Conductive Polyacrylonitrile Fiber Prepared by Copper Plating with L-Ascorbic Acid as Reducing Agent

WANG Yunhong(王運紅), TAN Youzhi(譚由之), XU Yajing(徐雅婧), GUO Lamei(郭臘梅), 3*，HUANG Liqian(黃莉茜), 3

1 Textile Inspection and Testing Institute, Jiangxi General Institute of Testing and Certification, Gongqing 332020, China2 College of Textiles, Donghua University, Shanghai 201620, China3 Key Laboratory of Textile Science & Technology, Ministry of Education, Donghua University, Shanghai 201620, China

Abstract: Conductive polyacrylonitrile fibers were prepared by electroless copper plating under weak alkaline conditions, with L-ascorbic acid as reducing agent. The influences of CuSO4·5H2O, L-ascorbic acid, 2,2′-bipyridine and K4Fe(CN)6 concentration on the conductivity and mass gain percentage of the fibers were studied. The morphological structure of the fibers was characterized by scanning electron microscopy(SEM), and the mechanical properties of the fibers were analyzed through the mechanical property test. The results showed that the optimal reaction conditions were as follows: 26 g/L CuSO4·5H2O, 26 g/L L-ascorbic acid, 12 mg/L 2,2′-bipyridine, 7 mg/L K4Fe(CN)6, and 38℃. The volume resistivity of the conductive PAN fibers prepared by the process was only 3.84×10-3 Ω·cm.

Key words: L-ascorbic acid; CuSO4·5H2O; redox; copper plating; conductive fiber

Introduction

The continuous development and integration of high-tech fields such as electronic technology and biotechnology with textile material technology has brought smart textiles into wide attention[1]. The fabric woven with conductive fibers can obtain good strain sensing performance, so it is one of the main preparation materials for flexible sensors[2-5]. Electroless plating which refers to the autocatalytic reaction occurring on the surface of the fiber without applying an electric current, is an important method for preparing conductive fibers. The coating thickness is uniform by electroless copper plating, and the operation is easy with simple process equipment without power supply. It has a wide range of applications and can be used on any shape of material surface[6-7].

In traditional electroless copper plating process, formaldehyde is typically used as reducing agent, but formaldehyde is volatile, which pollutes the environment and harms human health. Taking the protection of the environment and human health into account, finding a suitable reducing agent to replace formaldehyde has always been a research hotspot in electroless copper plating. A series of researches have been carried out on the process of electroless copper plating in domestic and overseas[8-9]. Reducing agents that can replace formaldehyde include hypophosphite[10], borohydride[11], hydrazine[12], dimethylamine borane[13], glyoxylic acid[14],etc., but there were still few research systems withL-ascorbic acid (L-AA) as reducing agent.

In this study,L-AA was used as reducing agent in order to replace the formaldehyde, and copper sulfate was used as a copper source to chemically deposit on the surface of polyacrylonitrile (PAN) fibers to prepare conductive PAN fibers. The influence law of its key factors was studied.

1 Experiments

1.1 Materials and instruments

PAN fibers used in this study were filaments with linear density of 3.5 dtex. SnCl2, AgNO3, CuSO4·5H2O, K4Fe(CN)6, HCl, NH3·H2O,L-AA (C6H8O6) and 2,2′-bipyridine (C10H8N2) were of analytical grade and produced by Sinopharm Group, Shanghai, China. Fixing agent (DE-2) was purchased from Shanghai Tiantan Additives Co., Ltd., China.

DKB-501A super constant temperature water tank (Shanghai Senxin Experimental Instrument Co., Ltd., China),UT39E+ digital multimeter (China Unitech Co., Ltd., Shenzhen, China), TM3000 tabletop scanning electron microscope (Hitachi, Japan), YG002C fiber fineness analyzer (Ningbo Textile Instrument Factory), electronic fiber strength tester (Laizhou Electronic Instrument Co., Ltd., China), SW-8A washing fastness tester (Wenzhou Bain Instrument Co., Ltd.,China), Bruker D8 advance X-ray diffractometer (Bruker Technology Co., Ltd., Germany) were applied in this study.

1.2 Process flow

The process flow for the preparation of the conductive PAN fibers is shown in Fig.1. First, the PAN fibers were pre-treated, including degreasing, sensitization, washing, and activation, and then the conductive fibers coated with copper layer were obtained by electroless copper plating. Finally, a fixing agent was used to fix the prepared conductive fibers.

