Thermoplastic composites in aerospace – the future looks bright

A significant milestone occurred in thermoplastic composites recently, and hardly anybody noticed. Gulfstream Aerospace (Savannah, GA, US) delivered its 300th Gulfstream 650 aircraft. This twin-engine business jet, which began production in 2012, is the first commercial airplane to use critical control surfaces made from thermoplastic composites. 

Airbus (Toulouse, France) has successfully employed thermoplastic composites on the leading edges of its A300-series aircraft for decades, but these are not critical control surfaces. If a leading edge falls off the plane, then the plane still lands without a problem and everybody stays safe. If a critical control surface fails, then probability of a catastrophic landing increases substantially. 

Thermoplastic composites were not considered for critical or major structural components in aircraft for many years. This was true for several reasons. First, thermosets are in the comfort zone for many — they’re structural and stable and have 40+ years of flight-allowable databases behind them. The application of continuous-fiber composites is almost completely structured around thermoset resins. Major composites manufacturers use autoclaves (and now OOA ovens), and other thermoset-driven capital equipment. Along with the thermoset-focused database and capital equipment, most composites engineers have lived in the thermoset comfort zone for their entire careers. They’ve designed or tailored a process around a handful of off-the shelf, flight-certified prepregs. Shop technicians are experts in vacuum bagging, bonding or other processes based on thermoset use. The customers only wanted to use thermosets, because they knew nothing about those “exotic” materials called thermoplastics. 

This comfort zone in a necessarily conservative community is a major reason for the aerospace industry’s slow progress in exploiting the advantages of thermoplastics. Even when a thermoplastic prepreg starts at less than 0.5% porosity (some of them do), and the AFP part made from the prepreg is at a similar porosity, some still want to put the final part into an autoclave to ensure consolidation. Heck, even some well-versed thermoplastic composites engineers like the security associated with ensuring consolidation via autoclave. If you find a thermoplastic composite in a database, it’s likely a PEEK that is autoclave-consolidated. When you do that, you lose the price advantage of thermoplastics.

Back to the G650. Its elevator and vertical tail rudder are made with carbon fiber/PPS composite and then assembled using induction welding via an FAA-certified process. That one sentence describes three milestones associated with the parts. First the elevator and tail rudder are critical for maintaining control of the aircraft, and the FAA would not certify them without substantial proof of performance. Second, the use of PPS — not a poly-ketone — on a critical part, was, when these structures were designed, almost inconceivable. Sure, PPS had been used on leading edges, but the resin only has a glass transition temperature (Tg) of 90°C. On a hot summer day in the Mojave desert, at a location on the plane near the engine exhaust, one can be sure the material surface temperature will come dangerously close to 90°C. Wouldn’t design of a critical control surface with such a low-Tg material create unnecessary risk?

Fortunately, PPS (and polyketones) are semi-crystalline polymers. The chain structure within the polymer enables them to retain a significant portion of their strength and stiffness above their Tg. In contrast, when a thermoset, such as epoxy, is exposed to temperatures above its Tg, it decomposes. PPS, in fact, has been used in underhood automotive applications at temperatures of more than 140°C for many years. An older composites engineer (like myself) would have had a hard time selecting a matrix material that could operate above its Tg. But some young, upstart engineer that “didn’t know any better” got it to work, and that was a major milestone.

Now for the third milestone. A major advantage of thermoplastics is that they can be welded, thereby eliminating the need for bonding and riveting and the cost and weight issues associated with each of these. For a welded, critical thermoplastic composite to be FAA-certified, it would have to be proven to meet spec every time. KVE Composites Group (The Hague, The Netherlands) developed the welding process for part manufacturer Fokker Technologies (The Hague, The Netherlands) using TenCate Advanced Composites (Nijverdal, The Netherlands) CETEX laminate prepreg. (Guess where? Yes, those A300-series leading edges.) And it was good enough to become FAA-certified. (As a side note, every thermoplastic composite engineer should thank God for the Dutch, but that’s a topic for another day.) 

So, despite the major technical milestone at Gulfstream that started production more than five years ago, why is the aerospace composites industry still operating in the thermoset comfort zone? One reason is an education gap: I sat on a SAMPE panel a couple of years ago with a professor from a major US university that has a heavy composites curriculum. One of his slides claimed there were no critical flight surfaces made with thermoplastic composites in production. When it was my turn, I showed the Gulfstream parts on a slide, and realized that I had lost a potential academic friend. He simply did not know. Had he been from a European university, he probably would have known.

The anti-thermoplastics bias in the US is not just from lack of knowledge, nor is it just because they’re outside of the comfort zone. Thermoplastic composites were overly hyped in the 1980s for military applications, and when they failed, as most entry-level technologies do on the first try, they got a real bad rap. Development of high-performance thermoplastic composites in the US was reduced to a crawl. In contrast, Airbus and the Dutch invested heavily in development of thermoplastic composites and began using tons of the material as early as the Airbus A320. By the way, Fokker is now  manufacturing a rudder similar to that already in production, intended for multiple Gulfstream aircraft.

Where will thermoplastics take us next? Because thermoplastic prepreg tapes allow for full automation of complex shapes, improved properties and full recyclability (although, not everyone I’ve talked to is convinced of this) and reduced cost, they are the way to go. I’ve recently heard industry experts claim that a fuselage made with thermoplastic composites via automated fiber placement will still have to be autoclaved to ensure full consolidation. This perspective neglects two key points. First, some aerospace-grade thermoplastic tape is made with very low porosity (<0.5%, and made in the US) and it’s only getting better. Second, given the recent, major advances in artificial intelligence-supported automation, real-time quality management of the AFP process is very real and very close. Why else would Toray (Tokyo, Japan), Boeing’s primary thermoset prepreg supplier, invest more than US$1 billion in thermoplastics specialist TenCate Advanced Composites (Morgan Hill, CA, US)? My prediction? The Fuselage of Tomorrow and/or the New Midsized Airplane will be made with thermoplastic composites, and it/they will be built by 2025.