Method for producing superabsorbent recycled polymer regenerated from used superabsorbent polymer having acid group, superabsorbent recycled polymer, cat litter, method for regenerating used superabsorbent polymer having acid group, and use of mixed solution of hydrophobic solvent and water to regenerate used superabsorbent polymer having acid group

Abstract

The purpose of the present disclosure is to provide a method which is for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, and by which a superabsorbent recycled polymer can be simply produced.　This method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group is characterized by comprising: a regeneration step for mixing the superabsorbent polymer, an alkali metal salt of carbonic acid, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution to regenerate the superabsorbent polymer into the superabsorbent recycled polymer; and a separation step for separating the superabsorbent recycled polymer from the mixed solution.

Description

A method for producing a superabsorbent recycled polymer regenerated from used superabsorbent polymers having acid groups, superabsorbent recycled polymer, cat litter, a method for regenerating used superabsorbent polymers having acid groups, and a mixed solution of a hydrophobic solvent and water for regenerating used superabsorbent polymers having acid groups.

[0001] This disclosure relates to a method for producing a superabsorbent recycled polymer regenerated from used, acidic superabsorbent polymers, the superabsorbent recycled polymer, cat litter, a method for regenerating used, acidic superabsorbent polymers, and the use of a mixed solution of a hydrophobic solvent and water for regenerating used, acidic superabsorbent polymers.

[0002] Studies are underway to regenerate used superabsorbent polymers. For example, Patent Document 1 discloses a method for regenerating a superabsorbent polymer, which involves extracting the superabsorbent polymer that has absorbed the absorbed liquid from a used body fluid absorbent article and performing washing and dewatering treatment, wherein the washing and / or dewatering treatment includes placing the superabsorbent polymer in an environment that discharges the absorbed liquid (Claim 1), and a method for regenerating a superabsorbent polymer according to Claim 1, wherein the operation of placing the superabsorbent polymer in an environment that discharges the absorbed liquid is an operation selected from an operation that changes the pH of the superabsorbent polymer, an operation that changes the temperature, and an operation that brings the superabsorbent polymer into contact with a hydrophilic organic solvent (Claim 2).

[0003] Japanese Patent Publication No. 2003-326161

[0004] Patent Document 1 does not describe the method relating to this disclosure. Therefore, the object of this disclosure is to provide a method for producing a superabsorbent recycled polymer, which can be easily produced, by regenerating a superabsorbent recycled polymer from a used superabsorbent polymer having acid groups.

[0005] The Disclosers have found a method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, comprising: a regeneration step of mixing the superabsorbent polymer, an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution and stirring the mixed solution to regenerate the superabsorbent polymer into the superabsorbent recycled polymer; and a separation step of separating the superabsorbent recycled polymer from the mixed solution.

[0006] The method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, as disclosed herein, allows for the easy production of a superabsorbent recycled polymer.

[0007] Specifically, this disclosure relates to the following embodiments: [Embodiment 1] A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, comprising: a regeneration step of mixing the superabsorbent polymer, an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution and stirring the mixed solution to regenerate the superabsorbent polymer into the superabsorbent recycled polymer; and a separation step of separating the superabsorbent recycled polymer from the mixed solution.

[0008] In the above manufacturing method, in the regeneration step, a superabsorbent polymer, an alkali metal salt of carbonic acid, a hydrophobic solvent, and water are mixed to form a mixed solution, and the mixed solution is stirred. The hydrophobic solvent has the function of dehydrating and sterilizing the superabsorbent polymer, as well as suppressing the swelling of the superabsorbent polymer by absorbing water. Water has the function of accepting the liquid (e.g., body fluid) dehydrated from the superabsorbent polymer by the action of the hydrophobic solvent and purifying the superabsorbent polymer (superabsorbent recycled polymer). The alkali metal salt of carbonic acid, together with water, converts the acidic groups of the superabsorbent polymer into alkali metal salts, thereby restoring the water absorption of the superabsorbent polymer. As a result, the above superabsorbent recycled polymer can be easily formed. In addition, although water is present in the mixed solution, the presence of a hydrophobic solvent and the stirring of the mixed solution suppress the absorption of water and swelling of the superabsorbent recycled polymer.

[0009] Subsequently, since the above manufacturing method includes a predetermined separation step, superabsorbent recycled polymers can be easily produced. The inventors of this application have confirmed that hydrophilic organic solvents are known to have a dehydrating effect on superabsorbent polymers. However, when attempting to regenerate superabsorbent polymers by adding an alkali metal salt of carbonic acid and water to a hydrophilic organic solvent containing superabsorbent polymers to form superabsorbent recycled polymers, it was found that the regenerated superabsorbent recycled polymers absorb water and swell, making it difficult to isolate the superabsorbent recycled polymers.

[0010] [Aspect 2] The method according to aspect 1, wherein in the regeneration step, the superabsorbent polymer is brought into contact with the hydrophobic solvent which may contain an alkali metal salt of carbonate, or a mixture of the hydrophobic solvent which may contain an alkali metal salt of carbonate and water, in order to form the mixed solution.

[0011] In the above manufacturing method, in the regeneration step, the superabsorbent polymer is brought into contact with a hydrophobic solvent which may contain an alkali metal salt of carbonate, or a mixture of the above hydrophobic solvent which may contain an alkali metal salt of carbonate and water, in order to form a mixed solution. Therefore, swelling of the superabsorbent polymer by absorbing water can be suppressed when forming the mixed solution, and consequently, superabsorbent recycled polymer can be easily manufactured.

[0012] [Aspect 3] The method according to aspect 1 or 2, wherein in the regeneration step, the hydrophobic solvent and the water are used in a mass ratio of 35 to 80:20 to 65 based on a total mass of 100 parts by mass of the hydrophobic solvent and the water.

[0013] In the above manufacturing method, since a hydrophobic solvent and water are used in a predetermined mass ratio during the regeneration step, the hydrophobic solvent and water can exert their functions more effectively. Specifically, the hydrophobic solvent can dehydrate and sterilize the superabsorbent polymer, and can also suppress the swelling of the superabsorbent polymer by absorbing water. Furthermore, water can help restore the water absorption capacity of the superabsorbent polymer by converting the acidic groups of the superabsorbent polymer into alkali metal salts. In addition, water can accept the liquid (e.g., body fluids) that the superabsorbent polymer held, and can purify the superabsorbent polymer (superabsorbent recycled polymer).

[0014] [Aspect 4] The method according to any one of aspects 1 to 3, wherein the alkali metal salt of the carbonic acid is selected from the group consisting of sodium bicarbonate, potassium bicarbonate, and lithium bicarbonate.

[0015] In the above manufacturing method, since the alkali metal salt of carbonic acid is a specific acidic salt, the pH of the superabsorbent recycled polymer when it absorbs liquid (hereinafter sometimes referred to as "pH at absorption") tends to become neutral to weakly alkaline, resulting in high safety when it comes into contact with the user's skin. Therefore, the above manufacturing method allows for the easy production of superabsorbent recycled polymers that are useful for a wide range of applications, such as items that come into contact with the user's skin, such as absorbent articles.

