Magnetorheological (MR) materials are a group of smart materials
whose rheological properties can be controlled by the application of an
external magnetic field. Up to now two main kinds of MR materials have
been reported. MR fluids and MR elastomers. MR fluids are well-known
smart materials, which exhibit Newtonian-like behavior in the absence of
a magnetic field and become a weak viscoelastic solid with a certain
yield stress when applying a magnetic field . However, the settlement
of magnetic particles has been a serious problem that affects the
performance of MR fluids.
MR elastomers can be thought of as a new generation of MR materials
that has distinct properties from those of MR fluids. They have
controllable field-dependent modulus while MR fluids have a
field-dependent yield stress. The obvious advantage of MR elastomers is
that the particles are not able to settle with time and then have stable
MR performance, MR elastomers also have a sensitive response to magnetic
fields and the time of response of MR elastomers is less than 10 ms .
MR elastomers hold promise in adaptive tuned vibration absorbers,
stiffness tunable mounts, automobile suspensions , and artificial
MR elastomers are mainly composed of magnetic particles and elastic
polymer matrix. The magnetic particles used are usually carbonyl iron
particles and the reported polymer matrix included soft silicone
elastomers [4-6], poly (vinyl alcohol) , gelatin , hard natural
rubber , and RTV polyurethane sealant . In general, magnetic
particles are first embedded in the uncured polymer and then the mixture
is cured under a strong magnetic field. Thus, the magnetic particles
form special chainlike structure in the direction of the magnetic field,
which results in the controllable shear modulus with the magnetic
fields. This kind of MR elastomers can be called anisotropic MR
elastomers. Up to now, the reported best relative effect for MR
elastomers based on silicon rubber can be up to 60% ; however, their
mechanical properties are poor. And that of the MR elastomers based on
natural rubber is about 30% and the absolute MR effect is ~0.7 MPa at a
magnetic field of 0.8 T . But the typical needed magnetic field is
very strong at about 8 X [10.sup.6] A/m , which makes its manufacture
complicated and difficult to be broadly applied.
Recently, Lokander and Stenberg [12, 13] studied the isotropic
magnetorheological rubbers that are prepared without applying magnetic
fields. They found that isotropic magnetorheological elastomers using
large irregularly shaped iron particles had a large absolute MR effect
that is about 0.4 MPa at 0.24 T. In the previous work, the new MR
elastomers based on PU/Si-rubber hybrid were studied . The maximum
increase in shear modulus of this kind of MR elastomers can be up to 0.5
MPa at 0.2 T and the relative MR effect is higher than that reported in
Isobutylene-isoprene rubber (IIR) has good thermal and oxidative
stability, chemical resistance, and high damping and has been applied
broadly, especially in vibration absorbers. To make practical MR
elastomers useable for adaptive tuned vibration absorbers, in this
paper, the isotropic MR elastomers are prepared using IIR as the matrix.
The MR effect, mechanical properties, and thermal properties of the MR
elastomers are studied here in detail.
The used spherical carbonyl iron particles are commercial FTF-4
type with the size range of 3-5 [micro]m bought from Hebao Nanomaterial.
IIR, carbon black (N660) and the other corresponding additives were
provided by Grandtour Tire (Anhui).
Magnetorheological (MR) elastomers based on isobutylene-isoprene
rubber (IIR) with different volume contents of carbonyl iron particles
were prepared by the common manufacturing procedure of rubber. Carbonyl
iron particles were mixed into the rubber together with vulcanization
system in a two-roll mixer. Then the mixtures were vulcanized at
175[degrees]C for 10 min. To compare the mechanical properties, a series
of samples containing carbon black instead of iron particles with the
same volume content were also prepared under the same conditions.
Tensile tests were performed on a Universal testing machine (WD-5D,
Changchun, China) with a crosshead speed of 50 mm/min at 25[degrees]C.
The average of five tests was reported here.
The magnetorheological effect was evaluated by measuring the
dynamic shear modulus with and without an applied magnetic field. The
schematic equipment set-up and the principle of the tests can be seen in
The magnetic field was made by electromagnet, and the magnetic
induction through samples was kept at 0.24 T. The samples tested were 30
X 10 X 2 [mm.sup.3] and sandwiched between a brass plate and an aluminum
The morphologies of MR elastomers with different volume contents of
carbonyl iron particles were observed using an XL30 ESEM at an
accelerating voltage of 10 kV. All the samples were coated with a thin
layer of gold before SEM observations.
The electrical resistance of MR elastomers was measured using plate
electrode of ZC-36-type high resistance meter and the corresponding
resistivity was calculated according to the instructions. For resistance
measurements, the quadrate samples with dimensions of 10 X 10 X 2
[mm.sup.3] were prepared.
The samples were analyzed by thermogravimetric analysis (TGA),
using a Netzsch STA-409c thermal analyzer under air flow from 25 to
600[degrees]C at the rate of 10[degrees]C/min.
RESULTS AND DISCUSSION
Magnetorheological Properties and Microstructure
Figure 1 shows the relative MR effect of MR elastomers with
different volume contents of iron particles. Here, the relative MR
effect meant the relative change of shear modulus of MR elastomers. It
can be seen that the MR effect increases in the beginning and decreases
when the content of iron particles is 15%.
