Preparation of Conductive Polyester Fibers Using Continuous Two-Step Plating Silver

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 Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan; [email protected] 2 College of Chemistry and Materials Engineering, Changzhou Vocational Institute of Engineering, Changzhou 213164, China; [email protected] 3 College of Textile and Clothing, Jiangnan University, Wuxi 214122, China; [email protected] (X.L.); [email protected] (C.S.) 4 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

Materials 2018, 11, 2033; doi:10.3390/ma11102033 www.mdpi.com/journal/materials Materials 2018, 11, 2033 2 of 16 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 (SnCl2·2H2O, ≥98.0%), silver nitrate (AgNO3, ≥99.8%), aqueous ammonia (NH3·H2O, 25.0–28.0%), glucose (C6H12O6, 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 Materials 2018, 11, 2033 3 of 16 tanks,Materials as shown2018, 11, inx FOR Scheme PEER REVIEW1. Thewhole process was operated under a dynamic condition. The3 fiberof 16 windingMaterials collecting 2018, 11 device, x FOR PEER at the REVIEW end of the equipment provided the drafting force. The winding3 of 16 speed waswas controlled controlled by by the the control control panel,panel, and the reaction reaction time time of of the the fibers fibers in ineach each plating plating tank tank could could be controlledwas controlled by different by the winding control panel, speeds and and the reactionthe numb timeer ofof the windings. fibers in eachThe platingsize of tank the couldgroove be in the be controlledcontrolled by by different differentwinding winding speeds speeds and and the the numb numberer of windings. of windings. The size The of the size groove of the in groovethe in equipment was 13.5 cm × 8.5 cm × 14.5 cm. The lining and the drum in the tank were made of the equipmentequipment was was 13.5 13.5 cm cm× × 8.5 cm ×× 14.514.5 cm. cm. The The lining lining and and the thedrum drum in the in tank the were tank made were of made of polytetrafluoroethylene (PTFE). polytetrafluoroethylenepolytetrafluoroethylene (PTFE). (PTFE).

SchemeScheme 1. The 1. equipment The equipment for for the the continuous continuous electrolesselectroless pl platingating of ofsilver silver under under a dynamic a dynamic condition. condition. (1) return wire rack; (2) polyester fiber (3) cleaning tank; (4) coarsening tank; (6) sensitizing tank; (8) (1)Scheme return wire1. The rack; equipment (2) polyester for the continuous fiber (3) cleaning electroless tank; plating (4) coarsening of silver under tank; a (6)dynamic sensitizing condition. tank; electroless silver-plating tank; (10) drying tank; (11) fibers winding collecting device; (12) control (8)(1) electroless return wire silver-plating rack; (2) polyester tank; (10) fiber drying (3) cleaning tank; (11) tank; fibers (4) coarseni windingng collecting tank; (6) sensitizing device; (12) tank; control (8) panel; (13) heating device; (5), (7), and (9) washing tank. panel;electroless (13) heating silver-plating device; (5),tank; (7), (10) and drying (9) washing tank; (11) tank. fibers winding collecting device; (12) control panel;Polyester (13) heating fibers device; were cleaned(5), (7), andusing (9) a washingmixed soluti tank.on of NaOH and lauryl sodium sulfate (LSS), 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 solution. The sensitizing of polymeric fibers was conducted using a Polyestera SnCl2 solution fibers towere make cleaned a Sn2(OH) using3Cl agel-layer mixed solution theon surface of NaOH of the and fibers. lauryl Then, sodium the sensitizing sulfate (LSS), SnClfollowed2 solutionfibers by were coarsening to put make into a the Snin 2 solutionNaOH(OH)3Cl solution.of gel-layerTollens’ The reagent on sensitizing the and surface glucose of of polymericfor the the fibers. silver fibers Then,deposition. was the conducted sensitizing using fibers werea SnCl put2 intosolution the solution to make of a Tollens’Sn2(OH) reagent3Cl gel-layer and glucose on the forsurface the silverof the deposition. 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. DepositionThe ofpolyester Silver by fibers Continuous with silver Electroplating deposition of electroless plating were connected with a negative 2.3.The Depositionwire polyester for electroplating, of Silver fibers by with Continuous as shown silver in deposition ElectroplatingScheme 2. ofIn detail, electroless a pure plating silver plat weree with connected the dimension with of a 80 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 Scheme2. In detail, a pure silver plate with the dimension of The1 V topolyester 10 V was fibersused as with an energy silver resource.deposition The of polyester electroless fibers, plating after being were coated connected with a with thin layera negative 80wire mm forof× silver40 electroplating, mm by ×electroless5 mm aswas plating, shown used were in as Scheme andirectly anode 2.used In material detail,for electroplating. a and pure a directsilver The currentplatelectroplatinge with with the solution a dimension voltage was ranging of 80 frommm 1 ×a V40 mixture to mm 10 × Vwith 5 wasmm a pH was used of usedabout as an as 9, energypreparedan anode resource. by material using 120 Theand g/L a polyester of direct sulfosalicylic current fibers, acid,with after 10 a g/Lvoltage being of potassium coatedranging with from a thin1 V layer tohydroxide, 10 ofV silverwas 30 used g/L by electrolessasof silveran energy nitrate, plating, resource. 50 g/L wereof ammoniumThe directly polyester acetate, used fibers, forand after electroplating.60 mL/L being of aqueouscoated The with ammonia. electroplating a thin layer solutionof silver was by aelectroless mixture with plating, a pH were of about directly 9, preparedused for electroplating. by using 120 g/L The of electroplating sulfosalicylic solution acid, 10 was g/L ofa potassium mixture with hydroxide, a pH of about 30 g/L 9, prepared of silver by nitrate, using 50120 g/L g/L of sulfosalicylic ammonium acetate,acid, 10 g/L and of 60 potassium mL/L of aqueoushydroxide, ammonia. 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.

