Charge patch mechanism

The measurement and evaluation of flocculation state is typically carried out over 10 min by means of programmed dosing, in which the flocculation value F is recorded during a stepwise increase in flocculant dosing. Figure 4 shows an example of a flocculation curve. The amount of flocculant required for optimal flocculation is that where F reaches an asymptotic limit (bridging mechanism) or a maximum (patch charge mechanism). 

Figure 7.5 Typical flocculation, mobility and adsorption data for flocculation by the patch charge neutralisation mechanism.

Figure 7.14. Sequence showing schematically (a) charge-stabilized particles, (b) particles flocculated by the patch flocculation mechanism, and (c) particles restabilized by steric repulsive (chain overlap) interactions at higher levels of polymer surface coverage...

For filter belt presses and centrifuges, resistance to shear and mechanical pressure is the most important parameter. In general, floes produced by charge patch neutralization are stronger than those produced by inorganic salts alone. 

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. 

Simple Charge Neutralization and Charge Patch Neutralization Oppositely charged polyelectrolytes reduce the particle surface charge density such that particles may approach each other sufficiently closely so that the attractive van der Waals force becomes effective. Flocculation caused by this mechanism should not be sensitive to the molecular weight of the polymer. 

Charge neutralization (simple and/or patch) is often the mechanism for flocculation by low molecular-weight polyelectrolytes. 

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