Processing characteristics and thermal stabilities of gel and sol of epoxidized natural rubber

The processing performances and aging behaviours of gel and sol separated from epoxidized natural rubber (ENR) using organic solvents were studied by using rubber processing analyzer (RPA), thermogravimetric analysis (TGA) and difference FT-IR method. As the gel with intermolecular ether structure is formed by the ring-opening reactions of a part of epoxy groups during the preparation and storage of ENR, the molar percentage of epoxy groups of gel is lower than that of sol. The gel shows higher elastic moduli to temperature and frequency responses, lower tan δ to frequency and strain response and higher elastic torque to strain response as comparing to those of sol. Therefore, the formation of gel will reduce processing performance of ENR. When the aging behaviours of gel and sol were analyzed by different methods, the aging reactions and their degrees were different because of the differences of aging conditions. Compared to gel, the sol is easy to be crosslinked, leading to a higher Δtan δ from the RPA analysis and it is also easy to be oxidized into small molecules, leading to lower onset temperature and apparent activation energy when being analyzed by TGA. However, the variations of functional groups of gel are more obvious than those of sol when analyzed by difference FT-IR method. As the mechanical properties of ENR will be varied with the variations of molecular structures, the formations of gel during preparation and storage of ENR will reduce the oxidative aging resistance of ENR.