Hybrid polyester composites using Ocimum gratissimum stem fiber and mussel shell derived calcium carbonate as reinforcements

Abstract The present study investigates the development of sustainable hybrid polyester composites reinforced with 30 vol% Ocimum gratissimum stem fiber and mussel shell-derived calcium carbonate (CaCO₃) for lightweight structural applications. The extracted natural fiber and bio-derived CaCO₃ particles were surface modified using 3-methacryloxypropyltrimethoxysilane (MPS) to enhance reinforcement–matrix compatibility. Hybrid composites containing untreated and silane-treated reinforcements with different CaCO₃ loadings (0, 1, 2, and 4 vol%) were fabricated through the hand lay-up technique and evaluated for density, mechanical, tribological, thermal, and morphological characteristics. The incorporation of hybrid reinforcements significantly improved the performance of polyester composites compared with neat polyester. Among the developed formulations, the silane-treated HT2 composite containing 30 vol% Ocimum gratissimum stem fiber and 2 vol% CaCO₃ exhibited the optimum balance of properties, achieving a tensile strength of 59 MPa, which represents a 78.8% improvement compared with neat polyester (33 MPa). The HT2 composite also demonstrated an 83.3% enhancement in flexural load, a 285.0% increase in impact strength, and a 17.6% improvement in hardness compared with the unreinforced polyester matrix. Furthermore, the HT2 composite exhibited superior tribological behaviour with 40.0% reduction in coefficient of friction and 62.8% reduction in wear rate, along with a 33.8% improvement in thermal conductivity due to enhanced reinforcement dispersion and reduced interfacial resistance. The improved performance of HT2 was attributed to effective silane-induced fiber–matrix bonding, uniform CaCO₃ dispersion, and efficient stress transfer mechanisms. Although HT4 exhibited the highest hardness and density due to increased CaCO₃ content, excessive filler loading promoted localized agglomeration and stress concentration, resulting in reduced overall mechanical and wear performance compared with HT2. The findings demonstrate that silane-treated Ocimum gratissimum stem fiber combined with optimized mussel shell-derived CaCO₃ reinforcement provides an environmentally sustainable composite system suitable for automotive, marine, construction, and general engineering applications.