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Synthetic Metals 159 (2009) 1082–1089

Contents lists available at ScienceDirect

Synthetic Metals

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ultifunctional cotton fabrics

lessio Varesanoa,∗, Annalisa Aluigia, Luca Floriob, Riccardo Fabrisb

CNR-ISMAC, Institute for Macromolecular Studies, C.so G. Pella, 16, 13900 Biella, ItalyIT IS “Q. Sella”, Via Rosselli, 2, 13900 Biella, Italy

r t i c l e i n f o

rticle history:eceived 4 March 2008eceived in revised form6 September 2008ccepted 21 January 2009

a b s t r a c t

Electrically conductive fabrics were produced by deposition of a thin film of doped polypyrrole on thesurface of cotton fibres. In situ oxidative chemical polymerisation were carried out in aqueous solutions ofpyrrole, oxidant and doping agents, at room temperature. Polypyrrole-coated fibres were characterizedby Light Microscopy, SEM, EDX, FTIR and TGA. Moreover, fabric samples were also evaluated for moistureregain, electrical resistivity, heat generation and antibacterial activity. PPy alters the combustion process

vailable online 23 February 2009

eywords:onductive textilesolypyrrolentibacterial activitylectrical conductivityeat generation

of cellulose fibres that maintain the fibrous shape after heating in air. Moreover, it seems that PPy is reallyan antibacterial agent, apart from the oxidant or dopant used. The results highlight potential applicationsas technical textiles with antistatic (low electrical resistance), heat generation, hygroscopy, antibacterialand high temperature resistance properties.

© 2009 Elsevier B.V. All rights reserved.

. Introduction

Intrinsically conducting polymers (ICPs) are �-conjugated poly-ers, such as polypyrrole (PPy) and polyaniline, that can be easily

roduced by chemical oxidative polymerisation in aqueous solu-ions of the monomers. After immersion in the polymerisation bath,

aterials are coated with an even and uniform layer of conductingolymer, and the presence of doping agents improves the electricalonductivity of the layer itself.

Electrically conductive textiles based on ICPs and processes forabricating them are known since the early 1990s [1–3] and appli-ations have been widely reported in literature [4–19]. PPy is onef the most suitable conductive polymer for deposition on textileaterials due to its excellent conductivity and relevant environ-ental stability [14,20]. The electrical properties of PPy were first

eported by Bolto et al. [21]. The first reference to an in situ chemi-al oxidative polymerisation of pyrrole is described by Bjorklundnd Lundström [22], who synthesised PPy from a dilute waterolution of ferric chloride on the surface of paper pulp. If ferric

hloride is used as the oxidising agent, Cl− ions act as dopinggents and permit the production of PPy with good conductiveroperties. Other oxidants (e.g. ammonium persulfate, ferric sul-

ate) provide SO4 anions but their effect on conductivity is one

∗ Corresponding author.E-mail address: a.varesano@bi.ismac.cnr.it (A. Varesano).

379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2009.01.036

order of magnitude lower [23]. In this case, additional doping agentshave to be dissolved in the polymerisation bath in order to improvethe electrical performances. A variety of doping agents suitablefor PPy have been proposed, especially those based on aromaticsulfonates such as p-toluene sulfonate, benzene sulfonate, naphtha-lene sulfonate, naphthalene disulfonate, anthraquinone sulfonateand anthraquinone disulfonate.

Antibacterial properties of some ICPs have recently been dis-covered [24,25]. In particular, it seems that the partially chargednitrogen atoms in the polymer backbone of PPy and polyaniline actagainst bacteria like quaternary ammonium salts.

Finally, ICPs improve the heat resistance of underlying substrates[26] and probably act as a flame retardant agent, but this propertyhas not been thoroughly studied or documented yet.

In this work, cotton fabrics were used as substrates for PPy depo-sition. Cotton is universally appreciated for its comfort, soft handle,water absorbency, strength and easy maintenance. Cotton swells ina high humidity environment, in water and in concentrated solu-tions of acids, salts and bases; it is attacked by hot dilute or coldconcentrated acid solutions. The swelling effect is usually attributedto the sorption of hydrated ions. The main chemical component ofcotton fibres is crystalline cellulose; acid hydrolysis of cellulose pro-

duces hydro-celluloses. Oxidation of cellulose can lead to two typesof so-called oxy-cellulose, depending on the environment in whichthe oxidation takes place. Moreover, cotton fibres are extremelysusceptible to any biological degradation such as mould, bacteriaand fungi.

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. Experimental

.1. Materials

The chemicals used were pyrrole, 97% (Fluka) as monomer;ron(III) chloride hexahydrate, 98% (FC, Fluka) ammonium per-ulfate, 98+% (APS, Sigma–Aldrich) and iron(III) sulfate hydrate,.a. (FS, Riedel-de Haën) as oxidants; 2,6-naphthalenedisulfoniccid, disodium salt, 97% (NDS, Sigma–Aldrich) as additional dop-ng agent. All chemicals were used as received, except for FS thatas exsiccated at 300 ◦C before use.

The fabric was a plain cotton cloth (Bleached Desized Cottonrint Cloth, Style 400) with a weight of 0.102 kg m−2, supplied byestfabrics Inc. (USA).

.2. Methods

PPy deposition was carried out by in situ chemical oxidativeolymerisation in an aqueous solution in sealed polypropyleneessels at room temperature for 4 h. The fabric was cut in squareamples of 10 cm × 10 cm (about 1 g of material) plunged in 50 mlf solution containing the oxidant and the additional doping agentwhen used). The concentrations of the oxidant were 0.066 M forC, and 0.033 M for both APS and FS; the concentration of NDSas 0.006 M. When the substrate was well wetted, the monomeras added drop-wise to the stirred bath, until the concentra-

ion 0.030 M was reached. After polymerisation, the fabrics werequeezed, rinsed in water, dried at room temperature and stored indark conditioned room at 20 ◦C and 65% RH for at least 24 h before

esting.PPy was also synthesised without the substrate at the same con-

itions reported above, in order to obtain a reference material foromparison. The powder obtained was filtered, washed with dem-neralised water, dried at room temperature for at least 24 h andransformed into pellets of 13.0 mm diameter, by pressing 400 �gf material at 900 MPa under vacuum. The pellets were stored inconditioned room at 20 ◦C and 65% RH for at least 24 h before

esting.The samples were labelled as F(xxx + yyy) for fabrics and

(xxx + yyy) for pellets, where xxx is the oxidant (i.e. FC, APS or FS)nd yyy is the additional doping agent (i.e. NDS) when used.

Light microscopic investigation was carried out using a LeicaMLP transmitted light microscope with Leica DC100 digital imag-

ng system. Scanning electron microscopic (SEM) investigation waserformed with a LEO (Leica Electron Optics) 435 VP SEM, at ancceleration voltage of 15 kV and a 20 mm working distance. Theabrics were mounted on aluminium specimen stubs with double-ided adhesive tape. In order to improve the quality of SEM images,he samples were sputter-coated with a 20 nm thick gold layer inarefied argon (20 Pa), using an Emitech K550 Sputter Coater, with aurrent of 20 mA for 180 s. The pellets were observed without met-llization. Energy dispersive X-ray (EDX) analysis were performedy an Oxford Instruments Model 7060 Link ISIS interfaced to a PCsing 4096 channels in the range of 10 keV. During EDX analysis,he SEM configuration was 400 pA probe current, 15 kV accelerationoltage and 20 mm working distance.

Fourier transform infrared (FTIR) spectra were acquired byeans of a Thermo Nicolet Nexus spectrometer, by attenuated total

eflection (ATR) technique with Smart Endurance accessory, in theange from 4000 to 550 cm−1 with 100 scansions and 4 cm−1 ofand resolution. ATR is a powerful tool for investigating evenness,

hickness and degree of coating because the infrared beam analysesnly a thin layer of the fibre surface.

The moisture regain of PPy was measured on the pellets, pro-uced as described above, by comparing their weight in wet and dryonditions. The pellets were stored in a conditioned room at 20 ◦C

als 159 (2009) 1082–1089 1083

and 65% RH for at least 24 h and then placed in a stove at 105 ◦C for4 h. This procedure has been repeated 3 times on the same sam-ples. The same method was used to measure the moisture regain ofPPy-coated fabric samples.

