Thermoforming of compostable PLA/PBS blends reinforced with highly hygroscopic calcium carbonate

Mineral fillers have always been used in engineered polymers to reduce the cost, promote the thermal and dimensional stability of the material, and contain its shrinkage and deformations generated by the conversion processes. Compounds based on compostable polymers are normally characterized by the presence of high levels of mineral fillers (up to 30 %), although mineral fillers with a limited ability to incorporate moisture are preferred. The bioplastic compounds are, in fact, based mainly on bio-derived polyesters, highly sensitive to hydrolytic degradation at high temperatures. Therefore, in the design of bioplastic compounds, the moisture content is kept strictly under control to avoid the onset of degradation mechanisms. Calcium carbonate is, however, the cheapest mineral filler and is broadly available on the market. It is the most widely used mineral filler in the design of engineered polymers based on polyolefins and polystyrenes. However, the use of calcium carbonate in the bioplastic compound segment is severely limited by the hygroscopicity of the carbonate. In this context, the use of calcium carbonate was studied, as a reinforcement element of binary blends between poly lactic acid (PLA) and polybutylene succinate (PBS). In particular, the role of the percentage of the mineral filler on the absorption of moisture in the compound has been studied. The thermo-rheological properties of the compound were also evaluated. Finally, the processability of the compound was studied in the thermoforming process of containers suitable for food contact on a prototype system (machine - mold), specifically designed to transform polyolefins. Experimental results have shown that a correct design of the bioplastic compound, even in the presence of a highly hygroscopic filler, can allow the development of a material suitable for the thermoforming process, without the need to make changes to the conversion plant and to the mold.