Thermoplastic composites: Poised to step forward

Thermoplastic composites (TPC) aren’t new to the aerospace sector, but the past couple of years have seen thermoplastic usage in commercial aircraft reach a tipping point. While TPCs have been used for some time for smaller parts such as clips and brackets, or smaller interior components, thermoplastics have been working their way into larger aircraft structures incrementally and now seemed poised to play a bigger role in the future of commercial aircraft. 

In March 2018, Toray Industries Inc. (Tokyo, Japan), the world’s largest carbon fiber manufacturer, acquired TenCate Advanced Composites (Morgan Hill, Calif., U.S. and Nijverdal, Netherlands) for €930 million (TenCate has since changed its name to Toray Advanced Composites). The move seemed to be an effort to strengthen Toray’s thermoplastics capabilities in preparation for the next wave of commercial aircraft development. Shortly after that announcement, Hexcel (Stamford, Conn., U.S.) and Arkema Inc. (King of Prussia, Pa., U.S.) announced a strategic alliance to develop thermoplastic composite solutions for aerospace, combining Hexcel’s skill in carbon fiber manufacture with Arkema’s polyetherketoneketone (PEKK) resins expertise. And over the course of the year, several other pieces of the thermoplastics puzzle seemed to fall into place.

In April 2018, Premium Aerotec GmbH (Augsburg, Germany) unveiled a demonstrator for an Airbus (Toulouse, France) A320 pressure bulkhead it had developed and manufactured using carbon fiber in a thermoplastic matrix. The demonstrator, which consists of eight welded segments, illustrates how the weldability of thermoplastics has the potential to enable larger aircraft components. (Learn more about Premium Aerotec’s A320 pressure bulkhead demonstrator).

In August 2018, Solvay (Alpharetta, Ga., U.S.), Premium Aerotec and Faurecia Clean Mobility (Columbus, Ohio, U.S.) launched IRG CosiMo (Industry Research Group: Composites for Sustainable Mobility), a consortium focused on the development of materials and process technologies aimed at enabling high-volume production of thermoplastic composites for the aerospace and automotive markets. The consortium combines companies along the entire thermoplastic composites process chain from materials to machinery to applications in automotive and aerospace. (Learn more about the IRG CosiMo consortium here).

Solvay has been partnering with GKN Fokker (Hoogeveen, Netherlands) to advance technology and further adoption of TPCs for large aerospace structures since June 2017. The company launched PEKK polymer production in September 2017 and then doubled its qualified UD thermoplastic tape capacity in 2018. Earlier in 2019, Solvay commissioned a dedicated TPC research lab in Alpharetta, Ga., U.S., aimed at the development of next-generation materials. Solvay plans to commence qualification of a new UD tape line in late 2019.

Teijin Ltd. (Tokyo, Japan) announced in January 2019 that its TENAX carbon fiber and carbon fiber/thermoplastic unidirectional pre-impregnated tape (TENAX TPUD) has been qualified by Boeing (Chicago, Ill., U.S.) for use as an intermediate advanced composite material for primary structural parts (Read the full news story here).

As these and similar technologies and materials progress, a picture of how the aerospace industry might start to look in the years and decades to come gradually comes into focus. The role of TPCs is becoming an increasingly larger part of that picture.

Fabricators are interested in taking advantage of the manufacturing benefits and fast processing times of thermoplastics, and in using TPCs to start making larger structures such as fuselage panels and ribs. In addition, thermoplastics boast high fracture toughness; good mechanical properties; recyclability; low flame, smoke and toxicity (FST), and can be stored at room temperature. And as OEMs and aerospace tier suppliers become more familiar with thermoplastics, they’re being used for more complex parts, welded assemblies and primary structures.

According to Steve Mead, managing director at Toray Advanced Composites (formerly TenCate), “[Major airframers] are really looking for a material solution that has the rate capabilities of aluminum and the weight capabilities of carbon fiber-based material — thermoplastics kind of bridge that gap.”

Processability of TPCs

A big part of why TPCs are finding their place in aircraft programs is their processability. Because thermoplastics are already fully polymerized, they have faster production rates than thermosets, which must undergo cure.

“When you look at the amount of time it takes to make a thermoset part today and compare it with the amount of time it takes to make a thermoplastic composite part, [thermoplastic] is  about 10 times faster,” says Mike Favaloro, president and CEO of CompositeTechs LLC (Amesbury, Mass., U.S.), a composite industry consultancy.

