Polymer powders mixing part I: Mixing characterization in rotating cylinders

In the production of rotomolded parts, the distribution of particle sizes in the form of polymer powder is highly important to produce parts of good quality. In the case of polymer blends, the particle size distribution becomes even more important to assure that good mixing occurs before the polymer melts. In this work, the mixing of free flowing polymer powders in a horizontal rotating cylinder has been studied. Special attention is given to the initial moments of mixing and the transition from a standstill to a fully developed rolling regime. The parameters studied include Froude number (rotational speed), cylinder loading, relative powder composition and particle sizes. For the first time, mixing dynamics are quantified using two image analyzes: grey-level co-occurrence matrix (GLCM) and Threshold methods. Using the experimental data, a model is proposed to follow the mixing dynamics and to determine the equilibrium time. The results also indicate that the Froude number is not enough to characterize the flow regime and the filling ratio must be accounted for.     

On the characterization of polymer powders mixing dynamics by texture analysis

In this work, the transverse mixing of free flowing particles in a horizontal rotating drum has been studied. Special attention is given to the initial moments of mixing and the transition from a standstill to a fully developed rolling regime. The parameters studied include rotational frequency (1.3 to 4.6 rpm), loading (15 to 50%), relative black/white powder content (1/3 to 2/3) as well as drum diameter (50 to 75 mm). To this end, four methods have been used to quantify dispersion: (1) Discrete Wavelet Transform (DWT), (2) Gray-Level Co-occurrence Matrix (GLCM), (3) Contact and (4) Entropy. Of these four methods, the GLCM algorithm was found best suited for this application: it is fast, reliable and easily interpretable despite requiring little user input. While Wavelet and Contact analysis methods also provide good results, Entropy systematically overestimates the degree of mixing for any given time. The results show that mixing dynamics from standstill can be modeled by a second-order dynamic based on the filling ratio and the number of drum rotations.     

RGB颜色模型应用于评价顶吹混匀时间的方法

针对评价顶吹宏观混匀时间的方法进行了气体顶吹搅拌水动力学实验研究,利用基于混合过程中示踪粒子的分布随时间演化规律的RGB颜色模型来确定搅拌容器内的宏观混匀时间。通过定义像素阈值分离每一像素,构建混匀像素比M值作为确定混匀时间的指标,观察M值的变化规律,利用3σ方法确定混匀时间。针对喷枪插入深度及流量,用量纲为1强度单位表述为0.5和1的实验工况一,当阈值分数X=90%时,测定混匀时间为13.30s。分析结果发现,RGB颜色模型能够基于混合过程中示踪粒子的分布情况确定混匀时间,且与贝蒂数法和电导率法测定的混匀时间偏差不超过10%。为解决在视觉上评价多相流混合效果等工程问题提供了一种新的思路,为提高ISA炉使用寿命、强化ISA炉冶炼生产以及优化ISA炉工艺过程提供了一定的实验依据。     

Predicting changes in the color of powders: Does Melamed's model fit to real industrial powders?

In 1963, Melamed proposed a model that expressed reflectance of a powder described as a population of spherical particles of unique diameter as a function of size, shape, and optical characteristics of the powder. This article shows how this model has been adapted to fit to industrial powders. An example of use for industrial quartz is given. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 413–419, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20058     

Instrument for ceramic particle dispersion II: Computational fluid dynamics analysis

A planetary-type mixer using a container equipped with stainless steel mesh has been developed. For various slurries (Al₂O₃ in water), each modeled as Newtonian fluid, the shear stress was calculated using computational fluid dynamics (CFD) under various mixing conditions and with different equipment properties. The meshed-geometry included more than 100,000 nodes with hexahedral cells in one zone, with quadrilateral cells in the remaining zones. The fluid viscosity, rotation rate, and mesh opening affected the maximum shear stress. The shear stress increased concomitantly with increasing fluid viscosity. The container rotation rate and the maximum shear stress share a proportional relation. For a fluid with 9.2 mPa s, the shear stress was 134 Pa or more for a 0.81 mm and larger mesh opening, as observed at the bottom of the container. Mesh having an opening smaller than 0.81 mm generated high shear stress on the mesh surface. The maximum shear stress increased with decreasing mesh opening size. The particle size distributions of the Al₂O₃ particles in the slurries after treatment by the mixer were estimated under conditions similar to those of the calculations. Results show peaks in the particle size distributions of the Al₂O₃ particles in the slurries before treatment at 0.2 and 2-70 μm because of the primary particle size and agglomerates. The amounts of the agglomerate decreased concomitantly with the decreased mesh opening size. When slurries pass through the small mesh openings, high shear stress is generated. That achieves the good dispersion of the sub-micron sized Al₂O₃ particles in the slurry.     

