P.Xue,K.H.Park,X.M.Tao ,W.Chen,X.Y.Cheng ElectricallyconductiveyarnsbasedonPVA/carbonnanotubes

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Electrically conductive yarns based on PVA/carbon nanotubes

P. Xue, K.H. Park, X.M. Tao

^*

, W. Chen, X.Y. Cheng

Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong Available online 2 December 2005

Abstract

Electrically conductive yarns from carbon nanotubes (CNTs) and polyvinyl alcohol (PVA) were prepared by two different methods, wet-spinning and coating processes. For the wet-spinning process, the CNTs/PVA mixed solution was extruded into a coagulant bath which contained high concentration coagulating agent. The coating process was carried out on five common fibers, including natural fibers and synthetic fibers, by using a blend of CNTs and PVA. Then, the conductive yarns were treated by an acetylization process in order to decrease the solubility of PVA to water. Their conductivity and the mechanical property were investigated. Additionally, the influence of acidic hydrolysis by strong acids used for the acetylization process on mechanical properties of the materials was investigated.

Keywords: Electrical conductivity; Mechanical property; PVA; Carbon nanotubes; Conductive ﬁbers

1. Introduction

Electrically conductive textile composites, representing a family of newly developed composites, have many potential applications, such as sensors, static charge dissipation, ﬁl- ters, electro-magnetic interference shield, and special purpose clothing acting as protection or dust and germ free purpose. Demand for electrically conductive textiles is increasing greatly in recent years.

Therefore, a number of researches have been carried out to produce electrically conductive textiles. Electrically conductive ﬁbers were produced by wet spinning[1], melt spinning[2]from conductive polymer polyaniline; or coating ﬁbers with electrically conductive materials such as metal powders [3,4], carbon black [5] or intrinsically conductive polymers [6–8]. Among the manufacturing processes, various coating techniques have been attractive due to simple in process and easy to handle. The textiles produced not only gain controllable electrical properties,

but also maintain their excellent physical properties of the textiles such as mechanical strength and ﬂexibility [9].

In addition to the electrically conductive materials men- tioned above, carbon nanotubes (CNT) have been found being outstanding. The research on CNT has revealed its unique atomic structure, very high aspect ratio, and extraordinary electrical, mechanical, electromechanical and chemical properties [10–13]. Carbon nanotubes have been proved to be an ideal reinforcement in macroscopic composites to improve electrical, mechanical, and physical properties of materials. Considerable research has been conducted toward this challenge[14–16].

Combining the outstanding characteristics of CNTs and excellent properties of textiles, this study is to develop CNT reinforced composite yarns and investigate their electrical and mechanical properties. Two different methods were applied to obtain conductive yarns, i.e. wet-spinning process and coating process. Variables affecting the performance of the conductive yarns were investigated. Five nature and synthetic substrates were used to study the effect of textile substrates on electrical and mechanical properties of CNTs reinforced conductive yarns.

doi:10.1016/j.compstruct.2005.10.016

* Corresponding author. Tel.: +852 2766 6470; fax: +852 2773 1432.

E-mail address:tctaoxm@inet.polyu.edu.hk(X.M. Tao).

www.elsevier.com/locate/compstruct

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2. Experimental 2.1. Materials

Multi-wall carbon nanotubes (MWCNTs), supplied from Shenzhen Nanotech Port Co. Ltd. with 5% of carbon impurities and catalysts, were used for wet-spinning and coating processes. The SEM micrograph of the MWNTs as-received is shown in Fig. 1. The length of MWNT is in the range of 0.5–500 lm and diameter of 40–60 nm. In order to prepare a homogeneous CNTs dispersion, CNTs were puriﬁed by a strong acid treatment procedure, then washed with water until the pH attained 7. The amount of puriﬁed CNTs, determined by the required weight ratio of CNTs to PVA powder, were dispersed in aqueous solution for further use, followed by two hours of ultrasonic vibration at the ambient temperature.

Polyvinyl alcohol (PVA) with molecular weight (M_w) of 88,000 and the degree of hydrolysis of 88%, supplied by ACROS, was used. The mixture of PVA and CNTs was prepared by dissolving the PVA in the CNTs aqueous solution. In order to obtain a high conductivity, the weight ratio of CNTs to PVA was increased up to 40%. The mixture was stirred continually at 60C under control in water evaporation until uniformity and viscosity were satisﬁed.

Seventeen percent sodium sulfate (Na2SO4) was used as the coagulating agent.

