The automotive industry uses the SAE J standard to evaluate how exterior plastics on automobiles weather. SAE J tests are performed in laboratories by auto manufacturers and parts suppliers. To understand SAE J, note that in some regions automobile exteriors will weather differently due to outdoor conditions. Know that SAE J standards were developed in the s and are still used for preliminary tests; but be aware that auto manufacturers conduct real outdoor testing to validate lab results and to develop new auto-exterior materials.
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The ability to predict longevity, or service life, is key to developing new automotive coating technologies for the original equipment manufacturers OEMs. While exposure testing is performed in natural field environments such as Florida, it is not cost or time efficient to solely use these results for automotive coating development.
In order to speed up the product development process, accelerated weathering tests are used to predict the longevity of the coating technologies in a fraction of the time that would be required for a field test. The acceleration factors can be as fast as 10 times, i. Table 2. Click image for larger view. The cycles are the same for both specifications, but can be run on different pieces of accelerated weathering equipment.
The accelerated methods usually require specific light sources, filters to optimize the UV exposure, specific cycles of light and dark time, specific control of the wetness and humidity, and specific control of the temperatures of either the chamber or the part or panel being tested.
The typical objective of these test methods is to accelerate the exposure relative to a summer in Florida. Florida has been established as a weathering benchmark due to the fact that the high temperatures, humidity and rainfall seen there appears to cause the most severe damage to automotive coatings as compared to other global environments.
Figure 1. While these accelerated methods are in significant use in the coatings industry, there are many flaws with these methods that can give accelerated results that do not correlate with the real field results.
Delamination failures and humidity-induced cracking failures have been seen on actual vehicles that were not shown in standard approved accelerated weathering tests Figures 1 and 2. The results seen in automotive OEM coatings might be as described in Table 2.
It is well known in the coatings industry that the light sources in these tests are not an exact match for sunlight. There are different intensities and distributions of wavelengths in the artificial light sources compared to sunlight that can provide different weathering results in accelerated weathering compared to field tests.
Recent work by Ford and 3M has potentially reduced this disparity, but there are other issues to be solved. It has also been revealed by many sources, including Ford1 and GE2, that the humidity and water control in the accelerated tests are not correct. Both GE and Ford have gotten better correlation to field test results by combining humidity-related tests such as water exposure or soak tests along with accelerated weathering techniques. Figure 2.
Factors in Accelerated Weathering While the current efforts are on the right track, there are still many more factors to control to bring the accelerated weathering tests into complete correlation with field results for automotive OEM coatings. In order to control all of the factors in accelerated weathering, one must know what the real field conditions are. Given that these accelerated tests are designed to match a Florida summer environment, weather data was collected in Jacksonville, FL, throughout the summer of from a weather station installed by Bayer MaterialScience Figure 3.
The reason for choosing this location is that one could look at this data in relationship to both the acid etch phenomena and overall weather conditions in an environment similar to that in South Florida, where most of the field test sites are located. A comparison of this data to actual data collected in South Florida showed the weather conditions to be comparable during the time period the data was collected.
A separate publication has evaluated this data versus the acid etch phenomena. Based on the weathering cycles and results seen with coatings, there are issues in virtually every aspect of the accelerated weathering tests. These include the following factors: intensity and distribution of light there appears to be a recent solution ; testing orientation; water - the proper amount of the correct type of water at the proper temperature; temperature - the proper maximum and minimum temperatures during the dark and light portions of the cycles; and time - the proper time duration to run the light and dark cycles, including the acceleration factor.
Intensity and Distribution of Light Since the light issue seems to be on the way to being resolved by work at Ford1 and 3M3, the first issue to address is the testing orientation.
While testing in a vertical orientation might have some logistical advantages in terms of equipment design, it may be difficult, if not impossible, to correlate the results for all types of coatings in a field environment. For example, architectural coatings that are used mostly in a vertical orientation might correlate with vertical accelerated tests. However, in automotive coatings, where the majority of failures are seen on horizontal surfaces, it may be very difficult to obtain correlation in a vertical testing orientation.
While most factors can be obtained in a vertical orientation equivalent to horizontal, such as temperature and light exposure, it may be difficult or impossible to match the effects of humidity and exposure of a horizontal surface to water sprayed in a test cabinet with a vertical orientation. There are the issues of gravity, where there might be more washing action but less penetration of water into a coating in a vertical orientation compared to a horizontal orientation with the opposite effect of less washing but more water on and inside the coating film.
Since the real issue of orientation is related to the ability to control the humidity and water exposure to test specimens, specific data must be generated to understand how to control this area of accelerated testing. Table 3. Water While data such as percent relative humidity and time of wetness are reported by various field test locations, data on the total amount of water contacting a test surface including both rain and dew , and the total amount of water inside the coating as a function of time and temperature has never been obtained or reported by the companies that perform the field testing service.
An aluminum panel was used to meet the output requirements for the load cell while having the rigidity to keep its geometry when exposed to water Figure 4.
In order to determine the relationship between current and mass, specific weights were placed on the panel to obtain an equation to convert the current output to mass units.
