Hydrophobic Stainless Steel Surfaces
For Low Maintenance, Energy Efficient Facades
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There is no doubting the longevity of stainless steel building facades. The Chrysler Building and numerous buildings that followed have stood the test of time in providing durable facades that can be readily cleaned to restore their original appearance. Cleaning a stainless steel facade can be done with mild detergent solutions.
Stainless steel has impressive solar reflectance, which is roughly equivalent to high performance white paint when measured in accordance with ASTM E1980, alternate method. There is one important difference; unlike paint coatings, as well as aluminum and titanium, stainless steel does not oxidize over time. It therefore provides sustainably superior solar reflectance in the form of a permanent radiant barrier based upon aged SRI data.
The best performance of all comes from the use of hydrophobic stainless steel surfaces that resist the accumulation of dirt, maximizing solar reflectance and minimizing the need to clean the facade. Accumulated surface soil absorbs more solar radiation than it reflects. It is therefore important to keep the stainless steel clean in order to maintain maximum solar reflection. The use of a hydrophobic surface will delay, if not eliminate the need to clean the building’s surface. Three stainless steel building roofs installed at least 10 years prior with a hydrophobic surface finish showed no degradation of solar reflectance when SRI measurements were taken.
A hydrophobic surface, as defined by the angle of incidence of droplets, repels water and has low surface tension, which tends not to attract contaminants and rinses cleanly. A permanent hydrophobic surface is achieved in stainless steel by micro-texturing the surface of a mill-finished coil with a surface pattern that has features smaller than the diameter of a human hair. There are several such hydrophobic stainless steel finishes available ranging from dull to bright.
This investigation concludes that maximum solar reflectance of a building facade can be achieved through the use of hydrophobic stainless steel surfaces.
It is first important to understand how stainless steel as a building material relates to solar energy, global warming, heat islands, cool roofs and SRI (solar reflectance index). Issues relating
Stainless steel, unlike every other outdoor metal people have day-to-day experience with, doesn't get burning hot in the sun. This is counter-intuitive. People are used to metals which oxidize and
Solar Reflectance Data
ASTM E 1980, alternate method for bare metals.
Sessile drop testing.
10 Year Aged SRI Measurements (ASTM E1980, alternate method).
Stainless steel with hydrophobic surface that was installed in a roofing application for at least 10 years at the sites below
When metals oxidize, i.e. corrode, their surfaces take on the physical properties of the oxide. Oxides have low reflectance and high emissivity. Thus, as metals corrode, their initial superior performance
Conclusion and Future Work
It is apparent from this investigation that it is possible to build facades that sustain themselves as self-cleaning, energy efficient surfaces that do not degrade over time. Future efforts will
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