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Chapter 8
Sustainable Development of Polysaccharide Polyelectrolyte Complexes as Eco-Friendly Barrier Materials for Packaging Applications Kai Chi and Jeffrey M. Catchmark* Department of Agricultural and Biological Engineering, The Pennsylvania State University, 226 Agricultural Engineering Building, Shortlidge Road, University Park, Pennsylvania 16802, United States *E-mail:
[email protected].
Interest in exploring high-performance eco-friendly barrier materials that could replace synthetic polymers whose composition and manufacturing processes present ecological challenges is growing. Polysaccharides are natural biopolymers already produced in large volumes for many industries including papermaking, textiles, and food production. Cellulose, starch, chitin, and their chemical derivatives, including carboxymethyl cellulose (CMC) and chitosan (CS) are among the highest volume, least expensive biopolymers produced. These polymers, however, are highly hydrophilic and do not possess adequate liquid barrier properties. Exceptional barrier behavior using these polymers has been achieved by combining them in polyelectrolyte complexation. Specifically, cationic CS and anionic CMC have been combined under high-shear homogenization to creat nanostructured particles that electrostatically coalesce during dehydration, forming a dense insoluble material. The current study demonstrates that this material is resistant to the penetration of grease (TAPPI T 559 cm-02, kit number 12), vegetable oil, and water. With the addition of rigid cellulose nanocrystals, the resulting materials exhibited improved mechanical and water vapor barrier properties. This work demonstrates that electrostatic complexation can be used to produce
© 2018 American Chemical Society
sustainable polysaccharide-based materials with unprecedented performance useful for replacing synthetics or higher cost alternatives in many high-volume applications including paper, food engineering, textiles, packaging, and construction.
Introduction Currently, the production of plastics is growing because of their widespread applications in every aspect of our everyday life. As of 2015, approximately 6.3 billion tonnes of produced plastic is estimated to have become waste, with 9% recycled, 12% incinerated, and 79% accumulated on our planet (1). It is predicted that roughly 12 billion tonnes of plastic waste will end up in landfills or the natural environment by 2050 (1), causing ecological threats such as ocean plastic pollution, microplastic contamination in tap water, and leachate infiltration. More unfortunately, approximately 99% of produced plastics are derived from petrochemicals, which are a limited and nonrenewable resource from economic and environmental perspectives (2, 3) and present several key negatives including price volatility, unreliable global supply, and greenhouse gas emissions (4, 5). As the largest plastics market, the packaging industry consumes roughly 44% of produced plastics (6). In the case of food packaging, traditional petroleum-based plastic packaging materials, such as polyethylene, polypropylene, polystyrene, and polyethylene terephthalate, are widely used because of their affordable cost and superior performance as compared to naturally occurring and biobased polymers. However, petroleum-derived polymer materials typically exhibit limited recyclability, poor biodegradability, and a large environmental footprint, all of which have posed significant waste disposal and environmental pollution issues. Therefore, increased interest in exploring sustainable, cost-effective, and ecologically compatible materials with excellent mechanical and barrier properties for the food packaging industry has driven research on the development of bioderived polymeric materials. Polysaccharide-based composites and blends have shown desirable mechanical performance, gas barrier properties, and antimicrobial properties, opening up new opportunities for scientists and engineers to develop novel packaging systems for food and other applications (7). Nonetheless, the hydrophilic nature of these polysaccharides has caused the resulting materials to have inferior mechanical, thermochemical, and barrier properties as compared to their petroleum-derived counterparts (8). In particular, the gas and water vapor barrier properties of some polysaccharide composite films are impaired at higher relative humidity (RH) conditions (9), which limits their further application as barrier coatings or films for certain food packaging applications. Many efforts have been devoted to developing polysaccharide polyelectrolyte complex materials with adequate barrier properties that could be promising alternatives to petrochemical polymers for food and other packaging applications (10–15). The oppositely charged polysaccharides could spontaneously form electrostatic crosslinking within the resulting materials, contributing to improvement in mechanical and barrier properties (16). Nanocellulose, a sustainable 110
