IndianJournalofFibre& TextileResearchVol.20, March1995, pp.27-33

SEM and IR studies on scouring and bleaching of linen

K K GoswamiResearchand DevelopmentDepartment,Jaya ShreeTextiles,Rishra 712 249, India

andN N Das & A K Mukherjee

IndianJute Industries'ResearchAssociation,17 Taratola Road, Calcutta 700 088, IndiaReceived7 April 1993; revisedreceived26 May 1994; accepted12 September1994

Sequential removal of wax,water-soluble substances, pectin, hemicellulose and lignin afforded a fabricthat mainly contained cellulosealong with very small amount of non-cellulosic constituents. The whitenessindexof the resultant fabric (59.4) wasset as a target for commercial production of bleached linen fabricsandwas well achieved by appropriate selection of scouring formulation at pre-bleaching stage and by propermanipulation of bleaching sequences.The durability parameters could be maintained at a satisfactory levelby controlled defibrillation of the filaments and optimum retention of the non-cellulosic constituents. Theentire chemical process was monitored using a Cambridge SEM (Stereoscan 250) and 250-30 HitachiSpectrophotometer.

Keywords: Bleaching, Linen, Scouring, Scanning electron microscopy

1 IntroductionLinen requires a lengthy and expensive bleaching

operation to furnish a satisfactory level of whiteness I.

Although complete removal of lignin imparts fullwhiteness, the process is prohibitive for industrialapplication since the fibre practically disintegratesinto ultimate cells in the process- - 3. Hence, optimumretention of lignin, which acts as inter-cellularcementing material, and other non-cellulosicconstituents (NCs) is essential. But the removal of theundesirable impurities from the fabric surface prior tobleaching is equally important. Most of theseimpurities are removed by scouring with Na2C03,NaOH or mixed alkali solutions, and furtherinclusion of a wetting agent improves the scouringefficiency+ - 8.

Various formulations have been suggested forbleaching of linen and allied ligno-cellulosic fibres,making use of acidic/alkaline NaOCI, NaCI02 orH202 solutions 1- 3.7-13, but peroxide does notproduce satisfactory whiteness on linen withoutpretreatments 12. The single-stage bleachingprocesses usually result in low levels of whiteness'Pand hence, two-stage and multi-stage bleachingprocesses have been developed to achieve variousdegrees of whiteness7.9.lo.14.1s. Irrespective of themode of bleaching operation, linen encounters aconsiderable loss in tensile strength owing to thedissolution of NCs.

The present work is aimed at improving theavailable techniques for bleaching linen with a view toachieve full whiteness, without causing appreciabledamage to the fibre through optimum retention ofNCs.

2 Materials and Methods

2.1 Fabric and ChemicalsThe representative grey fabric was made from

Belgium variety (Grade T-240) of fibre and had thefollowing structural specifications: weave, plain;mass, 220 g/rn"; ends/dm, 150; picks/dm, 158; andyarn count of 25 lea (66 tex) in both warp and weftdirections. Unless stated otherwise, reagents ofanalytical grade (mostly products of BDH or E.Merck) were used.

2.2 Instrumental and Analytical TechniquesThe infrared spectra of the fabric samples were

recorded by KBr pellet technique, using 250-30Hitachi infrared spectrophotometer. The samples forstudying the morphological features were preparedaccording to a reported method 16 and thephotomicrographs were taken using a CambridgeSEM, model Stereoscan 250. The reflectance offabrics was measured using a Diano ColorscanSpectrophotometer (a product of Milton Roy)equipped with Jay Pak 4800~II software for compu-

28 INDIAN 1. FIBRE TEXT. RES., MARCH 1995

tation of colour. Tensile strength was measured in thewarp direction only with an lnstron strength tester(model 1026) using 3.75 cm wide strips and 15.0 emgrip length, and the values were corrected for changesin thread density. Cuprammonium fluidity (Cu-f)was measured following British Specification BS3090-1959.

The reflectance values were recorded at 10°observer angle for computing yellowness index (YI)and whiteness index (WI) and at 2° for redness index(RT), using the following equations!":

YI = (128 X -106 Z)/Y, · .. (1)

where X, Y and Z are the tristimulus values.

