Solvents chlorofluorocarbons

Cleaning Supercritical fluids such as CO9 are being used to clean and degrease quartz rods used to produce optical fibers, produc ts used in the fabrication of printed circuit boards, oily chips from machining operations, and precision bearings in militaiy applications, and so on. Here, CO9 replaces convention chlorocarbon or chlorofluorocarbon solvents. 

The trimer of cyanogen chloride, (CNClls reacts with fluorine in the presence of arsenic pentafluoride in chlorofluorocarbon solvent forming the complex [C3N3Cl3F][AsF6]. 

This is a performance-based method that avoids the use of chlorofluorocarbon solvent. The method is applicable to aqueous matrices, using -hexane as the extraction solvent and gravimetry as the determinative technique. Because hexane is a hydrocarbon solvent, and if this solvent is employed for extraction, the method performance cannot be evaluated by IR measurement. The substances that may be determined by this method are relatively nonvolatile hydrocarbons, vegetable oils, greases, waxes, animal fats, and related materials. The method permits the use of other extraction solvents also, provided that the QC criteria are met. 

The basic tenet of cleaning is like dissolves like. Polar solutions dissolve polar materials and vice versa for nonpolar solutions. Crystals cannot dissolve in oils, and grease cannot be cleaned by water. Lubricants based on chlorofluorocarbons require chlorofluorocarbon solvents for cleaning. Thus, for effective cleaning, you must know what you are cleaning to have an effective plan of (cleaning) attack. 

Methods for determining TPH in aqueous samples are discussed above in Section 3.3.1. The overall method includes sample collection and storage, extraction, and analysis steps. Sampling strategy is an important step in the overall process. Care must be taken to assure that the samples collected are representative of the environmental medium and that they are collected without contamination. There are numerous modifications of the EPA, American Public Health Association (APHA), and American Society for Testing and Materials (ASTM) methods discussed above. Most involve alternate extraction methods developed to improve overall method performance for TPH or replacement of the chlorofluorocarbon solvents. SPE techniques have been applied to water samples (Schrynemeeckers 1993). Solvent extraction methods with hexane (Murray and Lockhart 1981 Picer and Picer 1993) or methylene chloride (Mushrush et al. 1994) have been reported as well. 

Solvents, like contaminants, may be polar or nonpolar. As a general rule, polar solvents dissolve polar residues while nonpolar solvents dissolve nonpolar residues. Thus, ionic residues such as chlorides, salts, acids, acid fluxes, and alkalis are best dissolved and removed with polar solvents such as water, isopropyl alcohol, ethanol, or methylethyl ketone. Greases, oils, silicones, rosin flux, and low-molecular-weight monomers are best dissolved and removed with solvents such as hydrocarbons, Freons , hydrochloro-fluorocarbons, xylene, terpenes, and naphtha. To remove both polar and nonpolar residues, a two-step process using both types of solvents may be used or, more conveniently, an azeotrope mixture of the two solvents can be used in a one-step process. Most of the chlorofluorocarbon solvents (Freons ) and their azeotropes with alcohols, methylene chloride, or ketones are being phased out due to their high ozone-depletion potentials. Solvent blends and azeotropes of hydro-fluoroethers and hydrochlorofluorocarbons (HCFC) are now replacing these solvents. 

The use of supercritical fluids represents a way to replace conventional solvents. Recall that a supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid, qqo (Section 11.4) Water and carbon dioxide are the two most popular choices as supercritical fluid solvents. One recently developed industrial process, for example, replaces chlorofluorocarbon solvents with liquid or supercritical CO2 in the production of polytetrafluoroethylene ([CF2CF2] , sold as Teflon ). Though CO2 is a greenhouse gas, no new CO2 need be manufactured for use as a supercritical fluid solvent. 

Methylene chloride has been the solvent of choice for many semivolatile analyses due to its high extraction efficiency, low cost, and specification by many regulatory methods. In the past chlorofluorocarbon solvents, such as trichlorofluoroethane, have been used for oil and grease analyses, because of their low human toxicity, and spectral qualities (i.e., no absorption at 2930 cm the wavelength generally used for sample measurement). However, owing to the detrimental effects of chlorofluorocarbons on stratospheric ozone, their use is being phased out. 

A major area of concern in chemical processes is the use of volatile organic compounds as solvents for reactions. The solvent is not generally consumed in the reaction, but there are unavoidable releases to the atmosphere even in the most carefully controlled processes. Further, the solvent may be toxic or may decompose at least to some extent during reaction, thus creating wastes. The use of supercritical fluids ("Chemistry at Work" box. Section 11.4) represents a way to replace the conventional solvent by CO2, a nontoxic gas already present in the atmosphere that can be recycled. Du Pont chemical company, for example, has invested in a production facility to make polytetrafluoroethylene, — [CF2Cp2] — (Teflon M) and copolymers with tetrafluoroethylene in liquid or supercritical CO2. In this case the CO2 replaces chlorofluorocarbon solvents, which, aside from their costs, have harmful effects on Earth s ozone layer (Section 18.3). 

Worldwide limits for solvents in the atmosphere were discussed and set (and subsequently updated) at a conference in Montreal in 1988. The protocol issued after the conference, the Montreal Protocol, laid down a timetable for solvent use reduction for chlorofluorocarbon solvents and specifically for carbon tetrachloride and methyl chloroform. The use of the former to be eliminated by 1997, and the latter to be reduced by 70% by the year 2000, and then to be totally banned by 2005. 

The preferred polymerization medium was a saturated fluorocarbon or chlorofluorocarbon solvent, although an aqueous medium could also be used. The solvent system was used to control the reaction conditions and increase the polymerization reaction rate. The reaction medium could also improve melt processability and increase the thermal stability and chemical resistance of the polymer. The reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization regimes. Azo compounds, peroxy compounds, ultraviolet radiation, or high-energy ionizing... 

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