Method for producing recycled pulp fibers, method for degrading super absorbent polymer, and method for evaluating cleanliness degree 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

Method for producing recycled pulp fiber, method for decomposing superabsorbent polymer, and method for evaluating the cleanliness of recycled pulp fiber

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

[0002] Studies are being conducted on recovering recycled pulp fibers from used sanitary goods. For example, Patent Document 1 describes a method for recovering pulp fibers from used sanitary goods containing pulp fibers and a polymer absorbent, the method comprising the steps of immersing the used sanitary goods in ozone water to decompose the polymer absorbent, discharging the ozone water in which the decomposed polymer absorbent has been dissolved to obtain a residue of the sanitary goods from which the polymer absorbent has been removed, and stirring the residue of the sanitary goods from which the polymer absorbent has been removed in an aqueous solution containing a disinfectant or in water to clean the residue of the sanitary goods and decompose it into its constituent elements.

[0003] JP 2014-217835 A

[0004] The method described in Patent Document 1 involves decomposing a polymeric absorbent using ozone water, which poses problems such as the need for a large ozone generator for generating ozone in order to carry out the method. Furthermore, recycled pulp fibers obtained by treatment with ozone water tend to be altered to a certain extent from the original pulp fibers, such as by reducing the amount of lignin and hemicellulose and lowering the molecular weight. While reducing the amount of lignin and hemicellulose and lowering the molecular weight of recycled pulp fibers is desirable depending on the application of the recycled pulp fibers, it may also be preferable for the recycled pulp fibers not to be altered from the original pulp fibers depending on the application of the recycled pulp fibers.

[0005] Therefore, the present disclosure aims to provide a method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and superabsorbent polymers, which method allows for easy decomposition and removal of the superabsorbent polymers and efficiently forms recycled pulp fibers that are resistant to deterioration.

[0006] The present inventors have discovered a method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, the method comprising: an ultraviolet treatment step of 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; and a recycled pulp fiber recovery step of recovering the recycled pulp fibers.

[0007] The method of the present disclosure for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer can easily decompose and remove the superabsorbent polymer, and can efficiently form recycled pulp fibers that are resistant to deterioration.

[0008] Specifically, the present 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, the method comprising: an ultraviolet treatment step of 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; and a recycled pulp fiber recovery step of recovering the recycled pulp fibers.

[0009] In the above method, the superabsorbent polymer is decomposed using specific ultraviolet rays in the ultraviolet treatment step. The ultraviolet rays decompose water to generate hydroxyl radicals, which oxidatively decompose the superabsorbent polymer, dissolving the oxidatively decomposed superabsorbent polymer in an aqueous solution. Because the ultraviolet generator that generates ultraviolet rays can be made smaller than the ozone generator that generates ozone, the system related to the recycled pulp fiber manufacturing method can be made compact and the superabsorbent polymer can be easily decomposed.

[0010] Furthermore, even if ozone generation from the ozone generator is stopped, ozone remains in the atmosphere (e.g., in water) for a certain period of time, so it takes a certain period of time for it to become harmless, and in order to recover recycled pulp fibers, it is necessary to either perform a treatment to render the ozone harmless (remove the ozone) or wait for a certain period of time.On the other hand, ultraviolet rays disappear and become harmless immediately when irradiation from the ultraviolet generator is stopped, so the recycled pulp fiber recovery step following the ultraviolet treatment step can be carried out quickly (efficiently).

[0011] Furthermore, the inventors of the present application have found that ultraviolet light rapidly decomposes superabsorbent polymers while not degrading pulp fibers. This makes the resulting recycled pulp fibers highly useful in applications that utilize the properties of pulp fibers. As a result, the method for producing recycled pulp fibers according to the present disclosure can easily decompose and remove superabsorbent polymers, while efficiently producing recycled pulp fibers that are resistant to degradation. Ultimately, this method can contribute to achieving the Sustainable Development Goals (SDGs).

[0012] [Aspect 2] The method according to Aspect 1, wherein the ultraviolet light has a wavelength of 290 nm or less. In this method, since the ultraviolet light has a predetermined wavelength, recycled pulp fibers that are resistant to deterioration can be efficiently formed.

