Clean air act

In a bid to cut ozone-depleting chemicals, GKN Aerospace develops an improved material for aircraft fuel bladders that can be RF welded. Siobhan Wagner reports.

Engineers at GKN Aerospace in Portsmouth, have developed an improved material for aircraft fuel bladders as part of an initiative to reduce and remove volatile organic compounds (VOCs) from fuel tank manufacturing.

The material is created using a new tank assembly process that removes ozone-depleting chemicals used in traditional rubber manufacture, and eliminates most adhesives and the solvents they contain.

Manufactured from a flexible, single-skin thermoplastic polymer, rather than traditional nitrile rubber, the bladder incorporates a crash and puncture-resistant textile based on a woven aramid fibre.

Traditionally, manufacturers thought they could only achieve these features using multiple layers of composites with adhesives joining the bladder to the protective textile layers. The drawbacks of this method are that it uses solvent-based adhesives and a final lacquer coating to protect the exterior of the bladder — both of which produce VOCs.

The material also contains an integrated fuel barrier to provide the same level of fuel tolerance as traditional tanks.

Another advantage is that because the material is polyurethane-based it can be welded using radio frequency (RF) technology.

‘Radio frequency welding is an attractive process for us because it offers a cold welding technique, which means we are not applying heat directly to the materials but inducing heat through RF waves,’ said John Pritchard, site director for GKN. ‘So the assembly operators are not handling hot welding equipment and the heat exposure of the materials is controlled.’

Frank Bamford, senior vice-president of business development and strategy at GKN, said his group estimates that the use of this material could lead to a 60 per cent reduction in the use of VOCs and a 30 per cent reduction in manufacturing time. In addition, the weight of each fuel bladder could be reduced by approximately five per cent through the removal of the adhesive coatings that are used in traditional construction.

Pritchard said there has been a drive to reduce VOCs across the aerospace industry since the 1989 Montreal Protocol on substances that deplete the ozone layer, an international treaty designed to protect the ozone layer.

‘This has driven the industry to seek innovations that allow development of less damaging technologies,’ he said. ‘The highest profile change in the aerospace industry was the end of 1.1.1.trichloroethane as a cleaning material. However, the removal of other VOCs within manufacturing processes is just as important.’

GKN’s Portsmouth facility has been developing and manufacturing aircraft fuel tanks since 1939 and aircraft fuel bladders since the early 1940s.

The company began been making an effort to reduce VOCs in the fuel bladder manufacturing process several years ago when it determined that the largest amount of VOC release at the site was related to the manufacture of the bladders.

Pritchard said the primary aim of the single skin material is to provide fuel bladders for helicopters which require a crash-survivable bladder — in other words, a bladder that does not rupture during an otherwise survivable accident. He said it is an ideal solution for air ambulances, police helicopters, aerial survey and the military.

The company launched the new bladder material at the Heli-Expo in Houston, Texas, last month, and is now taking orders from customers.