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| Picture 5.2 A compressed gas propellant produces less pressure as the product is used up.
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Aerosol cans are under pressure. Never take a real can apart or try to look inside |
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Liquid or gas |
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There are two types of propellant:
- a compressed gas (e.g. nitrogen or carbon dioxide)
- a liquid (liquefied gas) (e.g. butane, isobutane, propane)
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Advantages of compressed gas propellants |
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The advantages of these are:
- they don't burn (although other liquids in the aerosol may)
- they are readily available and cheap.
A disadvantage, however, is that the pressure in the can reduces over the lifetime of the aerosol. Partly because the available volume inside the can increases (as the product is used up) and partly because some of the gas is released with the product (see page 9).
Because the disadvantage outweighs the advantages, only a few modern aerosols use a compressed gas.
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Picture 5.3 liquefied propellants maintain a constant pressure.
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Vapour pressures |
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Most modern aerosols use a liquefied gas as the propellant. The gas has been turned into a liquid by putting it under pressure. Inside the can, its vapour produces enough pressure to force the product out. As the product level drops, more propellant evaporates to maintain a constant pressure in the space above the product. Most liquefied gas propellants are mixtures of simple hydrocarbons such as butane and propane. The drawback is that they are flammable.
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CFCs |
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For some time, a group of compounds known as chlorofluorocarbons (CFCs) were used. Whilst they were not flammable, scientists found that they were damaging the ozone layer at the top of the atmosphere and the UK aerosol industry stopped using them in 1989.
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