A Multimodal Experimental Assessment of Thermo‐Oxidative Aging in Tyre‐Grade Rubber Mixes

This work investigates the thermo‐oxidative aging of three anonymized industrial natural‐rubber‐based tyre mixes subjected to circulating air at 75°C for up to 21 days. Uniaxial tensile testing, dynamic mechanical analysis (DMA), oxygen elemental analysis using a CHNSO elemental analyzer and through‐thickness nanoindentation were combined within a common aging protocol to compare bulk mechanical, viscoelastic, chemical, and spatially resolved aging signatures. Tensile testing showed that aging reduced rupture strain much more strongly than rupture stress, identifying loss of extensibility as the more sensitive tensile aging signature. DMA revealed formulation‐dependent changes in stiffness, dissipation and frequency sensitivity, while CHNSO analysis showed progressive bulk oxygen uptake. Nanoindentation showed substantial aging‐induced stiffening together with the development of surface‐to‐core mechanical heterogeneity. Importantly, within the present experimental dataset, the different aging descriptors did not provide identical formulation rankings. Mix C showed the greatest increase in average indentation reduced modulus, whereas Mix A developed the largest absolute surface‐to‐core modulus contrast. Bulk oxygen uptake and spatial stiffness heterogeneity therefore provide complementary rather than interchangeable information about thermo‐oxidative aging. Overall, the results show that a single bulk property can obscure important differences in aging response and support a multimodal assessment based on extensibility, viscoelastic drift, oxygen uptake, and spatial stiffness heterogeneity.