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What are Carbon Black's Pellet Properties?

In its fluffy form as it originates at the carbon black reactor, carbon black agglomerates easily with agitation. This propensity is employed in the formation of pellets from the fluffy material. In the most common method called "wet process", the fluffy black is mixed with water, followed by high speed agitation as a mass to form pellets and then passed through a dryer. With this method pellets are formed which for most rubber grades are too soft for bulk rail shipments. For these grades, a binder containing a sugar component is commonly premixed into the pelletizing water. Moderate usage is necessary because such binders can create very hard, tough pellets.

Although pellet properties are not used as a guide for choosing the grade of carbon black to be used in a given compound, depending upon the mixing characteristics of the compound, they can certainly have a direct bearing on the achievement of ultimate properties and upon batch to batch uniformity.

What is Carbon Black Surface Activity?

In addition to particles and aggregates, a third major parameter of carbon black is what has become known as"surface activity". It can be shown experimentally that a given carbon black sample possessing a high reinforcement potential, upon being subjected to sufficient heat, will lose some of it's ability to reinforce rubber. Since it's morphology is changed very little with this treatment, such a sample can be described as having lost "surface activity". The loss of reinforcement is evidenced by a reduction in cure rate, modulus tensile strength, abrasion resistance, treadwear and other physical properties. The degree of change will depend upon the severity of treatment. This, in a negative but practical way, demonstrates what is meant by "surface activity", which is that portion of reinforcement due to chemical linkages at the rubber/ carbon interface as opposed to morphological construction. This surface effect is what makes carbon black more than just a filler.

What are Carbon Black Agglomerates?

A fourth basic concept is agglomeration. For fundamental reasons, carbon black aggregates tend to coalesce with physical agitation. This propensity for association is the characteristic that allows formation of pellets. In Fig. 1 an agglomerate is also labelled. Agglomerates consist of loosely bound but not fused aggregates.

The degree with which agglomeration takes place directly impacts the ability of the end user to disperse a carbon black in his formulation, be it rubber or some product such as ink. Even with the high intensity ultrasonic agitation with which the sample of Fig. 1 was prepared, complete agglomerate disassociation apparently was not realized. With perhaps the single exception of conductive compounds, ultimate physical properties are achieved only with complete dispersion of agglomerates into their constituent aggregates. In the real world, however, this desirable condition is rarely achieved.

*see Fig. 1

What are Carbon Black Particles?

The average particle size of commercially available rubber type carbon black grades ranges from about 10 to 500 nanometers. Within this fifty to one spread exists the possibility of a very large number of grades, each providing the rubber formulation a unique set of properties. In general, the larger the particle size, the lower the cost and the poorer the reinforcement potential of the material.

Just as the average particle size is important, so also is the distribution of particle sizes. Distributions of particle size can be narrow or broad or even bimodal, each type influencing the rubber properties in it's own subtle way. To further confuse the picture, broad distributions may occur, for example, as narrow distributions within single aggregates. To date, convenient means of measuring all types of distribution do not exist; however, the manufacturer can exercise a degree of control, as suggested by the example just mentioned, through astute manipulation of various process factors and conditions.

Q What are Carbon Black Aggregates?

As is apparent in Fig. 1, individual carbon black aggregates occur in what appear to be random constructions of randomly sized particles, but when dealing with aggregates, enormous numbers are involved and it is the mean effect of all these individual entities that must be controlled.

The morphology of aggregates is just as complex as that of particles. Aggregate sizes can vary as can the distribution of those sizes. They can occur as semi-spherical groupings of particles or they can occur as groupings with a distinctly long dimension. Aggregates can be of a very dense, solid construction or of an open lattice-like configuration. This last parameter can be termed aggregate density and will have a direct bearing upon aggregate count per unit mass. Aggregate count can be envisioned as a primary factor in many of the observed changes in rubber compounds with changes in carbon black morphology. As the count per unit mass increases, aggregate interstitial spacing is reduced which affects the mobility of that portion of elastomer that bridges the space. Again, convenient methods for distinguishing between many of these aggregate characteristics have not as yet been devised.

In general, those properties of aggregates which are independent of particle size have been lumped into the term " structure". For a higher structure, these would include the propensity of aggregates to have a higher particle count and for particles to be joined into more chain-like clusters, having a narrow core. The cores can include random branching.



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