Liquid-phase degradation of poly(vinyl chloride)

The main kinetic regularities of the thermal and thermal-oxidative degradation of poly(vinyl chloride) (PVC) in the liquid phase have been analyzed in comparison with the solid-phase degradation of PVC. The thermal (in an N₂ atmosphere) degradation of PVC in the form of dilute solutions is characterized by a number of essential distinctions, the primary peculiarity consisting of a lower dehydrochlorination rate due to the retarded reaction of the formation of bond polyconjugated systems. The same is observed for the thermal degradation of PVC plasticized with di- and polyesters. On the contrary, in an oxygen-containing atmosphere the solvents promote PVC decomposition. Accelerated PVC degradation under such conditions is due to the solvent oxidation that causes the appearance, within the system, of products activating the PVC macromolecular decomposition. The intensified degradation processes are accounted for, in the first place, by an increased reaction rate of the statistical (by the random law) detachment of HCl from normal macromolecular units. In general, the kinetics of the thermal-oxidative liquid-phase degradation of PVC is determined by the partial pressure of oxygen in the reaction zone, by the quantity of the solvent introduced into the polymer, as well as by the oxidative stability of the solvent. It has been shown that an effective stabilization of PVC in the liquid phase, particularly in the systems highly plasticized with esters, can be achieved through stabilizing the solvent, first of all, rather than the polymer, against oxidative decomposition. In this case the PVC dehydrochlorination rate decreases sharply and may reach an essentially lower value than under similar conditions of the solid-phase thermal degradation of PVC. PVC stabilization with respect to the reaction of HCl elimination achieved through the solvent stabilization against thermal-oxidative decomposition has been called the effect of “echo-stabilization” of PVC.     

Oxidative degradation of poly(vinyl chloride)

Rates of oxygen consumption and formation of oxidation products were determined in γ-initiated oxidation of poly(vinyl chloride) (PVC) at 25°C. Data are given for five dose rates and correlated on the basis that the overall oxidation is the sum of two reactions that are first order and half order in rate of initiation. The principle oxidation product is hydroperoxide, formed by the half-order reaction, whereas alcohol and carbonyl compounds result mostly from first-order reactions. These products account for 77–80% of the total oxygen absorption. The presence of oxygen was found to increase twofold the rate of dehydrochlorination of irradiated PVC. A reaction scheme is developed which assumes that a significant fraction of the interactions of tertiary peroxy radicals is nonterminating and yields alkoxy radicals which propagate or decompose by β-scission. This decomposition occurs mostly by splitting off a chlorine atom with formation of a ketone and, to a lesser extent, by C? C cleavage which accounts for the observed decrease in molecular weight of the oxidized PVC. Polystyrene was found to be much more resistant to oxidation than PVC. A classification of some commercial polymers in function of their susceptibility to oxidation is proposed.     

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.     

Radiation-induced oxidative degradation of poly(vinyl chloride)

Gas evolution and oxygen consumption in the γ-irradiation of PVC were studied. The gas evolution and the oxidative degradation are retarded by the presence of plasticizers and stabilizers. The G(HCI) and G(H₂) and 8 and 0.24 for the irradiation of pure PVC under vacuum and 0.02 and 0.14 for that of plasticized PVC, respectively. Gas evolution increases in the presence of oxygen, specially for the pure PVC. The G(–O₂) values for the pure and plasticized PVC are 30 and 12, respectively. The dependence of gas evolution and oxygen consumption on the oxygen pressure is more pronounced for the plasticized PVC than pure PVC because the oxygen diffusion is controlled.     

Photooxidative degradation of carboxylated poly(vinyl chloride)

Thin films of poly(vinyl chloride), PVC, and carboxylated poly(vinyl chloride), C-PVC, containing 1.8% of carboxyl groups were exposed to high energy ultraviolet radiation (λ = 254 nm) in laboratory conditions. The photochemical reactions in irradiated samples were studied by FTIR and UV–Vis spectroscopy, gel permeation chromatography and gravimetric estimation of insoluble gel. It was found that photodegradation and photocrosslinking in C-PVC is accelerated, whereas photodehydrochlorination is retarded comparing to these processes in PVC. Photooxidation investigated on the base of reaction leading to formation of hydroxyl groups is also more efficient in modified PVC. However, the total amount of carbonyl groups is much lower in UV-irradiated C-PVC than that in PVC. It indicates that competitive reactions (destruction of carboxyl groups and formation of new carbonyls) occur simultaneously in C-PVC chains. The influence of carbonyl groups on photochemical processes can be explained by an efficient Norrish I and II reactions as well as by energy transfer from absorbing species to weak chemical bonds.     

