Meaning
Long-term thermal stability of electrical and mechanical plastics is evaluated using a standardized indexing system established by Underwriters Laboratories. This rating is designated as the UL 746b RTI and it represents the maximum continuous use temperature at which the material will retain half of its critical properties over a long period. The index provides a baseline for selecting safe polymers for high-temperature applications.
It does not measure the short-term melting point or heat deflection temperature of the resin.
Rating Definition
The value represents the temperature where a polymer can operate continuously for one hundred thousand hours before losing half of its strength. A high UL 746b RTI means the plastic is highly resistant to thermal oxidation and can be used in demanding environments like automotive engine bays.
Testing Procedure
Determining this value involves placing multiple batches of polymer specimens in ovens at different elevated temperatures to accelerate their thermal degradation. Technicians periodically remove specimens over several months to test their tensile strength, impact resistance, and dielectric properties. The rate of property decay is plotted on a logarithmic scale and extrapolated using the Arrhenius equation to calculate the temperature at which the material will reach the half-strength threshold at the target service duration.
Material Sourcing
Sourcing resin with the correct thermal rating is necessary to obtain safety certifications for electrical appliances. Moulding companies must verify that the raw material has a valid listing on the yellow card database provided by the testing laboratory. Using regrind can degrade the polymer and compromise this rating, so moulders must carefully control the proportion of recycled material in certified production runs to avoid failing a safety audit.
