Coalescence primary particle formation

THE COLLISION-COALESCENCE MECHANISM OF PRIMARY PARTICLE FORMATION... 

Aerosol Reactors Commercial and Pilot Scale 332 Flame Reactors 332 Pyrolysis Reactors 334 Electron-Beam Dry Scrithhirif 335 Evaporation-Condensation Generators 336 The Collision-Coalescence Mechanism of Primary Particle Formation 338... 

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. 

There is another limitation on the applicability of this analysis. It holds when particle collision leads to coalescence and not to the formation of. solid primary particles and their aggregates. The assumption of coalescing particles usually holds best during the early stages of particle fonnation. In the later stages, for highly refractory (low vapor pressure)... 

The collision-coalescence mechanism of particle growth discussed in this chapter is thought to control primary particle size in Hame reactors. The emphasis is on the synthesis of transition metal oxide particles, which are important in the manufacture of pigments, addili ve.s, and ceramic powders. Also discussed are the factors that determine the formation of necks between particles and particle crystallinity. As demands on product quality become more stringent, more research will be needed on particle size, unifonnity. crystallinity, and aggregate formation. 

Industrial flame reactors are operated at high particle concentrations and high gas temperatures. As a result, particle collision rates are high primary particle size is determined by the relative rales of particle collision and coalescence (Ulrich, 1971). The collision/coalescence mechanism for particle formation is based on a series of steps assumed to proceed as follows ... 

In the emulsion polymerization of VC, the formation of polymer particles may occur by a coagulative nucleation process [26,27]. In favor of this approach is the mutual insolubility of VCM and PVC Here, the primary particles formed undergo coalescence whereby large true polymer particles are formed. In addition, the formation of primary particles during polymerization favors the growth of particles by association of small with large particles. The latex particles were also formed after the disappearance of the micelles [92]. Thus, the polymerization in the monomer-saturated aqueous phase produces the new particles. The precipitation of oligomer radicals within the monomer droplets may increase the number of particles. In favor of this hypothesis is the formation of less reactive (occluded) radicals. 

In the emulsion polymerization of VC, the formation of polymer particles by the coagulative nucleation mechanism is expected to be operative. As polymerization proceeds, the formation of primary particles takes place. In this process, the primary particles undergo coalescence to form large polymer particles and/or they associate with large particles. 

In a similar method, the trisodium citrate can be replaced by PVP. In a study carried out by Shin et al. [30], transmission electron microscopy (TEM) findings showed that both the amount and the molecular weight of the PVP in the irradiated solution affected the average size of the nanoparticles produced. A three-step mechanism for the formation of these nanoparticles has been postulated. First, the silver ions interact with the PVP, after which nearby silver atoms that have been reduced by y-radiation aggregate to form primary particles. Finally, nearby primary particles coalesce to form larger aggregates which are termed secondary particles. 

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