Indian Journal of Textile ResearchVol. 5. June 1980. pp. 29-36

A Model for the Crosslinked Cotton Fibre

B R MANJUNATH* & A VENKATARAMAN

The Bombay Textile Research Association, Bombay 400086

Received 6 April 1979; accepted 3 January 1980

The loss in tensile properties of cotton fibre subsequent (0 crosslinking has been shown to be due to rigidification of the fibrcstructure resulting in embrittlement and not to the degradation of the polymer by acid liberated or present during the reaction.as believed by some workers. This rigidification of the structure can be achieved by fusing the neighbouring fibrils either bycovalent linkages or by simple hydrogen bonding. A simple model has been proposed for the crosslinked fibre based on the I

helical structure of the cotton fibre. The predicted losses in tensile properties, based on this model, will be greater for G.herbaceum type than for G. barbadense type cottons.

Cotton fibre in its natural state does not possessexcellent elastic properties. When the fibre is subjectedto external stress, a certain amount of induced strainsets in the fibre permanently. Resin finishing has beenan answer to this problem for some time. However, theobserved improvement in the elasticity of the resinfinished fibre is invariably accompanied by adeterioration in the tensile properties of the cottonfibre. Low strength and low elongation make thecrosslinked fibre a poor textile material. Themechanism which causes this lowering in strength andelongation of the cotton fibre has been the subject ofsome controversy, with some workers holding the viewthat a significant part of this loss is due to hydrolysisfrom the acid catalysts. This has resulted in a continuedsearch for milder and milder catalysts which cause littledegradation of the cellulose chain.

This necessitates the understanding of the structureof a crosslinked cotton fibre in order to understand itsbehaviour under applied stress and circumscribing thescope of the resin finishing treatments as applied tocotton. The present work is directed towards this end.A structure crosslinked by covalent linkages has beencompared with one extensively crosslinked by simplehydrogen bonds.Materials and Methods

The cotton used in the present work was G.barbadense type.

Preparation of crosslinked samples-For crosslink-ing treatments, Egyptian Mcnoufi cotton was treatedwith formaldehyde of different concentrations (5, 10,IS, 20 and 25~;;;)using concentrated hydrochloric acidas catalyst in the ratio of 100: 3. The .reatment periodwas 10 min and after that the sample was squeezed toremove excess liquid and dried at 50 C. The sampleswere then cured at 150C for 5 min, washed and dried.'Present addre-,s: Textile Committee. Worli, Bomba) 4()OO'X.

The above crosslinked samples were treated with20% alkali for 10 min at 20°C, washed thoroughly withwater, soured with dilute acetic acid for 10 min andagain washed and dried at 100°C.

Preparation of highly ordered cellulose II sample-Cotton samples known to possess an extensivelyhydrogen bonded network were used 1. To obtain thesesamples, cotton fibres were repeatedly swollen in 20'%,)aqueous solution of alkali for 10 cycles and then in 120 °aqueous alkali solution for another 40 cycles. Eachcycle of treatment was carried out for 1 hr at ° C inslack condition in nitrogen atmosphere and after thatthe sample was washed thoroughly in water and driedin nitrogen atmosphere to avoid any degradation.

X -ray examination-X-ray examination of the fibreswas carried out on a Philips diffractometer with aGeiger counter using Cu K, radiation. For thediffractometer scans, the samples were finely cut, sievedthrough a 300 mesh screen and made into rectangularpellets weighing exactly 100 mg with the aid of a specialdie.

Lateral order-The variation in the lateral order inthe cellulose IIsamples was measured by studying theresolution of IO] and 002 interferences which generallyoverlap (Fig. 1). Then a factor R, which is inverselyproportional to the resolution, was computed bytaking the ratio of twice the height of the minimum tothe sum of the heights of peaks

2 mlR=----hi +hz

Thus. when R = I. resolution and hence order I..

completely l\,~t and when R =0, order is maximum.This method was followed throughout this study as anindex of lateral order? .

