Effect of inorganic flame retardants on the thermo-mechanical and fire-resistant properties of pineapple leaf fiber composites

Epoxy composites reinforced with pineapple leaf fiber (PALF) composites were fabricated and modified with three distinct flame retardants, namely aluminum hydroxide (Al(OH) 3 ), magnesium hydroxide (Mg(OH) 2 ), and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), at filler concentrations of 20, 40, and 60 wt.%. The samples were characterized using Limiting Oxygen Index (LOI), Shore D hardness, Raman spectroscopy, and thermo-mechanical analysis (TMA) as per the ASTM standards. The flame-retardant-modified composites exhibited significant improvement in LOI values compared to the neat composite (20% LOI), achieving 23–27% LOI with Al(OH) 3 , 24–30% LOI with Mg(OH) 2 , and 24–27% LOI with DOPO. The UL-94 vertical flammability testing showed that 40 wt.% of flame retardants achieved a V-0 rating, indicating superior self-extinguishing behavior. Scanning electron microscopy images of filler materials modified PALF composites displayed significant improvements. The use of Al(OH) 3 and Mg(OH) 2 filled the interfacial gaps and acted as barrier layers, helping to reduce the fiber exposure. DOPO-filled PALF composites observed with phosphorus-rich char precursors and porous protective structures. These features of flame-retardant filler materials improved char formation, suppressed the volatile release, and provided better condensed-phase protection. The Shore D hardness results observed with a moderate reduction in the hardness of modified composites compared to the neat composite. Raman spectral evaluation supported the char formation and water vapor release in Mg(OH) 2 -modified composites, supporting its superior barrier action and alignment with LOI performance. TMA exhibited that the 40 wt.% Mg(OH) 2 observed to have a higher glass transition temperature (Tg) modified than that of the neat PALF composite. The reduction in coefficient of thermal expansion at the glassy and rubbery regions confirms that modified composites effectively restrict the polymer chain and provide better dimensional stability. Mg(OH) 2 -modified PALF composites exhibit significantly better flame-retardant performance compared to neat PALF, which indicates their potential for use in fire-safe, lightweight materials.