Decorative or protective paint coatings need to have good adhesion to be effective. No single theory can describe adhesion, but several basic mechanisms are known to define it. When it comes to coating systems, adhesion is provided primarily by three mechanisms: adsorption, chemical and mechanical interlocking.
This article will review the rationale behind these mechanisms and reveal the properties of coating systems to achieve good adhesion. When considering these properties, the paint system is viewed as a simple three-part system consisting of the paint film, the interface between the film and substrate, and the substrate itself.
Adhesion
The importance of paint The durability and performance of a coating depend on two fundamental properties: cohesion and adhesion. Cohesion is the internal strength of a material, which is determined by the strength of the molecular forces in the bulk. It is usually measured by conventional tensile and elongation tests (ASTM D638).
Adhesion is the strength of the bond formed between one material and another. An illustration of some common types of bond and adhesive coating failures is given in Figure 1. Binder failure is usually in the paint film itself (abrasion, cracking due to aging, dissolution in solvents, etc.), although it can also be within the substrate (e.g. fiber failure in wood). Adhesive failure can be blister formation at the interface, lift of the paint film, or any other condition resulting from low adhesion at the interface. Adhesion testing methods depend on the degree of information required (research, field inspection, quality control), the type of coating system and the type of substrate (rigid, flexible, weak, strong).
Long-lasting protective coatings require cohesion and adhesion. Adhesion-related failures will determine the life of a coating system. Good adhesion occurs when:
Molecules in the paint film wet or flow freely on the substrate and come into intimate contact with the substrate, forming an interfacial bond (a process called adsorption)
Chemical bonds formed at the interface between coating and substrate
The paint film penetrates the asperities on the substrate surface, creating a mechanical interlock once the paint dries
All three of these mechanisms need not be present to form a good adhesion. Depending on the specific paint system, substrate and application method, different mechanisms may occur. However, good wetting or adsorption is generally required.
adsorption
Adsorption theory states that adhesion is caused by molecular contact between two materials and the resulting surface forces. Adsorption of the binder from the coating molecules on the substrate and the resulting attractive forces arise, often referred to as quadratic or van der Waals forces. In order for these forces to develop, the individual surfaces must not be more than 5 Angstroms apart. Therefore, the paint film needs to form an intimate molecular contact with the substrate surface.
The process of establishing continuous contact between the liquid paint film and the substrate surface is called "wetting". Figure 2 shows good and poor wetting properties of a paint film spread over a surface. Wetting is good when the adhesive flows into the depressions and crevices of the substrate surface; poor wetting is when the adhesive spans surface irregularities. Obtaining intimate contact of the adhesive with the surface essentially ensures that interfacial defects are minimized or eliminated. At a minimum, poor wetting results in: (1) a reduction in the actual contact area between the coating and the adherend; (2) a rise in stress at the blister or air pocket along the interface.
For good wetting to occur, the substrate needs to have a higher surface energy than the liquid paint film. Metal, glass, and some polymers have higher surface energies than most paint binders, so wetting is not an issue. However, adequate wetting will be prevented if the substrate is contaminated with lower surface energy materials such as shop oil. Also, if the substrate is contaminated with loose particulate matter, the contamination becomes a weak boundary layer that is prone to cohesive failure, severely weakening the entire coating system. Certain plastic substrates, such as polypropylene, fluoroplastics, and silicone rubber, have very low surface energies, and these substrates require some sort of surface preparation process to increase the surface energy. Common surface treatment processes are chemical,
chemical bonding
Certain paint systems are formulated using binders with functional groups that can chemically bond with compatible substrates. In these applications, the formation of covalent chemical bonds occurs at the interface. These strong and long-lasting bonds are often the result of intimate contact or adsorption of the adhesive on the surface followed by a chemical reaction.
Coating systems containing reactive functional groups such as hydroxyl or carbonyl groups tend to adhere more strongly to substrates containing similar groups. Hydroxyl bonding is one of the reasons epoxy and polyurethane base polymers are often used in structural coating formulations.
Perhaps an example of a chemical bond widely used in the coatings industry is an adhesion promoter or coupling agent. These multifunctional chemicals provide a "molecular bridge" between the molecules in the substrate and the paint film, as shown in Figure 3. One end of the adhesion promoter molecule has the functionality to react with the paint and the other end will react with the substrate. Forms a strong and long-lasting bond when the adhesive cures.
Organosilanes are examples of widely used adhesion promoters. They are used as additives in paint formulations and primers on glass and metal substrates to promote adhesion, improve moisture resistance, and reduce the potential for interfacial corrosion.
mechanical interlock
At one point, bonding was thought to occur simply by the paint film flowing and filling the pores, holes, cracks and micropores in the substrate. The paint film is held mechanically as it hardens. This theory of bonding still holds sway, especially on surfaces like wood, concrete and even metal and plastic. It explains why one of the common surface treatments is abrasion or mechanical roughening.
The surface of a solid material is never truly smooth; instead, it consists of a maze of peaks and valleys. According to the mechanistic theory of adhesion, in order to work properly, the paint film needs to penetrate irregularities on the surface, move entrapped air at the interface, and lock mechanically to the substrate.
One way surface roughness aids adhesion is by increasing the total contact area between the coating and the adherend. If interfacial or intermolecular attractive forces are the basis for adhesion, increasing the actual contact area will increase the total energy of the surface interaction proportionally. Thus, mechanical interface theory generally teaches that roughening of a surface is beneficial because it: (1) gives the substrate "teeth" and (2) increases the total effective area over which adhesion can develop.
Another benefit of mechanical interlocking is that the rough surface will provide a crack propagation barrier. Note that in Fig. 4, when the wedge is driven to the edge of the smooth interface, little energy dissipation is required to separate the adherends and a clean separation can be achieved. In this case, the substrate can simply be "unpacked". However, if surface roughness is present, tortuous interfaces between adherent materials will act as "path breaks" between separated adherends. These offsets dissipate energy and increase the ultimate strength of the joint.
Thus, in many cases, the physicochemical and mechanical interlocking forces of adhesion work together in the same joint. In these cases, the actual work of the adhesion is equivalent to the action produced by the adhesion mechanism (van der Waals forces) in addition to the action produced by the mechanical mechanism (elastic deformation).
