Air & Oxygen
MONTFORT A. JOHNSEN, PH.D.
TECHNICAL EDITOR
Good and bad for aerosols...
Today, virtually every aerosol has some air in the dispenser. The amount varies with the fill
volume, the degree of vacuum crimping (if any), the volatility of the content and other factors.
It can be useful on occasion, but is more likely to be either neutral or problematical.
Ordinary tropospheric air contains over 50 ingredients, if one includes those present in parts
per billion (ppb.) The more important ones are listed as follows:
COMPOSITION OF TROPPSPHERIC DRY AIR
INGREDIENT VOLUME BASIS WEIGHT BASIS
Nitrogen 78.09% 75.03%
Oxygen 20.95 23.01
Argon 0.93 1.28
Carbon Dioxide 0.04 0.06
Neon 0.00018 0.00013
Helium 0.00005 0.00003
Carbon Monoxide 0.00003 0.00003
Krypton 0.00001 0.00003
Ozone 0.00007 0.00001
Xenon, et al. nil nil
The most obvious effect of trapped air is that it adds
pressure to that of the aerosol formulation. For instance,
in an extreme case, if an empty can is atmospherically
crimped (no vacuum), and the can is then filled by a
through-the-valve (T-t-V) gasser to 80% of its capacity,
the partial pressure of air will typically be about 24psi.
(70°F). On the other hand, if it is filled with the product
to 80%, using a through-the-cup (T-t-C) gasser, and
then atmospherically crimped, the partial pressure of
air will be in the order of 5psi (70°F), since the amount
of trapped air will be much less. In most cases, the can
will have been filled with the liquid concentrate, prior
to gassing, resulting in partial pressures of air that are
intermediate between the two extremes.
The industry has gradually converted to the wide-spread use of vacuum crimping. Unofficial standards
for high-speed production lines anticipate that cans will be evacuated to 20 +/- 2” of mercury (a perfect
vacuum would be 29.92” of mercury, at sea level). The process removes about 64% of the trapped air, as
currently practiced. The partial pressure due to air then decreases linearly.
One might guess (or calculate) that the compression of the air-filled head space volume would follow
Boyle’s Law—that for a 50% decrease in volume a 10psi pressure of air would double to 20psi. This is
incorrect, since the liquid content dissolves most of the trapped air. The amount dissolved increases as the
pressure increases. Conversely, the amount decreases as the temperature rises. This happens, but the effect
is very small compared to the greater solvency of air due to the increased pressure.
Every aerosol product (concentrate plus propellant) will have a unique degree of air solvency, preventing
any generalizations. As aerosol dispensers are used, the head space volume increases. Some of the air then
flows from the liquid phase, but the overall effect is a lowering of the total pressure. The attenuation of
trapped air will reduce the delivery rate slightly, but will have almost no effect upon particle size distribution.
Such other aspects as propellant loss into the head space, propellant fractionation of blend, use of
vapor-tap valves and so forth affect spray characteristics to a greater degree than the decrease in the partial
pressure of air.
36 SPRAY March 2018