20-25 ☌) without the use of chiller units. 70 ☌), but it is difficult for equipment to control at the lower temperatures (approx. It is possible to approach and maintain temperatures near the average panel/part temperatures expected in many global regions (approx. There are some limitations in accelerated weathering equipment pertaining to temperatures. While the original thought was that this could accelerate real failures, the facts are that these unnatural failures lead to higher-cost coating products that are actually improperly formulated for real-world exposure conditions. Ford and 3M were both successful in their work with glass manufacturers to develop a proper filter combination.(3,4) It is well known that unnatural coating failures can be generated using light sources that do not duplicate sunlight, as well as testing at a higher irradiance and intensity light than is found outdoors (Figure 8). Given that SPD data existed for sunlight, the challenge was to determine the filter composition that yielded the same SPD after the Xenon light passed through it. The first change that was obvious to start with in accelerated weathering was the development of a light filter that would successfully match the spectral power distribution (SPD) of sunlight. These issues have led to the inability to properly produce and predict almost every potential defect known in outdoor exposure, including delamination/peeling, cracking, erosion, gloss loss/oxidation and color change/fade (Figures 6-7).
As mentioned by many previous authors,(2) there are issues with the entire methodology of accelerated weathering, including the filters, light usage, temperatures, humidities, water exposure and the time designated for each cycle step. The test procedures have not kept up with the changes in technology. The correlation between current accelerated weathering test results and actual service/outdoor testing of automotive coatings in use today is poor. Analytical techniques used to evaluate failure modes have advanced in their ability to predict coating failures. The service life expectation for automotive coatings is now 10 years or longer. Coatings today are more complex, and the modes of failure extend beyond gloss loss and color change/fade. The accelerated methods appeared to correlate well to outdoor testing based on these criteria. Gloss loss and color change/fade were the primary modes of failure and, in many cases, failure occurred in less than two years of outdoor testing. The accelerated weathering tests used today were developed using single-coat acrylics and high-solids enamels. It does not seem possible to test all potential organic and inorganic materials that could come in contact with an automotive coating. In the case of this type of testing, the challenge would seem to be overwhelming. There does not appear to be a good test to determine overall coating resistance to materials such as bug residue, tree sap and pollen from various sources. Many of the tests used to determine cracking potential of a coating do not accurately predict outdoor performance. There are numerous types of scratch test methods and pieces of equipment used in the coatings industry however it is difficult to point to a single test that correlates well to actual outdoor damage.
While there are many tests that can predict defects with good field correlation, there are other areas where correlation has been difficult. Also, a newly developed etch test has very good correlation to actual outdoor acid etch damage to automotive coatings.(1) For example, a Gravelometer has been used successfully by customers for many years using multiple types of media to predict various types of chipping that could happen to automotive coatings. Many tests have been developed in the past that can be used to generate a specific defect and predict the field performance of a coating system. Prior Test Procedures and Field CorrelationĪny accelerated laboratory test must be able to correctly duplicate and predict the type, degree and severity of defect that is seen in actual field conditions.