Fabrication and dynamic viscoelastic properties of MR elastomers with silicone oil

Abstract

Isotropic and anisotropic silicone rubber-based magnetorheological (MR) elastomers with various concentrations of silicone oil were fabricated, and their dynamic viscoelastic properties were measured. The tested results showed that the silicone oil played an important role in the MR elastomers. The sample with a higher concentration of silicone oil had lower zero-field storage modulus and larger storage modulus increasing rate. For the MR elastomer without silicone oil, the anisotropic sample showed higher storage modulus increasing rate than its isotropic counterpart. However, for the MR elastomers with 15% weight fraction of silicone oil, the isotropic sample dominated in the storage modulus increasing rate.

Keywords

Magnetorheological (MR) elastomer silicone rubber silicone oil dynamic viscoelasticity storage modulus

1. Introduction

Magnetorheological (MR) Elastomers are smart materials whose rheological properties can be controlled by the applied external magnetic field [1,2]. Under a stronger applied magnetic field, the MR elastomers perform a higher stiffness [3]. Because of their unique properties, MR elastomers have recently been used in a variety of applications, such as adaptive tuned vibration absorbers, dampers, sensors and so on [4–6].

The main components of MR elastomer were magnetically polarised particles, non-magnetic solid or gel matrix, and extra ingredients [3]. Iron particles were used as the filler materials for the MR elastomers due to their high permeability, low remnant magnetisation and high saturation magnetisation [7]. The size of iron particles played a significant role in MR elastomers. Böse and Röder [8] found that the storage modulus increasing rate of the MR elastomer with 40 μm iron particles showed an order of magnitude higher than that with 5 μm iron particles. Particle shape of the polarised particles was also important in MR elastomers [9]. Song et al. [10] compared the MR elastomer with spherical particle ranged from 6–10 μm and the Fe nanowires with 15 μm length and approximate 300 nm diameter, respectively. They found that the off-state dynamic stiffness of the 30 wt% nanowire-based MR elastomer was 3.5 times of that of the 30 wt% sphere-based MR elastomer. The matrix of the MR elastomer was usually natural rubber, silicone rubber or polydimethylsiloxane (PDMS) [11]. The extra ingredients, such as silicone oil and graphite powder [9] are commonly used to adjust the mechanical and chemical properties, or electrical performance of MR elastomer. Previous studies [2,9] proved that the addition of silicone oil could effectively enhance the stiffness change of MR elastomers.

MR elastomers can be categorised into two groups, namely isotropic and anisotropic MR elastomers, which attribute to different alignments of polarised particle inside the MR elastomers. In isotropic MR elastomers, the polarised particles disperse randomly in the matrix, so this kind of MR elastomer shows homogeneous performance in all directions [12]. For anisotropic MR elastomer, the polarised particles were arranged during the fabrication process to align with the direction of the applied magnetic field, which was usually perpendicular to the flat sample of MR elastomer [9]. When the anisotropic MR elastomers were measured, the shear directions were perpendicular to the iron particle alignment in most of the cases [9,13].

In this paper, authors will fabricate both isotropic and anisotropic silicone rubber-based MR elastomers with various weight fractions of silicone oil (0%, 5%, 10% and 15%). SEM observation is applied to show the iron particle dispersion in the samples. The dynamic viscoelastic properties of the MR elastomers are measured by a rheometer under pure shear. Experimental results confirm that the silicone oil plays a very important role in both isotropic and anisotropic MR elastomers. The sample with higher silicone oil concentration shows a lower zero-field storage modulus and a higher storage modulus increasing rate.

