A background to flocculation and the use of Lansperse dispersing agents to help prevent flocculation in aqueous coating applications.
Definition of Flocculation
Flocculation: the grouping together of colloidal particles to form a precipitate which may float in the liquid.
In terms of organic pigment dispersions once a stabilised pigment dispersion is added to a coating that has a high level of filler and a low level of resin then the dispersant strips off the organic pigment and attaches to the inorganic filler causing instant flocculation. The larger the surface area the pigment has (carbon black and Violet 23) the more likely these will flocculate as they are more difficult to stabilise.
Some typical dispersing agents and surfactants used for wetting and stabilisation include the following:
The dynamics of flocculation v stabilisation
Flocculation can occur when dispersants with opposing electrical charges are present in the same dispersion/coating .i.e. Anionic – Cationic +/- this can be used as an advantage when colouring paper pulp weak anionic pigment dispersions are flocculated on to the paper pulp using cationic flocculants.
Formulating and stabilisation
The Lansperse range of products for aqueous systems acts as both a wetting and dispersing agent for pigment particles, particularly in decorative emulsion paints and water-based inks.
These types of dispersing agents help stabilise the aqueous dispersions of organic pigment particles by the steric stabilization mechanism. Most of the time, they are used in combination with an anionic type dispersing agents such as Lansperse DS80, which provides stabilisation of the dispersion by the electrostatic stabilisation mechanism.
Flocculation is the recombination of dispersed pigment particles that were not properly stabilised in the pigment dispersion. Flocculation is undesirable at it detracts from hiding and colour development. Flocculation is reversible by applying a low degree of shear. In the picture below, the phthalocyanine (EU) blue pigment is flocculated. Upon rubbing with a finger, the deeper blue colour returns.
Stabilised pigment dispersion — Flocculated pigment dispersion