Guide to Aerosols
WHY DO WE NEED A PROPELLANT
The contents of the aerosol are made up of two components:
1. The product, in the form of a liquid, emulsion or suspension.
2. The propellant, which can be a liquefied gas, or even a compressed gas.
The propellant is the driving force (or, you could say, the ’engine’), behind the aerosol.
Liquefied propellants are gases that exist as liquids under pressure. Because the aerosol is under pressure the propellant exists mainly as a liquid, but it will also be in the head space as a gas. As the product is used up as the valve is opened, some of the liquid propellant turns to gas and keeps the head space full of gas. In this way the pressure in the can remains essentially constant and the spray performance is maintained throughout the life of the aerosol. The propellant is an essential element in the formulation.
Compressed gas propellants really only occupy the head space above the liquid in the can. When the aerosol valve is opened the gas ’pushes’ the liquid out of the can. The amount of gas in the headspace remains the same but it has more space, and as a result the pressure will drop during the life of the can. Spray performance is maintained however by careful choice of the aerosol valve and actuator.
In both of the examples above, you will see that the can is not full to the top with liquid. This is for safety reasons, as there must always be sufficient space for the propellant gas to occupy, under all likely storage conditions. If all the space in the can was full of liquid, there would be the possible danger of the can bursting. The actual amount of liquid that can be filled into an aerosol can is controlled by legislation, and there has to be a greater head space when using compressed gas propellants - see the section on aerosol cans.
Liquefied Petroleum Gas (LPG)
Aerosol propellant grade LPG consists of high purity hydrocarbons derived directly from oil wells, and as a by-product from the petroleum industry.
They consist of a mixture of propane, isobutane and n-butane. These propellants are used in most aerosols today, and have been used for many years in household aerosol products.
These gases are flammable, and this is reflected in the classification of aerosols which contain them.
Di Methyl Ether
This is an alternative liquefied propellant, and is more common in personal care products, and some air fresheners.
These liquefied propellant gases used to be very common prior to the discovery that they were affecting the ozone layer. They are no longer used in consumer aerosols in the western world. They are however permitted in inhalation aerosols, as used in the treatment of asthma.
Non-soluble compressed Gasses. (e.g. Compressed Air and Nitrogen)
These are sometimes seen in consumer products, and are an environmental alternative to LPG.
Soluble compressed Gasses (e.g. Carbon Dioxide)
This is another alternative to LPG, but has limited use, mainly with alcoholic systems, such as air treatment products, deodorants and personal care products.