Looking to lighten up aircraft interiors? Try natural fibers!

Aircraft OEMs are striving to make their products more fuel-efficient, with improved engines and ever-increasing use of composite materials, including in interiors. Composites have long had a role in cabin finishes, because of their high strength to weight ratio and thus contribution to higher aircraft fuel efficiency. As we have reported in CW in the past (e.g., Chris Red’s market outlook article here: http://www.compositesworld.com/articles/composites-in-aircraft-interiors-2012-2022), interior composite components can include seats, floor boards, bulkheads and cabin dividers, lavatories, galleys, wall and ceiling panels and stowage bins. While obviously not as structural as aircraft airframes, nevertheless composite materials for interiors must have strength and meet very stringent flame, smoke and toxicity (FST), heat release, burn rate/flame spread, crash safety and other performance mandates published in the US Federal Aviation Administration’s Federal Aviation Regulations (FAR), Part 25. Equivalent European regulations have been formulated by the European Aviation Safety Agency (EASA). A wide variety of interiors materials, including epoxy, phenolic and high-performance thermoplastic resins, cores, reinforcements, adhesives, potting compounds, decorative films, veils and much more, are available from many suppliers. And it’s a multi-billion dollar industry, as airlines seek to differentiate interior looks and amenities in new aircraft, and as older craft get refurbished.

With that said, well-established, qualified materials remain at the forefront, so truly new products are somewhat rare. But, a few new products have popped up recently, along with news from two European consortia exploring new material approaches. First, BASF (Ludwigschafen, Germany) has introduced an expanded polypropylene foam trademarked Neopolen P reFLAM, which was tested in an institution certified by the aviation industry, in accordance with the specification CS25.853. It is a second-generation version of the company’s expanded polypropylene foam Neopolen. The material owes its enhanced level of fire safety to its special flame-retardant additives, and offers a further improvement in thermal insulation properties while meeting the requirements of European REACH and those of the ROHS Directive and its amendments. The material is approved for molding densities from 36g/l to 75g/l in accordance with applicable requirements. The certification allows Neopolen P reFLAM to be used in numerous applications with the highest fire safety requirements, particularly in the aviation sector.

"The level of interest in new, lightweight and at the same time flame-retardant materials is very high, particularly in the aviation industry", says Carsten Junghans, the key account manager, BASF’s Specialty Particle Foams Europe. He points out the foam’s high thermal insulation characteristic and almost unlimited scope for forming. "Neopolen P reFLAM now provides manufacturers with a foam material which combines these requirements and enables a wide variety of different applications."

Although first introduced last year at the Aircraft Interiors show, SABIC’s (Riyadh, Saudi Arabia) new offerings include two new LEXAN clear sheet products. The thermoplastic polycarbonate LEXAN XHR2000 sheet, with 80% light transmission, has the highest level of light transmission available in an OSU-compliant sheet material and meets typical industry flame, smoke, toxicity (FST) requirements (FAR25.853, BSS7239, ABD0031). (A note: OSU refers to Ohio State University’s accepted rate of heat release test, which measures heat evolution energy during burning, reported as peak heat release in kilowatts per square meter, and total heat release in kilowatts per minute per square meter; the FAA’s regulatory peak/total requirement number is 65/65.)  According to SABIC, its patent pending LEXAN LIGHT F6L300 sheet, the lightest thermoplastic sheet option available today, provides long-awaited solutions to airlines’ quest for differentiated cabin interior designs while also helping to take out significant weight, resulting in more a fuel efficient aircraft. For instance, cabin dividers can be made with the clear LEXAN XHR2000 sheet and LEXAN LIGHT material has been used to create aircraft sidewalls.

