Semi-Batch Emulsion Copolymerization of Vinyl Acetate and Butyl ...

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Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2619 Figure 6. The effect of initiator addition policy on (a) the instantaneous monomer conversion and (b) the average particle size (surfactant S1 concentration 2 wt.-%). Figure 5. The effect of surfactant concentration on the particle size distribution. Surfactant S1 concentration: (a) 0.06, (b) 0.9, and (c) 1.8 wt.-% to monomers (initiator addition policy: 0.25– 0.25 wt.-%). initial amount of initiator or the amount of initiator continuously fed to the reactor was varied. Figure 6a and 6b depict the time evolution of the instantaneous monomer conversion and average particle size for various initiator addition policies. Notice that the rate of polymerization and the final monomer conversion increase as the total amount of initiator added to the reactor (e.g., initially charged and continuously fed) increases up to the total recipe value of 0.5 wt.-%. An increase in the initiator amount initially charged or/and in the initiator amount continuously fed to the reactor above the respective recipe values (e.g., of 0.25 wt.-% initially charged and 0.25 wt.-% continuously fed) led to severe destabilization and reduction of the final monomer conversion. It was observed that when the initiator amount initially charged was increased above its recipe value of 0.25 wt.-%, destabilization occurred early in the polymerization (Figure 6b). On the other hand, when the initiator amount continuously fed was increased above its recipe value, destabilization occurred later in the reaction. In all other cases, the effect of charged and fed initiator amounts could not be distinguished as evidenced by the experimental results obtained with 0.12 wt.-% initially charged and 0.13 wt.-% continuously fed initiator amounts (see Figure 6a and 6b). The extent of particle destabilization caused by an increase in the initiator concentration was less significant in the presence of surfactant S2 (Figure 7) and insignificant for surfactant S3 (Figure 8). The time evolution of the PSD at constant surfactant concentration and different initiator addition policies is depicted in Figure 9. The bimodal to unimodal evolution
2620 N. Lazaridis, A. H. Alexopoulos, C. Kiparissides Figure 7. The effect of the initial initiator concentration on the average particle size (surfactant S2: 1 wt.-%). Figure 8. The effect of the initial initiator concentration on the average particle size (surfactant S3: 1 wt.-%). of PSD which was observed with the nominal initiator addition policy (Figure 9a), was also observed for the case of an increased initiator addition policy (e.g., 0.45 wt.-% initially charged and 0.25 wt.-% continuously fed) (Figure 9c) but led to a narrower PSD located at larger particle sizes. However, when the amount of initiator continuously fed increased from 0.25 wt.-% to 0.53 wt.-%, the PSD remained bimodal and evolved to a multi-modal PSD located at larger particle sizes (Figure 9b). Other investigators [4] have reported a similar reduction in the average particle size as a result of a decrease in the initiator concentration in the presence of anionic or/and cationic surfactants. This behavior was attributed to the decrease of the ionic strength of the reaction medium with the concomitant improvement of the stability of the primary particles. It should be noted that even with nonionic surfactants the ionic strength could influence the steric stabilization of the particles by changing either the thickness of the adsorbed surfactant layer or the surfactant surface coverage. In order to elucidate whether or not the ionic strength of the reaction medium was the main mechanism for the Figure 9. The effect of initiator addition policy on the particle size distribution (surfactant S1, 1 wt.-%; initiator addition policy (a) 0.25–0.25 wt.-%, (b) 0.25–0.53 wt.-%, (c) 0.45–0.25 wt.-%). observed destabilization of latex particles at high initiator concentrations, three experiments were carried out by varying the initiator and electrolyte concentrations but keeping the total ionic strength of the reaction medium constant. The ionic strength of the medium, I, was calculated based on the concentrations of the ionic species of initiator (1:2) (C I : Na 2 S 2 O 8 ), and those of electrolyte (1:1) (C E : NaHCO 3 ). Accordingly, the total ionic strength of the reaction medium will be equal to: I = 3 N C I + C E . It
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