Surface functionalization of wool using 172 nm UV excimer lamp

Surface Functionalization of Wool Using 172 nm UVExcimer Lamp

Deepti Gupta, Shantanu Basak

Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas 110016, New Delhi, India

Received 9 June 2009; accepted 17 January 2010DOI 10.1002/app.32245Published online 12 May 2010 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Wool is an important natural fiber having acomplex surface structure which makes it difficult to dyeand process it. It has a hydrophobic surface due to thepresence of disulphide cystine crosslinks and a layer offatty acids on the fiber surface. Conventional chemicaltreatments used to improve the hydrophilicity of wool of-ten have an adverse effect on the feel and handle of thisfiber. In this study, effect of monochromatic UV excimerradiation of 172 nm on the performance properties such aswetting, surface chemistry and surface morphology ofwool has been studied. Irradiation reduces the wettingtime of wool nearly by a factor of 10, even after a short ex-posure time of 1 minute, irrespective of the atmosphereused. Wetting time continues to decrease with increasingtime of exposure and at 15 min of exposure, the absorp-tion becomes instant. However, no significant change wasobserved in the ATR spectra of treated and untreated sam-ples. SEM images show ablation and etching of the sur-

face. The well defined scaly contour is not seen and thereare deep striations on the surface of samples treated innitrogen atmosphere. In case of samples exposed in O2

atmosphere, micropores can be seen on the surface. Sam-ples treated in nitrogen atmosphere for 15 min shownearly 100% exhaustion of acid dye as compared to 73%for untreated samples after 20 min of dyeing. Treatedwool shows increased saturation dye uptake as well asimproved rate of dyeing. Since these changes are restrictedto surface and do not affect the bulk properties, these find-ings can have a significant effect on the commercial dye-ing and finishing procedures used for wool and can beused to design cleaner and more efficient processes infuture. VC 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117:3448–3453, 2010

Key words: dyes; surface modification; proteins; textile;UV excimer

INTRODUCTION

Wool is an important natural textile fiber but has acomplex surface structure which makes it difficult todye and process it. It has a hydrophobic surface dueto the presence of a large number of disulphide cys-tine crosslinks in the exocuticle and a layer of fattyacids on the fiber surface. Together, these act as a bar-rier against penetration of dyes and chemicals into thefiber. Several chemical treatments such as chlorina-tion have been developed to enhance the hydrophilic-ity of wool surface, but these treatments often useharsh chemicals which affect the fiber propertiesadversely and also are polluting in nature.

In the last few years, several studies have beenundertaken on use of low temperature plasma (LTP)as a physical method for modifying the surfaceproperties of wool. Treatment with O2 plasmaincreases the surface free energy and enhances thewetting and wicking behavior of wool.1 It has beenshown2 that O2 plasma leads to enhanced saturation

dye uptake as well as a faster rate of dye uptake bytreated wool when dyed with acid, reactive andchrome dyes. These effects are attributed to thechange in the morphology of wool surface due tocreation of microcraters or striations as well as par-tial modification of the scale structure on irradiation.Chemistry of the fiber surface also gets modifieddue to development of groups such as AC¼¼O,CAO, and ACOOH and NH2. An increase in thenitrogen content and a decrease in the carbon andsulphur content on the surface is observed in theXPS analysis.3 Presence of other polar groups suchas cysteic acid and sulphonic acid, created due tothe cleavage of cystine linkages has also beenestablished.4

The encouraging lab results obtained with plasmatreatment are difficult to replicate at industrial scaledue to the limitations of plasmas with respect toconsistency and uniformity of treatment in continu-ous processing. A more effective alternative can bethe UV excimer lamps which are very promising forsurface modification of textiles for large area indus-trial applications.UV irradiation has recently been used to modify

the surface properties of silk fiber to increase thehydrophilicity of silk5,6 and creation of interesting

Correspondence to: D. Gupta ([email protected]).

Journal ofAppliedPolymerScience,Vol. 117, 3448–3453 (2010)VC 2010 Wiley Periodicals, Inc.

multifunctional effects.7 Hydrophilicity of polyestercan be enhanced significantly and it can be dyedwith basic dyes due to creation of anionic groups-COOH on the surface after UV irradiation.8 Surfaceproperties of wool fibers modified using wet chemi-cal chlorination, laser, O2 plasma and UV ozonehave been compared.9 Maximum values for surfaceoxygen were observed for UV treatment and 100%of surface sulphur was oxidised to SVI state whichwas much better than all other methods used. How-ever, there is scant literature available on the effectof UV excimer radiation on wool fiber.

