materials Article
Preparation of Conductive Polyester Fibers Using Continuous Two-Step Plating Silver Changchun Liu 1,2 , Xuelian Li 2 , Xiaoqiang Li 3 , Tianze Xu 4 , Chunyu Song 3 , Kenji Ogino 1, * and Zhijie Gu 1, * 1 2 3 4
*
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan;
[email protected] College of Chemistry and Materials Engineering, Changzhou Vocational Institute of Engineering, Changzhou 213164, China;
[email protected] College of Textile and Clothing, Jiangnan University, Wuxi 214122, China;
[email protected] (X.L.);
[email protected] (C.S.) School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China;
[email protected] Correspondence:
[email protected] (K.O.);
[email protected] (Z.G.); Tel.: +81-42-388-7404 (K.O.)
Received: 21 September 2018; Accepted: 17 October 2018; Published: 19 October 2018
Abstract: Polyester fibers are used in various fields, due to their excellent mechanical and chemical stability. However, the lack of conductivity limits their application potential. In order to prepare conductive polyester fibers, silver is one of the most widely used materials to coat the surface of the fibers. This work aimed to prepare silver-coated polyester fibers by a continuous two-step method, which combined the operations of continuous electroless plating and electroplating. Meanwhile, we designed specialized equipment for the continuous plating of silver on the polyester fibers under a dynamic condition. The mechanical property, washability, electrical resistivity, and electrical conductivity of the resultant conductive polyester fibers obtained from different silver-plating conditions were also characterized. The results demonstrated that the conductive fibers prepared by continuous two-step silver plating equipment, had good electrical conductivity with better mechanical properties and washability. Keywords: continuous two-step silver plating; electroplating; electroless plating; polyester fiber; dynamic condition
1. Introduction Polyester fibers are widely used in various fields, because of their excellent physical and mechanical properties and chemical stability. However, normal polyester fiber has a low moisture regain [1,2], higher electrical resistance [3,4], and can easily accumulate large amounts of electric charges on the surface [5,6], which results in fibers repelling each other, clothing absorbing dust and clinging onto the body, and can even produce electrical shocks or ignite flammable substances. Polyester fibers’ easily generating static electricity and poor conductivity, limit their applications in apparel and home furnishings [7–9]. Therefore, it is necessary to improve the electrical conductivity of the fibers. Plating a layer of silver on the surface of a polyester fiber will preserve the excellent properties of the polyester. Meanwhile, it also has the excellent electrical conductivity [10], ductility [11,12], catalytic activity [13,14], and antibacterial deodorization properties of metallic silver [15–18], and the material can also obtain a certain metallic luster and decorative effect [19–22]. The electroless silver plating method is widely used in applications to the surface metallization of fibers, due to its simple operation process, uniform coating, and controllable thickness [23–26]. However, the surface of the fiber has no catalytic activity. Therefore, using electroless plating on the
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surface of the fiber to achieve metal deposition requires pretreatment to impart a certain catalytic activity to the surface of the fiber [27,28]. The conventional pretreatment process mainly includes the four steps of cleaning, coarsening, sensitization, and activation, and the activation is an important step to determine the catalytic activity of the substrate [29–31]. The activation of the fibers often uses palladium with high reactivity, such as the sensitization–activation two-step method [32–34], the colloidal palladium activation method [35,36], the liquid ion palladium activation method [37,38], and so on. Due to the highly toxic and expensive nature of palladium, as well as the great potential of polluting the plating layer [39,40], these types of methods are not conducive to the preparation and application of the electroless silver-plated fibers. It has been reported that in the process of pretreatment, silver nitrate has been used by replacing the palladium compound as an activation solution for electroless plating to form a silver catalytic center on the surface of the fiber [41], while nitrate is also considered as a potentially hazardous [42]. NaClO-HCl silver activation system also proved to be a good applicability in the silver-coating industry [43]. But these activation steps all prolong the time of electroless silver-plating, which is not conducive to production. In this work, the conventional activation step in the pretreatment process was integrated into the subsequent electroless silver plating step, reducing the number of steps and using palladium-free, cyanide-free materials. To the best of our knowledge, the study of fiber electroless silver plating has mainly focused on chemical plating under static conditions where the technology tends to be more mature [44–49], but that the study of the dynamic continuous silver plating of fibers has been seldom reported. In addition, due to the shorter reaction time, the electroless silver-plated fibers have a thin silver coating on the surface with very poor durability. Therefore, further electroplating treatment is generally required [50,51]. The first and best developed process in the electroplating industry is cyanide electroplating silver plating. The plating solution forms a coordination complex with CN− and Ag+ , which has high stability and good dispersion of the plating solution. The silver plating layer using cyanide electroplating has high bonding fastness to the substrate [52,53], but cyanide is highly toxic, so cyanide electroplating silver must be replaced [54–56]. This work aimed to prepare silver coated polyester fibers by a continuous two-step method, which combined the operations of continuous electroless plating and electroplating. In the electroless silver-plating step, the polyester fiber, after being pretreated by the sensitization washing, was directly immersed into the electroless plating solution for electroless plating silver. In the second step, the electroless silver-plated fiber was subjected to a cyanide-free electroplating silver treatment by using sulfosalicylic acid as the plating solution. The whole process was continuous, and specialized continuous industrial silver-plating equipment was also designed. 2. Experimental 2.1. Materials Polyester fibers (210D/36F) were purchased from Vekstar Textile (Shanghai) Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH, ≥96.0%), potassium hydroxide (KOH, ≥85.0%), lauryl sodium sulfate (LSS, >97.0%), stannous chloride dihydrate (SnCl2 ·2H2 O, ≥98.0%), silver nitrate (AgNO3 , ≥99.8%), aqueous ammonia (NH3 ·H2 O, 25.0–28.0%), glucose (C6 H12 O6 , AR), sulfosalicylic acid dihydrate (≥99.0%), ammonium acetate (≥98.0%), and hydrochloric acid (36.0–38.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. 2.2. Deposition of Silver on Polyester Fibers by Electroless Plating under the Dynamic Condition The general processes of preparing a conductive metal layer on polymeric fibers or plastics are mainly composed of two parts: (1) the pretreatment including cleaning, coarsening, sensitizing, and activating; and (2) the deposition of silver through the redox reaction. In this study, both the pretreatment and the silver deposition were continuously operated in a series of self-made reactive
