The quantitative measurement of the thermal stabilities of low-MW oils

Measuring the thermal degradation of low-MW oils presents special problems, which can lead to misleading results. In this study of the degradation behaviour of some polyisobutylene oils, the most important of these problems have been found to be: (1) In a pyrolysis experiment, some of the volatiles measured can arise from direct volatilisation of lower oligomers in the oil. This effect leads to erroneously high values for the rate constants (specific rates) of these lower oligomers. (2) Both of the degradation mechanisms occurring (random chain scission, and depropagation with transfer) result in a progressive decrease in the average MW of the residue. It can be shown that this results in an anomalous increase in the rate constants for the intermediate oligomeric species. (3) The highest oligomers detected are of low volatility, and for this reason there may be problems in estimating their quantitative yields during the defined pyrolysis period. This leads to an anomalous decrease in the rate constants for the highest oligomeric species. Although in principle it would be possible to study and correct for each of these effects, this is arduous and unnecessary in the light of the following recommendation: Since the rate of monomer formation is totally unaffected by any of the above problems, and since the monomer yield is influenced by both of the mechanisms which are leading to all products, the rate constant for monomer formation provides a quantitative inverse index for comparing the stabilities of low-MW oils. (C) 1998 Elsevier Science Ltd. All rights reserved.