Steel Vs

S4PAKSWISSIANS

Steel vs. Plastics: The Competition for Light-Vehicle Fuel Tanks Peter J. Alvarado As weight and cost savings drive changes in performance criteria for automotive materials, original equipment manufacturers (OEMs) are taking a harder look at the historically terne-coated steel used for gas tanks. This article compares steel and plastic for gas tank uses according to performance attributes and a competitive analysis. Legislation issues and current OEM activity are included.

INTRODUCTION Historically, terne-coated steel (an 8% tin-lead coating) has been the mainstay for automotive gas tanks; however, several issues are changing the performance criteria that must be met and, thus, threaten the application of steel products. The drivers for a material change are legislation, increased required part life to ten years/241,350 kilometers, permeability, weight, packaging, safety, and cost. In this article, the performance attributes of the plastic and steel alternatives are reviewed from an original equipment manufacturer (OEM) perspective in the critical areas of manufacturability, cost, design, weight, safety, corrosion, and recyclability. A comparative analysis of the various plastic and steel alternatives indicates that steel remains a cost-effective material that meets all of the required performance criteria. A more specific cost comparison of the new plastic tanks (i.e., multilayer or barrier coated) with the new steel tanks is still required. Many of the drivers such as lead reduction, clean fuels, permeability, and weight are a direct result of legislative and regulatory pressures described in the Issues sidebar. \

Figure 1. The 1998 and 2003 projections of plastics usage in fuel tanks, based on the Delphi VII report.

PLASTIC FUEL TANKS Since the mid-1980s, automakers have been displacing coated-steel fuel tanks with plastic ones. During the 1993 model year, approximately 2.7-3 million cars and trucks built in North America used nonmetallic tanks. This represents 22-25% of the market,

S4PAKSWISSIANS compared to 16% in 1990. By comparison, the European market uses 70-90% plastic tanks, and the Japanese market uses 5% plastic tanks4 (Figure 1). Although plastic-tank applications have experienced some reversals as a result of the stricter permeation standards, some experts believe their usage will gain momentum by the end of the decade as new plastics technology is converted to commercially feasible operations. The Delphi VII report by the University of Michigan indicates that experts predict plastic tanks will capture 28% of the North American market by the end of 1996 and up to 50% of the market by 2000.5 However, this projection needs to be tempered with the higher manufacturing cost and recyclability issues of the multilayer plastic tanks that will be required to meet the stricter permeation standards. This projection is considered as the worst-case scenario for steel if the industry fails to provide the OEMs with a cost-effective steel alternative that meets all of the performance criteria. Table I indicates the production volume of vehicles built in North America;6 Table II shows the estimated number of plastic gas tank units (according to Delphi VII projections) and their impact on steel shipments. As of 1993, the steel industry lost the opportunity to ship about 34,473 tonnes of steel as a result of plastics gains in gas-tank applications. In the worst-case scenario, continued acceptance of plastic gas tanks will increase steel's loss to 71,667 tonnes in 2000—an additional 43,544 tonnes. This means that annual shipment of 125,191 tonnes of steel will be reduced by better than a third to a total of about 81,646 tonnes per year. Table I. Actual and Projected Car and Light Truck Production in North America (in thousands)6 1992 1993 1994 1995 1996 1997 1998 1999 2000 Cars 7,474 8,141 9,050 9,775 9,960 10,125 10,280 10,358 10,435 Light 5,033 5,709 6,535 6,749 6,923 7,008 7,088 7,103 7,117 Trucks Total 12,507 13,850 15,585 16,524 16,883 17,133 17,368 17,461 17,552

Table II. Projected Plastic Gas Tank Share Based on the Delphi VII Scenario Steel Change Change Share Steel No. Lost Incre. Cum. (%) of Tanks (tonnes) (tonnes) (tonnes) 20 2,501,000 28,365 22 3,047,000 34,552 6,187 6,187 24 3,740,000 42,414 7,862 14,049 26 4,296,000 48,717 6,303 20,352 28 4,727,000 53,605 4,888 25,240 30 5,140,000 58,284 4,679 29,919 32 5,558,000 63,022 4,738 34,657 34 5,937,000 67,319 4,297 38,954 36 6,319,000 71,651 4,332 43,286 Incre.—incremental; Cum.—cumulative.