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

[0014] Generally, superabsorbent polymers absorb water in an aqueous solution, swell, and increase the viscosity of the solution. Therefore, in order to decompose a superabsorbent polymer using ozone, it is common to dehydrate the superabsorbent polymer using a dehydrating agent such as an acid or a polyvalent metal salt in order to allow the ozone to penetrate into the aqueous solution. In the above method, the superabsorbent polymer is decomposed using a specific ultraviolet ray, so that the superabsorbent polymer can be decomposed and removed even if it is not dehydrated. As a result, the dehydration step of 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 Aspect 3, wherein the aqueous solution having a solids concentration of 0.01 to 0.1% by mass is stirred in the ultraviolet treatment step. In the method, the aqueous solution having a predetermined solids concentration is stirred in the ultraviolet treatment step. 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 is dehydrated in the ultraviolet treatment step. In this method, since the superabsorbent polymer is dehydrated in the ultraviolet treatment step, 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 the aqueous solution having a solids concentration of 0.1 to 4.0% by mass is stirred in the ultraviolet treatment step. In the method, the aqueous solution having a predetermined solids concentration is stirred in the ultraviolet treatment step. 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, further comprising a dehydration step of dehydrating the superabsorbent polymer before the ultraviolet treatment step. Since the method includes a predetermined dehydration step before the ultraviolet treatment step, the viscosity of the aqueous solution can be reduced, and the ultraviolet treatment step can be carried out efficiently.

[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 this method, since the recycled pulp fibers have a predetermined carboxyl group amount, deterioration of the recycled pulp fibers is minimal, and the recycled pulp fibers have high utility value in applications that utilize the properties of the pulp fibers.

[0020] [Aspect 9] The method according to any one of Aspects 1 to 8, wherein the recycled pulp fibers have an increased amount of carboxyl groups of 0.035 mmol / g or less based on the pulp fibers. In this method, the recycled pulp fibers have a predetermined increased amount of carboxyl groups based on the pulp fibers, so that the recycled pulp fibers are less likely to deteriorate and have high utility value in applications that utilize the properties of the pulp fibers.

[0021] [Aspect 10] The method according to any one of Aspects 1 to 9, wherein the recycled pulp fibers have a degree of polymerization of at least 300. In this method, since the recycled pulp fibers have a predetermined degree of polymerization, 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 the pulp fibers.

[0022] [Aspect 11] The method according to any one of Aspects 1 to 10, wherein the recycled pulp fibers have a degree of polymerization reduction of 400 or less, based on the pulp fibers. In this method, the recycled pulp fibers have a predetermined degree of polymerization reduction, based on the pulp fibers, so that the recycled pulp fibers are less likely to be altered, and the recycled pulp fibers have high utility value in applications that utilize the properties of the pulp fibers.

[0023] [Aspect 12] The method according to any one of Aspects 1 to 11, wherein the recycled pulp fibers have an alkali solubility of 20% by mass or less. In this method, the recycled pulp fibers have a predetermined alkali solubility, so that the recycled pulp fibers are less likely to be altered, and the recycled pulp fibers have high utility in applications that utilize the properties of the pulp fibers.

[0024] [Aspect 13] The method according to any one of Aspects 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 this method, the recycled pulp fibers have a predetermined increase in alkali solubility, based on the pulp fibers, so that the recycled pulp fibers are less likely to be altered and have high utility value in applications that utilize the properties of the pulp fibers.

[0025] Aspect 14: The method according to any one of Aspects 1 to 13, further comprising recovering the oxidatively decomposed superabsorbent polymer in the recycled pulp fiber recovery step. In the method, both the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer are recovered in the recycled pulp fiber recovery step. This allows the method to easily decompose and remove the superabsorbent polymer, efficiently form recycled pulp fibers that are resistant to deterioration, and also recycle the oxidatively decomposed superabsorbent polymer, thereby contributing to the achievement of the Sustainable Development Goals (SDGs).

[0026] Aspect 15: The method according to Aspect 14, wherein the recycled pulp fiber recovery step further recovers the oxidatively decomposed superabsorbent polymer by solid-liquid separation of the aqueous solution containing the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer. In this method, the aqueous solution that has been subjected to the ultraviolet treatment step is subjected to solid-liquid separation to recover the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer, thereby enabling efficient recovery of the recycled pulp fibers and the oxidatively decomposed superabsorbent polymer.

