Chlorinated fluorinated hydrocarbons CFCs

There are liquid PBAs that are volatile and change from a liquid to a gaseous state when heated to the plastic processing temperatures. They are short-chain chlorinated and fluorinated aliphatic hydrocarbons (CFCs). Although they can be used over a wide temperature range and at low (atmospheric) pressures, they have been gradually discontinued due to their role in the reduction of stratospheric ozone.249 Other PBAs are reviewed in Table 8.2. 

How is the book arranged In the first few chapters we examine the properties of dmgs and excipients in the solid state and in solution. Gases also are treated because of their importance in the design and use of therapeutic pressurised aerosols, which until recently have been derived from chlorinated fluorocarbons (CFCs), but now are based on volatile fluorinated hydrocarbons (HFAs). 

Fluorine- chlorine- hydrocarbons (CFCs) 17 5 Aerosol/propellants, cooling, cleaning, fire suppressants, foaming agents... 

The chief candidates as CFC replacements are partly fluorinated hydrocarbons with minimal or zero chlorine content. The presence of hydrogen in the molecule confers reactivity in the troposphere, because it allows the attack of OH upon the molecule (see Chapter 3). The example of HCF-134a is shown in reaction (42)... 

Compared to TCE, fluorinated chlorinated hydrocarbons (CFC) offer benefits to dry cleaning because of their lower boiling points and their more gentle action to dyestuffs and fabrics. So since 1960 these solvents have had some importance in North America, Westem Europe, and the F ar East. They were banned because of their influence on the ozone layer in the stratosphere by the UNESCO s Montreal Protocol in 1985. 

Trichlorotrifluoroethane (CFC 113) is generally a stable molecule not prone to the reactivity that is often shown by the chlorinated hydrocarbons. The solvent blends that contain the fluorinated hydrocarbon and an alcohol display some metal reactivity which is inhibited by adding nitromethane as a stabilizer (e.g., the Freon blends discussed in Table 14.5). The vapors of a fluorinated hydrocarbon undergo thermal degradation in a high temperature oven or flame. 

As substitutes for the chlorine containing CFCs and HCFCs, partially fluorinated hydrocarbons such as R23, R125, R134a, R143a, and R152a are investigated. They do not contain chlorine, therefore their impact on the ozone layer is negligible. 

The CFC replacements need to be nontoxic, nonflammable and have significantly lower, or zero ozone depletion potentials. Many organic- and aqueous-based systems, that do not contain chlorine or fluorine, have been developed for some applications while others use hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Unlike hydrocarbon catalysis, the presence of hydrogen, chlorine and fluorine in the same molecule creates a very large number of... 

Fluorocarbons (FCs) Organic compounds analogous to hydrocarbons in which one or more hydrogen atoms are replaced by fluorine. FCs were once used in the United States as a propellant for domestic aerosols and are now found mainly in coolants and some industrial processes. FCs containing chlorine are called chlorofluorocarbons (CFCs). These are believed to be modifying the ozone layer in the stratosphere and are responsible for allowing more harmful solar radiation to reach the Earth s surface. 

Hydrofluorocarbons have been proposed as CFC and HCFC substitutes since 1987, after the Montreal Protocol, with the aim of decreasing the ozone depletion. At present, these compounds are industrially produced by reaction of chlorinated hydrocarbons with HF. Environmental concerns have been raised regarding such processes, due to the co-production of huge amounts of HCl and the possible generation of dioxins and chlorinated compounds [1]. Due to their contribution to the greenhouse effect, fluorinated alkanes have often been replaced by other materials (e.g., natural refrigerants) in several UE countries... 

The gas-phase chemistry reviewed in this chapter shows how the pattern of substitution of chlorine and fluorine atoms on the parent hydrocarbon affects its atmospheric degradation. Although numerous reaction details need to be investigated and some discrepancies resolved, a core of reliable data is available from which one can deduce which HFC or HCFC may have deleterious atmospheric consequences and which potential substitute appears environmentally acceptable. However, the work is not finished. This work has dealt only with the gas-phase chemistry. Research is still needed into the heterogeneous chemistry of HFC and HCFC degradation products and into the biological effects that they may have. It is then up to the atmospheric modelers to combine this scientific information with emission scenarios and meterology to ascertain the feasibility of a particular CFC substitute. 

The term fluorocarbons is defined here as carbon compounds containing fluorine. If they contain only fluorine, they are sometimes referred to as FCs. The compounds may also contain chlorine, bromine, or hydrogen. Other descriptive names include chlorofluorocarbons (CFCs), fluorinated compounds, and halogen-ated hydrocarbons. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (FIFOs) are compounds in this family that contains hydrogen. Unless otherwise specified, the term fluorocarbons in this monograph applies to FCs, CFCs, HCFCs, and HFCs. 

A subfamily of chlorinated hydrocarbons contains fluorine as well as chlorine and is called the chlorofluorocarbons or CFCs. Two examples of CFCs are dichlorodifluoromethane and trichlorofluoromethane. 

Chlorofluorocarbons CFCs and hydrochlorofluorocarbons HCFCs are all anthropogenic species and are the causative agents of ozone layer destruction as well as greenhouse gasses. CFCs are the molecules in which aU the hydrogen atoms of hydrocarbons are substituted by chlorine and fluorine atoms. They do not have absorption bands in the tropospheric actinic flux region and also do not react with OH radicals. Therefore, they do not have any dissipation process in the troposphere, and can be photolyzed only after they reached to the stratosphere. On the other hand, HCFCs is molecules in which at least one of chlorine or fluorine atom of CFCs is substituted by hydrogen atom. Since HCFCs react with OH radicals, they are removed in the troposphere, but a portion of them reach the stratosphere and photolyzed, similar to CFCs. 

Among halocarbons (halogenated hydrocarbons), bromochlorofluorocarbons in which at least one chlorine or fluorine atom of chlorofluorocarbons is substituted by bromine is called halons. Halons, as with CFCs, do not have any dissipating process in the troposphere, and reach to the stratosphere and are photolyzed, which affects the ozone layer destruction. Among halons, Cp2ClBr (halon—1211) and CFsBr (halon-1301) have the highest atmospheric concentratiOTis, and these two compounds are taken up here. 

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