Q. How many hours in a QUV Weathering Tester equals a year of outdoor exposure?
This is a simple question, but unfortunately there is no simple answer. It is theoretically impossible to have a single magic number that you can multiply by QUV exposure hours to compute years of outdoor exposure. The problem is not that we just haven’t developed the perfect weathering tester yet – no matter how sophisticated or expensive you make your weathering tester you still won’t find the magic factor. The biggest problem is the inherent variability and complexity of outdoor exposure situations. The relationship between QUV exposure and outdoor exposure depends on a number of variables including :
- The geographical latitude of the exposure site (closer to the equator means more UV).
- Altitude (higher means more UV).
- Local geographical features such as wind to dry the test samples or the proximity of a body of water to promote dew formation.
- Random year-to-year variations in the weather, which can cause degradation to vary as much as 2:1 in successive years at the same location.
- Seasonal variations (i.e. winter exposure may be only 1/7th as severe as summer exposure).
- Orientation of the sample (5° South, vs. vertical North).
- Sample insulation (outdoor samples with insulated backing often degrade 50% faster than non insulated samples.
- Operating cycle of the QUV (hours of UV and hours of condensation).
- UV operating temperatures (hotter is faster).
- The particular material tested.
Obviously it is logically meaningless to talk about a conversation factor between hours of accelerated weathering and months of outdoor exposure. The one is a constant condition, whereas the other is variable. Looking for a conversion factor requires pushing the data beyond the limits of its validity.
In other words : Weathering data is comparative data
Nevertheless, you still can get excellent durability data from the QUV, but you must realise that the data you get is comparative data, not absolute data. The most you can ask from laboratory weathering is reliable indications of the relative ranking of a material’s durability compared to other materials. In fact, the same thing can be said about Florida exposure tests. Nobody knows how a year on an outdoor “Black Box” exposure at 5° South compares to a year on a house or a car. Even outdoor testing gives you only relative indications of actual service life.
Comparative data, however, can be very powerful. For instance, you might find that a slightly altered formulation has over twice the durability of your standard material. Or you might find that among several suppliers offering what look like identical materials, some fail very quickly, most fail in a medium length of time, and a few fail only after prolonged exposure. Or you might find that a less expensive formulation has equivalent durability of your standard material which has given acceptable performance over, say 5 years, of actual service.
Here is a good example of the power of comparative data. A coatings manufacturer was developing a new type of clear coating. Initial QUV tests caused severe cracking in 200 to 400 hours – much sooner than conventional coatings used for the same purpose. However, after 3 years of continual reformulation and re-testing in the QUV, the coating was improved so that various formulations could withstand 2,000 to 4,000 hours in the QUV – much better than the conventional coatings. Subsequently parallel tests in Florida showed a similar 10:1 increase in durability. Yet if the coatings chemists had waited for the Florida data before changing their formulations, they would today still be back in the early stages of reformulation and the coating wouldn’t be the commercial success that it now is.
On the other hand : If you still insist on a “Rule of Thumb” conversion factor, find it empirically.
Despite the impossibility of a universal factor, hundreds of labs have successfully developed their own internal “Rule of Thumb” for converting their QUV hours into outdoor exposure hours. However, it is important to remember that these rules of thumb were developed from empirical comparisons of the lab’s own QUV tests with their own outdoor exposures. Furthermore, the rule of thumb conversions are valid only for :
- the specific material tested
- the specific set of QUV time cycles and temperatures
- the specific outdoor exposure site and sample mounting procedure.
If you have outdoor experience with your materials, it shouldn’t take more than a few months to develop your own QUV rule of thumb. If you don’t have experience with your own materials, it may be possible to work with competitive materials that do have a history of outdoor service. The World’s Most Widely Used Weathering Tester In addition, it is important to remember : ” “Correlation means “Rank Correlation”. When someone asks “How does the QUV correlate with outdoors?”, what they really should ask is “How well do rankings of materials’ durability in the QUV duplicate the rankings of materials outdoors?”. To measure rank correlation, we recommend Spearman’s Rho, a statistical measure which is easy to compute and which does not require the type of strong assumptions about the data that are required by linear correlation measures. A study of QUV and Florida durability rankings of 27 automotive coatings produced rank correlation of up to .89 between QUV rankings and Florida rankings. The rank correlation between different Florida exposures was .88 to .95. In other words, the QUV can reproduce Florida rankings almost as well as Florida can reproduce itself.
The World’s Most Widely Used Weathering Tester
In addition, it is important to remember : “Correlation means “Rank Correlation”.
