A new and potentially revolutionary weathering standard has been published by ASTM International. Titled “Xenon Arc Exposure Test with Enhanced Light and Water Exposure for Transportation Coatings,” ASTM D7869 is the result of a decade of research by a consortium of leaders in the automotive, aerospace, coatings, and weathering testing industries. The new standard was written by subcommittee D01.27 on accelerated testing of paints and coatings. Douglas Grossman, subcommittee chairperson and president of Q-Lab Corporation, calls ASTM D7869 “The most thoroughly researched weathering test procedure ever developed.” What makes this new standard so special?
To answer this question, it is important to look at today’s most commonly used weathering test method in the automotive coatings industry, SAE J2527. First published in 2004 to supplant SAE J1960, J2527 has been widely adopted and adapted by global OEMs and also is referenced by numerous non-transportation industries. While both methods shared the same test conditions, the old J1960 could be performed in only a small number of commercially available xenon test chamber models. Recognizing that this hardware-based approach to test standards made progress difficult, SAE took the important step of re-writing the standard. The updated version included a technically robust description of test conditions instead of listing particular models and the required programming steps for each.
The old SAE method was widely adopted despite not having demonstrated good correlation to outdoor tests in south Floridafor some critical coating performance properties. For example, accelerated tests performed under the SAE method failed to produce delamination of coatings that experienced catastrophic and extremely costly failures on consumers’ vehicles. J2527 does, however, generally correlate well to Floridatest results for color and gloss. Thus, in the early 2000s it appeared that xenon arc weathering was reasonably useful to coatings formulation chemists, but it needed a technological boost to accommodate the increasing demands of the 21st Century.
Research Part 1: Improving the Light Spectrum
This boost came from a series of projects throughout the 2000s, some of which had roots in the late 1990s. The earliest of these projects took a careful look at the light spectrum of the old SAE tests, which included ultraviolet light wavelengths not present in natural sunlight. As performance-based standards began to replace older hardware-based ones, xenon optical filter system definitions emerged. The legacy optical filters of the old SAE tests, often called “Quartz/Boro” filters, fell into a category called “Extended UV” filters. Those having a better match to natural sunlight, such as the Daylight-Q optical filter from Q-Lab, were classified as “Daylight” filters. The SAE tests were revised to accept these more realistic optical filter types while continuing to reference the Extended UV type.
While these definitions provided a significant improvement over the older standards, coatings experts were often frustrated by a lack of reproducibility of results when using various “Daylight” filters. Subtle differences in the short-wave portion of the spectrum, especially the cut-on wavelength, sometimes created different results when using a Daylight-Q versus a Daylight B/B system, for example.
ASTM D7869 addresses this problem with a much tighter definition of the optical filter system. Based on work done by Ford and 3M Corporation, this new optical filter matches the spectrum of south Florida more closely than other Daylight filters. They are currently sold under trade names “Daylight-F” and “Right Light,” although any manufacturer of xenon arc weathering chambers is free to adopt the new spectrum.
Research Part 2: Importance of Moisture
Although critical, the challenge of creating a better spectral match toFloridasunlight was simple compared to the daunting task of re-creatingFloridamoisture in an accelerated weathering chamber. Two of the companies that developed the new method were BASF and Q-Lab. They had previously worked together to develop a xenon arc method to reproduce acid etch of automotive clear coats, published as ASTM D7356. During this work, much was learned about measuring and characterizing moisture from dew formation and rainfall in the harshFloridaclimate.
Later, BASF partnered with Bayer to further investigate the critical role of water in accelerated testing of automotive coatings. As the research gained momentum, Boeing and Atlas became active participants in the efforts lead by Ford. Honda of America, which had separately been doing similar work, began sharing data and joined in the discussions.
Moisture uptake of coated panels inFloridaand their subsequent drying outdoors proved to be driving factors of degradation mechanisms previously unseen in the SAE accelerated method. The research showed that the SAE J2527 cycle, despite extensive use of water spray and high humidity, failed to result in water uptake sufficient to mimic theFloridaexposures.
Research Part 3: Developing the New Cycle
Although the emphasis in xenon arc testing is usually on the light exposure, the new cycle starts with a 4 hour dark period with water spray and high humidity. The duration and temperature were chosen because they achieved the maximum amount of water uptake that was seen in theFloridaexposures. This happens at night with clear skies, when maximum dew formation occurs. A critical departure from the SAE J2527 cycle is the absence of any water spray during light exposure. Water spray occurs only during dark portions of the cycle, again because wetness inFloridais caused mainly by dew formation at night.
On days when maximum dew formation occurs, the early morning hours cause the coatings to dry out as the sun rises. The new cycle mimics this by transitioning from the dark period to bright light with an intermediate step at relatively low irradiance and black panel temperature. The black panel temperature is 50°C in this step because coatings inFloridaare nearly dry before they experience any temperatures higher than this.
The cycle continues with a high temperature and irradiance step to mimic the bright sunshine and heat ofFlorida, followed by another low irradiance step to serve as a transition period similar to the end of the day, prior to night fall and dew formation. Another dark period of water spray precedes a subcycle of short steps designed to cause rapid thermal cycling. These steps were important in recreating the cracking and delamination that some coatings experienced in theFloridaexposures completed in this study. The subcycle ends with a period of darkness with no water spray and comparatively low temperature and relative humidity. Excluding this “relaxation” period may cause excessive, unnatural cracking or other unrealistic degradation.
New Performance Criteria for Xenon Arc Test Chambers
The research revealed some new performance criteria for xenon arc test chambers that were previously ignored or under-emphasized in test standards. First, the water delivery system must be calibrated to ensure the chamber is capable of delivering enough water to the test specimens. Without enough water, the benefits of the new cycle will go unrealized, and correlation to outdoor results will suffer. One of the researchers developed a simple yet reproducible method for measuring the water delivery, using a standardized sponge and weight scale. This method is described in the standard.
Also new is the requirement to reposition specimens in the chamber, regardless of the type of weathering tester. The research demonstrated that uniformity of results is the result of a complex interaction of several factors, not just light. In practical terms, this means that repositioning specimens is critical in both rotating rack and flat array testers.
The New State-of-the-Art in Weathering Testing
ASTM D7869 is the fruit of years of research, discussions, and experimentation. It represents a leap in weathering testing technology because, for the first time, a laboratory test was able to replicate multiple degradation mechanisms in a variety of coatings materials and systems. The new standard holds the promise of accelerating product development by providing fast and reliable results. When companies have confidence that their laboratory results match what will occur in their products’ actual service life, they can speed up development of more durable, cost effective, and environmentally friendly products. Weathering testing is now ready for the 21st Century.
ASTM D7869 is available for purchase at www.astm.org. For a limited time, Q-Lab Corporation has purchased electronic copies of the new standard and is distributing them on a complimentary basis to customers. Visit www.q-lab.com/newASTM for details.