Indian Journal of Fibre & Textile ResearchVol. 20, December 1995, pp. 185-191

Mercerization and crimp formation in jute

K P Sao & A K JainJute Technological Research Laboratories, 12 Regent Park. Calcutta 700 040. India

Received I August 1994; revised received 26 December 1994; accepted 14 February 1995

The mercerization effectsof aqueous NaOH solution of varyingconcentrations on the isolated ultimatecellsand the fibrestrands ofjute havebeenstudied.It isobservedthat the treatment with higherconcentrationof alkali (12% and above) results in a decrease in crystallinity index and deterioration in overall molecularorientation along with the change in crystalline lattice from cellulose I to cellulose II. The changes in thesupermolecular structure and morphology of cellulosein the ultimate cellsappear to bemainly responsiblefor the development of crimp injute. A qualitative mechanismfor the three-dimensionalcrimp formation injute has also been discussedin termsof rearrangement of internal molecularforcesin celluloseon merceriza-tion.

__,words: Crimp, Jute fibre, Mercerization

1 IntroductionThe mercerization of jute fibre by treatment with

strong aqueous sodium hydroxide solution at roomtemperature is a well-known process for the diversifi-cation of its end-uses'. It has been reported+' that themercerization of jute fibre is accompanied by loss inweight, longitudinal shrinkage, decrease in breakingstrength and an increase in extension at break. Themost important feature is the development of crimp inthe fibre", An X-ray study' has shown that there is apartial conversion of native cellulose I lattice to cellu-lose II. Jute differs from the pure cellulosic fibres like.cotton and ramie in that it contains a considerableamount of non-cellulosic materials of which ligninand hemicellulose are the most important":". Thecharacteristic features of jute originate from these lig-nin and hemicellulose molecules which probablyform some complex cross-bonding with the cellulosemolecules. The fibre strand is multicellular and cons-ists of ultimate cells (av. length, '" 2 mm and av~width,""20 urn) cemented together with non-cellulosic mat-erials". It has been reported" -10 that in mercerizedjute the ultimate.cells shrink longitudinally and thecell wall thickness increases along with a decrease inlumen width. The development of crimp in fibre hasbeen related by different workers to non-uniform dis-solution of cementing materials", swelling of ultimatecells8•1o and differential shrinkage of cellulose 1and cell-ulose II structures 5• In the present work, the effect of al-kali of varying concentrations on both ultimate cellsandfibre strands of jute has been studied in detail for abetter understanding of mercerization and crimp for-mation.

2 Materials and Methods

2.1 MaterialsJute fibre strands of both caplularis (JRC 212) and

olitorius (JRO 632) varieties were taken from the mid-dle portion of the reeds. The fibres were cleaned, suit-ably combed and then treated with various concentr-ations of aqueous NaOH solution at room temperat-ure ( '" 30°C) following the procedure described earli-er". The treated samples were washed, air dried andused for the study.

The ultimate fibre cells of both the jute varietieswere separated following the method described ear-lier!". These isolated ultimate cells were then treatedwith different concentrations of alkali as describedabove. The treated ultimate cells were washed till freeof alkali and then air dried or preserved in glycerol forfurther studies.

2.2 Methods

2.2.1 IR StudiesThe IR spectra of the samples were measured by

KBr disk technique using a Shimadzu IR 440 spectro-photometer following the procedure described ear-lier6•11.

2.2.2 Measurement of Crimp FrequencyThe crimp frequency in the fibre was.measured fol-

lowing the procedure described earlier!",

2.2.3 MeaSurement of Sonic Velodty in Fibre StrandThe sonic velocity measurements were carried out

186 INDIAN J. FIBRE TEXT. RES .. DECEMBER 1995

using a dynamic modulus tester (PPM-5). From theslope of spacing between the transducers versus tran-sit time curves the sonic velocity (c) was calculated.Ten fibres were taken for each sample and for eachfibre, the average often slopes was determined. Thedynamic modulus 13 (E) in g/den was calculated by thefollowing equation:E= 11.3 c2

where c is the sonic velocity in krn/s.

2.2.4 Measurement of Cell DimensionsCell width, lumen width and length of the ultimate

cells were measured using a projection microscope asdescribed elsewhere'"!".

