1. How aerosols work
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Single compartment cans P.4
There are two ways of storing the propellant and product inside an aerosol can: together in a single compartment or separately in two compartments. On this page we’ll look at the structure of a single compartment can.

In a single compartment can, the product and propellant are mixed. In some cases, the propellant may even act as the solvent for the product. In other cases, the product is dissolved in water or another suitable solvent. Let’s have a look inside a can.

Cut away diagram
single compartment aerosol can
single compartment aerosol can
single compartment aerosol can
single compartment aerosol can
single compartment aerosol can single compartment aerosol can
single compartment aerosol can
single compartment aerosol can
Picture 1.5 A single compartment aerosol can.
Aerosol cans are under pressure. Never puncture a real can or try to look inside
1. The product. This is a solution or suspension of the substance that we want to spray. In this case, it is an aqueous solution of a water-soluble glue.

2. The propellant. In this case, the propellant is a liquefied mixture of butane and propane (LPG). The gas has been put under pressure until it turns into a liquid (this happens at about 2.1 x 105 Pa). The space above the liquefied gas is taken up by butane or propane vapour. It is the pressure from this vapour that pushes the product out of the can. As the product gets used up, more of the LPG will turn into vapour, keeping the pressure constant.

3. The actuator. When you press down on the actuator, it opens ...

4. ... the valve and allows the product to be pushed out. The structure of the valve and the nozzle help determine the features of the spray.

5. The dip tube. The dip tube goes down 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.

The features of the spray

In designing an aerosol, the manufacturers have to consider the effects of a number of variables. Different aerosols have a number of different features – or output variables. For example:

  • angle of spread
  • droplet size
  • speed of spray.

Some of these features are related to each other. And they are all related to the way that the contents of the aerosol are formulated – or input variables. These will include:

  • proportions of product and propellant
  • type of propellant
  • solvent for product
  • size of hole in valve stem
  • shape and size of aperture in nozzle.
For example, in a single compartment can, propellant can be used to break up the spray once it has left the can. So this will produce a finer spray of smaller droplets. As smaller droplets make the spray feel warmer (see page 10), this sort of formulation is useful for body sprays and deodorants.

Filling the cans
You can see the case study for filling single compartment cans on page 6.
Testing
Once the cans have been filled, they pass through a warm water bath at 50 °C. This raises the pressure in the cans to 8 x 105 Pa (about 8 atmospheres) and tests for any leaks in the can (see page 19).

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Question 3
a) Look at the variables below. Which of these might break the spray up more if it were increased?
  1. The volatility of the propellant
  2. The proportion of propellant
  3. The size of the hole in the stem

Click shift/return to get a line break in your answer
b) Imagine you wanted to make a finer spray without increasing the flow rate. How could you achieve this using the variables above?

Summary                                           Close
  • single compartment cans usually use a liquefied gas propellant
  • the amount of propellant and product help determine the features of the spray
  • the cans are filled then tested in a single line