Synthesis of Three Novel Intumescent Flame Retardants Having Azomethine Linkages and Their Applications in EVA Copolymer

Three novel intumescent flame retardants (IFRs), polyphosphate esters (designated as PDP, PEP, and PPP), were synthesized by reacting phenyl dichlorophosphate with three bis-hydroxy azomethine monomers through interfacial polycondensation. The polyphosphate esters were characterized by hydrogen nuclear magnetic resonance (H-1 NMR), Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). Thermal stability and flammability properties of ethylene vinyl acetate copolymer (EVA)/30%IFRs blends were investigated by thermogravimetric analysis (TGA), limited oxygen index (LOI), and microscale combustion calorimeter (MCC). The EVA/IFRs blends exhibited high thermal stabilities, and char yields at 600 degrees C were 11-19% in nitrogen. The results from MCC indicated that the addition of PPP to EVA reduced the PHRR by about 33%, while the LOI values increased from 19 to 22. Scanning electron microscope (SEM) observation results indicated that the existence of the tight charred layer on the surface of the residues was responsible for the improvement of the flame retardancy of EVA.