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Materials 2018, 11, in x FOR PEER REVIEW of 16 tanks, as shown Scheme 1. The whole process was operated under a dynamic condition. The3 fiber 2018, 11,device x FOR PEER REVIEW 3 of 16 speed windingMaterials collecting at the end of the equipment provided the drafting force. The winding was controlledby bythe thecontrol controlpanel, panel, and and the in in each plating tank could be was controlled the reaction reactiontime timeofofthe thefibers fibers each plating tank could was controlled by thewinding control panel, andand the reaction time of the fibers in each plating tank could be in the controlled by different speeds the number of windings. The size of the groove be controlled byby different speedsand and number of windings. size of the in controlled differentwinding winding speeds thethe number of windings. The sizeThe of the groove in groove the was 13.5 cm ×× 8.58.5cm ×× 14.5 cm. The lining and the drum ininthe tank were made of theequipment equipment was 13.5 cm cm 14.5 cm. The lining and the drum the tank were made of equipment was 13.5 cm × 8.5 cm × 14.5 cm. The lining and the drum in the tank were made of polytetrafluoroethylene (PTFE). polytetrafluoroethylene (PTFE). polytetrafluoroethylene (PTFE).
The equipment continuouselectroless electroless plating of of silver under a dynamic condition. SchemeScheme 1. The1.equipment for for thethe continuous plating silver under a dynamic condition. (1) return wire rack; (2) polyesterfiber fiber (3) (3) cleaning tank; (4) coarsening tank; (6) sensitizing tank; (8) tank; (1)Scheme return wire rack; (2) polyester cleaning tank; (4) coarsening tank; (6) sensitizing 1. The equipment for the continuous electroless plating of silver under a dynamic condition. electroless silver-plating tank; (10) drying tank; (11) fibers winding collecting device; (12) control (8)(1) electroless silver-plating tank; (10) drying tank; (11) fibers winding collecting device; (12)tank; control return wire rack; (2) polyester fiber (3) cleaning tank; (4) coarsening tank; (6) sensitizing (8) panel; (13) heating device; (5), (7), and (9) washing tank. panel; (13) heating device; (5), (7),(10) anddrying (9) washing tank.fibers winding collecting device; (12) control electroless silver-plating tank; tank; (11)
panel;Polyester (13) heating device; (5), (7), and (9)a washing tank. of NaOH and lauryl sodium sulfate (LSS), fibers were cleaned using mixed solution
Polyester fibers were cleaned using a mixed solution of NaOH and lauryl sodium sulfate (LSS), followed by coarsening in NaOH solution. The sensitizing of polymeric fibers was conducted using followed by coarsening in NaOH The sensitizing of fibers was conducted Polyester fibers to were cleaned using mixed solution of polymeric NaOH lauryl sodium sulfateusing (LSS),a a SnCl 2 solution make a Snsolution. 2(OH) 3Cl a gel-layer on the surface of theand fibers. Then, the sensitizing SnCl solution to make a Sn (OH) Cl gel-layer on the surface of the fibers. Then, the sensitizing fibers were put into the of Tollens’The reagent and glucose for the silverfibers deposition. 2 fibersby 3 solution. followed coarsening in2solution NaOH sensitizing of polymeric was conducted using were put into the solution of Tollens’ reagent and glucose for the silver deposition. a SnCl2 solution to make a Sn2(OH)3Cl gel-layer on the surface of the fibers. Then, the sensitizing 2.3. Deposition of Silver by Continuous Electroplating
fibers were put into the solution of Tollens’ reagent and glucose for the silver deposition. 2.3. Deposition Silver by Continuous Electroplating Theof polyester fibers with silver deposition of electroless plating were connected with a negative wire for electroplating, as shown inElectroplating Scheme 2. In detail, a pure silver plate with the dimension of 80 2.3.The Deposition of Silver Continuous polyester fibersby with silver deposition of electroless plating were connected with a negative mm × 40 mm × 5 mm was used as an anode material and a direct current with a voltage ranging from wire for electroplating, as shown in Scheme 2. In electroless detail, a pure silver plate with the dimension of The polyester silver deposition plating were connected a negative 1 V to 10 V wasfibers used aswith an energy resource. Theof polyester fibers, after being coated with awith thin layer 80wire mm for × 40 mm × 5 mm was used as an anode material and a direct current with a voltage ranging electroplating, asplating, shownwere in Scheme In detail, a pure silver with thesolution dimension of silver by electroless directly 2. used for electroplating. The plate electroplating was of 80 from tomm 10 ×V used as an resource. The fibers, after being coated mixture with a pH of used about 9, energy prepared by using 120 g/Lapolyester of sulfosalicylic acid, 10 of potassium mm1×aV40 5 was mm was as an anode material and direct current with a g/L voltage rangingwith froma g/L of silver nitrate, 50 g/L were of ammonium acetate, andafter 60 mL/L of aqueous ammonia. thin by electroless plating, used for electroplating. The electroplating 1 Vlayer tohydroxide, 10of V silver was30 used as an energy resource. Thedirectly polyester fibers, being coated with a thin layer solution was mixture with a pHwere of about 9, prepared by using 120 g/L sulfosalicylicsolution acid, 10was g/L of silver by aelectroless plating, directly used for electroplating. Theof electroplating of apotassium hydroxide, 30 g/L of silver nitrate, 50 g/L of ammonium acetate, and 60 mL/L of mixture with a pH of about 9, prepared by using 120 g/L sulfosalicylic acid, 10 g/L of potassium aqueous ammonia. hydroxide, 30 g/L of silver nitrate, 50 g/L of ammonium acetate, and 60 mL/L of aqueous ammonia.
