Hydrophobic sol-gel coatings on bio-based materials – influence of catalyst and solvent concentration

Abstract

The impact of depositing sol-gel coatings on the hydrophobicity of a bio-based material was investigated in this study. Bio-based materials have tendency to absorb large amounts of water due to their highly porous structure and presence of hydrophilic hydroxyl groups in their structure. For this purpose, two sets of silica sols were prepared to study the influence of catalysts and solvent concentration on the hydrophobicity of hemp shiv. The first set of silica sols were prepared using different acidic (hydrochloric acid and nitric acid) and basic (sodium hydroxide and ammonium hydroxide) catalysts while keeping the concentration of precursors and solvents constant. The hydrophobicity of sol-gel coated hemp shiv increased significantly when using acid catalysed sols resulting in water contact angles of up to 100° using sessile drop method. Therefore, the next set of silica sols were prepared with acidic catalysts using higher concentration of solvent. Hemp shiv coated with ethanol diluted sols showed better hydrophobicity when compared to undiluted sols. This difference in hydrophobicity can be attributed to the change in surface roughness. It was found that silica sols containing higher concentration of solvent provide a uniform coating layer covering the hemp shiv entirely. In contrast, undiluted sol coatings developed minor cracks on the hemp shiv surface as observed under 3D optical profilometer. Therefore, the use of diluted hydrophobic silica sols offers potential for treatment of extremely hydrophilic bio-based materials by sol-gel technology. For practical application of coatings on bio-based materials, diluted silica sols are of interest due to longer shelf life, reduced cost and lower environmental impact of precursors.