[0016] Furthermore, when a normal salt of carbonic acid, such as sodium carbonate, is used, (i) the pH of the superabsorbent recycled polymer tends to become alkaline during liquid absorption, and (ii) the superabsorbent recycled polymer tends to have poor water absorption. In relation to (i), when a superabsorbent polymer with an alkaline pH during liquid absorption is washed with water, it absorbs the washing water, making it difficult to adjust the pH during liquid absorption by washing. In relation to (ii), the normal salt of carbonic acid tends to react quickly with the superabsorbent polymer (especially with the surface of the superabsorbent polymer) and absorb water, which tends to shorten the time that can be spent in the regeneration step. As a result, the reaction between the acid groups present inside the superabsorbent polymer and the alkali metal salt of carbonic acid does not proceed sufficiently, and the superabsorbent recycled polymer tends to have poor water absorption.

[0017] [Aspect 5] The method according to any one of aspects 1 to 4, wherein the hydrophobic solvent is a non-flammable solvent.

[0018] In the above manufacturing method, since the hydrophobic solvent is a non-flammable solvent, the hydrophobic solvent will not ignite in the regeneration step and separation step, making the above manufacturing method highly safe. Furthermore, if desired, the above manufacturing method will also be highly safe, as the hydrophobic solvent will not ignite when (i) drying the superabsorbent recycled polymer or (ii) sterilizing, bleaching, etc., the superabsorbent polymer using an oxidizing agent.

[0019] [Aspect 6] The method according to any one of aspects 1 to 5, wherein in the regeneration step, the superabsorbent polymer has a moisture content of 35 ± 10% by mass.

[0020] In the regeneration step of the above manufacturing method, the superabsorbent polymer has a predetermined moisture content. Therefore, the adhesive properties of the superabsorbent polymer suppress adhesion between the polymers themselves, while also suppressing fluctuations in the ratio of hydrophobic solvent and water in the mixed solution. If the moisture content of the superabsorbent polymer is high, the proportion of water in the mixed solution will increase, and the superabsorbent polymer (superabsorbent recycled polymer) may absorb water and swell. Thus, the above manufacturing method allows for the easy production of superabsorbent recycled polymer.

[0021] [Aspect 7] The method according to aspect 6, further comprising a moisture content adjustment step of adjusting the moisture content of the superabsorbent polymer before the regeneration step.

[0022] The above manufacturing method further includes a predetermined moisture content adjustment step before the regeneration step. This suppresses adhesion between superabsorbent polymers due to their tackiness, while also suppressing fluctuations in the ratio of hydrophobic solvent and water in the mixed solution during the subsequent regeneration step. Thus, the above manufacturing method allows for the easy production of superabsorbent recycled polymers.

[0023] [Aspect 8] The method according to any one of aspects 1 to 7, further comprising a dehydration step of contacting the superabsorbent polymer with a polyvalent metal ion source capable of supplying polyvalent metal ions, and dehydrating the superabsorbent polymer before the regeneration step.

[0024] In the above manufacturing method, a predetermined dehydration step is further included before the regeneration step, which allows for the removal of the liquid (e.g., bodily fluids) absorbed by the superabsorbent polymer. Furthermore, if the polyvalent metal ion source is strongly alkaline, such as quicklime or slaked lime, the superabsorbent polymer can be sterilized. As a result, the above manufacturing method allows for the easy production of superabsorbent recycled polymer.

[0025] [Aspect 9] The method according to aspect 8, further comprising a washing step of washing the superabsorbent polymer with an acidic aqueous solution after the dewatering step and before the regeneration step.

[0026] The above manufacturing method further includes a predetermined washing step after the dehydration step and before the regeneration step. This cleans the superabsorbent polymer, reduces the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and adjusts the water absorption of the superabsorbent recycled polymer separated in the separation step.

[0027] [Aspect 10] The method according to any one of aspects 1 to 9, wherein the superabsorbent recycled polymer contains 10.0 to 17.0% by mass of polyvalent metals and 6.0 to 10.0% by mass of alkali metals.

[0028] In the above manufacturing method, the superabsorbent recycled polymer contains predetermined amounts of polyvalent metals and alkali metals, so it can absorb liquids at a predetermined rate.

[0029] [Aspect 11] A superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, characterized in that the superabsorbent recycled polymer contains 10.0 to 17.0% by mass of polyvalent metals and 6.0 to 10.0% by mass of alkali metals.

[0030] The above-mentioned superabsorbent recycled polymer contains predetermined amounts of polyvalent metals and alkali metals, and can therefore absorb liquids at a predetermined rate.

[0031] [Aspect 12] The superabsorbent recycled polymer according to Aspect 11, wherein the superabsorbent recycled polymer in a dry state is mixed with 200 times its mass of physiological saline solution, and the physiological saline solution has a pH of 6.0 to 12.0.

[0032] The above-mentioned superabsorbent recycled polymer, when mixed with a predetermined amount of physiological saline solution, results in a physiological saline solution with a predetermined pH. Therefore, the pH of the superabsorbent recycled polymer during liquid absorption tends to be neutral to slightly alkaline, ensuring high safety when it comes into contact with the user's skin. For these reasons, the above-mentioned superabsorbent recycled polymer is easy to use in a wide range of applications, such as in items that come into contact with the user's skin, such as absorbent articles. However, superabsorbent recycled polymers with a high pH during liquid absorption may cause skin damage when wet or damp.

[0033] [Aspect 13] The superabsorbent recycled polymer in a dry state is the superabsorbent recycled polymer according to Aspect 11 or 12, which absorbs deionized water 20 times its mass in 120 seconds or less. Since the superabsorbent recycled polymer is excellent in 20-fold water absorption time, it is easy to use in a wide range of applications, for example, applications such as articles that come into contact with the user's skin, such as absorbent articles.

[0034] [Aspect 14] The superabsorbent recycled polymer according to any one of Aspects 11 to 13, which is for cat litter. Since the superabsorbent recycled polymer is for cat litter, preferably for the shell layer of cat litter, it can absorb cat urine at a predetermined rate.

[0035] [Aspect 15] A cat litter comprising a core layer and a shell layer, characterized in that the superabsorbent recycled polymer according to any one of Aspects 11 to 14 is provided in the shell layer.

[0036] The above cat litter can absorb cat urine at a predetermined rate.

[0037] [Aspect 16] A method for recycling a used superabsorbent polymer having an acid group, comprising a recycling step of mixing the superabsorbent polymer, an alkali metal salt of carbonic acid, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution to recycle the superabsorbent polymer into a superabsorbent recycled polymer. The above recycling method has the same effect as Aspect 1.

[0038] [Aspect 17] Use of a mixed solution of a hydrophobic solvent and water for recycling a used superabsorbent polymer having an acid group, comprising a recycling step of mixing the superabsorbent polymer, an alkali metal salt of carbonic acid, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution to recycle the superabsorbent polymer into a superabsorbent recycled polymer. The above use has the same effect as Aspect 1.

[0039] The following details describe in detail the method for producing a superabsorbent recycled polymer regenerated from used, acidic superabsorbent polymers (hereinafter sometimes simply referred to as the "production method"), the superabsorbent recycled polymer, cat litter, the method for regenerating used, acidic superabsorbent polymers (hereinafter sometimes simply referred to as the "regeneration method"), and the use of a mixed solution of a hydrophobic solvent and water for regenerating used, acidic superabsorbent polymers (hereinafter sometimes simply referred to as the "use").

