In the context of increasing global environmental awareness and increasingly stringent regulations on volatile organic compound (VOC) emissions by various countries, powder coatings, with their core advantage of being completely solvent-free, exhibit extremely low VOC emissions during the curing process. This effectively avoids the air pollution and health hazards caused by solvent evaporation in traditional solvent-based coatings, making powder coatings one of the most representative environmentally friendly coating systems in the coating industry, widely used in automotive, home appliance, building materials, hardware, and other fields.
Leveling properties of coatings are a key indicator for measuring the appearance quality of the coating film, directly affecting the visual effect and market competitiveness of the product. Leveling properties specifically refer to the smoothness and evenness of the coating surface after application (such as electrostatic spraying, fluidized bed dip coating, etc.) and the baking and curing process. A coating with excellent leveling properties should completely eliminate or significantly reduce irregularities such as orange peel (a textured surface resembling orange peel), brush marks (marks left by manual brushing; although powder coatings are mostly sprayed, improper operation can still result in similar textures), ripples (wave-like undulations on the surface), and pinholes (small depressions formed due to uneven surface tension). It should achieve a uniform, smooth, and flawless visual effect. Leveling properties are particularly crucial for products with extremely high aesthetic requirements, such as automotive bodies and high-end appliance casings, where they are a core factor determining product quality.
In actual production and testing, the methods for evaluating coating leveling properties are mainly divided into two categories: subjective observation and semi-quantitative measurement. Direct visual observation is the most basic and convenient method. During operation, the test sample and a standard sample (a reference sample with a determined leveling grade) should be compared under the same lighting conditions (such as a standard light source box). The leveling properties of the test sample are judged by observing the smoothness, number, and type of defects on both surfaces. However, this method has significant limitations: due to differences in individual visual sensitivity and evaluation standards, the same sample may lead to different conclusions depending on the observer, resulting in strong subjectivity. It is only suitable for preliminary screening or scenarios with low precision requirements, and cannot meet the needs of industries such as the automotive industry, which have strict quantitative standards for coating quality.
To address this issue, the automotive industry widely uses wavelength scanning to characterize the surface state of coatings. This method has a semi-quantitative effect and can more objectively and accurately assess leveling properties. Its principle is to use specialized equipment to emit light of different wavelengths to scan the coating surface. By detecting the reflection and scattering of light, a numerical value reflecting the surface smoothness is calculated. Specifically, this method uses long wavelengths (wavelength range 10–0.6 mm) and short wavelengths (wavelength range 0.6–0.1 mm) for scanning: long wavelengths mainly reflect larger-scale surface irregularities (such as ripples, obvious orange peel), while short wavelengths focus on micro-scale surface unevenness (such as fine pinholes, slight textures). The final measured values range from 0 to 100. Lower values indicate less surface undulation in both long and short wavelength ranges, better smoothness, and superior leveling properties. This semi-quantitative method effectively avoids the bias of subjective observation, providing a reliable basis for unified standards and precise control of coating quality in the automotive industry.
The factors affecting the leveling properties of powder coatings are complex and diverse. Based on industry practice and research, they can be mainly summarized into three core aspects: leveling aids, the melt viscosity of the powder coating, and the baking process. These three interact and jointly determine the final leveling effect of the coating.
First, leveling aids are key additives for optimizing the leveling properties of powder coatings. Adding appropriate types and dosages of leveling aids in the formulation design of powder coatings can significantly improve the surface condition of the coating. Commonly used leveling agents on the market include GL588 from Nanhai Company, H98 from Laisi Company, and PV88 from Worlee-Chemie Company. These agents share a common mechanism of action: when the powder coating reaches its melting temperature during baking, the leveling agent rapidly migrates to the coating surface, reducing the surface tension of the molten coating, breaking down localized stresses caused by uneven surface tension within the coating, and promoting rapid and uniform flow of the coating before curing into a film. This process effectively fills micro-depressions formed during spraying, eliminating defects such as orange peel, brush marks, and ripples, while inhibiting the formation of pinholes, ultimately resulting in a smooth and even coating film. It is important to note that the amount of leveling agent added must be strictly controlled. Excessive addition may lead to problems such as loss of gloss and decreased adhesion, while insufficient addition will not achieve the desired leveling effect. Therefore, precise adjustments are necessary based on the resin type and powder formulation.
