FLUOROPLASTICS

Also termed fluoropolymers, fluorocarbon resins, and fluorine plastics, fluoroplastics are a group of high-performance, high-price engineering plastics. They are composed basically of linear polymers in which some or all of the hydrogen atoms are replaced with fluorine, and are characterized by relatively high crystallinity and molecular weight. All fluoroplastics are natural white and have a waxy feel. They range from semirigid to flexible. As a class, they rank among the best of the plastics in chemical resistance and elevated-temperature performance. Their maximum service temperature ranges up to about 260°C. They also have excellent frictional properties and cannot be wet by many liquids. Their dielectric strength is high and is relatively insensitive to temperature and power frequency. Mechanical properties, including tensile creep and fatigue strength, are only fair, although impact strength is relatively high.

PTFE, FEP, and PFA

There are three major classes of fluoroplastics. In order of decreasing fluorine replacement of hydrogen, they are fluorocarbons, chlorotriflu-oroethylene, and fluorohydrocarbons. There are two fluorocarbon types: tetrafluoroethylene (PTFE or TFE) and fluorinated ethylene propy-lene (FEP). PTFE is the most widely used flu-oroplastic. It has the highest useful service temperature, 260°C, and chemical resistance.

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 processi-ble by techniques commonly used with other thermoplastics.

PTFE resins are opaque, crystalline, and malleable. When heated above 341°C, however, they are transparent, amorphous, relatively intractable, and they fracture if severely deformed. They return to their original state when cooled.

The chief advantage of FEP is its low melt viscosity, which permits it to be conventionally molded. 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 37.8°C. Perfluoroalkoxy (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.

Teflon is a tetrafluoroethylene of specific gravity up to 2.3. The tensile strength is up to 23.5 MPa, elongation 250 to 350%, dielectric strength 39.4 x 106 V/m, and melting point 312°C. It is water resistant and highly chemical resistant. Teflon S is a liquid resin of 22% solids, sprayed by conventional methods and curable at low temperatures. It gives a hard, abrasion-resistant coating for such uses as conveyors and chutes. Its temperature service range is up to 204°C. Teflon fiber is the plastic in extruded monofilament, down to 0.03 cm in diameter, oriented to give high strength. It is used for heat- and chemical-resistant filters. Teflon tubing is also made in fine sizes down to 0.25 cm in diameter with wall thickness of 0.03 cm. Teflon 41-X is a collodial water dispersion of negatively charged particles of Teflon, used for coating metal parts by elec-trodeposition. Teflon FEP is fluorinated ethyl-enepropylene in thin film, down to 0.001 cm thick, for capacitors and coil insulation. The 0.003-cm film has a dielectric strength of 126 x 106 V/m, tensile strength of 20 MPa, and elongation of 250%.

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 1000 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 equaled 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 18°C 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 109 Hz. Heat-aging tests at 300°C for 6 months show no change in this value. Dissipation factor of PTFE remains below 0.0003 up to 108 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 (Table F.4).

CTFE or CFE

Chlorotrifluoroethylene (CTFE or CFE) is stronger and stiffer than the fluorocarbons and has better creep resistance. Like FEP and unlike PTFE, it can be molded by conventional methods.

TABLE F.4

Properties of Fluoroplastics

ASTM or							Modified
UL Test	Property	PTFE	FEP	PFA	PVDF	CTFEa	ETFE
			Physical
D792	Specific gravity	2.13-2.24	2.12-2.17	2.12-2.17	1.75-1.78	2.13	1.70
D792		13-12.3	13.0-12.7	13.0-12.7	15.7-15.6	—	16.3
D570	Water absorption, 24 h,			0.03	0.04	0.01
	1/8-in. thk (%)
			Mechanical
D638	Tensile strength (psi)	3,350	3,000	4,000	5,200-7,400	5,430	6,500
D638	Elongation (%)	300	300	300	100-300	125	275
D638		0.5	—	—	1.6	1.86	1.2
D790	Flexural strength (psi)	No break	No break	No break	No break	10,700	No break
D790		0.5-0.9	0.95	0.95	2.0	2.54	2.0
D256	Impact strength, Izod	3.5	No break	No break	3-4	3.1	No break
	(ft-lb/in. of notch)
D785	Hardness, Rockwell	—	—	—	—	S 85	R50
	Shore D	50-65	55	60	80	79	—
			Thermal
C177		1.7	1.4	1.8	0.7-0.9	1.83	1.65
D696		5.5-8.4	4.6-5.8	6.7	8.0-8.5	4.8-15	5.2
D648	Deflection temperature (°F)
	At 264 psi	132	24	118	195	167	165
	At 66 psi	250	158	164	300	265	220
UL94	Flammability rating	V-0	V-0	V-0	V-0	V-0	V-0
			Electrical
D149	Dielectric strength (V/mil) Short time, 1/8-in. thk	500-600	500-600	500-600	260	490	400-500
D150	Dielectric constant At 1 kHz	2.1	2.1	2.1	7.5	2.45	2.6
D150	Dissipation factor At 1 kHz	0.00005	0.00005	0.0003	0.019	0.0247	0.0008
D257
	At 73°F, 50% RH
D495	Arc resistance (s)
			Optical
D542	Refractive Index	1.350	1.344	1.350	1.42	1.435	1.403
D1003	Transmittance (%)
	1-mil film	—					—
			Frictional
	Coefficient of friction Against steel (100 psi, 10 fpm)	0.050	0.330	0.214	0.14	—	0.400

