Effect of morphology and extent of infiltration on the cohesivity and dispersion mechanisms of particle agglomerates

A unified approach to predict the tendency for dispersion of particle agglomerates, inclusive of a wide range of particle and agglomerate properties, is presented. This framework is applied to analyze the behavior of three prototypical materials (fumed silica, calcium carbonate and titanium dioxide) across a range of agglomerate packing densities. Simulations of dispersion phenomena, which employ our previously developed solution for liquid-bridge interactions for wet interparticle contacts and the Rumpf model for the tensile strength of the dry and wet portions of the agglomerate, have been performed. Various mechanisms of dispersion are predicted for various conditions of agglomerate density and extent of fluid infiltration. These range from an adhesive mechanism at the wet-dry interface for sparse materials to a cohesive mechanism by erosion as agglomerate density increases. The results correspond well with the results of earlier experimental studies involving the same materials.     

Analysis of the kinetics of agglomerate erosion in simple shear flows

A model for the erosion kinetics of particle agglomerates in simple shear flows has been developed. The erosion rate is taken to be proportional to the difference between hydrodynamic and cohesive forces and to the rotation velocity of the dispersing agglomerate. For dispersion under identical hydrodynamic conditions, the model predicts faster erosion for larger agglomerates. Moreover, at equivalent hydrodynamic stress, erosion is enhanced at higher shear rates. Dispersion experiments using silica agglomerates of various densities and liquid low molecular weight polymers (e.g., poly(dimethyl siloxane)) of different viscosities were conducted in an oscillatory shear flow device. The experimental results validate the erosion model; the general shape of the erosion kinetic curve, and the effect of viscosity, shear rate and agglomerate size on dispersion kinetics are well predicted. The model can also predict erosion for agglomerates with fractal structure.     

Prediction of the dispersion of particle clusters in the nano-scale—Part II, unsteady shearing responses

A modeling approach to predict and enhance understanding of the dispersion phenomenon is presented. The discrete/distinct element method (DEM) is adopted to study the behavior of single spherical agglomerates, immersed in a simple shear flow field, in response to shearing under steady or dynamic/oscillatory flow conditions. The effects of hydrodynamic forces, which result from both the straining and rotating components of the flow, and cohesive forces of interaction, comprised of short-range van der Waals attractive and Born repulsive forces, are considered. Comparative results of simulated dispersion of nano-size silica agglomerates in response to steady and unsteady shearing are found to be in good agreement with reported experimental trends. The current three-dimensional model allows us to probe and predict the dispersion phenomenon as a function of processing conditions, agglomerate structure/morphology, and material properties and interaction forces.     

Prediction of the dispersion of particle clusters in the nano-scale—Part I: Steady shearing responses

A modeling approach to predict and enhance understanding of the dispersion phenomenon is presented. The discrete/distinct element method is adopted to study the behavior of single spherical agglomerates, immersed in a simple shear flow field, in response to shearing under steady or dynamic/oscillatory flow conditions. The effects of hydrodynamic forces, which result from both the straining and rotating components of the flow, and cohesive forces of interaction, comprised of short-range van der Waals attractive and Born repulsive forces, are considered. The results of simulated distortion and dispersion of nano-size silica agglomerates in response to steady shearing are used to demonstrate the functionality of the three-dimensional simulation. Simulated results are found to be in good agreement with reported experimental trends. The current model allows us to probe and predict the dispersion phenomenon as a function of processing conditions, agglomerate structure/morphology, and material properties and interaction forces.     

Selection of dispersants for the dispersion of carbon black in organic medium

The dispersion of the carbon black pigment in non-aqueous medium requires the use of dispersant additives. Two carboxylic dispersants of different chemical nature and molar masses have been investigated regarding this purpose. Their adsorption behavior has been evaluated by means of adsorption isotherms. The molecular interactions taking place at the surface have been inferred from IR and ¹³C RMN spectroscopies. The rheological behavior of the dispersions has considerably been improved by the presence of the copolymer dispersant of higher molar mass. It might adsorb at the surface by hydrogen bindings while taking a flat conformation with tails and possibly few loops that contributed to the steric stabilization of particles. The consequences of the deflocculation of the suspension on the colorimetric properties are an enhanced tinting strength and improved color stability.     