[0040] A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, as described herein, comprises the following steps: - A regeneration step (which may be referred to as the "regeneration step") in which the superabsorbent polymer is mixed with an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution, and the mixed solution is stirred to regenerate the superabsorbent polymer into the superabsorbent recycled polymer. - A separation step (which may be referred to as the "separation step") in which the superabsorbent recycled polymer is separated from the mixed solution.

[0041] The manufacturing method relating to this disclosure may further include the following steps as optional: - A moisture content adjustment step (hereinafter referred to as the "moisture content adjustment step") prior to the regeneration step, for adjusting the moisture content of the superabsorbent polymer; - A dehydration step (hereinafter referred to as the "dehydration step") prior to the regeneration step, for contacting the superabsorbent polymer with a polyvalent metal ion source capable of supplying polyvalent metal ions to dehydrate the superabsorbent polymer; - A washing step (hereinafter referred to as the "washing step") after the dehydration step and prior to the regeneration step, for washing the superabsorbent polymer with an acidic aqueous solution; - A drying step (hereinafter referred to as the "drying step") after the separation step, for drying the separated superabsorbent recycled polymer.

[0042] The regeneration method and use relating to this disclosure include the following steps: - A regeneration step (hereinafter sometimes referred to as the "regeneration step") in which the superabsorbent polymer is regenerated into the superabsorbent recycled polymer by mixing the superabsorbent polymer with an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution.

[0043] [Regeneration Step] In the above regeneration step, the used superabsorbent polymer having acidic groups, an alkali metal salt of carbonate, a hydrophobic solvent, and water are mixed to form a mixed solution, and the superabsorbent polymer is regenerated into the superabsorbent recycled polymer by stirring the mixed solution.

[0044] The superabsorbent polymer having the acid group described above is not particularly limited as long as it is used in the art as a superabsorbent polymer having an acid group, and examples include those containing a carboxyl group, a sulfo group, etc., with those containing a carboxyl group being preferred. Examples of superabsorbent polymers containing a carboxyl group include polyacrylate-based and polymaleate-based polymers, and examples of superabsorbent polymers containing a sulfo group, etc., include polysulfonate-based polymers.

[0045] The above-mentioned used superabsorbent polymer containing acid groups can be prepared, for example, by extracting the superabsorbent polymer containing acid groups that has absorbed bodily fluids from used sanitary products. In used sanitary products, not all of the superabsorbent polymer contained in the absorbent material has absorbed bodily fluids; the above-mentioned used superabsorbent polymer containing acid groups only needs to have absorbed a portion of bodily fluids. The above-mentioned sanitary products are not particularly limited as long as they contain a superabsorbent polymer containing acid groups, and examples include disposable diapers, disposable shorts, sanitary napkins, panty liners, incontinence pads, bed sheets, pet sheets, etc.

[0046] The above-mentioned used superabsorbent polymer having acidic groups preferably has a moisture content of 35 ± 10% by mass, more preferably 35 ± 7% by mass, and even more preferably 35 ± 5% by mass. This suppresses adhesion between the superabsorbent polymers due to their tackiness, while also suppressing fluctuations in the ratio of hydrophobic solvent and water in the mixed solution. In this disclosure, the moisture content is measured using Kett's FD-720 infrared moisture meter. Specifically, about 5 g of the sample is placed in the sample dish of the FD-720, the set temperature is set to 150°C, and the automatic stop mode is selected to measure the moisture content.

[0047] Examples of alkali metal salts of carbonate include sodium carbonate, potassium carbonate, and lithium carbonate. Examples of sodium carbonate include sodium bicarbonate and sodium carbonate. Examples of potassium carbonate include potassium bicarbonate and potassium carbonate. Examples of lithium carbonate include lithium bicarbonate and lithium carbonate. Preferably, the alkali metal salt of carbonate is sodium bicarbonate, potassium bicarbonate, or lithium bicarbonate. As a result, the pH of the superabsorbent recycled polymer tends to become neutral to weakly alkaline when it absorbs liquid, ensuring high safety when it comes into contact with the user's skin.

[0048] The hydrophobic solvent described above is not particularly limited as long as it is hydrophobic. Note that hydrophobicity means that it is not miscible with water in any ratio. Preferably, the hydrophobic solvent is a non-flammable solvent. This prevents the hydrophobic solvent from igniting during the regeneration and separation steps, resulting in a safer manufacturing method. Furthermore, if desired, the manufacturing method can also be made safer by preventing the hydrophobic solvent from igniting during (i) drying of the superabsorbent recycled polymer or (ii) sterilization, bleaching, etc., of the superabsorbent polymer using an oxidizing agent.

[0049] Examples of the hydrophobic solvents mentioned above include fluorinated organic solvents and aromatic organic solvents. Examples of the non-flammable solvents include the fluorinated organic solvents. Examples of the fluorinated organic solvents include hydrochlorofluoroolefin solvents (e.g., 1-chloro-2,3,3-trifluoropropene) and hydrofluoroolefin solvents (e.g., 1,1,1,3,3-pentafluorobutane, 1,3,3,3-tetrafluoropropene). Examples of the aromatic organic solvents include aromatic hydrocarbons, such as benzene-based aromatic hydrocarbons, such as toluene and xylene. The hydrophobic solvents have the function of dehydrating and sterilizing superabsorbent polymers, as well as suppressing swelling of superabsorbent polymers by absorbing water. Furthermore, since the fluorinated organic solvents are non-flammable, they are preferred from the viewpoint of safety and ease of handling.

[0050] The water mentioned above is not particularly limited and includes deionized water, tap water, industrial water, etc. The water has the function of receiving the liquid (e.g., body fluid) dehydrated from the superabsorbent polymer by the action of a hydrophobic solvent and purifying the superabsorbent polymer (superabsorbent recycled polymer).

[0051] When forming the above mixed solution, the order in which the superabsorbent polymer, the alkali metal salt of carbonic acid, the hydrophobic solvent, and the water are mixed is arbitrary. However, it is preferable to form the mixed solution so that the superabsorbent polymer and the water do not come into contact in the absence of the hydrophobic solvent. This is from the viewpoint of suppressing the swelling of the superabsorbent polymer due to water absorption.

[0052] To form the above mixed solution, it is preferable to contact the superabsorbent polymer with (i) the hydrophobic solvent that does not contain the alkali metal salt of carbonate, (ii) the hydrophobic solvent that contains the alkali metal salt of carbonate, (iii) a mixture of the hydrophobic solvent that does not contain the alkali metal salt of carbonate and water, or (iv) a mixture of the hydrophobic solvent that contains the alkali metal salt of carbonate and water. This makes it possible to suppress the superabsorbent polymer from absorbing water and swelling when forming the mixed solution.

