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Fighting Ice with Specialty Coatings

THURSDAY, MAY 17, 2018

By Allen Zielnik


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We are just coming out of what has been, for much of North America, a particularly severe winter. The East Coast suffered a parade of four nor’easters and bombogenesis storms in March, and the southern region of the United States had record-setting snowfall. The storms started early in October and continued through mid-April, with heavy snows and frigid temperatures in the Great Lakes and upper plains. Snow and ice took a heavy toll on critical transportation, utility and communications infrastructure.

Whether you’re a climate change believer or denier, the predictions are that our weather is changing, with more extreme fluctuations becoming the norm. If so, the question for our industry becomes: Can specialty coatings such as icephobic products help to minimize the damage?

© iStock.com / Fabian Gysel

Anyone who has flown in wintry weather knows the process of spraying an aircraft with de-icing fluid just prior to takeoff, and pilots are cognizant of the dangers of ice formation while in flight.

Icephobic surfaces repel ice or prevent ice formation due to the surface morphology, it’s patterned topography on a microscale. This is a different mechanism from anti-icing surfaces that are formed by the application of coatings. Most anti-icing coatings are also hydrophobic in nature, while icephobic surfaces need not be hydrophobic. In either case, these surfaces work by lowering the adhesion strength between the ice and the surface. But this leads to other questions, such as: How is ice formed on surfaces in the first place?

The Severity of the Icing Problem

Anyone who has flown in wintry weather knows the process of spraying an aircraft with de-icing fluid just prior to takeoff, and pilots are cognizant of the dangers of ice formation while in flight. Likewise, many of us have had to scrape ice of our automotive windshields and used salt to deice the sidewalks. Falling ice can be dangerous, with many cities having to cordon off streets and sidewalks for safety from the building ledges above. Ice can also fall off bridge superstructures and overhead traffic signage. Ice can build up on communications antennas and disrupt signals, a fact the author knows all too well with home satellite television.

Further, ice buildup on overhead electrical transmission lines could lead to mechanical line failure or insulator flashovers, resulting in major outages and tower damage, such as the series of five successive ice storms that hit Eastern Canada in January 1998 that toppled thousands of transmission pylons and power poles. A more recent issue related to the rapid growth of wind farms for renewable energy is ice formation on the rotor blades, which can be thrown off at high velocity or cause imbalance of the rotors leading to structural failure.

What Kind of Ice Would You Like With That?

Ice formation on—and adhesion to—surfaces is surprisingly complex. Ice formation from supercooled moisture is a common mechanism. Water droplets do not freeze as soon as the air temperature drops below 0 degrees Celsius due to the latent heat release as water changes phase from liquid to solid. In fact, water can remain liquid until about -20 degrees Celsius under the right conditions. These supercooled water droplets can form during a temperature inversion below an advancing warm front, or where a relatively warm air mass overruns colder air. It can start as snow, but then melts as it passes through a warm upper-air layer, then is supercooled below 0 degrees Celsius as it passes through a subzero near-surface layer. This creates freezing drizzle or freezing rain when the water droplet strikes a surface close to or below freezing and instantly forms a clear glaze of ice.

© iStock.com / Gudella

Field-applied coatings present a special problem, as the spin coating and vapor deposition techniques often used in the preparation of icephobic surfaces are unsuitable.

The conditions needed for freezing rain to occur are quite specific, but common in the U.S. and Canada. At other times or locations, we are more likely to see rain falling onto already-frozen surfaces, or wet surfaces that freeze as temperatures drop overnight. Both mechanisms generally give rise to clear ice. Rime ice is a white ice that forms when water droplets in fog freeze to cold surfaces. Hard rime has a comb-like appearance, while less dense soft rime appears more spiky or feathery.

Ice pellets, also known as sleet, are small translucent balls of ice that are smaller than hailstones. They generally do not freeze into a solid mass, unless mixed with freezing rain, but can accumulate up to two inches and is heavier than snow. Lastly, graupel is produced under some conditions where snow crystals encounter supercooled droplets. When accretion continues sufficiently and the original snow crystal is no longer identifiable, graupel is produced, often in connection with ice pellets. Graupel is fragile, and generally falls apart when touched. It is more common in high-altitude climates and is both denser and more granular than snow owing to its rimed exterior, and fresh layers of graupel are unstable on slopes, and act as ball bearings below subsequent snowfalls, leading to avalanches.

De-Icing Techniques—Rx for The Symptoms     

The traditional methods of de-icing surfaces rely on three basic methods: thermal, mechanical and chemical. De-icing chemicals work by lowering the freezing point, such as with salt, de-icing fluids, etc., and have issues with application costs, longevity and environmental consequences. Mechanical means, such as mechanical vibration, are costly to implement and of limited effectiveness. Thermal techniques such as electrical heating, are expensive and protect limited areas. Further, none of these techniques prevents ice formation and accumulation.

