Nonoxidative thermal degradation

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 ... 

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. 

Sometimes the term reversion is applied to other types of nonoxidative degradation, especially with respect to rubbers not based on isoprene. For example, thermal aging of SBR (styrene-butadiene rubber), which can cause increased crosslink density and hardening, has been called reversion, since it can be the result of overcure and can also degrade a product s usefulness. 

To characterize the thermal stability of PNCs, TGA data was collected on a TA Instruments Q50, at a heating rate of lO Cmin . Samples were heated up to 800 °C under a flow of 25 ml min nitrogen to study nonoxidative degradation, and under a flow of 25 ml min air to study oxidative degradation. 

To characterize the polymerization behavior of FA and to investigate how the presence of MMT influences this polymerization, FTIR spectra were collected before and during the resiniflcation process. The dispersion of the MMT in the PFA matrix is shown both directly and indirectly. The direct evidence consists of the XRD patterns of the FA-MMT suspension, which was used to monitor the process of intercalation and exfoliation of the MMT at various stages of resinifica-tion. The dispersion is indirectly evidenced in increased thermal stability of the MMT-PFA nanocomposite, as measured by TGA. The thermal stability is discussed and compared to the pure polymer and to the CW-PFA nanocomposites. In addition, the important differences between oxidative and nonoxidative degradation of the NaMMT-PFA nanocomposite is discussed, and a mechanism is proposed to explain the difference in terms of acid-catalyzed degradation. 

The behavior of 30BMMT is also similar compared to the case of nonoxidative degradation in that above 370 °C, 30BMMT-PFA shows the highest thermal stability of all three PNCs. Table 6.5 shows weight retention at 500 and 800 °C for cured... 

Degradation can occur through nonoxidative routes where the differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) thermograms will be identical in air and imder an inert atmosphere such as nitrogen. 

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