Surface aspects of some sulphur-vulcanised elastomers, with particular reference to surface modification, as studied by ESCA
x-ray photoelectron spectroscopy (ESCA) has been used to investigate the surface chemistry of a variety of elastomers, as a function of cure conditions. The surface composition of thickness on a depth scale in the range < 50Å depends on the cure conditions and the bulk formulations of polyisoprene (Natsyn 2200), styrene/butadiene (Solprene 1204) and acrylonitrile/butadiene (Krynac 34/50) elastomers. In all cases, higher levels of antioxidant and accelerators have been detected at the surface than in the bulk, whilst the level of zinc for Natsyn 2200 and Solprene 1204 systems increases with ESCA depth profile into the bulk. The samples of Krynac 34/50 indicated higher levels of zinc and cadmium in the surface regions and, in one case, the level of zinc at the surface is approximately the same as in the bulk. Zinc and cadmium in the surface regions are predominantly present as sulphides. A use of the 'swelling' data has been made, in conjunction with the ESCA data, to estimate the sulphur functionality. Particular emphasis has been placed on the elaboration of surface crosslink functionality, using inductively coupled radio-frequency glow discharges, excited in oxygen and hydrogen. Model studies to establish the changes in surface chemistry as a function of reaction time in plasmas have been carried out in conjunction with the type 1, Natsyn 2200, elastomers. The results show that the reactions with oxygen and hydrogen plasmas are by no means simple. Modification by the oxygen containing plasmas is extensive but confined to the outermost monolayer or so. The rate and extent of oxidation is a strong function of the initial surface chemistry of the samples. The level of sulphur in di- and trisulohide models is essentially the same before and after oxygen plasma treatments and the tetrasulphide model indicated a loss of sulphur by a factor of two. The optimum cured type 1, Natsyn 2200, sample did not indicate any tendency for a loss of sulphur under comparable conditions. For hydrogen plasmas, the level of sulphur in a disulohide model system remains essentially the same, whereas the trisulphide model and the type 1, Natsyn 2200 systems indicated a loss of sulphur-A thermal oxidation study has also been carried out on the optimum cured type 1, Natsyn 2 200, elastomers and the results are complementary in some respects to oxygen plasma treatments .Finally, the 'sulphur-vulcanised' elastomers have been treated with the thiol-amine reagents for determining the relative proportion of mono-, di- and polysulphide cross links but the results are additionally complicated by the incorporation of thiol and amine into the elastomers.