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.     

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.     

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.     

Photosensitized dehydrochlorination of poly(vinyl chloride). I. Effect of commercial additives

A commercial formulation of thermal antioxidant has been shown to photosensitize the dehydrochlorination of poly(vinyl chloride). The species responsible was shown to be a p-alkyl-substituted phenol, and this was confirmed using p-cresol as a model compound.     

Thermal dehydrochlorination of poly(vinyl chloride). I. Effect of iron oxide on the rate of dehydrochlorination

The thermal degradation of poly(vinyl chloride) (PVC) was studied by following the rates of dehydrochlorination at temperatures between 180°C in pure nitrogen and air flow. Iron oxide accelerates the elimination of hydrogen chloride from PVC. The accelerating effect depends on the concentration of the oxide, and it has a maximum. This work tried to explain these behaviors. A mechanism of dehydrochlorination is suggested for polymer containing iron oxide.     

Modification of poly(vinyl chloride). XIX. Electroplating of poly(vinyl chloride)

As a preliminary treatment in the PVC-electroplating procedure, treatment with dimethylformamide followed by sensitization leads to a finely roughened and a highly hydrophilic surface with reducing power. This is caused by the formation of an ionic complex compound between dimethylformamide and tin(II) chloride absorbed in the PVC surface. A much more finely and deeply etched surface which exhibits higher adhesion through the mechanical interlocking effect is obtained with the PVC blends containing the plasticizer with a low value of interaction parameter and with a solubility parameter approximate to that of PVC. Adhesion of the metal layer to the PVC surface thus obtained is improved about 1.5 times by thermal aging at 120°C for 20 min.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

Photothermal imaging of dehydrochlorination in poly(vinyl chloride) films

Impulse (time domain) laser photopyroelectric effect spectrometry uses the time dependence of heat diffusion in an irradiated sample to infer optical and thermal property depth distributions in the sample. This method was used in conjunction with a recently reported method of inverse problem theory to recover the depth dependence of optical absorption in photodegraded thin films of poly(vinyl chloride) (PVC) on a depth scale of a few tens of microns. The thin films were photodegraded under broadband ultraviolet light prior to photothermal analysis to induce subsurface profiles of visible absorption arising from the depth dependence of light initiated dehydrochlorination reactions in the films. Optical depth profile reconstructions could be made in samples that had undergone mild degradation only and where the absence of significant thermal effusivity changes accompanying degradation could be confirmed. Reconstructed optical transmission profiles were compared with optical transmission micrographs and were shown to resolve features that arise from the depth dependence of photo-induced dehydrochlorination reactions in the films. The extent of these latter reactions is controlled by the depth dependence of the oxygen concentration in the films.     

Poly(ethylene glycol)s catalyzed two-phase dehydrochlorination of poly(vinyl chloride) with potassium hydroxide

Poly(ethylene glycol)s [HO(CH₂CH₂O)_nH, where n > 3] are highly active and selective in catalyzing dehydrochlorination of poly(vinyl chloride) in organic–aqueous hydroxide two-phase systems. Their catalytic activity and stability are much higher than those of widely used quaternary ammonium or phosphonium compounds. Poly(vinyl chloride) can be extensively dehydrochlorinated within half an hour at room temperature. The products are polyacetylene-like and have long polyene sequences according to their UV/visible, FT-Raman, and FT-infrared spectra. They can be doped by iodine to conductive states, with conductivities of 1–4 S cm⁻¹. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2463–2469, 1998     

Modification of poly(vinyl chloride). XXXIV. Crosslinked poly(vinyl chloride) via paste-processing

A novel PVC-crosslinking technique using 6-dibutylamino-1,3,5-triazine-2,4-dithiol (DB) was applied for a paste processing to produce a crosslinked PVC product. The paste formulation recommended in the present study consisted of 100 parts of PVC (Zeon 121), 60 parts of dioctyl phthalate, 0.2 parts of MgO, and 6 parts of a 50% solution of DB-Na in butylcarbitol, which gave a highly crosslinked and transparent sheet with an excellent stability for thermal discolouring. The increasing viscosity behaviour of the paste during storage is explained by the effect of interparticle attracting forces of DB-Na which coordinates to the ether oxygen atoms in the glycol derivatives adsorbed on the surface of PVC particles. The increased viscosity can be reduced by addition of 3 parts of N-butyl-benzene-sulfonamide. The tension-distortion properties at elevated temperatures were remarkably improved at the crosslinked product compared with the uncrosslinked. The mechanical properties of the two crosslinked products produced via paste processing and roll-blending are compared in regard to the differences of the uniformity of crosslinking units.     