[0027] [Aspect 16] The method according to Aspect 14 or 15, wherein the oxidatively decomposed superabsorbent polymer is recovered from the liquid component in accordance with its molecular weight. In the above method, the oxidatively decomposed superabsorbent polymer is recovered in accordance with its molecular weight, and therefore the oxidatively decomposed superabsorbent polymer can be recycled for an appropriate application in accordance with the molecular weight of the oxidatively decomposed superabsorbent polymer.

[0028] [Aspect 17] The method according to any one of Aspects 14 to 16, wherein the oxidatively decomposed superabsorbent polymer is reused for adhesive applications, coating applications, clothing finishing applications, water treatment applications, corrosion inhibition applications, or other superabsorbent polymer applications. In this method, the oxidatively decomposed superabsorbent polymer is reused for a predetermined application, which can further contribute 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 the superabsorbent polymer, the method comprising: an ultraviolet treatment step of irradiating the aqueous solution containing the pulp fibers and the superabsorbent polymer with ultraviolet light having a wavelength of 380 nm or less to oxidatively decompose the superabsorbent polymer, and dissolving the oxidatively decomposed superabsorbent polymer in the aqueous solution.

[0030] The above method has the same effect as that of the first embodiment.

[0031] [Aspect 19] A method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers, comprising: a pre-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers recovered from the sanitary products and whose cleanliness is to be evaluated; an ultraviolet treatment step of irradiating an aqueous solution containing the recycled pulp fibers whose cleanliness is to be evaluated with ultraviolet light having a wavelength of 380 nm or less to the recycled pulp fibers whose cleanliness is to be evaluated, thereby forming recycled pulp fibers after ultraviolet irradiation; and a post-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers after ultraviolet irradiation.

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

[0033] The following are detailed descriptions of (i) a method for producing recycled pulp fibers from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products (hereinafter, sometimes simply referred to as a "method for producing recycled pulp fibers"), (ii) a method for decomposing a superabsorbent polymer in an aqueous solution containing pulp fibers and a superabsorbent polymer obtained from sanitary products (hereinafter, sometimes simply referred to as a "method for decomposing a superabsorbent polymer"), and (iii) a method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and a superabsorbent polymer (hereinafter, sometimes simply referred to as a "method for evaluating the cleanliness of recycled pulp fibers" or "method for evaluating cleanliness"). The method for decomposing a superabsorbent polymer will be described in detail below.

[0034] [Method for Producing Recycled Pulp Fiber] The method for producing recycled pulp fiber from a mixture containing pulp fiber obtained from sanitary products and a superabsorbent polymer according to the present disclosure includes the following steps: - An ultraviolet treatment step (hereinafter sometimes referred to as the "ultraviolet treatment step") in which an aqueous solution containing the mixture is irradiated with ultraviolet light having 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 fiber; - A recycled pulp fiber recovery step (hereinafter sometimes referred to as the "recycled pulp fiber recovery step") in which the recycled pulp fiber is recovered.

[0035] The method for producing recycled pulp fibers according to the present disclosure may further include the following optional steps: A dehydration step (hereinafter sometimes simply referred to as the "dehydration step") in which the superabsorbent polymer is dehydrated prior to the ultraviolet treatment step.

[0036] <Ultraviolet Ray Treatment Step> The sanitary product is not particularly limited as long as it contains pulp fibers and a highly absorbent polymer, and examples thereof include disposable diapers, urine absorption pads, sanitary napkins, bed sheets, pet sheets, etc. The sanitary product includes sanitary products that have been used by a user and have absorbed the user's excrement, sanitary products that have been used by a user but have not absorbed the user's excrement, sanitary products that have been discarded unused, defective products during manufacturing, etc.

[0037] The pulp fibers are not particularly limited as long as they are usable in sanitary products, and examples thereof include wood pulp (e.g., softwood pulp, hardwood pulp), crosslinked pulp, and non-wood pulp.

[0038] Examples of the superabsorbent polymer include those used in sanitary 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 products of sodium carboxymethyl cellulose. Examples of synthetic polymer-based superabsorbent polymers include polyacrylate-based, polysulfonate-based, maleic 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 Polymers).

[0039] The method for obtaining an aqueous solution containing a mixture containing pulp fibers and a superabsorbent polymer from the sanitary product is not particularly limited, and can be obtained, for example, by cutting a used sanitary product, obtaining a mixture containing pulp fibers and a superabsorbent polymer from the absorbent body, and dispersing the mixture in water. Alternatively, for example, a used sanitary product can be cut in water to obtain an aqueous solution containing a mixture containing pulp fibers and a superabsorbent polymer from the absorbent body.

