Surface studies on the additive migration and diffusion in the windowseal rubber component influencing adhesion to coating

As rubber formulations are complex mixtures of different additives, many of them often migrate and diffuse to the surface, resulting in poor adhesion of windowseal rubber to coating. The present work was conducted to identify and quantify the diffusion behavior of additives of different rubber samples [masterbatch and final automotive ethylene propylene diene (EPDM) rubber compounds] using different complementary techniques such as contact angle measurements, X-ray photoelectron spectroscopy (XPS), size exclusion chromatography (SEC), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The influence of different methods of processing (milling and extrusion) on the diffusion behavior and change in rubber surface chemistry was investigated. Contact angle measurements using scanning Wilhelmy and sessile drop methods over prolonged time mainly showed an increase in the polar component of the surface energy for the samples, although the total surface energy did not change significantly. XPS analysis showed that in masterbatch samples, the main additive excluded on the surface was the zinc salt of fatty acid. The diffusion coefficient of zinc stearate was found to be 1.083 x 10 -7 cm 2 /s from contact angle measurement, which is in good agreement with other stearate diffusants in the rubber matrix. Sulfur diffusion over time was more pronounced in the milled final sample. SEC indicated a mixture of additives with different molecular weights extracted from the sample surfaces, such as PE wax, oil, and zinc salt of fatty acid (Aktiplast), indicating that these particular additives migrate to the surface.