Researchers tested a sulphur polyphosphonate PIN flame retardant in PET achieving UL 94 V-0 (3.2 mm) at 8% loading but with dripping. The polysulfonyldiphenylene phenylphosphonate (PSPPP, a polyphosphonate) was synthesised by reacting phenylphosphonic dichloride with a sulfonyldiphenol (bisphenol S – BPS), then catalysing polymerisation. An 8% loading of PSPPP in PET (polyethylene terephthalate) increased LOI from 20.5% to 29.7% and improved UL 94 rating (3.2 mm) from V-2 to V-0 (no ignition, but with dripping). Inclusion of PSPPP reduced smoke release and increased char. The fire protection effect is considered to be a combination of char formation, transesterification reactions between PSPPP and PET increasing char barrier effect and gas phase actions of phosphorus radicals. It is not clear what if any contribution to fire performance was brought by the sulphur.
“A Sulfur-Containing Polyphosphonate Flame Retardant for Polyethylene Terephthalate”, X. Bo et al., Combustion Science & Technology, 2022, https://doi.org/10.1080/00102202.2022.2109153 Chemical diagram of BPS: Wikipedia.