilizing polymer derived from the recycled water absorbent resin may be added before or after the start of polymerization. From the viewpoint of uniformity of the polymer component, it is preferable to add the solubilizing polymer derived from the recycled water absorbent resin before the start of polymerization.

[0158] When the solubilized polymer derived from the recycled water absorbent resin is added in the "gel crushing step", the solubilized polymer derived from the recycled water absorbent resin may be added before crushing or during crushing. It may also be added in portions. From the viewpoint of uniformity of the gel component, it is preferable to add the solubilized polymer derived from the recycled water absorbent resin before gel crushing.

[0159] When the solubilized polymer derived from the recycled water absorbent resin is added in the "gel drying step", the hydrous gel and the solubilized polymer derived from the recycled water absorbent resin may be mixed before or after drying.

[0160] When the solubilized polymer derived from the recycled water absorbent resin is added in the "classification step", the dried polymer and the solubilized polymer derived from the recycled water absorbent resin may be mixed before classification, or may be mixed after classification. From the viewpoint of uniformity of particle size, it is preferable to add the solubilized polymer derived from the recycled water absorbent resin before classification.

[0161] When the solubilized polymer derived from the recycled water absorbent resin is added in the "surface cross-linking step", the water absorbent resin dried in advance may be mixed with the solubilized polymer derived from the recycled water absorbent resin, or may be mixed after the surface cross-linking. From the viewpoint of uniform mixing of the surface cross-linking liquid and the water absorbent resin, it is preferable to use the solubilized polymer derived from the powdered recycled water absorbent resin and subject all the surface cross-linked materials (i.e., the particulate dried polymer and the solubilized polymer derived from the powdered recycled water absorbent resin) to surface cross-linking in the form of powder. In addition, when the solubilized polymer derived from the recycled water absorbent resin is mixed after the surface cross-linking, it is preferable to use the solubilized polymer derived from the surface cross-linked recycled water absorbent resin as the solubilized polymer derived from the recycled water absorbent resin.

[0162] In the third manufacturing method, the solubilizing polymer derived from the recycled water absorbent resin may be added at any of the above timings. The total amount of the solubilizing polymer derived from the recycled water absorbent resin to be added may be added at one time at any timing, or may be added in portions at multiple timings. The solubilizing polymer derived from the recycled water absorbent resin may be added in different forms depending on the timing of addition.

[0163] In the third manufacturing method, the ratio of the solubilized polymer derived from the recycled water absorbent resin to all the water absorbent resin raw materials is not particularly limited, but is preferably 1% by mass or more and 60% by mass or less, more preferably 3% by mass or more and 50% by mass or less, and even more preferably 5% by mass or more and 40% by mass or less. In other words, in the third manufacturing method, it is preferable to add the solubilized polymer derived from the recycled water absorbent resin so that the ratio of the solubilized polymer derived from the recycled water absorbent resin to all the water absorbent resin raw materials is within the above range. Note that "all the water absorbent resin raw materials" is as described above. Also, the ratio is the ratio of the solid content of the solubilized polymer derived from the recycled water absorbent resin to the solid content of all the water absorbent resin raw materials.

[0164] [3-2-4] Colorability (yellowness index / YI value) of the water absorbent resin obtained by the third production method The YI value of the water-absorbent resin obtained by the third manufacturing method is preferably 20 or less, more preferably 16 or less, and even more preferably 12 or less. The lower limit of the YI value is not particularly limited and may be 0. The YI value of the water-absorbent resin is an index showing the colorability of the water-absorbent resin, and the YI value of the water-absorbent resin within the above range means that the coloring of the water-absorbent resin is further suppressed. The YI value of the water-absorbent resin can be measured by the method described in the Examples.

