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 * A Brief Review of Teflon (Polytetrafluoroethylene): a few examples of the physical properties and applications.**

Introduction

Teflon, also known as polytetrafluoroethylene or PTFE, is an unusual compound possessing unusual physical characteristics. For instance, the compound is chemically inert, resistant to highly corrosive chemicals, very stable at high temperatures, and has an extremely low dielectric factor. Because of these unique characteristics, Teflon has found application to a wide variety of uses, both industrial and in biomedicine.

Discovery The discovery of Teflon is an interesting story that warrants mentioning. Roy Plunkett, having recently completed his doctorate at Ohio State University, was performing research at the Jackson Laboratory of E.I. DuPont de Nemours and Company in Penns Grove, New Jersey. His goal was to prepare fluorochlorohydrocarbons, specifically tetrafluoroethylene. In order to address a solution to a current research problem, he needed a large quantity of tetrafluoroethylene, which had been previously only made in minute quantities in the laboratory. Plunkett devised a process in which to convert dichlorotetrafluoroethane to tetrafluoroethylene.. Upon vaporization of the tetrafluoroethylene from a small cylinder in one experiment, it was discovered that the flow of gaseous tetrafluoroethylene had stopped prematurely and that there was still material left in the cylinder. It was discovered that the remaining material was a white powder instead of gas, and, as Plunkett explains, “It was obvious immediately to me that the tetrafluoroethylene had polymerized and the white powder was a polymer of tetrafluoroethylene.” [1] Synthesis The monomer was first synthesized from tetrafluoromethane using and electric arc wire. The method included bromination and dehalogenation with zinc in order to separate tetrafluoroethylene from the reaction products [2]. The monomer was also prepared by the dechlorination of dichlorotetrafluoroethane, in which zinc was used to prepare tetrafluoroethylene [3] [2] [4]. As mentioned previously above, the first synthesis of polytetrafluoroethylene in bulk amount was performed by Plunkett [1] by using dichlorotetrafluoroethane under high pressure at room temperature to make tetrafluoroethylene (Figure 1). Subsequent methods were developed in which to create a fast and controllable procedure for polymerizing PTFE in the presence of additional reagents under high pressure, such as water and compounds that help initiate polymerizations, such as ammonium, sodium, hydrogen peroxide, or oxygen. Safety precautions were necessary to control the polymerization reaction because the monomer is highly unstable and can revert back to carbon and carbon tetrafluoride, yielding a violent exothermic reaction that is capable of explosion [2]. In one subsequent synthesis, a hydrogen peroxide solution was charged into a pressure bomb along with tetrafluoroethylene. The bomb was shaken under high heat for 17hrs in order to hasten polymerization of PTFE. The polymer was then separated from the aqueous phase using filtration and drying to yield a white powder [5].

Physical Properties

Structure Representations of the two dimensional structures of the monomer, tetrafluoroethylene (Figure 2A), the Teflon polymer (PTFE)(Figure 2B), and the repeating subunit of the polymer (Figure 2C) are shown below. X-ray diffraction studies were used to discover the three dimensional structure of Teflon at or near room temperature [6]. However, PTFE undergoes structural changes at room temperature as well [6]. Conformational transitions were found to occur at 20oC and 30oC and higher temperatures that are still below the melting temperature of 327oC. Below 20oC, the x-ray diffraction data indicate Teflon exists in a helical configuration with a zigzag ethylenic structure with a 180° twist every 13 carbons[7]. The resulting crystal unit cell is triclinic (see Figure 3 below). Above 20°C, however, the unit cell changes to a hexagonal structure (see Figure 3 below) with a small untwisting of helical conformation. These changes in helical conformations have an impact on chain packing as the helical twisting structure shifts from 13 to 15 carbon atoms per 180° twist below and above, respectively, the 20°C transition point. Further uncoiling of helical structure continues as temperature is increased towards melting point [6] [8].

By examining the X-ray diffraction patterns of PTFE at temperatures above and below its melting point, a highly crystalline structure is seen below the melting point while crystalline structure is absent at 330 oC [5]. The melting point of PTFE is at 327 oC, which is the point at which the compound transitions from highly crystalline solid. Additionally, added evidence of its melting point was discovered when it was found that the polymer can’t be maintained in an oriented state above it’s melting temperature of 327 oC [5]. For most polymers above their melting temperature, they succumb to viscous flow and a rapid deformation while for PTFE above its transition point, the polymer does not undergo viscous flow and succumbs to plastic deformation at a very slow rate [5].

