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.     

On the thermal degradation of poly(vinyl chloride). II. The effect of atmosphere

The early stages of the thermal degradation of PVC were studied. Two commercial, suspension-polymerized resins were thermally treated at different temperatures and oxygen contents. Dehydrochlorination kinetics were followed by conductometric measurements and the formation of polyene sequences by ultraviolet-visible spectroscopy. Crosslinking and chain scission were followed by gel chromatography (GPC) and viscometry. No chain scission was observed in nitrogen atmosphere and no crosslinking in oxygen. Degradation in air proceeded by both reactions. The rate of dehydrochlorination for one of the investigated polymers increased linearly with the logarithm of the oxygen pressure. In nitrogen, an increasing degradation temperature was found to give both an increasing crosslinking and less discoloration. In oxygen, chain scission reactions showed a slight temperature dependence. The temperature effect on the discolorations was similar to that in nitrogen. The main difference between the investigated resins was the amount of internal doubled bonds in the original polymers, the ratio being 2:1. The higher amount resulted in a higher rate of dehydrochlorination, a larger extent of chain scission in oxygen, and a lower extent of crosslinking in nitrogen. Both in oxygen and nitrogen, the obtained results are consistent with allylic mechanisms. In nitrogen, the polyene formation supposedly proceeds by a unimolecular reaction and crosslinking by an intermolecular nonradical dehydrochlorination. In oxygen, radical reactions are superposed and may lead to chain rupture via β-scissions of alkoxy radicals.     

On the influence of HCl on the thermal degradation of poly(vinyl chloride)

The thermal degradation of PVC was studied at 190°C in pure nitrogen and nitrogen containing 10, 20, and 40% HCl (by volume). The rate of dehydrochlorination was determined by gravimetry. Degradations in nitrogen were followed with conductometry in addition. Changes in molecular weight distribution and degree of long-chain branching (LCB) were determined by gel permeation chromatography–viscometry and polyene sequence distribution by UV spectroscopy. The rate of dehydrochlorination increases with the HCI content of the atmosphere. The rate of molecular enlargement also increases but only as a result of the increased dehydrochlorination rate. The increase in M _w and LCB are thus related to the extent of conversion only. Changes in the UV spectra indicate that the increase in rate of dehydrochlorination is mainly due to increased propagation rate in atmospheres containing less than 10% HCI. At higher HCI contents an increase in initiation rate is noted. It is suggested that this, at least in part, is due to the fact that HCI, by forming a cyclic transition state, catalyzes the random elimination of HCI. This process, in turn, is promoted by the presence of polyene sequences.     

On the thermal degradation of poly(vinyl chloride). IV. Initiation sites of dehydrochlorination

A set of poly(vinyl chloride) samples were investigated with respect to their thermal stability. The dehydrochlorination rates were measured in nitrogen at 190°C by use of a very accurate, conductometric method. For all polymers studied, a significant molecular weight increase was observed after 0.4% conversion. With only one exception, samples exhibiting higher degradation rates showed higher absorptions (350–450 nm) after 0.4% dehydrochlorination. In addition, the relative amount of short polyene sequences was found to be higher for polymers with higher degrees of branching. The dehydrochlorination reaction was predominantly initiated at sites of internal unsaturation (allylic chlorines), but an initiation at tertiary chlorines and unstable end-groups could not be completely omitted. Head-to-head units, extraneous impurities, and syndiotactic sequences were found to be of minor importance in this respect.     

Thermal behavior of PES/PEES copolymers with different sequence distribution: Comparative study of the kinetics of degradation

The degradation of three aromatic thermoplastics copolymers of polyethersulfone (PES) and polyetherethersulfone (PEES) with the same composition but different sequence distribution was performed in a thermogravimetric analyzer in both nitrogen and air environments, in isothermal and dynamic heating conditions. The obtained results suggest that the degradations started by random chain scission under all the experimental conditions. Under N₂ flow, branching and crosslinking were superimposed on the initial process, whereas complete oxidative degradation occurred in air. The apparent activation energies associated with the first degradation stage were evaluated and correlated with the sequence linkages present in the copolymer chains. The obtained values indicated that the chemical reactions occurring under nitrogen were different from those in air. In addition, a comparison of activation energies of PES/PEES copolymers with different sequence distribution was also reported. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Photochemical reactions of polyene sequences produced during the degradation of poly(vinyl chloride)

Photosensitized reactions of tetrahydrofuran (THF) and dichloromethane (DCM) solutions of polyene sequences introduced into poly(vinyl chloride) molecules by chemical degradation have been investigated. The distribution of polyenes produced following further thermal degradation in DCM were shifted toward longer sequences than were observed for the same reaction in THF. Benzophenone-sensitized bleaching of the polyene absorption in aerated THF solutions was characterized by induction periods, but the presence of oxygen had little effect on the same reaction in DCM. The bleaching process seems to involve reaction of the polyenes with the tetrahydrofuranyl radical formed by abstraction of the α-hydrogen from THF by triplet benzophenone. No induction periods were observed for benzoin-sensitized reactions, but the rate of reaction was faster in DCM than in THF. The differences are discussed in terms of the difference in reactivity of the alkyl and alkoxy radicals.     

