Effect of phosphorous-modified silica on the flame retardancy of polybutylene terephthalate based nanocomposites

The effect of silica treatment by three phosphorous compounds (phosphoric acid, dihydrogen ammonium phosphate and diammonium phosphate) on the thermal and fire behaviour of polybutylene terephthalate (PBT) composites was investigated using Thermogravimetric analysis (TGA), Pyrolysis Combustion Flow Calorimeter (PCFC) and cone calorimeter. It was found that with only 10 wt% of treated silica incorporated into PBT, which corresponds to only 0.2 wt% of phosphorus in composites, the fire behaviour of materials was dramatically improved with a 60% decrease of peak of Heat Release Rate (pHRR). The incorporation of modified silica also leads to a fire growth rate index of 4.3 kW/(m2.s) (i.e. a 40% decrease). Those phenomena were related to the expansion of composites during their combustion and to their rheological behaviour. Indeed, the higher the viscosity at low frequencies is, the better the fire behaviour is, due to the induced barrier effect. A decrease of time to ignition (TTI) was also observed with the addition of the various silicas (up to 18s with 10 wt% of neat silica), which was explained by the appearance of bubbles after the PBT melting during cone calorimeter tests leading to a fast increase of the surface temperature.