[0165] [3-2-5] Water absorption properties of the water-absorbent resin obtained by the third manufacturing method The water-absorbing properties of the water-absorbing resin obtained by the third manufacturing method are preferably CRC of 25 g / g or more, more preferably 30 g / g or more, even more preferably 35 g / g or more, and even more preferably 40 g / g or more. Also, AAP (0.3 psi) is preferably 10 g / g or more, more preferably 15 g / g or more, more preferably 20 g / g or more, more preferably 22 g / g or more, even more preferably 24 g / g or more, and even more preferably 26 g / g or more. The CRC and / or AAP (0.3 psi) of the water-absorbing resin in the above range means that the water-absorbing resin has excellent water absorption. The CRC and AAP (0.3 psi) of the water-absorbing resin can be measured by the method described in the Examples.

[0166] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. EXAMPLES

[0167] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples. Examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention.

[0168] (Measurement and evaluation methods) The methods for measuring and evaluating each physical property in the examples and comparative examples are as follows.

[0169] (a) CRC The "CRC" of a water-absorbent resin is an abbreviation for "Centrifuge Retention Capacity" and indicates the absorption capacity (unit: g / g) of a water-absorbent resin for 30 minutes under no pressure with respect to a 0.90% by mass aqueous solution of sodium chloride. The specific method for measuring the CRC of a water-absorbent resin (recycled water-absorbent resin) was as follows.

[0170] 0.200 g of the water-absorbent resin was uniformly placed in a bag (85 mm x 60 mm) made of nonwoven fabric (manufactured by Nangoku Pulp Kogyo Co., Ltd., product name: Heatlon Paper, model: GSP-22) and heat-sealed. The bag (containing the water-absorbent resin) 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 pulled out and drained for 3 minutes using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122) at a centrifugal force (250 G) described in Edana ABSORBENCY II 441.1-99, and the mass W1 (g) of the bag was measured. In addition, the same operation was performed without using the water-absorbent resin, and the mass W0 (g) at that time was measured. From the measured values ​​of W1 and W0, the CRC of the control water-absorbent resin was calculated according to the following formula (1): CRC of absorbent resin (g / g) = (W1-W0) / (mass of absorbent resin (g))-1···Equation (1).

[0171] (b) AAP0.3 The "AAP" of a water-absorbent resin is an abbreviation for "Absorption Against Pressure" and indicates the absorbency (unit: g / g) against a 0.90% by mass aqueous sodium chloride solution. Also, "AAP0.3" means the AAP under a pressure of 0.3 psi.

[0172] The AAP0.3 of a specific water-absorbent resin (recycled water-absorbent resin) was measured in accordance with NWSP 242.0.R2(15) except that the pressure condition was changed from 0.7 psi to 0.3 psi. More specifically, a large excess of 0.9 mass% sodium chloride aqueous solution was used, and 0.9 g of the water-absorbent resin was subjected to pressure of 2.07 kPa (21 g / cm) for 1 hour. 2After swelling under a pressure of 0.2 psi (0.3 psi), the AAP (absorbency against pressure) was measured.

[0173] (c) Solids concentration of the solubilized polymer aqueous solution The method for measuring the solids concentration of the aqueous solution of the solubilized polymer obtained by solubilizing the water absorbent resin was as follows. The aqueous solution of the solubilized polymer with a mass of Wa (g) was placed on an aluminum dish with a mass of Wb (g) and dried in a hot air circulating oven at 180°C for 3 hours. After drying, the total mass Wc (g) of the solubilized polymer and the aluminum dish was measured. The solids concentration of the aqueous solution of the solubilized polymer was calculated according to the following formula (2).

[0174] Solids concentration of solubilized polymer aqueous solution (mass%) = {(mass Wc - mass Wb) / mass Wa} × 100 Equation (2) (d) Mass average molecular weight of the solubilizing polymer The weight average molecular weight (Mw) of the solubilized polymers obtained in the examples and comparative examples was measured according to the following procedure.

