The Pharmaceutics and Compounding Laboratory

When liquefied gases (CFC, HCFC, HFC, hydrocarbons) are used as propellants, one of two systems can be formulated. The two phase system is the simplest system. Here the product concentrate is dissolved or dispersed in liquefied propellant and solvents creating a homogenous system. The propellants exist in both the liquefied phase and the vapor phase. When the aerosol valve is actuated, some liquefied propellant and solvent containing the product concentrate is emitted from the container. These aerosols are designed to produce a fine mist or wet spray by taking advantage of the large expansion of the propellant when it enters room temperature and atmospheric pressure. The two phase system is commonly used to formulate aerosols for inhalation or nasal application.

A three phase system (i.e., a heterogeneous system) is made up a layer of water immiscible liquid propellant, a layer of propellant immiscible liquid (usually water) which contains the product concentrate, and the vapor phase. This type of system is used when the formulation requires the presence of a liquid phase that is not propellant miscible. When the aerosol valve is actuated, the pressure of the vapor phase causes the liquid phase to rise in the dip tube and be expelled from the container. If the product is to maintain the liquefied gas reservoir, the dip tube must not extend beyond the aqueous phase. Sometimes it is desirable to have some liquefied propellant mixed with the aqueous phase to facilitate in the dispersion of the spray or to create a foam. In this case, the container should be shaken immediately prior to use.

If CFCs, HCFCs, and HFCs are used as the propellants, they will reside on the bottom of the container since their density is greater than water. The dip tube will then need to end somewhere in the middle of the container. If hydrocarbons are used as the propellants, they will reside on the aqueous layer since their density is less than water. In this case, the dip tube can be extended through the liquid propellant all the way down to the bottom of the container. Thus an important characteristic of any aerosol is the density of the propellant, propellants, or blend of propellants.

Foam aerosols are a three phase system in which the liquid propellant is emulsified with the product concentrate. When the valve is actuated, the emulsion is forced through the nozzle and the entrapped propellant reverts to the vapor phase and whips the emulsion into a foam when it reaches the atmosphere. To facilitate the formulation of a foam, some aerosols are shaken prior to use to disperse some of the propellant throughout the product concentrate. If a dip tube is present, the container is used while being held upright. If there is no dip tube, the container must be inverted prior to use.

Foam products operate at a pressure of about 40 to 50 psig at 70°F and contain about 4 to 7% propellant. Generally, a blend of propane and isobutane is used for foam aerosols. Contraceptive foam aerosols use A-31 as the propellant. Other foams use P-152a since it will produce a more stable foam and is less flammable than hydrocarbons. Other propellants that have been used include the compressed gases nitrous oxide and carbon dioxide. Typical products include whipped creams and toppings and several pharmaceutical and veterinary products.

Aerosols using compressed gases as the propellant operate essentially as a pressure package. The pressure of the gas forces the product concentrate out of the container in essentially the same form as it was placed in the container. Only the product concentrate is expelled; the compressed gas remains in the container occupying the headspace. The pressure drops in the container as the product concentrate is removed and the gas expands to occupy the newly vacated space. The pressure will continue to drop as the product concentrate is expelled. Therefore, the initial pressure in these containers is higher than used in liquefied gas aerosols and is usually 90 to 100 psig at 70°F. The amount of product left in the container after the pressure is exhausted varies with the viscosity of the product and loss of pressure due to gas seepage.

Depending on the nature of the formulation and the type of compressed gas used, the product may be dispensed as a semisolid (solid stream) foam or spray. Semisolid aerosols are used to dispense more viscous concentrates such as dental creams, hair dressings, ointments, creams, cosmetic creams, and foods.

In barrier pack systems, the propellant is physically separated from the product concentrate. The propellant pressure on the outside of the barrier serves only to push the contents from the container. In the piston type system, a polyethylene piston is fitted into the container. The product concentrate is placed into the upper portion of the container and a compressed gas or hydrocarbon gas is placed on the other side of the piston. The gas pushes against the piston and pushes the product concentrate out of the container when the valve is actuated. As the rises in the container, it scrapes against the side of the container which helps dispense most of the product concentrate.

This system is used to dispense cheese spreads, cake decorating icings, and ointments. Since these product concentrates are semisolid and viscous, they emit from the container as a lazy stream rather than a foam or spray. The piston type system is limited to viscous materials since liquids tend to pass around the edges of the piston into the gas compartment.

A collapsible plastic bag fitted into a container is another type of barrier pack system. In some systems, the bag is a thin walled aluminum pouch. The product concentrate is placed in the bag and the propellant surrounds the bag. The bag is accordion pleaded to prevent the gas from pinching it closed. These types of systems are used to dispense liquids as fine mists or streams, and semisolids as streams. They system can also be used for topical creams, ointments, or gels.

Gels that foam after being dispensed are placed in both the piston type and collapsible plastic bag type of system. The dispensed gel contains a low boiling liquid such as isopentane or pentane. The liquid will vaporized when the gel is placed in the warmth of the hands, producing the foaming gel.