Greffages mecanochimiques sur le polychlorure de vinyle. III. Stabilitéa thermique des polymègeres modifiéas

The mastication of poly(vinyl chloride) (PVC) in presence of monomers causes important changes of the thermal stability of the polymer when the last one possesses a basic character (e.g., 2-vinylpyridine). The dehydrochlorination rate is greatly increased, even if the monomer is present in only small amounts. In the most cases (styrene, acrylic ester) a better thermal stability is observed with respect to the acceleration of the dehydrochlorination, which is retarded or even suppressed. Infrared spectroscopy and differential thermal analysis show that the improved stability is due to an improved crystalline organization. When mastication causes the polymerization of the monomer (methyl methacrylate), degradation of the PVC part and depolymerization of the rafted part are observed simultaneously; this fact supports a radical mechanism for the thermal degradation of the PVC.     

Sur la dégradation thermique du polychlorure de vinyle. VI. Etapes initiales—etude générale préliminaire

A differential conductimetric apparatus allowing the continuous titration of HCl carried by a stream of gas with a limit sensitivity of 40 nanograms is described and applied to study the initial steps of poly(vinyl chloride) thermal degradation. The powdered sample is placed in a quartz cell which may be closed and detached and placed at a chosen time interval in a reflexion diffuse UV spectrometer. The dehydrochlorination process begins just above T_g; the isotherms under argon show, after a variable time, a steady state with a zero order and an activation energy of 22 kcal. For a dehydrochlorination ratio (DR) less than 1000 ppm, the shape of the UV visible spectrum does not depend on the DR, and for the same DR it does not depend on the degradation temperature. A plot of the reflectance ratio versus the DR shows a coloration process in two steps: a rapid step followed by a slower step. The phenomenon, which does not depend on the physical state of the sample, tends to disappear if the amount of polymer is lowered and if the rate of flow of the gas is increased. It suggests that the HCl formed upon the dehydrochlorination process remains partly dissolved in the polymer. Further proof of the dissolution of HCl in the polymer is given, and a through method of titration is proposed. It is suggested that the initial steps of the dehydrochlorination follows an ionic mechanism, and might be partially reversible and diffusion controlled, although the autocatalysis which involves a rather high dehydrochlorination ratio and the presence of a large amount of dissolved HCl might initiate the radical process.     

Degradation of poly(vinyl chloride). I. Kinetics of thermal and radiation-induced dehydrochlorination reactions at low temperatures

The dehydrochlorination reaction arising thermally and from exposure to γ-radiation has been followed, under vacuum, in the temperature range from 80° to 130°C by measuring the pressure of the evolved volatiles. The catalytic action of HCl, which was recently established, has been observed also at these low temperatures. In agreement with previous data, a free-radical mechanism has been accepted to be operating in the radiation-induced reaction, which has been found to be linearly dependent on dose rate and essentially independent of temperature. Assuming the thermal dehydrochlorination to proceed according to the same mechanism, its activation energy, lying within the range of values reported in the literature, represents the activation energy for the thermal process of radical formation by dissociation of normal and anomalous structures in PVC macromolecular chains. Since this value appears to be substantially constant in the temperature range from 90° to 240°C, it can be established whether the dissociation reactions of all the PVC structures are regulated by the same activation energy or, more simply, only one of these structures is concerned.     

Investigation of the effects of nonisothermal suspension polymerization of vinyl chloride on the fusion and degradation behavior of poly(vinyl chloride)

