Development and Performance Evaluation of Stretchable Silver Pastes for Screen Printing on Thermoplastic Polyurethane Films

Abstract

Efficient, stretchable wiring electrodes are achieved when the resistance change during expansion and contraction is minimal. Herein, we prepared silver pastes specifically designed for screen printing on thermoplastic polyurethane films; they exhibit minimal resistance changes. The pastes were prepared using silver particles with sizes of 2 and 7 μm as well as a mixture of 2 and 7 μm silver particles (50:50 wt%). These pastes were analyzed using methods such as rheological measurements, thermogravimetric analysis, printability tests, tensile and torsion tests, and light-emitting diode (LED) tests. The most promising results were obtained when exclusively using 2 μm silver flake particles. The pastes demonstrated a viscosity of 24,880 cps, a thixotropic index value of 2.82, excellent printability, and consistent resistance measurements even after 100% stretch, thus indicating exceptional tensile properties. Moreover, the pastes exhibited substantial stability, with no change in brightness after the attachment of seven LEDs at 20% tension.

1. Introduction

In recent years, the focus on the research and development of electronic components has intensified, particularly for stretchable electronic devices that go beyond flexible electronics [1,2,3,4]. These devices are a new generation of electronics that keep functioning without losing their characteristics, even when substantially stretched or bent, and can retain their properties when external forces are removed [5]. Stretchable electronic devices that extend beyond conventional rigid electronics patterned on glass substrates and flexible electronic devices based on flexible substrates, such as polyethylene terephthalate and polyimide (PI), have created a new field. The development of stretchable electrode patterns for wearable devices and stretchable printed circuit boards (PCBs) has been an area of active research, with various types of stretchable substrates (films) and electrodes being examined [6]. Stretchable substrates can generally be divided into two categories: urethane series and silicon series. Clearflex and thermoplastic polyurethane (TPU) are some of the most popular stretchable films of the urethane series [7]. However, Clearflex is not suitable for contact printing methods, such as screen printing, owing to its sticky surface upon production. For effective printing, a primer layer must be applied to Clearflex to reduce its stickiness [8]. Therefore, the TPU from the urethane series is generally considered preferable. Stretchable substrates aim to minimize resistance changes due to strain, and as such, their forms are very diverse, including wavy patterned structures [9], porous mesh structures [10], and polypyrrole/polyurethane elastic substrates [11]. Stretchable electrodes include silver nanowires [12], graphene [13], conductive polymer (PEDOT: PSS) electrodes [14], nanothick silver single-foil electrodes [9], carbon nanotube (CNT) electrodes [15,16,17], and hybrid electrodes [18]. Despite exploring various methods for forming stretchable substrates and electrodes, most of these studies were primarily research-oriented and paid little attention to commercialization and mass production. Therefore, a simple, cost-effective method for creating stretchable substrates and easily forming electrode patterns is needed. This study addresses this need, with a focus on commercialization and mass production [19,20,21,22]. Our research aimed to optimize screen printing conditions and the size of silver particles for the application of silver pastes to currently mass-produced TPU films. Producing an ideal silver paste for these TPU films is critical. We prepared a silver paste with excellent storage stability, suitable for screen printing on TPU films. The flake-type silver powder employed in producing the silver paste comprised particles with average sizes of 2 and 7 µm as well as a 50:50 wt% mixture of 2 and 7 µm particles, aiming to minimize the resistance change due to the strain post screen printing and curing on a TPU film. We assessed the dispersibility and screen printability of the prepared silver paste by measuring its rheological properties (i.e., viscoelasticity). In the case of stretchable PCB electrodes, our research suggests that a thick coating can minimize resistance changes due to stretching. We optimized the particle composition ratio of silver powder, deeming it most suitable for stretchable TPU films.

2. Materials and Methods

2.1. Preparation of Silver Pastes

Silver pastes are inks in which silver powders are dispersed in a binder (resin + solvent), necessitating a binder as an integral component. The binder should maintain its stretchability after a silver paste is printed and cured.

As such, we require a polymer resin with a low glass transition temperature (T_g) and high molecular weight (M_w). Herein, we used a high-M_w saturated polyester resin (SKYBONES-365, SK chemicals Co., Ltd., Republic of Korea) with an M_w of 40,000 and a T_g of 16 °C. Notably, this resin is produced in a sheet form owing to its low T_g, unlike general polymers that are in a pellet form. The resin was dissolved in gamma-butyrolactone at a concentration of 50:50 wt% via stirring at 70 °C for 24 h.

We then produced a silver paste using the prepared binder and flake-type silver powder (particles with sizes of 2 and 7 µm and a 50:50 wt% mixture of 2 and 7 µm particles), which have different morphological sizes (Figure 1 and

Table 1. Formulation of silver pastes.

