Roll-to-Roll Processed Cellulose Nanofiber Coatings - ACS Publications

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Roll-to-roll processed cellulose nanofiber coatings Vinay Kumar, Axel Elfving, Hanna Maarit Koivula, Douglas W. Bousfield, and Martti Toivakka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00417 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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Industrial & Engineering Chemistry Research

Roll-to-roll processed cellulose nanofiber coatings Vinay Kumar1*, Axel Elfving1, Hanna Koivula2, Douglas Bousfield3, Martti Toivakka1 1

Laboratory of Paper Coating and Converting, Centre for Functional Materials (FUNMAT), Åbo

Akademi University, 20500 Turku, Finland 2

Department of Food and Environmental Sciences, University of Helsinki, PL 66, Agnes

Sjöberginkatu 2, 00014 Helsinki, Finland 3

Department of Chemical and Biological Engineering, University of Maine, ME 04469 Orono,

USA. *Corresponding Author: Email address: [email protected] KEYWORDS: Cellulose nanofibers, roll-to-roll manufacturing, barrier coating, slot geometry

ABSTRACT: Current environmental concerns have encouraged the food packaging industry to search for bio-based barriers produced from renewable material sources. One candidate for renewable barrier applications is nanocellulose which has been found to possess excellent barrier properties, especially against grease and oxygen. However, most of the research presented so far has been based on small, batch-produced films or coatings of nanocellulose. Reports on continuous processing of nanocellulose into films or coatings, which is required for large-scale, low-cost production, are few. The current work presents a roll-to-roll coating process of cellulose nanofiber (CNF) suspensions on paperboard for renewable barrier applications. The coating

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apparatus used herein provides a low effective viscosity to enable the processing of the highly viscous nanocellulose suspensions into a coating on paper web. Impact of various process parameters on the coating quality and runnability are discussed. Strength (tensile and burst) and barrier properties (air permeability, water and heptane vapor transmission rate, and mineral oil and grease barrier) of the CNF-coated paperboards are promising and mostly superior to what has previously been reported.

1. INTRODUCTION Cellulose nanofiber (CNF) has become one of the most exciting nanomaterials appealing to scholars from wide-ranging scientific and technical backgrounds. Environmental concerns related to petroleum-derived polymers and need for sustainable development have further strengthened the interest in widely available, renewable, biodegradable and nontoxic bio-based materials. CNFs are isolated from wood and plant cell walls via various chemical, enzymatic, and/or mechanical means. Exhibiting lateral dimensions in the nanometer range and lengths reaching several microns, CNFs possess excellent properties, such as high aspect ratio, high specific strength, flexibility, large specific surface area, and thermal stability, combined with biodegradability and biocompatibility 1-6. These remarkable properties make CNFs suitable for a wide range of applications, such as reinforcing phase in composite materials packaging 21

11-16

, rheology modifiers for suspensions

17-20

7-10

, barriers in

, filters for water treatment technologies

, and flexible platforms for biomedical sensors and printed electronics applications 22-26. CNFs

can be formed into structurally different materials such as solids, films, gels (aerogel) or foams depending on the target application

27

. It is crucial to be able to process CNFs into end-use


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products in economical and environmentally friendly ways for fully exploiting the capabilities of this material. CNF films have recently caught the attention of materials community for their remarkable properties, such as mechanical robustness and excellent barrier against oxygen and fat

27-33

.

These properties are essential for numerous food packaging applications 5. CNF films when applied as a coating layer can potentially provide strength and barrier functions to a package. Application and benefits of CNF as a coating material have been covered in some recent reviews 34, 35

and doctoral theses

various other materials

36, 37

. CNF can be applied in pure form

12, 15, 17, 18, 45-48

28, 38-44

, or it can be mixed with

to produce a coating layer. CNF coating has been

previously used to either provide the barrier function in packaging 12-14, 28, 39, 40, 48, or improve the surface properties of paper for better printability 41-43, 46, 47. All the previous studies have reported CNF coatings produced in a batch manufacturing process, except in one instance, where Kinnunen et al.