•Lattice transformation= To assess the restrictionsImposed upon the fibre structure at different degrees of

INDIAN J. TEXT. RES .. VOL. 5, JUNE 1980

crosslinking, the crossnnked samples were mercerizedfor 10 min at O'C. When native cellulose is swollen inalkali, the lattice undergoes transformation fromcellulose I to cellulose II.The fractions of cellulose Iand II in the resultant samples are indicated by therelative intensities of IO] and 002 peaks in the X-raydiagram. When some restriction is put upon thestructure in any form, and in this case by covalentlateral linkages, the transformation is incomplete.Therefore, in the present study, the restriction,imposed upon lattice transformation was assessed bythe relative intensities of lOT and 002 peaks and alsothe half-width of the 002 peak in the cellulose Idiagrams.

Electron microscopy-Cotton samples were beatenin a blender for 45 min mounted on carbon grids,shadowed with platinum at an angle of approx. 28".

10 15 20 25DIFFRACTOMETER ANGLE(29·)

Fig. I--Diffractometer scan illustrating the method for computing R

Cross-sections were also photographed employing thelayer expansion technique.

Mechanical properties-For the present study, onlybundle strength and elongation were determined on astelometer at 1/8 in gauge length. No studies werecarried out on single fibres.

Results and DiscussionAs is well known, the crosslinked fibres lost their

strength and at higher degrees of crosslinking theywere extremely brittle and hence no strengthmeasurements could be carried out. The fibres lostabout 50% of their strength at about 0.5% boundformaldehyde level. This is in accordance with the dataalready reported in literature '.

The lattice transformation data on the other handwere quite interesting (Fig. 2). It is seen that atextremely low degrees of crosslinking, there was littlehindrance to lattice transformation. At about 0.3%bound formaldehyde level, while the crosslinks did notprevent the disruption of the cellulose Ilattice, they didnot allow the formation of cellulose II lattice. Thisresults in a highly skewed structure, with little evidenceof order. At increasing levels of bound formaldehyde,the crosslinks gradually prevented the disruption ofcellulose I lattice itself. Above about 1% boundformaldehyde, the X-ray diagram became invariantand was identical with that of the original untreatedmaterial. The variation in half-width of the 002 peakwith increasing degrees of crosslinking is shown inFig.3.

It can be inferred that at low degrees of crosslinking,the hydrogen bonds in the structure can be broken

(a) RAW COTTON (b) RAW COTTON (e) CROSSLINKED ANDALKALI SWOLLEN ALKALI SWOLLEN

BOUND HCHO0·31 'Yo

«n CROSSLI NKEDAND ALKALISWOLLEN

BOUND HCHO0·62%

(e)

CROSSLtNKEDAND ALKALISWOLLEN

10 15 20 25

(f)

CROSSLINKEDAND ALKALISWOLLEN

Fig. 2 --Diffractograms of crosslinked and subsequently alkali swollen samples indicating the restriction imposed on lattice transformationwith increasing degree of crosslinking. The first scan is that of an untreated sample

30

MANJUNATH & VENKATARAMAN: A MODEL FOR THE CROSSLI KED COTTO FIBRE

\ ~~~SSLINKED\RCERIZED

O~

0••••.•••••

4

;:r;"'3l-e-cUJa:m~ 2r~~N WNTROlo 1o

CROSSLINKED

o ,,0 2·0BOUND FOR MAL DE HYDE ••,.

Fig. 3-Variation in half-width of the 002 peak upon crosslinkingand then swe1ling

easily. but their reformation in an orderly manner so asto equalize the applied stress is not possible. At higherdegrees of crosslinking, the structure is so stabilized bythe covalent linkages that the hydrogen bonds cannotbe ruptured at all, as the neighbouring elements areimmobilized in relation to one another. The lack of anychange whatsoever in the X-ray diagram of crosslinkedsamples in comparison with those of the untreatedshows that the formation of cross links takes place onlybetween molecular assemblies and not between themolecules within the assembly. This view tallies withthat of Ollagnier" on thermodynamical con-siderations. It could, therefore, be deduced that incotton, crosslinks are effective in preventing slippagebetween fibrils.

