Oil furnace process carbon blacks have been produced which differ widely in the extent of their primary chainlike aggregation. The effect of the carbon black structure on rubber properties and road wear was evaluated in oil-extended SBR passenger tire tread compounds. The decrease in primary particle aggregation increases the tendency to form strong, dense, secondary agglomerates which require mixing techniques designed to insure that they are well dispersed. The behavior in rubber of a series of oil furnace blacks differing primarily in their structure is most apparent in processing, shrinkage, modulus and hardness. There are small effects on rebound and flex cracking resistance but no significant effects on tensile strength and heat buildup. Carbon black structure can be reduced to some extent without encountering significant loss in road wear resistance under normal wearing conditions. Some decrease in road wear performance occurred under very severe road test conditions. The use of higher black loadings of the normal structure blacks gave good wear resistance under both normal and severe service conditions.
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.
The observed existence of a linear log-log relationship between nonskid loss and mileage in a great number of widely different types of tests may be exploited in order to realize substantial savings in the road testing of tires for tread wear. In many evaluations it is common practice for the Gates Rubber Co. to test tires to one half initial nonskid depth or to some arbitrary mileage only. The regression line and mileage to baldness are then determined. Of course, the greater the amount of data, and the more accurately controlled the conditions of testing, the more reliable will be the mileage to baldness. If in tread wear evaluations tests were terminated prior to baldness, about $180 could be saved for each passenger tire tested, and $490 for each truck tire, for every 10,000 miles. One investigation, to determine the effect of three different tire designs on tread wear, was made at a total cost of $2200. If all tires had been tested to baldness, the cost would have been approximately $4600. In comparison with former common practice, the application of the proposed method of analysis of tire wear measurements can result in more reliable tread wear evaluations, more economical utilization of the tire test fleet, and a more rapid turnover of ideas.
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