A review of the relationship of tin content and ligand structure on stability of rigid PVC compositions

Abstract

Historically, the measure of thermal performance on given PVC compositions has been approximated by the metal content of the chosen heat stabilizer. However, advances in tin and ligand chemistry necessitate the evaluation of thermal stabilizers on criteria other than metal content. Many factors including the nature of the metallic species and relative activity of the organic ligands must be considered. A discussion will be given on the facets of heat stabilizer compositions and their effects upon PVC formulations and processing. Performance criteria for organotin stabilizers will be outlined based on kind and ratio of alkyl groups, ligand type and organic co-stabilizers. The take-home point will be that tin content alone, in most formulations, is not the most important determinant in PVC heat stabilizer effectiveness. VC was first observed over 150 years ago. But it P was not until the early 1930's that this polymer came into industrial use. It was at this time, George Wick of I. G. Farben found that processing of PVC could be improved considerably by using particular additives and new technological steps. Moreover, he also discovered that properties of finished PVC articles could be influenced by proper selection of processing conditions. As a result, numerous applications of PVC were established. The different types of processing aids and other additives which are available for processing PVC mixtures are found in Fig. 1 . Stabilizers and lubricants are essential ingredients; this paper is concerned with stabilizers and their role in PVC processing.Early on, lead salts and soaps were the stabilizer of choice, followed over the next 20 years by barium/ cadmium soaps, calcium/zinc soaps and culminating with the organotin compounds.The principal reason for selecting stabilizers is for their effective thermal properties, but they also need to fulfill the following requirements: optimum dispersion in PVC compatible with all other ingredients of formulation with no tendency to plate-out low volatility tasteless and odorless in the final article physiologically harmless regarding workplace atmosphere and end product migration good light stability low ecotoxicological potential permanent maintenance of physical, chemical, and electrical properties of finished PVC article N o known compound possesses by itself all these characteristics to the extent different applications require. Consequently, optimum stabilizers have to be designed for the specific end-use. In general, mixtures of several stabilizing components are necessary. Since the 1950's. when organotin mercaptide compounds were first used, a great many variations around the central tin atom have been utilized (see Fig. 2). The tin sites in organotin heat stabilizers are tetravalent and normally contain alkyl groups and ligands. It is the combination of alkyl groups and ligand groups which individually and collectively effect the stabilizing capacity of specific organotin heat stabilizers.The effect of alkyl groups on the stabilizing efficiency of various organot...