Abstract:

The abundant hydrophilic hydroxyl groups present on paper fibers surfaces limit its applicability in barrier packaging. Currently, most petroleum-based materials used to enhance the barrier properties of paper are poorly degradable. In contrast, alkali lignin exhibits inherent biodegradability, hydrophobicity, and flame retardancy, offering distinct advantages for the development of sustainable barrier packaging materials. To investigate the influence of alkali lignin on the hydrophobic performance of packaging paper, alkali lignin (A-Lig) was used as the raw material in this study. Esterification reactions were carried out using palmitoyl chloride and stearoyl chloride, yeilding lignin palmitate (Lig-P) and lignin stearate (Lig-S), respectively. Subsequently, coating solutions were formulated based on the three types of lignin (A-Lig, Lig-P, and Lig-S) and sprayed onto base paper to fabricate superhydrophobic paper. The chemical structures, micromorphologies, and thermal properties of A-Lig, Lig-P, and Lig-S were systematically characterized using a suite of analytical techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Meanwhile, the microstructure, hydrophobic properties, and mechanical strength of both the base paper and coated papers comprehensively evaluated through static contact angle, rolling angle, water absorption, surface water stability, self-cleaning property, and mechanical performance measurements. The results showed that the hydroxyl groups in the esterified lignin were effectively substituted, with aliphatic chains successfully grafted onto the lignin backbone. Additionally, the relative content of C-O bonds decreased, while the thermal stability of the modified lignin was reduced. The static contact angles of the base paper (Bas-P), A-Lig-coated paper (Lig-P1), Lig-P-coated paper (Lig-P2), and Lig-S-coated paper (Lig-P3) were measured as 45.9°, 83.5°, 150.8°, and 151.6°, respectively. Notably, the rolling angles of Lig-P2 and Lig-P3 were 9.3° and 3.5°, respectively, satisfying the established criteria for superhydrophobicity. Moreover, a petal-like micro-nano hierarchical roughness was observed on the surfaces of Lig-P2 and Lig-P3. Compared with Bas-P, Lig-P2 and Lig-P3 exhibited a slight decrease in tensile strength but a pronounced increase in elongation at break, indicating improved flexibility of the paper.

Key words: lignin, esterification modification, superhydrophobic, packaging paper