Article Preparation and Properties of Carbon /Carbon Nanotube Wet-Laid Composites

Suhyun Lee 1, Kwangduk Ko 1, Jiho Youk 2, Daeyoung Lim 1 and Wonyoung Jeong 1,*

1 Human Convergence Technology Group, Korea Institute of Industrial Technology, Ansan 15588, Korea; [email protected] (S.L.); [email protected] (K.K.); [email protected] (D.L.); [email protected] (W.J.) 2 Department of Chemical Engineering, Inha University, Incheon 22212, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-8040-6233

 Received: 23 July 2019; Accepted: 24 September 2019; Published: 30 September 2019 

Abstract: In this study, carbon nanotubes (CNTs) were introduced into carbon fiber (CF) wet-laid composites as functional nano-fillers to fabricate multi-functional composites with improved mechanical, electrical, and thermal properties. It was considered that the wet-laid process was most suitable in order to introduce filler into brittle and rigid carbon fiber substrates, and we established the conditions of the process that could impart dispersibility and bonding between the fibers. We introduced polyamide 6 (PA6) short fiber, which is the same polymeric material as the stacking film, into carbon fiber and CNT mixture to enhance the binding interactions between carbon fiber and CNTs. Various types of CNT-reinforced carbon fiber wet-laid composites with PA6 short fibers were prepared, and the morphology, mechanical and electrical properties of the composites were estimated. As CNT was added to the carbon fiber nonwoven, the electrical conductivity increased by 500% but the tensile strength decreased slightly. By introducing short fibers of the same material as the matrix between CNT–CF wet-laid nonwovens, it was possible to find optimum conditions to increase the electrical conductivity while maintaining mechanical properties.

Keywords: carbon fiber-reinforced composites; carbon nanotubes; polyamide 6; wet-laid process; electrically conductive materials

1. Introduction Among the fiber-reinforced composites, carbon fiber (CF)-reinforced composites are used widely in the aerospace, automobile, sports, medical instrument, and construction industries as well as in the military because of their high mechanical properties, impact resistance, thermal resistance, chemical stability, and lightweight nature [1–11]. Generally, the properties of carbon fiber-reinforced composites are determined not only by the carbon fiber content, fiber length, fiber orientation, and fiber matrix adhesion, but also by the inherent characteristics of the matrix [6,10,12]. Carbon fiber-reinforced composites exhibit excellent in-plane properties, but they typically show poor out-of-plane performance that is dominated by the polymer matrix [8]. Without a surface treatment with a polymer matrix, carbon fibers have poor wettability and adsorption with most polymers because they have a non-polar surface and are comprised of highly crystallized, graphitic basal planes with inert structures [6]. Among the matrix polymers available, polyamide 6 (PA6) is considered a good candidate as a carbon fiber-reinforced composite matrix, mainly because of its various mechanical properties, low cost, and easy handling [5,6,13,14]. One impregnation technique for obtaining polyamide continuous reinforcement composites is the use of interfacial polycondensation where the reinforcement is impregnated as it is passed and pulled

Polymers 2019, 11, 1597; doi:10.3390/polym11101597 www.mdpi.com/journal/polymers Polymers 2019, 11, 1597 2 of 11 across the polyamide reaction chamber. Another technique is impregnation by hot press molding, where the reinforcement is impregnated by a polymer matrix under heat and pressure [5]. Carbon nanotubes (CNTs) are strong nanomaterials with high mechanical, thermal, and electrical properties [7–10,15]. CNTs have recently been considered for use as reinforcements in advanced composites because of their high strength and conductivity compared to existing fibers [16–22]. Chou et al. [8] reported the potential of CNT–CF hybridization. They grafted CNTs on the carbon fiber using a direct growth method and measured the interfacial shear strength in single-fiber composites. The interfacial shear strength was improved considerably because of the increased surface area of the CNT-grafted carbon fiber. Zhao et al. [17] presented a synthesis approach by grafting CNTs onto carbon fibers via a layer-by-layer method. They demonstrated that the presence of CNTs improves the interfacial properties and mechanical interlocking between the carbon fibers and epoxy matrix effectively. Shin et al. [19] also incorporated high concentrations of CNTs into carbon fiber-reinforced epoxy composites and suggested that the insertion of a CNT mat between the carbon fiber fabric leads to the highest electrical conductivity and thermal conductivity. In addition, several studies have focused on developing an effective CNT–CF hybridization method, such as the growth of CNTs on the carbon fiber surface by chemical vapor deposition (CVD) [7,23], coating of CNTs on the carbon fiber surface by electrophoresis [8,20] or spraying [9] and CNTs dispersed in a matrix [10,18,19]. On the other hand, CNTs are difficult to disperse homogeneously and good adhesion between CNTs, carbon fiber, and polymer matrix is problematic because CNTs tend to agglomerate and entangle with each other due to their strong Van der Waals bonds [10,24,25]. Therefore, it is important to improve the dispersion of CNTs in carbon fiber-reinforced composites. This study developed carbon fiber-reinforced composites with improved mechanical properties and electrical conductivity using CNTs. In particular, the present study suggests that by adding PA6 fibers to the CNT–CF mixture, which was prepared for nonwoven fabric, the interfacial bonding strength of the CNT–CF-nonwoven fabric is enhanced by PA6 fiber melting through the film stacking process. For this purpose, CF, CNTs, and PA6 fiber were dispersed in water and a wet-laying process was used to produce the CNT–CF-nonwoven fabric. The fabricated CNT–CF-nonwoven composite was infiltrated with a PA6 film by a hot press molding process for film stacking to improve the toughness, environmental resistance, and usability. The mechanical properties, electrical conductivity, and thermal properties were then observed and compared according to the contents of CNTs and PA6 fiber.

