Polytetrafluoroethylene steam

Gases and liquids permeate fluoropol5miers to different extents depending on variables such as temperature, pressure, and the composition of the processing fluid. An increase in temperature accelerates the rate of permeation into the polymers. Thermal cycling can cause a part to stress-crack or form blisters because of successive evaporation and condensation of permeated chemicals. Steam is a well-known permeant of polytetrafluoroethylene and can create blisters upon cycling. 

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). 

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