CONICET | Buscador de Institutos y Recursos Humanos

Polymeric materials are widely used for shock and vibration isolation applications. Rubber and polymers cover a wide range of requirements of stiffness and damping, however sometimes the good isolation properties of polymeric materials are strongly dependent on temperature. For this reason, it is very important the development of polymeric matrix composites which can modify their damping capacity and elastic modulus controlled from external sources. Among the wide groups of engineering polymers which can be used as matrix for composite applications, polypropylene charged with magnetic inclusions provides a polymeric matrix which exhibits modifiable damping and modulus, depending of the applied magnetic field. Then, this material can be used as a polymeric spring with controllable rheologic response, which can be very useful for engineering applications where the change in temperature of in-service polymeric components give rise to an undesirable behaviour. The behaviour of damping, Q-1, and dynamic shear modulus, G´, has been studied in polypropylene charged with either different volume fraction or size of magnetite (Fe3O4) particles, as a function of the applied magnetic field at 318K (±0.25 K). An increase of the alternating (AC) magnetic field oscillating with 50 Hz, leads to an increase of the damping. In addition, during the subsequently decreasing alternating magnetic field, the damping decreases, but a hysteretic behaviour appeared. In fact, the damping of the decreasing part of magnetic field amplitude is found to be smaller than during the previously increasing amplitude part of the treatment with the alternating magnetic field. Subsequent magnetic treatment cycles, lead to successively decreasing damping. In contrast, during the increase of a direct (DC) magnetic field, the damping decreases and the elastic modulus increases. The behaviour of the damping and the elastic modulus during the application of an oscillating magnetic field (AC) is discussed on the basis of the development of both, a new zone with different rheological characteristics than the matrix but of the same material (self inclusion), and/or a deteriorated or damaged zone (chain?s cuts) of the polymer matrix in the neighbourhood of the magnetite inclusion. These effects are promoted by the movement or small relative rotation of the magnetite particles related to the surrounding matrix controlled by the oscillating field. The behaviour of the damping and elastic modulus during the application of a direct (DC) magnetic field is discussed on the basis of the increase of the internal stresses into the polymer matrix due to the promotion of the magnetomechanical stresses. The polypropylene samples were charged with 30, 50, and 60 volume percentage of Fe3O4 particles. Samples with 50 vol.% magnetite containing particles of mesh 40 - 63 m and mesh 63 - 80 m were also studied. Q-1 and G?, values were measured in a mechanical spectrometer working in torsion in forced mode at resonant frequencies of around 10Hz. Q-1 was determined by measuring the relative half width of the squared resonance peak for a specimen driven into forced vibration. Samples were parallelepiped of about 2mm x 2.2mm x 30mm and the maximum oscillating strain on the surface of the sample was 5x10-5. The magnetic field was produced by a water-cooled coil positioned at the place of the sample and its direction was parallel to the torsion axis. The maximum amplitude of the used alternating magnetic fields (50Hz) was HAC = 30 kA/m, the value of the maximum used direct magnetic fields was HDC = 15 kA/m. The constant measuring temperature was realised by an electrical heater Thermocoax (Philips) which is non magnetic and has compensated windings for eliminating the magnetic field of currents through the heater.