*2.2. Soft-Magnetic Particles*

MREs are fabricated by adding high magnetic permeability particles to a viscoelastic material [49]. As such, the magnetic particles added to the MR elastomer play an important role in the MR effect. In general, the conditions of good magnetic particle for MR materials require high saturation magnetization, low remnant magnetization, and high, short-term inter-particle attraction. Of the many magnetically permeable particles, the CI which was discovered by BASF in 1925 produced by thermal decomposition of iron pentacarbonyl (Fe(CO)5) is the most studied and used as MR particle [50–53]. The reason is that as one of the soft-magnetics, it has the advantage of high saturation magnetization, easy magnetization and demagnetization, and no magnetic hysteresis. Because of these properties, it has been used in many fields such as MRFs [54], MR foams [55], and MR greases [56]. CI particles are soft magnetic microparticles being categorized in two groups based on their shape, which are spherical

and flake-typed with a size of 1–7 μm and 5–50 μm, respectively [50,54]. In the case of MRF, due to the sedimentation problem caused by high density difference between medium oil and CI particles, their sedimentation problem has been improved by making their density relatively low by being coated with several organic materials. For example, coating polymers such as poly(methyl methacrylate) (PMMA) [6], PS [7], polyaniline [44], and polycarbonate [57] not only reduce the density of CI particles but also prevent chemical oxidation of CI surfaces. However, unlike MRFs, particle sedimentation is not a problem for MREs. The problem with MREs is the compatibility between the magnetic particles and the elastomeric medium. The reason is that CI particles are hydrophilic, whereas elastomer matrices are generally hydrophobic. Therefore, many studies have been conducted to increase the surface compatibility of the CI matrix with the elastomer matrix.

Among various surface modification methods, the treatment of CI particles with silane coupling chemicals is known to be a very economical and efficient way to increase the affinity between the CI particles and the medium [58]. An et al. [58] studied (3-aminopropyl) triethoxysilane (APTES) modified CI particles to improve compatibility with the elastomer matrix (Figure 2b). They fabricated a natural rubber based anisotropic MRE using CI particles surface modified with APTES. And they measured the properties of MRE according to the surface treatment. In order to coat the CI surface with APTES, hydrochloric acid was used for the pre-treatment. This enables the CI surface to be coated by activating the OH group to polymerize APTES onto the CI surface. Figure 2b is a SEM image, showing that the surface of the CI has a rough surface by APTES coating. When CI is coated with APTES in this way, it has NH2 group on the surface of CI, making CI particles to have a good affinity with natural rubber.

**Figure 2.** SEM image of (**a**) pure carbonyl iron (CI), (**b**) CI/(3-aminopropyl) triethoxysilane (APTES) [58], (**c**) CI/poly(methyl methacrylate) (PMMA) [59], (**d**) CI/poly(glycidyl methacrylate) (PGMA) [60], (**e**) CI/poly(fluorostyrene) [61], (**f**) CI/poly(trimethylsilyloxyethyl methacrylate) (PHEMATMS) [62].

Many studies have been carried out on coating CI particles with PMMA and applying them to MRFs [6,63]. The PMMA-coated core–shell structured CI particles have a lower density than neat CI particles, resulting in better dispersion stability in the fluid. Li et al. [59] have applied CI/PMMA core–shell particles to MREs rather than MRFs (Figure 2c). The rough surface represents how the CI particles were covered by PMMA particle. Fabricating MREs with CI/PMMA core–shell particle results in lower Payne effects by increasing bond strength between matrix and particles. CI particles were coated with PMMA by emulsion polymerization. First, the surface of the CI particles was activated with acetic acid and then dispersed in distilled water. Sodium lauryl sulfonate is used as a stabilizer to

suspend the CI particles, and the CI surface is coated with PMMA using MMA as a monomer and ammonium persulfate as an initiator. As shown in the Figure 2c, PMMA coated CI surface can be confirmed. Using the MRE made of CI/PMMA core–shell particles, the compatibility between particle and matrix is increased and the relative motion between particle and medium is reduced, resulting in smaller and more stable loss factors.

