Meeting composite surface needs

With its core competencies focused on structural adhesives and surface treatments, Henkel's Bjorn Ballien discusses the solutions the company has on offer for aircraft exterior surfaces. Mike Richardson reports.   Henkel exhibited a portfolio of products and applications for its aerospace customers during the recent JEC World show in Paris, including lightweight surfacing films for composite materials that offer a smooth, aerodynamic surface and advanced protection against lightning strikes. The company unveiled its lightweight Loctite EA 9845 Aero surfacing films which can be co-cured with epoxy based composite prepregs at 120-175°C to provide an excellent surface finish under autoclave and even vacuum only cure. The surfacing and lightning strike versions of these films protect the underlying composite materials and are said to enhance environmental resistance to UV and paint stripper exposure. Loctite EA 9845SF Aero is said to offer up to 30% weight savings compared to current surfacing films, whilst a reduction in manufacturing cost and process time is obtained through the elimination of filling and sanding steps during surface preparation. “Our products are designed to improve the surface quality of monolithic and honeycomb composite parts, and offer a durable, paintable surface with minimal surface preparation,” begins Bjorn Ballien, Henkel's EMEA business development manager aerospace. “The surfacing film is a thin epoxy layer that is applied to the exterior of the aircraft to protect the composite and to fill pin holes and voids to create a surface that can be painted on. “Aircraft exterior composite materials can sometimes contain surface texture and small defects, which during the life of the aircraft could be exposed to different chemicals, like paint strippers, and to repair processes. Applying a thin layer of epoxy improves the surface quality and acts as a protective layer, which also improves the paintability. In certain industries it's fashionable to see the aesthetic appearance of carbon fibre weave, but the aerospace industry does not follow this trend because the composite material's surface must be optimised for aerodynamics and to take coatings of primer and paint.” <Lightening meets lightning> Meanwhile, Loctite EA 9845LC Aero is claimed to provide the same high quality surface finishing with the lightning strike protection. This film includes lightweight, expanded copper or aluminium mesh for maximum electrical conductivity and protection of aircraft composite parts. The use of an integrated lightning strike solution reduces the number of materials to be inventoried and provides manufacturing cost reduction. As with the surfacing films the Loctite EA 9845LC Aero lightning strike materials are said to provide excellent surface finish when cured under autoclave or vacuum only conditions. The lightning strike films are available in a variety of film and foil weights to meet the requirements of different lightning strike zones on the aircraft. “Henkel's new generation of Loctite EA 9845LC Aero offers improved surface qualities,” states Ballien. “The product is designed to completely fill and smooth out any potential void or defect. In addition to surfacing, this product provides lightning strike protection. If a predominantly composite-built aircraft is struck by lightning there needs to be a safe method of conducting the electrostatic charge away. Henkel has produced a film that combines with thin copper or aluminium foils that are embedded into the surfacing film itself. Up until now, lightning strike films have required heavier surfacing film areal weights and significant re-work to fill and smooth the surface to meet aerodynamic surface requirements. The Loctite EA 9845 provides the same high quality surface finish and lightning strike protection when cured with only vacuum pressure.” Ballien adds that the new generation of Loctite EA 9845LC Aero also offers airframe OEMs potential time and cost savings in order to meet the increasing production rates. “In the past the airframer would need to use a much thicker film in combination with the copper or aluminium foils, because the thickness of epoxy layer needed to be sufficient enough to prevent the metallic foil from protruding out of the surface,” notes Ballien. “Due to the high surface quality and controlled-flow ability, it completely encapsulates the foils and is much better than the previous versions. Customers can now obtain a much lighter weight version – up to a 30% reduction in weight.” <Keeping UV in the shade> According to Ballien, all Henkel's products undergo the same rigorous qualification and testing requirements, plus there is a huge amount of mechanical testing that examines erosion resistance, fluid resistance, and chemical exposure, etc. “A key difference between Loctite EA 9845LC Aero and the previous generation of surfacing films currently being used is that the new Loctite EA 9845 Aero offers an enhanced ultraviolet (UV) resistance,” he continues. “During aircraft production, large exterior aircraft surfaces and components are stored in factories under UV lighting, or stored outside in direct sunlight. All the epoxy substances will change colour and the surface can degrade after a certain amount of UV exposure. Our new product has been developed with improved resistance to UV exposure, so the colour and surface characteristics won't change in a factory environment. “Another key environmental factor with this product is that if you consider the lifecycle of the aircraft, the first paint job the aircraft receives isn't always the one that sees it through its entire lifecycle. Airline operators may update livery or re-paint the entire aircraft. When an aircraft is sold or leased, the new operator will often change the paint schemes. Prior to repaint these aircraft go through paint stripping on the MRO side of the industry. The first generation of surfacing products weren't developed to resist the paint strippers. The paint stripper contained chemicals that could attack the surface, whereas our new surfacing film is resistant to paint stripper.” If Henkel's product really is so good, I finish by asking Ballien whether there is the possibility that customers can simply use copious amounts of 9845LC to hide any imperfections. “The ideal is that eventually we'll be able to manufacture composites parts so perfectly that they won't even need any surfacing products. However, the reality is that composite materials will always need to be protected – so these films serve an important purpose,” he concludes. “The first generation of films often contained quality issues after the cure cycle, such as pin holes, voids and other defects that required a lot of rework. The part would often need to be repaired using another adhesive and potting material to touch up all these defects. Loctite EA 9845 Aero will drastically reduce the amount of additional manual labour involved with the surface film of an aircraft.” www.henkel-adhesives.com