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Numerical Model of Bubbling Thermoplastics. (Abstract/Presentation)


pdf icon Numerical Model of Bubbling Thermoplastics. (Abstract/Presentation) (1863 K)
Butler, K. M.

NIST SP 998; May 2003.

Keywords:

fire growth; fire spread; thermoplastics; numerical models; equations

Abstract:

Thermoplastic materials such as polypropylene (PP), polyethylene (PE), and polystyrene (PS) are widely used in household furnishings, and therefore constitute a large fraction of the fuel load during a fire. The combustible gases generated during polymer decomposition contribute directly to the growth and intensity of the fire. In order to accurately predict the development of a fire and when or even whether flashover will take place, therefore, it is critical to determine the rate at which gases are released from these materials. The tendency of thermoplastic materials to bubble during intense heating has a profound effect on this process. The behavior of thermoplastic materials during pyrolysis and combustion is highly complex. As the temperature rises, the molecules in a thermoplastic solid become increasingly mobile, until the material becomes a viscoelastic fluid. Chemical bond breaking reduces the average molecular weight of the polymer, further reducing the viscosity. Eventually, the polymer fragments are small enough to constitute gas molecules. The gas does not escape instantaneously but diffuses through the surrounding material, collecting within any bubbles that have nucleated within the polymeric melt. The bubbles transport the volatile gases at a rate that is dictated by bubble dynamics. Upon reaching the surface, the bubbles may not release their gases immediately but instead develop a thin film that takes time to drain and rupture. The result is an insulating layer that reduces the transport of heat into the interior, thus slowing the gasification. As they burst, the bubbles may expose a larger region near the surface to chemical attack from the surrounding gases (such as oxygen) due to entrainment and, for highly viscous melts, to the distortion of the surface geometry.