Cellulose Nanofibril-Based Coatings of Woven Cotton Fabrics for

Research Article pubs.acs.org/journal/ascecg

Cellulose Nanofibril-Based Coatings of Woven Cotton Fabrics for Improved Inkjet Printing with a Potential in E‑Textile Manufacturing Oleksandr Nechyporchuk,*,† Junchun Yu,‡ Vincent A. Nierstrasz,‡ and Romain Bordes*,† †

Department of Chemistry and Chemical Engineering, Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ‡ Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, SE-501 90 Borås, Sweden S Supporting Information *

ABSTRACT: The roughness of woven fabrics strongly limits print quality, which is particularly critical in printing of conductive circuits on fabrics. This work demonstrates the use of wood-derived cellulose nanofibrils (CNFs) mixed with a plasticizer as coatings of woven cotton fabrics for inkjet printing using (i) conventional water-based pigment inks and (ii) conductive silver nanoparticle inks. CNFs, being similar in nature to cotton, introduced minimal alteration to woven cotton fabrics by preserving their visual appearance as well as their mechanical properties. We also showed that the use of CNF-based coatings facilitated ink droplet settling on the substrate, which ensured high quality with the potential of higher printing speed production. The coatings of CNFs plasticized with glycerol enabled concentrating the pigment on the surface of the fabric, preventing its penetration into the fabric depth, which allows for increasing the resolution of the printed pattern. When used for color ink printing, it enhanced the print chroma and permitted reducing the amount of deposited ink, yielding similar color lightness. The CNF coatings allowed for substantial reduction of the amount of silver ink when printing the conductive tracks on fabrics. Furthermore, the nature of the coating imparts flexibility to the conductive layer, while maintaining electric signal quality, even when folded. This study provides a platform for manufacturing sustainable and disposable e-textiles. KEYWORDS: Cellulose nanofibrils, Nanofibrillated cellulose, Nanocellulose, Fabric coating, Inkjet printing, Smart textiles, Electronic textiles (e-textiles)

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INTRODUCTION The development of electronic devices integrated into fabrics, known as electronic textiles (e-textiles) or, more generally, smart textiles, is a subject of increasing interest. Such products aim at offering advanced functionalities to the conventional textiles, while keeping such features as flexibility, foldability, and being lightweight. The e-textiles include wearable displays, light-emitting diodes (LEDs), electromechanical actuators, power storage devices, etc.1−3 The introduction of conductive elements into woven fabrics is not trivial. It is usually performed by weaving or knitting the conductive filaments (fibers, yarns, or threads) into the textile structure.3 Such an approach, however, lacks process flexibility and is limited in terms of conductive circuit design. Printing techniques that allow conductive paths to be deposited on fabric surfaces offer broader opportunities, more in line with the requirements of developing complex circuitry. Rotary and flatbed screen printing have been by far the most common technologies for printing on fabrics. However, due to a global trend of shortening of average run lengths and increase in speed of inkjet printheads, inkjet technology is becoming more attractive lately.4 Inkjet printing allows printing on © 2017 American Chemical Society

demand and gives opportunities of maskless and noncontact printing. Such features make inkjet a promising platform for development of innovative smart textiles, while being well aligned with the economic constraints of small volume production. Another aspect where inkjet is becoming more attractive is textile dyeing (bulk coloring). In comparison with conventional deying techniques, inkjet has a lower environmental footprint.5 It also offers higher flexibility compared to novel methods of waterless dyeing using supercritical carbon dioxide.6 In terms of electronic textile production, screen printing is appropriate for the deposition of relatively thick conductive layers using paste-like inks, which remain on the surface of the fabric due to high ink viscosity.7 In comparison, the lowviscosity inks used in inkjet printing are much easier to penetrate into the fabric depth, and the thickness of the deposited ink layer is much lower compared to that achieved by screen printing. This generates difficulties in achieving good Received: January 18, 2017 Revised: April 11, 2017 Published: May 5, 2017 4793

