Emulsion polymerization histories

The early history of redox initiation has been described by Bacon.23 The subject has also been reviewed by Misra and Bajpai,207 Bamford298 and Sarac.2,0 The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when initiation is required at or below ambient temperature and drey are frequently used for initiation of emulsion polymerization. 

Figure 1. Conversion-time histories in batch PVC emulsion polymerization for different initiator and emulsifier concentrations.

Emulsion polymerization has a history of half a century in science and much more in industry. There seem to be few undissolved subjects in general emulsion polymer-... 

Polyfvinyl acetate) (PVAc) latexes produced by batch and continuous emulsion polymerization were used in this study. Details for the apparatus and the polymerization procedure can be found in Penlidis et al. (6,12,K3). Samples taken during the reaction were subsequently analyzed to follow conversion- and particle growth-time histories. The batch experimental runs were designed to yield similar conversion-time histories but different particle sizes. Conversion was measured both off-line, by gravimetric analysis, and on-line using an on-line densitometer (a U-tube DPR-YWE model with a Y-mode oscillator with a PTE-98 excitation cell and a DPR-2000 electronic board by Anton Paar, Austria). A number of runs were repeated to check for reproducibility of the results. Four batch runs are described in Table I below and their conversion histories are plotted in Figure 1. 

Specific turbidity histories are also plotted vs. dimensionless time for a continuous emulsion polymerization run the samples were withdrawn from the second reactor of a continuous train where the first reactor is a small seeding reactor. Part A of Figure 3 shows the particle size behaviour during start up all monomer, water, initiator and soap feedrates were kept constant until the process reached a steady state. In part B, the soap concentration in the seed reactor was increased a decrease in the particle size was expected and it is clearly shown from the specific turbidity measurements. 

Figure 1. Conversion histories of the batch vinyl acetate emulsion polymerizations (similar recipes only the emulsifier concentration is different).

Since the polymerization conditions for styrene and methyl methacrylate often are chosen such that a in the early stages of polymerization (after N has become constant) lies between 10 2 and 1, it is understandable that gel-effect usually is important in emulsion polymerization of these monomers and therefore must be accounted for since it completely dominates the shape of the conversion history. 

Systems Where Radical Desorption is Negligible. Styrene and methyl methacrylate emulsion polymerization are examples of systems where radical desorption can be neglected. In Figures 4 and 5 are shown comparisons between experimental and theoretical conversion histories in methyl methacrylate and styrene polymerization. The solid curves represent the model, and it appears that there is excellent agreement between theory and experiment. The values of the rate constants used for the theoretical simulations are reported in previous publications (, 3). The dashed curves represent the corresponding theoretical curves in the calculation of which gel-effect has been neglected, that is, ktp is kept constant at a value for low viscosity solutions. It appears that neglecting gel-effect in the simulation of styrene... 

Figures 4, 5, 6. Comparison between experimental and theoretical conversion histories in emulsion polymerization of methyl methacrylate, styrene, and vinyl acetate. (—), model with gel-effect (—), model without gel-effect.

IPN s and related materials) in fact) have a long history. For example) IPN s were first synthesized to produce smooth sheets of bulk polymerized homopolymers (11), IPN s were next used as solution polymerized ion exchange resins. (12) 13) Further development of IPN s included the syntheses oT interpenetrating elastomer networks (lEN s) and simultaneous interpenetrating networks (SIN s) (14). lEN s consist of a mixture of different emulsion polymerized elastomers which are both crosslinked after coagulation. SIN s are formed by the simultaneous polymerization of mixed monomers by two noninterfering reactions (3 ) 16). 

The technique of emulsion polymerization is characterized by the formation of the polymer in the form of a latex. The particle size distribution (PSD) of the latex and the molecular weight distribution (MWD) of the contained polymer are two important measurable parameters of the latex. Not only do they influence the end-use behavior of the product, but they also reflect the growth history of the emulsion polymerization process. In what follows, we review the theories that have been developed to describe the PSD and MWD-of emulsion polymers. 

The population balance equations are very general and may be applied to batch, semicontinuous, and continuous emulsion polymerizations. Furthermore, both seeded and ab initio polymerizations are comprehended by Eq. (5) in all (or part) of the three commonly considered polymerization intervals. The following sections show how the different possibilities are reflected in different functional forms of the elements of the matrices O and K and of the vector c. It should be remembered, however, that certain conceivable situations are not comprehended by Eq. (5) for example, if the monomer molecules are not freely exchanged between the latex particles so that the monomer concentration inside each latex particle is determined by its growth history. 

In Chap. 1, a small overview of emulsion polymerization and miniemulsion polymerization is given, followed by some history of preparation of hybrid latex particles. 

We have tested this hypothesis in the inverse emulsion polymerization of acrylamide (A Am). A typical recipe ineluded A Am as the monomer dissolved in water, cyclohexane as the continuous phase, sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) or sorbitan monooleate (Span-80) as the surfactant and MMT or LRD as typical nascent clays. Both oil- and water-soluble azo compounds, such as 2,2 -azobisisobutyronitrile (AIBN) and 2,2 -azobis[2-methyl-A-(2-hydroxyethyl)propionamide] (VA-086), were used as initiators. Span-80 appeared to lead to more stable inverse emulsions in the presence of clay than AOT. We first noticed a different kinetic behavior during polymerization in the presence of clay platelets. The conversion time history for the AAm inverse emulsion polymerization in the presence of MMT showed a significant decrease in the rate of polymerization in comparison with those without clay. With increasing concentration of clay particles, lower rates of polymerizations and lower final monomer conversions were found, together with increased retardation of the polymerization. The clay platelets in the inverse emulsions might act as diffusion barriers for monomer and/or initiator. 

History. The first attempts to desensitize RDX were reported by Frankel and Carle ton (Refs 1 thru 5) who made use of polymeric materials such as polyurethanes to coat expl crysts by means of emulsion or soln techniques. The first true PB-RDX was developed in 1952 at the Univ of Califs Los Alamos Lab and consisted of RDX coated with polystyrene plasticized with DOP (Refs 6 21). Since then the Lawrence Livermore Lab has evolved a series of PBX formulations, many of which are listed in Tables 3,4 5. These compns are described in Ref 77... 

Acids were an early exception to the no water rule. It was recognized that aqueous solutions of acids would inhibit swelling of clays and shales as well as dissolve any acid-soluble minerals contained in a formation. By 1933 commercial well stimulation with hydrochloric acid was of great interest. A whole separate methodology and treatment chemistry has since evolved around acidizing and fracture acidizing(54). Water emulsions, mainly emulsified acids, and gelled acids thickened with polymeric additives were applied early in the history of well treatment. 

In regard to the solubility of the monomer in its polymer, the diffusion of vinyl chloride into PVC varies with the physical state of the resin. Earlier work frequently dealt with studies involving PVC films more recently, powdered resins were studied. In the latter case, variations in diffusion rates were found to depend on the method of polymerization (emulsion vs, suspension method) as well as on the physicochemical parameters [53]. The equilibrium solubility of vinyl chloride monomer in poly(vinyl chloride) was found to be a function of polymer type, polymer history, time, temperature, and the VCM partial pressure [7]. Above atmospheric pressure, with the ratio of the partial pressure of VCM to the initial partial pressure of the monomer greater than approximately 0.5, the solubility of vinyl chloride is 0.300 gm per gram of poly(vinyl chloride). At lower pressures, the solubility shows a distinct decrease with temperature. Kuchanov and Bort [36] state that the solubility of VCM in PVC varies between 22.1% and 23.7% between 30° and 60°C. 

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