With these criteria set forth, the major CFC's producers, such as Dupont, Allied Signal, ICI, Elf Atochem (France), Solvay (Germany), Ausimont (Italy), Asahi Glass, Mitsui-Dupont, Daikin and Showa Denko (all of Japan), searched the tables of known fluorocarbons in an attempt to match these properties.   The effort was launched shortly after Rowland and Molina's landmark publication in 1974. From these tables, which are extensive, only a few compounds approximate the physical properties of the existing CFC's. The following table identifies these.

* - Hydrofluorocarbon

All of these have been used as CFC replacements. But, with the limitations of the updated Montreal Protocol, the HCFC's serve only a temporary role even though their ODP's are low (0.02-0.11). Only four of those listed have no ODP - HFC-134a, -125, -152a and -43-10mee. And one of these - HFC-152a - is flammable. HFC-43-10mee is rather expensive to prepare because it requires a large amount of F, using two fully fluorinated alkenes as raw materials.

HFC-134a shows a good vapor pressure match for the most widely used refrigerant, CFC-12. For this reason, almost all the major refrigerant producers listed have made concerted efforts to commercialize this material under the trade names listed below.

CFC-125 is not as good a match for the important physical properties of CFC-115 which had a much smaller market than CFC-12. While HFC-125 is being used to replace CFC-115, blends of O3-compatible HFC's are also widely accepted. These, however, suffer from changing performance as the lower boiling components are lost owing to leakage.

A most important aspect in the replacement of CFC's is the toxicity of the candidates. Toxicity testing of chemicals is time-consuming and quite expensive. Therefore, some of the world's major producers of CFC replacements have cooperated to share this task. Of the tabulated list of alternative compounds, only one - HCFC-123 - has proven to be significantly more toxic (TLV* = 50ppm in air) than the already commercial CFC's and HCFC-22 which have TLV's > 1000ppm. Keep in mind that any fluorocarbon gas or vapor can displace air, particularly in low lying areas and enclosed spaces, and therefore can cause suffocation.

HFC-134a has become the refrigerant of choice to replace CFC-12 in most refrigeration and auto air conditioning systems. (For the time being, HCFC-22 is still in use for window and central air conditioning.) There is a nitch market for HFC-125.

Having identified these two key HFC's as the refrigerants for the 21st century, the challenge for the chemists and engineers was to develop practical commercial processes for them.

*- Threshold Limit Value - a measurement used by toxicologists to quantify
toxicity of gases and vapors.