Relative temperature index

The relative temperature index (RTI) according to UL 746B is a measure of the thermal aging resistance of a plastic at elevated temperatures . The RTI is defined as the temperature after which it takes as long for a material (Candidate B) when stored in air as for a comparable other material (Control A) at its already known RTI temperature until a specified property is reduced to 50%. its original value has fallen.

The RTI is available for different material thicknesses and typically the following parameters:

• Dielectric strength (RTI Elec)
• Tensile Strength (RTI Str)
• Impact resistance (RTI Imp)

determination

Storage attempts

The RTI value is determined from a large number of samples stored in air at different temperatures and for different times in the oven. After the end of storage, the specified property is determined using a (mostly destructive) material test method and plotted as a function of the storage time. As a result of the aging process, the initial value will drop after a sufficiently long storage period and will ultimately only be 50% of the initial value. This time k is determined graphically or mathematically by interpolation with a 3rd degree polynomial or another algorithm suitable for the application. The service life is then considered to have been reached and the test series can be ended.

Two different schemes can be used to select suitable storage temperatures and times: with the so-called fixed time frame , four sampling times between 1,000 and 5,000 hours are fixed from the start, while samples in steps of 10 K at temperatures of 40 K and more are required to achieve sufficient data must be stored above the expected RTI.

This process cannot be used for many thermoplastics because they melt when overheated or change their shape and properties in some other way than aging. In these cases, the fixed temperature scheme is used, in which (at least) four storage temperatures are to be selected so that at the highest temperature 50% of the initial value is reached in 500 to 1,000 hours and at the lowest temperature after at least 5,000 hours. There is a predefined schedule for this. However, since the exact value of the time k is not known beforehand, the expansion dates must be adapted to the actual course of the aging curve. It is also common to store some sample sets (so-called delay sets ) only later in order to be able to determine additional support points of the aging curve when their course can already be roughly identified. Thus, k can be determined more precisely over a sufficiently large number of nearby support points.

evaluation

The calculation of the RTI is based on the Arrhenius equation, which is often used for aging processes :

${\displaystyle k=A\cdot e^{\frac {-E_{A}}{R\cdot T}}}$

By taking the logarithm and replacing the parameters X = 1 / T , a straight line equation is obtained, the parameters of which are determined by linear regression from the at least four k and T value pairs. This also enables an extrapolation for even lower temperatures and the associated longer but practically unrealizable storage times.

Once the parameters of the Arrhenius straight line have been determined for material A (control) and B (candidate), the RTI of B is calculated from the known value for A with the same correlation time according to the approach

${\displaystyle a_{\rm {A}}+{\frac {b_{\rm {A}}}{\rm {RTI_{A}}}}=a_{\rm {B}}+{\frac {b_{\rm {B}}}{\rm {RTI_{B}}}}}$

calculated. The same evaluation is also possible graphically using the Arrhenius graph .

Correlation time

The correlation time indicates the expected service life at the RTI temperature of material A (control). According to UL746B, a maximum of 60,000 hours are considered sufficient for electrical engineering applications. However, it is often significantly lower.

literature

• UL 746B, 5th Ed .: Polymeric Materials - Long Term Property Evaluations (engl.)
• N. Navarro: Predicting Elevated Temperature Ratings of Polymeric Materials in C. White et al .: Service Life Prediction of Exterior Plastics , New York: Springer (2014), ISBN 9783319060330 (English)
• Over Time and Under Heat, Polycarbonates Hold Up : Plastics Eng. 24 (2011), p. 24 ff.