Heat Degradation of PVC Stabilized by Treatment with Alkylaluminum Compounds

Thermal degradation of PVC treated with alkylaluminum compounds has been studied. Four PVC samples of different molecular weights have been treated with Me₃Al, and Et₃A₁, and the dehydrochlorination rates of the polymers were determined at 190 and 220°C under a nitrogen atmosphere. The alkylaluminum-treated low molecular weight samples show marked increase in thermal stability, i. e., slower rate of dehydrochlorination right from the beginning of degradation, whereas with the higher molecular weight samples stabilization becomes pronounced only after a few percent of dehydrochlorination. The color of R₃Al-treated samples was much lighter (yellowish) than those of controls (dark brown) at 1% HCl loss. The average polyene sequence lengths formed during the early stages of dehydrochlorination are found to be much shorter with RsAl-treated PVC than with virgin samples. It appears as though polyene sequences which arose by zipping- initiation from allylic and/or tertiary chlorine sites are longer than those which form by random initiation along the chain. The autocatalytic (i. e., HC1-catalyzed) dehydrochlorination observed with virgin PVC disappears after treatment with R3A1. The HCl-catalyzed dehydrochlorination is minimized when thin films are used instead of powdery samples, which may be due to higher rates of HC1 diffusion through thin films. Autocatalysis of dehydrochlorination is affected by the concentrations of double bonds and HCl and the length of polyene sequences. Interaction between polyenes and HC1 by hydrogen transfer may lead to the re-initiation of unzipping, thus lengthening the polyene sequences.     

FT-RAMAN SPECTROSCOPIC STUDY OF THE FORMATION OF POLYENES DURING THERMAL DEGRADATION OF POLY(VINYL CHLORIDE) AND POLY (N-VINYL-2-PYRROLIDONE) BLENDS

In situ degradation of poly(vinyl chloride)/poly (N-vinyl-2- pyrrolidone) (PVC/PVP) blends has been studied by Fourier-transform Raman spectroscopy. PVP acts as a destabilizer in the thermal degradation of PVC as manifested by the reduction of temperature for the onset of degradation and the time the polymer is held at a particular temperature for this onset. Increasing the amount of PVP in the blends decreases the dehydrochlorination temperature and time. In blends containing high PVP concentrations, polyene bands dominate over nondegraded PVC bands. Maximum polyene lengths of around 35 were achieved. High PVP ratios and prolonged degradation at higher temperatures resulted in a decrease in mean polyene length due to crosslinking and oxidative scission.     

Kinetics of the Alkaline Dehydrochlorination of Polyvinyl Chloride)

From previous investigations of the alkaline dehydrochlorination of PVC it is well known that polyene sequences are formed. In this paper the true overall kinetics of the dehydrochlorination reaction between PVC and alcoholic KOH in tetrahydrofuran solution was studied at 9.5°C by measuring the rate of disappearance of KOH. Titrations of the hydroxide ions consumed and the chloride ions evolved showed close agreement. IR spectra of the samples did not show any evidence of substitution by hydroxide ions. Small amounts of DMSO present in the solvent were found to increase the reaction rate markedly. The kinetic scheme was discussed and the first rate constant was determined. Preliminary results of reactions of the polyene sequences with various reagents are presented. These reactions were made in order to introduce new functional groups into the PVC chains.     

Thermal degradation of graft copolymers of PVC prepared by mastication with styrene and methyl methacrylate,and of further PVC mixtures and vinyl chloride copolymers

Graft copolymers prepared by mastication of PVC in the presence of styrene or of a styrene/ methyl methacrylate mixture, have been studied by thermogravimetry, estimation of hydrogen chloride, thermal volatilization analysis, and flash pyrolysis/g.l.c. The degradation behaviour of PVC/ polystyrene mixtures, vinyl chloride/styrene random copolymers, a random copolymer of methyl methacrylate and styrene, and PVC/poly-α-methylstyrene mixtures has also been studied. The graft copolymers resemble the PVC/methacrylate graft copolymers previously studied in showing retardation of the dehydrochlorination reaction, but contrast with them in yielding chain fragments but no monomer during HCl production. Some stabilization of the second component at higher temperatures is also found. PVC/polystyrene mixtures behave in the same way as the corresponding graft copolymers, but vinyl chloride/styrene copolymers show reduced stability towards both dehydrochlorination and monomer production compared with the homopolymers. PVC/poly-α-methylstyrene mixtures yield some monomer concurrently with HCl loss, and display marked retardation of the latter reaction. Stabilization of the second polymer at higher temperatures is again observed. Many of these results add further strong support to the view that chlorine atoms are involved as chain carriers in the thermal dehydrochlorination of PVC.     

