INSIGHTS ON CARBON BLACK FUNDAMENTALS History

Carbon black is one of the oldest manufactured materials. Early uses

can be traced back to ancient China, the early Egyptians and the

production of Indian Inks. Early demand for carbon black was

driven by the invention of movable type used in fifteenth century

printing. The discovery that carbon black reinforces natural rubber

and thereby greatly increasing the longevity of tires in the early

nineteenth century thrust the material into the modern age.

Today carbon black is found in all aspects of modern life. It is used in inkjet printer ink,

as reinforcements for natural and synthetic rubber, it is the active agent in electrically

conductive plastics and is used as a pigment and tinting-aid in paints, coatings, news

paper inks and cosmetics to name a few. Carbon black is ubiquitous.

Production

There are three significant processes for the manufacture of carbon black: the furnace

process, the channel process and the acetylene process. The furnace process is the most

prevalent by far accounting for over 80% of capacity. The products made by each process

have unique characteristics. For example, the acetylene process produces a carbon black

with very low structure (particle complexity) and the particles have higher graphitic

content than those produced by the furnace process. The remainder of this overview will

concentrate on carbon blacks manufactured by the furnace process.

Although there are differences among the processes they all involve combusting a fuel in

a controlled atmosphere environment. In the furnace process “resid” or “decant” oils, low

cut fractions from the oil refining process, are combusted in a controlled atmosphere at

high temperatures. Typical processing temperatures are in the order of 800C to 1200C.

The best way to envision a carbon black reactor is to imagine a very large blow torch in a

ceramic tube with insufficient oxygen to cause combustion. If too much oxygen is present

soot formation takes place.

In the furnace process, fuel is atomized and sent to the reaction chamber where

carbonization starts. Oxygen levels are maintained below the level where soot formation

takes place. The atomized fuel undergoes reaction to eliminate non-carbonaceous

materials and the carbon atoms begin to bond to nearby neighbors within the droplet

starting the solidification process. It is this carbonized droplet that forms the most

fundamental carbon black particle, called the “primary particle.” As the carbonizing fuel

droplet leaves the reaction chamber and progresses down the path of the furnace it may

come in contact with other carbonizing droplets and under the appropriate conditions they

bond to one another to form a carbon black “aggregate.” The carbon black primary

particles fuse to form a coalesced mass. One can view a carbon black aggregate particle

as a “bunch” of grapes. Each individual grape is a primary particle and the “bunch” is the

aggregate particle.

The aggregates travel down the flight of the reactor. During this flight the reaction can be

quenched by water addition or temperature control. Both are used to selectively modify

the surface chemistry, size and complexity of the aggregate particle. After quenching the

particles travel the flight of the reactor where they are captured in bag filters or cyclones

separators. The particles at this stage are referred to as “fluffy” and have very low bulk

density.

Some degree of densification is required to convert the fluffy to a more useable form.

Densification can be achieved though pelletization processes, where the fluffy is mixed

with binders or water under low sheer conditions. Various densification processes are

used in the manufacturing of carbon black pellets and include vacuum rollers, pin

pelletizers and stirred tanks.

Fundamental Properties

The utility and ultimate economic value of carbon black is determined through a complex

mix of carbon chemistry, surface energy and particle physics. The most important

properties include the surface area, primary particle size,

structure (complexity of composition), surface chemistry and

binder chemistries used in the pelletization process. Frequently, tradeoffs and

compromises are made between desired end-use performance and the ability to disperse

carbon black.

Primary Particles: The smallest unit of a carbon black particle, the primary

particle, has dimensions of size, graphitic content, shape and crytallinity.

Although the majority of processes manufacture near-spherical shaped primary

particles some processes produce primary particles having aspect ratios higher

than those of true spheres. The higher aspect ratio leads to higher surface area per

unit volume and provides more wetable surface area improving ease of dispersion

and increasing electrical conductivity. Primary particle attributes influence color,

electrical conductivity and UV blocking performance of the carbon black.

Aggregates: Carbon black aggregates are complex clusters of fused primary

particles. Aggregates have dimensions of size, shape, void volume and structure.

