Biodegradation behavior of three-layer sheets based on gelatin and poly (lactic acid) buried under indoor soil conditions

The biodegradation of three-layer sheets composed by glycerol-plasticized bovine gelatin film as the inner layer and coated with two outer layers of poly(lactic acid) (PLA) was examined under indoor soil burial conditions during 120 days, using the natural soil microflora as degrading medium. The residual degraded samples of the multilayer and individual components were taken regularly from the soil to determine water uptake, weight loss, variations in thermal properties and morphology. The multilayer sheet exhibited reduced water uptake as compared to the gelatin counterpart, which was associated with the protection of the middle gelatin layer against water by the hydrophobic PLA layer at both sides. The gelatin layer almost disappeared after 25 days, while the pure PLA layer suffered marginal weight loss on day 120 in soil. On the contrary, the multilayer sheet was degraded to a much greater extent, leading to an overall mass reduction of approximately 20% at the end of the experiment. The presence of gelatin in the multilayer seems to enhance water availability around the three-layer sheet soil micro-environment, inducing gelatin hydrolysis (chemical and enzymatic) which favors the abiotic hydrolysis of PLA, and, in turn, possibly stimulates the action of more active soil microorganisms against PLA. This was experimentally confirmed by the presence of filamentous (actinomycete and fungi) microbes on the surface of the multilayer. The glass transition temperature of the degraded multilayer samples slightly increased while the degree of crystallinity augmented significantly up to 78% on day 120, evidencing the crystallization of the amorphous PLA induced by bio/degradation in soil.