Scheme 2. The equipment for electroplating silver on polyester fibers.
Scheme fibers. Scheme2.2.The Theequipment equipmentfor forelectroplating electroplating silver silver on on polyester polyester fibers.
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2.4. Characterization Fourier-transform infrared (FTIR) spectra were obtained on a Spectrum Two IR Spectrometer (PerkinElmer, Waltham, MA, USA). The surface morphology of different fibers was examined with a Hitachi 3400N scanning electron microscope (SEM, Hitachi, Japan) at an accelerating voltage of 15 kV. XRD patterns of different fibers were measured with a D8 ADVANCE X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a tube voltage of 40 KV, tube current of 1100 mA, a scanning range from 3 to 90 degrees, and scanning speed of 4 degrees per minute. The breaking strength and elongation at the break of the fibers were characterized by a YG020B Electronic Single Yarn Strength Tester (Changzhou No. 2 Textile Machine Co., Ltd., Changzhou, China) with a clamping length of 250 mm and a pre-tension of 10 CN. The resistivity of the conductive polyester fibers was measured with a Fluke 175C True RMS Digital Multimeter Shenzhen Xu Da Century Technology Co., Ltd. (F175C, Shenzhen, China). Ten readings at different locations of the fibers at a distance of 10 cm were taken randomly and the average was recorded. 3. Results and Discussion 3.1. Mechanism Discussion of Electroless Plating under the Dynamic Condition 3.1.1. Pretreatment on Polyester Fibers The processes of pretreatment on polyester fibers for electroless plating have been thoroughly investigated in previous studies [44,57–60]. To the best of our knowledge, only three steps are needed for the pretreatment including the cleaning, coarsening, and sensitizing. The activation step can be integrated into the subsequent electroless plating step. The aim of cleaning is to remove the oil and/or other impurities that are located on the surface of the fibers in the processes of production and transportation. In this study, we used a mixture of 0.07 g/mL NaOH and 0.002 g/mL LSS to clean the surface of the polyester fibers. Both the NaOH and LSS could remove the oil from the surface of the polyester fibers, and the LSS also could serve as the surfactant to reduce the surface tension of the polyester fibers. The SEM image of the polyester fibers after cleaning is shown in Figure 1b. The second step of pretreatment was coarsening. Coarsening is done to increase the roughness of the surface of the fibers by chemical etching and the chemical reaction between the plating solution and fibers to form uniform pores and grooves on the surface, thus forming the “locking effect” and enhancing the hydrophilicity of the substrate [28]. Therefore, the bonding strength between the coating and the substrate will be improved. In this work, the surface of the fibers was chemically etched by the NaOH solution, which should have an appropriate concentration. Too high a concentration will cause serious damage to the fiber surface and a high loss rate of the fiber mass, greatly reducing the mechanical properties of the fiber, while too low a concentration cannot meet the need of coarsening. In this study, the coarsening condition was that the fibers were coarsened in 100 g/L NaOH solution at 80 ◦ C for 5 min. Figure 1c shows the SEM image of the coarsened polyester fibers where there were more pits and holes on the surface of the fiber, and the roughness obviously increased.
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Figure 1. SEM Figure 1. SEM images images of of polyester polyester fibers: fibers: (a) (a) before before any any treatment; treatment; (b) (b) after after cleaning cleaning with with NaOH NaOH and and lauryl sodium sulfate (LSS); (c) after coarsening with NaOH; and (d) after sensitizing sensitizing with with SnCl SnCl22.
The next step of pretreatment pretreatment was was sensitizing. sensitizing. In In this this work, work, aamixed mixedsolution solutionof of30 30g/L g/L of SnCl SnCl22 mL/Lof ofhydrochloric hydrochloricacid acid was wasused used as asthe thesensitizing sensitizing solution. solution. After After the the fibers fibers were were immersed immersed and 40 mL/L (OH)33Cl Cl thin thin films films with with aa slightly slightly soluble soluble gel gel were were formed by the in the sensitizing solution, Sn22(OH) hydrolysis of stannous chloride during subsequent water washing [61]. This gel-like substance was adsorbed on the fiber surface to ensure the uniform adsorption of divalent tin ions onto the gel’s gel's facilitated the uniform uniform deposition deposition of of silver silver particles particles during during electroless electroless silver silver plating. plating. surface, which facilitated Therefore, sensitization mainly mainly occurs in the water washing process, and the reaction process is combine to form a slightly soluble gelatinous shown in Equations (1) and (2). Sn(OH)Cl and Sn(OH)22 combine (OH)33Cl. Cl. substance, Sn22(OH) SnCl2 + 2H2 O → Sn(OH)2 + HCl (1) SnCl2 + 2H2O → Sn(OH)2 + HCl (1) SnCl2 + 2H2 O → Sn(OH)Cl + HCl (2) SnCl2 + 2H2O → Sn(OH)Cl + HCl (2) Figure 1d shows the SEM image of the sensitized polyester fiber surface, showing that the Figure 1d surface shows was the SEM image of thewith sensitized fibercontinuous surface, showing that the polyester fiber obviously covered a morepolyester uniform and gel-like film. polyester fiber surface was obviously covered with a more uniform and continuous gel-like film. 3.1.2. Electroless Silver Plating on Polyester Fibers 3.1.2. Electroless Silver Plating on Polyester Fibers In the electroless plating solution, Ag(NH3 )2 + is first reduced by Sn2+ adsorbed on the surface of In the solution, 2+ is firstof reduced byshown Sn2+ adsorbed on the of the fiber to electroless form metalplating Ag particles, the Ag(NH reaction3)formula which is in Equation (3). surface Thus, the the fiber to form Ag particles, the reaction of which is shown ion in Equation (3). Thus, Ag particles are metal deposited on the surface of theformula fiber where the stannous is dispersed. The the Ag Ag particles deposited on activity the surface of the fiber where the stannous is dispersed. Ag particles haveare strong catalytic and become the catalytic center of theion electroless silver The plating + in theand particles strong catalytic activity become the catalytic center of the electroless silver plating reaction. have Similarly, [Ag(NH ) ] silver ammonia solution is reduced to metal Ag particles with 3 2 reaction. Similarly, [Ag(NH )2]+ surface in the silver ammonia solution reducedcenter to metal particles with glucose, and deposited on 3the of the fiber around theiscatalytic to Ag obtain a metallic glucose, andthe deposited the surface of theisfiber around the catalytic center to obtain silver silver layer, reactionon formula of which shown in Equation (4). Therefore, in thisa metallic non-activated layer, reactionsilver formula of which is shown in Equation (4). fiber Therefore, in this direct direct the electroless plating process, the sensitized-washed will first formnon-activated Ag particles on the electroless silver plating sensitized-washed fiber will firstthe form Ag particles on has the surface of the fiber as the process, catalyticthe center of the whole reaction, so that surface of the fiber surface the fiber as the catalytic center of the whole reaction, so that the surface of the fiber has catalyticofactivity. catalytic activity. 2[Ag(NH3 )2 ]+ + Sn2+ → Sn4+ + 2Ag↓ + 4NH3 (3) 2+→ Sn4+ + 2Ag↓ + 4NH3 2[Ag(NH )2]+ + → SnRCOONH C6 H12 O6 + 2[Ag(NH 3 )23]OH 4 + 3NH3 + 2Ag↓ + H2 O
