Emulsifier polymer rate dependence

The investigation of polymer rate dependence on Initiator concentration C. (with ionic strength of the solution equalized) and emulsifier concentration C (for various molecular structures of emulsifier) pennltted us tb establish that it can be described by the following equation ... 

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 rate at which a polymer adsorbs to an interface is one of the most important factors determining its efiScacy as an emulsifier [3,11,19]. The adsorption rate depends on the molecular characteristics of the polymer (e.g. size, flexibility, conformation, and interactions), the nature of the bulk liquid (e.g. viscosity, polarity), and the prevailing environmental conditions (e.g. temperature and fluid flow profile). It is often convenient to divide the adsorption process into two stages (i) movement of the emulsifier molecules from the bulk liquid to the vicinity of the interface, and (ii) attachment of the emulsifier molecules to the interface (Figure 5.9). In practice, emulsifier molecules are often in a dynamic equilibrium between... 

In eqn (3], Fi is the instantaneous copolymer composition referred to monomer 1, and rj and r2 are the monomer reactivity ratios for monomer 1 and 2 (terminal model), respectively. During intervals I and II, the concentration of the monomers in the polymer partides are governed by the partitioning of the monomers among monomer droplets, polymer particles, and aqueous phase. In interval III, there are no droplets and the monomer is mostly located in the polymer particles. The concentration of the monomers in the polymer partides depends on the relative values of mass transfer and polymerization rates. Except for poorly emulsified, highly water-insoluble systems, mass transfer is much faster than polymerization rate, and hence the concentration of monomers in the different phases is given by the thermodynamic equilibrium. 

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. 

The choice of coagulant for breaking of the emulsion at the start of the finishing process is dependent on many factors. Salts such as calcium chloride, aluminum sulfate, and sodium chloride are often used. Frequentiy, pH and temperature must be controlled to ensure efficient coagulation. The objectives are to leave no uncoagulated latex, to produce a cmmb that can easily be dewatered, to avoid fines that could be lost, and to control the residual materials left in the product so that damage to properties is kept at a minimum. For example, if a significant amount of a hydrophilic emulsifier residue is left in the polymer, water resistance of final product suffers, and if the residue left is acidic in nature, it usually contributes to slow cure rate. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

The steady-state conversion data for VA and MMA, shown in Figures 1, 2 and 9, are qualitatively consistent with proposed mechanisms if one considers the transfer of free radicals out of particles and the gel effect. If radicals are completely free to move into and out of polymer particles, one would expect, in the absence of a gel effect, that Rp would depend on the squcure root of initiation rate and would not depend at all on the emulsifier concentration. Ley et al (17) demonstrated that free radicals do transfer out of particles in PVA and PMMA emulsions, and that the transfer rate is considerably higher for vinyl acetate than for MMA. 

When an emulsifier is used, its type and concentration primarily affects the number of latex particles formed, which in turn determines the rate of polymerization and, depending also on the rate of initiation, the molecular weight of the polymer formed. Although the physical properties of the polymer are primarily dependent on its molecular waght and molecular weight distribution, the properties of the latex depend on its concentration, average particle size, particle size distribution, and the viscosity of the aqueous phase, which may be enhanced by addition of a thickener—a water-soluble polymer not adsorbed by the polymer phase which does not affect the course of the reaction,... 

S6). It depended on the variation of the number of latex particles formed iV with temperature. Unfortunately, they have overlooked the fact that the particle growth rate fi which appears to the power —f in the Smith-Ewart expression for the number of latex particles formed coitains the propa gation rate constant which is temperature dependent. It has also recently been realized that another factor on which JV depends, the area occupied by a surfactant molecule at the polymer-water interface Og, is also temperature dependent- Dunn et al. (1981) observed that the temperature dependence of N in the thermal polymerization of styrene differed from different emulsifiers. It seems unlikely that the differences ran be wholly explained by differing enthalpies of adsorption of the emulsifiers and, if not, this observation implies that the energy of activation for thermal initiation of styrene in emulsion depends on the emulsifier used. Participation of emulsifiers in thermal initiation (and probsbly also in initiation by oil-soluble initiators) is most probably attributable to transfer to emulsifier and desorption of the emulsifier radical frcan the micelle x>r latex particle into the aqueous phase the rates of these processes are likely to differ with the emulsifier. 

To be effective, a plasticizer must partition from the solvent phase into the polymer phase and subsequently diffuse throughout the polymer to disrupt the inter-molecular interactions.The rate and extent of this partitioning for an aqueous dispersion have been found to be dependent on the solubility of the plasticizer in water and its affinity for the polymer phase. The partitioning of water-soluble plasticizers in an aqueous dispersion occurs rapidly, whereas significantly longer equilibration times are required for water-insoluble plasticizing agents. For aqueous-based dispersed systems, water-insoluble plasticizers should be emulsified first and then added to the polymer. Sufficient time must be allowed for plasticizer uptake into the... 

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