Properties and End-uses of Carbon Black
Properties and End-uses of Carbon Black
Carbon black is used in many products and articles we use and see around us on a daily basis, such as:
- Rubbers
- Plastics
- Coatings
- Tires
- Inks
Thus, the requirements for the carbon black are different for each application and influence the specific properties in the final application.
For the coatings market, there is a wide range of carbon black grades available. This can make it difficult to choose the most suitable carbon black for your final application.
For example, when aiming for automotive paint with a blue undertone, the carbon black of choice will have a high jetness. However, normally these types of carbon black grades are the most difficult to disperse correctly into the desired particle size.
The carbon black producers are addressing these issues by developing specialty carbon black grades that have been surface-modified and/or are pre-treated to overcome these difficulties.
Carbon Black Applications
Let’s discuss the most important parameters to consider while selecting the suitable carbon black for your formulation and how they influence the other properties...
How Carbon Black is Produced?
How Carbon Black is Produced?
The properties of the carbon black are influenced by the
method of preparation. The different processes used for carbon black production are discussed below.
- Furnace Black Process: It is the most common method which uses
(aromatic) hydrocarbon oil as the raw material. Due to its
high yield and possibility to control the particle size and structure, it is most suitable for mass production
of carbon black.
In the reactor the conditions (e.g. pressure and temperature)
are controlled to provide a number of reactions. The most important
reactions include:
- Particle nucleation
- Particle growth
- Aggregate formation
Water injection rapidly reduces the temperature and ends the reaction. The primary particle size and structure of the carbon black is controlled by tuning the conditions in the reactor and the time allowed before the reaction is quenched.
- Thermal Black Process: It is the most common method used for
carbon black production after the furnace black process. It is a
discontinuous or cyclical process.
This process uses natural methane gas as raw material. When the natural gas is injected into the furnace at an inert atmosphere, the gas decomposes into carbon black and hydrogen.
The carbon black produced
using this method has the largest particle size and the
lowest degree of aggregates or structure. Due to the nature of the raw material, this carbon black is the
purest form available on the industrial scale.
- Channel Process: This process uses partially combusted fuel which is brought into contact with H-shaped channel steel. It is not the most used method anymore because of its:
- Environmental issues
- Increased natural gas price
- Low yield
The benefit of this process is that it provides carbon black with a lot of functional groups.
- Acetylene Black Process: This process uses acetylene gas as raw material. It produces mainly
high structure and higher crystallinity, making this type of carbon black suitable for
electric conductive applications.
- Lampblack Process: It is the oldest industrial process for making carbon black. It uses
mineral/vegetable oils as its raw material.
Recovered Carbon Black from End-of-life Tires
Recovered carbon black or (r)CB is a fast-expanding market.
Recovered carbon black is obtained through the
pyrolysis process of end-of-life tires. The importance of companies in the production and use of recovered carbon black is three-fold:
- The growing global problems arising with end-of-life tires (ELT)
- Companies shifting strategy to fulfill the targets ensuring a green economy
- Price changes of regular carbon black due to fluctuations in oil pricing
Recovered Carbon Black Obtained from ELT
Depending on the composition, the content of carbon black in tires can be up to 30%. Next to carbon black, the tires consists:
- Rubber
- Rubber processing additives
- Metal
- Textile
- Fillers such as silica
The amount of silica depends on the type of tire, for example winter or summer tire, racing tire, or tire for agricultural vehicles, and will not be separated from the
carbon black during the pyrolysis process, which will result in higher ash content.
In a typical car tire, up to 15 different types of carbon blacks can be used,
each attributing to the different properties required. This blend of carbon
blacks will then also be the make-up of the final (r)CB composition. Besides tires, other sources that can be used are
rubber conveyor belts or other technical rubber products.
The presence of inert conditions in the pyrolysis process is important so that no additional carbon black is being produced.
The main differences in the properties of recovered carbon black are:
- The ash content is higher for (r)CB caused by the fillers being used in tire production.