SchemeScheme 2. 2.The The equipment equipment for for electroplatingelectroplating silversilver onon polyester fibers. 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 Figure1b. 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. Figure1c 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. Materials 2018, 11, 2033 5 of 16 Materials 2018, 11, x FOR PEER REVIEW 5 of 16

Figure 1. SEM images of po polyesterlyester fibers: ﬁbers: ( a)) before before any any treatment; ( b) after cleaning with NaOH and

lauryl sodium sulfate (LSS); (c) after coarsening with NaOH;NaOH; andand ((d)) afterafter sensitizingsensitizing withwith SnClSnCl22.

The next step of pretreatment waswas sensitizing.sensitizing. InIn thisthis work,work, aa mixedmixed solutionsolution ofof 3030 g/Lg/L of SnClSnCl22 and 40 mL/L of of hydrochloric hydrochloric acid acid was was used used as as the the sensitizing sensitizing solution. solution. After After the the fibers fibers were were immersed immersed in the sensitizingsensitizing solution, Sn2(OH)(OH)33ClCl thin thin films films with with a a slightly slightly soluble soluble gel gel were formed by the hydrolysis of stannous chloride during subsequent water washing [[61].61]. This gel-like substance was adsorbed on the fiber fiber surface to ensure the uniform adsorption of divalent tin ions onto the gel’sgel's surface, which facilitated the uniform deposition of of silver silver particles particles during during electroless electroless silver silver plating. plating. Therefore, sensitization mainly occurs in the water washing process, and the reaction process is shown in Equations (1) and (2). Sn(OH)Cl and Sn(OH) 2 combinecombine to form form a slightly slightly soluble gelatinous substance, Sn2(OH)(OH)33Cl.Cl. SnCl2 + 2H2O → Sn(OH)2 + HCl (1) SnCl2 + 2H2O → Sn(OH)2 + HCl (1)

