Recent progress in understanding rubber processing through a rheological approach

Abstract

The rheology of rubber compounds is a very complex subject because in addition to the particular viscoelastic behaviour of the polymer, strong interactions take place between the fillers and the elastomer. The exact role of these interactions in affecting the flow behaviour of compounds is still largely unknown but progress has been made recently in firstly recognizing their importance and secondly in designing suitable experimental approaches to take them into account.

Rheological studies on rubbers find their justification as far as they allow a better understanding of rubber processing to be gained and hence an improved control of the operation, ultimately leading to quality improvements, lower scrap levels and a more efficient use of energy. With respect to these goals, it can be said that key progress has been made in the rheology of mixing. Elongational flow fields, while transient, are essential in dispersive mixing mechanisms and accordingly more attention should be given to the behaviour of rubber compounds in the earlier period of extensional deformation. In addition, it is now clear that within the internal mixer, spiralling flow fields are involving most of the displaced mass of material and it can be suspected that such flow situations are more efficient in breaking and dispersing filler agglomerates than any other type of flow.

In rheological terms, the extrusion of rubber compounds appears to be largely different from the extrusion of thermoplastic materials. Firstly the non-slippage assumption is surely not valid at all for (carbon black) filled rubber compounds, which brings additional concerns, particularly when short dies are used, as in factory conditions. Furthermore, the flow of rubber compounds through convergences is characterized by the absence of recirculating vortices, which in fact seems a paradox, since vortices have generally been associated with a high elastic character of the fluid. It may be hypothesized that the presence of elastic fillers allow elastic energy consumption processes to control the flow situation in convergent geometries. A clearer understanding of these peculiar aspects of rubber extrusion would certainly lead to a better control of the process.