Principles of Paint Strong Adhesion on Substrate Surface

When the coating is applied to the substrate, adhesion is generated during the drying and curing process. It is generally believed that these forces exist between the coating and the substrate: covalent bonding force, hydrogen bonding force, dispersion force, dipolar force, induction force, etc. between molecules. The magnitude of these forces depends on the surface and the properties of the binder (resin, polymer, base). In a broad sense, these forces can be divided into two categories: principal valence forces and secondary valence forces. The covalent bond force is the principal valence force, which has a much higher adhesion than the secondary valence force. The subvalent force is based on a much weaker physical force represented by hydrogen bonding

These forces are more common on substrates with polar groups such as carboxyl groups and less frequently on non-polar surfaces such as polyethylene substrates. When two dissimilar materials reach “close” contact, the two free surfaces in the air disappear, forming a new interface. The nature of the interface interaction determines the strength of the bond between the coating and the substrate, and the degree of this interaction is basically determined by the wettability of one phase by the other phase.

Several influencing factors for the adhesion between paint and substrate

1. Mechanical cohesion
The surface of the substrate often has microscopic roughness or porosity that the paint can penetrate into these recessed areas before curing. After curing, the “little hook” or “wedge” structure formed by the paint physically connects the paint film to the substrate, enhancing adhesion. This coating mechanism is suitable for coating penetration when applied to a substrate containing holes, holes, cracks, or holes. When the substrate is grooved and filled with cured paint, it is more difficult to remove the coating due to mechanical action, similar to putting two tenoned pieces of wood together. For example, sandblasted or sandblasted metal or wood surfaces can be firmly adhered to in this way.

2. Chemical bond binding
Polymer molecules in paint (such as groups containing amino, hydroxyl and carboxyl groups) chemically react with atoms on the surface of the substrate (such as oxygen atoms, hydroxide groups) to form covalent bonds or hydrogen bonds. Covalent bonds can occur at the interface between the coating and the substrate, which is more likely to occur in thermoset coatings, which have the strongest bonds and are most durable, but require reactive chemical groups to bond firmly to the substrate and coating. Because the interface layer is thin, the chemical bonds on the interface are difficult to detect.

However, as discussed below, interfacial bonding does occur, which greatly improves bond strength. Some surfaces, such as painted surfaces, wood, composites, and some plastics, have a wide variety of chemical functional groups that, under the right conditions, can form chemical bonds with the coated decoated material. For example, epoxy resins can chemically interact with aluminum surfaces, and phenolic resins can bond to metals at high temperatures, and this chemical bonding is an important source of adhesion.

Van der Waals force adsorption

When the paint is extremely close to the surface of the substrate (about 0.3-0.5nm), the instantaneous dipole moment between the molecules creates an attraction, known as the van der Waals force. Although a single van der Waals force is weak, the interaction of a large number of molecules can form significant adhesion.

However, the attachment of van der Waals forces alone is easily affected by environmental factors. The degree to which a coating wettes a solid surface is determined by the angle of contact to induce an inter-dipole attraction, known as the London force or dispersion force, which is a type of van der Waals force and also contributes to adhesion, and for some substrate/coating systems, these forces provide most of the attraction between the paint and the substrate.

Diffusion

If the substrate and paint are both polymer materials and the solubility parameters are similar, the paint molecules will diffuse with the substrate molecules, causing the interface to gradually disappear and form a uniform mixed layer, thereby improving adhesion. For example, oil-based coatings may adhere well to plastic substrates through diffusion.