Analysis of thermal degradation kinetics and carbon structure change between waste macadamia nut shell and PET plastic


Thermal degradation kinetics of co-pyrolysis of polyethylene terephthalate (PET) blended and macadamia nut shell was investigated using a large-scale customised thermogravimetric analysis (TGA) and 13C solid state nuclear magnetic resonance (NMR). Blending ratios ranging 20 to 80 wt% PET and macadamia nut shell were analysed at 3, 5 and 8 °C /min heating rates up to 1273 K in the presence of N2 atmosphere with a flow rate of 1 L / min.

The kinetic data was correlated with 13C solid state NMR. The results indicated that two synergistic effects between PET and macadamia nut shell during co-pyrolysis occurred, characterised by an enhanced carbon yield of the co-pyrolysis products. The secondary reaction occurring between primary products of macadamia nut shell and PET was identified as the cause of the synergistic effect, this effect varied with weight fraction of macadamia nut shell in the blend and heating rate.

At low temperatures, hydrogen transferred from PET stabilised radicals on primary pyrolysis products of macadamia nut shell. At high temperatures, the rate of PET degradation increased due to interaction with radicals formed on char, reducing activation energy but increasing reaction order. The measured changes in activation energy and reaction order imply that the thermal degradation mechanism of co-pyrolysis is different to that of the individual components. 13C Solid-state NMR was used to correlate molecular changes during pyrolysis with the kinetics.

The NMR results indicated presence of radicals on macadamia nut shell altered the degradation behaviour of PET leading to growth of polycyclic aromatic hydrocarbons (PAHs) through cross-linking reaction.