Layered double hydroxides as PVC thermal stabilisers : the effect of particle size and concentration

Abstract

Layered double hydroxides have shown promise as PVC thermal stabilisers, however how the properties of these clays effect their stabilising abilities is not fully understood. The purpose of this research was to help understand the relationship between the LDH clays properties and their ability to thermally stabilise PVC. The main property that was analysed was the particle size of the LDH clays. The particle size was varied by milling the clays. The effect of concentration of the clays was also tested. Two separate LDH clays were synthesised, a magnesium aluminium clay (MgAl) and a calcium aluminium clay (CaAl). These clays were then milled to four different particle sizes and characterised. XRD analyses was done to understand the crystal structure of the LDH s. PSA ensured that four different particle sizes had been reached, BET was done to analyse the surface area of the clays. Samples were also observed using SEM to understand their morphology and acid reactivity tests were done to observe the reactivity of the clays. The clays were added to a PVC dry blend, which was processed and then thermally degraded via three testing methods. The three instruments used was: Torque Rheometer, Testing oven and a PVC Thermomat. Characterisation showed that the two LDH s were successfully synthesised and milling had successfully reduced the particle size. We also noted the expected platelet shape from SEM. Reduction in particle size had little effect on the surface area of the particles. Results from the thermal degradation tests indicated that particle size had very little effect on the thermal stability of the PVC. This could have been due to bad dispersibility or agglomeration and it is suggest that further tests be done to determine whether dispersibility was a problem. An increase in concentration of the LDH s increased the stability time of the PVC in the majority of the thermal degradation tests. The CaAl LDH severally discoloured during processing and the MgAl caused only slight degradation. The MgAl LDH outperformed the CaAl LDH in the majority of the tests, the exception was the PVC Thermomat. This tests for HCl evolution, and the CaAl LDH may have performed better as it was shown to be much more reactive with acids. It was noted that different degradation tests led to different results, this important as a degradation method that is suitable for the application of the PVC should be used.