It is envisioned that next-generation space exploration vehicles will have integral liquid hydrogen and liquid oxygen cryogenic fuel tanks that not only contain fuel, but function as load-carrying structures during launch and flight operations. Traditionally, metallic tanks have been used for housing cryogenic fluids. The advantages of such tanks include high strength, high stiffness, and low permeability. Presently, it appears that the replacement of traditional metallic cryogenic fuel tanks with polymer matrix composite tanks may decrease weight significantly, and hence, increase load-carrying capabilities (ref. 1).
However, the tanks must be able to withstand flight loads and temperatures ranging from -250 to 120 °C, without loss of cryogenic fuel due to microcracking or delamination. Research of the NASA Glenn Research Center has led to the development of epoxy-clay nanocomposites with up to 70-percent lower hydrogen permeability than that of the base epoxy resin. Filament-wound carbon-fiber-reinforced tanks made with this nanocomposite had a fivefold lower helium leak rate than the corresponding tanks made without clay. Use of these advanced composites would eliminate the need for a liner in composite cryotanks,thereby simplifying construction and reducing propellant leakage.
Helium permeability of Epon 826 with various amounts of layered silicate in comparison to the base resin (clay). ODA, organically modified clay from Michigan State University (East Lansing, MI); 30B, Closite 30B--organically treated clay from Southern Clay Products (Gonzales, TX).
The significant enhancements in barrier performance that are typically reported for clay nanocomposites depend on the level of separation of the silicate. A high level of dispersion creates a maximum path length for the permeating gas, thereby slowing the leak rate. As shown in the preceding bar chart, addition of layered silicate into Epon 826 decreased permeability by 30- to 70-percent in comparison to the base resin. This reduction is common when the clay is simply added to the matrix because the resulting morphology is a combination of intercalated and exfoliated silicate layers, randomly arranged throughout the sample. It has been assumed that orienting the clay layers within the sample would produce the lowest permeability.
Nanocomposite matrix (left and center) and neat resin (right) test tanks.
Test tanks (see the photograph) were prepared, and the helium permeability was measured at room temperature and 25 psi. The nanocomposite matrix tanks showed a fivefold decrease in the helium leakage rate in comparison to the tank composed of a neat resin matrix.
The coefficient of thermal expansion is an important parameter to consider when developing materials for composite cryogenic tanks. The primary cause of microcracking is the difference in the coefficient of thermal expansion (CTE) of the matrix and that of the reinforcing carbon fibers. The results (shown in the next bar chart) indicate up to a 25-percent decrease in the CTE for the nanocomposite samples. A decrease in the CTE of the nanocomposite can be attributed to the fine dispersion and rigidity of the clay platelets in the epoxy matrix, which can inhibit the expansion of polymer chains as the temperature increases (ref. 2).
CTE of nanocomposite samples on Epon 826 compared with that of the base resin (clay).
Impact testing has shown that the nanocomposites have a decreased toughness in comparison to the base resin. This is commonly observed when a rigid filler is added. In this case, however, the decrease is minimal. The final bar chart shows the notched impact results for the neat resin and nanocomposites.
Izod impact resistance of notched nanocomposite samples of Epon 826 with D230 curing agent compared with the base resin (clay).
In summary, dispersion of layered silicate clays in two separate epoxy matrices reduced both the resin permeability and the CTE. The resin toughness varied. However, these results, as well as preliminary data from the composite tank, suggest that nanocomposite materials may be well suited for linerless composite tanks.
Last updated: October 10, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
