Leveling agents can be roughly divided into two categories. One type works by adjusting the viscosity of the paint film and leveling time. These leveling agents are mostly high boiling organic solvents or their mixtures, such as isophorone, diacetone alcohol, etc; Another type of leveling agent works by adjusting the surface properties of the paint film, and what people generally refer to as leveling agents mostly refer to this type of leveling agent. This type of leveling agent migrates to the surface of the paint film through limited compatibility, affecting surface properties such as interfacial tension of the paint film, resulting in good leveling of the paint film. According to their different chemical structures, there are three main types of leveling agents: acrylic, organosilicon, and fluorocarbon.
effect:
The main function of coatings is decoration and protection. If there are flow and leveling defects, it not only affects the appearance but also damages the protective function. If shrinkage occurs, it will cause insufficient thickness of the paint film, and the formation of pinholes will lead to discontinuity of the paint film, which will reduce the protective effect of the paint film. During the construction and film-forming process of coatings, some physical and chemical changes may occur, which, along with the properties of the coating itself, will significantly affect the flow and leveling of the coating.
After coating application, new interfaces will appear, usually the liquid/solid interface between the coating and the substrate, and the liquid/gas interface between the coating and air. If the interfacial tension between the liquid/solid interface of the coating and the substrate is higher than the critical surface tension of the substrate, the coating cannot spread on the substrate, naturally resulting in leveling defects such as fish eyes and shrinkage cavities.
The evaporation of solvents during the drying process of paint film can cause temperature, density, and surface tension differences between the surface and interior of the paint film, which in turn lead to turbulent motion inside the paint film, forming the so-called Benard vortex. Benard vortex can cause the production of orange peel; In systems containing more than one pigment, if there is a certain difference in the mobility of pigment particles, Benard vortices are also likely to cause floating and blooming, and vertical surface construction can lead to silk lines.
During the drying process of the paint film, insoluble particles may sometimes be generated, which can lead to the formation of surface tension gradients and often result in the formation of shrinkage cavities in the paint film. For example, in a cross-linked curing system, the formula contains more than one resin. During the drying process of the paint film, as the solvent evaporates, poorly soluble resins may form insoluble gel particles. In addition, in formulations containing surfactants, if the surfactant is incompatible with the system, or if its concentration changes during the drying process due to solvent evaporation, resulting in a change in solubility and the formation of incompatible droplets, a surface tension difference will also be formed. All of these may lead to the formation of shrinkage cavities.
During the construction and film formation process of coatings, the presence of external pollutants may also cause leveling defects such as shrinkage cavities and fish eyes. These pollutants are usually oil stains, dust, paint mist, water vapor, etc. from the air, construction tools, and substrates.
The properties of the coating itself, such as construction viscosity, drying time, etc., can also have a significant impact on the final leveling of the paint film. Excessive construction viscosity and short drying time often result in poorly leveled surfaces.
Therefore, it is necessary to add leveling agents and adjust the properties of the coating during construction and film formation to help the coating achieve good leveling.
Purpose:
For a determined formula system, the appropriate type of leveling agent should be selected based on the properties of the formula and the desired performance of the leveling agent.
1. Solvent based coating system
Acrylic leveling agents are commonly used in primer and intermediate coat formulations. If degassing and substrate wetting are required, it is advisable to choose medium or high molecular weight acrylic leveling agents. In primer, if stronger substrate wettability is required, it is recommended to use organic silicon leveling agents and modified acrylic leveling agents that can significantly reduce surface tension (such as fluorine modified acrylic leveling agents and phosphate modified acrylic leveling agents). If organic silicon leveling agents and fluorine modified acrylic leveling agents have side effects such as foam stabilization and affecting interlayer adhesion, phosphate modified acrylic leveling agents should be used.
In the formulation of topcoat and transparent paint, the requirements for the appearance of the paint film are relatively high. Generally, low molecular weight acrylic leveling agents can be used to achieve good leveling performance and avoid fogging in the paint film. In cross-linked curing systems, the use of acrylic leveling agents containing reactive functional groups often results in better leveling performance and improves the physical and chemical properties of the paint film. If better fluidity or smoothness and scratch resistance are required for the paint film, silicone leveling agent is necessary. In this case, it is best to use a combination of silicone leveling agent and acrylic leveling agent.
It should be pointed out that during vertical surface construction, the organic silicon leveling agent provides leveling performance while effectively reducing the tendency of coating sagging. In addition, silicone leveling agents should be used with caution in the formulation of metallic glitter paint, as they may cause uneven arrangement of flake aluminum pigments and result in uneven paint film color.
2. Powder coating system
The leveling process of powder coating is divided into two stages. The first stage is the melting of powder particles, and the second stage is when the powder particles melt and flow into a smooth paint film. Powder coatings do not contain solvents and do not generate surface tension gradients during film formation. Leveling is more related to substrate wetting.
Powder coatings often use acrylic leveling agents. If the leveling agent is in liquid form, it usually needs to be pre made into a masterbatch before use. There are also acrylic leveling agents made into powders, specifically used for powder coatings. These products adsorb liquid acrylic leveling agents onto silica powder, and some low-grade leveling agents are adsorbed with calcium carbonate.
If powder coatings require smoothness and anti scratch properties, organic silicon leveling agents should be used. There are already organic silicon leveling agents specifically used for powder coatings that are made into powders. Be careful to avoid the formation of shrinkage when using organic silicon leveling agents.
3. Water based coating system
The water-based coating system is divided into water-soluble system and latex system.
In water-soluble systems, it is necessary to strongly reduce the surface tension of the system. The most commonly used leveling agents are organosilicon leveling agents and fluorocarbon leveling agents, which have the same effect as their application in solvent based coating systems. Of course, if a truly smooth surface is required, acrylic leveling agents for water-based systems are necessary.
For latex systems, the film-forming mechanism is completely different, and viscosity does not change with the evaporation of solvents. The use of leveling agents in the formula may improve the substrate wettability of the coating, while acrylic leveling agents can improve the smoothness of the paint film. However, the main flowability of the coating is mainly controlled and adjusted by adding rheological control agents.
