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Zocher and von Fischer (S3), Kroeger (16), van Geel and Eymers (8), and others also substantiate the conception of a fiber structure. Chemical work by Staudinger (26) on molecular chains of rubber, and by Pummerer and collaborators 1

A literature review has been given dealing chiefly with the different factors that influence and regulate the dispersion of pigments in rubber, especially agglomeration and flocculation. Suggestions have been offered in regard to the validity of different physicochemical dispersion laws for the system rubber—pigment. The nature of agglomeration and flocculation of carbon black and zinc oxide has been described. Experimental evidence has been presented showing changes taking place in the dispersion of pigments during vulcanization. The distribution of carbon-black pigments in highly compounded treads has been discussed. The particles appear to assume a loose network formation which perhaps is an indication of the reënforcement of the system.

Many rubber technologists have already shown the importance of the dispersion of pigments in order to obtain the maxima physical properties of rubber mixtures. In a recent publication on the physical properties of gas black Carson and Sebrell state that they do not know of any article based on tests which deals with the relations between the dispersion of gas black and the properties of corresponding mixtures. Wiegand has already shown, in discussing mixtures highly loaded with gas black, that an incomplete dispersion of the pigments is no longer possible if the consistency of rubber falls below a definite value. He states that the lustre on the surface of a sample such as is used to determine tensile strength is a good method of estimating the degree of dispersion. Hauser upholds the idea that certain pigments attain a maximum dispersion during milling. In two preceding communications I studied the distribution of gas black in vulcanized and unvulcanized mixtures. I showed that changes in dispersion occur during milling as well as during vulcanization, and I discussed the theoretical possibility of obtaining the maximum dispersion and reënforcement. On the contrary, I am not concerned in these articles with the actual physical properties of the mixtures examined. In the present work, I wish to attempt to establish the relations between the dispersion of gas black and certain physical properties of rubber mixtures, whether vulcanized or not. The dispersion was determined by means of the microscope on freshly cut surfaces of mixtures vulcanized and unvulcanized, using a Leitz vertical illuminator and a Zeiss arc lamp as the source of light. Magnified about 300 times, the aggregates of gas black appear like a non-homogeneous black mass, while on the smoother and more homogeneous surface of the rubber the reflection is so increased that the field remains lighted.