Recycling of superabsorbent fibers by UV irradiation in a flow system

Abstract

Superabsorbent fibers (SAF) in the feed stream are converted to soluble polymers by UV irradiation in a flow system. The total UV energy used to convert the SAF to soluble polymers is less than about 50 MJ / kg SAF.

Description

[Technical Field]

[0001] The present invention generally relates to the recycling of superabsorbent polymers (SAF) using UV irradiation with short residence times in a flow system. More specifically, a feed stream containing SAF is fed to the flow system, and a product stream containing essentially soluble polymer is produced. The concentration of SAF in the feed stream is greater than about 1 wt. % and the total UV energy used to convert the SAF to soluble polymer is less than about 50 MJ / kg SAF. [Background technology]

[0002] Recycling absorbent hygiene products (AHPs) (i.e., baby diapers, feminine protective pads, and adult incontinence pads) is good for the environment and is required to meet the sustainability goals of many consumer companies. These goals include using 100% recycled materials and zero consumer and manufacturing waste to landfills. In addition to these goals, successful recycling helps the environment, stimulates economies, improves human health and water quality, and generates energy needed by consumers in developing regions of the world.

[0003] The main component of AHPs is typically superabsorbent polymer (SAP), with other components being adhesives, cellulose fibers, polyethylene, polypropylene, and polyester. SAP is a water-absorbent, water-swellable, and water-insoluble powdery solid that is a crosslinked, partially neutralized homopolymer of glacial acrylic acid. SAPs have an exceptionally high capacity to absorb aqueous liquids, such as contaminated water or urine. An alternative to poly(acrylic acid)-based SAPs is superabsorbent fiber (SAF). These fibers generally contain relatively high amounts of comonomers (compared to superabsorbent polymer particles). Therefore, like SAPs, there is also a need to recycle SAFs used in AHPs.

[0004] Recycling AHPs involves cleaning them from soils accumulated during their use and separating the various components into recycled material streams. More specifically, the recycled SAF material stream can be used in less demanding applications than AHPs (because recycled SAF has inferior properties compared to virgin SAF; for example, agricultural or horticultural water retention agents and industrial waterproofing agents) and / or converted into essentially non-crosslinked, slightly branched, or linear soluble polymers. For example, the soluble polymers can be 1) used as is in applications such as water treatment or corrosion protection, or 2) esterified and then used in adhesives, coatings, etc., or 3) used as is in SAF manufacturing, such as SAF fiber spinning, or 4) converted to SAF (see item 3) and blended with virgin SAF. The first two sets of uses are part of an effort to recycle SAF into other products by replacing unused compounds with compounds derived from recycled SAF, while the last two sets of uses are part of a circular economy for SAF, i.e., recycling SAF back into SAF. In all cases, the aim is to achieve the same properties as virgin material.

[0005] Although no references have been identified addressing the decomposition of SAFs that are not purely acrylic acid based, non-limiting examples of processes for producing a purified and separated stream of spent SAP from recycled AHP are disclosed and claimed in U.S. Patent Nos. 9,095,853 (B2), issued August 4, 2015, and 9,156,034 (B2), issued October 13, 2015, both of which were assigned to Fater SpA (Pescara, Italy).

[0006] Most SAPs are based on poly(acrylic acid) and are crosslinked network materials. Non-limiting examples of procedures used to produce SAPs from glacial acrylic acid and a crosslinker are disclosed in U.S. Pat. No. 8,383,746 (B2), issued February 26, 2013, and assigned to Nippon Shokubai Co., Ltd. (Osaka, Japan), and U.S. Pat. No. 9,822,203 (B2), issued November 21, 2017, and assigned to BASF SE, based in Ludwigshafen, Germany.

[0007] There are many references to attempts to decompose or depolymerize linear polymers, but only a few references to efforts to depolymerize SAPs. Typical forms of energy used in these attempts (as a single form of energy or in combination) are ultrasound, UV, mechanical (i.e., in the presence of extensional / stretching forces, e.g., Caruso, MM, et al., Chem. Rev., 109 (2009), 5755-5798), heat (e.g., McNeill, IC, and Sadeghi, SM, Polymer Degrad. Stability, 29 (1990), 233-246), and microwave. UV is used exclusively in conjunction with free radical generators such as hydrogen peroxide (H2O2). Examples using UV and hydrogen peroxide are described in Kaczmarek, H., et al., Die Angew. Makrom. Chemie, 261 / 262 (1998), 109-121, and Mierzwa, J.C. and Rodrigues, R., Adv. Oxidation Proc. Wastewater Treatm, Capter 2 (2018), 13-48. Examples using ultrasound, UV, and H2O2 are described in Prajapat, A.L., and Gogate, P.R., Ultrasonics Sonochemistry, 32 (2016), 290-299. The main theme from all these references regarding the degradation or depolymerization of linear polymers is that preferential scission occurs at the midpoint of the polymer chain, high molecular weight chains are degraded at a faster rate than low molecular weight chains, and there is a minimum molecular weight below which degradation or depolymerization will not occur.

[0008] More recently, Li, X., and Cui, Y., J. Appl. Polym. Sci., 108 (2008), 3435–3441, and Shukla, N.B., and Madras, G., J. Appl. Polym. Sci., 125 (2011), 630–639, attempted to degrade poly(acrylic acid)-based SAPs using UV irradiation in a static system. In the first study, the crosslinker was N,N-methylenebisacrylamide (NMBA), and in the second, it was ethylene glycol dimethacrylate (EGDMA). Weight loss, swelling capacity, and residual weight fraction were three parameters used to track degradation progress over irradiation time. The time scale of UV irradiation required to achieve zero swelling capacity or zero residual weight fraction was reported to be approximately 120 minutes, which is not commercially viable. At a fixed temperature close to room temperature (e.g., 30°C), UV irradiation power higher than 1500 W does not further increase weight loss, but at higher temperatures (e.g., 90°C), UV irradiation power higher than 1500 W slowly increases weight loss. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 9,095,853(B2) [Patent Document 2] U.S. Patent No. 9,156,034(B2) [Patent Document 3] U.S. Patent No. 8,383,746 (B2) [Patent Document 4] U.S. Patent No. 9,822,203(B2) [Non-patent literature]

