Thermal Degradation during Processing

Organic phosphites, phosphates, and phosphonites are used to prevent metal-ion-catalyzed thermal and thermal-oxidative degradation of polyesters, because they form insoluble complexes that inhibit degradation. Typically, stabilizer loads range 

Additional stabilizers effective against thermal degradation are, e.g., 5-hydroxy isophthalic acid (it reduces acetaldehyde formation) and a-diethyl succinate. 

Phosphor compounds are utilized for thermal-oxidative and thermal stabilization. Phenols, amines, and thio ethers are also used to stabilize polyesters. 

Besides the commercially available stabilizers, a number of highly effective stabilizers can be found in the patent literature. Several combinations of stabilizers exhibit synergetic effects in particular the combinations thio ether/sterically hindered phenols and phosphite/sterically hindered phenols, respectively. The combination trimethyl hexyl borate/triphenyl phosphite is another known synergetic stabilizer. 

For the improvement of thermal-oxidative resistance, the effectiveness of the compounds used depends largely on their chemical structure and on the point in time when they are added during the manufacture of PET. A temperature of 280 °C and a strong vacuum during polycondensation can result in the loss of antioxidants 

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ACS polymers, developed primarily in Japan, are grafts of acrylonitrile and styrene onto elastomeric chlorinated polyethylene. Although the polymer has good weathering properties it is somewhat susceptible to thermal degradation during processing and to date these polymers have been of limited interest. 

Frequently, different classes of light stabilizers are combined to optimize stabilizing efficiency [112]. For example, UVAs and HALSs, used in combination, often provide better photostability than either class alone. Light stabilizers are also used in combination with additives that protect the polymers against thermal degradation during processing, such as hindered phenols and phosphates [113]. Consequently, various bifunctional and trifunctional photostabilizers have been synthesized and some have been selected for use in commercial applications (see Chart 9.15). 

Figure 4.62 Cyclical transition state of polyesters under thermal degradation during processing 4.3.7.1 Polybutylene Terephthalate...

For chlorine containing polymers, as PVC, thermal degradation during processing is important. In this case HCl is split off the chain, which catalyzes further decomposition [17-21]. 

Another source of volatility is the presence of relatively unstable linkages in the backbone of the polymer which can thermally degrade during processing and fabrication. These bonds may be present even if the polymer has stable end groups directly from polymerization. These linkages usually do not involve tetrafluoroethylene in FEP and arise from two... 

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