Materials and Design 32 (2011) 4247–4256

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Materials and Design

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Electro-conductive and elastic hybrid yarns – The effects of stretching,cyclic straining and washing on their electro-conductive properties

A. Schwarz a,⇑, I. Kazani a, L. Cuny b, C. Hertleer a, F. Ghekiere b, G. De Clercq b, G. De Mey c,L. Van Langenhove a

a Ghent University, Department of Textiles, Technologiepark 907, 9052 Zwijnaarde, Belgiumb University College Ghent, Department of Textiles, Voskenslaan 362, 9000 Gent, Belgiumc Ghent University, ELIS, Sint-Pietersnieuwstraat 25, 9000 Gent, Belgium

a r t i c l e i n f o

Article history:Received 10 January 2011Accepted 12 April 2011Available online 27 April 2011

Keywords:Filament windingElectricalDesign for reliability

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.04.021

⇑ Corresponding author. Tel.: +32 9 264 5408; fax:E-mail address: Anne.Schwarz@UGent.be (A. Schw

a b s t r a c t

Electro-conductive yarns can be produced in various ways and can obtain very different properties interms of conductivity, touch, as well as strength and elasticity. In this research, it was focussed on man-ufacturing elastic and electro-conductive yarns (el2-yarns) via hollow spindle spinning. All yarns com-prised elastic core yarns, based on rubber, around which electro-conductive winding yarns, based onsilver, copper and stainless steel, were wound.

This paper presents the yarn’s electrical characteristics while stretching and after exposure to cyclicstraining and washing.

Analyzing the el2-yarn’s electro-conductive properties upon elongation, revealed that their electricalresistance remains constant over elongation levels up to 100%. Furthermore, it is shown that both, cyclicstraining and washing, decrease the yarn’s electrical performance.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Textiles can be distinguished from other material structures bytheir conformability and compliance. Thanks to these propertiesthey are very interesting to host electronic components that needto be applied close to the human body. Electronic components,such as electrodes to record an ECG [1,2] or the respiration rate[3,4], antennas [5,6], transmission lines [7,8], or heating elements[9], require electro-conductive yarns to increase textile compatibil-ity. However, electro-conductive yarns often suffer from poorelasticity [10], unstable electrical properties [11] and fragility.Therefore, they often face problems as they are deformed andsubjected to stress during application, e.g. by body movements.This will change their resistance [12] or causes yarn breakageand hence, their reliability cannot be guaranteed.

Consequently, there is a need for electro-conductive yarns thatare elastic and do not change their electro-conductive propertieswhen being stressed.

Elasticity can be achieved in electro-conductive textiles on dif-ferent levels. On fibre level, extruding elastic polymeric fibresloaded with electro-conductive particles is a prominent example.Most likely, rubber-based or thermoplastic elastomers are com-bined with conductive particles, such as carbon black [13,14]. On

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+32 9 264 5831.arz).

yarn level spinning is often applied. Here, research and productdevelopment of twisting or wrapping electro-conductive filamentsand yarns around an elastic core yarn is predominantly industry-driven, including companies as Textronics, Zimmermann orBekaert. Laminating and dry bonding are popular methods to com-bine elasticity and electro-conductive properties on a fabric level[15]. Thermoplastic polyurethane films are used as substrate mate-rial onto which meander-shaped copper structures were printed.Due to the ‘‘wavy’’ shape of the copper and its integration withthe polyurethane substrate, the composite system provides elasticresponses to applied strain, without inducing significant strain inthe copper structure [16]. Another solution to achieve conductivityon fabric level is a metallic deposition on a fabric [17].

In our research, the hollow spindle technology was applied toproduce electro-conductive and elastic yarns (el2-yarns) [18].Our aim was to obtain electro-conductive yarns with enhancedelastic and stable electro-conductive properties. The yarns had tomeet the following requirements:

� While processing, the materials are easily manageable.� Process is easily upscalable.� Elastic properties of the el2-yarn are comparable to the ones of

elastomeric yarns.� Electrical properties of the el2-yarn are in the range of metal

conductors.� Electro-mechanical properties of the el2-yarn are constant.