Electrical resistivity of the PPy powder was measured on thepressed pellets using the four-probe method, using a Lucas Labs Pro-4 Four Point Resistivity System connected to a Keithley SourceMeterInstrument Model 2611, with the applied current set to 1 × 10−4 A,at room temperature. Ten measurements were carried out for eachsample. The electrical resistivity of PPy coated fabrics was measuredaccording to EN 61340-5-1:2001, using a Vermason ConcentricRing Probe Model 222003 connected to a Keithley SourceMeterInstrument Model 2611, with the applied voltage set to 10 V. Heatgeneration was measured in a conditioned laboratory at 20 ◦C and65% RH, by means of an electrical circuit composed of a Metrelpotentiometer, a digital Multimeter Escort 170 used as a voltmeterand a Supertester 680G ammeter. Measurements were carried outon samples of 10 cm × 10 cm connected to the electrical circuitusing eight equidistant alligator clips on each side. The surfacetemperature was measured with a Raytek Raynger ST infrared ther-mometer.

Antibacterial activity was evaluated following the UNI EN ISO20645:2004 procedure using Escherichia coli (Gram negative) ATCC8739. For the lower layer, about 10 ml of nutrient medium (yeastextract agar) free from bacteria were poured into sterilized Petridishes and congealed. For the upper layer, 150 ml of agar were inoc-ulated with 1 ml of bacteria working culture (1–5 × 108 cfu/ml). Theinoculated nutrient medium was vigorously shaken to distributethe bacteria evenly, then 5 ml were poured into each Petri dish andcongealed. Circular test specimens with a diameter of 25 ± 5 mmwere cut from the PPy coated fabrics and placed in the Petri dishesensuring that there was a good contact between specimen and agarfor the whole incubation period. The Petri dishes were incubatedfor 18 h at 37 ± 1 ◦C. After the incubation period, the plates wereobserved and the specimens removed from the agar.

Thermogravimetric analysis (TGA) was performed by a MettlerToledo TG50 calorimeter equipped with a TC15 TA Controller. Thecalorimeter cell was flushed with 100 ml min−1 air. About 4 mg ofsample were used in each test using 70 ml aluminium oxide (Al2O3)crucibles. The runs were performed from 30 to 500 ◦C with heatingrate 20 ◦C min−1. The TGA data were collected and elaborated usingthe Mettler Toledo STARe System.

Mechanical tests were performed at constant elongation rate(100 mm min−1) using an Instron 5500R Dynamometer interfacedto PC software Instron Merlin Version 8.12. Three measures per sam-ples were carried out on fabrics in warp direction with 5 cm × 25 cmof size accordingly to UNI EN ISO 13934-1.

3. Results and discussion

3.1. PPy deposition

To obtain information about the influence of the oxidationagents on the final products, cotton fabrics were coated with PPy byoxidative chemical polymerisation in aqueous media, using threecommonly used oxidation agents: FC, APS and FS. With FC and FS,PPy is synthesised by the redox reaction between pyrrole and ferricions (Fe3+) in which ferric ions are reduced to ferrous ions. Using APSthe oxidative component is persulfate ion (S2O8 ) which is reducedto SO4 .

Moreover, using FC, PPy embeds chloride ions as counter-ionsthat are good doping agents, whereas the PPy produced with FSand APS embeds SO4 and needs the addition of a dopant to reachthe desired conductivity [23]. Both FC and FS give a high acidic pH tothe water solutions due to the production of ferric complexes with

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in water. The particles are more regular and smaller when APSis used as oxidant (Fig. 2(b)). Moreover, the roughness seems tobe increased by the presence of NDS in the polymerisation bath(Fig. 2(c)).

ig. 1. PPy-coated cotton fibres cross-section (F(FC) sample) by optical microscopy.n the box a typical bean-shaped cotton fibre is enlarged (2×). Arrows point to: *PPyayer (black), **cellulose (white), ***lumen (black).

H−. On the contrary, solutions of APS have a relatively low pH, dueo the hydrolysis equilibrium of ammonium ions and water.

In all cases, after the deposition processes described above, theinsed and dried fabrics were uniformly black. The weight increasef the fabrics (obtained as the difference between the weight afterhe PPy deposition and the initial weight) is greater when addi-ional dopant was added to the polymerisation bath. In particular,n F(FC) samples the 9% final weight increase is due to the presencef PPy, whereas the evaluated amount of PPy on F(APS + NDS) and(FS + NDS) was 12% and 13%, respectively. Considering that molec-lar weight of NDS is higher than that of chloride ions, the nature ofhe doping agent embedded as counter-ion in the polymer matrixlays an important role on the amount of the coating layer. More-ver, the conditions (pH, redox potential, ions and complexes) of theolymerisation solution, due to the nature of the chemicals used,ould also have an influence on the amount of PPy deposited on theabric surface, considering that the deposition process is a resultf the affinity of the forming oligomers of PPy for both the fabricurface and the solution.

