Poly(methyl methacrylate)‐coated carbonyl iron particles and their magnetorheological characteristics

Magnetorheological fluids (MRFs) are types of suspensions that contain magnetic particles and a carrier fluid, and are considered as semi‐active smart materials. By tuning the strength of an external magnetic field, like other traditional MRFs, a carbonyl iron (CI)–poly(methyl methacrylate) (PMMA) particle‐based MRF can change reversibly from a fluid‐like state to a solid‐like state within milliseconds. In the research reported, CI particles were encapsulated with PMMA via emulsion polymerization. After the polymerization, the fabricated CI–PMMA composite particles were dispersed in a suspension medium to prepare MRF. The synthesized CI–PMMA composite particle‐based MRF showed a shear stress of 60 kPa at the magnetic field strength of 0.6 T, and a greatly enhanced anti‐sedimentation stability. Copyright © 2010 Society of Chemical Industry     

Compressible magnetorheological fluids

A novel compressible magnetorheological fluid (CMRF) has been synthesized with additives that provide compressibility to the fluid. This CMRF has been designed to provide an elastic component to a magnetorheological fluid (MRF) that can be used as a springless damper. CMRF provides controllable compressibility to the MRF. The controllability of the fluid is achieved by the use of magnetic particles and an external magnetic field, and the fluid is made compressible by the addition of suspended compressible polymer particles. The compressibility of the fluid has been characterized with force–displacement measurements. This CMRF has controllable off‐state viscosity and high shear yield stress. The incorporation of polymeric particles into the MRF also decreases the settling of iron particles and improves the redispersion of the fluid. To make the fluid more redispersible, the surface of the iron particles is coated with a high‐temperature fluorinated polymer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

磁流化床稳定性分析

应用Foscolo的颗粒床模型分析了磁流化床稳定性,得到了磁流化床的稳定判据;根据得到的判据分别对层流和湍流情况分析了磁场对磁流化床稳定性的影响。     

Magnetic field sensitive functional elastomers with tuneable elastic modulus

The main purpose of the present work was to establish the effect of external magnetic field on the elastic modulus. We have prepared poly(dimethyl siloxane) networks loaded with randomly distributed carbonyl iron particles. It was found, that the elastic modulus of magnetoelasts could be increased by uniform magnetic field. In order to enhance the magnetic reinforcement effect, we have prepared anisotropic samples under uniform magnetic field. This procedure results in formation of chain-like structures from the carbonyl iron particles aligned parallel to the field direction. The effect of particle concentration, the intensity of uniform magnetic field as well as the spatial distribution of particles on the magnetic field induced excess modulus were studied. It was established that the uniaxial field structured composites exhibit larger excess modulus compared to the random particle dispersions. The most significant effect was found if the applied field is parallel to the particle alignment and to the mechanical stress. A phenomenological approach was proposed to describe the dependence of elastic modulus on the magnetic induction. The magnetic field sensitive soft materials with tuneable elastic properties may find usage in elastomer bearings and vibration absorber.     

Development and characterization of hydrocarbon polyol polyurethane and silicone magnetorheological polymeric gels

Magnetorheological polymeric gels (MRPG) have been developed for use in semi‐active magnetorheological fluid (MRF) dampers and other magnetorheological (MR) devices. The novel MRPGs are prepared by suspending iron particles in polymeric gels. Off‐state (i.e, no applied magnetic field) viscosity and settling behavior can be controlled through the selection of polymeric gels. In this study, tunable rheological properties were investigated with a piston‐driven flow type rheometer with a shear rate varying from 20 s^?1 to 6,000 s^?1. Silicone MRPG (with 84.5 wt % iron particles) has controllable viscosity and a high shear yield stress over a wide range of shear rates. Silicone MRPG (79.5 wt % iron particles) has the lowest viscosity of those studied. Polyurethane MRPG has the lowest settling rate. The order of addition of magnetic particles and polymer during the polymerization process affects the MRPG final off‐state apparent viscosity (80% increase in apparent viscosity for silicone MRPG polymerized after adding iron particles). This indicates that polymer gels modify the surface properties of the magnetic particles, causing interaction among particles. The dynamic shear yield stress is higher for fluids with better dispersion stability. Polyurethane MRPG, which has the lowest settling rate, has a high dynamic yield stress (23 kPa at 350 mT). Both dynamic and static shear stress values of the MRPGs were found to be similar in magnitude (5–8 kPa at 120 mT for silicone MRPG with 84.5 wt % iron particles and polyurethane MRPG), indicating that MRPGs can provide consistent performance in devices. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1176–1182, 2004     