Fig.1 Preparation process of conductive PAN fibers

1.3 Preparation of conductive PAN fibers

1.3.1Pretreatmentforcopperplating

First, 2.5 g/L washing powder solution was used to soak the PAN fibers for 10 min to remove the oily impurities on the surfaces of the fibers; next, the fibers were put in SnCl2solution (20 g/L) at room temperature (25-28 ℃), sensitized for 10 min to adsorb reductive Sn2+on the fibers surfaces; and then put the fibers into AgNO3solution (0.025 mol/L) for activation at room temperature for 10 min to generate catalytic active center[6].

1.3.2Copperplating

Copper plating solutions were prepared with a certain concentration, and the pH values of the solutions were adjusted to 9 with NH3·H2O. Pre-treated fibers of 0.2 g were immersed in 50 mL copper plating solution for copper plating, in a constant temperature water bath at (35.0 ± 0.2) ℃ for 1.5 h.

1.3.3Fixtreatment

The adhesion of the copper layer on the surfaces of the fibers was weak after electroless deposition. Inspired by the dye fixing principle, the cross-linking reaction type fixing agent DE-2 was used to fix copper on the fibers. The fix agent solution of 2% (mass fraction) was prepared and the fibers were soaked in it for 3 min[15]. Then the fibers infiltrated with the fixing agent dried at 120 ℃.

1.4 Characterization

Scanning electron microscopy (SEM) was used to observe the surfaces of the fibers. X-ray diffraction (XRD) was used to characterize the coating composition. The stability of the plating solution was characterized by the stability time which is the time required for the solution changing from clear to turbid during the chemical deposition.

The digital multimeter was performed to measure the resistance of the fibers, and 10 values were taken to average. The fiber fineness analyzer was used to measure the diameter of the fibers, and 50 values were taken to average. The volume resistivity of the fiber was calculated by

(1)

whereρ(Ω·cm) is the volume resistivity,R(Ω) is the fiber resistance value measured by the digital multimeter,D(cm) is the fiber diameter,S(cm2) is the fiber cross-sectional area, andL(cm) is the length of the fiber measured by the multimeter.

Mass gain percentage was measured under standard temperature and humidity conditions. The electronic balance was used to measure the mass of the fibers, and the mass gain percentage of the fibers was calculated by

(2)

whereW(%) is the mass gain percentage,mFBC(g) is the mass of the fibers before copper plating,mFAC(g) is the mass of the fibers after copper plating.

Mechanical performance test was performed by the electronic fiber strength meter to test the strength and elongation at break of the fibers. Referred to GB/T 14337—2008 test method for tensile properties of short chemical fiber, the mechanical performance test conditions were set as follows: the tensile gauge of 20 mm, the tensile speed of 20 mm/min, the correction factor of 1, and drawing numbers of stretching of 50 times for each sample. The results were averaged.

The washability test was performed to characterize the adhesion fastness of the coating, referred to GB/T 3921—2008, and test A for color fastness to soaping was selected. The mass loss percentage was used to characterize the adhesion fastness of the coating, which can be calculated by

(3)

whereη(%) is the mass loss percentage,mFBW(g) is the mass of the fibers before washing, andmFAW(g) is the mass of the fibers after washing.

2 Results and Discussion

In the electroless copper plating, the influence factors include the concentration of various substances, the reaction temperature, time and pH. The influences of the concentration of copper source, reducing agent, and stabilizer in the plating solution on the performance of the coating were mainly discussed in this article.

2.1 Influence of CuSO4·5H2O

The chemical deposition withL-AA as the reducing agent can be described by the following reactions.

Cu2++C6H8O6+2OH-→Cu+C6H6O6+2H2O,

(4)

[Cu(NH3)4]2++C6H8O6+2OH-→
Cu+C6H6O6+4NH3+2H2O.

(5)

It can be seen from Eqs. (4) and (5) that there is no generation of H2during the reaction, which avoids the formation of micropores on the copper coating due to hydrogen generation, and thus improves the quality of the coating. Besides, Cu2+is the main reactant in the copper plating for PAN fibers to obtain conductivity, and Cu2+concentration has the greatest impact on the result. As the concentration of CuSO4·5H2O increases (other conditions: 28 g/LL-AA, 10 mg/L 2,2′-bipyridine, 8 mg/L K4Fe(CN)6), the mass gain percentage of the fibers gradually increases and the volume resistivity of the fibers decreases (Fig.2(a)), while the stability timetof solution decreases (Fig.2(b)). When the CuSO4·5H2O concentration is 10 g/L, the fibers mass gain percentage is only 24%, and the volume resistivity of the fibers is relatively large, about 4.03×102Ω·cm, which means less Cu coated on the fibers. When the concentration of CuSO4·5H2O increases to 25 g/L, the fibers volume resistivity lowers to 4.34×10-2Ω·cm and the fibers mass gain percentage rises to 50%. After that, with the increase of CuSO4·5H2O concentration, the stability of the plating solution drops rapidly.

