New Flexible Protective Coating for Printed Smart Textiles

applied sciences

Article New Flexible Protective Coating for Printed Smart Textiles

Valérie Bartsch 1,* , Volkmar von Arnim 1, Sven Kuijpens 1, Michael Haupt 1 , Thomas Stegmaier 1 and Götz T. Gresser 1,2

1 DITF, Deutsche Institute für Textil- und Faserforschung Denkendorf, Körschtalstraße 26, 73770 Denkendorf, Germany; [email protected] (V.v.A.); [email protected] (S.K.); [email protected] (M.H.); [email protected] (T.S.); [email protected] (G.T.G.) 2 ITFT, Institut für Textil- und Fasertechnologien-University of Stuttgart, Keplerstraße 7, 70174 Stuttgart, Germany; [email protected] * Correspondence: [email protected]

Featured Application: Thin and ﬂexible protective coating for printed smart textiles.

Abstract: In the field of food packaging, the addition of exfoliated layered silicates in polymers has been established to improve the polymers’ gas barrier properties. Using these polymers as coatings to protect smart textiles from oxidation and corrosion while maintaining their textile properties should significantly extend their lifetime and promote their market penetration. The aim of this study was to print new polymer dispersions containing layered silicates to protect screen-printed conductive structures, and to test the resulting samples. For this, appropriate printing parameters were determined by statistical design of experiments. According to these results, conductive structures were printed and protected with the selected coating. The abrasion resistance and the continuity of the protective layer of the printed samples were then measured. A continuous protective coating of approximately 70–80 µm thickness was applied on a conductive structure. The printed samples showed a very high resistance to abrasion (unchanged by 85,000 abrasion cycles) while remaining flexible and presenting 2 a lower water vapor permeability (<2.5 g/m d) than the coatings commonly used in the textile field. Keywords: protective coating; smart textiles Citation: Bartsch, V.; von Arnim, V.; Kuijpens, S.; Haupt, M.; Stegmaier, T.; Gresser, G.T. New Flexible Protective Coating for Printed Smart Textiles. Appl. Sci. 2021, 11, 664. https:// 1. Introduction doi.org/10.3390/app11020664 The global smart textiles market is expected to grow by 30.4% between 2019 and 2025, reaching a value of $5.55 billion in 2025 [1]. The development of smart textiles is becoming Received: 29 November 2020 increasingly important because of their potential applications for improving health, safety Accepted: 19 December 2020 and comfort. Developments are moving in the direction of innovative, intelligent technical Published: 12 January 2021 textiles and garments that combine sensory, actuator and electronic functions [2–5]. In most of these application fields, like in the automotive sector, high durability requirements have Publisher’s Note: MDPI stays neu- to be met by the smart textiles [6]. tral with regard to jurisdictional clai- Screen-printing technology has been well established in the fields of electronics and ms in published maps and institutio- textiles [7,8]. That is why the production of textile-based sensors regularly makes use nal affiliations. of this technology [4,9–12]. Screen printing reduces the number of production steps and manual operations. A high degree of both automation and electronic integration is possible for the economic and variable production of smart textiles. Copyright: © 2021 by the authors. Li- A major obstacle in the industrial implementation and marketing of smart textiles censee MDPI, Basel, Switzerland. developments has been the susceptibility of the conductive structures—especially in flexible This article is an open access article textiles. Problems result from mechanical damage resulting from, for example, bending, distributed under the terms and con- abrasion and chemical damage as a result of water entrapment and oxidation. The typical ditions of the Creative Commons At- metallic materials used in the conductive structures—such as steel, copper and, to a limited tribution (CC BY) license (https:// extent, silver—are susceptible to corrosion. Contact with oxygen forms at least oxide layers, creativecommons.org/licenses/by/ which reduce electrical conductivity and increase contact resistance. The ingress of moisture 4.0/).

Appl. Sci. 2021, 11, 664. https://doi.org/10.3390/app11020664 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 16