2.2. Fabrication process 2.2.1. Wet-spinning process

Wet-spinning process started with the prepared PVA/

CNTs solution at room temperature. A table-top wet spinning apparatus was assembled. A syringe pump immersed in a coagulant bath which contained 17% sodium sulfate aqueous solution, as shown in Fig. 2. The syringe pump extruded the PVA/CNTs solution through a needle with an internal diameter of 0.5 mm. The PVA/CNTs solution

precipitated as a gel initially at the extrudate-coagulant interface, but progressively throughout the extrudate. The coagulated filaments passed over a guide to a roller at windup speed of 1.2 m/min. The processing variables that affect the performance of the produced products are viscosity of the spinning solution, composition, concentration and temperature of the coagulation bath, and drawing applied during spinning. Due to many steps in wet-spinning process being involved in the formation of fibers, therefore, at current stag the study will concentrate on the as-spun fibers (i.e. fibers in the state just after coagulation). Based on the optimization of the initial stage of the wet-spinning process, a complete wet-spinning technique will be more efficient in approaching the intrinsic limits of the material.

2.2.2. Coating process

Five kinds of yarns, including nature and synthetic, were used for coating in investigating the eﬀect of textile substrates. They are: 100% cotton yarn, silk yarn, wool/nylon mixed yarn, polyester yarn, and polypropylene (PP) yarn.

Table 1shows the chemical structures of the materials used in this study, as well as the surface morphology of the fibers with a magnification of 10 k·. Various spun yarns and filament yarns were washed and dried prior to coating process.

The substrate yarns were immersed in the prepared PVA/CNTs solution, then removed the excess material and dried at ambient condition for overnight. In this way, the CNTs bound by PVA were immobilized on the surface of the fibers, so as for yarn to obtain the electrical property. The adhesive force between the coating and the fiber surface is greatly dominated by chemical bonding of both materials and the characters of the surface. In this connection, PVA is one of the best choices to be used in coating process on common fibers including natural and man-made fibers, because more functional groups in chemical structure exist for PVA, which could interact with the substrate fibers. A schematic drawing is depicted in Fig. 3.

In nature, PVA can be easily dissolved in water; therefore an acetylating treatment to obtain the isolation of the hydrophilic group would be processed. In this study, the indissolubility in water was achieved by a hardening process with interlacing the hydroxyl groups of PVA with

Fig. 1. SEM micrograph of the MWNTs as-received.

take-up roller

coagulation bath CNT/PVA conductive fiber

Fig. 2. Schematic drawing of a table-top wet-spinning apparatus.

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Table 1

Chemical structure of the materials and the surface morphology of the ﬁbers used in this study Poly vinyl alcohol (PVA)

CH₂-CH OH

n

–

Cotton ^O

CH₂

OH OH

O n OH

Polyester (PET)

C O

O CH₂CH₂ C

C O

n

Nylon [ HN(CH₂)₆NHCO(CH₂)₄CO ]_n

Silk

Polypropylene CH₂CH

CH₃ n

Wool

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aldehydes and acids under acetyl formation, especially in multi-phase formaldehyde baths under high temperature with subsequent washing, drying, etc. In the multi-phase formaldehyde baths, sulfuric acid, sodium hydroxide, 99% zinc sulfate, and 37% formaldehyde solution, from Advanced Technology and Industrial Co., Ltd., were used.

Composition of the hardening baths for PVA/CNTs- coated fibers was given in Table 2. The temperature of these baths was set at 65C.Fig. 4 illustrated the scheme of the acetylization of PVA. In order to obtain an acetylating reaction only at the surface of coated fibers, the acetylating process was carried out in seconds. Therefore, the hydroxyl groups of PVA still remain in the inside of the coated fiber. The schematic drawing of interactions of PVA/CNTs-coated fiber after acetylation is given in Fig. 5.

2.3. Characterization

The surface structure of yarns was observed by a scan- ning electron microscope (SEM) for high magniﬁcation observation. The conductivity was measured with a mul- timeter (Keithley, 2010). Tensile tests were carried out by an Instron Mechanical Testing System. A single piece of yarn was attached to a paper frame before testing. The crosshead speed was 5 mm/min and the gage length of each specimen was 20 mm. At least ﬁve specimens from each kind of yarn were tested. The average value was taken and standard deviations were calculated. All electrical and mechanical tests were carried out at 20C and 65%

RH.

3. Results and discussion 3.1. Conductivity

By wet-spinning process, the conductive PVA/CNTs composite monofilaments were obtained with the average diameter of 0.5 mm. Unfortunately, electrical resistance of the composite monofilaments was very low, about several MX/cm for 10 wt% of CNTs to PVA, and was about several tens kX/cm for 40 wt% of CNTs to PVA. More- over, the conductivity was not uniform along the filament.