Mechanism of action of some stabilizers in the thermal degradation of poly(vinyl chloride)

The dehydrochlorination of PVC under vaccum has been studied at 170–200°C by a volumetric method. The accelerating effect of HCl is related to its interaction with the forming polyene units of macromolecules. A mechanism is proposed. The effects of various stabilizers, such as organic salts of Ca, Ba, Cd, trialkyl- and dialkyltin, trialkyl phosphites, and mixtures of phosphites with metal salts upon the rate of dehydrochlorination, polymer crosslinking, and electron absorption spectra of PVC during degradation in evacuated ampoules were investigated. The stabilizing activity of these compounds depends primarily on the effectivity of absorption of HCl and destruction of polyene units by these compounds.     

Cyclopentadienylation of PVC: Characterization and thermal and thermooxidative degradation studies

PVC has been cyclopentadienylated by two conventional basic, LiCp and NaCp, and a new acidic, Me₂CpAl, cyclopentadienylating agent. PVCs treated with basic cyclopentadienylating agents undergo severe random dehydrochlorination and exhibit a significant decrease in thermal and thermooxidative stability. In contrast, according to ozonization and degradation experiments, Me₂CpAl does not cause dehydrochlorination during cyclopentadienylation. The thermal stability of PVC treated with relatively high concentrations of Me₂CpAl and Me₃Al at 25°C markedly increases due to substitution of labile chlorines in PVC with methyl groups. Initial thermal dehydrochlorination behavior of virgin PVC and samples treated with Me₂CpAl at ?30°C are similar. In contrast, thermooxidative stability decreases on Me₂CpAl treatment at ?30°C; this is attributed to ease of oxidation of pendant cyclopentadienyl groups; that is, the formation of peroxy radicals that may initiate dehydrochlorination by attacking unchanged repeat units in PVC. Acceleration of thermal dehydrochlorination disappears and the length of polyene sequences is reduced on Me₂CpAl and Me₃Al treatment. These observations are attributed to differences in rates of protonation-deprotonation; that is, rates of reinitation of zipping of treated and untreated PVCs during thermal degration. The effect of traces of aluminum residues on degradation of modified PVCs, however, cannot be neglected.     

Deshydrochloruration du polychlorure de vinyle par le tertiobutylate de potassium sous atmosphere inerte. Chromatographie et spectroscopie ultraviolette

We investigated complete dehydrochlorination of PVC by t. BuOK in THF under inert conditions. We observed a modification of macromolecular chains giving three distributions studied by GPC with online u.v. spectroscopic analysis of eluted macromolecules. At low conversion rate, polyene macromolecules appear at the same degree of polymerisation as PVC. At higher conversion, much heavier products are formed and the polyene sequences are longer and do not change with conversion, on the other hand, scission products appear with comparable unsaturation but with very low molecular masses.     

Degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers. Effect of internal unsaturation on the stability of Saran copolymers

The thermal degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers containing a constant 9 wt % methyl acrylate and small but varying amounts of phenylacetylene has been examined in the solid phase and in bibenzyl solution. Thermally promoted degradative dehydrochlorination, largely uncomplicated by methyl chloride formation, readily occurs at temperatures approaching 200°C. Incorporation of phenylacetylene into the polymer structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene induces the formation of polyene segments during the polymerization so that all the terpolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing internal unsaturation arises from an increased rate of initiation for the degradation reaction. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. Thus random double bonds have been identified as the principal defect sites responsible for the facile degradation of Saran copolymers. Species which promote the degradation of Saran polymers probably do so by facilitating the introduction of double bonds into the structure. The ratio of hydrogen chloride to stilbene formed for degradation of the terpolymers in bibenzyl solution is ca. 35:1. This is strongly reminiscent of PVDC degradation and suggests that for degradation of either the homopolymer or Saran copolymers the chain-carrying allylic radical pair does not dissociate to any appreciable extent as dehydrochlorination occurs.     

The influence of tacticity on the thermal degradation of PVC

Based on experimental results on the kinetic zip length of polyene formation and on the polyene distribution in degraded PVC it is deduced, using quantitative model conceptions, that both the configuration and the conformation of the PVC macromolecule may influence the initiation, propagation and termination of thermal dehydrochlorination. In this way it is possible to explain the low ‘basic’ stability of PVC as being determined by the tacticity.     

Thermal stability of graft modifications of PVC and related materials

The grafting of polyisobutylene and polybutadiene onto PVC by use of alkylaluminum compounds gives graft copolymers which are superior in thermal stability to unmodified PVC as determined by dehydrochlorination measurements at a low degree of decomposition. The alkylation of toluene with PVC also exhibits high heat stability. The improved thermal stability observed for the modified samples is ascribed to the existence of an induction period and/or lower dehydrochlorination rates. PVC grafted with polybutadiene in the absence of cobalt cocatalyst exhibits autocatalytic behavior and is less stable than the unmodified PVC.     

