Dispersants
Dispersants (also known as dispersants ) are additives that enable or stabilize the dispersion , i.e. the optimal mixing of at least two, actually immiscible phases (substances). This includes the formation of emulsions and especially suspensions , both of which are part of the so-called dispersions . While emulsions are a fine mixture of two liquids, such as oil and water , a suspension is a sufficiently stable suspension of very small solids in a liquid. In coating materials, these can be pigments or fillers that are incorporated into a liquid ( dispersing medium , also millbase ).
Preparation of a dispersion
When producing a dispersion, it is important to wet the agglomerates and aggregates of the solids with liquid ( binder , solvent or water), to distribute them as possible into individual particles and to stabilize them. Wetting agents and dispersants help . They reduce the interfacial tension and thus promote wetting . Dispersants are suitable surfactants which have pigment-affine groups that attach to the pigments and protect them against agglomeration . This leads to a lower tendency for the pigments or paint to sediment . There is electrical and steric stabilization.
Types of stabilization
There are two possible mechanisms of stabilization and a combination of both.
With steric stabilization , the “pigment-loving” (pigment-affine) heads of the surfactant sit on the pigment, while the long chains protrude into the dispersing medium. If two pigment particles meet, their surfaces cannot adhere to one another because they are kept at a distance by the surfactants ( entropic repulsion ).
With electrostatic stabilization , the dispersing additive carries an electrical charge . For example, the pigment-affine group can be uncharged and the other part negative. In the case of a monomolecular shell around the pigment grain, the negatively charged parts of the surfactant protrude into a polar solvent such as. B. alcohols or water and form a protective cover. These surfactants can have a different structural structure .
The electrosteric stabilization combines both mechanisms. The electrical charges that are responsible for the repulsion are localized at the end of the chains protruding into the dispersing medium.
Selection parameters
The choice of the dispersing additive depends on both the pigments used and the dispersing medium. Steric stabilization works best in non-polar solvents, and electrical stabilization in polar solvents.
literature
- Artur Goldschmidt, Hans-Joachim Streitberger: BASF manual paint technology . Vincentz, Hannover 2002, ISBN 3-87870-324-4 .