Synergistic effect of hybrid fillers on the thermal degradation of natural rubber: stability and kinetic studies

The present study discusses the thermal degradation behavior of natural rubber (NR) reinforced with hybrid fillers. Effect of different combinations of carbon-based conductive fillers on the thermal properties of NR is analyzed, and its degradation mechanisms are evaluated. The fillers used for preparing the composites were conductive carbon black (CCB), multi-walled carbon nanotubes (MWCNT) and reduced graphene oxide (RGO). The fillers were treated with an ionic liquid, 1-ethyl-3-methylimidazolium chloride in order to improve the interaction between the fillers and matrix in the composites. Two step processing method was adopted for the preparation of the composites: rubber and filler mixing using an internal mixer at 100 °C with specified time and rotation followed by compounding in a two-roll mill. The composites were characterized using various techniques and the thermal degradation was studied using Thermogravimetric analyzer in different heating rates. Delayed thermal degradation of NR is observed in the presence of modified hybrid fillers. The thermograms of NR- hybrid filler systems at the heating rate of 20 °C min⁻¹ showed the dominant weight loss percentage in the temperature range of 380–400 °C. Kinetics of the thermal degradation is investigated using the models such as Coats–Redfern, Kissinger, Kissinger–Akahira–Sunose, and Flynn–wall–Ozawa. Furthermore, the activation energy derived from the kinetic models revealed the better thermal degradation stability of NR in the presence of ionic liquid-modified fillers and hybrid filler system of CCB–MWCNT–RGO. Among the different models used, Kissinger is the more appropriate model for calculating the activation energy of the degradation of NR in the presence of carbonaceous fillers because the calculation is based on the maximum degradation temperature.