Efficient Wood Hydrophobization Exploiting Natural Roughness Using Minimum Amounts of Surfactant-Free Plant Oil Emulsions

Abstract

Wood in service requires protection from excessive moisture. Herein, we demonstrate that efficient surface hydrophobization can be provided with small amounts of biobased oils, benefitting from the hierarchical roughness inherent to wood surfaces. The developed technique involves coating spruce wood with surfactant-free emulsions based on tung oil, linseed oil, or a linseed oil-based long oil alkyd resin. The ζ-potential of the emulsions was determined by electrophoretic mobility measurements. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and spectrophotometry were used to study coated surfaces. XPS measurements confirmed the presence of the tung oil coatings. Tung oil emulsions were effective at concentration levels as low as 0.04 wt % oil content, roughly equivalent to 0.04 g m –2 and led to static water contact angles reaching up to >130°. SEM imaging and AFM measurements provide evidence that the micro- and nanostructures inherent to wood enhance the hydrophobization effect of the obtained coatings. A further benefit of the method lies in only minimal effects of the coating on the surface color and gloss. Thus, the mass-efficient process following several of the principles of green engineering led to improved water repellency while not affecting the visual appearance of the coated wood.

Birch Wood Surface Characterization after Treatment with Modified Phenol-Formaldehyde Oligomers

“…Spruce wood coated with surfactant-free emulsions based on tung oil, linseed oil, or a linseed oil-based long oil alkyd resin demonstrated high water repellency with CA between 100 and 130 • and had minimal effect on surface gloss and did not change the color of coated wood [28]. Synthetic and natural epoxy resins and a rosin acid derivative as crosslinking agent have been used to prepare wood coatings which represented good adhesion to the wood, increased hardness and anti-fungal resistance which was ensured by limited water penetration into coated specimens [29].…”

“…It seems that UV light caused splitting or destruction of long aliphatic chains in M-PF resin structure, and hydrophilic destruction products were formed on the surface of wood. However, this theoretical consideration needs to be confirmed in future tests, because also other researchers have not focused on CA changes after exposure to intensive UV light [25,26,28].…”