Preparation of lignin-based epoxy resins with tunable properties through constructing a partially ordered crosslinking network

In this work, lignin-based epoxy resins with tunable thermomechanical properties were prepared through constructing a partially ordered crosslinking network, which is designed on the curing behavior of phenyl glycidyl ether (PGE) and phenolated lignin-based epoxy resin (PLBER) with methyl nadic anhydride. Curing kinetics analysis illustrated that the introduction of PLBER could substantially reduce the activation energy (Ea) of curing reaction, indicating that PLBER have higher reactivity than PGE. Epoxy resins exhibited improved flexural strength, impact toughness, storage modulus, and glass transition temperature with the increase in lignin content. Based on the atomic force microscopic images of the thermosets, the highly crosslinked lignin-based epoxy resin was embedded in the epoxy matrix to form a heterogeneous network structure, which confirms that PGE reacted with PLBER as an active monomer to form a partially ordered crosslinked network, where PLBER represents the high-density region in the epoxy network. Moreover, the incorporation of lignin decreased the initial degradation temperature (Td5%) of epoxy resins, whereas Td50% (the temperature at 50% weight loss), Tmax (the temperature at the maximum rate of weight loss), and char yield (Char800) were improved with the increase in lignin content. This result may be attributed to the abundant β–O–4 bonds in lignin, which are degraded rapidly to form a protective carbon layer at the initial stage of decomposition, thereby slowing down the decomposition rate of epoxy resins.