The Damping Properties of Flaky Magnetorheological Elastomer
Norhiwani binti Mohd Hapipi1, a , Saiful Amri Mazlan1, b* , Mumtaz Hana Ahmad Khairi1, c , and Norzilawati Mohamad1, d
1 VSE Research Laboratory, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra (Jalan Semarak), 54000 Kuala Lumpur, Malaysia
Keywords: magnetorheological elastomer, damping, loss factor, flaky-like, anisotropic, elastomer
Abstract. This paper aims to investigate the damping properties of flaky-like carbonyl iron particle (CIP) magnetorheological elastomer (MRE). The damping properties of MRE is mainly dependent on the strength of magnetic field. Anisotropic MRE was fabricated under various magnetic fields strength (70, 210, 345, and 482 mT) and its damping property prior to frequency-dependent was measured using a rheometer. Firstly, the flaky CIP was first synthesized from spherical CIP using a ball-milling method. The microstructure of flaky CIP was observed using low vacuum scanning electron microscope. Subsequently, two types of MREs which are isotropic and anisotropic were fabricated using 70 weight percent (wt.%) of flaky CIP. The experimental results showed that the anisotropic MRE has lower damping factor than isotropic MRE. Meanwhile, the damping factor increases with the increase of frequency.
Recently, materials with high damping ability are desired in various real-time applications specifically in vibration control and noise, automotive vehicle, and structures for civil engineering 1. MRE is one of the promising materials that recently attracted increasing attention due to their unique damping properties. Theoretically, MRE is one of the smart materials where their rheological properties can be changed reversibly, rapidly and continuously by controlling the external magnetic field 2. MRE consists of a non-magnetic rubbery matrix like elastomer, where the magnetic particles such as iron particles are embedded. Furthermore, MRE is a viscoelastic material which has a controllable field-dependent shear modulus. The field-dependent shear modulus is the result of the existence of the field-induced dipole magnetic force between the magnetic particles 3. The particles inside the matrix are uniformly dispersed without the presence of external magnetic field during curing process (isotropic). However, by applying the magnetic field during curing the particles will form chain-like structures to the direction of the field (anisotropic). According to Chokkalingam et al. 4, anisotropic MRE provides larger damping and stiffness properties.
The damping factor can be described as the loss factor (tan ?) that used for characterization of the efficiency of material’s damping properties 5. There are two main sources of damping; one is the damping of the matrix materials itself and the other one is from the interaction between the particles and the matrix 6,7. Materials with low damping have high vibration reduction and better in transferring the vibration 8,9. Lots of studies have been done to investigate the stiffness and damping properties of MRE 3,10,11. However, the previous study only limited in investigating the stiffness and damping properties of MRE consists of spherical shaped CIP. Some of them, have done a modification on the magnetic particles to improve the adhesion of particle-matrix interaction. For example, Chen et al. 12 studied the surface modification of the CIP in MRE towards vibration absorbing performance. In their study, they indicated that sol-gel method is an efficient way to develop the MRE with excellent mechanical performance with higher MR effect and low loss factor. Recently, researchers found out that, by altering the shape of the CIP from spherical-like (SL) to flaky, some properties like microwave-absorbing can be improved as the flaky particles have an easy plane anisotropic structures 13. Furthermore, according to Zheng et al. 14 flaky shape and size are important parameters for absorbing property. However, there are limited study on the rheological properties related to the flaky MRE. Therefore, this paper aims to study the frequency-dependent of the flaky MRE along with the effect of the magnetic field strength during curing. The rheological properties were investigated by using a rheometer.
High energy milling on the CIPs. The spherical CIP supplied by BASF Company, German with an average diameter of 6 µm were used as magnetic particles. To synthesize the flaky shaped CIPs, the SL CIPs were undergone milling process using rotary ball mill. It involves two steps which are pre-mill and milling. Firstly, the spherical CIPs were pre-milled for 10 hours with the ball-to-powder weight ratio of 10:1. Next, pure ethanol (2.5 vol%) was added as the control agent to avoid particle adhesion and to improve the efficiency in producing flaky shaped CIP. The ball-to-powder ratio for the second step is 20:1 and the milling time is further for another 40 hours with speed of 380 rpm.
