April, 06, 2011 - Substantial amounts of hydrocarbons leak through the walls of plastic fuel tanks in gasoline engines to the outside and pollute the atmosphere – a phenomenon known as permeation. LANXESS is therefore working on special blow-moldable grades of polyamide 6 that act as an extremely effective barrier against gasoline fuels. One example of such a material is Durethan TP 142-011. “The polyamide 6 is so effective in suppressing fuel permeation that test results are well below the upper limit for motorcycle tanks, which was recently tightened considerably by the U.S. Environmental Protection Agency (EPA). It is also impact-resistant, a quality it lends to the blow-molded tanks it is used to make,” said Maik Schulte, a development engineer at LANXESS. The material is an especially good alternative to high-density polyethylene, which has been used to make many plastic tanks for gasoline engines until now. It also has considerable advantages over other tank materials, such as aluminum, sheet steel and multi-layer plastic composites. 

Plastic fuel tanks are used not only in the gasoline engines of cars, motorcycles, boats and other watercraft, but also in a number of combustion machines such as lawnmowers, leaf blowers and mobile power generators. This diversity explains why the EPA (Environmental Protection Agency) has reduced the upper limit for fuel permeation from the plastic tanks of motorcycles and other smaller gasoline engines to 1.5 grams per square meter of interior surface per day (g/m2/d). The UN Economic Commission for Europe also plans to dramatically cut the upper limit for plastic motorcycle tanks in the near future and to align itself to the EPA’s specifications. 

Given the new reduced upper limit, high-density polyethylene (HDPE) has reached its limit in terms of permeation. Its effectiveness as a barrier against hydrocarbons can be improved by subsequently fluorinating the blow-molded tanks made from it, but due to the very aggressive nature of fluorine, this additional production step is both cost-intensive and complex in terms of the equipment and safety procedures involved.

A single-material solution for tanks

The new polyamide 6 is a single-material solution for blow-molded tanks. It therefore provides a cost-effective alternative to producing tanks made of multi-layer plastic composites, which contain a polar layer of ethylene-vinyl alcohol copolymer (EVOH) as a permeation barrier. The various layers are produced through coextrusion blow molding, which is also expensive and technically challenging. Compared with sheet steel and aluminum, polyamide offers the advantage of allowing much more design freedom when it comes to the often complex geometries of tanks. Functions such as fasteners can be integrated directly, which makes subsequent assembly easier. It is also more cost-effective as – unlike with steel sheet, for example – no forming, punching or welding is needed. 

Resistant to E85 fuels

Unlike aluminum in particular, the plasticizer-free and non-reinforced thermoplastic has the advantage of not being affected by ethanol-based biofuels. Because ethanol attracts water, particles caused by corrosion often form in aluminum tanks – the interior surfaces of which have no protective coating – and these can, in turn, damage the engine and its peripheral units. “We expect our new polyamide tank material to be resistant to E85 fuels, too,” said Schulte. These types of fuel contain 85 percent ethanol.

Source: Lanxess