bionanomaterial with many extraordinary features (such as robust mechanical properties, biodegradability, renewability, and high surface area) (17), is emerging as a barrier property enhancement filler or barrier coating for paper and paperboard packaging materials (18). Improved water vapor and oxygen barrier properties have been accomplished by incorporating nanocellulose into various biopolymer or synthetic polymer matrices (9). However, the incorporation of nanocellulose into a binary polyelectrolyte complex system has not been reported, and little is known about the role of nanocellulose in the structure−property relationship of ternary polyelectrolyte complex systems. Specifically, in the case of the paper and paperboard packaging industry, the uncoated lignocellulosic-based paper and paperboard exhibit desirable properties such as low cost, sustainability, recyclability, and biodegradability, yet their porous microstructure and hygroscopic properties have resulted in insufficient barrier properties against water, oxygen, and oil. Usually, the surfaces of paper and paperboard are coated with unsustainable materials such as fossil-based waxes, latex, plastics, and aluminum to develop high-barrier packaging materials at the expense of eco-friendliness and biodegradability. Replacing these coatings with compostable bioderived materials would make a significant impact on the disposable packaging problem and the environment. This chapter presents our recent research on developing polysaccharide polyelectrolyte complex materials as novel barrier coating materials for paper and paperboard packaging applications. Exceptional barrier performance has been achieved by subjecting multipolysaccharide systems to polyelectrolyte complexation coupled with a high-shear homogenization process, opening up new opportunities for high-performance eco-friendly biomaterials in paper, packaging, food engineering, and many other applications.
Experimental Section Materials High purity chitosan (CS) (ChitoClear, Primex Inc, Iceland) was obtained with an average molecular weight (MW) of 214 kDa and a degree of deacylation of 90%. The carboxymethyl cellulose (CMC) sodium salt (lot # 419273) had an average MW of 90 kDa and a degree of substitution of 0.7 according to the supplier (Sigma-Aldrich, St. Louis, MO). Lab-made cellulose nanocrystals (CNC) were extracted from microcrystalline cellulose (Avicel PH-101, Sigma-Aldrich) by a sulfuric acid hydrolysis process. The conditions of acid hydrolysis and subsequent purification processes were detailed in previous studies (19, 20). All chemical reagents including toluene, n-heptane, formic acid, and sodium hydroxide were used as received. Pristine paperboard substrate was kindly provided by Southern Champion Tray (Chattanooga, TN). It was coated on one side with clay, and the uncoated side was used for the coating experiments. High quality nanopure water with a resistivity of 18.2 MΩ/cm (Millipore Milli-Q UF Plus) was used to prepare sample solutions or dispersions. 111
Preparation of Polysaccharide Polyelectrolyte Complex Coating Materials The polysaccharide polyelectrolyte complex (PPC) coating materials were prepared by a high-shear homogenization process. Initially, a 4.5% (w/v) polysaccharide solution was prepared by dispersing CS (or CMC) powder into nanopure water, followed by the addition of formic acid to adjust the solution pH to different values (pH = 4 and pH = 5.7). The polysaccharide solution was then magnetically stirred at 60°C overnight to ensure complete dissolution. For binary PPC coating materials, CS and CMC solutions were combined and subjected to immediate high-shear homogenization by a homogenizer (T25 Ultra-Turrax, IKA) at a speed of 20,000 rpm for 20 min. For ternary PPC coating materials, the desired amount of 1% (w/v) CNC stock suspension was added into CMC solution, followed by stirring at 50°C for at least 2 h. The CMC/CNC mixture was subsequently added into the pre-prepared CS solution and homogenized under exactly the same conditions as the binary PPC materials. The CNC contents in ternary PPC materials were designed to be 5 and 10% (w/w) of the total solid content of the coating materials. In addition, the pH of ternary PPC materials was fixed at 5.7. Thus, the resulting PPC coating materials included CS/CMC_pH=4, CS/CMC_pH=5.7, CS/CMC/CNC5% and CS/CMC/CNC10%. All PPC materials were degassed for 5 min to remove the air bubble formed during the homogenization process before either casting to form free-standing films or coating on the paperboard substrates.
