Experimental investigations on the plastic memory effect of PTFE compounds

Due to their tribological characteristics, chemical inertness and temperature stability PTFE compounds are used for rotary shaft seals in automotives. In order to prepare a numerical simulation of the sealing system, thermomechanical experiments were carried out to study the temperature dependent hysteresis effects of this material. It was observed that the hysteresis effects were not only caused by the plastic and viscoelastic deformations, but also by the so called plastic memory effect (PME). The PME is of special interest, because it is possible to increase the inelastic deformation of a mechanically strained specimen through a temperature treatment to higher values. These additional inelastic deformations can be recovered through another temperature treatment in the unstrained state. The experimental investigations on this special kind of hysteresis were performed on tensile test sampler (90% PTFE, 5% short cylindrical glass fibres, 5% MoS2) using an electromechanical test machine in combination with a temperature chamber. The results show that: . It is impossible to implement the PME without a temperature treatment. . The total strain and the activation temperature have a strong influence on this effect. . It is possible to implement and activate the PME several times on the same test sample. With these results and other experiments like relaxation tests, we have the physical background necessary to formulate a material law describing the thermomechanical behaviour of PTFE and its compounds.