Paste	Fillers	Binders	Dispersants	Curing Agent	Solvents
Paste 1	72	13	1	1	13
Paste 2	72	13	1	1	13
Paste 3	72	13	1	1	13

). Initially, a blocked isocyanate-type curing agent (A/A6627, EONANOCHEM Co., Ltd., Republic of Korea) and a dispersing agent (DISPERBYK-180, BYK Co., Germany) were mixed, then the silver powder was gradually added to the mixture. The resulting mixture was subjected to premixing using a paste mixer (PDM-300, DaewhaTech Co., Ltd., Republic of Korea). The primarily dispersed silver paste then underwent secondary dispersion via three-roll milling (TRM-6.5, KyungYongMachinery Co., Ltd., Republic of Korea) to ensure a uniform distribution of silver particles within the binder, resulting in the final silver paste.

2.2. Characterization of the Printed Silver Paste

To confirm minimal changes in the physical properties (specifically, ductility) of the binder before and after curing, thermogravimetric analysis (TGA) measurements were conducted. The silver paste was subsequently screen printed onto a 150 µm urethane film and cured at 130 °C for 30 min. The electrode coating film was then scraped to prepare samples for performing TGA (TMA Q400, TA Instruments, New Castle, DE, USA) measurements in the air (heating rate: 10 °C/min). The objective of the TGA measurements was to ascertain the presence or absence of residual solvents in the prepared samples. The resulting silver paste was screen printed onto a urethane film. A 250-mesh stainless steel mesh with a photosensitive emulsion film thickness of 40 µm was employed as a screen plate, which was laminated to a total thickness of 80 µm using the combination plate-making method. The pattern length was 100 mm, and the line width was 0.8 mm. The silver paste was then printed using a semiautomatic screen printer (FORCE 2525, Minogroup, Tokyo, Japan). The electrode coating film printed on TPU film was cured at 130 °C for 30 min in an oven dryer (J-RCO, JISICO Co., Ltd., Seoul, Republic of Korea) to produce a stretchable electrode film. Strain changes in the prepared stretchable electrode film were assessed using a fatigue tester, and resistance fluctuations were continuously measured using a capacitance multimeter (Figure 2). During this procedure, the strain rate was maintained at 0.4 mm/s. Changes in the electrical conductivity and surface structure alterations were observed and photographed using an optical microscope (SV-55, Sometech Vision Co., Ltd., Seoul, Republic of Korea) under 10% strain application.

3. Results and Discussion

3.1. Hysteresis of TPU Films

The hysteresis property of a TPU film was evaluated using a stress test involving stretching of the film (Figure 3a). This assessment is crucial for stretchable PCB applications, motivating our experiment. The hysteresis evaluation included five cycles. Notably, the hysteresis characteristic was substantial during the first stretch but less noticeable from the second cycle onward, though it remained evident. The presence of this hysteresis characteristic suggests a tensile recovery of the cured film. A distinction exists between elastic recovery and plastic recovery concerning such recoverability. Unlike elastic recovery, plastic recovery does not result in 100% recovery, which can pose a potential issue [23,24,25].

3.2. Rheology Properties of Silver Pastes

Table 2. Viscosity value of silver pastes according to shear rate.

Shear Rate (1/s)	Paste 1	Paste 2	Paste 3
1	488,900 ± 20	88,780 ± 20	145,700 ± 10
5	70,250 ± 20	58,050 ± 10	68,780 ± 30
10	46,470 ± 10	40,940 ± 30	47,330 ± 20
50	24,880 ± 30	27,720 ± 20	28,880 ± 20
99.7	20,940 ± 20	14,390 ± 10	23,550 ± 10
TI 5/50	2.82	2.09	2.38

and Figure 3b present the results of the change in the viscosity of three silver pastes prepared with different particle sizes in response to the shear rate. Paste 2, which has the largest particle size, exhibits unusual behavior [26]. Considerable hysteresis occurs depending on the increase and decrease in the shear rate, which is attributed to the subpar dispersibility of the silver particles in the binder [27]. Figure 3c presents the results obtained by measuring the storage and loss moduli in relation to shear stress. Notably, the three-dimensional network structure is easily disrupted, leading to increased fluidity [28]. This issue with dispersibility is consistent with the viscosity measurement results. The three-dimensional network structure of the silver particles collapses at a stress of approximately 200 Pa, suggesting excellent binder dispersibility and the formation of a dense network owing to small silver particles.

