Gas-Phase and Condensed-Phase Flame-Retardant Mechanisms in Polyurethane Coatings Modified with ATH, Borax and Carbonate for Wood Protection

The development of multifunctional, environmentally friendly flame-retardant polymer coatings remains a significant challenge in materials engineering. In this study, polyurethane (PU)-based composite coating incorporating alumina trihydrate (ATH), borax, and carbonat additives was designed to enhance fire performance and mechanical properties. The structure – property relationships of the developed systems were systematically investigated using Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). Fire behavior was evaluated by Limiting Oxygen Index (LOI) and UL-94 vertical burning tests. The optimized formulation exhibited a marked increase in LOI (25.5%) compared to neat PU (18.5%) and achieved a V-0 rating without dripping. Significant reductions in toxic gas emissions were also observed, indicating improved smoke suppression efficiency. Mechanical analysis revealed substantial enhancement in tensile performance, confirming improved structural integrity of the composite system. The improved performance is attributed to the combined action of inorganic additives, promoting char formation in the condensed phase and radical quenching in the gas phase. These findings provide valuable insights into the design of high-performance flame-retardant polymer coatings for advanced structural and construction applications.