Synthesis, characterization of a DOPO-based polymeric flame retardant and its application in polyethylene terephthalate

A high molecular weight DOPO-based flame retardant (DP-DE) containing phosphorus linked pendent groups was synthesized by a two-step esterification and polycondensation reaction, and characterized by Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance (13C NMR) and gel permeation chromatography (GPC). After added into polyethylene terephthalate (PET) via melt-blending method, the flame retardancy of DP-DE were monitored by thermo-gravimetric analysis (TGA), limiting oxygen index (LOI) and underwriter laboratories (UL-94) test. The results showed that DP-DE display good flame retardancy in PET. And the thermal stability, char residue yield and LOI values of the composites increased with the increase of DP-DE content. The flame retarded mechanism of DP-DE was investigated by pyrolysis-gas chromatogram-mass (PY-GC-MS), in situ FTIR, scanning electron microscope (SEM) and laser Raman spectroscopy (LRS). DP-DE was evidenced to release phosphorus acid and its derivatives, which catalyze carbonization and produce compact carbonaceous layers. Through protecting the polymeric materials substrate from further degradation and inhibiting the pyrolysis degree, both the amount and thermal stability of carbonaceous layers of DP-DE were improved, and hence enhancing the flame retarded performance of PET/DP-DE composites.