Method for producing recycled pulp fibers, method for decomposing superabsorbent polymers, and method for evaluating the cleanliness of recycled pulp fibers.

Abstract

The purpose of the present disclosure is to provide a method, which is for producing recycled pulp fibers from a mixture comprising pulp fibers and super absorbent polymer obtained from sanitary products, and with which it is possible to easily degrade and remove super absorbent polymers and efficiently form recycled pulp fibers that are not readily modified.　This method for producing recycled pulp fibers from a mixture comprising pulp fibers and super absorbent polymer obtained from sanitary products is characterized by comprising: an ultraviolet ray treatment step in which an aqueous solution comprising said mixture is irradiated with ultraviolet rays including rays of a wavelength of 380 nm or less, the super absorbent polymer is oxidatively degraded, the oxidatively degraded super absorbent polymer is dissolved in the aqueous solution, and the recycled pulp fibers are formed; and a recycled pulp fiber recovery step in which the recycled pulp fibers are recovered.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing recycled pulp fibers from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products, a method for decomposing a superabsorbent polymer in an aqueous solution containing pulp fibers and a superabsorbent polymer obtained from sanitary products, and a method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and a superabsorbent polymer. [Background technology]

[0002] There are plans to recover recycled pulp fibers from used hygiene products. For example, Patent Document 1 describes a method for recovering pulp fibers from used sanitary products containing pulp fibers and a polymer absorbent, the method comprising the steps of: immersing the used sanitary products in ozonated water to decompose the polymer absorbent; discharging the ozonated water containing the decomposed polymer absorbent to obtain a residue of the sanitary products from which the polymer absorbent has been removed; and washing the residue of the sanitary products from which the polymer absorbent has been removed by stirring it in an aqueous solution containing a disinfectant or in water, thereby decomposing the residue of the sanitary products into its constituent elements. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2014-217835 [Overview of the project] [Problems that the invention aims to solve]

[0004] The method described in Patent Document 1 involves decomposing the polymer absorbent using ozonated water, which presents challenges such as the need for a large ozone generator to produce ozone. Furthermore, recycled pulp fibers obtained by treatment with ozonated water tend to undergo some degree of alteration from the original pulp fibers, such as a decrease in lignin and hemicellulose content and a reduction in molecular weight. While a decrease in lignin and hemicellulose content, as well as a reduction in molecular weight, is desirable for certain applications of recycled pulp fibers, it may also be preferable for recycled pulp fibers to not undergo any alteration from the original pulp fibers.

[0005] Accordingly, the present disclosure aims to provide a method for producing recycled pulp fibers from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products, which allows for easy decomposition and removal of the superabsorbent polymer and efficiently forms recycled pulp fibers that are less prone to deterioration. [Means for solving the problem]

[0006] The Disclosers have discovered a method for producing recycled pulp fibers from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products, the method comprising: an ultraviolet treatment step in which an aqueous solution containing the mixture is irradiated with ultraviolet light including a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer, dissolve the oxidatively decomposed superabsorbent polymer in the aqueous solution and form recycled pulp fibers; and a recycled pulp fiber recovery step in which the recycled pulp fibers are recovered. [Effects of the Invention]

[0007] The method for producing recycled pulp fibers from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products, as disclosed herein, allows for easy decomposition and removal of the superabsorbent polymer, and efficiently forms recycled pulp fibers that are less susceptible to deterioration. [Modes for carrying out the invention]

[0008] Specifically, this disclosure relates to the following aspects: [Aspect 1] A method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, A UV treatment step involves irradiating an aqueous solution containing the above mixture with ultraviolet light containing a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer, dissolving the oxidatively decomposed superabsorbent polymer in the aqueous solution, and forming the recycled pulp fibers. A recycled pulp fiber recovery step in which the above recycled pulp fibers are recovered, The above method, characterized by including the following.

[0009] In the above method, the superabsorbent polymer is decomposed using a predetermined ultraviolet light in the ultraviolet treatment step. The ultraviolet light decomposes water to generate hydroxyl radicals, which oxidatively decompose the superabsorbent polymer, and the oxidatively decomposed superabsorbent polymer is dissolved in the aqueous solution. Since ultraviolet light generators can be made smaller than ozone generators, the system for manufacturing recycled pulp fibers can be made more compact, and superabsorbent polymers can be easily decomposed.

[0010] Furthermore, even when ozone generation from an ozone generator is stopped, ozone remains in the atmosphere (e.g., in water) for a certain period of time, requiring a predetermined time for detoxification. Therefore, in order to recover recycled pulp fibers, it is necessary to either detoxify the ozone (remove the ozone) or wait for a certain period of time. On the other hand, ultraviolet light disappears immediately and becomes harmless when irradiation from an ultraviolet generator is stopped, allowing the recycled pulp fiber recovery step following the ultraviolet treatment step to be carried out quickly (efficiently).

[0011] Furthermore, the inventors of this application have found that ultraviolet light rapidly decomposes superabsorbent polymers while being less likely to alter pulp fibers. As a result, the resulting recycled pulp fibers have high utility value in applications that utilize the properties of pulp fibers. Based on the above, the method for producing recycled pulp fibers according to this disclosure can easily decompose and remove superabsorbent polymers, and efficiently form recycled pulp fibers that are less prone to deterioration. Consequently, the above method can contribute to achieving the Sustainable Development Goals (SDGs).

[0012] [Aspect 2] The method according to embodiment 1, wherein the ultraviolet light described above includes wavelengths of 290 nm or less. In the above method, since ultraviolet light includes wavelengths of a specific type, recycled pulp fibers that are less prone to deterioration can be efficiently formed.

