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Right: Q-Lab’s Q-Sun Xe-3 is a xenon arc test chamber accelerated weathering machine

temperatures. The difference between black and white speci- mens without backing can be up to 15°C, while for materials that have an insulated backing it can be even greater - up to 25°C. The effect of heat behind window glass can also be significant, where interior components inside cars can reach temperatures of up to 100°C. This can accelerate ageing and weathering, although this is offset a little by the fact that window glass can filter a lot of UV radiation,” he says. “The third force of weathering, and the one that is most com- monly overlooked, is water. Water can cause all sorts of failure routes in plastics, both chemical and physical. Water can be responsible for chalking, erosion of surfaces, blistering, swelling, delamination and facilitating other reactions,” says Francis. There are three ways that water can manifest

itself in nature, Francis explains. The first is water in the air, measured as relative humidity. This can affect products indoors and outdoors and lead to physical stress. Second is rainfall, where the main effect is on the surface of the material and gener- ally involves washing away surface layers, chalking and dirt removal, and removal of components in the plastic that might protect it from UV light. Rainfall can also produce thermal shock. For example, it has been observed in Florida that rainfall can reduce the temperature of panels from 70°C to 20°C in a matter of minutes, which can put a lot of stress on the material. The third water exposure route is dew – moisture

from the atmosphere that forms in small drops on any cool surface. Dew is important for polymer degradation for two reasons – it has a high oxygen content and long dwell times. This means that materials can be wet for long periods of time, typically six hours a day and in some tropical climates up to 16-18 hours per day. This makes dew the main source of outdoor wetness and the most influential of water-based weathering effects.

Directional decisions Laboratory-based accelerated weathering tests are intended to provide a tool for directional decision making. “Weathering science is often difficult to quantify,” says Francis. “However, they can be used to made decisions better and/or faster, as well as reducing the risk of making bad decisions or

58 COMPOUNDING WORLD | December 2017

making them too slowly. There is no one outcome for a weathering test. It depends on what you want to learn and how long a test you are willing to conduct. We break this down into classes of tests, based on how long they take, how realistic they are and how sophisticated.” The most common are quality

control tests, resulting in a pass or fail. The test time is defined and short and the results are com- pared to a given specification. A qualification/validation acceler- ated test also results in a pass or fail. The test time is defined and medium-long in duration and the results are compared to reference materials or specifications. A correla-

tive accelerated test provides rank-ordered data, with the test being open-ended and medium length. Results are compared to natural exposure on a benchmark site. Having this outdoor exposure data necessarily lengthens the duration of testing, but can provide more useful and realistic weather- ing test data. Finally, a predictive test results in a service life acceleration factor. The test is open- ended and long and results are compared to natural exposure in a service environment. There are two major test architectures in accelerated weathering, defined by how they

Schematic showing the layout of key components in Q-Lab’s Q-Sun Xe-3 xenon testing system Source: Q-Lab


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