Polylactide compositions: I. Effect of filler content and size on mechanical properties of PLA/calcium sulfate composites

Starting from calcium sulfate (gypsum) as a fermentation by-product of lactic acid fermentation, novel high performance composites have been produced by melt-blending polylactide (PLA) and previously dried calcium sulfate hemihydrate in a Brabender bench scale kneader at 190 °C. Due to PLA sensitivity towards hydrolysis, it has first been demonstrated that formation of ß-anhydrite II (AII) by adequate thermal treatment of calcium sulfate is a prerequisite. Then, the effect of filler content and mean diameter on thermal, mechanical and impact properties has been examined together with the morphology of the resulting materials. It shows that high tensile performances and impact strength are maintained up to a filler content of 20 wt% without any increase of PLA crystallinity. Interestingly enough and provided that AII particles with a mean diameter of ca. 10 µm were considered as PLA fillers, tensile and impact properties proved to be maintained at a very acceptable level at filler content as high as 50 wt%. Such remarkable mechanical behavior can be accounted for by the excellent filler dispersion throughout the polyester matrix and much favorable interactions between CaSO4 particles and ester functions of PLA chains as evidenced by the use of predictive mathematical models for composite mechanical properties and SEM-BSE imaging of fractured surfaces.