2.2.5 SEM StudiesFibres and ultimate cells were mounted on the alu-

minium stubs using a conducting adhesive tape andthen coated with a thin silver film ( - 200 ~ thickness)in a vacuum coating unit. These samples were obser-ved on a Hitachi scanning electron microscope (mo-del S430) and representative photographs weretaken.

3 Results and DiscussionFigs 1and 2 show respectively the variations in the

absorbance of the IR bands near 895 cm -I (~-linka-ge) and 1425 em -I (CH2 symmetrical deformation),measured as a ratio to CH stretching band near 2900em - 1 with the concentration of alkali. These bandsare sensitives-!' to the change in lattice from celluloseI to cellulose IT • It is seen that though the absorbance,values of ultimate cells and fibre strands ofjute differquantitatively, 'the nature of variation IS similar inboth the cases. The quantitative difference in the abs-orbance values may be attributed to the non-cellulo-sic constituents such as lignin and hemicellulose pres-ent in the jute fibre strand. However, it is observedthat the major change in the absorbance of thesebands occurs above 12% of NaOH concentration.Table 1shows the change in IR crystallinity index ofthe ultimate cells and fibre strands with the alkali con-centration. The crystallinity index (C.I.) was calcula-ted by the absorbance ratios al425 em./aS95 em--'

(O'Connor et al.14) and a1370 em-./a2900 em-' (Nelsonand O'Connor"). Nelson and O'Connor argued 15 thatthe absorbance ratio a 1370 em-./a2900 em-I can be appl-ied to both cellulose I and cellulose II and also tosamples containing mixed lattice as the band near1370 em -1 (CH bending vibrationsjdepends on thecrystallinity and not on the lattice type. However, injute fibre, both cellulose and xylan may contribute toth~~_~~n<!6.It is interesting toobserve that the C.!.

value is lower in the case of fibre strands compared i.othat in case of ultimate cells (Table I). This may be dueto the presence of non-cellulosic constituents like lig-nin and hemicellulose in the fibre strands. For bothultimate cells and fibre strands of jute, the C}. valuedecreases with the increase in alkali concentration.The decrease in c.I. value appears to be associatedwith the change in _crystalline lattice from celluloseI to cellulose II as observed above (Figs I arid 2).Table I further shows that there is practically no crimpin the fibre below 12% NaOH treatment, but the

Fig.

O,6~ 0 - Jute FibreIr .:J,t.:":2 : :< I t I I I Io~~r-~__~~~~ __-+~__~ __~

o 8 10 12 14 16 18NaOH Cone. (wi Iw I ) •• ,.

I-Effect of NaOH conc. on the IR absorbance ratio(a895 em-,/aZ900 em' .) in jute .

O,7~-----------------------------'0- Jule Fibre6 -Jute Ultimate Cello

~ 0,6a:e5 0..D

!c 0.4

Fig. 2--EfTect of NaOH cone. on the IR absorbance' ratio(a1425 em .,/a~900 cm "! ) injute

Table I-Infrared crystallinity index of alkali-treated jute

NaOH·conc.

%

IR crystallinity index CrimpfrequencyNo.remUltimate cells Fibre strands

at370 cm " ' a-142S cD\-1( U1370 em-I/ QI425 cm.-I;

a2900 cm " ' aS95 em-I a2900 cm "! a895 crn "!

0 0.570 1.712 0.522 1.4908 0.572 1.761 0.435 1.470

to 0.574 1.735 0.421 1.}81

12 0.516 1.539 0.410 \.J34 2.014- 0.512 0.848 0.375 1.037 2.216 0.504 0.743 0.353 0.919 3.218 0.504 0.717 0.350 0.902 J.8

SAO & JAIN: MERCERIZA nON & CRIMP FORMA nON IN JUTE 187

crimp frequency increases with the further increase inNaOH concentration. It is evident from the IR spec-tra of ultimate cells (Fig. 3) that the hydroxyl band"reduces and shifts slightly towards higher wave num-ber on mercerization. This apparently indicates wea-kening or breaking of some hydrogen bonds!".

