Effect of metering systems and drying methods on the barrier properties of paper coated with multiple layers of cellulose nanofibres

Developing environmentally friendly barrier coatings as an alternative to petroleum-based polymer packaging is a challenge. Cellulose nanofibres (CNF) films have exhibited great potential as water and gas barrier layers. Recycled paper was multi-layer coated with three commercial CNF (mechanical (M-CNF), TEMPO (T-CNF) and carboxymethylated (C-CNF)) using two metering systems (wound rod and blade micrometer) and two drying methods (tension-free contact drying and air drying). The coatability and rheological behaviour of the CNF suspensions were analysed. The gas, water and grease barrier properties of the CNF coated papers were determined and compared with the results of paper coated with cationic starch. The three CNF suspensions and the starch suspension were successfully coated using both metering systems. In the multi-layer coating, air drying yielded higher coat weights than contact drying, and the difference was more noticeable in M-CNF and T-CNF coated samples. A lack of uniformity observed in CNF coatings increased with the number of coating layers. M-CNF coated samples had a whitish surface with a uniform appearance, but coating was insufficiently compact to provide barrier properties. T-CNF coating exhibited a cracked surface and consequently provided no barrier properties. Drying stresses caused breakage of T-CNF coating, which could be explained by the drop in apparent viscosity at much lower rheological shear stresses in the T-CNF suspension. C-CNF showed successful coatability and provided barrier properties to paper with both metering systems and drying methods. Air permeance decreased considerably and the water wettability of the paper decreased by 60% compared to the reference paper, even with a single coating layer (about 1 g·m−2). In addition, a greaseproof paper (grease resistance >1800 s and kit no >10) was obtained with multi-layer C-CNF coating. Although water vapour permeability at 90 % RH and 20 °C decreased slightly with a single layer of CN