SnCl2 + 2H2O → Sn(OH)Cl + HCl (2) SnCl2 + 2H2O → Sn(OH)Cl + HCl (2) Figure1d shows the SEM image of the sensitized polyester fiber surface, showing that the polyesterFigure fiber 1d surfaceshows wasthe SEM obviously image covered of the withsensitiz a moreed polyester uniform andfiber continuous surface, showing gel-like film.that the 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+ 2+ In the electroless plating solution, Ag(NH3)2 is first reduced by Sn adsorbed on the surface of the fiberIn the to electroless form metal plating Ag particles, solution, the Ag(NH reaction3) formula2+ is first ofreduced which isby shown Sn2+ adsorbed in Equation on the (3). surface Thus, the of Agthe fiber particles to form are metal deposited Ag particles, on the surface the reaction of the fo fiberrmula where of which the stannousis shown ionin Equation is dispersed. (3). Thus, The the Ag particlesAg particles have are strong deposited catalytic on activitythe surface and becomeof the fiber the catalyticwhere the center stannous of the ion electroless is dispersed. silver The plating Ag + particlesreaction. have Similarly, strong [Ag(NH catalytic3)2 ]activityin the and silver become ammonia the catalytic solution center is reduced of the to electroless metal Ag particlessilver plating with reaction.glucose, andSimilarly, deposited [Ag(NH on3 the)2]+ surfacein the silver of the ammonia fiber around solution the is catalytic reduced center to metal to Ag obtain particles a metallic with glucose,silver layer, and thedeposited reaction on formula the surface of which of the is fiber shown arou innd Equation the catalytic (4). Therefore,center to obtain in this a metallic non-activated silver layer,direct the electroless reaction silver formula plating of which process, is shown the sensitized-washed in Equation (4). fiberTherefore, will first in this form non-activated Ag particles ondirect the electrolesssurface of thesilver fiber plating as the process, catalytic the center sensitized-w of the wholeashed reaction, fiber will so thatfirst theform surface Ag particles of the fiber on hasthe surfacecatalytic of activity. the fiber as the catalytic center of the whole reaction, so that the surface of the fiber has + 2+ 4+ catalytic activity. 2[Ag(NH3)2] + Sn → Sn + 2Ag↓ + 4NH3 (3) + 2+→ 4+ C6H12O6 + 2[Ag(NH2[Ag(NH3)23]OH)2] + →SnRCOONH Sn + 42Ag+ 3NH↓ + 4NH3 + 2Ag3 ↓ + H2O(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 efficiency,Continuous which silver is significantly plating on fibersdifferent under from a dynamic that under condition a static can facilitatecondition. deposition Under the efficiency, same pretreatmentwhich is significantly conditions, different we performed from that silver under plating a static contrast condition. experiments Under in thea static same beaker pretreatment under a dynamicconditions, condition we performed and obtained silver platingthe contrast contrast chart experiments of the deposition in a static rate beakerof silver under particles a dynamic under dynamiccondition and and static obtained conditions the contrast at different chart ofplating the deposition times (Figure rate 2). of silverThis demonstrated particles under that dynamic the weight and gainstatic of conditions the fibers at under different the platingstatic condition times (Figure was2 ).obviously This demonstrated lower than that that the under weight the gain dynamic of the condition,fibers under and the the static deposition condition rate was became obviously lower lower and thanlower that with under the theincrease dynamic of plating condition, time, and while the thedeposition deposition rate rate became of silver lower particles and lower under with the the dynamic increase condition of plating was time, higher while and the had deposition less influence rate of onsilver time. particles The self-made under the test dynamic equipment condition allowed was higherthe fibers and hadto fully less make influence contact on time. with The the self-made reaction liquidstest equipment under the allowed dynamic the transmission, fibers to fully thus make achiev contactingwith a better the reactionsilver plating liquids effect under than the under dynamic the statictransmission, condition, thus which achieving may be a because better silver the fibers plating mainly effect reacted than under with thethe staticsilver condition,ammonia ion which around may them.be because When the the fibers silver mainly was plated reacted under with a thestatic silver condition, ammonia the ion concentration around them. of silver When ammonia the silver ions was nearplated the under fibers adecreased static condition, with the the progress concentration of the reac oftion, silver soammonia the deposition ions nearrate of the the fibers silver decreased particles decreasedwith the progress continuously. of the reaction,However, so the the dynamic deposition transmission rate of the condition silver particles facilitated decreased the increase continuously. in the diffusionHowever, rate the of dynamic silver ammonia transmission ions conditiononto the surfac facilitatede of the the fibers, increase so inthe the deposition diffusion rate rate of of silver silver particlesammonia had ions little onto change. the surface of the fibers, so the deposition rate of silver particles had little change.

