Plastics Technology Laboratories  

Thermal Index

UL 746B

Click on the picture for a larger view

A Thermal Index of a material is an indication of that material's ability to retain a particular physical property above a certain level after exposure to elevated temperatures and/or extended periods of time. The Thermal Index value can be thought of as the maximum temperature below which the material will retain sufficient properties (typically 50% of the original value) for the intended application over a reasonable product lifetime. Thermal Index is an important property for materials that will be used in applications that must withstand significant spans of time at elevated temperatures - for example, outdoor applications and electronic components and housings. There can be more than one Thermal Index for a material, each based on a different physical property.

Test Procedure:
Critical physical properties, and either multiple temperatures or multiple times (a minimum of 4 temperatures or times) must first be defined to satisfy the intended purpose of the testing. A control material, with demonstrated end use performance, may be used to enhance the reliability of the data analysis. The test samples are aged in hot air recirculating ovens. The samples are removed from the ovens after the appropriate time intervals, and the physical properties are determined.

Specimen size:
Specimens for testing are dependant upon the physical property or properties that are the test criteria. More common physical properties include tensile strength; flexural strength; tensile, izod or charpy impact; dielectric strength, and flammability.

UL 746B suggests several methods for extrapolating the discrete data generated in testing to the appropriate time that defines the thermal index temperature, but, in general, kinetics equations are employed to predict property maintenance over extended time at end use temperatures.

Equipment Used:
Aging ovens
Physical property test equipment and fixtures

Intertek PTL Overview
Contact Us
Services & Tech Info
Industry Segments
Industry Segments


**Please note that this test description is intentionally generic in nature and aimed at providing a descriptive summary to enhance test understanding. For more information please contact a Intertek PTL Technical Representative at . Due to copyright restrictions, we are not able to provide copies of standards. Standards can be obtained from appropriate standards authorities.

Back to Testlopedia