Method for producing water absorbent resin and method for producing water absorbent resin particles

Abstract

Disclosed is a method for producing a water absorbent resin, the method including a dehydration step for dehydrating a water absorbent resin for recycling, which includes water and a water absorbent resin. The dehydration step is performed so that the weight-average molecular weight of the water absorbent resin for recycling after dehydration is 1,000,000 or more. After the dehydration step, the water absorbent resin for recycling after dehydration is placed in an environment in which the weight-average molecular weight thereof is 300,000 or more after 7 days have elapsed.

Description

Method for producing water-absorbent resin and method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing a water-absorbent resin and a method for producing water-absorbent resin particles.

[0002]

[0003] Generally, disposable sanitary products are constructed by enclosing pulp fibers and a water-absorbent resin between a water-impermeable cover sheet and a water-permeable nonwoven fabric, and the water-absorbent resin absorbs water and swells to absorb bodily waste. After use, such sanitary products are disposed of by incineration or landfilling, but recently, research has been conducted into the recovery and reuse of components from used sanitary products. For example, Patent Document 1 discloses a water-absorbent resin decomposition technology that focuses on cleavage of crosslinks within the water-absorbent resin, and a technology for recovering the water-soluble recycled polymer obtained by decomposing the water-absorbent resin as an aggregate by crosslinking with multivalent metal ions.

[0003] Japanese Patent Application Laid-Open No. 2020-49398

[0004] When a recycled water absorbent resin recovered from used sanitary products is regenerated, the recycled water absorbent resin is dehydrated and inactivated. According to the inventor's findings, if the structure of the water absorbent resin contained in the recycled water absorbent resin is destroyed by dehydration, the weight-average molecular weight of the recycled water absorbent resin obtained after dehydration (the weight-average molecular weight of the polymer obtained by solubilizing the water absorbent resin contained in the recycled water absorbent resin (cleaving the crosslinked structure)) becomes too small. If the weight-average molecular weight of the recycled water absorbent resin becomes small, when the polymer whose crosslinked structure has been cleaved in the recycled water absorbent resin is crosslinked again to produce water absorbent resin particles, the crosslinking of the water absorbent resin particles becomes insufficient, resulting in an increase in the amount of elution or an increase in the amount of crosslinking agent used for re-crosslinking, making it difficult to reuse the recycled water absorbent resin.

[0005] Furthermore, even if the weight-average molecular weight of the recycled water absorbent resin obtained after dehydration is relatively large, if the structure of the water absorbent resin contained in the recycled water absorbent resin is damaged during dehydration, the structure of the water absorbent resin is gradually destroyed, and the weight-average molecular weight becomes smaller. Therefore, when the recycled water absorbent resin after dehydration is stored for a certain period (for example, 7 days), the weight-average molecular weight of the recycled water absorbent resin may become too small, making it difficult to reuse it.

[0006] One aspect of the present invention relates to a method for producing a water-absorbing resin that is easily reusable from a water-absorbing resin for recycling.

[0007] One aspect of the present disclosure includes, for example, the following [1] to [6]. [1] A method for producing a water absorbent resin, comprising a dehydration step of dehydrating a water absorbent resin for recycling containing water and a water absorbent resin, wherein the dehydration step is carried out so that the weight-average molecular weight of the water absorbent resin for recycling after the dehydration step is 1,000,000 or more, and wherein, after the dehydration step, the water absorbent resin for recycling after the dehydration step is placed in an environment in which the weight-average molecular weight after 7 days is 300,000 or more. [2] The production method according to [1], wherein the dehydration step is a step of contacting the water absorbent resin for recycling with at least one substance selected from the group consisting of an acid component, a polyvalent metal salt, and an alcohol. [3] The production method according to [1] or [2], wherein the temperature of the environment is 70°C or less. [4] The manufacturing method according to any one of [1] to [3], further comprising a storage step of storing the dehydrated water absorbent resin for recycling, wherein the environment in which the dehydrated water absorbent resin for recycling is stored in the storage step is an environment in which the weight average molecular weight of the dehydrated water absorbent resin for recycling is maintained at 300,000 or more after 7 days of storage. [5] The manufacturing method according to [4], wherein the storage step is a step of storing the dehydrated water absorbent resin for recycling for 1 hour or more after the dehydration. [6] A manufacturing method of water absorbent resin particles, comprising: a preparation step of obtaining a water absorbent resin by the manufacturing method according to any one of [1] to [5], a cleavage step of obtaining a polymer by cleaving a crosslinked structure of a crosslinked polymer contained in the water absorbent resin, and a crosslinking step of crosslinking the polymer.

[0008] According to one aspect of the present invention, there is provided a method for producing an easily reusable water-absorbing resin from a water-absorbing resin for recycling.

[0009] Fig. 2 is a cross-sectional view showing an example of an absorbent article having an absorbent body. Fig. 3 is a schematic view showing a treatment tank for cleaving the cross-linked structure of a water-absorbent resin.

[0010] The present invention is not limited to the following examples.

[0011] In this specification, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. "Physiological saline" refers to a 0.9% by mass aqueous solution of sodium chloride.

[0012] One example of a method for producing a water-absorbent resin includes a dehydration step of dehydrating a recycled water-absorbent resin containing water and a water-absorbent resin. In this method, the dehydration step is carried out so that the weight-average molecular weight of the recycled water-absorbent resin after dehydration is 1,000,000 or more, and after the dehydration step, the recycled water-absorbent resin after dehydration is placed in an environment where the weight-average molecular weight after 7 days is 300,000 or more. The recycled water-absorbent resin may be a used water-absorbent resin (i.e., a water-absorbent resin that has become gelatinous due to absorption), an unused water-absorbent resin (e.g., waste water-absorbent resin generated during the production process of a water-absorbent resin), or a mixture of both.

[0013] Examples of absorbent articles containing recycled water-absorbent resins include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste treatment materials. The absorbent articles may be used. The recycled water-absorbent resin recovered from used absorbent articles may form a gel due to absorption of liquid during use.

[0014] The recycled water-absorbing resin contains water and a water-absorbing resin. The recycled water-absorbing resin may contain fibrous materials such as cellulosic fibers and synthetic fibers.

[0015] The water-absorbing resin contains, for example, a polymer (crosslinked polymer) of an ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide.

[0016] The crosslinked polymer may contain a monomer other than the ethylenically unsaturated monomer. The proportion of the ethylenically unsaturated monomer units (particularly, (meth)acrylic acid and salts thereof) in the crosslinked polymer may be 70 to 100 mol % based on the total amount of monomer units. The proportion of (meth)acrylic acid and salts thereof in the ethylenically unsaturated monomers may be 70 to 100 mol %.

[0017] The water-absorbing resin contains, for example, a crosslinked polymer having a poly(meth)acrylic acid structure.As such crosslinked polymer, for example, the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid to obtain a polymer, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond.