Each of these dimensions determine the ultimate utility of the carbon black and

goes to provide competitive grade differentiation among carbon black

manufacturers. The size of the aggregate influences the color aspect of the carbon

black and its tinting strength. The shape and structure influence dispersability and

to some extent electrical conductivity. Void volume influences wetability and is a

critical concern in applications where the carbon black will be used in a liquid

medium such as a coating, paint or ink.

Inter-particle Attractions and Agglomerates: Carbon black particles are small.

For example, large aggregates are the size of red blood cells and aggregates

composed of small primary particles are roughly the size of tobacco mosaic virus.

Primary particles can be as small as 5nm and aggregates can be as small as 50nm.

It is this small size that makes carbon black so useful in pigmentation – a little

goes a long way. The small size of carbon black and its

triboelectic properties results in carbon black obeying laws

of bi-polar particle physics. Carbon black is subject to van der Waals forces.

Van der Waals Forces: Owing to the small

size of carbon black particles, inter-particle

interactions are subject to van der Waals

forces. Referring to Figure I, as the distance

between two carbon black particles

decreases (moving from right to left on the

ordinate scale of Figure I) the repulsive forces decrease, represented by

negative potential energy Eµ until they reach the maximum attraction at

the minimum of the potential well (the valley on the abscissa axis.)

Moving the particles closer together meets with significant potential

energy resistance as indicated by the sharp rise in the energy dimension on

the ordinate axis. Particles trapped in the potential well are called

agglomerates. Once the carbon black aggregates have agglomerated it

requires energy to separate them.

The ability to break up the agglomerates into the constituent aggregates and

achieve adequate dispersion is critical to achieving desirable end-use performance

in many applications. A significant body of literature has been developed on the

science surrounding dispersion of carbon black in various medium, however, for

industrial applications dispersion remains an art with experience practitioners

having the advantage of know how, technique and trade secrets.

Color Properties

Owing to the variety of shape and size, carbon black can exhibit a range of color

properties. Important color properties include jetness, mass tone and tinting strength. The

delicate interaction between primary particle size, surface area and aggregate size

determines the ultimate color, whereas, structure influences the dispersability of carbon

black and thus determines the level of color achievable in the

host matrix. Carbon black is added to plastics not only to

make plastics black but also to change the tint characteristics of other colors.

Color properties of carbon black are measured using colorimeters and have values of L,

a*, b*. Typically, carbon black is compounded into the host plastic matrix at

concentrations near 1-2% and the color values measured on a molded plack specimen.

The L value (white = 100, black = 0) measures color density, the b* value is indicative of

the yellow/blue balance (positive values indicate yellow and negative values indicate

blue) and the a* value is reflective of the red/green balance (positive values indicating red

and negative values indicating green.)

The “L” value can be correlated to the ratio of particle size to surface area and is a useful

tool for comparing competitive grades of carbon black. The b* is indicative of jetness and

correlated to the L value.

Jetness. Carbon black is added to plastics to impart a black color the

measurement of which is referred to as “jetness.” More jet blacks appear blacker

than those having less jet characteristics. Jetness is a complex function of surface

area, primary particle size and degree of dispersion. Carbon blacks possessing

smaller primary particle sizes tend to impart a higher degree of jetness than those

having larger primary particles. Trial and error has resulted in empirical

relationships between the carbon black properties and jetness.

Masstone. When compounded into plastics carbon black can impart colors

ranging from a bluish black to a brown/black undertone. This color range is

referred to as the masstone of the black and is strongly correlated to particle size

and the scattering of light in the host plastic matrix. Owing to differences in

refractive index and light scattering for different plastics two different plastics

containing well-dispersed carbon black can have the same jetness but differ

greatly in masstone.

Tinting Strength. An important property of carbon black is

its ability to modify the visual appearance of other colors. The tinting strength is a

measure of the effectiveness of the carbon black. There are a variety of methods

used in measuring tinting strength but the most prevalent is ASTM D 3265. In this

method, carbon black is added to a mixture of zinc oxide in dispersion medium

(soy oil for example) and the reflectance values are measured relative to the zinc

oxide standard. Tinting strength increases with decreasing primary particle size

and decreases with aggregate structure complexity. Tinting strength reaches a

maximum for primary particle sizes of less than 20nm.

