March 22, 2000
UPTON, NY - Painting a bridge can be a costly and time-consuming undertaking, especially if the paint job doesn't last. So scientists have been working on ways to test paint durability before the brushes even get wet. At a March 22 session at the American Physical Society meeting in Minneapolis, scientists from the University of Missouri at Kansas City, who worked in collaboration with physicist Bent Nielsen of the U.S. Department of Energy's Brookhaven National Laboratory, will present findings that could lead to the development of an extremely sensitive and quick durability test.
The technique is called positron annihilation. Essentially, the scientists bombard small painted samples of metal with a beam of positrons, or positively charged electrons. When these "antielectrons" interact with the electrons in the molecules of the paint, they annihilate one another and send out gamma rays that give the scientists information about the molecules in the paint. The technique can detect nanometer-scale holes and defects in the paint molecules; free radicals, which indicate the presence of broken chemical bonds; and cross linking, which may make the paint brittle.
"These experiments show that this technique is extremely sensitive to detecting damage early," says Brookhaven's Nielsen - well before the formation of any visible cracks in the paint. "So you can test the paint on a much shorter time scale - a day instead of half a year. That's a big advantage," Nielsen says.
The scientists typically test the paint samples before and after exposure to ultraviolet (UV) light, one of the components of sunlight known to damage bridge coatings. The more sensitive the paint is to UV damage, the less durable the paint would be on a bridge exposed to sunlight day after day. They've also exposed samples to UV light during the positron annihilation test to see if they could detect the damage as it occurred. In both cases, the damage increased with UV exposure time, and was most severe near the surface of the paint.
In addition to laying the foundation for a quick paint durability test, the detailed observations made possible by positron annihilation may also help scientists learn more about the fundamental mechanisms of paint degradation. That knowledge, in turn, may eventually lead to the development of more durable paints.
Brookhaven was a pioneer in developing positron beams in the late 1970s and early 1980s. Positron emission tomography (PET) scanning, a medical technique used to learn about the function of body organs such as the brain, works on a similar principle, Nielsen says.
This paper will be presented at session L36 on March 22, 2000, at 10 a.m. in the Exhibit Hall of the Minneapolis Convention Center.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
Note to local editors: Bent Nielsen lives in Port Jefferson, New York.
Last updated 5/28/99 by Public Affairs