Solvent-Free Lignin-Silsesquioxane wood coating formulation with superhydrophobic and Flame-Retardant functionalities

There is a tremendous motivation to develop eco-friendly formulas for superhydrophobic and flame-retardant coatings. Presently used coating materials, particularly fluorinated compounds and their organic solvents, are potentially toxic. Here, we demonstrate that a silsesquioxane-grafted kraft lignin (WSL) and aluminum phosphate (AP) binder, i.e., an aqueous sustainable formula, can produce a flame retardant and superhydrophobic coating that is highly resistant to water and solvents. The chemical reaction of softwood kraft lignin (SKL) and aminopropyl/methyl silsesquioxane (WAPMSS) was studied comprehensively. NMR and XPS analyses confirmed the conversion of hydroxyl groups of SKL to Si-O-C via polycondensation. The product exhibited negligible wettability and was very hydrophobic. The dip coating of stain-grade pine wood species in the best formula containing WSL and AP dispersion (1/1 wt./wt.) rendered wood with superhydrophobic (with a water contact angle of (WCA) of 158°) and flame-retardant (with a limited oxygen index (LOI) of 27.2 %) functionalities. The exposure of coated wood to different liquids and high temperatures, as well as abrasion, touching, and knife-cutting analyses, confirmed the excellent durability of the coating formulation on wood. This paper demonstrates an eco-friendly pathway to produce a sustainable wood coating formulation with superhydrophobic and flame-retardant features.