Mode of action: formation of a solid charred surface layer of phosphorus compounds and in specific cases interruption of the radical chain process in the gas phase.
Phosphorous-containing flame retardants primarily act in the solid phase of the polymer. The flame retardant is transformed into phosphoric acid by thermal degradation, and water is released from the substrate in the solid phase.
A protective layer is developed by the polyphosphoric acid formed and by subsequent charring. The protective layer consists of interpenetrating networks of carbon and phosphorous oxides.
Specific phosphorus flame retardants such as the metal phosphinates may also act in the gas phase by the formation of P and PO radicals interrupting the radical chain mechanism of the cumbustion process.
Important phosphorous-containing flame retardants are:
Resorcinol bis (diphenyl phosphate) (RDP) is mainly used to flame retard PC/ABS blends.
Triaryl phosphates are used for many applications in thermoplastics.
Metal phosphinates are increasingly used in glass fiber reinforced polyamides and polybutylene terephthalate.
9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivates are used in polyester fibers and increasingly in printed circuit boards.
Trischloropropyl phosphate (TCCP) is mainly used in polyurethane flexible foams.
Ammonium polyphosphate (APP) is applied in intumescent formulations, polypropylene and thermosets such as unsaturated polyesters.
Red phosphorous is mostly used in polyamides.