Particles formation

As we mentioned in Section 2.1 (Fig. la), there are three major models for particle formation in emulsion polymerization. According to these models, polymer particles are formed  

When a free radical in the aqueous phase enters a monomer-swollen emulsifier micelle and polymerization proceeds therein (micellar nucleation). 

When the chain length of a free radical growing in the aqueous phase exceeds its solubility limit and precipitates to form a particle nucleus (homogeneous nucleation). 

However, when the resultant polymer particles become unstable and coagulate, then whatever the mechanism of particle formation is, the final number of polymer particles produced is determined by a limited coagulation between existing polymer particles (coagulative nucleation). 

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One of the areas critical to the MCVD process was understanding the chemistry of the oxidation reactions. It was necessary to control the incorporation of Ge02 while minimizing OH formation. Additionally, understanding the mechanism of particle formation and deposition was critical to further scale-up of the process. 

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. 

Studies of the particle—epoxy interface and particle composition have been helphil in understanding the mbber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in mbber-modifted epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and -butyl acrylate—acryhc acid copolymers (309), have been used for toughening epoxy resins. 

Fig. 27. Phase contrast photomicrographs showing particle formation via phase inversion.

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettahle monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal hahit in particle formation. Minimize surface roughness in milhng. 

Particle conservation in a vessel is governed by the particle-number continuity equation, essentially a population balance to identify particle numbers in each and every size range and account for any changes due to particle formation, growth and destruction, termed particle birth and death processes reflecting formation and loss of particulate entities, respectively. 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

Each particle formation process is now considered in turn. 

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

Figure 6.2 Particle formation via aggregation and agglomeration adapted from Low and White, 1975)...

Tailoring of the particle size of the crystals from industrial crystallizers is of significant importance for both product quality and downstream processing performance. The scientific design and operation of industrial crystallizers depends on a combination of thermodynamics - which determines whether crystals will form, particle formation kinetics - which determines how fast particle size distributions develop, and residence time distribution, which determines the capacity of the equipment used. Each of these aspects has been presented in Chapters 2, 3, 5 and 6. This chapter will show how they can be combined for application to the design and performance prediction of both batch and continuous crystallization. 

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. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

The mass transfer effect is relevant when the chemical reaction is far faster than the molecular diffusion, i.e. Ha > 1. The rapid formation of precipitate particles should then occur spatially distributed. The relative rate of particle formation to chemical reaction and/or diffusion can as yet be evaluated only via lengthy calculations. 

The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model was then applied to the precipitation of CaC03 via CO2 absorption into Ca(OH)2aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution. 

For liquid-liquid crystal precipitation systems where the particle formation processes are fast, mixing becomes an important determiner of performance with a subtle interplay of micro- and mesomixing, which changes as scale of... 

The combination of non-ideal phase behaviour of solutions, the non-linearity of particle formation kinetics, the multi-dimensionality of crystals, their interactions and difficulties of modelling, instrumentation and measurement have conspired to make crystallizer control a formidable engineering challenge. Various aspects of achieving control of crystallizers have been reviewed by Rawlings etal. (1993) and Rohani (2001), respectively. 

Jones, A.G., Hostomsky, J. and Waclii, S., 1996. Modelling and analysis of particle formation during agglomerative crystal precipitation processes. Chemical Engineering Communications, 146, 105-130. 

Pratola, F., Simons, S.J.R. and Jones, A.G., 2000. Micro-Mechanics of Agglomerative Crystallization Processes. Proceeding of Advances in particle formation, American Institute of Chemical Engineers National Meeting, November 2000, Paper 22 g. 

Shekunov, B. Yu., Baldyga, J. and York, P., 2001. Particle formation by mixing with supercritical antisolvent at high Reynolds numbers. Chemical Engineering Science, 56(7), 2421-2433. 

Wachi, S. and Jones, A.G., 1995. Aspects of gas-liquid precipitation systems for precipitate particle formation. Reviews in Chemical Engineering, 11, 1-51. 

The Smith-Ewart theory has been modified by several researchers [13,20-24]. These researchers argued against the Smith-Ewart theory that (1) the particle formation also occurs in the absence of micellar structure, (2) the predictions on particle number with the Smith-Ewart theory are higher relative to actual case. 

Various kinetic models on particle formation were proposed by different researchers. These may be classified as follows (1) radical absorption mechanisms by Gardon [28-34] and Fisch and Tsai [13], (2) micellar nucleation newer models by Nomura et al. [35,36] and by Hansen and Ugelstad [37], (3) homogeneous nucleation by Fistch and coworkers [13,38,39]. 

When the emulsion polymerization is conducted in the absence of an emulsifier, this process is termed emulsifier free or soapless emulsion polymerization [68-73]. In this case, the particle formation occurs by the precipitation of growing macroradicals within the continuous... 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

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. 

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