This may be resulted from the non-homogenous dispersion of the CNTs and their aggregation. The further eﬀorts are on going in our group, such as increasing some chemical functional group to CNTs to improve the dispersion of CNTs and enhance the chemical bonding. Both eﬀorts are very important for high conductivity and good spinning ability.

Fig. 6gives two typical SEM photographs of the PVA/

CNTs-coated conductive yarns. Their geometrical, physical parameters and add-on are listed inTable 3and the linear electrical resistivity is presented inTable 4 for the case of 30 wt% of CNTs to PVA. It can be seen that the polyester yarn, PP yarn, and silk yarn, with smaller in diameter, lower in density, and higher in add-on which was measured by the weight of the yarn before and after coating process, have higher conductivity than the others. It seems that the diameter of ﬁbers which consist of yarns do not contribute much to the conductivity of coated yarns. In fact, the lower in density and the smaller in diameter of yarns usually lead to a higher add-on, therefore, the add-on is one of the most important factors for controlling the conductivity of yarns.

It was also found the conductivity of filament yarn base was higher than that of short fiber spun yarn base, which could be explained by their coating quality. The geometrical structures of a spun yarn and a filament yarn are quite different, as shown inFig. 7. It is clearly seen that the structure of the spun yarn is not geometrically well organized as that of the filament yarn. The well organized structure should help to obtain a uniform coating layer during coating process.

OH OH

X X

Fiber

Hydrogen bonding

Fig. 3. Interaction between PVA and substrate ﬁbers.

Table 2

Composition of the hardening baths for PVA/CNTs-coated ﬁbers (%)

Composition 1st bath 2nd bath 3rd bath

Sulfuric acid 10.0 20.0 5.0

Sodium sulfate 23.0 20.0 –

Zinc sulfate 3.0 3.0 –

Formaldehyde 3.5 3.5 16.5

Water 60.5 53.5 78.5

OH OH

X X

Fibers

O O

H₂

C Acetylated part

Non-acetylated part

Fig. 5. Schematic drawing of interactions of PVA/CNTs-coated ﬁbers after acetylation.

CH₂-CH OH

CH₂-CH + HCHO OH

CH₂-CH O CH₂-CH

O C H₂ Fig. 4. Scheme of the acetylization of PVA.

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Moreover, it was also found that the conductivity of the yarns by coating process is much higher than that of the composite monoﬁlament, obtained by wet-spinning process, under the same CNTs concentration.

3.2. Mechanical tensile test under simple tension

The degradation of mechanical property comes from the change in molecule chains. This may occur when acting with some chemicals such as acid or alkali or under some environmental factors (UV, radiation, etc.). In our study, the strong acid (sulfuric acid) was used for the acetylization treatment on PVA in order to decrease its dissolubility in water. Therefore, it is necessary to understand the eﬀects of the acetylating treatment by hardening process, as well as by the coating on the performance of the yarns. Tensile tests were carried out for three categories of samples:

untreated yarns, yarns with acetylating treatment by hardening process, and PVA/CNTs-coated yarns after hardening process.

Fig. 7. Geometrical structures of filament yarn and short fiber spun yarn. (a) PP filament yarn; (b) spun cotton yarn.

Table 3

Geometrical, physical parameters and add-on of various conductive yarns

Material Yarn form Diameter of ﬁber

(lm)

Density of yarn (mg/cm)

Diameter of yarn (lm)

Added on (%)

100% cotton Short ﬁber spun 15.10 0.62 655 72.9

Silk Filament 13.95 0.32 417 158.0

Wool/nylon Short ﬁber spun 20.20/23.26 0.84 785 56.3

Polyester Filament 18.60 0.19 374 250.0

Polypropylene Filament 21.86 0.16 308 150.0

Table 4

Conductivity of various conductive yarns with the weight ratio of 1:1 (CNTs to PVA powder)

Material Resistance (kX/cm)

100% Cotton 2.87

Silk 1.17

Wool/nylon 48.67

Polyester 0.25

PP 0.54

Fig. 6. SEM photographs of two typical coated yarns. (a) Polyester yarn; (b) cotton yarn.

