Richard E. Lyon and Marc L. Janssens

This report provides an overview of polymer flammability from a materials science perspective and describes currently accepted test methods to quantify burning behavior. Simplifying assumptions about the gas and condensed phase processes of flaming combustion provide mathematical relationships between polymer properties, chemical structure, flame resistance, and fire behavior that can be used to design fire-resistant plastics.

Richard E. Lyon Ph.D & David Blake

The heat release rate of objects burning in a relatively large, simply ventilated cargo compartment is reconstructed from the oxygen consumption history of the exiting gas stream, assuming perfect mixing of the combustion gases in the compartment. The model was calibrated using a premixed propane gas burner to generate a variety of well-defined heating histories. Qualitative agreement between actual and computed heat release rate histories is obtained when the duration of the burning is on the order of 1/2 of the mixing time of the compartment. This research supports efforts by the Federal Aviation Administration to develop new certification requirements for aircraft cargo compartment fire detectors.

Michael Burns, William M. Cavage, Robert Morrison, & Steven Summer

Extensive development and analysis has illustrated that fuel tank inerting could, potentially, be cost-effective if air separation modules, based on hollow-fiber membrane technology, could be packaged and used in an efficient way. To illustrate this, the Federal Aviation Administration (FAA) has developed a prototype onboard inert gas generation system that uses aircraft bleed air to generate nitrogen-enriched air (NEA) at varying flows and purities during a commercial airplane flight cycle. A series of ground and flight tests were performed, in conjunction with National Aeronautics and Space Administration (NASA) aircraft operations personnel, designed to evaluate the FAA inerting system used in conjunction with a compartmentalized center wing tank (CWT). Additionally, the flammability of both the CWT and one inboard wing fuel tank was measured. The system was mounted on a Boeing 747, operated by NASA, and used to inert the aircraft CWT during testing. The inerting system, CWT, and the number 2 main wing tanks were instrumented to analyze the system performance, fuel tank inerting, and flammability.

The results of the testing indicated that the FAA prototype inerting system operated as expected. Using a variable-flow methodology allowed a greater amount of NEA to be generated on descent when compared to the simple dual-flow methodology, but it had no measurable effect on the resulting average ullage oxygen concentration after each test, while improving inert gas distribution by decreasing the worst bay oxygen concentration when three similar tests were compared. The highest average ullage oxygen concentration observed on any flight test correlates directly with the worst bay oxygen concentration, illustrating the importance of maintaining a low average ullage oxygen concentration in good inert gas distribution. Oxygen diffusion between the bays of the tank was relatively rapid, and overnight dispersion of the ullage oxygen concentration was measured to be very small. Flammability measurements showed trends very similar to what was expected based on both experimental and computer model data. The equilibrium data agreed favorably with data from both the Fuel Air Ratio Calculator and the Condensation Model, while transient data trends matched closely with the Condensation Model with some discrepancies in total hydrocarbon concentration magnitude at altitude.

Patricia Cahill

This report discusses the flammability tests conducted on aviation and nonaviation electrical wiring that were performed to re-evaluate the effectiveness of the current Federal Aviation Administration (FAA)-mandated 60° Bunsen burner flammability test requirement for aircraft wiring. The evaluation included a 60° flammability test, an intermediate-scale vertical flammability test, and an intermediate-scale cabin attic flammability test. Test results showed that the 60° single wire Bunsen burner flammability test may not be adequate to qualify wire when bundled and subjected to a severe ignition source.

Huiqing Zhang

The intrinsic relationships between polymer structure, composition, and fire behavior have been explored to develop new fire-safe polymeric materials.

Three milligram-scale methods (pyrolysis-combustion flow calorimetry (PCFC), simultaneous thermal analysis, and pyrolysis gas chromatography/mass spectrometry (GC/MS)) have been combined to fully characterize the thermal decomposition and flammability of polymers and polymer composites. Thermal stability, mass loss rate, char yield, and properties of decomposition volatiles were found to be the most important parameters in determining polymer flammability. Most polymers decompose by either an unzipping or a random chain scission mechanism with an endothermic decomposition of 100-900 J/g. Aromatic or heteroaromatic rings, conjugated double or triple bonds, and heteroatoms such as halogens, N, O, S, P, and Si, are the basic structural units for fire-resistant polymers. The flammability of polymers can also be successfully estimated by combining the pyrolysis GC/MS results or chemical structures with the thermogravimetric analysis.

The thermal decomposition and flammability of two groups of inherently fire-resistant polymers—poly(hydroxyamide) (PHA) and its derivatives and bisphenol C (BPC II) polyarylates—have been systematically studied. PHA and most of its derivatives have extremely low heat release rates and very high char yields upon combustion. PHA and its halogen derivatives can completely cyclize into quasi-polybenzoxazole structures at low temperatures. However, the methoxy and phosphate derivatives show a very different behavior during decomposition and combustion. Molecular modeling shows that the formation of an enol intermediate is the rate-determining step in the thermal cyclization of PHA. BPC II-polyarylate is another extremely flame-resistant polymer. It can be used as an efficient flame-retardant agent in copolymers and blends. From the PCFC results, the total heat of combustion of these copolymers or blends changes linearly with composition, but the change of maximum heat release rates also depends on the chemical structure of the components.

The flammability of various polymers and polymer composites measured by PCFC; cone calorimeter, ASTM E1354; and the Ohio State University (OSU) calorimeter, ASTM E906, were also compared. For pure polymers, there was a relatively good correlation between different methods. However, for polymer composites with inert fillers or flame-retardant additives, the OSU and cone calorimetries are more suitable evaluation methods.