Surface Modification of Ethylene Propylene Diene Terpolymer Rubber by Plasma Polymerization Using Organosilicon Precursors

The effect of atmospheric pressure plasma-enhanced chemical vapor deposition on ethylene propylene diene terpolymer (EPDM) with the precursors hexamethyldisiloxane (HMDSO) and tetraethyl orthosilicate (TEOS) on roughness, chemical composition, as well as wetting and friction properties has been investigated. For the first time, topography analyses like scanning electron microscopy, white light interferometry, digital microscopy, as well as surface analytical methods by using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were combined with contact angle and friction experiments to obtain a detailed analysis of plasma polymer surfaces. This work shows that different plasma coatings can be utilized to tailor wettability and surface energies and reduce friction of EPDM rubber, which are important for various applications. Wettability investigations have shown that both coatings are more polar compared to the untreated surface but less polar than the surface-activated EPDM without precursors. The carbon content decreased, and the content of oxygen and silicon increased after plasma polymerization, as shown by XPS investigations. ToF-SIMS investigations have revealed that the ion spectra of both coatings are very similar with a comparable surface chemistry. A lower penetration depth is considered for the contact angle measurements in contrast to the other surface-sensitive methods. The surface energy of the activated EPDM surface without precursors increases significantly compared to the untreated EPDM because of the incorporation of polar groups in the elastomer surface. Both coatings with the corresponding precursors also have a higher surface energy compared to the uncoated EPDM, whereas the TEOS coating reveals a higher surface energy than the HMDSO coating. However, both coatings have lower surface energies than the activated EPDM. The coefficient of friction and the stick-slip phenomenon can be significantly reduced using plasma polymer coatings based on organosilicon precursors sliding on glass substrates. The lowest friction values with the absence of stick-slip on EPDM were achieved by using the precursor TEOS as the friction partner.