Degradation of Superabsorbent Polymers via Oxidative Degradation

Abstract

The present invention discloses a method for degrading a crosslinked and poly(acrylic acid)-based superabsorbent polymer (SAP) into a soluble polyacrylic acid polymer. The degradation is achieved using an oxidatively water-soluble salt comprising at least one cation and at least one anion.

Description

Technical Field

[0001] The present invention generally relates to the oxidative degradation of poly(acrylic acid)-based superabsorbent polymers (SAPs) that are available for the recycling of, in particular, post-consumer use SAPs. Oxidative water-soluble salts are used to degrade the SAPs. Background Art

[0002] In view of the global challenges regarding sustainability, there is a need to recycle absorbent hygiene products (AHPs), such as diapers and pants, and feminine hygiene products. Many consumer companies are committed to using 100% recycled materials and sending zero consumer and manufacturing waste to landfills. In addition to these goals, successful recycling benefits the environment, stimulates the economy, improves human health and water quality, and generates energy that is needed by consumers in the world's developing regions.

[0003] The main component in AHPs is typically a superabsorbent polymer (SAP), while the minor components are adhesives, cellulose fibers, polyethylene, polypropylene, and polyester. Recycling of AHPs involves cleaning the dirt that accumulates during their use and separating the various components into recycling material streams. More specifically, the recycled SAP material stream can be used in applications that require less than the AHPs (since recycled SAP has poorer properties compared to virgin SAP), and / or can be converted into substantially linear or branched non-crosslinked poly(acrylic acid) (PAA). This PAA can then be used as a feedstock for various applications. For example, the PAA can be used as such in applications such as water treatment or corrosion inhibition; or it can be used or further esterified and then used in adhesives, coatings, etc. These applications are part of the effort to recycle SAP into other products by replacing virgin acrylic-based compounds with compounds derived from recycled SAP. In all cases, the aim is to achieve the same properties as the virgin material.

[0004] Recycled SAP can be post-consumer recycled (PCR) SAP or post-industrial recycled (PIR) SAP. Non-limiting examples of methods for generating a recycled SAP material stream from recycled AHPs are disclosed and claimed in U.S. Patent 9,095,853 B2, published on August 4, 2015; and 9,156,034 B2, published on October 13, 2015; both of which are assigned to Fater S.p.A, Italy.

[0005] Most SAPs are based on poly(acrylic acid) and are crosslinked network materials. Non-limiting examples of procedures for generating SAPs from acrylic acid and crosslinking agents are disclosed in U.S. Patent 8,383,746 B2, published on February 26, 2013 and assigned to Nippon Shokubai Co., Ltd., Japan; and U.S. Patent 9,822,203 B2, published on November 21, 2017 and assigned to BASF SE, Germany.

[0006] There are many references regarding attempts to degrade or depolymerize linear polymers, while there are only a few references regarding efforts to depolymerize SAPs containing branched PAA crosslinked into a polymer network. The typical forms of energy used in these efforts (as a single form or combination of energies) are ultrasound, UV, mechanical (i.e., in the presence of tensile / elongation forces; example: Caruso, M.M. et al., Chem. Rev., 109 (2009), 5755 - 5798), heat (example: McNeill, I.C. and Sadeghi, S.M., Polymer Degrad. Stab., 29 (1990), 233 - 246), and microwaves.

[0007] However, there is a need to degrade post - industrial recycled (PIR) SAPs (e.g., derived from unused AHP that is picked out during manufacturing and not provided to consumers; or derived from SAP manufacturing methods, such as when the SAP does not meet the required performance criteria), and particularly a need to degrade post - consumer recycled (PCR) SAPs derived from post - consumer AHP (i.e., after the AHP has been used). Thus, there is a need to degrade or depolymerize recycled SAPs into soluble linear or branched poly(acrylic acid) (PAA) in a short time frame, at low energy and power per unit mass of SAP, and under mild conditions (e.g., relatively low temperature) without using chemicals that are considered to pose environmental problems. The need for low energy per unit mass of SAP stems from the fact that recycling of SAP and its degradation or depolymerization into PAA is beneficial only when the energy consumed during the conversion of SAP to PAA is less than the energy used to prepare fossil - derived acrylic acid (petroleum - AA) from propylene (which is approximately 50 MJ / kg AA). The PAA produced from recycled SAP can then be derivatized into materials for other applications such as adhesives, coatings, water treatment, fabric care, etc. SUMMARY OF THE INVENTION

[0008] The present invention relates to a method for degrading a crosslinked and poly(acrylic acid) - based superabsorbent polymer (SAP) into a soluble polyacrylic acid polymer. The method comprises the following steps:

[0009] a) Provide SAP,

[0010] b) Provide an oxidation water-soluble salt comprising at least one cation and at least one anion;

[0011] c) Provide an aqueous carrier such as water or physiological saline (i.e., a solution containing 0.90 wt% NaCl per liter of water),

[0012] d) Mix the SAP with the oxidation water-soluble salt and the aqueous carrier,

[0013] e) Heat the mixture to a temperature of 30°C to 200°C to degrade the SAP into a soluble polyacrylic acid polymer (PAA).

[0014] The oxidation water-soluble salt can be dissolved in the aqueous carrier in method step d), or preferably before method step d).

[0015] The at least one anion can be selected from: persulfate, peroxymonosulfate, peroxydicarbonate, peroxydiphosphate, peroxydiborate, and mixtures and combinations thereof. Preferably, the at least one anion is persulfate. Description of the Drawings

[0016] Figure 1 Pictures are shown before and after the degradation step of Example A17, i.e., the left picture shows the sample after step 5 of the following experimental procedure, and the right picture shows the sample after the completion of the experimental procedure.

