Polymerization, emulsion

Polymerization in aqueous emulsions, which has been widely developed technologically, represents a special case of free radical chain polymerization in a heterogeneous system [52-58]. Most emulsion polymerization systems 

The polymer emulsion (or latex) has a much smaller particle size than the emulsified monomer, by several orders of magnitude. 

The polymerization rate is much faster than that of the undiluted monomer, by one or two orders of magnitude. 

It is obvious from the foregoing facts that the mechanism of emulsion polymerization involves far more than the mere bulk polymerization of monomer in a finely divided state. In fact, the very small particle size of the latex, relative to that of the original monomer emulsion, indicates the presence of a special mechanism for the formation of such polymer particles. 

Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves the polymerization of monomers in the form of emulsions (i.e., colloidal dispersions). The process bears a superficial resemblance to suspension polymerization (Sec. 3-13c) but is quite different in mechanism and reaction characteristics. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. 

Emulsion polymerization is one of the major examples where detergents are applied to create microreactions. For instance, to polymerize styrene (which is insoluble in water), an initiator is added to the aqueous phase. The polymer (polystyrene [PS]) is formed, and the suspension is stabilized by using suitable emulsifiers. The latex thus formed is used in various industrial applications. 

Polystyrene can be prepared as follows A mixture of styrene, detergent (Na-dodecanoate), and water is agitated ultrasonically to produce a fine emulsion. On the addition of hydrogen peroxide (initiator), PS is obtained as a polymer, which can be extracted after filtration. The polymer molecular weight is determined by various methods (such as light scattering and osmotic pressure). 

Emulsion polymerizations vary greatly, and no single reaction mechanism accounts for the behavior of all the important systems. Useful insights 

Before describing a qualitative picture of emulsion polymerization a note on monomer solubility and type of surface active agents is in order. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. The solubility must be less than about 0.004 mol/L, as otherwise the aqueous phase will become a major locus of polymerization and the system will then not be typical emulsion polymerization. At the same time the monomer must be slightly soluble as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (see later). 

Surfactants or emulsifiers play an important role in the emulsion process. They are composed of ionic hydrophilic end and a long hydrophobic chain. Examples are  

Sodium alkyl aryl sulfonate QiH2n-i-i Cationic detergent 

Emulsion polymerizations are carried out in one liquid phase dispersed within another. The monomer or a solution of the monomer is dispersed with the aid of an emulsifier in the homogeneous phase and polymerized, for example, with free radical initiators. The product is a colloidal dispersion of the polymer. Since dispersions have lower viscosity than the melt, they can be handled much better. Also, the temperature control is easier. Typical emulsion polymers are poly(methyl methacrylate), poly(methacrylic acid), polystyrene, and poly(vinyl chloride). Two special applications of emulsion polymerization are the making of well-defined dispersion particles that may contain only one or few polymer molecules, and the possibility to make better defined molecular sizes by controlling the growth periods. 

The precision of the molar mass is further enhanced with periodic initiation with intermittent illumination. During the dark period, the droplets with a growing radical produce a molecule of mass determined by the length of the dark period. During the light period, all growing molecules are terminated by the excess of free radicals. 

initiator, generates 10 radicals per. .. second and cm in the water phase 

Intermittent initiation starts and stops polymerization by diffusion of many or no radicals into each droplet so that at any time 50% droplets grow and 50% are dead. 

Emulsion polymerization is an important type of free radical polymerization which is performed in aqueous systems. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. The surfactants form micelles, and when monomer is added to the system they tend to penetrate and 

Emulsion polymerization is a technique of polymerization where polymer formation occurs in an inert medium in which the monomer is sparingly soluble (not completely insoluble). Traditionally, water is the inert medium and the initiator is chosen such that it is water soluble. Monomers undergoing step-growth reaction do not require any initiation and are not polymerized by this method. Emulsion polymerization is commonly used for vinyl monomers undergoing addition polymerization and even among these, those that polymerize by the radical mechanism are preferably polymerized by this method. Water-based emulsions for ionic polymerizations are uncommon because of high-purity requirements. This discussion is therefore restricted to the polymerization of monomers following the radical mechanism only. 

