The degradation of poly(lactic acid) (PLA) during thermal-mechanical processing was studied and the influence of processing conditions on degradation rate was determined by size exclusion chromatography coupled with multi-angle light scattering (SEC-MALS). A two-parameter model accounting for both chain scission and recombination processes was used to describe the experimentally observed molar mass distribution. The degradation and recombination rate constants were determined for undried and dried PLA. It was highlighted that the effect of processing temperature (in the 170-210 °C range), processing time (until 30 min) and shear rate (rotor speed varying from 0 to 150 rpm) on molar mass reduction can be relatively well simulated insofar as self-heating related to the mechanical energy conversion into heat was taken into account. The influence of melt processing on the thermal behaviour of PLA was also investigated using temperature modulated differential scanning calorimetry (TMDSC). It was evidenced that the molar mass reduction affects the crystallizability of PLA. Cold crystallization temperature progressively decreases with decreasing molar mass and the metastable a' phase is formed in place of the stable ? phase. The a' phase can be partially converted into ? form during melting giving rise to a double-melting peak. The two peaks can be separated using reversing and non-reversing signals confirming that recrystallization of the a' form occurs. (Résumé d'auteur

A method to determine the overall energy balance of neat PLA processing in a batch mixer was proposed. Energy consumption calculations rely upon shear rate and viscosity calculations using a double-Couette approximation. In the case of PLA mixing, it was found that the energy consumption is increased with an increasing rotor speed and decreased with an increasing temperature. During the melting phase the energy consumption is not strongly affected by the process parameters and the energy efficiency is close to 1. During the mixing phase, for a given viscosity, the energy efficiency is increased with increasing rotor speed or temperature and a method to optimise the process parameters to obtain a low viscosity with limited degradation and high energy efficiency was proposed. Moreover a comparison is given between the double-Couette approximation and conventional rheometry (rotational and capillary). Conventional rheometry translates into an overestimation of the viscosity especially at low temperature. This overestimation could probably be explained by the high sensitivity of rheology to molar mass reduction. (C) 2014 Elsevier Ltd. All rights reserved