Aeronautics and polymers: plotting a course for simplicity
In the space of a few decades, polymers have significantly reduced aircraft weight and improved engine efficiency. For some years now, aircraft manufacturers have been working to clean up their industry, leading to new technological advances such as hydrogen fuel cells and the recycling of composites and polymers.
Recyclability: you can bank on composites
Carbon/epoxy composites are still the Swiss Army knife for high-tech industries looking to cut down on weight. It is found in aircraft fuselages and wings, racing boats, Formula 1 cars and even top-of-the-range running shoes…
Replacing thermosetting resins with thermoplastic resins, which are better suited to a circular economy, in the design of fibre-based composite materials is one of today’s major challenges.
Though this composite possesses some extraordinary qualities, it is not without its faults, particularly from an environmental standpoint. In the vast majority of cases, the carbon fibres are impregnated with a thermosetting resin, usually epoxy, polyester or vinyl ester. However, once they have been polymerised, thermosetting resins cannot be melted down and remoulded, and recycling them is complicated.
This is a drawback in an age that values a circular economy…
Aircraft manufacturers are looking to replace them with thermoplastic resins such as polyamides, polybutylene terephthalate or PEEK—reversible resins. They can be remoulded once heated and melted down, and, more importantly, they can be recycled. Another benefit is that they do not emit volatile compounds when the fibres are impregnated, making them easier to handle. As for their thermomechanical properties, they are often comparable with those of “thermosets” (this is particularly true with PEEK). They would be the perfect solution if it were not for their high viscosity (1,000 times higher than that of thermosetting resins at the processing temperature), which makes it more difficult to impregnate the fibres. This is one of the issues that is gripping the composites industry at the moment; every lab is looking for a way to lower the melting point to reduce viscosity and make it easier to manufacture parts, all without negatively affecting the temperature of use (termed thermostability by scientists). This is vital research for the polymer industry because the metal industry, and the alloy industry in particular, is making rapid progress and is just waiting to claw back the top spot.
Composites: super cool
Engineers working on composites are currently trying to find a more energy-efficient way of solidifying them. At present, parts are “cooked” in autoclaves, a kind of giant pressure cooker. This is a long and fairly expensive process, given the high cost of the equipment.
Many research centres are looking at less expensive and more flexible alternatives to autoclaves, such as processes that involve infusion or injection of liquid resin directly onto a fibrous preform (a sort of fibre-reinforced skeleton).
Manufacturing processes are also being called into question. Researchers are looking to simplify them and, most importantly, make them cheaper.
In an ideal world, you would be able to carry out the entire operation at room temperature and still produce parts of the same quality. According to some research centres, such a solution is just around the corner…
Polymers on the mend
To further improve composite materials, the European Union (EU) has launched and financed the HIPOCRATES project, which is dedicated to the development of self-repairing materials. Little short of magic, these are materials capable of repairing microcracks and small breakages without human intervention. This property would be all the more important within the aviation industry since aircraft wings and fuselages are subjected to micro-shocks (hail, for example) every day. This is no cause for concern as aircraft are designed to withstand these conditions, but over time it causes composites to wear out a little more quickly, requiring multiple human interventions to repair them and keep them in good condition. Designing structural composite materials based on self-repairing polymers is a real challenge. To be effective, these repairs must be carried out quickly and, above all, remain stable.
Two different self-repair methods have been tested. The first involves adding microcapsules containing self-repair agents and a catalyst to the composites. In the event of a microcrack, the capsules break and release the repair agent, which then comes into contact with the catalyst. The result is polymerisation, which encapsulates the crack and prevents it from developing. The second method uses reversible polymers. The resin is simply enriched with a new chemical element capable of filling a crack after being stimulated by an external signal (heat, radiation or electrical induction). Not much more is known about this at the moment because this research is being kept secret. All we know is that two new composites have been successfully tested… The research team hopes to see practical applications within five years, the time required to carry out all the necessary testing…
Smart polymers are a thing of the not-so-distant future. Research into self-repairing composites is rapidly advancing.
Europe has also funded research into a hydrophobic coating that will do away with the need for windscreen wipers on cockpit windscreens. Windscreen wipers might seem insignificant, but they weigh a few kilograms and increase aircraft drag. Admittedly, they do not consume a huge amount of fuel, but it all adds up… This coating is essentially a skin made from a polyurethane silane gel subjected to hydrolysis. It is painted onto the windscreen, and once dry, it leaves behind a solid layer that easily repels rainwater. The coating also includes a transparent, heat-conducting layer which helps eliminate water when applied to the surface of the glass. Watch this space…
Doing away with windscreen wipers would save a few kilograms and improve aircraft aerodynamics. Researchers are looking to replace them with a hydrophobic polymer coating applied to the windscreen.
Propelled back into service
An electric-powered jumbo jet is the thing of dreams. While we wait for it to become a reality, we have to make do with small electric aircraft, usually two-seaters (and more often than not still in the testing phase). All these planes have one thing in common: their ultra-light construction made from composite materials. The projects are numerous and concern all manufacturers, including the biggest like Airbus. However, we will not be seeing a commercial aircraft flying through the air in total silence any time soon. Aircraft manufacturers are looking for a real breakthrough innovation in the field of batteries; in the meantime, engine manufacturers like Safran are seeking to innovate and improve traditional jet fuel engines. The company has developed the Open Rotor, a new type of engine that is much larger than current jet engines and is designed to be installed near the tail of the fuselage. The engine fairing has been removed, and two huge 4-metre propellers that rotate in opposite directions have been installed. In the world of aviation, these are known as counter-rotating propellers. Aircraft structure will have to be completely redesigned. According to Safran, the noise emitted by the Open Rotor would be equivalent to that generated by the LEAP engine. This is due to the design of the blades, which have been manufactured in 3D by weaving looms derived from the textile industry. As for the composite material made from carbon fibres, it is flexible enough to eliminate all the vibrations specific to propeller engines. Thanks to this modern technology, fuel consumption could be cut by a further 15–25%.
With Open Rotor, Safran is reinventing the propeller. Thanks to Open Rotor, engine fuel consumption continues to decrease without any performance loss.
When the first engine-powered aircraft took to the skies at the beginning of the 20th century, who could have imagined that 40 years later, they would break the sound barrier? The aeronautics industry is constantly reinventing itself to meet ever-increasing demand, and in an age when environmental issues are paramount, it is hard to say what aircraft will look like in thirty years’ time. One thing is certain: they will have to be significantly more economical in terms of resources and CO2 emissions. The same goes for polymers, which have always played a key role in the aircraft industry’s quest for performance and fuel efficiency.