The water-soluble initiator commonly used is potassium or sodium persulfate, and the usual recipe for emulsion polymerization is 200 parts by weight of water, 100 parts by weight of the monomer, and 2-5 parts by weight of a suitable emulsifier [1,2]. The monomer should be neither totally soluble nor totally insoluble in the water medium and must form a separate phase. The emulsifier is necessary to ensme that the monomer is dispersed uniformly as in a true emulsion [3-8]. The polymer that is formed from emulsion polymerization is in the form of small particles having an average diameter around 5 pm. The particles form a stable emulsion in water. Their separation can be effected only through the 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

The emulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, and some monomer), emulsified monomer droplets, the monomer-swollen micelles, and monomer-swollen particles. Water is the most important ingredient of the emulsion polymerization system. It is inert and acts as the locus of initiation (the formation of primary and oligomeric radicals) and the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particle micelles to particles. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. 

The emulsifier provides sites for the particle nucleation and stabilizes growing or the final polymer particles. Even though conventional emulsifiers (anionic, cationic, and nonionic) are commonly used in emulsion polymerization, other non-conventional ones are also used they include reactive emulsifiers and amphiphilic macromonomers. Reactive emulsifiers and macromonomers, which are surface active emulsifiers with an unsaturated group, are chemically bound to the surface of polymer particles. This strongly reduces the critical amount of emulsifier needed for stabilization of polymer particles, desorption of emulsifier from particles, formation of distinct emulsifier domains during film formation, and water sensitivity of the latex film. 

The most commonly used water-soluble initiator is the potassium, ammonium, or sodium salt of peroxodisulfates. Redox initiators (Fe2+salt/peroxodisul-fate, etc.) are used for polymerization at low temperatures. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in emulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. This results from the immobilization of oil-soluble initiator in polymer matrix, the cage effect, the induced decomposition of initiator in the particle interior, and the deactivation of radicals during des orption/re-entry events [14, 15]. 

It is usual to consider the course of emulsion polymerization to proceed through three intervals [16,17]. The particle number increases with time in Interval I, where latex particles are being formed, and then remains constant during Intervals II and II. The monomer concentration in particles is in equilibrium with a monomer saturated aqueous solution. Swelling is limited only by the opposite force of the particle surface/water tension. Hence, the concentration of monomer in the particles is usually taken as constant up to the point where free monomer droplets disappear. In Intervals I and II, the monomer concentration 

In emulsion polymerization, monomers are polymerized in the form of emulsions and polymerization in most cases involve free-radical reactions. Like suspension polymerization, the emulsion process uses water as the medium. Polymerization is much easier to control in both these processes than in bulk systems because stirring of the reactor charge is easier due to lower viscosity and removal of the exothermic heat of polymerization is greatly facilitated with water acting as the heat sink. Emulsion polymerization, however, differs from suspension polymerization in the nature and size of particles in which polymerization occurs, in the type of substances used as initiators, and also in mechanism and reaction characteristics. Emulsion polymerization normally produces polymer particles with diameters of 0.1-3//. Polymer nanoparticles of sizes 20-30 nm are produced by microemulsion polymerization (Antonietti et al., 1999 Ytldiz et al., 2003). 

Sodiiun laurate CH3(CH2)ioCOO Na Sodium alkyl aryl sulfonate 

Anionic and cationic detergent molecules may thus be represented by — and tively, indieating hydroearbon (hydrophobic) chains with ionic (polar) end groups 

The emulsion polymerization technique is a heterophase polymerization technique in which three phases can be distinguished the water phase, the latex particle phase and the monomer droplet phase (the latter is usually present during part of the polymerization reaction). The product of an emulsion polymerization is a latex a submicrometer dispersion of polymer particles in water. Non-aqueous dispersions of latex particles also exist. 

