Key factors influencing marching modulus in silica-silane compounds

Silica-silane became a very successful filler system for passenger car tire treads, as it considerably improves tire performance, mainly in terms of rolling resistance. However, this technology has its drawbacks in terms of processing: longer mixing cycles, 2 or 3 mixing stages, as well as influences on the curing behavior of the compound. One specific problem related to the latter is a marching modulus, which makes determination of the correct curing time difficult. As a consequence, the properties of silica compounds might be sub-optimal. Earlier studies describe various causes of marching modulus in silica compounds: filler-filler interaction [1], degree of silanization [2], filler-polymer and polymer-polymer coupling as well as sulfur donated by the silane [3,4]. These are also key factors in order to suppress marching behavior. In this study, filler-filler and filler-polymer interactions are varied and studied in depth on their influence on marching modulus and interference with other properties and processing parameters. Several types of silane coupling agents are used in order to vary the molecular structure of the filler-filler and filler-polymer interactions, and different silanization temperatures are applied to vary the degree of silanisation. These variations are correlated to the Marching Modulus Intensity (MMI) as well as bound rubber content and physical properties. In order to differentiate the influence of filler-filler and filler-polymer interaction on MMI of the silica-filled rubber compounds, the curing behavior was monitored in a rheometer at different strains. The results of this study clearly show, that both factors - filler-filler as well as filler-polymer interactions – are the main influencing parameters for the marching modulus, and are on their turn related to the degree silanization. A proper choice of compounding and processing parameters allows to suppress the marching modulus.