A UV Curable Conductive Ink for the Fabrication of Textile-based

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Applications of Polymer, Composite, and Coating Materials

A UV Curable Conductive Ink for the Fabrication of Textilebased Conductive Circuits and Wearable UHF RFID Tags Hong Hong, Jiyong Hu, and Xiong Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06432 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

A UV Curable Conductive Ink for the Fabrication of Textile-based Conductive Circuits and Wearable UHF RFID Tags Hong Hong a, Jiyong Hu * a,b, Xiong Yan a

a. Key Laboratory of High Performance Fibers & Products, Ministry of Education, Donghua University, Shanghai 201620, China b. Key Laboratory of Textile Science &Technology, Ministry of Education, Donghua University, Shanghai 201620, China

Supporting Information

ABSTRACT Textile is a kind of emerging substrate for wearable printed electronics to realize recyclable smart products by versatile and low-cost screen printing. It is necessary for the after-printed high temperature curing to get high conductivity, whereas most of common fabrics have poor temperature endurance. Meanwhile, both rough surface and porous structure of fabrics are not beneficial to obtain high-resolution and high-quality circuits. In this work, the ultraviolet (UV) curable conductive inks with low-temperature and short-time curing were developed for screen-printing e-textiles, and the rheological behavior of conductive inks with different polymer contents was characterized in order to determine the ink formulation suitable for screen-printing on

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fabrics. To demonstrate the usability of the developed ink in fabricating e-textiles, the conductive lines with different widths as well as the antenna for UHF RFID tags were screen-printed on plain nylon woven fabrics. The geometric morphology and the electrical properties of the printed conductive line were evaluated. The results showed that the screen-printed conductive lines have the minimum line width of 0.2 mm, the highest conductivity of 6.02 × 106 S m−1, and good bending endurance at a bending radius of 5 mm. And also, the feasibility of UV curable conductive ink for fabric-based electronic device was confirmed by the screen-printed antenna of UHF RFID tags, and the reading distance after five cycles of washing is still over 3.0 m. Generally, this work developed a kind of low-temperature curing ink characterized by direct screen-printing on common fabric and high electrical conductivity after curing, and it will facilitate the use of textiles as the screen-printed substrates for flexible and wearable electronic devices. KEYWORDS: UV curing, low-temperature, screen printing, conductive ink, silver nanoflakes, e-textiles

1. INTRODUCTION Textile-based electronics (e-textiles) have drawn considerable research interest during the past few decades,1-4 due to the rapid development in wearable electronic technology. The flexibility of textile substrate contributes to the integration of wearable electronics and skin, improves the monitoring effects5 and has significant application prospects for future wearable electronics,3 strain sensors,4,


6

flexible

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display,7 and antennas.8 Various methods have been developed to manufacture e-textiles, including weaving, knitting and embroidering of conductive wires into fabrics or garments, but many disadvantages such as cumbersome manufacturing processes, limitations of the type of fabric substrates and inevitable damage to the natural properties textiles, have hindered the advancement of e-textiles.6 Currently, printing technologies such as inkjet,9 soft-lithography,8 flexographic,10 and screen printing11-12 have been adopted to fabricate the electronic circuits. The screen printing not only enables versatile pattern design and large-scale production with low cost and eco-friendly fabrication procedures, but also can be easily applied to various surfaces, such as plastic,13 paper,14 polymer,15 and textiles.3, 16-17 The key to realizing wearable screen-printed electronics is functional inks. Various kinds of conductive materials have been used as conductive inks, including metal nanoparticles,18-20 carbon nanotube,21 graphene,22 and conductive polymers.23 Among them, silver nanoparticle has been recognized as more suitable material for conductive circuits in printed electronic devices because of its high electrical conductivity, oxidation resistance and good compatibility. However, some previous researches have confirmed that high temperature sintering process was essential to ensure high surface electrical conductivity, which is a requirement for most of e-textiles. For example, Kim24 worked out the conductive silver inks for electrode printing on cellulose film, but it was required to be sintered at 200 ℃, which would seriously affect the nature of cellulose film. Unfortunately, high sintering temperature with regard to e-textiles will cause thermal shrinking, mechanical degradation, and



