Despite their durability, soy-based pavement markings don't make the grade.
WHO: Missouri DOT (MoDOT)
PROJECT: One-year test application of soy-based paint
APPLIED: October 2007
THE LATEST: About four years ago, a product manager at Kansas City-based distributor Cook Composites and Polymers Co. heard Cargill Inc. had developed a new type of paint consisting of polymers derived from soybean oil.
That gave him an idea.
“They wanted us to provide a test deck to see how it would perform under highway traffic loads,” says Missouri DOT (MoDOT) Chemical Laboratory Director Todd Bennett.
In its role as caretaker of 32,400 lane-miles of roads, the agency places more than 1.5 million gallons of water-based paint each year.
So MoDOT asked the Missouri Soybean Merchandising Council to fund a plan to test the paint. The demonstration project consisted of placing and monitoring 12-foot-wide test strips that run from the shoulder to the center stripes on 40 feet of concrete and asphalt pavement on Route 63.
The markings passed the agency's retroreflectivity standard of 175 milli-candelas/square meter/lux on the wheel path, though not exactly to the same level as Ennis Paint, Sherwin-Williams, and Vogel Paint fast-dry waterborne formulations. It also held up against Missouri's freeze-thaw cycles; wind, rain; and the grinding of snow plows.
But the verdict was in even before the paint had dried.
The agency requires pavement paint to dry in less than 10 minutes, and the soy-based paint took more than 20.
“It's too bad because it offered good durability,” Bennett says. “We knew the formulation wasn't ideal.” Unfortunately, that particular flaw isn't the fault of the lab but of nature itself.
Bennett didn't calculate how much the agency would save — or spend — if it were to switch formulations. Water-borne paint prices vary from year to year depending on the availability of ingredients; in 2009, for example, an acrylic resin shortage drove prices up. In general, though, solvent-based formulations like the soy-based paint cost more initially but require less maintenance.
“The characteristics of soybean oil don't lend themselves to a 10-minute, lacquer-like drying time,” explains Dennis Ryer, Cook Composites and Polymers' product manager for liquid coating resins. “There's no technical approach to improve the drying time enough to meet the department's requirements. They're pretty tough, but also pretty typical across the country.”
The reason: The polymers in any oil-based paint are soft and require oxygen to cure. That takes time. “It needs the air to cure, and you can't speed that up,” Ryer says.
“The paint would've been very environmentally friendly, and we like to take advantage of those types of renewable resources,” Bennett adds.