Improvement of adhesion of conductive polypyrrole coating on wooland polyester fabrics using atmospheric plasma treatment
Saurabh Garg a, Chris Hurren b, Akif Kaynak c,∗a Department of Applied Chemistry, Delhi College of Engineering, University of Delhi, New Delhi, India
b Centre for Material and Fiber Innovation, Deakin University, Geelong 3217, Australiac School of Engineering and Information Technology, Deakin University, Geelong 3217, Australia
Received 25 October 2006; accepted 12 December 2006Available online 23 January 2007
bstract
In this paper wool and polyester fabrics were pretreated with atmospheric plasma glow discharge (APGD) to improve the ability of the substrateo bond with anthraquinone-2-sulfonic acid doped conducting polypyrrole coating. A range of APGD gas mixtures and treatment times werenvestigated. APGD treated fabrics were tested for surface contact angle, wettability and surface energy change. Effect of the plasma treatment on
he binding strength was analyzed by studying abrasion resistance, surface resistivity and reflectance. Investigations showed that treated fabricsxhibited better hydrophilicity and increased surface energy. Surface treatment by an APGD gas mixture of 95% helium/5% nitrogen yielded theest results with respect to coating uniformity, abrasion resistance and conductivity.
Conducting polymer-coated textiles can be made to possesscombination of desirable properties, such as high conduc-
ivity, flexibility, strength and structural variations. Reversibleesponse of these materials to external stimuli, such as stress,emperature and exposure to electromagnetic radiation suggestotential applications in the fields of sensors [1,2], actua-ors, electromagnetic shields and absorbers [3,4], and heating5]. In addition, recently developed techniques of printingonductive patterns of soluble conductive alkyl polypyrrolesPPy) on textiles enable production of electronic textiles6,7].
The key impediments for commercial application of conduc-ive polymer coated textiles have been degradation and poordhesion of the coating to the fiber surface. Increasing the surface
nergy of the fiber could increase the polymer–fiber interactionnd improve the binding strength. One way of modifying the sur-ace energy of a textile is by treatment with atmospheric plasma,
hich is favorable over other methods for several reasons. Itchieves surface modification whilst maintaining the bulk prop-rties (penetration is only to a depth of ≈1000 A [8]). It canroduce different radicals and reactive groups on the surfacey the use of different gas mixtures. The low treatment tem-erature avoids deterioration of delicate organic samples. Therocess is water free and avoids the need for costly effluent treat-ent. With an increasing ecological and economical concern,
lasma treatment provide an environmental friendly and lownergy alternative to improved surface wettability [9,10], shrinkesistance [11,12] and dyeing properties [10,13] of textiles.
Atmospheric plasma treatment generates different plasmaonstituents like electrons, ions, free radicals, meta-stables andV photons. These either directly or indirectly participate inlasma-chemical reactions which introduce reactive groups andree radicals onto the surface, thus improving the adhesion ofhemicals and polymers mostly by improved physical inter-ction. These interactions are normally seen in the form ofydrogen bonds, Van der Waals forces or dipolar interactions
14].
There are three types of atmospheric pressure plasmas:orona discharge, glow discharge and dielectric barrier atmo-pheric pressure glow discharge (APGD). Of these APGD is
Mrics was cut to produce a 38 mm diameter sample. The sample
2 S. Garg et al. / Synthet
referred as it generates uniform and stable discharge with thebility to be made into a continuous process [15]. Experimentalarameters including treatment time, discharge power, treatmentas and distance between electrodes can control the effectivenessf atmospheric plasma treatment. Mainly, oxygen [16], nitrogen16,17], air [17], water vapor [12], or mixtures of nitrogen andydrogen [16] have been used in wool plasma treatment. Thereave been many investigations on atmospheric and low temper-ture plasma treatment on wool fabrics and their dyeing ability13], but to the best of our knowledge no work has been pub-ished on polymerization of PPy on APGD treated wool andolyester (PET) fabric.
