The family of thermoplastics are extremely inert, paraffinic thermoplastic polymers that have some or all of the hydrogen replaced with fluorine. The family of materials includes polytetrafluoroethylene (PTFE, commonly called TFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), polychlorotrifluoroethylene (CTFE), poly (ethylene-chlorotrifluoroethylene (ECTFE) copolymer, ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), and copolymers of halogenated and fluorinated ethylenes.
PTFE, FEP, and PFA: Their high melt viscosity prevents PTFE resins from being processed by conventional extrusion and molding techniques. Instead, molding resins are processed by press-and-sinter methods similar to those of powder metallurgy or by lubricated extrusion and sintering. All other fluoroplastics are melt processible by techniques commonly used with other thermoplastics.
PTFE resins are opaque, crystalline, and malleable. When heated above 648 °F, however, they are transparent, amorphous, relatively intractable, and they fracture if severely deformed. They return to their original state when cooled.
FEP resins offer nearly all of the desirable properties of PTFE, except thermal stability. Maximum recommended service temperature for these resins is lower by about 100 °F. PFA fluorocarbon resins are easier to process than FEP and have higher mechanical properties at elevated temperatures. Service temperature capabilities are the same as those of PTFE.
PTFE resins are supplied as granular molding powders for compression molding or ram extrusion, as powders for lubricated extrusion, and as aqueous dispersions for dip coating and impregnating. FEP and PFA resins are supplied in pellet form for melt extrusion and molding. FEP resin is also available as an aqueous dispersion.
Properties: Outstanding characteristics of the fluoroplastics are chemical inertness, high and low-temperature stability, excellent electrical properties, and low friction. However, the resins are fairly soft and resistance to wear and creep is low. These characteristics are improved by compounding the resins with inorganic fibers or particulate materials. For example, the poor wear resistance of PTFE as a bearing material is overcome by adding glass fiber, carbon, bronze, or metallic oxide. Wear resistance is improved by as much as 1,000 times, and the friction coefficient increases only slightly. As a result, the wear resistance of filled PTFE is superior, in its operating range, to that of any other plastic bearing material and is equalled only by some forms of carbon.
The static coefficient of friction for PTFE resins decreases with increasing load. Thus, PTFE bearing surfaces do not seize, even under extremely high loads. Sliding speed has a marked effect on friction characteristics of unreinforced PTFE resins; temperature has very little effect.
PTFE resins have an unusual thermal expansion characteristic. A transition at 65 °F produces a volume increase of over 1%. Thus, a machined part, produced within tolerances at a temperature on either side of this transition zone, will change dimensionally if heated or cooled through the zone.
Electrical properties of PTFE, FEP, and FPA are excellent, and they remain stable over a wide range of frequency and environmental conditions. Dielectric constant, for example, is 2.1 from 60 to 10(to the 9th power) Hz. Heat-aging tests at 572 °F for six months show no change in this value. Dissipation factor of PTFE remains below 0.0003 up to 10(to the 8th power) Hz. The factor for FEP and PFA resins is below 0.001 over the same range. Dielectric strength and surface arc resistance of fluorocarbon resins are high and do not vary with temperature or thermal aging.
Applications: PTFE resin applications can be classified in five categories:
- Fluid conveying systems -- gaskets, molded packings and seals, piston rings, and bellows.
- Static and dynamic load supports -- bearings, ball and roller-bearing components, and sliding bearing pads.
- Release surfaces -- sheet for preventing adhesion, pressure-sensitive tapes, and heat-shrinkable roll covers.
- Electrical and electronic -- insulation for coaxial cable, fixture and motor lead wire, hookup and panel wiring, industrial signal and control cable, and for standoff and feedthrough components.
- Thermal system components -- ablative shields.
FEP resin applications include wire and cable insulation for computer and electronic systems, telephone and alarm systems, and business-machine interconnects, FEP resin is also supplied as extruded sheet and film for release surfaces, roll covers, linings for chemical-processing tanks, and piping. A concentrate is available for Freon-blown-foam wire coating.
PFA resins are used for high-temperature wire and cable insulation, heat-shrinkable tubing and roll covers, chemical-resistant linings for process-equipment components, and in semiconductor processing equipment.
Materials do not adhere readily to the slippery surface of FEP, PFA, and PTFE parts. Surfaces can be chemically etched, however, to permit bonding with adhesives. Thus, low-friction surfaces of fluorocarbon tape or film can be bonded to steel, aluminum, rubber, or other materials. FEP and PFA parts can be heat sealed to themselves, to PTFE parts, or to metals at low pressure and temperatures above 590 °F.