Thermal stability of poly(vinyl chloride) (PVC) samples polymerized under a temperature trajectory was studied from the point of view of morphological and microstructural characteristics. The results are compared with those of the PVC samples obtained by polymerization at constant temperature having the same K value. The Brabender^® plastograph data indicated that the final PVC synthesized with the temperature trajectory showed lower fusion time and higher thermal stability time. The nonisothermal condition also increased the degree of fusion of the final PVC resin, reflecting lower temperature/time required to process it. It was found that the thermal stability of nonisothermally produced PVC as characterized by dehydrochlorination rate decreased (improved) with the increasing monomer conversion until a minimum value was reached that corresponded to the conversion at the pressure drop. However, the dehydrochlorination rate remains almost constant with conversion for an isothermal grade PVC resin. Although the evolution of the number of internal double bonds as well as extent of discoloration of PVC with conversion shows a decreasing trend, the labile chlorine concentration exhibits a maximum at early conversion. The reason for the former can be explained by the temperature dependence of reactions forming defect structures, which are kinetically controlled and thus favored at higher temperatures. The latter, however, can be explained because of the increasing importance of transfer reactions to polymer with increasing polymer concentration. Finally, the results from differential thermogravimetry verify an improvement in thermal stability of the final PVC prepared by using a temperature trajectory during vinyl chloride monomer suspension polymerization. J. VINYL ADDIT. TECHNOL., 23:259–266, 2017. © 2015 Society of Plastics Engineers     

Studies of the solid-state thermal degradation of PVC. I. Autocatalysis by hydrogen chloride

The thermal degradation of virgin and HCI-treated PVC in powder form, as well as of PVC films of different thicknesses, has been studied as a function of time and temperature. The rate of dehydrochlorination was determined conductimetrically and from the polyene sequence distributions as obtained by UV spectroscopy. Increases in the rate of dehydrochlorination, ranging between 30 and 45%, were observed at all temperatures for the samples pretreated with HCI, while the corresponding activation energies were found to be lower by about 20%. For the PVC films, the rate increased with thickness, i.e., with longer residence time of evolved HCI within the sample. The results offer insight regarding the autocatalytic role of evolved HCI.     

Vergleich von thermisch und photolytisch ausgelöstem Abbau von PVC-Preßlingen und -Folien

Comparative investigations of PVC discs and foils concerning their thermal and photolytical behaviour showed an expiration of thermal degradation in the mass, whilst the photo processes injure only the surface of a sample. The investigations of polymer samples with different masses have demonstrated a constant rate of thermal dehydrochlorination in nitrogen, which was not influenced by the polymer mass. The degradation experiments in air resulted in an increased HCl-elimination with increasing degradation time. Oxidation reactions which occur in the presence of air cause an accelerated degradation and therefore the dehydrochlorination reaction is appointed as an additional reaction and not in competition with the HCl-elimination process. The influence of UV-light on PVC-discs and foils efforted a much higher dehydrochlorination rate; the energy of the rays is able to stimulate all types of chlorine atoms at the surface. The photolytical injury of the surface increased with degradation time resulting in a decrease of the degradation rate. The degraded and cross-linked material is insoluble in all solvents.     

Thermal Degradation of Poly (vinyl chloride)-Stabilization Effect of Dichlorotin Dioxine

Abstract

Thermal degradation of polyvinyl chloride (PVC) and the effect of dichlorotin dioxine (DCTD) on this process have been investigated using dehydrochlorination (DHCl) and thermogravimetric techniques. A significant decrease in the rate of degradation was observed when PVC was mixed with a small amount which was maximum with 0.25 pph of DCTD and thermal decomposition temperature with DCTD was found to be higher than that of pure PVC. The value of apparent activation energy of the dehydrochlorination process has been calculated and a suitable mechanism for the stabilizing action of DCTD on PVC has been purposed.     

Effect of poly(vinyl chloride)/chlorinated polyethylene blend composition on thermal stability

Blends of poly(vinyl chloride) (PVC) with different ratios of chlorinated polyethylene (CPE) were degradated by the thermogravimetric method under dynamic conditions (50–600°C) in an inert atmosphere. The effect of the miscibility and composition of the PVC/CPE blends on the thermal stability were investigated. DSC curves of the blends show neither a shift of the PVC glass transition temperature nor a shift of the CPE melting temperature, which means that these blends are heterogeneous. The characteristics of the TG curves were determined, some of which (T_1%,T_5%, Δm₁) can be used as indicators of the thermal stability of the blend. The apparent activation energy of PVC dehydrochlorination in the blends was also calculated. Comparison of the experimental TG curves and TG curves predicted by the additivity rule indicates the existence of the components' interaction in the PVC/CPE blends. The addition of CPE improves the thermal stability of PVC for all the investigated blends in the temperature range where α_calc is greater than α_exp. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 166–172, 2000     