3.3. TGA Results of Silver Pastes

Preserving the flexibility of stretchable conductive pastes is essential even after the electrode coating is cured. Specifically, if the resin is employed as a binder, the electrode coating hardens after being cured with blocked isocyanate; numerous minute cracks appear upon stretching the electrode, resulting in a rapid change in its resistance. Hence, maintaining the flexibility of the pastes is crucial even after resin curing. Figure 4 presents the TGA measurement results to understand the thermal properties of the resin before and after curing. In Figure 4, “Curing” refers to the process of adding a curing agent to the binder resin, while “Noncuring” corresponds to the results of TGA measurements for the resin samples that were cured at 130 °C for 30 min without adding a curing agent. These results indicate that the thermal properties of the cured and uncured coating films were nearly identical. This suggests that the flexibility of the coating film was retained even after curing. Therefore, the binder selected in this study was proven to be an extremely suitable resin for stretchable electrode pastes.

3.4. Tensile Test Results of Silver Pastes

The electrode pattern, printed on the urethane film, was cured at 130 °C for 30 min. Following this, the electrode film was attached to a fatigue tester to measure the changes in its resistance ((R − R₀)/R₀). Optical micrographs before and after applying a 40% strain are presented in Figure 5. The resulting resistance change due to strain is depicted in Figure 5. From the data, it is evident that for Paste 2, which was produced using large silver particles, the resistance change escalates rapidly as the strain increased from approximately 30% to 60%. For Paste 3, a combination of large and small silver particles, the resistance changes due to strain grow more gradually compared to Paste 2, but it sharply increases from approximately 80% strain. In the case of Paste 1, which comprises small silver particles, the packing density of the particles is high. However, the packing densities of Paste 2 and Paste 3 are lower than that of Paste 1. These lower packing densities account for the substantial change in the resistance due to strain.

3.5. Optical Image of Silver Pastes with Mechanical Tests

Table 3. Optical images of the screen-printed electrode surface under different strains.

	0%	20%	40%	60%	80%	100%
P1
P2
P3

shows the pattern surfaces of the silver pastes produced from three types of silver powders with differing particle sizes. The samples were photographed using an optical microscope at 150× magnification after being printed and cured on a urethane film. Paste 1 exhibited a relatively smooth surface, with the pattern showcasing sharp edges. Conversely, Paste 2 demonstrated a clear-screen mesh shape. This mesh shape can be predicted to result in microcracks under strain. For Paste 3, its surface smoothness was akin to that of Paste 1, but the pattern edges were not as smooth, suggesting inferior printability.

3.6. TGA Results of Silver Pastes

Table 3 presents the results of observing structural changes in the electrode surface using an optical microscope while gradually increasing the strain during the tensile test. Paste 2 exhibited fine cracks at approximately 40% strain. Similarly, Paste 3 demonstrated fine cracks at or below approximately 60% strain. In contrast, Paste 1 did not reveal any fine cracks, even when the strain was increased up to 100%. These observations are consistent with the changes in the resistance due to strain. Specifically, a sudden change in the resistance was noticed at 40% strain for Paste 1 and at or below 60% strain for Paste 2, which was associated with the formation of fine cracks in the electrode. Figure 6 presents the results of measuring the resistance change in the TPU films through a twist measurement (twist: 45°/45°). The rate of resistance change peaks at the first iteration, similar to the hysteresis behavior observed in the TPU substrate; this peak continuously increases in subsequent cycles (Figure 5).

4. Conclusions

Wearable or stretchable devices and sensors necessitate PCB electrodes for operation. Herein, we investigated conductive materials (silver) and substrates (film) to optimize stretchable PCBs. We manufactured silver pastes using three types of flake-type silver powder, known for their excellent particle interconnectivity, as an electrode material to construct stretchable PCB electrodes. The three types were the following: silver particles with average sizes of 2 and 7 μm and a 50:50 wt% combination of 2 and 7 μm particles. When the maximum particle size was 7 μm, we found suboptimal dispersion upon measuring the viscoelasticity of the resulting silver paste, rendering it unsuitable for stretchable electrodes. When the average particle diameter was approximately 2 μm (range: 0.5–4 μm), optimal screen printability and a minimal resistance change were observed upon stretching. Along with evaluating the stretchability of the TPU film, we also assessed its resistance change via twist evaluation. The TPU film exhibited minimal resistance change due to twisting. Therefore, the TPU film emerged as the most suitable substrate for stretchable PCB electrodes. However, considering the hysteresis and recoverability of each film, a stretch ratio of approximately 20% or less was found to be optimal for TPU films. The stretchable PCB, optimized in this manner, is deemed applicable in the healthcare and beauty care sectors. The results of light-emitting diode lighting at a typical elongation rate of 20% as well as a photograph of the actual applied product are presented in Figure 7. Thus, this study obtained a method for producing silver pastes with regard to TPU films as well as performed application tests.