Dispersion of Carbon Black in Rubber: Revised Calculation Procedure

Carbon black, the most important reinforcing filler for rubber, must be well dispersed if it is to function effectively. The state of dispersion of the carbon black can be studied effectively by microscopic examination of a thin (2µ) section of the rubber compound, using transmitted light, as described by Tidmus and Parkinson. Under the microscope, agglomerates of undispersed carbon black are visible. These agglomerates, which may range in size up to 100µ or even larger, are sectioned more or less cleanly by the glass knife, and appear as black or dark brown islands in a light brown field. While this method was originally proposed for qualitative estimation of the degree of dispersion, a quantitative calculation was subsequently developed by Leigh-Dugmore, by means of which the percentage of carbon black dispersed below a certain size was calculated. This calculation has been helpful in permitting a quantitative comparison of stocks of widely different visual appearance. As experience has accumulated with the Leigh-Dugmore method, some puzzling observations have been made. Frequently, the modulus level of a stock of a given dispersion was found to be higher than the proportionate modulus of a stock of 100% dispersion. Furthermore, stocks have occasionally been prepared with a measured dispersion rating of less than zero (see below). It thus seemed worthwhile to re-examine the Leigh-Dugmore calculation.