Adhesion by Chemical Bonding

Absorption and Adsorption are not the sam e Absorption is the incorporation (sometimes even consumption) of a matter into a medium (light is absorbed/consumed by a pigment, gas is ab-sorbed/dissolves into a liquid), whereas Adsorption is the adhesion of matter onto a—usually solid—surface (dust on furniture, steam on windscreen, vapours on any solid surface...) Adsorption is further subdivided in chemisorption, in which the matter is bound to a surface by chemical bonds, and phy si sorption, in which the bonding is only a physical effect. The transition between both is fluent. 

The stress of CVD-W films can vary, depending on the deposition conditions [Joshi et al.51, Clark et al.52, Blumenthal et al.148, Sivaram et al.149), by one order of magnitude (ie. from 3xl09 to 13xl09 dyne/cm2) and is mostly tensile. Experience has shown that for a plug process the stress is seldom a problem since the majority of the film is removed during the etch back process. Loss of adhesion is usually not observed in the blanket-plug process. When the interfaces between the different films are clean the adhesion will be formed by chemical bonds (1-2 eV) instead of (weak) physical forces (ca. 0.2 eV).To remove a film with an adhesion of 1 Ev per... 

Another important use for polyisocyanates is in adhesives, usually as additives to rubber cements. For example, in rubber-to-metal adhesion, a chemical bond probably forms between the rubber and metal by reaction of the isocyanate groups with active hydrogens in the rubber and with the hydrated oxide layer on the metal surface. 

In cases in which surface treatment produces a smooth oxide (intentionally or unintentionally) bond performance is controlled by chemical bonds across the oxide-epoxy interface. This situation can arise, for example, when an FPL adherend is rinsed in fluorine-contaminated water or is exposed to fluorine vapor. ( 8) The oxide-epoxy bonds are relatively weak and are readily disrupted by moisture.(, 50) As a result, bond failure is rapid upon exposure to humid conditions, and the crack propagates along the adhesive-oxide interface. In cases in which a smooth oxide is formed intentionally, coupling agents such as silanes can be used to improve durability. This is discussed in detail in Chapter 9 by E. Pleuddemann in the accompanying volume, Fundamentals of Adhesion. 

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

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