This article is mostly derived from the book “Coloring of Plastics: Fundamentals – Colorants – Preparations, chap.5” by Albrecht Müller.
Carbon black (C) – Pigment black 7 / 77266 / CAS 1333.86.4
carbon black (CI pigment black 7) dominates the market for black shades. Black pigments are used either for pure black shades or for darkening of colored shades.
Carbon black, pure carbon, is a very fine pigment with a very high tinting strength. Carbon black is classified as an inorganic pigment, for example in the German standard DIN 55944, in spite of the fact that carbon is one of the main components in the whole of organic chemistry.
Carbon black is manufactured by several production processes. The most important are the furnace black process and the channel black process; other processes of less importance are the lamp black process, the thermal black process, and the acetylene black process.
carbon blacks vary widely in particle size depending on the process by which they are made. Channel or impingement black is made by the impingement of smoky flames from tiny jets on iron channels; the deposited black is scraped off by moving the channels over stationary scrapers. Furnace blacks are made in refractory chambers by incomplete combustion of any of various types of gaseous or liquid hydrocarbons. Thermal blacks are produced in the absence of air when hydrocarbons are decomposed by contact with heated refractories. Lampblack, the oldest known black pigment, is produced by burning oil, usually coal-tar creosote, in shallow pans, in a furnace with the draft regulated to give a heavy smoke cloud. Acetylene black is produced in refractory chambers in the absence of air by the decomposition of acetylene gas preheated to 800° C (1,500° F). It is used in applications requiring high electrical conductivity, such as dry cells.
General properties
The properties of carbon black are determined by the particle size distribution, the structure (coalescence of primary particle into aggregates), the specific surface area and the chemical composition of the pigment surface.
The primary particle size of carbon black is in the range of 10–300 nm, it is a very fine grained pigment.
The specific surface area varies widely. A coarse grade of carbon black has a specific surface area as small as 10m²/g, while the finest grades can have a specific surface area as large as 1200m²/g. During the handling of very fine grades of carbon black it is necessary to take precautions. One aspect is to avoid a heavy soiling of the workers, machines, and the surroundings by the fine dust, and another involves wetting problems during incorporation in a polymer melt. Very fine carbon black is difficult to wet with the polymer melt, as this requires not only very effective dispersing agents but also a high shear during incorporation. Only in the case of a complete dispersal is it possible to make the most of the full
color depth of carbon black, which is an important feature when carbon black is used to darken a colored shade in addition to maintaining consistency of the exact shade from production to production. The color depth of carbon black increases with decreasing particle size. On the other hand, a coarser carbon black is easier to disperse in a polymer melt than a very fine grade. In the case of dispersal problems it may be advisable to use a coarser carbon black. Although the color depth of a coarser carbon is lower the visual impression is a higher color depth because of the more complete dispersion. The dependencies of typical properties of carbon black on the particle size are shown in Fig2.
Fig 2. Effect of particle size on properties of carbon black.
As mentioned previously the color depth of carbon black increases with decreasing particle size. At constant concentration the number of pigment particles increases at the same time, which results in a higher light absorption and less scattering. The absorption rate of very fine grades of carbon black can reach nearly 99.5%, which is a very “deep” black. Consequently a coarser carbon black absorbs less light and is scattered more; the visual impression is that of a “lighter” black. The bluish or brownish tone of carbon black is the result of this interaction between light absorption and light scattering, which depends on the particle size. Whether a bluish or brownish tone is noticeable depends on the angle of vision. A bluish tone appears when looking through a transparent part, colored with a coarse carbon black, and the brownish tone appears when looking on top of the surface of the same part. In case of a very fine grade of carbon black it is the reverse. The exact shade depends not only on the particle size but also on its structure.
During the production process the primary particles of carbon black, almost spherically shaped, coalesce into aggregates in the form of chains or clusters. The type of aggregates is called the structure of carbon black. The dependence of some typical properties of carbon black on the structure is shown in Fig3.
fig 3. Dependence of typical properties of Carbon Black on the structure.
Carbon black is manufactured by a partial combustion of hydrocarbons; therefore the surface of the particles contains oxygen, bound to the surface in the form of acidic or basic functional groups. The amount of surface oxides and their composition depends on the production process and the raw material. The amount of surface oxides can be increased by an oxidative after treatment. The structure and number of oxygen-containing functional groups on the surface of carbon blacks influence the application properties. The chemical composition of the carbon black surface is called surface chemistry.
Many polymers are degraded by UV radiation under atmospheric conditions. In those plastics, especially in polyolefins, carbon black acts as a stabilizer by absorbing the UV radiation. The stabilizing action of carbon black increases with decreasing particle size and with increasing concentration up to approx. 2–3%. A medium-fine and highly structured grade of carbon black is the preferred grade for such an application.
Another property of carbon black is its electrical conductivity. Depending on the requirements the plastic part has to fulfill, this electrical conductivity is either disruptive or required. The electrical conductivity depends on the type of production process, as well as on the specific surface area and structure, and of course on the concentration of carbon black in the finished part. If required, special grades of carbon black are marketed to lend polymers antistatic or electrically conductive properties. The necessary concentrations, however, are much higher than usually applied for the coloring of polymers.
Every substance with a very high specific surface area adsorbs lager amounts of gaseous or liquid components. Grades of carbon black with a large specific surface area have a remarkable adsorption capacity for solvents, binders, polymers, and additives; therefore interactions cannot be excluded. The adsorption of stabilizer or antioxidants can cause problems in polymer systems, such as a reduced light
fastness or weather resistance. The adsorption of dispersing agents leads to problems regarding wetting and dispersing of carbon black, and an adsorption of the polymer melt increases its viscosity. In rubber carbon black can influence the speed of vulcanization.
Application
More than 90% of the total amount of the carbon black produced is
used as reinforcing filler in rubber for tires and other technical rubber goods. The
use as colorant is approx. 2–4% of the produced quantity, nevertheless this is a
large quantity considering that the yearly production is about 6 million tons of
carbon black.
In comparison to carbon black the other black pigments play no important role in
coloring of plastics. Their main disadvantage is the low tinting strength. They are
used if carbon black cannot be used for any reason or for tinted shades. Generally
considered, for the tinting of a color only very small amounts of a colorant are
necessary, and on the other hand a very even distribution of a small quantity in a
larger mixture is difficult to achieve. In such a case a low tinting strength can even
be an advantage. If such a pigment is used for tinting, the necessary quantity
increases and mixing problems decrease. In addition, these other black pigments,
based on metal oxides, are much easier to disperse in a polymer melt than carbon
black.
Reference:
Albrecht Müller, Coloring of Plastics: Fundamentals – Colorants – Preparations, chap.5