There is evidence that PTFE is a linear, non-crosslinked polymer: it is highly crystalline, it can be oriented, and it has a sharp transition point [5]. It is unlikely that PTFE has chain branching due to the strong C-F bonds that are unlikely to break [5]. It is believed that there is restricted rotation of the carbon atoms in the fluorocarbon chain due to the strong electrostatic repulsion of fluorine atoms on neighboring CF2 groups. Secondly, the size of the fluorine atoms are large enough to cause steric hindrance, preventing rotation. Since the polymer is a chain of repeating CF2 groups, the restriction of rotation would extend through the entire polymer. It is thought that this restricted rotation may lead to very long segments of the polymer chain, preventing viscous flow, even under high heat [5].

Chemical Inertness Polytetrafluoroethylene (PTFE) is not susceptible to attack from high corrosive chemicals, such as hydrochloric acid, hydrofluoric acid, chlorosulfonic acid, and boiling nitric acid. Even at elevated temperatures, PTFE is resistant to attack by sulfuric acid [5]. Other reagents, such as aqua regia, boron trifluoride, and boiling solutions of sodium hydroxide do not have an appreciable effect on PTFE. Of all reagents tested for corrosive against PTFE under 300 oC, only molten alklali metals can attack PTFE [5]. Apparently, molten sodium can remove fluorine atoms from carbons in polymer chains. Other reagents, such as chlorine, bromine, and iodine have been found to be insufficient to cause attack on PTFE, while fluorine, under certain conditions, such as high pressure, can attack the polymer chain [2].

Thermal Stability Polytetrafluoroethylene is highly resistant to prolonged heating at elevated temperatures. When the polymer was kept at 300 oC for a period of one month, the tensile strength dropped only 10-20% while for longer periods at 250oC, there was no appreciable decrease in tensile strength [2]. Above the transition point for PTFE, the loss of crystalline structure does not prevent the polymer from remaining stable in form. The polymer can even be baked for several hours at 390 oC without significant loss in weight[9]. PTFE is also highly unusual at very low termperatures. For instance, it does not become brittle by chilling and can remain flexible at -100 oC, while at -185 oC, the polymer can be cracked [2]. Teflon is even stable at very high temperatures above its melting point [9]

Solubility (PTFE) is highly insoluble in organic solvents, such as hydrocarbons, esters, and phenols even at elevated temperatures above 327 oC [5]. Other polymers, such as polyethylene, are insoluble at room temperature but can become soluble when heated to 70 oC. The long segment structure of PTFE, due to its lack of free rotation on neighboring CF2 groups, is also thought to prevent solubility because motion of the chains are also necessary for the separation of polymer chains for a non-polar crystalline polymer to dissolve [5]. Several hundred solvents were used to try to solubilize polytetrafluoroethylene, including halogenated hydrocarbons, ketones, and esters, even at their boiling points. PTFE, however, is insoluble in all solvents tested [2].

Other Physical Properties (PTFE) has an dielectric loss factor of 0.0002, which is extremely low for a solid substance and is comparable to other polymers with extremely low dielectic loss factors, such as polystyrene and polyethylene [5], [2]. The loss constant for PTFE is also constant over a wide range of frequencies, extending from 60 cycles to 3,000 megacycles [2]. Teflon also shows enhanced absorption of microwaves within cylindrical holes in Teflon film [10]. The heat capacity and entropy studies of the powder and other forms show remarkably similar values over wide temperatures from 0-360 degree C

Applications

Industrial Because of Teflon’s extremely low coefficient of friction, it has been used in resin form as a lubricant and preservative coating for metals [11]. When used in bulk or sintered mixtures, however, Teflon demonstrated excessive flow under high loads which was further perturbed by heating due to friction between the contact surfaces. So, in order to make Teflon into a practical lubricant, a process was necessary in which to deposit it as a thin film and used in dry form. A method was found that incorporated a dispersion of Teflon resin to adequately coat metal surfaces, such as steel, brass, and aluminum. After allowing to dry, and heating to 725 oC, the Teflon left a thin, continuous film. Since it was found that the heat step at 725 oC could soften metals such as brass, a new process was developed in which high-frequency induction heating was used to adequately coat metals such as brass and aluminum without any significant decrease in the tensile strength of the metal. One of the first industrial applications of this metal coating technology were the Teflon coating of ammunition cartridges, leading to reduced gun malfunctioning and increased corrosion resistance to rain, humidity, and salt water. Even the weapons, themselves, such as guns and cannons, were Teflon-coated. The result was an increased firing rate when synthetic diester lubricating oil was used in conjunction with a Teflon-coated gun. Teflon also found application in the highly corrosive environment of submarines in which Teflon coatings were used to lubricate fuel meters and periscope bearings [11].

Teflon also found use in building double-junction reference electrodes that were used with organic solvents [12]. It was found that the reference electrode, rather than the indicator electrode, produced the largest uncertainty measurements when the electrode was used with nonaqueous solvents. Common internal electrodes made out of silver or silver chloride, for example, are unsuitable in aprotic solvents because they stabilize halo complexes relative to free halide ions. In order to solve this problem, the liquid-junction assembly was improved by forming it within a tightly rolled piece of Teflon tape. This results in minimizing the absorption of ions which can be critical when very dilute solutions in water can be difficult to measure with accuracy[12].