[0040] In the aqueous solution, the superabsorbent polymer may or may not have been dehydrated using a dehydrating agent. If the superabsorbent polymer has not been dehydrated, the dehydration step of the superabsorbent polymer can be omitted, and the resulting recycled pulp fibers are less likely to contain the superabsorbent polymer and its residue, and the dehydrating agent and its residue. If the superabsorbent polymer has been 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.

[0041] When the superabsorbent polymer is dehydrated with a dehydrating agent, the superabsorbent polymer is preferably dehydrated to have a water absorption capacity of 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. When the superabsorbent polymer is dehydrated, the superabsorbent polymer is preferably dehydrated to have a water absorption capacity of 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 the superabsorbent polymer to be efficiently decomposed and removed while maintaining a high concentration in the aqueous solution. The dehydrating agent will be described in the optional dehydration step section.

[0042] When the superabsorbent polymer is dehydrated with a dehydrating agent, the aqueous solution preferably has a solids 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. The aqueous solution preferably has a solids 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 the superabsorbent polymer to be efficiently decomposed and removed while maintaining a high concentration of the aqueous solution. The solids concentration is particularly preferred when the aqueous solution is continuously stirred.

[0043] When the superabsorbent polymer is not dehydrated with a dehydrating agent, the aqueous solution preferably has a solids 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. The aqueous solution preferably has a solids 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 the superabsorbent polymer to be efficiently decomposed and removed. The above solids concentration is particularly preferred when the aqueous solution is continuously stirred.

[0044] In this specification, the solids concentration is calculated by subtracting the moisture content from 100. The moisture content is measured using an infrared moisture meter FD-720 manufactured by Kett Corporation. Specifically, approximately 5 g of sample is placed on the sample tray of the FD-720, the temperature is set to 150°C, the automatic stop mode is selected, and the moisture content of the sample is measured.

[0045] The ultraviolet light used in the ultraviolet treatment step is not particularly limited as long as it has a wavelength of 380 nm or less, and preferably includes ultraviolet light with a wavelength of 290 nm or less, more preferably 260 nm or less, and even more preferably 200 nm or less. The ultraviolet light preferably includes ultraviolet light with a wavelength of 100 nm or more, more preferably 150 nm or more, and even more preferably 160 nm or more. This allows for highly energy-efficient decomposition of the superabsorbent polymer and suppression of deterioration of the recycled pulp fibers.

[0046] Examples of sources of ultraviolet light include low-pressure mercury lamps, high-pressure mercury lamps, deep ultraviolet LEDs, etc. Examples of trade names of sources of ultraviolet light include a deep ultraviolet light generator manufactured by Nikkiso Giken Co., Ltd., UV-LED irradiation flowing water sterilization module: PAQ-15ESET (treatment capacity: 8 liters per minute).

[0047] The ultraviolet treatment step can be carried out by, for example, 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 (air), while stirring the aqueous solution.

[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, the recycled pulp fibers can be recovered from the aqueous solution using, for example, a screen having a plurality of openings.

[0049] The recycled pulp fibers 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, which minimizes deterioration of the recycled pulp fibers and makes the recycled pulp fibers highly useful in applications that utilize the properties of pulp fibers.

[0050] The recycled pulp fiber has an increase in carboxyl group content 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, based on the pulp fiber before the ultraviolet treatment step, which minimizes deterioration of the recycled pulp fiber and makes the recycled pulp fiber highly useful in applications that utilize the properties of the pulp fiber.

[0051] In this specification, the amount of carboxyl groups, C (mmol / g), of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows: (1) Approximately 0.4 g of pulp fibers is added to a container containing 170 mL of deionized water, and the pulp fibers are dispersed in the deionized water. (2) 10 mL of 0.01 M NaCl is added to the container. (3) 0.1 M HCl is added to the container to adjust the pH to 2.8.

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

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

[0054] The recycled pulp fibers have a degree of polymerization of preferably 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. This reduces deterioration of the recycled pulp fibers, making them highly useful in applications that utilize the properties of pulp fibers.

