Torrefaction of Coffee Husk Flour for the Development of Injection-Molded Green Composite Pieces of Polylactide with High Sustainability

Coffee husk, a major lignocellulosic waste derived from the coffee industry, was first ground into flour of fine particles of approximately 90 µm and then torrefied at 250 °C to make it more thermally stable and compatible with biopolymers. The resultant torrefied coffee husk flour (TCHF) was thereafter melt-compounded with polylactide (PLA) in contents from 20 to 50 wt% and the extruded green composite pellets were shaped by injection molding into pieces and characterized. Although the incorporation of TCHF reduced the ductility and toughness of PLA, filler contents of 20 wt% successfully yielded pieces with balanced mechanical properties in both tensile and flexural conditions and improved hardness. Contents of up to 30 wt% of TCHF also induced a nucleating effect that favored the formation of crystals of PLA, whereas the thermal degradation of the biopolyester was delayed by more than 7 °C. Furthermore, the PLA/TCHF pieces showed higher thermomechanical resistance and their softening point increased up to nearly 60 °C. Therefore, highly sustainable pieces were developed through the valorization of large amounts of coffee waste subjected to torrefaction. In the Circular Bioeconomy framework, these novel green composites can be used in the design of compostable rigid packaging and food contact disposables.