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In a fire, the thermal and chemical environment is changing constantly. Therefore, it is difficult to simulate the conditions experienced by a material in a fire situation. The four commonly accepted variables are flammability, ignitability, smoke and toxic gas emission. The chemical structure of the PEEK polymer is highly stable and requires no flame retardant additives to achieve low flammability and ignitability values. The composition and inherent purity of PEEK polymer results in excellent smoke and toxicity performance.
 Flammability
The flammability of a material may be defined as the ability to sustain a flame upon ignition from a high energy source in a mixture of oxygen and nitrogen. The recognized standard for the measurement of flammability is the Underwriters Laboratory test UL94. This involves the ignition of a vertical specimen of defined geometry and measures the time for the material to self-extinguish. The average time from a repeated ignition sequence is used to classify the material. Natural 450G has been rated as V-0 (0.057 inch thickness) which is the best possible rating for flame retardancy.
Ignitability
The ignitability of a material may be considered in terms of the minimum concentration of oxygen which will just sustain a flame ignited from a high energy source (ASTM D2863-95). A comparative bar chart of the limiting oxygen indices for a range of engineering polymers is shown in Figure 22.
Figure 22 - Limiting Oxygen Index for a Range of Engineering Polymers
The data charted in Figure 22 show that natural PEEK™ polymer has a similar limiting oxygen index (35%) to other high performance materials.
Smoke Emission
The current standard for the measurement of smoke produced by the combustion of plastic materials is ASTM E662-95. This uses the National Bureau of Standards (NBS) smoke chamber to measure the obscuration of visible light by smoke generated from the combustion of a standard geometry sample in units of specific optical density. The test may be carried out with either continuous ignition (flaming) or interrupted ignition (non-flaming). A comparative bar chart of the specific optical density for a range of engineering plastics is shown in Figure 23.
Figure 23 - Specific Optical Density for a Range of Engineering Thermoplastics Measured in Flaming Mode for 0.126 Inch Thick Samples
The data in Figure 23 show that natural PEEK polymer has the lowest value of specific optical density of all the materials tested.
Toxic Gas Emission
The emission of toxic gases during combustion of a polymer cannot be considered purely as a function of the material. The component geometry, heat release, conditions of the fire, and the synergistic effects of any toxic gases affect the potential hazard of the material in an actual fire situation. PEEK polymer, like many organic materials, produces mainly carbon dioxide and carbon monoxide upon pyrolysis. The extremely low concentrations of toxic gases emitted have been evaluated using the UK Ministry Of
Defense test standard MOD NES 713. This procedure involves the complete combustion of a 0.22 lb. sample in a 35.3 cubic feet volume and subsequent analysis of the toxic gases evolved. The toxicity index is defined as the summation of the concentration of gases present normalized against the fatal human dose for a 30 minute exposure. PEEK polymer 450G gives a 0.22 index with no acid gases detected.
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