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26 POWDER COATING, April 2018 New developments in infrared reflective inorganic pigments According to the US Department of Energy (energy.gov), air conditioners use about 6 percent of all the electricity produced in the US and release ap- proximately 117 million metric tons of carbon dioxide into the air each year. While nearly half of the solar energy is from the infrared (IR) region, which is invisible to bare eyes, if coatings are able to reflect the IR portion of the sunlight and thus reduce heat absorption, en- ergy consumption will be reduced. Therefore, IR reflective pigments are becoming more attractive. They can be utilized in roof, window, and automo- bile coatings to provide temperature control and also protect heat-sensitive materials from thermal degradation. IR reflective pigments are based on the theory of diffuse reflection. When inci- dent light penetrates into the powder, it is reflected multiple times by grain boundaries of numerous particles be- fore exiting the surface at many angles, thus minimizing penetration. They are usually formulated from pure metal particles or inorganic complexes. Coating reflectivity depends on the particle size, refractive index, shape, distribution, and the polymer matrix, and is quantified by the Total Solar Reflectance (TSR) defined as the per- centage of irradiated energy that is re- flected by an object. The rule of thumb is that to achieve the highest reflectiv- ity, the particle size should be at least half the wavelength of the light to be reflected. Traditional white pigment titanium dioxide (TiO 2 ), for example, is optimized to scatter visible light with sizes in the 200 to 350 nanometer (nm) range. Infrared radiations range from 700 to 2,500 nm wavelength, and the heat-producing near infrared radiation ranges from 700 to 1,100 nm. Therefore, to reflect the near in- frared light, particle size should be at least 350 to 550 nm. New developments show that doping TiO 2 with other elements improves their reflectance and stability by creat- ing defects in the crystal lattice. Other pigment materials, such as silica, coated mica, cadmium stannate, zinc oxide, calcium carbonate, or a combi- nation thereof, can also be utilized to provide IR reflectance. As the pig- ments reflect only in the IR region, they can be formulated to have various color options. There are numerous commercial prod- ucts already on the market, but the op- timum pigment combination, optical properties, blending, and dispersion protocols are still to be optimized for broader applications. PC Editor's note For further reading, visit Powder Coat- ing magazine's website at www.pcoat ing.com and search the Article Archive by keyword, subject, organization, author, or issue date. All articles listed in the archive are available for free download to registered users. Marek W. Urban is J.E. Sirrine Founda- tion endowed chair and professor in the Department of Mate- rials Science and Engineering at Clem- son University, 299 A Sirrine Hall, Clemson, SC 29634; 864/656-1155; mareku@clemson.edu. Until 2012, he was a professor of polymer science and engineering in the School of Polymers and High Performance Materials at the University of Southern Mississippi-Hat- tiesburg (USM). After 13 years at North Dakota State University, he joined USM where he established and directed the National Science Foundation Materials Research Science Engineering Center (MRSEC) on Stimuli-Responsive Poly- meric Films and Coatings. He is co- director of the formally National Science Foundation Cooperative Research Center in Coatings and recently renamed Inter- face. He received an MS in chemistry from Marquette University, Milwaukee, Wis., and a doctorate in chemistry and chemical engineering from Michigan Technological University, Houghton, Mich. He has also done postdoctoral work in macromolecular science at Case Western Reserve University, Cleveland. Urban has written more than 300 research papers, several patents, and three books, and has edited nine books. He has won numerous awards, includ- ing the Technical Focus Speaker and Roon Foundation Awards (1999) from the Federation of Societies for Coatings Technologies (FSCT), and an outstand- ing research faculty award from USM. His research on self-healing coatings has been featured by numerous media including the New York Times, Forbes magazine, BBC, NBC, Discovery Chan- nel, USA Today, Yahoo, and many other TV and radio stations around the world. Current research interests include self- healing polymers and coatings, under- standing physico-chemical processes gov- erning coatings responsiveness, the development of heterogeneous stimuli- responsive polymeric coatings, and mol- ecular level processes that govern self- repaired coatings. Marek W. Urban, Ph.D. Clemson University Ying Yang, Ph.D. Clemson University Powder Coatings Clinic