DOI: 10.1021/acssuschemeng.7b00200 ACS Sustainable Chem. Eng. 2017, 5, 4793−4801

Research Article

ACS Sustainable Chemistry & Engineering

In this work, we aim to explore the potential of wood-derived CNFs as a base coating for textiles, focusing on improving the inkjet printing process with a potential in e-textile manufacturing. The CNF film-forming properties on woven cotton fabrics are examined, and the influence of plasticized CNF coatings with different basis weight, as well as printing with various ink droplet volume, on the printed layer properties are investigated. Color water-based pigment ink is first assessed as a proof of concept that CNF networks control the pigment penetration to the depth of fabric. Then, conductive silver nanoparticle ink is tested to produce printed circuits on fabrics coated with CNFs and a plasticizer with the objective of enhancing the conductivity of the printed paths. The flexibility and foldability of the produced e-textile is also evaluated.

conductivity of the printed path, and high fabric roughness becomes one of the main challenges for the print quality.8 Sufficient ink thickness can be thus achieved by multiple passes in the inkjet printer. Roughness reduction together with control of ink penetration to the fabric bulk are thus essential for the implementation of such technologies since they can reduce the amount of deposited conductive inks, which are generally expensive. Recently, cellulose nanofibrils (CNFs) have gained an increasing interest for use in printed electronics as substrates or as constituents in functional conductive ink formulations.9 CNFs are biobased and biocompatible nanomaterials produced by disintegration of cellulosic fibers that are traditionally used in papermaking, textile, and other industries. CNFs are potentially low-cost materials,10 generally having a diameter of 3−50 nm and a length of few micrometers, and are composed of alternating crystalline (ordered) and less crystalline (disordered) regions. 11 Manufacturing of CNFs is generally performed by mechanical disintegration of microscopic cellulose fibers with preliminary enzymatic hydrolysis (e.g., using endoglucanase) or chemical surface modification (e.g., carboxylation, carboxymethylation, or quaternization) that are used to facilitate the individualization of nanofibrils.11 CNFs have been widely reported previously as paper strength-enhancing additives or coatings,12−14 fillers in composite materials,15−17 rheology modifiers,18,19 emulsion stabilizers,20,21 freestanding films,22−25 aerogels,26−28 and hydrogels.29,30 In addition to the above applications, CNFs were used as substrates for printed electronics.31−35 ChingaCarrasco et al.31 reported the use of various grades of CNFs as film substrates for printed conductive circuits manufactured by inkjet printing of silver inks. CNFs produced by mechanical disintegration without chemical surface modification or with carboxylation or carboxymethylation pretreatments were examined. It was shown that the highest print resolution was achieved for chemically pretreated CNFs, owing to the lower surface roughness of the films. Moreover, higher print resolution was achieved by reducing the wettability of the film surfaces through grafting with hexamethyldisilazane.31 Hseigh et al.32 demonstrated the advantageous use of CNF nanopapers compared to conventional pulp papers for fabrication of conductive circuits by gold sputtering or inkjet printing using silver nanoparticle inks or particle-free metalloorganic decomposition (MOD) silver inks. They reported a drastic decrease in electrical resistance (from 6340 to 34 Ω for gold sputtering) for equally deposited circuits on nanopapers compared to those for traditional papers. This was attributed to the ability of CNFs to produce smooth and low porous surfaces. Composites of CNFs and inorganic filler particles36,37 or acrylic resins/CNFs38 were also proposed as substrates for flexible electronic devices. The aforementioned results suggests that CNFs may be suitable to modify the surface of fabrics that may be subsequently used as substrates for printed electronics. The use of CNFs as coatings in woven fabrics has not been extensively studied yet. Some works describe the use of such coatings to improve the print quality of nonwoven and woven synthetic fiber sheets39,40 and recently woven cotton fabrics.40 However, the influence of the CNF coatings on pigment penetration into the fabric bulk still requires investigation. Compared to various elastomer coatings widely used for fabrics, development of all-cellulose products applying wood-derived CNFs on cotton fabric appears very attractive.

