Organic radicals stabilization in natural rubber: Discerning the influence of thermo-oxidation using chemically modified or unmodified lignin as antioxidant.

• Lignin inserted by co-precipitation in natural rubber latex. • Improvement of compatibility between lignin and natural rubber through in-situ silylation. • Effect of lignin content in natural rubber and property retention after thermo-oxidative aging. • Enhancement of natural rubber resistance to thermo-oxidative aging determined by oxidation induction temperature. • Reduction of radicals produced by thermo-oxidation in natural rubber/lignin compounds. We have implemented a lignin compatibilization strategy through silylation and elaborated NR compounds with lignin and silylated lignin. Then, the lignin's contribution to the preservation of physico-mechanical properties and stability of the materials obtained against thermo-oxidation was studied. Thus, our work considered the indirect study of the antioxidant activity on NR compounds by determining the physico-mechanical properties before and after accelerated thermo-oxidative aging, which allowed us to determine that the increase in crosslinking density during the initial hours of accelerated aging was the main observed cause associated with a higher tensile strength retention rate (TSRR: 13.4 %) in NR compound with 5 phr of lignin (CL5) versus a TSRR: 4.7 % for the compound with 5 phr of silylated lignin (CL5Si1). On the other hand, the direct study of the antioxidant capacity by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using oxygen atmosphere indicated greater thermal stability in NR compounds when lignin is not silylated. The maximum degradation temperature for the CL5 compound was 341.6 °C, while for the CL5Si1 compound it was 328.6 °C. The oxidation induction temperature (OITtemp) in the CL5 compound was 190.9 °C with a peak at 221.0 °C, while for CL5Si1 it was 170.3 °C with a peak at 194.1 °C, indicating again that unsilylated lignin is more suitable as an antioxidant. Finally, through Electron Paramagnetic Resonance (EPR), we determined that the radical quenching effect due to the presence of lignin was significant as the lignin content increases, with a reduction in radical presence close to 50 % compared to a reference compound without lignin. [ABSTRACT FROM AUTHOR]