Application Development Engineer
Ticona, the technical polymers business of Celanese AG
Designers of automotive fuel systems have an increasingly tough task. They must ensure these systems function well for a decade or more in the face of aggressive fuels and other insults in the auto environment. Materials must be dimensionally stable to prevent microleaks and retain mechanical properties so they withstand physical, vibratory, and other loads.
Designers often rely on plastics for this task. Compared to metal, plastics can weigh less, improve crash safety, and consolidate multiple parts into one. They also need less secondary finishing, ease assembly, and make possible complex geometries in gas caps, valves, tanks, seals, lines, and other components.
PLASTICS IN FUEL APPLICATIONS
Automakers have traditionally turned to a variety of polymers when building auto fuel-system components:
Polyamides, including nylon 6/6, high-temperature nylon (HTN), and nylon 12, top the material choice list. Nylon 6/6 is economical and has good strength and toughness. HTN has better dimensional stability, chemical resistance, and impact strength than nylon 6/6, but not as good as that of polyphenylene sulfide. Nylon 12 exhibits barrier properties, impact resistance, and low permeability in extruded multilayer parts.
Acetal copolymer (POM) and homopolymer (POM-H), provide dimensional stability, chemical resistance, and low permeability to fuel.
High-density polyethylene (HDPE) is economical and has good impact strength and stands up well to road salts.
Polybutylene terephthalate (PBT) has good dimensional stability and generally serves where temperature resistance and permeability are not important.
Polyphenylene sulfide (PPS) withstands temperature extremes and harsh chemicals and is dimensionally stable. It also boasts low permeability to fuel components.
Applications include fuel-sending unit flanges and low-heat control valves made from POM, POM-H, and PBT as well as external fuel tanks from HDPE. Additionally, fuel filler pipes come from nylon or HDPE, while fuel lines in the U.S. tend to be multilayered and have nylon 12 as a major component.
Use of the right polymer helps ensure fuel systems will retain their strength, stiffness, impact resistance, dimensions, and other properties for the life of a vehicle. A review of test data is essential for an informed material choice. Until recently, most plastics in fuel systems were immersion-tested for 500 hr or less. This is changing, however, as designersrealize they must know how plastics withstand-prolonged exposure to fuels containing ethanol, methanol, and other oxygenates.
New immersion numbers are becoming available. One example is an extended database from resin manufacturer and supplier Ticona on how plastics and fuels interact. It tabulates results from an independent laboratory for fuels based on the SAE J1681 protocol. They include fuels simulating nonoxygenated gasoline (Fuel C), oxygenated fuels containing ethanol (TF1, TF2, CE22A, and CE85A), oxygenated fuels containing methanol (CM15A, CM25A, and CM85A), and a sour gas gasoline (Fuel CAP).
The study evaluated nine fuels and three plastics at typical 149°F (65°C) fuel-tank temperatures and five fuels and four plastics at typical 250°F (121°C) engine-compartment temperatures. Physical and mechanical measurements on each sample took place seven times during the 5,376-hr (32-week) study and followed SAE J1748 test protocols and appropriate ISO and ASTM procedures.
Most of the plastics evaluated retained their integrity during the study. They generally swelled 1 to 3% over the first 170 to 500 hr after immersion in fuels and had relatively little dimensional change after that. The one exception, POM-H, may not be suitable for extended life with some aggressive fuels.
At 149°F POM best resists oxygenated and nonoxygenated fuels. In fuel C, for example, unfilled POM-H had 40% more dimensional change than unfilled POM. Filled PBT had the greatest weight change of the filled grades. Tensile modulus and strength held steady or declined only slightly over the life of the study.
In CM15A at 149°F, the weight of filled and unfilled POM-H fell from an initial gain of about 2.5 and 3.5% to losses of about 2.5 and 1.5% in the last half of the study, respectively. Filled PBT dropped even more and ended the study at an 8% weight loss.
Dimensional change for unfilled POM-H rose sharply by over 2% after 168 hr and then started a steady decline to near zero by the end. All other materials rose initially and then held relatively steady.
In fuel CAP at 149°F, weight and dimension for unfilled and filled POM-H increased at first and ended the study by dropping 4 and 15%, respectively. Dimension and weight for its POM counterpart also rose initially, but only modestly, before holding steady. The tensile strength at break for unfilled POM held its original value during the test, while unfilled POM-H lost 15% and the filled POM-H materials lost between 50 and 90% of their initial values.
