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Journal of Coatings Technology and Research

, Volume 15, Issue 4, pp 875–883 | Cite as

Development of graphite-based conductive textile coatings

  • Patricia Schäl
  • Irén Juhász Junger
  • Nils Grimmelsmann
  • Andrea Ehrmann
Article

Abstract

Conductive yarns and coatings are necessary for a broad variety of smart textile applications, such as sensors, data transmission lines, or heated fabrics. The main problems of such conductive textile elements are abrasion and washing resistance. Since different findings with respect to these properties are reported in the literature for similar coatings, the required optimization is impeded. In a recent study, the washing resistance of different graphite–polyurethane coatings with graphite contents between 25% and 33% on cotton, linen, viscose, and polyester woven fabrics was compared, using two different graphite particle sizes on diverse textile substrates. It was found that not only the graphite particle dimensions and graphite concentration strongly influence the longevity of the coatings, but also the textile substrates which were coated with the conductive mass. This means that conductive coatings cannot be optimized without knowledge of the planned application.

Keywords

Conductive coating Graphite Polyurethane Washing resistance 

Introduction

In the area of smart textiles, conductive coatings, finishings, or yarns are often necessary. Energy and data are transported through conductive submission lines.1, 2, 3 Data are transferred to external receivers using textile antennas.4 Measurement of vital signs, such as ECG or pulse, is performed using conductive electrodes.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 Textile pressure sensors17,18 or elongation sensors19, 20, 21, 22, 23, 24 also necessitate conductive textile materials.

On the other hand, textile coatings used nowadays typically suffer from low washing and abrasion resistance. Several research groups have thus investigated new ideas to create novel conductive textile materials. Wire-shaped supercapacitors, e.g., were produced by coating carbon nanotubes on silver-covered yarns.25 Polyethylene terephthalate (PET) fibers were made conductive by coating them with reduced graphene oxide and silver nanoparticles.26 Similarly, conductive graphene/poly(vinyl alcohol) shells around polyurethane yarns were used to create strain sensors.27 While in these examinations, washing fastness was not tested or was found not satisfying for long-term use, another investigation using reduced graphene on a silk surface found washing resistance of the coatings to be acceptable.28 This material, however, is problematic due to the necessary use of sodium hydrosulfite for the conversion from graphene oxide into the black, graphitic form of reduced graphene since this chemical is noxious and can even explode if not treated carefully. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) coatings, on the other hand, can be easily prepared and reach high conductivities but are known to be unstable under UV radiation.29, 30, 31

In an earlier investigation, the conductive silicone rubber “Powersil 466 A+B” (Wacker Chemie AG, Munich, Germany) was shown to withstand a number of washing cycles32; however, this material necessitates polymerizing and curing at 200°C, a temperature which can damage or even destroy several textile materials.

Graphite, on the other hand, is a stable and conductive material which does not oxidize during washing, the opposite of silver coatings, does not necessitate high-temperature treatment and is thus a good candidate for textile coatings. Nevertheless, only a few examinations have yet dealt with graphite-based conductive coatings,33 partly as a base for comparison with new coatings, based on effect pigments which can, e.g., be used for electromagnetic shielding.34,35 No detailed examination of the influence of graphite particle shape or dimensions, graphite content and other parameters of conductivity and washability of such coatings was found in the scientific literature.

Because of their stability and suitability for textile coatings, different graphite/polyurethane mixtures were prepared on diverse textile materials and tested with respect to their washing fastness on a variety of woven fabrics from different textile materials.

Experimental methods and materials

Graphite/polyurethane (PUR) dispersions were prepared combining bredderpox®R15GB-Flex PUR resin, bredderpox®R15GB-Flex PUR hardener and diluting agent Verdünner XA (all purchased from Breddermann Kunstharze, Schapen/Germany) with different amounts of graphite (“coarse” particles with diameters 15–20 µm or “fine” particles with diameters 3–5 µm, respectively, purchased from Algin, Neustadt Glewe/Germany). The pastes always contained 6.25 g resin, 1.8 g hardener, and 5.82 g diluting agent, combined with 4.62 g/5.13 g/5.66 g/6.23 g/6.83 g of graphite. These values correspond to graphite concentrations between 25% and 33% in steps of 2% with respect to the overall mass, or concentrations of 36.5%/38.9%/41.3%/43.6%/45.9% with respect to the two-component PUR system (resin + hardener). These concentrations were chosen since preliminary tests had shown that lower graphite contents resulted in immeasurably high electrical resistances, while higher concentrations led to brittle coatings.32

As a benchmark, the conductive silicone rubber Powersil 466 A+B was tested additionally.

