Cool black for hot days

The Story

Drivers who can't find a shady parking spot in the middle of summer know what to expect: the black instrument panel, seats and steering wheel grow unbearably hot in the scorching sun. Dark surfaces of building roofs and facades also attract the heat, but light surfaces remain distinctly cooler. The reason for this difference: dark surfaces absorb the incident sunlight and convert it into heat, while light surfaces reflect most of the incoming energy.

Although these physical principles at first appear unalterable, innovative pigments from BASF nevertheless make it possible for surfaces to heat up much less in the sun despite their dark color: Paliogen® Black, Lumogen® Black and Sicopal® Black. In contrast to carbon black, the standard black pigments, they reflect most of the invisible near infrared (NIR) radiation which accounts for more than 50 percent of the total incident solar energy. “Because they swallow up the visible light completely like any conventional black pigment, the optical impression of blackness is preserved,” explains Ruth Bauer, marketing expert for special effect pigments at BASF.

In this way Paliogen® Black and Lumogen® Black reflect up to 45 percent and Sicopal® Black up to 30 percent of the total incident solar radiation energy. By comparison: for carbon black, this value known as “total solar reflectance (TSR)” is less than five percent. “In practical trials, the lower NIR absorption compared to other black pigments results in a temperature decrease of up to 20 degrees Celsius on the surface,” says Ruth Bauer. “This provides benefits not only for humans. The lower temperatures also mean that there is less strain on the material.”

But Paliogen® Black, Lumogen® Black and Sicopal Black offer heat-reducing benefits not only as pigments in purely black surfaces: paints, coatings and plastics in almost all other color shades also contain greater or lesser amounts of black pigments. If the BASF black pigments are used instead of carbon black, these colors also heat up much less in the sun. However, they exert this effect in different ways: Paliogen® Black and Lumogen® Black from the class of organic pigments, initially allow the NIR radiation to pass almost unhindered. What happens to it then depends on whether the substrate reflects or absorbs radiation.

“This is why Paliogen® Black L 0086 only exerts its effect in coatings with a reflective substrate or in combination with reflective pigments,” continues Bauer. “But then it has a higher TSR value. As an organic pigment, it has high coloring capability.” The new Sicopal® Black K 0095, on the other hand, reflects the NIR radiation on its own and thus functions independently of the substrate – although the effect can be further optimized by a reflective substrate. The inorganic pigment is particularly weather, temperature and chemical resistant.

Besides being used in automotive components such as the leather seats of the new BMW Cabrio, roofs and building facades are the main applications for these special pigments. In the “BASF House” in Nottingham, UK, inaugurated at the end of January 2008 as a model project showcasing the benefits of modern construction chemicals in a low-energy house, they are incorporated in the coating of the metal roof. “The low carbon roof is made of lightweight steel with a BASF Coatings AG coating infused with specially selected pigments that have solar heat reflectant properties – creating a colored cool roof,” explains Dr. Mark Gillot, Co-Director of the Institute of Sustainable Energy Technology, and research and project manager for the six research houses with comprise the ’Creative Energy Homes’ project on the University of Nottingham campus. “The coating is applied by the coil coating technique in which flat metal strips are coated and then shaped. By lowering the home’s heat absorption through the roof it remains cooler, energy consumption is lower in summer and the lifetime of the roof is extended because of the lower temperature strain,” adds Dr. Gillot.

“But roofs treated with this technique not only protect the occupants of individual houses against the summer heat,” comments Anne Heimes-Scheller, Product Manager for Industrial Coatings at BASF Coatings AG. “When applied extensively over large areas, our innovative coatings can also counteract the urban heat island effect, the overheating of entire metropolitan areas during the summer.” The principle of the coating that absorbs less solar radiation is attracting particular interest in Asia, the USA and the Mediterranean region. This is due both to the warmer climate and the construction techniques that use much less insulating material. While in the past it was mainly high energy-consuming air conditioners that provided tolerable indoor temperatures in these regions, in future these innovative roof coatings will also be counteracting overheating – and completely without electricity.

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The Prospects

The demand for NIR-reflecting black pigments for paints and coatings is constantly growing. There are many applications for these “cool paints”, ranging from roofs through facades to metal containers for international shipping traffic to protect their contents against the searing tropical sun. And their use is not restricted to coatings, because the pigments are also suitable for coloring plastics. As representatives of the organic pigments, the Lumogen® Black range of pigments are recommended for use in plastics because of their greater temperature stability. This opens up many possibilities for plastic paneling, window frames and not least, the entire interior trim of vehicles.

BASF offers its customers a special service in the form of its “CoolSim” computer tool: this simulation program allows BASF experts to work out for each desired color shade which pigment composition is optimal for the intended use in order to obtain the coolest possible surface – an enormous advantage for users who then don't have to conduct their own experiments.

The Info Box

From light to color

Visible and invisible radiation

What humans perceive as color is strictly speaking just a small part of the broad spectrum of electromagnetic radiation. The difference compared to other forms of this radiation lies in its wavelength: if it is between 400 (violet) and 700 (red) nanometers, it can be perceived by the human eye. If it is below, in the ultraviolet range, or above, in the infrared range, it remains invisible.

The solar spectrum

Ultraviolet accounts for only three percent of solar radiation intensity, while 39 percent of the radiation energy is found in the visible light spectrum. The largest portion of 58 percent is present in the near infrared (NIR) region at wavelengths between 700 and 2,500 nanometers. The NIR content of solar radiation therefore contributes most to heating up irradiated surfaces.

Organic and inorganic pigments

Pigments are color-providing particles that are insoluble in solvents and polymers. Like carbon black, they may consist of organic carbon compounds or inorganic metal oxides, such as chromium titanates. Their color is determined by what portion of the incident visible sunlight they reflect or absorb. For example, a white pigment reflects all the spectral components of visible light, whereas a black pigment absorbs them completely. Colored pigments, on the other hand, only reflect that part of the light that corresponds to their color.

Special effect pigments

Another important area of activity for BASF is special effect pigments which not only provide color but can also add a silky luster or beaming sparkle to surfaces. And even single pigments can produce colour travels over several shades depending on the observation angle. These pigments are always flake shaped and generally consist of a carrier material coated with metal oxides.