Fig.2 Effects of CuSO4·5H2O concentration on: (a) mass gain percentage and volume resistivity of fibers; (b) stability time of solution

As the concentration of CuSO4·5H2O increases from a low level, the amount of Cu2+in the solution rises, and the probability and quantity of Cu2+reacting with the reducing agent increase, so more coppers form on the PAN fibers, and thus the volume resistivity of the fibers drops. When the concentration exceeds 25 g/L, there would be more elemental coppers deposit on the fibers, and fewer catalytic activation centers remain. When the concentration continues increasing, it is easy to cause the accumulation of particles on the surface of the fiber resulting in a rough surface. In addition, the higher concentration of the substance in the solution leads to poor stability of the solution, thus further increasing of the concentration of CuSO4·5H2O has less positive influence on the volume resistivity of the conductive fibers. Besides, a higher concentration of CuSO4·5H2O will increase the cost. Therefore, the concentration of CuSO4·5H2O is selected as 25 g/L.

2.2 Influence of L-AA

L-AA in the plating solution is another element that affects the conductivity of PAN fibers. The conductivity of PAN fibers can be seen from Fig.3. When the concentration ofL-AA is low (other conditions: 25 g/L CuSO4·5H2O, 10 mg/L 2, 2′-bipyridine, 8 mg/L K4Fe(CN)6), the mass gain percentage of the fibers is low as well, and the volume resistivity is high. Under this condition, the Cu2+attached to the fibers is not completely reduced to Cu, resulting in less conductive coppers on the final fibers.

Fig.3 Effects of L-AA concentration on: (a) mass gain percentage and volume resistivity of fibers; (b) stability time of solution

With increasingL-AA, the generated copper increases, so the electrical resistivity of the fibers decreases. The volume resistivity of the fibers decreases to 1.13×10-2Ω·cm when the concentration ofL-AA is 26 g/L. Under this condition, the fibers mass gain percentage reaches 50%, and stability time of solution is 35 min. Afterwards, the mass gain percentage of the fibers continues to increase, and the conductivity of the fibers decreases (Fig.3(a)). This is because the higher concentration ofL-AA leads to more reduced copper elements, but excessively high concentration ofL-AA causes much higher reaction rate as seen from Fig.3(b), and thus the quality of the coating layer gets worse and the fibers surfaces get much rougher, which results in the lower conductivity of the fibers. Therefore, the optimal concentration ofL-AA is 26 g/L.

2.3 Influence of 2, 2′-bipyridine

In the copper plating solution, a small amount of 2,2′-bipyridine is mainly used to reduce the reaction rate and stabilize the plating solution[16]. It can be seen from Fig.4, as the concentration of 2,2′-bipyridine increases (other conditions: 25 g/L of CuSO4·5H2O, 26 g/L ofL-AA, 8 mg/L of K4Fe(CN)6), both the mass gain percentage of the fibers and the stability time of solution increase at first and then decrease.

Fig.4 Effects of 2,2′-bipyridine concentration on: (a) mass gain percentage and volume resistivity of fibers; (b) stability time of solution

While the corresponding volume resistivity of the fibers first decreases and then increases with the increase of 2, 2′-bipyridine concentration, and the optimal value is obtained when the concentration of 2,2′-bipyridine is 15 mg/L. That is because 2,2′-bipyridine is used as a stabilizer to reduce the deposition rate, making the coating fine and compact. However, when the concentration of 2,2′-bipyridine is too high (greater than 15 mg/L), the deposition rate will be too slow. Under this condition it is difficult to form dense surfaces on the fibers, which causes the mass gain percentage to drop rapidly, and the volume resistivity increases greatly. Therefore, after comprehensive consideration, the optimal concentration of 2,2′-bipyridine is selected as 15 mg/L.

2.4 Influence of K4Fe(CN)6

In the copper plating solution, the addition of a small amount of K4Fe(CN)6can improve the stability of the plating solution to a certain extent[17]. As shown in Fig.5, when K4Fe(CN)6is not added, the stability of the plating solution is poor. Moreover, although the mass gain percentage of the fibers is high, the volume resistivity is not optimal.

Fig.5 Effects of K4Fe(CN)6 concentration on: (a) mass gain percentage and volume resistivity of fibers; (b) stability time of solution

The addition of K4Fe(CN)6can make the reduction peak potential of Cu2+shift negatively, inhibit the precipitation of elemental copper in the solution to some degree, so as to increase the stability of the plating solution, improve the quality of the coating, make the appearance of the coating brighter, and enhance the fibers conductivity.

