Method for detecting recycled water-absorbent resin

Abstract

A recycled water-absorbent resin is detected from components constituting at least a part of an object to be treated obtained by subjecting a used absorbent article to recycling treatment.SOLUTION: In the method for detecting a recycled water-absorbent resin, a recycled water-absorbent resin in a component constituting at least a part of an object to be treated obtained by subjecting a used absorbent article to a recycling treatment is detected by observing fluorescence when the component is irradiated with light.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a recycling technology for used absorbent articles. [Background technology]

[0002] Technologies for recycling used absorbent articles are known (see, for example, Patent Documents 1 and 2). When components constituting an absorbent article are separated and recycled separately, there are cases where components derived from the water-absorbent resin are mixed with components such as pulp that are not derived from the water-absorbent resin. Since the components derived from the water-absorbent resin have special properties such as swelling and water absorption, it is preferable to remove them from the components not derived from the water-absorbent resin. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-249711 [Patent Document 2] Japanese Patent Application Publication No. 2020-195994 Summary of the Invention [Problem to be solved by the invention]

[0004] However, there is no known technique for detecting components derived from a water-absorbent resin that have been mixed with components not derived from the water-absorbent resin.

[0005] An object of the present invention is to detect a recycled water-absorbent resin from components constituting at least a part of a treated material obtained by recycling used absorbent articles. [Means for solving the problem]

[0006] In a method for detecting recycled water absorbent resin according to one embodiment of the present invention, the recycled water absorbent resin in a component is detected by observing fluorescence when light is irradiated onto the component constituting at least a part of a treated material obtained by recycling a used absorbent article.

[0007] In a method for producing a recycled water absorbent resin according to one embodiment of the present invention, a component constituting at least a part of a treated material obtained by subjecting a used absorbent article to a recycling treatment is irradiated with ultraviolet light having a wavelength of 350 nm or more and 370 nm or less, and fluorescence is observed to detect the recycled water absorbent resin in the component, and the detected recycled water absorbent resin is taken out from the component. [Effects of the Invention]

[0008] According to the present invention, a recycled water-absorbent resin can be detected from components constituting at least a part of a treated material obtained by subjecting a used absorbent article to a recycling treatment. [Brief explanation of the drawings]

[0009] [Figure 1] 1A and 1B are schematic diagrams for explaining the mechanism of thermal decomposition of a polyacrylic acid-based water-absorbent resin presumed in one embodiment of the present invention, in which (A) shows an example of a polyacrylic acid-based water-absorbent resin before decomposition, and (B) shows an example of a decomposition product of the polyacrylic acid-based water-absorbent resin. [Figure 2] The images were taken of the processed object using a 10x lens, with (A) showing an image taken in bright field mode and (B) showing an image taken in DAPI mode. [Figure 3] The images were taken of the processed object using a 4x lens, where (A) shows an image taken in bright field mode, and (B) shows an image taken in DAPI mode. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, one embodiment of the present invention will be described. In this embodiment, the term "structural unit" is used to refer to both a functional group that constitutes a compound and a structural unit that constitutes a polymer.

[0011] <Recycled water-absorbent resin> The recycled water-absorbent resin according to the present embodiment is a component derived from a polyacrylic acid-based water-absorbent resin (hereinafter also referred to as "water-absorbent resin") contained in a treated material obtained by subjecting used absorbent articles to a recycling treatment. The recycling treatment includes, for example, at least one treatment selected from a heat treatment, an oxidation treatment, a treatment with an acid agent, and a treatment with an alkaline agent. Examples of the oxidation treatment include an ozone treatment using ozone gas or ozone water as an oxidizing agent, and a hydrogen peroxide treatment using hydrogen peroxide as an oxidizing agent. Examples of acid agents that can be used in the acid-agent treatment include ascorbic acid, hydrochloric acid, sulfuric acid, hypochlorous acid, sodium hypochlorite, etc. Examples of alkaline agents that can be used in the alkaline-agent treatment include sodium hydroxide, calcium hydroxide, etc. By such a recycling treatment, the decomposition of the water-absorbent resin contained in the used absorbent article proceeds, thereby producing a recycled water-absorbent resin.

