In new equipment manufacturing (OEM) coating, the appearance of the coating becomes more and more important. Therefore, one of the main goals of the coatings industry is to optimize the performance of coatings according to the final requirements of users, which also includes satisfactory surface appearance. The surface condition affects the visual effect through factors such as color, gloss, haze and surface structure. Gloss and image clarity are often used to control the appearance of the coating. However, even if the coating film with high gloss is used, the fluctuation of its surface also affects the appearance of the entire coating film. At the same time, it is believed that the gloss measurement can not control the visual effect of fluctuation. This effect is also called "orange peel".
Orange peel or micro wave is a corrugated structure with a size between 0.1mm and 10mm. On the high-gloss coating surface, people can see wavy, light and dark areas. Two different levels of fluctuations can be distinguished: long fluctuations, also called orange peels, which are fluctuations that can be observed at an interval of 2 to 3 distances; the other is called short fluctuations or micro fluctuations, which are observed at a distance of about 50 cm. To the volatility.
It should be pointed out that sometimes in order to cover the surface defects of the substrate or obtain a special coating surface appearance, a certain degree of fluctuation or ripple structure is purposely designed. Therefore, "orange peel" can be defined as "the wavy structure of a high-gloss surface", which makes the surface of the paint layer produce a streak, unleveled visual appearance. The control of the visual appearance of the powder coating film (gloss, haze, leveling orange peel) is very important, especially in the assembly of parts sprayed in different fields.
This article briefly introduces the influence of rheological force on film flow and surface appearance during the film forming process of powder coatings. Discussed how to avoid the formation of orange peel, and how to judge and compare the phenomenon of orange peel and other aspects.

1) Factors affecting the flow and appearance of the coating film in powder coatings

In industrial coatings, the phase change of powder coatings during preparation and film formation is unique. Due to the lack of solvents to wet and improve the fluidity of the coating film, powder coatings are more difficult to remove surface defects than liquid coatings. Although the main components of the two are similar, they are based on very different mechanisms compared to liquid coatings and thermosetting powder coatings.
Powder coating is a solvent-free uniform system. During the preparation process, the pigment and other components are dispersed and partially encapsulated in the low-molecular solid resin through melt mixing. The use of powder coating is to transfer the powder to the substrate through the air (the powder is suspended in the air), and then attach it to the substrate through the charge. Heating at a predetermined temperature, the powder particles are melted, gathered together (coalesced), flowed (filmed), and then leveled. During this period, a viscous liquid stage wets the surface), and finally chemical crosslinks form high The molecular weight coating film, this is the film forming process of powder coatings.

2) Factors affecting the flow and appearance of the coating film

The film forming process can be divided into three stages: melting and coalescence, forming a coating film, and leveling.
At a given temperature, the most important factors to control the speed of melting and coalescence are the melting point of the resin, the viscosity of the powder particles in the molten state, and the size of the powder particles. In order to achieve the best flow effect, the melt coalescence should be completed as quickly as possible to allow a longer time to complete the leveling phase. The use of curing agent shortens the time required for flow and leveling, so the coating film formed by those extremely reactive powders often exhibits orange peel.
The key factors affecting the flow and leveling of the coating film are the melt viscosity of the resin, the surface tension of the system and the film thickness. In turn, the melt viscosity depends especially on the solidification temperature, solidification speed, and heating rate.
The various factors mentioned above, together with the particle size distribution and film thickness, are usually determined by the required coating film performance, coated objects and powder application conditions.
The power of flow and leveling in powder spraying comes from the surface tension of the system, which has also been mentioned earlier. This force is the opposite of the intermolecular gravitational force applied to the coating film. As a result, the higher the melt viscosity, the greater the resistance to flow and leveling. Therefore, the difference between the surface tension and the intermolecular gravitational force determines the level of the coating film.
For coatings with good fluidity, obviously, the surface tension of the system should be as high as possible, and the melt viscosity should be as low as possible. These can be achieved by adding additives that can increase the surface tension of the system and using low-molecular-weight and low-melting resins.

The coating prepared under the above conditions can have excellent fluidity, but due to its high surface tension, it will cause shrinkage, and at the same time, it will sag due to the lower melt viscosity, and the corner coating is poor. In actual work, the surface tension and melt viscosity of the system are controlled within a specific range, so that a qualified surface appearance of the coating film can be obtained.
The effect of surface tension and melt viscosity on the flow of the coating film is shown in Figure 2. As can be seen in the figure, too low surface tension or too high melt viscosity will prevent the flow of the coating film, resulting in poor fluidity of the coating film, and when the surface tension is too high, shrinkage cavities will appear during the film formation process. Too low melt viscosity will deteriorate the physical storage stability of the powder, poor corner coating during construction, and sag during construction on the facade.

In summary, it is obvious that the final surface conditions, defects and deficiencies (such as orange peel, poor fluidity, shrinkage, pinholes, etc.) of the obtained powder coating film are closely related to each other, and are also in the film formation process. Controlled by the rheological force involved in the phase change.
The powder particle size distribution also affects the surface appearance of the coating film. The smaller the particles, due to the lower heat capacity of larger particles, so their melting time is shorter than that of larger particles, coalescence is also faster, and the surface appearance of the coating film is better. The melting time of large powder particles is longer than that of small particles, and the coating film formed may have an orange peel effect. Powder electrostatic application method (corona discharge or friction discharge) is also a factor that causes orange peel.

3) How to reduce or avoid the orange peel effect

Promote flow and leveling to reduce or avoid orange peel. The system uses lower melt viscosity, prolonged leveling time during solidification and higher surface tension can improve flow and leveling. Controlling the surface tension gradient is an important parameter to reduce orange peel. At the same time, it is necessary to control the uniform surface tension of the coating film surface to obtain the smallest surface area.
In actual work, flow promoters or leveling agents are often used to improve the appearance of the coating film to eliminate surface defects such as orange peel, shrinkage holes, and pinholes. Flow promoters with good performance can reduce melt viscosity, thereby helping to melt mixing and pigment dispersion, improve the wettability of the substrate, flow and level the coating, help eliminate surface defects and facilitate the release of air. The relationship between the amount of flow modifier and the effect should be investigated. Insufficient dosage will cause shrinkage and orange peel, while excessive dosage will cause loss of gloss, haze, and adhesion problems to the upper layer. Usually, the flow modifier is added during premixing. They are either made into resin masterbatch (the ratio of resin to the additive is 9/1 to 8/2), or adsorbed on an inorganic carrier in powder form. The amount of this additive in powder coatings is 0.5 to 1.5% (in the effective polymer calculated based on the base material), but the effect may be good when the concentration is low.
Among the flow modifiers, polyacrylate resins are the most widely used, such as polybutyl acrylate ("Acronal 4F"), ethyl acrylate-ethylhexyl acrylate copolymer and butyl acrylate-hexyl acrylate copolymer. They can be used in a wide range of concentrations. Generally, polyacrylates have little effect on surface tension, and they can help the coating to form a relatively constant uniform surface. Compared with those additives that reduce surface tension (such as siloxane, etc.), they do not reduce surface tension, so they can be used to accelerate leveling. Additives that reduce surface tension include surfactants, fluorinated alkyl esters, and siloxanes. They are very sensitive to the amount added. Benzoin is a degassing agent and also has the effect of reducing surface tension. It is widely used to improve the surface appearance of powder coatings.