Aspects of the Fire Behavior of Thermoplastic Materials.
Aspects of the Fire Behavior of Thermoplastic Materials.
(5860 K)
Ohlemiller, T. J.; Shields, J. R.
NIST TN 1493; NIST Technical Note 1493; 158 p. January
2008.
Keywords:
thermoplastics; fire behavior; fire growth;
polypropylene; polystyrene; polyurethane foams; test
methods; flammability; heat release rate; upholstered
furniture; flammability tests; viscosity; temperature;
thermogravity; experiments; gasification; kinetics;
flammability testing; furniture calorimeters
Abstract:
Thermoplastic polymers pose unique new challenges
(resulting from the movement of burning melt material)
for the understanding and control of fire growth on
objects that incorporate them. Both full density
thermoplastics such as polypropylene and polystyrene, as
well as low density polyurethane foams are examined in
this study, which has two overall goals: (1) assessment
of potential test methods for quantitatively judging the
flammability hazard of a thermoplastic (focused most
specifically on polyurethane foams) and (2) fostering
the development and validation of a model of fire growth
over generic configurations of thermoplastic materials
suggested by their end product use (particularly
upholstered furniture). One such configuration is that
used in the flammability test method, but early model
development steps emphasize simpler configurations and
materials. A critical aspect of modeling these materials
is an adequate description of the viscosity of the
polymer melt as a function of temperature; the viscosity
can vary by several orders of magnitude. A procedure for
deriving an empirical description of
viscosity for full density thermoplastics, dependent
only on temperature, is given but it requires
extrapolation of melt viscosity out to burning
temperatures. The procedure may be stymied by the
complex behavior of polyurethane foam melts, indicating
a need for further work. Gasification kinetics of the
material, also needed in the fire growth model, are
derived here from thermogravimetry for four full density
thermoplastics. The more complex degradation behavior of
polyurethane foam requires further work to derive these
kinetics. The modeling process has been proceeding in
stages of increasing complexity in conjunction with an
outside contractor. The current stage focuses on
two-dimensional, non-flaming melting plus gasification
at heat fluxes comparable to those seen in fire growth.
The present study has produced data on four full density
thermoplastics and several polyurethane foams in this
configuration which serves to test the developing model.
Among the experimental results is the flux-dependent
fraction of mass lost as melt flow from the heated
sample surface. For polyurethane foams of varied
composition, these results were found to vary over a
wide range. This helped in understanding the relative
fire growth behavior of these same foams when tested as
roughly 30 cm by 60 cm slabs under the National
Institute of Standards and Technology (NIST) Furniture
Calorimeter. Similar scale tests were also done with
thin sheets of polypropylene (the fire growth
configuration which the model will attempt to predict
first); these tests revealed complex flow dynamics in
the melt pool fire and provided clues about the role of
a pool fire in the overall fire growth process. As a
result of the large scale foam tests we have proposed a
tentative test configuration for polyurethane foams that
are intended for use in upholstered chairs and a
validation test series is being planned. The question of
whether this test can be scaled down to use smaller
amounts of foam is addressed; the scaling is difficult
because of a mis-match in transient behaviors.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899