Cool Coatings for Aluminium Building Materials

So-called "cool roofing" materials refer to roof materials with properties to keep the surface cold under sunlight exposure. Such "cold" materials can have a dark colour in the visible region, but "white" (highly reflecting) in the near infra infrared region of the solar spectrum. Approximately half part of the energy in solar radiation is situated in the near infrared region. Since infrared radiation is not visible to the human eye, the reflection of infrared energy from the sun can be attained without altering the actual surface colour. Hence, cool roofing materials absorb significantly less solar heat than traditional materials of the same colour and the surface temperature can therefore be significantly decreased. While traditional dark roofs can reach temperatures of 60-70°C or more in the summer sun, a cool roof under the same conditions could stay more than 30°C cooler. Benefits of cool building materials include: - Improved indoor thermal comfort of buildings - Reduced cooling costs for air-conditioning and reduced CO2 emissions - Improved system lifetime - Reduction of urban heat island effect More than 40% of Europe's energy is used within building. The project "Cool Coatings for Aluminium Building Materials" will contribute to reduced energy consumption in buildings. The project is also based on use of recycled aluminium, which consumes only 5% of energy compared to primary aluminium. The project will hence contribute to the requested "green shift" in Europe. The project had a time frame of four years, 2015-2018, and resulted in a completely new type of roof and façade panels, manufactured of coated aluminium. The new product is not only "cool", but also aesthetical and functional. New coating systems based on special pigments and organic polymers are developed and tested. These coatings have "cool" properties and are capable to reflect the sun energy in the near infrared (NIR) region and simultaneously take advantage of the exceptional reflection properties of the aluminium substrate. One of the challenges in the project work was to formulate the coating systems in the relevant colour shades requested by the market, like grey or terracotta, keeping the "cold" properties intact. Several colours, typically used in building industry have been produced and tested for weather resistance, in accelerated tests and outdoor exposure Florida. Pilot production in full scale was completed, using two selected "cool" coatings, applied by coil coating on recycled Al, at one of the coating lines at Hydro Holmestrand. The materials have good reflection properties, additional to good outdoor performance. Long term outdoor testing is on-going. Panels are exposed at Bohus-Malmön, Sweden, for corrosion, and Florida, for UV test. To maximize the cooling effect, the project has also worked to reduce thermal emittance of material reverse side. Consequently, line produced materials were coated on reverse side with a lacquer which contributes to lower thermal emittance. Use of aluminium substrate is also in this aspect an important advantage. Testing the cooling effect of "cool" materials is performed at "Zero Emission Building" laboratory Toulouse, showing that the outside surface stays cold, contributing to keep inside temperature lower, compared to standard materials in same colours. Further on, testing of cool effect for "cool" materials with insulation is initiated. Type and thickness of insulation are selected from typical specifications for buildings in south Europe. Aluminium substrate had an important role in the "cool" materials. Optimisation of the surface pre-treatment to achieve higher sun reflection and corrosion performance have been central and important part of the project work. Full scale trials performed on the coating line at Hydro Holmestrand, with optimized pre-treatment, confirm excellent performance of re-cycled aluminium for this application. Regarding energy consumption in buildings, it has been investigated how climate and use of insulation influence energy savings when using the new "cold" materials from the project, both in roof and façade. Not surprisingly, the greatest effect is for buildings located in warm climate. For well-insulated buildings in Nordic climate, the effect of cool roofing is much lower. Conclusion : The results of this project show that re-cycled aluminium, in combination with "cool" coatings, is highly suitable to produce "cold" materials for roofing and facades. The new "cold" materials have both attractive appearance and good outdoor performance. They achieve higher sun reflection in the NIR-region, compared to standard materials in similar colours, confirming the capability to reject large amount of sun warm. The high cooling effect of the new "cool" materials is mainly based on the good reflection properties of the aluminium substrate used in the project and the new cool coatings.

New developed cool coated aluminium for roof & facade panels is a product which combines the cool function additional to the aesthetic aspect. The new product is differentiating and outperforming existing steel cool products and can contribute to acquire increasead market share in the segment of metal roofing / facade. The new product is based on recycled aluminium, with improved reflection and corrosion properties,coated with a novel cool coating, resulting in high reflective roof or facade panels, with ability to reduce the indoor building temperature, decrease energy use for air conditioning, reduce the CO2 emissions, reduce material thermal stress and increase the system lifetime, contribute to reduction of the urban heat island effect. Use of recycled aluminium, in synergy with the cool coating makes the cool coated aluminium materials an important contributor to the green shift.

The underlying idea of this project is to develop aluminium building products that will differentiate from and outperform existing steel products and thereby acquire an increased market share in the increasingly important market segment of metal roofing. This can be achieved by inventing a new coating system for cool roofing. The coating will be applied in HARPs existing coil coating production line. Further development and understanding of the pre-treatment process is essential to meet the main goal and the market request for increased use of recycled aluminium. A specific challenge for the project is to develop coatings with selected colours and corresponding TSR (total solar reflectance) values that are higher than what is achieved for cool coil coatings today. On the interior surface of the roofing material a low thermal emittance may be desired (depending on the design of the roofing system). A second target for the project is to develop a low-emission coating for the reverse side of the sheet metal.