Carboxylated Lignin as an Effective Cohardener for Enhancing Strength and Toughness of Epoxy

It is demonstrated that pristine or functionalized lignin can be used in epoxy as a cohardener or comonomer; however either unsatisfactory mechanical properties or low lignin content remains a challenge in utilizing the sustainable biomass to replace petrochemical product. In this study, carboxylic acid‐modified kraft lignin (lignin–COOH) is synthesized and used as a cohardener for epoxy with loading content of up to 10.0 wt%. With incorporation of 10.0 wt% of lignin–COOH, the resulting composite exhibits increments of 16%, 13%, 20%, and 95% on tensile modulus, flexural modulus, tensile strength, and toughness respectively, in contrast to neat epoxy. The good dispersion of lignin–COOH in epoxy, rigid aromatic structure of lignin, and the reduced crosslink density in the composite can simultaneously contribute to the high mechanical performance, which is verified by the thermal and mechanical analysis results. It suggests that lignin can be converted to effective alternative curing agents for epoxy thermosets. Cured epoxy incorporated with carboxylic acid–modified kraft lignin as a cohardener at moderate loading contents exhibits simultaneous enhancement of tensile modulus, flexural modulus, tensile strength, and toughness, which can promote the utilization of this sustainable biomass as an alternative feedstock in practical thermosetting applications in terms of substituting petrochemical product.