Mass spectrometric methods for assessing the thermal stability of liquid polymers and oils: study of some liquid polyisobutylenes used in the production of crankcase oil additives☆
Introduction
The initial target in the present work is to devise a reliable and quantitative method of assessing the thermal stability of liquid polyisobutylene (PIB) over temperature ranges covering all practical usage. This procedure can then be used to assess and compare the thermal performance of different PIB samples, and also products derived from these samples for use in producing dispersants. The ultimate target is to use the results to assist in improving the performance of materials of this kind.
Previous work from this research group on the thermal stability of liquid polymers has included measurements at a single specified temperature of the rate constants for the formation of a variety of volatile products from the thermal degradation of a liquid PIB [1]. The conclusion from that study was that whilst the rate constant for monomer evolution provides an inverse index of the thermal stability of the polymer at that temperature, the measured rate constants for the evolution of the individual oligomers cannot be used for this purpose. This situation arises for several reasons, one of which is that volatile oligomeric products are produced not only by thermal decomposition, but also by direct evaporation of the original components from the lower end of the molecular weight distribution of the polymer [2].
To obtain a proper understanding of liquid polymer behaviour at elevated temperatures, it is therefore important to distinguish between the effects of evaporation and degradation, and this quest is the starting point of the present study. The specific objectives are then to assess the temperature at which the onset of degradation is detectable, and also to study the patterns of the degradation behaviour at temperatures above this. The ultimate targets are to relate the thermal stability properties of the liquid polymers or oils to their structures, and thence to consider whether their stabilities can be improved by making appropriate structural changes.
Section snippets
Materials
The polymer samples compared in Section 3 are referred to by the numbers 54, 57, 49 and 52, which are based on the reference numbers of the suppliers (Lubrizol Research Centre, Derby, UK). They are PIB oligomeric liquids, and have similar molecular weight averages except for sample 54, for which the average is lower. The samples differ in the following respects:
54—‘PIB type I’ [Lubrizol OS 148054]
A colourless viscous liquid.
An example of the type I (‘new technology/high reactivity’) PIB samples
Comparisons of the PIB and PIBSA samples
In this section the thermal behaviour characteristics of the samples listed in Section 2.1 are compared in several respects.
Temperature sequence pyrograms
It has been shown that using pyrolysis-GC to provide temperature sequence pyrograms can give a good general picture of the changeover from evaporation to degradation as the temperature of an oil increases. The method has the advantage of chromatographically resolving all the volatile products. This helps in the choice of key degradation products (monomer, in the case of polyisobutylene) which are characteristic of the degradation processes. These key products can then be used to distinguish
Acknowledgements
The authors are grateful to the Lubrizol Company for contributions towards the maintenance grants of R.D., Y.L. and M.R., and for the Company's consistent and constructive interest in this research.
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Separations of petroleum products involving supercritical fluid chromatography
2012, Journal of Chromatography ACitation Excerpt :Direct spectroscopic methods, such as ultraviolet or infrared spectrometry [63] and NMR [64], on carbon or hetero-elements, offered rapid and general information on the lubricant but these methods generally suffer from low specificity. Chromatographic methods, GC [65–67] and LC [68–70] were used for the separation and the identification of additive (most often polymers additives) but they could not directly provide the total determination of the composition of a lubricant [71,72]. Elution of heavy compounds in GC is difficult or impossible and degradation of some additives is likely to occur, while HPLC lacks resolution and both universal and sensitive detector.
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Submitted in July 2001 to the Journal of Analytical and Applied Pyrolysis as a contribution towards the special issue honouring Professor Shin Tsuge.