Determining the components of mixing energy when preparing rubber compounds in instrumented internal mixers

An experimental procedure has been developed to assess the various components of the mixing energy when preparing carbon black filled rubber compounds. A microprocessor based instrument developed for laboratory mixers allows the measurement and recording of all the relevant parameters of the operation, which are stored on diskette, loaded in a Lotus 123 spreadsheet, and further processed with the appropriate macro‐programs. Using compounds based either on ethylene‐propylene rubber, EPDM, or natural rubber, NR, and constant level of different carbon blacks, experiments were made using single pass mixing procedures, typical of factory processes. The position of the ram during the process has been identified as an important parameter of the process, and a method to correct the actual mixing energy by the effective batch volume has consequently been developed. With this correction and the data treatment described, it is possible to split the overall mixing energy into its various components, i.e., the energy to masticate the rubber alone, the energy to fragment and disperse the filler, and the extra mastication energy due to the presence of the filler. The energy to fragment and disperse various types of carbon black is dependent on the structure and the particle size of the filler considered. Depending on the compounding formulation, 65 to 80% of the total mixing energy is associated with the mastication of the pure gum, 15 to 20% accounts for the extra mastication energy due to the presence of the filler, and 5 to 15% is needed to fragment and disperse the filler.