Cellulose Cold Plasma REDUCTION

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2492 Langmuir 1991, 7, 2492-2497

Reduction and Oxidation of Cellulose Surfaces by Means ofCold Plasma

C. M. ilbert Carlsson' and G6ran StrBm

Insti tute f or Surface Chemistry, Box 5607, S-114 86 Stockholm, Swede n

Received August 6, 1990. In Final F orm: Decem ber 5 , 1990

Hydrogen and oxygen plasma treatm ent of two cellulose materials, a filter paperof pure celluloee anda greaseproof paper with a fairly high surface contentof wood resin, has been studied with ESCAas wellas by contact angle or water absorption. The hyd rogen plasma treatm ent reducea the hydroxyl groupson the cellulose and creates low molecular weight materials. Dueto th e lower polarity, water absorptionis reduced. T he oxygen plasma treatm ent of the pur e cellulose both oxidizes and reduce s the surface. Th eresin-rich paper, which has a hydrophobic nature, shows improved water wettability after both hydrog enand oxygen plasma treatments.

I n t r o d u c t i o n

Surface modification of syn thetic and natu ral polymersis of considerab le imp ortanc e in a wide range of indus trialapplications. It has been shown tha t cold plasma trea t-me nt is a powerful technique t o obtain tailor-made surfaceprope rties while retaining th e bulk proper ties of differe ntmaterials.1J

A cold plasma arises when a low-pressure gas a t roomtem pera ture is subjected to an electric discharge, e.g., aradio frequency or a microwave field.3As a whole theplasma is neutral. It consists mainly of neutral speciesaswell as small fractions of free electrons, ions, radicals,excited molecules, an d photons. T he activated speciesinterac t with the surface in different ways. T he final resultdepends on the gas used, treatment time, an d parametersassociated with t he equ ipment, suchas gas flow, energyin pu t, a nd g e ~ m e t r y. ~ , ~

Plasma tre atm ent of synthetic polymers has been usedextensivelyto improve wettability,z6adhesion,z6and other

ma terial properties.' Stu die s on cellulose materials havealso been reported. Westerlind et reported improvedadhesion between a resin-rich paper a nd polyethylene afteroxygen and ammonia plasma treatment. Benerito etfound improved water an doil wettabilityof cotto n celluloseafter treatment with argon, nitrogen, or air plasma.Improved water wettability of cellulose strips after dif-ferent plasma treatm ents has been reported by Chang Tangand Bosisio.lo

Although a number of publications have shown theeffects of plasma treat me nts, the chemistry involved isstill poorly understood. T he aim of this workis to carefullystud y the effect of oxygen and hydrogen plasma on thechemical composition and wettability of two cellulosematerials. T he first is a filter paper, made from a verypure cotton cellulose. T he second is a greaseproof pape r

(1) Kinloch, A. J. Adhesion an d Adhesiues; Chapman and Hall:

(2) Wu, S. Polymer Interface and Adhesion; Marcel Dekker. NewLondon, 1987; Chapter 4.

York; 1982; Chapter 9.(3 )Yaeuda, H.PloamoPolymerization;AcademicPrecle: ondon, 1985.(4 ) Gerencer, L. J. Polym. Mater. Sci. Eng. 1990, 62, 125.(6) Cleich , H.; Criens, R. M.: Mosl6. H. C.: Leute. U. Znt. J. Adhes.

Adhes. 1989, 9 , 8 8 .

Collord Interface Scr. 1974, 47, 610.(G).Weeterdahl, C: A. L.; Hall, J. R.; Schram, E. C.; Levi, D. M. J.

(7) Triolo, P. M.; ndrade, J. D. J. Biomed. M a t e r. Res. 1983,17,129.(8) Weeter lind, B.; Larseon, A.; Rigdahl, M. Int. J. Adhes. Adhes. 1987,

CI . a .I , 1 4 1 .

(9 ) Benerito , R. R.; Ward, T. L.; Soignet, D. M. ; Hinjcea, 0. Text. Res.

