Carbon black: How to select the right grade for plastics?

Carbon black is used in many products and articles we use and see around us daily, such as:
 

• Plastics, rubbers & tires
• Coatings & inks

Each product has a specific property that can be modified and enhanced by carbon black additives. The most important role of a carbon black is:

• Reinforcement of elastomers to enhance their mechanical properties (modulus, tensile and tear strengths, abrasion resistance…)
• Light protection
• Coloring
• Conductivity enhancement
• To reduce cost
  • By decreasing the rubber weight (and its cost) used to obtain the same mechanical performances
  • By increasing the functional performances as abrasion resistance

Thus, the requirements for the carbon black are different for each application and influence the specific properties in the final application. For the plastics and rubbers, there is a wide range of carbon black grades available in our Master Catalog. This can make it difficult to choose the most suitable carbon black for your final application.
 

For example, when aiming for 'piano blacks' for rich, luxurious finishes in automotive interiors, the carbon black pigment is often a material of choice. 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

You should consider important parameters while selecting the suitable carbon black grade. It is also essential to study how these parameters influence the other properties. Let's begin by discussing the processes used in the production of carbon black pigments.

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

Furnace black process 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 are controlled in two ways. The first is by tuning the conditions in the reactor. The second is by adjusting 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 in an inert atmosphere, the gas decomposes into carbon black and hydrogen. 

The carbon black produced using this method has:
 

• Largest particle size and
• 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. This makes 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, you need to understand the fundamental differences between the available carbon blacks in our Master Catalog.
 

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

Carbon black with smaller particle size

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. 


 

Carbon black with high particle size

The higher particle size improves the viscosity and dispersibility properties within the application. They have a more brownish undertone. They are generally more suitable for rubber and tire applications.


 

Structure

Physically, the carbon blacks are organized in three structural levels:
 

• Primary particles characterized by sizes ranging from 10 to 500 nm.
• Aggregates of particles characterized by size ranging from 40 to 600 nm.
• Agglomerates of aggregates.

Chemically, carbon blacks are more or less pure carbon as shown in table below.

 

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, and
• The level of branches in the aggregates.

Structure of carbon black

High-structured aggregates give improved dispersibility and increased viscosity. On the other hand, they will affect the blackness with several important in-rubber properties.
 

Select from 80+ carbon black grades with improved dispersibility on our platform.

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 number 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 number of oxygen-containing groups on the surface.

Note: During the surface oxidation of carbon black, carboxyl groups are formed on the surface. This leads to a low pH of the carbon black. This could even 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.