Thermal degradation of poly(vinyl chloride) blends

Poly(vinyl chloride) was mixed with various poly(methacrylate)s and polycarbonates by combined precipitation from common solutions. The thermal stability of the samples was measured at 180°C under nitrogen, the HCl evolved was detected by conductometry. UV-Vis-spectra of degraded samples were measured to investigate the influence of the poly(methacrylate)s on the lengths of polyenes formed during the degradation of poly(vinyl chloride). The experiments show that the nature of the ester group is the dominating factor for the thermal stability of poly(vinyl chloride) in these blends. Poly(n-butylmethacrylate) exhibits the best stabilization for poly(vinyl chloride) in this series. Polycarbonates with a higher glass transition temperature than the temperature of degradation destabilize poly(vinyl chloride). Stabilization experiments with dibutyltin-bis(isooctylthioglycolate) show a costabilizing effect of the poly(methacrylate)s and polycarbonates.     

Molecular aggregation in poly(vinyl chloride)

PVC synthesized in a bulk polymerization at ?25°C was fractionated by preparative GPC. The eluent THF solutions consist largely of dissociated molecules. Following precipitation into methanol, recovered PVC fractions contain aggregated molecules and are partially insoluble in THF. The soluble portions produce distorted chromatograms characteristic of molecular aggregation. The aggregate fraction increases with molecular weight. Heating recovered PVC fractions at 90°C in THF was adequate to dissociate the aggregates. Following recovery, fractions of PVC do not show crystalline x-ray diffraction patterns. The PVC fractions were crystallizable by subsequent annealing at 150°C to approximately 15% crystallinity, largely independent of molecular weight.     

Molecular aggregation in poly(vinyl chloride)

Molecular weight distributions and molecular aggregation for poly(vinyl chloride) (PVC) polymerized in bulk at ?10, ?30, and ?50°C have been measured using gel permeation chromatography. The aggregate content in PVC polymerized at ?50°C was found to be 87 wt-%. These spherical aggregates of mean diameter of 5000 A are formed preferentially from PVC chains having high molecular weights and long syndiotactic sequence lengths. A temperature of 200°C was used to disintegrate these aggregates into single PVC molecules. In disagreement with measurements of M _n and M _w published in the literature, our measured values do not reach a minimum but rather increase continuously with decreasing temperature of polymerization. This disagreement is most probably due to the phenomenon of molecular aggregation in PVC.     

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.     

Vinyl chloride formation from the thermal degradation of poly(vinyl chloride)

The volatile products from the thermal degradation of poly(vinyl chloride) (PVC) resins and compounds are shown to contain trace amounts of vinyl chloride. Data presented show the effect of temperature and resin type on the amount of vinyl chloride formed. At the maximum temperatures involved in PVC processing which may reach 210°C, vinyl chloride monomer (VCM) evolution amounts to less than 1 ppm (resin basis). A technique employing a thermogravimetric balance and charcoal adsorption of volatiles is described for studying thermal degradation of PVC. The volatiles are analyzed for vinyl chloride by gas chromatography. Peak identity was confirmed by mass spectrometry.     

Vinyl chloride formation from the thermal degradation of poly(vinyl chloride)

The volatile products from the thermal degradation of poly(vinyl chloride) (PVC) resins and compounds are shown to contain trace amounts of vinyl chloride. Data presented show the effect of temperature and resin type on the amount of vinyl chloride formed. At the maximum temperatures involved in PVC processing which may reach 210°C., vinyl chloride monomer (VCM) evolution amounts to less than 1 ppm (resin basis). A technique employing a thermogravimetric balance and charcoal adsorption of volatiles is described for studying thermal degradation of PVC. The volatiles are analyzed for vinyl chloride by gas chromatography. Peak identity was confirmed by mass spectrometry.     

Studies in the thermal degradation of poly(vinyl chloride)

Thermal degradation of poly(vinyl chloride) (PVC) was studied in nitrogen atmosphere in the presence of rubber seed oil and epoxidized rubber seed oil, barium and lead soaps of rubber seed oil, and epoxidized seed oil at various temperatures. The rate of dehydrochlorination at 1% degradation and the time required to attain 1% degradation were used to assess the effect of the thermal susceptibility of PVC to dehydrochlorination. It was found that epoxidized rubber seed oil, the metal soaps of rubber seed oil, and epoxidized rubber seed oil markedly enhance the thermal stability of PVC. The order of increasing stabilizing influence was metal soaps of epoxidized rubber seed oil > metal soaps of rubber seed oil > epoxidized rubber seed oil > rubber seed oil. © 1993 John Wiley & Sons, Inc.     

