Flame-retardant action of polysulfonyldiphenylene phenylphosphonate(PSPPP) on polyethylene terephthalate (PET) was investigated employing X-rayphotoelectron spectroscopy(XPS or ESCA)

thermogravimetry and Fourier transform infrared spectroscopy techniques. The flame retardant PSPPP widens the temperature range of PET decomposition

and reduces the amount of combustible volatile products within the range of combustible temperature of PET. PSPPP can accelerate thedegradation of PET below and near the ignition temperature TI of PET

which is favorable for melt dripping so as to enhance the loss of heat and material from the surface of the burning PET.Some degraded products of PET/PSPPP flame retardant system are charred above TI of PET so as to limit to a certain extent heat and mass transfer across the phase boundary by creation of an insulating charred layer on the surface of the burning PET.