The synthesis of novel Schiff base antioxidants to promote anti-thermal aging properties of natural rubber

There is of great interest in promotion of anti-thermal aging properties of natural rubber (NR) to improve the applicability. In this study, two novel Schiff base antioxidants (SBAOs) for NR were synthesized utilizing 4-aminodiphenylamine with 3,5-Di-tert-butyl-2-hydroxybenzaldehyde or cinnamic aldehyde in an ethanol medium. IR, 13 C-NMR, and 1 H-NMR confirmed the structures of SBAOs. Addition of SBAOs improved the rheometric properties, mechanical properties and thermal oxidative stability of NR vulcanizates. Introduction of SBAOs in NR increased the apparent activation energy of thermal oxidative degradation according to Kissinger and FWO methods. Anti-thermal aging performance of SBAOs for NR is related to the structures. The C=N double bonds in SBAOs improve the electron density of Ar–OH and/or Ar–NH–Ar structures, benefiting the release of active hydrogen. The active hydrogen could capture free radicals initiated during the thermal oxidative aging process. The lone pair electrons on nitrogen atom are also beneficial to delay or terminate free radical reaction. NR with SBAOs showed high mechanical properties of the tensile strength, tensile stress at 100% elongation and Shore A hardness compared to commercial BHT and 4010 during aging 96 h. It indicates potential applications of SBAOs as efficient antioxidants for NR.