1. How aerosols work
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Bicompartmental cans P.5
Bicompartmental cans allow us to keep the propellant separate from the product. This means that the propellant has no effect on the break up of the spray. This can be useful in, for example, shaving gels. In this case, we want to push the gel out without forming any droplets. This is easier if the propellant is in a separate compartment from the gel. It is also useful where the propellant might react with the product.

There are two main methods:

  • bag-in-can
  • bag-on-valve

Let's have a look inside a can of shaving gel, which uses the bag-in-can method.

Bicompartmental aerosol can cut away diagram Bicompartmental aerosol can cut away diagram
Cut away diagram
Bicompartmental aerosol can cut away diagram Cut away diagram
Bicompartmental aerosol can cut away diagram
Bicompartmental aerosol can cut away diagram Bicompartmental aerosol can cut away diagram
Cut away diagram
Bicompartmental aerosol can cut away diagram Cut away diagram
Cut away diagram
Cut away diagram Cut away diagram
Cut away diagram
Cut away diagram Cut away diagram
Cut away diagram
Cut away diagram Cut away diagram
Picture 1.6. A bicompartmental aerosol can.
Aerosol cans are under pressure. Never puncture a real can or try to look inside
1. The product is kept separate by putting it in a nylon bag inside the can.

2. The propellant surrounds the bag, producing an inwards pressure to push the product out.

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 nylon bag is usually crimped into the join between the can and the valve cup. Often, it is softened with steam so that it is more flexible and crimps more easily.

The aerosols are fully assembled (except for the actuator) before they are filled.

Filling the cans

The most common bicompartmental cans are filled at both ends.

The propellant is injected through a bung in the base of the can using a type of hypodermic syringe. Flammable propellants (like butane and propane) are put into cans in a special gassing house (see page 6). The top of the can is attached to a vacuum to collapse the bag whilst the propellant is injected.

Once the propellant has been injected, the product is forced in through the valve at the top. About 200g of product is forced into the can in 2 seconds at a pressure of about 6.0 MPa (about 60 atmospheres). This means that the fastest lines can fill about 250 cans per minute.

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 4
a) The density of the product is 1.25 g cm-3. What is the volume of 200g of the product (value given to 3 SF)?  cm3
b) The delivery pipe has a diameter of 2 cm2. What is its cross sectional area?  cm2
c) What is the length of 200 g of product whilst it is in the pipe?  cm
d) The product is delivered in 2 seconds. How fast is it travelling?  cm s-1

Summary                                           Close
  • the product is inside a nylon bag which is surrounded by pressurised gas propellant
  • they are mostly filled at both ends
  • they are tested once they have been filled