Fig. 1. Design of an el2-yarn: Conductive winding yarns are wound around anelastic core yarn in S- and Z-direction.

Table 2Overview of production parameters and appearance of el2-yarns.

Yarn Count(tex)

Twist 1(tpm)

Twist 2(tpm)

Appearance

Silver-basedC1W9 788 5000 5000

C2W9 516 12,000 12,000

C4W9.a 483 5000 5000

C4W9.b 595 6000 6000

C5W9 569 5000 5000

C2W10W11.a 678 12,000 11,000

C2W10W11.b 601 6000 6000

C2W10W12 592 6000 6000

C2W10W13.a 592 6000 8000

C2W10W13.b 619 5500 7000

4248 A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256

� Electrical properties of the el2-yarn are stable when� repetitively stretched by 25%� being washed 25 times

In this paper our analysis on the yarn’s stability and reliability ispresented. Hence, we measured their electrical resistance duringand after different scenarios: (A) in stretched state, (B) after cyclicstraining, and (C) after washing, and discuss the results in the fol-lowing sections. According to the knowledge of the authors, thisstudy on the effects of stretching, cyclic straining and washingon the electro-conductive and elastic yarns is novel and has notbeen addressed in any other research papers.

Copper-basedC2W5 353 12,000 12,000

ClW6.a 528 2700 3200

ClW6.b 459 5000 5000

C2W6.1 415 12,000 12,000

C2W6.2 502 12,000 12,000

C3W6 549 5000 5000

C4W6 599 5000 5000

C5W6 386 4000 4000

2. Experimental

2.1. Materials – el2-yarns

In total, 27 el2-yarns were produced via the hollow spindle pro-cess and used for the experiments. They were all designed accord-ing to the same principle: While elastic, non-conductive yarnswere used as core yarns, electro-conductive yarns or filamentsand, in a few cases, non-conductive yarns were applied as windingyarns (Fig. 1).

For the core yarn, different naked and covered rubber yarns,varying in fineness and different elasticity levels were chosen. Asconductive winding materials copper, silver and stainless steelwere selected. For the latter preference was given to staple and fil-

Table 1Overview of selected input yarns.

Material Count (tex) Diameter (lm)

Core yarnsC1 Naked rubber 600 860C2 Naked rubber 560 800C3 Rubber covered with polyamide 291 690C4 Rubber covered with polyamide 306 606C5 Rubber covered with polyester 307 632

Winding yarnsW1 Stainless steel staple fibre yarn 186W2 Stainless steel multifilament yarn 110W4 Stainless steel filamen 10W5 Copper monofilament, 30 lm, soft 7W6 Copper monofilament, 50 lm, soft 17W7 Copper monofilament, 70 lm, soft 34W8 Copper-coated Zylon yarn 300W9 Silver-coated copper filament, 56 lm 21W10 Silver-coated Zylon yarn 160W11 Polyamide filamentW12 Polyester filamentW13 Tactel� multifilament yarn 69

ament yarns. In the case of copper and silver, monofilaments aswell as metallized poly(p-phenylene-2,6-benzobisoxazole) (PBO)multifilament yarns (Zylon�) were chosen. In some cases non-con-ductive winding yarns were used as well. These were polyester andtwo different polyamide yarns. Table 1 gives an overview of theyarns that were used as input material for core and for windingyarns.

C2W7.a 493 12,000 12,000

C2W7.b 650 14,000 14,000

C2W8W11 942 5000 6000

C2W8W12 830 5000 6000

Steel-basedC1W1 1193 650 450

C4W1 1365 450 400

C1W2.a 787 600 700

C1W2.b 886 900 900

C2W4 353 12,000 12,000

C4W4 426 5000 5000

Fig. 2. Test set-up to measure the electro-mechanical properties of the el2-yarn.

Fig. 3. Cyclic straining testing instrument.

A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256 4249

The combination of different input yarns and variations inproduction parameters lead to 27 el2-yarns. Table 2 shows the pro-duction parameters and photographs of all el2-yarns.

