Polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) has a chemical structure which can be designated by (CF2)k. From its resemblance to the chemical structure of polyethylene it might be thought that the spectra of these two polymers should be quite similar. They do in fact resemble each other, but there are also important differences. This is a consequence of the fact that the PTFE chain configuration is quite different from that of polyethylene, and also the intramolecular forces are undoubtedly significantly different in the two cases. As we shall see, the spectrum is moderately well understood, but not in quite as great detail as that of polyethylene. This is primarily a result of the lack of Raman data on the polymer and certain key polarization data in the infrared. 

The spectrum of a stretched PTFE specimen is shown in Fig. 7 [Liang and Krimm (111)], covering the range from 70 to 3300 cm-1, with polarization data at frequencies above 350 cm-1. Reflection spectra have been obtained in the 1000 to 1600 cm 1, region [Robinson and Price (187)1, and the spectrum has been recorded up to frequencies of about 3700 cm-1 in a recent study [Moynihan (141)1- The band descriptions are listed in Table 8 together with the assignments, which will be discussed below. (In some cases the band positions represent mean values of those quoted in Liang and 

Raman spectrum of PTFE has been obtained as yet, so that an attempt has been made to infer the Raman active fundamentals from data on n-C7F16 [A. P. I. Spectra (7), No. 196] and from combination bands [Moynihan (74/)]. These, as well as the infrared, fundamentals are shown in Table 9. 

The basic crystalline chain structure of PTFE has been worked out from x-ray diffraction studies [Bunn and Howells (29)]. It consists of a helical arrangement of CF2 groups along the chain with 13 CF2 groups in the identity period of the helix (see Fig. 8). The two-fold axis of each CF2 group is perpendicular to the helix axis. 

The polymer undergoes a phase transition at 19° C. which seems to be accompanied by a change in the identity period, there now being 15 CF2 groups in the repeat [Pierce, 

Polytetrafluoroethylene. Black atoms are carbon turquoise atoms are fluorine. Gray stick indicates double 

Polytetrafluoroethylene has the lowest coefficient of friction of any known substance. The coefficient of friction is a measure of how easily one substance slides over the surface of a second substance. Polytetrafluoroethy-lene s low coefficient of friction means that nothing will stick very well to its surface, accounting for Teflon s primary use in the manufacture of non-stick products. 

Polytetrafluoroethylene was invented in 1938 by Roy J. Plunkett (1910-1994) quite by accident. As a research chemist at DuPont s Jackson Laboratory, Plunkett was studying compounds that might be used for refrigerants. He kept the compounds in steel tanks and was surprised on one occasion to find that the gas he wanted did not leave the storage tank when the valve was opened. He cut the tank open to see what had happened to the gas and found a waxy white material. Upon analysis, the material turned out to be polytetrafluoroethylene. The gas stored in the tank, the potential refrigerant, was tetrafluoroethy-lene. It had undergone polymerization spontaneously within the tank, making it possible for Plunkett to discover one of the most remarkable synthetic products in the world. 

Because Teflon does not that makes the Teflon  

Perhaps the best known application of polytetrafluor-oethylene is in kitchen utensils with non-stick coatings, such as pots, pans, and spatulas. Polytetrafluoroethylene is also used to coat fibers to make them water-repellant and stain-resistant. Water will bead up and roll off the surface of clothing and other materials coated with polytetrafluoroethylene instead of penetrating the fabric and possibly 

Since PTFE is an inert polymer which is virtually non-extractable once processed it is of limited interest to a wet chemist , and only two areas will be considered here. These are the determination of mixed filler systems in granular PTFE and the separation, identification and determination of nonionic and anionic surfactants in PTFE aqueous dispersions. 

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. It is today produced by a number of chemical manufacturers such as Du Pont (Teflon), ICI (Fluon), Hoechst (Hostaflon TF), Rhone-Poulenc (Soreflon), Montecatini (Algoflan), Nitto Chemical-Japan (Tetraflon) and Daikin Kogyo-Japan (Polyflon). 

