Thermo-oxidative Stabilisation

As with all organic polymers, exposure of aromatic polyesters to heat and oxygen can, especially over long time periods, result in degradation of the polymer. This thermo-oxidation manifests as discoloration of materials, loss of physical properties, and complete failure of the substrate. To prevent (or more likely control) such processes it is necessary to incorporate additives which can protect the host polyester against the effects of heat and oxygen antioxidants. 

As may be deduced from the discussions undertaken in Chapter 3, the most damaging reactions within the autoxidation cycle posited for polymers are initiation and chain branching, and propagation. These are also the reactions where it may be possible to utilise specific chemical compounds to interfere with the processes involved. 

Antioxidants are usually roughly divided into two categories  

Primary, or chain-breaking antioxidants interfere with the chain propagation step, i.e., the reactions  

Secondary, or preventative antioxidants react with the hydroperoxides responsible for chain initiation and chain branching. 

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From the available studies it would appear that hindered phenols in combination with phosphites, provided both components are sufficiently non-volatile and thermally stable, would be the preferred option for the thermo-oxidative stabilisation of aromatic polyesters. 

Whereas the use of thermo-oxidative stabilisers can be regarded as largely optional for aromatic polyesters unless particularly harsh conditions are likely to be met in service, the light stabilisation of these polymers is more of a requirement in any situation where an article made thereof is going to be exposed to short-wavelength light, or a combination of this with oxygen. 

Transition metals compounds stabilise PAI at both low and high temperatures, but sharply accelerate thermo-oxidation of methylene chains, for example, PE. 

PF foams offer to the construction industry a useful combination of physical and flammability characteristics. They generate low levels of smoke in most fire tests. It was reported that a cupric complex of a macro heterocyclic compound is an effective stabiliser for thermo-oxidative decomposition of PF foams above 200 °C. An amount of 0.25-5.0 pph permits significant decrease of ignitability [178]. 

Figure 2.5 shows the effect of the residual amount of stabiliser on the thermo-oxidative stability of LDPE/Chimassorb 944 exposed to various testing environments [154]. The differences in the oxidation behaviour of the polymeric matrix (as measured by DSC) are related to the differences in the consump-tion/migration rate of the stabiliser and the amount of stabiliser remaining in the polymeric matrix (as measured by UV spectroscopy). 

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