Grain analysis of highly disperse powders using a scanning photosedimentograph

A scanning photosedimentograph built by specialists at Ural State Engineering University is described. Results of grain analyses of various refractory materials conducted by means of the device are presented. Data are given for alumina, cement, and quartzite, bauxite, and mullite slips. Experimental dependences of the error of measurement, on the concentration of the solid phase are derived. Grain compositions obtained on the photosedimentograph are compared with data obtained on microscreens, and the results are shown to be in satisfactory agreement. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp. 19–22, April, 1998.     

Visualizing isostatic pressing of ceramic powders using finite element analysis

Cold isostatic pressing, where a rubber bag is filled with ceramic powder, sealed and subjected to hydrostatic pressure, is a method of forming ceramic components with near-net shape. Cracking of the ceramic compact after pressing is one problem associated with the pressing of complex shapes. One mechanism responsible for the cracking of components is the interaction of the rubber bag with the component during the final stages of decompression where the elastomer can deform significantly and impose non-uniform loadings on the compact. Visualization of the detachment process and the stresses induced in the ceramic compact offer the opportunity for the design of press tooling which minimizes the potential for cracking of the components. In this paper, both 2D and 3D finite element models are developed to investigate this problem. The effect of different contact conditions between the compact and the rubber bag is discussed, and the distribution of stresses resulting from the interaction of the compact and the rubber tooling presented. These indicate methods for alleviating the stresses within the compact through suitable tool design.     

Behaviour of gas-fluidized beds of fine powders part II. Voidage of the dense phase in bubbling beds

The collapse rate technique has been used to evaluate the average dense phase properties in vigorously bubbling beds of fine powders. The results of experiments on 13 air/solid systems are used to correlate the average dense phase voidage, $\text{?}$_D, in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. $\text{?}$_D increases as the particle density and mean particle size decrease, and as the fraction of fine <45 μm, gas viscosity and gas density increase.The gas velocity, U_D, through the dense phase of a bubbling fluidized bed has been calculated from $\text{?}$_D assuming Darcy's law and can now be predicted. Since it is less than the minimum bubbling velocity, the improved performance of fluidized bed reactors when the gas/solid properties are changed so as to increase $\text{?}$_D can be attributed more to smaller bubbles splitting and coalescing frequently rather than to the small amount of extra gas passing through the dense phase.     

A quantitative analysis of the mixing of two segregating powders of different density in a gas-fluidised bed

The mixing/segregation behaviour of binary mixtures of powders differing in density has been studied experimentally. The mixing index M can be related to the fluidising velocity U by means of the logistic equation:

where

U_TO in the gas velocity at which M = 0.5, and U_F is the lower of the two minimum fluidisation velocities. U_TO, the take-over velocity, is related to the size, density, shape and proportion of the two powders and the bed aspect ratio. A complex empirical equation including all these factors has been obtained.If there is no density difference between the powders, the logistic equation can still be used provided U_TO is evaluated experimentally.     

Effects of adhesion on mixing homogeneity part I: ordered adhesion—random adhesion

According to Hersey, adhesion of a finely powdered, monosized ingredient to a course, monosized diluent may yield ordered powder mixtures of a higher degree of homogeneity than conforming to random mixtures.However, this concept implies the adherence of an identical (or almost identical) number of the fine particles to each of the carriers (‘ordered adhesion’). If, on the contrary, a random variation of the number of adhering fines per diluent particle is assumed (‘random adhesion’), pseudorandom mixtures can be formed at best. This type of mixture shows the degree of homogeneity as well as the statistical properties of fully randomized systems.To produce ordered powder mixtures, it is thus the basic requirement that ordered but not random adhesion must be achieved during mixing. So far, no mechanism has been established that may yield ordered adhesion.     

