Thermal deformation resistance

Thermal deformation resistance and Vicat softening point state information about the short-timed resistance of a plastic under pressure against high temperature. With the help of standard tests, each temperature (DTUL) at which the sample meets a certain deflexion or complies with the pressure of a needle up to a penetration depth of 1mm (Vicat), is determined.

During these tests the sample is treated with a controlled tensile and bending stress. The higher the values are the higher is the thermal resistance of the material in the application area. The mechanical properties of all thermoplastics change with the impacted temperature. This procedure is slow and nearly linear for amorphous polymers like Lexan® polycarbonate and Ultem® polyetherimide. Valox® thermoplastic polyester shows a distinctive dependency to the temperature as it is a crystalline polymer. Diagram 3 illustrates the tensile modulus as function of the temperature.

GE films often are a good choice for high temperature applications because of their high thermal stapability.

The wide softening range of Lexan® and Ultem® simplifies the thermal forming. Screen printed Lexan® film can be cured at temperatures up to 120°C.

RTI (heat ageing)

The relative temperature index (RTI) of plastics for electrical applications means, according to UL: “the maximum service temperature at which the material remains to its critical properties in fungible limits for longer terms.” The values in chart 10 are based on a 100,000 hours period of use. The RTI test is essential for product certifications according to UL. The antiquitated phrase of “continuous operating temperature” should be avoided.


As Lexan® and Ultem® are isotropic and amorphous polymers, they are relative dimensionally stable at high temperatures. Short-timed tests (30 min) in the laboratory did not show noticeable changes of measurement. Lexan® was tested up to 135°C and Ultem® up to 150°C. Lexan® films with a thickness of 0.075 up to 0.125mm shrink about 0.5% at 150°C (glass temperature). Thicker qualities shrink about 1% to 2% at the same temperature. At temperatures above 150°C the shrinking increases clearly. Lower temperatures do not have effects to Lexan® films as they remain to their low temperature toughness up to at least -135°C.