Thermal Degradation of Vinylchloride/α-Olefin Copolymers

Abstract

The preliminary results obtained for the thermal degradation of vinylchloride/α-olefin copolymers are, in part, unexpected. Decrease of the initial polyene length with the increase of comonomer content is a reasonable consequence of the shorter viny chloride sequence lengths, but the constancy of these lower values in the course of further degradation indicates, that some secondary reactions of the polyenes (e.g. cyclization) must be absent here. The decrease of the HCl elimination rate in case of copolymers differs from the dependence given by the weak sites model. Formally, the rate is proportional to a netative power of conversion, the value of the exponent depending on the composition. These results indicate the necessity of further research for clearing up the unknown details of PVC degradation mechanism.     

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.     

Isothermal degradation of cis-1,4-polyisoprene vulcanizates

The isothermal degradation of high cis-1,4-polyisoprene vulcanizates having different crosslinking structures was investigated by a measurement of weight loss of sample, IR and NMR, gel permeation chromatography (GPC), and gas chromatography (GC). The degradation behavior of dicumy peroxide-cured sample is similar to that of the uncrosslinked one. On the other hand, a sulfur-cured sample is very different from the other samples. At the initial stage of the degradation, weight loss in this sample is faster than that in an uncrosslinked one. Furthermore, a microstructural change in this polymer occurring by thermal degradation is also very much larger than microstructural changes in the others. This will be attributed to polythiyl radicals produced by the scission of polysulfide crosslinkages. Although the polyisoprene chain undergoes random scission along the main chain above 473 K under inert gas or in vacuo, a molecular weight distribution of the residue after thermal degradation was broadened as the degradation proceeded. That is to say, the fragments having enormous molecular weight increase together with the production of lower molecular weight compounds. This indicates the occurrence of crosslinking reaction and the addition of polymer radicals to carbon–carbon double bonds in another polymer molecule in the thermal degradation process. Such reactions are thought to take place in the crosslinked polymers, in particular the sulfur-cured polymer, in larger quantities. Thermal degradation mechanisms were discussed in some detail.     

Thermomechanical behavior of poly(vinyl chloride) in the process of degradation

The thermomechanical behavior of poly(vinyl chloride) (PVC) was investigated during its thermal degradation by using torsional braid analysis. In thermomechanical behavior as a function of temperature, the relative rigidity G_r decreased initially with increasing temperature, then began to increase passing through a minimum at about 200°C, and finally decreased at 340°C. Increase in G_r from 200°C was caused by formation of a conjugated polyene chain accompanied by dehydrochlorination and by crosslinking reaction, and decrease in G_r at 340°C was related to scission reactions of the crosslinking network by oxidation. In the change in logarithmic decrement Δ, three peaks were observed: at 90°C, coinciding with the glass transition of the polymer; at about 200°C, due to the melting transition of crystallites, and at about 300°C, due to a loss of mechanical energy in the rheological transition of the polymer from a liquid state to a glassy state passing through a viscoelastic region. The thermomechanical properties of PVC with different molecular weights were also measured, and the effect of molecular weight G_r and Δ are discussed. In isothermal measurements of the relative rigidity in air, exponentially increasing curves were observed as a function of time. These curves were analyzed kinetically as a first-order reaction, and an activation energy of 22.7 kcal/mole was obtained.     

Heat stabilization of segmented copoly(ether ester)s

Oxidative degradation of segmented copoly(ether ester)s can be suppressed by interference of the following basic processes: a. degradative radical chain reactions. Effective inhibition is possible by means of primary antioxidants. b. initiation of the radical chain reactions by means of hydroperoxides formed. Such initiation reactions can be suppressed by inactivation of the hydroperoxides with the aid of secondary antioxidants. c. polyester segmental chain scission by formic acid formed during oxidative degradation of the polyether segments. By scavenging the formic acid, or its precursor formaldehyde, this serious process can be restricted. Combined addition of stabilizers, simultaneously influencing the basic processes a., b. and c. can lead to synergism. Examples of the activity of the different stabilizers are given.     

