Plastic fuel tanks are today a common feature of not only cars, motorcycles, boats, and other watercraft but also a wide range of machinery powered by combustion engines. The latter include lawnmowers, leaf blowers, and mobile power generators. Plastic fuel tanks offer a host of advantages. Yet not all materials reliably prevent the escape of gasoline vapor.
A host of advantages
Plastic fuel tanks are highly popular with designers. There are a number of reasons for this. In the first place, plastic tanks can be shaped in almost any way and made to fit perfectly alongside surrounding parts. Moreover, the production of plastic tanks is cheaper, because it does not involve complex welding seams, as do metal tanks. At the same time, plastic tanks are generally lighter in weight, which eases handling, whether for a motorcycle or a lawnmower. And last but not least, plastic doesn’t rust.
One small drawback
Plastic tanks do have one small drawback: They are not completely impermeable. While there is no danger of a puddle of gasoline accumulating under the vehicle, a plastic fuel tank is unable to prevent the slow escape of gasoline vapor. This phenomenon is known as permeation and is a result of the polymer’s macromolecular structure. It can lead to substantial amounts of hydrocarbons being released into the atmosphere.
This explains why the U.S. Environmental Protection Agency (EPA) has now lowered the maximum permissible permeation of fuel from plastic tanks in motorcycles and other small items of equipment with gasoline engines. The maximum limit now stands at 1.5 grams per square meter of tank interior surface per day (g/m²/d). The UN Economic Commission for Europe is expected to follow the EPA’s lead here and likewise substantially reduce the maximum limit for motorcycle fuel tanks made of plastic.
Performance polymer blocks gas leakage
LANXESS is therefore developing a number of high-performance polymers that are highly impermeable to gasoline vapor. One such material is the new plastic Durethan BC 550 Z DUSXBL. “Polyamide 6 suppresses permeation so effectively that this still falls well within the much more stringent maximum limit implemented by the U.S. Environmental Protection Agency for motorcycle tanks. At the same time, it has a high impact strength, which means that tanks made of this material are very resistant to knocks,” explains Maik Schulte, a development engineer at LANXESS. First and foremost, the material offers an excellent alternative to high-density polyethylene, which is still used to fabricate many fuel tanks for gasoline engines.
Because of its permeability characteristics, high-density polyethylene has problems meeting the more stringent maximum limit for motorcycle tanks. The tank’s permeability to hydrocarbons can reduced by subsequent fluorination. Yet this involves an extra manufacturing step, which is complicated and costly on account of the highly aggressive nature of fluorine.
The newly developed performance polymer from LANXESS also offers substantial advantages compared to other tank materials such as aluminum, steel sheet, and multilayer plastic composites.
A single production process
The new polyamide 6 is formed into tanks by means of a process known as blow molding — a single-material solution par excellence, since no additional material is required for sealing. The new material also provides a cost-effective alternative to tanks made of multilayer plastic composites with a permeation barrier of EVOH (ethylene-vinyl alcohol copolymer). In the latter case, the layers of the composite are laminated in a technically demanding and comparatively expensive process.
Compared to steel sheet and aluminum, polyamide 6 also permits much greater freedom of design when it comes to the often complicated geometry of tanks. Similarly, elements such as mounting fixtures can be directly integrated, which facilitates subsequent assembly. And in contrast to steel sheet, for example, no forming, punching or welding is required, which also helps reduce costs.
Suitable for biofuels
Of equal significance is the fact that this new material offers a response to one of the most pressing problems of this century — namely, a reduction of our dependence on fossil fuels through the greater use of fuels produced from renewable materials.
Here at LANXESS, we have therefore taken care to ensure that the new polyamide 6 is compatible with the ethanol concentrations currently used in biofuels. In fact, initial studies show that the material is suitable even for gasoline with an ethanol concentration of 85 percent. “We expect our new polyamide tank material to be resistant to E85 fuels,” confirms Schulte.
Unlike aluminum in particular, the plasticizer-free and non-reinforced thermoplastic Durethan BC 550 Z DUSXBL has the advantage of not being affected by ethanol-based biofuels. Ethanol attracts water, which means that aluminum tanks without a protective coating on their inner walls are susceptible to corrosion. The particles caused by this corrosion can damage the engine and the auxiliary units.
In addition, the new material is a good alternative to duroplastic composites, which are a popular choice for fabricating fuel tanks for boats. Contrary to the experience with such composites, the polymer matrix is not dissolved by ethanol. As a result, with Durethan BC 550 Z DUSXBL there is no longer any danger of a buildup of deposits on the intake valves and resultant engine damage.