Aerospace adhesives

TECHNOLOGY ASSESSMENT ASSOCIATES P.O. Box 1362, Amherst NH 03031-9939, (603) 673-0312. A number of studies on Adhesives in Electronics, Bonding and Joining Composites, Markets for Adhesive Tapes and Films, Conductive Adhesives, Aerospace Sealants, and Growth Markets for Adhesives. 

In the following sections/ we shall discuss adhesion chemistry/ adhesion physics / radiation-curable adhesiveS/ high-temperature adhesiveS/ anaerobic and structural adhesiveS/ hot-melt adhesives/ film adhesiyes/ waterborne adhesives/ aerospace structural adhesiveS/ conventional sealants/ advanced aerospace sealants/ and adhesives and sealants for solar collectors. 

Applications adhesives, aerospace structures, audiovisual equipment, barrier films, bobbins, cameras, capsules for electronic devices, coatings, composites, connectors and sockets, electric motor components, fiber optic connectors, fuel cells bipolar plates, Information storage devices, lamp sockets, LED, microwave cookware, precision molded components, printers and copiers parts, SMT components, sporting goods, under-bonnet automotive components, watches ... 

The performance of the classifier has been verified using a number of practical applications, such as civil engineering [3], inspection of aerospace composite structures, ball bearings and aircraft multi-layer structures. Here we present shortly some results, focusing on detection of disbonds in adhesively joint multi-layer aerospace structures using Fokker Bond Tester resonance instrument, details can be found in [1]. 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Epoxy stmctural adhesives are used in an extraordinarily wide range of appHcations. They are available in essentially all of the forms discussed above, except for primer—Hquid combinations or as room temperature curing Hquids. The highest technology appHcation for epoxies is in aerospace stmctural... 

Phenohc resins are the oldest form of synthetic stmctural adhesives. Usage ranges from bonding automobile and other types of brake linings to aerospace apphcations. These adhesives have a reputation for providing the most durable stmctural bonds to aluminum. Because of volatiles, however, and the need for high pressures, the phenohc resins are used less as adhesives than the epoxy resins. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

The most common aerospace adhesives include nitrile epoxy systems, epoxy polyamides, epoxy phenolics, and various unmodified epoxies [18]. Polyimides... 

Allbericci, P, Aerospace applications. In Kinlock, A.J. (Ed.), Durability of Structural Adhesives. Applied Science Publishers, London, 1983, Ch. 8. 

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

Although the acrylate adhesives are readily available and studies have shown that they can produce reasonable bonding properties, they have the disadvantages of having high shrinkage, high fluid absorption, and low service temperatures. Acrylate adhesive applications would be limited. The development of EB-curable epoxy adhesives would have applications in the aerospace and automotive industry and potential wider uses. The most immediate application for these resin systems is composite repair of commercial and military aircraft. 

The aerospace field is a broad one and has a complex history. A comprehensive review of structural adhesive applications on currently flying aerospace vehicles alone could fill its own book. Hence this chapter will concentrate on the aerospace commercial transport industry and its use of adhesives in structural applications, both metallic and composite. Both primary structure, that is structure which carries primary flight loads and failure of which could result in loss of vehicle, and secondary structure will be considered. Structural adhesives use and practice in the military aircraft and launch vehicle/spacecraft fields as well as non-structural adhesives used on commercial aircraft will be touched on briefly as well. 

While the use of adhesive bonding is widespread among all aerospace firms, the author is most familiar with history and practice at The Boeing Company and will of necessity draw primarily on that resource. Adhesive bonding use at other firms will be included as much as possible. 

As polymer chemistry advanced in the 1930s and 1940s, stronger and more durable synthetic adhesives such as early phenol, resorcinol and urea formaldehydes began to supplant natural glues in wood aircraft manufacture. Around this time however, metal began to replace wood as the dominant material for aircraft manufacture. Aerospace adhesives research and development moved on to focus on metals, primarily aluminum, as the substrates of interest. 

De Bruyne himself was undoubtedly no small factor in the spread of adhesive bonding through the European aerospace community. In addition to co-inventing the Redux family of adhesives, he was involved in the conceptual development of the Mosquito and numerous other aircraft and sponsored (and taught) a series of educational sessions on the benefits of structural bonding during the post-war years. 

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