Abstract
Superhydrophobic surface coatings have enormous potential to protect wood from excessive water uptake. Unfortunately, such coatings often require toxic chemicals or complex fabrication methods, and their mechanical stability is very limited. As an alternative, highly water-repellent wood surfaces with mean water contact angles (WCAs) of 160° were prepared by straightforward spray-deposition of alkyl ketene dimer (AKD) wax microparticles. While water repellency was unaffected by static loading with a cast iron weight, prolonged rubbing resulted in a strong reduction in WCA to 125° since the spherical wax microstructure was largely destroyed by the acting shear forces. Thermal treatment of such a degraded surface at 80 °C led to an almost full recovery (WCA 155°) of water repellency. Microscopy and thermal analysis revealed that exposure to temperatures above the melting range and subsequent slow cooling enable self-assembly of AKD into leaf-shaped microstructures, which are responsible for the high WCA observed. It is proposed that the thermally induced self-recovery ability will greatly enhance the utility and service life of AKD-based superhydrophobic wood surfaces and make them suitable for multiple practical applications.
Highlights
Research on highly water-repellent surfaces, commonly defined by static water contact angles (WCAs) of 150° or higher (Feng et al 2002; Yan et al 2011), is motivated by early publications on artificial superhydrophobic surfaces (Onda et al 1996; Shibuichi et al 1996) as well as an increasing understanding of fundamental biological hydrophobisation strategies
The particle size distribution, the polydispersity index (PDI), and the ζ-potential of the alkyl ketene dimer (AKD) wax particles within the dispersion were investigated by means of dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Panalytical GmbH, Kassel, Germany) instrument
The aqueous AKD dispersion used for spraying was examined by means of dynamic light scattering
Summary
Research on highly water-repellent surfaces, commonly defined by static water contact angles (WCAs) of 150° or higher (Feng et al 2002; Yan et al 2011), is motivated by early publications on artificial superhydrophobic surfaces (Onda et al 1996; Shibuichi et al 1996) as well as an increasing understanding of fundamental biological hydrophobisation strategies. Wood protection by means of superhydrophobic surface coatings has attracted increasing attention for several years (Liu et al 2011; Wang et al 2011a, c). Thin layers with sophisticated chemistry and architecture that hinder liquid water uptake by wood have many potential advantages compared to bulk wood modification, for example rapid application, reduced consumption of chemicals, negligibly small weight gain, and the preservation of mechanical and optical wood properties to a large extent. In many use cases it requires protection against excessive water uptake to prevent damage from dimensional instabilities or biological decay. Superhydrophobic surfaces dramatically reduce the wetting of hygroscopic wood substrates and have the ability to minimize the adsorption of liquid
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