Sintering of PLLA powders for rotational molding

This paper is aimed to study the suitability of poly-lactic acid (PLLA) for the production of components by rotational molding. To this purpose, the sintering behavior of PLLA powders was studied by thermomechanical analysis (TMA), in order to identify the onset and endset temperatures of sintering and the onset temperatures of degradation. The results indicate that sintering of PLLA is characterized by two different steps, namely powder coalescence and void removal. The first process is fast, occurring just above the melting temperature, whereas the second one occurs at much higher temperatures. Finally, at higher temperatures, degradation involves the formation of gas in the bulk of the polymer, leading to a decrease of the bulk density. The different phenomena occurring during heating of PLLA powders were interpreted by means of dimensionless numbers. The use of such approach allowed identifying the processing window for PLLA powders, defined as the difference between the endset of sintering and the onset of degradation. In agreement with experimental results, the dimensionless analysis confirmed that wider processing windows are obtained for slower heating rates of PLLA powders. On the other hand, it is well known that potential materials for rotational molding should be characterized by an adequate toughness, essentially related to de-molding of parts. Therefore, PLLA was mixed with two different plasticizers, a non-biodegradable one, i.e. di-ethyl-hexyl-phthalate (DEHP) and a biodegradable one, i.e. poly-ethylene glycol (PEG). The plasticizers were responsible of a reduction of viscosity and therefore a faster sintering process. On the other hand, the decrease of thermal stability due to the addition of plasticizer is expected to significantly decrease the width of the processing window.