Polyaramid fibres

If degradation is due to sequential or synergistic mechanisms, then a simple extrapolation may not be reasonable. For example, in accelerating the rate of oxidation of polyethylene it may be necessary to identify the induction time and subsequent degradation time separately and to produce Arrhenius diagrams for each. The total time to failure is the sum of the two times. Figure 9.3 shows an instance of where unknowing extrapolation of the short-term results in tests on polyaramid fibres could have led to overestimates of lifetime and premature failure. 

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. 

More recently, Wilczynski, Ward and Hine [24] have proposed an inverse calculation method where the elastic constants of a fibre can be estimated from fibre resin composite and the elastic constants of the resin. The method was confirmed by measurements on polyethylene/epoxy and carbon fibre/epoxy resin composites. It has been applied [25] to the determination of the elastic constants of a new organic fibre, poly 2,6-dimidazo[4,5-6 4 5 -e]pyridinylene l,4(2,5-dihydroxy)pheny-lene (PIPD). This fibre is a lyotropic liquid crystalline fibre with a very high Young s modulus of 285 GPa and a much higher tensile strength (5.21 GPa) and compressive strength (5OOMPa) than other polyaramid fibres such as Kevlar. 

Cofacially stacked polymeric phthalocyanines (Figure 17.4) have been incorporated into high-strength polyaramid fibres (for example, for Kevlar) and doped after extrusion to yield stable conductive fibres which demonstrate conductivities up to 5 S cm with metallike properties, i.e. dcr/dT is negative [146,147]. Polymeric phthalocyanines are also covalently linked to... 

It should be noted that certain organic fibres such as the polyaramids (e.g. Kevlar, Nomex) are affected by UV light. This is of importance on thin (single ply) unpigmented sheeting reinforced with these materials, where properties can deteriorate markedly on exposure [48]. 

The blade has a true composite construction, as shown in Figure 8.27, with areas of unidirectional glass and carbon fibre, woven glass cloth trailing edge skins laid at 45 over a polyaramid paper honeycomb core, and a titanium erosion shield. The polymeric matrix for the fibres is epo) resin. The metal blades that it replaces were retired after 9400 hours in service, whereas the composite blades do not need replacement during the 40000 hour life of the airframe. Safety is improved, and overall running costs reduced. 

Poly(l,3-phenylene isophthalamide) (polyaramide (4.64) deteriorates rapidly on exposure to terrestrial sunlight. Fibres made from this polymer suffer loss of breaking strength and become yellow [369-371, 378, 2131, 2132]. 

Another interesting composite is that obtained by cospinning of metal phthalocyanines (MPc) or metallo-Pc polymers, e.g, [Si(Pc)0] , with the polyaramid Kevlar. This process produces fibres with conductivities of up to IScm" with good long-term stability and excellent mechanical properties. 

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