by Angela Spivey

Researchers learn how to get more paint on the product and less on the painter.

ecilia Tan flips a switch, and air flows through a wind tunnel at 70 feet per minute—it feels like a brisk ocean breeze. Then she turns on her mannequin. With a remote-control box a little smaller than a toaster, she adjusts his right arm, moving it up, back, then down a little. After she has him aimed, she hits the switch that operates his trigger finger, which turns on a spray gun she’s taped in his hand.

Tan, a Ph.D. student in environmental sciences and engineering, is part of a team studying how spray paint acts when it’s released into an enclosed, ventilated area—exactly what happens when workers paint cars and airplanes. The typical “spray booth” is 12 feet wide, 12 feet tall, and four or five feet deep, though some can take up whole buildings. Such booths usually have an air intake system that’s separate from the rest of the building. This setup is designed to keep contaminated air away from others in the building and the spray painters. But it doesn’t always work that way, says Mike Flynn, professor of environmental science and engineering.

Under some conditions, the fresh air can actually carry contaminated residue back into a worker’s breathing zone, Flynn says. Some workers wear masks or respirators to filter out the residue, but not all do. And even masks can’t eliminate all contaminants. “You want to use ventilation to get as much of the contaminant out of there as you can, and then a respirator can add extra protection,” Flynn says.

Spray painters can be exposed to some nasty stuff. Paint residues contain chromium—a known carcinogen—as well as isocyanates, which cause asthma-like symptoms. “Once people are sensitized to isocyanates, it’s hard for them to do their job anymore because even very low levels can trigger an asthmatic response,” Flynn says. And solvents such as toluene and xylene release vapors that depress the nervous system, causing a drunken feeling.

Flynn and Tan study how much paint actually gets on the intended target, how much ends up floating in the air, and where that air goes. The results vary depending on where the worker stands, how tall he is, the velocity of the air, and the power of the spray gun. Many scientists model these variables using computers. Flynn’s team does too, but they go a step further; to improve the computer models, they test them using simulated spray painting.

hat’s where the mannequin comes in. Flynn got the local Belk department store to donate it, and then took it to Randy Goodman, the shop supervisor in the School of Public Health, for some improvements. The mannequin isn’t the friendliest looking guy and has been known to startle a housekeeper and others who’ve come upon him unexpectedly. “He needs some hair,” Goodman says.

Goodman designed the mechanics, while Clif Burgess, the instrument maker in the shop, made some of the parts. Goodman cut open the mannequin’s back and put in three gear motors (the same kind of motor that pulls paper through a copy machine). The motors, some switches, a chain drive, and other hardware make the mannequin’s arm move back and forth and up and down, hold a spray gun, even paint by pulling the spray gun’s trigger. Goodman also installed sampling tubes in the mannequin’s mouth to measure the amount of paint residue that ends up in the breathing zone.

There are plenty of mechanical details here, but what Flynn is really getting at is simple in its essence—air. That’s what carries the paint residue to a spot where it could be inhaled. The team tracks where the air moves, how fast, in what patterns. The computer models, which use computational fluid-dynamics software to solve the equations that govern airflow, are good because they’re flexible, Flynn says. They can be used to easily vary small details, such as the size of the particles generated by the spray gun. The models account for many of the possible variables using simple shapes, such as a cylinder to represent the worker. Then the mannequin lets the researchers verify the models by repeatedly simulating different conditions.

Next: more paint on the object, less in the air
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