Ultrasonic welding using tie-layer materials. part II: Factors affecting the lap-shear strength of ultrasonic welds

The influence of tie-layer Melt Flow Index on the lap-shear strength of ultrasonic welds in oriented polypropylene (OPP) has been evaluated. The tie-layer Melt Flow Index was varied from 0.03 dg/min to 2600 dg/min; the highest lap-shear strength properties were obtained using tie-layers that had melt flow index values between 30 and 100 dg/min. When using low Melt Flow Index tie-layers, hot spot formation and concomitant changes in fusion zone and heat-affected-zone dimensions produced stress concentrations that promoted failure in oriented polypropylene material away from the bondline region. When very high Melt Flow Index (2600 dg/min) tie-layers were used, the mode of failure during lap-shear testing was a mix of cohesive, in oriented polypropylene, and adhesive failure. The molecular weight of material at the bondline was not markedly affected by the thermal cycle produced during ultrasonic welding. Only the flash ejected when using low Melt Flow Index tie-layers exhibited any evidence of degradation; it is suggested that the ejected flash may have been degraded because of a combination of thermal, cavitation, and thermo-oxidative processes.     

Characterization of feeder effects on continuous solid mixing using fourier series analysis

Variance reduction ratio (VRR) is generally considered as an important index in characterizing the continuous powder mixing. Although the capacity of the mixer to smooth out feeder fluctuations can be expressed by the VRR, few studies are performed quantitatively in this area. The feeder effects are investigated on the solid mixer through the Fourier series analysis. The VRR is deduced quantitatively using the Fourier series of the feed rate variability and the residence time distribution (RTD), which facilitate the explicit decomposition of VRR into intensities of different frequency components. It provides a novel model to determine whether the integrated feeder‐mixer system satisfies specific solid mixing performance criteria, and provides guidelines of system improvement. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Waterborne latex coatings of color: II. Surfactant influences on color development and viscosity

A matrix of coating variables, nonassociative versus associative thickeners, different latex median particle sizes, individual surfactants and colorants [carbon black (CB), red, and yellow pigments], was examined for their influence on variances in coatings rheology and color development. Within the different coating groups, the variable of interest in this study was the surfactant added to the colorant formulation. In all three colorant formulations, sodium dodecyl sulfate (an anionic surfactant) provided poorer color development (CD) than in applied formulations containing an equivalent nonylphenol oxyethylene (EO) surfactant. In CB formulations, nonionic surfactants with higher EO content provide improved color development at low (2 mM) concentrations, but near equality in CD is achieved with low EO surfactants at higher concentrations. In contrast to CB formulations, red and yellow colorants exhibit good color development with high EO content nonionic surfactants only at low nonionic surfactants concentrations. This variance appears to be related to the interactions of surfactants with inorganic pigments (talc and laponite) in the colorant formulation. The coating’s rheology is related to latex, thickeners, and surfactant components of the paint, as has been noted in previous studies, but not to the nature of the color pigment. The viscosity of the hydroxyethyl cellulose (nonassociative type) and HEUR (associative type) thickened paint decreased with colorant addition due to dilution effects. There were no unusual deviations with the NP(EO)_x surfactants, except when a large hydrophobe nonionic surfactant [e.g., C₁₈H₃₇(EO)₁₀₀] is added. In HEC thickened coatings, the viscosity decreases when C₁₈H₃₇-(EO)₁₀₀ is in the colorant due to that surfactant inhibiting depletion flocculation. In the C₁₈H₃₇(EO)₁₀₀ coatings containing the HEUR thickener, significant increases in viscosity were observed, above the dilution values observed with the colorant addition. This is related to the viscosity maximum in the low concentration of HEUR with the C₁₈H₃₇(EO)₁₀₀ surfactant. Color development is independent of the viscosity profile of the coating. Presented in part at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2003 in Philadelphia, PA.     

An analysis of ball-and-race milling part II. The babcock E 1.7 mill

A mathematical model was developed for a ball-and-race mill based on specific rates of breakage and primary fragment distributions. The model includes internal classification of particles falling back into the race and external classification due to the built-on classifier. It was demonstrated that the normalized primary fragment distribution produced in a pilot-scale Babcock E-type mill of 17 in. race diameter was the same as in the Hardgrove laboratory test mill and that the specific rates of breakage varied with particle size in the same manner. Steady-state continuous tests on the pilot-scale mill showed that breakage rates depended on the rate of feed, since the mill pulled less power at low feed rates. This effect plus the residence time effect gave coarser product size distributions at low and high feed rates than at a medium feed rate. Model simulations based on parameters measured in the Hardgrove mill correctly predicted the product size distribution from the E-type mill.     

