Polytetrafluoroethylene (PTFE) is a synthetic material accidentally invented in the late 1930s while a chemist was endeavoring to develop a new type of perfluorethylene-based refrigerant. Rather than achieving a chlorofluorocarbon, the scientist was surprised to find that the perfluorethylene used in the process reacted with the iron content of its container and polymerized under pressure.
Tensile Properties:The tensile breaking stress and breaking strain are used extensively for quality control purposes, but they are unsatisfactory quantities for design purposes for two reasons: firstly, and most importantly, PTFE should never be used at strains beyond the yield point (the point at which the load-deformation curve has a distinct change of slope) and secondly, the point of fracture is dependent on specimen shape and is therefore not useful for predicting behaviour in practice.
The tensile load-extension curves obtained with specimens of PTFE depend on crystallinity, molecular weight, the size, shape and perhaps the structure of the original particles and the severity of faults remaining after fabrication. Furthermore they depend, as is usual with thermoplastics, on test temperature and straining rate. Because of these complications the data here can only be indicative of general behaviour. The general trends of behaviour in tension for PTFE as a function of temperature. These are typical curves from which the yield stress can be derived, though less precisely than is possible for most other plastics materials.
High Voltage Uses of PTFE:With regard to high voltage applications it has been known for a long time that in the presence of surface discharges failure occurs by erosion, as PTFE is a nontracking material. Parr and Scarisbrick have compared the behaviour of a wide range of polymeric insulators by the IEE tracking test using electrolyte, and by an ASTM dust-fog test (D21 32-62T). They found that PTFE was one of the erosion class which showed a long life, i.e. >1000 hours in the dust-fog test. Thus PTFE has useful surface characteristics for exploitation in outdoor applications.
For bulk insulation high quality fabrication will be required in order to produce structures with the very low level of porosity and internal voiding demanded by high voltage applications. Tests by means of electronic discharge detectors can be made to ensure freedom from the damaging discharges which may occur in voids. Alternatively it is possible to reduce the discharges by impregnating the PTFE with dielectric liquids or with a high pressure gas so as to fill, at least partially, any voids in the polymer. In consequence, values for dielectric strength obtained from tests conducted in oil may be misleadingly high for poorly fabricated PTFE due to impregnation of any voids present by the oil.
Post time: Nov-24-2019