Cryogenic Temperatures and High Performance Plastics
Cryogenics is the study of the production and behavior of materials at very low temperatures. A cryogenic environment exhibits temperatures below -150°C. Many modern industries use cryogenics in a wide variety of applications. Some of these applications include cryogenic fuels, spacecraft hardware as well a machinery for medical and bio-science. Applications include freezers and magnetic resonance imaging (MRI), particle accelerators, and superconducting magnets. Curbell Plastics® recently published a white paper by Dr. Keith Hechtel on the effect of cryogenic temperatures on some common high performance plastics. This article will briefly summarize some of the white paper’s main points.
Generally speaking, all materials exhibit greater hardness and stiffness when exposed to cryogenic temperatures. For example, the compressive modulus of PTFE increases from 100 kpsi to 900 kpsi when cooled from room temperature to 20°K (-424°F). However, materials at cryogenic temperatures become more brittle and have a lower Izod impact strength and tensile elongation. Tensile elongation describes how far a material will bend under pressure before snapping. Plastics are already more prone to snapping than many metals. Therefore special precautions must be taken when using plastics in a design for a cryogenic environment that is subject to a high amount of pressure or impact.
It is important to consider the high CTE or coefficient of thermal expansion of plastics when designing plastic components for a cryogenic application. Basically, plastics tend to shrink more than other materials when cooled and expand more when heated. For example, when materials are cooled from room temperature to nearly 0°K, PTFE contracts by 2.2% while aluminum contracts by less than 0.5%. This shrinkage can cause real problems in applications where metal and plastic components must remain in close contact. Hechtel also points out that carbon and glass fibers can be used to mitigate this shrinkage issue.
Friction and Wear
Cryogenic processing or hardening has been used since the 1960’s to increase the wear resistance of steel. The same basic principles apply to plastics as well. Generally speaking, the harder the material, the lower the friction and wear. Hechtel uses the example of an athletic shoe versus a dress shoe-the athletic shoe has a softer sole and creates more friction against the ground. Plastics become harder as temperatures drop and therefore exhibit less friction. Plastic bearings, for example, have the added benefit of showing good resistance to wear even without lubrication. This property is especially important in cryogenic temperatures. Many oils and other lubricants may perform less effectively at these very low temperatures.
For more information and a large array of illustrative charts please check out the original white paper.
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