Both the graphite/polyurethane coatings and the Powersil coatings were applied using a doctor’s knife after adhesive tape had been glued on the respective textile fabric to define the coated area of 10 mm width and 150 mm length as well as the coating layer height. For each combination of coating and fabric, four samples were prepared on which three different ranges of 100 mm length each were marked. In this way, each result could be averaged over twelve measurements.

Table 1 shows the characteristics of the four woven fabrics used in this investigation. Material, thickness, and areal weight are varied, allowing for the qualitative detection of the influences of all three parameters in this first-principle study.
Table 1

Thickness and areal weight of textile fabrics used for coating

Name

Material

Thickness/mm

Areal weight/(g/m2)

Warp threads/cm

Weft threads/cm

CO

Cotton

0.32

94

30

22

VI

Viscose

0.17

67

24

40

LI

Linen

0.34

141

26

24

PES

Polyester

0.11

68

40

50

Washing tests were performed using a household washing machine Miele Primavera with heavy duty detergent at 40°C and spin cycling with 1400 min−1. In this study, we have performed 10 washing cycles to find the most promising material combinations and develop them further. With the best samples, 50 washing cycles are planned to be performed, to cover 1 year in the lifecycle of a typical garment.

For resistance measurements, a Mastech PM334 digital multimeter was used with a maximum measurable resistance of 30 MΩ. For all immeasurable values, thus a resistance of 30 MΩ was assumed.

Microscopic images of the coating surfaces were taken using a confocal laser scanning microscope (CLSM) VK-9700 by Keyence. All CLSM images depicted in this article have the same nominal magnification of 2000×.

Results and discussion

For a first optical examination of the coatings, CLSM images of the samples were made. Figure 1 shows the CLSM images of graphite coatings on linen after production. The coatings on other fabrics have the same optical appearance and are thus not depicted here. While the difference between fine and course graphite flakes is obvious, the difference between the smallest and the largest amount of graphite is hardly visible. In the case of the higher graphite concentrations, the surfaces seem to be brighter on average, suggesting that the orientation of the flake-shaped graphite is more parallel to the surface in these cases and thus showing stronger reflections. This apparent higher orientation may contribute to the resistance of the coated layers. In all cases, the large number of graphite flakes visible at the surface suggests that washing will impede the coating by separating the flakes from the bulk coating material.
Fig. 1

CLSM images of graphite coatings on linen before washing

This was tested by taking CLSM images again after 10 washing cycles. The results are depicted in Fig. 2. As expected, these images partly show areas in which no graphite flakes are visible anymore. However, most of the surface is still coated by graphite, the percolation paths necessary for conductivity still seem to exist.
Fig. 2

CLSM images of graphite coatings on linen after 10 washing cycles

A different situation is visible for the Powersil coatings. Here, the surface is completely smooth before washing (Fig. 3). After 10 washing cycles, the surface is principally not rougher than before, but some fine defects are visible. These can be attributed to damages due to collisions with rough materials during washing, such as the pleats occurring in the linen or cotton samples. These defects, however, are only visible on a microscopic scale. With the naked eye, no differences between Powersil coatings before and after washing are visible.
Fig. 3

