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Importance of Leveling in Paints and Coatings

Leveling in Paints and Coatings
  1. What is Leveling?
  2. Why does leveling matter for paints & coatings?
  3. What are the Factors that Impact Leveling?
  4. Popular Methods to Measure Leveling of Films
  5. How to obtain desired leveling?


What is Leveling?


End-users’ decisions are strongly influenced by paint’s quality and appearance. A flat smooth coated surface gives a better glossy appearance as well as provides adequate performance.

Leveling is the property of coatings that determines the aesthetics of the coated surface. It is the ability of the coating to eliminate surface defects that occur after the application, which often cannot be seen immediately after application. Some examples of observed eliminate surface defects as may be noticed in poorly leveled paint films are:

  • Orange peeling
  • Structured surface
  • Craters
  • Fisheyes
  • Crawling
  • Brush marks
  • Pinholes, etc.

Most of these defects are the result of local surface tension differences of the involved raw materials, of substrate contaminants or of surface tension differences as the result from solvent evaporation.

Orange Peel Caused by Poor Leveling
Crater Extending Right Down to the Substrate
Fisheyes Caused by Poor Leveling
Type of Defects Due to Poor Leveling

Therefore, coating systems need good flow and leveling to have a smooth appearance and is typically achieved using leveling agents in the coating formulation. Proper usage of flow and leveling agents can significantly correct these defects.


Why does leveling matter for paints & coatings?


Optimum leveling performs many functions in a coating as it promotes the quality of spreading of the applied paint layer into a flat, smooth, homogeneous film. Leveling additives are often incorporated into the formulation enabling the application of thinner coatings, improving performance while gaining material savings...get assistance in finding the best suitable leveling agent for your project through expert-led selection tips.

Good flow and leveling are the key properties for coatings resulting in superior finishes in end applications, such as (not limited to):

  • General industrial coatings
  • Can and coil coatings
  • Wood finishes
  • Automotive OEMs (original equipment)

Leveling Agents – When to Use Them?


If a coating formulation effectively dries or cures without coating defects, leveling agents will not improve the coating formulation. Leveling agents should be tried in the following situations.

  • Shifting to or reformulating water-borne and high-solid coating. The coating often has a higher surface tension while reformulating to eliminate VOC (Volatile Organic Compounds) level (low solvents). Therefore, reformulated coating may require leveling agents to ensure coating cures without any surface defects.

  • Moving from controlled laboratory conditions to field. A coating may behave differently in field leading to surface defects. Hence, leveling agents might help coating to perform well for surfaces with defects or dirt.

If you already have the suitable additive options with you, it might be worth taking our course to see hands-on practical examples related to on surface defects and potential solutions to prevent them.

Solving Coatings Film Defect_SG_Prop


What are the Factors that Impact Leveling?


In simple terms, leveling is a measure of the ability of coating to flow out after application to eliminate any surface irregularities. It is strongly dependent on coating’s flow properties, surface tension, application parameters and drying conditions. Now that you've learned what leveling is, let's walk through the factors that influence leveling in paints and coatings.

Leveling and Surface Tension Go Hand in Hand


Controlling the leveling and flow of a coating is all about controlling the surface tension of the system. Surface tension, with symbol γ, commonly defined as an unbalanced force, which acts in a material to adapt the smallest possible surface under the set conditions. These unbalance forces occur due to the cohesive forces between the molecules in the surface layer that are not evenly distributed to all sides compared to molecules in the inner phase.

  • The term “surface” is used for the boundary between a condensed phase (liquid or solid) and a gas or a liquid.
  • The term “interface” is used to describe the junction between the two phases.

Go deeper into the underlying science of “Surface Tension” to see how it determined overall effectiveness of any coating.

Almost all surface defects are related to surface tension and by fine-tuning or correcting the surface tension or surface tension gradient of the systems, many surface defects including poor leveling or orange peel appearance can be solved. Lowering a surface tension of a coating will reduce the attraction forces locally on the surface between nearby molecules. This will eventually allow better flow.

Solving Surface Tension Issues

The factors contributing to a good leveling is not just simply reducing the surface tension as low as possible. Leveling can also depend on other factors like viscosity of the paint, application method and horizontal or vertical position. It is important to understand that coating effects like substrate wetting, foam formation, flow and leveling and slip are interdependent properties.

Substrate Wetting and Leveling


All coatings contain interfacially surface active substances such as wetting agents and surface modifiers. They act at interfaces like:

  • Liquid/air (e.g. foam)
  • Liquid/solid (e.g. substrate wetting) or
  • Liquid/liquid (e.g. compatibility)

The wetting of the substrate and leveling of the liquid film depend on the surface tension of the coating as well as on the surface tension of the substrate to be coated.

A paint will ‘wet’ a substrate when the substrate has an equal or higher surface tension than the liquid itself. Therefore, poor wetting results from the higher surface tension of paints as compared to surface tension of the substrate.

Contact angle is one of the common ways to measure the wettability of a surface or material.

