| Allen Zielnik,CHENG Ge,MA Xu-dong.Test Method Development for Improved Laboratory Accelerated Weathering of High Performance Coatings[J],46(4):101-113 |
| Test Method Development for Improved Laboratory Accelerated Weathering of High Performance Coatings |
| Received:September 20, 2016 Revised:April 20, 2017 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2017.04.018 |
| KeyWord:weathering test accelerated ageing outdoor exposure automotive coating SAE J2527 ASTM D7869 |
| Author | Institution |
| Allen Zielnik |
Atlas Material Testing Technology LLC, Chicago 60056-6039, United States |
| CHENG Ge |
Atlas Material Testing Technology LLC, Chicago 60056-6039, United States |
| MA Xu-dong |
Atlas Material Testing Technology LLC, Chicago 60056-6039, United States |
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| Abstract: |
| To improve correlation and predictive ability of a laboratory accelerated weather test to that of outdoor subtropical exposure. A consortium of several companies, including Ford, Boeing, BASF & Atlas etc., cooperated in the methods development over almost 10 years. Over twenty automotive basecoat/clearcoat coating systems of various known outdoor weathering performance, and a number of monocoat systems, were investigated. Samples were subject to natural and accelerated weathering using various test cycles. A variety of spectroscopic techniques were employed to follow several key peaks in the coatings that are markers for photo-oxidation and hydrolysis degradation and could be followed over time. Microtomy was also used to provide 5-micron thick sections through the weathered coating systems to follow both the chemical changes as well as changes to the UV absorber concentration. It was found that both the qualitative and quantitative spectral match to terrestrial outdoor solar radiation, particularly in the UV cut-on wavelength, were critical factors in altering the coating degradation chemistry. A special light source filter was developed to overcome the limitations of the systems that were in use. Next, the effects of moisture delivery in both quality and quantity were compared to Florida exposures and found to be deficient. Laboratory test cycles were modified until both moisture absorption and desorption match the diurnal pattern of outdoor exposure, with additional compensation for the normal excursions of natural conditions. These included periods of long water soaks to allow coating saturation, followed by drying cycles and thermal shock events. Lastly the effects of specimen temperature were investigated for both thermomechanical stress as well as the effect on coating hydrodynamic volume during diurnal cycles, and a stepped irradiance and temperature profile was established to better mimic the thermal effect on secondary coating degradation reactions as well as physical weathering. The new test cycle produced significant improvements in the correlation of the coatings degradation chemistry to outdoor exposures across all coatings systems tested, and reproduced the physical changes to the coating systems including delamination and adhesion loss, cracking, and blistering similar to Florida exposure. The test was approximately 40% faster in producing the same effects as two and five year Florida weathering exposure compared to existing methods. The final test cycle was established as ASTM D7869-13 standard practice for xenon arc exposure test with enhanced light and water exposure for transportation coatings. |
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