Utilization of Lignin-Derived Small Molecules: Epoxy Polymers from Lignin Oxidation Products

Much effort has been invested in developing methods for producing small molecules from lignin as a way to source feedstock chemicals from renewable sources. Significant progress is being made, and methods for deconstructing lignin are producing good yields of small, mononuclear aromatic products - sufficient amounts to enable studies of the potential use of these compounds as replacements for compounds currently produced from petroleum. To investigate the use of lignin products in epoxies, we begin with aromatic acids that can be produced from lignin, treat them with epichlorohydrin to make glycidyl ethers, and investigate the thermal and mechanical properties of cured mixtures of these compounds with a commercial epoxy resin (EPON 826) and an anhydride curing agent (NMA). While most of the lignin-modified epoxy polymers exhibit good physical and thermal properties, the polymer prepared from p-hydroxybenzoic acid (compound 6) has a higher glass-transition temperature (T_g = 159 °C) than do thermosets made with other lignin-derived materials, such as vanillic acid diglycidyl ether (compound 4) and matches the T_g of cured samples of the commercial EPON-826/NMA epoxy system. This is significant, as p-hydroxybenzoic acid is readily available by simple hydrolysis of several different lignins and functions as a drop-in replacement for 50% of the BPA-based material in this commercial system without significant degradation of material properties. The use of lignin-derived small molecules in high-value systems such as epoxies may help improve the economics of biorefineries.