Relative temperature index

Over the years many attempts have been made to provide some measure of the maximum service temperature which a material will be able to withstand without thermal degradation rendering it unfit for service. Quite clearly any figure will depend on the time the material is likely to be exposed to elevated temperatures. One assessment that is being increasingly quoted is the UL 746B Relative Temperature Index Test of the Underwriters Laboratories (previously known as the Continuous Use Temperature Rating or Index). 

Such a value for relative temperature index will be specific to a particular grade of a polymer, sometimes even to a specific colour. The difference between grades of a particular species of polymer can be substantial, depending both on the variation in the inherent stability of a material between differing manufacturing methods and also on the type and amount of additives used. It is possible to obtain from the Laboratories a Generic Temperature Index to cover a species of material but this will usually be considerably lower than for many of the individual grades within that species. 

Table 9.1 Some collected values for Relative Temperature Index (RTI) (Unless otherwise stated, data are for mechanical without impact and for unreinforced grades)...

ISO 2578 [3] calculates a Temperature Index (TI) which is the temperature at which the chosen threshold is reached in (usually) 20,000 hours. The Relative Temperature Index (RTI) is a comparative value with a reference material. The HIC is the halving interval the temperature change needed to halve the time to the end point from the TI. 

Heat deflection temperature— ASTM D648. This data is dangerous in the respect that it often is the only temperature data provided on a resin data sheet, which leaves the impression that it is a reliable indicator of the limit to which the product can be used (see the section on Relative Temperature Index later in this chapter). It is really nothing more than the temperature at which a given load (66 or 264 Ib/in ) will deflect a specimen an arbitrary amoinit. Other temperature tests are also used, and some are described in the following sections. Results of those tests are usually available from the resin manufacturer. 

See also relative temperature index aging thermal degradation. 

Glass transition temperature Continuous-use temperature UL relative temperature index Vicat softening temperature Heat capacity Resistivity, volume Resistivity, surface... 

Relative temperature index 90°C UL-746B (mechanical without im-... 

Fig. 1.8 Relative temperature index (RTI) cloud of known formulations and prediction of RTI on new formulation using similar compound ingredients in the cloud of several nylon formulations...

Relative temperature index RTI is the temperature index of a test material for the time span corresponding to the temperature index of a reference material, when both materials are subjected to the same aging and evaluation processes in a comparative test [252]. 

Temperature indices determined according to UL 746 B [240] are relative values obtained in comparative tests with a well-known similar material. The relative temperature index RTI according to UL 746 B is the temperature in°C at which, after a certain time (originally 60,000 h, extrapolated), the initial value of a property has decreased by 50%. Therefore, this is a relative rating, because the values are always compared to a reference specimen. Numerically they agree fairly well with the temperature limits extrapolated at 40,000 h exposure time for half-lifetimes according to lEC 60216 [101], [501]. 

Table 5.48 Relative temperature index (RTI) according to UL 746 and temperature index (TI) at 5000 h and 20,000 h, respectively. Thermal Endurance Profile (TEP) according to lEC 216 and upper temperature limits (without mechanical load) for various polyamides short-term loaded once or several times for less than an hour, long-term permanent load over years [ 14], [86]...

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