Axial dispersion measurements of Bodenstein >100,000 flows through nano-channels etched on flat surfaces

We demonstrate the possibility to generate ultra-rapid, low-axial dispersion flows through flat-rectangular nano-channels, i.e., channels with a thickness as small as . This is achieved by resorting to the very basic, yet often overlooked shear-driven flow principle. With the recent improvements in micro-channel manufacturing possibilities, the major advantage of this flow type, i.e., the possibility to impose quasi unlimitedly large flow velocities through sub-micron thin channels without running into conflict with Posiseuille's pressure-drop law, can now be fully exploited. Demonstrating the possibility to generate rapid tracer flows with Bodenstein numbers of over 100,000, the present study proves the practical feasibility of this novel micro-channel concept. As it opens the road towards an unprecedented range of short radial diffusion times, the system has potential applications in the conduction of ultra-rapid chromatographic separations, the conduction of short time-scale binding and reaction kinetics measurements in diffusion-free conditions, and the conduction of combinatorial drug and catalyst screening at unprecedented throughput frequencies.     

Convective diffusion of particles deposition under electrostatics from turbulently-mixed conditions

A theoretical model is presented to describe inertialess particle deposition onto smooth surfaces under the influence of electrostatic force. Both Boltzmann and combined diffusion and field charging mechanisms are investigated. A modified Fick's law equation accounting for Brownian and turbulent diffusion, spatially-independent external force, i.e. gravitational and Coulombic force, is presented based on the simplified three-layer model. The results show that the concentration boundary layer thickness is very thin and the previous three-layer model can be further simplified. The Coulombic force influences the particle deposition significantly and for considerably high charge and electric field, deposition is independent on turbulent intensity. The model predictions agree very well with the literature DNS results.     

A numerical investigation of aerosol dynamics in a wall-less reactor

This paper describes a numerical investigation of aerosol formation during silane decomposition in a wall-less reactor. The wall-less reactor is amenable to numerical investigation because the homogeneous chemical reactions leading to the formation of solid particles are isolated from heterogeneous effects, such as occur at the walls of a laminar flow aerosol reactor. The flow/heat transfer and gas-phase chemical kinetics are simulated utilizing separate one-way coupled models. The aerosol dynamics model is based on a simplified sectional model originally developed by Okuyama et al. This model is modified to allow for the simulation of particle growth via condensation. Simulations have been performed which indicate that particle growth via condensation may be an important process. Additionally, the effects of total reactor pressure, temperature and inlet silane concentration on the dynamics of the aerosol population have been investigated. Conditions which result in the formation of larger and more numerous particles have been identified.     

纳米炭黑分散方法和含量对低碳镁碳材料力学性能的影响

采用加KH-550偶联剂和高速搅拌的方法,将不同量的纳米炭黑N220均匀分散在酚醛树脂中,制成纳米炭黑-酚醛树脂复合结合剂,研究了纳米炭黑含量对复合结合剂黏度及其经1500℃炭化后碳结构的影响,并采用这些复合结合剂制备了w(C)=3%的低碳镁碳试样。研究了纳米炭黑含量(相对于酚醛树脂的质量分别为0、2.5%、5%、10%和15%)对低碳镁碳试样力学性能的影响,同时还与将纳米炭黑以预混合粉方式加入的低碳镁碳试样的力学性能进行了对比。结果发现:1)随着纳米炭黑含量的增加,复合结合剂的黏度迅速增大;2)纳米炭黑-酚醛树脂复合物经1500℃炭化后的碳结构呈光学同向性,但其微气孔数量减少,微气孔尺寸减小,碳结构的石墨化程度提高;3)随着纳米炭黑加入量的增加,低碳镁碳试样的常温抗折强度、高温抗折强度和常温耐压强度逐渐增大;4)将纳米炭黑直接分散在细粉中加入时,试样的常温抗折强度、高温抗折强度和常温耐压强度均比以纳米炭黑-酚醛树脂方式加入时差。     

