Surface Tension: A Property to Determine Coating’s Effectiveness

1. Surface Properties of the System
2. Fundamentals of Surface Tension
3. Importance of Contact Angle & Sustrate Wetting in Coatings Technology
4. What are the Popular Methods & Standards to Measure Surface Tension?
5. How Coating Defects are Related to Surface Tension?
6. Typical Coating Properties Governed by Surface Tension
7. Role of Surfactants in Coating Formulations

Surface Properties of the System

When it comes to a coated surface, good appearance is one of the most notable features to consider. In order to obtain a flat, smooth defect-free surface, most of the coating systems are formulated to display good wetting, leveling and flow. Surface tension is the key parameter which determines good leveling and wetting of a coating.

Surface tension is involved in almost every way in coatings technology i.e., dispersing, wetting, leveling, adhesion, etc. Infact, the appearance of surface defects on the coated surface is also governed by the surface tension of the materials (coating, substrate...) involved.

There is no denial in saying that surface tension (& properties) are amongst the key decisive factors for the longevity and quality of the applied coatings. Defect-free appearance & color, corrosion behavior, electrical conductivity, wetting behavior, adhesion of coatings and many other properties are determined by surface properties of the system involved.

Here, you will learn about the surface properties of coating system with special focus on the role played by “surface tension” in determining coating’s effectiveness. Let’s begin with the fundamentals of surface tension...

Fundamentals of Surface Tension

Formulating defect-free coatings is all about controlling the surface properties. For the better understanding of surface tension, let us first begin with cohesion.

Cohesion is the interaction between the molecules that are alike. Cohesive forces are intermolecular forces that resist liquid separation. These cohesive forces among liquid molecules are responsible for the phenomenon of surface tension.

Surface tension (γ) is commonly defined as an unbalanced force (given in units dyn/cm or mN/m) which acts in a material to adapt the smallest possible surface under the set conditions. These unbalanced 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.


In coating systems, interfacial surface tension is also important. Interfacial surface tension is defined as surface tension at the surface separating two non-miscible materials, such as liquid & gas. The understanding of interfacial surface tension in paint technology helps to solve many technical problems related to paint defects.

So, when a liquid coating is applied on the surface, surface tension forces come into play to redistribute the coating layer. If the surface tension of a liquid coating is higher than that of the substrate the coating, the coating will obtain the lowest possible common surface with the substrate. Thus, the coating will not wet the surface properly leading to surface defects.

Now when you have learnt about surface tension fundamentals, learn how contact angle and surface tension are related?

Importance of Contact Angle & Substrate Wetting in Coatings Technology

The contact angle θ is the angle between the intersection of the liquid-solid interface and the liquid-vapor interface at the three-phase contact line. It is an indication for the ability of the liquid to spread on the substrate, respectively a possibility to determine the dimension for the surface tension relative to a reference.

Solid / liquid (S/L) interfaces exhibit a different "surface tension" than the individual surfaces. The relationship between the S/L surface tensions can be derived from the Equation of Young:





Where:

• γsv represents surface tension of solid respectively
• respectively γlv represents surface tension of liquid and γsl represents interfacial tension solid /liquid
• θ is the contact angle

The interfacial tension can be determined by measuring the contact angle. Partial wetting occurs if the contact angle θ is > 0 < 90; spontaneous and complete wetting at θ = 0, implying γs > γl (surface tension solid is larger than surface tension liquid).

» Discover the range of information that can be extracted from Contact Angle Measurements, a simple & far-too-often misused test



Therefore, substrate wetting depends primarily on the surface tension of the paint and on the surface tension of the substrate to be coated. To allow for proper wetting and adhesion between the layers, the surface tension of the coating must be lower than the substrate. By comparison:

• Solvent borne systems have naturally low surface tensions and can easily wet most substrates.
• Water-based systems often have high surface tension resulting in poor wetting and require use of suitable raw materials or substrate treatment for effective substrate wetting. (discussed later)

The concepts of surface energy, surface tension, and wetting and contact angle phenomena are of exceptional importance to coating technology. Their understanding is vital for the proper formulation and application of coating.

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What are the Popular Methods & Standards to Measure Surface Tension?

There are several methods to measure the surface tension of a liquid. Two main methods are discussed below.

Static Surface Tension

Du Noüy Ring Method or commonly referred as “static surface tension method” is one of the easiest techniques for measuring the surface tension of liquids.

This method involves dipping of a platinum-iridium ring into the liquid and slowly lifting a ring, so that a lamella is formed at the air interface. The force needed to pull this lamella is a direct measure of the surface tension of the liquid. The Wilhelmy Plate method is similar, using a plate.



When this method works perfectly to compare the surface tensions of surfactants in waterborne, or of clear solvent borne coatings, it is not suitable for pigmented systems due to the presence of pigment hinders lamella stability.

