Adhesion is the set of physicochemical phenomena that occur in the molecular attraction between two materials of different nature placed in contact. Thus the force depends on the nature of the materials, temperature, and the pressure between the surfaces.
Adhesion is due to intermolecular forces of the same kind as that causing cohesion. While the term “adhesion“ refers to the action of attraction between molecules of different types, the term “cohesion“ refers to the action of attraction between molecules of the same type.
Although it occurs systematically, this phenomenon is often not very evident: in most cases, the objects we are dealing with do not spontaneously adhere to each other, because the intensity of the forces involved is very small. Adhesion, in fact, is due to interactions whose range of action is of the order of intermolecular distances: physical bonds – intermolecular forces – between the surfaces of bodies in contact and, sometimes, even real chemical bonds between molecules belonging to different bodies.
In order for two macroscopic objects to adhere with an appreciable intensity, a contact must be established between them at the microscopic level. However, mechanical reasons, due to the irregularities of the surface, and the impurities between the molecules, first of all moisture, make this contact difficult to achieve between two solids; on the other hand, some experiments have shown that, if the separation distance falls below a critical value – typically between 1 and 10 nm – samples of mica, gold, rubber and gelatin, smooth on an atomic scale, adhere spontaneously to each other.
Different is the case with a liquid and a solid, between which it is easy for molecular contact to be established that makes adhesion very intense; the phenomena of capillarity, or the very fact that liquids wet solids, are proof of this. For the most part, the adhesives commonly used in industry and in daily life are in the liquid phase at the time of application and only then solidify, precisely so that they can adequately wet the substrate and subsequently increase their cohesion: among the many examples we can cite vinyl glues, which, applied in water emulsion, solidify following the evaporation of the liquid medium, and instant universal adhesives based on cyanoacrylates, which polymerize very quickly in the presence of moisture on the substrate.
Adhesion is also of great importance in cell biology, as it represents the mechanism by which cells join together or with the extracellular matrix, particular proteins, called adhesion molecules, play a fundamental role in the formation of tissues and organs during embryogenesis, in the immune response and in the development of some diseases such as tumors. However, it is mainly technological applications, first and foremost bonding, that fuel interest in adhesion.
Among the major consumers of adhesives are high-tech industries such as aeronautics, aerospace, automotive and electronics, where welding and riveting have been partially replaced by adhesive joints, which make it possible to easily join different materials together, ensure a more uniform distribution of stress and are more resistant to fatigue, as well as being lighter and more aesthetically pleasing. Adhesives are also widely used in the wood industry, construction, footwear, packaging, sports, etc.; more recently their use has been introduced in dentistry and surgery. An in-depth knowledge of adhesion, as well as enabling the quality of adhesives to be improved, is useful in the manufacture of those products – paints, coatings, composite materials, etc. – whose performance depends on the properties of the adhesive. – whose performance depends on the properties of the interface between two materials.
The origins of adhesion science date back to the first studies on intermolecular forces and fracture mechanics, but it is only since the 1970s that this discipline has gained its own autonomy. Although it has always been in a secondary position with respect to technological applications, in recent years the gap between the science and technology of adhesion is rapidly closing. Adhesion is a complex phenomenon, which involves several disciplines, such as physical chemistry of surfaces and interfaces, macromolecular science, materials science, rheology and fracture mechanics, and is therefore difficult to understand in its generality.
The science of adhesion is, as we have seen, a rather recent discipline, dealing with a phenomenon that is quite complex, and so it should come as no surprise that there is no single theory of adhesion. It can be said that almost every pair of materials represents a particular case, to be described through a specific model, but the fact that there are some traits common to several systems has led to the formulation of theories capable of establishing criteria of good adhesion for entire classes of materials. As already mentioned, very general aspects are thermodynamics and rheology, which allow the interpretation of a large number of adhesive phenomena. Less universal, but equally worthy of attention, is the role of chemical bonds, which, when present, contribute significantly to the tightness of an interface. The electrostatic theory, which at the origin of adhesion invoked a transfer of electrons between adhesive and substrate, is now outdated: it is applicable only in very few cases and completely neglects the rheological aspects.
The thermodynamic theory has very general validity, due to the fact that London’s dispersive forces are universally present, and is one of the most widely used in the study of adhesion. The first formulation of a thermodynamic theory of adhesion is due to Louis Houghton Sharpe and Harold Schonhorn and is based on the hypothesis that physical interactions between molecules are sufficient to generate adhesion between two materials. These forces, in order to be effective, require that the adhesive and the substrate are in intimate contact, which is usually the case if the adhesive is applied in liquid form on a solid substrate; a necessary condition for adhesion is therefore that the adhesive wets the substrate. In describing the wetting process, thermodynamic theory establishes some wetting criteria and relates the thermodynamic work of adhesion to the surface energies of individual materials, so that this theory is also known as the wetting model.
A description of the rheological aspects is of fundamental importance in the science of adhesion, because it makes it possible to relate so-called practical adhesion to so-called fundamental adhesion.