Epoxy syntactics and structural adhesives can now meet a range of critical and demanding applications for the world’s aerospace manufacturing and repair industries. Dr Phillippe Christou looks at some of the latest product developments and applications in this field.

Epoxy adhesives and syntactics are now widely used in the manufacture, assembly and repair of interior and exterior aircraft components and all must be able to meet, indeed exceed, the demanding qualifications of the world's leading aircraft manufacturers.

These products must not only deliver excellent mechanical performance and ease of use, but must also comply with the aerospace industry’s fire and smoke regulations and environmental standards regarding the use of flame retardant compounds. To do this, many adhesives and syntactics are today self-extinguishing and exhibit the low flame, smoke and toxicity characteristics required for interior component manufacture and repair.

Adhesives and syntactics are used in the aerospace industry for a range of applications but the most common ones include: manufacture and repair of aircraft interior and exterior components; flexible bonding of engineering plastic substrates; reinforcing of honeycomb structures including overhead luggage lockers, floor panels, galley and lavatory cabinets, flight control surfaces and landing gear doors; manufacture and repair of sandwich structures; dampening of engine and noise vibration and general impact resistance.

To heighten productivity in component manufacturing and repairs, pre-formed cured epoxy inserts are also available. These can reduce the time required to reinforce honeycomb sections by up to 95percent, eliminating all weighing, mixing, gelling and curing procedures. It also minimises end-user exposure to reactive materials during the reinforcing procedure.

The minimal void content produced using these inserts rather than conventional techniques gives greater integral strength, optimises compressive performance, and its precision application reduces the amount of reinforcing material required, thereby reducing part weight.

Two recent areas of product development have seen the introduction of a new epoxy based aerofoil syntactic paste for engine vibration dampening; a bromine-free*, flame retardant epoxy syntactic paste for high strength reinforcement in honeycomb core parts.

At JEC2006, an epoxy based aerofoil fill syntactic paste was launched to the aerospace industry by Huntsman Advanced Materials. Developed by the Araldite team in conjunction with engineers from British aircraft engine manufacturers, Rolls Royce and the Engineering Department at the UK’s University of Sheffield, Araldite1641A/B won the JEC Composites Innovation Award (Aeronautics category) at the show.

This lightweight adhesive solution is designed for injection into the hollow fan vanes of gas turbine engines on commercial aircraft. The syntactic is designed to absorb vibration and noise over a wide operational temperature range. Araldite1641A/B is a very low density two component syntactic paste and it can be used as a replacement for traditional metal reinforcements in aerospace engine components such as outer guide vanes where it not only significantly reduces vibration but enhances overall robustness as well.

Cascade Aerospace Inc, of British Columbia, Canada is one of the leading aircraft maintenance companies in the country, with a reputation for providing fast, durable repairs on a variety of aircraft components.

Among the latest products now being used at Cascade, is a new low density, bromine-free, flame-retardant epoxy syntactic paste from Huntsman – Epocast 1633-A41/B. It is being used for the high-strength reinforcement of the fasteners in overhead baggage lockers and other honeycomb core parts.

Cascade switched to Epocast 1633-A41/B in response to the latest environmental standards governing the formulations used in flame-retardant epoxies. This lightweight syntactic is the product of several years’ research aimed at eliminating the chemical compounds octabromodiphenyl ether (OBDE) and pentabromodiphenyl ether (PBDE) that have been used by manufacturers to produce flame-retardant compounds.

In addition to sound environmental credentials, Epocast 1633-A41/B epoxy syntactic products exhibit a compressive strength of 45MPa and are qualified to Airbus AIMS 08-08-001-04 and BoeingBMS5-28, Type18, Class2 specifications.

The syntactic paste used by Cascade is grey although light blue, orange and black formulations are available.

Because of virtually continuous heavy use, overhead lockers are among the aircraft parts that need most frequent repair. Damage is found when the lockers are removed during the strip down on a fuselage inspection. While locker repairs are generally cosmetic, airlines want to keep interiors looking smart and pleasing to passengers.

As part of its refurbishment programme on the lockers, Cascade reinforces honeycomb core at points that will be used to mount the locker in the aircraft. Using a manual dispensing gun, a technician fills a pre-drilled hole, covered with an insert tab, with Epocast1633 epoxy resin and hardener. Syntactic paste is added until the material flows from the other side of the tab, indicating that the cavity is full and the reinforcement complete. The paste has a two-five minute work life and sets quickly at room temperature to increase productivity.

After the epoxy syntactic cures, the insert tab is removed, excess epoxy is sanded over, the unit can then be repainted before being put back in place.

The 50ml dual cartridge used for dispensing the epoxy syntactic ensures thorough mixing of the components and neat dispensing of the paste with minimal waste. Cartridges can also be resealed and used again with a new nozzle.

Epoxy adhesives and syntactics now play a critical role in the aerospace industry. These joining and reinforcing systems have enabled manufacturers to use more lightweight structural components and an increasing range of dissimilar materials, while still conforming to the highest performance standards.

*Bromine-free as required by EU Directive 76/769/EEC relating to restrictions on the marketing and use of certain dangerous substances and preparations (PBDE or OBDE).

Dr Phillippe Christou is with Huntsman Advanced Materials (Switzerland) GmbH, Basel, Switzerland. www.huntsman.com