Basic Aerosol Technology

Basically the valve operation is either push down or tilt.

The vast majority of aerosol valve technology lies in the hands of a relatively small number of valve manufacturers. As expected the majority of these are American. The major valve manufacturers in the world are:

• PRECISION VALVE CORPORATION
• SEAQUIST - VALOIS
• SUMMIT PACMGING SYSTEMS INC.
• COSTER TECHNOLOGIE
• AEROSOL RESEARCH AND DEVELOPMENT (COPE ALLMAN)

The basic components of a valve are illustrated in Figure 1. some of the terminology is unique to the aerosol industry.

Generally the nozzle fits over the stem but in some instances it fits into the stem - in this case it is called a female valve.

The latest developments are sleeve gaskets to replace the flowed in compound and laminated mounting cups.

• solvent based or
• water based.

• single phase or
• two phase - which require shaking of the product before spraying.

• Space spray - 50-90% propellant
• Surface spray - 5-15% propellant

This is probably the major concern for all formulators. During the development of a formulation there are a number of basic technicalities which have to be observed. For example, in anhydrous formulations the moisture level of raw materials must be checked.

Shelf testing is obviously a necessity - if possible shelf test samples should be filled on a production line. It must be remembered that corrosion requires the presence of an electrolyte, two electrodes and a potential difference between the electrodes. The electrolyte is the solution of the product and the two electrodes (in a tinplate container) are the tin and iron. However, as tin and iron are very close in their electrochemical behaviour, slight variations in their environment such as the level of oxygen can determine which metal is the anode and which is the cathode. Normally corrosion occurs at the anode.

As the area of tin which is exposed is large compared to the amount of iron, you can see why the polarity is critical. If the large area (i.e. tin) cathode is coupled to the small area anode (iron) the corrosive attack is concentrated and pinholing results. If the system was reversed general detinning occurs rather than pinholing.

Aluminium can be rather unpredictable in terms of corrosion, particularly as it is a very active metal which readily takes on a tenacious film of inert aluminium oxide when exposed to air, giving rise to a rather unique and complex set of corrosion properties.

Products can be successfully shelf tested and even be manufactured satisfactorily for many years yet occasionally corrode. Unfortunately in many cases the reason for the failures is never determined despite considerable time and expense being spent on investigation of the problem.

General
The presence of air (oxygen) in an aerosol product, apart from causing corrosion problems, can also significantly increase the pressure inside should large amounts be present. Water quality should be at least considered as presence of too many or different metal ions or other impurities may affect compatibility with the container.

It should be acknowledged that internal can linings (usually epoxies) and side stripes on the weld are never perfect and should not be relied upon to protect a container. In some instances "concentration" can occur through the pinholes in a lacquer, thus allowing a lacquered can to corrode or pinhole faster than a plain can.

In Australia, flammability of an aerosol is measured by the flame test (see Figure 2), whilst in Europe, the percentage of flammable materials governs the classification. In the latter case, any adjustment by way of spray rate and/or valve configuration does not affect the flammability classification which determines such items as wording on labels, warehousing, transport etc.

The gas ozone provides the earth with an effective UV (ultraviolet) shield. The majority of ozone is located in the stratosphere, about 10 to 50 km above the earth's surface. It is in constant movement because it is determined by a balance between photochemical processes that produces it and other processes that destroy it. The quantity therefore varies with latitude, longitude, night and day, seasons, year by year and possibly with cycles of solar activity.

The concern about CFC is that prolonged release might deplete the stratospheric ozone layer. This hypothesis was raised in 1974 by Rowland and Molina and current evidence is that global ozone levels are decreasing very slightly. The ozone problem arises because CFCs, released at the Earth's surface predominantly in the major cities of the northern hemisphere, are sufficiently stable to resist breakdown by oxidation in the lower atmosphere. Thus they are transported by the weather systems into the southern hemisphere and also into the upper atmosphere where they are broken down by intense ultraviolet radiation. Not only does this photolysis of CFCs release chlorine which is capable of enhancing the natural ozone destruction processes, but it does so in a region of the atmosphere close to where most of the ozone is found. Atmospheric models indicate that continued use of CFCs at current levels will cause small but significant losses of ozone over the entire globe, with substantial losses in Antarctica and possibly in the Arctic as well. The Montreal Protocol (under the auspices of the United nations Environment Program) was agreed to in September 1987. it called for, using 1986 consumption as base figures, for CFCs,

• freeze at 1986 levels from mid 1989.
• reduce these levels by 20% by mid 1993.
• further reduction by 30% (i.e. total 50%) by mid 1998.

• in December 1987 endorse the protocol but phase down at a faster rate.
• in April 1988 to phase out CFC (except for essential use Government defined products) by the 31 December 1989.

There was a meeting in London in June 1990 to review the Montreal Protocol. It revised the Protocol for CFCs (in total) to now be,

• 1995 50% phase out
• 1997 85% phase out
• 2000 100% phase out

CFCs can also contribute to the Greenhouse effect by absorbing infrared radiation.

Carbon dioxide is the major gas of concern - it is expired by man, it is also formed when forests and other items are burnt.

The natural gas (hydrocarbons) used in aerosols, do not persist in the atmosphere for very long and the contribution to the Greenhouse Effect is, in effect, so small as to be of no consequence.

To put the issue in perspective, the greenhouse contribution through the per capita use of 10 aerosol products per year is the equivalent of driving an average family car an extra 15 km in a whole year.

1. Fraser, P.J. (Ed), Environmental, Health and Economic Implications of the use of Chlorofluorocarbons as Aerosol Propellants and and Possible Substitutes, 1987.
   A joint Australian Environment Council/National Health and Medical Research Council Document.
2. Australian Environment Council, "Strategy for Ozone protection", August 1989.
3. Johnsen, M.A., "The Aerosol Handbook", Second Edition, 1982.
4. Herzka, A.,(Ed), "International Encyclopaedia of Pressurized Packaging (Aerosols)" 1966.