OXIDATIVE DAMAGE AND RECOVERY OF SILICONE-RUBBER SURFACES .1. X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY

The oxidative degradation of silicone rubber surfaces in air plasmas o btained by RF or corona discharges and the subsequent recovery process were studied by X-ray photoelectron spectroscopy (XPS or ESCA). Using relatively short treatment times (5 min), the surface oxygen content was found to increase and that of carbon to decrease. Within 1 day som e recovery toward the original composition was observed, but it was fa r from being complete. Angle-dependent measurements have shown an almo st total recovery in the topmost layer. The degree of surface degradat ion of a solvent-extracted sample was much higher while its recovery w as much smaller than the corresponding features of the nonextracted sa mple. According to GC and GC-MS analyses the extract contained a mixtu re of cyclic, and, in a minor quantity, linear dimethylsiloxane oligom ers. Based on the above results the following steps were proposed for the oxidative damage and the subsequent recovery processes on silicone rubber surfaces: first the majority of surface methyl groups is remov ed and an oxidized layer containing Si atoms bound to 3 or 4 oxygens a ppears. The surface is later covered by a very thin (2-3 nm thick) ''s ilicone oil'' layer due to migration of low-molecular-weight component s from the bulk. This diffusion-controlled migration step plays a more important role in the recovery process than the eventual reorientatio n of the newly formed polar groups from the surface toward the bulk. T he proposed model is discussed in the light of published data. (C) 199 4 John Wiley & Sons, Inc.