Paint It Cool! Solar Heat Management in Paints and Coatings
'Cool' colors for hot days
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. All dark surfaces that are exposed to sunlight heat up strongly, while light surfaces remain distinctly cooler.
This is because dark surfaces absorb incident sunlight and convert it into heat, while light surfaces reflect more of the incoming energy.
Although these physical principles at first appear insurmountable, innovative pigments and preparations from BASF, for example
Paliogen® Black L 0086
Sicopal® Black L 0095
Xfast® Black 0095
Luconyl® NG Black 0095
make it possible to formulate surface coatings that significantly reduce the heating effect in sunlight, despite their dark color.
Carbon black, the most commonly used black pigment, strongly absorbs the invisible near infrared (NIR) radiation that accounts for more than 50 % of total incident solar energy. In contrast, BASF's ‘cool’ black pigments reflect or transmit the same radiation with very low levels of absorption. Because they swallow up the visible light completely, like any conventional black pigment, the optical impression of blackness is preserved. For carbon black, the “total solar reflectance” (TSR) is less than 5 percent, whereas Paliogen® Black or Sicopal® Black, for instance, achieve TSR values in excess of 30% (achievable TSR value is dependent on color, formulation and substrate), even in black or very dark colors. In practical terms, this improved TSR performance can result in a temperature decrease of up to 20°C on the surface. This not only provides benefits from a comfort perspective, but the lower temperature also results in reduced demand from the material itself.
Paliogen Black L 0086, Sicopal Black L 0095, Xfast Black 0095 and Luconyl NG Black 0095 pigments and preparations offer heat-reducing benefits in more than pure black applications: paints and coatings in almost all other color shades also contain greater or lesser amounts of black pigment. If BASF's functional black pigments are used instead of carbon black, these colors also heat up significantly less in the sun. This effect is achieved in different ways:
Paliogen® Black is an organic pigment and essentially transparent to NIR radiation. This results in the NIR radiation passing through the pigment with virtually no absorption. What then happens depends on whether the substrate reflects or absorbs the transmitted radiation. To achieve the desired solar reflectance, Paliogen Black L 0086 should be used to overcoat a reflective substrate or formulated in combination with reflective pigments. As a result, in lighter coatings in combination with NIR reflective pigments, e.g., titanium dioxide, even higher TSR values can be achieved, which makes this product unique in its performance in the market. As with most organic pigments, it has very high tinctorial strength.
Sicopal Black L 0095, Xfast Black 0095 and Luconyl NG Black 0095, on the other hand, self-reflect NIR radiation and can thus function independently of the substrate – although the effect can be further enhanced by using a reflective substrate. These inorganic pigments are particularly weather, temperature and chemical resistant.
BASF offers the broadest pigment portfolio in the market, including a full range of pigment solutions for solar heat management.
Our solutions provide the greatest benefits through:
Flexibility of color design, yet compliance with regulations
Increased reflectivity of coating and therefore reduced surface temperature
Lower cooling requirement, resulting in lower energy usage
Longer lifetimes for coating and substrate through reduced temperature strain
“BASF House” – building a sustainable future house
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 functional pigments. In the low-energy "BASF House" in Nottingham, UK, inaugurated at the end of January 2008 as a model project showcasing the benefits of modern construction chemicals, they are incorporated in the coating used on the metal roof.
The low carbon roof is made of lightweight steel with a coating formulated using specially selected pigments that have solar heat reflectant properties – creating a colored 'cool roof'. 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, as a result energy consumption in summertime is lower and the lifetime of the roof is extended because of the lower temperature strain.
But roofs treated with this technique not only protect the occupants of individual houses against the summer heat. When applied extensively over large areas, the innovative functional pigments in the coating can also counteract the urban heat island effect – the overheating of entire metropolitan areas during the summer. The principle of a coating that absorbs less solar radiation is attracting particular interest in Asia, the USA, the Middle East and the Mediterranean regions. 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 of buildings.
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From light to color
Visible and invisible radiation
What humans perceive as color is strictly speaking just a small part of the broad electromagnetic radiation spectrum. The differences between the various types of radiation lies in the wavelength: if the wavelength 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 radiation accounts for only three percent of solar radiation intensity, while 39 percent of the radiation energy is found in the visible light region. The largest portion, 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 heat-up effects in irradiated surfaces.