[0053] In the above regeneration step, the hydrophobic solvent and water are used in a mass ratio of preferably 35-80:20-65, more preferably 45-80:20-55, and even more preferably 55-80:20-45, based on a total mass of 100 parts by mass of the hydrophobic solvent and water. This allows the hydrophobic solvent to dehydrate and sterilize the superabsorbent polymer, and to suppress swelling by absorbing water. The water can also help restore the water absorption of the superabsorbent polymer by converting the acidic groups of the superabsorbent polymer into alkali metal salts. Furthermore, the water can accept the liquid (e.g., body fluids) that the superabsorbent polymer held, and purify the superabsorbent polymer (superabsorbent recycled polymer).

[0054] The ratio of the superabsorbent polymer to the hydrophobic solvent is not particularly limited, as long as it can achieve the effects of this disclosure. The hydrophobic solvent is preferably 4.0 g / g or more, more preferably 8.0 g / g or more, and even more preferably 10.0 g / g or more, relative to the superabsorbent polymer in a dry state. Furthermore, the hydrophobic solvent is preferably 50.0 g / g or less, more preferably 40.0 g / g or less, and even more preferably 30.0 g / g or less, relative to the superabsorbent polymer in a dry state. This allows for efficient reaction between the acidic groups of the superabsorbent polymer and the alkali metal salt of carbonic acid, while suppressing swelling due to water absorption by the superabsorbent polymer. Note that 4.0 g / g or more of the hydrophobic solvent relative to the superabsorbent polymer in a dry state means that the hydrophobic solvent is 4.0 times or more by mass relative to the superabsorbent polymer in a dry state.

[0055] In this disclosure, the superabsorbent polymer and superabsorbent recycled polymer may be collectively referred to as "superabsorbent polymer, etc." In this disclosure, the superabsorbent polymer, etc. in a dry state can be obtained by drying the superabsorbent polymer, etc. at 120°C for 120 minutes. In this disclosure, the mass of the superabsorbent polymer, etc. in a dry state may be simply referred to as the dry mass.

[0056] The ratio of the superabsorbent polymer to the alkali metal salt of carbonate may vary depending on the water absorption capacity that the superabsorbent recycled polymer should have, for example, the water retention ratio. The alkali metal salt of carbonate is preferably 0.5 g / g or more, more preferably 0.7 g / g or more, and even more preferably 1.0 g / g or more, relative to the superabsorbent polymer in a dry state. Alternatively, the alkali metal salt of carbonate is preferably 2.0 g / g or less, more preferably 1.7 g / g or less, and even more preferably 1.5 g / g or less, relative to the superabsorbent polymer in a dry state. This allows the superabsorbent recycled polymer to have water absorption capacity suitable for applications such as cat litter (especially the skin layer), toilets (e.g., emergency toilets), water bags, soil conditioners, etc. Furthermore, the fact that the alkali metal salt of the carbonate is 0.5 g / g or more relative to the superabsorbent polymer in a dry state means that the alkali metal salt of the carbonate is 0.5 times or more by mass relative to the superabsorbent polymer in a dry state.

[0057] The stirring of the above mixed solution is preferably such that one of the hydrophobic solvent and water is dispersed in the other, i.e., it becomes emulsified. If the stirring is performed, for example, by rotating the treatment tank containing the mixed solution, the rotation of the treatment tank itself is preferably 20 to 200 rpm, and more preferably 40 to 100 rpm. If the stirring is performed, for example, by rotating a stirring blade inside the treatment tank containing the mixed solution, the rotation of the stirring blade is preferably 100 to 2,000 rpm, and more preferably 200 to 1,000 rpm.

[0058] The manufacturing method relating to this disclosure may include at least one of the following steps prior to the regeneration step: an optional dehydration step, an optional washing step, and an optional moisture content adjustment step.

[0059] [Dehydration Step] In the optional dehydration step, the used superabsorbent polymer having acidic groups is brought into contact with a polyvalent metal ion source capable of supplying polyvalent metal ions to dehydrate the superabsorbent polymer. This allows the liquid absorbed by the superabsorbent polymer, such as bodily fluids, to be expelled. Furthermore, if the polyvalent metal ion source is strongly alkaline, such as quicklime or slaked lime, the superabsorbent polymer can be sterilized. The dehydrated superabsorbent polymer may be referred to as dehydrated superabsorbent polymer.

[0060] In the dehydration step described above, the acidic groups (e.g., -COOH) of the superabsorbent polymer having acidic groups are crosslinked with polyvalent metal ions (e.g., calcium ions), the superabsorbent polymer having acidic groups is dehydrated, and a dehydrated superabsorbent polymer is formed.

[0061] Examples of the above-mentioned polyvalent metal ions include alkaline earth metal ions, transition metal ions, and post-transition metal ions. Examples of the above-mentioned alkaline earth metal ions include beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions. Examples of the above-mentioned transition metal ions include iron ions, cobalt ions, nickel ions, and copper ions. Examples of the above-mentioned post-transition metal ions include aluminum ions.

[0062] When the above-mentioned polyvalent metal ions are alkaline earth metal ions, examples of polyvalent metal ion sources capable of supplying the above-mentioned polyvalent metal ions include hydroxides of alkaline earth metals (e.g., calcium hydroxide, magnesium hydroxide), salts of alkaline earth metal hydroxides and acids (e.g., calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate), and oxides of alkaline earth metals (e.g., calcium oxide, magnesium oxide), with calcium chloride being preferred.

[0063] The above-mentioned acids are not particularly limited, but examples include inorganic acids and organic acids. Examples of inorganic acids include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid being preferred from the viewpoint of not containing chlorine and cost. Examples of organic acids include those having an acid group, such as a carboxyl group or a sulfo group. Organic acids having a sulfo group are called sulfonic acids, and organic acids having a carboxyl group but not a sulfo group are called carboxylic acids. From the viewpoint of protecting equipment, organic acids having a carboxyl group, and especially carboxylic acids, are preferred.

[0064] Examples of the above organic acids include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid (all carboxylic acids having multiple carboxyl groups), gluconic acid (C6), pentanoic acid (C5), butanoic acid (C4), propionic acid (C3), glycolic acid (C2), acetic acid (C2), glacial acetic acid, formic acid (C1) (all carboxylic acids having one carboxyl group), methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid (all sulfonic acids).

[0065] When the above-mentioned polyvalent metal ions are transition metal ions, examples of polyvalent metal ion sources capable of supplying these ions include transition metal hydroxides (e.g., iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide), transition metal hydroxides and acid salts, and transition metal oxides (e.g., iron oxide, cobalt oxide, nickel oxide, copper oxide). Examples of the above-mentioned acids include those listed under "Alkaline Earth Metal Hydroxides and Acid Salts".

[0066] Specific examples of the above-mentioned transition metal hydroxides and acid salts include inorganic salts and organic salts. Examples of the inorganic salts include iron salts such as iron chloride, iron sulfate, iron phosphate, and iron nitrate; cobalt salts such as cobalt chloride, cobalt sulfate, cobalt phosphate, and cobalt nitrate; nickel salts such as nickel chloride and nickel sulfate; and copper salts such as copper chloride and copper sulfate. Examples of the organic salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate.

[0067] When the above-mentioned polyvalent metal ions are post-transition metal ions, examples of polyvalent metal ion sources capable of supplying these ions include post-transition metal hydroxides (e.g., aluminum hydroxide), post-transition metal hydroxides and acid salts, and post-transition metal oxides. Examples of the above-mentioned acids include those listed under "Alkaline Earth Metal Hydroxides and Acid Salts."

