As you already know, polymers and metals react to heat very differently. In this blog post, we are going to look at some of the key thermal properties of polymers and discuss the test standards used for determining these properties.

Glass Transition Temperature and Melting Point (ASTM D 3418)

The glass transition temperature for a polymer is usually referred to as its Tg. Above the Tg, an amorphous polymer will become rubbery and soft, which implies a severe loss of mechanical strength and other key properties. In short, a polymer should not be used above its Tg.

The melting point is, of course, at a higher temperature than the glass transition temperature. At the melting point, a semi-crystalline thermoplastic changes from a solid to a liquid. Thermosets do not melt, but rather char or burn, and thus do not have a melting point. Note that the Tg and melting for a polymer are both determined in accordance with the ASTM D 3418.

Heat Deflection Temperature (ASTM D 648)

Another key thermal property for polymers is the heat deflection temperature, sometimes called the heat distortion temperature. The heat deflection temperature provides a measure of how well a polymer can perform in elevated temperatures for short periods of time while under a load.

This temperature is based on the ASTM D 648 test method. In this method, a ½” thick test specimen is arranged so that a specified loading induces simple bending stress. A thermometer measures the increase in temperature and a deflection gauge measures how much the test specimen has deflected. The temperature at which the specimen deflects 0.010 inches is the HDT for that polymer.

Some people may confuse the continuous service temperature with the heat deflection temperature. The continuous service temperature is a bit more complex. While the heat deflection temperature applies only to short term increases in temperature, the continuous service temperature applies to long term service (defined as about 10 years). In addition, the heat deflection temperature focuses primarily on deflection while the continuous service temperature lies where the polymer can still retain at least 50% of its physical properties.

Both the heat deflection temperature and continuous service temperature are very important considerations when choosing a polymer for extreme temperature applications.

Coefficient of Linear Thermal Expansion (E 831 TMA)

The coefficient of linear thermal expansion, often abbreviated CTE or CLTE, is a ratio that represents the change in linear dimensions of a polymer when it is exposed to a unit change in temperature.

This is a very important property when selecting a polymer for applications that involve changes in temperature. Polymers can expand or contract significantly more than metals, with the most dimensionally stable polymers approaching the CTE of aluminum. Keep in mind that the CTE of a
polymer can be increased through additives or fiber reinforcements.

Conclusion

When selecting a polymer for an application, it is important to keep in mind that they behave differently from metals and different properties must be considered. For polymers that will be used in elevated temperatures, these include the glass transition temperature, melting point, heat deflection temperature, and continuous service temperature. When a polymer will be exposed to changes in temperature, the coefficient of linear expansion is an important consideration.

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