Inkjet Printing of Reactive Silver Ink on Textiles - ACS Applied

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Functional Inorganic Materials and Devices

Inkjet Printing of Reactive Silver Ink on Textiles Hasan Shahariar, Inhwan Kim, Henry Soewardiman, and Jesse S. Jur ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18231 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Inkjet Printing of Reactive Silver Ink on Textiles Hasan Shahariar1, Inhwan Kim1, Henry Soewardiman2, Jesse S. Jur*1 1Fiber

and Polymer Science Program, NC State University;

2Mechanical

and Aerospace Engineering, NC State University

Corresponding author email address: [email protected]

Abstract Inkjet printing of functional inks on textiles to embed passive electronics devices and sensors is a novel approach in the space of wearable electronic-textiles (E-textiles). However, achieving functionality such as conductivity by inkjet printing on textiles is challenged by the porosity and surface roughness of textiles. Nanoparticle based conductive inks frequently cause the blockage/clogging of inkjet printer nozzles making it a less than ideal method for applying these functional materials. It is also very challenging to create a conformal conductive coating and achieve electrically conductive percolation with the inkjet printing of metal nanoparticle inks on rough and porous textile and paper substrates. Herein, a novel reliable and conformal inkjet printing process is demonstrated for printing particle-free reactive silver ink on uncoated polyester textile knit, woven and nonwoven fabrics. The particle-free functional ink can conformally coat individual fibers to create a conductive network within textile structure without changing the feel, texture, durability and mechanical behavior of the textile. It was found that the conductivity and the resolution of the inkjet-printed tracks are directly related with the packing and the tightness of fabric structures and fiber sizes of the fabrics. It is noteworthy that the electrical conductivity of the inkjet-printed conductive coating on pristine PET fibers is improved by an order of magnitude by in-situ heat curing of the textiles surface during printing as the in-situ heat curing process minimizes the wicking of the ink into the textile structures. A minimum sheet resistance of 0.2 ± 0.025 Ω/□ and 0.9 ± 0.02 Ω/□ on polyester woven and polyester knit fabric is achieved, respectively. These findings aim to advance E-textile product design through integration of inkjet printing as a low-cost, scalable and automated manufacturing process.

Keywords E-textiles, inkjet printing, particle-free reactive ink, silver coating, conductive pattern, bending, washability. 1|Page ACS Paragon Plus Environment

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Introduction Electronic textiles (E-textiles) have gained great interest in research, especially for wearable applications.

1-5

The adoption of new materials, machines and processes that are required for E-

textile manufacturing is challenging the manufacturing growth of this sector. Of particular concern is the need for facile patterning process for developing complex electronic architectures on textiles. While standard dip coating processing of the textile in conductive material solutions can easily incorporate conductivity in textiles, 6,7 it is challenging to create conductive patterns on textile by this process. Printing technology is observed as a facile process for physically integrating heterogenous material on textiles. In particular, screen printing has already been adopted to print electronics on textiles by depositing thick-paste metal inks.8-11 Screen-printing is widely used for fast prototyping methods to develop sensors,

12,13

and other electronics

14,15

onto various

substrates16,17, but has significant limitations. Screen-printing requires multiple processing steps and the viscous pastes results in a great amount of ink waste and post-process cleaning of the screen. The thick pastes used in screen printing also alters the properties of textile substrates, which change the feel of textiles. The screen-printed conductive thick paste can easily crack and lose the functionality by the mechanical deformation of textiles. Thus, a film lamination or thick surfacesmoothing coating is required to screen print conductive pastes on textiles. The use of conductive yarn for making conductive patterns on textile also requires multiple expensive processing steps such as of metallization of yarn followed by the mechanical incorporation (sewing, embroidery) of the conductive yarns to locally place a functional pattern on textile. On the contrary, inkjet printing is a single step automated manufacturing process which uses much less ink than that of screen printing and can deposit a thin layer of conductive or functional layers on a select pattern region of a surface. Figure 1 shows a comparative analysis of processing steps required in conductive yarn-based knitting process, embroidery and sewing process with the single step inkjet printing process for metalizing textile patterns. Inkjet printing processes requires very dilute and low volume functional fluids to continuously jet picolitre size droplets on the substrates. Primarily, functional metal nanoparticle inks are used for inkjet printing conductive patterns on polymeric films and textiles.18,19 Inkjet printing nanoparticle inks can result in the inkjet printer nozzles to clog, and thus the inks are limited to sufficiently small particle sizes in order to flow through the nozzles. Moreover, due to the high surface roughness and porosity of the textile, a large number of print passes for high density metal particles are needed to create a conductive path20 or, an ink 2|Page ACS Paragon Plus Environment

