Flexible and Heat-Resisting Lignin-Based Epoxy Resins by Hardwood Kraft Low-Molecular-Weight Lignin as a Sustainable Substitute for Bisphenol A

Finding renewable substitutes for bisphenol A (BPA) for producing epoxy resin is of high practical significance. While the low cost and high yield of lignin make it a promising candidate, its high molecular weight, low reactivity, and high dispersity limit its applications. Here, we utilized a low-molecular-weight (low-Mw) lignin mainly consisting of syringaresinol and stilbenes with low dispersity and rich hydroxyl groups as a partial substitute for BPA to synthesize epoxy resins. The resulting lignin-based epoxy resins were designed under three different reaction conditions to yield lignin replacement amounts of 20, 40, and 60%. The epoxy resins with a 20% substitute amount exhibited the best properties with enhanced elongation (239%), tensile stress (144%), and thermal stability (48%) compared to the BPA-based epoxy resin. This study revealed that, besides the epoxy value, low-Mw lignin derivatives including syringaresinol, stilbenes, and aldoketones are essential in improving the elongation. Especially, the newly proposed possible open-loop mechanism of syringaresinol (β–β) indicates that the cross-linked network formed by the furan ring-opening after curing could further reinforce the epoxy resins’ mechanical properties. It provides a new sustainable substitute for BPA to design flexible and heat-resistant lignin-based epoxy resins.