Carbon Black

	Carbon Blacks
	Color Index	Pigment Black 6, 7, 8
	Formula	C (Carbon)

Structure

Carbon blacks are manufactured by thermal oxidative dissociation of hydrocarbons such as mineral oils or natural gases. They consist of elemental carbon, but there is a wide variety of products available depending on the starting materials and method of manufacture.

Carbon blacks are classified according to an internationally recognised system, which signifies the manufacturing process and the jetness. The first two letters designate the strength of the pigment: high color (HC), medium color (MC), regular color (RC), and low color (LC). The final letter describes the manufacturing process: furnace (F) and the channel/gas process (C) (for example: HCF = high color furnace).

Channel blacks are produced by burning petroleum gases in an atmosphere deficient in oxygen. This process is not very used because it has disastrous consequences on the environment.
Furnace blacks are produced by the thermal oxidative process: hydrocarbon oils are burned in a limited supply of air.

Properties

Properties of carbon blacks
Property	Carbon Blacks
Light Fastness	excellent
Resistance to solvents	good
Chemical Stability	excellent
Heat Stability	excellent

Color

The particle size determines the intensity of blackness, known as jetness. Smaller particles giving higher values.

Surface area
Carbon blacks differ by their surface area.

	Average surface area
	Channel blacks	110 m2/g
	Furnace blacks	80 m2/g

Problems

Absorption

The light dusty form of carbon blacks allows them to be converted to an easy to disperse pellet form by the dry pelleting process. Over a period of time carbon blacks absorb active ingredients in a paint formulation, such as the metal soaps used as driers in air-drying alkyds. This absorbtion can lead to problems... However, doubling the dosing of drier content is a common way of compensating for this issue.

Flocculation

Following dispersion small particles of carbon black have a tendency to flocculate. To minimize this tendency it is necessary to take utmost care when diluting paints. Only small calculated additions should be made with thorough homogenization between additions.

Dispersion

Carbon blacks can be difficult to disperse as they have very small particles with a correspondingly large surface area. Improved dispersions can be attained by oxidizing the surface of the pigment or by adding small quantities of organic groups such as carboxylic acids. Such groups decrease the pH of the aqueous extract and aid the wetting of the particles.

Viscosity of the paint

Particles of carbon form clusters which are often compared to a cluster of grapes, held together by forces ranging from weak physical attraction to chemical bonds. This structure affects the dispersibility, the jetness, gloss and, above all, the viscosity of the paint.

Uses

Carbon black is one of the oldest of pigments. It was used 27000 years ago for cave paintings, also around 2500 BC by the Egyptians in inks and for the printing process during the Middle Ages.

The rubber industry is the biggest user of carbon blacks by a large margin. In comparison, the paint industry is only a modest user. The printing inks industry uses as well significant quantities.
Finer particle blacks are used for high-quality finishes such as in automotive paints on account of their higher jetness. Medium size blacks are used for intermediate quality paints, whereas the coarser pigments are used for decorative paints.
For tinting purposes, coarser grades are usually used as their lower strength meaning they are easier to control.

Price

The price of carbon blacks varies depending on particle size, increasing as the particle size is decreasing. However, carbon black is a relatively cheap pigment. Speciality grades, made for specific applications, are of course more expensive.