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Any development in the aviation industry focuses on making airplanes safer and more comfortable while also reducing their weight and environmental impact. 20 years ago, only two percent of a commercial jet’s construction was made of composite materials. Nowadays, these materials account for more than 50 percent. Metal parts are increasingly being replaced by novel composite materials, where adhesive bonding happens to be the preferred joining method.
As a pioneer of composite technologies, Austrian aviation company FACC Operations has been a leader in design, development, and production of components and systems made of composite materials for the aviation industry. Reportedly, its lightweight composite components are installed in more aircraft models than any other manufacturer in the field. Its range of products includes structural components for the fuselage and wings such as aerodynamic winglets, panels and covers in the engine area, as well as complete passenger cabins for commercial aircraft, business jets, and helicopters.
FACC produces for all large aircraft manufacturers such as Boeing, Airbus, Bombardier, Embraer, COMAC, as well as for all important engine manufacturers and sub-suppliers.
As far as the interior of passenger aircraft is concerned, OEMs demand lightweight, robust overhead stowage bins, entertainment systems, lavatories, and other amenities. Passengers seek comfort and an attractive design. In the case of the FACC luggage compartments used in the Airbus A380 and the A320 family, the Interior division relies on composite materials in the form of honeycomb panels. Aramid fibers—better known as Nomex or Kevlar—coated with phenolic resin make the material light and durable. A transparent, decorative film of polyvinyl fluoride is then applied to the surface. Apart from being dirt-repellent and chemical-resistant, this decorative film gives the surface a pleasant feel and provides an attractive appearance.
The composite sandwich panels are lightweight and maintain high torsion resistance during manufacturing in their flat, half-finished state. Further machining will be required for bent or angled structures. This process takes place at the FACC Interiors plant, located in the Austrian town Ort im Innkreis. Using the so-called "ditch-and-pot" technology, the honeycomb structure is cut from the back of the composite sandwich panel. This allows the panel to then be easily bent or angled. Afterwards, the resulting cavity is filled with adhesive. Once cured, the adhesive will permanently keep the sandwich panel in its desired shape.
FACC had a set of demands for their adhesive: A two-component material with a mixing ratio of 1:1 to simplify mass production. In addition, high strength is crucial since the panels must withstand the weight of heavy, shifting luggage. Epoxy-based products meet these requirements very well. Due to the manual production process plus any necessary final touches on each component, the processing time of a 100-gram batch should be at least 30 minutes, while curing time until initial strength at room temperature should not exceed five hours.
The double-chamber cartridge dispensing system allows the thixotropic epoxy resin to be quickly and easily applied just as a one-component adhesive would. The adhesive fully cures at room temperature within the given parameters. Curing can be significantly accelerated by increasing the temperature to as high as +60 °C, thus allowing production cycle times to be reduced without affecting final strength.
After intensive testing, FACC opted for DELO-DUOPOX AB8390, specifically developed by DELO for aircraft interiors and especially designed for ditch-and- pot applications on sandwich panels. Further tests proved the versatility of this construction adhesive since it would also be suitable for bonding of load-bearing joints with inserts.
DELO-DUOPOX AB8390 not only fulfills all these requirements, but also, as a standalone product, it complies with the specifications prescribed by aviation authorities. This includes, for example, fire protection in aircraft interiors, proven by the Flame-Smoke-Toxicity (FST) test. In addition, the adhesive complies with RoHS Directive 2015/863/EU.
The flame-retardant adhesive features good strength and high chemical resistance at temperatures ranging from -55 °C to +85 °C, as is routine for aircraft interiors. For example, the tensile shear strength of two sand-blasted, bonded standard specimens made of aluminum is 25 MPa. This value is almost reached even after 500 hours of storage at +70 °C in a reference fluid for Skydrol (tri-n-butylphosphate).
DELO-DUOPOX AB8390 is extremely colorfast, keeping its white color even after 1,000 hours of storage at +70 °C and an air humidity of 85 %. FACC can thus rely on consistent colors to maintain the appearance of their aircraft components such as those used for luggage storage compartments.
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