Thermal decomposition of poly(vinyl chloride) and chlorinated poly(vinyl chloride). II. Organic analysis

A concerted study of poly(vinyl chloride), chlorinated poly(vinyl chloride), and poly(vinylidene chloride) polymers by spectroscopy, thermal analysis, and pyrolysis-gas chromatography resulted in a proposed mechanism for their thermal degradation. Polymer structure with respect to total chlorine content and position was determined, and the influence of these polymer units on certain of the decomposition parameters is presented. Distinguishing differences were obtained for the kinetics of decomposition, reactive macroradical intermediates, and pyrolysis product distributions for these systems. It was determined that chlorinated poly(vinyl chloride) systems with long-chain ? CHCI? units were more thermally stable than the unchlorinated precursor, exhibited increasing activation energy for the dehydrochlorination, and produced chlorine-containing macroradical intermediates and chlorinated aromatic pyrolysis products. The poly(vinyl chloride) polymer was relatively less thermally stable, exhibited decreasing activation energy during dehydrochlorination, and produced polyenyl macro-radical intermediates and aromatic pyrolysis products.     

Photosensitized dehydrochlorination of poly(vinyl chloride). III. Reaction of thermally degraded poly(vinyl chloride) with hydrogen chloride

Films prepared from thermally degraded poly(vinyl chloride) were photolyzed in the presence of hydrogen chloride. When the light was filtered through a Pyrex disk, the absorbance in the region between 270 nm and about 415 nm decreased and reached a minimum value but the absorbance increased at wavelengths shorter than 270 nm and longer than 415 nm. Maximum bleaching occurred at a wavelength which depended on that of the irradiating light. When the Pyrex filter was omitted, an additional slower photodehydrochlorination reaction was superimposed on the photobleaching reaction. Photolysis of hexane or ethanol solutions of 1,8-diphenyloctatetra-1,3,5,7-ene and hydrogen chloride showed a similar reaction involving bleaching of the absorption of the polyene structure. The problems of using absorbance as an indication of the extent of the photodegradation of poly(vinyl chloride) are discussed.     

Photosensitized dehydrochlorination of poly(vinyl chloride)

The mechanism of the benzophenone-photosensitized degradation of films of poly-(vinyl chloride) was studied, both under vacuum and in the presence of oxygen. Initial rapid increases in absorption in the 340 nm region strongly implicate the ketyl radical in the initiation process which involves the abstraction by triplet benzophenone of a methylenic hydrogen from poly(vinyl chloride). Dehydrochlorination then occurs by a chain mechanism, and the conjugated polyene structure produced photosensitizes further initiation and, in the presence of oxygen, photo-oxidation of the polymer.     

Thermal decomposition of poly(vinyl chloride) and chlorinated poly(vinyl chloride). I. ESR and TGA studies

Significant effort has been made in the past by many workers to investigate the mechanism of thermal decomposition of poly(vinyl chloride) (PVC). The presence and role of free radicals has been controversial in this regard. Our data on PVC and chlorinated PVC systems demonstrate the existence of macroradicals in the early stage of thermal decomposition under inert and oxidative atmospheres. Data from conventional thermogravimetric experiments are used in conjunction with the electron spin resonance findings.     

Thermal dehydrochlorination of poly(vinylidene chloride)

The kinetics of the early stages of thermal degradation below 1% dehydrochlorination of emulsion-polymerized poly(vinylidene chloride) (PVDC) is studied by the variation of the pH value of potassium hydroxide aqueous solution between 160 and 190°C in the presence of air and other gas streams. The results turned out that the thermal degradation of PVDC can be divided into three stages, which correspond to an induction period, a period with conversion below 0.1% dehydrochlorination, and that with conversion ranging from 0.1 to 1%. For the induction stage, the induction time depends upon the types of environment gas and degradation temperature. Both of the second and the third stages are zero-order reactions, which also result in the discoloration and crosslinking of the neat polymer. The average apparent activational energy of the zero-order degradation reaction was about 21 kcal/mol, which is independent of the types of environment gas. The whole degrading kinetics data can be well explained by the mechanism of a free-radical-induced dehydrochlorination. The viscosity of the degraded sample increases rapidly with degradation and becomes insoluble in regular solvents. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2035–2044, 1999     

Thermal degradation of poly(vinyl chloride). II. Degradation mechanism based on decomposition energetics

On the basis of the measured activation energies for radical and HCl generation in four poly(vinyl chlorides) of controlled structures, an overall mechanism of degradation is proposed. The mechanism is a chain reaction involving a β-chloro elimination from a radical-containing polymer molecule. Molecules containing tertiary C? Cl bonds are shown to undergo a fast initiation. Those containing tertiary C? H bonds are also demonstrated to be highly unstable, but the instability is not associated with the initiation step.     

