CompositesWorld
Issue link: https://cw.epubxp.com/i/450689
CompositesWorld.com 25 NEWS N E W S N S N E W S E N W S W H 2 Tanks for FCVs charter and a few changes in members (for a list of consortium participants, see "Learn More"). Members of both programs reportedly were chosen not only for their technical expertise but also their willingness and capability to form a viable supply chain should research lead to a commercial product. After a market study yielded initial pressure vessel design speci- fcations, the DuraStor team moved on to materials and processes. A patent survey showed that most intellectual property in this arena involved thermoset composites, but one team member familiar with the permeation resistance of inexpensive ther- moplastic liner materials convinced the team to explore them, because they not only weigh less than metals but also process faster and ofer greater impact strength and reprocessability/ recyclability than thermosets. Further, thermoplastics can be remelted, ofering the opportunity to produce liner and overwrap separately, then join them to create a monolithic tank that could avoid fatigue issues seen in existing metal/composite hybrids and facilitate recycling. By eliminating all metals except for coupling hardware, which could be isolated, hydrogen embrittlement and galvanic corrosion could be reduced or eliminated. "In the lead up to these programs, we had many discussions with automakers who clearly were interested in fuel-cell technology, but lacked confdence in the long-term performance of current hydrogen-storage vessels," explains Dr. Matthew Turner, head of R&D;, EPL Composite Solutions Ltd., project lead for the DuraStor and HOST programs. "Our consortium members believe that ther- moplastic composites can ofer better durability than thermosets." DuraStor evaluated three semi-crystalline thermoplastic resins (supplied by consortium member Celanese, Dallas, TX, US) known for good mechanical properties and high chemical and permeation resistance: polybutylene terephthalate, polyphenylene sulfde, and polyoxymethylene (POM, or acetal). POM was chosen based on prior work by two team members and its long history of use in auto- motive fuel-handling components and low-pressure fuel storage. Te liner would be neat POM to optimize permeation resistance, but the overwrap had to be reinforced to maximize the vessel's hoop strength and burst- and puncture-resistance. Te team selected continuous-strand carbon fber rather than E-glass, because the former ofers greater strength-to-weight and is already used, with epoxy, by many tank manufacturers. Unidirectional tapes, considered the most practical fber form, ensured very low void content and no dry spots, both critical to permeation resistance. Process selection began with the hollow liner. Because leakage was unacceptable, the team wanted to produce liners in one piece rather than join multiple parts. To manage costs, the process needed to be inexpensive at research volumes and scalable to auto- motive production volumes. Blowmolding, casting and rotomolding (rotational molding) were considered. Te latter was selected based on its low tooling costs; its low forming stresses (which reduce post- mold warpage); its ability to produce large components with good surface fnish and controlled wall thickness; and the fact that threads could be molded in for coupling hardware (which allows fuel fow in and out) and inserts could be used to minimize post-mold fnishing. Although rotomolding cycles are slower than most other thermo- plastic forming methods, family tools and use of rotary stations can increase output at a relatively low cost vs. other options. Filament winding, already used by tank manufacturers, would be the overwrap process but the team would likely have to modify winding heads to handle preimpregnated thermoplastic tapes. With materials and processes in hand, the team conducted parallel testing projects. Some members did small-scale mechanical testing (per ISO 527-3, ISO 178 and ASTM D3410M) and permeation testing (per ISO 5869) on neat, injection molded POM and vacuum/ autoclave-cured POM/carbon laminates. Another group designed the liner, its coupling hardware and the rotomolding tool. Several liner designs were trialed between 2011 and 2013. Some molded liners from each batch were cut up and subjected to further mechanical and permeation testing. It was important to validate the accuracy of preliminary tests results — which were conducted on injection- molded samples with a POM grade and additive package designed for injection molding — by testing samples produced in the roto- molding process, using a similar but not identical POM grade with a diferent additive package. Te correlation was quite good and the team concluded that formulation and processing diferences didn't adversely afect polymer behavior. Initial results In 2014, the DuraStor team's frst public reports of its 2010-2013 research indicated that liner and overwrap materials meet or "Our consortium members believe that thermoplastic composites can ofer better durability than thermosets." Hydrogen can be compressed and held under very high pressures, or liquefed and held at cryogenic temperatures. Compression and cryogenic refrigeration increase H 2 energy density per unit of volume, but both are energy-intensive operations, and the vessels for both must be made of materials that can withstand high pressure and prevent hydrogen loss via permeation. The resulting robust containers don't "package" well in commercial trucks, let alone on passenger cars. Further, the cryo option requires continuous energy input to keep hydrogen at -253°C. In the world of ground transportation, it's impractical for any use other than large-scale H 2 storage at vehicle refueling stations. A third option would be quite welcome in automotive circles. It's technically possible to store hydrogen in a solid state at, notably, room temperature and normal atmospheric pressure. The method involves absorption or adsorption of hydrogen into certain metal halides. This method, however, is fraught with challenges and remains commercially impractical. For the present, auto OEMs are focusing on compressed hydrogen gas, which currently ofer the best cost vs. beneft ratio and the fewest practical barriers to implementation. H 2 storage methods SIDE STORY