INFLUENCE OF ANTIOXIDANTS INTERCALATED LAYERED DOUBLE HYDROXIDES (AO-LDHs) ON THE THERMO-OXIDATIVE STABILITY OF PLA AND PE BASED NANOCOMPOSITES

Layered double hydroxides (LDHs) are excellent host matrices for storage and delivery of antioxidants (AOs) [1, 2]. The layered structure protects AOs against degradation, reduces migration and allows a gradual release of AOs by ion-exchange and diffusion. In this work, two hindered phenolic antioxidant-modified LDHs (AO-LDHs) were successfully prepared by intercalation of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid (IrganoxCOOH) and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox). It was found that by anchoring the phenolic moieties to the LDH layers the thermal degradation of AOs is prevented, and that the DPPH free radical scavenging capacity of the free AOs is maintained and even improved in the case of IrganoxCOOH-LDH. A small amount of the two AO-LDHs was dispersed into poly(lactic acid) (PLA) and polyethylene (PE) by solution mixing and the thermo-oxidative stability of the composites was compared with that of neat polymers and polymers containing free AOs. In particular, SEC analysis of PLA based samples after a controlled period of aging indicates that both the AO-LDHs protect the PLA from chain scission. The oxidation induction time (OIT, DSC) carried out at 230°C in the case of PLA and at 200°C in the case of PE samples shows also the beneficial effects of the presence of AO-LDHs. The data suggest that a barrier effect due to the LDH lamellae can be excluded and that the antioxidant ability of the AO-LDHs is likely a function of the AO chemical structure, of the composites morphology, and of the spacing between the AO functionality and the polymer chains. Finally, a preliminary migration test carried out on films of PLA/IrganoxCOOH-LDH and PLA/Trolox-LDH confirmed the low tendency of the AO in AO-LDHs (embedded in PLA) to migrate, due to a robust ionic anchoring of AO between the LDH layers.