The surface tension (ST) of a coating describes how the molecules/particles at the surface of a coating interact with each other. Typically, it is measured in dynes per centimetre (dyn/cm) or millinewtons per metre (mN/m). A high ST indicates strong cohesive forces at the surface. In the context of liquid coatings, this determines how the coating behaves if applied to a substrate with respect to wetting, adhesion, film thickness and overall quality. Coating manufacturers must carefully consider the factors and control parameters that influence the performance characteristics, properties and appearance that their products provide. Some of these key considerations and control parameters are discussed below.

Key Considerations

Wetting: ST affects the ability of a liquid coating to spread and wet the surface of a substrate. The lower the ST, the better a coating can wet and adhere to the substrate. Achieving good wetting is essential for coatings to form a smooth and uniform layer without irregularities such as pinholes, bubbles, beads or droplets.

Substrate Selection: Different substrates have different surface energies, which can influence how well a coating wets and adheres to them. Coatings with a ST that matches or is slightly lower than the surface energy of the substrate tend to have good adherence. Therefore, ST and energy-matching can be essential for achieving good adhesion.

Drying and Curing: ST can influence how quickly a coating dries or cures. A high ST can lead to a slow drying time.

Coating Thickness: ST can affect the thickness of the coating film. A coating with a high ST tends to form a thick film. A coating with a low ST may yield a thin film. Typically, spray coatings with a low ST can be sprayed readily in a uniform manner.

Durability and Performance: The ST of a coating can influence its long-term performance. Appropriate wetting and adhesion are crucial for a coating to resist environmental stresses such as moisture, chemicals and physical abrasion.

Overall Finish: ST can influence the final appearance of a coating, thereby affecting gloss and smoothness. Therefore, control of ST is crucial for achieving the desired finish.

Control Parameters

Modification of the ST of a coating is a common strategy for ensuring successful coating and achieving the desired performance on different substrates. Often, experimentation and testing are necessary to “fine tune” the coating for a particular application. Careful consideration of the coating formulation, substrate properties and application conditions are crucial. Some of the methods available to coating specialists for modifying, testing and controlling ST are discussed below.

Selection of Formulation Components

  • Surfactants are chemicals that can lower the ST of a coating. They contain hydrophilic (water-attracting) and hydrophobic (water-repelling) groups. Such groups can reduce the cohesive forces between coating molecules at the surface, allowing the coating to spread readily. Surfactants are used commonly to improve wetting and adhesion on various substrates.
  • Solvents: The choice of solvent can influence ST markedly. Different solvents have different values of ST. Selection of solvents with appropriate ST characteristics can enable “tailoring” of the coating to the substrate. For example, using solvents with a low ST can help the coating spread on substrates with low surface energy.
  • Resins and Polymers: Some resins and polymers have high surface energy and tend to increase the ST of the coating. Others have low surface energy and tend to reduce the ST, which can improve the ability of the coating to spread and adhere to substrates with low surface energy. The molecular weight and chain length of the resin or polymer can also influence ST.

Blending and Dilution

  • Mixing ratio: Controlling the mixing ratio of the various components in a formulation is a fundamental and versatile method used by coating manufacturers. This strategy allows tailoring of the ST of the coating to meet the specific requirements of different applications and substrates. The solvent, resin, polymer ratio and additive quantity can be tailored.
  • Dilution: The solvents selected for dilution should be compatible with the existing coating formulation. Compatibility issues can lead to problems such as phase separation, precipitation or changes in the physicochemical properties of the coating. One must choose solvents that are chemically compatible with the resins and additives in the coating.

Temperature Control

  • ST is dependent upon temperature. Most coatings have a low ST at high temperatures. At high temperatures, the kinetic energy of the molecules in a coating increases, leading to weak intermolecular forces at the surface. One must monitor and control the temperature carefully within an acceptable range because excessive heating can lead to evaporation of volatile components, changes in the curing/drying characteristics of the coating and potential safety hazards.

Use of Additives

  • Specific additives (e.g., flow and levelling agents) can be included in the formulation to influence ST. These additives can help improve the wetting and spreading of the coating.

Application Methods

  • The method of application of the coating can also influence its ST. Spray coating, brush application or roll coating can affect how the coating interacts with the substrate.

Testing and Verification

  • One must carry out tests to determine the optimal ST for a specific coating–substrate combination. This approach can involve using equipment to measure ST for assessment of the behaviour of the coating on different substrate samples and adjusting the formulation accordingly.  This can help determine the optimal combination of components and concentrations to achieve the desired ST.

To conclude, the goal of achieving the appropriate ST is to ensure that the coating wets and adheres well to the substrate while also providing other necessary properties (e.g., durability and chemical resistance). This approach requires a balance between various formulation factors (including ST modification) to achieve the desired coating performance. This is where ITAC can help. By selecting the appropriate formulation components and strategies, and through careful experimentation and testing, our experts can achieve the desired ST of a coating for optimal application and end-use performance.

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