A big advantage of thermoplastic automated fiber placement (AFP) compared to thermoset AFP — particularly given the lack of cure cycle — are higher production rates due to faster processing time. There are cost savings to be found in in-situ lamination and out-of-autoclave (OOA) post-consolidation. Plus, taking the autoclave out of the equation allows for the development of larger structures.

David Leach, director of business development for ATC Manufacturing (Post Falls, Idaho, U.S.), acknowledges that the unit cost of thermoplastics exceeds the cost of thermosets, but argues that TPC material prices will come down. Further, he says, processing efficiencies offer an opportunity to reduce costs today. The general consensus in the composites industry is that OOA thermoplastic processes, right now, offer costs savings of more than 30 percent compared to thermosets. 

“Thermoplastics are finding their way into programs even after planes have gone into production,” Leach points out. “It’s a testament to the cost benefits of thermoplastics.”

The potential of high-performance matrix polymers extends beyond what is currently available on the market. Doug Brademeyer, head of Ultra-Polymers Materials at Solvay, says the company is working both internally and with partners to develop tailored polymers that are customized for the different fabrication processes.

“We are excited by these tailored PAEK solutions and can rapidly bring these to commercialization in our world scale assets, based on customer needs,” says Brademeyer.

Low-melt PAEK

With aircraft OEMs and suppliers scrambling for higher production rates and shorter cycle times, processability is key. Polyetheretherketone (PEEK) has long been the favored thermoplastic polymer since it has the biggest database and is the most widely qualified. But according to Favaloro, low-melt polyarlyetherketone (LM PAEK) offers some advantages, especially for automated processing methods like ATL. 

“PEEK is processible via stamp forming and continuous molding, but LM PAEK processes at a lower temperature, has a lower working viscosity which allows for better automated processing, and has a lower degree of crystallinity which reduces residual molding stresses,” he says. “The ultimate goal is to use an ATL machine to lay [the tape] down and be done with it — you need the right degree of crystallinity, a good window and good laydown speeds.”

LM PAEK has a wide process window of 350-385ºC. For reference, polyphenylene sulfide (PPS) processing temperatures range from 330-350°C, while polyetherketoneketone (PEKK) and PEEK processing temperatures are 380°C and 400°C, respectively.

“The material has gotten so much traction because of its processability,” says Scott Unger, chief technical officer at Toray Advanced Composites. Toray Advanced Composites collaborated with Victrex (Lancashire, U.K.) to produce Cetex TC1225, a unidirectional tape using LM PAEK.

“The intent with the development of TC1225 was to create a product that processed easily at temperatures close to that used for PPS, had a favorable cost position for the end user and had the mechanical and fluid resistance properties of PEEK,” says Unger. “With TC1225 LM PAEK, I feel that we accomplished all of those goals.” 

Cetex TC1225 is currently undergoing qualification by the National Center for Advanced Materials Performance (NCAMP, Wichita, Kan., U.S.). In addition, Toray says there are two major airframer-based qualifications in the works for the material, as well as a couple of qualifications programs based on emerging markets such as air taxis and urban air mobility.

Tapes using LM PAEK are reportedly yielding improved laydown speeds. Tim Herr, director of Aerospace SBU at Victrex, says, “The laydown rates we can achieve for both in-situ AFP and out-of-autoclave consolidated AFP are unprecedented.” He indicates that 60 meters per minute can be achieved on oven-consolidated panels; 20 meters per minute reportedly is possible with in-situ consolidation.

In terms of quality, Unger claims that low-melt PAEK offers the ability to get the same laminate quality using in-situ fiber placement as with a fiber-placed laminate that’s been put through a post-fiber placement oven consolidation.

Composites bonding

The weldability of TPCs is a big advantage of the material for use in developing aircraft. Fusion bonding/welding offers an alternative to mechanical fastening and the use of adhesives, both of which are methods employed for joining thermoset composite parts. 

Stephen Heinz, product development director at Solvay, says, “Joining and welding plays a major role in the future of assembly and has the potential to cut costs and improve the reliability of aerostructures. Companies like GKN Fokker are taking the lead in demonstrating welding.”

 GKN Fokker (Hoogeveen, Netherlands) has been working to develop TPCwelding for some time, having started experimenting with resistance welding of thermoplastics in the 1990s. The company has been using thermoplastic welding processes to join leading-edge internal ribs and skins. At JEC World 2019, the company showcased an area-ruled thermoplastic composite fuselage panel manufactured using Solvay’s APC (PEKK-FC) UD tape. The panel is the result of a joint R&D project between GKN Fokker and Gulfstream Aerospace (Savannah, Ga., U.S.). The part is reportedly the lowest-cost composite panel, due to simple “butt-jointed” orthogrid stiffening and fully welded frames.

