|dc.description.abstract||Material with high damping capability is used to reduce vibration in structures. Magnetorheological elastomers (MREs) are a new group of damping materials which consist of an elastomeric matrix containing a suspension of magnetically permeable particles. Damping occurs mainly by the viscous flow of the rubber matrix and inclusion of magnetic particles in the rubber enables additional damping through magnetic particle interaction and interfacial damping. The aim of this thesis was to produce MREs based on iron sand and natural rubber that have good damping performance for potential use in vibration damping.
Dynamic Mechanical Analysis (DMA) was carried out in an isothermal shearmode to measure the changes in material properties caused by vulcanization in order to assess the optimum cure time of rubber compounds to ensure the best damping performance. The results revealed that the shear storage modulus (G′), shear loss modulus (G′′) and tan δ all reflect the vulcanization process, however, tan δ gave the best representation of the level of vulcanization. Indeed, tan δ was able to be used to derive the optimum cure time for rubber compounds and showed good agreement with the results using conventional methodology.
The Taguchi method was employed to investigate the effect of a number of factors, namely, iron sand content, iron sand particle size and applied magnetic field during curing on tan δ and energy dissipated during hysteresis tests. The data were then statistically analysed to predict the optimal combination of factors and experiments were then conducted for verification. It was found that the iron sand content had the greatest influence on tan δ when measured over a range of frequency (0.01-130Hz at 0.5% strain amplitude and at room temperature) as well as on the energy dissipated during the hysteresis tests. The iron sand content and magnetic field were also found to influence the width of the peak in tan δ as a function of temperature (studied over the range -100 to 50ºC at 1Hz and 0.5% strain amplitude). However, none of the factors showed significant influence on tan δ for the plateau region from 1.0-4.5% strain amplitude at 100Hz and at room temperature, which is likely to be due to breakdown of weak interactions between iron sand and rubber at low strain amplitudes and therefore, damping being dominated by the viscous flow of the rubber matrix and friction of rubber chains and iron sand. Evidence from SEM micrographs of MRE sections showed that isotropic MREs had uniform particle distribution and that alignment of magnetic particles occurred for anisotropic MREs as a consequence of an applied magnetic field. However, obvious gaps between iron sand and rubber were evident, suggesting weak interaction between iron sand andrubber.
Bis-(3-triethoxysilylpropyl) tetrasulphane (TESPT) was employed for surface modification of iron sand. The amount of TESPT was varied at five levels (2, 4, 6, 8 and 10wt%) relative to iron sand content to assess the optimum amount of coupling agent for interfacial bonding and damping performance. Evidence that coupling had occurred between iron sand and TESPT was identified by Raman Spectroscopy and the grafting percentage was determined by thermogravimetric analysis. Crosslink density assessment by swelling testing provided evidence that the tetrasulphane group of TESPT formed crosslinks with the rubber chains. The results exhibited the advantages of TESPT as a coupling agent between iron sand particles and rubber and also revealed that 6% TESPT content produced the highest crosslink density. It was found that the silane coupling agent improved the amount of energy dissipated during hysteresis tests as well as tan δ over the range of frequency and strain amplitude explored. The results also revealed that with silane treated iron sand, tan δ increased with increasing magnetic field up to a saturation point at 600 mT. However, the presence of coupling agent and formation of different lengths of aligned particles did not strongly affect the peak height and width of the tan δ versus temperature curves.
Tan δ and energy dissipated during hysteresis testing of isotropic and anisotropic MREs containing silane modified iron sand particles were compared with existing antivibration rubbers. The chosen antivibration rubbers for comparison contained different contents of carbon black filler (30, 50 and 70 phr) in a natural rubber matrix. Energy absorption for comparative samples was generally higher than isotropic and anisotropic MREs over the range of frequency and strain amplitude explored, as well as in hysteresis testing and this was believed to be largely due the presence of carbon black in the existing antivibration rubber formulations. Further assessment was carried out on materials that were the same as the anisotropic MREs except they had additions of carbon black. The energy absorption was generally found higher than comparative samples with the same carbon black contents, supporting the use of iron sand to improve damping. However, this trend was found to reverse at around Tg, which is considered to be due to the segmental motion of rubber chains being by far the most significant influence on energy absorption in the glass transition zone.
A model was developed to include viscous flow of the rubber matrix, interfacial damping and magnetism-induced damping to give the total damping capacity of MREs. The proposed model was assessed experimentally using a series of isotropic and anisotropic MREs. Comparison between tan δ with predicted damping capacity showed that the predicted damping capacity matched the experimental trends with average percentage difference of 8.1% and 21.8% for MREs with modified iron sand and unmodified iron sand, respectively.||