hand hardening of fabric substrate, especially made of heat-sensitive fibers.25 To solve this problems, Wang18 developed a new conductive ink and screen printed on PET fabrics, and then treated with a solvent system consisting of dichloromethane (DCM) and diallyldimethylammonium chloride (DADMAC) to construct conductive pathway at room temperature. It is well known that halides have very strong interactions with silver nanoparticles and make it possible for conductive ink to cure at room temperature, but poor conductivity could not reach the requirements of some printed electronics. Therefore, considering the nature of textiles, it is necessary to develop a conductive silver ink with low-temperature processing, high electrical conductivity and reliability. Relatively, the UV curing process is conducted under a low temperature. Tang26 introduced UV curable conductive inks comprising of spherical silver nanoparticles and silver nanoflakes, which could be screen-printed on PCB. However, the adhesion force between nanosilver and substrate was poor, causing the poor durability and mechanical properties of printed conductive patterns, and also the conductivity of the printed patterns was much lower than that of typical conductive inks. In addition, due to the intrinsic porous structure and texture characteristic of the textile27, it is difficult to uniformly deposit conductive nanometal inks on the fabric surface, and even the conductive lines deposited on the fabric surface can be discontinuous and have poor mechanical deformation. Significantly, it is necessary to especially develop a kind of UV-curing conductive ink for common fabrics as printing substrates.


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Here, we report a novel type of UV curable silver nanoflakes ink for screen-printed conductor on nylon woven fabrics, as illustrated in Figure 1. The rheological properties of inks were investigated to emphasize its influence on the quality of conductive lines. And meanwhile, lines with different widths were screen-printed, the printing resolution and the minimum line width achievable by a fabric screen were discussed, and the relationship between different line widths and electrical performance and the mechanically bending durability were systematically analyzed. In addition, to demonstrate the feasibility in printing e-textiles, this study fabricated a fully printed antenna for UHF RFID tags using the UV-curing conductive ink by screen printing, and characterized the reading performance and the washability of tags.

Figure 1. Schematic diagram of applying UV-curing conductive ink on woven fabric.

2. EXPERIMENTAL SECTION 2.1 Materials Silver nanoflakes were supplied by Suzhou Tanfeng Graphene Technology Co., Ltd., China. (~180 nm, ≥99.99 %). Polyurethane acrylate (PUA, ≥99.8%), trimethylolpropane triacrylate (TMPTA, ≥99.0%), tripropylene glycol diacrylate



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(TPGAD, ≥99.0%), 1-(2-hydroxy-5-methylphenyl) -1-propanone (1173, ≥99.0%), and γ-(2, 3-Epoxy-propoxy) propytrimethoxysilane (KH-560, ≥99.0%) were purchased from Chengdu Guangju Technology Co., Ltd., China. Antifoaming agent (BYK-555), and leveling agent (BYK-333) were obtained from BYK additives & instruments, German. All materials and chemical agents were analytical grade and used without further treatment. The nylon woven lining fabric was selected as the substrate, and its properties are listed in Table 1. Table 1. Specifications and properties of nylon woven fabric. Fiber

Yarn

content

density

Nylon

290 T

GV (g m-3) 58.52

Thickness

Pore size

Roughness

(mm)

(um)

(Ra)

0.0542

5.94

3.47

2.2 Preparation of UV-curing conductive inks UV-curing conductive inks generally consist of conductive micro- or nanoparticles/flakes, polymers, photoinitiators, diluents, and additives. In order to prepare the UV-curing conductive inks, silver nanoflakes with high aspect-ratio were employed as the conductive filler in the present study, which could fuel high conductivity compared with spherical silver nanoparticles.3 PUA was chosen as polymer due to its chemical and environmental stability, and it provides the flexibility and softness of the conductive lines screen-printed by the conductive inks. 1173 was selected and used as a photoinitiator to initiate the curing of conductive inks. TMPTA and TPGAD functioned as diluents to adjust the rheological behavior of conductive inks suitable for the screen printing. The addition of KH-560 could improve the adhesion of conductive inks to the fabrics by modifying the surface of nanosilver.