[0018] The shape of the recycled water absorbent resin is not particularly limited, and if it is a used gel, it may be in the form of an irregular lump, and if it is an unused one (generally a dry powder), it may be in the form of an irregular crushed shape, flakes, granules, etc.

[0019] In the dehydration step, the recycled water absorbent resin is dehydrated to obtain a dehydrated recycled water absorbent resin. In the dehydration step, the recycled water absorbent resin may be inactivated to reduce its water absorption performance. Due to the inactivation, even if the dehydrated recycled water absorbent resin is brought into contact with water, the dehydrated recycled water absorbent resin is less likely to absorb water than before dehydration.

[0020] The dehydration of the recycled water absorbent resin can be carried out, for example, by contacting the recycled water absorbent resin with at least one substance selected from the group consisting of an acid component, a polyvalent metal salt, and an alcohol. The dehydration of the recycled water absorbent resin may be carried out by contacting the recycled water absorbent resin with at least one substance selected from the group consisting of an acid component and a polyvalent metal salt, from the viewpoint of sufficiently inactivating the recycled water absorbent resin. The dehydration of the recycled water absorbent resin may be carried out by contacting the recycled water absorbent resin with an acid component, from the viewpoint of making it easier to carry out the cleavage step described below, or by contacting the recycled water absorbent resin with a polyvalent metal salt, from the viewpoint of enabling storage in a high-temperature environment in the storage step described below.

[0021] The acid component may be an inorganic acid, an organic acid, or a combination of both. The inorganic acid may be, for example, at least one selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and boric acid. The organic acid may be, for example, at least one selected from the group consisting of acetic acid, succinic acid, citric acid, malic acid, lactic acid, and fumaric acid. The acid component may be at least one of citric acid and sulfuric acid, from the viewpoints of suppressing destruction of the structure of the water absorbent resin, suppressing excessive reduction in the weight-average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time after dehydration, and facilitating the production of a water absorbent resin that is easily reusable. Alternatively, citric acid may be used because of its chelating effect and bactericidal effect.

[0022] The acid component to be brought into contact with the recycled water-absorbing resin may be in a liquid state such as an aqueous solution, or in a solid state such as a powder.

[0023] When the acid component is liquid, the pH of the acid component may be 0.1 or more, 0.3 or more, 0.5 or more, 0.7 or more, 0.8 or more, 0.9 or more, 1.0 or more, or 1.1 or more from the viewpoint of sufficiently dehydrating the recycled water absorbent resin, and from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time after dehydration from becoming too small, and making it easier to produce a water absorbent resin that is easy to reuse, the pH of the acid component may be 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, 2.5 or less, 2.4 or less, or 2.3 or less. The pH of the acid component may be 0.1 to 5.0, 0.3 to 4.5, 0.5 to 4.0, 0.7 to 3.5, 0.8 to 3.0, 0.9 to 2.5, or 1.0 to 2.4.

[0024] The amount of the acid component to be contacted with the recycled water absorbent resin (solid content) is, relative to 100 parts by mass of the water absorbent resin contained in the recycled water absorbent resin, from the viewpoint of sufficiently dehydrating the recycled water absorbent resin, 50 parts by mass or more, 100 parts by mass or more, 150 parts by mass or more, or 200 parts by mass or more, suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time from dehydration from becoming too small, from the viewpoint of making it easier to produce a water absorbent resin that is easy to reuse, 4000 parts by mass or less, 3000 parts by mass or less, or 2000 parts by mass or less. From these viewpoints, the amount of the acid component to be contacted with the recycled water absorbent resin (solid content) may be 50 to 4000 parts by mass, 100 to 4000 parts by mass, 150 to 3000 parts by mass, or 200 to 2000 parts by mass, relative to 100 parts by mass of the recycled water absorbent resin.

[0025] The temperature when the acid component is contacted to dehydrate the recycled water absorbent resin may be 0 ° C. or higher, 10 ° C. or higher, or 15 ° C. or higher from the viewpoint of sufficiently dehydrating the recycled water absorbent resin. From the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin after the dehydration step and after storage for a certain period of time from dehydration from becoming too small, and making it easier to produce a water absorbent resin that is easy to reuse, the temperature may be 100 ° C. or lower, 80 ° C. or lower, 60 ° C. or lower, 40 ° C. or lower, or 30 ° C. or lower. From these viewpoints, the temperature when the acid component is contacted to dehydrate the recycled water absorbent resin may be 0 to 100 ° C., 10 to 60 ° C., 10 to 40 ° C., or 15 to 30 ° C. The temperature when the acid component is contacted to dehydrate the recycled water absorbent resin may be 20 ° C. or higher, 30 ° C. or higher, 40 ° C. or higher, 50 ° C. or higher, 60 ° C. or higher, 70 ° C. or higher, or 80 ° C. or higher.

[0026] The polyvalent metal salt is a salt of a divalent or higher metal, and the divalent or higher metal may be, for example, at least one selected from the group consisting of calcium, aluminum, magnesium, titanium, chromium, manganese, iron, cobalt, nickel, cadmium, and lead.

[0027] The polyvalent metal salt may be an inorganic salt of a polyvalent metal or an organic salt of a polyvalent metal. Examples of inorganic salts of polyvalent metals include chlorides of polyvalent metals such as calcium chloride, aluminum chloride, and magnesium chloride; hydroxides of polyvalent metals such as calcium hydroxide, aluminum hydroxide, and magnesium hydroxide; and carbonates of polyvalent metals such as calcium carbonate, aluminum carbonate, and magnesium carbonate. Examples of organic salts of polyvalent metals include carboxylates of polyvalent metals such as calcium formate and calcium acetate. The polyvalent metal salt may be a chloride of a polyvalent metal or calcium chloride, from the viewpoints of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing excessive reduction in the weight average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time after dehydration, and facilitating the production of a water absorbent resin that is easy to reuse. The polyvalent metal salt may be used alone or in combination of two or more types.

[0028] The polyvalent metal salt to be brought into contact with the water-absorbing resin for recycling may be in a liquid state such as an aqueous solution, or in a solid state such as a powder.

[0029] When the polyvalent metal salt is a liquid, the pH of the polyvalent metal salt suppresses destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppresses the weight average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time after dehydration from becoming too small, and makes it easier to produce a water absorbent resin that is easy to reuse. From the same viewpoint, it may be 1.0 or more, 3.0 or more, 5.0 or more, 5.5 or more, 6.0 or more, 6.5 or more, or 7.0 or more. From the same viewpoint, it may be 13.0 or less, 12.5 or less, 12.0 or less, 11.5 or less, 11.0 or less, 10.5 or less, or 10.0 or less. The pH of the polyvalent metal salt may be 1.0 to 13.0, 3.0 to 12.5, 5.0 to 12.0, 5.5 to 11.5, 6.0 to 11.0, 6.5 to 10.5, or 7.0 to 10.0.

