According to EPA statistics, approximately $40 billion is spent annually in the U.S. to air-condition buildings. The incorporation of solar reflective pigments in paint can decrease the cost to air condition buildings in the U.S. by more than $8 billion.
When exposed to sunlight, it is commonly known that light colors, especially white, remain cooler than darker surfaces. Darker colors, especially black, absorb infrared light energy, resulting in warming of the substrate. The amount of light energy absorbed is dependent on color.
Other factors that determine an object's temperature in an outside environment, in addition to it’s color and solar reflectivity, include it’s emissivity, convection and conduction. To illustrate further, Figure 1 indicates at an ambient air temperature of 20° C, a white object will remain at about 20°C, whereas a black object will be about 35° C for the coating surface of a steel building.
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However this same black coating will have a surface temperature of about 65°C if the coated substrate is wood, plastic or isolated steel sheets. Key definitions follow:
Total Solar Reflectance - the amount of solar radiation that is reflected by a surface, measured as a percentage.
Thermal Emittance - the ability of a material to dissipate heat away from itself, or rather, to shed heat.
Convection - exchange of energy with air above the substrate.
Conduction - exchange of energy with the layer of the substrate directly below the surface.
To better understand the phenomena of why colors display different heating/cooling characteristics in sunlight, it is essential to examine the natural light spectrum. Solar radiation reaches the earth’s surface in three distinct wavelength packets. These packets of light include Ultra Violet light (UVA 280 – 315 nm, and UVB 315 – 400 nm), Visible light (400 nm – 700 nm) and Infrared light (near IR and far IR) between 700 and 2500 nm. The human eye sees light primarily in the visible portion of the light spectrum resulting in color. Some animal species, such as birds can see light in the UV portion of the spectrum.
Figure 2 illustrates the natural spectrum of solar radiation. 5% of the light energy is UV light, 46% is visible light, and the remaining 49% is infrared light energy. Pigments can absorb or reflect solar infrared energy resulting in 1) heat build-up of the coated substrate if the pigment absorbs IR energy (for example conventional darker pigments); or 2) little or no increase in temperature if the pigment reflects IR light (for example white and lighter colors).
Solar infrared energy (700 – 2,500 nm) is different than infrared energy emitted by hot objects in interior spaces, such as heaters. Infrared energy is found in the far infrared range beyond 1,200 nm.
Total Solar Reflectance (TSR) is the measurement of how much of the sun’s energy an object reflects. TSR can be measured using a Solar Spectrum Reflectometer. White pigments such as titanium dioxide have a high solar reflectivity due to their white color, while conventional darker pigments have a lower TSR depending on how dark the pigment is.
Solar reflective colored pigment technology uses specially designed doped ceramic (mixed metal oxides) pigments to achieve a higher degree of TSR than do conventional pigments of the same color. For example, the TSR of a dark brown conventional coating is about 8%, whereas the same color using solar reflective pigment technology will be about 28%. Typically, every one percent increase in TSR is accompanied by a one degree F drop in surface temperature. Colors using conventional pigments have a TSR in the range of 10 – 20%, whereas using SR pigments results in colors which have a TSR of 25% and higher.
As Figure 3 illustrates, the TSR of Solar Reflective Paints is not affected by gloss or sheen as these later two properties are a measure of visible light reflectance and are independent of solar IR reflectance.
Environmental benefits of Solar Reflective Paint Technology in architectural applications such as exterior roofs and sidewalls include:
- Less heat build up in urban areas
- Lower levels of smog and ozone
- Lower energy requirements to air condition buildings means less pollution produced by fossil fuel burning power paints
In addition to lower cooling costs, the performance advantages of solar reflective paint technology include the following:
- Improved chalk and fade resistance
- Slower degradation of the resin system (lower heat)
- Improved thermal and dimensional stability of the substrate means less stress on the substrate
The use of solar reflective pigments can provide coatings with a TSR of about 25% for a black color to about 67% for lighter colors.
Solar reflective pigments should not be over ground in the pigment dispersion step as this may cause a color shift. Since SR pigments are mixed metal oxide pigments, their density is somewhat higher than many conventional pigments. More care must be taken in the formulation to avoid hard settling. Also, when mixing two or more SR pigments together, the actual SR of the paint quite often results in a lower SR than would be anticipated based on the weighted average of the TSR of the combined pigments.
Visual opacity (based on visual light transmission) does not equate to IR opacity (based on IR light). Accordingly, the TSR contrast ratio should be determined to ensure that the coating offers adequate TSR at the applied film thickness, or heat build-up in the substrate will result due to transmitted IR light through the coating to the substrate. Accordingly, care must be taken to apply the proper film thickness based on TSR opacity and not visual opacity. If good IR opacity is not achieved, when the coating is applied over a white versus a gray or black substrate containing carbon black, the coated substrate will display different capacities to reflect IR light.
Pigment particle size is also very important to provide optimum IR reflectance. For highest reflectivity, the pigment particle size should be more than half the wavelength of the light to be reflected. Thus for reflecting infra red light of 700-1400 nm wave length, particle size should be at least 0.35 to 0.70 microns. Accordingly, over grinding may result in lower IR reflectivity. Lastly, care must be taken not to inadvertently contaminate an SR formulation with a pigment such as carbon black from the milling process.
In summary, solar reflective paint is currently used in multiple applications, including commercial and residential roofs and sidewalls, automotive, industrial and military. There are a number of agencies in the U.S. as well as globally to incentivize and support the use of solar reflective coatings including: Energy Star, LEED Green Building Rating Council, and the Cool Roof Rating Council. A few suppliers of IR reflective pigments include Shepherd and Heubach.
For additional information concerning material selection to formulate solar reflective coatings, please navigate to www.ulprospector.com.
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