People have intentionally tested coatings for their resistance to environmental exposure for just over 100 years, and unintentionally since the dawn of mankind. Light-fastness and weatherability testing for color and appearance properties such as fading, color shift and gloss loss, as well as physical performance such as adhesion or cracking is necessary for many reasons. These tests may be used to evaluate new ingredients, formulations, technologies and applications; to benchmark competitive products; to meet customer, industry or regulatory requirements; or to understand product liability and warranty issues, among other needs.
However, the need for better (e.g., faster, more predictive) weatherability testing of materials and products is growing due to several significant factors. Key among these are:
· frequent changes in formulation, such as pigment chemistry and low-VOC content, brought about by environmental and health safety legislation or cost-reduction initiatives;
· sourcing or producing ingredients or products from new suppliers or regions, such as “Chindia”;
· new “functional” roles for coatings in addition to decoration and protection;
· increased harsh environmental exposure to products, such as the use of more natural day-lighting in buildings, the 'mobile' generation, the globalization of product distribution and end use;
· increased customer expectations of quality, especially for premium brands; and
· faster time to market, less time and resources for product testing, competitive pressures.
Let's explore some of these issues more closely.
Formulation Changes
Regulatory requirements have forced coatings formulation changes. One example are the EU Directives restricting certain colorants such as azo dyes, cadmium and hexavalent chromium pigments and others; VOC reduction legislation is also a significant driver. For example, one contract office furnishings company had major fade and hue shift coloration problems on their coated file cabinets when they switched from traditional to ‘green’ colorants that had poor light-fastness.Raw materials or finished products are often being sourced from new suppliers, often with unexpected consequences. One premium manufacturer of window fashions started to source white, painted wooden window blinds from China.
Some production batches were fine, but random lots exhibited severe yellow discoloration once installed. The problem was traced to the coating manufacturer sourcing inferior grades of white titanium dioxide pigment from the open market to save money.
Sometimes products are used in new and innovative applications, such as in markets or geographies that were not anticipated. A kitchen cabinet manufacturer faced a major problem when white, pantry cabinet doors started to yellow once installed in a new home development project. The color shift of the doors was relatively minor but very noticeable against the ‘pure white’ – a different coating formulation – of the wood frame. The problem quickly surfaced once the cabinets were installed during home construction in a southern U.S. state with high solar radiation, outside of the manufacturer’s normal northern distribution territory.
Even coatings not intended for long-term direct weather exposure can have problems. One supplier of pre-primed exterior wood trim boards sold through big-box home improvement stores had major problems in the New England states. Made from low-cost, fast-growing pinus radiata from South America, the product had a factory-applied white primer coating. Unfortunately the primer looked like a finish coat and the product was widely installed as finished house trim with dire consequences of severe warping, cupping and cracking. As most coating failures are the result of improper use or installation, it is advisable for manufacturers to test for the lack of “robustness” of a product to improper application, such as poor substrates, bad weather, etc., or to test surface preparation requirements for specific coatings.
Product Testing
Comparative TestingThere are a number of exposure testing tools that can be used to evaluate the weather durability of coatings. However these tools can be used in various ways to elicit information from your coating. For example, the easiest test to understand is comparative testing, or the “put it out and see what happens” approach. In this case, various test specimens are exposed to natural, artificial or accelerated weathering conditions and evaluated or compared at various points, usually until failure. The resulting information is usually limited, however, and often raises rather than answers questions. For example, how would the test samples rank in other climates? How and why did they fail?
Experimental Design
The next level, designed experiments, can test a range of factors – both product formulation and climate factors – in an organized, methodical approach. This minimizes testing that is often difficult, time consuming or expensive to accomplish, and provides more usable information. It requires more information and discipline to design and implement but can provide data such as “interaction effects” not obtainable in comparative testing.
Forced Degradation
Lastly, “forced degradation” testing, using stress conditions that may be more severe than will be seen in actuality, can be used to detect product susceptibilities or weaknesses and determine the “robustness” of the product to various conditions.
The amount of information resulting from testing is proportional to the effort put into it. Proper testing can provide data invaluable for improving the product, reducing material costs, improving processes, developing new markets and applications, determining product warranties and lifetime expectations, and providing competitive information to drive the business.
