Properties of PTFE Resins

Note Because of the very large total number of commercial grades and frequent changes due to product developments or discontinuation, the listings cannot ever be complete. However, they illustrate the basic properties of individual classes of thermoplastic elastomers. The formats vary by individual suppliers therefore, it was not possible to have a completely uniform format. For further details the vendor should be contacted. 

PTFE Resin Reduction Ratio Range Specific Gravity Average Particle Size, pm Thermal Instability Index Extrusion Pressure, MPa Reduction Ratio Bulk Density 

Examples of Selected PTEE Micropowders (Nominal Values) 

Product Average Bulk Density, g/L Average Particle Size, gm Specific Surface Area, m2/g Melting Peak Temp, °C 

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Table 3.31. Effects of Oven-Aging at 300°C on the Electrical Properties of PTFE Resins ...

Copolymeis of ethylene [74-85-1] and tetiafluoioethylene [116-14-3] (ETFE) have been alaboiatory curiosity for more than 40 years. These polymers were studied in connection with a search for a melt-fabricable PTFE resin (1 5) interest in them fell with the discovery of TFE—HFP (FEP) copolymers (6). In the 1960s, however, it became evident that a melt-fabricable fluorocarbon resin was needed with higher strength and stiffness than those of PTFE resins. Earlier studies indicated that TFE—ethylene copolymers [11939-51 -6] might have the right combination of properties. Subsequent research efforts (7) led to the introduction of modified ethylene—tetrafluoroethylene polymer [25038-71-5] (Tefzel) by E. I. du Pont de Nemours Co., Inc, in 1970. 

These materials were first introduced by Du Pont in 1956 and are now known as Teflon FEP resins. (FEP = fluorinated ethylene-propylene.) Subsequently other commercial grades have become available (Neoflon by Daikin Kogyo and Teflex by Niitechim, USSR). These copolymers may be regarded as the first commercial attempt to provide a material with the general properties of PTFE and the melt processability of the more conventional thermoplastics. 

The volume resistivity of polytetrafluoroethylene remains unchanged even after a prolonged soaking in water, because it does not absorb water. The surface arc-resistance of PTFE resins is high and is not affected by heat aging. They do not track or form a carbonized path when subjected to a surface arc in air [39]. The electrical properties of PTFE are summarized in Table 3.6. 

In this section, heat and temperature related or dependent properties of polytetrafluoroethylene resins are discussed. These include thermal stability, thermal expansion, thermal conductivity, and specific heat (heat capacity). These characteristics are important to both design and use of PTFE parts. 

Copolymers of tetrafluoroethylene were developed in attempts to provide materials with the general properties of PTFE and the melt process-ability of the more conventional thermoplastics. Two such copolymers are tetrafluoroethylene-hexafluoropropylene (TFE-HFP) copolymers (Teflon FEP resins by Du Pont FEP stands for fluorinated ethylene propylene) with a melting point of 290°C and tetrafluoroethylene-ethylene (ETFE) copolymers (Tefzel by Du Pont) with a melting point of 270°C. These products are melt processable. A number of other fluorine containing melt processable polymers have been introduced. 

Fluorinated ethylene-propylene resin (FEP, PFEP) n. This member of the fluorocarbon family is a copolymer of tetrafluoroethy-lene and hexafluoropropylene, possessing most of the desirable properties of PTFE, yet truly meltable and, therefore, process-able in conventional extrusion and injection-molding equipment. It is available in pellet form for those operations and as dispersions for spraying and dipping. 

Engineering problems involved in the production of TFE seem simple as compared with those associated with polymerization and processing of PTFE resins. The monomer must be polymerized to an extremely high molecular weight in order to achieve the desired properties. The low molecular weight polymer does not have the strength needed in end use applications. 