[0030] The amount of polyvalent metal salt to be contacted with the recycled water absorbent resin (solid content) is, relative to 100 parts by mass of the water absorbent resin contained in the recycled water absorbent resin, from the viewpoint of sufficiently dehydrating the recycled water absorbent resin, 50 parts by mass or more, 100 parts by mass or more, 150 parts by mass or more, or 200 parts by mass or more, from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, by increasing the weight average molecular weight, it may be 4000 parts by mass or less, 3000 parts by mass or less, or 2000 parts by mass or less. From these viewpoints, the amount of polyvalent metal salt to be contacted with the recycled water absorbent resin (solid content) may be 50 to 4000 parts by mass, 100 to 4000 parts by mass, 150 to 3000 parts by mass, or 200 to 2000 parts by mass, relative to 100 parts by mass of the recycled water absorbent resin.

[0031] The temperature when the recycled water absorbent resin is dehydrated by contacting the polyvalent metal salt may be 0 ° C. or higher, 10 ° C. or higher, or 15 ° C. or higher from the viewpoint of sufficiently dehydrating the recycled water absorbent resin, and from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin after dehydration and after storage for a certain period of time after dehydration from becoming too small, and making it easier to produce a water absorbent resin that is easy to reuse, it may be 100 ° C. or lower, 80 ° C. or lower, 60 ° C. or lower, 40 ° C. or lower, or 30 ° C. or lower. From these viewpoints, the temperature when the recycled water absorbent resin is dehydrated by contacting the polyvalent metal salt may be 0 to 100 ° C., 10 to 60 ° C., 10 to 40 ° C., or 15 to 30 ° C.

[0032] The alcohol may be, for example, at least one selected from the group consisting of methanol, ethanol, 2-propanol, t-butanol, isobutanol, pentanol, and hexanol. The amount of the alcohol to be contacted with the recycled water absorbent resin may be 1,000 to 1,000,000 parts by mass, or 10,000 to 100,000 parts by mass, relative to 100 parts by mass of the water absorbent resin contained in the recycled water absorbent resin.

[0033] The dehydration step is carried out so that the weight-average molecular weight of the recycled water absorbent resin after dehydration is 1,000,000 or more. That is, the weight-average molecular weight of the recycled water absorbent resin at the time of dehydration (weight-average molecular weight after dehydration) is 1,000,000 or more. The weight-average molecular weight of the recycled water absorbent resin at the time of dehydration means the weight-average molecular weight of the polymer measured after the dehydration of the recycled water absorbent resin is completed and the dehydrated recycled water absorbent resin is solubilized (the crosslinked structure is cleaved). Furthermore, the weight-average molecular weight of the recycled water absorbent resin at the time of dehydration (weight-average molecular weight of the polymer) is a value measured within 24 hours from the time when the dehydration of the recycled water absorbent resin is completed, and it is sufficient that at least one of the measured values ​​of one or more weight-average molecular weights measured within 24 hours is 1,000,000 or more.

[0063] Note that, normally, the weight average molecular weight of a recycled water absorbent resin is reduced by dehydration, but in the dehydration step in the method for producing a water absorbent resin according to one embodiment, not only the case where the weight average molecular weight of the recycled water absorbent resin reduced by dehydration becomes 1,000,000 or more, but also the case where the weight average molecular weight of the recycled water absorbent resin is increased by dehydration to become 1,000,000 or more is included.

[0034] In this specification, the weight average molecular weight of the water absorbent resin for recycling means a weight average molecular weight measured by light scattering GPC for the polymer obtained by solubilizing the water absorbent resin contained in the water absorbent resin for recycling to obtain a decomposition liquid containing the polymer. Specifically, it is measured by the method described in the examples below.

[0035] The weight average molecular weight after dehydration may be 1.1 million or more, 1.2 million or more, 1.3 million or more, 1.4 million or more, 1.5 million or more, 1.6 million or more, 1.7 million or more, 1.8 million or more, 1.9 million or more, or 2 million or more, or 3 million or less, 2.8 million or less, 2.6 million or less, 2.5 million or less, or 2.4 million or less. The weight average molecular weight after dehydration may be 1 million to 3 million, 1.2 million to 2.8 million, 1.4 million to 2.6 million, 1.6 million to 2.5 million, or 1.8 million to 2.4 million.

[0036] The swelling ratio of the recycled water absorbent resin after dehydration may be 2 times or more, 4 times or more, 6 times or more, 8 times or more, 10 times or more, 12 times or more, 14 times or more, 16 times or more, 18 times or more, 20 times or more, or 22 times or more, or 30 times or less, 28 times or less, 26 times or less, 24 times or less, 23 times or less, 22 times or less, 20 times or less, 18 times or less, 16 times or less, 14 times or less, 12 times or less, 10 times or less, 8 times or less, or 6 times or less. The swelling ratio of the recycled water absorbent resin after dehydration can be calculated by dividing the mass [g] of the recycled water absorbent resin after dehydration by the pure content [g] of the water absorbent resin in the recycled water absorbent resin, and specifically, it is calculated by the method described in the examples described later. The swelling ratio of the recycled water absorbent resin after dehydration may be 2 to 30 times, 2 to 26 times, 2 to 24 times, 4 to 30 times, 4 to 26 times, or 4 to 24 times.

[0037] The recycled water absorbent resin after dehydration is placed in an environment in which the weight-average molecular weight becomes 300,000 or more after 7 days have passed. By placing the recycled water absorbent resin after dehydration in such an environment, a water absorbent resin that is easy to reuse can be produced. Note that, although the weight-average molecular weight of the recycled water absorbent resin usually decreases after 7 days have passed since dehydration, the above-mentioned environment not only includes a case in which the weight-average molecular weight of the recycled water absorbent resin, which has decreased after 7 days have passed since dehydration, is maintained at 300,000 or more, but also a case in which the weight-average molecular weight of the recycled water absorbent resin increases to 300,000 or more after 7 days have passed since dehydration.

[0038] The temperature of the environment in which the recycled water absorbent resin after dehydration is placed (environmental temperature) may be 90°C or less, 80°C or less, 70°C or less, 60°C or less, or 50°C or less, from the viewpoint of preventing the weight average molecular weight of the recycled water absorbent resin from becoming too small. The environmental temperature may be, for example, 0°C or more, 5°C or more, 10°C or more, 20°C or more, 25°C or more, 30°C or more, 40°C or more, or 50°C or more. The environmental temperature may be 0 to 90°C, 0 to 80°C, 5 to 90°C, or 5 to 80°C.

