Powder coatings, with their air-dispersion coating characteristics, possess multiple advantages such as high production efficiency, excellent film performance, good environmental protection, and outstanding economic efficiency. They maintain an annual growth rate of over 10% in the global coatings market, becoming one of the most promising coating categories. Throughout the entire powder coating baking and film formation process, many common problems such as pinholes, orange peel, and delamination are inextricably linked to surface tension. This article, based on surface tension theory and combined with practical film formation phenomena, discusses these issues, hoping to provide some reference for colleagues engaged in powder coating technology research.

1. The Three Stages of Powder Coating Film Formation

Powder coatings exist in solid powder form under normal conditions and cannot directly adhere to form a film. They must undergo a complete process of "melting-leveling-curing" to form a strong coating film. This process can be clearly divided into three core stages:

The first stage is the aggregation process: initially dispersed individual powder particles gradually aggregate and fuse under the influence of heat, forming a continuous but uneven initial film layer. The second stage is the leveling process: this uneven initial film layer slowly flows under continuous heating, eventually forming a smooth and flat liquid film surface. The third stage is the curing process: the molten liquid coating undergoes a cross-linking reaction, its viscosity continuously increases, and it eventually solidifies into a hard and dense coating film, completing the entire film formation process. Changes in surface tension continuously affect these three stages, directly determining the final quality of the coating film.

2. Various Coating Defects Caused by Surface Tension and Countermeasures

Surface tension is the intermolecular attraction of a liquid surface, which causes the liquid surface area to shrink. During the melting and film formation of powder coatings, even slight changes in surface tension can cause defects. Below, we analyze the causes of the four most common surface tension-related defects and share practical countermeasures.

2.1 Orange Peel Defect

The essence of orange peel defect is the localized eddy effect generated by the flow of liquid coating during the powder coating film formation process, also known as Bénard vortices. Its core cause is the change in viscosity during powder melting, which in turn alters the surface tension: high-viscosity, low-surface-tension coatings sink to the center of the vortex, ultimately forming depressions in the coating film; while low-viscosity, high-surface-tension coatings rise to the periphery of the vortex, forming protrusions, ultimately exhibiting an orange peel-like texture.

To mitigate the orange peel effect, three approaches can be taken:

First, standardize the spraying and baking processes, ideally controlling the thickness of each coating layer to 50-70 μm. Excessively thick coatings or excessively rapid baking temperatures will exacerbate the orange peel phenomenon. Simultaneously, appropriately extend the melt-leveling time to allow sufficient space for the coating to flow and level.

Second, adjust the powder melt viscosity. Optimizing the formulation to increase the viscosity of the powder coating during melting can increase the flow resistance of the coating and reduce eddy generation.

Thirdly, precise selection of leveling agents is crucial. High-quality leveling agents must possess both wetting and leveling effects: at around 100℃, wetting is dominant, requiring low surface tension to promote fusion; when the temperature exceeds 150℃, leveling is dominant, requiring appropriately increased surface tension to improve smoothness. Therefore, leveling agents are usually formulated from two or more materials.

2.2 Pinholes

Pinholes are a typical defect caused by surface tension differences, specifically manifested as circular depressions on the coating surface. Electron microscopy reveals that most pinholes are vortices formed by insufficiently wetted small particles and surrounding incompatible resin, with a prominent small dot at the center of larger depressions. The formation logic is that the coating spontaneously flows from low surface tension points to high surface tension points. These low surface tension points may be dust or oil droplets introduced during processing, or insufficiently wetted powder particles in the coating.

The key to preventing pinholes lies in "controlling contamination and strengthening wetting": First, strictly maintain the cleanliness of the entire processing environment to avoid introducing low surface tension points from impurities, which is the foundation for preventing pinholes. Second, select a suitable wetting agent to enhance the coating's ability to wet and disperse impurity particles, preventing particles from becoming the core of pinholes. Finally, appropriately increase the viscosity of the powder coating to suppress abnormal flow of the coating liquid and reduce the diffusion and deepening of pinholes.

2.3 Pinholes

Pinholes refer to penetrating pores formed when gases inside the coating fail to escape successfully after passing through the nearly sealed high-viscosity elastic resin layer during the melting and curing process of powder coating. These gases mainly come from three sources: first, low-molecular-weight substances contained in the coating raw materials; second, volatiles adsorbed on the surface of the coated workpiece; and third, reaction products generated during the curing reaction of the powder coating.

From the perspective of surface tension, bubble formation requires overcoming surface tension to generate new surface area. When the surface tension of the coating film is low, less energy is required to form stable bubbles, making bubble clusters easier to form. Under natural conditions, the internal pressure of small bubbles is higher than that of large bubbles. This pressure diffuses through adjacent interfacial membranes to the larger bubbles, causing the small bubbles to gradually shrink and the large bubbles to continuously grow, eventually thinning and rupturing the bubble membrane. If the coating has already entered the curing stage and lost its fluidity before the bubble ruptures, pinholes will form.

Reducing pinholes can be achieved through two core measures: First, strictly control the surface treatment quality and spraying process. The workpiece surface must be thoroughly cleaned to ensure no small molecules such as stains or spots adhere. For large workpieces such as cast iron, preheating is recommended to remove adsorbed volatiles. Air compressors should be drained regularly to remove moisture and prevent water from mixing into the coating and causing bubbles. During electrostatic spraying, the coating thickness should not exceed 100μm to allow for gas escape. Second, appropriately select defoamers. Benzoin (also known as benzoin) is commonly used as a defoamer in powder coatings, and there are also some special-purpose permeable agents. The defoaming mechanism can be simply summarized as follows: the defoamer first contacts the bubble, then spreads at the bubble interface, subsequently enters the bubble or displaces the bubble membrane, ultimately causing the bubble to rupture.

2.4 Other

Besides the three common defects mentioned above, surface tension can also affect the adhesion between the coating and the substrate, and even cause spot defects. When the surface tension (or surface energy) of the powder coating and the substrate are mismatched, if the difference is small or the coating is too thick, it may lead to decreased coating adhesion; if the difference is too large, a severe mismatch will directly cause delamination. Furthermore, spot defects are prone to occur when using metal powder to formulate powder coatings, mostly because low surface tension substances are introduced during the spraying process, disrupting the surface tension balance of the coating liquid.

3. Summary

Surface tension is one of the core factors determining the film quality of powder coatings. A deep understanding of the mechanism of surface tension and precise control of surface tension in conjunction with the inherent characteristics of powder coatings are crucial for improving coating performance, reducing defects, and expanding application scenarios. With continuous innovation in coating technology, new products such as UV curing, low-temperature curing, and ultra-weather-resistant coatings have emerged in the field of thermosetting powder coatings. The successful application of these new technologies also requires precise control of surface tension.

In the future, as the industry's understanding of surface tension deepens and breakthroughs continue in technologies such as formula optimization and process upgrades, powder coatings will be applied in more high-end fields, and their environmentally friendly and efficient advantages will be more fully realized, injecting stronger momentum into the green transformation and upgrading of the coating industry.