Formaldehyde-Free Method for Incorporating Lignin into Epoxy Thermosets

A series of liquid and curable lignin-containing epoxy prepolymers were prepared for making renewable epoxy thermosets. First, lignin was modified to phenolated lignin (PL) in a solvent-free reaction. PL was subsequently co-oligomerized with salicyl alcohol (SA) in water without the use of formaldehyde to obtain fully bio-based polyphenols (PL-SA). Glycidylation of lignin-based polyphenols yielded exclusively liquid epoxy prepolymers without production of solid co-products. The liquid epoxy prepolymers (with lignin content up to 21 wt %) were curable with amine hardener (diethylenetriamine) to generate homogeneous thermosets, which required no epoxy co-prepolymer. The structural evolution from starting monomers to epoxy thermosets was followed by nuclear magnetic resonance and Fourier transform infrared spectroscopy. Compared to common syntheses in which lignin is glycidylated prior to being blended with epoxy co-prepolymers, the herein reported methodology conferred networks with increased α-relaxation temperature (106–114 vs 96 °C), storage modulus (1843–2151 vs 1828 MPa), cross-link density (8.2–16.0 vs 5.4 mmol/cm3) and tensile properties (stress of 66.9–68.1 vs 28.7 MPa, and strain of 3.3–3.7 vs 1.4%). Moreover, bio-based thermosets exhibited comparable or superior thermomechanical properties to conventional bisphenol A (BPA)-based counterpart. By producing liquid-phase lignin-containing epoxy prepolymers, this study provides a formaldehyde-free method for incorporating lignin into epoxy thermosets without the need for additional co-prepolymers.