Polytetrafluoroethylene, 413 Table

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

Plastics. Almost all commercial plastics find some use both dry and lubricated for sliding at low speeds and light loads the most commonly used thermoplastics are nylon, acetal resins, and polytetrafluoroethylene (PTFE). Typical thermosetting resins for bearing appHcations are phenoHcs, polyesters, and polyimides. Table 8 compares the characteristics of plastic bearing materials with those of graphite, wood, and mbber which find use in somewhat similar appHcations. 

Powdered Teflon for use in pyrots is covered by US Mil Spec MIL-P-48296IPA) (1 May 1974), Polytetrafluoroethylene (TFE) . Three classes of material are specified (1,2 3). The requirements are purity, 99.4% min infrared spectrum, peaks consistent with figure shown color, TFE shall be opaque and the color shall range from white to gray moisture, 0.05% max ash, 0.1% max mp, 337° 10°C packing density, Class 1 — 1.18 0.13g/cc, Class 2 - 1.25 0.02g/cc, Class 3- 1.14 0.09g/cc particle size by sieve analysis, Class 1 — 95 15 microns, Class 2 — 237 27 microns, Class 3 — 200 30 microns particle size distribution by sieve analysis, as specified in Table 1... 

Table 1. Comparison of the respective evolution of the compliancies of a natural artery on the one hand and of an expanded polytetrafluoroethylene prosthesis on the other hand when the blood pressure increases...

Oxidation is also dependent on the permeability of the polymer to oxygen. Table 10.1 lists the permeabilities of selected polymers to oxygen. Because bulk oxidations are dependent on the permeability to oxygen, crystalline polymer forms are more resistant to oxidation than amorphous forms. Also, the very nature of the molecules present in the chains affects the tendencies toward oxidation. Thus the fluorine atom in polytetrafluoroethylene (ptfe)... 

The thermal data on possible condis crystals are collected in Table 8. Substantial entropy gains are observed at the disordering transition, but variations are large, depending on the amount of conformational mobility gained. When compared to the total entropy of fusion, the listed entropies of disordering vary from 30 %, for polytetrafluoroethylene, to close to 100% for the polyphosphazenes. 

The physical properties of polytetrafluoroethylene have been reviewed by Doban, Sperati, and Sandt and by Renfrew and Lewis, and some average properties for well molded samples are listed in Table 1. 

Many important applications of polytetrafluoroethylene depend on its superb electrical properties tabulated in Table 3. These properties have been attributed to its highly symmetrical structure (Doban, Sperati, and Sandt). Complete fluorination of the carbon chain results in an exact balance of the electrical dipoles which is manifested in a very low dielectric constant and electrical loss factor. These two properties are virtually independent of the frequency from 60 to 109 cycles per second... 

A list of typical commercial pervaporation membranes [23] is given in Table 3.1. Commercial hydrophilic membranes are very often made of polyvinyl alcohol (PVA), with differences in the degree of crosslinking. Commercial hydrophobic membranes often have a top layer in polydimethyl siloxane (PDMS). However, a wide variety of membrane materials for pervaporation can be found in the literature, including polymethylglutamate, polyacrylonitrile, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, polyacrylic acid, and many others [24]. A comprehensive overview of membrane materials for pervaporation is given by Semenova et al. [25],... 

As an example, Table 2 compares the results obtained using a smooth Cd cathode and a hydrophobicized Cd electrode prepared from powdered Cd, carbon, acetylene black A-437E, and polytetrafluoroethylene (PTFE) [31]. The current efficiency and current density increased for smooth Cd... 

TABLE 1. Single-step experimental parameters used in preparing low molecular weight granular polytetrafluoroethylene using either ethane or chloroform as chain transfer agents. 

TABLE 2. Physical properties of polytetrafluoroethylene formed in the presence of chain transfer agents ethane and chloroform. 

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. 

The compns are based upon a copolymer of vinylidene fluoride and perfluoropropylene (Viton A) with polytetrafluoroethylene (Teflon) and various metals and oxidizers. A number of extrudable formulations are listed in Table 15 (Ref 57). The major advantages claimed for these proplnts are high density impulse, compatibility with highly energetic proplnt ingredients and resistance to high centrifugal forces... 