PLASTICS TECHNOLOGY

S4PAKSWISSIANS High-density polyethylene (HDPE) has been the resin of choice for plastic gas tanks, and production capacity has been on the increase. Kautex of Canada built a new plant in Avilla, Indiana, to meet anticipated increases in demand for plastic automotive fuel tanks. Production was scheduled for 400,000 tanks in 1994 and eventually will be boosted to between 600,000 and 700,000 units per year.7 These plastic tanks are currently being used on Chrysler's Jeep Cherokee and T300 trucks. Belgium-based Solvay is the exclusive supplier of plastic fuel tanks to General Motors' Saturn Division. Solvay has also expanded their Canadian subsidiary in Blenheim, Ontario, and installed two new blow-molding machines to make HDPE tanks for the Chrysler's LH series and Viper sports car. Chrysler expects to sell 300,000 LH vehicles, all with HDPE tanks that offer more volume capacity than steel tanks. Monolayer-HDPE tanks offer long-term structural integrity but will not meet future permeation requirements. Chrysler started to switch in 1995 to multilayered HDPE to meet the more stringent SHED test. The emergence of new technologies has enabled the increase of plastic gas tanks. These new technologies can be grouped into either multilayer or barrier types.

Multilayer Technology Some manufacturers see multilayer tank technology as the answer to stricter emission standards. Ford uses six-layer fuel tanks made of HDPE and, at one point, considered a $110 million investment in machinery and equipment to produce the tanks (which would have been the first commercial use of coextruded tanks). However, Ford decided to increase steel-tank stamping capacity at Dearborn; the Explorer and the new F-150 truck (PN96) gas tanks have been switched back from plastic to zinc-nickel coated steel.8 Kautex is supplying Ford's facility in Milan, Michigan, with this six-layer technology. The tanks are designed to meet California's stricter evaporative fuel standards and consist of an inner layer of HDPE joined by an adhesive layer and barrier layer of polyamide or ethylene-vinyl alcohol copolymer. An additional adhesive layer is joined by a layer of "regrind" and an outer layer of HDPE. Walbro Automotive Corporation began commercial production of multilayer plastic fuel tanks for 1995 models. Annual production of these multilayer tanks is expected to reach 500,000 units by 1998. Their tank uses a barrier layer of ethylene-vinyl alcohol (EVOH) that is sandwiched between two layers of HDPE.9

LEGISLATION AND REGULATORY ISSUES A number of federal and state legislative and S4PAKSWISSIANS

Barrier Technology

regulatory actions have been initiated to promote the use of cleaner automotive fuels such as reformulated gasoline, methanol, ethanol, and natural gas. In 1988, the U.S. Department of Transportation introduced a Clean Fuels Grant Program for local transit authorities. The Alternative Motor Fuels Act of the same year established production incentives and a federal purchase plan for alternative fuel vehicles. Later, the Clean Air Act Amendments of 1990 established fleet mandates to reduce emissions from the primary source of air pollution (i.e., vehicles). Two years later, the Energy Policy Act of 1992 encouraged the use of alternative-fuel vehicles or flexible-fuel vehicles (FFVs) to reduce oil imports and increase the use of U.S. sourced and renewable fuels. In addition, future increases in Corporate Average Fuel Economy (CAFE) will add pressure to lighten every vehicle component, including gas tanks. These issues and stricter U.S. Environmental Protection Agency (EPA) fuelpermeation standards are challenging the designs and materials used in vehicle fuel tanks.

