Polytetrafluoroethylen surfactant

Dispersion Resins. Polytetrafluoroethylene dispersions in aqueous medium contain 30—60 wt % polymer particles and some surfactant. The type of surfactant and the particle characteristics depend on the appHcation. These dispersions are appHed to various substrates by spraying, flow coating, dipping, coagulating, or electro depositing. 

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... 

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. 

FLUORINECOMPOUNDS,ORGANIC - FLUORINATED AROMATIC COMPOUNDS] (Vol 11) [FLUORINECOMPOUNDS,ORGANIC - POLYTETRAFLUOROETHYLENE] (Vol 11) [SURFACTANTS] (Vol 23)... 

Tetrafluoroethy lene polymerized in the presence of the chain transfer agent ethane was used to prepare elastomeric grandular polytetrafluoroethylene. The average reaction time was roughly 90 minutes and occurred in the absence of the surfactant perfluoro-octanoic acid. 

More recently, W. L. Gore Associates, Inc. [31] have used expanded polytetrafluoroethylene (ePTFE) porous films and PFSA resin solutions to produce a composite membrane. In this process, the PFSA solution was brushed onto both sides of the ePTFE film so as to impregnate and substantially occlude the interior volume of the film a nonionic surfactant (5%, w/v) was then added into the PFSA solution as a penetrant. The surfactant was removed by soaking in isopropanol after drying at 140 °C (this procedure was repeated several times so as to fully occlude the interior... 

In dishwashing, one must consider soil and surfactant adsorption to both polar and nonpolar surfaces. Metals (aluminum, stainless steel, carbon steel, cast iron, silver, and tin), siliceous surfaces (china, glass, and pottery), and organics (polyethylene, polypropylene, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and wood) present a wide variety of surface characteristics. They span the range of high interfacial free energy (metals and many ceramics) to low interfacial free energy (hydrocarbon polymers) surfaces [27,28],... 

Another approach to the classification of the applications of polytetrafluoroethylene dispersions is the nature of thermal treatment of the fabricated part. Some articles are sintered, some are not sintered but heated to remove the water and surfactant. In some application, the parts are neither sintered nor heated high enough to remove the surfactant. Table 5.11 summarizes the process-based categorization of dispersion applications. 

Polytetrafluoroethylene dispersions may contain one or more surfactants (and other additives) such as perfluoroammoniumoctanoate, also known as C8 in parlance of the industry. There are health hazards associated with some of the surfactants such as C8. 

Finally, it is vitally important to exhaust the fumes of the ovens properly, thus avoiding exposure to the by products of the decomposition of surfactants, additives, and polytetrafluoroethylene. 

The most important polymer in this group is polytetrafluoroethylene (PTFE). Because PTFE has such a high heat of polymerization (172kJmol ) it is polymerized in water (at 75 °C), using air (O2) or persulphates as catalysts. Either granular or dispersion (using fluorocarbon surfactants) polymerization techniques can be used. 

Dispersion Polymerization - This technique is a heterogenous regime where a significant amount of surfactant is added to the polymerization medium. Characteristics of the process include small uniform polymer particles which may be unstable and coagulate if they are not stabilized. Hydrocarbon oil is added to the dispersion polymerization reactor to stabilize the polytetrafluoroethylene emulsion. Temperature and agitation control are easier in this mode than suspension polymerization. Polytetrafluoroethylene fine powder and dispersion are produced by this technique. 

When disperse phase of the coarse emulsion wets the membrane wall and suitable surfactants are dissolved in both liquid phases, the process results in a phase inversion namely a coarse OAV emulsion is inverted into a fine W/O emulsion (Figure 6.1c), and vice versa (Suzuki et al, 1999). The main advantage of this method is that a fine emulsion can be easily prepared from a low concentration coarse emulsion at high rates. For polytetrafluoroethylene (PTFE) membrane filters with a mean pore size of 1 im, the maximum dispersed phase volume fraction in phase-inverted emulsions was 0.9 and 0.84 for O/W and W/O emulsions, respectively (Suzuki et al., 1999). Flow-induced phase-inversion (FIPI) phenomenon was observed earlier by Akay (1998) who used a multiple expansion-contraction static mixer (MECSM) consisting of a series of short capillaries with flow dividers. Hino et al. (2000) and Kawashima et al. (1991) inverted a W/O/W emulsion made up of liquid paraffin. Span 80 (a hydrophobic surfactant), and Tween 20 (a hydrophilic surfactant) into a W/ O emulsion by extrusion through polycarbonate membranes with a mean pore sizes of 3 and 8 im. Inside the membrane pores, surfactant molecules are oriented with their hydrophobic groups toward the wall surface and with hydro-phihc groups toward the solubilized water molecules as a result of a lamellar structure formed inside the pores. The structure ruptured at the pore outlets. 

As discussed in Chapter 10, there are various F-containing polymers that can be used as binders. For example, polytetrafluoroethylene (Pl FF) can be used as a water-based dispersion, and P F can be used after dissolution in organic electrolytes. However, to make a dispersion of PTFE, some surfactants are needed, which will produce some side reactions with electrode materials of lithium-ion batteries. Unfortunately, the dispersion is not stable at elevated temperature and after long-term storage. As a result, PVDF is preferred instead of PTFE since the electrochemical stable window of PVDF is also broader than that of other F-containing polymers, and it is very stable with many positive and negative electrode materials. Its reaction with li metal only starts above 200°C, which is above the normal temperature scope for lithium-ion batteries. As shown in... 

Fluorinated surfactants facilitate coplating of polytetrafluoroethylene and metals onto a metal substrate [43]. A cationic fluorinated surfactant adsorbed onto the polymer particles imparts a positive charge and thus allows the polymer and metal to be electrolytically coplated. 

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