Slot Die Coating of Nanocellulose for Barrier Applications

Martti Toivakka 1 , Rajesh Koppolu1, Tiffany Abitbol2, Agne Swerin2,3, Johanna Lahti4 and Jurkka Kuusipalo4

1 Paper Coating and Converting, and Center for Functional Materials, Åbo Akademi University, Turku, Finland

2 Bioscience and Materials – Surface, Process and Formulation, RISE Research 7 Institutes of Sweden, Stockholm, Sweden

3 Division of Surface and Corrosion Science, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden

4 Paper Converting and Packaging, Tampere University of Technology, Tampere, Finland

Corresponding author: Martti.Toivakka@abo.fi

IntroductionSearch for bio-based barriers produced from renewable material sources has accelerated in recent years. One candidate for renewable barrier applications is nanocellulose, which has been found to possess excellent barrier properties, especially against oils, grease and oxygen. Numerous reviews (Abitbol et al. 2016; Brodin et al. 2014; Dufresne 2013; Isogai 2013; Lavoine et al. 2012; Lindström et al. 2015) and book chapters (Aspler et al. 2013; Bardet and Bras 2014) cover various fundamental aspects of nanocellulose manufacturing and properties. However, most of the research presented so far has been based on small, batch-produced free-standing films of nanocellulose. Reports on continuous processing into films or coatings, which is required for large-scale and low cost production, are few. Similarly, high deformation rate rheology of nanocellulose, which is relevant for high speed coating operations, has not received much attention. The current work aims at understanding nanocellulose suspension rheology at high shear rates and demonstrates continuous roll-to-roll coating on paperboard with slot die geometry. Various barrier properties of the nanocellulose-coated paper, including mineral oil, water and heptane vapor, and oxygen barrier, are examined to illustrate potential future applications.

ExperimentalFigure 1 shows electron micrographs of CNF (cellulose nanofibers, also known as nano- or microfibrillated cellulose) and CNC (cellulose nanocrystals) used in the current work. CNF was produced at the Process Development Centre of the University of Maine (Orono, USA) using mechanical treatment as described in detail in Kumar et al. (2014). Without chemical pre-treatment, the fibril diameter varies greatly, from less than 100 nm to microns, and the length can reach tens of microns. Much smaller sized nanocellulose can be produced by using, e.g., TEMPO-mediated oxidation or phosphorylation prior to defibrillation. CNC was obtained from Melodea Ltd. as a 3 wt% aqueous suspension at pH 4. Dissolving pulp was used as the source for CNCs, and sulfuric acid for the hydrolysis step.

Nanocellulose suspensions are typically gel-like, and demonstrate very high viscosities and yield stress already at low solids concentrations, typically less than 10 wt-%. The high viscosity, the

large amount of water to be dried and water absorption into substrate for absorptive surfaces such as paper, challenge the use of nanocellulose as coating material.

Figure 1. Transmission electron micrographs of CNF (left) and CNC (right)

Due to experimental problems, viscosity of CNF suspensions has usually been reported only for very low shear rates, typically up to 100 s-1. Sample ejection from the measurement gap, potentially due to repeated shear, is a common problem when measuring with rotational viscometers. In order to reach higher shear rates, the current work utilizes slot rheometry with a simple custom-built slot die. By measuring the inlet pressure and the outflow, the viscosity of the test material can be calculated with well-known equations for narrow gap flow as detailed in Kumar et al. 2016A. Corrections to account for fluid acceleration, slot end/exit effects and non-Newtonian behavior can be applied in a manner similar to capillary viscometry. Capillary viscometry as such is unsuitable for most nanocellulose materials due clogging of the capillary entrance, which is less of a problem for slot rheometry.

Slot die coating technique is well-known in production of, e.g., magnetic tapes, pressure-sensitive adhesives, photovoltaics and functional coatings. Fundamentals of the technique can be found, e.g., in Schweizer 1997. However, slot die coating has not become a common technique for paper coating, one potential reason being the swelling of base paper fibers, which complicates coat weight control when applying thin wet films. This is not a problem when coating of low solids content fluids, for which the wet coating layer is thicker, as is the case with nanocellulose suspensions. Coating on impermeable and non-swelling webs does, of course, not suffer from this limitation.

