1、 Definition of leveling agent
Leveling agent is a commonly used coating additive that can promote the formation of smooth, even, and uniform coatings during the drying process. Different types of coatings are suitable for different leveling agents.
2、 Principle of Leveling Action (Migration Mechanism)
As a leveling agent, it usually needs to meet the following two requirements:
1. It has a certain compatibility with the system.
2. The surface tension should be lower than the system.
Both of these factors will affect the migration of leveling agents. Only when the leveling agent spontaneously migrates to the surface of the paint film can it be called leveling. The migration principle also applies to the migration of other auxiliary devices.
Migration principle of leveling agent
The migration orientation of leveling agents follows the "minimum energy principle" in physics, that is, from places with high energy to places with low energy. When the coating is applied to the surface of the substrate, two interfaces are formed, namely the "substrate film interface" and the "film film air interface".
Therefore, the total energy of the system=(substrate surface tension - membrane surface tension)+(membrane surface tension - air surface tension).
When leveling agent is added to the system, if the leveling agent migrates to the "film air interface", the total energy of the system=(substrate surface tension - film surface tension)+(leveling agent surface tension - air surface tension), due to the surface tension of the leveling agent<The surface tension of the paint film and the total energy of the system decrease, and this process can occur spontaneously.
However, if the leveling agent migrates to the "substrate film interface", the total energy of the system=(substrate surface tension leveling agent surface tension)+(film surface tension air surface tension), due to the surface tension leveling agent<The surface tension of the paint film increases the total energy of the system, and this process cannot occur spontaneously.
Of course, from the perspective of molecular motion theory, when a leveling agent is added to the system, due to the random motion of the molecules, a portion of the leveling agent molecules will move to the "matrix paint film interface", but this portion is flat. The amount of medication is very small and can be ignored compared to the total amount added.
3、 The principle of organic silicon leveling agent
The structure of organic silicon leveling agent is mainly composed of the most common polyether modified silicone oil, and its structural formula can be expressed as follows:
- （SiO（CH3）2）m - （SiOCH3（CH2CH2O）x（CH2CHCH3O）y）nR
The m segment represents the unmodified part of silicone oil, which belongs to the part with limited compatibility; Section n is the modified part and belongs to the compatibility section; X is polyethylene oxide in the polyether modified segment. Alkane fraction; Y is the polyepoxypropane moiety in the polyether group; m. The four values of n, x, and y determine the performance exhibited by the leveling agent.
1. Compatibility: The compatibility of silicone leveling agents mainly depends on the value of m/n. The smaller the value of m/n (i.e., the lower the content of incompatible segments), the better the compatibility. When the value of m/n is fixed, the larger the value of x/y, the better the compatibility, because the compatibility of polyethylene oxide exceeds that of propylene oxide.
2. Texture: With a fixed xy value, the texture of silicone leveling agents mainly depends on the m/n value. The larger the m/n value, the better the texture; The same m/n value, the larger the absolute value of m, the better the hand feel; From this point of view, it can be seen that for silicone leveling agents, pursuing good compatibility and good hand feel is often contradictory. In order to achieve these two goals, m/n can usually only be selected within a very small range.
3. Flattening ability: The influence of m, n, x, and y on the leveling effect is relatively complex. Generally speaking, a m/n value between 1 and 2 has a good leveling effect. However, for a fixed m/n value, the larger the value of x+y, the better the leveling effect.
4. Foam stability: generally, at x, y, m/n> 3 or m/n
5. Repaintibility: Generally speaking, the larger the m+n value and the higher the m/n value, the more likely the recoating problem is to occur, and the type of polyether end group R group will also be recoated. Sex has an impact.
6. The orientation ability of extinction powder: According to experiments, the orientation ability of extinction powder is greatly affected by the m/n value and the absolute value of m. The larger the absolute value of m, the better the directional ability of the extinction powder.
