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In powder coating applications, high-gloss powder coatings are highly favored for giving workpieces a bright and refined appearance. However, in actual use, they often suffer from the troublesome problem of "hazing"—the coating surface loses its proper translucency and gloss, presenting a hazy, foggy appearance. This seemingly minor flaw can seriously lower the product's grade, especially in fields with stringent appearance requirements such as home appliances, furniture, and automotive parts, potentially leading to product defects and economic losses for companies. At its root, this hazing phenomenon is not insurmountable; it is essentially an optical problem: numerous pinholes or fine textures on the coating surface scatter light, causing the coating to lose its smooth and translucent texture. Eliminating these pinholes and fine textures resolves the hazing problem.
A common explanation in the industry for pinholes and fine textures in high-gloss powder coatings is "excessive volatile substances in the powder coating." Undeniably, if the volatile content in the powder exceeds the standard, these substances will volatilize during the baking and film-forming process, potentially leaving pores and pinholes inside the coating. But is this really the case? We found the answer through a series of experiments. The experiment targeted all high-gloss powders produced in a certain stage (covering different types such as blended and pure polyester), baking them at 200℃ for 4 hours and testing their volatile content. The results showed that regardless of whether the final coating of these powder coatings had pinholes or fine textures (some powder coatings had excellent surface smoothness), their volatile content was generally stable at around 0.4%, with a maximum of only 0.47%. This data indicates that pinholes in powder coatings caused by volatile substances are extremely rare in actual production and can almost be ignored. So, what is the real culprit behind pinholes and fine textures? After long-term observation, statistics, and analysis, we found that the mutual penetration and interference that occurs between different polyester materials after extrusion is the root cause of the vast majority of problems. Moreover, this problem is more likely to occur in small-batch prototyping—the smaller the batch size, the more significant the impact of differences in gel time between powders on the coating, and the more severe the interference. However, a practical solution is to re-extrude the mixed powder, which can significantly reduce this penetration interference, and sometimes even eliminate it completely.
In actual production, there are two perplexing phenomena regarding pinholes and fine textures in high-gloss powder coatings. We have found answers to these questions by combining film-forming principles.
Sometimes, even when using epoxy resin from the same manufacturer and the same type of polyester, pinholes and fine textures on the coating surface appear intermittently during large-scale production of the same powder coating, with unstable frequency and quantity. Further observation revealed that this intermittent problem is more likely to occur when the volume concentration of pigments and fillers in the powder is high. This phenomenon is actually closely related to the extrusion mixing effect and material properties.
After Cooling Another strange phenomenon occurred during the sample coating process: some high-gloss panels appeared smooth and flat with perfect gloss when first removed from the oven, but as the temperature gradually decreased, pinholes or fine lines slowly appeared on some panels. This problem of "appearing only after cooling" puzzled many technicians until we delved into the film-forming process of thermosetting powders and found the key reason. Unveiling the Film-Forming Process To understand the causes of pinholes and fine lines, we must first understand how thermosetting powders form coatings. The entire film-forming process can be divided into several key stages: When the powder is heated to a specific temperature, it begins to melt. At this time, due to uneven heat transfer within the powder and differences in surface tension in different areas, the melting powder generates countless tiny turbulences (also known as "Bénal cells"). As the temperature continues to rise, the powder melts further, intensifies the turbulence, and gradually flows outwards and flattens. When the temperature reaches the cross-linking reaction threshold of the resin, the resin begins to undergo chemical cross-linking, and the viscosity of the coating gradually increases. As the temperature continues to rise or the holding time is extended, the coating viscosity increases, and the intensity of turbulence and the rate of surface tension equilibrium gradually decrease until they stop completely—at this point, the powder enters a "gel" state, and the coating is essentially set. Finally, if the temperature continues to rise or is maintained at a high temperature for a period of time, the cross-linking reaction of the resin reaches its limit, and the coating is completely cured. When the workpiece is removed from the oven, the coating cools down as the ambient temperature decreases, and the entire film-forming process ends. It is this complex film-forming process that provides a "breeding ground" for pinholes and fine lines. Based on the film-forming mechanism, we can clearly analyze the causes of three key problems.
