Suspensions and Sediments. Part II. Behavior of Concentrated Suspensions

Abstract

The behavior of concentrated suspension, the phenomenon of hindered settling, and structural characteristics of the settling suspensions are described.

The use of interface sedimentation rate the settled volume in calculating the volume of associated fluid carried down on sedimenting particles is outlined by employing a theory which assumes that particle-particle association is the factor responsible for variation of sedimentation behaviour from one suspension system to another.     

Nucleation of Flocs in Dilute Colloidal Suspensions

In contrast to earlier theories which assume that coagulation of particles in dilute colloidal suspensions occurs by random collisions, we present evidence that coagulation is limited by a nucleation and growth phenomenon, typical of a phase transformation. We show that suspensions of well-dispersed particles, which are almost indefinitely stable, flocculate rapidly when seeded with pregrown flocs. The nucleation argument is supported further by the observation that flocs which are grown at low ζ potential, where the particle strongly attract, do not peptize when the pH is changed to high ζ potential, where the attraction is weak. The experiments were carried out with aqueous dispersions of alumina particles. The time-dependent change in cluster sizes was measured by in situ photon correlation spectroscopy. The observations raise the question of agglomeration as a phase transformation, involving a change in entropy, when the system moves from a dispersed to a flocculated state.     

Retention of Liquid by Solids in Suspensions and Sediments

Abstract

The effects of associated immobile liquid on the settling behavior of particles in solid-liquid suspensions are investigated. The effects are studied with respect to such sedimentation parameters as suspension porosity ?, final sediment volume fraction u_s, internal void fraction of the flow unit u_i, Steinours parameter A, and McKay's packing factory which corrects for the effective volume and density of the flow units in a sedimenting suspension. It is shown that the degree of liquid association with the settling particles is a function of their physicochemical characteristics. Suspensions containing clays and carbonates, owing to their ionic nature and loose packing behavior, absorb large amounts of liquids and result in larger settled sediments of a compressible nature. On the other hand, systems like glass beads-liquid suspensions absorb little associated liquid and virtually result in noncompressible settled sediments due to their rigid spherical shapes and close packing. The addition of hydrocolloids (such as natural gums) provides ionic stability to suspensions by giving them low cmpressibility and low values of u_s, u_i, A, and p.     

Fouling of Thin-Channel and Tubular Membrane Modules by Dilute Suspensions

Abstract

In this paper, fouling of thin-channel and tubular ultrafiltration (UF) membrane modules by dilute suspensions have been studied theoretically. A hydrodynamic analysis of fluid-particle system is presented to describe the role of dilute suspensions in fouling such membrane modules. The present analysis assumes that for very dilute suspensions, only inertial effects-are-important for particulate fouling. Particle trajectory history and hence the fouling is computed from equations of motion for the particles, where the fluid-flow is given by the full solution of Navier-Stokes equation. To simulate the flux decline due to build up of foulant layer on the membrane walls, it is assumed that the deposition of particles on the membrane surface at discrete time interval is a steady state event and thus formulating the fouling problem as an infinite series of successive steady state events. Present simulation results indicate that inertial effects are important and under positive wall permeation flux conditions, particles are encouraged to migrate towards the membrane wall causing so-called ‘membrane fouling’ by the particulars.     

Dispersant-Binder Interactions in Aqueous Silicon Nitride Suspensions

The interaction of dispersant and binder on the surface of particles was studied to identify the effect of these additives on aqueous ceramic powder processing. Poly(methacrylic acid) (PMAA) and poly(vinyl alcohol) (PVA) were used as the dispersant and binder, respectively. The adsorption isotherms of the organic additives on silicon nitride were determined. The adsorption of PMAA was differentiated from PVA in the mixed additive system via ultraviolet spectroscopy. The electrokinetic behavior of silicon nitride was measured by using an electrokinetic sonic amplitude analyzer. As the PMAA concentration increased, the isoelectric point (pH_iep) of silicon nitride shifted from pH 6.7 ± 0.1 to acidic pH values. The magnitude of the shift depended on the surface coverage of PMAA. PVA did not affect the pH_iep of suspensions but did cause a moderate decrease in the near-surface potential. Finally, the rheological behavior of silicon nitride suspensions was measured to assess the stability of particles against flocculation in aqueous media; this behavior was subsequently correlated with the electrokinetic and adsorption isotherm data.     

Modeling of Dynamic Processes in Dilute and Dense Gas Suspensions (Review)

Combustion, Explosion, and Shock Waves - A review of investigations dealing with mathematical and numerical modeling of shock wave and detonation processes in gas suspensions of fine-grained inert...     