[0039] The humidity of the environment in which the recycled water absorbent resin is placed after dehydration (environmental humidity) may be 100 RH % or less, 80 RH % or less, or 60 RH % or less.

[0040] The atmospheric gas in the environment in which the dehydrated water-absorbing resin for recycling is placed may be, for example, air, an inert gas such as nitrogen, or the like.

[0041] The method for producing water-absorbent resin particles may include a storage step of subjecting a recycled water-absorbent resin to a dehydration step and then storing the dehydrated recycled water-absorbent resin. The environment in which the recycled water-absorbent resin is stored after dehydration in the storage step may be an environment in which the weight-average molecular weight of the recycled water-absorbent resin is maintained at 300,000 or more when the recycled water-absorbent resin after the dehydration step has been stored for 7 days. Note that "storage" means storing the recycled water-absorbent resin in a container or the like under a specific environment for 1 hour or more after dehydration. "Under a specific environment" means an environment at a specific temperature and specific humidity, and more specifically means an environment maintained at a storage temperature of ±5°C and a storage humidity of ±5% RH, as described below.

[0042] In the storage step, for example, the dehydrated water absorbent resin for recycling is filled into a container, and the water absorbent resin for recycling is stored at a specific temperature and in a specific atmosphere. The material of the container for storing the water absorbent resin for recycling may be, for example, metal, resin, glass, etc. When the material of the container is metal, examples of the metal include stainless steel and copper. When the material of the container is resin, examples of the resin include PE, PP, PS, PVC, fluorine-based resin, etc. The external shape of the container for storing the water absorbent resin for recycling may be, for example, a prismatic shape or a cylindrical shape, or may be an irregular shape such as a bag.

[0043] The temperature (storage temperature) of the environment in which the recycled water absorbent resin after dehydration is stored may be determined depending on the components that have been brought into contact with the recycled water absorbent resin in the dehydration step. When the recycled water absorbent resin is brought into contact with an acid component in the dehydration step, the storage temperature may be, for example, 70°C or lower, from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin from becoming too small after storage for a certain period of time, and making it easier to produce a water absorbent resin that is easy to reuse. When the recycled water absorbent resin is brought into contact with a polyvalent metal salt in the dehydration step, destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin is suppressed more than when the acid component is contacted, and therefore the storage temperature may be, for example, 90°C or lower.

[0044] The storage temperature may be 90°C or lower, 80°C or lower, 70°C or lower, 60°C or lower, or 50°C or lower, from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the recycled water absorbent resin, suppressing the weight average molecular weight of the recycled water absorbent resin from becoming too small after storage for a certain period of time, and making it easier to produce a water absorbent resin that is easy to reuse. The storage temperature may be, for example, 0°C or higher, 5°C or higher, 10°C or higher, 20°C or higher, 25°C or higher, 30°C or higher, 40°C or higher, or 50°C or higher. That is, the storage environment may be an environment in which the recycled water absorbent resin after dehydration is stored at the above storage temperature. The storage temperature may be 0 to 90°C, 0 to 80°C, 5 to 90°C, or 5 to 80°C.

[0045] The humidity at which the recycled water absorbent resin is stored after dehydration (storage humidity) may be 100 RH % or less, 80 RH % or less, or 60 RH % or less.

[0046] The atmospheric gas for storage may be, for example, air, an inert gas such as nitrogen, or the like.

[0047] The time for storing the dehydrated water absorbent resin for recycling (storage time) may be the time until the cleavage step described below is carried out. In the storage step, for example, the dehydrated water absorbent resin for recycling may be stored for 1 hour or more, 3 hours or more, 5 hours or more, 7 hours or more, 10 hours or more, 15 hours or more, or 24 hours or more after dehydration, or may be stored for 170 hours or less, 160 hours or less, 150 hours or less, 140 hours or less, 130 hours or less, or 120 hours or less after dehydration. The storage time may be 1 to 170 hours.

[0048] The environment in which the recycled water absorbent resin is stored after dehydration in the storage step is an environment in which the weight average molecular weight of the recycled water absorbent resin is maintained at 300,000 or more when the recycled water absorbent resin is stored for 7 days after dehydration. That is, the weight average molecular weight of the recycled water absorbent resin (weight average molecular weight after storage) when the recycled water absorbent resin is stored for 7 days after dehydration in the storage step is 300,000 or more. The weight average molecular weight of the recycled water absorbent resin after 7 days of storage means the weight average molecular weight of the recycled water absorbent resin measured within 24 hours after storing the recycled water absorbent resin for 7 days after the dehydration step, and it is sufficient if at least one of the measured values ​​of one or more weight average molecular weights measured within 24 hours is 300,000 or more. The weight average molecular weight after storage is measured by the same method as the weight average molecular weight after dehydration.

[0049] The weight average molecular weight after storage may be 500,000 or more, 700,000 or more, 800,000 or more, 900,000 or more, 1 million or more, 1.1 million or more, 1.2 million or more, 1.3 million or more, 1.4 million or more, 1.5 million or more, 1.6 million or more, 1.7 million or more, 1.8 million or more, 1.9 million or more, or 2 million or more, or 3 million or less, 2.8 million or less, 2.6 million or less, 2.5 million or less, 2.4 million or less, or 2.3 million or less. The weight average molecular weight after storage may be 500,000 to 3 million, 500,000 to 2.6 million, 500,000 to 2.4 million, 700,000 to 3 million, 700,000 to 2.6 million, 700,000 to 2.4 million, 800,000 to 3 million, 800,000 to 2.6 million, or 800,000 to 2.4 million.

[0050] Weight average molecular weight after dehydration (Mw １) relative to the weight average molecular weight (Mw ２ ) ratio (Mw ２ / Mw １ ) may be 0.3 or more, 0.4 or more, 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more, or 0.95 or more. ２ / Mw １ is preferably larger, since the weight average molecular weight of the recycled water absorbent resin does not change significantly between after dehydration of the recycled water absorbent resin and after storage, and therefore damage to the structure of the water absorbent resin contained in the recycled water absorbent resin is suppressed.

[0051] A water-absorbent resin can be obtained by the above-described method for producing a water-absorbent resin. The obtained water-absorbent resin can be used for producing water-absorbent resin particles. That is, another embodiment of the present invention is a method for producing water-absorbent resin particles, comprising a preparatory step of obtaining a water-absorbent resin by the above-described production method.

[0052] The crosslinked structure of the crosslinked polymer of the water-absorbent resin can be cleaved by contacting the water-absorbent resin with an acid component (e.g., sulfuric acid), a base component (e.g., sodium hydroxide), an oxidizing agent, a reducing agent, etc. That is, in the cleavage step, the water-absorbent resin may be contacted with at least one selected from the group consisting of an acid component, a base component, an oxidizing agent, and a reducing agent, thereby cleaving the crosslinked structure of the crosslinked polymer to obtain a polymer.

