How do levelling agents level the paint?
1、Flattening agent definition
Leveling agent is a common coating additives, it can promote the coating in the drying process to form a flat, smooth, uniform coating film, different types of coatings applied to the leveling agent different.
2、The principle of leveling (migration mechanism)
As a levelling agent, generally need to meet the following two points:
2.1, and the system has a certain degree of compatibility.
2.2, surface tension needs to be lower than the system.
These two points affect the migration of the levelling agent, only the levelling agent spontaneously migrate to the surface of the paint film, can be called the occurrence of levelling effect. The migration principle also applies to the migration of other additives.
Migration principle
The migration of levelling agents follows the “lowest energy principle” in physics, i.e. they flow spontaneously from a place of high energy to a place of low energy. When the paint is applied to the substrate surface, two interfaces are formed, the “substrate – paint film interface” and the “paint film – air interface”.
Therefore, the total energy of the system = (surface tension of the substrate – surface tension of the paint film) + (surface tension of the paint film – surface tension of the air).
When a levelling agent is added to the system, if the levelling agent migrates to the “film-air interface”, then the total system energy = (substrate surface tension – film surface tension) + (levelling agent surface tension – air surface tension), as the levelling agent surface tension < film surface tension, the total system energy is reduced and the process can The process can proceed spontaneously.
However, if the levelling agent migrates to the “substrate – paint film interface”, the total energy of the system = (substrate surface tension – levelling agent surface tension) + (paint film surface tension – air surface tension), as the levelling agent surface tension < paint film surface tension, so the total energy of the system is increased and the process cannot proceed spontaneously.
Of course, from the point of view of molecular movement theory, when the levelling agent is added to the system, due to the irregular movement of molecules, there will be part of the levelling agent molecular movement to the “substrate – paint film interface”, but the amount of this part of the levelling agent is very small, compared with the total amount added can be negligible.
3、The principle of silicone leveling agent
The structure of silicone leveling agents is based on the most common polyether modified silicone oil, whose structural formula can be expressed in the following formula:
-(SiO(CH3)2)m-(SiOCH3 (CH2CH2O)x(CH2CHCH3O)y)nR
Where m chain segment indicates the unmodified part of the silicone oil, belongs to the chain segment whose compatibility is restricted; n chain segment is the modified part, belongs to the compatible chain segment; x is the polyethylene oxide part of the polyether modified chain segment; y is the polyethylene oxide part of the polyether group; m, n, x, y four values determine the performance shown by the leveling agent.
3.1、Compatibility: The compatibility of silicone levelling agents mainly depends on the value of m/n. The smaller the value of m /n (that is, the lower the content of incompatible chain segments), the better the compatibility, and the larger the value of x/y under the fixed value of m/n, the better the compatibility, which is because the compatibility of poly(ethylene oxide) exceeds that of poly(propylene oxide).
3.2, feel: in the case of fixed xy value, the feel of silicone levelling agent also depends mainly on the value of m/n, the larger the value of m/n, the better the feel; m/n value of the same case, the larger the absolute value of m, the better the feel; from this point can be seen, for silicone levelling agent, the pursuit of good compatibility and good feel is often a contradiction, in order to achieve both, usually m/n can only be selected within a small range.
3.3, leveling ability: m, n, x, y values on the impact of leveling effect is more complex, in general, m/n values between 1 to 2 when there is a better leveling effect, and for a fixed m/n value of the case, the greater the value of x + y, the better the leveling effect.
3.4, stable foam: in general, in the usual x, y value, m/n> 3 or m/n < 1/4 can get basically unstable foam effect, in layman’s terms is when the silicon content is very high or very low, the leveling agent is not stable foam, and if made into a very high silicon content varieties, you can get a feel excellent, not stable foam, but slightly less compatible leveling agent, if If the silicon content is very low, the performance of the levelling agent is very good compatibility, not stable foam, but the feel of the silicone levelling agent is not enough. 5.
3.5. recoatability: in general, 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 to the type of polyether end group R group will also have an impact on recoatability.
3.6, the directional ability of the matting powder: according to the test, the directional ability of the matting powder is greatly influenced by the value of m/n and the absolute value of m. The larger the absolute value of m, the better the directional ability of the matting powder.
Causes
Paint surfaces are prone to phenomena such as shrinkage, orange peel, surface roughness and lack of gloss, which seriously hinders the promotion and application of new technologies such as powder, water-based and high-solid coatings. Levelling agents are used in paints to reduce these phenomena, making the surface of the paint film smooth and decorative.
To achieve this, the surface tension is related.
Surface tension
Within a liquid, the molecular gravitational forces on any molecule are balanced. But on the surface of the liquid, the gravitational force of the liquid phase molecules on the surface molecules is greater than the gravitational force of the gas phase molecules on the surface molecules, and thus there is an attraction towards the inside of the liquid, and this is the surface tension. When the levelling agent is added, the levelling agent can float to the surface of the paint film to homogenise it, which is one of the levelling mechanisms.
Shrinkage and orange peel
The reason for this is that if the surface tension of the liquid is different at two points on the wet paint film, the paint material will flow from one point to the other, thus creating shrinkage, orange peel etc. The acrylic levelling agent can reduce surface tension and interfacial tension to increase wettability, and can form a single molecular layer on the surface of the paint to ensure that the surface tension in the surface section is homogenised.