Thermoplastics In The Aerospace Industry

Interview conducted by G.P. ThomasNov 14 2013

AZoM talks to Tom Smith and James Mondo from TenCate about the unique properties and applications of thermoplastics, especially with regards to the aerospace industry.

Thermoplastic polymers are increasingly being used in fibre reinforced composites in a variety of applications – could you please tell us a little about what a thermoplastic is and what fibres they are used with to produce composites?

A thermoplastic resin differs from a thermoset resin (epoxy for example) in that is can be re-formed multiple times to the desired shape. Thermoset composites (i.e. epoxies) on the other hand permanently lock into place after cure.

Once they are cured, they cannot be re-formed or re-shaped. A thermoplastic composite, however, can be re-heated and once it is above its softening temperature it can be re-formed. This characteristic also provides a part fabricator with the ability to use fast cycle times to heat the thermoplastic composite, form it, and shape it in seconds or minutes versus hours that it would take a thermoset to cure.

From a fiber perspective, thermoplastic resins and thermoset resins use much the same carbon fibers. In a unitape form, advanced thermoplastic unitapes are offered in either a standard modulus or intermediate modulus. The fabrics are much the same i.e plain weave or 5-harness for example and on the glass side, commonly S-2 glass is used in unitapes, or 7781 fiberglass fabrics. For industrial applications, lower cost carbon fibers can also be used similar to thermosets.

TenCate is at the forefront of innovation involving thermoplastic composites – how have you seen the use of thermoplastics grow over the last few decades?

In aerospace, thermoplastic composites first took their lead being used on commercial aircraft for leading edges, interior components, engine pylons, access doors, aircraft flooring and a variety of molded parts. The initial drivers for usage were their impact resistance (durability) and for interior components their inherent flame resistance.

Further, in common aircraft rib stiffened structure (like leading edges), resistance welding techniques allowed a strong fusion weld of the rib to the skin eliminating the need for an adhesive layer.

In the last five years, we have seen continuing advances in thermoplastics where carbon fiber skins and ribs are induction welded by robots, clips and brackets are formed and shaped in cell manufacturing environments and increased interest in thermoplastic composites has been experienced in non aerospace applications like automotive, electronics, oil and gas and recreational applications.

What are some of the key reasons for this growth? For example, what are some of their advantages compared to their thermoset counterparts?

Thermoplastic composites offer a number of advantages over their thermoset counterparts:

• Inherent flame retardancy (PEEK, PPS, PEI)
• High degrees of impact toughness
• Low moisture absorption
• Ability to be stored at room temperature indefinitely
• 250-300°F (121-177°C) service temperatures hot/wet that rival typical epoxy thermoset composites
• Ability to use fusion welding techniques such as induction or resistance welding and eliminate both mechanical fasteners and secondary adhesives. Saving weight and cost.
• Very fast part forming cycle times i.e. seconds and minutes vs hours

With respect to more common thermoplastic polymers such as Nylon, PET, Polycarbonate, Polypropylene and Polyethylene we find that each of these specific polymers finds its way into specific applications because of the unique polymer properties.

Nylon is found in under the hood automotive applications because of its heat resistance and its solvent resistance. Plus it is a very tough polymer that can withstand the stresses found in automotive parts while still providing competitive prices and competitive rapid part forming capabilities.

Polycarbonate and polycarbonate/ABS blends are found in electronic enclosure applications because of their high surface quality, impact resistance and UL flame ret ardency. High density polyethylene is commonly used with either glass or carbon fiber in pipe overwrapping applications in oil and gas.

What are the main industrial applications of thermoplastic composites and which of these are TenCate involved in?

Thermoplastic composites are used in both high end applications such as aircraft components to high volume applications such as footwear and orthotics.

Oil and gas applications make use of high end thermoplastics because of their ability to resist corrosion and high temperatures for down hole applications to umbilical’s used in offshore oil drilling. Also, high density polyethylene advanced composites are used as an overwrap of pipe to provide additional reinforcing strength while providing a durable corrosion resistance outer layer.

Emerging applications include composite enclosures for smart phones, tablets and hand held devices to allow these devices to achieve lighter weight or thinner profiles while retaining the rigidity needed to protect the device from damage. Further, secondary processes like injection overmolding can be utilized to form detail clips and brackets necessary for the device details.

