The development of a technique for mixing rubber compounds in an internal mixer to a rheological setpoint

The research evaluated the means in which mild variations in the polymer properties could be modified during the mixing process so that the final mixed compound met the desired rheological parameters. The initial work developed a model to measure the viscosity of the material in the internal mixer during the mixing process. The model assumed the mixer could be treated as two concentric rotary viscometers. The model had a rotor speed dependence which was caused by the assumption not being valid. However, this led to the work continuing in a modified form. It was felt that at a given rotor speed and temperature the torque measured on the rotors would be proportional to the viscosity of the material. This was found to be correct and therefore work continued using this principle. Programs were developed for internal mixers with variable speed rotors and fixed speed rotors. The programs were evaluated using natural rubber with different viscosities and were found to be capable of modifying the polymers to obtain a final mixed compound to a target viscosity. The control program was also used to mix a series of batches at different processing conditions. The rheological properties of these batches were compared to a similar group mixed to a specification based on unit work. The result was the control system gave more rheologically uniform material.