Rheological properties and screen printability of UV curable conductive ink for flexible and washable E-textiles

doi:10.1016/j.jmst.2020.06.033

Abstract

Abstract:

As a critical component for the realization of flexible electronics, multifunctional electronic textiles (e-textiles) still struggle to achieve controllable printing accuracy, excellent flexibility, decent washability and simple manufacturing. The printing process of conductive ink plays an important role in manufacturing e-textiles and meanwhile is also the main source of printing defects. In this work, we report the preparation of fully flexible and washable textile-based conductive circuits with screen-printing method based on novel-developed UV-curing conductive ink that contains low temperature and fast cure features. This work systematically investigated the correlation between ink formulation, rheological properties, screen printability on fabric substrates, and the electrical properties of the e-textile made thereafter. The rheological behaviors, including the thixotropic behavior and oscillatory stress sweep of the conductive inks was found depending heavily on the polymer to diluent ratio in the formulation. Subsequently, the rheological response of the inks during screen printing showed determining influence to their printability on textile, that the proper control of ink base viscosity, recovery time and storage/ loss modulus is key to ensure the uniformity of printed conductive lines and therefore the electrical conductivity of fabricated e-textiles. A formulation with 24 wt% polymer and 10.8 wt% diluent meets all these stringent requirements. The conductive lines with 1.0 mm width showed exceptionally low resistivity of 2.06 × 10 ^-5 Ω cm Moreover, the conductive lines presented excellent bending tolerance, and there was no significant change in the sample electrical resistance during 10 cycles of washing and drying processes. It is believed that these novel findings and the promising results of the prepared product will provide the basic guideline to the ink formulation design and applications for screen-printing electronics textiles.

Key words: Conductive ink, Rheology, Screen printing, UV curing, Washable E-textiles

Hong Hong, Jiyong Hu, Kyoung-Sik Moon, Xiong Yan, Ching-ping Wong. Rheological properties and screen printability of UV curable conductive ink for flexible and washable E-textiles[J]. J. Mater. Sci. Technol., 2021, 67: 145-155.

Figures/Tables 16

Fig. 1. Schematic diagram of fully flexible and washable textile-based circuits printed with UV curable conductive ink.

Table 1 Formulation for UV-curing conductive inks.

Ingredients (g)	CI-1	CI-2	CI-3	CI-4
PUA	1.6	2.0	2.4	2.8
TPGDA	0.94	0.74	0.54	0.34
TMPTA	0.94	0.74	0.54	0.34
1173	0.4	0.4	0.4	0.4
KH-560	0.088	0.088	0.088	0.088
BYK-333	0.012	0.012	0.012	0.012
BYK-555	0.02	0.02	0.02	0.02
Ag nano-flakes	6.0	6.0	6.0	6.0

Fig. 2. Schematic diagram of screen printing in a cross-sectional view. a) The conductive ink was placed on the screen and the squeegee moved downward. b) The squeegee scraped the conductive ink and pressed the screen at the same time.

Fig. 3. a) Photograph of the prepared CI-4. b) XRD pattern of nano-silver flakes used for the preparation of UV curable conductive ink. c) Size distribution of nano-silver flakes by intensity.

Fig. 4. a) Viscosity of UV curable conductive inks at shear rates ranging from 0.1 to 1000 s-1. b) Rheological behavior of UV curable conductive inks in simulated printing process.

Table 2 Viscosity of different conductive inks at different shear rates.

	0.1 s^-1 at 20 s	200 s^-1 at 50 s	0.1 s^-1 at 80 s	0.1 s^-1 at 110 s	Recovery at 80 s	Recovery at 110 s
CI-1	85.52 Pa·s	2.78 Pa·s	22.39 Pa·s	51.80 Pa·s	26.18 %	60.57 %
CI-2	117.53 Pa·s	3.79 Pa·s	44.26 Pa·s	81.22 Pa·s	37.65 %	69.10 %
CI-3	176.29 Pa·s	5.57 Pa·s	83.12 Pa·s	153.32 Pa·s	47.14 %	86.97 %
CI-4	309.49 Pa·s	8.14 Pa·s	169.91 Pa·s	254.94 Pa·s	54.90 %	82.37 %

Fig. 5. a) Storage and loss modulus as a function of shear stress. b) Stress at G′=G″ for different conductive inks.

Table 3 Oscillatory stress sweep parameters in the LVE region.

Ink	LVE shear stress (Pa)	LVE G′ (Pa)	LVE G″ (Pa)	G′/ G″
CI-1	0.15-0.40	20.59	15.29	0.74
CI-2	0.12-2.51	147.72	93.86	0.63
CI-3	0.25-6.30	983.61	494.49	0.50
CI-4	0.15-10.00	7154.95	2330.92	0.32

Fig. 6. a-d) SEM images of printed lines with different UV-curing conductive inks: a) CI-1, b) CI-2, c) CI-3, and d) CI-4. e) SEM image of the edge of conductive line printed with CI-3. d) Local SEM images of conductive lines printed with CI-3.

Fig. 7. a-d) Elemental mapping images of conductive lines printed with different conductive inks: a) CI-1, b) CI-2, c) CI-3, and d) CI-4.

Fig. 8. a) Printed line widths of different conductive inks after curing. b) Schematic diagram of conductive ink penetrating into the gap between the screen and the fabric substrate.

Fig. 9. a) Electrical resistance of the conductive lines at different lengths. b) Electrical resistivity of the printed lines using different conductive inks. c) The selected elemental mapping images of the conductive lines printed with CI-3. d) EDS spectrum of the conductive line printed with CI-3.

Table 4 Content of elements distributed in conductive lines after UV curing.

Element (wt%)	CI-1	CI-2	CI-3	CI-4
Ag	93.76	89.39	88.94	82.38
C	3.65	5.69	6.39	9.50
O	2.25	4.57	4.26	7.48
N	0.34	0.34	0.41	0.63
Total	100.00	100.00	100.00	100.00

Fig. 10. a) Electrical resistance change of conductive lines printed with different inks during 500 bending cycles. b) Relative electrical resistance change of conductive lines during 500 bending cycles.

Fig. 11. a) Electrical resistance change of conductive lines printed with different inks during 20 washing cycles. b) Relative electrical resistance change of conductive lines during 20 washing cycles.

Table 5 Comparison of electrical resistivity, flexibility and washability of present work with reported screen-printed silver conductive inks.

Silver	Substrates	Curing	Resistivity (Ω cm)	Bending	Washing	Reference
Micro flakes 56 wt%	PDMS	80℃/30 min	1.35 × 10^-3			[1]
Micro fakes 80 wt%		150℃/30 min	1.0 × 10^-5	10% /1000 cycles		[29]
Nanoparticles 77 wt %	polyimide films	200℃/30 min	5.5 × 10^-6	50 % /4 mm/1000 cycles ^a		[22]
Nanoparticles 30 wt%	Cotton fabric	60℃/30 min	2.0 × 10^-3	700 %/10 mm/300 cycles		[30]
Nanoparticles 50 wt%	PET fabric	40℃	1.97 × 10^-1		1700 % /20 cycles ^b	[8]
Nanoparticles 44 wt%		UV	1.0 × 10^-4			[31]
Nanoparticles 60 wt%	PCB	UV/7 s	2.21 × 10^-4			[32]
Nano flakes 60 wt%	Coated fabric	UV/20s	2.06 × 10^-5	30%/10 mm/500 cycles	600 %/20 cycles	Present work

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