Despite its wide application as a suspended particle in magnetorheological (MR) fluids along with its extensive investigations on MR characteristics and dispersion stability, the pristine carbonyl iron (CI)-based MR fluid still possesses several drawbacks, including severe sedimentation of the CI particle due to its high density. To resolve these problems, we prepared poly(methyl methacrylate) (PMMA) coated-CI composite particles, possessing different shell thicknesses onto iron particles. The polymer coated composite magnetic particles (CIPMMA) were synthesized via an in-situ dispersion polymerization method using CI dispersions in methyl methacrylate monomer, and then adopted as dispersed phase of MR fluids. The CIPMMA was found to potentially resolve several problems of CI only based MR fluids such as severe sedimentation problem and poor dispersion stability. Flow and viscoelastic properties of the MR fluids were analyzed via a rotational rheometer equipped with a magnetic field suppler using measuring system of a parallel plate in both steady and oscillatory shear modes. Main focus was put on studying the effect of shell thickness in the internal structure formation and sedimentation characteristics. The MR characteristics including a yield stress were found to be affected by the CI contents in the composite particles.
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