Background and Objectives: Paper, as a cellulose-based, renewable, and biodegradable material, plays a significant role in the packaging industry and various other industrial applications. However, its porous structure and the abundance of hydroxyl groups in cellulose confer an inherently hydrophilic character, which leads to performance deterioration and functional limitations under humid conditions. Therefore, imparting hydrophobicity is essential to enhance performance and broaden the application range of paper. Although petroleum-based polymer coatings can improve water resistance, they pose environmental concerns and complicate paper recycling processes. Consequently, the development of a sustainable and environmentally compatible approach to achieve hydrophobicity without compromising recyclability is of considerable importance. The objective of this study is to produce hydrophobic paper through surface coating with deep eutectic solvents based on stearic acid and fatty alcohols as a bio-based and efficient system.
Materials and Methods: Kraft paper with a basis weight of 95 g/m² was selected as the base paper. For surface modification, deep eutectic solvents based on stearic acid as the hydrogen bond donor and cetyl alcohol or lauryl alcohol as the hydrogen bond acceptor were prepared at molar ratios of 1:1, 1:2, and 1:3. The formulations were prepared according to the molar mass of the components and applied to the paper surface at a coating weight of 10 g/m². After the coating process, the samples were evaluated in terms of wettability behavior, barrier performance, and mechanical properties. For this purpose, dynamic contact angle, water vapor permeability (WVP), tensile strength, bending resistance (Taber), and bursting strength were measured. Fourier transform infrared (FTIR) spectroscopy was used to identify chemical bonds, structural changes, and functional groups.
Results: Fourier transform infrared (FTIR) spectroscopy results confirmed that the stearic acid–alcohol eutectic systems were stabilized through non-covalent interactions. The enhancement of aliphatic bands and the reduction in the contribution of hydroxyl groups indicate the formation of a hydrophobic layer without the occurrence of any new chemical reactions. This surface modification approach is considered an effective and green chemistry–compatible strategy for improving the hydrophobicity of paper.The dynamic contact angle measurements revealed that no stable water droplet was formed on the uncoated paper surface and no measurable contact angle could be recorded, indicating its highly hydrophilic nature due to the abundance of hydroxyl groups and the highly porous structure of cellulose. In contrast, coating with deep eutectic solvent systems containing long alkyl chains led to a significant increase in the contact angle and transformed the surface behavior from hydrophilic to hydrophobic. The stearic acid–cetyl alcohol system exhibited superior performance compared to the stearic acid–lauryl alcohol system, which was attributed to the longer alkyl chain length and consequently lower surface energy of cetyl alcohol. In both systems, the 1:1 molar ratio demonstrated the highest efficiency. Moreover, coating with deep eutectic solvents resulted in a pronounced reduction in the water vapor transmission rate. This improvement was attributed to partial filling of surface pores, reduction of open voids, and the formation of a relatively continuous low-polarity layer composed of alkyl chains. Mechanical evaluations indicated that bursting strength, tensile strength, and bending resistance were not significantly affected, with only slight increases observed. These minor improvements were attributed to reduced moisture uptake, enhanced surface cohesion, and more uniform stress distribution in the coated structure.
Conclusion: Overall, the results demonstrated that the deep eutectic solvent coating effectively transformed the surface wettability from hydrophilic to hydrophobic and enhanced the barrier properties without significantly compromising the fibrous structure or mechanical performance of the paper. This approach can provide an effective pathway for the development of high-performance and environmentally sustainable paper materials.