Controllable Fabrication And Characterization Of Robust Superhydrophobic Coatings On Wood Surface

Fabrication of superhydrophobic coatings on wood substrate can minimize wood–water interactions and thus avoid damages associated with water absorption,such as distortion,cracking and degradation.However,the fragile surface roughness of the nanostructures and the week interfacial interaction between the substrates and the structural layers limits the practical application of superhydrophobic wood.Hence,fabrication of superhydrophobic coatings on wood substrate with the mechanical stability and durability is essential for superhydrophobic wood.In this thesis,inspired by the lotus effect,organic-inorganic hybrid superhydrophobic films with high durability are fabricated on the intrinsically heterogeneous wood substrates by using environment-friendly and chemically stable inorganic SiO2 and TiO2 nanoparticles as inorganic fillers and transparent,adhesive polymers as organic matrices.The main contents and results are as follows:(1)Fabrication of robust,damage-tolerant superhydrophobic coatings on wood surfacesSuperhydrophobic coatings were fabricated on wood surfaces via a two-step process by first applying a primer coating of transparent epoxy resin to cover the naturally micro-grooved surface structure,and subsequently construction of superhydrophobic nanocomposite film using silica/epoxy resin/fluorinated alkylsilane(FAS).The surface morphology and chemistry of the superhydrophobic coatings were characterized by field emission scanning electron microscopy(FE-SEM),atomic force microscopy(AFM)and fourier transform infrared spectroscopy(FTIR).The hydrophobicity,oleophobicity,self-cleaning effects,mechanical durability and UV stability of the coated wood were tested.The results indicated that well-developed microstructures with dual-scale roughness patterns and the low-surface-energy FAS in the coatings are believed to be responsible for the good superhydrophobicity(with water contact angle of 153° and sliding angle less than 4°),oleophobicity(with glycol contact angle of 146° and sliding angle less than 11°)and self-cleaning property.The water contact angles and sliding angles of the coated wood exhibited negligible changes after sandpaper abrasion and water spray impact,exhibiting remarkable mechanical durability,which can be attributed to the strong bonding between expoxy resin and SiO2 as well as the roughness-regenerating capability of the coating.Meanwhile,the coated wood retained its original color after UV-irradiation,exhibiting excellent durability against ultraviolet(UV)irradiation.(2)Fabrication of semitransparent,durable superhydrophobic coatings on varnished woodSuperhydrophobic wood was successfully obtained by depositing superhydrophobic coating on varnished wood via a simple dip-coating method based on polydimethysiloxane(PDMS)/ SiO2 nanocomposites.The surface morphology of the superhydrophobic coatings were characterized by FE-SEM and AFM.The hydrophobicity,transparency and mechanical durability of the coated wood were tested.The results indicated that the surface microstructure and roughness of the hybrid coatings on wood are governed by the mass ratio of SiO2 to PDMS,resulting in tuneable wettability of the wood surface.When the nano-SiO2 particle content reached a critical level(at a particle loading of 28~35%),dual-scale features with micro-scale roughness superimposed with a nanostructure can be formed in the surface coatings,resulting in a transition from hydrophobicity to superhydrophobicity.In spite of slightly declined optical transparency of the hybrid coated surface,the coated wood retained essentially its original colour and aesthetic appearance.The as-prepared superhydrophobic coating on wood exhibited remarkable mechanical durability against sandpaper abrasion,which can be attributed to the strong PDMS-SiO2 bonding as well as the roughness-regenerating capability of the coating.(3)Fabrication of self-healing superhydrophobic coatings on wood substrateSuperhydrophobic wood was fabricated by simply spraying a waterborne perfluoroalkyl methacrylic copolymer(PMC)emulsion mixed with Ti O2 nanoparticles onto polydimethylsiloxane(PDMS)pre-coated wood.The surface morphology and chemistry of the superhydrophobic coatings were characterized by SEM and X-ray photoelectron spectroscopy(XPS).The hydrophobicity,mechanical durability and self-healing ability of the coated wood were characterized.The results showed that well-developed microstructures with dual-scale roughness patterns and the low-surface-energy materials in the coatings are believed to be responsible for the exceptional repellency of the coated wood toward water as well as oil.The liquid contact angle remained around 150° when the surface tension of the liquid was greater than 30.1 mN m-1.The superhydrophobic coating on the rigid wood was durable enough to withstand repeated sandpaper abrasion whilst retaining its superhydrophobicity.Even if the PMC could be easily decomposed under UV irradiation,resulting in an increased contact angle hysteresis,the damaged surface could restore its superhydrophobicity(with water sliding angles less than 10o)by a simple heat treatment.In the process of heat treatment,thermodynamically driven by minimizing the surface free energy,the underlying hydrophobic PDMS migrated onto the surface replenishing the essential low-surface-energy substances.