Reduction of air (oxygen) in aerosols
As is well known, trapped air increases aerosol pressures, may oxidize some sensitive ingredients and may
influence the rate of can corrosion.
Techniques for reducing head-space air include vacuum crimping, purging and sparging. Vacuum
crimping is the most popular by far. The unofficial industry standard is to apply a vacuum of 20 ± 2"
of mercury (508 ± 51 mm Hg.), although many fillers set their controls to about 18”–19” for economic
reasons. This eliminates about 62% of the air in the head space. There is always a slight increase of headspace
air, as the aerated concentrate unloads a portion of its dissolved air into the gas phase. Air readily
dissolves in stirred water to the extent of 15.64 mL per liter at 86°F (30°C). The dissolved air then contains
33 volume-% oxygen at 86°F, since oxygen is more soluble than nitrogen. The head-space oxygen
available to participate in possible corrosion reactions is about 27.4 = mg per 100mL at 77°F (25°C) and
760mm Hg. Vacuum crimping and other methods decrease this amount. If the oxygen supply is totally
depleted (which is rare) any corrosion of the aerosol can will usually stop, unless there are alternative
oxidizing species in the formulation.
Purging has been used on occasion, sometimes with vacuum crimping to further reduce the trapped
air or oxygen content. The purging gas is usually nitrogen or carbon dioxide. On the production line,
streams of purging gas may be directed into moving cans using flexible tubes. This is wasteful since a
large portion of gas is lost between cans. Alternatively, in-line or rotary machines can be applied. In a
tunnel device (proposed by G.B. Diamond of the now defunct Diamond Aerosol Corp.), carbon dioxide
is filled into a compact, flap-ended tunnel. Cans enter the tunnel and the head-space air is displaced by
the much heavier carbon dioxide. By adjusting operational conditions, well over 90% of the air can be
removed. In most cases, the retention of very small levels of carbon dioxide is not a problem. The use of
another heavier-than-air gas, such as argon or Solstice HFO1234ze, is an untested possibility.
Sparging is an inefficient technique for head-space air reduction, but it has been used commercially,
always with vacuum crimping, for water-based insecticides and various other aerosol products. It involves
inserting a multi-perforated ball that is filling with nitrogen into the concentrate feed line. By adjusting
the nitrogen pressure to about 10–20 psi greater than the line pressure, the “bubbler ball” forces the gas
into the concentrate. When the product reaches the filler bowl, the pressure drops to atmospheric and
most of the nitrogen bubbles out, taking most of the air (oxygen) with it.
The air content of saturated water is 17.08mL per liter at 77°F (25°C), equal to 5.78mL per liter of
oxygen. Some fillers prefer to heat their tanks of de-ionized water to about 160°F, (71°C) “to remove the
air.” Actually, at 160°F, the solubility of oxygen is still significant, at 3.68 mL per liter (even in 212°F
water, 2.70 mL per liter remains). It follows that heating water to 160°F is rather inefficient
in reducing the already very small amount of oxygen that it may contain.
Because of the challenges presented by oxygen species and associated free radicals, many
cosmetic products include various antioxidants in their formulas. They act by removing free
radicals (including lipid peroxyl radicals) and chelating metal initiators. Antioxidants also
reduce those substances produced by singlet oxygen species, such as hydroperoxides, thus
preventing them from further degradations.
There are many causes leading to skin damage. They include exposure to ozone, nitrogen
dioxide, UV light and the endogenous production of singlet oxygen species. The skin is also
subject to stress factors, such as atopic dermatitis, icythyosis, psoriasis and pruritus. There
are three types of antioxidants. The lipid soluble variety includes vitamins A and E, as well
as their relatives, carotenol, BHA (substituted anisole mixture), BHT (substituted toluene),
hydroqinone and many others. The water soluble category includes uric acid, ascorbic acid
(vitamin C), glutathione and others. The final group includes certain enzymes, such as
superoxide dismutase (SOD), mentioned earlier.
“Airosols” & “Nitrosols”
Both purified air and nitrogen are in use as propellants, although nitrogen is favored since
it is almost totally chemically inert at ambient temperatures. The few exceptions can be
a) Metallic lithium reacts to form lithium nitride (Li3N).
b) A few transition-metal complexes can slowly form.
c) Some nitrogen-fixing bacteria exist.
d) Symbiotic reactions occur on certain plant roots, such as peas and beans.
The more a wasp and
hornet spray is used,
the more the can’s
pressure decreases and
it’s “reach” lessens.
June 2018 Spray 29