materials testing | PVC weatherability
goes through the refl ective surface and substrate (the transmitted light is no longer available to build heat in the test material since only refl ected light reaches the target area).
Figure 5: High UV-Vis, low Vis-NIR spectral refl ectance of new device mirrors
Figure 5 shows the spectral refl ectance of the new device mirrors. The mirrors maximise the shorter wavelength solar UV and visible irradiance in the target area while minimizing longer wavelength visible and IR irradiance on test materials. High UV-Vis irradiance causes photo degradation of test material. High Vis-IR irradiance causes little photo degradation. Reducing the long wavelength Vis-IR irradiance reduces the heat contributed by these longer wavelengths resulting in lower exposure temperatures for most materials. The spectral selectivity of the refl ective elements represents the key to reducing exposure temperatures for most materials on the new exposure device. Researchers named the new device “Low Temperature EMMA” (EMMA stands for ‘equatorial mount with mirrors for acceleration’).
The results Figure 6. Exposure temperature comparison for decking materials
reduce heat caused by concentrated solar irradiance absorbed by test specimens. However, convective cooling is not the only way to reduce exposure tempera- tures. The energy balance of materials under solar irradiance provides several alternatives for affecting material exposure temperature. It ‘balances’ the energy absorbed by a material with the energy radiated away, convectively cooled, or conductively cooled. In one example, effi cient heat sinks attached to
materials’ unexposed side may conductively cool materials. However, reducing the energy available for absorption by the test material represents another alternative for maintaining reasonable test exposure temperatures. Filtering out longer wavelengths of visible and NIR reduces total solar energy without reducing the concentrated solar UV. Special mirrors can selectively fi lter out wavelengths only responsible for heating the specimen while concentrating the UV-Vis wavelengths responsible for natural material weather- ing degradation. Figure 4 shows a new low temperature device with ‘cold mirrors’ installed on the collector3,4,5
. The mirrors appear transparent because much of the visible light 48 COMPOUNDING WORLD | June 2014
Decking temperature comparison: Figure 6 shows temperature data from thick commercially available composite decking materials. Researchers inserted thermocouple probes just beneath the cap stock (exposed surface) of the decking material and con- nected temperature measurement specimens to data loggers. Three identical specimen sets included four dark colours (black, gray, brown, and walnut). The black specimen was obtained by spray painting one material with black paint. The three specimen sets were
simultaneously exposed on: 1) a standard G90 device; 2) the new Low Temperature EMMA device; and, 3) a simple unaccelerated horizontal backed exposure through solar noon on a typical summer day near summer solstice in Phoenix, Arizona on 07/29/13. The data show that the new Low Temperature EMMA
maintains exposure temperature signifi cantly lower than the standard G90 exposure for the decking specimen sets. In fact, the new Low Temperature EMMA device maintains exposure temperatures slightly lower than temperatures observed for horizontal backed exposure specimens! Black standard temperature (BST) and black panel
temperature (BPT) comparison: Figure 7 shows black standard temperature data obtained using Xenosensive detectors. The detectors measure BSTs using a black coated steel panel on top of a 5 mm thick insulating polymer backing. A resistive temperature device (RTD) sandwiched between the black panel and insulating polymer measures the BST. Three identical detectors were simultaneously exposed on: 1) a standard G90 device; 2) the new Low Temperature EMMA device; and,
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