The role of rubber rheology in tire tread extrusion – A review
Ch. Hopmann, A. Limper, S. Kammer, M. Schön, M. Drach and M. Facklam
purpose of this review paper is to demonstrate the importance of rheology in rubber processing. Using the economically significant example of the extrusion of treads for vehicle tires, typical challenges in the processing of rubber are presented and their rheological origins are examined. The structure-property relationships between the components of a rubber compound and the rheological properties are considered in order to understand the cause-and-effect relationships. The most important mathematical models are presented to describe the complex flow behaviour of rubbers. In this context, shear and elongation viscosity, viscoelasticity, time-temperature superposition, wall slip and crosslinking kinetics are discussed. Important methods for characterizing the rheological properties are then discussed in the context of the rheological phenomena relevant for tread extrusion. Particular attention is paid to phenomenological rheometry in laboratory extrusion, high pressure capillary rheometry, oscillation rheometry and elongation rheometry. Finally, it is summarised what the causes of the typical processing problems are, with which parameters these can be described and with which methods these can be characterised. It is shown, that the typical processing problems, such as die swell and melt fracture, cannot be explained by the frequently used shear viscosity alone. Rather, the normal stresses occurring under the complex stress conditions of the viscoelastic rubber must be taken into account to explain these phenomena.
Keywords: die swell, elongation viscosity, extrusion, melt fracture, rheology, rheometry, rubber, shear viscosity, tire tread, viscoelasticity