Structural adhesives applications

Structural applications of rubber base adhesives were also obtained using rubber-thermosetting resin blends, which provided high strength and low creep. The most common formulations contain phenolic resins and polychloroprene or nitrile rubber, and always need vulcanization. 

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. 

Adhesives are not used Just for structural applications on modern aircraft. In fact, the number of non-structural applications of adhesives vastly outnumbers the structural applications. Adhesives are used for everything from assembling lavatory walls to attaching the No Smoking sign to cabin partitions. Just a sampling of adhesive types and applications are discussed below. 

The thermal stability of epoxy phenol-novolak resins is useful in adhesives, structural and electrical laminates, coalings, castings, and encupsulalions for elevated temperature service. Filament-wound pipe and storage lanks. liners liir pumps and other chemical process equipment, and conosion-resisiant coatings are typical applications using the chemically resistant properties of epoxy novolak resins. 

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in structural applications. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

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. Figure 1.4 illustrates the degree to which adhesive bonding is used in modem aircraft. 

These hybrid epoxy adhesives are generally used for demanding structural applications such as in the aerospace industry where the optimal properties from each component are desired. For example, epoxy is generally used to provide good adhesion and processing characteristics. They are blended with the following resins to provide additional improvements in the properties noted. 

Solid epoxy adhesive formulations can be processed to either a thermoplastic or a thermoset state. Solid epoxy resins of exceptionally high molecular weight (e.g., phenoxy) can be used without any degree of cure as a hot-melt type of adhesive. However, fully crosslinked, thermoset systems are generally employed in structural applications. 

One of the most popular uses of radiant curing is the advancement (viscosity increase) or crosslinking of pressure-sensitive adhesives. These applications have been satisfied mostly with acrylate-based adhesive systems. With epoxy-based adhesives, the main applications are electrical and electronic components, the bonding of large aerospace structures such as composites, and the bonding of transparent substrates such as glass and plastic. 

The values are within the limits for electronic application. The seal is thermally stable up to 180 °C. The investigations show that by tailoring chemical structures of sol-gel derived organically modified hetero polysiloxanes, mechanical and adhesive structures can be tailored for needs of application. 

High adhesive strength of synthetic adhesives, making them the only suitable choice for structural applications... 

A family of acetylene-terminated phenyl quinoxalines have been synthesized by the Polymer Branch of the Materials Laboratory. ( 1) These phenyl quinoxalines are remarkable for their thermooxidative stability and resistance to moisture. These materials have potential for structural applications as adhesives or composite matrix resins.(2) The feature of moisture resistance makes the materials especially attractive for bonding aluminum. However, problems arise from the fact that aircraft aluminum alloys (and their surface oxiges) are altered by exposures to temperatures above 177 C (350 F) and this is much lower than the polymerization temperatures of the acetylene-terminated oligomers. 

Even though the strength was lower than that of recent high-performance structural adhesives, we might say that the dismantlable adhesive has enough strength for semi-structural applications. 

The strength of these systems is typically adequate for most structural applications. However, the deleterious effect moisture has on the bonding between the metal and adhesive must be taken into account if epoxy resins are to be used for bonding steel in moist or humid environments. This places a severe limitation on the number of structural bonding applications where epoxide adhesives could be used. 

Adhesion properties of epoxies, complete reactivity with no volatiles during cure, and minimal shrinkage make the materials outstanding for adhesives, particularly in structural applications. The most commonly known adhesive applications involve the two-component liquids or pastes, which cure at... 

From the chemists has come the classification of adhesives and sealants by chemical type, and from the mechanical engineers the classification as either structural or non-structural. Neither is a pure system, since many adhesives and sealants have more than a single chemical base resin, and many structural -based resin systems are used in non-structural applications. In a chapter entitled Structural Adhesives we are told that the term structural adhesive came into general use in the 1960 1970 period, but to this day all definitions are inadequate [9, Chap. 7]. Adhesive manufacturers and their advertising... 

MUF resin can be used as a cold-setting wood laminating adhesive for glulam and fingerjointing by the use of adequate acid hardeners. In all semiexterior and protected exterior structural applications where a clear/invisible glue line is preferred for aesthetic reasons then a MUF adhesive is preferred to the classical PRF adhesives used for this purpose. It is then more a question of fashion cycles, but notwithstanding this MUF resins have taken a considerable hold today in Europe (contrary to North America where PRFs are by far preferred) and confidence in them for this application has been steadily growing. 

Although PVA adhesives for nonstructural applications and formaldehyde-based resins for structural applications have price advantages over epoxies and offer excellent... 

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