Flying higher on plastics

C&I Issue 1, 2008

The issue of climate change is now firmly established  at the top of the public agenda. Developments such the award of the Nobel Peace prize to Al Gore and last month’s climate change talks in Bali ensure that the issue is never far away from the news headlines. The aviation industry has come under particular scrutiny. Greenhouse gas emissions from air travel account for 3.5% of the total ‘human activities’ contribution to climate change, but they have increased by 87% since 1990.

And as a greater proportion of the world’s population acquires the wealth to travel, the aviation business is set to expand even more. In September, Boeing predicted that China’s domestic air travel market will grow nearlyfive-fold in the next 20 years.

All of this means that the aviation industry is under great pressure to deliver improvements in CO2 emissions. The complete solution – an alternative source to petroleum for power generation – is being developed, but the widespread use of cleaner fuels seems far off at present. However, substantial emissions savings can still be made in other areas, as a new generation of planes shows.

One of these new aircraft is the Boeing Dreamliner, set to have its first flight at the end of March 2008. Both the Dreamliner and the recently launched Airbus A380 show marked improvements in both passenger comfort and environmental performance, enabled by the use of high performance plastics. Roughly half of the Dreamliner and 25% of the A380 consist of glass-, carbon- or quartz-reinforced resins, so-called plastics composites.

Advanced composite components in planes make for a significant improvement in the ecological impact of flying. The Dreamliner, for example, will be 20% lighter than a comparable plane of this size, reducing fuel consumption and greenhouse gas emissions by 20%. And the A380, says Airbus, will use less than 3 litres fuel per passenger per 100 km.

But both these new aircraft are not just more economical, they also set new standards in terms of flying experience and passenger comfort (C&I 2007, 22, 10). Composite materials can withstand higher pressure and thus permit increased air pressure in the cabin. At a cruising altitude of 35 000ft, the atmosphere in the Dreamliner amounts to 6000ft, the equivalent of being in Mexico City, instead of an altitude of 8000ft in traditional planes. This should diminish fatigue from flying and help to alleviate jet lag problems as well as about 50 further symptoms, according to Boeing.

While an even lower cabin atmosphere would be technically feasible, studies have shown that this doesn’t bring additional benefits for passengers. Moreover, air at altitudes of 6000ft also contains 10% more available oxygen than at 8000ft, which helps to reduce drowsiness. In addition, more humidity will be possible in the cabin air, causing less dry eyes and dehydration for passengers. Last, but not least, the Dreamliner will have bigger windows so that passengers can make the most out of their trip above the clouds.

Plastics composites show exceptional properties in that they are particularly strong and able to withstand great pressure. At the same time, the material is lighter than traditional materials used for aircraft structures. It is this combination of characteristics that enables advances in both environmental performance and passenger comfort. Moreover, plastics composites are more resistant to corrosion, which enables higher humidity in the cabin. Boeing foresees that the lower level of material fatigue and corrosion associated with composites will lead to a decrease in maintenance costs of 30%. While traditional airplanes need to make their first heavy maintenance check after six to eight years, the lower potential for damage with composite materials means that the Dreamliner will need this only after 12 years.

Composite materials combine two material components, with the matrix or binder holding together a cluster of fibres of a much stronger material, the reinforcement. The matrix, mostly plastics, not only binds the fibres together, but also protects them from damage by stress. Thermosetting plastics, malleable polymers that form a strong plastic once heat is applied, are widely used for matrix applications. Epoxy resins dominate the market for reasons of price, but cyanate ester and polyimide resins are also possible, while even engineering thermoplastics can sometimes be used. For highly specialised purposes, ceramics, carbon and metals can figure as the matrix. Ceramics, for instance, are employed in space rocket components where resistance to high temperatures is critical.

Reinforcements for plane structures mainly consist of carbon fibres, which result in very strong materials, but quartz and glass reinforcement also feature in the A380. Glass fibres and phenolic resins, for instance, are combined to produce parts of the passenger cabin of Airbus’ latest aircraft, with a view to keeping down smoke development in case of fire. The nose of the Airbus A380 consists of quartz-fibre reinforced plastics. Another very strong reinforcement material is aramid, a polymer fibre that is also used in bullet-proof vests and helmets.

The flooring in aircraft is reinforced with aramid in order to withstand the extreme pressure per square inch exerted by high heels. The reinforcement can also be supplemented by a honeycomb made of plastics, aluminium or paper to make so-called sandwich composites that show particular bending stiffness.

By carefully choosing the combination of reinforcement and matrix material, engineers can produce properties that exactly match the requirements for a particular structure or purpose. The direction of the fibres, for instance, determines in which direction the material can sustain stress. Aircraft wings can thus be made to bend in one direction, but not in others.

The use of composites has also changed the way planes are manufactured. Before the introduction of composite materials, airplanes used to be built by creating an aluminium skeleton reinforced by steel. Stringers and spars were riveted together and then large, thin sheets of aluminium were fastened on with rivets. To join the parts together, holes are drilled, by hand or by laser-guided machines, and then each hole would be filled with a fastener that joins the parts together. Once the body sections were complete, they would be linked together and filled with insulation, wiring and windows.

The production of composites marks a departure from a loud and dirty shop floor. Hannes Schlosser, an engineer at Austrian supplier FACC, which manufactures composite structures for both Boeing and Airbus, explains that plane parts are produced in a clean room that ensures constant temperatures and air humidity and prevents the inclusion of dust in the final product. Air or dust inclusions can threaten the stability and resistance of the composite. Final plane parts are therefore scanned by ultrasound to ensure consistent quality.

To make the composite, pre-soaked layers of fabric are laid down in precise patterns on moulds (mandrels). The matrix, often a liquid resin, is then sprayed or pumped in before the pieces are placed in vacuum-tight containers. The mould is then baked for between 1.5 and 2.5 hours at temperatures of 120 or 180°C in big ovens, so-called autoclaves, where pressure of between 2 and 10 bar is applied.

During this process, the resins ‘cure’ into a strong form that cannot be melted again, whereas the pressure ensures that they enclose the fibres tightly. The curing process transforms the resin into a plastic as molecular chains link into a rigid, three-dimensional structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. Aromatic amines are the most important catalysts for the curing process for epoxy resins.

Plastics have also come to replace some of the rivets in joining different plane parts together. To fill the space between thicker sandwich composites whose edges aren’t flat, epoxy resins in various densities are applied and then cured to make a stable bond.

Composites started to appear in planes in the 1970s and have steadily increased their share of aircraft structures since. For its next project, the A350XWB, scheduled for 2013, Airbus foresees using 50% composite materials. But the trend has only just started. With almost 200 orders for the A380 and more than 700 orders for the Boeing Dreamliner, the era of plastic planes is set to continue, to the benefit of both passengers and the environment.

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