Paint Surface Tension: Importance, Factors, and its Control

Paint surface tension refers to the cohesive forces that tightly pack the paint molecules. It defines how well the paint flows and adheres to the surface. Water has a high molecular cohesion of 72 mN/m due to its polar nature. Its positive and negative ends create strong cohesive forces and do not blend easily. Likewise, a droplet runs and does not stick well on high surface tension substrates (metal, oil, glossy surface). Unlike water-based paints, oil-based paints have surfactants in their formulation to lower the surface tension. Keep the surface tension of paint lower than the substrate to avoid bubbles and pinholes.

Why Is Surface Tension Important in Paint Formulation?

Here is why understanding surface tension is important in paint formulation:

Wetting and Adhesion: A paint with high surface tension does not spread easily on high-energy substrates like metal. Low surface tension paints (contact angle 0 oC) adhere better to the substrate.
Spreading and Coverage: Unlike highly resistant paints, low surface tension paints provide full coverage as they stick to the surface well. They hide ink stains and contrasting colors without any smudge marks.
Leveling and Smooth Finish: High surface tension paints do not settle on the substrate well, which leads to frequent orange peels. Adjust the interfacial tension of paints to achieve a smooth texture.
Penetration into Substrates: High surface tension primers and sealers do not soak into high-energy substrates. It increases the chances of craters, cissing, and telegraphy. Adjust the mixing ratio of additives and lower the paint surface tension to minimize this risk.

What Factors Affect Surface Tension in Paints?

Here are the factors that affect the paint surface tension:

Solvent or Water Content: Water affects the surface tension of paint due to its strong hydrogen bonding. It attracts the paint molecules inward and doesn’t let them spread easily. High solvents (acetone, water, neopentane) are directly linked to high surface tension.
Surfactants and Wetting Agents: Adding fatty alcohol sulfates, alcohol ethoxylates, and wetting agents ( methylcellulose, acacia, and pectin) in paint reduces the surface tension. Adjust the quantity of these surfactants to control surface tension gradients.
Pigments and Additives: Inorganic paint pigments do not dissolve in non-polar solvents, which causes poor adhesion. For example, when chromium oxide (Cr₂O₃) and iron oxides are mixed with non-polar solvents, they change the interfacial behaviour of paints.
Temperature and Humidity: High temperature increases the movement of molecules and pushes them away. It lowers their cohesive forces, like shear rate does the viscosity, and reduces the paint surface tension. Humidity levels also indirectly ruin the surface tension gradient. High moisture in the air affects the paint drying time and leveling.

How to Control Surface Tension in Paint?

Here is how to control the surface tension in paint:

Flow Agents: Add flow agents like dispersants to control the paint surface tension. They are amphiphilic molecules that break the intermolecular bridges of paint to lower the surface tension. Flow agents are used in automotive coatings and architectural paints for smoother application and enhanced finish.
Use Silicone-Based Additives: Add silicone-based additives with low molecular weight to reduce the paint surface tension. They contain hydrophilic and silicone components to improve leveling and paint performance. Mix the right ratio, as silicone-based additives also cause contamination if overused.
Pre-Treat Surfaces: Pre-treating the surface with the right primers and sealers helps to control the paint surface tension. Use flame treatment, high-frequency electrical discharge, and chemicals to control the surface tension of substrates.
Balance Resin-Solvent Ratio: A right mixing ratio of polar resin with polar solvents and inorganic binders with non-polar turpentine affects the surface tension of paint.

Why Is Surface Tension Control Important in Specific Paint Applications?

Here is why controlling surface tension is important before painting any substrate:

Automotive Finishes: High surface tension paints ruin the leveling of automotive parts. Metal and iron often have high interfacial energy. They require low surface tension paints that settle on the substrate with 0% contact angle.
Wood Coatings: High surface tension paints like acrylic, latex, and chalk paints do not soak into wood without preparation. Control the surface tension with additives to paint chairs, bed frames, and decorative items.
Architectural Paints: Adjust the surface tension of paint for a uniform coverage on fences, railings, and gates. Low-density paint works best for painting rough walls, ceilings, and trims.
Industrial Coatings: Epoxy or polyurethane bases are used on storage tanks, machinery, and pipelines to protect industrial equipment from rust. Mix them with a polar solvent for a uniform film without fisheyes.

How is the Surface Tension of Paint Measured?

Here are the effective methods to measure the surface tension of paints:

Du Noüy Ring Method: It is the most common method to measure the static surface tension of liquids or semi-solids. Heat a platinum-iridium ring on a Bunsen burner and pour the paint sample into a small beaker. Suspend the ring in the sample and then gently lift it until a lamella is formed. The tensiometer measures the pulling force, which is used to calculate paint tension. Use the formula γ=F/2⋅π⋅R for accurate values in mN/m or dynes/cm.
Wilhelmy Plate Method: Unlike the ring method, the Wilhelmy method uses a plate to measure the surface tension of paints. Hang this rectangular plate in the sample. Tensiometer measures the downward force that paint applies on the plate to sink it. Put in the values in the formula γ=F/L to find the static surface tension of paint.
Bubble pressure method: It is used to measure the dynamic liquid film tension. Take a test tube and pour the paint sample into it. Dip a capillary tube with a manometer to apply an air pressure. Record the maximum air pressure required to pop out bubbles from the manometer. Use the formula γ=Pmax⋅R/2 to get the ST of paint.
Contact angle measurement: This method is used in laboratories to test the wetting abilities of paint on different substrates. A camera or sensor examines the droplet to measure its contact angle. Adjust the paint formulation if your paint has poor wetting ( contact angle >90 oC).