The following article appears in the journal JOM, 48 (7) (1996), pp. 22-25. JOM is a publication of The Minerals, Metals & Materials Society

Steel vs. Plastics: The Competition for Light-Vehicle Fuel Tanks

INTRODUCTION

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

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 Trucks	5,033	5,709	6,535	6,749	6,923	7,008	7,088	7,103	7,117
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
Share (%)	Steel No. of Tanks	Steel Lost (tonnes)	Change Incre. (tonnes)	Change Cum. (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

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

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 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) fuel-permeation standards are challenging the designs and materials used in vehicle fuel tanks. Flexible-Fuel Vehicles The automotive industry has been pursuing ways to meet the legislative and regulatory pressures to reduce emissions in the form of FFVs capable of running with methanol/gasoline mixtures. However, testing has verified that neither bare nor painted terne will resist the corrosion effects of alcohol (methanol or ethanol)-based fuels for the expected life of the vehicle—241,350 km or ten years. The actual methanol fuel considered was M85 (85% methanol, 15% gasoline); however, this fuel has recently lost popularity because, although it costs about the same as gasoline, it offers only half the energy 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 the engine are expected to be recyclable. The EPA currently uses the sealed housing evaporative determination (SHED) test to measure pollution caused by fuel evaporation. All major vehicle-manufacturing nations except Japan have accepted SHED as the evaluation method of choice. The current SHED requirement of 24 grams maximum of hydrocarbons in 24 hours has been tightened to the more restrictive level of two grams per day.3 The material implications for steel are tighter demands on the integrity of tank seams and filler neck joint areas. Implications for plastics are the need to switch to harder-to-recycle material systems and more expensive processes to meet the stricter SHED test.

Barrier Technology

Solvay Automotive of Troy, Michigan, has developed technology to keep HDPE tank emissions down to 0.2 g/24 h or less, even with methanol-containing fuels.¹¹ Using technology called Solvay-optimized fluorination, Solvay equals or exceeds the permeation performance of coextrusion with EVOH barrier resin. Solvay continues to add multilayer capacity to 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

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

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.

Design Features and Weight

TANK MATERIALS AND ORIGINAL EQUIPMENT MANUFACTURERS

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). General Motors General Motors has an ongoing corrosion test program that includes two-side painted Zn-Ni, hot-dipped galvanneal, and hot-dipped tin. Zn-Ni-coated tanks are currently used on the J (Cavalier/Sunfire) and N platforms (Grand Am/Skylark) and the new fullsize van (GMT 600) launched in November 1995. The company's current plans include a conversion of all of their current terne fuel tanks to either Zn-Ni or galvanneal fuel tanks by 2000.

The average gas tank for a compact automobile (e.g., Nissan Altima) can boast weight savings of up to 30% versus a similar steel tank.¹⁴ However, Cadillac claims that although their plastic tanks allow design flexibility with increased safety, they do not achieve any weight savings over steel tanks.¹⁵ 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

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 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

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 Advantages: Low cost at high volumes, recyclable, materials cost, and permeability Disadvantages: Shape flexibility, ineffective corrosion protection from methanol fuel, lead-containing coating Electrocoated Zn-Ni and Galvanneal Advantages: Low cost at high volumes, recyclable, effective inside and outside corrosion protection, material cost, and permeability Disadvantages: Weldability and shape flexibility Hot-Dipped Tin Advantages: Low cost at high volumes, recyclable, effective inside and outside corrosion protection, material cost, permeability, and weldability Disadvantage: Shape flexibility Stainless Steel Advantages: Corrosion, recycable, and permeability Disadvantages: Cost at all volumes, formability, and joinability
Plastics HDPE Advantages: Shape flexibility, low tooling costs at low volumes, weight, and corrosion resistance Disadvantages: High tooling costs at high volumes, high material cost, permeability, and recyclability Multilayer and Barrier HDPE Advantages: Shape flexibility, low tooling costs at low volumes, weight, corrosion resistance, and permeability Disadvantages: Higher tooling costs at high volumes, higher material cost, and harder to recycle

Recyclability

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

COMPETITIVE ANALYSIS

References

For more information, contact P.J. Alvarado, U.S. Steel, 201 W. Big Beaver Road, Suite 1400, Troy, Michigan 48084-4171; (810) 740-3484; fax (810) 740-5179; e-mail ALVASTEEL@AOL.COM.