.2. Fibre cross-section

The cross-section of the cotton fibre is bean-shaped. Each cot-on fibre is composed of concentric layers composed of crystallineellulose fibrils. The innermost part of the fibre (called lumen) isomposed of the cell nucleus and protoplasm.

Fig. 1 shows the cross-section of cotton fibres coated with ahin layer of PPy. The fibres have the typical bean-shaped cross-ection with fineness ranging from 12 to 20 �m. From this picture,e can state that PPy deposits on the fibre surface without a fullenetration into the fibre bulk, contrary to the case of man madeellulose-based fibres (e.g. viscose, lyocell) in which PPy also visi-ly penetrates the fibre bulk [11,12]. This different behaviour coulde explained considering that the high crystallinity and moleculareight of cellulose in cotton fibres prevent the forming oligomers of

Py from penetrating the fibre bulk, whereas man-made cellulosebres usually have more amorphous structure and lower molecular

eight. Bhat et al. [18] stated that PPy diffuses in the amorphous

egion of the cellulose structure. They carried out X-ray diffractionnalysis on pure cotton and PPy treated cotton fibres. We can notonfute this assertion, but in our opinion, the amount of the pen-trated PPy (if presents) is likely not significant comparing optical

als 159 (2009) 1082–1089

microscope cross-sections of both PPy treated man-made cellulosefibres [11,12] and PPy treated cotton fibres (Fig. 1).

3.3. PPy morphology

Fig. 2 shows the morphology of the PPy powders observed bySEM. The powder is composed of quasi-spherical particles (with adiameter of about 300 nm) bonded each other in irregular agglom-erates. The agglomerates exhibit the typical three-dimensional anddendritic structure of PPy obtained by chemical polymerisation

Fig. 2. PPy powders by SEM: (a) P(FC), (b) P(APS + NDS), and (c) P(FS + NDS) samples.

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are very similar. In the range of 4000–2000 cm−1, they show thecharacteristic tail of the electronic absorption [27] related to theelectrical conductivity of PPy [23]; whereas, the band at 1535 cm−1

is assigned to C–C and C–N stretching vibrations in the pyrrole

Fig. 3. Pristine cotton fibres.

Fig. 3 shows cotton fibres before the PPy deposition. The fibresave irregular or flatten cross-sections; the surface appears quitemooth at low magnification (no granular structures), but at highagnification a intense roughness is visible with small wrin-

les.The pictures in Fig. 4 show the surfaces of PPy-coated fibres.

he fibres show a very uniform film-like dense layer, however theoughness of the pristine cotton is still visible. The coating appearsuite smooth in the sample F(FC) as shown in Fig. 4(a); on theontrary, both the samples F(APS + NDS) and F(FS + NDS) show aough surface, see Fig. 4(b) and (c). SEM examination revealed thathe addition of NDS influenced the morphology of the chemicallyynthesised PPy coating increasing the size of PPy agglomerates ofarticles deposited on the fibre surface.

.4. Elemental analysis

The presence of extraneous elements (i.e. different from car-on and nitrogen) can be qualitatively evidenced by a simple EDXnalysis of the PPy pellets and coatings [14]. Fig. 5 shows theDX spectra; the spectra (a) refer to PPy produced by FC (samples(FC) and F(FC)). The peak of chlorine (ClK� 2.62 keV) is visible inhe spectra of both fabric and pellets. Therefore, the Cl− ions arencorporated into the polymer during polymerisation. The spectrab) and (c) refer to PPy produced with APS and FS, respectively.he presence of sulphur is proved by the peak SK� at 2.31 keVainly due to the presence of NDS embedded in the PPy dur-

ng polymerisation, although it is possible that some SO4 ionsncorporated at the same time in the polymer matrix. The spec-ra of the fabrics are complicated by the overlap among the peaksf sulphur and gold used for sample preparation (AuM� 2.12 keVnd AuM� 2.20 keV). The fabrics were indeed sputter-coated withold to achieve a high acquisition rate, whereas the pellets alreadyossess sufficiently high volume conductivity to be analysed asroduced.

