Outdoor Correlation & Real World Life

We are often asked "how does laboratory testing correlate with the real outdoors?" or "how long will my product last outside?". Whilst these seem simple questions there are no simple definitive answers.

It is not possible to directly correlate laboratory accelerated weathering with real outdoor exposure. The main reason for this is that the outdoor environment is inherently variable throughout the day, across the seasons and is highly dependent on the specific geographic location on the Earth’s surface. In addition, other outdoor factors not recreated in the Laboratory environment, such as wind, rain and pollution, may contribute to the degradation of a material. See more under our uv map. Even in locations like Florida the day the day variation can be great. Testing in these locations has shown variations can be as great as 7 times when testing duplicate samples.

Laboratory UV testing can be easily replicated and is consistent with respect to UV intensity, temperature and moisture. Different standards specify different simulated weather cycles, we commonly test to ISO 4892 which is generally accepted to give a good simulation to outdoor exposure. However, other cycles may give different correlations.

The QUV or Xenon arc provides a constant level of intense UV radiation, and therefore simulates a constant application of midday sun. This means that the laboratory conditions produce a significant acceleration factor, particularly when used in conjunction with heat and moisture. The significance of this acceleration factor is also dependent on the material type. Porous or open weave materials such as textiles will have higher acceleration factors as the moisture generated during the test can easily penetrate the samples. Plastics and solid substrates, with non-porous surfaces will have lower acceleration factors, and require longer tests for the same amount of UV degradation.

However, having said all this, over the 15 years of testing experience we have been able to generate some very rough rules of thumb for clients. The approximate acceleration factor obtained with the standard fluorescent tube type laboratory test for plastic or non porous materials tested in accordance with ISO 4892 is very roughly 10 – 20 times. Hence a 500 hour test approximately equates to 7- 14 months of exposure in a hot, humid location near the equator, such as Florida. Hence we normally recommend clients start at 500 hours as this will quickly determine if you have a poor performing material. If it performs well at 500 hours it is easy to continue to longer durations. The most reliable method is to test until you reach a defined failure point.

A 500 hour test is also a good starting point for a series of comparative tests. Rather than trying to determine a quantitative set of results for a single material type, which is often a lengthy and complex process, a qualitative comparison of a range of materials can be easily performed. Using a control or a sample that has a well known extensive service history as a benchmark increases the value of the data produced by such a test. This is often the best option when trying to compare the performance of various suppliers. Ranking the performance of your new suppliers against historical ones ensures that you compare on a like for like basis.

Testing your products at several different exposure durations is better than doing a single duration. It will help identify more accurately any progressive degradation in the material or any sudden changes. For onerous outdoor applications e.g. tropical zones, longer duration testing should be considered. As an example the cable industry will screen materials at the 500 hour and 1000 hour point and will look for minimum rentention of properties of 70% and 60% retention respectively.

Whilst direct correlation of artificial UV weathering to real life is difficult the data generated is extremely valuable and is beneficial in terms of both product design or any potential liability issues. There is no substitute for testing your product and in many areas Materials Technology staff will have experience of testing similar products and will be able to provide useful advice and recommendations.

Call us now to discuss your specific needs.