Preparation of paper-based conductive pattern for 3D printing

With the development of printed electronics technology, the demand for printed conductive inks had also put forward higher standards. The shortcomings of traditional printed circuit boards, such as complex technology, consumables, and pollution, had also been resolved with the development of 3D printing technology. In this paper, low-content silver nanoparticles were blended with graphene alcohol slurry and dissolved in a non-polluting solvent such as absolute ethanol. The synergistic effect of the composite of the hexagonal structure of the graphene sheet and the silver nanoparticles improved the ink’s performance of electrical conductivity, adding water-based polyurethane/acrylic resin system to improve the adhesion between ink and paper. The conductive ink can be used to 3D print circuit circuits on paper. After drying, the resistance was measured. After the battery, diode, and switch were connected, a circuit diagram was made. After pressing the diode emits was lighted. This work is expected to be applied to flexible circuit boards to provide basic research.


Introduction
With the trend of increasingly integrated and miniaturized electronic printing technology, traditional circuit board printing technology was unable to meet the needs of the market. Modern electronic products are expected to have good flexibility. In the future, electronics printed will have a major impact on the photovoltaics, lighting, displays, smart cards and packaging, providing more convenient lifestyles. Since the discovery of graphene, the two-dimensional structure of graphene had given a good thermal and electrical conductivity, and had been widely used in various industries. Graphene was currently used as a reinforcement material for the preparation of functional composites. The inorganic nanoparticles can reduce the interaction of graphene sheets. Graphene's large specific surface area can make an excellent carrier for nanoparticles, which can maximize the synergistic effect of composite materials. Researchers at the University of Cambridge developed graphene conductive inks that was used for 3D printing [1]. Researchers at Northwestern University in the United States developed graphene inks in the inkjet printing, and made highly conductive flexible electrode [2].
Currently, graphene conductive inks were mainly used in the field of electronic devices and energy storage devices [3][4][5][6], and by spraying, spin coating, printing and other technologies they were applied as conductive lines, conductors, resistors, sensors, etc. It was necessary of selecting the conductive phase of the ink, the preparation of the ink, and the treatment method before and after printing. According to the actual needs of the product, a relatively suitable conductive ink was chosen. Graphene inks in the electronics industry were mainly used by gravure printing [7], screen printing [8], inkjet printing [9,10], 3D printing [11] and so on. In gravure printing and screen printing the manufacturing process required many processes from early drawing, photo plate making, film formation to final cleaning and inspection. It not only consumed a lot of time and materials, but also generated a lot of waste liquid during the etching process, which caused the pollution. For inkjet printing, the ink layer's thickness can be adjusted by changing the ink composition or printing speed, however, it is necessary to design two-dimensional circuit [2]. The 3D printing can reproduce the designed 3D graphics by printing 2D graphics layer by layer, and it can accurately control the thickness of each ink layer. Here a high viscosity inks are developed in 3D printers made in China and we need to find the right parameters to print out the designed circuit thickness by modulating the ink.
The glass [12], plastic [13], paper [14], ceramics [15], and textiles [16] can be used as substrates for circuits. As an insulating layer, paper based circuit board had significant advantages such as low cost, portability, environmental protection, and recyclability, which can be used in flexible electronic products. 3D printing was a rapid prototyping method for the paper based circuits, and it was to print the pre-designed circuit diagram on paper, which had many advantages such as simple process, fast forming, less waste, low cost, environmental friendliness and so on.
Conductive polymers [17], carbon nanotubes [18], and graphene inks [19] can all be used as conductive components of the inks. However, organic inks usually exhibited low conductivity, poor chemical and thermal stability, while metal-based inks had good conductivity but high cost. Graphene-based composite inks were widely used in flexible electronics because of their low cost, good electrical conductivity, and stable thermal conductivity. Although nano-silver showed excellent performance in many aspects, nano-materials were easy to agglomerate. Graphene had a large specific surface area and active groups such as hydroxyl, carboxyl and epoxy groups on its surface, which can be used as a carrier of nano-silver. The synergistic effect of the composite material made the conductive ink exhibit good electrical conductivity [20]. In the ink conductive filler particles was of nanoscale, so the ink had low temperature sintering properties. The ink dried quickly and had good adhesion and bending properties when printed on paper. The ink had the advantage of low cost compared to commercially available water-based inks, and the printing process was simple and green, so it can be used for paper-based flexible circuits by 3D printing. This research will provide opportunities for 3D printable conductive inks to create printed electronics.

