DSM thermoplastics featured in CNG and hydrogen tanks

Royal DSM (Singapore) reports that a combination of two of its thermoplastics technologies has resulted in high-performance pressure vessels that are well 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 percent.  

Countries with major natural gas reserves are looking at the advantages of using CNG as an alternative fuel which reduces the CO₂ emission by 15 percent compared to current solutions in petrol or diesel.

A traditional steel tank of 40 liters/10.6 gal weighs around 60 kg/132 lb, while a composite Type IV tank with DSM's Akulon (polyamide) Fuel Lock liner can weigh as little as 20 kg/44 lb. Every 10 kg/22 lb removed from a vehicle translates roughly into a reduction in CO₂ emissions from the vehicle on the road of 1g/km.

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 blowmolded 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 is said to further improve the already strong gas barrier of polyamide 6, and also provides it with extremely high impact resistance at low temperatures (down to -60°C/-76°F). 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 reportedly 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 blowmolding of liners for large pressure vessels for heavy duty vehicles such as buses and trucks. It is normally difficult to make large blowmoldings in polyamide 6, owing to the polymer’s relatively low melt strength, but this grade has sufficient melt strength to create a stable parison for tanks longer than 2m/6.6 ft, enabling high precision in control of the wall thickness.

Type V pressure vessels weigh up to 70 percent less than steel tanks and can be lighter than Type IV pressure vessels. DSM says they are more durable than steel, have better chemical resistance (no corrosion) and are 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 [Hasselt, Belgium] 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 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.”