Aqueous graphene dispersions for paper packaging

Abstract: Graphene is widely touted as the thinnest and the most versatile material available. As an atomically thin layer of carbon atoms arranged in a hexagonal configuration, graphene has a combination of technologically important properties, such as thermal and electrical conductivity, mechanical strength, and impermeability to gases. From an industrial perspective on applications, water as a dispersing media for graphene offers safer handling and environmental benefits compared with conventional organic solvents. However, the high surface tension of water and the attractive forces between graphene surfaces drive the sheets to aggregation. Although surfactants have been an important stepping stone in the advancement of aqueous graphene dispersions, these surface-active molecules are often needed in excess and have adverse effects on coatings during film formation. These challenges limit the industrial relevance of graphene as an effective barrier in composites. In general, gas barriers against both oxygen and water vapour, made from a single coating formulation, is seemingly a holy grail for the packaging industry. In this thesis work, the aim was to gain a fundamental understanding of aqueous graphene dispersions for gas barriers used in paper packaging. Biobased materials were systematically investigated as dispersing agents for graphene based on dispersing conditions and functional barrier performance. Flavin mononucleotide (FMN), a food additive, dispersed graphene using a relatively low amount of FMN and showed intriguing spectroscopic signatures of π-π interactions with graphene. Starch nanoparticles (SNPs) realised concentrated and stable aqueous graphene dispersions for composite films. The SNP-stabilized graphene sheets in starch films lowered the gas permeability of both oxygen and water vapour simultaneously by over 70% under all the conditions tested. In general, a combined gas barrier performance is unusual for both bioplastics and common petrochemical-based plastics used in the packaging industry. Motivated by the graphene network leading to the extraordinary barrier performance, the aqueous SNP-graphene dispersion was modified for inkjet printing. The printed patterns were flexible and electrically conductive in the order of 104 S m-1 that is on par with the highest reported values in the literature. These surfactant-free aqueous SNP-graphene dispersions have the potential and versatility for paper-based gas barriers with integrated electronics. Multifunctional composite films made from these dispersions, when optimized, could become competitive with commercial plastics, and meet the current and future demands of the packaging industry.