Synthesis And Rheology Of Magnetorheological Fluids Based On High Viscosity Linear Polysiloxane With Focus On Sedimentation

Magnetorheological fluid(MRF) mainly consists of micro-sized magnetic particles, carrier fluid, and additives. With a magnetic field, MRF varies from fluid to semi-solid phases. The particles form into magnetic chains very rapidly and reversibly with remarkably changing the rheological property of MRF. Such benefits motivate MRF for applications like seismic and automobile dampers, jet polishing etc. MRF is not widely commercialized currently, where a key reason is the settling issue caused by a large density difference(~7 times) between the particles and carrier. The sedimentation of MRF deteriorates the performance and can be a severe issue after long-term downtime. Although methods like using additives, particles surface treatment, and raising carrier viscosity were taken to improve the stability, the issue has not been solved very effectively. Based on Stokes law, this study proposes high viscosity linear polysiloxane(HVLP) as the carrier of MRF(i.e., HVLP MRF), which holds a much higher stability than traditional MRFs as it is able to show no sedimentation for at least 365 d.MR materials can be classified into three types, i.e., MR fluids, elastomers and gels, among which the emerging MR gels are not studied sufficiently. HVLP MRF is essentially a MR gel, which features that the very high zero-shear viscosity of HVLP enables high sedimentation stability, and the significantly reduced viscosity at high shear rate helps to maintain a good MR effect. Besides studies on the rheology under various conditions and verification of long-term stability of HVLP MRF, Kynch theory was adopted for giving profound insights into sedimentation characteristics of MRF.(1) Three HVLPs with different zero-shear viscosities were synthesized, as well as batches of HVLP MRFs for studies on different topics. Besides, a group of traditional MRFs were prepared for its sedimentation study.(2) MR gels usually present different rheological behaviors from general MRFs. For HVLP MRF, the helically entangled structure of HVLP is a key factor for the special behaviors, such as the magnetized layer structure, zero-field yield stress, and the post-yield transitional region. With study on rheology of HVLP MRF at low shear rate(0.1~400 s-1), these behaviors are considered an outcome from the superimposition effect of rheology of HVLP and a coupling effect between the polymer and magnetic chains. The shear thinning of HVLP was verified and illustrated by the microstructural deformation behavior of polymer chains under varying shear rates. Besides, the helical structures of HVLP leads to a compressibility that a 20 wt% HVLP MRF showed a compression ratio of 20.51% at 141.5 MPa. This study focuses on the effects of particle concentration, HVLP viscosity and temperature on the rheology of HVLP MRF. The yield stress could be 3.2 times that of Lord MRF. HVLP MRF has a good temperature stability so that the yield stress at 25? was able to maintain about 80% of that at 70?.(3) High shear test is a simulation of many MRF applications. The behavior of HVLP MRF at high shear rate(103~104 s-1) was studied by using a concentric rotational cylinder viscosimeter. Three factors were considered:(a) particle concentration,(b) comparison with Lord MRF, and(c) rheological differences under high and low shear rates. The results showed that: zero-field yielding and shear thinning behaviors existed at high shear rate, and the yield stress was still much greater than that of the Lord MRF.(4) Testing and analysis of sedimentation of MRF were carried out using a vertical axial inductance monitoring system(VAIMS). The particle concentration in the entire MRF column was scanned by the VAIMS. The evolution of sedimentation zones versus time was obtained to give a direct view of the MRF settling process. The comparisons between three settling models proved Vesilind model the best for settling velocityconcentration relationship. Based on Kynch theory, concentration propagation velocity(CPV) was deduced that gives profound insights into the MRF sedimentation. Finally, the obtained solids flux provides a theoretical support for development of more stable MRFs, e.g., the particle concentration should be at least 21 vol%.(5) The helically entangled structure of HVLP is also a key contribution to the sedimentation stability of HVP MRF. Another truth is that HVLP has linear polymer chains with length up to 1~4 ?m close to the particle size(1~10 ?m). The mechanisms can be stated as:(a) HVLP linear chains form into stable spatial micro-entanglements with particles;(b) it is the very high zero-shear viscosity of HVLP, or the zero-field yield stress of HVLP MRF that leads to the high stability. Using VAIMS, long-term(365 d) stability of HVLP MRF was verified by tests. The results showed that, without additives:(a) 32 vol% HVLP MRFs showed no sedimentation when HVLP zero-shear viscosity was higher than 140 Pa·s, and(b) HVLP MRFs with HVLP zero-shearv iscosity of 440 Pa·s showed no sedimentation when particle concentration was greater than 20 vol%.