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US-12617922-B2 - Degradation of superabsorbent polymer via oxidative degradation

US12617922B2US 12617922 B2US12617922 B2US 12617922B2US-12617922-B2

Abstract

A method for degrading crosslinked and poly(acrylic acid)-based superabsorbent polymer (SAP) into soluble polyacrylic acid polymers is disclosed. Degradation is achieved with an oxidative water-soluble salt comprising at least one cation and at least one anion.

Inventors

  • Arsen SIMONYAN

Assignees

  • THE PROCTER & GAMBLE COMPANY

Dates

Publication Date
20260505
Application Date
20230123
Priority Date
20190823

Claims (19)

  1. 1 . A method for degrading crosslinked, poly(acrylic acid)-based superabsorbent polymer (SAP) into soluble polyacrylic acid polymers, comprising: a) providing SAP; b) providing an oxidative water-soluble salt comprising at least one cation and at least one anion, wherein the at least one anion is selected from the group consisting of: peroxydisulfate, peroxymonosulfate, peroxydicarbonate, peroxy diphosphate, peroxydiborate, and mixtures thereof; c) providing an aqueous carrier; d) mixing the SAP with the oxidative water-soluble salt and the aqueous carrier to form a mixture; e) heating the mixture to a temperature of from about 30° C. to about 200° C. to degrade the SAP into soluble polyacrylic acid polymers; f) providing a mesh and capturing the SAP on the mesh; g) degrading the SAP captured on the mesh, wherein the SAP is captured by the mesh until the SAP has degraded sufficiently to pass through the mesh; wherein a ratio of the oxidative water-soluble salt to SAP is from about 0.001 g to about 0.05 g of oxidative salt per 1 gram of SAP.
  2. 2 . The method of claim 1 , wherein the oxidative water-soluble salt is dissolved in the aqueous carrier prior to method step d) or in method step d).
  3. 3 . The method of claim 2 , wherein at least about 50 weight % of the aqueous carrier, with the oxidative water-soluble salt dissolved therein, is absorbed into the SAP in method step d) and/or e).
  4. 4 . The method of claim 1 , wherein the amount of aqueous carrier provided in method step c) enables the SAP provided in step a) to swell to at least about 20% of the SAP's Centrifuge Retention Capacity (CRC) upon absorption of all aqueous carrier provided, wherein the CRC is measured according to EDANA method NWSP 241.0.R2.
  5. 5 . The method of claim 1 , wherein the cation is selected from the group consisting of: Li+, Na+, K+, Rb+, Cs+, NH4+, organically substituted ammonium, Ca2+, Mg2+, Sr2+, Ba2+, Al3+, transition metal cations of 1+ to 3+ oxidation state, and mixtures thereof.
  6. 6 . The method of claim 1 , wherein at least 50% by total weight of the oxidative water-soluble salt is an alkaline peroxydisulfate.
  7. 7 . The method of claim 1 , wherein the method further comprises the step of adding hydrogen peroxide.
  8. 8 . The method of claim 1 , wherein the ratio of aqueous carrier to SAP is from about 2 g to about 20 g of aqueous carrier per 1 gram of dry SAP.
  9. 9 . The method of claim 1 , wherein the SAP provided in step a) is dry SAP or is swollen to from about 0.05 g to about 10 g per 1 gram of dry SAP.
  10. 10 . The method of claim 1 , wherein the SAP provided in step a) has a CRC value of about 10 g/g to about 50 g/g as measured according to EDANA method NWSP 241.0.R2.
  11. 11 . The method of claim 1 , wherein the mixed SAP, oxidative water-soluble salt and the aqueous carrier are maintained at the temperature of method step e) for about 10 minutes to about 3 hours.
  12. 12 . The method of claim 1 , wherein the temperature to which the mixture is heated in step e) is at least about 10° C. below the decomposition temperature of the oxidative water soluble salt.
  13. 13 . The method of claim 1 , wherein the SAP is provided in step a) in particulate form.
  14. 14 . The method of claim 1 , wherein additives are added in method step d) and wherein the total amount of additives is not more than about 10 weight % based on the weight of the aqueous carrier.
  15. 15 . The method of claim 1 , providing the SAP as a continuous stream.
  16. 16 . The method of claim 1 , wherein the aqueous carrier and the oxidative water-soluble salt are provided as a spray.
  17. 17 . The method of claim 1 , wherein the mixture is free of additional oxidative water-soluble salt.
  18. 18 . A method for degrading crosslinked, poly(acrylic acid)-based superabsorbent polymer (SAP) into soluble polyacrylic acid polymers, comprising: a) providing SAP; b) providing an oxidative water-soluble salt comprising at least one cation and at least one anion, wherein the at least one anion is selected from the group consisting of: peroxydisulfate, peroxymonosulfate, peroxydicarbonate, peroxydiphosphate, peroxydiborate, and mixtures thereof, wherein the oxidative water-soluble salt is substantially free of other anions; c) providing an aqueous carrier; d) mixing the SAP with the oxidative water-soluble salt and the aqueous carrier to form a mixture; and e) heating the mixture to a temperature of from about 30° C. to about 200° C. at a pH of from about 3 to about 7 to degrade the SAP into soluble polyacrylic acid polymers, f) passing the mixture through a mesh so that the soluble polyacrylic acid polymers pass through the mesh and retaining the SAP that has not degraded on the mesh until it degrades, wherein a ratio of the oxidative water-soluble salt to SAP is from about 0.001 g to about 0.05 g of oxidative salt per 1 gram of SAP, and wherein the soluble polyacrylic acid polymers comprise an average molecular weight, Mw, of from about 10 kDa to about 10 MDa.
  19. 19 . A method for degrading crosslinked, poly(acrylic acid)-based superabsorbent polymer (SAP) into soluble polyacrylic acid polymers, consisting essentially of: a) providing SAP; b) providing an oxidative water-soluble salt comprising at least one cation and at least one anion, wherein the at least one anion is selected from the group consisting of: peroxydisulfate, peroxymonosulfate, peroxydicarbonate, peroxy diphosphate, peroxydiborate, and mixtures thereof; c) providing an aqueous carrier; d) mixing the SAP with the oxidative water-soluble salt and the aqueous carrier to form a mixture; and e) heating the mixture to a temperature of from about 30° C. to about 200° C. to degrade the SAP into soluble polyacrylic acid polymers, f) passing the mixture through a mesh so that the soluble polyacrylic acid polymers pass through the mesh and retaining the SAP that has not degraded on the mesh, wherein a ratio of the oxidative water-soluble salt to SAP is from about 0.001 g to about 0.05 g of oxidative salt per 1 gram of SAP.

