High Temperature Plastics

Definition:

For plastics, the definition of ‘high temperature’ is taken to mean ‘any temperature above 135°C and it is true that the majority of available plastics are suitable only for use at temperatures below this value. These plastics are generally called the ‘commodity’ plastics and constitute by far the largest volume of plastics used in the world today.
Despite this, the last few years have seen a rise in the importance of ‘engineering’ plastics and these have significantly improved performance at temperatures above 135°C. The table at right gives the approximate upper limit for the service temperature of a range of plastics families and the engineering plastics show significant improvements in service temperature over the commodity plastics.

Service Temperature:

Assigning a "maximum service temperature" to any plastic should be undertaken with care. At high temperatures plastics not only soften but can also start to thermally degrade. A plastic that softens at a high temperature but which starts to degrade at a much lower temperature can only be considered for applications below the temperature at which it starts to degrade. Specifying the service temperature also requires knowledge of the thermal degradation performance of the material.

The physical ‘softening point’ of a plastic is defined largely by the type of plastic being used. For amorphous polymers (such as Ultem®, PMMA or PS) the important temperature is Tg – the glass transition temperature. For highly crystalline polymers (such as PTFE) the important temperatureis Tm – the melting point. In either case the exact definition of the "softening point" will depend on the test method used.

Test Methods

There are two basic methods for assigning a value to the performance of plastics at high temperatures:

Vicat Softening Temperature (VST) - ASTM D 1525 (ISO 306) This test measures the temperature at which a plastic starts to soften rapidly. A round, flat-ended needle of 1 mm2 cross section is placed on the surface of the test specimen under load and the temperature is raised at a uniform rate. The Vicat Softening Temperature (VST) is the temperature at which the penetration reaches 1 mm.

Deflection Temperature Under Load (DTUL) - ASTM D 648 (ISO 75) This test measures short term performance under load at elevated temperatures for a by measuring the effect of temperature on stiffness. A defined surface stress is applied to the standard test specimen and the temperature is raised at a uniform rate. Note: When ISO 75 is used the result is referred to as the Heat Distortion Temperature or Heat Deflection Temperature (HDT).

Time-Temperature Superpostion:

Any mechanical property of a plastic is governed by the principles of time-temperature superposition - based on the original work of Williams Landel and Ferry (WLF). shows that time and temperature can have the same (but inverse) effect - the strength of a plastic at high rates of loading and low temperatures can be effectively the same as the strength at low rates of loading and higher temperatures.

This means, fortunately, that information from testing at high temperatures and at fast rates can be used to estimate the properties at lower temperatures and at slower rates. Unfortunately, it also means that the effective service temperature of a plastic can vary significantly with the rate of loading. Apparently small load application rates at high temperatures can have the same effect as large load application rates at lower temperatures.