Supramolecular magnetorheological polymer gels

Novel magnetorheological fluids—supramolecular magnetorheological polymer gels (SMRPGs)— were investigated. Supramolecular polymer deposited on the surface of iron particles was suspended in the carrier fluids. The supramolecular network was obtained by metal coordination between terpyridine monomers and zinc ion. These SMRPGs had such advantages as controllable off‐state viscosity, a reduced iron particle settling rate, and stability. The viscoelastic behavior of SMRPGs with small‐ and large‐amplitude oscillatory shear was investigated using the amplitude and frequency sweep mode. The effects of strain amplitude, frequency, and magnetic field strength on the viscoelastic moduli were measured. The linear viscoelastic (LVE) strain range was obtained by the oscillation and static stress strain methods. The maximum LVE value was equal to the preyield strain point, 0.3%. Microstructural variation of SMRPG is proposed as an explanation of the rheological changes in the oscillation tests. The results of this research indicate that off–state viscosity and particle settling can be controlled by adjusting the concentration of supramolecular polymer gel. Dynamic yield stress significantly increased with an external magnetic field up to ～23,500 Pa under a magnetic flux density of 500 mT. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2464–2479, 2006     

DEM‐simulation of the magnetic field enhanced cake filtration

Using the discrete element method, we simulate numerically the cake formation and growth in magnetic field enhanced cake filtration to give further insight on the mechanisms of the structuring of the filter cake due to the interaction of magnetic, hydrodynamic, and mass forces. The motion of the discrete particles is obtained by applying the three‐dimensional Newton's equations to individual particles, allowing for both external forces (gravity, applied magnetic field) and particle–particle interactions calculated using the modified DLVO‐theory. Continuous liquid phase flow is assumed as one‐dimensional. The simulation results compare favorably with reported experimental data, 1 and can be used to delineate the regimes associated with different liquid flow and magnetic field effects that are observed experimentally. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Measuring the yield stress of a particulate suspension under high electric fields

Certain classes of particulate suspensions, consisting of semi-conducting, solid particles dispersed in an insulating carrier liquid, show a dramatic increase in flow resistance when placed under an external electric field. These so-called “electro-rheological fluids” have potential application in engineering devices such as tunable vibration damping systems. Under the field, a yield stress is induced in the fluid, and the characterisation of this quantity is essential for the design of many engineering devices. A new method for determining the yield stress is presented, which involves subjecting the sample to a constant shear stress and monitoring its shear rate after a step change in the electric field. It is found that this step field method can reproducibly determine the field-induced yield stress of a suspension of silica particles in silicon oil.     

Organic/inorganic hybrid of polyaniline/BaTiO3 composites and their electrorheological and dielectric characteristics

Barium titanate (BaTiO₃) inorganic particles which possess large electronic resistance and excellent dielectric properties were employed to synthesize conducting polyaniline (PANI)/BaTiO₃ composites via an in situ oxidative polymerization, since conducting PANI/inorganic composites have been considered as a superior candidate of electrorheological (ER) fluids because of their physical properties, unique structure, and the combined merits of the two phases. The influence of the fraction of BaTiO₃ particles in the as‐synthesized composites on the physical properties (morphology and crystal structure) and the ER behaviors were examined. Yield stress data obtained were analyzed based on the universal yield stress equation as a function of applied electric field and it was found that the universal yield stress equation collapses these data onto a single curve independent of BaTiO₃ particle concentration. Their shear stresses under an applied electric field were also found to be fitted well with the Cho–Choi–Jhon model. In addition, the investigated dielectric spectra were found to be useful to interpret the differences in the ER performances for the PANI/BaTiO₃ composite based ER fluids. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Magnetically assisted gas–solid fluidization in a tapered vessel: First report with observations and dimensional analysis

The article presents first experimental results on gas–solid fluidization in a tapered bed in presence of an external transverse magnetic field that creates a novel branch in magnetically assisted fluidization. Phase diagrams similar to those used to describe cylindrical beds have been created to distinguish the bed regimes occurring under the action of two principle macroscopic variables such as field intensity and gas flow rate. A detailed analysis and parallelism to the bed behaviour exhibited by non‐magnetic spouted beds of cohesive particles have been performed. Principle process variables such as bed depth, field intensity, particle size, cone angle have been detected. A dimensional analysis utilizing a “pressure transform” of the initial set of variables has been applied to develop scaling relationships. Examples of scaling experimental data pertinent to the minimum spouting point and involving the magnetic granular Bond number have been developed.     