In situ study of the dynamics of erosion of carbon black agglomerates

With the help of a counter‐rotating transparent rheometer equipped with an optical microscope, the erosion of single, commercial carbon black agglomerates suspended in a polyisobutylene fluid was accurately recorded in situ. We observed that clouds of very small aggregates were leaving the agglomerate at 45 and 225° in the shear gradient–flow direction plane, where the stress is maximum. The distribution of the arrached aggregates is more efficient when the carbon black agglomerate is more asymmetric. A spherical agglomerate of critical radius R₀ will reach a radius R_t at time t following R ? R = at, a being a constant. This implies that the rate at which the number of aggregates is leaving the agglomerate is a constant, independent of the size of the agglomerate. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1627–1629, 2001     

不同炭黑在胶印黑墨中适用性探讨

综述了炭黑原生粒子的粒度、原生聚集体的结构对其在胶印黑墨体系中分散难易的影响,便于原材料——炭黑的选择。     

炭黑生产工艺的进展

介绍了较为成熟的炭黑生产工艺,讨论了其各自在工业应用方面的前景,并对近年来涌现出的一些具有可应用前景的工艺方法进行了简要评述,同时也归纳了近几年炭黑生产工艺的进展表现。     

Observation of carbon black agglomerate dispersion in simple shear flows

Experiments aimed at studying the mechanisms of agglomerate breakup due to the application of a simple shear flow field were performed in a cone and plate transparent device. Spherical compacts of carbon black (diameters 1-2 mm) in a range of different porosites were used in the experiments. Two distinct breakup mechanisms, denoted as “rupture” and “erosion”, were observed. The critical stress for erosion was found to be smaller than that for rupture. Once erosion starts, it continues for very long times. Rupture occurs shortly after reaching a critical stress and concludes abruptly. For this analysis of rupture, the dimen-sionless group $\alpha {\rm = \{ }\eta {\rm .}\mathop \gamma \limits^{\rm .} {\rm /K'}\phi ^{\rm 4} {\rm \} }$, which is the ratio of applied stress to cohesive strength, was found to be a significant parameter for determining the final particle size distribution. The size analysis of fragments produced by shearing pellets for 1 minute showed a lognormal distribution function.     

Mechanism for the growth of multiwalled carbon-nanotubes from carbon black

Multiwalled carbon-nanotubes have been grown from carbon black by solid-state transformation at the anode of a modified high-temperature arc-furnace without a catalyst. A mechanism for the solid-state transformation of carbon black into nanotubes is proposed. The migration of pentagon and heptagon defects present in carbon black to regions of high tensile-stress is key to the growth mechanism. The growth process can be broken into two stages. The basic mechanism for both stages is the same; only the source of the tensile stress that drives the nanotube growth differs. In the initial stage of growth the necks between carbon-black particles are lengthened into short nanotubes by thermal forces. Electrostatic forces present in the plasma of the high-temperature arc-furnace drive the subsequent extension of the short nanotubes to multiple-micron lengths.     

Cationic photopolymerization of epoxides containing carbon black nanoparticles

The active centers responsible for cationic photopolymerizations are essentially non-terminating, and continue to propagate after the illumination is ceased. In this contribution, the mobility of the long-lived cationic active centers is investigated for the cure of epoxides containing carbon black nanoparticles. Concentration profiles for the cationic active centers produced during illumination were coupled with an analysis of the active center reactive diffusion during the post-illumination period, revealing that migration of the active centers leads to cure beyond the illuminated depth. A kinetic analysis yielded predicted cure times for coatings of varying thickness and carbon black loading, showing good agreement with experimental results obtained for photopolymerizations of cycloaliphatic diepoxide coatings containing a monodisperse carbon black with mean hydrodynamic radius of 29.2 nm. These results indicate that the long lifetimes and reactive diffusion of cationic active centers may be used for effective curing of coatings containing carbon black nanoparticles. This comprehensive approach could be applied to other opaque nanocomposite systems.     

