Nanocellulose Coatings for Use in

Article pubs.acs.org/IECR

Multilayered Alkyd Resin/Nanocellulose Coatings for Use in Renewable Packaging Solutions with a High Level of Moisture Resistance Christian Aulin*,†,‡ and Göran Ström† †

Innventia AB, Box 5604, SE-11486 Stockholm, Sweden Wallenberg Wood Science Center, Royal Institute of Technology, SE-10044 Stockholm, Sweden

‡

ABSTRACT: A surprisingly simple and rapid methodology for large-area, lightweight, and thin laminate coatings with remarkable moisture barrier properties is introduced. Commercially available paperboards are coated with thin layers of nanocellulose. The nanocellulose coating induces a surface smoothening effect on the coated sheets as characterized by environmental scanning electron microscopy and white light interferometry. A moisture-protective layer of renewable alkyd resins is deposited on the nanocellulose precoated sheets using a water-borne dispersion coating process or lithographic printing. Through an auto-oxidation process, the applied alkyd resins are transformed into moisture sealant layers. The moisture barrier properties are characterized in detail by water vapor permeability measurements at different levels of relative humidity. The water vapor barrier properties of the nanocellulose precoated substrates were significantly improved by thin layers of renewable alkyd resins. The effect of the alkyd resin properties, coating technologies, and base paper substrates on the final barrier performance of the sheets were studied. It was found that the nanocellulose coating had a notable effect on the homogeneity and barrier performance of the alkyd resin layers and in particular those alkyd resin layers that were applied by printing. The concept is environmentally friendly, energy-efficient, and economic and is ready for scaling-up via continuous roll-to-roll processes. Largescale renewable coatings applicable for sustainable packaging solutions are foreseen.

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intermolecular hydrogen bonds.14 Nanocellulose materials can be isolated by chemical/enzymatic and homogenization treatments from the cell walls of wood and plants,15−19 where they are responsible for structural strength. NFC forms a remarkable emerging class of nature-derived nanomaterials because of its extraordinary mechanical properties, combining high stiffness of up to ca. 140 GPa and its expected strength in the GPa range with a lightweight character (density ca. 1.5 g/ cm3).20 Since NFC is derived from wood or plant sources, it is globally abundant and renewable and represents a resource that does not interfere with the food chain or require petrochemical components. Consequently, NFC is emerging as one of the most promising sustainable building blocks for future advanced materials. So far the main interest in NFC has been to generate strong and tough nanopapers and nanocomposites by adding small amounts to polymeric matrices or robust foams and aerogels.21−24 Flexible and transparent NFC sheets with low thermal expansion have been used as substrates for organic light-emitting diode displays, highlighting the possibilities of developing new forms of electronics based on degradable and renewable materials.25 In order to expand the use of nanocellulose as a gas barrier in large-volume packaging applications in high-moisture environments, the hydrophilicity of the nanofibers must be decreased. Recently, the use of surfactants has enabled the production of

INTRODUCTION There is growing interest in using agriculture and food industry byproducts to develop biodegradable materials to replace petroleum-based polymers in packaging applications. Furthermore, the use of nanotechnology in food packaging is expected to grow rapidly over the next few years as further globalization increases demand for shelf life-enhancing packaging. Applications of nanotechnology include improved barrier, mechanical, and antimicrobial properties as well as the incorporation of nanosensors for traceability and the monitoring of the condition of foodstuffs during transport.1 In recent years, a lot of effort has been aimed at developing new biobased polymer containing films and nanocomposites which can act as barrier materials in packaging.2−10 Unlike synthetic plastics, under dry conditions the films of natural polymers exhibit good barrier properties against oxygen and grease due to their high cohesive energy density. However, natural polymers are hydrophilic in nature, and films produced from these materials are often hygroscopic, resulting in the partial loss of their barrier properties at high humidity levels.4,11 The gas permeability of polysaccharide materials can increase by orders of magnitude as humidity increases. Since most food applications demand materials that are resistant to moisture as well, the major challenge is to overcome the inherent hydrophilic behavior of these biomaterials.12,13 Nanocellulose, also referred to as nanofibrillated cellulose (NFC) or microfibrillated cellulose (MFC), exhibits diameters in the nanometer range and lengths reaching several micrometers. These nanofibers are composed of aligned β-D-(1→ 4)glucopyranose polysaccharide chains, which form native cellulose I crystals in which the parallel chains form strong © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2582

July 6, 2012 January 29, 2013 January 31, 2013 January 31, 2013 dx.doi.org/10.1021/ie301785a | Ind. Eng. Chem. Res. 2013, 52, 2582−2589

