Pentaerythritol derived bicyclic phosphates as halogen-free flame retardants for thermoplastic polyurethane

A series of pentaerythritol derived flame retardant additives (FRs labeled FR1, FR2, FR3, and FR4) were synthesized and blended with thermoplastic polyurethane (TPU) to enhance its flame retardancy. The chemical structures of the FRs were established by FTIR, ¹H, ¹³C and ³¹P NMR spectroscopy. The flammability of the TPU blends was analyzed through UL-94 vertical burning and limiting oxygen index (LOI) test. It is found that for FR4-TPU blends with 2% of phosphorus content, the LOI value reached up to 31.2%, and no melt dripping with a V-0 rating was obtained by UL-94, indicating its superior flame retardant properties. Thermogravimetric analysis (TGA) of TPU blends showed that the char yield increases from 3.9% in neat TPU to 20% with the addition of 2% of FR3 in TPU. Based on findings from scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), a possible mechanism for the flame retardant property of blends has been proposed. The improved flame retardant properties of TPU blends can be attributed to the formation of a continuous and highly entangled char. This work provides a suitable approach for synthesizing promising phosphorus-based flame retardants from readily available raw materials that can be used to enhance the flame retardancy of TPU.