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Bunsen Burner Test UL 94

The UL 94 (Underwriters’ Laboratory) tests use a small (~15 mm) Bunsen burner type flame to ignite the endmost inch (25 mm) of a 6 in. (150 mm) strip of plastic material, either horizontally oriented (for the lowest HB classification) or vertically oriented, for the higher V-2, V-1, and the highest V-0 classifications). If flaming travels to the other end of the test strip, a failure is recorded. In the vertical test, if flaming extinguishes within 10 s, the V-0 classification is achieved, if it continues for 30 s, V-1 is achieved, unless there are flaming drips, which lower the classification to V-2.

Although widely regarded as the simplest of flammability tests, the UL 94 horizontal and vertical tests involve several interacting physical processes which are inadequately reflected in the final classification of HB, V-2, V-1, and V-0. As the flame propagation is either horizontal or vertically upward, flame dilution, and hence flame dilation is likely to have a smaller effect than energy absorption through endothermic decomposition or solid phase heat capacity, since a greater portion of the heat of combustion is fed back to the polymer, than in the LOI test. Unlike the LOI, dripping can be disadvantageous in the UL 94 test; flaming drips limit the classification to V-2 at best. Solid phase fillers and residues will almost always reduce dripping. Endothermic decomposition with release of water or carbon dioxide must coincide with fuel release to be effective in flammability reduction - in the UL 94 test, once the first flame is extinguished, it is followed by the second 10 s application - after which the endothermic water loss is exhausted, and the heat capacity of the protective residue may make the most important contribution.

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