The discovery of PTFE (1) in 1938 opened the commercial field of perfluoropolymers. Initial production of PTFE was directed toward the World War II effort, and commercial production was delayed by Du Pont until 1947. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of chemically identical polymer. Suspension polymerization produces a granular resin, and emulsion polymerization produces the coagulated dispersion that is often referred to as a fine powder or PTFE dispersion. 

Manufacturers of PTFE include Daikin Kogyo (Polyflon), Du Pont (Teflon), Hoechst (Hostaflon), ICI (Fluon), Ausimont (Algoflon and Halon), and the CIS (Fluoroplast). India and The People s RepubHc of China also manufacture some PTFE products. 

Preparation. The manufacture of tetrafluoroethylene [116-14-3] (TEE) involves the following steps (2—9). The pyrolysis is often conducted at a PTFE manufacturing site because of the difficulty of handling TFE. 

Pyrolysis of chlorodifluoromethane is a noncatalytic gas-phase reaction carried out in a flow reactor at atmospheric or sub atmospheric pressure yields can be as high as 95% at 590—900°C. The economics of monomer production is highly dependent on the yields of this process. A significant amount of hydrogen chloride waste product is generated during the formation of the carbon—fluorine bonds. 

A large number of by-products are formed in this process, mostly in trace amounts more significant quantities are obtained of hexafluoropropylene, perfluorocyclobutane, l-chloro-l,l,2,2-tetrafluoroethane, and 2-chloro-l,l,l,2,3,3-hexafluoropropane. Small amounts of highly toxic perfluoroisobutylene, CF2=C(CF2)2, are formed by the pyrolysis of chlorodifluoromethane. 

When PTFE is degraded by ionizing radiation under vacuum, the amount of toxic compounds, such as HE and CO, decreases. The threshold value of absorbed dose for vacuum degradation of PTFE was found to be about 50-60 kCy.i  

copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HEP), has physical and chemical properties similar to those of PTFE, but it differs from it in that it can be processed by standard melt processing techniques. 

As pointed out earlier, the fluorine atoms are too large to allow planar zigzag structure, which confers rigidity on the polymer [20]. The PTFE molecule has a regular folded structure, which produces a laminar crystal [16]. 

The true densities of crystalline and amorphous PTFE differ considerably, and 100% crystalline PTFE densities of 2.347 at 0°C (32°F) and 2.302 at 25°C (77°F) were calculated from x-ray crystallographic data [21]. The density decrease of about 2% between these temperatures includes the decrease of approximately 1% due to the transition at 19°C (66°F), which results from a slight uncoiling of the helical conformation of molecules on heating through the transition. By contrast, the density of amorphous PTFE is not affected by the transition at 19°C, and values around 2.00 have been reported from extrapolations of specific volume measurements to zero crystallinity [15]. 

Effects of structural changes on properties, such as specific heat, specific volume, or dynamic mechanical and electrical properties, are observed at various tern- 

Type ofTransition Temperature, °C Region Affected Technique Used 

First order 19 Crystalline, angular displacement causing disorder Thermal methods, x-ray, NMR 

Major polymer applications wear reduction, friction reduction, film, tubes, gaskets, valve and pump parts, tank lining, laboratory equipment, filtration membranes, bearings, piston rings, seals, non stick coating, electric insulation applications, Gore-Tex membranes 

Typical fillers glass fiber, carbon fiber, graphite, metal powders (bronze), molybdenum sulfide, boron nitride, carbon black, Ni-Zn ferrite 

Typical concentration range glass fiber - 15-25 3% graphite - 20-30 wt% 

Special methods of incorporation fillers are premixed with powdered resins and then molded 

Special considerations high temperature deflection decreases with glass fiber loading and after heating to 200°C the effect of reinlbrcement disappears 

