Use of pyrolysis-GC–MS to assess the thermal degradation behaviour of polymers containing chlorine II. Thermal stability characteristics of Neoprene/chlorobutyl rubber composites, before and after artificial ageing

Abstract

Some composites containing a cross-linked blend of Neoprene and “chlorobutyl rubber” have been artificially aged at temperatures of 35 and 80°C for up to 3 months. Overall rate constants for their degradation at 387°C, before and after the ageing, have been measured by pyrolysis-GC–MS in order to assess whether the ageing has had any effect on their thermal stability. The results show that the low-temperature ageing treatment has caused no detectable deterioration in the thermal stability of the composites as measured at the higher temperature. The mean value of the first order rate constant, determined from a reproducibility test using 21 data points, is 0.0205±0.0036 s −1 at 387°C (68% confidence limit). The scatter is attributed to macro inhomogeneity in the composition of the samples. With respect to the different batch samples, the mean values of the specific rates of degradation of the two samples were found to lie within the standard deviation of the overall scatter. The effects of the artificial ageing have also been assessed by comparing the resolved gas chromatograms of the many products obtained from the 387°C pyrolyses of the control and the aged samples. These chromatograms are virtually identical, and this suggests that not only is the thermal stability unaffected, but also that the detailed mechanisms of the degradation have not changed with ageing. Mass spectra and mass chromatograms have also been examined in order to assess the general structures present within the wide variety of degradation products. High-yield low molecular weight components include isomeric butenes (butene-1, butene-2, and 2-methyl-propene-1), but it is difficult to distinguish between these because of the similarities in their mass spectra. There is also evidence for oligomeric butenes and many other aliphatic hydrocarbons (both saturated and unsaturated, linear and cyclic) with molecular weights up to several hundred. Other products include hydrogen chloride and chlorinated hydrocarbons, plus a few other hetero-compounds which can be related to known minor additives in the composites. Hydrogen chloride has also been established as a product at its detection limit when 100 μg of the chlorobutyl rubber component alone were heated at 100°C for 10 s, despite the fact that this material contains less than 2 wt% of chlorine.