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Defoamer (BYK-555) could help prevent foam formation when conductive inks were strongly agitated with magnetic stirrer and provide effective defoaming performance during screen printing process. BYK-333 was used as leveling agent to reduce the surface tension of the conductive inks, thus improving the wettability of fabric surface in screen printing conductive inks. Table 2. The formulation of UV curable conductive inks. Ingredients (wt%)

UV Ink-1

UV Ink-2

UV Ink-3

UV Ink-4

PUA

16

20

24

28

TMPTA

9.4

7.4

5.4

3.4

TPGAD

9.4

7.4

5.4

3.4

1173

4

4

4

4

KH-560

0.88

0.88

0.88

0.88

BYK-555

0.2

0.2

0.2

0.2

BYK-333

0.12

0.12

0.12

0.12

Ag nanoflakes

60

60

60

60

First, PUA, TMPTA, TPGAD, 1173, KH-560, BYK-555 and BYK-333 were mixed in the proportion listed in Table 2, and stirred at 2000 rpm for 1h to prepare four different UV curable solutions. Then, 60 wt% of silver nanoflakes were added to the four UV curable solutions to make four different conductive inks: UV Ink-1, UV Ink-2, UV Ink-3 and UV Ink-4. The mixtures were agitated vigorously with magnetic stirrer at 2000 rpm for 2 hours to formulate viscous UV-curing conductive inks which can be applied to screen printing, as shown in Figure 2.



Figure 2. Photo of the as-prepared UV curable conductive ink-4. 2.3 Screen printing and UV curing The nylon woven fabrics as screen-printing substrates were firstly washed with ethanol, then rinsed in deionized water for 15 minutes, and finally the fabrics were dried in an oven at 60 ℃ . A simple screen printing method with semi-automatic Desktop screen printer (Shenzhen Kaimao mechanical and equipment factory, China) was performed to print the UV curable conductive ink on fabric substrate. Figure 3 displays a cross-sectional view of the screen printing process, and the conductive ink was placed on the top of fabric screen. The specifications of the fabric screen are 350 mesh count, 0.04 mm mesh diameter, 0.03 mm mesh opening, and 0.02 mm emulsion thickness, and a polymer squeegee has Shore hardness, 60. During screen printing, the fabric substance was held on a vacuum plate, the printing speed was 17.0 cm/s, the squeegee with the screen formed an angle ≈ 85°, and after the screen printing, the samples were cured under UV at 3 w cm-2 intensity for 20 s.

Figure 3. Cross-sectional illustration of direct screen printing on fabric.


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2.4 Fabrication of conductive lines and UHF RFID tags Screen-printed line widths are one of basic elements that determine the printing resolution of conductive lines and its application to flexible electronics. Therefore, lines with different widths were screen printed on fabric substrates, the length is 50 mm and the widths are 5, 4, 3, 2, 1, 0.5, 0.4, 0.3, and 0.2 mm, respectively, as shown in Figure 4a. Currently, the printing resolution can achieve as high as 0.2 mm by a fabric screen, which can satisfy the requirements of certain flexible electronic application. (a)

(b)

Figure 4. (a) Screen-printed pattern of conductive lines on fabric substrate. (b) Antenna structure for UHF RFID Tag (unit: millimeter). To further demonstrate the good prospective application of the UV-curing conductive ink in wearable electronic devices, a fully screen-printed antenna for UHF RFID tag was fabricated. The antenna structure is shown in Figure 4b, and it is a short straight dipole antenna28 with a total length of 0.25 λ. The antennas were screen-printed on fabric substrates, and after the screen printing, they were cured under UV for 20 s. Then, the tag integrated circuit (IC), which is Alien H3 series RFID IC, was attached to bridge the narrow gap of the antenna by anisotropic conductive adhesives. Finally, the reading performance of tags before and after



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washing processes was studied, respectively. 2.5 Characterization and measurement The size distribution of silver nanoflakes was taken with Particle Size & Potential Analyzer (ZEN 3600, UK). The pore size of the fabric was obtained by Poremeter (PMI CFP-1100AI, USA). The roughness of the fabric was measured by Optical Profilometer (Wyko NT9100, USA). The surface and cross-sectional SEM morphology of conductive lines were taken using a scanning electron microscopy (TM3000, Japan). The cross-section was sampled by Ion Milling (IM 4000 plus, Hitachi High Technologies, Japan). According to AATCC 76-2005 (electrical surface resistivity of fabrics), the four-wire method with a digital multimeter (34970A, USA) was carried out to measure the electrical resistance of printed lines.

