PVC weatherability | materials testing
Henry Hardcastle of Atlas Material Testing Technology introduces a new application development and weathering device designed to
overcome obstacles to using Fresnel concentrators for studying the performance of vinyl materials
Low temperature outdoor accelerated weathering devices – a new application development
Vinyl and other temperature sensitive materials have a great tradition in outdoor applications when proper constituents are compounded correctly. However, compounders can’t see far into the future of 10 to 20 year outdoor performance when making changes for improvement. Accelerated weathering test methods help com-
pounders assess outdoor weathering performance more quickly and researchers have successfully accelerated weathering for many materials by increas- ing light irradiance in testing exposures. However, a major constraint exists for temperature sensitive materials; increasing irradiance results in increasing material exposure temperature. Irradiance and material temperature typically co-vary – when one variable goes up, so does the other variable.
The problem ASTM G901
represents one confi guration of an outdoor
accelerated weathering device currently used by material testing researchers. Horizontal and vertical tracking pivots keep the device pointing at the sun
throughout the day. A set of ten fl at mirrors refl ect ten images of the sun coincident on material test speci- mens mounted in the target area. Mirrors of the current G90 device refl ect the majority of the entire solar spectrum (approximately 290 nm to 2,500 nm). A typical total refl ectance spectrum for these
mirrors is shown in Figure 1. Mirrors refl ect and concentrate solar irradiance on test specimens including Ultra Violet (UV), Visible (Vis) and Near Infra-Red (NIR) portions of the solar spectrum. Concentrating light from UV, Vis, and NIR portions of the solar spectrum on absorbent materials can build signifi cant heat in the test specimens which may reach unacceptably high exposure temperatures. In order to convectively cool the test specimens exposed to the highly concentrated solar irradiance, G90 devices employ blowers to force ambient cooling air across the specimens. This cooling method, although effective for many test materials, presents some signifi cant temperature limitations for other materials and conditions, such as thick, highly absorbing, insulating specimens. For example, researchers may
Figure 1. Full solar spectral refl ectance of current ASTM G90 mirrors
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Figure 2. Exposure temperature effect on appearance
June 2014 | COMPOUNDING WORLD 45
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