[0053] The acid component may be an inorganic acid, for example, at least one inorganic acid selected from the group consisting of acetic acid, nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and boric acid, or at least one inorganic acid selected from sulfuric acid and hydrochloric acid. When the water-absorbent resin is contacted with the acid component, the temperature may be 80 to 140°C.

[0054] The base component may be an inorganic base, for example, at least one inorganic base selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or at least one inorganic base selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide. When the water-absorbent resin is brought into contact with the base component, the temperature may be 50 to 120°C.

[0055] Specifically, when the crosslinked structure of the crosslinked polymer is cleaved by a basic component, for example, the crosslinked structure can be cleaved by stirring a solution containing a water-absorbent resin and a basic component at a temperature of 50 to 150°C and in an environment of pH 10 or more for 1 to 50 hours.

[0056] When the water-absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the polymer obtained by cleaving the crosslinked polymer has a poly(meth)acrylic acid structure. When the water-absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the crosslinked polymer may be cleaved by contacting the water-absorbent resin with a base component, from the viewpoint of easily producing water-absorbent resin particles having high water absorption performance and a small soluble content by avoiding cleavage of the poly(meth)acrylic acid structure.

[0057] The weight average molecular weight of the polymer obtained in the cleavage step may be 300,000 or more, 500,000 or more, 700,000 or more, 1,000,000 or more, 1,300,000 or more, 1,500,000 or more, 1,700,000 or more, or 1,900,000 or more. The weight average molecular weight of the polymer may be 10,000,000 or less, 8,000,000 or less, 6,000,000 or less, 4,000,000 or less, 3,000,000 or less, or 2,000,000 or less. The weight average molecular weight of the polymer may be 300,000 to 10,000,000, 500,000 to 6,000,000, or 1,000,000 to 3,000,000. The weight average molecular weight of the polymer is measured by GPC (gel permeation chromatography) under the following conditions. (Conditions) Autosampler: Autosampler AS-11 (manufactured by FLOM Corporation). Degasser: Gastr AG-16 (manufactured by FLOM Corporation). Solution delivery unit: LC-10AD (Shimadzu Corporation). Column: OHpak SB-807HQ, SB-806HQ, SB-804HQ (Shodex series, Resonac Corporation). Detector: Triple Detector TDA 302 (Viscotec). Eluent: NaNO ３(0.2 mol / L) / methylparaben (2 mmol / L) / distilled water. Measurement conditions: injection volume 500 μL, flow rate 0.5 mL / min, column / detector temperature 40°C, dn / dC 0.2270. A sample solution adjusted to contain 0.01 g of polymer was placed in a 300 mL beaker, and the above eluent was added to bring the total volume to 100 mL. The mixture was stirred at 250 rpm for 1 hour. If the pH of the sample solution was not neutral, it was adjusted to pH 7 using hydrochloric acid (Nacalai Tesque, 1 mol / L hydrochloric acid) or aqueous sodium hydroxide solution (Nacalai Tesque, 1 mol / L). After stirring, the sample solution was filtered through a 0.8 μm filter syringe and the filtrate was measured by GPC.

[0058] The method for producing water-absorbent resin particles may include a step of performing solid-liquid separation (solid-liquid separation step) to remove impurities in the polymer solution after cleaving the crosslinked structure of the crosslinked polymer to obtain a polymer. In the solid-liquid separation step, for example, the polymer solution may be subjected to filtration or ultrafiltration. In the solid-liquid separation step, the pH of the polymer solution may be adjusted to near neutral before filtering the polymer solution.

[0059] The method for producing water-absorbent resin particles may include a step of diluting, heating, adding a salt (a salt containing a monovalent cation), or the like, from the viewpoint of decreasing the viscosity of a polymer solution after obtaining the polymer by cleaving the crosslinked structure of the crosslinked polymer and before carrying out the crosslinking step.

[0060] The method for producing water-absorbent resin particles may include a step of sterilizing the polymer solution after the crosslinked structure of the crosslinked polymer is cleaved to obtain the polymer and before the crosslinking step is carried out.

[0061] In the crosslinking step, for example, a solution containing a crosslinking agent capable of forming a covalent bond with a functional group of a polymer is prepared, and the polymer is crosslinked via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the polymer solution may be gelled. In this case, the entire polymer solution loses fluidity, and a gel containing the crosslinked polymer and water is formed. When crosslinking proceeds to the extent that a gel is formed, it is particularly easy to obtain water-absorbent resin particles having high water absorption capacity and a small amount of dissolved matter.

[0062] Examples of functional groups possessed by the polymer include carboxyl groups. When the polymer has carboxyl groups, the polymer is crosslinked by a reaction between the carboxyl groups and a crosslinking agent and / or a reaction between the carboxyl groups themselves. The covalent bond may be at least one selected from the group consisting of an ester bond, a thioester bond, an amide bond, an ether bond, and a carbon-carbon bond. The polymer may be crosslinked, for example, via at least one group selected from the group consisting of a carboxylic acid ester group, a thioester group, an amide group, an acid anhydride group, an oxyalkylene group, and an oxyarylene group.

[0063] Examples of the crosslinking agent include aliphatic polyhydric alcohols such as (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, and pentaerythritol; glycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; bisacrylamide compounds such as N,N'-methylenebis(meth)acrylamide; allylated starch; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0064] The amount of the crosslinking agent may be 0.0001 parts by mass or more, 0.001 parts by mass or more, or 0.005 parts by mass or more, and may be 10 parts by mass or less, 10 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the polymer. The amount of the crosslinking agent may be 0.0001 to 15 parts by mass, 0.001 to 10 parts by mass, or 0.005 to 5 parts by mass, relative to 100 parts by mass of the polymer.

[0065] The reaction temperature when crosslinking the polymer is appropriately set depending on the type, amount, etc. of the crosslinking agent used, but may be 50° C. or higher, 80° C. or higher, or 100° C. or higher, and may be 220° C. or lower, 200° C. or lower, or 180° C. or lower. The reaction temperature when crosslinking the polymer may be 50 to 220° C., 80 to 200° C., or 100 to 180° C.

[0066] The reaction time for crosslinking the polymer is appropriately set depending on the type and amount of the crosslinking agent used, the reaction temperature, etc., but may be 1 to 200 minutes, or 5 to 150 minutes.

[0067] The method for producing water-absorbent resin particles may include a surface cross-linking step of obtaining a cross-linked polymer through a cross-linking step and then surface cross-linking the cross-linked polymer. The surface cross-linking can be performed, for example, by adding a cross-linking agent (surface cross-linking agent) for performing surface cross-linking to the cross-linked polymer and causing a reaction.

[0068] The surface cross-linking agent may be a compound having two or more reactive functional groups that are reactive with the cross-linked polymer. The surface cross-linking agent may be the same as or different from the cross-linking agent in the cross-linking step.