Relationship Between Wetting and Contact Angle
Wetting Conditions and Their Relation to Contact Angles 


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

Contact Angle: Decisive Surface Data from a Single Test

Low Contact Angle vs High Contact Angle


θ < 90° : low contact angle θ > 90° : high contact angle
Favorable wetting - Positive influence on the leveling Unfavorable wetting - Negative influence on the leveling
The paint will spread over the surface The paint will minimize its contact with the substrate
Good wetting of the paint

Good wetting of the paint
Poor wetting of the paint

Poor wetting of the paint

Both processes, wetting and leveling, have opposing requirements in terms of surface tension.

  • If the surface tension is too high, poor wetting occurs, along with the formation of possible defects such as craters.
  • If the surface tension is too low, poor leveling occurs and can cause orange peel.


Rheology/Viscosity


Ease of leveling can be improved through rheology changes. Rheology adjustment can be done by using solvents. High boiling point solvent reduces not only the viscosity of the paint but also allow a slower viscosity increase after application through their low evaporation speed. Thus, allowing more time for leveling.

Use of polymeric leveling agents is another method to modify the rheology of the surface layer of the paint film. For example, polyacrylates, which typically show a moderate effect on surface tension of the paint, but create a physical uniform surface, hence reducing local surface-tension differences, followed by a homogeneous leveling.

Rheology & Viscosity Made Easy

Additives in the Formulation


Leveling is related to main components in a paint formulation like:

  • Solvents
  • Binders
  • Pigments, and
  • Extenders

Changes in binder composition, for instance by including CAB (Cellulose Aceto Butyrate) typically contributes to improved leveling of solvent-borne industrial and automotive systems.

The choice of used extender largely effects leveling of waterborne systems. For instance, using kaolin, the plate nature of kaolin particles provides a good balance of flow and leveling as these particles tend to align parallel to 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.

Leveling agent working

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.

Related Read: Troubleshooting Solvent Substitution Challenges in Coatings

Application Method


Application methods can influence leveling and lead to surface defects. Some brushes, rollers and direct roller coaters can produce an uneven paint surface. As the paint dries out, these non-uniformities tend to flatten out with the means of leveling leaving a protective and aesthetically pleasing smooth finish.

Other methods that produce a smooth surface – spraying, curtain coating, reverse roller coating – need to maintain that smoothness during the baking and curing process.

Curing Time


Leveling also depends on time. If a coating applied at different thickness, it would need different time to achieve the final flat surface. Thus, the solvents that are used in the formulation are important in order to allow the time to achieve the desired paint film leveling. The time needed for leveling is directly proportionate to viscosity and inversely proportionate to surface tension and film thickness. 


How to measure leveling?


Leveling can be determined from the liquid as well as the dry film. Leveling of the liquid film is determined for instance, by evaluation the "flow-out" ability, using an instrument, such as the "leveling tester".

The leveling testers are designed and engineered following International Testing Standard e.g. ASTM D4062 - 11(2016) Standard Test Method for Leveling of Paints by Draw-Down Method.

This test method covers the laboratory determination of the relative leveling of water and solvent-reducible architectural paints in white and light tints by comparing the ridges produced in a draw-down film to a series of plastic leveling standards.

Leveling test procedure includes:

  • Drawing the paint to be tested over a plane substrate (test chart), producing 5 pairs of streaks of various film thicknesses.
  • Holding the test panel in a horizontal position, and see which of the pairs of streaks converge.
  • Generally, the gap depth of that pair of streaks is indicated, where the intervals between the streaks are slightly visible.

Leveling of dry films is determined by wave scan measurements, determining the waviness of the coating film at a range of wavelengths of approx. 0.1 to 30 mm and DOI (distinctness of image).


How to obtain desired leveling?


Leveling agents/flow modifiers are added to coating formulations to reduce the surface tension of the coating and interfacial tension between coating and substrate thus delivering superior surface qualities and optimal coating finish. They also help to optimize the viscosity during film formation process.

The working mechanism of these additives is to eliminate local surface tension differences, resulting from solvent evaporation or presence of impurities, at the paint film surface. Moreover, they improve substrate wetting and spreading. Complete substrate wetting is essential for optimal leveling and occurs at the condition if surface tension of the substrate is larger than the surface tension of the liquid paint (γs > γl).

There are several base chemistries available when selecting a leveling agent. If you already know the suitable chemistry for your system, choose the suitable grade from our coatings, paints and inks material selector for tech profile, ask for samples or discuss your case with producer’s tech staff.

Else, get on board and understand what the key points are that you should consider while finding the suitable leveling agent for your formulation through our exclusive guide for free.

Leveling Agents Selection Tips for Coatings


Leveling Agents/Flow Modifiers for Paints, Coatings and Inks


View a wide range of leveling agents / flow modifiers available today, analyze technical data of each product, get technical assistance or request samples.


2 Comments on "Importance of Leveling in Paints and Coatings"
Alberto N Aug 27, 2021
Excellent article.
Ketan D Jul 22, 2021
Very nice and detailed article.

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