Interactions between PVC and binary or ternary blends of plasticizers. Part I. PVC/plasticizer compatibility

Solid–gel transition temperatures, t_m, were measured for PVC in binary and ternary mixtures of plasticizers. Data were used to investigate interactions between plasticizers, and their effect on t_m and polymer–plasticizer interaction (χ) values. The UNIFAC-FV method was used to predict Gibbs free energy of mixing, and χ for a range of plasticizers, and interactions for one quaternary mixture. Synergy was observed when one or more branched phthalates were mixed with ODPP, but none when three phthalates were mixed. Mixtures of DOP with aliphatic ester plasticizers again produced synergy. Equations were produced to enable interaction coefficients of different plasticizer blends to be predicted. It was found that interaction coefficients increase as the difference in molar mass between the two plasticizers increases. Calculated χ values were generally similar to experimental values. The lower limit of Gibbs free energy of mixing required for plasticization of PVC using monomeric plasticizers was calculated to be about −0.9 J/g, and that for polymeric plasticizers −0.15 J/g. Equations were produced enabling Gibbs free energy of mixing to be predicted from the amount of C atoms in the plasticizer.     

Hydrophobic Interactions in Silane‐Treated Silica Suspensions and Bitumen Emulsions

In the extraction of bitumen from Canada's oil sands reserves, bitumen droplet coalescence is an important factor in determining the efficacy of the separation process. In a simplified analysis of bitumen droplet interactions, the relative contributions of classical DLVO and non‐DLVO forces occurring between a bitumen droplet and surface‐treated planar collector were evaluated using the impinging jet cell technique. Initial experiments were conducted using a system consisting of silane‐treated silica particles and a hydrophobic collector. This system is known to exhibit attractive non‐DLVO interactions as was clearly demonstrated by the present study. The attachment of bitumen droplets onto two different hydrophobic collectors was then examined. Results of these experiments indicate that bitumen droplet interactions involve an additional repulsive force that may be attributable to the heterogeneous nature of the droplet surface.     

Physical studies of ionically-terminated polytetrahydrofuran polymers. I. Dilute solution behavior

A series of tetrahydrofuran polymers, terminating at one end with a pyridinium bromide group, were prepared; their hydrodynamic behavior contrasted with the equivalent methoxyterminated polytetrahydrofurans. This article reports viscometry studies of these polytetrahydrofurans in cyclohexane, toluene, tetrahydrofuran, methyl ethyl ketone, and cyclohexanone.     

Removal Of Cadmium From Dilute Solutions By Hydroxyapatite. I. Sorption Studies

Abstract

The removal of toxic metals (such as cadmium) was investigated by using hydroxyapatite, an effective inorganic sorbent, at the ultrafine particle size range. In bench-scale experiments performed batchwise, the influence of the main sorption parameters were examined (i.e., solution pH, sorbent and cadmium concentrations, and temperature) and comparison was attempted between demineralized and tap water. Typical adsorption isotherms of the Langmuir type were calculated; ζ-potential measurements of the hydroxyapatite particles and the release of calcium (during the process) were also examined and related to possible mechanisms occurring during the cadmium removal process.     

Measuring color-matching functions. Part I

A visual colorimeter was designed to implement a new model for the determination of color-matching functions developed in the Munsell Color Science Laboratory. An instrument utilizing laser primaries was originally developed. The instrument was later improved to incorporate CRT primaries and seven interference filters to make matches to simulated daylight in a 2° bipartite field. The system was designed to minimize the strain on observers. Color-matching functions of naïve observers can be measured in approximately 30 minutes. Results from an individual observer correlate well with data collected on the National Research Council of Canada's Trichromator. Part II of this article gives the color-matching functions results for 18 observers and 20 repetitions by a single observer.     

Anisotropy in injection-molded ethylene-propylene-diene rubbers. Part I

Anisotropy and molecular orientation are well-known phenomena in the field of thermoplastics. In the case of rubber materials only a few studies have described anisotropy. Injection molding has been shown to give rise to higher anisotropy than compression molding. The anisotropy in the rubber material is assumed to be due to molecular orientation and is strengthened by carbon black. In order to understand the mechanism of anisotropy in rubber materials, an extensive study has been performed. In this paper, results from two injection-molded ethylene-propylene-diene (EPDM) rubbers, compounded both with and without carbon black, are presented. The polymers had different molecular weight distributions and the compounds were injected into center-gated 1- and 4-mm-thick disks. The properties measured in different directions were mechanical, dynamic mechanical, and swelling. These measurements show that anisotropy can be a very important factor to take into account. The origin of anisotropy is presumably the molecular orientation which arises during the filling of the mold with the rubber melt.     