Preparation of PPC Films Various binary and ternary PPC films were fabricated using a casting/ evaporation method. Generally, the degassed PPC dispersion was casted into a glass petri dish. After a 2-day evaporation at 35°C and 20% RH, the PPC film was peeled from the glass petri dish and further dried in a vacuum oven (25 psi) at 60°C for 8 h to remove the residual water. All film samples were conditioned in a desiccator at ambient temperature for at least 3 days before any further analysis.
Preparation of PPC-Coated Paperboards PPC-coated paperboard samples were prepared through a dip coating process according to our previous study (11). The uncoated, porous paperboard substrate was immersed in a freshly prepared PPC dispersion for 5 min. Afterward, the coated sample was removed, allowed to drain, and dried in an oven at 140°C for 15–20 min. The individual polysaccharide–coated paperboards were prepared using the same protocol and were considered control samples. The coating weight was defined as the weight difference per square meter between the initial (before coating) and final weight (after coating and oven drying) of the paperboard substrate. The pristine and coated paperboard samples were stored in a desiccator at room temperature and 20% RH before the morphology and liquid barrier performance analysis. 112
Characterization The surface charge content of individual polysaccharides (amino group of CS, carboxylate group of CMC, and sulfate group of CNC) was determined by conductometric titration. Conductivity and pH values were simultaneously monitored using a SevenExcellence pH/conductivity meter (Mettler Toledo, Columbus, OH) with an InLab 730 conductivity probe. Sample preparation, titration conditions and processes, and the calculation of surface group content have been described in previous studies (11, 17). The particle size of PPC coating materials was measured by a laser diffractometer (Mastersizer 2000, Malvern Panalytical) at 25°C. At least three measurements were taken for each sample. The viscosity of PPC coating materials was determined using a Zahn cup (type 2). Zeta potential measurement of various PPC coating materials was conducted using a Zetasizer Nano ZS (Malvern Panalytical) at 25°C. Five runs were performed for each sample, and the data was fitted using the Smoluchowski model. Scanning electron microscopy (SEM) morphology of PPC coating materials was carried out with a Nova NanoSEM 630 field emission scanning electron microscope (FEI) operating at an accelerating voltage of 3 kV, a beam current of 42 pA, and a working distance of 4 mm. Before SEM imaging, PPC coating dispersions were diluted (~0.001 wt %), lyophilized, and sputter coated with a thin layer (~3–5 nm) of iridium. For PPC film samples, the surface and cryofractured cross section morphologies were observed using the same SEM instrument and operation conditions as previously described. Tensile tests of PPC films were performed using a dynamic mechanical analyzer (Q800, TA Instruments Inc., New Castle, DE). Rectangular strips with the dimensions of 50 mm × 5 mm × 0.05 mm (length × width × thickness) were cut from films and tested at a strain ramp rate of 5%/min and a gauge length of 10 mm until break at ambient conditions. For each sample, at least five specimens were used for tensile testing. The swelling of PPC films was determined by immersing the film strips (20 × 50 mm2) in water. The films were removed from water at various times ranging from 1 to 24 h, blotted between filter papers to remove excess surface liquid, and reweighed at various times up to 24 h. The degree of swelling (DS) was calculated using the following formula: DS (%) = (weight of swollen film − weight of initial dry film)/(weight of initial dry film) × 100. At least three replicates were measured for each sample. The water vapor transmission rate (WVTR) of PPC films was determined gravimetrically following the ASTM Standard E 96 procedure with slight modification, according to our previous study (11). For PPC-coated paperboard samples, the surface and cross section morphologies were characterized by SEM imaging. The morphology of individual polysaccharide (CS and CMC)–coated paperboard was also observed. In addition, the liquid (grease, water, and vegetable oil) barrier properties of individual polysaccharide– and PPC-coated paperboards were analyzed at room temperature. The grease resistance property was evaluated by the standard method for paper and paperboard known as the Kit test (TAPPI T 559 cm-02), as described in a previous study (11). The water and vegetable oil resistance tests were performed by depositing a small volume of liquid (~10 μL) on the surface 113
of coated paperboard samples. Different locations on the coated paperboard were selected, and the results were reported as the average of five measurements.
