Hydrophobic Coatings

What is a hydrophobic coating?

The terms hydrophobic and superhydrophobic are becoming more popular, especially with regard to coatings and surface treatments.

But what do these terms mean?

“Phobic” comes from the Greek root “phobos,” meaning “fear”, meanwhile “hydro” refers to water. It follows that the literal meaning of hydrophobic is “afraid of water.” The term hydrophobic commonly refers to a surface that repels water and water-based materials.

When water and water-based materials come in contact with a hydrophobic surface, individual drops of will bead up and will roll off or slide off the surface with ease. Larger amounts of water will pool together, the edges pulling back from the hydrophobic surface and move as one sheet.

This effect can also be described as dewetting, where the water pulls away leaving the surface totally dry. A classic example is the leaf of the lotus plant. The surface of the leaves is so repellent to water that the surface is said to be “superhydrophobic.” Water seems to form near perfect spheres when coming in contact with the leaf surface.

With the potential to impart properties such as stay-dry, easy-to-clean, waterproof, anti-corrosion, and reduced-hydro-drag, hydrophobic coatings and treatments are finding their way into more mainstream applications.

Hydrophobic Grass Photo

How can the degree of hydrophobicity be measured?

This can be determined by measuring how “spherical” a water drop is as it sits on the hydrophobic surface. As the hydrophobicity of the surface increases, the high surface tension of the water pulls itself away from the surface more and more into a sphere.

If a line is drawn tangent to the drop as it contacts the surface, then the angle (θ) between that line, and a line parallel with the surface, is said to be the contact angle (CA). Surfaces are considered to be hydrophobic if the contact angle is between 80o and 120o.

An example of historic importance is polytetrafluoroethylene (Teflon) with a contact angle of 109o. Polytetrafluoroethylene is well known for its repellent, non-stick and non-friction surface. Surfaces with a contact angle from 120o to 180o are considered superhydrophobic.

Theoretically, the maximum hydrophobicity is 180o and in this case, a drop is perfectly spherical, contacting the surface at a singular, almost imperceptible point. Water and water-based materials will appear to bounce off a superhydrophobic surface. It seems the superhydrophobic surface is exerting a force against the water droplet and pushing it away, however that is not the case. There is simply no attraction between the surface and the water droplet.

If there was an attraction, the surface would be considered hydrophilic. This means “water-loving.”

Hydrophilic surfaces attract water and allow it to level out or wet out.

This property is used, for example, on the glass of self-cleaning windows.

Instead of water forming discrete droplets, it forms sheets which quickly flow down and off the glass, taking some of the dust and dirt on the window with it.

To measure the contact angle, an instrument called a goniometer is used. “Goni” is the Greek prefix for “angle.”

Measurement

Hydrophilic

Surfaces with a contact angle of 10o-80o

Super-hydrophilic

Surfaces with a contact angle of 0o-10o

This instrument deposits a drop of liquid onto the surface of the substrate and measures the angle between a line tangent to the drop and a line parallel to the surface. Newer equipment will deposit the drop automatically and then measure the angle using a video camera and specialized software.

More sophisticated equipment and software programs enable the researcher to analyze the surface in different ways. In fact, surface science, or the study of surface chemistry and its properties, is expanding rapidly.

Hydrophobicity and hydrophilicity are important considerations when applying most coatings, inks, and paints. Coatings like polyethylene or polypropylene, for example, will not stick to hydrophobic surfaces. However, it is possible to chemically modify surfaces to allow coatings to wet out and adhere to it.

Other surfaces may be sanded to become more hydrophilic. In some cases, a special primer may be used to impart a hydrophilic surface. Printers often check the CA to see how readily a surface will accept a particular type of ink.

 

What makes a surface hydrophobic?

Chemical make-up

To understand the chemical make up of the surface, consider the chemical nature of water. Water is a polar molecule, which means that it carries a partial charge between its atoms of oxygen and hydrogen.

Oxygen, as an electronegative atom, draws the electrons of each bond closer to its core, thus creating a more negative charge. Therefore, any materials with a charge, be it negative or positive, will be able to interact with water molecules to dissolve. For example, salt easily dissolves in water. This is due to the charges of the sodium and chlorine ions.

So essentially, a hydrophobic material or coating, will have molecules on its surface that do not have a charge, meaning they are non-polar. By lacking a charge, these molecules do not have any charge-to-charge interactions that will allow them to interact with water.

Instead, they repel them. Examples of molecules that are non-polar may contain atoms of silicon, fluorine, chlorine or carbon. In many instances, these atoms are formed into polymeric molecules that form a wall of non-polarity at the surface.

Topographical make-up

Another way to achieve a hydrophobic surface is by means of a 3-dimensional structure. Water with high surface tension is attracted to itself and readily forms droplets when exposed to air.

If there are discreet projections or channels on the surface, whereby the water droplet flows around the projections, the surface is said to be in the Wenzel state. When the water droplet sits on top of the projections, with air contained within the 3-dimensional surface, most of the water is in contact with air.

The water will be pulled back into its spherical shape. The surface is said to be in the Cassie-Baxter state. This is the surface that will lead to a high degree of hydrophobicity. Additionally, when the surface is covered with non-polar molecules, the surface can achieve superhydrophobicity.

A well-studied example of this is the lotus leaf. When examined under magnification, the lotus leaf has a 3-dimensional structure.

The projections here are covered with highly hydrophobic waxy coating yielding a superhydrophobic surface. Today, scientists are researching ways to create micro or nano-structure surfaces to achieve superhydrophobicity.

However, with their textured surface, these coatings are prone to rapid breakdown due to abrasion and other mechanical stress. Some coatings must be reapplied frequently to maintain repellency. This limits their practical application.

Nanoslic is a Hydrophobic Coating

NanoSlic coatings are hydrophobic with water contact angles between 105o-107o. These coatings achieve this high degree of water repellency through a chemical reaction that takes place during the curing phase.

As the solvent dries, highly non-polar molecules align themselves and form the coating’s surface. Below this layer, the bulk of the coating polymerizes into an inert silica-based structure.

At the substrate surface, the polymer chemically bonds to hydroxyl groups on the substrate. The advantage of this approach is that a high degree of adhesion and abrasion resistance is achieved. Therefore NanoSlic coatings are an excellent choice for the most demanding applications and challenging environments.