Structure–Property Relationship in Bio‐Based Epoxy Novolacs Through Phenol Modification

ABSTRACT Bio‐based epoxy resins offer a renewable substitute for conventional petrochemical systems. In this work, phenol‐modified cardanol novolacs (PMCN) were synthesized with phenol content ranging from 0% to 40%. It was subsequently epoxidised through insitu performic acid route converting the cardanol side‐chain unsaturation into oxirane rings. A series of epoxidised resins (EPMCN‐1 to EPMCN‐5) were prepared. The chemical structures of the liquid PMCN precursors were analyzed by 1 H 1 NMR, FTIR, and GPC, while FTIR and 1 H 1 NMR confirmed successful oxirane formation in the EPMCN resins. The resins were self‐curable using 2‐methylimidazole in catalytic amounts, through reaction between the phenolic hydroxyl and oxirane groups. Gel content, a measure of crosslink density, peaked at 94.3% (EPMCN‐4) and fell to 88.6% at the highest phenol loading (EPMCN‐5). Thermal stability (TGA/DTG), char yield, and the char‐derived limiting oxygen index rose with phenol content, reflecting the increasing aromatic fraction. Dynamic contact‐angle relaxation followed the Kohlrausch–Williams–Watts model, with the stretching exponent β falling from ~1 to 0.58, indicating an increasingly broad distribution of surface relaxation rates. SEM surface morphology followed the same compositional trend. Simple phenol modification provides a sustainable route to tune the structural, thermal, and surface properties of bio‐based cardanol epoxy thermosets (79.7%–96.3% bio‐based carbon content).