The effect of the molecular weight on thermal degradation of fractionated PVC

A commercial PVC and another one prepared in bulk at 40°C were fractionated by means of fractional solution method using cyclohexanone-methanol mixtures. Both fractionations resulted in two sets of fractions. Their thermal degradation at 175, 180 and 185°C was followed by means of a conductivity cell which allows a continuous titration of the evolved HCl. The results showed that the lower the molecular weight the higher the degradation rate was; nevertheless, this relationship is only valid up to molecular weights of 60 000 to 90 000, from which the degradation rate appeared to be independent on the molecular weight. The UV-visible spectra of the degraded polymers suggest a similar polyene sequence distribution for all the fractions which accounts for a similar mechanism of degradation in contrast to what happens to fractions with different tacticity.     

Influence of crystallinity in the curing mechanism of PVC plastisols

The influence of the crystalline areas observed in poly(vinyl chloride) (PVC) the mechanical and thermal properties of PVC plastisols was studied. Several industrial‐degree PVC resins were used to obtain a broad range of molecular weights and processing conditions for PVC plastisols. The gelation process was fully studied at different temperatures and was related to the existence of crystalline areas at high temperatures, even near the glass transition. A simple explanation of the phenomena observed during the gelation of plasticized PVC is proposed, according to the variation in the mechanical and thermal properties at different temperatures. The final gelation was obtained at 140–150°C, which was a lower temperature than those at the beginning of the thermal degradation process. The thermodynamic aspects of the gelation of plasticized PVC were mainly controlled by the PVC resin properties, whereas the plasticizer only influenced the diffusion and stability of the material. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 538–544, 2004     

Partially dehydrochlorinated PVC. I. Structure

PVC made by suspension and anionic polymerization was partially dehydrochlorinated by three different methods. In the first one, a solution in THF and in the second one, particles swollen by dioxane were treated with alcoholic KOH at low temperatures. In the third method, a solution of PVC in DMF was heated to 130°C or refluxed (153°C). The reactions were followed by UV spectrophotometry and the products analyzed by GPC. Furthermore, the products were ozonolyzed and the molecular weights of the polymeric residues determined by GPC. Treatment with KOH in THF yielded products with randomly distributed polyene sequences. At higher degrees of dehydrochlorination a slight decrease in molecular weight was observed. The polymers were shown to be built up by PVC segments with an average degree of polymerization between 70 and 80, interspaced by polyene segments each with relatively few conjugated double bonds in sequence (fewer than 15). The reaction of KOH with particles swollen in dioxane was slower but yielded products with the same molecular structure. Thermal dehydrochlorination in DMF gave rise to long polyene sequences. They were fewer in number as evidenced by the higher molecular weights of the residues after ozonolysis.     

Hydrogen chloride evolution from the heating of poly(vinyl chloride) compounds

The dehydrochlorination and the thermal decomposition of five PVC materials was studied using two techniques: a batch analytical method, combining ion chromatography and atomic absorption and continuous thermogravimetry. The temperatures studied ranged from 60°C to 120°C, over a period of almost one year (50 weeks). It was found that a very large proportion of the soluble chloride emitted early on by the PVC materials into the liquid phase is not hydrogen chloride. None of the five materials tested emitted significant amounts of HCl at temperatures lower than 105°C. The emissions at 90°C after 50 weeks ranged from <0.01 Φg/g to 23.62 Φg/g. Furthermore, one of the materials tested emitted virtually no HCl, even at 105°C, as the amount of HCl measured was almost indistinguishable from the normal background of the analytical instruments (37.30 Φg/g after 50 weeks). Numerical calculations of kinetic reaction rates and extrapolation of the results to use temperatures (40°C) indicate that properly stabilized PVC compounds will be unlikely to lose 1% of the mass of PVC as HCl until 2 billion years have passed. This number has no physical meaning as such and may be incorrect by a vast margin, but it clearly indicates that a 1% loss of HCl is unlikely to occur during the useful lifetime of a commercial product into which the PVC material has been fabricated. This is a conservative estimate, which ignores the much higher activation energy for dehydrochlorination at temperatures below the glass transition temperature (ca. 85°C). Copyright © 2005 John Wiley & Sons, Ltd.     