The main focus of this work is to examine the effect of gasixtures in the APGD treatment on the binding strength ofPy coatings on PET and wool fabrics. Properties such as sur-ace resistivity, abrasion resistance and change in colour wereeasured for the coated fabrics.
. Experimental
.1. Materials
Two fabrics were used for the experimental work. One was acoured, unbleached and undyed 100% wool with a 2/1 brokenwill weave. The wool fabric was 228 g/m2 and had 38 ends/cmn the warp and 32 picks/cm in the weft direction. The secondabric was an undyed plain weave 100% polyester (PET) fab-ic that had been scoured and bleached. The PET fabric was12 g/m2 and had 48 ends/cm in the warp and 46 picks/cm in theeft direction. Each sample was cut to the dimension of 400 mm
quare for plasma treatment. Fabric specimens were conditionedn ambient laboratory conditions (65% RH and 21 ◦C) beforereatment.
.2. Atmospheric plasma treatment
APGD treatment was conducted on a Sigma technologiesnternational APC 2000 atmospheric glow plasma machine.hree different gas mixtures were used. These were: (i) helium
He), 14 lstp/min; (ii) helium, 14 lstp/min and acetylene (A5%),.7 lstp/min (5%); (iii) helium, 14 lstp/min and nitrogen (N5%),.7 lstp/min (5%). The fabric was attached to the treatmentoller and rotated past the plasma source. Treatment was doneor 5, 25 and 50 revolutions of the treatment roller. On eachevolution of the treatment roller the fabric was exposed tohe plasma source for 0.848 s. The power of the treatmentlasma was 970 W and the frequency of the power supply was0 kHz.
.3. Contact angle and wettability
Contact angle and wettability measurements were conductedsing KSV CAM100 contact angle and surface tension tester.
he droplet used was distilled water and was 180 pixels inize. Five specimens of wool and PET for each of the APGDreatments were tested. For each measurement a 15 mm squareample was cut from the treated fabric and mounted onto a
wpfm
tals 157 (2007) 41–47
lass slide using double-sided tape. Specimens were cut fromandom areas of the treated fabric. Wettability time was alsoalculated simultaneously using the KSV CAM100 software.he interval of the frames captured by camera was varied from3 ms to 1 s depending upon the rate of absorption. For treatedET fabrics frame interval was set at 33 ms due to the fastate of absorption. For treated wool fabrics the frame intervalas set at 1 s. All the readings were taken within 2 h of plasma
reatment.
.4. Conductive coating
Polymerization was carried out in an aqueous solu-ion that contained 0.045 mol/l pyrrole (Sigma–Aldrich),.018 mol/l anthraquinone-2-sulfonic acid sodium salt (AQSA)onohydrate 97% (Sigma–Aldrich), 0.1 mol/l ferric chloride
exahydrate and 0.25 g/l Albegal FFA (Ciba Specialty Chem-cals) [18,19], resulting in a black PPy coating on the fabricurface. Each coating was carried out on two 3.0 g fabric sam-les at a liquor ratio of 50:1. All stock solutions were made upo 10% strength of the original chemical strength apart fromhe pyrrole (5%) and AQSA (1%). The monomer, dopant andxidant were stored at 2 ◦C after being diluted. The fabric wasoated in a 450 ml stainless steel dye pots. Coating was carriedut for 2 h at 10–15 ◦C in a Rapid H240 rotary dyeing machine.insing was conducted using cold tap water while the sampleas opened out.
.5. Surface resistivity
Resistivity measurements were carried out on all conduct-ng polymer coated fabrics before and after abrasion testing.esistance was measured 10 times on each side of each sam-le and averaged. Two square copper electrodes measuringmm × 6 mm, separated by 8 mm, were pressed onto fabric by.856 N of force. The resistance was measured with a Fluke 83II Multimeter. Each measurement was multiplied by a factorf 1.33 to convert the measured resistance to surface resistivitysheet resistance, Rs). The formula for calculating the surfaceesistivity is given in the following equation:
s = R
(L
W
)(1)
here Rs is the sheet resistance in ohms/square, R the resistancen ohms, L the distance between electrodes and W is the widthf the each electrode.