2.2. Methods

The el2-yarns were evaluated by quantifying their electricalresistances during and after different scenarios as mentioned ear-lier. To measure the electrical resistance of the el2-yarns a four-point probe technique was used. The four-point probe is a set-upused to measure resistance of a thin film or diffusion layer on aninsulating material [19]. The four-point probe station consisted ofan Ampèremeter, type Solartron Schlumberger 7150 Plus DigitalMultimeter (Resolution �0.1 mA), a DC current source (RS PL-ser-ies), and a voltmeter, type Keithley 195A Digital Multimeter (Res-olution 0.001 mV). The four probes were arranged in linear fashion,with the two outer probes connected to the current supply, and theinner probes to the voltmeter.

The measurements were carried out in an unconditioned roomto correspond to the yarn’s end application conditions.

2.2.1. StrainingThe resistance under straining was measured as a function of

current input and voltage output (Fig. 2). A slider was used, intowhich the relaxed, unstrained el2-yarn with a defined distance of5 cm was clamped. The el2-yarn was then elongated by movingone clamp. For each straining level and el2-yarn type 15 measure-ments were taken. The results were analyzed by using Analysis ofVariance (ANOVA).

2.2.2. Cyclic strainingThe el2-yarns underwent cyclic straining, which constantly

strained and relaxed them. The level for cyclic straining was care-fully chosen. One criterion for exclusion were strain-levels causingfailure in the el2-yarn if applied as a constant force. Moreover, ithad to be a realistic strain-level to simulate later application sce-narios. These considerations yielded into a cycling level of 25%.

Hence, the samples were cycled 2150 times with a cycled strainfrom 0 to 25%. The instrument used was especially designed forcyclic straining experiments on yarns (Fig. 3). The instrument oper-ates on the principle of constant deformation rate where the exten-sion applied on the yarn increases with time at a constant velocity.The yarns were clamped vertically between an upper and a lowerjaw. The movement was carried out by the lower jaw, while theupper one remained fixed. The instrument allows to clamp tenyarn samples next to another. All yarns tested were cut in sampleswith a length of 20 cm.

The electrical properties of the cyclically loaded samples weremeasured using the four-point probe set-up as described above.For each el2-yarn type 15 measurements were recorded.

The data was then analyzed using appropriate statistical proce-dures: the F-test followed by the t-test. The t-test assesses the sta-tistic difference of means from two groups. A probability value ofp < 0.05 was used to identify significant values for statistical testsperformed on the data.

2.2.3. WashingThe el2-yarns underwent a washing procedure in order to

investigate their long term behaviour. The samples were washed25 times at 40 �C for 40 min per cycle according to the interna-tional standard ISO 6330:2000 with the selected procedure 6Afor a type A washing machine, type Wascator FOM71 CLS. In addi-tion to the samples, the machine was filled with cotton fabrics toreach its standard load of 2 kg. Conclusively, the yarn’s resistancewas measured with the four-point-probe. 15 measurements weretaken for each el2-yarn. To find a trend in the results, ANOVAwas applied.

3. Results and discussion

3.1. Electrical resistance under straining

The graphs in Figs. 4–6 illustrate that the electrical resistanceappeared to remain constant when stretching the produced yarns.

With the goal to determine whether the el2-yarn’s electricalresistance remains constant under elongation, ANOVA was applied.

Fig. 4. Electro-mechanical properties of silver-based el2-yarns.

Fig. 5. Electro-mechanical properties of copper-based el2-yarns.

Fig. 6. Electro-mechanical properties of stainless steel-based el2-yarns. Values for C1W4 and C2W4 are not shown in the table as their values are significantly higher than theothers.

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Table 3El2-yarns onto which 100% elongation had no influence on their electrical resistance.

EI2-yarn F p-Value Fcrit

Silver-basedC1W9 0.068 0.997 2.323C2W9 2.103 0.073 2.323C4W9.a up to 60% 1.833 0.152 2.769C4W9.b 1.232 0.301 2.323C5W9 2.067 0.078 2.323C2W10W11.a 1.22 0.307 2.323C2W10W11.b 0.634 0.675 2.323C2W10W12 0.626 0.601 2.769C2W10W13.a 1.023 0.409 2.323C2W10W13.b 2.005 0.086 2.323

