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

doi:10.1016/S0165-2370(02)00032-3

Journal of Analytical and Applied Pyrolysis

Volume 64, Issue 2, September 2002, Pages 207-227

https://doi.org/10.1016/S0165-2370(02)00032-3 Get rights and content

Abstract

The onset temperature of thermal degradation of a liquid polymer or oil, and the characteristics of the pyrolysis at higher temperatures, can be assessed by studying the volatiles produced. However, this approach requires that the true thermal degradation products are assessed, and not any components which are simply evaporated from the sample. It is shown that degradation and evaporation behaviour can be distinguished by utilising the principle that thermal degradation produces components with structures which differ from those of components which are simply evaporated. This approach underlies the following two methods which have been used to study the thermal behaviour of liquid polyisobutylene (PIB). (1) In the temperature-sequence gas chromatography (GC) method, the same sample is subjected to a sequence of 10-s pyrolyses at increasing temperatures, and for each temperature the complete chromatogram of the volatile products is recorded. This method gives a good general picture in which it is demonstrated that at lower temperatures only evaporation occurs, but as the temperature is increased, degradation becomes increasingly important. The method has the potential to provide good characterisation of the volatile products, but its disadvantage is that not enough temperature steps can be taken to allow the onset of degradation and its development with temperature to be defined with acceptable accuracy. (2) In the direct mass spectrometry (MS) of volatiles method, a thin film of the sample is subjected to a continuous (small-stepped) temperature programme, during which helium carrier gas sweeps the volatiles continuously into the source of a MS scanning at the rate of 10 mass spectra per second. Appropriate plotting of selected ion currents of chosen fragment ions against time is used to reveal first the direct evaporation, and then the onset and progress of the degradation. Because of the large number of small temperature steps used, the degradation onset temperature is better defined by this method. The second of these methods has been used to compare the thermal behaviour of two liquid PIB samples which had been produced using different catalysts, and also their corresponding PIB-succinic anhydrides (PIBSA). (The latter are used as starting materials for the production of crankcase oil dispersant additives). The results have been interpreted in terms of general degradation mechanisms, on the basis of which a proposal is made for increasing the stability of the materials.

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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C.S. Pattenden, Effect of radiation on the thermal stability of polyisobutylene, PhD thesis, University of Birmingham,...
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An improved (thermocouple-feedback) pyrolysis-GLC technique and its application to study polyacrylonitrile degradation kinetics
Eur. Polym. J.
(1982)

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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.

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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☆

Abstract

Introduction

Section snippets

Materials

Comparisons of the PIB and PIBSA samples

Temperature sequence pyrograms

Acknowledgements

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

Polym. Degrad. Stabil.

An improved (thermocouple-feedback) pyrolysis-GLC technique and its application to study polyacrylonitrile degradation kinetics

Eur. Polym. J.