WI = L-3b · .. (2)

where L = 10 Y and b 7(Y -0.847 Z)/Y

· .. (3)RI ;::::R655 - R535

2.3 ScouringThe fabrics were scoured with (I) Na2C03( 109/I)

and NOPCO 1535, a proprietary surfactant (2g/I),and (2) Na2C03 (lOg/I), NaOH(2g/l) and samesurfactant (2g/I), keeping the fabric-to-liquor ratioat 1:30(w/v), at 95± 1°Cfor 30rnin incase of(l) and for2 h in case of(2). Scoured fabrics were neutralized withdil. acetic acid and washed successively with hot andcold water.

2.4 BleachingThe scoured fabrics were subjected to single-stage

bleaching by NaOCI (B), NaCI02 (B2) or H202 (B3)and sequential 2-stage (B2-B3) and 3-stage(B2-B)-B3) bleaching. The single-stage bleachingconditions are given in Table 1. Identical procedureswere adopted for individual operations inmulti-stage bleaching. The concentrations ofbleaching agents in the stock solutions wereestimated 19 before use. Each treated fabric wasair dried, manually flattened and conditioned (67%RH; 27"C) prior to experimentation and evaluation.

3 Results and DiscussionThe quality parameters of grey and treated linen

fabrics are given in Table 2. The grey fabric shows ahigh degree ofYI and RI and a poor WI. Sequentialremoval of wax (T-I), water-soluble substances(T-II) and pectins (T-III) from the fabric results inmarginal improvement in WI, while subsequentremoval of alkali-soluble substances (T-IV) resultsin a significant increase in WI. The change may beattributed to the partial removal of lignin along withhemicellulose. Remarkable improvement in WI isobserved upon delignification through T-V. This isaccompanied by a sharp decrease in both YI and RI, asteep rise in cuprarnmonium fluidity and a cumulativeloss of '" 50% in tensile strength. Thus, extensiveremoval ofNCs produces a high degree of WI ( '" 75%with respect to white standard tile, Tables 2 and 3), butthe process causes intensive defibrillation of thefilaments and excessive degradation of cellulose as

Table I-Bleaching conditions

Bleaching conditions

BiChemicals" Sodium hypochlorite

(av. C12, 4.0%)

B~

Hydrogen peroxide(4.0%)

Sodium silicate(84"Tw, 1.0%)

Sodium carbonate(1.0%)

Sodium chlorite(av. C12• 6.0%)

Reaction parameters"Temperature, ·C 40 85 95pH 9.5-10.0 10.2 3.Sf

Duration, min 120 90 60a Bleaching followed by antichlor treatments? and water' wash.b Bleaching followed by successive washes with hot and cold water and then neutralization by acetic acid.C Bleaching followed by neutralization with sodium carbonate and water wash.d Concentrations expressed on the weight of fabric.e As per optimized conditions!".f Adjusted by the addition of formic acid and tri-sodium phosphate .

•

29GOSWAMI et al.: SCOURING AND BLEACHING OF UNEN

Table 2-Quality parameters of linen fabrics treated with different chemicals

Treatment Reflectance" Weight Cuprammoniumloss fluidity%

Loss in tensilestrength", %

Sample No.

YI

46.5± 1.543.6± 1.738.2± 1.537.4± 1.23I.0± 1.7

12.9±0.93S.0±0.5

34.6±0.5

WI

11.4± 1.612.6± 1.615.7 ± 1.313.0±1.222.7± 1.259.4± 1.820.1 ±O.S21.6±0.2

RIGrey fabricsTo NilT, T-IT2 T-IIT3 T-IIIT4 T-IVr, T-VT 6 Scouring recipe IT 7 Scouring recipe 2Scoured fabricsT6T6T6T6T6T7T7T7T7T7White std.tile

a Av. of 10 readings (± indicates 95% confidence interval).b Av. of 5 readings.T-I -Soxhletting(6h)with benzene; T-II-Refluxing(6h) with water;·T-III- Refluxing(6h) with 0.5% ammonium oxalate; T-IV-Refluxing (6h) with 2.0% NaOH (refs 25 & 26); and T-V - Delignification through a series of treatment in tendem (ref.22).