炭黑在橡胶中分散技术研究进展

对于炭黑补强橡胶复合材料,平衡生热、补强以及磨耗之间的关系是炭黑应用的瓶颈问题。炭黑在橡胶基体中的分散性是影响其补强的关键因素之一,从增强炭黑与橡胶之间的相互作用角度出发,炭黑在橡胶中分散技术的研究进展主要体现在炭黑改性、橡胶改性以及液相复合技术方面。重点阐述炭黑的表面改性、聚合物基体的功能化改性以及炭黑与橡胶湿法混炼制备母胶,旨在促进炭黑在橡胶中均匀纳米分散,提高炭黑对橡胶的补强作用,获得性能优异的炭黑/橡胶复合材料。     

油炉法炭黑的工程设计

主要介绍了油炉法炭黑的流程特点及主要生产设备。     

Scaling of the viscoelasticity of highly filled carbon black polyethylene composites above the melting point

Different types of linear low-density polyethylene and ethylene butylacrylate copolymers were mixed with various types of carbon black in amounts between 25 and 40% by weight. Viscoelastic properties were measured using dynamic mechanical analysis applying a frequency sweep. Typically, the complex modulus approaches asymptotically a constant value at small frequencies, which is referred to as ‘yield modulus’. These results were analysed using a scaling approach according to which the complex modulus and the frequency are normalised by the yield modulus and the quotient of the yield modulus and the polymer viscosity, respectively. Thus a master curve is achieved for nearly all samples independent of the polymer and carbon black type and loading. A similar scaling behaviour has been observed earlier for differently concentrated suspensions of carbon black in Newtonian liquids, but not for filled polymers and different carbon blacks. Thus, contributions from polymer and carbon black to the compounds' viscoelastic properties are discussed.     

Enhancement of Hydrocyclone Classification Efficiency for Fine Particles by Introducing a Volute Chamber with a Pre‐Sedimentation Function

A new hydrocyclone was designed with a volute chamber positioned prior to the inlet. Since the volute chamber prior to the inlet has a pre‐sedimentation function due to the centrifugal sedimentation effect, coarse particles are concentrated on the outer side and fine particles are concentrated on the inner side as the particles reach the entrance of the hydrocyclone. Consequently, coarse particles in the hydrocyclone are easily separated into the underflow and fine particles are easily transferred into the overflow. As a result, the separation or classification performance of the hydrocyclone was improved effectively. Compared with the traditional type of hydrocyclone, this new type of hydrocyclone with a volute chamber before the inlet was shown to possess a much higher classification efficiency for fine particles.     

Numerical study of particle dispersion in the wake of gas-particle flows past a circular cylinder using discrete vortex method

A numerical investigation on the particle dispersion in the wake of particle-laden gas flows past a circular cylinder at Reynolds number of 10⁵ is presented. In the numerical method, the Discrete Vortex Method with the diffusion velocity model is employed to calculate the unsteady gas flow fields and a Lagrangian approach is applied to track individual particles. A dispersion function is defined to represent the dispersion scale of the particle. The distributions of gas velocities and vortex blobs, the trajectories and dispersion functions as well as distributions for particles with various Stokes numbers ranging from 0.01 to 1000 are obtained. The numerical results show that: (1) very small sized particles with St = 0.01 can distribute both in the vortex core and around the vortex periphery, whereas intermediate sized particles with St = 1.0, 10 are distributed around the vortex periphery, and very large sized particles with St = 1000 do not feel the gas flow; (2) only at small Stokes number (St = 0.01, 0.1) the particles do not impact with the cylinder; (3) the particle's dispersion intensity decreases precipitously as St is increased from 0.01 to 10.