ULTRA SMALL-ANGLE X-RAY SCATTERING STUDY OF FLOCCULATION IN SILICA-FILLED RUBBER

The flocculation of silica during vulcanization is monitored using the ultra small-angle X-ray scattering technique for two different types of silica: a highly dispersible silica (HD) and a conventional silica (CV), mixed into a blend of S-SBR and BR rubbers. The cutoff length of the silica aggregate R-ss and the mass fractal dimension D-m, which indicate the degree of flocculation of aggregates, are estimated according to the modified unified equation. The aggregate radius R-a is estimated to be related to the lower cutoff length R-ss, indicating the radius of gyration of the mass-fractal structure. For both silicas, R-a increases during vulcanization. For the CV silica, an increase of D-m is observed, whereas no significant increase of D-m can be seen for the HD silica. The R-a of CV is relatively high compared with that of HD. On the other hand, the CV silica shows a relatively lower D-m compared with that of HD. These results indicate that CV has a larger size of aggregates and lower degree of agglomeration of its aggregates. The presence of di(tri-ethoxy-silyl-propyl)tetrasulfide (TESPT) as coupling agent between the silica and rubber decreases the aggregate radius of silica. However, in the absence of TESPT, a low mass-fractal dimension, which means a low degree of agglomeration of aggregates, is observed. This results from a lower mobility of silica aggregates, depending on the size of the aggregates. The silica loading also has an influence on the flocculation process. The aggregate radius increases as the silica loading is increased. At the same time, a higher mass-fractal dimension, and therefore also a higher degree of agglomeration, can be seen at higher silica loading.