Coating, coating, spraying, painting pretreatment

Coatings play an extremely important role in daily life and industrial production. Coatings and paints mainly play the role of decoration, protection and marking on the surface of substrates.

Coating adhesion is one of the important indicators of coating performance. Poor coating adhesion will cause the coating to fall off, crack, and become unusable. Therefore, ensuring good adhesion between the coating and the substrate is an important part of the coating process. However, the prerequisite for the coating and the coating to fully play the above role is that there is good interface bonding between the coating and the substrate to which it is attached. The size of the adhesion is generally used to measure the quality of this bonding.

The adhesion of a coating refers to the ability of the coating to bond to the substrate through physical, chemical, mechanical action, etc. Intuitively speaking, it is the force required to remove the coating from the substrate. The harder it is to remove the coating from the substrate, the better the adhesion of the coating on the substrate, the stronger the practicality and durability of the coating, and the better the corresponding function can be played. If the coating falls off from the substrate easily, it will not play a role, but will accelerate the aging, corrosion and wear of the material.

Factors affecting coating adhesion

Substrate surface roughness

The surface roughness of the substrate is an important factor affecting the adhesion between the substrate and the coating. Generally speaking, the greater the surface roughness, the stronger the adhesion between the coating and the substrate. However, the roughness should not be too large, because defects are easily formed at the interface between the coating and the substrate, which will lead to internal stress concentration and failure.

Appropriately increasing the surface roughness of the substrate can increase the contact area between the coating and the substrate. On the one hand, it increases the mechanical attachment points between the coating and the substrate, and to a certain extent, it also increases the physical adsorption (van der Waals force) and chemical adsorption between the coating and the substrate.

Plasma treatment is an important method to change the roughness of the microstructure of the solid surface. The etching effect of plasma will form a large number of micron-level grooves on the surface of the material, increasing the surface roughness. The rough surface profile can significantly increase the contact area between the coating and the substrate, which is conducive to the formation of more mechanical interlocking between the coating and the substrate interface. The change in surface roughness before and after plasma treatment is shown in Figure 1 below:

Figure 1: Before plasma treatment, the surface is relatively smooth. After plasma treatment, more grooves appear on the surface, and the roughness increases.

Substrate surface wetting

The necessary condition for the coating to adhere to the substrate is its wetting and spreading on the solid surface. The lower the surface tension of the coating, the higher the surface tension of the substrate, the better the coating's wettability to the substrate, and the surface tension of the coating must be as lower as possible than the surface tension of the substrate.

In order for the coating to wet the substrate, the smaller the contact angle, the better. Therefore, the surface tension of the coating should be much lower than the surface tension of the substrate. Especially after the organic matter is adsorbed on the surface, the surface tension of the material is greatly reduced, making it difficult to wet. Therefore, surface treatment must be performed before coating to increase the surface tension of the substrate, which is conducive to the wetting of the coating on the substrate.

When plasma is used to treat certain polymer materials (such as polyethylene and polypropylene), the high-energy plasma will cause the polymer molecules to break, oxidize or cross-link, thereby improving the wettability of the material surface. The change in surface contact angle before and after plasma treatment is shown in Figure 2 below:

Figure 2: Water naturally beads up on the untreated surface, but spreads out (lower contact angle) on the plasma treated and activated surface on the right.

Substrate surface activation state

For polymer materials such as plastics, most substrates have low surface energy, low surface tension, and few active groups on the surface that can participate in the reaction. It is particularly important to improve the adhesion of such materials by activating the surface.

Plasma treatment can introduce hydrophilic groups such as carboxyl-COOH, hydroxyl-OH and other polar groups to the substrate surface, increasing the activity of the substrate surface. The generated polar functional groups are chemically bonded with the active groups of the substrate to form covalent bonds, so that the coating and the substrate form good adhesion. The changes in surface polar groups before and after plasma treatment are shown in Figure 3 below:

Figure 3: After plasma treatment, the C-C group is significantly reduced, while the C-O group and C=O group are increased, indicating that after the active particles in the plasma interact with the material matrix, more oxygen-containing groups are introduced on the material surface, forming a chemical bond between the material surface and the coating.