CFD simulation for biomagnetic separation involving dilute suspensions

Full‐Eulerian simulation of the separation of magnetic particles carried by a Newtonian fluid through a planar channel under the influence of a magnetic field is presented. The simulation is based on the application of the Navier–Stokes and concentration equations. The scheme for the magnetic separation of particles is achieved by applying an external magnetic dipole field. The hydrodynamic and magnetophoretic interactions between the particles and the carrier fluid are analysed. Analysis of the competing tendencies of mass transfer indicates that the magnetophoresis migration of magnetic particles is dominant over the molecular diffusion. This dominance becomes more evident at lower diffusivities leading to a drastic magnetic separation confined within a small region in the proximity of the magnetic field source. © 2012 Canadian Society for Chemical Engineering     

Anisotropic mechanical behavior of magnetically oriented iron particle reinforced foams

Reinforced foams were prepared by exposing a polyurethane matrix filled with iron particles to a magnetic field during the foaming process. The magnetic field induced an alignment of the iron particles along the field direction, giving rise to columnar structures similar to fibrils, as observed by SEM and microtomographic 3D reconstructions. The anisotropic reinforcement induced by the fibrils improved the mechanical performances, yielding a threefold increase of both elastic modulus and yield stress in the alignment direction, whereas minor effects were observed in the transversal direction. In this case, the mechanical properties were comparable with those of randomly filled foams or, in some cases, of unfilled foam. The reinforcing efficiency of fibrils was evaluated through a theoretical model, based on the combination of the mechanics of foams with two micromechanical models for aligned short fibers composites (Halpin‐Tsai and Cox‐Krenchel). The theoretical predictions based on the Halpin‐Tsai equations showed a good agreement with the experimental data, whereas the model derived from Cox‐Krenchel equations overestimated data. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Model filled polymers: The effect of particle size on the rheology of filled poly(methyl methacrylate) composites

The effect of size of crosslinked monodisperse spherical polymer particles on the steady shear and dynamic rheology of filled poly(methyl methacrylate) (PMMA) composites was studied for PMMA and polystyrene (PS) particles in the range from 0.1 to 1.3 micron particle size. For PMMA matrices filled with crosslinked PS particles, reduction in filler size increases non‐Newtonian behavior. Particle size effects on the rheology of filled PMMA were much less pronounced for PMMA filler. The rate of growth of steady shear viscosity with aging time was much larger for PMMA filled with PS particles than with PMMA particles. The apparent yield stress of filled PMMA composites was estimated from Casson plots. The yield stress was negligible for PMMA filler but increased with decreasing particle size for PS filler. We suggest that PS particles are rejected by the PMMA matrix and form clusters, causing large enhancements in viscosity and moduli. Polym. Eng. Sci. 44:452–462, 2004. © 2004 Society of Plastics Engineers.     

AGGREGATION IN MAGNETIC FLUIDS AND MAGNETIC FLUID COMPOSITES

Many of the properties of magnetic fluids and magnetic fluid composites can be explained by the presence of aggregates of particles. In this paper, the experimental methods by which these properties can be studied are reviewed. Theoretical methods of predicting these properties, and the particle size and magnetic field dependence of the formation of aggregates are also presented.     

Aggregation Experiments on Fine Fly Ash Particles in a Gradient Magnetic Field

Aggregation experiments were conducted on two kinds of fly ash particles in the size range of 0.023–9.314 μm in a gradient magnetic field produced by permanent magnetic rings. The two types of fly ash particles were obtained from Dongsheng and Datong coal combustion. The effect of particle size, total particle mass concentration, particle residence time in the magnetic field and gas velocity were examined. Experimental results showed that the removal efficiencies in a gradient magnetic field are much higher than those in a uniform magnetic field. The total and single‐sized particle removal efficiencies can be improved by increasing the total particle mass concentrations and the particle residence time in the magnetic field or reducing the gas velocity. Mid‐sized particle removal efficiencies are higher than those of the larger and smaller ones. With the increase in total particle removal efficiencies, the particle size corresponding to the maximum values of single‐sized particle removal efficiencies and the particle number median diameters both decrease. Both the single‐sized and total removal efficiencies for the particles from the Dongsheng coal combustion are higher than those from the Datong coal combustion.     

Visualization and simulation of the transfer process of index‐matched silica microparticle inks for gravure printing

A combined experimental and computational study of the transfer of transparent index‐matched silica‐particle inks between two flat plates is presented for gravure printing applications. The influence of printing speed and initial ink droplet size on the ability to accurately transfer ink during the printing process is explored systematically. Smooth interface volume of fluid simulations show the same trends as the ink transfer observed in experiments over a wide range of printing speeds and for inks having different silica particle loadings. Our calculations indicate that for ink droplets with characteristic dimensions in the vicinity of 10 μm, which are of particular interest for gravure printing applications, ink transfer improves significantly due to the diminishing effect of gravity, and the increased importance of capillary forces at small length scales. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1419–1429, 2017     

