Synergistic interactions between nanocellulose and clay minerals for paper packaging coatings

Nanocomposites based on biopolymers and clay minerals are gaining attention due to their improved mechanical, thermal, and barrier properties.¹ In this study, we investigated the interactions between cationic cellulose nanofibrils (cCNF) and anionic layered silicates, montmorillonite (MMT) or kaolin (KAO), to develop functional coatings for paper packaging. Zeta (ζ) potential measurements showed good colloidal stability of the pristine suspensions of cCNF and MMT (+52 and −34 mV, respectively), while KAO suspension presented poor stability (−18 mV). The addition of MMT to cCNF maintained stability up to 70 wt.% clay (ζ > +42 mV), but a charge inversion (to −24 mV) occurred at 80 wt.% clay, indicating that the neutral point lies between 70 and 80 wt.% clay. In contrast, cCNF+KAO mixtures did not show charge inversion in the proportions studied. Quartz crystal microbalance with dissipation monitoring experiments showed strong interfacial interactions between cCNF/MMT and cCNF/KAO. The increase in dissipation upon the adsorption of the MMT layer onto cCNF was smaller than that observed when KAO adsorbed onto cCNF. This suggests that the cCNF/MMT structure is more compact and rigid, while the cCNF/KAO layer is looser and more viscoelastic.² Mixtures of cCNF and clays in different proportions were applied as coatings to paperboard substrates. Scanning electron microscopy images showed that films formed on the paper reduce both roughness and porosity near the surface. Water vapor transmission rate measurements showed a reduction of 5% for cCNF+MMT and 4% for cCNF+KAO compared to uncoated paper. Oil/grease resistance tests revealed a significant improvement with cCNF+MMT coatings, reaching kit test #12. Oxygen transmission rate experiments (23 ºC, dry) demonstrated a dramatic reduction of at least 96% for cCNF+MMT coatings compared to uncoated paper. The results showed that cCNF/clay-based coatings enhance the barrier properties of paper, expanding the use of this type of material in packaging. Further studies are needed to better understand the interactions between cCNF and anionic clays, as well as to optimize the coating formulation for improved moisture resistance.

Acknowledgment: FAPESP (Grant #2021/11967-6), FUNDEPAG, CCD Circula.

References

1. L. Alves et al. Adv. Colloid Interface Sci. 2019, 272, 101994.

2. M. Ghanadpour et al. Applied Materials Today, 2017, 9, 229−239.