The emulsion polymerization technique usually contains a micelle-forming surfactant and a water-soluble initiator in combination with a water-insoluble monomer. Polymerization takes place in the monomer-swollen micelles and latex particles. Therefore, the term emulsion polymerization is a misnomer the starting point is an emulsion of monomer droplets in water, and the product is a dispersion of latex particles. In the case of microemulsion polymerization, the monomer droplets are made very small (typical particle radius is 10-30 nm) and they become the locus of polymerization. In order to obtain such small droplets, a co-surfactant (e.g. hexanol) is usually applied. A microemulsion is thermodynamically stable 

Functional polysiloxanes were also prepared via anionic polymerization of cycles with three or four SiO(CH3)R units, where R = CH2CH2CF3 [170], phenyl [171] or vinyl [172]. Similar kinetic features as for the anionic polymerization of D4 were 

However, the PAn products were not easily recovered and generally were isolated by breaking the emulsion (e.g., by adding acetone), to precipitate the ES. A major development with this method was therefore the discovery of a direct synthesis of an ES that is highly soluble in organic solvents by workers at Monsanto.117 The method uses a reverse emulsion procedure involving initial formation of emul- 

The PAn/DNNSA is highly soluble (it is not a dispersion) in nonpolar organic solvents such as xylene and toluene, common solvents used in many paints. It has a molecular weight (Mw) 22,000 and an electrical conductivity of 10 5 S cm-1. Interestingly, treatment of a PAn/DNNSA film with methanol or acetone leads to a marked (five orders of magnitude) increase in conductivity, which is believed to arise from extraction of excess DNNSA dopant causing an increase in polymer crystallinity. 

Several types of surfactants can be used in emulsion polymerization and a summary of the various classes is given in Table 4.2. 

The role of surfactants is twofold, firstly to provide a locus for the monomer to polymerize and secondly to stabilize the polymer particles as they form. In addition, surfactants aggregate to form micelles (above the critical micelle concentration) and these can solubilize the monomers. In most cases a mixture of anionic and nonionic surfactant is used for optimum preparation of polymer latexes. Cationic surfactants are seldom used, except for some specific applications where a positive charge is required on the surface of the polymer particles. 

Several other anionic surfactants are commercially available such as sulfosuccinates, isethionates and taurates and these are sometimes used for special applications. 

Alkyl trimethyl ammonium chloride, where R contains 8-18 C atoms, e.g. dodecyl trimethyl ammonium chloride, 

N-alkyl betaines which are derivatives of trimethyl glycine (CH3)3NCH2COOH (that is described as betaine). An example of betaine surfactant is lauryl amido propyl dimethyl betaine Ci2H25CON(CH3)2CH2COOH. These alkyl betaines are sometimes described as alkyl dimethyl glycinates. 

The maleic Surfmers were tested in core-shell emulsion polymerization of styrene/butyl acrylate in comparison with a standard nonreactive surfactant (nonyl phenol reacted with 30 mol of EO - NP30). While the methacrylic-derived Surfmer was completely incorporated during the polymerization (although about one-third of it was buried inside the particles) the NP30, the maleic Surfmer and the allylic and vinyl Surfmers were not incorporated and could be extracted with acetone (for the last two probably because of the formation of acetone-extractable oligomers due to a chain transfer behavior) [31]. 

Recently Uniqema has introduced commercially a Surfmer under the trade name of Maxemul 5011. Maxemul is produced by esterification of an unsaturated fatty anhydride with a methoxy PEG such that the reactive group is close to the hydrophilic moiety [ 34 ]. Stable latexes with a solid content of 52% were produced in the seeded emulsion polymerization of film-forming methyl methacrylate/butyl acrylate/acrylic acid (3% Surfmer on monomers, constant monomer feeding rate over 4 h, potassium persulfate/sodium metabisulfate redox initiator). The latexes were stable to electrolytes but not to freeze-thaw. 

It was estimated that, if all the Surfmers contributed to stabilization, the surface coverage would be close to 20% at the end of the process. When Surfmer burial is considered, the minimum surface coverage is in the region of 14.7-15.0 % [35]. The authors have also studied the influence of the addition procedure on the evolution of the Surfmer conversion and concluded that, despite the low reactivity due to the presence of the alkenyl double bond, the incorporation could be increased to 72% from the original 58% obtained with a constant feeding rate. A mathematical model able to describe Surfmer polymerization was used in the optimization process [36]. 