Crosslinking-property relationships in PMR polyimide composites. Part I

This study examines for the first time how matrix crosslinking affects the composite physical and mechanical properties of a graphite fiber reinforced PMR polyimide composite during long-term isothermal aging. Unidirectional composite specimens of Celion 6000/PMR-P1 were isothermally exposed at 288°C in air for various time periods up to 5000 h. The matrix crosslink densities were estimated from the kinetic theory of rubber elasticity and shifts in the glass transition temperatures (T_gs). The T_g, coefficient of thermal expansion, density, weight loss, moisture absorption, and elevated temperature flexural and interlaminar shear properties were also determined. Several linear relationships were found between the matrix crosslink density and composite physical and mechanical properties. The T_g, initial weight loss and density, and elevated temperature interlaminar shear strength increase with an increase in crosslink density. Conversely, the initial moisture absorption and coefficient of thermal expansion decrease with increasing crosslink density. As expected, the elevated temperature flexural strength and modulus show no direct correlations with crosslink density. Further, after achieving the highest matrix crosslink density, several of the composite properties begin to decrease rapidly. These findings suggest that time-temperature dependent nature of attaining the maximum matrix crosslinking is closely linked to the onset of the composite property degradation. Though much more work is needed, a fundamental understanding of the relationships between matrix crosslinking and composite physical and mechanical property can provide a scientific basis for the prediction of the extent of composite service life not only for PMR polyimides but also for other thermosetting matrix resins, such as epoxies and bismaleimides.     

Hot embossing in microfabrication. Part I: Experimental

The relationship among processing conditions, material properties, and part quality in hot embossing was investigated for three optical polymers: polycarbonate (PC), polymethyl methacrylate (PMMA), and polyvinyl butyral (PVB). A series of systematic embossing experiments was conducted using mold inserts having either single or multiple feature depths. The feature dimensions varied from 90 to 3000 μm. The processing conditions studied include embossing pressure, thermal cycles, and heating methods. The displacement profile, replication accuracy and molded‐in stresses were measured experimentally. It was found that for isothermal embossing, both replication accuracy and birefringence pattern depend strongly on the processing conditions. For non‐isothermal embossing, the molded parts showed excellent replication as long as the feature transfer was completed. The flow pattern under isothermal embossing resembles a biaxial extensional flow. Under non‐isothermal embossing, the polymer deformation involves an upward flow along the wall of mold features, followed by downward compression and outward squeezing. Rheological characterization and hot embossing analysis are presented in Part II.     

骨胶工艺及设备

《明胶科学与技术》自1981年创刊至今已整整25年了。在这25年中,明胶界广大的工程技术人员、技术工人和从事明胶科学研究的专家、学者,为明胶科学与工艺学以及设备的研制与改进做了大量的工作,为我国明胶工业的发展作出了重大的贡献,其中有相当部分的成果先后刊载在已出版的各期《明胶科学与技术》上。在回顾这25年的奋斗历程的时候,编辑部信息组拟就已发表的内容分若干专题,编写成综述,供读者阅读参考,也借以弥补因许多过期期刊售缺而给读者带来的不便。随着时代的前进,各专业的科学与技术也随之发展,明胶行业也不例外,这些即将发表的综述不可避免地会跟不上这一形势。编辑部热诚地欢迎明胶界内外的同仁和朋友们给予补充与纠正。     

Numerical Analysis of the Coupling Between the Flow Kinematics and the Fiber Orientation in Eulerian Simulations of Dilute Short Fiber Suspensions Flows

This paper focuses on an accurate evaluation of short fibers suspensions models coupling the flow kinematics with the fiber orientation evolution. In coupled models the flow kinematics is usually solved using the finite element method, where the fiber orientation is introduced in the constitutive equation through its value in some points (nodes or integration points). In this paper we will compare in a simple steady shear flow, the exact solutions of the extra‐stresses associated with the fibers' presence with the numerical simulations obtained using both the method of characteristics and the discontinuous Galerkin's method to solve the equation governing the generalized gradient evolution, in order to avoid the introduction of any closure relation. The error introduced if a quadratic closure relation is considered in the constitutive equation will be also quantified.     

Polystyrenes with macro-intercalated organoclay. Part I. Compounding and characterization

Nanocomposites of polystyrene (PS) were prepared using a melt compounding or co-solvent method. Two commercial PS were used, and two organoclays—one prepared in this laboratory (COPS), and the other commercial Cloisite^® 10A (C10A). The COPS is a product of clay intercalation with a copolymer of styrene and vinyl benzyl tri-methyl ammonium chloride. According to the XRD diffraction data, the clay platelets in COPS and its PNC with PS were relatively well dispersed, i.e. with the interlayer spacings of d₀₀₁=7-8 nm. By contrast, d₀₀₁ in PNC prepared with C10A was only 4 nm. However, the number of clay platelets per stack in PS/COPS was found to be significantly larger than that in PS/C10A, viz. m=3-12, compared to m=2-6. The scanning and transmission electron microscopy indicated that in the PS matrix COPS existed in form of large, immiscible domains.