Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Mixed Layer Model for Pyrolysis of Bubbling Thermoplastic Materials (NISTIR 6242)



Kathryn M. Butler


For many thermoplastic materials exposed to a strong heat flux from above, a layer of bubbles forms in the melted region near the upper surface. The action of these bubbles is observed to range from slow growth and bursting to vigorous boiling behavior. Although the bubbles appear to have a significant effect on the macroscopic thermal and mechanical properties of the material, the mechanisms by which a bubbling layer affects heat and mass transfer are not well understood. Since the thermal conductivity of a gas is much lower than the thermal conductivity of a liquid, we might expect the transport of heat to be slowed considerably in the presence of bubbles. Alternatively, vigorous boiling behavior could be mixing the upper layer of melted thermoplastic material, resulting in a nearly uniform temperature throughout the bubling layer. How would these seemingly contradictory mechanisms affect heat and mass transfer, and which dominates behavior during pryolysis?
NIST Interagency/Internal Report (NISTIR) - 6242
Report Number


fire research, fire science, fire suppression, thermoplastics, pyrolysis, heat flux, bubbles


Butler, K. (1998), Mixed Layer Model for Pyrolysis of Bubbling Thermoplastic Materials (NISTIR 6242), NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed June 23, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created October 1, 1998, Updated November 10, 2018