Award-winning LSG Sky Chefs and Norduyn1 in-flight trolleys, certified as the lightest in the world, incorporate SABIC’s ULTEM thermoplastic polyetherimide (PEI) resin in the extrusion profiles and door latch, while ultra-tough NORYL modified polyphenylene (PPE) thermoplastic resin replaces metal in the frame and other components. The ability to custom-color ULTEM resin allows Norduyn and LSG Sky Chefs to offer trolleys in the relevant airlines’ brand colors. A new passenger service unit (PSU) engineered and supplied by PECO Manufacturing (Clackamas, OR, US) for the new Boeing 737 uses LEXAN and ULTEM resins. PECO used an integrated structural design which consolidated parts in order to reduce the footprint of the PSU. The use of ULTEM 9085 resin enabled thinner walls, resulting in a 30% smaller unit with considerable weight savings for the airline.

Now, sustainability and “green” interiors are entering the conversation. A European project called “Cayley,” which concluded about two years ago, brought together Boeing Research and Technology Europe (Madrid, Spain), Invent GmbH (Braunschweig, Germany), Aimplas  (Valencia, Spain) and Lineo (St.-Martin du Tilleul, France) and was aimed at industrializing environmentally-friendly interior panels made with renewable polymers or recyclable thermoplastic sheets and natural fibers, namely flax. Lineo's linen uni tape, called FlaxTape, and FlaxPreg linen/epoxy prepregs intended for transport interiors (cars, trains and aircraft interiors), reportedly are 35% lighter than carbon fiber/epoxy prepreg tapes. Lineo also offers a sandwich product, called Simbaa, as an alternative to traditional sandwich constructions.

A spinoff of flax grower parent LSM, Lineo is reportedly gearing up for large volume production. Francois Vanfleteren, company CEO, told European Plastics News at the conclusion of the Cayley project that Lineo's UD tape production line for the project makes tape that is 40cm wide, but when scaled up for series production 1m wide tape will be possible. PSA Peugeot Citroën is reportedly already starting to incorporate the material into its automotive platforms.

Pedro Martin, material scientist in the materials and fuel cells department at Boeing Research & Technology Europe, has given presentations about Cayley, and reported that Boeing is working with the partners on the flax sandwich panel made with epoxy, which can be used for cabin sidewalls, while other partners are investigating PP and PLA. "Fire resistance is the most challenging issue to overcome when working with natural fibers," Martin has told European Plastics News. During the project research, flax fabrics were treated with halogen-free flame retardants and used to produce a full-scale sidewall panel for a 737 interior in a vacuum bag process. At laboratory scale, the biocomposite achieved compliance with FAA and EASA fire resistance requirements. Curing time for the thermoset resin is reportedly too long, and shelf life for the prepreg needs to be increased. The project had a target to produce panels in an automated one-step process at a rate of one panel every 15 minutes.  

CW contacted Lineo, and got some additional information from Lineo’s R&D engineer Bruno Dellier. “Our FlaxPreg product meets the FAA’s FAR 25.853a self-extinguishing characteristics,” he notes. A video showing FlaxPreg’s fire resistance is available on the company’s Web site: http://www.lineo.eu/#!applications/vstc2=property-no3. Dellier goes on to tell CW that while Boeing and Airbus have shown interest in using the material, for the moment “the qualification process has not yet started.” When asked about the price of FlaxPreg compared to a glass/epoxy unidirection prepreg, he says that the price is similar.

A second natural fiber for aircraft consortium dubbed “DesAIR “Design of Environment Friendly Structures for Aircraft” was an R&D consortium project that brought together 4 Portuguese organizations – Almadesign (AD), Amorim Cork Composites (ACC), Instituto De Ciência e Inovação em Engenharia Mecânica e Engenharia Industrial (INEGI) and Universidade da Beira Interior (UBI) – with the intent of developing new high performance composite solutions for aircraft interiors, integrating natural materials, including cork,and developing specific manufacturing processes. This research appears to be inactive at the moment but interesting from the standpoint that major groups, including aircraft OEM Embraer, were involved.

Will natural fiber products perform well enough be be adopted in interiors, in the future? Given the substantial weight savings over traditional reinforcements, their on-par cost, renewable nature and life cycle benefits, they look like serious contenders. Indeed, Lineo had a stand at this year’s Paris Air Show, and was mentioned in Aviation Week magazine as an innovative company to watch. We’ll keep our eyes and ears open.