Results and Discussion PPC Coating Materials In this study, various binary and ternary polysaccharide-based coating materials were developed by combining them in polyelectrolyte complexation. During the complexation process, strong electrostatic association via Coulombic attraction is believed to be the major interaction between cationic CS and anionic cellulose materials (CMC and CNC), forming a physically crosslinked network structure. Other intermolecular interactions such as hydrogen bonding and van der Waals forces could also exist because of the similar saccharide structure. Many internal (polysaccharide backbone structure, MW, degree of substitution of ionic groups, etc.) and external parameters (pH, ionic strength, charge molar ratio, etc.) can impact the formation, structures, and properties of PPC materials (21). Previous studies have demonstrated that the charge molar ratio between polycations and polyanions is the most important factor that can determine the structure and properties of PPCs (21, 22). The 1:1 charge molar ratio can ensure the majority of available surface ionic groups of polysaccharides electrostatically interact with their counterparts, contributing to a high degree of crosslinking within the resulting materials. The evolution of surface charge content of individual polysaccharides as a function of solution pH is shown in Figure 1a. CS has glucosamine and acetyl glucosamine randomly distributed on its backbone. The pKa of CS is ~6.0–6.3, resulting in the protonation of amine groups in acidic conditions with cationic charges. CMC, on the other hand, is a weak polyanion and can be deprotonated at a pH higher than its pKa (~4.5–5). At different pH values, the charge molar ratio between CS and CMC was different. Therefore, two different solution pH values (pH = 4 and 5.7) were chosen for binary PPC material composed of CS and CMC in order to understand the influence of the degree of ionic crosslinking on the structure and properties of the resulting materials. More cationic charges derived from protonated amine groups of CS existed at pH = 4, while the 1:1 charge stoichiometry was achieved when the pH reached 5.7. CNC had anionic sulfate groups on its surface because of the esterification of surface hydroxyl groups in the acid hydrolysis process. The amount of sulfate groups, 0.23 mmol/g, was stable from pH = 3 to 10, as the pKa of sulfate is ~1.5–2. Considering such a large difference in the density of ionizable groups, CNC was incorporated into the CS/CMC blend by weight fraction to fabricate ternary PPC materials. Properties such as particle size, viscosity, and zeta potential of various PPC coating materials were characterized (Figure 1b) before applying the coating on paperboard substrates. These parameters are important, as they are the main factors that determine the formation, surface coverage, and properties of the resulting coating layer on the paperboard surface. For binary PPC materials, an increase of pH from 4 to 5.7 induced larger particle size (49.5 μm vs 93.7 μm), lower viscosity (93.2 cSt vs 70.8 cSt), and smaller zeta potential value (20.4 114
mV vs 8.1 mV), implying increased electrostatic interaction between oppositely charged CS and CMC and therefore a high degree of crosslinking within the resulting material. The particle size distribution (PSD), determined by using the formula PSD = [Dv (90) − Dv (10)]/Dv (50), exhibited an increase from 2.81 to 3.23 (data not shown in Figure 1), indicating a wider PSD in the CS/CMC blend at pH = 5.7. PPC coating material with a wider PSD is believed to be beneficial to the effective coverage of porous paperboard substrates, as smaller particles could fill up spaces between large particles. With the addition of CNC, the ternary PPC materials showed continuously increased particle size and lowered viscosity and zeta potential. At 10% CNC addition, large particles (101 μm) were formed, causing a decrease in the viscosity (61.2 cSt) of coating materials. It could be explained that these large complex aggregates displayed reduced surface area of the hydrophilic polysaccharide and thus reduced the viscosity in water. Therefore, the inherent high viscosity of individual polysaccharide (especially for CS) was lost when polyelectrolyte complexation occurred. This particle size–viscosity relationship has been depicted in a previous study (10), and the current results are