Calibrations were done every two weeks during the exposure to verify the load cell output Table 3. The load cell output was recorded as one data point every five minutes from May 25 until August 30, While there were a couple of events that interfered with the load cell output, very good data was collected from June 30 - August 25, 56 days. The data from the load cell can actually tell us the total volume of water contacting the panel surface, the temperature of the panel with water on the surface, and the amount of time that the panel was wet.
The factors of wind and debris on the panel interfered with the ability to gain the actual amount of water inside the coating layers. In the day period from June 30 - August 25, , the load cell recorded a total water weight of 41 gallons. It is known that much of this is the same water standing on the surface of the panel for increments of time.
There are many instances in the field where the coating surface is totally covered by water at temperatures from 70 - degrees F approximately 20 - 50 degrees C for time periods up to 18 hours hours is a typical value.
These instances occur typically when the panel collects dew in the evening, eventually evaporating off the coating surface as the sun rises and the panel temperature increases. This takes place at little or no solar radiation. Table 4. In visually watching this step in an actual J process, it is obvious that panels do not ever approach total coverage with water. This results in an order of magnitude less water uptake by the coating film. While it is impossible to directly compare the vertical spray from J with horizontal field exposure, it can be reasonable to perform water soak experiments in the laboratory that match the exact field conditions and compare them to water uptake values found in the light and dark cycles of the J process.
This value is low compared to soak tests and regular humidity tests. Temperature and Time A variety of temperatures and times were evaluated that match specific field conditions when the load cell panel was totally covered with water. It appears that the majority of the water uptake and humidity-related damage occurs during the hours with dew sitting on a coating at ambient temperature.
The last three columns of Table 4 show the start of water uptake from a rain event, and the values in red in Table 5 show the significant changes, such as the point where the water finally has totally evaporated off the panel surface.
While a coating can uptake more water in a shorter time at a higher temperature, the amount of time that a coating sees when it is saturated with water at temperatures of 50 degrees C and higher is quite low.
Examples of some of the water soak data and uptake data are as shown in Figures This data indicates that there is insufficient water exposure to coatings in a J or J accelerated weathering test. It might be possible to modify the test cycles to achieve the proper water uptake even in a vertical orientation. Additional work should be done to determine if a vertical vs. The goal should be a match of the water uptake with the accelerated weathering cycle versus the comparable soak data that simulates field conditions.
While the time of wetness in the field vs. Table 5. Water Temperature The temperature of the applied water also has to be considered. Data from a Florida summer indicates a typical water temperature of approximately 20 - 25degrees C.
The varying water temperature will have an effect on the panel temperature as a function of time, and also the ability of an accelerated device to control the panel temperature. More stress could be induced into a film when very cold water is sprayed onto a hot coating surface vs.
Also, as the water temperature and resulting coating temperature is lower, there will be less water penetration into the coating film. While it is certainly convenient to use deionized water in the spray applications for accelerated weather testing, there is no such thing as deionized water falling as rain or condensing as dew. The real water in the field has varying ion concentrations and pH values.
Most of the dew and rainfall collected and analyzed during the summer of in Jacksonville was acidic. The average pH of the dew collected was approximately 6. The average pH from all of the water collected was 6. While it would be inconvenient from both a mechanical and logistical perspective to use water other than deionized water, we are probably missing the effects of hydrolysis reactions among others, by not using water that is more representative of the Florida environment.
There is etch-type damage that is found on actual units as well as field exposure panels. The use of some acidic spray could duplicate this damage. Panel Temperature While the temperature of the applied water has been discussed, another critical factor is the panel temperature. While the highest panel temperature measured in Jacksonville in the summer of was Assuming an average of This in itself indicates that 70 degrees C may be too high for an accelerated test.
However, one can argue that since the maximum temperature is this high, it is proper to use it as the accelerated test temperature. Also, there has been data recorded that shows the real temperatures on actual vehicles to be higher as well. The issue might not be the maximum temperature as much as the rates of temperature change.
In the Florida weather data, the panel temperatures indicate that the surface cools to the ambient temperature every night. This value is typically degrees C. So for the approximately This would be a condition of low mechanical stress for most coatings. There could be some minor reactions taking place due to the fact that water does sit on the panels most of the night. While it is known that it takes more water and more time for the actual horizontal surfaces of a vehicle to cool to ambient temperature than it does for a panel, those surfaces do eventually cool to ambient temperature.
In contrast, the weather data from Florida shows an approximate 20 degrees C drop before a typical rain event takes place. Typically clouds form first and the panel temperature drops before rain occurs. While one can argue that this is because there might be an instantaneous circumstance where water would hit a panel at 70 degrees C, this will happen very seldom under actual field conditions.
Out of more than 30 hours of rainfall recorded by the weather station in Jacksonville, the panel temperature was over degrees F 38 degrees C for only 10 minutes 0. Water hitting a coating surface at a lower temperature causes much less mechanical stress than when the panel at a higher temperature is sprayed with water. This data indicates that more water is needed at slightly lower panel temperatures in order for a sufficient amount of water to soak into the coating film without unnatural mechanical stresses to the film in the process.
Table 6. Cycle Time The cycle time also needs to be addressed.
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