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TESTING | WEATHERING


Right: Q-Lab’s proven QUV fluorescent UV weathering testing system


deliver light – xenon arc and fluorescent UV. “The xenon arc tester can have two basic geometries,” Francis says. “One is a rotating drum or rack where the lamp is vertical in the chamber and the specimens rotate about the lamp, while the other is a flat array where the lamps shine down from above.” The spectrum of the xenon


arc tester is meant to mimic natural outdoor sunlight. However, Francis says xenon lamps have very short wave- length UV light – below 250 nm – which can cause unrealistic degradation in many durable materials. So optical filters are used to provide a more realistic light spectra. These include daylight, window and extended UV filters. When setting up a test it is crucial to understand what the service environment will be and choose the correct filter, he says. Black panel temperature control is used to


approximate the maximum specimen surface temperature and can be used in combination with chamber air temperature sensing and control. “In order to optimise acceleration, the maximum service temperature should be used, but it should not be exceeded to minimise error,” Francis says.


Wet chamber options Chamber air temperature control is required by some test methods and needed for control of relative humidity. Xenon arc testers can generate and control relative humidity. However, for many


durable materials, relative humidity makes little difference compared with spray and condensation. Water spray is the main water delivery method in a xenon arc tester, with front spray being the primary method al- though back spray, dual spray with soap or acid, and immersion are all other options. “Xenon arc testers offer the best simulation of


full-spectrum sunlight, but the lamps do age, which can affect results over time. Although xenon arc testers have many advantages, there are greater costs and maintenance requirements, as well as issues of complexity, compared with fluorescent UV testers,” he claims. “In fluorescent UV equipment, the specimens form the wall of the tester, which results in their backs getting cooled by laboratory air. This means that it is easy to form condensation on the front sur- face, which is a critical feature in fluorescent UV testers,” he says. “The light spectrum is almost exactly analogous to the xenon arc tester. Fluores- cent UV testers do not simulate the full spectrum of sunlight like xenon testers - just the UV portion that is typically most damaging to plastics. By choosing appropriate lamps, different light spectra can be achieved in fluorescent UV testers. UVA-340 lamps replicate daylight UV, UVA-351 lamps simulate UV light behind window glass, UVB-313EL/FS-40 lamps for extended UV light, and cool white lamps to simulate indoor office lighting. The lamps do not generate radiant heat so there


Schematic showing the typical configuration of a fluorescent UV testing machine Image: Q-Lab


60 COMPOUNDING WORLD | December 2017


is very little difference in colour temperature – the temperature for a black or white specimen will not vary much. The ability to produce hot condensation is one of the main advantages of a fluorescent UV tester, according to Francis, who says that conden- sation is the most rigorous, realistic and best way to simulate water in accelerated weathering. “Tap water can be used and is naturally distilled by the tester, which means you cannot deposit debris on the specimens. It is also possible to spray to examine the effects of erosion of surfaces and thermal shock,” Francis says. “In fluorescent UV testing, UVA-340 is the best simulation of short wavelength UV light, while UVB-313 is faster, harsher and more realistic. The spectrum delivered by UV lamps is stable – no matter how long you run a UV lamp, its spectrum will not change. The


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PHOTO: Q-LAB


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