ULTRA SMALL-Angle X-RAY Scattering STUDY of Flocculation In Silica-FILLED RUBBER

Filler dispersion plays an important role for the physical properties of rubber. Silica-reinforced rubber compounds tend to flocculation/demixing during the heating period of the vulcanization process, which negatively influences the properties. This flocculation was studied using ultra small angle X-ray scattering (USAXS).

The radius of the aggregates R_a and their mass-fractal dimension D_m, which indicates the degree of aggregation and agglomeration, were estimated for the highly-dispersible (HD) silica Zeosil 1165MP and the conventional (CV) silica Ultrasil VN3 in a 80 phr reinforced tire tread compound. The aggregate radius of the CV silica is relatively high and increases faster compared to HD silica as vulcanization proceeds. This translates into a flocculation rate of CV silica which is relatively fast compared to that of HD silica.

For the CV-type silica, the mass-fractal dimension is relatively low but increases during vulcanization, whereas the HD silica shows no significant change of D_m. The form-factor profile R_ss^-1 of the CV silica is higher than that of the HD silica, indicating that the CV silica has larger aggregates and shows less agglomeration. For the compound with highly dispersible silica, two different slopes at high and low scattering angle q are visible, characteristic for mass-fractal structures and primary particles with their surface fractal dimension. The power-law value p at high scattering angles does show a value of 3.3, also indicating that the surface of the particles is not smooth.  

The results of this study confirm that silica flocculation predominately is a physical phenomenon based on the diffusion mobility of the particles.