High shear devices

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

For laboratory investigations of miniemulsions, a variety of high-shear devices have been used, although sonication has been the most popular. Soni-cation, however, may not be very practical for the large-scale production of commercial miniemulsion polymers. An effective alternative to sonication is also driven by the need to design an efficient miniemulsion polymerization process. A continuous process places greater demand on the shear device in terms of energy consumption and dissipation. 

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

Emulsion liquid membranes can be effectively demulsified by high shear. A variation on this is to employ centrifugation as a first step, followed by processing in a high shear device [46]. 

Besides bubble columns, other reactor types are investigated. Using a high-shear device creates a high air/oxygen—cumene interface, which increases CHP formation by increased oxygen mass transfer [52]. 

For the mechanical high-shear devices, there is quite a bit of empirical and proprietary data on how these devices disperse sohds, liquids, and gasses in a continuous medium. It turns out that many types of these devices have become inherent tools in a particular industry and that the performance is empirically studied and correlated to give the resulting final product. 

A recent development, where high-shear devices such as ultrasound and high-pressure homogenizers were used to reduce droplet size and the nanoreactor diameter to 30-100 nm, thus allowing the formulation of different types of nanocapsule, forms the main subject of this chapter. 

F. Ganachaud, and J.L. Katz, Nanoparticles and nanocapsules created using the ouzo effect Spontaneous emulsification as an alternative to ultrasonic and high-shear devices, ChemPhysChem, 6 (2), 209-216, 2005. 

Liquid polymers can be chemically crosslinked to form thermosets. Materials in this category include epoxies, embedding compounds, coating materials, urethanes, silicone polymers, and others. Due to the inherently low viscosity of liquid polymers, they are compounded using a variety of mixing systems, including high shear devices. 

A typical formulation used in mini-emulsion polymerization consists of water, monomer mixture, co-stabilizer, surfactant and initiator. Hie key difference between emulsion polymerization and mini-emulsion polymerization is the utilization of a low molecular mass compound as the co-stabilizer and also the use of a high-shear device (ultrasound, etc.). Mini-emulsions are critically stabilized, require a high-shear to reach a steady state and have an interfacial tension much greater than zero (Koul c/ /.,2011 Winkelmann and Schuchmann, 2011). 

Note If this yield is still not being achieved, further work is necessary to determine the cause since further increases in mixing would not appear to be effective. A very high shear device such as a rotor-stator or Waring blender could be tested to determine if the reaction is still too fast (DaM too large) to realize the maximnm possible yield (minimum Xs). 

In low shear mixers, resins, plasticizers, and stabilizers should be melted first and thermoplastic rubber crumb added incremently. Fillers should be added last. In high shear devices, the order of time of adding the various components is also important to minimize mixing cycle time. Rubber crumb should be added first with resins, plasticizers and fillers added in increments which do not disrupt the mixer action. Stabilizers should be introduced before the crumb receives appreciable shearing action in any case. Presoaking plasticizers into the thermoplastic crumb will speed mixing. 

Formulations based on thermoplastic rubbers can be made into emulsions by dispersing solutions or melts into water containing appropriate surface active agents. Normally, any organic solvent present would be stripped from the emulsion. High shear devices, such as colloid mills or centrifugal pumps, have been successfully used for the emulsifying step. 

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