Other than through alkylene oxide chemistry, monomeric Surfmers have been produced from polyvinyl alcohol [37] and saccharides [38]. 

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. 

However, some important distinctions between emulsion and suspension polymerizations are as follows  

Emulsions usually are composed of small particles (0.05-5 p.m) compared with suspensions containing particles of 10-1000 xm in diameter. 

Water-soluble initiators are used rather than monomer-soluble ones. 

The end product usually is a stable latex—an emulsion of polymer in water rather than a filterable suspension. 

Because of these couditions, the mechauism of polymerization is basically different. The essentials of an emulsion polymerization system are a monomer, a surface-active agent (surfactant), an initiator, and water. Initially, the surfactant is in the form of micelles, spherical or rodlike aggregates of 50-100 surfactant molecules with their hydrophobic tails oriented inward and their hydrophilic heads outward. These micelles form whenever the concentration of surfactant exceeds a rather low critical micelle concentration. The manner in which the critical micelle concentration for a given surfactant is measured is instructive for our purposes. If a dilute 

The section on suspension polymerization indicated the differentiation between suspension and emulsion (or latex) polymerizations. Emulsion polymers usually are formed with the initiator in the aqueous phase, in the presence of surfactants, and with polymer particles of colloidal dimensions, i.e., on the order of 0.1 gm in diameter [17]. Generally, the molecular weights of the polymers produced by an emulsion process are substantially greater than those produced by bulk or suspension polymerizations. The rate of polymer production is also higher. As a large quantity of water is usually present, temperature control is often simple. 

Typical emulsion polymerization recipes involve a large variety of ingredients. Therefore, the possibilities of variations are many. Among the variables to be considered are the nature of the monomer or monomers, the nature and concentration of surfactants, the nature of the initiating system, protective colloids and other stabilizing systems, cosolvents, chain-tranfer agents, buffer systems, short stops, and other additives for the modification of latex properties to achieve the desired end properties of the product. 

In the preparation of a polymer latex, the initial relationship of water, surfactant, and monomer concentration determines the number of particles present in the reaction vessel. Once the process is underway, further addition of monomer does not change the number of latex particles. If such additional 

From the preparative standpoint, there are two classes of initiating systems. 

The thermal initiator system. This system is made up of water-soluble materials that produce free radicals at a certain temperature to initiate polymerization. The most commonly used i materials for such thermal emulsion polymerizations are potassium persulfate, sodium persulfate, or ammonium persulfate. 

It has been common practice for many years to regard the course of a conventional emulsion polymerization reaction as being divided into the following three more-or-less distinct intervals  

Interval I This is the stage of the reaction in which the entities which will later grow into the particles of the eventual polymer colloid are brought into existence. It is often referred to as particle nucleation or locus nucleation . Interval II This is a Stage during which polymerization occurs within the loci formed during Interval I, and in which excess monomer is present as a separate droplet phase. 

Interval III In this final stage, polymerization continues within the loci which were formed during Interval I, and which grew during Interval II, but excess monomer is no longer present as droplets. 

Recent work has done little to cast doubt upon the essential correctness of this division for the majority of reactions which are classed as emulsion polymerizations. 

This is an important method that, in addition to improved thermal control, results in a higher rate of production, higher molecular weights and a narrow distribution. The product may be utilized as an emulsion like rubber latex or coatings. Otherwise the emulsion must be destroyed, and the polymer carefully dried and separated, because retention of residues from the emulsifier (detergent) may cause the deterioration of properties (mainly for electrical uses). 

The kinetics of emulsion polymerization differ completely from that of bulk or suspension. The soap (detergent) is used not only as a stabilizer but mainly as locus of the polymerization—so-called soap micelles. These consist of an array of 20-100 molecules of soap creating a micellar stmcture of 25-50 A in length (radius). The initiator is water soluble and the free radicals and the monomer molecules diffuse into the hydrophobic interior of the micelles while water is attracted to the hydrophilic exterior zone. Thus, the micelles serve as the core of growing polymer particles. At a later stage the micellar structure disappears, and the process continues in the polymer particles swollen by monomers, leaving the soap as a protective layer on the particle surface. The concentration of soap dictates the molecular weight and rate of production. 

