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Aerosols and Pressure

Getting the right vapour pressure

The picture below is the ingredients list for a dry shampoo aerosol. It contains a blend of three liquid propellants that are all above their boiling points at 20 ºC.


The propellant is a mixture of three liquified gases: butane, isobutane and propane.
Chemical engineers blend the liquids to get exactly the right pressure in the can.

If just single propellants were used  the vapour pressure at 20 ºC would be:

PROPANE      8 bar  ( That is 8 x atmospheric pressure, which is too high)

BUTANE         2 bar  ( Too low)

ISOBUTANE  3 bar  (Too low)

By blending the propellants a pressure that is just right can be created. The higher the propane content, the higher the pressure.

This chart shows how the vapour pressure depends on temperature. The higher the percentage of propane the higher the pressure.


The other ingredients are perfumes, alcohol ( as a solvent ) and conditioners. The iron oxide ingredients are pigments to add a dark brown colour.

Try to find the same information about temperature and propellants on this can. This is an anti-perspirant product. The ingredients are different to the dry shampoo.
The propellants are the same, but the percentage of each will depend on the pressure required by the design engineers.

Storing the liquid propellants

The liquified gases are stored in strong tanks like these. These are kept away from the aerosol filling building. The tanks have safety systems to detect gas leaks and automatic spray systems to prevent fire.


1. Why are the storage tanks painted white?

2. Why is propane added to the blend of propellants?

3. From the graph what is the vapour pressure of pure butane at 0ºC

4. Which of these statements is true about the graph?
           a. The graph shows a linear relationship between temperature and pressure.
           b. The graph is non-linear. The rate of pressure increase rises with temperature.

5. Compare the pressure increase for 100% propane (the top line) from 10 to 20 ºC and from 30 to 40 ºC.

6. For a 50/50 mix (the green line,) what would you recommend to can designers as the maximum safe working pressure for the can at 50 ºC


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When wet is dry

For dry shampoo and deodorants the product is partly dry. Rice starch dissolved in water would turn into a gooey mess like wallpaper paste. The rice starch and powder iron oxide colourings are held in suspension in the propellant, but stay 'dry' because the propellant contains no water.
Cans like this must be shaken vigorously before each use to mix the product into the propellant.

Some of the ingredients DISSOLVE in the propellant. Others are held in SUSPENSION. Products in suspension will eventually settle if they have a density higher than the propellant. Shaking the can before each use mixes the ingredients again.
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