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Introduction

The single most important requirement for most coatings is that the system adheres strongly to the substrate after film formation has completed. Adhesion is defined as the resistance of the coating to being removed from the substrate.

Complete Wetting

Coatings are most often applied as liquid paints. For good 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 is, by definition, the situation in which the contact angle is 0°. 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.

Key Aspects of Adhesion

Several aspects, next to complete wetting, must be taken into consideration when good adhesion is required. This applies to both ‘normal’ adhesion, referring to a coating adhering on a substrate, and intercoat adhesion, the adhesion of a coating on another coating.

A variety of adhesion mechanisms can be used to obtain good adhesion¹. Some of these mechanisms are highly effective and others may help.

Two mechanisms result in excellent adhesion.

First, diffusive adhesion is possible when paint components are able to cross the substrate-paint interface during film formation, giving interfacial mixing. The concept of diffusion can be used when a plastic object must be coated. Superior adhesion is obtained when polymeric resin molecules from the paint can diffuse sufficiently deep into the plastic and entangle with polymeric tails and chains of the plastic.

Secondly, superior adhesion can be obtained when the coating is chemically bonded, via covalent bonds, to the substrate; the substrate and the coating are then said to be coupled². The covalent bonds form at the interface during film formation, when chemical groups of the binder system in the paint react with chemical groups on the surface of the substrate.

Specific additives, called adhesion promoters, can be used to chemically bond coatings to, for example, metals^3,4.

A pre-condition for obtaining and maintaining excellent adhesion is that the covalent interfacial bonds are connected to the binder system in the coating. Also, the coating must be sufficiently flexible to be able to deal with stresses that are applied to the system. This flexibility requirement is especially true for the part of the coating that is near the interface.

An example of using the combination of diffusive adhesion and coupling is when a liquid topcoat is applied on a liquid primer being still wet. Superior intercoat adhesion can be obtained when the two paints are miscible to some degree and when the paints are based on the same crosslink mechanism. This concept is called wet-on-wet application.

A crucial pre-condition for long-term strong adhesion is that the mechanical properties of the complete system, consisting of substrate, coating and interface, are OK. The system should have no internal stress and the system must be able to dissipate any force that is applied. Especially, the region near the substrate-coating interface must be strong but not rigid. This implies, for example, that the crosslink density within the coating, and especially near the interface, should not be too high.

Additional Mechanisms

Other adhesion mechanisms can assist in obtaining good adhesion.

A rough surface has a higher effective surface area than a smooth surface. Surface roughness, that can be obtained via pre-treatment like sanding, etching or abrasive blasting, is beneficial for adhesion.

Secondly, liquid paint can penetrate into the pores of porous substrates, like stone and wood. Penetration may improve adhesion because of mechanical anchoring.

Assuring strong intermolecular interactions between substrate and coating can be important, especially when metals are coated. Preferably, strong physical bonds, like hydrogen bonds, are used. It is risky to use hydrogen bonding as main adhesion mechanism. The reason is that other molecules, that are able to form hydrogen bonds, can compete with the hydrogen bonds that give adhesion. A well-known example is that some metal coatings show excellent adhesion under dry conditions but fail when the metal-coating interface comes in contact with water.

It is important to realize that the complete system, consisting of substrate, coating(s) and interface(s), governs how strong or weak the (intercoat) adhesion is.

References

1. A Guide to Providing Perfect Coating Adhesion, Ron Lewarchik, 29 November 2019.
2. Co-dependent: Adhesion on Steel via Covalent Bonding, Jochum Beetsma, 2 March 2018.
3. Superior Coatings Performance with Organosilane Components, Ron Lewarchik, 10 August 2018.
4. Adhesion Promoters 101, Marc Hirsch, 19 May 2016.

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