A method for regenerating cellulose from a mixture containing a water-absorbent polymer and pulp fibers.

Abstract

This invention provides a method for producing regenerated cellulose from a mixture of superabsorbent resin and pulp fibers, which are generated as so-called scraps in the manufacturing process of absorbent materials for absorbent articles. [Solution] The present invention provides a method for producing regenerated cellulose, comprising the steps of: dissolving the pulp fibers by mixing a mixture of a water-absorbent resin and pulp fibers with an ionic liquid; preparing a pulp fiber solution by separating the water-absorbent resin from a liquid containing the water-absorbent resin and dissolved pulp fibers; and regenerating cellulose from the pulp fiber solution, wherein the ionic liquid is a salt made of an acid with an acid dissociation constant of 4.26 or higher at 25°C in water, and the mixture of the water-absorbent resin and pulp fibers is characterized in that when the mixture containing the water-absorbent resin and pulp fibers is sucked onto the outer surface of a suction drum and deposited to produce an absorbent body, the material that falls out from the outer surface of the suction drum is used.

Description

[Technical Field]

[0001] This invention relates to the recycling of pulp fibers, and more specifically, to a method for regenerating cellulose from a mixture of pulp fibers and superabsorbent resin, which are generated as so-called scraps when manufacturing absorbent materials for absorbent articles. [Background technology]

[0002] Due to the aging population, the number of people using adult disposable diapers for caregiving is increasing. Globally, the number of people using child disposable diapers is also increasing. In recent years, the amount of disposable diapers used has increased rapidly. These disposable diapers and other hygiene products are soiled with waste after use and are therefore discarded without being reused. Discarded used hygiene products are usually incinerated. However, with the increasing amount of used hygiene products being discarded, attempts are being made to recycle them from an environmental protection perspective. In order to recycle used hygiene products, their constituent components such as pulp fibers are separated and recovered from the used products.

[0003] Patent Document 1 discloses a method for processing used sanitary products, comprising at least the steps of disintegrating the sanitary products and dispersing them in water, and separating and recovering the fibers and SAP contained in the sanitary products, characterized in that a polyvalent metal salt and an acidic substance, which are crosslinking agents, are added in the step of disintegrating the sanitary products and dispersing them in water. Patent Document 1 also discloses that in the step of separating and recovering the fibers and SAP contained in the sanitary products, the process is carried out using a screen or cleaner to first recover the SAP, and then recover the fibers from the dispersion containing the fibers.

[0004] Patent Document 2 discloses a method for recovering pulp fibers from used absorbent articles containing pulp fibers and superabsorbent polymers, the method comprising the steps of: treating the used absorbent articles with an aqueous organic acid solution having a pH of 2.5 or less at a temperature of 80°C or higher to inactivate the superabsorbent polymers; separating the pulp fibers and inactivated superabsorbent polymers from the used absorbent articles after treatment with the aqueous organic acid solution; treating the mixture containing the separated pulp fibers and inactivated superabsorbent polymers with an oxidizing agent (such as ozone) to decompose, reduce the molecular weight of, and solubilize the inactivated superabsorbent polymers; and separating the pulp fibers from the mixture treated with the oxidizing agent.

[0005] Patent Document 3 discloses a decomposition method characterized by contacting a water-absorbing polymer with a decomposition agent containing a periodate as an oxidizing agent, thereby decomposing the water-absorbing polymer.

[0006] Patent Document 4 discloses a method for decomposing a water-absorbing polymer in a water-absorbing polymer or a water-absorbing material containing the same, characterized in that a decomposition agent containing at least one alkaline compound and at least one persulfate compound which is an oxidizing agent is added to the water-absorbing polymer, and the water-absorbing polymer is decomposed in the presence of water.

[0007] Patent Document 5 discloses a method for recycling used disposable diapers, characterized by crushing the used diapers, then placing the crushed used diapers into a polymer decomposition tank where a polymer decomposition agent is mixed with water and stirred, and then decomposing the superabsorbent polymer contained in the diapers into monomers in the polymer decomposition tank before separating and recovering the pulp components contained in the diapers. Calcium chloride is described as an example of the polymer decomposition agent. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2013-150976 [Patent Document 2] Japanese Patent Publication No. 2018-021283 [Patent Document 3] Japanese Patent Publication No. 2001-316519 [Patent Document 4] Japanese Patent Publication No. 2003-321574 [Patent Document 5] Japanese Patent Publication No. 2000-84533 [Overview of the project] [Problems that the invention aims to solve]

[0009] Absorbent articles such as disposable diapers, incontinence pads, light incontinence pads, sanitary napkins, breast pads, and pet pads use absorbent materials containing superabsorbent resin and pulp fibers. One method of manufacturing these absorbent materials involves drawing a mixture containing superabsorbent resin and pulp fibers onto the outer surface of a suction drum (sometimes called a "forming drum") and depositing it to produce the absorbent material. The outer surface of the suction drum is provided with suction areas corresponding to the shape of the absorbent material. These suction areas are made of screens that can draw in the mixture containing superabsorbent resin and pulp fibers.

[0010] Pulp fibers are short fibers with a distribution in fiber diameter. Some pulp fibers are large enough to pass through the screen in the suction area. Superabsorbent polymers are also granular, and some superabsorbent polymers are also permeable to the screen in the suction area.

[0011] Absorbent polymers and pulp fibers that pass through the screen without accumulating on the surface of the suction area are discarded as industrial waste. From the perspective of resource conservation and improving production efficiency, it is important to reuse these materials. However, there is a problem in that the absorbent polymers and pulp fibers that pass through the screen in the suction area are in a mixed state, making it difficult to separate them. Furthermore, the pulp fibers that pass through the screen in the suction area are difficult to reuse because of their short fiber length.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a method for regenerating cellulose from a mixture containing a water-absorbent resin and pulp fibers that has permeated the outer surface of a suction drum when producing an absorbent material by sucking a mixture containing a water-absorbent resin and pulp fibers onto the outer surface of a suction drum and depositing it on the outer surface of the suction drum during the manufacturing process of an absorbent material for an absorbent article. [Means for solving the problem]

[0013] The present invention, which solves the aforementioned problems, is a method for producing regenerated cellulose, comprising: a dissolution step of mixing a mixture of a water-absorbent resin and pulp fibers with an ionic liquid to dissolve the pulp fibers; a water-absorbent resin separation step of separating the water-absorbent resin from the liquid containing the water-absorbent resin and dissolved pulp fibers to prepare a pulp fiber dissolution solution; and a step of regenerating cellulose from the pulp fiber dissolution solution, wherein the ionic liquid is a salt made of an acid with an acid dissociation constant of 4.26 or higher at 25°C in water, and the mixture of the water-absorbent resin and pulp fibers is used when, in the process of producing an absorbent material for an absorbent article, the mixture containing the water-absorbent resin and pulp fibers is sucked onto the outer surface of a suction drum and deposited to produce an absorbent material, and the material that falls out from the outer surface of the suction drum is used.

[0014] The essence of this invention lies in enabling easy solid-liquid separation of a water-absorbent resin and dissolved pulp fibers by dissolving only the pulp fibers from a mixture of the water-absorbent resin and pulp fibers using an ionic liquid. Furthermore, since water-absorbent resins used in sanitary products are generally salts of cross-linked polymers of polyacrylic acid, by using a salt of an acid with an acid dissociation constant of 4.26 or higher at 25°C in water as the ionic liquid, ion exchange between the water-absorbent resin and the ionic liquid can be suppressed, thereby preventing changes in the quality of the ionic liquid. [Effects of the Invention]

[0015] According to the present invention, in the manufacturing process of the absorber of an absorbent article, regenerated cellulose can be produced from a mixture of a water-absorbing resin and pulp fibers that is generated as so-called end materials. At the same time, the water-absorbing resin can be recycled.

Brief Description of the Drawings

[0016] [Figure 1] Schematic diagram for explaining the manufacturing apparatus and method of the absorber. [Figure 2] Flowchart of the method of the present invention. [Figure 3] Explanatory diagram showing the change in the aggregated state of the water-absorbing resin.

Embodiments for Carrying Out the Invention

[0017] The present invention includes a dissolution step of mixing a mixture of a water-absorbing resin and pulp fibers with an ionic liquid to dissolve the pulp fibers, and a water-absorbing resin separation step of separating the water-absorbing resin from the liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution solution. And a step of regenerating cellulose from the pulp fiber dissolution solution. As the ionic liquid, a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water is used. As the mixture of the water-absorbing resin and pulp fibers, in the manufacturing process of the absorber of an absorbent article, a mixture containing a water-absorbing resin and pulp fibers is suctioned onto the outer peripheral surface of a suction drum and deposited to form an absorber, and the material that has fallen off from the outer peripheral surface of the suction drum is used.

[0018] The method for producing regenerated cellulose of the present invention is as follows. 1) As a mixture of a water-absorbing resin and pulp fibers, in the manufacturing process of the absorber of an absorbent article, when a mixture containing a water-absorbing resin and pulp fibers is suctioned onto the outer peripheral surface of a suction drum and deposited to form an absorber, the material that has fallen off from the outer peripheral surface of the suction drum is used. 2) A dissolution step of mixing the mixture of the water-absorbing resin and pulp fibers with an ionic liquid to dissolve the pulp fibers. 3) A water-absorbent resin separation step, in which the water-absorbent resin is separated from a liquid containing the water-absorbent resin and dissolved pulp fibers to prepare a pulp fiber solution, 4) The process includes a step of regenerating cellulose from the pulp fiber dissolution solution (cellulose regeneration step).

[0019] 1) Regarding mixtures of superabsorbent polymer and pulp fibers Absorbent articles such as disposable diapers, incontinence pads, light incontinence pads, sanitary napkins, breast pads, and pet pads use absorbent materials containing superabsorbent resin and pulp fibers. In the present invention, as the mixture of superabsorbent resin and pulp fibers, in the manufacturing process of the absorbent material for absorbent articles, the mixture containing superabsorbent resin and pulp fibers is sucked onto the outer surface of a suction drum and deposited to produce the absorbent material, and the material that falls off from the outer surface of the suction drum is used.

[0020] One method for manufacturing absorbent materials for absorbent articles involves sucking a mixture containing a water-absorbent resin and pulp fibers onto the outer surface of a suction drum and depositing it to produce the absorbent material. Figure 1 is a schematic diagram showing an example of an absorbent material manufacturing apparatus and an example of a manufacturing method for the absorbent material. The absorbent material manufacturing apparatus 1 includes a suction drum 3, a supply duct 7 for supplying a mixture 5 of water-absorbent resin and pulp fibers, and a suction belt type conveyor 9.