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receptive thick coating/smoothing layer is required to be deposited on rough textiles before inkjet printing, thus limiting the soft and pliable properties of textile materials.21-26 The durability of inkjet-printed such metal lines on textile is not well-studied and understood in prior literatures. Recently, the particle-free reactive metal inks synthesized using modified tollen chemistries have shown promising characteristics, such as achieving high conductivity using low sintering temperatures (~1000 C). 22,27-29 The conductive metal exists as a complex ion dissolved in a waterbased solvent system which only solidifies once heated. Although inkjet printing of conductive silver nanoparticle ink, reactive ink on textiles has been reported in literature,19,25,28 the wetting mechanism of the diluted ink on the textile fibers, the process modification of inkjet for porous substrates, and the durability of the deposited conductive layers has not been well discussed. The inkjet process was only demonstrated to deposit functional materials only on the top layer of densely packed fabric structures, where layers of metal particles added up on the textile surface to create conductive network which might not be durable.18-23,27-29 The modification of inkjet process was not developed to coat the surface area of individual fibers in the textile structure. The coating of the individual fibers around their surfaces in the fabric structure is required to create conductive percolation in loosely structures textiles such as knit and to enhance the durability of the printed layers. Limited research has been performed towards understanding the process conditions of printing particle-free inks on textile, particularly toward understanding how to print on native textiles without any pretreatment coatings to better facilitate ink reception. This work utilizes inkjet printing with in-situ heat curing to deposit reactive silver ink onto various textile substrates without an interfacial layer. The process was studied by changing the textile substrate, modifying the number of layers of reactive ink printed, and comparing printing with and without in-situ heating. To print a pattern conformally on textile fibers, the ink properties such as viscosity and surface tension was adjusted with the surface energy of the print media (textiles) to spread and adhere ink to the surface of the textile fibers. Additionally, the ink was selected with a sintering temperature higher than the printer-platen temperature (textile substrate temperature in in situ heating) during the inkjet process so that the particle free reactive ink can adequately wet the fibers before being solidified. The conductive traces are analyzed with optical microscopy, SEM with EDS mapping, XRD analysis and sheet resistance measurements to determine the conditions necessary to inkjet print conductive pathways. Furthermore, validation of the 3|Page ACS Paragon Plus Environment


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conductive coating applicability to withstand the common conditions of textile use (mechanical deformations, washability) is presented.

Experimental The particle-free conductive ink is a class of commercially available organometallic reactive compounds which are soluble in an aqueous vehicle. For example, the ink presented in this work is a silver salt in an aqueous amine solution. At room temperature, the amine compound can form a dissolvable complex ion with silver salt to form a particle-free solution. The reactive metal inks can then react to form Ag once heated to a temperature when the silver complex can be reduced, shown at scheme 1. The other bi-components of the reaction are released as volatile gases. The general form of the reaction is shown in Figure 2 (a). The chain length of R1 and R2 can be altered to tune the sintering temperature of the ink. In collaboration with Liquid X Printed Metal, a silver reactive ink with an optimized sintering temperature of 140°C was developed for inkjet printing.30,31 The curing temperature should be such that the ink remains stable at low temperatures to prevent curing at the capillary inkjet nozzle during firing droplets. At the same time, the sintering temperature should not be so high as to degrade or burn the polymer-based textile fibers. It was found that the ink with of 140°C sintering temperature can be jetted for prolong time without any clogging the inkjet nozzle. The physical properties of the ink such as surface tension, viscosity was adjusted and optimized for stable droplet generation. The physical properties of the reactive silver ink are listed in Table 1.

