From aquatic waste to flame-retardant composites: Fish scale-reinforced epoxy for sustainable material design

This study explores the viability of Labeo rohita fish scales, a renewable aquatic waste, as eco-friendly reinforcements in epoxy composites. Composites with varying scale loadings (0–20 g) were fabricated using a two-stage layering technique to retain the scales’ hierarchical structure. Mechanical, water absorption, and flame-retardant properties were systematically evaluated. The major finding of this work is the exceptional flame-retardant property achieved, with the burning rate decreasing by 70% (from 23.76 to 7.00 mm/min at 20 g scales), which is attributed to hydroxyapatite’s heat-shielding and collagen’s char-forming capabilities. However, a critical trade-off was observed in mechanical performance: tensile strength declined by 76% with initial scale addition, although a partial recovery in flexural strength at 15 g (38.85 MPa) suggests an optimal loading threshold. Furthermore, the composites maintained surface integrity, with Shore D hardness remaining stable (86.00–87.13), but faced limitations in humid environments due to increased water absorption (up to 2.52%). Despite the mechanical trade-offs, this study successfully demonstrates that untreated fish scales are highly effective as sustainable flame-retardant fillers. This positions the developed composites as promising materials for non-structural applications where fire safety is paramount, such as automotive interiors, partition panels, and packaging.