Before understanding adhesion promoters, we must first understand what adhesion is.

Adhesion: The phenomenon of adhesion between a solid surface and the interface of another material through molecular forces. The coating film and the substrate can be combined together through mechanical bonding, physical adsorption, hydrogen bonding and chemical bonding, mutual diffusion and other effects. The adhesion generated by these effects determines the adhesion between the paint film and the substrate. This adhesion should be the sum of various bonding forces (adhesion forces) between the paint film and the substrate.
It is the key property of coatings to play the role of protection, decoration and special functions. Even if the coating itself has excellent physical and chemical properties, it will not have much practical value if it cannot firmly bond with the substrate surface or the base coat. This shows the importance of adhesion in coating performance.
When the paint film adhesion is poor, measures such as grinding the substrate, reducing the coating construction viscosity, increasing the construction temperature, and drying can be taken to improve the mechanical bonding force and diffusion effect, thereby improving the adhesion.

Generally an adhesion promoter is a substance that enhances the bond between two surfaces, making the bond stronger and longer-lasting.
Adding adhesion promoters to the coating system is also one of the most effective ways to improve adhesion.

Adhesion promoters have four modes of action:
Chemical anchoring for both the paint film and the substrate;
Chemical anchoring for the paint film and physical wrapping for the substrate;
Physical wrapping for the paint film and chemical anchoring for the substrate;
Physical wrapping for both the paint film and the substrate.

Classification of common adhesion promoters
1. Organic polymer adhesion promoters. Such adhesion promoters usually contain substrate anchoring groups such as hydroxyl, carboxyl, phosphate, or long-chain polymer structures, which improve the flexibility of the paint film and enhance the adhesion of the paint film to the substrate.
2. Silane coupling agent adhesion promoters. After the coating with a small amount of silane coupling agent is applied, the silane migrates to the interface between the coating and the substrate. At this time, when it encounters moisture on the surface of the substrate, it can be hydrolyzed to form silanol groups, and then form hydrogen bonds with the hydroxyl groups on the surface of the substrate or condense into Si-O-M (M represents the substrate surface) covalent bonds; at the same time, the silanol groups between the silane molecules condense with each other to form a network structure covering film.

Factors to consider when selecting adhesion promoters
System compatibility;
Storage stability;
Influence on the basic physical and chemical properties of coatings;
Surface treatment of substrates;
Combining with other raw materials to optimize coating formulations.