Study of Glow Wire Ignition Temperature (GWIT) and Comparative Tracking Index (CTI) performances of engineering thermoplastics and correlation with material properties

Recent regulation IEC 60335-1 ed.4 (2008) was introduced for materials used in electric appliance, establishing new limits in glow wire ignition temperature (GWIT) performance for materials used for electric connectors. Development of new products with high GWIT is possible, but the main issue is to keep good mechanical properties and processability, as well as tracking resistance (CTI). Only few patents and no scientific publication exist about glow wire test performance of polymers. In this work we report GWIT and CTI properties for three engineering thermoplastic polymers (PBT, PET and PC). We have also studied the phenomena involved in this test, treating the phenomena with the parameterization approach already used in the studies of the fire behaviours of polymers. PC, PBT and PET filled with 30% w/w glass fibres have been tested, and material properties that can be related to GWIT and CTI performance have been measured by TGA, Laser flash, Py-GC/MS. CTI seems to be predictable with the char formation tendency of the materials, so PBT show a higher tracking resistance than PET and PC. Polycarbonate is the only that pass the test glow wire (GWIT higher than 775°C) but generally GWIT performance is not directly related with degradation temperature, since PET is more thermal stable compared with PBT, but less stable in glow wire test. The ignition process, together with the unsteady heat and mass transfer process characteristic of glow wire test, are affected by many parameters at the same time. That’s why it is not easy to relate results of TGA, Laser flash, Py-GC/MS with the glow wire ignition temperature of the materials tested, but the whole of these properties can give useful indication