Polytetrafluoroethylene types

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. 

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. 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see Lubrication and lubricants). SiHcon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

Shock-resistant, flexible hose (Figure 40.32) assemblies are required to absorb the movements of mounted equipment under both normal operating conditions and extreme conditions. They are also used for their noiseattenuating properties and to connect moving parts of certain equipment. The two basic hose types are synthetic rubber and polytetrafluoroethylene (PTFE), such as DuPont s Teflon fluorocarbon resin. 

Many rubber compounds have a tendency to stick in the mould cavity after vulcanisation and require some type of mould release agent. The substances used are surface-active materials such as detergents, soaps, wetting agents, silicone emulsions, aqueous dispersions of talc, mica and fatty acids, applied by spray or brush. Alternatively, dry types based on polytetrafluoroethylene or polyethylene, usually carried in a solvent, can be aerosol applied. An alternative is the addition of an incompatible material to the rubber compound which will bleed to the rubber surface during vulcanisation. 

Conveyor systems are applied in a number of areas in the rubber industry. The types used can range from simple canvas belt conveyors used for haul-off from conventional extruders, to systems used for transport and cooling of profile products, both in and emerging from continuous vulcanisation units. The latter types have to be resistant to the temperatures used in such systems and are variously constructed from glass fibre-reinforced polytetrafluoroethylene or a silicone rubber covered belt. 

The EPR spectrum of irradiated polytetrafluoroethylene can be interpreted as arising from radicals of the type shown below ... 

Which materials are best for cell design depends essentially on the type of electrolyte used. Because HF acid is quite common in the electrochemistry of silicon, materials resistant to HF are preferable. Polyvinyl chloride (PVC), polypropylene (PP), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) can be used for the cell body. PVC is a good choice for most designs because it is inex-... 

Fluorocarbon elastomers represent the largest group of fluoroelastomers. They have carbon-to-carbon linkages in the polymer backbone and a varied amount of fluorine in the molecule. In general, they may consist of several types of monomers poly-vinylidene fluoride (VDF), hexafluoropropylene (HFP), trifluorochloroethylene (CTFE), polytetrafluoroethylene (TFE), perfluoromethylvinyl ether (PMVE), ethylene or propylene.212 Other types may contain other comonomers, e.g., 1,2,3,3,3-pentafluropropylene instead of HFP.213 Fluorocarbon elastomers exhibit good chemical and thermal stability and good resistance to oxidation. 

The vacuum pyrolysis of thin samples of polytetrafluoroethylene follows first-order kinetics with monomer as the major decomposition product in the temperature range from 360—510° C (Siegle, Muus, and Lin). The rate constant does not depend on either the molecular weight or the type of polymer and is characterized by an activation enthalpy of 83.0 kcal/mole and a frequency factor of 3 x 1019 sec-1. The melt viscosity decreases during pyrolysis. 

The formation of coagulum is observed in all types of emulsion polymers (i) synthetic rubber latexes such as butadiene-styrene, acrylonitrile-butadiene, and butadiene-styrene-vinyl pyridine copolymers as well as polybutadiene, polychloroprene, and polyisoprene (ii) coatings latexes such as styrene-butadiene, acrylate ester, vinyl acetate, vinyl chloride, and ethylene copolymers (iii) plastisol resins such as polyvinyl chloride (iv) specialty latexes such as polyethylene, polytetrafluoroethylene, and other fluorinated polymers (v) inverse latexes of polyacrylamide and other water-soluble polymers prepared by inverse emulsion polymerization. There are no major latex classes produced by emulsion polymerization that are completely free of coagulum formation during or after polymerization. 

The PEFC was first developed for the Gemini space vehicle by General Electric, USA. In this fuel cell type, the electrolyte is an ion-exchange membrane, specifically, a fluorinated sulfonic acid polymer or other similar solid polymer. In general, the polymer consists of a polytetrafluoroethylene (Teflon) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group, which is an outstanding proton conductor. Hydration of the membrane yields dissociation and solvation of the proton of the acid group, since the solvated protons are mobile within the polymer. Subsequently, the only liquid necessary for the operation of this fuel cell type is water [7,8],... 

The family of FPs, also called fluorocarbon plastics, is based on polymers made of monomers composed of fluorine and carbon may also include chlorine atoms in their structure. Specific types include polytetrafluoroethylene (PTFE), polytetrafluoroethylene-cohexafluoro-propylene or fluorinated ethylene propylene (FEP), polytrafluoroethylene-coperfluoropropylvinyl ether (PFA), ethylenetetrafluoroethylene (ETFE). polychlorotrifluoroethylene (PCTFE), ethylene-chlorotri-fluoroethylene (ECTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), perfluoromethylvinylether (PFMV), perfluoroalkoxy (PFA), etc. 

Non-polar plastics such as polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), and polystyrene (PS) are more difficult to adhere to. 1,3-butylene glycol diacrylate has shown some efficiency in promoting adhesion to these types of substrates. 

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