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Abstract Summary: The flame retardant mechanisms of red phosphorus, magnesium hydroxide and red phosphorus combined with magnesium hydroxide were studied in high impact polystyrene by means of comprehensive decomposition studies and combustion tests. The study is intended to illuminate prerequisites and the potential of red phosphorus as a fire retardant for hydrocarbon polymers in the condensed phase and in the gas phase. Thermal and thermo‐oxidative decomposition, decomposition kinetics and the product gases evolved were characterized using thermogravimetry coupled with Fourier transform infrared spectroscopy and mass spectroscopy, respectively. Fire behaviour was investigated with a cone calorimeter using different external heat fluxes, whereas the flammability was determined by limited oxygen indices. The combustion residues were analysed using XPS. Red phosphorus reduced the heat release in HIPS due to radical trapping in the gas phase. Magnesium hydroxide influenced fire behaviour by heat sink mechanisms, release of water and the formation of a magnesia layer acting as a barrier. The combination of both flame retardants in HIPS nearly resulted in a superposition. A slight synergy in barrier characteristics was due to the formation of magnesium phosphate, whereas a slight anti‐synergism occurred in flammability and in the gas phase action. The latter effect is controlled by a decreased fuel rate due to the barrier layer rather than by an initiation of red phosphorus oxidation in the condensed phase. Heat release rate and total heat release at various external heat fluxes for HIPS (dotted = 70 kW · m −2 , dashed = 50 kW · m −2 , solid = 30 kW · m −2 ). magnified image Heat release rate and total heat release at various external heat fluxes for HIPS (dotted = 70 kW · m −2 , dashed = 50 kW · m −2 , solid = 30 kW · m −2 ).
Braun et al. (Tue,) studied this question.