Characterization of the abiotic degradation of oxo-biodegradable polypropylene obtained from transition metal-free pro-oxidant

Abstract

The pro-oxidant additives used in the production of oxo-biodegradable polymers are generally organic salts of transition metals added during the processing of polyolefins. Transition metals accelerate the photooxidation and thermal oxidation of macromolecules, producing oxygenated fragments which are then used as nutrients by microorganisms in the biodegradation process. However, the use of these additives can lead to contamination of the environment by metals, which come free from the reactions. The aim of this study was to obtain an oxo-biodegradable polypropylene by processing flat films with distinct levels of benzoin, an organic substance free of transition metals, which is biodegradable. After extrusion (t = 0 h), samples were submitted to OIT analysis, which showed that benzoin favors the thermal oxidation process. Subsequently, the samples were subjected to accelerated ageing in a UV chamber and were characterized by dilute solution viscometry (DSV), FTIR, TGA, DSC and SEM. Results of the DSV analysis revealed a significant reduction in the viscosimetric average molar mass of the samples containing the additive, which was accompanied by a considerable increase in carbonyl groups, according to the carbonyl indexes calculated from the FTIR spectra. Thermogravimetric analysis of the aged samples containing benzoin indicated a greater loss of thermal stability and the DSC thermograms revealed a considerable reduction in the melting temperature of the oxo-biodegradable PP. The SEM images of the aged samples indicated that the presence of the additive favored the appearance of regions containing flaking and erosion. In general, the results clearly indicated that benzoin accelerated the photooxidation process during ageing by UV irradiation. Unlike organic salts of transition metals, benzoin seems to act in the initiation step of photo- and thermal oxidation, as demonstrated at the end of this study.