Formulation of conductive ink
Most conductive inks were composed of conductive fillers, solvents, connecting materials, additives [21]. The conductive fillers were uniformly dispersed in the solvent, and there was a certain gap between the filler particles. After high-temperature sintering, solvent volatilization, and metal melting, the filler particles were close to each other and combined with resin to form a conductive network. In water-based conductive ink organic solvents were replaced with water, eliminating the evaporation of organic solvents. In addition, its preparation process was simple, environmentally friendly and safe. Here the water-based polyurethane resin and water-based acrylic emulsion is connecting material that can improve the ink's adhere and the ink's viscosity, and it was the ink filmforming material when drying. In this paper, only the best mixing ratio of the two resins is tested.
Pneumatic 3D printers required inks with suitable viscosity, which was from 2 P to 10 P. Micro-pen direct writing technology was used in the printer.
The graphene alcohol slurry was purchased from Anjude Nano Technology Co., Ltd., and the silver powder was purchased from Lijia Metal Co.
After several experiments, a conductive ink was formulated for the JD200 Proo 3D printer (Xi'an Rite-3D Technology Co., Ltd., China). The ink used a small amount of silver nanopowder (about 200 nm) and graphene alcohol slurry, as shown in figure 1. The mixed solvent's volume ratio of deionized water, anhydrous ethanol, propylene glycol was 1:3:2, then silver powder and graphene alcohol slurry were added in the mixed solvent according at the mass ratio of 1:1. The mixture was stirred with a glass rod, and ultrasonically dispersed for 30 min. The water-based polyurethane resin, water-based acrylic emulsion was added, whose content was totally 3 wt% in the entire conductive ink. After stirring and ultrasonic dispersion, the ink was prepared as showed in figure 2, and the ink composition ratio is shown in table 1. A viscometer and a pH meter was used to measure the viscosity and pH of the ink respectively.

Testing of paper properties
In this paper, photo paper was selected as the substrate material for the circuit board. A computer controlled Buick Smoothness Tester (DC-BKP10000) was used to test the smoothness of paper. A dynamic penetration analyzer (PDA.C-02) was used to test the permeability of paper. A paper and paperboard ink absorption tester (YM-20) was used to test the paper ink absorption. The paper folding tester was used to test the paper folding resistance.

Conductive inks for 3D printing
The prepared conductive ink was printed on the photo paper using the JD200 Proo 3D printer (China, Xi'an Ruite 3D Technology Co., Ltd.). After drying for a period of time at room temperature, measuring the resistance with a multimeter.
(1)Three-dimensional design of the circuit board The circuit diagram was designed using UG software, and the file format was saved as STL format. The set line width was 0.35 mm, and the set line length was 30 mm, and the ink layer thickness was 0.15 mm.
(2)Slicing of circuit boards The 3D model file created in UG NX 10.0 was loaded into the slicing software, sliced, and saved in Gcode format.

(3)Printing of circuit boards
After starting the printer, the speed was adjusted to 8 mm s −1 . The pressure was initially adjusted to about 0.15 MPa, then slowly increases to find the optimal printing pressure.

(4)Sintering of circuit boards
After the circuit board is printed, it was sintered at 30°C.

(5)The performance test of the board
The printed line samples were tested for resistance, bending property, friction resistance, and adhere property. The resistance was tested with a multimeter, and the bending performance was tested after bending 60°to the ink film layer. The adhere was tested by the cotton cloth that wrapped with a kilogram of weight and that was damped with 95% alcohol, and the line printed by 3D printer was wiped back and forth . The number of times before the ink falls off can evaluate the adhesion of the ink.

Results and discussion
The volatilization of the solvent reduced the spacing between the conductive particles and graphene, forming a conductive network. Free electrons can move along the direction of the applied electric field to form a current [22]. Graphene had a layered network structure, and its crystal lattice was a regular hexagon. These carbon atoms formed a π bond through hybridization, in which π electrons can move freely. Therefore, in the direction perpendicular to the crystal plane the π bond in the structure played a major role in the conductive process of graphene [23,24].