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MATERIALS

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METHODS

Unbleached cotton fabric with 2/1 twill weave, a basis weight of (112 ± 1) g/m2, and a thickness of 0.22 mm (determined according to ASTM D1777-96) was kindly provided by Eton Fashion AB (Sweden). Glycerol (≥99.5%) and hydrochloric acid (37%) were purchased from Sigma-Aldrich Sweden AB. Nanocellulose. CNFs in the form of aqueous suspensions with a solids content of 3.3 wt % were kindly provided by Stora Enso AB (Sweden). The CNFs were produced by means of mechanical fibrillation of softwood pulp (ca. 75% of pine and 25% of spruce, containing 85% of cellulose, 15% of hemicellulose, and traces of lignin, as determined by the supplier) and had an average nanofibril diameter of 7 ± 3 nm and a length of ca. 1 μm, as determined from height profiles of atomic force microscopy images (Figure S1, Supporting Information). They had a charge density of (20.7 ± 0.6) μeq/g at pH 5.2 (measured using a particle charge detector PCD-02 (Mütek Analytic GmbH, Germany) titrated using polydiallyldimethylammonium chloride). Inks. Water-based cyan pigment ink (VelvetJet) was provided by Bordeaux Digital PrintInk, Ltd. (Israel). It had a solids content of ca. 30 wt %, viscosity of 10.5 mPa s at a shear rate of 10,000 s−1 and temperature of 35 °C, and surface tension of 31.1 ± 1.2 mN/m at 25 °C. The pigment average particle size was determined as 110 nm at 90° by means of dynamic light scattering using an N4 Plus submicron particle size analyzer (Beckman Coulter, USA). An Epson T2631 photo black ink cartridge was purchased from Seiko Epson Corporation (Japan). Water-based silver nanoparticle ink (NBSIJMU01) was purchased from Mitsubishi Paper GmbH (Germany). The ink had a silver content of 15 wt %, viscosity of 2.30 ± 0.50 mPa s at 25 °C, and surface tension of 31.0 ± 3.0 mN/m, as reported by the supplier.

Fabric Coating. The initial CNF suspension was diluted with deionized water to 1 wt %, and 100 g of the suspension was homogenized using Heidolph DIAX 900 (Heidolph Instruments, Germany) equipped with 10 F shaft at power 2 (11,600 rpm). The formulation containing a plasticizer was prepared in a similar way by adding glycerol to CNFs to reach 1/10 wt/wt ratio prior to homogenization. Coating of woven cotton fabrics was performed via spraying of the above formulation using a Cotech Airbrush Compressor AS18B (Clas Ohlson AB, Sweden) at a pressure of 3 bar. The coatings were deposited in 2, 4, or 6 runs with intervals of 20 min. The samples were dried under a fume hood at ambient temperature (ca. 20 °C). As a result, an increase in basis weight by 8.1, 13.4, and 20.4 g/m2 was obtained for the fabric coated with CNFs and glycerol, as measured by gravimetry. Inkjet Printing with Cyan ink. Printing on fabrics uncoated or coated with CNFs/glycerol was performed using a custom-made inkjet printer Urtidium B200 (VdW-Consulting bvba, Belgium) equipped with a piezoelectric printhead Konica Minolta KM1024i, allowing a print resolution of 360 dpi. The ink was passed through a nylon 4794


