Upper flocculation temperature

Figure 2 The upper critical flocculation temperature is shown plotted against the applied pressure. refers to the PMMA latex while, o refers to the PVAc latex. The solid line represents 9 conditions for a solution of PIB + 2-methylbutane while the dashed line represents the theoretically calculated UCFT as a function of pressure.

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." ... 

Alternatively, the 0-condition may be reached by increasing the temperature. Here the interactions between the medium and the segments of the polymer decrease because of the decreasing density of the medium, and segment-segment interactions dominate when an upper flocculation temperature (u.f.t.) is reached. This is analogous to the lower consolutc behaviour of those polymer solutions which undergo a phase separation as the temperature is raised. 

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. 

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 critical factor in the development of the syndrome is the size of the ultrafine zinc oxide particles produced when zinc is heated to temperatures approaching its boiling point in an oxidizing atmosphere." The particles must be small enough (zinc oxide powder is either inhaled or taken orally. Only freshly formed fume causes the illness, presumably because flocculation occurs in the air with formation of larger particles that are deposited in the upper respiratory tract and do not penetrate deeply into the lungs. ... 

The first strategy, studies of incipient instability, provides insights into the thermodynamic factors that control stability. Further, they show how optimum stability can be imparted and allow the bounds of stabilization (e.g. the upper and lower limits to temperature stability) to be determined. Necessarily, however, incipient instability studies examine dispersions in what might be described as their death throes. This could well change the very properties that we wish to study in order to understand how stability is imparted. It is therefore important to complement incipient instability studies with measurements on robustly stable dispersions. In this and the two subsequent chapters, the phenomenology of incipient flocculation will be discussed. The presentation of the properties of robustly stable dispersions will be deferred until Chapter 13. 

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