Industrial & Engineering Chemistry Research

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EXPERIMENTAL SECTION Materials. In order to study the impact of the surface structure of the substrate formation on the resulting barrier properties of the alkyd resin layer, two cellulose-based packaging materials were used. A highly open surface structure was represented by the uncoated side (bleached sulfate pulp) of a commercial one-side coated paperboard from Stora Enso (Fors Mill, Sweden) with a grammage of 204 g/m2 and a thickness of 312 μm. This substrate is named substrate O (as in open) hereinafter. The other substrate had a more closed surface structure and was represented by a surface-treated side of an unbleached kraft paper with a grammage of 191 g/m2 and a thickness of 263 μm. This substrate is named substrate C (as in closed) hereinafter. A commercial low-density polyethylene (LDPE) film was used as a reference substrate. Nanocellulose was prepared by a carboxymethylation16 pretreatment of the fibers. In brief, the dissolving pulp (Domsjö Dissolving Plus, Domsjö Mills, Sweden) was first dispersed in deionized water at 10 000 revolutions in an ordinary laboratory reslusher. The fibers were then solventexchanged to ethanol by washing in ethanol four times with an intermediate filtration step. The fibers were thereafter impregnated with a solution of monochloroacetic acid in isopropanol. This carboxymethylation reaction was allowed to continue for 1 h. Following this carboxymethylation step, the fibers were filtered and washed in three steps: first with deionized water, then with acetic acid (0.1 M), and finally with deionized water. The fibers were then impregnated with a NaHCO3 solution (4 wt % solution) to convert the carboxyl groups to their sodium form. Finally, the fibers were washed with deionized water and drained on a Buchner funnel. After these treatments, the anionic fibers were homogenized using the high-pressure fluidizer (Microfluidics Ind., U.S.). Cellulose slurries containing a 2% pulp fiber suspension in deionized water were processed through the homogenizer. The thus prepared nanocellulose dispersion was further diluted through homogenization to a final concentration of 0.85%. The density of a dry nanocellulose film has previously been determined to be 1.57 g/cm3.4 Four commercially available alkyd resins (DSM, Netherlands) were used in the coating trials (Table 1). The alkyd

stable cellulose whisker suspensions in toluene and cyclohexane.26,27 Alternatively, chemical modifications to the surface hydroxyl groups have also been studied using silane reagents,28,29 fluorination,30 and acetylation.31 Furthermore, frequently used methods to improve the strength and water resistance as well as barrier properties of natural polymers are to blend them with inorganic fillers, such as montmorillonite.12,13,32 Despite a large amount of work on the modification of nanocellulose fibrils and substrates and corresponding oxygen permeability measurements, few studies report on the water vapor permeability of nanocellulose and the use of moisture protective layers on nanocellulose films and coatings.33,34 Alkyd resins have been widely used in coating applications, such as paint, coating, adhesives, binder for composites, etc. Alkyd resins are polyesters whose principal building blocks are fatty acids derived from vegetable oils (e.g., linseed oil, soybean oil, and tall oil), polyhydric alcohols (e.g., glycerol) and a dicarboxylic acid (e.g., phthalic acid).35 They can be dissolved in organic solvents, such as white spirit, and cast from the solution to create glossy coatings or emulsified in water and used to create waterborne coatings.35,36 The alkyd molecule has pendant unsaturated hydrocarbon chains which originate from the vegetable oils used during production. Alkyd resins are referred to as drying oils since they polymerize and solidify under exposure to oxygen in a process of auto-oxidation followed by polymerization.37 The rate of this curing process depends on the unsaturation of the fatty acids, access to oxygen, temperature, and added catalytic driers such as soaps of cobalt and magnesia.38,39 Under the right conditions, the hardening process can be completed within a few hours. Although alkyd resins typically have a low molecular weight and a glass transition temperature far below ambient conditions, the auto-oxidative cross-linking reactions result in excellent film formation, transparency, a sharp increase in hardness over time, and notable hydrophobic layer characteristics. The protective or architectural coating properties of the alkyd resins can be correlated with material morphology and the individual components in the resin. Accordingly, alkyd resins based on tall oil and linseed oil were applied as moisture protective layers on top of nanocellulose precoated paper substrates. The alkyd resins were applied using a single-step water-based coating or printed directly onto the precoated papers. Their hydrophobicity, biodegradability, and renewability characteristics make them very promising candidates in combination with nanocellulose for commercial use and up-scaling. The effect of the alkyd resin components, e.g., type of oil and oil length, on the water vapor barrier properties of the unmodified nanocellulose precoated papers was studied. Furthermore, the improved surface smoothness of the paper substrates generated by the nanocellulose coating was found to be beneficial to the formation of the moisture protective alkyd resin layer. The surface morphology and roughness of the coated and uncoated sheets was characterized in detail by environmental scanning electron microscopy (E-SEM) and white light interferometry. We report on multilayer-structured barrier coatings on paper that can be obtained by using simple, quick, and scalable concepts including printing and water-based dispersion coatings.

