Fluoroplastic

Fluoroplastic

 

in the USSR, the name used in industry for any one of a series of fluorine-containing plastics, which are homopolymers of fluorine derivatives of ethylene or copolymers of ethylene fluorine derivatives and, for example, fluoroolefins, olefins, or perfluoroalkyl vinyl ethers. The most important are polytetrafluoroethylene (PTFE), which accounts for 85 percent of the world production of fluoroplastics, and polychlorotri-fluoroethylene (PCTFE). Both are white, crystalline substances that exhibit good chemical and thermal stability, good resistance to cold, weatherproofness, nonflammability, and a number of valuable physical properties.

PTFE, [—CF2—CF2—]n, has a molecular weight ranging between 5 × 105 and 2 × 106 and a density of approximately 2.2 g/cm3 (at 20°C). It surpasses platinum, quartz, graphite, and all synthetic materials in chemical stability; it is resistant to the action of strong oxidizing and reducing agents, acids, alkalies, and organic solvents and is decomposed only by alkali metals that are melted or dissolved in liquid ammonia or by fluorine gas and chlorine trifluoride (at temperatures of about 150°C). In polyfluorinated hydrocarbons it starts to swell at temperatures above 327°C. PTFE is characterized by a tensile strength of 14–35 meganewtons/m2 (140–350 kilograms-force/cm2), by a relative elongation of 250–500 percent, by unusually high dielectric properties (the tangent of the dielectric loss angle is 0.0002–0.00025 at 60 hertz to 1 megahertz), which are virtually not affected by frequency and temperature, and by high arc resistance (250 sec). It does not change in water, liquid fuels, and oils. It is resistant to tropical climates, as well as to the action of fungi; it is physiologically inert. PTFE retains some elasticity at temperatures as low as –269°C; it exhibits cold flow under load, has low adhesion, and is not stable to radiation. Upon melting (at 327°C), it becomes transparent; it decomposes at 415°C without passing through the viscous state.

PCTFE, [—CF2—CF2—]n, has a molecular weight ranging between 56,000 and 360,000 and a density of 2.09–2.16 g/cm3 at 25°C (crystallized samples). It is chemically stable to the action of oxidizing agents, alkalies, and strong acids; it swells in many ethers and halogen derivatives of hydrocarbons and dissolves in aromatic hydrocarbons at temperatures above their boiling points. It is characterized by a compressive strength of up to 500 meganewtons/m2, or 5,000 kilograms-force/cm2 (for roasted samples), good dielectric properties at low frequencies (the tangent of the dielectric loss angle is 0.024 at 1 kilohertz), high arc resistance (greater than 360 sec), low cold flow, and low water and gas permeability. It melts at 210° and at 240°–270°C passes to the viscous state. It decomposes at 270°C; however, its mechanical properties already deteriorate considerably at 170°–200°C. PCTFE is serviceable at temperatures of –196° to 130°–190°C.

Copolymers of tetrafluoroethylene and hexafluoropropylene, as well as copolymers of tetrafluoroethylene and perfluoropropyl vinyl ether, combine high chemical and thermal stability with good workability; because of the high viscosity of the melt, the second copolymer is suitable for use as a high-temperature adhesive for fluoroplastics. Copolymers of tetrafluoroethylene and perfluoroolefins, which contain a sulfo group, are thermally and chemically stable cation-exchange resins that surpass all other hard ion-exchange resins with respect to acidity; they are successfully used as membranes for fuel cells. Copolymers of tetrafluoroethylene and ethylene and copolymers of tetrafluoroethylene and vinylidine fluoride (as well as polyvinyl fluoride and polyvinylidene fluoride) have lower chemical stability than the aforementioned homopolymers; however, they do have a number of other valuable qualities, including high strength and good technological properties.

Fluoroplastics are obtained by free-radical polymerization or by the copolymerization of the corresponding monomers. They are processed using the methods adopted for thermoplastics, such as compression molding and extrusion, with the exception of PTFE, which is processed by the cold preforming of powder under a pressure of 25–35 meganewtons/m2, or 250–350 kilograms-force/cm2, with subsequent baking at 360°–380°C.

Fluoroplastics are used in the production of films, conveyor belts, antifriction materials for bearings and packings operating without lubricants, fibers, fabrics, laboratory ware, chemically stable coatings, and metal plastics. Low-molecular-weight PTFE is used as a chemically stable lubricant. Products made from fluoroplastics are used in electrical and radio engineering, aeronautical and rocket engineering, machine building, the chemical and atomic industries, cryogenic technology, the food-processing industry, and medicine.

In the USSR, fluoroplastics are produced under the name ftorlon: PTFE is manufactured as ftorlon-4 and PCTFE as ftorlon-3. In the United States, PTFE and PCTFE are known as Teflon and Kel-F, respectively.

REFERENCES

Ftorpolimery. Moscow, 1975. (Translated from English.)
Entsiklopediia polimerov, vol. 3. Moscow, 1977.

S. V. SOKOLOV

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