The properties needed for aircraft tire treads are significantly different from the ones required for passenger car or truck tires, for which improvements mainly focus on a better balance of rolling, wet skid and wear resistance. Aircraft tires experience severe operation conditions during service: The temperature can reach above the critical temperature of polymers, thus accelerating tread wear. To reduce the peak temperatures of the tire tread during departure and landing, the material of the tread should have a low hysteresis. Moreover, the tires need to be able to withstand high loads and impact during landing. Therefore they also require outstanding strength properties and reversion resistance.
This paper focuses on the influence of the processing sequence of ZnO and DPG on the properties mentioned above. It is based on a blend of natural and butadiene rubber reinforced with hybrid carbon black-silica fillers. From this study, we found that the ZnO addition sequence has a strong influence on the properties of rubber compounds, while the DPG addition sequence has almost no effect. The incorporation of ZnO in the first stage of mixing results in low Mooney viscosity, better silica dispersion, slow cure rate but high reversion resistance and high mechanical properties. When ZnO was omitted during the first stage of mixing and added in the last stage of mixing, it resulted in high filler-polymer interaction, which is beneficial for high stress-strain properties. However, the latter compounds were prone to reversion under prolonged vulcanization leading to a significant reduction of the modulus at 300% elongation and crosslink density. Finally, the addition sequence of ZnO and DPG does not influence the hysteresis.
The energy loss (hysteresis) of the compounds is influenced by the filler-filler and filler-polymer interaction as well as crosslink density. These properties vary with mixing procedure; i.e., adding ZnO in the first stage of mixing shows low filler-filler interaction but also low filler-polymer interaction, while adding ZnO in the last stage of mixing results in high Payne effect but also high filler-polymer interaction. In case of the degree of crosslinking, all compounds exhibit a similar level.