[0175] (Preparation of measurement samples) First, the solubilized polymer was dissolved in the following solvent to prepare a solution with a concentration of 0.1% by mass. The obtained solubilized polymer solution was then passed through a filter (GL Sciences: GL Chromatodisk, aqueous 25A, pore size 0.2 μm) to prepare a measurement sample.

[0176] Solvent: Aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM sodium dihydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 to 6.38). Using the measurement sample, GPC measurement was carried out under the following measurement conditions.

[0177] (GPC measurement) The above-mentioned measurement sample was subjected to GPC measurement using a Viscotec TDAmax manufactured by Malvern Instruments. The measurement device was equipped with a size exclusion chromatograph, a refractive index detector, a light scattering detector and a capillary viscometer. The measurement device and measurement conditions were as follows.

[0178] Pump, autosampler: Viscotec GPCmax (Malvern) Guard column: OHpak SB-G (Showa Denko Co., Ltd.) Column: OHpak SB-806MHQ (Showa Denko K.K.), two columns connected in series Detector: Viscotec TDAmax (Malvern) Solvent: Aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM sodium dihydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 to 6.38). Flow rate: 0.5ml / min Injection volume: 100μl.

[0179] The water used in the GPC measurements was pure water from which impurities had been thoroughly removed. The GPC measurements were performed under conditions where the detector baseline was stable, particularly when there were no noise peaks in the light scattering detector.

[0180] The measuring device was calibrated using polyoxyethylene glycol (mass average molecular weight (Mw): 21966, molecular weight distribution (Mw / Mn): 1.0, differential refractive index (dn / dc): 0.132, solvent refractive index: 1.33) as a standard sample.

[0181] The differential refractive index (dn / dc) of the solubilized polymer to be measured was set to 0.12, and the solvent refractive index was set to 1.33. Data collection and analysis of the refractive index, light scattering intensity, and viscosity were performed using software (Viscotek OmniSEC4.7.0 (registered trademark)).

[0182] Based on the data obtained from the refractive index (RI), light scattering intensity (angle: 7°) (LALS), and viscometer (DP) measurements, the mass average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of the solubilized polymer were calculated.

[0183] (e) Coloring evaluation of water absorbent resin (YI value / yellowness index) The coloring evaluation of the recycled water absorbent resins obtained in the Examples and Comparative Examples and the water absorbent resins using the recycled water absorbent resins as part of the raw material was carried out using a spectrophotometer (Spectrophotometer SE7700) manufactured by Nippon Denshoku Industries Co., Ltd.

[0184] Specifically, the YI value (Yellow Index) of the water-absorbent resin was measured by the spectrophotometer under the conditions of room temperature (20°C to 25°C) and humidity of 50 RH%. Reflection measurement was selected as the measurement setting condition, and the measurement diameter was LAV (28 mm). In addition, the attached 35Φ×15H round cell, 35Φ×15H cell case, and standard white board for powder were used.

[0185] [Production Example 1] A reaction vessel was charged with 244.9 g of acrylic acid, 0.71 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass % aqueous solution of trisodium diethylenetriaminepentaacetate (DTPA·3Na), 103.7 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 389.0 g of deionized water, and the mixture was mixed to prepare an aqueous monomer solution (a').

[0186] Next, the monomer aqueous solution (a') was cooled while stirring. When the liquid temperature reached 40.0°C, 100.9 g of a 48.5 mass% aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare a monomer aqueous solution (a). At this time, the temperature of the monomer aqueous solution (a) rose to 78.2°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5 mass% aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved and became a transparent homogeneous solution.

