Roll-to-roll processing of nanocellulose into coatings

Paper and paperboard consumption is growing worldwide. However, the demand for traditional graphic paper grades is dwindling in mature markets, e.g. Europe, due to an increasing influence of the electronic media. For a sustainable growth, it is essential for the paper industry to expand its product portfolio to aim for novel value-added applications that can utilize the paper industry raw material, e.g. pulp, in unconventional ways. Nanocellulose, a product of the wood biomass itself, shows a considerable promise in this context. Paper, modified with barrier and functional coatings, is attracting interest as a potential sustainable solution to the environmental concerns associated with petroleum-derived plastics commonly used in various packaging applications. Excellent oil and gas barrier properties along with functionalization possibility of nanocellulose films and coatings seem valuable in proliferating the use of such paper. It is vital to produce nanocellulose films and coatings on a large scale to reap these benefits. However, nanocellulose suspensions cannot be coated or formed into films easily using the traditional coating and metering constructions. This is due to either clogging caused by the aggregation of nanocellulose fibers or the excessively high viscosity of nanocellulose suspensions, resulting in coating and film defects, poor coating/film quality, and poor process runnability. Besides, these suspensions contain large amounts of water, usually more than 95 wt%, which is challenging to dry in a continuous process.
In this work, coating of nanocellulose suspensions is enabled by utilizing their apparent shear thinning behavior, which extends to high shear rates in pressure-driven flow through a slot-die. The resulting low effective process viscosity of the suspensions allows forming a wet film and coating application onto paper. The slot-die is used in an unconventional manner, where it acts as both a shearing and a metering element. A conventional slot-die operation does not involve post metering, i.e. all the material coming out from the slot is transferred to the substrate. The process developed in this work successfully demonstrates continuous roll-to-roll coating of nanocellulose onto paper-based substrates. The resulting coatings improved barrier (air, grease, mineral oil and heptane) properties significantly, along with enhancing strength properties.
The utility of the developed process was also demonstrated for coating of nanographite suspensions, with similar characteristics as nanocellulose suspensions, onto paper-based substrate. The novel coating technique allowed depositing thick conductive coating layers, which was otherwise challenging using the conventional coating techniques.
There are still challenges that need to be addressed to speed up the potential upscaling of this coating technique. For example, the pumping requirements to feed the slot-die limit the maximum solids content of the suspension to be coated. Furthermore, it is rather difficult to manufacture, with precision, wide slot-dies required for industrial scale. Nevertheless, this process shows some promise in enabling the use of nanocellulose at the industrial scale, which may help improve the forest-based industry profitability, and contribute to curbing the menace of climate change and achieving a sustainable planet.