38

reported very thin CNF coating layers applied using foam coating. CNF

composite papers produced using a web-forming process were also reported recently by Rantanen et al.

49

. However, to the authors’ best knowledge, roll-to-roll processing of CNF as

thick coating layer suitable for barrier functions in a package has not been reported before. There are several obstacles in processing CNFs into coating using conventional techniques: firstly, CNF suspensions have very high viscosity even at low mass concentrations, which makes their transformation into a wet thin film challenging; secondly, ample drying capacity is required due to the typically low solids concentration of the CNF suspensions; and thirdly, paper-based substrates may not withstand such large amounts of water during coating leading to web breaks and runnability issues. In this work, we report coating of CNF suspension on a packaging paperboard in a roll-to-roll manufacturing process for the first time. The shear thinning behavior


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of CNF suspensions is utilized to make their processing possible via custom-built slot geometry. The coating process is reported in detail, and various parameters affecting the coating quality and runnability are discussed. The objectives were to understand the impact of slot gap, wet coating layer thickness, coating speed, CNF suspension concentration, and addition of carboxymethyl cellulose (CMC) on the coating quality and runnability. The investigated process holds promise for large-scale roll-to-roll manufacturing of CNF coated substrates for various applications, thereby rendering the remarkable CNF properties useful. 2. EXPERIMENTAL SECTION 2.1 Materials CNF suspension was produced by the Process Development Centre of the University of Maine (Orono, USA) through mechanical treatment as described previously

27

. In brief, the bleached

softwood Kraft pulp was circulated through a refiner with specialized plates until the fines content reached over 90%, as measured with a standard fiber size analyser MorFi by Techpap (Saint-Martin-d’Hères, France). The SEM and TEM images of CNF in Figure 1 show the level of fibrillation achieved with the mechanical treatment. The diameter of CNFs seems to vary from a few nanometres to hundreds of nanometres, and the length goes up to several microns. Carboxylate content of the CNFs was 0.31–0.34 mmol/g determined in earlier studies

27

. For

coating purpose, the CNF suspension was diluted to mass concentrations 1%, 2% or 3%. CMC (FINNFIX 4000G) was supplied by CPKelco (Äänekoski, Finland). The degree of polymerisation and degree of substitution for CMC were 2000 and 0.8, respectively. The substrate used for coating was a packaging paperboard (Grammage: 178 ± 4 g/m2 and Thickness: 190 ± 5 µm) produced from recycled pulp fibers. A pigment coated packaging paperboard


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(Grammage: 195 ± 3 g/m2 and Thickness: 205 ± 4 µm) was also used to study the CNF coating coverage.

Figure 1. SEM (left) TEM (right) images of CNFs. 2.2 Coating equipment Rotary Koater manufactured by RK PrintCoat Instruments Ltd. (Hertfordshire, UK) with some modifications was used to coat CNF suspensions. The base machine was equipped with one 5 kW infrared drying unit and two 10 kW air dryers with adjustable air flow and operating temperatures up to 200°C. To aid the drying process, three small infrared heaters (1 kW each) and four air dryers (2 kW each) were fitted onto the machine. Operating speeds for the machine were 1-30 m/min. A custom-made slot die (slot width: 74 mm, slot length: 34 mm, slot gap: 500 or 1000 µm, and distribution channel diameter: 16 mm) was used as coating applicator, as shown in Figure 2. It was attached on an adjustable rail installed parallel to the axis of a backing roll. The rail movement perpendicular to the backing roll allows for precise control of the distance between the slot die and the substrate referred to as Slot-Web gap (SWG). Fine adjustment of the SWG enables the slot die to be used as a metering device to control wet coating thickness. Coating suspension is fed into the slot die from an air-pressurized container. Excess coating


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material metered off is collected in a tray underneath the slot die. The substrate width used was 10-15 cm and the width of the coated area was approximately 7 cm.

