At Themost commonly used gases, or propellants, are

At Themost commonly used gases, or propellants, are

At one time or another, most of us have probably used an aerosol can.With just a push of the button, we easily and quickly apply just the rightamount of furniture polish, bathtub cleanser, or underarm deodorant. It isobvious to us that the aerosol can is more convenient than the old-fashioned jars of liquid polish or cans of powdered cleanser, which canspill or be applied too thickly.

At the same time, however, most of usprobably do not know how the aerosol can works. We accept its ease andconvenience without realizing that the aerosol can is quite a complexcontainer.An aerosol is not a simple cleaner or polish; it is a colloidalsystem.

That means it consists of finely divided liquid or solidparticles, called a product, dispersed in and surrounded by a gas. Themost commonly used gases, or propellants, are chlorinated hydrocarbons,butane, propane, isobutane, vinyl chloride, and nitrogen. (Tom Harris)Nitrogen is used particularly for products when the taste or smell of thepropellant is not desired in the product, for example, in toothpaste orwhipped cream.To complicate things further, the propellant, or gas, is present ineither a liquid or a gaseous state. (Tom Harris) If the propellant isliquid, it is mixed with the product in the form of a solution.

An exampleof this type of aerosol is hair spray. The spray or lacquer is usuallydissolved in alcohol and then completely mixed with the propellant. If thepropellant is present in a gaseous state, the container usually is abouthalf-filled with the product and half-filled with a propellant such asnitrogen. The propellant gas forms a separate layer over the product.

Aerosol toothpaste is a good example of this type of aerosol. The contentsof the can, then, are either a product-propellant solution or a layer ofproduct topped by a layer of propellant. In both cases, the propellant gasis not simply put in a jar but is forced into the can under high pressure,about 90-120 pounds per square inch.The releasing mechanism for an aerosol can is also complicated.

Allaerosol cans are fitted with a valve and a siphon tube, which allow for therelease of the product. (Tom Harris) The valve is either a spray valve ora foam valve, depending on the contents of the can. The valve is force-fitted into the neck of the dispenser under high pressure (about 3/4 tons),so that a strong, gas-tight seal is formed. This prevents the propellantin the can from escaping. The valve is connected to a siphon tube, whichhangs down inside the can to the bottom of the can. When the valve buttonis pressed, the propellant vaporizes immediately, and its pressure forcesthe liquid through the siphon tube and out the nozzle. The liquid comesout of the can in the form of a mist or foam.

What appears to be a simple push of a button, then, is really acomplex accomplishment of modern physics and chemistry. The insectrepellant that sprays a fine mist neatly and quickly was not simply pouredinto a bottle; it was packaged very carefully, with specific andcomplicated equipment. The next time we pick up a familiar aerosol can andwonder how it works, we should remember that the ease and convenience itgives us is the result of someone or someone else doing a lot of work.