At 250°F, linear PPS had the least weight and dimensional change and the greatest tensile strength retention, especially with the more aggressive fuels. It was superior to HTN in the more aggressive fuel blends and equal to or better than HTN in milder blends. Linear PPS was more consistent than HTN and nylon 6/6, both of which lost retained tensile strength over long exposures. Linear PPS and HTN were generally superior to nylon 6/6.
In fuel CAP, nylon 6/6 had the greatest initial weight gain and then dropped to 3.5% by 5,376 hr. Both PPA and HTN had a similar pattern and ended the study with a 3 and 5% loss, respectively. Only linear PPS held steady at between a 3.0 and 3.5% increase after an initial weight gain.
In CE22A, nylon 6/6 had the greatest weight gain and linear PPS the lowest. All materials held more or less steady after an initial weight gain. Nylon 6/6 also suffered the greatest loss in tensile strength.
In CM15A, nylon 6/6 again had the greatest weight gain. Other resins rose initially and held steady, with PPS showing the least gain. HTN and nylon 6/6 had the greatest dimensional change at first, but fell off late in the study. PPA and linear PPS remained steady after an initial rise. Linear PPS retained 80% of its initial tensile strength by the end of the study, while the other materials had fallen to between 30 and 40% of their original values.
Overall, the study found most of the plastics underwent relatively small and steady changes once they adapted to a fuel and so remain good candidates for use in today's fuel systems. POM performed best at 149°F, while linear PPS topped the list at 250°F.
In addition to these long-term fuel immersion data, Ticona also has long-term continuous-use data on acetal copolymer, PBT, and linear PPS materials per UL746B testing protocol.
PLASTICS AND FUEL CELLS|
The fuel cell is a possible successor to the internal combustion engine. If projections are correct, fuel-cell-based cars may make up 20% of the global fleet 20 yr from now. Plastics should play a significant role in making this possible.
Fuel cells generate electricity by converting hydrogen and oxygen to water, electricity, and heat. Proton exchange membrane (PEM) fuel cells, most likely to be used in autos, use pure hydrogen or hydrogen released from a material such as methanol or natural gas. Each PEM cell has a polymer electrolyte membrane coated with platinum catalyst between a flat anode and cathode. Many of these low-current cells are bolted into stacks to yield the voltage desired.
Plastics will likely serve in PEM-cell membranes, bipolar plates, end plates, manifolds, pumps, and portions of the heating, cooling, electrical, fuel supply, and waste-removal systems.
PEM-cell membranes need excellent ion conductivity, a relatively high-use temperature, good mechanical properties, and chemical resistance. One new membrane material, polybenzimidazole, operates at temperatures up to 374°F (190°C) and thereby has better energy efficiency than current membranes, which operate below 212°F. This is important for the next generation of PEM cells that may function at 356°F (180°C) or more.
Thermoplastics in bipolar and end plates provide needed dimensional stability, mechanical strength at elevated temperatures, flame retardancy, and resist the effects of deionized water as well as gaseous and liquid fuels. A Ticona design-engineering study found that injection-molded bipolar and end plates made of linear PPS and/or LCP reduce plate costs as much as 50% of those of metal or thermosets. The new materials also weigh 30% less.
ADAPTING TO NEW NEEDS
Plastics slated for current and future fuel systems must allow for a wide variety of reactive fuels, longer vehicle life, elevated temperatures, and many other factors. The call for higher-performing plastics is challenging resin suppliers. That's because they must adapt their products so the right materials are on hand to cope with any fuel under any set of circumstances.
Consider the adoption of biodiesel fuels (i.e., those having several percent of rapeseed methyl ether) in Europe. These fuels are more acidic and so more reactive than standard diesel blends. Given this reactivity, automakers considered switching from acetal to more costly polymers in emission control valves, tank-mounted valves, and fuel-sender units. A new grade of POM, Hostaform C13031 XF, works well in temperatures to 212°F (100°C) and withstands continuous use in these applications. The new grade also has greater elongation at break and thus better impact and crash resistance.
