Cellulose nanofibers for grease and air barrier properties enhancement: A comparison on recycled and food contact paper samples

The application by spray coating of low amounts (2-4 g/m²) of cellulose nanofibers (CNF) onto three different paper substrates, a recycled one (RC, 130 g/m²) and two food contact samples (FC45, 45 g/m² and FC100, 100 g/m²), allowed to investigate the impact of CNF typology and mechanical pre-treatment on the barrier performance against grease and air permeation. Two CNF additives were produced and characterized (TEM, XRD, FT-IR), differing for the production process: TEMPO-mediated oxidation (TOCNF) and enzymatic hydrolysis (ECNF). The crucial role of homogenization cycles to achieve nanoscale dimensions was correlated to the barrier response. TOCNF-based coatings performed better than ECNF ones, reaching a complete grease barrier (kit 12) both for RC and FC100, even at the lower consistencies. The addition of glycerol as plasticizer in formulations allowed to obtain a similar result for FC45, but just in the case of TOCNF-based coatings. A significant decrease of water permeability was also observed when applying TOCNF coating to RC (Cobb₆₀ value from 138 g/m² to 40.3 g/m²) and a complete air barrier was measured at the higher consistency of both CNF as detected by Gurley test. SEM analysis of coated samples disclosed the effect of coatings on porous substrates.