Figure 2. Coating application unit. SWG is the gap between the slot die and the web. 2.3 Water retention and rheology of CNF suspensions ÅA-GWR (Åbo Akademi Gravimetric Water Retention Device) 50 and pressure filtration tests were performed on the CNF suspensions to determine their water retention ability, which plays a critical role in the coating and drying process. Pressure filtration measurements were performed at 12 bars, and the results are given as g/m2 of filtrate amount obtained over time. Shear viscosity measurements with parallel plate geometry were carried out using Paar Physica MCR300 rheometer (plate diameter 50 mm, gap 1 mm). The apparent (process) viscosity of CNF suspension at high shear rates was obtained from the slot die by gravimetrically measuring the


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outflow for known feeding pressures. The apparent viscosity was calculated based on wellknown slot flow equations while applying appropriate corrections for entrance effects 51, 52. 2.4 Characterization of coated paperboards All coated boards and the substrate were calendered with a laboratory soft nip calender, keeping the back side towards the soft roll, using a line load of 60 bar and temperature of 60°C. All samples were then conditioned (23°C, 50% RH) for at least 24 hours before testing. The thickness of each coated paperboard was measured using a Lorentzen & Wettre (L&W) Micrometer SE-250 (Kista, Sweden) at ten different areas of the sample, and average was used to determine thickness in microns. Grammage was determined by weighing a known size of the coated paperboard. Due to the varying grammage of the substrate, exact coat weights with thin coatings could not be determined precisely. However, the coat weights for thicker coatings reached up to 16 g/m2. Moisture content of the coated substrates was determined following the TAPPI standard (T550). L&W Tensile strength tester SE-060 (Kista, Sweden) was used to measure the coated paperboard’s tensile strength index (TSI), strain at break (SaB) and tensile energy absorption index (TEI). The tester used a 200 N load cell and a strain rate of 12 mm/min on a 15 mm wide and 150 mm long strip of the coated sample. An average is reported from ten parallel measurements. Burst strength index (BSI) of the coated paperboards was measured according to standard method SCAN (P24) using an L&W Müllen-type burst strength tester (Kista, Sweden). The result is given as an average of six parallel measurements. Water vapor transmission rate (WVTR) tests were performed according to the ASTM Standard (E96/E96M-05) method. Anhydrous calcium chloride (CaCl2) was placed into a cup to maintain 0% relative humidity (RH) inside the cup, the test sample was placed over the mouth of the cup, and then, molten wax was applied to seal the sample over the rim of the cup. The cup was left in


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a climate controlled room (23°C, 50% RH) for 24 hours, and the increase in weight of cup due to water vapor permeation through the test sample was used to determine the WVTR. The average from three parallel measurements is reported. A similar procedure was used for determining heptane vapor transmission rate (HVTR), as described by Miettinen et al. 53. For this, the CaCl2 salt was replaced with 20 ml of heptane onto a sponge (to reach a liquid/gas equilibrium as quickly as possible), and the decrease in weight of cup due to evaporation of heptane through the test sample was continuously tracked for two hours. The results were then extrapolated to 24 h time. HVTR was determined based on an average of three parallel measurements for each sample. Air permeability of the coated samples was determined using an L&W Air permeability tester SE-166 (Kista, Sweden) with a measurement range of 0.003—100 µm/(Pa·s). Bendtsen tester from M.C. TEC (JN Giessen, The Netherlands) was also used to determine the air permeability. Five parallel measurements were performed on different areas of each sample. Grease resistance of the coated paperboards was determined using the KIT Test following the TAPPI standard (T 559). The capability of the coatings to function as barrier materials against mineral oil was studied qualitatively using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). A Perkin Elmer Spectrum One FT-IR Spectrometer (Norwalk CT, USA) was used similarly to the procedure used by O’Neill et al. 54. The diameter of the measuring head was 5 mm. Transmittance spectra of offset printing ink mineral oil PKWF 4/7 (Haltermann, Germany) and the coated side of the paperboard samples were first obtained to form references in the wavenumber range 600-4000 cm-1. Four wavelength sweeps were made in each measurement. For the barrier tests, a drop of the mineral oil was placed on the back (uncoated) side, and the transmittance spectra from the coated side were obtained at the following times: 0 (reference), 1, 3, 25 and 50 h after placing the drop on the surface. The drop