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Figure 1. Mechanical properties of woven cotton fabrics noncoated and coated with CNFs or CNFs/glycerol (10/1 wt/wt) with different basis weight (a, b, c) and visual appearance of CNF (d) and CNFs/glycerol (e, f) freestanding cast films (ca. 55 g/m2) that were 180° folded and unfolded; films folded one (d, e) or three (f) times. syringe filter of 0.45 μm before feeding the printhead. Fabric specimens were printed by varying the ink volume per droplet ejected from a single nozzle, viz., 49, 101, and 145 pL. The printed substrate was subsequently cured in the oven at 150 °C for 5 min. Inkjet Printing with Silver Nanoparticle Ink. Uncoated and coated fabrics were printed with conductive silver nanoparticle ink using an Epson Expression Premium XP-600 inkjet printer (Seiko Epson Corporation, Japan). The original Epson cartridges were replaced with compatible ones filled with conductive inks through a nylon filter of 5 μm. The fabric samples were mounted on a CD tray for printing. Each fabric sample was processed by three printing cycles. Before printing, the fabric was sprayed with 0.05 M HCl, which was necessary to disrupt the silver nanoparticle stabilizing agent and to achieve the required conductivity of the printed circuit without a thermal sintering step. Printing of Resolution Test Chart. An Epson Expression Premium XP-600 inkjet printer (Seiko Epson Corporation, Japan) was used to print the resolution test chart on uncoated and coated fabrics using Epson T2631 photo black ink. The chart was designed with 1.2−5.2 line pairs per mm, where one line pair contained one printed line and one blank line. Scanning Electron Microscopy (SEM). Fabric specimens were examined using a LEO Ultra 55 field emission gun (FEG) SEM (Carl Zeiss, Germany), operating at an acceleration voltage of 2−3 kV. The specimens were glued on stubs using carbon tape and were coated at the edges with PELCO conductive liquid silver paint to improve the conductivity and finally were sputtered with an Au layer of ca. 10 nm. Cross sections of the specimens were prepared by cutting the fabric with a fresh razor blade stroke with a hammer. Optical Microscopy. Optical microscopy images of the fabric cross sections and top surfaces were taken using a Zeiss Axio Scope.A1 (Carl Zeiss, Germany) microscope equipped with Zeiss AxioCam MRc5 digital camera. ZEN 2012 acquisition software was used for image processing. Atomic Force Microscopy (AFM). The AFM was performed in a tapping mode using NTEGRA Prima equipped with an NSG01 cantilever (NT-MDT, Russia) to examine the morphology of CNFs. The CNF suspensions were diluted to the concentration of 10−2 wt %, and a droplet was placed on a freshly polished silicon wafer substrate and dried. The AFM height images were then processed in Gwyddion software. Colorimetry. Color coordinates of the prints were measured in the CIE L*a*b* color space by Datacolor Check II spectrophotometer (Datacolor, USA). The measurements were performed using a D65 light source at a 10° observer. Data processing was performed using the Datacolor TOOLS 2.1 software. The color coordinates were characterized by the values of L* representing the lightness, which varies from 100 (white) to 0 (black), a* and b* representing the

chromatic components, where + a* is the red, − a* is the green, + b* is the yellow, − b* is the blue directions, and a 0 value for both a* and b* represents a grayscale. The components a* and b* can be expressed by a single chroma parameter C* determined as [(a*)2 + (b*)2]1/2. Tensile Testing. Mechanical testing was performed according to the ASTM D5034-09 (2013) method. Instron 5565A (Norwood, MA, USA) equipped with a static load cell of 5 kN and pneumatic clamps with a pressure of 5 bar was used for the measurements. Data processing was performed using Bluehill software. Rectangular specimens with a length of 150 mm and a width of 20 mm were cut parallel to warp direction along the threads. The specimens were conditioned at least 12 h before the measurements at a temperature of 23 °C and a relative humidity of 60%. Each specimen was fixed in the clamps around steel pins to avoid the slippage. The measurements were performed at a constant extension rate of 300 mm/min at the gauge length of 20 mm. Seven measurements were performed for each sample, and the average values were calculated. The terms related to the force and deformation properties were used as determined by ASTM D4848-98 (2012). The Young’s modulus values were determined from the linear viscoelastic region after passing the fabric toe region. Contact Angle. A dynamic angle tester DAT 1100 (Fibro System AB, Sweden) was used to measure the angle of the ink or deionized water of a volume of ca. 3 μL that is in contact with the fabric as a function of time. Electric Signal Analysis. A soundcard-based virtual oscilloscope system Soundcard Oscilloscope V1.46 (Christian Zeitnitz, Germany) was used to generate and to record the complex signals with a frequency of 0.5−20 kHz through the inkjet-printed conductive paths on fabrics. In addition, measurements were carried out using a SHT75 humidity sensor (Sensirion AG, Switzerland). The readings were done using an open source platform Arduino Uno, which used the 2-wire library to dialogue with the 4-pin sensor. The electrical resistance was measured using an A830L digital multimeter.

