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Abstract

Studies have been made of the effects of a number of metal compounds on the thermal degradation and oxidation of polybutadiene and polypropylene. It was hoped that the results would throw light on the action of burning rate catalysts on the breakdown of the polymeric binders used in solid propellants.

In the Introduction, the mechanisms of the thermal and oxidative degradation of polybutadiene and polypropylene are discussed. Knowledge of the mechanisms involved in the gas phase oxidation of hydrocarbons is used to supplement the information obtained from the few studies of the high-temperature oxidation of polyolefins. The different modes of antioxidant action are reviewed and the limited information about the effects of metal oxides on polymer oxidation is discussed.

In the Experimental Section the materials and procedures used are described and the construction and
operation of the thermobalance, hot-stage microscope and surface-area-measurement apparatus are discussed.

In the Results and Discussion Section the experimental measurements made on the pyrolysis and oxidation of polybutadienes (particularly carboxy-terminated polybutadiene (CTPB)) are described and their significance
is considered. The pyrolysis of CTPB was shown to be considerably more complex than that of simple polyolefins, such as polyethylene, owing to the inherent unsaturation of the former polymer and metal chelates had no effects due to their thermal instability at the pyrolysis temperatures used. The oxidation of CTPB takes place in four well-defined stages. A surface film is formed which protects the polymer from oxidation until the rate of formation of volatile breakdown products exceeds their rate of diffusion out of the polymer; this film ultimately remains as a char. Because oxidation is normally diffusion-controlled, the catalytic effects of metal chelates could not be examined. However the chelates act as antioxidants, lengthening the induction period, and their effectiveness is partly dependent on their thermal stability, particularly for the 3,5-diisopropylsalicylato chelates. The oxidation of CTPB, when thinly coated on metal oxides, was found not to be diffusion-controlled and some oxides (especially MnO, and CoO) exhibited very marked catalytic effects. A correlation appears to exist between the width of the forbidden zone of semiconductor oxides and their catalytic activity.

Studies of the pyrolysis of polypropylene confirmed the relatively simple thermogravimetric behaviour of this polymer. As with CTPB, metal chelates had no effect on the rate of breakdown. The oxidation of polypropylene was found to be even more complex than that of CTPB, owing partly to the overlapping of the successive stages and partly to the large effect of sample size. Although it appeared that a surface film was formed, this was of a very different nature to that formed from CTPB. Oxidation was still diffusion-controlled so that no catalytic effects of the metal chelates could be observed. Nevertheless the chelates functioned as antioxidants and bis(stilbene dithiolato) nickel was exceptionally effective in this respect, acting as a powerful catalyst for the nonradical decomposition of polypropylene hydroperoxides.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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