[0012] In this embodiment, the absorbent article refers to an article that contains a water-absorbent resin and is capable of absorbing moisture. Examples of absorbent articles include disposable diapers, sanitary napkins, urine absorption pads, panty liners, pet toilet sheets, drip sheets, moisture absorbents, deodorizers, etc. Note that a "drip sheet" refers to a water-absorbing sheet that absorbs excess moisture (body fluids, etc.) from food ingredients such as meat and fish. Furthermore, in this embodiment, the absorbent article is preferably an absorbent hygiene product that absorbs body fluids such as urine and menstrual blood. Examples of absorbent hygiene products include disposable diapers, sanitary napkins, urine absorption pads, panty liners, etc.

[0013] The polyacrylic acid-based water-absorbent resin contained in used absorbent articles before being recycled is a crosslinked polymer having structural units derived from acrylic acid. The crosslinked polymer referred to here means a three-dimensional network structure in which polymer chains, which are the main structure of the polymer, are linked within or between polymer chains, and does not have a weight-average molecular weight. The polyacrylic acid-based water-absorbent resin according to this embodiment is at least one type of water-absorbent resin, and may contain multiple water-absorbent resins.

[0014] The polymer having acrylic acid-derived structural units may be, for example, at least one selected from a homopolymer of acrylic acid, a copolymer of acrylic acid, and salts thereof. Examples of the homopolymer of acrylic acid and salts thereof include polyacrylic acid, polyacrylic acid / polyacrylate, and polyacrylate. Examples of the copolymer of acrylic acid include, but are not limited to, poly(vinyl alcohol / acrylate) copolymer and starch-acrylate graft copolymer.

[0015] Furthermore, the "salt" constituting the water absorbent resin includes at least one salt selected from, for example, alkali metal salts (sodium salts, potassium salts, lithium salts, etc.), alkaline earth metal salts (calcium salts, magnesium salts, barium salts, etc.), ammonium salts (quaternary ammonium salts, quaternary alkylammonium salts, etc.), etc. From the viewpoint of obtaining stable water absorption, it is advantageous for the salt constituting the water absorbent resin to include an alkali metal salt, and more advantageous for it to include a sodium salt.

[0016] The decomposition mechanism of the water-absorbent resin estimated in this embodiment will be described using the schematic diagrams of Figures 1(A) and (B). In these figures, the water-absorbent resin is a crosslinked product of polyacrylic acid / sodium polyacrylate, the linear structure is a polymer chain including a polymer main chain and a crosslinked chain, "COOH" is a carboxy group derived from acrylic acid, and "COONa" is a sodium salt of a carboxy group derived from acrylic acid.

[0017] As shown in Figure 1(A), a water-absorbent resin forms a three-dimensional network structure in which multiple main chains are cross-linked by cross-linking chains containing hydrogen bonds, ester bonds, etc. It is believed that when such a water-absorbent resin is subjected to a recycling process, the molecules that form the three-dimensional network structure are cleaved. In this process, relatively weak bonds such as hydrogen bonds and ester bonds are thought to become unstable and more likely to cleave. Furthermore, when the water-absorbent resin is a copolymer of multiple types of monomers, it is thought that structural units resulting from unstable monomers are preferentially cleaved. Thus, in this step, by taking advantage of the fact that the water-absorbent resin has unstable sites, it is thought that the three-dimensional network structure can be decomposed while leaving the acrylic skeleton of the water-absorbent resin intact, producing a decomposition product such as that shown in Figure 1(B).

[0018] The recycled water-absorbent resin according to the present embodiment includes a decomposition product of the water-absorbent resin and an intermediate product in the process of decomposing the water-absorbent resin to become the decomposition product. That is, in the example shown in Figures 1(A) and 1(B), the recycled water-absorbent resin according to the present embodiment includes a decomposition product shown in Figure 1(B) and an intermediate product in the process of decomposing the water-absorbent article shown in Figure 1(A) to become the decomposition product shown in Figure 1(B). Hereinafter, the intermediate product in the process of decomposing the water-absorbent resin to become the decomposition product will also be simply referred to as an intermediate product of the water-absorbent resin.