[0013] [Aspect 3] The method according to embodiment 1 or 2, wherein the superabsorbent polymer in the UV treatment step is not dehydrated.

[0014] Generally, superabsorbent polymers absorb water in aqueous solutions, swell, and increase the viscosity of the solution. Therefore, in order to decompose superabsorbent polymers using ozone, it is common practice to dehydrate the polymer using dehydrating agents such as acids or polyvalent metal salts to allow the ozone to penetrate the aqueous solution. In the above method, the superabsorbent polymer is decomposed using a predetermined ultraviolet light, so the superabsorbent polymer can be decomposed and removed even if it has not been dehydrated. As a result, the dehydration step for the superabsorbent polymer can be omitted, and the resulting recycled pulp fibers are less likely to contain the superabsorbent polymer and its residue, as well as the dehydrating agent and its residue.

[0015] [Aspect 4] The method according to embodiment 3, wherein the aqueous solution having a solid content concentration of 0.01 to 0.1% by mass is stirred in the ultraviolet treatment step described above. In the above method, in the ultraviolet treatment step, an aqueous solution having a predetermined solid content concentration is stirred. Therefore, the pulp fibers and the superabsorbent polymer in the aqueous solution can be efficiently stirred, and as a result, the superabsorbent polymer can be decomposed and removed.

[0016] [Aspect 5] The method according to aspect 1 or 2, wherein the superabsorbent polymer in the ultraviolet treatment step is dehydrated. In the above method, since the superabsorbent polymer in the ultraviolet treatment step is dehydrated, the aqueous solution can be reduced to a predetermined moisture content, and the superabsorbent polymer can be efficiently decomposed and removed while maintaining a high concentration of the aqueous solution.

[0017] [Aspect 6] The method according to aspect 5, wherein in the ultraviolet treatment step, the aqueous solution having a solid content concentration of 0.1 to 4.0% by mass is stirred. In the above method, in the ultraviolet treatment step, an aqueous solution having a predetermined solid content concentration is stirred. Therefore, the pulp fibers and the superabsorbent polymer in the aqueous solution can be efficiently stirred, and as a result, the superabsorbent polymer can be efficiently decomposed and removed.

[0018] [Aspect 7] The method according to aspect 5 or 6, including a dehydration step of dehydrating the superabsorbent polymer before the ultraviolet treatment step. Since the above method includes a predetermined dehydration step before the ultraviolet treatment step, the viscosity of the above aqueous solution can be reduced, and the ultraviolet treatment step can be efficiently carried out.

[0019] [Aspect 8] The method according to any one of aspects 1 to 7, wherein the recycled pulp fibers have a carboxyl group amount of 0.075 mmol / g or less. In the above method, since the recycled pulp fibers have a predetermined amount of carboxyl groups, there is little deterioration of the recycled pulp fibers, and the recycled pulp fibers have high utility value in applications that utilize the properties of pulp fibers.

[0020] [Aspect 9] The method according to any one of embodiments 1 to 8, wherein the recycled pulp fibers have an increase of 0.035 mmol / g or less of carboxyl groups based on the pulp fibers. In the above method, the recycled pulp fibers have a predetermined increase in carboxyl groups relative to the pulp fibers, resulting in less deterioration of the recycled pulp fibers and giving them high utility value in applications that utilize the properties of pulp fibers.

[0021] [Aspect 10] The method according to any one of embodiments 1 to 9, wherein the recycled pulp fibers have a degree of polymerization of 300 or more. In the above method, the recycled pulp fibers have a predetermined degree of polymerization, resulting in minimal deterioration of the recycled pulp fibers and giving them high utility value in applications that utilize the properties of pulp fibers.

[0022] [Aspect 11] The method according to any one of embodiments 1 to 10, wherein the recycled pulp fibers have a reduction in the degree of polymerization of 400 or less, based on the pulp fibers. In the above method, the recycled pulp fibers have a predetermined decrease in the degree of polymerization relative to the original pulp fibers. Therefore, the recycled pulp fibers undergo minimal deterioration, and thus have high utility value in applications that utilize the properties of the original pulp fibers.

[0023] [Aspect 12] The method according to any one of embodiments 1 to 11, wherein the recycled pulp fibers have an alkali solubility of 20% by mass or less. In the above method, the recycled pulp fibers have a predetermined alkali solubility rate, resulting in minimal deterioration of the recycled pulp fibers and giving them high utility value in applications that utilize the properties of pulp fibers.

[0024] [Aspect 13] The method according to any one of embodiments 1 to 12, wherein the recycled pulp fibers have an increase in alkali solubility of 16% by mass or less, based on the pulp fibers. In the above method, recycled pulp fibers have a predetermined increase in alkali solubility relative to pulp fibers, resulting in less deterioration of the recycled pulp fibers and giving them high utility value in applications that utilize the properties of pulp fibers.

[0025] [Aspect 14] The method according to any one of embodiments 1 to 13, wherein the above-mentioned recycled pulp fiber recovery step further recovers the superabsorbent polymer that has been oxidatively decomposed. In the above method, both recycled pulp fibers and oxidatively decomposed superabsorbent polymers are recovered in the recycled pulp fiber recovery step. As a result, the above method can easily decompose and remove the superabsorbent polymers, efficiently form recycled pulp fibers that are less prone to deterioration, and also recycle the oxidatively decomposed superabsorbent polymers, thereby contributing to the achievement of the Sustainable Development Goals (SDGs).