2. Materials and Methods

2.1. Materials Carbon fiber (T700; 12K, 800TEX) was purchased from Toray (Tokyo, Japan) and cut into 10-mm-long pieces using a Fidocut machine (Schmidt & Heinzmann, Bruchsal, Germany). CNTs (Hanwha Chemical Co., Ltd., Seoul, Korea) were used multi-wall carbon nanotubes with a purity of 97%, 10–15 nm in diameter, and 180–200 µm in length. Kolliphor P188 as a surfactant was purchased from Sigma-Aldrich (St. Louis, MO, USA). Polyvinyl alcohol (PVA) from Kurary (Tokyo, Japan) was prepared as a . Polyamide 6 (PA6) fiber, 0.4 mm in length of 0.4 denier, and films, 58 g/m2 in weight and 0.06 mm in thickness, were obtained from Sam Young Chemical (Seoul, Korea). All reagents were used as received.

2.2. Fabrication of CNT–CF-Nonwoven Two grams of carbon fiber (0.1 wt %) and 0.2 g of PVA (0.01 wt %) were dispersed in 2 L of water. CNTs (0–20 wt % with respect to carbon fiber loading), PA6 staple fiber (0, 50, and 100 wt % with respect to CNTs loading), and surfactant (0.1 wt % with respect to water) were then added in sequence to the mixture and stirred at 200 rpm for 24 h. A small amount of surfactant and PVA were added to disperse the carbon fibers and CNTs and to provide the minimum binding force for web formation. The CNT–CF-nonwoven fabric was then made using a wet laying process, as shown in PolymersPolymers2019 2019, ,11 11,, 1597 x FOR PEER REVIEW 3 ofof 1111

CF-nonwoven fabric was then made using a wet laying process, as shown in Figure 1. The wet laying Figureprocess1. is The environmentally wet laying process friendly, is environmentally using water as friendly, a solvent; using the waterquantity as aof solvent; carbon thefiber quantity and CNTs of carbonadded fibercan andbe controlled CNTs added and can all be thermoplastic controlled and poly all thermoplasticmers can be polymersapplied. After can be the applied. Afterwere thedisintegrated fibers were and disintegrated dissociated and through dissociated the throughpulper, the pulper,web was the manufactured web was manufactured through a through wet-laid a wet-laidprocess. process.Finally, the Finally, nonwoven the nonwoven fabric samples fabric sampleswere fabricated were fabricated through throughthe drying the process drying processin an oven in anfor oven 24 h. for 24 h.

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof thethe process:process: CF, carboncarbon fiber;fiber; CNT,CNT, carboncarbon nanotube;nanotube; PA6,PA6, polyamidepolyamide 6;6; PVA,PVA, polyvinyl polyvinyl alcohol. alcohol.

2.3.2.3. FilmFilm StackingStacking byby HotHot PressPress MoldingMolding AA filmfilm stacking stacking process process using using hot hot press press molding molding was wascarried carried out in out a steel in mold. a steel First, mold. the CNT– First, theCF-nonwoven CNT–CF-nonwoven fabric was fabricheated was to 260 heated °C and to held 260 ◦atC this and temperature held at this for temperature 20 min. Subsequently, for 20 min. it Subsequently,was pressed under it was 40 pressed ton of pressure, under 40 tonand ofheld pressure, for 10 min and with held PA6 for 10 film. min The with mold PA6 was film. then The cooled mold wasto 70 then °C. cooled to 70 ◦C.

2.4.2.4. CharacterizationCharacterization The morphology of the CNT–CF-nonwoven composites was observed by field emission scanning The morphology of the CNT – CF-nonwoven composites was observed by field emission electron microscopy (FE-SEM; SU8010, Hitachi, Tokyo, Japan). Prior to the examination, the sample scanning electron microscopy (FE-SEM; SU8010, Hitachi, Tokyo, Japan). Prior to the examination, the surfaces were sputter-coated with platinum. The mechanical properties of the CNT–CF-nonwoven compositessample surfaces with various were sputter-coated CNTs and PA6 fiberwith contents platinum. were The characterized mechanical according properties to ASTMof the D638 CNT using–CF- anonwoven universal testingcomposites machine with (Instron various 3343, CNTs Norwood, and PA6 MA, fiber USA). contents The sample were characterized size was 100 mm according20 mm to × forASTM testing. D638 Theusing gauge a universal length testing and crosshead machine (Ins speedtron were 3343, 60 Norwood, mm and MA, 10 mmUSA)./min, The respectively. sample size Thewas electrical 100 mm × conductivities 20 mm for testing. of the The CNT–CF-nonwoven gauge length andcomposites crosshead speed were characterizedwere 60 mm and using 10 amm/min, 4-point proberespectively. machine The (CMT-100S, electrical conductivities Advanced instrument of the CNT–CF-nonwoven technology, Suwon, composites Korea). Thermogravimetric were characterized analysisusing a (TGA;4-point Q500, probe TA instrument,machine (CMT-100S, New Castle, Advanced DE, USA) instrument was performed technology, to assess theSuwon, degradation Korea). temperatureThermogravimetric and the analysis weight (TGA; fraction Q500, of theTA CNT–CF-nonwoveninstrument, New Castle, composites DE, USA) at awas heating performed rate ofto 10assess◦C/min the indegradation a nitrogen temperature atmosphere. and the weight fraction of the CNT–CF-nonwoven composites at a heating rate of 10 °C/min in a nitrogen atmosphere. 3. Results 3. Results 3.1. Analysis of the Structure of the CNT–CF-Nonwoven Composites 3.1. AnalysisCNT–CF-nonwoven of the Structure composites of the CNT with–CF-Nonwoven different structures Composites were prepared from carbon fibers and CNTs to examine the effects of the mechanical properties and electrical conductivity according to the CNT–CF-nonwoven composites with different structures were prepared from and manufacturing process. CNTs to examine the effects of the mechanical properties and electrical conductivity according to the Figure2 presents a schematic diagram of the CNT–CF-nonwoven samples. The structure of the manufacturing process. CNT–CF-nonwoven composites was examined using two methods. One was making a single layer Figure 2 presents a schematic diagram of the CNT–CF-nonwoven samples. The structure of the of nonwoven fabric by mixing carbon fibers and CNTs, simultaneously. The other was making each CNT–CF-nonwoven composites was examined using two methods. One was making a single layer layer of nonwoven fabric by carbon fibers and CNTs, separately, and integrating them with a PA6 film. of nonwoven fabric by mixing carbon fibers and CNTs, simultaneously. The other was making each For this preliminary experiment, CF (2 g) and CNT (1 g) were dispersed in water, and then prepared in layer of nonwoven fabric by carbon fibers and CNTs, separately, and integrating them with a PA6 film. For this preliminary experiment, CF (2 g) and CNT (1 g) were dispersed in water, and then

