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Abstract

The degradation of PLA proceeds through hydrolysis of the ester linkages in the polymer backbone. This leads to a significant reduction of molecular weight and thus to a deterioration of mechanical properties. However, up to now there are no studies of PLA-hydrolysis performed under practical conditions and long-term storage. In this work, the influence of molecular weight, granule size, crystallinity, air humidity and temperature on the hydrolytic degradation of PLA was investigated. Two commercially available PLA types with different molecular weights were used in this study. Three different granule sizes were prepared through compounding with a co-rotating twin-screw extruder. The granules were annealed at defined temperatures so that crystallinity degrees in three different magnitudes were generated. Finally, all granules were stored at five different temperatures in the range from 10 to 50�ºC and a relative humidity of 33 and 75 % for six months. During this time, the viscosity of the samples was monitored with a rotational rheometer. A simple linear function was used to define the relationship between the weight average molecular weight and the zero viscosity in logarithmic form. By fitting the zero viscosity in the simplified kinetic equation, the degradation rate constants for each sample at each temperature were determined and used afterward in the Arrhenius equation to calculate the activation energy of the hydrolytic degradation for each sample. As most significant influences on the hydrolytic degradation of PLA, temperature and air humidity were identified. The degradation rate constants and therefore the rate of the hydrolytic degradation, increase significantly with increasing temperature and air humidity. Furthermore, small granules, high crystallinity and high molecular weight increase the activation energy and thus reduce the rate of PLA degradation during the long-term storage.

Biography

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