Components of Coating Colors
With respect to mass the most important component is the pigment with a total amount of almost 18 V 106 t per year dry material worldwide. There may be only one kind of pigment in a coating color, or more commonly a combination of several types e. g. clay, calcium carbonate, talc, titanium dioxide (more details see 2.2.2). The share of pigment in the dry coating is about 85–95 wt. %.
In the dry form, pigment particles form clusters, in which the parts of the clusters are more or less tightly attached to each other. More tightly attached ones are called “aggregates”. Aggregates and primary particles together can also form clusters that are less tightly bound – so-called “agglomerates” or “flocs”. The purpose of dis¬persing is to make a dispersion where neither agglomerates nor aggregates exist and only primary particles are present. Primary particles are evenly distributed in water, and the system stays stable for a certain time.
Disruption of aggregates, disaggregation, is typically an irreversible process, af¬ter which particles cannot be bound as tightly as they used to be. Instead of form¬ing aggregates, disaggregated particles tend to form agglomerates. Disruption of agglomerates is called “deflocculation”. Deflocculation is a reversible process. Ag¬glomerates re-form when the force for deflocculation is removed.
The process of dispersion can be divided into three stages: wetting, disruption of particle clusters and stabilization. Wetting means that all the external surfaces of pigment particles must come into contact with water. Air must also be displaced from the internal surfaces between pigment particles in agglomerates and ag¬gregates. Wetting is usually not a problem with paper pigments except for talc which is not spontaneously wet by water.
When talc is dispersed, a separate surfac¬tant is required to provide proper wetting. Disruption of particle clusters, disaggrega¬tion, is accomplished by mechanical energy. Disruption can be performed using crushing mills, kneading mixers, or high-speed mixers. Crushing mills are needed when the shear forces induced by mixers are not sufficient for disaggregation. Although dispersing can be performed in mixers, there is a certain difference between mixing and dispersing. Mixing does not change the size and surface area of particles, while dispersing changes both. Stable pigment dispersions with the highest solid content can be achieved only by using dispersants.
When pigment clusters are broken down by dispersing, the surface area of pigment in the disper¬sion increases, there is more surface for particles to interact with each other, and the viscosity in the dispersion increases rapidly. Use of dispersants stabilizes de-flocculated particles in the dispersion and hinders their interaction. The dispersant must be mixed with water at once, when the breakdown of pigment clusters starts. Only then can interaction and therefore reagglomeration of particles be avoided. For this reason, dispersant is added to water at the beginning of dispersing, even before the pigment is added.