[0010] [Non-Patent Document 1] Caruso,MM,et al.,Chem.Rev.,109(2009),5755~5798)

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0011] Therefore, there is a need to recycle AHPs and their main component (which for some AHPs is SAF). Recycling SAF requires that the recycled SAF be decomposed into soluble polymers on a short-term scale, with low energy and power per unit mass of SAF, and under mild conditions, such as room temperature, thereby avoiding chemical degradation, such as decarboxylation, of the decomposed SAF. The requirement for low energy per unit mass of SAF arises from the fact that recycling SAF and its decomposition into soluble polymers is only beneficial if the energy consumed during the conversion of SAF to soluble polymer is lower than, for example, the energy used to make the same fossil-derived soluble polymer. In the case of acrylic acid from propylene (petro-AA), the energy required is approximately 50 MJ / kg AA. The soluble polymers produced from recycled SAF can then be incorporated back into virgin SAF (thus increasing its recycled content and supporting a circular economy for SAF) and / or derivatized into materials for other applications, such as adhesives, coatings, water treatment, and fabric care. [Means for solving the problem]

[0012] In one embodiment of the present invention, a method is provided for degrading superabsorbent fiber (SAF) into a soluble polymer, the soluble polymer comprising 5% to 75%, preferably 10% to 75%, or 10% to 70% by weight of polymerized acrylic acid monomer units, based on the total weight of the soluble polymer. As used herein, the term "acrylic acid monomer units" excludes derivatives of acrylic acid monomers, such as esters and amides, and also excludes methacrylic acid monomer units and their salts.

[0013] The method includes flowing a feed stream containing the SAF through a UV irradiation zone, irradiating the feed stream with UV, and producing a product stream at an end of the irradiation zone that contains the soluble polymer, wherein the feed stream contains the SAF at a concentration greater than about 1 wt %, the feed stream has a residence time in the UV irradiation zone of less than about 120 seconds, and the UV used to convert the SAF to the soluble polymer requires a total UV energy of less than about 50 MJ / kg SAF.

[0014] Also provided is a method for degrading superabsorbent fiber (SAF) into a soluble polymer, the soluble polymer comprising 5% to 75%, preferably 10% to 75%, or 10% to 70% by weight of polymerized acrylic acid monomer units, based on the total weight of the soluble polymer. The method includes passing a feed stream comprising the SAF through a UV irradiation zone, irradiating the feed stream with UV, and producing a product stream at an end of the UV irradiation zone comprising a soluble polymer, the feed stream comprising water and SAF at a concentration greater than about 1% by weight, the feed stream having a residence time in the UV irradiation zone of less than about 120 seconds, the UV used to convert the SAF to the soluble polymer requiring less than about 16 MJ / kg SAF total UV energy, and the soluble polymer having a weight average molecular weight of less than about 1,000,000 g / mol.

[0015] The present invention further relates to a method for degrading superabsorbent fiber (SAF) into a soluble polymer, the soluble polymer comprising 5% to 75% by weight, preferably 10% to 75% by weight, or 10% to 70% by weight, of polymerized acrylic acid monomer units, based on the total weight of the soluble polymer. The method includes passing a feed stream containing the SAF through a UV irradiation zone, irradiating the feed stream with UV, and producing a product stream at an end of the UV irradiation zone comprising a soluble polymer, the feed stream comprising water and SAF at a concentration greater than about 5% by weight, the feed stream having a residence time in the UV irradiation zone of less than about 120 seconds, the UV used to convert the SAF to the soluble polymer requiring less than about 16 MJ / kg SAF total UV energy, and the soluble polymer having a weight average molecular weight of less than about 1,000,000 g / mol.

[0016] The soluble polymer may be water-soluble. The soluble polymer preferably has a solubility in water at 25° C. of more than 5 g of soluble polymer per 100 g of water, or more than 15 g of soluble polymer per 100 g of water, or from 20 g to 120 g of soluble polymer per 100 g of water, or from 35 g to 100 g of soluble polymer per 100 g of water.

[0017] The soluble polymer obtained by the method of the present invention can be subjected to the NMR content method described below. The NMR content method can be used to determine the molar ratio of protons of functional groups with different NMR signals, such as alkene terminal moieties, alkoxy groups (-O-CHR1-, where R1 is H, alkyl, aryl, heteroaryl, alkoxy, or halogen), aliphatic groups (-CHR1-, where R1 is H, alkyl, aryl, heteroaryl, or halogen), and / or other 1H-NMR active groups. Given that the spectra obtained by the NMR content method allow the determination of various functional groups due to their different NMR signals, the NMR content method also allows the determination of many monomer units other than acrylic acid monomer units contained in the soluble polymer.

[0018] The soluble polymers obtained by the methods of the present invention may have a 3.6:CH ratio of at least 0.04, as measured by the NMR content method described herein. The soluble polymers may have a 3.6:CH ratio of at least 0.1 or at least 0.2, as measured by the NMR content method described herein. The soluble polymers may have a 3.6:CH ratio of 0.7 or less, as measured by the NMR content method described herein.

[0019] The soluble polymers obtained by the present invention may have a content of at least 5% "% 3.6 ppm" as measured by the NMR content method described herein. The soluble polymers may have a content of at least 10% or at least 15% "% 3.6 ppm" as measured by the NMR content method described herein. The soluble polymers may have a content of 75% or less "% 3.6 ppm" as measured by the NMR content method described herein.

[0020] The soluble polymers obtained by the methods of the present invention may have an alkene content "% Alkene" of 0.31% or less, as measured by the NMR content method described herein. DETAILED DESCRIPTION OF THE INVENTION

[0021] I. Definition As used herein, the term "SAF" refers to superabsorbent fibers. The SAF of the present invention can absorb at least 7 times their dry weight, preferably at least 10 times their dry weight, of a 0.9 wt. % saline solution at 25°C, as measured according to the Centrifuge Retention Capacity (CRC) test method described herein. The typical absorption mechanism is osmotic pressure. SAF absorbing water or an aqueous solution becomes softer and gel-like.

