Structural adhesives definition

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... 

Pressure-sensitive adhesive masses, in one sense the opposite of structural adhesives (but also requiring permanence within their own performance definition), have been reported for both UV and E-beam processes where telechelic polybutadiene/acrylonitrile liquids are concerned. 

Structural bond—A bond that stresses the adherend to the yield point, thereby taking full advantage of the strength of the adherend. On the basis of this definition, a dextrin adhesive used with paper (e.g., postage stamps, envelopes) which causes failure of the paper, forms a structural bond. The stronger the adherend, the greater the demands placed on the adhesive. Thus, few adhesives qualify as structural for metals. A further requirement for a structural adhesive is that it be able to stress the adherend to its yield point after exposure to its intended environment. 

By definition, a structural bond involves the formation of a load-bearing joint between high-strength materials, typically metal, wood, ceramic, and certain plastics. The first step in the selection of a joining method should be a comparison of the relative merits of the available techniques.In addition to structural adhesives, these include a number of mechanical fastening methods, such as screws, bolts, nails, staples, and rivets, as well as metal fusion processes. Many of these techniques are usually associated with the joining of metal structures. 

EN 12701 2001 Structural adhesives - Storage - Definitions of words and phrases relating to the product life of structural adhesives and related materials... 

It is very often convenient to use SEM to examine failure surfaces of joints as indicated above. The micrograph of O Fig. 9.2 is the failure surface of an aluminum substrate bonded with a structural adhesive. Although on the basis of the microscopy one would tend to classify this as an interfacial failure, there are a few small islands of adhesive left on the metal side of the failure that vary in size from a few micrometers up to around 100 pm. As the polymer is an insulating material, it will generally charge during electron microscopy and this is evident in the micrograph of Fig. 9.2 as darker contrast around the adhesive residue. As Chap. 10 will indicate, the definition of the locus of failure is a rather complex task and depends on the level... 

All of the above conclusions have been drawn for isothermal conditions without considering a thermal fluctuation under creep conditions but which in practice represent the usual case during the service life of structural adhesive joints. Considering temperature effects in creep-dependent lifetime, prediction can either follow a worst case scenario in which the definition of the load-dependent fracture envelope or the test for compKance with predefined strain limits is carried out at the maximum temperature to he expected during service life. This conservative approach is likely to lead to excessive contingency reserves. 

Over the last decade advances have occurred very rapidly in the area identified as composite materials. In general, a composite material is the combination of any two or more materials, one of which has superior mechanical properties but is in a difficult to use form (e.g. fiber, powder, etc.). The superior component is usually the reinforcement while the other component serves as the matrix in which the reinforcement is dispersed. The resultant composite is a material whose properties are near those of the reinforcement element but in a form which can be easily handled and can easily function as a structural element. Included in this definition are all of the reinforced materials including particulate, fiber, flake and sheet reinforcements. Adhesive joints for, example, would be a planar or two dimensional composite 1). 

The type of adhesion dealt with in the examples in the second paragraph above and Fig. 1 is mechanical or structural while for the lithographic resist adhesion requirements described in this paper a more practical definition of adhesion, one first proposed by Mittal [16], is being referenced and used. Resist patterning layer-substrate adhesion is required only to process or pattern a particular device layer. After the circuit layer is patterned, the resist layer is removed and does not become an integral part of the circuit, as opposed to a PI interlevel metal dielectric layer which does. As such, it is not required to possess high mechanical adhesion strength. In fact, the resist layer must be quantitatively removed after the circuit required layer has been patterned. If the resist layer adheres too well and becomes difficult to remove, it actually interferes with successful circuit fabrication. 

What is accomplished by adhesion promotion treatments in IC manufacturing should actually be referred to as wafer substrate preparation, and not adhesion. Adhesion in the structural sense, as experienced in airplane composite material parts attachment, is not accomplished by silane wafer processing treatments except for the PI applications discussed early in this paper. The term adhesion, as it is used here, refers to a more practical definition—that is, resist image adhesion. Nevertheless, this type of adhesion is essential to the huge international semiconductor business, and the early silane work of Plueddemann and others was essential to early wafer adhesion process development. 

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