Flame spread behaviour of Polydimethylsiloxane (PDMS) membranes in 1 g and μg environments

Diffusion flame behaviour and silica ash (SiO2 ) production were experimentally studied for various Poly-
dimethylsiloxane (PDMS) membrane thicknesses (0.125 mm to 1.0 mm) in normal gravity and during mi-
crogravity flight experiments. The flames were established on vertical samples (300 mm in length) and
subjected to either opposed or concurrent forced flows (both laminar and turbulent), assimilating the
NASA Test 1 that is in use for spacecraft material selection. The opposed flame spread rate was observed
to be steady and could be estimated using classical theory. Under concurrent flow, the flame spread rate
was only steady for very high forced flows. The opposed flame-spread rate ranged from 0.5 to 1.5 mm/s,
while the concurrent case ranged between 0.1 and 12 mm/s. The transport of silica ash (SiO2) was found
to affect the heat balance of the concurrent flame spread in a manner that resulted in unsteady flame
spread. For opposed flame spread, on the other hand, the transport of silica ash showed to be irrele-
vant. The extinction behaviour for the concurrent flame spread was heavily dominated by the transport
of silica-ash, while for opposed flames, extinction was due to kinetics (at high forced flows). In micro-
gravity environments, the transport and deposition of silica ash is anticipated to dominate flame spread
and near-limit as well. These results suggest that silica-based products might be less flammable in mi-
crogravity than other similar materials such as common thermoplastics (PP or PE) used as wire jackets.