Emulsions homopolymerization

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Considerable work has been done to understand emulsion homopolymerization from a mathematical modeling viewpoint beginning with Smith and Ewart (i) in 1948. Significant contributions to homopolymerization theory have been recently added by the models of workers such as Min and Ray (2.), Rawlings and Ray ( ,), Hansen and Ugelstad (2), Gilbert and Napper (A), and Feeney et al.(8-9). For other work in the field the reader is directed to the review of Penlidis et al. (2.). ... 

General. In this section, a mathematical dynamic model will be developed for emulsion homopolymerization processes. The model derivation will be general enough to easily apply to several Case I monomer systems (e.g. vinyl acetate, vinyl chloride), i.e. to emulsion systems characterized by significant radical desorption rates, and therefore an average number of radicals per particle much less than 1/2, and to a variety of different modes of reactor operation. 

Other Monomer Systems. Very slight modifications are required to make the model applicable to emulsion homopolymerization of vinyl chloride (VCM). An initial study on PVC reactors has been reported in (69) and some more recent results following will finely illustrate the case. 

M.M. Reis, M. Uliana, C. Sayer, P.H.H. Araujo and R. Giudici, Monitoring emulsion homopolymerization reactions using ET-Raman spectroscopy, Braz. J. Chem. Eng., 22, 61-74 (2005). 

Ozdeger et al. studied the role of the nonionic emulsifier Triton X-405 (octyl-phenoxy polyethoxy ethanol) in the emulsion homopolymerization of St [99] and n-butyl acrylate (n-BA) [ 100], and in the emulsion copolymerization of St and n-BA [101]. In the emulsion homopolymerization of St, they noted two separate nucleation periods, resulting in bimodal PSDs. Although the total concentration of the emulsifier was maintained at a level above its CMC based on the water phase in the recipe, the portion of the emulsifier initially present in the aqueous phase was below the CMC due to partitioning between the oil and aqueous phases. Due to the nature of this emulsifier, the first of the two nucleation periods was attributed to homogeneous nucleation, while the second was... 

It is clear from Eq. 1 that the monomer concentration in a polymer particle is one of the three key factors that control the particle growth rate, and accordingly, the rate of polymerization. In emulsion polymerization, the course of emulsion polymerization is usually divided into three stages, namely. Intervals I, II and III. In Intervals I and II of emulsion homopolymerization, the monomer concentration in the polymer particles is assumed to be approximately constant. In Interval III, it decreases with reaction time. Two methods are now used to predict the monomer concentration in the polymer particles in emulsion homopolymerization empirical and thermodynamic methods. 

Jagodic et al. (1975, 1976) also studied the effect of the HLB of emulsifying agents in the emulsion homopolymerization of methyl methacrylate, ethyl acrylate, and acrylonitrile and in the emulsion copolymerization of methyl methacrylate and ethyl acrylate, methyl methacrylate and... 

Emulsion homopolymerization of styrene Maximization of monomer conversion and minimization of deviation of average molecular weight and particle size from desired values. Adapted GA for MOO Model parameters were estimated based on experimental data. A decision support system was also developed to rank the Pxeto-optimal solutions. Massebeuf et al. (2003)... 

Massebeuf, S., Fonteix, C., Hoppe, S. and Pla, F. (2003). Development of new concepts for the control of polymerization processes multiobjective optimization and decision engineering. I. Application to emulsion homopolymerization of styrene, J. Appl. Polym. Sci, 87, pp. 2383-2396. 

Open-loop control strategies were developed and implemented to allow for reduction of transition times during grade transitions in continuous high-impact styrene polymerizations [61]. Similar strategies were also used to control the MWDs in emulsion homopolymerizations and to control the copolymer composition and the MWDs simultaneously in non-hnear emulsion polymerizations [36,37,182]. 

Seeded emulsion polymerization can be used with batch, semi-continuous, or continuous polymerization to give the desired value of N, In batch or semi-continuous emulsion polymerization, seeding ensures batch-to-batch reproducibility of the final particle size in continuous emulsion polymerization, it ensures the reproducibility, not only of the final particle size, but also of the conversion of the exit stream. Seeded emulsion polymerization is equally adaptable to emulsion homopolymerization and copolymerization. Moreover, two-stage or multiple-stage polymerizations can be used to produce core-shell latex particles the variation of the process type---batch, semi-continuous, continuous----as well as the para-... 

Sajjadi [85] investigated the diffusion-controlled nucleation and growth of particle nuclei in the emulsion homopolymerizations of styrene and methyl methacrylate. The polymerization starts with two stratified layers of monomer and water containing surfactant and initiator, with the water layer being stirred gently. In this manner, the rate of transport of monomer becomes diffusion-limited. As a result, the rate of growth of particle nuclei is reduced significantly, and more latex particles can be nucleated in emulsion polymerization. 

Desorption of Free Radicals in Emulsion Homopolymerization Systems... 

The Smith and Ewart-Stockmayer-O Toole treatments [48-50] (see Chapter 4) that are widely used to calculate the average number of free radicals per particle (n) are based on the assumption that the various components of the monomer-swollen latex particles (e.g., monomer, polymer, free radicals, chain transfer agent, etc.) are uniformly distributed within the particle volume. A latex particle in emulsion homopolymerization of styrene involves uniform distribution of monomer and polymer within the particle volume except perhaps for a very thin layer near the particle surface. In the case of free radicals, this uniform distribution would only hold in a stochastic sense. However, as illustrated in Eq. (8.1), free radicals are not distributed uniformly in the latex particles when water-soluble initiators are used to initiate the free radical polymerization. The assumption of uniform distribution of free radicals in the latex particles would be valid only if the particles are very small or chain transfer reactions are the dominate mechanism for producing free radicals. If such a nonuniform free radical distribution hypothesis is accepted, the very basis of the Smith and Ewart-Stockmayer-O Toole methods might be questioned. Despite this potential problem, the Stockmayer-O Toole solutions for the average number of free radicals per particle have been used for kinetic studies of many emulsion polymerization systems. The theories seem to work reasonably well and have been tested extensively with monomers such as styrene. 

Emulsion homopolymerization or copolymerization of MMA is usually carried out in a pressurized batch reactor with a water-soluble initiator and surfactant. The polymerization temperature may be varied from 85 C to 95 C to achieve high conversion. Bacterial attack, common in acrylic polymer... 

Different approaches are used to prepare polymer particles with attaching to surface-functionalized groups. In majority of the cases, they consist of step-batch or -semibatch polymerizations in dispersed media, being among them pulsion polymerization (emulsifier-free or not) the most used polymerization process (i) emulsion homopolymerization of a monomer containing the desired functional group (functionalized monomer), (ii) emulsion copolymerization of styrene (usually) with the functionalized monomer, (iii) seeded copolymerization to produce composite functionalized latexes, and (iv) surface modification of preformed latexes. 

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