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RESULTS AND DISCUSSION The printing quality on woven fabrics can be improved by using coatings that force the pigment or colorant to concentrate and settle on the surface rather than to spread within the depth of the fabric. The desired coating should act as a support for printing ink while preserving the fabric flexibility. CNFs have the same nature as cotton, thus they can impose minimal alteration to woven cotton fabrics when used as coatings. However, the inherent brittleness of CNF films has to be overcome. In this work, glycerol was used as a plasticizer to impart flexibility to the CNF coatings. 4795


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Figure 2. Water (a) and pigment ink (b) contact angle measurements on woven cotton fabrics with different amounts of coated CNFs/glycerol. Droplet volume as a function of time is indicated in the top right corner of the photographs.

Figure 3. SEM images showing the top surface view of noncoated (a) and coated (b, c, d, e) woven cotton fabrics with 8.1 g/m2 (b), 13.4 g/m2 (c), and 20.4 g/m2 (d, e) of CNFs/glycerol and the cross section view of the fabric coated with 20.4 g/m2 of CNFs/glycerol (f).

without compromising the overall mechanical resistance of the CNF coating. The effect of the introduction of glycerol on the mechanical properties is also reflected by the improved foldability of the freestanding cast films. The photographs of the CNF films (with a thickness of ca. 40 μm, measured using a digital micrometer (Model IDC-112MB; Mitutoyo Co, Japan), and a basis weight of ca. 55 g/m2) without and with glycerol, which were folded and unfolded for 180° one time, are shown in Figure 1d and e, respectively. The film without glycerol breaks, whereas the plasticized film does not fall apart. Moreover, the addition of the plasticizer allows us to fold/unfold the film several times without rupture (Figure 1f). Therefore, CNFs/ glycerol coatings were further used to study the effect of the coating on printability of fabrics. Figure 2a shows the contact angle of a droplet of water on the surface of the fabric in the time interval of up to 10 s. The higher contact angle on the noncoated fabric can be explained by the presence of lubricant residues (e.g., tallow or mineral oil) widely used in fabric weaving processes44 that impart some hydrophobicity to cellulose. Therefore, the water droplet stays on the surface of the fabric without distinct penetration, reflected by the practically nonchanged droplet volume. As expected, the contact angle is significantly reduced when the

Figure 1a−c shows mechanical properties of the fabrics coated with CNFs and the CNFs/glycerol (10/1 wt/wt) mixture. The breaking force of coated textiles remains practically unchanged at a low amount of deposited formulations (Figure 1a), whereas it tends to decrease slightly as the coating amount increases. Despite the large error bars, CNF/glycerol coatings seem to have slightly higher values of breaking force compared to CNF ones. The elongation at break remains constant for the fabric treated with CNFs/glycerol (Figure 1b) compared to the coatings of CNFs alone, elongation of which decreases progressively. In addition, Young’s modulus values (Figure 1c) indicate that the stiffness of the coated textile increases with a higher amount of applied formulations. These results demonstrate the reinforcing capacity of CNF coatings both with and without plasticizer. CNF/glycerol coatings at a lower basis weight result in enhancement of Young’s modulus while keeping constant both the breaking force and the elongation at break, thus suggesting that toughness of the coated fabrics slightly increases. The use of glycerol allows reducing brittleness of the coatings exposed to elongation. These conclusions are in agreement with the results found for other polysaccharides, for example, starch, chitosan, or cellulose derivatives.41−43 The introduction of the plasticizer is believed to enhance sliding of the nanofibers 4796


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Figure 4. Optical microscope images of noncoated (top) and coated (bottom) with 8.1 g/m2 CNFs/glycerol (10/1 w/w) fabrics. The images of cross section (a, b; left) and top view (a, b; right) of fabrics printed with cyan pigment ink volume of 49 pL/droplet (a) and 145 pL/droplet (b). The fabrics with printed patterns having two line pairs per mm (c).

