Surface characterization of synthetic vulcanized rubber treated with oxygen plasma

Abstract

The surface of a synthetic vulcanized styrene–butadiene rubber (R2) was treated in an oxygen plasma to improve adhesion in joints prepared with a one-component solvent-based polyester–urethane adhesive. The modifications produced on the rubber surface by plasma treatment were assessed using advancing and receding contact angle measurements, x-ray photoelectron spectroscopy, (XPS), infrared-attenuated total reflection spectroscopy and scanning electron microscopy. Adhesion was obtained from T-peel tests of treated R2 rubber/polyurethane adhesive joints. Several experimental variables were considered, such as the radio-frequency power and the length and lifetime of the plasma treatment. The treatment in the oxygen plasma produced a noticeable decrease in contact angle, which can be mainly ascribed to the creation of CO and CO moieties on the rubber surface. Advancing and receding contact angles only differed by ∽10°. Depending on the experimental conditions used, some ablation was produced on the surface, which was more noticeable as the length and power of the treatment increased. An adequate performance of adhesive joints was obtained using a power of 50 W and a time for oxygen plasma treatment of <10 min. The changes in the rubber surface remained for 2 h after plasma treatment, as indicated by the variation in peel strength and XPS data. Although improved adhesion was obtained by treating the rubber in an oxygen plasma, the T-peel strength values are not sufficient to assure technical use, probably due to the migration of waxes and zinc stearate to the surface once the treatment was carried out. Finally, sulfur oxidation was produced by the plasma treatment, and for severe conditions solid crystals of a sodium salt of an oxidized sulfur compound (sodium sulphate or an organic sulphate) appeared on the treated rubber surface. © 1998 John Wiley & Sons, Ltd.