Chlorofluorocarbon advantages

Over the next two years, Midgley and his group discovered and patented other chlorofluorocarbons and the halons, a class of bromofluoro-carbon compounds that are the world s best Are fighters. At the time, their remarkable stability seemed like an advantage. In the 1970s scientists were able to determine that CFCs and halons—which are so stable that they remain in the atmosphere for long periods of time—deplete the ozone layer 15 miles above Earth. 

Until recently, only three chlorofluorocarbon (CFC) propellants, namely CFCs 11, 12 and 114 (Table 1), had been approved worldwide for use in medical MDIs. Their widespread acceptance was due to their ability to substantially meet the ideal propellant properties. All the CFC MDIs that are currently marketed employ CFC 12 as the major constituent mixed with either CFC 11 or with a mixture of CFC 11 and CFC 114. These mixtures of propellants closely obey Raoult s law and therefore the blend selected can be used to give a defined vapor pressure (Table 1). The inclusion of CFC 11 in the formulation also offered advantages in that it increased the solvency of most propellant systems, thereby facilitating the dissolution of surfactants in suspension formulations. By virtue of it being a liquid below 24° C, it was used as the primary dispersion medium for either suspending or dissolving the drug. 

The advantages of these aromatic polyesters are lower cost than conventional polyether polyols, better flame retardance, and high-temperature resistence. However, their disadvantages include compatibility problems with chlorofluorocarbons and quality deviations in viscosity and hydroxyl values. In order to improve the compatibility problems, amine-based polyether polyols have been blended. 

Halocarbons have the further advantage of reducing the viscosity of the reaction mixture and, where used as the main blowing agent instead of the carbon dioxide produced by the isocyanate-water reaction, cheaper foams are obtained since less isocyanate is used. The reader should, however, note the comments made about the use of chlorofluorocarbons and their effect on the ozone layer made in Section 27.5.4. 

There also are numerous advantages chlorofluorocarbons offer structural plastic designers and molders. As the transportation and construction industries become more energy-minded, more of those structural components are being shifted to plastic from metal. The optimal material of construction frequently is cellular plastics. Reduced weight and lower costs of most cellular plastic parts are possible without sacrificing the necessary strength and other attributes. 

Tetrafluoroethane (hydrofluorocarbon-134a or HFC-134a, b.p. -25°C), which was developed as a replacement for the chlorofluorocarbons that were banned because of ozone-depleting effects, is approved in the UK for the production of natural food flavor extracts. It can be applied to optimize the extraction of plant materials and provides an environmental advantage, as well as health and safety benefits [55,56]. 

Of particular relevance to this chapter is the use of CO2 in polymer synthesis, in the manufacture of polymethylmethacrylate and polystyrene (Xerox) and for the production of fluoropolymers (DuPont). One of the main drivers for the latter was the phasing out of the chlorofluorocarbons (CFCs) used in the original process. The main advantage to this application is not necessarily the avoidance of the use of CFCs (although this is important), but the superior polymer processing properties made possible by the relative volatility of CO2 and its ease of removal. 

VDF is a colorless, flammable, and nearly odorless gas that boils at -82°C its freezing point is -144°C [4]. VDF exhibits several advantages it is a nontoxic, environmentally friendly gas in contrast to chlorotrifluoroethylene (CTFE), nonexplosive contrarily to tetrafluoroethylene ( IFF) or trifluoroethylene (TrFE). Various syntheses to prepare VDF are reported in a review [5]. In fact, the chlorination of chlorofluorocarbon (CFC) 152 (i.e., l,l-difluoroethane)to l-chloro-l,l-difluoroethane, followed... 

Lubricants which are not based on petroleum are classed as synthetic and include silicones, poly glycols, polyphenyl ethers, fluoro compounds, chlorofluorocarbon polymers, and phosphate esters. The major advantage of these fluids is that they are tailor-made for a specific function and usually require few additives. 

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