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January 2000 |
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The response and resistance of products to the effects of weathering is of concern in a wide variety of industries. Typical examples include automobiles (exterior finish, paint, upholstery, plastics, trims), fabrics (clothing, carpeting, wall coverings, outdoor furniture), building materials (lumber, roofing), architectural coatings (paint, vinyl siding), toys, bicycles, and camping equipment, to name just a few. Simply exposing a product under test to the elements, however, is not always the ideal way to perform this type of environmental testing, as the exact exposure conditions cannot be controlled and the process cannot be accelerated. On the other hand, accurately simulating outdoor exposure in the laboratory, especially the effects of natural sunlight, poses its own set of challenges. This article reviews the technology currently in use to perform accelerated weathering, and in particular examines the use of thin-film coatings to better match the output of xenon arc lamps to the natural solar spectrum. Anatomy of a Weathering Chamber The three major factors that determine damage to products outdoors are sunlight, moisture (in the form of both humidity and rain), and temperature. Thus, properly simulating outdoor exposure requires the ability to produce a given level of these parameters with accuracy, repeatability, and reproducibility. In this context, repeatability means the consistency of response of a particular testing station when given a specific input, and reproducibility means the unit-to-unit consistency for a specific input. When these three goals are achieved, then a test can be closely correlated with true outdoor exposure. This also enables a manufacturer to make an "apples-to-apples" comparison of the results of testing on various product formulations to determine which one will work best under a certain set of conditions.
The photo of an actual weathering test chamber, the Atlas Ci4000 Weather-Ometer®, illustrates how such testing is implemented in practice (Figure 1). A high-intensity xenon arc lamp line source sits at the center of a stainless steel chamber. Specimens of the materials under test are placed on a barrel-shaped rack that rotates about the center of the lamp. This shape and motion of the rack are designed to enhance irradiation uniformity. A blower, sitting atop the unit, is used to draw air through the chamber. An array of water nozzles is available to produce "rain." Additionally, atomized water can be introduced into a compressed-air stream to enable any arbitrary chamber humidity within a certain range to be reached. The lamp output is intended to simulate sunlight; some of it is channeled through a light pipe to an ultraviolet (UV) bandpass filter, and then on to a photodetector. This signal is then used in a closed-loop system to stabilize source output. The lamp is monitored in the UV for two reasons. First, most material degradation occurs because of UV exposure, so maintaining constant UV output is critical to achieving high accuracy, repeatability, and reproducibility. Second, the lamps themselves tend to degrade more quickly in the UV than they do in the visible or infrared (IR) portions of the spectrum. A unique patented feature of the Weather-Ometer is its ability to independently control both chamber air temperature and the "Black Panel" temperature, which is an industry standard that is meant to represent worst-case surface temperature of the parts under test (due to heating by the light source). Air temperature is continuously monitored and regulated by opening or closing a damper, which in turn limits the ability of the blower fan to draw air through the chamber. Typically, the Black Panel temperature is measured by a resistance temperature device welded to a metal panel that is painted black and placed in the sample rack. In operation, air temperature, sample temperature, humidity, and part irradiance can be held constant at various levels to simulate the type of exposure experienced at different locations (northern versus tropical latitudes) or during different seasons at a given location; alternately, exposure parameters can be cycled to simulate diurnal variations or the effects of specific weather patterns. Besides allowing precise control of exposure parameters, another important benefit of this type of environmental testing is that it enables acceleration of the weathering process. Obviously, when testing a product designed to withstand ten years of exposure, it is not practical to take that long to perform the experiment. Accelerated testing is accomplished through two means. First, the percentage of time a part is exposed to sunlight during a 24-hour period can be increased up to 100 percent. In the real world, even during daylight hours, the changing angle of the sun alters the level of exposure experienced out in the open. Thus, maintaining a noontime light level over an entire day of testing results in far more than a twofold increase in exposure. The second method of producing acceleration is to use light levels that are greater than "one sun." This can also be accompanied by higher air temperature and humidity.
