FROM recent x-ray diffraction data (2), it is possible to calculate a limiting density value for a few commercial carbon blacks. The values thus obtained are considerably higher than those generally accepted (4). The experimental values have, for the most part, been determined

Recent publications (•>, 11, 13) concerning tic• behavior of carbon black in GR-S compounds de-S'-ribed a group of carbons as "structure carbons". These blacks arc characterized by their ability to impart smoothness to uncured stocks at normal tread stock loading, and high modulus to i•ured compounds. Such properties were attributed to structure-forming tendencies -that is, to the ability of the carbon particles to maintain a chainlike grouping (illustrated in Figure 1 as a secondary aggregate) when dispersed in rubber media, as contrasted to nonstructure blacks, which tend to disperse as single discreet particles. The structure of the primary particle was thoroughly defined by x-ray diffraction studies (.(). No theoretical interpretation has been offered to describe rigidly the contribution of structure to modulus; it was found necessary to introduce a particle-shape factor to account, for modulus characteristics of carbon blacks (,v).Shawinigan acetylene black is a commercial carbon black exhibiting high modulus which has been attributed to a higli structure tendency. However, it seems reasonable to assume that structure is not a property unique to any single type of carbon black but is possessed to some degree by all blacks. Modulus properties of a black may be taken as some indication of the extent of structure.

It was suggested that carbon blacks owe some of their characteristic properties to a tendency for individual particles to cluster into chainlike groupings or secondary aggregates. Results of mechanical processing experiments show that application of mechanical work altered or destroyed secondary aggregates. This conclusion was made since resistance to compression had been permanently overcome. Analytical measurements confirmed the supposition that secondary aggregates had been altered or destroyed, since oil absorption and liquid retention values markedly decreased with mechanical work. Finally, properties of compounded GR-S stocks were seriously altered—namely, smoothness of milled stock, high modulus, and enhanced electrical conductivity. It is concluded that these properties were originally caused by the presence of secondary aggregates or structure units. Alternative interpretations of these data are possible—namely, it might be concluded that it was not possible to disperse the densified material to the same extent as the original carbon black. However, this hypothesis contends that a large particle size material is being dispersed. Therefore, a decrease in tensile strength should have been noted. There is also some meager evidence suggesting that the chemical nature or activity of the carbon black surface was altered, so that its contribution to modulus was limited. It is believed, however, that most of the evidence offered by the data presented here preponderantly supports the structure theory of high modulus blacks. The authors consider these results as preliminary in the study of the structure properties of carbon blacks. They appear to be of sufficient interest to be brought to the attention of those studying reinforcement. Data available at present do not permit an interpretation of the contribution of structure to properties of “smooth-out” modulus and electrical conductivity of compounded rubber stocks.

Carbon blacks are composed of spherical particles which are to varying degrees arranged in chainlike structures. This type of particle association, which is readily seen in electron photomicrographs of most carbon blacks, can be termed “primary structure”. The use of the term “structure” to describe interparticle association must not be confused with the basic intraparticle structure of an atomic crystallographic nature. There is strong evidence that primary structure units, and possibly individual particles, can further associate or flocculate in fluid or elastomeric systems. This is a secondary type of structure formation which can be readily disrupted under the influence of mechanical strain. Some investigators have used the term “structure” to describe this strain-sensitive flocculation behavior. It is suggested here that carbon blacks possess both primary structure features and the ability to form secondary structures by flocculation in dispersed systems. The tendency to form secondary structures is probably greater with carbon blacks possessing a high degree of primary structure. Unless otherwise specified, the term “structure” in this paper will be used in the sense of primary structure. The structure of carbon blacks is thought to originate in the flame by the agglomeration of growing carbon nuclei and particles. The appearance of electron micrographs of carbon blacks lends some support to the assumption of simultaneous agglomeration and growth processes. Carbon blacks having a broad particle size distribution are characterized by carbon black chains, where each chain is composed of particles of the same size, rather than a randomized distribution of various sized carbon black particles. Thus, these chainlike structures must result from the continued growth of agglomerates formed from neighboring carbon particles at the same stage of their growth history. This process results in a chemical fusing of these particles as layers of new carbon are deposited on the surfaces of actively growing agglomerates.