Thermal Stability and Flammability Behaviors of Phosphorus/Graphene Oxide Co-Modified Waterborne Polyurethane Coatings: An Experimental Study

To enhance flame retardancy of waterborne polyurethane (WPU) coatings, this paper proposes a co-modification method using modified graphene oxide (SiO2@GO) and a phosphorus flame retardant (P-). SiO2@GO refers to graphene oxide (GO) with an attached silicon dioxide (SiO2) layer, while the phosphorus flame retardant (P-) in this work is THPO, a reactive flame retardant used as a chain extender. The influence of component additions on flame retardancy was systematically investigated. Modified WPU coatings (P-SiO2@GO/WPU) were prepared using THPO and SiO2@GO as flame-retardant chain extenders. The morphology, structure, and thermal stability of P-SiO2@GO/WPU were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). At 2% SiO2@GO, coatings showed enhanced hydrophobicity (water repellency) and thermal stability. With 4% phosphorus flame retardant (P-), the limiting oxygen index (LOI, a measure of flame retardancy) reached 32.2%, and the heat release rate was 32.4% lower than before modification. A continuous, dense P/Si-containing carbonaceous ceramic-like barrier layer was formed, effectively blocking the release of combustible gases and the transfer of heat, thereby demonstrating excellent flame retardancy. This synergistic P-SiO2@GO/WPU modification offers theoretical support and practical guidance for optimizing and enhancing the flame-retardant performance of WPU coatings.