Investigations on poly(vinyl chloride). II. Pyrolysis of chlorinated polybutadienes as model compounds for poly(vinyl chloride)

The pyrolysis of chlorinated polybutadienes (CPB) was investigated by using a pyrolysis gas chromatograph. CPB corresponds to poly(vinyl chloride) (PVC) constructed with head–head and tail–tail linkages of the vinyl chloride unit. Benzene, toluene, ethyl-benzene, o-xylene, styrene, vinyltoluene, chlorobenzenes, naphthalene, and methylnaphthalenes were detected in the pyrolysis products from CPB above 300°C, and no hydrocarbons could be detected at 200°C. The pyrolysis products from CPB were similar to those from PVC and new products could not be detected. Lower aliphatics, toluene, ethylbenzene, o-xylene, chlorobenzenes, and methylnaphthalenes were released more easily from pyrolysis of CPB than from PVC; amounts of benzene, styrene, and naphthalene formed were small. These results support the conclusion that recombination of chlorine atoms with the double bonds in the polyene chain takes place and that scission of the main chain may depend on the location of methylene groups isolated along the polyene chain during the thermal decomposition of PVC.     

Structure of chlorinated poly(vinyl chloride). V. Molecular parameters of chlorinated poly(vinyl chloride)

The molecular parameters of samples of chlorinated poly(vinyl chloride) (CPVC) and chlorinated β,β-dideuterated poly(vinyl chloride) (β,β-d₂-CPVC) were determined by gel permeation chromatography (GPC), light scattering, osmometry, and viscometry. Comparison of GPC, light scattering, osmometric, and viscometric data resulted in a discussion of the possibility of degradation and the causes of changes in the solution properties in chlorination of PVC and ββ-dideuterated poly(vinyl chloride) (ββ-d₂-PVC). The results obtained are discussed in relation to the mechanism of chlorination of PVC.     

Thermo-oxidative dehydrochlorination of rigid and plasticised poly(vinyl chloride)/poly(methyl methacrylate) blends

The aim of this work was to study the thermo-oxidative dehydrochlorination of rigid and plasticised poly(vinyl chloride)/poly(methyl methacrylate) blends. For that purpose, blends of variable compositions from 0 to 100 wt% were prepared in the presence (15, 30 and 50 wt%) and in the absence of diethyl-2-hexyl phthalate as plasticiser. Their miscibility was investigated by using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Their thermo-oxidative degradation at 180 ± 1 °C was studied and the amount of HCl released from PVC was measured by a continuous potentiometric method. Degraded samples were characterised, after purification, by FTIR spectroscopy and UV-visible spectroscopy. The results showed that the two polymers are miscible up to 60 wt% of poly(methyl methacrylate) (PMMA). This miscibility is due to a specific interaction of hydrogen bonding type between carbonyl groups (CO) of PMMA and hydrogen (CHCl) groups of PVC as shown by FTIR analysis. On the other hand, PMMA exerted a stabilizing effect on the thermal degradation of PVC by reducing the zip dehydrochlorination, leading to the formation of shorter polyenes.     

Study of the thermal dehydrochlorination of poly(vinylidene chloride) and poly(vinyl chloride) by ESR spectroscopy

The evidence for a radical elimination of hydrogen chloride during the thermal degradation of homopolymers and copolymers of vinylidene chloride is summarized and confirmed by an ESR spectroscopic study of the degradation residues. However, sufficient differences in the degradation characteristics exist between these polymers and those of vinyl chloride to suggest that a radical process alone is not sufficient. No evidence of a radical process can be obtained from an ESR spectroscopic analysis of the elimination. The paramagnetic character of the degraded polymer is attributed to the polyene structure produced on dehydrochlorination.     

Thermal dehydrochlorination of poly(vinyl chloride)—epoxidized butadiene/styrene triblock copolymer blends

