Surprisingly, plastic can perform quite well, as long the plastic products are created with cryogenic freezing in mind.
The beginning of plastic materials in cryogenic environments
The study of plastics in cryogenic environments started all the way back in the 1960s. The research was conducted by NASA, as they wanted to see how plastic would perform in environments below 150 degrees Celsius. They were invested in the findings because, at the time, NASA was looking for ways to improve the seals, pumps, valves, and bearings in their spacecrafts.
Based on that research, and continued research throughout the decades, plastics are now used in more applications than ever. From plastics in aerospace to the military and plastic products that can enhance workplace safety, plastic is everywhere, and it’s used in cryogenic environments too. Today, plastic is used in liquid hydrogen storage and processing equipment, spacecraft hardware, particle accelerators, cryogenically cooled supercomputers, pharmaceutical freezers, and superconducting magnets.
So, what exactly does plastic offer over other materials, and where does plastic fall short?
No matter what the material, as it approaches subzero temperatures, it’s going to become harder, stiffer, and more brittle. For example, PTFE increases from 100 kpsi to 900 kpsi when cooled from room temperature to 20°K (-424°F).
That doesn’t mean that plastic products cannot be used. Creating plastics for cryogenic environments just requires that higher loads be placed on the material. That way it is able to conform to a mating metal part in order to achieve a seal. Today, PTFE is commonly used to create gaskets and seals that are used in a wide variety of applications because, with the right techniques, it can be used in subzero temperatures.
The materials must be carefully considered when using plastic with other materials, like metal. When designing machinery that operates in cryogenic temperatures, it must be created to withstand a relatively high CTE (coefficient of thermal expansion) of polymers, especially compared to the CTEs in average environments. If the plastic part and other parts don’t shrink at the same rate of cooling, it can spell disaster. This is a common problem because, in general, plastic shrinks more quickly than other materials. For example, PTFE contracts by 2.2 percent, while aluminum contracts by less than 0.5 percent when cooled from room temperature to nearly 0°K.
Fortunately, this problem can be corrected. Plastics that will be used in a cryogenic environment are often filled with other materials like glass fibers, graphite, or molybdenum disulfide. These additives enable the plastic to shrink at the same rate as other materials.
Friction and wear
When it comes to friction, plastic definitely has a leg up on more traditional building materials, like metal. Polymer bearing materials are usually able to function without lubricants, which is a good thing in subzero temperatures. It isn’t uncommon for lubricants to freeze in extreme cold temperatures, the lubricant can cause problems in a vacuum environment, and it can raise concerns over contamination.
Wear is a bigger problem. Harder materials wear more slowly than softer materials, which is why metal might come to mind before plastic. However, with the process of cryogenic processing or hardening, plastics can be made harder. Combined with the fact that cryogenic temperatures make materials harder, plastic can perform surprisingly well in extremely cold temperatures.
It is important to mention that not all hardening processes work well in a cryogenic environment. Additives that may increase a material’s hardness at room temperature won’t increase their hardness in extreme cold temperatures. For example, graphite fillers that are added to PTFE may make the plastic more durable at room temperature, but they aren’t effective in a cryogenic environment.
The search for the perfect cryogenic plastic continues
Because plastics are lightweight and more affordable than other materials, the search continues for the perfect cryogenic plastic. To learn more about plastic materials in cryogenic environments, give us a call. As your fastener experts, we can help you find the perfect components for your next project.