Define the term chain length. If two polymers have the same chain length, do they also have the same molecular weight  

In the case of a polymer made by condensation, is the repeating vmit identical to the sum of the monomers  

What is the molecular weight of a cross-linked polymer. How are cross-linked polymers dissolved  

ABS products with high impact strengths and relatively high surface gloss may be produced by using traditional emulsion polymerization techniques. 

ABS compositions with bimodal particle size distributions of the grafted rubber can be prepared by emulsion graft polymerization techniques. The preparation of ABS types by emulsion polymerization consists in brief of (13)  

Preparation of an aqueous elastomeric polymer emulsion of colloidally dispersed small particles with a particle size of 0.15-0.22 p, 

Conventional anionic emulsifiers are alkyl sulfates, alkyl sulfonates, aralkyl sulfonates, soaps of saturated or unsaturated fatty acids and alkaline disproportionated or hydrogenated abietic or tall oil acids (14). 

In the agglomeration step, the latexes are partially agglomerated using a core/shell agglomerating agent latex, which consists of an elastomeric 1,3-butadiene/slyrene copolymer core and an ethyl acrylate/methacrylic acid copolymer shell. This partial agglomeration operation should not be confused with a coagulation operation where the emulsion is fully destabilized (13). 

Surfactant-polymer systems have additional technological significance because surfactants are normally used in emulsion polymerization processes. 

FIGURE 14.12. Polymer-surfactant interactions are important in many areas of polymer science and technology, especially emnlsion polymerization. In snch processes surfactants and micelles perform several dnties snch as emulsification of monomers (a), solnbihzation of growing oUgomeric free radical chains (b), and stabilization of growing and final polymer particles (c). 

In cases where there is little affinity of the polymer for water, as for polystyrenes or polyalkylacrylates and methacrylates, little effect of surfactant on water solubihty would be expected. The action of the surfactant on such latex systems is then limited to its action as a monomer solubilizer during preparation and an adsorbed stabilizer afterward. 

The complex relationships that can exist between polymers and surfactants raises a great many questions concerning the interpretation of data obtained from such mixed systems. They also open the door to possible new and novel applications of such combinations, however, and will no doubt provide many interesting hours of experimentation and thought for graduate students and industrial researchers in the future. 

Adsorption isotherms of polymers on surfaces usually exhibit a high affinity character. That is, at low polymer concentration virtually all the polymer is adsorbed, with very little left in solution (often immeasurable quantities). It is also common to find that the adsorption process is very slow and that adsorbed polymer cannot be readily removed by washing with the same solvent used for adsorption. Explain these observations using logical physical reasoning at the molecular level and, where possible, thermodynamic arguments as support. 

The preceding discussion of free-radical addition polymerization has considered only homogeneous reactions. Considerable polymer is produced commercially by a complex heterogeneous free-radical addition process known as emulsion polymerization. This process was developed in the United States during World War II to manufacture synthetic rubber. A rational explanation of the mechanism of emulsion polymerization was proposed by Harkins and quantified by Smith and Ewart after the war, when information gathered at various locations could be freely exchanged. Perhaps the best way to introduce the subject is to list a typical reactor charge  

0-1 part chain-transfer agent (monomer soluble) 

The first question is. What s the soap for Soaps are the sodium or potassium salts of organic acids or sulfates  

Despite the fact that most of the monomer is present in the droplets, the swollen micelles, because of their much smaller size, present a much larger surface area than the droplets. This is easily seen by assuming a micelle volume to drop volume ratio of 1/10 and using the ballpark figures given above. Since 

Redox systems, so called because they involve the alternate oxidation and reduction of a trace catalyst, are a newer and more efficient means of generating radicals. For example, 

Vinyl monomers may be polymerized at favorable rates in an aqueous medium containing an emulsifier and a water-soluble initiator. A typical simple Tecipe would consist of the following ingredients with their proportions indicated in parts by weight 100 of monomer, 180 of water, 2 to 5 of a fatty acid soap, and 0.1 to 0.5 of potassium persulfate. Cationic soaps (e.g., dodecylamine hydrochloride) may be used instead of the fatty acid soap, and various other initiators may replace the persulfate (e.g., hydrogen peroxide and ferrous ion, or a water-soluble organic hydroperoxide). 