Polymers 2019, 11, 1597 4 of 11

Polymers 2019, 11, x FOR PEER REVIEW 4 of 11 separate layers or in a mixed layer. The prepared CF and CNT web was completed as a composite usingprepared a PA6 in filmseparate stacking layers process. or in a mixed layer. The prepared CF and CNT web was completed as a composite using a PA6 film stacking process.

Figure 2. Schematic of structure of the CNT–CF-nonwoven samples. Figure 2. Schematic of structure of the CNT–CF-nonwoven samples. Table1 lists the properties of CNT–CF-nonwoven composites according to the structure. The thicknessTable 1 lists and the weight properties of the of PA6–CFCNT–CF-nonwove/CNT–PA6n and composites PA6–CF–PA6–CNT–PA6 according to the samplesstructure. were The higherthickness than and those weight of PA6–CF–PA6. of the PA6–CF/CNT–PA6 These results and were PA6–CF–PA6–CNT–PA6 attributed to CNT, which samples has a large were volume higher fractionthan those with of aPA6–CF–PA6. high aspect ratio These [16 results]. were attributed to CNT, which has a large volume fraction with a high aspect ratio [16]. The electricalTable conductivity 1. Properties of of the CNT–CF-nonwoven carbon fiber-reinforced according composites to composite was structure. low because the PA6 film in the composites is an insulator that interferes with the flow of electrons. The introduction of Electrical Conductivity Characteristics Thickness (mm) Weight (g/m2) Tensile Strength (MPa) Modulus (MPa) CNTs into the carbon fiber-reinforced composites was expected(S/cm) to enhance the electrical conductivity PA6–CF–PA6 0.288 181.6 27.75 1.63 72.65 12.46 3499 275 because they have outstanding electrical properties and± can promote the ±formation of an electrical± PA6–CF/CNT–PA6 0.365 263.3 120.48 21.14 46.21 7.01 2946 213 ± ± ± networkPA6–CNT–PA6–CF–PA6 in the composites0.325 [8,19]. The conductivi 305.0ty of 33.11the CNT–CF-nonwoven3.67 69.41 14.66 samples was 3295 improved539 ± ± ± by the insertion of CNTs to 120.48 S/cm for PA6–CF/CNT–PA6 and 33.11 S/cm for PA6–CNT–PA6– CF–PA6.The electricalIn particular, conductivity the conductivity of the carbon of PA6–CF/CNT–PA6 fiber-reinforced composites composed was of lowcarbon because fibers the and PA6 CNTs film inas thea single composites layer iswas an improved insulator that significantly, interferes withcompared the flow to ofPA6–CNT–PA6–CF–PA6, electrons. The introduction which of CNTs was intocomposed the carbon of layers fiber-reinforced of carbon composites fibers and was CNTs. expected This towas enhance attributed the electrical that the conductivity network of because highly theyinterconnected have outstanding PA6–CF/CNT–PA6 electrical properties benefits andfrom can the promote formation the of formation conducting of an paths electrical spanning network the inrelatively the composites large regions, [8,19]. as The opposed conductivity to the PA of the6–CNT–PA6–CF–PA6, CNT–CF-nonwoven which samples are wascomposed improved individual by the insertionlayers of carbon of CNTs fibers to 120.48 and CNTs S/cm forwith PA6–CF a PA6 /filmCNT–PA6 layer [8]. and 33.11 S/cm for PA6–CNT–PA6–CF–PA6. In particular, the conductivity of PA6–CF/CNT–PA6 composed of carbon fibers and CNTs as a single Table 1. Properties of CNT–CF-nonwoven according to composite structure. layer was improved significantly, compared to PA6–CNT–PA6–CF–PA6, which was composed of layers of carbon fibers and CNTs. This was attributedElectrical that the network Tensile of highly interconnected Thickness Weight Modulus PA6–CFCharacteristics/CNT–PA6 benefits from the formation of conductingConductivity paths spanningStrength the relatively large (mm) (g/m2) (MPa) regions, as opposed to the PA6–CNT–PA6–CF–PA6, which(S/cm) are composed individual(MPa) layers of carbon fibers andPA6–CF–PA6 CNTs with a PA6 film0.288 layer [8]. 181.6 27.75 ± 1.63 72.65 ± 12.46 3499 ± 275 OnPA6–CF/CNT–PA6 the other hand, in the case0.365 of the mechanical 263.3 120.48properties, ± 21.14 the tensile 46.21 strength± 7.01 and 2946 modulus ± 213 decreasedPA6–CNT–PA6–CF–PA6 due to the CF /CNT0.325 mixture. The 305.0 mechanical 33.11 properties ± 3.67 of 69.41 the carbon± 14.66 fiber-reinforced 3295 ± 539 composites depend mainly on the adhesion between the fibers and the matrix [6]. PA6–CF–PA6 and PA6–CNT–PA6–CF–PA6On the other hand, showedin the similar of the tensile mechanic strengthal properties, and modulus. the tensile This means strength that and the adhesionmodulus forcedecreased among due the to carbon the CF/CNT fiber, CNTs mixture. and PA6 The film mechan interfaceical is properties similar. On of the the other carbon hand, fiber-reinforced the mechanical propertiescomposites of depend PA6–CF mainly/CNT–PA6 on the were adhesion drastically betw lowereen the than fibers those and of thethe othermatrix samples. [6]. PA6–CF–PA6 Several studies and reportedPA6–CNT–PA6–CF–PA6 that the fabrication showed of carbon similar fiber tensile and strength CNT composites and modulus. is extremely This means diffi cultthat andthe adhesion imposes severalforce among limitations, the carbon such as fiber, an inhomogeneous CNTs and PA6 distribution film interface and pooris similar. bonding On with the theother matrix hand, [7, 16the]. mechanicalFrom the properties results, of these PA6–CF/CNT–PA6 manufacturing methodswere drastically limit the lower reinforcing than those strength of the becauseother samples. of the weakSeveral physical studies adsorptionreported that between the fabrication the carbon of carb fiberson fiber and CNTs.and CNT Therefore, composites in thisis extremely study, PA6 difficult fiber wasand introducedimposes several to the limitations, CNT–CF mixture such as for an nonwoveninhomogeneous composites distribution to prevent and thepoor deterioration bonding with of the mechanicalmatrix [7,16]. properties of composites and realize multi-functionality, such as electrical conductivity. PA6 From the results, these manufacturing methods limit the reinforcing strength because of the weak physical adsorption between the carbon fibers and CNTs. Therefore, in this study, PA6 fiber was introduced to the CNT–CF mixture for nonwoven composites to prevent the deterioration of the