Structural defects in polyvinylchloride—I. Internal unsaturation as initiation sites for dehydrochlorination

To obtain double bonds in PVC, copolymerizations of vinyl chloride (VC) with phenylacetylene (PA) and dimethylester of 2-butyne dioic acid (ADCE) were carried out. Copolymer compositions, as measured by i.r., were compared with ozonolytically evaluated chain scission numbers. Only the PA-units seemed to be incorporated at random in the PVC-chain. Phenyl groups conjugated to the double bonds led to a striking increase in thermal dehydrochlorination. Ozonolysis on degraded PVC samples provides insight into the mechanism of the dehydrochlorination, revealing the formation of additional unsaturated sites after degradation and permitting the estimation of average polyene sequence lengths.     

Microstructure and Thermal Properties of Ethylene/Vinyl Chloride Copolymers

Abstract

The microstructure of ethylene/vinyl chloride (E/VC) copolymers, prepared by partial reduction of PVC with a variety of reagents, was analyzed using carbon-13 NMR spectroscopy. The effect of the reduction mechanism on the microstructure and stereochemistry of the copolymers was studied. The reduction with tri-n-butyltin hydride (TBTH) produced copolymers with higher degrees of alternating monomer units than copolymers obtained by reduction with lithium aluminum hydride (LAH). Reduction with lithium triethylborohydride (SH) produced blockier copolymers. The correlation of monomer sequence and distribution with thermal stability and dehydrochlorination rate in these copolymers was investigated. Partial reduction of PVC by all reagents was shown to produce E/VC copolymers with improved thermal stability compared to PVC. In the series of copolymers produced by TBTH reduction, improvement in thermal stability increased with an increase in E content and reached a maximum at 40% E content but dropped steadily thereafter. All copolymers from SH reduction showed a considerable drop in their dehydrochlorination rate compared to PVC. The stabilization effect reached a maximum at ～7 mol% E content and stayed constant throughout.     

Isothermal crystallization of propylene-1-decene copolymers

The isothermal crystallization and subsequent melting behavior of one propylene homopolymer and three propylene-1-decene copolymers with different comonomer contents prepared by metallocene catalyst were studied using differential scanning calorimetry (DSC). It is found that the Avrami exponent of the propylene copolymers decreases gradually with the increase of comonomer content, from 3.0 for the propylene homopolymer to 1.4 for the copolymer with 7.83 mol% 1-decene units. Higher comonomer content also weakens the dependence of crystallization rate constant and crystallization halftime on temperature. Double melting peaks, which correspond to α and γ crystal phases, respectively, are observed for all copolymers under isothermal crystallization. The result shows that higher crystallization temperature is favorable to the segregation of α and γ crystal phases, resulting in higher proportion of γ crystal phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Degradation of PVCs obtained by controlled chemical dehydrochlorination

Thermal and thermo-oxidative degradation of poly(vinyl chloride)s (PVCs) containing increased concentrations of allylic chlorines, PVC(A)s, prepared by controlled chemical dehydrochlorination with potassium-t-butoxide (t-BuOK) have been studied. The introduction of small amounts of internal allylic chlorines into PVC significantly decreases the thermal and thermo-oxidative stability of the resin. A linear relationship exists between the initial rates (V_HCl)₀ of thermal and thermooxidative dehydrochlorination of solid PVC(A)s and the concentration S of internal allylic chlorines. Both the slope and the intercept of the thermo-oxidative (V_HCl)₀ vs. S plot are higher in oxygen than those obtained in nitrogen at the same temperature; this finding is attributed to fast oxidation of polyenes, and to peroxy radicals formed during polyene oxidation, which initiate subsequent HCl loss by attacking normal repeat units in PVC. The extent of HCl loss as a function of time during thermal degradation of PVC(A)s in intert solvent shows a rapid initial phase followed by a slower stationary phase. The first phase is due to dehydrochlorination involving the labile chlorines, while the stationary phase indicates random initiation of HCl loss at normal? CH₂? CHCl? repeat units. Initial rates of HCl loss increase with S, while the rates of HCl loss during the stationary phase are independent of S. The rate constant of initiation of HCl loss at internal allylic chlorines is almost four orders of magnitude higher than that of random initiation; however, the former is still orders of magnitude lower than that of chain propagation. Quantitative analysis of UV-visible spectra of PVC(A)s degraded in solution suggests geometric polyene distribution. The average length of polyenes decreases as the extent of HCl loss increases and reaches a constant value of ca. 3 at ca. 1% HCl loss for all the investigated PVC(A) samples.     

