How does an aerosol bomb work?
The active substance is either dissolved or held in suspension after
agitation in a liquid gas that has a boiling point below room temperature
and is therefore under pressure. When you press the valve, the pressure from
the liquid gas forces the mixture up the pipe on the inside of the can to
the valve, where it is directed to where it is needed. On contact with the
outside air, the pressure is relieved and the gas boils off violently in the
nozzles, forcing the active substance to break up into minute particles to
form the mist. The nozzle design can change the form of the spray radically,
according to the application.
As a general rule, there is more gas than active substance in an aerosol
bomb.
History of the aerosol bomb.
The aerosol bomb was a product of the 1939-45 war. It was first produced
as an insecticide spray, with DDT, as a hygiene measure for use in field
hospitals. It was called a bomb because they looked just like this, a black
cylindrical object with convex ends, the two halves being welded together
round the middle. The propellant gas was CFC-12,
more usually used as a refrigerant. This was chosen as being non-toxic,
non-flammable, odourless and relatively cheap.
The household sprays, in simpler tinplate cans, became popular in the
1950s for hairsprays, deodorants, paint, furniture polish etc., as well as
insecticides. These mostly used CFC-11 or a mixture of CFC-11 and -12 as
propellant, because of pressure concerns with the weaker cans at high
temperatures.
By the mid 1970s, nearly all the millions of cans produced daily, for an
ever widening range of active products, used CFCs as propellants.
As a result of the theory of CFCs possibly causing
ozone depletion, bearing in mind that all the
gas was emitted to the atmosphere in use, various bodies studied the
possibility of reducing the use of CFCs in this application. In 1978, a
number of countries mandated that CFCs should no longer be used as aerosol
propellants (with a few exceptions). This was gradually followed by many
other countries so that, by the mid-1980s, most CFC use was already phased
out in developed countries.
This placed the fillers in a tizzy, as all the other gases had some
disadvantage or another. Butane and propane were flammable; carbon dioxide
and nitrous oxide required higher pressures and were more chemically
reactive; tetrafluoromethane was not reactive but required higher pressures
with a relatively low dissolving power for other substances. Each of these
required a complete redesign of the product; the can, valve and nozzle
design, as well as the liquid formulation, generally had to be changed. New
gases were also developed, such as HCFC-22 and
HFC-134a, but the former was also
ozone-depleting and latter a bad greenhouse
gas.
Today, most aerosol bombs use any of the above gases as a propellant,
with butane and propane for most applications, even with the problem of
flammability. These gases were otherwise ideal, solubilising a wide range of
active products.
Of course, there were other non-aerosol solutions to some of the
problems: window cleaning and surface cleaning products in plastic spray
bottles, roll-on anti-perspirants, air-pumped insecticides and so on.
The exceptions
There is one class of aerosol that still uses CFCs: metered dose
inhalers, such as may be used by severe asthmatic and chronic emphysema
patients. For most patients with tracheal and bronchial disease, powder
inhalers were very successful, but these were insufficient for patients with
reduced alveolar function. The main problem here was that the propellant gas
had to be totally non-toxic and unmetabolisable. CFC-12 was ideal, as it
also was sufficiently active that the medication droplets could be
dimensioned so that the suspension could reach the alveoles. For many MDIs,
a compromise was successfully sought with fluorocarbon gases but a few
formulations still require the use of CFCs.