FIGURE 3 46 Schematic representation of a surfactant molecule with a hydrophilic head and a hydrophobic tail. 

FIGURE 3-47 Two-dimensional schematic representation of a spherical micelle. 

Water-based latex paints first appeared on the scene in the late 1940s and started a mini-revolution. 

They were more environmentally friendly and did not have the high viscosity problems associated with their solvent-based cousins. 

FIGURE 3-49 BUNA rubber, a styrene/butadiene random copolymer made by emulsion polymerization manufacture, circa 1940s (Courtesy Bayer). 

In such a system, the distance among particles is about 10-15 nm, and each particle would contain about 600 macromolecules of 10 gmol molecular weight. The dispersed system is thermodynamically unstable, and kinetic stability is provided by emulsifiers (ionic and non-ionic) and by incorporation of hydrophilic groups into the polymer. 

In a broad sense, polymer dispersions include both synthetic polymer dispersions and natural rubber (Table 6.1 ).The yearly production of synthetic polymer dispersions is about 10% of the overall polymer consumption [1]. Synthetic polymer dispersions are produced by emulsion polymerization. About half of these polymers are commercialized as waterborne dispersions. Carboxylated styrene-butadiene copolymers, acrylic and styrene-acrylic latexes and vinyl acetate homopolymer and copolymers are the main polymer classes (Table 6.2). The main markets for these dispersions are paints and coatings (26%), paper coating (23%), adhesives (22%) and carpet backing (11%) [2]. Polymer dispersions have also found an interesting market niche in biomedical applications (diagnosis, drug delivery and treatment [3]). 

A substantial part of the synthetic polymer dispersions is commercialized as dry products. These include styrene-butadiene rubber (SBR) for tires, nitrile rubbers, about 10% of the total poly(vinyl chloride) production, 75% of the total acrylonitrile-butadiene-styrene 

Waterborne dispersions Carboxylated styrene-butadiene polymers Vi nyI acetate polymers Acrylic and styrene acrylic polymers 

Commercialized as dry polymer Styrene-butadiene rubber Nitrile rubbers (NBR and HNBR) 

An emulsifier (surfactant) is a molecule that posses both polar and non-polar moities, i.e., it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration (CMC), they associate spontaneous to form aggregates - micelles (Fig. 1 A). The formation of micelles is controlled by the chemical equilibrium between emulsifier monomers and larger micellar aggregates. At low concentrations the emulsifier dissolves as free monomers but as soon as the emulsifier concentration exceeds the CMC the monomer concentration remains roughly constant and the emulsifier aggregates into micelles. 

In aqueous solutions, at concentrations not too large with respect to the CMC, say in the range CMC to 10 CMC, ionic emulsifiers form spherical or close to spherical micelles [47]. Micelles are responsible for many of the processes such as  

Emulsion polymerization involves dispersion of a relatively water-insoluble monomer (e.g., styrenes, alkyl methacrylates, etc.) in water with the aid of 

1) The first step includes the formation of primary radicals and their transformation to the surface active oUgomeric radicals through the addition of monomer units to the growing radical. 

2) The second step involves the entry of ohgomeric (surface active) radicals into the monomer-swollen micelles (micellar nucleation) or the precipitation of growing radicals (homogeneous nucleation) from the aqueous phase [49-52]  

The kinetic chain length (o) can be calculated according to the following equation  

Example 9.12 constitutes one of the simplest possible illustrations of what is sometimes termed polymer reaction engineering. Even a minor complication, such as substitution of the parameters of Example 9.1 or the inclusion of chain transfer would necessitate a numerical solution. While the basic principles are there, additional detail is beyond the scope of this chapter, but you might wish to consider the application of these principles to a nonisothermal reactor, etc. 