Thermooxidative degradation of poly(vinyl chloride)/acrylonitrile–butadiene–styrene blends

The thermal degradation process of poly(vinyl chloride)/acrylonitrile–butadiene–styrene (PVC/ABS) blends was investigated by dynamic thermogravimetric analysis in the temperature range 50–650°C in air. The thermooxidative degradation of PVC/ABS blends of different composition takes place in three steps. In this multistep process of degradation the first step, dehydrochlorination, is the most rapid. The maximal rate of dehydrochlorination for the PVC blends containing up to 20% ABS-modifier is achieved at average conversions of 23.5–20.0%, i.e., at 13.5% for the 50/50 blend. The apparent activation energies (E = 103–116 kJ mol⁻¹) and preexponential factors (Z = 2.11 × 10⁹−3.45 × 10¹⁰min⁻¹) for the first step of the degradation process were calculated after the Kissinger method. © 1996 John Wiley © Sons, Inc.     

Kinetic model for thermal dehydrochlorination of poly(vinyl chloride)

In this paper, a novel method for calculating degradation kinetics is presented. The method has been applied to the thermal dehydrochlorination of two different samples of PVC. It has been observed that this dehydrochlorination is complex and involves two different processes. A model that accounts for the entire dehydrochlorination is proposed. This model involves nucleation and growth and diffusion controlled mechanisms. The kinetic parameters are obtained from linear heating rate, isothermal and sample controlled thermal analysis experiments. Kinetic results obtained from the macroscopic thermal analysis measurements demonstrate the correlation between the kinetics of the thermal dehydrochlorination of PVC and the structure of this macromolecule.     

Das thermische verhalten von strahlenchemisch hergestelltem PVC. II. Auswertung der DTA- und TG-kurven

The thermal behaviour, especially the thermal dehydrochlorination of PVC which was polymerized by y-initiation, was investigated by simultaneous dynamic DTA, TG and DTG. The dehydrochlorination reaction occurs in two successive degradation steps, the first one having the reaction order n = 1 and the second one n = 0. The IR analysis of the residual polymer shows that, in a first step, the dehydrochlorination attacks predominantly the crystalline, syndiotactic regions; in a second step the amorphous parts are dehydrochlorinated. The influence of tacticity and molecular weight on the thermal behaviour is discussed.     

Kinetics and mechanism of the thermal degradation of poly(vinyl chloride)

The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution, and rate of polyene sequence formation. These results are accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence. Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation. Mean rate constants and activation parameters for random initiation, propagation, and termination reactions of the PVC degradation chain were calculated by simulation. Activation enthalpy/entropy correlations for the experimental data available for dehydrochlorination of chloroalkanes and chloroalkenes in the gas and in the liquid phase or nonpolar solvents and elementary reactions of PVC degradation show that initiation is an HCl elimination through a transition state of four centers requiring a synperiplanar conformation of the >CH–CCl< group, whereas propagation is a dehydrochlorination through a transition state of six centers requiring a cis configuration of the double bond.     

Continuous dosing of fast initiator during vinyl chloride suspension polymerization: Thermal stability of PVC resin

Thermal stability of poly(vinyl chloride) (PVC) produced using continuous dosages of a fast initiator method was investigated in terms of morphological and microstructural characteristics. The results were compared with the properties of the PVC prepared by conventional polymerization method. The Brabender^® Plastograph and DSC results showed a lower fusion time, higher stable time, and greater degree of fusion for PVC obtained by polymerization using initiator continuous dosage method. Also, chemical analysis indicated that the PVC produced under an initiator continuous dosage system have lower structural defects, that is, branch numbers, internal double bonds, labile chlorine, and tacticity index, thereby improving the thermal stability of PVC resin. The results obtained from dehydrochlorination and thermogravimetry analysis confirm the improvement of thermal stability of PVC chains synthesized with continuous dosages of a fast initiator. Moreover, it was found that the concentration of microstructural defects and the dehydrochlorination rates of the PVC samples prepared by both processes increase with monomer conversion, particularly after critical conversion. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44480.     

