Getting the choice right for aerospace interiors adhesives

Senior application development engineer, Andrew Marks of 3M looks at the key product selection criteria for OEMs components suppliers in the aerospace sector when selecting adhesives for interiors applications.
Strict regulations govern all materials used in aircraft interiors, right down to the type of adhesive used in forming the components and structure of an aircraft cabin. Among others, there are three main drivers which govern all materials used in aircraft interiors: passenger safety, weight management and aesthetics. Similarly to other products, the critical requirements for adhesives are that they ensure the safety and comfort of passengers, deliver durability and performance of individual interior components and ensure compliance with industry regulations. From the interiors manufacturer's point of view, selecting the correct adhesives can also contribute to production efficiency and greater speed of throughput. Flammability regulations relate to all aspects of aircraft interiors, both visible and invisible. Aircraft interiors – wall, floor and ceiling panels, seating structures, galley structures and so on – are largely made from composite panels chosen for their light weight and high strength-to-weight ratio. These are fabricated from face sheets made from phenolic resin and fibreglass, aluminium or aramid fibre, with a honeycomb core structure of Nomex, aluminium or aramid fibre and finished with decorative laminates and veneers or paint. In the process flow for the construction of interior components made from composite panels, adhesives play a key role at all levels, from core splicing to insert potting, edge filling, panel assembly, veneers' application and reinforcement for fixtures and fittings.  <Meeting safety requirements> From the safety aspect, there is a range of regulations which impact on the choice of materials for aircraft interiors designed to ensure passenger safety and reduce environmental risk. The Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) are responsible for the regulations that cover fire testing requirements for flame retardancy (FR) and fire, smoke and toxicity (FST). The US regulations focus primarily on flame retardancy, the FAA regulation applying to compartment interiors being 14CFR 25.853. This sets out the increasingly stringent, specific fire testing requirements composite structures bonded with adhesives and void fillers must undergo. While the EASA tends to adopt FAA wording for FR/FST requirements, there is a difference in regulatory emphasis between the US and Europe. In Europe there are additional regulations relating to toxicity levels as well as REACH health and safety concerns which have to be taken into account. REACH is the regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals which entered into force in 2007. Its main aim is to ensure a high level of protection both to human health and the environment from risks posed by chemicals.  REACH places restrictions on the manufacture, placement on the market and use of specific substances known as SVHCs (substances of very high concern and the list contains over 1,000 substances in over 59 categories. Some, such as Benzene, heavy metal and hexavalent chromium (Cr VI) have limitations on their use and are being phased out while others are completely banned. These regulations mean that some materials available in the US cannot be used in Europe and OEMs need to take account of the fact that any aircraft flying into Europe is governed by European legislation so substances not permitted under REACH cannot be used regardless of their acceptability in other territories. This not only inflicts additional restrictions to OEMs and components manufacturers tasked with selecting products for interiors, but also pushes material suppliers to reinvent their offering to ensure regulatory compliance.  As a forward-looking industry supplier, 3M follows a strategy of developing solutions that meet both current and future regulatory requirements looking at a 10 to 20 year horizon to take into account chemicals which are likely to be phased out or banned. In response to REACH, for example, 3M has already developed halogen and heavy-metal compound free FST systems. Scotch-Weld Structural Adhesive 7246-2 B/A FST by 3M, for example, was the first standalone FST compliant, non-halogenated and heavy-metal compound-free paste adhesives designed specifically for interior bonding applications. <The weight factor> When manufacturing composite panels, the choice of void filler can make a critical difference to the weight of the finished panel. From an operational point of view, aircraft weight is a key factor influencing fuel consumption which impacts both on airline profitability and the environment. Depending on the specification of the aircraft, the weight of the interior components could be higher than the weight of the airframe and non-structural non load-bearing components offer the greatest opportunity to reduce weight.  