Thermal and Hydrolytic Stabilisation

Aromatic polyesters, in particular poly(ethylene terephthalate) (PET), are subject to thermal degradation processes during polycondensation, melt processing and, to a lesser extent in service, in situations where oxygen is absent or subject to limited availability. 

As described in Chapter 2, heating of aromatic polyesters to high temperatures in the absence of oxygen can lead several consequences, 

Most purely thermal or hydrolytic damage will take place during melt processing of an aromatic polyester, so this chapter will concentrate on the means of preventing degradation and, due to the nature of these materials as condensation polymers, on ways and means of repairing degraded materials. 

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As discussed in Chapter 2, the chain ends of polyester molecules exert considerable influence on the thermal stability of these polymers. In particular, carboxylic acid chain ends which are present in the as-synthesised polymer and which build up with thermal degradation and hydrolytic reactions, can severely restrict the stability of the polymer. It is therefore unsurprising that a great deal of effort has been put into finding additives and processes which can control or eliminate such moieties. Additives of this type are known as end-cappers , although some are also referred to as hydrolytic stabilisers . 

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