Degradation of superabsorbent polymer via oxidative degradation

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.

What is claimed is: 
     
         1 . A method for degrading crosslinked and 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;   c) providing an aqueous carrier;   d) mixing the SAP with the oxidative water-soluble salt and the aqueous carrier; 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.   
     
     
         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 . 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 . The method of  claim 3 , wherein the SAP has absorbed a total amount of liquid of from about 5 g to about 25 g per about 1 gram of SAP, wherein the total amount of liquid is the sum of the amount of liquid comprised in the SAP provided in step a) and the amount of aqueous carrier absorbed in method steps d) and e). 
     
     
         5 . 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 CRC upon absorption of all aqueous carrier provided, wherein the CRC is measured according to EDANA method NWSP 241.0.R2. 
     
     
         6 . The method of  claim 1 , wherein the at least one anion is selected from the group consisting of: peroxydisulfate, peroxymonosulfate, peroxydicarbonate, peroxydiphosphate, peroxydiborate and mixtures and combinations thereof. 
     
     
         7 . The method of  claim 1 , wherein the cation selected from the group consisting of: Li+, Na+, K+, Rb+, Cs+, NH4+, organically substituted ammonium, Ca2+, Mg2+, Sr2+, Ba2+, A13+, transition metal cations of 1+ to 3+ oxidation state, and mixtures and combinations thereof. 
     
     
         8 . The method of  claim 1 , wherein the oxidative water-soluble salt comprises at least about 50 weight-% of alkaline persulfate by total weight of the oxidative watersoluble salt. 
     
     
         9 . The method of  claim 1 , wherein the method further comprises the step of adding hydrogen peroxide. 
     
     
         10 . The method of  claim 1 , wherein the ratio of 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. 
     
     
         11 . 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. 
     
     
         12 . 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. 
     
     
         13 . 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. 
     
     
         14 . 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. 
     
     
         15 . 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 watersoluble salt. 
     
     
         16 . The method of  claim 1 , wherein the SAP is provided in step a) in particulate form. 
     
     
         17 . 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. 
     
     
         18 . The method of  claim 1 , wherein the SAP provided in step a) is postconsumer recycled SAP derived from post-consumer absorbent-hygiene products and wherein the method further comprises the step of:
 (i) isolating the post-consumer recycled SAP from post-consumer absorbent-hygiene products such that the post-consumer SAP comprises less than about 20% by weight of solid contaminations from the post-consumer absorbent-hygiene products, wherein step (i) is carried out prior to step a).   
     
     
         19 . The method of  claim 1 , wherein the method further comprises the method step:
 f) separating the soluble polyacrylic acid polymers in an aqueous solution from other compounds and components in the mixture obtained by step c).