in agreement. The smaller zeta potential indicated that some CNC might participate in the ionic complexation with CS. However, it should be noted that other intermolecular interactions (like hydrogen bonding and van der Waals forces) between CNC and CMC or CS might also exist because of their similar saccharide structures. The morphology of PPC materials is shown in Figure 1c,d. Fiber-shaped particles were found for binary PPC prepared at pH = 4 (Figure 1c), whereas a mixture of fiber-shaped and platelet-like particles was observed for ternary PPC with 10% CNC content (Figure 1d). In addition, larger particles can be seen in Figure 1d, which supports the particle size measurement. The morphology of PPC particles is more dependent on the homogenization and lyophilization processes. Under the same production process and sample preparation procedure, ternary PPC material with 10% CNC displayed larger, platelet-like particles, suggesting the participation of CNC in the interaction with CS and CMC to form larger particles. The formation of fiber-shaped particles in this study is hypothesized to originate from the homogenization process. The application of immediate high-shear blending opens up a new process element in which the PPC materials were sheared during the ionic complexation process, yielding isolated fiber-like particles. PPC Films The properties of solution-casted PPC films were characterized as represented in Figure 2. Figure 2a–c depicts the surface and cross section morphologies of various PPC films analyzed by SEM. All PPC films were smooth and homogeneous as observed at the surface and cross section, independent of the preparation pH and addition of CNC. No pores or voids could be detected in the films, suggesting that the films were dense and compact. The addition of CNC, even at 10% weight fraction (Figure 2c), did not cause any discernable phase separation, indicating good compatibility between CNC and the CS/CMC blend. The distribution of CNC in the CS/CMC blend is difficult to observe 115
without the help of energy dispersive electron spectroscopy analysis with sulfur mapping of CNC. An evenly distributed sulfur element was observed in PPC film with the addition of 10% CNC as reported in our previous study (11), suggesting the uniform distribution of CNC in PPC film. Such uniform distribution of rigid CNC contributes to the nanoreinforcing effect of CNC, resulting in an improvement in the mechanical and water vapor barrier properties of PPC films. It is also worth mentioning that the ternary PPC films were prepared by initially mixing anionic CNC and CMC, followed first by homogenizing with CS under high-shear conditions and finally by solution casting into films. Both CMC and CNC have negative charges and similar backbone structures, yet they possess a large difference in charge density and backbone rigidity. This work hypothesizes that the high surface charge (almost 10 times larger than CNC based on conductometric titration results) (11) and flexible CMC polymer chain segments might be more prone to electrostatic interaction with the cationic CS polymer chains, thus forming a densely entangled polymer network. By contrast, the rigid and much less charged CNC was unable to wrap around the CS polymer chains and form the crosslinked network structure. Therefore, the ternary PPC films developed in this study might be based on the individualized distribution of CNC in a matrix of CS/CMC with a crosslinked structure.
Figure 1. Characterization of PPC coating materials: (a) surface charge content of individual polysaccharides (CMC, CS, and CNC) as a function of pH; (b) particle size, viscosity, and zeta potential of PPC coating materials prepared at various conditions, including binary PPC prepared at pH = 4 (CS/CMC_pH = 4) and 5.7 (CS/CMC_pH = 5.7) and ternary PPC prepared at pH = 5.7 and different CNC loading levels (5 and 10%); SEM images of freeze dried CS/CMC_pH = 4 (c) and CS/CMC/CNC10% (d). (a) and (b) reproduced with permission from reference (11). Copyright 2018 Elsevier. 116
Figure 2. Characterization of free-standing PPC films: SEM surface and cross section (inset) morphologies of CS/CMC_pH = 4 (a), CS/CMC_pH = 5.7 (b), and CS/CMC/CNC10% (c); tensile (d), water swelling (e), and water vapor barrier (f) properties of various PPC films. (a–d) and (f) reproduced with permission from reference (11). Copyright 2018 Elsevier.
