Thermosetting powder coating manufacturing and film forming process

Powder coating

Powder coating

After melting, the active chemical bond of the powder coating undergoes cross-linking reaction, the molecular weight increases sharply, the viscosity increases continuously, and finally solidifies into a hard coating film.
This process is a solidification process.

4.
Brief description of surface tension

The force that acts on the surface of the liquid to reduce the surface area of ​​the liquid is called the surface tension of the liquid.
The reason for it is that there is a thin layer on the surface where the liquid contacts the gas, called the surface layer.
The molecules in the surface layer are sparser than the inside of the liquid, and the distance between the molecules is larger than the inside of the liquid.
The interaction between the molecules is manifested by gravitational force.
Usually powder coatings The surface energy of the particles when they are melted is similar to that of ordinary liquids, so the film forming process of powder coatings can be analyzed.

5.
Several related surface tension issues of powder coatings

5.
1 Orange peel

Orange peel is a local vortex effect of liquid flow in the film forming process of powder coatings.
This is due to the change in surface tension caused by the viscosity change of the powder coating when the powder coating is melted.
This forms: the coating liquid with high viscosity and low surface tension sinks to The middle of the vortex (that is, the later recessed part); the low viscosity and high surface tension coating liquid rises to the periphery of the vortex (that is, the later convex part).
In order to reduce the impact of orange peel, the following measures can be taken:

5.
1.
1 Regular spraying and baking

If the first coating is too thick or the baking temperature is too fast, the orange peel will be obvious.
The first coating should be 60-80um.
In addition, the melting and leveling time should be extended appropriately, which is beneficial to the leveling of the powder coating.

5.
1.
2 Increase the viscosity of powder coatings

From the above reasons for the formation of orange peel, it can be seen that increasing the viscosity of the powder coating during melting can increase the flow resistance during melting and reduce the orange peel.

5.
1.
3 Appropriate selection of leveling agent

Generally, the leveling agent selected for powder coatings should have two effects, namely wetting effect and leveling effect.
The wetting effect is increased to the system when the powder is at about 100℃; when the leveling is combined, the surface tension should be as small as possible at this time , That is, the leveling effect above 150℃ is the main one.
The surface tension of the powder should be larger to increase the leveling.
Therefore, the leveling agent generally contains more than two materials.

5.
2 Shrinkage

Shrinkage refers to the special defects caused by points of low surface tension on the surface when the powder coating is formed into a film.
If observed with an electron microscope, most of the shrinkage cavities are vortexes formed by a small part of the particles that are not fully wetted and the surrounding incompatible resin, that is, there is a protruding small vortex in a larger sinking vortex.
The spot coating film flows from a low surface tension point to a high surface tension point to form shrinkage holes.
Generally, the low surface tension point can be dust, oil droplets, etc.
during processing, or it may be a powder that is not sufficiently wetted.
In order to reduce the impact of shrinkage, the following measures can be taken:

5.
2.
1 Clean the entire processing environment

In order to avoid the impact of shrinkage due to external environmental reasons, special attention should be paid to the cleanliness of the entire working environment.

5.
2.
2 Appropriate selection of some wetting agents

Some wetting agents can effectively wet and disperse the particles that cause shrinkage.

5.
2.
3 Increase the viscosity of powder coatings

5.
3 Pinhole

Pinhole refers to the process of powder coating from melting to solidification, the gas in the powder coating passes through the high-viscosity, nearly closed elastic resin layer from the bottom layer to the coating surface without escaping the defects formed by general powder coatings The gas is caused by low-molecular substances in the raw materials, volatiles adsorbed on the surface of the coated workpiece, caused by the powder spraying process, or defects generated during the curing of certain types of powder coatings.
These bubbles are melting In the state of the coating film, a certain number of air bubbles are formed.

In order to reduce the impact of pinholes, the following measures can be taken:

Strictly control the surface treatment quality and spraying process of the wheel hub.

The surface-treated wheel hub must not contain stains, spots and other small molecules attached to the surface of the wheel hub.
If it is a large workpiece such as cast iron, it is recommended to preheat it first.
In addition, the air compressor should be frequently drained.
When electrostatic spraying, a certain film thickness should be controlled, and it is recommended not to exceed 100um.

6 Conclusion

We are convinced that the development of thermosetting powder coatings must be the general trend.
With the continuous innovation of products (such as low-temperature curing, super weather resistance, etc.
), the application of powder coatings has expanded a broader field and continuously promoted the development of the entire powder coating industry.
(Source: Global Coatings Network) (For more information, please log in: Global Coatings Network http:// )