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size for the mineral oil was typically around 20 µl. On the porous structure of the paperboard, the drop spread over an area of approximately 4-7 cm2. Three parallel measurements were performed for all the samples. When the coating functions as an effective barrier, the measured spectra will not show any peaks originating from the mineral oil. The measuring head was cleaned using isopropanol and lint-free wipes between the measurements. Print penetration tests were performed on the coated paperboards to determine their surface porosity, which gives a qualitative idea of the coating coverage. An IGT AIC2-5 tester by IGT Testing Systems (Amsterdam, The Netherlands) was used according to the standard method IGT-W24. Surface and cross-section images of the coated paperboards were obtained using a Leo (Zeiss) 1530 Gemini scanning electron microscope. A thin layer of gold was sputtered on the specimen before imaging. SEM was operating in secondary electron mode, and the surface images were obtained at five different magnifications from a working distance of 11 mm using an acceleration voltage of 10 kV. 3. RESULTS AND DISCUSSION 3.1 Coating process The coating process required optimization of several process parameters, such as CNF concentration, slot gap, SWG, coating speed, and CMC addition for rheology control. Figure 3 shows the coating quality development as a function of change in coating speed, CNF concentration, slot gap and SWG. The first step was to determine the CNF concentration suitable for coating. High concentration is preferable due to less drying capacity required and high coat weight achieved; however, our system was limited in terms of pressure required for feeding the high concentration material through the narrow slot gap. Therefore, only 1 and 2% CNF suspensions were initially used during process optimization. From Figure 3, one can observe


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that 1% CNF suspension is not picked up well by the moving web, especially at low speeds (1 and 2 m/min). Therefore, 2% CNF was chosen for optimizing other process parameters. The slot gap was then selected based on two considerations: (1) pressure required to feed the material through the narrow gap; (2) uniform flow without any blockage. A slot gap of 1000 µm was selected instead of 500 µm, as it allowed for using lower pressures, especially at 2% CNF concentration, and posed less hindrance to the flow caused by the presence of some large aggregates in the CNF suspension. It can be clearly seen from Figure 3 that the 1000 µm slot gap allows for more uniform flow of 2% CNF material compared with the 500 µm slot gap. SWG was optimized after the CNF concentration and slot gap were finalised. SWG is one of the most important parameters, as it acts as a metering element and allows for controlling the wet coating thickness and hence the final coat weight. The maximum value of SWG was decided based on the drying capacity required at a particular speed. For example, SWG could be up to 700 µm at a speed of 3 m/min to produce a fully dry coating. The feeding pressures varied from 0.2-2 bar depending on the slot gap and CNF concentration used for coating. Even higher feeding pressures might allow for higher CNF concentrations to be used for coating.


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Figure 3. Coating quality using different parameters. Images are from the start-up. The rheological properties of the coating material play a vital role in a coating process. CNF suspensions have unique and complex rheological properties

52, 56-59

with highly shear thinning

apparent viscosity (see Figure 4), which can be utilized for processing them into thin films/coatings. Analysis of slot coating of yield stress fluids is minimal in the literature

60

. Slot

coating of power law liquids has been investigated by Bhamidipati et al. 61. Based on the process findings and the complex rheological nature of CNF suspensions, it was concluded that once the