CTFE: Sensitivity to processing conditions is greater in CTFE resins than in most polymers. Molding and extruding operations require accurate temperature control, flow channel streamlining, and high pressure because of the high melt viscosity of these materials. With too little heat, the plastic is unworkable; too much heat degrades the polymer. Degradation begins at about 525 °F. Because of the lower temperatures involved in compression molding, this process produces CTFE parts with the best properties.
Thin parts such as films and coil forms must be made from partially degraded resin. Degree of degradation is directly related to the reduction in viscosity necessary to process a part. Although normal, partial degradation does not greatly affect properties, seriously degraded CTFE becomes highly crystalline, and physical properties are reduced. Extended usage above 250 °F also increases crystallinity.
CTFE plastic is often compounded with various fillers. When plasticized with low-molecular-weight CTFE oils, it becomes a soft, extensible, easily shaped material. Filled with glass fiber, CTFE is harder, more brittle, and has better high-temperature properties.
Properties: CTFE plastics are characterized by chemical inertness, thermal stability, and good electrical properties, and are usable from 400 to -400 °F. Nothing adheres readily to these materials, and they absorb practically no moisture. CTFE components do not carbonize or support combustion. Up to thicknesses of about 1/8 -in., CTFE plastics can be made optically clear. Ultraviolet absorption is very low, which contributes to its good weatherability.
Compared with PTFE, FEP, and PFA fluorocarbon resins, CTFE materials are harder, more resistant to creep, and less permeable; they have lower melting points, higher coefficients of friction, and are less resistant to swelling by solvents than the other fluorocarbons.
Tensile strength of CTFE moldings is moderate, compressive strength is high, and the material has good resistance to abrasion and cold flow. CTFE plastic has the lowest permeability to moisture vapor of any plastic. It is also impermeable to many liquids and gases, particularly in thin sections.
Applications: Molded and extruded CTFE resin applications include components for handling and containing corrosive liquids (diaphragms, valves, sight glasses); seals, gaskets, O-rings, valve seats, and packings for liquid-oxygen and hydrogen equipment; and flexible-circuit laminations, wire insulation, jacketed cable, coil bobbins, and other electrical components. CTFE materials are FDA-approved for use in food-handling equipment. Thin, optically clear CTFE moldings are used for infrared windows in missiles, radome covers, oil-reservoir covers, and gage faces.
PVDF: Polyvinylidene fluoride, the toughest of the fluoroplastic resins, is available as pellets for extrusion and molding and as powders and dispersions for corrosion-resistant coatings. This high-molecular-weight homopolymer has excellent resistance to stress fatigue, abrasion, and to cold flow. Although insulating properties and chemical inertness of PVDF are not as good as those of the fully fluorinated polymers. PTFE and FEP, the balance of properties available in PVDF qualifies this resin for many engineering applications. It can be used over the temperature range from -100 to 300 °F and has excellent resistance to abrasion.
PVDF can be used with halogens, acids, bases, and strong oxidizing agents, but it is not recommended for use in contact with ketones, esters, amines, and some organic acids. Oxygen index is 44.
Although electrical properties of PVDF are not as good as those of other fluoroplastics, it is widely used to insulate wire and cable in computer and other electrical and electronic equipment. Heat-shrinkable tubing of PVDF is used as a protective cover on resistors and diodes, as an encapsulant over soldered joints.
Valves, piping, and other solid and lined components are typical applications of PVDF in chemical-processing equipment. It is the only fluoroplastic available in rigid pipe form. Woven cloth made from PVDF monofilament is used for chemical filtration applications.
A significant application area for PVDF materials is as a protective coating for metal panels used in outdoor service. Blended with pigments, the resin is applied, usually by coil-coating equipment, to aluminum or galvanized steel. The coil is subsequently formed into panels for industrial and commercial buildings.
A recently developed capability of PVDF film is based on the unique piezoelectric characteristics of the film in its so-called beta phase. Beta-phase PVDF is produced from ultrapure film by stretching it as it emerges from the extruder. Both surfaces are then metallized, and the material is subjected to a high voltage to polarize the atomic structure.
When compressed or stretched, polarized PVDF generates a voltage from one metallized surface to the other, proportional to the induced strain. Infrared light on one of the surfaces has the same effect. Conversely, a voltage applied between metallized surfaces expands or contracts the material, depending on the polarity of the voltage.
Representative commercial fluoroplastics:PTFE, Du Pont (Teflon TFE), Allied-Signal (Halon, TFE), ICI Americas (Fluon) FEP, Du Pont (Teflon FEP) PFA, Du Pont (Teflon PFA) CTFE, 3M (Kel-F), Allied-Signal (Plaskon CTFE) PVDF, Pennwalt (Kynar), Atochem (Foraflon) PVF, Du Pont (Tedlar) ETFE, Du Pont (Tefzel) ECTFE, Allied-Signal (Halar)