Polymer-bridging flocculation, mechanism

Fig. 3.2 Schematic representation for the mechanism of polymer-bridging flocculation. (From ref. [8])...

Weakly flocculated systems have been studied by Buscall et al. (165), Heath and Tadros (166), Goodwin et al. (167), Patel and Russel (168, 169), Otsubo (170-172), Buscall et al. (51,173), Woutersen and de Kruif (174), and Nakai et al. (175). The weak flocculation can be obtained by several means, including secondary-minimum flocculation, depletion flocculation, polymer-bridging flocculation, and incipient flocculation. Details of the various mechanisms of interparticle attraction can be found in Russel et al. (27) and Somasundaran and Yu (176). Normally, flocculated systems have a solid volume fraction of no less than 0.2. When 0 < 0.2, an attractive system will settle down quickly. When 0 > 0.2, a flocculated suspension can be maintained easily for a period of time for a steady shear measurement to be completed. 

Swenson, J., M.V. Smalley, and H.L.M. Hatharasinghe. 1998. Mechanism and strength of polymer bridging flocculation. Physical Review Letters 81 (26) 5840-5843. 

This type of mechanism is likely to be partly operative in systems containing inorganic electrolytes as, for example, in the case of aluminium species. Some polyelectrolytes may also induce flocculation by charge neutralisation but the adsorbed polymer may also be able to bridge from one particle surface to another ( polymer bridging ). 

A polymer molecule may also simultaneously adsorb onto both surface, hence causing flocculation by polymer-bridging mechanism [8], Figure 3.2 shown this behavior. 

Stock composition, kinetics of adsorption and hydrodynamic shear dictate the point at which a cationic polymer is added to a papermaking furnish in order to induce flocculation. Flocculation of cellulose fibers in turbulent flow proceeds very rapidly and is completed in less than two seconds.120-123 Flocks form due to charge interactions through a patch-type or a bridging-type mechanism. However, these flocks will be sensitive to shear force and deflocculation and reflocculation might occur. 

Because of these considerations it is often necessary to deliberately reflocculate the particles but now in a controlled fashion. This can be brought about by the addition of a low level of an adsorbed bridging polymer (see above) or else the use of a fully dissolved nonattached polymer that brings about depletion flocculation. This latter phenomenon occurs due to the exclusion of dissolved polymer from between the particles, resulting in a net attraction between particles. The use of both mechanisms of polymer induced flocculation have been employed by Tadros et al. to promote resuspendability of dispersed particles by forming a loosely flocculated structure. 

Figure 7.13. Sequence showing schematically (a) charge-stabilized particles, (b) particles flocculated by the polymer bridging mechanism, and (c) particles restabilized by steric repulsive (chain overlap) interactions at higher levels of polymer surface...

Colloidal aggregates may be formed by different mechanisms. For instance, particles surrounded by an electrical double layer coagulate either in the primary or secondary minimum by the addition of salt. Polymer-coated particles may flocculate by polymer bridging or by changing the composition of the medium such that the polymer coats become insoluble. 

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