FigureFigure 2. 2. DepositionDeposition rate rate of of silver silver particles particles under under (a) (a) dynamic, dynamic, and and (b) (b) static static conditions. 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 clampDuring and then the extendedelectroplating along process, the radial the direction silver la ofyer the was fibers. first Indeposited this process, onto the the newly fibers deposited near the clampsilver wasand then gradually extended coated along onto the the radial surface direction of the of fibers the fibers. and was In this closely process, combined the newly with deposited the metal silversilver was coated gradually by the coated previous onto electroless the surface plating, of the whichfibers and became was theclosely effective combined cathode with area the inmetal the silverelectroplating coated by process. the previous The new electroless coating also plating, reduced which the became fiber resistivity the effective and increased cathode thearea cathode in the electroplatingarea. In addition, process. due The to the new smaller coating diameter also redu andced longer the fiber length resistivity of the polyesterand increased fiber, the the cathode specific area.surface In areaaddition, of the due fiber to wasthe smaller larger, resultingdiameter inand a morelonger complex length of change the polyester in the cathode fiber, the area specific of the surfacefiber [62 area]. If theof the traditional fiber was constant larger, current resulting method in a wasmore used complex to electroplate change in the the fibers, cathode the cathode area of areathe fiberwould [62]. change, If the andtraditional the cathode constant current current density method in the was system used would to electroplate be unstable, the whichfibers, wouldthe cathode cause areathe surfacewould ofchange, the fibers and tothe fluctuate cathode greatly,current whichdensity is in not the conducive system would to the be formation unstable, of which high-quality would causecoatings. the Assurface such of conditions the fibers are to comparatively fluctuate greatly, easier which to control is not in co thenducive constant to the voltage formation method, of inhigh- this qualitystudy, we coatings. used a constantAs such voltage conditio powerns are supply comparatively method to easier control to the control fiber electroplating in the constant experiment. voltage

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Materialsmethod,2018 in, 11this, 2033 study, we used a constant voltage power supply method to control the 7fiber of 16 electroplating experiment.

3.2.2. InfluenceInfluence of Control Voltage onon ElectroplatingElectroplating SilverSilver For the same electroplating time of 4 min, the influenceinfluence of the plating voltage on fiber fiber resistivity and the rate of weightweight gain is shownshown in FigureFigure3 3.. WithWith thethe increaseincrease inin the the electroplating electroplating voltage,voltage, thethe fiberfiber rate of weight gain always increased, while the resistivity of the fiberfiber graduallygradually decreased, and after the voltage rose toto aboveabove 1.51.5 V,V, thethe resistivityresistivity diddid notnot substantiallysubstantially decrease.decrease. ThisThis isis becausebecause withwith the increase in voltage, the deposition rate of silver particles on the fiber fiber surface was accelerated, accelerated, resulting in the increased rate of weightweight gain of the fiber,fiber, andand thethe silversilver particlesparticles depositeddeposited on thethe fiberfiber surface were closely combined with the existing silver layer, resulting in the decrease of the fiberfiber resistivity. WhenWhen thethe voltagevoltage continuescontinues toto increase,increase, thethe depositiondeposition raterate ofof silver particles will continue toto increase;increase; too fast aa depositiondeposition rate will easilyeasily leadlead toto thethe accumulationaccumulation of particlesparticles on the silversilver layer,layer, whichwhich makesmakes thethe surface rough and uneven, resu resultinglting in a a slow slow decline decline in in resistivity. resistivity. However, However, ifif thethe voltagevoltage isis tootoo high,high, itit isis easyeasy toto "scorch""scorch" thethe fibers.fibers. When the plating voltage reaches 2.6 V, thethe “scorch”“scorch” phenomenonphenomenon occursoccurs inin thethe fiberfiber coating.coating. WhenWhen the voltage is lower, thethe depositiondeposition rate of silver particlesparticles is is slow, slow, and and the the silver silver plating plating efficiency efficiency is also is lower.also lower. Therefore, Therefore, the best the control best voltagecontrol rangevoltage is range 1.5–2.0 is V. 1.5–2.0 V.