Zum mechanismus der thermischen abspaltung von chlorwasserstoff aus polyvinylchlorid. 6. Mitt.: Untersuchungen an reduziertem polyvinylchlorid

The reduction of atactic poly(vinyl chloride), PVC, with lithium aluminum hydride in tetrahydrofuran is a random process; in polymers containing long syndiotactic chain segments the latter withstand reduction longest. The reduced polymers contain CH₃-groups, the amount of which can be determined by i. r. spectroscopic compensation against polymethylene. Furthermore, the i. r. spectra indicate the existence of trans-vinylene groups; depending upon the conditions of polymerization, both these groups are found to be present in amounts of 5 to 15 per 1000 carbon atoms. The experimental results of thermal degradation of PVC samples reduced to different extents are in agreement with the assumption that dehydrochlorination is primarily caused by carbon double bonds within the PVC chains.     

True stabilization: A mechanism for the behavior of lead compounds and other primary stabilizers against PVC thermal dehydrochlorination

A free-radical mechanism by which lead compounds are thought to inhibit thermal dehydrochlorination of polyvinyl chloride (PVC) is described. This mechanism is called “true stabilization” in order to distinguish it from simple hydrogen chloride (HCl) scavenging, which is a well-known and sometimes important function of all primary stabilizers used in PVC. In true stabilization, it is thought that stearate or other aliphatic carboxylate groups (initially from lubricants) react with reservoirs of basic lead compound such as the carbonates, dibasic lead phosphite, the phthalates, or the sulfates, to give mobile carboxylates of lead. These latter salts then react with chlorine atoms released by the hot PVC, giving chlorides of lead and aliphatic carboxylate free-radicals. Hence the chlorine atoms are trapped and unable to propagate free-radical dehydrochlorination. Also it is thought that the aliphatic carboxylate free-radicals esterify PVC molecules at methylenic carbon atoms (from which hydrogen atoms have been removed by chlorine atoms giving HCl and free-radical sites in the polymer chain). Thus, unpaired electrons on the carboxylate free-radicals and on these methylenic carbon atoms in the PVC molecules are paired, so that the neighboring chlorine atoms in polymer chloromethylenic groups are stabilized. Hence loss of chlorine atoms in the free-radical dehydrochlorination of PVC is prevented. The pendant aliphatic carboxylate groups dissociate from the PVC molecules taking chloromethylenic hydrogen atoms to form acid molecules, and leaving chlorine atoms in relatively stable vinyl type groups. The aliphatic carboxylic acids react with more of the basic lead compound reservoir, giving mobile carboxylates of lead which can enter further reactions as just described. Thus, the true stabilizing mechanism is continuous and cyclic, while the reservoir of basic lead is available, and the PVC thermal dehydrochlorination will be retarded to almost negligible rates in favorable circumstances. It is thought that the behaviors of metal soaps and of organo-tin stabilizers may be encompassed within the general true stabilization concepts of free-radical exchanges and PVC esterifications described above. In these ways they also would retard PVC thermal dehydrochlorination. However, they are neutral compounds and have no basic reservoir which can react with carboxylic acids in the manner described above for lead stabilizers. Hence they are not able to confer long term stability on PVC in the way that basic lead stabilizing regimes do.     

