Activation of pine kraft lignin by Fenton-type oxidation for cross-linking with oligo(oxyethylene) diglycidyl ether

Abstract

Abstract Mechanically stable hydrogels featuring water absorption capacities of up to 75 gH2O ggel -1 can be obtained by cross-linking of activated technical lignins with poly(ethylene) glycol diglycidyl ether under strong alkaline conditions. Fenton oxidation prior to cross-linking by hydrogen peroxide and catalytic amounts of ferrous chloride has been found to be superior to an alkaline H2O2 pre-treatment with respect to gel formation, water sorption, and rheological properties of the resulting oligo(oxyethylene) lignin gels. Purified pine kraft lignin undergoes in the course of Fenton oxidation hydroxylation of both aliphatic and aromatic moieties. This is the main reason for the enhanced cross-linking density obtained after treatment with poly(ethylene) glycol diglycidyl ether. The oxidative changes have been demonstrated by principal component analysis of Curie point pyrograms, wet chemical methods, FT-IR, and 31P NMR spectroscopy. Cleavage of side-chains, radical 5,5′-coupling of phenylpropane units, formation of carbonyl and carboxyl groups, and cleavage of aromatic rings were observed. These structural changes may increase or decrease the water sorption capability of the cross-linked products.