CLSM images of Powersil coatings on linen before and after 10 washing cycles

Figure 4 shows the washing cycle dependence on the resistance of Powersil coatings on the four different textile fabrics. Opposite to the former investigation with thicker lines,32 the electrical resistance was significantly increased by washing for the coatings on viscose and polyester, while the increase was much smaller on cotton and nearly negligible on linen. This finding cannot be attributed to the coating thicknesses which were found to be between 100 and 120 µm dry film thickness, independent of the substrate. Instead, the structures of the woven fabrics and the fibers can be assumed to play an important role. On the one hand, the natural fibers CO and LI have rougher surfaces and more uneven cross sections than the man-made fibers VI and PES. On the other hand, CO and LI fabrics are thicker and have lower thread densities (calculated as warp threads/cm · weft thread/cm, cf. Table 1), resulting in visibly looser fabrics with larger open areas in which the coating masses can penetrate. This can be expected to result in a better form-locking adhesion between coating and textile fabric as well as additional coating mass in the textile substrate. Since no percolation paths can be formed inside the textile (they would regularly be interrupted by warp and weft threads), this part of the coating does not directly contribute to the overall conductivity; however, it may shelter the actual coating layer on top of the fabric from washing influences on the backside and thus result in “undisturbed” coating layers directly on top of the natural fiber fabrics.
Fig. 4

Resistance versus number of washing cycles, measured for Powersil coatings on cotton (CO), viscose (VI), linen (LI), and polyester (PES) fabrics

The absolute resistance values depicted here are significantly larger than those known from a typical copper wire or similar “hard” conductive paths. Instead, such coatings can be used for heating a textile, as pressure or elongation sensors, but also as coatings on ECG sensors, etc., in which the current flows through the coating parallel to the surface normal, i.e., only along a short distance of some ten to hundred micrometers.

The washing cycle dependence of the resistance of the fine graphite coatings is depicted in Fig. 5. Starting with the highest graphite concentration of 33%, significant differences are visible between the coatings on the different textile fabrics. Opposite to Fig. 4, here PES and CO show the best results, while on viscose the coated lines break after the 8th washing cycle, resulting in a strongly increased resistance. For a graphite content of 31%, the results are similar. Especially for this graphite concentration, resistances measured on CO and PES are comparable to those found for Powersil coatings, with graphite on PES even showing lower resistances after ten washing cycles. Proceeding to smaller graphite concentrations, the coatings on PES start showing higher resistance values, while the coatings on viscose are improved and now withstand more washing cycles. Finally, for the lowest graphite concentration of 25%, there is no measurable conductivity left on the PES fabric, while on cotton, the resistance even decreases during washing to values significantly below those measured with Powersil. This shows that the self-produced fine graphite coatings with relatively high graphite contents are well-suited for use on CO or PES, while for coatings on LI, lower graphite concentrations are ideal to prevent breaks due to washing.
Fig. 5

Resistance versus number of washing cycles, measured for all fine graphite coatings on cotton (CO), viscose (VI), linen (LI), and polyester (PES) fabrics. In the last diagram, the resistance values of the coating on PES were continuously too high to be measured

Apparently, the probability of complete breaks is larger on viscose and linen fabrics and especially given for higher graphite contents. This can be attributed to the strong pleats which are often built in these two materials and which heavily stress the coating, especially if the graphite content is near the maximum possible one.

On the other hand, for the lowest graphite content coatings on polyester the resistance is out of measurement range, i.e., the resistance must be higher than 30 MΩ. This finding can be explained by the visibly reduced layer height of the graphite coatings on PES which could not be avoided although coating was always performed with the same vertical force on the doctor’s blade. In particular, the coatings with smaller amounts of graphite were nearly completely sunk in the PES fabric, most probably due to its strong hydrophilicity.

Interestingly, in some cases, the resistance is decreased after the first washing cycles. This may be due to an incomplete polymerization process after coating and will be examined again during the evaluation of the coatings with course graphite.

The results of the washing tests with course graphite flakes are shown in Fig. 6. Generally, all resistances are larger than in the case of the fine graphite flakes. The breaks in the conductive lines on viscose, however, now have a higher probability for smaller amounts of graphite, while high contents of coarse graphite help in getting washable conductive coatings on viscose. For CO, smaller graphite contents are more stable than higher ones. Additionally, the coatings on linen are in most cases now advantageous compared with those on cotton and also more stable for smaller graphite contents. Polyester, however, is again a problematic substrate, especially for smaller graphite contents.
Fig. 6

Resistance versus number of washing cycles, measured for all coarse graphite coatings on cotton (CO), viscose (VI), linen (LI), and polyester (PES) fabrics. In the last three diagrams, the resistance values of the coating on PES were continuously too high to be measured

As realized before, the conductivity of the coarse graphite coatings on cotton and linen is increased again for smaller amounts of graphite during the first washing cycles. One possible explanation is an incomplete polymerization 1 day after sample preparation when the first measurement took place.