The influence of the tacticity on thermal degradation of PVC. III. Verification by ozonization of degraded samples

Two sets of ozonization reactions, for 1 h and 24 h respectively, have been carried out at ?20°C on degraded samples of PVC having different contents of tactic sequences. The evolution of the polyene sequences distribution with the ozonization was followed by UV-Visible spectroscopy. The number of chain scissions of ozonized samples was calculated from the number average molecular weight measurements before and after ozonization. The observed increase of short polyenes relative to the long polyenes with ozonization was found to depend markedly on the content of syndiotactic sequences. On the other hand, the number of chain scissions after total ozonization was found to be the higher the lower the content of tactic sequences in PVC samples is.     

Thermal dehydrochlorination of pure PVC polymer: Part I—thermal degradation kinetics by thermogravimetric analysis

Thermal degradation of PVC occurs in two stages, with each stage subdivided into two substages. The first refers to the dehydrochlorination, where hydrochloric acid is formed, and giving polyene structures. Hitherto, the degradation mechanism and action of hydrochloric acid as a catalyst during the dehydrochlorination stage are poorly known. Recently, the importance of the tacticity has gained attention for its influence on the dehydrochlorination mechanism. The present work focused on the dehydrochlorination stage, studying the molecular structure by FTIR analysis and the kinetic parameters by TGA analysis in Nitrogen atmosphere, based on three mathematical methods: Friedman, Kissinger, and Flynn-Wall-Ozawa. The sample was a pure homopolymer obtained by suspension polymerization. The dehydrochlorination kinetics follows a first order reaction model and occurs by nucleation and growth. The dehydrochlorination begins with the loss of very labile chlorine atoms present in defective and isotactic molecular segments. The formed HCl acts as a catalyst in the degradation. Following 40% conversion, a drop in Ea is observed. After that, chlorine atoms present in syndiotactic and atactic sequences, are released and, added to the large number of polyene chain sequences, and an increase in Ea is observed up to 60% conversion, where the dehydrochlorination stage is concluded.     

Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AM1) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans double bond is formed and no hydrogen chloride is present to catalyze trans-cis isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn or 1,4-cis mechanisms, promoting a so-called catastrophic, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination because of a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.     

Network formation in poly(ethylene terephthalate) by thermooxidative degradation

Gelation of poly(ethylene terephthalate) by heating at 263°–300°C was investigated. Under nitrogen flow, crosslinks were scarcely formed. However in air, degradation and crosslinking were common, and these were accelerated by purging gaseous and sublimable degradation products out of the system with a stream of air. The main component of the sublimate was terephthalic acid. Infusible and insoluble gel was treated with methanol at 260°C, and then the methanolysis products were separated into two parts. The methanol-insoluble part exhibited a polyene structure with ester groups, and the methanol-soluble part contained dimethyl terephthalate, ethylene glycol, and some 1,2,4-butanetriol. To clarify the relation between the crosslinking and the formation of vinyl ester groups, the degradation of vinyl methyl terephthalate was studied. Thermoxidative degradation of linear polyesters other than poly(ethylene terephthalate) was also studied. Poly(ethylene isophthalate) and poly(ethylene sebacate) were easily gelated. However, poly(trimethylene terephthalate) and poly(neopentyl terephthalate) were scarcely gelated. The primary reaction leading to crosslinking is assumed as follows. At first, the random scission of polyester chain may take place forming carboxylic acids, vinyl esters, aldehydes, etc. After accumulation of vinyl esters to some extent, vinyl polymerization of the esters takes place and network structures are formed.     

Parameters affecting the grafting reaction and side reactions involved in the free‐radical melt grafting of maleic anhydride onto high‐density polyethylene

The parameters affecting the grafting reaction and side reactions in free‐radical melt grafting of maleic anhydride (MA) onto high‐density polyethylene with the aid of 2,5‐dimethyl‐2,5‐di(t‐butyl peroxy)hexane peroxide(DTBPH) have been studied using an internal mixer. MA grafting degree of the maleated samples was measured with titrometry and FTIR spectroscopy methods. The extent of chain‐branching/crosslinking side reactions was evaluated with gel content and MFI determination. The flow behavior and melt viscoelastic properties of the samples were measured using a rheometric mechanical spectrometer. DTBPH and MA concentrations, reaction temperature, rotor speed, the type and concentration of coagents were among the studied parameters. The results show that MA and DTBPH concentration has a major role on the grafting reaction, chain‐branching/crosslinking side reactions and also the grafts microstructure in the final product. The reaction temperature has a complex effect on the maleation reaction. Increasing the rotor speed causes an increase in MA grafting degree of the samples and reduces the competitive side reactions. By using Gaylord additives, gel formation reduces at the expense of a dramatic decrease in the grafting degree. MA grafting degree is increased by the use of comonomers in the reaction and this is accompanied with a decrease in crosslinking side reaction when the vinyl type styrene comonomer is used. The results of processing torque in combination with the measurements of the melt viscoelastic property and gel content of the samples provide a great insight into understanding the gel formation mechanism. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Controlled degradation and crosslinking of polypropylene induced by gamma radiation and acetylene

