Q. When should I choose a) the QUV accelerated weathering tester vs. b) the Q-SUN xenon-arc lightfastness tester vs. c) Q-Lab outdoor testing?
- The QUV weathering tester is fast and economical. Fluorescent UV lamps provide the best simulation of solar UV. However, the QUV lacks the longer wavelengths necessary for testing certain materials.
- Short-wave UV: The QUV tester provides the best possible simulation of sunlight in the short-wave UV region, making it ideal for testing durable materials such as coatings, roofing and plastics. Its fluorescent UV lamps are spectrally stable, enhancing reproducibility and repeatability.
- Condensation: The QUV tester's condensation system (100% RH) is the most realistic acceleration of outdoor moisture attack. Penetrating moisture may cause damage such as blistering in paints.
- The Q-SUN xenon arc tester reproduces the full spectrum of sunlight, including UV, visible and infrared. It is especially useful for testing dyes, pigments, textiles, inks and indoor materials. However, xenon arcs are inherently less stable than fluorescent lamps and water spray is less realistic than the QUV tester's condensation cycle.
- Full-spectrum sunlight: Xenon arc lamps produce the full spectrum of sunlight. With optical filters, the Q-SUN tester can reproduce daylight and sunlight through window glass exposure conditions.
- Humidity and moisture: The Q-SUN tester is excellent for light stability testing of humidity-sensitive materials such as textiles, inks, cosmetics and pharmaceuticals. With superior moisture dwell properties, it is also the only xenon tester suitable for accelerated 'Jacksonville' acid etch testing of automotive clearcoats.
- Q-Lab Outdoor Testing
- Q-Lab Arizona and Florida represent the world standards for sunlight and moisture. They allow the most realistic prediction of product performance. While some exposure tests can take years to complete, there are techniques that can be used to gain useful data in the short term.
- Natural Florida Weathering: Florida subtropical exposures have abundant sunlight, high temperatures and lots of moisture. Most materials should undergo natural Florida benchmark exposures. Even though natural tests can take several months or years to complete, results are the most realistic, and provide benchmark data for accelerated testing.
- Natural Arizona Weathering: Arizona desert exposures lack moisture, but have even more UV sunlight than Florida and extreme, fluctuating temperatures that may cause thermal shock.
- Q-TRAC Natural Sunlight Concentrator: In just one year, specimens can receive as much natural sunlight as 5 years in Florida.
Q. How many hours in a Q-SUN Xenon Test Chamber or 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 weathering tester 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 tester 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 uninsulated samples).
- Operating cycle of the tester (hours of light and hours of wetness).
- Operating temperatures of the tester (hotter is faster).
- The particular material tested.
- The Spectral Power Distribution (SPD) of the laboratory light source.
Obviously, it is logically meaningless to talk about a conversion factor between hours of accelerated weathering and months of outdoor exposure. 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 accelerated weathering testers. But you must realize that the data you get is comparative data, not absolute data. The most you can ask from laboratory weathering are 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 in 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 to your standard material that 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. This is much sooner than conventional coatings used for the same purpose. However, after 3 years of continual reformulation and retesting in the QUV tester, the coating was improved so that various formulations could withstand 2,000 to 4,000 hours in the QUV tester - much better than the conventional coatings. Subsequent 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 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 conversion factor, hundreds of labs have successfully developed their own internal 'Rule of Thumb' for converting their Q-SUN or QUV tester 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 accelerated tests with their own outdoor exposures. Furthermore, the rule of thumb conversions are valid only for:
- The specific material tested.
- The specific set of lab tester time cycles and temperature.
- 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 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.
Many labs have successfully developed their own 'Rule of Thumb' for converting Q-SUN or QUV tester hours into exposure hours.
In addition, it is important to remember: 'Correlation' means 'Rank Correlation.' When someone asks, 'How do the accelerated testers correlate with outdoors?' what they really should ask is 'How well do rankings of materials' durability in the accelerated testers duplicate the rankings of materials outdoors?' To measure rank correlation, we recommend Spearman's rho, a statistical measure that 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 tester and Florida durability rankings of 27 automotive coatings produced rank correlations of up to .89 between QUV tester rankings and Florida rankings. The rank correlation between different Florida exposures was .88 to .95. In other words, the QUV tester can reproduce Florida rankings almost as well as Florida can reproduce itself.
See Technical Bulletin LU-0833 for more information.
Q. How many Langleys or joules or Watts/m2 do the Q-SUN and QUV testers produce?
This question sounds straightforward, but it is based on some erroneous assumptions. Generally the person asking this intends to take the light output of the testers (expressed in Langleys, joules, or Watts/m2) and divide it by the intensity of outdoor sunlight to get a magic factor for converting accelerated tester 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 photodegradation is not the total light dosage in joules, but rather, how those joules are distributed with respect to wavelength. A joule of UV light (short wavelength), for instance, can be more damaging than a joule of visible or infrared light (longer wavelength), depending on the material you are testing.
