Environmental friendly and sustainable gas barrier on porous materials: Nanocellulose coatings prepared using spin- and dip-coating

Highlights

• Oxygen barrier coatings solely based on nanosize cellulose were developed.

• The coating thickness varied from hundreds of nanometers for spin-coated layers, to micrometers for dip-coated ones.

• Nanosize cellulose coatings showed better oxygen barrier than polymers such as PE and PS in dry conditions.

• Thin nanocellulose coatings were sensitive for humidity and long term storage.

Abstract

In this study, environmental friendly and sustainable coatings of nanocellulose (NC) were prepared using spin- and dip-coating methods, on two different porous cellulose substrates. Microscopy studies showed that spin-coating technique was suitable for the substrate with smaller pore size, while the dip-coating was suitable for the substrate with larger pore size. The coating thickness ranged from some hundreds of nanometers for the spin-coated layers, to some micrometers for the dip-coated ones. It was also seen that the contact angle increased with the coating thickness and roughness. NC coating resulted in low oxygen permeability (between 0.12 and 24 mL ∗ μm/(m² ∗ 24 h ∗ kPa)) at 23% RH, but at 50% RH the oxygen permeability was too high to be measured, except for the dip-coated sample with 23 μm thickness. Also, it was seen that eight month storing reduced the barrier properties of the coatings when compared with fresh materials. These results indicate that NC coatings have a great potential as sustainable alternative coating on paperboard.

Introduction

Nowadays, more than half of the packaging materials used for food and drugs are made of petroleum-based polymers, such as low and high-density polyethylene (LDPE and HDPE). These types of non-biodegradable polymers are selected because of their good gas-barrier properties, mechanical strength and flexibility, along with their low cost and ease of manufacturing, which are crucial properties for packaging applications. However, petroleum-based polymers are associated with high levels of solid residues in land and water bodies (contamination), along with a high dependency on fossil fuel sources. Their poor biodegradability has resulted in increased plastic pollution in oceans from 5.8 million metric tons (MT) in 1975 to 275 MT in 2010, and the prediction for 2025 is one order of magnitude higher, if the current tendency continues [1]. Due to the environmental problems that non-biodegradable packaging materials are causing, interest in the use of biobased materials has increased in recent years. Ideally, the packaging material should be effective in preventing oxygen from penetrating into food, while being environmentally friendly and sustainable [2], [3]. To meet these demands nanomaterials, such as clay, have been studied as functional additive to improve gas barrier properties of polymers [4], [5]. Also, interest in biopolymers such as poly (lactic acid) [6], [7], starch [8] and cellulose acetate [9] has increased during recent years. Apart from polymers, paperboard is a widely used material for packaging applications and has the advantage of being biobased. However, its oxygen transmission rate (OTR) is very high > 400,000 mL ∗ m^− 2 ∗ 24 h^− 1 [10], compared to for example polyethylene 7800–2600 mL ∗ m^− 2 ∗ 24 h^− 1 [11]. To improve the gas barrier, multilayered structures consisting of paper, polymer or metals are used, but the problem of using non-biodegradable material still exists. Therefore, a biobased coating, such as nanocellulose, could present the ideal solution to improve the barrier properties of paper for packaging applications at low relative humidity (RH), while maintaining biodegradability, sustainability and low gas permeability. Aulin et al. [12] and Lavoine et al. [13], [14] showed that coatings of microfibrillated cellulose (MFC) prepared using a simple rod-coating method improved the mechanical properties and the gas barrier of paper. Similar results were obtained by Syverud et al. [15], where MFC coatings, made with a dynamic sheet former, revealed an increase in the tensile index and elongation at break of the coated paper. However, these studies also showed that increasing the RH had a negative effect on the mechanical and barrier properties of the coatings. Nanocellulose coatings can be produced with simple rod-coating or using sheet forming, as mentioned above, but also using a layer-by-layer (LbL) assembly [16], [17], [18]. The most common LbL techniques are spin-coating (S-C), and dip-coating (D-C). S-C has proved to be a reliable method for the preparation of reproducible and smooth thin films and the technique is based on the removal of the liquid phase from the material suspension using high-speed spinning. The benefit of the S-C technique is that this method is widely used for the production of very thin coatings (below 10 μm). In S-C, a diluted suspension is placed on the coating substrate and rotated at high speed, while the fluid spins off the edges coating the substrate at the same time that the solvent evaporates [19]. D-C is a method that produces anisotropic coatings on the top of the substrates. The D-C coatings can be as thin as required and this is easily controllable with the concentration of the suspension used. The substrate is immersed into a reservoir of the coating suspension to deposit a layer of material and withdrawn [19].

Coatings of cellulose nanofibers and nanocrystals have shown to decrease the OTR of the coated substrate [12]. However, it is interesting to note that most of the studies are done on self-standing films of cellulose (fibers and/or crystals) or coatings on polymeric substrates [8], [9], [20], [21], [22], [23], [24] but very few on porous substrates such as paper board [12], [13], [14], [15], [18]. These studies demonstrated how the gas barrier properties of NC coatings improved with the increase in the grammage and density of the coating. Our previous study, about cellulose nanocrystal-coatings on porous substrates, showed that thin films of only 300 nm increased the oxygen permeability of the substrates from completely permeable to values as low as 0.54 (mL ∗ μm)/(m² ∗ 24 h ∗ kPa) [18].

The main goal of this study was to investigate the use of nanosized cellulose as a environmental friendly and sustainable coating on two different porous substrates using S-C and D-C. We investigated how the morphology of the substrates and the coating method affected the thickness, surface characteristics and barrier properties. Additionally, it was of the interest to gain knowledge how the storage time affected the coatings and their barrier properties. Even though there exists some research on microfibrillated cellulose coatings on paper using bar coating method [12], [13], [14], [15], the study of coatings of pure nanocellulose on porous substrates using S-C and D-C is a new field of research that is worth investigating. The findings of this study are believed to have a great value, since they support the use of biobased and sustainable materials in packaging applications.