Figure 3. InfluenceInﬂuence of voltage on the resistivity andand thethe raterate ofof weightweight gain.gain.

3.2.3. InfluenceInfluence of Electroplating TimeTime onon ElectroplatingElectroplating SilverSilver Figure4 4 presents presents the the SEM SEM images images of of thethe silver-platedsilver-plated layer layer on on the the surface surface of of thethe fiberfiber forfor differentdifferent platingplating times. The silver silver particles particles were were first first unifor uniformlymly deposited deposited on on the the surface surface of of the the fiber fiber to to form form a acontinuous continuous dense dense silver silver coating coating (Figure (Figure 4b).4b). With prolonged electroplating time,time, thethe depositiondeposition ofof silver particles increased and the coating on the surface of the fiberfiber becamebecame thicker.thicker. However, withwith tootoo long a time, the deposited silver particles tended toto accumulate inin certaincertain partsparts ofof the coating and caused thethe coatingcoating toto becomebecome rougherrougher (Figure(Figure4 4c).c).

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Figure 4. SEM images of polyester fibers for different electroplating time: (a) 0 min; (b) 4 min; Figure(c) Figure8 min. 4. SEM4. SEM images images of polyester of polyester fibers fibers for different for different electroplating electroplating time: (time:a) 0 min; (a) 0 (b min;) 4 min; (b) (c4) min; 8 min. (c) 8 min. The effects of different platingplating timestimes onon the resistivityresistivity andand thethe rate of weight gain of the polyester fibersfibers wereThe effects investigated of different under plating the sametimes platingon the resistiv voltage ity (1.6 and V). the Therate ofresults weight are gain shown of the in polyester Figure 5 5.. WithWithfibers prolongedprolonged were investigated electroplatingelectroplating under time, the same the rate plating of weight voltage gain (1.6 of V). the The fiberfiber results increased are shown constantly, in Figure whilewhile 5. thetheWith resistivityresistivity prolonged decreaseddecreased electroplating continuously.continuously. time, the The rate resistivityresistivity of weight remainedremained gain of the basicallybasically fiber increased unchanged constantly, after 4 while minmin ofof the resistivity decreased continuously. The resistivity remained basically unchanged after 4 min of electroplating. Different Different plating plating ti timesmes can can produce produce different different coating coating thicknesses, thicknesses, but but if ifthe the coating coating is electroplating. Different plating times can produce different coating thicknesses, but if the coating is istoo too thick, thick, the the flexibility flexibility of of the the fibers fibers will will deterior deteriorate,ate, so so in in this this work, work, the electroplating time was too thick, the flexibility of the fibers will deteriorate, so in this work, the electroplating time was controlled atat 44 min.min. controlled at 4 min.

Figure 5. Influence of electroplating time on resistivity and the rate of weight gain. Figure 5. InfluenceInﬂuence of electroplating time on resistivityresistivity andand thethe raterate ofof weightweight gain.gain.

3.3.3.3. Silver Silver Composition Composition Analysis Analysis Figure 6 shows the infrared spectra of the polyester fibers before and after silver plating. It can Figure6 6 shows shows thethe infraredinfrared spectraspectra ofof thethe polyesterpolyester fibers fibers before before and and after after silver silver plating. plating. ItIt cancan be seen that the positions of the absorption peaks of the polyester fibers did not change before or after be seen that that the the positions positions of of the the absorption absorption peaks peaks of the of the polyester polyester fibers fibers did didnot change not change before before or after or silver plating, which indicates that the basic structure of the polyester fiber is not affected by silver aftersilver silver plating, plating, which which indicates indicates that the that basic the basicstructure structure of the ofpolyester the polyester fiber is fiber not isaffected not affected by silver by silverplating. plating. Meanwhile, Meanwhile, the intensity the intensity of the ofinfrared the infrared absorption absorption peaks of peaks the silver-plated of the silver-plated fiber became fiber plating.weaker Meanwhile, as the surface the intensityof the fiber of wasthe infraredcoated with absorption a dense peakssilver layer,of the whichsilver-plated reflects fiberpart ofbecame the became weaker as the surface of the fiber was coated with a dense silver layer, which reflects part of weakerinfrared as thelight surface and weakens of the thefiber infrared was coated absorption with intensitya dense silverof the fiber.layer, In which addition, reflects no other part newof the the infrared light and weakens the infrared absorption intensity of the fiber. In addition, no other new infraredabsorption light peaksand weakens were observed the infrared on the absorptionspectrogram, intensity indicating of thatthe fiber.there wasIn addition, no other impurityno other onnew absorption peaks were observed on the spectrogram, indicating that there was no other impurity on absorptionthe surface peaks of the were fiber. observed on the spectrogram, indicating that there was no other impurity on thethe surfacesurface ofof thethe fiber.fiber.

