Kinetics of partially depolymerized lignin as co-curing agent for epoxy resin.

Lignin, which is crosslinked by ether and carbon‑carbon linkages, is a highly branched polymer consisting of phenyl propane units. And owing to the abundant phenolic hydroxyl groups and its structural heterogeneous characteristics, lignosulfonate, considered as a renewable aromatic macropolymer, can be used as co-curing agent in EP thermoset systems. In this study, kinetics of lignosulfonate (LS) and partially depolymerized lignosulfonate (DLS) as co-curing agents for applications to EP thermoset were discussed with the differential scanning calorimetry (DSC) measurement. The activation energy (Ea) and dynamic modeling of curing reaction were obtained by the methods of non-isothermal kinetic analysis reported by Kissinger and Ozawa. The glass transition temperature (Tg) was determined by DSC and mechanical properties of epoxy resins were carried out using the Universal Testing Machine. The LS dropped the exothermic peak temperature and Ea of curing reaction of EP thermoset. Those of DLS were more active than the higher molecular weight lignosulfonate. The EP thermoset cured with DLS (EP-DLS) exhibited much higher reactivity, and the curing reaction could be occurred at lower temperature. The Tg of EP-DLS has risen by 10 °C compared with the case without lignosulfonate. The tensile strength, elongation at break and impact strength of EP-DLS rose by 28.7%, 52.5% and 53.5%, respectively, than those of the epoxy resin cured with LS (EP-LS).Molecular weight of lignosulfonate could be an important factor of the curing reaction of EP thermoset. The high performance EP thermoset could be obtained by partially depolymerized lignin as co-curing agents.