^a Crystalline compound. Below and above 135°F.

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 274°C. 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. The 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 121°C 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°C. 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 3.2 mm, CTFE plastics can be made optically clear. Ultraviolet absorption is very low, which contributes to its good weatherability.

Compared with PTFE, FEP, and PFA fluo-rocarbon 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.

The fluorohydrocarbons are of two kinds: poly-vinylidene fluoride (PVF2) and polyvinyl fluoride (PVF). Although similar to the other fluo-roplastics, they have somewhat lower heat resistance and considerably higher tensile and compressive strength.

Except for PTFE, the fluoroplastics can be formed by molding, extruding, and other conventional methods. However, processing must be carefully controlled. Because PTFE cannot exist in a true molten state, it cannot be conventionally molded. The common method of fabrication is by compacting the resin in powder form and then sintering.

PVF2, 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 -73 to 149°C 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.

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.

PFA, ECTFE, and ETFE

The following three fluoroplastics are melt pro-cessible. Perfluoroalkoxy (PFA) can be injection-molded, extruded, and rotationally molded. Compared to FEP, PFA has slightly greater mechanical properties at temperatures over 150°C and can be used up to 260°C.

Ethylene-chlorotrifluoroethylene (ECTFE) copolymer resins also are melt processible with a melting point of 240°C. Their mechanical properties — strength, wear resistance, and creep resistance, in particular — are much greater than those of PTFE, FEP, and PFA, but their upper temperature limit is about 165°C. ECTFE also has excellent property retention at cryogenic temperatures.

Ethylene-tetrafluoroethylene (ETFE) copolymer resin is another melt-processible flu-oroplastic with a melting point of 270°C. It is an impact-resistant, tough material that can be used at temperatures ranging from cryogenic up to about 179°C.

One of the advantages of the coploymers of ethylene and TFE — called modified ETFE — and of ethylene and CTFE — called ECTFE — compared with PTFE and CTFE is their ease of processing. Unlike their predecessors, they can be processed by conventional thermoplastic techniques. Various grades can be made into film or sheet, into a monofilament, or used as a powder coating; all grades can be heat-sealed or welded.

Although these resins have lower heat resistance than PTFE or CTFE, they offer a combination of properties and processibility that is unattainable in the predecessor resins. Maximum service temperature for no-load applications is in the range of 149 to 199°C for ETFE and ECTFE, compared with 199°C for CTFE and 288°C for PTFE. Glass reinforcement increases these values by 10°C.

Both tensile strength and toughness of these resins are higher than those of the other fluo-ropolymers; they are rated "no break" in notched Izod tests. The modulus of ECTFE is higher than that of ETFE up to about 100°C; above 150°C, ETFE has a higher modulus. Deflection temperature of both resins is similar, with ECTFE slightly higher (116°C, compared with 104°C, at 0.44 MPa, and 77°C compared with 71°C at 1820 MPa). Hardness of ETFE is Rockwell R50; that of ECTFE is R93; see Table F.4. The limiting oxygen index (LOI) of ETFE is 31; that of ETCFE is 60. (LOIs of PTFE, FEP, and CTFE are over 95.)

As with other fluoroplastics, these resins are compatible with most chemicals, even at high temperatures. ETFE is not attacked by most solvents to temperatures as high as 199°C. ECTFE is similar to 121°C, but is attacked by chlorinated solvents at higher temperatures. ETFE has better chemical stress-crack resistance.

Applications for these resins include wire and cable insulation, chemical-resistant linings and molded parts, laboratoryware, and molded electrostructural parts.