Appl. Sci. 2021, 11, 664 oxide layers, which reduce electrical conductivity and increase contact resistance. The2 of in- 14 gress of moisture accelerates and intensifies these corrosion processes and leads to a loss of the metals’ electrical conductivity. This limits their high application potential for printed electronics in smart textiles [13–15] and prevents applications in humid environ- mentsaccelerates [16–19]. and intensifies these corrosion processes and leads to a loss of the metals’ electricalBarrier conductivity. requirementsThis for limits coatings their highwith applicationlow water vapor potential and for oxygen printed diffusion electronics rates in gosmart far textilesbeyond [the13– 15coating] and preventsrequirements applications for electrical in humid insulation. environments The coatings [16–19 established]. in textileBarrier technology, requirements such foras polyurethanes, coatings with low polyacrylates water vapor and and silicones, oxygen generally diffusion meet rates go far beyond the coating requirements for electrical insulation. The coatings established electrical insulation requirements (especially for the low-voltage power supplies com- in textile technology, such as polyurethanes, polyacrylates and silicones, generally meet monly used in smart textiles), and ensure waterproofing. However, the coatings do not electrical insulation requirements (especially for the low-voltage power supplies commonly meet the barrier requirements for water vapor and oxygen in electronic assemblies sensi- used in smart textiles), and ensure waterproofing. However, the coatings do not meet the tive to oxidation and water vapor. On the other hand, the coating materials commonly barrier requirements for water vapor and oxygen in electronic assemblies sensitive to oxida- used in electrical engineering do not maintain the typical properties of textiles (flexibility, tion and water vapor. On the other hand, the coating materials commonly used in electrical shape adaptation, textile feel, etc.). Therefore, the development of new coatings is needed engineering do not maintain the typical properties of textiles (flexibility, shape adapta- to meet the requirement of smart textiles by protecting the sensor structures efficiently. tion, textile feel, etc.). Therefore, the development of new coatings is needed to meet the Few polymers are effective barriers against both water vapor and oxygen [20].Several requirement of smart textiles by protecting the sensor structures efficiently. approaches for flexible barrier layers are described in the scientific literature [21]. For ex- Few polymers are effective barriers against both water vapor and oxygen [20].Several ample,approaches highly for flexible flexible barrier layers arewere described realized in by the multilayer scientific literaturepolyurethane-EVOH [21]. For ex- combinationsample, highly [22,23]. flexible On barrier the other layers hand, were coatings realized with by increased multilayer water polyurethane-EVOH vapor and oxygen barrierscombinations have been [22,23 achieved]. On the by other incorporatin hand, coatingsg layered with silicates increased into aqueous water vapor polymer and oxy-dis- persionsgen barriers or polymer have been melts achieved [24]. by incorporating layered silicates into aqueous polymer dispersionsThis study or polymer focuses meltson using [24 ].polymeric coating systems which have not yet been es- tablishedThis as study textile focuses coatings on usingto print polymeric and test a coating new thin systems and flexible which coating have not for yet printed been smartestablished textiles as with textile a coatingsbarrier effect to print against and testwater a new vapor. thin For and this, flexible we coatingdeveloped for printedseveral polymericsmart textiles systems with with a barrier diffus effection-inhibiting against water polyurethane vapor. For dispersions this, we developed mixed with several lay- eredpolymeric silicates. systems The most with diffusion-inhibitingappropriate coating polyurethanewas involved dispersions in the fabrication mixed withof a printed layered smartsilicates. textile. The The most abrasion appropriate resistance coating of wasthe printed involved textile in the samples fabrication as well of a as printed their insula- smart tiontextile. properties The abrasion were tested. resistance of the printed textile samples as well as their insulation properties were tested. 2. Materials and Methods 2.1.2. Materials Methodology and Methods 2.1. Methodology The methodology applied to this study is described in Figure 1. First of all, a polymer containingThe methodology layered silicates applied was tochosen, this study depending is described on its inflexibility Figure1 .and First its of water all, a vapor poly- permeability,mer containing to layeredbe used silicatesas protection was chosen,for a printed depending conductive on its structure. flexibility Then, and itsprinting water parametersvapor permeability, were determined to be used to apply as protection the chosen for polymer a printed as conductive a homogenous structure. layer. These Then, parametersprinting parameters were used were to determinedprint an encapsulat to applyed the multilayered chosen polymer conductive as a homogenous structure, layer. and theThese structure’s parameters abrasion were usedresistance to print as anwell encapsulated as the continuity multilayered of the protection conductive layers structure, were tested.and the Depending structure’s on abrasion the results, resistance new multilayered as well as the samples continuity wereof printed. the protection layers were tested. Depending on the results, new multilayered samples were printed.

Figure 1. Methodology of the study. Figure 1. Methodology of the study.

2.2. Choice of the Protective Coating 2.2.1. Water Vapor Permeability of the Diffusion-Inhibiting Protective Barrier Layer The water vapor permeability was determined according to ISO 15106 Part 3. The mea- surements were performed with the water vapor permeability tester WDDG from Brugger at 85 rel. h and 23 ◦C. Appl. Sci. 2021, 11, 664 3 of 14

The water vapor permeabilities were measured on thin polymeric ﬁlms applied on a polyester sheet with a thickness of 25 µm. The pure PET sheet had a water vapor permeability of 6.6 g/m2·d. Conforming to the diffusion model for multilayer systems— according to which the permeation resistance may be regarded as the sum of the individual resistances of the different layers—the total water vapor permeability is dominated by the more effective barrier layer.

2.2.2. Layered Silicates Two kinds of layered silicates have been considered to reduce the water vapor permeability of the coating: LAPONITE-SL25 (BYK-Chemie GmbH) and NTS-5 (Topy industries). These liquid dispersions were mixed with the polyurethane dispersion Takelac WPB-341 A (Mitsui Chemicals), which presents sufficient water vapor barrier properties for food packaging. The addition of LAPONITE-SL25 resulted in a significant increase in the viscosity of the mixture. For this reason, the amount of LAPONITE added remained at 14% whereas NTS was added at about 50%. A thin film of each mixture was applied on a PET sheet, and its water vapor permeability was measured. According to Figure2, the addition of NTS layered silicates strongly reduced the water vapor permeability of the dried film beneath 1 g/m2 d and confirmed that layered silicates can be used to improve barrier properties. On the other hand, the addition of these layered silicates caused the film to become much more brittle and to require a long drying time, which reduces its suitability with smart textiles applications or with an economically viable printing process. The addition of LAPONITE layered silicates did not have a significant impact on the water vapor permeability of the dried film, but nevertheless affected screen- printing rheological paste properties and managed to form a flexible film with a reasonable Appl. Sci. 2021, 11, x FOR PEER REVIEWdrying time. For this reason, LAPONITE-SL25 layered silicates were further used4 of 16 in

this study.

