By: Thierry Dubois, published in Aviation Week & Space Technology, Sep 1, 2014
Thermoplastic composites are gradually gaining ground over thermosets and metal. Although they offer numerous benefits, the trend has been relatively slow, as the aerospace industry’s investment in thermoset production tooling is recent. But thanks to aerospace’s growing need for higher volumes, the move may now accelerate, piggybacking on the automotive sector’s shift to thermoplastics.
The difference between thermoplastics and thermosets lies in the resin (the matrix) rather than the fibers (the reinforcement). When heated, a thermoplastic resin softens and melts; when cooled down, it can resolidify without losing any property. The solidification process involves no chemical curing. Advantages of thermoplastics include notably faster manufacturing and better performance.
Some 15-20 years ago, thermoplastics were tried and rejected, including using them in the wing of Boeing’s X-32 demonstrator for the Joint Strike Fighter program. At the time, however, suitable processing technologies such as fiber placement and thermoforming were still unavailable. Moreover, the expected demand for thousands of military aircraft dwindled down to hundreds. Thus, the advantage thermoplastics have in faster manufacturing for higher volumes was no longer of interest.
The latest advancements show that thermoplastics can be used in more, increasingly large primary structure components. Fokker manufactures the horizontal tail of the in-development AgustaWestland AW169 medium twin helicopter. This yields a 15% weight savings over previous composite technology, Fokker claims, thanks to the stiffness of the material. The Dassault Falcon 5X, scheduled to perform its first flight next year, and the in-service Gulfstream G650 business jets have their rudder and elevators made of thermoplastics.
At the laboratory stage is a full horizontal tailplane, including the stabilizer and elevators. “We have manufactured a demonstrator for an aircraft the size of a bizjet,” says Richard Cobben, Fokker Aerostructures’ technology vice president. A 10% weight reduction can be expected compared to thermoset material, he asserts.
Airbus appears relatively conservative on the A350 XWB, which uses thermoplastic clips and brackets. The A380 has thermoplastic J-nose leading edges, adds Christian Weimer, a composite-material expert with Airbus Group Innovations.
EASA has certified Expliseat’s new economy seat that incorporates a thermoplastic resin developed by TenCate.
On the Boeing 787, Montreal-based Marquez supplies thermoplastic air ducts for personal service units. Designed with glass fiber, the part is said to be much lighter and is much quicker to produce than comparable ones—5 min. versus 6 hr. Boeing did not answer Aviation Week requests for more details on thermoplastics on the 787.
Aerostructure specialist Daher-Socata in July announced it had completed the construction of a lighter, cheaper carbon-fiber wingbox demonstrator dubbed Ecowingbox. Both thermoset and thermoplastic resins were tried, even for the main spar. Eventually, the main spar was made with a thermoset resin. Thermoplastics can be found elsewhere in the wingbox, such as the stringers.
In cabin interiors, thermoplastics have been increasingly used for the thinner part design they enable, their straightforward manufacturing and their excellent behavior in flame, smoke and toxicity tests.
They are now beginning to be chosen for more critical features. The European Aviation Safety Agency (EASA) in April certified Expliseat’s titanium seat, which passed 16g crash tests yet is twice as light as the nearest competition, according to its designers. It features a composite-titanium structure, for which Netherlands-based TenCate supplied a thermoplastic resin. The seat weighs 8.8 lb. per passenger—it is offered as integrated three-seat assemblies—and divides the part count by 10.
Fokker’s Cobben sees two main benefits in thermoplastics. First, the very high toughness of their matrices, which allow the laminates to be thinner and thus create lighter products. Second, the stamp-forming and welding techniques that can be used with thermoplastics are lower-cost processes.
Tim Greene, Greene Tweed’s product manager, composites, provides more details on the production side. In aerospace thermoplastics, Greene Tweed specializes in manufacturing complex geometric parts, notably by compression molding.
Thermosets have a defined heat-up rate, curing time and cool-down rate. Therefore, the material dictates how long it takes to manufacture a part. “There is very little you can do to cut a lengthy cure cycle, which can be many hours,” Greene says.
Thermoplastics have no such defined cycle, however; so the driver is the equipment—tooling, press, etc. Greene Tweed emphasizes that many thermoplastic processing techniques take place out of an autoclave—an expensive, pressurized oven that often causes production bottlenecks. This creates a potential for faster processing.
Moreover, once a production process has been set up, it can be considered as very reliable, without any deviation, Airbus’s Weimer says. This may be different from thermoset pre-impregnated fiber manufacturing.