[0021] A mixture 5, which consists of pulp fibers and a water-absorbent resin in a predetermined ratio, is air-conveyed to the suction drum 3 through the supply duct 7.

[0022] The suction drum 3 shown in Figure 1 has a hollow cylindrical shape. The outer surface of the suction drum 3 is provided with suction areas and non-suction areas corresponding to the shape of the absorbent material. The suction areas on the outer surface of the suction drum 3 are composed of a screen with multiple openings so that a mixture containing, for example, a water-absorbent resin and pulp fibers can be sucked up. The inside of the suction drum 3 can be depressurized and is configured to suck up the air around the suction drum 3. The suction drum 3 sucks up the conveyed mixture containing the water-absorbent resin and pulp fibers, and the mixture of water-absorbent resin and pulp fibers accumulates on the outer surface of the suction drum 3, forming a continuous body 11 containing the water-absorbent resin and pulp fibers.

[0023] Pulp fibers are short fibrous and have a distribution in fiber length and fiber width. Some pulp fibers are large enough to pass through the screen in the suction area of ​​the suction drum 3. Superabsorbent resin is granular and generally irregular in shape and has a particle size distribution. Some superabsorbent resin is large enough to pass through the screen in the suction area of ​​the suction drum 3. Therefore, when an absorbent body is made by sucking and depositing a mixture containing superabsorbent resin and pulp fibers onto the outer surface of the suction drum 3, there is a mixture 19 of pulp fibers and superabsorbent resin that permeates from the outer surface of the suction drum 3 into the interior. In this invention, "what falls out from the outer surface of the suction drum when an absorbent body is made by sucking and depositing a mixture containing superabsorbent resin and pulp fibers onto the outer surface of the suction drum" means the mixture of pulp fibers and superabsorbent resin that permeates from the outer surface of the suction drum into the interior. Conventionally, the mixture 19 of pulp fibers and water-absorbent resin that permeates from the outer surface of the suction drum 3 into the interior was discarded as industrial waste because it was difficult to separate the two. However, in the present invention, cellulose is regenerated from this mixture 19 of water-absorbent resin and pulp fibers.

[0024] A carrier sheet 13 on which a continuous body 11 containing a water-absorbent resin and pulp fibers is placed is conveyed from a carrier sheet roll 12. The carrier sheet 13 covers the pulp fibers and water-absorbent resin in the final absorbent article, forming the absorbent body. The carrier sheet 13 is breathable and permeable to liquids. Hot melt adhesive 15 is applied to the upper surface of the carrier sheet 13 by an adhesive application device 17.

[0025] The carrier sheet 13, to which the hot melt adhesive 15 has been applied, is transported on a suction belt conveyor 9 toward the suction drum 3. The suction belt conveyor 9 is driven by rollers 10 and can suck from the underside (from below in the drawing). The continuum 11, which has accumulated on the suction drum 3, is sucked through the carrier sheet 13 to the suction belt conveyor 9 and pressed against the carrier sheet 13 by the suction drum 3. The continuum 11 moves from the surface of the suction drum 3 onto the carrier sheet 13 and is bonded to the carrier sheet 13 by the applied hot melt adhesive 15.

[0026] A liquid-permeable sheet is further laminated onto the continuous body 11 of pulp fibers and water-absorbent resin laminated on the carrier sheet 13, as needed, and then cut into the desired shape to produce an absorbent body.

[0027] The screen having multiple openings that constitute the suction area on the outer surface of the suction drum 3 is not particularly limited, but is preferably 20% or more, more preferably 25% or more, and preferably 40% or less. By setting the size of the screen in the suction area within the above range, the yield of the absorbent material can be increased. As the screen, a metal mesh screen or a circular-hole screen having the above-mentioned opening ratio can be used.

[0028] If the screen having an opening is a circular-perforated screen, the plan view shape of the holes is preferably circular. The diameter of the holes in the circular-perforated screen is preferably 0.1 mm or more, more preferably 0.3 mm or more, preferably 0.8 mm or less, and more preferably 0.5 mm or less. This is because if the diameter of the holes in the circular-perforated screen is within the above range, the yield of the absorbent material can be increased.

[0029] If the screen having an opening is a mesh screen, the mesh opening is preferably 0.05 mm or larger, more preferably 0.1 mm or larger, more preferably 0.25 mm or smaller, and more preferably 0.2 mm or smaller. This is because if the mesh opening of the mesh screen is within the above range, the yield of the absorbent material can be increased.

[0030] The pulp fibers used in this invention are used in the manufacture of absorbent materials for absorbent articles. The pulp fibers are not particularly limited as long as they are fibers composed of cellulose molecules capable of forming a cotton-like mat that is an absorbent material. "Cellulose" is (C6H 10 It is a polymer compound represented as O5)n (where n is a natural number). Examples of the pulp fibers include cellulose fibers such as chemical pulp and dissolved pulp obtained from wood, and artificial cellulose fibers such as rayon and acetate. Examples of raw pulp for the pulp fibers include non-wood pulp such as wood pulp, cotton pulp, or straw pulp. As for wood pulp, softwood kraft pulp is preferred from the viewpoint of forming a cotton-like mat.

[0031] The pulp fibers are short fibrous, and there is a distribution in fiber length and fiber width.

