Engineering plastics from lignin, 23. Network formation of lignin‐based epoxy resins

Abstract

Abstract The network‐forming behavior, gelation, and vitrification of lignin‐polyether‐copolymer‐epoxide/amine resins were determined using a recently described dynamic mechanical thermal analysis (DMTA)‐based method. Six resins were studied which differed in lignin content, polyether arm length, and molecular weight. Glass transition temperature ( T g )‐cure time master curves could be constructed for cure temperatures ranging from 90 to 200°C; and semiexperimental vitrification plots were derived for each system based on the knowledge of activation energy and cure time at a reference temperature. The point at which the cure process becomes diffusion‐controlled (i.e., vitrification) happens at lower extents of conversion for lignin‐rich fractions. Near vitrification, the cure events change from n ‐th‐ to second‐order kinetics, and this indicates a reduction in polyether‐arm mobility. This suggests that the crosslinking of lignin centers with polyether arms produces a network in which the reactant‐carrying flexible arms do not have sufficient free volume to react. The greatest increase in T g during crosslinking is about 100°C for all molecular weight fractions, and this suggests that high‐ T g networks can only be achieved with high‐ T g prepolymers.