Cooperation between chemical and physical networks in crosslinked and plasticized PVC

The effect of crosslinking of plasticized PVC on the mechanical properties has been investigated. Crosslinking was performed by using a dithioltriazine, and penetration as a function of temperature was used to follow the changes in mechanical properties. Changes in the melting behavior were detected by differential scanning calorimetry. Even rather low levels of gel content gave a substantial reduction in penetration in the interval 60–110°C, while a more complete reaction prevented major penetration up to at least 200°C. The penetration curve of heavily crosslinked materials showed several plateaus and the transitions could be related to melting of different crystallites formed by annealing at different temperatures. The physical network formed by the crystallites constitutes an important complement to the chemical network in plasticized PVC crosslinked by dithioltriazine. Both the amount and the melting temperature of the crystallites increased after annealing. The introduction of the chemical network further improved the heat resistance of the crystallites. The presence of the two networks will therefore show a synergistic effect with respect to the mechanical properties at increased temperatures.     

Structure of the carbon chain polymer in unsaturated polyesters

The curing behaviour of thick laminates was simulated by curing thin polyester sections isothermally at various temperatures. Unsaturated polyesters with different ratios of styrene to unsaturations in the polyester were cured with benzoyl peroxide and subjected to hydrolytic degradation. The molecular size and structure of the carbon chain polymer was examined by size exclusion liquid chromatography and high resolution ¹³C FT n.m.r. analysis and was found to depend on the reaction temperature. The overall values of the molecular weights ranged from 12 000 to 36 000, which is a magnitude lower than those reported earlier. There was a similar tendency for molar ratios of styrene to unsaturations in the resin varying between 1.25 and 2.00. In the region 60°C–80°C the molecular weight increased with increasing temperature and then, from a maximum at 80°C–90°C, the molecular weight decreased with cure temperature. For the molar ratio of styrene to unsaturations of 2.25 the molecular weight decreased with increasing cure temperature. The average styrene sequence lengths were not found to deviate from the ratio expected from the stoichiometry in the resin before curing except for temperatures below 70°C and above 120°C.     

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.     

Radiation-induced polyene formation of poly(vinyl chloride). I. Effect of conformations

PVC samples with different syndiotacticities were degraded at temperatures in the range of 10° to 115°C by UV radiation. The average length of polyenes increases with increase in syndiotacticity of PVC. It was found that by comparison with the long tactic sequences, on the basis of Bernoullian statistics, the propagation reaction of the polyenes at room temperature is terminated when there are more than three racemic dyads in the tactic sequences. At room temperature, the deformation of about 1.3 Å of the PVC main chain takes place, and at temperatures above the glass transition temperature the propagation rate of the polyenes is competitive with the relaxation time between the (tt) and the (tg) conformations.     

Über die Spaltung von PVC mit Alkali in Tetrahydrofuran

With the objective of scanning the molecular structure of PVC, its reaction with alkali in tetrahydrofurane which has been published previously was reexamined in detail. It was found that, stirring of a 1% solution of PVC in THF with KOH/MeOH at room temperature or 60°C for 90 min yields a product which has only two-thirds of the original K-value. If reacted at room temperature 16 molecules of HCl per 100 VC-units will be eliminated, obtaining at the same time 12 double bonds and the introduction of 2 oxygen atoms. If the splitting is performed at 60°C, 30 molecules of HCl per 100 VC-units will be eliminated obtaining 15 double bonds and the introduction of 10 oxygen atoms. Analysis by gelpermeation chromatography in combination with balance of materials indicates that parallel to chain splitting an increase in molecular weight takes place possibly by intermolecular elimination of HCl. As compared to reactions with low molecular weight compounds the possibility will be discussed that the splitting of the chain in THF could have proceeded at a head-to-head-addition site after HCl elimination followed by saponification (retroaldolreaction).     