[0069] The reactive functional groups of the surface cross-linking agent may be carbonate groups, alcoholic hydroxyl groups, epoxy groups, halogeno groups in haloepoxy compounds, isocyanate groups, oxetanyl groups, oxazoline groups, or combinations thereof. Carbonate groups are considered two reactive functional groups because they can react with two other molecules.

[0070] Examples of surface cross-linking agents having a carbonate group include alkylene carbonates (ethylene carbonate, etc.). Examples of surface cross-linking agents having an alcoholic hydroxyl group include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin, and hydroxyalkylamide compounds (bis[N,N-di(β-hydroxyethyl)]adipamide, etc.). Examples of surface cross-linking agents having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether. Examples of haloepoxy compounds having an epoxy group and a halogeno group include epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of surface crosslinking agents having an isocyanate group include 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Examples of surface crosslinking agents having an oxetanyl group include 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol.

[0071] The amount of the surface crosslinking agent may be 0.001 parts by mass or more, 0.005 parts by mass or more, or 0.01 parts by mass or more, and may be 5 parts by mass or less, 3 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the crosslinked polymer. The amount of the crosslinking agent may be 0.001 to 5 parts by mass, 0.005 to 3 parts by mass, or 0.01 to 1 part by mass, relative to 100 parts by mass of the polymer.

[0072] A regenerated water-absorbent resin containing a crosslinked polymer is formed by a method including removing water from a crosslinked polymer obtained through an optional surface crosslinking step. When the reaction liquid to be subjected to the crosslinking step is an aqueous solution, the water-absorbent resin particles are formed by a method including a drying step of removing water from a block of crosslinked polymer formed by gelling the reaction liquid itself, drying the crosslinked polymer to form a dried product, and a pulverizing step of pulverizing the dried product.

[0073] The method for drying the crosslinked polymer may be a common method such as a squeezing method such as centrifugation, dehydration using an organic solvent, natural drying, heat drying, air drying, freeze drying, or a combination thereof. The heating temperature for drying may be 80 to 220°C, 90 to 200°C, or 100 to 180°C from the viewpoint of efficiently removing water.

[0074] The moisture content of the dried product may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The moisture content of the dried product means the proportion of moisture in the dried product based on the total amount of the dried product. When a crosslinked polymer containing water is heated at 200°C for 2 hours, the difference in mass of the crosslinked polymer before and after heating can be considered to be the moisture content of the dried product.

[0075] The method for producing water-absorbent resin particles may include a step of crushing the crosslinked polymer before drying it to form a crushed product containing structures of a certain size. By forming a crushed product, water can be efficiently removed. The structures constituting the crushed product may be, for example, elongated structures, granular structures (particles), or a combination thereof. The crushed product may include a plurality of structures having a shape that can pass through a circular hole with a diameter of 10 mm or 7 mm. The elongated structures may be curved, and as long as their maximum width is 10 mm or less, they can be said to have a shape that can pass through a circular hole with a diameter of 10 mm. The granular structures (particles) may be irregular in shape or may have a shape that can pass through a circular hole with a diameter of 10 mm while changing direction. Examples of crushing devices for crushing the crosslinked polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and farmer mills.

[0076] In the pulverization step of pulverizing the dried product, the dried product is pulverized to form water-absorbent resin particles. The pulverization method is not particularly limited. In the pulverization step of pulverizing the dried product, the dried product can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill.

[0077] The method for producing water-absorbent resin particles may include a step of classifying the water-absorbent resin particles obtained by pulverization. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the water-absorbent resin particles after classification may be pulverized and classified again.

[0078] The classification method is not particularly limited, and may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not pass through by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing an air flow.

[0079] The median particle size of the crosslinked polymer powder obtained through pulverization and, if necessary, classification may be, for example, 200 to 500 μm, or 300 to 500 μm. The particle size distribution may be adjusted by mixing two or more powders obtained by classification and having different median particle sizes.

[0080] Fig. 1 is a cross-sectional view showing an example of an absorbent article having an absorbent body containing water-absorbent resin particles. The absorbent article 100 shown in Fig. 1 comprises a water-absorbent sheet 50 having a film-like absorbent body 10, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40.

[0081] The water-absorbent sheet 50 includes an absorbent body 10 containing a powder of water-absorbent resin particles 1, and two core wrap sheets 20a and 20b. The absorbent body 10 is disposed inside the core wrap sheets 20a and 20b. The absorbent body 10 maintains its shape by being sandwiched between the two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b may consist of two sheets, a single folded sheet, or a single bag. A sheet member that does not have any other components on the outside of the core wrap sheets 20a and 20b that wrap the absorbent body 10 is sometimes referred to as a water-absorbent sheet.

[0082] The absorbent body 10 is a component that mainly contains a powder of water-absorbent resin particles 1 and is retained to have a certain shape. The absorbent body 10 may contain fibrous material 3 in addition to the powder of water-absorbent resin particles 1, or may not contain fibrous material 3. The content of the water-absorbent resin particles 1 in the absorbent body 10 may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less, based on the mass of the absorbent body 10.

[0083] The thickness of the absorbent body 10 may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less, or may be 0.1 mm or more, or 0.3 mm or more. The thickness of the absorbent body 10 may be 0.1 mm or more and 20 mm or less. The mass per unit area of ​​the absorbent body 10 is 1000 g / m ２ Below, 800g / m ２ or less than 600 g / m ２ or less, and ２ It may be more than that.

[0084] The fibrous material 3 can be, for example, a cellulosic fiber, a synthetic fiber, or a combination thereof. Examples of cellulosic fibers include comminuted wood pulp, cotton, cotton linters, rayon, and cellulose acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may also be a hydrophilic fiber (e.g., pulp).

[0085] The absorbent 10 may further contain inorganic powder (e.g., amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the water-absorbent resin particles 1 contain inorganic particles, the absorbent 10 may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles 1.

[0086] The water-absorbent sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the absorbent body 10. An adhesive layer may be interposed between the core wrap sheets 20a, 20b on both sides and the absorbent body 10. The adhesive 21 is not particularly limited, and may be, for example, a hot-melt adhesive.

[0087] The core wrap sheets 20a, 20b may be, for example, nonwoven fabrics. The two core wrap sheets 20a, 20b may be the same or different nonwoven fabrics. The nonwoven fabric may be a nonwoven fabric made of short fibers (i.e., staple) (short fiber nonwoven fabric) or a nonwoven fabric made of long fibers (i.e., filaments) (long fiber nonwoven fabric). The staple may have a fiber length of, but is not limited to, typically several hundred millimeters or less.