Automotive and other transportation applications are looking for advanced composites both in thermoplastic and thermoset form to lighten the weight especially with hybrid and battery powered vehicles where lightweight allows the range of the vehicle to be increased.

Thermoplastic composites have an advantage over thermoset composites because of their rapid processing in high volume applications along with their durability and strength.

Focussing on the aerospace industry, why are thermoplastic composites now so prevalent and what benefits do they have over more traditional materials?

As the aerospace industry moves to composite fuselages and wings, many of the structural aluminum clips and brackets holding the interior major assemblies are changing to thermoplastics composites. Thermoplastic composites offer a combination of strength, fire resistance and galvanic corrosion resistance that offers weight and process savings.

There exists a variety of consortiums focused on the assembly of large structure using thermoplastics composites including major parts of the aircraft. The indefinite shelf stability of thermoplastics eliminates the limitations of out time and the need for refrigerated storage that is a normal part of thermoset technologies.

Thermoplastic composites are amenable to a variety of processes from rapid thermoforming and press forming to traditional autoclave processing and tape and fiber placement techniques. Thermoplastics are recyclable and may be re-formed to shape if they are not made to the original specification.

In addition, fusion welding techniques allow the thermoplastic composite bond to be as strong as the polymer eliminating the need for secondary adhesives or mechanical fasteners.

Different aerospace environments of course require different composite properties. What are some of the compositional differences between thermoplastic composites being used in the interior and exterior of an aircraft?

Typically semi crystalline polymers such as PEEK, PEKK and PPS are used on the exterior of aircraft as a result of their inherent resistance to solvents. PEI and polycarbonates( for more cost competitive applications) are more commonly used in interiors because they are fire retardant, but not necessarily resistant to some of the harsh solvents use on aircraft exteriors and engines. PEI is exceptionally tough and durable and has high heat resistance and would be found in galley’s and stowage bins, while polycarbonates would be more cost competitive for secondary interior structures like window shades, trays, carts and decorative liners and window enclosures.

What are some of the special finishes available for the TenCate Cetex® thermoplastics range and what aerospace applications benefit from these?

From a very basic perspective, TenCate can provide thermoplastics in several shades ranging from natural to white to gray. We offer three standard surface textures rough, brushed and smooth. Rough surfaces act more as the bond layer between honeycomb and the skin, while the brushed or glossy smooth surface is used for interiors.

One of the great advantages of thermoplastic polymers is their ability to be remoulded and reshaped at high temperatures into complex shapes. How does this influence the production process of TenCate Cetex® thermoplastic composites?

One of the intrinsic benefits of using thermoplastic composites is their ability to be rapidly formed in minutes and sometimes seconds depending upon the part. This is one of the important features in industrial applications which expect very fast cycle times. If a part is not exactly formed to the desired shape, it is possible to re-process that part and re-form it to the proper shapes eliminated scrap and waste. One of the product forms that TenCate Cetex thermoplastic composites are offered in are reinforced thermoplastic laminates (RTL).

In this form, TenCate fabricates a laminate of four by 12 foot and as many at 25 plies each oriented to a customer’s specification. This laminate is consolidated under high pressure and temperature such that the resins fully encapsulate each of the fibers. The laminate is then inspected through non destructive inspection and certified much in the same way a composite typical part is inspected and certified. This RTL product form allows the user to thermoform their parts seconds and minutes to the desired shape.

For more complex shapes with varying cross sections and integral ribs, TenCate has developed thermoplastic bulk molding compounds using chopped carbon thermoplastic unitapes. Further for those users that would prefer a final part, TenCate has developed tooling, fabrication and part design expertise which can be deployed or TenCate can work with a customer and a vendor to deliver turnkey manufacturing for high volume parts.

Could you tell us a little more about the different resin types that are used in the TenCate Cetex® thermoplastics range?

The TenCate Cetex® thermoplastic composite product offering ranges from the engineering thermoplastics of PEEK, PPS and PEI for high temperature structural applications to the more performance oriented thermoplastics such as polycarbonate, nylon, polypropylene, polyethylene and PET.

We also supply PMMA which is used in the recreational footwear industry for inner soles on high end basketball sneakers for support and lightweight to hockey skate footboard for rigidity and strength.

