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. The Fig. 7 also clearly shows that for both epoxy systems containing DOL-epoxy resin and DKL-epoxy resin, a common trend can be observed: Tg decreases with increasing the amount of lignin-based epoxy resin in the epoxy systems up to 50 wt%, which could be explained by the lower epoxy content of lignin-based epoxy resins in contrast with the DGEBA, leading to a lower cross-link density in the bio-based epoxy systems. Afterwards with further increasing the amount (>50 wt%) of lignin-based epoxy resins in the epoxy systems, the Tg increases slightly for the DOL-epoxy resin containing systems, but drastically for the DKL-epoxy resin containing systems, which could be attributed to the greater hydroxyl content in the DKL-epoxy resin that could promote the curing process via the etherification and homopolymerization reactions [31] and [32], leading to a higher cross-linking density of the cured resin samples.