FigureFigure 2. 2. InfluenceInﬂuence of of layered layered silicates silicates on on the the water water vapor vapor permeability permeability of of protective protective films. ﬁlms.

2.3. Printing Process 2.3.1. Printed Materials To efficiently protect a conductive pattern, it was encapsulated with two protective layers. Thus, the printed sample was made up of five layers (Figure 3). The first and fifth layers served as a flexible primer and topcoat using the soft polyurethane Takelac WS 5000 (Mitsui) combined with layered silicates LAPONITE-SL25 (BYK), acting as an intermediate layer between a flexible textile and a rigid barrier layer to protect the whole system from damage. The second and fourth layers were barrier layers, and were mainly composed of the rigid polyurethane Takelac WPB-341A (Tg 115 °C) combined with the soft polyurethane Takelac WS 5000 (Tg 65 °C) and LAPONITE-SL25 layered silicates as well as with a thickening agent Rohagit SD 9523. The third layer was the conductive structure, composed of the silver-containing paste 200-05 (Dico Electronic). The composition of the protective layers is detailed in Table 1. Appl. Sci. 2021, 11, 664 4 of 14

2.3. Printing Process 2.3.1. Printed Materials To efficiently protect a conductive pattern, it was encapsulated with two protective layers. Thus, the printed sample was made up of five layers (Figure3). The first and fifth layers served as a flexible primer and topcoat using the soft polyurethane Takelac WS 5000 (Mitsui) combined with layered silicates LAPONITE-SL25 (BYK), acting as an intermediate layer between a flexible textile and a rigid barrier layer to protect the whole system from damage. The second and fourth layers were barrier layers, and were mainly composed of the rigid polyurethane Takelac WPB-341A (Tg 115 ◦C) combined with the soft polyurethane Takelac WS 5000 (Tg 65 ◦C) and LAPONITE-SL25 layered silicates as well as with a thickening agent Rohagit SD 9523. The third layer was the conductive structure, Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 16 composed of the silver-containing paste 200-05 (Dico Electronic). The composition of the protective layers is detailed in Table1.

Conductive paste (layer 3)

Diffusion-inhibiting paste (layers 2 and 4)

Flexible paste (layers 1 and 5)

Figure 3. Top view and cross section of a printed sample. Figure 3. Top view and cross section of a printed sample.

Table 1. Composition of the protective layers. Table 1. Composition of the protective layers. Component Flexible Layers 1/5 Barrier Layers 2/4 Component Flexible Layers 1/5 Barrier Layers 2/4 Takelac WPB-341A / 43 g Takelac WPB-341A / 43 g Takelac WS-5000 74.0 g 30 g LAPONITE-SL25Takelac WS-5000 9.0 g 74.0 g 11 g 30 g Rohagit SD 9523LAPONITE-SL25 / 9.0 g 3 g 11 g Water Rohagit SD 9523 17.0 g / 14 g 3 g Water 17.0 g 14 g For each printing step, the flow curve of the paste—and thus its rheological behavior under shear forces—was measured with an Anton Paar MCR 301 rotary rheometer. Both For each printing step, the ﬂow curve of the paste—and thus its rheological behav- pastes used as protective layers had a viscosity of between 30 and 90 Pa.s at a shear rate ior under shear forces—was measured with an Anton Paar MCR 301 rotary rheometer. of 10 s−1. The conductive paste showed a higher viscosity of 239 Pa.s at a shear rate of 10 Both pastes used as protective layers had a viscosity of between 30 and 90 Pa.s at a shear s−1. rate of 10 s−1. The conductive paste showed a higher viscosity of 239 Pa.s at a shear rate of 10 s−1. 2.3.2. Print Pattern A U-shaped pattern with two contact surfaces was designed to test the protective properties of the coating. For each coating step, we carefully kept the contact spots free of coating. The layers were printed on a polyester weaving fabric. Layer 1 was in contact with the fabric. Layer 5 was printed last.

2.3.2. Print Pattern A U-shaped pattern with two contact surfaces was designed to test the protective properties of the coating. For each coating step, we carefully kept the contact spots free of coating. The layers were printed on a polyester weaving fabric. Layer 1 was in contact with the fabric. Layer 5 was printed last.

