Heat resistance is divided up into short-term heat resistance, i.e. the ability of the material to withstand peak temperatures, and long-term heat aging. The latter indicates the ability of the material to withstand loss of properties as a result of thermo-oxidative degradation.

Heat Deflection temperature
The Heat Deflection Temperature Under Load (HDTUL) gives an indication if the materials ability to withstand peak temperatures under load. It is measured by putting a flexural load of 1.8 or 0.45 MPa on a test bar, and than heating up the bar at a controlled rate of temperature increase (2°C per minute). The HDTUL is defined as the temperature where the deflection exceeds a specified value (about 0.3 mm).

Reltive Temperature Index
The Relative Temperture Index (RTI) as defined in UL 746 gives an indication of the resistance of a material to long-term heat aging. The RTI is given for three categories: electrical, mechanical with impact and mechanical without impact. The RTI is defined as the temperature at which, after an exposure of 60,000 hours, a property has dropped of to 50% of its original value. For the electrical listing, usually the di-electric strength is chosen as the test parameter, for mechanical with impact usually the Izod impact strength or the tensile impact test, and for mechanical without impact usually the tensile strength is taken as the reference. The RTI depends on the wall thickness; lower wall thickness reduces the RTI.
 
To ensure that the results are not influenced by temperature fluctuations of the oven, a reference material with known RTI values is tested alongside the trial material. The RTI is measured relative to the reference material, hence the name Relative Temperature Index. RTI listing is extremely expensive, and is not carried out for most materials. On the UL yellow cards, many materials carry only the intrinsic performance listing of a base polymer. In the table below both the actual listed value of our products is mentioned as well as an estimated real value.