Analysis of the time course of degradation for fully biobased nonwoven agricultural mulches in compost-enriched soil

The time course of degradation for biodegradable and fully biobased plastic agricultural mulches prepared from polylactic acid (PLA) and PLA–polyhydroxyalkanoate (PHA) blends using meltblown (MB) nonwovens processing technology in compost-enriched soil during a 45 week burial duration was investigated under controlled conditions in a greenhouse to better understand the underlying mechanism for degradation. Mulches underwent an initial 4 week period during which the tensile strength decreased by ≥ 60%. For MB-PLA, between weeks 4 and 12, the crystallinity increased and the molecular weight slightly increased. After 12 weeks, the molecular weight decreased linearly. At 28 weeks, the crystallinity increased significantly, corresponding to an observed increase of macroscopic fragmentation. For MB-PLA+PHA, the mass fraction of PHA decreased significantly (by 11.6%) during the first 4 weeks; subsequently; the PHA fraction decreased linearly. Between 17 and 22 weeks, a large increase of crystallinity for the PLA component and a decrease of molecular weight occurred. After 22 weeks, the molecular weight and crystallinity of PLA increased nearly linearly. These results suggest that during the first 4–12 weeks, microorganisms begin to utilize the more readily available components of the mulches as carbon sources: amorphous PLA regions and PHA for MB-PLA and PLA+PHA, respectively, and that microbial activity is greater and occurs more strongly for PHA. After the initial induction period, depolymerization occurs slowly and steadily for both MB plastic mulches.