Hydrophobic Coatings

What is a Hydrophobic Coating?

Water Drops on Hydrophobic
Surface of Grass

The terms hydrophobic and super hydrophobic 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” and “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 de-wetting. Where the water pulls away the surface seems to remain totally dry. A classic example is the leaf of the lotus plant. The surface of the leaves are so repellent to water that the surface is said to be “Super hydrophobic.” Water seems to form near perfect spheres when coming in contact with the leaf surface.

 

 

Water on Lotus Leaf

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.
But how can the degree of hydrophobicity be measured? This can be determined by measuring how “spherical” a water drop as is 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 inperceptible point . Water and water based materials will appear to bounce off a superhydrophobic surface. Although it seems that the super hydrophobic surface is exerting a force against the water droplet, pushing it away, that is not what is happening. Actually, there is simply no attraction between the surface and the water droplet.

Measurement of Contact Angle

If there was an attraction, the surface would be considered hydrophillic. Hydrophyllic means “water loving.” Hydrophillic surface attract water and will allow it to level out or wet out. This property is used, for example, on self-cleaning windows. Instead of the water forming discreet droplets on the glass, it will form sheets and quickly flow down and off the window, taking some of the dust and dirt on the window with it. Surfaces with a contact angle of 10o-80o are considered to be hydrophillic. Surfaces with a contact angle of 0o-10o are considered to be super hydrophillic.

To measure the contact angle, an instrument called a goniometer is used. “Goni” is the Greek prefix for “angle.” This instrument will deposit a drop of liquid onto the surface of the substrate and measure 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 properties is expanding rapidly.

 

Hydrophobicity and hydrophillicity are important considerations when applying most coatings, inks and paints. If a surface is hydrophobic, for example, polyethylene or polypropylene, most coatings will not stick to it. It is possible though to chemically modify this surface to allow coatings to wet out and adhere to it. Other surfaces may be sanded to become more hydrophillic. In some cases a special primer may be used to impart a hydrophillic surface. Printers often check the CA to see how readily a surface will accept a particular type of ink.

Goniometer

What makes a surface hydrophobic? Two factors, the chemical and topographical make-up of the surface. 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 ions sodium and chlorine.

 

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.

Water Molecule
Red (-)
Blue (+)

Another way to achieve a hydrophobic surface is by means of a 3-dimensional structure. As we discussed above, water with its 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 pull 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 super hydrophobicity.

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 super hydrophobic surface. Today, scientists are researching ways to create micro or nano-structure surfaces to achieve super hydrophobicity. 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 has limited in their practical application.

A droplet resting on a solid surface and surrounded by a gas forms a characteristic contact angle θ. If the solid surface is rough, and the liquid is in intimate contact with the solid asperities, the droplet is in the Wenzel state. If the liquid rests on the tops of the asperities, it is in the Cassie-Baxter state.

Nanoslic is a Hydrophobic Coating

NanoSlic coatings are hydrophobic with water contact angles in the 105o-107o range. 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.