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

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 (Tg)‐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 Tg during crosslinking is about 100°C for all molecular weight fractions, and this suggests that high‐Tg networks can only be achieved with high‐Tg prepolymers.