In the current study effect of 172 nm UV excimerradiation on the properties of wool fiber have beenreported. The effect of time and atmosphere (O2, Airand N2) of irradiation on the physico–chemical prop-erties of wool has been studied. Data is also pre-sented for wetting and dyeing properties of wool.

EXPERIMENTAL

Materials

Scoured woven wool fabric having plain weave(GSM-96, EPI-73, and PPI-66) was used for the study.It was scoured using 0.5 g/L Lisapol N (nonionic sur-factant) at 80�C for 30 min with MLR of 1 : 40 andrinsed with hot water followed by cold water. It wasthen air dried and stored in a desiccator over P2O5.

Excimer UV radiation chamber

Xenon Excimer UV lamp (Xeradex 20W/L40/120/SBSX46/KF50) and the high voltage power supply(DBD 110V/230V 50 Hz/60 Hz) for the lamp wereprocured from Messrs Radium Lampenwerk Wip-perfurth, Germany. The lamp emits almost mono-chromatic light in VUV region (k ¼ 172 nm) withthe irradiation power of 50 mW/cm2. The lamp wasmounted in a chamber as shown in Figure 1.

Irradiation of the sample

The scoured wool fabric samples of size (10 cm � 2cm) were irradiated for 1, 5, 10, and 15 min, under

atmospheric pressure in Air, O2, and N2 atmosphereat a distance of 5 mm from the lamp.

Characterization studies

Treated wool samples were tested for change in per-formance properties such as wetting and dyeing behav-ior and for change in surface topography brought aboutby UV exposure. All samples were conditioned for 24 hin standard atmosphere before testing.Wetting time was measured as the time taken for

the absorption of a water droplet by the fabric. Fabricwas held horizontally at its two ends, with clamps,keeping it free of folds. A drop of water was releasedonto it from a syringe from a height of 40 mm. Timetaken for the complete absorption of drop by the fab-ric was recorded using a stop watch. This test wasrepeated on five different parts of the fabric and theaverage wetting time was calculated. Schematic forthe procedure used is shown in Figure 2.Changes in surface morphology of wool fiber sur-

face were studied with the help of a high resolution(up to 3 nm) scanning electron microscope (ZEISSEVO50) using SE detector, 20 kv power and goldcoating of 100 Angstroms. ATR analysis was carriedout to study the change in molecular composition ofwool on exposure to radiation. Perkin–Elmer 16PCFTIR spectrometer in ATR reflection mode with azinc selenide crystal was used for the purpose.

Studies on dyeability of wool

Dyeing of wool with acid dye

Wool samples were dyed with Acid dye Navimillyellow 56N at 2% shade (on weight of fiber) at 60�C

Figure 1 Schematic diagram of irradiation chamber(a) Stainless steel chamber (b) wool fabric.

Figure 2 Schematic of the setup used for measuring the wet-ting time of the fabric. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

SURFACE FUNCTIONALIZATION OF WOOL 3449

Journal of Applied Polymer Science DOI 10.1002/app

using material to liquor ratio of 1 : 40 for varyinglengths of time in a shaking water bath. pH of thebath was maintained at 5 using 0.5 g/L of aceticacid.

Exhaustion of dye bath

Dye bath exhaustion was measured by sampling theconcentration of dye bath before and after dyeingfor 20, 30, 60, 90, and 120 min. Absorbance of thedye solution was measured using UV-Vis spectro-photometer (Perkin–Elmer, Lambda 25 ) at the 425nm (kmax). Extent of dye bath exhaustion (% E) wascalculated using the following equation:

% E ¼ A0 � At

A0� 100 (1)

where A0 is the absorbance of dye solution beforedyeing and At is the absorbance of dye solution afterdyeing for t minutes.

Shade depth of dyed fabric

After dyeing, the shade depth in terms of K/S valuesof dyed wool samples was measured with GretagMacbeth computer color matching instrument usingD65 light source and 10� observer. K/S are valuescalculated from reflectance data according to theKubelka–Munk equation given below:

K=S ¼ ð1� RÞ2=2R (2)

where K is the absorption coefficient, S is the scatter-ing coefficient, and R is the reflectance of the dyedfabric at the wavelength of maximum absorption.

RESULTS AND DISCUSSION

Wool fabrics were exposed to 172 nm UV excimerlamp in air, O2 and N2 atmosphere. Effect of irradia-tion on the performance properties such as wetting,dyeability, surface chemistry, and surface morphol-ogy of wool has been studied. Results of the studiesare reported later.