[0019] In the recycled water absorbent resin according to the present embodiment, the structural unit derived from acrylic acid may constitute a salt. The salt may be a salt derived from the water absorbent resin before decomposition, or may be a salt derived from a treating agent used in the decomposition treatment. Specific examples of the salt include at least one salt selected from the above-mentioned alkali metal salts (sodium salt, potassium salt, lithium salt, etc.), alkaline earth metal salts (calcium salt, magnesium salt, barium salt, etc.), ammonium salts (quaternary ammonium salt, quaternary alkyl ammonium salt, etc.), etc.

[0020] The recycled water absorbent resin according to this embodiment has a cyclic structure produced in a decomposition reaction of a polyacrylic acid-based water absorbent resin. The recycled water absorbent resin having a cyclic structure is, for example, at least one selected from a compound (including a polymer) having a functional group with the cyclic structure as a skeleton, and a polymer containing a cyclic structure in the polymer chain. Among these, the recycled water absorbent resin having a cyclic structure can be produced by a ring-closing reaction between functional groups of adjacent structural units of the water absorbent resin, or an addition polymerization reaction to an unsaturated bond, which will be described later.

[0021] Furthermore, the cyclic structure typically has an aromatic ring, such as a benzene ring, which is a cyclic hydrocarbon, or a heteroaromatic ring containing oxygen. It is believed that a recycled water-absorbent resin having an aromatic ring is produced by a reaction involving ring closure, such as dehydration condensation or dehydrogenation, between carboxyl groups of adjacent structural units derived from acrylic acid during the decomposition process of the water-absorbent resin. Chemical formula 1 shows an example of a cyclic structure that can be contained in the polymer chain of the recycled water-absorbent resin according to this embodiment. The structural formula of Chemical formula 1 shows an example in which an aromatic ring (benzene ring) is contained between two structural units derived from acrylic acid. However, examples of the cyclic structure are not limited to this.

[0022] [ka]

[0023] The inventors of the present application have discovered that fluorescence is observed when such a recycled water absorbent resin is irradiated with light. This is thought to be due to the fact that the recycled water absorbent resin has an unsaturated bond in the vicinity of the cyclic structure (for example, a position forming a π-conjugated system with the cyclic structure), particularly at the position between the carbon adjacent to the cyclic structure and the carbon adjacent to the unsaturated bond, thereby enhancing the light absorption characteristics.

[0024] The recycled water-absorbent resin according to the present embodiment has the structure described above, and thus can be used as a functional material having a function other than water absorption. For example, the recycled water-absorbent resin according to the present embodiment can be used as a fluorescent agent. Examples of materials that can contain the recycled water-absorbent resin according to the present embodiment as a fluorescent agent include paints, paper, fibers, detergents, cleaning aids, etc. Furthermore, examples of uses of the recycled water-absorbent resin according to the present embodiment as a fluorescent agent include gardening, interior decoration, air fresheners, deodorizers, and ice packs.

[0025] Furthermore, the recycled water absorbent resin according to the present embodiment can be used as a dispersant, an ultraviolet absorbent, etc. Examples of materials that can contain the recycled water absorbent resin according to the present embodiment as a dispersant include paints, paper, detergents, cleaning aids, deodorizers and / or deodorants, electronic substrates, electronic components, batteries, rechargeable batteries, cement, concrete, asphalt, foundry sand, runners, cosmetics, etc. Examples of materials that can contain the recycled water absorbent resin according to the present embodiment as an ultraviolet absorbent include paints, paper, fibers, cosmetics, topical skin preparations, plastic products, etc.

[0026] Note that the decomposition product of the recycled water absorbent resin is water-soluble, while the intermediate product of the recycled water absorbent resin is water-insoluble. Therefore, by filtering a liquid in which the recycled water absorbent resin is dissolved in water, the decomposition product and the intermediate product of the recycled water absorbent resin can be separated. In other words, the decomposition product of the recycled water absorbent resin passes through the filter paper as a solute in the aqueous solution, and the intermediate product of the recycled water absorbent resin remains on the filter paper as a residue.

