| Abstract: | The gasification behavior for a wide range of polydimethylsiloxane fluids in a nitrogen atmosphere was investigated. Part 1 of this study addressed the measurement of the energy (global heat of gasification) required for the gasification of a wide range of dimenthylsiloxanes. Several significant corrections were required to reconcile measured gasification energy(s) with calculated heat(s) of gasification based on fundamental thermochemical data. The identification of the dominant mode(s) of gasification via the characterization of pyrolysis products provided a firm basis and rationale for understanding and directing efforts at quantifying these correction factors. In Part 2, the gasification products were identified and quantified at various stages of the gasification process corresponding to ignition, fire growth, and steady-state burning. Pyrolysis of methylated siloxanes occurs via two modes: (1) the volatilization of short chain and intermediate chain length species native to the polymer, and (2) the volatilization of short chain and intermediate chain length species resulting from thermal degradation via siloxane rearrangement. The former process is the dominant gasification mechanism for short chain oligomers and low viscosity fluids (η<10 cS) and the latter process is dominant in all higher molecular weight polymers (η>100 cS). Both gasification mechanisms are evident in all polymers (η>20 cS); the dominant mechanism is dependent upon polymer size and distribution thereof, the gasification stage, and the presence of trace catalysts in the polymer. Because of their structural similarity, the combustion of all gasification products emanating from PDMS regardless of the stage of the pyrolysis process or the dominant mode of gasification will result in virtually identical combustion products, i.e. SiO2, CO2, and H2O. Copyright © 1998 John Wiley & Sons, Ltd. This paper was written under the auspices of the US Government and is therefore not subject to copyright in the US. |