[0022] "Superabsorbent fiber" ("SAF") is used herein to refer to a superabsorbent polymer material in fiber form. Superabsorbent fibers have a length and a cross-section. The length is the maximum dimension of the fiber when it is, or would be, laid flat and straight on a surface so that any curvature or crimp in the fiber disappears and the fiber assumes a generally rod-like configuration. The cross-section is perpendicular to the length. For purposes of this specification, a fiber is a material having a maximum and a minimum dimension, the ratio of the maximum to the minimum dimension being at least 10:1, preferably at least 15:1, and even more preferably at least 20:1; i.e., the maximum dimension (also referred to as the length) of a superabsorbent fiber is at least 10 times, or at least 15 times, or at least 20 times the minimum dimension (also referred to as the width) of the fiber. When a superabsorbent fiber has a cross-section that varies along the length of the fiber, the maximum dimension of the cross-section (determined along the length of the fiber) is taken as the width of the fiber when calculating the ratio of the maximum to the minimum dimension.

[0023] As used herein, the term "soluble polymer" refers to a non-crosslinked, slightly branched or linear polymer that contains 5% to 75% by weight of acrylic acid as a monomer unit and may have a degree of polymerization of 2 or higher. For the purposes of the present invention, there is no distinction between a polymer and an oligomer when it comes to a soluble polymer.

[0024] As used herein, the term "degradation" refers to the conversion of SAF into a soluble polymer through partial depolymerization, decrosslinking, molecular backbone disruption, or any combination of the above processes. For purposes of the present invention, the terms degrading, recycling, and conversion are used interchangeably, as long as they refer to the conversion of SAF into a soluble polymer. Degradation also essentially preserves the carboxyl groups of SAF, and therefore, the product soluble polymer contains these carboxyl groups. Note that complete depolymerization of SAF should result in the monomers originally contained in the SAF.

[0025] As used herein, the term "virgin SAF" refers to SAF made from virgin monomers, which are the feedstocks used today to make SAF. Virgin monomers can be produced from either fossil-derived or biologically derived materials (non-limiting examples of biological materials are lactic acid, 3-hydroxypropionic acid, glycerin, biopropylene, carbon dioxide, and sugars). Virgin SAF does not contain more than about 1% by weight of recycled SAF.

[0026] As used herein, the term "used SAF" refers to SAF that has already been industrially produced and / or commercially used, for example, in baby diapers, feminine pads, adult incontinence pads, or other articles and / or applications. Used SAF can be post-consumer SAF, post-industrial SAF, or a combination of both. Unless otherwise specified in the present invention, SAF refers to either "used SAF" or "virgin SAF."

[0027] As used herein, the term "degraded SAF" refers to SAF that has been degraded into soluble polymers. For purposes of the present invention, the terms "degraded SAF" and "soluble polymers" are used interchangeably.

[0028] As used herein, the term "recycled SAF" refers to SAF that contains at least 1 wt. % degraded SAF (or equivalent soluble polymer) that was incorporated into the SAF while it was being manufactured from acrylic acid and comonomers using typical manufacturing methods. Thus, recycled SAF is a blend of virgin SAF and at least 1 wt. % degraded SAF.

[0029] As used herein, the term "feed stream" refers to a body of fluid flowing in a particular direction and supplied to a flow system.

[0030] As used herein, the term "product stream" refers to the body of fluid produced from a flow system when a feed stream is fed into the flow system.

[0031] As used herein, the term "UV irradiation zone" refers to the area of ​​the flow system that is irradiated by a UV source, e.g., an Hg lamp. For purposes of the present invention, UV refers to radiation between 10 nm and 400 nm, which includes the ranges of vacuum UV (VUV; 10 nm to 200 nm), far UV (FUV; UV C; 200 nm to 280 nm), mid UV (MUV; UV B; 280 nm to 315 nm), and near UV (NUV; UV A; 315 nm to 400 nm).

[0032] As used herein, the term "wall shear stress" refers to the shear stress on the wall of a flow system using the viscosity μ of the feed stream, assuming no slip at the wall. If the flow system is a circular tube with a uniform radius R, the wall shear stress (τ w ) is expressed by the following formula:

[0033]

number

[0034]

number

[0035] As used herein, the term "viscosity ratio" or "viscosity reduction ratio" refers to the ratio of the viscosity of the product stream to the viscosity of the feed stream. The viscosity of the feed stream is typically measured using a parallel plate fixture in oscillatory mode, and the reported complex viscosity typically corresponds to a frequency of 1 rad / s. The real and imaginary parts of the complex viscosity represent the dynamic viscosity and storage viscosity, respectively. To calculate the viscosity ratio, we use the dynamic viscosity, i.e., the real part of the complex viscosity. The viscosity of the product stream is measured using either a cup-and-bob fixture in steady mode or a parallel plate fixture in oscillatory mode. When the viscosity is measured in steady mode using a cup-and-bob fixture, the reported viscosity is typically measured at a frequency of 4 s. -1These viscosity measurement techniques are well known to those skilled in the art. For purposes of this invention, the negative decimal logarithm of the viscosity ratio indicates, by several orders of magnitude, the extent of SAF degradation to soluble polymer, as it is accepted by those skilled in the art that the lower the viscosity of a given soluble polymer solution, the lower the molecular weight of the soluble polymer at a given concentration.

[0036] As used herein, M n is the number average molecular weight expressed in g / mol or equivalently in Da, and M w is the weight average molecular weight expressed in g / mol or equivalently in Da, and M z is the z-average molecular weight expressed in g / mol or equivalently Da, and PDI is M w / M n is the polydispersity index, defined as

[0037] As used herein, the term "ply glue" refers to an adhesive used to glue two or more plies together. For example, a ply glue for a paper towel product is used to glue two paper plies together to form a two-ply paper towel product.

[0038] II.Feed stream Unexpectedly, it has been found that UV-based degradation of SAF to soluble polymers (i.e., essentially no decarboxylation) is much more pronounced when the SAF feed stream is flowing while being irradiated, compared to when the SAF is static while being irradiated, at the same residence time. While not wishing to be bound by any theory, Applicants believe that stresses generated in the flowing feed stream cause bond stretching, 1) reducing the UV energy required to decompose the SAF into soluble polymers and / or 2) accelerating the rate of SAF degradation.