Teflon has also found use in photochemical reactors [13]. Teflon has an advantage over quartz in the construcntion of postcolumn photochemical reaction detectors within high-performance liquid chromatography. Teflon’s superior qualitites over quartz are seen in increased light transmission efficiency at low wavelengths. Teflon has also found application as UV irradiation of natural water samples: Teflon can be used for trace metal studies because the hydrophobic surface prevents adsorption of metals and it can be cleaned easily of all trace metal contaminants by acid washing. However, there is a significant caveat to using Teflon in photochemical reactors. When Teflon coils are used, a high background conductance can occur, especially when fluoride and hydrogen ion release is expected[13].

BioMedicine Teflon has also found utility in the biomedicine arena. For instance, it has been used in a paste form for endoscopic injection into the lamina propria behind the submucosal ureter in order to treat vesicoureteric reflux (VUR) [14]. Injection of the paste allows patching under bladder mucosa below affected ureteric orifice. A needle is used to push the paste into the orifice to form a plug. In one study, 116 out of 123 patients were discharged from the hospital on the same day they had the procedure. The success of the procedure was evaluated with ultrasonography and cystourethrography after 3 months and yearly thereafter. Out of 191 ureters treated with Teflon paste for VUR, 149 or 78% had reflux eliminated. Only one patient suffered complications as a result of the initial procedure, suffering left flank pain[14].

Teflon has also been used as stents in the treatment of mammary and pulmonary artery fistula [15]. Internal mammary artery (IMA) to pulmonary artery (PA) fistulas are quite rare and may occur usually due to a genetic predisposition. The IMA-to-PA fistulas may also be acquired under circumstances in which a complication of coronary artery bypass grafting (CABG) occurs that may be present s myocardial ischemia. In one case report, a 63-year old man with ischemic cardiomyopathy was evaluated for congestive heart failure (CHF) and recurring angina after previously sustaining a myocardial infarction a year earlier. The patient underwent stenting of his left anterior descending artery followed by CABG. However, 5 months later, he suffered a second myocardial infarction and subsequent angioplasty was unsuccessful. After several complications and unsuccessful procedures later, a Teflon-covered balloon-expandable stent was used to treat the LIMA-to-PA fistula. The Teflon-covered stent is used for coronary perforations of native vessels and when bypass grafts exceed 2.75mm in diameter. The stent is constructed of a thin piece of Teflon between two stainless steel stents. As a result of having 4 Teflon stents inserted, the patient suffered no recurrent angina and was without evidence of fistula connection. However, in this case the patient ended up dying due multiple complications. Even though the patient died, it was found that the Teflon stents had served their purpose and that the key advantage of the stents over traditional stents is that myocardial ischemia from the fistula connections can be treated percutaneously without procedural risk and was concluded as a useful treatment for future therapeutic options[15].

Another interesting use of Teflon in biomedicine, is the attachment of antibacterial surfaces to polytetrafluoroethylene[16]. There is a search for biomaterials that can be implanted and come into contact with the human body. Since all biomaterials are subject to bacterial attack, which can have damaging effect for the host, a material was sought that was resistant to such bacterial attack. One way was to chemically modify the surface of Teflon in order to create a bacteria resistant surface while still possessing the other chemically inert properties that make it useful for implantation. For this reason, enhanced PTFE (ePTFE) has been developed and implemented to function as vascular grafts, mitral valve tendon replacements or in orthopedic surgeries in plastic and reconstructive surgeries. However, even ePTFE is still subject to bacterial attack, such as Staphylococcus aureus. For this reason, one group attempted to attach the antibacterial agent Penicillin to ePTFE in order to prevent bacterial attack after implantation. Specifically, the ePTFE surface is treated with microwave plasma reactions in presence of maleic anhydride which is followed by surface hydrolysis, leading to the formation of acid groups which can be esterified with reactions of penicillin in presence of polyethylene glycol. The result is that ePTFE coating with Penicillin was shown to be highly effective as an antimicrobial agent toward gram-positive bacteria such as Staphylococcus aureus [16].

Another example of a biomedical use of Teflon is as an implantable chip holding lithium crystals in order to make safe, repeated measurements of pO2 levels in tissues, which is important in monitoring for diseases that are characteristic of unusual oxygen levels [17].

Conclusion

As has been shown, Teflon was a revolutionary material in that it was able to have so much application in both industrial and biomedical settings. The unusual physical characteristics, such as chemical inertness, thermal stability, low dielectric loss factor, and low coefficient of friction, allows new uses to be found for Teflon.

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