Ticona takes cost and weight out of fuel cells
Development work by Ticona, the technical polymers business of Celanese AG, indicates that injection-molded end plates made of glass-reinforced Fortron polyphenylene sulfide (PPS) can produce "significant" cost and weight savings in proton exchange membrane (PEM) fuel cells that integrate the end plates and the adjacent insulating plates into one functional unit.
PEM fuel-cell stacks typically have sets of separate stainless steel or aluminum end plates and insulating plates at both ends of the stack. End plates hold the bipolar plates in a stack together and compress the gaskets between them. Insulating plates have bored manifolds that feed fuel, air, and deionized water (for cooling) to the stack. Even with automated machining, the two sets of plates on each stack can cost $300 to $500, according to Ticona.
Actual savings from integrating the plates is between $100 and $120 per stack, the company said, and the switch from metal to plastic can cut stack weight at least 10 lb (4.5 kg). Injection molding reduces manufacturing time to less than two minutes per plate versus the several hours needed for machined plates.
Fuel Cell Components and Integrators, Inc. believes Fortron PPS is a good candidate material for end plates. "From our experience, it molds well, replicates fine features, and has little warpage compared to other plastics we've evaluated," said Bernie Rachowitz, company President. "In use, it is stiff, strong, and hard, has low creep, and looks and feels like metal."
The material has the purity needed to satisfy known electrochemical demands in fuel cells, according to Ticona, which says tests show it has little effect on the conductivity of fluids in contact with it. Fortron PPS grade 1140L4 generates only a 2 µS/cm change in conductivity in a 50% glycol solution after seven days. Maintaining low conductivity in the end plates and minimizing ionic impurities that might leach into the electrolyte are key to continued fuel stack efficiency, in the view of Ami El Agizy, Ticona Market Development Manager for fuel cells. Fortron PPS has good dimensional stability and retains its mechanical properties at temperatures of over 200°C (393°F). "This is well above the 80°C at which PEM fuel cells now operate and the 150°C projected for the next PEM generation."
The material also has the rigidity needed to tolerate the high stresses that go along with compressing PEM fuel stacks, as well as long-term resistance to deionized water, oxygen, propane, natural gas, and other hydrocarbon fuels, El Agizy added. In addtion, he added, PPS is a good insulator, has a low coefficient of thermal expansion, absorbs little water, and is inherently flame-retardant.
Ticona offers both short- and long-glass-fiber-reinforced PPS grades. Smaller fuel-cell stacks place relatively low stress on the end plates, and short-fiber-reinforced Fortron 1140L4 PPS is useful for these applications. Larger stacks may require Celstran PPS-GF, which contains long glass fibers chemically coupled to the plastic matrix using a patented pultrusion process that fully impregnates them. The fully wetted fibers improve the polymer's mechanical properties.
As part of its fuel-cell activities, Ticona/Celanese is a member of the U.S. Fuel Cell Council (USFCC), an industry association dedicated to fostering the commercialization of fuel cells in the U.S.
- Patrick Ponticel
It's what's on the inside that counts
Beginning in 2004, CARB LEV II evaporative emission standards will require a 75% reduction from the current 2.0 g/day evaporative emissions requirement (following a diurnal plus hot soak SHED test). Significant reductions are also required under EPA Tier 2 regulations. Dyneon LLC products for fuel system applications have achieved durability and SHED test results that meet both current and future standards. The 15-year/150,000-mi durability requirements for CARB LEV II in 2004 are also within the current performance expectations of the company's fluoropolymer product portfolio, which is branded Dyneon.
While other suppliers try to provide all the materials used in a fuel line, Minnesota-based Dyneon focuses its efforts on the fuel line's barrier, which can often increase the strength, durability, and stability of an entire system. The interior wall of a fuel line is a fuel system's initial defense against permeation, chemical attack, temperature, and shock, according to Mark Geis, New Business Development and Automotive End-Use Manager for fuel containment systems, Dyneon. "This barrier can significantly contribute to how well the fuel system does in environmental and industry testing," he said. "By choosing the right materials for the job, OEMs are afforded the opportunity to safely build an entire fuel system around what they know is a reliable core."