[0017] Figure 2 Pictures are shown before and after the degradation step of Comparative Examples C1 and C2, i.e., the left picture shows the sample after step 5 of the following experimental procedure, and the right picture shows the sample after the completion of the experimental procedure. It can be seen that no degradation occurred in Examples C1 and C2. Detailed Description

[0018] Definition

[0019] As used herein, "superabsorbent polymer" refers to an absorbent material that is a crosslinked and poly(acrylic acid)-based superabsorbent polymer capable of absorbing at least 10 times its own weight of 0.9% saline solution, as measured using the Centrifugal Retention Capacity (CRC) test method (EDANA method NWSP 241.0.R2). These polymers are typically used in particulate form ("superabsorbent polymer particles"). The term "particulate" refers to granulates, fibers, flakes, spheres, powders, sheets, and other shapes and forms known to those skilled in the art of superabsorbent polymer particles.

[0020] The superabsorbent polymer particles can be spherical superabsorbent polymer particles, or oval superabsorbent polymer particles, or irregular-shaped superabsorbent polymer particles, or fibrous superabsorbent polymer particles, namely, slender needle-shaped superabsorbent polymer particles.

[0021] As used herein, the term "poly(acrylic acid)" or "PAA" refers to a substantially uncrosslinked poly(acrylic acid) molecule having acrylic acid as monomer units. In contrast to SAP, PAA is water-soluble. The PAA of the present invention can be linear or branched. Due to the reaction mechanism of the present invention, the PAA of the present invention can contain oxygen-containing side groups or end groups. The PAA can also contain reaction products of a very small amount of crosslinking agent that is used for degradation when preparing the SAP provided to the method of the present invention. Based on the amount of acrylic acid, the SAP provided for the present invention can contain less than 0.2 mol% of the reaction products of the crosslinking agent. For the purposes of the present invention, PAA includes polymers and oligomers of acrylic acid. Preferably, the PAA is a polymer having an average molecular weight Mw of at most 10 MDa, more preferably at most 10 MDa. More preferably, the PAA is a polymer having an average molecular weight of at least 1 kDa, or at least 5 kDa, or at least 10 kDa.

[0022] As used herein, the term "degradation" refers to the conversion of SAP into PAA via the action of depolymerization, de-crosslinking, main chain breakage of molecules, or any combination of the above actions. For the purposes of the present invention, the terms degradation, recycling, and conversion can be used interchangeably as long as they refer to the conversion of SAP into PAA.

[0023] As used herein, the terms "absorbent hygiene product" and "AHP" refer to devices that absorb and contain body exudates, and more specifically refer to devices that are placed in close contact with or adjacent to the body of the wearer to absorb and contain various exudates discharged from the body. Absorbent articles include diapers (e.g., baby diapers and diapers for adult incontinence), absorbent pants, absorbent pads, feminine care absorbent articles such as sanitary napkins or pantiliners, etc. The term "exudates" includes, but is not limited to, urine, blood, vaginal secretions, sweat, and feces. Preferred AHPs of the present invention are diapers, absorbent pants, and / or feminine care absorbent articles. The AHP can be disposable, and the post-industrial or post-consumer SAP provided for the method of the present invention can be derived from the AHP.

[0024] "Disposable" is used in its ordinary sense and refers to an article that is disposed of or discarded after a limited number of uses (e.g., less than 20 uses, less than 10 uses, less than 5 uses, or less than 2 uses) over different time periods. If the disposable absorbent article is a diaper, pants, sanitary napkin, menstrual pad, or wet wipe for personal hygiene, the disposable absorbent article can (and most commonly) be intended to be disposed of after a single use.

[0025] Degradation method

[0026] Unexpectedly, it has been found that SAP can be degraded into soluble PAA, particularly in the form of polymers of acrylic acid, by mixing SAP with an oxidizing water-soluble salt (hereinafter referred to as "salt"). The salt contains at least one cation and at least one anion. The SAP and the salt are mixed with an aqueous carrier such as water or physiological saline.

[0027] By heating the mixture to a temperature of 30°C to 200°C (hereinafter referred to as "high temperature"), at least some of the anions decompose into free radicals.

[0028] The high temperature can be at least 35°C, or at least 40°C, or at least 50°C, or at least 60°C, or at least 70°C, or at least 80°C. The high temperature can be less than 190°C, or less than 180°C, or less than 150°C. Generally, at high temperatures above 200°C, the SAP begins to decompose and decay in an uncontrolled manner, which is not desirable for the present invention.

[0029] Upon heating, the hydrogen atoms of the salt, more specifically: the hydrogen atoms of one or more anions of the salt are abstracted, and the anions form free radicals. The high temperature to which the mixture is heated can be at least 10°C lower than the decomposition temperature of the salt (resulting in free radical formation), or the mixture can be heated to a high temperature at least equal to the decomposition temperature, or heated to a high temperature at least 10°C higher than the decomposition temperature of the salt.

[0030] As used herein, "decomposition temperature" is the 10-hour half-life temperature in water, which is, for example, 69°C for ammonium persulfate and 60°C for potassium persulfate.

[0031] Thus, the choice of the optimal temperature range depends particularly on the choice of the salt, since different salts (especially different anions) have different decomposition temperatures. The free radicals formed can react with the SAP, for example, by reacting with the aliphatic C-H groups contained in the polymer chains of the SAP. As a result of this free radical reaction, the polymer chains of the SAP are broken, and carbon-centered free radicals are formed at the broken SAP polymer chains. The reaction can also occur at the carboxyl groups of the SAP, which also results in carbon-centered free radicals. Alternatively or in addition, the reaction can occur at a nitrogen atom, which can be contained in the crosslinking agent used for the initial preparation of the SAP. If the reaction occurs at the nitrogen atom, nitrogen-centered free radicals rather than carbon-centered free radicals are formed.