The goal, where possible, is to prevent ice from forming and adhering at sub-zero temperatures, and this requires understanding the adhesion of the ice-surface interface where highly polar ice molecules strongly interact with the solid. The strong adhesion of ice to materials is mainly a property of the ice–solid interface. This physical adhesion involves three distinct forces: hydrogen bonding, van der Waals forces, and most importantly, electrostatic interaction. For the latter, charges on ice molecules induce equal and opposite charges on conductors such as metals, while the induced charge and resulting electrostatic attraction is smaller with insulators (dielectrics).

In addition to the electrostatic forces, the surface roughness plays a significant role in ice adhesion. To better understand this function, we need to understand hydrophobicity and superhydrophobicity, and it is easier to start with the opposite, hydrophilicity. If a water drop is placed on a hydrophilic surface, the drop will not retain is shape, but rather will spread and flatten out, appearing like a contact lens placed dome-side up. We can measure the contact angle by drawing a tangent line from the drop’s edge, like determining the slope of a pile of sand. For hydrophilic surfaces this angle is 0-90 degrees.

© iStock.com / eugenesergeev

The goal, where possible, is to prevent ice from forming and adhering at sub-zero temperatures, and this requires understanding the adhesion of the ice-surface interface where highly polar ice molecules strongly interact with the solid.

However, if placed on a hydrophobic surface such as waxed kitchen paper, the drop will ride high and mostly retain its shape. It will appear more like an underinflated basketball sitting on a surface. If a tangent line is drawn from the edge of contact, the contact angle will be 90-150 degrees and the diameter of the footprint will be smaller than the diameter of the drop.

Superhydrophobicity simply means that the contact angle is 150-180 degrees, making it extremely “non-wettable.” This is primarily achieved by making the surface non-polar, such as coating with a wax, so it doesn’t interact with polar water (oil and water don’t mix). The non-polar surface has low surface energy due to low intermolecular forces of attraction. In addition, features such as microtexture, which traps air, increase hydrophobicity. Ideally, an icephobic coating would be both non-polar superhydrophobic and possess optimum microtexture to minimize surface contact with the ice. While the latter is difficult to achieve with a typical film-forming coating, this is an active area of product research and development as the combination of low surface energy and morphology could offer the most effective approach for icephobic coatings.

Field-applied coatings present a special problem, as the spin coating and vapor deposition techniques often used in the preparation of icephobic surfaces are unsuitable. One innovative approach being developed at the University of Michigan involves modifying common rubbery elastomers, not for any hydrophobicity, but rather for their physical ability to reduce ice adhesion through a process called interfacial cavitation. As even a small force can deform a rubbery surface, the electrostatic attraction is reduced, breaking the adhesive bond. Since neither hydrophobicity nor delicate microtextured surfaces are required, the elastomeric coatings are quite durable and resistant to in-service environmental and mechanical stresses.

Most field-applied commercial icephobic coatings today are based on superhydrophobic and low surface energy principles and use several different base chemistries such as dimethyl siloxane, energy fluorodecyl polyhedral oligomeric silsesquioxane (POSS), other silca-siloxane based materials or are lubricant-infused. However, other technologies could be available.

One issue is that there are no standards that completely define or measure icephobicity. While the term implies low adhesion force between the ice and a solid surface, some use shear strength and some normal stress to measure it. If the term is used to define the ability to prevent ice formation on the surface, this depends on the time for a supercooled droplet to nucleate and form ice, which depends on several external factors. Yet another definition uses the impact bounce of water droplets at the temperatures below the freezing point. So, a universal standard for measuring coating or surface effectiveness is still lacking, as is a test method for ice release from a surface.

Even though standards and test methods have yet to be firmly established, the effectiveness of both manufactured ice-resistant surfaces and applied coatings is a key area of research and development. While perhaps not perfect, the commercial products available today could provide at least some degree of improved safety and reliability for today’s modern infrastructure and can only improve with research into man’s battle with ice.

ABOUT THE BLOGGER

Allen Zielnik

Allen Zielnik has 42 years of experience in both chemical and physical instrumental methods of analysis of materials. He has been with Atlas Material Testing Technology for the past 22 years, specializing in the effects of solar radiation, weather, and the environment on the durability and performance of materials and products, including coatings. A frequent speaker at various worldwide technical symposia, he is the author of more than 120 publications and conference presentations. Zielnik has degrees in electronics engineering and analytical chemistry.

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Tagged categories: icephobic; Protective coatings; Weathering

Comment from Marc Hirsch, (5/24/2018, 8:47 AM)

Really thorough article on an important topic, Allen.


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