Developing chitin nanocrystals for flexible packaging coatings

Highlights

• Formation of stable water-based acrylic resin/TEMPO-oxidized chitin nanocrystals (TOCNs) coating formulations.

• The potential use of TOCNs as a rheological modifier in water-based coating formulations.

• Fabrication of multilayered BOPP/TOCN laminates with oxygen-barrier function.

• Addressing issues of the high sensitivity of TOCNs to moisture and the poor wettability to the hydrophobic surface.

• Unreduced optical transparency when chitin nanocrystals were applied as a coating on flexible packaging.

Abstract

This study assessed the applicability of chitin nanocrystals employed in combination with an existing coating material intended for flexible packaging. The 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) oxidized chitin nanocrystals (TOCNs) were applied 1) as an additive in a water-based acrylic resin (WBAR) that was then coated onto the surface of a biaxially oriented polypropylene (BOPP) film, and 2) as a neat layer in multilayered BOPP laminates bonded by a WBAR adhesive layer. The results indicated that the flow behavior and shear viscosity of the TOCN/WBAR system were dependent on TOCN contents. The TOCNs as a dispersed phase in the acrylic resin matrix did not improve the oxygen barrier property of the resulting coated BOPP. By contrast, the neat continuous TOCN coating layer improved the oxygen barrier property of the laminates of BOPP and TOCNs bonded by the acrylic resin, a 44% oxygen transmission rate reduction for a laminate with a 8.33-$\text{μ}\text{m}$ TOCN layer compared to the laminate without a TOCN layer. The inclusion of the TOCNs maintained the optical transparency of the resulting films.

Introduction

Much attention has been paid to renewable materials that can be potential to be used as alternative replacements for petroleum-based products. Chitin has gained tremendous attention because it has been recognized as an underutilized biopolymer; this biopolymer exists as a major supporting component in crustaceans, fungi, and insects (Goodrich & Winter, 2007; Kumar, 2000; Nair & Dufresne, 2003). Among various types of chitin, α-chitin is the most ubiquitous, which can be obtained in a large quantity from food waste such as crab, shrimp and lobster shells in the canning industry (Kumar, 2000). Approximate 6 $\sim$ 8 million tonnes of these waste shells are generated annually around the world, and they contain 15 $\sim$ 40% α-chitin (Yan & Chen, 2015). Efficient utilization of chitin as a green component in sustainable products not only can benefit our environment but also can create new added-values of chitin.

Chitin on the nanoscale possesses attractive features such as high specific area, high stiffness, high strength and low density in addition to its nontoxicity, renewability, biodegradability, biocompatibility (Farrán et al., 2015; Ifuku, Morooka, Nakagaito, Morimoto, & Saimoto, 2011; Wu, Zhang, Girouard, & Meredith, 2014). Chitin nanomaterials have been applied as green functional components such as reinforcing and antimicrobial agents for natural and synthetic polymers (Butchosa et al., 2013; Li et al., 2015; Shams, Ifuku, Nogi, Oku, & Yano, 2011). To our knowledge, little attention has been paid to the applications in coatings intended for packaging applications, especially in water-based coating systems that are more environmentally friendly than solvent-based coating systems. Mechanical properties and other performance of water-based coatings are often inferior to those of solvent-based coatings (Tan et al., 2016). The incorporation of nanomaterials from polysaccharides to water-based coating systems could improve mechanical performance while maintaining optical transparency (Grüneberger, Kunniger, Zimmermann, & Arnold, 2014; Tan et al., 2016; Trovatti et al., 2010). TEMPO-mediated oxidation followed by mechanical disintegration can yield water-dispersible TEMPO-oxidized chitin nanocrystals (TOCNs) while retaining high crystallinity (Fan, Saito, & Isogai, 2008). Water-dispersible TOCNs with high stiffness and strength might serve as a potential reinforcing and barrier component in water-based coating systems.

Biaxially oriented polypropylene (BOPP) is a commonly used polymer for food packaging. Poor oxygen barrier properties and low surface energy are two limitations of BOPP (Mirabedini, Arabi, Salem, & Asiaban, 2007; Mokwena & Tang, 2012), which might be overcome by surface coatings with barrier and other functional components or lamination with other barrier polymers (Lange & Wyser, 2003). Conventional oxygen barrier polymers, either in the form of coatings or film laminates for food packaging, are non-degradable and petroleum-based products; common synthetic barrier polymer for packaging include such as polyvinyl alcohol (PVOH), polyvinylidene chloride (PVDC), ethylene vinyl alcohol (EVOH) copolymers (Duan et al., 2013; Hong & Krochta, 2004; Mokwena & Tang, 2012). Sustainable barrier materials based on polysaccharide nanomaterials attract growing interests in recent years. TEMPO-oxidized cellulose nanofibers exhibited excellent oxygen barrier performance when applied to PLA film under the dry environment (Fukuzumi, Saito, Iwata, Kumamoto, & Isogai, 2009). Similarly, TEMPO-oxidized α-chitin nanowhisker coated polylactic acid (PLA) films also exhibited excellent oxygen barrier properties at 0% relative humidity (Fan, Fukuzumi, Saito, & Isogai, 2012). Fukuzumi et al. (2009) and Fan et al. (2012)’s studies indicated that cellulose nanofibers and chitin nanowhiskers are promising barrier materials, even extremely thin coating layer (0.1 ∼0.4 $\mu$ m) to a PLA substrate could impart excellent barrier properties when tested at 0% relative humidity. However, the poor wettability on hydrophobic polymer surfaces and the high sensitivity of either cellulose or chitin nanomaterials to moisture restrict their use in practical applications. For example, hydrophobic polymeric substrates need to be surface-hydrophilized, like plasma-treated, so as to increase the wetting of hydrophilic polysaccharide nanomaterials on their surface (Fan et al., 2012; Fukuzumi et al., 2009). Moreover, surprisingly, chitin nanofiber coated- (∼8 μm coating thickness) and cellulose nanocrystal coated (∼7 μm coating thickness) PLA films did not exhibit any improved barrier performance whentested at 50% relative humidity (Satam et al., 2018). Satam et al. (2018) also proposed that a spray-assisted layer-by-layer coating of chitin nanofibers and cellulose nanocrystals could overcome adverse effect of moisture to oxygen barrier and improve oxygen barrier properties at high relative humidity. Herein, we attempt to adopt a conventional lamination method that was widely used in food packaging industry, in which hydrophilic barrier materials are wrapped into water-resistant hydrophobic polymeric substrates using a “tie layer” as a bonding (Mokwena & Tang, 2012).

In this paper, the initial attempt was to apply varying TOCN contents to an existing coating system water-based acrylic resin intended for packaging coatings. We investigated their rheological behavior and their coating performance on BOPP substrates including surface energy and oxygen transmission rates. Subsequently, the attempt was to apply TOCNs as a neat barrier layer combined with the existing coating system water-based acrylic resin as a “tie layer” to be laminated with hydrophobic BOPP films so as to address the drawbacks of TOCNs as a dispersed phase in the water-based resin coatings. A multilayered structure was constructed. The effects of the thickness of the TOCN layer on the oxygen transmission rate (OTR) of BOPP laminates were determined. Finally, the optical transparency of water-based acrylic resin/TOCNs coated BOPP films, and BOPP/TOCNs laminates were also determined.