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Rain repellant coatings

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Bryan Koene, Ph.D.
Director of Advanced Materials Group

Imagine attempting to land a multimillion dollar aircraft on a landing strip barely wider than the wingspan of the plane.  Now try to do this while the landing strip is moving – not only forward at 35 knots, but pitching back and forth at ever changing angles.  This is part of the job description for our highly trained US Navy pilots that must take off and land on moving aircraft carriers in the middle of the ocean on a daily basis.  As if this isn’t difficult enough, they may do all of this in the midst of a storm where wind gusts can blow the craft side to side as torrential rains sheet across the canopy, completely obscuring the pilots view.  Navy pilots certainly need nerves of steel to protect themselves, their fellow sailors and airmen, not to mention the extremely expensive weapon systems they are controlling.

[caption id="attachment_3419" align="aligncenter" width="300"] Figure 1. F35 Landing on an aircraft carrier (Photo:  http://www.wallsfeed.com/wp-content/uploads/2013/01/Aircraft-Carrier-Lockheed-Martin-F-35-Lightning-Ii.jpg)[/caption]

Maintaining a high level of visibility with aircraft canopies and windshields is exceedingly difficult under inclement operational conditions.  Unlike most commercial aircraft, fighter jets do not possess windshield wipers due to their operational requirements of high speeds and for the low observable (stealth) constraints of modern aircraft.  A durable, surface coating on the windshield to shed water would be an ideal solution to maintain visibility.  Off the shelf, commercially available rain repellent treatments have poor durability and weather resistance, and they cannot be applied to polymeric transparent substrates, such as aircraft canopies/windshields.  For this reason, Luna has been working with the U.S. Navy, Naval Air Systems Command (NavAir) to develop an extremely durable, transparent coating with very low surface energy to repel water and other liquids in order to keep aircraft canopies clear during rainfall.

There have been many academic and commercial efforts focused on the development of hydrophobic and superhydrophobic coatings that will shed water quickly off surfaces.  Whereas many technologies and coatings have demonstrated the primary goal of achieving watershedding properties, they are limited mainly to academic interest since they lack mechanical and environmental durability or have been too difficult or expensive to apply.  Luna’s rain repellent coating is a simple treatment solution that can be applied by the manufacturer to polycarbonate or acrylic-based canopies, polyurethane, metals, and glass.  This thin, hard coating is >99% transparent across visible spectrum with less than 0.5% haze.  The advanced coating is tough and abrasion-resistant, with a hydrophobic, low surface energy providing excellent watershedding properties (Figure 2) to meet the requirements of aircraft canopies. 

[caption id="attachment_3420" align="aligncenter" width="358"] Figure 2. Transparent hydrophobic coating demonstration with water spray: Coated side – business card is viewable throughout course of the experiment; Uncoated side - Water sheets over glass obscuring visibility.[/caption]

In June, 2013, Luna successfully coated two full-scale F-16 canopies within a transparency manufacturing and coating plant to demonstrate the scalability of the treatment and application process (Figure 3).  These coatings have demonstrated improvements over the current qualified polyurethane coatings with watershedding properties, abrasion-resistance, chemical-resistance (acid, cleaners, oil, jet fuel, etc.), adhesion, salt spray, and weatherability.

[caption id="attachment_3418" align="aligncenter" width="619"] Figure 3. F-16 canopy coated with Luna’s hydrophobic coating.[/caption]

Whereas these coatings have been developed by Luna for protecting multimillion dollar equipment, they are inexpensive enough to be applied to common use items such as eyeglasses for water, oil, and dirt protection.  The applications are endless for a low surface energy coating that can prevent surface fouling including displays, signs, automotive windshields, skylights, solar panels, etc.  This technology is complementary to our fluid repellent textile treatment that we discussed in our April 24 blog post (https://lunainc.com/water-required-self-cleaning-clothing/).