[0032] Preferably, more than 90% of the pulp fibers are distributed between 5 μm and 7 mm in length, and the length-weighted average fiber length is in the range of 2 mm to 3 mm. This is because if the length-weighted average fiber length of the pulp fibers is within the above range, it is possible to easily form a cotton-like mat, which is the absorbent material, and to manufacture an absorbent article that balances absorption rate and backflow properties.

[0033] Preferably, the fiber length distribution (volume distribution) of the pulp fibers is distributed in the range of 5 μm to 4 mm, with 80% or more of the fibers being in that range.

[0034] Preferably, the fiber width (volume distribution) of the pulp fibers is distributed within the range of 5 μm to 70 μm, with 80% or more of the fibers being in that range.

[0035] In this invention, it is preferable that the minimum fiber length of the pulp fibers is smaller than the minimum diameter of the opening of the screen in the absorption region of the suction drum. It is also preferable that the maximum fiber length of the pulp fibers is larger than the maximum diameter of the opening of the screen in the absorption region of the suction drum.

[0036] The superabsorbent resin used in this invention is not particularly limited as long as it is used as an absorbent material in an absorbent article. The superabsorbent resin is granular, generally amorphous, and has a particle size distribution.

[0037] More than 90% of the number of superabsorbent resin particles are distributed between 75 μm and 1000 μm in size. The median particle size of the superabsorbent resin is not particularly limited, but is preferably in the range of 105 μm to 850 μm, and more preferably in the range of 150 μm to 710 μm. This is because if the median particle size of the superabsorbent resin is within the above range, the superabsorbent resin will not escape from the top sheet of the absorbent article, and the texture and feel of the absorbent article will not be affected.

[0038] The water-absorbing resin is not particularly limited, but examples include one made of a crosslinked polymer mainly composed of acrylic acid, in which at least a portion of its carboxyl groups are neutralized.

[0039] The content of the acrylic acid component constituting the crosslinked polymer is preferably 90% by mass or more, more preferably 95% by mass or more, preferably 99% by mass or less, and even more preferably 97% by mass or less.

[0040] The cations that neutralize at least a portion of the carboxyl groups of the crosslinked polymer are not particularly limited, but examples include alkali metal ions such as lithium, sodium, and potassium, and alkaline earth metal ions such as magnesium and calcium. Among these, it is preferable that at least a portion of the carboxyl groups of the crosslinked polymer are neutralized with sodium ions. The neutralization of the carboxyl groups of the crosslinked polymer may be performed by neutralizing the carboxyl groups of the crosslinked polymer obtained by polymerization, or the crosslinked polymer may be formed using monomers that have been neutralized in advance.

[0041] The degree of neutralization of the carboxyl groups in the crosslinked polymer is preferably 50 mol% or more, more preferably 55 mol% or more, and even more preferably 60 mol% or more. Furthermore, there is no particular upper limit to the degree of neutralization, and all carboxyl groups may be neutralized. The degree of neutralization can be calculated using the following formula. Degree of neutralization (mol%) = 100 × "Number of moles of neutralized carboxyl groups in the crosslinked polymer" / "Total number of moles of carboxyl groups in the crosslinked polymer (including neutralized and unneutralized groups)"

[0042] The superabsorbent resin used in the absorbent material of absorbent articles is preferably made of a crosslinked polymer mainly composed of acrylic acid, and the crosslinked structure (crosslinked chain portion) is preferably formed by a crosslinking reaction between the carboxyl group of acrylic acid and a polyvalent glycidyl compound.

[0043] There are no particular limitations on the shape of the water-absorbing resin, and examples include irregularly shaped crushed, flake-shaped, pearl-shaped, and rice-grain-shaped powders. Of these, the irregularly shaped crushed form is preferred.

[0044] In a mixture of water-absorbent resin and pulp fibers, the mixing ratio of water-absorbent resin to pulp fibers (water-absorbent resin / pulp fibers, mass ratio) is not particularly limited, but is preferably 1 / 7 to 1 / 1, and more preferably 1 / 5 to 1 / 2.

[0045] Figure 2 is a flowchart illustrating the general method for producing regenerated cellulose according to the present invention. In this invention, as a mixture of water-absorbent resin and pulp fibers, in the manufacturing process of an absorbent article, when an absorbent material is produced by sucking and depositing a mixture containing water-absorbent resin and pulp fibers onto the outer surface of a suction drum, the material that falls off the outer surface of the suction drum is used, and cellulose is regenerated by the process shown in Figure 2.

[0046] 2) Regarding the dissolution process for dissolving pulp fibers In the dissolution process, a mixture of the water-absorbent resin and pulp fibers is mixed with an ionic liquid. The ionic liquid used in the dissolution process of the present invention is not particularly limited as long as it dissolves the pulp fibers.

[0047] The aforementioned ionic liquid is a salt that is liquid at room temperature (approximately 25°C). Since it is a salt, the ionic liquid has both a cation and an anion portion.

[0048] Examples of cations in ionic liquids include imidazolium type, quaternary ammonium type, pyridinium type, pyrrolidinium type, and phosphonium type.

[0049] Examples of imidazolium types include 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-methyl-3-propylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-hexyl-3-octylimidazolium, 1-decyl-3-methylimidazolium, 1-dodecyl-3-methylimidazolium, 1-benzyl-3-methylimidazolium, and 1-ethyl-2,3-dimethylimidazolium.