[0068] Specific examples of the above-mentioned post-transition metal hydroxides and acid salts include inorganic acid salts and organic acid salts. Examples of the inorganic acid salts include aluminum salts such as aluminum chloride, aluminum sulfate, aluminum phosphate, and aluminum nitrate. Examples of the organic acid salts include aluminum lactate, aluminum acetate, and aluminum stearate.

[0069] In the dehydration step described above, for example, a used superabsorbent polymer having acidic groups can be brought into direct contact with a polyvalent metal ion source capable of supplying polyvalent metal ions to form a dehydrated superabsorbent polymer. Alternatively, in the dehydration step, a used superabsorbent polymer having acidic groups can be dehydrated and a dehydrated superabsorbent polymer can be formed by stirring the used superabsorbent polymer having acidic groups in a dehydration tank containing an aqueous solution of a polyvalent metal ion source capable of supplying polyvalent metal ions for approximately 5 to 60 minutes, depending on the temperature.

[0070] The aqueous solution containing the polyvalent metal ion source in the dehydration step may or may not be heated. If the aqueous solution containing the polyvalent metal ion source in the dehydration step is not heated, there are no particular restrictions on its temperature, and it can be, for example, room temperature (25°C) to 40°C or less than 60°C.

[0071] In the dehydration step described above, the temperature of the aqueous solution containing the polyvalent metal ion source is preferably higher than room temperature, more preferably 60 to 100°C, even more preferably 70 to 95°C, and even more preferably 80 to 90°C, if heating is performed. If the solubility of the polyvalent metal ion source in water is low, heating can efficiently supply polyvalent metal ions from the source and promote the formation of the dehydrated superabsorbent polymer.

[0072] In the dehydration step described above, the used superabsorbent polymer having acidic groups is dehydrated so that the dehydrated superabsorbent polymer has a moisture content of preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less. This reduces the amount of waste remaining in the dehydrated superabsorbent polymer and makes it easier to form a superabsorbent recycled polymer from the dehydrated superabsorbent polymer. However, if the moisture content of the dehydrated superabsorbent polymer is reduced too much, it may become difficult to form a superabsorbent recycled polymer in the subsequent regeneration step.

[0073] In the dehydration step described above, when a used, acidic, dehydrated superabsorbent polymer is brought into contact with an aqueous solution containing a polyvalent metal ion source, for example, when a used, acidic, superabsorbent polymer is added to an aqueous solution containing a polyvalent metal ion source, the concentration of the polyvalent metal ion source in the aqueous solution containing the polyvalent metal ion source is preferably 1.0 to 30.0% by mass, more preferably 3.0 to 25.0% by mass, and more preferably 5.0 to 20.0% by mass. This makes it easier for the dehydrated superabsorbent polymer to have the desired moisture content.

[0074] [Washing Step] In the optional washing step, after the dehydration step and before the regeneration step, the dehydrated superabsorbent polymer is washed with an acidic aqueous solution to form a post-wash dehydrated superabsorbent polymer. By washing the dehydrated superabsorbent polymer with an acidic aqueous solution, at least a portion of the acid groups [e.g., (-COO)2Ca] of the dehydrated superabsorbent polymer that were crosslinked by polyvalent metal ions (e.g., calcium ions) can be returned to free acid groups (e.g., -COOH). This cleans the superabsorbent polymer and reduces the amount of polyvalent metal crosslinking the acid groups of the superabsorbent polymer, thereby adjusting the water absorption of the superabsorbent recycled polymer formed in the regeneration step.

[0075] The above-mentioned acidic aqueous solution refers to an aqueous solution containing an acid. Examples of acids include the hydroxides and acid salts of the above-mentioned alkaline earth metals.

[0076] If the above acid is an organic acid having a carboxyl group, the organic acid may have one or more carboxyl groups per molecule, and it is preferable that it has multiple carboxyl groups. By doing so, the organic acid is more likely to form a chelate complex with polyvalent metal ions, such as calcium, and it becomes easier to remove polyvalent metal ions that were crosslinking the acid groups of the dehydrated superabsorbent polymer, or polyvalent metal ions that were attached to the dehydrated superabsorbent polymer.

[0077] The acid used to wash the dehydrated superabsorbent polymer returns the acid groups that were crosslinked by polyvalent metal ions back to free acid groups, thus affecting the acid dissociation constant (pK) of the acid groups in the superabsorbent polymer. a Acid dissociation constant (pK) smaller than (in water) a It is preferable to have (in water).

[0078] If the above acid has multiple acid groups, for example, if the above acid is a dibasic acid or a tribasic acid, the acid dissociation constant (pK) of the above acid is a The largest acid dissociation constant (pK) among those in water. a (In water) The acid dissociation constant (pK) of the acid group of the superabsorbent polymer a, in water) is preferably smaller, and when the superabsorbent polymer has a plurality of acid groups, the acid dissociation constant (pK a , in water) of the largest acid dissociation constant (pK a , in water) among the plurality of acid groups of the superabsorbent polymer is smaller than the smallest acid dissociation constant (pK a , in water). This is from the viewpoint of returning the acid groups of the superabsorbent polymer to free acid groups.

[0079] In the present disclosure, the acid dissociation constant (pK a , in water) can adopt the values described in the Electrochemical Handbook edited by the Electrochemical Society. According to the Electrochemical Handbook, the acid dissociation constants (pK a , in water, 25 °C) of the main compounds are as follows. [Organic acids] - Tartaric acid: 2.99 (pK a1 ), 4.44 (pK a2 ) - Malic acid: 3.24 (pK a1 ), 4.71 (pK a2 ) - Citric acid: 2.87 (pK a1 ), 4.35 (pK a2 ), 5.69 (pK a3 ) [Inorganic acids] - Sulfuric acid: 1.99 (pK a2 )

[0080] The acid dissociation constant (pK a , in water) of an acid not described in the Electrochemical Handbook can be determined by measurement. As a device capable of measuring the acid dissociation constant (pK a , in water) of an acid, for example, the Compound Physical Property Evaluation Analysis System, T3 manufactured by Sirius can be mentioned.

[0081] In the above washing, the ratio of the dehydrated superabsorbent polymer to the acidic aqueous solution is not particularly limited, but preferably, the acidic aqueous solution is used in a mass ratio of preferably 5 times or more, more preferably 7 times or more, and even more preferably 10 times or more with respect to the dehydrated superabsorbent polymer. Thereby, while cleaning the superabsorbent polymer, the amount of polyvalent metal crosslinking the acid groups of the superabsorbent polymer is reduced, and it becomes easier to adjust the water absorption of the superabsorbent recycled polymer formed in the subsequent regeneration step.

[0082] The above acidic aqueous solution preferably has a predetermined pH. The predetermined pH is preferably 5.0 or less, more preferably 4.0 or less, even more preferably 3.0 or less, and even more preferably 2.0 or less. Alternatively, the predetermined pH is preferably 1.0 or higher, more preferably 1.2 or higher, even more preferably 1.5 or higher, and even more preferably 1.8 or higher. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the subsequent regeneration step. In this specification, pH (excluding "physiological saline pH" described later) refers to the value at 25°C. Furthermore, pH (excluding "physiological saline pH" described later) can be measured, for example, using a twin pH meter AS-711 manufactured by Horiba, Ltd.