Degradation of poly(vinyl chloride)

The decomposition of poly(vinyl chloride) below 155°C has been examined by thermogravimetric analysis. Degradation is enhanced by irradiation with 1 MeV electrons. Later stages of isothermal weight loss for thermal and radiolytic decomposition follow 3/2-order kinetics and a similar reaction scheme is inferred. A free radical mechanism for dehydrochlorination involving allyl and polyenyl radicals is postulated.     

Molecular aggregation in poly(vinyl chloride). III. Role of molecular structure

The present study offers a visualization of the process of molecular aggregation in PVC polymerized at low temperatures in bulk. “Kinks” along with the chains are responsible for molecular entanglement with subsequent microcrystallization of the syndiotactic sequences. The crystallites act as bonding nuclei. The “kinks” are the result of the different chain conformations of isotactic and syndiotactic sequences. The number of “kinks” increases on lowering the polymerization temperature both because the chains are longer and also because of the more frequent switching between isotactic and syndiotactic placements.     

Degradation of poly(vinyl chloride). II. Kinetic investigation of thermal and radiation-induced polyene formation at low temperatures

The degradation arising from purely thermal treatments as well as with accompanying exposure to γ-radiation has been followed under vacuum by visible and ultraviolet spectroscopy in the temperature range from 80° to 130°C. As corroborated by extensive literature reports, absorbance changes have been considered suitable to provide valuable information on the unsaturation sequences produced. In agreement with the results obtained in the same conditions by following the evolution of HCl,¹ a catalytic action has been ascribed to HCl depending on the efficiency of removal of HCl. Furthermore, both the purely thermal and radiation-induced uncatalyzed degradations yield length distribution functions that are constant with time and have been found to be accounted for by the previously formulated free-radical mechanism, also from a quantitative point of view.     

Stabilization of poly(vinyl chloride). III. Synergism between metal soaps and masking agents on the stabilization of poly(vinyl chloride)

The stabilization mechanism by synergetic metal soaps containing complementary colors was previously reported. With increased heating times, the color of heated poly(vinyl chloride) (PVC) films containing Cd/Ba and Zn/Ca synergetic soaps markedly deviated from the polyene color. These color deviations usually decreased the thermal stability of PVC. Discoloration from polyene color to blue appeared especially on PVC films containing Zn/Ca synergetic soap and was concomitant with a marked decrease in thermal stability. The stabilization of PVC containing synergetic metal soaps can be improved by masking or removing the excessive color. In this work, the addition of various masking agents, such as ethylenediaminetetraacetic acid, o-phenanthroline, triethanolamine, urea, N,N′ -dimethylolurea, melamine, stearylamide, and lactams, to PVC containing synergetic metal soaps was investigated. It was shown that these masking agents do markedly slow down the discoloration of PVC.     

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.     

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.     

The effect of amines on the dehydrochlorination of poly(vinyl chloride)

A study has been made of the effect of amines, particularly those used as catalysts in the preparation of polyurethane foams, on the dehydrochlorination of poly(vinyl chloride) (PVC). It has been shown that when PVC is heated at a temperature of 80°C in the presence of amines the levels of dehydrochlorination produced are considerably greater than those found if no amine is present. This has been attributed to a chemical dehydrochlorination reaction and could well have implications with respect to the physical and mechanical properties of the polymer for certain of its commercial applications.     

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.     

Degradation of poly(vinyl chloride) to small molecules

By treatment with alkali in 2-methoxyethanol, poly(vinyl chloride) is converted to an insoluble network structure. On subsequent oxidation with 65% HNO₃, a water-soluble mixture of acids is obtained. Per 1000 carbon atoms of the original polymer chain, approximately 4 moles succinic acid, 2 moles glutaric acid, and 1 mole adipic acid are found. In spite of the fact that the occurrence of succinic acid is in the same order as tail-to-tail polymerization in PVC, it must be assumed that unexpected aggregation of more than two CH₂ groups is due to rearrangement during the alkali treatment or the oxidation procedure.