[0187] Next, 12.1 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred aqueous solution of monomer (a), and the mixture was immediately poured into a stainless steel vat-shaped container in an open-air system. The time from the start of the second neutralization to the pouring of the aqueous solution of monomer (a) into the vat-shaped container was 55 seconds, and the vat-shaped container was a hot plate, which was heated until the surface temperature reached 40°C after the aqueous solution of monomer (a) was poured. The polymerization reaction started 60 seconds after the aqueous solution of monomer (a) was poured into the vat-shaped container. The polymerization reaction proceeded while the reactants expanded and foamed in all directions while generating water vapor. After the reaction, the reactants shrunk to a size slightly larger than the vat-shaped container. After 3 minutes had passed since the start of the polymerization reaction, the reactants were taken out as a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (S1). The series of operations was carried out in an open-air system.

[0188] The hydrogel (1) obtained by the polymerization reaction was cut into strips and crushed with a meat chopper having a die diameter of 7.5 mm. The crushed hydrogel (1) was spread on a 50-mesh wire screen and dried with hot air at 190°C for 60 minutes to obtain a dried polymer (1).

[0189] Next, the dried polymer (1) was further pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an irregularly pulverized particulate dried polymer (S1).

[0190] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained particulate dry polymer (S1), and the mixture was heat-treated at 100°C for 45 minutes to perform surface crosslinking of the particulate dry polymer (S1).

[0191] The surface-crosslinked particulate dried polymer (S1) was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (S1). The properties of the obtained water-absorbent resin (S1) were measured and evaluated. The results are shown in Table 1.

[0192] [Production Example 2] 100 parts by mass (200 g) of the water absorbent resin (S1) obtained in Production Example 1 were added to a reaction vessel equipped with a stirrer, and 3000 parts by mass (6,000 g) of artificial urine (a solution consisting of 1.9 mass% urea, 0.80 mass% sodium chloride, 0.10 mass% magnesium chloride hexahydrate, 0.10 mass% calcium chloride dihydrate, and 97.1 mass% ion-exchanged water) were added, and the mixture was left at room temperature for 24 hours to obtain a simulated used water absorbent resin (1).

[0193] [Comparative Example 1] 10,000 g of ethanol was added to the reaction vessel containing the above-mentioned simulated used water absorbent resin (1), and the mixture was stirred for 15 minutes to dehydrate, after which the contents of the reaction vessel were filtered through a 100-mesh stainless steel wire screen. 800 g of the resulting filtrate (solid content concentration 24.6% by mass) was continuously fed into a meat chopper, pulverized, and then dried in an oven at 190°C for 2 hours.

[0194] Next, the dried product was pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a comparative recycled water absorbent resin precursor (C1) in an irregularly pulverized form.

[0195] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained comparative recycled water absorbent resin precursor (C1), and the mixture was heat-treated at 100°C for 45 minutes, thereby performing surface crosslinking of the comparative recycled water absorbent resin precursor (C1).

[0196] The comparative recycled water absorbent resin precursor (C1) after surface cross-linking was cooled and mixed uniformly with an aqueous solution consisting of 1 part by mass of water and 0.01 part by mass of trisodium diethylenetriaminepentaacetate. After drying at 60°C for 1 hour, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain a comparative recycled water absorbent resin (C1). The physical properties of the comparative recycled water absorbent resin (C1) obtained were measured and evaluated. The results are shown in Table 1.

[0197] [Example 1] 10,000 g of ethanol was added to the reaction vessel containing the above-mentioned simulated used water absorbent resin (1), and the mixture was stirred for 15 minutes to dehydrate, after which the contents of the reaction vessel were filtered through a 100-mesh stainless steel wire screen. 800 g of the resulting filtrate (solid content concentration 24.6% by mass) and 39.4 g of a 0.50% by mass aqueous hydrogen peroxide solution as a bleaching agent were continuously fed into a meat chopper (bleaching process), crushed, and then dried in an oven at 190°C for 2 hours.

[0198] Next, the dried product was pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a recycled water-absorbent resin precursor (1) in an irregularly pulverized form.

[0199] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained recycled water absorbent resin precursor (1), and the mixture was heat-treated at 100° C. for 45 minutes, thereby performing surface crosslinking of the recycled water absorbent resin precursor (1).

