Polystyrene latex flocculation

Figure 8. Relationship between critical flocculation temperature(CFT) and concentration of HPC for polystyrene latex.

Studies on orthokinetic flocculation (shear flow dominating over Brownian motion) show a more ambiguous picture. Both rate increases (9,10) and decreases (11,12) compared with orthokinetic coagulation have been observed. Gregory (12) treated polymer adsorption as a collision process and used Smoluchowski theory to predict that the adsorption step may become rate limiting in orthokinetic flocculation. Qualitative evidence to this effect was found for flocculation of polystyrene latex, particle diameter 1.68 pm, in laminar tube flow. Furthermore, pretreatment of half of the latex with polymer resulted in collision efficiencies that were more than twice as high as for coagulation. 

Kitahara and Ushiyamaf flocculated a polystyrene latex of radius 665 A with KC1. The stability ratio Wwas found to vary with the KC1 concentration as follows ... 

Recent studies in our laboratory (3) using polystyrene latex dispersions stabilised by the above "comb" dispersing agent have shown that polymers such as poly(ethylene oxide) induce wealc flocculation above a certain critical concentration of free polymer which was dependent on the molecular weight of the chain. 

Interpretation of rheological results The trends in the variation of Xg with are similar to those obtained recently (3) using a model polystyrene latex dispersion. The 1 values obtained in the present system are also close to tSose obtained with the model dispersion(0.017, 0.008 and 0.005 for PEO 20,000, 35,000 and 90,000 respectively). As mentioned before the sharp increase in x above + indicates that at the onset of flocculation the dispersions show marked viscoelasticity. The flocculation obtained at corresponds to the onset of the "semidilute" region, p, i.e., where the polymer coils in solution begin to arremge themselves in some... 

If a polystyrene latex that is stabilized solely by an electrostatic mechanism is coagulated by the addition of electrolyte, that coagulation is usually irreversible to subsequent dilution. In contrast, sterically stabilized dispersions can usually be flocculated by the addition to the dispersion medium of a nonsolvent for the stabilizing moieties mere dilution of the concentration of the nonsolvent to a suitably low value is often sufficient to induce the particles to redisperse spontaneously. 

Fig. 16.9. The dependence of the critical concentration of free hydroxyethyl cellulose upon its molecular weight for the depletion flocculation of polystyrene latex particles curves 1, in the absence of anionic surfactant 2, in the presence of nonionic surfactant (after Sperry et al., 1981).

Finally, we note that Sperry (1982) has recently used the theory of Asakura and Oosawa (1954 1958) to explain the results obtained by Sperry et al. (1981) for the flocculation of polystyrene latex particles by hydroxyethyl cellulose. Although semi-quantitative agreement was claimed between theory and experiment, it should be noted that the theory contains too many adjustable parameters to allow a satisfactory comparison to be made with experiment. 

In summary the depth of the secondary minimum increases with electrolyte concentration for particles of the same size and also increases with particle size. The possibility of reversible flocculation has been examined using a series of polystyrene latexes containing particles of increasing diameter namely, 210 nm, 340 nm, 781 nm and 1.01 pm [46]. The ccc was determined optically for each sample by the addition of sodium chloride. The sample was then dialysed against distilled water and, after 20 h, it was examined optically. The results are shown in Figure 3.21. In the case of the smallest particles the turbidity ratio (turbidity measured at two wavelengths) was either the same or lower than that obtained immediately before dialysis. This indicated that for this system the... 

Rheological studies of sterically stabilised concentrated polystyrene latex dispersions under conditions of incipient flocculation... 

Key words Rheology, concentrated dispersions, sterically stabilised dispersions, polystyrene latex, incipient flocculation... 

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