Optimization of the mechanical properties of peanut shell powder/macadamia nutshell powder/reinforced poly(lactic acid) biocomposites using the mixture design

The development of biocomposites from agricultural waste has received a great deal of attention in the context of sustainability and the circular bioeconomy. While the use of natural fillers and single agricultural waste fillers to reinforce polylactic acid (PLA) composites has been widely investigated, dual filler reinforcement of PLA using multiple waste-derived particulate fillers has received limited attention. This study aims to optimize the mechanical properties of PLA biocomposites reinforced with peanut shell powder (PSP) and macadamia nutshell powder (MNSP) using a mixture design approach and analysis of variance (ANOVA). The biocomposites were fabricated through twin-screw co-rotating extrusion followed by injection moulding, and evaluated using tensile, flexural and impact tests according to ASTM standards. The mixture design enabled systematic evaluation of the effects and interactions of PSP, MNSP and PLA compositions. The optimal experimental formulation composition of 5 wt% PSP, 5 wt% MNSP, and 90 wt% PLA exhibited superior mechanical properties. The predicted optimum mechanical properties are as follows: tensile strength of 48.08 MPa, tensile modulus of 1730.24 MPa, flexural strength of 88.12 MPa, flexural modulus of 5749.46 MPa, and Izod impact strength of 43.20 kJ/m 2 . The biocomposite was characterised using SEM, EDS and FTIR. The results demonstrate that this method effectively produces sustainable, dual-filler PLA biocomposites that have improved mechanical performance.