The Recent Evolution of Fuel System Materials

Forty years ago steel was the material of choice for fuel systems. Not only was steel an excellent barrier against fuel vapour but the piece price of components, such as fuel tanks, was also very competitive when produced in large volumes.

Notwithstanding the advantages of using steel fuel system components, the weight penalty and ever present potential of leakages and corrosion paved the way for the introduction of plastics in the 1970’s. As market and legislative demands increased new plastic materials were developed which have improved fuel system performance in all the key areas: Safety, permeability, weight, packaging, cost and durability to name a few.


These high performance plastics hold several advantages over steel:



• Easily formed, plastics allow for innovative design and manufacturing techniques
• High impact strength plastics, retain integrity after an impact thereby reducing the risk of leakage
• Plastics do not corrode
• Replacing steel with plastic generally saves weight thereby improving fuel-efficiency

The move away from steel resulted in replacement materials such as high-density polyethylene (HDPE) and several polyamides (in particular polyamide 12 also known as nylon 12 or PA12) finding widespread use in fuel systems. This proliferation led to a steady increase in consumption of plastics in the automotive sector.

Despite HDPE, used in fuel tank manufacture, accounting for approximately 44% of the plastic used in fuel systems; fuel lines, vapor control canisters and fuel modules drive the demand for a wide range of other plastics.


Image credit: Plastics news graphic by ScottMerryweather

However these plastics, in particular PA12, are not only used in the automotive industry: Several other high growth industries, such as the photovoltaic industry and offshore pipeline manufacturers, are also large consumers of engineering plastics.

This combined demand for PA12 increased to a point where in 2012 the global capacity was stretched to the limit. The two largest suppliers, Germany’s Evonik Industries AG and French competitor Arkema SA, accounted for about half of the world's supply of PA12, according to Analyst Tim Urguhart of research firm IHS Automotive.

With the global supply already stretched, a massive explosion and fire at Evonik Industries’ Marl facility in Germany, in March 2012, left the automotive industry facing a potential crisis.

Consequences of the 2012 Evonik fire

Not only was Evonik a major global supplier of PA12, but they also produced an estimated 40 percent of the world's supply of Cyclododecatriene (CDT), which is used to make laurolactam which, in turn, is used as a monomer in PA12.

Shortly after the incident, industry executives met in the Detroit suburb of Southfield to form the Automotive Industry Action Group, looking for consensus on alternatives that could fill in for Nylon-12 until supplies were back to normal. The meeting comprised Detroit’s Big Three automakers as well as international competitors such as Volkswagen and Honda, along with key suppliers DuPont Automotive and Delphi.

In a statement released after the meeting TI Automotive CEO, Bill Kozyra, confirmed: "It is now clear that a significant portion of the global production capacity of PA12 has been compromised, thus threatening uninterrupted production of certain components".

With suppliers typically carrying a PA12 inventory of two weeks, the industry was facing a crisis that required alternative materials to avert the looming disaster.
Although there are automotive tubing substitutes for nylon 12, including nylon 6, nylon 10 and nylon 10/10, testing and validating parts made with these replacement compounds could take months: Especially as these were mostly safety critical components.

However, even before the disruption, robust demand for the resin had pushed prices higher, and many suppliers and automakers were already working on alternative materials, according to IHS.

Alternative PA12 replacement materials

Following the 2012 events DuPont CEO, Ellen Kullman, announced several alternative high-performance materials: Materials suitable for applications in mono- and multi-layer fuel lines, fuel vapor tubes, PCV hoses, air brake hoses and tubing, brake line coatings, vacuum brake tubes and fuel line connectors.

The alternative materials being referred to included: The Zytel long chain polyamide product family comprising 612 and renewably sourced 610 and 1010; Hytrel thermoplastic elastomers, as well as Zytel polyphthalamide (PPA) and Vamac ethylene acrylic elastomer (AEM).
The Zytel Long Chain Polyamides (LCPA), which DuPont claims combine outstanding heat resistance with chemical and hydrolysis resistance, are of particular interest. With attributes such as high temperature resistance, rigidity and lower permeability to fuel and gases Zytel LCPA is ideally suited to fuel system applications.
Furthermore Zytel is also available in renewably sourced (RS) grades.


Image credit: DuPont industries

When compared to similar materials Zytel holds several advantages:

• Consistent supply of raw materials from multiple sources, including up to 63% renewable content in the RS grades of PA610 and PA1010.
• Similar properties/functionality to PA12
• Chemical resistance and ZnCl/CaCl stress cracking resistance that is comparable to PA11/PA12 and better than toughened, plasticized PA612 & PA610
• Excellent yield stress/stiffness balance, better than PA11/12
• Blow moulding compatible

Another alternative to PA12 is the Genestar polyamide engineering plastic produced by American company Kuraray. Developed from a unique C9 monomer Genestars properties include low water absorption, high heat resistance, high chemical resistance, and excellent dimensional stability.

With applications that range from electrical to automotive components it offers a superior cost-performance alternative to traditional materials such as ETFE, PA6, and PA12.

