Evaluating the potential of thermoplastic polymers for cryogenic sealing applications: strain rate and temperature effects
Cryogenic fuels, such as liquid hydrogen and liquid natural gas, emerge as versatile and sustainable energy carriers that are revolutionising various industries including aerospace, automotive, marine, and power generation. Thermoplastic polymers can be a suitable alternative to metal seals in cryogenic fuel systems. However, there is limited study about the behaviours of thermoplastics at cryogenic temperatures, especially at liquid hydrogen temperature of 20 Kelvin (K). This paper measured the tensile properties and coefficient of thermal expansion of three popular thermoplastics: PTFE, PEEK and UHMWPE at room temperature (RT), 77 K and 20 K and at four strain rates. Further microscopic analysis was also conducted to understand the failure mechanisms occurring when combining reduced temperature with varying strain rate. The tensile strength of each polymer increased from RT to 77 K and decreased from 77 K to 20 K. Elastic modulus tended to increase, and the strain recorded at failure decreased when reducing temperature from RT to 20 K. From microscopic observation of PEEK and UHMWPE, a reduction in temperature from 77 K to 20 K resulted in a larger instantaneous fracture, with multi-faceted fracture surfaces containing many small mirror like and opaque or misty sub-regions within the fracture zone. For PTFE, the surface morphology exhibited an insensitivity to the increase in strain rate at cryogenic temperatures, and the microscopy showed how the size of dimples found within the fracture interface became smaller when temperature was reduced from 77 K to 20 K. Finally, PEEK was found to contract much less than PTFE and UHMWPE at 20 K, in agreement to it having the highest glass transition temperature of the three polymers, which is normally a good indicator when attempting to identify polymers that will tend to exhibit smaller contraction at cryogenic temperatures.