Fluorinated ethylene propylene (FEP) is an effective alternative to PTFE Teflon. Sharing many of the properties of PTFE, e.g. can be heat-shaped, but being less permeable to gases and also more transparent. In fact, FEP is the most transparent of all resins in the Teflon family. 

The degree to which radiation exposure affects FEP resins is determined by the energy absorbed, regardless of the type of radiation. Changes in mechanical properties depend on total dosage, but ate independent of dose rate. The radiation tolerance of FEP in the presence or absence of oxygen is higher than that of PTFE by a factor of 10 1. 

Mech nic lProperties. Extensive Hsts of the physical properties of FEP copolymers are given in References 58—63. Mechanical properties are shown in Table 3. Most of the important properties of FEP are similar to those of PTFE the main difference is the lower continuous service temperature of 204°C of FEP compared to that of 260°C of PTFE. The flexibiUty at low temperatures and the low coefficients of friction and stabiUty at high temperatures are relatively independent of fabrication conditions. Unlike PTFE, FEP resins do not exhibit a marked change in volume at room temperature, because they do not have a first-order transition at 19°C. They ate usehil above —267°C and are highly flexible above —79°C (64). 

Because of its excekent combination of properties, processibkity, and relatively low price compared to other fluoropolymers, PVDF has become the largest volume fluoropolymer after PTFE consumption in the United States has grown from zero in 1960 to about 6200 metric tons in 1991 (186). About 49% of the consumed volume is PVDF modified by copolymerization with 5—12-wt % HFP to enhance flexibkity. In 1992, Hst price for homopolymer powders was 15.32/kg, and for pekets 15.42/kg the reported market price was 14.09—14.22/kg (187). In the United States, almost ak PVDF is suppHed by Ausimont USA, Inc., Elf Atochem North America, Inc., and Solvay Polymers, Inc. Ausimont and Elf Atochem are producers Solvay is an importer of the resin. Smak amounts of resin are imported from Germany by Huls America, Inc, and from Japan by Kureha Chemical Industry Co., Ltd. PVDE producers and their trademarks are Hsted in Table 4. 

The commercial polymers are mechanically similar to PTFE but with a somewhat greater impact strength. They also have the same excellent electrical insulation properties and chemical inertness. Weathering tests in Florida showed no change in properties after four years. The material also shows exceptional non-adhesiveness. The coefficient of friction of the resin is low but somewhat higher than that of PTFE. Films up to 0.010 in thick show good transparency. 

Polytetrafluoroethylene (Teflon) (PTFE) is the most corrosion-resistant thermoplastic polymer. This polymer is resistant to practically every known chemical or solvent combination and has the highest useful temperature of commercially available polymers. It retains its properties up to 500°F (260°C). Because of its exceedingly high molecular weight PTFE is processed by sintering. The PTFE resin is compressed into shapes under high pressure at room temperature and then heated to 700°F (371°C) to complete the sintering process. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Table 2. Mechanical and Electrical Properties of Commercial PTFE Resins [7]...

Mechanical properties of FEP are in general similar to those of PTFE with the exception of the continuous service temperature, 204°C (400°F) as compared with that of PTFE (260°C or 500°F). Unlike PTFE, FEP does not exhibit a marked volume change at room temperature because it is lacking the first-order transition at 19°C. FEP resins are useful above -267°C ( 1490F) and are highly flexible above -79°C (-110°F).42... 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

Perfluoroalkylvinyl ethers form an important class of monomers in that they are used as comonomers for the modihcation of the properties of homofluoropolymers in addition to their broad nse in copolymers with TFE and other monomers. They are capable of snppressing the crystallization of PTFE efficiently, which imparts usefnl mechanical properties to lower molecular weight of polytetrafluoroethylene polymers. Copolymers of PAVEs and tetrafluoroethylene are thermally stable as PTEE homopolymers. Commercially significant monomers are perfluoropropylvinyl ether and perflnoromethylvinyl ether (PMVE), used for the production of a variety of perflnoroalkoxy resins. 

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