DEVELOPMENT OF SYNERGISTIC HEAT STABILIZERS FOR PVC FROM ZINC BORATE-ZINC PHOSPHATE

The importance of flame-retardant and smoke-suppressed poly(vinyl chloride) (PVC) compositions is increasing gradually in the polymer industry since PVC releases smoke and toxic gases (hydrogen chloride, HCl) during heating at temperatures above 140°C with the result of dehydrochlorination reaction. In this study, the synergistic effects of zinc borate (ZB)-zinc phosphate (ZP) on the thermal stability of PVC were investigated using thermal techniques. The induction and stability time values of PVC plastigels were obtained at 140° and 160°C. The results revealed that PVC plastigels having only ZP and ZB retarded dehydrochlorination of PVC compared with the unstabilized sample. However, the plastigels with both ZB and ZP had a superior synergistic effect on char formation of PVC. Since the induction periods of the samples having both ZB and ZP were higher than those of the unstabilized samples having only ZB or only ZP, the synergistic effect was observed.     

Emulsionspolymerisation von vinylchlorid, 10. Einfluß wasserlöuslicher oligomerer auf das dehydrochlorierungsverhalten des polymeren

The dehydrochlorination of different PVC samples is investigated using a thermoanalyzer coupled with a conductometric HCl detection system. The investigations are carried out in a non isothermal manner. The investigated PVC samples are containing different amounts of low molecular weight water soluble oligomers. The higher the amount of such oligomers, the higher the HCl loss of the PVC at low temperatures. Formation of these oligomers during the polymerization process is discussed in terms of polymerization recipes, conversion, and monomer concentration.     

Studies on photodegradation of poly(vinyl chloride), part 3

PVC films were degraded by outdoor exposure, irradiation by a weatherometer and heating by an electric oven. In PVC films subjected to outdoor exposure and to irradiation by the weatherometer at a low temperature, e. g. sample surface temperature of 0°C, photo-oxidation and scission of the main chain were caused, but dehydrochlorination or formation of solvent-insoluble substances were not observed. In contrast, PVC subjected to irradiation by the weatherometer at a high temperature, e. g. sample surface temperature of 80°C, showed a degradation tendency similar to that of PVC heated at 200°C in the electric oven, and decomposition of the thermal degradation type characterized by dehydrochlorination, formation of polyenes and formation of solvent-insoluble substances were observed.     

Effect of copolymers modifying PVC on its physical and mechanical properties and its UV-radiation resistance. III. Changes in thermal stability of PVC films modified by MMA/MA,VC/VAC,MBS or ABS induced by UV-irradiation

The influence of UV-radiation on thermal stability of PVC films, depending on the type and quantity of modifying copolymers, was studied. It was found that UV radiation can induce various changes in thermal stability of PVC decreasing its dehydrochlorination temperature T⁰₁ while increasing temperature of decomposition T⁰₂. Particularly distinct changes in sampels containing VC/VAC indicate that even 1% of this copolymer accelerates the photodehydrochlorination and the processes which decrease the temperature of decomposition of PVC. Smaller changes than in pure PVC in thermal parameters in samples containing ABS after irradiation show that this copolymer increases the PVC resistance to UV radiation. Films containing MBS, ABS, MMA/MA reveal the largest changes in T⁰₁ after irradiation in the case of a content of 1% of these copolymers in the samples. In the case of bigger quantity (3 to 10%) the changes are smaller and may even diversify their direction. These copolymers, contrary to VC/VAC, make PVC films a little turbid indicating the limited compatibility of both components. Hence, the conclution is that the structure of a film, depending on compatibility of components and changing with their differing concentration, has decisive influence on the thermal and photochemical processes.     

Two-Stage pyrolysis model of PVC

The pyrolysis of polyvinyl chloride (PVC) was examined with a thermal gravimetric analyzer (TGA). The experiments were carried out over the temperature range of 400-800 K at three heating rates of 1, 2, and 5.5 K/min and in a nitrogen atmosphere. The results indicate that the entire process of PVC pyrolysis under the experimental conditions of this investigation consists of two distinct pyrolysis stages, namely, the thermal dehydrochlorination and the breakdown of the intermediate products produced after the dehydrochlorination stage. The corresponding activation energy, pre-exponential factor, and reaction order were determined. A two-stage pyrolysis model, which is composed of four reactions including a number of independent, consecutive and competitive reactions with volatiles and solid products, was developed. This kinetic model gives good agreement with the experimental results.