Flocculation temperature dispersions

As another criterion of stability, a critical flocculation temperature(OFT) was measured. The measurement of CFT was carried out as follows the bare latex suspension was mixed with the polymer solution of various concentrations at 1+8 °C by the same procedure as in the adsorption experiments. Then, the mixture in a Pyrex tube(8 ml, U.0 wt %) was warmed slowly in a water bath and the critical temperature at which the dispersion becomes suddenly cloudy was measured with the naked eye. 

Figure 3. Critical flocculation temperature (T) versus log (particle volume fraction ) for the two Si02 g PDMS dispersions in bromocyclohexane O, S15/PDMS5 x, S15/ PDHS3.

Figure 5 shows the variation of Xg with temperature at two C values (0.20 and 0.25 mol dm 3). In both cases Xg is essentially zero until a critical temperature is reached, above which xg increases rapidly with increasing temperature reaching a maximum above which there is a tendency forXg to fall again with further increase in temperature. The critical temperature corresponding to the abrupt increase inTg is 20 and 25°C for C equal to 0.25 and 0.20 mol dm, respectively This temperature may be identified with the critical flocculation temperature (CFT) of the concentrated dispersion. 

FIG. 13.14 Critical flocculation temperature of aqueous poly(vinyl acetate) dispersion stabilized by poly(oxyethylene) indicated by a sharp change in absorbance with temperature. (Redrawn with permission from D. H. Napper, in Colloid and Interface Science (M. Kerker, R. L. Rowell, and A. C. Zettlemoyer, Eds.), Academic Press, New York, 1977.)... 

Table 8.4 Classification of sterically stabilised dispersions and comparison of critical flocculation temperatures (c.f.t) with theta-temperatures112 (By courtesy of Academic Press Inc.)...

TABLE 10.6 Comparison of Critical Flocculation Temperatures (CFT) with 0 Temperatures for Some Aqueous and Nonaqueous Dispersions"... 

Filler particles can be modified to decrease flocculation. Kaolin particles modified by a graft of poly(ethylene oxide) showed an increase in the upper critical flocculation temperature. Stabilization of particle dispersion was due to an enhanced steric stabilization." ... 

Reversible flocculation of aqueous dispersions stabilized sterically with POE nonionics can be accomplished by increasing the temperature. With increase in temperature, the hydrogen bonding of the POE chains to water is reduced and the chains tend to aggregate, flocculating the dispersion. Upon reducing the temperature, the chains hydrate again and the particles redisperse. 

A decrease in temperature may also lead to flocculation at a lower flocculation temperature (l.f.t.), partly because the Brownian motion can no longer maintain the dispersed state and partly... 

Experimental observation of selective flocculation. Croucher and Hair (1980a) have demonstrated the phenomenon of selective flocculation in mixtures of heterosterically stabilized dispersions. They prepared particles of poly(vinyl acetate) stabilized by polystyrene and polyacrylonitrile particles stabilized by polyisobutylene, both in cyclopentane. The value of X23 for polystyrene and polyisobutylene is known to be positive (Hyde and Tanner, 1968), corresponding to mutually incompatible polymers. The poly(vinyl acetate) particles stabilized by polystyrene exhibited both upper and lower critical flocculation temperatures whereas the polyisobutylene stabilized particles only flocculated on heating. 

When a mixture of the two different types of particles dispersed in cyclopentane was cooled to the lower critical flocculation temperature (280 K) of the poly(vinyl acetate) particles stabilized by polystyrene, those... 

The coagulants (typically multivalent counterions) commonly used are the salts of aluminium and iron and salts or the bases of calcium and magnesium. The transition from stabilized to destabilized emulsions on changing the temperature is very sharp at the critical flocculation temperature (CFT) when nonionic surfactants are used. Generally, aqueous dispersions destabilize upon increasing temperature, while non-aqueous dispersions destabilize with decreasing temperature. 

The principal consideration in selecting an appropriate soluble polymeric component for the steric barrier is that it should be freely soluble in the dispersion medium to be usedo This had been recognised empirically for some years before Napper (4) undertook quantitative studies which showed that sterically stabilised latexes flocculated at or around the theta point for the soliible polymer in the dispersion medium. Later studies have shown that both upper and lower critical flocculation temperatures may occur in organic solventso... 

The consistency or the viscosity q of most paint formulations decreases with increasing temperature. However, in some cases the viscosity may show an increase with increasing temperature when the latter reaches a critical value. In most cases this is due to sudden flocculation of the paint above a critical temperature (referred to as the critical flocculation temperature, CFT). This flocculation may result Irom a decrease in solvency of the chains to worse than 0-solvent above a critical temperature. Alternatively, the flocculation may occur as a result of desorption of the dispersant at high temperature due to the sudden incretise in its solubility. Due to the above changes in... 

Clearly, current aggregation models can reliably predict simple effects of salt and some effects of polymers, but additional work is warranted to develop the capability to predict aggregation and dispersion of particulates of all types (solid, gas and liquid) under real-life conditions of varying concentrations of flocculants or dispersants, hydrodynamic perturbation and local pressure and temperature variation in addition to the effects of any magnetic or sonic fields. 

In other words, the lower the mass of the particle, the higher its velocity, because the average energy of any particle at a given temperature is constant, kT. A dispersed particle is always in random thermal motion (Brownian motion) due to coUisions with other particles and with the walls of the container (4). If the particles coUide with enough energy and are not well dispersed, they will coagulate or flocculate. 

Latex particles may flocculate when injected in a reservoir with high formation temperatures. When the particles flocculate, shrink, and harden, they form a more effective blocking agent than the dispersed, expanded, and softer particles [1654]. 

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