[0083] The above acidic aqueous solution preferably has a predetermined temperature. The predetermined temperature is preferably 15°C or higher, more preferably 20°C or higher, even more preferably 25°C or higher, and even more preferably 30°C or higher. Alternatively, the predetermined temperature is preferably 90°C or lower, more preferably 80°C or lower, even more preferably 70°C or lower, and even more preferably 60°C or lower. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the subsequent regeneration step.

[0084] In the above washing step, the method for washing the dehydrated superabsorbent polymer with the acidic aqueous solution is not particularly limited, and can be carried out, for example, by immersing the dehydrated superabsorbent polymer in the acidic aqueous solution. Furthermore, when immersing the dehydrated superabsorbent polymer in the acidic aqueous solution, the acidic aqueous solution may be stirred. The immersion time for the dehydrated superabsorbent polymer in the acidic aqueous solution can be, for example, 0.5 to 2.0 hours.

[0085] In the washing step described above, the dehydrated superabsorbent polymer after washing preferably has a moisture content of 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less. This reduces the amount of residual waste, unreacted polyvalent metal ion sources in the dehydration step, and polyvalent metal ions with crosslinked acid groups in the dehydration step, and also restores the water absorption properties of the dehydrated superabsorbent polymer after washing, making it easier to form a superabsorbent recycled polymer from the dehydrated superabsorbent polymer after washing. However, if the moisture content of the dehydrated superabsorbent polymer after washing is reduced too much, it may become difficult to form a superabsorbent recycled polymer in the subsequent regeneration step.

[0086] [Moisture Content Adjustment Step] In the optional moisture content adjustment step, (i) the moisture content of the superabsorbent polymer may be adjusted before the regeneration step, (ii) the moisture content of the dehydrated superabsorbent polymer may be adjusted after the dehydration step and before the regeneration step, or (iii) the moisture content of the dehydrated superabsorbent polymer after washing and before the regeneration step may be adjusted. This suppresses adhesion caused by the superabsorbent polymer, the dehydrated superabsorbent polymer after washing, or the dehydrated superabsorbent polymer (hereinafter, "the superabsorbent polymer, the dehydrated superabsorbent polymer, or the dehydrated superabsorbent polymer after washing" may be referred to as "superabsorbent polymer, etc.") having adhesive properties, while allowing the reaction between the superabsorbent polymer and the alkali metal salt of carbonic acid to proceed accurately in the next regeneration step.

[0087] The method for adjusting the moisture content is not particularly limited and includes, for example, natural drying, heat drying, reduced-pressure drying, immersion drying in a hydrophilic organic solvent, microwave drying, and freeze-drying of the superabsorbent polymer.

[0088] For example, the above-mentioned heat drying method involves drying the superabsorbent polymer, etc., in a dryer at 60 to 200°C for 1 to 24 hours. For example, the above-mentioned natural drying method involves drying the superabsorbent polymer, etc., at room temperature for 12 to 24 hours. Natural drying can also be carried out under forced air. For example, the above-mentioned vacuum drying method involves drying the superabsorbent polymer, etc., under reduced pressure at 60 to 80°C for 4 to 12 hours. For example, the above-mentioned immersion drying method involves immersing the superabsorbent polymer, etc., in a solvent for 5 to 60 minutes. From the viewpoint of adjusting the moisture content, it is preferable that the solvent has a purity of 95% by mass or more and a water content of less than 5% by mass.

[0089] In the moisture content adjustment step described above, the superabsorbent polymer preferably has a moisture content of 35 ± 10% by mass, more preferably 35 ± 7% by mass, and even more preferably 35 ± 5% by mass. This suppresses adhesion between the superabsorbent polymers due to their tackiness, while allowing the reaction between the superabsorbent polymer and the alkali metal salt of carbonic acid to proceed accurately in the subsequent regeneration step.

[0090] [Separation Step] In the above separation step, the superabsorbent recycled polymer is separated from the mixed solution. The separation of the superabsorbent recycled polymer from the mixed solution can be carried out using a known solid-liquid separation apparatus, such as a rotary drum screen, an inclined screen, a vibrating screen, etc.

[0091] [Drying Step] In the drying step as desired above, the separated superabsorbent recycled polymer is dried. In the drying step, for example, the separated superabsorbent recycled polymer can be dried in a dryer at 60 to 200°C for 1 to 24 hours.

[0092] The moisture content of the superabsorbent recycled polymer after the drying step is preferably 10% by mass or less, more preferably 5% by mass or less. This makes it possible to prevent stickiness of the superabsorbent recycled polymer, ensure absorption capacity, suppress mold growth, and reduce transportation costs.

[0093] The above superabsorbent recycled polymer contains polyvalent metals in a dry state, preferably in a ratio of 10.0% by mass or more, more preferably 11.5% by mass or more, and even more preferably 13.0% by mass or more. Furthermore, the above superabsorbent recycled polymer contains polyvalent metals in a ratio of preferably 17.0% by mass or less, more preferably 15.5% by mass or less, and even more preferably 14.0% by mass or less.

[0094] The above-mentioned superabsorbent recycled polymer contains alkali metals in a dry state, preferably in a ratio of 6.0% by mass or more, more preferably 7.0% by mass or more, and even more preferably 7.5% by mass or more. Furthermore, the above-mentioned superabsorbent recycled polymer contains alkali metals in a ratio of preferably 10.0% by mass or less, more preferably 9.0% by mass or less, and even more preferably 8.5% by mass or less. By containing polyvalent metals and alkali metals within the above ranges, the above-mentioned superabsorbent polymer can absorb liquids at a predetermined rate.

[0095] The ratio of polyvalent metals in the superabsorbent recycled polymer can be adjusted, for example, by the amount of polyvalent metal ion source in the dehydration step, the contact time with the polyvalent metal ion source, etc., during the washing step. The ratio of alkali metals in the superabsorbent recycled polymer can be adjusted, for example, by the amount of alkali metal salt of carbonate in the regeneration step.

[0096] In this disclosure, the ratio of polyvalent metals and alkali metals in the superabsorbent recycled polymer is measured as follows: [Pretreatment] (1) Accurately weigh out approximately 1 g (mass: m (g)) of superabsorbent recycled polymer into a conical beaker. (2) Add 5 mL of nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for measuring hazardous metals) and 10 mL of hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for measuring hazardous metals) to the conical beaker using a graduated pipette, and heat the contents of the conical beaker to decompose them.

[0097] (3) After disassembling the contents of the conical beaker, 0.5 mL of nitric acid (for measuring harmful metals) is added to the conical beaker using a pipette, and then approximately 5 mL of ultrapure water is added. The ultrapure water is produced from tap water using a pure water production system (Merck KGaA, Elix Essential UV5), and then from the above pure water using an ultrapure water production system (Merck KGaA, Simplicity UV). Hereinafter, it will simply be referred to as "ultrapure water".

