Catalytic effect of nanosized ZnO and TiO2 on thermal degradation of poly (lactic acid) and isoconversional kinetic analysis

Thermal features and thermal decomposition of poly(lactic acid) (PLA) hybridized with nanosized ZnO and TiO2 have been investigated in inert nitrogen by means of multiple heating-rate thermogravimetric analysis. Experimental results show that PLA hybrids become thermally destabilized as compared to pure PLA and ZnO exhibits stronger effect than TiO2. Evolutional FTIR spectra demonstrate that PLA and its two hybrids exhibit relatively different thermal dependences. Isoconversional kinetic analysis has been performed with various differential and integral methods and the activation energies thus calculated over the whole pyrolysis range suggest that the metal oxide added has led to greatly decreased activation energy for PLA to undergo pyrolysis and resulted in substantially higher decomposition rate constant. Such catalytic effect is found to be more considerable for ZnO than TiO2. By using the Coats-Redfern method, the D3 model is determined to be the most probable reaction model for PLA decomposition. Using the isoconversional activation energy, pre-exponential factor and D3 model, the temperature-dependent mass conversion curves under different heating rates have been successfully recast, leading to very satisfactory calculation performances for the description of pyrolysis de-composition of PLA and its two metal oxide hybrids.