High Temperature Plastics
Published on Aug 15, 2016
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.
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.
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).
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.
Assessing the performance of a plastic in a high temperature application is therefore a complex task. In real life, essential factors such as the rate of loading, the load itself and the nominal surface stress will inevitably be different from the testing conditions. The application is the important thing and detailed knowledge of this is necessary to specify the correct material. The Decreasing Options Despite the importance of the application details, it is immediately evident from the chart that as the specified service temperature increases then the number of possible and suitable plastics materials rapidly decreases.
At room temperature the designer can choose from almost any of the available plastics but above 135°C there are a very limited number of plastics that are suitable. Engineering plastics offer substantial performance improvements at high temperatures but often at substantially increased costs compared to the commodity plastics. Despite this, engineering plastics can also offer substantial product improvements over traditional materials due to their easier processing, easier handling and improved application.
The Available Materials
The Ashby chart shows the clear differentiation between the engineering plastics, such as PTFE and PEEK™ and the commodity plastics such as PVC and PMMA. For high temperature applications, the most suitable materials, in ascending order of maximum service temperature, are:
---> PTFE (260°C)
---> PFA (260°C)
---> PEEK™ (260°C)
---> PEI (Ultem®) (180°C)
It may appear that the designer has limited choices but these plastics more than adequately cover a wide range of temperatures and can be very successfully used at high temperatures. The Available Processes Fortunately for the designer the engineering plastics can be processed by the same methods as for the commodity plastics. The exception to this is PTFE, which requires special techniques due to the extremely low co-efficient of friction. This means that extrusion of the engineering plastics is possible to create a range of extruded products suitable for use at high temperature. The extrusion process allows the production of tight tolerance tubing in a variety of formats such multi-lumen tubing, tubing with integral monofilament, heat shrinkable tubing and in some cases lay-flat tubing. Fillers such as radio-opaque products, glass fibers, carbon or other special fillers can be used to further modify tubing products for specific applications. Extrusion also allows the design and production of custom profiles for specific customer applications.
High temperature plastics have a variety of applications and are often used as to replace more conventional materials to reduce weight, cost or meet a specific application requirement. Some typical applications for high temperature plastics include:
Aircraft/aerospace electrical components
Automotive applications (under-hood).
Wire insulation for extremely high temperature applications, cable couplings and connectors.
Electrical/electronic applications at high service temperatures.
Medical tubing or other products that require sterilization.
Monofilament for the production of woven products for filters, belting and meshes.
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