Carbon Black Dispersion

Carbon black is a particulate form of industrial carbon. The particles are aggregates of even finer material. Carbon black is produced by thermal cracking of a hydrocarbon raw material such as natural gas or fuel oil. Special carbon blacks are found useful for color, conductivity and UV protection applications. They are used in coatings - pigmenting latex paints, water-borne printing inks, textile printing pastes, and paper. Properties such as blackness, surface area, particle size, structure, oil absorption, tinting strength and electrical resistance affect performance.

Carbon black also possesses a couple of unattractive traits. It is fluffy, owing to a very low density, and it is very dusty. Therefore, large volume containers are necessary to transport relatively small weights. As a result, dry conveyance, metering and weighing is a very dusty, dirty undertaking.

An economical, convenient means of transporting, measuring, and introducing carbon blacks without the associated mess is as aqueous carbon black dispersions. Dispersions are prepared by incorporating the black into a dispersing medium, water in the case of aqueous dispersions.

Preparing dispersed carbon black proceeds through three phases. First, mixing displaces the air between the aggregates with water and wets the black's surface. Next mechanical shear is applied, usually by ball milling, to break down the carbon black aggregates to the primary black structure. By reducing the particles to their primary structure, the full coloring potential of the black is developed. The third phase is stabilizing the dispersion.

Dispersants
Dispersants are added to facilitate the milling process and to stabilize the resulting dispersion.

Among the most potent carbon dispersants are the anionic oxy-lignins derived from sodium lignosulfonates. They products offer:

Increased

• Milling Speed
• Carbon Black Loading

• Initial Viscosity
• Gelling
• Free Water

The structure of the primary aggregates determines the properties of each grade of carbon black. Low structure blacks have few primary particles in compact structures. High structure blacks have many primary particles fused into long, complex chains. The combination of particle size, structure, and surface oxidation determine the properties such as coloring ability, UV absorption, and conductivity.

Benefits
In order to fully develop a given carbon black's useful coloring properties, the optimum dispersant requirement, or DAR (Dispersing Agent Requiement), must be determined. The objective is to obtain the lowest dispersion viscosity and to prevent flocculation or settling. The DAR for a given carbon black concentration is defined as the minimum amount required to obtain a fluid dispersion. The higher structured blacks have larger surface areas and require greater treatments of dispersant. They will also give higher viscosities, which limits the carbon black loading values.

Figure 1, below, illustrates that determining the DAR is essential for an optimal dispersion. Too little or too much dispersant can compromise the systems stability as indicated by the higher viscosity.

Figure 1 - effect of sodium Lignosulfonate on Viscosity of Carbon Black Dispersions

How The Products Work
Carbon Black dispersants are oxidized, partially desulfonated, highly purified sodium lignosulfonates. They function by adsorbing onto the black's surface and imparting steric and electrostatic stabilization mechanisms.

As illustrated in Figure 2, they are most effective in a pH range of 9 to 11.5. They function as both grinding aids and dispersion stabilizers, effecting improved fluidity at high carbon black concentrations. They lower both initial viscosity and gelling on standing. They also reduce or eliminate free water. Reduced milling times and higher loadings both serve to lower manufacturing costs.

Figure 2 - Effect of pH on Viscosity of Carbon Black Dispersions

The Lignin Institute can assist lignin users with information and questions on lignin products. LI is a non-profit organization based in Atlanta, GA.

5775 Peachtree-Dunwoody Road . Building G, Suite 500 . Atlanta, GA 30342 . (404) 252-3663 . E-mail: li@kellencompany.com . Internet: http://www.lignin.org