Residual lignin affects production and properties of TEMPO-oxidized cellulose nanofibrils from partially delignified sugarcane bagasse

This study investigated the production of cellulose nanofibrils (CNFs) from partially delignified sugarcane bagasse (SCB), focusing on the effects of residual lignin on TEMPO oxidation and CNF properties. Through a comprehensive assessment of SCB processing into CNFs, we explored the correlations between initial lignin content (15–24 wt%), the amount of NaClO (25–50 mmol/g substrate), and the resulting chemical, morphological, and surface modifications. Regardless of the initial lignin content, oxidizing agent level, or type of mechanical treatment (ultrasonication or microfluidization), CNFs with average lengths of ~ 600–800 nm were obtained. However, in substrates with higher initial lignin content (24 wt%), increasing the NaClO concentration from 25 to 50 mmol/g substrate enhanced fibrillation but reduced the average CNF length from 813 to 665 nm. In contrast, no significant differences in fibrillation were observed when using 25 or 50 mmol NaClO/g substrate in samples with lower initial lignin content (15 wt%). Overall, TEMPO oxidation reduced lignin levels to 5–7.5 wt%, despite substantial differences in the starting materials. Substrates with higher initial lignin content yielded lower amounts of carboxylated groups, likely due to the oxidizing agent being consumed in lignin removal rather than cellulose oxidation. Morphological characterization of the substrates before and after TEMPO oxidation highlighted the critical role of this step in modifying fiber structure, which directly correlated with fibrillation efficiency. Zeta potential and rheological analyses highlighted the potential of CNFs produced from a single-step delignification process for high-value applications such as rheology modifiers, hydrogels, and films.