In the samples repeatedly treated with alkali, it wasseen that the lateral order increased enormously, asevidenced by the X-ray diagram. In Fig. 4, the scan oftherepeatedly treated sample is shown along with that ofordinarily swollen sample which shows poor order.The improvement in resolution and hence order withincreasing number of cycles of treatment is shown inFig. 5. The inflection point at about 10 cycles oftreatment is d.ue to a change in the concentration of thealkali solution used in the treatment. This has beendealt with in an earlier paper'. This shows that lateralorder reaches an optimum level after some time and thediffraction pattern becomes relatively invariantthereafter. From this we can conclude that hydrogenbonding increases during the treatment and reaches anoptimum level.

Although it is now confirmed that the lateral orderimproves greatly due to multiple swelling treatments,X-ray diffraction data cannot unambiguously clarifyhow this lateral order is achieved. The lateral order canbe improved in any of the following three ways:(i) increase ill! the size of the aggregates, (ii).growingperfection within the ordered aggregates, and

(iii) simultaneous effect of both (i) and (ii).It is, however, interesting to evaluate the dimensions

of the aggregates or 'crystallite' by applying Scherrer'sformula, without attaching great physical significanceto the derived values.

0.9 ).Dhkl = B cos e

where B is the peak half width, &, the Bragg angle and ;"the wavelength of the radiation used. The Dhkl valuesfor untreated and treated fibres are given in Table I. Itis seen that the aggregate size increases in all the threedirections.

Electron microscope observations-The relativelyeasy separation of unmodified cotton fibres into

Table I-Dimensions Normal to the CrystallographicPlanes

101 002LOI

Untreated

Sample su bjected to 50 cyclesof treatment

40A 45A 60A

80A 115A 95A

COTTONREPETITIVELYTREATED INALKALI FOR50 CYCLES

COTTON TREATED IN 20 "I.~ ALKALI FOR 1 CYCLE

cOJ

t~~.,-:>-

'"zUJ>-Z

(al (b)L-

1'-0---'15--...I

2-0--2-'-5--' 10 15 2 a 25

DIFFRACTOMETER ANGLE (29°)

Fig. 4-Diffractograms of (a) alkali swollen sample for I cycle; and(b) repeatedly alkali swollen sample

SUDAN COTTON50 CYCLES TREATED-1>- 20 % NoaH

-0-12 % NoOH

\,'--4IIIII

\~ I

o O---O_C-O~J10 20 30 4c S:

NUMBE R OF CYCL E S 0 F IRE AT MH.T

0'60

'"'"<{

IJJa,

~ O'SOooz<{

I~o~ 0·400::ou,IIIUJ:::>::i. 030>0::.

o

fig. 5-··Yarialion in the resolution of 1'01 and 00] peaks ill thecellulose II diagrams with increasing cycle, of swelling trcatmeru-,

] I

INE>IAN J. TEXT. RES., VOL. 5, JUNE 1980

Characteristic fragments by beating in water is obviousfrom Fig. 6. The fibrillar nature of cotton is easilyobservable with individual long fibrils separated fromone another. The beating of crosslinked cotton fibresalso yields fragments, but they differ from the untreatedones in several respects. The beaten fibre consists ofshort and jagged segments which show little fibrillarseparation (Fig. 7). Even with continued beatingfibrillar separation is difficult to achieve. Most of thesegments consist of two or three layers of cell wallwhich have not been separated. This appearance ofcrosslinked fragments confirms the low extensibilityand brittleness of the treated fibres. Fragments withsuch characteristics result because of (i) mutual

Fig. 6-Electron micrograph of untreated cotton showingfibrillation (low magnification)

Fig. 7-Fragments of crosslinked fibre (low magnification)

32

•,

bonding of neighbouring microfibrils and, therefore,cell wall, and (ii) poor slippage between theneighbouring elements leading to well defined lateralshearing effects over short lengths.

