Surface chemical analysis of raw cotton fibres and associated materials

R. Mitchell1, C. M. Carr2,*, M. Parfitt3, J. C. Vickerman3 and C. Jones41CSMA Ltd, Queens Road, Penkhull, Stoke-on-Trent, ST4 7LQ, UK; 2Textiles & Paper, School ofMaterials, The University of Manchester, Manchester, M60 1QD, UK; 3School of Chemical Engineering &Analytical Sciences, The University of Manchester, Manchester, M60 1QD, UK; 4Unilever Research, PortSunlight Lab, Merseyside, L63 3JW, UK; *Author for correspondence (e-mail: chris.carr@manchester.ac.uk)

Received 21 December 2004; Accepted in revised form 17 June 2005

Key words: Cotton, Cellulose, Surface analysis, Time of Flight Secondary Ion Mass Spectrometry(ToF-SIMS), Textile processing, X-ray Photoelectron Spectroscopy (XPS)

Abstract

The surface chemical composition of raw unscoured cotton was successfully investigated by the surfaceanalytical techniques X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion MassSpectrometry (ToF-SIMS). The presence of non-cellulosic material at the fibre surface was established anddetermined to be a complex mixture of fatty acids, alcohols, alkanes, esters and glycerides. The effect ofscouring and bleaching was to reduce the surface concentration of these materials but even after aqueousprocessing some non-cellulosic material residue was still detected at the fibre surface.

Introduction

Cotton fibres are composed of a secondary wall(typically 90% by weight) and an outer primarywall / cuticle (Peters 1963; Vigo 1994; Wakelynet al. 1998). Whilst the secondary wall is essentiallycellulosic in nature, the primary wall / cuticle con-sists of approximately only 55% cellulose, theremainder attributed to proteins, pectins and wax.Due to these surface contaminants raw cotton iswater repellent, exhibiting little water absorbency.Hence, to ensure uniform and rapid absorbency,acceptable dyeability and uniform colour it isimportant to process the raw cotton fibres by ascouring process (Peters 1963; Peters 1967;Wakelyn et al. 1998; Buchert et al. 2000), whichremoves the hydrophobic waxes and ‘dirt’

incorporated into the fibres. The distribution of thewax within the primary wall is uncertain but anoverall reduction in its surface concentration anddisruption of the primary wall is important forsubsequent processing (Peters 1967; Peters 1967;Johansson et al. 2004). Whilst the exact nature ofthe impurities at the cotton fibre surface are still notfully established, the fatty acids in the cotton waxaid in the scouring and emulsification of the otherinsoluble impurities from the fibre. Thus proteins,ash and pectins (i.e. methyl esters of polygalactu-ronic acid), are reported to be degraded, solubilisedand effectively removed from the fibre (Peters1967). However, since the wax acts as a lubricantduring fibre spinning and fabric processing, it isbelieved to be beneficial not to completely removeall of the surface wax (Taylor 1997).

Cellulose (2005) 12:629–639 � Springer 2005

DOI 10.1007/s10570-005-9000-9

In this study, the nature of the cotton fibre isinvestigated by the surface analytical techniques,X-ray Photoelectron Spectroscopy (XPS) andTime-of-Flight Secondary Ion Mass Spectrometry(ToF-SIMS), in order to characterize the surfacewax/ cellulose polymer and establish the efficiencyof the scouring process.

Experimental

Materials

Raw cotton fibres were kindly supplied by ShilohMills, Oldham, UK and originated from the USA.‘‘Standard’’ fatty acids, alcohols, alkanes, trigly-cerides and fatty esters were obtained from SigmaChemicals, UK.

Scouring and Bleaching

The raw cotton was scoured in a 3.0 g/l solution ofsodium hydroxide at a temperature of 100�C andliquor to goods ratio of 10:1 for 120 min. Thescoured fibres were rinsed in hot water (80�C) andair-dried.

The scoured fibres were bleached in a solutioncontaining 7 g/l sodium silicate, 1.2 g/l sodiumhydroxide, 1.8 g/l sodium carbonate, 5 g/l hydro-gen peroxide and 1 g/l wetting agent for 60 min, at85�C, and a liquor ratio of 10:1. The bleachedfibres were rinsed in hot water, neutralised in0.5 ml/l acetic acid for 10 min, rinsed and air-dried.