[0032] Without being bound by theory, it is believed that the following reaction scheme exemplarily shows the process of degradation of SAP into soluble PAA (i.e., the following "crosslinking polymer product"):

[0033]

[0034] (Generation of sulfate radicals and hydroxyl radicals in an aqueous medium)

[0035]

[0036] Wherein R is H, or an alkaline cation, an ammonium cation, or a cross-linked residue.

[0037] The mixture can be maintained at a high temperature for 10 minutes to 5 hours, preferably 10 minutes to 4 hours, more preferably 10 minutes to 3 hours. From an economic perspective, a shorter time is preferred. Shorter treatment times can be obtained, for example, by a higher salt concentration, a higher temperature (however, below 200 °C) and / or by an optimized mixing of the SAP and the salt. The time for which the mixture is held at a high temperature also depends on the desired degree of degradation (i.e., the average molecular weight of the PAA obtained by the process). Generally, once the SAP has degraded such that no or only trace amounts of insoluble SAP are present, indicating that all of the SAP has decomposed into soluble PAA, the mixture may no longer need to be maintained at a high temperature and the temperature can be reduced to room temperature (25 °C) or lower.

[0038] The SAP, the salt and the aqueous carrier can be mixed, for example, by pre-mixing the salt and the aqueous carrier such that the salt is partially or completely dissolved in the aqueous carrier. Then, the aqueous carrier in which the salt is dissolved can be mixed with the SAP, such as by spraying the aqueous carrier having the dissolved salt onto the SAP. After spraying the aqueous carrier having the dissolved salt onto the SAP, the mixture may or may not be further mixed, depending, for example, on the amount of SAP, i.e., the thickness of the SAP layer. If the aqueous carrier having the dissolved salt is sprayed onto a thin layer of SAP such that the SAP is properly contacted with the aqueous carrier and the dissolved salt, further mixing may not be required.

[0039] As an alternative to pre-mixing the aqueous carrier with the salt to dissolve the salt in the aqueous carrier, the aqueous carrier and the salt can also be provided separately to the SAP such that the salt is only dissolved in the aqueous carrier after having been mixed with the SAP. Importantly, the salt must be able to dissolve in the aqueous carrier after contact with the SAP or preferably before contact with the SAP.

[0040] Before mixing the aqueous carrier with the salt and the SAP, the aqueous carrier can be pre-heated to a high temperature. Such pre-heating can accelerate the degradation process. Alternatively, however, the aqueous carrier can be pre-heated to a temperature below the high temperature before mixing with the salt and the SAP. Still further alternatively, the aqueous carrier can not be pre-heated before mixing with the salt and the SAP and heating to the high temperature is completed after mixing the aqueous carrier, the salt and the SAP.

[0041] If the salt is dissolved in the aqueous carrier before mixing with the SAP, the aqueous carrier can be preheated to a temperature below the high temperature to avoid premature formation of free radicals by the anions, such that the anions degrade by self-decomposition and are subsequently no longer available for degrading the SAP after mixing with the SAP. However, if the salt is dissolved in the aqueous carrier only for a short time or immediately before mixing with the SAP, the aqueous carrier can be preheated to the high temperature before mixing with the SAP. The preheating can accelerate the time for the salt to dissolve in the aqueous carrier.

[0042] Alternatively or in addition, the SAP can be preheated to the high temperature or a temperature below the high temperature before mixing with the aqueous carrier and the salt. The preheated SAP can result in a shorter swelling time of the SAP, thus accelerating the absorption of the aqueous carrier and the dissolved salt into the SAP particles, enabling faster degradation. The faster absorption of the dissolved salt into the SAP can also improve the uniform dispersion of the dissolved salt within the SAP, which can contribute to more uniform degradation and thus avoid undegraded SAP fragments remaining in the mixture.

[0043] Still alternatively, the mixture obtained in method step d) can be heated to the high temperature only after at least 50 wt%, or at least 70 wt%, or at least 90 wt%, or all of the aqueous carrier in which the salt is dissolved has been absorbed into the SAP. However, some preheating to a temperature below the high temperature may have been completed previously.

[0044] The ratio of the salt to the SAP can be from 0.001 g of salt to 0.05 g of salt / 1 g of SAP, or can be from 0.005 g of salt to 0.03 g of salt / 1 g of SAP, or can be from 0.01 g of salt to 0.03 g of salt / 1 g of SAP.

[0045] The ratio of the aqueous carrier to the SAP can be from 2 g to 20 g of aqueous carrier / 1 g of SAP, or can be from 5 g to 15 g of aqueous carrier / 1 g of SAP.

[0046] The SAP can be provided as dry or swollen to less than 20 g, or less than 15 g, or less than 10 g, or less than 5 g of liquid (such as water or saline) / g of SAP in method step a).

[0047] The total amount of liquid absorbed into (i.e., contained in) the SAP in method step e) - including the liquid contained in the swollen SAP provided in method step a) (if the SAP is not provided as dry SAP) and the amount of aqueous carrier absorbed into the SAP in method step e) and thus contained in the SAP in method step e) - can be from 2 g to 25 g per 1 g of SAP, or can be from 2 g to 20 g per 1 g of SAP, or can be from 5 g to 15 g per 1 g of SAP, or can be from 8 g to 13 g per 1 g of SAP. As used herein, "dry SAP" means that the SAP has a liquid content (referred to as "water content") of less than 0.20 g / g of SAP, preferably less than 0.15 g / g of SAP. The water content of the SAP is measured according to the EDANA water content test method NWSP 230.0.R2(15) or via a moisture analyzer (HX204, obtained from Mettler Toledo, drying temperature 130 °C, starting superabsorbent weight 3.0 g (±0.5 g), stop criterion 1 mg / 140 s). If the water content of the superabsorbent polymer particles is greater than 3 wt%, the SAP is dried to a moisture content of <3 wt%, for example in an oven at 105 °C for 3 hours or for example at 120 °C for 2 hours.

