Polyvinyl chloride stabilization methods

The authors demonstrated that polyvinyl chloride segments of these graft and block copolymers are more stable than PVC homopolymers prepared according to traditional ways. These last ways lead to disproportionation reactions giving unsaturations at the ends of polymers and these ends are responsible for the poor thermal stability of the polymers. In the present method, termination occurring essentially by transfer reaction, no unsaturations are observed and the authors showed an improvement in stability of the PVC segments of about 20 °C (Scheme 39). 

BS 2782. Method 641 A, Determination of dimensional stability at lOO C of flexible polyvinyl chloride sheet, 1983. 

BS 2782 Method I.IOA. Determination of the thermal stability of polyvinyl chloride by the Congo red method. 1991. 

In the oxidation of polyvinyl chloride, for example, a vital role is played by splitting out of HCl molecules and the formation of double bonds. The oxidation of polyamides proceeds without any appreciable induction period and differs sharply in character from the oxidation of hydrocarbons. The thermooxidative destruction of polysiloxanes also proceeds uniquely. Condensation polymers - epoxide resins, polyary-lates, and polycarbonates begin to be oxidized at comparatively high temperatures, and thus far no methods of stabilizing them are known. 

Some peculiarities of the thermal and thermooxidative decomposition of polyvinyl chloride depend on the conditions of its production. Thus, it is known that samples of the polymer produced by initiating the polymerization of vinyl chloride with ultraviolet irradiation possess higher stability in comparison with samples produced in polymerization under the action of chemical agents [26, 27]. Reversibility of the process of dehydrochlorination in the decomposition of samples of polyvinyl chloride produced by the latex method is noted, while in the process of decomposition of suspension polymer, the phenomenon of reversibilily is not observed [21]. It has been shown that the rate of dehydrochlorination of the latex polymer is significantly higher than that of the suspension polymer under the same conditions [21]. It has been established that the polymerization of vinyl chloride in the presence of oxygen leads to the formation of unstable peroxide groups, which can initiate decomposition of the polymer [28, 29]. It is noted that an extremely substantial influence on the stability of polyvinyl chloride is exerted by the purity of the monomer, as well as the presence of impurities of metals of variable valence [28]. 

Copolymersof vinyl chloride with vinylidine chloride, produced by the suspension method, are more stable than the latex copolymers. The rates of thermal decomposition of polyvinyl chloride and the copolymer of vinyl chloride with vinylidine chloride, produced by the latex method, are practically the same for the latex copolymer, just as for the latex homopolymer, reversibility of the process of dehydrochlorination is observed [21]. In an investigation of the stability of copolymers of vinyl chloride with vinyl acetate, vinylidine chloride, and with vinylisobutyl ether in nucleophilic substitution reactions, it was found that the copolymer with vinyl acetate is the least stable to the action of alcoholic alkali the copolymers with vinylidine chloride and vinylisobutyl ether proved more stable [56]. The stability of the copolymer of vinyl chloride witii methyl acrylate is substantially increased when the degree of homogeneity of the copolymer with respect to composition is increased, and when monomers with a smaller content of impurities are used, as well as when the copolymerization is conducted in the presence of chain carriers [57, 58]. 

There are indications that the introduction of small amounts of a second monomer during the process of polymerization leads to a disturbance of the regular structure of the basic chain of the macromolecule and to increased stability for high-molecular compounds that decompose with high yields of the monomer [59], However, the information on the increase in the thermal stability of pols inyl chloride by co-polymerization with small amounts of glycidyl methacrylate [21] cannot serve as a basis for a general evaluation of the effectiveness of this method of stabilizing polyvinyl chloride. 

Three methods can be used in principle to slow down the thermal and thermooxidative decomposition of polyvinyl chloride and vinyl chloride copolymers 1) suppression of chain reactions developing during the process of thermal and thermooxidative decomposition 2) the creation of conditions imder which the substances formed during the process of breakdown prevent more profound decomposition of the polymer, and 3) the creation of conditions under which decomposition proceeds reversibly. In practice, the first method is used in the overwhelming majority of cases in the stabilization of halogen-containing high-molecular com-poimds, while the substances used as stabilizers, as a rule, are de-celerators of chain decomposition reactions. 

The high activity of organotin stabilizers is responsible for their use in the production of colorless transparent objects from nonplasticized compositions, as well as in the reprocessii of polyvinyl chloride by the extrusion method. In both cases the necessary condition is high thermal stability of the compositions. 

Figure 5.81 Influenoe of the buffer system on residual stability (Congo red method) in polyvinyl chloride films after UV irradiation with Xenotest 1200 (OSRAM Vitalux lamps), distance 30 cm, formulation 100 ml S-PVC, 54 ml DOP, I ml ESO, 3 ml Ba/Cd stabilizer [202]...

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