Femtosecond laser patterning for enhanced hydrophobicity of modified nanocellulose coatings

Abstract

The hydrophobic properties of spray-coated nanocellulose coatings containing modified cellulose microfibrils (mCMFs) or cellulose nanofibrils (mCNFs) that are hydrophobized by deposition of organic nanoparticles with encapsulated wax, can interestingly be improved after surface patterning using ultra-short (femtosecond) pulsed laser processing. In this study, it is demonstrated how the parameters for laser patterning are tuned to create reproducible biaxial grid patterns with optimized resolution and increased hydrophobicity under different power settings (36 to 40 %), repetition rates (250, 500 kHz), hatch pitch sizes (20 to 50 μm) and number of processing layers. A relatively small laser processing window (39 % laser power, 500 kHz rate) was identified resulting in an increase in sessile water contact angles from 118° to 158° (mCMF coatings) or 98° to 122° (mCNF coatings), corresponding with contact angle hysteresis of 28° (mCMF coatings) or 23° (mCNF coatings). For different pattern geometries, experimental water contact angles could be predicted from trends following the theoretical Cassie-Baxter equation. Moreover, the structural changes in hydrophobic nanocellulose coatings after laser processing were demonstrated through spectroscopic analysis, indicating the enhanced hydrogen bonding and progressive development of more ordered cellulose structures under moderate or optimized power settings. In future, the femtosecond laser patterning can be applied as industrially scalable technology to tune hydrophobic properties and performance of nanocellulose coatings by selecting given pattern geometries and processing conditions.