[0048] To ensure that the salt is available to effectively degrade the SAP, it is desirable that a significant amount of the aqueous carrier in which the salt is dissolved is absorbed into the SAP in method steps d) and e). At least 50 wt%, or at least 60 wt%, or at least 75 wt%, or at least 90 wt%, or 100 wt% of the aqueous carrier in which the salt is dissolved provided in step c) can be absorbed into the SAP. Absorption of the aqueous carrier in which the salt is dissolved in method steps d) and e) means that the aqueous carrier in which the salt is dissolved can only be absorbed in method step d) (which would be the case especially when 100 wt% is absorbed), or mainly in method step e) (which can be the case if heating has already started when the SAP, salt, and aqueous carrier are mixed), or a portion of the aqueous carrier in which the salt is dissolved is absorbed into the SAP in method step d) while another portion of the aqueous carrier in which the salt is dissolved is absorbed into the SAP in method step e).

[0049] The SAP provided in method step a) can have a centrifuge retention capacity (CRC) value of from 10 g / g to 50 g / g, or from 10 g / g to 40 g / g. If drying is required for the recycling of the AGM, the CRC is measured as measured according to the CRC test method (EDANA method NWSP241.0.R2).

[0050] If the SAP provided in method step a) is post-consumer recycled SAP, the SAP (sample) must first be dried, and then the CRC of the sample is measured to determine the CRC of the SAP.

[0051] At least one anion of the salt may be selected from: persulfate, monopersulfate, peroxydicarbonate, peroxydiphosphate, peroxydiborate, or a mixture thereof.

[0052] At least one cation of the salt is not critical because the cation does not dissociate into free radicals. Thus, the choice of cation does not directly affect the degradation process because the cation does not form free radicals. At least one cation may be selected to have sufficient solubility in the aqueous carrier and it should be obtained at a relatively low cost. At least one cation may be selected from: Li + , Na + , K + , Rb + , Cs + , NH 4 + , organically substituted ammonium, Ca2+, Mg2+, Sr2+, Ba2+, Al3+, transition metal cations in the 1+ to 3+ oxidation states, or a mixture thereof (e.g., a combination of different salts with different cations). Most preferably, one or more basic cations and NH 4 + cations.

[0053] Based on the total weight of the salt, at least 50 wt%, or at least 60 wt%, or at least 70 wt%, or at least 80 wt%, or at least 90 wt%, or 100 wt% may be basic persulfate.

[0054] Hydrogen peroxide may be added in the method. Hydrogen peroxide may help to increase the per-time yield of PAA, i.e., the degradation rate. Hydrogen peroxide may also help to decolorize the decomposed pollutants. Hydrogen peroxide may be added as a separate aqueous solution to the SAP, or it may be added to the aqueous carrier before mixing with the SAP, with or without the salt dissolved in the aqueous carrier. The amount of hydrogen peroxide used in the method of the present invention may be 10 wt% to 200 wt% based on the weight of the salt, or may be 20 wt% to 100 wt% based on the weight of the salt, or may be 30 wt% to 80 wt% based on the weight of the salt.

[0055] Method step e) may be carried out at a pH of 3 - 7. Generally, no additional special measures are required to obtain a pH within this range. Persulfate radicals are less stable, for example, at a pH above 7.

[0056] Additives can be used in the method of the present invention. For example, small molecular weight alcohols such as methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, propylene glycol, glycerol, or mixtures thereof can be added to the aqueous carrier provided in method step c), or to the mixture in method step d). These additives can support the initial wettability of the aqueous carrier and the salts dissolved therein on the SAP. They can also improve the stability of the aqueous carrier against bacterial contamination. Other additives such as antibacterial additives can also be added. The total amount of the additives can be no more than 10% by weight, or no more than 8% by weight, or no more than 5% by weight, or no more than 3% by weight based on the weight of the aqueous carrier.

[0057] The method of the present invention can be carried out in a continuous method or a batch method. Generally, from a commercial / cost perspective, the continuous method is usually preferred. In the continuous method, the SAP can be provided, for example, in a continuous stream on, for example, a carrier tape, and the aqueous carrier and the salt (and optionally hydrogen peroxide) can be mixed with the SAP, for example, by spraying the aqueous carrier and the salt onto the SAP. The mixture of the SAP, the salt, and the aqueous carrier can be transferred to a tape after method step d) (for example, after the aqueous carrier in which the salt is dissolved has been partially or completely absorbed into the SAP), and heated in a continuous or batch manner.

[0058] Alternatively, the aqueous carrier can be provided in a batch container or a similar container (wherein the salt is dissolved therein before or after the aqueous carrier is provided into the container). Then, the SAP can be added to the container that has been filled with the aqueous carrier and the dissolved salt, and the SAP can be made to absorb the aqueous carrier and the salt dissolved therein, and the mixture can be heated to a high temperature simultaneously or subsequently.

[0059] For SAP, especially if provided as dry SAP particles, air tends to "be trapped" between the particles, i.e., in the voids between the SAP particles when the SAP particles absorb liquid and swell. Therefore, the swollen SAP tends to "float" in the liquid. As the SAP degrades, the dissolved PAA can sink in the container, where it can be (continuously) removed. To avoid the removal of undegraded or partially degraded SAP together with the PAA (since some SAP can sink in the container), a sieve or the like can be installed in the container to prevent undegraded or fully degraded SAP particles from further sinking to the bottom of the container, because they will be trapped in the sieve until they are more fully degraded and can pass through the sieve.

[0060] Alternatively, the mixture of the SAP, the salt, and the aqueous carrier can also be stirred so that the swollen SAP particles sink towards the base surface of the container, and the soluble PAA, i.e., the product of the method, can be removed from the upper part of the container.

