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The Aerospace Industry - Feeding Ground for Structural Adhesive Development

Adhesives have always played a significant role in the aircraft / aerospace industry primarily because they offer a low weight, fatigue resistant, and aerodynamically sound method of assembly. Adhesive bonding is also less labor and cost intensive when applied to large structures such as those commonly utilized in the aerospace industry. Structural adhesives, especially, account for the greatest market share of all the adhesives used in aerospace applications.

In the aerospace market a distinction is made between primary and secondary structural applications. Joint failure in a primary structure will result in the loss of the aircraft. Whereas, failure in a secondary structure will result in only localized damage. Structural adhesives are used in both applications.

The primary types of epoxy adhesives that are used in the aerospace industry are structural film adhesives and liquid and paste adhesives. The film applications include metal-to-metal and honeycomb bonding. Advanced composite-to-composite bonding using film adhesive is gaining in significance because of the rapid growth rate and use of these materials. Liquid and paste epoxy adhesives are used for a variety of assembly operations including liquid shimming and aircraft maintenance and repair.

The early aerospace adhesives were primarily based on epoxy resin chemistry. However, unique applications requiring high temperatures and fatigue resistance has forced the development of epoxy-phenolic, epoxy-nitrile, epoxy-nylon, and epoxy-vinyl adhesives specifically for this industry. The aerospace industry has led in the development and utilization of these epoxy-hybrid adhesives.

Over the past several decades, significant advances have been made in developing epoxy based adhesive having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high temperature applications.

There are many examples of the evolution of epoxy structural adhesives. Often the aerospace industry will take the lead in incorporating these new technologies into their manufacturing system and then other industries (automotive, for example) will become attracted to their value proposition. Several examples of this development activity are:

• Heat activated epoxy adhesives for use with induction heating apparatus can lead to increased assembly line productivity.
• Epoxy adhesives suitable for use on oily metal can reduce the number of manufacturing operations.
• Adhesion promoters such as dithiooxamides can be included in epoxy formulations to improve adhesion and durability.
• Epoxy resins have been modified with phosphorous to introduce flame retardance.
• Siloxanes have been used to modify both epoxy adhesives and adherends in order to improve adhesion and durability.

Structural adhesive bonding in the aerospace industry has primarily been through the initiative and development of applications for military. For example, in 1975 the U.S. government sponsored a program known as the Primary Adhesively Bonded Structures Technology (PABST) to extend the use of adhesive bonding for aerospace and to optimize manufacturing. Furthermore, many of the standards and specifications that are used in the industry come from military and / or government sources. These include Mil-A-25463 (bonding honeycomb structures) and MMM-A-132 (bonding metal-to-metal airframe structures). These standards define the physical (temperature, fatigue, etc.) and chemical environments (e.g., salt spray, humidity, jet fuel, hydraulic oil, etc.) to which aerospace adhesives will be exposed and their minimum requirements.

More recently, alternative adhesive chemistries (both organic and inorganic) have been explored that provide revolutionary improvements in adhesive properties. Some of these unique chemistries are indicated in Table 1. New chemistries are needed to break through limitations in temperature stability, fracture toughness, damage resistance, and long term durability.

The aerospace industry is also concerned about processing time and efficiency. Figure 1 illustrates the massive effort that it takes to incorporate adhesive bonding in aircraft manufacture. Alternative cure technologies are being developed for "cure-on-demand". These are designed to expedite curing and reduce flow times. They include UV, e-beam, and dual cure concepts.

Figure 1: Production line for the Boeing 787 Dreamliner. (Courtesy Boeing Company) (SpecialChem Fig. Ref.: 787 Dreamliner)

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References

1. Blohowiak, K., "Adhesive and Sealant Technology Needs for the Aerospace Industry", Adhesives & Sealants Council Exposition, April 17, 2012.

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