Poly(lactic acid) degradation in soil or under controlled conditions

The fate of a racemic lactic acid polymer (PLA50) was investigated by allowing parallelepiped plates of PLA50 to age in liquid medium containing a mixed culture of Fusarium moniliforme and Pseudomonas putida at 30°C. Microbial activity was monitored by measuring pH, lactic acid formation by high-performance liquid chromatography, and esterase activity in supernatant. Degradation of the plates was monitored by weighing, size exclusion chromatography, and visual examination. Under the selected conditions, pH increased up to 8, whereas it remained constant in the microorganism-free control. No significant lactic acid formation or esterase activity was detected in the supernatant during the experimental period (32 weeks). For the first 17 weeks, the water absorption rate of the plates was the same in both the microbial and the control media. Subsequently, plates absorbed more water in microbial than in control media. Typical surface/center differential degradation of plates was observed until 17 weeks, but it reduced progressively. At 32 weeks, specimens appeared completely disintegrated in the microbial medium. PLA50 plates were also buried in the soil for 8 weeks. After recovery, plates were allowed to degrade 8 weeks under controlled conditions. Scanning electron microscopy of these plates showed the development of some filamentous fungi at the surface and into the bulk of plates. Five fungal strains were isolated which appeared to be able to assimilate PLA50 oligomers in mixed cultures. It was assumed that PLA50 plates had been first degraded by chemical hydrolysis, followed by the bioassimilation of degradation by-products in both experiments. © 1996 John Wiley & Sons. Inc.