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Wednesday, 03 August 2011 04:35

Fluorocarbons

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The fluorocarbons are derived from hydrocarbons by the substitution of fluorine for some or all of the hydrogen atoms. Hydrocarbons in which some of the hydrogen atoms are replaced by chlorine or bromine in addition to those replaced by fluorine (e.g., chlorofluorohydrocarbons, bromofluorohydrocarbons) are generally included in the classification of fluorocarbons—for example, bromochlorodifluoromethane (CClBrF2).

The first economically important fluorocarbon was dichlorodifluoromethane (CCl2F2), which was introduced in 1931 as a refrigerant of much lower toxicity than sulphur dioxide, ammonia or chloromethane, which were the currently popular refrigerants.

Uses

In the past, fluorocarbons were used as refrigerants, aerosol propellants, solvents, foam-blowing agents, fire extinguishants and polymer intermediates. As discussed below, concerns about the effects of chlorofluorocarbons in depleting the ozone layer in the upper atmosphere have led to bans on these chemicals.

Trichlorofluoromethane and dichloromonofluoromethane were formerly used as aerosol propellants. Trichlorofluoromethane currently functions as a cleaning and degreasing agent, a refrigerant, and a blowing agent for polyurethane foams. It is also used in fire extinguishers and electric insulation, and as a dielectric fluid. Dichloromonofluoromethane is used in glass-bottle manufacture, in heat-exchange fluids, as a refrigerant for centrifugal machines, as a solvent and as a blowing agent.

Dichlorotetrafluoroethane is a solvent, diluent, and cleaning and degreasing agent for printed circuit boards. It is used as a foaming agent in fire extinguishers, a refrigerant in cooling and air-conditioning systems, as well as for magnesium refining, for inhibiting metal erosion in hydraulic fluids, and for strengthening bottles. Dichlorodifluoromethane was also used for manufacturing glass bottles; as an aerosol for cosmetics, paint and insecticides; and for the purification of water, copper and aluminium. Carbon tetrafluoride is a propellant for rockets and for satellite guidance, and tetrafluoroethylene is used in the preparation of propellants for food-product aerosols. Chloropentafluoroethane is a propellant in aerosol food preparations and a refrigerant for home appliances and mobile air conditioners. Chlorotrifluoromethane, chlorodifluoromethane, trifluoromethane, 1,1-difluoroethane and 1,1,-chlorodifluoroethane are also refrigerants.

Many of the fluorocarbons are used as chemical intermediates and solvents in varied industries, such as textiles, drycleaning, photography and plastics. In addition, a few have specific functions as corrosion inhibitors and leak detectors. Teflon is used in the manufacture of high-temperature plastics, protective clothing, tubing and sheets for chemical laboratories, electric insulators, circuit breakers, cables, wires and anti-stick coatings. Chlorotrifluoromethane is used for hardening metals, and 1,1,1,2-tetrachloro-2,2-difluoroethane and dichlorodifluoromethane are used to detect surface cracks and metal defects.

Halothane, isoflurane and enflurane are used as inhalation anaesthetics.

Environmental Hazards

In the 1970s and 1980s, evidence accumulated that stable fluorocarbons and other chemicals such as methyl bromide and 1,1,1-trichloroethane would slowly diffuse upward into the stratosphere once released, where intense ultraviolet radiation could cause the molecules to release free chlorine atoms. These chlorine atoms react with oxygen as follows:

Cl + O3 = ClO + O2

ClO + O = Cl + O2

O + O3 = 2O2

Since the chlorine atoms are regenerated in the reaction, they would be free to repeat the cycle; the net result would be a significant depletion of stratospheric ozone, which shields the earth from harmful solar ultraviolet radiation. The increase in ultraviolet radiation would result in an increase in skin cancer, affect crop yields and forest productivity, and affect the marine ecosystem. Studies of the upper atmosphere have shown areas of ozone depletion in the last decade.

As a result of this concern, beginning in 1979 nearly all aerosol products containing chlorofluorocarbons have been banned throughout the world. In 1987, an international agreement, the Montreal Protocol on Substances that Deplete the Ozone Layer, was signed. The Montreal Protocol controls the production and consumption of substances that can cause ozone depletion. It established a deadline of 1996 for totally phasing out the production and consumption of chlorofluorocarbons in developed countries. Developing countries have an additional 10 years to reach compliance. Controls were also established for halons, carbon tetrachloride, 1,1,1-trichloroethane (methyl chloroform), hydrochlorofluorocarbons (HCFCs), hydrobromofluorocarbons (HBFCs) and methyl bromide. Some essential uses for these chemicals are allowed where there are no technically and economically feasible alternatives available.

