Water-to-monomer ratio

Figure 7 The effect of monomer-to-water ratio on the variation of monomer conversion by the polymerization time in the emulsion polymerization of styrene. KPS = 1.65 mM SDS = 15.4 mM reaction volume = 300 ml stirring rate = 250 rpm temperature = 70°C.

The process usually starts with the polymerization of a small proportion of the reagents at a very low monomer to water ratio (the seed stage), followed by the feeding of the remaining monomer (which may take several hours) and of other materials (if needed) once the conversion in the reactor has reached 70% or more. The in-reactor conversion will then depend upon the rate of polymerization compared to the rate of feed. If the reaction is continued under the so-called monomer-starved conditions, the in-reactor conversion is kept at a high 80-90%, which reduces the polymerization rate. To compensate, temperature is raised however, then the initiator depletes faster and more has to be added during the reaction. 

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) ... 

The influence of the monomer to water ratio on the polymerization rate was studied with sodium lauryl sulfate as the emulsifier. The conversion curves for the case of 3% emulsifier are shown in Figure 5. In Figure 6 the linear conversions for ten minutes of irradiation at 0.175 Mrads per hour are plotted against the water-monomer ratio for 1, 3, and 5% emulsifier. All three sets of data show a linear dependence of the rate on the ratio, in other words, the rate per cubic centimeter of water phase is independent of the monomer-water ratio. 

At,a fixed monomer to water ratio N a l/r, this leads to Rn =... 

Acres and Dalton (1963a), using the dioctyl ester of sodium sulfosuc-cinate as the emulsifier, found the intensity exponent of the rate to vary with the monomer to water ratio from 0-22 to 0.34 but to reach the classical Smith-Ewart value of 0.4 when extrapolated back to zero monomer concentration. They interpreted this result in terms of the special role of the hydrogen atans arising from the radiolysis of water. 

Ozygen was found to inhibit the reaction in the sense that an induction period was introduced. After this the steady-states rates were not affected. Increasing the rate of stirring first increased the renclion rate which then levelled off above about 600 rpm. No effect of the monomer to water ratio was found. Both the rate and tbe number average molecular weight were... 

Furthermore, the authors pointed out that they obtained in the emulsion polymerization of styrene (monomer to water ratio 1 2) with an inisurf concentration of 5.4 X 10 mol/1 water in the presence of an alkylated poly (oxyethyl-ene) emulsifier (alkyl chain length C16 -18 and 20 oxyethylene units 4% by weight related to water) the same overall rate of polymerization as with water-soluble initiators in the concentration range 10 to 10 mol/1 water. The polymer produced in the presence of inisurf has a molecular weight of some of 10 g/mol mainly due to the lowered termination rate constant. 

In Experiment 1, 9.28 moles of allyl acetate per liter of latex was emulsified with 0.087 mole of sodium lauryl sulfate, buffered with 0.45 mole of sodium pyrophosphate, and initiated with 0.0920 mole/liter of potassium persulfate. In experiment 2, again 9.28 moles of allyl acetate per liter of latex was polymerized in the presence of 0.087 mole of sodium lauryl sulfate, 0.45 mole of sodium pyrophosphate, and 0.366 mole/liter of potassium persulfate. Since the MW of allyl acetate is 100, the above information implies that the basic monomer to water ratio is an unlikely 928 gm of monomer to approximately 70 gm of water. If indeed these are the experimental facts, then the fact that the polymers produced resembled those produced in bulk or in solution is not surprising. A reaction mixture consisting of nearly 93% pure monomer, naturally would be expected to produce a polymer similar to one produced from a pure (i.e., bulk) monomer and not one similar to an emulsion polymer. Compositions of less than 60% monomer in water would ordinarily be expected to produce latices. Perhaps the data in question refer to a ratio of 9.28 moles of monomer to one liter of water. 

Figure 7 Experimental (a) and model (b) molar mass chemical composition distribution (MMCCD) of a high-conversion (95 mol.%) STY/MA emulsion copolymer (Is = 0.33, monomer-to-water ratio (MM) = 0.5,1wt.% n-dodecylmercaptan, 50 °C, M =110000gmol ). Reprinted from van Doremaele, G. H. J. Geerts, F. H. J. M. aan de Meulen, L. J. German, A. L. Polymer 99Z, 33,1512-1518. ...

Other Components. The smaller the particle size, at a given phase ratio, the more difficult it is to ensure colloidal stability (cf Fig. 5). This means that for aqueous heterophase polymerizations in the order suspension < microsuspension < emulsion < miniemulsion < microemulsion, the stabilizer concentration increases. Contrary to the simple polymerization of st5Tene in water, polymerization recipes for industrially important polymer dispersions comprise up to six monomers, frequently more than two emulsifiers, more than one initiating system, and a few other aids like biocides, defoaming agents, plasticizers for supporting film formation (39). The monomer-to-water ratio is adjusted in such a way that a solid content results typically between 40 and 60% or even higher. The amoimts of surfactants and initiator (mainly peroxodisulfate) are typically between 0.5 and 2% (w/w) relative to the monomers and 0.5% (w/w) relative to water, respectively. 

When both monomers are sparsely water soluble, for example, the monomer pairs styrene-butadiene and styrene— -butyl acrylate, the effect of the monomer to water ratio on the monomer composition in the droplet phase as well as in the particle phase is negligible. So for sparsely water-soluble monomer pairs, the monomer to water ratio has hardly any influence on the instantaneous and cumulative copolymer composition. 

As a conclusion, it can be stated that for monomer pairs of which one monomer is sparsely water soluble and one is moderately water soluble or even completely miscible with water, the concentration ratio of the monomers in the particle phase strongly depends on the volume ratio of the phases involved. However, the effect of the monomer to water ratio, M/W, is only important for too small values of A4/W. Thus, for industrial recipes M/W > 1 and the influence of M/W on the copolymer composition will be negligible. 

The time-dependent behavior of emulsion polymerization arises due to variation in monomer concentration, changes in the number of polymer particles Nt, or both. We have aheady observed that N, changes due to nucleation in stage I of emulsion polymerization and this normally ends at about 10-15% conversion. However, when the monomer-to-water ratio MIW) is high or the monomer is more than sparingly soluble, the constancy of N, cannot be assiuned up to conversions as large as 50%. If the monomer droplets are sufficiently small, they also become the loci of particle formation and, in such circumstances, the Smith-Ewart theory is inadequate to explain the experimental phenomena. We now present the outline of a mathematical model of emulsion polymerization that is... 

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