logo      1. How aerosols work
Contents and operation P.3
The parts of an aerosol
The basic components of an aerosol are:
  • a container with . . .
  • a product that is put under pressure by . . .
  • a propellant and pushed out through . . .
  • a controlling valve.

However, each of these features can vary enormously. We looked at cans on the last page. Now let's see what's inside.

The product
The product is usually in a solution or a suspension. Sometimes, it is actually dissolved in the liquefied propellant.
LPG gas storage tanks
Picture 1.3. LPG storage tanks. Notice the water sprinkler system above the tanks.

The propellant
There are two types of propellant:
  • liquefied gas
  • compressed gas

Liquefied gas propellants are more common. These are made by compressing gases that can be liquefied. This allows a large volume of gas to be stored in a small space.

The most common liquefied gas propellants are mixtures of butane and propane (known generally as LPG - liquefied petroleum gas).

Liquefied gases are an integral part of the contents of an aerosol and often act as a solvent for the product. As well as producing the pressure that pushes the product out of the can, they help with the break up of the spray once it has left the can. The big advantage of liquefied gases is that the pressure in the can remains constant throughout the lifetime of the aerosol.

Compressed gases such as nitrogen, nitrous oxide, air and carbon dioxide are used sometimes, mainly for industrial and car products such as de-icers. Nitrous oxide is used in aerosol cream.

As compressed gases do not turn to liquid, there is a limit on the amount that can be included in the can. There usually isn’t enough to help with the break-up of the spray. Also, the pressure in the can will reduce as the contents of the can run down, so great care needs to be taken with the formulation so that the last of the product can be sprayed out.

Valve construction sequence
Picture 1.4. The valve assembly.
Aerosol cans are under pressure. Never take a real can apart or try to look inside
The valve
The valve has a number of components:

A plastic stem with a hole in it. The product passes out through this hole when the valve is opened. The valve is kept closed by a . . .

rubber gasket that slips over the stem. When closed, this seals the hole in the stem. But it can bend downwards to reveal the hole (see next paragraph). The stem and the rubber ring sit in a . . .

housing. When the actuator is pushed down, the ring is bent downwards and uncovers the hole - letting the product through. Inside, there is a spring that pushes the stem back up when the actuator is released; the rubber ring then re-seals the hole in the stem.

The valve design has a big impact on the aerosol spray. Changing the size of the hole in the stem can alter the flow rate. Along with the size and shape of the nozzle, the flow rate will affect the spread of the spray.

The dip tube. The dip tube goes down almost to the bottom of the can to make sure it is always dipped in the product and that you always get all of the product out.

In the can
Often the product and propellant are mixed in a single compartment can. However, in some cases, the propellant is kept separate from the product. You can find out more about single compartment cans and bicompartmental cans on the following two pages.
Question 2
Chlorofluorocarbons (CFCs) used to be used as propellants. The aerosol industry stopped using them in 1989. Why was this?

Click shift/return to get a line break in your answer

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Summary                                           Close
  • there are two types of propellant:
  • liquefied gas - usually LPG put under pressure until it liquefies; often acts as a solvent as well
  • compressed gas - e.g. nitrogen or carbon dioxide