- A blend of carbon black properties as a result of the carbon black used in the tire.
- Residual hydrocarbons on the carbon black surface, depending on the quality of the pyrolysis process.
To understand how the properties of (r)CB influence the final applications and to know which carbon black is used in which category, we need to understand the fundamental differences between the available carbon blacks.
Key Properties of Carbon Black
Key Properties of Carbon Black
Primary Particle Size
The first parameter to consider is the primary particle size of the carbon black. The primary particle size can vary
from 15 nm up to 300 nm. Some furnace blacks have a particle size of even as small as 8 nm.
Primary Particle Size of Carbon Black
Small particles result in higher jetness caused by a high surface area. They also provide:
- Better weatherability
- UV-fastness
- Better conductivity
On the downside, the smaller particle sizes lead to higher viscosity and require more energy for dispersing. These types generally have a
blueish undertone and are used in the automotive industry where high jetness is required.
Whereas, the higher particle sizes improve the viscosity and dispersibility properties within the application. They have a more
brownish undertone and are generally more suitable for the
rubber and tire applications.
Structure
Already during the production process, aggregates are being formed from the primary particles. The structure of the carbon black is determined by:
- How the aggregates are shaped?
- The level of branches in the aggregates.
Structure of Carbon Black
High structured aggregates give improved dispersibility and
increased viscosity, but on the other hand, they will affect the
blackness with several important in-rubber properties.
| Influence on Properties |
Particle Size Decreases |
Structure Increases |
| Viscosity |
↑ |
↑ |
| Hardness |
↑ |
↑ |
| Modulus |
- |
↑ |
| Elongation at Break |
↓ |
↓ |
| Swelling after Extrusion |
- |
↓ |
| Dispersibility |
↓ |
↑ |
| Impact Resilience |
↓ |
- |
| Tensile Strength |
↑ |
- |
Influence on Properties for Particle Size and Structure
Surface Chemistry
Another important aspect of carbon black is surface chemistry. Depending on the production process, the functional groups on the surface of the carbon black will be different. The
type and amount of functional groups will play a big role in the affinity within the application it is being used.
In general, when talking about surface chemistry, it is meant the level of oxygen-containing groups on the surface. For certain applications, the carbon black is further oxidized to increase the amount of oxygen-containing groups on the surface.
Specifically, in ink and coating applications, this will be beneficial to improve the dispersibility, pigment wetting, rheology and overall performance in the selected system.
Note: During the surface oxidation of carbon black, carboxyl groups are formed on the surface, leading to a low pH of the carbon black. This could cause incompatibility in certain coating systems.
Analysis Methods
A number of tests are normally done to further specify the properties and analyze the carbon black used. The table below shows an overview of the most important test properties for carbon black and their corresponding ASTM methods.
| Property |
Unit |
Test Method |
| BET Surface Area |
m2/g |
ASTM D6556 |
| Statistical Thickness Surface Area, STSA |
m2/g |
ASTM D6556 |
| Oil Absorption Number |
cm3/100g |
ASTM D2414 |
| Pellets Hardness (average) |
g |
ASTM D5230 |
| Pour Density |
kg/m3 |
ASTM D1513 |
| Sieve Residue, 325 mesh |
% |
ASTM D1514 |
| pH |
- |
ASTM D1512 |
| Moisture Content |
% |
ASTM D1509 |
| Ash Content |
% |
ASTM D1506 |
| Sulfur Content |
% |
ASTM D1619 |
Properties of Carbon Black and Their Corresponding ASTM Methods
Carbon Black for Coatings and Inks
Carbon Black for Coatings and Inks
When carbon black is used in coating or ink applications the following properties are the most important:
- Pigmentation
- Viscosity
- Protection
Tint Strength
Tint strength is the ratio, expressed as tint units, of the reflectance of a standard paste to a sample paste, both prepared and tested under specified conditions.
As described in the test method
ASTM D 3265-19b, a carbon black- zinc oxide paste is prepared, either by using an
automatic muller apparatus or the Speedmixer® (DAC 150 FVZ).
