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.
Highlights
As of 2012, the annual generation of municipal solid waste is about 1.3 billion tons around the world and is predicted to reach about 2.2 billion tons by 2025.1 Food packaging amounts to 20% of the total municipal solid waste, and depending on the country, 40−85% of food packaging ends up either in landfills or in the oceans.[2,3] This poses an urgent need to replace packaging that contains nonbiodegradable, fossil fuelbased plastics with bio-based and biodegradable alternatives.Nanocellulose, polylactic acid (PLA), starch, methylcellulose, chitosan, soy protein, gelatin, and poly-hydroxyalkanoate (PHA) are some examples of such biomaterials that could potentially be used to replace plastics in various packaging applications
Extrusion coating of low-density polyethylene (LDPE) done on the nanocellulosecoated paperboard was used as a reference to compare barrier properties with PLA-based coatings
Cationic starch and corona treatment were used to improve the adhesion at nanocellulose/baseboard and nanocellulose/PLA interfaces, respectively
Summary
Nanocellulose, polylactic acid (PLA), starch, methylcellulose, chitosan, soy protein, gelatin, and poly-hydroxyalkanoate (PHA) are some examples of such biomaterials that could potentially be used to replace plastics in various packaging applications. Nanocellulose has been attracting interest as a promising biomaterial for packaging applications.[16] Depending on the source and the processing technique, nanocellulose consists of microfibrillated cellulose (MFC) with diameters of 20−60 nm, lengths up to a few micrometers, and crystallinities of 60−70% on the one end of the spectrum[17] and cellulose nanocrystals (CNCs) with diameters of 5−20 nm, lengths of 100−500 nm, and crystallinities close to 90% on the other end of the spectrum.[18] Nanocellulose-based films and coatings exhibit excellent barrier against oxygen, grease, and mineral oils.[19−23] nanocellulose is sensitive to moisture, with most of the barrier properties degrading when the relative humidity (RH) approaches 90%.24. Nanocellulose-based films and coatings exhibit excellent barrier against oxygen, grease, and mineral oils.[19−23] nanocellulose is sensitive to moisture, with most of the barrier properties degrading when the relative humidity (RH) approaches 90%.24 Most of the nanocellulose suspensions have high viscosity and yield stress at already low Received: January 15, 2019 Accepted: March 4, 2019 Published: March 4, 2019
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