Thermodynamic and multi–step kinetic analysis of slow pyrolysis of natural rubber–silanised cellulose composites with 30–55 phr filler content

Pyrolysis is a promising thermochemical process for waste reduction and energy recovery. Natural rubber (NR) composites filled with 30, 45, and 55 phr silanised cellulose (CELS) were prepared and characterised by SEM and FTIR techniques. Thermogravimetric curves for heating rates of 2, 4, 6, 8, 10, and 20 °C·min −1 were measured in an inert gas. Kinetic parameters were determined by isoconversional kinetic analysis using the Friedman model-free method and a model-based method. By applying the generalised master plot method, it was found that the pyrolysis process follows an autocatalytic mechanism involving two kinetically independent, parallel pathways, each pathway consisting of two sequential steps. The results show that silanisation of cellulose has a positive effect on composite thermal stability, but only up to a specific content of CELS. At high loadings, the resulting silica-rich ash can act as a solid acid catalyst, accelerating secondary cracking reactions during pyrolysis. Innovative approaches for determining the formal thermodynamic parameters have been presented. The first method is based on the Eyring equation and the knowledge of E α = f (α) and A α = f (α) from the model-free method, providing the thermodynamic parameters as a function of the entire conversion range, α. The second method is based on the results of model-based kinetic analysis. The method makes it possible to determine these parameters for individual steps of a multi-step model and, thus, to compare the energy demand, spontaneity, and change in disorder of the system in the transition state for these steps.