[0098] (4) After allowing the contents of the conical beaker to cool, transfer the contents of the conical beaker to a 50 mL volumetric flask. Add 0.25 μL of mixed internal standard solution (Yttrium standard solution, Y1000, concentration: 1,000 mg / L, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to the volumetric flask using a pipette, and then make up the volumetric flask with ultrapure water to prepare the test sample. If the contents of the volumetric flask are turbid, filter the contents of the volumetric flask with filter paper (Quantitative filter paper No. 5C, manufactured by ADVANTEC) after making up the volumetric flask, and use the filtrate as the test sample. (5) Perform steps (1) to (5) using the same amount of ultrapure water as the superabsorbent recycled polymer instead of the superabsorbent recycled polymer to prepare the blank sample.

[0099] [Preparation of Calibration Standard Solution] (1) Measure 0.5 mL of nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for measuring heavy metals) with a pipette and add it to four 50 mL volumetric flasks. Next, measure 0.25 μL of mixed internal standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., yttrium standard solution, Y1000, yttrium concentration: 1,000 mg / L) with a pipette and add it to the four 50 mL volumetric flasks containing the nitric acid. (2) If the metal to be measured is sodium as an alkali metal, take 0.05 mL, 0.1 mL, 0.2 mL, and 0.5 mL of sodium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Na1000, sodium concentration: 1,000 mg / L) with a pipette and add them to the four 50 mL volumetric flasks to which the nitric acid and mixed internal standard solution were added in (1).

[0100] [Measurement] (1) Following the operating procedure of the ICP mass spectrometer (manufactured by Thermo Fisher Scientific Inc., iCAPQc), a calibration curve is prepared from the ionic intensity ratio (ionic intensity of sodium mass number / ionic intensity of yttrium mass number) and concentration of the calibration standard solution. The measured mass number of sodium is 23, and the mass measurement number of yttrium is 89. (2) The ionic intensity ratio (ionic intensity of sodium mass number / ionic intensity of yttrium mass number) of the test sample is determined, and the concentration of sodium is calculated from the calibration curve.

[0101] (3) If the output of the test sample exceeds the upper limit of the concentration range of the calibration curve, dilute the test sample at the specified dilution ratio: Z (times), remeasure from the pretreatment, and calculate the sodium concentration using the following formula: Concentration = {((Ionic intensity ratio of test sample - a) / b) × Z} - {(Ionic intensity ratio of blank sample - a) / b} a: slope of the calibration curve b: intercept of the calibration curve (4) Determine the sodium concentration of the blank sample and confirm that it is below the lower limit of the concentration range of the calibration curve. (5) Divide the sodium concentration by m (g) and round the value according to JIS Z 8401:2019 "Rounding of numerical values". Use two significant figures, up to the lower limit of quantification.

[0102] The above describes the method for measuring the concentration when the metal to be measured is sodium as an alkali metal. However, if the metal to be measured is not sodium as an alkali metal, the sodium standard solution can be changed as follows and the measurement can be performed in the same manner. If the metal to be measured is potassium as an alkali metal, the sodium standard solution is changed to potassium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Ka1000, potassium concentration: 1,000 mg / L). If the metal to be measured is lithium as an alkali metal, the sodium standard solution is changed to lithium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Li1000, lithium concentration: 1,000 mg / L).

[0103] If the metal to be measured is calcium as a polyvalent metal, the sodium standard solution should be changed to a calcium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Ca1000, calcium concentration: 1,000 mg / L). The number of mass measurements for calcium should be 43 and 44. If the metal to be measured is magnesium as a polyvalent metal, the sodium standard solution should be changed to a magnesium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Mg1000, magnesium concentration: 1,000 mg / L). The number of mass measurements for magnesium should be 24 and 25.

[0104] If the metal to be measured is aluminum as a polyvalent metal, the sodium standard solution is changed to aluminum standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Al1000, aluminum concentration: 1,000 mg / L), and 0.001 mL, 0.004 mL, 0.010 mL, and 0.040 mL are taken with a pipette and added to each of the four 50 mL volumetric flasks to which nitric acid and mixed internal standard solution were added in (1). The number of mass measurements for aluminum is 27. If the metal to be measured is iron as a polyvalent metal, the sodium standard solution is changed to iron standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Fe1000, iron concentration: 1,000 mg / L), and 0.001 mL, 0.004 mL, 0.010 mL, and 0.040 mL are taken with a pipette and added to each of the four 50 mL volumetric flasks to which nitric acid and mixed internal standard solution were added in (1). The mass measurements for iron are 54 and 56.

[0105] After mixing the superabsorbent recycled polymer in its dry state with 200 times its mass of physiological saline solution, the physiological saline solution preferably has a pH of 6.0 or higher, more preferably 7.0 or higher, and even more preferably 8.0 or higher. Alternatively, after mixing the superabsorbent recycled polymer in its dry state with 200 times its mass of physiological saline solution, the physiological saline solution preferably has a pH of 12.0 or lower, more preferably 11.5 or lower, and even more preferably 11.0 or lower. This makes it easier for the pH of the superabsorbent recycled polymer to become neutral to weakly alkaline when it absorbs liquid, thus increasing safety when it comes into contact with the user's skin. In this disclosure, the pH of the physiological saline solution after mixing the superabsorbent recycled polymer in its dry state with 200 times its mass of physiological saline solution may be referred to as "physiological saline pH".

[0106] In this disclosure, the pH of the physiological saline solution after mixing the superabsorbent recycled polymer with 200 times its mass is measured as follows: (1) Measure 100 g of physiological saline solution at 25°C into a beaker. The physiological saline solution is an aqueous solution of 0.9% by mass sodium chloride (first-grade reagent). (2) Add 0.50 g of the superabsorbent recycled polymer to the beaker. (3) Place the beaker on a magnetic stirrer, insert a stirring bar into the beaker, and stir the contents of the beaker for 5 minutes. (4) After stirring for 5 minutes, read the pH value of the solution in the beaker to two decimal places using a pH meter (HPH-130, manufactured by Denki Kagaku Keiki Co., Ltd.). (5) Repeat steps (1) to (4) a total of 5 times using different physiological saline solutions and different superabsorbent recycled polymers, and round the average value to one decimal place to adopt it as the pH of the physiological saline solution.

[0107] In a dry state, the superabsorbent recycled polymer absorbs 20 times its mass of deionized water in preferably 120 seconds or less, more preferably 90 seconds or less, and even more preferably 60 seconds or less. Furthermore, in a dry state, the superabsorbent recycled polymer absorbs 20 times its mass of deionized water in preferably 10 seconds or more, and more preferably 20 seconds or more. This makes the superabsorbent recycled polymer easier to use in a wide range of applications, such as articles that come into contact with the user's skin, such as absorbent articles. In this disclosure, the time it takes for the superabsorbent recycled polymer in a dry state to absorb 20 times its mass of deionized water may be referred to as the "20x water absorption time".

[0108] In this disclosure, the time it takes for the superabsorbent recycled polymer to absorb 20 times its mass of deionized water is measured as follows: (1) A sample is formed by drying the superabsorbent recycled polymer at 120°C for 120 minutes. (2) 1.0 g of the sample and 20.0 g of deionized water are weighed out in a constant temperature room at 25°C. (3) 1.0 g of the sample is placed at the bottom of a beaker with an inner diameter of 5.0 cm. (4) 20.0 g of deionized water is added to the beaker and a stopwatch is started to measure the time it takes for all droplets on the surface of the sample to disappear, and this is defined as the 20-fold absorption time. (5) The 20-fold absorption time is measured a total of 10 times with different samples, and the average value is adopted.

