A fleeting model
Vehicle paint has nothing in common with a butterfly wing – at first glance. The developers at Wörwag know better. How nanostructures make new effects possible.
The orange paint of the sports car parked near the Wörwag office in Zuffenhausen shines and glitters in the blazing sun. Depending on your viewing angle, the color shifts from yellowish to reddish. This color effect also depends on the angle of the sun’s rays. It’s the same principle as applies with the structural colors of butterflies. For industry, nature is like a giant test lab.
Over the course of thousands of years, many plants and animals have adapted to perfectly fit their environment, securing their survival. It’s therefore no surprise that researchers and technicians look to nature for answers to complex technical challenges. As a paint and coatings manufacturer, Wörwag can learn a lot from nature. In nature, colors serve as signals to warn or attract, and the appearance can differ depending on the surface structure and refraction of light.
Since the 1960s, scientists have been studying a field known as bionics – where biology meets technology – which is a relatively young field of research. Instead of trying to copy nature, it is an attempt to learn from it and to draw conclusions: As with butterfly wings, different color effects on vehicles can be created with nanostructures.
Nano means very small: The thickness of such structures is in the range one tenth of a micron. A micron is 0.001 millimeters.
Surface structure of a Wörwag coating in close-up
It all began with “fish silver”
Vehicles paintwork consists of up to four layers of paint. The color is determined by the base coat. This first coat contains the color pigments and mica, which contributes to the pearlescent effect. Natural shine and shimmer have fascinated people for a long time.
The oldest pearlescent pigment, called “fish silver”, was made in the 17th century from the scales of the white fish – a type of carp. The scales were milled in water until the shimmering material could be separated out. It was a huge effort: 100 tons of fish were required for two kilograms of fish silver. So the search for a more efficient production method began.
Today, a special raw material is used to produce the pearlescent effect. Woerwag uses mica, which comes from India. The mineral is mined, then it is cleaned, ground, and coated with titanium dioxide, which is a metal oxide. Both mica and titanium dioxide are semi-transparent. Some of the light penetrates the outer layer of titanium dioxide, while some is reflected.
The portion of light that penetrates is refracted at the interface with the mica. The waves of the returning light overlap, resulting in amplification or attenuation at different wavelengths in a process called interference. This phenomenon results in the iridescence or pearlescence that we also observe with butterfly wings. In the coatings industry, this is called the color flop effect. Whether the light is reflected at this boundary layer of mica or titanium dioxide, or goes through unbroken, depends on the angle of incidence of the light. If the titanium dioxide layer is very thin, the effect is silverwhite.
Paint with color flop effect
Thicker layers result in darker interference colors. From matte mother-of-pearl shimmers to the full spectrum of rainbow colors, anything is possible. Although we don’t have the roof tile structures of butterfly scales for car paintwork, the principle is the same.
Obviously the color experts have a strong professional interest in the array of color found in nature. Either in the house for butterflies or on vacation in Central America, the colorful diversity of butterflies is more than impressive. Still Wörwag would not go as far as nature does with iridescent colors for car paintwork. People want the color flop effect for vehicles – but in moderation. After all, you still have to like your car ten years later.
The article was first published in the Wörwag customer magazine “finish” in 2013.
