The optimal mixing conditions for silica-filled NR compounds dictate the need to proceed at a high temperature, i.e., 150 °C, to achieve a sufficient degree of silanization. On the other hand, natural rubber is prone to degradation due to mechanical shear and thermal effects during mixing, particularly at long exposure times. The present work investigates NR rubber degradation during mixing in relation to prolonged silanization times. The Mooney viscosity and stress relaxation rates, bound rubber content, storage modulus (G’), and delta δ were investigated to indicate the changes in the elastic/viscous responses of NR molecules related to rubber degradation, molecular chain modifications, and premature crosslinking/interaction. In Gum NR (unfilled), an increase in the viscous response with increasing mixing times indicates a major chain scission that causes a decreased molecular weight and risen chain mobility. For silica-filled NR, an initial decrease in the Mooney viscosity with increasing silanization time is attributed to the chain scission first, but thereafter the effect of the degradation is counterbalanced by a sufficient silanization/coupling reaction which leads to leveling off of the viscous response. Finally, the higher viscous response due to degradation leads to the deterioration of the mechanical properties and rolling resistance performance of tire treads made from such silica-filled NR, particularly when the silanization time exceeds 495 s.

Mixing silica-reinforced rubber for tire tread compounds involves high shear forces and temperatures to obtain a sufficient degree of silanization. Natural Rubber (NR) is sensitive to mastication and chemical reactions, and thus, silica–NR mixing encounters both mechanical and thermal degradation. The present work investigates the degradation phenomena during the mixing of silica-reinforced NR compounds in-depth. The Mooney stress relaxation rates, the dynamic properties with frequency sweep, a novel characterization of branch formation on NR using Δδ values acc. Booij and van Gurp-Palmen plots, together, indicate two major competitive reactions taking place: chain scission or degradation and preliminary cross-linking or branch formation. For masticated pure NR and gum compounds, the viscous responses increase, and the changes in all parameters indicate the dominance of chain scission with increasing dump temperature. It causes molecular weight decrease, broader molecular weight distribution, and branched structures. Different behavior is observed for silica-filled NR compounds in which both physical and chemical cross-links are promoted by silanization and coupling reactions. At high dump temperatures above 150 °C, the results indicate a significant increase in branching due to preliminary cross-linking. These molecular chain modifications that cause network heterogeneity deteriorate the mechanical properties of resulting vulcanizates.

Various commercial silica‐silane coupling agent combinations are used these days for synthetic rubber‐based passenger car tire treads with high silica‐loadings. For truck tire treads, much lower silica‐loadings are employed in combination with Natural Rubber (NR). The present study examines the impact of various commercial silane coupling agents on degradation phenomena during mixing of silica‐filled NR compounds by analyzing changes in viscoelastic response, with a focus on different silane microstructures. A novel approach using delta–delta (Δδ) values to quantify branch formation caused by degradation‐induced chain modifications has been adopted. Unfilled NR compounds primarily undergo polymer chain scission from thermo‐mechanical and thermo‐oxidative influences. During mixing, bis‐(3‐TriEthoxySilylPropyl) Tetrasulfide (TESPT) silane creates silica‐rubber coupling and lightly crosslinked materials partly based on polysulfidic bonds. The lightly crosslinked rubber essentially sustains the reinforcement level in the compounds, counterbalancing the degradation effect. However, predominant degradation at 180°C significantly deteriorates the mechanical and dynamic properties of compounds containing both TESPT and TESPD (bis‐(3‐Triethoxysilylpropyl) disulfide). OTPTES (3‐OctanoylThioPropylTriEthoxySilane) and MTCO (Mercapto‐ThioCarboxylate Oligomer) silane‐based compounds exhibit notable plasticization effects along with degradation. The choice of OTPTES in particular demonstrates excellent thermal stability, preserving mechanical properties even till 180°C. MTCO, with one reactive mercaptan group, couples efficiently with rubber at low dump temperatures, 130°C and 150°C, enhancing vulcanizate properties. At high dump temperatures of 170°C and 180°C, though, MTCO releases plasticizing moieties during mixing, leading to inferior final properties. The present study highlights the intricate balance between rubber degradation, crosslinking, branching, network formation, rubber‐filler interactions, and plasticization effects specifically in silica‐filled NR compounds for truck tire tread applications.