Emulsion polymerization principles
Emulsion polymerization is a process employed to convert a variety of unsaturated organic carbon compounds into large chains through radical chain polymerization. In this process, monomers polymerize in the form of emulsions (i.e. colloidal dispersions) using an inert medium in which the monomer is moderately soluble (not totally insoluble) [Odian, 2004, Yildirim, 2000]. Normally, the inert medium used is water and produces a milky fluid called latex, while the initiator used is selected such that it is water-soluble. Latexes are liquids (generally aqueous) in which polymer particles are dispersed. Specific performance characteristics such as chemical resistance, durability and dimensional stability can be improved by adding latexes to commercial products. It is important to note that there is a great diﬀerence between suspension and emulsion polymerization which is represented in the phase where the reaction takes place.
In the case of suspension polymerization, the reaction occurs in the monomer or organic phase, while for emulsion polymerization, the reaction takes place in the water phase or inert medium [Yildirim, 2000]. In the case of emulsion polymerization, the reaction does not occur in the initial monomer droplets and the kinetics observed is totally diﬀerent from the kinetics in bulk polymerization because there is a mass transfer between the phases in which the initiator and the monomer are present [Dotson et al., 1996].
Uses of emulsion polymerization
Commercial polymerizations of vinyl acetate, chloroprene, various acrylate copolymers, and copolymerizations of butadiene with styrene are carried-out by emulsion polymer-izations. The process presents important advantages mainly related to the physical state of the emulsion which facilitates the control of the reaction. For example, viscosity and heat transfer problems are less significant than in bulk polymerization. The product of an emulsion polymerization can be used directly without additional separation steps [Odian, 2004]. Some of the common market applications include paints and coatings (26%), pa-per and paperboard (24%), adhesives (23%), carpet backing (10%) and floor polishes and other markets (17%) [Urban and Takamura, 2002].
Technical interest of emulsion polymerization
One of the most important things to consider is the kinetic diﬀerence of emulsion poly-merization with respect to other polymerizations. This is a unique process that presents the advantage of being able to achieve simultaneously high molecular weights at high reaction rates as a result of the segregation of radicals by compartmentalization within polymerizing particles [Gentric, 1997, Kumar and Gupta, 1998, Odian, 2004]. Normally, there is an inverse relationship between the polymerization rate and the final polymer molecular weight and, in consequence, to obtain large molecular weights, it is necessary to importantly decrease the polymerization rate by lowering the reaction temperature or the initiator concentration. However, in the case of emulsion polymerization, there is no such dependence.
Polymers production has increased importantly since World War II when several large industrial plants were built, and, particularly since the 1950s, when the production has grown exponentially. Around 1960, the production of synthetic polymers reached about 8 million of metric tons while, around 2000, about 189 million of metric tons were produced being 20 times larger than 40 years before [Urban and Takamura, 2002]. In 2012, a global production of polymers of 288 million tons was reported with an increase of 2.8% compared to 2011 [PlasticsEurope, 2013]. The three main polymer classes are polyolefins, polyvinyl chloride and polystyrene. These three classes account for 64% of synthetic polymers [Ur-ban and Takamura, 2002]. Annual production of synthetic polymer dispersions, obtained by emulsion polymerization, represents about 4-10% of the total consumption (Figure 2.1) [Asua, 2007, Urban and Takamura, 2002]. In 2011, the world emulsion polymer demand was 10.3 million metric tons (dry basis) with 24% for North America, 25% for Western Eu-rope, 20% for China, 19% for Other/Asia/Pacific, and 12% for all other countries. Global demand for emulsion polymers is expected to rise 5.1%/yr to 13.3 million metric tons (dry basis) in 2016 [Freedonia, 2012]. Among the most important applications of emulsion polymers, are the adhesives. World formulated adhesive consumption in 2009 was 16.6 billion pounds valued at USD$20.6 billion. The adhesive industry is expected to recover from the 2009 global recession with a 4.5%/yr rate of growth projected through 2014. In 2009, water-based technology accounted for 54% of the adhesive poundage [Kusumgar, 2010]. The most important classes of polymer dispersions are styrene-butadiene copoly-mers (37%), vinyl acetate homopolymers and copolymers (28%), and polyacrylates (30%) [Urban and Takamura, 2002]. Polyvinyl acetate polymers and acrylics were by far the lead-ers and, in 2009, they accounted for over 80% of all water-based adhesives. As water-based materials, emulsion polymers have an expanding market due to their good environmental profile represented in lower emissions of volatile organic compounds during cure. Figure 2.2 shows the global adhesives consumption by year and its projection through 2014. Figure 2.3 shows the incomings related with that consumption [Kusumgar, 2010]. In Colombia, according to the report of the Administrative National Department of Statistics (Depar-tamento Administrativo Nacional de Estadfstica- DANE in Spanish) , 4404 metric tons of vinyl acetate polymers and copolymers, valued at USD$6.73 million, were exported between J anuary and March of 2011. In 2012, for the same period of time, 6758 met-rie tons were exported accounting for USD$9.28 million. The main destination countries of the plastic raw materials produced in Colombia are United States, Spain, Panama, Venezuela, Mexico, Turkey, Ecuador, Peru, Brazil, among others. Finally, the manufac-turers of polymer dispersions are distributed in more than 500 companies around the world with BASF, DOW Chemical and Rohm & Hass as leaders. They have an annual production capacity of more than 1 million metric tons (wet basis) and cover 20% of the total market. In Table 2.1, the main companies for the global market of aqueous polymer dispersions are reported.
