Antistatic Fibers for High-Visibility Workwear: Challenges of Melt-Spinning Industrial Fibers

materials

Article Antistatic Fibers for High-Visibility Workwear: Challenges of Melt-Spinning Industrial Fibers

Rudolf Hufenus 1,*, Ali Gooneie 1 , Tutu Sebastian 2, Pietro Simonetti 1, Andreas Geiger 2, Dambarudhar Parida 1 , Klaus Bender 3, Gunther Schäch 3 and Frank Clemens 2

1 Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland; [email protected] (A.G.); [email protected] (P.S.); [email protected] (D.P.) 2 Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland; [email protected] (T.S.); [email protected] (A.G.); [email protected] (F.C.) 3 EMS-CHEMIE AG, Business Unit EMS-GRILTECH, Via Innovativa 1, 7013 Domat/Ems, Switzerland; [email protected] (K.B.); [email protected] (G.S.) * Correspondence: [email protected]; Tel.: +41-58-765-7341

Received: 19 May 2020; Accepted: 8 June 2020; Published: 10 June 2020

Abstract: Safety workwear often requires antistatic protection to prevent the build-up of static electricity and sparks, which can be extremely dangerous in a working environment. In order to make synthetic antistatic fibers, electrically conducting materials such as carbon black are added to the fiber-forming polymer. This leads to unwanted dark colors in the respective melt-spun fibers. To attenuate the undesired dark color, we looked into various possibilities including the embedding of the conductive element inside a dull side-by-side bicomponent fiber. The bicomponent approach, with an antistatic compound as a minor element, also helped in preventing the severe loss of tenacity often caused by a high additive loading. We could melt-spin a bicomponent fiber with a specific resistance as low as 0.1 Ωm and apply it in a fabric that fulfills the requirements regarding the antistatic properties, luminance and flame retardancy of safety workwear.

Keywords: bicomponent melt-spinning; safety workwear; nanocomposite; antistatics; carbon black

1. Introduction Antistatic properties in safety workwear are required in order to reduce its electrical resistivity. If friction-induced charge cannot dissipate across the fabric surface and discharge, it may build up and create sparks and static electricity, which can be highly dangerous. Therefore, antistatic clothing is required to prevent fire and explosions while working with flammable liquids and gases, and to avert damage to sensitive electrical components. Antistatic properties can be achieved by a surface treatment that absorbs moisture, resulting in a thin conductive film on the fabric [1,2]. The commercial fiber Resistat® [3], a polyamide 6 (PA6) fiber with a carbon black (CB) coating, is one such example. In respective garments, the treatment can fade or rub off, and the resistivity is influenced by the atmospheric humidity and can increase in dry environments. In order to make synthetic fibers intrinsically antistatic or conductive, fillers such as CB, carbon nanotubes (CNTs), graphene or metal powders are often used as electrically conductive additives [1,4,5]. Typically, the mechanical properties of synthetic fibers change significantly when conductive fillers are used. Since fibers with fillers have very low tenacity, bicomponent fibers with an antistatic compound as a minor element are an interesting approach [6,7]. Respective bicomponent fibers are already commercially available, e.g., “Belltron” (side-by-side) by Kanebo [8] or “Antistat” (core-sheath)

Materials 2020, 13, 2645; doi:10.3390/ma13112645 www.mdpi.com/journal/materials Materials 2020, 13, 2645 2 of 15 by Perlon [9]. In this study, we take Belltron B31 [8] as the reference, with the fiber having an electrical resistivity of ~66 Ωm, and the conductive part of the bicomponent fiber one of ~6 Ωm. In this study, these resistivities are applied for comparison with bicomponent fibers or antistatic compounds, respectively. Another problem with carbon-based fillers is that they lead to unwanted dark colors in the resulting fibers. This reduces the visibility of the corresponding luminous fabrics used for high-visibility (hi-viz) workwear. Garments with hi-viz features are meant to protect workers exposed to risks from vehicles and heavy equipment. Higher visibility can be achieved by using a bicomponent fiber approach, when only a small part of the conductive phase is connected to the surface of the fiber, thus reducing the appearance of the black color. The goal of our study was to achieve good electrical conductivity and simultaneously attenuate the undesired dark color. To enable stable fiber melt-spinning, good electrical conductivity should be realized with low carbon content. In general, the electrical resistivity of a polymeric material decreases with an increasing content of a conductive filler like carbon black [10]. Gulrez et al. give a good overview of different conductive fillers (e.g., CB, CNTs, graphite, graphene and metal nanoparticles) in polyolefin-based thermoplastics [11]. The formation of the conductive pathways of the fillers is described by the percolation curve, which illustrates the relationship between the quantity of added CB and the achieved electrical resistivity [12]. As soon as the particles start to form an interconnected network, i.e., percolation, the electrical resistivity drops dramatically. The shape of the percolation curve is mainly affected by the state of dispersion, morphology, content, and intrinsic properties of the conductive filler [10]. It is also well known that the polymer–polymer, polymer–filler and filler–filler interactions play an important role in the equilibrium microstructure of the (nano)composite and hence control the achieved electrical conductivity [13]. Theoretical and experimental investigations suggest that mixing carbon nanoparticles with different shapes and aspect ratios can lead to developing more continuous networks and thus increase the electrical conductivity at a fixed carbon content [10,13–16]. Molecular simulations have shown that by increasing the aspect ratio, the equilibrium microstructure of nanoparticles in a polymer matrix changes from a random dispersion to a self-assembled morphology and eventually to a bridging self-assembled network [14]. Besides the particle shape, a range of electrical conductivity can be achieved by varying the amount of filler added to the polymer. In case of CNTs and carbon nanofibers (CNFs), enhanced polypropylene (PP) with a high electrical conductivity can be achieved at relatively low filler loading [11]. Moreover, mechanical models have been developed to address the overall behavior in such polymer (nano)composites [17]. Based on this knowledge, researchers have already investigated the combination of CNT and CB fillers in polymers [18]. In this study, a bicomponent melt-spinning technique to grant antistatic properties to synthetic fibers is reported. First, different conventional approaches to achieve sufficient electrical conductivity with the lowest possible carbon content are assessed, scrutinizing combinations of CBs and CNTs in polyamide, as well as highly loaded CB compounds blended with an immiscible neat polymer (double-percolated conductive network) [19–21]. Secondly, elaborate melt-spinning trials are presented, performed on a custom-made pilot line to determine what material and cross-sectional combinations result in fibers with low electrical resistivity, attenuated blackness and sufficient tensile properties. Finally, the bicomponent antistatic fiber concept is transferred to industrial-scale production and followed through to a prototype antistatic hi-viz safety jacket.

2. Materials and Methods

2.1. Polymers and Compounds The neat polymers used to dilute antistatic masterbatches by compounding were the low-densitiy polyethylene (LDPE) 1700 MN 18 C, the medium-density polyethylene (MDPE) 1020 FE 30, the high-density polyethylene (HDPE) 2055 MN, the polyamide 6 (PA6) Grilon A26, and the polyamide 12 Materials 2020, 13, 2645 3 of 15

(PA12) Grilamid L 20. The base polymer used for ﬁber melt-spinning was the PA6 Grilon F 34 NL. The commercial antistatic compounds included in this study were the CB-based pigment masterbatches Grilamid L 20 EC, Palamid Black 00-6405 and the polyethylene (PE) compound ColColor RKK E 40/FP, as well as the CNT-containing masterbatch PlastiCyl PA1502. Grilamid FE 11384 is an in-house antistatic compound developed by EMS-CHEMIE (Domat/Ems, Switzerland); compounds No. 6081 and 6082 are 50:50 and 85:15 mixtures of Grilamid L 20 EC and PlastiCyl PA1502, respectively, while compound No. 6083 is a 90:10 mixture of Grilamid FE 11384 and PlastiCyl PA1502. Compounds No. 6237 and 6739 are 50:50 mixtures of ColColor RKK E 40/FP with Grilon F 34 NL and Sabic LDPE, respectively, prepared on a co-rotating 36 L/D twin-screw extruder (Collin, Maitenbeth, Germany) by Empa. The pigment masterbatch Palamid White 00-2305 was used to attenuate the dark color of the carbon. Details of the compounds are given in Table1.

Table 1. Polymers and compounds used in this study. Carbon black (CB), carbon nanotube (CNT) and

TiO2 content are speciﬁed by the provider.

CB CNT TiO Base 2 Code/No. Product Name Provider Content Content Content Polymer (wt%) (wt%) (wt%) LDPE LDPE 1700 MN 18 C Total LDPE - - - MDPE MDPE 1020 FE 30 Arkema MDPE - - - HDPE HDPE 2055 MN Total HDPE - - - 887 Sabic LDPE Sabic LDPE - - - 5432 Grilon A26 EMS-CHEMIE PA6 - - - 5528 Grilon F 34 NL EMS-CHEMIE PA6 - - - 5793 Grilamid L 20 EMS-CHEMIE PA12 - - - 6065 PlastiCyl PA1502 Nanocyl PA12 - 15 - 6066 Grilamid L 20 EC EMS-CHEMIE PA12 25 - - 6067 Grilamid FE 11384 EMS-CHEMIE PA12 17 - - 6081 L-2597 KTI-2 EMS-CHEMIE PA12 12.5 7.5 - 6082 L-2598 KTI-3 EMS-CHEMIE PA12 21.25 2.25 - 6083 L-2599 KTI-5 EMS-CHEMIE PA12 15.3 1.5 - 6107 Palamid Black 00-6405 BASF PA6 40 - - 6111 ColColor RKK E 40/FP Evonik PE 40 - - 6183 Palamid White 00-2305 BASF PA6 - - 30 6237 - - PE/PA6 20 - - 6739 - - PE/PE 20 - -