[0039] Typical properties of SAF are mechanical properties, swelling capacity, and centrifuge retention capacity (CRC), measured according to the test methods described herein. SAF also contains more than 25% to less than 95% by weight of other comonomers (i.e., other than acrylic acid monomers). Suitable comonomers include, for example, methyl acrylate, methyl methacrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, or monomer groups including / consisting of polymers and copolymers such as polyacrylamide, polyvinyl alcohol, ethylene-maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, and vinyl sulfonic acid, polyvinylamine, polyallylamine, and polyvinylpyridine. Other suitable polymers in SAF include hydrolyzed acrylonitrile-grafted starch, acrylic acid-grafted starch, and isobutylene-maleic anhydride copolymers, and mixtures thereof. SAF is crosslinked to render the material substantially water-insoluble. Preferred monomers are methyl (meth)acrylate and / or monomers containing hydroxyl groups, such as hydroxyalkyl (meth)acrylates, e.g., hydroxyethyl methacrylate, tripropylene glycol monoacrylate, 5 glyceryl monoacrylate, etc. The SAF can comprise / consist of a polymer formed from two types of monomers (including acrylic acid) or, more preferably, three or more types of monomers.

[0040] The SAF may have a sodium level of greater than about 8 wt. % Na, or 10-20 wt. % Na, or 15-18 wt. % Na. In yet another embodiment of the invention, the SAF has a sodium level of less than 12 wt. % Na (for the avoidance of doubt, the terms "weight-%)," "wt. %," and "wt. %" are used interchangeably herein).

[0041] The SAF provided by the method may be i) in the form of loose fibers, or ii) in the form of a nonwoven web, or a combination of i) and ii). When the SAF is in the form of a nonwoven web, the nonwoven web may consist of or comprise the SAF. The nonwoven web comprising the SAF and provided by the method may comprise at least 50% by weight of the SAF, based on the total weight of the nonwoven web, or may comprise at least 60% by weight, or at least 70% by weight, or at least 80% by weight, or at least 90% by weight of the SAF, based on the total weight of the nonwoven web.

[0042] The nonwoven web may further comprise additional components such as synthetic fibers, natural fibers (e.g., cellulose fibers), or combinations thereof. The synthetic fibers that may be included in such nonwoven webs may be polyolefin fibers (e.g., polyethylene, polypropylene, or combinations thereof), or PET fibers, or a combination of polyolefin and PET fibers.

[0043] The non-SAF components of the nonwoven web may, but need not, be removed before subjecting the SAF to the process of the present invention. Alternatively, the nonwoven web containing the SAF may be subjected to the process of the present invention (as an intact nonwoven web, cut, shredded, or comminuted) along with the non-SAF components of the nonwoven web.

[0044] The nonwoven web may be cut into small pieces or ground or otherwise shredded to make the SAF more accessible to the oxidized water-soluble salt before or during the process of the present invention. Alternatively, the nonwoven web may be subjected to the process "as is."

[0045] In one embodiment of the present invention, the feed stream comprises SAF. In another embodiment of the present invention, the feed stream comprises SAF and water. In yet another embodiment of the present invention, the feed stream comprises SAF and ethylene glycol (EG). In yet another embodiment of the present invention, the feed stream comprises SAF, water, and ethylene glycol. The water in the feed stream can be RO water, regular tap water, or water containing dissolved inorganic salts at various salt concentrations. A non-limiting example of water containing salt is a 0.9 wt% solution of sodium chloride. Other salts with monovalent cations but higher ionic strength can be used to reduce the viscosity of the feed stream or allow for the use of higher SAF concentrations. A non-limiting example of a viscosity-reducing salt is sodium sulfate.

[0046] The feed stream can also include any free radical-generating compound. Non-limiting examples of such compounds include hydrogen peroxide (HO), persulfates (e.g., sodium persulfate or potassium persulfate), perborates, perphosphates, percarbonates, diazo compounds, ozone, organic free radical initiators (e.g., di-tert-butyl peroxide (DTBP)), combinations thereof, and the like. In one embodiment of the present invention, the feed stream includes SAF and HO. The feed stream can also include a UV photocatalyst, such as titanium dioxide (TiO).

[0047] In one embodiment of the invention, the feed stream comprises SAF and H2O2. In another embodiment of the invention, the feed stream comprises SAF and H2O2 solution.

[0048] In one embodiment of the invention, the feed stream comprises SAF at a concentration greater than about 1 wt%. In another embodiment of the invention, the feed stream comprises SAF at a concentration greater than about 5 wt%. In yet another embodiment of the invention, the feed stream comprises SAF at a concentration greater than about 10 wt%. In yet another embodiment of the invention, the feed stream comprises SAF at a concentration of about 2.5 wt%. In one embodiment of the invention, the feed stream comprises SAF at a concentration of about 5 wt%.

[0049] In one embodiment of the invention, the feed comprises SAF and an HO solution, wherein the SAF has a concentration of about 2.5 wt%, the HO solution has a concentration of 97.5 wt%, and the HO solution has a concentration of less than about 3 wt%. In another embodiment of the invention, the feed comprises SAF and HO, wherein the SAF has a concentration of about 5 wt%, the HO solution has a concentration of about 95 wt%, and the HO solution has a concentration of less than about 3 wt%. In yet another embodiment of the invention, the feed comprises SAF and an HO solution, wherein the SAF has a concentration of about 2.5 wt%, the HO solution has a concentration of 97.5 wt%, and the HO solution has a concentration of about 3 wt%. In another embodiment of the invention, the feed comprises SAF and an H2O2 solution, wherein the concentration of SAF is about 5 wt%, the concentration of the H2O2 solution is about 95 wt%, and the concentration of H2O2 in the H2O2 solution is about 3 wt%.