[0026] [Aspect 15] The method according to embodiment 14, wherein, in the recycled pulp fiber recovery step described above, the oxidatively decomposed superabsorbent polymer is further recovered by solid-liquid separation of the aqueous solution containing the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer. In the above method, the aqueous solution that has undergone the ultraviolet treatment step is subjected to solid-liquid separation to recover recycled pulp fibers and oxidatively decomposed superabsorbent polymers, thus enabling efficient recovery of recycled pulp fibers and oxidatively decomposed superabsorbent polymers.

[0027] [Aspect 16] The method according to embodiment 14 or 15, wherein the superabsorbent polymer, which has been oxidatively decomposed from the liquid component, is recovered according to its molecular weight. In the above method, the oxidatively decomposed superabsorbent polymer is recovered according to its molecular weight, thus allowing the oxidatively decomposed superabsorbent polymer to be recycled for appropriate uses according to its molecular weight.

[0028] [Aspect 17] The method according to any one of embodiments 14 to 16, wherein the superabsorbent polymer that has been oxidatively decomposed is reused for adhesive applications, coating applications, clothing finishing applications, water treatment applications, corrosion inhibition applications, or superabsorbent polymer applications. The above method allows for the reuse of oxidatively decomposed superabsorbent polymers for specific applications, thereby contributing to the achievement of the Sustainable Development Goals (SDGs).

[0029] [Aspect 18] A method for decomposing a superabsorbent polymer in an aqueous solution containing pulp fibers obtained from sanitary products and a superabsorbent polymer, A UV treatment step involves irradiating the aqueous solution containing the pulp fibers and the superabsorbent polymer with ultraviolet light including a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer and dissolve the oxidatively decomposed superabsorbent polymer in the aqueous solution. The above method, characterized by including the following.

[0030] The above method has the same effect as in Embodiment 1.

[0031] [Aspect 19] A method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers, A pre-treatment mass measurement step for measuring the dry mass of recycled pulp fibers to be evaluated for cleanliness, recovered from the above-mentioned sanitary products. A UV treatment step is performed to form recycled pulp fibers after UV irradiation, by irradiating an aqueous solution containing recycled pulp fibers whose cleanliness should be evaluated with ultraviolet light containing a wavelength of 380 nm or less, thereby forming recycled pulp fibers after UV irradiation. A post-treatment mass measurement step in which the dry mass of recycled pulp fibers after UV irradiation is measured. The above method, characterized by including the following.

[0032] Since the above method includes a predetermined pre-treatment mass measurement step, a predetermined ultraviolet treatment step, and a predetermined post-treatment mass measurement step, the cleanliness of recycled pulp fibers can be easily measured.

[0033] This disclosure describes in detail the following: (i) a method for producing recycled pulp fibers from a mixture containing pulp fibers and superabsorbent polymers obtained from sanitary products (hereinafter sometimes simply referred to as the "method for producing recycled pulp fibers"), (ii) a method for decomposing superabsorbent polymers in an aqueous solution containing pulp fibers and superabsorbent polymers obtained from sanitary products (hereinafter sometimes simply referred to as the "method for decomposing superabsorbent polymers"), and (iii) a method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers (hereinafter sometimes simply referred to as the "method for evaluating the cleanliness of recycled pulp fibers" or the "cleanliness evaluation method"). The method for decomposing superabsorbent polymers will be explained together in the section on the method for producing recycled pulp fibers.

[0034] [Method for manufacturing recycled pulp fibers] The method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, as disclosed herein, includes the following steps: - An ultraviolet treatment step (hereinafter sometimes referred to as the "ultraviolet treatment step") in which an aqueous solution containing the above mixture is irradiated with ultraviolet light containing a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer, dissolve the oxidatively decomposed superabsorbent polymer in the aqueous solution, and form the recycled pulp fibers. • A recycled pulp fiber recovery step (hereinafter sometimes referred to as the "recycled pulp fiber recovery step") for recovering the recycled pulp fibers mentioned above.

[0035] The method for producing recycled pulp fibers relating to this disclosure may further include the following steps as optional steps: • A dehydration step (hereinafter sometimes simply referred to as the "dehydration step") is performed to dehydrate the superabsorbent polymer before the UV treatment step described above.

[0036] <UV treatment step> The above-mentioned sanitary products are not particularly limited as long as they contain pulp fibers and superabsorbent polymers, and examples include disposable diapers, incontinence pads, sanitary napkins, bed sheets, pet sheets, etc. The above-mentioned sanitary products include sanitary products used by a user that have absorbed the user's excrement, sanitary products used by a user that have not absorbed the user's excrement, unused sanitary products that have been discarded, defective products from the time of manufacture, etc.

[0037] The pulp fibers mentioned above are not particularly limited as long as they are used in sanitary products, and examples include wood pulp (e.g., softwood pulp, hardwood pulp), cross-linked pulp, and non-wood pulp.

[0038] Examples of the superabsorbent polymers mentioned above include those used in hygiene products, such as starch-based, cellulose-based, and synthetic polymer-based superabsorbent polymers. Examples of starch-based or cellulose-based superabsorbent polymers include starch-acrylic acid (salt) graft copolymers, saponified starch-acrylonitrile copolymers, and crosslinked sodium carboxymethylcellulose. Examples of synthetic polymer-based superabsorbent polymers include polyacrylate-based, polysulfonate-based, maleate-anhydride-based, polyacrylamide-based, polyvinyl alcohol-based, polyethylene oxide-based, polyaspartate-based, polyglutamate-based, polyalginate-based, starch-based, and cellulose-based superabsorbent polymers (SAP, Super Absorbent Polymer).