Another factor is fuel-permeation rates. Polymers are increasingly chosen for their ability to control fuel permeation through part walls. A comparison of permeation rates for selected plastics in fuel systems found that linear PPS had the lowest rate at 0.12 gm-mm/m2-day, followed by POM at 0.35 gm-mm/m2-day. HDPE had the highest permeation at 65 gm-mm/ m2-day.
Other polymers being developed have better properties in such areas as dimensional stability, and thermal and chemical resistance. Electrostatic dissipative grades of POM, for example, limit static discharge and the possibility of thermal events as fuel is pumped into a tank. These grades are finding their way into fuel caps, sending units, and pumps, as well as filler necks, valves, and flanges.
Automakers can also switch to higher performingresins as requirements change. A shift to linear PPS from POM, for example, more than doubles a part's thermal ability. Another possibility is to add more effective barrier materials (liquid-crystal polymers (LCPs) in place of polyvinyl alcohol) to reduce permeability in fuel tanks. Yet another option is fluorinating, a process often used with monolayer HDPE diesel tanks on passenger vehicles in Europe. Here, designers lower permeation rates by adding a thin, inner layer of polytetrafluoroethylene after the tanks are blow molded.
The need to reduce evaporative emissions is also driving consideration of nontraditional tank fabrication methods including special overmolding techniques, vacuum forming, and rotational molding. And joint design is being influenced by stricter emission limits leading some designers to laser weld adjacent parts rather than using gaskets or snap fits.
New fuel-tank designs have begun to integrate vapor-management components such as fill-limit vent valves and rollover valves within the tank. In so doing, designers are turning to highly stable polymers having the integrity to withstand a decade or more of continuous immersion in fuel.
Alterations in a fuel system can require that polymers perform at higher levels than anticipated. The adoption of plastic fuel tanks allows more freedom for tank shapes than was possible with steel. Because plastic is a thermal insulator the conversion to plastic raised fuel-tank temperatures to about 149°F versus roughly 104°F (40°C) in steel tanks. Also, the tighter layouts possible with plastic limited air circulation and the amount of heat carried away by moving air. The rise in tank temperature makes fuel more reactive and thus demands use of more stable plastics.
Environmental regs for emissions
Environmental regulations are key drivers when it comes to plastics in fuel systems. Such laws mandate the use of more reactive fuels to cut exhaust emissions and require control of hydrocarbon evaporative emissions. In the U.S., the Clean Air Act of 1990 called for reformulated gasoline containing oxygenates. Initially, methyl tertiary-butyl ether was the oxygenate of choice, but health and groundwater concerns have been forcing a shift to other additives such as ethanol. While oxygenates reduce emissions, they can weaken plastics and change their dimensions over time.
Limits on evaporative emissions have grown increasingly strict. A good example is the amendment that California's Air Resource Board added to its Low Emission Vehicle (LEV) law in 1998 for LEV Phase II (LEV II) and partial zero emission vehicles (P-ZEV). The LEV II standard starts being phased-in during 2004 and will be completed by 2010. It limits emissions in each passenger vehicle to no more than 0.5 gm/day.
At the same time, P-ZEV is being implemented as a phase-in and limits emissions to no more than 0.3 gm/day. This phase-in of P-ZEV applies to a yet-to-be-determined percentage of vehicles made for sale within California. In Europe, EURO III (Euro 2000) set whole-car permeation levels below 2 gm/day and for fuel systems from gas cap to fuel rail at 0.5 gm/day maximum.
In meeting LEV II, P-ZEV and EURO III, designers must eliminate microleaks between components having tolerances as tight as a few thousandths of an inch, use less permeable polymers, and find new ways to package the fuel system. The latter involves new designs such as placing a shield around fuel tanks to capture emissions or inclosing within the tank fuel filters and other elements normally found outside.
Other regulations also affect plastics used in fuel systems. For instance, the U.S. National Highway Transportation Safety Authority FMVSS 301 recently required that fuel systems withstand crash speeds of 50 mph, up from 30 mph. This impacts fuel-system design and fosters a need for stronger and tougher plastics.
Alfmeier Corp., (864) 299-6300,
Siemens VDO Automotive AG, 49 (0) 69 4 08 05-0
Ticona, the technical polymers business of Celanese AG, (800) 833-4882,