Formulation and properties' evaluation of PVC/(dioctyl phthalate)/(epoxidized rubber seed oil) plastigels

Epoxidized rubber seed oil (4.5% oxirane content, ERSO) was prepared by treating the oil with peracetic acid generated in situ by reacting glacial acetic acid with hydrogen peroxide. The thermal behavior of the ERSO was determined by differential scanning calorimetry. The effect of the epoxidized oil on the thermal stability of poly (vinyl chloride) (PVC) plastigels, formulated to contain dioctyl phthalate (DOP) plasticizer and various amounts of the epoxidized oil, was evaluated by using discoloration indices of the polymer samples degraded at 160°C for 30 min and thermogravimetry at a constant heating rate of 10°C/min up to 600°C. The thermal behavior of the ERSO was characterized by endothermic peaks at about 150°C, which were attributed to the formation of network structures via epoxide groups, and at temperatures above 300°C, which were due to the decomposition of the material. Up to 50% of the DOP plasticizer in the PVC plastisol formulation could be substituted by ERSO without a marked deleterious effect on the consistency of the plastigel formed. In the presence of the epoxidized oil, PVC plastigel samples showed a marked reduction in discoloration and the number of conjugated double bonds, as well as high temperatures for the attainment of specific extents of degradation. These results showed that the ERSO retarded/inhibited thermal dehydrochlorination and the formation of long (n > 6) polyene sequences in PVC plastigels. The plasticizer efficiency/permanence of ERSO in PVC/DOP plastigels was evaluated from mechanical properties' measurements, leaching/migration tests, and water vapor permeability studies. The results showed that a large proportion of DOP could be substituted by ERSO in a PVC plastisol formulation without deleterious effects on the properties of the plastigels. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

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.     

PVC Mixtures with Processing Aids

Abstract

The mechanical properties (dynamical, tensile) and morphology by electron microscopy have been studied on PVC mixtures with processing aids of different compatibility with PVC (copolymers of methylmethacrylate, polystyrene, polyvinylacetate) in the temperature range of 20-150°C. It has been found and interpreted that the mechanical properties of PVC in the temperature range of 100-120 °C are favourably influenced by addition of a processing aid, which is compatible with PVC, has a sufficiently high molecular weight and its glass temperature is about 20°C higher than that of PVC.     

Reaction rate and MWD changes in crosslinking of PVC,with dithioltriazine

The reaction rate of crosslinking of PVC with dithioltriazine has been studied by following gel formation and changes in the molecular weight distribution (MWD). Compounding was performed on a roll mill at 145°C and crosslinking by heat treatment at 180 or 90°C. In this system crosslinking is executed by the thiolate anion, formed in situ by reaction with MgO. We have studied the catalyzing effect of several polyols in order to achieve a more efficient reaction. Most likely, these catalysts work by chelating the Mg²⁺ ions, thus increasing the nucleophilic character of the thiolate. With the most efficient ones, ditrimethylolpropane and HO(CH₂CH₂)_6-7H, complete crosslinking can be obtained in 3 min at 180°C, i.e., at processing temperatures. We also followed the changes in the MWD before gelation at a considerably lower temperature, 145°C, and found an extensive molecular enlargement even after 5-10 min. Most surprisingly, μM_n increased up to 100% without formation of insoluble material. By ¹H-NMR measurements on low molecular weight extracts, we have shown this to be due to a fast and selective reaction with allylic chlorine in the unsaturated end groups, ～ CH₂? CH?CH? CH₂Cl, formed in the mechanism of chain transfer to monomer. Due to this reaction, formulations with too high reactivity may crosslink during processing, which calls for a careful balancing of the reactivity for each processing case.     

Antiplasticization and transition to marked nonlinear viscoelasticity in poly(vinyl chloride) (PVC)/poly-ε-caprolactone (PCL) blends

