Fuel Resistant Plastics
Mike Braeckel, Dwight Smith, Joseph G. Tajar and John Yourtee
a business of Celanese AG
Summit, New Jersey
(Excerpts appeared in Advanced Materials
& Processes - August 2000 - pages 37ff)
Limits on emissions and the need to simplify production have led manufacturers to replace steel with plastic in many fuel components, from gas caps to fuel rails. This fuel module is made of Ticona Celcon ® acetal copolymer because of its superior performance in aggressive fuels.
Without plastics, automakers would have been hard pressed to develop fuel systems that simultaneously withstand aggressive fuels, reduce vehicle weight, aid impact resistance, and enable complex geometries. Furthermore, given the complexity of fuel systems, changes in one area often affect other areas. This is the case when hot fuel from the engine compartment is returned to the tank. Plastic was introduced for fuel tank construction to improve chemical resistance and impact, and to enable tighter auto layouts versus steel tank designs. However, fuel temperatures rose sharply higher in the plastic tanks, because plastic is a good insulator, and because tighter layouts reduce the air flow that dissipates heat. As a result, fuel temperatures in tanks may reach 65....C (150°F) in plastic tanks, versus about 40....C (105°.F) in steel tanks, and can rise to 120°C (250°F) or more in the engine compartment. The higher temperatures also make fuel more reactive and require more stable plastics.
Environmental laws have also made fuel more reactive. The Clean Air Act of 1990 called for reformulated gasoline, which led to the addition of the oxygenate methyl tertiary butyl ether (MTBE). Because this chemical can cause plastics to swell, automakers either adjusted tolerances to compensate for expansion, or found MTBE-stable plastic grades. However, recent MTBE health and groundwater-contamination concerns make it likely MTBE may be replaced by other oxygenated materials such as ethanol. Unfortunately, many of these also are aggressive toward plastics.
Environmental initiatives have affected plastics in autos in other ways for the past few decades. For example, limits on evaporative emissions that began in the 1970s, have tightened since then until in the 1990s they drove changes in polymers for fuel caps, valves, charcoal canisters, seals, and other parts (Table 1 and 2). Such limits continue to tighten. The California Clean Air Board’s Low Emission Vehicle II (LEV II) standard, scheduled for 2003, will reduce vehicle evaporative emissions from 2 g/day to 0.5 g/day, and make dimensional stability and permeability more crucial issues. In Europe, EURO 2000, which will take effect in 2005, also will severely limit evaporative emissions and highlight similar issues.
Table 1 — Properties of fuel system plastics
Acetal Dimensional stability, chemical resistance, and low fuel permeability
Nylon 6/6 Good impact and other mechanical properties High temperature nylon (HTN) easier processing and better dimensional stability, chemical
resistance and impact than nylon 6/6, but not as good as PPS
Nylon 12 extruded multilayer parts to provide barrier properties, impact resistance and low permeability Aliphatic polyketone Impact and thermal resistances that fall between acetal and nylon.
Polyphenylene sulfide Excellent high temperature and chemical resistance, as well as excellent dimensional stability, erosion resistance and low permeability
High density polyethylene (HDPE) Good economics, as well as impact resistance and chemical resistance to road salts.
Polybutylene terephthalate (PBT) Dimensional stability, but should be used where temperature resistance and permeability are not important.
Properties needed for fuel applications
Plastics in fuel systems must perform at a consistently high level under demanding conditions for the life of a car. Several chemical, physical, mechanical and thermal properties are important for plastics to survive long-term fuel contact.
Chemical resistance relates to a change in chemical structure or composition, which generally causes a loss in performance.
Dimensional stability is an essential property because many fuel-system parts have tight tolerances. This factor is also important where different plastics or a plastic and a metal meet, since their different dimensional stabilities can affect seals.
Mechanical properties include impact, tensile, and compressive strength, as well as elongation and strength at break. Parts must be designed to retain sufficient integrity to do their jobs over the life of a vehicle.
Coefficient of thermal expansion is one of a number of temperature-related effects that include chemical reaction rates and mechanical property changes.
Permeability is now under intensive study in light of the LEV II standards. The industry is evolving standard tests and measurements.
Extended auto fuel study
As auto design life reaches 150,000 mi. or 15 years, it is essential to know how fuel system plastics withstand prolonged exposure to the new generation of more-reactive fuels. Unfortunately, tests on plastics for fuel contact are typically limited: some are run for just 48 hours, and others rarely exceed 500 hours (three weeks).
Recognizing the need for more extensive design verification, Ticona tested combinations of nine fuel blends and seven plastics at two temperatures (65° and 121°C) for more than 5000 hours each (Table 3). The lower temperature simulated fuel tank conditions, and the higher one simulated conditions under the hood. Initial results from this program, which lasted two years in total, support the continued use of most plastics now found in auto fuel applications. The study also indicated that one material (acetal homopolymer) might not be suitable for extended life with some aggressive fuels.