Components of an emulsion polymerization
A typical emulsion polymerization is performed using several components to form a recipe. Water, free radical polymerizable monomers, emulsifiers and/or protective colloids and initiators form the basic recipe. Also, other auxiliary components such as chain transfer agents, bases, acids, biocides, buffers, etc. , could also be used [Urba n and Takamura, 2002]. There are four main components : the monomer, the dispersion medium, the emulsifier and the initiator. UsuaUy, water is used as the dispersing medium where the other various components are dispersed by means of the emulsifier and/or protective colloid, mixing them in appropriate amounts within a certain temperature range. Emulsion of monomer droplets is formed in a continuous dispersion medium and the initiator is responsible for the polymerization of monomer molecules.
Sorne important characteristics of the ingredients are described below [Yildirim , 2000] :
1. Monomer : Sorne of the substances commonly used as monomers in this process include acrylic and methacrylic acid and their organic esters ( ethyl and butyl acry-lates and methacrylates) , vinyl acetate, acrylonitrile, butadiene and styrene. AU of them are used in making rubbers and also it is common to utilize more than one mono mer to obtain copolymers. The ratio of water to mono mer(s) is generally in the range 70/30 to 40/60 (in a weight basis) [Odian, 2004]. The largest part of the monomer is dispersed as monomer droplets which are stabilized by surfactant molecules absorbed on their surfaces. Monomers are important because they influ-ence and define the properties of the films produced from the corresponding polymer dispersions. The most important properties defined in the selection of the monomer are glass transition temperature, water absorption capacity and elasticity, as well as another secondary properties such as chemical stability, crosslinking or hydrophilic properties related with the presence of comonomers [Urban and Takamura, 2002].
2. Dispersion medium : The medium by excellence used in emulsion polymerization is water because of its low price and environmental advantages (nonflammable, non-toxic, relatively odorless). Moreover, it is a convenient medium to remove the re-action heat released during the polymerization. The quality of the water used is important because sometimes the presence of ions in uncontrolled concentrations can cause interference with both the initiation process and the action of the emulsi-fier [Yildirim, 2000]. Furthermore, in a commercial product, water is very expensive to ship and therefore some high solid latexes have been developed.
3. Emulsifier : It is also known as surfactant or soap and its action is due to its molecules that have both hydrophilic end group and hydrophobic long segments (dodecyl, hex-adecyl or alkyl-benzene). The hydrophilic group may be cationic or anionic. There is also a set of nonionic emulsifiers or protective colloids such as polyvinyl alco-hols, polyvinyl-pyrrolidone, alkylpolyglycol ethers, etc., that can be used [Dotson et al., 1996, Urban and Takamura, 2002]. These surface-active agents solubilize the monomer to a certain extent, facilitate the formation of the emulsion of the organic monomer phase and the water phase, and finally stabilize the polymer-water emul-sion product by electrostatic or steric means, or by some combination of the two [Dotson et al., 1996]. The concentration of surfactant exceeds its critical micelle concentration (CMC) and that excess of surfactant molecules aggregate themselves to form micelles, which are small colloidal clusters [Kumar and Gupta, 1998]. Many industrial formulations use surfactant at concentrations higher than CMC, which is normally low (approximately 0.001 mol/l) [Dotson et al., 1996]. In a micelle, many surfactant molecules are fixed with their hydrocarbon side pointed toward the in-terior of the micelle and the ionic extremity toward the aqueous phase. Surfactant defines the way particles are formed because of its eﬀect on the nucleation mecha-nism. In consequence, the quantity of surfactant is used to control the latex particle size distribution [Odian, 2004, Yildirim, 2000].