(3) (4)
C6H12O6 + 2[Ag(NH3)2]OH → RCOONH4 + 3NH3 + 2Ag↓ + H2O
(4)
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3.1.3. Influence of the Transmission Condition on the Continuous Electroless Plating Silver 3.1.3. Influence of the Transmission Condition on the Continuous Electroless Plating Silver Continuous silver plating on fibers under a dynamic condition can facilitate deposition Continuous plating on fibers underfrom a dynamic condition can facilitate deposition efficiency, efficiency, whichsilver is significantly different that under a static condition. Under the same which is significantly different from that under a static condition. Under the same pretreatment pretreatment conditions, we performed silver plating contrast experiments in a static beaker under a conditions, we performed silver plating contrast in a static under a dynamic dynamic condition and obtained the contrast chartexperiments of the deposition rate beaker of silver particles under conditionand andstatic obtained the contrast chart of the deposition rate 2). of silver particles under dynamic and dynamic conditions at different plating times (Figure This demonstrated that the weight static conditions at different plating times (Figure 2). This demonstrated that the weight gain of the gain of the fibers under the static condition was obviously lower than that under the dynamic fibers under thethe static condition was obviously lower that under dynamic condition, the condition, and deposition rate became lower and than lower with the the increase of plating time,and while deposition raterate became lower and lower with increasecondition of platingwas time, whileand thehad deposition rate of the deposition of silver particles under thethe dynamic higher less influence silver particles under thetest dynamic condition was higher and had less make influence on time. on time. The self-made equipment allowed the fibers to fully contact withThe theself-made reaction test equipment allowed thetransmission, fibers to fullythus make contact with the reaction liquids under theunder dynamic liquids under the dynamic achieving a better silver plating effect than the transmission, thus achieving a better silver plating effect than under the static condition, which may static condition, which may be because the fibers mainly reacted with the silver ammonia ion around be because reacted witha the silver ammonia around them. Whenammonia the silver ions was them. Whenthe thefibers silvermainly was plated under static condition, theion concentration of silver plated static condition, concentration of silver ions near the fibers near theunder fibersadecreased with thethe progress of the reaction, soammonia the deposition rate of the silverdecreased particles with the progress of the reaction, sothe thedynamic deposition rate of the condition silver particles decreased continuously. decreased continuously. However, transmission facilitated the increase in the However,rate the of dynamic transmission facilitated the fibers, increase diffusion rate rate of of silver silver diffusion silver ammonia ionscondition onto the surface of the so in thethe deposition ammoniahad ionslittle ontochange. the surface of the fibers, so the deposition rate of silver particles had little change. particles
Figure 2. 2. Deposition Deposition rate rate of of silver silver particles particles under under (a) (a) dynamic, dynamic, and and (b) (b) static static conditions. conditions. Figure
3.2. Influence of Electroplating Process Conditions on Deposition of Silver 3.2. Influence of Electroplating Process Conditions on Deposition of Silver 3.2.1. Influence of Power Supply Method on Electroplating Silver 3.2.1. Influence of Power Supply Method on Electroplating Silver During the electroplating process, the silver layer was first deposited onto the fibers near the During the extended electroplating thedirection silver layer was firstIn deposited ontothe thenewly fibersdeposited near the clamp and then alongprocess, the radial of the fibers. this process, clamp and then extended along the radial direction of the fibers. In this process, the newly deposited silver was gradually coated onto the surface of the fibers and was closely combined with the metal silver gradually onto the surface of the which fibers and was the closely combined witharea the in metal silver was coated by the coated previous electroless plating, became effective cathode the silver coated by the previous electroless plating, which became the effective cathode area in the electroplating process. The new coating also reduced the fiber resistivity and increased the cathode electroplating process. new coating also reduced the fiber resistivity and increased area. In addition, due The to the smaller diameter and longer length of the polyester fiber, the thecathode specific area. In area addition, to was the smaller diameter in and longer length of the polyester fiber, the specific surface of thedue fiber larger, resulting a more complex change in the cathode area of the surface area of the fiber was larger,current resulting in a was moreused complex change in cathode area ofarea the fiber [62]. If the traditional constant method to electroplate thethe fibers, the cathode fiber [62]. If theand traditional constant current method usedwould to electroplate the which fibers,would the cathode would change, the cathode current density in thewas system be unstable, cause area would change, and the cathode current density in the system would be unstable, which would the surface of the fibers to fluctuate greatly, which is not conducive to the formation of high-quality cause the As surface the fibers tocomparatively fluctuate greatly, which is notinconducive to the formation of in highcoatings. such of conditions are easier to control the constant voltage method, this quality coatings. As such conditions are comparatively easier to control in the constant voltage study, we used a constant voltage power supply method to control the fiber electroplating experiment.
