Chlorofluoro carbons

One air pollution problem of a global nature is the release of chlorofluoro-carbons used as propellants in spray cans and in air conditioners and their effect on the ozone layer high in the atmosphere. (See Chapter 11.)... 

The half-life, f1/2, of a substance is the time needed for its concentration to fall to one-half its initial value. Knowing the half-lives of pollutants such as chlorofluoro-carbons allows us to assess their environmental impact. If their half-lives are short, they may not survive long enough to reach the stratosphere, where they can destroy ozone. Half-lives are also important in planning storage systems for radioactive materials, because the decay of radioactive nuclei is a first-order process. 

Here, X may be H, OH, NO, Cl, or Br. Cl from the photodissociation of chlorofluoro-carbons such as CFC-12 (CCI2F2) is the main known catalyst currently acting in the so-called... 

Uses. In the manufacture of chlorofluoro-carbons, which in turn are primarily used as refrigerants formerly used widely as a solvent, also as a grain fumigant and in fire extinguishers. Because of toxicity consumer uses have been discontinued and only industrial use remains. 

The depletion of the ozone layer increases by the catalysis of chlorofluoro carbon (CFC) with ultraviolet lights, which decompose CFC gases into free radicals of chlorine (Cl ), as a catalyst. 

Expandable polymeric blends with SAN are conventionally manufactured by incorporating a blowing agent, such as chlorofluoro-carbons. Some of these blowing agents are known to be environmental pollutants. 

Chlorine atoms in the stratosphere catalyze the destruction of Earths ozone layer. As we explore further in Chapter 17, evidence tells us that chlorine atoms are generated in the stratosphere as a by-product of human-made chlorofluoro-carbons (CFCs), once widely produced as the cooling fluid of refrigerators and air conditioners. Destruction of the ozone layer is a serious concern because of the role this layer plays in protecting us from the sun s harmful ultraviolet rays. One chlorine atom in the ozone layer, it is estimated, can catalyze the transformation of 100,000 ozone molecules to oxygen molecules in the 1 or 2 years before the chlorine atom is removed by natural processes. 

An important piece of evidence that some of this chlorine comes from the breakdown or CFCs was the unusually high levels of fluorine compounds detected in the Antarctic stratosphere. Whereas chlorine compounds come from a number of natural sources, fluorine compounds in nature are relatively rare. The source of this stratospheric fluorine, therefore, is most likely chlorofluoro-carbons. In addition to elevated fluorine levels, evidence of ozone depletion... 

Lennard-Jones parameters used for chloro- and fluoro- and chlorofluoro-carbons in zeolites. 

Partial atomic charges for a selection of chloro-, fluoro- and chlorofluoro-carbons. 

One of the success stories of in situ measurements in the stratosphere is the confirmation that the rapid loss of ozone over Antarctica each October is indeed caused by photochemistry related to the release of chlorofluoro-carbons at the surface of the earth. Ground-based measurements of the primary chlorine culprit, CIO, and 03 have given a similar picture (4), but not with the fine detail possible from the in situ techniques, as shown in... 

In 1975 he joined Hoechst AG Frankfurt, Germany. He worked there at the Institute of Industrial Toxicology and was in charge of the safety assessment of Acesulfam, an artificial sweetener. Further projects consisted of fluorocarbons as replacements for chlorofluoro-carbons. He contributed to the development of several pesticides. 

The importance of this process was not recognized until the mid-1970s when it was realized that Cl could be produced from the photodegradation of chlorofluoro-carbons (CFCs), which had become very widely used as refrigerants and for other applications for which an inert gas was needed. This realization was a big deal, and it eventually earned Mario Molina, F. Sherwood Rowland, and Paul Crutzen the Nobel Prize for chemistry in 1995. 

The toxic materials may be sohd, liquid, or gas. The sohd toxic materials are radioactive substances and metals such as Pd, Cd, As, Cr, Al, and others in various forms. The chemicals are mostly in liquid and gaseous forms. For example, diethyl bromide, chlorofluoro carbons (CFCs), trichlorethane, or trichloromethane are liquids whereas phosgene, chlorine, carbon monoxide, hydrogen cyanide, and isocyanate are gases. 

Another well known group of fluorine compounds is rhe chlorofluoro-carbons (CFCs). For many years, the CFCs were used for a wide variety of industrial purposes, including refrigeration, cleaning systems, and as... 

Physical blowing agents may be classified as CFCs (chlorofluoro-carbons), HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbon ethers) and non-fluorine-containing organic liquids. These fluorinated blowing agents can also be used in foaming polyisocyanurate foams, polyoxazolidone foams, and polyurea foams. 

Volatilization of a low-boiling liquid, either by the heat liberated by an exothermic reaction, or by externally applied heat. Commonly used liquids are chlorofluoro-carbons (CFCs). This is the most widely used technique in the production of rigid polyurethane foams. However, due to the ozone depletion problem in the stratosphere, they must be phased out and industry is presently searching for alternative blowing agents. 

OCS, N20 and even CH4 have long residence times. The CFCs (chlorofluoro-carbons, Fig. 3.4b refrigerants and aerosol propellants) also have very limited reactivity with OH. Gases like these build up in the atmosphere and eventually leak across the tropopause into the stratosphere. Here a very different chemistry takes place, no longer dominated by OH but by reactions which involve atomic oxygen (i.e. O). Gases that react with atomic oxygen in the stratosphere can interfere with the production of 03 ... 

Information about the true density of a powder can be used to predict whether a compound will cream or sediment in a metered dose inhaler (MDI) formulation. The densities of the hydrofluoroalkane (HFA) propellants, 227 and 134a, which are replacing chlorofluoro-carbons (CFCs) in MDI formulations, are 1.415 and 1.217 g/cm , respectively. Therefore, suspensions of compounds that have a true density less than these figures will cream (rise to the surface), and those that are denser will sediment. Those that match the density of the propellant will stay in suspension for a longer period (Williams III et al. 1998). It should be noted, however, that the physical stability of a suspension is not merely a function of the true density of the material. 

In recent years ozone in the stratosphere has been depleted at an alarmingly fast rate by chlorofluoro-carbons (CFCs). A CFG molecule such as CFCI3 is first decomposed by UV radiation ... 

Hydrochlorocar- bons/HF Hydro- chlorofluoro- carbons Fluorinated metal oxides Room temp 18... 

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