Aerosol cans are common household items, used for a variety of products from air fresheners to paints. These dispensers rely on a specialized gas, known as a propellant, to function. Understanding how this gas moves within the can reveals its internal engineering.
Key Components of an Aerosol Can
An aerosol can operates as a pressurized system, composed of several distinct parts. The main container, often made of metal like tinplate or aluminum, is designed to withstand high internal pressure. At the top of the can is the valve assembly, which controls the release of the product and includes elements such as a stem, spring, and gasket.
Extending from the valve assembly down to the bottom of the can is a thin, straw-like component called the dip tube. This tube allows the liquid product concentrate to be drawn upwards. The final component is the propellant, a gas that provides the force to expel the product.
Propellants and Their Properties
Propellants are substances, either gases or liquefied gases, that generate high vapor pressure inside the sealed can. This pressure is essential for driving the product out. There are two primary categories of propellants: compressed gases and liquefied gases.
Compressed gas propellants, such as nitrogen or carbon dioxide, remain in a gaseous state within the can. Their main limitation is that the internal pressure steadily decreases as the product is dispensed. This can lead to a weakening spray over time.
Liquefied gas propellants, including hydrocarbons like propane and butane, or hydrofluorocarbons, are more common. These propellants exist as a liquid under the high pressure inside the can, but they readily vaporize into a gas when that pressure is released. This vaporization continuously replenishes the gas, maintaining constant pressure and ensuring a consistent spray until nearly all the product is used.
The Mechanics of Aerosol Discharge
When a user presses the actuator button, it depresses the valve stem, opening a pathway. This action immediately creates a pressure differential between the high-pressure environment inside the can and the lower atmospheric pressure outside. The high-pressure propellant then rapidly expands.
As the propellant expands, it pushes the product concentrate up through the dip tube. This mixture of product and propellant is forced through the small opening of the nozzle. The sudden drop in pressure at the nozzle causes the propellant to expand further and, in the case of liquefied gases, to flash into a gaseous state. This rapid expansion atomizes the product, breaking it into tiny droplets or forming a fine mist or foam.