The fragments obtained by beating ordinarilymercerized cotton fibre are shown in Fig. 8. Thefibrillar nature of cotton is very much evident withsingle separated microfibrils.

A fragment obtained from the samples subjected tomultiple swelling treatments is shown in Fig. 9. Withthe conspicuous absence of single separatedmicrofibrillar structure, it is seen that this bears astriking resemblance to the fragments of crosslinkedsample.

Fig. 8-Electron micrograph of mercerized cotton fibre showingfibrillation ,.

Fig. 9-Fragments of fibre swollen repeatedly in alkali solution (lowmagnification). The distance between markers represent 1 J1

MANJUNATH & VENKATAR'AMAN: A MODEL FOR THE CROSSLINKED COTTON FIBRE

Electron micrographs of untreated and repeatedlytreated cotton samples at high magnification areshown in Figs 10and 11.The fusion or hornification offibrils is very much in evidence, with the absence offibrillation in repeatedly treated samples. It is all themore important to note here that this has beenachieved by hydrogen bonding only and not throughcovalent linkages. Layer expansion data (Figs 12-15)also confirm this.

Based on the size of the aggregates obtained from X-ray diffraction data, one can judge that on an averageabout 4 elementary fibrils have fused to form one largeaggregate. This conclusion, however, should be treated

Fig. lO-Untreated cotton (high magnification)

Fig. ll-Repeatedly swollen cotton (high magnification). Thedistance between markers represent 0.1 J1

•,

with caution, as the imperfection within each aggregatehas not been taken into account. But viewed generally,such a fusion of 4 elementary fibrils would reduce theavailable fibrillar surface area by one-fourth. Buttaking into account the fact that there was somehydrogen bonding between the neighbouring fibrilseven initially, this would mean that the sorption wouldbe lowered considerably in comparison to ordinarilymercerized fibre. This has been borne out by sorptionstudies'.

Mechanical properties-The mechanical proper-ties of these hydrogen-bonded crosslinked fibres werestudied to find out how they compare with those of

Fig. 12-Untreated cotton: Cross-section by layer expansiontechnique ( x SOOO)

Fig. 13-Untreated cotton: Cross-section by layer expansiontechnique ( x 22(00)

33

IND1AN J. TEXT. RES., VOL. 5, JUNE 1980

covalent crosslinked samples. The fibres after beingsubjected to. multiple swelling treatments were brittle.It is seen from Fig. 16 that a tatal strength lass of aver50% is encountered for these fibres. The loss jn .elongation is also.of the order of 50% in relation to.theelongation of the samples treated in 20% alkali for 1cycle (Fig. 17). It is also. interesting to. nate that theserates of decrease bear a striking relationship with theincrease. in the lateral order or hydrogen bandingshown in Fig. 5.

These data on the tensile properties of a samplewhich has, been crosslinked extensively by hydrogenbonding and where no molecular degradation hasoccurred are, very similar to. those. of the covalentcrosslinked cotton fibres extensively reported in theliterature, particularly bY'Rebe.nfeld5 and Orr et a1.6.

Proposed ModelThe cellulase molecules in cotton fibre farm fibrils

and these fibrils are. laid in spirals which change theirdirection at irregular intervals. The fibrils are held

Fig. 14-Repeatedly swollen sample: Cross-section by, layerexpansion technique (x 75(0)

Fig. IS-Repeatedly swollen sample- Cross-section by, layerexpansion, technique (x 22QOQ).

34

•,

together by weak hydrogen bands. The hydrogenbonds within each fibril have much higher energy andenable the cellulose molecules to form a lattice. Eachstructural element-s-a molecule, a fibril-s-is subjectedto.many diverse forces. In the normal unstrained state,these forces balance one another, reaching anequilibrium. However, when an external stress isapplied, this equilibrium E1 is disturbed. Some lateralbonds are strained and same others break down.During this deformation, same relative motion of theneighbouring elements takes place. The freed bondingforce (mostly hydrogen bonds and some Van der Waalsforces) tend to attain equilibrium E2 in their newenvironment. When the applied stress is againremoved, the equilibrium E2 is once again disturbed,with the strained bonds trying to pull the structure toits original configuration and the newly formed bondspreventing it. Therefore, the resultant configurationwill depend upon the new equilibrium E3, which is,established. But some permanent deformation isalways retained by the structure.

