About 75% of the North American coil coating industry is dedicated to building products. Since the lifetime of these products is measured in decades, as opposed to merely years, the weathering performance of the coatings used for this market is critical. Understanding how coatings perform, therefore, is essential. There are many approaches to studying weathering performance, and one of the more interesting techniques to accelerate the weathering process involves the devices described in these two ASTM standards:
- G90 Practice for Performing Accelerated Outdoor Weathering of Nonmetallic Materials Using Concentrated Natural Sunlight
- D4141 Practice for Conducting Black Box and Solar Concentrating Exposures of Coatings
These devices are branded Q-Trac® by Q-Lab Corporation and EMMAQUA® by Ametek (the parent company of Atlas Material Testing Solutions), which are the more familiar descriptions of these weathering devices than the original “Fresnel reflector,” a name not even used in the titles of the two standards above.
But before discussing this type of natural sunlight-accelerated exposure, let’s go back about 200 years and allow me to introduce you to Augustin-Jean Fresnel (pronounced FRAY-nel). He was a civil engineer and physicist whose work led to the acceptance of the wave theory of light. Light is a strange thing. Sometimes it acts like a wave, and this wave-like characteristic is what explains light diffraction (the fuzziness as light passes by straight edges), interference (colorful oil slicks), and polarization (sunglasses). But sometimes it acts like a particle, which is commonly called a photon.
Monsieur Fresnel was also instrumental in the development of the Fresnel lens. This is the type of lens used in lighthouses in the past; today, GPS is used to guide ships safely. The genius of these lenses is that they take normal diffuse illumination (just about any light source that you routinely encounter produces diffuse light), and, through a complex series of lenses, this diffuse light becomes collimated (i.e., is made parallel), which can be seen for miles, as shown below. (NOTE: Although M. Fresnel was a wave guy, the picture below is illustrated with light depicted as particles traveling in straight lines.)
A Fresnel-type weathering apparatus is the same concept, but in reverse. The light from our sun is already collimated, and a Fresnel reflector gathers and focuses the sunlight. (Okay, the light from the sun is not truly collimated, but after travelling 93 million miles to reach us, the sun’s rays are essentially parallel when striking Earth.)
Courtesy Q-Lab Corporation
Courtesy Q-Lab Corporation, with NCCA annotation
The light from the sun is collected by 10 mirrors and reflected upwards onto the target. From the standpoint of reflected energy, the 10 mirrors direct about “8 suns” of energy onto the panels (i.e., 8 times more solar energy hitting the target than if the panels were placed on a rack facing the sun).
NOTE: “8 suns” does not necessarily mean that there is an 8-times acceleration factor. There is indeed acceleration, but the acceleration factor depends on many things, especially the material and its properties being tested.
To maximize the amount of sunlight collected, these Fresnel devices track the sun. We all know that the sun rises in the east and sets in the west, and a Fresnel device watches the sun and follows it during the day. The sun, however, is higher or lower in the sky depending on the season. At the summer solstice it is at its highest, and at the winter solstice the sun is at its lowest position in the sky. All of this is taken into consideration by the photocells and servo motors on each device.
The mirrors used in a Fresnel device get dirty; imagine that job cleaning each and every mirror. And each mirror assembly must be measured a few times each year to make sure that it complies with the G90 (see method above) reflectance requirements. To enhance the lifetime and cleanliness of the mirrors, the entire Fresnel assembly is flipped so the mirrors are facing downward each night, and then set aright early each morning.
Fresnel devices are always located in a desert environment to maximize the UV dose (no clouds, no rain). Phoenix is great at providing vast amounts of heat and sunlight, two important components that are considered by coatings formulators as they create coatings to overcome degradation. But another factor—moisture—hardly exists in the desert. (Fresnel devices only function well in the desert where scattering of sunlight is slight because the air is so dry.) It is common, therefore, to spray the panels each day. Water can be sprayed onto the panels during the exposure cycle when the panels are really hot. Because this may create water spots (even though the water being sprayed is deionized), many prefer a nighttime wetting cycle. Recall that the Fresnel assembly is flipped upside down during the evening. That means the mirrors are facing the ground and the panels are facing upward. Water is sprayed onto these panels (and they are reasonably cool in the evening). Because evaporation is slow, water has a chance to imbibe into the coating. Research done by the Ford Motor Company showed that coatings saturated with water degrade more quickly than dry coatings, so this nighttime wetting cycle in the desert is a good idea for the coil coating industry.
Careful correlation work done by NCCA 25 years ago showed that Fresnel devices are the best at accelerating weathering, but the data also said that real-time weathering was still needed. The correlation with Fresnel devices was good, but not good enough to replace the standard method that we have depended upon over the years: real-time Florida weathering.
NCCA Technical Director