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I've heard that in the past, spray cans (aerosol cans) contained Freon. I found that the Montreal Protocol made the usage of Freon illegal due to its effect on the ozone layer, but I can't find the reason for why Freon was used in the first place. I understand its role in refrigeration systems, but I fail to see a functionality for spray cans.

So why would anyone use Freon in spray cans, what was its contribution? I assume that it affected the pressure inside somehow, but it's just a guess.

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Spray cans work by containing a propellant which has a boiling point a small amount below room temperature. When the can is sealed the liquid and vapour reach equilibrium at a relatively modest pressure. When the spray valve is opened this pressure forces out the contents as an aerosol and more of the propellant evaporates as the internal pressure drops.

This has two advantages firstly the can doesn't need to be anywhere near as strong as if the propellant was a fully pressurised gas and it provides a fairly constant pressure untill the propellant is exhausted without needing mechanical regulators which would add cost and complexity.

This is pretty much the same principal as a butane lighter.

So what you need in a propellant is something with a very specific boiling point which provides the required vapour pressure at the expected operating temperature.

As mentioned in the comments butane and/or propane are now often used as propellants in aerosol type cans indeed 'canned air' dusters used for cleaning computers etc is often just butane.

One downside of butane is that there is an inherent cooling effect as the liquid evaporates and the gas expends which can cause a significant cooling effect and, as the boiling point of butane is a bit on the low side this can cause significant cooling of the contents of the can and a consequent drop-off in pressure.

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    $\begingroup$ And if it is non flammable and cheap that is a nice bonus. Freon was both, the propane they often use today, not so much on the flammability thing. $\endgroup$
    – Dan Mills
    Feb 18, 2017 at 14:01
  • $\begingroup$ Thank you. So basically it's a mixture between the scent and the Freon. So one would have sprayed both on one's skin, right? $\endgroup$
    – Physther
    Feb 18, 2017 at 14:14
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    $\begingroup$ @DanMills: Being non-toxic and odorless is also pretty useful. :) And yeah, there are very few compounds that satisfy all those criteria, or even most of them. Water has too high a boiling point, ammonia too low (and it stinks to high heaven). In between you mostly find short-chain hydrocarbons (which are flammable) and their halogenated derivatives (which, as noted, tend to be bad for the ozone layer). $\endgroup$
    – Ilmari Karonen
    Feb 18, 2017 at 15:33
  • $\begingroup$ I think you mean a small amount below room temperature... $\endgroup$
    – user_1818839
    Feb 18, 2017 at 21:54
  • $\begingroup$ @Brian Drummond...yes well spotted $\endgroup$
    – Chris Johns
    Feb 18, 2017 at 22:26

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