2.3.3. Screen-Printing Technology Protective layers as well as the conductive structure were applied by screen-printing. For this, we used a Schenk Variprint Easy screen-printing carousel with three different print stations equipped with a polyester mesh 32–100 PW and 8 aluminum pallets The screen-printing process first consists of a flooding step followed by a printing step. During the flood stroke, the lifted mesh of the stencil is filled with paste without being in contact with the substrate. During the printing stroke, the paste in the screen is then transferred to the substrate. This process makes it possible to print an exactly defined amount of paste on a textile. Before printing the whole protected conductive structure, we determined the optimum print parameters for the protective layers using a response surface method experimental design. Because both protective polymer dispersions had a similar rheological behavior, only the flexible paste was used to determine the print parameters. To limit the number of tests, the following printing parameters were fixed on the basis of existing preliminary tests and experience [25–27]: - Flood bar angle: Usually, a squeegee angle of 75◦ is used. - Squeegee pressure during flooding: 1.5 bar. - Flooding speed: 10 mm/s. - Flood bar type: aluminum. - Printed paste: paste for the flexible layer. All the protective pastes have a similar viscosity. It is expected that the parameters determined for the flexible layer can also be applied to the diffusion-inhibiting paste. - The influence of the following parameters on the printing process was investigated: - Off-contact distance: 2–6 mm. A jump of 2 mm ensures that the screen fabric does not come into contact with the substrate during flooding. For jumps larger than 6 mm, very high squeegee pressures are required to bring the screen fabric into contact with the substrate during printing. This could damage the screen fabric. - Squeegee hardness: 80 Shore (hard), 60 Shore (soft). Squeegee hardness influences the amount of paste applied to the substrate. A soft squeegee usually applies more paste. - Squeegee pressure during printing: kissing-print + 0–0.5 bar. The kissing-print cor- responds to the pressure required for punctual contact of the screen fabric with the substrate. This pressure is highly dependent on the off-contact distance (Table2). Therefore, only the additional pressure to the kissing-print was considered in the experimental design. - Squeegee speed during printing: 10–30 mm/s. The speed of the printing squeegee influences the amount of material applied to the substrate and the print quality, among other things.

Table 2. Dependence between the off-contact distance and the kissing-print pressure.

Off-Contact Distance (mm) 2 3 4 6 Kissing-print-pressure (Bar) 1.5 1.6 1.7 1.8

The paste quantity, the homogeneity of the layers and the print quality (complete print) were defined as target values and were optically evaluated in order to determine the quality of the samples. A statistically significant model was created for each target figure. A total of 40 trials were carried out to determine the print parameters. Appl. Sci. 2021, 11, 664 6 of 14

2.3.4. Drying Process After each print, a drying process was performed with a QC 5050 MW medium wave ﬂash cure dryer (Calmat). The optimum drying parameters were determined in another study on the basis of design of experiments [28]. A drying time of 99 s and the IR-Radiator power P04 were used to dry all pastes without damaging the fabric.

2.4. Testing 2.4.1. Pinhole Test In order to check the continuity of the protective layers, a pinhole test was performed Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 16 with the printed samples. Electrolysis of an NaCl solution was performed using the coated Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 16 conductive textile as cathode (Figure4). A standard microcable was used as anode.

FigureFigure 4. 4. PinholePinhole test.test. test.

ThisThe method following enables enables electrochemical visible visible detection detection reactions of ofany any tookpinhole pinhole place or crackor at crack the in electrodes the in thecoating coating whenthrough through current flowed. thethe formationformationThis method ofof HH2 2enables bubblesbubbles in visiblein the the uncoated uncoated detection areas. areas. of The any The formation pinhole formation orof hydrogencrackof hydrogen in thebubbles coatingbubbles through thewaswas formationassessed qualitativelyqualitatively of H2 bubbles but but not not in quantitatively. quantitatively. the uncoated areas. The formation of hydrogen bubbles was assessedPrior to testing,testing, qualitatively wewe checked checked but that notthat current quantitatively. current could could flow flow through through the theprint print conductive conductive path.path. PriorOne side to testing, ofof thethe conductive conductive we checked pattern pattern that wa currentwas connecteds connected could with ﬂowwith a voltage througha voltage supply. thesupply. printThe Theother conductive other path. side of the pattern was connected with a standard microcable, whose stripped end was Oneside of side the of pattern the conductive was connected pattern with was a standard connected microcable, with a voltage whose supply.stripped Theend otherwas side of immersed in the NaCl solution as well as the anode. By increasing the voltage, H2 bubbles theimmersed pattern in was the NaCl connected solution with as well a standard as the anode. microcable, By increasing whose the stripped voltage, endH2 bubbles was immersed shouldshould occur atat thethe stripped stripped end end of of the the microcable microcable (Figure (Figure 5). 5). in theBecause NaCl the solution conductive as well layer as of the print anode. patte Byrn contains increasing silver the particles, voltage, the Hvoltagebubbles should Because the conductive layer of the print pattern contains silver particles, the2 voltage occurwas applied at the before stripped the print end ofpattern the microcable was immersed (Figure in the5 saline). solution to protect the was applied before the print pattern was immersed in the saline solution to protect the silver particles from oxidation. silver particles from oxidation.

FigureFigure 5. 5. ControlControl of ofthe the current current flow ﬂowthrough through the print the sample. print sample.

Figure2.4.2. Scanning 5. Control Electron of the current Microscopy flow through (SEM) the print sample. SEM images were obtained with a Hitachi Tabletop Microscope TM-1000 in order to 2.4.2.observe Scanning the structure Electron of the Microscopy printed layers (SEM) and any disruption in the coating. SEM micros- copySEM was usedimages in addition were obtained to the pinhole with a testHitachi to check Tabletop the continuity Microscope of the TM-1000 applied layers.in order to observe the structure of the printed layers and any disruption in the coating. SEM microscopy was used in addition to the pinhole test to check the continuity of the applied layers. Appl. Sci. 2021, 11, 664 7 of 14

Because the conductive layer of the print pattern contains silver particles, the voltage was applied before the print pattern was immersed in the saline solution to protect the silver particles from oxidation.

