Gasification of Silicone Fluids Under External Thermal Radiation. Part 1. Gasification Rate and Global Heat of Gasification.
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899