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The bonding method of the coating and the influencing factors of the coating adhesion

1. Formation of adhesion of coatings

The adhesion between the paint film and the substrate can be determined by mechanical bonding, physical adsorption, hydrogen bonding, chemical bonding, and mutual diffusion.

2. The bonding method of the coating

2.1. Mechanical bonding force

Firstly, the surface of any substrate cannot be smooth. Good roughness not only makes it easier to wet, but also forms physical anchors. Some substrates, such as graphite and refractory materials, may have certain depressions or gaps, and thinning coatings can penetrate to form physical anchors to ensure adhesion.

2.2. Van der Waals force adsorption

According to calculations, when the distance between two ideal planes is 10A, the attractive force between them can reach 103-104N/cm2 under the action of van der Waals force, and 104-105N/cm2 under the distance of 3-4A. This value far exceeds the strength that the best structural adhesive can achieve at present, but this is only an ideal situation. Even after precision polishing, the contact between the two planes is less than one percent of the total area. Of course, liquid coatings will be better, provided that the coating is completely wetted before curing, but it will also be much lower than the theoretical value, mainly because various defects will appear in the coating during the curing process. Adhesion is not the sum of various forces, but depends on the force in the weakest part of the local area.

If the coating is only bonded by van der Waals forces, it is only a physical adsorption effect, which is easily replaced by water vapor in the air. Therefore, if the coating is only bound by physical adsorption, its strength is not enough.

2.3. Chemical and hydrogen bonding

When the polymer contains amino, hydroxyl, and carboxyl groups, these groups are prone to form hydrogen bonds with the oxygen atoms or hydroxyl groups on the surface of the substrate, resulting in stronger adhesion.

The active groups on the resin can also react with metals to form chemical bonds. For example, phenolic resin can react with aluminum, stainless steel, etc. at higher temperatures, and epoxy resin can also react with aluminum surfaces to a certain extent. The combination of chemical bonds is of great significance for adhesion. The application of coupling agents (surface modifiers) can illustrate this point. Adding coupling agents to coatings requires one end group to react with the coating and the other end group to react with the substrate. For example, the most commonly used silane coupling agent, X3Si (CH2) nY, where X is a functional group that can hydrolyze into hydroxyl groups and react chemically with inorganic surfaces, and Y is a functional group that can react chemically with coatings.

2.4. Diffusion effect

For example, when oil-based polymer coatings are applied to a substrate that is also a polymer, the molecules in the coating will diffuse and dissolve with the molecules on the substrate, and the interface will eventually disappear. Polymer miscibility requires certain conditions, such as similar solubility, a certain free volume, and the possibility of kinetics. Raising the temperature can also promote diffusion.

2.5. Electrostatic effect

When metal comes into contact with organic coatings, the metal has low electron affinity and is prone to losing electrons, while organic paint films have high electron affinity and are easy to obtain electrons. Therefore, electrons can move from the metal to the paint film, causing a contact potential at the interface and forming a double layer to generate electrostatic attraction.

3. Factors affecting coating adhesion

3.1. The influence of coating viscosity

When the viscosity of the coating is low, the surface tension will be small, making it easy to spread and flow into the grooves and pores of the substrate to form mechanical anchors.

Generally, dried paint has better adhesion than air dried paint, one of the reasons being that the viscosity of the coating is very low at high temperatures.

3.2. Wetting condition of substrate surface

Usually, pure metal surfaces have high surface tension, while coatings generally have low surface tension. However, in reality, metal surfaces may form oxides and adsorb low surface tension objects such as organic matter. It will affect the wetting situation, so we will emphasize the removal of oil, rust, and scale in the substrate treatment.

3.3. Surface roughness

Improving surface roughness can increase mechanical bonding and also facilitate surface wetting.

3.4. Influence of internal stress

The internal stress, adhesion, and strength of the paint film are in opposition to each other. If the internal stress is too high, the paint film may be damaged or detached from the substrate.

There are two sources of internal stress in coatings:

3.4.(1) The stress generated by volume shrinkage during the curing process of the coating;

3.4.(2) The thermal expansion coefficients of coatings and substrates are different, resulting in thermal stress during temperature changes.

Solvent evaporation and chemical reactions can cause volume shrinkage. Among them, due to the overflow of small molecules during the reaction, the condensation reaction shrinks the most severely. In the addition reaction, the two double bonds change from van der Waals forces to covalent bonds, greatly reducing the atomic distance and resulting in a significant volume shrinkage rate of up to 10%. The small shrinkage rate during the curing process of epoxy resin is an important reason for the good adhesion of epoxy coatings.

When the thermal expansion coefficients of the paint film and substrate are different, the stress generated during temperature change is proportional to the temperature change. Therefore, if the thermal stress is severe, the curing temperature should not be too high.