An aerosol system is a suspension of fine solid particles or liquid droplets dispersed within a gas medium. This state is maintained inside a pressurized container, allowing the product to be released in a controlled manner. A propellant is the substance, either a compressed or liquefied gas, that provides the necessary force to expel the product concentrate from the can.
The Core Function of Propellants
The primary purpose of the propellant is to create and sustain high internal pressure, which functions as the energy source for dispensing the product. When the actuator valve is pressed, a pathway is briefly opened, and this stored pressure instantly forces the product mixture out of the container.
The internal pressure drives the product concentrate up a narrow tube, known as the dip tube, and through the small opening of the valve system. The sudden release from high pressure to atmospheric pressure causes the mixture to atomize, or break apart, into tiny particles. The specific design of the valve and the physical properties of the propellant determine whether the output is a fine mist, a steady stream, or a stable foam.
Propellants maintain pressure to ensure consistent product delivery throughout the can’s lifetime. This pressure maintenance is important so that the last application is dispensed with a force similar to the first. The energy stored ensures nearly all of the contents can be evacuated before the pressure drops too low to be effective.
Major Classes of Propellant Chemistry
Propellants are categorized into two major classes based on their state inside the can and how they maintain pressure: compressed gases and liquefied gases. Compressed gases, such as nitrogen (N2) or carbon dioxide (CO2), remain entirely gaseous within the container. These gases rely solely on the initial pressure charge, meaning the internal pressure steadily drops as the product is used and the headspace volume increases.
Because their pressure decreases over time, compressed gases are often favored for products that require a coarser spray, like certain food products or foams, or where non-flammability is paramount. Liquefied gases, conversely, are stored as both a liquid and a gas in phase equilibrium under pressure. Common examples include hydrocarbons like propane, butane, and isobutane, which are the most widely used propellants today.
The advantage of a liquefied gas is its ability to maintain a nearly constant vapor pressure until the liquid phase is depleted. As the product is dispensed, the liquid propellant vaporizes into the expanding headspace, continually replenishing the gas pressure. This phase change mechanism ensures a consistent spray pattern and force throughout the entire use of the product, making them ideal for fine mist applications like hairsprays and deodorants.
Historical Shift and Modern Environmental Alternatives
The history of aerosol propellants is marked by a significant shift driven by environmental science and international regulation. Chlorofluorocarbons (CFCs) were once the standard propellant due to their non-flammable, non-toxic, and highly effective properties. However, scientists discovered that these compounds were migrating to the stratosphere and actively destroying the Earth’s protective ozone layer.
This discovery led to the 1987 signing of the Montreal Protocol, a landmark international treaty designed to phase out ozone-depleting substances globally. The swift and coordinated action resulted in manufacturers transitioning away from CFCs to more environmentally responsible alternatives.
The immediate shift was largely toward hydrocarbon propellants, which do not deplete the ozone layer. While hydrocarbons are effective and affordable, they are flammable and contribute to smog formation, leading to the development of other alternatives. Hydrofluorocarbons (HFCs) and the newer generation of hydrofluoroolefins (HFOs) were developed as non-flammable options with lower global warming potential (GWP). Hydrocarbons remain the dominant choice for consumer goods, but regulations promote the use of ultra-low GWP alternatives for specific applications, such as medical inhalers.