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3.2.2. Influence of Control Voltage Voltage on onElectroplating ElectroplatingSilver Silver For the same electroplating time of 4 min, the influence of the plating voltage on fiber resistivity and the rate of weight weight gain is shown shown in Figure Figure 3. 3. With the the increase increase in in the the electroplating electroplating voltage, voltage, the fiber rate of weight gain always increased, while the resistivity of the fiber gradually gradually decreased, and after the voltage rose to to above above 1.5 1.5 V, V, the theresistivity resistivity did didnot notsubstantially substantially decrease. decrease. This This is is because because with accelerated, with the the increase increase in in voltage, voltage, the deposition rate of silver particles on the fiber surface was accelerated, resulting in the increased rate of weight gain of the fiber, fiber, and and the the silver silver particles particles deposited deposited on the fiber fiber surface surface were were closely closely combined combined with the existing silver layer, layer, resulting resulting in in the decrease decrease of the fiber resistivity. resistivity. When When the the voltage voltage continues continues to to increase, increase, the the deposition deposition rate rate of of silver particles will continue to increase; too fast a deposition deposition rate will easily lead to the the accumulation accumulation of particles on the silver layer, slow decline decline in in resistivity. resistivity. However, However, layer, which which makes makes the surface rough and uneven, resulting in a slow if the voltage is too high, it is easy to "scorch" the fibers. When the plating voltage reaches 2.6 V, the “scorch” “scorch” phenomenon phenomenon occurs occurs in in the the fiber fiber coating. coating. When the voltage is lower, the the deposition deposition rate of silver and thethe silver plating efficiency is also Therefore, the bestthe control silver particles particlesisisslow, slow, and silver plating efficiency is lower. also lower. Therefore, best voltage control range is range 1.5–2.0isV.1.5–2.0 V. voltage
Figure 3. 3. Influence of voltage voltage on on the the resistivity resistivity and and the the rate rate of of weight weight gain. gain. Figure Influence of
3.2.3. 3.2.3. Influence Influence of of Electroplating Electroplating Time Time on on Electroplating ElectroplatingSilver Silver Figure the fiber fiber for for different different Figure 44 presents presents the the SEM SEM images images of of the the silver-plated silver-plated layer layer on on the the surface surface of of the plating times. The silver particles were first uniformly deposited on the surface of the fiber forma plating times. The silver particles were first uniformly deposited on the surface of the fiber totoform acontinuous continuousdense densesilver silvercoating coating(Figure (Figure4b). 4b).With Withprolonged prolonged electroplating electroplating time, time, the the deposition deposition of of silver silver particles particles increased increased and and the the coating coating on on the the surface surface of of the the fiber fiber became became thicker. thicker. However, However, with with too too long long aa time, time, the the deposited deposited silver silver particles particles tended tended to to accumulate accumulate in in certain certain parts parts of of the the coating coating and and caused caused the the coating coating to to become become rougher rougher (Figure (Figure 4c). 4c).
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Figure 4. SEM images of polyester fibers for different electroplating time: (a) 0 min; (b) 4 min; (c) 8 min. Figure 4. SEM images of polyester for different electroplating time: (a) 0(b)min; (b) (c) 4 min; Figure 4. SEM images of polyester fibersfibers for different electroplating time: (a) 0 min; 4 min; 8 min. (c) 8 min.
The effects effects of different different plating plating times times on on the the resistivity resistivity and and the the rate rate of weight gain of the polyester The The effects of different plating times on the resistivity and the rate weight gain of thein polyester results are shown Figure fibers were investigated under the same plating voltage (1.6 V). The The of results shown in Figure 5. 5. fibers were investigated under the same plating voltage (1.6 V). The results are shown in Figure 5. With prolonged electroplating time, the rate of weight gain of the fiber increased constantly, while With prolonged electroplating while prolonged electroplating time, the rate of weightremained gain of the fiber increased constantly, continuously. theWith resistivity decreased continuously. The resistivity basically unchanged after 4while min of the resistivity decreased continuously. The resistivity remained basically unchanged if after 4coating min of is the electroplating. Different Different plating platingtimes timescan canproduce producedifferent differentcoating coatingthicknesses, thicknesses,but but if the coating electroplating. Different plating times can produce different coating thicknesses, but if the coating is too thick, the flexibility of the fibers will deteriorate, so in this work, the electroplating time was is too thick, the flexibility of the fibers will deteriorate, so in this work, too thick, the flexibility of the fibers will deteriorate, so in this work, the electroplating time was controlled at at 44 min. min. controlled controlled at 4 min.