Besides the classical polymer introduced by Merrifield (1%-crosslinked chloromethylated polystyrene), a broad variety of polymeric supports is available for SPPS and some of the most popular resins are summarized in Table 1. The chemical structures of some selected resins are presented in Figure 1 and electron micrographs of several examples are displayed in Figure 2. In addition to the solid supports listed in Table 1, there are several other carriers used in peptide synthesis such as the gel-type and macroporous poly(meth-acrylates), coated surfaces like polystyrene films on polyethylene (PEt) sheets, polystyrene-coated polyethylene or polytetrafluoroethylene, and modified glass surfaces. (For recent reviews on polymeric carriers see refs . )... 

TABLE 23.3 Properties of Typical Polytetrafluoroethylene (Teflon) Resin... 

Polytetrafluoroethylene, molybdenum disulfide, graphite, and aramid fibers reduce the frictional coefficient. These may be used as single friction additive, in combination with other fillers, and in combination with silicone oil. Table 5.17 illustrates effect of PTFE on the frictional properties of different polymers. 

In Table I are summarized the results obtained for different sample Nafion perfluoroethylene membrane (NAF), acrylic acid irradiation grafted polytetrafluoroethylene (PTFE), sulfonated styrene irradiation grafted fluorinated ethylene propylene copolymer (RAI), and sulfonated polysulfone (SPS). 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

When an automotive chemist chooses a polymer for dynamic sealing, he or she will have to consider the conditions present in Table 6.1. The two main polymers utilized today for these applications are the thermoset polyimides and polyetheretherketones. Polytetrafluoroethylene is also used for less severe applications. Wear test data are collected on an apparatus in which three circular pins of the polymer being examined... 

Two typical slurry coating formulations are shown in Table V. Formulation A, which consists of PPS, TiCb, water, and a dispersing agent, is suitable for the production of multiple coats and for a variety of other applications. Formulation B contains polytetrafluoroethylene, and can be used to produce single coat nonstick surface coatings, or it can be applied as a top coat to a surface already coated with Formulation A. 

Table VII shows the effect of long-term aging of coatings at 500°F in air. The weight loss after 10 weeks exposure is less than 1% in the formulation containing a small amount of polytetrafluoroethylene.

Table 19-23 lists substances which have been studied seriously as components of self-lubricating composites. The matrix materials contribute properties such as mechanical strength, resistance to corrosion, etc. The function of a polymeric plastic such as polytetrafluoroethylene... 

The only requirement for an additive to qualify as filler for PTFE is that it should be able to withstand the sintering temperatures of polytetrafluoroethylene. Sintering involves exposure to temperatures close to 400°C for several hours, which excludes a great many materials. Characteristics of the filler such as particle size and shape and the chemical composition of the filler affect the properties of compound. A list of most common fillers and descriptions of their important characteristics can be found in Table 3.6. 

Mechanical properties. Polytetrafluoroethylene retains excellent properties at very low and high temperatures. Table 3.11 provides summary of some of the mechanical properties of three different compounds containing 65% bronze, 15% carbon, and 25% glass fiber at different temperatures. Properties of unfilled PTFE have been listed for comparison. 

Deformation under load of all filled polytetrafluoroethylene compounds decreases in comparison to unfilled resin, as seen in Table 3.13. Combinations of carbon and graphite reduce deformation the most at room and at elevated temperatures. The next effective filler in reducing deformation under load is bronze at 60% by weight. Hardness is increased by the addition of additives, particularly bronze, carbon, and graphite (Table 3.14). 

Electrical properties. Fillers and additives significantly increase the porosity of polytetrafluoroethylene compounds. Electrical properties are affected by the void content as well as the filler characteristics. Dielectric strength drops while dielectric constant and dissipation factor rise. Metals, carbon, and graphite increase the thermal conductivity of PTFE compounds. Tables 3.19 and 3.20 present electrical properties of a few common compounds. 

Polytetrafluoroethylene has excellent chemical resistance properties. The effect of incorporation of additives on chemical properties depends on the t) e of the filler and the specific chemicals. In general, chemical properties of filled PTFE compounds are not as good as those of the unfilled resin. Table 3.21 shows the effect of a number of chemicals on car-bon/graphite, glass, and bronze compounds. 

Resin manufacturers have long recognized the excessive deformation of polytetrafluoroethylene in applications where parts such as gaskets and seals experience high pressures. Copolymers oftetrafluo-roethylene with small amoimts of other fluorinated monomer are known as Modified PTFE resins and have been reported to exhibit reduced deformation under load. Examples of the properties of some of the commercial products can be seen in Tables 3.22-3.24 and Figs. 3.20 and 3.21. Significant reduction in deformation under load can be achieved, particularly at elevated temperatures and pressures. 

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