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RTI


Relative temperature index (RTI)


One of the most important tests to determine a given plastic’s fitness for long-term ambient heat is the UL Standard 746B, Polymeric Materials, Long-Term Property Evaluation. As a data point, it’s shortened to Relative Thermal Index (RTI). Underwriters Laboratories Inc. (UL) developed RTI to test the deterioration of insulating materials in electrical devices over time. Maximum service temperature for a material where a class of critical property will not be unacceptably compromised through chemical thermal degradation. This spans over the reasonable life of an electrical product, relative to a reference material having a confirmed, acceptable corresponding performance defined RTI.

RTI electrical

The electrical RTI is associated with critical electrical insulating properties.

RTI mechanical impact

The mechanical impact RTI is associated with critical impact resistance, resilience and flexibility properties.

RTI mechanical strength

The mechanical strength (mechanical without impact) RTI is associated with critical mechanical strength where impact resistance, resilience and flexibility are not essential.

 



RTI gives an indication of the aging temperature that a material can endure for 100,000 hours and still retain at least half of the initial property being measured. Different materials’ properties decay at dissimilar rates. As a result, one caveat in using the data “is that you could track the wrong properties,” cautions James Galipeau, laboratory manager for Plastics Technology Laboratories in Pittsfield, MA. “You don’t want to use test results on impact as an indication of flexural modulus or bending results.” RTIs indicate a given plastic’s electrical properties and specific mechanical properties with and without impact. UL gives RTI ratings for each thickness of the plastic tested.

Sets of test specimens are placed in ovens at four different preset temperatures. At intervals, specimens are removed and tested for specific mechanical or electrical properties. The results are plotted on a time versus property graph until the property being tested declines to 50 percent or less of its initial value. The 50 percent value is referred to as the “half-life” for that property. The half-lives are then plotted against the reciprocal of the absolute aging temperature, resulting in a straight line that can be extrapolated to indicate the half-life of a property at other temperatures. That line is called an “Arrhenius” type of plot. Results are compared to a material with known aging performance, hence the term Relative Thermal Index or RTI.

Here’s how the test applies to design of a plastic part, say a toaster oven. Start with a requirement for how long the oven would actually operate. “Most manufacturers would want a warranty of at least 1,000 hours,” comments Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. “So you go back to the RTI for the appropriate mechanical property and find the point that gives you 1,000 hours of use and still maintains 50 percent of the original property value. And then you design in a safety factor of two, three, or four times – which you should do.”

You must also consider the possibility of heat aging in the presence of water or some type of chemical environment. Hydrolytic stability, or resistance to attack by water, is particularly important because water is ubiquitous and can be very aggressive to many types of plastics. One test is to immerse test samples in boiling water and then test mechanical properties after 10 days.

Plastics obviously face potential for a wide variety of chemical attack when used in moving equipment or devices. At times in the past, there was inadequate testing for known exposures, such as when certain plastics were first introduced for exterior automotive applications in Europe. It’s a dicey proposition due to the difficulty in predicting the exact effect of chemical exposure on a polymeric component because the reagent, the exposure time, the temperature of the reagent, the temperature of the polymeric component and the stress on the component all affect the extent of attack and any change in performance. The only reliable method to predict performance is to test a prototype part under actual conditions. Plastics producers perform screening tests to provide general guidance and compare materials.