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Figure 6. FTIR spectra of the polyester fibers ﬁbers (a (a)) before and (b) after silver plating.

The XRD patterns of the polyeste polyesterr fiber fiber before and after silver plating plating are are shown shown in in Figure Figure 77.. Figure 77aa demonstratesdemonstrates thatthat thethe polyesterpolyester fibersfibers beforebefore silversilver platingplating hadhad threethree diffractiondiffraction peakspeaks ofof polyester at at 18°, 18◦ ,22°, 22◦ and, and 26°, 26 respectively,◦, respectively, and and had had no other no other crystal crystal diffraction diffraction peaks. peaks. Figure Figure 7b shows7b showsthat there that were there strong were diffraction strong diffraction peaks at peaks 38°, 44°, at 3864°,◦, 77°, 44◦, and 64◦ ,82°, 77◦ ,which and 82 corresponded◦, which corresponded to the 111, to200, the 220, 111, 311, 200, and 220, 222 311, crystal and plane 222 crystal diffraction plane peaks diffraction of the peakselemental of the silver elemental crystallites, silver respectively. crystallites, respectively.The diffraction The peaks diffraction of the peaks elemental of the elementalsilver were silver sharp were and sharp narrow, and narrow,and there and were there no were other no otherdiffraction diffraction peaks, peaks, indicating indicating that the that polyester the polyester fiber surface fiber surface was coated was coated with metallic with metallic silver silverwith good with goodcrystallinity crystallinity and high and purity high purity [63]. The [63 ].characteristic The characteristic diffraction diffraction peaks of peaks the polyester of the polyester fibers in fibers Figure in Figure7b were7b obviously were obviously weakened, weakened, which which also indicated also indicated that the that silver the silverlayer layeron the on fiber the surface fiber surface was more was moreuniform uniform and denser. and denser.

Figure 7. XRD patterns of the polyester fiber ﬁber (a) before and (b) after silver plating.

3.4. Mechanical Properties Analysis The mechanical properties of the polyester fibers ﬁbers at different processing stages were tested by the Electronic Single Yarn StrengthStrength TesterTester (YG020B,(YG020B, ChangzhouChangzhou No.No. 2 2 Textile 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 shownMaterials in Table2018, 111,. x FOR PEER REVIEW 10 of 16

TableTable 1. 1.Comparison Comparison of of the the mechanicalmechanical propertiesproperties of of different different fibers. ﬁbers.

