Chlorofluorocarbon

The chlorofluorocarbons (CFCs), chlorofluorocarbons or chlorofluorocarbon gases are derivatives of the saturated hydrocarbons obtained by replacing hydrogen atoms with fluorine and/or chlorine atoms mainly.

CFCs are a family of gases that are used in various applications, mainly in the refrigeration industry, and as an aerosol propellant. They are also present in thermal insulators. CFCs have a long persistence in the atmosphere, from 51 to about 200 years. With the passage of time they reach the stratosphere, where they dissociate due to the action of ultraviolet radiation, releasing chlorine and this begins the process of destroying the ozone layer. CFC is the generic name for a group of chlorine-, fluorine-, and carbon-containing compounds used as cold-producing agents and as propellant gases in aerosols. Its multiple applications, its volatility and its chemical stability cause its accumulation in the upper atmosphere, where its presence causes the destruction of the protective ozone layer.

CFC molecules obtained by substitution of hydrogen atoms by fluoride or chlorine atoms

It is now known that the appearance of the "hole" of ozone over southeast Antarctica, at the beginning of the austral spring, is related to the photochemistry of the CFCs present in various commercial products (freon, aerosols, paints, etc.).

Types of CFCs according to their commercialization

• CFC-12 is a gas at room temperature. Until recently, it was widely used in car air conditioners, where they were released into the atmosphere during use and service. A special equipment is currently used to capture CFCs (and their modern substitutes) when the air conditioners of the cars have completed their service.

After World War I it was discovered that by vaporizing CFC-12 in a liquid state, it could be used to create bubbles in rigid foam plastics. The tiny embedded bubbles of CF₂Cl₂ make these products good thermal insulators, as this gas is a poor conductor of heat. However, CFC-12 is released immediately during the formation of foam sheets, such as the white trays used to package fresh meat products, and formerly to contain hamburgers in fast food restaurants.

• The CFCl compound₃, called CFC-11, is a liquid that boils at temperature near the environment. The CFC-11 was used to form holes in soft foam products, such as pillows, padded carpets, cushions and seats and car fillers. This compound has also been applied to make rigid urethane foam products used as insulating in refrigerators, freezers and in some buildings. The use of insulating foam products increased in the last quarter of a century due to interest in energy conservation.

• The other CFC that causes great environmental concern is 1,2-dichloro-1,1,2,2-tetrafluoroethane, called CFC-114. This compound has been widely used to clean the fat, glue and weld residues in electronic circuit panels after their manufacture, consumed about 2 kilograms per square meter. Many manufacturers have changed their manufacturing processes in order not to use any type of cleaning fluid. CFCs do not possess any tropospheric sink in such a way that all their molecules ascend to the stratosphere. This vertical transport process in the atmosphere is not affected by the fact that the mass of these molecules is greater than the average mass of nitrogen and oxygen in the air, since the differential force of gravity is much lower than that of the constant collisions of other molecules that randomize the directions of molecules, even the past. Through this transport, the molecules of CFC, finally, migrate to the medium and high parts of the stratosphere where there is sufficient UV-C of the still unfilled sunlight to decompose photochemically such molecules, thereby freeing chlorine atoms.

• Carbon tetrachloride, CCl₄, is a decreasing ozone substance (DOS). Commercially, it has been used as a solvent and as an intermediate in the manufacture of CFC-11 and CFC-12, losing a certain amount to the atmosphere during its production. Its application as a dry cleaning solvent has been interrupted in most developed countries, although until very recently its practice was still ongoing in many other countries.

• Methylchloroform, CH₃--CCl₃, or 1.1.1- trichloroethane, was produced in large quantities and used in metal cleaning, so that a large part was released into the atmosphere. Although, about half of this amount has been eliminated from the troposphere by reaction with the hydroxyl radical, the rest survives enough time to migrate to the stratosphere. Currently methyl chloroform and carbon tetrachloride contribute, together, to about half of the contribution of CFCs to stratosphere chlorine.

Degradation of the ozone layer

It has been proposed that the mechanism through which CFCs attack the ozone layer is a photochemical reaction: when light falls on the CFC molecule, a chlorine atom with a free electron is released, called the chlorine radical, very reactive and with great affinity for ozone, which breaks the latter molecule. The reaction would be catalytic; the proposed theory estimates that a single chlorine atom would destroy up to 100,000 ozone molecules. Some claim that CFC remains for more than a hundred years in the upper layers of the atmosphere, where ozone is found, but this is impossible since CFC molecules have a molecular weight that varies between 121.1 and 137.51 while the density of the atmosphere is 29.01, so the few Freon molecules that reach the stratosphere fall back to earth in a short time.

The studies by Fatbian, Borders and Penkett (ref: P.Fabian, R. Borders, S.A. Penkett, et al., “Halocarbons in the Stratosphere.” Nature, pp. 733-736) demonstrated that Freons F- 11 and F-12 peaked at 29 to 32 km elevation, where their concentrations ranged from 0.1 to 10 ppb (parts in a billion). Considering that the energy required for UV radiation to dissociate the CFC molecule must be equal to or greater than that of the UV-C band (286-40 nanometers), and this radiation is totally absorbed by oxygen above the 45 km high, the radiation necessary to dissociate the CFCs does not reach the height where the first molecules are found.

