Influence of Adhesion and Cohesion of Coating Film on Properties
The adhesion of the coating film refers to the firmness of the coating film and the surface of the coated substrate, which is produced by the molecular polar groups of the polymer in the coating and the polar groups of the polar molecules on the surface of the coated substrate. mutual attraction and physical bonding. The cohesion of the coating film is the ability to make the particles in the coating film stick together to form a continuous and complete coating film, which is produced by the mutual attraction between adjacent molecules inside the coating film. Adhesion and cohesion affect the performance of the coating film at the same time. If the adhesion is not good, the coating film will easily peel off and fail. When the cohesion is not enough, the coating film itself is easily damaged and cracks occur. In the T-bend test of the pre-coated coil coating , the sample should be bent and pulled with a certain sticky tape to observe whether it has cracks and peeling off. If cracks appear, it means that the cohesion of the coating film is destroyed; if the coating film is stuck by the tape, it means that the adhesion with the substrate is not enough.
1) The main influencing factors of adhesion Adhesion is greatly affected by the variety and surface state of the substrate. The adhesion of the coating film can be greatly improved by proper surface treatment of . At the same time, it reduces the surface tension of the coating, improves the wettability and
Increasing the polarity of the paint can also improve adhesion. It is based on this principle that the adhesion . For the composite coating film on the bottom surface, the interlayer adhesion between the primer film and the top paint film should also be considered, and the influence of the primer greater, that is, the so-called secondary crosslinking problem of the primer. The degree of crosslinking of the primer film during the first baking should not be too high , otherwise the primer film will be too dense, which will affect the interlayer adhesion with the topcoat. However, if the functional to have different reactivity, the functional groups with high activity will react first when the primer is cured, so that the primer film is partially cross-linked and cured, and then the functional groups with slightly lower activity will be released in the primer film. When the topcoat is baked and cured, the cross-linking reaction is carried out for the second time, so that the primer is completely cured to improve the adhesion between the bottom layers.
2) The main factors affecting the cohesion of the coating film The main factors affecting the cohesion of the coating film are: the pigment-base ratio of the coating, the properties , the thickness of the coating film, and curing crosslinking.
The addition of pigments will reduce the cohesion of the coating film. Take the topcoats of two colors commonly used in coil coatings: white gray and sea blue as an example. In the same base material system, the T-bend of white gray is often not as good as that of sea blue, generally 1~2T worse. This is precisely because of the white gray The ratio of pigment to base is much higher than that of sea blue, and its cohesion is reduced, which is manifested as a decrease . Furthermore, when the critical pigment volume concentration (CPVC) is reached or exceeded, the cohesion of the coating film will decrease sharply, because there are too many pigments in the coating film, the structure is loose, and there is not enough binder resin to bond them together, resulting in a decrease in cohesion. Low. In addition, when the pigment is dispersed, adding a suitable dispersant improves the wettability and dispersibility, which helps to improve the cohesion of the coating film and increases the molecular weight of the base resin, which can improve the cohesion of the coating film. Increasing the resin functionality to increase the crosslink density can also improve the cohesion of the coating film. The glass transition temperature (Tg) of the base resin affects the cohesion of the coating film. When the external temperature is higher than the Tg of the coating film, the free volume in the coating film increases and the coating film becomes softer. The deformation of the coating film after being stressed at a temperature lower than its Tg is brittle cracking. If the T-bend performance is observed, the coating film will produce cracks . The coating film thickness increases and the cohesion increases. Macroscopically, it is shown that as the film thickness increases, the mechanical properties of the coating film, such as T-bend and wipe resistance, increase.
When the coating is cured, due to the evaporation of the solvent and the occurrence of the crosslinking reaction, the volume shrinkage often occurs, the internal stress , and the cohesion of the coating film is maintained. The epoxy resin system commonly used in coil coating primers has good adhesion to the substrate due to the presence , but its cohesion is low and the volume shrinkage is small, and the stress can only be released through cracking. It is manifested in T- bend performance, that is, epoxy primers tend to be more prone to cracks. If the coating film has good adhesion and high cohesion, the coating film will not easily produce cracks. If it is subjected to external stress, the general coating film will lose its adhesion, which is reflected in the T-bend performance. The coating film is more likely to stick off and not easy to crack crack.
The degree of cross-linking of the primer should not be too high. If the cross-linking of the primer is too good, the paint film is too hard, and the pigment-base ratio of the primer is often high. When the cohesive strength is low, it cannot resist the volume shrinkage of the topcoat film. The released stress can easily cause cracking of the primer film
Effect of Crosslink Density of Topcoat on Properties
The crosslinking density of the topcoat in the precoated coil coating has a great influence on some important properties of the coating film, such as MEK wipe resistance, stain resistance hardness, T-bending and scratch resistance. Generally speaking, the higher the cross-linking density, the higher the compactness of the coating film, so that the MEK wipe resistance, stain resistance, hardness and scratch resistance of the coating film will be improved, but the T-bend performance may often decrease .
The factors affecting the cross-linking density of the topcoat film mainly include the functionality of the base resin, the degree of branching, the amount of curing conditions. The functionality of the base resin increases, and if there is enough cross-linking resin to react with it, the coating film will be denser . The higher the chaining degree of the resin, the easier it is to form a network structure, the denser the coating film, and the higher the crosslinking density. The type of crosslinking resin also has a certain influence on the crosslinking density of the obtained coating film. The commonly used cross-linking agents in coil coatings are amino resins (especially methylated melamine resins) and blocked polyisocyanate compounds. For the same base resin, the cross-linking density of the coating film cured with amino resins is generally higher than that of blocked polyisocyanate compounds. Type isocyanate resin cured coating film is high.
