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High-gloss powder coatings, with their high gloss, excellent decorative and protective properties, are widely used in high-end coating fields such as furniture, home appliances, automotive parts, and architectural decoration. However, in actual production and application, hazing is a common quality problem in high-gloss powder coatings, seriously affecting the appearance and market competitiveness of products. Hazing is essentially an optical effect, specifically manifested as a hazy, foggy visual effect on the surface of the high-gloss coating, losing its proper specular gloss. The core reason for its formation is the presence of numerous pinholes or fine textures on the coating surface that are difficult to detect with the naked eye. These tiny defects strongly diffuse incident light, preventing the light from forming regular specular reflections, thus presenting a hazy visual appearance. From a technical perspective, the key to solving the hazing problem lies in eliminating pinholes and fine textures on the coating surface, restoring the surface smoothness of the coating, and ensuring that light can achieve uniform specular reflection. In industrial production, haze not only reduces the appearance grade of products but may also lead to customer doubts about product quality. This is especially true in high-end home appliances and automotive interiors, where appearance requirements are stringent; even slight haze can result in product scrapping, causing direct economic losses to companies. Therefore, in-depth research into the causes of haze and the development of targeted solutions are of significant practical importance for the high-quality development of the high-gloss powder coating industry.
Pinholes and fine textures on the surface of high-gloss powder coatings are the direct cause of haze. Regarding their causes, various viewpoints have existed in the industry, with "excessive volatile matter in powder coatings" being a widely circulated explanation. Undeniably, when powder coatings contain excessive volatile matter (such as residual solvents, moisture, low molecular weight additives, etc.), during the baking and curing process, the volatile matter will vaporize upon heating and attempt to escape from the molten coating. If the escape channels fail to close in time, pinholes will form on the coating surface. To verify the accuracy of this viewpoint, industry researchers conducted a specific experiment: selecting all high-gloss powder coating products produced during a certain production stage, covering the two main types—hybrid (epoxy-polyester) and pure polyester—and measuring the volatile matter content according to standard testing methods at 200 degrees Celsius for 4 hours. The experimental results showed that the volatile matter content of all tested samples remained relatively stable at around 0.4%, with the highest value being only 0.47%. Furthermore, this value was unrelated to the presence of pinholes or fine textures in the samples—even high-gloss powder coatings with excellent surface smoothness and no haze exhibited volatile matter content within the same range. This experimental result indicates that pinholes caused by excessive volatile matter are extremely rare in high-gloss powder coatings and can be almost ignored. The traditional viewpoint that attributes haze problems to volatile matter clearly fails to grasp the core issue and cannot provide effective guidance for solving problems in actual production.
Through long-term production practice observation, extensive data statistics, and in-depth technical analysis, industry researchers have found that pinholes and fine textures on the surface of high-gloss powder coatings are mainly caused by the following three core factors, rather than excessive volatile matter:
This is the primary cause of pinholes and fine textures. During the production of high-gloss powder coatings, when different batches and types of polyester resins are intermingled after extrusion, differences in the chemical structure and reactivity of various polyesters lead to mutual interference during film formation. This interference is particularly pronounced in the small-scale sampling stage due to the small feed rate and difficulty in ensuring uniform material mixing; the greater the difference in gelation time, the more severe the mutual interference.
It is worth noting that if this mixed powder with intermingling interference is re-extruded, the mixing uniformity of the material will be significantly improved, the interfacial differences between different polyesters will be dispersed and reduced, or even completely eliminated, and the resulting pinhole and fine texture problems will be significantly improved. This phenomenon further confirms that material interpenetration interference is one of the core causes.
In production practice, it has been found that high-gloss powder coatings with higher pigment and filler volume concentrations (PVC) are more prone to pinholes and fine textures. The essence of this phenomenon is closely related to the kneading and mixing effect of the extruder: when the pigment and filler volume concentration is high, if the extruder's mixing capacity is insufficient, it will lead to uneven dispersion of pigments and fillers in the resin matrix—some powder particles have low pigment and filler content, while others have excessively high content. During baking and curing, powder particles with lower pigment and filler content melt more easily to form turbulence, while particles with excessively high pigment and filler content are difficult to form effective turbulence due to the barrier effect of the pigments and fillers, and may even fail to melt completely. This difference in melting state forms a significant interface within the coating film, resulting in an uneven surface or wrinkles after curing, which appear as pinholes or fine textures to the naked eye.
