Surface Modification of SBS Rubber by Low-Pressure Inert Gas Plasma for Enhanced Adhesion to Polyurethane Adhesive

Abstract

Surface modification studies of styrene–butadiene–styrene (SBS) elastomer by inert gas plasmas for improving the adhesion to polyurethane (PU) adhesives are presented in this paper. The plasma was generated by an RF discharge (13.56 MHz, parallel plate reactor) in inert gases (argon or helium) under low pressure.To characterize the changes to the SBS surface, peel tests, infrared spectroscopy analysis and contact angle measurements were utilized. Compared with non-treated surface, inert gas plasma treatments were found to result in a significant increase (4–6 times) in adhesion (peel) strength. The maximum value of the peel strength was attained in a very short time of the plasma treatment (5–15 s), after which the value decreased.For relatively longer times (a few minutes), in most cases a re-increase of the peel strength was observed. Functional groups, such as –OH, >C=O, C–O–C, were detected on the plasma-treated SBS surface. The concentration profiles of –OH and >C=O groups vs plasma treatment time (t) revealed maxima, position of which coincided with the peel strength maximum. In contrast, the C–O–C concentration increased systematically with t Furthermore, the dispersion γd s and polar γp s components of the surface free energy increased also monotonically with t approaching constant levels. These results confirmed the chemical nature of the bonding between the SBS surface and polyurethane adhesive. They also suggested that three main effects were associated with the inert gas plasma treatment process, namely: formation of free radicals, crosslinking and probably plasma etching of the SBS surface. Then, after contact with air, subsequent reactions with O2 and H2O vapor took place. On this basis, a kinetic model was proposed, which was capable of describing the strength of SBS–PU joints as a function of t. The model indicated that ion bombardment was a possible mechanism for the free radical formation on the SBS surface treated by inert gas plasmas. © Koninklijke Brill NV, Leiden, 2012