As one way to drive down weight in aircraft interiors applications, 3M has developed a line of lightweight, low-density void-fillers designed to fill, close and reinforce honeycomb and sandwich structures. The weight of the void filler can be reduced by effectively ‘adding air' in the form of microspheres as an alternative to conventional fillers; this is achieved thanks to the 3M Glass Bubbles Technology, engineered hollow glass microspheres with a high strength to density ratio. A combination of epoxy resin and Glass Bubbles Technology can deliver a density range from 033 to 0.75g/cm³. Both one-part and two-part structural and semi-structural epoxy fillers are FR and FST compliant as standalone products, meeting FAR/JAR 25.863 regulations and ABD 0031 Airbus Directive requirements, and are available in halogen and antimony trioxide-free options. <Adding to the aesthetics> Modern aircraft interiors do not just have to be supremely functional, they also have to look good. As well as other interior products, adhesives can play a part in ensuring that suppliers products create an aesthetically pleasing interior. Used as an alternative to mechanical fixings, adhesives provide a smooth, clear surface without any exposed parts. Additionally, they offer benefits in terms of performance. Adhesives are lighter in weight than mechanical fixings and bond strength enables the distribution of the load over a wider area. From the manufacturer's perspective, self-adhesive finishing materials can be applied quickly with minimal pressure and to accelerate work flow, components can be pre-taped for speed of application. For instance, 3M has developed the VHB Tape 5958 FR, a solution that replaces rivets and thus offers all of these performance and appearance benefits. One disadvantage of many adhesives is that they are dark in colour so might require additional surface coatings to conceal, hence increasing the number of manufacturing steps and the overall process costs. To overcome this, 3M has developed a Scotch-Weld Epoxy Adhesive EC-2815 B/A FR which is a white interiors adhesive that prevents see-through, eliminating the need for extra treatment. <The manufacturer's perspective> To meet the ultimate needs of the customer, when selecting adhesives for the manufacture of interior components, OEMs and parts manufacturers have to address all of the areas outlined above – observance of regulations for the safety of passengers and the safeguard of the environment, elimination of excess weight for operational efficiency and the overall aesthetics and comfort demanded of aircraft interiors. With an increasing demand from airlines for modern fuel-saving craft, there is a huge order backlog across the industry. Heightened demand puts pressure on any organisation within the supply chain to accelerate deliveries to help speed up production. For interiors manufacturers, the choice of adhesives can impact significantly on manufacturing efficiency and speed of production, which in turn can significantly add to profitability.  There are a number of factors that affect productivity in bonding/filling operations:  speed and flexibility of curing, mix efficiency, filling capability and materials properties. Carefully selecting solutions with the right performance properties is central to having a positive impact on manufacturing efficiency.  For instance, with two-part void fillers and adhesives, the speed of curing influences the rate at which subsequent steps in the manufacturing process can be achieved. For high-volume small bonding operations such as insert potting, a faster curing adhesive is advantageous whereas larger and more complex panel assemblies require a greater open time to allow for correct positioning or alignment. So despite the emphasis on production speed, using a fast curing adhesive is not always the answer. The solution is to select an adhesive that cures at an appropriate rate at which it will achieve handling strength, as different applications and volumes will require different handling times. Having understood these needs, 3M has developed a portfolio of interiors adhesives which offer solutions for all kind of curing time requirements. For example, Scotch-Weld Epoxy Adhesive EC-3531 FR reaches handling strength in 20 to 30 minutes, whereas Scotch-Weld Structural Adhesive EC-7246 FST B/A takes 4 to 5 hours to handling strength.  When using two-part systems, choosing filling and bonding solutions with the optimum curing cycles can help streamline manufacturing and speed throughput. Alternatively, if manufacturing processes allow, using one-part systems, core filling of boards can be achieved during the original panel manufacturing process. One-part void fillers and adhesives are designed to co-cure at the same temperatures as the resins in the composite skin during initial panel manufacture. This means that the cured filler is already in place when the panel comes downstream for routing or drilling and insert bonding operations offering opportunities both to eliminate process steps and to reduce process time. Choosing the right method of application can also make a significant contribution to production efficiency. Machine or hand-held dispensing equipment saves time when dealing with adhesives or void fillers in large quantities for high-volume applications and also offers the additional benefit of reducing the operator's exposure to the adhesive. When dealing with two-part adhesives, conventional hand mixing not only takes time but can be inaccurate whereas a pneumatic cartridge dispenser ensures the correct mix, a cleaner operation and avoids problems such as air entrapment, shrinkage and slump. Using the right adhesives and choosing the most effective delivery method can substantially simplify many aircraft interior manufacturing tasks. The correct choice allows operators to complete assembly speedily and efficiently whilst meeting the over-riding demand for passenger safety and, of course, meeting the airlines' desire for aircraft interiors that are appealing to their passengers. www.3m.co.uk/aerospace