Synthesis and modification of hydrotalcite as a thermal stabiliser for poly(vinylchloride)

In recent years there has been considerable interest in the use of synthetic hydrotalcites as stabilisers for poly(vinyl chloride) (PVC). Hydrotalcites are essentially hydrated magnesium-aluminium-hydroxy-carbonates: a typical formula being Mg6Al2(OH)16CO3.4H2O. Substitution of a divalent by a trivalent cation in the brucite structure creates a positive charge that needs to be counterbalanced by the presence of an anion, and this is usually carbonate. Hence, hydrotalcites are anionic clays, as opposed to the more common montmorillonite-like clays or so called cationic clays. These double-layered hydroxides have the capacity to undergo anion exchange reactions and so the carbonate can be substituted for other anions such as chloride, sulphate or nitrate. The characteristic of anion exchange means that, commercially, synthetic hydrotalcites are used for catalysis, ion scavenging, purification processes and the stabilisation of PVC formulations. This thesis present result of a project to synthesise and modify hydrotalcites to produce ultra-fine particles in recognition of the fact that the benefits of hydrotalcite could be enhanced with smaller particle sizes. The work has proceeded via two routes: firstly, by experiments to modify and intercalate the hydrotalcite with a surfactant in a way analogous to that used to montmorillonite clays, and secondly by novel synthesis methods. Two different synthesis reactions were used: one based on Urea and the second by a controlled pH method. Different drying technologies were used to obtain the optimum particle morphology and size. It was found that ultra-fine hydrotalcite particles gave an improvement in PVC thermal stability, although addition of higher levels of the hydrotalcites caused a reversal in this trend. The Haake Rheometer was used extensively to study the behaviour of the polymer clay nanocomposite formulations and to investigate the pros and cons of the addition of nanometre particles. Further work using an autoclave has resulted in particles of nanometre-sized dimensions with enhanced crystallinity and very high surface area which are promising for future work on PVC clay nanocomposites.