In order for laboratory weathering to simulate accurately the effects of real-world exposure, it is essential that the output spectrum of the light source closely match the solar spectrum. The xenon arc lamp source approximates solar radiation well; however, its output does contain some intense spectral lines in the IR and also extends deeper into the UV than the sun does (Figure 2). The IR spikes become particularly problematic during accelerated testing -- greater than one sun exposure levels -- because they produce excessive product heating. Historically, absorptive glass filters have been the solution for eliminating the unwanted UV and IR portions of the xenon arc lamp spectrum. IR absorption filters, however, are notoriously unstable in the critical UV, and significant changes in UV transmission challenge efforts to achieve repeatability and reproducibility. Thin-film coatings are inherently more stable, and can also be designed to block the IR spikes in the xenon lamp output more precisely. In the case of the Atlas Ci4000 Weather-Ometer, however, the filters are in the form of long concentric cylindrical tubes that surround the lamp. Water is flowed in through the inner tube and back out through the outer tube to cool the lamp. Thus, a thin-film coating for this application must be deposited on a small-radius tube and also be capable of surviving both high flux levels and direct exposure to the continual abrasive flow of cooling water. Traditional evaporative coating technology cannot meet these demands for a number of reasons. First, it is difficult to uniformly coat the entire surface of highly curved parts using evaporation methods. More importantly, this type of film is relatively porous, and a porous film has poor resistance to heat and mechanical wear, and also tends to absorb water, causing a shift in its spectral characteristics. Sputtering is an alternative coating method that provides denser films and also enables uniform deposition on highly curved parts. Sputtering is conducted in a chamber filled with an inert gas at low pressure. Targets of various conductive materials are placed around the periphery of the chamber. A high voltage is applied to the targets, ionizing the gas around them to form a plasma. These ions are then accelerated into the target, causing atoms to sputter off. The sputtered atoms fill the chamber, and some are deposited on the surfaces of the optics. The low-pressure gas randomizes the movement of the sputtered atoms, leading to uniform deposition on all surfaces regardless of their orientation relative to the material source. Furthermore, the high energy of the sputtered atoms produces coatings that are inherently denser than is achieved through evaporation. If oxygen is introduced into the machine during sputtering, it will react with the atoms of the material to produce oxides. This process, called reactive sputtering, yields coatings with even better mechanical and thermal characteristics. Recently, Deposition Sciences Inc. has developed a new approach to reactive sputtering called the MicroDyn® process. In MicroDyn, sputtering is augmented by a microwave plasma that forms a wider range of oxygen species (ozone, etc.) to enhance the reactive process. The net result is faster, more efficient deposition, with greater control and variation of the deposited material's refractive index. Using MicroDyn sputtering, it has been possible to produce a filter that supplies the necessary spectral characteristics, while delivering sufficient mechanical density to withstand prolonged exposure to water, heat, and high flux levels. Specifically, a multilayer, broad-bandpass filter with a cut-on deep in the UV and a cut-off in the IR is deposited on the outside diameter of the inner tube. The cut-off of the filter is placed to provide the precise attenuation of IR wavelengths required; the UV cut-on is placed at a wavelength much lower than required. The outer tube is then fabricated from sharp cut-on absorptive glass, which only transmits above the necessary UV wavelength, but has relatively low IR absorption for maximum stability. This arrangement enables the overall transmission to be optimized on both the long- and short-wavelength ends of the spectrum simultaneously. Laboratory weathering enables manufacturers in a broad range of industries to improve their products' durability and lifetime. Recent developments in coating technology have now enabled weathering systems with both improved solar accuracy and greater reliability. The authors of this article are Robert Crase, lighting operations manager of Deposition Sciences Inc., and Kurt P. Scott, general manager of laboratories and calibration services for Atlas Electric Devices. For further information, contact Deposition Sciences at 386 Tesconi Court, Santa Rosa, CA 95401; (707) 573-6785; fax (707) 579-0731; e-mail: bob.crase@depsci.com; or Atlas Electric Devices at 4114 N. Ravenswood Ave., Chicago, IL 60615; (773) 327-4520; fax: (773) 327-5787; e-mail: kscott@atlas-mts.com.
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