[0088] The core wrap sheets 20a, 20b may be a thermal bonded nonwoven fabric, an air-through nonwoven fabric, a resin bonded nonwoven fabric, a spunbonded nonwoven fabric, a meltblown nonwoven fabric, an airlaid nonwoven fabric, a spunlace nonwoven fabric, a point bonded nonwoven fabric, or a laminate containing two or more types of nonwoven fabric selected from these.

[0089] The nonwoven fabric used as the core wrap sheets 20a, 20b can be made of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include fibers containing synthetic resins selected from polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamides such as nylon, and rayon. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the nonwoven fabric may be polyolefin fibers, polyester fibers, or a combination thereof. The core wrap sheets 20a, 20b may also be tissue paper.

[0090] The water-absorbent sheet 50 may be used to manufacture various other absorbent articles. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste disposal materials. The absorbent bodies that make up these absorbent articles often move or deform due to the movements of the user of the absorbent article, etc.

[0091] The liquid-permeable sheet 30 is positioned as the outermost layer on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is positioned outside the core wrap sheet 20b while in contact with the core wrap sheet 20b. The liquid-impermeable sheet 40 is positioned as the outermost layer on the opposite side of the absorbent article 100 from the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is positioned outside the core wrap sheet 20a while in contact with the core wrap sheet 20a. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have main surfaces that are wider than the main surface of the water-absorbent sheet 50, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wrap sheets 20a, 20b. However, the size relationships among the absorbent body 10, the core wrap sheets 20a, 20b, the liquid-permeable sheet 30, and the liquid-impermeable sheet 40 are not particularly limited and may be appropriately adjusted depending on the intended use of the absorbent article, etc.

[0092] The liquid-permeable sheet 30 may be a nonwoven fabric. The nonwoven fabric used as the liquid-permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From that viewpoint, the liquid-permeable sheet 30 may be a nonwoven fabric having a hydrophilicity of 5 to 200 as measured in accordance with the Pulp and Paper Testing Method No. 68 (2000) of the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may also be 10 to 150. For details of Pulp and Paper Testing Method No. 68, see, for example, WO2011 / 086843.

[0093] Hydrophilic nonwoven fabrics may be made from fibers with moderate hydrophilicity, such as rayon, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin and polyester fibers. Methods for obtaining nonwoven fabrics containing hydrophilized hydrophobic chemical fibers include spunbonding a mixture of hydrophobic chemical fibers and a hydrophilizing agent, adding a hydrophilizing agent to the hydrophobic chemical fibers, and impregnating a spunbond nonwoven fabric obtained from hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0094] The basis weight (mass per unit area) of the nonwoven fabric used as the liquid-permeable sheet 30 is 5 to 200 g / m from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and from the viewpoint of increasing the liquid permeation rate of the absorbent article. ２ , 8 to 150 g / m ２ , or 10 to 100 g / m ２The thickness of the liquid-permeable sheet 30 may be 20 to 1400 μm, 50 to 1200 μm, or 80 to 1000 μm.

[0095] The liquid-impermeable sheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out through the liquid-impermeable sheet 40. The liquid-impermeable sheet 40 may be a resin sheet or a nonwoven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The nonwoven fabric may be a spunbond / meltblown / spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. The liquid-impermeable sheet 40 may be a composite sheet of a resin sheet and a nonwoven fabric (e.g., a spunbond nonwoven fabric or a spunlace nonwoven fabric). The liquid-impermeable sheet 40 may be breathable to reduce stuffiness during wear and to alleviate discomfort to the wearer. For example, a low-density polyethylene (LDPE) resin sheet can be used as the breathable liquid-impermeable sheet 40.

[0096] In order to ensure flexibility and not impair the wearing comfort of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 is set to 10 to 50 g / m ２ may be.

[0097] The present invention will be described in more detail below with reference to examples.

[0098] <Evaluation Method> The weight average molecular weight and swelling ratio were measured according to the following evaluation method. The measurement results are shown in Table 1.

[0099] [Weight-average molecular weight] As shown in FIG. 2, a reflux condenser, a nitrogen gas inlet pipe, and a stirrer (a 150 mL round-bottom cylindrical separable flask equipped with a stirring blade having four inclined paddle blades with a blade diameter of 40 mm, manufactured by EYELA) were prepared. 1 g of the recycled water-absorbent resin immediately after dehydration or the recycled water-absorbent resin immediately after storage for 7 days was weighed out in solids into the separable flask, and 4.0 g of 30% sodium hydroxide was added. Then, ion-exchanged water was added so that the total amount of the contents of the separable flask became 100 g. Stirring was started at 300 rpm, and nitrogen gas was blown into the separable flask at 200 mL / min for 15 minutes to replace the atmosphere in the separable flask with nitrogen. Thereafter, the temperature was raised using a personal organic synthesis reaction apparatus (manufactured by EYELA), and the internal temperature was maintained at 80 ° C. for 6 hours to obtain a decomposition solution containing a polymer that is a solubilized (crosslinked structure cleaved) water-absorbent resin. The time when the decomposition liquid was obtained was 7 hours after the time when the recycled water absorbent resin was finally obtained by filtering through a 75 μm standard sieve as described below, or after the recycled water absorbent resin was stored for 7 days. After obtaining the decomposition liquid, the weight average molecular weight of the polymer contained in the decomposition liquid was immediately measured by light scattering GPC under the following conditions, and the obtained weight average molecular weight was used as the weight average molecular weight of the recycled water absorbent resin. The time when the measurement of the weight average molecular weight was completed was 8.5 hours after the time when the recycled water absorbent resin was finally obtained by filtering through a 75 μm standard sieve as described below, or after the recycled water absorbent resin was stored for 7 days. In addition, the ratio of the weight average molecular weight of the recycled water absorbent resin after storage to the weight average molecular weight of the recycled water absorbent resin after dehydration (weight average molecular weight of the recycled water absorbent resin after storage / weight average molecular weight of the recycled water absorbent resin after dehydration) was calculated. (Conditions) Autosampler: Autosampler AS-11 (manufactured by FLOM Corporation). Degasser: Gastrr AG-16 (manufactured by FLOM Corporation). Liquid delivery unit: LC-10AD (manufactured by Shimadzu Corporation). Column: OHpak SB-807HQ, SB-806HQ, SB-804HQ (Shodex series, manufactured by Resonac Corporation). Detector: Triple Detector TDA 302 (manufactured by Viscotec).Eluent: NaNO. ３ (0.2 mol / L) / methylparaben (2 mmol / L) / distilled water. Measurement conditions: injection volume 500 μL, flow rate 0.5 mL / min, column / detector temperature 40°C, dn / dC 0.2270. A sample solution adjusted to contain 0.01 g of polymer was placed in a 300 mL beaker, and the above eluent was added to bring the total volume to 100 mL. The mixture was stirred at 250 rpm for 1 hour. If the pH of the sample solution was not neutral, it was adjusted to pH 7 using hydrochloric acid (Nacalai Tesque, 1 mol / L hydrochloric acid) or aqueous sodium hydroxide solution (Nacalai Tesque, 1 mol / L). After stirring, the sample solution was filtered through a 0.8 μm filter syringe and the filtrate was analyzed by GPC.

