Adhesion in coating paints refers to the ability of a paint or coating to firmly adhere to the surface of a substrate, such as metal, wood, concrete, plastic, or other materials. Regarding coating paints, there are three adhesion mechanisms that can occur: mechanical adhesion, chemical adhesion, and diffusion adhesion.
Mechanical Adhesion
Mechanical adhesion refers to the ability of a coating or adhesive to adhere to a substrate through physical anchoring, micro-anchoring, or interlocking. Unlike chemical adhesion, which involves the formation of chemical bonds between the coating and the substrate, mechanical adhesion is based on physical and geometrical forces that enhance the bonding and cohesion between materials. Similarly, mechanical adhesion occurs at a superficial level compared to diffusion adhesion.
Chemical Adhesion
This type of adhesion is based on the formation of chemical bonds and intermolecular forces between the components of the coating or adhesive and the surface of the substrate, resulting in a chemical union and intimate molecular interaction between materials.
Diffusion Adhesion
Diffusion adhesion is an adhesion mechanism that occurs when molecules or atoms from one material diffuse into the surface of another material, creating an intimate union and molecular interpenetration between the two materials. This type of adhesion is common in polymeric coatings and is based on the ability of polymers to penetrate and diffuse into the surface of the substrate, forming a mechanical and chemical bond between the coating and the substrate.
Wetting Capacity is the Basis of All Good Adhesion
For any of these adhesion mechanisms to occur, the resin must be capable enough to penetrate the pores of the substrate. This is called wetting capacity or wetting the substrate. It is of no use to have a resin that forms strong chemical bonds with the substrate molecules if it cannot interact sufficiently with it, and penetration allows it to interact with many more substrate molecules than if it only remains on the superficial contact zone. The degree of penetration depends on the porosity of the substrate, the viscosity of the resin, pressure, and duration.
Porosity allows for more effective interpenetration and interlocking of the coating molecules within the pores and irregularities of the substrate surface. Therefore, it is highly recommended to texture and roughen the substrate surface before applying the first paint layer.
The viscosity of the resin or the first layer is crucial since the resin must have slight movement to be able to penetrate the pores. Sometimes additives are used to encourage this slight movement and improve adhesion.
Pressure refers to external forces that push the liquid resin into the pores. Atmospheric pressure is one of them: air molecules tightly packed pushing the resin. Therefore, an application at sea level will have better adhesion than one at thousands of meters with all other factors equalized.
Duration refers to the time the resin has to penetrate as much as possible. In the case of thermosetting resins, they have a limited time in which they are liquid before reacting with the hardener and becoming solid forever. The longer they remain in the liquid state, the deeper they can penetrate.