[0039] The method for obtaining an aqueous solution containing a mixture of pulp fibers and a superabsorbent polymer from the above-mentioned sanitary products is not particularly limited. For example, it can be obtained by cutting up used sanitary products, obtaining the mixture containing pulp fibers and a superabsorbent polymer from the absorbent material, and dispersing the mixture in water. Alternatively, for example, used sanitary products can be cut up in water to obtain an aqueous solution containing the mixture containing pulp fibers and a superabsorbent polymer from within the absorbent material.

[0040] In the above aqueous solution, the superabsorbent polymer may or may not be dehydrated using a dehydrating agent. If the superabsorbent polymer is not dehydrated, the dehydration step for the superabsorbent polymer can be omitted, and the resulting recycled pulp fibers are less likely to contain the superabsorbent polymer and its residue, as well as the dehydrating agent and its residue. If the superabsorbent polymer is dehydrated, the above aqueous solution can be reduced to a predetermined moisture content, and the superabsorbent polymer can be efficiently decomposed and removed while maintaining a high concentration in the aqueous solution.

[0041] When the superabsorbent polymer has been dehydrated with a dehydrating agent, it is preferable that the superabsorbent polymer has been dehydrated to have a water absorption ratio of preferably 50 times or less, more preferably 30 times or less, even more preferably 25 times or less, and even more preferably 20 times or less. Furthermore, when the superabsorbent polymer has been dehydrated, it is preferable that the superabsorbent polymer has been dehydrated to have a water absorption ratio of preferably 1 time or more, more preferably 2 times or more, even more preferably 3 times or more, and even more preferably 4 times or more. This allows for efficient decomposition and removal of the superabsorbent polymer while maintaining a high concentration in the aqueous solution. The dehydrating agent will be explained in the section on the desired dehydration step.

[0042] When the superabsorbent polymer has been dehydrated with a dehydrating agent, the aqueous solution preferably has a solid content concentration of 4.0% by mass or less, more preferably 3.5% by mass or less, and even more preferably 3.0% by mass or less. Furthermore, the aqueous solution preferably has a solid content concentration of 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more. This allows for efficient decomposition and removal of the superabsorbent polymer while maintaining a high concentration in the aqueous solution. The above solid content concentration is particularly preferable when the aqueous solution is continuously stirred.

[0043] If the superabsorbent polymer has not been dehydrated by a dehydrating agent, the aqueous solution preferably has a solid content concentration of 0.1% by mass or less, more preferably 0.075% by mass or less, and even more preferably 0.05% by mass or less. Furthermore, the aqueous solution preferably has a solid content concentration of 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.03% by mass or more. This allows for efficient decomposition and removal of the superabsorbent polymer. The above solid content concentrations are particularly preferable when the aqueous solution is continuously stirred.

[0044] In this specification, the solid content concentration is calculated by subtracting the moisture content from 100. The moisture content is measured using Kett's FD-720 infrared moisture meter. Specifically, approximately 5g of the sample is placed in the FD-720's sample dish, the temperature is set to 150°C, and the automatic stop mode is selected to measure the moisture content of the sample.

[0045] The ultraviolet light used in the ultraviolet treatment step is not particularly limited as long as it includes wavelengths of 380 nm or less, preferably 290 nm or less, more preferably 260 nm or less, and even more preferably 200 nm or less. Furthermore, the ultraviolet light preferably includes wavelengths of 100 nm or more, more preferably 150 nm or more, and even more preferably 160 nm or more. This allows for the decomposition of superabsorbent polymers with high energy efficiency while suppressing the deterioration of recycled pulp fibers.

[0046] Examples of ultraviolet light sources include low-pressure mercury lamps, high-pressure mercury lamps, and deep ultraviolet LEDs. An example of a product name for an ultraviolet light source is the PAQ-15ESET (processing capacity 8 liters / minute) deep ultraviolet light generator / UV-LED irradiated water sterilization module manufactured by Nikkiso Giken Co., Ltd.

[0047] The above ultraviolet treatment step can be carried out by irradiating an aqueous solution containing a mixture of pulp fibers and a superabsorbent polymer with ultraviolet light, for example, from within the aqueous solution or from outside the aqueous solution (from the air), while stirring.

[0048] <Recycled pulp fiber recovery step> In the recycled pulp fiber recovery step, recycled pulp fibers are recovered. In the recycled pulp fiber recovery step, for example, recycled pulp fibers can be recovered from the aqueous solution using, for example, a screen having multiple openings.

[0049] The recycled pulp fibers described above preferably have a carboxyl group content of 0.075 mmol / g or less, more preferably 0.070 mmol / g or less, even more preferably 0.065 mmol / g or less, and even more preferably 0.060 mmol / g or less. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0050] The recycled pulp fibers described above have an increase in carboxyl groups of preferably 0.035 mmol / g or less, more preferably 0.030 mmol / g or less, even more preferably 0.025 mmol / g or less, and even more preferably 0.020 mmol / g or less, relative to the pulp fibers before the UV treatment step. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0051] In this specification, the carboxyl group content: C (mmol / g) of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows. (1) Add approximately 0.4 g of pulp fiber to a container to which 170 mL of deionized water has been added, and disperse the pulp fiber in the deionized water. (2) Add 10 mL of 0.01 M NaCl to the container. (3) Add 0.1M HCl to the container and adjust the pH to 2.8.

[0052] (4) Add 0.05 M NaOH to the container at a rate of 0.1 mL / min to 0.2 mL / min until the pH reaches 11, and monitor the electrical conductivity of the contents of the container. (5) Plot the amount of 0.05M NaOH added on the X-axis and the electrical conductivity on the Y-axis, and determine the amount of 0.05M NaOH added (V(mL)) at which the electrical conductivity becomes constant. (6) The pulp fibers are recovered by filtration, and their oven-dry mass: m1 (g) is measured.