Characterization Methods

Carbon black manufactured by commercial processes is a complex mixture of particles.

Industrial grades of carbon black exhibit a distribution of aggregate sizes and shapes,

each aggregate comprised of a distribution of different size and shaped primary particles.

Consequently, property measurements of carbon black are a statistical average around a

mean value of the bulk sample.

Surface area and structure are the key properties that influence the utility and value of

carbon black in many applications. For example, surface area influences the ability of

carbon black to absorb UV radiation. Structure influences the ease or difficulty of

dispersion and electrical conductivity properties. It is the balance of surface area and

structure that determines the utility of the particular grade of carbon black.

Surface Area Measurements: A variety of methods exist to measure surface area of

carbon black, each has its unique application either in the manufacturing quality control

environment or in laboratory confirmation studies. The most prevalent methods for

surface area measurement include CTAB (cetyltrimethylammonium bromide adsorption),

Iodine adsorption and Nitrogen number. Recent advances in high resolution microscopy

and inexpensive high speed computers has enabled the use of particle size analysis with

automated image classification, however, this method is relegated to research labs.

CTAB. The CTAB method is used primarily in laboratory

environments for measuring the surface area of carbon black. The method is

described in ASTM D 3765 and essentially involves measuring the isotherm-of-

adsorption of CTAB by carbon black in an aqueous dispersion. The CTAB

method is particularly relevant to the plastics market because it measures the

surface area available for wetting by the plastic matrix unlike nitrogen porosity

measurements that measure the surface area of excluded volume, unavailable to

wetting.

Iodine Number. In the manufacturing environment, where the target carbon black

grade properties are known, the Iodine method (ASTM D 1510) is used to

measure surface area. The method is useful for quality control but is influenced by

active surface chemistry, unreacted feedstock, oils and binders. The reported

iodine number should not be used as a guide for grade selection when developing

a plastic compound or end use article.

Nitrogen Surface Area. Surface area as determined by BET nitrogen absorption

techniques (ASTM D3037) reflects the true surface area including occluded

volume and the porosity of the carbon black primary particles. The method is

particularly useful when the carbon black will be dispersed in a liquid medium

such as in ink or coating applications. The correlation between nitrogen surface

area measurements and CTAB can be used to gain insight on the porosity of the

carbon black. Porosity is an important characteristic in grade selection and quality

control for inks and coatings.

Structure Measurements: The structure of carbon black influences the dispersability in

plastics, electrical conductivity and to some degree the color of the finished article.

Structure is a measure of the complexity of the carbon black aggregate particle and

reflects how the constituent primary particles are connected. Carbon black particles vary

in shape from the spherical particles found in thermal blacks

to more complex chains and clustered forms common in

furnace blacks. The more complex shapes form internal voids, nooks and crannies.

Liquid adsorptive measurements are used to characterize the structure of carbon blacks.

Aggregates that are more spherical in shape adsorb less material than those exhibiting

more complex shapes having more voids. Liquid adsorption methods provide insight on

the average bulk property and do not provide information on the distribution of properties

constituting the average.

DBP Method. The most frequently used method of characterizing the structure of

carbon black is the adsorption of dibutylpthlate, DBP (ASTM 2414-90.) The

method is based on measuring the torque of carbon black powder as DBP is

added. At the point of full absorption, correlating to full surface area coverage, the

mixture reaches a plasticization state and the torque rises quickly at this point the

level of DBP added is recorded. Higher levels of DBP addition correlate to higher

structure.

Crushed DBP Method. Industrially manufactured carbon black exhibits a

distribution of particle sizes and shapes. Consequently, smaller aggregates can

occupy the occluded volume of larger aggregates. Additionally, owing to van der

Waals forces, carbon black aggregates can agglomerate into clusters which can

influence the DBP measurement. Both of these phenomena reduce the apparent

structure as measured by the DBP method. The crushed DBP method was

developed to address these phenomena.

The method (ASTM D 3493) involves subjecting the carbon black to several

cycles of high pressure, typically 165MPa, before conducting the DBP

measurement. The high pressure serves to break agglomerates and separate the

particles captured in the occluded volumes of larger aggregates.