Tran Van Hoang

Thermal dehydrochlorination and stabilisation of poly(vinylchloride) in solution: Part VI—Dihydropyridine as organic costabiliser with Zn-Ca stearates

Abstract 1,4-Dihydropyridine diester derivative (DHP) has been used in the stabilisation of PVC carried out in trichlorobenzene (TCB) at 187°C in the absence and in the presence of zinc and calcium stearates. The kinetics of HCl evolution as well as of chloride ion formation on consumption of the metal soaps were studied by coulometry. DHP, used alone, can slow down the rate of degradation of PVC, but it does give a synergistic effect in combination with Zn and Ca stearates. Studies carried out by FTIR, UV and NMR have shown clearly that DHP, as secondary stabiliser, is highly sensitive to a reduction reaction by either heat or by chemical agents, especially by Zn stearate and Zn chloride. This property explains the improvement of both initial colour and long term stabilisation: the first effect is due to the partial hydrogenation of the polyene sequences, the second to the ability of its oxidised derivative (the pyridinic form) to complex Zn chloride strongly, avoiding the catastrophic blackening after full consumption of the metal soaps. Finally, the possibility of substitution of DHP on allylic chlorine in PVC was also discussed.

Thermal dehydrochlorination and stabilization of poly(vinyl chloride) in solution: Part III—Zn-Ca stearates as stabilizers

Abstract The thermal degradation of poly(vinyl chloride) (PVC) in trichlorobenzene (TCB) solution at 187°C has been studied in the presence of various mixtures of zinc and calcium stearates, using coulometric titration of chloride ions, HCl evolution measurements and spectroscopic analysis (FTIR and UV) of the degraded polymer at various times. The substitution reaction, initially suggested by Frye and Horst1,2 is clearly demonstrated, although the very small amount of stearate groups involved with respect to the number of vinyl chloride units makes it difficult and requires the use of high performance equipment and careful recording conditions. Many of the known features of the stabilization of PVC with mixed metal soaps are also observed. In addition, a transient complex between the metal stearates is observed which cannot be separated from the polymer because it shows the same solubility behavior. Evidence for secondary reactions between metal chlorides and stearic acid is given. Finally, the rates of the processes are very dependent on the amounts of structural defects in the initial polymer.

Polyvinylchloride stabilisation with organotin compounds—Part III: Effect of lewis acidity and co-ordination of organotin chlorides upon their catalytic activity towards dehydrochlorination

Abstract The catalysis of the E2 dehydrochlorination of a model compound for the allylic chlorine atoms in PVC involves the co-ordination of this chlorine to the tin atom. It is enhanced by an increase in the Lewis acidity of the tin chloride compound (scaled by Mossbauer spectroscopy), but may be inhibited if other groups, internal or external, are competing for the co-ordination. HCl may be one of these competitors, although, in some cases, it breaks the weakest tin—carbon bonds, giving rise to stronger prodegradant Lewis acids.

Infrared evidence for the grafting of ester groups on polyvinyl chloride stabilised by Zinc and Calcium stearates

SummaryCareful FTIR analysis of PVC samples heated in trichlorobenzene solution at 187°C in the présence of mixture of zinc and calcium stearates for various times were carried out. A special accessory allowing identical flushing of several samples with dry nitrogen was used. The samples were thoroughly purified before examination. The very weak carbonyl absorption, after an initial decrease, increases as a sharp band near 1734 cm-1 progressively shifting towards lower wave numbers and then decreases with simultaneous broadening. A corresponding C-H band is seen near 2850 cm-1.

Thermal dehydrochlorination and stabilisation of poly(vinylchoride) in solution: Part II—Effects of HCl readdition reaction

Abstract The role of HCl during the thermal degradation of PVC has been investigated. It was shown that HCl catalyses the degradation as well as itself adding to the conjugated sequences in the degraded PVC. There is competition between these two reactions. At the beginning of the dehydrochlorination, the first reaction is dominant, while readdition becomes more important as double bonds are formed. Metal or organometallic chlorides may significantly influence these effects. The reverse reaction is catalysed by organotin chlorides of high Lewis acidity. These retardation effects are a pure consequence of the partial and random reversible thermal decomposition of PVC; they do not result from the configurational rearrangement by the readdition reaction nor by the internal isomerisation assisted by tin compounds.

