Correlation of low voltage tracking behaviour with chemical structure

Thermogravimetric analysis (TGA) can give valuable information regarding thermal degradation of polymers and has been described as giving a qualitative guide to tracking properties: non tracking polymers leave little or no residue when subjected to TGA in a nitrogen atmosphere, whereas polymers which track produce a conductive char in nitrogen, which slowly volatilises above 700°C if the atmosphere is changed to air. As part of a long term programme to develop new high performance electrical insulation systems, the authors have investigated the use of the TGA technique and developed an empirical expression which quantitatively relates tracking behaviour as determined by the comparative tracking index (CTI) method of ASTM D3638 to char index/char residue as determined by TGA. The char index may in turn be related to the chemical structure of the polymer repeat unit by use of the Van Krevelen technique (1975) which assigns chemical group contributions to the overall char residue