Figure 2 shows the experimental setup for continuous slot die coating of nanocellulose used in the current work. A custom-built slot die (slot width=74 mm, slot length=34 mm, slot gap=500-1000 µm, lip lengths=5 mm) is placed at three o’clock position against the backing roll. For a chosen slot gap, the coating process can be controlled by changing the web speed (1-50 m/min), feeding pressure (0-10 bar) and the Slot-Web Gap (SWG, 0- 2000 µm). Occasional large aggregates in the coating material, especially when using CNF, require the use of relatively large slot gaps to avoid clogging of the slot entrance. At the same time, high flow rates through the slot are needed to produce low apparent (process) viscosity and uniform flow. Therefore, to obtain low coat weights at relatively low coater speeds the slot die has to be run in an unconventional mode in which part of the coating material is metered off. For well-controlled metering, it was found to be beneficial to offset the slot die 6 mm below the center line of the backing roll. This creates a converging geometry between the substrate and the slot die lips, which enables coat weight

control by changing the SWG. The split, i.e. the percentage of nanocellulose coated on web vs. being metered off, is determined by the balance between the pressure-driven extrusion flow forcing the coating both upward and downward, and the shear-drag of the moving paper web pulling the coating along upwards. In the converging narrow gap geometry the pressure-driven flow scales to the third power of the gap distance, whereas the shear drag upward is directly proportional to the inverse of the gap distance. Closing the SWG, increasing the feeding pressure and lowering the web speed all reduce the relative proportion of coating transferred to the web. The final coat weight, which depends on the web speed and the amount of coating that is not metered off, increases with increasing SWG, feeding pressure and lowering of the web speed.

Figure 2. Experimental setup for slot die coating of nanocelluose.

ResultsFigure 3 shows apparent viscosity of 3% CNF suspension over 6 decades of shear rate. The low shear rate viscosity measured with parallel plate geometry agrees reasonably well with the high shear rate data from the slot geometry. Noteworthy is the extension of the apparent shear thinning behavior to high shear rates, where the apparent viscosity approaches 10 mPas. The shear-thinning in the pressure-driven gap flow can be explained by dynamic yield behavior, where a non-yielding center plug is surrounded by a yielded, flowing suspension layers at the boundaries.

Utilizing the slot die geometry, a recycled fiber paperboard was coated with CNF suspension at 2% solids concentration using 5 pph carboxymethyl cellulose (CMC, FINNFIX 4000G, CPKelco) as an additive. CMC improves the CNF dispersion and increases the water retention. For the CNC, commercial pigment coated paperboard, TrayformaTM Special (Stora Enso) was used as the substrate. The mineral pigment coating has smaller surface pore size than uncoated paperboard, which improves the coating hold-out of the fine CNC material. Since CNC forms brittle films, 20 pph sorbitol was used as plastisizer. In addition, to improve the CNC coating adhesion, a thin (< 1 µm) primer layer of cationic starch was used. The dry coat weights for CNF was varied from 1 to 16 g/m2 and for CNC from 1 to 7 g/m2. All the coated samples 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. Samples were conditioned (23°C, 50% RH) for at least 24 hours before testing. In order to study the influence of CNC coating on OTR (Oxygen Transmission Rate) some CNC-coated samples were afterwards extrusion coated with ca. 18 µm layer of either LDPE (low density polyethylene) or PLA (polylactic acid) at Tampere University of Technology pilot scale converting line.

Figure 3. Apparent viscosity of CNF suspension with parallel plate and slot geometries (Kumar et al. 2016A).

Figure 4 shows example cross-sections of CNF-, CNC- and CNC+PLA-coated samples. The coatings appear uniform, although the CNF coating, which has relatively large fibrils, is difficult to separate from the underlying uncoated paperboard with similar, although larger fibers. Some selected barrier performance results for the nanocellulose coatings are given in Figure 5 and Table 1. CNF, with its relatively large fibrils, appears to require a minimum coat weight of ca. 6 g/m 2 in order to provide reasonable barrier. Air permeance is below the detection limit for the used two instruments, WVTR (water vapor transmission rate) levels out at ca. 100 g/m2/day, grease barrier with KIT test is 8 or above, and full barrier for mineral oil is obtained. CNC coating, which consists of smaller cellulose particles, improves the barrier properties of the baseboard considerably already at low coat weights. The potential of using thin CNC coatings for oxygen barrier is illustrated in Table 2, which shows reduction of OTR by 99% and 97%, for LDPE and PLA extrusion coatings, respectively. The grease barrier improves 9-fold for the LDPE and two-fold for the PLA.

Concluding remarksThe current work measured high shear rate viscosity of nanocellulose suspensions utilizing slot geometry. The results agree with those obtained with parallel plate devices and demonstrate that, in pressure-driven flow, the highly shear-thinning behavior observed with rotational instruments at low shear rates extends to high shear rates. The highly shear-thinning process viscosity in slot flow enables roll-to-roll coating of viscous, gel-like nanocellulose materials on paperboard and other surfaces. Provided there is a full coverage of the substrate, the barrier properties improve thereby considerably. Challenges that need to be addressed to provide a way to commercialization of nanocellulose-based barriers are related to the low solids concentration

of the CNF and CNC materials as well as their moisture sensitivity. Commercial opportunities for these type of barrier coatings can be found in food service packaging, e.g. hamburger wrappings, pizza boxes, paper plates etc.