As the core film-forming substance in powder coatings, the quality stability of resin directly affects the final coating effect. In actual production, if the content of reactive groups (such as epoxy value and carboxyl value) varies significantly between different batches of purchased resin, it will lead to a mismatch between the formulation design and the actual raw material performance. This results in some unreacted thermoplastic resin remaining in the coating after curing. Furthermore, if the production equipment (such as extruders and grinding mills) is not thoroughly cleaned during the sample production process, residual cleaning resin will mix into the new formulation system, leading to an excess of certain resin components. These unreacted or excess thermoplastic resins, during the coating cooling process, will generate a shrinkage stress difference with the surrounding thermosetting resin due to their much higher shrinkage rate than fully cross-linked thermosetting resins. This will form unevenness or wrinkles on the coating surface, ultimately manifesting as pinholes or fine textures. This also explains why some high-gloss panels have a smooth surface when first taken out of the oven, but gradually show a hazy effect as the panel cools. It is worth mentioning that even when using epoxy resin from the same manufacturer and the same type of polyester, the presence and quantity of pinholes and fine textures may vary during mass production of the same powder coating. This phenomenon is the result of multiple factors, including slight batch-to-batch differences in resin, fluctuations in equipment mixing effects, and changes in the temperature and humidity of the production environment, further highlighting the meticulous requirements of the high-gloss powder coating production process.
To deeply understand the formation logic of the haze phenomenon, it is necessary to analyze it in conjunction with the film-forming process of thermosetting powder coatings. The film formation of thermosetting powder coatings is a complex physicochemical process, mainly divided into the following stages:
After the powder coating is heated to its melting point, it begins to transform from a solid state to a molten state, forming a uniform molten coating.
As the temperature continues to rise, the molten coating generates countless tiny turbulent flows (also known as Bénard's pockets) under the influence of heat exchange and surface tension differences. This stage is crucial for the smoothing of the coating surface.
As the temperature rises or the holding time increases, the molten resin begins to undergo a cross-linking reaction. The system viscosity gradually increases, and the turbulent flow rate gradually slows down until it stops, and the coating enters a gelation state.
When the temperature reaches the complete curing temperature, the cross-linking reaction rate of the resin tends to peak, forming a three-dimensional network structure, and finally curing into a film.
After the workpiece is removed from the furnace, the coating gradually cools to room temperature, completing the entire film formation process. From the perspective of film formation mechanism, the formation of haze is directly related to the stability of the turbulence and cooling stages: When powders with different curing rates are mixed, some powders first enter the gelation and curing state, while others remain in the turbulence stage, forming a distinct interface. After curing, this interface is fixed, resulting in unevenness or wrinkles. If the interface size is extremely small, it is difficult to detect with the naked eye; if the interface size increases, it appears as pinholes; if the interface further expands, it manifests as fine textures. Secondary extrusion can neutralize the differences in curing rates between different powders, reducing the dispersion of the interface and thus improving the haze problem. During the cooling stage, the difference in shrinkage rates between unreacted thermoplastic and thermosetting resins can disrupt the smoothness of the coating surface, forming micro-defects and subsequently causing haze.
Based on the core causes of hazing, and drawing on practical production experience, targeted solutions can be developed from four dimensions: equipment cleaning, raw material selection, formula optimization, and process control.
Equipment cleaning is crucial to avoid material cross-contamination, especially when changing product types (particularly major product categories). A strict cleaning process must be implemented. The focus of cleaning should be on all parts that come into contact with materials after the pressing rollers, including core components such as the extruder barrel, screw, grinding chamber, and classifier rollers. A triple cleaning method of "physical cleaning + solvent cleaning + blank material rinsing" is recommended: first, use tools to remove residual material clumps from the equipment; then, clean the inner walls of the equipment with a special solvent; finally, use blank resin (pure resin without pigments or fillers) for extrusion rinsing to ensure no residual material remains in the equipment, thus preventing cross-contamination between different batches and types of materials from the source.
