Surface treatment with UV-ozone to improve adhesion of vulcanized rubber formulated with an excess of processing oil

An excess of processing oil was intentionally added in the formulation of vulcanized styrene-butadiene rubber for analyzing the effect on the treatment with UV radiation combined with ozone (UV-ozone) in improving the adhesion to waterborne polyurethane adhesive in footwear. Due to the excess of processing oil in the rubber, poor adhesion was expected. Both the length of the treatment and the distance between the UV radiation source to the rubber surface were studied, and the effects of the treatment on the surface chemistry, wettability and surface energy, and topography of the rubber were analyzed. The treatment of the rubber with UV-ozone removed hydrocarbon moieties and zinc stearate from the surface, and surface oxidation (C-O, C=O and COO- groups formation) occurred. As a consequence, improved wettability and increased surface energy (mainly due to the polar component of the surface energy) were obtained. The increase in the length of treatment and for a distance of 3–5 cm from the rubber to the UV radiation source favored the effectiveness of the UV-ozone treatment. Besides, whereas the UV-ozone treatment for 3 min produced the ablation of surface contaminants mainly, longer lengths of treatment (i.e., 6–9 min) caused roughness and cracks on the rubber surface. The UV-ozone treatment also caused heating of the surface. Finally, the treatment with UV-ozone increased the adhesion of the rubber to waterborne polyurethane adhesive, the highest adhesion was obtained in the joints made with UV-ozone treated rubber for 3 and 6 min at a UV radiation source-rubber surface distance of 5 cm.