Suspensions formation process

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. 

Once the pulp fibres have been refined to the necessary degree, they are then formed into a sheet of paper on the paper machine. The paper formation process itself is essentially a fast filtration process and involves the delivery of a dilute fibre suspension in water on to a woven endless plastic wire belt, through which it drains to form a wet fibre network. The Fourdrinier paper machine is the most well-established system for forming the wet web, but there are now many variations of this basic principle. A schematic diagram of the Fourdrinier formation process is shown in Figure 5.15. 

The methods of disintegration rely entirely upon increasing the dispersity of a solids which process can, at least theoretically, be stopped at any instant resulting in the formation of a suspension of definite dispersity but one that is not necessarily stable. The processes of suspension formation by methods of condensation on the other hand are more complicated, owing to the fact that unless the resulting colloidal suspension possesses at least some degree of stability the process of condensation once set in operation will not cease but proceed until the transformation to the macrocrystalline structure is complete. 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

The polyurethane (PU) can be considered an environment-friendly material because the urethane bond resembles the amide bond, which implies possible biodegradability. It can be used in various elastomer formulations, paints, adhesives for polymers and glass, and artificial leather as well as in biomedical and cosmetic fields. Polyurethane spheres were prepared from 20/40% of PU prepolymer solution in xylene [91]. PU droplets were formed in water with the SPG membrane of different pore size (1.5-9.5 pm) and then polymerized to form the final microspheres. Finally, spherical and solid PU particles of 5 pm were obtained after the removal of the solvent. In another study, Ma et al. reported the formation of uniform polyurethane-vinylpolymer (PUU-VP) hybrid microspheres of about 20 pm, prepared using SPG membranes and a subsequent radical suspension polymerization process [92], The prepolymers were solubilized in xylene and pressed through the SPG membrane into the continuous phase containing a stabilizer to form uniform droplets. The droplets were left for chain extension at room temperature for some hours with di- and triamines by suspension polymerization at 70 °C for 24h. Solid and spherical PU-VP hybrid particles with a smooth surface and a higher destructive strength were obtained. 

The suspension polymerization process allowed the formation of capsules of l-30 rm consisting of migrin oil as core and polyurea as wall material. The latter was formed by interfacial polycondensation reactions between different diisocyanates and emulsified ethylenediamine [106],... 

Figure 2 presents some SEM pictures taken directly from suspensions of the fluffy precipitate of [Pt(NH3)4](HC03)2 in ethanol before addition of TEOS and representing the formation process of the templating Pt-salt nanofibers. Figure 2a shows the salt as it was received from the supplier. The micrograph 2b was taken after dissolution of the salt in water and re-precipitation with ethanol and, finally, micrograph 2c after the addition of TEOS and calcination. 

The propellants used were some years agochloro-fluorocarbons (CFCs) (11 and 12) in various proportions (65 35 or 50 50). The manufacturing process includes the suspension formation in liquid propellant 11 in a cooled and hermetically closed mixer containing drug and lubricants. This mixture is introduced in the cans that are closed by a special powder valve, and propellant 12 was then injected through the valve. The upper part of the valve has often a special aperture through which a gaseous phase cleans the valve as well as the actuator. 

The solids formation process is much different if suspensions or slurries are dried. 

The topic of suspension polymerization has been reviewed by several authors at different times, with different emphases [24, 242-248]. In a suspension polymerization process, the monomer (or monomers in the case of a copolymerization), which is relatively insoluble in water, is (are) dispersed as liquid droplets. Dispersion stability is maintained with the help of a stabilizer and vigorous stirring. The final product, once the continuous (usually aqueous) phase has been removed, consists of solid polymer particles (beads). The initiators used in this process are usually soluble in the liquid monomer. The terms pearl and bead polymerization are also used for the suspension polymerization process when particle porosity is not required. The major aim in suspension polymerization is the formation of an, as uniform as possible, dispersion of monomer droplets in the aqueous phase, with controlled... 

In general, in previous discussions of free-radical polymerizations, we have attempted to draw a sharp distinction between suspension- and emulsion-polymerization processes. This distinction is quite readily apparent in the case of monomers which are quite insoluble in water, such as styrene. In that case, by use of monomer-soluble initiators and a variety of suspending agents, the suspension-polymerization process leads to the formation of spherical particles which can be separated by filtration. 

PGSS This acronym refers to particles from gas-saturated solutions (or suspensions) . This process consists of dissolving an SCF into a liquid substrate, or a solution of the substrate(s) in a solvent, or a suspension of the substrate(s) in a solvent followed by a rapid depressurization of this mixture through a nozzle causing the formation of solid particles or liquid droplets according to the system. 

Figure 5.7. Schematic representation of the sphere formation process and some TEM images of the samples obtained from the suspensions with different water contents. Water content (vol%) (a) 20%, (b) 30%, (c) 40%, and (d) 50%. The scale bars in the figures are 100nm. Source From Li et al., 2006a.

In general, the suspension polymerization can be distinguished into two types, namely, the bead and powder suspension polymerization [4]. In the former process, the polymer is soluble in its monomer and smooth spherical particles are produced. In the later process, the polymer is insoluble in its monomer and, thus, precipitates out leading to the formation of irregular grains or particles. The most important thermoplastic produced by the bead suspension polymerization process is PS. In the presence ofvolatile hydrocarbons (C4—C6), foamable beads, the so-called EPS, are produced. On the other hand, PVC, which is the second largest thermoplastic manufactured in the world, is an example of the powder type suspension polymerization. 

To improve the structure-dynamics relationships of CLs, the effects of applicable solvents, particle sizes of primary carbon powders, wetting properties of carbon materials, and composition of the catalyst layer ink should be explored. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules and, therefore, control the catalyst layer formation process. Mixing the ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition will increase the interfacial area between ionomer and Pt/C nanoparticles. The choice of a dispersion medium determines whether ionomer is to be found in the solubilized, colloidal, or precipitated forms. 

The model scheme for the formation of three-dimensional macroporous structures (Scheme 3.1) shows the steps that occur in the suspension polymerization process. Each particle (that can be seen as a microreactor ) is composed of monomer solution, initiator, and diluent (I). This organic phase is suspended in an aqueous phase containing the suspension stabilizer. 

Figure 4 Simulated (a) bubble formation time and (b) instantaneous gas flow rate through the orifice during the bubble formation process in the liquid-solid suspension (i)o = l-63 mm, Kc = 650 cm, 8s = 0.18). (From Yang et al., 2000a.)...

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