Hazards

The fluorocarbons are, in general, lower in toxicity than the corresponding chlorinated or brominated hydrocarbons. This lower toxicity may be associated with the greater stability of the CF bond, and perhaps also with the lower lipoid solubility of the more highly fluorinated materials. Because of their lower level of toxicity, it has been possible to select fluorocarbons which are safe for their intended uses. And because of the history of safe use in these applications, there has mistakenly grown up a popular belief that the fluorocarbons are completely safe under all conditions of exposure.

To a certain extent, the volatile fluorocarbons possess narcotic properties similar to, but weaker than, those shown by the chlorinated hydrocarbons. Acute inhalation of 2,500 ppm of trichlorotrifluoroethane induces intoxication and loss of psychomotor coordination in humans; this occurs at 10,000 ppm (1%) with dichlorodifluoromethane. If dichlorodifluoromethane is inhaled at 150,000 ppm (15%) , loss of consciousness results. Over 100 fatalities have been reported from the sniffing of fluorocarbons by spraying aerosol containers containing dichlorodifluoromethane as propellant into a paper bag and inhaling. At the American Conference of Governmental Industrial Hygienists (ACGIH) TLV of 1,000 ppm, narcotic effects are not experienced by humans.

Toxic effects from repeated exposure, such as liver or kidney damage, have not been produced by the fluoromethanes and fluoroethanes. The fluoroalkenes, such as tetrafluoroethylene, hexafluoropropylene or chlorotrifluoroethylene, can produce liver and kidney damage in experimental animals after prolonged and repeated exposure to appropriate concentrations.

Even the acute toxicity of the fluoroalkenes is surprising in some cases. Perfluoroisobutylene is an outstanding example. With an LC50 of 0.76 ppm for 4-hour exposures for rats, it is more toxic than phosgene. Like phosgene, it produces an acute pulmonary oedema. On the other hand, vinyl fluoride and vinylidene fluoride are fluoroalkanes of very low toxicity.

Like many other solvent vapours and surgical anaesthetics, the volatile fluorocarbons may also produce cardiac arrhythmia or arrest under circumstances where an abnormally large amount of adrenaline is secreted endogenously (such as anger, fear, excitement, severe exertion). The concentrations required to produce this effect are well above those normally encountered during the industrial use of these materials.

In dogs and monkeys, both chlorodifluoromethane and dichlorodifluoromethane cause early respiratory depression, bronchoconstriction, tachycardia, myocardial depression and hypotension at concentrations of 5 to 10%. Chlorodifluoromethane, in comparison to dichlorodifluoromethane, does not cause cardiac arrhythmias in monkeys (although it does in mice) and does not decrease pulmonary compliance in monkeys.

Safety and health measures. All fluorocarbons will undergo thermal decomposition when exposed to flame or red-hot metal. Decomposition products of the chlorofluorocarbons will include hydrofluoric and hydrochloric acid along with smaller amounts of phosgene and carbonyl fluoride. The last compound is very unstable to hydrolysis and quickly changes to hydrofluoric acid and carbon dioxide in the presence of moisture.

The three commercially most important fluorocarbons (trichlorofluoromethane, dichlorodifluoromethane and trichlorotrifluoroethane) have been tested for mutagenicity and teratogenicity with negative results. Chlorodifluoromethane, which received some consideration as a possible aerosol propellant, was found to be mutagenic in bacterial mutagenicity tests. Lifetime exposure tests gave some evidence of carcinogenicity in male rats exposed to 50,000 ppm (5%), but not 10,000 ppm (1%). The effect was not seen in female rats or in other species. The International Agency for Research on Cancer (IARC) has classified it in Group 3 (limited evidence of carcinogenicity in animals), There was some evidence of teratogenicity in rats exposed to 50,000 ppm (5%), but not at 10,000 ppm (1%), and there was no evidence in rabbits at up to 50,000 ppm.

Victims of fluorocarbon exposure should be removed from the contaminated environment and treated symptomatically. Adrenaline should not be administered, because of the possibility of inducing cardiac arrhythmias or arrest.

Tetrafluoroethylene

The principal hazards of tetrafluoroethylene monomer are its flammability over a wide range of concentrations (11 to 60%) and potential explosivity. Uninhibited tetrafluoroethylene is liable to spontaneous polymerization and/or dimerization, both of which reactions are exothermic. The consequent pressure rise in a closed container can result in an explosion, and a number of such have been reported. It is thought that these spontaneous reactions are initiated by active impurities such as oxygen.

Tetrafluoroethylene does not present much of an acute toxic hazard per se, the LC50 for 4-hour exposure of rats being 40,000 ppm. Rats dying from lethal exposures show not only damage to the lungs, but also degenerative changes in the kidney, the latter also being exhibited by other fluoroalkenes but not by fluoroalkanes.

