Laboratory composting of extruded poly(lactic acid) sheets

Composting of extruded poly(lactic acid) (PLA) in combination with pre-comp osted yard waste in a laboratory composting system was studied. Yard waste and PLA mixtures containing 0%, 10%, or 30% PLA (dry weight basis) were pla ced in composting vessels for four weeks. Exhaust gases were analyzed for c arbon dioxide concentration twice per week. After the first week, significa ntly greater (P < 0.5) amounts of carbon dioxide were generated in vessels with 10% or 30% PLA than in control (0% PLA) vessels. Data indicated that m icrobial degradation of PLA occurred. There was no significant difference ( P > 0.05) in carbon dioxide emission between 10% and 30% PLA mixtures. Comp ost pH dropped (from 6.0 to 4.0) after 4 weeks of composting for 30% PLA, b ut remained unchanged (6.3) for 0% or 10% PLA. Most likely, in the case of 30% PLA, substantial chemical hydrolysis and lactic acid generation lowered the compost pH. The lowered pH likely suppressed microbial activity, thus explaining the lack of difference in carbon dioxide emissions between 10% a nd 30% PLA mixtures. Gel permeation chromatography showed a notable decreas e in PLA molecular weight as a result of composting. It was demonstrated th at PLA can be efficiently composted when added in small amounts (<30% by we ight) to pre-composted yard waste. (C) 2000 Elsevier Science Ltd. All right s reserved.