(10) Chan Tang, T. W.; Boeieio, R. G . Tappi 1980,63,111.J. 1981,51, 224.

containing fairly highamounts f wood resins made fromsulfite pulp. In a subsequent paper wewill discuse theeffect of these plasma treatments on the adhesiontopolye hylene.

E x p e r i m e n t a l S e c t io nMa terials. The filter paperused was a Munktell filter paper

made from pure cotton.The greaaeprcmf paper was made of bleachedsullite pulp

without any additives. It contains0.5% extractives, .e, reaidualwood resin,as determ ined by extraction with dichloromethaneusing the SCAN-C7:62 standard method.

Plarm a Treatme nt. For hydrogen plasma treatment a P h -maprep 100 eactor from Nanotech Inc. wasused. It operatesat 13.52 MHz. The gas flow was 10 mL/min (STP) nd thepower 100 W. The preaaure was approximately100 Pa.

The oxygen plasma treatment was carried outin a reactorbuilt at the Institu te for Surface Chemistry. It operates in thekilohertz range. The p b m a treatmen t was performed undertwo different conditions. When5 Wand 20 mL/min (STP) ereused the pressure was8.7 Pa and the generator was operating at

350 kHz, while a t 20W

and 5mL/min

(STP) he p m u r e was4.4 Pa and the frequency275 kHz.ESCA. TheESCAmeasurement8 were made with a b yb old -

HeraeusLH 000 electron spectrom eter equipped withan Al K aX-ray source. The analyzed area was 1 X 10 m m nd the exitangle from the sam pleto the detector was90. Peak ntensitieswere determined by integrating the area under thepeaks and thesensitivityfactorsused were0.21 for C 1s and 0.64 for 0 s.1* Thecurve fitting to the C 1s peaks was done with a Gaussian curve-fitting program.

Contact Angle Measurementra n d Absorption Studies.The contact angles were measured on10-pL dropleta placed onthe samples. The liquidsusedwere deionized and distilled waterand diiodomethane.

The absorption times were measured by placing a0.10-mLdroplet on the sample andobserving he time required fortotalabsorption.

R e s u l t s a n d D i s c us s io nE S C A S p e c t r u mof Pure Cellulose. Figure l ashows

th e C 1s signal originating from the filter paper. Th eresolved signal shows four peaks, which correspondtocarbons in different oxidative status.13J4 TheC1 peakrepresents unoxidized carbons, theCZ arbons with oneoxygen bond, th eCB arbons with two oxygenbonds, ndthe CC arbons with three oxygen bonds. Pu re cellulose

_ _ _ _ ~ ~ ~ ~ ~

(11) Carleeon, C. M. G.; trt)m, G. Surf . Interface A d . 1991,17,611.(12) Herder, P. C. TheeL, Trita-FYK-8801, Stockholm, weden, 988,

(13) Dorrie, G. M.; ray, D. C. Cellul. Chem. Technol. 19 78, 12,9 .(14) Gray, D. C. Cellul. Chem. Technol. 1978, 12,736.

pQ 80-82.

0743-7463 91 2407-2492$02.50 0 0 1991 American Chemical Society


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Cold Plasma OxidationlReduction of Cellulose Surfaces

Figure 1. Curve-resolved ESCAC 1s signal of (a) untreated and(b ) 1-min and (c ) 8-min hydrogen plasma treated filter paper.Thepeak markedC1consistsof unoxidizedorganic carbon (CH x),the Co peak contains carbons with one bondto oxygen (C-O),the Cs peak collects carbonswith two bonds to oxygen(C-0 and0-C-O), and the C, peak contains carbons with three bondst o oxygen (0-C=O).