2.2. Extrusion and Melt-Spinning The polymer compounds were homogeneously mixed using the microcompounder Minilab (Thermo Fisher Scientific, Karlsruhe, Germany) to produce undrawn filaments with high carbon filler content (Figure1a). Here, the mixture was fed with a top-feeder into a co-rotating conical twin-screw extruder at a speed of 100 rpm and mixed at a temperature of 250 ◦C. The mixture was cycled towards a backflow slit die channel, equipped with two pressure sensors. After reaching the equilibrium in the pressure sensors, the valve was switched towards a die with diameter 0.5 mm. The filaments were extruded onto a conveyor belt and collected. For the experiments, masterbatches No. 6107 and 6111, both with 40 wt% CB, were diluted with the thermoplastics listed in Table1, to investigate the e ffect of the CB content on the resistivity. To investigate the effect of additional CNTs, CB masterbatch No. 6107 and CNT masterbatch No. 6065 were diluted with PA12 (No. 5793). Details of the respective compounds and the resulting extrudates are given in Table S1 of Supplementary Materials. Materials 2020, 13, 2645 4 of 15

Materials 2020, 13, x FOR PEER REVIEW 4 of 15

Materials 2020, 13, x FOR PEER REVIEW 4 of 15 Feed hopper Spin Melt Feed pack pump hopper Spin Melt pack pump

Screw extruder

Quenching Screw extruder chamber Quenching chamber Godets

Godets

Pressure sensor 1 Winder Pressure Microcompounder sensor 1 Winder b) Pressure Microcompounder sensor 2 b) Pressure sensor 2 Take-up

a) Quenching Take-up Figure 1. Schematic drawingQuenching of (a) the micro compounder with a backflow channel and quenching a)Figure 1. Schematic drawing of (a) the micro compounder with a backflow channel and quenching andand take-up take-up units, unit ands, and (b ()b of) of the the pilot pilot melt-spinningmelt-spinning plant plant (diameters (diameters of of godets godets and and winder winder spool spool are are 100Figure mm). 1. By Schematic way of illustration, drawing of the (a) colorlessthe micro polymer compounder is represented with a backflow in yellow. channel and quenching 100 mm).and take By-up way unit ofs illustration,, and (b) of the the pilot colorless melt-spinning polymer plant is represented (diameters of in godets yellow. and winder spool are 100 mm). By way of illustration, the colorless polymer is represented in yellow. TheThe bicomponent bicomponent fibers fibers were were melt-spunmelt-spun in a custom custom-made-made pilot pilot plant plant (Figure (Figure 1b)1,b), originally originally built by Fourné Polymertechnik (Alfter, Germany), and described in detail elsewhere [22]. Two types built by Fourné Polymertechnik (Alfter, Germany), and described in detail elsewhere [22]. Two types of spinneretsThe bicomponent were applied, fibers yielding were eithermelt- spun“wedge in ” a (Figurecustom 2a)-made or “ sandwichpilot plant” (Figure(Figure 2b) 1b) fiber, originally cross- of spinnerets were applied, yielding either “wedge” (Figure2a) or “sandwich” (Figure2b) fiber sections.built by FournéThe major Polymertechnik component of (Alfter, the bicomponent Germany), fiberand describeconsistedd inof detailPA6 (polymer elsewhere No. [22]. 5528) Two with types 3 cross-sections.of spinnerets The were major applied, component yielding either of the “ bicomponentwedge” (Figure fiber 2a) or consisted “sandwich of” PA6 (Figure (polymer 2b) fiber No. cross 5528)- wt% and 7 wt% of white masterbatch (MB No. 6183), resulting in 0.9% and 2.1% TiO2 content in the withmajorsections. 3 wt% fiber and The part,7 major wt% respectively. ofcomponent white masterbatchSix of different the bicomponent antistatic (MB No. fibercomponents 6183), consisted resulting were of inPA6 introduced 0.9% (polymer and (13 2.1% No. or TiO 205528) vol%)2 content with as 3 in theminor majorwt% and component fiber 7 wt% part, ofs respectively. (Figurewhite masterbatch 2c): PA6 Six diwithff (MBerent 40 No. wt% antistatic 6183), CB (No. resulting components 6107), in PA12 0.9% were withand introduced 2.1%25 wt% TiO CB2 content (13(No. or 6066), 20in the vol%) as minorPA12major with componentsfiber 21.25 part, wt% respectively. (Figure CB and2c): 2.25 Six PA6 differentwt% with CNT 40antistatic (No. wt% 6082), CB components (No. PA12 6107), with were PA1212.5 introduced wt% with CB 25 and wt%(13 7.5or CB 20wt% (No.vol%) CNT 6066), as PA12(No.minor with 6081), component 21.25 PA12 wt% withs CB(Figure 15.3 and wt% 2c): 2.25 CBPA6 wt% and with CNT1.5 40wt% (No.wt% CNT 6082),CB (No. (No. PA12 6083), 6107), with and PA12 a 12.5 50:50 with wt% blend25 CBwt% of and PECB 7.5containing(No. wt% 6066), CNT (No.40PA12 6081), wt% with CB PA12 with 21.25 with PA6 wt% 15.3 (No. CB wt% 6237).and CB2.25 and wt% 1.5 CNT wt% (No. CNT 6082), (No. PA12 6083), with and 12.5 a 50:50wt% CB blend and of 7.5 PE wt% containing CNT 40 wt%(No. CB 6081), with PA12 PA6 with (No. 15.3 6237). wt% CB and 1.5 wt% CNT (No. 6083), and a 50:50 blend of PE containing Wedge Sandwich 40 wt% CB with PA6 (No. 6237). CB 40 CNT Wedge Sandwich ) blend

t CB

w 40 CNT

(

) t blend

(

o 20t

o r 20

a 10

1596 1694 a 10

C 0 6107 6066 6082 6081 6083 6237 a) b) c) 1596 1694 Antistatic compound 0 6107 6066 6082 6081 6083 6237 a) Figure 2. Illustration of the antistaticb) compounds: optical microscopyc) images of the two types of fiber Antistatic compound cross-sections produced: (a) wedge, and (b) sandwich. (c) Carbon content and type of filler in melt- Figure 2. Illustration of the antistatic compounds: optical microscopy images of the two types of fiber Figurespun 2. bicomponentIllustration fibers. of the antistatic compounds: optical microscopy images of the two types of cross-sections produced: (a) wedge, and (b) sandwich. (c) Carbon content and type of filler in melt- fiber cross-sections produced: (a) wedge, and (b) sandwich. (c) Carbon content and type of filler in spun bicomponent fibers. melt-spun bicomponent fibers.

Materials 2020, 13, 2645 5 of 15

The two polymer composites were fed from two separate single-screw extruders (Fourné Polymertechnik, Alfter, Germany; length to diameter ratio of 25, major component: 18 mm diameter, minor component: 14 mm diameter). Metering pumps (Mahr, Göttingen, Germany) with nominal throughputs of 5.4 and 0.8 cm3/min (bicomponent ratio, 87:13), as well as 4.8 and 1.2 cm3/min (bicomponent ratio 80:20), transferred the melts into the spin pack. The respective spin pressures and processing temperatures can be found in Table S2. The bicomponent ﬁber exited the spinneret into the quenching chamber, where it was cooled by air in order to solidify before drawing. Finally, the ﬁber was taken up, drawn by three heated godets (100 mm diameter, decreasing temperatures: 85, 50 and 30 ◦C), and spooled onto a bobbin (100 mm diameter). The take-up velocity was 300 m/min, and the draw ratio, namely the ratio between the winder speed and that of the take-up godet, was varied within the range 1–4 (resulting in respective winding speeds of 300–1200 m/min).

2.3. Physical Properties The resistivity of the filaments produced by the microcompounder, i.e., their length and cross-section specific electrical resistance, was evaluated with a four-point probes method that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements when the resistance is low. The distance between the four probes, connected to a multimeter (SourceMeter 2450, Keithley Instruments, Cleveland, OH, USA), was 10 mm. A two-point measurement was applied to assess the resistivity of the melt-spun bicomponent fibers, as their electrical resistance was high enough in all cases. Here, the fibers were fixed with a conductive silver paste (Acheson, Agar Scientific, Stansted Essex, UK) on two probes with a distance of 10 mm, connected to the multimeter. The tensile characteristics of the melt-spun fibers were determined using a Statimat ME+ (Textechno Herbert Stein, Mönchengladbach, Germany) with a 10 N load cell. A series of 10 specimens were tested for each fiber type using a gauge length of 100 mm and a cross head speed of 200 mm/min. The fineness was calculated from the weight of a 100 m long fiber sample. To study the fiber cross-section, fibers were embedded in epoxy resin and cross-sections prepared with the polisher EcoMet 250 Pro (Buehler, Esslingen am Neckar, Germany). Optical microscopy pictures were taken using a VHX-1000 (Keyence, Mecheln, Belgium) system. Scanning electron microscopy (SEM) was conducted on a Hitachi S-4800 SEM (Hitachi High-Technologies, Krefeld, Germany). As prepared, the fiber samples were coated with Au/Pd (5 nm) prior to analysis. The surface color of a fiber bobbin was measured using the dual-beam spectrophotometer Datacolor 550 (Datacolor, Rotkreuz, Switzerland) fitted for reflectance measurements, in a wavelength range between 360 nm and 700 nm. The bobbin was attached to an integrating sphere (diameter 152 mm) and illuminated through a slit of defined size, in order to detect the spectrum of the reflected light.

