Decomposition of PVC

Since that time increasing numbers of experiments have been carried out using analytical techniques of improved sensitivity and most workers now agree that there is overwhelming evidence that HC1 does catalyze the theimal decomposition of PVC both in the presence of oxygen and under inert conditions. Among the first group of workers whose results showed a positive... 

The difference of the polyolefins and the PVC in the catalytic degradation can be seen in Fig. 3 the decomposition of PVC takes place at least in two steps, the first is due to the release of HC1 with 300 °C characteristic temperature, the second peak corresponds to the pyrolysis of the residual hydrocarbon framework (440 - 470 °C temperature range). 

Decomposition of PVC proceeds via loss of HCl and by autoxidation these lead to embrittlement and to coloration, due at least in part to the formation of conjugated polyenes. 

Although the exact mechanism of the thermal, heat-induced decomposition of PVC remains unknown, most chemists agree that the chlorine atoms present in the polymer play an important role. Lead salts are added to PVC both to provide anions less reactive than chloride and to provide lead ions to combine with the released chloride ions. As a beneficial side effect, the lead... 

Also, the impact on the environment in the pyrolysis of waste plastics must be taken into consideration. If a PVC material is contained in the reactant, the hydrochloric acid is evolved during decomposition of PVC which causes air pollution. Thus, a system is needed... 

The decomposition of PVC leads to the formation of HCl, which is neutralized by bringing the hot gases into contact with a solid absorbent. This results in a CaCl2-fraction that has to be landfilled. The purified gas is cooled, most of the hydrocarbon condensed as distillate feedstock. The remaining light hydrocarbon gas is compressed, reheated and returned to the reactor as fluidizing gas. Part of the stream could be used as fuel gas for heating the reactor, but as it is olefin-rich, recovery options are underway. 

In their study of decomposition of PVC, they reported that metal oxides with a large enough metal ion radius such as iron oxide are able to dechlorinate the PVC by attracting chlorine and weakening of C-Cl bonds in PVC and Chloro-organic compounds. Iron oxide initially acts as catalyst and under the reaction conditions it is converted to iron chloride by reacting with HCL The iron chloride phase is also active for dechlorination of chloroorganic compounds. 

A stabilizer is added to retard the decomposition of PVC during curing, which is carried out at 150-175 C. Because of low thermal conductivity, several hours are required to raise the central portion of the propellant grain to curing tern-perature. The portions of the grain close to the source of heating may show a tendency to decompose and stabilizers are added to inhibit the decomposition. It is advisable that the stabilizer should be able to bind the hydrogen chloride which would decompose the polymer. However propellants with aluminium powder show better thermal conductivity and hence the time of cure can be reduced. 

Some studies show that pyrolysis of certain polymer blends can be influenced by the migration of a small molecule or a small radical formed from one type of polymer and affecting the other type. For example, poly(methyl methacrylate) (PMMA) in blends with poly(vinyl chloride) (PVC) shows higher resistance to heat. The thermal decomposition of PVC generates HCI, which interacts with the PMMA forming anhydride units in the middle of PMMA chains, as shown below ... 

PVC is relatively unstable to heat, and at temperatures as low as 200° C begins to decompose. The decomposition of PVC has a complex chemical mechanism [1 ] and takes place in two steps, the first being dehydrochlorination, and the second being the... 

Depending on the polymer sample (and presence of various stabilizers), as well as the heating conditions, thermal decomposition of PVC may show some variations. The curve showing the variation of weight loss % as a function of temperature (TG curve) for a 3.5 mg PVC sample with M = 85,000 is shown in Figure 6.3.1. The heating was done between 30° C and 830° C at a rate of 10° C/min. in air. 

Both free radical and molecular elimination mechanisms seem to participate in PVC degradation. After the initial formation of some HCI molecules, these seem to have a catalytic effect on the decomposition of PVC. which can be explained by reactions of the type [25] ... 

As a consequence of the low temperature removal of HC1, thermal decomposition of PVC is a two-step process dehydrochlorination of the polymer to form a polyene macromolecular structure followed by cracking and decomposition of the polyene. 

Dehydrochlorination, brought about by free radical intermediates, is characteristic of both thermal and radiolytic decomposition of PVC [Salovey, 1973]. HCl is the major gaseous product of irradiation. Its formation depends on temperature at T = -196°C, H is the major product but HCl is also formed [Ohnishi et al., 1962]. As the temperature increases, the yield of HCl increases, first linearly up to T -60°C, and then at higher rates from T -40°C to +30°C. At T = 30 to 70°C the rate of HCl formation again increases [Ohnishi et al., 1962]. 

The thermal decomposition of PVC involves the complete elimination of HQ, leading to the formation of macromolecular residues with polyene sequences. The latter then rearrange and decompose to yield sizeable amoimts of aromatic hydrocarbons. 

The mechanisms concerning the chemical processes occurring in the thermal decomposition of PVC have been somewhat controversial in the past, but at present Scheme 5.4 is widely accepted in the literature. ... 

The use of carbon black pigmentation/stabUization in thermoplastics for external exposure is widely applicable but has to be undertaken with care since it increases radiant heat absorption and hence the possibility of distortion. The type, particle size and degree of dispersion of the carbon are important for optimum efficacy. Additions in the order of 1-3% are generally sufficient for this purpose but in rubbers, where the carbon black also has a reinforcing function, considerably more is used. Other pigments have varying effects, an example being iron oxide which protects polyolefins yet catalyses the decomposition of PVC. 

Figure 9.9. TG and DTG curves. Experimental (dotted line) and theoretical curves (full lines) for plasti-sols containing 65 phr of DBF (a), DOP (b) and DIDP (c), at 5°C/min. [Reprinted from Polymer Degradation and Stability, Vol 53, Beltran M., Marcilla A., PVC-plasticizer interactions during the thermal decomposition of PVC plastisols. Influence of the type of plasticizer and resin, p. 261-268, 1996, with permission from Elsevier Science],...

Vol 53, Beltran M., Marcilla A., PVC-plasticizer interactions during the thermal decomposition of PVC plastisols, Influence of the type of plasticizer and resin, p. 261-268, 1996, with permission from Elsevier Science],... 

Thermogravimetric analysis has been widely employed to characterize polymers by their decomposition behavior. Marcilla and Beltran presented a series of papers " in which the behavior during the decomposition of PVC, plasticizers and plastisols was compared to each other, showing a clear dependence on the concentration and the type of plasticizer used. 

Thermal decomposition of PVC is believed to be linked to the loss of plasticizers. Activation energy of thermal degradation is given by equation ... 

Coloured pigments have also been used, such as lead oxide, chromium oxide and red iron oxide. However, heavy metals are considered inappropriate for many applications. A few inorganic pigments (notably cadmium sulphide and the anatase form of titanium dioxide) can even act as UV sensitisers, aggravating the degradation. Iron pigments cannot be used in PVC compositions either, because of the risk of forming ferric chloride, which is a catalyst for the decomposition of PVC. 

FIGURE 4.13 Adsorption isotherms of benzene vapors on PVDC charcoal before and after pore blocking by decomposition of PVC. (After Sharma, N., Ph.D. thesis submitted to Panjab Univ., Chandigarh, India, 1991. With permission.)... 

The above considerations have a possible interpretation in terms of mechanism. Chain initiation followed by unzipping may occur randomly at existing 2,2,4,4-tetra-chloro moieties throughout a PVDC chain. Is it possible that 2,2,4-trichloro moeities could start decomposition of PVC The critical nature of hydrogen chloride in the initiation reaction may depend upon its ability to form 2,2,4-trichloro moeities by addition, elimination and readdition of HCl at double bonds at or near chain ends. 

There are no reports in the literature concerning the ease of dehydrochlorination of 2,2,4-trichloro moieties but certainly none of the model compounds that have been studied has lost hydrogen chloride at temperatures where incipient decomposition of PVC occurs. It may be that the function of hydrogen chloride in the initiation of PVC is merely the creation of a structure that is analogous to that already present in PVDC. 

The addition of chalk increases thermal stabihty of the filled PVC sample, indicated by a longer induction period of dehydrochlorination. Also, the rate of HCl-elimination is lower in the presence of calcium carbonate. Other investigations with unstabihzed PVC samples have shown that the filler acts as a trap for the split-off hydrogen chloride, but it has no influence on the decomposition of PVC. This is confirmed by the UV-spectra of PVC which was heat-treated in the absence and in the presence of chalk. Under both conditions, the same unsaturated sequences in PVC are formed, as shown in Figure 8 (curve a and b). On the contrary, in the presence of stabilizers (curve c) practically no polyene sequences are formed during the induction period. 

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