Nonoxidative thermal

In keeping with earlier observations (19,98), the nonoxidative thermal dehydrochlorination of PVC has been shown recently to be facilitated by preliminary photodegradation of the polymer (10,99). The thermal sensitivity enhancement increases with decreasing wavelength of irradiation (10) and undoubtedly results from the photolytic formation of thermally labile defect sites (10). 

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

Tt is known that the principal pathway for nonoxidative thermal - degradation of poly (vinyl chloride) (PVC) involves the evolution of HC1 and the accumulation of unsaturation in the polymer chains (1,2,3,4), as shown in Equation 1 ... 

Reversion (when defined as the loss of cross-Unks during nonoxidative thermal vulcanizate aging) is a problem associated mainly with natural rubber or synthetic isoprene polymers. It can occur only under severe conditions in butadiene rubber in SBR, instead of the softening associated with the nonoxidative aging of natural rubber, one can observe... 

The structure of the diamine component has a greater effect on the (nonoxidative) thermal than on the thermooxidative stability. 

The onset temperature of decomposition of alkyl quaternary ammonium-modified montmorillonite, in nonoxidative thermal degradation, is about 180°C. Initial degradation of the surfactant follows either a Hoftnann elimination or an Sn2 nucleophilic substitution mechanism. Both mechanisms can affect the performance of high-processing-temperature nanocomposites and, in general, the thermal stability and combustion behavior of nanocomposites. In particular, Hofmann elimination generates acidic sites on the layered silicate that can act as a protonic acid catalyst on polymer decomposition. " Imidazolium and phosphonium salts exhibit improved thermal stability compared to ammonium salts.Alkylimidazolium salt-modified layered silicates were used successfully to prepare organoclays that exhibit an onset of decomposition temperature up to 392°C. 

Straightforward reversion to monomers is not convenient with some of the commodity polymers, namely, polyethylene, PP, and PVC. An omnibus solution is to use some sort of nonoxidizing thermal treatment that wiU produce a low-molecular-weight product. The terms chemical recycling and plastics-to-chemicals are used for such processes. 

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