Phosphorus PIN flame retardants cover a wide range of inorganic and organic compounds and include both reactive products which are chemically bound into polymers or onto natural fibres as well as additive products which are integrated into the material by physical mixing only.

They act mainly by formation of a char layer at the material surface (solid phase), stopping the contact between fuel and air. Combined with intumescence, foam-like char also provides a thermal protection effect. Phosphorus flame retardants can also have some gas phase effect (phosphorus radicals)

Char formation results in generally less smoke, because there is no gas phase forced incomplete combustion and because release of volatile compounds is limited by the barrier effect. Phosphorus flame retardants generate considerably less acidic and corrosive gases than halogenated flame retardants.

Phosphorus (P) PIN FRs include a wide range of inorganic and organic compounds. These can have different phosphorus oxidation states (from –3 to +3). They can be reactive, additive or polymeric. Some P-FRs bring other functions in addition to fire safety (e.g. plasticisers).

Widely used P-FRs include inorganic (poly)phosphates, (poly)phosphonates, phosphinates, phosphate esters, DOPO* derivates, and red phosphorus for example.

Naturally occurring phosphorus-rich compounds, such as phytate (from plant seeds) can provide phosphorus PIN FRs, but this is today at the R&D stage because of challenges around quality, supply and cost.

Phosphorus-containing flame retardants are widely used in standard and engineering plastics, polyurethane foams, thermosets, coatings, textiles (for both synthetic fibres and natural fibres) and in timber products.