The tensile properties, including tensile strength (TS), Young’s modulus (E), and elongation at break, of various PPC films are depicted in Figure 2d. A high level of ionic crosslinking within the CS/CMC blend should improve the strength and stiffness of the film. This assumption is supported by the tensile data obtained for binary PPC films with varied degrees of electrostatic crosslinking. PPC film prepared at pH = 5.7 had a TS and E of 43.2 MPa and 3.1 GPa, respectively, which were 51 and 57% higher than those of PPC film prepared at pH = 4. Several previous studies have reported a similar beneficial influence of ionic complexation on the mechanical properties of PPC materials (13, 15). With the incorporation of CNC, both the TS and E values of the resulting PPC films significantly increased, which is primarily attributable to the uniform distribution and interface compatibility of rigid, highly crystalline CNC within the CS/CMC matrix, as evidenced by the energy dispersive electron spectroscopy data (11). Nanocellulosic materials (including CNC, cellulose nanofibrils, and bacterial nanocellulose) with high crystallinity (~70–90%), a high Young’s modulus (~100–130 GPa), and a large surface area (several hundred m2/g) are well known for their exceptional nanoreinforcing efficiency in many polymer nanocomposites (17, 23, 24). The distribution and interface compatibility of CNC within the polymer matrix are two key elements of its reinforcing efficiency. A high loading level of CNC (>10 wt %) in the PPC materials could cause CNC self-aggregation, impairing both TS and E, as evidenced in our recent study (11). CS/CNC film was also prepared as another control sample to elucidate the role of CMC in the 117
ternary PPC films. The TS and E of CNC/CS film were 45.2 ± 3 MPa and 3.3 ± 0.3 GPa, respectively, and were inferior to those of ternary PPC films with the same CNC content. This result supports our hypothesis that the high level of electrostatic crosslinking between CS and CMC might play a more significant role in improving the stiffness and strength of PPC films. The swelling kinetics of PPC films is shown in Figure 2e. All films showed rapid swelling within 1 h, and no significant increase in DS was observed after 3 h. Binary PPC film with pH = 4 represented the maximum DS at any time over other PPC films. The CS and CMC control films were also tested for DS at the same conditions (data not shown here). Both individual polysaccharides were completely dissolved within 2 h. The lowest DS was observed for binary PPC films prepared at pH = 5.7, possibly because of the high level of crosslinking within the films. Films with stronger crosslinking could result in fewer available hydrophilic groups, such as hydroxyl groups, that can bind water. For ternary PPC films, the incorporation of CNC negatively impacted the DS. Indeed, CNC is an extremely hydrophilic material and can absorb a large amount of water. Overall, it seems that PPC material can become resistant to water swelling after polyelectrolyte complexation occurs. The WVTR of PPC film samples is shown in Figure 2f. WVTR is a critical parameter for materials used in food packaging, as the transportation of water vapor from the surrounding environment (especially at higher moisture conditions) into food products can significantly impact the shelf life of those products. Generally, electrostatic crosslinking and the addition of evenly distributed crystalline CNC were found to be beneficial to the improvement of WVTR in our study. Binary PPC film with a high level of crosslinking (pH = 5.7) exhibited significantly decreased WVTR (from 32,000 to 13,220 g μm m-2 d-1). On the other hand, with incorporation of 10 wt % CNC, the WVTR of PPC film was further decreased by 40% (from 13,220 to 7982 g μm m-2 d-1). Many factors, including temperature, pressure, crystallinity, wettability, film density, and structure and pore size, can impact the WVTR of a film sample. The homogeneous, defect-free, and densely packed PPC films, as observed from SEM analysis, could block effective travel for the diffusion of water vapor. The