Table 1. Chemical Components and Characteristics of the Alkyd Resins According to the Supplier commercial name water based Uradil AZ514 Z-60 Uradil AZ515 Z-60 Uradil AZ516 Z-60 oil based Uradil AD14 W-75

code

origin of fatty acid

oil length (%)

wLO73 wTO73 wTO63

linseed oil tall oil tall oil

73 73 63

oLO68

linseed oil

68

resins had an oil length, i.e., weight percent of fatty acid to resin, of 63−73%. The water-based dispersion consisted of alkyd resin emulsified in water to a solid content of 60 ± 1 wt %. The oil-based dispersion consisted of an alkyd resin dissolved in white spirit to a solid content of 75 ± 1 wt %. The alkyds resins were used as received. According to the supplier, the density of cured alkyd resins is 1.08 g/cm3. Methods. Dynamic Light Scattering (DLS) Measurements. The size of the particles in the aqueous alkyd resin emulsions was determined using a Zetasizer ZEN3600 particle character2583

dx.doi.org/10.1021/ie301785a | Ind. Eng. Chem. Res. 2013, 52, 2582−2589


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Environmental Scanning Electron Microscopy (E-SEM) and Energy Dispersive X-ray Analysis (EDAX). A Philips XL30 E-SEM-FEG (environmental scanning electron microscopefield emission gun) electron microscope was used to analyze the surface texture of the coated and uncoated substrates. The surfaces of the paper substrates were coated with a thin conducting layer of gold and imaged in the high vacuum mode using an SE detector (secondary electrons). The acceleration voltage was 5 kV and working distance was approximately 7 mm. X-ray analyses were carried out using the Philips XL30 ESEM-FEG unit in low vacuum mode with a pressure of 0.3 mbar in the sample chamber. The instrument is equipped with a BSE detector (backscattered electrons), EDS (energy dispersive spectrometer), X-ray detector, and INCA X-ray analysis system from Oxford Instruments. Working conditions were as follows: acceleration voltage 15 kV and working distance 10 mm. No conductive coating was used for analyzed samples. The energy dispersive spectrometer detects the energy of all of the elements occurring in the analyzed materials/compounds simultaneously and, together with the INCA X-ray analysis system, gives semiquantitative results for the chemical composition of all of the elements present in the analyzed material. Water Vapor Permeability. The water vapor transmission rate (WVTR) tests were performed with a Mocon Permatran 3/33 apparatus (Mocon, Minneapolis, U.S.A.) in accordance with ASTM F 1249-05; the test conditions were 23 °C at 50% and 80% relative humidity (RH) and 37.8 °C at 90% RH, respectively. The sample area was 5 cm2 and the partial pressures of the water vapor were 0.01387, 0.02219, and 0.05826 atm at 50, 80, and 90% RH, respectively.

