Thermo-oxidative instability

Early work [119-124] showed that the thermo-oxidative instability of carbon fibers affected the stability of high temperature laminates at 300° C. 

A second interesting feature of the first wave of research in this area was that very few oxidation systems were investigated. This was partly because fewer well characterised homogeneous transition metal complex catalysts were known and certainly their chemistry was not well understood and secondly because their was an implicit belief that polymers would be unsuitable as supports in oxidation reactions because of their own intrinsic thermo-oxidative instability. The situation was reviewed in 1988 [133]. 

These full synthetics have been all PAO or ester based, and a mixture of ester plus PAO. The addition of PAO or ester to petroleum based engine oils for improved oxidative stability has shown average quality results. The addition of lower levels of PAO or ester base stock <15 wt % to petroleum based formulations show little or no improvement in the thermo-oxidative engine test. The predominant automotive synthetic base stocks (PAO, diesters, polyol esters) do not show any hydrolytic instability in engine oil applications. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

It is no surprise that a combination of different agents that cause polymer instability increases the degradation rate. For example, the presence of heavy metals (left behind catalyst or brought in with, for example, clay nano-particles) enhances photo- or thermo-oxidative degradation. Dynamic flow behavior of the polymeric nanocomposites (PNC) with either PS or PP matrix... 

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