The effect of particle size of fly ash (FA) on the interfacial interaction and performance of PVC/FA composites

In this study, the effect of particle size of fly ash (FA) on the interfacial interaction between the filler particles and the polymer matrix is investigated. Structural and physical characterization of FA with different particle sizes show that its chemical composition is highly dependent on the particle size. The mechanical, dynamic‐mechanical, structural, and microstructural properties of the composites are evaluated. Interfacial interaction between FA particles and the polymer matrix is assessed experimentally using a nanoindenter and numerically using two different models developed by Pukanszky and Kubat. The composites reinforced with smaller particles exhibit better mechanical, viscoelastic, and microstructural properties. Structural and interfacial studies show that, although the characterized amount of silicon oxide in the small particles is lower than the large particles, the concentration of –OH group in SiO₂ is particle‐size and surface‐area dependent. Therefore, smaller particle inclusions result in better interfacial interaction and improved properties. This observation is consistent with the numerically estimated interfacial interaction. J. VINYL ADDIT. TECHNOL., 25:134–143, 2019. © 2018 Society of Plastics Engineers     

Universal stress‐sensing dimeric anthracene‐based mechanophore particle fillers incorporated into polyurethane thermoset matrices

Mechanochemistry, in which mechanical forces induce chemical changes, can allow for targeted damage detection by way of embedded mechanophore units, which emit a measurable signal change correlating to an applied force. In this work, we successfully created stress‐sensing, functional composites by employing microparticles of the mechanophore dimeric 9‐anthracene carboxylic acid in a thermoset polyurethane matrix. The goal being to study the application of the particles as universal stress‐sensing fillers in network polymer matrix composites, after previously evaluating the particles in an epoxy matrix. Under a compressive force, there is bond breakage in the mechanically weak cyclooctane photodimers, such that there is reversion to the fluorescent anthracene‐type monomers. This fluorescent emission was then correlated to the applied strain, and the precursors to damage were detected with a noticeable signal change at a strain of only 2%, which was attributed to increased interactions between the matrix and the particles, with possible surface grafting occurring. This early damage detection was additionally possible at very low particle loadings of 2.5 and 5 wt%, with the 5 wt% loading showing enhanced material properties, due to particle reinforcement. Overall, the stress‐sensitive particle filler allows for facile addition of advanced functionality to these ubiquitous thermoset composites. POLYM. ENG. SCI., 57:901–909, 2017. © 2016 Society of Plastics Engineers     

High‐density polyethylene reinforced with submicron titania particles

Submicron titania particles were prepared by means of two different synthetic procedures in order to obtain different particle size (diameter ranging from 20 to 350 nm), shapes, and morphologies (amorphous or crystalline). Titania particles were surface modified with octadecylsilane in order to improve their compatibility with respect to polymeric matrices. High‐density polyethylene (HDPE)–titania composites were prepared by melt blending by using an internal mixer. The obtained composites were mechanically characterized in quasi static and creep tensile conditions. The presence of submicron titania particles (1 %vol) led to a significant increase of elastic modulus (20–25%) with respect to the unreinforced HDPE together with a slight increase of yield stress and a decrease of ultimate elongation. An interesting reduction for both elastic and viscoelastic creep compliance components was also evidenced. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Preparation and properties of a p‐aramid fabric composite impregnated with a magnetorheological fluid for body armor applications

A polyurethane (PU)‐magnetorheological fluid (MRF)/p‐aramid fabric composite was fabricated, and its mechanical properties were subsequently investigated. The contribution of the PU‐MRF matrix to the impact resistance of the system was then discussed. MRFs consist of stable suspensions of magnetite particles within a carrying fluid. Therefore, when an external magnetic field is applied, the MRFs exhibit drastic and reversible changes in rheological properties as a result of the field‐induced ordering of the particulate phase. We then attempted to develop new and enhanced bulletproof materials by incorporating MRF and PU in a p‐aramid fabric. It was found that when a magnetic field was applied, the mechanical properties of the PU‐MRF/p‐aramid fabric composite improved. It was also found that adding a PU matrix improves the impact performance of the PU‐MRF/p‐aramid fabric composite, relative to a neat p‐aramid fabric and a MRF/p‐aramid fabric composite with similar areal density. The improved impact performance of the PU‐MRF/p‐aramid fabric composite appears to be because the PU film and MRF enable different energy absorbing mechanisms, including particle friction, fabric/matrix debonding, matrix cracking, and delamination, which are not observed in neat p‐aramid fabric systems. The findings of this study are thought to be important from a design viewpoint of soft armors. POLYM. ENG. SCI., 55:729–734, 2015. © 2013 Society of Plastics Engineers