[0027] <Detection method for recycled water absorbent resin> In addition to recycled water-absorbent resin, used absorbent articles contain components derived from the pulp and various fiber sheets that make up the absorbent articles. When used absorbent articles are separated into these components and recycled separately, recycled water-absorbent resin may become contaminated with other components. Because recycled water-absorbent resin has unique properties, such as swelling and water absorption, it is preferable to remove the contaminated recycled water-absorbent resin in order to effectively utilize the other components. In particular, for sanitary products, which are a type of sanitary product, Japan has established quality standards for sanitary products as set forth in the Sanitary Treatment Product Manufacturing and Sales Approval Standards (Yakusei-Hatsu 0628-4, issued June 28, 2021). Article 2.4, Law 1, states, "When this product (excluding materials protecting the fixative) is irradiated with ultraviolet light (wavelength: 350-370 nm) in a dark place, no fluorescence is observed, or if fluorescence is observed, the fluorescence is not stronger than that of the standard fluorescent product." Therefore, it is preferable to remove fluorescent recycled water-absorbent resin so that it falls below the standard. It is also desirable to effectively utilize recycled water-absorbent resin that has been removed from other components.

[0028] As described above, the inventors of the present application have found that fluorescence is observed when a recycled water absorbent resin is irradiated with light. Therefore, in the detection method for recycled water absorbent resin according to this embodiment, the recycled water absorbent resin mixed in a component constituting at least a part of a treated material of a used absorbent article is detected by observing the fluorescence when the component is irradiated with light. In the detection method according to this embodiment, the recycled water absorbent resin can be detected from at least one component extracted from the treated material, and the recycled water absorbent resin can also be detected directly from the treated material.

[0029] The light irradiated to detect the recycled water absorbent resin is typically ultraviolet light having a wavelength of 350 nm or more and 370 nm or less. However, the wavelength of the light irradiated to detect the recycled water absorbent resin can be determined arbitrarily, and may be, for example, in the visible light region (blue, green, red, etc.).

[0030] <Method for manufacturing components constituting absorbent articles> By extracting the recycled water absorbent resin detected by the above-mentioned detection method from components constituting at least a part of the treated material of used absorbent goods, it is possible to produce a recycled water absorbent resin that can be used as a fluorescent agent, a dispersant, an ultraviolet absorber, etc., as described above.

[0031] The method for extracting the recycled water absorbent resin can use existing techniques for separating and recovering water absorbent resin, and examples thereof include separation by size using a sieve or screen, separation by specific gravity using an air current or water current, optical separation using an air gun, and separation by precipitation in an aqueous solution of an acid, base, or neutral salt, etc. Optical separation, which allows for simultaneous detection and separation, is particularly preferred.

[0032] Furthermore, when the recycled water-absorbent resin according to the present embodiment is used in applications requiring water absorption, the water absorption capacity, defined as the weight ratio of absorbable water to its own weight, is preferably 5 times or more, and more preferably 20 times or more. Therefore, the manufacturing method according to the present embodiment may include a step of subjecting the recycled water-absorbent resin to a re-crosslinking treatment. Examples of the re-crosslinking treatment of the recycled water-absorbent resin include a treatment in which calcium chloride or polyethyleneimine is reacted with the recycled water-absorbent resin in water, and a treatment using an existing water-absorbent resin crosslinking agent, such as a treatment in which an internal crosslinking agent having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule is copolymerized or reacted.

[0033] Furthermore, since the utility value of the component from which the recycled water absorbent resin has been removed in the treated material of used absorbent articles also increases, the production method according to this embodiment not only produces the recycled water absorbent resin extracted from the component, but also the component from which the recycled water absorbent resin has been extracted, at the same time. For example, by extracting the recycled water absorbent resin detected by the detection method according to this embodiment from the recycled pulp extracted from the treated material, the recycled water absorbent resin and recycled pulp are produced at the same time.

