Black coating? Is black a color you can love? This achromatic color is certainly a mainstay for the arts, fashion, design and the automotive industry. And yet the fact that black isn’t simply black is something the experts at Wörwag are well aware of. After all, some black magic is usually needed when creating these coatings.
Black trousers, a black shirt – Falk Tragor had the appropriate outfit on when he arrived at the preliminary discussions. “I made a special effort”, as the Technical Director Industrial Coating explains. Ottfried Klein, who is responsible for the Top Coats for Transport Vehicles division, recommends wearing a yellow tie with the outfit to set a counterpoint. Ramona Baumgärtel, head of the Development Laboratory for Top Coats, counts black among her favorite colors to wear. And Wolfgang Fritz from product development explains why he doesn’t actually like black: “I used to have a black e-bike but I crashed while riding it.” Four experts on the color black, and four different standpoints. The color is always in fashion, and yet black isn’t simply black – it varies just as much as fashion advice does.
Everything starts with carbon black. The black powder is produced in industrial processes, and is one of the fifty chemicals with the highest production volumes worldwide. A good eight megatons (1 megaton = 1 million tons) are produced every year by the combustion of oils or gases, with around eighty per cent of this output being used to make rubber tires. A small proportion is used as a pigment in the manufacture of black paints and dyes. The particles of carbon black range between 10 and 300 nanometers in size, which is less than one thousandth of the diameter of a hair. This is why anyone who touches the particles ends up spending a long time scrubbing their hands clean again.
The darkest black of them all
The way we perceive a color depends, among other things, on the surface properties. However, as an achromatic color, black has assumed a special role, as a small side note on physics helps to clarify. Our eye perceives the color black when a surface reflects only an extremely small amount of light. This means the retina of our eye no longer picks up any more light waves in the visible spectrum. What this means when taken to extremes has been shown by British researchers. They have developed a paint-like nanotechnology that makes objects coated with it appear like black holes, and makes photos of them look like a poorly executed montage. Practically all the light is absorbed, with even laser beams vanishing into a void. Things don’t get darker than this. It looks even darker than Vanta black, which was previously considered the blackest black ever made. And a color that had been proved to absorb 99.965 per cent of the light striking it – including the ultraviolet and infrared radiation.
However, the small but subtle differences are also of great importance when developing high-quality coatings. Depending on the surface structure, the little light that is reflected can also be ‘distorted’. This is why the choice of measuring geometry is decisive when gauging shades of black. “Manual coloring using black requires extensive experience”, as Tragor adds. The finer the pigment, the more pronounced the bluish cast. The coarser the pigment is, the more yellow shimmers through the black. Sure instincts are the road to success. This is also true of the brightness – but caution needs to be exercised. If there is too much white, then the eye is quick to perceive the final color as grey.
Biggest challenge are single-coat solutions
The untrained eye only detects these nuances when making a direct comparison. Yet no-one lines up cars with different shades of black in a row. A comparison is easier to make using color tables: high gloss, silk gloss, silk matt, dull matt. Wörwag stocks a black that is suitable for just about all coating systems and purposes: powder coatings, UV coatings for interiors, and coating systems comprised of one to three layers. Of these, single-layer cover coats represent the biggest challenge.
Wörwag has developed a high gloss black called R4220 that can also be applied to a suitable substrate in a single layer. A coating is considered high gloss when it features more than 80 gloss units. R4220 features almost 90. This means that anyone viewing the coating will easily be able to see themselves mirrored in the surface. But there’s one critical issue from the perspective of development: the more gloss, the more focused the reflection of the light needs to be. “Added to this is the fact that the substrate must be suitable”, Baumgärtel adds. “The degree of gloss and the appearance also depend on the surface of the workpiece and any pretreatment.” R4220 is also extremely resistant to the effects of packaging, weather and scratching. “The trick is the choice of binding agent”, the head of the laboratory reveals.
Amongst other applications, R4220 is being used on the VW ID.3, delivery of which is planned from 2020 on. The original plan was to coat its roof with a black base coat and then clear coat. However, VW then decided on R4220 as a single-layer cover coat. This not only saves Volkswagen a process step, but also a painting booth and a dryer. It plans to make high-gloss black roofs the distinguishing feature of the entire VW electric fleet.
From the market to the museum
Developing the coating system in itself is not, however, sufficient. When using black colors in particular, diligence in production is not the only step that matters. The way the customer uses the coating is also decisive for the quality of the finish. “Cleanliness and conscientiousness are just as important for coating processes as they are in manufacturing”, Klein confirms. “You can see every flaw.”
In 2018, one in every four new vehicles was black. Almost all over the world. The popularity of black is much higher in the interior. Eighty per cent of all interiors are black, where along with soft haptics, high gloss is also in increasing demand. Black UV piano finish is one example. It is attractive to look at, however presents challenges in production. Klein recalls how lasering plastic components for illuminated operating controls caused problems. The particles that swirl down when doing so became lodge in the base coat. “Only cleaning it using CO2 after lasering solved the problem.”
300 to 400 new colors every year
Wörwag isn’t reinventing the color black. Customers almost always provide clear specifications, which then determine the exact color that Fritz subsequently uses. The head of Top Coats development division develops anywhere from 300 to 400 new colors every year together with his team. They are usually universal top coats that do not require clear coat. Following application, forced drying at 80 °C normally suffices. They are used to coat plastic add-on parts for passenger cars, commercial vehicles, agricultural and construction machinery, for example, as well as bicycles and shavers. 1C clear baking varnish is, in contrast, used for vehicle bodies. It cures at 140 °C. “We have ten standard shades of black in the drawer, most of them matt, a few high gloss”, Fritz explains. “We use them as a basis to build up on.”
When power coatings are used, the structure of the coating plays a major role – far more so than in other production lines. Tragor pulls four color tables painted black out of his bag, and turns them toward the window. The light is reflected differently on each table. This is how differences in their surfaces are made visible, from virtually high gloss to clearly structured surfaces. “What is known as a hammertone finish is largely typical for powder in industrial machinery”, Tragor explains, “and we can control these differences using additives. They structure or matt the surface.” Additives such as wax or minerals in powder coatings prevent the finish from becoming excessively smooth, and cause the light to scatter. A very fine structure results in a surface with a matt appearance.
Powder Coating in the MoMA New York
There are two shades of black which Tragor is particularly pleased with: an acrylic coating for rails on car roofs, and a polyurethane coating that is used by USM Haller to refine the finish on modular furnishings. In particular, the shelf units made by the Swiss company are considered design classics and have even been exhibited in the Museum of Modern Art in New York. “We can be proud of the powder coating system for USM Haller”, as Tragor underscores, “it remains unique on the market.”
Speaking of museum exhibits: the Russian artist Kazimir Malevich (1878–1935) created Black Square, a painting which represents the absolute zero of the visual arts.
A black square in which just about everything merges together – forms and colors. It is one of the most iconic paintings of the 20th century. And it is as hotly debated as black is in fashion. The experts in seeing black at Wörwag are likely to have an opinion on this as well.
The article “Why Wörwag likes black” was first published in the Wörwag customer magazine “finish” in 2019.