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Figure 1: a) Shows different steps for processing existing process of making conductive patterns on fabric using conductive thread/yarn; b) shows the single step process of patterning conductive structure on textile by inkjet printing process.

Scheme 1. The formation of silver from the organometallic amine compound when exposed to heat.



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Figure 2: (a, b & c) the systematic of process flow for inkjet printing with reactive Ag ink on textile surfaces, & d) the XRD characterization of inkjet printed conductive ink on PET knit compared with pristine knit samples. Table 1: Characteristics of reactive Ag ink used for inkjet printing Ink characteristics

Specifications

Appearance

Clear, water -based

Viscosity (cP) at 25o C

11

Density (g/mL)

1.1-1.3

Surface tension (mN/m)

30-34

Diaminopropane; Metallic 10-30% ;10-30%; 10-30% (silver) salt; Diaminoethane


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To demonstrate the robustness of the particle-free based inkjet processing on textiles, three very different types of textile materials and structures were studied – plain-woven fabric made with PET (polyethylene terephthalate) yarns, single jersey knit fabric made with PET yarns and Evolon® nonwoven fabric made with bi-component PET and polyamide fibers. The surface free energy of the polyethylene terephthalate and polyamide are ~44 mN/m and ~46 mN/m, respectively32, which is higher than the surface tension of reactive silver ink. This helps the reactive ink to spread onto the PET and polyamide fibers. Reactive silver ink was also observed to not appropriately reduce on cellulose-based substrates (such as cotton) as the abundant –OH groups trap the silver its ionic form to prevent metallization. As a result, the reactive silver ink results in a non-conductive material that is brown in color on cellulosic material even with extending heating. The polymer types are confined to polyester and polyamide because the reactive silver ink has a good adhesion to the surface and functional group of polyester and polyamide.

Inkjet printing process The experimental setup for printing is provided in Figure S1 (supporting information). Printing was performed using a Botfactory Squink thermal drop-on-demand inkjet printer (90 drops-perinch, DPI), with a printing surface area of 127mm x 127mm and an adjustable Z-direction to accommodate for thick substrates. The ink cartridge, with 12 nozzles, is made from polycarbonate and has a foam insert to facilitate the printing of dilute reactive inks but prevents nanoparticle inks from being printed. The reactive silver ink was deposited into the cartridge using a syringe and needle to penetrate the foam. A syringe filter with the porosity of 0.22µm was used to filter the ink before using for inkjet printing. The drop volume of the printer was fixed to 35 picoliters. A rectangular print pattern of 40 mm X 4 mm was printed on the fabric substrates. Two different modes of printing were selectively chosen for the experiment that allowed for an examination printing with and without in-situ heating of the textile substrate. When printing with in-situ heating, the software allows the plate to be heated to 80°C, on which the substrate is placed. The number of layers printed was varied from up to eight print passes, adjustable via the Bot Factory software. After printing, the printed traces were heat cured at 140o C for 300 seconds. Herein, samples in which only the final heat curing process was performed are denoted as ‘ex-situ’, whereas samples in which an in-situ heating was performed with the final curing process are 7|Page ACS Paragon Plus Environment


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denoted as ‘in-situ’. This curing process was recommended by the ink manufacturer and was done once all layers were printed onto the textile substrate.

Characterization Optical transmission microscopy and 3D laser scanning confocal microscope (Keyence, model VK-X1000) were used to observe the ink coverage on the textile substrates. To further analyze the ink microstructure, ink coverage, and the ink conformity on fibers, printed samples were characterized by Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) mode. The fabric samples were prepared for the cross-sectional SEM images by dipping them in liquid nitrogen for 5 minutes followed by bisecting with a sharp razor blade. The confirmation of reduced metal silver on the fabric samples was characterized by X-ray diffraction (XRD). Sheet resistances were calculated by measuring the resistance of each sample using the two-point probe method and measuring the length and width of each sample. The electromechanical performance of the printed conductive line on PET knit fabric was tested with Instron, (model 5566) Mechanical Tester. The durability of the conductive tracks was also analyzed by multiple cycles of washing and drying following AATCC 61 for accelerated washing.

