With global warming, hot summers have become more and more frequent. Reflective roofs or cool roofs offer an ecological solution to decrease the interior temperature of buildings by up to 6° C, effectively reducing the use of air conditioning and therefore the energy bill. However, to be effective, these white roofs must retain their whiteness for a long time. Kynar Aquatec® PVDF emulsion has been specially designed for very long-lasting reflective paints. White roofs, protected by a UV-resistant, dirt-resistant and cleanable Kynar Aquatec® latex-based topcoat, offer a solar reflectivity throughout the life of the coating that is far superior to that of traditional paints.
Seen from the sky, the roof of the Géant Casino in Valence Sud is immaculately white, almost blinding. But most of all, it is particularly effective in fighting against the heating of the building during hot weather. This cool roof is the largest in France. 18,000 m² covered with reflective white paint, a technology designed to reflect all the radiation that hits the buildings, and thus limit the heat under the roof and ultimately reduce the use of air conditioning in the building - up to -30%. Most importantly, this roof is finished with Kynar Aquatec® emulsion, a PVDF (Polyvinylidiene Fluoride) latex which optimizes and prolongs the whiteness, ensuring optimal reflectivity and efficiency. Arkema's recently developed Kynar Aquatec® latex has inherited the outstanding resistance properties of Kynar 500® FSF® PVDF resin, which has been the benchmark for coating metal surfaces in large buildings around the world for several decades.
A reflective paint made of Kynar Aquatec® features the outstanding properties of Kynar 500® PVDF resin
Let’s leave the South of France, heading instead towards the Louvre Pyramid in Paris. Since the structure was built in 1988, Kynar 500® PVDF resin has been protecting the steel structure of this architectural masterpiece from bad weather and pollution. This fluoropolymer combined with various additives acts as an invisible shield (topcoat) applied to the primary layer of paint (base coat). To its record of accomplishments, we can add the roof of Wimbledon’s Centre Court, the Pearl Tower in Shanghai, and the Renaissance Center in Detroit, along with a great many buildings and skyscrapers around the world.
Kynar 500® PVDF resin was developed more than 50 years ago to meet the durability requirements of metal coatings used in architecture. “This technology has so many qualities that our engineers wanted to develop an PVDF/acrylic latex to formulate a paint that can be applied, no longer just on metal in the factory using lacquering and curing, but directly on-site with traditional painting tools,” explains Agnès Beaugendre, head of Arkema’s fluoropolymer paint development. And they did it. The latex developed was soon to become a champion of cool roofing.
Kynar Aquatec®, a water-based PVDF/acrylic latex that enables highly efficient and durable solar reflective coatings
This innovation was made possible thanks to a decisive advance during its synthesis. Arkema’s R&D succeeded in creating a PVDF/acrylic emulsion without using solvents other than water! The resultant formulated coating, when used as a topcoat combines PVDF’s exceptional resistance to aging, soiling, and mildew with the advantage of a water-based acrylic emulsion that can dry at room temperature.
“The first samples of these novel paints that were subjected to aging under the sun in Florida are almost 20 years old and have hardly changed in appearance or integrity. These tests, correlated with those carried out in the accelerated aging chamber, show that white paint with Kynar Aquatec® PVDF/acrylic binder does not turn ‘flour-like,’ contrary to what can be seen after three or four years with many conventional acrylic paints. As a result, this novel latex helps to combine reflectivity and longevity,” says Agnès Beaugendre. This latex “benefits from the characteristics of PVDF, which has almost unbreakable carbon-fluorine bonds. In other words, it would take an energy much higher than natural levels of radiation to degrade our polymer and yellow it, for example.”
Preventing the soiling of white roofs
The durability of cool roof paints made with Kynar Aquatec® latex has many advantages. As an example, it reduces the amount of dirt on the roof from pollution and facilitates cleaning at a lower cost. “This is essential because what determines the quality of the cool roof coating is its ability to reflect UV and infrared rays over time,” emphasizes Agnès Beaugendre. “A paint with degraded film, on the other hand, will develop a roughness allowing stains and micro-organisms to become attached. The uniformity of the PVDF/acrylic latex based coating and its unchanged surface make it less vulnerable to dirt retention and make it easier for the roof to be cleaned by rainwater. It’s like the difference between a cloth and an oilcloth: on one, the dirt and stains become attached. On the other, they slide off,” says Agnès Beaugendre.
When it comes to implementation, a PVDF/acrylic latex based paint can be applied to all types of materials (bitumen, cement, synthetic membrane). “The substrate must be well prepared and cleaned. Above all, the application of the two-layer system – the primer + topcoat – must be done on a durable medium. The idea is not to repaint a roof that is too damaged, especially since we are offering a solution expected to last at least 20 years.”
Moreover, Arkema’s Encor® Flex acrylic emulsion is an undercoat that is perfectly tailored to use with Kynar Aquatec® emulsion.
A reflective roof paint with reasonable cost
The cool roof topcoat based on Kynar Aquatec® PVDF/acrylic latex is applied using conventional tools (spray guns, brushes). “What is important,” says an Arkema representative, “is that the topcoat is applied according to our recommendations. You have to be very rigorous. In particular, adhere to the temperature and humidity ranges that we recommend. We always recommend that our customers work in collaboration with trained and certified application specialists.”
The cost announced by Cool Roof France, the leading company on the French market, amounts to around €20/m². The operation includes cleaning of the substrate, application of the primer and the topcoat. The cost of the PVDF/acrylic latex based topcoat is much lower than the cost of preparation of the surface to be coated, as it is not necessary to apply a thick layer to benefit from its properties. “We are talking between 50 and 75 microns. It’s three to four times less than with a conventional acrylic paint. So you use less paint. This, added to the fact that repainting less often generates savings,” explains Agnès Beaugendre. “Finally, using water as a solvent significantly reduces VOCs. The novel latex also does not contain fluorosurfactants or other highly regulated compounds such as alkylphenol ethoxylates (APEOs).”
All these arguments demonstrate a significant added value and contribute to an optimization of the building's maintenance costs. With a Kynar high performance cool roof coating, roof life is extended, and it provides better protection for the membrane that ensures waterproofing of the roof. Add to this an improvement in the building’s energy balance and a reduction in urban heat islands. What could be better than that?