Magnetorheological elastomers – possibilities and limitations

The mechanical properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by an applied magnetic field. Solid MR materials consist of magnetically polarisable particles, generally iron, in an elastomer matrix. The required iron concentration is high, about 30% by volume. Problems and solutions will be discussed related to MR-technology. Finally long-term properties of MR-constructions will be discussed and results from chemilumininescence measurements shown. INTRODUCTION Magnetorheological (MR) elastomers are controllable composites that consist of magnetically polarisable particles in an elastomer matrix and are used for e.g. vibration control and in clutch applications. MR elastomers belong to a group of smart materials that have seen an increased interest during recent years.The idea of changing the mechanicalical properties of materials by applying a magnetic field is not new. In the late 19th century viscosity change was seen when an electric field was applied across a mixture a mixture of glycerin, paraffin, and castor oil. A small and reversible change in viscosity was observed. Winslow discovered a similar but larger effect using powdered oil dispersions in 1947 (1). Winslow called the fuids “electro-viscous” (2), but the term “electrorheological” was later considered as to be more appropriate to describe the behaviour of the materials in an electric field. At about the same time, J. Rabinow discovered a similar effect of an applied magnetic field, which he named magnetorheology (MR) (3). During the 1950 ́s many patents based on these phenomena, especially MR were published. However, none of them had any commercial success due to the small difference between the activated and non-activated states of the materials, coupled with sedimentation problems. The interest in ER and MR decreased. However in the 1980 ́s with new faster generations of computers, new applications, such as controllable dampers were made possible, and again the ER and MR seemed interesting (4). The possible applications of ER and MR fluids are numerous. For example, clutches, brakes, dampers, and shock absorbers have been suggested for both ERand MR-fluids (5). It was long believed that that electricity was easier to use than magnetism in the theoretical modelling of the material systems (6). During the last decade the interest has shifted to MR since it has been more successful in commercial applications. EU-PROJECTS ON MR-TECHNOLGY: INDDAMPERS Title: Industrial novelty dampers by development of advanced materials with Magnetorheological elastomers – possibilities and limitations B. Stenberg, M. Lokander and T. Reitberger Department of Fibre and Polymer Technology, KTH, SE-100 44 Stockholm, Sweden. Department of Chemistry, Nuclear Chemistry, KTH, SE 100 44 Stockholm, Sweden. high performance under electromagnetic rheological stimulation. Contract No. G5RD-CT-1990-00125. KTH was the coordinator of this project. In this project many possible applications were analysed. The transport sector (air, train, car) was focused on. Crosslinked rubber to reduce the sedimentation of the heavier iron particles was chosen. Mattias Lokander presented his licenciate thesis in 2002 (Title: Performance of isotropic magnetorheological rubber materials) and he will defend his doctoral thesis October 1 this year.