Figure 4. Crosssectional SEM images of CNF (left), CNC (middle), and CNC+PLA coatings.

Figure 5. Barrier properties of the CNF coated paperboard. Clockwise from the top left: air permeability, water vapor transmission rate at 50% RH, mineral oil (PKWF 4/7, Haltermann, Germany) barrier measured with ATR-FTIR, and heptane vapor and grease barrier (KIT) (Kumar et al. 2016B).

Table 1. Barrier properties for heptane (HVTR) and water vapor (WVTR) for the CNC coatings (Koppolu et al. 2018).

Coat weight (g/m2)

HVTR (g/m2/day)

WVTR (g/m2/day)

Baseboard 1710 ± 78 174 ± 51.4 131 ± 35 89 ± 82.0 187 ± 38 90 ± 23.5 21 ± 13 85 ± 4

6.7 18 ± 11 59 ± 3

Table 2. Oxygen transmission rate (OTR) and grease barrier improvement with 2.5 g/m2 CNC pre-coating.

LDP

E PLALDPE+CN

CPLA+CN

C

OTR (cm3/m2/d)*599

0 334 18 13Grease Penetration (days) 5 23 44 47*23 °C / 50 % RH

AcknowledgmentsVinay Kumar and Aayush Kumar Jaiswal are thanked for helping out with coating of the nanocelluloses. Dr. Hanna Koivula is thanked for conducting the mineral oil barrier measurement.

References Abitbol T, Rivkin A, Cao Y, Nevo Y, Abraham E, Ben-Shalom T, Lapidot S and Shoseyov O (2016) Nanocellulose, a

tiny fiber with huge applications. Curr Opin Biotechnol 39:76–88. Aspler J, Bouchard J, Hamad W, Berry R, Beck S, Drolet F and Zou X (2013) Review of nanocellulosic products

and their applications. In: Dufresne A, Thomas S, Pothen LA (eds) Biopolymer nanocomposites: processing, properties, and applications, 1st edn. Wiley, Hoboken, pp 461–508

Bardet R and Bras J (2014) Cellulose nanofibers and their use in paper industry. In: Oksman K, Mathew AP, Bismarck A, Rojas O, Sain M (eds) Handbook of green materials: processing technologies, properties and applications, 1st edn. World Scientific Publishing Company, New Jersey, pp 207–232

Brodin FW, Gregersen OW and Syverud K (2014) Cellulose nanofibrils: challenges and possibilities as a paper additive or coating material—A review. Nord Pulp Pap Res J 29:156–166.

Dufresne A (2013) Nanocellulose: a new ageless bionanomaterial. Mater Today 16:220–227. Isogai A (2013) Wood nanocelluloses: fundamentals and applications as new bio-based nanomaterials. J Wood

Sci 59:449–459. Koppolu R, Abitbol T, Kumar V, Jaiswal A, Swerin A and Toivakka M (2018) Continuous roll-to-roll coating of

cellulose nanocrystals onto paperboard, Cellulose, 25(10), 6055–6069. Kumar V, Nazari B, Bousfield DW and Toivakka M (2016A) Rheology of Microfibrillated Cellulose Suspensions in

Pressure-driven Flow. Appl Rheol, 26(4), 43534. Kumar V, Elfving A, Koivula H, Bousfield DW and Toivakka M (2016B) Roll-to-roll processed cellulose nanofiber

coatings. Ind Eng Chem Res, 55(12), 3603–3613. Kumar V, Bollström R, Yang A, Chen Q, Chen G, Salminen P, Bousfield D and Toivakka M (2014) Comparison of

nano- and microfibrillated cellulose films. Cellulose 21:3443–3456. Lavoine N, Desloges I, Dufresne A and Bras J (2012) Microfibrillated cellulose—Its barrier properties and

applications in cellulosic materials: a review. Carbohydr Polym 90:735–764. Lindström T, Naderi A and Wiberg A (2015) Large scale applications of nanocellulosic materials - a

comprehensive review. J Korea Tech Assoc Pulp Pap Ind 47:5–21. Nazari B, Kumar V, Bousfield DW and Toivakka M (2016) Rheology of Cellulose Nanofibers Suspensions:

Boundary Driven Flow. J Rheol, 60(6), 1151–1159. Schweizer P and Kistler SF (Eds.) (1997) Liquid Film Coating: Scientific principles and their technological

implications. Springer. ISBN 978-94-011-5342-3.