Hydrogen fluoride, pyrolysis

Because PTFE resins decompose slowly, they may be heated to a high temperature. The toxicity of the pyrolysis products warrants care where exposure of personnel is likely to occur (120). Above 230°C decomposition rates become measurable (0.0001% per hour). Small amounts of toxic perfiuoroisobutylene have been isolated at 400°C and above free fluorine has never been found. Above 690°C the decomposition products bum but do not support combustion if the heat is removed. Combustion products consist primarily of carbon dioxide, carbon tetrafluoride, and small quantities of toxic and corrosive hydrogen fluoride. The PTFE resins are nonflammable and do not propagate flame. 

Vlayl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions. It was first prepared by reaction of l,l-difluoro-2-bromoethane [359-07-9] with ziac (1). Most approaches to vinyl fluoride synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (2—5) or utilizing catalysts (3,6—10). Other routes have iavolved ethylene [74-85-1] and HF (11), pyrolysis of 1,1-difluoroethane [624-72-6] (12,13) and fluorochloroethanes (14—18), reaction of 1,1-difluoroethane with acetylene (19,20), and halogen exchange of vinyl chloride [75-01-4] with HF (21—23). Physical properties of vinyl fluoride are given ia Table 1. 

The first step is set up to produce hydrogen fluoride and the second yields trichlo-romethane (chloroform). Chloroform is then partially fluorinated with hydrogen fluoride to chlorodifluoromethane using antimony fluoride as catalyst in the third step. Finally, in the fourth step, chlorodifluoromethane is subjected to pyrolysis in which it is converted to tetrafluoroethylene. The pyrolysis is a noncatalytic gas-phase process carried out in a flow reactor at atmospheric or subatmospheric pressure and at temperatures 590 to 900°C (1094 to 1652°F) with yields as high as 95%. This last step is often conducted at the manufacturing site for PTFE because of the difficulty of handling the monomer.9... 

The shock tube technique employed in the pyrolytic decomposition of polyfluorohydro-carbons22,27,28 showed that the elimination of molecular hydrogen fluoride is the predominant reaction. Yet, a side process of C—C bond breaking becomes important as the temperature is increased beyond 1300 K. Several fluoroethanes have been found to react by molecular dehydrofluorination in chemical activation process29 and Table 2 summarizes the kinetic parameters for the gas-phase pyrolysis of this type of compound. In the case of 1,1,2-trifluoroethane, three olefin products were obtained (equations 3-5). 

Poly[tetrafluoroethene-co-trifluoro(trifluoromethoxy)ethene] also decomposes at lower temperatures than perfluorinated macromolecules, and the major products of pyrolysis include hydrogen fluoride [65], tetrafluoroethene, hexafluoroethane and perfluoro(methyl vinyl) ether [64]. 

If the PVC content of the plastic waste is low, the hydrogen chloride produced can be absorbed either directly in the fluidized bed to which calcium oxide or magnesium oxide is added, or in a separately connected fluidized bed. This method has proved satisfactory, at least for the absorption of hydrogen fluoride in the pyrolysis of PTFE-containing plastic wastes and for hydrogen sulfide in the pyrolysis of rubber. 

At substantially higher temperatures (300°C-400°C), pyrolysis of satin in the air leads almost exclusively to propylene and methyl phosphonofluoiidic acid. Over platinized alumina, there are formed stoichiometric amounts of phosphoric acid, water, hydrogen fluoride, and carbon dioxide (Baier and Weller, 1967). 

The synthesis of this monomer involves nine steps starting with perfluoroglutaryl chloride, as indicated in Figure 2 (3). Most of the steps give good yields except for the selective pyrolysis step (Figure 2, compounds V-VI), where the yields are only 10-20 . By-products are perfluoro(butenyl vinyl ether) and the hydrogen fluoride adducts of both the desired acid and perfluoro(butenyl vinyl ether). 

Vinylidene fluoride, CH2=CF2, is obtained by the pyrolysis of 1,1-difluoro- 1-chloroethane, which in turn is produced from acetylene, vinylidene chloride, or 1,1,1-trichloroethane by reaction with hydrogen fluoride. Because of its low boiling temperature, —84°C, vinylidene fluoride is suspension or emulsion polymerized under pressure. Considerable head-head linkage quantities are produced in these polymerizations. 

In the first stage, vinylidene chloride undergoes addition with hydrogen chloride at about 30°C and atmospheric pressure in the presence of a Friedel-Crafts type catalyst. The resulting trichloroethane is then treated with hydrogen fluoride at about 180°C and 30 atmospheres in the presence of antimony pentachloride to give chlorodifluorethane. Pyrolysis of this product yields vinylidene fluoride Vinylidene fluoride is a gas, b.p. —84°C. 

The density of the a-polymorph is 1.98 g cm amorphous PVDF has a density of 1.68 g cm . Thus, commercial samples with a density of 1.75-1.78 g cm have 45% crystaUinity. The a-polymorph melts at 170 °C however, the processed polymer, because of its polymorphism, displays no sharp melting point but melts between 150 and 190 °C. The thermal decomposition becomes significant at T > 300°C. Pyrolysis of PVDF yields hydrogen fluoride, the monomer C2H2F2 and C4F3H3 [12]. Up to 600 °C, pyrolysis also yields polyaromatic structures by cyclization of polyenic intermediates formed through HF ehmination [16]. This is a particular advantage over PTFE, which is less likely to yield carbonaceous products. Thus in obscurant applications, PVDF is preferred over PTFE as a fluorine source (see Chapter 11). 

In the first step, chloroform is converted into chlorodifluoromethane by catalytic vapor phase fluorination reaction with anhydrous hydrogen fluoride and in the second step, chlorodifluoromethane is subjected to a noncatalytic gas phase pyrolysis at temperature 590-900°C and at atmospheric or subat-mospheric pressures to obtain tetrafluoroethylene in about 95% yield. 

Treatment of the imine CFC1 NF with hydrogen fluoride at ISCC has been claimed to yield A -fluorotrifluoromethylamine, CFj-NHF. This amine was also found as a by-product in material (b.p. - 5-5 °C) obtained by heating the imine with mercuric fluoride at 125 °C and thought to possess the structure CF,-N CF-NFj on the basis of elemental analysis, and mass, n.m.r. (no data quoted), and i.r. (vc-s 1670 cm ) spectroscopic analysis one of the products (b.p. -4 to -3°C) of co-pyrolysis of perfluoro(methylene-methylamine) with nitrogen trifluoride over caesium fluoride in a steel tube... 

Reaction of acetone with hydrogen sulfide in the presence of acidified ZnCl2 at 25° C gives a good yield of a product composed of60-70% hexamethyltrithiane and 30-40% of 2,2-propanedithiol. Thioacetone can be obtained by pyrolysis of either of these compounds. The trithiane is pyrolyzed either on quartz rings heated to 500-650° C at 5-20 mm (30) or by means of a hot wire (32). The dithiol is pyrolyzed on sodium fluoride pellets heated to 150° C at 11 mm (50). In both cases the pyrolysate is immediately collected in a trap cooled to — 78° C. 

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