Continuous Processing of Nanocellulose and Polylactic Acid into Multilayer Barrier Coatings

Abstract

<p>Recent years have seen an increased interest towards utilizing bio-based and biodegradable materials for barrier packaging applications. Most of the above said materials usually have certain shortcomings that discourage their adoption as a preferred material of choice. Nanocellulose falls into such category. It has excellent barrier against grease, mineral oils and oxygen, but poor tolerance against water vapor, which makes it unsuitable to be used at high humidity. In addition, nanocellulose suspensions' high viscosity and yield stress already at low solids content, and poor adhesion to substrates create additional challenges for high-speed processing. Polylactic acid (PLA) is another potential candidate that has a reasonably high tolerance against water vapor, but rather poor barrier against oxygen. The current work explores the possibility to combine both these materials into thin multilayer coatings onto paperboard. A custom-built slot-die was used to coat either microfibrillated cellulose (MFC) or cellulose nanocrystals (CNCs) onto pigment-coated baseboard in a continuous process. These were subsequently coated with PLA using a pilot scale extrusion coater. Low-density polyethylene (LDPE) was used as a reference extrusion coating. Cationic starch pre-coating and corona treatment improved the adhesion at nanocellulose/baseboard and nanocellulose/PLA interfaces, respectively. Water vapor transmission rate for nanocellulose + PLA coatings remained lower than the control PLA coating, even at a high relative humidity of 90% (38 oC). The multilayer coating had 98% lower oxygen transmission rate compared to just PLA coated baseboard and heptane vapor transmission rate reduced by 99% in comparison to baseboard. Grease barrier for nanocellulose + PLA coatings increased 5-fold compared to nanocellulose alone and 2-fold compared to PLA alone. This approach of processing nanocellulose and PLA into multiple layers utilizing slot-die and extrusion coating in tandem has the potential to produce a barrier packaging paper that is both 100% bio-based and biodegradable.</p>