Influence of Lignin Features on Thermal Stability and Mechanical Properties of Natural Rubber Compounds

The aim of this work is to elucidate the relationship between lignin main features and its behavior in natural rubber compounds, in particular focusing on thermal stability and mechanical properties. Five lignins obtained from different sources and through different extraction processes were characterized in terms of purity, sulfur content, molecular weight distribution (GPC), and qualitative and quantitative functional group distribution (FTIR and 31P NMR). Then, the lignins were incorporated in natural rubber by two different approaches, namely coprecipitation and dry-mixing. Thermal stability and mechanical properties of lignin/natural rubber blends were investigated in both masterbatches and vulcanized compounds. The oxidation induction time (OIT) was used to determine the thermal stabilization of the lignin/NR masterbatchs, while tensile stress–strain properties of the compounds were evaluated after vulcanization. It was found that differences in the chemical and morphological characteristics of lignin influence its antioxidant and reinforcement capability. The addition of lignin to vulcanized compounds demonstrated the possibility to improve mechanical properties hypothetically through a tandem mechanism of protection and reinforcement.