The Science of Spray Paint
Those ubiquitous, cylindrical cans you have sitting around the garage represent a pretty simple design. They're usually constructed from light gauge steel or aluminum and include a valve, a dip tube and a pea, each of which we'll discuss later in this section.
Spray paint works the way it does because it's actually a combination of paint and a gas propellant. During the manufacturing process, the cans are first filled with paint and then injected with the gas propellant. The valve is then crimped, trapping the highly compressed compound within the confines of the can. And this is the genius behind spray paint's efficiency. Chlorofluorocarbons were the propellant of choice for most aerosol-delivered products, including spray paint, until 1978, when it was banned after it was discovered that it contributed to the depletion of the ozone layer [source: Martineau]. So the industry turned to hydrocarbons – organic compounds made up of hydrogen and carbon – as an alternative until the 1980s when it was determined they may cause smog (butane, propane and methane are a few common examples of hydrocarbons) [source: Practice Greenhealth]. Today, most aerosols use hydrofluorocarbons ,which consist of hydrogen, fluorine and carbon, and present a much smaller risk to the environment because they do not contain potentially harmful elements like chlorine.
So, how does all of this work together?
When the nozzle is pressed, the paint-propellant mixture rushes up the dip tube in response to the sudden, dramatic change in pressure that causes the propellant in the can to expand, forcing the paint through the tube and out through the nozzle. The dip tube acts like a straw, with one end attached to the valve and the other end near the bottom of the can. Once through the dip tube, the nozzle and free of the pressurized can, the propellant expands and the paint atomizes into a fine, even mist. These tiny paint particles (not actually atoms) are evenly distributed because they are positively charged, repelling each other as they leave the can. And because the object being painted is oppositely charged, spray paint adheres to the surface and can be applied in even, smooth layers with almost zero waste.
The pea, a small metal ball bearing, maintains the propellant-paint mixture inside the can. By shaking the can, the pea agitates the mixture so you can be sure the two components are combined properly. This is where spray paint gets its signature rattle. If the paint and propellant are allowed to settle, they will come out in uneven proportions, causing the paint to clump.
This combination of mechanics, chemistry and physics has turned spray paint into one of the most useful tools for home improvement. But the benefits are not limited to household use, so keep reading and discover some heavy-duty applications of spray paint.