CO2 laser-induced pulsating regression behavior of GAP at sub-atmospheric pressures

Linear pyrolysis of glycidyl azide polymer (GAP) by CO2 laser heating was s tudied experimentally in argon, nitrogen or oxygen at subatmospheric pressu res with laser heat fluxes of 1 to 20 W/cm(2). Surface temperature variatio ns were measured by fine chromel-alumel thermocouples pressed onto the samp le surfacer and pulsating regression caused by an abrupt exothermic decompo sition was observed. The behavior is very similar to the chuffing phenomeno n. The momentary exothermic decomposition reaction was accompanied by the e volution of N-2 resulting mainly from breaking of the azide bond. The surfa ce temperature in one cycle is characterized by three critical temperatures : liquefying (T-1), abrupt thermal decomposition threshold (T-ad), and maxi mum surface (T-ms) temperatures. Maximum surface temperatures are 700 to 80 0 K, which are equivalent to the surface temperature observed during self-s ustained combustion controlled chiefly by exothermic reaction at the decomp osition surface. As the surface heat flux increases, the period of the puls ating regression decreases, being only slightly dependent on the ambient ga s. If the period becomes shorter than the characteristic time of GAP, then the pulsating regression will transition into steady regression assisted by laser irradiation. SEM photographs of the quenched surfaces reveal the pre sence of a liquid layer with a thickness of 7 to 8 mum, about 1/30 of the m omentary regression distance. at the GAP surface just before the abrupt the rmal decomposition. Spatial temperature profiles at various critical times were established by using two thermocouples embedded inside the GAP sample. Phenomenological modeling is attempted to explain the pulsating phenomena for GAP pyrolysis. From the phenomenological modeling. a subsurface layer w ith a relatively small temperature gradient is thought to exist at the GAP surface. In view of SEM photographs of quenched surfaces, the surface layer of the whole momentary regression distance Delta1 is thought to change int o a liquid layer as soon as the abrupt thermal decomposition starts. One of the causes of nonself-sustained combustion of GAP in inert gases of a few atmospheres would be pulsating decomposition. At 66.7 kPa of oxygen gas the pulsating decomposition very often transitions into sustained combustion. (C) 2001 by The Combustion Institute.