Many powders used in practice as pigments, fillers, etc. contain primary particles that have dimensions less than 1 micrometer and hence high specific surface area. AS particle size decreases the cohesiveness of the powder (the strength of the forces t&t cause agglomeration) increases, and this affects the operation of various processes such as the dispersion of powder in liquids, granulation. fluidization and handling of powders.
Powders normally contain some aggregates of primary particles which are attached to other aggregates and/or primary particles to form agglomerates. Aggregates are groups of primary particles joined such that the total surface area is reduced over that of the individual units. Both manufacturing procedure and storage conditions are responsible for the particular state a powder achieves.
To aid in handling, powders are often further agglomerated which is generally referred to as size enlargement. This process encompasses many different operations and can be subdivided into the following three classes which may overlap [ 11: (a) those where particles are brought into close contact, usually in the presence of a liquid, so that short range interparticle forces take effect. Examples are pelletizing of fertilizers and clustering of colloidal particles in a loose fashion so that they may be subsequently readily disperse& fb) those that involve mechanical compaction, such as tabletting of pharmaceuticals and briquetting of soft metals, and (c) those that depend on heating the powder, as in the sintering of ores and ceramics.
We are interested in the dispersion of powders containing submicron particles in liquid media. This process involves a number of stages: (a) incorporation of the powder in the liquid. (b) wetting the surface of the primary particles, (c) breakdown of agglomerates and aggregates into primary particles, and (d) stabilization of the dispersed primary particles. In practice these stages overlap. The efficiency of the first three collectively is termed the dispersibility of a powder/liquid system. With an increase in powder cohesiveness comes a lowering of the efficiency of this process.
In the dispersion of powders in liquids two types of wetting occur: in the incorporation stage only the external surface of the agglomerates needs to be wetted, but for disagglomeration the liquid must penetrate the internal structure of the agglomerates. Theoretical analysis of these wetting phenomena suggest that the requirements in terms of surface tension and contact angle differ for each stage This work aims at defining the critical values of these parameters for carbon black in aqueous media.