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SPECIALTY PLASTICS Polytetrafluoroethylene (PTFE). The high thermal stability of the carbon-fluorine bond has led to considerable interest in fluorine-containing polymers as heat-resistant plastics and rubbers.
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The high thermal stability of the carbon-fluorine bond has led to considerable interest in fluorine-containing polymers as heat-resistant plastics and rubbers. The discovery of PTFE by Plunkett in 1938 gave an impetus to the study of fluorine containing polymers.
General Description In addition to the presence of stable C-F bonds, the PTFE molecule possesses other features which lead to materials of outstanding heat resistance, chemical resistance and electrical insulation characteristics and with a low coefficient of friction. However the mechanical properties are average.
Characteristic properties are -1 almost universal chemical resistance. insolubility in all known solvents below 300°C, high thermal stability continuous service temp. range -270 to +260°C - low adhesion, low coefficient of friction,
Characteristic properties are - 2 Outstanding electrical and dielectric properties, Resistant to stress cracking High weather resistance Limited use in structural components because of the low modulus of elasticity. These properties can be modified by compounding with reinforcements or wear-reducing additives.
Preparation of monomer: Synthesis is based on fluorspar (CaF2), sulfuric acid and chloroform (CHCl3). CaF2 + H2SO4 CaSO4 + 2HF Treatment of CHCl3 with HF yields MCFM, (monochlorodifluoromethane) a gas boiling at -40.8oC. CHCl3 + 2HF CHClF2 + 2HCl MCFM may be converted to TFE by pyrolysis by passing through a platinum tube at 700oC.
Structure-Property relationship PTFE cannot exhibit the planar zigzag formation of the crystalline regions of the polyethylene macromolecule. The larger fluorine atoms hinder each other so that they can only find space along the C-C backbone in a spiral arrangement.
Structure-Property relationship Below 19°C,one twist comprises 26 Carbon atoms, above this temperature, 30. This transition causes a change in volume of 1 %. The compact structure leads to exceptionally high chemical and thermal resistance. The intermolecular forces of PTFE are not large. Hence the high melting temperature, low mechanical strength & stiffness.
Structure-Property relationship C-F bond is very strong (504 kJ mol-1). Because two fluorine atoms are attached to a single carbon atom there is a reduction in the C-F distance from 1.42oA to 1.35oA. Since the only other bond present is the stable C-C bond, PTFE has very high heat stability, even when heated above its crystalline melting point of 327oC.
Structure-Property relationship The high crystallinity & low intermolecular forces render PTFE resistant to all solvents. Only near the crystalline melting region (gel temp.) of 327 °C fluorine-containing liquids such as per fluorinated kerosene act as solvents.
The properties of PTFE moldings are considerably influenced by the processing conditions and polymer grade. Particle shape and size determine the processability and especially the number of voids in the molding. Molded PTFE materials exhibit high toughness; this applies at temperatures down to - 200 °C.
The molecular weight influences the crystallinity and hence the physical properties. Crystallinity and pore fraction are, however, also influenced by the processing conditions. The average molecular weight of commercial PTFE grades is 4x105 to 9x106. The degree of crystallinity of the polymer reaches 94%. After processing, cooling conditions determine the crystallinity of the molding. Slow cooling leads to higher crystallinity. High crystallinity affects the physical properties.
Additives PTFE does not require any kind of stabilizer. Additives are only used to impart specific properties to moldings such as freedom from maintenance for bearings. Graphite increases abrasion resistance and has proved effective particularly where self-lubricating properties of parts exposed to water must be maintained, e.g., PTFE sliding bearings. Bronze improves creep strength, thermal conductivity and wear resistance of moldings.
Availability Wide range of grades which differ in their particle size and free flowing characteristics to suit the method of processing such as compression molding, extrusion or paste extrusion. Numerous special compounds are also available containing additives such as graphite, bronze, lead, MoS2, glass fiber, etc.