3. Results and Discussion

3.1. Assessment of Different Approaches to Reduce the Resistivity of Compounds Before discussing the properties of melt-spun polymer fibers, we establish an assessment of potential approaches to produce conductive compounds for later use in fibers. The electrical conductivity of polymers with carbon fillers is first and foremost influenced by the filler content. Upon diluting the PA6 compound containing 40 wt% CB (No. 6107) with neat PA12 (No. 5793), the electrical resistivity increases (Figure3a). Figure3a shows that, by adding CB in steps of 5 wt%, the resistivity of PA gets roughly 100 times lower per step, until the percolation threshold is reached and the resistivity levels out. Without drawing the filament, a good enough resistivity, compared to the conductive part of the commercial fiber Belltron B31 (~6 Ωm), is reached at about 24 wt% CB. Follow-up fiber spinning experiments revealed that a filler content above 20 wt% resulted in the clogging of the filters and spinneret over time. Materials 2020, 13, x FOR PEER REVIEW 6 of 15

screw extruder Collin (Table S1). Upon mixing the same compound (No. 6739) a second time in the same way, the resistivity increases a thousand-fold to 0.32 ± 0.15 kΩm (Table S1), which can be explained by the breakdown of percolated networks of aggregated CB. Compounding a similar material combination (50 wt% LDPE with 50 wt% 6111) in the microcompounder leads to an abrupt increase in resistivity up to 7.8 ± 0.4 MΩm (Table S1, Figure 4a). This trend shows the significant effect Materialsof intensive2020, 13 ,mixing 2645 on the formation and breakdown of percolated networks of conductive CB fillers6 of 15 in the polymer matrix, as explained by the so-called “aggregate-network” model [23].

103 106 20wt% CB

)

m 104 16wt% CB

(

( 2 4wt% CNT y 10

i 18wt% CB

t 2 2wt% CNT

s 10 i

r 14wt% CB

a 0 6wt% CNT

c 10 i 1

t 10

E E 10-2 12wt% CB 8wt% CNT 10-4 100 0 10 20 30 40 0 10 20 30 40 a) CB content (wt%) b) Relative CNT content (wt%)

FigureFigure 3. 3.Electrical Electrical resistivity resistivity of of ( a(a)) polyamide polyamide (PA)(PA) filamentsfilaments produced produced by by microcompounding microcompounding,, as asa a functionfunction of of CB CB content; content; and and (b (b)) compounded compounded/extruded/extruded filamentfilament as as a a function function of of relative relative CNT CNT addition addition withwith respect respect to to the the total total carbon carbon content content (i.e., (i.e., CB CB+ + CNT),CNT), which is is kept kept at at 20 20 wt%. wt%. For For clarification, clarification, the the absolute CB and CNT concentrations of the respective compounds are stated in the graph. The error absolute CB and CNT concentrations of the respective compounds are stated in the graph. The error bars represent double the standard deviation (some of the error bars are too small to be visible; see bars represent double the standard deviation (some of the error bars are too small to be visible; see Table S1). For comparison, the resistivity of the conductive part of the commercial bicomponent fiber Table S1). For comparison, the resistivity of the conductive part of the commercial bicomponent fiber Belltron B31 is plotted as dashed line. Belltron B31 is plotted as dashed line.

108 102 Upon diluting a PE compound containingPE 40 wt% CB (No. 6111) with PE (No. 887) in order PE/HDPE to achieve 20 wt% CB in the mixture,PE/LDPE the resistivity reaches 0.23 0.01 Ωm after once mixing in the

) 106 ) ± twin-screwm extruder Collin (Table S1).PE/ UponMDPE mixing them 10 same1 compound (No. 6739) a second time

( PE/PA12 (

y y

in thet same way, the resistivity increases a thousand-foldt to 0.32 0.15 kΩm (Table S1), which can i 4 PE/PA6 i

v v ±

i 10 i

t be explained by the breakdown of percolated networkst of aggregated CB. Compounding a similar

i i 0

s s 10

e material combination (50 wt% LDPE with 50 wt% 6111)e in the microcompounder leads to an abrupt

r r

l 2 l

a 10 a

increasec in resistivity up to 7.8 0.4 MΩm (Table S1, Figurec 4a). This trend shows the signiﬁcant e ﬀect

i i

t ± t

c of intensive mixing on the formation and breakdown ofc percolated-1 networks of conductive CB ﬁllers

l l 10 0 in theE polymer10 matrix, as explained by the so-called “aggregate-network”E model [23].1661-1665 Fillers’ properties such as their particle size and aspect ratio are known to influence1636-1640 the electrical 1631-1635 conductivity10-2 of polymer (nano)composites [14–16]. In particular,10-2 the dispersion quality, aspect ratios of 10 15 20 25 30 35 40 45 0.5 1 1.5 2 2.5 3 3.5 carbon nanoparticles, and formation of nanoparticle networks with continuous pathways for efficient a) CB content (wt%) b) Draw ratio (DR) electron transport are largely interconnected. Various studies have shown the synergistic effect of a Figure 4. (a) Electrical resistivity of compounded/extruded filament of PE masterbatch No. 6111, CB–CNT hybrid filler on the electrical conductivity of the polymer composite [24]. Thus, we evaluated blended with different polymers. PE = No. 6111; PE/PE: 50% No. 6111 diluted in HDPE, LDPE and different CB–CNT combinations between 5 and 40 wt% carbon, with 0–100 wt% CNT content. Due to MDPE, respectively; PE/PA12: 37.5%, 40% and 50% No. 6111 diluted in No. 5793; PE/PA6: 37.5%, 40% the higherand 50% area No. to 6111 weight diluted ratio in No. of 5432. CNTs, The a miscible lower fillercompounds loading show is a needed very high to resistivity, reach the whereas percolation threshold,the whichimmiscible results blends in a reveal reduced a resistivity resistivity in the (Figure range3 ofb). the Figure value3 b of shows the con that,ductive with part high of the enough CNT loading,commercial the bicomponent overall carbon fiber content Belltron toB31 reach (~6 Ωm, a good dashed enough line). (b resistivity) Electrical canresistivity be below of selected 20 wt%. Polymermelt-spun nanocomposites bicomponent fibers based with 13 on vol% CB–CNT antistatic mixtures compound still (Table retain S3) as their a function undesired of draw blackratio color despite(DR). their For advantage comparison, of higher the resist conductivityivity of the commercial at lower carbon bicomponent contents. fiber In Belltron an attempt B31 (~66 to further Ωm) is reduce the blacknessplotted ofas thedashed extrudates line. The whileerror bars preserving represent the double electrical the standard conductivity, deviation the (see PA6 Tables and S1 PE and compounds S3); containingsome 40 of wt%the error CB bars (No. are 6107 too small and 6111,to be visible. respectively) were diluted with either neat polyethylenes (HDPE, MDPE or LDPE) or polyamides (PA6 and PA 12, No. 5432 and 5793, respectively). As a result, carbon particles mainly accumulated in one phase or at the interface of the polymers and thus formed the so-called double-percolated conductive networks [20,25,26]. The concept of double-percolated polymer blends seems to work as long as the polymers are immiscible (Figure4a). Mamunya [ 27] suggested that, among immiscible polymers, CB tends to accumulate more in the polymer with lower surface tension. The distribution of CB in two different polymer phases can be predicted based on their interfacial tension and wetting behavior [26]. The distribution tendency of CB in immiscible polymer Materials 2020, 13, x FOR PEER REVIEW 6 of 15

screw extruder Collin (Table S1). Upon mixing the same compound (No. 6739) a second time in the same way, the resistivity increases a thousand-fold to 0.32 ± 0.15 kΩm (Table S1), which can be explained by the breakdown of percolated networks of aggregated CB. Compounding a similar material combination (50 wt% LDPE with 50 wt% 6111) in the microcompounder leads to an abrupt increase in resistivity up to 7.8 ± 0.4 MΩm (Table S1, Figure 4a). This trend shows the significant effect of intensive mixing on the formation and breakdown of percolated networks of conductive CB fillers in the polymer matrix, as explained by the so-called “aggregate-network” model [23].

103 106 20wt% CB

)

m 104 16wt% CB

(

( 2 4wt% CNT y 10

i 18wt% CB

t 2 2wt% CNT

s 10 i

r 14wt% CB

a 0 6wt% CNT

c 10 i 1

t 10

E E 10-2 Materials 2020, 13, 2645 12wt% CB 7 of 15 8wt% CNT 10-4 100 0 10 20 30 40 0 10 20 30 40 blendsa) found in literatureCB content is summarized (wt%) in Table2 (seeb) also [ 28]).Relative Based CNT on content the surface (wt%) energies of PE [29Figure], PA6 3. [29 Electrical,30], PA12 resistivity [30,31 ]of and (a) polyamide CB [21], as (PA well) filaments as their produced temperature by microcompounding dependencies, we, as estimated a the wettingfunction parameters of CB content; to and determine (b) compounded/extruded the distribution filament of CB in as our a function blends of [ 31relative]; see CNT Table addition2. We predict that CBwith selectively respect to locatesthe total either carbon at content the interface (i.e., CB + of CNT), PE/PA6 which or is in kept the at PE 20 phase wt%. For of PEclarification,/PA12 blends. the Thus, a double-percolatedabsolute CB and conductiveCNT concentrations network of canthe respective be envisioned compounds in both are immiscible stated in the blends. graph. The In consequence,error high conductivitybars represent could double be the achieved standard at deviation lower carbon (some of contents. the error However,bars are too the small transfer to be visible of this; see concept to bicomponentTable S1). antistatic For comparison, ﬁbers the failed, resistivity as is explainedof the conductive in the part next of section.the commercial bicomponent fiber Belltron B31 is plotted as dashed line.

