A Sustainable Perspective for Macadamia Nutshell Residues Revalorization by Green Composites Development

Macadamia is a nut widely used globally in food, personal hygiene products, and human health. After removing the nut, high amounts of the shell residual, which is an underutilized material with high economic interest. This work presents a sustainable alternative to revalorize the macadamia nutshell residues (MR) developing the green composites. Polypropylene (PP) composites were prepared using different MR content (5, 10, 15, 20, 25, and 30 wt%) as a filler. The samples were investigated by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis, scanning electron microscopy (SEM), mechanical tests (tensile and flexure) before and after water absorption tests, and Life Cycle Assessment (LCA). FTIR results indicated that the MR acted only by physical impediments, without chemical interactions between the filler and matrix. Thermal analysis showed a significant increase (similar to 20 degrees C) in PP thermal stability after MR addition, probably due to the filler high lignin content, and SEM photomicrographs indicate that high MR contents resulted in little cracks and voids in the interface. However, the defects did not influence the mechanical performance, and the green composites showed superior elastic modulus (2401 MPa) than the pristine PP (1516 MPa) and similar tensile strength (similar to 30 MPa), confirmed by the statistical analysis and corroborating with the theoretical percolation network (25 wt%). The composites showed low water absorption (up to 0.3%), reflecting in downward variations in the wet samples' mechanical behavior. It evidences the possibility to use these samples for water applications. LCA indicated that higher MR contents promoted lower environmental impacts than the classical handling of this residue, and the results indicated that 30 wt% MR is the better environmental option. However, the PP/MR-25% showed reduced environmental impacts and associated with other properties improvements, considered the better sample. This work shows the MR revalorization as a sustainable approach to expand green composites applications. Graphic