Polymers 2019, 11, 1597 5 of 11

Polymers 2019, 11, x FOR PEER REVIEW 5 of 11 is a semi-crystalline thermoplastic polymer generally used in many tribological applications because mechanical properties of composites and realize multi-functionality, such as electrical conductivity. of the balance between its various mechanical properties and interfacial interaction [6]. By dispersing PA6 is a semi-crystalline thermoplastic polymer generally used in many tribological applications the PA6 short fiber in the CNT–CF-nonwoven fabric, the mechanical and electrical properties of the because of the balance between its various mechanical properties and interfacial interaction [6]. By composites are expected to improve as the PA6 fiber melts inside the nonwoven composites and acts as dispersing the PA6 short fiber in the CNT–CF-nonwoven fabric, the mechanical and electrical a binderproperties between of the the composites carbon fibers are expected and CNTs to throughimprove a as film the stacking PA6 fiber process. melts inside the nonwoven compositesCNT–CF-nonwoven and acts as a composites binder between were the manufactured carbon fibers according and CNTs tothrough various a film CNT stacking and PA6 process. contents. Table2 listsCNT–CF-nonwoven the final sample composites codes of each were CNT–CF-nonwoven manufactured according sample. to various CNT and PA6 contents. Table 2 lists the final sample codes of each CNT–CF-nonwoven sample. Table 2. Preparations of the CNT–CF wet-laid composites.

Sample Code CNTTable 0 wt 2. % Preparations CNT 1 of wt the % CNT– CNTCF wet-laid 5 wt % composites. CNT 10 wt % CNT 20 wt % PA fiberSample 0 wt Code % CNTP1-C0 0 wt % CNT P1-C1 1 wt % CNT P1-C5 5 wt % CNT P1-C1010 wt % CNT 20 P1-C20 wt % PAPA fiber fiber 50 wt 0 %wt % P2-C0P1-C0 P2-C1P1-C1 P1-C5 P2-C5 P1-C10 P2-C10 P1-C20 P2-C20 PA fiber 100wt % P3-C0 P3-C1 P3-C5 P3-C10 P3-C20 PA fiber 50 wt % P2-C0 P2-C1 P2-C5 P2-C10 P2-C20 2 PA fiber* PA6 and100 CNT:wt % wt % withP3-C0 respect to carbonP3-C1 fiber. PA6 filmP3-C5 for film stacking process:P3-C10 58g /m , 0.06P3-C20 mm. * PA6 and CNT: wt % with respect to carbon fiber. PA6 film for film stacking process: 58g/m2, 0.06 mm. 3.2. Morphology of CNT–CF-Nonwoven Composites According to the PA6 and CNT Contencts 3.2. Morphology of CNT–CF-Nonwoven Composites According to the PA6 and CNT Contencts Figure3 shows surface and cross section images of the CNT–CF-nonwoven according to various CNT contentsFigure 3 without shows surface the PA6 and fiber. cross In section the cross-sectional images of the CNT–CF-nonwoven image, it can be seen according that CNTs to various are well distributedCNT contents among without the carbon the PA6 fibers fiber. according In the cross-sect to theional CNT image, content. it can On be the seen other that hand, CNTs the aresurface well imagesdistributed show thatamong the the PA6 carbon film deposited fibers according by the filmto the stacking CNT content. process On did the not other adhere hand, uniformly the surface to the surfaceimages of show the CNT–CF-nonwoven that the PA6 film deposited composite by the as thefilm CNT stacking content process was did increased, not adhere and uniformly the formation to of poresthe surface increased. of the This CNT–CF-nonwoven means that PA6, whichcomposite can serveas the as CNT a matrix content between was theincreased, carbon and fibers the and CNTs,formation did not of penetratepores increased. into the This CNT–CF means nonwoventhat PA6, which sufficiently. can serve as a matrix between the carbon fibersThe and interfacial CNTs, did adhesion not penetrate between into fibers the andCNT–CF polymer nonwoven matrix sufficiently. plays an important role in improving The interfacial adhesion between fibers and polymer matrix plays an important role in the mechanical behavior of carbon fiber-reinforced composites [10]. Therefore, to strengthen the improving the mechanical behavior of carbon fiber-reinforced composites [10]. Therefore, to adhesion between the carbon fiber and CNTs with the polymer matrix, various contents of PA6 strengthen the adhesion between the carbon fiber and CNTs with the polymer matrix, various short fibers were introduced into the CNT–CF-nonwoven. Figure4 shows the morphology of the contents of PA6 short fibers were introduced into the CNT–CF-nonwoven. Figure 4 shows the CNT–CF-nonwoven samples according to the contents of PA fibers. morphology of the CNT–CF-nonwoven samples according to the contents of PA fibers. AsAs shown shown in in Figure Figure4, 4, unlike unlike the the P1-C10 P1-C10 sample sample withwith no PA6 short fiber fiber introduced, introduced, in in P2-C10 P2-C10 andand P3-C10, P3-C10, which which contained contained the PA6the shortPA6 fiber,short thefiber, PA6 the polymer PA6 polymer was distributed was distributed evenly throughout evenly thethroughout CNT–CF-nonwoven the CNT–CF-nonwoven composites. composites. This might beThis due might to the be homogeneousdue to the homogeneous support of support the carbon of fibersthe carbon resulting fibers from resulting the good from dispersibility the good dispersibility and interfacial and interfacial interactions interactions between between the CNTs theand CNTs PA6 matrixand PA6 [10]. matrix Some studies[10]. Some have studies indicated have that indicated the interfacial that the interaction interfacial mechanismsinteraction mechanisms of CNTs, carbon of fiber,CNTs, and carbon PA6 matrix fiber, canand be PA6 ascribed matrix mainly can be toascribed three aspects: mainly to (i) three the Van aspects: der Waals (i) the interaction Van der Waals among theinteraction CNTs; (ii) among chemical the reaction CNTs; (ii) of chemical CNTs with reaction carbon of fibersCNTs andwith the carbon PA6 fibers matrix; and and the (iii) PA6 mechanical matrix; interlockingand (iii) mechanical between theinterlocking CNTs and between polymer the matrix CNTs [and17]. polymer matrix [17]. TheseThese results results confirmed confirmed that that thethe polymerpolymer matrixmatrix was formed formed inside inside the the CNT–CF-nonwoven CNT–CF-nonwoven materialmaterial by by the the introduction introduction of of PA6 PA6 short short fibers fibers throughthrough thethe wet-laid process. process. P1-C0