Sample BreakingBreaking Strength Strength (CN) (CN) Elongation Elongation at Breakat Break (%) Sample Original polyester fiber 1041.81 23.57(%) CoarseningOriginal treated polyester polyester fiber fiber 911.23 1041.81 16.35 23.57 ElectrolessCoarsening silver-plated treated polyester polyester fiber fiber 934.25 911.23 18.81 16.35 ElectroplatedElectroless silver-coated silver-plated polyester polyester fiber fiber 950.31 934.25 19.24 18.81 Electroplated silver-coated polyester fiber 950.31 19.24 The results show that the fiber breaking strength and the elongation at break of the coarsened polyesterThe fiber results were show reduced that the by fiber 12.48% breaking and 30.63%,strength respectively,and the elonga whention at compared break of the to thosecoarsened of the originalpolyester polyester fiber were fiber. reduced The breaking by 12.48% strength and 30.63% and elongation, respectively, at break when of compared the fibers to after those electroless of the silveroriginal plating polyester were fiber. 2.53% The and breaking 15.78%, strength higher and than elongation those before at break electroless of the fibers plating, after respectively.electroless 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 fiber was was covered covered with with a dense a dense silver silver layer. layer. Furthermore, Furthermore, the metallic the metallic silver silver had good had goodductility. ductility. So, So,the the silver silver layer layer can can share share part part of the of extra the extra tensile tensile force forcefor the for fiber the when fiber stretched, when stretched, resulting resulting in the in theincrease increase of the of breaking the breaking strength strength of the of silver-pla the silver-platedted fiber. The fiber. electroplated The electroplated metallic metallic silver layer silver layercompletely completely covered covered the thefibers fibers and and slightly slightly enha enhancednced the the mechanical mechanical properties properties of ofthe the fibers. fibers. However,However, due due to to the the thinner thinner silver silver layer, layer, there there waswas lessless of an increase in in the the breaking breaking strength strength of ofthe the silver-platedsilver-plated fiber. fiber.

3.5.3.5. Washing Washing Fastness Fastness Analysis Analysis TheThe silver silver layers layers on on the the electroless electroless oror electroplatedelectroplated silver-coated polyes polyesterter fibers ﬁbers have have a great a great likelihoodlikelihood of of being being removed removed by by the the act act of of washing. washing. Therefore,Therefore, it it is is necessary necessary to to examine examine the the washing washing fastness.fastness. The The resistivity resistivity changes changes and and the the SEMSEM imagesimages of the the electroless electroless and and electroplated electroplated silver-coated silver-coated polyesterpolyester ﬁbers fibers are are shown shown in in Figures Figures8 and8 and9, 9, respectively. respectively.

FigureFigure 8. Influence8. Influence of of washing washing times times on on the the resistivity resistivity ofof different fibers: fibers: (a) (a) el electrolessectroless silver-plated silver-plated fiber;fiber; (b) (b) electroplated electroplated silver-plated silver-plated fiber. 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 Figure8, 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 rapiddeposition deposition of silver of silver particles particles on the on thesilver silver surface surface of ofthe the electrol electrolessess silver-plated silver-plated fiber fiber during during the electroplating process, resulting in a thicker and denserdenser silver layer, which improved the washability of the electroplated silver-platedsilver-plated fiber.fiber. Figure9 9 shows the the SEM SEM images images of ofthe the electroless electroless silver-plated silver-plated fibers fibers and the and electroplated the electroplated silversilver-platedplated fibers after fibers washing after washing 30 times. 30 After times. washing, After washing, the silver the layer silver on the layer surface on the of surfacethe electroless of the electrolesssilver-plated silver-plated fiber severely fiber peeled severely off, peeledwhile off,only while some only loose some silver loose particles silver peeled particles off peeled from the off fromsurface the of surface the electroplated of the electroplated silver-plated silver-plated fiber, an fiber,d the andmain the body main of body the silver of the layer silver was layer in was good in goodcondition. condition. This may This maybe because be because the shorter the shorter time time of continuous of continuous electroless electroless plating plating silver silver caused caused a athinner thinner coating coating layer layer and and a aweaker weaker binding binding force force be betweentween the the silver silver layer layer and and the the fiber, fiber, while the electroplating after continuous electroless plating significantlysignificantly increased the thickness of the silver layer, andand thethe bindingbinding forceforce betweenbetween thethe silversilver layerlayer andand thethe fiberfiber waswas alsoalso obviouslyobviously enhanced. enhanced.

Figure 9.9. SEM images of conductiveconductive polyester fibersfibers after being washedwashed for 3030 times:times: (a)) electrolesselectroless silver-plated fibers;fibers; ((bb)) electroplatedelectroplated silver-platedsilver-plated fibers.fibers.

