Some mechanistic insights in the behaviour of thiol containing antioxidant polymers in lignin oxidation processes

Abstract

Modified polyethylene glycol polymers are described in which thiol containing groups have been added via a simple esterification step. The resultant thiol end-capped polyethers retain the antioxidant behaviours of the monomeric thiols, but mitigate essentially completely the undesirable properties of the monomers, including offensive odour, volatility, and insolubility in aqueous solvents. This concept substantially enhances the number of potential practical applications of the thiol containing polymers as compared to the monomeric substances. A class of polyethylene glycols substituted with thiolactic acid or thioglycolic acid is described with potential in the pulp and paper industry, for the inhibition of radical induced oxidative processes including brightness reversion in lignin containing papers. Several techniques are used to elucidate aspects of the mechanism by which such thiols are effective: a novel photoacoustic technique demonstrates the scavenging effects of the thiol end capped polyethylene glycols towards alkoxy radicals. ESR studies demonstrate the ability of these thiol polymers to quench both the exogenous “phenoxy” radicals present in CTMP and TMP papers as well as the enhanced radical signal resulting from irradiation of these papers. The strong uv absorbance of these relatively persistent phenoxy radicals may decrease the optimal initial brightness of papers made from mechanical pulps.