Particle formation stages

Stable particles in sufficient number, all the oligo-radi-cals and nuclei generated in the continuous phase are captured by the mature particles, no more particles form, and the particle formation stage is completed. The primary particles formed by the nucleation process are swollen by the unconverted monomer and/or polymerization medium. The polymerization taking place within the individual particles leads to resultant uniform microspheres in the size range of 0.1-10 jjLvn. Various dispersion polymerization systems are summarized in Table 4. 

Marangoni interfacial stresses which slow the dynamics of wetting. Additional variables which influence adhesion tension include (1) impurity profile and particle habit/morphology typically controlled in the particle formation stage such as crystallization, (2) temperature of granulation, and (3) technique of grinding, which is an additional source of impurity as well. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

The water solubilities of the functional comonomers are reasonably high since they are usually polar compounds. Therefore, the initiation in the water phase may be too rapid when the initiator or the comonomer concentration is high. In such a case, the particle growth stage cannot be suppressed by the diffusion capture mechanism and the solution or dispersion polymerization of the functional comonomer within water phase may accompany the emulsion copolymerization reaction. This leads to the formation of polymeric products in the form of particle, aggregate, or soluble polymer with different compositions and molecular weights. The yield for the incorporation of functional comonomer into the uniform polymeric particles may be low since some of the functional comonomer may polymerize by an undesired mechanism. 

Particle formation in the early stages of a batch reaction is normally quite rapid. Hence the particle surface area produced is able to adsorb the free emulsifier quite early in the reaction (2 to 10% conversion) and particle formation ceases, or at best slows to a very low rate. Particles formed in the beginning of the reaction would have approximately identical ages at the end of the batch reaction. These particles would be expected to be nearly the same size unless flocculation mechanisms, stochostic differences, or secondary nucleation factors are significant. 

As a rule, short nucleation times are the prerequisite for monodisperse particle formation. A recent mechanistic study showed that when Pt(acac)2 is reduced by alkylalu-minium, virtually all the Pt cluster nuclei appear at the same time and have the same size [86]. The nucleation process quickly consumes enough of the metal atoms formed initially to decrease their concentration below the critical threshold. No new metal cluster nuclei are created in the subsequent diffusion-controlled growth stage. 

If one chooses a chelating agent with a relatively high stability constant, such as EDTA, one may find some ideal separation of nucleation and growth stages as a decisive prerequisite for monodispersed particles formation, since a drastic change of supersaturation can be expected in a system with metal ions around the stoichio-... 

By reducing the solvent-power of a dense gas in several stages, fractionation of the product and unreacted reactants is possible. Fractionation is also possible by extracting the mixture, usually with the same dense gas as used in reaction, but under different process conditions. In all downstream processing schemes, various particle-formation techniques [31] or chromatographic techniques can be integrated. 

This model is based on the particle formation during polymerization where the polymer particles are sterically stabilized by graft-copolymerized PEO chains on the particle surface. In the later stage the polymer particles were supposed to grow in size mainly by copolymerization of monomers occluded in the particles which may favor the substrate monomer (styrene) over the macromonomer as compared to the composition in the continuous phase. 

In 1965 Dunn and Taylor confirmed the theory for vinyl acetate polymerization (15), and proposed, in the light of the presumed importance of rapid coagulation during the earliest stages of reaction, that the "DLVO" theory for colloid stability (16) be applied. Fitch proposed a kinetic basis for a quantitative theory and observed that for observation of particle formation kinetics, "fast" reaction techniques must be used because "particle formation occurs in a matter of seconds or even less (17)". 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

Our final goal in the present paper is to devise an optimal type of the first stage reactor and its operation method which will maximize the number of polymer particles produced in continuous emulsion polymerization. For this purpose, we need a mathematical reaction model which explains particle formation and other kinetic behavior of continuous emulsion polymerization of styrene. 

Some polymer-composition vs. conversion curves were obtained for the copolymerizations with different f s (Figure 2), and all of them seem to intersect the ordinate at 1.0. From the initial slope of the curves and the monomer ratio in the aqueous phase the monomer reactivity ratio was calculated, but the calculation resulted in a negative r2. Therefore, it was concluded that the copolymerization could not be regarded as a homogeneous one even just after the beginning of the reaction. The first stage was considered to be a transitional stage to establish the particle formation. 

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