3.6. Conductive Properties Analysis Figure 1010bb shows the polyester fiberfiber prepared byby usingusing thethe self-madeself-made continuouscontinuous silver plating equipment. The winding bobbin made by the continuous silver plating equipment was well-formed, and the silver-plated fiberfiber hadhad aa silver-whitesilver-white metallicmetallic luster.luster. We measured the resistivity of the polyester fibersfibers before before and and after after silver-plating. silver-plating. The resistivitiesThe resistivities of the originalof the polyesteroriginal polyester fiber, electroless fiber, 10 −2 silver-platedelectroless silver-plated fiber, and electroplated fiber, and electr silver-platedoplated silver-plated fiber were 9.5 fiber× 10 wereΩ ·9.5cm, × 1.5 10×10 Ω10·cm,Ω 1.5·cm, × and10−2 −4 2.3Ω·cm,× 10 and Ω2.3·cm, × 10 respectively.−4 Ω·cm, respectively. The conductivity The conductivity of the polyester of the fiberpolyester was significantlyfiber was significantly improved byimproved silver-plating. by silver-plating.

Figure 10. Photographs of winding bobbins of polyester fibers: (a) before silver plating; (b) after silver plating.

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deposition of silver particles on the silver surface of the electroless silver-plated fiber during the electroplating process, resulting in a thicker and denser silver layer, which improved the washability of the electroplated silver-plated fiber. Figure 9 shows the SEM images of the electroless silver-plated fibers and the electroplated silver- plated fibers after washing 30 times. After washing, the silver layer on the surface of the electroless silver-plated fiber severely peeled off, while only some loose silver particles peeled off from the surface of the electroplated silver-plated fiber, and the main body of the silver layer was in good condition. This may be because the shorter time of continuous electroless plating silver caused a thinner coating layer and a weaker binding force between the silver layer and the fiber, while the electroplating after continuous electroless plating significantly increased the thickness of the silver layer, and the binding force between the silver layer and the fiber was also obviously enhanced.

Figure 9. SEM images of conductive polyester fibers after being washed for 30 times: (a) electroless silver-plated fibers; (b) electroplated silver-plated fibers.

3.6. Conductive Properties Analysis Figure 10b shows the polyester fiber prepared by using the self-made continuous silver plating 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 −4 MaterialsΩ·cm, and2018, 112.3, 2033 × 10 Ω·cm, respectively. The conductivity of the polyester fiber was significantly12 of 16 improved by silver-plating.

FigureFigure 10. PhotographsPhotographs of of winding winding bobbins bobbins of ofpolyester polyester fibers: ﬁbers: (a) before (a) before silver silver plating; plating; (b) after (b) silver after Materialssilverplating. 2018 plating., 11 , x FOR PEER REVIEW 12 of 16

In this study, thethe luminescenceluminescence experimentexperiment ofof aa smallsmall bulbbulb waswas alsoalso conducted.conducted. As shown in Figure 1111,, the two ends of the silver-plated fiber fiber we werere connected to two wires and fixed fixed on insulating paper with an an insulating insulating tape. tape. After After the the wires wires conne connectedcted the the LED LED bulb bulb to the to thepower power supply supply (3 V), (3 the V), thesmall small bulb bulb emitted emitted an anobvious obvious bright bright light light and and the the brightness brightness was was constant constant and and stable stable for for 10 10 min, min, which indicated that the prepared fiberfiber surface was uniformly and continuously covered with the silver plating layer, andand thethe fiberfiber couldcould conductconduct electronselectrons andand hadhad goodgood electricalelectrical conductivity.conductivity.

Figure 11. The luminescence experiment of a small bulb.

4. Conclusions In this study, aa continuouscontinuous two-steptwo-step silversilver platingplating methodmethod was developed with polyester ﬁbersfibers used asas thethe substrate substrate material, material, which which combined combined the th operationse operations of continuous of continuous electroless electroless plating plating under aunder dynamic a dynamic condition condition and continuous and continuous electroplating. electroplating. Furthermore, Furthermore, we designed we specializeddesigned specialized equipment forequipment the continuous for the continuous plating of silver plating on of the silver polyester on the ﬁbers. polyester According fibers. toAccording the machine to the conditions, machine theconditions, pretreatments, the pretreatments, electroless silver-plating, electroless silver-plating, and electroplating and silverelectroplating plating process silver plating were improved 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 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.

Materials 2018, 11, 2033 13 of 16 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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