WITHDRAWN: Surface hydrophobization of pulp paper and nanofibrillated cellulose(CNF) films using blocked diisocyanates

Abstract

Cellulose fibers and cellulose nanofibrillated (CNF) stand at the forefront of sustainable material innovation, thanks to their unique structural properties that pave the way to produce remarkable all-cellulose products. Despite their promising attributes, challenges such as high hydrophilicity and lower durability in wet conditions highlight the need for simple and cost-effective hydrophobization techniques. In this study, we explore the potential of a novel two-step hydrophobization process of pulp paper and CNF films using blocked isocyanate chemistry. 4,4-Methylenebis (phenyl isocyanate) (MDI) was employed along with phenol and linear chain alcohols to produce blocked diisocyanates adducts. Alkylic groups with chain lengths varied from 3 to 18 carbons was produced and characterized through FT-IR, Liquid ¹H-NMR and TGA. The hydrophobization process involved dipping samples of pulp paper and CNF films in the adduct solution for a few seconds followed by heating at 170 °C. The resultant hydrophobized papers and films were analyzed employing FT-IR, scanning electron microscopy (SEM), atomic force microscopy (AFM), and water contact angle (WCA). The analyses revealed that the effectiveness of the hydrophobization was intricately linked to the length of the adduct moiety and the inherent roughness of the cellulose surfaces. Hydrophobized pulp paper exhibited WCAs ranging from 109° to 144° reaching near superhydrophobic state in comparison to WCA 0° observed for the hydrophilic ones. Similarly, hydrophobized CNF films showed WCAs between 93° and 114°, significantly higher than the 50° of the pristine CNF films. Once both cellulose surfaces were treated with the same adducts, the difference in WCA values from pulp paper to CNF films is attributed to surface roughness: pulp paper, with a rougher surface of 75 nm, had higher WCAs, whereas the CNF films, with a smoother surface near 20 nm, had lower WCAs. This study not only sheds light on the critical role of chemical modification in enhancing the water resistance of cellulose-based materials but also opens new avenues for the development of cellulose products with enhanced durability and sustainability.