Dynamic Surface Tension

For dynamic or fast application processes like printing, rolling, etc., it can be interesting to understand also the “dynamic surface tension” behavior of a system. The dynamic surface tension is measured with the “Maximum Bubble Pressure Method” that measures the ability of the surfactant to adsorb rapidly at the air / liquid interface.

A bubble pressure tensiometer produces gas bubbles at a specified rate and blows them through a capillary, which is submerged in the sample liquid. In this process, the pressure required to generate the new interface within the liquid passes through a maximum, which is directly related to the dynamic surface tension.

Apart from the popular methods for surface tension mentioned above, there are several ASTM standards that are related to surface technology and may be useful to those investigating surface properties & coatings.

ASTM Standard	Title
ASTM C813	Hydrophobic Contamination on Glass by Contact Angle Measurements
ASTM D971	Interfacial Tension of Oil Against Water by the Ring Method
ASTM D3825	Dynamic Surface Tension by the Fast-Bubble Technique
ASTM D5725	Surface Wettability and Absorbency of Sheeted Materials Using an Automated Contact Angle Tester
ASTM D7334	Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement
ASTM D7490	Measurement of the Surface Tension of Solid Coatings, Substrates and Pigments Using Contact Angle Measurements
ASTM D7541	Standard Practice for Estimating Critical Surface Tensions

How Coating Defects are Related to Surface Tension?

There are many ways in which surface properties are involved in coating defects. The control and prevention of surface-tension-driven defects in coating films requires a knowledge of surface properties and film formation processes.

Some of the common defects and their plausible causes include:

Defect	Causes	Possible Preventative Measures
Crating	Craters are small bowl-shaped depressions and can be formed due to contamination with low surface tension materials that are not compatible with the coating system.	To solve crater problems, the surface tension of the system should be lowered with proper wetting agents to allow the system to wet the contaminating material.
Pin holes	Pinholes result from slow rising micro air bubbles that are unable to separate from the substrate.	The right combination of defoaming agent and wetting agent is must to avoid pin holes.
Orange Peel	It is surface bumpiness or waviness that resembles orange peel.	Improve the leveling by reducing the surface tension can reduce orange peel.

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Typical Coating Properties Governed by Surface Tension

As we have discussed, the surface tension value gives an indication of how well the coating will spread on the substrate. Efficiently controlling tensions at different interfaces can help you achieve better balance in paint properties (wetting, adhesion, foaming, leveling, etc.) and thus can help you get the desired coatings performance. Learn about these properties in detail and their relationship with surface tension here:

• Leveling
• Foaming
• Adhesion
• Wetting
• Coalescing & film formation

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Role of Surfactants in Coating Formulations

Surfactants or interfacial tension modifiers are widely used in coating formulations. They are also often classified according to the composition of their "head" into four primary groups: anionic, cationic, non-ionic, and amphoteric (dual charge)

• Anionic surfactants are sulfonate, carboxylate, and phosphate ester.
• Cationic surfactants are typically amine derivatives, such as quaternary ammonium compounds.
• Non-ionic surfactants classes majorly include are polyglycol ether derivatives, such as alkyl- and alkyl-aryl polyethylene glycol ether (alkyl PEG), polypropylene glycol ether and block copolymers of polyethylene glycol and polypropylene glycol.
• Commonly used amphoteric surfactants include betaines, amine oxides

Some examples of how surfactants are used in coatings formulations & their function are discussed in brief below.

Wetting and dispersing agents: To reduce the interfacial tension between pigment and liquid medium. In the pigment dispersion process, surfactants enable complete wetting of the pigment particles i.e., to replace the adsorbed air onto pigment particle by the liquid medium in the mill base. Because of complete wetting the viscosity of the mill-base is reduced (reduction of "apparent pigment volume fraction") and the milling process optimized.

Leveling agents: To adjust interfacial tension between the liquid paint and substrate or air. Complete substrate wetting - i.e., spontaneous spreading of the liquid over the substrate is essential for leveling and occurs at the condition γsv > γlv. Leveling agents are used in coating formulation to fulfill this condition. The term "substrate" also includes impurities such as dust or oily particles, present on the coated substrate. Leveling agents strongly contribute to an appealing surface of the coating and avoid the formation of defects such as cratering, fisheyes, pinholes, and orange-peel film appearance.

Defoamers: To modify interfacial tension between paint and air. The function of surfactants as defoamer, anti-foaming agent or de-airing agent is related to interference with foam stabilizing surfactants. Strong intra-molecular forces between surfactant molecules play a dominant role in foam stabilization. Surfactants, demonstrating weak intra-molecular interactions at high surface activity replace foam stabilizing components at the air/ liquid interface and therefore reducing foaming

Obviously, the selection process of surfactants for a particular coating application is very complex and requires detailed analysis of all ingredients and processes involved.

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