[0050] Examples of pyridinium-type pyridinium include 1-ethylpyridinium, 1-butylpyridinium, 1-butyl-3-methylpyridinium, 1-butyl-4-methylpyridinium, 1-hexyl-4-methylpyridinium, and 1-octyl-4-methylpyridinium.

[0051] Examples of quaternary ammonium compounds include trimethylpropylammonium, tributylmethylammonium, methyltrioctylammonium, tetrabutylammonium, N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium, and N-oleyl-N,N-di(2-hydroxyethyl)-N-methylammonium.

[0052] Examples of pyrrolidinium-type pyrrolidinium include 1-ethyl-1-methylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, and 1-butyl-1-methylpyrrolidinium.

[0053] Examples of phosphonium-type phosphorylation include tetrabutylphosphonium and tributyldodecylphosphonium.

[0054] The ionic liquid used in this invention is a salt of an acid with an acid dissociation constant of 4.26 or higher in water at 25°C. That is, the anionic portion of the ionic liquid used in this invention is a proton (H) from an acid with an acid dissociation constant of 4.26 or higher in water at 25°C. + It is an anion formed by removing ().

[0055] As the acid component constituting the anion part of the ionic liquid, it is preferable that the acid is of the same degree or weaker than acrylic acid which is a constituent of the water-absorbing resin powder. That is, as the acid constituting the anion part, it is preferable to use an acid having an acid dissociation constant equal to or higher than that of acrylic acid. By using a metal salt of an acid that is the same as or weaker than acrylic acid, acid substitution between the acid component constituting the anion part of the ionic liquid and the acid component (acrylate) contained in the water-absorbing resin component is prevented. The acid dissociation constant of acrylic acid at 25 °C in water is 4.26. In the present invention, as the ionic liquid, it is preferable to use a salt of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water, more preferably a salt of an acid having an acid dissociation constant of 4.30 or more, and even more preferably a salt of an acid having an acid dissociation constant of 4.50 or more. The upper limit of the acid dissociation constant of the acid constituting the anion component is not particularly limited, but 12 is preferable and 10 is more preferable. For example, for phenol with an acid dissociation constant of 9.95, an effect is recognized, but for ethanol with an acid dissociation constant of 16, no effect is recognized. Acid (H n X n- ) The acid dissociation constant pKa is represented by the following formula. pKa = -log(1 / Ka), Ka = [H + n ·[X n- / [H n X n-

[0056] In the case of an acid that dissociates in multiple stages, it is preferable that at least one-stage acid dissociation constant is in the range of 4.26 to 12, more preferably in the range of 4.3 to 11, and even more preferably in the range of 4.5 to 10. Also, it is preferable that the dissociation constants of all stages are in the range of 4.26 to 12, more preferably in the range of 4.3 to 11, and even more preferably in the range of 4.5 to 10.

[0057] ​​Specific examples of acids with an acid dissociation constant of 4.26 or higher in water at 25°C include, for example, adipic acid (4.26), azelaic acid (4.39), p-anisic acid (4.48), o-aminobenzoic acid (second row: 4.95), p-aminobenzoic acid (4.85), isovaleric acid (4.58), isonicotinic acid (second row: 4.87), isobutyric acid (4.63), octanoic acid (4.89), valeric acid (4.64), quinaldic acid (second row: 4.75), citric acid (second row: 4.35), crotonic acid (4.69), and cinnamic acid (trans) (4.44). Examples include acetic acid (4.56), cyclohexanecarboxylic acid (4.70), (R,R)-tartaric acid (second row: 4.44), p-hydroxybenzoic acid (4.58), pimelic acid (4.31), 2,6-pyridinedicarboxylic acid (third row: 4.68), propionic acid (4.67), hexanoic acid (4.63), heptanoic acid (4.66), o-benzenedicarboxylic acid (second row: 4.93), malonic acid (second row: 5.28), butyric acid (4.63), malic acid (3.24), levulinic acid (4.44), and carbonic acid (6.35).

[0058] Specific examples of ionic liquids used in the present invention include alkylimidazolium-alkylcarboxylic acid, alkylimidazolium-alkylpropionic acid, alkylimidazolium-alkylbutyric acid, alkylimidazolium-alkyl phosphoric acid, and alkylammonium-hexafluorophosphate, with alkylimidazolium-alkylcarboxylic acid or alkylimidazolium-alkyl phosphoric acid being preferred.

[0059] In the dissolution process, it is preferable to mix the mixture of water-absorbent resin and pulp fibers with an ionic liquid, and more preferable to mix the mixture of water-absorbent resin and pulp fibers with an ionic liquid and water. When water is further mixed in the dissolution process, the ratio of ionic liquid to water (ionic liquid / water, mass ratio) is preferably 80 / 20 or more, more preferably 85 / 15 or more, even more preferably 90 / 10 or more, preferably 99 / 1 or less, more preferably 97 / 3 or less, and even more preferably 95 / 5 or less. By setting the ratio (mass ratio) of ionic liquid to water within the above range, the pulp fibers become easier to dissolve.

[0060] Furthermore, in the dissolution process, the amount of pulp fibers to be recycled is preferably 3 parts by mass or more, more preferably 4 parts by mass or more, even more preferably 5 parts by mass or more, preferably 35 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, per 100 parts by mass of the ionic liquid. If the amount of pulp fibers to be recycled is within the above range, the solubility of the pulp fibers will be good. In addition, the viscosity of the ionic liquid will decrease, and the workability will be improved.