108 102 PE PE/HDPE PE/LDPE

) 106 ) m PE/MDPE m 101

( PE/PA12 (

y y

t t i 4 PE/PA6 i

i 10 i

i i 0

s s 10

r r

l 2 l

a 10 a

c c

i i

c c -1

l l 10 0 E 10 E 1661-1665 1636-1640 1631-1635 10-2 10-2 10 15 20 25 30 35 40 45 0.5 1 1.5 2 2.5 3 3.5 a) CB content (wt%) b) Draw ratio (DR)

FigureFigure 4. 4. ((aa)) Electrical Electrical resistivity resistivity of of compounded/extruded compounded/extruded filament filament of of PE PE masterbatch masterbatch No. No. 6111, 6111, blended with different polymers. PE = No. 6111; PE/PE: 50% No. 6111 diluted in HDPE, LDPE and blended with different polymers. PE = No. 6111; PE/PE: 50% No. 6111 diluted in HDPE, LDPE and MDPE, respectively; PE/PA12: 37.5%, 40% and 50% No. 6111 diluted in No. 5793; PE/PA6: 37.5%, 40% MDPE, respectively; PE/PA12: 37.5%, 40% and 50% No. 6111 diluted in No. 5793; PE/PA6: 37.5%, and 50% No. 6111 diluted in No. 5432. The miscible compounds show a very high resistivity, whereas 40% and 50% No. 6111 diluted in No. 5432. The miscible compounds show a very high resistivity, the immiscible blends reveal a resistivity in the range of the value of the conductive part of the whereas the immiscible blends reveal a resistivity in the range of the value of the conductive part of the commercial bicomponent fiber Belltron B31 (~6 Ωm, dashed line). (b) Electrical resistivity of selected commercial bicomponent fiber Belltron B31 (~6 Ωm, dashed line). (b) Electrical resistivity of selected melt-spun bicomponent fibers with 13 vol% antistatic compound (Table S3) as a function of draw ratio melt-spun bicomponent fibers with 13 vol% antistatic compound (Table S3) as a function of draw ratio (DR). For comparison, the resistivity of the commercial bicomponent fiber Belltron B31 (~66 Ωm) is (DR). For comparison, the resistivity of the commercial bicomponent fiber Belltron B31 (~66 Ωm) is plotted as dashed line. The error bars represent double the standard deviation (see Tables S1 and S3); plottedsome of as the dashed error line.bars are The too error small bars to representbe visible. double the standard deviation (see Tables S1 and S3); some of the error bars are too small to be visible.

3.2. Physical Properties of Bicomponent Fibers Based on the results presented so far, it appears that the most convenient approach to overcome the intrinsic resistivity of synthetic fibers is to add carbon particles in high concentrations to the polymer matrix and melt-spin them into bicomponent fibers. In bicomponent melt-spinning, two polymers of different chemical/physical natures are extruded from one spinneret to form a single fiber [6]. This approach, with an antistatic compound as a minor element, prevents a severe loss of the tensile strength of the melt-spun fibers caused by a high additive loading. A design of experiment (DoE) was chosen that covers a theoretical maximum of 336 combinations. In the end, 110 different bicomponent fibers were melt-spun and analyzed (Table S3). The sandwich cross-sections were regular in shape for all respective fibers (Figure2b or Figure S1a–c). The shape of the wedge cross-sections, on the other hand, varied strongly as a function of the viscosity differences of the two compounds constituting the bicomponent fibers (Figure2a or Figure S1d–i). For all cross-sections, the shape was maintained when the fibers were drawn (Figure S1a–f). TiO2 was used to partially bury the conductive element inside a dull bicomponent fiber, and to attenuate its undesired dark color. As an effect of increased draw ratio, we noticed that the scattering efficiency of the TiO2 pigments decreased with increasing spatial dispersion, while the coverage of the carbon black was largely unaffected (Figure5). Materials 2020, 13, 2645 8 of 15

Table 2. Distribution tendency of CB in immiscible polymer blends. Polymers considered: polymethyl methacrylate (PMMA), polypropylene (PP), polyoxymethylene (POM), polyvinylidene ﬂuoride (PVDF), ethylene vinyl acetate (EVA), polystyrene (PS) and acrylonitrile butadiene styrene (ABS). Materials 2020, 13, x FOR PEER REVIEW 8 of 15 Distribution Blend Composition Reference 3.2. Physical Properties of Bicomponent Fibers Tendency of CB PMMA PP CB Interface Sumita et al. [20] Based on the results presented so far, it appears that the most convenient approach to overcome PMMA HDPE CB Interface Sumita et al. [20] the intrinsicLDPE resistivity of synthetic PP fibers CB is to add carbon LDPE particles in highMamunya concentration [27] s to the polymer matrixLDPE and melt- POMspin them into CB bicomponent Interface fibers. In bicomponent Mamunya melt [27-spinning,] two polymers of differentHDPE chemical/physical PVDF nature CBs are extruded HDPE from one spinneret Feng to et form al. [ 32a single] fiber [6]. This approach,PA66 with an antistatic PP compound CB as a minorPA66 element, prevents Zoldan a severe et al. loss [33 of] the tensile strength of PA6the /melt6-9-spun fibers PP caused by CB a high additive PA6/ 6-9loading. Tchoudakov et al. [34] A designPP of experiment EVA (DoE) was CB chosen that EVA covers a theoretical Huang et maximum al. [35] of 336 PP PS CB PS Tan et al. [36] combinations. In the end, 110 different bicomponent fibers were melt-spun and analyzed (Table S3). LDPE EVA CB LDPE Yu et al. [37] The sandwichPS cross-sections PMMA were regular CB in shape for all PS respective fibers Pan (Figure et al. 2b [38 or] Figure S1a– c). The shapeHDPE of the wedge PPcross-sections, CB on the other HDPE hand, varied strongly Xu et al.as [a39 func] tion of the viscosity differencesPP of the two PA6 compounds CB constituting PA6 the bicomponent Chen fibers et (Figure al. [40] 2a or Figure S1d–i). For allABS cross-sections, PA6 the shape was CB maintained PA6when the fibers were Wu drawn et al. [41 (Figure] S1a–f). TiO2 wasPS used to partially PA6 bury the CBconductive element PA6 inside a dull bicomponent Xu et al. [39] fiber, and to attenuate itsPMMA undesired dark PA6 color. As an CBeffect of increased PA6 draw ratio, we noticed Xu et al. that [39] the scattering PE PA12 CB PE This study efficiency of the TiO2 pigments decreased with increasing spatial dispersion, while the coverage of PE PA6 CB Interface This study the carbon black was largely unaffected (Figure 5).

30 1.0 1.5 2.0 2.5 3.0 3.5 4.0 DR 1690 25 1696 1676

) 1682

% 20 0.9wt% TiO2

(

i 1676 1677 1678 1679 1680 1681 1682

t 15

R 10

5 2.1wt% TiO2

0 1690 1691 1692 1693 1694 1695 1696 400 450 500 550 600 650 700 a) Wave length (nm) b)

Figure 5. Grayness of selected filaments as a function of the draw ratio (DR) and TiO2 content: (a) Figure 5. Grayness of selected filaments as a function of the draw ratio (DR) and TiO content: (a) reflectivity (in %) measured directly on the bobbins; (b) photograph of fiber bobbins. 2 reflectivity (in %) measured directly on the bobbins; (b) photograph of fiber bobbins. The tensile properties of selected fibers are listed in Table S4. Additionally, Figure 6a shows the The tensile properties of selected fibers are listed in Table S4. Additionally, Figure6a shows the load–strain behavior of melt-spun bicomponent fibers, which is typical for PA6 fibers [42,43]. In the load–strain behavior of melt-spun bicomponent fibers, which is typical for PA6 fibers [42,43]. In the undrawn state (DR 1), the fibers show considerable plastic deformation under stress. With an undrawn state (DR 1), the fibers show considerable plastic deformation under stress. With an increasing increasing draw ratio, the fibers gain in stiffness and resilience. A draw ratio of 4 is about the drawmaximum ratio, the that fibers can gainbe achieved in stiffness without and resilience.breaking the A drawfiber, ratiowhich of corr 4 isesponds about the to maximuman elongation that o canf be300 achieved% of the without undrawn breaking (DR 1) the fiber. fiber, These which findings corresponds indicate to thatan elongation the antistatic of 300% compound of the undrawnhas an (DRindustrially 1) fiber. These tolerable findings impact indicate on the thattensile the properties, antistatic compoundas long as its has proportion an industrially is small tolerable (at or below impact on20 the vol%). tensile As properties,expected, the as specific long as tensile its proportion strength (i isn smallcN/tex) (at is orreduced below in 20 comparison vol%). As to expected, a pristine the specificPA6 filament tensile strength(Table S4), (in mainly cN/tex) since is reducedthe antistatic in comparison compound tocontributes a pristine to PA6 the weight filament but (Table not to S4), mainlythe tensile since strength. the antistatic Tex, a compound direct measure contributes of the linear to the fiber weight density, but is not the to mass the tensilein grams strength. per 1000 Tex, m a directof the measure fiber. of the linear fiber density, is the mass in grams per 1000 m of the fiber. Figure 6b also illustrates that the influence of the cross-section type on the load–strain behavior is small. The maximum tensile strength seems to be slightly higher with the wedge cross-section, most probably due to a smaller influence on the integrity of the fiber (see Figure 2a). Furthermore, the increased stiffness of the sandwich fiber with a higher antistatic portion can be explained by a mobility constraint of the load-bearing part by the antistatic fraction. Such a constraint seems not to affect the wedge fibers, most probably because of the above-mentioned smaller influence on the load– strain behavior.

Materials 2020, 13, 2645 9 of 15 Materials 2020, 13, x FOR PEER REVIEW 9 of 15

45 45 DR 4 sandwich 80:20 40 40 inlet 80:20

) ) inlet 87:13

x 35 x 35

e e sandwich 87:13

t t

/ /

N 30 N 30 c DR 3 c

( (

h h

t 25 t 25

g g

n n

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r 20 r 20 t DR 2 t

s s

e e

l l

i 15 i 15 s DR 1 s

n n

e 10 e 10

T T

5 5

0 0 0 100 200 300 400 0 50 100 150 a) Elongation (%) b) Elongation (%) FigureFigure 6. 6.Average Average tensile tensile stress–strain stress–straincurves curves ofof selectedselected fibers fibers containing containing 13% 13% of of PA12 PA12 with with 15.3 15.3 CB CB andand 1.5 1.5 CNT CNT as as the the antistatic antistatic component: component: ((aa)) sandwichsandwich cross cross-section-section fiber fiber at at different different draw draw ratios ratios (1: (1: 1647,1647, 2: 1649,2: 1649, 3: 3: 1651, 1651, 4: 4: 1653); 1653); ( b(b)) wedge wedge (80(80/20:/20: 1623, 87/13: 87/13: 1628) 1628) and and sandwich sandwich (80/20: (80/20: 1644, 1644, 87/13: 87/13: 1650)1650) cross-section cross-section fibers fibers with with draw draw ratio ratio 2.5. 2.5.