[0200] The recycled water absorbent resin precursor (1) after surface cross-linking was cooled and uniformly mixed with an aqueous solution consisting of 1 part by mass of water and 0.01 part by mass of trisodium diethylenetriaminepentaacetate. After drying at 60°C for 1 hour, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water absorbent resin (1). The physical properties of the obtained recycled water absorbent resin (1) were measured and evaluated. The results are shown in Table 1.

[0201] [Example 2] 10,000 g of ethanol was added to the reaction vessel containing the above-mentioned simulated used water absorbent resin (1), and the mixture was stirred for 15 minutes to dehydrate, after which the contents of the reaction vessel were filtered through a 100-mesh stainless steel wire screen. 800 g of the resulting filtrate (solid content concentration 24.6% by mass) and 59.0 g of a 0.50% by mass aqueous hydrogen peroxide solution as a bleaching agent were continuously fed into a meat chopper (bleaching process), pulverized, and then dried in an oven at 190°C for 2 hours.

[0202] Next, the dried product was pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a recycled water-absorbent resin precursor (2) in an irregularly pulverized form.

[0203] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained recycled water absorbent resin precursor (2), and the mixture was heat-treated at 100° C. for 45 minutes, thereby performing surface crosslinking of the recycled water absorbent resin precursor (2).

[0204] The recycled water absorbent resin precursor (2) after surface cross-linking was cooled and mixed uniformly with an aqueous solution consisting of 1 part by mass of water and 0.01 part by mass of trisodium diethylenetriaminepentaacetate. After drying at 60°C for 1 hour, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water absorbent resin (2). The physical properties of the obtained recycled water absorbent resin (2) were measured and evaluated. The results are shown in Table 1.

[0205] [Example 3] 10,000 g of ethanol was added to the reaction vessel containing the above-mentioned simulated used water absorbent resin (1), and the mixture was stirred for 15 minutes to dehydrate, after which the contents of the reaction vessel were filtered through a 100-mesh stainless steel wire screen. 800 g of the resulting filtrate (solid content concentration 24.6% by mass) and 59.0 g of a 0.50% by mass aqueous hydrogen peroxide solution as a bleaching agent were continuously fed into a meat chopper (bleaching process), pulverized, and then dried in an oven at 190°C for 2 hours.

[0206] Next, the dried product was pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a recycled water-absorbent resin precursor (3) in an irregularly pulverized form.

[0207] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained recycled water absorbent resin precursor (3), and the mixture was heat-treated at 100° C. for 45 minutes, thereby performing surface crosslinking of the recycled water absorbent resin precursor (3).

[0208] The recycled water absorbent resin precursor (3) after surface cross-linking was cooled and uniformly mixed with an aqueous solution consisting of 1 part by mass of water and 0.01 part by mass of trisodium diethylenetriaminepentaacetate. After drying at 60°C for 1 hour, it was passed through a JIS standard sieve with an opening of 710 μm, and further, 0.50 part by mass of titanium dioxide (manufactured by Kanto Chemical Industry Co., Ltd.) was uniformly mixed (white pigment addition step) to obtain a recycled water absorbent resin (3). The physical properties of the obtained recycled water absorbent resin (3) were measured and evaluated. The results are shown in Table 1.

[0209] [Table 1]

[0210] In the table, the amounts of bleach and white pigment added are shown relative to 100 parts by mass of the simulated used water absorbent resin.

[0211] (summary) Table 1 shows the water absorption properties and YI values ​​(yellowness index) of the recycled water absorbent resins produced in Comparative Example 1 and Examples 1 to 3.

[0212] The comparative recycled water absorbent resin (C1) produced in Comparative Example 1 has water absorption properties equivalent to those of the water absorbent resin (S1) produced in Production Example 1, but on the other hand, it has a very high YI value (yellowness index), and is therefore unsuitable for product applications.

