Synergistic flame-retardant effects of SiO2 sol and intumescent flame retardant in wood flour-polypropylene composites

Wood plastic composites (WPCs) are widely used green materials composed of a polymer matrix with wood as a filler. However, due to the flammable nature of the substrates used in WPC, there is a fire hazard associated with their use. To address this, this study developed WPCs by adding SiO₂ sol and an intumescent flame retardant (IFR), which included ammonium polyphosphate, melamine, and double pentaerythritol. This approach aimed to investigate their combined effects on thermal stability and flame retardancy. FTIR and laser particle size analysis confirmed the successful incorporation of SiO₂ sol and its adhesion to wood flour. The WPC with addition of both SiO₂ sol and IFR exhibited excellent flame-retardant performance, with a limiting oxygen index of 39.0%, a UL 94 V-0 rating, and a char residue of 28.1%. Cone calorimeter tests showed that the char layer expanded to approximately 4 cm, significantly reducing the total heat release and heat release rate. The formation of a silicon-containing char layer from SiO₂ sol, coupled with an intumescent char layer from the cellulose-IFR reaction, created a composite multi-layered structure that effectively insulated against heat. These findings suggest that the developed WPCs exhibit enhanced flame retardancy, offering potential for application in fire-safe building materials.

Graphical abstract