When K4Fe(CN)6concentration is 8 mg/L (other conditions: 25 g/L CuSO4·5H2O, 26 g/LL-AA, 15 mg/L 2,2′-bipyridine), the results are as follows: the fiber mass gain percentage is 56%, stability time of solution is 40 min, and the fiber volume resistivity is 1.05×10-2Ω·cm. Continuing to increase K4Fe(CN)6concentration will reduce the stability of solution, cause particle agglomeration on the surfaces of the fibers, and thus deteriorate the quality of the coating. Therefore, the concentration of K4Fe(CN)6is selected as 8 mg/L.

2.5 Orthogonal optimization experiment

In order to further optimize the process, an orthogonal experiment was designed. In the experiment, temperature and concentrations of CuSO4·5H2O,L-AA, 2,2′-bipyridine, and K4Fe(CN)6were selected as five factors for orthogonal design, and 4 levels were selected for all factors, as shown in Table 1. The L16(45) orthogonal experiment conditions and the results of the fiber volume resistivity are shown in Table 2.

Table 1 Factors and levels of the orthogonal experiment

Table 2 Results of the orthogonal experiment

According to the results in Table 3, the order of the factors affecting the final fiber volume resistivity is as follows: 1)L-AA, 2) temperature, 3) CuSO4·5H2O, 4) 2,2′-bipyridine, 5) K4Fe(CN)6. The best combination is A3B2C1D2E4,i.e., 26 g/L CuSO4·5H2O, 26 g/LL-AA, 12 mg/L 2,2′-bipyridine, 7 mg/L K4Fe(CN)6, and 38 ℃. The volume resistivity of the fibers prepared under these conditions is 3.84×10-3Ω·cm.

Table 3 Range analysis of orthogonal experiment data

2.6 Mechanical property

Fibers labeled No.1-No.4 were prepared under the process conditions in section 2.4 respectively. As shown in Fig.6, there is no significant change in the breaking strength and breaking elongation of the fibers before and after copper plating. The difference in the existing data is more likely to be the error in the fiber sample itself. It shows that during the series of pre-treatments and copper plating, the mechanical properties of the fibers are hardly damaged.

Fig.6 Mechanical properties of the fibers before and after copper plating: (a) fiber breaking strength; (b) fiber elongation at break

The washing durability of the copper-plated fibers before and after the fixative treatment was tested separately. After ten consecutive washings, the mass loss percentages are 4.30% and 1.53%, respectively. The results show that the copper coating on the fibers has good adhesion fastness, and the fastness is improved after the fixative treatment.

2.7 XRD and SEM analyses

The XRD pattern of the copper-plated fibers is shown in Fig.7. Strong diffraction peaks appear when the diffraction angles 2θare 43.4°, 50.6°, and 74.1°, which correspond to the crystal planes of elemental copper of (111), (200), and (220), respectively. And there are no other peaks in Fig.7, indicating that the substance contained in the plating layer is mostly copper element with few impurities.

Fig.7 XRD of copper-plated fibers

The morphology of the PAN fibers obtained by different treatments is shown in Fig.8, in which the left ones are SEM images and the right ones are the pictures taken by a mobile phone. It can be seen from Fig.8(a) that there are individual small bright spots on the fibers, which means that there are some dust and impurities on the surfaces of the fibers without any treatments. Some small bright spots are evenly distributed on the fibers after activation, as shown in Fig.8(b), which are some catalytic activition centers formed by elemental silver. A bright copper conductive layer is attached to the fibers after the chemical deposition, but there are more agglomerated particles on the surfaces, as shown in Fig.8(c). The SEM image of the fiber after the fixation treatment shown in Fig.8(d) indicates that the particle agglomeration on the surface of the fiber is obviously reduced compared with the one in Fig.8(c).

3 Conclusions

(1) The chemical deposition withL-AA as reducing agent only needs to be carried out under the condition of weak alkaline (pH=9), and the damage to the fiber is weak. Moreover, there is no generation of H2during the reaction, which avoids the formation of micropores on the copper coating because of H2release, and thus improves the quality of the coating.

(2) With volume resistivity of the fibers as the evaluation index, the optimal copper plating condition is as follows: 26 g/L CuSO4·5H2O, 26 g/LL-AA, 12 mg/L 2,2′-bipyridine, 7 mg/L K4Fe(CN)6, and 38 ℃.

(3) The volume resistivity of the conductive PAN fibers prepared under the optimal process conditions is 3.84×10-3Ω·cm. The plating layer is mainly copper element, and the bonding force between the plating layer and the fibers is good. After the treatment, the obtained PAN fibers still have good mechanical property and flexibility, for which the obtained PAN fibers can be used as materials for flexible electronics in the medical care, motion detection, communications and other fields.