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Leveling agent is a commonly used coating additive, which can promote the coating to form a flat, smooth and uniform coating film during the drying and film-forming process. Different types of coatings are suitable for different leveling agents.
As a leveling agent, it generally needs to meet the following two points:
1. It has a certain solubility with the system.
2. The surface tension must be lower than the system.
Both points affect the migration of the leveling agent. Only when the leveling agent spontaneously migrates to the surface of the paint film can it be called a leveling effect. The migration principle is also applicable to the migration of other additives.
Migration principle: The migration orientation of the leveling agent follows the "minimum energy principle" in physics, that is, it spontaneously flows from a high energy place to a low energy place. When the paint is applied to the surface of the substrate, two interfaces are formed, namely the "substrate-paint film interface" and the "paint film-air interface".
Therefore, the total energy of the system = (substrate surface tension-paint film surface tension) + (paint film surface tension-air surface tension).
After the leveling agent is added to the system, if the leveling agent migrates to the "paint film-air interface", then the total energy of the system = (substrate surface tension-paint film surface tension) + (leveling agent surface tension-air surface tension). Since the surface tension of the leveling agent is less than the surface tension of the paint film, the total energy of the system is reduced, and the process can proceed spontaneously. However, if the leveling agent migrates to the "substrate-paint film interface", the total energy of the system = (substrate surface tension-leveling agent surface tension) + (paint film surface tension-air surface tension). Since the surface tension of the leveling agent is less than the surface tension of the paint film, the total energy of the system is increased, and the process cannot proceed spontaneously.
Of course, from the perspective of molecular motion theory, when the leveling agent is added to the system, due to the irregular movement of molecules, some of the leveling agent molecules will move to the "substrate-paint film interface", but the amount of this leveling agent is very small and can be ignored compared with the total amount added.
The structure of silicone leveling agent is mainly based on the most common polyether modified silicone oil, and its structural formula can be expressed by the following formula:
—(SiO(CH3)2)m—(SiOCH3 (CH2CH2O)x(CH2CHCH3O)y)nR Wherein the m segment represents the unmodified part of the silicone oil, which is a segment with limited compatibility; the n segment is the modified part, which is a compatible segment; x is the polyethylene oxide part in the polyether modified segment; y is the polypropylene oxide part in the polyether group; the four values of m, n, x, and y determine the performance of the leveling agent, and the different sizes of these four values determine the performance of the leveling agent.
1. Compatibility: The compatibility of silicone leveling agent mainly depends on the value of m/n. The smaller the value of m/n (that is, 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. This is because the compatibility of polyethylene oxide exceeds that of polypropylene oxide.
2. Feel: When the xy value is fixed, the feel of silicone leveling agent also mainly depends on the value of m/n. The larger the value of m/n, the better the feel; when the m/n value is the same, the larger the absolute value of m, the better the feel; from this point, it can be seen that for silicone leveling agent, the pursuit of good compatibility and good feel is often a contradiction. In order to achieve both, m/n can usually only be selected within a small range.
3. Leveling ability: The influence of the values of m, n, x, and y on the leveling effect is relatively complex. Generally speaking, when the value of m/n is between 1 and 2. there is a better leveling effect. For a fixed m/n value, the larger the value of x+y, the better the leveling effect.
4. Foam stability: Generally speaking, under the usual values of x and y, m/n>3 or m/n<1/4 can obtain a basically unstable foam effect. In layman's terms, when the silicon content is very high or very low, the obtained leveling agent is unstable. If it is made into a variety with a high silicon content, you can get a leveling agent with excellent feel, unstable foam, but slightly poor compatibility. If it is made into a variety with a very low silicon content, the performance of the obtained leveling agent is a silicone leveling agent with good compatibility, unstable foam, but insufficient feel.
5. Recoatability: Generally speaking, the larger the value of m+n and the higher the value of m/n, the more likely it is to have recoatability problems. In addition, the type of polyether end group R group will also affect the recoatability.
6. Orientation ability of matting powder: According to the test, the orientation ability of matting powder is greatly affected by the value of m/n and the absolute value of m. The larger the absolute value of m, the better the orientation ability of matting powder.