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Choosing the right adhesives for aerospace interiors

08 September 2015

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. Andrew Marks examines the key product selection criteria.

Three main drivers govern all materials used in aircraft interiors: passenger safety, weight management and aesthetics. Apart from safety, the critical requirements for adhesives are that they deliver durability and ensure compliance with industry regulations. From the interiors manufacturer’s point of view, selecting the correct adhesives can also result in productivity gains.

Aircraft interior structures 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. Adhesives play a key role at all levels in the construction of interior components, from core splicing to insert potting, edge filling, panel assembly, veneer application and reinforcement for fixtures and fittings.

Meeting safety requirements
Various regulatory bodies dictate the choice of materials for aircraft interiors.  The Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) are responsible for the regulations that cover fire testing requirements for flame retardant (FR) properties and fire, smoke and toxicity (FST). The US regulations focus primarily on flame retardant properties (FAA regulation 14CFR 25.853 sets out the fire testing requirements that 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 (Registration, Evaluation, Authorisation and Restriction of Chemicals) criteria, which have to be taken into account.

REACH places restrictions on the manufacture, placement on the market and use of specific substances known as SVHCs (substances of very high concern). The list contains over 1,000 substances in over 59 categories, some of which 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.

As a a supplier of adhesives to this sector, 3M’s strategy has been to develop products 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, the company developed halogen and heavy-metal compound free FST systems. Its Scotch-Weld structural adhesive 7246-2 B/A FST, for example, was the first standalone FST compliant, non-halogenated and heavy-metal compound-free paste adhesives formulated specifically for aircraft interior bonding applications.

When manufacturing composite panels, the choice of void filler can make a critical difference to the weight of the finished panel. 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.

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. Its ‘Glass Bubbles’ technology (engineered hollow glass microspheres with a high strength to density ratio) in combination with an epoxy resin can deliver a density range from 033 to 0.75g/cm.

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 – and they frequently 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 using a product such as 3M’s VHB tape 5958 FR to replace rivets.

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. 3M’ Scotch-Weld epoxy adhesive EC-2815 B/A FR, which is a white interiors adhesive that prevents see-through, avoids the need for extra treatment.

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. 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 to achieve handling strength, as different applications and volumes will require different handling times.  3M has developed a portfolio of interiors adhesives which cater for a variety 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 four or five hours to reach handling strength.

As an alternative to two-part systems - and if the manufacturing process allows it - using one-part systems will enable the core filling of boards to 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.

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.

Andrew Marks is a senior application development engineer at 3M

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