Wettability of wool

Wettability of a surface can be defined as the timetaken for a drop of water to be completely absorbedby a fabric and it depends on the hydrophilicity ofthe surface being wetted. Wool as a fiber is charac-terized by a hydrophilic core protected by an outersurface rendered hydrophobic by the presence of athin (0.9 nm) layer of fatty acids on its epicuticularsurface.9 This layer does not get removed even dur-ing the scouring of wool and the fiber continues tobe hydrophobic on the surface thus making it diffi-cult to dye and process it using aqueous industrialprocesses.Table I shows the results of wettability of wool

when exposed to 172 nm UV radiation in air, oxygenor nitrogen atmosphere. Irradiation reduces the wet-ting time of wool nearly by a factor of 10, even afteran exposure of 1 minute, irrespective of the atmos-phere used. The effect of UV exposure is immediate.Wetting time continues to decrease with increasingtime of exposure. At 15 min of exposure, the absorp-tion becomes instant, thus indicating that fiber sur-face is highly hydrophilic.Several physical and chemical reactions are

responsible for the observed hydrophilicity of woolafter UV exposure. The first phenomenon is com-monly known as the ozone–oxygen cycle.9 Oxygenfrom the atmosphere absorbs high-energy photonsfrom the excimer to form highly reactive excitedoxygen O (1D) either directly or first forming O3

(ozone) and then dissociating into O (1D) and O2.The presence of nitrogen in air plays an importantrole in the production of such reactive excited oxy-gen species.10,11 The excited oxygen O (1D) in turnoxidizes the fatty layer and reacts with the fiber sur-face to form oxygen containing polar groups such ashydroxyl (AOH) and carbonyl (AC¼¼O).12

However, by far the most important oxidationreaction is the oxidation of disulphide protein link-ages (ASASA) to give SVI in the form of anionic sul-phonic acid groups (ASO3H)9,13 accompanied by in-termediate products like cystine monoxide andcystine dioxide.13 Amount of SVI on wool surfacehas been shown to have a linear relationship with itswetting behavior.3,9 In Table I also samples treatedin O2 atmosphere show the least wetting time upto10 min of exposure. Creation of these reactive sitesmakes the fiber hydrophilic and more receptive todyes and chemicals particularly those carrying reac-tive nucleophilic groups.14 Surface functionalizationby creation of oxygen rich groups coupled with thedestruction of the fatty barrier layer of wool makesthe fiber surface hydrophilic on exposure to UVradiation.In N2 atmosphere, the increase in hydrophilicity is

due to the severe etching of the surface and also the

TABLE IEffect of atmosphere of irradiation on wettability of wool

Time of irradiation(min)

Wetting time (s)

Air O2 N2

0 280 – –1 22.2 16.1 295 1.48 1.25 5

10 1.00 0.79 0.8915 0.46 0.48 0.48

3450 GUPTA AND BASAK


creation of NH2 groups.15 This effect has been dis-cussed later with respect to the dyeing studies.

ATR analysis

Treated and untreated wool samples were subjectedto ATR analysis with a view to confirm the presenceof functional groups mentioned above. ATR investi-gations provide information about the chemical na-ture of the outer 10�6 m of modified fiber surfaces.However, no significant change is observed in thespectra of samples treated with UV, Figure 3. Thestrongest peaks in all spectra occur at around 1627

cm�1 and 1511 cm�1 and are attributable to primaryand secondary amide (ACONH). Absorbancearound 1000–1450 cm�1 indicates sulphur-oxygenband of sulphonic acid.9,15

Physical changes in the surface of wool

SEM micrographs of the untreated and UV treatedwool fabric in air, O2 and N2 atmosphere are shownin Figure 4. The sharp, well defined contour ofscales seen in the untreated wool sample (A) seemto have smoothened out significantly on irradiationin all atmospheres and the surface of treated fibersappear to be less waxy. Surface topography of sam-ples treated in nitrogen atmosphere (D) appears tobe significantly different from the other samples.The typical scale geometry of wool is not visible andthere are deep striations on the surface. The fibersurface appears to have been etched and roughenedby irradiation. Reason for this typical appearance isunknown at this stage. Micropores can be seen onthe sample exposed in O2 atmosphere. The samplesthus seem to have undergone ablation and etchingdue to UV exposure but this change in surface mor-phology has only a minimal effect on the wettingbehavior. Increase in surface hydrophilic groupsalong with surface roughening may significantly

Figure 4 SEM micrographs (x3000) of wool fibre exposed to UV Excimer lamp for 15 min. (A) Control (B) Air (C) O2

and (D) N2 atmosphere.