The effect of the type of epoxidized butadiene/styrene block copolymer [ESBS; linear (B/S) or radial (E(B/S)_n), containing 0–27% of epoxy groups] on the thermal dehydrochlorination of poly (vinyl chloride) (PVC)-ESBS blends (ESBS content 10%) was investigated in the temperature range 170–180 °C, under a non-oxygen atmosphere. Thermal stability of the PVC-ESBS blends was estimated on the basis of induction time, t₀, and maximum rate of hydrochloride emission, V_max from the system. It was found that, for a similar degree of epoxidation of the SBS copolymer, the induction time i.e. the time after which emission of HCl begins, is longer in the series PVC < PVC-SBS < PVC-EB/S < PVC-E(B/S)_n, and the same is true for thermal stability. However, the maximum rate of emission of HCl is lowest in the case of PVC-EB/S blends, in the range of molar ratios from 0.5 to 2.0 × 1O⁻². On the basis of the dependence V_max = f(EB/VC), it was found that there is a certain content of epoxidized butadiene (EB) units in a mixture which causes the optimum stability of poly (vinyl chloride) during heating. During thermal destruction of the PVC-ESBS blends, the HC1 evolved undergoes addition both to epoxy fragments and to double bonds. The degree of conversion of EB units in time t₀ is 20% at 170 °C and 30% at 180 °C. The glass transition temperature, T_g, of PVC in the PVC-ESBS blends shifts towards higher temperatures by about 6–8 °C, proving the existence of crosslinking processes during moulding of the blends. The epoxidized butadiene/styrene radial block copolymer, E(B/S)_n, is a better thermal stabilizer of PVC than the linear EB/S copolymer.     

Thermal degradation of poly(vinyl chloride)

This paper presents an initial attempt at describing poly(vinyl chloride) (PVC) thermal degradation through a semi-detailed and lumped kinetic model. A mechanism of 40 species and pseudocomponents (molecules and radicals) involved in about 250 reactions permits quite a good reproduction of the main characteristics of PVC degradation and volatilization. The presence of the two step mechanism—the first step of which corresponds to dehydrochlorination and the second to the tar release and residue char formation—are correctly predicted both in quantitative terms and in the temperature ranges. The model was validated by comparison with several thermo gravimetric analyses, both dynamic at different heating rates, and isothermal. When compared with the typical one step global apparent degradation models, the approach proposed here spans quite large operative ranges, especially when it comes to predicting product distributions. The initial results of these product predictions, even though quite preliminary, are encouraging and confirm the validity of the model.     

Investigations on poly(vinyl chloride). I. Evolution of aromatics on pyrolysis of poly(vinyl chloride) and its mechanism

Poly(vinyl chloride) (PVC) alone or mixed with 10 wt-% and 50 wt-% TiO₂, SnO₂, ZnO, and Al₂O₃ were pyrolyzed by using a pyrolysis gas chromatograph. Benzene, toluene, ethylbenzene, o-xylene, styrene, naphthalene, and various chlorobenzenes were identified. No hydrocarbons could be detected in pyrolysis products of any samples at 200°C. More aromatic hydrocarbons than aliphatic hydrocarbons are released from the PVC–TiO₂ system and in preheated PVC. The contrary result is observed in the PVC–ZnO and PVC–SnO₂ systems. Aromatics having methyl endgroups are easily released from the PVC–ZnO and PVC–SnO₂ systems and at elevated pyrolysis temperature, because methylene groups are easily isolated along the chain by ZnO, SnO₂ and the heating. The release of ethylbenzene o-xylene, and chlorobenzenes suggests a repeated dehydrochlorination and recombination of HCl and Cl₂ to double bonds along the chain. Possible decomposition mechanisms of PVC are discussed.     

Structure and solution properties of radical-initiated poly(vinyl chloride). II. Correlation of solution properties

The results of detailed measurements of osmotic pressure, light scattering, and viscosity of poly(vinyl chloride) solutions are used to establish the molecular weight dependence of [η] and A₂, to estimate the unperturbed dimensions of the poly(vinyl chloride) molecule, and to analyze critically the [η]–M correlations published hitherto.     

Investigations on poly(vinyl chloride). III. Effects of metal oxides upon thermal decomposition of poly(vinyl chloride)

Thermal decomposition mechanisms of poly(vinyl chloride) (PVC) and the effects of a few metal oxides on the pyrolysis of PVC were previously reported. In the present work, 33 metal oxides were investigated to determine their effects on the thermal decomposition of PVC, by using a pyrolysis gas chromatograph. Most acidic oxides accelerate the recombination of chlorine atoms with double bonds, since PVC containing these metal oxides easily release lower aliphatics, toluene, ethylbenzene, o-xylene, and chlorobenzenes. On the other hand, most basic metal oxides, such as oxides of alkaline earths or silver, inhibit the recombination. These tendencies observed in the thermal decomposition of PVC agree with the contributions of corresponding metal salts to the dehydrochlorination of PVC proposed by other workers. This means that thermal decomposition or dehydrochlorination of PVC is affected by irregularities in head-to-tail linkages formed by the recombination of chlorine atoms during heat treatment of PVC.