Weatherability Testing Tools
Products are usually tested for weatherability by exposing them to the expected service environment or, more precisely, environments. This may be in real-world situations, at formal private or commercial exposure test sites or under controlled, laboratory-simulated conditions. For outdoor exposures, “trade” panels of various substrates, or prepared test coupons such as painted metal panels or special samples can be used. These would generally be affixed to south-facing (in the northern hemisphere) exposure racks of various standard types and angled from near horizontal to vertical, depending on a number of factors such as product end-use orientation (roof vs. wall coatings, for example), exposure test site location latitude and time of year (solar zenith angle). Samples may be exposed directly to the sun and weather or protected behind glazing of different types to evaluate indoor use.To provide ‘special case’ conditions, outdoor exposures can be conducted in specialty cabinets at elevated temperature and humidity levels, or mounted on follow-the-sun tracking racks to accelerate the sun exposure. Special solar tracking and concentrator systems employing mirrors for sun exposure acceleration are also possible. Additional tests such as salt spray corrosion, water immersion or freeze/thaw cycling can often be combined or alternated with conventional exposures.
Outdoor test exposure commercial sites are available in a variety of climates and geographies but tend to be in tropical and subtropical or dry desert locations between latitudes of 15º to 35º north and south of the equator. The humid, wet locales, such as benchmark south Florida, have particularly proven to be very severe for organic coatings. But local conditions can never be exactly replicated or simulated, so Atlas, for example, has created a Worldwide Exposure Network of 20+ sites, the latest being in Chennai, India.
Outdoor tests are time consuming, and the exposures relatively uncontrolled, but absolutely necessary to eventually validate any accelerated weathering testing. There have been advances in outdoor accelerated weathering techniques such as the ASTM G 90 concentrated Fresnel reflector systems. Originated by Atlas’ DSET Laboratories Arizona test site, the EMMA® and EMMAQUA® devices use tracking mirrors to concentrate natural solar radiation onto the test samples. They have proved especially useful in testing high-durability coatings systems such as automotive, aerospace and architectural coatings. There have been significant advances in both temperature control and moisture delivery with these devices. AAMA 624-07, Voluntary Specification Performance Requirements and Test Procedures for High Performance Organic Coatings on Fiber Reinforced Thermoset Profiles, is one new standard that takes advantage of the new temperature-controlled Emmaqua as an alternative to traditional five-year south Florida outdoor exposure testing.
Work continues on ultra-high natural sunlight concentration to further speed test time. Atlas, for example, now has an experimental, utra-accelerated UV concentrator system using natural sunlight available for advanced research studies. Early proof of concept feasibility was originally presented and published in a U.S. government study on coatings in 1999.1
The real-world exposure performance of products at test sites is an essential and time-honored method of durability testing and produces the most reliable results. The realities of business, however, often require other approaches. Real-time exposure site testing, even when accelerated such as through the use of tracking racks or mirror concentrator systems, can still be a lengthy process. Sometimes the range of possible end-use environments is too great to manage through outdoor testing. To deal with these realities, the use of laboratory “artificial weathering” (sometimes referred to as ‘laboratory accelerated weathering’) is used, often in conjunction with ‘natural’ exposures to develop the interim data necessary to make product business decisions.
Matching Sunlight
Laboratory artificial weathering instruments are available in a range of size capacities and environmental simulation capabilities. One of the significant differences among them is the nature of the light source used and how well it reproduces the intended target such as outdoor solar radiation. This is important not only in terms of visible light but also in the accuracy in reproducing the ultraviolet and infrared portions of the spectrum as well in both intensity and spectral match. No artificial light source perfectly matches sunlight but some come quite close. A properly filtered (by the manufacturer) xenon arc discharge lamp is considered the ‘gold standard’ for matching sunlight or sunlight filtered through glass.In addition to the light, artificial weathering also needs to include moisture delivery (in terms of relative humidity and/or rain spray) and temperature controls of both the air and actual specimen temperatures in order to properly reproduce the intended service environment. Laboratory artificial weathering is highly repeatable and reproducible, making it a good choice for relative product comparisons. Likewise, it offers acceleration over outdoor testing time, typically by a factor of four to eightfold. But by its nature, laboratory weathering cannot reproduce the full complement of weather and climate factors nor their cycles and natural variability.
