Bonds, main chemical types

As discussed in Chapter 2, for the formation of intrinsic adhesion forces across the adhesive/substrate interface it is a necessary, though sometimes insufficient, requirement that intimate interfacial contact occurs between the adhesive and substrate. This means that at some time in the bonding operation the adhesive must be in a liquid form, ideally with a relatively low viscosity. However, in service most adhesive joints carry some mechanical loads, even if only those resulting from the weight of the substrates, and to be able to transfer mechanical loads efficiently from one substrate to the other requires that the adhesive is a solid with an adequate stiffness and strength. These conflicting requirements mean that once the adhesive has established interfacial contact, i.e. completely wet the substrate, it must then harden or set . In this chapter the methods by which the adhesive may be transformed from a liquid state to a solid state will be considered and this classification will also be used to introduce briefly some of the main chemical types of adhesive to those unfamiliar with this aspect of adhesives technology. 

Adsorption phenomena are divided into two main categories, physical (van der Waals or dispersion forces) and chemical (analogous to valence-bonding). The former type results in small... 

The main mechanism by which activated carbon removes impurities is one of physical adsorption, this being a reversible process. Consequently one can expect that desorption of the impurities will render the carbon surface available again for adsorption. Regeneration of spent activated carbon is not only important from the point of view of restoring the adsorption capacity of the carbon, but also because in many cases the recovery of the adsorbed species is important. If the adsorption is of chemical type (chemisorption), the formation of a bond between the carbon and the adsorbate makes the process non-reversible, and even if desorption is possible the desorbed species will be different to those originally adsorbed. Additionally, adsorption (especially in liquid phase) is often accompanied by precipitation of species which cannot be removed by simple desorption. 

In view of the enormous chemical diversity of the enolate-bound metals that cover the range in the periodic table from lithium to tin, it is not surprising to realize enormous differences in enolate structures. As a consequence, structural studies have to answer very different questions concerning mainly the type of bonding, stereochemistry, the role of ligands at the metal, aggregation, and solvation. 

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