ization system (Malvern Instruments Ltd., U.K.) with a 633-nm red laser collecting the scattered light at an angle of 173°. The particle diameter was measured as the z-average size. Aqueous Nanocellulose/Alkyd Resin Dispersion Coating. Each substrate was single- or double-coated with an aqueous nanocellulose dispersion (Na-form) with a concentration of 0.85 wt %, using a rod coater (K101 Control Coater, RK Print Coat Instruments Ltd., U.K.) and dried under ambient conditions. Immediately before the coating procedures, the nanocellulose dispersion was mechanically stirred at 1000 rpm using a four-blade propeller stirrer (RW 20, IKA, Germany). A coat weight of 3 ± 0.1 g/m2 was applied in each coating. The wet coat weight of nanocellulose was measured and the dry coat weight was calculated based on the nanocellulose dispersion concentration. Based on the density of nanocellulose, the average coating thicknesses of nanocellulose were calculated to be 2 μm (3 g/m2) and 4 μm (6 g/m2), respectively. The papers were stored at 23 °C and 50% RH for at least 24 h before further alkyd resin coating or testing. Some of the nanocellulose precoated sheets were further coated with aqueous alkyd resin dispersions. A coat weight of 20 ± 0.3 g/m2 was applied in each coating. Based on the density of the cured alkyd resin films, the coating thickness of alkyd resin was calculated to be 19 μm. Transfer of an Oil-Based Alkyd Resin through Laboratory Printing, Drying, and Curing. The oil-based alkyd was transferred to the substrate using an instrument referred to as the ink surface interaction tester (ISIT).40 This technique is widely used to study interactions between a paper surface and a printing ink. It requires the fluid to have a certain tackiness. Thus oil-based alkyd resins can be used but not water-based alkyd resins. The technique enables the transfer of thin layers, and the amount of the fluid transferred is determined gravimetrically. The fluid (in our case the oil-based alkyd resin) is first evenly distributed over the surfaces of a set of metallic rollers. The mass of the metallic roller with the film of the fluid is determined. This roller is then placed in the ISIT, and the fluid is transferred to the paper in a “printing” nip. The mass of the roller is measured again and the transferred amount is calculated. Coat weights of 1.4−14 g/m2 were applied in each printing step. The alkyd resin coated boards were dried at 60 °C for at least 2 weeks before performing the water vapor permeability evaluation. This was to ensure that complete hardening was reached without using catalytic driers, although the hardening process of the alkyd layer is most probably completed within a few hours under optimal conditions. Based on the density of the cured alkyd resin films, coating thicknesses of 1−13 μm were applied in each printing step. Topography. Topography/surface roughness of the uncoated and coated substrates was characterized in detail using a white light interferometry instrument, MicroProf (Fries Research Technology, Germany). White light is focused through a lens with chromatic aberration generating different colors which focus at different distances. The color composition of the reflected light is analyzed and translated into height positions. Twenty profiles with a length of 40 mm in the CD direction were measured on each of the samples. The profiles were placed 2 mm apart in the MD direction. Resolution was 2.2 μm in the CD direction and 5 nm in the z-direction. The height variations (μm standard deviation) which represent the roughness were divided into six wavelength bands, ranging from 0.016 to 1 mm.

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RESULTS AND DISCUSSION Analysis of Material. The surface treatment of substrate C was not revealed by the supplier but subjected to standard analysis at our laboratory. The E-SEM surface-image (Figure 1b) showed a relatively closed, nonporous fiber structure. The X-ray analysis showed that the main elements were carbon (9.4 at. %), oxygen (62.6 at. %), alumina (12.6 at. %), and silica (14.6 at. %). Traces of sodium, potassium, calcium, titanium, and sulfur were found but with a content less than 0.3 at. %. No magnesia was detected. The surface analysis suggests that the pretreatment was a thin coating of a polymer containing platy kaolinite clay. Most other minerals contain significant amounts of magnesia, potassium and/or calcium. The average particle diameter and polydispersity index (PDI) of the alkyd resins was determined using DLS. The size distribution of the alkyd resins was found to be monomodal and the corresponding PDI very low, indicating a high level of monodispersity (Table 2). The particles size and PDI were slightly higher for Uradil AZ514 Z-60 compared with Uradil AZ515 Z-60 and Uradil AZ516 Z-60. Influence of the Nanocellulose Coating on the Resulting Paperboard Surface Texture and Surface Topography. The surface structures of the uncoated and nanocellulose-coated substrates were studied using E-SEM. The results of this study are summarized in Figure 1, which show micrographs of the paperboards with different coat weights of nanocellulose and alkyd resin. Substrate O (Figure 1a) shows a very open, porous network of randomly crossed fibers. A nanocellulose coat weight of 3 g/ 2584



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< 1 μm), which is not possible using air-leak techniques such as Bendtsen and PPS. Figure 2 shows the surface roughness at

Figure 2. Topography spectrum of substrates O and C coated with 3 and 6 g/m2 of nanocellulose, respectively. Substrate O (▲) with 3 (◆) and 6 g/m2 (■) of nanocellulose precoating and substrate C (Δ) with 3 (◇) and 6 g/m2 (□) of nanocellulose precoating.