Availability Dispersions are available for coating. The materials are supplied as powdery molding compounds for compression molding and extrusion, dispersions for manufacturing repelling coatings on all types of substrate and as semi-finished products or stock shapes such as blocks, rods, profiles, tubes, hoses and film.
Mechanical Properties-1 PTFE is tough and resilient Transition Temperatures: - An unusual phenomenon for plastics is the phase transition, i.e. the temperature spatial re-arrangement of molecular chains. - A small transition is observed at 19 °C and a first order transition (melting) at about 327 oC. - Above this temperature the material does not exhibit true flow but is rubbery.
Mechanical Properties-2 Long-term Behavior: PTFE deforms under extended mechanical stress like other plastics. The degree of deformation can be significantly influenced by compounding the basic material with fillers and/or reinforcements. Creep is linked with a progressive orientation of the structure. As a result of the increasinghardness, the rate of deformation decreases with time.
Mechanical Properties -3 Dynamic strength values are lower than static. The fatigue limit drops with temperature, the stress cycle frequency & with stress peaks. PTFE is nevertheless suitable for components stressed, for example, in the pulsating tensile stress region as in machined bellows. PTFE is relatively soft. Hardness can be considerably increased by fillers. Processing conditions also affect hardness.
Mechanical Properties - 4 Because of the high carbon/fluorine bond energy and low polarizability of the fluorine atoms, PTFE exhibits significantly smaller intermolecular forces than other polymers, hence its anti-adhesive characteristics. Wettability by water is hindered by the high contact angle (128°).
Mechanical Properties - 5 PTFE has the lowest coefficient of friction of all solids because of the low inter-molecular forces. The coefficients of friction of filled compounds are generally lower than those of unfilled PTFE and usually lie between 0.1 and 0.25.
Mechanical Properties - 6 Abrasion Resistance of PTFE is very high because of the weak inter-molecular forces and also because the molded material is produced by sintering and not by melting. Wear of filled compounds is, however, lower.
Thermal Properties -1 Thermal stability of PTFE is superior to that of other commercial plastic including other fluoro-polymers. Noticeable degradation sets in above 350°C. Stabilization against thermal and photo-oxidative degradation is unnecessary. Continuousservice temperature range is -260°C to +260°C
Thermal Properties - 2 Between 325 and 340°C, white crystalline PTFE changes into a transparent amorphous substance with a reversible change in volume of 30%. Moldings retain their shape at this temperature although strength is considerably reduced. This behavior precludes the use of conventional molding methods and a process similar to that of sinter metallurgy is used.
Electrical Properties The Volume Resistivity of 1018 cm is almost independent of temperature up to 150 °C. It does not fall noticeably on extended immersion in water. The high surface resistivity of 1017 still exceeds 1012 even in air with 100% relative humidity and leads to high electrostatic charge. Processing thus requires a high degree of cleanliness and dirty particles cause defects. PTFE has the lowest dielectric constant and lowest dissipation factor of all plastics due to its symmetrical structure,
Optical Properties The refractive index for visible light is 1.35. Thin layers of unfilled, natural PTFE are transparent, thick films are opaque white. PTFE films 100 mm thick transmit 80% of incident light in the range 300 to 350 nm. In the infrared region of 8 to 8.7 mm, almost 100% of the incident radiation is absorbed.
Resistance to Chemicals PTFE has almost universal chemical resistance. It is not, resistant to elemental fluorine, chlorotrifluoride or molten alkali metals. No chemicals dissolve PTFE at Room Temp. At temperatures above 300 °C, chemically related fluorine-containing hydrocarbons cause reversible swelling of PTFE.
Weathering Resistance PTFE is distinguished by high resistance to UV and weathering and can be recommended without reservation for outdoor use. This also applies to weathering over many years under extreme climatic conditions. PTFE requires no stabilizers.
Resistance to High Energy Radiation PTFE is not a radiation resistant plastic. Degradation (& not cross-linking) of PTFE occurs particularly in the presence of oxygen. Tetrafluoroethylene is released but no elemental fluorine is liberated during decomposition although corrosive secondary products can be formed. Flammability PTFE is self extinguishing. It is classified V-0 to UL94. The oxygen index is 95%.
