# Development of Effective Infrared Reflective Coatings
## 1 Introduction  
In cases of surfaces exposed to sunlight, solar energy can be transmitted, reflected, or absorbed. When the frequency of the incoming light is near to the electron energy levels of a sun-exposed material, the electrons will absorb the light wave energy and the electrons' energy state changes. In other words, the absorbed light is converted into thermal energy. The absorption of light depends on the nucleus and on electrons. In its transmission, light moves through a substrate, and at the reflection of different wavelengths, the angle of incidence of the light is equal to that of the reflection on smooth surfaces; consequently, the light bounces back from the surface [1]. More solar energy absorbed requires more cooling for the interior. Solar irradiation comprises three regions: ultraviolet (280-400 nm, only 5% of the sun's energy), visible (400-700 nm, 43% of the sun's energy), and near infrared (700-2500 nm, 52% of the sun's energy). Near infrared light plays an important role in heat generation [2, 3]. Absorbed sunlight increases temperature and requires more cooling energy [4, 5], additionally it can cause physical damage. Energy consumption could be diminished not only by cooling but also by using special infrared reflective coatings. When solar reflectance increases, surface temperature diminishes; solar radiation is reflected rather than absorbed. Consequently, an objectretains a more comfortable thermal condition without air conditioning [6,7]. In the case of traditional coatings, the main consideration is often its aesthetic property and not its infrared reflective character. When infrared reflective coatings are used, the most important features are their heat reduction and energy savings, as well as improving indoor comfort. Of course, aesthetic appearance is also important. The proper selection of an infrared reflective coating needs systematic screening of its refractive index, transparency, electric resistance, thermal conductivity, etc. The use of color pigments goes back to antiquity; they formed the basic part of all paints and decoration [8, 9, 10]. Pigments should be stable and chemically inert to stand up against aggressive environmental conditions and in order to retain their color. Generally, inorganic pigments are small particles that are insoluble in solvents and binders [11]. A lot of color pigments (e.g., white, blue, yellow) that are responsible for the color of objects absorb visible light and reflect others selectively (especially in the near infrared waves). Black pigments absorb and store heat not only in the visible but also in the near infrared wavelength, too [12]. Generally used infrared reflecting pigments applied in the paint industry are summarized in Table 1.  
Coatings of metals (silver, aluminum, gold) are very useful because of their high reflectivity in the infrared region; they are mainly used for windows. Metal oxides (TiO\({}_{2}\), ZnO) act in very thin polymer films blended with other materials. Compatibility and durability are important factors in the selection of materials [26]. TiO\({}_{2}\) (rutile, anatase) is mainly applied in white pigments [27,28]. The application of TiO\({}_{2}\), together with other pigments, could enhance infrared reflective efficacy [29,30]. The usefulness and effectiveness of zinc oxide [31,32], and the importance of bismuth vanadate [33] and bismuth titanate [34,35], as well as of the rare earths [36,37], has also been demonstrated. A very useful review summarizes the optical properties of near infrared reflective inorganic pigments published in 2019 [7]. Other publications describe and sort the most important pigments (according to their color) active in the near infrared light region [38, 39, 40, 41]. Though organic pigments are brighter and more transparent than inorganic ones, their light resistance is much less, and they can migrate in thermoplastics.