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Selecting Carbon Black for Plastics

Carbon black is a highly engineered form of carbon widely used in plastics to achieve a spectrum ranging from gray to deep black. Over the time, the properties of carbon black pigment have been modified to achieve required properties in the final product, such as increased tinting strength, improved the level of jetness or blue undertone and conductivity.

Explore the different carbon black production processes and the properties to consider while selecting the right carbon black for your formulations by reviewing:

  1. How Carbon Black is Produced?
  2. Key Properties of Carbon Black
  3. Carbon Black for Plastics
  4. Finding the Right Carbon Black Grade for Your Application

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Properties and End uses of Carbon Black


Carbon Black Selection for PlasticsCarbon 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 Products
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?


The properties of the carbon black are influenced by the method of preparation. The different processes used for carbon black production are discussed below.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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 from End-of-life Tires
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


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.

Particle Size
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
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 in Plastics


In plastics, the carbon black provides three main properties:

  • Color
  • UV protection
  • Conductivity

The carbon blacks are used to produce masterbatches that are further used in the final preparation of the plastics. During the production of the masterbatch, the carbon black must have good tinting properties, resulting in the desired color with minimal use of carbon black with good dispersibility, ensuring low energy needed to provide good dispersion of the carbon black.

When the masterbatch is used in the final application, the carbon black must spread easily from the base polymer to create an even result, good dilutability.


Food Contact Regulation


For certain applications, specialty carbon blacks are needed which comply with the food contact regulations as set by the FDA (The U.S. Food & Drug Administration).

The applicable purity requirements for compliance with U.S. FDA regulations are:

  • Total PAHs should not exceed 0.5 ppm
  • Benzo(a)pyrene should not exceed 5.0 ppb

As a result of a new Food Contact Notification (FCN) submitted by Cabot to FDA (FCN 1789), FDA-compliant specialty carbon blacks can be used as a colorant for polymers with no specified upper limit.

The Commission Regulation EU No. 10/2011 is applicable in all the countries of the European Union.

The purity requirements and specifications for compliance are:

  • Toluene extract ≤ 0.1%2
  • Cyclohexane extinction at 386 nm < 0.02 for 1 cm cell or < 0.1 for 5 cm cell
  • Benzo(a)pyrene ≤ 0.25 mg/kg (250 ppb)
  • Primary particles of 10-300 nm, Aggregates of 100-1200 nm, Agglomerates 300nm+
  • In the final food contact item, a maximum of 2.5% carbon black by weight is allowed

US Food Contact Regulations


Carbon Black for Coatings and Inks


When carbon black is used in coating or ink applications the following properties are the most important:



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


JetnessThe 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
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


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
Treated Surface oxidation to provide better dispersibility, high volatile content, acidic pH
Food contact Indirect food contact - FDA regulation
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

Carbon Blacks by Birla Carbon


Carbon Black in Rubbers and Tires


The official classification of carbon black used in rubbers is described in ASTM D17651.

  • The first number indicates the particle size, where
    • The N100 series has the smallest having a particle size of 11-19 nm (average)
    • The N900 series has the largest particle size of 201-500 nm (average)
  • The second and third digit are arbitrary numbers but can be used to describe the functionality or structure of the carbon black.

Here, N stands for the ‘normal’ cure of a rubber compound.

The channel carbon blacks were (predominantly) slow curing, and these grades of carbon black were indicated with the S prefix.

In tires, mainly types from N115 to N375 are being used, and all have a specific contribution to the final performance of the tire.

  • For the liners within tires, the carbon blacks with a larger particle size from N660 to N990 are being used.
  • For technical rubbers, in general, the larger particle sizes are used starting at N550 with specific addition of N3030.


Commercially Available Carbon Blacks






 »  Also Read: How Plastics Industry Trends Drive Development of New Carbon Blacks!


References

  1. https://www.astm.org/Standards/D1765.htm
  2. https://www.iso.org/standard/45776.html
  3. http://www.carbonstandard.com/


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