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Fig. 8presents typical tensile curves for three samples: PP yarn, wool/nylon mixed yarn, and silk yarn, with and without the acetylating treatment. From the tensile responses of all the yarns studied, it is known that the acetylating treatment did not significantly affect the mechanical property for 100% cotton, polyester, and PP yarns. Especially for PP yarn, there was negligible difference in tensile property because it is very stable to acid. However, for the wool/

nylon mixed yarn, the acetylating treatment greatly reduced the material strength (almost 50%) and the ultimate strain (about 16%), because nylon can be hydrolyzed rapidly with strong acid at high temperature. Degradation scheme of nylon 66 by acid hydrolysis is given inFig. 9. For silk yarn, the treatment obviously reduced the YoungÕs modulus.

Generally, the level of degradation of polymer chain by

chemicals will vary with the applied fiber. For wool/nylon mixed yarn, the cleavage of polymer main chain of both materials will easily occur under strong acid, resulting in obvious degradation in performance. Moreover polypropylene has strong resistance to both acidic or alkali chemicals. For silk yarn, the degradation by acidic chemicals is slower than that of other natural fibers. The opening of side chain bonding can be occurred first; its stability on acidic chemical is situated between PP yarn and wool/nylon yarn.

The eﬀect of coating on mechanical property is under- stood from the load vs. displacement curves obtained from the tensile tests.Fig. 10gives the typical curves for the samples of PP yarn, wool/nylon mixed yarn, and silk yarn, with and without coating. All the samples are tested after acetylating treatment. As expected, coating materials enhanced the YoungÕs modulus, but the eﬀect was not obvious. The reduction in ultimate strain and increase in strength were

0 1 2 3 4 5 6

0 10 15 20

Displacement (mm) (a)

(b)

(c)

P (N)

untreated PP yarn treated PP yarn

0 2 4 6 8 10 12

0 10 15 20 25

Displacement (mm)

P (N)

untreated yarn treated yarn

untreated silk yarn treated silk yarn 0

2 4 6 8 10

0 2 4 6 8

P (N)

5

Fig. 8. Load vs. displacement relationship for the PP yarn, wool/nylon mixed yarn, and silk yarn under tensile loading. (a) PP yarn; (b) wool/

nylon mixed yarn; (c) silk yarn.

R' [HN(CH₂)_x NHCO(CH₂)_y-2CO]_n- R H⁺

R'HN(CH₂)_x NH₂ + HOOC(CH₂)_y-2CO - R Fig. 9. Degradation scheme of Nylon 66 by acid hydrolysis.

0 1 2 3 4 5

0 5 10 15 20

P (N)

After coating Before coating

0 1 2 3 4 5 6

0 5 10 15

P (N)

After coating Before coating

After coating Before coating 0

2 4 6 8 10

0 2 4 6 8

P (N)

(a)

(b)

(c)

Fig. 10. Load vs. displacement relationship for the coated wool/nylon mixed yarn, the coated silk yarn and coated polyester yarn under tensile loading. (a) PP yarn; (b) wool/nylon mixed yarn; (c) silk yarn.

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negligible. Except the wool/nylon mixed yarn, all the studied yarns can mostly retain the mechanical characteristics of a real textile after coating and treatment. These textile properties are very important because the conductive ﬁbers would be transformed into textile structures by weaving, knitting or other manufacturing processes for smart and intelligent textiles.

4. Conclusion

In this study, two different methods, wet-spinning and coating processes, have been adopted to obtain conductive fibers by using the carbon nanotubes. The conductivity of the obtained CNTs/PVA composite fiber by the wet-spinning process was about tens kX/cm for 40 wt% of CNTs to PVA. This performance will be improved by increasing some chemical functional group to CNTs to improve their dispersion and enhance the chemical bonding. Therefore, the following conclusions are drawn only from the conductive yarns produced by the coating technique:

• Coating technique is a simple and practical method to obtain conductive yarns. Beneﬁting from the characteristics of MWCNT, the linear resistivity of PVA/CNTÕs- coated yarn can be reduced to 250 X/cm.

• Electrical conductivity of PVA/CNTs-coated yarns obtained varied with the substrates. PVA/CNTs-coated polyester yarn, PP yarn, and silk yarn have much better electrical properties than the other conductive yarns due to low density, small diameter of the yarn and high add- on, while preserving their original strength and flexibil- ity. Moreover, the conductivity of filament yarn base was higher than that of short fiber spun yarn base.

• Among the yarns investigated, the tensile property of wool/nylon mixed yarn decreased greatly after acetylization treatment comparing with that of other ﬁbers. For PP yarn, the degradation in tensile behavior is negligible, because PP is very stable to acid.

Acknowledgements

The authors wish to acknowledge the Innovation and Technology Commission of the Hong Kong SAR Govern-

ment for the partial support under ITF project ITS/71/02 and the postdoctoral fellowship from The Hong Kong Polytechnic University.

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