Numerical Model of Bubbling Thermoplastics. (Abstract/Presentation)
Numerical Model of Bubbling Thermoplastics.
Butler, K. M.
NIST SP 998; May 2003.
fire growth; fire spread; thermoplastics; numerical
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.