Results and Discussion XRD analysis of inkjet printed PET knit sample showed three distinct diffraction peaks at 38.2o, and 44.4o in Figure 2 (d), which represent the [111] and [200] Miller indices of cubic face-centered silver. The pristine PET knit fabric in Figure 2(d) did not show any characteristic silver peaks. No silver oxide was observed in the XRD analysis. Figure 3(a) shows the sheet resistance of the printed lines on all the selected fabric surfaces with respect to the number of layers of ink printed for both the in-situ and ex-situ curing. For the following discussion, Figures 3 (b-d) provide microscopy analysis various fabrics as observed from the upper and bottom surfaces after eight inkjet printed passes. A key observation from these images is the penetration depth of the final conductive coating with and without the in-situ heating. It can be confirmed from the images that the in-situ heat curing process helped to minimize the wicking (in both in-plane and through-plane directions) and improved the printed resolution. Due to the high porosity and surface roughness, the knit fabric with only ex-situ heating required a higher number of print passes (6 layers) to achieve low sheet resistance. Evolon® demonstrated a generally similar trend with respect to 8|Page ACS Paragon Plus Environment

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performance although the increased compactness of the fiber structure and finer diameter of the fibers increased the capillary force to allow the ink to flow both in the in-plane and through-plane directions. The penetration of the ink in the z-direction of the fabric thickness disrupted the electrical percolation network with the same amount of ink volume used. In contrast, the samples the underwent the additional in-situ curing process resulted in significantly lower resistance, presumably due to the in-situ annealing evaporating the water-based solvent before the ink penetrates dip in to the textile structure and localizing a thicker conductive coating on the near surface fibers. On the contrary, increasing the temperature of the fabric helps of improve wetting property of the ink as the surface tension and the viscosity of the ink decreases with temperature33, which helps to coat the fiber conformally, which helps to decrease the sheet resistance and improve the durability of the printed patterns. In general, it is observed that the in-situ heat cured traces on Evolon® and knit require fewer print passes to achieve a sheet resistance two orders of magnitude lower than samples that were just ex-situ heat cured. This large discrepancy was not observed in the woven PET samples, as the large fiber diameter and the tight fabric structure (low porosity) of the structure was able to reduce the overall ink penetration in the fabric, resulting in an increased percolation network on the surface of the fabric. Thus, the woven fabric achieved the lowest sheet resistance of about ~ 1 Ω/�� after 4 print passes for both in-situ and ex-situ heat cured procedures. Still, in-situ curing process did show some reduction of the of the sheet resistance (0.2 Ω/��).



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Figure 3: (a) Measured sheet resistances corresponding to the number of layers of reactive silver particle-free reactive ink printed on various substrates (knit PET, woven PET & Evolon®) with the comparison of in-situ and ex-situ heat curing.; b), c) and d) shows the optical images of printed traces on knit, Evolon® and woven fabric (face & back side) for in-situ and ex-situ heat curing process. The scale bar is equivalent to 2 mm.

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Figure 4: Transmission optical microscopy at 10X resolution of in-situ heat cured (a-c) Evolon®, PET knit and woven fabrics whereas Figure (d-f) shows the corresponding interface areas between the printed and non-printed par of fabrics Figure 4 shows the magnified optical images using a 3D laser confocal microscope of silver traces produced by inkjet printing of the particle-free reactive inks. It can be seen that the knit fabric has the most open structure and the Evolon® nonwoven has a very dense, randomly oriented fabric structure. The woven fabric has a tighter structure than the knit and it is constituted with fibers that have a largest diameter than that of other substrates. All the fabric samples, Evolon® fabric, PET knit, and woven fabrics seem to have good coverage of the ink on the surface as observed in Figure 4 (d), (e), and (f). The edge pattern of the coating, visible in in Figure 4 (d), (e) and (f) demonstrate that the ink was coated on the fibers of the fabric without blocking the open structure of textile fibers. This is the unique phenomenon of inkjet printing with particle-free reactive ink. Figure 5 shows the SEM images of the inkjet printed substrates. The images also compare the ink coverage on the fibers for the in-situ heat-cured and the ex-situ heat-cured samples with a control native fabric (unprinted area). The comparatively brighter portion of the fibers are the indication of silver conductive ink in the SEM images. Figure 5 (d, e & f) shows the SEM images of the ink coverage on Evolon® substrate. Evolon® has a more distributed coverage of ink on the fibers than that on the fibers of knit fabric. Interestingly, the ink resides on the edges of fibers on Evolon® fabrics, shown in both Figure 5 (b) &(c). This migration of ink is possibly due to the cylindrical 11 | P a g e ACS Paragon Plus Environment


