Ammonium polyphosphate modified epoxy–polyurethane system for high-performance bamboo composites with improved fire safety

Bamboo is a renewable and eco-friendly material increasingly recognized as a sustainable alternative to traditional materials like wood and metals due to its light weight, high strength, and rapid growth. However, its high cellulose content makes it highly flammable, posing significant fire risks. To address this issue, this study introduces a flame-retardant treatment by incorporating ammonium polyphosphate (APP) into a polyurethane-toughened epoxy (EP-TPU) resin system, along with alkali pretreatment of bamboo fibers to enhance resin impregnation. Experimental results demonstrate that APP incorporation significantly improves flame retardancy and thermal stability. Mechanical properties decrease with increasing APP content; however, the composite containing 20% APP maintains mechanical performance exceeding industrial non-load-bearing standards while exhibiting enhanced flame resistance. Thermogravimetric analysis (TGA) confirmed that APP promotes char formation, thereby improving thermal stability. Cone calorimetry tests show a reduction in both peak heat release rate (PHRR) and total heat release (THR) with higher APP content. Specifically, the 25% APP composite showed the lowest PHRR, while the 30% APP composite achieved the lowest THR and smoke production. In UL-94 vertical burning tests, the 20% APP-modified composite self-extinguished within 2 seconds, attaining a high flame-retardant rating. These results present a viable strategy for fabricating high-performance, flame-retardant bamboo composites with potential for broad practical applications.