Thermoplastics also allow for automated welding. However, while welding (which can involve ultrasound, resistance, induction, vibration, etc.) is widely used in automotive manufacturing, it is not in widespread use in aerospace. Only a few production applications can be found. Some techniques are not mature in aerospace, partly because of the higher requirements and higher temperatures, Greene says.
The butt-joining technique that involves only the resin of the parts was discovered 10 years ago, Fokker’s Cobben remembers. It has been tested and validated on fuselage panels, under the Tapas joint research project with Airbus, TenCate and other partners.
Thermoplastics inherently have better toughness and are more repairable. This opens the door to using them in impact-prone areas. New, toughened thermosets already include thermoplastic particles to improve their impact resistance, Greene says.
“Thermoplastics are more repairable, but we are still looking for the Holy Grail—remelting the resin in the damaged area on the ramp,” Cobben explains. It is do-able in principle but not so easy in practice. The resin softens, so a technician would need some tooling to support the part.
The idea of using thermoplastic composites for those components that are exposed to collisions has limits, however. One could think of using them on a single-aisle fuselage, as such aircraft can be found on a busy apron several times a day. But a difficulty is inherent in the size of the fuselage. The smaller the diameter, the thinner the fuselage and therefore the greater sensitivity to impact, Cobben says.
Finally, another benefit is thermoplastics’ recyclability. Reheating the matrix polymer allows it to be remolded into something else. But the recycled part generally should be used only for non-structural application, as reforming moves the fibers around.
All these benefits are worth investing in if production volumes are high enough. “Thermoplastic resins are more expensive, but prices could decrease sharply if demand picks up, including demand from the automotive sector,” notes Jean-François Maire, director of the materials and structures department at France’s Onera aerospace research center. This is why Fokker’s Cobben sees Japan’s push for thermoplastics in automotive manufacturing as “good for us because it increases volumes.”
But thermoplastics need to make their case versus thermosets. “There is a reluctance to change, especially if OEMs and suppliers have invested in their current development and manufacturing processes using thermosets,” says Kim Choate, Sabic’s director of marketing for mass transportation and innovative plastics. Sabic is a Saudi Arabian petrochemical group.
Two projects that emerged recently in the car industry may have implications for aircraft. Sabic and Switzerland’s Kringlan Composites have developed the world’s first thermoplastic composite wheel for cars. Sabic and Kringlan are looking at applications in other industries in which weight reduction is a key driver, such as aerospace.
In France, Onera and mechanical engineering center Cetim have manufactured the first fully composite automotive wishbone suspension. The part combines light weight, high mechanical performance and swift production, according to its promoters. It is made of a thermoplastic matrix reinforced with carbon fibers.
The weight is cut to 4.4 lb. from 7 lb. However, Maire could not provide a price estimate. As it is a prototype, only the cost of the material may be representative—$9.50. At least the cost of the conventional wishbone suspension, made of metal, is known—$57. Producing one item takes just 8 min. Onera engineers hope to reuse these technologies in aerospace.
In Enschede, Netherlands, the Thermoplastic composites research center (TPRC) opened its laboratory in 2012 and has attracted Boeing, Fokker, TenCate, Daher-Socata and the university of Twente. The TPRC’s activities have taken place in aerospace so far, but automotive has just been added.
Thus the future of thermoplastics in aircraft looks tightly linked to their use in cars. And the ongoing aircraft production ramp-up is spurring interest.
But some challenges remain. For example, today’s thermoplastics are not compatible with thickness variations, Airbus’s Weimer says. According to Sabic’s Choate, they may not be the best alternative in certain instances, such as supersonic applications that require extreme heat tolerances, or when very strong chemical resistance or an extremely high strength-to-weight ratio is required. Knowledge still has to be gained in durability and behavior against fatigue, Maire adds.
Maybe the main problem is a chicken-and-egg issue. The lack of infrastructure and understanding results in a perceived risk, Greene believes. “This slows down thermoplastic adoption,” he says. So will thermoplastics eventually take over thermosets? Experts agree that thermosets will not be replaced for specific environments or when only a small number of parts needs to be manufactured. Greene sees the main potential for his company being the replacement of complex-shape metallic parts for secondary and semi-structural applications—covers, enclosures, fairings, etc. “We are looking at the remaining 50% or so of metal in an aircraft; the idea is not to replace thermosets but to replace the remaining metal,” he says.
aviexmax 