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shape of the fine fibers. Additionally, the ink areas on Figure 5 (c) correspond to the in-situ heat cured printed area on Evolon®, seems denser and more concentrated than ex situ heat cured substrate shown in Figure 5 (b). From Fig, 5 (d- f), we can observe that the silver coverage of the fibers on knit PET substrates is improved with in-situ curing. Furthermore, Figure 5 (g- i) demonstrates that the silver coverage on the woven fabric is more uniform than that of knit and Evolon®. Additionally, there is not a noticeable visual difference of ink coverage between the ex-situ and in-situ cured samples on woven fabric. This explains the result in Figure 3 (a) that the sheet resistance of conductive pattern on woven fabric is very similar for both ex-situ and in-situ heat cured samples. Figure 6 provides a pictorial representation of how a droplet is able to spread on cylindrical fibers with different diameters. In a short summary, the inkjet drop can spread longer in length on a fiber when the diameter of the fiber is higher than the droplet and vice-versa, given the surface energy of the fibers is similar. This phenomenon can be further explained by the following equation,34 𝐾2 = (𝑥22 ― 𝑎2𝑥21)𝑥22

………… (ii)

where, x2 and x1 are the diameter of fiber, a is a constant related to the surface energy of the fiber, and k is the capillary length. From, equation (ii), we can see that whenever, x2 (fiber diameter) > x1 (drop diameter), the value of capillary length of the fluid is higher for a constant wetting behavior of fiber surface. As a demonstration of this phenomena, Figure 7 shows the detailed orientation of silver ink on the surface of fabric using EDS to show the residing location of the silver. The fiber diameter of Evolon® varies from 3-7 µm which is much smaller than the diameter of ink droplet (32 µm). As a result, the ink mostly resides at the edges and junctions between the fibers instead of spreading on the surface of fibers, as shown in Figure 7 (a) & (d). It can be predicted that the drop impacting on the multiple fibers and their crossover points experience a capillary force (created by the capillary spacing between fibers) that draws the ink to fiber junctions. It is noted, that increasing the surface energy and decreasing the droplet size would help to conformally coat the fibers with conductive ink. From Figure 7 (b), (c), (e) & (f) the ink drop tends to spread more on when impacting the knit and woven fabric structure. In this case, the larger diameter of the fibers (~20-30 µm) is comparable to the ink droplet size and helps facilitate the spreading of the drop. 12 | P a g e ACS Paragon Plus Environment

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Figure 5: Shows the SEM images (a) native Evolon® fabric (b) Evolon® fabric with ex situ heat curing (c) Evolon® fabric with in situ heat curing and (d), (e), (f) & (g), (h), (i) are the corresponding SEM images on PET knit and woven fabrics showing the silver coated areas.



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Figure 6: (a) Shows that the ink droplet trends to move to the junctions of the fibers when the drop diameter is larger than the fiber diameter; b) shows that the drop can spread when the fiber diameter is larger than the diameter of ink droplet.

