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PEEK polymer flammability and the inadequacy of the UL-94 classification


Terrence Richard Hull, Centre for Fire and Hazards Science, School of Forensic and Investigative Science, University of Central Lancashire, Preston, PR1 2HE, U.K.



Fire safety is dependent on reliable information on material properties, particularly relating to burning behaviour. The Underwriters Laboratory UL-94 test is a widely used simple Bunsen burner test for vertically upward flame spread. Aryl polyetheretherketones (PEEK) are polymers of exceptional thermal stability, typically decomposing at around 600°C and forming 50% carbonaceous char residue. Tests on seven PEEK polymers, and two related materials, in independent laboratories have revealed large inconsistencies in both the final broad classification and the scatter within each set of test results. In many cases, this variance is so large that if samples from the same batch of many of the materials were repeatedly submitted to test laboratories, this would ultimately result in one set remaining below the maximum burn time criteria, and so meeting the least flammable V-0 rating. Initial data are presented indicating that a larger ignition source actually results in shorter burning times and more consistent burning behaviour. The reported behaviour of PEEK indicates that the inconsistencies reported here are not a function of inconsistencies in the material itself but rather a consequence of the low applied heat flux of the test method being very close to the critical heat flux for ignition of the PEEK polymer, which is rather high. With higher applied heat fluxes, this generates sufficient heat for a protective char to form, creating an effective barrier to further flame spread. Copyright © 2011 John Wiley & Sons, Ltd.