Influence of prolonged silanization time during mixing of silica-reinforced natural rubber

To ensure a satisfactory silanization reaction, the mixing condition of silica-filled NR compound is obligatorily operated at high temperatures i.e., 150 °C in which degradation of natural rubber molecules occurs as observed in our previous work. Besides the mixing temperature, the duration of the silanization is an additional factor that contributes to the degradation. Therefore, this work aims to investigate rubber degradation during mixing due to prolonged silanization time. Silica-filled NR compounds are prepared at various silanization times using gum NR as reference. To monitor the degradation, dynamic properties of uncured samples are characterized to investigate chain scission via changed viscous behavior and preliminary crosslinking/interaction via elasticity change. In gum NR, rising viscous responses with longer mixing time are observed as an incline in delta-delta (Δδ). This indicates chain scission resulting in shorter molecular chains and higher chain mobility. In case of silica-filled NR, increasing viscous behavior is observed only at certain increasing silanization times due to a combination of increased polymer chain scission from degradation and decreased filler-filler interaction from silanization. Thereafter, leveling off of viscous responses are noticed which is attributed to the overshadowing effect of degradation by a sufficient silanization/coupling reaction. Furthermore, the mechanical properties of the vulcanizates are characterized. In gum NR, increasing mixing times lead to deteriorations of mechanical properties which is due to chain scission or degradation. Besides, changes of the mechanical properties with prolonged silanization times are less in filled NR which is attributed to competitive reactions between degradation and silanization. Inferior tensile strength is observed only for prolonged silanization times to over 500 secs.