Color Stabilization of Polyvinyl Chloride) with Heat Treatment

Stabilization of polyvinyl chloride) (PVC) containing metal soaps was investigated by psychophysical colorimetry. A color difference observed among heated PVC films containing various metal salts depends on coloration of the π-complex of polyene with metal chloride converted from the metal salt added and that the stabilization effect of synergistic soaps should be based on an effect of complementary colors set up among a polyene color and metal chloride-polyene complex colors. These conclusions are well supported by colorimetry of heated PVC films containing various dyes. The color of heated PVC films containing Zn/Ca and Cd/Ba synergetic soaps markedly deviated from a polyene color with increased heat treatments, owing to greater degree of coloration of Zn complex and Cd complex, respectively. These color deviations usually decrease the thermal stability of PVC. The thermal stability of PVC was markedly improved by the use of synergetic soaps together with masking agent such as triethanolamine, urea, N,N′-dimethylol-urea, and vinylpyridine-methylmethacrylate copolymer, owing to the masking effect of these nitrogen-containing compounds. These masking agents did not slow down the dehydrochlorination of PVC.     

Photodegradation and photo-oxidation of poly(vinyl chloride) in solution

Quantum yields of dehydrochlorination and of main chain scission were determined for photolysis of PVC in solution in tetrahydrofuran and 1,2-dichloroethane. The observed auto-accelerated degradation results from an increased absorption of light by the growing polyene structures. The presence of oxygen enhances the efficiency of dehydrochlorination, chain scission and crosslinking processes. From the constant quantum yields, it is inferred that energy transfer occurs from the absorbing polyenes and that initially present unsaturations are responsible for initiation of the degradation.     

Kinetics of the dehydrochlorination of poly(vinyl chloride) in the presence of NaOH and various diols as solvents

The dehydrochlorination of PVC in the presence of NaOH was investigated in different diols. Diethylene glycol (DEG), triethylene glycol (TEG), and propylene glycol (PG) were found to be effective in accelerating the dechlorination of PVC. The dehydrochlorination was promoted in the order TEG > DEG > PG, which was in agreement with the compatibility between PET and the diol. Compatibility resulted in an improved penetration of the PVC particle by the solvent, leading to the acceleration of the dehydrochlorination. The dehydrochlorination of PVC in NaOH/diol followed first-order kinetics, confirming the progress of the reaction under chemical reaction control. The apparent activation energies were 82 kJ mol⁻¹, 109 kJ mol⁻¹, and 151 kJ mol⁻¹ for TEG, DEG, and PG, respectively. The lower the activation energy became the faster the dehydrochlorination of PVC proceeded.     

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.     

ENVIRONMENTAL STABILITY OF THE POLYENE PREPARED BY DEHYDROCHLORINATION OF POLY(VINYL CHLORIDE)

A high-quality polyene can be obtained by exensive dehydrochlorination of poly(vinyl chloride) (PVC) in aliquid/solid two-phase system. The liquid phase is a tetrahydrofuran solution of PVC containing a small amount ofpoly(ethylene glycol) with molar mass of 400 g as a phase transfer catalys. The solid phase is potassium hydroxide particles.The structure of the polyene is polyacetylene-like and has a long conjugated C=C sequence and a narrow dispersity ofpolyene sequences according to its FT-infrared and Raman spectra. The environmental stability of the polyene was alsostudied by IR, Raman spectra and elemental analysis. Experimental results demonstrated that the polyene was susceptible toair and could be changed into a material containing high concentrations of hydroxyl and carbonyl groups. The polyenesequences were shortened and its dispersity became broader due to the effect of dioxygen.     

A reexamination of the degradation of polyvinylchloride by thermal analysis

Degradation of polyvinylchloride has been reexamined in the light of its DT-DSC-TG analytical behavior up to a temperature of 1000°C in an inert atmosphere. Four distinct stages of degradation have been identified. The first stage is almost eventless with no change in weight for untreated PVC samples. The second stage is almost exclusively dehydrochlorination. The third stage appears to be a structural reorganization involving such processes as cis-trans isomerization, aromatization, and crosslinking. The fourth stage appears to be a structural degradation and is associated with the evolution of hydrocarbons. The role of liberated hydrogen chloride has been better appreciated in catalyzing the above secondary reactions on the polyene residue obtained on partial or total dehydrochlorination. The effect of the particle dimension and chemical and physical pretreatments of the samples, such as low temperature dehydrochlorination by an alkali and vacuum heat treatment, respectively, on the degradation pattern has been studied. © 1994 John Wiley & Sons, Inc.