[0053] (7) The amount of carboxyl groups in pulp fibers: C (mmol / g) is given by the following formula: C(mmol / g) = (V × 0.05 / 1000) / m1 It is calculated by [method]. pH will be measured using a pHashion pH meter, model C-62, manufactured by AS ONE Corporation. Electrical conductivity will be measured using a portable electrical conductivity meter (CM-31P type), manufactured by Toa DKK Corporation.

[0054] The recycled pulp fibers described above preferably have a degree of polymerization of 300 or more, more preferably 350 or more, even more preferably 400 or more, even more preferably 450 or more, and even more preferably 500 or more. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0055] The recycled pulp fibers described above have a degree of polymerization reduction of preferably 400 or less, more preferably 300 or less, even more preferably 200 or less, and even more preferably 150 or less, relative to the pulp fibers before the UV treatment step. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0056] In this specification, the degree of polymerization (DP) of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows. (1) According to "6.4.1 Viscosity ratio" of "Dilute cellulose solution - Method for determining the intrinsic viscosity number - Copper ethylenediamine method" specified in JIS 8215:1998, the viscosity ratio η of pulp fibers r Measure (=η / η0).

[0057] (2) The degree of polymerization (DP) of the pulp fibers is calculated using the following formula. Specific viscosity:η sp =η r -1 Intrinsic viscosity:[η]=ηsp / (100 × c(1 + 0.28η) sp )) Degree of polymerization: DP=175×[η] The above formula is based on the description on page 101 of the Wood Science Experiment Manual (edited by the Japan Wood Research Society, Bun'eido Publishing, 2000), where c represents the cellulose concentration (g / mL).

[0058] The above recycled pulp fibers preferably have an alkali solubility of 20% by mass or less, more preferably 17% by mass or less, even more preferably 14% by mass or less, and even more preferably 12% by mass or less. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0059] The recycled pulp fibers described above have an increase in alkali solubility of preferably 16% by mass or less, more preferably 13% by mass or less, even more preferably 10% by mass or less, and even more preferably 7% by mass or less, relative to the pulp fibers before the ultraviolet treatment step. As a result, the recycled pulp fibers undergo little deterioration and have high utility value in applications that utilize the properties of pulp fibers.

[0060] In this specification, the alkali solubility of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows. (1) Immerse a pulp fiber (approximately 1.0 g) with a mass of m² (g) in 50 mL of 5 M sodium hydroxide solution and let it stand for 1 hour. (2) The sodium hydroxide aqueous solution is subjected to a centrifuge to separate the supernatant from the precipitate, and the precipitate is obtained by filtration, neutralized, and then dried at 70°C. (3) Allow the precipitate to dry completely and measure its dry mass: m3 (g). (4) Alkali solubility: S (mass%) is given by the following formula: S(mass%)=100×(m2-m3) / m2 It is calculated by [method].

[0061] In the manufacturing method described herein, in the recycled pulp fiber recovery step, in addition to the recycled pulp fibers, the oxidatively decomposed superabsorbent polymer can be further recovered. This makes it possible to recycle the oxidatively decomposed superabsorbent polymer, thereby contributing to the achievement of the Sustainable Development Goals (SDGs). This manufacturing method is also referred to as "a method for producing (recovering) recycled pulp fibers and oxidatively decomposed superabsorbent polymers from a mixture containing pulp fibers and superabsorbent polymers obtained from sanitary products."

[0062] The recovery of the oxidatively decomposed superabsorbent polymer can be carried out by methods known in the art, for example, by solid-liquid separation of an aqueous solution that has undergone an ultraviolet treatment step, separating it into a solid component containing recycled pulp fibers and a liquid component containing the oxidatively decomposed superabsorbent polymer. Furthermore, methods for recovering the oxidatively decomposed superabsorbent polymer from a liquid component containing the oxidatively decomposed superabsorbent polymer include evaporation of water, filtration, and water extraction.

[0063] Furthermore, when recovering oxidatively decomposed superabsorbent polymers from an aqueous solution that has undergone a UV treatment step, or from a liquid component containing oxidatively decomposed superabsorbent polymers, the oxidatively decomposed superabsorbent polymers can also be recovered according to their molecular weight. This allows the oxidatively decomposed superabsorbent polymers to be recycled for appropriate applications according to their molecular weight.

[0064] For recovery according to molecular weight, known techniques can be employed without restriction, such as recovery using a microfiltration membrane. Other methods for recovery according to molecular weight include fractional precipitation (e.g., non-solvent addition method), fractional dissolution (e.g., column method), gel permeation chromatography (GPC), dissolution metric method, ultracentrifugation, adsorption method, molecular distillation, diffusion method, and thermal diffusion method. The above molecular weights include number-average molecular weight and weight-average molecular weight.

[0065] The oxidatively decomposed superabsorbent polymer can be reused, for example, in adhesive applications, coating applications, clothing finishing applications, water treatment applications, corrosion inhibition applications, or other superabsorbent polymer applications. Examples of the above-mentioned adhesive applications include adhesives for bonding plies such as paper, paper towels, and toilet paper. Furthermore, examples of the above-mentioned adhesive applications include adhesives for bonding paper cores to paper, paper towels, and toilet paper.

