Paint it black

Solar-reflecting pigments promise lower roof heat absorption to keep homes cooler, cut energy consumption and extend the roof’s lifetime. Siobhan Wagner reports

Black pigments designed to reflect the sun’s heat could decrease the amount of stress on materials and save energy if used in coatings on domestic roofs.

Developed by BASF, the pigments differ from carbon black — the standard black pigments — because they reflect, rather than absorb, most of the invisible near infrared (NIR) radiation, which accounts for more than 50 per cent of the total incident solar energy.

The company is offering three grades — Paliogen Black, Lumogen Black and Sicopal Black — which have different NIR reflecting capabilities.

Paliogen and Lumogen both reflect up to 45 per cent of the total incident solar radiation energy, and Sicopal up to 30 per cent — and nearly all of the NIR, which is responsible for heat build-up.

By comparison carbon black has a known total solar reflectance (TSR) of less than five per cent. In trials, the lower NIR absorption compared to other black pigments resulted in a temperature decrease of up to 20ºC on the surface. So if these pigments were used in a roof coating they would not only be able to reduce material strain by releasing thermal expansion, but would also be able to keep homes cooler and save energy used by air-conditioning.

However, the three grades exert this effect in different ways. Paliogen and Lumogen, 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.

In this way Paliogen, which is most suited for coatings applications, only exerts its effect with a reflective substrate or in combination with reflective pigments. but it still has a high TSR value and a high colouring capability as an organic pigment.

Lumogen pigments are ideal for colouring plastics, where a multi-layer system is used. In addition to their NIR properties they are claimed to have good temperature stability and other resistance properties. This means they could be used for plastic panelling, UPVC window frames and even the entire interior trim of vehicles.

Sicopal Black reflects the NIR radiation on its own and thus functions independently of the substrate. This is said to make it suitable for use in both coatings and plastics applications, and the inorganic pigment has excellent weather, temperature and chemical resistance.

John Martin of BASF said: ‘In the plastics market they [the pigments] can help prevent heat expansion, allowing for larger parts, and in some cases improving the lifespan of parts susceptible to degradation due to higher temperatures.’

He added: ‘It is now possible by pigment selection to control the thermal expansion of materials, by control of the temperature build-up. You can therefore get equal coefficients of expansion from different colours.’

BASF worked with Nottingham University to demonstrate the pigments on the roof of a test house, where the coatings were applied to a low-carbon roof made of lightweight steel.

‘By lowering heat absorption through the roof the home remains cooler, energy consumption is lower in summer and the roof’s lifetime is extended,’ said Mark Gillot, co-director of the Institute of Sustainable Energy Technology, and project manager for six research houses on the university’s campus.

While keeping homes cool might not be a big issue in the UK, Martin said the technology is drawing much interest from certain parts of Asia, the US and the Mediterranean.

When applied extensively over large areas, the coatings are also claimed to counteract the urban heat island effect — the overheating of entire metropolitan areas during the summer.