Figure 3 ATR spectra of UV treated wool in differentatmospheres (A) Control (B) O2 (C) N2 and (D) Air.



contribute to the adhesive forces between the fibersurface and water molecules leading to improveddyeability. Wool fibers thus prepared can beexpected to show better dyeability as well as betteradhesion for coatings and finishes.

Dyeability of wool

As wettability of wool is seen to increase appreci-ably on irradiation, it is expected that its dyeingbehavior would also be modified. Anionic acid dyeNavilan yellow 56N was used to dye the wool sam-ples irradiated for different time periods in air, O2

and N2 atmosphere. Exhaustion of dye bath and thecolor value of the dyed fabric were used to evaluatethe dyeability of fibers. Results for rate of dyeuptake at 60�C for untreated sample and samplestreated with UV for 15 min in different atmospheresare shown in Figure 5.

All treated samples show higher uptake of dye atthe end of dye cycle as compared to untreated one.The rate of dye uptake is also significantly higherfor all treated samples as seen by the amount of dyetaken up after various intervals of dyeing. Nearly100% dye is exhausted within 20 min of dyeing forsample treated in N2 atmosphere as compared to73% for untreated wool. Samples treated in air andO2 atmosphere show an identical trend. However,samples treated in nitrogen exhibit much higherdyeability. At exhaustion too, 100% of the dye is ex-hausted in nitrogen treated sample as compared to84 and 93% for untreated sample and samplestreated in air and oxygen atmosphere respectively.

Change in color strength value (K/S) of dyedfabric as a function of exposure time is shown inTable II. There is a direct relation between the lengthof exposure to UV and the amount as well as therate at which the color depth increases on fabric.Maximum depth is observed for samples treated inN2 atmosphere.

Improved dye uptake by all treated samples canbe attributed to the destruction of the lipid barrier

layer and decrease in the number of disulphidegroups (SAS) in the keratin that act as a barrier tothe diffusion of aqueous solutions. Enhanced uptakeof dye by samples treated in N2 atmosphere can beattributed to the creation of additional ANH2 groupson the fiber which provide cationic sites for theattachment of acid dye in acidic pH.16–18 This can bea very critical finding in dyeing and finishing ofwool where irradiation could be used to shorten thedye cycles or reduce the temperature of dyeing,while at the same time reducing the consumption aswell as wastage of dyes. All these benefits comewithout any loss of fabric strength or fabric handle(common in chemical treatments) as the changes arerestricted to nano meter level at the fiber surface.

CONCLUSION

Monochromatic UV irradiation of 172 nm can mod-ify the surface properties of wool to an appreciableextent. 15 min of exposure time brings about notablechanges in the surface morphology as well as sur-face chemistry of wool as seen by the SEM analysisand the improved wetting and dyeing behavior ofall treated samples. Most dramatic effects areobserved for treatment in N2 atmosphere for 15 min.SEM of these samples shows deep striations on thesurface and the scale structure is greatly modified. It

Figure 5 Uptake of Acid dye by wool treated with UVradiation for 15 min.

TABLE IIK/S value of wool fabrics treated with UV and dyed

with Acid dye

Time ofirradiation

Dyeingtime (min)

K/S

Atmosphere

Air O2 N2

0 min20 7.45330 7.87860 8.74490 8.750120 9.011

1 min20 7.89 7.82 7.4730 9.02 9.21 9.5060 9.98 9.50 10.0890 10.43 10.34 10.25120 13.46 10.95 12.27

5 min20 10.23 10.70 12.3830 10.62 11.52 13.2660 11.55 11.92 13.6890 12.71 12.18 13.81120 13.55 12.55 14.93

15 min20 11.12 11.27 13.3830 11.21 11.77 15.4060 12.35 12.14 16.1990 12.83 12.34 16.62120 14.02 12.77 17.06

3452 GUPTA AND BASAK


possibly leads to creation of additional aminogroups as corroborated by increased uptake ofanionic dyes for these samples. It may hence be con-cluded that UV radiation has the potential to beexploited in design and development of cleaner,more effective and more efficient commercial proc-esses for dyeing and finishing of wool.

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Surface functionalization of wool using 172 nm UV excimer lamp