[0055] The recycled pulp fibers 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, based on the pulp fibers before the ultraviolet treatment step. This reduces deterioration of the recycled pulp fibers, making the recycled pulp fibers highly useful in applications that utilize the properties of the 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) The viscosity ratio η of the pulp fibers is determined in accordance with "6.4.1 Viscosity ratio" of "Cellulose dilute solution - Intrinsic viscosity number measurement method - Copper ethylenediamine method" specified in JIS 8215:1998. r (=η / η0) is measured.

[0057] (2) The degree of polymerization (DP) of the pulp fiber is calculated using the following formula: Specific viscosity: η sp =η r -1 Intrinsic viscosity: [η] = η sp / (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'ei-do Publishing, 2000), where c represents the cellulose concentration (g / mL).

[0058] The 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, which minimizes deterioration of the recycled pulp fibers and makes the recycled pulp fibers highly useful in applications that utilize the properties of the pulp fibers.

[0059] The recycled pulp fibers 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, based on the pulp fibers before the ultraviolet treatment step, which minimizes deterioration of the recycled pulp fibers and makes the recycled pulp fibers highly useful in applications that utilize the properties of the pulp fibers.

[0060] In this specification, the alkali solubility of pulp fiber and recycled pulp fiber (hereinafter sometimes simply referred to as "pulp fiber") is measured as follows: (1) The mass: m2 (g) of pulp fiber (approximately 1.0 g) is measured, immersed in 50 mL of 5 M aqueous sodium hydroxide solution, and allowed to stand for 1 hour. (2) The aqueous sodium hydroxide solution is centrifuged to separate the supernatant and the precipitate, and the precipitate is collected by filtration, neutralized, and then dried at 70°C. (3) The precipitate is dried completely, and its dry mass: m3 (g) is measured. (4) The alkali solubility: S (mass%) is calculated using the following formula: S (mass%) = 100 × (m2 - m3) / m2.

[0061] In the manufacturing method according to the present disclosure, in the recycled pulp fiber recovery step, in addition to the recycled pulp fibers, an oxidatively decomposed superabsorbent polymer can be further recovered. This makes it possible to recycle the oxidatively decomposed superabsorbent polymer, further 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 polymer from a mixture containing pulp fibers and a superabsorbent polymer obtained from sanitary products."

[0062] The oxidatively decomposed superabsorbent polymer can be recovered by a method known in the art, for example, by subjecting the aqueous solution that has been subjected to the ultraviolet treatment step to solid-liquid separation to separate it into a solid component containing recycled pulp fibers and a liquid component containing the oxidatively decomposed superabsorbent polymer. Methods for recovering the oxidatively decomposed superabsorbent polymer from the liquid component containing the oxidatively decomposed superabsorbent polymer include evaporation of water, filtration, and water extraction.

[0063] Furthermore, when recovering the oxidatively decomposed superabsorbent polymer from the aqueous solution that has been subjected to the ultraviolet treatment step or the liquid component containing the oxidatively decomposed superabsorbent polymer, the oxidatively decomposed superabsorbent polymer can also be recovered according to its molecular weight, thereby allowing the oxidatively decomposed superabsorbent polymer to be recycled for an appropriate application according to its molecular weight.

[0064] For the recovery according to molecular weight, known techniques can be adopted without limitation, for example, recovery using a microfiltration membrane. Further, for the recovery according to molecular weight, fractional precipitation methods (for example, non-solvent addition methods), fractional dissolution methods (for example, column methods), gel permeation chromatography (GPC), dissolution measurement methods, ultracentrifugation methods, adsorption methods, molecular distillation methods, diffusion methods, thermal diffusion methods, etc. The molecular weight may be number average molecular weight, weight average molecular weight, etc.

[0065] The oxidatively decomposed superabsorbent polymer can be reused for, for example, adhesive applications, coating applications, clothing finishing applications, water treatment applications, corrosion inhibition applications, or other superabsorbent polymer applications. Examples of such adhesive applications include adhesives for bonding plies of paper, paper towels, toilet paper, etc. Also, examples of such adhesive applications include adhesives between paper cores and paper, paper towels, toilet paper, etc.

[0066] The coating application includes a raw material for coating. The superabsorbent polymer application includes repolymerizing the oxidatively decomposed superabsorbent polymer with other acrylic monomers, acrylic monomers containing hydroxyl groups, etc. to form a superabsorbent polymer. The superabsorbent polymer formed in the superabsorbent polymer application can be used in fields where superabsorbent polymers are usually 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] The manufacturing method according to the present disclosure may further include, as an optional step, a dehydration step of dehydrating the superabsorbent polymer before the UV treatment step. This can reduce the amount of excrement contained in the superabsorbent polymer, as well as the viscosity of the superabsorbent polymer and, in turn, the viscosity of the aqueous solution containing the mixture of pulp fibers and the superabsorbent polymer, allowing the UV treatment step to be performed in a state where the aqueous solution has a low moisture content (high solids 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 the superabsorbent polymer (or the sanitary product itself) in an aqueous solution containing the dehydrating agent.

[0070] Examples of the dehydrating agent 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 are less likely to leave ash on the 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 allows the water absorption capacity of the superabsorbent polymer to be sufficiently reduced, reducing the risk of equipment corrosion and reducing the need for large amounts of alkaline chemicals for neutralization during wastewater treatment.

[0071] Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid being preferred from the standpoint of not containing chlorine and being cost-effective. Examples of the organic acid include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, and ascorbic acid, with acids capable of forming complexes with metal ions contained in excrement, such as hydroxycarbonate-based organic acids such as citric acid, tartaric acid, and gluconic acid, being particularly preferred. Examples of metal ions contained in excrement include calcium ions. This is because the chelating effect of acids capable of forming complexes with metal ions contained in excrement traps and removes the metal ions in excrement. Furthermore, citric acid is expected to have a high stain removal effect due to its cleaning effect.

[0072] In the dehydration step, the superabsorbent polymer is dehydrated so that the superabsorbent polymer has a water absorption capacity 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 water absorption capacity is measured as follows: (1) A superabsorbent polymer is placed in a mesh and hung for 5 minutes, water adhering to the surface is removed, and its mass before drying: m4 (g) is measured. (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 capacity (g / g) is calculated using the following formula: Water absorption capacity (g / g) = 100 x m4 / m5.

[0074] It should be noted that the optional dehydration step can be carried out simultaneously with the UV treatment step, thereby allowing for efficient implementation of the method according to the present disclosure.

[0075] [Method for Evaluating the Cleanliness of Recycled Pulp Fibers] A method for evaluating the cleanliness of recycled pulp fibers recovered from sanitary products containing pulp fibers and superabsorbent polymers according to the present disclosure includes the following steps: A pre-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers recovered from the sanitary products and whose cleanliness is to be evaluated (hereinafter, may be referred to as the "pre-treatment mass measurement step"), An ultraviolet treatment step of dispersing the recycled pulp fibers whose cleanliness is to be evaluated in water and irradiating the recycled pulp fibers whose cleanliness is to be evaluated with ultraviolet light having a wavelength of 380 nm or less to form recycled pulp fibers after ultraviolet irradiation (hereinafter, may be referred to as the "ultraviolet treatment step"), and A post-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers after the ultraviolet irradiation (hereinafter, may be referred to as the "post-treatment mass measurement step").

[0076] <Pre-treatment Mass Measurement Step> In the pre-treatment mass measurement step, the dry mass of the recycled pulp fibers recovered from the sanitary goods and to be evaluated for cleanliness is measured. The dry mass is measured after the recycled pulp fibers to be evaluated for cleanliness are dried at 120°C for 10 minutes.

[0077] <Ultraviolet Treatment Step> The "ultraviolet treatment step" in the cleanliness evaluation method according to the present disclosure is the same as the "ultraviolet treatment step" in the recycled pulp fiber manufacturing method, and therefore a description thereof will be omitted.

[0078] <Post-treatment mass measurement step> In the post-treatment mass measurement step, the dry mass of the recycled pulp fiber after ultraviolet irradiation is measured. The dry mass is measured after the recycled pulp fiber after ultraviolet irradiation is dried at 120°C for 10 minutes.

[0079] By comparing the dry mass of the recycled pulp fiber whose cleanliness is to be evaluated with the dry mass of the recycled pulp fiber after ultraviolet irradiation, it is possible to determine the amount of substances that are decomposed by a specified ultraviolet ray, such as superabsorbent polymers, that the recycled pulp fiber whose cleanliness is to be evaluated contains.

