The single most important requirement for most coatings is that the system adheres strongly onto the substrate after film formation has completed. Adhesion is defined as the resistance of the coating to being removed from the substrate.
Coatings are most often applied as liquid paints. For good coatings adhesion, it is crucial that the liquid fully spreads over the surface that must be coated. This pre-condition for obtaining adhesion, called complete wetting, is governed by the surface tension of the liquid and the surface energy of the solid.
Complete wetting will occur when the surface tension of the liquid is lower than the surface energy of the solid. Two approaches can be used when the pre-condition of complete wetting is not obeyed:
- Raise the surface energy of the solid via pre-treatment. In some cases, simply cleaning the surface with organic solvent can be sufficient as pre-treatment.
- Lower the surface tension of the paint by using an additive called wetting agent (EU).
Key Aspects of Adhesion
Several aspects have to be taken into consideration when good adhesion is required. This applies to both normal adhesion, referring to a coating on a substrate, and intercoat adhesion, the adhesion of a coating onto another coating.
A variety of adhesion mechanisms can be used in order to obtain good adhesion:
- The approach of assuring strong intermolecular interactions between substrate and coating is important, especially when metals are coated. Preferably strong physical bonds, like hydrogen bonds, are used.
- Diffusive adhesion is possible when paint components are able to dissolve into the substrate during film formation. Diffusion can be used when plastic objects are coated.
- A rough surface has a higher effective surface area than a smooth surface. Surface roughness is beneficial for adhesion. It can be obtained via pre-treatment like sanding or abrasive blasting.
- When a substrate is porous, like stone, liquid paint can penetrate into the pores. In many cases penetration gives strong adhesion because of mechanical anchoring.
- Superior adhesion is obtained when the coating is chemically bonded, via covalent bonds, to the substrate. Substrate and coating are then said to be coupled. Specific additives, adhesion promoters (EU) based on silane chemistry, are used to bond coatings onto metals.
It depends upon the total system, consisting of both substrate and coating, what adhesion mechanisms can be used for obtaining good adhesion.
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