Rotational Molding of Poly(lactic acid): Effect of Polymer Grade and Granulometry

This paper is aimed at studying the effect of the grade and the granulometry of poly(lactic acid) (PLA) on its processability by rotational molding. Two PLA grades were considered: a low melt flow rate (MFR), high viscosity, material, characterized by an excellent quenchability, leading to a completely amorphous structure of rotomolded samples; a high MFR, low viscosity, material, characterized by a lower quenchability, leading to a semicrystalline structure of rotomolded samples. Three different granulometries were considered: a coarser one, that is, the pellets as received, intermediate size pellets produced by extrusion followed by pelletizing, and a powder obtained by grinding of as received pellets. Besides the different dimensions, the intermediate size pellets also experienced a supplementary extrusion step, which induced some degradation in the material. For both grades and three granulometries, void-free prototypes were obtained, which indicate a very efficient sintering process, attributed to the low viscosity of all PLA grades. As a consequence, the modulus of the rotomolded samples was found to be unaffected by the PLA grade or granulometry. In contrast, the strength was shown to be more significantly dependent on quenchability than on the grade. To obtain an adequately high strength, an amorphous structure must be developed during cooling. Finally, the supplementary extrusion step experienced by the intermediate size pellets was shown to significantly decrease the strength of prototypes, as a consequence of the thermal degradation induced by this additional processing step.