A solid material dispersed in a liquid requires an additive to make the dispersion process easier and more stable – this is the role of the dispersing agent, or dispersant.
Dispersants deflocculate solids and thus significantly reduce the viscosity of a dispersion or paste. As a result of this effect, solid loading – the amount of dispersible powdered material – can be increased accordingly. The dispersion phase is possibly the most time- and energy-consuming stage of the production process. This is due to the different surface tensions of the liquids (e.g., resin, solvents) and the solids (e.g., fillers, additives). A dispersing additive is necessary to produce stable formulations and ensure storage stability (no viscosity instability, no separation). BASF offers three groups of dispersing agents: high-molecular-weight (Efka® 4000 Series), low-molecular-weight (Efka® 5000 and Efka® 6000 Series) and polyacrylate polymer dispersants (Dispex®, Pigmentdisperser and Ultradispers® range).
Commercially available pigments and extenders are supplied to paint factories as agglomerates. The grinding process incorporates these solid materials into the liquid vehicle or grinding resin. During this process, the agglomerates are transformed into a dispersion of particles, which can be as small as the primary particle size.
The process consists of three stages:
The Efka® 1000 range offers grinding resins for solvent-based as well as water-based coatings.
Ionic dispersing agents act by coating the surfaces of inorganic mineral particles as the result of an attraction to the positive charges that develop on particles when in contact with water, a process known as ‘adsorption’. Adsorption imparts a net negative charge to the particle surface so that the particles are repelled from each other.
When a mineral is dispersed in water using Dispex products, this negative attraction reduces the interaction between particles, thus lowering the viscosity of the mineral slurry or paint mill-base. The slurry then remains fluid until a fairly high quantity of the mineral has been added (70 - 80% mineral). The Dispex addition gives the slurry better flow properties and it remains stable for longer.
By making the polymer chain more hydrophobic, interaction with minerals such as TiO2 and ZnO is improved, as is the water-resistance and gloss of a formulated coating.
Dispex dispersing agents enable bulk minerals to be supplied in a stable slurry form that continues to remain fluid over a period of time. These minerals are suitable for a wide variety of applications. China clay (or kaolin), for example, is commonly used in coating formulations and as a filler in the paper industry, and also in slip casting of ceramics.
Another mineral often supplied in slurry form is calcium carbonate, which must normally be ground to a particular particle size in a process greatly assisted by the use of a dispersing agent. Calcium carbonate is widely used in the paint industry and also for paper production. Numerous other inorganic (and organic) minerals and pigments are also dispersed in water-based slurries for a variety of applications.
The amount of dispersing agent required depends on the actual surface area of the mineral or combination of minerals to be dispersed. The smaller the particle size, the higher the actual surface area, hence the larger the area the dispersant has to cover. A dispersion curve can be plotted by simply measuring the reduction in viscosity as the dispersing agent is added incrementally to a mineral slurry, the minimum viscosity indicating the optimum dosage level.
For a paint mill-base, the addition of 40% more dispersant is recommended, once the optimum dose has been determined. This will ensure a safety margin should the particle size distribution of the raw materials vary more than expected.
Both types of dispersant improve the wetting process and result in shorter grinding times. The conventional low-molecular-weight types are categorized according to their chemical structure as anionic, cationic, electroneutral and nonionic.
Their effectiveness is determined by the adsorption of the polar group onto the pigment surface and the behavior of the non-polar chain in the medium surrounding the particle.
The molecular weight of these products is low – usually between 1,000 and 2,000 g/mol. Molecules with only one polar group attach themselves to the pigment surface while their non-polar chains extend into the resin. If the molecules have more than one polar group, they arrange themselves in such a way that the free polar groups form hydrogen bonds and build a physical structure with the pigments. Shear forces are often responsible for breaking down this structure. This principle is called controlled flocculation. It is mainly used for heavily filled systems such as primers and surfacers for industrial and maintenance coatings.
The polar groups, which contain positively and negatively charged ions such as metallic oxides, have the strongest affinity for inorganic pigment surfaces. Unfortunately this type of attachment is not effective with organic pigments, which consist of uncharged, covalently bonded carbon, hydrogen, oxygen and nitrogen atoms.
This led to the development of high-molecular-weight dispersing agents. These additives are linear or branched molecules with a polyurethane or polyacrylate structure and molecular weights between 5,000 and 30,000 g/mol. They have pendant anchoring groups, which adsorb onto the surface of the organic pigment particle. Adsorption is achieved through:
It is strong, because the dispersant is bound to numerous sites on the surface at the same time. This makes them effective for inorganic pigments, too.
The remainder of the polymeric backbone is sufficiently large to create an effect called steric stabilization. Permanent deflocculation is achieved with a minimum distance of 200 Angstroms inter-particle separation.
BASF's pigment wetting and stabilization technology is also highly effective in aqueous systems. Conventional dispersants that build up a charged double layer around the pigment are generally used to stabilize pigments in water. This electrostatic repulsion prevents the pigments from re-agglomerating. However, this only works well with single pigments. The stabilizing charge can easily collapse when subjected to external influences such as impurities and other ions, or if other pigments with different zeta potentials are added. BASF's high-molecular-weight dispersants ensure steric stabilization, which is far stronger and much more effective. The amount of polymeric dispersant used is very important, since performance depends on optimal saturation of the pigment surface by the dispersant. BASF's data sheets and pigment lists provide guidance on the appropriate amounts.
BASF supplies two different chemical families of high-molecular-weight dispersants: