Photodegradation and Biodegradation by Bacteria of Mulching Films Based on Ethylene-Vinyl Acetate Copolymer: Effect of Pro-Oxidant Additives

摘要

The photodegradation (432 h under irradiation of Xe-Lamp-solar filter) of an ethylene vinyl acetate (EVA) copolymer with vinyl acetate content of 9% was studied, and the effect of iron and calcium stearates was evaluated using different techniques such us attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), gel permeation chromatography (GPC), and thermal analysis methods (DSC and TGA). A re-arrangement in crystallization and consequent decrease in thermal stability were found through differential scanning calorime-try (DSC) and thermogravimetric analysis (TGA), which were in agreement with the chain scission tendency. The presence of Ca and Fe pro-oxidants additives in EVA films increased the ketone carbonyl formation and decreased the ester absorption band of the acetate respect to the pure EVA, as it was evidenced by the significant changes in Carbonyl Indexes found by FTIR. The activity of stearates has been also evaluated by chemiluminescence, where the temperature-ramping tests under nitrogen showed the formation of a peroxide peak at lower temperature. The lower stability of the films containing pro-oxidants was evidenced by the values of oxidation induction time (OIT) determined by DSC. The results were supported by GC-MS, where the concentration of extracted products identified in the EVA containing pro-oxidants was significant and a much greater decrease in molecular weight was determined by GPC, which confirmed the development of degradation for EVA with Ca and Fe stearates in comparison to pure EVA. Biodegradation of photodegraded EVA films were studied at 45C during 90 days using a mixture of Bacillus (MIX) (B. cereus, B. megaterhtm, and B. subtilis) and, in parallel, by Bre-vibacillus borstelensis as reference strain. Biodegradation of EVA-films was studied by Chemiluminescence, ATR-FTIR and GC-product analysis and the data confirm more efficient biodegradation on the materials containing pro-oxidants. The chemiluminescence emissions due to decomposition of oxidation species was observed at lower temperatures on the biodegraded samples. Also, the drastic decrease of carbonyl index and the disappearance of photo-generated low molecular products with biodegradation were more efficient on the biodegraded films containing pro-oxidants. EVA mineralization was evaluated by carbon dioxide measurement using indirect impedance technique. Biodegradation by B. borstelensis and MIX at 45℃ was similar and exhibited a pronounced difference between the pure photodegraded EVA film (around 15% of mineralization) and the corresponding photodegraded films containing Ca and Fe stearates where biodegradation extent reached values of 23-26% of biodegradation after 90 days.