[0034] <Examples and Comparative Examples> [Recycled water-absorbent resin] (Sample preparation) In Examples 1 to 8, the following samples were prepared by simulating recycled water-absorbent resins by subjecting water-absorbent resins (sodium polyacrylate crosslinked body, manufactured by Sanyo Chemical Industries, Ltd.) to treatments equivalent to recycling treatments. Example 1 In a small tubular furnace, 5 g of the water-absorbent resin was heat-treated in a nitrogen atmosphere at 350°C for 30 minutes. 1 g of the heat-treated material was dissolved in 50 g of water, and the resulting solution was filtered to obtain a filtration residue. The obtained filtration residue was dried under reduced pressure at 60°C for 24 hours, and the obtained dried product was used as a sample. Example 2 In a small tubular furnace, 5 g of the water-absorbent resin was heat-treated in a nitrogen atmosphere at 350°C for 5 minutes. 1 g of the heat-treated material was dissolved in 50 g of water, and the resulting solution was filtered to obtain a filtration residue. The obtained filtration residue was dried under reduced pressure at 60°C for 24 hours, and the obtained dried product was used as a sample. Example 3 In a small tubular furnace, 5 g of the water-absorbent resin was heat-treated at 375°C for 5 minutes in a nitrogen atmosphere. 1 g of the heat-treated material was dissolved in 50 g of water, and the resulting solution was filtered to obtain a filtration residue. The obtained filtration residue was dried under reduced pressure at 60°C for 24 hours, and the obtained dried product was used as a sample. Example 4 5 g of the water-absorbent resin was heat-treated in a small tubular furnace in a nitrogen atmosphere at 400° C. for 5 minutes, and the resulting treated material was used as a sample. Example 5 In a small tubular furnace, 5 g of the water-absorbent resin was heat-treated in a nitrogen atmosphere at 400°C for 5 minutes. 5 g of the heat-treated material was dissolved in 20 g of water, and 5 g of calcium chloride was added to the solution. The liquid was dried under reduced pressure at 60°C for 24 hours, and the resulting dried product was used as a sample. Example 6 In a small tubular furnace, 5 g of the water-absorbent resin was heat-treated in a nitrogen atmosphere at 400°C for 5 minutes. 5 g of the heat-treated material was dissolved in 20 g of water, and 15 g of an aqueous polyethyleneimine solution (30 wt%) was added to the solution. The liquid was dried under reduced pressure at 60°C for 24 hours, and the resulting dried product was used as a sample. Example 7 5 g of the water-absorbent resin was subjected to ozone exposure treatment for 1 hour. 1 g of the treated material after the ozone exposure treatment was dissolved in 50 g of water, and the liquid was filtered to obtain a filtration residue. The obtained filtration residue was dried under reduced pressure at 60°C for 24 hours, and the obtained dried product was used as a sample. Example 8 5 g of the water-absorbent resin was exposed to ozone water for 1 hour. 1 g of the object to be treated after the ozone water exposure treatment was dissolved in 50 g of water, and the solution was filtered to obtain a filtration residue. The obtained filtration residue was dried under reduced pressure at 60°C for 24 hours, and the obtained dried product was used as a sample.

[0035] In Comparative Examples 1 to 6, the following samples were prepared. Comparative Example 1: A water-absorbent resin (sodium polyacrylate crosslinked body, manufactured by Sanyo Chemical Industries, Ltd.) that had not been subjected to a treatment equivalent to a recycling treatment was used as a sample. Comparative Example 2: 20 g of sodium polyacrylate aqueous solution reagent (45 wt % Mw 8,000, manufactured by Wako Pure Chemical Industries, Ltd.) was dried under reduced pressure at 60° C. for 24 hours, and the resulting dried product was used as a sample. Comparative Example 3: 9 g of calcium chloride was added to 20 g of sodium polyacrylate aqueous solution reagent (45 wt %, Mw 8,000, manufactured by Wako Pure Chemical Industries, Ltd.), and the resulting liquid was dried under reduced pressure at 60°C for 24 hours to obtain a dried product, which was used as a sample. Comparative Example 4: 20 g of sodium polyacrylate aqueous solution reagent (45 wt %, Mw 8,000, manufactured by Wako Pure Chemical Industries, Ltd.) was mixed with 30 g of polyethyleneimine aqueous solution (30 wt %), and the resulting liquid was dried under reduced pressure at 60°C for 24 hours, and the resulting dried product was used as a sample. Filter paper: The filter paper used was No. 5C, 185 mm, manufactured by Advantec Toyo Co., Ltd. Standard paper: Prepared in accordance with 9. Reagents and test solutions, standards, standard solutions, measuring instruments and equipment, (2) Standards, Fluorescent Standards 2, of the Sanitary Hygiene Product Quality Standards. 7 mg of fluorescent brightener (Fluorescent-351) was accurately weighed and placed in a brown measuring flask. Water was added to make exactly 500 mL and stored in the dark. 0.35 mL of this solution was taken, 0.1 g of sodium chloride was added, and water was added to make 50 mL. Filter paper (cut to 8 cm x 15 cm) was immersed in this solution for 1 hour, occasionally turning it over, and then dried on the filter paper to serve as the standard paper.

