Toward a deeper understanding of the thermal degradation mechanism of nanocellulose

Understanding the thermal degradation process of cellulose-nanofibers (CNF) is necessary for developing high-value added CNF-based materials with e.g. fire retardant properties or high thermal stability. This study compares the thermal degradation behavior of softwood pulp before and after TEMPO-mediated oxidation, and that of the respective CNF and TEMPO-oxidized CNF with either sodium carboxylate (T-CNF COONa) or carboxylic acid surface groups (T-CNF COOH). The cellulose materials were charred under slow pyrolysis (i.e. at 4 °C min−1) from 50 °C to 600 °C using a thermogravimetric analyzer (TGA). The TGA was coupled to a mass spectrometer (MS) to analyze the volatile products evaporated during the heating process. The chemical structure and composition of the resulting chars after heating at different temperatures were assessed by infrared spectroscopy and X-ray diffraction. Fibrillation of the unmodified pulp resulted in a decrease in thermal stability of the obtained CNF, whereas TEMPO-oxidized pulp and T-CNF showed similar thermal behavior. Compared with the starting cellulose pulp, TEMPO-oxidized materials showed a reduced thermal stability. The presence of either sodium carboxylate or carboxylic acid groups influenced in particular the process of thermal degradation of the CNF. While COOH groups enhanced by 14% the thermal stability of CNF, COONa groups promoted the char formation by 27%. Tuning the counterion of T-CNF proved to be a straightforward approach for tailoring the thermal degradation behavior of the materials. The combinations of TG-MS, IR, and XRD finally resulted in the proposal of a degradation pathway for each investigated material.