It was considered that for isotropic MR elastomers, the change of
shear modulus was due to the deformation of MR elastomers under magnetic
fields. And the deformation was the accumulative results of that of
every part of matrix resulted from the dragging of iron particles to the
direction of magnetic fields. When the content of iron particles was
less, the accumulative results were less which resulted in the small MR
effect. With the increase of iron particles, the MR effect would improve
accordingly. However, up to a certain content of iron particles a
network of iron particles formed in the matrix. The modulus mainly
depended on the network and the potential of deformation under magnetic
fields was little with little content of IIR matrix and contributed
little to the change of modulus. So, the MR effect would decrease at a
certain content of iron particles.
Figure 2 shows the microstructure of MR elastomers based on IIR
with different contents of iron particles. It can be seen that when the
content of iron particles surpassed 15%, a network of iron particles had
formed in the matrix. To further prove the existence of the network of
iron particles, the volume resistivity of MR elastomers with different
contents of iron particles was tested in Fig. 3. After 15 vol%, there
was a rapid decrease of volume resistivity that indicated the formation
of a connected network throughout the bulk of the rubber matrix. So,
according to the assumption mentioned earlier, the trend of MR effect
changing with the content of iron particles was understandable.
The mechanical properties including tensile strength, hardness, and
elasticity of MR elastomers with different contents of iron particles
were studied and compared with the same content of carbon black. Figure
4 shows the variation of tensile strength with different contents of
fillers. It can be seen that the tensile strength of MR elastomers
increased with the increase of iron particles until the content of 27%.
However, their tensile strength was far smaller compared with those with
the same content of carbon black. At 15 vol% of fillers, the tensile
strength of IIR containing carbon black was about four times of that of
MR elastomers. This might be due to the larger size of carbonyl iron
particles and the weak interaction with the matrix for their polar
surface properties. The weak interaction with the matrix can be seen
from the microstructure in Fig. 2. The gaps between iron particles and
matrix can be seen clearly. So, it can be concluded that if MR
elastomers were to be put into application, the reinforcement of the
material should be considered.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Figure 5 shows the variation of elongation at break with the
content of filler. It can be seen that the elongation at break of IIR
filled with carbon black had a maximum at 15 vol%, but that of MR
elastomers increased always with the content of iron particles and the
elongation was larger at the same volume content. This further proved
that the interaction between iron particles and IIR matrix was weak and
could not prevent the movement of molecular chains.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Figure 6 shows the dependence of shore hardness on the filler
content. The curve reveals that the hardness increases almost linearly
with the content of fillers and the degree of increase of hardness of MR
elastomers is still smaller than that of IIR containing carbon black.
Figure 7 shows the variation of elasticity with the content of
fillers. It can be seen that the elasticity of IIR is low and it
decreases with the content of fillers. And at the same volume content,
the value of elasticity of the MR elastomers was almost the same with
that of IIR elastomers containing carbon black. It can be deduced that
the elasticity mainly depended on the content of IIR matrix and had less
relationship with the size and properties of fillers.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Thermal stability is an important factor which affects the service
life of rubber. The effect of iron particles on the thermal properties
of IIR was investigated by TG analysis and the result can be seen in
Figure 8 shows that the mass of pure iron particles was kept
constant within tested temperature in [N.sub.2] atmosphere, and so the
mass loss of MR elastomers was entirely due to the thermal degradation
of IIR. The temperature at 5% mass loss of IIR of MR elastomers with
different contents of iron particles and that of samples for comparison
are listed in Table 1. It can be seen that the temperature increases
with the sequence 5% CB < 5% Fe < 15% Fe, which indicates that IIR
containing iron particles had better thermal properties compared with
IIR filled with carbon black and the thermal stability was improved by
increase of iron particles.
MR elastomers based on IIR rubber were prepared by the common
manufacturing procedure of rubber. The MR effect varied with the content
of iron particles and a maximum of 20% in MR effect at 15 vol% was
obtained. The relationship between MR effect and microstructure was
discussed in detail.
The mechanical properties including tensile strength, hardness, and
elasticity were also studied. The results showed that tensile strength
increased with the content of iron particles; however, the tensile
strength was far lower than that of IIR filled with the same content of
carbon black due to their larger size and weak interaction with the
matrix. The hardness increased with the content of iron particles and
the elasticity decreased with the content of iron particles.
TGA measurements suggested that the thermal stability of MR
elastomers based on IIR can be improved by increasing the content of
The authors would like to thank Dr. M. Gong and Mr. F. Yu (Lab of
Mechanical and Material Science of USTC) for their attributions to the
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Yinling Wang, Yuan Hu
State Key Laboratory of Fire Science, University of Science and
Technology of China, Hefei 230026, People's Republic of China
Yinling Wang, Huaxia Deng, Xinglong Gong, Peiqiang Zhang
CAS Key Laboratory of Mechanical Behavior and Design of Materials,
Department of Mechanics and Mechanical Engineering, University of
Science and Technology of China, Hefei 230027, People's Republic of
Wanquan Jiang, ZuyaoChen
Department of Chemistry, University of Science and Technology of
China, Hefei 230026, People's Republic of China
Correspondence to: Y. Hu; e-mail: firstname.lastname@example.org; X. L. Gong:
Contract grant sponsor: Chinese Academy of Sciences and Specialized
Research Fund for the Doctoral Program of Higher Education; contract
grant numbers: 2001 vCB409600, 20030358014.
TABLE 1. Temperature at 5% mass loss ([degrees]C).
Sample Temperature at 5% mass loss
5% CB 347.4
5% Fe 359.7
15% Fe 375.4