Fluoroelastomer

a synthetic fluorine-containing rubberlike polymer characterized by high thermal stability, nonflammability, and resistance to the action of many corrosive media.

The most widely used fluoroelastomers in industry are copolymers of vinylidene fluoride and hexafluoropropylene or chlorotrifluoroethylene (types SKF-26 and SKF-32; see Table 1), which are stable during storage. They are white nontoxic products, with a density of 1.80–1.86 g/cm³, a glass-transition temperature of approximately –20°C, and a molecular weight of 10,000 to 1 million. Their principal vulcanizing agents are diamines and their derivatives, which react with macromolecules at the methylène groups (—CH₂—) to form relatively weak C—N linkages. For this reason, greater residual deformations accumulate in such fluoroelastomers, particularly when they are used in a stressed, usually compressed, state. Elastomers with less residual compressive strain can be prepared by radiation vulcanization, in which case strong C—C linkages are formed between the macromolecules, as well as by using dipotassium bisphenol salts as vulcanizing agents. Elastomer mixtures based on fluorine usually contain fillers, such as soot and calcium fluoride, as well as oxides of magnesium and calcium, which bind the HF; the separation of HF from the macromolecule, which can result from the extreme temperatures during fluoroelastomer processing or from the use of the fluoroelastomer products at high temperatures, causes the corrosion of metallic equipment.

The tensile strength of SKF-26 and SKF-32 elastomers is 16–27

Table 1. Types of fluoroelastomers and their macromolecular structure
Elastomer (trade names)	Structure of macromolecule
Copolymer of vinylidene fluoride and hexafluoropropylene (SKF-26, USSR; Viton, USA)
Copolymer of vinylidene fluoride and chlorotrifluoroethylene (SKF-32, USSR; Kel-F, USA)
Copolymer of vinylidene fluoride and perfluoroethyl vinyl ether (SKF-260, USSR)
Copolymer of tetrafluoroethylene and trifluoronitrosomethane
Copolymer of tetrafluoroethylene and perfluoromethyl vinyl ether (SKF-460, USSR; ECD-006, USA)
Perfluoroalkylenetriazine
Perfluoroalkyl acrylate
Fluorosiloxane (SKTFT, USSR)

meganewtons/m² (160–270 kilograms-force/cm²), and the relative elongation is 250–500 percent. The elastomers are resistant to fuels, oils, various oxidizing agents, and acids but are not stable to the action of alkalies, ketones, Freons, and ionizing radiation. SKF-26 elastomers are serviceable for long periods at temperatures of 200°–250°C, while SKF-32 elastomers are serviceable at 175°–200°C; for shorter periods they are serviceable at temperatures of 300° and 250°C, respectively. The one serious drawback of such elastomers is low resistance to cold temperatures. Elastomers resistant to such temperatures may be obtained by using copolymers of vinylidene fluoride and perfluoromethyl vinyl ether (SKF-260). The glass-transition temperature for such fluoroelastomers is about –40°C.

Elastomers that are resistant to alkalies, all solvents, and oxidizing agents, including fluorine, are obtained from completely fluorinated elastomers, such as copolymers of tetrafluoroethylene and trifluoronitrosomethane; vulcanization of such elastomers involves the introduction into the macromolecule of a third co-monomer containing functional groups, such as carboxylic groups. These elastomers are further characterized by good resistance to cold, although their thermal stability does not exceed 175°C because of the weakness of the N—O bond. More thermally stable elastomers are obtained from copolymers of tetrafluoroethylene and perfluoromethyl vinyl ether (SKF-460). The good mechanical properties of these elastomers (tensile strength, 20–25 meganewtons/m², or 200–250 kilograms-force/cm²; relative elongation, up to 230 percent) remain unchanged after storage in air at a temperature of about 300°C for one month. One advantage of SKF-460 elastomers is their low residual compressive strain, even under operating conditions at temperatures of about 250°C.

Elastomers made from perfluoroalkylenetriazine, which are resistant to acids and oxidizing agents but decompose in alkalies, exhibit the greatest thermal stability (up to 370° for short-term use and up to 300°C for long-term use). Rubbers based on certain fluoroelastomers of this type are resistant to cold temperatures, as low as –50°C.

Perfluoroalkyl acrylate elastomers are considerably inferior to other fluoroelastomers with respect to thermal and chemical stability but exhibit good resistance to oils and water. Oil-resistant fluorosiloxane rubbers (SKTFT) have properties similar to those of the rubber-like polyorganosiloxanes, the macromolecule of which does not contain a fluorine atom.

The principal method of synthesizing fluoroelastomers is the free-radical polymerization of monomers in emulsion. Fluoroelastomers are used mainly in the manufacture of packings to be used in contact with oils, oxidizing agents, and other corrosive media at 200°C and higher. Perfluoroalkyl acrylate latexes are used to impregnate fabrics used in the production of protective clothing. Fluoroelastomers are costly to produce and consequently are used mainly in the chemical industry and in aviation and space technology.