Solid polytetrafluoroethylene

New materials also emerged. Nylon, developed brilliantly by W. H. Carothers and his team of research workers for Du Pont as a fibre in the mid-1930s, was first used as a moulding material in 1941. Also in 1941 a patent taken out by Kinetic Chemical Inc. described how R. J. Plunkett had first discovered polytetrafluoroethylene. This happened when, on one occasion, it was found that on opening the valve of a supposedly full cylinder of the gas tetrafluoroethylene no gas issued out. On subsequently cutting up the cylinder it was found that a white solid, polytetrafluoroethylene (PTFE), had been deposited on the inner walls of the cylinder. The process was developed by Du Pont and, in 1943, a pilot plant to produce their product Teflon came on stream. 

The constant, which is a property of the particular liquid-solid system, can be calculated from the molecular properties of the two substances dipole moment, polarizability, ionization energy, and molecular radius. Free energies have been calculated from contact angle data for the following solids polytetrafluoroethylene, polyhexafluoropro-p y 1 e n e, poly(trifluoromonochloro)ethylene, and a monolayer of perfluorolauric acid. 

Tests on a number of polymers (Fig. 13) exhibit varying degrees of pressure dependence [10]. The trend is as expected and has been observed by others [19,30]. In general, the increase is gradual. Andersson and Backstrom [31] observed high-pressure transitions for solid polytetrafluoroethylene, but no such transitions were observed for the materials shown in the figure. 

Both Yc Ys of polymers decrease slightly with increasing temperature. However, systematic studies of the effect of temperature (and also of molecular weight and crystallinity) on and Yg are very scarce. For polymer melts both the experimental and calculated32 values of -dy/dT are typically 0.04-0.1 mJ.m 2.K l. Using the equation of state approach combined with contact angle measurements as a function of temperature, Neumann obtained the value 0.06-0.07 for solid polytetrafluoroethylene. The contact angle for a polymer-liquid pair may increase or decrease with temperature (e.g. reference 29). ... 

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.

Surface Protection. The surface properties of fluorosihcones have been studied over a number of years. The CF group has the lowest known intermolecular force of polymer substituents. A study (6) of liquid and solid forms of fluorosihcones has included a comparison to fluorocarbon polymers. The low surface tensions for poly(3,3,3-trifluoropropyl)methylsiloxane and poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl)methylsiloxane both resemble some of the lowest tensions for fluorocarbon polymers, eg, polytetrafluoroethylene. 

Besides the spontaneous, complete wetting for some areas of application, e.g., washing and dishwashing, the rewetting of a hydrophobic component on a solid surface by an aqueous surfactant solution is of great importance. The oil film is thereby compressed to droplets which are released from the surface. Hydrophobic components on low-energy surfaces (e.g., most plastics) are only re wetted under critical conditions. For a complete re wetting of a hydrophobic oil on polytetrafluoroethylene (PTFE) by an aqueous solution, the aqueous solution-oil interface tension must be less than the PTFE-oil interface tension... 

Several solid materials, such as organics dissolved In plastics (,22,23), phosphors sintered with polytetrafluoroethylene (30), phosphors (31), and lumogen-T (23), have been suggested as calibration standards. But most of the publications suggesting these materials (except Ref. 31) have not Included digital data for the corrected spectra. Additional Information, precautions, and pitfalls to be aware of In the use of various standards have been summarized In Reference 11 and the references cited therein. 

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

Since many polymeric materials are used as clothing, household items, components of automobiles and aircraft, etc. flammability is an important consideration. Some polymers such as polytetrafluoroethylene and PVC are naturally flame-resistant, but most common polymers such as PE and PP are not. Small-scale horizontal flame tests have been used to estimate the flammability of solid (ASTM D-635), cellular (ASTM D-1692-74), and foamed (ASTM D-1992) polymers, but these tests are useful for comparative purposes only. Large-scale tunnel tests (ASTM E-84) are more accurate, but they are also more expensive to run than ordinary laboratory tests cited before. 

Fluorine is a reactive, almost colorless gas of F2 molecules. Most of the fluorine produced by industry is used to make the volatile solid UF6 used for processing nuclear fuel (Section 17.12). Much of the rest is used in the production of SF6 for electrical equipment and to make fluorinated carbon compounds, such as Teflon (polytetrafluoroethylene). Most fluoro-substituted hydrocarbons are relatively inert chemically they are inert to oxidation by air, hot nitric acid, concentrated sulfuric acid, and other strong oxidizing agents. 

Esveld et a/.81,82 developed a continuous dry media reactor (CDMR) for pilot-scale applications. It consisted of a multi-modal tunnel microwave cavity operating at a frequency of 2.45 GHz with a power range from 0 to 6 kW irradiated on a surface of 0.6 m2. Temperatures of up to 250°C were achieved. A web conveyor travelling at 17 cm min-1 transported the solid-phase reaction mixture to the oven in low, open Pyrex supports closely packed on a polytetrafluoroethylene (PTFE)-coated glass fibre. An open flat bed process was employed to facilitate easy evaporation. 

Teflon (polytetrafluoroethylene) does not leach out plasticisers and can be purchased in substantial blocks, sheets and rods of various thicknesses. The preferred method is to machine a trough from a solid block of this material. This is a practicable procedure if a good milling machine is available but one is limited to a rather shallow trough. This does not matter if it is to be used for the study of monolayers at the air/water interface only but is a severe limitation if it is to be used to make LB films. Attempts have been made to use a metal trough and coat it using a Teflon aerosol. Such attempts have not proved very successful. 

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