Leveling agents are very important in the process of coating application, as they can promote the formation of a flat, smooth, and uniform coating film during the drying and film-forming process of the coating. What are the specific effects of leveling agents on coating films? Next, the editor of Jianyi Chemical will introduce it in detail to everyone!
An important factor affecting the quality of coating is the surface tension of the coating. Secondly, the construction viscosity, construction conditions, wet film thickness, and surface drying time of the coating have a certain impact on the quality of the coating film.
In painting, the wettability of the coating on the substrate and the leveling of the wet film are closely related to the surface tension. Only when the surface tension of the coating is lower than the critical surface tension of the substrate, can it wet the substrate and promote the coating to spread smoothly on the substrate. The leveling process of wet film is the process of homogenizing the surface tension of the coating film. During the drying process of the wet film after coating, due to the continuous evaporation of solvents, the surface tension of the coating layer increases, viscosity and density increase, and temperature decreases, resulting in tension and density differences between the surface and inner layers of the coating. This drives the low surface tension coating enriched with solvents to move towards the high surface tension coating layer and spread on the surface of the coating, achieving surface tension uniformity; The surface coating will sink to the bottom of the coating due to gravity. The surface flow and up and down convection of wet film are repeatedly carried out, resulting in local turbulent fluid flow and the formation of uneven structures on the surface, such as the Bernard vortex. This approximately hexagonal vortex structure causes the coating to exhibit phenomena such as orange peel or floating color. When the wet film loses effective flow due to solvent evaporation and increased viscosity, if the surface tension of the coating film has not yet reached uniformity, the flowing coating film cannot continue to level and becomes a surface defect.
The main reason for shrinkage is that when the surface tension of the coating is greater than the critical surface tension of the substrate, the wet film cannot fully wet the substrate and cannot spread into a uniform coating. When the substrate is contaminated with oil, moisture, dust, or fingerprints, it can lead to a decrease in surface tension at the contaminated area, causing the paint mist of the wet film and dust particles in the air to retract and form shrinkage holes.
In addition, the scattered paint mist and dust particles in the air during construction, as well as insoluble particles generated by differences in solubility or other reasons of the coating, will form low surface tension points in the wet film, causing the wet film with higher surface tension to be unable to wet these points and resulting in shrinkage. The effective method to prevent shrinkage of coating film is to use anti shrinkage and leveling agents that can strongly reduce the surface tension of the coating, so that the surface tension of the coating is lower than that of the substrate or pollutants in the coating film, helping the wet film fully wet the substrate and evenly distribute, and making the surface tension of the coating film reach uniformity as soon as possible. The main reason for the drawbacks such as orange peel, brush marks, and floating colors is the difference in surface tension and density caused by solvent evaporation in the wet film, which leads to surface flow and up and down convection in the wet film. Additionally, due to poor leveling of the coating or rapid solvent evaporation, it is difficult to achieve uniform surface tension within the effective flow time, resulting in a tendency for the surface of the wet film to become flat. When the fluidity of the wet film is hindered due to the increase in viscosity, defects such as orange peel, brush marks, and floating color will form on the coating that has not yet been leveled.
Excessive surface tension, poor leveling, high viscosity, influence of construction conditions and environment, or improper selection of diluents in coatings can all lead to these drawbacks. The main method to reduce or eliminate coating defects such as bark, brush marks, and floating colors is to add leveling agents to the coating, which can appropriately reduce the surface tension of the coating, prolong the leveling time of the wet film, and effectively reduce the surface tension difference between the surface and interior of the coating, shorten the time of surface flow and upward and downward convection, help the surface tension to become uniform as soon as possible, and thus prevent the occurrence of the above surface defects.
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.