Figure 5. Influence of electroplating time on resistivity resistivity and and the therate rateof ofweight weightgain. gain. Figure 5. Influence of electroplating time on resistivity and the rate of weight gain.
Silver Composition Analysis 3.3. Silver Composition Analysis 3.3.3.3. Silver Composition Analysis Figure 6 shows the infraredspectra spectra ofthe the polyester fibers fibers before and after silver plating. It It can Figure Figure 66 shows shows the the infrared infrared spectra of of the polyester polyester fibers before before and and after after silver silver plating. plating. It can can be seen that the positions of the absorption peaks of the polyester fibers did not change before or after be seen that thatthe thepositions positionsofofthe the absorption peaks of the polyester fibers not change before or be seen absorption peaks of the polyester fibers did did not change before or after silver plating, which indicates that the basic structure of the polyester fiber is not affected by silver after plating, indicates that the basic structure the polyester not affected by silversilver plating, whichwhich indicates that the basic structure of theof polyester fiber isfiber not is affected by silver plating. Meanwhile, the intensity of the infrared absorption peaks of the silver-plated fiber became silver plating. Meanwhile, the intensity of the infrared absorption peaks of the silver-plated fiber plating. Meanwhile, the intensity of the infrared absorption peaks of the silver-plated fiber became weaker as the surface of the fiber was coated with a dense silver layer, which reflects part of the became weaker as the surface of thewas fiber was coated a dense silver which reflects of weaker as of the the fiber with intensity awith dense silver layer,layer, reflects partpart of the infrared the lightsurface and weakens infraredcoated absorption of the fiber. Inwhich addition, no other new the infrared light weakens infrared absorption intensityofofthe the fiber.In In addition, no no other other new infrared light andand weakens thethe infrared new absorption peaks were observed on the absorption spectrogram,intensity indicating that fiber. there wasaddition, no other impurity on absorption peaks were observed on the spectrogram, indicating that there was no other impurity on absorption peaks were observed on the spectrogram, indicating that there was no other impurity on the surface of the fiber. the the surface surface of of the the fiber. fiber.
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Figure 6. FTIR fibers (a) (a) before before and and (b) (b) after after silver silver plating. plating. Figure 6. FTIR spectra spectra of of the the polyester polyester fibers
The XRD patterns of the polyester polyester fiber before and and after after silver silver plating plating are are shown shown in in Figure Figure 7. 7. Figure 7a demonstrates that the polyester fibers before silver plating had three diffraction peaks of ◦ , respectively, polyester at 26°, andand hadhad no other crystal diffraction peaks.peaks. FigureFigure 7b shows at 18°, 18◦ ,22°, 22◦and , and 26respectively, no other crystal diffraction 7b ◦ ◦ ◦ ◦ ◦ shows thatwere there were diffraction strong diffraction at 38 44 , and 64 ,82°, 77 ,which and 82 , which corresponded that there strong peaks atpeaks 38°, 44°, 64°,, 77°, corresponded to the 111, to the 111, 200, 220, andplane 222 crystal planepeaks diffraction of the elemental silverrespectively. crystallites, 200, 220, 311, and 222311, crystal diffraction of the peaks elemental silver crystallites, respectively. Thepeaks diffraction of the elemental silver were sharp and narrow, andwere thereno were no The diffraction of thepeaks elemental silver were sharp and narrow, and there other other diffraction indicating that the polyester fiber surface was coated with metallic with diffraction peaks,peaks, indicating that the polyester fiber surface was coated with metallic silversilver with good good crystallinity andpurity high purity [63]. The characteristic diffraction peaks of the polyester fibers in crystallinity and high [63]. The characteristic diffraction peaks of the polyester fibers in Figure Figure were obviously weakened, also indicated that the layer silveron layer thesurface fiber surface was 7b were7bobviously weakened, which which also indicated that the silver theon fiber was more uniform and denser. more uniform and denser.
Figure 7. 7. XRD fiber (a) (a) before before and and (b) (b) after after silver silver plating. plating. Figure XRD patterns patterns of of the the polyester polyester fiber
3.4. Mechanical Properties Properties Analysis Analysis 3.4. Mechanical The properties of of the the polyester polyester fibers fibers at at different different processing stages were were tested tested by by The mechanical mechanical properties processing stages the Electronic Single Yarn Strength Tester (YG020B, Changzhou No. 2 Textile Machine Co., Ltd., the Electronic Single Yarn Strength Tester (YG020B, Changzhou No. 2 Textile Machine Co., Ltd., Changzhou, China). Each group of samples was tested ten times and averaged. The results were shown in Table 1.