0

Figure 2. Struc ture of cellulose.is a homopolysaccharide composed of 8-D-glucopyranoseunits.l6 As can be seen from Figure2, each un it containsfive carbons with one bondto oxygen and one carbon withtwo bonds to oxygen. T hu s one expects a curve-resolvedESCA C 1s signal to consist of only two peaks (CZ nd C3).T h e ca rb on co mp os itio n, C ~ / C ~ / C ~ / C I ,or the purecellulose is expected to be 083:17:0. However, th e carboncomposition for the filter paper, given by the ESCAmeas ureme nts, was foun d to be 8:72:17:3. Th e appearanceof the two peaks C1 and C4 may be due to eithercontamination of the filter paperor a chemical change inth e cellulose structu re. If one interpr ets t he C1 an d C4peaks a s due to chemical changes, then calculations withthe carbon composition of the filter paper suggest th atone carbon per two glucopyranose unitshas been reducedto a m ethylene or methyl group,and tha t approximatelyone per five glucopyranose units has been oxidizedto acarboxylic acid or carboxylate group.

H y d r o ge n P l a s m a Tr e a t m e n tof Filter Paper. Afterexposure to a hydrogen plasma th e surface compositionof the filter paper changes. Th is can be seen in the shapeof the C 1s signals shown in par tsb and c of Figures 1,which correspond t o1- an d 8-min exposu re, respectively.T h e Cz carbon decreases and the C1 increases afterprolonged exposure to th e hydrogen plasma.

Figure 3 shows the change in the O/ C atomic ratio an dthe different carbon peaks as a function of treatm ent time.

Th e different carbon peaks are given as percent of th eC1s signal. The O/C ratio decreases from 0.75 to aboutone-third of th at value. Carbon singly bonded to oxygendecreases sharply an d carbon w ith two bonds to oxygendecreases slightly. A high increase is observed for unox-idized carbons while a fairly mod erate increase is observedfor carbons with three bonds to oxygen. T hese da tastrongly suggest a dehydrox ylation of th e cellulose mol-ecule, since the C Zcarbons are predominantly bound tohydroxyl groups. T he increase in th e C4 peak can beexplained by two processes. Mos t probablyit is a resultof a reaction between radicals in the surfac e with oxygen

(15) Sjbtr6m, E. Wood C h e m k t r y : Fundamentals and Applications;Academic Press: New York, 1981.

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0,o J

801

0 - c - 0

>-c=o

- .

0 4 8 12 1.6

Duration of treatment / minFigure 3. Hydrogen plasma trea tme ntof filter aper. Changesin theO/C atomic ratio and in the amount of th e 8f feren t carbonsdue to time of treatment.

0 Untreated

0 After plasma treatmentAfter plasma treatment

098

0,6i n

0 and extractiondLT 0,4

cl c2 c3 c4Figure 4. Effect of hydrogen plasma treatm ent and extraction(ethanoland dichloromethane) n the compositionof filter paperas characterizedby ESCA.

when the sample is removed from the plasma reactor.Another explanation might be th at oxygen removed fromthe cellulose becomes activated in the plasma an d the nreacts with t he cellulose creating C4 groups.

The ESCA spectra show several indications that themolecular structure of the cellulose is degraded uponprolonged exposure to a hydrogen plasma. In Figure3 i t

is shown that the O /C ratio reaches a level of0.25. Thiscorrespon ds to 1.5 oxygen per glucopyranose unit, w hilea stru cture with th e two ether bonds stil l present and allthe hydroxyl groups removed would contain two oxygenatoms per unit. Figure 3 also shows th at th e CZ peakdecreases to below 33%. This shows tha t the carbonswith one ether bond are lost to some degree. T he C3 peakdecreases, indicating th at some of the carbons w ith twoethe r bonds are changed to some other carbon type. T hu swe conclude th at neith er of these decreases can take placewithout a ring opening or a chain scissoring.

T o study the degradation fur ther, a hydrogen plasmatreate d filter paper was extracted with ethanol an d withdichloromethane. Figure4 shows th e changes in the C 1 ssignal due to t he extraction. Th e C1 peak, which increased


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2494 Langmuir, Vol. 7 , No. 11 , 1991

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Carlsson and Str6m

n

/

, ~ I . , . I . , . /

0 1 2 3 4 5

Duration of treatment / minFigure 5. Absorption time for 0.10-mL water droplets onhydrogen plasma treate d filter papers. After 8-min plasmatreatment the absorptiontime is >500 a.

during plasma treatm ent, is reduced after th e extraction;and th e Cz peak is increased. After the extraction, theamount of C1 and CZ n th e treated paper is not totallyrecovered. Th us we conclude tha t the hydrogen plasmatreatment produces low molecular weight degradationproducts th at are less polar than the extracted surfaceand th is surface is in turn less polar th an th e untreatedfilter paper.

