There are plastics that have different behaviors to smoke, going from no smoke to large amounts of smoke. Whether a plastic gives off light or heavy smoke and toxic or noxious gases depends on the plastic used, its composition of additives and fillers, and the conditions under which its burning occurs. Some plastics burn with a relatively clean flame, but some may give off dense smoke while smoldering.

Smoke is recognizcd by firefighters as being in many ways more dangerous than actual flames. It obscures vision, making it impossible to find safe means of egress, thus often leading to panic and not being able to rescue victims. Smoke from plastics, wood, and other materials usually contains toxic gases such as carbon monoxide (CO), which has no odor, often accompanied by noxious gases that may lead to nausea and other debilitating effects as well as panic, warning the fire victim of danger. With only CO the victim would die whereas the start of a fire with noxious gases could alert a person that a fire has started and to leave the area.

One of the most stringent and most widely accepted tests is UL 94 that concerns electrical devices. This test, which involves burning a specimen, is the one used for most flame-retardant plastics. In this test the best rating is UL 94 V-0, which identifies a flame with a duration of 0 to 5 s, an afterglow of 0 to 25 s, and the presence of no flaming drips to ignite a sample of dry, absorbent cotton located below the specimen. The ratings go from V-0, V- 1, V-2, and V-5 to HB, based on specific specimen thicknesses. Details on fire testing and evaluating plastics are provided by UL who have extensive history on the effect and evaluation of fire as it effects plastics and other materials.

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