In addition, the amount of UV in sunlight varies quite a bit, which can have a tremendous effect on the weathering of samples. 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 (TUV), such as the 'UV Langley' or 'UV joule,' may be misleading because the same reasoning applies: within the UV, shorter wavelengths generally cause faster degradation to durable materials.
Here is an example of the wrong conclusions you can get from using Langleys, joules, or even TUV to evaluate accelerated weathering testers. The QUV tester 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? Not always. Many materials will degrade slower with UV-A lamps because the UV they produce is longer wavelength UV. In the Q-SUN tester, you will find these same variations depending on the filters used.
Another reason you can't compare the light intensities of the Q-SUN or the QUV testers with sunlight is that such procedures completely ignore 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 color 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 an accelerated tester, and it is possible to have a wide range of temperatures in outdoor exposure. Temperature has a profound effect on the speed of photodegradation. We observe in our accelerated testers that in some cases a 10°C increase in test temperature can double the speed of degradation.
For more information, see Q-Lab Corporation technical bulletin LU-8030, Errors Caused by Using Joules to Time Laboratory and Outdoor Exposure Tests.
See Technical Bulletin LU-0833 for more information. Q. What is the conversion factor between hours in a QUV tester and hours in a Q-SUN xenon arc test chamber?
This is another simple question with no simple answer. The shapes of the SPD curve are different for each type of tester. Therefore, there is no mathematically valid procedure for computing a ratio of photodegradation power. In addition, different filters can be used in xenon testers, which make it even more difficult to make comparisons to a QUV tester.
In the same way, it is difficult to compare either one of these testers to a carbon arc tester. Again, the SPD curves are different. Results may vary depending on filters used, and also the kind of carbon arc used ('sunshine' vs. 'enclosed').
Furthermore, the testers use moisture mechanisms that are fundamentally different.
Finally, laboratory weathering is material-dependent. A material that is vulnerable to visible light and longer-wave UV will usually degrade much more quickly in a xenon tester. But a material that is vulnerable to shorter-wave UV will usually degrade much more quickly in a QUV tester.
See Technical Bulletin LU-0833 for more information.
Q. What is Cyclic Corrosion Testing?
Cyclic corrosion testing is intended to be a more realistic way to perform salt spray tests than traditional, steady state exposures. Because actual atmospheric exposures usually include both wet and dry conditions, it makes sense to pattern accelerated laboratory tests after these natural cyclic conditions. Research indicates that, with cyclic corrosion tests, the relative corrosion rates, structure and morphology are more similar to those seen outdoors. Consequently, cyclic tests usually give better correlation to outdoors than conventional salt spray tests. They are effective for evaluating a variety of corrosion mechanisms, including general, galvanic, and crevice corrosion.
Cyclic corrosion testing is intended to produce failures representative of the type found in outdoor corrosive environments. CCT tests expose specimens to a series of different environments in a repetitive cycle. Simple exposures like Prohesion may consist of cycling between salt fog and dry conditions. More sophisticated automotive methods call for multi-step cycles that may incorporate immersion, humidity, condensation, along with salt fog and dry-off. Originally, these automotive test procedures were designed to be performed by hand. Laboratory personnel manually moved samples from salt spray chambers to humidity chambers to drying racks, etc. More recently, microprocessor controlled chambers have been used to automate these exposures and reduce variability.
See Technical Bulletin LF-8144 for more information.
Q. Why should I do weathering or light stability testing at all? What is my ROI?
The Big Payback on Weathering & Light Stability Testing: Quite often weathering and light stability testing yields huge returns on investment. Here are some of the returns you can get from a relatively modest outlay on testing.
Catastrophes Avoided: A new product or a reformulated product could fail very quickly when subject to sunlight and weather. The best protection against this is to test it before introduction. Even established products can fail if a production batch is made improperly or made with a bad shipment of some component material. What is the cost of a recall of a new product or a production batch that fails catastrophically in the field?
Big Savings on Material Costs: Maybe your product could be just as durable with less expensive materials – for instance a lower cost pigment, or the same pigment from a lower cost supplier. Maybe you could get the same performance with a reduced amount of some expensive additive. Or maybe a totally new polymer could lower your cost without hurting durability. The only way to know is to test. How much would a 1% savings on material cost be worth? What about 5%, or 10%?
Enter New Markets: To break into a new market you need to meet the customer's expectations for durability. You can hope that your product has the required durability, or you can test for it. If you find that your product's durability needs further development, you can use testing to improve it. You can re-design and re-test until you get the durability required to break into the market. What is the dollar value of being able to enter a new market?
Expand Existing Products: Even a relatively small change to a product, like a new color, can make a big difference in light stability. To take advantage of those small enhancements that can make a big difference to a product line, you need to evaluate the durability by testing it. What is it worth to extend your product line, without compromising durability?