[0061] The obtained solution in which PAA is dissolved can be transferred, for example via a pump, to different containers, pipes, or any other suitable device for any post-treatment that may be desired for the solution. Possible post-treatments are filtration, desalination, or many other treatments.

[0062] The energy consumption of the degradation process depends especially on the high temperature. The higher the high temperature, the higher the energy consumption per time (i.e., a shorter treatment time at a higher high temperature may overall require less energy compared to a relatively lower high temperature at a relatively longer treatment time). For example, for a batch process in an adiabatic container (i.e., where the process of heating to the high temperature is only done once), for a high temperature of about 100 °C, the energy consumption is about 3.5 MJ / kg of dry AGM.

[0063] SAP provided in method step a)

[0064] The SAP provided in method step a) can be in particulate form.

[0065] The SAP provided in the method can be native SAP, post-consumer recycled SAP (PCR SAP), post-industrial recycled SAP (PIR SAP), or a combination of those materials. "Post-consumer SAP" and "post-consumer recycled SAP" (PCR SAP) are used interchangeably herein and as used herein refer to SAP that has been included in an AHP and the AHP has been used by a consumer (e.g., worn by an incontinence user). After use, the AHP is recycled and the PCR SAP is separated from the AHP. However, for the method of the present invention, it is not necessary to purify the SAP such that the post-consumer SAP provided for the method of the present invention does not contain other components of the post-consumer AHP.

[0066] "Post-industrial SAP" and "post-industrial recycled SAP" (PIR SAP) are used interchangeably herein and as used herein refer to SAP that may or may not be included in an AHP. The PIR SAP has not been used previously, e.g., it is not included in an AHP that has been used by a consumer. Instead, the PIR SAP can be derived from AHPs that have been selected during production, e.g., because they are defective. The PIR SAP can also be selected during SAP production, e.g., because they do not meet the required performance goals (such as capacity, whiteness, etc.). Thus, for the latter case, the PIR SAP was not previously included in an AHP.

[0067] Typical properties of SAP are mechanical properties, swelling ability, saline flow conductivity (SFC), absorption against pressure (AAP), residual monomers, extractables, and cylinder retention capacity (CRC). Also, for the purposes of this invention, the SAP can include other comonomers such as itaconic acid, acrylamide, etc. The amount of the comonomer can be less than 2.0 wt%, or less than 1.5 wt%, or less than 1.0 wt%, or less than 0.5 wt% based on the total weight of the dried SAP.

[0068] SAP is typically prepared using homogeneous solution polymerization methods or by multiphase polymerization techniques such as inverse emulsion or suspension polymerization. The polymerization reaction is typically completed in the presence of a relatively small amount of difunctional or polyfunctional monomers such as N,N'-methylenebisacrylamide, triacrylate, ethylene glycol di(meth)acrylate, triallylamine, etc. The difunctional or polyfunctional monomer compounds are used to lightly crosslink the acrylate polymer chains so that the SAP is water-insoluble but water-swellable. In addition, the SAP can be surface-crosslinked after polymerization by reaction with a suitable crosslinking agent such as di / polyepoxides, di / polyols, di / polyhaloalkanes, etc. The SAP provided for the method of this invention can be in particulate form. The particulate form can be produced from a mass of material using any typical size reduction technique such as grinding.

[0069] The SAP can be completely unneutralized (in which case, DN = 0), completely neutralized (in which case, DN = 100%), or partially neutralized. In one embodiment of the invention, the SAP has a DN greater than about 50%. In another embodiment of the invention, the SAP has a DN of about 65% to about 75%. In yet another embodiment of the invention, the SAP has a DN greater than about 75%. In still yet another embodiment of the invention, the SAP has a DN below about 50%.

[0070] The SAP provided to the method of the present invention may be in dry form or may be partially swollen with water, brine or urine (e.g., urine in PCRSAP). Thus, the SAP may be swollen with water, brine or urine to 0.05 g / g to 20 g / g, preferably 0.05 g / g to 15 g / g, more preferably 0.10 g / g to 10 g / g, more preferably 0.20 g / g to 5 g / g, and even more preferably 0.50 g / g to 2 g / g. Completely dry (i.e., 0 g / g of water, brine or urine) may not be very advantageous for the method of the present invention because completely dry SAP takes longer to absorb the aqueous carrier in which the salt is dissolved. On the other hand, SAP that is overly swollen (or even completely swollen) when provided to the method may also result in an increase in time until the salt dissolved in the aqueous carrier is absorbed into the SAP. The SAP provided to the method may have an absorption capacity CRC of 10 g / g to 50 g / g (measured as the centrifuge retention capacity "CRC" according to the EDANA method NWSP 241.0.R2).

[0071] The amount of the aqueous carrier provided in method step c) may be such that the SAP provided in step a) can swell to at least 20%, or at least 30%, 50%, or at least 60%, or at least 70%, or at least 80% of its CRC when absorbing all of the provided aqueous carrier. If the SAP is not provided as dry but pre-swollen (see further details below), less aqueous carrier is required to obtain the desired degree of SAP loading, i.e., the desired CRC.

[0072] Upon absorption of the liquid, the polymer chains within the polymer network of the SAP begin to disentangle. Such disentanglement will make the polymer network more accessible to the radicals formed by the anions of the salt. Thus, the degradation is improved. If the amount of the aqueous carrier provided in step c) does not allow the SAP to swell to at least 20% of its CRC when absorbing the aqueous carrier, the polymer chains within the polymer network of the SAP may not be sufficiently disentangled, resulting in slower or generally less effective degradation.

