The Third Dimension of Carbon Black Structure

Structure is probably the most difficult characteristic of carbon black to define. Electron microscopy has shown that the individual particles of a black are fused together into a variety of shapes, collectively defined as the structure. Here, particle is used to refer to that entity which is somewhat spherical in shape and is the smallest “building block” of structure. These particles have also been called “domains” of rotational graphitic layers and “nodules”. The larger unit resulting from the fusion of these particles forms an “aggregate” or a “primary structure” unit. A collection of aggregates, held together by some other force (e.g , van der Waals force), is called an “agglomerate” or “secondary structure”. With dry black, it is difficult to completely eliminate all secondry structure. However, main emphasis in this study was on primary structure, because it has the greater influence on rubber properties and because secondary structure is greatly reduced in good rubber mixing. Many recent characterization studies have been concerned with making direct measurements on primary structure units (aggregates) by automated image analysis techniques, a pencil following device, and a comparative chart method. In the work described here, a different approach has been taken to viewing carbon black primary structure units, an approach using the scanning electron microscope combined with stereoscopic methods. This initial report will be mainly concerned with a description of the techniques and results on several individual primary structure units. It is suggested that the efforts described here provide a realistic view of carbon black primary structure and that a third dimension term is essential for primary structure characterization.