different wavelength bands for uncoated substrates and substrates coated with 3 and 6 g/m2, respectively. The nanocellulose coating is accompanied by a significant decrease in surface roughness at all wavelengths, e.g., the smoothness of the paperboards was significantly improved by the nanocellulose coating already at very low coat weights (3 g/m2). A further increase in coat weight (6 g/m2) resulted in minor improvements in surface smoothness. This is in agreement with the surface texture analysis with E-SEM, indicating unchanged surface texture associated with the double-coating. The surface smoothing effect of nanocellulose allows for the material to be used as a general primer; a first layer coating, which is important in technical applications such as paper pigmentcoating. Surface smoothness is also essential for various printing operations, where printability is dependent on the topography of the underlying substrate. The open substrate had a higher surface roughness than the closed substrate but after nanocellulose coating, both substrates exhibited rather similar surface roughness profiles. The open substrate actually became slightly smoother in the fine scale region. Nanocellulose coating thus has a high potential to smooth and densify the quite rough and open structure of an uncoated paper surface already at very low coat weights. Water Vapor Permeability of the Alkyd Resin/Nanocellulose Coated Paperboards. The applicability of biobased polymer films in packaging may be enhanced when they are applied as coatings on a backing material, thereby serving as a layer improving the gas barrier properties of the layered packaging. Coatings of nanocellulose and renewable alkyd resin were accordingly applied onto commercially used packaging board in a multilayer structure. Coating techniques, alkyd resin properties and nanocellulose precoating affected the water vapor permeability results. Oil-based alkyd resins are tacky which make it possible to deposit thin and uniform films through lithographic printing. A linseed oil-based alkyd resin (named oLO63) was applied at coat weights of 1.4 to 14 g/m2 to the open substrate (substrate O) before and after the substrate had been precoated with nanocellulose. As shown in Figure 3, there was a decrease in WVTR at 23 °C and 50% RH with an increased alkyd resin coat weight and also from nanocellulose coating only. The

Figure 1. Top view E-SEM micrographs (magnification ×500) of (a) substrate O, (b) substrate C, (c) nanocellulose-coated substrate O (coat weight 3 g/m2), (d) nanocellulose-coated substrate C (coat weight 3 g/m2), (e) alkyd resin/nanocellulose-coated substrate O (coat weight 20 g/m2), and (f) alkyd resin/nanocellulose-coated substrate C (coat weight 20 g/m2). The scale bar is 50 μm.

Table 2. Chemical Components and Characteristics of the Alkyd Resins commercial name

particle diameter (nm)

polydispersity index (PDI)

Uradil AZ514 Z-60 Uradil AZ515 Z-60 Uradil AZ516 Z-60

446 403 398

0.205 0.155 0.179

m2 resulted in a continuous film formation of the coating layer (Figure 1c) and a hardly visible fiber structure in the base paper. As expected, substrate C (Figure 1b) revealed a closer and less porous structure than substrate O. As for substrate O, when a coat-weight of 3 g/m2 was applied to substrate C, the network structure became more diffuse and a continuous nanocellulose film was formed. An additional nanocellulose coating, 3 g/m2, was applied on both single-coated substrates and resulted in unchanged surface textures (not shown). ESEM images reveal highly uniform and featureless alkyd resin coatings on top of the nanocellulose precoated paperboards (Figure 1e,f). The perfect uniformity of the applied alkyd resin layers is a result of the rod coating technique in combination with the auto-oxidation of the alkyd resin emulsions. The rod coating technique seems to be most suitable for spreading the viscous dispersion onto the paper surfaces with adjustable coating thickness. The alkyd resin film formation on the paper surface is not limited to the alkyd resin water-borne emulsion, as printing also allows for very homogeneous alkyd resin films with identical surface textures as for the alkyd resin dispersion surfaces (not shown). In order to fully study the influence of nanocellulose coating on the resulting paperboard surface roughness, a technique based on white light interferometry was used. The instrument allows the measurement of small-scale roughness (wavelengths 2585



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for more homogeneous deposition and a further reduction in WVTR. This clearly shows the importance of rendering the surface smooth and nonporous in order to maximize the hold out of the alkyd resin layer. In order to obtain a more optimal situation, the substrate was changed to one that originally had a more closed surface (i.e., substrate C). The surface of the substrate was further improved by nanocellulose precoating and the alkyd resin used was now water-based in order to render the system more environmentally friendly. The WVTR of substrate C decreased with single (3 g/m2) and double (6 g/m2) layered coatings of nanocellulose (Table 3). At 23 °C and 50% RH, the WVTR was decreased from 90 to 29.4 g/m2 day by 3 g/m2 nanocellulose and to 30.7 g/m2 day by 6 g/m2 nanocellulose. A similar reduction in WVTR was obtained with the nanocellulose coating at 23 °C and 80% RH. Thus it appears that a dense nanocellulose film is achieved already at a low coat weight. Substrate C was further coated with alkyd resin dispersion before and after it had been precoated with nanocellulose, and the ability of the alkyd resin to act as a moisture protective layer was again studied by measuring the WVTR. An alkyd resin coat weight of 20 g/m2 was used in order to create a dense alkyd resin layer and to avoid the formation of pinholes in the layer. Remarkable increases in moisture barrier properties were observed for all of the alkyd resin coated samples (Table 3). At 23 °C and 50% RH, the WVTR of the nanocellulose precoated substrates was reduced by approximately 1 order of magnitude by top-coating with wTO73 and even more with wLO73, whereas the reduction in WVTR was approximately 50% using wTO63. The WVTR of bare substrate C was reduced by approximately 2 orders of magnitude by nanocellulose precoating followed by wLO73 top-coating. Similar trends can be observed at 23 °C and 80% RH. Coating with alkyd resins only, without nanocellulose precoating also gave significant improvements, but not to as large extent as when the substrate had been precoated.

