Editor’s Note: This is the third article in a three-part series.
Coatings are utilized for their ability to visually beautify and functionally protect many types of materials such as metals, woods and composites. Recent advances in specialty infrared reflective pigmentation technologies allow the coatings formulator, and thus the original equipment manufacturer or contractor, to impart an additional functionality to their products. This functionality enhances the overall value of the products. This functionality is the ability to reflect invisible heat energy while achieving a desired visible color. The reflection of heat energy results in a lower temperature substrate when exposed to sunlight. The lower substrate temperature has a direct impact on reducing the cooling load in a building, for example, when used in a roofing application. The higher infrared reflectivity of the substrate results in other benefits such as reduced warpage, reduced thermal cycling, reduced chemical degradation and the improved comfort of people in direct contact with the substrate.
In this final part of the series the application of infrared reflective pigmentation technology will be reviewed.
Infrared Reflective Pigmentation Technologies - Keeping Objects CoolInfrared reflective pigmentation technology allows the user to achieve a specific color space in the visible light range while reflecting incident light in the near-infrared (NIR) range of the electromagnetic spectrum. The reflection of light in the NIR reduces the temperature of the coated object by lowering overall energy absorption from terrestrial solar irradiance (less absorbed energy equals less energy converted to heat).
Traditional colorants, such as carbon black, efficiently absorb visible light as the mechanism to achieve a dark color space. Additionally, carbon black pigmentation also absorbs strongly throughout the ultraviolet and near-infrared wavelengths of light. Approximately 53% of the terrestrial solar irradiance is infrared energy; therefore, coatings and substrates containing carbon black have significant energy absorption properties. This high degree of energy absorption is ultimately converted to heat, causing environmentally undesirable phenomenon such as Heat Islands. Heat Islands commonly occur in modern urban centers where ambient temperatures rise well above those of the rural surroundings due to a large absorption of incident solar radiation.1
An object containing carbon black, exposed to sunlight, will cause the temperature to rise as much as 100 °F above ambient temperatures. The resultant heat build-up of a roof, or building surface pigmented with carbon black increases heat transfer into the structure, and increased cooling loads on a structure. Increased utility costs are directly related to increased cooling loads.2
When designing an infrared reflective coating, maximizing total solar reflectance and emissivity, and minimizing all contamination by infrared absorbing materials is a necessity. Contamination must be eliminated from infrared reflective dispersions in order to achieve optimal reflectivity properties.
Impact of Visual Color on Total Solar ReflectanceSince visible light consists of approximately 42% of the solar terrestrial irradiance, the visible color of infrared pigmentation will dictate the maximum achievable total solar reflective benefit. All things being equal, the darker the visible color, the lower the total solar reflectance. Rutile titanium dioxide continues to be the base pigmentation of choice in highly reflective white coatings. Titanium dioxide exhibits excellent light scattering and infrared reflectance due to its high refractive index, however, the primary particle size is optimized to scatter light most efficiently in the 500-550 nanometer region.
When formulating a black color space with infrared reflective pigmentation, one will encounter a maximum theoretical limit on the achievable total solar reflectivity of around 47% using specialty synergistic infrared reflective technology. Using newer mixed metal oxide pigments to achieve a black color space yields a total solar reflectance from 25-34%.
The performance properties of the coating will dictate the optimal pigment selection for the coating. In cases where color stability is necessary, inorganic pigments should be considered. In these cases accelerated weathering testing is strongly urged to prove the stability of the selected pigmentation and its performance in the coating system.
Optimization of Infrared Reflective PigmentationPigment Dispersions
When dispersing infrared reflective pigmentation, several items must be considered: optimal particle size, stability, elimination of contaminants and quality assurance.
There exists an optimal particle size for all pigment dispersions. The optimal particle size maximizes the physical properties of the pigmentation such as color development and color strength, stability and light scattering. If an inorganic infrared reflective pigment is not dispersed properly, color strength will be weak, requiring a greater-concentration pigment to achieve the desired color (and opacity will not be at its optimal level).
In-can shelf stability of inorganic infrared-reflective pigments is difficult due to their high specific gravity and larger primary particle size. Care must be taken to avoid hard settling and, in some cases, separation can occur over time.
Most producers of pigment dispersions have visible light spectrophotometers readily available to conduct color testing. Unfortunately, cases may occur where infrared-reflective pigment dispersions pass visible color specifications, but have a greatly reduced infrared reflectivity. The danger is that an out-of-specification infrared-absorbing material could be errantly approved, and performance properties will suffer greatly, voiding warranties resulting in an unpleasant business situation. A small amount of infrared-absorbing material such as 0.01% carbon black based on total weight can significantly reduce infrared reflectivity.3
Quality assurance must be undertaken with instrumentation that can certify the infrared performance, or total solar reflectance. Such instrumentation is the UV-VIS-NIR spectrophotometer, or the Devices and Services Solar Spectral Reflectometer.
IR Reflective CoatingsCASE 1: Infrared Reflecting Gray versus Traditional Gray
To demonstrate the performance advantage of infrared pigmentation technology, a traditional gray coating was compared to an infrared-reflective gray coating (Table 1). Both gray coatings have a similar light to dark value. When evaluated using the CIE LAB system, the resulting Delta L* is 0.032, indicating the grays are achromatically equivalent (Figure 1). The total solar reflectance of the traditional gray coating is 17.5%, and the total solar reflectance of the infrared-reflective gray coating is 47.1%. While the visible color difference is negligible, the total solar reflectance indicates a raw 29.6% improvement of the IR reflective gray versus the traditional gray. The net result is a reduced predicted heat build-up when coated panels are evaluated using the ASTM D 4803 heat build test method. The visible and infrared reflectance of the panels is shown in Figures 1 and 2.