Processing PTFE can only be processed by special methods: - Pre-forming under pressure followed by free sintering or pressure sintering; - ram extrusion, - paste extrusion, - coating followed by sintering, - impregnation.
Processing Processing on presses and extruders involves three steps: compression (compaction), sintering, cooling. Only suspension polymers are suitable as compression moldable and ram extrusion powders since they are coarse particled. Paste extrusion can only be carried out with powders obtained by coagulating PTFE dispersions (emulsion polymers).
Processing PTFE in the form of waterborne dispersions is used for coating and impregnating various substrates. The melt characteristics of PTFE differ considerably from those of other thermoplastics with the exception of very high molecular weight PE-HD-UHMW. A melt viscosity of 1010-1011 poises has been measured at 350oC which is important for processing,
Processing This is why compression moldings retain their shape even at sinter temperatures of 400°C although with reduced strength. Free-flowing molding powders are used with hard to fill molds and automatic presses; materials with poorer free-flowing characteristics are used to produce high quality moldings. Compression molding of preforms takes place at room temperature at pressures of 10 to 35 bar. Pressures of 30 to 100 bar are used for compounds depending on the amount and type of filler.
Processing The preforms are subsequently sintered by heating slowly from 330+15°C to between 370 and 400°C. Free sintered moldings are not pore-free. In pressure sintering, the work piece cools in the mold under pressure. It is also common to shape pre-sintered preforms on impact molding presses.
Processing Tube up to 250 mm dia. and 0.1 to 4 mm, wall thickness as well as cable and wire sheathing are produced from emulsion polymer by the paste extrusion method. The material is mixed with 18 to 25% petroleum ether to form a kneadable mass which is fed into the ram extruder. The lubricant is evaporated off in a sinter section heated to 380 °C. Porous thread-sealant tapes are not sintered but Calendered and dried.
Processing PTFE films are either peeled from a sintered ring or cast from dispersions. PTFE dispersions are also used to impregnate asbestos or glass fiber cloth. Semi-finished products can be formed under pressure at temperatures around the crystalline melting point (330+ 15°C).
Welding Because it does not form a melt, PTFE cannot be welded in the classical sense by melting the surfaces to be joined with or without the use of welding rod. Peeled film can be welded under pressure (2 to 3 bar) after first warming to 380 to 390°C. Using 50 to 100 mm thick film, welded joints can be obtained at low pressures of 0.02 to 0.03 bar and temperatures of 360 to 380 °C without pretreatment. Such joints can have a quality factor of 0.1 to 1.0.
Bonding The only way of bonding fluoropolymers is adhesion bonding with two component adhesives based on epoxide resins or cyanoacrylate adhesives. The surfaces must be pretreated with sodium in liquid ammonia or sodium napththyl. Decorating Decorating with fluorine-containing printing inks is common.
Typical Applications Packaging, static and dynamic seals, expansion elements, bellows, piston rings, tubes, hoses, fittings, cable insulation, insulating film, crucibles, maintenance-free bearings, coatings with repellant surfaces, substrates for printed circuits and impregnated asbestos, glass fiber or aramid fiber cloth.
O-rings & Valves Unsintered Tape Seals & gaskets Bellows Typical Applications
Manufacturers (Hindustan Fluoropolymers Ltd.) Fluon (AGC Chemicals) Daikin-Polyflon (Daikin America, Inc.) Teflon (Du Pont de Nemours, US) Dyncon (Dyncon, US) Algoflon (Solvay Solexis)
Further Reading BRYDSON J.A, Plastics Material, Butterworth Heinemann, oxford, New Delhi (2005) DOMININGHAUS H., Plastics for Engineering, Hanser publishers, Munich, New York (1998) CHARLESS A . HARPER, Modern Plastics Hand Book McGraw –Hill, New York (1999) MARGOLIS J. M., Engineering Thermoplastics, Marcel-Dekker, New York (1985)
