Suppressed degradation by stabilizers during mixing of silica/silane-filled natural rubber

Aging antioxidants and processing stabilizers, i.e., quinoline (TMQ) and phenylenediamine (6PPD) derivatives, a radical chain terminator (TEMPO), a phenylphosphite (TBPP) and a hydroxyphenylpropionate (PTBHPP) derivative, were considered potential degradation suppressors during mixing for silica-reinforced Natural Rubber (NR) compounds. Thus, the influences of these stabilizers on rubber degradation during mixing were investigated. Results show that TMQ and 6PPD give higher viscosities and larger elastic responses compared to the reference compound, attributed to their ability to suppress rubber degradation during compounding leading to maintained molecular weight. Additionally, these two antioxidants enhance filler-rubber interaction due to a boosting effect on the reaction between silane and rubber. TEMPO appears to terminate chain radicals of rubber during mixing, leading to permanent breakdown of the chains and so lower molecular weight. TBPP does not work in this system as its melting point is higher than the mixing temperature. Results from a model compound study suggest that TMQ slightly reduces the silanization efficiency in the compounds, while other stabilizers do not intervene. However, this interference does not give an adverse effect to the final properties. A practical silica-filled NR compound containing a blend of TMQ/6PPD shows improved mechanical properties of the vulcanizates, highlighting the essential role of these antioxidants added during mixing to suppress rubber degradation. PTBHPP does not noticeably help in preventing rubber degradation during mixing, but it provides a significant positive effect on long term aging stability.