When different types of powders are mixed, problems arise if the curing rates of the components differ significantly: during baking, the component that cures faster enters the gel and solidification state first, while the component that cures slower remains in a turbulent flow state. A distinct "interface" forms between them. Once the coating is fully cured, this interface is permanently fixed, leading to unevenness or wrinkles on the coating surface. Under light, these uneven areas create shadows: if the interface size is small, the bumps or wrinkles are not obvious and are difficult to detect with the naked eye; if the interface size is slightly larger, pinholes will form; if the interface size is large enough, fine lines will appear. Re-extruding the mixed powder allows for more uniform dispersion of different components, reducing or even eliminating these interfaces, thus reducing the formation of pinholes and fine lines.
The kneading and mixing effect of the extruder directly affects the uniformity of the powder. Especially when the volume concentration of pigments and fillers in the powder is high, if the extruder mixing is insufficient, the pigments and fillers cannot be uniformly dispersed in the powder. After powdering, some particles have low pigment and filler content, while others have high content. During baking, particles with low pigment and filler content are more likely to generate turbulence, flowing smoothly and evenly; while particles with high content are difficult to generate turbulence, or even unable to flow. The interface formed between these two will cause bumps or wrinkles on the coating surface, ultimately forming pinholes or fine lines.
The stability of raw materials is also a crucial factor that cannot be ignored. We found that the content of reactive groups in different batches of purchased resin varied significantly, which could easily lead to discrepancies between the actual effect of the formulation and the original design. For example, after the coating has cured, varying degrees of unreacted thermoplastic resin may remain; another example is that if residual resin is not thoroughly cleaned after cleaning the extruder and grinding equipment during sample production, it can be mixed into the new powder during grinding, resulting in excessive levels of certain types of resin in the powder. Furthermore, thermoplastic resins and thermosetting resins have different shrinkage rates—thermoplastic resins shrink much more than thermosetting resins when the temperature changes. When the coating cools from a high temperature, this difference in shrinkage can cause unevenness or wrinkles on the coating surface, leading to pinholes or fine lines. This is the core reason why "the panel is smooth when it comes out of the oven, but problems appear after cooling."
Pinholes and fine lines in high-gloss powder coatings are very common in the industry, but they can be effectively reduced or even eliminated by finding the right methods. Based on production practice, we have summarized the following practical countermeasures:
Especially when changing product types, the equipment must be thoroughly cleaned, with a focus on cleaning the area behind the pressure rollers, especially the grinding chamber. Residual resin and pigments, if mixed into the new batch of powder, can easily cause formula imbalance, leading to pinholes or fine lines.
In selecting pigments and fillers, prioritize products from reputable manufacturers with stable quality. These materials are not only heat-resistant but also have good dispersibility, reducing mixing problems caused by uneven material distribution. When purchasing resin, pay attention to batch-to-batch stability, and choose products with minimal fluctuations in reactive group content.
The amount of pigments and fillers added should not be blindly pursued for optimal effect; it needs to be determined in conjunction with the mixing capacity of the extruder. If the extruder's kneading effect is mediocre, the volume concentration of pigments and fillers should be appropriately reduced to avoid problems caused by uneven dispersion. At the same time, the reaction characteristics of the resin should be fully considered during formula design to reduce the residue of unreacted resin after curing.
When dealing with coating problems, one cannot rely solely on surface phenomena. A comprehensive analysis is needed, considering factors such as materials (e.g., resin batch, pigment type), processes (e.g., extrusion count, baking temperature), and environment (e.g., workshop humidity, dust). Accumulating experience through repeated practice is crucial for quickly and accurately pinpointing the root cause of the problem and adjusting solutions promptly. In short, the haze problem in high-gloss powder coatings, while seemingly complex, is actually traceable. By understanding the causes and addressing them accordingly, the coating can maintain a smooth, glossy, high-quality appearance, safeguarding product competitiveness.
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