Th e adhesive streng th of these trea ted filter papers topolyethylene was studied. A detailed report will bepublished.11 It was found t ha t even though th e hydrogenplasma treatme nt drastically reduced th e adhesive strength,it was regained after extraction of the filter paper (i.e.,removal of degradation prod ucts). We th us conclude th ata reduced polarity of th e paper su rface, which correlatesto a reduced surface free energy, does not decrease theadhesive strength as long as the surfacedoes not carry lowmolecular weight material. If ther e is low molecular weightmaterial on t he surface, the adhesion drops drastically.Th e observed failure is in this case the n presumably due

to a weak bo undary layer16 forme d by t he low molecularweight degradation products.The wettability of the hydrogen plasma treated, but

not extracted, filter papers was studied by measuring theabsorption time for 0.10-mL water droplets. Due to thesurface roughness and th e fast absorption of water on anuntreated filter paper it was not possible to measurecontact angles on the papers treate d for less than8 min.As can be seen in Figure 5, the absorp tion tim e increaseswith the plasma trea tme nt time. After an 8-min plasmatreat me nt, the absorption was too slow to be m easuredaccurately due to evaporation. Th e filter paper absorbswater slowly, an d spread ing occurs laterally in the pap er,yet the dro plet maintains a constant contact angle, overth e 8-min me asure me nt period, of -115'. Th is shows

tha t the hydrogen plasma treatm ent gives the paper surfacea hydrophobic character.ESCAS p e c t r u m o f G r e a se p r o ofPaper. Th e grease-

proof paper contains high am ounts of wood resins in th esurface." Th e resins are detected mainly as aliphatichydrocarb on, Le., in th e C1 peak in ESCA . Hence, theESCA C 1s signal in Figure 6a shows a different shapecompared to the untreated filter paper in Figure la.

H y d ro g en P l a s m a Tr e a t m e n tof GreaseproofPaper.During exposure to a hydrogen plasma th e greaseproofpaper shows changes similar to he filter paper,C1 ncreasesand CZdecreases (Figure 6b, c). Due to th e wood resins

(16) Bikerman, J. J. J . Appl. Chem. 1961, 11 , 81 .(17) Abereon, G .M. APPI Spec. T ech. Assoc. Publ.1970, N o. 8,282.

Figure 6. Curve-resolved ESCAC 1s ignal of (a) untrea ted and(b) 2-min and (c ) 8-min hydrogen plasma treated greaseproofpaper.

l'O1

0,o J

73E

0 - c - 0

O-C=O0-

0 4 8 1 2 16Duration of tre8tment / min

Figure 7. Hydrogen plasma treatmentof filter paper. Changesin the O/C atomic ratio an d the differentcarbons due to timeof treatment.

in the greaseproof paper surface, the O /C ratio is only0.45. Figure 7 hows tha t there is a weak maximum in th eO /C ratio after 2-3-min treatm ent time corresponding toa minimum in th e amount of unoxidized carbon. Th is isprobably due to removal of wood resins from th e surfacecaused by a slight sp uttering effect of this low molecularweight material. Figure8 shows electron micrograph s ofuntreated, hydrogen plasma treated, and oxygen plasmatrea ted greaseproof paper. Although it is quite difficultto interpr et surface roughness fromSE Mpictures of pape rsurfaces, a comparison of the hydrogen plasma treatedand untreated paper suggests th at th e hydrogen plasmasomewhat smooths the surface. Another explanation tothe O /C maximum m ight be that radicals formed duringthe plasma treatment may react with the atmosphericoxygen when the sample is moved out of the plasmachamber.

