Thermal, rheological and mechanical behavior of PLA and PP biocomposites filled with Brewer’s spent grain (BSG): A comparative analysis of polymer-filler interactions

This work is focused on composites based on Brewer’s spent grain (BSG), a by-product from beer production, with two thermoplastic polymers with different properties, Polylactic Acid (PLA) and Polypropylene (PP). The effects of BSG concentration were evaluated using Thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC), melt dynamic rheology, tensile testing, and Scanning Electron Microscopy (SEM). TGA showed that BSG tend to interact with PLA, particularly at high temperatures during compounding, thereby accelerating PLA degradation and promoting chain cleavage as evidenced by size exclusion chromatography. This interaction affects the thermal transitions of the composites and mechanical properties, as evidenced by the ductile behavior observed in the micrographs. Conversely, BSG seems to act as an inert filler in the case of PP matrix. Rheological analysis suggested particle flocculation when PLA is mixed with above 30% (wt./wt.). In contrast, PP demonstrated consistent flow behavior with increasing BSG content, which is highly advantageous for extrusion and injection molding. PP appears to promote a good dispersion of the BSG particles, likely due to its sufficiently high viscosity. The tensile strength of both polymers was negatively affected by BSG, likely due to poor interfacial adhesion, as indicated by the microvoids in the SEM images. However, a steady elastic modulus with increasing BSG content for both polymers demonstrated their potential to maintain mechanical integrity when subjected to stress. This study provides a better understanding of BSG-filled PP and PLA composites, with opposite behavior, also contributing for the development of sustainable materials for packaging, home furniture, or automotive applications.