Mixing of Carbon Black with Rubber. II. Mechanism of Carbon Black Incorporation

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Abstract
The present study leads to postulating the following mechanism for carbon black incorporation. During the first stage of incorporation, carbon black agglomerates become encapsulated by the polymer. At this stage, the interstices within these agglomerates are still filled with air, giving a very weak, crumbly composite. The rubber becomes forced into these interstices during the second stage of incorporation. As the rubber is being forced through the narrow channels between the aggregates, bound rubber is being formed. The immobilized layer of the polymer tends to reduce the effective cross section of the channels through which more rubber must pass before reaching the inner part of the agglomerates. Thus, as the activity of carbon black increases, e.g., higher bound rubber, the incorporation time increases. At a given carbon black activity, the effective thickness of the immobilized polymer layer would increase with increasing molecular weight of polymer, but be independent of the oil loading. This agrees with experimental observation that the incorporation time increases with molecular weight of the polymer, while oil loading has no effect other than that associated with the lowering of bound rubber content. The average size of channels through which rubber must pass depends also on the carbon black morphology. The size of these channels would be expected to increase with increasing structure of carbon black, and (to a lesser extent) with increasing particle size. These parameters have the expected influence on the incorporation time; since the incorporation time decreases with increasing DBPA value and decreasing tint.

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