[0050] In one embodiment of the present invention, the feed comprises SAF and an HO solution, wherein the SAF has a concentration of about 2.5 wt%, the HO solution has a concentration of 97.5 wt%, and the HO solution has a concentration of about 0.3 wt%. In another embodiment of the present invention, the feed comprises SAF and HO, wherein the SAF has a concentration of about 5 wt%, the HO solution has a concentration of about 95 wt%, and the HO solution has a concentration of about 0.3 wt%. In yet another embodiment of the present invention, the feed comprises SAF and an HO solution, wherein the SAF has a concentration of about 2.5 wt%, the HO solution has a concentration of 97.5 wt%, and the HO solution has a concentration of about 0.03 wt%. In another embodiment of the invention, the feed comprises SAF and an H2O2 solution, wherein the concentration of SAF is about 5 wt%, the concentration of the H2O2 solution is about 95 wt%, and the concentration of H2O2 in the H2O2 solution is about 0.03 wt%.

[0051] In one embodiment of the invention, the feed comprises SAF and an H2O2 solution, wherein the concentration of H2O2 in the H2O2 solution is less than about 3 wt%. In another embodiment of the invention, the feed comprises SAF and H2O2, wherein the concentration of H2O2 in the H2O2 solution is less than about 0.3 wt%. In yet another embodiment of the invention, the feed comprises SAF and an H2O2 solution, wherein the concentration of H2O2 in the H2O2 solution is less than about 0.03 wt%.

[0052] The viscosity of the feed stream is typically measured using a parallel plate fixture in oscillatory mode, and the reported complex viscosity typically corresponds to a frequency of 1 rad / s. Depending on the SAF concentration, the complex viscosity of the feed stream can be higher than 200 Pa.s (or equivalently, 200,000 cP). The feed stream can be in the form of a solution or a gel, depending on the concentration of the SAF.

[0053] The non-renewable energy usage (NREU) for producing acrylic acid (AA) from fossil-derived propylene is estimated to be approximately 50 MJ / kg AA. Therefore, any successful recycling attempt of SAF desirably consumes less energy than NREU to produce AA, i.e., less than approximately 50 MJ / kg SAF, to be environmentally and commercially competitive with fossil-derived virgin materials. For purposes of NREU, the SAF is assumed to be fully unneutralized (DN=0).

[0054] III. Flow System Typically, the feed stream is in fluid communication with the flow system via a pipe or channel and a pump. Non-limiting examples of the pipe or channel are glass, metal, alloy (such as stainless steel), and polymer pipes. The pipe or channel can have any cross-sectional shape, such as circular, rectangular, oval, or diamond. The cross-sectional area of ​​the pipe or channel may be the same or vary in size along the flow direction. A non-limiting example of a varying cross-sectional shape of the pipe is a corrugated pipe, which can subject the feed stream to expansionary stresses as it flows down the pipe. These expansionary stresses can be beneficial to the breakdown of SAFs that are part of the feed stream. Shear stresses can also be beneficial to the breakdown of SAFs. Expansionary and / or shear stresses can be imparted to the feed stream by static mixers or other mixing elements disposed inside the pipes and / or channels through which the feed stream flows.

[0055] In one embodiment of the present invention, the wall shear stress τ w In another embodiment of the present invention, the wall shear stress τ w In yet another embodiment of the present invention, the wall shear stress τ w In yet another embodiment of the present invention, the wall shear stress τ w is at least about 10 kPa.

[0056] Non-limiting examples of pumps are centrifugal pumps (such as axial, radial, and mixed flow pumps) and positive displacement pumps (such as reciprocating, rotary, piston, diaphragm, gear, peristaltic, screw, and vane pumps). A flow system may use one or more pumps.

[0057] IV.UV irradiation UV irradiation can be accomplished by exposing the flow system to either the sun, any typical laboratory or plant UV system, or a combination of both. Typical UV sources in a laboratory or plant are LEDs, lasers, gas discharge lamps, incandescent lamps, etc. UV radiation breaks down the SAF into soluble polymers. A typical additional benefit of UV radiation is its sanitizing and sterilizing effect on the SAF feed and product streams.

[0058] The feed stream is irradiated by UV (i.e., exposed to UV radiation) as it flows through the irradiation zone and has a residence time within the irradiation zone. In one embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 20 minutes. In another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 10 minutes. In yet another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 5 minutes. In one embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 120 seconds. In another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 60 seconds. In yet another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 30 seconds. In yet another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 15 seconds. In one embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 10 seconds. In another embodiment of the present invention, the residence time of the feed stream in the UV irradiation zone is less than about 5 seconds.

[0059] UV total energy is the electrical energy supplied to the UV unit, and UV irradiance energy (or simply UV energy) is the UV energy dissipated in the feed stream. Calculations of UV total energy and UV irradiance energy are exemplified in Methods Section VIII (which are well known to those skilled in the art).

[0060] In one embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 30 MJ / kg SAF. In another embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 20 MJ / kg SAF. In yet another embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 10 MJ / kg SAF. In yet another embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 5 MJ / kg SAF. In one embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 1 MJ / kg SAF. In another embodiment of the present invention, the UV irradiation energy used to convert SAF into a soluble polymer is less than about 0.5 MJ / kg SAF.

[0061] In one embodiment of the invention, the total UV energy used to convert the SAF to a soluble polymer is less than about 50 MJ / kg SAF. In another embodiment of the invention, the total UV energy used to convert the SAF to a soluble polymer is less than about 32 MJ / kg SAF. In yet another embodiment of the invention, the total UV energy used to convert the SAF to a soluble polymer is less than about 16 MJ / kg SAF. In yet another embodiment of the invention, the total UV energy used to convert the SAF to a soluble polymer is less than about 10 MJ / kg SAF. In one embodiment of the invention, the total UV energy used to convert the SAF to a soluble polymer is less than about 2 MJ / kg SAF.

[0062] UV irradiation can be carried out at room temperature or any other temperature. UV irradiation can be carried out under atmospheric pressure, vacuum, or high pressure. Furthermore, other processes such as microwave heating, IR heating, ultrasonic irradiation / cavitation, extrusion, and stretching can be carried out before or after UV irradiation.

[0063] V. Production logistics UV irradiation of the feed stream produces a product stream at the end of the irradiation zone. In one embodiment of the invention, the product stream comprises a soluble polymer. In another embodiment of the invention, the product stream comprises a soluble polymer and an SAF.