Copper-basedC1W6.a 1.868 0.109 2.323C1W6.b 0.245 0.941 2.323C2W5 2.139 0.767 2.401C2W6.2 2.233 0.059 2.323C3W6 1.371 0.243 2.323C4W6 0.032 0.1 2.409C5W6 1.729 0.138 2.332C2W7.a 1.449 0.215 3.323C2W7.b 0.725 0.607 2.342C2W8W11 0.729 0.604 2.323C2W8W12 0.997 0.425 2.323

Stainless steel-basedC1W1 1.03 0.41 2.32C4W1 0.5 0.77 2.32C1W2.a 1.28 0.28 2.32C1W2.b 1.63 0.16 2.32C2W4 1.53 0.19 2.32C4W4 1.72 0.14 2.32

A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256 4251

It quantified the relation between the electrical resistances at in-creased elongation levels. The variable factor was the elongationlevel, being 0%, 20%, 40%, 60%, 80% and 100%. For all yarns, it canbe reported that p > 0.05. Hence, the results tell that the probabilityis small (less than 5%) that the elongation on the samples influ-enced their resistance (Table 3).

The elastic deformation of the core yarn upon stretching impliesan increase of twist length. However, the fact that the electricalresistance is constant over elongation levels up to 100% shows thatthe dimensions of the winding yarn itself are not changed. No plas-tic deformation took place, since this would lead to a change inresistance.

Fig. 7. Electrical resistance of silver-based el2

3.2. Electrical resistance after cyclic straining

The data obtained for silver-, copper- and stainless steel-basedel2-yarns are diverse and therefore they are separately analyzed.

3.2.1. Silver-based el2-yarnsThe obtained values for the electrical resistance before and after

cyclic straining are depicted in Fig. 7. The graph supposes that theel2-yarns’ resistance slightly increases after cyclic straining.

As comparison of the mean resistance values before and aftercyclic straining, a single factor ANOVA is made (Table 4). The t-testindicates that cyclic straining does not lead to the same meanresistance for most samples. Exceptions are samples C2W9 andC2W10W11.a. For them, the decrease is not significant (p > 0.05).

The differences in resistance for the el2-yarns based on silvermonofilaments can be attributed to their production parameters.If the core yarn was not drafted to its maximum elongation duringproduction, the winding yarns were not tightly twisted around it.This allowed them to slide along the core yarn axis during cyclicstraining.

And yet another phenomenon could be noticed. The constantsliding of the non-conductive winding yarns over the silver-coatedyarns during cyclic straining had an abrasive effect on the silverlayer (Fig. 8).

3.2.2. Copper-based el2-yarnsBased on the results obtained for the silver-based yarns, the

copper-based yarns are expected to increase in resistance aftercyclic straining. To compare the mean resistances before and afterstraining is the objective of this study, a single factor ANOVA(paired t-test) is made. The null hypothesis states that the cyclicstraining results in a higher yarn resistances (Table 4).

The results represented in Fig. 9 indicate that not an overallconclusion can be drawn on the mean value of the copper samples.For samples C2W5, C1W6.a, C1W6.b and C3W6 it is suggested thatthe electrical resistance was influenced by the cyclic straining, asp < 0.05 in these cases. For the three latter el2-yarns, the differencein resistance before and after cyclic straining is very highly signif-icant (p < 0.001). For the other el2-yarns, there is no evidence thattheir resistance differs before and after cyclic straining (p > 0.05).

When looking at factors affecting the sample’s electrical resis-tance, winding yarn breakages could be excluded. Also, thoroughexamination of microscopic photographs of the yarns, taken beforeand after cyclic straining, revealed no visual changes in their struc-

-yarns after 2150 straining cycles at 25%.