Fabrication and characterization of MRE. The room temperature vulcanization (RTV) silicone rubber, NS625 was used as a non-magnetic matrix. The silicone rubber was mixed thoroughly with 70 wt. % of flaky CIPs by using a mechanical stirrer for 20 minutes at room temperature in order to obtain a uniform mixture. Subsequently, the curing agent was added to the mixture with the ratio of 20:1 and stirred for another 2 minutes. The mixture was poured into a customized mould with a thickness of 1 mm in the presence of magnetic field. The strength of the magnetic field was varied from 0, 100, 300, 500 and 700 mT. A Low Vacuum Scanning Electron Microscopy (JEOL JSM-IT300) was used to observe the morphology of the flaky CIP. An oscillatory shear test was conducted using a rotational Rheometer (MCR 302, Anton Paar). The damping properties of flaky-like anisotropic MRE was measured using a rheometer at a dynamic strain of 0.02%, a constant frequency of 1 Hz and under the various magnetic flux densities start from 0 up to 800 mT.
Result and discussions
Morphology observation. Figure 1 illustrates the morphology of the CIP particles before; (a) spherical and after milling process; (b) flaky CIP, for 40 hours. Obviously, the particles are flattened which means that the particles are gradually grounded into flaky shaped. According to Han et al. 13, the flaky particle has a higher complex permeability that makes it as a good candidate for absorbing microwave applications.
Figure 1. Morphologies of carbonyl iron particles (a) spherical, (b) flaky-like.
Rheological properties. Figure 2 shows the frequency dependent of the loss factor (tan ?) for spherical (Iso-SL) and flaky (Iso-PL) isotropic MRE. It can be seen, loss factor increases with the increasing of the frequency. MRE with flaky particles, Iso-PL has lower loss factor than Iso-SL MRE. The maximum loss factor achieved for Iso-SL and Iso-PL is 0.210 and 0.186 respectively at the frequency, 100 Hz. This result indicates that the flaky CIP has better interfacial force between particles and matrix that reduce the friction between the particles and matrix.
Figure 2. A relationship between loss factor and frequency for Iso-SL and Iso-PL MRE.
Figure 3 shows the frequency dependent of the loss factor (tan ?) for isotropic MRE and anisotropic MRE with the same particles content of 70 wt.%. Anisotropic MRE was cured under magnetic field of 345 mT during crosslinking process. The results indicate that the loss factor decreases sharply at low frequency at 1 Hz for both isotropic and anisotropic MRE. Then the value increases gradually with the further increasing of frequency. In addition, the loss factor of isotropic is larger than the anisotropic MRE for the entire frequency range. The maximum loss factor achieved is 0.186 and 0.090 for isotropic and anisotropic MRE, respectively as the frequency is 100 Hz. It shows that the loss factor for isotropic MRE which is cured at 0 mT seems to linearly increases with the increases of frequency. This phenomenon is due to the weak bonding between the particles and matrix that causes the relative motion and friction between particles and matrix to increase 15. The anisotropic MRE has small loss factor than isotropic MRE due to the stronger bond between the particles and matrix resulting in smaller friction indirectly leads to less energy dissipation 16.
Figure 3. A relationship between loss factor and frequency for isotropic and anisotropic flaky MRE.
Figure 4 presents the variation of loss factor (Tan ?) over frequency for flaky MRE cured under different magnetic field strengths. For anisotropic MRE, as the frequency increases, the loss factor of MRE increase gradually. This is because, after the sample has been cured with an external magnetic field, the particles will form chain-like structures and strengthen the interaction force between the particles and the matrix. According to Benxiang et al. 17, by curing the MRE sample under external magnetic field, the magnetic particles tend to agglomerate in the matrix and cause the matrix to restrained by the particles. The restrained particle chains will reduce the friction between the particles and matrix, thus decrease the energy dissipation. In addition, the loss factor decreases with the increasing of magnetic field strength. This is due to the degree of magnetic particles agglomerate between the particle and matrix increases that lead to smaller loss factor.
Figure 4. A relationship between loss factor and frequency for anisotropic flaky MRE cured under differences magnetic field during curing.
The influenced of magnetic field strength on the damping properties in term of frequency of flaky anisotropic MRE was studied. The results showed that the loss factor of spherical-like MRE is lower than the MRE with flaky particles. The maximum peak loss factor is 0.210 and 0.186 for Iso-SL and Iso-PL MRE respectively. Moreover, the damping property of anisotropic flaky MRE is lower than the isotropic one. It showed that the MRE sample cured with the presence of magnetic field had a better particle-matrix bonding that reduces the friction force between the particles. A further study with more focus on a comprehensive study on the rheological properties of flaky shaped MRE is therefore suggested.