Vinyl chloride polymers and copolymers are known to deteriorate under the influence of heat, so resulting in alteration of the physical properties thereof and discoloration which precludes use in transparent and lightly colored articles. This thermal, nonoxidative degradation involves the sequential loss of hydrogen chloride from adjacent monomer units and results in the formation of conjugated polyene structures which impart undesirable color to the vinyl chloride polymer.
Efforts to obviate these limitations have included the use of stabilizing compositions of the MY.sub.2 type wherein M is a metal cation, for example R.sub.2 Sn.sup.+2 (R=alkyl), Ba.sup.+2, Cd.sup.+2, Zn.sup.+2, Pb.sup.+2, Ca.sup.+2, and so forth, and Y is an organic anion. The most effective stabilizers arising from this class of compounds are those classified as organotin compounds. It has been theorized that these MY.sub.2 stabilizers react with hydrogen chloride, and since hydrogen chloride is known to catalyze PVC degradation, the efficacy of these stabilizers is due in part to hydrogen chloride scavenging. However, the stabilizers are also believed to have the ability to undergo rapid and selective metathetical reactions which destroy certain PVC structural anomalies that are the active degradation sites. Accordingly, in addition to reducing the rate of color development in PVC by interrupting polyene growth, the MY.sub.2 stabilizers should manifest a true chemical stabilization effect by reducing the rate of formation of total chloride.
Despite conflicting theories advanced by workers in the art relative to the described stabilization mechanism, the MY.sub.2 stabilizers have been used extensively in industry. Although considered satisfactory from the standpoint of stabilization, toxicity of the heavy metal residues and ecological considerations have stimulated further evaluation of the subject compositions and have generated a search for alternatives.
In accordance with the present invention, this end is attained by means of a novel process which yields a product that is free from toxic heavy metal residues characteristic of most of the prior art. Additionally, the polymer so prepared evidences enhanced stability with respect to the unstabilized compositions, and such stability has been found comparable to that provided by many prior art vinyl chloride stabilizers.
Briefly, the inventive technique involves heating a vinyl chloride polymer in solution with an organotin compound, precipitating the vinyl chloride polymer from the reaction product, and subjecting the resultant precipitate to solvent extraction to remove heavy metal residues.