Air Products and Chemicals of Allentown, Pennsylvania, has commercialized a fluorine-based barrier technology that enables plastic fuel tank manufacturers to meet more stringent emissions standards. The SHED tests completed in 1992 on Airoguard plastic tanks produced by Kautex of Canada indicated hydrocarbon permeation rates as low as 0.1 g/24 h— significantly lower than rates for tanks using previously available barrier technology. The performance of the Airoguard tanks compares with multilayer extrusion tanks while maintaining the long-term structural integrity of Flexible-Fuel Vehicles monolayer tanks.10 The automotive industry has been pursuing ways to Solvay Automotive of Troy, Michigan, has meet the legislative and regulatory pressures to reduce emissions in the form of FFVs capable of running with developed technology to keep HDPE tank methanol/gasoline mixtures. However, testing has emissions down to 0.2 g/24 h or less, even verified that neither bare nor painted terne will resist with methanol-containing fuels.11 Using the corrosion effects of alcohol (methanol or ethanol)based fuels for the expected life of the vehicle— technology called Solvay-optimized fluorination, Solvay equals or exceeds the 241,350 km or ten years. permeation performance of coextrusion The actual methanol fuel considered was M85 (85% with EVOH barrier resin. Solvay methanol, 15% gasoline); however, this fuel has recently lost popularity because, although it costs about continues to add multilayer capacity to the same as gasoline, it offers only half the energy meet Chrysler's requirements. Aero Tec Laboratories of Ramsey, New Jersey, has developed a semiflexible safety fuel tank made from an undisclosed olefin compound of polymers and antidiffusion-barrier additives.12 The tank can be used for race cars and military vehicles as well as general automotive aftermarkets.

STEEL FUEL TANKS North American auto manufacturers are currently supplied with tanks comprising a steel substrate coated with either terne or zinc-nickel. In all, about 125,191 tonnes of steel substrate per year are applied to gas tanks.

density by volume. In the case of ethanol, the EPA and others estimate wholesale costs between $1.50 and $2.10 per gallon.1

More recently, natural gas has pushed high methanol and ethanol fuels to a very distant second and third in popularity. Natural gas is affordable—$0.70 worth provides the same energy of one gallon of gasoline. Additionally, it produces no evaporative emissions and meets ultralow tailpipe emissions.2 Furthermore, there is already a pipeline infrastructure which, coupled to a low-cost compressor, would allow owners to refill at home. However, compressed natural gas cylinders are bulky and heavy, thus reducing space and range. Natural-gas vehicles, like electric vehicles and hybrids, stress the need for weight reduction to achieve the same performance of driveability and range as current automobiles. So do hydrogen-powered vehicles, which represent another alternative that requires heavy tanks. The need for lightening this type of vehicle also increases the possibility of alternative materials substitution for steel parts in other components throughout the vehicle. Permeability/Evaporative Emissions Three types of evaporative emissions are being studied: refueling vapor, running (non-tailpipe) emissions, and permeation. Also, because of environmental pressures, the materials used to contain the fuel and deliver it to

S4PAKSWISSIANS Stainless steel tanks have been tested, and although effective in flexible fuels, they are difficult to form without severe breakage occurring during stamping. Also, stainless steel is expensive, with an estimated cost ratio to terne steel exceeding 5:1. The electrocoated zinc-nickel product is painted on both sides with an aluminum-rich epoxy. Industry accelerated tests on the corrosion of painted zinc-nickel confirm that it will meet a ten-year life in current fuels and flex-fuels and resist external corrosion. Testing the characteristics of painted galvanneal (zinc-iron alloy coated steel) have found it effective for resisting corrosion on both the inside and outside surfaces of the tank. General Motors has a product undergoing testing. Hot-dipped tin has also been found to be effective for resisting all fuels, but it does require a paint coating for exterior protection from road-induced corrosion. This product welds faster than painted terne and has a better potential for good solderability than painted galvanneal and zinc-nickel coated steel substrates, permitting the attachment of fuel filler tubes and other lines.

PERFORMANCE ATTRIBUTES: PLASTIC VERSUS STEEL Manufacturability Terne plate holds a materials cost advantage over HDPE: $0.66-0.79/kg vs. $0.861.08/kg.13 The cost of the material is not the only driver; consideration includes the net cost of the fabricated tank and its reliability within the total fuel system of the vehicle, including the tank, filler tube, level control, baffles, the housing reservoir for the sending unit, and other assorted tubing, fittings, and seals from the tank to the engine. All of these components must function properly with the various fuel types and for the life of the car. Unforeseen corrosion can easily contaminate the fuel delivery system and cause costly repairs. Manufacturing costs for either tank material seem conflicting, depending on the source. Nevertheless, due to the invested capital of OEMs on stamping, welding, and assembly equipment for metal tanks, their cost structure indicates a lower cost per piece on steel tanks versus plastic ones, with the latter usually being outsourced (except for some Ford models). Plastic tanks are formed by blowing a thick continuous tube of HDPE within a mold that determines the final shape of the virtually seamless part, which could include the filler neck. The blowing molds are cast from aluminum and cost considerably less than machined steel dies used to stamp steel tanks. In general, four or more molds are integrated into one rotary style blow-molding machine to achieve the desired productivity (i.e., one station blows while the other one cools). Typically, the OEMs outsource the plastic tanks to various suppliers who bid for the business. The plastic tank manufacturer also has to either chlorinate or fluorinate the plastic to retard permeation, and both processes can be highly toxic if mishandled. This represents additional OSHA requirements, which add to the cost of the tank.

