Biodegradation of poly(lactic acid)/starch/coir biocomposites under controlled composting conditions

The aim of this work was to investigate the aerobic biodegradation of a composite under controlled composting conditions using standard test methods. Composite was formed by poly(lactic acid) (PLA), with and without the addition of maleic anhydride (MA), acting as coupling agent, thermoplastic starch (TPS) and short natural fibre (coir). For comparison its starting materials, such as TPS and matrix (containing 75 wt% of PLA and 25 wt% of TPS), were also tested.At the end of the incubation period, TPS appeared to be the most bio-susceptible material being totally biodegraded and the matrix showed a higher level of biodegradation (higher amounts of evolved CO₂) than PLA, probably due to the TPS domains preferentially attacked by microorganisms and increasing the percentage of carbon dioxide produced. Fibres seemed to play a secondary role in the process as confirmed by the slight differences in carbon dioxide produced. The compatibilised composite revealed a lower percentage of evolved CO₂ than the uncompatibilised one. Finally, the degradation results were confirmed by thermal properties' changes of tested materials at different incubation times, as monitored by thermal analysis, and by the scanning electron microscopy (SEM) analyses of the compost aged samples. SEM micrographs showed the formation of patterns and cracks on the surface of the materials aged in the compost evidencing a profound loss of structure. Moreover, an extended biofilm (evident also with optical microscopy observation) was detected on the biodegraded materials, thus indicating the growth of a large number of bacteria and fungi on their surfaces.