And then the

conductivity was calculated with the electrical resistance and initial geometry of screen-printed patterns. Rheological behavior of prepared conductive inks was measured with a parallel-plate rheometer (ARES, TA Instruments, USA) with diameter of 50 mm and gap height of 1.0 mm. The conductive ink was pre-sheared at a shear rate of 0.1 s

−1

for 30 s before each of tests. The shear viscosity of conductive inks was measured by the steady-state flow step test at shear rates varying from 0.1 to 1000 s −1. In order to characterize the effect of rheological behaviors on properties of printed lines, the peak hold step test was carried out within three intervals to simulate the screen-printing process. The first interval represents the ink on the screen mesh fabric before screen printing, and the shear rate was 0.1 s −1 for 30 s. Then, in the second interval, 200 s−1


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shear rate was applied for 30 s to simulate the dynamic behavior of ink during print stroke. Finally, the conductive ink was left for 140 s at 0.1 s−1 in the third interval to show the structure recovering. In addition, the oscillation stress sweep step test was carried out at a constant frequency of 1 Hz and a stress range of 1 to 1000 Pa. This test is typically used to estimate structure modification of conductive ink during printing process, and it can identify the linear visco-elastic (LVE) region. All rheological measurements were performed at 25 °C. To characterize the flexibility of conductive lines printed on fabrics, a bending test was performed for the conductive lines with different widths. The bending test was carried out at a cylindrical rod with a radius of 5 mm, as shown in Figure 5. The electrical resistance before and after bending was measured and the relative resistance ratio (ΔR) was calculated by: ΔR =

𝑅1 ― 𝑅0 𝑅0

(1)

where R0 and R1 are the electrical resistance of conductive lines before and after bending, respectively.

Figure 5. Bending test at a cylindrical rod with a radius of 5 mm.

3. RESULTS AND DISCUSSION 3.1 Rheological behavior of the conductive inks



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The rheological behavior of the UV-curing conductive inks with different contents of polymer and diluent was first studied to determine how to affect the ink properties and the screen printing quality. Figure 6a shows the viscosities at different shear rates. Obviously, all conductive inks exhibit a shear thinning thixotropic behavior, and the viscosity of conductive ink decreases with an increase of the shear rate. In addition, the UV-curing conductive ink with higher polymer content displays higher viscosity at the same shear rate. In fact, high viscosity and shear thinning propriety are very important for conductive inks, which could prevent silver nanoparticles form depositing during storage. Table 3. Viscosities of UV-curing conductive inks at different shear rates corresponding to the screen-printing process steps. 0.1 s-1

200 s-1

0.1s-1

0.1 s-1

Recovery

Recovery

@ 20 s

@ 50 s

@ 80 s

@ 110 s

@ 80 s

@ 110 s

UV Ink-1

85.52 Pa s

2.78 Pa s

22.39 Pa s

51.80 Pa s

26.18%

60.57%

UV Ink-2

117.53 Pa s

3.79 Pa s

44.26 Pa s

81.22 Pa s

37.65%

69.10%

UV Ink-3

176.29 Pa s

5.57 Pa s

83.12 Pa s

153.32 Pa s

47.14%

86.97%

UV Ink-4

309.49 Pa s

8.14 Pa s

169.91 Pa s

254.94 Pa s

54.90%

82.37%

By the peak hold step simulating test of conductive ink in three intervals, Table 3 and Figure 6b show the viscosities at different shear rates. The UV Ink-1 had the lowest viscosity at a shear rate of 0.1 s

−1

at 20 s, and respectively only 26.18% and

60.57% of initial state recovered at 80 s and 110 s. The viscosity would considerably decrease when the squeegee stroke the conductive ink, hence allowing conductive ink to flow through the fabric screen. However, the conductive ink was inclined to produce small bubbles during the screen printing when the viscosity was low, as is