Secondly, the melt viscosity of the powder coating has a direct impact on the leveling properties and must be considered in conjunction with the curing reaction characteristics. For the most widely used thermosetting powder coatings, the melting and flow process is not simply a change in physical state, but is accompanied by a cross-linking and curing reaction between the resin and the curing agent. Increased temperature not only melts the powder into a fluid, but also accelerates the cross-linking reaction, leading to a gradual increase in the viscosity of the coating system. Specifically, the higher the temperature, the faster the curing reaction and the faster the viscosity of the system increases. This directly shortens the effective flow time of the coating. If the flow time is too short, the coating loses its fluidity due to increased viscosity before it has fully leveled, significantly limiting the leveling properties of the final film. Therefore, the initial melt viscosity of the powder coating cannot be used to judge the final leveling properties of the film; parameters such as the heating rate and cross-linking curing temperature in the baking process must also be considered. For example, if the curing temperature is set too high, although it can increase production efficiency, it may lead to insufficient leveling due to a rapid increase in viscosity. If the curing temperature is too low, it may prolong the curing time, affecting the production rhythm, and may also cause sagging of the coating due to excessive flow time (although the risk of sagging in powder coatings is lower than that in liquid coatings, it still needs to be controlled).
Third, the baking process is a crucial step in controlling the leveling properties of powder coatings, with the heating rate being particularly critical. During baking and curing, powder coatings inevitably undergo a temperature rise from room temperature to the curing temperature. The rate of this rise alters the dynamic viscosity of the coating, thus affecting the leveling effect. Experimental data shows that regardless of the heating rate, the overall trend of the dynamic viscosity of powder coatings with temperature changes is generally consistent: in the initial stage of heating, the increasing temperature causes the powder to gradually melt, and the system viscosity continuously decreases with increasing temperature, reaching a minimum at a certain temperature point; subsequently, as the temperature continues to rise, the cross-linking curing reaction accelerates, the molecular chains gradually form a network structure, and the system viscosity rapidly increases with increasing temperature. The difference in heating rate mainly manifests in two aspects: First, a faster heating rate results in a lower dynamic viscosity at its minimum value—lower viscosity means better fluidity of the coating, allowing it to more fully fill surface defects. Second, a faster heating rate corresponds to a higher temperature at which the viscosity reaches its minimum value—higher temperatures provide more energy for coating flow and are also more conducive to the subsequent curing reaction, avoiding insufficient leveling due to asynchronous flow and curing. Therefore, in actual production, optimizing the heating rate (such as using stepped heating or adjusting oven power) can maximize the leveling properties of the coating film while ensuring curing quality.
Powder coatings, with their environmental advantages of zero organic solvents and low VOC emissions, have become an important direction for the green development of the coating industry. Leveling property, as a core indicator for measuring the appearance quality of the coating film, is directly related to the application value of the product. The evaluation of leveling property requires a combination of subjective observation and semi-quantitative wavelength scanning methods to achieve accurate assessment. The influencing factors include leveling agents, melt viscosity, and baking process: leveling agents promote flow by reducing surface tension; melt viscosity needs to be analyzed in conjunction with the dynamics of the curing reaction; and the baking process (especially the heating rate) optimizes the leveling effect by controlling the viscosity change trend. In practical applications, synergistic optimization of the formulation (selection of additives and resins) and the process (baking parameters) is necessary to fully leverage the advantages of powder coatings, obtain high-quality coatings that combine environmental performance and appearance quality, and meet the high-quality coating needs of different industries.
