Degradation of PLA/ZnO and PHBV/ZnO composites prepared by melt processing

Composites of polylactide (PLA) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and ZnO nanoparticles (nZnO) were prepared by melt processing. During extrusion and moulding nano ZnO formed aggregates with sizes between 0.5 and 5 μm in PLA and between 0.5 and 15 μm in PHBV. Nano ZnO acted as a disruptor of PLA crystallization process and shifted the polymer glass transition temperature to lower temperatures. This was explained by degradation of PLA polymer chains during melt processing. SEC, FTIR and 1H NMR confirmed that PLA degradation was correlated to nZnO concentration. The effect of nZnO on crystallization of PHBV matrix was much less intense which was shown by TGA. On the other hand, PHBV showed significantly lower thermal stability than PLA. ZnO participated as a reactant and an accelerator in the degradation reaction of PLA and at a smaller extent with PHBV. The results of this study revealed that addition of pure nZnO in concentrations higher than 0.1 wt.% is not recommended for the preparation of PLA/nZnO composites by melt processing while in the case of PHBV the nZnO concentration may be higher but it should not exceed 1.0 wt.%. The exposure time of these materials to high temperatures during melt processing should also be minimized.