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demonstrated by some voids observed on the cured conductive lines in Figure 7a. Furthermore, the rough edge of conductive line printed on fabric with UV Ink-1 indicates the irregular and excessive diffusion of conductive ink, therefore it is not suitable for screen printing on fabric substrate. On the contrary, the UV Ink-4 had high viscosity, and it decreased from 309.49 to 8.14 Pa s when the shear rate increased to 200 s −1. After reducing the shear rate in the third interval, the viscosity recovered quickly to 254.94 Pa s at 80 s. In this case, levelling may not be completed with the UV Ink-4 after screen printing, thus resulting in the formation of small pores, as shown in Figure 7d. Moreover, because of the high viscosity of the UV Ink-4, the fabric easily adheres to the screen during printing and can not automatically separate from the screen, which affects the quality of the printed lines. Among these four conductive inks, the UV Ink-2 and UV Ink-3 exhibited relatively appropriate rheological behavior for screen printing. However, the viscosity of UV Ink-2 only recovered 37.65% and 69.10% of initial values at 80 s and 110 s, respectively, thus there are some voids and zigzag burrs along the edges of printed conductive lines. The viscosity of UV Ink-3 recovered to 83.12 and 153.32 Pa s at 80 s and 110 s, and this may allow levelling and formation of smooth and well defined conductive line. Furthermore, the viscosity recovery of UV Ink-3 was higher than 80% at 110 s, indicating that the ink has good elasticity.29 In most cases, the viscosity of the typical conductive inks based on silver micro or nanoparticles/flakes, even if the content of their solids is more than 70 wt%, can hardly exceed 70% of the



recovery.30-31 Comparatively, the uniform surface and clear edge of conductive lines printed on fabric with UV Ink-3 indicate its suitability for screen printing, though minute zigzag burrs on the edges of printed lines was observed. The reason for the zigzag edges of printed conductive lines is partially the rheological behavior of the conductive inks. Secondly, the woven fabrics are multi-scale and regular porous structure and rough surface, and it makes the surface energy of fabric have more or less regular variation, thus leading to different ink spreading through the inter-yarn and inter-fiber gap from that across yarn or fiber surface. Finally, during the printing process, there is still a gap between the screen and the fabric, and the periodic structure of both screen and fabric can cause the ink suspended infiltration and the formation of zigzag burrs on the edge of the printed conductive line. UV Ink-1 UV Ink-2 UV Ink-3 UV Ink-4

100

10

1

0.1 0.1

1

10

-1

Shear rate (s )

100

1000

(b) 1000 Viscosity (Pa.s)

(a) 1000 Viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

10 UV Ink-1 UV Ink-2 UV Ink-3 UV Ink-4

1

0.1

0

30

60

90

120

Time (s)

Figure 6. (a) Viscosity as a function of shear rate for the UV-curing conductive inks. (b) Thixotropic behavior of different conductive inks during screen printing.


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Figure 7. SEM images of conductive lines printed on woven fabrics with different conductive inks: (a) UV Ink-1, (b) UV Ink-2, (c) UV Ink-3, and (d) UV Ink-4. Oscillatory rheological test for the UV Ink-3 was also performed to determine the visco-elastic region, the storage modulus (G′) as well as loss modulus (G″). Figure 8a shows the curves of G′ and G″ of UV Ink-3 as a function of shear stress, and each curve can be divided into three regions32. Region I at the beginning of stress sweep step test is defined as the LVE region, and represents the endurable maximum deformation of the ink without destroying its structure.[34] In this region, the silver nanoflakes keep in close contact with each other and can be recovered elastically to any applied strain or stress,33 thus the structure of the conductive ink is not dependent on shear stress. The factor tan δ, which is the ratio of G″ to G′, is showed as a function of shear stress in Figure 8b, and relevant to the cohesiveness of conductive ink. In the LVE region, the value of tan δ is calculated to be ≈0.51. Durairaj et al.34 concluded that the ink with tan δ less than 0.28 was highly viscous and was hard to release



during screen printing process, and that with tan δ higher than 0.55 leads to bridge formation between lines and poor line edge definition. Then, Region II in Figure 8b shows that G′ and G″ values decreased continuously, and the conductive ink structure was gradually destroyed when the shear stress increased. In this region, G′ is higher than G″ which means that the conductive ink still exhibits a dominant elastic behaviour and shows a higher liquid characteristic with the increase of shear stress. Region III started from the cross-over point, the value of G″ became larger than that of G′, and the rheological behavior of conductive ink gradually changed from solid-like to liquid-like behavior. The measured cross-over stress at G′ = G″ was 157.36 Pa, which is comparable with the conductive pastes in previous study.35 Moreover, the stress at G′ = G″ can be used to evaluate the conductive ink cohesiveness. The higher the stress at G′ = G″, the more soild-like the conductive ink is, which could influence the flow of conductive ink through fabric screen, subsequently poor ink release from the screen.