Figure 5. Colorimetric properties of noncoated and coated textiles nonprinted and printed with various ink volume per droplet: (a) CIE L*a*b* color space showing change in chromatic components, (b) lightness, and (c) chroma at different volumes of ink per droplet.

inkjet printing. Thus, much less water is required to be absorbed by the coating during the printing process. The contact angle measurements suggest that CNF-based coatings may be beneficial for higher speed inkjet printing on fabrics due to better surface wetting and faster ink droplet settling. SEM images of noncoated and coated woven cotton fabrics with various amounts of CNFs/glycerol are shown in Figure 3a. It can be seen that at 8.1 g/m2 the coating covers the surface of the fabric, even though some gaps remain not covered. The pores become completely filled when increasing the amount of the coating to 20.4 g/m2, resulting in a smooth layer that reduces the roughness of the fabric. These results are in good agreement with the previous studies showing the smoothening effect of CNF coatings on paper47−49 or on synthetic nonwoven fabric mats.39 Despite the nanoscale dimensions of CNFs, the coating mainly remains on the fabric surface and does not penetrate deep between the threads of the woven fabric (Figure 3f). This is likely to occur due to the entangled structure of the CNFs (Figure S1, Supporting Information). Since the CNF suspension was produced by mechanical fibrillation without chemical surface modification pretreatments, the nanofibrils are physically entangled. The size of these agglomerates decreases from ca. 300 μm to ca. 100 μm when exposed to shear flow,19 as during spraying. However, this size is still large enough for deep transfer into the fabric. Instead, these entanglements form a continuous film on the surface of the fabric.

CNF-based coatings are applied. Additionally, the droplet absorption rate becomes faster. The above indicates both better surface wettability and better water penetration rate when the coated fabrics are used. The contact angle decreases further with the higher basis weight of the coating, which occurs due to the progressive introduction of more hydrophilic material. When cyan pigment ink is used for contact angle measurements, the surface wetting becomes overall better (Figure 2b) compared to that when using water, which is explained by lower surface tension of the ink. Due to better wetting, the ink penetrates faster into the substrate. However, the ink absorption rate becomes slower for the coated fabric compared to the uncoated one, indicated by a slower decrease in the droplet volume. Such phenomena may be explained by the formation of filter cake,45 which is likely to occur when using CNF-based coatings with nanoscale fibril networks. In such a process, pigment particles fill the pores, aggregate, and hinder water transfer through the substrate. When a noncoated fabric is used, the rate of ink penetration into the fabric is higher due to higher porosity of the substrate. It was previously reported that by decreasing the ink contact angle on substrates, wider printed patterns are obtained by inkjet printing; thus, the printing resolution decreases.46 On the other hand, the surface roughness plays an important role for the printed pattern resolution. It is also noteworthy that the ink volume per unit area used for contact angle measurements is much higher than the maximum volume commonly used during 4797


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Figure 6. Manufacturing of inkjet-printed paths using silver nanoparticle ink on cotton woven fabrics coated with CNFs/glycerol (8.1 g/m2): (a, b) with connected LED and battery, (c) with connected digital humidity sensor, signal processing circuit, and LED display, and waveform of the signals at (d) 2 kHz and (e) 5 kHz recorded from the printed path (right path in a) on the fabric and corresponding frequency spectra in the insets.

The inset in Figure 3f illustrates the lamellar organization of the CNF coatings. Such a structure is not an artifact produced by the application of several spraying passes but is an intrinsic property of CNFs to self-assemble, which was also previously reported for CNF films prepared by casting/evaporation or vacuum filtration methods22,25,50 from surface modified50 and nonmodified22,25,51 CNFs. Optical microscopy was then performed in order to investigate whether the CNF network prevents the pigment transfer to the fabric bulk. Figure 4a and b shows the cross section and top view images of noncoated and coated CNF/ glycerol fabrics that were inkjet printed with different amounts of water-based cyan ink, viz., 49 pL/droplet (Figure 4a) and 145 pL/droplet (Figure 4b). When printed on noncoated fabric, irrespective of the droplet volume, the ink penetrates deeply into the fabric structure. An increase in the amount of ink results in a more saturated color. The printing on coated fabrics shows a striking difference since the ink pigment is localized on the surface of the CNF-based coating without penetrating the fabric threads. This observation is in agreement with the contact angle measurements (Figure 2) that suggested the filter cake formation phenomenon and pigment localization on the surface of the coating. This hypothesis is also supported by the pore size of the CNF layer that is in the same range as the pigment particles. The increase in the droplet size, as for the noncoated textile, leads also to a higher color saturation. The printing resolution also benefits from the CNF-based coatings, as demonstrated in Figure 4c, which shows the visual appearance of the printed pattern with two line pairs per millimeter. The resolution test chart with the wide range of line pairs (1.2−5.2) per millimeter printed on coated and noncoated fabrics is shown in Figure S2 in the Supporting Information. In order to quantify the reduction of pigment penetration and its influence on color properties, colorimetry measurements were carried out using various cyan ink amount on noncoated and coated textiles with different coating basis weight. Figure 5a demonstrates the CIE L*a*b* color space representing the change in chromatic components by varying