Measurements of mechanical damping (tan δ) in the temperature range of ?120° to +120°C at 110 Hz, of uniaxial tensile creep at 25.0° ± 0.5°C covering creep times from 10 to 1000 sec, and of impact strength at 21°C have been carried out for a series of physical PVC/PCL blends in the composition range of 0%–12% by weight of PCL in the blend. With increasing PCL content in the blend, the α-peak of PVC was shifted to lower temperatures and became broadened. The β-peak of PVC was also shifted to lower temperatures and was markedly suppressed. The tensile creep compliance of approximately linear viscoelasticity showed a maximum decrease of 10%, and the impact resistance was reduced 3.5 times when 5% and 12% by weight of PCL, respectively, was blended with PVC. There was also a considerable increase (25%) in stress level at which the transition from approximately linear to markedly nonlinear viscoelasticity occurred when up to 5% by weight of PCL was added to the PVC. These results are attributed to the antiplasticizing effect of PCL on PVC. They support the importance of β-mechanism in the stress-activated processes proposed to be responsible for the appearance of nonlinear viscoelasticity in glassy polymers, and they are in agreement with the pseudocrosslinking concept of antiplasticization. By comparing the antiplasticization behavior of PVC/PCL blends with that of PVC/DOA and PVC/DOS from reported data, it was possible to obtaing an idea of the level of compatibility in the PVC/PCL blends. The results suggest that PCL is partially compatible with PVC.     

Silane cross-linking of PVC. I. Grafting of mercaptoalkylalkoxysilanes onto PVC: Properties of the grafted and cross-linked product

The reaction of poly(vinyl chloride) (PVC) with mercaptoalkyltrialkoxysilanes yielded silane-grafted PVC that can be cross-linked by the hydrolytic mechanism. The grafting of the silanes on the plasticized and unplasticized PVC was carried out at 180° C during processing in the presence of basic lead-containing stabilizers. The reaction is favorably affected by the presence of plasticizers and lubricants containing polar, preferably ester, groups. The silane-grafted and cross-linked polymers have satisfactory thermal stability. The results of strength measurements at elevated temperatures after cross-linking by water indicate that the material obtained has greatly improved parameters that are suitable for a number of applications. © 1993 John Wiley & Sons, Inc.     

Properties of aminosilane grafted moisture curable poly(vinyl chloride) formulations

The reaction of plasticized PVC with [N-(2-amino ethyl)-3aminopropyl trimethoxy silane] yielded silane-grafted PVC that was crosslinked by a hydrolytic mechanism. It was found that crosslinking resulted in improvement of mechanical properties (tensile strength and elongation at break) specially at temperatures higher than 80°C. Properties at 130°C were substantially enhanced, showing a fourfold increase in ultimate tensile strength (UTS), and a twofold increase in elongation at break. There was a unique relationship between UTS at 130°C and gel content irrespective of processing conditions, with UTS starting to increase sharply around 30% gel content, and reaching a plateau at about 60%. Also, the softening temperature was improved. The crosslinked material had a higher softening temperature (up to 165°C, compared with 95°C for the compound without crosslinking agent), indicating that the hydrolytic crosslinking of PVC yielded a material with improved properties.     

Sample pretreatment for aggregate-free PVC molecular weight measurement by size-exclusion chromatography

The procedure of aggregate-free poly(vinyl chloride) (PVC) solutions for size-exclusion chromatography (SEC) at room temperature using tetrahydrofuran (THF) as the mobile phase is proposed. PVC was dissolved in 1,2,4-trichlorobenzene (TCB) at 130 or 140°C for 6 h, precipitated in methanol, filtrated, and dried. The pretreated PVC was again dissolved in THF and SEC measurements were performed at room temperature. High molecular weight (MW) PVC required a higher pretreatment temperature of 140°C. The existence of aggregates sometimes could not be observed by a refractive index detector, but a light-scattering detector attached to the SEC columns could detect them clearly. The slope of the relationship between the MW and retention volume at the high MW region was steep compared with that at the other part of the peak and it was also a good indicator of the existence of aggregates. The pretreatment of PVC resulted in the decrease in MW averages, which was attributed to the disappearance of aggregates and not to the degradation of PVC. The pretreatment at 150°C resulted in the degradation of PVC. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1801–1809, 1998     

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.     

The effect of shear on fusion and mechanical properties of rigid PVC pipes

The shear level was increased during twin screw extrusion of PVC at different melt temperatures by inserting a hole plate in front of the screw tips. The variation of shear level did not significantly affect the capillary pressure in capillary viscometry at 135°C. However, the falling weight impact strength was markedly influenced. In internal water pressure tests at 60°C premature failure was obtained for pipes extruded at 198°C at the highest shear level.