Effect of Global Climate Variables on Weathering Testing
The automobile industry has experienced tremendous growth in the past several years and is expected to expand further in 2016 due to strengthening economies around the world. Until recently, growth has been particularly rapid in many emerging regions, and today, China and the United States are the two largest automobile markets worldwide. This globalization is driving automakers to seek volume efficiencies through implementation of identical, larger-scale manufacturing processes at multiple sites around the world, which in turn has led to global vehicle platforms with global specifications. As a result, automotive coatings today must meet long-term outdoor durability and performance requirements with respect to delamination, gloss loss, cracking, and blistering, regardless of location and climate.
“More and more frequently we are receiving questions from manufacturing sites around the world as to the expected performance of coatings in their locations given the results of long-term weathering testing in Florida,” says Mark Nichols, technical leader, Paint and Corrosion Research with Ford Research and Advanced Engineering. There were not any established models that enable this type of prediction, however. “This issue is actually a real concern given the global nature of the car industry today and the wide variation in climates around the world,” Nichols notes. So, he and his colleagues set out to develop one based on simple coating properties and local climate variables, focusing initially on the two largest markets—the United States and China.
Since cars are sold and kept where people live, the first step in developing the model was to establish population-weighted climate data. For instance, in China most people live along the east coast where it is cooler and wetter, and very few are located in the western parts of the country where it is hot and sunny. The 40 largest cities in the United States and the 30 largest cities in China (representing approximately 50% and 17% of each country’s population, respectively) were used for the study. Geographic and climate data for each location was then gathered from a variety of sources (U.S. Department of Energy, NASA, etc.), including the latitude and longitude of the city center, its elevation, and solar radiation (UV radiation was assumed to be directly proportional to total radiation and the angle of exposure was assumed to be horizontal), temperature, and moisture (rainfall) levels.
. . . automotive coatings today must meet long-term outdoor durability and performance requirements with respect to delamination, gloss loss, cracking, and blistering, regardless of location and climate.
Evaluation of this data revealed populated areas in the United States in general have harsher climates than the populated areas in China. “Notably, the climate in Seattle, which is one of the mildest in the United States, is on par with the harshest climates in populated places in China,” states Nichols. He attributes these results to the fact that in the eastern part of China it is often overcast or rainy, and thus the UV exposure is significantly reduced. On the other hand, a large number of people in the United States live in high solar radiation areas, such as Florida, Arizona, and southern California.
Next, the researchers considered different failure modes that are dependent on different coating properties, and concluded that coating thickness is of significance for many of them. Thinner coatings will allow more transmission of UV radiation through an absorbing coating layer to a photolabile substrate, while thicker coatings are more likely to undergo stress-induced cracking. To develop their model, knowledge of the variability of coating thickness on one vehicle and from vehicle to vehicle was therefore required in order to determine its impact on coating durability. This data was collected by taking daily measurements of the thickness of the clearcoat at 80 sites on randomly chosen vehicles (all the same body style) that were all painted in the same spray booths.
Several failure models based on simple to complex failure modes were then developed to predict the failure time of a coating relative to the failure time in a known location. The use of automotive coatings (and thus the number of vehicles) was assumed to correspond directly to the population in a given area. For most of the models, test results obtained in southern Florida were found to be a good standard for natural weathering exposure, because the conditions in this area are among the harshest identified. For the Denver cracking failure mode, which results from exposure to very low temperatures, testing results obtained in Denver were used as the standard.
Importantly, the variation in coating thickness was found to have a significant impact on coating performance for failure modes that are influenced by the thickness of the coating, according to Nichols. Two examples include delamination of a coating from a cathodic electrocoat and that of a clearcoat from a low-durability basecoat. When average coating thickness data was used for these failure modes, both early failures and longer lifetimes were not accurately predicted. In one particular case, coatings on as many as 25% of vehicles in the United States were expected to fail earlier than coatings on cars in southern Florida. For cracking failures, the differences were even greater, with as many as 80% of vehicle coatings in the United States lasting for less time than coatings on cars in southern Florida. In both cases, the numbers are much lower for China due to the milder climate.
Gloss loss is a different story, however. The loss of gloss is largely due to photooxidation and hydrolysis of the coating surface followed by removal of the degraded material by liquid water, including both dew and rainfall. Moist climates are therefore harsher on gloss than dry, sunny climates. It is not surprising, then that the researchers found China to be a harsher environment for gloss loss than the United States due to the higher fraction of the population that resides within relatively wet regions. It should be noted that although pollution in many Chinese cities is a significant problem, the researchers did not include the effects of acid rain in the study. “Acid rain is a short-term phenomenon and can rapidly damage coatings regardless of their age. We consider it to be a separate issue and do not consider it as a factor for long-term weathering testing,” Nichols explains. He does note, however, that gloss loss under severe pollution conditions may be exacerbated by acid etching. Particulate pollution such as that observed in many Chinese cities does, on the other hand, acts like cloud cover with respect to reduction of UV exposure, and thus may have a positive influence on coating lifetimes due to dampening of the harshness of the climate.
Overall, the findings of the study revealed that southern Florida appears to be a good location for the long-term weathering testing of automotive paint systems. Although it is not the harshest climate with respect to UV exposure and temperature, it is very aggressive for failure modes that involve liquid water. In China, however, much of populated areas have climates that are harsher than those in the United States with respect to gloss loss. As mentioned above, intense pollution may aggravate this failure mode, but reduce the impact of UV exposure. Failure due to exposure to cold temperatures also appears to be more common in China than in the United States (Denver cracking). The influence of film thickness variability was also shown to be significant and suggests that in the future, exposure testing should be conducted not just at target film builds, but at minimum and maximum thicknesses as well.
Natural, long-term weathering testing is currently underway in the United States and China at several locations with varying climates using a range of coatings that have been shown to exhibit poor to excellent performance and different failure modes. The results will be used to evaluate the ability of the new model to accurately predict coating lifetimes for different coating systems in different geographical locations.
For more details on the study, see: Nichols, M.E. and Frey, J.F., “Statistical modeling of coating lifetimes in disparate
environments,” J. Coat. Technol. Res., 12 (1) 49-61 (2014).