Plastics for Extreme Temperature Applications

Cryogenic biotechnology machinery, spaceflight hardware, and low temperature physics equipment are sometimes required to operate at extremely cold temperatures.

When designing plastic components for use in cryogenic conditions, it is generally beneficial to select plastic materials that exhibit ductile behavior at low temperatures and have low rates of thermal expansion. For cold temperature applications where heat transfer is a concern, it is also important to select a plastic material with the right thermal conductivity. At Curbell, we can talk with you about the variability in the thermal conductivities of plastics at cryogenic temperatures, with some materials functioning as thermal insulators and others having relatively high thermal conductivity.

Plastics wear at different rates (and via different mechanisms) at low temperatures—so if an application requires a plastic part to function as a bearing in cryogenic conditions, it is important to select a material that has been engineered to have low friction and good wear properties at cold temperatures.

Selecting a plastic material for use in a high temperature environment requires careful review of material properties data. The following are some of the specific factors to consider:

• Strength and modulus– Plastics will soften (lose strength and modulus) at elevated temperatures. It is important to evaluate the mechanical properties of the materials being considered at the low end and high end of operating temperature range.
• Thermal expansion– Plastics tend to have much higher rates of thermal expansion compared with metals. The coefficient of thermal expansion should be carefully reviewed to insure that CTE mismatch between plastic parts and mating metal parts does not cause the device to malfunction due to dimensional changes.
• Creep strain and stress relaxation– Plastics exhibit increased creep strain and stress relaxation at elevated temperatures. These factors should be considered carefully during the design process.
• Degradation– Plastics may degrade over long periods of time in high temperature environments, especially if oxygen is present. Thermal degradation curves should be reviewed when selecting polymers for use in elevated temperature environments.

Aircraft, spacecraft, and other devices must function throughout a broad operating temperature range, from very low to very high temperatures. When plastics are to be used in these conditions, designers must be careful to select materials that will function reliably throughout the required temperature range. For example, the rate of thermal expansion for some plastics such as PTFE will vary considerably depending on the temperatures involved. This complicates design when a plastic part must function in direct contact with mating metal components. Some plastics including DuPont™ Vespel® tend to have very consistent rates of thermal expansion throughout a wide temperature range, simplifying part design. Vespel® formulations with additives and fillers such as Vespel® SCP-5050 are engineered to have extremely low CTEs, close to those of industrial metals.

In addition to thermal expansion rates, our technical experts can help you review various considerations for using plastics in wide temperature ranges. For example:

• At cold temperatures, plastics tend to be harder, stiffer and more brittle.
• At elevated temperatures, plastics tend to be softer and more ductile. Read an article to learn more.
• Plastics tend to have reduced chemical resistance at elevated temperatures.
• Plastic bearings can be particularly sensitive to wide operating temperature ranges, since plastic components will undergo changes in their dimensions, mechanical properties, and friction and wear behavior when they are heated or cooled.