Figure 3. Water vapor transmission rate at 23 °C and 50% RH vs coat weight for an alkyd resin-coated substrate O. (■) Nanocellulose precoated substrate O and (□) substrate O.

WVTR decreased from 517 to 73 g/m2 day with the application of 3 g/m2 of nanocellulose followed by 14 g/m2 of alkyd resin. It is somewhat unexpected that nanocellulose alone improves water vapor barrier properties of the paperboard. Nanocellulose is highly hygroscopic and associated with an increased moisture uptake with increasing RH.4 Water molecules adsorbed to the surface or in the amorphous zones of the fibrils may disrupt hydrogen bonding and weaken the fibril/fibril joint formation, resulting in increased mobility of the fibril network. This could create additional sites for the permeation of, e.g., oxygen and water vapor and increased mobility of penetrating molecules within the polymer network. The reduction in WVTR is probably due to surface densification and the partial closure of the surface pores by the nanocellulose. Water molecules might be strongly adsorbed to the surface or in the amorphous zones of the nanofibrils which could result in lower water vapor transmission through the nanocellulose film. It is clear that pretreatment by nanocellulose has a strong impact on WVTR for thin alkyd resin layers, but the effect is reduced as the thickness of the alkyd resin layer increases. A precoated nanocellulose layer of 3 g/m2 efficiently creates a dense, smooth layer for the alkyd resin printing step that allows

Table 3. Water Vapor Transmission Rate (WVTR, g/m2 day) and Water Vapor Permeability (WVPcoating, g mm/m2 day atm) of Nanocellulose (NFC)/Alkyd Resin Coatings on Substrate C at 23°C and 50% RH, 23°C and 80% RH and 37.8°C and 90%, Respectively WVTR

a

base substrate and coatings

23 °C, 50% RH

23 °C, 80% RH

substrate C before treatment wLO73, 20 g/m2 wTO73, 20 g/m2 wTO63, 20 g/m2 NFC, 3 g/m2 NFC, 3 g/m2 and wLO73, 20 g/m2 NFC, 3 g/m2 and wTO73, 20 g/m2 NFC, 3 g/m2 and wTO63, 20 g/m2 NFC, 6 g/m2 NFC, 6 g/m2 and wLO73, 20 g/m2 NFC, 6 g/m2 and wTO73, 20 g/m2 NFC, 6 g/m2 and wTO63, 20 g/m2 PLA7 arbinoxylan41 cellophane42 polystyrene42 LD-PE

90.0 9.4 12.6 11.9 29.4 2.0 4.2 13.2 30.7 0.8 3.1 12.5

224 28.9 34.1 36.0 104 17.2 17.1 19.2 107 12.7 16.3 17.9

WVPcoating 37.8 °C, 90% RH 603 202

>1200 104

23 °C, 50% RH

23 °C, 80% RH

37.8 °C, 90% RH

1707a 14.0 19.6 18.3 6.2 3.0 6.5 22.8 13.2 1.3 5.2 23.5 117 201

2655a 27.7 33.6 35.8 17.1 18.8 18.3 21.2 36.2 13.6 17.8 19.7

2722a 64.2

7.3

7.2

>40 36.6

2326 68 6.9

WVPcoating based on a substrate thickness of 263 μm 2586



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thickness of the alkyd resin/nanocellulose multilayer is less than 25 μm, the results imply that high moisture barrier properties, especially related to the wLO73 nanocellulose and wTO73 nanocellulose coatings, were achieved and that this application can be an effective solution to extend the application of paper based materials in e.g. the food packaging area, thus avoiding the use of oil-derived products. Due to different WVTR measuring methods, bare substrate thickness and permeability, it is difficult to compare the WVTR presented here with previously published data. Therefore, to assist comparability and to further evaluate the effect of alkyd resin/nanocellulose coating on the moisture barrier properties, the water vapor transmission rate (WVTRcoating) and hence the water vapor permeability of the coating layers (WVPcoating) were determined (Table 3). The WVTR of the alkyd resin/ nanocellulose coating, WVTRcoating, and base substrate, WVTRsubstrate, was calculated from the WVTR of the multilayer-coated substrates, WVTRtot, according to eq 147