2.4.2. Scanning Electron Microscopy (SEM) SEM images were obtained with a Hitachi Tabletop Microscope TM-1000 in order to observe the structure of the printed layers and any disruption in the coating. SEM microscopy was used in addition to the pinhole test to check the continuity of the applied layers.

2.4.3. Abrasion Resistance The abrasion resistance was evaluated with a Martindale abrasion tester according to the standard DIN EN ISO 12947 part 2 and 4. Two samples with conductive pattern and without protective coating as well as two ◦ Appl. Sci. 2021, 11, x FOR PEER REVIEWsamples with protective coating were conditioned under 65%8 rel. of 16 h. and 20 C for 24 h and then tested simultaneously. The samples were ﬁrst loaded with 9 kPa and scrubbed with a standard scouring wool fabric at a scrubbing speed of 3000 T/min. After 50,000 rubbing tours,2.4.3. Abrasion the load Resistance was increased to 12 kPa. The abrasion resistance was evaluated with a Martindale abrasion tester according to the standard DIN EN ISO 12947 part 2 and 4. 3. ResultsTwo samples with conductive pattern and without protective coating as well as two 3.1.samples Determination with protective coating of Printing were conditioned Parameters under for 65% the rel. Protective h. and 20 °C for Layers 24 h and then tested simultaneously. The samples were first loaded with 9 kPa and scrubbed with a standardThe scouring results wool of fabric the experimentalat a scrubbing speed design of 3000 T/min. are shown After 50,000 in rubbing Table 3. The best results were obtainedtours, the load with was aincreased small off-contactto 12 kPa. distance of >2 mm. To obtain a homogeneous layer, a slow

squeegee3. Results speed and high squeegee pressure was required. 3.1. Determination of Printing Parameters for the Protective Layers TableThe 3. resultsPrinting of the parameters experimental derived design are from shown the in experimentalTable 3. The best design. results were obtained with a small off-contact distance of >2 mm. To obtain a homogeneous layer, a slow squeegeeOff-Contact speed and high squeegeeSqueegee pressure Pressure was required. Squeegee Speed Squeegee Hardness Distance (mm) (bar) (mm/s) (Shore) Table 3. Printing parameters derived from the experimental design.

Off-Contact Distance3 Squeegee Pressure 2.1Squeegee Speed Squeegee 10 Hardness 60 (mm) (bar) (mm/s) (Shore) 3 2.1 10 60 Printing tests were carried out with these parameters (Figure6), and they conﬁrmed the resultsPrinting tests of thewere experimental carried out with these design. parameters (Figure 6), and they confirmed the results of the experimental design.

Figure 6. Print result when using the print parameters determined by the experimental de- Figuresign. 6. Print result when using the print parameters determined by the experimental design.

3.2.3.2. P Printedrinted Samples Samples for Testing for Testing MultilayeredMultilayered conductive conductive samples were samples printed in were a single printed process using in a the single printing process using the printing parameters determined previously for the five layers (Table 4). In order to characterize parametersthe properties of determined the obtained sample, previously nine samples for thewere five printed layers simultaneously. (Table4). Each In order to characterize the propertiessample was then of theused obtainedto carry out sample,a specific test nine (e.g. samples, pinhole test, were abrasion printed test or simultaneously.SEM Each sample microscopy). In this way, the influence of external conditions such as temperature or hu- wasmidity then did not used affect to the carry results out obtained a specific. The resulting test (e.g., printing pinhole is shown test, in Figure abrasion 7. No test or SEM microscopy). Indifference this way,s could the be observed influence visually of externalbetween the conditionssamples. such as temperature or humidity did not affect the results obtained. The resulting printing is shown in Figure7. No differences could be observed visually between the samples. Appl. Sci. 2021, 11, 664 8 of 14

Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 16 Table 4. Optimal printing and drying parameters used for each layer of the sample to be tested.

Off-Contact Distance (mm) 3 Table 4. Optimal printing and drying parameters used for each layer of the sample to be ◦ tested. Flood bar angle ( ) 75 Flood bar pressure (bar) 1.5 FloodingOff-Contact Distance (mm) 3 Flood bar speed (mm/s) 10 Flood bar angle (°) 75 Flood bar type Aluminum Flood bar pressure (bar) 1.5 Flooding Flood bar speed (mm/s)Squeegee angle (◦) 10 75 Flood bar typeSqueegee pressure (bar) Aluminum 2.1 Printing Squeegee angleSqueegee (°) speed (mm/s) 75 10 Squeegee pressure (bar) 2.1 Printing Squeegee hardness/type Shore A Hardness 60/Silicone rubber Squeegee speed (mm/s) 10 Squeegee hardness/typeDryer power Shore A Hardness 60/Silicone rubber P04 Drying Dryer power Drying time (s) P04 99 Drying Drying timeDryer–textile (s) distance (mm) 99 69 Dryer–textile distance (mm) 69

FigureFigure 7. 7. PrintedPrinted samples. samples. 3.3.3.3. Tests Tests 3.3.1.3.3.1. Pinhole Pinhole Test Test In the first print sample tested, the respective protective layers and the conductive layer Inwere the each ﬁrst printed print once sample as single tested,layers. During the respectivethe pinhole test, protective hydrogen bubbles layers and the conductive layerformed were on theeach surface printed of the print once sample as (Fig singleure 8a), layers. which can During be explained the pinhole by a hole test,in hydrogen bubbles formedthe protective on the coating. surface This was of theconfirmed print by sample SEM images, (Figure which8a), showed which the can presence be explained by a hole in of pores in the protective layers (Figure 9). the protective coating. This was conﬁrmed by SEM images, which showed the presence of Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16 pores in the protective layers (Figure9).