(a)10000

(b) 100

1000

Region III

100 Region I

G''/G'

G', G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Region II

10 1 0.1 0.1

10

1

G' G'' 1

10

100

Shear stress (Pa)

1000

0.1 0.1

1

10

100

Shear stress (Pa)

1000

Figure 8. (a) Oscillatory stress sweep for UV-curing conductive Ink-3. (b) Factor tan δ (G ″/ G ′) as a function of shear stress. 3.2 Surface morphology of screen-printed lines The printability and uniformity of conductive ink on fabric substrates are


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generally decided by the distribution silver of nanoflakes in PUA matrix, hence the surface morphology of conductive lines were characterized. Figure 9a shows all of conductive lines screen-printed on the fabric substrates using UV Ink-3. The SEM image in Figure 9b exhibits that the conductive line has uniform, regular and clear edge. In addition, the conductive lines are metallic color and continuous without holes, even for the narrowest conductive line with width of 0.2 mm, which is essential to the good quality and reproducibility of printed flexible electronics, such as RFID tags. However, some voids are still observed in the printed line, and these voids could be caused by the bubbles produced during screen printing as well as evaporation of solvent in curing process. Moreover, the printed lines were cured under UV for 20 s, which resulted in partial decomposition of the additives to produce dense and compact structure of silver nanoflakes for higher conductivity,36 as shown in Figure 9c.

Figure 9. (a) Optical image of lines screen-printed on fabric substrate with different widths. (b) SEM image of the sharp and uniform edge of conductive line after curing. (c) SEM image exhibiting the dense and compact structure of silver nanoflakes in the conductive line. In order to further study the distribution of silver nanoflakes in the printed line as well as solid content of silver nanoflakes after curing, the local area was tested by energy dispersive spectrometer (EDS), and Figure 10 shows the corresponding element mapping and the EDS spectra. The corresponding element mapping of



printed lines in Figure 10a exhibits that silver nanoflakes were uniformly distributed throughout the PUA with no obvious aggregation, which mainly attributed to the appropriate rheological behavior of UV Ink-3 as well as the interaction between PUA matrix and silver nanoflakes. Besides, the EDS spectra in Figure 10b shows that the content of silver nanoflakes of printed lines after curing is 78.57 wt%. During the curing of conductive ink, solvent evaporation and PUA shrinkage would make the silver nanoflakes closer and improve the electrical conductivity. All these evidences demonstrate the better printability of UV Ink-3 as well as the uniform mixing of silver nanoflakes in the PUA matrix. (a)

(b)

Figure 10. (a) Element mapping of the screen-printed lines on woven fabrics by UV Ink-3. (b) The EDS spectra of the printed lines on woven fabrics. 3.3 Geometrical properties of screen-printed lines Figure 11a shows the cross-section of printed conductive line with 1.0 mm width, and the thickness of conductive line was obtained according to the cross-sectional SEM images. Figure 11b displays the measured thickness of conductive lines with different line widths, and observably it increases with an increase of the line pattern width. It is mainly because of the fabric screen structure as well as the screen printing process. When the screen-printed line pattern width


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increases, the relative restriction of the screen mesh openings is reduced, and thus more silver nanoflakes pass through the screen mesh, causing the increase of the printed line thickness, as is similar to the screen-print patterns with silver nanowires ink.32 The aspect ratio (thickness to width ratio) of conductive lines fluctuated randomly between 0.0025 and 0.004 in Figure 11c, and lacks a correlation between the width and the thickness of printed line, which is partially related to the rough and porous surface of fabric substrate as well as its texture structure.6 In addition, the formation of non-uniform printed lines is more common due to the relatively high viscosity and pseudoplastic thixotropic behaviour of conductive inks.37 Generally, it is difficult to uniformly deposit nanometal inks on the surface of fabric substrates.

