The potential for a new generation of solar reflective coatings. Lilian Bacco, Huntsman, Latin America

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1 The potential for a new generation of solar reflective coatings Lilian Bacco, Huntsman, Latin America Background In the coatings world, the growing global demand for energy reduction presents opportunities and provides the impetus for innovation. Apparent scope for a new generation of solar-reflective coatings is a prime example. The potential for energy savings enabled by these coatings is becoming increasingly recognized. The scene is set for new formulations that add more value and offer more choice than conventional solar-reflective coatings. In our view, cool roof and wall technology for buildings is probably the best current illustration of how solar-reflective coatings can play an effective role in helping to combat the rise in energy consumption, but the time is arguably right for exploring other potential applications for these coatings. Marine, automotive and aerospace products are likely candidates to benefit from a new generation of solar-reflective coatings. Who is to say the list ends there? How can solar reflective technology help reduce energy consumption? According to the International Energy Agency (IEA), buildings currently account for as much as 40% of primary energy consumption in most IEA member countries (1). The World Business Council for Sustainable Development (WBCSD) states that growing prosperity means that more people expect to have air conditioning to combat heat. The WBCSD also suggests that the energy consumed by heating, ventilation and air conditioning in buildings can account for more than 37% of total energy consumption and that by using cool roofing, 6-16% of cooling energy use could be saved depending on the climate (2). We are facing a challenge to reduce energy consumed by air conditioning and this could present an opportunity to the coatings industry. Solar reflective coatings can help to reduce the internal cooling load of a building. They do this by reflecting a portion of the sun s energy thereby reducing the temperature of the exterior surface and the amount of heat transfer into the roof or wall cavity and finally into the building s internal space. This reduces the need for air conditioning, which can result in reduced electricity consumption along with reductions in associated greenhouse gas emissions generated by power creation. A presentation given by Oak Ridge National Laboratory in 2009 (3) suggests that the cooling load of a building could be attributed to several factors (shown in Figure 1). Figure 1. Factors affecting the cooling load of a building Other 16% Windows 33% Internal gains 27% Roof 14% Walls 10% The pie chart indicates that there is a substantial opportunity for solar reflective roof and wall coatings to reduce the cooling load of a building by reflecting solar energy. There is a wide variety of research estimating the potential energy savings that could be realized using a cool roof; the United States

2 Department of Energy (DoE) has highlighted that typical energy consumption of a building with a low rise roof could potentially equate to per cent of total electricity used in the building (4) ; according to Levinson and Akbari, increasing the solar reflectance index of a roof from 0.20 to 0.55 could yield annual cooling energy savings of 5.02kWh/m 2 across the USA which would reduce carbon dioxide emissions by 3.02kg/m 2 (5). The benefits of using solar reflective coatings on walls appear to be less widely publicized but one study undertaken by Oak Ridge National Laboratory estimates that using solar reflective coatings on walls could save annual electricity cooling requirements by 4-9% when compared to walls without solar reflective coatings (3). While there is usually an increase in demand for heating in winter, when solar reflective coatings are introduced into appropriate locations and on the right building types, according to Levinson and Akbari this so called winter penalty is usually insignificant in comparison to the potential benefits of reduced cooling load (5). This is because in general the sun s winter rays carry little energy, are available for only a few hours each day and are often diluted by cloud cover. There can be many other potential advantages derived from the use of solar reflective coatings, these include: The potential to reduce air-conditioning unit size as a result of reduced cooling loads (10) Increased comfort for building occupants, particularly where air-conditioning is not installed (10) Diminished peak demand effects on the power grid, ranging from avoiding blackouts to financial savings for peak period consumers and reduced infrastructure requirements (5) Increased life expectancy of the roof system (10) Mitigation of the urban heat island effect which helps to reduce the formation of smog (6) Emerging standards for solar reflectance The United States introduced solar reflective coatings in the 1990s. They were used in cool roofing installations to carry out energy-saving research. This started the Cool Roof Rating Council (CRRC) program which now partners with the Energy Star rating scheme and has minimum performance specifications for roofs to be classified as cool. In Europe, the European Cool Roof Council (ECRC) has been formed to make an important contribution to mitigating climate change through increasing the energy efficiency of buildings, and is promoting the certification of cool roof products and their use across Europe. Also, in Europe, AFNOR(DTU59.1) has included a minimum solar reflectance value for all facade coatings used in France. In Asia, the Chinese Government is currently exploring the benefit of solar reflective technologies and is considering drafting new legislation to enforce their use. Japan is promoting cool roofing by issuing the JIS K standards for roof coatings with high solar reflectance. Solar reflectance the basics The sun s energy, reaches Earth as UV, visible and infrared radiation (demonstrated in figure 2). In order to obtain the highest solar reflectance, a white roof coating should be painted onto the exposed surface. However, not everyone wants white. Increased solar reflectance can be achieved in colored coatings by using specially designed infrared reflecting pigments. In order to increase the solar reflectance of a coating we need to concentrate on reflecting the infrared energy because for a coating to be a specific color it must reflect and absorb parts of the visible spectrum.