Isotactic polypropylene (iPP) undergoes crosslinking and extensive main chain scissions when submitted to irradiation. The simultaneous irradiation of PP and acetylene is able to control chain scission and produce grafting. The grafted PP further reacts with PP radicals resulting in branching and crosslinking. In this work, commercial polypropylenes (iPP) of different molecular weights were irradiated with a ⁶⁰Co source at dose of 12.5 kGy in the presence of acetylene in order to promote the crosslinking. The mechanical and rheological tests showed a significant increase in melt strength and drawability of the modified samples obtained from resins with high melt flow index. The characterization of the molecular modifications induced by gamma irradiation of isotactic polypropylenes under acetylene atmosphere proved the existence of branching, crosslinking and chain scission in a qualitative way. The G′ and G″ indicated the presence of LCB in all samples. Therefore, PP irradiation under acetylene was proved to be an effective approach to achieve high melt strength polypropylene (HMSPP).     

Photoinitiated crosslinking of low density polyethylene. V: Orientation in stretched samples

Low density polyethylene (LDPE) is extruded, stretched, and photocrosslinked at different temperatures with different stretch rations in a continuous production line. Three different sequences are adopted: crosslinking before drawing, crosslinking after drawing, and crosslinking during drawing. The effect of drawing is studied by measuring the tensile properties. The increase of tensile strength and the decrease of elongation at break after stretching are related to chain orientation in crystalline morphology measured by X-ray diffraction. Thermal analysis by differential scanning calorimetry (DSC) supports these results. Owing to chain relaxation, the effects of orientation by stretching are significant only when the samples are drawn at a temperature not much higher than the melting point. Annealing of the drawn samples at a temperature above the melting point shows that the orientation in a crosslinked sample is retained for a much longer time than in an uncrosslinked sample. For the crosslinked samples, it is found that crosslinking after or during drawing gives a material with lower shrinkage upon heating; however, crosslinking before drawing gives a heat-shrinkable material.     

Influence of tacticity on thermal degradation of PVC. V. Relation between the nature of labile conformations and the polyene distribution in the degraded polymer

Four poly(vinyl chloride) (PVC) samples, 1, 2, 3, and 4, were prepared in bulk using 2,2′-azodiisobutyronitrile at 90°C and 60°C and the same initiator together with UV irradiation at 0°C and ?50°C, respectively. Fractions were obtained from samples 2,3, and 4 by extracting with acetone, sample 1 being completely soluble in this solvent. The whole PVC's as well as their acetone soluble and insoluble fractions, were characterized by determining the intrinsic viscosity, the osmometric molecular weight, and the tacticity; then they were thermally degraded up to conversion of 0.3% in powder state. The values of degradation rate for both the insoluble fractions and the whole PVC's were in agreement with some prior results on the influence of syndiotactic sequences on propagation step. The soluble fractions proved to be very unstable in comparison with the insoluble, which, despite the lower molecular weight of the former, seems to obey their higher content of isotactic conformations. The fine polyene distribution in the degraded polymers was carried out by UV–visible spectroscopy. The results reveal the occurrence of two different polyene distributions, and allow for the peculiar one of the soluble fractions to be related to polyenes consisting of two sequences of trans conjugated double bonds separated by a single cis double bond. The results clearly shows that there are two mechanisms for initiation of the PVC degradation, depending on whether it occurs by random unstable structures or by the normal GTTG isotactic or TTTG heterotactic triads. Moreover, these two initiation processes are proved to give rise to different types of polyenes, which accounts for the occurrence of two unlike mechanisms of propagation.     

Catalytic effect of hydrogen chloride on the degradation of poly(vinyl chloride)

The distribution of polyenes which results from the chemical dehydrochlorination of poly(vinyl chloride) has been studied in dichloromethane (DCM) and tetrahydrofuran (THF) solvents. A higher percentage of longer polyenes is formed in DCM than in THF. On the addition of trifluoroacetic acid to DCM solutions of the polyenes, new species are formed with strong absorptions in the region 500–800 nm. The absorptions are probably due to polyenylic ions formed by protonation of the polyenes, and the interrelated changes in their intensities can be explained by the migration of short polyene sequences along the polymer chain with the formation of longer sequences. The species are extremely photosensitive and are bleached in a few seconds with light from a medium-pressure mercury lamp. The relevance of these experiments to the effect of HCl on the polyene distribution and on the rate of photocrosslinking is discussed.