Table 2 shows that the sonic velocity and dynamicmodulus values are slightly higher for olitorius varietycompared to that for capsu/aris variety. The sonicvelocity in fibres is related to the state of overall mole-cular orientation with respect to fibre axis'3. The con-siderable decrease in both sonic velocity and dynamicmodulus at higher NaOH concentrations (18% and25%) indicates the overall disorientation of'molecu-lar chains on mercerization. The marginal decrease insonic velocity at lower NaOH concentration (8%)may be due to the disorientation of molecular chainsas a result of intercrystalline swelling.

The dimensional changes in the ultimate cells ofboth capsularis and olitorius jute with increasing con-centration of alkali are shown in Table 3. The celllength and the lumen width decrease while the cellwall thickness increases with the increase in alkaliconcentration as expected. Table 3 also shows a decr-ease in the cell width at the lower NaOH concentrat-ion (8%). This may be due to the dissolution of nitrog-enous solid residues often present within the lumen 17

Table 2-Sonic velocity (c) and dynamic modulus (E) in alkali-treated jute fibres

and change in cross-sectional shape of lumen as aresult of intercrystalline swelling in the cell wall. Athigher NaOH concentration, a slight increase in thecell width (outward swelling) is observed in the-case ofcapsularis jute. Mazumder et al." correlated this beh-aviour to the better crimp formation in capsularis ascompared to that in olitorius variety. However, thethick cell wall and narrow lumen in the ultimate cellsof capsularis jute (Table 3) may result in an outwardswelling of cell wall on mercerization.

The scanning electron micrographs of ultimatecells and fibre strands of jute are shown in Figs 4-7.Fig. 4a shows the multicellular nature of jute fibre inwhich the ultimate cells are seen to be cemented toget-her to form the technical fibre strand. In mercerizedjute fibre (Fig. 4b), the ultimate cells are more clearly

Wavele n9th J ~

2~.5~ ~3r·0 4~.0100r

70

60

50CIIuc.2 40-EIII 3c0•..~~

NaOH capsularis jute olitorius juteconc.

% c. km/s E. g/den c. kmjs E. g/den 00 5.05 288.18 5.64 359.45 4000 3500 3000 25008 4.27 206.03 4.99 281.37 Wave number, em'

18 3.21 116.44 3.01 \02.38 Fig. 3-IR spectra of ultimate cells of jute showing change in the25 2.76 86.08 2.51 71.19 hydroxyl band on mercerization [(a) raw, and (b) mercerized]

Table 3-Dimensions of alkali-treated ultimate cells of jute

Jute sample NaOH cone. Cell length Cell width Lumen width Cell wall Change in Change in cell% mID IJDl 11111 thickness cell length wall thickness

11111 % %

0 1.98 IS.2 3.0 6.1capsularis 8 1.80 14.9 2.3 6.3 -9.1 3.3variety 18 1.68 IS.9 1.5 7.2 -IS.2 18.0

2S 1.60 IS.6 1.2 7.2 -19.2 18.00 1.95 16.0 3.8 6.1

olitorius 8 1.8S IS.3 3.0 6.2 -S.1 1.6variety 18 1.73 15.9 1.7 7.1 -11.3 16.4

2S 1.63 IS.7 1.6 7.1 -16.4 16.4

188 INDIAN J. FIBRE TEXT. RES., DECEMBER 1995

depicted. This is evidently due to removal of somenon-cellulosic constituents by the action of alkali.Fig. 4b also shows that the dissolution of cementingmaterials between the ultimate cells is non-uniform.Fig. 5 shows the surface topography of untre-ated and mercerized jute fibres at higher magnificati-on. The mercerized jute fibre shows less surface debris

and more clear fibrillar structure as compared to untr-eated fibre. It also shows less compact structure offibrils as compared to untreated fibre, probably dueto swelling.