This leads to the issue of standardized test methods or specifications. There are literally thousands of product specifications and test standards referencing artificial weathering. Most do not claim to predict real long-term weathering performance but do serve as a minimum basis for relative comparison and performance. It is important to realize that just as there is no single climate or weather condition, there is no ‘one size fits all’ test method able to reproduce all climates and applicable to all material chemistries. However, most standard methods and specifications act as if there were. In fact, most methods don’t purport to reproduce any specific climate, but they can serve as a common basis for comparison or set minimum performance levels. By controlling the environmental factors in the laboratory artificial weathering test it is possible to simulate a variety of weather and climate end-use conditions, at least within the capabilities of the available equipment. By properly choosing the ‘boundary conditions’ of the tests, you may understand how your product will behave in real life in a shorter period of time.
For example, you may find that a particular coating loses gloss under hot and humid conditions but yellows in hot dry conditions. Or you may determine that a coating fades and chalks in the tropics but cracks and crazes in the desert sun – and all without leaving the relative comfort of the laboratory.
Some materials are very sensitive to a particular weather element, such as UV radiation. If the artificial weathering test is a poor match to sunlight in the UV, the test may make the product appear better or worse than it actually is. In either case we have a lack of correlation, a failure of the test to predict real-world performance. The old philosophy in testing was to achieve acceleration through unnaturally severe exposures in the belief that it would more quickly achieve the degradation that would be seen under normal conditions. While this is sometimes true, it often is not and the results can be surprising. The move of the automotive industry from monocoat paint to basecoat-clearcoat systems is a telling example. These early systems were passed as stable by very severe UV exposure tests, but quickly resulted in catastrophic clearcoat delamination on many thousands of vehicles. The unnatural conditions of the harsh test changed the degradation chemistry resulting in changes that did not result in clearcoat delamination. The lesson: do a different test and expect a different result.
The particular wavelength sensitivity of a coating to solar radiation is principally a function of the binder formulation and the specific constituent chemical bonds. But in addition, any additives or contaminants may also be photoreactive and contribute to degradation. Colorants may also degrade from the effects of visible light exposure resulting in fading or hue shift. And, lastly, different coating colors and substrates will attain different surface temperatures from the infrared heating effects of the sun, which can in turn affect the rate of degradation and influencing factors such as moisture migration into the coating. So it is critical to ‘get the light right’ in weatherability testing or you have a test you can’t rely on.
‘Getting the light right’ is usually the domain of the equipment manufacturers and considerable effort is expended on this effort. This is also a real problem with many traditional standard laboratory test methods in that they either make it difficult to understand the requirements or simply have not kept pace with developments in testing equipment. For example, improved UV cut-on and infrared reduction was obtained with the introduction of Coated Infrared Absorbing (CIRA) filter technology by Atlas in 2003 but has been incorporated in only a few standards. Atlas’ newest filter technology and best spectral match to sunlight to date is RightLight™, now commercially available for xenon Weather-Ometers®.
Light Control
Getting the light right with optical filter improvements is only part of the story. All lamps and filters have ageing characteristics that affect the light spectral quality. If we know the specific wavelength spectral sensitivity of a material under test, it would be beneficial to measure and control the light at that specific point or wavelength range. Or perhaps we want to use additional light filtering, such as through architectural or automotive glazing and need to analyze what light gets to the test specimens. For these applications, Atlas has introduced the first major innovation for light control in laboratory weathering instruments in 30 years - FSM™ (Full Spectrum Monitoring). With an FSM spectroradiometer equipped Weather-Ometer, one can monitor and control the light at any wavelength or wavelength range (in one nanometer increments) within the 250 - 800 nm band.But the light spectral quality and measurement isn’t the only area of technology improvements in laboratory weathering. Just as different colors warm to different temperatures from the sun outdoors, the same should be true in laboratory weathering testing. If not, the results can be non-predictive. Current technology usually measures the worst case (based on a black coated reference panel) with the assumption that other colors will be at their correct corresponding temperature. But this isn’t always so based on specific instrument settings and performance; this can be a problem, particularly with special effects or infrared absorbing or reflecting pigments.
So, to actually monitor the exact test sample temperatures, Atlas has developed S3T™ (Specific Sample Surface Temperature) technology. This allows Weather-Ometer temperature monitoring or control by using the actual test specimens rather than a generic black panel. It allows direct non-contact temperature measurement of samples while under light exposure. These advancements facilitate improved service life predictions based on both true test specimen temperatures and specific light dosages (with FSM), rather than estimates or theoretical set points. With technological advances comes the ability to understand how to test in a meaningful way. With this understanding comes better long-term predictability and the ability to make product decisions faster and with greater confidence.
For more information, visit www.atlas-mts.com.
