Introduction to Coating Leveling and Standards
Paint industry:
1、 Definition of Flattenness
Leveling, as described in 2.150 of GB/T 5206-2015/ISO 4618: 2014 "Terminology and Definitions for Paints and Varnishes", refers to the ability of a coating to spread after application, thereby minimizing any surface roughness caused by the application process.
Specifically, after coating application, there is a process of flow and drying to form a film, which gradually forms a flat, smooth, and uniform coating. Whether the coating can achieve a smooth and even characteristic is called leveling.
The motion of wet coating film can be described by three models: ① Distribution flow contact angle model on the substrate; ② A sine wave model that flows from an uneven surface to a flat surface; ③ The Benard vortex in the vertical direction. They correspond to the three main stages of wet film leveling - spreading, pre leveling, and post leveling, during which factors such as surface tension, shear force, viscosity changes, and solvents play important roles.
2、 Related standards
At present, there are no leveling related standards in China. The currently effective relevant standards include ASTM D4062-2024 "Test Method for Determining the Flatness of Paint by the Stretching Method".
GB/T 1750-1979 "Determination of Flatness of Coatings" (abolished) provides practical methods for determining flatness, including brushing and spraying methods. The brushing method requires checking how long it takes for the coating to recover from the brush marks to a smooth state.
ASTM D2801-1969 (1981) "Method of Test for Leveling Characteristics of Paint by Draw Down Method" (now discontinued) uses a special scraper to scrape the paint into strip-shaped coatings of different thicknesses, observe their leveling properties, and provide a rating method from 0 to 10.
3、 The manifestations and causes of poor leveling
1. Poor performance of leveling
(1) Shrinkage cavity
There are low surface tension substances (pore fouling sources) in the coating, which have a surface tension difference with the surrounding paint. This difference promotes the formation of pores, causing the surrounding liquid fluid to flow away from it and form depressions.
(2) Orange peel
After drying, the surface of the coating presents many semi-circular protrusions and ripples similar to orange peel, which is called the orange peel phenomenon.
(3) Flow hanging
The wet coating film is driven by gravity to cause flow marks, which is called sagging.
2. Factors affecting leveling
(1) The influence of surface tension of coatings 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 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 shrinkage, shrinkage porosity, and fisheye.
(2) The influence of solubility on flatness
During the drying process of the paint film, sometimes insoluble adhesive particles are generated, which can form a surface tension gradient and lead to the formation of shrinkage cavities.
In addition, in formulations containing surfactants, if the surfactants are incompatible with the system, or if their concentration changes during the drying process due to solvent evaporation, resulting in a change in solubility and the formation of incompatible droplets, a difference in surface tension will also be formed. All of these may lead to the formation of shrinkage cavities.
(3) The influence of wet film thickness and surface tension gradient on flatness
Benard vortex - The evaporation of solvents during the drying process of paint films can create temperature, density, and surface tension differences between the exterior and interior of the paint film, which in turn lead to turbulent motion inside the paint film, forming the so-called Benard vortex.
The paint film problems caused by the Benard vortex are not limited to orange peel. In systems containing more than one pigment, if there is a certain difference in the mobility of pigment particles, the Benard vortex is likely to cause floating and blooming, and vertical surface construction may also cause silk lines.
(4) The influence of construction technology and environment on leveling
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 poor leveling of the surface.
