The role of leveling agents
There are many types of leveling agents. Different types of leveling agents are used in different coatings. The most commonly used leveling agent in oily coatings is acrylate polymer, which is usually used in solvent-based coatings and powder coatings, especially in powder coatings It is necessary to use additives in the production and application process. The most commonly used leveling agent in water-based coatings is polyurethanes, which are widely used especially in high-grade latex paints. Other leveling additives include silicones and associative alkali swelling and leveling thickeners.
                                             
After the coating is applied, there is a flowing and drying film forming process, and then a flat, smooth and uniform coating film is gradually formed. Whether the coating film can achieve flatness and smoothness is called leveling. Shrinkage refers to the irregularity formed on the coating film, such as small bowl-shaped depressions, which makes the coating film lose its flatness. It is often centered around a drop or a small piece of impurities, and a ring-shaped edge is formed around it.

From the perspective of leveling, it is a special "spot-type" flow unevenness, which is generated on the surface of the coating film. Its shape can be divided into flat type, crater type, dot type, open bottom type, and bubble from the performance. Formula etc.

The synthesis of acrylic leveling agents is generally carried out by polymerizing butyl acrylate with amines and alkyl esters. The polymerization temperature is controlled at 80-90 ° C. The solvent is toluene, xylene, cyclohexane, etc. The relative molecular weight of the polymer is controlled at 4000 ～ 10,000, the narrower the molecular weight distribution, the better the leveling effect.

Acrylic leveling agents are mainly used in the production and construction of powder coatings. Leveling agents are commonly used to improve the appearance of coating films and eliminate surface defects such as orange peel, shrinkage holes, pinholes, and edge shrinkage. The leveling agent eliminates surface defects of the coating film by reducing or changing surface tension and interfacial tension, and by promoting uniformity of the surface tension during curing. A high-quality leveling agent can reduce the melt viscosity of the system, thereby helping melt mixing and pigment dispersion, improving the wettability of the substrate, improving the flow and leveling of the coating, helping to remove surface defects and being beneficial to The release of air.

In powder coatings, the most commonly used leveling agents are acrylate homopolymers and copolymers and modified polysiloxanes. Polyacrylate leveling agents are less sensitive to excess and contamination than polysiloxanes. Therefore, the most popular systems used in the coatings industry are still acrylate polymers.
                                         
Silicone leveling agents can be used in oily coatings. Some varieties can also be used in water-based coatings. Such leveling agents are prone to quality defects due to poor compatibility with the paint and its low surface tension, such as improper dosage or improper matching. In recent years, some additive manufacturers have made various modifications to the silicone-based leveling agents to increase the compatibility and increase the scope of application year by year. And because of its small amount, the effect is significant. Therefore, in terms of consumption and development of new varieties, silicone leveling agents may have relatively large development in the future.

Polyurethane leveling agent is generally used in water-based coatings, especially in emulsion-containing systems. It has both leveling and thickening properties. It is prepared by reacting polyethylene oxide with isocyanate and then fat modification. This leveling agent has been produced in many domestic auxiliaries factories, and some varieties are equivalent to foreign levels. With the increase in domestic production level and variety of auxiliaries, people ’s understanding of leveling agents has gradually increased, and the consumption will Will increase year by year.

It is also worth noting that many varieties of thickeners have certain leveling properties through fat modification or other means. Such as alkali swelling and associative thickener, small molecular weight fat-modified hydroxyethyl cellulose, etc. For a certain formula system, the appropriate leveling agent should be selected according to the properties of the formula and the desired performance of the leveling agent Variety.

1. Solvent-based coating system
In primer and middle coat formulations, acrylic leveling agents are commonly used. If degassing and substrate wetting are required, a medium or high molecular weight acrylic leveling agent should be selected. In the primer, if stronger substrate wettability is required, consider using silicone leveling agents and modified acrylic leveling agents (such as fluorine-modified acrylic leveling agents and phosphate ester modifiers) that can significantly reduce surface tension. Acid acrylic leveling agent), if the silicone leveling agent and fluorine modified acrylic leveling agent appear to stabilize the foam, affecting interlayer adhesion and other side effects, phosphate ester modified acrylic leveling agent should be used.

In topcoat and transparent paint formulations, the appearance of the paint film is relatively high. Generally, a low molecular weight acrylic leveling agent can be used. In this way, good leveling properties will be obtained, and haze will not easily occur in the paint film. In cross-linking curing systems, acrylic acid leveling agents containing reactive functional groups are often used to obtain better leveling properties, while improving the physical and chemical properties of paint films. If a paint film is required to have better flowability or slip and scratch resistance, a silicone leveling agent is necessary. In this case, it is best to use a silicone leveling agent in combination with an acrylic leveling agent.

It should be noted that, when applied in a vertical plane, the silicone leveling agent provides leveling performance and can effectively reduce the tendency of the coating to sag. In addition, in metal glitter paint formulations, silicone leveling agents should be used with caution, because it may cause uneven arrangement of flake aluminum pigments and cause uneven paint film color.

2. Powder coating system
The leveling process of powder coatings is divided into two stages. The first stage is the melting of the powder particles, and the second stage flows into a flat paint film after melting. Powder coatings do not contain solvents, do not produce surface tension gradients during film formation, and leveling is more related to substrate wetting.

Powder coatings often use acrylic leveling agents. If the leveling agent is in a liquid state, it is generally used in advance as a masterbatch. There are also powdered acrylic leveling agents, which are specifically used in powder coatings. This type of product absorbs liquid acrylic leveling agents on silica powder, and some low-level leveling agents use calcium carbonate.

If the powder coating needs slipping and anti-scratch resistance, it is necessary to use a silicone leveling agent. There are already silicone leveling agents for powder coatings that are made into powder. Care should be taken when using silicone leveling agents to avoid the formation of craters.

3.Water-based coating system

Water-based coating systems are divided into water-soluble systems and latex systems.

In water-soluble systems, it is necessary to strongly reduce the surface tension of the system. The most commonly used is a silicone leveling agent, a fluorocarbon-based leveling agent, which functions the same as they are used in solvent-based coating systems. Of course, if a truly flat surface is required, an acrylic leveling agent for an aqueous system is necessary.

For latex systems, the film-forming mechanism is completely different, and the 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. The acrylic leveling agent can improve the flatness of the film, but the main flow properties of the coating are more controlled and adjusted by adding rheology control agents. .