CASE 2: Traditional Green versus Two Infrared-Reflecting
Three green coatings were created to further demonstrate the use of infrared reflective pigmentation in blended colors. The first green (Green 1) was created using standard colorants. The second Green color match (Green 2) was created using infrared reflective pigmentation technology, and the third green color match (Green 3) was created using synergistic infrared reflective technology. All three green color matches are visually equivalent in color.
The total solar reflectance of the traditional green coating (Green 1) is 7.6%, and the total solar reflectance of the infrared reflective green coating (Green 2) is 20.2% (Table 2). The total solar reflectance of the third coating (Green 3) is 36.8%. While the visible color differences were negligible, the total solar reflectance values show a significant difference, which has a pronounced reduction of predicted heat build-up when tested using ASTM D 4803 (Table 2).
IR Reflective TechnologyExample
Traditional black asphalt shingles are well known for their ability to absorb heat in direct sunlight. Black asphalt shingles exhibit a total solar reflectance of 3.5%, that is, they absorb over 96.5% of the incident terrestrial solar radiation. This energy absorption heats the surface to exceptionally high temperatures. On a clear summer day, the temperature of the roof can well exceed 180 °F. This absorbed energy is conducted, convected and radiated into the building structure, driven by the large temperature gradient. Assuming an average terrestrial solar irradiance of 900 watts per square meter (an average value derived from the integral of the ASTM solar irradiance spectrum), this translates to energy absorption of 868.5 watts/M2 or 868.5 joules/(sec*M2). Ultimately this results in a hotter building, requiring additional HVAC equipment to cool the rooms at a greater energy burden.
Using specialty-formulated infrared reflective pigmentation technology, a similar black roofing color space can be achieved as compared to the traditional black asphalt shingle. By using specialty infrared reflective pigmentation, the coatings formulator can create a coating with a total solar reflectance of approximately 29% in a deep base. This greater solar reflectance results in a total absorption of 71% of the incident terrestrial solar irradiance, or around 639 watts/square meter of absorbed energy. There remains a 26% raw reduction in energy absorption (234 watts/square meter) versus the asphalt shingle roof. This translates into a cooler roof and a cooler building with a lower HVAC demand.
A second approach utilizes specialty synergistic infrared reflective technology. This allows the formulator to have the visual effect of a dark color while achieving a higher solar reflectance than the asphalt shingle and the infrared reflective black pigmentation mentioned in the previous paragraph. This produces a shade of black with a total solar reflectance of 47%, and net energy absorption of 53%, or 477 watts/square meter, resulting in a 47% reduction in absorption of energy based on the energy absorption of the asphalt shingle.
Avoiding Warpage and Thermal CyclingThe primary challenge in the pigmentation of exterior thermoplastic substrates is achieving a dark color space while avoiding excessive heating, resulting in warping of the thermoplastic. The structural integrity of a thermoplastic part will be reduced or fail completely if the temperature of the part exceeds the softening point of the base material. Traditional pigmentation must be used with apprehension with most exterior thermoplastic applications. Infrared reflective technologies are utilized to maintain the heat build of the thermoplastic part below that of the softening point.
cycling occurs due to the expansion and contraction of materials caused by the
repeated exposure to temperature fluctuations from day through night, etc. When
materials are heated, they naturally expand, and then contract when they are cooled.
Lowering the heat build by maintaining a high solar reflectance and high
emissivity minimizes the effects of thermal cycling and, therefore, minimizes
excessive stressing of structural components. This should improve the longevity
of these components.
ConclusionsInfrared reflective pigmentation technologies can be utilized to minimize heat build in objects exposed to sunlight, resulting in local energy efficiencies and physical performance property enhancements, while achieving a visually appealing color. The energy efficiencies gained by utilizing infrared-reflective products add value to existing products incorporating the new pigmentation technologies.
Maximizing the total solar reflectance and emissivity of a surface in a given color space results in the lowest temperature and the greatest energy savings benefit. One must avoid contamination of infrared reflecting products with highly absorptive products and must consider the impact of the entire coating system and substrate to maximize performance.
Infrared-reflective pigmentation technologies have been proven to reduce thermoplastic warping while achieving darker colors than previously attainable utilizing traditional pigmentation.
For more information, contact David M. Hyde, Coatings Process Development Engineer, at firstname.lastname@example.org; Benjamin Arnold, Market Development Manager, at email@example.com; or Elizabeth Campbell, Product Development Manager, at firstname.lastname@example.org.
References1 Pomerantz, M.; Pon, B.; Akbari, H.; Chang, S. The Effect of Pavements’ Temperatures on Air Temperatures in Large Cities, Report No. LBNL-43442, April 2000, Lawrence Berkeley National Laboratory, Berkeley, CA.
2 Akbari, H.; Konopacki, S. The Impact of Reflectivity and Emissivity of Roofs on Building Cooling and Heating Energy Use, LBNL-41941, pp 2-3.
3 Rabinovitch, E.B.; Quisenberry, J.G.; Summers, J.W. Predicting Heat Buildup Due to the Sun’s Energy. J. of Vinyl Technology 1983, Vol. 5, No. 3.