Figure 3. Cont.

Polymers 2019, 11, 1597 6 of 11 PolymersPolymers 2019 2019, 11, ,11 x ,FOR x FOR PEER PEER REVIEW REVIEW 6 of6 of11 11 P1-C10 P1-C10

P1-C20 P1-C20

FigureFigureFigure 3.3. 3.CrossCross Cross section section section images imagesimages of of ofthe the the CNT CNT CNT–CF-nonwoven–CF-nonwoven–CF-nonwoven composites composites composites with with various with various various CNT CNT contents: CNTcontents: contents: (a )( a) (a) P1-C0; (b) P1-C10; and (c) P1-C20 with magnification 1k, 5k, and 30k, respectively. P1-C0;P1-C0; (b )( bP1-C10;) P1-C10; and and (c )( cP1-C20) P1-C20 with with magnification magnification ×1k, ×1k, ×5k,× ×5k, and× and ×30k, ×30k, respectively.× respectively.

Figure 4. Surface and cross section images of the CNT–CF-nonwoven composites with various PA6 FigureFigure 4. 4.Surface Surface and and cross cross section section images images of ofthe the CNT CNT–CF-nonwoven–CF-nonwoven composites composites with with various various PA6 PA6 contents: (a) P1-C10; (b) P2-C10; and (c) P3-C10; with magnification 30k. contents:contents: (a )( aP1-C10;) P1-C10; (b )( bP2-C10;) P2-C10; and and (c )( cP3-C10;) P3-C10; with with magnification magnification ×30k. ×30k.× 3.3. Mechanical Properties of the CNT–CF-Nonwoven Composites 3.3.3.3. Mechanical Mechanical Properties Properties of ofthe the CNT CNT–CF-Nonwoven–CF-Nonwoven Composites Composites The mechanical properties of the CNT–CF-nonwoven composites were examined by applying a tensileThe load.The mechanical mechanical Figure5 propertiesshows properties the of tensile of the the CNT–CF-nonwo strengthCNT–CF-nonwo of theven CNT–CF-nonwovenven composites composites were were examined compositesexamined by by applying according applying a toa thetensiletensile CNTs load. load. and Figure PA6Figure fiber 5 shows5 shows contents. the the tensile tensile strength strength of of the the CNT–CF-nonwoven CNT–CF-nonwoven composites composites according according to to thethe CNTsThe CNTs tensile and and PA6 strengthPA6 fiber fiber contents.of contents. the CNT–CF-nonwoven composite increased with increasing PA6 short fiber content.TheThe tensile tensile For strength thestrength samples of of the the withoutCNT–CF-nonwoven CNT–CF-nonwoven CNTs, the tensile co compositemposite strengths increased increased were with 64.24, with increasing increasing 68.60, and PA6 PA6 89.03 short short MPa forfiberfiber P1-C0, content. content. P2-C0, For For andthe the samples P3-C0, samples respectively.without without CNTs, CNTs, This the the tendency te nsiletensile strengths strengths was the were same were 64.24, until64.24, 68.60, up 68.60, to and 20 and wt 89.03 89.03 % ofMPa CNTsMPa (37.84,forfor P1-C0, P1-C0, 92.63, P2-C0, andP2-C0, 128.59 and and P3-C0, MPaP3-C0, forrespectively. respectively. P1-C20, P2-C20, This This tendency andtendency P3-C20, was was the respectively) the same same until until because up up to to 20 the20 wt wt PA6 % %of shortof CNTs CNTs fiber reduced(37.84,(37.84, 92.63, the92.63, empty and and 128.59 space128.59 MPa and MPa facilitatedfor for P1-C20, P1-C20, load P2-C20, P2-C20, transfer and and inP3-C20, P3-C20, the composites. respectively) respectively) In because addition, because the thethe PA6 mechanicalPA6 short short propertiesfiberfiber reduced reduced of the the the carbon empty empty fiber-reinforcedspace space and and facilitated facilitated composites load load transfer aretransfer closely in in the relatedthe composites. composites. to the interfacialIn In addition, addition, bonding the the betweenmechanicalmechanical the properties carbon properties fiber of of surfacethe the carbon carbon and fiber-reinforced polymer fiber-reinforced matrix, composites whichcomposites is relatively are are closely closely low related related because to to the of the theinterfacial interfacial non-polar surfacebondingbonding of between the between carbon the the fiberscarbon carbon [13 fiber ,26fiber]. surface Carbonsurface and fibersand polymer polymer have matrix, some matrix, chemical which which is groups, isrelatively relatively such low low as because carboxyl,because of of the epoxy, the ether,non-polarnon-polar and hydroxylsurface surface of groups,ofthe the carbon carbon which fi bersfi easilybers [13,26]. [13,26]. form Carbon chemicalCarbon fibers fibers bonds have have with some some the chemical amidechemical group groups, groups, of such the such matrix as as carboxyl, epoxy, ether, and hydroxyl groups, which easily form chemical bonds with the amide group PA6carboxyl, [6]. Therefore, epoxy, ether, as theand contents hydroxyl of groups, PA6 increased, which easily the form chemical chemical bonding bonds between with the theamide carbon group fiber of the matrix PA6 [6]. Therefore, as the contents of PA6 increased, the chemical bonding between the andof the PA6 matrix matrix PA6 became [6]. Therefore, stronger dueas the to contents the increased of PA6 number increased, of functional the chemical groups bonding capable between of chemical the carboncarbon fiber fiber and and PA6 PA6 matrix matrix became became stronger stronger due due to to the the increased increased number number of of functional functional groups groups bonding with the carbon fiber, resulting in an increase in tensile strength. capablecapable of of chemical chemical bonding bonding with with the the carbon carbon fiber, fiber, resulting resulting in in an an increase increase in in tensile tensile strength. strength.

