Propellants hydrofluorocarbons

Sprays. Aerosol spray emulsions are of the water-in-oil type. The preferred propellant is a hydrocarbon or mixed hydrocarbon—hydrofluorocarbon. About 25 to 30% propellent, miscible with the oil, remains in the external phase of the emulsion. When this system is dispensed, the propellant vaporizes, leaving behind droplets of the w/o emulsion (Fig. 2b). A vapor tap valve, which tends to produce finely dispersed particles, is employed. Because the propellant and the product concentrate tend to separate on standing, products formulated using this system, such as pesticides and room deodorants, must be shaken before use. 

The implication of CFCs in the destruction of stratospheric ozone (Section 6.2.6.) has resulted in their virtual elimination as aerosol propellants, except in MDIs, an essential lifepreserving application, where they will continue to be used until hydrofluorocarbon-based formulations are introduced. 

The active ingredients in MDIs are usually water-soluble and chlorofluorocarbon- or hydrofluorocarbon-insoluble. Some CFC and HFA formulations use ethanol as a suspending agent by using an ethanol-insoluble salt form of the drug. Since the vehicle in MDIs must be propellant-based, a product with the drug suspended in the propellant may be the most stable dosage form. 

In the early 1990s, chlorofluorocarbon refrigerants and propellants were largely replaced by hydrofluorocarbons and hydrochlorofluorocarbons in order to reduce stratospheric ozone depletion. The hydrochlorofluorocarbons and hydrofluorocarbons, unlike the older chlorofluorocarbons, are unstable in the... 

The replacement of chlorofluorocarbon (CFC) propellants with the non-ozone-depleting hydrofluorocarbons (HFCs) merit mention for two reasons. First, it illustrates how environmental impact can be an important selection criterion at a time when green issues are high profile. Second, HFCs were developed and evaluated for safety and delivery capability by a consortium of pharmaceutical companies, with costs shared and evaluation programs defined by prior agreement between end-users and propellant manufacturers. Such collaboration could be employed usefully in the future to develop novel excipients for delivery or targeting. The benefits would undoubtedly accrue to all. 

The first MDI products were developed by Riker Laboratories and marketed in 1956, using a newly patented design of metering valve. In most countries the MDI is now established as the principal dosage form of inhalation drug therapy for bronchial asthma and chronic obstructive pulmonary disease (COPD). Since its introduction, MDI technology has evolved steadily. However, with the phase-out in the commercial use of chlorofluorocarbon (CFC) propellants, which have been the mainstay of pharmaceutical MDIs, the pace of MDI technology development has accelerated with the transition to hydrofluorocarbon (HFC) propellants. ... 

Difluoroethane, a hydrofluorocarbon (HFC), is an aerosol propellant used in topical pharmaceutical formulations. Difluoroethane may be used as a vehicle for dispersions and emulsions. 

Heptafluoropropane (P-227) is classified as a hydrofluorocarbon (HFC) aerosol propellant since the molecule consists only of carbon, fluorine, and hydrogen atoms. It does not contain any chlorine and consequently does not affect the ozone layer, nor does it have an effect upon global warming. It is therefore considered as an alternative propellant to CFCs for metered-dose inhalers (MDIs). While some of its physical and chemical properties are known, little has been published in regard to its use as a replacement for CFCs in MDIs. 

Tetrafluoroethane is a hydrofluorocarbon (HFC) or hydro-fluoroalkane (HFA) aerosol propellant (contains hydrogen, fluorine, and carbon) as contrasted to a CFC (chlorine, fluorine, and carbon). The lack of chlorine in the molecule and the presence of hydrogen reduces the ozone depletion activity to practically zero. Hence tetrafluoroethane can be considered as an alternative to CFCs in the formulation of metered-dose inhalers (MDIs). It has replaced CFC-12 as a refrigerant since it has essentially the same vapor pressure. Its very low Kauri-butanol value and solubility parameter indicate that it is not a good solvent for the commonly used surfactants for MDIs. Sorbitan trioleate, sorbitan sesquioleate, oleic acid, and soya lecithin show limited solubility in tetrafluoroethane and the amount of surfactant that actually dissolves may not be sufficient to keep a drug readily dispersed. 

Table 5 Physicochemical Properties of Chlorofluorocarbon and Alternative Hydrofluoroalkane Propellants such as Hydrofluorocarbons...

Because CFC had many essential functions in all aspects of our daily life (for example refrigerants, foaming agents, or propellants for aerosol cans), subsequent to the Montreal Protocol an intensive search for potential replacements was initiated. CFC replacements so far include hydrofluorocarbons (HFC for example CF3CFH2, marketed as HFC-134a), hydrochlorofluorocarbons (HCFC), and partially fluori-nated ethers (for example CH3OCF3). These substances are much less stable to attack by radicals in the lower atmosphere and thus cannot reach the stratosphere where they would deplete the ozone layer [30]. 

Hydrofluorocarbons andHydrochlorofluorocarbons. The properties of HFC and HCFC propellants are given in Table 3. Propellant 22 is nonflammable and can be mixed to form nonflammable blends. Some of these propellants are scheduled for phase-out by 2015—2030. 

Hydrocarbon aerosol hair sprays contain an alcohol-hydrocarbon solvent-propellant system, a synthetic polymeric resin, a base to neutralize the resin if it is a carboxylic acid-containing resin, plasticizer(s), fragrance, and, in some cases, surfactant(s) to improve the spreading characteristics of the polymer. Most of the new low-VOC aerosol hairsprays contain alcohol-water as the solvent system and dimethyl ether as the propellant. Together the alcohol-dimethyl ether content must be below 55% (54.5%). For cost considerations, dimethyl ether is the propellant of choice, although Hydrofluorocarbon 152-A is exempt as a VOC and provides acceptable, but expensive formulations with previously used resins [54]. In Europe and other parts of the world, there are no VOC limits, and hydrocarbon-alcohol systems with virtually no water are widely used. 

As an interim measure, hydrochlorofluorocarbons (HCFCs) can be used in refrigerants in place of CFCs. While less harmful to the environment than CFCs, HCFCs are still ozone-depleting (they are classified as Class IT ozone-depleters) and will be phased out by 2020. Hydrofluorocarbons appear to have little or no ozone-depleting effect and can also be used in refrigerants and aerosol propellants. 

MFC 152a Hydrofluorocarbon 152A (INCI) Propellant 152a R 152a... 

Freon 23 Halocarbon 23 HCFC 23 MFC 23 Hydrofluorocarbon 23 Methane, trifluoro Methyl trifluoride Propellant 23 R 23 Refrigerant 23 Empirical CHF3... 

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