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Hydrophobic, Hydrophilic: How Coatings Affect The Behavior of Water On Acrylic

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Two types of coatings can be applied to acrylic to change the way water behaves when it comes in contact with the plastic: hydrophobic and hydrophilic. Without a coating, acrylic is neither hydrophobic nor hydrophilic. Replex doesn’t apply such coatings in-house, but we occasionally contract third-party contract coaters to apply scratch-resistant coatings.

Hydrophobic Coatings

Hydrophobic coatings have extremely low surface tension. When water touches a surface coated with a hydrophobic coating, it beads into little balls. A hydrophobic commercial product that many are familiar with is the windshield product, Rain-X™. Even in heavy rain, windshields coated with Rain-X™ don’t require wipers because incoming water beads up and rolls off the windshield.

Hydrophilic Coatings

Hydrophilic coatings manipulate water in exactly the opposite way as hydrophobic coatings. They have very high surface tension, so water physically can’t form drops. Instead of beading, water spreads out in a thin, consistent layer. These are often called “anti-fog” coatings because foggy mirrors are the result of small, round drops of water forming and clinging to the surface. Hydrophilic coatings prevent this process. Instead, water forms a thin sheet, making the surface appear dry, even though it is completely covered with water.

The Two Coatings In Nature And Industry

Both hydrophobic and hydrophilic coatings are found throughout nature, from the water-repellent lotus leaf to the bumps on the desert stenocara beetle, which attract condensation. Industries of all kinds attempt to mimic these properties. When water flows over a surface to cool it, for example, hydrophilic coatings assure maximum contact between water and surface. On the surfaces of condensers in desalination plants, on the other hand, hydrophobic coatings allow all droplets to slide off and be replaced with new ones.

Hydrophobic and hydrophilic coatings can be expensive and difficult to apply. Before any manufacturer orders one or the other, it’s worth spending some time understanding what the coatings do, and what applications require their presence.

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