FigureScanning6b also electron illustrates microscopy that the p influenceictures of ofselected the cross-section sandwich fiber type surfaces on the load–strain(Figure 7) show behavior the is small.following. The The maximum surface of tensile the antistatic strength compound seems to shows be slightly a higher higher roughness with the than wedge that of cross-section, pure PA6, mostwhich probably can be dueexplained to a smaller by the tendency influence of on CB the to integrityagglomerate of thewithin fiber the (see polymer Figure matrix.2a). Furthermore, When the thebase increased polymer sti offfness the antistatic of the sandwich compound fiber and with the amain higher fiber antistatic component portion are miscible can be explained(Figure 7a,b), by a the sandwich filling is perfectly integrated in the fiber, but if they are immiscible (Figure 7c,d), the mobility constraint of the load-bearing part by the antistatic fraction. Such a constraint seems not two components tend to separate, which could lead to fibrillation when a mechanical stress is applied. to affect the wedge fibers, most probably because of the above-mentioned smaller influence on the However, no transversal cracks were found in the sandwich fillings of the drawn fibers (Figure 7b,d), load–strain behavior. thus the increasing electrical resistivity at higher draw ratios (Figure 4b) cannot be explained by a Scanning electron microscopy pictures of selected sandwich fiber surfaces (Figure7) show the rupture of the antistatic compound. following. The surface of the antistatic compound shows a higher roughness than that of pure PA6, which can be explained by the tendency of CB to agglomerate within the polymer matrix. When the base polymer of the antistatic compound and the main fiber component are miscible (Figure7a,b), the sandwich filling is perfectly integrated in the fiber, but if they are immiscible (Figure7c,d), the two components tend to separate, which could lead to fibrillation when a mechanical stress is applied. However, no transversal cracks were found in the sandwich fillings of the drawn fibers (Figure7b,d), thus the increasing electrical resistivity at higher draw ratios (Figure4b) cannot be explained by a rupture of the antistatic compound. The increase in the resistivity when the fiber is melt-drawn can be ascribed to a spatial dilution of the carbon filler (Figure4b). Here, a draw ratio (DR) of 3 still gives low enough resistivity values. As this is also a reasonable DR for PA6, we mainly consider this DR for the following analysis. As long as the carbon content is above a certain value, the resistivity is only affected by the conductivity of the carbon composite and not by its content ratio within the bicomponent fiber, as shown in Figure4b. In other words, it is sufficient that only a small portion of the bicomponent fiber is conductive. At the same bicomponent ratio (20:80), the shape of the cross-section has no influence when only CB is used (Figure8a). However, when CNTs are added, the resistivity is reduced in case of the wedge structure. This is probably due to the spinneret channels, which are comparatively narrower to achieve the wedge cross-section. The resulting higher shear improves the dispersion state of the CNTs by breaking the CNT aggregates, leading to lower resistivity [44,45].

Materials 2020, 13, 2645 10 of 15 Materials 2020, 13, x FOR PEER REVIEW 10 of 15

a) b)

c) d)

FigureFigure 7. 7.SEM SEM imagesimages of of selected selected fiber fiber surfaces. surfaces. (a,b ()a Sandwich,b) Sandwich fibers fibers with withantistatic antistatic compound compound No. No.6066 6066 (25% (25% CB CB in inPA12), PA12), draw draw ratios ratios 1 (No. 1 (No. 1581, 1581, a) and (a)) 2 and (No. 2 (No.1583, 1583,b). (c, (db))). Sandwich (c,d) Sandwich fibers with fibers withantistatic antistatic compound compound No. No.6237 6237 (50:50 (50:50 mixture mixture of PA6 of PA6and andPE with PE with40% 40%CB, draw CB, draw ratios ratios 1 (No. 1 1669, (No. 1669,c) and 2 (No. 1671, d). Arrows indicate the antistatic compound. Materials(c)) and 2020 2, (No.13, x FOR 1671, PEER (d)). REVIEW Arrows indicate the antistatic compound. 11 of 15

5 6 10The increase in the resistivity when the fiber is melt10 -drawn can be ascribed to a spatial dilution of the carbon filler (Figure 4b). Here, a draw 6066 ratio (DR) of 3 still gives low enough resistivity values. 6081 6083 5 As this4 is also a reasonable DR for PA6, we mainly consider10 this DR for the following analysis. As

) 10 )

m long as the carbon content is above a certain value, them resistivity is only affected by the conductivity 4

( ( of the carbon composite and not by its content ratio within 10 the bicomponent fiber, as shown in Figure

y y

t 3 t

i i

v 10 v

4b.i In other words, it is sufficient that only a small portioni of the bicomponent fiber is conductive.

t t

s s

i i 3

s At the same bicomponent ratio (20:80), the shape s of10 the cross-section has no influence when only

e e

r r

CBl is2 used (Figure 8a). However, when CNTs are added,l the resistivity is reduced in case of the

a 10 a

c c 2

i i

r r 10

wedget structure. This is probably due to the spinnerett channels, which are comparatively narrower

c c

e e tol achieve1 the wedge cross-section. The resulting higherl shear improves the dispersion state of the E 10 E 1 CNTs by breaking the CNT aggregates, leading to lower10 resistivity [44,45]. Figure 8b shows the influence of the type of filler on the resistivity. With 40 wt% CB (6107), even with 0 0 10 10 a DR ofwedge 3, thesandwich percolationwedge thresholdsandwich iswedge far exceededsandwich and the resulting6107 resistivity6066 6082 is 25 times6081 lower6083 than6237 that ofa) the commercial Typefiber. of With cross-section 25 wt% CB (6066), the b)dilution of the carbonType fillerof carbon at a fillerdraw factor of 3 increased the resistivity by a factor of ~1000, whereas the twin-screw extruded undrawn filaments had Figure 8. Resistivity of melt-spun bicomponent fibers (DR 3) as a function of the (a) fiber cross-section previouslyFigure 8. shownResistivity sufficient of melt-spun conductivity bicomponent (Figure fibers 3a). The (DR combination 3) as a function of ofCB the and (a) CNTs, fiber cross-section on the other (fibers with 20 vol% antistatic compound) and (b) type of carbon filler (sandwich fibers with 13 vol% hand,(fibers resulted with 20in vol%good antistaticconductivity compound) (i.e., low and enough (b) type resistivity) of carbon at filler an overall (sandwich carbon fibers content with 13below vol% 25 antistatic compound). For comparison, the resistivity of the commercial bicomponent fiber Belltron wt%antistatic (6081–6083). compound). Considering For comparison, the approach the resistivitywith a 50: of50 the polymer commercial–polymer bicomponent blend with fiber carbon Belltron black B31 in B31 (~66 Ωm) is plotted as a dashed line. The error bars represent double the standard deviation (some only(~66 oneΩ ofm) the is plottedimmiscible as a dashedpolymers line. (6237), The error the conductivity bars represent was double lost after the standard the melt deviation-spinning (somethe blend. of of the error bars are too small to be visible; see Table S3). Wethe believe error that bars this are toois due small to tobreakup be visible; of the see conductive Table S3). domains within the nonconductive polymer in the narrowIn conclusion, channels we of thefound spinneret, that the which carbon led content to losing can the be double reduced-percolation. by applying a mixture of CB and Figure8b shows the influence of the type of filler on the resistivity. With 40 wt% CB (6107), even CNTs in the antistatic compound, and by improving the dispersion state of CNTs in the narrow melt- with a DR of 3, the percolation threshold is far exceeded and the resulting resistivity is 25 times lower flow channels of a bicomponent spinneret [45]. On the other hand, the grayness can be attenuated by

a combination of a low carbon composite ratio in the bicomponent fiber and a high TiO2 loading in the major polymer component. Such an approach would also guarantee good mechanical behavior of the melt-spun fibers.

3.3. Prototype Production of Antistatic Hi-Viz Workwear Upscaling trials on an industrial-scale spinning plant, using the sandwich cross-section approach, showed that all CNT-containing composites led to severe spinning instabilities, finally resulting in filter and spinneret blockage. The main reason for this setback is that the industrial-scale spin-packs and spinnerets are not typically optimized for such a kind of materials. Since developing a new design for the processing equipment was beyond the scope of this research, we were forced to resort to our CNT-free material for industrial melt-spinning. In consequence, the final upscaling trials were performed with the 40 wt% CB compound (No. 6107), the only feasible conductive composite left in this study. For both the antistatic composite and main bicomponent fiber polymer (No. 5528), PA6 was used for the upscaling melt-spinning trials at EMS-CHEMIE AG (Domat/Ems, Switzerland). Due to co- extrusion viscosity mismatch, adding TiO2 to the base polymer also led to spinning instabilities; in consequence, the final upscaling trials were performed without TiO2. The lowest bicomponent ratio achievable with the available industrial-scale melt-spinning plant over a prolonged time was 43 vol% of antistatic compound. Although the processing conditions were far from optimal, we still proceeded to the production of the respective antistatic staple fibers (fineness, 1.2 ± 0.3 tex; nominal length, 51 mm). The resulting fibers reached a resistivity of less than 0.1 Ωm, which is three orders of magnitudes better than that of the commercial fiber Belltron B 31 (~66 Ωm; nominal fineness, 0.3 tex) that we took into consideration as a reference. The overall carbon black content in the fiber, calculated from the CB content of the masterbatch and the bicomponent ratio of the fiber, was ~17 wt%. In the next step, the Rieter Spin Center (Winterthur, Switzerland) produced a staple fiber yarn by ring spinning, having a fineness of 40 tex. The yarn was composed of 95 wt% flame retardant polyester staple fibers (Trevira CS, fineness 0.33 tex, nominal length 60 mm) and only 5 wt% of above antistatic staple fiber, resulting in a CB content of ~0.9 wt% concerning the yarn. With this antistatic yarn, Jenny Fabrics AG (Niederurnen, Switzerland) produced a plain weave fabric (Figure 9), with