For the preparation of the carbon black-zinc oxide paste, pre-determined raw materials are being used, such as:
- Industry tint reference black (ITRB2)
- A specific zinc oxide (lot number 11), and
- Greenflex ESO (epoxidized soybean oil)3
The reference paste is set as 100, and all the carbon blacks used are compared to this paste. This means when carbon black has a
tinting strength of 80, it will give a less black color when using the same amount.
Jetness
The jetness (Mc) is the
color-dependent black. It is indicatively measured as b* using a colorimeter (where b* is directly related to the L-value) and is not to be confused with blackness. The
jetness is influenced directly by the primary particle size.
The lower the primary particle size, the higher the jetness.
Blackness, on the other hand, is a degree of blackness, directly related to the
reflectance. In the case of high jetness pigments, it can be even below 1%.
In general, jetness is determined according to procedure DIN 55979 - determination of the black value of carbon black, where the
residual reflection is measured. In this method, the blackness is used as an indication of the jetness.
The combination of blackness and jetness will tell you the undertone, where:
| Blackness/Jetness Combination |
Color of Undertone |
| Blackness > Jetness |
Brown |
| Blackness = Jetness |
Neutral |
| Blackness < Jetness |
Blue |
Conductivity
There are various carbon blacks in the market that can provide anti-static or conductive properties. The main properties which will influence the conductive properties of the carbon black are:
- Specific surface area
- Structure
- Surface chemistry
Most of the conductive carbon blacks available in the market have higher surface areas and structures and can contain a significant volume of micropores.
Conductivity is measured by the surface resistivity of the conductive film presented in Ω/square or in volume resistivity of Ω-cm.
A better conductivity performance of a conductive carbon black will aid in adding the appropriate loading of carbon black to achieve the minimum required surface resistivity for the application.
Surface Resistivity in Ω/square
In the final selection, to prepare a conductive or dissipative coating, a balance in the carbon black properties has to be found.
As the high surface area will give you a more conductive coating but these blacks, therefore, have a higher oil absorption number, causing more binder or wetting agents to be used for optimal dispersion, and more energy is required to disperse the carbon black to achieve the desired particle size. Next to this, the level of surface resistivity required will then determine the amount of carbon black needed.
Having learnt about the production processes and properties of carbon black, let's explore the parameters to consider while selecting the carbon black for specific
coatings and ink applications.
Finding the Right Carbon Black Grade for Your Application
Finding the Right Carbon Black Grade for Your Application
With regard to coating applications, we need to consider the following parameters:
- Tinting strength
- Jetness
- Ease of use
- Dispersion time
- Dispersion loading
- Viscosity
- Physical form: powder or pellets
- Price
- Final requirements of application such as:
- Indirect food contact
- UV protection
- Conductivity
In the table below an overview is given of the different types which are available:
| Type of Carbon Black |
Description |
Examples |
| High color |
Highest jetness |
|
| Medium color |
Medium-high jetness for masstone |
|
| Low viscosity |
Provide good stability and dispersibility |
|
| Multi-purpose |
All-purpose grade for use in both tinting and masstone |
|
| Tinting |
Provide high tint strength and desired undertone |
|
| Conductive |
Conductive carbon black |
- VULCAN® XC72Rb
- Hiblack® 40B2c
- Printex® 85c
- Conductex 7055b
|
| Treated |
Surface oxidation to provide better dispersibility, high volatile content, acidic pH |
|
| Food contact |
Indirect food contact - FDA regulation |
- Raven FC1b
- BLACK PEARLS® 4750a
- Printex® F 80c
|
| Recovered carbon black |
Recovered using rubber pyrolisis, high ash content |
|
| a: Cabot; b: Birla Carbon; c: Orion Engineerd
Carbons; d: Black Bear; e: Mitsubishi
Chemical; f: ShanDOng Emperor-Taishan Carbon; g: Spring
Green |