[0073] For the degradation method of the present invention, it may be preferable to use post-consumer SAP rather than virgin SAP: the polymer chains in the polymer network of SAP that has been previously swollen and then at least partially redried have been disentangled. It is believed that re-swelling and thus re-disentanglement is faster relative to the swelling of virgin SAP. It has been found that SAP that has been previously swollen and then redried has a higher CRC when swollen after redrying compared to the CRC measured when swelling SAP from virgin SAP for the first time.

[0074] If post-consumer SAP is provided for the method of the present invention in a partially swollen form, given that drying post-consumer SAP is time-consuming and energy-consuming, it is also advantageous that the SAP used in the method does not need to be completely dried. However, post-consumer SAP can be sterilized before being provided to the method of the present invention.

[0075] If post-consumer SAP or post-industrial SAP is separated from AHP to be provided for the method of the present invention, the SAP does not necessarily need to be purified so that no other components of AHP are present. On the contrary, it has been found that the SAP may be contaminated by other components of AHP such as synthetic fiber materials or membranes (e.g., fibers, sheets, membranes, and fiber layers), cellulose fibers, adhesives, inks, dyes, surfactants, etc. The amount of these contaminants may not exceed 20% by weight of the mixture of SAP and the contaminants, or may not exceed 15% by weight, or not exceed 10% by weight, or not exceed 5% by weight, or not exceed 2% by weight, or not exceed 1% by weight.

[0076] If post-consumer SAP is still swollen, for example, with urine or other liquids, when calculating the amount of contaminants by weight of the mixture of SAP and the contaminants, the urine or other liquids contained in the SAP are not taken into account.

[0077] If SAP is provided as dry SAP for the present invention, the average particle size of post-consumer SAP can optionally be reduced, for example, by grinding, milling, or other suitable means. The D50 average particle size of the dry SAP provided for the present invention (whether PCR SAP, PIR SAP, or SAP manufactured natively) can be from 100 μm to 1,000 μm, as measured according to ISO method 13322-2. The particle size distribution (PSD) of the dry SAP can be from 40 μm to 5,000 μm, or from 50 μm to 2,000 μm, or from 50 μm to 1,000 μm, or from 50 μm to 800 μm.

[0078] If SAP is provided in a pre-swollen form, for example, as post-consumer SAP that is undried or only partially dried after recycling, the SAP can be subjected to comminution to increase the surface area of the SAP, which can enable faster absorption of the aqueous carrier in which the salt is dissolved. Such faster absorption can in turn lead to faster degradation of the SAP. Comminution can be accomplished, for example, by wet milling.

[0079] A smaller particle size can contribute to the rapid and uniform absorption of the dissolved salt into the SAP, resulting in faster and more complete degradation of the SAP.

[0080] Optional method step f) of separating the soluble polyacrylic acid polymer in the aqueous solution from the other compounds and components in the mixture:

[0081] Once the SAP has been decomposed into PAA, the PAA can be separated from the mixture of (possibly remaining undecomposed) SAP, salts, aqueous carrier, and optionally additional components such as hydrogen peroxide. The mixture may still contain a certain amount of undecomposed SAP, which may be present as a solid insoluble component in the mixture.

[0082] The PAA can be extracted from the mixture via a variety of methods. Non-limiting examples of these methods are water evaporation, filtration of PAA, water extraction, etc. Also, the salts can be removed via any desalination technique known to those skilled in the art. Non-limiting examples of desalination methods are membrane methods (e.g., reverse osmosis, forward osmosis, electrodialysis reversal (EDR), nanofiltration, etc.), freeze desalination, solar desalination, geothermal desalination, ion exchange, wave power desalination, etc. The same techniques can generally also be applied to remove other small molecular weight compounds in the mixture, e.g., other typical compounds of post-consumer AHP, such as adhesives, inks, dyes, surfactants, and degradation products of these compounds.

[0083] For example, filtration can be used to eliminate solid compounds and components from the mixture, i.e., method step d) for separating the soluble polyacrylic acid polymer in the aqueous solution from other compounds and components in the mixture obtained by step c). The solid compounds and components can be the remaining insoluble SAP and other components of post-consumer AHP, such as synthetic fiber materials or membranes (fibers, sheets / membranes / fiber layers) and cellulose. It is noteworthy that polyolefins (e.g., polypropylene, polyethylene) contained in other components of post-consumer AHP (such as synthetic fiber materials or membranes) are insoluble or non-swellable in the aqueous carrier. They can only react with salts to a negligible extent, i.e., the polyolefins are not degraded or only insignificantly degraded by the method of the present invention. The same applies to PET, which can also be contained in synthetic fiber materials or membranes. Therefore, those materials will remain as solid components in the mixture and can be filtered out.

[0084] For example, PEG (i.e., another typical component in post-consumer AHP) contained in surfactants is degraded by the method of the present invention. However, PEG is usually degraded into relatively small molecular weight molecules, which are significantly smaller than the molecular weight of PAA. Therefore, the small molecular weight reaction products of PEG can be separated from the soluble PAA polymer, for example, by the above techniques.

[0085] Alternatively or in addition, the mixture of PAA, (possibly remaining part of) SAP, salts, and aqueous carrier (which may contain compounds of post-consumer AHP) can also be mixed in a co-solvent in which PAA is insoluble, so that the PAA will precipitate to separate it from the mixture. Before such mixing in the co-solvent, the solid compounds in the mixture can be removed by filtration.

[0086] PAA obtained by this method

[0087] The PAA obtained by the degradation method of the present invention may have different molecular weights. The PAA may or may not contain oligomers. Preferably, the PAA does not contain oligomers, that is, the PAA only refers to polymers. The average molecular weight Mw of the PAA may be at most 10 MDa or at most 5 MDa. The average molecular weight Mw of the PAA may be at least 10 kDa or at least 20 kDa. The PAA may be linear or branched. However, the PAA is not cross-linked and thus they are water-soluble.

