Particle formation rate, emulsion

As we discussed in Sect. 3.1.1, Hansen et al. [15] made significant improvements to the concept of the radical capture efficiency proposed by Nomura et al. [ 14]. Taking this concept into consideration, they examined the effect of radical desorption on micellar particle formation in emulsion polymerization [ 65 ]. Assuming that radical entry is proportional to the x power of the micelle radius and the polymer particle radius, they proposed the following general expression for the rate of particle formation ... 

Usually particle formation by initiation in the monomer droplets droplet nucleation) is not considered important in conventional emulsion polymerization. This is because of the low absorption rate of radicals into the monomer droplets, relative to the other particle formation rates. However, when the monomer... 

Stalagmometric determination of the SDDS adsorption at the aqueous solution-ethyl acrylate interface dependence on the rate of drop formation (volume 0.03 cm ) were carried out in our laboratory by Vasilenko. The measurements showed that establishment of adsorption equilibrium at the CMC occurs at drop formation periods of 15-20 sec, ije., at surface formation rates not exceeding 10 m sec. Adsorption kinetics acquires considerable importance in analysis of the mechanism of particle formation during emulsion polymerization, when tbe rate of organic phase formation and the rate of adsorption layer formation may be commensurate. 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

Acrylonitrile (Figure 9) shows two periods of almost constant but different absolute reaction rates, followed by a period of first-order reaction rate at a high conversion. This monomer is somewhat similar to vinylidene chloride since it also does not swell in its own polymer. On the other hand acrylonitrile has a water solubility roughly three orders of magnitude higher than vinylidene chloride or styrene and even higher than methyl acrylate (see Table I). We therefore have to assume particle formation in the aqueous phase, as was done for methyl acrylate emulsions. 

Vinyl caproate in emulsion polymerization behaves like styrene in most respects. The rate is first order in monomer. In the range of 1015 to 10"16 particles/cc, it depends on Np to the 0.75 power. This is higher than that for styrene in this range, Rp oc Np°5, indicating that there is less diffusion into the aqueous phase for vinyl caproate. However, the mechanism of particle formation for vinyl caproate may not fit the Smith-Ewart mechanism because of the high chain transfer rate to monomer. 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

Utilizing the seed latex polymerization method to avoid the occurance of new particle formation, the kinetic treatment of an emulsion polymerization is quite straight forward. Assuming that all the particles are the same size, the rate of polymerization,... 

The utility of the analytical predictor method of dead-time compensation to control of conversion in a train of continuous emulsion polymerizers has been demonstrated by simulation of the vinyl acetate system. The simulated results clearly show the extreme difficulty of controlling the conversion in systems which are operated at Msoap-starvedM conditions. The analytical predictor was shown, however, to provide significantly improved control of conversion, in presence of either setpoint or load changes, as compared to standard feedback systems in operating regions that promote continuous particle formation. These simulations suggest the analytical predictor technique to be the preferred method of control when it is desired that only one variable (preferably initiator feed rate) be manipulated. 

Assuming that classical chemical kinetics are valid and that the crosslinking reaction rate is proportional to the concentrations of polymer radicals and pendant double bonds, it was shown theoretically that the crosslinked polymer formation in emulsion polymerization differs significantly from that in corresponding bulk systems [270,316]. To simplify the discussion, it is assumed here that the comonomer composition in the polymer particles is the same as the overall composition in the reactor, and that the weight fraction of polymer in the polymer particle is constant as long as the monomer droplets exist. These conditions may be considered a reasonable approximation to many systems, as shown both theoretically [316] and experimentally [271, 317]. First, consider Flory s simplifying assumptions for vinyl/divinyl copolymerization [318] that (1) the reactivities of all types of double bonds are equal, (2) all double bonds... 

The kinetics and mechanisms of particle growth and polymer structure development are comparatively well understood compared to those of particle nucleation. Therefore, the rate of polymerization and the properties of the polymer produced can be (roughly) estimated as long as the number of polymer particles produced is known (for example, in seeded emulsion polymerization). However, the prediction of the number of polymer particles produced is still far from being an estabUshed technique. Therefore, further efforts are needed to qualitatively and quantitatively clarify the effects of numerous factors that affect the process of particle formation in order to gain a more quantitative understanding of emulsion polymerization. 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

Recently, Ugelstad et al. l969i proposed a semiempirtcal rate coefficient for radical desorption in vinyl chloride emulsion polymerization. On the other hand, Nomura et al. (1971, 1976) have derived a rate coefficient for radical desorption theoretically with both stochastic and deterministic approaches and have successfully applied it to vinyl acetate emulsion polymerization. They also pointed out that radical desorption from the particles and micelles played an important role in micellar particle formation, Fiiis et al. 1973 also derived the rate coefficient for radical desorption in a different way. Lift et al. (1981) discussed in more detail the chemical reactions incorporated in the physical process of radical desorption in the emulsion polymerization of vinyl acetate. 

In this chapter, the polymerization rate equations for emulsion polymerization will be reviewed briefly. Then, the rate coefficient for radical desorption from tbe panicles will be derived theoretically, and the effect of rathcal desorption on the rate of emulsirai polymerization and the micellar particle formation will he discussed. 

Let us consider a seeded emulsion polymerization where no particle formation occurs and hence the number of polymer particles is constant. At a steady state, the overall rate of radical entry into the particles is expressed by... 

Models for emulsion polymerization reactors must account for particle formation and particle growth. If these two phenomena can be handled in a satisfactory manner one can predict the polymerization rate, the number of particles formed, and the particle size distribution. The model presented below was first developed by Gershberg and Longfield (1961). It is based on the concepts developed for batch reactors by Smith and Ewart (1948) in their Case 2 model. 

---