A Sulfur-Containing Polyphosphonate Flame Retardant for Polyethylene Terephthalate

A sulfur-containing polyphosphonate compound, polysulfonyldiphenylene phenylphosphonate (PSPPP), was prepared based on diphenylphosphine and bisphenol S, and applied to flame retarded polyethylene terephthalate (PET). The flame retardancy and thermal stability of PSPPP/PET composites were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter and thermogravimetric analysis (TGA). LOI value increased from 20.5% for pure PET to 29.2% for flame retardant PET with 8 wt.% PSPPP, and 8%PSPPP/PET passed UL-94 V-0 rating. The cone calorimeter test displayed that PSPPP not only inhibited the maximum combustion strength of PET composites but also suppressed the smoke release. TGA results indicated that PSPPP significantly enhanced the char yield of composites and the thermal stability of residues at higher temperature. The flame retardant modes of action of PSPPP were confirmed by experiments using TGA-Fourier transform infrared (TGA-FTIR), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), Hot-Stage FTIR, and scanning electron microscope (SEM). PSPPP played a role of flame retardant in condensed phase and gas phase. During combustion, PSPPP continuously released phosphorus-oxygen-free radicals with quenching effect, thus playing the gas phase flame retardant effect. On the other hand, the transesterification reactions between PSPPP and PET promoted the formation of dense carbon layer and led to high barrier effect.