2. Fabrication and measurement of MR elastomers

The components for fabricating the silicone rubber-based MR elastomers are carbonyl iron particles, silicone rubber, and silicone oil. Carbonyl iron particles (CIP CS, BASF) are spherical and 6.0–7.0 μm in diameter. Silicone rubber is made from a base material and curing agent (KE-1241 and CLA-9, Shin-Etsu Chemical Co. Ltd.) at 10:1 weight ratio. Silicone oil (378364, Sigma-Aldrich Co. Ltd.) has a viscosity of 0.0001 m²/s at room temperature. In the fabrication process, carbonyl iron particles, silicone rubber and silicone oil at certain concentrations were mixed and stirred sufficiently in a beaker; then the mixture was placed in a vacuum to eliminate the involved air bubble. The vacuumed mixture was poured into a nonmagnetic mould with the internal dimension as 12 mm width, 2 mm depth, and 65 mm length. Aiming to have the isotropic MR elastomers, the mould was kept rotated by a servo motor at 60 rpm to avoid the settlement of the iron particles. For anisotropic samples, the mould was placed in a magnetic field at 0.3 T. For both isotropic and anisotropic MR elastomers, the mould was heated with a heat gun to 80 °C for 60 minutes to accelerate the curing. The components of MR elastomers with various silicone oil and silicone rubber concentrations are summarised in Table 1. The abbreviations ISO and ANI in the sample names represent the isotropic and anisotropic MR elastomers, respectively. After the fabrication, an SEM was employed to observe the microstructure of both isotropic and anisotropic MR elastomers, which is shown in Fig. 1. In Figs 1a and 1b, the carbonyl iron particles disperse evenly in the matrix because isotropic MR elastomers were fabricated without a magnetic field. In the anisotropic samples (see Figs 1c and 1d), the iron particles form aligned structure along the direction perpendicular to the MR elastomer sheet surface. This is due to the magnetic field applied to the MR elastomers in their curing process.

Table 1
Concentrations of MR elastomers

Sample type Carbonyl iron Silicone rubber Silicone oil

Isotropic Anisotropic wt% vol% wt% vol% wt% vol%

0%ISO 0%ANI 70 26.29 30 73.71 0 0.00

5%ISO 5%ANI 70 25.51 25 59.59 5 14.90

10%ISO 10%ANI 70 24.77 20 46.30 10 28.93

15%ISO 15%ANI 70 24.07 15 33.75 15 42.18

Sample type	Carbonyl iron	Silicone rubber	Silicone oil
0%ISO	0%ANI	70	26.29	30	73.71	0	0.00
5%ISO	5%ANI	70	25.51	25	59.59	5	14.90
10%ISO	10%ANI	70	24.77	20	46.30	10	28.93
15%ISO	15%ANI	70	24.07	15	33.75	15	42.18

Fig. 1.

SEM images of isotropic and anisotropic MR elastomers: a, 0%ISO; b, 15% ISO; c, 0% ANI; d, 15% ANI.

Fig. 2.

a, Testing device for MR elastomer; b, Detailed dimension of MR elastomer in testing.

As shown in Fig. 2a, the testing device for MR elastomers mainly consists of a conventional electromagnet to apply a magnetic field to the MR elastomers, a sliding system to oscillate an oscillating plate between the magnetic poles of the electromagnet, and an excitation system to sinusoidally oscillate the sliding system at 0.3 Hz. After the MR elastomer samples had been cured, they were cut to be 50 mm in length. As seen in Fig. 2b, two pieces of MR elastomers of 12 mm in width, 2 mm in thickness, and 50 mm in length were installed to two gaps of 2 mm between two magnetic poles of the electromagnet and the oscillating plate of 1.0 mm in thickness. They were glued by double-sided tape between the surfaces of magnetic poles and the oscillating plate to ensure the perfect connection. So, any sliding or rotation between the MR elastomers and the oscillating plate did not occur, and this ensured the MR elastomers to be measured in pure shear.

3. Results and discussion

All the MR elastomers were tested at the equilibrium state, and a PC was used to collect the data. The hysteresis loops of shear stress versus shear strain for four MR elastomers (0%ISO, 15%ISO, 0%ANI, 15%ANI) at various shear strain amplitudes (10%, 20%, 30% and 40%) are shown in Fig. 3.

Fig. 3.

Hysteresis loops of shear stress versus shear strain for at four shear strain amplitudes under magnetic field ( B = 0. 64 T): (a) 0%ISO, (b) 15%ISO, (c) 0%ANI, (d) 15%ANI.

Fig. 4.

Hysteresis loops of shear stress versus strain for 15%ISO under various magnetic flux densities and at different strain amplitudes: (a) shear strain amplitude ϵ_a = 10%, (b) shear strain amplitude ϵ_a = 40%.

Fig. 5.

Hysteresis loops of shear stress versus strain for all isotropic MR elastomers at various shear strain amplitudes under zero-field ( B = 0 T).

