The mechanism of fire retardancy by phosphonated polystyrene

Rates of volatilisation and styrene evolution from isothermal degradations (330–360 °C) of radical polystyrene (RPS), telechelic chloromethoxyphosphonated polystyrene (TPPS) and their mixtures were measured by a coupled thermogravimetric (t.g.) gas-liquid chromatographic (g.l.c.) technique, and degradation products analysed.

Phosphonated groups substituted at chain ends (TPPS), strongly reduce the rate of degradation of the polymer whereas the same groups are inactive when substituted on aromatic rings.

The rate of volatilisation (T > 350 °C) of RPS is reduced by addition of TPPS (> 20%). At increasing temperature and amount of additive, the mixtures tend to behave like the pure additive.

Analysis of degradation products shows that during volatilisation of TPPS and its mixtures with RPS, phosphorus tends to remain in the residue while chlorine is completely volatilised.

The oxygen index (O.I.) of mixtures of TPPS (> 20%) and RPS increases regularly with the amount of TPPS in a similar way in O₂/N₂ and N₂O/N₂ atmospheres, showing that the flame retardancy mechanism of PPS is chiefly a solid phase action. This is confirmed by a correlation found between O.I. and the rate of volatilisation or styrene evolution from the polymers and mixtures investigated.