Description

FIELD OF THE INVENTION The present invention generally relates to oxidative degradation of poly(acrylic acid)-based superabsorbent polymer (SAP), especially useful for recycling of post-consumer used SAP. An oxidative water-soluble salt is used to degrade the SAP. BACKGROUND OF THE INVENTION Recycling of absorbent-hygiene products (AHPs), such as diapers and pants, feminine hygiene articles, is needed in view of the global challenges regarding sustainability. Many consumer companies strive towards using 100% recycled materials, and having zero consumer and manufacturing waste go to landfill. In addition to these goals, successful recycling benefits the environment, stimulates the economy, improves people's health and water quality, and generates energy needed by consumers in developing regions of the world. A major component in AHPs is typically the superabsorbent polymer (SAP), whereas minor components are adhesives, cellulose fibers, polyethylene, polypropylene, and polyester. Recycling of AHPs involves cleaning of the AHPs from the soils accumulated during their use, and separating the various components into recycled material streams. More specifically, the recycled SAP material stream can be used in applications less demanding than AHPs (since the recycled SAP has inferior properties compared to virgin SAP) and/or can be converted to essentially linear or branched, non-crosslinked poly(acrylic acid) (PAA). Then, this PAA can be used as a feed material to various applications. For example, the PAA can be used as-is 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-acid-based compounds with compounds derived from recycled SAP. In all cases, the objective is to achieve the same properties as virgin materials. Recycled SAP can be either post-consumer recycled (PCR) SAP or post-industrial recycled (PIR) SAP. Non-limiting examples of processes that produce recycled SAP material streams from recycled AHPs are disclosed and claimed in U.S. Pat. No. 9,095,853 B2, issued on Aug. 4, 2015; and 9,156,034 B2, issued on Oct. 13, 2015; both assigned to Fater S.p.A, Italy. Most SAPs are based on poly(acrylic acid) and are crosslinked network materials. Non-limiting examples of procedures used to produce SAPs from acrylic acid and crosslinkers are disclosed in U.S. Pat. No. 8,383,746 B2, issued on Feb. 26, 2013, and assigned to Nippon Shokubai Co., Ltd, Japan; and U.S. Pat. No. 9,822,203 B2, issued on Nov. 21, 2017, and assigned to BASF SE, Germany. There are many references on attempts to degrade or de-polymerize linear polymers and only a few references on efforts to depolymerize SAPs, which comprise branched PAA cross-linked into a polymer network. The typical forms of energy used in these efforts (either as single forms of energy or in combination) are ultrasound, UV, mechanical (i.e., in the presence of extensional/elongational forces; example: Caruso, M. M., et al., Chem. Rev., 109 (2009), 5755-5798), thermal (example: McNeill, I. C., and Sadeghi, S. M., Polymer Degrad. Stability, 29 (1990), 233-246), and microwave. However, there is a need to degrade post-industrial recycled (PIR) SAP (e.g. derived from un-used AHPs which are sorted out during manufacturing and not provided to consumers, or derived from SAP manufacturing processes, e.g. when the SAP did not meet the required performance criteria) and, especially, to degrade post-consumer recycled (PCR) SAP derived from post-consumer AHPs (i.e. after the AHPs have been used). Thus, there is a need to degrade or de-polymerize recycled SAP into soluble, linear or branched, poly(acrylic acid) (PAA) in short time scale, with low energy and power per unit mass of SAP, and at mild conditions, e.g. relatively low temperature and without use of chemicals considered environmentally problematic. The requirement for low energy per unit mass of SAP stems from the fact that the recycling of SAP and its degrading or de-polymerization to PAA is beneficial only if the energy spent during the converting of SAP to PAA is less than that used to make fossil-derived acrylic acid (petro-AA) from propylene, which is about 50 MJ/kg AA. The PAA produced from recycled SAP can then derivatized into materials for other applications such as adhesives, coatings, water treatment, fabric care, etc. SUMMARY OF THE INVENTION The invention relates to a method for degrading crosslinked and poly(acrylic acid)-based superabsorbent polymer (SAP) into soluble polyacrylic acid polymers. The method comprises the steps of: a) providing SAP,b) providing an oxidative water-soluble salt comprising at least one cation and at least one anion;c) providing an aqueous carrier, such as water or physiological saline (i.e. a solution with 0.90% by weight of NaCl per liter of water),d) Mixing the SAP with the oxidative