[0088] The PAA obtained by the method of the present invention can be used, or derivatized into materials for other applications such as adhesives, coatings, water treatment, etc. In one embodiment of the present invention, the as-received or derivatized PAA is used in adhesives. In yet another embodiment of the present invention, the as-received or derivatized PAA is used in fabric care applications. In still yet another embodiment of the present invention, the as-received or derivatized PAA is used in water treatment applications.

[0089] Example

[0090] Experimental procedure

[0091] Several samples of SAP were subjected to the method of the present invention. The SAP used in all examples is polyacrylic acid-based SAP, with a capacity (CRC) of 27.6 g / g, a water content of 0.4%, and an average particle size D50 of 398 μm, as measured according to ISO method 13322-2 (particle size distribution PSD is 63 - 710 um). The anti-pressure absorption (AAP) of the SAP is 25.5 g / g, as determined by the EDANA method WSP 442.2-02. In the deviation from EDANA WSP 442.2-02, a pressure of 0.7 psi is applied (while the pressure specified by the EDANA method is only 0.3 psi).

[0092] The deionized water used hereinafter is MilliporeQ. The conductivity was measured using a laboratory conductivity meter COND 70 instrument (without CELL, #50010522, equipped with Cell VPT51-01 C = 0.1, obtained from XS Instruments) or via LF 320 / Set (#300243, equipped with 325, obtained from WTW), and the conductivity is <160 μS / cm at 0 °C. Therefore, similar equipment for measuring conductivity can be used. The deionized water used in the examples represents the aqueous carrier. The actual amount of deionized water (= aqueous carrier) in the samples is shown in the "m_w_total" column in Table 1.

[0093] Unless otherwise indicated, experimental procedures were carried out in a climate control chamber under standard conditions of 23 °C ± 2 °C temperature and 45% ± 10% relative humidity.

[0094] Procedure :

[0095] 1. A 0.5 wt% potassium persulfate (KPS) stock solution was prepared by completely dissolving 5.0 g of dry salt (Sigma-Aldrich, >= 99.0% purity, stock number 216224-500G) in 995.0 g of deionized water (i.e., the aqueous carrier) by stirring in a 1 L plastic bottle (prepared from HDPE, Nalgene TM ). Complete dissolution of the KPS salt was observed when no visible salt crystals remained in the solution.

[0096] 2. A 1.0 wt% hydrogen peroxide (HPO) stock solution (30.0 g) was prepared by adding 1.0 g of 30 wt% HPO (also known as strong hydrogen peroxide, Sigma-Aldrich, stock number 216763-500ML) to 29.0 g of deionized water in a 40 ml glass vial with a plastic snap cap and used fresh (within 24 hours).

[0097] 3. For all examples, on a balance, an amount of 2.00 g ± 0.02 g weight "M0" of dry SAP was measured into a 40 ml volume glass vial.

[0098] 4. Separately, in a 100 ml glass beaker (Pyrex), a corresponding amount of the aqueous carrier with diluted salt (as described in point 2 above) having a mass "Ms0" for each example was prepared, which contained a 0.5 wt% corresponding oxidation salt stock solution ("m_salt_solution") and additional deionized water ("m_water") (i.e., an additional amount of the aqueous carrier) in a corresponding volume in ml (the density of all solutions was 1.0 g / ml), and (for those examples containing HPO as indicated in Table 1) a 1.0 wt% hydrogen peroxide solution ("m_HPO"), measured by an Eppendorf pipette equipped with a 10 ml plastic pipette tip, such that the effective final weight concentrations given in Table 1 were obtained, i.e., "w_salt" for the salt and "w_HPO" for HPO.

[0099] 5. Add the amount "Ms0" of the aqueous carrier in which the salt is dissolved, prepared as per point 4, to the dry SAP in the glass vial such that all the SAP particles come into contact with the aqueous carrier in which the salt is dissolved. The amount "Ms0" for each example is given in Table 1. If necessary, gently shake by hand to improve the wetting and uniform swelling of all the SAP particles in each sample. Typically, it takes 10 seconds to 60 seconds until the entire amount of the aqueous carrier in which the salt is dissolved is absorbed into the SAP. Thus, the amount of the aqueous carrier with dissolved salt in the SAP is Ms0 / M0.

[0100] 6. Then, close the sample in the glass vial with a snap-on plastic cap and let it stand for 10 minutes.

[0101] 7. Preheat the circulating oven (Binder GmbH, Germany, Modell FED 720) to the temperature shown in the "Temperature T" column in Table 1 (i.e., "high temperature"). When the temperature is reached, place the corresponding vial with the sample on the aluminum tray and start the stopwatch.

[0102] 8. After the time shown in the "Time" column in Table 1 has elapsed, remove the corresponding sample from the glass vial and let it cool for 10 minutes.

[0103] 9. Then, transfer the sample from the glass vial to a centrifuge vial (a plastic 100 ml centrifuge vial with a screw cap). Place the centrifuge vial in a laboratory centrifuge (Multifuge X1m Thermo Scientific TM , equipped with a BIOshield TM 720 rotor) and centrifuge at 5000 rpm for 30 minutes (for this setting, equivalent to 4528 g-force). Centrifugation precipitates any remaining undegraded and insoluble portions of the SAP from the clarified solution formed during degradation. When no liquid formation is observed, centrifugation is not performed, e.g., in Comparative Examples C1 to C4.

[0104] 10. For all centrifuged samples, decant the clarified solution into a separate glass vial (40 ml) and thus separate it from any remaining undegraded and insoluble portions of the SAP.