100 parts (by weight) monomer (water insoluble) 180 parts water 

2-5 parts fatty acid surfactant (emulsifying agent) 

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In mass polymerization bulk monomer is converted to polymers. In solution polymerization the reaction is completed in the presence of a solvent. In suspension, dispersed mass, pearl or granular polymerization the monomer, containing dissolved initiator, is polymerized while dispersed in the form of fine droplets in a second non-reactive liquid (usually water). In emulsion polymerization an aqueous emulsion of the monomer in the presence of a water-soluble initiator Is converted to a polymer latex (colloidal dispersion of polymer in water). 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The production of organic polymeric particles in tire size range of 30-300 nm by emulsion polymerization has become an important teclmological application of surfactants and micelles. Emulsion polymerization is very well and extensively reviewed in many monographs and texts [67, 68], but we want to briefly illustrated tire role of micelles in tliis important process. 

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

Several polymerization techniques are in widespread usage. Our discussion is biased in favor of methods that reveal additional aspects of addition polymerization and not on the relative importance of the methods in industrial practice. We shall discuss four polymerization techniques bulk, solution, suspension, and emulsion polymerization. 

The fourth and most interesting of the polymerization techniques we shall consider is called emulsion polymerization. It is important to distinguish between suspension and emulsion polymerization, since there is a superficial resemblance between the two and their terminology has potential for confusion A suspension of oil drops in water is called an emulsion. Water-insoluble monomers are used in the emulsion process also, and the polymerization is carried out in the presence of water however, the following significant differences also exist ... 

Emulsifying agents which are soaps or detergents play a central role in the emulsion polymerization process. 

The surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

Figure 6.10 Schematic representation of the distribution of surfactant in an emulsion polymerization. Note the relative sizes of suspended particles. [From J. W. Vanderhoff, E. B. Bradford, H. L. Tarkowski, J. B. Shaffer, and R. M. Wiley,Chem. 34 32(1962).]...

In an emulsion polymerization experiment at 60°C the number of micelles per unit volume is 5.0 X 10 hter and the monomer concentration in the micelle... 

In this example the number of micelles per unit volume is exactly twice the stationary-state free-radical concentration hence the rates are identical. Although the numbers were chosen in this example to produce this result, neither N nor M are unreasonable values in actual emulsion polymerizations. 

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. 

There is a great deal more that could be said about emulsion polymerization or, for that matter, about free-radical polymerization in general. We shall conclude our discussion of the free-radical aspect of chain-growth polymerization at this point, however. This is not the end of chain-growth polymerization, however. There are four additional topics to be considered ... 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Acrylates are primarily used to prepare emulsion and solution polymers. The emulsion polymerization process provides high yields of polymers in a form suitable for a variety of appHcations. Acrylate polymer emulsions were first used as coatings for leather in the eady 1930s and have found wide utiHty as coatings, finishes, and binders for leather, textiles, and paper. Acrylate emulsions are used in the preparation of both interior and exterior paints, door poHshes, and adhesives. Solution polymers of acrylates, frequentiy with minor concentrations of other monomers, are employed in the preparation of industrial coatings. Polymers of acryHc acid can be used as superabsorbents in disposable diapers, as well as in formulation of superior, reduced-phosphate-level detergents. 

Emulsion Polymerization. Emulsion polymerization is the most important industrial method for the preparation of acryhc polymers. The principal markets for aqueous dispersion polymers made by emulsion polymerization of acryhc esters are the paint, paper, adhesives, textile, floor pohsh, and leather industries, where they are used principally as coatings or binders. Copolymers of either ethyl acrylate or butyl acrylate with methyl methacrylate are most common. 

The surfactants used in the emulsion polymerization of acryhc monomers are classified as anionic, cationic, or nonionic. Anionic surfactants, such as salts of alkyl sulfates and alkylarene sulfates and phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes, are most common (87,98—101). Mixed anionic—nonionic surfactant systems are also widely utilized (102—105). 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

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). 

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