Royal DSM says a combination of two of its most innovative thermoplastics technologies has resulted in high performance pressure vessels that are ideally suited for use as lightweight fuel tanks for automobiles running on compressed natural gas (CNG) or hydrogen.

With a solution for both the inner liner and the outer tape reinforcement, DSM is able to reduce the weight of the tank by up to 70%.

A traditional 40-liter steel tank weighs around 60 kg (132 lbs), while a composite Type IV tank with Akulon Fuel Lock liner can weigh down to 20 kg (44 lbs).

Type IV pressure vessels are based on plastics and continuous fiber reinforcements, unlike Type I, II and III pressure vessels, which contain metal components. DSM demonstrated a tank with a liner blow molded in its Akulon Fuel Lock, a polyamide 6-based engineering plastic with very high barrier to hydrocarbons, at the Fakuma plastics processing exhibition in Friedrichshafen, Germany, in mid-October.

The tank can be wrapped in a tape based on thermoset or thermoplastic resins, including its EcoPaXX polyamide 410.

Akulon Fuel Lock contains an additive formulation that further improves the already strong gas barrier of polyamide 6, and also provides it with extremely high impact resistance at low temperatures (down to -60°C). The permeation of HDPE liners is too high to allow the installation of composite Type IV tanks incorporating such liners inside a vehicle. However, Akulon Fuel Lock liner material reduces emissions by a factor of at least 150 compared to HDPE and therefore enables the use of Type IV tanks inside the car.

The Akulon Fuel Lock portfolio has been expanded with a grade that is suitable for blow molding of liners for large pressure vessels for heavy duty vehicles such as buses and trucks. It is normally difficult to make large blow moldings in polyamide 6, owing to the polymer’s relatively low melt strength, but this grade has sufficient melt strength to create a stable parison (a rounded mass formed by rolling a molten material immediately after removal from the furnace) for tanks beyond a length of 2m, enabling high precision in control of the wall thickness.

In current Type IV pressure vessels, the tape reinforcement comprises either glass or carbon fibers in a thermosetting polymer, such as an epoxy or unsaturated polyester. DSM is cooperating in the development of next-generation Type V pressure vessels. These are made by winding a tape, developed by DSM, of continuous fiber reinforced with a thermoplastic, such as the company’s EcoPaXX polyamide 410, or another grade of Akulon.

Type V pressure vessels weigh around 70% less than steel tanks and can be lighter than Type IV pressure vessels too. They are more durable than steel, they have better chemical resistance (no corrosion), and they are also fully recyclable. EcoPaXX has the additional advantage that it has a zero carbon footprint from cradle to gate, owing to the fact that the polymer is made entirely from renewable resources.

“In the process developed by Covess for making Type V pressure vessels, you can balance weight, performance, and economics by using glass, carbon or even hybrid fibers,” says Tony Vanswijgenhoven, Director of Covess, a specialist in advanced thermoplastic composite vessels for a wide range of applications, who is working closely with DSM. “Whatever choice you make, it always works out lighter than the existing comparable type IV tanks.”