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Gasification of Silicone Fluids Under External Thermal Radiation. Part 1. Gasification Rate and Global Heat of Gasification.


pdf icon Gasification of Silicone Fluids Under External Thermal Radiation. Part 1. Gasification Rate and Global Heat of Gasification. (1612 K)
Austin, P. J.; Buch, R. R.; Kashiwagi, T.

Fire and Materials, Vol. 22, 221-237, 1998.

Keywords:

silicones; gasification; thermal radiation; degradation products; heat of vaporization; thermal degradation

Abstract:

Transient gasification rates and fluid temperatures were measured for polydimethylsiloxane fluids ranging in viscosity from 0.65 cS to 60 000 cS in a nitrogen atmosphere at external radiant fluxes from 20 kW/m2 to 70 kW/m2. A detailed energy balance for each fluid sample was conducted to determine its global heat of vaporization. Two major energy loss corrections were identified and quantified. The absorption of incident radiation by the volatile products from short chain oligomers was measured and found to substantially reduce the incident flux to the sample surface; the energy loss due to re-radiation was determined to be a substantial factor in reducing the net heat flux to the sample for long chain length fluids. Other energy losses, e.g. heat loss to the substrate, were observed but were less significant. The average gasification rate for each fluid increased linearly with increasing external radiant flux. The global heat of gasification increases with an increase in the chain length (molecular weight) for the siloxane oligomers. These agreed well with calculated values. The global heat of gasification for 50 cS fluid is about 1200 kJ/kg and its value remains nearly constant for all higher molecular weight dimethylsiloxanes. Pyrolysis rates for siloxane fluids are very sensitive to trace catalysts. Measurements of the global heat of gasification for ultra-clean polymers resulted in significantly higher values (3000 kJ/kg). The gasification of siloxanes occurs via two modes or combinations thereof: (1) volatilization of molecular species native to the polymer, and (2) volatilization of thermal degradation products. The former process dominates for low molecular weight siloxanes and the latter process dominates for high molecular weight siloxanes. For the intermediate molecular weight siloxanes, both volatilization and degradation processes occur.