[0061] The mixture of water-absorbent resin and pulp fibers with an ionic liquid can be performed, for example, using a tank equipped with a stirrer.

[0062] The duration of the dissolution process is preferably 4 hours or more, more preferably 5 hours or more, even more preferably 6 hours or more, preferably 24 hours or less, more preferably 12 hours or less, and even more preferably 10 hours or less.

[0063] The temperature of the dissolution process is preferably 40°C or higher, more preferably 45°C or higher, even more preferably 80°C or higher, preferably 120°C or lower, more preferably 100°C or lower, and even more preferably 90°C or lower. The temperature of the dissolution process is the temperature of the ionic liquid, or a mixture of water and an ionic liquid. When mixing with water, care should be taken to prevent evaporation.

[0064] 2) The process of separating the superabsorbent resin from the mixture obtained in the dissolution process and obtaining a pulp fiber solution containing dissolved pulp fibers (superabsorbent resin separation process)

[0065] The mixture obtained in the dissolution step of the present invention contains a water-absorbent resin and dissolved pulp fibers. In the water-absorbent resin separation step, the water-absorbent resin is separated from this mixture to prepare a pulp fiber solution containing dissolved pulp fibers.

[0066] Methods for separating the superabsorbent polymer include, for example, decantation, vacuum filtration, pressure filtration, and atmospheric pressure filtration of the mixture. Specifically, it is preferable to allow the mixture to stand or centrifuge to precipitate the superabsorbent polymer, followed by decantation, vacuum filtration, pressure filtration, and atmospheric pressure filtration. It is even more preferable to separate the polymer by centrifugation followed by vacuum filtration and pressure filtration. If necessary, the separated superabsorbent polymer (cake) may be redispersed in an ionic liquid or a mixture of water and an ionic liquid, washed, and separated again.

[0067] 3) About the cellulose regeneration process The present invention provides a method for producing regenerated cellulose, comprising the step of regenerating cellulose from the pulp fiber dissolution solution. Specifically, by bringing the pulp fiber dissolution solution into contact with water, the cellulose becomes insoluble and precipitates. The "cellulose" to be regenerated is (C6H 10 This means that the polymer compound is represented by O5)n (where n is a natural number), and its shape (such as fibrous or membranous) is irrelevant. It is preferable to use neutral water as the water.

[0068] The mass ratio of pulp fiber dissolving solution to water (pulp fiber dissolving solution / water) is preferably 80 / 20 or higher, more preferably 82 / 18 or higher, even more preferably 85 / 15 or higher, preferably 99 / 1 or lower, more preferably 97 / 3 or lower, and even more preferably 95 / 5 or lower.

[0069] In a preferred embodiment of the present invention, cellulose is regenerated into fibers. As a method for regenerating cellulose into fibers, for example, fibrous cellulose can be regenerated by continuously releasing a pulp fiber dissolution solution into water using a needle-shaped or tubular member such as a syringe.

[0070] Pulp fibers that permeate the outer surface of the suction drum are difficult to utilize because of their short fiber length. However, according to the present invention, the fiber length can be increased by regenerating fibrous cellulose.

[0071] In another preferred embodiment of the present invention, cellulose is regenerated in the form of a film or sheet. A method for regenerating cellulose in the form of a film or sheet is to apply a pulp fiber dissolving solution onto a film or substrate to form a film, and then introduce or immerse the film directly into water.

[0072] It is preferable to wash, dehydrate, and dry the regenerated cellulose as needed. It is also preferable to cut the dried cellulose into the desired size.

[0073] The regenerated cellulose obtained by this invention is of high quality and has few limitations on its reuse applications. Therefore, it can be expected to be applied in various fields. Examples of reuse applications include fibrous articles, film articles, and molded products. Fibrous articles can be divided into short fibers and long fibers. Short fibers can be used as pulp for disposable diapers, airlaid nonwoven fabrics, and thermal bond nonwoven fabrics by blending them with other raw cotton. Examples of use include paper substitutes for business cards and calendars, face masks, cooking paper, wet wipes, hand towels, and wet wipes. Long fibers can be used in spunbond nonwoven fabrics and textiles. Examples of use include gauze, adhesive bandages, oil filters, and drip sheets. Examples of film articles include packaging materials and laminates. Examples of molded products include fixtures and cutlery.

[0074] The present invention also includes a method for regenerating the superabsorbent resin separated in the superabsorbent resin separation process. Superabsorbent resin is used in hygiene products as a powder. When superabsorbent resin powder absorbs water, it swells and exists as aggregates of superabsorbent resin powder. These aggregates of superabsorbent resin powder contain a large amount of water, which makes them difficult to dry. In particular, even if they are dried as is, only the surface dries, and the inside of the aggregates remains dry.

[0075] In the present invention, it is preferable to regenerate the aggregates of the water-absorbent resin powder separated in the water-absorbent resin separation step by processing them as follows.