Figure 7: Shows the SEM images inkjet printed (a) Evolon® nonwoven (b) PET knit fabric and (c) PET woven fabric; and (d), (e), (f) show the corresponding EDX images (pink color represents silver). One of the great advantages of particle free reactive silver ink over particle based is that the porous, cylindrical shaped textile fibers can be conformally coated by inkjet process. This process is very similar to a dyeing process where the dyes can be coated just on to the fiber surface and can also diffuse to the fiber to some extent. When the droplets of the inkjet nozzles impinge on the fibers, the adjacent droplets of particle free reactive ink settle down on the fiber surfaces, spread and wet fiber over time forming conformal films around the fiber surfaces. This conformal films of reactive ink around the fiber surfaces get sintered to form elemental silver upon annealing process. The mechanism of creating conductive pathways with the reactive ink on textiles is fundamentally different than that of particle-based ink. Figure S2 in the supporting information shows the difference in the wetting properties of the particle-based and the particle-free conductive inks in the inkjet printing process. The particle-colloidal system is very different since the density of the ink is much higher due to high loading of metal nanoparticles. Thus, the gravitational force is much higher, and the particles starts filling up the fiber gaps when the ink is printed on the textile surface.35 14 | P a g e ACS Paragon Plus Environment

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The Figure 8 (a) and (b) provides cross-sectional SEM imaging of the silver coating on the knit and woven fabric, respectively. Both figures show that the silver is conformally coated around the fiber surfaces without filling the areas spaced among fibers. The thickness of the Ag after seven print passes ranged from 150 nm to 1 µm, which can be influenced by a number of factors including interlacing points between fibers, fiber’s waviness and the porosity in the textile. Thus, inkjet printing with particle free reactive silver ink retains the feel/hand of pristine textile fabric.

Figure 8: SEM cross-section of the conformal coating of Ag ink during inkjet printing on a) the woven PET and b) knit PET fabrics. An electromechanical analysis of printed patterns on PET knit fabric was performed to understand the change of mechanical function of the fabric after printing. In this analysis, the PET woven fabric and Evolon® nonwoven is not stretchable because of the inherent fabric structure. Therefore, only the PET knit fabric was examined. An interconnect pattern of 30 mm x 4 mm was printed with 7 repeating print passes on the PET knit fabric. Figure 9 (a) shows the infrared images and the optical images of the printed interconnect while it was stretched to 180% of the initial length (30 mm) using the Instron mechanical tester. The printed conductive pattern with interconnects were connected to a power source to generate resistive joule-heating. Additionally, the real time change of the resistance was recorded when the conductive pattern was stretched using the digital multimeter. The experimental setup image is shown in the supporting information at Figure S3. It is noteworthy that the interconnect kept heating when the fabric was stretched to 100% of the initial length. This proves the current is flowing through the path of fabric in the stretched condition. The knit loops of the fabric get closer and create closer pack of conductive fibers, as shown in the optical images in Figure 9 (a). This characteristic of knit structure helps to 15 | P a g e ACS Paragon Plus Environment


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decrease the resistance while the knit fabric is stretched. Figure 9 (b) shows the change of electrical resistance of the interconnect printed on a knit fabric with the increase of strain rate. Figure 9 (c) compares the load-elongation curve for pristine knit fabric with inkjet printed interconnects on the same fabric. The data clearly suggests that there is no significant difference in the mechanical properties of textiles after the demonstrated inkjet printing process. The result confirms that inkjet printing of particle-free reactive ink does not alter the structural and physical properties of textile (see demonstration in supporting information). Figure 9 (d) shows the change of normalized resistance over 10,000 bending cycles for inkjet printed conductive knit. The fabric was bent in the course direction along the continuous knit loops. There has no significant increase or change of the resistance observed over 10,000 bending cycles, which is unprecedented for printed conductive textiles. This example confirms a significant advantage of inkjet printing of particle free reactive ink directly on knit textile (without any coating or film lamination) over any other process.