FigureFigure 8. Pinhole 8. Pinhole test on testprinted on samples. printed Conductive samples. layer Conductivecoated with single layer layer ( coateda) and with single layer (a) and double double layer (b) of protective coating. layer (b) of protective coating.

Figure 9. SEM image (cross section) of a sample coated once.

To improve the barrier properties of the coating, protective layers were printed twice (double layer) to increase their thickness (Figure 10) and thus minimize the presence of continuous pores in the coating. In this case, the formation of hydrogen bubbles on the surface of the printed sample could not be detected during the pinhole test (Figure 8b). These results suggest that the developed diffusion-inhibiting pastes can be applied to textiles by screen-printing as flexible protection for conductive patterns, and thus suggest that the pastes can be implemented in the fabrication of smart textiles. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16

Appl. Sci. 2021, 11, 664Figure 8. Pinhole test on printed samples. Conductive layer coated with single layer (a) and 9 of 14 double layer (b) of protective coating.

Figure 9. SEM image (cross section) of a sample coated once. Figure 9. SEM image (cross section) of a sample coated once. To improve the barrier properties of the coating, protective layers were printed twice To improve(double the barrier layer) properties to increase of theirthe coat thicknessing, protective (Figure layers 10) and were thus printed minimize twice the presence of (double layer) continuousto increase their pores thickness in the coating. (Figure In 10) this and case, thus the minimize formation the of presence hydrogen of bubbles on the continuous poressurface in the of coating. the printed In this sample case, couldthe formation not be detected of hydrogen during bubbles the pinhole on the test (Figure8b). surface of the printedThese results sample suggest could not that be the de developedtected during diffusion-inhibiting the pinhole test (Figure pastes 8b). can be applied to Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 16 These results suggesttextiles that by screen-printingthe developed diffus as ﬂexibleion-inhibiting protection pastes for conductive can be applied patterns, to tex- and thus suggest tiles by screen-printingthat the pastes as flexible can be protecti implementedon for conductive in the fabrication patterns, of smartand thus textiles. suggest that the pastes can be implemented in the fabrication of smart textiles.

Figure 10. SEM images of samples single-layer coated (a) and double-layer coated (b). Figure 10. SEM images of samples single-layer coated (a) and double-layer coated (b).

3.3.2. Martindale3.3.2. Abrasion Martindale Test Abrasion Test A MartindaleA abrasion Martindale test was abrasion performed test was to evaluate performed the toabrasion evaluate resistance the abrasion of the resistance of developed protectivethe developed coating protective according coating to the parameters according todescribed the parameters in Section described 2.4.3. The in Section 2.4.3. results of the Thetest resultsare shown of the in testTable are 5.shown in Table5.

Table 5. Influence of a coating once printed on a conductive structure.

Tours Load (kPa) Without Coating With Coating

0 9

610 9

Slight abrasion of the conductive structure No significant change Appl.Appl. Sci. Sci. 2021 20212021,, 11,, ,,1111 xx ,,FORFOR xx FORFOR PEERPEER PEERPEER REVIEWREVIEW REVIEWREVIEW 11 1111of 17ofof 1717

FigureFigure 10. 10. SEM SEM iimages iimages of ofsamples samples single singlesingle-layerlayer--layerlayer coated coatedcoated (a ) (( aanda)) andand double double-layerlayer--layerlayer coated coatedcoated (b )((.b. ))..

3.33..2.23...2.2 Martindale.. Martindale Abrasion Abrasion Test Test

Appl. Sci. 2021, 11, 664 A AMartindale Martindale abrasion abrasion test test was was performed performed to toevaluate evaluate the the abrasion abrasion resistance resistanceresistance10 of of ofofthe 14 thethe developeddeveloped protective protective coating coating according according to tothe the parameters parameters described described in in Section Section 2. 4.32.4.3.. The.. The resultsresultsresults of ofofthe thethe test testtest are areare shown shownshown in ininTable Table 5.. 5..

Table 5. Inﬂuence of a coating once printed on a conductive structure. TableTable 5. 5.Influence InfluenceInfluence of ofofa coating aa coatingcoating once onceonce printed printedprinted on onon a conductive aa conductiveconductive structure structurestructure.. ..

ToursToursTours LoadLoad Load (kPa) (kPa)(kPa) WithoutWithout Without Coating Coating With WithWith Coating Coating

00 00 9 99 9

610610610610 9 99 9

Appl.Appl. Sci. Sci. 2021 2021, 11,, ,11 x ,,FOR xx FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 12 12of of17 17

ToursTours LoadLoad (kPa) (kPa)(kPa) SlightSlightSlight abrasion abrasion abrasionWithoutWithout of ofthe of the the conductiveCoating conductiveCoating conductive structure structure structure NoNoNo significant signiﬁcantWith significantWith Coating Coating change change

161016101610 9 9 9

DistinctDistinctDistinct abrasion abrasion abrasion of of the the conductive conductive structure structure NoNo significant signiﬁcant significant change change