role of electrostatic crosslinking in the improvement of WVTR has been previously reported (11, 15). It is speculated that ionic interaction could enhance the cohesive energy density of a PPC film, therefore largely inhibiting the diffusion of water vapor (18). Improved water vapor barrier properties of PPC films after the incorporation of highly crystalline CNC could possibly be attributed to the synergetic roles of impermeable crystalline regions of CNC and a dense percolating nanoparticle network, which together increase the tortuousness of the diffusion path water vapor must take to permeate the film. Furthermore, other intermolecular interactions between CNC and a CS/CMC matrix, possibly including ionic bonding, hydrogen bonding, and van der Waals forces, could lead to the formation of a densely packed network structure, higher cohesive energy density, and lower free volumes with lower permeability to water vapor. High levels of CNC content (>10 wt %) in PPC films were found to impair the water vapor barrier property in our recent study (11) because of the aggregation behavior of CNC. 118
PPC-Coated Paperboards The surface and cross section morphologies of uncoated and PPC-coated paperboards were analyzed by SEM as shown in Figures 3 and 4. As a control, the morphology of paperboard coated with individual polysaccharide (CS and CMC) was also evaluated. The pristine, uncoated paperboard (Figure 3a,a1) exhibited a porous structure with pore size ranging from 10 to 100 μm. Many holes and cavities were seen on the surface of the pristine paperboard. In addition, small pores (~5 μm) were present (not shown here) at the cellulose fiber surface. The porous characteristic of pristine paperboard leads to poor barrier performance against water, oxygen, oil, and grease. Individual polysaccharides were deposited on the paperboard substrate at a coating weight of ~8 g/m2, which could be considered a starting point for well-coated paperboard intended for barrier applications (25). In the case of CS-coated paperboard (Figure 3b,b1), partial coverage of the substrate was observed. The number of holes was largely decreased, yet some small pores (~10 μm) still existed. Further, the covered surface was not smooth and showed a dented feature that can be seen in the inset of Figure 3b. It seems that the CS solution penetrated the paper sheet and thus could not form a continuous film on the paperboard substrate. This result is unexpected, as the CS solution used for coating had a solid content of 3.5% and high viscosity. The CMC-coated paperboard showed homogeneous and void-free surface characteristics. The CMC coating’s cross section image displayed a layered structure (~10–15 μm in thickness), implying the formation of a continuous film on the paperboard.
Figure 3. SEM images of surface (a–c) and cross section (a1–c1) morphologies of pristine paperboards (a and a1) and CS- (b and b1) and CMC-coated (c and c1) paperboards with the coating weight of 8 g/m2. 119
Deposition of the PPC coating materials onto cellulosic paperboard substrate led to different surface features as seen in Figure 4. A lower coating weight (4 g/m2) was applied for CS/CMC prepared at pH = 4. The pores within the paperboard were filled by the coating material, yet the covered surface showed roughness and dented features (see white arrows in the inset of Figure 4a). Thus, a higher coating weight is needed for a smooth and fully covered surface, which could ensure the formation of continuous film on the substrate. With the coating weight of 8 g/m2, all PPC-coated paperboard samples exhibited cavity-free and smooth surface features, indicating the formation of a continuous layer of coating material on the paperboard substrates. The appropriated particle size and viscosity of PPC materials are believed to be of great importance in forming homogeneous, continuous, and compact barrier layers on the paperboard substrate (11). In the case of ternary PPC with a 10% CNC-coated paperboard sample, larger particles (white arrows in Figure 4d) were observed, in agreement with the previous particle size data.