The isolated ultimate cells before and after mercer-ization treatment are shown in Figs 6a and 6b respect-ively. The untreated cells are mostly straight in nature

Fig. 4-Scanning electron micrographs of jute fibre: (a) raw, and (b) mercerized

Fig. S-Scanning electron micrographs showing fibrillar structure in jute fibre: (a) raw, and (b) mercerized

SAO & JAIN: MERCERIZATION & CRIMP FORMATION IN JUTE 189

with the tapered ends. However, mercerized ultimatecells' are mostly curved in nature. This crimped nat-ure of the mercerized ultimate cells indicates that theformation of crimp in jute is mainly associated withthe swelling of cellulosell. The fibrillar structure ofuntreated and mercerized ultimate cells at higher ma-gnification is shown in Figs 7a and 7b respectively. Asexpected, the fibrillar structure is more clear in ultim-

ate cells as compared to that in fibres. In the untreatedultimate cells, the fibrillar structure consists mostly ofstraight fibrils running along the fibre axis while in themercerized ultimate cells it is comparatively distort-ed. It may be noted that cellulose II is thermodynami-cally more favoured structure than native cellulose I(ref. II). Stockmann'v-t? reported that elementaryfibrils have built-in stresses in native cellulose. As

Fig. 6-,-Scanning electron micrographs of ultimate cells of jute: (a) raw, and (b] mercenzed

Fig. 7-Scanning electron micrographs showing fibrillar structure in ultimate cells of jute: (a) raw, and (b) mercerized

190 INDIAN J. FIBRE TEXT. RES., DECEMBER 1995

CelluloseChain

I Longitudinal force due tot relaxation of stress

Twisting force due toconformational change

-- Transverse force dueto sw~lling

tFig. 8-lnternal molecular forces in cellulose on intracrystalline

swelling and conversion from cellulose I to cellulose II

discussed above, the intracrystalline swelling due toNaOH causes a reduction in the interaction betweenthe adjacent cellulose chains. Thus, the relaxation ofstresses and the disorientation of molecular chainsmay cause the longitudinal shrinkage offibrils. Furt-her, during conversion from cellulose I to cellulose II,the chain conformation is reported20•21 to changefrom the bent (cellulose I) to the bent and twisted form(cellulose II). These changes in internal forces mayproduce a supermolecular texture, leading to the dist-orted fibrillar structure as observed above.

It may be concluded from the above observationsthat the intracrystalline swelling of cellulose and thechange in lattice from cellulose I to cellulose II seem tobe the essential prerequisites for the formation ofcrimp. Swelling and change in lattice from cellulose Ito cellulose II may cause a rearrangement of internalmolecular forces as shown in Fig. 8. These forces mai-nly include the longitudinal forces due to stress-rela-xation and shrinkage, twisting forces due to change inconformation of molecules and the transverse forcesdue to change in lateral packing of chain molecules.The above rearrangement of molecular forces toget-her with the differential transverse swelling 1 0 of cellwall due to the variation in cell wall thickness alongthe length of the ultimate cell may lead to the develop-ment ofa three-dimensional crimp injute. Moreover,in partially crystalline jute, shrinkage will be moreprominent in the amorphous regions than in the com-paratively rigid crystalline regions of cellulose. Thus,a lower crystallinity and smaller crystallite size in thecellulose structure of jute may lead to a better crimpcurvature.

4 ConclusionsMercerization and crimp formation in multicellu-

lar jute fibre depend essentially on the change in supe-rmolecular structure and morphology of cellulose inthe ultimate cells. The alkali of concentration below12% affects mainly the non-cellulosic constituentsadhering to the ultimate cells and may cause inter-crystalline swelling of cellulose structure without devel-oping any crimp. At higher concentrations, alkali pe-netrates the crystalline structure of cellulose, result-ing in the change in lattice from cellulose I to celluloseII, decrease in the crystallinity, deterioration in over-all molecular orientation and development of crimpin fibre. T-he IR crystallinity index decreases andcrimp frequency increases with the increase in alkaliconcentration. The development ofthree-dimensio-nal crimp in jute may be attributed mainly to the rearr-angement of internal molecular forces which is cau-sed by longitudinal shrinkage due to stress relaxationand molecular disorientation, twisting of chain mole-cules due to change in conformation on conversionfrom cellulose I to cellulose II, and the transverse for-ces due to the change in the lateral packing of molecu-lar chains. In the partially crystalline cellulose struct-ure of jute, a low crystallinity along with the change inlattice from cellulose I to cellulose II may give bettercrimp curvature or frequency.

AcknowledgementThe authors are grateful to Dr S N Pandey, Direc-

tor, JTRL, for his keen interest in the work and encou-ragement.

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