Emulsion polymerization completion

Because of the large number of mechanistic processes opo-ative in emulsion polymerizations, complete theories for the PSD are necessarily complex. Nevertheless, they can be formulated by a population balance approxch. Much remains to he done, however, to clarify the basic colloid science that underpins the nucleation process in Interval I. The experimental challenge in evaluating the predictions of the theory for PSDs resides not only in the attainment of agreement with experiment but also in showing that such agreement is not merely fortuitous but arises from the correct mechanistic scheme. Considerable experimental work will be required to establish the validity of mechanistic assumptions for any particular monomer. 

Emulsion polymerization as a continuous operation has been described in the patent literature an has found industrial application. One technical process mentioned uses a coiled pipe of such dimensions that at a certain temperature the monomer emulsion polymerizes completely during its pa sage. If necessaiy, a pipe system can be built that is heated to different temperatures at its various sections. Other continuous processes can be carried out in polymerization towers if the monomer has a lower density than water and the polymer has a higher one. The monomer emulsion is added at the top of the tower and agitated by stirring devices, with turbulence limited to the upper parts of the tower. As the polymerization proceeds and the polymer sinks to the bottom of the tower, new monomer is introduced at the top and latex removed at the base. Instead of one tower, a battery of interconnected reactors can be advantageously used in a similar procedure. 

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). 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

Emulsion Polymerization. In this method, polymerization is initiated by a water-soluble catalyst, eg, a persulfate or a redox system, within the micelles formed by an emulsifying agent (11). The choice of the emulsifier is important because acrylates are readily hydrolyzed under basic conditions (11). As a consequence, the commonly used salts of fatty acids (soaps) are preferably substituted by salts of long-chain sulfonic acids, since they operate well under neutral and acid conditions (12). After polymerization is complete the excess monomer is steam-stripped, and the polymer is coagulated with a salt solution the cmmbs are washed, dried, and finally baled. 

The aqueous emulsion polymerization can be conducted by a batch, semibatch, or continuous process (Fig. 5). In a simple batch process, all the ingredients are charged to the reactor, the temperature is raised, and the polymerization is mn to completion. In a semibatch process, all ingredients are charged except the monomers. The monomers are then added continuously to maintain a constant pressure. Once the desired soflds level of the latex is reached (typically 20—40% soflds) the monomer stream is halted, excess monomer is recovered and the latex is isolated. In a continuous process (37), feeding of the ingredients and removal of the polymer latex is continuous through a pressure control or rehef valve. 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

Since the 1940s, alkanesulfonates have served as emulsifiers in the emulsion polymerization of vinylchloride. Nevertheless, the detailed mechanisms of this process are not yet completely known. An important advantage of using alkanesulfonates is that they produce latices with a high solid content, which can be effectively processed by spray drying [90]. 

The Emulsion Polymerization Model (EPM) described in this paper will be presented without a detailed discussion of the model equations due to space limitations. The complete set of equations has been presented in a formal publication (Richards, J. R. et al. J. AppI. Poly. Sci . in press). Model results will then be compared to experimental data for styrene and styrene-methyl methacrylate (MMA) copolymers published by various workers. 

Polychlorotrifluoroethylene (PCTFE) is ordinarily prepared by emulsion polymerization. A polymer suitable for thermal processing requires coagulation, extensive washing, and postpolymerization workup. Coagulation to provide a filterable and washable solid is a slow, difficult process and removal of surfactant is an important part of it. Complete removal may be extremely difficult depending on the extent of adsorption to the polymer particles. Consequently we set out to develop a suspension polymerization process, which would be surfactant-free and afford an easily isolated product requiring a minimum of postreaction workup. 

Many commercially important polymers are produced via emulsion polymerization. This is also one of the most common methods to produce dye-doped beads. A dye is added to the mixture of monomers prior to initiating the polymerization and is either noncovalently entrapped or is copolymerized. The second method ensures that no leaching will occur from the particle but requires modification of the dye (typically by providing it with a double bond). This method is most common for preparation of pH-sensitive beads where a pH indicator is entrapped inside cross-linked polyacrylamide particles. The size of the beads can be tuned over a wide range so that preparation of both nano- and microbeads is possible. Despite thorough washing the surfactants are rather difficult to remove completely and their traces can influence the performance of some biological systems. 