Polymers 2019, 11, 1597 7 of 11 Polymers 2019, 11, x FOR PEER REVIEW 7 of 11

FigureFigure 5. 5. MechanicalMechanical properties properties of of the the CNT CNT–CF-nonwoven–CF-nonwoven composites. composites.

On the other hand, in terms of the effects of CNTs, the tensile strength of P1, which was On the other hand, in terms of the effects of CNTs, the tensile strength of P1, which was CNT– CNT–CF-nonwoven without a PA6 short fiber, decreased gradually with increasing CNT content. CF-nonwoven without a PA6 short fiber, decreased gradually with increasing CNT content. The The tensile strengths were 64.24, 62.23, 46.91, and 37.84 MPa for P1-C0, P1-C5, P1-C10, and P1-C20, tensile strengths were 64.24, 62.23, 46.91, and 37.84 MPa for P1-C0, P1-C5, P1-C10, and P1-C20, respectively. The reason for this result can be seen in the cross section images in Figures3 and4. CNTs respectively. The reason for this result can be seen in the cross section images in Figures 3 and 4. CNTs occupied the space between the carbon fibers in the nonwoven composite instead of PA6, which shows occupied the space between the carbon fibers in the nonwoven composite instead of PA6, which shows a lack of interaction between the carbon fiber and CNTs [12]. Therefore, the medium of facilitated a lack of interaction between the carbon fiber and CNTs [12]. Therefore, the medium of facilitated load load transfer was not present in the composites. This can also be explained by the increased stress transfer was not present in the composites. This can also be explained by the increased stress build-up build-up around the carbon fibers due to the introduction of CNTs on the composites. CNTs can act as around the carbon fibers due to the introduction of CNTs on the composites. CNTs can act as defects defects and degrade certain properties, such as strength [27]. As a result, the tensile strength of the and degrade certain properties, such as strength [27]. As a result, the tensile strength of the CNT–CF- CNT–CF-nonwoven composite decreased with increasing CNT content. nonwoven composite decreased with increasing CNT content. In the case of composites containing both CNTs and PA6 short fiber, however, CNTs and PA6 had In the case of composites containing both CNTs and PA6 short fiber, however, CNTs and PA6 a positive effect on the tensile strength of the CNT–CF-nonwoven composites. As shown in Figure5 had a positive effect on the tensile strength of the CNT–CF-nonwoven composites. As shown in Figure for P2 and P3, the tensile strength of the CNT–CF-nonwoven composite increased with increasing CNT 5 for P2 and P3, the tensile strength of the CNT–CF-nonwoven composite increased with increasing content. In addition, the tensile strength was improved further as the content of PA6 was increased. CNT content. In addition, the tensile strength was improved further as the content of PA6 was The maximum value was achieved by the introduction of 20 wt % CNTs and 100 wt % PA6 short increased. The maximum value was achieved by the introduction of 20 wt % CNTs and 100 wt % PA6 fibers, and the strength increased from 64.24 MPa to 128.59 MPa. Bekyarova et al. [8] reported that short fibers, and the strength increased from 64.24 MPa to 128.59 MPa. Bekyarova et al. [8] reported the addition of CNTs plays a significant role in reinforcing the polymer matrix. According to their that the addition of CNTs plays a significant role in reinforcing the polymer matrix. According to their research, the introduction of ~0.25% CNTs in the CF/epoxy composites enhanced the interlaminar research, the introduction of ~0.25% CNTs in the CF/epoxy composites enhanced the interlaminar shear strength by 27%. The incorporation of CNTs at the composite interface can increase the specific shear strength by 27%. The incorporation of CNTs at the composite interface can increase the specific surface area of the reinforcement filler significantly, which results in an improvement in the Van der surface area of the reinforcement significantly, which results in an improvement in the Van der Waals interactions at the composite interface, such as PA6 [18]. Therefore, in this study, the surface Waals interactions at the composite interface, such as PA6 [18]. Therefore, in this study, the surface area that can be conjugated with the polymer matrix increased with the introduction of CNTs into area that can be conjugated with the polymer matrix increased with the introduction of CNTs into the the . Because the PA6 polymer formed a strong chemical bond between the CNTs composite material. Because the PA6 polymer formed a strong chemical bond between the CNTs and carbonand carbon fiber, fiber, the theinterfacial interfacial interaction interaction was was enhanced enhanced considerably, considerably, and and the the tensile tensile strength waswas improvedimproved [26]. [26]. In In addition, addition, as as CNTs CNTs are are inserted inserted to to the the carbon carbon fiber-reinforced fiber-reinforced nonwovennonwoven compositescomposites withwith PA6 PA6 fibers, fibers, the mechanical stiffness stiffness of the sa samplesmples can be increased due to the entangled CNTs, resultingresulting in in an an increased increased tensile tensile strength strength of of the the CNT–CF-nonwoven CNT–CF-nonwoven composites [9]. [9]. Overall,Overall, the the presence presence of of CNTs CNTs bound the carboncarbon fibersfibers and polymerpolymer matrixmatrix together,together, whichwhich enhancedenhanced load load transfer transfer at at the the interface interface [20,23]. [20,23].