After longer exposure times th e effect of th e hydrogenplasma on both greaseproof paper a nd filter paper seemsto be the same. Th e O/C ratio and the amounts of differentcarbons level out a t approximately the sam e amounts inboth cases; see Figures3 and 7.

Since the greaseproof paper is very smooth, contact


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Cold Plasma OxidationlReduction of Cellulose Surfaces

Figure 8. Scanning electron micrograph of (from top t o bottom)(a) untreated, (b )hydrogen plasma t reated , and (c) oxygen plasmatreated greaseproof paper. The scale shows the dimension of 0.1mm.

angles could be measured with both water and di-iodomethane (Figure 9). The contact angle of di-iodomethane remains constant, showing tha t the disper-sion par t of the surface free energy does not change. T hedecreasing contact angle of water with longer hydrog enplasma exposu re s surprising, since the ESCA resu lts showa decrease in the surface polarity. However, the con tactangle of water is not only determined by the averagepolarity of the local distribution of the hydrophobicmaterial.l8J9 If the hydrogen plasma removes hydrophobicpatche s of wood resin, it is not unlikely tha t the advan cing

Langmuir, Vol. 7, N o. 11, 1991 2495

1 0 jo ! ' 1 . 1 . '

0 4 8 1 2 1 6

Duration of treatment / minFigure 9. Contact angles for (m) water and (A) iiodomethaneon hydrogen plasma treated greaseproof paper.

n

Figure 10. Curve-resolved ESCA C 1s signal of (a) untreatedfilter paper and filter paper following 2-min treatme nt in oxygenplasma a t (b) 5 W, 20 mL/min (STP) and (c) 20 W, 5 mL/min(STP)

contact angle decreases upon hydrogen plasma tre atme nt,althoug h th e average surface becomes less polar. Ano therexplanation to the low contact angles might be theform ation of surface-active deg radatio n products, whichreduces the surface tension of the water. These areprobably formed from degrada tion products of the woodresins since this effect is not seen on the pure cellulose ofthe filter paper.

When the hydrogen plasma trea ted greaseproof paperwas laminated with polyethylenea slight, but not verypronounced decrease was observed in the adhesivestrength.

Oxygen Plasm a Treatment of Filter Paper. Figure10 shows the carbon peakof the filter paper af ter oxygenplasma exposure a t wo different conditions.A t ow powerand high flow rate th e changes in the surface seem to besmall. But, a t the higher power and lower flow rate theshape of the carbon peak changes dramatically. Th eam oun t of C1 increases to a high level and ther e is someformation of C4. As can be seen in Figures11 and 12 , thechanges occur in the firs t15 s and th en level out. T heeffect follows the changes that have been observed insynthetic polymers.20p21 Since all carbon in cellulose isbound t o at east one oxygen, oxygen incorporation in m ostcases would cause degr adation of the molecule. T hetrem endo us increase of C1 mig ht be d ue to scissoring inthe cellulose. Radicals forming during the plasma tre at-men t process might term inate, creating double bonds orcross-linking. The increase of C4 indicates that someoxygen is inco rpora ted, forming carboxylic acid groups.

(18) Dettre, R. H.; Johnson, R. E. , Jr. J . Phys. Chem. 1965,69,1507.(19) Johnson, R. E., Jr.; Dettre, R. H. J . P hys . Chem. 1964,68,1744.(20) Gerenser, L. J. J . Adhes. Sci. Technol. 1987, 1, 303.(21) Clark, D. T.; Dilks, A. J . Polym. Sci. 1979,17,957.


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Langmuir, Vol. 7, No. 11,1991

80-

Bo -

0-e-0

e- eI0 0 1 2 3 4 0 - C . 0

Duration of treatment / min

Figure 11. Oxygen plasma treated (5 W, 20 mL/min (STP))filterpaper. Changes in theO/C atomic ratio and the differentcarbons due to time of treatment.

"O l

t0-

L e-0

0 c- em 0-e-0

0 - e 4

00 1 2 3 4

Duration of treatment / minFigure 12. Oxygen plasma treated (20 W, 5 mL/min (STP))filterpaper. Changesin the O/C atomic ratio an d the differentcarbons due to time of treatment.

