Characterisation of rotationally moulded polymer liners for low permeability cryogenic applications in composite overwrapped pressure vessels
Murray, Brendan R.
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Composite overwrapped pressure vessels (COPVs) have become a critical component in satellite and space applications due to their ability to store highly permeating fuels at high pressures and under cryogenic conditions. Their main drawback is the associated cost of manufacturing the inner metallic liner which has hindered their integration across other forms of industry. Polymers have been identified as a prospective replacement material for titanium liners in COPV applications. The work presented here assesses the ability of polymer materials to fulfil the requirements of a functioning COPV liner (via helium permeability testing and cryogenic environment testing) while also developing a manufacturing method for COPV production using a modified rotational moulding process and laser assisted tape placement of a thermoplastic carbon fibre tape. Prospective liner materials have been permeability tested to assess their ability to resist permeation from helium gas. Permeability coefficients have been measured from helium leak rates and used to rank prospective materials based on their ability to resist permeation. These measured coefficients have then been used to predict the ability of each material tested to perform as a permeation barrier in a demonstrator COPV, with PA11 and PVDF showing significant promise. These results have been verified by permeability testing PA11 samples at a 5 bar (0.5 MPa) pressure difference (which is indicative of the final operating conditions) and has shown acceptable levels of fuel containment. The effect of variations in processing conditions on permeability in the chosen manufacturing method has also been assessed, with internal void contents and defects giving higher permeation coefficients and valuable insights into liner formations studies. The effect of air voids on permeability has also been assessed with a three dimensional finite element mass diffusion analysis of void distributions in rotomoulded samples. Using polyethylene, a microscopic study of the effects of temperature on void distributions has shown lower void contents in samples manufactured at higher temperatures. The correlation between the maximum forming temperature and the void volume fraction and void radius has been used to assess the forming conditions which produce the lowest permeability. The finite element analysis has shown that higher void contents lead to an increase in permeability. Models with larger voids have lower leak rates than models with smaller voids (for equal void volume fractions) due to the smaller voids forming leak paths throughout the sample. This has led to recommendations for using higher forming temperatures for polymer liner formations to reduce void contents and hence permeability. A modified rotational moulding tool has been used to produce demonstrator liners for proof of concept testing for the current project. The tooling was designed and fabricated as part of this project and has proven versatile in the production of polyethylene, nylon, polyvinyl-difluoride and poly(ether ether ketone) liners using the same tooling for each formation. The use of segregated heating lines has increased control of temperature distributions (specifically in the flange region, the area of highest heat loss) and has increased the dimensional accuracy of the formed parts. This has been demonstrated via thermal imaging analysis and has also been confirmed via wall thickness consistency measurements in demonstrator parts produced using the aforementioned tooling. A number of material properties have also been measured with thermal and dynamic mechanical analyses. This has been coupled with studies of bonding properties between the polymer liner and the thermoplastic carbon fibre overwrap, which has been applied using the laser assisted tape placement process. The effect of cryogenic cycling on cracking in the liner-overwrap bond region has been assessed in detail with X-ray CT scanning, while nano-indentation has been used to map the effects of the laser welding process on the bond between the polymer liner and carbon fibre overwrap.
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