HCFC-FREE

In the discussion of ecotoxicity the perhalogenated chlorofluorocarbons (CFCs), hydrogen-containing hydrochlorofluorocarbons (HCFCs) and the chlorine-free hydrofluorocarbons (HCFs) have been for several years in the headlines (the depletion of the ozone layer in the stratosphere, the greenhouse or global warming effects in the troposphere).75... 

Stratospheric HF is believed to be the dominant stable reservoir of free fluorine atoms released from photolysis of CFCs and HCFCs [20]. The only route for removal of HF from the stratosphere is diffusion down to the troposphere where it may be rained out HF therefore has a long stratospheric lifetime. Concentrations of tropospheric fluorine have increased in recent decades, showing a rise from below 0.5 ppbv in 1970 to over 2.0 ppbv in 1995, although there is evidence from satellite studies of the mesosphere of a slowdown in the rate of increase of atmospheric fluorine [21]. The total atmospheric reservoir thus contains approximately 7.5 million tonnes of fluorine [22]. 

Cr203 will also catalyze the approach to equilibrium from the reverse direction HCFC-124 will disproportionate over Cr203 to give a mixture of HFC-125 and HCFC-123, free of the a and b isomers [73]. 

CFCs have been phased out in recent years and replaced by hydrogen-containing hydrochlorofluorocarbons (HCFCs), such as CHCI2CF3 these compounds are more likely to be destroyed by atmospheric reactions before they reach the ozone layer. Also HFCs (hydrofluorocarbons) cannot produce chlorine free radicals - for this reason CH2FCF3 is now used as a domestic refrigerant. 

The obvious chemical consideration behind the choice of HFCs and HCFCs as replacements is the susceptibility of C—H bonds to attack by hydroxyl free radicals (present in the troposphere). This simple consideration masks the complex reaction chemistry involved, some of which is still a matter of dispute or ambiguity. Two excellent recent reviews by Banks summarise the current position [4, 5] and should be consulted for an authoritative view. As an illustration, a model for the tropospheric degradation of the alternative CHCIF2 (HCFC-22) is presented in Figure 8.1. 

The fluorinated polymer (PFA-C, PPMA-C ) was dissolved in HCFC-225 (concentration 1 wt%), and a Si-wafer was coated with this solution by the spin-coating method (2000 rpm, 30 seconds). The static contact angle against water and methylene iodide, surface free energy [27], and dynamic contact angle against water for the PFMA-Cy thin films are summarized in Table 19.2. For the PFMA-Cj, with y < 6,... 

The role of the CFCs and HCFCs is that they could provide an important source of chlorine containing species and of chlorine free radicals to the stratosphere. They act as carriers for the transportation of chlorine atoms into the stratosphere. 

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