(b) 16

(c) 0.005

14 12

0.004

10

Aspect ratio

Printed line thickness (μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4

0.003

0.002

2 0

0.2

0.3

0.4

0.5 1

2

3

4

5

0.001

0.2

0.3

Printed line width (mm)

0.4

0.5 1

2

3


4

5

Figure 11. (a) Cross-sectional SEM image of the printed line with 1.0 mm width. (b) Thickness of printed lines with different line pattern width. (c) Aspect ratio of the printed lines. All the conductive lines were printed with UV Ink-3 at 25 ℃.



3.4 Electrical properties of screen-printed lines The measured electrical resistance of conductive lines for different length and various line widths are showed in Figure 12a. Significantly, it shows a high linear correlation between line length and resistance (R>0.997), and the electrical resistance decreases as the printed line width increases. These results are well fitted to the definition of electrical resistance: 𝑅 = 𝜌𝐿 𝐴 = 𝜌𝐿 𝑤𝑡, where ρ , L , A, w, and t are the resistivity, length, cross-sectional area, width and thickness of the screen-printed line, respectively.11 Furthermore, the electrical conductivity was calculated according to the resistance definition (Table S1, Supporting Information), and Figure 12b shows the average conductivity of the printed lines with different widths. Apparently, the electrical conductivity of all printed lines is higher than 106 S m−1, and the highest electrical conductivity is 6.02 × 106 S m−1, which is much better than previously published works, as listed in Table 4. However, the electrical conductivity of printed line is still an order of magnitude lower than that of the bulk silver (6.3 × 107 S m−1), and it is attributed to the 21.43 wt% organics in the conductive line after curing, as shown in Figure 10b. 70 60 50 40 30

(b) 8x10

W= 0.2 mm 0.3 mm 0.4 mm 0.5 mm 1.0 mm 2.0 mm 3.0 mm 4.0 mm 5.0 mm

20 10 0

1

6

7x106

Conductivity (S/m)

(a) 80 Resistance (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

2

3

Length (cm)

4

5

6x106 5x106 4x106 3x106 2x106

0.2

0.3

0.4

0.5 1

2

3


4

5

Figure 12. (a) Electrical resistance of the printed conductive lines as a function of


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


line widths. (b) Calculated electrical conductivity of the printed lines as a function of line widths. Table 4. Comparison of electrical conductivity of present work with previous reported screen-printed conductive inks. Electrical conductivity

Solid content

Substrate

Curing

Silver flakes 70 wt%

LTCC

875℃/60 min

3.33 × 107

[32]


Glass

200℃/60 min

3.03 × 106

[38]


Paper

160℃/50 min

5.26 × 106

[39]


Silicone

160℃/30 min

1.51 × 106

[8]

Glass

150℃/5 min

3.33 × 106

Fabric

80℃/30 min

7.38 × 104

[41]

Fabric

60℃/30 min

5.0 × 104

[20]

UV

1.0 × 106

[40]

PCB

UV/7 s

4.52 × 105

[27]

Fabric

UV/20 s

6.02 × 106

Present work

Silver nanoparticles 65 wt% Silver flakes 56 wt% Silver nanoparticles 30 wt% Silver nanoparticles 44 wt% Silver nanoparticles 60 wt% Silver nanoflakes 60 wt%

(S m −1)

Reference

ET-4F Youte New Material

Table 4 compares the electrical conductivity of screen-printed lines with various conductive inks based on silver micro or nanoparticle/flakes sintered or cured at different conditions.8, 18, 26, 31, 38-41 Comparatively, the conductive lines screen-printed using UV Ink-3 show higher electrical conductivity than most of the previously reported conductive inks, and have shorter curing time as well as lower curing temperature. In fact, it is well known that silver nanoflakes have higher surface contacts and hence the dense and continuous conductive networks can be formed.



