A major trend in the aerospace industry is a move toward greater use of thermoplastics vs. “traditional” thermoset epoxies. Thermoset composites become plastic at the time they are made into structures—polymerized in the jargon of the industry. Thermoplastics are already polymerized when combined with fibers or made into parts.
“To date, thermosets have been used predominantly in the industry. For example, the fuselage of the Boeing 787 is an epoxy-based polymer,” said Mike Favaloro, Ticona Technical Marketing Manager for Fortron PPS Composites, Americas, a supplier of resins. Thermosets are generally favored for a variety of reasons, especially on commercial aircraft.
“Thermoset composites have been used for 30 to 40 years in aerospace. People are now comfortable designing critical structures with thermoset composites,” he said.
Favaloro pointed out the advantages that argue for a harder look at thermoplastics—energy savings, recyclability, and cost of materials. In terms of energy, aerospace thermosets often require expensive autoclaves. Heating for hours may be required even for those compounds that are cured out-of-autoclave.
“Energy savings can be a factor of 10 or more with thermoplastics,” said Favaloro, since a thermoplastic part is typically heated for only a few minutes during forming. Thermoplastics such as Ticona’s Fortron polyphynelene sulphide (PPS) can be recycled many times without loss of properties, according to Favaloro. The raw cost of thermoplastic resins, such as Ticona’s PPS, may be an order of magnitude cheaper than thermosets or other thermoplastics such as polyetheretherketone, or PEEK, he added.
Acceptance of a new material hinges on fundamental economic advantages, according to Jim Mondo, Vice President of TenCate, a supplier of both thermoset and thermoplastic prepreg fabrics and tapes. “Aerospace designers will assume you can meet their performance requirements,” he said. “For a new material to be accepted, it has to lead to a lighter weight or lower cost part or both.”
TenCate has developed a number of thermoplastic prepreg fabrics and tapes that are now flight-qualified with Airbus, Boeing, and others, according to Mondo.
The cost advantages of thermoplastics may go beyond resin cost since it offers easier processing and manufacturability. Thermoplastic prepregs do not require special refrigeration and have virtually unlimited shelf life, a practical benefit in handling that translates to reduced cost. In addition, since thermoplastic parts are formed by simply heating, melting, and cooling, they can be welded like metals.
An example of weldability is evident in the new Gulfstream G650, expected to enter service in 2012. It uses TenCate’s Cetex thermoplastic CFRP for rudders and elevators. Using thermoplastics in such primary structures on a commercial aircraft is relatively new. How they are made is just as interesting.
Fokker Aerostructures, Gulfstream, KVE Composites Group, TenCate Aerospace Composites, and Ticona won the 2010 JEC Aeronautics Innovation Award for the process used to make these structures. The ribs are press-formed and then induction-welded using robots, eliminating fasteners. “Eliminating fasteners saves weight. It also saves cost by eliminating the labor [and tooling] needed to drill all those precision holes,” explained Mondo.