Continuous Processing of Nanocellulose and Polylactic Acid into Multilayer Barrier Coatings

Abstract

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 extrusion coater. Low-density polyethylene was used as for reference extrusion coating. Cationic starch precoating and corona treatment improved the 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 multilayer coating had 98% lower oxygen transmission rate compared to just the PLA-coated baseboard, and the heptane vapor transmission rate reduced by 99% in comparison to the baseboard. The 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% biobased and biodegradable.

Recent Developments in Cellulose Nanomaterial Composites

“…[ 113 ] Likewise, Koppolu CNM‐coated paperboard followed by a thin film of poly(lactic acid) (PLA), reducing oxygen and water transmissions. [ 114 ] In a recent work, Kim et al. took advantage of LbL assembly to design a highly flexible, visible light and radio frequency transparent coating on a poly(ethylene terephthalate) PET film by blending negatively charged CNF with positively charged chitin nanowhiskers.…”

Nanocellulose Bio-Based Composites for Food Packaging

“…In general, the coating of paperboard with NC based layers reduced the oxygen permeability and improved the grease resistance, but a high water vapour permeability was still observed [ 50 , 51 , 52 , 53 , 54 ]. To reduce the high water vapour permeability of NC, combination with, for example, polypyrrole and PLA were reported [ 53 , 55 ]. Polypyrrole particles were added to NFC suspension before coating the paperboard.…”

“…A decrease in oxygen permeability and an increase in the mechanical properties were obtained after coating [ 53 ]. In another approach, a multilayer coating onto paperboard was produced using PLA as the outer layer, while the CNC was the intermediate one, together yielding low oxygen permeability and water vapour permeability [ 55 ]. Transparent multilayer films were produced from NFC thin layers coated on amorphous PLA and semi crystalline PLA substrates.…”

Enhancing the Oxygen Barrier Properties of Nanocellulose at High Humidity: Numerical and Experimental Assessment

“…Moreover, they can be broken down into microplastics and negatively affect the food chain and human health [21]. The protection of the CNF layer on paper with a layer of PLA was described [22] and excellent results were obtained with this bio-based polymer that is expected to be compostable, at least industrially. Methods to help decrease the water sensitivity of CN films were reviewed and the authors suggest further work into chemical modifications of CN materials and the use of layers to protect the CN from moisture [23].…”