An aerosol can is a self-contained dispensing system that stores a product and a gas under pressure in a sealed metal container. The gas, known as the propellant, is the driving force that expels the product when the valve is opened. This system relies on the stored energy of the compressed propellant to dispense the contents in a fine mist, spray, foam, or stream. Aerosol cans are used for a wide range of products, including paints, lubricants, deodorants, and cooking sprays.
How Propellants Function in Aerosol Systems
The propellant’s function is to maintain the internal pressure that pushes the product out of the can through the valve and nozzle. This high-pressure environment allows the liquid product to be atomized into tiny droplets upon release. Without the propellant, the can’s internal pressure would quickly drop after the first use, resulting in an uneven and eventually non-functional spray.
Aerosol systems are designed around two distinct propellant mechanisms: liquefied gas or compressed gas. The propellant must also be chemically compatible with the product formulation, which is a major factor in the choice of gas. In some systems, like bag-on-valve technology, the propellant surrounds a flexible bag that holds the product, keeping them physically separate.
The Main Chemical Classes of Modern Propellants
The gases used in modern aerosol cans are broadly categorized into liquefied and compressed propellants, each offering different performance characteristics. The most common propellant type in use today is the liquefied gas, which is often a blend of hydrocarbon compounds.
Liquefied Gases (Hydrocarbons)
Liquefied gas propellants are stored under pressure as a liquid that converts to a gas as the product is dispensed. The most common examples are hydrocarbons, specifically propane, butane, and isobutane. These gases maintain a consistent pressure level throughout the life of the can because the liquid evaporates to fill the empty space as the product is used. The drawback of hydrocarbon propellants is their high flammability.
Other liquefied gases include Dimethyl Ether (DME), which acts as a solvent in water- and alcohol-based formulations. DME is moderately flammable and is often used in hair sprays and industrial cleaners. The constant pressure provided by these systems results in a consistent and fine spray pattern.
Compressed Gases
Compressed gas propellants are gases that remain in a gaseous state inside the can, even under high pressure. The examples used in current aerosol systems are nitrogen (N2) and carbon dioxide (CO2). These gases are non-flammable and inert, making them a safer choice in terms of fire hazard.
However, compressed gas systems suffer from a pressure drop as the product is used. Since the gas does not change phase, the pressure inside the can decreases steadily as the contents are expelled. This results in a spray that becomes progressively weaker over time, often leaving some product unusable at the bottom of the can. CO2 and N2 are frequently used for food aerosols like whipped cream, as well as for some specialized industrial and pharmaceutical sprays.
The Shift Away from Environmental Hazards
The modern landscape of aerosol propellants is a result of a shift away from earlier gases that were found to be environmentally damaging. Historically, Chlorofluorocarbons (CFCs) were widely used as propellants because they were non-flammable and provided excellent, consistent pressure performance.
Scientific findings in the 1970s revealed that CFCs were rising into the upper atmosphere and destroying the ozone layer, which protects the Earth from harmful ultraviolet radiation. This led to a major international response, most notably the 1987 Montreal Protocol, an agreement designed to phase out ozone-depleting substances.
The industry quickly transitioned to alternatives, primarily hydrocarbon propellants, which do not deplete the ozone layer. Subsequently, other fluorinated compounds, Hydrofluorocarbons (HFCs), were adopted in some applications as a replacement for CFCs. While HFCs do not harm the ozone layer, they are powerful greenhouse gases with a Global Warming Potential (GWP) hundreds to thousands of times greater than carbon dioxide. International efforts are now focused on phasing down HFCs, further encouraging the use of low-GWP alternatives like hydrocarbons and compressed gases.