X-ray Photoelectron Spectroscopy (XPS)(Ratner and Castner 1997)

XPS measurements were performed using aSurface Science Instruments (SSI) M-Probe spec-trometer operating at a base pressure of 3 · 10)9

Torr. The samples were irradiated with mono-chromatic Al Ka X-rays (1486.6 eV) using anelliptical X-ray spot size of 1000 lm · 400 lm anda power of 180 W. Survey spectra were recordedwith a pass energy of 150 eV, from which thesurface chemical compositions were determined.In addition, high-resolution carbon (1 s) spectrawere recorded with a pass energy of 25 eV, from

which the carbon chemical states were determined.All binding energy values were calculated relativeto the carbon (1 s) photoelectron at 285 eV.Charge compensation for these electricallyinsulating materials was achieved using a beam ofca. 4 –9 eV electrons at a flood current of ca.0.1 mA, with an electrically grounded 90% trans-mission nickel mesh screen positioned ca. 1 mmabove the sample surfaces. The standard take-offangle used for analysis was 35�, producing amaximum analysis depth in the range of 3 –5 nm.

Time-of-Flight Secondary Ion Mass Spectrometry(ToF-SIMS) (Vickerman and Swift)

ToF-SIMS spectra were acquired under staticconditions using a PHI 7000 instrument operatingat a base pressure of less than 4.0 · 10)8 Torr. Theinstrument was equipped with a reflectron analy-ser, a Cs+ ion source (8 keV; pulse length 1.25 ns)and a pulsed electron flood source (50 –70 eV) forcharge compensation. Both positive and negativesecondary ion mass spectra were acquired fromareas measuring 250 lm · 250 lm over the massrange 0 –1000. For each sample analysis, the totalprimary ion dose was less than 1 · 1012 ions/cm2,which lies below the threshold for static SIMS of1 · 1013 ions/cm2.

Results and discussion

The surface chemistry of fibres can have animportant influence on their processing and end-use properties (Vigo 1994; Volooj et al. 2000;Parfitt et al. 2003). In previous ToF-SIMS studiesthe nature of the keratin fibre surface has beeninvestigated and the durability of the covalentlybound surface lipids (in the outer 3 –5 nm) to the‘clean’ wool or cashmere surface examined (Voloojet al. 2000). In this class of proteinaceous fibres,the major lipid component is 18-methyleicosanoicacid, bound predominantly to the keratin surfaceprotein via a thioester linkage, which imparts theassociated hydrophobicity to the fibre. Whilecotton is classified as a cellulosic material it alsohas natural impurities, which although notconsidered to be covalently bound to the fibre, stillneed to be removed to convert the cotton from itshydrophobic raw state to a commercially

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acceptable and ‘clean’ absorbent form. Cotton hasto be pre-processed, by scouring and bleaching, toremove a range of natural, non-cellulosic impuri-ties, in order to allow effective, uniform dyeing,improve whiteness, fibre quality and impart waterabsorbency (Peters 1967; Wakelyn et al. 1998;Buchert et al. 2000).

Examination of the carbon (1 s) spectrum of theraw cotton shows the major chemical state occursat a binding energy of 285.0 eV attributed to C –C,C –H bonding only, indicating that it is notcellulosic in nature, Figure 1. Theoretically, forcellulose, the XPS spectrum should only exhibitcarbon (1 s) peaks at binding energies of 286.6 eV,

Figure 1. C (1s) XPS spectrum of raw cotton fibres.

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C–O bonding, and 288 eV, O –C–O bonding, witha relative intensity of 5:1, respectively. PreviousXPS studies have reported comparable surfacecompositional data for raw cotton and identifiedthe ‘‘contaminant’’ as non-cellulosic (Soignet et al.1976; Ahmed et al. 1987; Donaldson 1989,Buchert et al. 2001). In this study the effect of

scouring was to impart instantaneous wetting ofthe cotton in water. In addition, scouring alsoproduces an obvious effect on the carbon (1 s)XPS spectrum, where a reduction in the spectralcontribution of the 285.0 eV component, fromapproximately 62% raw cotton to 22% in thescoured cotton, is apparent, Figure 2. In contrast

Figure 2. C (1s) XPS spectrum of scoured cotton fibres.

