Phenolation Prior to Epoxidation of Kraft Lignin and Their Impacts on Mechanical Properties of Epoxy Adhesive

Abstract

Numerous efforts focus on renewable epoxy resin development, with lignin emerging as a viable green alternative to fossil‐derived resources. Being Earth's second‐most abundant organic molecule, lignin, rich in phenolic groups, shows promise as a substitute for bisphenol A in diglycidyl ether of bisphenol A (DGEBA) production. This work successfully modifies Kraft lignin (KL) via one‐step epoxidation (KLE) and phenolation prior to epoxidation (KLPE), significantly increasing oxirane ring content. Characterization via FTIR and 1H NMR spectroscopy validates the modifications, yielding epoxy equivalent weights of 862.1 and 214.6 g mol−1 for KLE and KLPE, respectively. Incorporating 20 wt% KLE into adhesive maintains tensile and lap shear strength compared to 100 wt% DGEBA adhesives, with an increase in elastic modulus, indicating its potential as a sustainable resin substitute. However, phenolation prior to epoxidation slightly reduces mechanical properties despite higher oxirane ring concentration. These results shed some light on the impact of both modification protocols on the mechanical properties of epoxy adhesives, supporting the development and properties optimization of sustainable epoxy adhesives.