Chinese scientists have discovered a way to combine high-tech light-absorbing nano-technology with more conventional water-based carbon pigment spray to coat car bodies with a durable ultra-black finish. The result is a black that creates a ‘black-hole’ effect of virtually no reflected light at all.
The Chinese color research is being fueled by a frantic trend in its domestic car market where black is definitely this year’s color of choice.
Where once white took two-thirds of car sales in China, black has surged in popularity to almost a quarter of the market, meaning 7.5 million new black vehicles will roll onto Chinese roads this year. This scale dwarfs European and North American national markets combined, creating an enormous economy of scale for manufacturers of black paints. Black cars are currently the most highly desired as the ultimate symbol of status and luxury, and in a strange twist on Henry Ford’s famous maxim, in China it’s now the blacker the better.
Nano-tech black has been used with limited success over the last decade or so by creating microscopic atomic-level ‘forests’ of vertical tubes of carbon. We reported on the technique in 2019 when a BMW concept X6 sported the new ultra black and was described as ‘the blackest car in the world.’ The industry has been on the hunt for practical ways to achieve this effect ever since.
The nano-black proved hard to apply and too fragile for car coverings. The new Chinese hybrid coating mixes in some more conventional carbon-black pigment without losing the effect and most importantly, can be sprayed on to car bodies in the normal way. The Shanghai team showed off a car model sprayed in the super-black coating recently. The process doesn’t seem to have diluted any of the nano blackness, it still absorbs an incredible 99.90% of visible light wavelengths. Even more promising for the auto industry, the coating showed excellent long-term stability when the researchers exposed it to water and humidity tests.
"In China, car color has become a key selling point," says research chemist with the Color Technology, Group Core R&D Shanghai, Nipsea Group, Zhiwei Liu. "Deep black finishes have long been the premium choice and signature color for luxury cars due to their elegant appearance, powerful visual impact, and luxurious undertone."
Automotive companies, he says, are actively pursuing "mass-processable ultra-black coating solutions with extreme blackness." In the future, the researchers may use gradient refractive index technology with the new hybrid black. This GRIT technique creates graduated layers through the coating that reduce surface reflection and soak up even more light.
Let's have a quick recap on the tech involved here. A Vertically Aligned Carbon Nanotube (VACNT) array coating, often referred to as a ‘CNT forest,’ is a high-tech coating created on a microscopic scale. A nanometer is one billionth of a meter, so we’re dealing with stuff the size of atoms here. The layer has billions of carbon nanotubes tightly packed and standing perpendicular to the underlying surface. This creates the unique level of light absorption. VACNT arrays are some of the blackest materials in existence. Their porous structure traps 98% to 99.9% of light across the UV, visible and far-infrared spectrums.
Way back in 1980 Douglas Adams’ science-fiction novel The Restaurant at the End of the Universe describes a spaceship so black that "light just seems to fall into it." It was used to block out the sun during rock concerts for dramatic effect. The idea of blackness like that has been pursued by scientists for decades.
It seemed to have arrived when in 2014 the UK-based Surrey NanoSystems developed Vantablack, a coating made from vertically aligned carbon nanotubes. It gave objects an almost 2D appearance. It became famous when artist Anish Kapoor acquired exclusive artistic rights to use it. We covered the early progress, and in 2016 the same technology was used for a very different application: reducing light reflections within an ultra-sensitive space telescope.
A few years later, when BMW presented its Vantablack-coated X6 it drew widespread attention, but many observers suspected the practical limitations of its delicate, expensive nature would prohibit its use on an industrial scale.
This new Chinese hybrid material seems to have overcome those limitations. It allows interactions between the two carbon materials to let pigment particles arrange themselves along the nanotubes in a rough landscape of microscopic peaks and valleys acting as optical traps. Combined with the already strong light-absorbing properties of carbon black, it enables the mixed coating to absorb more than 99% of visible light.
The Shanghai researchers say the coating can be applied using conventional automotive spray-coating techniques. It also passes humidity, water-resistance and adhesion tests, making it more applicable than any predecessors.
The research has been published in the journal Matter & Light.
Whether the new coating is truly the blackest black may be difficult to determine. Experts say that the biggest challenge at that extreme level of blackness is actually measuring it. Some ultra-black coatings now enter the realm of what scientists call 'the four nines' – that’s 99.99% light absorption.
Source: Cell Press via EurekAlert
