Glow wire ignition temperature (GWIT) and comparative tracking index (CTI) of glass fibre filled engineering polymers, blends and flame retarded formulations

Engineering thermoplastic polymers such as polyamides, polycarbonates, semi-crystalline aromatic polyesters and their blends are widely used as insulating materials in electrical and electronic appliances. Flame retardants are often employed in the formulation of these materials, since good performance in terms of ignition and tracking resistance, evaluated by Glow Wire Tests (GWIT) and Comparative Tracking Index (CTI) are required in these applications. In this paper, a comparative evaluation of GWIT and CTI performances has been simultaneously performed for a wide set of glass fibre filled materials chosen among engineering thermoplastics and their blends. Some flame retarded formulations have been also tested, in order to screen the effects of various additives. Useful indications have been obtained on the effect of each polymer and additives on GWIT and CTI properties. In addition, interesting synergies have been observed, especially by blending polyesters and polyamides. Thermogravimetric measurements of char yields have been successfully related with CTI behaviour. The presence of additives changes the structure of the carbonaceous residue, hence the conductivity of the tracks. Neat polycarbonate passed the GWIT test but not CTI, while poly(butylene terephthalate) showed the best balance of GWIT and CTI performance among the pure resins tested. Blending polycarbonate with polyester did not improve significantly GWIT performance, but had a negative effect on tracking resistance. Polyesters/polyamide blends were dominated by polyester in GWIT, but they showed synergistic effects in CTI.