[0100] [Swelling ratio] The mass of the recycled water absorbent resin after dehydration was measured, and the swelling ratio was calculated by the following formula: Swelling ratio [times] = Mass of recycled water absorbent resin after dehydration [g] / Pure content of water absorbent resin in recycled water absorbent resin [g]

[0101] (Example 1) A commercially available disposable diaper (Merrys Pants, Smooth Air Through, L size, manufactured by Kao Corporation) was prepared, and a water-absorbent resin for recycling containing a crosslinked polymer containing acrylic acid and an acrylate salt as monomer units was collected from inside the disposable diaper.

[0102] 10 g (100 parts by mass) of recycled water absorbent resin was added to 4000 g of physiological saline solution (liquid temperature 25 ° C.) contained in a 5 L polybeaker, and stirred at 100 rpm for 10 minutes using a three-one motor (manufactured by Shinto Scientific Co., Ltd., HEIDON BLh1200) equipped with two 75 mm paddle blades. After stirring, the mixture was filtered through a standard sieve with a mesh size of 75 μm, and the swollen recycled water absorbent resin remaining on the sieve was collected. Subsequently, the swollen recycled water absorbent resin was added to 4000 g of 1% by mass citric acid aqueous solution (40,000 parts by mass, liquid temperature 25 ° C.) contained in a 5 L polybeaker, and stirred at 100 rpm for 10 minutes using the above-mentioned three-one motor, followed by dehydration (inactivation) treatment. After filtering through a 75 μm standard sieve, the recycled water absorbent resin on the sieve was poured into 2 L of ion-exchanged water contained in a 5 L polybeaker and washed by stirring at 100 rpm for 3 minutes using the above-mentioned Three-One motor. Thereafter, the recycled water absorbent resin was filtered through a 75 μm standard sieve and dehydrated to obtain 224.13 g of recycled water absorbent resin. The swelling ratio of the recycled water absorbent resin (mass of the recycled water absorbent resin after dehydration) / (pure content of the water absorbent resin in the recycled water absorbent resin (in this example, the mass of the recycled water absorbent resin collected from the diaper was considered to be the pure content of the water absorbent resin)) was 22.4 times (= 224.13 / 10). Furthermore, the weight-average molecular weight of the recycled water absorbent resin after dehydration was 1,979,000.

[0103] Next, the water absorbent resin for recycling after dehydration was placed in a 100 mL glass beaker, wrapped, and stored for 7 days in a thermo-hygrostat (Espec Corporation, LH-113, temperature 50°C, humidity 50RH%). The weight average molecular weight of the water absorbent resin for recycling after storage for 7 days was 1,719,000. The starting point of the 7 days (storage period) was the time when the water absorbent resin for recycling was finally obtained by filtering through a standard sieve with an opening of 75 μm as described above.

[0104] (Examples 2 to 9, Comparative Examples 1 and 2) The same operation as in Example 1 was performed, and the conditions were changed as shown in Table 1 to obtain a dehydrated water absorbent resin. In addition, the weight average molecular weight of the recycled water absorbent resin after dehydration, the weight average molecular weight of the recycled water absorbent resin after storage for 7 days, the ratio of the weight average molecular weight of the recycled water absorbent resin after storage to the weight average molecular weight of the recycled water absorbent resin after dehydration, and the swelling magnification were measured. The measurement results are shown in Table 1.

[0105]

[0106] In Examples 1 to 9, the weight average molecular weight of the recycled water absorbent resin was sufficiently large after dehydration and after storage for a certain period of time after dehydration, whereas in Comparative Examples 1 and 2, the weight average molecular weight of the recycled water absorbent resin was sufficiently large after dehydration, but the weight average molecular weight after storage for 7 days was small. That is, in Examples 1 to 9, it was possible to produce a water absorbent resin that is easily reused from the recycled water absorbent resin.

[0107] Furthermore, from comparisons between Examples 1 and 3 and Comparative Example 1, and comparisons between Example 4 and Comparative Example 2, it is seen that when an acid component is used as an additive, the weight-average molecular weight of the recycled water absorbent resin after storage for 7 days tends to decrease as the temperature of the environment in which the recycled water absorbent resin is placed after dehydration (storage temperature in the storage step) increases. On the other hand, from Examples 8 and 9, it is seen that when a polyvalent metal salt is used as an additive, the weight-average molecular weight of the recycled water absorbent resin after storage for 7 days is sufficiently large even if the temperature of the environment in which the recycled water absorbent resin is placed after dehydration (storage temperature in the storage step) increases.

[0108] DESCRIPTION OF SYMBOLS 1...water-absorbent resin particles, 10...absorbent body, 20a, 20b...core wrap sheet, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 50...water-absorbent sheet, 100...absorbent article, 110...round-bottom cylindrical separable flask, 120...four-inclined paddle blade, 130...nitrogen gas inlet pipe, 140...reflux condenser, 150...oxygen concentration meter, 160...gas outlet pipe, 170...stirring motor, 180...shaft holder, 185...stirring shaft, 190, 195...three-way cock, 200...treatment tank.

Claims

1. A method for producing a water absorbent resin, comprising: a dehydration step of dehydrating a water absorbent resin for recycling containing water and a water absorbent resin; the dehydration step is carried out so that the weight average molecular weight of the water absorbent resin for recycling after the dehydration is 1,000,000 or more; and after the dehydration step, the water absorbent resin for recycling after the dehydration is placed in an environment in which the weight average molecular weight after 7 days is 300,000 or more.

2. The method according to claim 1, wherein the dehydration step is a step of contacting the recycled water absorbent resin with at least one substance selected from the group consisting of an acid component, a polyvalent metal salt, and an alcohol.

3. The manufacturing method according to claim 1, wherein the temperature of the environment is 70° C. or less.

4. The method according to claim 1, further comprising a storage step of storing the dehydrated recycled water absorbent resin, wherein the environment in which the dehydrated recycled water absorbent resin is stored in the storage step is an environment in which the weight average molecular weight of the dehydrated recycled water absorbent resin is maintained at 300,000 or more after storage for 7 days.

5. The method according to claim 4, wherein the storage step is a step of storing the dehydrated recycled water absorbent resin for one hour or more after the dehydration.

6. A method for producing water-absorbent resin particles, comprising: a preparation step of obtaining a water-absorbent resin by the production method according to any one of claims 1 to 5; a cleavage step of obtaining a polymer by cleaving a crosslinked structure of a crosslinked polymer contained in the water-absorbent resin; and a crosslinking step of crosslinking the polymer.

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