Definition of leveling

The leveling property of a coating is described as the ability of the coating to flow after application, thereby maximizing the elimination of any surface unevenness caused by the application process. Specifically, after the coating is applied, there is a process of flow and drying, and then a flat, smooth, and uniform coating film is gradually formed. Whether the coating can achieve a flat and smooth property is called leveling.

The movement of wet coating can be described by three models:

① spreading flow-contact angle model on substrate;

② sine wave model of flow from uneven surface to flat surface;

③ Benard vortex in vertical direction. They correspond to the three main stages of wet film leveling – spreading, early and late leveling, during which surface tension, shear force, viscosity change, solvent and other factors play an important role in each stage.

Poor leveling performance

(1) Shrinkage holes
There are low surface tension substances (shrinkage hole sources) in the coating film, which have a surface tension difference with the surrounding coating. This difference promotes the formation of shrinkage holes, causing the surrounding liquid fluid to flow away from it and form a depression.

(2) Orange peel
After drying, the surface of the coating shows many semicircular protrusions, similar to the ripples of orange peel. This phenomenon is called orange peel.

(3) Sagging
The wet coating film is driven by gravity to form flow marks, which is called sagging.

Factors affecting leveling

(1) The effect of coating surface tension on leveling.
After coating application, new interfaces will appear: the liquid/solid interface between the coating and the substrate and the liquid/gas interface between the coating and the air. If the interfacial tension of the liquid/solid interface between the coating and the substrate is higher than the critical surface tension of the substrate, the coating will not be able to spread on the substrate, and leveling defects such as shrinkage, shrinkage cavities, and fisheyes will naturally occur.

(2) The effect of solubility on leveling.
During the drying process of the paint film, some insoluble particles are sometimes produced, which in turn form a surface tension gradient and lead to the formation of shrinkage holes. In addition, in the formulation containing surfactants, if the surfactant is incompatible with the system, or during the drying process, as the solvent evaporates, its concentration changes, resulting in changes in solubility, forming incompatible droplets, and forming surface tension differences. These may lead to the formation of shrinkage holes.

(3) The effect of wet film thickness and surface tension gradient on leveling.
Benard vortex – The evaporation of solvent during the drying process of the paint film will produce temperature, density and surface tension differences between the surface and the inside of the paint film. These differences will lead to turbulent motion inside the paint film, forming the so-called Benard vortex. The paint film problems caused by Benard vortices are not only orange peel. In systems containing more than one pigment, if there is a certain difference in the mobility of the pigment particles, Benard vortices are likely to cause floating and blooming, and vertical surface application will also cause silk lines.

(4) The effect of construction technology and environment on leveling.
During the construction and film-forming process of the coating, if there are external pollutants, it may also cause leveling defects such as shrinkage holes and fish eyes. These pollutants usually come from oil, 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.) will also have a significant impact on the final leveling of the paint film. Too high construction viscosity and too short drying time usually produce a poorly leveled appearance.

Nanjing Reborn New Materials provides leveling agents including Organo Silicone and Non-silicon ones that match BYK.