>

Plastics with low temperature properties

Most engineering plastics are generally well suited to temperatures below zero, although the extent to which this is the case depends mainly on the material and the specific application conditions.

Due to the specific nature of thermoplastics, it is difficult to determine a low-temperature threshold that can be clearly defined using standard test methodology. When thermoplastics are used in conditions where the temperature is increasing, there are some specific physical properties, such as glass transition temperature (Tg) and crystalline melting temperature (Tm), that strongly influence mechanical behaviour and service life. By using standard tests, it is possible to evaluate the permanent loss of properties due to thermal ageing and oxidation.

Conversely, however, when temperature decreases, there is no requisite permanent loss of properties due to exposure to low temperatures. Even though general increases in stiffness and decreases in impact strength can be observed, these characteristics will generally return to “normal” when the material is heated up again. 

Here a distinction must be made between amorphous and semi-crystalline thermoplastics. An amorphous material should not be subjected to mechanical wear above the glass transition temperature (the temperature at which polymers change from a hard state to a flexible state), as the mechanical strength decreases sharply. Semi-crystalline materials, on the other hand, can still demonstrate certain mechanical strength beyond the glass transition temperature due to the presence of certain crystalline areas within the polymer. 
For both semi crystalline and amorphous thermoplastics, a service temperature in the negative range is not precisely defined, and depends largely on practical application requirements, as a result, it can only be found through actual testing.

Nevertheless, years of polymer usage give a reasonable indication on the lower application temperature for each material, as reported in the following table for Ensinger materials.

The application of these negative service temperatures is heavily influenced by the type of mechanical load present during use: impacts or vibrations can lead to premature component failure.

Materials that have been modified using reinforcement fibres tend to demonstrate more brittle behaviour. Modification with fillers should consequently be viewed more critically at low temperatures.

When it comes to cryogenic temperatures, down to -200°C and below, only very few polymers can be used successfully. These true “low temperature polymers” include: