Influence of antioxidants on the silanization reaction during mixing of silica-filled NR compounds

Commonly, in tire tread formulations, antioxidants like TMQ and/or 6PPD are added in order to reduce rubber degradation caused by oxidation and aging throughout their service life; especially in silica-filled natural rubber compounding where high mixing temperatures are needed to ensure the optimal silanization reaction between a silane coupling agent and the filler. The chemical structure of the antioxidants is generally composed of an amine which possibly promotes or otherwise interferes with the silica-silane coupling reaction.
The present work studied the influence of TMQ and/or 6PPD in silica-filled NR compounds on the degree of silanization and degradation, compared to a control compound without antioxidants. Model compound studies are also conducted using formulations derived from the practical compounds. In the model compounds, released ethanol is used to monitor the degree of silanization.
Adding TMQ alone to a compound shows a small decrease in released ethanol indicating a reduction in silanization. The interference of TMQ with the silanization is confirmed in practical compounds, in which those with TMQ show a higher Mooney viscosity and a larger Payne effect indicating high filler-filler interaction. On the other hand, using TMQ and/or 6PPD shows a benefit in decreased rubber degradation, as seen in increases in the elastic behavior and rubber-rubber and rubber-filler interactions in the compounds, evident from Mooney stress relaxation, storage modulus at 100% strain, and bound rubber content analyses. Finally, higher mechanical properties of vulcanizates while using TMQ/6PPD prove the major role of antioxidants in suppressing rubber degradation also during mixing.