the ink amount per droplet and printing on noncoated and coated fabrics with 8.1 g/m2 of CNFs/glycerol. The color properties of coated and noncoated fabrics before printing are very similar (Figure 5a) and tend toward yellow, representing the color of the unbleached cotton fabric. Therefore, the presence of a CNF-based coating does not deteriorate the visual aspect of the fabrics. When the coatings are applied on fabrics, which are further printed with cyan ink, there is an enhancement of the print chroma. Therefore, the use of a CNF-based coating can increase the color gamut, that is, the capacity in reproducing a larger set of colors. This can be explained by (i) a reduction in the pigment penetration to the fabric depth and (ii) a decrease in the substrate roughness and hence the light scattering. The variation of the chroma parameter C* as a function of coating basis weight is shown in Figure 5b. It can be seen that the chroma increases already at 8.1 g/m2 for all the amounts of ink per droplet. As the amount of deposited coating further increases, no distinct changes in chroma are observed, which is in line with Figure 3 showing no drastic fabric surface coverage after the coating of 8.1 g/m2. On the other hand, Figure 5c indicates that the lightness L*, that is, the perceived brightness of an object, continues decreasing further with an increase in the amount of coating, which is in agreement with previous reports.40 It can be seen that similar values of L* can be obtained for fabrics coated with 8.1 g/m2 printed with 49 pL/ droplet and noncoated fabrics printed with 101 pL/droplet, as a result of the increased pigment particle density on the surface. The above results, which showed a reduction in the pigment penetration to the fabric depth by application of the CNF/ glycerol coatings, were further utilized to print conductive paths on the cotton fabrics. The use of silver nanoparticle-based ink has become relatively common for specific substrates, such as synthetic polymers, and often require a sintering step in order to enhance the interparticle contacts needed to achieve proper conductivity. In the case of textiles, however, the highly porous nature of the material does not allow us to reach a sufficient particle density necessary to achieve good conductivity values when using a low amount of ink. 4798


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Figure 7. SEM images at different magnification showing the silver nanoparticle pigment printed on (a, b) noncoated woven cotton fabrics, showing that the pigment does not create a uniform layer, and on (c, d) CNFs/glycerol-coated woven cotton fabrics (8.1 g/m2), where the pigment resides well on the surface of the coating and forms a uniform layer.

Figure 6a shows an example of the circuitry that was inkjet printed using only three passes with silver nanoparticle ink on the fabric with 8.1 g/m2 CNF-based coating. When no coating was applied on the fabric, the sheet resistance of the printed pattern was >83.3 kΩ/sq (above the threshold of the multimeter). When the coating was used, the sheet resistance became significantly lower, viz., 2.9 ± 0.3 Ω/sq. Thus, the use of CNF-based coating reduced significantly the printed path resistance and allowed us to manufacture the demonstration circuit composed of an inkjet-printed conductive path, LED, and battery. Figure 6b and the video (Supporting Information) show that such circuitry operates well when the fabric is largely bent or folded several times. In the present case, the CNF layer enables us to maintain the interparticle cohesion, which preserves the printed path conductivity. Additionally, the electric signal quality was assessed using an oscilloscope by sending sine wave signals with a frequency of 0.5−20 kHz through the printed path (right path, Figure 6a). The recorded signals for the frequencies of 2 and 5 kHz are shown in Figure 6d and e, respectively. No considerable distortions of amplitude could be noted. Another more complex demonstration of the e-textile is given in Figure 6c, where the signal quality is evaluated via communication of the digital humidity sensor SHT75 (Sensirion AG, Switzerland) with a microcontroller through the inkjet-printed conductive paths on the fabric using an advanced data coding on two wires (I2C). The information from the sensor is processed by an Arduino Uno platform to be displayed on an LCD screen. In such a case, the transferred signal is of binary nature with pulse lengths typically ranging from 15 to 250 ns. This illustrates that the conductive paths are suitable for advanced electronic applications, whereas conductive paths of low qualities would not enable proper data decoding.