II.3±0.411.3±0.6

9.3±0.59.9±0.58.4±0.33.7±0.1

1O.0±0.19.8±0.2

o0.66.51.9

14.74.59.5

16.0

Nil2.702.832.953.42

12.272.002.20

48.4Control

11.8

B)B2B,

B2- rinse - B3B2-B,-B3

B3B2B,

B2- rinse - B3B2-B,-B3

27.S±0.524.7±0.925.6± 1.817.6±0.313.2±0.115.7±0.222.6±0.323.2±0.216.3±0.412.5 ±O.II

6.3

37.0±0.642.0± 1.342.9±0.2.53.8±0.262.7±0.34O.6±0.344.0±0.645.0±0.155.8±0.563.1 ±O.I

79.4

12.1±0.211.6±0.49.4±0.9

5.3±0.13.1 ±0.2

1O.8±0.21O.8±0.39.3±0.95.1 ±0.13.I±O.1

-0.4

10.811.8

12.215.416.817.218.118.521.522.8

2.842.954.865.11

7.493.103.228.795.37

11.40

11.512.214.018.724.322.121.6

27.727.336.2

Table 3-Chemical composition of linen at various stages of bleaching process

Composition, %SampleNo.

WI Wax' Water-soluble Pectin" Hemicellulose" Lignin"substances"

(A) (B)

11.4 0.62 6.5 1.88 14.0 4.53 2.6059.4 0 0.8 0 3.8 2.01 0.26

(100) (87) (100) (74) (56) (90)20.1 0.59 5.4 1.22 13.0 4.44 2.55

(5) (17) (35) (12) (2) (2)21.6 0.56 2.9 0.82 10.0 4.37 2.53

(10) (55) (56) (32) (4) (3)

Grey & scouredToTs

Scoured & bleachedT6 (B2- B3) 0.84

(68)0.87(67)0.82(68)

• 0.80(69)

0.48 3.1 0.77 10.5 2.54(23) (52) (59) (29) (44)0.46 2.8 0.73 11.5 2.16(26) (57) (61) (22) (52)0.38 2.2 0.70 7.4 2.52(39) (66) (63) (50) (44)0.28 1.8 0.66 7.1 2.11(55) (72) (65) (52) (53)

a Determined by soxhletting with benzene (6h). C Determined by refluxing (6h) with 0.5% ammonium oxalate.b Determined by refluxing (6h) with water. d Determined by refluxing (6h) with 2% NaOH (refs 21&22).

e Estimated by (A) a series of treatments in tandem according to Turner= with some modificationss-: and (B) H2S04 method->,Figures in parentheses indicate removal efficiency (11) of non-cellulosic constituents (NC)11 = [100 - (%NC in sample/%NC in grey fabric) x 100]

53.9

62.7

55.8

63.1

30 INDIAN J. FIBRE TEXT. RES., MARCH 1995

evident from its cuprammonium fluidity and loss intensile strength. The inference is in conformity withthe morphological changes observed during thesetreatments. The SEM photomicrograph of grey linen(Fig.1A) shows that the ultimate cells are tightlybound to one another by intercellular substances.Deposits of impurities are clearly visible on thesurface of filaments. These impurities might haveoriginated from the residual cell wall debris thatremained adhered to the fibre during the preceedingprocess-" of fibre extraction. The fabric surface showsgradual improvements in T I through T 4 as the surfacebecomes more smooth and free from deposits.However, the filament separation in T4 (Fig.IB) ismore prominent, presumably due to the partialremoval of intercellular encrusting materials.Removal oflignin to the extent of90% in T5 causesexcessive decementation and promotes in profounddefibrillation and severe damage to the surface(Fig.IC).

Notwithstanding the physico-chemical changes,the WI of T5 was set as a target in the subsequentstudies. The T-I - T-V sequence is not a commer-cially viable proposal because of (a) high costinvolvement, and (b) excessive deterioration in thedurability parameters. Hence, the subsequent studieswere focussed on manipulation of the chemicalprocessing to achieve the target WI, without causingdamage to the textile properties to an appreciableextent.

The inherent impurities hinder the reactions atbleaching stage and, therefore, are removed byscouring. The treatment not only minimizes impuri-ties but also facilitates better penetration of thebleaching agents. Scouring with recipes (1) and (2)solubilized 9.5°11.and 16% of NCs respectively.Although the second recipe slightly improves thewhiteness (Table 2), the fabric suffers greatercellulosic degradation, as evidenced from thecuprammonium fluidity values. The enhanceddegradation may be attributed to the prolongedexposure of the fabric at higher alkalinity. Thephotomicrographs of T 6 and T 7 (Figs 1D and 1E)reveal that both the scouring recipes are equallyeffective in removing the non-fibrous impurities,since the deposits of these impurities, which werefrequently visible over the filament surface of theparent fabric, are conspicuously absent in the scouredfabrics.

