Epoxy Adhesives on Selected Substrates

This chapter identifies and discusses various epoxy adhesives and the processes that have been used to successfully bond or seal specific substrates. There are only a few materials that epoxy adhesives will not bond well. These uncooperative substrates are most notably low-surface-energy plastics, such as the polyolefins, fluorocarbons, and silicones. However, even these materials can be bonded effectively with epoxy adhesives if a prebond surface treating process is used to change the nature of the substrate surface. Of the other substrate materials, there are some that epoxy adhesives will bond more effectively than others. Table 16.1 lists substrates that generally provide excellent epoxy adhesive joints. 

The selection of the proper epoxy adhesive formulation depends on 

The physical nature of the bulk substrate material (porosity, modulus, thermal expansion coefficient, etc.) 

The physical and chemical nature of the surface (surface energy, weak boundary layer attachment, resistance to corrosion, etc.) 

Optimal joints can generally be fabricated by the correct combination of epoxy adhesive formulation (i.e., type of resin, curing agent, fillers, modifiers) and surface treatment process. 

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To reduce the thermal stress for components as well as for thermoplastic base substrates, conductive adhesives and selective soldering can be used. In working with adhesives, good interconnections with a maximum temperature of only about lOCfC can be established. This temperature is necessary to harden the epoxy material. In the selective soldering technique, the heat is transferred only to places where interconnections have to be made between the component s termination and the pad on the substrate. This reduces the overall thermal stress situation for the whole 3D MID. 

An additional consideration for the selection of an adhesive system is that robust bonds must be formed to all surfaces involved in the interconnection. Materials commonly found include metallizations on the substrate and components (e.g., gold, solder, copper, aluminum, and indium tin oxide), polymer substrates and coatings (e.g., polyimide, polyester, epoxy, and acrylic adhesives), and chip passivation layers (e.g., Si02 and SI3N4). Adhesion promoters may be required. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

There are a wide range of adhesives which can be used to join concrete and metallic materials to FRP composite adherents, including epoxies, polyurethanes, acrylics and cyanoacrylate, but the one that is generally used for polymer composite plate bonding to concrete and metallic substrates is one of the epoxy group of adhesives. There are a number of epoxies on the market but the one selected must be compatible with the two adherents and the curing conditions the manufacturers advice should be sought for the most relevant adhesive to use in particular circumstances. 

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