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.     

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.     

Mathematical models of the initial stage of the thermal degradation of poly(vinyl chloride). II. The thermal degradation of poly(vinyl chloride) with effective removal of HCl

The dehydrochlorination of different samples of PVC under vacuum with continuous removal of HCl by freezing, has been studied at 180–210°C. The comparison of the kinetic curves of the dehydrochlorination of various samples of PVC which were obtained by us and other investigators, with the theoretical curves for the thermal degradation of idealized PVC in the absence of HCl has been carried out. This had made it possible to evaluate the influence of unstable fragments present in the original polymer on the initial rate of PVC degradation quantitatively. It has been shown that the distinction between the stationary rates of the dehydrochlorination of various samples of PVC is determined by the difference of the values of the average length of dehydrochlorination chain, l_av. The most probable interval of the values of l_av has been ascertained to be 4–12. It is established that the most probable value of the constant of the rate of dehydrochlorination of normal links of PVC, k₀, is 2.1 × 10^?7?2.5 × 10^?7 s^?1 at 200°C. © 1993 John Wiley & Sons, Inc.     

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.     

Thermal degradation of poly(vinyl chloride) synthesized with a titanocene catalyst

PVC was synthesized using a trichloroindenyltitanium-methylaluminoxane catalyst at room temperature, and its degradation was monitored along with a commercial sample at 160, 170 and 180 °C under air or nitrogen atmosphere. The process was followed by HCl evolution, yellowing index, colour formation and thermogravimetric analysis. The produced polymer had a lower molecular weight and higher surface area, compared with a commercial PVC, while ¹H NMR and T_g values show minimal differences between materials. The HCl evolution degradation studies indicate that produced PVC has a lower thermal resistance than commercial PVC, while TGA reveals the opposite behaviour. Yellowing index and colour evaluation give evidence that nitrogen atmosphere and high surface area in produced PVC allow the polyene growth, whereas low surface area and air atmosphere generate shorter polyenes and chromophoric species. Differences in degradation performance are thought to be due to chemical origin, inherent morphology and differences in instrumentation.     

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.     

Dehydrochlorination of Poly(vinyl Chloride) in Pyridine. 1. Effect of Conditions

Poly(viny1 chloride) (PVC) was dehydrochlorinated thermally in pyridine solution under N₂ atmosphere and the effect of variation of reaction time, temperature, and concentration of PVC in pyridine was studied. The extent of dehydrochlorination (or conversion, x%) increases with an increase in reaction time and temperature, and with a decrease in the concentration of PVC. Incomplete precipitation of dehydrochlorinated PVC (DHPVC) occurs by nonsolvent (methanol). During dehydrochlorination there is no HCl evolution as it forms a pyridine hydrochloride complex which is supposed to act as a catalyst for dehydrochlorination. A possible mechanism has been proposed. Chain scission and cross-linking reactions are responsible for the molecular weight changes that take place during the reaction.     

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.     

Initial steps of thermal degradation of PVC prepared at various temperatures

PVC samples were obtained by bulk polymerization initiated with AIBN and ultraviolet irradiation at 40, 25, 0, ?30, and ?50°C. They were characterized by ¹³C NMR measurements, infrared spectroscopy, GPC in hexamethylphosphoramide and B.E.T. surface area measurements. Their thermal degradation was studied between 110 and 185°C by using continuous titration of HCl evolved through a conductimetric cell. The ultraviolet spectra were recorded at various steps of the degradation. At high degradation temperatures, the more syndiotactic the polymer, the longer the polyene average sequences are. The amount of HCl evolved is minimum for a polymerization temperature of 0 or 25°C, depending on the degradation temperature and on the morphology of the polymer. The results are discussed in terms of chemical factors (tacticity distribution, molecular weight) as well as of physical factors (morphology, interval viscosity).     

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.     

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 thermal degradation of poly(vinyl chloride)

Extensively studied thermal degradation of polyvinyl chloride (PVC) occurs with formation of free hydrogen chloride and conjugated double bonds absorbing light in visible region. Thermogravimetric monitoring of PVC blends degradation kinetics by the loss of HCl is often complicated by evaporation and degradation of plasticizers and additives. Spectroscopic PVC degradation kinetics monitoring by absorbance of forming conjugated polyenes is specific and should not be affected by plasticizers loss. The kinetics of isothermal degradation monitored by thermal gravimetric analysis in real time was compared with batch data obtained by UV/Visible absorption spectroscopy. Effects of plasticizer on kinetics of polyene formation were examined. Thermal degradation of PVC films plasticized with di-(2-ethylhexyl) phthalate (DEHP) and 1,2,4-benzenedicarboxylic acid, tri-(3-ethylhexyl) ester (TOTM) was monitored by conjugated double bonds light absorption at 350 nm at 160, 180, and 200 °C. Plasticizer-free PVC powder degradation kinetics and that of plasticized films were also obtained thermogravimetrically at temperatures ranging from 160 to 220 °C. Plasticizer-free PVC powder degradation and spectroscopically monitored degradation of plasticized PVC films occurred with the same apparent activation energy of ≈150 kJ mol⁻¹. No difference in degradation kinetics of films plasticized with DEHP and TOTM was detected.     