The EDX spectra reported in Fig. 5 also show peaks assigned toarbon (CK� 0.27 keV), oxygen (OK� 0.52 keV) and nitrogen (NK�.39 keV). Obviously, carbon is largely present in both cellulose andPy, oxygen belongs to cellulose or sulfates (i.e. NDS, SO4 ), whereas

itrogen belongs to PPy only. It is worth noting that the iron peakFeK 6.3 keV, FeL 0.7 keV), as a residue of oxidative polymerisa-ion, was not found in the spectra (a) and (c). This is particularlymportant because iron can act as antibacterial agent, therefore thentibacterial properties observed in the following tests (Section 3.9)re due to PPy action.

als 159 (2009) 1082–1089 1085

3.5. Infrared spectroscopy

Fig. 6 shows FTIR spectra of uncoated and PPy-coated cotton fab-rics. The spectrum (a) refers to uncoated fabrics. It is characterizedby the broad bands at 3400 and 2900 assigned to O–H stretchingvibration in the presence of H-bonds, C–H stretching vibration and astrong absorption band with a maximum at 1030 cm−1, as a result ofthe overlapping bands attributed to functional groups of cellulose,namely the C–C, C–O and C–O–C stretching vibrations.

The spectra of PPy-coated fabrics (b), (c) and (d) in Fig. 6

Fig. 4. PPy-coated cotton fibres by SEM: (a) F(FC), (b) F(APS + NDS), and (c)F(FS + NDS) samples.

1086 A. Varesano et al. / Synthetic Metals 159 (2009) 1082–1089

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Table 1Moisture regain and resistance measurements.

Sample Pellets Fabrics

Moistureregain (%)

Surface resistance(�/sq.)

Moistureregain (%)

Surface resistance(�/sq.)

FC 22.5 19.0 7.3 4.25

the theoretical moisture regain of coated fabrics were calculated as7.6% for F(FC), 7.1% for F(APS + NDS) and 6.8% F(FS + NDS), that arein good agreement with the measured values.

ig. 5. EDX spectra of (a) F/P(FC), (b) F/P(APS + NDS), and (c) F/P(FS + NDS) samples.

ing and the band at 1300 cm−1 is attributed to C–H and C–N in-lane deformation modes. In the region from 1250 to 1000 cm−1,ellulose has a broad intense band that appears attenuated in thepectra of coated fabrics. Since the infrared beam of the ATR tech-ique analyses only a thin layer of the fibre surface, the reductionf the absorption band of cellulose on coated samples is a sign ofhe excellent evenness (fibres are fully covered) and high thicknessf the coating. Moreover, in that range PPy has a band at about040 cm−1 assigned to C–H and N–H in-plane deformation vibra-ions, and a band at about 965 cm−1 assigned to C–C out-of-planeing-deformation. Some different features can be observed in theand at 1155 cm−1 between spectrum (b) and spectra (c) and (d).n particular, the band appears sharper in the spectra (c) and (d).his could be attributed to the presence of the sulfate group thatt is expected to have an absorption band at about 1170 cm−1. Theeak at 1097 cm−1 is assigned to the in-plane deformation vibrationf NH+ groups which are formed in the PPy chains by protona-ion [23]. Finally, the peaks at 780 and 660 cm−1 are attributedo C–H out-of-plane ring deformation and C–H rocking vibration,espectively.

.6. Moisture regain

In order to evaluate the hygroscopic properties of PPy, pelletsere subjected to wetting and drying processes. The results are

ig. 6. FTIR spectra of (a) cotton fabric, (b) F(FC), (c) F(APS + NDS), and (d) F(FS + NDS)amples.

APS + NDS 11.3 9.8 7.2 2.17FS + NDS 7.7 4.5 7.0 2.18

reported in Table 1. It seems that PPy produced in different waysexhibits different affinity to humidity. In particular, it is supposedthat the presence of organic counter-ions (i.e. NDS) in the polymermatrix gives to PPy more hydrophobic properties; on the contrary,Cl− produces a more hygroscopic PPy. Cotton fibres have a moistureregain of 6.5% (measure performed using the procedure describedabove). The moisture regains of PPy-coated fabrics are reported inTable 1. As can be seen, the presence of PPy increases the hygro-scopic properties of the whole fabric. Considering the PPy contenton the fabrics, the moisture regain of the pure PPys and pure cotton,

Fig. 7. Current–voltage curves, plot (1). Heat generation, plot (2).