661066106610 9 9 9

Conductive structure nearly completely removed ConductiveConductive structure structure nearly nearly completely completely removed removed NoNo significant signiﬁcant significant change change

5050,000,,000 T byT by 9 k9Pa kPa + + 8585,000,,000 3535,000,,000 T byT by 12 12 kPa kPa

HoleHole in inthe the uncoated uncoated fabric fabric ConductiveConductive structure structure completely completely removed removed NoNo significant significant change change in inthe the coated coated structure structure

A Aslight slight abrasion abrasion of of the the conductive conductive pattern pattern of of the the uncoated uncoated samples samples was was observed observed afterafter only only 610 610 cycles cycles. After.. After After 50 50 50,000,,000 abrasion abrasion cycles cycles with with a aload load of of 9 9kPa kPa, no,, no significant significant changechange could could be be detected detected in in the the coated coated samples. samples. These These samples samples were were then then loaded loaded with with 12 12 kPakPa and and scrubbed scrubbed again. again. After After an an additional additional 35 35,000,,000 cycles cycles, a,, holea hole was was formed formed in in the the un- un- coatedcoated areas. areas. However, However, the the coated coated area area remained remained unchanged. unchanged. Th Theseese results results suggest suggest that that thethethe developed developed coating coating system system can can be be used used as as efficient efficient protection protection from from abrasion abrasion for for printed printed conductiveconductive pattern patterns. s..

4. 4.Discussion Discussion 4.14.1. Abrasion.. Abrasion Resistance Resistance TheThe samples samples with with uncoated uncoated printed printed conductive conductive patterns patterns show showeded a lowa low abrasion abrasion re- re- sistancesistance. Because.. Because of of its itsits high highhigh viscosity viscosityviscosity, the,, thethe conductive conductiveconductive paste pastepaste remained remainedremained at atatthe thethe surface surfacesurface of of ofthe thethe Appl.Appl. Sci. Sci. 2021 2021, 11, ,11 x ,FOR x FOR PEER PEER REVIEW REVIEW 12 12of 17of 17

ToursTours LoadLoad (kPa) (kPa) WithoutWithout Coating Coating WithWith Coating Coating

16101610 9 9

DistinctDistinct abrasion abrasion of ofthe the conductive conductive structure structure NoNo significant significant change change

66106610 9 9 Appl. Sci. 2021, 11, 664 11 of 14

Table 5. Cont.

Tours Load (kPa)ConductiveConductive structure structure Without nearly nearly Coating completely completely removed removed NoNo significant With significant Coating change change

50,5000050,000,000 T byT T by by9 k 9 9Pa k kPaPa + + + 8585,000,85000,000 35,3535,000000,000 T byT T by by12 12 12kPa k kPaPa

HoleHoleHole in in inthe the the uncoated uncoated uncoated fabric fabric fabric ConductiveConductive structure structure completely completely removed removed NoNoNo significant signiﬁcant significant change change change in inthe the coated coated structure structure

AA slightAslight slight abrasion abrasion abrasion of of the ofthe the conductive conductive conductive pattern pattern pattern of of the ofthe the uncoated uncoated uncoated samples samples samples was was was observed observed observed afterafterafter only only only 610 610 cycles. 610 cycles cycles After. After. After 50,000 50 50,000 abrasion,000 abrasion abrasion cycles cycles with cycles with a loadwith a of loada 9 load kPa, of of no 9 kPa9 signiﬁcant kPa, no, no significant significantchange couldchangechange be could detected could be be detected in detected the coated in inthe samples.the coated coated samples. These samples. samples These These weresamples samples then were loaded were then then with loaded loaded 12 kPa with with and 12 12 scrubbedkPakPa and and again.scrubbed scrubbed After again. anagain. additional After After an an additional 35,000 additional cycles, 35 35, a000 hole,000 cycles wascycles, formeda, holea hole was in was the formed uncoatedformed in inthe areas. the un- un- However,coatedcoated areas. theareas. coatedHowever, However, area the remained the coated coated unchanged.area area remained remained These unchanged. unchanged. results suggest Th Theseese thatresults results the suggest developed suggest that that coatingthethe developed developed system coating can coating be usedsystem system as can efﬁcient can be be used used protection as asefficient efficient from protection protection abrasion from for from printedabrasion abrasion conductive for for printed printed patterns.conductiveconductive pattern patterns. s.