Hydrogen Halide-Catalyzed Thermal Decomposition of Poly(vinyl Chloride)

The thermal decomposition of solid PVC was studied in the presence of added hydrogen chloride and hydrogen bromide over the temperature range 170–210°C. Under certain conditions the decomposition was shown to be dependent in a first-order manner on the hydrogen halide pressure. These gases acted as catalysts, increasing the rate of HCl evolution and the degree of discoloration but not producing longer polyene sequences. Activation energy for the HCl-catalyzed process was found to be similar to that of the uncatalyzed decomposition of PVC. A unified mechanism is presented for an overall process consisting of three steps: random generation of a single carbon-carbon double bond in the cis configuration; 1,4-elimination of HCl via a six-centered transition state yielding a polyene; HCl- or HBr-catalyzed isomerization of the polyene formed by HC1 elimination to regenerate the initial structure. Hydrogen chloride catalysis is seen as an integral part of the overall process.     

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.     

Comparative non-isothermal kinetic analysis of thermal degradation of poly(vinyl chloride) prepared by living and conventional free radical polymerization methods

Non-isothermal kinetics of the thermal degradation of poly(vinyl chloride) (PVC) prepared by a living radical polymerization (LRP) method was performed and compared with the results obtained from PVC prepared by the conventional free-radical process (FRP). Both differential and integral isoconversional methods were applied for determining the apparent activation energy of the dehydrochlorination stage. This study made clear noticeable differences in the thermal degradation of the PVC samples under analysis. The newly synthesized LRP-PVC material has a better thermal stability and presents substantial differences in the macroscopic kinetics of the dehydrochlorination process compared with conventional FRP-PVC. These differences were assessed in quantitative terms on the basis of the kinetic triplet [E_a,A,f(α)].     

Mechanical and thermal properties of nanosized titanium dioxide filled rigid poly(vinyl chloride)

Nano-sized rod-like titanium dioxide (TiO2) filled rigid poly(vinyl chloride) (PVC) nanocomposites were prepared by using injection-molding method. Vicat, Charpy impact and tensile tests as well as thermogravimetric and dynamic mechanical analyses were used to characterize the structure and properties of the nanocomposites. The results showed that nano-TiO2 could improve Vicat softening temperature and also improve thermal stability of PVC during the stages of dehydrochlorination and formation of carbonaceous conjugated polyene sequences, which can be ascribed to restriction of the nanoparticles on the segmental relaxation as being evidenced by raises in glass transition and β-relaxation temperatures of PVC upon filling TiO2. Addition of TiO2 nanoparticles less than 40 phr (parts per hundreds of resin) could significantly improve impact strength of the composites while the TiO2 agglomeration at high contents leads to a reduction in impact toughness.     

Photodehydrochlorination of carbon monoxide–vinyl chloride copolymer

In order to examine effect of the carbonyl group in carbon monoxide–vinyl chloride copolymer, poly(CO–VC), photoirradiation with a high-pressure mercury lamp on the copolymer was carried out. Poly(CO–VC) had a rate of dehydrochlorination three times that of PVC, and the reaction involved a decrease in chlorine content. Also there was a marked change in the ultraviolet spectra of the photoirradiated films. However, no pronounced change of molecular weight was observed, but a change in R_f in TLC was observed clearly. These facts confirmed that photoirradiation of poly(CO–VC) produced a structural change by dehydrochlorination without serious decrease of molecular weight. In addition, photodehydrochlorination of the copolymer or PVC film was followed kinetically, and after ozonolysis of the dehydrochlorinated polymers, the number-average molecular weights were measured. From the results of degree of dehydrochlorination and molecular weight, the number average of conjugated double bonds or carbonyl groups was estimated. A mechanism for dehydrochlorination process by photo-irradiation is suggested.     

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.     

聚氯乙烯结构缺陷和热不稳定性之间关系的研究——Ⅰ.在单体不饱和蒸汽压下聚氯乙烯的合成及其热稳定性与分子量之间的关系

聚氯乙烯(PVC)容易发生脱氯化氢的热降解。研究者普遍认为,这是由于PVC分子链中存在结构缺陷而引起的。对究竟何种结构缺陷是引起PVC热不稳定的主要根源这一重要问题,虽然已有许多研究者进行过研究,但由于他们得到的结果往往各不相同,因而迄今未能达到一个明确一致的结论。如Minsker认为羰基烯丙基氯结构是引发脱HCl的主要原因。但Starnes却持相反意见。Hjertberg在其报告中指出叔氯     

聚氯乙烯结构缺陷和热不稳定性之间关系的研究——Ⅰ.在单体不饱和蒸汽压下聚氯乙烯的合成及其热稳定性与分子量之间的关系

Five PVC samples (U-PVC) were prepared in water suspended system at monomer unsa-turation vapor pressure. The molecular weigh's and dehydrochlorination rates (De-HCl) of these samples and five commercial PVC samples (S-PVC) were determined. Experimental data showed that the thermal stabilities and molecular weights of U-PVC samples decreased wifh decreasing the monomer unsaturation vapor pressure (Pm/Po) in the preparation process. It was found also that there was a good linear correlation between 1/Mm, De-HCl and Pm/Po This result indicated the important role of the terminal structural defect in the dehydrochlorination of PVC.The mechanism of formation of terminal structural uefects was discussed.