Materials 2020, 13, 2645 11 of 15 than that of the commercial fiber. With 25 wt% CB (6066), the dilution of the carbon filler at a draw factor of 3 increased the resistivity by a factor of ~1000, whereas the twin-screw extruded undrawn filaments had previously shown sufficient conductivity (Figure3a). The combination of CB and CNTs, on the other hand, resulted in good conductivity (i.e., low enough resistivity) at an overall carbon content below 25 wt% (6081–6083). Considering the approach with a 50:50 polymer–polymer blend with carbon black in only one of the immiscible polymers (6237), the conductivity was lost after the melt-spinning the blend. We believe that this is due to breakup of the conductive domains within the nonconductive polymer in the narrow channels of the spinneret, which led to losing the double-percolation. In conclusion, we found that the carbon content can be reduced by applying a mixture of CB and CNTs in the antistatic compound, and by improving the dispersion state of CNTs in the narrow melt-flow channels of a bicomponent spinneret [45]. On the other hand, the grayness can be attenuated by a combination of a low carbon composite ratio in the bicomponent fiber and a high TiO2 loading in the major polymer component. Such an approach would also guarantee good mechanical behavior of the melt-spun fibers.

3.3. Prototype Production of Antistatic Hi-Viz Workwear Upscaling trials on an industrial-scale spinning plant, using the sandwich cross-section approach, showed that all CNT-containing composites led to severe spinning instabilities, finally resulting in filter and spinneret blockage. The main reason for this setback is that the industrial-scale spin-packs and spinnerets are not typically optimized for such a kind of materials. Since developing a new design for the processing equipment was beyond the scope of this research, we were forced to resort to our CNT-free material for industrial melt-spinning. In consequence, the final upscaling trials were performed with the 40 wt% CB compound (No. 6107), the only feasible conductive composite left in this study. For both the antistatic composite and main bicomponent fiber polymer (No. 5528), PA6 was used for the upscaling melt-spinning trials at EMS-CHEMIE AG (Domat/Ems, Switzerland). Due to co-extrusion viscosity mismatch, adding TiO2 to the base polymer also led to spinning instabilities; in consequence, the final upscaling trials were performed without TiO2. The lowest bicomponent ratio achievable with the available industrial-scale melt-spinning plant over a prolonged time was 43 vol% of antistatic compound. Although the processing conditions were far from optimal, we still proceeded to the production of the respective antistatic staple fibers (fineness, 1.2 0.3 tex; nominal ± length, 51 mm). The resulting fibers reached a resistivity of less than 0.1 Ωm, which is three orders of magnitudes better than that of the commercial fiber Belltron B 31 (~66 Ωm; nominal fineness, 0.3 tex) that we took into consideration as a reference. The overall carbon black content in the fiber, calculated from the CB content of the masterbatch and the bicomponent ratio of the fiber, was ~17 wt%. In the next step, the Rieter Spin Center (Winterthur, Switzerland) produced a staple fiber yarn by ring spinning, having a fineness of 40 tex. The yarn was composed of 95 wt% flame retardant polyester staple fibers (Trevira CS, fineness 0.33 tex, nominal length 60 mm) and only 5 wt% of above antistatic staple fiber, resulting in a CB content of ~0.9 wt% concerning the yarn. With this antistatic yarn, Jenny Fabrics AG (Niederurnen, Switzerland) produced a plain weave fabric (Figure9), with only one out of eight yarns being antistatic (none of the warp yarns, only every fourth of the weft yarns), the rest being neat flame-retardant staple fiber yarn (Trevira CS, fineness 37 tex). Thus, the final CB content of the fabric was ~0.1 wt%. Finally, the fabric was finished with a luminescent dye by AG Cilander (Herisau, Switzerland). With this fabric, a hi-viz safety workwear jacket was produced by Hüsler Berufskleider AG (Sirnach, Switzerland) and positively tested regarding the most relevant requirements, i.e., surface resistivity (EN 1149-5 [46]), luminescence (EN 471 [47]) and flame retardancy (UL94 [48]). Materials 2020, 13, x FOR PEER REVIEW 12 of 15

only one out of eight yarns being antistatic (none of the warp yarns, only every fourth of the weft yarns), the rest being neat flame-retardant staple fiber yarn (Trevira CS, fineness 37 tex). Thus, the final CB content of the fabric was ~0.1 wt%. Finally, the fabric was finished with a luminescent dye by AG Cilander (Herisau, Switzerland). With this fabric, a hi-viz safety workwear jacket was produced by Hüsler Berufskleider AG (Sirnach, Switzerland) and positively tested regarding the Materials 2020, 13, 2645 12 of 15 most relevant requirements, i.e., surface resistivity (EN 1149-5 [46]), luminescence (EN 471 [47]) and flame retardancy (UL94 [48]).

Warp yarn

Weft yarn

Antistatic yarn

Figure 9. Plain weave fabric with every fourth of the weft yarns being antistatic, the rest being neat Figure 9. Plain weave fabric with every fourth of the weft yarns being antistatic, the rest being neat flame-retardant staple fiber yarns. ﬂame-retardant staple ﬁber yarns.

4.4. Conclusions Conclusions TheThe use use of antistatic antistatic fibers fibers in safety in safety workwear workwear is an established is an established technology technology to prevent to friction prevent- friction-inducedinduced sparks sparks that can that harm can harm delicate delicate equipment equipment or provoke or provoke fire fire and and explosions explosions in in combustible combustible environments.environments Despite. Despite their their blackness, blackness, especiallyespecially objectionable in in hi hi-viz-viz applications, applications, and and its its negative negative influence on the tensile properties of fibers, carbon (nano)particles (CB and CNTs) are still the most influence on the tensile properties of fibers, carbon (nano)particles (CB and CNTs) are still the most promising candidates for antistatic compounds. In this study, we assess the potential of various promising candidates for antistatic compounds. In this study, we assess the potential of various conventional methods of implementing such particles in polymers, in order to increase the electrical conventional methods of implementing such particles in polymers, in order to increase the electrical conductivity of the host. A rather comprehensive overview of these approaches is offered, which conductivity of the host. A rather comprehensive overview of these approaches is offered, which reveals the challenges of upscaling such materials from the lab to pilot to the industrial production reveals the challenges of upscaling such materials from the lab to pilot to the industrial production of of melt-spun fibers. melt-spunWe show fibers. that the embedding of a conductive compound into a bicomponent melt-spun fiber, as We show that the embedding of a conductive compound into a bicomponent melt-spun fiber, as a a minor component within a TiO2-pigmented main polymer, can result in a fiber with reduced minorgrayness component and still within-sufficient a TiO conductivity2-pigmented and main tensile polymer, strength. can It result is discussed in a fiber that with the reduced combination grayness of andCB still-su and CNTfficient within conductivity one polymer, and as tensile well as strength. the blending It is of discussed a CB compound that the with combination a pure immiscible of CB and CNTpolymer, within can one both polymer, lead to as reduced well as electrical the blending resistivity of a CB at compound maintained with carbon a pure content. immiscible However, polymer, the canattempt both leadto melt to- reducedspin respecti electricalve antistatic resistivity compounds at maintained in a bicomponent carbon content. approach However, resulted the either attempt in tosevere melt-spin processing respective instabilities antistatic or in compounds total failure in of a the bicomponent anticipated electrical approach performance. resulted either in severe processingNevertheless, instabilities we orcould in total successfully failure of develop the anticipated and produce electrical an antistatic performance. fiber at the industrial scaleNevertheless, with an electrical we could resistivity successfully below 0.1 develop Ωm, which and is produce about a anthousand antistatic times fiber better at thethan industrial that of scalethe with existing an electricalcommercial resistivity fibers. Out below of these 0.1 Ω fibers,m, which we produced is about a a thousand staple fiber times yarn better and finally than that a ofwoven the existing fabric, commercialwhich was finished fibers. with Out a of luminescent these fibers, dye. we From produced this fabric, a staple a safety fiber workwear yarn and jacket finally a wovenwas tailored, fabric, which which fulfilled was finished the relevant with astandards luminescent regarding dye. Fromresistivity, this fabric,luminescence a safety and workwear flame jacketretardancy. was tailored, which fulfilled the relevant standards regarding resistivity, luminescence and flame retardancy.We conclude by emphasizing that there is still a lot of space for improvement. To realize accordiWe concludengly optimized by emphasizing antistatic that fibers there on is a still large a lot scale, of space for for instance, improvement. an auxiliary To realize extruder accordingly with optimizedcomparably antistatic small fibers throughput on a large would scale, be for required, instance, as an well auxiliary as a special extruder spinneret with comparably that takes small the throughputviscosities wouldof the carbon be required, composite as well and asthe a TiO special2-pigmented spinneret main that polymer takes the into viscosities account. of the carbon composite and the TiO -pigmented main polymer into account. Supplementary Materials:2 The following are available online at www.mdpi.com/xxx/s1, Table S1: Electrical Supplementaryresistivity of filaments Materials: producedThe following in this study are available by compounding online at http:in a twin//www.mdpi.com-screw extruder/1996-1944, Table S2/:13 Pilot/11/ 2645melt/-s1, Tablespinning: S1: Electrical average spin resistivity pressures of filaments and processing produced temperatures in this study, Table by S3 compounding: Electrical resistivity in a twin-screw of fibers produced extruder, Tablein this S2: study Pilot melt-spinning:by melt-spinning average, Table spinS4: Tensile pressures properties and processing of selected temperatures, fibers produced Table in S3:this Electrical study, Figure resistivity S1: ofMicroscopic fibers produced images in thisof selected study byfiber melt-spinning, cross-sections Table produced. S4: Tensile properties of selected fibers produced in this study, Figure S1: Microscopic images of selected fiber cross-sections produced.

