Solvent-Free, Highly Transparent, and Strong Cellulose Films from Carnauba Wax/Waterborne Polyurethane for Sustainable Packaging

Abstract Conventional transparent paper suffers from limited optical transmittance, poor water resistance, and low mechanical strength, and its fabrication typically depends on organic solvents, which cannot meet the long-term requirements of high-end packaging. In this work, we fabricated a biodegradable, highly transparent Carnauba wax/Waterborne Polyurethane (CW/WPU) composite paper via a green, solvent-free aqueous emulsion impregnation process using cellulose paper as the substrate, and systematically elucidated the synergistic mechanism among cellulose, CW, and WPU. The aqueous CW/WPU system uniformly disperses and infiltrates the paper pores, forming a dense, continuous structure (RMS = 7.31 nm) that suppresses interfacial light scattering and thus achieves a high transmittance of 87.79% at 550 nm. The continuous WPU network establishes strong interfacial adhesion and hydrogen bonds with the cellulose substrate, imparting excellent mechanical strength and toughness (tensile strength = 37.6 MPa). Meanwhile, the composite exhibits enhanced water vapor barrier performance (WVTR = 125.5 g/(m2·d)) while retaining biodegradability in natural soil. This entirely green, solvent-free strategy overcomes the optical, mechanical, and barrier limitations of conventional cellulose paper, providing a pathway to replace plastics in packaging for consumer electronics displays and protective equipment.