Image credit: Tencate

Could you tell us about a recent success story involving the Cetex® range?

There have been several interesting developments in the past few years and many of these have won JEC innovation awards. Here is a summary:

Gulfsteam G650 Rudder and Tail Elevator – Fokker Aerostructures designed and built the new G650 tail elevator and rudder from TenCate Cetex thermoplastics composites and induction welded the ribs to the skins without the use of fasteners or adhesives, saving 15% of weight and 10% of the cost of thermoset based designs.

Aircraft Seat Frames - Cutting Dynamics, Inc worked with TenCate Cetex thermoplastic unitapes in a ¼ inch slit tape that was secondarily braided by A&P Technologies and used to fabricate lightweight composite commercial aircraft seats saving weight over the traditional aluminium design while still meeting the 16g loading specified by the FAA. Engineers at A&P designed a braided preform that exactly meets the geometry and mechanical requirements of the seat frame. The composite seat frames can deliver a 30 percent weight savings vs. conventional aluminum frames.

Agusta Westlands AW169 helicopter – Fokker Aerostructures used a TenCate Cetex thermoplastic to fabricate the first all thermoplastic horizontal composite tailplane. The full-scale development of the horizontal tail started in July 2011. By the end of 2012, four AW169 helicopter had been fitted with the new horizontal tail. The AW169 horizontal tailplane has a length of 3 meters and spans tip-to-tip. Weight reduction is achieved by the stiffness of the thermoplastic material. Fokker has designed and developed this integrated solution as a co-consolidated, single-piece torsion box.

Image credit: Tencate

The prevalence of composites in some aircraft can lead to higher fuel efficiency – do you feel your composites have a role to play in making the aerospace industry more environmentally friendly?

TenCate Advanced Composites believes that thermoplastic composites do play a role in helping the environment. Thermoplastics composites are inherently reformable and are much more able to be recycled than thermoset composites. Additionally some thermoplastics are inherently flame retardant thereby eliminating the need for secondary flame retardants which may be harmful to humans or the environment.

Today’s aircraft interiors are required to meet OSU flame/smoke toxicity testing of 65/65 while TenCate’s thermoplastic composites well meet that standard with very low results of 20/20 without the use of secondary flame retardant chemicals.

Further, thermoplastics allow room temperature storage indefinitely which minimizes any need for out time concerns, the need for refrigerated storage and shipment and of course the ability to form in seconds or minutes vs multi hour autoclave cycles with thermosets which also saves energy and time in part fabrication.

What does the future hold for TenCate and its range of thermoplastic composites? Do you foresee any major breakthroughs on the horizon?

There is a great deal of innovation and investment being directed at thermoplastic composites both in the industrial and aerospace world. Changes in many aspects of the economy from electric powered vehicles to all composite fuselages are driving changes from metals to composites and where this is taking place, thermoplastic composites are leading the way.

As thermoplastic composites elevate from aerospace to industrial applications we will see continued breakthroughs on the processing side of thermoplastics much as we have seen in the automation of thermoset composites with automated tape laying and fiber placement equipment. This will improve the economics even more for thermoplastic part fabrication and develop an entire new round of growth in applications.

About Tom Smith and James Mondo

Tom Smith is currently president of TenCate’s Performance Composites group focused primarily on thermoplastic composites in industrial applicatons. Tom has been  have been running thermoplastic composite companies (PMC, Baycomp and PMC China) for over 15 years.  TenCate Performance Composites is a  leader in the development of materials and parts manufacturing technologies and is  one of the largest volume producer of components today.  Tom has given numerous presentations on thermoplastic composites to the community.

Dr. James Mondo is currently the VP of Thermoplastic Technology and the Thermoplastic Composite Product Manager at TenCate Advanced Composites in Morgan Hill, CA.  Jim oversees the advancement of thermoplastic unidirectional prepregging technology, new product development, aerospace and defense qualifications, and serves as the point of contact with key thermoplastic composite organizations. Prior to joining TenCate, Jim was the President and CEO of Automated Dynamics, a manufacturer of thermoplastic and thermoset automated fiber placement equipment and a manufacturer of thermoplastic composite structures for industrial, oil and gas applications. Jim received his Ph.D. in Organic Chemistry from Yale University in 1982