3.4. Electrical Properties of the CNT–CF-Nonwoven Composites

The electrical conductivity was examined by measuring the surface resistance using a 4-point probe system. The electrical conductivity was affected by the carbon fiber and CNT structure quality within the matrix, contents, dispersion, and the compatibility among the carbon fiber, CNTs, and the

Polymers 2019, 11, 1597 8 of 11

3.4. Electrical Properties of the CNT–CF-Nonwoven Composites The electrical conductivity was examined by measuring the surface resistance using a 4-point Polymers 2019, 11, x FOR PEER REVIEW 8 of 11 probe system. The electrical conductivity was affected by the carbon fiber and CNT structure quality within the matrix, contents, dispersion, and the compatibility among the carbon fiber, CNTs, and the polymer matrix [24]. Figure 6 shows the electrical conductivity of the CNT–CF-nonwoven composites polymer matrix [24]. Figure6 shows the electrical conductivity of the CNT–CF-nonwoven composites according to the CNTs and PA6 fiber contents. according to the CNTs and PA6 fiber contents.

FigureFigure 6. 6. ElectricalElectrical properties properties of of the the CNT CNT–CF-nonwoven–CF-nonwoven composites. composites.

As in P1-C0, the electrical conductivity of pure carbon fiber nonwoven was 15.97 S/cm. The electrical As in P1-C0, the electrical conductivity of pure carbon fiber nonwoven was 15.97 S/cm. The conductivity increased to 32.99 S/cm with increasing PA6 short fiber content, which was attributed to electrical conductivity increased to 32.99 S/cm with increasing PA6 short fiber content, which was the binding effect of the PA6 matrix in forming a channel for ion transfer between the carbon fibers. attributed to the binding effect of the PA6 matrix in forming a channel for ion transfer between the The introduction of CNTs in the carbon fiber-reinforced composites resulted in significant carbon fibers. improvement of the electrical conductivity of the CNT–CF-nonwoven composites as a result of The introduction of CNTs in the carbon fiber-reinforced composites resulted in significant the reinforcement of the electron transport channels [8]. The electrical conductivities of the improvement of the electrical conductivity of the CNT–CF-nonwoven composites as a result of the CNT–CF-nonwoven according to the CNT content were 22.24 S/cm, 57.43 S/cm, and 70.23 S/cm reinforcement of the electron transport channels [8]. The electrical conductivities of the CNT–CF- for P1-C5, P1-C10, and P1-C20, respectively. The high electrical conductivity was attributed to the nonwoven according to the CNT content were 22.24 S/cm, 57.43 S/cm, and 70.23 S/cm for P1-C5, P1- formation of excellent conductive CNT networks in the carbon fiber nonwoven composites. CNTs C10, and P1-C20, respectively. The high electrical conductivity was attributed to the formation of became a perfect auxiliary conductive network before the formation of the main conductive framework excellent conductive CNT networks in the carbon fiber nonwoven composites. CNTs became a of the carbon fibers. In addition, the unique electrical properties of the CNTs also enhanced the perfect auxiliary conductive network before the formation of the main conductive framework of the conductivity of the carbon fiber-reinforced composites. Moreover, the CNTs provide free charges in carbon fibers. In addition, the unique electrical properties of the CNTs also enhanced the conductivity the surroundings because of their specific surface area and low surface energy [21]. Therefore, as the of the carbon fiber-reinforced composites. Moreover, the CNTs provide free charges in the CNT content in the nonwoven composites increases, the CNTs strengthen the role of the conductive surroundings because of their specific surface area and low surface energy [21]. Therefore, as the bridge for electron flow inside of the composite, thereby improving the electrical conductivity. CNT content in the nonwoven composites increases, the CNTs strengthen the role of the conductive The electrical conductivity of the CNT–CF-nonwoven composites containing both CNT and PA6 bridge for electron flow inside of the composite, thereby improving the electrical conductivity. short fibers increased with increasing CNT and PA6 content. The maximum value was achieved by The electrical conductivity of the CNT–CF-nonwoven composites containing both CNT and PA6 81.25 S/cm at P3-C20. The conductive network depended mainly on the connection conductivity among short fibers increased with increasing CNT and PA6 content. The maximum value was achieved by the carbon fiber, CNTs, and PA6 polymer matrix [21]. The homogeneous dispersion of CNTs between 81.25 S/cm at P3-C20. The conductive network depended mainly on the connection conductivity the carbon fibers with the polymer matrix leads to extensive overlap between the CNTs that establish among the carbon fiber, CNTs, and PA6 polymer matrix [21]. The homogeneous dispersion of CNTs three-dimensional conductive paths [20]. This mechanism was enhanced further as the content of between the carbon fibers with the polymer matrix leads to extensive overlap between the CNTs that CNTs and PA6 fiber increased. As a result, the electrical performance of the CNT–CF-nonwoven establish three-dimensional conductive paths [20]. This mechanism was enhanced further as the increased significantly with increasing CNT and PA6 short fiber content. content of CNTs and PA6 fiber increased. As a result, the electrical performance of the CNT–CF- From the above results, it can be concluded that the conductive network composed of nonwoven increased significantly with increasing CNT and PA6 short fiber content. CNT–CF-nonwoven with PA6 short fibers may have some synergetic effect among the carbon fiber, From the above results, it can be concluded that the conductive network composed of CNT–CF- CNTs, and PA6 matrix. Therefore, the transmission of electrons in the CNT–CF-nonwoven composite nonwoven with PA6 short fibers may have some synergetic effect among the carbon fiber, CNTs, and PA6 matrix. Therefore, the transmission of electrons in the CNT–CF-nonwoven composite could be realized in three ways: (i) CF-to-CF, (ii) CNT-to-CNT, and (iii) CF-to-CNT. In addition, the PA6 short fibers provided a more rigid binding effect between the carbon fiber and CNTs. These are the main reasons for the observed positive effect of CNTs and PA6 on improving the electrical conductivities of the CNT–CF-nonwoven composites [24].

