Emulsion polymerization description

Emulsion Polymerization. Poly(vinyl acetate)-based emulsion polymers are produced by the polymerization of an emulsified monomer through free-radicals generated by an initiator system. Descriptions of the technology may be found in several references (35—39). 

As is clear from Eq. l,the rate of particle growth (R /Nr) is proportional to the monomer concentration, [M]p and the average number of radicals per particle, n, respectively. Thus, n is one of the basic parameters that characterize the kinetic behavior of particle growth in an emulsion polymerization system. Early researchers devoted their efforts to deriving a quantitative description of n by solving Eq. 3 for n defined by Eq. 2 [4,119,120]. 

From recent investigations()( 5 ), it is generally concluded that polymerization tcdces place exclusively in the polymer particles in vinyl acetate emulsion polymerization. As a detailed description of the role of polymer particles was presented in the previous paper(4 ), a brief explanation will therefore be reviewed here. 

Description of suspension polymerizations fits most appropriately in Chapter 10, where polymer reaction engineering is the topic. This chapter focuses on dispersion and emulsion polymerizations. 

Simple sequential processes frequently do not yield particles with the planned architectures. This is because of the complexity of emulsion polymerizations and because a system in which different polymers coexist with water will tend to rearrange toward the composition with the lowest overall surface energy. Theoretical descriptions of such phenomena [17-19] are based on the concept that the final state of the system consisting of polymer I, polymer 2, and water (labeled phase 3) depends on the three interfacial tensions yi2, Y2i and y2.i. and the corresponding interfacial areas. The equilibrium state of the three phases is determined by the minimum value of the surface free energy, Gy. 

A brief description is given below of the emulsion polymerization process of a monomer which is slightly soluble in water. This picture, which is now gen-... 

This picture of emulsion polymerization mechanisms, mainly due to Harkins (29, 30), is generally accepted as is borne out by its description in several recent textbooks on polymerization (4, 10, 13, 24, 68). An extensive review has been given by Gerrens (26). 

That the average number of radicals per particle should be about 1/2 can also be seen by considering that, on entry of a new radical into a very small particle containing one polymer particle, termination between the two radicals occurs after a time lapse very small compared with the time interval between successive entries of new radicals under the conditions usually prevalent in emulsion polymerization. Hence, the particle contains either one or no radical and the average number of radicals per particle is 1/2. This simple description was first given by Smith and Ewart (58) and will be referred to as the Smith-Ewart rate theory. It follows from n = 1/2 that [R ] = N/2N, ... 

Table 1 Description of the three intervals of emulsion polymerization...

When water-soluble initiators and surface-active agents are used, relatively stable latices are formed from which the polymer cannot be separated by filtration. In the case of vinyl acetate, the distinctions are more blurred. Our description of Procedure 3-3 above represents a transitional situation between a solution and a suspension process since the product separated from the reaction medium. Between the true suspension and the true emulsion polymerization, we find, according to Bartl [4], the processes for formation of reasonably stable dispersion of fine particles of poly(vinyl acetate) using reagents which are normally associated with suspension polymerization. The product is described as creme-like. The well-known white, poly(vinyl acetate), household adhesives may very well be examples of these creamy dispersions. The true latices are characterized by low viscosities and particles of 0.005-1 /im diameter. The creme-like dispersions exhibit higher viscosities and particle diameters of 0.5-15 fim. 

In order to understand the quantitative relations governing emulsion polymerization kineties, it is necessary to give a qualitative description of the process. 

A number of questions need to be resolved from the qualitative description of emulsion polymerization given in the previous section. For example, it is necessary to consider whether the diffusion of monomers to the polymer particles is high enough to sustain polymerization given the low solubdity of monomer in the aqueous phase. It is also important to know the average radical concentration in a polymer particle. Also, the validity of the assumption that only the monomer-polymer particles capture the radicals generated by the initiator needs to be established convincingly. The answers to these questions were pro vided by Smith and Ewart and this forms the basis for the quantitative treatment of the steady-state portion of emulsion polymerization. 

The Smith-Ewart kinetic theory of emulsion polymerization is simple and provides a rational and accurate description of the polymerization process for monomers such as styrene, butadiene, and isoprene, which have very limited solubility in water (less than 0.1%). However, there are a number of exceptions. For example, as we indicated earlier, large particles (> 0.1 to 0.5 cm diameter) may and can contain more than one growing chain simultaneously for appreciable lengths of time. Some initiation in, followed by polymer precipitation from the aqueous phase may occur for monomers with appreciable water solubility (1 to 10%), such as vinyl chloride. The characteristic dependence of polymerization rate on emulsifier concentration and hence N may be altered quantitatively by the absorption of emulsifier by these particles. Polymerization may actually be taking place near the outer surface of a growing particle due to chain transfer to the emulsifier. 

Poly-(R)-3-hydroxyalkanoates (PHAs) are linear biopolyestos produced by a wide variety of bacteria as a reserve of carbon and energy [39]. Recently De Koning and Maxwell [40] drew an analogy between the conventional emulsion polymerization process and the biosynthesis of PHAs. Based on this Kuija et al. [41,42] made a more quantitative description of the accumulation of poly-(R)-3-hydroxybutyrate (PHB) in Alcaligenes eutrophus. 

Emulsion polymerization involves the emulsification of monomers in an aqueous phase, and stabilization of the droplets by a surfactant. Usually, a water-soluble initiator is used to start the free-radical polymerization. The final product is a dispersion of submicrometer polymer particles, which is called latex. The locus of polymerization is the micelle. Typical applications are paints, coatings, adhesives, paper coatings and carpet backings. The latex particles can have different structures (see Fig. 2). Excellent text books on the applications and structure-property relationships exist [11-15]. Besides a full description of the kinetics and mechanism of emulsion polymerization [16], a textbook adapted for use as material for people entering the field is also available [17]. 

The developments of CRP in aqueous dispersed systems have been reviewed several times in the last 10 years [22-30]. Since an exhaustive description of the achievements in the field is not the main target of this article, the reader is invited to refer to those well-documented review articles. Only a brief overview will be given here and all references concerning miniemulsion and emulsion polymerizations (the main processes discussed in this review) can be found in the above-mentioned papers. 

This chapter basically focused on the scientific and industrial importance of the emulsion polymerization and vinyl acetate based emulsion polymers from past to present. Firstly, the basic issues of conventional emulsion polymerization were given. Its ingredients, kinetics, and mechanisms were explained in detail. Other emulsion polymerization methods including micro-, mini- and inverse-emulsion polymerization were mentioned, followed by the description of main emulsion polymerization processes comprising batch, semi-... 

Theoretically, the description of the evolution of the CLD for an emulsion polymerization process is complex as not only the number of radicals has to be tracked per particle but also the chain lengths of these radicals. For FRP, however, the computational effort can be reduced in case a so-called zero-one system is obtained, in which the polymer particles either contain no radicals or only one radical at a given time. 

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