Oxygen P lasma Tre a tm ent o f GreaseproofPaper.Figures 13 an d 14 shows the effect of oxygen plas ma onthe surface composition of th e greaseproof paper. Allchanges occur during the first15 s. Th e reatment reducesthe content of unoxidized carbons and increases theamoun t of carbons with two or th ree bonds to oxygen. TheO/C ratio increases from0.455 to -0.6.

Carlsson and Strom

Figure 13. Curve-resolvedW CA C l a signalof (a) untreatedand (b) 16-8 and (c) 60-8 oxygen plasma treated greaseproofpaper.

- I\

0 1 5 3 0 4 5 6 0

Duration of treatment / IFigure 14. Oxygen plasma treated (20 W, 5 mL/min grease-proof paper. Changes in theO /C atomic ratio and the differentcarbons due to time of treatment.

Figure8 suggests that th e oxygen plasma treatm ent hasa spu ttering effecton the material.

The contact angles with water were very difficulttodeterm ine since th e water droplets swell the oxygen plasmatreate d greaseproof paper. T he result of this swelling islower contact a ngle values an d irregularly shaped droplets.Therefore, the conta ct angles on oxygen -treated grease-proof paper can only be roughly estimated. Th e values liebetween 10' and 30' for papers treated for15 s and below10' for papers treated for30 s or more.

It is well-known that oxygen plasma treatment canproduce hydroperoxide group^.^^*^ In order to identifysuch groups inour material, treated a nd untreated grease-proof papers were exposedto sulfur dioxide, which tagsthe group by formation of a sulfona te group. Figure15shows he ESCAS 2 p signal for both untreated and trea tedpaper. Th e treated paper showed a sulfur signal corre-sponding to approximately3 hydroperoxide groups per100 lucopyra nose units.

T he oxygen plasm a treate d greaseproof paper showeda s harp increase in adhesive strength when laminated to

(22) Briggs, D.; endall, C. R. nt . J . Adhea. Adhea. 1982,2, 13.(23) Gerensar,L. J.; Elman, J. F.; ason,M. .; ochan, J. M. olymer

1985,26, 1162.


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I I i1310 1 3 1 5 1320

Figure 15. ESCAS 2p signal from oxygen plasma treated anduntreated greaeeproof paper, both exposed to SOZ.

polyethylene.11 Th is is probably due to t he fo rma tion ofhydroperoxides, which decompose homolytically at thetempera ture used during lamin ationa and then may formcovalent bonds t o th e polyethylene surface.

(24)bmn, 8. w.; haw, R n Organic Peroxides; Swem, D., d.;Wdry Intancrenw: New Yorir, 1970; Vol.1, p 106.

Langmuir, Vol. 7 , No. 11, 1991 2497

ConclusionsThe results show th at chemical changes occur during

cold plasma treatm ent of cellulose materials and tha t theyaffect the wettability and the adhesion of the material.

Th e hydrogen plasm a reduces the hydroxyl groups onthe cellulose and low molecular weight materials areformed. The water wettability of a pure cellulose isreduced du e to the lower polarity of th e surface, but fora paper w ith high am ounts of resins in the surface thewettability is improved.

The oxygen plasma treatm ent both oxidizes and reducesthe surface. Among the oxygen-containing groups incor-porated, hydroperoxides are formed. T he water wetta-bility of th e greaseproof paper is improved after th e plasm atreatment.

Acknowledgment, We thank Professor Per Steniusfor valuable comments on the manuscript and Pia F il dtfor skilful work on th e sc anning electron microscope. Wealso thank the Swedish National Board for TechnicalDevelopment and Bo R ydins Foundation for ScientificResearch for financial support.

&&try NO. Ha, 1333-74-0; 0 2 , 7782-44-7; H20,7732-18 -5;CH212, 75-11-6; cellulose, 9004-34-6.

Cellulose Cold Plasma REDUCTION

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