Thermal Decomposition Temperature (Td): This property varies greatly with the chemical composition of materials, from the mid 300ºC range for many epoxy systems to over 400ºC for some polyimides. Td is the temperature at which a material begins to degrade thermally. Some data sheets will list a Td as the temperature at which a material has lost 5% of its original weight due to decomposition. A better indicator of performance would be the onset temperature, at which significant weight loss begins to occur. By the time a material, has lost 5% of its weight to decomposition it may well be unusable in many applications.