Viscosity
The viscosity of the conductive ink was measured at 25°C and 200 rpm for 1 min using a cone plate viscometer, and the viscosity was about 5 P. Viscosity determines the formation of ink droplets to a certain extent, and the viscosity standard of the conductive ink should be matched with the 3D printer used. In general, the 3D printer used to print circuit boards was divided into two types of pneumatic and piezoelectric. Here the machine used in this case is pneumatic 3D printer.

pH value
The pH value of the ink measured using the ink pH meter was 8.1. The ink was weakly alkaline and will not damage the stability of the binder, which was conducive to the adhesion and drying of the ink. The configured conductive ink was printed on paper, and the drying method of the conductive ink was mainly resin oxidation drying. The acidity and alkalinity of the ink affected the oxidation drying. Acid environment affects the rate of oxidative polymerization of resin. When the ink was acidic, the rate of oxidative polymerization of resin oxidation is slow, so the drying is slow. The ink was weakly alkaline, and the rate of oxidative polymerization of resin oxidation is fast, so the drying is fast [25].

Print quality analysis of circuits 3.2.1. Performance analysis of paper-based circuit board
The substrate of the circuit board had a great influence on the application field and quality of the printed electronics. The choice of substrate was related to the particle size of the conductive filler in the ink, the printing method, the choice of printer, and the ablation temperature. The selection of different substrates produced different properties. Commonly substrates were plastic, glass, ceramic, paper, etc. Photo paper generally had a bottom and top coating, and the surface was smooth, so it is easy for nano-silver particles and graphene to closely align, thus forming a continuous conductive layer. Therefore, the photo paper was chosen as the substrate material for the circuit board, as shown in table 2.
The higher the smoothness of the paper, the less the micro pits and holes on the surface of the paper, thus making the paper less absorbent of ink, that is, the less the ink absorption of the paper. To a certain extent, the more water-based ink penetrating into the paper, the lower the solid density of the water-based ink. Dynamic penetration which was too strong or too weak will affect the printing performance.
As showed in table 2, wettability characteristic parameter 'Max' refers to the time after the paper sample is completely wetted and the signal intensity reaches its maximum, expressed in milliseconds. A30 represents the strength of the paper's ability to absorb water. the larger the A30, the stronger the paper's absorption of ink, the deeper the ink penetration in the paper. Surface porosity characteristic value t95 reflects the size of its pore development degree when the paper's absorption no longer depends on the influence of surface water resistance, but directly depends on its surface openness with 10%-20% test solution in penetration test. As showed in figure 3, t[s] represented the time, I[%] represented the ultrasonic intensity. The working principle of PDA dynamic permeameter is to compare the ultrasonic strength of paper and paper wetted by solution. By the characteristic curve the speed of liquid penetration can be reflected.
The greater the absorption performance of the paper, the faster the paper absorbing the ink, that was, the faster the penetration of the ink into the paper. Therefore, the more absorbent the paper, the easier it was to dry.