[0105] 11. Measure the net weight of the clarified solution "Ms". Via a 5 ml plastic syringe, measure an aliquot portion of the clarified solution with a mass of "m_a" into a pre-weighed 10 ml empty (without snap cap) glass vial with a weight of "m_sc". Then, place the 10 ml vial with the clarified solution in a vacuum oven (Heraeus Vacutherm type, Thermo Scientific TM)Evaporate for 3 hours to ensure significant evaporation of water. Weigh the dried polymer residue and calculate the degradation yield "Y%" using its mass "Mp" via the following formula:

[0106] Y = 100*(Mp×Ms) / (m_a×M0), in wt%

[0107] Therefore, the yield Y represents the ratio of the extracted soluble polymer, which is the product of the SAP degradation solution, to the amount of the initial dried SAP. Considering that SAP is a crosslinked network of polyacrylic acid, the extracted soluble polymer is substantially soluble polyacrylic acid.

[0108] 12. The test conditions of sample A8 are the same as those of sample A18** to determine the reproducibility of the test procedure.

[0109] Details and results are given in Table 1.

[0110]

[0111] The dimensions and values disclosed herein should not be construed as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to represent the recited value and a range functionally equivalent around that value. For example, a dimension disclosed as "40 mm" is intended to represent "about 40 mm".

[0112] Unless expressly excluded or otherwise limited, each document cited herein, including any cross-referenced or related patent or application, is incorporated herein by reference in its entirety. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone or in any combination with any one or more other references anticipates, suggests, or discloses any such invention. Further, when any meaning or definition of a term in this invention conflicts with the same term's meaning or definition in a document incorporated by reference, the meaning or definition assigned to the term in this invention shall govern.

[0113] Although specific embodiments of the invention have been illustrated and described, it will be apparent 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. Accordingly, it is intended that all such changes and modifications that fall within the scope of the invention be covered by the appended claims.

Claims

1. A method for degrading a crosslinked and poly(acrylic acid)-based superabsorbent polymer (SAP) into a soluble polyacrylic acid polymer, comprising: a) providing the SAP, b) providing an oxidation water-soluble salt comprising at least one cation and at least one anion; wherein the at least one anion is selected from: persulfate, peroxymonosulfate, peroxydicarbonate, peroxydiphosphate, peroxydiborate, and mixtures and combinations thereof; c) providing an aqueous carrier, d) mixing the SAP with the oxidation water-soluble salt and the aqueous carrier, and e) heating the mixture to a temperature of 30°C to 200°C to degrade the SAP into a soluble polyacrylic acid polymer, wherein the method further comprises the step of adding hydrogen peroxide.

2. The method according to claim 1, wherein the oxidation water-soluble salt is dissolved in the aqueous carrier before or during method step d).

3. The method according to claim 2, wherein in method step d) and / or e), at least 50% by weight of the aqueous carrier in which the oxidation water-soluble salt is dissolved is absorbed into the SAP.

4. The method according to claim 3, wherein the SAP has absorbed a total amount of liquid of 5 g to 25 g / 1 g of SAP, wherein the total amount of liquid is the sum of the amount of liquid contained in the SAP provided in step a) and the amount of aqueous carrier absorbed in method steps d) and e).

5. The method according to any one of claims 1-4, wherein the amount of aqueous carrier provided in method step c) enables the SAP provided in step a) to swell to at least 20% of the CRC of the SAP when all of the provided aqueous carrier is absorbed, wherein the CRC is measured according to the EDANA method NWSP 241.0.R2.

6. The method according to any one of claims 1 - 4, wherein the cation is selected from: Li + , Na + , K + , Rb + , Cs + , NH 4 + , organically substituted ammonium, Ca 2+ , Mg 2+ , Sr 2+ , Ba 2+ , Al 3+ , transition metal cations in an oxidation state of 1+ to 3+, and mixtures and combinations thereof.

7. The method according to any one of claims 1-4, wherein the oxidation water-soluble salt comprises at least 50% by weight of basic persulfate based on the total weight of the oxidation water-soluble salt.

8. The method according to any one of claims 1-4, wherein the ratio of the oxidation water-soluble salt to the SAP is 0.001 g to 0.05 g of oxidation salt / 1 g of SAP.

9. The method according to any one of claims 1-4, wherein the ratio of the aqueous carrier to the SAP is 2 g to 20 g of aqueous carrier / 1 g of dry SAP.

10. The method according to any one of claims 1-4, wherein the SAP provided in step a) is dry SAP or swollen to 0.05 g to 10 g / 1 g of dry SAP.

11. The method according to any one of claims 1-4, wherein the SAP provided in step a) has a CRC value of 10 g / g to 50 g / g, as measured according to the EDANA method NWSP 241.0.R2.

12. The method according to any one of claims 1-4, wherein the mixed SAP, oxidation water-soluble salt, and the aqueous carrier are maintained at the temperature of method step e) for 10 minutes to 3 hours.

13. The method according to any one of claims 1 - 4, wherein the temperature to which the mixture is heated in step e) is at least 10 °C lower than the decomposition temperature of the water-soluble oxidized salt.

14. The method according to any one of claims 1 - 4, wherein the SAP is provided in particulate form in step a).

15. The method according to any one of claims 1 - 4, wherein an additive is added in method step d), and wherein the total amount of the additive does not exceed 10% by weight based on the weight of the aqueous carrier.

16. The method according to any one of claims 1 - 4, wherein the SAP provided in step a) is post-consumer recycled SAP derived from post-consumer absorbent hygiene products, and wherein the method further comprises the following steps: (i) separating the post-consumer recycled SAP from the post-consumer absorbent hygiene product such that the post-consumer SAP contains less than 20% by weight of solid contaminants from the post-consumer absorbent hygiene product, wherein step (i) is carried out before step a).

17. The method according to any one of claims 1 - 4, wherein the method further comprises the following method step: f) separating the soluble polyacrylic acid polymer in the aqueous solution from the other compounds and components in the mixture obtained by step c).

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