Modified layered double hydroxides as PVC heat stabilisers

Abstract

Hydrotalcite (HTC) was intercalated with different aromatic carboxylic acids via two synthesis methods: reconstruction and co-precipitation. The reconstruction method involves the rehydration of the products of LDH calcination. The co-precipitation method involves the addition of a base to solutions containing a mixture of the MII and MIII ions found in the metallic layers of an LDH. The intercalated compounds were then compounded with flexible grade PVC to see if these compounds had any effect on the heat stability of the PVC. Complete intercalation of these stabilisers did not occur; however layered double hydroxides did form for almost every synthesis. The organic acids that were to be intercalated were also present in every synthesised stabiliser. Neat hydrotalcite was the best overall stabiliser with an early stability time of 32.40 min and a final or long term stability time of 106.51 min. The best modified layered double hydroxide (LDH) in terms of early stability was 4-hydroxybenzoic acid + HTC synthesised with the reconstruction method. This stabiliser had an early stability time of 25.40 min. The best performing modified stabiliser in terms of late stability was salicylic acid + HTC synthesised with the co-precipitation method. This stabiliser had a late stability time of 71.32 min. The highly activating nature of the hydroxyl substituent group should make hydrotalcites intercalated with hydroxybenzoic acids good free radical scavengers. The substituent group positions that give the best PVC heat stability are the ortho and para positions. The pKa 2 value for an organic acid may be used as a selection parameter for intercalation into hydrotalcite. If a high pKa 2 value organic acid is intercalated into hydrotalcite, the resulting compound will have good PVC heat stabilisation properties.