[0064] In one embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 5,000,000 g / mol. In another embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 2,000,000 g / mol. In yet another embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 1,000,000 g / mol. In yet another embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 500,000 g / mol. In one embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 300,000 g / mol. In another embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 200,000 g / mol. In yet another embodiment of the invention, the soluble polymer has a weight average molecular weight of less than about 100,000 g / mol.

[0065] In one embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 1,000,000 g / mol to about 5,000,000 g / mol. In another embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 500,000 g / mol to about 2,000,000 g / mol. In yet another embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 100,000 g / mol to about 1,000,000 g / mol. In yet another embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 150,000 g / mol to about 500,000 g / mol. In one embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 90,000 g / mol to about 300,000 g / mol. In another embodiment of the present invention, the soluble polymer has a weight average molecular weight of about 20,000 g / mol to about 200,000 g / mol. In yet another embodiment of the present invention, the soluble polymer has a weight average molecular weight of from about 10,000 g / mol to about 100,000 g / mol.

[0066] In one embodiment of the invention, the soluble polymer has a polydispersity index (PDI) of less than about 10. In another embodiment of the invention, the soluble polymer has a PDI of less than about 6. In yet another embodiment of the invention, the soluble polymer has a PDI of less than about 4. In yet another embodiment of the invention, the soluble polymer has a PDI of less than about 2. PDI is the ratio of weight average molecular weight to number average molecular weight, and these molecular weights are measured by GPC (described in Methods Section VIII), as is well known to those skilled in the art.

[0067] The viscosity of the product stream is typically measured using either a parallel plate fixture in oscillatory mode or a cup-and-bob fixture in steady mode. Reported oscillatory viscosities are typically equivalent to 1 rad / sec, and reported steady-state viscosities are typically equivalent to 4 sec. -1 Depending on the concentration and molecular weight of the soluble polymer, the viscosity of the product stream can be as low as 1 mPa.s (or equivalently 1 cP; i.e., the viscosity of water).

[0068] The ratio of the viscosity of the product stream to the viscosity of the feed stream is the viscosity reduction ratio (or simply viscosity ratio), which indicates the extent of SAF degradation into soluble polymers by the UV flow system. The negative decimal logarithm of the viscosity ratio measures the order of magnitude of viscosity change between the feed stream and the product stream. In one embodiment of the invention, the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 6. In another embodiment of the invention, the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 4. In yet another embodiment of the invention, the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 2.

[0069] The soluble polymers from the product stream can be derivatized into materials for various applications such as adhesives, coatings, water treatment, etc. In one embodiment of the invention, the soluble polymers from the product stream, either as is or derivatized, are used as adhesives. In yet another embodiment of the invention, the soluble polymers from the product stream, either as is or derivatized, are used in fabric care applications. In yet another embodiment of the invention, the soluble polymers from the product stream, either as is or derivatized, are used in water treatment applications.

[0070] In one embodiment of the invention, the soluble polymer from the product stream is used as a ply glue in a paper product. In another embodiment of the invention, the soluble polymer from the product stream is used as a ply glue in a paper towel product. In yet another embodiment of the invention, the soluble polymer from the product stream is used as a ply glue in a toilet paper product. In yet another embodiment of the invention, the soluble polymer from the product stream has an M of greater than about 350 kDa. w In one embodiment of the present invention, the soluble polymer from the product stream has an M of about 400 kDa to about 500 kDa and is used as a ply glue in paper products. w It is used as a ply glue in paper products.

[0071] In another embodiment of the present invention, the soluble polymer from the product stream is used as a glue between a paper core and a paper towel product. In yet another embodiment of the present invention, the soluble polymer from the product stream is used as a glue between a paper core and a toilet paper product.

[0072] Soluble polymers can be extracted from the product stream via a number of processes. Non-limiting examples of these processes include water evaporation, soluble polymer filtration, water extraction, etc. Additionally, salts present in the product stream from the use of SAF in AHP can be removed by any desalination technique known to those skilled in the art. Non-limiting examples of desalination processes include membrane processes (e.g., reverse osmosis, forward osmosis, electrodialysis reversal (EDR), nanofiltration, etc.), freeze desalination, solar desalination, geothermal desalination, ion exchange, wave desalination, etc.

[0073] VI. Recycled SAF The soluble polymer from the product stream can be fed to a process to produce SAF, thus producing recycled SAF. In one embodiment of the present invention, the soluble polymer is used to produce recycled SAF.

[0074] In one embodiment of the present invention, the recycled SAF has a CRC, measured using the Centrifuge Retention Capacity CRC test method described herein, of about 7 g / g to about 45 g / g. In another embodiment of the present invention, the recycled SAF has a CRC, measured using the Centrifuge Retention Capacity CRC test method described herein, of about 10 g / g to about 35 g / g. In yet another embodiment of the present invention, the recycled SAF has a CRC, measured using the CRC test method described herein, of about 15 g / g to about 35 g / g.

[0075] VII. Method NMR content method The NMR content method detects alkene terminal moieties, alkoxy groups (-O-CHR1-, where R1 is one of H, alkyl, aryl, heteroaryl, alkoxy, or halogen groups), aliphatic groups (-CHR1-, where R1 is one of H, alkyl, aryl, heteroaryl, or halogen groups), and / or other groups. 1 It is used to determine the molar ratio of functional groups with different NMR signals, such as H-NMR active groups.

[0076] This method uses proton NMR spectroscopy to analyze samples of soluble materials in deuterated water and identify different 1The peaks in the H-NMR domains are integrated and ratios are taken to identify functional groups or different 1 Determine the mole percentage of protons in each H-NMR domain.

[0077] The soluble polymer is dried in a vacuum oven (Heraeus Vacutherm model, Thermo Scientific™) at 40° C. and 10-50 mbar pressure for 3 hours. Any low molecular weight alcohols, esters, or ethers are removed from the soluble polymer.

