Leveling agents are commonly used coating additives that promote the formation of a smooth, even, and uniform coating film during the drying process. They effectively reduce the surface tension of the coating liquid, improving its leveling and uniformity. They improve the permeability of the coating liquid, reduce the likelihood of spots and blemishes during brushing, increase coverage, and result in a uniform and natural film. They are mainly surfactants and organic solvents. There are many types of leveling agents, and the types used vary depending on the coating. In solvent-based coatings, high-boiling-point solvents or butyl cellulose are used. In water-based coatings, surfactants or polyacrylic acid, carboxymethyl cellulose, etc., are used.
The main functions of coatings are decoration and protection. Defects in flow and leveling not only affect the appearance but also impair the protective function. For example, pinholes can cause insufficient film thickness, and craters can lead to discontinuities in the film, both of which reduce the protective properties of the coating. During application and film formation, coatings undergo physical and chemical changes. These changes, along with the inherent properties of the coating itself, significantly affect its flow and leveling.
After application, new interfaces emerge, typically the liquid/solid interface between the coating and the substrate, and the liquid/gas interface between the coating and air. If the interfacial tension at the liquid/solid interface between the coating and the substrate exceeds the substrate’s critical surface tension, the coating cannot spread properly, resulting in leveling defects such as fisheyes and pinholes.
During the drying process, solvent evaporation creates temperature, density, and surface tension differences between the film’s surface and interior. These differences lead to turbulent flow within the film, forming Benard vortices. Benard vortices can cause orange peel texture; in systems containing multiple pigments, if the pigment particles exhibit varying degrees of mobility, Benard vortices can also cause floating color and mottling, and vertical surface application can result in streaking.
During the drying process of paint films, insoluble colloidal particles are sometimes generated. The formation of these particles leads to surface tension gradients, often resulting in pinholes within the paint film. For example, in cross-linked curing systems containing more than one resin, the less soluble resin may form insoluble colloidal particles as the solvent evaporates during drying. Additionally, in formulations containing surfactants, if the surfactant is incompatible with the system, or if its concentration changes during drying due to solvent evaporation, leading to changes in solubility and the formation of incompatible droplets, surface tension differences can also contribute to pinhole formation.
External contaminants present during application and film formation can also cause leveling defects such as pinholes and fisheyes. These contaminants typically originate from air, oil, dust, paint mist, and moisture from application tools and the substrate.
The properties of the paint itself, such as application viscosity and drying time, also significantly affect the final leveling of the paint film. Excessively high application viscosity and excessively short drying time usually result in poorly leveled surfaces.
Therefore, it is necessary to add leveling agents to adjust the properties of the coating during application and film formation, thereby helping the coating achieve good leveling.