The Examination of Diffusing Materials on Rubber Surfaces by Photoacoustic Infrared Spectroscopy
Abstract Minor amounts of diffusing materials can be beneficial or detrimental to the performance of elastomeric components. Methods to study these materials have been nonexistent or extremely time-consuming and insensitive for routine use. This report shows that photoacoustic detection Fourier-transform infrared spectroscopy (PAS-FTIR) is a simple method for in situ characterization of diffusing materials in elastomers. In this study, PAS-FTIR was used to monitor the appearance and diffusion of two vulcanization by-products to the surface of a carbon-black-filled natural rubber matrix. The compounds were identified from the PAS-FTIR spectra as a salt of stearic acid (most likely zinc stearate) and dimorpholinyl thione. Three rubber formulations, differing only in the particle size and surface activity of the carbon blacks used, were monitored by PAS-FTIR for a 135-day period to investigate the effect of carbon black on the diffusion of the compounds cited above. Diffusion of the dimorpholinyl thione was not affected by the carbon-black properties presented in this study. Larger carbon-black particle size delayed the appearance of zinc stearate on the vulcanizate surface.