Performance of a multifunctional additive as a coupling agent for silica filled natural rubber compounds

Abstract

Suitability of a multifunctional additive (MFA), a widely available cheap natural polymer belonging to the group polysaccharides, as a coupling agent for silica filled natural rubber (NR) compounds was investigated using a solid tyre compound. The test compounds were prepared by replacing currently used silane coupling agent, Si-69, with the MFA in different proportions ranging from 10 – 50% by weight. A compound containing only Si-69 as the coupling agent was also prepared to use as the control. Rheological properties of test compounds were assessed by using a Oscillating Disc Rheometer (ODR 100) at 1500C. The effectiveness of the MFA as a coupling agent for silica in NR compounds was evaluated through investigation of mechanical properties such as tensile and tear strength of silica filled NR vulcanisates containing MFA.  
Rheological properties of the above compounds indicate that there is a marked influence of MFA on the cure time,t90,  cure rate index(CRI), scorch time (ts2) and maximum torque (MH) of the rubber compounds. The compound became quite scorchy as the 50% of Si-69 was replaced by MFA and its cure time decreased by about 6 minutes. MH was increased by about 4 units resulting in an increase in CRI. 
Evaluation of technical properties of the vulcanisates of tread compounds reveals that, about 10-20% of Si-69 can be replaced with the MFA without adversely affecting mechanical properties such as tensile strength, tear strength and modulus. Aging properties vucanisates having 10% of Si-69 replaced with MFA were within acceptable regions though that tends to decline with increasing the MFA content.
It appears that the added MFA is capable of functioning as a coupling agent for silica filled compounds but this coupling action gets masked with increasing its concentration in the rubber compound. This may be a consequence of cure acceleration effect imposed on the rubber compounds by the MFA as revealed by rheological properties.