Component Recognition

UL Standard 746B, Standard for Polymeric Materials - Long-Term Property Evaluations, determines the thermal index (TI) of a single material. A material’s TI indicates the maximum temperature at which it will retain its properties over a period of time (“end-of life”).

UL defines “end-of-life” as the point at which a material’s mechanical and/or electrical properties reach 50% of their original value or when the material’s dielectric strength declines to less than 300 V/mil. For new sheet insulation, that time is 100,000 hours (11.4 years).

Since it is not practical to test materials for 100,000 hours, accelerated aging techniques are used. Materials are aged at four elevated temperatures selected to produce an end-of-life in the material in two, three, six and 12 months. The end-of-life data at the four elevated temperatures are plotted and an Arrhenius equation is developed based on the data.

Using this equation, the material’s TI at 100,000 hours can be predicted. The TI is limited by IEEE and UL to selected values, such as 130°C, 155°C, 180°C, 200°C and 220°C. Thus, a material whose end-of-life is projected to be 100,000 hours at 191°C would be assigned a TI of 180°C.

DuPont™ Nomex® brand paper has been assigned a TI of 220°C and is consequently known in the industry as a 220°C material.

Systems Recognition

UL Standard 1446, Standard for Systems of Insulating Materials - General, determines the relative thermal index (RTI) of a combination of materials used in the insulation system of electrical equipment. Systems Recognition demonstrates the compatibility and durability of the combination of insulation materials over time at a specific temperature.

Typically, insulation systems are tested using motorettes. The motorettes are aged at three elevated temperatures, each differing by 20°C. The lowest and highest temperatures are selected to produce end-of-life in the insulation system in at least 5,000 hours and at least 100 hours, respectively.

The aged materials are exposed to vibration, cold shock and moisture. Then, while still saturated with moisture, they are electrically stressed to ensure the integrity of the insulation system. This cycle continues until the system reaches its end-of-life.

A control system with a known life is tested in the same manner as the new system to account for variations in test conditions. The end-of-life data of the new and control systems are plotted and Arrhenius equations are developed for each.

The life projection of the control system, typically 20,000 to 40,000 hours, is used to determine the RTI of the new system with the same life expectancy. The RTI is limited by IEEE and UL to selected values such as 130°C, 155°C, 180°C, 200°C and 220°C.

Thermal aging testing

The best way to compare the thermal performance of sheet insulations is to heat them alone—without resins, varnishes or enamels—and measure the resulting change in properties.

The graph below shows the extent of degradation after aging at 180°C. Nomex® paper and NMN laminates based on Nomex® paper outperform other materials including two DMD laminates that are being promoted as 180°C materials.

Click on the insulation paper types below and drag the slider to the right to see their appearance at various stages of aging. Note: Nomex® paper exhibited virtually no degradation during this exposure time.

Performance when the heat is on

Nomex® paper is UL-recognized in insulation systems with temperature classes as high as 220°C.

Laminates of Nomex® (NMN) are UL-recognized as the major insulation in many systems, including 180°C systems. The use of Nomex® in systems of a lower temperature class than the major component material assures an extra margin of safety for the system.

If you require superior toughness for 180°C and lower thermal classes, laminates of Nomex® are your best choice.