Use of Citrus Peel Waste as Bio-Fillers in Polyester Resin Composites: Analysis of Mechanical Properties

In the context of global trends in sustainability and the circular economy (CE), this article aims to investigate the potential of microparticles derived from citrus peel waste (grapefruit, key lime, lemon, and orange), constituting approximately 50% of the fruit weight, as eco-friendly bio-fillers in polymer composites, thereby reducing the consumption of petrochemical resins. The composites were fabricated by gravity casting using polyester resin (PR) as the matrix at filler concentrations of 2.5%, 5%, and 10% by weight. Functional properties were assessed using static tensile testing (measuring Peak Load, Peak Stress, and Young's modulus) and Shore D hardness testing. The incorporation of unprocessed fillers generally decreased tensile strength (Peak Stress REF: 31.48 MPa), attributed to poor interfacial adhesion. The lowest Peak Stress value was recorded for the 2.5O composite (16.04 MPa). The exception was the 10K composite (10 wt.%key limee), which achieved a Peak Load (1.28 kN) nearly identical to the neat resin (1.29 kN), although the Peak Stress remained lower due to the reduced effective cross-sectional area. Stiffness (Young's modulus REF: 3.26 GPa) increased by more than 10 wt.% for 5G (3.63 GPa), indicating effective reinforcement at this concentration. A key positive finding was a universal increase in Shore D hardness across all biocomposites (REF: 78.4 ShD), with a maximum of 83.8 ShD for 10L (lemon), a typical response to rigid fillers that suggests enhanced surface resistance. The results suggest that citrus peel waste could be considered for non-structural applications where surface durability and efficient waste management are priorities.