Theory of Adhesion and its Practical Implications A Critical Review

The quality and durability of a coating is directly related to the nature of adhesion. Chemists tend to associate adhesion with the energy liberated when two surfaces meet to form an intimate contact termed as an interface. In other words, adhesion may be defined as the energy required to dismantle the interface between two materials. Physicists and Engineers usually describe adhesion in terms of forces, with the force of adhesion being the maximum force exerted when two adhered materials are separated. There are many theories regarding the mechanism of adhesion such as adsorption (van der Waals forces), electrostatic, diffusion (entanglement of polymers with a substrate), chemical bonding, mechanical interlocking etc. all of which may play a significant role in interfacial bonding. The energy required to separate the adhesive (coating) and the substrate is a function of the adhesion level i.e. interactions at the interface, but it also depends on the mechanical and viscoclastic properties of the coating materials. It is definite that all the mechanisms mentioned could affect bond strength and adhesion. Because of the complexity of adhesion phenomena, there are many models for it. None of them, on its own, can fully explain adhesion. However, each model describes a part of the complex processes involved in adhesion. For optimum adhesion it is therefore absolutely essential to ensure good wetting by the coating material applied, thus creating ideal conditions for causing the film forming agent molecules to approach the substrate. In general, for good substrate wetting the surface tension of the coating material (p) should be lower than the surface tension of the substrate (s), or they should at least be equal. The real reason for insufficient substrate wetting is the too high surface tension of the liquid coating, however, other factors will also influence how strongly this defect shows up. It can only be performed indirectly by measuring the contact angle of droplets of liquid which have been applied to the solid surface being examined. A measurement of contact angle has been discussed briefly. The range of chemicals used as adhesion promoters includes silanes, silicones, titanium compounds, zirconates, amides, imines, phosphates, and specially modified polymers. Furthermore, there are binders, plasticizers, and additives (e.g., wetting agents) which though intended for other purposes, have the secondary effect of providing good adhesive strength. In terms of adhesive strength, it is in many cases the overall formulation of a paint or other coating material which is decisive. The mode of action of silanes has been briefly reviewed as these are the future commercially viable coupling agents (adhesion promoters) for external coating of pipelines. These coupling agents like epoxy and amino silanes are often applied as very thin layers on substrates such as steel/aluminium before an adhesive is applied. In many cases, only TOFSIMS is able to characterize the very thin layer of the coupling agent on the substrate.