Effect of amine structure, thermal treatment and stoichiometry on the adhesion Properties of Bio-based Non-Isocyanate Polyurethanes (NIPUs) for footwear substrates

Non-isocyanate polyurethanes (NIPUs) synthesized through cyclic carbonate–amine reactions are emerging as sustainable alternatives to conventional polyurethane adhesives for footwear bonding. In this work, solvent-free NIPU hot-melt adhesives (PHUs) were prepared from diglycerol dicarbonate and either triethylenetetramine (TETA) or Priamine® 1074. The influence of cyclic carbonate-to-active amine hydrogen equivalent ratio (1:0.8, 1:1.0 and 1:1.2) and thermal treatment (25 and 110°C) was systematically investigated. Structure–property relationships were studied by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and oscillatory parallel-plate rheology, 1 H NMR, XRD and GPC, while adhesion performance was evaluated by T-peel tests on SBR–SBR and SBR–leather joints representative of footwear substrates. TETA-based formulations showed more advanced hydroxyurethane formation, lower residual carbonate contribution, and higher glass transition temperatures, leading to increased cohesive strength and superior peel strength. Notably, stoichiometric and amine-rich systems cured at 110°C achieved peel forces above 3.5 N/mm, reaching values close to 7 N/mm on SBR–SBR joints and meeting the minimum requirement of EN 15,307:2015 for upper-to-sole bonding. Overall, these results demonstrate that amine structure, stoichiometry and thermal treatment are key parameters for designing high-performance isocyanate-free NIPU hot-melt adhesives for sustainable footwear applications.