Assessment of PVC stabilisation using hydrotalcites - Raman spectroscopy and other techniques

The stabilising effect of hydrotalcites on rigid (UPVC) and flexible (PPVC) has been investigated for samples cut from compression moulded sheets oven aged at 180 °C using UV-visible and Raman spectroscopies, solubility, and X-ray diffraction, and by observing their discolouration. Hydrotalcite improves the long term stability of PVC and inhibits crosslinking. When crosslinking does not occur longer unsaturated sequences that produce red colouration in degraded samples of UPVC containing hydrotalcite are detected by Raman spectroscopy. For UPVC the best result was observed with 2 phr hydrotalcite; severe degradation occurred when the hydrotalcite concentration was increased to 5 phr. PPVC is generally much more stable, with significant ageing only being observed after ≥120 min. After 120 min stability increased as hydrotalcite concentration increased; the reduction in the concentration of unsaturation containing 14 conjugated sequences in the presence of Alcamizer P93 hydrotalcite (double layered magnesium/aluminium hydroxide) was particularly striking. X-ray diffraction measurements made on UPVC samples suggested that initially there is some exfoliation of the hydrotalcite layered structure, and subsequently intercalation is also observed. This suggests that the PVC chains are separating the hydrotalcite layers, and destroying order. It is proposed that this causes separation of chains, preventing the formation of crosslinks.