Oxidative aging of natural rubber (NR) leads to the deterioration in the physical and mechanical properties. Using various antioxidants during compounding is a common way to improve the aging resistance of NR. The most widespread used antioxidants are phenolic and amine-based synthetic antioxidants and 2,2,4-trimethyl-1,2-dihydroquinoline in rubber industry. However, synthetic antioxidants cause some environmental problems during their production and tend to be replaced by natural alternatives. In this study, henna has been evaluated as a natural antioxidant in NR-based rubber compounds for efficient and conventional sulfur vulcanization systems. Performance of henna was evaluated in terms of rheological, mechanical, aging, and temperature scanning stress-relaxation properties of the compounds. It has been concluded that henna could be used as an alternative antioxidant for sulfur vulcanization of natural rubber matrix, compensating little deteriorations in vulcanizate properties. Short- and long-term performances of henna were also found to be related to vulcanization system.

Unsaturated chain structure of natural rubber makes it a poor defense against thermo-oxidative aging. Synthetic antioxidants are commonly used in rubber compound recipes to prevent/retard aging of the rubber material during its service life. However, synthetic antioxidants cause some negative effects on human and environmental health; they tend to be replaced by natural alternatives. In this study, the short- and long-term antioxidant effects of henna and its basic active components, lawsone and gallic acid, have been investigated individually for natural rubber cured with semi-efficient sulfur vulcanization system. The composition of henna was determined by gas chromatography-mass spectrometry (GC-MS) analysis. Qualitative and quantitative analysis were performed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) to highlight structural changes on aged vulcanizates. The authors attempted to correlate the suggested aging mechanism with rheological, mechanical, and morphological properties. Results showed that both lawsone and gallic acid were impressively successful regarding their anti-oxidation activity. In addition, henna, which contains a sufficient amount of lawsone and gallic acid, has been suggested as a competitive natural alternative to the common synthetic stabilization system for natural rubber, considering its sustainable commercial abundancy.