Characterization of plastic-bonded explosives by pyrolysis gas chromatography and multivariate data analysis part II: Aging studies

Application of Curie point pyrolysis gas chromatography (Py-GC) and principal component analysis (PCA) in combination with a new normalization method could show the presence of patterns in PCA score plots not visible with the commonly used normalization method. In the first four principal components (PC's), two patterns were observed which could be attributed to aging and the crosslink density (CD). Aging of PBX samples has a complex effect on the peak loadings on PCI and are difficult to reveal. This in contrast to the peak loadings on PC3 and PC4 where both PC's are dominated by only one compound (respectively, C₅H₆ and cyclohexene). This means that the formation of both these compounds proceeds independently from each other, i.e., by different mechanisms. It was shown that aging is slowed down in samples containing more of the antioxidant Flexzone. From the correlation matrix a few compounds were found to have very high correlations (> 0.98). This was the case for methane, propane and n-butane in both unaged and aged samples indicating that these compounds do not originate form moieties that are affected by aging. Also, cis- and trans-butene have such high correlations and were formed in a 0.86:1 ratio, probably through the same mechanism. The absolute intensities decrease upon aging suggesting that both compounds originate form those parts of the polymer that are sensitive to aging. PC2 was related to saturated and unsaturated hydrocarbon compounds. The unsaturated compounds result from the elimination of water (free OH groups) from samples with CD values below 1.     

Flow,suspension, and mixing dynamics in DASGIP bioreactors: Part 1

The bioreactor flow environment has a significant impact on process performance, especially in stem cell cultures. The work of Correia et al found intermittent agitation modes to improve induced pluripotent stem cell (iPSC)-cardiomyocyte differentiation yields; however, to date, the impact within the flow has not been fully characterized. This work aims to characterize the flow dynamics occurring within a commercially available DASGIP bioreactor, equipped with a two-blade paddle impeller, operating under different agitation modes and for two bottom geometries. The paddle impeller configuration generated an axial flow profile due to a large impeller D/T and blade confinement with the bioreactor wall. The application of intermittent agitation was shown to induce two transient spikes in flow velocity and shear stress, the amplification of which increased with dwell duration. Marginally increasing the dwell duration was shown previously to increase differentiation yields, therefore it can be stipulated that introduction of these spikes was favorable toward cardiogenic differentiation.     

Polymer weld strength predictions using a thermal and polymer chain diffusion analysis

A new simplified model for strength prediction of welded polymer-polymer interfaces is proposed. Two schools of thought exist for strength prediction of polymer-polymer interfaces: microscopic analysis of polymer chain behavior, and macroscopic analysis of bulk polymer properties during welding. The microscopic analysis is based on De Gennes' reptation theory for macromolecules (1), and has been described extensively by both analytical and empirical techniques. Reptation models are based on constant temperature interfaces, which are not found in any actual welding process. Conventional macroscopic analyses of welding empirically relate strength to important thermal process parameters, such as power density and heating time, but do not address the behavior of the polymer chains. Little interaction exists between these two schools of thought. This study seeks to combine these two areas, using reptation theory to explain the polymer chain interactions on a macromolecular level, and relate the interface thermal signature to strength prediction. The result of the new model is a method for strength prediction that takes into account fundamental materials properties as well as the engineering conditions imposed in a realistic welding process to predict weld strength. The model is adapted to run for similar conditions as two separate empirical endeavors. The results show that the model is effective in predicting overall trends, which emphasizes the importance of examining heat transfer effects in any polymer welding process.     

Thermodynamic analysis of an adsorption-based desalination cycle (part II): Effect of evaporator temperature on performance

The potential to use waste heat to co-generate cooling and fresh water from saline water using adsorption on silica recently prompted a detailed study of the performance of adsorption-based desalination (AD) using a thermodynamic model (Wu et al., 2010). Further study has now revealed that the nature of the cycles and the performance are significantly influenced by the evaporator temperature relative to the cooling water temperature. This paper reports this study. Following an enumeration of all possible thermodynamic cycles for an AD system that arise from varying the evaporator temperature relative to the cooling water temperature, the effects of operating water temperatures and the silica gel water vapour capacity on the fresh water productivity and cycle energy consumption are presented and discussed.