Best practices in weathering - European Coatings€¦ · and accelerated in test devices such as the fluorescent UV ("QUV"), or xenon-arc test chambers. Such equipment can provide

Quelle/Publication: European Coatings Journal

Ausgabe/Issue: 04/2009

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Best practices in weathering

Manufacturers need to know how well their productswill perform in the environment in which they are used.Testing for weathering and light stability is essential.Thus, outdoor exposure testing and accelerated testingin the laboratory are widely used for research andproduct development, quality control, or materialcertification. Is one more reliable than the other? Canthey be correlated?

Outdoor and accelerated testing comparedMichael J. Crewdson*Warren D. Ketola 1)Most weathering damage is caused by light (especiallyshort-wavelength UV), high temperature and moisture. Anyone of these factors may cause deterioration. They maywork also together to cause even more damage.In the laboratory, these natural elements can be reproducedand accelerated in test devices such as the fluorescentUV ("QUV"), or xenon-arc test chambers. Such equipmentcan provide fast and reproducible results. Acceleratedweathering is a useful tool for product research and itis a requirement of many international specifications.Timepressures in product development may drive manufacturersto choose only accelerated testing and not outdoorexposures. However, variables exist which, withoutcomparative data, may lead to incorrect results or faultyconclusions.A well-planned weathering programme, incorporating bothoutdoor exposures and accelerated weathering, allowsmeasurement of the rank correlation between laboratoryand real-world results. Rank correlation is the amount ofagreement in how two sets of data rank performance. Anexample of the rank correlation statistic is given later in thispaper.Researchers must be cautious about trying to find an"acceleration factor" that will magically calculate howmany hours in a weathering test device equal how muchoutdoor exposure for all materials. There is too muchinherent variability in weathering testing and the response ofmaterials to the effects of light, heat, and moisture for sucha factor to be reliable.

Too many variablesA major problem in comparing outdoor and laboratorytests is that the weathering-chamber environment is wellcontrolled, whilst nature is not. Outdoor exposure tests aresubject to a number of variables. They include:››› Daily light/dark cycle and weather changes››› Latitude of the exposure site (more UV closer to theequator)››› Altitude (higher means more UV)››› Local conditions (e.g. a constant wind dries the testsamples)››› Random year-to-year variations in the weather(degradation can vary significantly in successive years atthe same location)

››› Seasonal variations (winter exposure may be less severethan summer exposure) (Figure 1)››› Orientation of thesample (many orientations can be used, ranging from 5°facing the equator to vertical facing away from equator)››› Sample insulation (outdoor samples with insulatedbacking often degrade 50 % faster than non-insulatedsamples)››› Variable properties of test materialsAccelerated testing has fewer variables. These may becontrolled by using correct operating and maintenanceprocedures:››› Laboratory test equipment operating cycle(light/dark/wet cycles are usually specified by a standard)››› Temperature used in the laboratory test (hotter is faster)››› Variability in the test equipment light source (spectral-power distribution, manual or automatic irradiance control,lamp and filter aging)››› Variable properties of test materialsNo matter how carefully the conditions are set, laboratoryand outdoor exposures are different one is controlled, theother is unpredictable. Not all of the factors of weatheringcan be re-created in the equipment used for acceleratedtests. There are cyclic conditions in nature that laboratorytests can closely simulate, but not match. The best practiceis to always use outdoor weathering to verify acceleratedtesting. Without this verification, results and predictionsfrom accelerated testing are ultimately just "guesses".

Start testing outdoorsIt is important to begin testing outdoors as soon aspossible. With a correct plan and execution, an outdoortest can produce meaningful data in a relatively shorttime. A successful outdoor weathering programme startswith good experimental design, use of control materialsand test repetition. An outdoor location should be chosenthat has the most intense weather for the required testconditions. Southern Florida provides a hot, wet and high-UV environment (Figure 2). Desert locations in Arizonaare very hot and very dry, with higher annual total UVthan Florida. Other outdoor climate selections are available,depending on how the product under test will be used.Any material can be tested outdoors. Choose anappropriate test for the material’s expected end-useenvironment. Natural, direct exposures may be combinedwith additional factors such as salt spray for corrosiontesting, or freeze/thaw cycles to produce thermal shock.Under-glass exposures permit testing of interior materials.Concentrator systems use mirrors to increase the intensityof solar radiation by 5 to 7.5 times for acceleratedoutdoor testing (Figure 3).It is important to consider criticalformulation variables when setting up the weatheringexperiment and to make sure that critical variables areincluded in each test. An equal number of specimensshould be exposed in each test and the exposure periodsshould be similar, taking into account the time of year.Mounting, handling and analysis methods must be repeatedconsistently to enable test comparisons to be reliable.