Thermal degradation of poly(vinyl chloride) in alkyl maleates

The thermal degradation of poly(vinyl chloride) has been investigated in diethyl, dibutyl and dioctyl maleate under nitrogen from 160°C to 210°C. The rate of dehydrochlorination was highest in dibutyl maleate and lowest in dioctyl maleate. The apparent activation energy calculated from the average value of the rate constant at each temperature for diethyl, dibutyl and dioctyl maleate was found to be 83.6 kJ mole^?1, 112.7 kJ mole^?1, and 150.5 kJ mole^?1, respectively. The rates of dehydrochlorination and the activation energy observed in alkyl maleates are higher than those observed in ethyl benzoate. In the visible range spectra the absorption at higher wavelengths is considerably reduced in alkyl maleates.     

Influence of temperature on the ultrasonic degradation of poly(vinyl acetate) and poly(vinyl chloride)

This study investigates the effect of temperature on the ultrasonic degradation of polyvinyl acetate and polyvinyl chloride in chlorobenzene. The time evolution of molecular weight distribution was determined using gel permeation chromatography. Continuous distribution kinetics was used to obtain the degradation rate coefficients. The rate coefficients decrease with increasing temperature, and this is attributed to the increase of vapor pressure and the decrease of kinematic viscosity. The degradation rate coefficient also changes sharply near the glass transition temperature, indicating that this factor may play a role in the degradation process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 12: 2818–2822, 2003     

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.     

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.     

Comparison of dynamic and static degradation of poly(vinyl chloride)

The thermal degradation of PVC was measured under dynamic and static conditions. The UV and IR spectra, as well as the molecular weight distribution of the PVC samples, taken after different time intervals were measured. It was established that during the dynamic PVC degradation, performed in a mixing chamber, two stages with different degradation rates can be distinguished both in extinction and torque vs. time curves. While oxygen does not, dissolved HCl does play an important role in the dynamic degradation: HCl steps into reaction with the formed polyenes and has a catalytic action. The chemical degradation, particularly the crosslinking, produces the changes in the rheological behavior of the material. Results were compared with those obtained under static conditions in argon, air, and HCl atmosphere.     

Effects of lead salts on thermal poly(vinyl chloride degradation)

The effect of lead salts (lead stearate, tribasic lead sulfate and tribase/stearic acid) on the thermal degradation of PVC was studied in trichlorobenzene (TCB) solution and under dynamic conditions in a mixing chamber. It was established that lead stearate directly reduces the HCl elimination rate (mainly by diminishing the zip length) whereas the tribase is acting only as HCl acceptor. In presence of stearic acid the tribase becomes a true stabilizer. At the end of the induction period rapid increase of the zip length and transiently high HCl elimination rates were observed. The consequences of these on the possible mechanism of stabilization are discussed.     

The degradation of poly(vinyl chloride). II. Oxidative degradation in solution

The results of studies on the oxidative degradation of poly(vinyl chloride) in a solvent, triphenyl phosphate, are described and compared with results previously reported for the oxidative degradation of bulk polymer samples. A range of chain-breaking and peroxide-decomposing antioxidants of the type commonly used to stabilize polyolefins were not effective in reducing the rate of dehydrochlorination of poly(vinyl chloride).     

The solubility of vinyl chloride in poly(vinyl chloride)

The solubility of vinyl chloride monomer (VCM) in PVC powders has been studied by equilibrium vapor pressure and microbalance gravimetric techniques at temperatures from 30 to 110°C. At temperatures and VCM concentrations above the glass transition, the solubility closely follows the Flory-Huggins equation with χ = 0.98 and is independent of temperature and of the PVC type, molecular weight, or history. In the glassy state, the VCM solubility is higher than the Flory-Huggins value and shows pronounced dependence upon time and the PVC history. These results have been interpreted through the dual-mode sorption concept of Michaels, Vieth, and Barrie: Normal dissolution follows the Flory-Huggins relation, and the additional glassy-state solubility represents the contribution of a hole-filling process. Changes in solubility with time and sample history parallel well-known volume relaxation processes, indicating that vapor solubility measurements offer a direct and sensitive measure of the free-volume state of glassy polymers.