Leveling agent is a commonly used coating additive that can promote the formation of a flat, smooth, and uniform coating film during the drying and film-forming process of coatings. It is a common additive in paint construction. What is the function of leveling agent?
The function of leveling agent:
There are many types of leveling agents, and the types of leveling agents used in different coatings are also different. The most commonly used leveling agent in oil-based coatings is acrylic polymer, which is usually used in solvent based coatings and powder coatings, especially as a necessary additive in the production and application process of powder coatings. The most commonly used leveling agent in water-based coatings is polyurethane, especially widely used in mid to high end latex paints. Other leveling agents include organosilicon and associative alkali swelling leveling thickeners.
After coating application, there is a process of flow and drying to form a film, gradually forming a flat, smooth, and uniform coating. Whether the coating can achieve a smooth and even characteristic is called leveling. Shrinkage refers to the irregular, bowl shaped small depressions formed on the coating, which cause the coating to lose its flatness. It is often centered around a drop or small impurity, forming a circular edge around it.
From the perspective of leveling, it is a special "point like" unevenness that occurs on the surface of the coating. Its shape can be divided into flat, volcano like, point like, exposed bottom like, bubble like, etc.
The synthesis of acrylic leveling agents generally involves the polymerization of butyl acrylate with amines and alkyl esters, with a polymerization temperature controlled at 80-90 ℃ and solvents such as toluene, xylene, cyclohexane, etc. The relative molecular weight of the polymer is controlled at 4000-10000. The narrower the molecular weight distribution, the better the leveling effect.
Acrylic leveling agents are mainly used in the production and construction process of powder coatings. Common leveling agents are used to improve the appearance of coatings, eliminate surface defects such as orange peel, shrinkage, pinholes, and shrinkage edges. Leveling agents eliminate surface defects in coatings by reducing or altering surface tension and interfacial tension, as well as promoting uniformity of surface tension during curing. A high-quality leveling agent can reduce the melt viscosity of the system, thereby facilitating melt mixing and pigment dispersion, improving the wetting of the substrate, improving the flow and leveling of the coating, helping to remove surface defects, and facilitating air release.
The most commonly used leveling agents in powder coatings are acrylic homopolymers and copolymers, as well as modified polydimethylsiloxane. However, polyacrylic acid ester leveling agents are less sensitive to excessive use and contamination compared to polydimethylsiloxane. Therefore, the most popular system used in the coating industry is still acrylic polymer.
Organic silicone leveling agents can be used for oil-based coatings, and some varieties can also be used for water-based coatings. Due to poor compatibility with paint and its low surface tension, this type of leveling agent is prone to quality defects if used improperly or mixed improperly. In recent years, some additive manufacturers have made various modifications to organic silicon leveling agents, increasing their compatibility and application scope year by year. Also, due to its low dosage and significant effect. So, in terms of consumption and new product development, there may be significant growth in the use of organosilicon leveling agents in the future.
Polyurethane leveling agent is generally used for waterborne coatings, especially in systems containing lotion, which has both leveling and thickening properties. It is prepared by reacting polyepoxyethane with isocyanate and then fat modifying. At present, this leveling agent has been produced by many domestic auxiliary plants, some of which are equivalent to the foreign level. With the improvement of domestic auxiliary production level and variety, people's understanding of leveling agent is gradually improved, and the consumption will increase year by year.
It is also worth noting that many varieties of thickeners have a certain leveling property through fat modification or other means. For a given formulation system, suitable leveling agent varieties should be selected based on the properties of the formulation and the desired performance of the leveling agent, such as alkali swelling and binding thickeners, small molecular weight fat modified hydroxyethyl cellulose, etc.
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, organic silicon leveling agent is necessary. In this case, organic silicon leveling agent and acrylic leveling agent are used in combination.
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. One 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.