.6. Abrasion resistance
Abrasion resistance measurements were performed using aartindale abrasion tester. Each of the conductively coated fab-
as placed in the top holder of the Martindale under 9 kPa ofressure. Each sample was abraded against SDC wool abradantabric for 200 cycles. After abrasion each of the fabrics waseasured for resistivity and UV/vis reflectance.
ic Metals 157 (2007) 41–47 43
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S. Garg et al. / Synthet
.7. Reflectance
UV/vis reflectance was measured for each of the abradedamples using a Datacolor Spectraflash 600. Each reflectanceeasurement was made at a 10◦ angle from the horizontal.he percentage reflectance for the wavelength of 430 nm was
ecorded for each sample.
.8. Microscopy
Scanning electron microscope (SEM) observations wereade with a Leica S440W scanning electron microscope.
mages were taken at a magnification of 5000 times at a workingistance 10 mm and an EHT of 10 kV. All samples were mountednd gold sputtered under vacuum prior to observation.
PPy coated yarn was mounted in Technovit 7100 resinKulzer) before being sectioned into 10 �m slivers using a Slee062 microtome. Optical microscopy was conducted using anlympus BX51 optical microscope at a magnification of 1000
imes using differential interference contrast (DIC).
. Results and discussion
.1. Contact angle and wettability
The results of the surface contact angle tests for the wool fab-
ics after plasma treatment are given in Table 1. Contact anglesnd wettability times are presented with the initial droplet imageaptured as part of the test. The images in Table 1 can be com-ared with the image in Fig. 1 for a visual appreciation of the
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able 1ettability and surface contact angle results for plasma treated wool fabric
arameter Helium
passes
ettability (s) 14
5 passes
ettability (s) 1.18
0 passes
ettability (s) 0.21
Fig. 1. Water droplet profile on untreated wool fabric.
hange in surface contact angle with plasma treatment. Plasmareatment reduces the contact angle for all of the treatment gasessed over all of the treatment times when compared with thentreated sample. The rate of surface contact angle reductionnd hence the improvement in wetting time is proportional tohe number of treatment passes. Some of the treatment gassesave a better influence on the contact angle than others. A mix-ure of helium and acetylene is far less effective in improvingabric wettability than helium alone.
A mixture of helium and nitrogen is by far the best combi-
ation to use for improving wettability on wool fabrics. Variousow-pressure plasma treatments have shown that the use of nitro-en gas in the plasma treatment gas produces (–NH2) groups on
Helium + A5% Helium + N5%
300 16
7.0 0.495
1.4 0.099
44 S. Garg et al. / Synthetic Metals 157 (2007) 41–47
to pla
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ncawucnaof water from the coating liquor. The coating is subsequentlydeposited on a highly expanded fiber. During drying the fiber
Fig. 2. Drop placement and adsorption on
he surface of the fiber [16,20]. These groups form on the outer-ost surface of the fiber (F-layer), which without the addition of
he (–NH2) groups is very hydrophobic in nature [20]. The addi-ion of (–NH2) groups on the F-layer improves the hydrophilicityf the fiber surface and provides bonding sites for subsequentolymer coatings.
The wettability results for the PET fabric were similar to thosef the wool fabric. Each of the plasma gases had the effect ofeducing surface contact angle, however, the degree of improve-ent was indistinguishable between the gases. After 5 passes of
lasma treatment, the contact angle for the PET fabrics reducedrom 98◦ to 0. This was the same for all of the treatment gasombinations. Fig. 2 shows the first five images of the surfaceontact angle test for the 5 pass helium treated PET fabric. Theime interval of each of the frames is 33 ms. The images showhat as the droplet is placed onto the fabric it is already startingo be adsorbed before the needle is detached.
