Improvements of lanthanum complex on the thermal-oxidative stability of natural rubber

Mixed antioxidants composed of antioxidant IPPD and novel rare earth lanthanum complex were used as an additive to prepare natural rubber (NR) samples. The variations of macro-properties, surface characterizations, and internal groups were investigated by mechanical testing, X-ray photoelectron spectroscopy, and thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA/FT-IR), respectively, to study the thermal-oxidative stability of NR. The thermal-oxidative degradation kinetic parameters were determined by analyzing the thermogravimetric curves at different heating rates with two model-free methods, Kissinger method and Flynn–Wall–Ozawa method. The results all showed that, compared with pure antioxidant IPPD, the same mass of mixed antioxidants could indeed improve the thermal-oxidative stability of NR. Furthermore, based on the TGA/FT-IR results and quantum mechanics simulations, the autocatalytic, free radical chain reaction mechanism for the thermal-oxidative aging of NR was clarified, and the different function mechanisms of antioxidants IPPD and p-ASALa were also discussed. Except for functioning as a labile-hydrogen donor which is similar to antioxidant IPPD in protecting NR against autoxidation, p-ASALa has strong coordination abilities and large coordination numbers, resulting in the high efficiency in enhancing the thermal-oxidative stability of NR.