Ticona began the study by asking its auto OEM and Tier I fuel system customers what fuels and plastics they wanted to see studied. They chose seven plastics: acetal copolymer, acetal homopolymer, PPS, PBT, aliphatic polyketone, nylon 6/6, and high-temperature nylon (HTN).
The study also looked at many current and potential future fuel blends. Nine fuels were tested: three methanol blends; three ethanol blends, including TF1; TF2, an ethanol/methanol blend; and C and CAP fuels with aggressive water (i.e., water that contains highly reactive ions such as chloride) andperoxide. (Fuel that contains peroxide, a refining impurity that makes fuel more reactive, is often called sour gas.)
The wide range of methanol and ethanol concentrations evaluated reflects the practice by some gasoline distributors of cutting gasoline with less-expensive alcohol. The effects of this can vary.
For example, fuel blends with methanol are most aggressive in the 20% range.
Although the study began well before the July 1999 release date for SAE J1681, which sets standard fuels for screening plastics and elastomers, it anticipated many of the recipes contained in J1681.
The study also followed SAE J1748 protocols for testing and evaluating components in fuel tests, and evaluated mechanical and physical properties. Dimensional stability results showed that the resins tested generally swelled by about 1% to 3% during the first seven to 21 days of exposure, and changed relatively little after that. One exception to this pattern was acetal homopolymer (Figure 1), which swelled during the first seven days of exposure to CM15A (85% Fuel C and 15% aggressive methanol), held almost steady for the next 35 days, and then shrank steadily over the next 182 days until the study ended. The slope of the curve implies that shrinkage would continue unabated after this. The weight change data (Figure 2) followed the same pattern, suggesting that the fuel reacted with the homopolymer and altered its composition. The study also found that acetal copolymer had significantly less swelling and weight change than aliphatic polyketone in oxygenated and peroxide-containing fuels.
PPS had the best overall performance of the polymers studied in the 121°C (250....F) high temperature soaking test. It exhibited the lowest weight gain, the least dimensional change, and the highest retention of tensile strength over the range of fuels evaluated, with the differences accentuated with the more aggressive fuel blends. For example, in fuel CM15A, weight change (Figure 3) and dimensional change (Figure 4) were significantly lower for the PPS than for the HTNs and nylon 66.
All the nylons showed continual reduction in retained tensile strength with increased exposure time, whereas the PPS remained much more consistent (Figure 5). The study indicates that no quick answers are available in understanding how fuels and plastics interact, and that assumptions about these interactions should be supported by long-term testing. This is especially important for dimensional stability, because so many of today’s high-tech emission-control parts depend on tolerances of 0.05 to 0.076 mm (0.002 to 0.003 in.) Even modest changes in dimension can create problems. For example, a part 12.7 mm (0.5 in.) in diameter that swells just 2% would grow 0.25 mm (0.01 in.) and may exceed tolerances set for it. Swelling also can affect permeability and create microleaks.
Evaluating new fuel formulations will clearly be an ongoing task as fuel oxygenate content changes.
MTBE is due to be phased out by 2003 and replaced by other blending components, possibly TAME (tertiary amyl methyl ether), ETBE (ethyl tertiary butyl ether), and/or ethanol. Similar issues also apply to trucks. A commitment to bio-diesel fuels in Europe has led to the introduction of rapeseed oil (a.k.a., canola oil), commonly called “rape seed methyl ester,” or RSME. This makes fuel more acidic and aggressive to plastics at high temperatures.
Plastic manufacturers are improving their products to cope with the new fuels. They are lowering permeability while enhancing dimensional stability, chemical resistance, thermal capability and more. A good example of this is Ticona’s Hostaform C13031 XF acetal copolymer, which is designed to be stable during prolonged exposure to RSME-containing diesel fuels at relatively high system temperatures.
In meeting future auto fuel system needs, especially on a global basis, plastic suppliers must account for a wide variety of more aggressive fuels, longer vehicle life and higher temperatures. This will mean higher-performing plastics able to cope with any fuel under any set of conditions.
Existing resins can be reformulated to enhance their properties. Or automakers can move up the performance chain, switching from acetal copolymer to PPS to more than double thermal capabilities. In terms of this example, acetal has a long history in fuel system applications for emission control valves mounted on or within fuel tanks where temperatures are moderate. At higher underhood temperatures, PPS would be a more appropriate valve material. Other options are new grades of plastics with advanced performance characteristics.
For more information: Dwight Smith, Application Development Engineer, Fuel Systems, Ticona,
90 Morris Avenue, Summit, NJ 07901; telephone: 765/478-4826; e-mail: email@example.com;
Web site: http://www.ticona.com.