During or after the construction process, due to poor leveling, surface defects such as orange peel, roller marks, shrinkage holes, pitting, needle holes, etc. often occur, which not only affect the appearance of the coating, but also reduce or damage its protective function. Leveling agent is a commonly used coating additive, which can promote the formation of a flat, smooth, and uniform coating film during the drying and film-forming process of the coating.
In painting, the wettability of the coating on the substrate and the leveling of the wet film are closely related to the surface tension. When the surface tension of the coating is lower than the critical surface tension of the substrate, it can wet the substrate and drive the coating to spread smoothly on the substrate.
Another concept about surface tension is the gradient of surface tension. Due to the complex mixture of coatings, even on a uniform substrate, the distribution of surface tension is uneven due to solvent evaporation, wet film flow, and curing reactions. Many phenomena such as the Bernard vortex and edge contraction are related to this concept. After adding leveling agents, their compatibility and migration behavior directly affect the surface tension gradient. Suitable leveling agents can not only reduce the surface tension of the interface, but also help to achieve uniform surface tension; However, some leveling agents with very low surface tension sometimes cannot eliminate orange peel, also because they are ineffective in balancing the surface tension gradient. On the contrary, some resins that do not reduce surface tension or instead increase surface tension can act as adjuvants to balance the surface tension gradient. So in many cases, reducing surface tension alone is not enough to solve the problem, but efforts need to be made to achieve uniform surface tension.
Acrylic leveling agents are homopolymers or copolymers of acrylic esters with varying molecular weights. This type of leveling agent only slightly reduces the surface tension of the coating, but can balance the difference in surface tension of the paint film. By utilizing controllable incompatibility, it migrates to the surface of the paint film to form a monolayer, suppress solvent evaporation rate, improve surface fluidity, eliminate defects such as orange peel and brush marks, and achieve a smooth, mirror like effect on the paint film.
Fluorocarbon modified acrylic polymers have strong advantages in reducing coating surface tension, preventing film shrinkage, and improving flow and leveling properties due to their low surface tension. They are comprehensive non silicone resin leveling agents. However, its price is relatively expensive, and there is a tendency to stabilize foam and affect recoating properties. It can generally be used in combination with acrylic leveling agents and silicone leveling agents.
Organic silicone leveling agents are generally divided into polydimethylsiloxane, polydimethylsiloxane, and organic modified polysiloxane. Most of these leveling agents can strongly reduce the surface tension of coatings, improve the wettability of coatings on substrates, and prevent shrinkage porosity; It can reduce the surface tension difference caused by solvent evaporation on the wet film surface, improve the surface flow state, and shorten the coating leveling time. This type of leveling agent can also form a very thin and smooth film on the surface of the coating, thereby enhancing the smoothness of the coating surface.
Polydimethylsiloxane has poor compatibility with coatings, so it is basically not used in coatings now. Polyether modified polydimethylsiloxane is currently the most commonly used and diverse leveling agent. Its structural formula can be expressed as follows:
— (SiO(CH3)2)m— (SiOCH3 (CH2CH2O)x(CH2CHCH3O)y)nR
The dimethyl siloxane portion of the m segment is a segment with limited compatibility;
The n segment is the modified part and belongs to the compatible segment;
X is the polyethylene oxide portion in the polyether modified chain segment;
Y is the polyepoxypropane moiety in the polyether group;
m. The four values of n, x, and y determine the performance of the leveling agent, and the different sizes of these four values determine the different performance of the leveling agent.
Compatibility:The compatibility of organic silicon 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. In the case of a fixed m/n value, the larger the value of x/y, the better the compatibility. This is because the phase compatibility of polyethylene oxide exceeds that of polypropylene oxide.
feel:In the case of fixed values of x and y, the hand feel of organic silicon leveling agents also depends mainly on the value of m/n space. The larger the value of m/n, the better the hand feel; When the value of m/n is the same, the larger the value of m, the better the hand feel; From this point, it can be seen that for organic silicon leveling agents, pursuing good compatibility and good hand feel is often a contradiction. In order to achieve a balance between the two, m/n can usually only be selected within a small range.
Leveling ability:m. The influence of the values of n, x, and y on the leveling effect is quite complex. Generally speaking, the value of m/n between 1 and 2 has a better leveling effect, while for a fixed m/n value, the larger the value of x+y, the better the leveling effect.
Stability of foam:Generally speaking, under the usual values of x and y, m/n>3 or m/n< 1/4 of the leveling agent can achieve a basically unstable foam effect. Simply put, when the silicon content is high or low, the leveling agent obtained is unstable foam. If it is made into a variety with high silicon content, it can achieve excellent hand feel and unstable foam effect. However, for leveling agents with slightly poor compatibility, if it is made into a variety with low silicon content, the leveling agent obtained has good compatibility and unstable foam, but the hand feel may be lacking.
Repainting ability:Generally speaking, the larger the value of m+n and the higher the value of m/n, the more likely it is to cause recoating problems. In addition, the type of R group at the polyether end group can also affect recoating.
The directional ability of extinction powderAccording to the experiment, the ability of extinction powder orientation is greatly affected by the values of m/n and m. The larger the value of m, the better the ability of extinction powder orientation.