[0213] On the other hand, the recycled water absorbent resins of Examples 1 and 2, which were subjected to a bleaching step of adding hydrogen peroxide as a bleaching agent, maintained the same water absorption properties as the water absorbent resin (S1), while further improving the YI value (yellowness index). Moreover, the recycled water absorbent resin of Example 3, which was subjected to a white pigment addition step of adding titanium dioxide, a white pigment, in addition to the bleaching step, further improved the YI value (yellowness index). From these results, it was shown that a recycled water absorbent resin with suppressed coloring can be provided according to the manufacturing method of a recycled water absorbent resin according to one embodiment of the present invention, which includes a bleaching step and / or a white pigment addition step.

[0214] [Production Example 3] Into a reaction vessel equipped with an agitator, 100 parts by mass of the recycled water absorbent resin (3) obtained in Example 3, 733 parts by mass of deionized water, 149.3 parts by mass of a 15% by mass aqueous sodium carbonate solution as a pH adjuster, and 6.67 parts by mass of 30% by mass hydrogen peroxide as a decomposition agent were added and mixed with a stirrer.

[0215] Next, the reaction vessel containing the mixture was immersed in a thermostatic bath at 80° C. to initiate the decomposition reaction. After 60 minutes, it was confirmed that the gel-like matter had been completely solubilized.

[0216] Next, in order to eliminate hydrogen peroxide remaining in the system, 80.7 parts by mass of 15% by mass sodium carbonate was added and stirring was continued for 1 hour. After 1 hour, the solution was cooled to room temperature to obtain an aqueous solution of solubilized polymer (A) derived from recycled water absorbent resin (3). The aqueous solution of solubilized polymer (A) had a solid content of 9.37% by mass and a mass average molecular weight Mw of 483,000.

[0217] [Example 4] A reaction vessel was charged with 220.4 g of acrylic acid, 0.64 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 320.2 g of the aqueous solution of solubilized polymer (A) obtained in Production Example 3, 1.83 g of a 1.0 mass % aqueous solution of trisodium diethylenetriaminepentaacetate (DTPA·3Na), 93.4 g of a 48.5 mass % aqueous solution of sodium hydroxide, and 29.8 g of deionized water, and the mixture was mixed to produce an aqueous monomer solution (4').

[0218] Next, the monomer aqueous solution (4') was cooled while stirring. When the liquid temperature reached 40.0°C, 90.8 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare a monomer aqueous solution (4). At this time, the temperature of the monomer aqueous solution (4) rose to 78.2°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved and became a transparent homogeneous solution.

[0219] Next, 10.9 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred aqueous monomer solution (4), and the mixture was immediately poured into a stainless steel vat-type container in an open-air system. The time from the start of the second neutralization to the pouring of the aqueous monomer solution (b) into the vat-type container was 55 seconds, and the vat-type container was a hot plate, which was heated until the surface temperature reached 40°C after the aqueous monomer solution (b) was poured. The polymerization reaction started 70 seconds after the aqueous monomer solution (b) was poured into the vat-type container. The polymerization reaction proceeded while the reactants expanded and foamed in all directions while generating water vapor. After the reaction, the reactants contracted to a size slightly larger than the vat-type container. After 3 minutes had passed since the start of the polymerization reaction, the reactants were taken out as a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (4). The series of operations was carried out in an open-air system.

[0220] The hydrogel (4) obtained by the polymerization reaction was cut into strips and crushed with a meat chopper having a die diameter of 7.5 mm. The crushed hydrogel (4) was spread on a 50-mesh wire screen and dried with hot air at 190°C for 60 minutes to obtain a dried polymer (4).

[0221] Next, the obtained dry polymer (4) was further pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain irregularly pulverized particulate dry polymer (4).

[0222] A surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained particulate dry polymer (4), and the mixture was heat-treated at 100°C for 45 minutes, thereby performing surface crosslinking of the particulate dry polymer (4).