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Evaluation intervals must be scheduled so that rapidly-occurring changes are not overlooked.The greater the number of specimens, the better thedata analysis. The use of three or more replicates ofeach material under test allows mean and standarddeviation calculations and permits greater confidence in theconclusions derived from the results.The use of a control is very important. This is material whichreacts to weathering in a known way. It should not be astandard reference material such as the polystyrene chip,but a similar material to the one being tested for which thedata is reliable and repeatable. It is best to use controlmaterials with good and bad durability. The control can beused to compare different tests and different exposures andto normalize experimental results.Test repetition is vital to prove the reliability of the testmethod. The first step in writing a standard test is to provethat it is repeatable. The additional variable factors that canappear when testing need to be determined and measured.

Laboratory accelerated testingAccelerated weathering testing is used for R&D orproduct development and enables confidence in theselection between various material or process options whendeveloping a product, or between various product optionsfor an application.To determine which accelerated-weathering test conditionsto use, the type of degradation or failure that is mostcritical for your customer must be considered. For colourstability, a xenon-arc exposure device is normally used,because it provides the full wavelength spectrum of UV andvisible radiation known to cause fade and colour change(Figure 4). If surface integrity (such as gloss retention),or the retention of mechanical properties (such as impact,tensile, or flexural strength) are important, a fluorescent UVchamber with UVA340 lamps is generally used. It providesthe best possible simulation of short-wave UV, a radiationwhich is known to cause changes in physical properties.A wide variety of exposure conditions may be reproducedin either type of device. Here are some general principlesthat should help when selecting test conditions:››› High irradiance will produce faster results, but willprobably increase test equipment operation costs, becauselamps or filters will require replacement more frequently.››› A test temperature that is high, but not so high that it willcause unrealistic thermal degradation, should be used (70°C black-panel temperature is a reasonable starting pointfor many materials).››› The highest possible moisture stress is needed (waterspray, relative humidity, condensation time). In most cases,product failures occur faster in climates where dew causesmaterials to become wet for longer periods. Fluorescent UV/condensation exposure is the only accelerated weatheringtest that actually produces dew on the surface of materialsduring exposure.When comparing materials, a control provides a benchmarkfor evaluating the performance of new materials andassessing their durability for use in a product. Comparisonsof test materials must not be made with a material ofknown performance that was exposed at a different time.Test variability makes such comparisons extremely risky.A control material must be exposed side-by-side with testmaterials.

Replicate specimens of all the test materials and of thecontrol must be used. Using only one specimen of eachmaterial does not permit statistical analysis of the results;so one cannot be confident about differences between thematerials. Two replicates is an acceptable number, butthree or more is better.It is best to run the test until the materials fail. Anotherapproach is to stop once there is a statistically significantdifference between the test and control material. The bestmethod is to run the test until all the materials fail.

Comparing exposure resultsIn order to develop confidence in accelerated tests, it mustbe possible to compare their results with those from outdoorexposures hence, the critical need to start outdoor exposuretesting first. Ideally, there would be outdoor results formaterials or products with a range of performance fromgood to poor. When accelerated weathering tests are runon these materials, rank correlation is used to determinewhether the test conditions chosen will be useful for theapplication.Control materials should be exposed in each test andthe type and rate of degradation compared with the testmaterial. This will ensure that the accelerated test isproviding the correct results. Table 1 shows an example ofa balanced weathering test programmeThe degradation mode must be the same in all laboratoryand outdoor tests. Tests that produce different failuremodes will have poor correlation to outdoor tests, since theyare not affecting the same variable. All known degradationmodes should be evaluated.Non-destructive observations such as visual appearance,gloss and colour measurement can be measuredrepeatedly on the same sample. However, to obtaininformation such as chemical and internal properties forthe samples, destructive methods are needed and severalsamples should be used. Tensile strength, hardness andabrasion are also evaluated using destructive test methods.Statistical methods may be employed to assess thereliability of test results. Spearman rank-order correlationcan indicate whether an accelerated laboratory test is agood predictor of real-time, outdoor weathering results.