Line pattern on the circuit board
The 3D model of the circuit pattern was carried out by 3D software, and parameters such as line width, line length and thickness are set, as shown in figure 4. The 3D model file of the circuit pattern was sliced and layered, as shown in figure 4(a). The printing needle diameter was 0.31 mm, and the bottom plate temperature was 30°C. The air pressure of the six horizontal lines printed from top to bottom is respectively 0.  figure 4(b), for the line's printing, the suitable air pressure range was from 0.18 MPa to 0.201 MPa.Under the range's air pressures we can find the vertical line's printing quality is better than the horizontal line's, and the line's thickness is uniform. Here the longitudinal direction of the photo paper is vertical.
The thickness of some lines in figure 4 is not uniform because they are extruded and printed under different air pressure. In addition, because the solute is not fine enough, the smoothness of the printed lines is not good enough.
The 3D printer here mainly uses the principle of pneumatic pressure and needle tube extrusion to print ink. The ink has high viscosity and can print different single-layer thickness. The ink-jet printing is mainly carried out through the nozzle. According to the printing principle, the nozzle is divided into bubble type, piezoelectric  type, etc. The ink viscosity is small. The pattern in figure 4 can be printed easily using inkjet printing, but if you want to change the single layer thickness of printing, ink-jet printing is more troublesome.
In order to accurate control the thickness of the ink layer, it is necessary to adjust the best printing conditions for extrusion printing. Grinding the solute of ink before preparation is also the premise to promote good results.
As showed in figure 5, when the air pressure was 0.15 MPa, the circuit broke, and the resistance was infinite. When the air pressure was increased to 0.201 MPa, the circuit printed well. When increased to 0.254 MPa, the line printing become thicker. When the air pressure was adjusted back to 0.18 MPa, the circuit printed well again. The faster the printing speed, the thinner the printed circuit.
3D printed line's size deformation was small. The average line width was about 0.5 mm. The thickness of printed line was moderate at the right air pressure. The line length was set to 30 mm, and there is almost no deformation. The thickness of the ink layer printed on the photo paper was 0.1 mm, while the thickness of the 3D model design was 0.15 mm. The thickness of the ink on the photo paper was much smaller than the designed thickness, and the photo paper had a coating and capillary structure with little diffusion of conductive fillers, but some of the resin penetrated the capillary structure of the paper (figure 6), leading to a reduction in the thickness of the ink film [26]. Figure 7 showed the ink cross section of the paper, that proved some of the resin penetrated the capillary structure of the paper.
From figure 8, we can see the silver particles with diameter of about 200 nm are evenly distributed in the graphene layers which effectively solve the problem that silver particles are easy to agglomerate.

Bending performance of the circuit board
The bendability test [27] was performed. In figure 9, it was found that the conductivity of the line was good and relatively stable within 100 bending times. As shown in figure 10, the printed circuit resistance increased with the increase of the bending angle, and tends to be stable at 30°∼90°. This was caused by the more order arrangement of conductive particles under the action of external force during the bending process.

Adhere of the ink on the paper
The times wiped back and forth when the printed line ink fall off was used as the evaluation standard of ink adhesion. At the time, we can found the white cotton cloth adhered some black ink. There is no doubt that the addition of resin can improve the adhesion of ink on paper. But if there is no resin in the conductive ink, the ink can not pass through the needle of 3D printer. So here we didn't discuss the effect of the resin on the adhere.
When the mass of waterborne polyurethane accounts for 1% of the total ink mass, the ink begins to drop slightly when the wiping times are 3. When the mass of waterborne polyurethane is 2% of the total ink mass, the ink begins to drop when the wiping times are 5. When the mass of waterborne polyurethane is 3% of the total ink mass, the ink begins to drop when the testing times are 7 . So the content of waterborne polyurethane can improve the adhesion of ink. At the same time, we did the experiment of the relationship between the content of water-based acrylic resin and the adhesion of ink, and found that with the increase of acrylic acid content the adhesion of ink did not improve. Both acrylic resin and polyurethane can improve the viscosity of ink, but the viscosity of acrylic resin is higher than that of polyurethane with the same mass content.

Luminous experiment of the circuit
Patterns printed on photo paper using conductive inks were used to create an electronic circuit. The circuit consisted of three 1.5 V coin cell batteries, a printed ink electronic circuit, a switch and a surface-mounted LED that was placed between two ink lines and held in place with tape. Experiments were then conducted to test the  suitability of the conductive ink. When the circuit was turned on, the surface-mounted LED lit up and worked well, as shown in figure 11. This shows that it was effective to use the conductive pattern as an electronic circuit. This result provided some ideas for future paper-based circuits.

Conclusion
The preparation of the conductive ink in this work mainly used a small amount of silver powder and graphene alcohol slurry as the conductive filler. A mixture of ethanol and glycerol was used as a solvent to dissolve and disperse the conductive filler. Water-based polyurethane resin and water-based acrylic tree were used as connecting materials. Finally, the ink was modulated into conductive ink with moderate viscosity to meet the requirements of paper-based 3D printing for pneumatic 3D printers. The water-based conductive ink made by 3D printing saved energy and reduced pollution emissions, and can be used for printing circuit boards on photo paper. The prepared circuit boards had good bending and adhere properties, while their conductive properties were still good, and were expected to be used in electronic tags, paper batteries, smart cards, etc. In the future some additives such as defoamer, dispersant, and dustproof agent can be added to the ink to improve printability. These additives needs to be discussed later, and a single resin or other resin materials used as the ink binder needs to be discussed later.