HFC

Difluoroethane is a nonreactive and stable material. The liquefied gas is stable when used as a propellant and should be stored in a metal cylinder in a cool, dry place. 

Compatible with the usual ingredients used in the formulation of pharmaceutical aerosols. 

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

Under the terms of the Montreal Protocol, aimed at reducing damage to the ozone layer, the use of chlorofluorocarbons has been prohibited since January 1996. Since difluoroethane does not contain chlorine, there are no environmental controls on the use of this material as a propellant, since it does not deplete the ozone layer and is not a greenhouse gas. 

Difluoroethane is a liquefied gas and exists as a liquid at room temperature when contained under its own vapor pressure, or as a gas when exposed to room temperature and atmospheric pressure. The liquid is practically odorless and colorless. Difluoroethane is noncorrosive and nonirritating. 

---

Polymeric cation-exchange resins are also used in the separation of fmctose from glucose. The UOP Sarex process has employed both 2eohtic and polymeric resin adsorbents for the production of high fmctose com symp (HFCS). The operating characteristics of these two adsorbents are substantially different and have been compared in terms of fundamental characteristics such as capacity, selectivity, and adsorption kinetics (51). 

The 1990 Clean Air Act regulates the production and use of CFCs, hydrochlorocarbons, hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbon (HFC) substitutes. CFC and halon (Class I substances) usage is to be phased out in steps until total phaseout occurs on January 1,... 

Hjdrofluorocarbons andHjdrochloroJluorocarhons. 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. 

In this sequence the Cl also acts as a catalyst and two molecules are destroyed. It is estimated that before the Cl is finally removed from the atmosphere in 1—2 yr by precipitation, each Cl atom will have destroyed approximately 100,000 molecules (60). The estimated O -depletion potential of some common CFCs, hydrofluorocarbons, HFCs, and hydrochlorofluorocarbons, HCFCs, are presented in Table 10. The O -depletion potential is defined as the ratio of the emission rate of a compound required to produce a steady-state depletion of 1% to the amount of CFC-11 required to produce the 1% depletion. The halons, bromochlorofluorocarbons or bromofluorocarbons that are widely used in fire extinguishers, are also ozone-depleting compounds. Although halon emissions, and thus the atmospheric concentrations, are much lower than the most common CFCs, halons are of concern because they are from three to ten times more destmctive to O, than the CFCs. 

Reaction conditions must be controlled since HF is also an excellent polymerization catalyst. Controlled reaction conditions can alternatively lead to vinyl fluoride or to HFC-152a (CH2CHF2). The latter can be thermally cracked to form vinyl fluoride. 

Table 2. Physical Properties of Aliphatic Hydrofluorocarbons (HFCs) ...

HFC number Formula CAS Registry Number Molecular weight Boiling point, °C Melting point, °C Liquid density, g/mL at °C Liquid refractive index, at°C Critical temp, °C Critical pressure, MPa "... 

CFC-12 refrigerants HFC-134a HCFC-22 HFC-134a HFC-152a blends... 

CFC-113 cleaning agents blends / a2eotropes HCFC-225ca/cb HFCs... 

CFC-114 blowing agents and refrigerants HCFC-124 HCFC-142b blends / azeotropes HFCs... 

H-1211 fire extinguishant HCFC-123 HBFC-22B1 HBFC-124B1 PFC-51-14 HFCs... 

Table 9. Physical Property Comparisons of CFCs and Their HCFC or HFC Substitutes...

The FlCFC-123 alternative to CFG-11 is made by the fluotination of tetrachloroethylene with either hquid or gaseous HF (41). Further reaction of HCFC-123 with HF provides the HCFC-124 and HFC-125 alternatives. 

Since HFC-134a likely will be the single largest volume CFG alternative produced, many manufacturers around the world are in the process of or have plans to commercialize it, each under their own trade name. United States and foreign trademarks and manufacturers of CFG alternatives are Hsted in Table 11. 

Preparation. Thermal elimination of HCl from l-chloro-l,l-difluoroethane (HCFC-142b) [75-68-3] is the principal industrial route to VDF covered by numerous patents (8—19). Dehydrohalogenation of l-bromo-l,l-difluoroethane (20), or 1,1,1-trifluoroethane (HFC-143a) (21—25), or dehalogenation of l,2-dichloro-l,l-difluoroethane (26—28) are investigated alternative routes (see Fluorine compounds, organic-fluorinated aliphatic compounds). 

---