Guide to Aerosols
HOW AN AEROSOL WORKS
An aerosol contains two essential components:-
•The product, in the form of a liquid, emulsion or suspension.
•The propellant, which can be a liquefied gas, or even a compressed gas.
The ’Classic’ aerosol delivers the product in the form of a spray, which may be made up of small, medium or large droplets, depending upon the function of the aerosol. As an example, an air freshener will consist of very small droplets in order to disperse a fragrance into the air; a furniture polish will consist of larger droplets, as it is only necessary to deliver the spray to a surface. Other aerosol formats include, foams, gels, etc.
In all cases it is the propellant pressure within the can that pushes the product through the various components until it reaches the actuator and emerges as a spray, or one of the other formats.
The way in which the liquid is turned into a spray depends on a number of factors which include,
1. The valve specification (see appropriate page)
2. The actuator specification (see appropriate page)
3. The type and amount of propellant
Liquefied propellants are gases that exist as liquids under pressure. Because the aerosol is under pressure the propellant exists mainly as a liquid, but it will also be in the head space as a vapour. As the product is used up as the valve is opened, some of the liquid propellant turns to vapour and keeps the head space full of vapour. In this way the pressure in the can remains essentially constant and the spray performance is maintained throughout the life of the aerosol. The propellant is an essential element in the formulation.
When the liquid propellant emerges from the actuator the droplets immediately vaporise, and if the propellant is intimately mixed with droplets of the product, these will be ’blown’ into smaller droplets. The ultimate size of the droplets can be controlled by adjusting the amount of propellant, and its pressure, in the aerosol.
The design of the actuator is also important, as this will have a significant affect on the droplet size due to the mechanical action on the liquid, as it passes through the small holes and channels within the actuator.
If the product is a foam, or mousse, the liquefied propellant creates the bubbles in the liquid when it emerges from the actuator, or one of the special foam applicator used nowadays.
Compressed Gas Propellants
Compressed gas propellants really only occupy the head space above the liquid in the can. When the aerosol valve is opened the gas ’pushes’ the liquid out of the can. The amount of gas in the headspace remains the same but it has more space, and as a result the pressure will drop during the life of the can.
Unlike liquefied propellants, there is NO liquid to instantly vaporise when the product emerges from the actuator, and only the product is sprayed out.
The only means of creating the droplets required to form an acceptable spray, is by breaking up the liquid through mechanical action as it passes through both the valve and the actuator. The choice of these components is critical, and fortunately the suppliers of both valves and actuators are able to offer specifications which meet the necessary requirements.