SURFACE CHARACTERIZATION OF SYNTHETIC VULCANIZED RUBBER TREATED WITH OXYGEN PLASMA

The surface of a synthetic vulcanized styrene-butadiene rubber (R2) wa s treated in an oxygen plasma to improve adhesion in joints prepared w ith a one-component solvent-based polyester-urethane adhesive. The mod ifications produced on the rubber surface by plasma treatment were ass essed using advancing and receding contact angle measurements, x-ray p hotoelectron 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. S everal experimental variables were considered, such as the radio-frequ ency power and the length and lifetime of the plasma treatment. The tr eatment 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 mo ieties on the rubber surface. Advancing and receding contact angles on ly differed by similar to 10 degrees. Depending on the experimental co nditions used, some ablation was produced on the surface, which was mo re noticeable as the length and power of the treatment increased. An a dequate performance of adhesive joints was obtained using a power of 5 0 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 indicat ed by the variation in peel strength and XPS data. Although improved a dhesion was obtained by treating the rubber in an oxygen plasma, the T -peel strength values are not sufficient to assure technical use, prob ably due to the migration of waxes and zinc stearate to the surface on ce the treatment was carried out. Finally, sulfur oxidation was produc ed by the plasma treatment, and for severe conditions solid crystals o f a sodium salt of an oxidized sulfur compound (sodium sulphate or an organic sulphate) appeared on the treated rubber surface. (C) 1998 Joh n Wiley & Sons, Ltd.