Fabrics polyaramide

One can think of the PTFE dispersion coating process as impregnation of fibrous or porous materials such as glass fiber, woven glass cloth, and polyaramide fibers and fabrics. The composite product combines the properties of PTFE and substrate. The polytetrafluoroethylene coated or impregnated products are characterized by a number of common attributes, Table 5.9. 

The presence of moisture in the gas stream, which above 100 C will be present in the form of superheated steam, will also cause a rapid degradation of many fibres through hydrolysis, the rate of which is dependent on the actual gas temperature and its moisture content. Similarly, traces of acids in the gas stream can pose very serious risks to the filter fabric. Perhaps the most topical example is found in the combustion of fossil fuels. The sulphur that is present in the fuel oxidises in the combustion process to form SO, and in some cases, SO3 may also be liberated. The latter presents particular difficulties because, in the presence of moisture, sulphuric add will be formed. Hence, if the temperature in the collector were to be allowed to faU below the acid dew point, which could be in excess of 150°C, rapid degradation of the fibre could ensue. Polyaramid fibres are particularly sensitive to acid hydrolysis and, in situations where such an attack may occnr, more hydrolysis-resistant fibres, such as those produced from polyphenylene sulphide (PPS), would be preferred. On the debit side, PPS fibres cannot snstain continuous exposure to temperatures greater than 190 °C (or atmospheres with more than 15% oxygen), and where this is a major constraint, consideration would have to be given to more costly materials, such as polytetrafluoroethylene (PTFE). 

Yet another problem which may confront the engineer is the presence of very hot particles. Whether from combustion, drying, or other process, these particles have been known to be carried with the gas stream into the filtration compartment, where they present a serious risk of fire. (In certain conditions even ostensibly nonflammable polyaramid fibres have been found to ignite.) Consequently, if adequate particle screening is not provided, the fabric may require a special flame retardant treatment. 

Since important properties of the poly(p-aramide)s are quite different from those of Nomex , most apphcations are also different. Characteristic are applications as bullet-proof body armor, cut-fast gloves, substitute of asbestos, and as reinforcing component in concrete. They are also used as fibers in high performance tires for airplanes and racing cars, or as laminates for the constmction of boat hulls. Furthermore, poly(p-aramide)s are used for the production of sports articles, drum heads, brake pads, turbo hoses, optical fiber cables, or electrical-mechanical cables. Their fabrics serve as hot air filters and in combination with Nomex for the production of protective clothing and helmets. The total production of polyaramides amounts in 2008 to 60,000 tons. 

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