Facile and scalable fabrication of hydrophobic CNF-based composite films via michael addition for enhanced mechanical performance and water resistance

Cellulose nanofibers (CNFs) are promising renewable materials because high availability, strong mechanical performance, and biodegradability are provided. A major limitation is intrinsic hydrophilicity. Mechanical durability can also be insufficient. Use in moisture-resistant packaging and paper-based materials is therefore constrained. Conventional hydrophobization strategies for CNFs, especially PEI/fatty acid-based amine chemistry, are often plagued by complex workflows, poor scalability or an inevitable trade-off between hydrophobicity and mechanical properties. Herein, a facile, green and scalable route was developed to produce hydrophobic CNF/polyethyleneimine (PEI)/stearyl acrylate (SA) composite films. Dip-coating was coupled with a Michael addition reaction. Composite films were prepared by sequential dip-coating of CNF films in ethanolic PEI and SA solutions. Thermal ageing was then applied. Covalent crosslinking between PEI amino groups and SA unsaturated ester groups was thereby induced. Immobilization of the PEI/SA coating on the CNF surface was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). Film properties were evaluated by water contact angle (WCA) measurement, thermogravimetric analysis (TGA), tensile testing, folding endurance measurement, and visible-light reflectance spectroscopy. A hydrophobic surface was obtained (WCA ≈ 118.6°) and mechanical performance was increased (tensile strength ≈ 104 MPa; folding endurance ≈ 816 cycles). Thermal stability was improved, and the maximum pyrolysis temperature was increased by 14 °C. Visible-light reflectance was reduced. These outcomes were achieved with a low PEI/SA coating loading of 1.01 ± 0.05 g m⁻², the composite films achieved simultaneous enhancement of multiple key properties without performance compromise. This strategy is simple, environmentally benign, and scalable. Key limitations of CNFs are addressed, and broad potential in moisture-resistant packaging and other functional paper-based materials is supported. • Facile dip-coating/Michael addition yields hydrophobic CNF/PEI/SA films. • Enhanced hydrophobicity (WCA ∼118.6°) and mechanics. • Improved folding endurance broadens packaging/functional paper applications.