A. Varesano et al. / Synthetic Metals 159 (2009) 1082–1089 1087

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ig. 8. Contact zones of antibacterial tests. (a) F(FC), (b) F(APS + NDS), and (c)(FS + NDS) samples.

.7. Electrical resistivity

Electrical properties were measured on both pellets of PPy pow-er and PPy-coated fabrics. The surface resistances are reported inable 1. The most conductive PPy powder was produced using FSnd NDS, followed by those obtained with APS, NDS and then FC.t is worth noting that this kind of measurement is subject to errorue to differences in the thickness of pellets, ranging from 200 to00 �m, although the powder was accurately weighted before theellet production.

The measurements of the surface resistance are a reliableethod to evaluate electrical properties of conducting-coated tex-

ile materials. Moreover, the data are in perfect agreement with theeasures derived from the current–tension curves obtained during

eat generation tests (Section 3.8). The results reported in Table 1how that the F(APS + NDS) and F(FS + NDS) samples have almosthe same surface resistance, whereas sample F(FC) has a greateresistance. This means that the dopants have a higher influence onhe electrical performances with respect to the oxidants and, in par-icular, NDS works better than chloride ions. However, F(FC) havehe same order of magnitude in resistance of the samples treatedith the NDS dopant. In our experience, the use of both APS and

S without NDS increases the surface resistance of the fabrics to ateast one order of magnitude.

.8. Heat generation

Plot (1) in Fig. 7 reported the current–voltage values measureduring the heat generation tests. The relationship between thepplied voltage and the resulting electrical current is quadratic. It

s well known that the electrical resistivity of PPy decreases withncreasing the temperature, due to the mechanism of electrical con-uction in ICPs. As reported above, F(APS + NDS) and F(FS + NDS)xhibit the highest conductivity with respect to the F(FC) sample.

able 2hermogravimetric results on fabrics.

abric sample Degradation step Residual weightb (%)

Temperaturea (◦C) Weight loss (%)

otton 344 77.9 2.6(FC) 314 59.8 6.3(APS + NDS) 303 64.2 6.9(FS + NDS) 288 54.1 5.5

a Position of the inflection point from the weight loss curves.b Measured at 500 ◦C.

Fig. 9. First derivative of weight loss curves (from 200 to 400 ◦C) during thermogravi-metric analysis of (a) cotton fabric, (b) F(FC), (c) F(APS + NDS), and (d) F(FS + NDS)samples.

Heating devices (such as seats, mattress pads, blankets andclothing) are defined in terms of power demands per surface unit.Heaters that have contact with the human body (e.g. heated seat)typically require less than 100 mW in.−2 (15.5 mW cm−2) [4]. AsFig. 7 shows in the plot (2), the heating performances of the fab-rics produced in this work completely satisfy this requirement. Inparticular, they exhibit a surface temperature increase from 5 to 7 ◦Cwith a power demand of 15 mW cm−2. The temperature increaseswere evaluated as the differences between the actual temperatureand the starting temperature (at 0 V). Moreover, there is a linearrelationship between electrical power and temperature increase,and the slope of the relation is a function of the surface resistance.

3.9. Antibacterial properties

Antibacterial activity was evaluated against E. coli by puttingthe fabrics in contact with a culture of bacteria. The absence or thepresence of bacterial growth in the “contact zone” between agarand specimen and the eventual presence of an “inhibition zone”around the specimen was assessed. After the incubation period,no inhibition zone was observed: the colonies of bacteria grewaround the fabric. Removing the specimen from the agar (as themethod requires) we observed the absence of colonies under thecoated fabrics (contact zone), as Fig. 8 shows. Therefore, there isantibacterial activity on the fabric surface just by contact becausethe anti-bacteria agent (PPy) is fixed directly to the fabric, and,unlike small molecules, PPy cannot diffuse. The absence of bacterialgrowth, even without inhibition zone, may be regarded as a goodantibacterial effect.

