The glass transition temperature (Tg) of the cured epoxy systems was measured by DSC based on the inflexion point of the stepwise transition. Fig. 7 shows the Tg of the cured bio-based epoxy systems comprising DGEBA and various percentages of DOL-epoxy resin or DKL-epoxy resin. The results demonstrate that the amount of lignin-based epoxy resin in the bio-based epoxy systems has a significant effect on Tg. Tg of an epoxy system comprising the DKL-epoxy resin is higher than that with the DOL-epoxy resin, which might be attributed to the greater hydroxyl content in the DKL-epoxy resin than that in the DOL-epoxy resin. The hydroxyl group in lignin-based epoxy resin could promote the curing process via the etherification and homopolymerization reactions [31] and [32], as discussed previously, leading to a higher cross-linking density of the cured resin samples, hence an increased Tg