[0076] First, the aggregates of the superabsorbent polymer powder are mixed with a dehydrating agent to perform a dehydration treatment. This dehydration treatment removes the water-absorbing properties of the superabsorbent polymer powder. Divalent metal salts are preferably used as the dehydrating agent, and calcium chloride is more preferably used. By treating with a divalent metal salt, the carboxyl groups of the acrylic acid that make up the superabsorbent polymer powder are crosslinked. As a result, the volume of the aggregates of the superabsorbent polymer powder is reduced. Due to the change in the volume of the aggregates, water is discharged from the aggregates. This dehydration treatment reduces the water content of the superabsorbent polymer powder from approximately 98% by mass to approximately 70% by mass.

[0077] Next, a weak alkali metal salt is added to the agglomerates of the water-absorbent resin powder after dehydration. The weak alkali metal salt acts on the agglomerates of the water-absorbent resin powder. Treatment with the weak alkali metal salt causes an ion exchange reaction between the divalent metal ions and the alkali metal ions. As a result, the metal crosslinks are broken, making it easy to granulate the agglomerates of the water-absorbent resin powder. The granulated water-absorbent resin has an increased surface area, so it can be easily dried.

[0078] Figure 3 shows the change in the aggregation state of the water-absorbent resin powder. Figure 3(a) shows the aggregates of water-absorbent resin powder 1. Water-absorbent resin powder 1 undergoes metal crosslinking (-COO) due to the divalent metal ions of the dehydrating agent. - ·M 2+ · - It is aggregated by OOC-. Divalent metal ion M 2+ and alkali metal ions M + The metal bridge is broken by ion exchange between (-COO - M + M +- OOC-). As a result, the aggregates of the water-absorbent resin powder change into granular form (Figure 3(b)).

[0079] The amount of alkali metal salt of the weak acid added is preferably 1 part by mass or more, more preferably 3 parts by mass or more, preferably 100 parts by mass or less, and more preferably 30 parts by mass or less, per 100 parts by mass of the water-absorbing resin in a water-containing state treated with a dehydrating agent.

[0080] The mixing of the aggregated water-absorbing superabsorbent resin powder with the weak acid alkali metal salt is not particularly limited, but can be carried out using a rocking mixer such as the BHR type manufactured by Aichi Electric Co., Ltd. The mixing is not particularly limited, but is preferably carried out at a temperature range of 5°C to 130°C for 5 minutes to 6 hours. Dehydration may be carried out simultaneously with the mixing. The dehydration of the mixture is not particularly limited, but is preferably carried out at a temperature range of 80°C to 130°C for 5 minutes to 6 hours. It can be carried out under atmospheric pressure, reduced pressure, or pressurized pressure.

[0081] Upon mixing, the aggregates of the water-absorbing resin powder are transformed into granular or powdered water-absorbing resin. Since the granular or powdered water-absorbing resin powder has a large surface area, it can be easily dried.

[0082] The weak acid component of the alkali metal salt of the aforementioned weak acid is preferably an acid of the same degree as or weaker than acrylic acid, which is a component of the water-absorbent resin powder. That is, it is preferable to use an acid as the weak acid component that has an acid dissociation constant equal to or greater than that of acrylic acid. By using a metal salt of the same or weaker acidity as acrylic acid, the ion exchange reaction with the divalent metal salt used as a dehydrating agent becomes easier to occur. The acid dissociation constant of acrylic acid in water at 25°C is 4.26.

[0083] In other words, it is preferable to mix the aggregate of the water-absorbent resin powder after dehydration with an alkali metal salt of an acid having an acid dissociation constant of 4.26 or higher at 25°C in water. Examples of acids having an acid dissociation constant of 4.26 or higher at 25°C in water include acids that constitute the anionic portion of the ionic liquid used in the dissolution process.

[0084] Examples of alkali metals include lithium, sodium, and potassium. Specific examples of alkali metal salts of weak acids include sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium citrate acetate, and sodium succinate acetate. Furthermore, it is preferable that the alkali metal salt of the weak acid be in solid form. This is because adding and mixing a solid alkali metal salt of the weak acid facilitates the discharge of water from the aggregates of the water-absorbing resin powder due to osmosis.

[0085] The granular water-absorbing resin obtained by the above processing method can be dried to regenerate granular water-absorbing resin powder. The regenerated water-absorbing resin powder also exhibits excellent absorption performance. For example, the absorption ratio of the regenerated water-absorbing resin powder is 15 g / g or higher.

[0086] The recycled water-absorbing resin powder can be used, for example, as a soil conditioner or as a solid fuel. [Examples]

[0087] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples, and any modifications and embodiments that do not depart from the spirit of the present invention are all included within the scope of the present invention.

[0088] (Example 1) Using an airflow mixing device (Autec pad former), superabsorbent polymer and crushed softwood kraft pulp (Golden Isles, Georgia Pacific) were mixed, and a mat-like absorbent material was formed by suction onto a mesh member (a 0.198 mm mesh screen with multiple openings and an opening ratio of 34.3%). At this time, the mixture of pulp fibers and superabsorbent polymer that fell out of the mesh member was collected. An aqueous solution of an ionic liquid was prepared by mixing 95 parts by mass of 1-ethyl-3-methylimidazolium acetate (1-ethyl-3-methylimidazolium acetate) and 5 parts by mass of water as an ionic liquid. 95 parts by mass of the aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the mixture of pulp fibers and superabsorbent polymer was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fibers were dissolved, but the superabsorbent polymer was not dissolved.

[0089] The resulting mixture was filtered to separate the superabsorbent resin, and a pulp fiber solution containing dissolved pulp fibers was obtained.

