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