4.4. Discussion4.Discussion Discussion 4.1.4.14.1. Abrasion Abrasion. Abrasion ResistanceResistance Resistance TheTheThe samplessamples samples withwith with uncoateduncoated uncoated printedprinted printed conductiveconductive conductive patternspatterns patterns showedshow showeded aa low lowa low abrasionabrasion abrasion re-re- re- sistance.sistancesistance.Because Because. Because ofof its ofits highits high high viscosity, viscosity viscosity, the the, the conductive conductive conductive paste paste paste remained remained remained at at the atthe the surface surface surface of ofthe ofthe the fabric, enabling a high conductivity of the structure after printing. However, as demon- strated by Kazani et al. [29], the conductive structure was thereby more sensitive to abrasion. In the study performed by Kazani et al., a conductive pattern was screen-printed on different fabrics, and its abrasion resistance and washability were tested. For this, the square resistance was measured before and after 5000 abrasion cycles. A clear increase of the square resistance was observed after abrasion. To improve the washability of the printed fabrics in the study by Kazani et al., the conductive patterns were laminated with an 80 µm thermoplastic polyurethane film. This should also improve the abrasion resistance, even if it was not tested in their study. However, the use of a film twice as thick as the coating obtained by screen-printing in our study limits the flexibility of the textile, and it especially requires an additional process to produce the smart textile, which makes the manufacturing process less economically viable. Although our encapsulated structure also remained at the surface of the fabric, as seen in Figure 10, it showed a very high abrasion resistance. A Martindale abrasion test proved that a layer with a total thickness of approximately 20–40 µm of the developed coatings efficiently protected conductive patterns from abrasion. The coating can only be damaged by higher loads up to 12 kPa and more cycles than 85,000. This can be explained not only by the use of polyurethane—known for its good abrasion resistance—but also by the presence of layered silicates in the coating. Tabsan et al. [30] showed that adding a small amount of layered silicates (3 phr) in a rubber highly improved its abrasion resistance. To confirm this, abrasion tests should be performed with a similar TPU coating without layered silicates. Appl. Sci. 2021, 11, 664 12 of 14

4.2. Barrier Properties In a similar work [31], a conductive structure was encapsulated with two polyurethane layers. The ﬁrst layer had waterproof properties. The second layer had adhesive properties to prevent the conductive layer from cracking. The thickness of the printed coating (90 µm) was similar to our coating (70–80 µm), even if it required four printing steps instead of the two steps in our case. However, the coating’s water vapor permeability was not studied, so it remains unclear whether these coatings efﬁciently protected the conductive structure from water. Because water vapor barrier properties were not mentioned by Yang et al., we predict that the water vapor permeability of their coating is higher than the water vapor permeability of the coatings used in our study. In this case, the risk of damaging the conductive structure by oxidation would be higher. To date, no coatings with a low water vapor permeability have been used as printed layers to protect smart textiles.

4.3. Improvement of the Coating Due to the formation of pores during the printing/drying processes, a single coating step did not effectively protect the printed conductive layer. This could also explain the relatively high water vapor permeability measured with the single-layer coatings containing LAPONITE layered silicates. By repeating the printing process and thus applying a thicker protective coating (70–80 µm), the likelihood of pore formation was reduced. In this case, a pinhole test showed that a continuous coating was obtained on the printed conductive pattern. It can therefore be concluded that polyurethane dispersions mixed with layered silicates are suitable for the protection of printed smart textiles as long as the pore formation during the printing process is limited. The formation of these pores could be due to: - The elimination of the water present in the dispersion during the drying process, especially if the drying temperature exceeds 100 ◦C too early; or - Air entrapment during the printing process and paste preparation. In addition to avoiding the formation of gas bubbles by modifying the printing and drying parameters, it is possible to promote their removal. According to Stokes’ law (Equation (1)), the rising speed of the bubbles depends on the dynamic viscosity of the coating, among other things.

2 (ρb−ρ f ) 2 v = 9 µ gR (1)

where: v the rising speed of the bubble (m/s); g is the gravitational ﬁeld strength (m/s2); R is the radius of the gas bubble (m); 3 ρp is the mass density of the particles (kg/m ); 3 ρf is the mass density of the ﬂuid (kg/m ); µ is the dynamic viscosity of the coating (kg/(m × s)). A reduction in the solid content in the dispersion (and therefore a reduction in its viscosity) could improve the evacuation of gas bubbles and thus the barrier properties. The use of defoamer or de-aerator additives in the composition of the coating should also be tested to avoid the formation of pores. In this way, one could expect to obtain the same barrier properties with even thinner coatings.

5. Conclusions In this study, polyurethane dispersions were mixed with LAPONITE-SL25 layered silicates to obtain screen-printing pastes with low water vapor permeability. The screen- printing technology is suitable to apply these dispersions as a thin continuous coating. By applying a multi-layer coating made up of a rigid layer with high diffusion-inhibiting Appl. Sci. 2021, 11, 664 13 of 14

properties and a flexible layer, on and under a conductive structure, we were able to efficiently protect it from mechanical and chemical stresses like abrasion and oxidation. The use of screen-printing enables a cost-effective application of these coatings on conductive structures with complex geometries. It is conceivable to reduce the thickness of the coating while maintaining its barrier properties as long as the formation of pores during the printing/drying process is limited. While these coatings have proven to be effective in protecting printed structures, they could also be used to protect conductive textile structures with complex geometries such as embroidery, knitted and weaving fabrics. These coatings are therefore suitable for use in the interdisciplinary field of smart textiles.

Author Contributions: V.B.: scientific design of the process development (printing, testing), implementation and analysis of the screen-printed samples and tests, original writing and proof reading of manuscript; V.v.A.: scientific design of material development (coatings), implementation and analysis of developed coatings, co-writing and proof reading of manuscript; S.K.: implementation and analysis of the screen-printed samples; M.H.: proof reading of manuscript; T.S.: funding acquisition, project supervision, proof reading of manuscript; G.T.G.: proof reading of manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded via the AiF as part of the program to promote joint industrial research (IGF) by the German Federal Ministry of Economics and Energy. Grant number 19734. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: My special thanks go to all my colleagues at the DITF who were involved in this research project for their friendly and efficient cooperation. Conflicts of Interest: The authors declare no conflict of interest.

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