The quest for the self-cleaning car

  • 01-Apr-2015 04:14 EDT
self-cleaning.jpg

Water beads up on a super-hydrophobic surface, creating spherical droplets that roll along picking up dirt.

As the winter-that-wouldn’t-leave finally does so and many car owners at last dig out their buried cars, the next big issue is getting a car wash. But wouldn’t it be great if cars just shed road dirt, grime, and salt automatically?

Such a possibility could be approaching reality as chemists at University College London (UCL) and other universities have unveiled a new tough, self-cleaning paint system that maintains its dirt-shedding properties in everyday wear and tear. In tests, the resilient coating worked even after being wiped, scratched with a knife, and scuffed with sandpaper forty times.

The coating—a thin layer of titanium dioxide nanoparticles covered with a waterproof veneer that can be applied to steel, glass, and other surfaces with spray adhesives—may eventually find application as automotive paint, glass and lighting coatings, even as a protectant for the surfaces of solar cell panels. More development work will be needed to determine if the paint can meet the car industry’s requirement for a glossy surface and other needs, the researchers said, but the likes of Magna International and Land Rover have already inquired about their studies.

Self-cleaning paint

Self-cleaning surfaces work by being extremely repellent to water—a property known as super-hydrophobicity—but often stop working when they are damaged or exposed to oil and grease, said Claire Carmalt, UCL professor of inorganic chemistry, whose research focuses on the development of functional coatings.

Self-cleaning properties, she explained, are based on two effects that combine so that surfaces reject any contact with water. “To achieve a super-hydrophobic surface, what people might know as the lotus-leaf effect,” she said, “you need two things: a textured, rough microstructure with multiple tiny protrusions plus what we call a low-energy surface, one with a very low affinity for water—a waxy coating, for example.”

“Rather than spreading out like normal and wetting the surface, water on a super-hydrophobic surface beads up, creating spherical droplets like marbles that roll along picking up dirt, viruses, and bacteria along the way,” Carmalt said. “The droplets act like tiny miniature vacuum cleaners.”

Most coatings rub off

Unfortunately, most existing such ultra-waterproof coatings are mechanically weak. They typically rub off easily and fail to function when exposed to oil, she continued: “The biggest challenge for the widespread adoption is finding a way to make them tough enough to withstand everyday damage. But by pairing our paint with different adhesives, we’ve shown it is possible to make a robust self-cleaning surface. We used materials that are readily available so our methods can be scaled-up for industrial applications.”

The study, which was led by Ivan Parkin, head of UCL’s materials and inorganic chemistry research group, and included Carmalt, doctoral candidate Yao Lu, as well as researchers from Imperial College London and Dalian University of Technology in China, was recently published in Science magazine. (http://www.sciencemag.org/content/347/6226/1132.abstract?sid=bd3ee6e4-2734-4351-94d9-7eb99ee7b21a)

According to the report, they “have created an ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticles that forms a paint that can be sprayed, dipped, or extruded onto both hard and soft materials to create a self-cleaning surface that functions even upon emersion in oil. Commercial adhesives were used to bond the paint to various substrates and promote robustness.”

“We used two sizes of titanium dioxide nanoparticles to create the uneven surface,” Carmalt said. “A waxy surface—a low-energy surface with low water affinity—was then created by coating the particles with perfluorosilane, a relatively common chemical compound that produces a waterproof surface.”

The uneven titanium dioxide nanoparticles essentially form a scaffold that captures tiny air pockets that support the water droplets while the perfluorosilane rejects contact with water. “We found that material works relatively well when exposed to oil; the oil coating supported the water droplets,” she noted.

“Our paint worked extremely well for a variety of surfaces in tough conditions which were designed to simulate the wear and tear of materials in the real world,” Lu said. “For example, car paint frequently gets scuffed and scratched, and we wanted to make sure our paint would survive that.”

Previous dirt-proof paints

Water-resistant coatings for cars are not new. Nissan reported last year that it is testing a super-hydrophobic and oleophobic paint that is impervious to water and oils. The technology, sold by UltraTech under the name of Ultra-Ever Dry, was applied by the automaker’s engineers to a Nissan Leaf. And researchers at Eindhoven University of Technology in the Netherlands have been developing a surface coating called DynaClean that not only repels precipitation, but also adds some self-healing aspects.

Ultra-Ever Dry is not a paint, but a coating that is applied to the painted car in two steps. The coating consists of a base coat and a top coat. When applied, the aromatic solvents in the base coat evaporate and leave behind a polyisocyanate film, which bonds both to the substrate and the top coat. When the top coat is applied, the solvents evaporate and leave behind silica and a fluorocarbon coating.

One big drawback to Ultra-Ever Dry is that it is not clear. As applied, it is white and hazy, and so it is unusable on windshields or mirrors. In addition, it applies unevenly and its lifetime is uncertain as the chemicals are susceptible to exposure to the sun’s UV rays as well as detergents, alcohols, and solvents.

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