Montfort A. Johnsen, Ph.D.
Personal care formulations benefit most...
Contrary to popular belief, the formulation of most aerosols is a very complex business.
It is an art not taught in universities. Chemists and packaging engineers are
instructed by senior employees, by taking special seminars, reading manuals, reviewing
supplier literature, talking with supplier experts and by attending conventions and
trade shows. Literally hundreds of aspects must be considered, such as:
• Optimum choice of primary ingredients and percentage
• Optimum choice of support ingredients and percentage
• Ingredient compatibility—acute and long-term status
• Possible U.S. Patent infringement
• Dispenser compatibility
• Elimination of pathogens
• Control of other micro-organisms
• Selection of antioxidant, if needed
• Stable sources for ingredients (no obsolescence)
• Flammability profiling
• Finished product functionality and label claims
• Solution, emulsion or gel stability
• Fragrance and color stability
• Regulatory compliance—Federal and State
• Consumer acceptance
• Cost and Return-on-Investment
One of these many considerations is the possible effect of
trapped air (oxygen) on susceptible product ingredients. All
aerosols are filled in an atmosphere of air. Before the dispensers
are sealed, they always contain the concentrate and a headspace
full of air. An increasing number of
aerosols are “vacuum crimped,” meaning
that an 18" to 22" Hg. (partial) vacuum is
applied prior to crimping. In fact, many fillers prefer to use the 18" lower limit,
which removes 60% of the air (oxygen). Considering a head-space of 100mL
and a typical air density of air to be 0.1832g per 100mL, the 23.14 weight-%
of oxygen in that amount of air would be 0.0434g. If an 18" Hg. vacuum was
applied, the remaining oxygen would still amount to about 0.0174g. per 100mL
of head space. These amounts would seem to be parsimonious, but are still
sufficient to slowly cause significant oxidative chemical changes and malodor
problems in susceptible formulations.
Oxygen free radicals are capable of attacking various aerosol ingredients.
A favorite target is the double bond found in such triglycerides as vegetable
oils and some fatty animal products. Specific fatty acids include oleic, linoleic,
linolenic and elaidic acid. The rancidity of these oils and acids is well known.
It results from the disruption of the double bonds, with the formation of
peroxide and hydroperoxide intermediates, which degrade further into acids,
aldehydes, alcohols and ketones. Compounds such as caproic acid have a very
disagreeable, rancidic odor. The aldehydes have strong, biting odors that are
usually unpleasant. Some are further oxidized to the corresponding malodorous
carboxylic acids.
The oxidation process, once begun, almost always continues until the oxygen
initiator is used up. It can be considered as a chain reaction process that cannot
Technical Editor
The Role of Aerosol
Anti-oxidants
32 Spray June 2019