Vinyl acetate emulsion polymerization kinetics

The results definitely prove our hypotheses in the kinetic model for vinyl acetate emulsion polymerization (10), that vinyl radical, CH2=C-0Ac, is the major monomer radical formed and is a stable radical which reinitiates relatively slowly compared to the propagation step. 

The kinetics of vinyl acetate emulsion polymerization in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly (vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stability, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

The following are suninerized for kinetic behavior of vinyl acetate emulsion polymerization from the data presented here. 

Ciullo PA, Hewitt N (1999) The rubber formulary. Noyes, Norwich Coker K (2001) Modeling of chemical kinetics and reactor design. Gulf, Boston, MA Davis FJ (2004) Polymer chemistry a practical approach. Oxford University Press, Oxford Drobny JG (2001) Technology of fluoropolymers. CRC Press, Boca Raton, FL Durairaj RB (2005) Resorcinol chemistry technology and applications. Springer, Berlin Dutcher JR et al (eds) (2005) Soft materials structure and dynamics. Marcel Dekker, New York Erbil HY (2000) Vinyl acetate emulsion polymerization and copolymeiization with acrylic monomers. CRC Press, Boca Raton, FL... 

Several reports have been published on the in-line monitoring of vinyl acetate emulsion polymerization reactions in semibatch mode [22]. With appropriate models, this approach can provide good feedback about the polymerization reaction kinetics. Heat flow calorimetry (Hfc) is frequently used to... 

The emulsion polymerization of vinyl acetate (to homopolymers and copolymers) is industrially most important for the production of latex paints, adhesives, paper coatings, and textile finishes. It has been known that the emulsion polymerization kinetics of vinyl acetate differs from those of styrene or other less water-soluble monomers largely due to the greater water solubility of vinyl acetate (2.85% at 60°C versus 0.054% for styrene). For example, the emulsion polymerization of vinyl acetate does not follow the well-known Smith-Ewart kinetics and the polymerization exhibits a constant reaction rate even after the separate monomer phase disappears. The following observations have been reported for vinyl acetate emulsion polymerization [78] (a) The polymerization rate is approximately zero order with respect to monomer concentration at least from 20% to 85% Conversion (b) the polymerization rate depends on the particle concentration to about 0.2 power (c) the polymerization rate depends on the emulsifier concentration with a maximum of 0.25 power (d) the molecular weights are independent of all variables and mainly depend on the chain transfer to the monomer (e) in unseeded polymerization, the number of polymer particles is roughly independent of conversion after 30% conversion. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

Araki et at. (1967, 1969) carried out a more systematic study of the kinetics and other features of the y-iniliated emulsion polymerization of vinyl acetate using sodium lauryl sulfate as the emulsifier. This system had been thoroughly investigated with potassium persulfate as the initiator (Litt et cL. 1960,1970). Some post ei cts have been observed with vinyl acetate, particularly above 50% conversion (Friis, 1973 Sunardi, 1979). These effects had been used by Allen cr at. (1960,1962) for the possible synthesis of block and graft polymers and will be described later in this chapter. The half-life of the radicals in a vinyl acetate latex polymerization was determinad by Hummel et at. (1969) as 0.8 min at 53.8% conversion. Araki et fll. (1967, 1969) determined all the normal rate dependencies and included some seeded latex studies. Their results and those of other investigators are summarized in Table II together with those found with potassium persulfate initiation and those predicted by the Smith-Ewart Case 2 theory. The... 

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

An interesting variation on the use of azo groups-containing polymers is the use of polyacrylamide prepolymers as both initiator and emulsifying agent for the emulsion polymerization of vinyl acetate. The observed reaction kinetics were typical (see Fig. 4.7) for an emulsion polymerization and, in particular, the slope (a = 1) of the plot of lg Rp vs lg CPrep confirms the double function of these prepolymers very clearly. 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

Aizpurua et al. [96] have studied the kinetics of vinyl acetate miniemulsions stabilized with PS or PVAc. Guyot and coworkers [97] used PS as the costabilizer for the mini emulsion encapsulation of pigment. Samer [67] has used PMMA to stabilize MMA miniemulsions for continuous polymerization in a CSTR. 

Kinetics and Mechanism of the Emulsion Polymerization of Vinyl Acetate... 

Polymerization of styrene in an emulsion polymerization has been shown to follow a kinetics scheme as first described by Smith and Ewart. When the vinyl monomer is not a good solvent for the polymer (l.e. acrylonitrile or vinyl acetate) large deviations from Sraith-Ewart Theory kinetic predictions are observed. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

The kinetic behavior of emulsion polymerization is greatly affected by radical desorption from polymer particles. This has been shown by Dgelstad et al. (1969)> Litt et al. (1970), Harada et nl. (1971), Friis and Nyhagen (1973), and Nomura et al (1971). It is believed that the deviation of the kinetic behavior of the emulsion polymerization of water-soluble monomers such as vinyl acetate and vinyl chloride from the Smith and Ewart (1949) Case 2 kinetic theory is mainly due to dominant desorption of... 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

In a complex apparatus, Gimesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the water-cooled bulk polymerization of small particles. It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amoimt of imdesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes unique to this system and different from the conventional mechanism of suspension polymerization. 

This observation seems to be in line with the Smith-Ewart concepts. The adsorption of surfactants on the surfaces of latex particles influences the capture by the particles of low-molecular-weight polymers formed in the aqueous solution. This in turn affects the reaction kinetics and the formation of new particles. The number of free radicals per particle, which is usually considered to be constant during the major phases of an emulsion polymerization, seems to vary considerably during the polymerization of vinyl acetate [139]. 

Most commonly, in the emulsion polymerization of vinyl acetate, anionic surfactants are used either alone or in combination with a protective colloid. Typical examples of surfactants which have found application are Aerosol OT (sodium dioctylsulfosuccinate), alkyl aryl sulfonate salts (e.g., Santomerse-3), sodium lauiyl sulfate, etc. A study of the kinetics of the vinyl acetate polymerization in the presence of sodium lauryl sulfate indicated that the rate of polymerization was proportional to the square root of the initiator concentration and the 0.25th power of the number of particles. The number of particles were proportional to the 0.5th 0.05 power of the surfactant concentration but independent of the level of potassium persulfate. The intrinsic viscosity of the final polymer was said to be independent of the initiator concentration and of the munber of polymer particles. These observations were said to suggest that the mechanism of the vinyl acetate polymerization in emulsion resembles that of vinyl chloride [153]. 

The study of Nomura et al. [143] is perhaps one of the most detailed investigations on the kinetics and mechanism of the emulsion polymerization of vinyl acetate published. The work presents a detailed evaluation of the interactions of monomer, surfactant, and initiator concentration in the course of the... 

The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [170,171]. The variation in reactivity ratios may also by related to the following observations The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate—rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. 

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-... 

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