Barrier coating of grafted cellulose microfibers (CMF) presenting oleo- and hydrophobic properties for molded pulp trays

To replace per- and polyfluoroalkyl substances (PFAS) in packaging applications, alternative barrier coatings must be developed in accordance with safe-and-sustainable-by-design (SSbD) principles. Fiber-based packaging materials are currently a major source of PFAS as it provides excellent resistance to water, oil, and grease; however, the high potential for direct contact, environmental release, and persistence raises significant concerns. In this study, a sprayable and bio-based coating with combined hydrophobic and oleophobic properties was developed for molded pulp trays. The coating performance was tuned through the selection bio-based compounds with low surface energy in combination with the intrinsic microscale roughness of cellulose microfibers (CMF). Starting from cellulose pulp, the CMF were produced via sodium periodate oxidation followed by mechanical fibrillation using a Masuko grinder and/or high-pressure homogenizer to achieve different degrees of fibrillation. The process parameters for fibrillation, oxidation and subsequent grafting of octadecylamine (ODA) onto dialdehyde-modified cellulose microfibers (CMF) were optimized through design-of-experiments for best performance. The resulting aqueous suspensions were further optimized for spray processing with concentrations between 0.7 to 1.5 wt.-% and controlled application parameters (nozzle diameters 1.6 to 2.0 mm, pressure 1.5 to 2.0 bar) ensuring reproducible deposition and linear thickness growth of the coating. The coated pulp trays exhibited water contact angles exceeding 140° and oil contact angles above 100° with performance improving as coating thickness increases. Microscopic evaluation interestingly revealed that no continuous film layer was formed on the molded pulp trays, preserving intrinsic porosity of the fibrous network that enables better moisture regulation and enhances recyclability. The localized deposition of CMF on top of the fibrous network increases surface roughness, which contributes synergistically to the observed wetting resistance. Overall, the proposed approach provides a PFAS-free coating solution that is compliant with emerging regulations, while offering strong barrier performance and improved recyclability potential.