Semibatch emulsion polymerization

In emulsion polymerizations semibatch operation provides better control of the particle size of the product. The properties of the product polymers can be modified, also, by continuous or intermittent changes in the composition of the monomer feed in emulsion copolymerizations, where a given monomer can be preferentially concentrated in the interior or on the surface of the final particles, as described in Chapter 8. 

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). 

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. 

Polymerizable surfactants capable of working as transfer agents include thiosulfonates, thioalkoxylates and methyl methacrylate dimer/trimer surfactants. Thioalkoxylates with 17-90 ethylene oxide units were produced from ethoxylated 11 bromo-undecanol by replacing the bromine with a thiol group via the thiazonium salt route [8]. In the presence of water-soluble azo initiator the thio ended Transurfs (used at a concentration above the CMC) gave monodispersed latex particles in emulsion polymerization of styrene. However, the incorporation of the Transurf remained low, irrespective of the process used for the polymerization (batch, semibatch, seeded). The stability of the lattices when the surfactant and the transfer function were incorporated in the same molecule was better than when they were decoupled. 

Polymer emulsions can be produced by the direct and the inverse emulsion process. The direct emulsion polymerization can be performed in a batch, semibatch and continuous process. 

A particular emulsion polymerization yields polymer with Mp = 500,000. Show quantitatively how you would adjust the operation of a semibatch emulsion process to produce polymer with = 250,000 in interval II without changing the rate of polymerization, reaction temperature, or particle concentration. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

In semibatch emulsion polymerizations the polymer particles are kept monomer-starved to obtain higher rates of polymerization and to permit easier control of the rate and particle size distribution. There are two aspects to the control of PSD. The controlled addition of emulsifier during particle growth stabilizes the particles without further particle nucleation. The second aspect is related to the particle sticky stage which often occurs... 

Liotta, V. Sudol, E.D. El-Aasser, M.S. Georgakis, C. On-line monitoring, modeling, and model validation of semibatch emulsion polymerization in an automated reactor control facility. J. Polym. Sci. Pt. A Polym. Chem. 1998, 36 (10), 1553-1571. 

Crowley, T.J. Meadows, E.S. Kostoulas, E. Doyle, F.J. Control of particle size distribution described by a population balance model of semibatch emulsion polymerization. J. Process. Control 2000, 10 (5), 419-132. 

Types of Reactor Processes Batch Reactors Semibatch Reactors Continuous Reactors Emulsion Polymerization Kinetics Other Preparation Methods... 

The earliest polymerization processes were either batch mode or semibatch. The semibatch method was used for products, where the two monomers differed greatly in reactivity, as in Union Carbide s early Dynel, acrylonitrile-vinyl chloride, process. Bulk, solution, and emulsion polymerization processes have also been developed for acrylonitrile and its copolymers. However, in recent years nearly every major acrylic fiber producer has used a continuous aqueous suspension process, employing a redox catalyst, followed by a series of steps, which includes slurry filtration and polymer drying. 

The commercially used emulsion polymerization reactors (stirred-tank and continuous-loop) are designed to achieve perfect mixing. As will be discussed in Section 6.4.5, perfect mixing is not always achieved. Nevertheless, this flow model allows a good prediction of the emulsion polymerization reactor performance with a moderate mathematical effort, and it will be used here. Macroscopic balances (i.e., considering the reactor as a whole) are used. For the sake of generality, inlet and outlet streams are included in the balances. Both terms should be removed for batch operation, the outlet term should be eliminated in semibatch and both maintained in continuous processes. 

Calorimetry Semibatch emulsion polymerization of VAc/BA [66, 67] Epoxy-amine curing polymerization [86] Non-invasive, robust and almost continuous/ Requires state estimators and the values of the reactivity ratios for multimonomer systems All polymerization techniques... 

CHDF (capillary hydrodynamic fractionation) Semibatch emulsion polymerization of styrene, and VAc/BA [49, 123] PSD directly measured/Invasive, dilution or sampling loop required, non-robust for industrial environment, time delay Emulsion polymerization... 

Figure 8.5 Monitoring a semibatch emulsion polymerization reaction of VAc/BA by means of an open-loop observer based on calorimetric measurements, (a) Conversion and (b) copolymer composition.

Perri, M. (1988) Adaptive dynamic optimization of the semibatch emulsion polymerization process, PhD thesis, Georgia Institute of Technology. 

A. Vatansever, T. Inan, H. Dogan, A. Sirkecioglu, Synthesis of poly(butyl acrylate-co-methyl methacrylate)/montmoriUonite waterborne nanocomposite via semibatch emulsion polymerization. Journal of Applied Polymer Science 132 (32) (August 1, 2015) 42373. 

M.D. Besteti, A.G. Cunha, D.M.G. Freire, J.C. Pinto, Core/shell polymer particles by semibatch combined suspension/emulsion polymerizations for enzyme immobilization, Macromolecular Materials and Engineering 299 (2014) 135-143. 

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