[0066] One example of the coating applications mentioned above is the raw material for coatings. One application of the superabsorbent polymer described above is to repolymerize the oxidatively decomposed superabsorbent polymer with other acrylic monomers, acrylic monomers containing hydroxyl groups, etc., to form a superabsorbent polymer. Furthermore, the superabsorbent polymer formed in the above application can be used in fields in which superabsorbent polymers are normally used, such as absorbent articles.

[0067] The recovery and reuse of oxidatively decomposed superabsorbent polymers are disclosed in US2021 / 053028A, WO2021 / 042118A, WO2021 / 257431A, WO2021 / 257432A, WO2022 / 081523A, WO2022 / 081451A, WO2022 / 093672A, etc.

[0068] <Dehydration Step> The manufacturing method according to this disclosure may optionally further include a dehydration step of dehydrating the superabsorbent polymer before the ultraviolet treatment step. This reduces the amount of waste contained in the superabsorbent polymer and lowers the viscosity of the superabsorbent polymer, and consequently the viscosity of the aqueous solution containing the pulp fibers and the superabsorbent polymer, allowing the ultraviolet treatment step to be carried out in an aqueous solution with a low water content (high solid content).

[0069] The dehydration step can be carried out, for example, by adding a dehydrating agent to an aqueous solution containing a mixture of pulp fibers and a superabsorbent polymer, or by immersing the pulp fibers and superabsorbent polymer (or the sanitary product itself) in an aqueous solution containing a dehydrating agent.

[0070] Examples of the above-mentioned dehydrating agents include acids (e.g., inorganic acids and organic acids), lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, and aluminum chloride. Acids are preferred because they do not easily leave ash residue on recycled pulp fibers. When an acid is used as the dehydrating agent, the aqueous solution preferably has a pH of 2.5 or less, and more preferably 1.3 to 2.4. This sufficiently reduces the water absorption capacity of the superabsorbent polymer, reduces the risk of equipment corrosion, and reduces the need for large amounts of alkaline chemicals in the neutralization treatment during wastewater treatment.

[0071] Examples of the inorganic acids mentioned above include sulfuric acid, hydrochloric acid, and nitric acid, but sulfuric acid is preferred from the viewpoint of not containing chlorine and cost. Examples of the organic acids mentioned above include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, and ascorbic acid, but acids that can form complexes with metal ions contained in excrement, such as hydroxycarbonate organic acids like citric acid, tartaric acid, and gluconic acid, are particularly preferred. Calcium ions are an example of metal ions contained in excrement. This is because the chelating effect of acids that can form complexes with metal ions contained in excrement traps and removes metal ions in the excrement. Furthermore, citric acid can be expected to have a high dirt removal effect due to its cleaning effect.

[0072] In the dehydration step, the superabsorbent polymer is dehydrated so that it has a water absorption ratio of preferably 50 times or less, more preferably 30 times or less, even more preferably 25 times or less, and even more preferably 20 times or less, and preferably 1 time or more, more preferably 2 times or more, even more preferably 3 times or more, and even more preferably 4 times or more. By doing so, the viscosity of the aqueous solution can be maintained within a predetermined range, and the superabsorbent polymer can be efficiently decomposed.

[0073] The above water absorption ratio is measured as follows: (1) Place the superabsorbent polymer in a mesh and suspend it for 5 minutes to remove any moisture adhering to its surface, then measure its pre-drying mass: m4(g). (2) The superabsorbent polymer is dried at 120°C for 10 minutes, and its mass after drying: m5 (g) is measured. (3) The water absorption ratio (g / g) is given by the following formula: Water absorption capacity (g / g)=100×m4 / m5 It is calculated by [method].

[0074] Furthermore, the optional dehydration step can be performed simultaneously with the ultraviolet treatment step. This allows for efficient implementation of the method described herein.

[0075] [Method for evaluating the cleanliness of recycled pulp fibers] The method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers, as disclosed herein, includes the following steps: • A pre-treatment mass measurement step (hereinafter sometimes referred to as the "pre-treatment mass measurement step") that measures the dry mass of recycled pulp fibers to be evaluated for cleanliness, recovered from the above-mentioned hygiene products. - A UV treatment step (hereinafter sometimes referred to as the "UV treatment step") is performed to disperse the recycled pulp fibers whose cleanliness should be evaluated in water, irradiate the recycled pulp fibers whose cleanliness should be evaluated with ultraviolet light containing wavelengths of 380 nm or less, and form the recycled pulp fibers after UV irradiation. • A post-treatment mass measurement step (hereinafter sometimes referred to as the "post-treatment mass measurement step") which measures the dry mass of the recycled pulp fibers after the above-mentioned ultraviolet irradiation.

[0076] <Pre-processing mass measurement step> In the pre-processing mass measurement step, the dry mass of recycled pulp fibers recovered from sanitary products, whose cleanliness should be evaluated, is measured. The dry mass is measured after drying the recycled pulp fibers, whose cleanliness is to be evaluated, at 120°C for 10 minutes.

[0077] <UV treatment step> The "ultraviolet treatment step" in the cleanliness evaluation method relating to this disclosure is the same as the "ultraviolet treatment step" in the method for manufacturing recycled pulp fibers, so its explanation is omitted.

[0078] <Post-processing mass measurement step> In the post-processing mass measurement step, the dry mass of the recycled pulp fibers after UV irradiation is measured. The dry mass is measured after drying the recycled pulp fibers at 120°C for 10 minutes following UV irradiation.

