Formation of nanosized islands of dialkyl β-ketoester bonds for efficient hydrophobization of a cellulose film surface.

The efficient hydrophobization mechanism of a hydrophilic cellulose film surface with alkylketene dimer (AKD) was studied in terms of formation of β-ketoester bonds at AKD/cellulose interfaces and their nanosized distribution analysis. AKD-treated cellulose and nanocellulose films were sequentially extracted with chloroform, hot water, and dioxane/water. Atomic force microscopy and high-resolution secondary-ion mass spectrometry were used to analyze the surface structures of the AKD-treated cellulose films and those after the sequential extraction. The results showed that the AKD molecules had melted and transformed into spherical nanoparticles, ∼37 nm in diameter, on the film surface during heat treatment, forming "sea/island"-like structures; the film surface projection area comprised 99% hydrophilic cellulose and 1% hydrophobic AKD nanoparticles. Determination of the AKD contents in the films revealed that an extremely small amount of AKD/cellulose β-ketoester bonds were likely to form at the AKD/cellulose interfaces during heating, clearly contributing to the hydrophobic nature of the sequentially extracted cellulose films.