Aerospace adhesives are used through the interior, exterior, and engine compartments of an aircraft. They are used on countless parts, ranging from overhead lockers, to seat trays, to in-floor lighting, and more. The engine contains threadlockers, retaining compounds, hydraulic system thread sealants, and structural adhesives. Furthermore, exterior panels, wing spars, and door moldings all use adhesives of some type.
Like many other components in the aerospace industry, aerospace adhesives must be applied according to specifications such as Mil-Spec, ASTM, SAW, or an internal company standard such as BAC Standard (Boeing Aircraft Company) or PCM (Lockheed Martin). Apart from these standards, adhesive selection generally comes down to the substrates being used and the requirements of the bond. The most common types of aerospace adhesives are anaerobic adhesives, structural acrylics, epoxies, and cyanoacrylates. Let’s take a look at each.
Adhesives of this type are used for thread locking, retaining, thread sealing, and can form in-place gasketing. Anaerobics are most commonly used within the engine compartment, although thread lockers can be used throughout an aircraft to prevent loosening caused by vibrations.
Structural acrylics are a highly diverse adhesive. They are used in a broad range of applications such as bonding magnets in electric motors and attaching harness clips.
The main reason behind the use of adhesives is the need for strength and weight reduction. These needs are also driving the use of carbon fiber within aircraft and drones. In addition to permabond structural acrylics, materials such as Permabond ET5428 and ET5429 are ideal for bonding carbon fiber and many other types of substrate. Lightweight honeycomb edge fillers are frequently epoxy-based due to their very low density that helps keep the aircraft as light as possible.
Finally, cyanoacrylates are used not only to permanently bond various items, but also as a processing aid to instantly secure parts that are being bonded with more slowly-curing adhesives. They also serve as a quick fix for damaged panel interior trims. Because they take mere seconds to cure, the turnaround time is virtually zero.
Aerospace adhesives are also classified by their adhesion principles based on the chemical properties of the adhesive itself. Adhesives are either considered reactive or non-reactive. Reactive adhesives comprise one- and multi-part adhesives, natural adhesives, and synthetic adhesives, while non-reactive adhesives include drying adhesives, pressure sensitive adhesives, contact adhesives, and hot-melt adhesives.
Adhesives are often preferred over mechanical fasteners like bolts or screws, especially when weight is important to the structural integrity and function of an aircraft. There are certain cases where mechanical fastening must be used, but both mechanical and adhesive fastening are frequently both viable options. Adhesives also offer certain aesthetic design and function benefits. For example, adhesives allow smoother body shape, benefitting aerodynamics. Additionally, aerospace designs sometimes require the use of very fragile materials that cannot easily undergo welding or bolting. Adhesives are able to bond materials of all shapes, sizes, and fragilities.
The main drawback of adhesives when compared to mechanical fasteners is that they cannot withstand all types of pressures, temperatures, and stresses. Furthermore, in cases where the adhesive sheet must be cured, the curing method may not be compatible with other materials, resulting in unwanted wear and tear. Nevertheless, aerospace adhesives are a highly beneficial tool throughout aerospace manufacturing.
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