In 1987 an international agreement, the Montreal Protocol on Substances that Deplete the Ozone Layer, was signed to control the production and consumption of ozone-depleting substances. In this protocol, 1996 was established as the deadline to completely abandon the production and consumption of chlorofluorocarbons in developed countries. Developing countries have 10 more years to comply with this requirement. Controls were also established for halides, carbon tetrachloride, 1,1,1-trichloroethane (methyl chloroform), hydrochlorofluorocarbons (HCFCs), hydrobromofluorocarbons (HBFCs), and methyl bromide. These chemicals are only permitted for essential uses and provided there are no technically and economically viable alternatives.

In addition, the effectiveness of ozone destruction is increased if stratospheric clouds are present. This happens only in the cold of the polar night, when temperatures drop below 200 K and, in the Antarctic, 180 K or below. In the Antarctic spring, mainly in October and November, markedly low and waning amounts of ozone have been recorded since 1975. This phenomenon is known as the ozone hole. When the sun returns, the loss is quickly recovered.

Risks

Fluorocarbons are generally less toxic than the corresponding chlorinated or brominated hydrocarbons. This lower toxicity may be due to a greater stability of the C-F bond and, perhaps also, to the lower lipoid solubility of the more fluorinated substances. Thanks to their low level of toxicity, it has been possible to select fluorocarbons that are safe for their intended uses.

Actually, volatile hydrocarbons have narcotic properties similar to those of chlorinated hydrocarbons, although weaker. Acute inhalation of 2,500 ppm trichlorotrifluoroethane causes intoxication and psychomotor incoordination in humans, an effect also seen with concentrations of 10,000 ppm (1%) dichlorodifluoromethane. Inhalation of dichlorodifluoromethane at concentrations of 150,000 ppm (15%) causes loss of consciousness. More than 100 deaths related to fluorocarbon inhalation have been recorded as a result of aerosols containing dichlorodifluoromethane as a propellant being sprayed into a paper bag and subsequently inhaled. The TLV of 1000 ppm established by the American Conference of Government Industrial Hygienists (ACGIH) does not produce narcotic effects in humans.

Fluoromethanes and fluorethanes also do not produce toxic effects, such as liver or kidney damage, upon repeated exposure. Fluoralkenes, such as tetrafluoroethylene, hexafluoropropylene, or chlorotrifluoroethylene, can cause liver and kidney damage in experimental animals after prolonged and repeated exposure at appropriate concentrations.

However, the acute toxicity of fluoralkenes is surprising in some cases. Perfluoroisobutylene is a good example of this. With an LC50 of 0.79 ppm for four hours of exposure in rats, it is more toxic than phosgene. Like this last product, it produces acute pulmonary edema. On the other hand, vinyl fluoride and vinylidene fluoride are fluoralkanes with very low toxicity.

Like many other solvent vapors and anesthetics used in surgery, volatile fluorocarbons can also cause arrhythmia or cardiac arrest when the body releases an abnormally high amount of adrenaline (as in situations of anxiety, fear, excitement, or violent exercise). The concentrations required to produce this effect are far in excess of those typically found in industry.

In dogs and monkeys, both chlorodifluoromethane and dichlorodifluoromethane rapidly cause respiratory depression, bronchoconstriction, tachycardia, myocardial depression, and hypotension at concentrations between 5 and 10%. Chlorodifluoromethane, unlike dichlorodifluoromethane, does not cause cardiac arrhythmias in monkeys (although it does in mice) and does not reduce lung function either.

Health and safety measures. All fluorocarbons undergo thermal decomposition when exposed to the action of flame or red-hot metals. The decomposition products of chlorofluorocarbons are hydrofluoric and hydrochloric acids, along with smaller amounts of phosgene and carbonyl fluoride. This last compound is very unstable to hydrolysis and quickly transforms into hydrofluoric acid and carbon dioxide in the presence of moisture.

Mutagenicity and teratogenicity studies carried out on the three most important fluorocarbons from an industrial point of view (trichlorofluoromethane, dichlorodifluoromethane and trichlorotrifluoroethane) have given negative results.

Chlorodifluoromethane (R-22), once considered a potential aerosol propellant, was found to be mutagenic in bacterial mutagenesis studies. Lifetime exposure studies provided some evidence of carcinogenicity in male rats exposed to concentrations of 50,000 ppm (5%), but not to concentrations of 10,000 ppm (1%). This effect was not seen in female rats or in other species. The International Agency for Research on Cancer (IARC) has classified this substance in Group 3 (limited evidence of carcinogenicity in animals). Some evidence of teratogenicity was also obtained in rats exposed to 50,000 ppm (5%), but not 10,000 ppm (1%), nor in rabbits exposed to concentrations up to 50,000 ppm.