Improve Product Durability: Frequently small changes in your materials can make big improvements in durability. Often our clients make dramatic improvements in durability without increasing their costs. The only way to do this is to test different materials and combinations of materials. What is it worth to have a more durable product?
Reduce Warranty Claims: How much are warranty claims costing you? How much could you save by reducing your warranty claim rate?
Verify Suppliers' Claims: Each of your suppliers truly believes that they have the best combination of cost and durability. And each of them is probably right - for some set of circumstances. How do you know if what is 'good enough' to your supplier is good enough for you? To find out which supplier is right for your particular requirement, you need to test your suppliers head to head, either in your own lab or at an independent third party like Q-Lab.
Expand Market Share: A more durable product can expand your market share at the expense of your competitors. However, you can't develop better durability unless you test durability.
Stay Ahead of Government Regulation:
Environmental and safety regulations make it increasingly expensive to use many proven raw materials. And every year, even more materials become restricted. Redesigning with environmentally friendly materials is now a way of life. Some of these new materials won't compromise your durability, but some will. The only way to be sure is to test. How much can you save by picking materials that bypass costly regulations? How much would it cost if a new regulation forces you to use a substitute material with poor durability, or a material with high cost?
Outrun the Competition: The pace of innovation in materials keeps getting faster. Better and cheaper materials are introduced every day. If you don't take advantage of them, someone else will. The only way to evaluate these materials is to test them. What is the value of taking advantage of new materials before your competitors do? What is the cost of playing catch up?
Enhance Your Reputation: The products you sell now will affect your reputation far into the future. A premature field failure can haunt you for years to come. On the other hand, products with consistent durability will act as advertisements for your quality throughout their lifetime. An investment in durability testing can protect and enhance your reputation like few other investments. What is your good name worth to you?
Ensure Compliance With Durability Standards: Some customers require light stability test data as a pre-condition for buying your product. In that case you can't sell at all, until you produce test data either in your own lab, or in an independent contract testing lab.
Q: I've never done weathering testing before but I think I want to send in some specimens for testing, what do I do?
If you're almost ready to do some testing but you've never done this before and you're not sure how the process works, don't worry it's quite simple. There are a few important things to remember though to make it easier and get things rolling faster.
The first step is to contact us, and the telephone or email works equally well for a first time contact. The best number to call for new service is our Florida location at +1 (305) 245-5600 and let the receptionist know you want to discuss a new test. If you email us at firstname.lastname@example.org
asking for a call back, there are a few bits of information that you can include which will help us prepare.
- What is your product, and what material is it made of.
- What is the end use and where is the expected usage location
- What is the anticipated time of durability and what are the expected failure modes
- What are your objectives from the testing
One of our customer service people will contact you back or will talk with you immediately, and our aim is to determine the best test method for your needs. During this discussion we will ask you some more questions such as
- Are you testing to meet a specification or to improve durability
- What is your budget for testing
- What do you need to learn from the testing
- What time frame are you working within
We will run through the options available both for accelerated and outdoor testing, and together we will agree on which is the most suitable weathering test or tests for you. We will agree on the following specific items
- Test method to use, and the type of exposure
- The quantity of specimens to test
- The dimensions of your specimens and their suitability for testing
- The type and frequency of evaluations or measurements
- The total duration of exposure and whether interim removals are needed
- How the test is going to be paid for
Once this is all gathered, someone at Q-Lab will prepare a cost estimate for you which will also include all the parameters of the test. This will confirm the testing that we will perform for you and give you an idea of the costs involved. If you need to, you may contact us again and revise the test program. We will issue you with a revised cost estimate. exposure analysis.
When everyone is agreed that the test program and costs are acceptable, you should now go ahead and send the specimens to Q-Lab. Typically you can send your test specimens directly to the test site location where the testing will take place. There are a couple of items you will need to include in the box of specimens, or you can provide them to us separately. We need a purchase order and a list of the specimens to be tested. We cannot start the checking in process until we have a purchase order or pre-payment for the test. We also need a full ID list, preferably in a spreadsheet, so that we can transfer the labels directly into our computer system. This speeds up the entire process and reduces the chance for errors. If you send us the test information separately, please put a cover letter or note in the box, so we can connect the specimens to an order.
The process of setting up the test is known as "checking in", and we may need to contact you during this phase if there is any missing information. Our goal is to get all test specimens exposed very soon after their arrival, provided we have all the necessary information, and a purchase order. Once the specimens are exposed, you will shortly thereafter receive an email from us with your user account and password for our customer web site at www.myweathertest.com You will be able to get all your test documentation and reports from this website.
If you have any questions at any time during the set up of your test, you may call any of our customer service people in Florida, Arizona, or Ohio. Once the test has started you may contact either your customer service person, or talk directly to the lab technicians performing the test.
If you have any feedback for us regarding our service, please let us know. Or you can use the customer service feedback feature on our website.