[0223] The particulate dried polymer (4) after surface crosslinking was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water absorbent resin (4) containing the solubilized polymer (A) derived from the recycled water absorbent resin (3) as a part of the raw material. The physical properties of the obtained water absorbent resin (4) were measured and evaluated. The results are shown in Table 2.

[0224] [Table 2]

[0225] Table 2 shows the water absorption properties and YI value (yellowness index) of the water absorbent resin of Production Example 1, which does not use a solubilized polymer as a raw material (using only normal monomer raw materials), and the water absorbent resin of Example 4, which is a water absorbent resin using 10 mass% of a solubilized polymer with respect to all water absorbent resin raw materials. From Table 2, it can be seen that the water absorbent resin obtained in Example 4 is a water absorbent resin that has excellent water absorption properties and an excellent YI value that is comparable to that of normal water absorbent resins (in other words, coloring is sufficiently suppressed). That is, it was shown that by using a solubilized polymer derived from a recycled water absorbent resin produced by a method for producing a recycled water absorbent resin according to one embodiment of the present invention as a part of the raw material, a water absorbent resin in which coloring is suppressed can be provided while using a recycled water absorbent resin as a raw material. [Industrial Applicability]

[0226] According to one aspect of the present invention, a recycled water-absorbent resin with suppressed coloration can be provided. The recycled water-absorbent resin can be suitably used for various applications, particularly for sanitary products such as paper diapers, sanitary napkins, and adult incontinence products.

Claims

1. A method for producing a recycled water-absorbent resin from a water-absorbent resin recovered from a used absorbent article or a waste generated in a process for producing a water-absorbent resin, comprising the steps of: A method for producing a recycled water-absorbent resin, comprising: a bleaching step of bleaching the water-absorbent resin; and / or a white pigment adding step of adding a white pigment to the water-absorbent resin.

2. The method for producing a recycled water-absorbent resin according to claim 1, wherein the water-absorbent resin is bleached using an oxidizing bleach or a reducing bleach in the bleaching step.

3. The method includes a washing step of washing the water absorbent resin, a dehydration step of dehydrating the water absorbent resin, and a drying step of drying the water absorbent resin, The method for producing a recycled water absorbent resin according to claim 1, wherein the bleaching step is carried out after the dehydration step.

4. The method includes a washing step of washing the water absorbent resin, a dehydration step of dehydrating the water absorbent resin, and a drying step of drying the water absorbent resin, The method for producing a recycled water absorbent resin according to claim 1, wherein the white pigment addition step is carried out after the dehydration step.

5. The method for producing a recycled water absorbent resin according to claim 2, wherein in the bleaching step, the oxidizing bleach or the reducing bleach is added in an amount of 0.01 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the water absorbent resin.

6. The method for producing a recycled water absorbent resin according to claim 1, wherein in the white pigment adding step, the white pigment is added in an amount of 0.01 parts by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the water absorbent resin.

7. A method for producing a water-absorbing resin, comprising the steps of: producing a water-absorbing resin from a monomer constituting the water-absorbing resin as a raw material; and using a recycled water-absorbing resin obtained by the method for producing the water-absorbing resin according to any one of claims 1 to 6 as a part of the raw material.

8. The method for producing a water absorbent resin according to claim 7, wherein the recycled water absorbent resin accounts for 1% by mass or more and 60% by mass or less of all raw water absorbent resin materials.

9. A method for producing a water-absorbent resin, comprising the steps of: producing a water-absorbent resin from a monomer constituting the water-absorbent resin as a raw material; and using a solubilized polymer derived from a recycled water-absorbent resin obtained by the production method according to any one of claims 1 to 6 as a part of the raw material.

10. The method for producing a water absorbent resin according to claim 9, wherein the solubilized polymer derived from the recycled water absorbent resin accounts for 1% by mass or more and 60% by mass or less of the total water absorbent resin raw material.

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