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material has been shear thinned by passing through the narrow slot gap, it can be applied as a thin film on the moving web even at low coating speeds. This sheared state of material allows it to be easily metered with the SWG, thereby controlling the final coat weight. In a conventional coating process such as blade coating, the high shear rate to shear the material is achieved at high coating speeds only. It is difficult to dry the CNF coating at that high speed due to the presence of too much water. In the present process, the shearing is achieved in the slot gap itself and then the material is easily metered to a certain wet thickness depending on the drying capacity. This already sheared state of material also allows for coating application at low speeds. Addition of CMC can further affect the rheological behavior in a favorable way, as it reduces the low shear viscosity (see Figure 4) that helps pumping the high concentration CNF suspensions to the coating application unit with ease.

Figure 4. Viscosity curves of CNF suspensions at different CMC addition levels. "MCR pp" for results obtained using parallel plates, and "slot" for analysis of flow in the slot.


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Water retention, i.e. the ability of the coating material to resist dewatering into the substrate, is another important coating process parameter. CNF suspensions contain an ample excess amount of water, and therefore it is important to control the water retention for successful processing. Figure 5 shows the water retention capability of CNFs at 1 and 3% solids. The effect of CMC addition on water retention is also shown. ÅA-GWR results agree well with the pressure filtration results. Higher concentration of CNF suspensions leads to higher water holding capacity. CMC addition increases the water retention of CNF suspensions further, and the slow dewatering allows for CNF coating application to paperboard without causing web breaks, in addition to efficient drying. The dewatering evaluation method used in this work is a static one; whereas the coating condition shears the suspension, and, therefore, the dewatering properties immediately after shear, prior to the re-establishment of the static structure, may well differ strongly from the values given. CMC addition also affects the coatability to a large extent (see Figure 6). One can observe a clear development in the coating quality with increasing CMC levels. Herein, 5 pph seems to be the optimal CMC addition level. This development is instigated by the increased water retention of CNF suspension and favourable rheology for metering due to the presence of CMC. The long chain and high molecular weight CMC used here might help dispersing CNF by way of mechanical disentanglement and/or overall anionic charge increase in the system. The tendency of CNFs to phase separate is thus reduced leading to higher water retention. CMC could also be providing this dispersing effect in the system by decreasing the amount of free water (increasing the water phase viscosity) in the suspension as reported by Vesterinen et al.

62

. They propose a

change in the water-fiber interactions due to presence of CMC possibly leading to a change in


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the observed rheological behaviour. The CNF coated paperboard strength properties and air permeability values also benefit from the CMC addition.

Figure 5. ÅA-GWR (left) and pressure filtration (right) results for 1 and 3% CNF suspensions.

Figure 6. Impact of CMC addition on coatability and coating performance. 3.2 Coated paperboard characterization Upon optimizing various process parameters discussed in the previous section, a series of CNF coated paperboards were produced with different SWGs. For all the coated samples, 2% CNF with 5 pph CMC addition level was used at a uniform coating speed of 3 m/min. Total grammage and coat weight with increasing SWG are shown in Table 1. It must be noted that the coat weight determination at low SWGs is not as precise as at high SWGs due to large variation in


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grammage of the base paperboard itself. The high coat weight (> 10 g/m2) achieved in this work in a single step CNF coating has not previously been reported, except in one instance where Beneventi et al.44 used spray-on-wet coating technique to apply 3-40 g/m2 coat weights. The highest coat weights reported in a single coating step with CNF were below 1 g/m2 using foam coating

38

, 1-3 g/m2 using rod/bar coater

12, 28, 39, 40, 42, 43, 46

, 2.5 g/m2 using filtration and

deposition 48, and up to 8 g/m2 using a dynamic sheet former 32 and size press 37, 39, 40. Very high coat weights (> 20 g/m2) with a mixture of Microfibrillated cellulose (MFC) and Shellac using a bar coater and dynamic sheet former have been reported 15. The process reported herein enables thick coating application in a single layer, which can be further optimized by using higher concentrations of CNF. Table 1. Grammage and coat weights of the CNF coated paperboards Sample

