Bonding the Skies: A Practical Guide to Choosing and Using Aerospace Adhesives

Adhesives are used in various fields including engineering and non-engineering fields. One of the fields that extensively uses adhesives is the aerospace field. The adhesives used in the aerospace industry are termed “Aerospace Adhesives”, and they must possess special properties in order to be used in such a sensitive industry. 

In an aircraft, the adhesives are used in the internal components, external components and even in the engine components. For other aerospace applications, adhesives are selected according to the substrates in use and the bond characteristics. According to market research, it was found that the aerospace industry and the automotive industry share the usage of some types of adhesives [1].

Definition of Aerospace Adhesives

Aerospace adhesive is a term called on the adhesives used in the aerospace industry applications. There are numerous applications for the adhesives in the aerospace industry, such as thread sealing, retaining, aerodynamic smoothening, gasketing, …etc. 

Advantages of using adhesives in aerospace industry are numerous, such as:

• Protection against corrosion 

• Improvement of the distribution of stress

• Aerodynamic smoothening 

• Lowering aircraft weight

Types of Aerospace Adhesives

Aerospace adhesives can be classified into two categories: structural & non-structural adhesives. The main difference between the two categories is that structural adhesives bond components that experience large stresses that can even reach their critical point. 

On the other end, non-structural adhesives are used only to bond light-weighted components that do not carry stresses, such as decoration panels inside the cabin. An adhesive that can withstand 1000 psi or more of shear force can be considered as a structural adhesive.

Under these two classifications, there are various types of aerospace adhesives, some of these types can be listed as follows:

Anaerobic Adhesives

Anaerobic adhesives are adhesives formed of one part only, and they cure only in active metal surfaces, they usually fail to bond to plastics, rubber and glass; this is due to the requirement of oxygen absence in the bond line. This type of adhesive is used for locking and sealing of thread, retaining and gasketing. They are extensively used in the engine compartments, but also are used in locking of threads in various parts of aircraft to prevent bolt/nut loosening during vibrations. 

Structural Acrylics

This type of adhesives includes products that are based on methyl methacrylate, which can offer ultimate strength adhesive performance. They are used on plastics, metal surfaces, and aluminum. They also have the advantage of performing very well on oily surfaces. Structural acrylic adhesives provide a continuous bonding line, which in consequence can prohibit the leaking of fluids or dirt inside or outside of the bonded components.

Figure 1 – Structural Acrylic Application

In the aerospace industry, these adhesive types are used in electrical parts, such as to bond the electric motors magnets and electric harness clips attachments. 

Epoxies

Epoxy adhesives are the most frequently used adhesives in the aerospace industry. The reason for that is their ability to enhance the light weighting efforts. Because of their ability to withstand extreme conditions and variations, as well as adhering to stringent regulations, it proved to be the excellent adhesive type for aerospace applications. 

Epoxy adhesives can supplement, or even replace fasteners in bonding some light-weighted components, and with the extensive use of composite materials in the aerospace industry, the usage of epoxy rather than fasteners also increased throughout the aircraft, and specially in fuselage parts. 

Figure 2 – Metal Coating with Epoxy 

Epoxy not only decreases the weight of the aircraft when used to bond components, but also it enhances the stress distribution on that component. This is achieved by allowing a more consistent bond line, and thus the stress is distributed on a greater area instead of being concentrated on fasteners or rivets. 

Epoxies are also used in the construction of composite sandwich structures that are used in the fuselage skin panels in modern aircrafts. An example is the Henkel Hysol EA 9390 Adhesive Paste. They can provide excellent adhesion between the layers and great resistance to stress and corrosion [1]. 

Cyanoacrylates 

Cyanoacrylate adhesive is the industrial name of the super glue that is used in various engineering and non-engineering applications. This type of adhesives are used to bond various components. They have the advantage of fast curing and instant adhesion. That is why they are used in some cases as a processing aid to provide time to slower curing adhesion cures. These types of adhesives are used mainly in the interior cabin parts and decorations due to their fast-curing time which will result in lower turnaround time [2].

Selection of Aerospace Adhesives

There are numerous factors to consider when selecting adhesives to use in the aerospace industry. It’s very important to study the area of application of the adhesive from various perspectives before selecting the right option for the task. Factors that must be considered when selecting the adhesive can be listed as follows:

• Temperature resistance: adhesives used in the aerospace industry are often exposed to extreme temperature conditions. Some of the adhesives experience freezing temperatures (at high altitudes), such as the adhesives used externally on the fuselage and wing structures. An example is Scotch Weld EC2216B/A. Some other adhesives are exposed to very high temperatures, such as adhesives used inside engines and air-conditioning bays. An example is the EA9309 3.3NA AERO Epoxy Compound Adhesive.  So, careful study of the temperature variations of the component to be bonded is a must before selecting the adhesive.
• Chemical compatibility: one of the important factors before selecting the adhesive is its chemical compatibility with the component to be bonded. Neglecting this may result in selection of an adhesive that can chemically react with the component, and in consequence can corrode or degrade the component’s ability. As a result, the adhesive selected must be chemically stable with the component structure. 
• Bond strength requirement: this is the primary requirement to be considered when selecting an adhesive. For structural adhesives, the bond strength must be enough to withstand the stresses experienced on the component to be bonded. An example of an adhesive with a particular high bond strength is the Henkel Loctite EA 9394 C2 Hysol Adhesive.
• Environmental considerations: aircrafts fly at high altitudes where they experience extreme moisture and icy conditions, they also may land in areas with extremely dry atmosphere; so, adhesive selection must be taken with consideration to these varying environmental conditions. Sealants for example, must be able to prevent the moisture from penetrating, while also withstanding dry atmosphere. 

Safety Considerations

While using adhesives, personnel must keep some safety considerations in mind. Some types of adhesives have a strong odor and can be harmful to human respiratory systems, so staff must be strictly committed to safety precautions while using these types of adhesives. Also, protective gloves should be used when dealing with some adhesives, as they may cause harm to the skin. 

Selection of adhesive types is also a safety concern, as low-quality adhesives can result in damage to aircraft components and may threaten aircraft safety. That is why specifications were set for these adhesives. Compliance to these specifications can guarantee the reliability of the bond. Examples of these specifications are Mil, ASTM and SAE. Aircraft manufacturers such as Boeing and Lockheed Martin have internal company specifications also.

Future Trends & Innovations

The size of the adhesives market in the aerospace industry is growing very fast with the increase of aircraft industry orders. The market is expected to grow from $1.51 million to $1.63 million in the current year, with an annual increase of 7.82%. That is a reason to encourage the major companies to invest more in the adhesive and sealant product innovation [3].