Abstract
There is an ever-increasing interest towards utilizing nanocellulose as barrier coatings and films, with many companies moving towards pilot scale production of nanocellulose to be used primarily for barrier coatings. However, high suspension viscosity and yield stress, poor adhesion to substrates, poor moisture sensitivity, and additional drying infrastructure needed for large-scale processing of nanocelluloses are some of the challenges that need to be addressed before commercialization. The current work aims at understanding and addressing the above challenges and to develop high-throughput continuous processes required to convert nanocellulose suspensions into barrier coatings and films. Rheology of different types of nanocelluloses across a wide range of shear rates is evaluated with special attention on the influence of dispersants (carboxymethyl cellulose (CMC) and Sodium polyacrylate (NaPA)) on the suspension processability and coating quality. A slot-die applicator is used to apply nanocellulose suspensions as a thin layer on a paper substrate in a continuous process. For moisture protection, biodegradable polymers and dispersions are applied onto the nanocellulose-coated paper via extrusion or dispersion coating. The resulting multilayer structure is then evaluated for its barrier properties viz., oxygen, water vapor, mineral oils, and grease at different test conditions. CMC addition reduces the yield stress, increases water retention, and slows down structure recovery (post high-shear) for nanocellulose suspensions, and thus has positive influence on coating quality and barrier properties. A new Casson-power-cross model was proposed to explain the viscosity behavior of cellulose nanofibrils (CNFs) across a wide shear-rate region, and Herchel-Bulkley model explains the viscosity behavior of cellulose nanocrystals (CNCs). Water vapor permeance for multilayer coatings remained below the control single-layer moisture-barrier materials, and oxygen permeance values were similar or lower than that of pure nanocellulose films. Glycerol and sorbitol plasticizers further improve oxygen barrier and kaolin addition improves the adhesion at nanocellulose/thermoplastic interface. The results provide insight into understanding the various factors that influence the continuous processing of a wide variety of nanocellulose suspensions into biodegradable barrier coatings and will pave the way for industrial production of sustainable packaging.
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