Engineering surface roughness of nanocellulose film via spraying to produce smooth substrates

Due to the requirements of smooth cellulose substrates for printing electronic circuits to have proper conductance, nanocellulose (NC) have been used as a substrate for electronic material such as solar panels, supercapacitors and flexible electronics etc. Recently we developed a process to produce smooth NC films by spraying onto an impermeable substrate. The spray coated NC film has a shiny and glossy smooth surface where it was in contact with the substrate. In this study, we investigated the effect of i) substrate surface chemistry, ii) substrate roughness, iii) NC fibre diameter and iv) addition of carboxymethyl cellulose (CMC) in NC suspension on the roughness of the smooth surface of NC film. The three parameters controlling the roughness of the resultant NC films were the roughness of the substrate surface, the fibril diameter of the NC and CMC addition. Adding 1.5 wt. % CMC had only a small effect with very smooth substrates but had a larger effect with rougher substrates. The best roughness obtained is 336 ± 117 nm at a 259.1 μm × 259.9 μm inspection area when spraying of 1.5 wt. % CMC – 1.75 wt.% NC suspension on the super polished stainless steel (SSS). The controlling order of roughness of NC films was found to be 1.5 wt. % CMC addition in 1.75 wt.% NC suspension>base substrate > fibril diameter. The addition of CMC in the film decreases the surface roughness by reducing the friction experienced between the fibres. This reduction in friction allows the fibril network to form a smoother film when compared to a film without the presence of CMC. This study has shown that spraying of NC on base surfaces produce a smooth NC film in a rapid single step, with a smoothness that can be tailored across fibrils.