Particle Agglomeration Measurements

Particle agglomerate sizes can be determined using a laser diffraction particle size analyzer, such as a Brinkmann PSA-2010. With this instrument, the principle of particle size measurement is based on various light scattering angles generated by paiTicles of 

FIGURE 3.8. Particle size distribution change for a Hombikat UV-100 After sonication (—) and after one hour mixing in the reactor (—). (Reprinted with permission of M. SaUces, PhD Dissertation, Univereity of Western Ontario 2002). 

FIGURE 3.9. SEM micrographs showing the distribution of Degussa P25 particles for the 4.8-5.6-wl%) loading (Reprinted from Appl. Catal. B Environ., 1011, H. Ibrahim andH.I. de Lasa, Photocatalytic conversion of air borne pollutants Effect of catalyst type and catalyst loading on a novel Photo-CREC-Air unit, 1-13, Copyri t 2002, with pennission from Elsevier). 

For Hombikat UV-100, increasing the number of particles leads to a reduced number of larger-particle agglomerates (Ibraliim and de Lasa, 2002). The overall effect, as confirmed using SEM, shows that the total iiTadiated catalyst remains close to constant when the catalyst loading is increased. This phenomenon explains the close to constant photo degradation rate. 

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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). 

This dependency was also drawn onto Fig. 27 and, as can be seen, the particle sizes measured during this experiment fall more or less on the theoretical line further lowering of the viscosity yielded no agglomerates, as expected. 

The rate of particle agglomeration depends on the frequency of collisions and on the efficiency of particle contacts (as measured experimentally, for example, by the fraction of collisions leading to permanent agglomeration). We address ourselves first to a discussion of the frequency of particle collision. 

Capillary forces of attraction cause the hydrate-encrusted droplets to agglomerate. These capillary forces are a strong function of temperature at low temperatures the forces decrease between the particles, as measured by Taylor (2006). 

By electron microscopy the size of the primary particles in the aggregates is estimated to be about 10 nm. Particle size measurements using a nanosizer show the size of aggregates dispersed in a well wetting solvent to be in the range of 100 nm. Laser diffraction of fumed silica dispersed in air provides sizes of agglomerates larger than 5 pm. 

Another problem in PSD measurement is caused by particle agglomerates. A single large agglomerate may contain multiple small crystals. But this agglomerate will be counted as one large particle if it is not dispersed during measurement. Therefore, it is always... 

The thin film consisted of large particle agglomerates, and most of the agglomerates were disconnected. XRD measurements revealed (image not shown) an amorphous structure without distinct X-ray reflection peaks. In conclusion the molecules deposited by solution cast methods are randomly oriented on the Si/Ti02 substrate. 

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. 

In particle size measurements, one of the most inqKatant problems is the obtaining of a representative sample. It is the most difficult problem, and one that is often overlooked or undoestimated, especially for latexes. This problem increases in magnitude as the sample source increases in size, such that it is easier to obtain a representative sample from a 1-litre container than it is to obtain one from a 200-litre drum and, in turn, it is even more difficult to obtain a representative sample from a rail tank car. The reasons for this are obvious when one takes into consideration such factors as agglomeration, flocculation, settling, contamination and so forth. It is sufficient simply to call attention to this problem while collecting a sample for measurement as well as raising this question before measurements are made. 

These have the potential of eliminating a number of problems dealing not only with sample handling and preparation, but also with ultimate versus agglomerate size issues, and open the door to a whole new way of addressing particle size measurements. 

Particle size measurement with microscopy is a direct observation method, where individual particles are observed directly, so that their shape and the degree of agglomeration can be identified at the same time. It provides straightforward information on a ceramic powder. There are three types of microscopies optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Optical microscopes can be used for particle sizes larger than... 

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