Mechanical characterization of hybrid polymer matrix composite strengthened with coir fibers

Corn starch-polypropylene (CS–PP) hybrid composites reinforced with 0-30 wt.% chopped coir fibres (CFs) were fabricated by injection moulding and evaluated for physical, mechanical, thermal and microstructural performance. The theoretical density increased from 0.985 to 1.121 g/cm 3 with increasing CFs content, while the experimental density remained lower because of processing-induced voids. Porosity remained low up to 10 wt.% CFs but increased at higher fibre loading due to agglomeration, moisture retention and incomplete interfacial wetting. Tensile and flexural properties improved with CF addition and reached an optimum range at CPCF20-CPCF25, where better fibre-matrix interaction, fibre bridging and crack deflection promoted effective stress transfer. Impact strength increased from 2.0 to 4.1 J, while Shore D hardness increased from 61.8 to 78.8, confirming improved energy absorption and surface resistance. Thermal analysis showed initial degradation at T 5 %=271.212-281.2°C and T 10 %=285.785-296.014°C, with maximum degradation at 296.01-375.79°C for CPCF25 composite and final residue below 4%. However, thermal conductivity and flame propagation rate increased with CF loading, indicating the need for flame-retardant modification for automotive interior use. SEM fractography confirmed the transition from matrix-dominated failure to fibre-assisted toughening, followed by defect-controlled failure at excessive CF loading.