Influence of selected agents on crystallization power of polylactide

The influence of seven additives on the crystallization ability of polylactide (PLA), melt flow index (MVR) and mechanical tensile properties was studied. Pressed plates with a thickness of 0.8 mm were tested. Selected additives added in amounts of 0.5 and 1.0% were as follows: talc, sodium benzoate, mixtures of organic salts with amorphous SiO2 and zinc stearate, metal salt, phosphate salt, and potassium salt of 5-dimethylsulfoisophthalate (LAK-301 - nucleating agent developed for PLA). Non-isothermal crystallization measurements were performed at different cooling rates (0.3; 0.5; 0.7; 1.0 and 1.5 ° C). All nucleation agents increased the MVR of PLA except talc; the largest increase (9-fold and 24-fold) was the addition of metal salt. The additives did not fundamentally change the mechanical properties. All samples were rather brittle (the most brittle with LAK-301), the modulus of elasticity was around 1.2 GPa for all samples, the strength of PLA was increased the most by the addition of 1% talc (by 12%) and the elongation at break was increased by organic salt with SiO2. All samples with nucleating agents content of 1% were amorphous (crystalline content did not exceed 2%). Thus, the addition of reagents did not support the crystallization process during rapid cooling, even in the case of LAK-301. However, LAK-301 was acting as an excellent nucleating agent at slow cooling rates (1.5 °C / min and below). The nucleation activity of the additives decreased in the following order: LAK-301, organic salt with zinc stearate, talc, organic salt modified with amorphous SiO2 and phosphate salt. Samples with sodium benzoate and metal salt were crystallizing on cooling in several steps and it was not possible to use the method of Dobrev and Gutzow to evaluate the nucleation activity