At present, there are several theories trying to explain the phenomenon of adhesives’ adhesion on the substrates, and no unified theory can justify all cases now. It is required the use and combination of different theories to justify all cases.
Mechanical Theory
The mechanical theory explains the phenomenon of adhesion. It is directly linked to the porosity and surface roughness of the substrate with the degree of adhesion that can be obtained.
Any type of material if it is observed at the microscopic level, has a surface composed of valleys and ridges, the surface topography allows the adhesive to penetrate and fill the valleys, resulting in anchorage areas between the adhesive and substrate.
In addition to the roughness and porosity of the substrate surface, to generate adhesion anchor points, the adhesive needs to have a good filling power, and can penetrate into the valleys and pores substrate surface. The adhesive filling power is directly related to its viscosity.
Mechanical adhesion theory doesn’t account for the incompatibility that may exist between the adhesive and the substrate. It only takes into account the topography of the substrate and adhesive filling power, and so this theory can’t explain the adhesion between surfaces with low roughness or smooth, nor the lack of adhesion between rough substrates incompatible with adhesive.
Adsorption Theory
The adsorption theory explains the phenomenon of adhesion based on concepts such as contact angle, wet ability and surface tension.
When the adhesive has a lower surface tension compared to the substrate surface energy, it is capable of wetting the surface, generating a contact angle less than 90°, thus generating the adhesion between the adhesive and substrate.
Against mechanical theory and diffusion theory, adsorption theory explains the phenomenon of adhesion without penetration by the adhesive to the substrate. The adhesion is generated by the contact between the adhesive and substrate.
Chemisorption Theory
Chemisorption theory is an extension of the adsorption theory of adhesion, in which the adhesive has properly wet the substrate. The adhesion phenomenon arises when generating intermolecular or Van der Waals forces and chemical bonds between the adhesive and substrate. This theory can be used to explain the use of the compatible agents between the adhesive and substrate.
Diffusion Theory
The diffusion model explains the concept of adhesion by the compatibility between polymers and the movements that occur in the polymer chains. When two polymers are compatible, its polymer chains are able to mix up between them, resulting in partial penetration between the two materials, as a result of these penetrations anchorage areas and adhesion points take place.
The mobility and degree of penetration of the polymers is determined directly by their molecular weight so that short polymer chains have high mobility and penetrate into the other material before the long polymer chain. Rouse model and reptation model give a detailed explanation of the movements that occur between the polymer chains which produce the diffusion.
This theory can be used to explain the phenomenon of adhesion that occurs between polymeric materials, plastic welding, plastic binding with adhesives, etc.
Electrostatic Theory
The Electrostatic theory resembles the phenomenon of adhesion to a condenser, where the electrostatic charges of opposite sign attract each other causing the adhesion between the adhesive and substrate.
This model uses the concept of the electrical double layer to explain the formation of electrostatic charges.
The phenomenon of adhesion is currently studied in the research and development centers, due the importance of this phenomenon in the field of adhesives, since the complete knowledge of the phenomenon of adhesion allows the development of adhesives to attach any type of material under any condition.