NOPERA CONSIDE

There are mainly three types of inorganic black pigments used in the coating industry, carbon black, iron oxide black and mixed- phase metal oxide pigments. Carbon black pigments dominate the volume of black pigments used in the coating industry, while the others are used in much smaller quantities. Iron oxide black and mixed-phase metal oxide pigments will be more suitably discussed in later chapters on other iron oxide pigments and high performance pigments, respectively. Therefore, in this chapter, only carbon black is discussed.

3.3.2.1 Carbon black

Pigment Black 7; formula: C

Carbon black pigments are pure elemental carbon produced by partial combustion or thermal decomposition of liquid or gaseous hydrocarbons such as natural gas, heavy oil or acetylene under controlled conditions. Different grades of carbon black pigments are produced possessing a range of properties that are governed by the type of hydrocarbon used and the manufacturing conditions. Important characteristics of carbon black pigments are particle size, surface area, surface chemistry, and structure, which influ-

ence important properties of these pigments, such as color, tinting strength, conductivity and texture.

Carbon black pigments are very fine, having a particle size ran- ging from 5 nm to 500 nm. Generally, the smaller the particles, the better the jetness (intensity of blackness) and tinting strength. On the other hand, finer particles have very large surface area and therefore absorb a high amount of polymer and build up high vis- cosity at very low pigment loading. Primary particles of carbon black pigments are spherical, but they aggregate in the form of clusters or chains. In the industry, the extent of this aggregation is called structure. A higher degree of structure results in higher

viscosity and conductivity but provides a softer texture. Although all carbon black pigments are elemental carbon, they differ in their surface chemistry depending upon the manufacturing technique. As a result of oxidation at the surface during manufacturing, car- bon black pigments have varying amounts of complexes containing chemisorbed polynuclear oxygen on their surface. The amount of these complexes is normally termed the percent volatiles (deter- mined based on loss of mass at 950 °C in a closed platinum crucible). Higher volatile content increases dispersibility, jetness and gloss, while it lowers the conductivity. The oxygen complexes also reduce the pH of aqueous extracts of carbon black pigments.

Carbon black pigments are classified into six classes: lamp black, furnace black, thermal black, acetylene black, channel black and gas black. They all differ in their typical properties such as particle size, jetness, undertone and surface chemistry.

Lamp black is the oldest type of carbon black. In earlier days, it

was produced from vegetable oil and hence is also called vegetable black, but in modern processes, it is produced from a high-boiling aromatic oil. Lamp black has a larger particle size and low jetness, but due to its bluer undertone, is frequently of choice as a tinting black to produce gray coatings, where it also has the advantage of low flooding and floating owing to the larger particle size. Furn- ace black is synthesized by partial combustion of hydrocarbon

oil having a high aromatic content using a single flame in a clo- sed furnace in the presence of insufficient oxygen. The process

is highly efficient and has greater control of reactants. Furnace black has a finer particle size than other black pigments, except for channel black and gas black. This type of pigment is frequently oxidized in a subsequent step to improve its texture and jetness.

Thermal black pigments are produced from thermal decomposi-

tion of natural gas, or occasionally hydrocarbon oil, in the absence of air. They have a large particle size and almost negligible volatile content. Acetylene black is produced by thermal decomposition

of acetylene or acetylene mixed with light hydrocarbons. Once the reaction is started, it is maintained by exothermic decomposition of acetylene. This process is known to give products with higher conductivity. Channel black is produced by burning natural gas

or methane in insufficient air where the flame impinges on a metal surface. The process gives a very low yield of carbon black, and hence channel black is more expensive than other carbon black pigments. Channel black has a very fine particle size, high volatile content, acidic pH, high structure, the highest jetness and the high- est tinting strength. The Degussa gas black process is similar to

the process used to form channel black, but coal tar oils instead of natural gas are used that give higher yields and production rates. Therefore, most of the channel black process has been replaced by the gas black process for high quality oxidized blacks.

Extremely high hiding power and tinting strength of carbon black pigments make them widely accepted black pigments in paints and printing inks. Carbon black pigments are relatively economi- cal, but the prices are largely governed by particle size. Expen- sive, finer blacks are used in high quality coatings while coarser ones are preferred in low cost coatings. Carbon black is a strong absorber of UV light, which helps protect binders from degradation and hence is very useful in exterior coatings. Among the other important advantages of carbon black are excellent color stability, solvent resistance, acid resistance, alkali resistance and thermal stability. However, carbon black is difficult to disperse compared to other black pigments, although surface-treated, easily dispersible grades are available now. Carbon black pigments have a tendency to adsorb driers and other additives on their surfaces, requiring slightly higher dosages.