Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites

In this study, rice husk (RH, 15wt%) served as a carbonizing agent, and ammonium polyphosphate (APP) served as an acid and gas source. These were combined with polylactic acid (PLA) to develop a high-strength and flame-retardant PLA-based composite. The APP surface was modified with silane coupling agents (KH550 and KH570) to enhance the compatibility with the PLA matrix and improve both mechanical and flame-retardant properties. The composite was evaluated using UL-94 flame retardancy tests, limiting oxygen index (LOI) measurements, and mechanical properties assessments. The findings demonstrated that both PLA/RH-APP10% and PLA/RH-APP15% composites met the UL-94 V-0 standard. Increasing APP content enhanced flame retardancy but reduced mechanical strength. Compared to unmodified PLA composite, the PLA/KAPP5% composite exhibited an 18.7% increase in tensile strength, an elongation at break improvement from 3.26% to 4.09%, and a LOI of 27.9%. The silane modification significantly improved APP dispersion within the PLA matrix, increasing interfacial contact and improving overall mechanical properties. The flame retardancy improvements were attributed to reduced thermal decomposition rates and increased carbon residue formation.