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Selecting Carbon Black for Paints, Coatings and Inks

Adding carbon black (CB) particles to elastomeric polymers is essential to the successful industrial use of rubber in many applications, and the mechanical reinforcing effect of CB in rubber has been studied for nearly 100 years. Despite these many decades of investigations, the origin of stiffness enhancement of elastomers from incorporating nanometer-scale CB particles is still debated. It is not universally accepted whether the interactions between polymer chains and CB surfaces are purely physical adsorption or whether some polymer–particle chemical bonds are also introduced in the process of mixing and curing the CB-filled rubber compounds. We review key experimental observations of rubber reinforced with CB, including the finding that heat treatment of CB can greatly reduce the filler reinforcement effect in rubber. The details of the particle morphology and surface chemistry are described to give insights into the nature of the CB–elastomer interfaces. This is followed by a discussion of rubber processing effects, the influence of CB on crosslinking, and various chemical modification approaches that have been employed to improve polymer–filler interactions and reinforcement. Finally, we contrast various models that have been proposed for rationalizing the CB reinforcement of elastomers. 


Natural rubber composite has been continuously developed due to its advantages such as a good combination of strength and damping property. Most of carbon black (CB)/Natural Rubber (NR) composite were used as material in tyre industry. The addition of CB in natural rubber is very important to enhance the strength of natural rubber. The particle loading and different structure of CB can affect the composite strength. The effects of CB particle loading of 20, 25 and 30 wt% and the effects of CB structures of N220, N330, N550 and N660 series on tensile property of composite were investigated. The result shows that the tensile strength and elastic modulus of natural rubber/CB composite was higher than pure natural rubber. From SEM observation the agglomeration of CB aggregate increases with particle loading. It leads to decrease of tensile strength of composite as more particle was added. High structure of CB particle i.e. N220 resulted in highest tensile stress. In fact, composite reinforced by N660 CB particle shown a comparable tensile strength and elastic modulus with N220 CB particle. SEM observation shows that agglomeration of CB aggregates of N330 and N550 results in lower stress of associate NR/CB composite.


Carbon black is a highly engineered form of carbon widely used in paints as paint carbon black, coatings and inks 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.


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, ink carbon clack.


Thus, the requirements for the carbon black are different for each application and influence the specific properties in the final application.


For the coating carbon blacks 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.


How Carbon Black is Produced?


The properties of the carbon black are influenced by the method of preparation. The different processes used for channel 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:

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 high purity 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


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 conductive carbon blacks can be used, each attributing to the different properties required. This blend of environmental 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 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 rubber 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 plastic carbon black is used in which category, we need to understand the fundamental differences between the available carbon blacks.
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