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To protect and decorate. These properties of coatings are essential. 

It is admitted for many paints that a perfect appearance means also a perfect leveling. Poor leveling (like orange peel or ghosting) may not influence significantly the paint parameters in time,

as it is more aesthetic orientated. But the leveling agent itself may cause some severe issues.

A correct leveling agent helps in reaching the wanted leveling. But a wrong choice may lead to craters, pinholes, loss of gloss or even intercoat adhesion problems.

Leveling is strongly linked with the flow properties of the paint, and

the surface tension will play an important role on the paint appearance. This guide will focus on the paint leveling agents, and I invite you to read the Substrate Wetting Agent guide for a complete overview of the influence of the surface tension on the paint properties.

Leveling Agent: A Surface Tension Story!

Leveling is an important aesthetic parameter of the paint. To understand the leveling, it is important to understand the influence of the surface tension on the paint. Indeed, the paint has two interfacial areas, one with its substrate, one with the surrounding atmosphere. General surfactants are active on these interfaces, and depending on their chemistry, can help with the substrate wetting or be active at the paint / air interface.

The substrate wetting of a liquid on a solid substrate is defined by the surface tension, function of the contact angle. The contact angle θ is the angle between the intersection of the liquid-solid interface and the liquid-air interface at the three phase contact line.

A good substrate wetting is important to reach a good paint surface leveling ! 

Low Contact Angle vs High Contact Angle

Ideal theory, a perfect coating would have a contact angle θ = 0° regarding the substrate, which is not realistic. Nevertheless, we can consider reaching a good wetting when the contact angle θ < 90° , in other words, when the liquid has a lower surface energy than the substrate : γ (L) < γ(S)

θ < 90° γ (Liquid) <γ (Substrate)

• Decrease the surface tension of the liquid, improve the substrate wetting - positive influence on the leveling
• Equalize the surface tension at the liquid / air interface - improve the leveling
• Good flow means the avoidance of interfacial tension problems and surface texture issues & good leveling means the achievement of the lowest surface area possible.

Using specific surfactants like leveling agents will improve the surface aspect of the paint !

Surface Tension and Leveling

Of course, besides the surface tension, many parameters are influencing the paint leveling :

 

• Raw materials

• Application method

• Paint viscosity

• Film thickness

• Curing / drying conditions…

…are some examples. But the surface tension parameter plays also a critical role to avoid leveling defects such as the orange peel. In realistic conditions, it is not always possible to play with each parameters to reach the wanted leveling. But working on the surface tension parameters using solvents and additives will help in improving the paint surface quality.

Competing forces within the liquid

A paint drop is a combination of various materials, such as solvents, pigments or resins. Each of them have their own surface tension, and the general tension is caused by the unbalanced forces of liquid molecules at the surface. During the paint drying, the solvent (or water) evaporation will further unbalanced this state, creating a surface tension gradient at the paint surface.


 

Role of Solvents in Leveling

The solvents used in the formulation are really important to reach the required leveling. Too fast evaporation will lead to poor leveling, too low evaporation may increase the risk of contamination, surface defects, and reduce the production speed. But this is also linked with the film thickness: higher film thickness, more solvent to evaporate, longer drying time.

During the paint drying / curing, the leveling agent will move to the liquid (paint) / air interface and orientates itself at the surface. The possible reactive modification will stay in the liquid part and react with the polymers during the drying / curing.

The leveling agent, as a surfactant, will:

• Balance and harmonize this surface tension at the paint / air interface
• Reduce the surface tension gradient
• Improve the surface aspect.

Finding a balance is critical.

• Too low surface tension may be good for the substrate wetting, but may lead also to a wavy surface.
• Too high surface tension can cause adhesion and surface wetting issues.

Unbalanced forces at the surface will create waves, (long waves and short waves) which may lead to surface aspect issues such as the orange peel, loss of gloss and poor DOI.

Selecting the Right Leveling Agent

Which Leveling Agent for which Coating?

Leveling agent may be based on several chemistries.

Click on the links below to know more about the most common types of leveling agents:

• Silicone based agents (Polydimethylsiloxane modified - PDMS)
• Acrylate based agents (Polyacrylate)
• Fluorocarbon based agents (Modified Fluorocarbons)
• Hydrocarbon based leveling agents: less efficient than previous families

Nowadays, you can easily find leveling agent chemistries for solventborne and waterborne coatings. In case of powder coatings, acrylic based leveling agents are preferable due to their low contamination tendency.

Leveling Agents and their Effects in the Coating System

This summary table compares the global tendency of each leveling agent chemistry.

The formulator has a large choice in the selection the leveling agent. In the most exigent end-uses, the best solution is often achieved by a combination of technologies, such as modified acrylates with modified silicones.

The best leveling agent for a formulation will, of course, improve the leveling, without any influence on the other parameters and will not provoke any unwanted negative side-effect!

Some recommendation:

 

• Always check the all parameters when using a leveling agent, and not only the leveling and surface aspect. As surface active material, it can have an influence on the substrate wetting, the haze, the intercoat adhesion, or foam stabilization (especially in waterborne).
• Do not hesitate in testing variation of a product: not only in the dosage, but also try different molecular weight, and product modifications (such as EO/PO for PDMS for instance). Some variations may better fit your system and improve the leveling agent compatibility, reducing the loss of gloss and haze negative effects.