When someone asks “How does the QUV correlate with outdoors?”, what they really should ask is “How well do rankings of materials’ durability in the QUV duplicate the rankings of materials outdoors?”. To measure rank correlation, we recommend Spearman’s Rho, a statistical measure which is easy to compute and which does not require the type of strong assumptions about the data that are required by linear correlation measures. A study of QUV and Florida durability rankings of 27 automotive coatings produced rank correlation of up to .89 between QUV rankings and Florida rankings. The rank correlation between different Florida exposures was .88 to .95. In other words, the QUV can reproduce Florida rankings almost as well as Florida can reproduce itself.
Q. How many Langleys or joules or watts/m2 does the QUV produce?
The question sounds straight froward, but it is based on some erroneous assumptions. Generally the asker intends to take the light output of the QUV (expressed in Langleys, joules, or watts/m2) and divide it by the intensity of outdoor sunlight to get a magic factor for converting QUV exposure hours into outdoor exposure years. Unfortunately, there is no mathematically valid way to make such a calculation, because it runs counter to the most basic principles of accelerated weathering. (Not to mention that, by definition, the Langley refers only to the sun and not to other light sources). The result of such a calculation is at best meaningless, and at worst totally misleading.
One reason such a computation is invalid is that it ignores the effect of wavelength. What determines the amount of photo degradation is not the total light dosage in joules, but how those joules are distributed with respect to wavelength. A joule of UV light (short wavelength) is vastly more damaging than a joule of visible or infrared light (longer wavelength).
The QUV wavelengths that are crucial for weathering damage constitute only a tiny percentage (less than 5%) of the energy in sunlight. But Langleys and joules are measures of the total overall energy in sunlight. As such they are essentially measures of the harmless visible and infrared light. Because the UV in sunlight varies much more than the visible and infrared, Langleys and joules fail to reflect the wide variations in solar UV that occur from season to season, day to day, and in fact hour to hour. For this reason a number of studies have shown that in successive outdoor exposures where replicate samples received the same exposure in Langleys, there can be as much as a 7:1 variation in the amount of damage produced. In other words, the Langley is too inconsistent to be used as a standard measure of outdoor exposure. The conclusion is clear : The Langley may have valid uses, but certainly not in the field of laboratory weathering. Even a measurement of total UV, such as the “UV Langley” or “UV joule”, may be misleading because the same reasoning applies : even within the UV, the shorter wavelengths are much more damaging than the longer wavelengths. Here is an example of the wrong conclusions you can get by using Langleys, joules, or even UV joules to evaluate accelerated weathering testers. The QUV can use two types of lamps : UV-A lamps with a peak emission at a wavelength of 340 nm, or UV-B lamps with a peak at 313 nm. The UV-A lamps produce more joules (and more UV joules) than the UV-B lamps, so isn’t it reasonable to deduce that the UV-A lamps will produce faster degradation? Wrong. The UV-A lamps actually are usually substantially slower because the UV they produce is longer wavelength UV.
Another reason why you can’t compare the light intensities of the QUV and sunlight to compute a conversion factor, is that such a procedure completely ignores the effect of moisture. We find that for many materials the effects of rain and dew are more important than the effects of sunlight. This is often true even for phenomena like gloss loss and colour change which are sometimes considered to be UV induced changes. If you don’t take moisture into account, you can’t possibly come up with a magic conversion factor.
Finally, a conversion computation based on light intensity is invalid because it ignores the effect of temperature. It is possible to choose a wide range of temperatures in the QUV, and it is possible to have a wide range of temperatures in outdoor exposure. The temperature of UV exposure has a profound effect on the speed of photo degradation. We observe in the QUV that in some cases a 10°C increase in test temperature can double the speed of degradation.
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Q. What is the conversion factor between hours in a QUV and hours in a carbon arc or xenon arc tester?
This is another simple question with no simple answer. For instance, a “sunshine” carbon arc tester may produce degradation 5 times as fast as an “enclosed” carbon arc tester. Furthermore, 2 different models of sunshine carbon arcs can give very different results. Even if you limit yourself to one particular model of sunshine carbon arc, changing or removing the light filters can change the speed of degradation as much as 5 to 1. The situation is similar for xenon arc testers. Different manufacturers, different models, and different filter combinations all give varying results. This means it is impossible to address the question of how the QUV compares unless you specify the exact type of arc machine and the exact operating parameters. Even then it is difficult to make a quantitative comparison. The shape of the Spectral Energy Distribution curve is different for each tester, so there is no mathematically valid procedure for computing a ratio of photo degradation power. For instance, total light intensity in the QUV is only a small fraction of the intensity in a xenon arc machine, yet the QUV routinely produces photo degradation 2 to 4 times as fast. Furthermore, the QUV’s condensation system is fundamentally different from the spray system used in arc machines, so it is difficult to predict the exact amount by which the moisture acceleration in the QUV exceeds moisture attack in arc machines.