4. Thermal effects
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Heating and safety P.19
Thermal conductivity
The metal of aerosol cans is a good thermal conductor. However, the contents, which are a mixture of liquids and gases, will not always be so good. Even if the outside wall is at 60  °C, it will take time for the middle of the can contents to warm up. Although this means that the cans have to be in the water bath for about 2 minutes, it can also have advantages.
Photo of water bath
Picture 4.2. A water bath. You can see the line of cans goes up and down the water bath before leaving by the ramp on the left.

Shrink wrapping
Once the cans have been filled and packed onto cardboard trays, they are ‘shrink wrapped’. This involves wrapping a sheet of polythene around a tray of cans and heating it to shrink it. The polythene has to be heated to 190  °C to make it tight. Whilst heating the cans at all can be hazardous (see below), the shrink-wrapping is done in less than a second. Due to the thermal inertia of the metal can, there is not enough time to raise the temperature of the metal of the cans, let alone allow any heat to conduct through to the product. So the temperature inside the cans is unchanged.
Safety on the line
The shrink wrapping machine is part of the production line. However, unlike other parts of the line, the cans don’t go into the machine until there is a clear path through it. So, if the line stops, there won’t be any cans stationary inside the shrink wrapper.
Safety warning
All aerosol cans carry a safety warning about not exposing them to sunlight or throwing them onto a fire. Let’s see why.

There are volatile liquids inside – often a LPG propellant. If these are heated, then they will produce more vapour inside the can, which will make the pressure rise very quickly. A temperature rise of 30 °C can double the pressure inside. Even though the cans are tested, they will burst if the pressure goes up too far. A bursting can could be dangerous in its own right.

However, as well as being volatile, the propellant or other contents could also be flammable.

Even if the liquefied gas propellant has run out (i.e. the can is empty) or it uses a pressurised gas propellant, the can should still not be heated. This is because of the effect of the pressure law (see page 14). Raising the temperature of the gas inside will increase its pressure and, once again, the can might burst.

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Question 15
a) Explain why the high temperature of the shrink wrapper is safe as long as the cans pass through quickly.

Click shift/return to get a line break in your answer
b) There is a standard school experiment to compare the conductivities of metals. A copper rod and an aluminium rod each have a drawing pin attached to one end by a piece of wax. The other ends of the rods are heated. The drawing pin falls off the copper rod first - when the wax melts.

Assume that this experiment is done fairly and has the same results whenever it is repeated. Does it show that copper has a better conductivity than aluminium? Explain your answer.

Summary                                           Close
  • filled cans are heated in a water bath to check for leaks; they stay in for about 2 minutes to allow the temperature to rise
  • filled cans are shrink wrapped; this is is very fast so that the can contents don't have time to heat up