Another hazard relates to the toxic impurities formed during the preparation or pyrolysis of tetrafluoroethylene, particularly octafluoroisobutylene, which has an approximate lethal concentration of only 0.76 ppm for 4-hour exposure of rats. A few fatalities have been described from exposure to these “high boilers”. Because of the potential dangers, casual experiments with tetrafluoroethylene should not be undertaken by the unskilled.

Safety and health measures. Tetrafluoroethylene is transported and shipped in steel cylinders under high pressure. Under such conditions the monomer should be inhibited to prevent spontaneous polymerization or dimerization. Cylinders should be fitted with pressure-relief devices, although it should not be overlooked that such devices may become plugged with polymer.

Teflon (polytetrafluoroethylene) is synthesized by the polymerization of tetrafluoroethylene with a redox catalyst. Teflon is not a hazard at room temperature. However, if it is heated to 300 to 500 °C, pyrolysis products include hydrogen fluoride and octafluoroisobutylene. At higher temperatures, 500 to 800 °C, carbonyl fluoride is produced. Above 650 °C, carbon tetrafluoride and carbon dioxide are produced. It can cause polymer fume fever, a flu-like illness. The most common cause of illness is from lit cigarettes contaminated with Teflon dust. Pulmonary oedema has also been reported.

Fluorocarbon anaesthetics. Halothane is an older inhalation anaesthetic, often used in combination with nitrous oxide. Isoflurane and enflurane are becoming more popular because they have fewer reported side-effects than halothane.

Halothane produces anaesthesia at concentrations above 6,000 ppm. Exposure to 1,000 ppm for 30 minutes causes abnormalities in behavioural tests which do not occur at 200 ppm. There are no reports of skin, eye or respiratory irritation or sensitization. Hepatitis has been reported at sub-anaesthetic concentrations, and severe—sometimes fatal—hepatitis has occurred in patients repeatedly exposed to anaesthetic concentrations. Liver toxicity has not been found from occupational exposures to isoflurane or enflurane. Hepatitis has occurred in patients exposed to 6,000 ppm of enflurane or higher; cases have been also been reported from use of isoflurane, but its role has not been proven.

One animal study of liver toxicity found no toxic effects in rats repeatedly exposed to 100 ppm of halothane in air; another study found brain, liver and kidney necrosis at 10 ppm, according to electron microscopy observations. No effects were found in mice exposed to 1,000 ppm of enflurane for 4 hours/day for about 70 days; a slight reduction in body weight gain was the only effect found when they were exposed to 3,000 ppm for 4 hours/day, 5 days/week for up to 78 weeks. In another study, severe weight loss and deaths with liver damage were found in mice exposed continuously to 700 ppm of enflurane for up to 17 days; in the same study, no effects were seen in rats or guinea pigs exposed for 5 weeks. With isoflurane, continuous exposure of mice to 150 ppm and above in air caused reduced body weight gain. Similar effects were seen in guinea pigs, but not rats, at 1,500 ppm. No significant effect was seen in mice exposed 4 hours/day, 5 days/week for 9 weeks at up to 1,500 ppm.

No evidence of mutagenicity or carcinogenicity was found in animal studies of enflurane or isoflurane, or in epidemiological studies of halothane. Early epidemiological studies suggesting adverse reproductive effects from halothane and other inhalation anaesthetics have not been verified for halothane exposure in subsequent studies.

No convincing evidence of foetal effects was found in rats with halothane exposures up to 800 ppm, and no effect on fertility with repeated exposures up to 1,700 ppm. There was some foetotoxicity (but not teratogenicity) at 1,600 ppm and over. In mice, there was foetotoxicity at 1,000 ppm but not 500 ppm. Reproductive studies of enflurane found no effects on fertility in mice at concentrations up to 10,000 ppm, with some evidence of sperm abnormality at 12,000 ppm. There was no evidence of teratogenicity in mice exposed up to 7,500 ppm or in rats at up to 5,000 ppm. There was slight evidence of embryo/foetotoxicity in pregnant rats exposed to 1,500 ppm. With isoflurane, exposure of male mice at up to 4,000 ppm for 4 hours/day for 42 days had no effect on fertility. There were no foetotoxic effects in pregnant mice exposed at 4,000 ppm for 4 hours/day for 2 weeks; exposure of pregnant rats to 10,500 ppm produced minor loss of foetal body weight. In another study, decreased litter size and foetal body weight and developmental effects were found in the foetuses of mice exposed to 6,000 ppm of isoflurane for 4 hours/day on days 6 to 15 of pregnancy; no effects were found at 60 or 600 ppm.

Fluorocarbons tables

Table 1 - Chemical information.

Table 2 - Health hazards.

Table 3 - Physical and chemical hazards.

Table 4 - Physical and chemical properties.

 

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Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides