Emulsion polymerization reactions

An emulsion polymerization reaction follows three conventional steps, namely, initiation, propagation, and termination. These steps can be described by the conventional reactions that are valid for any free radical polymerization. Smith and Ewart [10] proposed that a forming latex particle in an ideal emulsion polymeriza-... 

Two different emulsion polymerization reactions were Investigated. One was the polymerization of acrylonitrile and methylacrylate (75/25 AN/MA) In the presence of an acrylonitrile elastomer (70/30 BD/AN) to produce a graft resin, llie second was the copolymerization oiE acrylonitrile and styrene (70/30 AN/S). Chromatographic analyses of latex solutions were conducted periodically during both types of polymerization reactions, using acetonitrile as latex solvent and chromatographic mobile phase. 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

The locus of reaction during an emulsion polymerization is nearly exclusively within particles in which the ratio of polymer to monomer is high enough so that the reacting fluid is quite viscous. Relative to bulk polymerization, this is beyond the start of the Mgel effect and one should expect the influence of restricted diffusion of the polymer to be felt during the entire emulsion polymerization reaction. The most common approach for treating this behavior in bulk polymerization is to treat the termination rate constant, k, as a function of conversion level... 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerization occurs in the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerization reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerizations are carried out in the presence of a plentiful supply of monomer. 

It has been advantageous to use the FT-Raman method to study various dynamic processes of interest in the paint industry. One such study was the study of an emulsion polymerization reaction whereby a FT-Raman system actually monitored the process (1). 

Additional regulators were identified by the author [1] in a subsequent investigation and used in emulsion polymerization reactions ... 

Schuller [150] and Guillot [98] both observed that the copolymer compositions obtained from emulsion polymerization reactions did not agree with the Mayo Lewis equation, where the reactivity ratios were obtained from homogeneous polymerization experiments. They concluded that this is due to the fact that the copolymerization equation can be used only for the exact monomer concentrations at the site of polymerization. Therefore, Schuller defined new reactivity ratios, TI and T2, to account for the fact that the monomer concentrations in a latex particle are dependent on the monomer partition coefficients (fCj and K2) and the monomer-to-water ratio (xp) ... 

Apart from intrinsic interest, the theoiy of compartmentalized free-radical polymerization reactions is of importance primarily because it is believed that most of the polymer which is form in the course of an emulsion polymerization reaction is formed via reactions of this type. The general sl pe of the conversion-time curve for many emulsion polymerization reactions suggests (see Fig. I) that the reaction occurs in three more-or-less distinct stages or intervals. The first of these, the so-called Interval I, is interpreted as the stage of polymerization in which the discrete reaction loci are formed. In the second and third stages—Intervals II and III—the polymerization is believed to occur essentially by compartmentalized free-radical polymerization within the loci which were formed during Interval I. 

The matter of the decay behavior of a seeded emulsion polymerization reaction following the cessation of the generation of new radicals in the external phase has recently been treated by Lansdowne et al. (1980) using the matrix ajqjroach. 

When the surfeclant concentration is high relative to the polymer concentration sufRdent sui ctant may be adsorbed on individual polymer molecules to prevent their coalescence to form latex particles or indeed to disperse prdbrmed polymer to form clear solutions in which the solute behaves as a po yelectro yte, But the influence of such effects on tbe course of emulsion polymerization reactions has not been elucidated. Sata and Saito (1952) showed that poly(vinyl acetate) preeptated from acetone solution with water could be solubibzed in sodium dodecyl sulfate solutions after removal of the acetone by dialysis. To obtain a clear solution at 20°C, a wdght of surfactant S-10 times that of pol ner was required. Althou this greatly exceeds the surfactant concentrations normally used in emulsion... 

Emulsion polymerization reactions are sometimes carried out with small seed particles formed in another reaction system. A number of advantages can he derived from using seed particles. In a batch reactor seed latex can he helpful hi controlling particle concentration, polymerization rate, particle morphology, and particle size characteristics. In a CSTR the use of a feed stream containing seed particles can also help to prevent conversion and/or surface tension oscillations, which are caused by particle formation phenomena, This factor will be discussed in more detail later in this chapter. 

The colloidal nature of the reaction media has a significant influence on the course of an emulsion polymerization reaction. A number of distinct phases exist during different intervals of a batch reaction. Chemical and physical phenomena within these phases and at the interfaces can be important in determining reaction kinetics and the properties of the latex product. 

Summary. All of the phases and the physical and chemical mechanisms discussed in this section are important during the course of an emulsion polymerization reaction. They influence the reaction kinetics and the properties of the latex produced. Not all of the phenomena that can occur are understood in a quantitative manner. Nevertheless, considerable advances have been made in the fundamental understanding and the commercial exploitation of emulsion polymerization processes. The remainder of this chapter will focus on reactor types and reaction kinetics. 

Emulsion polymerization reactions have also been studied in reactors consisting only of tubes. Such reactors offer the potential advantage of a large area for heat transfer per unit volume and hence a high polymerization rate. One potential problem with tubular reactors, namely plugging, has discouraged commercial use. A number of studies have been reported on once-through continuous tubular reactors but commercial reactors of this type have not been publicized. 

The kinetics of emulsion polymerization reactions are complex because of the numerous chemical and physical phenomena that can occur in the multicomponent, multiphase mixture. A large amount of literature exists on kinetics problems. The general references listed at the end of this chapter contain many important papers. The review paper by Ugelstad and Hansen (11) is a comprehensive treatment of batch kinetics. The purpose of the remainder of this chapter is to present the general kinetics problems and some of the published results. The reader should use the references cited earlier for a more detailed study. 

USE Bleaching and oxidizing agent promoter for emulsion polymerization reactions. Caution Highly irritating to skin, mucous membranes. 

The large number of variables, and the dependence of many of them upon conversion, usually means that it is necessary to compare experimera with predictions and to Iterate to values of the parameters which give best-fit correlation with experimental data. These control strategies are, therefore, specific to a particular emulsion polymerization reaction (i.e. a specific formulation and reaction conditions). While the principles of this approach can be applied to a range of emulsion copolymerizations, it is not possible to predict the optimum addition rate profiles directly and considerable experimental effort is required to establish optimum values of parameters for each system. 

An extensive literature exists on modelling and simulation of emulsion polymerization reactions, including CSTR systems. The early work of Omi et al. [7],... 

The polymer formed during the emulsion polymerization reaction is believed to have essentially the following type of structure ... 

Two general features of emulsion polymerization reaction systems for the production of carboxylated rubber latexes are of special interest The first b that the polymerization usually takes place under acidic conditions (c. pH 3-4) and not under the alkaline conditions which are usual for the production of non-functionalized synthetic rubber latexes. Polymerization is carried out under acidic conditions in order to encourage the carboxylic acid monomer to become copolymerized into the molecxiles of rubber being prcxluced. If the reaction is carried oui under alkaline conditions, then the carboxylic acid monomer is present mainly as a carboxylate salt which partitions strongly in favour of the aqueous phase If it polymerizes at all under these conditions, polymerization occurs mainly ir the aqueous phase of the reaction system, and the polymer molecxiles in whicl it becomes incorporated are far more hydrophilic than are the majority of the polymer molecxiles, which are produced in the latex particles. The requiremen... 

Spectroscopic Techniques for Continuous Monitoring of Emulsion Polymerization Reactions... 

Real time continuous monitoring of the emulsion polymerization reactions was achieved using transmission and reflectance techniques. Transmission spectra were obtained on-line using the automatic sampling and dilution system. The reflectance probes were immersed in the reactor and the spectra recorded as function of time. Figure 3 shows a schematic of the probes in the reactor. In all cases special care was taken to ensure that the tip of the probe was well immersed in the reacting mixture. The lattices prepared were also used to study the effect of particle concentration on the reflectance measurements. 

Figure 10. Left Transmission spectra reported at three different times during of a styrene emulsion polymerization reaction. Right Mean particle size as function of time for the reaction as a function of time.

Reflectance spectroscopy has the advantage that can be used both, in solid materials, and/or in concentrated dispersions. Although Uv-Vis reflection techniques have been used in some applications, " they have not been extensively tested for monitoring emulsion polymerization reactions. Yu et al used reflectance to monitor imidization reactions. Specular reflection spectra from polarized light combined with a Kramers-Kroning transformation were used to assess the surface orientation of PET sheets. NIR with monochromatic laser light has been used to monitor the particle size distribution. The main problem for the interpretation of... 

Figure 23. Left. Conversion measurements for an emulsion polymerization reaction. Data collected using probe configuration 1. Right Comparison between the conversion estimated using probe configuration 4 and gravimetric data.

Figure 24, Initial and final reflectance spectra obtained with probe 4, measured from a seeded emulsion polymerization reaction of styrene, concentration is approximately 30 %...

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