Although

traditional

heating-sintering

silver

inks

Page 22 of 28

showed

better

electrical

conductivity, the high temperature sintering processes make them impossible to be applied on common paper or fabric substrates for flexible printing electronics. Obviously, the present UV-curing conductive ink shows high conductivity with a relatively low curing temperature, and is a promising material for mass manufactured textile-based electronics. 3.5 Mechanical properties of screen-printed lines The bending tests were performed for the screen-printed conductive lines with different widths to further study their mechanical bending tolerance. Figure 13a shows the relative electrical resistance (ΔR) of printed lines during 500 bending cycles and Figure 13b further presents the relative resistance of printed lines after 500 times bending cycles. Apparently, the electrical resistance of printed lines with different widths increases moderately as the bending cycle increases, and it can be clearly found from Figure 13b that the printed lines with smaller width shows larger change of electrical resistance after 500 bending cycles. Relatively, the printed line with 0.2 mm width has the maximum resistance change, the resistance is 2 times of the original resistance, but it is still far lower than previously published results18. According to Figure 11b, the line with width of 0.2 mm has the thinnest thickness, and the lines with thinner thickness probably tend to crack during bending by the neutral plane theory of laminated plates, leading to larger resistance change.


Page 23 of 28

(b) 250

W= 0.2 mm 0.3 mm 0.4 mm 0.5 mm 1 mm 2 mm 3 mm 4 mm 5 mm

200 150 100

200

ΔR (%)

(a) 250

ΔR (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 50

50 0

150

50

100

200

300

Beding cycles

400

500

0

0.2

0.3

0.4

0.5

1

2

3

4


5

Figure 13. (a) Relative electrical resistance of the printed conductive lines on fabrics during 500 bending cycles at a bending radius of 5 mm. (b) Relative electrical resistance of printed lines with different width after 500 bending cycles. 3.6 Reading performance and washability of tags To demonstrate the application prospect of the UV Ink-3 in fabricating e-textiles, the antenna for UHF RFID tag was screen-printed, as shown in Figure 14a, and its washability was evaluated by machine wash at 40 ℃ for 38 min with 5 g/ L soaping agent and drying at 60 ℃

by terms of the ISO 6330:2012. All these washing

procedures were repeated 5 times. After each washing, the antenna was attached to the Alien H3 series RFID IC by anisotropic conductive adhesives, and the maximum reading distance was measured by a commercial UHF RFID reader. Generally, the average reading distance of the screen-printed tags is about 8.32 m, and it is much longer distance than its requirement used in textile product logistic and storehouse management. And also, as shown in Figure 14b, the average reading distance of tag with washed antenna has significant decrease from 8.32 m to 3.12 m after 5 washing cycles, whereas the reading distance is still over 3.0 m, which is in the reading range required by the textile storage and logistics management. This decrease of the reading performance is mainly attributed to the existing complex deformation



modes during washing process, and also some liquid or soaping agent, which were absorbed into both the antenna and the fabric substance during washing and dry process, can affect the reading distance.

(b) Reading distance (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

9 8 7 6 5 4 3 2

0

1

2

3

4

5

Washing times

Figure 14. (a) Screen-printed UHF RFID tag antenna inserted with IC. (b) Average reading distance of UHF RFID tags during 5 washing cycles.

4. CONCLUSION In this work, UV-curing conductive inks with 60 wt% silver nanoflakes for screen printing on common fabrics were developed and the usability in e-textile was demonstrated. The experimental results showed that the UV curable conductive ink containing 24 wt% polymer and 10.8 wt% diluent, has a viscosity of 176.29 Pa s at shear rate of 0.1 s-1 as well as the relatively appropriate rheological behavior suitable for screen printing. With this kind of conductive ink, the regular and clear-edged conductive lines with a resolution of 0.2 mm were obtained on fabric substrates, and also it demonstrated high electrical conductivity of 6.02 × 106 S m−1 and the excellent mechanical bendability, which is suitable for printed flexible electronics. In addition, the UV curable conductive ink was successfully used to screen-print the antenna for UHF RFID tags with woven fabric substance, and the average reading distance of the


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printed tags is up to 8.32 m. Overall, these results confirmed the usability of the developed novel ink for fabrication of e-textiles by screen printing technology with low curing temperature.

ASSOCIATED CONTENT Supporting Information Geometry dimensions and electrical resistance of the screen-printed conductive lines. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86 18019713776. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by the Fundamental Research Funds for the Central Universities under Grant number CUSF-DH-D-2018026 and 2232019G-02.

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Table Of Contents (TOC)


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A UV Curable Conductive Ink for the Fabrication of Textile-based

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