[0036] (Water absorption evaluation) The water absorption of the samples according to Examples 1 to 8 and Comparative Examples 1 to 4 was evaluated. Table 1 shows the water absorption capacity of the samples according to Examples 1 to 8 and Comparative Examples 1 to 4. All of the samples according to Examples 1 to 8 exhibited water absorption. Furthermore, when attention was paid to the water absorption capacity of the samples according to Examples 4 to 6, it was confirmed that re-crosslinking was promoted by the action of calcium chloride or polyethyleneimine aqueous solution.

[0037] [Table 1]

[0038] (Evaluation of fluorescence intensity) The fluorescence intensity was evaluated for the samples according to Examples 1 to 8 and Comparative Examples 1 to 4. Fluorescence photographs were taken using an all-in-one fluorescence microscope (model number: BZ-X710, manufactured by Keyence Corporation). The fluorescence intensity of each sample was evaluated using the following fluorescence intensities A, B, and C.

[0039] Fluorescence intensity A First, the sample was photographed as a color image in brightfield mode using a 10x lens with an exposure time of 1 / 100. The resulting image was converted to a monochrome image (Image → Type → 8-bit) in ImageJ (image analysis software). The image brightness was checked using a histogram of brightness levels from 0 to 255 by thresholding the brightness level (Image → Adjust → Threshold). The area calculated by setting the brightness threshold level to 1 to 254 was used as the brightfield area of ​​the sample. Next, the sample was photographed as a monochrome image (Green) in GFP mode (illumination light 450 nm to 490 nm) using a 10x lens with an exposure time of 1 / 350. The obtained image was analyzed using ImageJ (image analysis software) to separate the RGB image (Image → Color → Split channels), and the brightness of the green image was checked using a histogram of brightness levels from 0 to 255 at the threshold level (Image → Adjust → Threshold). The area obtained by setting the brightness threshold level to 10 to 255 was used as the GFP fluorescent area of ​​the sample. The fluorescence intensity A was then calculated using the following formula: [Fluorescence intensity A] (%) = 100 × [GFP fluorescent area of ​​sample] ÷ [bright field area of ​​sample]

[0040] Fluorescence intensity B First, the sample was photographed as a color image in brightfield mode using a 10x lens with an exposure time of 1 / 100. The resulting image was converted to a monochrome image (Image → Type → 8-bit) using ImageJ (image analysis software). The image brightness was then thresholded (Image → Adjust → Threshold) to check the histogram of brightness levels from 0 to 255. The area obtained by setting the brightness threshold level to 1 to 254 was used as the brightfield area of ​​the sample. Next, the sample was photographed as a monochrome image (Red) in TxRed mode (irradiation light 540 nm to 580 nm) using a 10x lens with an exposure time of 1 / 70. The obtained image was analyzed using ImageJ (image analysis software) to separate the RGB image (Image → Color → Split channels), and the brightness of the red image was checked using a histogram of brightness levels from 0 to 255 at the threshold level (Image → Adjust → Threshold). The area obtained by setting the brightness threshold level to 10 to 255 was used as the TxRed fluorescent area of ​​the sample. The fluorescence intensity B was then calculated using the following formula: [Fluorescence intensity B] (%) = 100 × [TxRed fluorescent area of ​​sample] ÷ [bright field area of ​​sample]

