UV-curable epoxy primer for adhesion in steel reinforced polymer composites

In continuous composite production, for example the continuous impregnation of steel wires or cords with a thermoplastic polyurethane matrix, an ideal and cost-effective solution to improve interfacial adhesion and durability would be to deposit and cure a primer, on-line, at high processing speeds. A UV-curable epoxy primer was investigated for this purpose; it is reticulated by cationic photo-polymerisation, possibly followed by dark-curing and/or post-cure during further processing. The primer base formulation was tailored using additives including a hyperbranched monomer for tailoring cross-linking and flexibility and trimethylolpropane oxetane to increase curing speed and reduce viscosity. Curing kinetics, coating mechanical properties, and adhesion and durability at the steel-TPU interface were assessed for several combinations of coating formulation and processing parameters (time, temperature, UV intensity). Pull-out tests were analysed by finite element methods to refine interpretation of the experimental results for process-induced stresses, interfacial shear strength and energy release rate. The results demonstrated the fast-curing capability together with adhesion and durability improvements between galvanized or stainless steel reinforcements and a thermoplastic polyurethane elastomer by forming physico-chemical bonds with both the polymer and the substrate, while maintaining low internal stresses at the interface. The proposed UV-curable primer outperformed conventional surface treatments and thermally cured aminosilane primers used as benchmark in terms of processing time, adhesion and durability.