Emulsion polymerization concentrations

The surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

In an emulsion polymerization experiment at 60°C the number of micelles per unit volume is 5.0 X 10 hter and the monomer concentration in the micelle... 

In this example the number of micelles per unit volume is exactly twice the stationary-state free-radical concentration hence the rates are identical. Although the numbers were chosen in this example to produce this result, neither N nor M are unreasonable values in actual emulsion polymerizations. 

Acrylates are primarily used to prepare emulsion and solution polymers. The emulsion polymerization process provides high yields of polymers in a form suitable for a variety of appHcations. Acrylate polymer emulsions were first used as coatings for leather in the eady 1930s and have found wide utiHty as coatings, finishes, and binders for leather, textiles, and paper. Acrylate emulsions are used in the preparation of both interior and exterior paints, door poHshes, and adhesives. Solution polymers of acrylates, frequentiy with minor concentrations of other monomers, are employed in the preparation of industrial coatings. Polymers of acryHc acid can be used as superabsorbents in disposable diapers, as well as in formulation of superior, reduced-phosphate-level detergents. 

Suspension- and emulsion-polymerized PVDF exhibit dissimilar behavior in solutions. The suspension resin type is readily soluble in many solvents even in good solvents, solutions of the emulsion resin type contain fractions of microgel, which contain more head-to-head chain defects than the soluble fraction of the resin (116). Concentrated solutions (15 wt %) and melt rheology of various PVDF types also display different behavior (132). The Mark-Houwink relation (rj = KM°-) for PVDF in A/-methylpyrrohdinone (NMP) containing 0.1 molar LiBr at 85°C, for the suspension (115) and emulsion... 

Initia.tors, The initiators most commonly used in emulsion polymerization are water soluble although partially soluble and oil-soluble initiators have also been used (57). Normally only one initiator type is used for a given polymerization. In some cases a finishing initiator is used (58). At high conversion the concentration of monomer in the aqueous phase is very low, leading to much radical—radical termination. An oil-soluble initiator makes its way more readily into the polymer particles, promoting conversion of monomer to polymer more effectively. 

Soap-starved recipes have been developed that yield 60 wt % soHds low viscosity polymer emulsions without concentrating. It is possible to make latices for appHcation as membranes and similar products via emulsion polymerization at even higher soHds (79). SoHds levels of 70—80 wt % are possible. The paste-like material is made in batch reactors and extmded as product. 

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

Ethoxylated andSulfatedAlkylphenols. Because these aLkylphenols degrade less readily than the sulfated alcohol ethoxylates, their anticipated expansion failed to materialize, although by 1965 they were widely used in retail detergent products. Sulfated alkylphenol ethoxylates are used in hospital cleaning products, textile processing, and emulsion polymerization. Sulfated alkyphenol ethoxylates are sold as colorless, odorless aqueous solutions at concentrations of >30%. The presence of ethylene oxide in the molecule increases resistance to hardness ions and reduces skin irritation. Representative commercial sulfated alkylphenol ethoxylates are given in Table 12. 

Propagation. The rate of emulsion polymerization has been found to depend on initiator, monomer, and emulsifier concentrations. In a system of vinyl acetate, sodium lauryl sulfate, and potassium persulfate, the following relationship for the rate of polymerization has been suggested (85) ... 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

Emulsion Polymerization. Emulsion polymerization takes place in a soap micelle where a small amount of monomer dissolves in the micelle. The initiator is water-soluble. Polymerization takes place when the radical enters the monomer-swollen micelle (91,92). Additional monomer is supphed by diffusion through the water phase. Termination takes place in the growing micelle by the usual radical-radical interactions. A theory for tme emulsion polymerization postulates that the rate is proportional to the number of particles [N. N depends on the 0.6 power of the soap concentration [S] and the 0.4 power of initiator concentration [i] the average number of radicals per particle is 0.5 (93). 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

The main purpose of pesticide formulation is to manufacture a product that has optimum biological efficiency, is convenient to use, and minimizes environmental impacts. The active ingredients are mixed with solvents, adjuvants (boosters), and fillers as necessary to achieve the desired formulation. The types of formulations include wettable powders, soluble concentrates, emulsion concentrates, oil-in-water emulsions, suspension concentrates, suspoemulsions, water-dispersible granules, dry granules, and controlled release, in which the active ingredient is released into the environment from a polymeric carrier, binder, absorbent, or encapsulant at a slow and effective rate. The formulation steps may generate air emissions, liquid effluents, and solid wastes. 

A semi-batch reactor has the same disadvantages as the batch reactor. However, it has the advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. Semi-batch reactors are also of value when parallel reactions of different orders occur, where it may be more profitable to use semi-batch rather than batch operations. In many applications semi-batch reactors involve a substantial increase in the volume of reaction mixture during a processing cycle (i.e., emulsion polymerization). 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

The rate of an ideal emulsion polymerization is given by Eqn (4). In this expression [/] is the initiator concentration, [ ] is the emulsifier concentration, and [M] is the concentration of monomer within the forming latex particles. This value is constant for a long reaction period until all the monomer droplets disappear within the water phase. 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

Flgure 4 The effect of initiator concentration on the variation of monomer conversion by the polymerization time in the emulsion polymerization of styrene. Styrene-water = 1/3 SDS = 15.4 mM reaction volume = 300 ml stirring rate = 250 rpm temperature = 70°C. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

---