Coupled thermo–hygro–mechanical ageing and chemical degradation mechanisms of polyurethane adhesive joints in bus structures: glass bonding

This study examines the coupled thermo–hygro–mechanical degradation mechanisms governing the durability of polyurethane Sikaflex®-256 adhesive joints used in bus glazing assemblies. Laminated glass substrates were treated with Sika® Activator-100 and Sika® Primer-206 G + P to evaluate the influence of surface chemistry on adhesion performance. Accelerated ageing at 70 °C and 100% relative humidity was conducted in accordance with ISO 9142, and moisture diffusion and interfacial degradation were assessed using mass uptake, Shore A hardness, FTIR, DMTA, SEM, and contact-angle measurements. FTIR deconvolution revealed hydrolytic scission and hydrogen-bond disruption, while DMTA showed a 3 °C–5 °C reduction in glass-transition temperature accompanied by moderate viscoelastic softening. Surface-energy analysis based on the Owens–Wendt method demonstrated that primer application preserved a balanced interfacial polarity, with total surface energy remaining close to 50 mN m−1 and effectively suppressing hydrophobic recovery after ageing. A multi-objective genetic-algorithm framework was employed to calibrate viscoelastic Prony parameters, deconvolute FTIR spectra, and optimize process variables. Pareto-optimal solutions indicated curing near 40 °C–45 °C with intermediate flash-off durations, resulting in moisture uptake of approximately 0.4% and cohesive retention exceeding 90%. Compared with conventional empirical or single-parameter approaches, this integrated experimental–computational framework quantitatively links molecular-scale degradation to macroscopic adhesion performance, providing a predictive basis for durability optimization in automotive glazing adhesives.