Polymers 2019, 11, x FOR PEER REVIEW 9 of 11

3.5. Thermal Properties of the CNT–CF-Nonwoven Composites

The thermal properties of the CNT–CF-nonwoven composites were examined by TGA to Polymers 2019, 11, 1597 9 of 11 determine the effect of the PA6 contents on the thermal degradation of the composites, as shown in Figure 7. The samples, P1-C10, P2-C10, and P3-C10 exhibited similar thermal behavior. The weight losscould in be the realized initial in threeprocess ways: (~100 (i) CF-to-CF, °C) was (ii) attributed CNT-to-CNT, to the and (iii)removal CF-to-CNT. of adsorbed In addition, water the and PA6 decompositionshort fibers provided of some a more oxygen-containing rigid binding effect functi betweenonal the groups carbon fiber[24]. andThe CNTs. weight These losses are thein mainthis temperaturereasons for the were observed 0.68%, positive0.87%, and effect 1.02% of CNTs for P1-10, and PA6P2-10, on and improving P3-10, respectively. the electrical The conductivities curves show of thatthe CNT–CF-nonwoventhermal degradation composites began to occur [24]. only after the materials absorbed a certain amount of heat. The heat initiates the degradation process and breaks down the matrix structure of the material by cutting3.5. Thermal its molecular Properties chain of the rupture CNT–CF-Nonwoven or scission [13]. Composites AThe sudden thermal decrease properties in ofthe the mass CNT–CF-nonwoven of the composites composites occurred were at approximately examined by TGA 350 to °C, determine which indicatedthe effect thermal of the PA6 degradation contents onof PA6 the thermal [6]. The degradationresidual mass of after the composites,degradation as was shown possibly in Figure carbon7. fiberThe samples,and CNTs. P1-C10, Owing P2-C10, to the andaddition P3-C10 of exhibitedPA6 short similar fiber, the thermal weight behavior. of the carbon The weight fiber and loss CNTs in the decreasedinitial process from (~100 47% ◦toC) 37%. was attributed to the removal of adsorbed water and decomposition of some oxygen-containingDespite the same functional content of groups carbon [24 fiber]. The and weight CNTs lossesin the innonwoven this temperature composites, were the 0.68%, mechanical 0.87%, propertiesand 1.02% forand P1-10, electrical P2-10, conductivity and P3-10, respectively.tended to in Thecrease curves with show increasing that thermal PA6 short degradation fiber content. began Therefore,to occur only the after use the of materials PA6 short absorbed fibers acan certain impa amountrt high of heat.mechanical The heat properties initiates theand degradation electrical conductivityprocess and breaks to the down CNT–CF-nonwoven the matrix structure composite of the material with the by same cutting CNT its molecularcontent compared chain rupture to the or CNT–CF-nonwovenscission [13]. without PA6.

Figure 7. TGA curves of the CNT–CF-nonwoven composites according to the PA6 contents. Figure 7. TGA curves of the CNT–CF-nonwoven composites according to the PA6 contents.

A sudden decrease in the mass of the composites occurred at approximately 350 ◦C, which indicated thermal degradation of PA6 [6]. The residual mass after degradation was possibly carbon fiber and CNTs. Owing to the addition of PA6 short fiber, the weight of the carbon fiber and CNTs decreased from 47% to 37%. Despite the same content of carbon fiber and CNTs in the nonwoven composites, the mechanical properties and electrical conductivity tended to increase with increasing PA6 short fiber content. Therefore, the use of PA6 short fibers can impart high mechanical properties and electrical conductivity to the CNT–CF-nonwoven composite with the same CNT content compared to the CNT–CF-nonwoven without PA6.

4. Conclusions Various types of carbon fiber-reinforced composites with improved mechanical properties and electrical conductivity were prepared with CNTs and PA6 short fibers using wet-laid and film stacking processes. The structure of the CNT–CF-nonwoven composite through wet-laid processing demonstrated more effective electrical conductivity when forming a single layer of a carbon fiber and CNT mixture, rather than forming carbon fiber and CNTs as individual layers. Therefore, the binding interaction Polymers 2019, 11, 1597 10 of 11 between the carbon fiber and CNTs was increased by introducing PA6 short fibers into the carbon fiber and CNTs mixture to improve the mechanical properties of the nonwoven composite. The morphology of the CNT–CF-nonwoven composites from SEM images show three-dimensionally distributed CNTs within the carbon fiber and polymer matrix. The mechanical properties of the nonwoven composites decreased with increasing CNT content only when CNTs were inserted. On the other hand, when CNTs and PA6 fiber were inserted simultaneously, the tensile strength increased with increasing CNT and PA6 fiber content because of the improved interfacial adhesion with matrix. The electrical conductivity of the composites was improved significantly with the addition of CNTs.

Author Contributions: Writing-original draft preparation, S.L.; investigation and data curation, K.K.; methodology, J.Y.; project administration, D.L.; supervision and writing-review & editing, W.J. Funding: This research was supported through the Korea Institute of Industrial Technology (kitech JA-19-0001) and Gyeonggi-Do Technology Development Program (kitech IZ-19-0003) as “Development of smart textronic products based on electronic fibers and .”. Conflicts of Interest: The authors declare no conflict of interest.

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