Table 4Overview of the statistical analysis of cyclic straining as well as the difference inresistance for all el2-yarns.

t p-Value Difference(X)

Increase inresistance (%)

Silver-basedC1W9 95.12 3.66 � 10�8 7.72 � 10�2 30.33C2W9 0.19 0.43 0.04 � 10�2 �0.50C4W9.a 7.43 0.01 � 10�1 1.71 � 10�2 6.55C4W9.b 5.65 0.002 2.65 � 10�2 10.19C5W9 10.16 0.02 � 10�2 4.04 � 10�2 17.90C2W1W11.a 1.00 0.17 0.01 � 10�2 �0.15C2W10W11.b 3.55 0.02 � 10�1 1.32 � 10�2 30.88C2W10W12 11.72 7.40 � 10�8 2.31 � 10�2 40.42C2W10W13.a 2.03 0.03 0.31 � 10�2 5.96C2W10W13.b 28.28 6.34 � 10�12 7.81 � 10�2 137.87

Copper-basedC2W5 2.47 0.02 221 � 10�4 8.21C1W6.a 66.34 1.55 � 10�7 1850 � 10�4 84.95C1W6.b 22.24 1.21 � 10�5 2280 � 10�4 64.46C2W6.2 0.28 0.39 1.68 � 10�4 0.18C2W7.a 0.80 0.22 �56.8 � 10�4 �5.52C2W7.b 0.36 0.36 �10.8 � 10�4 �2.45C3W6 67.31 1.46 � 10�7 2000 � 10�4 55.80C4W6 1.59 0.09 �104 � 10�4 �2.95C5W6 0.90 0.21 22.2 � 10�4 0.81C2W8W12 0.23 0.41 2.19 � 10�4 1.12C2W8W13 0.15 0.44 3.65 � 10�4 1.95

Stainless steel-basedC1W1 18.11 2.06 � 10�11 0.17 34.62C1W2.a 18.99 1.09 � 10�11 0.35 47.55C1W2.b 33.60 4.36 � 10�15 0.39 45.39C2W4 5.39 0.01 � 10�2 0.48 7.29C4W1 91.06 4.05 � 10�21 0.34 64.96C4W4 30.82 1.44 � 10�14 9.93 35.30

Fig. 8. Damaged silver layer of a winding yarn after cyclic straining.

Fig. 9. Electrical resistance of copper-based e

4252 A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256

ture (Fig. 10). However, as these yarns are based on thin copper fil-aments, it is suggested that the cyclic straining had an abrasive ef-fect on the filament, leading to a thinning down of the filaments’cross-section.

The samples not influenced by cyclic straining appeared to havetightly twisted winding yarns around their core yarns. Strainingthe el2-yarns by 25% was not sufficient to allow the winding yarnsto slide along the core yarn axis. Consequently, the samples did notchange their electrical resistance before and after cyclic straining.

3.2.3. Stainless steel-based el2-yarnsFor the steel-based yarns, the same hypothesis as for the cop-

per-based yarns was stated that cyclic straining increases theirresistance. Taking a look at the graph first (Fig. 11), an increasein electrical resistance after cyclic straining was noticeable for allsamples. The paired t-test revealed that the resistances are signif-icantly different before and after cyclic straining (p < 0.001) aslisted in Table 4.

This behaviour can be explained through damage of the metallicyarns, whereby electrical current flow is obstructed and the overallresistance increases.

3.3. Electrical resistance after washing

The effect of the washing on the el2-yarn depended on themetal used as winding yarns. While the electrical resistances ofthe silver-based yarns remained unchanged after washing, resis-tances of copper and stainless steel-based yarns changed after 25washing cycles. The dependency on washing was determined byapplying a paired t-test with a = 0.05. Samples were regarded asequal if p > 0.05. The statistical analysis as well as the differencesin resistance, expressed in X and in%, are listed in Table 5. The ef-fect of washing will be extensively investigated, distinguishing sil-ver, copper and stainless steel-based samples.

3.3.1. Silver-based el2-yarnsIn Fig. 12 the electrical resistances of corresponding unwashed

and washed silver-based el2-yarns are displayed. It shows that thecorresponding values were almost equal. After comparing theirmeans with statistics, it can be stated that the silver-based el2-yarn’s resistance remained stable after 25 washing cycles(p > 0.05), except for C2W10W12 and C2W10W13.b. For thesetwo el2-yarns the trend is observed that washing influences theirresistance (0.01 < p < 0.05).

The question arises why the two samples C2W10W12 andC2W10W13.b changed their electrical resistance after washing

l2-yarns before and after cyclic straining.

Fig. 10. Copper monofilament-based el2-yarns before and after cyclic straining (2150 cycles).

