Dehydrochlorination rates

As already indicated, the measurement of dehydrochlorination rates is not a practical way of assessing the effect of a stabiliser. Thus the congo red test sometimes specified in standards, in which a piece of congo red paper is held in a test tube above a quantity of heated PVC and the time taken for the paper to turn blue due to the evolution of a certain amount of hydrogen chloride, cannot be considered as being of much value. 

Hjertberg and coworkers [38-41] were able to correlate the amount of labile chlorine, tertiary and internal allylic chlorine, to the dehydrochlorination rate. They studied PVC samples with increased contents of labile chlorine, which were obtained by polymerization at reduced monomer concentration. According to their results, tertiary chlorine was the most important defect in PVC. In agreement with other reports [42,43], the results also indicated that secondary chlorine was unstable at the temperatures in question, i.e., random initiation would also occur. 

Jamieson and McNeill [142] studied the degradation of poIy(vinyI acetate) and poly(vinyI chloride) and compared it with the degradation of PVC/PVAc blend. For the unmixed situation, hydrogen chloride evolution from PVC started at a lower temperature and a faster rate than acetic acid from PVAc. For the blend, acetic acid production began concurrently with dehydrochlorination. But the dehydrochlorination rate maximum occurred earlier than in the previous case indicating that both polymers were destabilized. This is a direct proof of the intermolecular nature of the destabilizing effect of acetate groups on chlorine atoms in PVC. The effects observed by Jamieson and McNeill were explained in terms of acid catalysis. Hydrochloric acid produced in the PVC phase diffused into the PVAc phase to catalyze the loss of acetic acid and vice-versa. 

Insecticidal Activity and Dehydrochlorination Rates of Some Polychloro Insecticides... 

Table I. Dehydrochlorination-Rate Constants and Insecticidal Activity of DDT Analogs...

These data lead to the conclusion that, contrary to the hypothesis of Martin and Wain, there is no statistically significant relationship between dehydrochlorination rate and insecticidal activity in DDT analogs, at least as regards activity against A. quadri-maculatus larvae. Similar comparisons of insecticidal data against adult houseflies (11,12),... 

Table II. Dehydrochlorination-Rate Constants and Toxicity to Anopheles quadrimaculatus Larvae of Isomers of Benzene Hexachloride...

The dehydrochlorination rate increases substantially in a wet oxidation environment and this acceleration is believed to be due to peroxy radicals, which are formed by the straight oxidation of a hydrocarbon or a fraction of polymer. However, this also is yet to be confirmed experimentally. 

Measuring Methods. Chlorine content was determined by the oxygen flask method (2) on a polymer purified by precipitation from the solution in cyclohexanone. Thermal stability, as HC1 evolution, was determined according to ASTM method D-793-49, determining the quantity of HC1 evolved by the polymer maintained at 180 °C in a nitrogen atmosphere. From the slope of the straight line for the amount of HC1 evolved with time, the constant K for the dehydrochlorination rate (DHC) is deduced. 

Figure 6. Dehydrochlorination rate vs. temperature for a crude FVC/EPR and a PVC homopolymer...

The OH group of organic acid is characterised by an absorption band at 3400 cm 1, corresponding to the stretching vibration. The best heat stability and gelation effects are obtained with ESO for long heating times, which are confirmed by dehydrochlorination rate constants. 

Milled rigid sheets of poly (vinyl chloride) on heating at 185°C. lose weight at a rate which increases with time. By polymer fractionation procedures, it was shown the rate of hydrogen chloride loss increases as the content of tetrahydro-furan-insoluble resin increases. The insoluble resin content accumulates at a rate which depends, in part, on the additive present. This insolubilization reaction is catalyzed by cadmium compounds. The increased dehydrochlorination rate of the insoluble crosslinked resins may result from the susceptibility of the crosslinked structures to oxidation and from the subsequent thermal degradation of the oxidation products. The effects of various common additives on the rates of insolubilization and weight loss are described. 

The reverse relationship is implicit in a series of studies by Imoto, Otsu, and Ota (14, 15, 21) in which the dehydrochlorination rate of the polymer in air was observed to be greater than in nitrogen. Thus, the oxygen caused a reaction which accelerated the rate of hydrogen chloride loss. Therefore, dehydrochlorination was at least in part a result rather than a cause of oxidation. 

The effect of metal compounds on HC1 evolution has been noted by many authors. Thus, Hartmann (12, 13) pointed out that lead and di-butyltin compounds did not affect dehydrochlorination rate, but ferric salts and zinc chloride accelerated it. Rafikov and co-workers (22) reported that lead and calcium soaps promoted crosslinking. These investigators do not seem to have considered the hypothesis that dehydrochlorination is accelerated as a consequence of crosslinking. 

An additional example of neighboring group participation in the gas-phase pyrolysis of 2-substituted ethyl chlorides was the elimination kinetics of 2-dimethylaminoethyl chloride67. The magnitude of the effect of Me2N on the dehydrochlorination rate (Table 14) led to a similar consideration to that for CH3SCH2CH2C1 by assuming the transition state for the elimination as an intimate ion-pair as represented above. 

Figure 11.12. Dehydrochlorination rate of PVC with and without 10% calcium carbonate. [Adapted, by permission, from Braun D, Kraemer K, Recycling of PVC Mixed Plastic Waste, La Manta F P, Ed., ChemTec Publishing, Toronto, 1996.]...

Zeolites modifled with organic solvents increase the reastance and degree of radiation crosdinking of filled poly-vinyl-chloride. The dehydrochlorination rate of poly-vinyhchloride on y-irradiation decreases in the presence of zeolites due to the sorption of hydrochloric acid by the filler. Modification of zeolite with carbcMi tetrachloride and methjlene chloride decrease the swelling d ee of filled poly-vinyl-chloride also. 

Potential adverse effects products of reaction with HCl accelerate dehydrochlorination rate, plateout, influence on taste and odor or products in contact, fogging, staining, low chemical resistance, easy to extract and to be lost by migration... 

Table 10.10 shows the inflnence of the alcohol part of plasticizer degradation on plasticized PVC dehydrochlorination rate. Here thermal stabihty of films (PVC 100 phr, DOP 60 phr, and alcohol 2 phr) was also measured using kinetic determination of HCl evolution and expressed by thermostability nnmber A. It is evident that alcohols of higher molecular weight (above six carbon atoms) have very little inflnence on PVC thermal degradation rate. Only volatile xylene and isobutanol increase degradation with a rate comparable to the weakest acids. 

Figure 10.52 shows that addition of the plasticizer decreased dehydrochlorination rate of PVC. Crosslinking rate was reduced with the presence of plasticizers. Also, the carbonyl group formation was slower in the presence of a plasticizer. Comparison of data presented by the same authors for the effect of radiation above and below 290 nm stresses importance of wavelength selection on the results obtained. When studies were conducted with an unfiltered mercury lamp, presence of phthalates increased the carbonyl group formation. 

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