[0109] The uses of the above-mentioned superabsorbent recycled polymer are not particularly limited, but it is useful in applications where a predetermined water retention capacity is preferable, such as cat litter, emergency toilets, and water bags.

[0110] The cat litter described above includes a core layer and a shell layer, the shell layer containing the superabsorbent recycled polymer. This allows it to absorb cat urine at a predetermined rate. From this viewpoint, it is preferable that the cat litter is of a type that absorbs excrement, especially urine.

[0111] The present disclosure will be explained below with examples, but the present disclosure is not limited to these examples. [Manufacturing Example 1] Superabsorbent polymer that has absorbed excrement was recovered from used disposable diapers. Approximately 6 kg (dry mass: approximately 1.2 kg) of the recovered superabsorbent polymer was immersed for 60 minutes in a container containing 32 L of 4.0% by mass aqueous solution of slaked lime (Ca(OH)2), then 18 times the amount of deionized water was added to the container, and the contents of the container were then sieved to perform a dehydration step to obtain dehydrated superabsorbent polymer No. 1.

[0112] Dehydrated superabsorbent polymer No. 1 was immersed in 5 L of deionized water at room temperature, and the deionized water was stirred for 30 minutes to perform a washing step, thereby obtaining dehydrated superabsorbent polymer No. 1 after washing. After washing, dehydrated superabsorbent polymer No. 1 was dried at 110°C for 90 minutes to perform a moisture content adjustment step, thereby obtaining superabsorbent polymer No. 1 with a moisture content of approximately 35% by mass.

[0113] 373 g of deionized water was added to a container, and while stirring the contents of the container at 70 rpm using a dry cleaning test machine (Daiei Kagaku Seiki Seisakusho, DC-1A), 32.4 g of sodium bicarbonate (NaHCO3), 1,100 g of a fluorine-based solvent as a hydrophobic solvent (AGC Inc., AMOLEA® AS-300), and 50 g of superabsorbent polymer No. 1 (SAP) were added to the container in this order, and stirring was continued for 30 minutes to carry out the regeneration step. The contents of the container were separated into solid and liquid components, and dried at 120°C for 120 minutes to obtain superabsorbent recycled polymer No. 1.

[0114] [Production Examples 2-9 and Comparative Production Example 1] Superabsorbent recycled polymers No. 2-10 were obtained in the same manner as in Production Example 1, except that the amounts of superabsorbent polymer No. 1, hydrophobic solvent, water, and sodium bicarbonate were as shown in Table 1. [Comparative Production Example 2] Superabsorbent recycled polymer No. 11 was obtained in the same manner as in Production Example 1, except that the amounts of superabsorbent polymer No. 1, hydrophobic solvent, water, and sodium carbonate (Na2CO3) were as shown in Table 1.

[0115] [Comparative Production Example 3] Superabsorbent polymer No. 12 was obtained in the same manner as in Production Example 1, except that the amounts of superabsorbent polymer No. 1, hydrophobic solvent, and water were as shown in Table 1, and sodium hydroxide (NaOH) was used instead of sodium bicarbonate as shown in Table 1.

[0116] [Examples 1-11 and Comparative Examples 1-3] For each of the superabsorbent recycled polymers No. 1 to No. 12, the pH of the physiological saline solution and the 20-fold water absorption time were measured. The results are shown in Table 1. Note that for superabsorbent recycled polymers No. 10 and No. 11, as the regeneration step progressed, the superabsorbent recycled polymers No. 10 and No. 11 absorbed water, making it impossible to measure the pH of the physiological saline solution and the 20-fold water absorption time.

[0117]

Claims

1. A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, comprising: a regeneration step of mixing the superabsorbent polymer, an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution and stirring the mixed solution to regenerate the superabsorbent polymer into the superabsorbent recycled polymer; and a separation step of separating the superabsorbent recycled polymer from the mixed solution.

2. The method according to claim 1, wherein in the regeneration step, the superabsorbent polymer is brought into contact with the hydrophobic solvent which may contain an alkali metal salt of carbonate, or a mixture of the hydrophobic solvent which may contain an alkali metal salt of carbonate and water, in order to form the mixed solution.

3. The method according to claim 1 or 2, wherein in the regeneration step, the hydrophobic solvent and the water are used in a mass ratio of 35 to 80:20 to 65 based on a total mass of 100 parts by mass of the hydrophobic solvent and the water.

4. The method according to any one of claims 1 to 3, wherein the alkali metal salt of the carbonic acid is selected from the group consisting of sodium bicarbonate, potassium bicarbonate, and lithium bicarbonate.

5. The method according to any one of claims 1 to 4, wherein the hydrophobic solvent is a non-flammable solvent.

6. The method according to any one of claims 1 to 5, wherein in the regeneration step, the superabsorbent polymer has a moisture content of 35 ± 10% by mass.

7. The method according to claim 6, further comprising a moisture content adjustment step of adjusting the moisture content of the superabsorbent polymer before the regeneration step.

8. The method according to any one of claims 1 to 7, further comprising a dehydration step of contacting the superabsorbent polymer with a polyvalent metal ion source capable of supplying polyvalent metal ions, thereby dehydrating the superabsorbent polymer, prior to the regeneration step.

9. The method according to claim 8, further comprising a washing step of washing the superabsorbent polymer with an acidic aqueous solution after the dehydration step and before the regeneration step.

10. The method according to any one of claims 1 to 9, wherein the superabsorbent recycled polymer contains 10.0 to 17.0% by mass of polyvalent metals and 6.0 to 10.0% by mass of alkali metals.

11. A superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acidic groups, characterized in that the superabsorbent recycled polymer contains 10.0 to 17.0% by mass of polyvalent metals and 6.0 to 10.0% by mass of alkali metals.

12. The superabsorbent recycled polymer according to claim 11, wherein the superabsorbent recycled polymer in a dry state is mixed with 200 times its mass of physiological saline solution, and the physiological saline solution has a pH of 6.0 to 12.

0.

13. The superabsorbent recycled polymer according to claim 11 or 12, wherein the superabsorbent recycled polymer in a dry state absorbs 20 times its mass of deionized water in 120 seconds or less.

14. A superabsorbent recycled polymer according to any one of claims 11 to 13, for use as cat litter.

15. Cat litter comprising a core layer and a shell layer, characterized in that the shell layer comprises a superabsorbent recycled polymer according to any one of claims 11 to 14.

16. A method for regenerating a used superabsorbent polymer having an acid group, comprising a regeneration step of mixing the superabsorbent polymer, an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution to regenerate the superabsorbent polymer into a superabsorbent recycled polymer.

17. A method for using a mixed solution of a hydrophobic solvent and water to regenerate a used, acidic superabsorbent polymer, comprising a regeneration step of mixing the superabsorbent polymer, an alkali metal salt of carbonate, a hydrophobic solvent, and water to form a mixed solution, and stirring the mixed solution to regenerate the superabsorbent polymer into a superabsorbent recycled polymer.

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