Kwon et al. [60] coated poly(glycidyl methacrylate) (PGMA) on CI particles and applied them to MREs and studied their properties (Figure 2d). The surface of the CI/PGMA particles was observed to be rougher than those of the pristine CI particles. The process of producing core–shell particles using the dispersion polymerization method has the advantage of being simple in a single step process. Due to the hydrophobicity of PGMA, coating PGMA on magnetic particles has the advantage of increasing chemical a ffinity between the CI particles and the elastomer. PGMA coated CI particles increase the hardness of the MRE matrix, resulting in lower MR e ffects than MREs made from pure CI. PGMA coatings enhance the bond strength between the CI particles and the elastomeric medium, reducing the loss tangent.

Fuchs et al. [61] researched the coating of poly(fluorostyrene) on CI particles with the ATRP method and applied them to MREs (Figure 2e). By coating poly(fluorostyrene) on the CI surface, it was possible to prevent the oxidation of magnetic particles, one of the potential problems with MRE. It prevents mechanical property from decreasing due to the rapid oxidation of magnetic particles, making it possible to apply for a longer time as a vibration isolator.

Martin et al. [62] reported the characteristics of MREs by coating poly(trimethylsilyloxyethyl methacrylate) (PHEMATMS) on the surface of CI particles, in which the CI-g-PHEMATMS particles showed about 6.2% reduction in magnetization than neat CI particles due to the 10–20 nm coating thickness. However, it was possible to improve thermo-oxidation stability and anti-acid/corrosion properties through polymer coating. In the case of neat CI, wettability is low, which brings a strong reinforcing e ffect to the MRE. However, when CI is coated with PHEMATMS, mobility is improved inside the elastomer matrix, creating a plasticizing e ffect. As a result, MRE with CI-g-PHEMATMS particles shows lower G' and higher G" than MRE with neat CI. This results in a higher damping factor. In addition, the increase in particle mobility increases the relative MR e ffect, which is expected to be a more practical application.

In addition to CI particles, the e ffects of many types of magnetic particles such as CoFe2O4 [38], Ni [39], FeCO3 [40], and manganese zinc ferrite [41] have been studied. Siti et al. [42] fabricated MREs using CI and nano-sized Ni-Mg cobalt ferrites based on SR, and studied various properties including field-dependent viscoelastic characteristics of MREs. The experimental results showed that the G' and loss factor were increased in MRE containing Ni-Mg cobalt ferrite nanoparticles. This means that Ni-Mg cobalt ferrite nanoparticles increase the interaction between MRE and particles. On the other hand, Nordalila et al. [64] manufactured MRE using industrial waste nickel-zinc ferrite. Experiments were conducted to compare the degree of swelling according to the content of nickel-zinc ferrite. The thermal analysis of MRE and the MR e ffect of anisotropic and isotropic MREs were also performed.

Recently, in addition to these soft magnetic particles, studies have been made on MREs using hard magnetic fillers [21,65–67]. Conventional soft magnetic particle-based MREs aimed at maximizing the MR e ffect. But the MR elastomer fabricated with hard magne<sup>t</sup> behaves like a flexible permanent magnet. Due to these characteristics, hard magnet-based elastomers have been applied to other applications. When an input magnetic field is present, hard magnetic particles cause rotational motion, which can cause motion and force like a magnetic field-controlled actuator. Koo et al. [21] studied the actuation properties of hard magne<sup>t</sup> MRE (H-MRE) using barium hexaferrite, strontium ferrite, samarium cobalt, and neodymium magnet. It o ffers the possibility that MREs with a hard magne<sup>t</sup> base can be used as magnetically controlled actuators. Therefore, it can be noted that MREs were studied by applying various magnetic particles from soft magnetic to a hard magnet.
