Natural weathering,photo and thermal degradation of the two-phase system poly(vinyl chloride) and poly(isobutylene)

Natural weathering, photo-oxidative and thermo-oxidative degradation have been compared for poly(vinyl chloride) (PVC) and poly(isobutylene) (PIB) and nine of their blends covering the entire composition range. The degradation techniques used include heating in an air oven at 100°, u.v. irradiation and natural weathering. Studies on the morphology of the blends show that the state and mode of dispersion of the elastomer in films of the blend are influenced by composition. PIB is substantially less stable than PVC and does not act as a good modifier for PVC in freshly prepared samples. Considerably improved mechanical properties were obtained when the polyblends were partially degraded presumably because of the interactions between the polymers. The changes in the chemical properties were monitored by i.r. measurements. The presence of PIB in PVC up to 20% gave optimum stabilization to PVC whereas small amounts of PVC in PIB destabilized PIB considerably. The presence of PIB in a PVC-PIB blend leads to considerable suppression of dehydrochlorination at 100°. The blends became more stable than the homopolymers. The effect of PIB is chiefly to stabilize PVC; a mechanism is proposed. Equations have been developed to allow evaluation of the role of PIB as an effective stabilizer for PVC.     

Suspension polymerization of vinyl chloride and the processibility of poly(vinyl chloride)

The most important technological procedure in the production of PVC is the suspension polymerization of vinyl chloride, as processibility of the polymer may be influenced to a considerable extent by the choice of polymerization conditions. Structure heterogeneities in PVC powders manifest themselves in plasticized PVC by the occurrence of “fish eye” particles. This review concerns the formation and properties of these particles and discusses the causes of their difficult processibility. Also, the relation between polymerization process and PVC dehydrochlorination is discussed and a new mechanism of its initiation based on the reactivity of cisoid enone structures is proposed. These structures catalyze elimination of hydrogen chloride from regular units of PVC by an interchain enzyme-like mechanism giving rise to chloroallyl structures.     

Photo-oxidation of poly(vinyl chloride)

Hydrogen chloride is evolved at an increasing rate in the light-induced oxidation of poly(vinyl chloride) films. These accelerated kinetics were shown to result from an increased absorption of light by the polyenes formed, since the quantum yield of dehydrochlorination (Φ_HCl = 0·015) is independent of the extent of the reaction in the dose range investigated. Determination of the quantum yields of the different processes involved indicate that, for each scission of the polymer backbone, 11 molecules of hydrogen chloride are evolved while three carbonyl groups, two hydroperoxides and 0·4 intermolecular crosslinks appear on the polymer chain. A mechanism that involves β-scissions of the tertiary alkoxy radicals, resulting from non-terminating interactions of α-chloro-peroxy radicals, is suggested to explain the observed increase of the polymer degradation in the presence of oxygen.     

Radiolysis of poly(vinyl chloride)

Allyl free-radical intermediates are detected by ultraviolet absorption at 255 mu in poly(vinyl chloride) irradiated at ?196°C and stored at 25°C. In vacuum at 25°C, allyl radicals are converted into polyenyl free radicals and polyenes. From the nature of allyl radical decay in vacuum, radical chain transfer between polyenyl radicals and poly(vinyl chloride) is inferred. Allyl and polyenyl free radicals are scavenged by oxygen on post-irradiation storage in air.     

Pyrolysis of poly(vinyl chloride)

A pyrolysis–gas chromatography–mass spectrometric technique has been developed to study the thermal degradation of poly(vinyl chlorides) polymerized at different temperatures. Hydrogen chloride and benzene evolution during successive stages of pyrolysis have been quantitatively determined and correlated to the tacticity and molecular weight of the polymer. It was found that lowering the temperature of polymerization and molecular weight depresses benzene evolution and increases char weight but does not affect the HCl yield. It is suggested that the syndiotactic trans microstructure favored at low temperature of polymerization yields polyenes which cannot cyclize to form benzene. As the molecular weight decreases, the increase in number of vinyl chain ends facilitates pyrolytic crosslinking and char formation.     

Photo-oxidation of poly(vinyl chloride)

The photo-oxidation of PVC has been studied over the temperature range 30–150°C. Initiation with ultraviolet (2537A) radiation has been correlated with the presence of minute amounts of ozone. The contribution of atomic oxygen and singlet oxygen (¹Δ_g) molecules to the initiation mechanism is discussed. The β-chloroketones probably formed in the photo-oxidation of PVC, decomposed according to a Norrish type I reaction without loss of chlorine atoms. The gaseous products of the photo-oxidation of PVC at 30°C were carbon dioxide, carbon monoxide, hydrogen, and methane. Hydrogen chloride was obtained only when PVC was heated at high temperatures. When PVC was photo-oxidized and then heated at high temperature, benzene was obtained in addition to hydrogen chloride. The gaseous products from the photo-oxidations of model compounds, such as 4-chloro-2-butanone and 2,4-dichloropentane, were also compared with those from PVC. Hydrogen chloride was detected only after photo-oxidation at temperatures of 25°C or higher. Therefore, it was concluded that hydrogen chloride is mainly a product of thermal decomposition. Since unsaturation was not observed in photo-oxidized PVC films, the cause of discoloration is unclear. When PVC was modified by stabilizers or additives, the oxidative degradation was further complicated by side reactions with the additives.     

