Adhesive and mechanical characterization of urethane epoxy resins synthesized using glycidol and aliphatic diisocyanates

Abstract

This study investigated the synthesis, characterization, and performance of urethane epoxy resins derived from glycidol and two aliphatic diisocyanates-isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI)-combined with varying proportions of diglycidyl ether of bisphenol A (DGEBA). Solvent-free synthesis followed by two-step thermal curing process (80 degrees C / 2 h + 150 degrees C / 5 h) produced high-purity, well-crosslinked resins, as confirmed by the disappearance of the NCO peak at 2270 cm(-)(1) in FTIR spectra, indicating complete urethane linkage formation and improved thermal stability through dense network formation. Mechanical testing showed that HDI-based systems achieved the highest flexibility and toughness, with an impact strength of 74.7 kJ m(-)(2) and strain up to 28%, attributed to their linear molecular architecture enabling uniform crosslinking and efficient energy dissipation. IPDI-based systems exhibited maximum lap-shear strength of 26.9 MPa on stainless steel at 10 wt % DGEBA, demonstrating substrate-dependent performance requiring compositional optimization. HMDI-based formulations, examined only for structural comparison, showed poor compatibility with DGEBA and were not the focus of this study. Failure mode analysis revealed that cohesive failure dominated at higher strengths, confirming the link between cohesive integrity and improved adhesion. Collectively, the findings establish clear correlations between molecular structure, curing conditions, and thermo-mechanical performance, offering a framework for designing high-performance urethane epoxy adhesives with superior strength, flexibility, and thermal stability for aerospace and automotive applications.