For automotive applications, the company focuses solely on developing products with low permeation ratings, good cold-impact performance, no extractables, maximum flexibility, and ease of processing. "This translates into reduced cost, excellent performance, and enhanced sealability," said Geis.
Two Dyneon products—fluorinated ethylene propylene (Dyneon FEP) and the new Dyneon THV 800—provide some of the best barrier characteristics in the industry, according to the company. Permeation testing performed at 60°C (140°F) with CE10 fuel shows that Dyneon FEP achieves a rating of about 0.52 g•mm/m2•day (0.000068 oz•in/ft2•day), according to the company. "This is one of the lowest permeation constants on the market," Geis said. Dyneon THV 800 performs at 2.78 g•mm/m2•day (0.00036 oz•in/ft2•day), which is better than the industry standard of typical ETFE barriers. "Furthermore, recent micro-SHED testing supports these (and even lower) values when these materials are tested in a fuel-line/hose construction," noted Geis.
Effective in permeation resistance and field performance, modified polytetrafluoroethylene (Dyneon TFM PTFE) and Dyneon ETFE also produce optimum results in fuel line applications. Under the same permeation test conditions using cups, PTFE (Dyneon TFM 2100) rated 1.0 g•mm/m2•day (0.00013 oz•in/ft2•day) and ETFE (Dyneon ET 6235G) rated about 3.1 g•mm/m2•day (0.0004 oz•in/ft2•day). "When tested with various alternative, methanol-based fuels, our results are just as impressive," said Geis. "You'll find that in addition to these excellent permeation ratings, our products also offer strength and processing characteristics as good or better than competitive materials."
All four products offer good cold impact performance, showing no signs of damage or cracking at temperatures of -40°C (-40°F) or lower, according to Dyneon.
In high-heat conditions, Dyneon TFM PTFE performs measurably well at a continuous 250°C (482°F), according to the company. Dyneon ETFE material, in combination with Nylon 12 coverstock, also tests well in environments up to a continuous 90°C (194°F) and an intermittent 115°C (239°F). Dyneon FEP, which holds the industry's lowest permeation rating, in combination with a Nylon alloy coverstock can withstand up to a continuous 125°C (257°F) and an intermittent 150°C (302°F).
Competitively priced, these products offer increased flexibility over alternative materials, Dyneon says. And, in accordance with material specifications and recent industry concerns, its fluoropolymers used in automotive applications do not contain extractables and will not clog fuel systems.
In MY2002, Dyneon is further commercializing one of its newest fuel line product offerings as it becomes a part of the fuel bundle in one of the best-selling commercial and passenger vehicles in the United States. "This is exciting for us because it will be the first time we have been written into the specifications of such a large-scale fuel line project," said Geis.
- Patrick Ponticel
Delphi gets DuPont help in weight savings
Delphi chose DuPont's Zytel HTN high-performance polyamide (HPPA) to take weight out of the unit's tandem power piston and air valve. The tandem power piston converts and multiplies atmospheric air intake into hydraulic pressure that activates the brakes. Switching from a thermoset to HTN enabled engineers to reduce significantly the power piston's wall thickness, resulting in a 60% reduction in mass. The material meets Delphi's requirement for withstanding more than 3000 lb (13,000 N) of force in this application, and the reduced wall thickness allows for larger air passages, contributing to better airflow and improved packaging and assembly of mating components.
"We worked with our suppliers to push the envelope on geometric tolerances," said Michel Vermoesen of Delphi's Dayton Technical Center. "With support from DuPont, Delphi was able to achieve tolerance unknown in this industry."
The air valve is at the heart of Delphi's Generation II vacuum booster, and the new material delivers a 19% weight reduction while increasing strength by more than 15%, according to Delphi. The valve formerly was made of PPS. The valve regulates airflow into the vacuum booster, defines reaction ratio, and provides feedback to the driver under braking conditions. The application required a high-strength, dimensionally stable material that could be held to tight tolerance. The specifications also demanded exceptionally low warpage to ensure proper sealing under extreme braking conditions and temperatures.
In addition to supplying the material for Delphi's vacuum brake booster, DuPont provided advance support from the origin of the concept in the form of computer-based simulation studies to predict warpage and mold flow. DuPont also provided technical support to the component molders, DynaPlas Ltd. and Pixley Richards, Inc.
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