A Study of the Color Change of Automotive Coatings Subjected to Accelerated and Natural SAE Weathering Tests for Exterior Materials Durability

Exposure of automobile exterior materials such as coatings in the South Florida environment has become the de facto standard for testing their durability. In the early thirties, testing sites were established in this location by the automotive industry. Each automobile manufacturer applied different exposure techniques for their own acceptance criteria. SAE J1976 was developed in 1989 to standardize the various exposure types, however, the document still contains radically different test rack designs.

Even though the Florida exposure tests have not changed extensively the coatings have become much more durable. Today's service life expectation of more than 5 years means that the coating cannot be tested to failure in the Florida environment and allow the developer to bring a new technology to the marketplace in a timely and competitive manner. This has led to the use of accelerated tests to speed up the exposure process. There are several alternative tests specified by SAE including xenon arc Weather-Ometer® (SAE J1960), Fresnel reflector device (SAE J1961) and fluorescent testing device (SAE J2020).

The study reported here was carried out to draw conclusions regarding differences between the outdoor test racks designs and their relationship to the accelerated test methods, in order to identify the accelerated test providing the best correlation to Florida. Specimens were donated by three automotive coating manufacturers, representing a combination of coating types and colors in the form of coated test panels. Seventy specimens were exposed per test method. Performance evaluations including the measurement of color, were conducted several times during the exposure testing. Comparative color change data is reported for all of the test methods and statistical comparisons are made between each. the results showed that the best correlation was achieved when the light source most closely matched sunlight. All of the outdoor tests correlated well with each other.