Agglomerates primary

In commercial applications of submicron powdered materials as additives, fillens, and pigments, nonagglomerated or weakly agglomerated primary particles are usually desired. Quantitative, predictive methods for describing neck formation and the nature of the bonds between particles are not available. In the ab,sence of proven methods, we di.scu.ss some guidelines that may serve as a starting point for future research. 

Pigment powders consist primarily of particles, aggregates and agglomerates. Primary particles, which are true single crystals with a specific X-ray pattern, and aggregates consisting of primary particles that have... 

Pig. 9 Influence of the particle Reynolds number Rep on the temporal development of a the total number of particles Np considering both primary particles and agglomerates and b the total number of agglomerated primary particles with the particle size distribution as a pruameter (Table 1 Cases R and V)... 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

Characterization. The proper characterization of coUoids depends on the purposes for which the information is sought because the total description would be an enormous task (27). The foUowiag physical traits are among those to be considered size, shape, and morphology of the primary particles surface area number and size distribution of pores degree of crystallinity and polycrystaUinity defect concentration nature of internal and surface stresses and state of agglomeration (27). Chemical and phase composition are needed for complete characterization, including data on the purity of the bulk phase and the nature and quaHty of adsorbed surface films or impurities. 

Agglomeration a process where precipitation particles grow by coUision with other particles. Pigment agglomerates can be broken into smaller primary particles with the aid of mechanical shear. 

FIG. 20-71 Classification of agglomeration processes by agitation intensity and compaction pressure. Relative density is with respect to primary particle density and equals (1 — e) where e is the solid volume fraction. Reprinted from Granulation and Coating Technologies for High-Value-Added Industties, Ennis and Litster (1996) with permission of E G Associates. All rights reserved. 

Dispersion polymer obtained by coagulation of a dispersion. It consists of agglomerates with an average diameter of 450 pm made up of primary particles 0.1 pm in diameter. 

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