[0078] A fluid solution is prepared by dissolving less than 10% by weight of the sample in DO and adjusting the pH to 5.5-6.5. The solution is transferred to an NMR glass-grade tube and placed in the sample holder (bore) of a proton NMR instrument. An example of a suitable instrument is a Bruker NMR device with a 400 MHz field strength. Instruments from other manufacturers and with other field strengths, including "low-field" instruments operating as low as 60 MHz, can also be successfully used to perform this method. A nosy-presat sequence is used to acquire data and suppress residual water signals. Those skilled in the art will be familiar with the appropriate selection of other specific data collection parameters. Suitable parameters used with the exemplary 400 MHz Bruker instrument described above are a 4.1-second acquisition time (FID length), an 8-second relaxation time, a 90-degree pulse width, a 20 ppm spectral width, 64 k points in the FID, and 64 repeated scans. In the Fourier transform step, exponential apodization is used with 0.3 Hz line broadening, and the spectrum is absorbed stepwise. Spline baseline correction is used to ensure a flat baseline on either side of the integrated peak.

[0079] The following peak areas are typically used and integrated for content determination: 1) One of two terminal alkene protons at chemical shifts in the range of approximately 5-6 ppm, typically one terminal alkene proton at approximately 5.35 ppm + / - 0.5 ppm. (To confirm the identification of such proton peaks as terminal alkene protons, use standard edited 1 H- 13 Using the C HSQC sequence (e.g., according to W. Willker, D. Leibfritz, R. Kerssebaum & W. Bermel, Magn. Reson. Chem. 31, 287-292 (1993)), it can be determined that the alkene signals seen in the 1D-1H spectrum are both attached to the same methylene (secondary) carbon (-CH). The resulting integral is called "integral_alkene". 2) Alkoxy protons at chemical shifts in the range of about 3.2-3.8 ppm, typically about 3.6 ppm. The resulting integral is called "integral_3.6." If multiple signals appear around 3.6 ppm, i.e., in the range of about 3.2-3.8 ppm, the signal with the largest integral value is selected to obtain "integral_3.6." 3) Methylene protons of aliphatic CH groups, typically at about 1.5 ppm and 2.1 ppm, or about 1.8 ppm. The integral obtained is called "integral_CH." 4) Other aliphatic groups at chemical shifts in the range of approximately 1.0 to 2.6 ppm. 5) Different under the following conditions 1 Other groups or peaks of H-NMR domains can be analyzed similarly. a) It is a separated signal; b) have peak maxima at a distance of at least 0.5 ppm; 6) Identify peaks in the NMR spectrum corresponding to classes 1), 2), 3), and / or 4) and integrate them if present. If no such peaks are present, this is reported as no measurable content of classes 1), 2), 3), or 4), respectively. As known to those skilled in the art, the integral ranges from baseline (start of signal) to baseline (end of signal), or, in the case of broad and / or complex signals, the boundary of the integral originates from the start of the next adjacent signal.

[0080] The ratio "Ratio 3.6:CH" is calculated by the following formula: ratio 3.6CH=integral_3.6 / integral_CH The ratio 3.6CH is a unitless number and represents the ratio of protons at approximately 3.6 ppm.

[0081] The alkene content "% alkene" is calculated by the following formula: %Alkene=[integral_alkene / (integral_alkene+integral_3.6+integral_CH)]×100%

[0082] The methylene content "% CH" is calculated by the following formula: %CH=[integral_CH / (integral_alkene+integral_3.6+integral_CH)]×100%

[0083] The content of approximately 3.6 ppm protons (such as alkoxy protons) "%3.6 ppm" is calculated by the following formula: %3.6=[integral_3.6 / (integral_alkene+integral_3.6+integral_CH)]×100%

[0084] The ratio "Ratio 3.6:CH" is reported to the nearest 0.001.

[0085] Alkene, methylene, and proton contents at approximately 3.6 ppm are reported in % to the nearest 0.01%.

[0086] Centrifuge Retention Capacity (CRC) Test Method The capacity of superabsorbent fibers is determined in accordance with the Centrifuge Retention Capacity (CRC) test method set forth in EDANA NWSP 241.0.R2(19). In deviation from EDANA NWSP 241.0.R2(19), sampling of superabsorbent fibers, superabsorbent nonwovens, and / or superabsorbent cores (Chapter 8 of EDANA NWSP 241.0.R2(19)) is as follows:

[0087] The superabsorbent fibers, superabsorbent nonwoven fabric, and / or superabsorbent core are cut into small pieces with a maximum dimension of approximately 5 mm. Cutting can be done manually, for example, with scissors. Care is taken to ensure that the fibrous structure (core, nonwoven fabric, or bulk of fibers) is not significantly compressed before or during the cutting process. This ensures sufficient void space between the superabsorbent fibers, so that the superabsorbent fibers can be primarily wetted by the swelling medium over their entire surface area.

[0088] Further deviations or additions from EDANA NWSP 241.0.R2(19) in the procedures for superabsorbent fibers, superabsorbent nonwovens, and / or superabsorbent cores (Chapters 9.1-9.5 of EDANA NWSP 241.0.R2(19)) are as follows:

[0089] The sample for measurement is carefully taken, for example using laboratory tweezers, and placed in a tea bag. Using the laboratory tweezers, the fibers are carefully distributed into the tea bag to avoid clumps, and any clumps of fibers are carefully spread out.

[0090] When sealing the tea bag, care is taken to ensure that the sealed area is free of superabsorbent fiber, superabsorbent nonwoven, and / or superabsorbent core material, to ensure a complete and sufficiently strong seal of the tea bag.

[0091] All other aspects of the test method will be performed as described in EDANA NWSP 241.0.R2(19).

[0092] UV Total Energy Calculation The total UV energy is the electrical energy supplied to the UV unit and is based on the voltage and amperage of the UV unit and the residence time of the feed stream. If the voltage of the UV unit is 200 V and the amperage is 16 A, the power of the UV unit is 3200 W. The total UV energy per unit mass of SAF is calculated based on the total UV power and the amount of SAF in the feed stream that covers the entire length of the UV lamp. If the flow system tubes have a diameter of 0.368 cm, the number of flow system tubes that fit into the 15 cm length of the UV lamp is 40. If the flow rate of the feed stream in each tube is 5 mL / min and the SAF concentration is 2.5 wt%, assuming a density of the feed stream of 1 g / mL, the total UV energy per unit mass of SAF is calculated as 3200 (J / sec) × 60 (sec) / (40 × 5 (mL feed) × 0.025 (g SAF / g feed) × 1 (g feed / mL feed)) = 38.4 MJ / kg SAF.