Author Contributions: Conceptualization, R.H., K.B. and F.C.; methodology, R.H., A.G. (Ali Gooneie) and F.C.; validation, A.G. (Ali Gooneie); formal analysis, R.H. and F.C.; investigation, R.H., T.S., P.S., A.G. (Andreas Geiger), D.P., G.S. and F.C.; writing—original draft preparation, R.H.; writing—review and editing, R.H., A.G. (Ali Gooneie) and F.C.; visualization, R.H. and A.G. (Ali Gooneie); supervision, R.H., K.B. and F.C.; project administration, R.H.; funding acquisition, R.H., A.G. (Ali Gooneie), K.B. and F.C. All authors have read and agreed to the published version of the manuscript. Materials 2020, 13, 2645 13 of 15

Funding: This research was funded by Innosuisse (Switzerland), grant number 18816.1 PFIW-IW, and Zürcher Stiftung für Textilforschung (Switzerland), grant number 127. Ali Gooneie was supported by the Empa Internal Research Call 2018 in the framework of the “ALLFIN” project. Acknowledgments: The authors acknowledge the support of Mathias Lienhard in the compounding trials, Benno Wüst in the melt-spinning experiments, and Marion Höhener in the electrical resistivity measurements. We also thank EMS-CHEMIE AG, Rieter Spin Center, Jenny Fabrics AG, AG Cilander and Hüsler Berufskleider AG (all Switzerland) for donations and upscaling trials. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Article Antistatic Fibers for High-Visibility Workwear: Challenges of Melt-Spinning Industrial Fibers

Rudolf Hufenus 1,*, Ali Gooneie 1, Tutu Sebastian 2, Pietro Simonetti 1, Andreas Geiger 2, Dambarudhar Parida 1, Klaus Bender 3, Gunther Schäch 3 and Frank Clemens 2

Supplementary Materials

Table S1. Electrical resistivity of filaments produced in this study by compounding in a twin-screw extruder (Minilab microcompounder if not stated otherwise). Specified are the compounds with respective CB and CNT content, as well as mean value and standard deviation of the measured electrical resistivity in Ωm.

CB CNT CB CNT Electrical Base Polymer Compound Compound Content Content Resistivity (wt%) (wt%) - - 100% 6111 - - 40 - 0.043 ± 0.000 Ωm - - 100% 6107 - - 40 - 0.37 ± 0.01 Ωm 25% 5793 75% 6107 - - 30 - 1.0 ± 0.1 Ωm 37.5% 5793 62.5% 6107 - - 25 - 1.7 ± 0.2 Ωm 50% 5793 50% 6107 - - 20 - 0.29 ± 0.01 kΩm 62.5% 5793 37.5% 6107 - - 15 - 0.15 ± 0.02 MΩm 75% 5793 25% 6107 - - 10 - 11 ± 5 MΩm 87.5% 5793 12.5% 6107 - - 5 - 736 ± 151 MΩm 41.7% 5793 45% 6107 13.3% 6065 18 2 0.108 ± 0.003 kΩm 33.3% 5793 40% 6107 26.7% 6065 16 4 0.084 ± 0.002 kΩm 25% 5793 35% 6107 40% 6065 14 6 12.1 ± 0.6 Ωm 16.7% 5793 30% 6107 53.3% 6065 12 8 1.39 ± 0.08 Ωm 50% HDPE 50% 6111 - - 20 - 29.9 ± 1.4 MΩm 50% MDPE 50% 6111 - - 20 - 3.6 ± 0.4 MΩm 50% LDPE 50% 6111 - - 20 - 7.8 ± 0.4 MΩm 50% 5432 50% 6111 - - 20 - 0.252 ± 0.000 Ωm 60% 5432 40% 6111 - - 16 - 3.20 ± 0.09 Ωm 62.5% 5432 37.5% 6111 - - 15 - 8.05 ± 0.13 Ωm 50% 5793 50% 6111 - - 20 - 0.59 ± 0.01 Ωm 60% 5793 40% 6111 - - 16 - 1.10 ± 0.13 Ωm 62.5% 5793 37.5% 6111 - - 15 - 7.59 ± 0.70 Ωm 50% 5793 50% 6107 - - 20 - 289 ± 5 Ωm 50% 5432 50% 6107 - - 20 - 0.98 ± 0.04 MΩm 50% LDPE 50% 6107 - - 20 - 0.28 ± 0.32 MΩm 50% 887 50% 6111 - - 20 - 0.23 ± 0.01 Ωm 1

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- - 100% 6739 - - 20 - 0.32 ± 0.15 kΩm 1 1 Compounded with twin screw extruder Collin.

Table S2. Pilot melt-spinning: average (±2) spin pressures and processing temperatures. The temperature of the polymer melt was measured between extruder and metering pump as well as between metering pump and spin pack.

TiO2 Spinneret Anti-static Fibers No. Spin Pressure Polymer Melt Content Type Ratio (bar) Temperature (°C) Antistatic (wt%) (vol%) minor major minor major spin compound part part part part pack

20 1631-1635 85 145 255 276 281 6107 0.9 wedge 13 1636-1640 81 157 255 276 281 sandwich 20 1581-1586 60 54 256 275 286 6066 0.9 20 1587-1591 64 53 256 275 281 wedge 13 1592-1598 89 175 256 275 281 20 1600-1604 93 123 257 277 281 6082 0.9 wedge 13 1605-1609 91 138 257 277 281 20 1654-1660 122 138 256 268 274 sandwich 13 1661-1667 112 145 256 270 274 6081 0.9 20 1610-1614 111 137 257 276 282 wedge 13 1615-1619 107 151 257 276 282 20 1641-1646 195 140 256 260 259 sandwich 13 1647-1653 191 156 256 260 259 6083 0.9 20 1620-1624 185 166 256 270 285 wedge 13 1625-1629 176 176 257 270 285 20 1669-1674 98 150 256 271 277 0.9 sandwich 13 1676-1682 92 162 256 271 279 6237 20 1683-1688 95 149 256 272 278 2.1 sandwich 13 1690-1696 90 160 256 272 278

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Table S3. Electrical resistivity of fibers produced in this study by melt-spinning. Specified are the bicomponent cross-section (sandwich or wedge), the type of antistatic compound (Table 1), the volumetric ratio of the antistatic compound in the bicomponent fiber, the respective draw ratio as well as mean value and standard deviation of the measured electrical resistivity in Ωm.

Fiber Cross- Antistatic Antistatic TiO2 Draw Electrical No. Section Compound Ratio Content Ratio Resistivity (vol%) (wt%) (DR) 1581 sandwich 6066 20 0.9 1.0 0.73 ± 0.09 Ωm 1583 sandwich 6066 20 0.9 2.0 3.4 ± 1.0 Ωm 1584 sandwich 6066 20 0.9 3.0 31 ± 7 kΩm 1585 sandwich 6066 20 0.9 1.5 0.71 ± 0.12 Ωm 1586 sandwich 6066 20 0.9 2.5 231 ± 7 Ωm 1587 wedge 6066 20 0.9 1.0 2.9 ± 0.4 Ωm 1588 wedge 6066 20 0.9 1.5 0.50 ± 0.11 Ωm 1589 wedge 6066 20 0.9 2.0 2.5 ± 1.1 Ωm 1590 wedge 6066 20 0.9 2.5 30 ± 16 Ωm 1591 wedge 6066 20 0.9 3.0 32 ± 4 kΩm 1592 wedge 6066 13 0.9 1.0 2.9 ± 2.3 Ωm 1593 wedge 6066 13 0.9 1.5 0.88 ± 0.11 Ωm 1594 wedge 6066 13 0.9 2.0 6.2 ± 0.5 Ωm 1595 wedge 6066 13 0.9 2.5 0.16 ± 0.06 kΩm 1596 wedge 6066 13 0.9 3.0 13 ± 3 kΩm 1597 wedge 6066 13 0.9 3.5 36 ± 14 kΩm 1598 wedge 6066 13 0.9 4.0 0.10 ± 0.14 MΩm 1600 wedge 6082 20 0.9 1.0 1.15 ± 0.07 Ωm 1601 wedge 6082 20 0.9 1.5 0.88 ± 0.07 Ωm 1602 wedge 6082 20 0.9 2.0 2.5 ± 0.4 Ωm 1603 wedge 6082 20 0.9 2.5 9.5 ± 0.7 Ωm 1604 wedge 6082 20 0.9 3.0 74 ± 5 Ωm 1605 wedge 6082 13 0.9 1.0 1.0 ± 0.2 Ωm 1606 wedge 6082 13 0.9 1.5 1.0 ± 0.1 Ωm 1607 wedge 6082 13 0.9 2.0 3.0 ± 0.4 Ωm 1608 wedge 6082 13 0.9 2.5 16 ± 3 Ωm 1609 wedge 6082 13 0.9 3.0 65.8 ± 0.5 Ωm 1610 wedge 6081 20 0.9 1.0 0.80 ± 0.22 Ωm 1611 wedge 6081 20 0.9 1.5 0.90 ± 0.07 Ωm 1612 wedge 6081 20 0.9 2.0 2.9 ± 0.1 Ωm 1613 wedge 6081 20 0.9 2.5 4.1 ± 0.3 Ωm 1614 wedge 6081 20 0.9 3.0 9.4 ± 0.7 Ωm 1615 wedge 6081 13 0.9 1.0 1.6 ± 0.4 Ωm 1616 wedge 6081 13 0.9 1.5 2.0 ± 0.1 Ωm 1617 wedge 6081 13 0.9 2.0 6.2 ± 1.2 Ωm 1618 wedge 6081 13 0.9 2.5 9.9 ± 3.6 Ωm 1619 wedge 6081 13 0.9 3.0 33 ± 5 Ωm 1620 wedge 6083 20 0.9 1.0 2.2 ± 0.3 Ωm 1621 wedge 6083 20 0.9 1.5 3.1 ± 0.4 Ωm 1622 wedge 6083 20 0.9 2.0 3.0 ± 0.3 Ωm 1623 wedge 6083 20 0.9 2.5 3.4 ± 0.2 Ωm 1624 wedge 6083 20 0.9 3.0 4.5 ± 1.6 Ωm 1625 wedge 6083 13 0.9 1.0 4.4 ± 0.6 Ωm 1626 wedge 6083 13 0.9 1.5 2.3 ± 0.2 Ωm 1627 wedge 6083 13 0.9 2.0 4.6 ± 0.4 Ωm

