Blends of various fluorocarbon propellants are generally used for pharmaceutical aerosols. These mixtures help in obtaining the desirable pressure, delivery and spray characteristics. By varying the proportion of each component, any desired vapor pressure can be achieved.
The CFCs have been implicated in having ozone depleting effect, increasing the amount of UV radiations reaching the earth’s surface resulting with greenhouse and global warming effect. Depletion of ozone layer is also alleged to result in increased incidence of skin cancer. After the Montreal protocol signed in 1987, which was an effort to phase out the use of ozone depleting substances, including CFCs, pharmaceutical companies have come out with alternate substances to be used as propellants in medicinal products. Hydrocarbon propellants such as butane, propane and isobutane have replaced CFCs for topical pharmaceutical aerosols. These have lower solubility but higher flammability. Their cost is also lower in comparison to CFCs.
Hydrocarbon propellants and their various blends can be used in aerosol formulations. Their density is less than one and immiscible with water, hence making them suitable for three-phase systems. They generally possess better solubility characteristics than fluorinated hydrocarbons. They are chemically stable. They are not prone to hydrolysis, hence suitable for water-based formulations. Alternative propellants such as hydrochlorofluorocarbons (HCFCs) and Hydrofluorocarbons (HFCs) can also be used as replacement for CFCs in appropriate formulations.
A comparison between liquified gases and compressed gases as propellants is given in Table 12.2.
Table 12.2 Comparison of liquefied gases and compressed gases as propellants
| Liquefied gases | Compressed gases |
|---|---|
| The propellant is an integral part of the formula. | The propellant forms a separate phase on the surface of the product. |
| The spray consists of smaller and finer particles. | The spray results in large, wet particles. |
| The pressure is maintained throughout the life of the product. | There is a pressure drop seen during the product life. |
| Large temperature changes can affect pressure inside the container. | Pressure exerted by the gases is not affected by changes in temperature. |
| The pressure exerted is independent of propellant concentration and hence no pressure drop. | The pressure exerted is dependent on the propellant concentration and hence pressure drop. |
| Cost varies with the kind of propellant and generally of higher cost. | Less expensive. |
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