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Aspects of the Fire Behavior of Thermoplastic Materials.


pdf icon 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.