Oxidation of natural rubber-based magnetorheological elastomers
Introduction
The rheological properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by an applied magnetic field. Magnetorheological fluids, where magnetically polarisable particles are dispersed in a carrier oil, were introduced by Rabinow in 1948 [1]. Interest in solid analogues of magnetorheological fluids, where the carrier oil is replaced by a rubber or a gel, has increased recently. The MR effect in MR fluids is seen as a field-responsive yield stress, whereas in MR solids the shear modulus is field-responsive [2], [3]. In most MR solids, the polarisable particles, generally pure iron, have been aligned by an applied magnetic field prior to the curing of the matrix [4], [5], [6], [7]. However, although the particles have been aligned, the iron concentration needed in order to get a good MR effect is about 30% by volume [5], [7], [8]. It is possible to achieve a good MR effect without aligning the particles if the particles used have a relatively low critical concentration (the concentration where the particles are in touch with each other). In such cases, the actual iron concentration in the material has to be close to the critical concentration of the particles [9], [10], [11]. The largest magnetorheological effect in MR solids reported so far is about 60% [12].
The high iron concentrations required in order to get a substantial magnetorheological effect may influence the long-term stability of the materials. The surface of “pure” iron particles is covered with a thin layer of iron oxides. This results in large amounts of oxygen incorporated into the material. Furthermore, iron ions are known to enhance the oxidation of rubber materials [13], [14], [15], [16], [17].
In this paper, the oxidative stability of natural rubber-based magnetorheological elastomers has been studied by chemiluminescence and oven ageing. Natural rubber was chosen as the matrix because it has been successfully used in magnetorheological rubbers [4], [11], and because it is known to give a good chemiluminescence signal when oxidised [18].
Section snippets
Materials
The iron particles used were irregularly shaped pure iron particles from Höganäs AB, Sweden, ASC300 (particle size <60 μm), or spherical carbonyl iron CM from BASF (particle size ≈10 μm). The matrix material was conventionally cured natural rubber, SMR GP. The recipe was 100 phr rubber, 6 phr zinc oxide (ZnO), 0.5 phr stearine, 3.5 phr sulphur, and 0.5 phr MBT (phr = grams per hundred grams rubber). Some materials were stabilised by addition of 2 phr Irganox 1076. The iron particles were mixed into the
Results and discussion
Chemiluminescence curves at 120 °C for materials with different content of ASC300 particles are shown in Fig. 1. As can be seen, the time to reach the peak of the CL curve is shorter for the materials with iron. The number of counts per minute is a measure of the reaction rate, but the iron particles shields some of the light emitted, which causes the signal to be weaker when the iron content increases although the reaction is actually faster. The maximum in the CL curve corresponds to the
Conclusions
The oxidative stability of natural rubber decreases dramatically when large amounts of iron particles are incorporated in the matrix. This is probably due to the catalytic effect of iron ions on the decomposition of hydroperoxides, and the large amounts of oxygen on the surface of the particles. Conventional antioxidants can be used to prolong the lifetime of magnetorheological elastomers, but in order to get acceptable lifetime of the materials a careful selection of the antioxidant system,
Acknowledgements
The European Commission is gratefully acknowledged for financial support (Contract no: G5RD-CT-1999-00125).
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