Effect of agricultural waste particle size on the properties of composites based on flame-retardant polyester resin

This study explores the use of hemp stalks, an agricultural waste material, as a sustainable reinforcement in flame-retardant polyester (FRP) composites. The objective is to assess the influence of different hemp stalk particle sizes (0–1, 1–2, 2–4, and 4–8 mm) on the physical, mechanical, and thermal properties of the composites. Composites were fabricated using 80% hemp stalks and 20% halogen-free FRP resin by volume. The physical, mechanical and thermal performance of the composites were evaluated based on density, water absorption, thickness swelling, ultrasonic pulse velocity (UPV), compressive strength, thermal conductivity and thermal degradation properties determined by thermogravimetric analysis (TGA). The smaller particle sizes led to higher apparent density, thermal conductivity, water absorption, and thickness swelling, while compressive strength and UPV values decreased. The composite reinforced with 4–8 mm hemp particles showed the best performance, achieving the lowest thermal conductivity (0.0976 W m⁻¹K⁻¹) and the highest compressive strength (0.675 MPa). TGA results also indicated superior thermal stability for composites with larger particles. Hemp stalk particle size has a significant effect on the composite’s behaviour. Coarser particles enhance mechanical strength and thermal stability, while finer particles increase moisture sensitivity and thermal conductivity. These findings highlight the potential of agricultural by-products in the development of environmentally friendly and application-specific polymer composites.