Curing behaviour, mechanical and thermal properties of unmodified lignin–epoxy composites using a bio-based phenalkamine hardener

Developing sustainable epoxy systems requires the effective use of renewable components such as lignin, although challenges in compatibility and dispersion often limit performance. This study evaluates the direct incorporation of unmodified lignin (0–12 wt%) into a diglycidyl ether of bisphenol A (DGEBA) epoxy cured with a biobased phenalkamine hardener at 1:1 and 2:1 ratios. FTIR was used to assess chemical interactions, optical and camera-based imaging to examine dispersion, tensile testing to determine mechanical properties, and DSC to characterise curing behaviour. FTIR results showed that lignin hydroxyl groups participated in the epoxy–amine reaction, evidenced by changes in the O–H region and reduced epoxy ring vibrations. Morphological analysis revealed greater heterogeneity with increasing lignin contents, corresponding to reductions in tensile strength and modulus. Notably, the 8 wt% lignin formulation at the 2:1 ratio achieved improved performance (18.26 MPa tensile strength; 0.87 GPa modulus), indicating partial compatibilisation under resin-rich conditions. DSC analysis confirmed that lignin acted as a co-curing agent by lowering onset curing temperatures and increasing reaction enthalpy, while phenalkamine supported more flexible network development. Overall, this study demonstrates the potential of unmodified lignin as reactive bio-filler for scalable, partially biobased epoxy composites suitable for structural and protective applications.