Fig. 11. Electrical resistances of the stainless steel-based el2-yarns after straining them 2150 cycles for 25%.

Table 5Overview of the statistical analysis of washing behaviour as well as the difference inresistance for all el2-yarns.

t p-Value Difference(X)

Increase inresistance (%)

Silver-basedC1W9 0.75 0.23 �2.52 � 10�3 �0.99C2W9 1.11 0.14 �3.15 � 10�3 �4.31C4W9.a 1.05 0.16 3.16 � 10�3 1.21C4W9.b 1.58 0.07 4.07 � 10�3 1.56C5W9 0.29 0.39 �0.92 � 10�3 �0.41C2W10W11.a 0.79 0.24 �1.97 � 10�3 �2.67C2W10W11.b 1.55 0.07 2.41 � 10�3 5.63C2W10W12 2.69 0.009 2.91 � 10�3 5.09C2W10W13.a 1.23 0.12 1.10 � 10�3 2.13C2W10W13.b 1.85 0.04 3.63 � 10�3 6.41

Copper-basedC2W5 0.58 0.29 6.16 � 10�3 2.29C1W6.a 23.45 6.14 � 10�13 45.92 � 10�3 21.14C1W6.b 18.81 1.23 � 10�11 53.85 � 10�3 15.20C2W6.2 8.5 3.34 � 10�7 �4.92 � 10�3 �5.24C2W7.a 0.98 0.21 �1.15 � 10�3 �2.70C2W7.b 10.09 4.18 � 10�8 6.26 � 10�3 14.24C3W6 10.79 1.8 � 10�8 34.71 � 10�3 9.69C4W6 10.48 3.06 � 10�6 39.41 � 10�3 11.16C5W6 6.84 4.04 � 10�6 20.68 � 10�3 7.57C2W8W11 7.2 2.28 � 10�6 5.69 � 10�3 29.21C2W8W12 6.45 7.62 � 10�6 �0.96 � 10�3 �5.11

Stainless steel-basedC1W1 3.56 0.002 0.03 6.70C4W1 31.17 1.24 � 10�14 0.17 32.72C1W2.a 9.9 5.28 � 10�8 0.10 14.14C1W2.b 8.09 5.99 � 10�7 0.04 4.94C2W4 6.84 4.02 � 10�6 1.43 21.69C4W4 15.38 1.83 � 10�10 1.04 3.69

A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256 4253

while the other sample’s resistance statistically remainedunchanged.

Both el2-yarns were covered with silver-coated winding yarns.One hypothesis was that the yarn’s layers might have been dam-

aged due to mechanical actions during washing. After a closeexamination of microscopic pictures of the samples (Fig. 13) thisproved to be false.

It should be noted, although statistics shows that the samplesbehave different, the difference in their values is not high (Table5). However, a more detailed study is desired in the future.

3.3.2. Copper-based el2-yarnsDetermining the electrical resistances of the copper-based el2-

yarns revealed a degradation after 25 washing cycles, losing up to30% of their conductivity. However, they still possessed useful elec-trical resistances as displayed in Fig. 14. For almost all copper-based el2-yarns the hypothesis can be confirmed. With p < 0.001,the difference in resistance is highly significant before and afterwashing. Two exceptions are el2-yarns C2W5 and C2W7.a. Forthem, the difference in resistance is not significant before and afterwashing (p > 0.05).

For the copper-coated winding yarns (W12 and W13), an exam-ination of their surface showed no visible traces of corrosion afterwashing. However, the scanning electron microscopic (SEM) pho-tographs revealed some cracks in the coating surface after 25washing cycles. This resulted in loss of electro-conductive proper-ties (Fig. 15).

Another reason for the change in electrical resistance was thecopper’s chemical reaction with the components of the washingsolution. Copper easily forms hexa-aqua-copper(II)ions[Cu(H2O)6]2+ in solution. As the washing solutions contains 9.1%sodium carbonate, the precipitate copper(II)carbonate (CuCO3) isformed according to

2½CuðH2OÞ6�2þðaqÞ þ CO2�

3 ðaqÞ ! CuCO3 þ CuðOHÞ2 þ 2Hþ

þ 10H2O ð1Þ

Copper precipitates can also be formed due to a reaction of[Cu(H2O)6]2+ ions with hydroxide ions as the washing solution con-tains 2.8% of sodium soap. Hydroxide ions remove hydrogen ions

Fig. 12. Electrical resistances of the silver-based el2-yarns before and after washing remained the same.