An NMR and DSC-investigation of the poly(vinyl chloride)-dioctylphthalate system

It has been demonstrated that the system poly(vinyl chloride)-dioctylphthalate (PVC-DOP) gives rise to distinct component lines in the wide-line NMR spectrum. These lines correspond to very different degrees of segmental mobility of the material within a large interval of temperatures. We consider that these component lines correspond to at least two PVC-phases of the samples since no established phase separation occurs and since the narrow line registered at higher temperatures vanishes at room temperature. DOP itself exhibits a very narrow line at room temperature. The DSC-results indicate a variety of phases since the transition region occurs over a wider temperature interval when the amount of DOP increases. The T_g of PVC also decreases nearly lineary with the DOP-content. This result indicates an inhomogeneous structure of the system.     

Retardation of discoloration of poly(vinyl chloride) and decolorization of discolored poly(vinyl chloride) with diimide

Retardation of discoloration of poly(vinyl chloride) with diimide was studied in dimethylformamide at 130°C. with the use of p-toluenesulfonylhydrazide (PSH) as a source of diimide. A process was proposed that involved prolonging the induction periods of discoloration by inhibiting the development of conjugated polyene structure. The optimum proportion of PSH was one fourth of the poly(vinyl chloride), the best results. Furthermore, poly(vinyl chloride) discolored by thermal degradation in o-dichlorobenzene or gamma-ray irradiation under vacuum was decolorized in solution at 130°C. by addition of PSH. The decolorized poly(vinyl chloride) thus obtained was thermally stable compared with that obtained by oxidative methods.     

Dehydrochlorination of poly(vinyl chloride) by aqueous sodium hydroxide solution under two-phase conditions

The reaction of poly(vinyl chloride) powder with aqueous sodium hydroxide solution in the presence of quaternary ammonium or phosphonium halides yielded dehydrochlorinated products of conjugated polyene structure. The reaction was discussed in terms of a phase transfer catalytic mechanism between aqueous and polymer phases. Among the catalysts used tetrabutylammonium bromide was the best. To obtain the optimum conditions the effects of reaction temperature, the concentration of NaOH and the catalyst, and the molecular weight of poly(vinyl chloride) were investigated. Treatment of PVC films and solutions in tetrahydrofuran with aqueous NaOH solutions under two-phase conditions also produced dehydrochlorinated films and powders.     

Radiation effects on the thermal degradation of poly(vinyl chloride) and poly(vinyl alcohol)

Radiation effects on the formation of conjugated double bonds in the thermal degradation of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) were investigated. Thin films of PVC and PVA were either irradiated with γ-rays at ambient temperature (pre-irradiation) and then subjected to thermal treatment, or irradiated at elevated temperatures (in situ irradiation). An extensive enhancement of the thermal degradation was observed for the pre-irradiation of the PVC films, which was more effective than the effect of the in situ irradiation at the same absorption dose. For the PVA degradation, however, the effect of the in situ irradiation was larger than that of the pre-irradiation. The results were explained and related mechanisms were discussed based on radiation-induced chemical reactions and their individual contributions to the thermal degradation behaviors of the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3089–3095, 1998     

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.     

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.     

Thermal degradation of poly(vinyl chloride): effect of oxygen on the thermal degradation of poly(vinyl chloride)

On the basis of the simultaneously measured free radical generation and HCl evolution a mechanism accounting for the effect of the oxygen on the degradation of poly(vinyl chloride) (PVC) is proposed. The mechanism is based on the reaction of oxygen with radicals generated in PVC to form unstable peroxy radicals. In addition, it is proposed that oxygen reacts directly with conjugated polyenes to form peroxy linkage.     

NMR spectrum of poly(vinyl chloride)

The 100-MHz proton NMR spectra of commercial and laboratory-prepared poly(vinyl chloride) (PVC) have been measured in various solvents at high temperature (80–150°C). Tacticity in PVC was determined by the analysis of the β-proton spectrum. The spectrum was calculated assuming that the PVC chain consists of tetrad sequences of monomer units and that their distribution in the chain is described by a simple Bernoulli-sequence statistics with a P_m (the probability of isotactic placement) of 0.45 for commercial PVC polymerized at 50°C. Tacticity calibration curves based on measurements made for the polymer in pentachloroethane and β-dichlorobenzene were established, and they provide a simple method for the measurement of tacticity in PVC directly from the observed spectra. Excluding samples prepared in butyraldehyde solution, the formation of syndiotactic structures in PVC (prepared by free-radical polymerization) was found to be favored by lowering the polymerization temperature. This preference is due to an increase in the activation enthalpy of 510 cal/mole which is required for forming an isotactic placement in the chain during the propagation step.     

Antioxidative stabilization of poly(vinyl chloride)

Halogenated resins such as poly(vinyl)chloride undergo rapid degradation when exposed to heat. Such degradation is significantly increased by oxygen, and it is believed that the induction of the process of decomposition is caused by autoxidation of the polymer, followed by extensive dehydrohalogenation. Sulfur-containing peroxide decomposers allow primary stabilization of PVC with organic compounds which are not HCl acceptors but prevent free radical cross-linking of the polymer. This implies a discolored polymer melt (due to loss of HCl and formation of polyenes) which nonetheless exhibits good long term stability (no cross-linking). The stabilization of labile chlorine atoms results from the oxidation of various chain irregularities, and thus the elimination of the early discoloration can be achieved by the use of small quantities of monohydrocarbyltin co-stabilizers. The relative performance of various antioxidant systems are reported and possible stabilization mechanisms 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.     

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.