[0041] Fluorescence intensity C First, the sample was photographed as a color image in brightfield mode using a 10x lens with an exposure time of 1 / 100. The resulting image was converted to a monochrome image (Image → Type → 8-bit) in ImageJ (image analysis software). The image brightness was checked using a histogram of brightness levels from 0 to 255 by thresholding the image brightness (Image → Adjust → Threshold). The area calculated by setting the brightness threshold level to 1 to 254 was used as the brightfield area of ​​the sample. Next, the sample was photographed as a monochrome image (Blue) in DAPI mode (illumination 340 nm to 380 nm) using a 10x lens with an exposure time of 1 / 70. The resulting image was analyzed using ImageJ (image analysis software), and the RGB image was split (Image → Color → Split channels). The brightness of the blue image was checked using a histogram with brightness levels of 0 to 255 at the threshold level (Image → Adjust → Threshold). The area calculated by setting the brightness threshold level to 10 to 255 was used as the DAPI fluorescent area of ​​the sample. The fluorescence intensity C was then calculated using the following formula: [Fluorescence intensity C] (%) = 100 × [DAPI fluorescent area of ​​sample] ÷ [bright field area of ​​sample]

[0042] Table 2 shows the fluorescence intensities A, B, and C of the samples according to Examples 1 to 8 and Comparative Examples 1 to 4. Table 2 also shows the fluorescence intensities A, B, and C of the filter paper and the standard paper. High fluorescence intensities A, B, and C were obtained in all of the samples according to Examples 1 to 8. On the other hand, the high fluorescence intensities A, B, and C were low in all of the samples not subjected to the treatment equivalent to the recycling treatment of Comparative Examples 1 to 4, and almost no fluorescence was observed. This confirmed that recycled water absorbent resins can be successfully detected by observing the fluorescence when irradiated with light.

[0043] [Table 2]

[0044] [Absorbent articles] Next, we conducted an experiment to confirm that recycled water-absorbent resin can be detected from treated materials that have been recycled from used absorbent articles. Specifically, 15 kg of actual used disposable diapers collected from nursery schools were heated and stirred using a heating and stirring device to generate a powder-like treated material, and the treated material was photographed using an all-in-one fluorescence microscope (model number: BZ-X710, manufactured by Keyence Corporation).

[0045] Figures 2(A) and 2(B) are images of the same field of view of the processed object taken using a 10x lens. Figure 2(A) shows an image of the processed object taken in bright field mode, and Figure 2(B) shows an image of the processed object taken in DAPI mode. Figures 3(A) and 3(B) are images of the same field of view of the processed object taken using a 4x lens. Figure 3(A) shows an image of the processed object taken in bright field mode, and Figure 3(B) shows an image of the processed object taken in DAPI mode.

[0046] In the images shown in Figures 3(A) and (B), it can be seen that the recycled water absorbent resin is present in the bright colored areas where fluorescence is observed. This confirmed that the recycled water absorbent resin can be detected from the treated object obtained by recycling used absorbent articles. Therefore, based on this detection result, it is possible to extract only the recycled water absorbent resin from the treated object of used absorbent articles.

Claims

1. The recycled water-absorbent resin in a component is detected by observing the fluorescence emitted when the component constituting at least a part of the treated material obtained by subjecting a used absorbent article to a recycling treatment is irradiated with light. A method for detecting recycled water absorbent resin.

2. The light irradiation is ultraviolet light irradiation with a wavelength of 350 nm or more and 370 nm or less. A method for detecting the recycled water-absorbent resin according to claim 1.

3. The recycling treatment includes at least one selected from a heat treatment, an oxidation treatment, a treatment with an acid agent, and a treatment with an alkaline agent. A method for detecting recycled absorbent resin according to claim 1 or 2.

4. detecting a recycled water-absorbent resin in a component by observing fluorescence when ultraviolet light having a wavelength of 350 nm or more and 370 nm or less is irradiated onto a component constituting at least a part of a treated material obtained by subjecting a used absorbent article to a recycling treatment, The detected recycled water absorbent resin is removed from the components. A manufacturing method for recycled water-absorbent resin.

5. The recycled water-absorbent resin has a water absorption capacity of 5 times or more. The method for producing the recycled water-absorbent resin according to claim 4.

6. The recycled water absorbent resin extracted from the components is subjected to a re-crosslinking treatment. The method for producing the recycled water-absorbent resin according to claim 5.

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