The quality and stability of raw materials directly determine the consistency of powder coating performance. When selecting raw materials, the following principles should be followed: Resin suppliers should be reputable manufacturers with stable production capabilities and strict quality control. A quality agreement should be signed, requiring suppliers to provide test reports for each batch of products, with particular attention paid to the fluctuation range of key indicators such as reactive group content and gelation time. Pigments and fillers should be selected based on their excellent temperature resistance, good dispersibility, and high batch stability to avoid surface defects caused by uneven dispersion or insufficient thermal stability. A raw material inspection system should be established, with sampling and testing of each batch of incoming raw materials to ensure that their performance indicators meet the formulation design requirements. Unqualified raw materials are strictly prohibited from being used in the warehouse.
Formulation design must match the actual capacity of the production equipment:
Based on the kneading and mixing effect of the extruder, reasonably determine the volume concentration of pigments and fillers to avoid dispersion difficulties due to excessively high PVC; for equipment with weak mixing capacity, the PVC value can be appropriately reduced, or dispersants and other additives can be added to improve the dispersibility of pigments and fillers;
Optimize the curing system, select resin combinations with high gelation time matching to avoid excessive differences in curing rates between different resins; when mixing different batches of powder, conduct compatibility tests in advance to ensure consistent curing characteristics;
Adjust baking process parameters, reasonably set the baking temperature and holding time according to the curing curve of the powder coating to ensure sufficient cross-linking reaction of the resin and reduce the residue of unreacted thermoplastic resin; at the same time, control the cooling rate of the workpiece after exiting the oven to avoid excessive shrinkage stress differences due to excessively rapid cooling.
Accurately determining the cause of haze is a prerequisite for solving the problem. Therefore, a comprehensive sample analysis system integrating materials, processes, formulations, and environment needs to be established: When observing samples, not only should the surface haze phenomenon be considered, but also information such as the sample's baking parameters, raw material batches, and production equipment status should be recorded; Microscopic analysis (such as microscopic observation) should be performed on samples with haze to determine whether the defect is pinholes or fine textures, and then trace its formation stage; Sample analysis data should be summarized regularly to establish the correspondence between problems and causes, accumulate production experience, and improve the ability to quickly troubleshoot problems.
The haze phenomenon in high-gloss powder coatings is an optical effect caused by pinholes or fine textures on the coating surface. Its core cause is not, as traditionally believed, excessive volatile matter, but rather the instability of the film-forming process caused by factors such as the interference of different materials, uneven dispersion of pigments and fillers, and insufficient batch stability of resin. These factors affect the smoothness of the coating surface by influencing key stages of thermosetting powder coatings, such as melt turbulence, gelation and curing, and cooling shrinkage, thus producing haze. Solving the haze problem requires a closed-loop approach involving source control, process management, and end-of-pipe analysis: rigorous equipment cleaning to avoid material interference, selection of high-quality raw materials to ensure performance stability, formulation and process optimization to improve film quality, and scientific sample analysis to quickly pinpoint the problem. Although haze is relatively common in high-gloss powder coating production, understanding its formation mechanism and implementing targeted prevention and control measures can effectively reduce or even eliminate this quality defect. For powder coating companies, solving the haze problem not only improves product appearance quality and market competitiveness but also drives refined upgrades in raw material control, process optimization, and equipment management. As the requirements for appearance in the high-end coating field continue to increase, continuous and in-depth development of high-gloss powder coating performance optimization and quality control will become a crucial support for companies to achieve high-quality development. In the future, with continuous technological advancements, through the research and development of new raw materials, the upgrading of production equipment, and the innovation of formulation systems, the haze problem in high-gloss powder coatings will be more thoroughly solved, injecting new momentum into the industry's development.
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