“With thermoplastics, an orthogrid can be greatly simplified by ‘butt joining’ the grid to the skin,” explains Arnt Offringa, head of Thermoplastic Composites Technology Development for GKN Fokker. “The grid is now made up of just simple, flat preforms that are co-consolidated with the skin laminate to form a low-cost, integrally stiffened shell. Frames are welded onto the grid. These welds are loaded in shear, making it feasible to leave out all bolts.”

While welded thermoplastic structures have been used on aircaft for some time, the technology now seems well-poised for use in primary structures. Mike Favaloro believes aerospace fabricators and OEMs are gaining confidence with TPCs, particularly with process control. “On a 10-year horizon we’ll start to see it adopted much more,” he says. 

Read more about welding technology in Ginger Gardiner’s article “Welding thermoplastic composites.”

 

Going tool-less

Another innovation on the horizon that could enable acceleration of thermoplastics use is tool-less composites manufacturing. The concept, as the name implies, obviates the need for traditional molds and tooling, replacing them with robotics.

Aerospace manufacturer General Atomics Aeronautical Systems Inc. (GA-ASI, San Diego, Calif., U.S.) is developing such a process for the fabrication of thermoplastic composite structures. Composite Automation LLC (Cape Coral, Fla., U.S.), using Mikrosam (Prilep, Macedonia) equipment, worked with GA-ASI to develop the automation. The process uses two 6-axis robots working together to place thermoplastic tape. One robot consists of a standard unidirectional tape placement system that provides laser heating to perform in-situ consolidation of the thermoplastic material. The second robot provides support, working opposite the automated tape layer (ATL) to provide a movable tooling surface against which the ATL places tape. (To learn more, see “General Atomics Aeronautical developing tool-less thermoplastics composites process.”)

Recyclability

Another benefit of TPCs is recyclability. Because thermoplastic polymers can be remelted and reshaped, several companies are looking toward TPCs as a way to re-use materials.

One such recycling initiative, operated by the Thermoplastic CompositesApplication Center (TPAC, Enschede, Netherlands) and the Thermoplastic Composites Research Center (TPRC, Enschede, Netherlands), is focused on re-use of production scrap from TPC processing, from collection to shredding and reprocessing through to application. The TPC-Cycle project is working to develop an affordable, environmentally friendly recycling route for high-end, high-volume markets — all while producing a material that retains as many of the mechanical properties of the original thermoplastic materials as possible. The project boasts short cycle times, net-shape manufacturing and is said to enable the production of complex shapes.

The collaboration includes several industrial partners in the value chain, from material, manufacturing, design and application, including GKN Fokker, Toray Advanced Composites, Cato Composite Innovations (Rheden, Netherlands), Dutch Thermoplastic Components (Almere, Netherlands) and Nido RecyclingTechniek (Nijverdal, Netherlands).

 

The right material for the right job

So amid the din of excitement about these materials, the question that arises is, have TPCs arrived? Tier 1 and Tier 2 aerospace suppliers are investing in thermoplastics. There is more interest and investment from smaller and medium-size suppliers. Consortiums like IRG CosiMo are looking at both aerospace and automotive markets to advance process technologies to achieve high-volume production. 

“It’s the Trifecta,” says Mead, “OEMs are investing, machinery folks are investing, the right material has been developed. All of the components of the recipe are coming together.”

In larger scope, what does all of this mean when it comes to materials use on next-generation aircraft? After all, there are numerous materials competing for a spot on the aircraft of the future, and innovation isn’t slowing down — thermoset composites continue to evolve; aluminum and titanium will continue to play a role. 

“As airframers develop a qualification basis with thermoplastics, they now have a choice,” says Unger. “And that choice will be based upon selecting the right material for a given application which meets the production rate and cost requirements for the component or structure in question. As you look at commercial aviation going forward, what I believe you’ll see airframers doing is using the right material for the right job. If a material enables the most appropriate structure for the least cost and meets program build rate requirements, it will win its way onto the airplane.”

The Next Generation of Aerospace Manufacturing

With the commercial aviation industry projected to double in the next 20 years, meeting the demand for passenger and freight aircraft will require new technologies and unprecedented manufacturing rates.

Learn more about the materials and processes that will shape next-generation aircraft in a collection of stories from CompositesWorld, Modern Machine Shop and Additive Manufacturing, available to read or download for free. Get it here.