[0090] Ten mL of the obtained pulp fiber solution was filled into a 20 mL syringe with a needle tip diameter of 2 mm, and released into 500 mL of water to regenerate fibrous cellulose.

[0091] (Example 2) Using an airflow mixing device (Autec pad former), superabsorbent polymer and crushed softwood kraft pulp (Golden Isles, Georgia Pacific) were mixed, and a mat-like absorbent material was formed by suction onto a mesh member (a 0.198 mm mesh screen with multiple openings and an opening ratio of 34.3%). At this time, the mixture of pulp fibers and superabsorbent polymer that fell out of the mesh member was collected. An aqueous solution of the ionic liquid (1-ethyl-3-methylimidazolium butanate (1-ethyl-3-methylimidazolium butyrate)) was prepared by mixing 95 parts by mass of ionic liquid with 5 parts by mass of water. The aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the mixture of pulp fibers and superabsorbent polymer was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fibers were dissolved, but the superabsorbent polymer was not dissolved.

[0092] (Example 3) 95 parts by mass of ionic liquid (1-ethyl-3-methylimidazolium acetate (1-ethyl-3-methylimidazolium acetate)) and 5 parts by mass of pulp fibers were mixed. The ionic liquid was heated to 90°C and stirred for 1 hour to dissolve the pulp fibers. Water was added to 100 parts by mass of the resulting pulp fiber solution in amounts of 5, 10, 15, and 20 parts by mass. Pulp fibers precipitated when the pulp fiber solution / water ratio was 80 / 20. The results are shown in Table 1.

[0093] [Table 1]

[0094] (Example 4) 95 parts by mass of 1-ethyl-3-methylimidazolium butanate (1-ethyl-3-methylimidazolium butyrate) and 5 parts by mass of pulp fibers were mixed as an ionic liquid. The ionic liquid was heated to 90°C and stirred for 1 hour to dissolve the pulp fibers. Water was added to 100 parts by mass of the resulting pulp fiber solution in amounts of 5, 10, 15, and 20 parts by mass. Pulp fibers precipitated when the pulp fiber solution / water ratio was 85 / 15. The results are shown in Table 2.

[0095] [Table 2]

[0096] (Example 5) A mixture of pulp fibers and superabsorbent resin that fell out from the outer surface of a suction drum (a 0.198 mm mesh screen with multiple openings and an aperture ratio of 34.3%) during the manufacturing of Saracare Pad Big (Lot No.: 20240501) manufactured by Ribdo Corporation was collected. An aqueous solution of the ionic liquid was prepared by mixing 95 parts by mass of 1-ethyl-3-methylimidazolium acetate (1-ethyl-3-methylimidazolium acetate) with 5 parts by mass of water. The 95 parts by mass of the aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the mixture of pulp fibers and superabsorbent resin was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fibers were dissolved, but the superabsorbent resin was not dissolved.

[0097] (Comparative Example 1) A mixture was obtained in the same manner as in Example 1, except that 1-ethyl-3-methylimidazolium butanate (1-ethyl-3-methylimidazolium acetate) was replaced with 1-ethyl-3-methylimidazolium chloride (1-ethyl-3-methylimidazolium hydrochloride). In the resulting mixture, the pulp fibers were not completely dissolved, and the superabsorbent polymer was also not dissolved. [Industrial applicability]

[0098] The present invention can be suitably used as a method for regenerating cellulose from a mixture of pulp fibers and water-absorbent resin, which are generated as so-called scraps when manufacturing absorbent materials for absorbent articles.

Claims

1. A dissolution step in which a mixture of water-absorbent resin and pulp fibers is mixed with an ionic liquid to dissolve the pulp fibers, A water-absorbing resin separation step is performed to separate the water-absorbing resin from a liquid containing the water-absorbing resin and dissolved pulp fibers in order to prepare a pulp fiber solution. A method for producing regenerated cellulose, comprising the step of regenerating cellulose from the pulp fiber dissolution solution, wherein the ionic liquid is a salt made of water and an acid with an acid dissociation constant of 4.26 or higher at 25°C, and the mixture of water-absorbent resin and pulp fibers is used as the mixture of water-absorbent resin and pulp fibers, wherein in the process of producing an absorbent body for an absorbent article, the mixture containing water-absorbent resin and pulp fibers is sucked onto the outer surface of a suction drum and deposited to produce an absorbent body, and the material that falls out from the outer surface of the suction drum is used.

2. A method for producing regenerated cellulose according to claim 1, wherein the pulp fiber dissolution solution is brought into contact with water to regenerate cellulose.

3. A method for producing regenerated cellulose according to claim 1, wherein cellulose is regenerated into a fibrous form.

4. A method for producing regenerated cellulose according to claim 1, wherein cellulose is regenerated into a membrane.

5. A method for producing regenerated cellulose according to any one of claims 1 to 4, wherein water is further mixed in the dissolution step.

6. A method for producing a regenerated superabsorbent resin, comprising: a dehydration step of adding a dehydrating agent consisting of a divalent metal salt to the superabsorbent resin obtained in the water-absorbing resin separation step of claim 1 to dehydrate it; and a method for producing a regenerated superabsorbent resin of adding an alkali metal salt of an acid with an acid dissociation constant of 4.26 or higher at 25°C to the dehydrated superabsorbent resin and mixing it to form aggregates of the superabsorbent resin into granules, and drying the resulting granular superabsorbent resin.

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