Grammage (g/m2)

Coat weight approx. (g/m2)

Substrate

178.0 ± 3

0

SWG-50

179.1 ± 2.1

1

SWG-100

181.8 ± 1.8

4

SWG-200

182.8 ± 1.8

5

SWG-300

184.1 ± 1.9

6

SWG-400

186.8 ± 2.4

9

SWG-500

189.1 ± 3.1

11

SWG-600

190.4 ± 3.0

13

SWG-700

193.8 ± 2.9

16

Coating coverage


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Coating coverage is an important parameter that affects the coating quality and thereby the performance of the coating in desired barrier applications. There are various ways that can help determine the coating coverage qualitatively. However, it can be difficult to see clearly the CNF coating coverage on a paperboard substrate due to optical similarity between CNF coating and pulp fibers in the substrate. Therefore, to show clearly the contrast between the CNF coating and the substrate surface a pigment coated substrate, in addition to the paperboard, was also coated using the same process parameters. SEM images of two CNF coated paperboards (uncoated and pigment coated) with increasing SWG are shown in Figure 7 and Figure 8. Improvement in CNF coating coverage with increasing SWG can be easily seen, especially for the pigment coated substrate. Uniform coverage is achieved already at 300 µm SWG. It is difficult to differentiate between the two substrates above a SWG of 400 µm and higher. However, the surface structure and chemistry of the two substrates may influence the coating coverage to some extent. The CNF coating also seems to form a more closed structure compared with the base paperboard. This is also evident from the print penetration test results (see Figure 9), which quantify the surface porosity in terms of stain length. Higher stain length indicates a more closed surface. The stain length is already outside the measurement limit for 300 µm SWG sample. Therefore, it can be established from SEM images and the print penetration test results that the roll-to-roll process starts to provide a uniform and full coating coverage already at 300 µm SWG. There is potential that even a lower SWG will provide a full coating coverage, given that the substrate and/or CNF type can be optimized for this.


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Figure 7. SEM images of CNF-coated paperboards at 50, 100 and 200 µm SWG.


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Figure 8. SEM images of CNF-coated paperboards at 300, 400 and 500 µm SWG.


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Figure 9. Surface porosity results from print penetration test. Different SWGs correlate to different coat weights as in Table 1. Mechanical properties The CNF coated paperboards were characterized for their strength properties with increasing SWG. The strength properties clearly improve with CNF coating thickness (see Table 2). There is a slight improvement in strength properties even at low coat weights. The improvement in TSI and SaB at highest SWG is around 10% and 20%, respectively. Additionally, TEI and BSI are increased by 30% and 50%, respectively. Due to large specific surface area available for bonding in CNFs, they tend to form a stronger network compared with pulp fibers. The large pores on the surface of paperboard are also filled by CNFs forming a strong network by way of hydrogen bonding and mechanical interlocking between the paperboard surface and coating. The overall strength is, however, largely controlled by the base paperboard, and therefore the improvement in TSI is only minor with CNF coating. However, the improvement in stretch due to enhanced interlocking provided by CNFs leads to an overall increase in TEI and BSI. A small improvement in TSI of paperboard with shellac/MFC coating was reported by Hult et al.

15

too.

Applying multiple layers of nanocrystalline cellulose (NCC) with a dip coating technique was


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also reported to improve the strength properties of filter paper

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. A slightly higher level of

improvement in TSI and SaB (15 and 30%, respectively) with a lower MFC coat weight (8 g/m2) was seen by Syverud and Stenius 32. This was because the base paper used by them had a lower grammage and TSI compared with the present work. Amini et al.

48

and Beneventi et al.

44

also

reported improvement in strength properties with MFC coating. Contrarily, Lavoine et al.

