The Relative thermal index (RTI) is a characteristic parameter related to the ability of

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

materials to resist

thermal degradation Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...

. The RTI is part of the longterm thermal aging program (LTTA) described in the UL 746B standard from UL. During the process of the UL 746B program, the degradation in hot

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

of certain properties of the material like

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

and mechanic strength, is investigated with regards to thermal-aging. For a full study, the candidate material (B) is aged together with a reference material A (control) with already known RTI value in the same ovens. The RTI is the rounded temperature in degrees C, at which the properties of ''B'' have decreased to 50 percent of their initial value in about the same amount of time (correlation time) than it takes for ''A'' at its own RTI value. A maximum correlation time of 60.000 hours is considered acceptable for many electrical applications, however it may also become as low as 5.000 hours according to UL 746B. If a material has not been investigated (yet), the RTI shown is based on the generic class (

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

type) of the material. Though the RTI is an index, it is given in

Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

units. The UL 746B standard distinguishes between three sub-categories of the RTI: *''Electrical RTI'' which relates to electrical insulating performance. *''Mechanical impact RTI'' which relates to mechanical performance with

impact Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Impac ...

*''Mechanical strength RTI'' which relates to mechanical performance without impact There is also the ''RTI Elongation'' (by means of ''Elongation at break'') for

films A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...

and other nonrigid materials.

Literature

* UL 746B, 5. Ed.: ''Polymeric Materials – Long Term Property Evaluations'' * N. Navarro: ''Predicting Elevated Temperature Ratings of Polymeric Materials'' in C. White et al.: ''Service Life Prediction of Exterior Plastics'', New York: Springer (2014), * ''G. Mediratta, et al. Over Time and Under Heat, Polycarbonates Hold Up'': Plastics Eng. 67 (7) (2011), p. 24

References

{{Reflist Materials testing Threshold temperatures