[0079] By comparing the dry mass of recycled pulp fibers whose cleanliness should be evaluated with the dry mass of recycled pulp fibers after UV irradiation, it is possible to determine the extent to which the recycled pulp fibers whose cleanliness should be evaluated contained substances that are decomposed by a given amount of UV light, such as superabsorbent polymers. [Examples]

[0080] The following examples illustrate this disclosure, but this disclosure is not limited to these examples. [Example 1] A mixture of 1.0 g of pulp fiber (coniferous pulp fiber) and 0.5 g of superabsorbent polymer (Sumitomo Seika Co., Ltd., AquaKeep®, SA60S) was placed in a mesh bag (25 cm square, NBC Mesh Tech Co., Ltd., N-No.250HD). The mixture, along with the mesh bag, was immersed in 80 mL of physiological saline for 15 minutes to allow the superabsorbent polymer (SAP) to absorb the saline solution. Next, the mixture was immersed in a pH 2.0 aqueous sulfuric acid solution containing sulfuric acid as a dehydrating agent for 15 minutes to dehydrate the superabsorbent polymer and form mixture No. 1. The pulp fibers and superabsorbent polymer placed in the mesh bag were dried at 120°C for 10 minutes.

[0081] Mixture No. 1 in the mesh bag is placed under a low-pressure mercury lamp (manufactured by Sen Special Light Source Co., Ltd., UVL20PH-6, discharge tube power: 0.05W / cm²). 2 The mixture was placed in a photochemical reactor (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd., and 600 mL of a dispersed aqueous solution of pulp fiber and superabsorbent polymer with a solid content concentration of 0.25% by mass was formed using deionized water. The 600 mL dispersed aqueous solution was continuously stirred with a stirrer. Note that the container capacity of the GT500 refers to the minimum volume of contents.

[0082] Ultraviolet light was irradiated onto a dispersed aqueous solution from a low-pressure mercury lamp for 30 minutes to decompose the superabsorbent polymer. Next, the contents of the photochemical reactor were filtered, and the filter was washed with deionized water.

[0083] The filtrate, washed with deionized water, was dried at 120°C for 10 minutes, and the dry mass of the dried filtrate (m6 g) was measured. The percentage of remaining filtrate, R (mass %), is given by the following formula: R (mass%)=100×m6 / 1.5 It was calculated using the method described below. The results are shown in Table 1.

[0084] [Example 2] The superabsorbent polymer was decomposed in the same manner as in Example 1, except that the superabsorbent polymer was not dehydrated using a dehydrating agent. The remaining percentage of the filtrate is shown in Table 1.

[0085] [Reference example 1] The remaining percentage of the filtered material was measured in the same manner as in Example 1, except that only 1.0 g of pulp fiber (without 0.5 g of superabsorbent polymer) was placed in a mesh bag. The results are shown in Table 1. [Reference example 2] The remaining percentage of the filtered material was measured in the same manner as in Example 1, except that only 0.5 g of superabsorbent polymer (without 1.0 g of pulp fiber) was placed in a mesh bag. The results are shown in Table 1.

[0086] [Comparative Example 1] Mixture No. 1 was formed in the same manner as in Example 1. Mixture No. 1 in a mesh container and deionized water were added to a 2L ozone gas exposure tank to form 600mL of a dispersed aqueous solution of pulp fibers and superabsorbent polymer with a solid content concentration of 0.25% by mass. The ozone concentration was 50g / m³ from an ozone generator (Ecodesign Co., Ltd., ozone gas exposure tester: ED-OWX-2). 3 Furthermore, an ozone-containing gas (with dry air as the only gas) adjusted to a flow rate of 1 L / min was blown into the dispersed aqueous solution in the ozone gas exposure tank for 30 minutes to decompose the superabsorbent polymer. The remaining percentage of the filtered material is shown in Table 1.

[0087] [Comparative Example 2] The superabsorbent polymer was decomposed in the same manner as in Comparative Example 1, except that it was not dehydrated using a dehydrating agent. The remaining percentage of the filtered material is shown in Table 1.

[0088] [Reference example 3] The remaining percentage of the filtered material was measured in the same manner as in Comparative Example 1, except that only 1.0 g of pulp fiber (without 0.5 g of superabsorbent polymer) was placed in a mesh bag. The results are shown in Table 1. [Reference example 4] The residual rate of the filtrate was measured in the same manner as in Comparative Example 1, except that only 0.5 g of the superabsorbent polymer (excluding 1.0 g of pulp fibers) was placed in the mesh bag. The results are shown in Table 1.

[0089] [Table 1]

[0090] From Examples 1 and 2, and Reference Examples 1 and 2, it can be seen that when UV is used, all of the superabsorbent polymer is decomposed and all of the pulp fibers remain. On the other hand, from Reference Examples 3 and 4, when ozone is used, not all of the superabsorbent polymer is decomposed and some of the superabsorbent polymer remains, so it can be seen that the residual rate of the filtrate in Comparative Examples 1 and 2 is high.

[0091] [Example 3] Pulp fibers (softwood pulp fibers) were dispersed in deionized water to form 600 mL of an aqueous dispersion solution of 0.25% by mass of pulp fibers. 600 mL of the above aqueous dispersion solution was filled into a photochemical reaction apparatus (GT500, container capacity: 50 mL) manufactured by Global Top Chemical Co., Ltd. equipped with a low-pressure mercury lamp (manufactured by SEN Special Light Source Co., Ltd., UVL20PH-6, arc tube power: 0.05 W / cm 2 ), and the dispersion aqueous solution was continuously stirred with a stirrer. Ultraviolet rays were irradiated from the low-pressure mercury lamp to the above aqueous dispersion solution for a predetermined time, and the amount of carboxyl groups, degree of polymerization, and alkali dissolution rate at the predetermined time were measured. The results are shown in Tables 2, 3, and 4.