The high quality of signal transfer through the conductive paths could be explained by the silver nanoparticle layer morphology on the coated fabric, as shown in Figure 7. When no coating is applied, the layer of silver nanoparticles on the fabric is noncontinuous (Figure 7a,b), whereas when a CNFbased coating is present (Figure 7c,d), the silver nanoparticles remain on the surface of the coating and form a concentrated uniform layer, which finally results in highly conductive circuits. It is interesting to look further at the details of the dried ink droplets on the surface (Figure 7c). Owing to the presence of the CNF layer, the liquid of the picoliter-size ink droplet is drained and leaves a circular deposit of silver nanoparticles of ca. 25 μm in diameter. This implies that the resolution limit of the printed pattern could approach this specific size, allowing the conductive paths with a width as low as a few tens of micrometers to be produced.

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CONCLUSIONS This study shows that plasticized CNFs can be applied as a coating on woven cotton fabrics, resulting in web-like networks of nanofibrils that reduce the fabric roughness and preserve their visual appearance, being similar in nature to cotton. Compared to various elastomer coatings, development of an allcellulose product applying wood-derived CNFs on cotton fabric appears very attractive. The application of plasticized CNF coatings reduce the pigment penetration into the fabric bulk, while not lowering their mechanical properties. By enhancing the pigment density on the surface of the fabric, it is possible to improve the print quality or to reduce the amount of needed inks. We show that such an approach can be used to produce electronic textiles with inkjet-printed conductive paths made of silver nanoparticle ink. High conductivity of printed paths can be achieved with a low amount of printed ink, which is not possible on noncoated fabrics. The use of such coatings may be further extended for inkjet printing of supercapacitors or antennas. The use of inkjet 4799


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ACS Sustainable Chemistry & Engineering printing gives large flexibility in terms of printed pattern geometry and offers opportunities of printing on demand, compared to the other techniques based on weaving of the conductive elements into the fabric or screen printing. Moreover, inkjet printing allows multichannel printing with several different inks in one pass, depending on the number of installed cartridges. Additionally, nanocellulose coatings may be promising for better fabric recyclability due to limited coloring of the fabric structure. This may facilitate the transition from the traditional dyeing technology toward environmentally friendly and market-driven inkjet printing. There is still a limitation of using such an approach if washable fabrics are desired since there is not enough adhesion between the CNF coating and the fabric. Further work on improving the robustness of CNF coatings on fabrics, for example, using a cross-linking approach,35 can provide wider opportunities for industrial application of such technology. On the other hand, such coatings can be readily used for the production of sustainable and disposable electronic textiles and garments, for example, for medical and health care applications, where washability is not a limiting factor.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00200. Video of bending of the produced e-textile. (MPG) AFM and optical microscopy images of CNFs, and photographs of the fabrics with printed resolution test charts. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(O.N.) E-mail: [email protected]. *(R.B.) E-mail: [email protected]. ORCID

Oleksandr Nechyporchuk: 0000-0001-7178-5202 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Anders Mårtensson from Chalmers University of Technology for the support with SEM measurements and to Mats Johansson and Sina Seipel from the University of Borås for their assistance in inkjet printing and colorimetric measurements, respectively. J.Y. and V.N. are grateful for the financial support from KK-stiftelsen (The Knowledge Foundation) and TEKO for enabling this research.

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REFERENCES

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