Читать книгу Concise Handbook of Fluorocarbon Gases - Sina Ebnesajjad - Страница 29

Fluorocarbon elastomers are the largest group of fluoroelastomers. Different polymers are fluorinated to variable extents; most of fluorine is bonded to the carbon-carbon backbone of the macromolecule. Commercial fluoroelastomers are based on a number of monomers: tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PMVE), vinylidene fluoride (VDF), chlorotrifluoroethylene (CTFE), 1-hydro-pentafluoropropane (HPFP), ethylene, and propylene. Specific combinations of two or more of these monomers result in amorphous polymers with elastomeric behavior. A more complete list of monomers combined in significant commercial fluoroelastomers has been provided [17].

The first commercially available fluoroelastomer was Kel-F^® developed by the M.W. Kellog Co. in the late 1950’s. A well-known fluorinated elastomer was a copolymer of TFE and VDF developed by DuPont under the trade name Viton^® A. Later, Viton^® B was developed that was a terpolymer of TFE/VDF/HFP. Vinylidene fluoride based fluoroelastomers have been the most popular in the group. Fluorocarbon elastomers have grown extensively over time because of the need for high performance products.

Fluoroelastomers may be classified by their fluorine contents, 66%, 68%, & 70% respectively. Fluoroelastomers with higher fluorine content have higher fluids resistance due to higher fluorine content. Peroxide cured fluoroelastomers have inherently improved water, steam, and acid resistance. Fluoroelastomers are, generally, manufactured by emulsion polymerization process. The monomers are charged to a batch reactor and polymerized under elevated temperature and pressure in the presence of surfactants and additives. After polymerization has been completed the latex is discharged from the reactor, the polymer coagulated, washed, dried and prepared for shipping. Chemical formulas of major fluoroelastomers are shown below:

Fluoroelastomers provide high levels of resistance to chemicals, oil and heat, and service life at elevated temperatures, some exceeding 200°C. Fluoroelastomers are fabricated into a variety of shapes like hoses, seals, O-rings, shaft seals, diaphragms, vibration dampeners, expansion joints and electrical connectors and many others for demanding end uses in a number of industries:

Aircraft and aerospace

Automotive

Chemical processing and transportation

Oil and gas exploration and production

Petroleum refining and transportation

Pharmaceutical