[0092] [Example 4] The amount of carboxyl groups, degree of polymerization, and alkali dissolution rate at a predetermined time were measured in the same manner as in Example 3, except that the low-pressure mercury lamp was changed to a high-pressure mercury lamp (manufactured by SEN Special Light Source Co., Ltd., HL100GL-1, arc tube power: 8 W / cm 2 ). The results are shown in Tables 2, 3, and 4.

[0093] [Comparative Example 3] Pulp fibers (coniferous pulp fibers) and deionized water were added to a 2L ozone gas exposure tank to form 600mL of a dispersed aqueous solution of pulp fibers with a solid content concentration of 0.25% by mass. The ozone concentration was 50g / m³ from an ozone generator (Ecodesign Co., Ltd., ozone gas exposure tester: ED-OWX-2). 3 Furthermore, an ozone-containing gas (with dry air as the only gas other than ozone), adjusted to a flow rate of 1 L / min, was blown into the above-mentioned dispersed aqueous solution in an ozone gas exposure tank for 30 minutes. The ozone concentration in the dispersed aqueous solution was approximately 6 ppm. The amount of carboxyl groups, degree of polymerization, and alkali solubility were tracked at a predetermined time. The results are shown in Tables 2, 3, and 4.

[0094] [Table 2]

[0095] [Table 3]

[0096] [Table 4]

[0097] Table 2 shows that when pulp fibers are irradiated with ultraviolet light from low-pressure and high-pressure mercury lamps, the amount of carboxyl groups in the pulp fibers does not increase as much as when they are exposed to ozone, meaning that the pulp fibers are less likely to be oxidized. Furthermore, Table 3 shows that when pulp fibers are irradiated with ultraviolet light from low-pressure and high-pressure mercury lamps, the degree of polymerization of the pulp is less likely to decrease compared to when the pulp fibers are exposed to ozone.

[0098] Furthermore, Table 4 shows that when pulp fibers are irradiated with ultraviolet light from low-pressure and high-pressure mercury lamps, the alkali solubility rate does not increase as much compared to when pulp fibers are exposed to ozone. Note that the pulp fibers in Comparative Example 3 showed discoloration in the oven-dried precipitate.

[0099] Combining Tables 2-4 with Table 1 reveals that UV light is superior to ozone in its ability to decompose superabsorbent polymers and is less likely to alter pulp fibers.

Claims

1. A method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, A UV treatment step involves irradiating an aqueous solution containing the mixture with ultraviolet light having a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer, dissolving the oxidatively decomposed superabsorbent polymer in the aqueous solution, and forming the recycled pulp fibers. A recycled pulp fiber recovery step for recovering the recycled pulp fibers, The method, characterized by including the following:

2. The method according to claim 1, wherein the ultraviolet light includes wavelengths of 290 nm or less.

3. The method according to claim 1, wherein the superabsorbent polymer in the ultraviolet treatment step is not dehydrated.

4. The method according to claim 3, wherein in the ultraviolet treatment step, the aqueous solution having a solid content concentration of 0.01 to 0.1% by mass is stirred.

5. The method according to claim 1, wherein the superabsorbent polymer in the ultraviolet treatment step is dehydrated.

6. The method according to claim 5, wherein in the ultraviolet treatment step, the aqueous solution having a solid content concentration of 0.1 to 4.0% by mass is stirred.

7. The method according to claim 5, further comprising a dehydration step of dehydrating the superabsorbent polymer before the ultraviolet treatment step.

8. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have a carboxyl group content of 0.075 mmol / g or less.

9. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have an increase of 0.035 mmol / g or less of carboxyl groups based on the pulp fibers.

10. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have a degree of polymerization of 300 or more.

11. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have a reduction in the degree of polymerization of 400 or less, based on the pulp fibers.

12. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have an alkali solubility of 20% by mass or less.

13. The method according to any one of claims 1 to 7, wherein the recycled pulp fibers have an increase in alkali solubility of 16% by mass or less, based on the pulp fibers.

14. The method according to any one of claims 1 to 7, wherein in the recycled pulp fiber recovery step, the superabsorbent polymer that has been oxidatively decomposed is further recovered.

15. The method according to claim 14, wherein in the recycled pulp fiber recovery step, the oxidatively decomposed superabsorbent polymer is further recovered by solid-liquid separation of the aqueous solution containing the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer.

16. The method according to claim 14, wherein the superabsorbent polymer that has been oxidatively decomposed from the aqueous solution is recovered according to its molecular weight.

17. The method according to claim 14, wherein the oxidatively decomposed superabsorbent polymer is reused for adhesive applications, coating applications, garment finishing applications, water treatment applications, corrosion inhibition applications, or superabsorbent polymer applications.

18. A method for decomposing a superabsorbent polymer in an aqueous solution containing pulp fibers obtained from sanitary products and a superabsorbent polymer, A UV treatment step involves irradiating the aqueous solution containing the pulp fibers and the superabsorbent polymer with ultraviolet light including a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer and dissolve the oxidatively decomposed superabsorbent polymer in the aqueous solution. The method, characterized by including the following:

19. A method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers, A pre-treatment mass measurement step for measuring the dry mass of recycled pulp fibers to be evaluated for cleanliness, recovered from the aforementioned sanitary products. A UV treatment step involves irradiating an aqueous solution containing recycled pulp fibers whose cleanliness is to be evaluated with ultraviolet light containing a wavelength of 380 nm or less, thereby forming recycled pulp fibers after UV irradiation. A post-treatment mass measurement step in which the dry mass of the recycled pulp fibers after ultraviolet irradiation is measured, The method, characterized by including the following:

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