It can be seen that with the increase of applied shear strain amplitude, the hysteresis loops of all the MR elastomers show lower slopes, which represent the lower storage modulus. It is also noted that the hysteresis loops of MR elastomer without silicone (Figs 3a & 3c) are ellipse shape, which means that they were tested within the linear range. However, for MR elastomers with 15wt% silicone oil (Figs 3b & 3d), the shapes of the hysteresis loops are not a perfect ellipse, meaning that these MR elastomers behave non-linearly.

The effect of the applied magnetic field on 15%ISO, as an example, is shown in Fig. 4, where we see that the slope of the hysteresis loops has a higher value under a stronger magnetic field, meaning that the applied magnetic field can effectively enhance the stiffness of MR elastomers. Figure 5 compares the hysteresis loops for isotropic MR elastomer with various silicone oil concentrations. It can be seen that the hysteresis loop of MR elastomer with higher silicone oil has a lower slope, which means the silicone oil softens the matrix of the MR elastomers.

The storage modulus ( G′) can be calculated from the hysteresis loops of shear stress versus shear strain by the theory of rheology [14]. The storage modulus versus magnetic flux density at 10% shear strain amplitude for both isotropic and anisotropic MR elastomers is summarised in Fig. 6. It shows that the storage modulus of MR elastomer has a clear increasing trend with increasing the magnetic flux density, which is the unique property of MR elastomer. The addition of silicone oil contributes to enhancing the storage modulus change in MR elastomers when the magnetic field increases. Figure 6 also shows that the MR elastomer with higher silicone oil concentration has a lower zero-field storage modulus G ′ ₀ at B = 0 T. However, when the magnetic field reaches B = 0. 64 T, the storage modulus of the MR elastomers results in a similar level in spite of the different concentrations of silicone oil in the MR elastomers. For a particular MR elastomer tested at a certain shear strain amplitude, the maximum storage modulus G ′ _max is recorded at the highest magnetic flux density in the testing, which can be used to calculate the maximum increase of storage modulus Δ G ′ = G ′ _max − G ′ ₀. The storage modulus G ′ increasing rate is defined as Δ G ′∕ G ′ ₀, which can be used to state how much the stiffness changes for a particular MR elastomer.

Fig. 6.

Curves of storage modulus G ′ versus magnetic flux density B at 10% shear strain amplitude for the MR elastomer with various silicone oil concentration: (a) isotropic MR elastomers, (b) anisotropic MR elastomers.

Fig. 7.

Curves of G ′ increasing rate Δ G ′∕ G ′ (a) and zero-field storage modulus G ′ (b) versus shear strain amplitude ϵ_a for all MR elastomers.

Figure 7 summarises the G ′ increasing rate and zero-field storage modulus G ′ ₀ versus shear strain amplitude ϵ _a for all the MR elastomers. In Fig. 7a, it is seen that the G ′ increasing rate of all MR elastomers decreases with increasing the shear strain amplitude, and the silicone oil contributes to enhancing the G ′ increasing rate for isotropic and anisotropic MR elastomers, respectively. It also shows that the different types of particle alignment inside the MR elastomers play an important role to the dynamic viscoelastic properties of MR elastomers. For MR elastomers with 0% of silicone oil, the G ′ increasing rates of the anisotropic and isotropic samples are 63% and 18%, respectively. However, for MR elastomers with 15% weight fraction of silicone oil, the G ′ increasing rate of isotropic samples is 596%, which is higher than that of the anisotropic sample as 479%. This means that without silicone oil, the anisotropic MR elastomers show higher G ′ increasing rate than the isotropic samples; however, with high concentrations of silicone oil, the isotropic sample’s G ′ increasing rate dominated. Figure 7b shows that the silicone oil has a great impact on reducing the MR elastomers’ storage modulus. With a lower zero-field storage modulus, the G ′ increasing rate would reach a higher level for the MR elastomers with higher silicone oil concentration.

4. Conclusion

In this study, both isotropic and anisotropic silicone rubber-based MR elastomers with various concentrations of silicone oil were fabricated. Then the MR elastomers were tested to measure the dynamic viscoelastic properties in an oscillatory pure shear mode at different shear strain amplitudes and under different magnetic flux densities. The testing results indicated that the silicone oil contributed to lower the MR elastomers’ zero-field storage modulus G ′, and it also helped to enhance the G ′ increasing rates of both isotropic and anisotropic MR elastomers.

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