INVESTIGATION OF IGNITION OF THERMOPLASTICS THROUGH THE HOT WIRE IGNITION TEST

Abstract

The purpose of this study was to investigate the ignition phenomena of selected polymeric materials—Plexiglas® (PMMA), polyethylene, Lexan® (polycarbonate), and nylon using the Hot Wire Ignition Test. Underwriters' Laboratories (UL) prescribe this test for polymeric materials used in enclosures or insulation systems of electrical equipment. The main objective of this study was to identify the effect of specimen thickness on the ignition time. In addition, temperature changes at the surface of selected polymeric materials during the different phases of the ignition process were also investigated. The apparatus used for this test was specified by ASTM D 3874–90a—Standard Test Method for Ignition of Materials by Hot Wire Sources. It consisted of a supply circuit capable of maintaining a continuous 60-Hz power density of 0.26 W/mm over 12 in. (304.8 mm) of No. 24 AWG, Nichrome V wire and a variable transformer for adjustment of the voltage to achieve the desired current. Temperature measurements were also performed using a Cole-Parmer Digi Source® 12-channel scanning thermometer and 30 AWG chromel–alumel (type K) thermocouples. A linear relationship between time to ignition and thickness was verified for the four polymers of thicknesses 1/16 in., 1/8 in., 3/16 in., and 1/4 in. The different polymers evaluated in terms of ease of ignition from most ignitable to least ignitable were in the order: Plexiglas, polyethylene, Lexan, and nylon. From the temperature measurements performed in tests with Plexiglas, it appears that the major factor affecting the time to ignition is the rate of temperature rise during the decomposition process. One important recommendation is that temperature measurements similar to those performed with four thicknesses of Plexiglas should be performed with Lexan (polycarbonate), nylon, and polyethylene. These data would confirm the principal role played by the rate of temperature rise during the decomposition process on the time to ignition of thermoplastic materials.