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Figure 9: a) IR images (top) of the inkjet printed knit textiles under tensile tension while applying a DC voltage (1V) to generate resistive heating, and their optical microscope images (bottom); b) corresponding graph showing the decrease of the resistance under strain; c) tensile properties of the pristine (bare) and inkjet printed knit textiles ; and d) change of normalized resistance of inkjet printed conductive knit over the bending cycles Apart from the features such as conductivity, preserving the comfort of textile, the wash durability/fastness of the inkjet printed conductive patterns on textile fabric is an unavoidable requirement for many E-textiles applications, but is still rarely reported following industry standards. Figure 10 shows the change of resistance of the printed patterns on knit and woven fabric after 5, 10, 15, 20 and 25 regular wash cycles, following AATCC test method 61. The fabric swatch is dipped in a cylindrical canister where 0.24 gm of detergent powder is added in 150 ml of water. Fifty steel balls are also added in the canister to intensify the mechanical agitation. The canister is then rotated for 45 minutes at 49o C water bath in the washing machine. After the washing process, the sample is rinsed and dried at 50o C for 15 minutes. It needs to be mentioned that one accelerated wash cycle as defined by the method is equivalent to 5 regular wash cycles. The conductive pattern on nonwoven increased the resistance 50x after a single wash (2.3 Ω ± 0.2 to >100 Ω). The resistance of the conductive pattern on knit fabric increased 2x after 15 regular washing and drying cycles. After 20 washes, the resistance increased to > 1 kΩ. The conductive pattern on woven fabric showed the highest wash-durability showing little change of the through 15 washing cycles. The conductive woven sample showed a reasonable amount of change of resistance (~3.5x increase) after 25 wash cycles. This wash durability results of the inkjet printed conductive patterns on knit, woven and Evolon® further confirms that large diameter and high surface area of fibers in woven fabric enhance the adhesion of the conductive ink and hence improves the wash durability. On the other hand, the concentrated ink portions on to the fiber junctions on Evolon® are loosely attached to the fibers. Thus, it can be surmised that the ink particles were washed off during the first wash cycle. The XRD analysis of the washed knit sample after 15 cycles of washing (figure S4) does not show any characteristic peaks of silver oxides, which proves that the silver layers on the fabric do not oxide even after washing cycles. The increase of resistance in the washing process may happen due to the wearing of the silver layer by the mechanical agitation during the washing process. It can be summarized that wash durability of the demonstrated inkjet printed textiles largely depends on the physical structures of the fabric. 17 | P a g e ACS Paragon Plus Environment


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Selection of tighter structure (less porous) can result in reduced sheet resistance and washdurability of inkjet printed pattern of reactive particle free silver ink on textiles.

Figure 10: a) Shows the accelerated washing procedure following AATCC test method 61 and b) shows the change of resistance for knit (red) and woven (black) polyester fabric with different washing cycles.

Conclusion Inkjet printing of reactive silver ink on knit, woven and nonwoven fabrics is demonstrated without applying any additional surface coating or any modification. The reactive silver ink chemistry is compatible on the polyethylene terephthalate (PET) and bi-component polyamide (PA) and polyethylene terephthalate (PET) polymeric fibers. The particle free reactive ink can uniquely coat the textile fibers all over the surfaces without altering the inherent properties of textiles. The conductivity of the printed traces on the selected fabrics depends on the tightness of the fabric structures, fiber diameters, porosity, and surface energy. The addition of an in-situ heat curing during the printing process was shown to decrease the sheet resistance by two order of magnitude for PET knit and Evolon® fabric. For compatible textiles with fiber sizes smaller than the ink droplet, the ink has the tendency to reside at the junctions of the fibers rather than spreading longitudinally along the fiber length. However, the cured ink in the fiber edges and junctions creates a mesh like network to create conductive path. Additionally, the inkjet printing of reactive silver ink on knit textile achieved unprecedented conductivity, bend durability and wash durability 18 | P a g e ACS Paragon Plus Environment

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without changing the comfort and the pristine mechanical properties of the fabric. This work aims to serve as a future guide for understanding conductive inkjet printing on textiles through specific interjoined ink to textile properties such as a drop size of inkjet, surface tension, and drop spacing that can enhance the electrical and durability performance of conductive pattern on textiles.

Acknowledgement This work is supported by the US National Science Foundation through Nanosystems Engineering Research Center for Advanced Self-Powered Systems for Integrated Sensors and Technologies under Grant EEC 1160483. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

Supporting Information Demonstration of inkjet printing process on textile, an image showing the different wetting behavior of particle-based and particle-free ink of textile fibers during inkjet process, an image showing the experimental set-up to analysis the electro-mechanical and in-situ thermal properties of inkjet printed conductive interconnect on knit-textile, and the XRD analysis of inkjet printed conductive textile before and after the washing process.

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Inkjet Printing of Reactive Silver Ink on Textiles - ACS Applied

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