Polylactic acid and Polylactic acid-based nanocomposites photooxidation

The importance of photooxidation in promoting formation of anhydride functional groups and thus promoting hydrolysis/biodegradation of polylactic acid and PLA nanocomposites were elucidated. PLA based nanocomposites were prepared by adding 5 %wt filler content of sodium montmorillonite (ClNa), sodium montmorillonite partially exchanged with Fe (III) (ClFe), organically modified montmorillonite (Cl20A), unmodified sepiolite (SEP) and fumed silica (SiO2). The pure PLA and nanocomposites were UV-light irradiated in artificial accelerated conditions representative of solar irradiation (λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photo-oxidation were followed by IR and UV-visible spectroscopies. The infrared analyses of PLA photooxidation shows the formation of a band at 1845 cm-1 due to the formation of anhydrides. A photooxidation mechanism based on hydroperoxyde decomposition is proposed. The mechanism proposed is confirmed by an increase of anhydride formation rate: the main responsible of this acceleration was identified as transition metal impurities contained in the nanofillers as impurities and involved in the catalytic hydroperoxide decomposition.