Spearman rank correlationThis is carried out by ranking the order of performance forthe materials in each test and determining the differencesbetween the rank data by subtraction.The differences are squared and added up to give Thefollowing equation is used to calculate the Spearman rankcorrelation coefficient rs (N is the number of materialscompared).Rank order sorts the results in order ofperformance. For example, if ten materials are being tested,each material is assigned a rank number from 1 to 10in order of performance. Materials that are the same inperformance are assigned the same rank.The time to achieve 50 % loss of initial gloss is an exampleof a measurable target for rank order. In this case, rankingshould be done at the end of the test. If measuring aproperty or change in a property for ranking, this can takeplace at any time. The highest rank correlations occur whenthe difference in performance between the materials is thelargest.

Acceleration factor



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Accelerated (laboratory) and natural (outdoor) test resultsare often compared by looking at the respective exposuretime needed to produce the same degree of degradation.Ranking is used to verify the observation. Alternatively, themean values of the experimental data sets are compared.By correlating laboratory and outdoor test results, it ispossible to calculate an acceleration factor (AF). This is theoutdoor exposure time divided by the laboratory exposuretime needed to produce the same amount of change in thematerial.An acceleration factor for one material is often wronglyassumed to apply to other materials and may be incorrectlyused to develop warranties for product lifetime. The hugerange of material responses to the effects of light, heat, andmoisture means that an acceleration factor for one materialwill not apply to others. Thus, using one acceleration factorfor many different materials can lead to very large errors inthe estimates of actual times to fail in the real environment.

Tips for successful testingAccelerated weathering must be verified by outdoorweathering. Laboratory testing is a useful tool for productresearch and development. Without benchmark outdoordata, there are a number of variables that may lead to awrong conclusion.It is important to use as many replicates as possible for eachmaterial being evaluated in a weathering programme. Thespecimens under test should be examined and evaluatedoften and regularly. Control or reference specimens areuseful for comparison with historical data. Tests should berepeated to gain confidence that the observed weatheringeffect is understood.Relative costs should be considered in planning the testprogramme. Outdoor weather testing provides inexpensive,realistic data. The cost to test a useful set of samples can beas low as $500-$1000 USD a year. This small expense canresult in very useful benchmark data. Accelerated testingentails higher costs, but results are available much faster.The cost of not testing at all is potentially the highest. It canresult in recalls and replacements and damage a company’sreputation for producing reliable products.Fast and easy formulas do not exist. A single conversionfactor between hours of accelerated weathering and monthsof outdoor exposure would desirable, but is theoreticallyimpossible. The former is measured under constantconditions, whereas the conditions of the latter are variable.The search for a reliable conversion factor requires pushingthe data beyond the limits of its validity. Weathering data iscomparative, not absolute. í

REFERENCES[1] M. Crewdson, Outdoor Weathering Must VerifyAccelerated Testing (technical paper presented at TheWaterborne Symposium, 2008, New Orleans, Louisiana) [2]D. Grossman, Correlation Questions & Answers, Q-PanelLab Products Technical Bulletin LU-0833, 2002. [3] W.Ketola, Make the Most of Accelerated Laboratory Testingand Don’t Trust Acceleration Factors to Compare Lab &Outdoor Testing, Q-Lab Corporation LabNotes, 2008-2009.[4] ASTM G141, Standard Guide for Addressing Variabilityin Exposure Testing of Nonmetallic Materials. [5] ASTMG169, Standard Guide for Application of Basic StatisticalMethods to Weathering Tests.

* Corresponding Author:

Michael J. CrewdsonQ-Lab WeatheringResearch ServiceTel. +1 305 245 [email protected]

Results at a glance»› Accelerated weathering must be verified using outdoorweathering. »› Laboratory testing is a useful tool for productresearch and development, but without benchmark ofoutdoor data, there are a number of variables that maylead to a wrong conclusion.»› Statistical comparison ofaccelerated and outdoor weathering tests is essential.»›Reference specimens, a large sample size and repeatedtesting are the key to confidence in the results.»›Weathering analysis costs money. Not doing it costseven more.»› A single conversion factor for acceleratedv. outdoor exposure does not exist. Weathering data iscomparative, not absolute.



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Best practices in weathering - European Coatings€¦ · and accelerated in test devices such as the fluorescent UV ("QUV"), or xenon-arc test chambers. Such equipment can provide