Fig. 13. Silver-coated winding yarn before and after washing.

Fig. 14. Electrical resistances of the copper-based el2-yarns before and after washing changed in most cases.

4254 A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256

from the water ligands attached to the copper ion. Once a hydro-gen ion has been removed from two of the water molecules, a neu-tral complex is left that is insoluble in water by

½CuðH2OÞ6�2þ þ 2OH� ! ½CuðH2OÞ4ðOHÞ2� þ 2H2O ð2Þ

As copper precipitates are formed the amount of copper on theyarn will decrease. Consequently, the resistance will be higher dueto a decrease in cross-sectional area.

3.3.3. Stainless steel-based yarnsThe electrical properties of the stainless steel-based el2-yarns

were also influenced by the washing cycles. A comparison of the

electrical resistance of all stainless steel el2-yarns before and afterwashing is depicted in Fig. 16. From the comparison of their meanresistance values before and after washing, it is evident that thewashing of the steel-based el2-yarns is related to a higher electri-cal resistance (p < 0.05 for C1W1 and p < 0.001 for the other steel-based el2-yarns).

Although there was no visual damage (Fig. 17) on the yarn afterwashing, the chromium(III)oxide (Cr2O3) layer, being formed whensteel is exposed to air to protect it from oxidation, can be damaged.Even though the layer is impervious to water, it might get damageddue to mechanical actions during washing. Hydrocarbons presentin the washing solution penetrated through the damaged area, at-

Fig. 15. Copper-coated winding yarns before and after washing.

Fig. 16. Electrical resistances of the steel-based el2-yarns before and after washing remained the same.

Fig. 17. Stainless steel-based el2-yarn C4W1 before and after washing – no damage visible.

A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256 4255

tack the carbon in the metal and convert it to carbon dioxide. Thus,corrosion, in this case so-called crevice corrosion [20], is promotedand the electrical resistance increases.

4. Conclusions

In this paper our study on characterizing the electro-conductiveproperties of electro-conductive and elastic yarns (el2-yarns)

under different situations was presented: during straining, aftercyclic straining and after washing. The major conclusions are:

The el2-yarns electrical resistance remains constant overelongation levels up to 100%.

The el2-yarns can be stretched up to 100% of their originallength without a significant change in their electrical resistance.This implies, that the dimension of the core and winding yarns isnot changed.

4256 A. Schwarz et al. / Materials and Design 32 (2011) 4247–4256

The electrical resistance of el2-yarns increases slightly aftercyclic straining for 2150 times.

Depending on the production parameters as well as the natureof the input yarns, 2150 cycles of 25% elongation deteriorated theelectrical properties in some el2-yarns. If the core yarn is notstretched to its maximum during production, the winding yarn willslide along the core yarn’s axis during cyclic straining. The slidingof the winding yarn, in turn, causes an abrasive effect on the wind-ing yarn’s surface, damaging the conductive layer and increasingthe electrical resistance.

Silver-based el2-yarns are not affected by 25 washing cycles,while most of copper- and stainless steel-based el2-yarns showan increase in their electrical resistance.

The change in electrical resistance of copper-based el2-yarnsafter washing can be attributed to the precipitation of copper dur-ing washing.

For stainless steel-based el2-yarns, the increase in resistancecan be reasoned by the damage of the chromium(III)oxide layerdue to mechanical actions during washing, that promotes crevicecorrosion.

Acknowledgements

This work was carried out in the scope of the Tetra-project‘Electro-conductive and elastic yarns for smart textile applications’funded by the IWT (Flemish agency for innovation by Science andTechnology) as well as Bota, Elasta, Soieries Elite, Cousy, Medio’sand Cumerio. The authors would like to thank the IWT and theindustrial partners for their financial support. Furthermore, ourgratitude is expressed to Syscom Advanced Materials for theirdonation of silver and copper-coated Amberstrand� yarns.

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