Coalescence emulsion polymerization

F.M. Uhl, S.P. Davuluri, S.C Wong, D.C Webster, Organically modified mont-morillonite in UV-curable urethane-acrylate films , Polymer, 2004, 45, 6175-6187. J.Y. Kim, W.C Jung, K.Y. Park, K.D. Sub, Synthesis of Na montmorillonite/ amphilitic pol3Tirethane nanocomposite via hulk and coalescence emulsion polymerization , J. Appl. Polym. Sci., 2003, 89, 3130-3136. 

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

Emulsion polymerization is one of the major processes for the production of industrial polymers. It represents a sizable application for surface active agents, although manufacturers tend to minimize their use because of economic and environmental considerations (surfactants are usually more expensive compared to monomers and are mostly left in the liquor) and because of the negative effects on the final properties of the polymers and of their coalesced films. 

In certain chemical and biological as well in some unit operations, foaming occurs to such an extent that the process is severely impaired or even comes to a complete standstill. For example, chemical reaction systems tend to foam if a gas is formed in nascent state, because such minute gas bubbles do not coalesce to form larger ones and therefore remain in the system. Expulsion of residual monomers after emulsion polymerization often involves serious foaming problems because, in this case, very fine gas bubbles are formed in a material system which contains emulsifiers, i.e. foam producing surfactants. 

Oscillations in the number of polymer particles, the monomer conversion, and the molecular weight of the polymers produced, which are mainly observed in a CSTR, have attracted considerable interest. Therefore, many experimental and theoretical studies dealing with these oscillations have been published [328]. Recently,Nomura et al. [340] conducted an extensive experimental study on the oscillatory behavior of the continuous emulsion polymerization of VAc in a single CSTR. Several researchers have proposed mathematical models that quantitatively describe complete kinetics, including oscillatory behavior [341-343]. Tauer and Muller [344] proposed a simple mathematical model for the continuous emulsion polymerization of VCl to explain the sustained oscillations observed. Their numerical analysis showed that the oscillations depend on the rates of particle growth and coalescence. However, it still seems to be difficult to quantitatively describe the kinetic behavior (including oscillations) of the continuous emulsion polymerization of monomers, especially those with relatively high solubility in water. This is mainly because the kinetics and mech-... 

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. 

When the surfeclant concentration is high relative to the polymer concentration sufRdent sui ctant may be adsorbed on individual polymer molecules to prevent their coalescence to form latex particles or indeed to disperse prdbrmed polymer to form clear solutions in which the solute behaves as a po yelectro yte, But the influence of such effects on tbe course of emulsion polymerization reactions has not been elucidated. Sata and Saito (1952) showed that poly(vinyl acetate) preeptated from acetone solution with water could be solubibzed in sodium dodecyl sulfate solutions after removal of the acetone by dialysis. To obtain a clear solution at 20°C, a wdght of surfactant S-10 times that of pol ner was required. Althou this greatly exceeds the surfactant concentrations normally used in emulsion... 

Foams can occur in any chemical, biological or industrial process to such an extent, that process control is made considerably more difficult or even becomes impossible, Thus, for example, chemical reaction systems tend to foaming, if nascent ("in statu nascendi ) gas is produced in them, because such fine gas bubbles do not coalesce into larger ones and therefore remain in the system. Major foaming problems are often connected with the expulsion of the residual monomer after the end of emulsion polymerization (e.g. Buna" rubber manufacture), because here the finest gas bubbles are formed in a material system, which contains emulsifiers, e.g. foam-forming surface active substances. 

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-... 

Coalescence is also controlled by the condition of drop surfaces. Surfactants reduce the interfacial tension and help preserve drop stability, therefore affecting drop sizes. Surface-active materials are important in suspension/emulsion polymerization processes. 

Coalescence is the combining of drops either by collision or at a surface. It is desirable in some applications, while undesirable in others. It aids mass transfer and separation processes, but it is harmful to suspension and emulsion polymerization. Coalescence can occur when drops collide with one another or come to rest on surfaces and interfaces. Collisions between drops can result in either coalescence or rebounding. The impact creates transient forces that act on the colliding drops to thin the film separating them. As this film gets thinner, rupture (coalescence) occurs when the thickness reaches a critical value. If the film thickness does not reach this critical thickness during contact, the drops will depart without coalescing. 

Polymer Dispersions (Emulsion Polymers). Waterborne paints based on polymer dispersions (usually referred to as emulsion paints) are not water soluble. They are water-thinnable systems composed of dispersions of polymer particles in water (see Section 3.5). The particles consist of high molecular mass polymers (e.g., of styrene, butadiene, acrylate, or vinyl monomers) and are produced by emulsion polymerization. These waterborne paints also contain small amounts of organic solvents (< 5 wt %) that serve as film-forming (coalescing) agents that partially evaporate on drying. 

Figure 4.9 Effect of varying the phase ratio on the tuimber of latex particles formed with a constant emulsifier concentration in the emulsion polymerization of styrene at 50 °C [99]. At a phase ratio greater than monomer water 1 25 coalescence of primary particles is reduced increasing the number of ultimate particles formed. Reproduced by permission of the American Chemical Society...

No explanation has been offered as to why particular emulsifier mixtures produce monodisperse latexes. Latexes produced by emulsifier-ffee emulsion polymerization are monodisperse (although comparatively large in size). Hence it may be inferred that the coalescence rate should be at a maximum for minimal polydispersity. The emulsifier mixture which produces a monodisperse latex is that which is least effective in stabilizing the particles. 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

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