Dehydrochlorination of PVC

The presence of irregular sequences increases the rate of degradation considerably. The initial rates of degradation are proportional to the content of these irregularities. 

The degradation is autocatalytic, i.e., the hydrogen chloride formed initially, catalyzes the reaction further. Moreover, the degradation of PVC proceeds very fast in the presence of Lewis acids, such as FeCls, ZnCb, AICI3, etc. 

It has been proposed that the Lewis acids form intermediate complexes like 

When the dehydrochlorination reaction proceeds, a sequence of conjugated double bonds are formed that causes coloring. 

Alkyl tin compounds Mixed metal compounds /S-Diketones Epoxidized fatty acids Hydrotalcite compounds 

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Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

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). 

Table I presents the results of "isothermal" simultaneous thermoanalytical (STA) runs, at 573 K and 773 K, for all three products. Similar data, at a fixed heating rate is shown in Table II. One of the crucial parameters is the temperature of maximum weight loss rate, corresponding to the time when dehydrochlorination of PVC starts becoming important. This temperature is close to 573 K in all cases. In fact, at a relatively fast heating rate, almost no decomposition occurs at temperatures under 563 K. If the materials are heated at 573 K for a prolonged period, complete dehydrochlorination takes place, but no further stages of PVC decomposition occur. None of the three materials investigated decomposes completely until a temperature of ca. 773 K is attained. Even then only a certain fraction of the entire mass of the samples is volatilised, due to the presence of inorganic fillers in their composition.

In addition, Nakagawa et al. (1JL) have shown that dehydrochlorination of PVC was suppressed extremely by incorporation of DTC or MDTC group, while the mechanism was not fully obvious. 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

FIGURE 18.23 Comparison of the PBM for chain stripping with dehydrochlorination of PVC in constant heating rate TGA. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

The effect of dialkyltin maleates and laurates on the thermal dehydrochlorination of PVC has been compared in 1,2,4-trichlorobenzene solution by IR analysis, and showed that tin laurates are superior to tin maleates in replacing the labile chloride atoms in PVC. Attempts to trace intermediate monochlorotin derivatives in the case of maleates by polarography and Mossbauer spectroscopy were not conclusive. 

Photothermal dehydrochlorination of PVC as studied by derivative ultraviolet absorption and fluorescence analysis techniques has shown that carbonyl and hydroperoxide groups are more important in the initiation steps than... 

Free radical processes were first postulated by Arlman [166], Winkler [192] and Stromberg et al. [165]. The rate of dehydrochlorination of PVC was reported to be a 3/2 order reaction by Stromberg et al. [165] and the mechanism suggested was the following. 

Therefore, molecular, non-radical dehydrochlorination is suggested by many workers to be the most probable mechanism for dehydrochlorination of PVC. 

Figure 4.13 Degree of dehydrochlorination of PVC at 150°C as a function of time O untreated PVC, % HCl treated PVC.84...

This study (104) shows that the nonoxidative dehydrochlorination of PVC can be inhibited by anthracene to some extent. 

Blends of POM/TPU Ultraform ) must be reprocessed with care. When the processing stock contains regrind of different pellet size, the change in the flow properties may affect the product characteristics. Furthermore, since these blends are immiscible with other thermoplastics, contaminated regrind may lead to delamination. Contamination with other thermoplastics, especially with small amount of PVC, may cause sudden, uncontrollable decomposition engendered by dehydrochlorination of PVC. 

Thermal decomposition, viz. dehydrochlorination of PVC, unzipping of PMMA or POM, ester-group decomposition, carbonization of lignin, etc. 

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