How Long do I Need to Test?
In an
earlier post, we discussed the question by differentiating between three different test approaches or “why?” we run weathering tests. In this post, we will discuss the pros & cons of using radiant exposure as the basis for determining test duration.
Annual Radiant Exposure Data
Knowing the amount of global radiation reaching the earth is of great importance to many industries as well as climate research. Therefore, everyday it is measured in almost every country. At Atlas, we measure daily the UV irradiance (295-385 nm) as well as the global solar radiation (290-2500 nm) at our primary weathering testing sites in Homestead, Florida and New River, Arizona in the US, as well as Sanary, France and Chennai, India. This database supports all relevant weathering comparison studies.
The annual radiant exposure value in MJ/m² or kJ/m² needs to be translated in Ws/m² (1 MJ/m² =
106 Ws/m²). For irradiance, you enter the setpoint of your weathering device (using Daylight filters). This calculation requires that the spectral irradiance distribution, especially in the UV in the accelerated test (here xenon-arc radiation with Daylight filters) closely matches natural solar radiation.
As a result: inside a xenon weathering device which is set to 65 W/m² (300-400 nm) irradiance, an approximate equivalent amount of annual UV radiation like in Florida (approximately 350 MJ/m² at 300-400 nm) is achieved after approximately 1500 hours. If your accelerated test has a dark phase, this needs to be taken into account as well.
It is very important to note that the above calculation ignores the effect of temperature, moisture, and secondary weather effects. Therefore, this cannot be used to calculate acceleration factors and there is no guarantee that your materials which went through your xenon test will achieve the same level of degradation compared to the natural exposure.
Correlation to Natural Benchmark Exposures
Whenever you decide to do instrument weathering you should start in parallel complementary natural weathering testing - ideally at a benchmark site. Natural benchmark exposure data are invaluable to compare and understand product performance, and the good thing is that they don’t cost much. Why are they useful? Only by comparing instrument test results from your accelerated weathering test to the results from natural exposures can you verify the significance of your instrument test. Typically, correlation factors are determined by analyzing your specific material property change criteria such as gloss loss, color change, or cracking, to name a few. In case of insufficient correlation, you need to do further finetuning of the test settings until satisfying correlation is achieved. Once you have a well validated laboratory test, you can calculate “real” acceleration factors for your instrument test and answer the question: How long do I need to test? to simulate a specific number of months at the benchmark site or a comparable region.
More Information
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