Figure 4. SEM images of surface morphology of PPC-coated paperboards with magnifications of 200 and 600 times (inset): CS/CMC_pH = 4 with the coating weight of 4 (a) and 8 g/m2 (b); CS/CMC_pH=5.7 (c) and CS/CMC/CNC10% (d) with the coating weight of 8 g/m2. 120
The liquid barrier properties of individual polysaccharide– and PPCcoated paperboards are represented in Figure 5. Generally, the individual polysaccharide–coated paperboard samples exhibited very poor barrier performance against grease, water, and vegetable oil, as compared to paperboards coated with PPC materials. The hydrophilic nature, uneven coverage on paperboard, and high wettability may contribute to the penetration of these liquids. In comparison, the PPC materials developed in this study show significantly enhanced liquid barrier properties. In the case of paperboard coated with binary PPC materials prepared at pH = 4, improvements in grease (kit number of 6), water penetration (2 days), and vegetable oil penetration (2 days) resistance were observed. With a high degree of ionic crosslinking, paperboard coated with binary PPC materials prepared at pH = 5.7 showed the best barrier performance against the liquids studied here, exhibiting a kit number of 12 (comparable to a polyethylene–coated paper or paperboard) and water and vegetable oil penetration resistance up to 7 days. The addition of CNC into this binary PPC system could affect the liquid barrier performance of the resulting materials, especially at high CNC content. The addition of 10% CNC notably deteriorated the grease, water, and vegetable oil resistance performance of coated paperboard. Possible reasons could be the increased wettability and hydrophilicity and the existence of CNC-aggregated domains that introduce porosity after incorporating a high content of CNC. A recent study by Gicquel et al. (25) has revealed the poor resistance of CNC coating against grease, as evidenced by the quick penetration (5 s) of grease into the pure CNC-coated paper substrate. CNC is a highly hydrophilic material, and the increased content of CNC in the PPC coating material contributes to the water wettability of the resulting PPC barrier layer. Additionally, as depicted in our previous study (11), the larger particle size and decreased viscosity of PPC coatings induced by the incorporation of CNC could affect the microstructure of the coatings, possibly leading to the existence of mesopores or micropores within the coating layer. The aforementioned defects are vulnerable to liquid penetration and especially to water penetration, as water could act as a plasticizer to split up the hydrogen-bonded CNC aggregated domains that provide the space and pathway for liquid water. All ternary PPC material–coated paperboard was considered grease resistant, as the kit numbers were larger than 8. SEM surface and cross section morphologies of paperboards coated with CMC (Figure 3c,c1) and CS/CMC prepared at pH = 4 (Figure 4b) displayed defect-free, uniform surface coverage of the paperboard substrate with a continuous film having formed on the surface. However, the liquid barrier data from Figure 5 suggest that achieving a homogeneous and continuous layer on top of the paperboard substrate is not sufficient to resist liquid penetration. Therefore, this work hypothesizes that surface wettability and electrostatic crosslinking are key parameters that could dominate the barrier performance of PPC-coated paperboard. Theories of capillary penetration indicate that there is a link between the surface wettability and the permeation of liquid. Assuming that there are no severe defects, a liquid is unable to penetrate a barrier layer if it is difficult for the liquid to wet it (18). For CMC- and ternary PPC material–coated paperboards, the surface water wettability is high because of the hydrophilic nature of CMC 121
and CNC, thereby accelerating the penetration and migration of water through the paperboard. On the other hand, electrostatic crosslinking within the PPC materials could ensure a continuous and densely packed structure within the barrier layer.
Figure 5. Liquid (grease, water, and vegetable oil) barrier properties of polysaccharide control and PPC-coated paperboards. Reproduced with permission from reference (11). Copyright 2018 Elsevier.
Conclusions Sustainable, cost-effective, and ecologically compatible materials derived from renewable natural resources have attracted a tremendous level of attention as replacements for a broad array of high-volume commercial materials based on petroleum-derived compounds. In this work, novel binary and ternary PPC materials composed of CS, CMC, and CNC were successfully fabricated through a high-shear blending approach. The developed PPC film materials exhibited homogeneous, densely packed morphological characteristics and improved mechanical and water vapor barrier properties, which were ascribed to the uniform distribution and good interfacial compatibility of CNC within the electrostatically crosslinked CS/CMC matrix. PPC films with 10 wt % CNC content showed TS and a Young’s modulus of 60.6 MPa and 4.7 GPa, respectively, and a WVTR of 7982 g μm m-2 d-1. When applied as barrier coatings on porous paperboard substrate, the PPC materials functioned as efficient barrier layers that resisted the penetration of water, oil, and grease. A high level of electrostatically crosslinked PPC coating without added CNC could make the resulting coated paperboard 122
grease resistant (kit number of 12), as well as water and vegetable oil resistant up to 7 days. It is expected that such sustainable and ecologically compatible PPC materials may be competitive barrier alternatives to petroleum-derived polymers for food packaging and handling as well as many other product applications. Furthermore, such edible barriers may offer new approaches for creating high-performance foods where control over the transport of aqueous solutions and oils is important.
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