4. Initiator : Emulsion polymerization takes place by means of a radical mechanism. The initiator causes the formation of free radicals at elevated temperatures (60-100 C), and then the propagation of the polymer molecules is promoted. As initia-tor acts in the water-phase, it must be water-soluble. The initiators commonly used are potassium or sodium persulfate, hydrogen peroxide and 2,2’-azobis(2-amidinopropane) dihydrochloride [Dotson et al., 1996]. The common recipe for emul-sion polymerization is 100 parts by weight of the monomer, 200 parts by weight of water, and 2 to 5 parts by weight of a suitable emulsifier [Kumar and Gupta, 1998]. The initiator is selected according to its partitioning behaviour between the aqueous and oil phases, and with regard to its half-life time.
Sites of polymerization
In general, the locus of initiation depends on the nature of the initiator, the monomer solubility, and the structure of the interphase [Dotson et al., 1996]. The initiating radi-cals are produced in the water phase as a result of the low solubility of the initiator in the organic monomer. Therefore, strictly speaking, the site of polymerization is not the monomer droplets. It can be demonstrated experimentally that polymerization does not occur in the monomer droplets because they do not compete with micelles in capturing radicals produced in solution [Odian, 2004]. This can be explained because of the much smaller total surface area of the monomer droplets. Polymerization takes place essentially in the micelles. The micelles are the place to put in contact the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration with respect to the monomer in solution. While polymerization takes place, the micelles grow with the addition of monomer from the aqueous solution that, at the same time, is provided by dissolution of the monomer from the monomer droplets. A schematic representation of an emulsion polymerization system is shown in Figure 2.4.
Table of contents :
1.2 Outline of the thesis
2 Emulsion polymerization principles
2.1.1 Uses of emulsion polymerization
2.1.2 Technical interest of emulsion polymerization
2.1.3 Economic aspects
2.2 Components of an emulsion polymerization
2.3 Sites of polymerization
2.4 Advantages and drawbacks
2.5 Process description
2.6 Kinetics and mechanisms
2.6.1 Free radical polymerization
2.6.2 Rate of polymerization
2.6.3 Number of polymer particles
2.6.4 Average number of radicals per particle
2.6.5 Monomer concentration [M]
2.7 Other kinetic effects
2.7.1 Electrolyte concentration
2.7.2 Emulsifier structure
2.7.3 Nonionic emulsifiers
2.7.4 Monomer to water ratio
2.7.8 Gel effect
2.8 Reactor engineering
2.8.1 Batch reactor
2.8.2 Semibatch reactor
2.8.3 Continuous reactor
3 Modeling and simulation of emulsion polymerization
3.2 Modeling of emulsion polymerization
3.2.1 Initiator balance
3.2.2 Monomer balance
3.2.3 Average number of radicals per particle ¯n
3.2.4 Radicals in the aqueous phase
3.2.5 Monomer phase distribution
3.2.6 Moments of dead chains
3.2.7 Energy balance equations
3.3 Experimental validation of the model
4 Dynamic optimization of vinyl acetate polymerization
4.1 Dynamic optimization fundamentals
4.1.2 Problem definition
4.1.3 Analytical methods
4.1.4 Numerical methods
4.2 Dynamic optimization of emulsion polymerization
4.3 Case study : Vinyl acetate emulsion polymerization
4.3.1 Process operation
4.3.2 Minimization of batch time with T as control variable
4.3.3 Minimization of batch time with reactor T and qI as control variables
4.3.4 Minimization of batch time with T, qI and qM as control variables
5 Nonlinear geometric control and state estimation
5.1 Nonlinear geometric control fundamentals
5.1.1 Differential geometry concepts
5.1.2 Input/output linearization
5.1.3 Su-Hunt-Meyer linearization (or input/state linearization)
5.2 State estimation
5.2.1 State estimation principles
5.2.2 Extended Kalman filter (EKF)
6 Nonlinear control of vinyl acetate polymerization
6.1 Control of emulsion polymerization
6.2 Simulation of the nonlinear geometric control
6.2.1 Controller design
6.2.2 Extended Kalman filter
6.2.3 Simulations results for the current operation of the industrial reactor
6.3 Control under optimal conditions
6.3.1 Optimal temperature control with optimal feed policies (qI and qM)
7 Conclusions and perspectives