.2. Surface energy
Surface energy can be calculated by using the Young–Duprequation given in the following equation:
= γ(1 + cos θ) (2)
here γ is the surface tension of water at 20 ◦C and θ is theontact angle in degrees.
Fig. 3 shows the calculated surface energy values for the woolabric for each gas at 5 and 25 passes. An increase of over seven
imes is seen for the wool fabric with 25 passes of plasma treat-
ent. A similar increase was observed for the PET fabric, theurface energy of which changed from 62.69 to 145.2 mJ with thereatment of the fabric. The pure helium and helium/nitrogen gas
ig. 3. Surface energy of wool fabric samples for each of the treatment gasombinations.
ud
sma treated PET fabric at 33 ms intervals.
ombinations showed similar surface energies after 25 passes.his correlates with the wettability information.
.3. Microscopy
All previous work investigating plasma treatment of woolber [13,21] showed that the exocuticle layer of the fiber wasignificantly modified or partially removed by oxidation duringlasma treatment. This break down of the exocuticle enablesn easier passage of dye and chemical molecules into a woolber. Electron microscopy of the wool fiber treated in this workas shown that there has not been a significant modification ofhe exocuticle of the wool fiber (Fig. 4). The lack of surface
odification of the exocuticle layer defends the proposition thathe improved adhesion of the polymer layer could be caused byhe increase of amino groups (–NH2), other reactive groups andadicals on the fiber surface.
Optical microscopy of the sectioned wool fibers showed sig-ificant differences between the untreated and plasma treatedoated fiber. The plasma treated coated fiber (Fig. 5a) showsthin layer of conductive polymer bonded to the exocuticlehich is not present in the untreated fiber (Fig. 5b). Both thentreated and treated fibers show a significant amount of PPyoating that is not attached to the fiber. The PPy coating isot attached to the fiber because as the fiber is prepared forqueous coating it undergoes expansion due to the absorption
ndergoes significant contraction and this causes the coating toelaminate from the fiber surface for a significant part of the
Fig. 4. SEM image of a helium plasma treated wool fiber (50 passes).
S. Garg et al. / Synthetic Metals 157 (2007) 41–47 45
ated p
sPfiatoo
iewPtidph
3
ie
F
aear
mcot(cadtr
suc
Fig. 5. Cross sectional microscopy of the PPy co
urface. Plasma treatment does not stop the delamination of thePy coating entirely however, the mechanism is changed from aber/PPy delamination to a PPy/PPy delamination. This leavesthin layer of coating still attached to the whole surface area of
he fiber after contraction which provides an explanation for thebserved improvement in conductivity and abrasion resistancef plasma treated wool fibers.
PET fibers do not show the same delamination of the coat-ng from the fiber surface that is seen in the wool. This can bexplained by the level of hygral expansion of the PET fibersithin the coating parameters. The cross sectional area of theET does not significantly change when added to water at low
emperatures so drying after coating does not cause a delam-nation of the coating from the fiber surface. There was littleifference in the coating appearance of the PET with or withoutlasma treatment and due to this only the plasma treated imageas been shown in Fig. 6.
.4. Surface resistivity
One of the problems with coating wool with conductive PPys the unevenness of the coating. The epicuticle, which surroundsach cuticle cell of the wool fiber, consists of an outermost fatty
ig. 6. Cross sectional microscopy of the PPy coated plasma treated PET fibers.
sctAshp
F
lasma treated (a) and untreated (b) wool fibers.
cid monolayer and a protein matrix. Fatty acid chains are ori-nted away from the fiber to produce a “polyethylene-like” layert the fiber surface, thus making the epicuticle hydrophobic andesistant to the bonding of polymer coatings [22,23].
The wool fabrics that had undergone plasma treatment had aore uniform coating on their surface. The plasma treated PPy
oated fabrics were deeper black in color. Optical microscopyf a sectioned group of coated/plasma treated fibers showed thathis was due to a thin even coating of the whole fiber surfaceFig. 6). This improvement in coating coverage led to improvedonductivity, which is shown in Fig. 7. Surface resistivity ofll samples subjected to plasma treatment decreased and theecrease was proportional to the number of passes of plasmareatment. The treatment gas mixture of helium and nitrogenesulted in the lowest resistivity.