Tetrafluoroethylene boils at -76.3 C. It is not the only product from the above pyrolytic reaction of difluorochloromethane. Other fluorine byproducts form as well and the monomer must be isolated. The monomer polymerizes in water at moderate pressures by a free-radical mechanism. Various initiators appear effective. Redox initiation is preferred. The polymerization reaction is strongly exothermic and water helps dissipate the high heat of the reaction. A runaway, uncontrolled polymerization can lead to explosive decomposition of the monomer to carbon and carbon tetrafluoride  

Polytetrafluoroethylene is linear and highly crystalline. Absence of terminal CF2=CF-groups shows that few, if any, polymerization terminations occur by disproportionation but probably all take place by combination. The molecular weights of commercially available polymers range from 39,000-9,000,000. Polytetrafluoroethylene is inert to many chemical attacks and is only swollen by fluorocarbon oils at temperatures above 300 °C. The Tm value of this polymer is 327 °C and 7g is below -100 °C. 

The physical properties of polytetrafluoroethylene depend upon crystallinity and on the molecular weight of the polymer. Two crystalline forms are known. In both cases the chains assume helical arrangements to fit into the crystalloids. One such arrangement has fifteen CF2 groups per turn and the other has thirteen. 

Polyurethanes are sensitive to strong acids, strong alkalis, aromatics, alcohols, hot water, hot moist air and saturated steam. The hydrolytic stability of polyurethanes in applications must be considered carefully. However, polyurethanes are resistant to weak acids, weak alkalis, ozone, oxygen, mineral grease, oils and petroleum. There are doubts for the oxidation stability of polytetramethylene ether glycol based polyurethanes. Polycarbonate urethane is a promising substitute with good oxidation stability. 

Polytetrafluoroethylene, PTFE, is the polymerization product of tetraflu-oroethylene discovered in 1938 by R.J. Plunkett of Du Pont. The polymer is linear and free from any significant amount of branching. The highly compact structure leads to a molecule of great stiffness and results in a high crystalline melting point and thermal stability of the polymer. 

The weight average molecular weights of commercial PTTE are in the range 400 000 to 9 000 000. The degree of crystallinity of the polymer 

Apart from its good slip and wear characteristics the advantages of PTFE are a. almost universal chemical resistance, b. insolubility in all known solvents below 300 °C, c. high thermal stability, d. continuous service temperature range -270 to 260 °C, e. low adhesion, f. low coefficient of friction, g. outstanding electrical and dielectric properties, h. resistant to stress cracking and weathering, but limited use in structural components because of the low modulus of elasticity. 

Commercially, PTFE is produced from the monomer tetrafluoroethylene by two different polymerization techniques, namely, suspension and emulsion polymerization. These processes give two vastly different physical forms of chemically identical PTFE. While suspension polymerization produces granular PTFE resin, emulsion polymerization produces an aqueous PTFE dispersion and PTFE fine powders (after coagulating the dispersion). 

PTFE cannot be processed by the usual extrusion or injection molding methods, because the required processing temperatures cause increased degradation. Therefore, molded parts are produced by ram extrusion or by sintering of preforms above the melting point [599]. 

PTFE decomposes at high temperatures (above 400 °C) under formation of low-molecular components whose identity is influenced by variations in pressure, temperature, and overall composition. Corresponding to the considerably lower bond energy of the C-C-bonds in the backbone compared to the bond energy in C-F 

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Figure X-9 shows plots of cos 6 versus 7l for various series of liquids on Teflon (polytetrafluoroethylene) [78]. Each line extrapolates to zero at a certain 7l value, which Zisman has called the critical surface tension 7 since various series extrapolated to about the same value, he proposed that 7 was a quantity characteristic of a given solid. For Teflon, the representative 7 was taken to be about 18 and was regarded as characteristic of a surface consisting of —CF2 — groups.

Dekker A, Reitsma K, Beugeling T, Bant]es A, Fei]en J and van Aken W G 1991 Adhesion of endothelial-oells and adsorption of serum-proteins on gas plasma-treated polytetrafluoroethylene S/omaferfa/s 12 130-8... 