[0080] The present disclosure will be described below using examples, but the present disclosure is not limited to these examples. [Example 1] A mixture of 1.0 g of pulp fiber (softwood pulp fiber) and 0.5 g of superabsorbent polymer (AQUAKEEP (registered trademark), SA60S, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was placed in a mesh bag (25 cm square, N-No. 250HD, manufactured by NBC Meshtec Co., Ltd.), and the mixture, including the mesh bag, was immersed in 80 mL of physiological saline for 15 minutes to allow the superabsorbent polymer (SAP) to absorb the physiological saline. Next, the mixture was immersed in a sulfuric acid aqueous solution of pH 2.0 containing sulfuric acid as a dehydrating agent for 15 minutes to dehydrate the superabsorbent polymer, forming Mixture No. 1. The pulp fiber and superabsorbent polymer placed in the mesh bag were dried at 120°C for 10 minutes.

[0081] Mixture No. 1 in the mesh bag was heated under a low-pressure mercury lamp (UVL20PH-6, manufactured by Sen Special Light Source Co., Ltd., arc tube power: 0.05 W / cm 2 The mixture was loaded into a photochemical reactor (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd. equipped with a pulp fiber and superabsorbent polymer dispersion solution having a solids concentration of 0.25% by mass using deionized water. The 600 mL of the dispersion solution was continuously stirred with a stirrer. The container capacity of the GT500 refers to the minimum volume of the contents.

[0082] The aqueous dispersion was irradiated with ultraviolet light from a low-pressure mercury lamp for 30 minutes to decompose the superabsorbent polymer.The contents of the photochemical reactor were then filtered, and the filtrate was washed with deionized water.

[0083] The filtered product washed with deionized water was dried at 120°C for 10 minutes, and the dry mass (m6) (g) of the dried filtered product was measured. The residual rate (R) (mass%) of the filtered product was calculated using the following formula: R (mass%) = 100 × m6 / 1.5. 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 residual rate of the filtrate is shown in Table 1.

[0085] Reference Example 1: The residual rate of the filtrate was measured in the same manner as in Example 1, except that only 1.0 g of pulp fiber (not including 0.5 g of superabsorbent polymer) was placed in the mesh bag. The results are shown in Table 1. Reference Example 2: The residual rate of the filtrate was measured in the same manner as in Example 1, except that only 0.5 g of superabsorbent polymer (not including 1.0 g of pulp fiber) was placed in the mesh bag. The results are shown in Table 1.

[0086] Comparative Example 1 Mixture No. 1 was prepared in the same manner as in Example 1. Mixture No. 1 in the mesh and deionized water were placed in an ozone gas exposure tank with a volume of 2 L to prepare 600 mL of an aqueous dispersion of pulp fiber and a superabsorbent polymer with a solids concentration of 0.25% by mass. An ozone generator (Ecodesign Co., Ltd., Ozone Gas Exposure Tester: ED-OWX-2) emitted ozone at a concentration of 50 g / m3. 3 An ozone-containing gas (the gas other than ozone was dry air) adjusted to a flow rate of 1 L / min was blown into the aqueous dispersion in the ozone gas exposure tank for 30 minutes to decompose the superabsorbent polymer. The residual rate of the filtrate 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 the superabsorbent polymer was not dehydrated using a dehydrating agent. The residual rate of the filtrate is shown in Table 1.

[0088] Reference Example 3 The residual rate of the filtrate was measured in the same manner as in Comparative Example 1, except that only 1.0 g of pulp fiber (not including 0.5 g of superabsorbent polymer) was placed in the 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 superabsorbent polymer (not including 1.0 g of pulp fiber) was placed in the mesh bag. The results are shown in Table 1.

[0089]

[0090] It can be seen from Examples 1 and 2 and Reference Examples 1 and 2 that when UV was used, the superabsorbent polymer was completely decomposed and all of the pulp fibers remained. On the other hand, it can be seen from Reference Examples 3 and 4 that when ozone was used, the superabsorbent polymer was not completely decomposed and some of the superabsorbent polymer remained, resulting in a higher residual rate of the filtered matter in Comparative Examples 1 and 2.

[0091] [Example 3] Pulp fibers (softwood pulp fibers) were dispersed in deionized water to prepare 600 mL of a 0.25% by mass aqueous dispersion of pulp fibers. 600 mL of the aqueous dispersion was irradiated with a low-pressure mercury lamp (UVL20PH-6, manufactured by Sen Special Light Sources Co., Ltd., arc tube power: 0.05 W / cm). 2 The dispersion was loaded into a photochemical reaction apparatus (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd. equipped with a 1000-kJ / kg ethanol solution, and the aqueous dispersion was continuously stirred with a stirrer. The aqueous dispersion was irradiated with ultraviolet light from a low-pressure mercury lamp for a predetermined period of time, and the amount of carboxyl groups, degree of polymerization, and alkali solubility were measured at the predetermined period of time. The results are shown in Tables 2, 3, and 4.