The inhibition zone is the zone near to the sample in which thebacteria growth is limited or inhibited by small molecules or ionsthat diffuse in the medium from the sample. The absence of inhibi-tion zone confirms that no small molecules or ions (such as iron) are

Table 3Mechanical tests.

Fabric sample Tensile strength at break (N) Elongation at break (%)

Cotton 237.3 ± 7.1 5.3 ± 0.3F(FC) 278.5 ± 8.3 6.7 ± 0.1F(APS + NDS) 276.7 ± 8.7 6.9 ± 0.2F(FS + NDS) 273.2 ± 13.4 6.5 ± 0.2

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Fig. 10. PPy-coated cotton fibres after thermogravimetric analysis (up to 500 ◦C, inair) observed by SEM. (a) F(FC), (b) F(APS + NDS), and (c) F(FS + NDS) samples.

als 159 (2009) 1082–1089

present in the samples (as EDX pointed out, see Section 3.4), or theyhave no antibacterial action. Therefore the bacteria growth is inhib-ited just by the presence of positive charged PPy. A low diffusibilityis not a fault, on the contrary this gives some advantages: no dis-persion of active substances in the environment or on the skin, andno diminishing of antibacterial effect due to dilution of the activesubstance. Moreover, accordingly to FTIR analysis (Section 3.5) thatrevealed the presence of NH+ vibration modes, it seems that PPyacts against bacteria as the quaternary ammonium salts. Finally, thegood results obtained from all the samples produced with differentoxidants and doping agents, pointed out that the antibacterial prop-erty does not depend on either the nature of oxidation agent used inthe synthesis of PPy or the doping agent embedded in the polymermatrix.

3.10. Thermogravimetric analysis

The thermogravimetric behaviour of the fabrics is reported inTable 2. The results pointed out that the treatments of PPy coat-ing greatly reduce both the degradation temperature of celluloseand the weight loss associated to the degradation process. As Fig. 9shows, the burning step is broadened by the presence of PPy, there-fore the associated heat of combustion is spread in a wider rangeof temperatures. These findings point out that PPy alters the com-bustion process of cellulose, when coated with PPy.

Finally, the residual weight at 500 ◦C for all the coated samplesis more than double that of cotton. Further observations of the car-bonised samples by SEM analysis (Fig. 10) revealed the PPy coatedfabrics still maintain the initial fibrous shape and still have a goodconsistency, on the contrary of pure cotton fabric that produces avery little amount of char, impossible to handle and gather for anal-ysis. From pictures in Fig. 10, it is worth nothing that the celluloseinside the fibres is degraded by the high temperature in air, but PPythat covers the fibres has not reduced to ashes.

3.11. Mechanical tests

Results of the tensile tests are shown in Table 3. Mechanicalproperties of fabrics were significantly enhanced by the PPy treat-ment. It seems that the PPy layer significantly increased both tensilestrength at break and elongation at break of PPy coated fabrics withrespect of pure cotton fabric: strength increases of about 40 N andelongation of more than 1%. On the other hand, mechanical proper-ties of PPy coated fabrics are similar, not significant variations occuras a consequence of the polymerisation conditions.

4. Conclusions

Cotton fabrics have been coated with an even and uniform layerof PPy. Three different oxidants and an additional doping agent wereused. The samples were characterized by SEM and Light Microscopy,elemental analysis and FTIR spectroscopy. Moisture regain of thewhole was found to be increased by the presence of the PPy layer.The PPy-coated fabrics exhibited good electrical properties: lowelectrical resistivity and heat generation performances suitable forheating devices. These fabrics also showed good antibacterial effectby contact with Gram-negative bacteria. The absence of inhibitionzone is a sign that the antibacterial agent (i.e. PPy) is unable to dif-fuses in the medium and it is well linked to the fibre. Moreover,PPy produced by different oxidants and dopants showed the sameantibacterial efficiency.

Moreover, thermogravimetric analysis revealed that PPy altersthe combustion process of cellulose; in particular the burning stepwas broadened and took places at lower temperature, but thedegraded material still maintained the initial fibrous shape. Finally,PPy significantly increases tensile strength of cotton fabrics (of

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cknowledgements

The authors thank the Prof. Claudio Tonin for the help, andratefully acknowledge the Regione Piemonte for supporting thisesearch within the HI-TEX project (D.G.R. n. 227-4715).

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