Victims of fluorocarbon exposure should be evacuated from the contaminated area and receive symptomatic treatment. Adrenaline will not be administered to them, as there is the possibility of causing arrhythmias or cardiac arrest. HCFCs, HFCs. They make the owners of the equipment that use refrigerants need to take actions on what the industry offers in favor of the continuity of their refrigeration facilities, in favor of the environment.

Alternatives to CFCs

In recent years there has been a lot of effort to find alternatives to CFCs. Among them, the most studied have been hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These molecules contain, attached to the carbon atoms, hydrogen, chlorine and/or fluorine atoms. Hydroxyl radicals, present in the troposphere, easily degrade the C--H bonds of these compounds. At the same time, the presence of these Cl and Br substituent compounds gives them some of the advantageous properties of CFCs: low reactivity and fire suppression, good insulators and solvents, and boiling points suitable for use in refrigeration cycles. Some of the CFCs have already been replaced by these compounds. CHF₂Cl (HCFC-22) is a refrigerant that can replace CCl₂F₂ (CFC-12) in air compressors. domestic air conditioning and refrigerator systems. For the manufacture of polyurethane foam insulation, CH₃CFCl₂ (HCFC-141b) or CF₃CHCl_{2 can be used} (HCFC-123) instead of CCl₃F (CFC-11).

New technologies consider compounds other than HCFCs or HFCs as substitutes for CFCs. Both isobutane and dimethyl ether (mixed with water to reduce its flammability) can be used as aerosol propellants. Similarly, hydrocarbons have replaced CFCs as blowing agents in foam manufacturing. The rigid foams used to insulate the walls of refrigerators, initially made up of CFC-11 and currently HCFC-141b, will be replaced in the future with panels filled with a solid material and sealed under vacuum. The electronics industry is replacing CFCs, used as circuit cleaning solvents, with aqueous detergent cleaners, or is developing new printing systems that reduce the number of cleaning steps required.

Replacement of fluids used in air conditioning and refrigeration systems is more difficult. There are many alternatives in the offing. One of them is the application of substances already used in the past for these purposes, such as ammonia and hydrocarbons. However, its development has been slowed down by the problems of ammonia corrosion and the flammability of hydrocarbons.

There are currently air conditioning systems that do not require a compressor. These are based on the combination of an evaporative cooling system and a desiccant to dry cold air.

Currently, no suitable alternative has been found to halons, substances used to extinguish fires in enclosed spaces such as offices, airplanes, and military tanks. Since their production ceased in 1994, they have been subject to careful marketing, depending on the development of alternatives. Halons exhibit an attractive combination of low reactivity and effective fire suppression that is hard to match. CF₃I is currently the most promising candidate, since like CF₃Br (halon-1301) it is heavy enough to extinguish fires. The C--I bond is easily broken by the action of UV photons, even at ground level, therefore the molecule's lifetime is very short.

The shift, from peak CFC production to CFC substitution, is occurring faster than might have been predicted a few years ago.

Historical overview

CFCs arose from the need to search for non-toxic substances that could be used as refrigerants for industrial applications. Thomas Midgley was the one who discovered that these gases were harmless to humans, thus avoiding thousands of accidental poisonings. Since at the time when the use of CFCs was discovered there was not much information about ozone and the harmful effects of CFCs were unknown, Thomas Midgley himself died thinking that he had done a great service to humanity.

CFCs, also known commercially as freons, replaced ammonia and their use spread mainly in car air conditioners, refrigerators and industries. From 1950 they began to be used as boosting agents for atomizers, in the manufacture of plastics and to clean electronic components.

The discoverers of the threat posed by the use of CFCs were the American chemist F. Sherwood Rowland, from the University of California, the Mexican chemist Mario J. Molina from the Massachusetts Institute of Technology (MIT) and the Dutch Paul Crutzen, from the Max Planck Institute for Chemistry in Mainz, Germany, architects of these discoveries, who on October 11, 1995 received the Nobel Prize in Chemistry in recognition of their research in this field.

A clear example of the CFC problem, how the conflict developed and how it was resolved, is found in Carl Sagan's book Billions (chapter 10: "One is missing piece of heaven").

However, there were other experts who accepted the hypothesis of the action of CFCs on ozone. An example is the book by Rogelio Maduro and Ralph Schauerhammer, "The Holes in the Ozone Scare," published in 1992 by 21st Century Science Associates. ISBN 0-9628134-0-0. This informative book was contested at the time in magazine articles and its arguments do not correspond to the evidence that appeared in scientific journal articles (reviewed and contrasted).

The latest major review of the knowledge on the ozone hole was written and reviewed by some 300 scientists and was presented in Geneva on September 16, 2010, on the occasion of the International Day for the Preservation of the Ozone Layer. UN Ozone.

Measures to reduce the emission of CFCs

1. Avoid consumption of deodorants in aerosol; replace them with bar or ball.
2. Prefer hair fixers on gel.
3. Use deodorants or other products that come in pressure mechanical containers that do not contain CFCs.
4. Avoid environmental deodorants, maintaining good ventilation.
5. Try to use natural insecticides.
6. Avoid aerosols to shave; instead use soap or shaving cream
7. Avoid using products that do not have a guaranteed seal of not containing CFCs.