35

1

, ,CONTROL

>< 30 IJ,

~ \ '

'" 21x~ ~

~ 25 ~IJ,z 'IJ,~ "IJ, 0

t;; "'"UJ OQ~...J 0020z~CD

SUDAN COTTON,50 CYCLES TREATED

-IJ,- 20 PERCENT N,oOH-0-12 PERCENT NoOH

o

15~ ~ ~ ~ ~ __ ~~Wo 10 20 30 40

NUMBER OF CYCLES OF TREATMEN:T

so

Fig. 16-Fall in bundle strength of cotton fibres with repeatedswellings

30r------------------------------- __ -,SUDAN CQTrON

50 CYCLES TREATED

-IJ,- 20 % N,oOH-0-12% NoOH

~zo~. 20

'"zo...JW o o 015

o oo

oCONTROL10a------L-- __--~-- __~ ~ ~~

o 10 20 3'0 ..40 5QNUMBER OF CYCLES OF TREATMENT

Fig. l7.-Fall in elongation at break WIth repeated swellings

MANJUNATH & VENKATARAMAN: A MODEL FOR THE CROSSLINKED COTTON FIBRE

To make the network return to its originalconfiguration, the initial bonds should be strongenough to prevent excessive slippage. Therefore, themechanism of recovery depends primarily on lateralbonding.

Under axial stress, the cotton fibre behaves verymuch like a spring, with each fibril contributing to thisact by tending to align towards the fibre axis. In doingso, they have to undergo mutual slippage and a certainamount of deformation such as bending. Duringbending, the outermost chains in the fibrils suffermaximum deformation due to extension andcompression. The greater the distance of theseperipheral elements from the neutral axis, the greaterwill be the strain imposed on them, resulting in theirearly breakdown. Further, a certain amount offreedom to individual fibrils is necessary in order to slipagainst one another. For this to be possible, the lateralbonding between fibrils should be minimum. In such acase, the extension of the fibre will be greatest and thestrength will also be higher as the fibrils will tend toalign more towards the fibre axis and allow the primarylinkages to absorb a considerable portion of theimposed stress After all, the strength of a polymer isdictated largely by the primary valence forces along thechain. Therefore, if ()is the helix angle in a cotton fibre,cos ()determines the extent to which the primary bondsare contributing .to the fibre strength (Fig. 18). Onstretching, ()reduces to ()'at the breaking point; hence,cos ()' is the strength contribution.

In a crosslinked fibre or a repeatedly swollen fibre,where the fibrils have fused together to form very largeaggregates, the fused fibrils fail to bend easily and alignmore towards the fibre axis. ()"is the helix angle of thehornfied fibre at the breaking point and ()" is greaterthan ()'. This results in failure of the fibre at lowerstresses itself, as cos ()" is lower than cos ()'.Because ofthe failure ofthe helix to open up and reduced slippagebetween the bulky fibrils due to lateral bonding, theelongation at break is also lower.

That the strength and elongation losses in acrosslinked fibre are due to the failure of the helix toopen up is also borne out by a study of the stress-strain

a

NORMAL flBQE HORNIFIED FIBREDURING STRESSING DURING STRESSING

Fig. 18-Model of (a) untreated fibre: and (b) crosslinked fibre

V)V)wa:....V)

UNTR EHE 0

CROSSL INKED

STRAINFig. 19--Stress-strain diagram of untreated and crosslinkeu fibres

diagram of untreated and treated fibres (Fig. 19).Thestress-strain curve for untreated cotton fibre showsthat beyond the yield point, the curve bends towardsthe stress axis. This results from the spiralling fibrils inthe cotton as they bear more load near the point ofrupture, by aligning themselves towards the fibre axis.In the stress-strain diagrams of the crosslinked cottonsthis very characteristic curvature is conspicuouslymissing, confirming that the spiral fails to open up dueto increased lateral bonding. This is the main cause forthe strength losses in crossJinking and not aciddegradation.