3 Figure 2. Terrestrial solar reflectance An innovative response to customer demand Huntsman has engineered a new titanium dioxide based pigment that preferentially reflects infrared radiation from the sun compared to Huntsman s TIOXIDE pigments that are designed to preferentially reflect visible light. When mixed with colored pigments the result is higher infrared reflectance in an unprecedented range of colorful coatings and polymers including dark and vibrant shades. Huntsman has produced hundreds of high infrared reflecting starter formulations for customers. These formulations have been color matched to a shade specified by the customer and they include the new infrared reflecting pigment. Customers have reproduced these starter formulations in their coating systems, optimising the solar reflectance performance using their colored pigments and Huntsman s novel infrared reflecting pigment. Huntsman has used a Cary 5000 spectrophotometer to confirm uplift in solar reflectance. Huntsman has also created software to help formulators predict the solar reflectance value that could be achieved using the new infrared reflecting pigment combined with a range of preferred colored pigments, including iron oxides. (11) The example below demonstrates the outputs: Example Huntsman has produced a theoretical color match for terracotta (L* 51, a* 29, b* 24.5) using two different formulations: Formulation Pigments used Titanium Dioxide*** ALTIRIS 550 pigment PY42** Solar reflectance PR101** Maximum predicted 7 Example * * * Example * * * *pigment volume concentrations quoted are for the quantity of pigment that would be required in the dry paint film should these formulations be produced (contact Huntsman for more details) **the example uses widely available colored pigments, however, results could differ depending on the manufacturer. Huntsman has identified these pigments for their optical properties on the basis of results obtained using proprietary computer software. ***TIOXIDE pigments Minimum predicted 7

4 Our tests indicate that when using exactly the same colored iron oxide pigments and substituting the titanium dioxide for Huntsman s new infrared reflecting pigment, we find significant uplift in solar reflectance as shown in the table. Customers are advised to verify that any pigment combination suggested by Huntsman is suitable for their intended application. (12) Optimizing Infrared Reflection The crystal size of the novel pigment has been engineered to give rutile crystal structure with optimized infrared reflectance. The pigment incorporates a patented dense silica coating technology designed to give the product very low photocatalytic activity. There are two products, one for dark colors and one for medium colors. ALTIRIS 800 pigment is recommended for generating increased infrared reflection in dark colors that have an L* value greater than 25 and less than 50 where very little pigmentary titanium dioxide would typically be used in the production of the color. This product is engineered to give optimal near infrared / visible reflectance ratios and has exceptionally low tint strength. Whereas ALTIRIS 550 pigment is recommended to raise infrared reflection in medium shade colors with an L* value greater than 40. Increasing solar reflectance using primers To obtain the highest solar reflectance in coatings, primers can be used. The novel infrared reflecting pigment described here has been designed to reflect infrared energy and when used in a base layer or primer, under a topcoat with infrared transparency, it can increase the solar reflectance of the overall coating system. Huntsman has produced a theoretical demonstration based on light-scattering theory to demonstrate the potential benefits of the pigment in a primer layer. The model is based on an implementation of the QCA (8) framework, which extends Mie theory to include dependent scattering i.e. concentration effects. Output from the QCA model backscattering coefficient at a given wavelength is then used to calculate reflectance using modified Kubelka-Munk equations. (9) This approach can therefore be used to predict the potential amount of infrared energy reflected e.g. by a pigmented primer. Figure 3. Percentage of near infrared reflectance for different volumes of titanium dioxide based pigments (11) % near infrared reflection ( nm) Volume concentration (%) New infrared ALTIRIS reflecting Novel pigment Pigment pigment TIOXIDE Titanium Titanium Pigment dioxide dioxide pigment pigment This graph has been produced using the theory stated above and shows that when the new infrared reflecting pigment is used in a primer, the degree of infrared reflection from that layer is superior to the

5 results that could be achieved using Huntsman s TIOXIDE titanium dioxide pigments that are designed to preferentially reflect visible light. (11) This translates to a potential solar reflection uplift of up to 6 percentage points. Summary Huntsman s new infrared reflecting pigment can help achieve new levels of solar reflectance in coatings and polymers in a wide range of bright and vibrant colors. This could increase design flexibility for specifiers to choose the colors they want with the solar reflectance they desire. Increasing solar reflectance through the use of the infrared reflecting pigment could benefit many as it could help reduce energy needed to power air conditioning systems, associated costs and greenhouse gas emissions (5). Increasing the solar reflectance of a structure s surface has significant implications for not only energy efficiency but also product durability. Reduced thermal cycling stress and coating temperature can lower the probability of delamination or peeling of the coating. This can result in extended product life and reduced product replacement cycles. (10)

6 References WBCSD, Energy Efficiency in Buildings, Facts and Trends Full Report 3 André Desjarlais, (2009), Oak Ridge National Laboratory, Energy Efficiency Benefits of Cool Walls 4 La France, P. M., (2009) US, Department of Energy, Energy Efficiency and Renewable Energy, Proposed Cool Roof Project for APP, BATF Tokyo (slide 5), 7 October Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants; Ronnen Levinson and Hashem Akbari; 14 March Cooling the cities A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments; Mat Santamouris; July Theoretical predictions have been made for coatings of 75 micron thickness and solar reflectance values are given as follows: Our most cautious approximation of the maximum solar reflectance that could be achieved over a white substrate (we call this an infinitely thick film) Our approximation of the solar reflectance over a black substrate 8 Varadan VK et al., Radio Science, 17 (1982), Tunstall DF and Dowling DG, J. Oil. Col. Chem. Assocn., 54 (1971), Cool Roofs in Europe, Initiatives and Examples, 11 Our specialist research team prepares coatings containing ALTIRIS pigment and control samples using TIOXIDE pigment with the same RAL index. We then compare Total Solar Reflectance using a Cary 5000 spectrophotometer. 12 All information contained herein is provided "as is" without any warranties, express or implied, and under no circumstances shall the authors or Huntsman be liable for any damages of any nature whatsoever resulting from the use or reliance upon such information. Nothing contained in this publication should be construed as a license under any intellectual property right of any entity, or as a suggestion, recommendation, or authorization to take any action that would infringe any patent. The term "Huntsman" is used herein for convenience only, and refers to Huntsman Corporation, its direct and indirect affiliates, and their employees, officers, and directors. ALTIRIS and TIOXIDE are registered trade marks of the Huntsman Corporation or an affiliate thereof in one of more, but not all countries.