The improvement using nanocellulose is due to at least two effects. First, the layer of nanocellulose seals the surface and creates a surface layer with low porosity, and thus reduces the WVTR compared to the bare substrate. Second the layer smoothes the surface and thus allows for a more homogeneous alkyd resin deposition with high hold out. The second conclusion is also supported by the fact that WVTR became slightly lower when the coat weight of nanocellulose was increased from 3 to 6 g/m2. This increase in coat weight did not result in lower WVTR in absence of alkyd resin deposition but it made the surface smoother as is evident from Figure 2 and thus a more optimal deposition of the alkyd resin layer. Additional WVTR measurements on bare substrate C, nanocellulose precoated, alkyd resin-coated (wLO73) and wLO73/nanocellulose-coated substrates were performed at 37.8 °C and 90% RH. Major improvements in moisture barrier properties were observed, especially associated with the substrate that had been nanocellulose precoated. The WVTR of substrate C decreased from 603 to 104 g/m2 day by nanocellulose precoating (3 g/m2) followed by wLO73 topcoating. Nanocellulose precoating (3 g/m2) resulted in a WVTR of >1200 g/m2 day, above the detection limit of the instrument. The high WVTR value for the nanocellulose precoated substrate is probably related to a significant moisture uptake and corresponding swelling of the nanocellulose film. Nanocellulose films are associated with an exponential moisture uptake as a function of RH.4 Alkyd resin coating alone resulted in a WVTR of 202 g/m2 day. The WVTR at 23 °C and 50% RH for substrate O precoated with 3 g/m2 of nanocellulose followed by wLO73 was measured as 9.1 g/m2 day. A similar coating procedure on substrate C resulted in a WVTR of 2 g/m2 day (Table 3), indicating that a dense, nonporous surface is indeed essential for optimal barrier performance. Substrate C coated with wLO73/nanocellulose (0.8 g/m2 day) and wTO73/nanocellulose (3.1 g/m2 day), respectively, is shown to exhibit one of the lowest WVTR reported for biobased coatings and laminations at 23 °C and 50% RH. Hult et al. demonstrated a WVTR at 25 °C and 50% RH of 7 g/m2 day when laminating nanocellulose precoated greaseproof papers with shellac.33 Edlund et al. coated polyethylene terephthalate (PET) with wood hydrolysate containing 50% of chitosan and showed a WVTR at 23 °C and 50% RH as low as 0.16 g/m2 day.43 However, the authors used a rather thick PET film (38 μm). Previously published data on petroleumbased latex barrier coatings is very extensive. Earlier studies for example include cross-linked and carboxylated styrenebutadiene latex coating on commercial paperboard. The authors achieved a WVTR of < 10 g/m2 day at 23 °C and 50% RH with a coating thickness of ca. 150 μm.44 Earlier studies also include water-based coatings of commercial linerboard using coating formulations with different ratio of styrene-butadiene latex and kaolin clay.45 A clay/latex solid ratio of 1:1 and a coat weight of 15 g/m2 resulted in a WVTR of 18 g/m2 day at 23 °C and 50% RH. Another widely used method for creating a barrier layer is lamination or coextrusion with high-barrier polymers, such as PET in combination with polyethylene (PE). Common flexible packaging films based on coextrusion of PET/PE exhibit a WVTR of 4−6 g/m2 day at 23 °C and 80% RH.46 As far as WVTR is concerned, a material can be considered a “high moisture barrier” if values are lower than 5 g/m2 day (at 25 °C, 50% RH) for a 25 μm thick film).33 Given that the

1 1 1 + = WVTR coating WVTR substrate WVTR tot

(1)

