Surface modification of TEMPO-oxidized cellulose nanofibrils derived from rice straw waste using alkyl ketene dimer for enhanced hydrophobicity

In this study, unbleached rice straw soda pulp (UP) was used to produce highly reactive TEMPO-oxidized cellulose nanofibrils (TOCN) via TEMPO-mediated oxidation with sodium hypochlorite (NaClO) at 5 mmol g⁻¹ under pH 10 for 2 h. Alkyl ketene dimer (AKD) was then used to chemically modify the TOCN surface to overcome the inherent hydrophilicity arising from the abundance of hydroxyl groups in its structure. The influence of AKD loadings (1, 7.5, and 15 wt%) on surface modification was investigated through their corresponding films. The results showed that a high weight recovery ratio of 78.08% was achieved, with a carboxylate content of 1.08 mmol g⁻¹. The resulting TEMPO-oxidized cellulose nanofibrils (TOCN) showed a nanofibrillation yield of 74.44%, with fibril widths ranging from 3 to 10 nm (average width of 4.4 nm) and lengths on the micrometer scale. FTIR and XPS results support the formation of ester-related functionalities, which increased progressively with AKD loading, consistent with surface modification of TOCN by AKD to enhance hydrophobicity. The unmodified TOCN films exhibited rapid water absorption due to their intrinsically hydrophilic nature. In contrast, AKD-modified films demonstrated higher initial water contact angles and reduced contact angle decline over time. Higher AKD loadings, particularly at loadings of 7.5 and 15 wt%, resulted in more stable water contact angle values, confirming improved surface hydrophobicity and water resistance. AKD modification may offer a route to enhancing the surface hydrophobicity of TOCN, with possible relevance to moisture-related applications such as sustainable biopolymer packaging, water-resistant coatings, and paper-based products.