Infrared reflective colorant used in cold paint
With concerns about the heat island effect in large cities and the need to reduce energy consumption, there is growing interest in color pastes that reflect infrared rays to reduce heat build-up. In addition, many countries have begun to enact regulations requiring the use of “cold” coatings to reduce energy consumption. To meet these new requirements, coatings formulators are looking for raw materials to support their research and development.
We know that sunlight is the main source of infrared energy. When these light waves are absorbed by any outdoor surface, the surface becomes increasingly hotter. On a black surface, it can be hot enough to burn your skin. During the highest temperatures during the scorching summer sun, black asphalt roads are very hot because dark surfaces cannot reflect the infrared energy from the sun. Additionally, this effect can also lead to higher room temperatures in buildings with dark roofs. In buildings without air conditioning, excessive indoor temperatures can be dangerous. Using air conditioners to lower indoor temperatures will increase costs.
The use of infrared reflective pigments in paint is a method of reflecting infrared energy from the coating surface into the atmosphere. This reduces the surface temperature. While roof coatings are exactly where these colorants are used, infrared reflective colorants can also be used in other areas to benefit people and the environment. This article provides an overview of the technology and uses of infrared reflective colorants.
Total solar radiation
Solar radiant energy includes about 3% ultraviolet (UV), 44% visible rays and 53% infrared rays. Most of the energy from UV rays is absorbed by the atmosphere. In addition, the atmosphere scatters large amounts of blue light waves in visible light, which is why the sky appears blue. However, most of the energy of infrared light reaches the Earth’s surface, where it generates heat. Near-infrared light (approximately 700-2500 nanometers) contains most of the heat energy generated by sunlight. Figure 1 shows the solar radiation spectrum, including solar radiation in the upper boundary of the atmosphere, and the distribution of ultraviolet, visible light and infrared energy in solar radiation on the earth’s surface.
Usually the earth absorbs solar radiation during the day and transmits it back to space in the form of radiation at night. About 23% of the solar radiation reaching the Earth’s atmosphere is absorbed by the atmosphere, 31% is radiated back to space, and 46% is absorbed by the Earth’s surface. The earth’s surface will radiate the absorbed solar radiation energy back to space through heat exchange such as long-wave radiation, latent heat and sensible heat.
When incoming and outgoing radiation are in equilibrium, there is no change in the overall temperature of the Earth’s surface. However, as the Earth’s surface is covered with heat-absorbing materials such as black roofs, coated concrete, asphalt or other dark surfaces, the balance of incoming and outgoing radiant energy can change.
Counteract absorbed infrared radiation
To counteract the effects of absorbed infrared radiation, coatings reflect infrared light from the coated surface back into the atmosphere. Infrared reflective coatings can be formulated using a variety of pigments, metals (such as aluminum) or other raw materials to create an infrared reflective barrier layer. These raw materials reflect the near-infrared region of the spectrum where the most heat is generated. Due to the reflection of these infrared rays, a surface coated with infrared reflective paint will have a lower temperature than a surface with traditional paint.
One way to understand how this concept works.� is to consider why white appears in the visible light region. Because white reflects all wavelengths in the visible spectrum, the color appears white to the human eye. In fact, titanium dioxide, a raw material used in most white paints, reflects infrared radiation as well as visible light radiation. Conversely, materials that do not reflect visible light appear black. Carbon black also absorbs most wavelengths of infrared light and therefore becomes hotter than most other colors. If we could see the color of a material in the infrared wavelength range, a coating that reflects all infrared wavelengths would also appear white. Figure 3 depicts this phenomenon by showing gray in the visible and infrared regions formulated with non-IR reflective pigments such as carbon black and IR reflective pigments respectively.
People are craving more options when it comes to staining surfaces, and for many uses white paint is not suitable. Numerous coloring materials are now available that offer a wide range of colors and reflect infrared wavelengths. These materials have been formulated by Chromaflo Technologies into liquid dispersions, making them easy to use by coating formulators.
Color selection using infrared reflective colorants
When determining color options and infrared reflectance, the total solar reflectance (TSR) of the material needs to be considered. TSR is the amount of solar radiation reflected from the surface of a material, expressed as a percentage. At higher TSR values, the surface will absorb less solar radiation energy, resulting in a lower surface temperature. Changes in temperature are measured in degrees and are called Heat Build. The TSR value of white paint is about 75%. The reason is that titanium dioxide can reflect more than 90% of infrared rays between about 700-1300 nanometers, and can reflect about 30% of wavelengths of about 2500 nanometers. The TSR value of carbon black coating is about 5-10%. The TSR value of coloring materials used in paint ranges from approximately 25% to 70%. Mixing any infrared reflective colorant will change the TSR value of the pure colorant. Therefore, the correct choice of color paste is very important for a specific color space in order to ultimately determine the target TSR value of the color.
Currently, this technology has been used in roof coatings, building exterior wall coatings, and wood (door and window) coatings. The purpose of these applications is often focused on reducing the amount of heat absorbed by building surfaces in an effort to reduce the cost of energy used to cool the interior spaces of the building and protect the substrate from high temperatures. These technologies will also be applicable to other surface coatings such as those used on outdoor tanks, playground equipment, swimming pool decks, concrete surfaces, automotive interior parts and theme parks. In summary, any outdoor surface coating that would benefit from lowering surface temperatures would be an application for infrared reflective coatings.
Solvent-based and water-based formulations
Specially designed solvent-based and water-based infrared reflective colorants have broad compatibility with coatings. An example of a solvent-based infrared reflective colorant is the Spartacryl PM® range, which can be formulated to provide paint formulators with a universal colorant system for tinting and fully tinting most solvent-based industrial coatings. There is also a solvent-based low VOC colorant called Chroma-CHEM® 846. Offering a wide color space, excellent durability, lightfastness and chemical resistance, these pigments are dispersed in a unique proprietary acrylic resin base that provides excellent wetting and dispersion. The solvents used are a variable mixture of proprietary esters and propylene glycol monomethyl ether acetate.
Examples of water-based infrared reflective colorants are the Chroma-Chem50-990 series and 897 series. The former is a low-VOC pigment dispersion consisting of a blend of finely ground pigments dispersed in water, additives and a unique low-VOC paint base. The resulting colorants and water-based products have broad compatibility. To better meet the requirements of current VOC regulations, these colorants do not contain any added volatile organic compounds and are glycol-free. Product 897 is a high tinting strength, low VOC colorant designed for use in a variety of water-based industrial coatings. These pigments offer excellent durability, lightfastness, chemical resistance, and are APE-free.