[0093] UV irradiation energy calculation UV irradiance energy (or simply UV energy) is the UV energy dissipated in the feed stream and is calculated based on the total energy density of the lamp as shown in the examples below (as known to those skilled in the art). A UV PowerMAP® #20082105 A / B / C / V (EIT, Inc., Sterling, VA) was operated under the UV lamp at various speeds to measure the total energy density. For example, at a speed of 5.6 m / min, the energy density was UVA 488 MJ / cm. 2 , UVB 466MJ / cm 2 , and UVV 102MJ / cm 2 and 1,056 MJ / cm 2 The UV energy density is ignored in these calculations. Similarly, at a speed of 11.2 m / min, the respective energy densities are 249, 238, and 52 MJ / cm. 2 and the total energy density is 539MJ / cm 2Extrapolation is used for speeds outside the above speed range, and interpolation is used for speeds within the above speed range.

[0094] In the example used in the UV total energy calculation above, the flow rate of the feed stream through a 0.368 cm diameter tube was 5 mL / min. Therefore, the average linear velocity of the feed stream was 5 (cm 3 / min) / ((π×(0.368cm) 2 This speed is outside the range measured above, so the total energy density at 0.47 m / min is calculated as 12,565 Mj / cm. 2 Then, the UV irradiation energy per unit mass of SAF was calculated as 12.565 (J / sec.cm) 2 )×0.368(cm)×15(cm) / (0.025(g SAF / g supply)×π×((0.368 2 (cm 2 )) / 4)×15cm×1g supply / cm 3 ) = 1.7MJ / kg SAF.

[0095] Note that in the above example, the total UV energy is 38.4 MJ / kg SAF and the corresponding UV irradiance energy is 1.7 MJ / kg SAF. These UV energy values ​​indicate an energy efficiency of the UV unit of 1.7 / 38.4 = 4.4%.

[0096] Molecular weight distribution (MWD) analysis This is done using Tetra-detection gel permeation chromatography (GPC-T; Jordi Labs; Mansfield, MA) technique, as is well known to those skilled in the art. This technique uses a GPC column, a refractive index (RI) detector, a right-angle light scattering (RALS) detector, and a viscometry (DP) detector. Approximately 50 mg of sample is diluted with gentle stirring into 2 mL of 0.1 M NaNO3 containing 0.01 M Na2HPO4 to obtain a clear solution. These solutions are injected without further treatment. The instrument is then run with pullulan standards (M w= 61,330 Da) to verify its performance. w =68,162Da).

[0097] It should be understood that the foregoing description has been given for clarity of understanding only, and that no unnecessary limitations should be implied therefrom, since modifications within the scope of the invention will be apparent to those skilled in the art.

[0098] Dimensions and values ​​disclosed herein should not be understood as being strictly limited to the exact numerical values ​​recited. Instead, unless otherwise indicated, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as "40 mm" is intended to mean "about 40 mm."

[0099] All documents cited herein, including any cross-references or related patents or applications, are incorporated herein by reference in their entirety unless expressly excluded or otherwise limited. The citation of any document shall not be deemed to be prior art to any invention disclosed or claimed herein, or to teach, suggest, or disclose any such invention, either alone or in combination with any other reference(s). Furthermore, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.

[0100] While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

Claims

1. 1. A method for breaking down superabsorbent fibers (SAF) into soluble polymers for recycling, wherein the SAF is post-consumer SAF and is in the form of loose fibers, and the soluble polymer comprises 5% to 75% by weight of polymerized acrylic acid monomer units based on the total weight of the soluble polymer, the method comprising: passing a feed stream comprising the SAF through a UV irradiation zone; irradiating the feed stream with UV; and producing a product stream at an end of the UV irradiation zone comprising the soluble polymer, wherein the feed stream comprises the SAF at a concentration greater than about 1% by weight, the feed stream having a residence time in the UV irradiation zone of less than about 120 seconds, and the UV used to convert the SAF to the soluble polymer requires a total UV energy of less than about 50 MJ / kg SAF.

2. The method of claim 1 , wherein the residence time is less than about 60 seconds.

3. 3. The method of claim 1 or 2, wherein the UV total energy is less than about 16 MJ / kg SAF.

4. The method of any one of claims 1 to 3, wherein the feed stream comprises SAF and water.

5. The method of any one of claims 1 to 4, wherein the feed stream comprises SAF and hydrogen peroxide.

6. 6. The method of any one of claims 1 to 5, wherein the SAF has a sodium level greater than 8% by weight as Na.

7. 7. The method of any one of claims 1 to 6, wherein the SAF has a sodium level of about 10% to about 20% by weight as the amount of Na.

8. 8. The method of any one of claims 1 to 7, wherein the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 6.

9. 9. The method of any one of claims 1 to 8, wherein the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 4.

10. 10. The method of any one of claims 1 to 9, wherein the feed stream has a viscosity, the product stream has a viscosity, the ratio of the viscosity of the product stream to the viscosity of the feed stream is a viscosity ratio, and the negative decimal logarithm of the viscosity ratio is less than about 2.

11. The method of any one of claims 1 to 10, wherein the soluble polymer has a weight average molecular weight of less than about 2,000,000 g / mol.

12. The method of any one of claims 1 to 11, wherein the soluble polymer has a weight average molecular weight of at least 10,000 g / mol.

13. The method of any one of claims 1 to 12, wherein the soluble polymer has a polydispersity index (PDI) of less than about 4.

14. 14. The method of any one of claims 1 to 13, wherein the soluble polymer is used to produce recycled SAF and / or recycled superabsorbent polymer particles (SAP), the SAF and / or SAP comprising a concentration of soluble polymer, the soluble polymer concentration being less than about 30%.

15. 15. The method of any one of claims 1 to 14, wherein the SAF of claim 1 has a Centrifuge Retention Capacity of at least 15 g / g, measured using the Centrifuge Retention Capacity (CRC) test method set forth in Nonwoven Standard Procedure 241.0.R2(19) by the European Nonwovens Association.

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