Equation 1 is based on the assumption of well-defined and uniform coating layers with no defects and interpenetrating layers, thus the WVPs presented here are presumably slightly overestimated. The coated layers of substrate C (6 g/m2 of nanocellulose precoating followed by wLO73 top-coating) exhibited a WVP of 1.3 and 13.6 g mm/m2 day atm at 23 °C and 50% RH and 23 °C and 80% RH, respectively. Under tropical conditions, 37.8 °C and 90% RH, the coated layers of substrate C (3 g/m2 of nanocellulose precoating followed by wLO73 top-coating) exhibited a WVP of 36.6 g mm/m2 day atm. The WVP of LDPE was determined to be 7.3, 7.2 and 6.9 at 23 °C and 50% RH, 23 °C and 80% RH and 37.8 °C and 90% RH, respectively. Thus, at 23 °C and 50% and 23 °C and 80%, respectively, the WVP of the wLO73/nanocellulose coated layers are in the same range as LDPE, whereas at tropical conditions, the WVP of the alkyd resin/nanocellulose coated layers are higher, but in the same region as LDPE. At tropical conditions, the WVP of the wLO73/nanocellulose coated layers are lower than that reported for polystyrene (68 g·mm/m2·day·atm) 42 and cellophane (2326 g mm/m2 day atm).42 At 23 °C and 50% RH, the WVP of the wLO73/nanocellulose coated layers are much lower than that reported for other biobased packaging materials, including arbinoxylan (201 g mm/m2 day atm) 41 and polylactic acid (PLA) (117 g mm/m2 day atm).7 Surprisingly, the WVPcoating of nanocellulose at 23 °C and 50% RH is low; 6.2 and 13.2 g mm/m2 day atm at coat weights of 3 and 6 g/m2, respectively. As discussed earlier, nanocellulose seals the pores of the base substrate already at very low coat weights which might contribute to the low WVPcoating values. At 37.8 °C and 90% RH, the WVPcoating of nanocellulose is considerably higher; > 40 g·mm/m2·day·atm due to moisture-induced swelling of the nanocellulose layer. It is possible to draw conclusions regarding the impact of the chemistry of the alkyd resins on the barrier properties of the material. As expected, increased oil length (increased fatty acid content) improved the WVTR. An alkyd resin with a higher oil length has a higher density of fatty acid chains which will facilitate efficient polymerization and the formation of a highly cross-linked hydrophobic polymer layer. 2587


Industrial & Engineering Chemistry Research Comparisons could also be made between alkyd resin based on tall oil and linseed oil at the same oil length, i.e., 73. The results showed that the alkyd resin based on linseed oil was the most efficient one in terms of reducing WVTR. This is also expected and can be explained by the fatty acid composition and the degree of unsaturation in the two vegetable oils. About 50% of the fatty acid of linseed oil is linolenic acid, which contains three double bonds. The content of linolenic acid in tall oil is low, typically around 10% 35 or below.48 The content of oleic acid (one double bond) on the other hand is around 50% and that of linolic acid (two double bonds) is around 35%. Due to the high content of polyunsaturated fatty acids in linseed oil, the film of wLO73 would yield the most cross-linked structure. Practical use of this technology in the manufacturing of packaging materials involves removal of water after precoating with nanocellulose, removal of water after alkyd resin deposition, and preventing the alkyd surface from sticking to the backside of the packaging material in the roll in a roll-to-roll coating process. The latter problem has in fact been solved by the printing industry which frequently uses printing inks containing alkyd resins as a binder. Polyethylene or Teflon wax particles are used as additives in the ink. These particles serve as a distance block between sheets that otherwise would come into close contact. Furthermore, the printer uses a spray powder, normally starch-based, to further increase the space between the sheets. The drying issue of the nanocellulose film stresses the importance of low coat weight. Thus, achieving the smoothening effect already at a very low coat weight is indeed an advantage.

ACKNOWLEDGMENTS

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REFERENCES

The authors thank the Wallenberg Wood Science Centre for financial support. Joanna Hornatowska is gratefully acknowledged for her assistance with the E-SEM measurements. Anni Hagberg and Lucyna Lason are acknowledged for their laboratory assistance. Dr. Göran Flodberg is acknowledged for his valuable discussions.

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CONCLUSIONS An efficient path for the enhancement of the moisture barrier properties of paperboard and paper is presented. Nanocellulose and renewable alkyd resins were deposited on fiber based substrates using water-borne dispersion coatings or using a printing technique. The water vapor permeability properties were measured to quantify the barrier effect of the applied coatings. In addition, the surface texture and topography of the coated sheets were characterized in detail using environmental scanning electron microscopy and white light interferometry. The water vapor permeability of the paperboard and papers was substantially decreased with a multilayer coating of nanocellulose and alkyd resins and reached values considered as high barriers in packaging applications. The surface smoothness of the paperboard in different wavelength regimes is enhanced by the nanocellulose coating. The surface smoothness effect of the nanocellulose precoating step improves the homogeneity and uniformity of the deposited alkyd resin and thus the water vapor barrier performance. The alkyd resin with high oil length and high polyunsaturated fatty acid content exhibited the best barrier properties. The favorable combination of high moisture barrier properties and biobased, lightweight materials makes such coatings interesting for flexible high performance packaging applications.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2588



Article

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