39

reported a slight decrease in the TSI with MFC coating. They attributed this to the multiple drying and rewetting cycles of paper, as the coating was applied multiple times to gain sufficient coat weight. Thus, a coating with CNF can contribute to the strength of the paperboard to some extent depending on the coating process used. The roll-to-roll coating of CNF seems to be providing better results compared with the batch coating processes, such as bar coating 15, 39, size press 39, dip coating 36 and dynamic sheet former 15, 32, used in earlier studies. This could be due to a stronger network formed in a single thick layer compared with multiple thin layers by avoiding weakening of the substrate due to multiple wetting cycles. It must also be noted that the ample amount of water present in CNF may affect the strength properties of base paperboard itself, as it allows for stress release during wetting and shrinkage of base paperboard structure during subsequent drying. This was quantified by applying pure water at SWG-50 under the same process conditions as other coatings with CNF. The water treatment was found to reduce the strength properties to some extent, which is in agreement with what Lavoine et al.39 had observed. The reduction in tensile strength at SWG-50 also indicates that the water impacts the strength negatively. However, the strength contribution from CNF coating at SWG-100 and higher makes up for the strength reduction caused by water. Table 2. Strength properties of coated paperboards Sample

TSI (kNm/kg)

SaB (%)

TEI (J/kg)

BSI (kPa·m2/g)


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Substrate

49.01 ± 1.52

1.87 ± 0.12

603.7 ± 59.7

1.4 ± 0.1

SWG-50

47.73 ± 0.96

2.11 ± 0.08

650.6 ± 36.0

1.8 ± 0.1

SWG-100

50.85 ± 1.15

2.18 ± 0.04

722.2 ± 24.3

1.7 ± 0.1

SWG-200

50.12 ± 1.76

2.18 ± 0.12

708.8 ± 62.1

1.7 ± 0.0

SWG-300

50.12 ± 1.13

2.07 ± 0.08

677.2 ± 38.8

1.7 ± 0.1

SWG-400

51.97 ± 1.45

2.11 ± 0.11

721.5 ± 53.4

1.7 ± 0.0

SWG-500

52.60 ± 1.25

2.13 ± 0.09

739.1 ± 51.4

1.8 ± 0.0

SWG-600

51.72 ± 1.72

2.14 ± 0.16

731.6 ± 85.0

1.8 ± 0.1

SWG-700

53.61 ± 1.03

2.25 ± 0.10

798.2 ± 50.6

2.1 ± 0.2

Barrier properties CNF coated paperboards were also characterized for their barrier function against air, water vapor, heptane vapor and mineral oil. Figure 10 shows the WVTR and air permeability results for CNF coated paperboards with increasing SWG. A fully closed network formed by CNF coating reduces the WVTR by more than 85%. Reduction in WVTR here is higher than any of the previous studies 15, 36, 48. This is in accordance with our earlier results 27 from batch produced pure CNF films, which showed low WVTR compared to other nanocellulose films reported in literature. However, this reduction still does not make these coated boards suitable for moisture barrier applications. The hydrophilic nature of CNF coating is not able to reduce the WVTR so significantly. An addition of pigments

10

or application of alkyd resin layer on top of CNF

coating 12 might help improve the WVTR and other barrier properties further. CNF coating also reduces the air permeability drastically, as has been reported previously

15, 28, 32, 38-40, 44

. The

drastic drop in air permeability is caused by a closed coating structure provided by the CNF layer. For 300 µm and higher SWG, the values are below the detection limit of the instruments


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Page 22 of 35

indicating full and uniform coating coverage, as is evident from SEM images as well. The air permeability results from two different instruments agree well with each other. Grease barrier plays an important role in various packaging applications such as bakery products, pet foods, and fast foods. CNF coated boards are a potential candidate for such applications

28, 48

. The paper and board industry uses the KIT test method to characterize barrier

coated materials for grease resistance

53

. Figure 11 shows the KIT test results for CNF coated

paperboards. There is a clear improvement (from 0 to 10) in the KIT number with increasing SWG. Improvement in KIT number is higher than what has been previously reported for MFC 39, 40

and water-based barrier coatings reported by Miettinen et al.