Thermal dehydrochlorination and stabilization of poly(vinyl chloride) in solution: Part 1—Conditions for reliable experimental data

Abstract The detection of the HCl evolved on dehydrochlorination of poly(vinyl chloride) in trichorobenzene solution was carried out by coulometric titration; this method is not affected by the solvent carried by the stream of inert gas into the titration cell. This allows the use of a minimum path between the reactor and the titration cell so that the major source of error, which is the adsorption and desorption of HCl on, or from, the glassware, is practically suppressed. The shape of the dehydrochlorination kinetic curve changes from linear to concave downward, when the temperature is increased from 175°C to 195°C; between these temperatures a sigmoidal shape is observed.

The rôle of polyols as secondary stabilizers for poly(vinyl chlorides)

Abstract In the ternary stabilization system, zinc carboxylate-calcium carboxylate-polyol, for poly(vinyl chloride), benzyl ethers of polyols have been used to check the possible substitution of the polyol residue in the PVC backbone; GPC experiments with a uv detector give support for this substitution, but it is not confirmed by infra-red spectra. Coulometric titration of the chloride ions produced by reaction of the metal soaps with HCl or labile chlorine atoms of the PVC during heat treatment in the presence of polyols is correlated with the development of yellow and brown color. These experiments confirm that the polyols are useful mainly as long-term secondary stabilizers, the effect being related to the number of OH groups in the molecule.

Chloride formation and polymer coloration during thermal degradation of PVC stabilised with a Zn-Ca recipe

Abstract The part played by metallic chlorides, and especially zinc chloride generated during the stabilisation process of PVC with the zinc-calcium recipe, is analysed through a brief review of the mechanism established previously, a few additional data being presented. Correlations between polymer discoloration and chloride formation, estimated through a coulometric method based upon Ag + ion precipitation in the presence of chloride ions, are analysed for different short-term and long-term secondary stabilisers. They allow the exact part played by each component to be explained.

Study of the Stabilization of Polyvinyl Chloride) by Using Model Molecules. VI. Mechanism of Reactions of Aminocrotonate Esters

Abstract The mechanism of reaction of the β-aminocrotonate of butanediol (βACB) with 4-chloro-2-hexene (4C2H), a model compound for the allylic chlorine in polyvinyl chloride), was studied in THF or dichloroethane at 60°C by gas and liquid chromatography. The reaction, which needs ZnCl2 as a catalyst, leads to substitution products through the primary amine group and the hydrogen atom of the trisubstituted double bond. βACB reacts with HCl to give NH4Cl and a set of complex organic products. NH4Cl and the substitution products are able to complex ZnCl, inhibiting its catalytic activity. In combination with other stabilizers, βACB strongly induces the substitution reaction versus the dehydro-chlorination. In the polymer at 190°C, it increases very much the time of action of the stabilizers; it acts as an HC1 acceptor but also it may be substituted on the polymer even without catalysts. Synergistic effects are observed with epoxy compounds or indole derivatives.

Relation between optical and electrical properties of ion implanted PPV

Abstract Poly(p-phenylene vinylene) has been synthesized using the sulphonium precursor route. Heat treatments have led to the formation of free-standing PPV films, which have been implanted at increasing ion fluences in the energy range 50–150 keV. Optical absorption and specular reflectance have been studied in the wavelength domain 200–3000 nm. The progressive shift of the absorption edge from the UV towards the visible has been attributed to the narrowing of the band gap, whereas the significant increase of the reflectance in the near infrared has been interpreted by the development of aromatic domains. A permanent increase by more than 13 orders of magnitude of the conductivity has been obtained in correlation with the progressive narrowing of the band gap. The comparison between implantations performed with doping ions and rare gases, producing comparable damage, shows significant differences at energies lower than 80 keV for heavy ions (iodine and xenon) and lower than 150 keV for light ions (sodium and neon), providing evidence for a doping effect, which is screened by the radiation damage at high implantation energies. The magnitude of this effect is, however, limited (one order of magnitude) in comparison to the conductivity enhancement (13 orders of magnitude) resulting from the radiation damage. The irreversible structural changes induced in ion implanted PPV films lead to the conclusion that the electronic doping occurs in a new material, which may be compared to (DLC) diamondlike films involving a significant sp 2 component.