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the spectral contribution of the 286.5 eV specieshas risen from approximately 27% to 58% for thescoured fibres. Nevertheless the 285.0 eV‘‘contamination’’ is still clearly discernible. This isperhaps not entirely unexpected since even with‘pure’ cellulose materials, for example, Whatmanfilter paper and cellulose powder, there is alwaysspectral intensity at 285.0 eV, typically contribut-ing 10 –20% of the carbon (1 s) XPS spectralintensity, indicating either hydrocarbon contami-nation or polymer modification (Soignet et al.1976; Ahmed et al. 1987; Donaldson 1989;Buchert et al. 2001; Johansson et al. 2004).

The effect of aqueous bleaching of the scouredcotton fibres is to visually improve the whiteness ofthe cotton by destroying the yellow/‘creamy’chromophores. However, the carbon (1 s) XPSspectrum of the bleached cotton shows surpris-ingly little difference to the scoured cotton fibres,Figure 3. In particular, the lack of any furthersignificant reduction in the C –C, C –H (285 eV)component suggests strong substantivity andinertness of the bound organic material. Further,the absence of any increase in the carboxylic acidspecies, at a C (1 s) binding energy of 289.0 eV,suggests that as oxidation and chain scissionincreases with treatment time, the fibre loses thehighly oxidized, carboxylate-rich oligomericsurface material into the bleaching solution andthe apparent surface carboxylate concentrationremains unchanged. In contrast the increase inspectral intensity at 288 eV is due to the formationof surface carbonyl species, which in addition tothe O –C–O spectral component, contributes tothe observed spectral increase at the C (1 s) bind-ing energy of 288 eV.

While the XPS technique can provide usefulinformation about the fibre surface elementalcomposition, oxidation state and chemical envi-ronment, the complementary approach of utilizingToF-SIMS is beneficial in that the molecular nat-ure of the surface species can be established(Ratner and Castner 1997, Vickerman and Swift1990). The positive and negative ToF-SIMS spectraof ‘raw’ (untreated) cotton indicate the presence of acomplex mixture of chemical species at the fibresurface, Figures 4, 5 and 6. In order to effectivelycharacterize and establish the nature of thesecomponents a range of model/reference standardshave been analyzed. Previous wet chemical analysisof cotton wax extracts has reported the presence of

palmitic, stearic and oleic acids but also the pres-ence of longer chain C28 –C34 fatty acids (Cliffordand Probert 1924, Peters 1967; Hardin and Kim1998). In this study the negative ion spectrum of the‘raw’ cotton confirms the presence of carboxylicacids, the chain length extends from palmitic (C16,m/z 255)), through to C36 (m/z 535)), Figure 6,with the C28 and C30 species the most intense sig-nals in the spectrum. This suggests that these are themost abundant of the acids at the surface of the‘raw’ cotton fibres, which is in good agreement withthe reported wet chemical analysis (Clifford andProbert 1924, Peters 1967).

Other cotton wax fibre surface components re-ported in the literature are the hydrocarbons(alkanes), fatty alcohols and esters. In orderto establish their presence, or otherwise, n-triacontane (C30), n-triacontanol and associatedtriglycerides related to the individual fatty acidmixtures detected on the ‘raw’ cotton have beenanalyzed. It is evident from their ToF-SIMSspectra that the C30 alkane, alcohol and ester yieldcharacteristic ‘fingerprint’ data that allow thecotton fibre surface components to be established:

The triacontanoic acid spectra show signals atm/z 451) (C29H59CO2

)), m/z 453+ (C29H59CO2H2+)

and m/z 479+ (C29H59CO2H.Al+);The negative ion spectrum of n-triacontanol

shows signals at m/z 437) (C30H61O)) and m/z

435) (C30H59O));

The n-triacontane spectrum shows signals at m/z409+ (C29H61

+, dominant) and the molecular ion atm/z 423+ (C30H63

+), where the m/z 409+ signaldominates. This is either indicative of fragmenta-tion or a C29 impurity. In the negative ion spec-trum the signals observed at m/z 407) (C29H59

) ) andm/z 421) (C30H61

) ) again confirm this observation,and the C29 ion again dominates.

