  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.
