Abstract
The particle size of the pigment may be expressed in terms of several average diameters, depending on the method of measurement. In this work three of these averages were considered; namely, d1, which is the arithmetical mean, d3, which is the average diameter from which specific surface is calculated, and D, which is the average diameter from which the number of particles per gram is calculated. A series of samples was prepared and these three average diameters were determined These samples were then compounded with rubber and several physical properties measured. As a result of these tests it is shown that, in general, the physical properties vary directly as the specific surface and the number of particles per gram. However, if the number of particles per gram is maintained constant and the specific surface is varied, it was found that the physical properties vary directly as the specific surface. No such relationship was found when the specific surface was maintained constant and number of particles per gram varied. It is further pointed out that the only satisfactory means of measuring the d3 diameter, and therefore the specific surface, is by some microscopic method which determines the size-distribution curve for the pigment.