Emulsifier coverage

For a water-soluble monomer, the above reasoning does not apply. Nevertheless the number of particles is constant after 20% conversion of vinyl acetate (25). This suggests that oligomers and/or polymers of this monomer are easily adsorbed by particles at less than full emulsifier coverage. 

Table II shows that 45-100% of the sulfur incorporated into the polymer is present on the particle surface. In terms of the surface charge, these amounts cannot be neglected as a factor in latex stability, both during and after polymerization. (This accounts for the fact that polymerizations with hydrogen peroxide initiator and low emulsifier concentrations frequently coagulate at low conversions). For latexes B-1, B-2, and D-4, the charge density due to sulfate endgroups is, respectively, at least 4, 8, and 1.6 times the charge due to adsorbed emulsifier. For the other latexes, it amounts to 20-100% of the emulsifier adsorbed. Thus, this permanent charge is an important factor that has been overlooked in earlier studies of latexes at low emulsifier coverage (16, 17).

Poljmierisation to high solids content has been described in the patent literature (12). However for emulsion polymers a serious problem is one of mechanical stability during polymerisation. Palmgren (22) has studied the mechanical stability of monodisperse PVC latices as a function of particle size, emulsifier concentration, electrolyte concentration and residual monomer content. The mechanical stability of PVC latices of different particle size containing different surface concentrations of SDS emulsifier is shown in Fig 4. At a given emulsifier coverage on the latex, these data indicate that the latex stability decreases with increasing particle size. Latex... 

Fig. 3 -10 Soap titration. Determination of the emulsifier coverage of the polymer particles.

The final increase in particle size, shown in figure 7, is pro bably caused by limited flocculation,since particle coverage by emulsifier is very limited. Conductimetric titration of emulsifier shows that only a part of it is used for stabilizing particles Typi cal results are shown in figure 9. 

Curve 1 in figure 9 shows that SDS amount used for particle coverage is less than half the added emulsifier amount (4 g). Curve 2 shows that the actual particle coverage decreases as new particles are created. Data for curve 2 were calculated from Gardon (9)... 

Figure 9. Amount of emulsifier adsorbed (1) on the particles and actual coverage (2) vs. conversion (run P>)...

Figure 15. Coverage (1) and amount of adsorbed emulsifier on the particles (2)...

Constant composition copolymer may be produced in this process, but particle size changes during the process. Particle number increases continuously, again following polymer yield. Although emulsifier tends to be more completely used for covering particle, coverage itself remains weak. 

We have demonstxated that the coverage of particles by the anion-active emulsifier (sodium alkyl sul-phonate) in the polymethylacrylate latex does not exceed 30%, and reaches 4 in the polybutylacrylate latex (10). 

In Figure 3 the same stability data are plotted versus the surface concentration of emulsifier. The surface concentration is given as per cent of total coverage of the particle surface, as determined by adsorption titration. 

The area occupied by each adsorbed emulsifier molecule at the polymer-water interface (A ) was estimated from the mean particle size of the latices, determined by the analytical centrifuge, and the amount of emulsifier adsorbed at the interface corresponding to full coverage, determined by adsorption titration. 

Coagulation of polymer particles, as of any colloidal dispersion, depends on a number of factors, among them the stabilizing action of the emulsifier, the compatibility of soap and polymer, and the consistency of the polymer-monomer interior of the particles. When vinyl chloride is polymerized above the softening point of the swollen polymer, particles coalesce to such an extent that the soap coverage remains about 100% (33)—i.e., the number of particles decreases with conversion. Extensive coalescence after the period of particle formation occurs in the polyn erization of vinylidene chloride investigated by Sweeting and coworkers (21,32, 39). 

The reaction order 0.56 obtained from Rp°c [SDSJ0 56 indicates the emulsifier-flooded condition (the monomer droplet surface not saturated with SDS). However, the increased coverage of the droplet surface by emulsifier is accompanied by the enhanced homogeneous nucleation, i.e., Nw increases significantly with increasing [SDSJ. Monomer droplet nucleation predominates in the particle formation process for the run with the lowest [SDSJ (4 mM). By contrast, mixed modes of particle nucleation are operative in the polymerizations with... 

Different emulsifier compositions might require different homogenization parameters. For example, the maximal degree of dispersing was obtained with 500 bar and 3 cycles for poloxamer 188-stabilized systems [38], Homogenization with pressures of 1000 or 1500 bar did not result in further reduction of the particle size. In contrast, pressures of 1500 bar proved to be the best for lecithin- (Lipoid S75) stabilized systems. A possible explanation for this observation is the different velocity of the coverage of the new lipid surfaces. 

Adams, et al. [89] could find no evidence of differing efficiency of entry between charged and uncharged radicals or of any effect of the extent of surface coverage with emulsifier or of ionic strength collisional or diffusive entry models predicted entry rate coefficients which were too large. Subsequently Maxwell, et al. [60] concluded that the rate-determining process was the time required for an initiator radical to add two styrene residues in the aqueous phase and thereby acquire sufficient hydrophobic character to adsorb onto a latex particle. 

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