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Changzhou, China). Each group of samples was tested ten times and averaged. The results were shown in Table Materials 2018, 11,1.x FOR PEER REVIEW 10 of 16 Table 1. 1.Comparison ofdifferent differentfibers. fibers. Table Comparisonofofthe themechanical mechanical properties properties of Sample
Sample
Original polyester fiber Original polyester fiber Coarsening treated polyester fiber Electroless silver-plated Coarsening treatedpolyester polyesterfiber fiber Electroplated polyester fiber Electrolesssilver-coated silver-plated polyester fiber
Electroplated silver-coated polyester fiber
Breaking Strength (CN)(CN) Breaking Strength
Elongation at Break (%) Elongation at Break
1041.81 1041.81 911.23 934.25 911.23 950.31 934.25
(%) 23.57 23.57 16.35 18.81 16.35 19.24 18.81
950.31
19.24
The results show that the fiber breaking strength and the elongation at break of the coarsened Thefiber results show that the breaking strength respectively, and the elongation break of the coarsened polyester were reduced byfiber 12.48% and 30.63%, when at compared to those of the polyester fiber were reduced by 12.48% and 30.63%, respectively, when compared to those of the original polyester fiber. The breaking strength and elongation at break of the fibers after electroless original polyester breaking elongation at break of the fibers after respectively. electroless silver plating were fiber. 2.53%The and 15.78%,strength higherand than those before electroless plating, silver plating were 2.53% and 15.78%, higher than those before electroless plating, respectively. After After electroplating, they further increased by 1.72% and 2.29%, respectively. This was because electroplating, they further increased by 1.72% and 2.29%, respectively. This was because the fiber the fiber was hydrolyzed with a concentrated alkali at a high temperature during coarsening, and its was hydrolyzed with a concentrated alkali at a high temperature during coarsening, and its surface surface was damaged by etching. After calculation, the mass loss rate after coarsening was about 9.8%, was damaged by etching. After calculation, the mass loss rate after coarsening was about 9.8%, which which led to a decrease in the mechanical property. After the electroless silver-plating, the surface of led to a decrease in the mechanical property. After the electroless silver-plating, the surface of the thefiber fiberwas wascovered covered with a dense silver layer. Furthermore, the metallic had good ductility. with a dense silver layer. Furthermore, the metallic silversilver had good ductility. So, So,the thesilver silverlayer layer can share part of the extra tensile force for the fiber when stretched, resulting can share part of the extra tensile force for the fiber when stretched, resulting in the in the increase of breaking the breaking strength the silver-plated fiber.electroplated The electroplated silver increase of the strength of theofsilver-plated fiber. The metallicmetallic silver layer layer completely covered the fibers and slightly enhanced the mechanical properties of the fibers. completely covered the fibers and slightly enhanced the mechanical properties of the fibers. However, due to to the thinner an increase increasein inthe thebreaking breakingstrength strength However, due the thinnersilver silverlayer, layer,there therewas was less less of of an ofof thethe silver-plated fiber. silver-plated fiber. 3.5.3.5. Washing Fastness Analysis Washing Fastness Analysis The silver layers silver-coatedpolyester polyesterfibers fibershave have a great The silver layersononthe theelectroless electrolessor orelectroplated electroplated silver-coated a great likelihood of of being removed isnecessary necessarytotoexamine examinethe the washing likelihood being removedby bythe theact actof ofwashing. washing. Therefore, Therefore, itit is washing fastness. The resistivity changesand andthe theSEM SEM images images of the silver-coated fastness. The resistivity changes theelectroless electrolessand andelectroplated electroplated silver-coated polyester fibers shown Figures8 8and and9,9,respectively. respectively. polyester fibers areare shown ininFigures
Figure 8. Influence washingtimes timeson onthe theresistivity resistivity of of different different fibers: Figure 8. Influence ofof washing fibers:(a) (a)electroless electrolesssilver-plated silver-plated fiber; (b) electroplated silver-plated fiber. fiber; (b) electroplated silver-plated fiber.
In Figure 8, the resistivities of the electroless silver-plated fiber and electroplated silver-plated fiber increased with the increase of washing times, but the increase of resistivity of the electroplated silver-plated fiber was slower than that of the electroless silver-plated fiber, indicating that the electroplated silver-plated fiber had better surface fastness and washability. This was due to the rapid
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In Figure 8, the resistivities of the electroless silver-plated fiber and electroplated silver-plated fiber increased with the increase of washing times, but the increase of resistivity of the electroplated silver-plated fiber was slower than that of the electroless silver-plated fiber, indicating that the Materials 2018, 11, x FOR PEER REVIEW 11 of 16 electroplated silver-plated fiber had better surface fastness and washability. This was due to the rapid deposition of silver particles on the silver surface electrolesssilver-plated silver-platedfiber fiber during during the deposition of silver particles on the silver surface of of thethe electroless the electroplating silver layer, layer, which electroplating process, process, resulting resulting in in aa thicker thicker and and denser denser silver which improved improved the the washability washability of of the the electroplated electroplated silver-plated silver-plated fiber. fiber. Figure 9 shows the SEM images of the electroless silver-plated and the electroplated Figure 9 shows the SEM images of the electroless silver-plated fibersfibers and the electroplated silversilver-plated fibers after washing 30 times. After washing, the silver layer on the of the plated fibers after washing 30 times. After washing, the silver layer on the surface of surface the electroless electroless silver-plated fiber severely peeled off, while only some loose silver particles peeled off silver-plated fiber severely peeled off, while only some loose silver particles peeled off from the from theofsurface of the electroplated silver-plated fiber, thebody mainofbody of the layer silver was layerinwas in surface the electroplated silver-plated fiber, and theand main the silver good good condition. This may be because the shorter time of continuous electroless plating silver caused condition. This may be because the shorter time of continuous electroless plating silver caused a athinner thinnercoating coatinglayer layerand anda aweaker weakerbinding bindingforce forcebetween betweenthe thesilver silverlayer layer and and the the fiber, fiber, while while the the electroplating increased the the thickness electroplating after after continuous continuous electroless electroless plating plating significantly significantly increased thickness of of the the silver silver layer, layer, and and the the binding binding force force between between the the silver silver layer layerand andthe thefiber fiberwas wasalso alsoobviously obviouslyenhanced. enhanced.
9. SEM images of conductive conductive polyester fibers after being washed washed for 30 30 times: times: (a) electroless electroless Figure 9. silver-plated fibers; fibers; (b) (b) electroplated electroplated silver-plated silver-plated fibers. fibers.