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1628 wedge 6083 13 0.9 2.5 11 ± 1 Ωm 1629 wedge 6083 13 0.9 3.0 48 ± 4 Ωm 1631 wedge 6107 20 0.9 1.0 0.09 ± 0.02 Ωm 1632 wedge 6107 20 0.9 1.5 0.09 ± 0.01 Ωm 1633 wedge 6107 20 0.9 2.0 0.26 ± 0.04 Ωm 1634 wedge 6107 20 0.9 2.5 1.1 ± 0.2 Ωm 1635 wedge 6107 20 0.9 3.0 3.2 ± 0.2 Ωm 1636 wedge 6107 13 0.9 1.0 0.14 ± 0.02 Ωm 1637 wedge 6107 13 0.9 1.5 0.12 ± 0.04 Ωm 1638 wedge 6107 13 0.9 2.0 0.35 ± 0.03 Ωm 1639 wedge 6107 13 0.9 2.5 1.3 ± 0.3 Ωm 1640 wedge 6107 13 0.9 3.0 2.7 ± 0.6 Ωm 1641 sandwich 6083 20 0.9 1.0 3.4 ± 0.4 Ωm 1642 sandwich 6083 20 0.9 1.5 1.1 ± 0.1 Ωm 1643 sandwich 6083 20 0.9 2.0 3.7 ± 1.1 Ωm 1644 sandwich 6083 20 0.9 2.5 5.3 ± 0.6 Ωm 1645 sandwich 6083 20 0.9 3.0 61 ± 6 Ωm 1646 sandwich 6083 20 0.9 3.5 0.13 ± 0.02 kΩm 1647 sandwich 6083 13 0.9 1.0 3.7 ± 0.7 Ωm 1648 sandwich 6083 13 0.9 1.5 2.7 ± 0.6 Ωm 1649 sandwich 6083 13 0.9 2.0 5.5 ± 0.4 Ωm 1650 sandwich 6083 13 0.9 2.5 37 ± 3 Ωm 1651 sandwich 6083 13 0.9 3.0 0.11 ± 0.01 kΩm 1652 sandwich 6083 13 0.9 3.5 0.19 ± 0.02 kΩm 1653 sandwich 6083 13 0.9 4.0 1.1 ± 0.2 kΩm 1654 sandwich 6081 20 0.9 1.0 2.8 ± 0.6 Ωm 1655 sandwich 6081 20 0.9 1.5 1.2 ± 0.2 Ωm 1656 sandwich 6081 20 0.9 2.0 3.4 ± 0.2 Ωm 1657 sandwich 6081 20 0.9 2.5 12.4 ± 0.7 Ωm 1658 sandwich 6081 20 0.9 3.0 35 ± 1 Ωm 1659 sandwich 6081 20 0.9 3.5 63 ± 4 Ωm 1660 sandwich 6081 20 0.9 4.0 81 ± 9 Ωm 1661 sandwich 6081 13 0.9 1.0 3.1 ± 0.5 Ωm 1662 sandwich 6081 13 0.9 1.5 2.8 ± 0.1 Ωm 1663 sandwich 6081 13 0.9 2.0 7.9 ± 2.2 Ωm 1664 sandwich 6081 13 0.9 2.5 20 ± 2 Ωm 1665 sandwich 6081 13 0.9 3.0 62 ± 10 Ωm 1666 sandwich 6081 13 0.9 3.5 98 ± 11 Ωm 1667 sandwich 6081 13 0.9 4.0 0.19 ± 0.02 kΩm 1669 sandwich 6237 20 0.9 1.0 0.06 ± 0.04 kΩm 1670 sandwich 6237 20 0.9 1.5 37 ± 12 kΩm 1671 sandwich 6237 20 0.9 2.0 0.32 ± 0.01 MΩm 1672 sandwich 6237 20 0.9 2.5 0.25 ± 0.01 MΩm 1673 sandwich 6237 20 0.9 3.0 0.16 ± 0.01 MΩm 1674 sandwich 6237 20 0.9 3.5 0.15 ± 0.01 MΩm 1676 sandwich 6237 13 0.9 1.0 0.21 ± 0.05 kΩm 1677 sandwich 6237 13 0.9 1.5 7.4 ± 3.4 kΩm 1678 sandwich 6237 13 0.9 2.0 0.07 ± 0.02 MΩm 1679 sandwich 6237 13 0.9 2.5 0.19 ± 0.02 MΩm 1680 sandwich 6237 13 0.9 3.0 177 ± 4 kΩm 1681 sandwich 6237 13 0.9 3.5 172 ± 4 kΩm 1682 sandwich 6237 13 0.9 4.0 0.13 ± 0.03 MΩm

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1683 sandwich 6237 20 2.1 1.0 0.05 ± 0.02 kΩm 1684 sandwich 6237 20 2.1 1.5 0.24 ± 0.01 MΩm 1685 sandwich 6237 20 2.1 2.0 78 ± 29 kΩm 1686 sandwich 6237 20 2.1 2.5 287 ± 4 kΩm 1687 sandwich 6237 20 2.1 3.0 0.24 ± 0.01 MΩm 1688 sandwich 6237 20 2.1 3.5 0.27 ± 0.01 MΩm 1690 sandwich 6237 13 2.1 1.0 0.10 ± 0.03 kΩm 1691 sandwich 6237 13 2.1 1.5 0.35 ± 0.07 MΩm 1692 sandwich 6237 13 2.1 2.0 0.42 ± 0.03 MΩm 1693 sandwich 6237 13 2.1 2.5 0.25 ± 0.01 MΩm 1694 sandwich 6237 13 2.1 3.0 0.28 ± 0.01 MΩm 1695 sandwich 6237 13 2.1 3.5 0.19 ± 0.03 MΩm 1696 sandwich 6237 13 2.1 4.0 0.17 ± 0.04 MΩm

Table S4. Tensile properties of selected fibers produced in this study. Specified are the bicomponent cross-section (sandwich or wedge), the volumetric ratio of the antistatic compound in the bicomponent fiber, the respective draw ratio and resulting fineness, as well as mean value and standard deviation of measured tensile properties. For comparison, the tensile properties of a monocomponent filament (No. 1563), melt-spun from PA6 Grilon A26, are also stated.

Fiber Cross- Antistatic Draw Fineness Tensile Strain at No. Section Ratio Ratio (tex = mg/m) Strength Break (%) (vol%) (DR) (cN/tex) 1563 mono - 4.0 2.8 59.7 ± 1.4 52 ± 2 1620 wedge 20 1.0 12.7 12.0 ± 3.5 372 ± 28 1621 wedge 20 1.5 8.8 17.5 ± 2.7 223 ± 47 1622 wedge 20 2.0 6.3 21.2 ± 3.3 154 ± 13 1623 wedge 20 2.5 5.3 27.8 ± 3.4 99 ± 6 1624 wedge 20 3.0 4.5 44.8 ± 5.9 76 ± 5 1625 wedge 13 1.0 14.3 11.0 ± 1.1 374 ± 15 1626 wedge 13 1.5 9.5 16.9 ± 2.6 233 ± 21 1627 wedge 13 2.0 7.1 21.7 ± 1.9 153 ± 12 1628 wedge 13 2.5 5.8 27.1 ± 2.3 102 ± 7 1629 wedge 13 3.0 4.9 36.1 ± 2.9 73 ± 4 1641 sandwich 20 1.0 12.4 11.6 ± 4.6 290 ± 125 1642 sandwich 20 1.5 8.0 9.9 ± 1.5 195 ± 36 1643 sandwich 20 2.0 6.2 24.1 ± 8.4 172 ± 53 1644 sandwich 20 2.5 5.0 30.3 ± 3.9 105 ± 20 1645 sandwich 20 3.0 4.2 35.8 ± 3.8 75 ± 8 1646 sandwich 20 3.5 3.8 40.1 ± 15.7 46 ± 19 1647 sandwich 13 1.0 13.3 10.8 ± 1.4 368 ± 75 1648 sandwich 13 1.5 8.7 14.2 ± 1.2 228 ± 25 1649 sandwich 13 2.0 6.5 17.9 ± 2.4 134 ± 26 1650 sandwich 13 2.5 5.2 26.6 ± 3.6 105 ± 9 1651 sandwich 13 3.0 4.5 28.0 ± 2.3 69 ± 7 1652 sandwich 13 3.5 3.9 36.2 ± 2.9 54 ± 4 1653 sandwich 13 4.0 3.5 43.1 ± 2.1 39 ± 3

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DR 1 1581 DR 2 1583 DR 3 1584

a) b) c)

DR 1 1620 DR 2 1622 DR 3 1624

d) e) f)

DR 3 1591 DR 3 1604 DR 3 1635

g) h) i) Figure S1. Microscopic images of selected fiber cross-sections produced: (a–c) sandwich fibers with draw ratios 1–3; (d–f) wedge fibers with draw ratios 1–3; (g–i) different types of wedge cross-sections (all draw ratio 3).