Polytetrafluoroethylene degradation resistance

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

A variety of natural and synthetic materials are used throughout fuel and lubricant systems. Examples include transfer lines, hoses, fan blades, impellers, small gears, housings, and a host of supporting framework. Some plastics can be degraded by fuels, lubricants, additives, and various petroleum-based compounds. The most resistant material is polytetrafluoroethylene (PTFE). Ryton and Viton are less resistant, but are still quite stable in fuel and lubricant systems. Characteristics of PTFE and Ryton are shown below ... 

It should not be thought, however, that perfluorocarbons are completely inert toward combustion. Even the very inert perfluorocarbon polymer polytetrafluoroethylene [PTFE, Du Pont s Teflon F(CF2CF2)nF] is thermodynamically unstable in oxygen with respect to CO2 and CF4 (Exercise 12.6) and can burn in a 95% 02/5% N2 mixture at 0.1 MPa, although combustion is hard to initiate because of the nonvolatility of PTFE and the resistance of the thermal degradation products to oxidation. Conflagrations involving more reactive, volatile fluorocarbons such as perfluoro-toluene have been reported.15... 

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). 

Polytetrafluoroethylene (Teflon) belongs to the group of degrading polymers when irradiated. It is relatively radiation resistant in the absence of oxygen, but rapidly deteriorates in air or oxygen atmosphere. The decomposition takes place with chain mechanism with participation of alkoxy radicals. In practice, radiation-induced degradation is used to produce powdered Teflon. The process requires several hundred kGy dose and the powder is used as lubricant after blending with other materials. 

Fluoropolymers are very weather resistant. Polytetrafluoroethylene s weathering and light resistance are excellent. After 25 years of uninterrupted outdoor weathering in Florida, no signs of degradation could be detected on specimens made from FIFE [32]. 

Polytetrafluoroethylene Is highly resistant to thermal degradation and decomposes only very slowly under exclusion of air at 350 °C. With increasing temperature, its decomposition rate gradually increases however, decomposition proceeds rapidly only at 600 to 700 °C, creating mainly monomers. 

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