53

. However, Emilsson et al.

63

recently reported water-borne Poly (vinyl alcohol)-based barrier coatings with a KIT number of 12 at a low coat weight of 2.4 g/m2. Migration of mineral oil 53, 64, 65 from packaging material to food is a major concern to the food packaging industry. HVTR can be a good indicator of mineral oil barrier, as heptane is one of the components in various mineral oils used in printing inks 53. HVTR values of CNF coated paperboards are shown in Figure 11. There is a substantial reduction in HVTR with increasing SWG. No heptane vapour transmission was detected at SWG of 300 µm or higher. The reduction in HVTR observed here is greater than the reduction achieved using other water-based barrier coatings reported earlier 53. These results support the air permeability results and further confirm the uniformity of coating coverage at SWG 300 µm and higher.


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Figure 10. WVTR and air permeance of CNF coated paperboards. Different SWGs correlate to different coat weights as in Table 1.

Figure 11. HVTR and KIT results of CNF coated paperboards. Different SWGs correlate to different coat weights as in Table 1. Figure 12 shows the FTIR spectra of mineral oil barrier measurements for the SWG-400 sample. No mineral oil peak is seen even after 50 h, indicating that the improvement in mineral


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Page 24 of 35

oil barrier is substantial with the increased SWG. CNF coating at 400 µm and higher SWG seems impermeable to mineral oil. This good oil barrier makes these CNF coated paperboards suitable candidates for barrier packaging. The suggestion is supported by a report from Amini et al. 48 which also showed that CNF coating improved mineral oil and castor oil barriers.

Figure 12. FTIR spectra for mineral oil barrier measurements for SWG-400 coated paperboard. Impact of 3% CNF on coating A coating trial with 3% CNF was also carried out. The strength properties and air permeability of those coated paperboards with same coat weights are shown in Figure 13. One can clearly see that the 3% CNF coating performs at par or better than the 2% CNF coating. This indicates a positive outlook for future trials with even higher concentrations of CNF, as it will take less energy during the drying process making it suitable for large scale processing.


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Figure 13. 2% CNF coating (empty bar/circle) vs 3% CNF coating (filled bar/circle) comparison. 4. CONCLUSIONS A new method for roll-to-roll coating of CNF suspensions on paperboard has been developed and demonstrated successfully. Various parameters affecting the coating quality and runnability were identified and optimized. The complex rheological behavior of CNF suspensions is used to help process them into uniform coating layers. The coating apparatus shears the material and thereby enables application of thick and uniform CNF coatings at low speeds. Addition of CMC was found useful for improving the coatability of CNF suspensions by way of improving the water retention and modifying the low shear rheology. CNF coating improves the strength properties to some extent even at very low coat weights. The improvement in barrier properties


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Page 26 of 35

of CNF coated paperboards with increasing coating thickness was substantial. CNF-coated paperboards demonstrated excellent barrier against air, grease and mineral oil. Even WVTR improved significantly with the CNF coating. Although the process reported herein enables coating of CNF, with potential benefits for numerous end-use products, challenges remain related to scaling of the technology to high speed industrial operation. The main issue is the high drying energy requirement caused by the low solids concentration of the CNF suspensions. Coating at higher CNF concentrations and the impact of substrate and CNF type on the coating process should be addressed in future work. ACKNOWLEDGEMENTS The authors wish to express their gratitude to all those who have helped during this work. REFERENCES (1) Siró, I.; Plackett, D. Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose 2010, 3, 459-494. (2) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. Angewandte Chemie International Edition 2011, 24, 5438-5466. (3) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: current international research into cellulose nanofibres and nanocomposites. J. Mater. Sci. 2010, 1, 1-33.


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Table of contents Graphic:


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Roll-to-Roll Processed Cellulose Nanofiber Coatings - ACS Publications

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