Abstract
The different nanocellulose grades are promising for formulation of functional coatings and can be applied by spraying, but their lifetime and performance under demanding conditions requires robustness and appropriate mechanical characterization. In this study, the spraying conditions were optimized through design of experiments for native and hydrophobically modified nanocellulose suspensions. Spraying is most critical for native nanocellulose at low concentrations (1.5 to 2.3 wt.-%) due to its high viscosity, while surface-modified nanocelluloses could be sprayed at higher concentrations (4.0 to 7.0 wt.-%). The coating weight linearly increases with number of spraying layers while the apparent density becomes constant above 4 μm. The coating porosity and density are confirmed by topographical and microscopic evaluations, while the relationships with hydrogen bonding energy are demonstrated from FTIR spectra. According to nanoindentation testing (continuous stiffness measurements), mechanical properties show a transition from thickness-dependent values to thickness-independent values for coating thickness above 4 μm. The mechanical properties for non-modified nanocellulose coatings are in line with coating densities and hydrogen bonding, while the mCMF coatings interestingly did not show a significant reduction of mechanical properties. The robustness of sprayed nanocellulose coatings has been well quantified and controlled depending on nanocellulose grades and surface modification, while mechanical testing has successfully balanced the effects of substrate and coating roughness. Further developments of mechanically robust nanocellulose coatings will result in their application for functional and protective coatings (e.g., barrier, hydrophobic, antimicrobial and anti-corrosion coatings).
Published Version
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