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene, catalysed by oxygen, yields polytetrafluoroethylene (Tejlon) as a very tough horn-hke material of high melting point. It possesses excellent electrical insulation properties and a remarkable inertness towards all chemical reagents, including aqua regia. 

A recent innovation in IR sample preparation is the use of disposable sample cards made from thin sheets of either polyethylene (PE) or polytetrafluoroethylene (PTFE). 

Figure 3.16a shows the storage and loss components of the compliance of crystalline polytetrafluoroethylene at 22.6°C. While not identical to the theoretical curve based on a single Voigt element, the general features are readily recognizable. Note that the range of frequencies over which the feature in Fig. 3.16a develops is much narrower than suggested by the scale in Fig. 3.13. This is because the sample under investigation is crystalline. For amorphous polymers, the observed loss peaks are actually broader than predicted by a...

Figure 3.16 Some experimental dynamic components, (a) Storage and loss compliance of crystalline polytetrafluoroethylene measured at different frequencies. [Data from E. R. Fitzgerald, J. Chem. Phys. 27 1 180 (1957).] (b) Storage modulus and loss tangent of poly(methyl acrylate) and poly(methyl methacrylate) measured at different temperatures. (Reprinted with permission from J. Heijboer in D. J. Meier (Ed.), Molecular Basis of Transitions and Relaxations, Gordon and Breach, New York, 1978.)...

Eig. 6. Decomposition of polymers as a function of temperature during heating. A, polymethylene B, polytetrafluoroethylene C, silicone D, phenoHc resin ... 

Dry chlorine has a great affinity for absorbing moisture, and wet chlorine is extremely corrosive, attacking most common materials except HasteUoy C, titanium, and tantalum. These metals are protected from attack by the acids formed by chlorine hydrolysis because of surface oxide films on the metal. Tantalum is the preferred constmction material for service with wet and dry chlorine. Wet chlorine gas is handled under pressure using fiberglass-reinforced plastics. Rubber-lined steel is suitable for wet chlorine gas handling up to 100°C. At low pressures and low temperatures PVC, chlorinated PVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), poly(vinyhdene fluoride) (PVDE), and... 

AUoys of ceUulose with up to 50% of synthetic polymers (polyethylene, poly(vinyl chloride), polystyrene, polytetrafluoroethylene) have also been made, but have never found commercial appUcations. In fact, any material that can survive the chemistry of the viscose process and can be obtained in particle sizes of less than 5 p.m can be aUoyed with viscose. 

Polytetrafluoroethylene (PTFE) provides the most satisfactory electrical insulation. Concentric rings of PTFE and PTFE impregnated with calcium fluoride are used for the packing glands which support the anode and cathode posts. Rubber is used as the gasket material to form a seal between the cover... 

Aqueous hydrogen fluoride of greater than 60% maybe handled in steel up to 38°C, provided velocities are kept low (<0.3 m/s) and iron pickup in the process stream is acceptable. Otherwise, mbber or polytetrafluoroethylene (PTFE) linings are used. For all appHcations, PTFE or PTEE-lined materials are suitable up to the maximum use temperature of 200°C. PTEE is also the material of choice for gasketing. AHoy 20 or Monel is typically used for valve and pump appHcations. Materials unacceptable for use in HE include cast iron, type 400 stainless steel, hardened steels, titanium, glass, and siHcate ceramics. 

In some cases particles have been added to electrical systems to improve heat removal, for example with an SF -fluidized particulate bed to be used in transformers (47). This process appears feasible, using polytetrafluoroethylene (PTFE) particles of low dielectric constant. For a successful appHcation, practical problems such as fluidizing narrow gaps must be solved. 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

Fig. 2. The steric protection of the carbon backbone by fluorine of a polytetrafluoroethylene chain. The hehcal configuration with a repeat distance of 1.68...

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