Antioxidant and Anti–Aging Activity of Freeze–Dried Alcohol–Water Extracts from Common Nettle (Urtica dioica L.) and Peppermint (Mentha piperita L.) in Elastomer Vulcanizates

The research article aimed to examine the antioxidant nature of freeze–dried extracts from Urtica dioica L. and Mentha piperita L. and to present a deep characterization of their influence on the properties of natural rubber–based vulcanizates before and after simulated aging processes. Natural extracts were prepared in three solvent systems at selected volume ratios: water (100), methanol–water (50/50), ethanol–water (50/50), which were further lyophilized and used as additive to natural rubber mixtures. Freeze–dried materials were investigated by UV–VIS diffuse reflectance spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), thermal stability by thermogravimetric analysis (TGA). Antioxidant activity and total phenolic content (TPC) were also examined. Prepared samples were subjected to accelerated simulated degradation processes by using ultraviolet and thermo-oxidative aging. Vulcanizates resistance to degradation effects was determined by the study of cross-linking density (equilibrium swelling method), mechanical properties (tensile strength, elongation at break) and color change in comparison with the results of the reference samples. The research showed that analyzed extracts are characterized by a high content of polyphenols and antioxidant activity, thus have a protective influence on elastomer vulcanizates against damaging effects of aging processes, which consequently extends the lifetime of materials.