Initiators of degradation

Low levels of ultraviolet absorbers (typically 0.2-0.8 pphr [parts per hundred resin]) can also be useful in preventing initiation of degradation mechanisms. Modified benzophenones and benzotriazoles are in widest commercial use. 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

As discussed in Section III.G, the possibility of simultaneous initiation of degradation from normal units in the PVC chain is receiving increased attention. The work of Millan and coworkers who showed that the normal secondary chlorines in isotactic triads are also labile and can act as points of initiation for thermal degradation was also discussed. This favors simultaneous participation of normal chlorines along with defect sites in the thermal degradation of PVC. The content of nondefect labile structures in PVC is much higher than that of the defect structures [125]. 

There have been many studies on the thermal and thermo-oxidative degradation of PMMA.23 24 It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized structure and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.31... 

Figure 6. Arrhenius Activation Energy for the Initiation of Degradation of a Vinylidene Chloride/ Methyl Acrylate (Five Mole percent)/ 4-Vinylpyridine (0.1 Mole Percent) Terpolymer.

The weather resistances of the copolymers were about the same as that of cotton. The differences in mechanisms of the initiation of degradation of cellulose by rotting or by weathering probably accounts for these results (22, 23, 26, 41, 48, 49, 50, 51, 53). Apparently, under the conditions of weathering used, degradation of cellulose by microorganisms was not an important factor (30). 

A comparison was made between the mechanisms of action of the PVC costabiliser N-phenyl-3-acetylpyrrolidin-2,4-dione and existing costabilisers. The results obtained demonstrated that the performance of short-term costabilisers in calcium/zinc formulations was linked to their ability to complex zinc stearate prior to the initiation of degradation and that this prevented the early formation of prodegradant zinc chloride. 6 refs. 

Steps (1) and (4) are slow, so thermal oxidation is usually associated with an induction period. Additionally, thermal oxidation with O2 can be slowed with the use of an antioxidant, which competes with R H for hydro peroxy radicals. When atomic oxygen is present, however, oxidation begins immediately, without evidence for an induction period, and the erosion rate is unaffected by the addition of antioxidants. Therefore, the initiation of degradation was proposed to involve reaction with atomic oxygen ... 

Scheme 1 a Initiation of degradation of a polymer chain (R-H) by heat or light with the formation of a carbon-centered radical (R ). b Oxidation of the polymer chain by formation of a peroxyl radical (ROO ). c Formation of a hydroperoxide (ROOH) and R, leading to auto-oxidation... 

A significant amount of research has been carried out to determine the nature of the weak links in FR PS [83-102]. Various initiation of degradation... 

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