ContinuousProcessing of Nanocellulose and PolylacticAcid into Multilayer Barrier Coatings

摘要

Recent years have seen an increased interest toward utilizing biobased and biodegradable materials for barrier packaging applications. Most of the abovementioned materials usually have certain shortcomings that discourage their adoption as a preferred material of choice. Nanocellulose falls into such a 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 solid content and poor adhesion to substrates create additional challenges for high-speed processing. Polylactic acid (PLA) is another potential candidate that has reasonably high tolerance against water vapor but rather a poor barrier against oxygen. The current work explores the possibility of combining both these materials into thin multilayer coatings onto a paperboard. A custom-built slot-die was used to coat either microfibrillated cellulose or cellulose nanocrystals onto a pigment-coated baseboard in a continuous process.These were subsequently coated with PLA using a pilot-scale extrusioncoater. Low-density polyethylene was used as for reference extrusioncoating. Cationic starch precoating and corona treatment improvedthe adhesion at nanocellulose/baseboard and nanocellulose/PLA interfaces,respectively. The water vapor transmission rate for nanocellulose+ PLA coatings remained lower than that of the control PLA coating,even at a high relative humidity of 90% (38 °C). The multilayercoating had 98% lower oxygen transmission rate compared to just thePLA-coated baseboard, and the heptane vapor transmission rate reducedby 99% in comparison to the baseboard. The grease barrier for nanocellulose+ PLA coatings increased 5-fold compared to nanocellulose alone and2-fold compared to PLA alone. This approach of processing nanocelluloseand PLA into multiple layers utilizing slot-die and extrusion coatingin tandem has the potential to produce a barrier packaging paper thatis both 100% biobased and biodegradable.