3.6. Conductive Properties 3.6. Conductive Properties Analysis Analysis Figure Figure 10b 10b shows shows the the polyester polyester fiber fiber prepared prepared by by using using the the self-made self-made continuous continuous silver silver plating plating equipment. The winding bobbin made by the continuous silver plating equipment was well-formed, equipment. The winding bobbin made by the continuous silver plating equipment was well-formed, and We measured and the the silver-plated silver-plated fiber fiber had had aa silver-white silver-white metallic metallic luster. luster. We measured the the resistivity resistivity of of the the polyester fibers before and after silver-plating. The resistivities of the original polyester fiber, electroless polyester fibers before and after silver-plating. The resistivities of the original polyester fiber, 10 Ω ·cm, 1.5 × 10−2 Ω ·cm, and silver-plated fiber, and electroplated silver-platedsilver-plated fiber were 9.5fiber × 10were electroless silver-plated fiber, and electroplated 9.5 × 1010 Ω·cm, 1.5 × 10−2 − 4 2.3 × 10 respectively. The conductivity of the polyester was significantly improved −4 Ω·cm, respectively. Ω·cm, and Ω 2.3·cm, × 10 The conductivity of the fiber polyester fiber was significantly by silver-plating. improved by silver-plating.
Figure 10. Photographs of winding bobbins of polyester fibers: (a) before silver plating; (b) after silver
equipment. The winding bobbin made by the continuous silver plating equipment was well-formed, and the silver-plated fiber had a silver-white metallic luster. We measured the resistivity of the polyester fibers before and after silver-plating. The resistivities of the original polyester fiber, electroless silver-plated fiber, and electroplated silver-plated fiber were 9.5 × 1010 Ω·cm, 1.5 × 10−2 Ω·cm, and 2.3 × 10−4 Ω·cm, respectively. The conductivity of the polyester fiber was significantly Materials 2018, 11, 2033 12 of 16 improved by silver-plating.
Figure 10. Photographs of of polyester fibers: (a) before silver plating; (b) after Figure Photographsofofwinding windingbobbins bobbins polyester fibers: (a) before silver plating; (b) silver after plating. silver plating. Materials 2018, 11, x FOR PEER REVIEW 12 of 16
In As shown In this this study, study, the the luminescence luminescence experiment experiment of of aa small small bulb bulb was was also also conducted. conducted. As shown in in Figure 11, the two ends of the silver-plated fiber were connected to two wires and fixed on insulating Figure 11, the two ends of the silver-plated fiber were connected to two wires and fixed on insulating paper with an an insulating insulatingtape. tape.After Afterthe thewires wires connected LED bulb to the power supply (3 the V), paper with connected thethe LED bulb to the power supply (3 V), the small bulb emitted an obvious bright light and the brightness was constant and stable for 10 min, small bulb emitted an obvious bright light and the brightness was constant and stable for 10 min, which which indicated indicated that that the the prepared prepared fiber fiber surface surface was was uniformly uniformly and and continuously continuously covered covered with with the the silver plating layer, and the fiber could conduct electrons and had good electrical conductivity. silver plating layer, and the fiber could conduct electrons and had good electrical conductivity.
Figure 11. The luminescence experiment of a small bulb.
4. Conclusions 4. Conclusions In In this this study, study, aa continuous continuous two-step two-step silver silver plating plating method method was was developed developed with with polyester polyester fibers fibers used as the substrate material, which combined the operations of continuous electroless platingplating under used as the substrate material, which combined the operations of continuous electroless aunder dynamic condition and continuous electroplating. Furthermore, we designed equipment a dynamic condition and continuous electroplating. Furthermore, wespecialized designed specialized for the continuous plating of silver on of thesilver polyester fibers. According the machine conditions, equipment for the continuous plating on the polyester fibers. to According to the machine the pretreatments, electroless silver-plating, and electroplating silver plating process were improved conditions, the pretreatments, electroless silver-plating, and electroplating silver plating process
were improved accordingly, and the silver-plated conductive fibers were successfully prepared. The influence of the power supply method, control voltage, and electroplating time on electroplating silver was studied. The optimal conditions for electroplating silver conductive polyester fibers should include the power supply method having a constant voltage power, the best control voltage range of which is 1.5–2.0 V, and an electroplating time of 4 min. The Ag-coated polyester fibers were
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accordingly, and the silver-plated conductive fibers were successfully prepared. The influence of the power supply method, control voltage, and electroplating time on electroplating silver was studied. The optimal conditions for electroplating silver conductive polyester fibers should include the power supply method having a constant voltage power, the best control voltage range of which is 1.5–2.0 V, and an electroplating time of 4 min. The Ag-coated polyester fibers were characterized by SEM, FTIR, and XRD. Moreover, the mechanical properties and washing fastness of the electroplating silver-plating fibers were compared with the electroless silver-plating fibers. The results demonstrated that after the continuous two-step silver plating, the surface coating of the fiber was obviously thickened, and the surface silver particles were denser and continuous, with better mechanical properties and washability. The electrical resistivity reached 2.3 × 10−4 Ω·cm, and the conductivity was obviously improved. The luminescence experiment of a small bulb also showed that the conductive fiber prepared by the continuous two-step silver plating method had good electrical conductivity. Author Contributions: Conceptualization, C.L. and X.L.; methodology, C.L., X.L. and X.L.; software, C.L. and X.L.; validation, C.L., C.S. and T.X.; formal analysis, C.L. and T.X.; investigation, C.L. and C.S.; resources, C.L.; data curation, C.L., X.L. and X.L.; writing—original draft preparation, C.L. and X.L.; writing—review and editing, C.L. and X.L.; visualization, C.L.; supervision, K.O. and Z.G.; project administration, X.L.; funding acquisition, X.L. and Z.G. Acknowledgments: This research was funded by the China Postdoctoral Science Foundation (2017M611696), the Postdoctoral Science Foundation of Jiangsu Province (1701012B), the Jiangsu Province Higher Vocational Education High-level Key Professional Construction Project (2017SGJ-No. 17-80), the Program of Study Abroad for Young Scholars sponsored by the Jiangsu Education Department, and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015B178). Conflicts of Interest: The authors declare no conflict of interest.
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