THE ROLE OF OXYGEN IN THE IGNITION OF POLYSTYRENE BY A SMALL FLAME

The ignition of slabs of high-impact polystyrene by a lean hydrogen-oxygen flat flame was studied. The delays and initial rates of flame development after ignition are reported as functions of gas temperature and the separation between flame and polymer surface. The delays follow an Arrhenius-type expression with activation energy of 98 plus or minus 18 kJ mol** minus **1. The rates of flame development drop as the gas temperature increases. During long ignition delays the apparent heat transfer coefficient at the sample surface dropped from about 100 W m** minus **2 K** minus **1 to values close to that expected for a hot gas impinging at right angles on a cold surface. For short delays it was higher and more constant at about 100 W m** minus **2 K** minus **1. Although the surface temperature reached before ignition exceeded that required for nonoxidative pyrolysis, the polymer surface charred only when oxygen was present. It is concluded that both oxidative and nonoxidative pyrolysis contribute to the ignition of polystyrene.