TANK MATERIALS AND ORIGINAL EQUIPMENT MANUFACTURERS S4PAKSWISSIANS

Design Features and Weight Plastic tanks have the ability to meet packaging constraints with complex shapes, and design engineers have greater flexibility in the car design and styling without having to worry about fitting the gas tank. The plastic tank could virtually be made to fit whatever cavities are left by the design. Other attachments to and within the plastic tank require gasketed mechanical joints. However, plastic swells with constant exposure to organic liquids and vapor, thus making the joints very critical in the event of repairing the tank hardware.

Chrysler Chrysler made the decision to outsource plastic tanks in 1988 and, given recent developments in multilayer HDPE technology, they remain committed to plastic tanks. The listed advantages of plastic over steel according to Chrysler include lack of corrosion concerns, easier packaging, ability to design thickness where needed, and lower weight. Whether all of these attributes offset the higher unit cost of plastic tanks remains uncertain. Their plans indicate that all of their tanks will be made of plastic by the end of 1996. Eventually, all plastic HDPE or sulfonated HDPE tanks will have to be switched to the more expensive and slower-to-process coextruded ethylene-vinyl alcohol/HDPE layered tanks—a must for the tanks to pass stricter permeability standards. This mandatory switch represents an opportunity for steel tanks, which will be more cost competitive against the coextruded plastic tanks. Ford Ford's timetable calls for a switch to Zn-Ni-coated steel from terne-coated steel tanks in all their models by 1997. Ford is currently using Zn-Ni on the Bronco, Grand Marquis, Explorer, the new F-150 (PN96), and the new Taurus (DN101). The switch to Zn-Ni will also include the plastic tank in the current Escort/Tracer but other models with plastic tanks will remain the same (e.g., Ranger, Windstar, Aerostar, and Contour/Mystique).

The average gas tank for a compact automobile (e.g., Nissan Altima) can boast weight savings of up to General Motors 30% versus a similar steel tank.14 General Motors has an ongoing corrosion test program that includes two-side painted Zn-Ni, hot-dipped galvanneal, and However, Cadillac claims that hot-dipped tin. Zn-Ni-coated tanks are currently used on the J although their plastic tanks allow (Cavalier/Sunfire) and N platforms (Grand Am/Skylark) and design flexibility with increased the new fullsize van (GMT 600) launched in November 1995. safety, they do not achieve any The company's current plans include a conversion of all of weight savings over steel tanks.15 their current terne fuel tanks to either Zn-Ni or galvanneal fuel tanks by 2000. These two examples seem to contradict the general view that plastic's weight advantage increases with the size of the tank. On the other hand, the new permeability requirement is expected to diminish the weight advantage of plastics.

Safety One critical part of the performance criteria of the tank is its ability to meet crash requirements. Generally, plastic tanks are considered safer in crashes because they are seamless and, thus, not prone to failures in the vulnerable seam areas. They are not a source of sparks. Also, plastic tanks deform and have some ability to rebound back to shape. When steel tanks absorb energy and deform, the pressure within the tank increases as the volume decreases. This makes them vulnerable at welded or clamped areas where failure can potentially occur. The thermal properties of the chosen material are also an issue, especially due to the proliferation of injector fuel delivery systems, where a portion of the unused fuel delivered by the gas pump is returned to the gas tank at "engine-hot" temperatures. At the same time, the tank must withstand extreme temperatures in North America from -40°C