The production of synthetic rubbers by solution polymerization processes is a stepwise operation very similar in many aspects to production by emulsion polymerization. There are distinct differences in the two technologies, however. For solution polymerization, the monomers must be extremely pure and the solvent should be completely anhydrous. In contrast to emulsion polymerization, where the monomer conversion is taken to approximately 60%, solution polymerization systems are polymerized to conversion levels typically in excess of 90%. The polymerization reaction is also more rapid, usually being completed in 1 to 2 hours. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

In particular, if a latex is to be used for coatings, adhesives, or film applications, no silicone-base stopcock greases should be used on emulsion polymerization equipment. Although hydrocarbon greases are not completely satisfactory either, there are very few alternatives. Teflon tapes, sleeves, and stoppers may be useful, although expensive. 

Formulas for emulsion polymerization also include buffers, free radical initiators, such as potassium persulfate (KiSiOs), chain transfer agents, such as dodecyl mercaptan (G sSTT). The system is agitated continuously at temperatures below 100°C until polymerization is essentially complete or is terminated by the addition of compounds such as dimethyl dithiocarbamate to prevent the formation of undesirable products such as cross-linked polymers. Stabilizers such as phenyl Beta-naphthylamine are added to latices of elastomers. 

Of great importance to the polymerization is the smooth resolution of the w/o emulsions at the end of the prepolymerization. The transition of w/o emulsion polymerization to suspension polymerization (type o/w) must be complete and must occur without great effort. It has already been pointed out that the stability of w/o emulsions increased with de-... 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

The present review paper, therefore, refers firstly to the particle formation mechanism in emulsion polymerization, the complete understanding of which is indispensable for establishing a correct kinetic model, and then, deals with the present subject, that is, what type of reactor and operating conditions are the most suitable for a continuous emulsion polymerization process from the standpoint of increasing the volume efficiency and the stability of the reactors. 

The formation of coagulum is observed in all types of emulsion polymers (i) synthetic rubber latexes such as butadiene-styrene, acrylonitrile-butadiene, and butadiene-styrene-vinyl pyridine copolymers as well as polybutadiene, polychloroprene, and polyisoprene (ii) coatings latexes such as styrene-butadiene, acrylate ester, vinyl acetate, vinyl chloride, and ethylene copolymers (iii) plastisol resins such as polyvinyl chloride (iv) specialty latexes such as polyethylene, polytetrafluoroethylene, and other fluorinated polymers (v) inverse latexes of polyacrylamide and other water-soluble polymers prepared by inverse emulsion polymerization. There are no major latex classes produced by emulsion polymerization that are completely free of coagulum formation during or after polymerization. 

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]. 

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. 

The monomer can be polymerized either directly, that is undiluted (block or substance polymerization), or in the presence of a non-polymerizable solvent (solvent polymerization). In the first case there is a problem with dissipating the localized heat of reaction (traces of decomposition products result from overheating) and in the second case the solvent must be completely removed. Other possibilities are to suspend the monomers in dispersions, e.g. in water (suspension or pearl polymerization), or eventually using an emulsifier (emulsion polymerization). Emulsifiers and dispersants are considered to be processing aids for the production of polymers. 

To survey as completely as possible the grafting behavior of EVA copolymers toward various vinyl compounds, our investigations covered the grafting of vinyl acetate, vinylidene chloride, and acrylic and meth-acrylic esters. As polymerization processes, at first we preferred suspension polymerization to exclude the influence of solvents by terminating or transfer reactions during polymerization. Grafting by emulsion polymerization, in which the EVA copolymer was dissolved in the monomer before polymerization, was difficult because coagulate was formed as polymerization proceeded. 

A mathematical model of emulsion polymerization as a whole would be too complicated. Therefore Smith and Ewart divided the process into three phases, and appropriately simplified the situation in each of these. The start of polymerization and the monomer-polymer transformation up to the disappearance of micelles were designated as phase I, the subsequent time interval until the complete consumption of monomer droplets as phase II, and the remaining part of polymerization as phase IIIt. 

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