4.1 For the broad range of commercial rubber grade carbon blacks, tint strength is highly dependent upon particle size. Tint strength can be used as an indication of particle size; however, tint strength is also dependent on structure and aggregate size distribution. Therefore, differences in tint strength within grades of carbon black may reflect differences other than particle size.

Note 1: This test method was developed primarily for the characterization of N100, N200, and N300 series carbon blacks.

4.2 Tint strength values within the carbon black industry have been developed using a Automatic Muller apparatus which is used to prepare carbon black-zinc oxide pastes. A new mixing apparatus, SpeedMixer4 (DAC 150 FVZ), and a corresponding procedure have been extensively studied within D24 and shown to provide equivalent tint strength for all carcass or soft blacks and most tread blacks with the exception of higher surface area N100 types and specialty blacks. Therefore, it is the responsibility of the user of this alternate apparatus to ensure their products will adequately disperse. Disputes arising between a user and producer should be resolved using the Automatic Muller apparatus until ASTM develops adequate precision statements.

4.3 The term ITRB is used in the entire text for both, the original ITRB, used as the first reference material for tint testing, but which is now used up, and the successor reference material, ITRB2. Wherever required, like in calculations where it is crucial to differentiate between the original ITRB and ITRB2, this will be clearly mentioned in the text of the test procedure.