Another interesting material which has been used in place of PA12 comes from plastic specialist company ARkema. Branded as Rilsan HT, it is claimed that this thermoplastic from the polyphthalamide (PPA) family has superior long-term resistance to thermo-oxidative and chemical aging at high temperatures.

Currently used in the fuel filler neck of the Porsche 911 Turbo and Boxster, Rilsan HT is claimed to be the only PPA-based resin offering processing characteristics similar to aliphatic PA. Whilst having a renewable carbon content derived from non-food-crop vegetable feedstock, Rilsan HT is six times lighter than steel and three times lighter than aluminium.

Notwithstanding the wide range of plastics available for application in current fuel systems alternative gaseous fuels, such as compressed natural gas and hydrogen, will require new materials to meet the pressure and permeability demands of the proposed fuels.

With traditional steel alternative-fuel tanks the gas can migrate to the metal during extended use. This makes the metal brittle, fatiguing it to the point where gas would escape from the tank.

Higher quality steel can prevent embrittlement, but raises the cost and the weight of the tank: a premium steel tank holding 3 kg of hydrogen could itself weigh 400kg, making packaging extremely difficult.

New materials for alternative fuels

In October 2013 Rice University chemist, James Tour, unveiled a new material that could solve these problems. The new material, using an enhanced polymer, is far more impermeable to pressurized gas than previous plastic tanks; whilst far lighter than metal tanks.
The researchers led by Rice graduate student Changsheng Xiang produced thin films of the new composite material by solution casting graphene nanoribbons (GNRs) treated with hexadecane and thermoplastic polyurethane (TPU).

The tiny amount of treated GNRs accounted for no more than 0.5 percent of the composite’s weight. But the overlapping 200 to 300 nanometer-wide ribbons were nearly as effective as large-sheet graphene in containing the gas molecules. The GNRs’ geometry makes them far better than graphene sheets for processing into composites, Tour said.

Tour’s breakthrough "unzipping" technique for turning multiwalled carbon nanotubes into GNRs, first revealed in Nature in 2009, has been licensed for industrial production.
Notwithstanding the need for change; any new processes, materials and technologies have to be proven before they enter series production. Without extensive testing and validation problems are bound to occur.

Unexpected consequences of changing material

In April 2013 Chrysler announced a recall of about 30,000 Jeep Patriot and Jeep Compass four-wheel-drive vehicles from the 2012 model year.

The recall stems from the saddle-style gas tanks that go around the rear drive shaft. To maintain delivery, fuel is pumped from one side of the tank to the other through a fuel transfer tube, which was designed to be produced using PA12.


Image credit: Chrysler service bulletin

As a result of the worldwide shortage of PA12 Chrysler was forced to use an alternative material in these Compass and Patriot fuel tank tubes.

The new material required a higher manufacturing temperature than PA12 resulting in some of the hoses not being correctly formed, which in turn breached the fuel flow to the engine. The process was easily corrected, but the consequences of the material change, namely the recall, were not insignificant.

Despite several alternatives PA12 is still widely used in the automotive industry. To meet this ongoing demand Evonik has not only rebuilt the damaged facility in Marl but has commissioned a new production plant in Singapore, which will come online in 2015.

Performance polymers business unit head Gregor Hetzke said at an October 17 news conference, at K 2013, that Evonik anticipates nylon 12 resin demand to grow at an eight percent annual rate through 2018. The growth will come from a variety of nylon 12 applications in the automotive, oil and gas, solar and medical device markets.

Conclusion

With plastic fuel tanks having been in production for over forty years, there have been several technologies and materials developed to keep pace with the ever evolving requirements. But vehicle propulsion systems have never been this diverse with each system facing different challenges:

• Hybrid vehicles face noise problems when in EV mode, as well as heat from the battery packs
• Fuel systems have to operate with a wide variety of fuels and blended fuels without corrosion or evaporative emissions
• Natural gas vehicles need safe, reliable and cost effective cells that have low permeability and are lightweight

Fortunately with evolving sciences such as nanotechnology, and materials such as carbon fibre and graphene, it’s likely unique and unusual solutions to these challenges will reach series production in the years to come.


Sources:

• PlasFuelSys interview– Matthew Beecham
• Chicago tribune – Dupont, Dow to help global automakers avoid output crunch. (Bernie Woodall – Reuters)
• The Minerals, Metals & Materials Society – Report on the Competition for Light-Vehicle Fuel Tanks
• Rice University - Mix of graphene nanoribbons has potential for cars.
• Automotive News – Suppliers race against clock to avoid resin shortage (Dustin Walsh)
• NewsTribune.com - Chrysler recalls over 247,000 vehicles (Tom Krisher and Dee-Ann Durbin)
• http://www.puneplastics.org – Properties and applications of Rilson HT
• DuPont - Zytel LCPA resin specification homepage
• American chemistry council - Composite fuel tanks allowing alternative fuels to be carried onboard

Peter Els is a technical writer for Automotive IQ