Emulsion polymerization particle number

Even though the chemical reactions are the same (i.e. combination, disproportionation), the effects of compartmentalization are such that, in emulsion polymerization, particle phase termination rates can be substantially different to those observed in corresponding solution or bulk polymerizations. A critical parameter is n, the average number of propagating species per particle. The value of h depends on the particle size and the rates of entry and exit. 

In emulsion polymerizations, the number of particles containing n radicals, N , normally assumes a steady-state value early in the reaction. Therefore the rate of formation of such particles equals their rate of disappearance. A particle containing n radicals is formed either from a particle with n—1 radicals which... 

Fig. 6 Particle diameter for emulsion polymerization (A) number average and (B) distribution. (View this art in color at WWW. dekker. com.)...

What happens to (a) rate of emulsion polymerization, (b) number average degree of polymerization, and (c) polymer particle size, if more monomer is added to the reaction mixture during stage II polymerization Explain. [Ans. (a) No change (b) no change (c) increases.]... 

As alluded to earlier, one of the key advantages of miniemulsion polymerizations ova conventional emulsion polymerizaticxis was seen to be the direct ccHitrol over the resulting number of particles which coirld be achieved by controlling the initial numbo of monomer droplets. If the latter could be controlled in a reproducible fashion, then the irreprodudbilities associated with the creation of latex particles might be eliminated. In conventional emulsion polymerizations, the number of particles is chiefly controlled by the concentrations of surfactant and initiator. 

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). 

Soap. A critical ingredient for emulsion polymerization is the soap (qv), which performs a number of key roles, including production of oil (monomer) in water emulsion, provision of the loci for polymerization (micelle), stabilization of the latex particle, and impartation of characteristics to the finished polymer. 

Emulsion Polymerization. Emulsion polymerization takes place in a soap micelle where a small amount of monomer dissolves in the micelle. The initiator is water-soluble. Polymerization takes place when the radical enters the monomer-swollen micelle (91,92). Additional monomer is supphed by diffusion through the water phase. Termination takes place in the growing micelle by the usual radical-radical interactions. A theory for tme emulsion polymerization postulates that the rate is proportional to the number of particles [N. N depends on the 0.6 power of the soap concentration [S] and the 0.4 power of initiator concentration [i] the average number of radicals per particle is 0.5 (93). 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Medvedev et al. [57] extensively studied the use of nonionic emulsifiers in emulsion polymerization. The emulsion polymerizations in the presence of nonionic emulsifiers exhibited some differences relative to those carried out with the ionic ones. Medvedev et al, [57] proposed that the size of latex particles remained constant during the reaction period, but their number increased continually with the increasing monomer conversion. The use of nonionic emulsifiers in emulsion polymerization usually results in larger sizes relative to those obtained by the ionic emulsifiers. It is possible to reach a final size value of 250 nm by the use of nonionic emulsifiers in the emulsion polymerization of styrene [58]. 

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

Polymer Particle Balances (PEEK In the case of multiconponent emulsion polymerization, a multivariate distribution of pjarticle propierties in terms of multiple internal coordinates is required in this work, the polymer volume in the piarticle, v (continuous coordinate), and the number of active chains of any type, ni,n2,. .,r n (discrete coordinates), are considered. Therefore... 

The increase in iV, and therefore in the rate as well, with initial soap concentration is thus explained. Quantitative results agree approximately with the predicted three-fifths power dependence. The prediction of an increase in polymerization rate with also has been confirmed by experiments at variable initiator concentrations.t Most important of all, the actual number of particles N calculated from Eq. (35) agrees within a factor of two with that observed. It is thus apparent that the theory of emulsion polymerization developed by Harkins and by Smith and Ewart has enjoyed spectacular success in accounting for the unique features of the emulsion polymerization process. 

Since the same propagation rate constant applies to both bulk and emulsion polymerization, comparable rates of polymerization R must obtain when the number of emulsion particles is twice the number of radicals at the steady state in the bulk polymerization. An increase in the bulk rate at the given temperature can only be realized by an increase in the rate of initiation and, thus, an increase in the... 

Population Balance Approach. The use of mass and energy balances alone to model polymer reactors is inadequate to describe many cases of interest. Examples are suspension and emulsion polymerizations where drop size or particle distribution may be of interest. In such cases, an accounting for the change in number of droplets or particles of a given size range is often required. This is an example of a population balance. 

Many water-soluble vinyl monomers may be polymerized by the emulsion polymerization technique. This technique, which differs from suspension polymerization in the size of the suspended particles and in mechanism, is widely used for the production of a number of commercial plastics and elastomers. While the particles in the suspension range from 10 to 1000 nm, those in the emulsion process range from 0.05 to 5 nm in diameter. The small beads produced in the suspension process may be separated by filtering, but the latex produced in emulsion polymerization is a stable system in which the charged particles cannot be recovered by ordinary separation procedures. 

A variety of behaviors are observed for the polymerization rate versus conversion depending on the relative rates of initiation, propagation, and termination, which are in turn dependent on the monomer and reaction conditions (Fig. 4-2). Irrespective of the particular behavior observed, three intervals (I, II, III) can be discerned in all emulsion polymerizations based on the particle number N (the concentration of polymer particles in units of number of... 

The number-average degree of polymerization in an emulsion polymerization can be obtained by considering what occurs in a single polymer particle. The rate r,- at which primary radicals enter a polymer particle is given by... 

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