Study on the relationship between powder and surface tension of wheel hub

introduction

Powder coating is a solid coating in powder form. This is a type of paint that is usually dispersed with air as a carrier and applied to the surface of the wheel hub. Electrostatic spraying is usually adopted, which has attracted great attention from the entire coating industry with its high production efficiency, excellent coating performance and appearance effect, good ecological environmental protection and outstanding economy in the world. With an annual growth rate of more than 12%, it is one of the most promising paint varieties. In the film-forming process of powder coating, many phenomena are related to surface tension. For example: pinholes on the surface, orange peel, peeling of coatings, mismatch between paints and coatings, etc. To this end, the following focuses on analyzing and solving the problem from the surface tension of the powder.

1. The manufacturing process of powder coatings

The determination of powder coating formula is generally determined according to the technical requirements of the customer's product and its own production environment, and the formula design should fully consider the customer's requirements and the rationality of its own production.

2. Coating of powder coating

The coating production line is completed according to the technical requirements of the coated workpiece and the generation batch.

3. Powder coating film-forming process

After the powder coating is applied to the surface of the wheel, there are generally four processes in the film formation process:

3.1 Softening process

After the powder absorbs a certain amount of heat, the resin in the powder will become sticky and soften due to heat absorption.

3.2 Melting process

When the resin in the powder absorbs a certain amount of heat, the active chemical bonds in the resin open. The powder will then exist on the surface of the product in a molten state. This phase is extremely short.

3.3 Leveling process

When the powder exists on the surface of the product in a molten state, it flows from a continuous uneven surface to a relatively smooth and flat surface, which is the leveling process.

3.4 Curing process

After melting, the active chemical bonds of the powder coating increase sharply in molecular weight and viscosity through cross-linking reaction, and finally solidify into a hard coating film, which is the curing process.

4. Brief description of surface tension

Any force acting on the surface of a liquid, reducing the surface area of the liquid. This is called liquid surface tension. It occurs due to the presence of a thin layer on the surface where the liquid comes into contact with the gas, called the surface layer. The molecules in the surface layer are sparser than those inside the liquid, and the distance between the molecules is larger than that inside the liquid, and the interaction between the molecules is manifested as gravitational force, and the surface energy of the powder coating particles when they are melted is similar to that of the general liquid. Therefore, the powder coating film formation process can be analyzed.

5. Several surface tension problems related to powder coating

5.1 Orange peel

Orange peel is a local vortex effect of liquid flow during the film-forming process of powder coatings. This is due to the change in surface tension due to the change in viscosity during melting of the powder coating. In this way, a high viscosity and low surface tension coating sinks to the middle of the vortex (i.e., the later depression), and a low viscosity and high surface tension coating rises to the periphery of the vortex (i.e., the later convex part), and the following measures can be taken to reduce the effect of orange peel:

5.1.1 Regular spraying and baking

A coating that is too thick at one time or bakes that heats up too quickly can cause the orange peel to be noticeable. The primary coating should be 60-80um, and the melting leveling time should be appropriately extended, which is conducive to the leveling of 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 powder coating during melting can increase the flow resistance during melting and reduce orange peel.

5.1.3 Appropriate selection of leveling agent

Generally, the leveling agent used for powder coating should have two effects, namely wetting effect and leveling effect. The wetting effect is to increase the leveling of the system when the powder is around 1O0°C. At this time, the surface tension should be as small as possible, i.e. the leveling effect greater than 150°C is the main one. The surface tension of the powder should be a little larger to increase the leveling properties, so usually the leveling agent generally contains more than two materials.

5.2 Shrinkage porosity

Shrinkage pore refers to a special defect on the surface caused by a point of low surface tension during the formation of a powder coating. If observed with an electron microscope. Shrinkage porosity is mostly formed by a small number of insufficiently wetted particles and the surrounding incompatible resin, that is, in a large sag vortex, there is a protruding small dot coating film flowing from the low surface tension point to the high surface tension point to form the shrinkage porosity. In general, the low surface tension point can be dust, oil droplets, etc. during processing, or it can be powder that has not been sufficiently wetted. In order to reduce the impact of cratering, the following measures can be taken:

5.2.1 Cleanliness of the entire processing environment

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

5.2.2 Properly select some wetting agents

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

5.3 Pinholes

Pinhole refers to the process of powder coating from melting to curing, and the gas in the powder coating passes through the high-viscosity, nearly closed elastic resin layer from the bottom layer to the coating surface. The defects formed by the non-escape are generally caused by the low molecular substances brought in the raw materials, the volatiles adsorbed on the surface of the coated workpiece, the defects caused by the powder coating process, or the defects generated during the curing process of some types of powder coatings. These bubbles form a certain number of bubble groups in the molten state of the coating film. In order to reduce the effect of pinholes. The following measures can be taken: strictly control the quality of the surface treatment of the wheel hub and the spraying process. The surface treatment of the wheel hub is required to contain stains, spots and other small molecule substances attached to the surface of the hub. If it is a large workpiece such as cast iron, it is recommended to preheat it first, and the air compressor should release water frequently. When electrostatic spraying, it is necessary to control a certain thickness of the coating film, and it is recommended not to exceed 100um.

6. Conclusion

We firmly believe that the development of thermosetting powder coating is bound to be the trend of the times, with the continuous innovation of products (such as low-temperature curing, super weather resistance, etc.), for the application of powder coating to expand a broader field, and constantly promote the development of the entire powder coating industry.