Coalescence particle formation

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

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. 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

The initial step of the reaction, Eq. (7), provides the individual metal sulfide molecules via reaction of the M2+ ions with H2S. In the case of films derived from fatty acids, the two carboxylate functions, associated with the M2+ ion, are the sink for the two protons released from the reaction. The diffusion and coalescence of the individual MS molecules to give MS particles are depicted in Eq. (8). Despite an abundance of literature concerning Q-state particle formation in LB films, there has been little discussion relating to mechanistic aspects of how the nature of the LB support matrix effects the processes depicted in Eq. (7) and (8). The remainder of this section outlines the mechanistic and kinetic insights gained into these processes over the course of study of metal chalcogenide formation in LB films. 

Although emulsion polymerization has been carried out for at least 50 years and has enormous economic importance, the detailed quantitative behavior of these reactors is still not well understood. For example, there are many more mechanisms and phenomena reported experimentally than have been incorporated in the existing theories. Considerations such as non-micellar particle formation, non-uniform particle morphologies, polymer chain end stabilization of latex particles, particle coalescence, etc. have been discussed qualitatively, but not quantitatively included in existing reactor models. 

Coagulation of polymer particles, as of any colloidal dispersion, depends on a number of factors, among them the stabilizing action of the emulsifier, the compatibility of soap and polymer, and the consistency of the polymer-monomer interior of the particles. When vinyl chloride is polymerized above the softening point of the swollen polymer, particles coalesce to such an extent that the soap coverage remains about 100% (33)—i.e., the number of particles decreases with conversion. Extensive coalescence after the period of particle formation occurs in the polyn erization of vinylidene chloride investigated by Sweeting and coworkers (21,32, 39). 

Much remains to be done with soluble monomers. Coalescence and coagulation of oligomeric and polymeric molecules and indeed of particles have prevented the quantitative treatment of such systems. It might be useful to use seeded polymerizations which provide a means of separating the processes of particle formation and particle growth. 

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 PRIMARY PARTICLE 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 ... 

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... 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

Microsuspension and Inverse-microsuspension. In suspension polymerizations, particle formation occurs through a droplet breakup-coalescence mechanism, with the diameter controlled by the temperature, interfacial tension, agitation intensity and conversion. Suspension polymerizations have typically been characterized by an initiator soluble in the monomer phase and particle diameters in the 50-1000 pm range [40]. Smaller particles (0.2-20 pm) have been produced at higher agitation speeds (lower interfadal tensions) [41] and in such cases a prefix micro has been added to the nomenclature (microsuspension) to reflect both the dominant synthesis conditions (suspension) and the nominal particle size (1 micron). Therefore, microsuspension polymerization has historically referred to a subdomain of suspension polymerization occurring at smaller particle sizes. Based on an analogy to this nomenclature, inverse-microsuspension polymerization has been proposed for similar water-in-oil... 

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