In this research, kenaf/unsaturated polyester and kenaf fiber/CaCO₃ polyester hybrid composites were fabricated using hand lay-up method. The effect of surface treatment of kenaf and CaCO₃ on the mechanical properties, morphology, chemical structures and water absorption were studied. The Fourier Transform Infrared (FT-IR) spectroscopy was observed for untreated and treated kenaf fibers, and untreated and treated CaCO₃ particles. The untreated and treated kenaf fibers showed a very strong and broad absorption peak in the region 3200-3600cm-1 of hydrogen bonded O-H stretching vibration. However, the peaks 2918cm-1 and 2917cm-1 of the treated kenaf showed that stearic acid had successfully coated the surface of the kenaf fiber as well as 2918cm-1 in treated CaCO₃. The tensile strength of kenaf/ CaCO₃ polyester composites was found to have highest value with 6% CaCO₃ (70.1MPa) in comparison with treated kenaf fiber/unsaturated polyester composites (57.7MPa) and untreated kenaf fiber/unsaturated polyester composites (43.2MPa) at 20% fiber loading. The flexural strength and flexural modulus showed similar trend as tensile strength and tensile modulus (68.5MPa and 3741.4MPa respectively). The Scanning Electron Microscope (SEM) revealed that the surface of the treated fibers became rough after treatment due to the coated of SA onto the surface. The treatment enhances better interaction and adhesion of fiber and matrix. The results of water absorption study depicted that increasing the loading of kenaf fiber in the composites resulted in increasing water absorption, and hybrid composites attained equilibrium earlier than other composites due to the incorporation of CaCO₃ that prevent further water penetration followed by treated and untreated composites respectively.
