Flexible spray coating process for smooth nanocellulose film production

A novel rapid method for high throughput production of smooth nanocellulose (NC) films by spray coating was communicated recently. In this method, we employed spray coating to produce wet films on stainless steel plates moving on a conveyor, forming free-standing films with interesting structural, mechanical and surface properties upon drying. In this research, we investigate the range of mechanical and physical properties of nanocellulose films prepared by spraying. Furthermore, a comparison with NC films prepared via conventional vacuum filtration was conducted to evaluate the suitability of this method as an alternative film preparation process. One set of experiments was completed where the solids concentration of the suspension was fixed at 1.5 wt% and the conveyor velocity was varied, while two series of experiments were completed where the solids concentration of the suspension was varied and the conveyor speed was fixed at either 0.32 or 1.05 cm/s. By varying speed and solids concentration, spray-coating was found to allow efficient production of films with basis weights ranging from 38 to 187 g/m², with film thicknesses ranging from 58.4 to 243.2 μm, respectively. There was a universal linear relationship between the thickness and basis weight, independent of the process conditions. The optical uniformity of film was also noticeably dependent on the spraying process. The optical uniformity index of films, relative to vacuum filtered films, increased with conveyor speed at 1.5 wt% solids concentration and was independent of solids concentration at low speed. Forming at the higher speed of 1.05 cm/s produced a maximum in optical uniformity in the range 1.5–1.75%, with these films being more uniform than conventional films produced through vacuum filtration. The most uniform films produced by spraying also had a similar tensile index to films made via vacuum filtration. With an understanding of these parameters and effects, we demonstrate this method to be a more time efficient alternative method to produce uniform films where the properties can be tailored to the required application.