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Smith, Derek A. (1965) Characterisation of polymer decomposition in an inert atmosphere by thermal analysis. Doctoral thesis, Northern Polytechnic.
Use of the term "characterisation" by the chemist is often different from that by the technologist. Related to the thermal decomposition of polymers, a chemical characterisation may involve extensive product and residue analyses together with isothermal weight loss determination at many different temperatures which together are intended to establish the kinetic equations governing the component chemical reactions of the overall degradation process. In relatively simple cases, it may be possible to propose for each of the reactions a mechanism which is consistent with the experimental data. Using this approach, it may take many months to investigate the decomposition of a single material but, by way of reward, valuable specific information will be obtained about the detailed chemical changes taking place in the system. The results of such studies have been reviewed in several well-known books and in a comprehensive paper.
The technologist is less concerned with the detailed chemical mechanism of component reactions, but more with describing adequately the degradation process. This more general requirement, together with a usual need to complete the description in the shortest possible time, has led to the development of empirical "heat ageing" tests which, while often quite reproducible, provide little information about the specific nature of degradation processes.
Some years ago the author discussed the importance of a new type of technological evaluation procedure which at best could provide almost as much information as the chemical approach, but which might take little or no longer to carry out than relatively uninformative empirical tests. The procedure involves kinetic characterisation of the overall process or, more precisely, of the rate determining step of the process. An example of this approach is the characterisation of the thiazole-accelerated sulphur vulcanisation of natural rubber by Russell, Smith and Welding.
The work described in this thesis was similar in object but has been applied to a much more general problem. The problem was to characterise quickly but comprehensively the thermal degradation of polymers and this thesis records a number of exploratory investigations into the breakdown of a wide variety of polymers by the use of dynamic thermal analysis methods. The intention here was to review critically the efficacy of the new TA techniques in tackling these problems, rather than to seek complete solutions to the problems themselves.
"Characterisation" has been used here to indicate kinetic description of an overall degradation process and it would be misleading to relate the parameters used to any individual chemical reaction without first adducing supporting data in the form of tests for homogeneity and also product analyses. It is partly for this reason that opportunity has been taken to carry out conventional spectroscopic and other analytical determinations wherever the additional information seems to be required in order to support arguments based solely on thermal analysis data.
It appears possible to use these new techniques for a wide variety of applications from the crudest "batch reproducibility" tests on raw polymers and compounding ingredients to the more sophisticated supplementing and even replacing of "isothermal" tests in the establishment of chemical mechanism. In the course of this account, attempts are made to indicate the relationship of the present work both to the more "industrial" and to the more "academic" aspects of the subject but no attempt is made to review the vast field of polymer degradation studies from either viewpoint.
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