Preparation and toughening of mechanochemically modified lignin-based epoxy

In this work, succinylated lignin (SA-KL) was prepared by reacting a kraft lignin and succinic anhydride (SA) through a solvent-free ball milling process. A bisphenol A/F epoxy resin (DER 353) cured with a commercial alicyclic anhydride and SA-KL (~38 wt% in the composition) exhibited a flexural strength of 48.8 MPa and modulus of 3.62 GPa but a low impact strength of 6.7 kJ/m2 which were comparable to the properties of many anhydride cured epoxies. Toughening of the lignin-based epoxy resin with a carboxyl-terminated butadiene acrylonitrile (CTBN) liquid rubber was investigated. Introduction of the CTBN rubber significantly increased the impact strength and fracture toughness but resulted in reduction of flexural strength. A ductile-brittle transition was observed at 5–12 wt% CTBN rubber. Examination of the fracture surfaces by scanning electron microscopy suggests that rubber particle cavitation and massive shear yielding of the epoxy matrix were the dominant toughening mechanisms. Incorporation of CTBN had little change to the thermal stability of the lignin-based epoxy. This study introduces a green method for synthesis of lignin-derived polyacid and a viable approach for achieving balanced mechanical and thermal properties for lignin-based epoxy materials.