Abstract

Sustainable polyurethane (PU) coatings were synthesised from a 50/50 polyol blend of poly(ethylene glycol) (PEG3000) and modified used palm oil (mUPO) with toluene diisocyanate (TDI) at an NCO index of 100. Ethylene glycol (EG) was used as a chain extender (0–5 mol) to investigate its effects on structure and performance. The developed PUs are intended as bio-based adhesive and structural coatings for glass and polymer substrates, with a focus on bonding strength, flexibility, and thermal stability. FTIR spectra provided clear evidence of urethane linkage formation and intermolecular hydrogen bonding. Mechanical tests demonstrated improvements in adhesion strength and elongation with EG, reaching 26.2 MPa and 284%, respectively, at a concentration of 2 mol. SEM demonstrated rougher bonded surfaces at moderate EG, consistent with the enhanced toughness. Surface analysis revealed that a higher EG content promoted phase separation, increased the water contact angle, and reduced the surface energy from 127 to 12 mJ/m². DSC/TGA indicate improved hard-soft interactions and thermal stability at moderate EG, but performance decreased at higher EG due to plasticisation. In addition, the activation energy (E<inf>a</inf>), determined using the Coats–Redfern method, increased progressively from 22.58 kJ/mol (EG0) to 225.19 kJ/mol (EG5), confirming enhanced thermal stability with increasing EG content. These results suggest that EG is a crucial factor in tailoring environmentally friendly PU coatings that balance adhesion, flexibility, wettability, and stability.