S4PAKSWISSIANS to 79°C in-tank temperatures. The 79°C temperature not only exceeds the boiling point of the alcohol fuels, but also creates sagging problems for plastic (especially under the weight of a filled tank) while the extreme cold introduces potential cracking problems. As a result, OEMs resort to heavier gauge plastic, negating at least some of the weight advantage, and must also use support brackets and special shields against the heat of local sources like an inferior or perforated muffler or tailpipe. High ambient temperatures underneath the car remain a consideration. Plastic acts as an insulator to retard heat transfer to the fuel when compared to a steel tank. In the case of an under-car fire, plastic tanks will retard the rise in fuel temperature, but they will soften, sag, and eventually release the fuel. A steel tank does not sag in a fire; however, the fuel temperature may rise rapidly, perhaps resulting in over pressurization and release of fuel through a mechanical fitting. The American Iron and Steel Institute reports16 that a series of more than 75 tests undertaken by the National Fire Prevention Research Foundation and Factory Mutual Research Corporation indicated that plastic containers storing flammable or combustible liquids in general purpose warehouses fail abruptly when exposed to a small fire. This failure results in a rapidly developing spill fire that overpowers conventional sprinkler systems. The same tests conducted with flammable and combustible liquids stored in steel containers resulted in no spill fire, no excessive temperatures, no content involvement, and no significant loss of visibility due to smoke. The fires involving the steel containers extinguished themselves. These findings have led to a return to steel containers from plastics for safety and fire insurance cost reasons. It is not known if tests have been conducted by OEMs to compare the performance of steel and plastic tanks in under-car fire situations.

Corrosion Corrosion is a well-known concern on both the inside and outside surfaces of tanks. The outside surfaces and supporting structure are exposed to road chemicals, salt, mud, and gravel. The corrosion issue is critical with zinc-coated products that replace terne plate because of their sacrificial nature, which puts an even higher demand on the quality of the barrier film for both inside and outside surfaces. In contrast, the HDPE gas tanks are inert to the corrosive environments inside and outside the tank.

S4PAKSWISSIANS Table III. Competitive Materials Analysis for Gas Tanks Performance Attributes Recyclability, safety, corrosion resistance in methanol fuels, and weight Manufacturing Issues Cost, formability/shape flexibility, weldability Steel •

•

•

•

Terne-Coated Steel o

Advantages: Low cost at high volumes, recyclable, materials cost, and permeability

o

Disadvantages: Shape flexibility, ineffective corrosion protection from methanol fuel, lead-containing coating

Electrocoated Zn-Ni and Galvanneal o

Advantages: Low cost at high volumes, recyclable, effective inside and outside corrosion protection, material cost, and permeability

o

Disadvantages: Weldability and shape flexibility

Hot-Dipped Tin o

Advantages: Low cost at high volumes, recyclable, effective inside and outside corrosion protection, material cost, permeability, and weldability

o

Disadvantage: Shape flexibility

Stainless Steel o

Advantages: Corrosion, recycable, and permeability

o

Disadvantages: Cost at all volumes, formability, and joinability

Plastics •

•

HDPE o

Advantages: Shape flexibility, low tooling costs at low volumes, weight, and corrosion resistance

o

Disadvantages: High tooling costs at high volumes, high material cost, permeability, and recyclability

Multilayer and Barrier HDPE o

Advantages: Shape flexibility, low tooling costs at low volumes, weight, corrosion resistance, and permeability

o

Disadvantages: Higher tooling costs at high volumes, higher material cost, and harder to recycle

S4PAKSWISSIANS

Recyclability The Resource Conservation and Recovery Act discourages the use of materials that cannot be recycled and might end up in landfills.17 As a result, automotive-design engineers must not only meet customer, design, styling, cost, weight, and regulatory needs but also environmental criteria. All materials suppliers must show that their product is not only lighter and cost effective but also recyclable. In this respect, plastics must work the hardest to show that they are recyclable and have the ability to be recovered in vehicle disassembly in a cost-effective manner. To accomplish these objectives, automotive designers must develop prototypes that can be disassembled easily into the various material groups that have a recycling infrastructure. Despite progress in recycling, the proliferation of plastics in automotive applications faces some hurdles. •

The absence of a plastics recycling infrastructure. The infrastructure for recovering and recycling the ferrous content of cars is well established—70-80% of a typical passenger car is recoverable steel and iron.