Abrasion resistance and coating uniformity are correlatedince the improvement of abrasion resistance manifests as moreniform coating. The coating system used was a dynamic pro-ess where the fabric sample was tumbled through the coatingolution to facilitate PPy to surface interaction. During this pro-ess poorly fixed polymer can be abraded from the surface ofhe fabric as it contacts with the surface of the coating vessel.
fter the dynamic coating process, untreated wool fabrics had
mall patches of white on their surface where the PPy coatingad been abraded away. Such uneven patches were not visible inlasma treated fabrics due to improved abrasion resistance of the
ig. 7. Surface resistivity changes in wool fabric due to plasma treatment.
46 S. Garg et al. / Synthetic Metals 157 (2007) 41–47
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mwwafopaghoa
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ig. 8. Surface resistivity of the wool samples after 200 cycles of abrasion.
oating. The improved fastness of the coating may be attributedo formation of bonding sites on the epicuticle surface by thelasma treatment.
In contrast to wool fabrics, PPy coated PET fabrics showednly a slight improvement in appearance after plasma treat-ent. There was also only a negligible improvement in surface
esistivity with the values ranging between 140 and 160 �/�or all the samples. The surface energy of the untreated PETabric was higher than that of the untreated wool fabric. Theigher untreated surface energy of the PET fabric produced aighly uniform coating. Plasma treatment did not significantlymprove coating uniformity and hence the difference in resis-ivity between untreated and plasma treated PPy coated PETabrics was not significant.
.5. Martindale abrasion
Martindale abrasion testing has confirmed that plasma treat-ent has resulted in improved binding of PPy coating to both theool and PET fabrics. Fig. 8 shows the surface resistivity for theool fabric samples after 200 cycles of abrasion on a Martindale
brasion tester. Without plasma treatment the surface resistivityor the wool fabric increased to 6000 �/� and a large amountf the coating had been abraded from the fabric surface. Withlasma treatment the surface resistivity of the wool fabric afterbrasion was as low as 200 �/�. Each of the plasma treatment
ases had a different effect on the resistance to abrasion. Theelium and nitrogen mixture was far more effective than eitherf the other gas combinations. Surface energy results were ingreement with these observations.
stf
ig. 10. PET samples after 200 cycles of abrasion (a) untreated, (b) helium 50 passes
ig. 9. Surface resistivity of the PET samples after 200 cycles of abrasion.
The trends seen in the wool fabric samples were also seen inhe PET samples (Fig. 9). The difference between the wool andhe PET was that the untreated PET fabric had a better surfaceesistivity before abrasion (1800 �/�). This can be attributed tohe higher surface energy of the untreated PET fabric and theack of delamination of the coating. As seen with the wool fabriche helium and nitrogen treatment gas mixture produced the bestoating adhesion for the PET fabric.
Figs. 8 and 9 show that for the helium/5% nitrogen plasmareatment gas a shorter plasma treatment time (5 passes) canroduce good resistivity values after abrasion, which is compa-able to higher treatment times (25 and 50 passes) of the sameas mixture. This is a significant economic advantage as only ahort plasma treatment time is sufficient to provide substantialmprovement in the surface resistivity after abrasion.
The samples in Figs. 10 and 11 show visually the effectf abrasion on the samples. The samples that had no plasmareatment had a large amount of the coating abraded from theirurface. The helium/nitrogen treatment gas has given the bestbrasion resistance for both the wool and PET fabrics. The visualesults reinforce the resistivity results as a reduction in the con-uctive attached to the fiber is proportional to a reduction inesistivity.
.6. Reflectance
Reflectance measurements of the fabric before and after abra-ion testing showed a similar trend to the resistivity results. Ashe black coating was abraded from the surface of the sample theabric reflectance increased. As the reflectance measurements
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