Synthesis of Phosphorus–Sulfur-Containing Polyols for Intrinsic Flame Retardant Flexible Polyurethane Foams with Enhanced Mechanical Properties

The development and preparation of intrinsic flexible polyurethane foam (FPUF) with low-load flame retardancy and high mechanical properties are challenging. Herein, a reactive flame retardant, poly(ethylene methylphosphonothioate) (PEMPT), was synthesized by the polycondensation of methylphosphonothioic dichloride and ethylene glycol. Subsequently, PEMPT was chemically bound to the FPUF chain. When the PEMPT content was 2.5 wt % polyols, the FPUFs exhibited self-extinguishing properties in less than 3 s after removing the igniter and passed the TB 117-2000 vertical burning test. Furthermore, the flame retardant FPUF with only 10 wt % PEMPT loading (FPUF10) showed an oxygen index value of 23.5%. Also, its peak heat release and total heat release rates were reduced by 25.8 and 24.0%, respectively. Concurrently, the incorporation of PEMPT improved the compressive and reversible properties of the foams. These results indicate that PEMPT is a promising flame retardant to endow FPUF with excellent flame retardancy and mechanical properties.