•

The molding process for plastic fuel tanks. As is the case for other ap-plications, this process results in roughly 30% of plastic material ending as industrial waste.18

•

The necessary sorting of the various plastics types since mixing types can ruin the batch. This is not a problem with steel—the scrap industry recycles 10.8 million tonnes per year of shredded automotive iron and steel, which is used to make new steel products.19

•

The lack of technology that dismantlers can use to quickly collect various plastics. The current infrastructure of automobile dismantlers, shredders, and scrap-metal processors is steel dominated and relies on magnetic separation and inexpensive shredding equipment for efficient and low-cost processing. In the case of terne gas tanks, the units are removed from the car, flattened, baled, and sent either to special operators that can reclaim the lead or to the steel mill.

•

Cost. Recycled plastics are not cost competitive with virgin plastics.20

While the recycling of HDPE gas tanks is easier to tackle in terms of dismantling and avoiding type mixing, these tanks will not meet the new evaporative emissions standards. The barrier-type and multilayer tanks that will meet such standards can pose a bigger challenge to recycle in a cost-effective manner.

COMPETITIVE ANALYSIS Table III summarizes the advantages and disadvantages of steel and plastic with respect to the performance and manufacturing objectives for gas-tank applications. The comparative analysis of the performance attributes of the various plastic and steel alternatives indicates that steel products still represent a cost-effective material that meets all the required performance criteria of gasoline tanks. A description of current OEM tank activity is described in the sidebar.

S4PAKSWISSIANS

References 1. Timothy T. Maxwell and Jesse C. Jones, Alternative Fuels: Emissions, Economics and Performance (Warrendale, PA: Society of Automotive Engineers, 1995), pp. 29-42. 2. Robert Q. Riley, Alternative Cars in the 21st Century (Warrendale, PA: Society of Automotive Engineers, 1994), pp. 173-176. 3. Bundy International, "Fuel Supply Systems for a Healthier Environment", ed. Michael Scarlett, Automotive Technology International '94, pp. 37-40. 4. "Plastic Bounces Back in Fuel Tanks," Automotive News (January 30, 1995). 5. Delphi VII Forecast and Analysis of the North American Automotive Industry (Ann Arbor, MI: Office for the Study of Automotive Transportation, University of Michigan Transportation Research Institute, 1994), pp. 71-75. 6. Automotive Outlook: June 1995, (West Chester, PA: Autofacts, Inc.), pp. 4-1, 5-1. 7. "Canadian Blow Molder Expands to U.S.," Canadian Plastics (February 1993). 8. "Ford's Plastic Plant Eyes $110 Investment in Gas Tanks," Plastic Week (August 10, 1992). 9. "Supplier Profile: Walbro Entering Fuel Tank Market; Manufacturer Looks to Double Sales," Ward's Automotive Reports (August 24, 1992). 10. "Air Products Develops Technology that Helps Plastic Fuel Tank Manufacturers Meet More Stringent Emission Standards," Waste Treatment Technology News (December 1992). 11. "Gas Tank Molder Unfazed by CARB," Plastics World (May 1992). 12. "AeroTec Fuel Tanks Designed for Safety," Plastics News (April 12, 1993). 13. "Cadillac Unveils Shift from Terne to Plastics," American Metal Market (January 27, 1992). 14. "Plastic Gas Tank Field is Filling Up," Plastics News (May 11, 1992). 15. "Allante to Use Plastic Tank," Plastics News (January 13, 1992). 16. Steel, the Safe Solution for Flamable Liquid Storage, CN-06, (Washington, D.C.: AISI, 1990). 17. "Recycling and the Automobile," Automotive Engineering (October 1993), pp. 42-50. 18. Asian Autotech Report, 186, (3) (1994), p. 11. 19. "Steel Recycling Increases in '94," American Metal Market (April 5, 1995), p. 10. 20. "Missing Link; Lack of Infrastructure Poses Huge Hurdle to Plans to Recycle Auto Plastics," Automotive News (Supplier Spotlight) (March 14, 1994).