A Study of the Effects of Cashew Nutshell Epoxy Resin and Graphene Reinforcement on Soy Flour-based Bio-adhesive Performance

Abstract

This study presents a comparative evaluation of how cashew nutshell liquid (CNSL)-derived epoxy and graphene were independently used to tune the structure-property response of soy flour-based bio-resins. Soy flour proteins were alkaline-denatured and crosslinked in water, with resin formation shown by the color change and increased viscosity. The base resin was then modified with low CNSL epoxy concentrations or graphene nanoparticles to isolate the contribution of each modifier. Epoxy modification improved thermal stability, giving the highest onset degradation temperature of 209.3 C for EPRm_1, compared with 181.9 C for the neat resin and 202.3 C for GRm_0.5. Sol–gel analysis confirmed enhanced network formation, with gel content peaking at 64.51% for GRm_1. CNSL epoxy most effectively improved surface hydrophobicity, producing a maximum water contact angle of 81.55 , whereas graphene enhanced thermal resistance and stiffness-related behavior but increased apparent wettability at higher loading, reflecting the influence of filler distribution and surface heterogeneity. The neat resin retained the highest tensile strength of 6.75 MPa, whereas all formulations showed shear-thinning behavior and modified systems had high moduli. The CNSL epoxy mainly promotes hydrophobic network formation. Graphene enhances thermal and viscoelastic performance. Thus, these findings provide resin-level guidance for sustainable adhesives, coatings, and composite matrices while informing future research on bonded-joint performance, durability, and long-term service stability.