Each of these ions is clearly discernible in the‘raw’ cotton spectra confirming the complex mix-ture of species present within the wax-like layer onthe ‘raw’ cotton fibres. It should be noted thatthere is some mass overlap with simple long-chainhydrocarbons for some of the ester and alcoholsignals, which accordingly makes unequivocalidentification difficult.

In light of the numerous fatty acid signalsdetected at the cotton fibre surface, the presence ofa complex mixture of glycerides and esters is likely.As such, and as part of establishing furtherrelevant reference data, palmitic acid palmitate

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ester and tripalmitin triglyceride were analyzed tocharacterize the type of fragmentation behaviourexpected for material on the ‘raw’ cotton fibresurfaces. For the palmitic acid palmitate ester,molecular ions were detected at m/z 479+ and m/z479). However the most intense fragment ionswere detected at m/z 257+ and m/z 255) attributedto the palmitate moieties C15H31COOH2

+ andC15H31COO), respectively.

Similarly for the pure tripalmitin, weak molec-ular ions were detected at m/z 807+ and m/z 805),with the intense m/z 255) palmitate moiety againbeing the most intense fragment ion, Figures 7 and8. Other palmitic ions not specific to thetriglyceride were also observed. Even for ‘mixed’triglycerides such as stearic with palmitic combi-nations, the expected palmitate and stearate moi-eties will be detected as the base peaks (i.e. the

Figure 3. C (1s) XPS spectrum of scoured, bleached cotton fibres.

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most intense) with weak molecular ions. Thisaccordingly results in difficulties in unequivocalassignment of fatty acids in the lower mass range.Nevertheless analysis of the ‘raw’ cotton showsevidence of triglycerides based on stearate-,palmitate- and oleate-containing esters.

After scouring the relative intensity of thesignals associated with the wax / fatty acid-basedspecies is significantly lowered but is still evident,

Figure 9. This confirms the XPS assertion thatnot all of the material is removed during thescouring process, and that some of the material istenaciously bound to the cotton surface. With theremoval of most of the waxy layer, the cellulosicsignals, as might be expected, become more clearlydefined; for example the cellulose-specific m/z 87),113) and 221) are more intense for the scouredcotton relative to the ‘raw’ cotton, Figures 10 & 11.

Figure 4. Positive ion TOF-SIMS spectrum of raw cotton fibres.

Figure 5. Negative ion ToF-SIMS spectrum of raw cotton fibres.

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The proposed structures of these characteristic cel-lulosic ions are illustrated in Figure 12, (VickermanJ. C. et al. 1996).

The effect of the peroxide bleaching process is toremove further surface material, with the signalsassociated with the waxy-type layer are barely

Figure 6. Negative ion ToF-SIMS spectrum of raw cotton fibres. (* Fatty Acid, + Fatty Alcohol and � Alkane).

Figure 7. Negative Ion ToF-SIMS Spectrum of Tripalmitin.

Figure 8. Positive ion ToF-SIMS spectrum of tripalmitin.

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discernible in the ToF-SIMS spectrum, Figure 13.Associated with this decrease in surface ‘‘contam-inants’’ is the concomitant increase in the intensityof the cellulosic signals.

Conclusions

The complementary surface analytical XPS andToF-SIMS techniques have confirmed the presence

Figure 9. Negative ion ToF-SIMS spectrum of scoured cotton fibres.

Figure 10. Negative ion ToF-SIMS spectrum of raw cotton fibres. (* Cellulosic Signals).

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of a complex mixture of ‘‘wax-type’’ material onthe surface of ‘raw’ cotton fibres. This surfacecontamination has been shown to consist ofmainly long chain fatty acids with associated fattyalcohols and alkanes. In addition there is evi-dence of triglycerides and esters also being pres-ent in the surface cotton wax. The effect ofscouring and bleaching is to remove the majority

of these surface species but still a residual, tena-cious layer remains.

Acknowledgements

The authors gratefully acknowledge the financialsupport of Unilever Research and the technicaladvice of Phil Cohen.

Figure 11. Negative ion ToF-SIMS spectrum of scoured cotton fibres (* Cellulosic signals).

Figure 12. Proposed ToF-SIMS structural assignments of cellulosic materials.

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