[0092] Example 4 The low-pressure mercury lamp was replaced with a high-pressure mercury lamp (HL100GL-1, manufactured by Sen Special Light Sources Co., Ltd., arc tube power: 8 W / cm 2 The amount of carboxyl groups, the degree of polymerization, and the alkali solubility were measured at predetermined times in the same manner as in Example 3, except that the temperature was changed to 100°C. The results are shown in Tables 2, 3, and 4.

[0093] Comparative Example 3 Pulp fibers (softwood pulp fibers) and deionized water were placed in an ozone gas exposure tank with a volume of 2 L to form 600 mL of an aqueous dispersion of pulp fibers with a solid content of 0.25% by mass. An ozone generator (Ecodesign Co., Ltd., Ozone Gas Exposure Tester: ED-OWX-2) generated ozone at a concentration of 50 g / m. 3 An ozone-containing gas (the gas other than ozone was dry air) adjusted to a flow rate of 1 L / min was blown into the aqueous dispersion in the ozone gas exposure chamber for 30 minutes. The ozone concentration in the aqueous dispersion was approximately 6 ppm. The amount of carboxyl groups, degree of polymerization, and alkali solubility were monitored over a predetermined period of time. The results are shown in Tables 2, 3, and 4.

[0094]

[0095]

[0096]

[0097] Table 2 shows that when pulp fibers are irradiated with ultraviolet light from a low-pressure mercury lamp and a high-pressure mercury lamp, the amount of carboxyl groups in the pulp fibers is less likely to increase, i.e., the pulp fibers are less likely to be oxidized, compared to when the pulp fibers are exposed to ozone. Also, Table 3 shows that when pulp fibers are irradiated with ultraviolet light from a low-pressure mercury lamp and a high-pressure mercury lamp, 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 a low-pressure mercury lamp and a high-pressure mercury lamp, the alkali dissolution rate is less likely to increase than when pulp fibers are exposed to ozone.In addition, the pulp fibers of Comparative Example 3 had colored bone-dry precipitates.

[0099] Combining Tables 2 to 4 with Table 1, it can be seen that UV is superior to ozone in decomposing 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, comprising: an ultraviolet treatment step of 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 of recovering the recycled pulp fibers; The method, characterized in that it comprises:

2. The method of claim 1 , wherein the ultraviolet radiation comprises wavelengths of 290 nm or less.

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

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

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

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

7. The method of claim 5 , further comprising a dehydration step of dehydrating the superabsorbent polymer prior to 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 amount of 0.075 mmol / g or less.

9. The method of any one of claims 1 to 7, wherein the recycled pulp fibers have an increment of carboxyl groups of 0.035 mmol / g or less 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 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 of any one of claims 1 to 7, wherein the recycled pulp fibers have an increase in alkali solubility of 16% or less by weight based on the pulp fibers.

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

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 oxidatively decomposed superabsorbent polymer is recovered from the aqueous solution according to its molecular weight.

17. 15. The method of claim 14, wherein the oxidatively degraded superabsorbent polymer is reused for adhesive applications, coating applications, clothing 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 a sanitary product and the superabsorbent polymer, comprising: an ultraviolet treatment step of irradiating the aqueous solution containing the pulp fibers and the superabsorbent polymer with ultraviolet light having 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 in that it comprises:

19. 1. A method for evaluating the cleanliness of recycled pulp fibers recovered from hygiene products containing pulp fibers and superabsorbent polymers, comprising: a pre-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers recovered from the sanitary product and for which cleanliness is to be evaluated; an ultraviolet treatment step of irradiating an aqueous solution containing recycled pulp fibers whose cleanliness is to be evaluated with ultraviolet light having a wavelength of 380 nm or less to the recycled pulp fibers whose cleanliness is to be evaluated, thereby forming recycled pulp fibers after ultraviolet irradiation; a post-treatment mass measurement step of measuring the dry mass of the recycled pulp fibers after the ultraviolet irradiation; The method, characterized in that it comprises: