Control and optimisation of rubber mixing in an internal mixer

Some areas of current relevance to the rubber mixing industry have been explored with a view to maximizing productivity of mixing plant in conjunction with improvement in uniformity of mixea compound properties.

Simulation of batch temperature profile is adopted for scaling between BR and 3D Banbury mixers. The emerging procedures, based on work with three natural rubber compounds, gave rise to rubber from the laboratory mixer more ciosely resembling that from the production process, and hence reduced development costs.

Three dynamic test methods were also evaluated, showing the Monsanto MDR curemeter to give a good indication of dynamic performance.

A theoretical model for real-time control of the internal mixing process proposes mixing to specific torque targets for the mastication and carbon black incorporation stages of the mixing cycle, and to a heat history target for the latter curative distribution stage. Investigations of these two concepts were undertaken with a BR Banbury mixer. A number of uncertainties have prevented the reduction of mixer torque from following material viscosity with the level of precision required for torque control to present advantage over mixing to time. Heat history control was however found to be a viable technique for augmenting uniformity in scorch/cure characteristics of mixed rubber compound. This also demonstrates scope for mixing curatives at temperatures approaching 150°C, showing only a small increase in batch-to- batch variation compared to production mixing to time at just over 100°C.

Consequently heat history control has been applied to a production scale 3D Banbury mixer resulting in a reduction of batch-to-batch variation in scorch and cure times from 3.0% to 1.7% coefficient of variation.