To investigate the relative resistance change due to washing, Figs. 7, 8, and 9 show the ratios of the resistances after 10 washing cycles R10 and before washing R0. Values smaller/larger than 1 show a decrease/increase of the resistance due to washing. Values equal to 1 without error bars, however, depict the case that the resistance is not measurable from the beginning.
Fig. 7

Relative resistance change of Powersil coated samples, comparing the measurements after 10 washing cycles and before washing

Fig. 8

Relative resistance change of samples coated with fine graphite particles, comparing the measurements after 10 washing cycles and before washing

Fig. 9

Relative resistance change of samples coated with coarse graphite particles, comparing the measurements after 10 washing cycles and before washing

The Powersil coatings used as a benchmark show the most stable resistance increases which are nearly independent from the textile substrate—which can be expected since no particles are visible on the surface which could be washed out—but not the smallest. Interestingly, the resistance increase ratios are on the average larger for the fine graphite particles and for higher graphite concentrations, i.e., for smaller absolute resistances. On the other hand, no general correlation between the textile material and the resistance increase can be recognized.

These results show that on one hand, PUR coatings including finer graphite flakes result in higher conductivities. On the other hand, the resistances depend not only on the flake dimensions and the graphite contents, but also significantly on the textile substrates. This can even mean that coatings with the best performance on cotton have the worst conductivity on polyester, etc. Apparently, when preparing a conductive textile coating based on graphite, the planned application, i.e., the textile fabric to be coated, must be taken into account. Otherwise, contradictory results may be found in experiments, which explains the small amount of scientific literature available about this subject.

Finally, some remarks should be made with respect to the usability of such coatings in diverse applications. Firstly, each coating which is thicker than some 10 nm will more or less modify the haptics of the textile fabric, its drapability, and other mechanical properties. Second, the water vapor resistance will increase. Third, all graphite-based coatings will modify the color of the fabric.

Both coatings applied here are relatively flexible. Nevertheless, they increase the bending rigidity of the textile fabric and correspondingly decrease its drapability. Additionally, since the textile structure is completely covered (cf. Figs. 1, 2, 3), the haptic impression changes. This is of special relevance for the Powersil coating which feels sticky like rubber, while the PUR coatings with graphite have a more common surface, feeling similar to prints on T-shirts. Powersil is also less well-suited for large-scale applications on textile fabrics due to the significantly increased water vapor resistance—it is nearly completely impermeable for water vapor and air, while the PUR coating, however, does not show advantageous properties here. This means that both coatings should not be used on large areas of garments. For applications as ECG electrodes, as sensors or conductive paths in smart textiles, however, these properties are of reduced importance.

The color is an additional issue which must be taken into account. Due to the black color of graphite in most modifications, graphite-based coatings will always be black (Powersil) or dark-gray (PUR-graphite). This effect can be ignored in most smart textile applications in which clearly showing the electronic components is often part of the design concept, or medical applications in which the color of electrodes does not matter.

To conclude, such coatings are not ideal for large-scale applications on garments, but well-suited for typical smart textiles, electrodes, conductive paths, etc., where there is no objection to making functions visible.

Conclusions

To conclude, different textile substrates were coated with graphite/PUR layers. The influence of the graphite concentration and flake dimensions as well as the fabrics on the washing fastness of the conductive layers was studied. Unexpectedly, not only significant dependencies on the coating parameters, but also on the textile substrates were found. For CO and LI fabrics, relatively stable coatings could be created by optimizing graphite contents and particle dimensions. Some combinations of textile fabric/particle/concentration resulted in comparable or even more stable coatings than the Powersil material which was used as a benchmark. This means that graphite/PUR coatings must be tailored to match a specific textile material. Further research is necessary to gain more insight into all relevant fabric parameters and to strongly increase the washing fastness on most textiles under examination, e.g., by combining graphite particles of different sizes.

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Copyright information

© American Coatings Association 2018

Authors and Affiliations

  1. 1.Faculty of Engineering and MathematicsBielefeld University of Applied SciencesBielefeldGermany

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