In G. barbadense type cottons, the helix angle ()is lowinitially itself; therefore, the strength losses or.crosslinking are generally lower than those for Indiancottons which have larger helix angles. Of course, thebest type of cottons for crosslinking are those whichexhibit high strength and high elongation, like PimaS-l, wherein fibrillar size also plays an important role.

From the foregoing it would appear that the onlyway to minimize the strength losses is to orient thehelix better towards the fibre axis prior to thecrosslinking treatment so as to have a low value for einitially itself, which results in high strength retention.Although this solution may sound attractive, it has itsown disadvantages, as the material will have poorelongation even initially and it will reduce further uponcrosslinking. Good extensibility is needed to resist tearand abrasion. A tear in the fabric will propagate fasterand easier if the rigid threads fail to extend and,therefore, break individually.

Some efTortshave been made to crosslink cotton ...with long chain reactants., based on the conception thatthis will allow good extensibility and at the same timeimparting good recovery properties to the material.This view is not stereochemically sound, as a longbulky chain will hold the adjacent cellulose chains at adistance greater than that required for hydrogenbonding. Although covalent crosslinks help the

'. ...i/~ i

35

INDIAN J. TEXT. RES.. VOL. 5. JUNE 1980

structure to regain its original configuration to acertain extent, the main contribution towards thisphenomenon still comes from the hydrogen bonds incellulose. If the formation of these is prevented, therecovery properties fail to show any significantimprovement. Attempts in these directions havealready failed. Further, a long chain introduced in alinear network like that of cellulose will always orientitself along the length of the fibrils and not form alateral bridge. Therefore, fresh attempts to improve thetensile properties of resin finished fabric with newerand newer resins or milder and milder catalysts will notyield any significant dividends, but only marginalbenefits. One can agree with Nickerson's statementthat 'Strength loss is always directly proportional tocrease-recovery conferred by the treatment regardlessof the type of reagent used or degree of cure'",

Improvements, if any, will only be marginal and onehas to come to terms with this and strike a balancebetween recovery and tensile properties. Stretching thefibre and orienting the fibrillar network offer a partialsolution to this problem.

One can soften the fibre slightly by swellingtreatments subsequent to the crosslinking treatments.Apart from this, these high recovery properties are notcompatible with high tensile properties for cotton fibre

36

because of its very structure. One can, however,minimize the losses in the fabric by adjusting the yarnand fabric geometries suitably, which by far is the bestsolution.

AcknowledgementThe authors' grateful thanks are due to Dr Mary L.

Rollins and Miss Iness de Gruy of Southern RegionalLaboratory, USDA, New Orelans, and Dr S.M.Betrabet of Cotton Technological ResearchLaboratory, Bombay, for the electron micrographs ofthe treated materials. They also wish to thank Shri T.V.Ananthan, Director, BTRA, lor permission to publishthis paper.

ReferencesI Manjunath B R, Venkataraman A & Stephen T, Proceedings.

international symposium on textiles (Textile Association(India) Delhi Branch) 1973, 32.

2 Manjunath B R, Venkataraman A & Stephen T,J appl Polym Sci,17 (1973) 1091.

3 Frick J G (Jr), Kottes Andrews B A & David Reid J, Text ResJ, 30(1960) 495.

4 Ollagnier M, Chim Ind, 79 (1958) 770.5 Rebenfeld L, Text Res J, 32 (1962) 154.6 Orr R S, Burgis A W, Deluca L B & Grant J N, Text Res J, 31

(1961) 302; 31 (1961) 550.7 Nickerson R F, Am Dyestuff Reptr, 39 (1950) 46-50. 60.