Agglomerate measurement

As discussed in the previous chapters, single screw extruders are not good dispersive mixers, and therefore reliance is placed on suppliers to provide well dispersed additives in compounds and masterbatches. When it is considered necessary to test incoming materials or compare samples from different suppliers, which may, in particular, be required for carbon black masterbatch, there are a number of quality standards. These are mainly for carbon black in polyethylenes used in water pipes and cables, but can also be used for coloured pipes and cables where agglomerates can cause electrical failures. The tests can be divided between those that examine thin samples, and those which use extrusion filtering. 

The most common visual assessment for carbon black is made by comparing thin samples imder a microscope with a set of photomicrographs in the standard [4]. This requires matching the specimens appearance with those in the standards, as a specified number of agglomerates imder a certain size is permissible 

The specimen can be a microtomed section or a tiny piece pressed out between glass microscope slides on a hot plate. The hot pressing (sometimes referred to as the pinhead test) tends to smear striations, but is suitable for detecting agglomerates. 

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Barbosa-Canovas et al., 1987 Malave-Lopez et al., 1985 Yan and Barbosa-Canovas, 1997), and agglomerate strength (Adams et al., 1994). In particular, any agglomerate measurement will be affected by both the breakage properties of individual particles and the deformability of their assembly as a whole (Nuebel and Peleg, 1994). Equipment models used in different research projects are listed in Table I. 

Finally, another new laboratory technique, that is particularly important for the evaluation of agglomerates, measures porosity. Fig. 11.10 is the photograph of a Pore-master automatic pore size analyzer which is based on mercury porosimetry. While this technology is not new, the automatic features and computer control, including... 

A fundamental requirement in powder processing is characterization of the as-received powders (10—12). Many powder suppHers provide information on tap and pour densities, particle size distributions, specific surface areas, and chemical analyses. Characterization data provided by suppHers should be checked and further augmented where possible with in-house characterization. Uniaxial characterization compaction behavior, in particular, is easily measured and provides data on the nature of the agglomerates in a powder (13,14). 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Unvulcanized Latex and Latex Compounds. A prime consideration has to be the fluid-state stabihty of the raw latex concentrate and hquid compound made from it. For many years, the mechanical stabihty of latex has been the fundamental test of this aspect. In testing, the raw latex mbber content is adjusted to 55% and an 80 g sample placed in the test vessel. The sample is then mechanically stirred at ultrahigh speed (ca 14,000 rpm) by a rotating disk, causing shear and particle cohision. The time taken to cause creation of mbber particle agglomerates is measured, and expressed as the mechanical stabihty time (MST). 

Tableting, pressing, mol ding, and extrusion operations are commonly used to produce agglomerates of well-defined shape, dimensions, and uniformity in which the properties of each item are important and output is measured in pieces per hour (see Ceramics, ceramics processing Pharmaceuticals Metallurgy, powderp tallurgy Plastics processing).

Free-swelling tests are commonly used to measure a coal s caking characteristics. A sample of coal is packed in a cmcible or tube, without compaction, and heated at a fixed rate to about 800°C. Infusible coals distill without changing appearance or state of agglomeration. The fusible coals soften, fuse, and usually sweU. The profile of the resultant coke is compared to a series of reference profiles so that a swelling index can be assigned. The profiles represent indexes between 0 and 9. The best cokes come from coals having indexes between 4 and 9. 

Specification for Particulates Feed, recycle, and product from size reduction operations are defined in terms of the sizes involved. It is also important to have an understanding of the degree of aggregation or agglomeration that exists in the measured distribution. 

Bulk density is easily measured from the volume occupied by the bulk solid and is a strong func tion of sample preparation. True density is measured by standard techniques using liquid or gas picnometry Apparent (agglomerate) density is difficult to measure directly. Hink-ley et al. [Int. ]. Min. Proc., 41, 53-69 (1994)] describe a method for measuring the apparent density of wet granules by kerosene displacement. Agglomerate density may also be inferred from direcl measurement of true density and porosity using Eq. (20-42). 

Agglomerate porosity can be measured by gas adsorption or mercuiy porosimetiy. However, any breakage or compression of the granules under high pressure during porosimetiy can invahdate the results. 

It is notoriously diffieult to solve the so-ealled inverse problem and extraet kinetie data using the population balanee. Muralidar and Ramkrishna (1986) deseribe a proeedure to obtain agglomeration frequeneies from measured size distributions without the kinetie proeesses of nueleation, growth and disruption. The authors point out that even if the experimental data are very aeeurate, it is not always possible to estimate the aggregation frequeney satisfaetorily and to distinguish between different meehanisms. 

For stirrer speeds of 4.2, 8.4, 16.7, 25 and 33.4Fiz, agglomeration kernels obtained in this study vary from 0.01 to 183 s . Unfortunately, no other measured data for agglomeration of calcium oxalate analysed using Smoluchowski s kernel were found in the literature. The corresponding values reported by Wojcik and Jones (1997) for calcium carbonate, however, cover a range from 0.4 to 16.8s-. ... 

The model is able to predict the influence of mixing on particle properties and kinetic rates on different scales for a continuously operated reactor and a semibatch reactor with different types of impellers and under a wide range of operational conditions. From laboratory-scale experiments, the precipitation kinetics for nucleation, growth, agglomeration and disruption have to be determined (Zauner and Jones, 2000a). The fluid dynamic parameters, i.e. the local specific energy dissipation around the feed point, can be obtained either from CFD or from FDA measurements. In the compartmental SFM, the population balance is solved and the particle properties of the final product are predicted. As the model contains only physical and no phenomenological parameters, it can be used for scale-up. 

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

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