Application of single- and multi-stage bleachingsequences on scoured T 6 and T 7 substrates furnishedtwo separate series of fabrics. The efficacy ofbleaching in both the series, as envisaged from the WI,increases in the order B3 < B2 < BI < B2-B3

< B2-B1-B3, while the YI and RI increase in thereverse order. Increase in whiteness is accompaniedby increase in weight loss and cuprammonium fluidityvalues and decrease in tensile strength. Among thesingle-stage bleaching processes, the hypochloriteaffects the durability parameters most severely ..

A careful scrutiny of the data reveals that the initialscouring parameters have strong influence indetermining the properties of the fabric at the finalstage of bleaching process. The initial difference inwhiteness between T6and T 7 is carried over in all thebleaching processes, except in the B2-B1-B3sequence which gives fabrics of comparablewhiteness with either substrate. Samples ofT 7 seriessuffer greater loss of NCs (~17-23%) than thecorresponding samples ofT 6series ( ~ 10-17%). Thisphenomenon may be attributed to the enhancedopenness of the filaments in T7, which, in turn, makesthe reagents more accessible to the reaction sites. Theaugmented reactivity ofT 7 has detrimental effect onthe durability parameters of the bleached fabrics. Thecomparative values of cuprammonium fluidity in T 6

(2.8-7.5) and T 7 (3.9-11.4) series indicate a highertrend of cellulosic degradation in the latter series. Thisdegradative action has direct bearing on the tensileproperties and the observed tensile data agree wellwith the contension. The loss in tensile strength ofT 7

series is much higher (22-36%) as compared to that inT6 series (11.5-24%). The most important outcome ofthis investigation is that a whiteness exceeding thetarget value (59.4) could be achieved by theapplication ofB2-B1-B3 bleaching sequence, usingeither T6 or T 7 as substrate. The whiteness just fallshort of the target value in the two-stage bleachingsequence. Even then the process may be adopted toderive satisfactory functional properties by using T 7

and Br-B3 bleach combination sequence.

The surface characteristics of T 6 and T 7 samplessubjected to 82-B3 bleach combination sequence(Figs 1F and 1G respectively) are quite satisfactorysince none of them shows any sign of damages. TheSEM photomicrograph of T6 sample subjected toB2-B1-B3 bleach combination sequence (Fig.1H) issatisfactory in all respects, while the correspondingsample derived from T 7 shows surface damage atvarious places (Fig. I I). Considering all these aspects,the scouring recipe 1 has been found to have thefollowing advantages:

(i) The duration of scouring treatment per batch issubstantially low (30 min) compared to that for recipe2 (120 min) and for other formulations6-7 (180 min).The alkali consumption is also significantly low andhence causes reduction in the overall process cost.

GOSWAMI et al.: SCOURING AND BLEACHING OF LINEN

Fig. l-SEM photomicrographs or' differentlinen fabrics [A-Grey (To); B-Alkali extractediT.); C-DeJignified (T,); D-Optimallyscoured (To); E-Conventionally scoured (T7);F- To bleached with B, - B.1 sequence; G- T7bleached with B2- B3 sequence; H- T"bleached with B2- BI - B, sequence and 1-T,

bleached with B, - BI - B3 sequeuce

31

32 INDIAN J. FIBRE TEXT. RES., MARCH 1995

4~~--~--~--~~~~~--~~--~~~o 3500 3000 2500 2000 18001600 14001200 1000100 600 400

WINE NUMBER ,r.m-I

Fig. 2-Infrared spectrum of grey linen

5WAVELENGTH, J.Jm

6 7 8 9 10 11 12

2000 1800 1600 11,00 1200 1000 800WAVE NUMBER, em -1

Fig. 3-lnfrared spectra of treated linen fabrics [l-Delignified(T 5); 2-Optimally scoured (T 6); ~ptimally scoured (T 6) andbleached with B2- B. - B3 sequence; and 4-Conventionally

scoured (T 7) and bleached with B2- B. - B3 sequence

(ii) Either B2-B3or B2-B)-B3 sequence givessatisfactory whiteness without causing appreciabledeterioration of the durability parameters throughretention of NCs to a greater extent.

The entire process of scouring and bleaching wasalso monitored by infrared spectroscopy. The IRspectrum of grey linen (Fig.2) shows the characteristicbands of a typical ligno-cellulosic substance. Thenotable absorption bands are: 3400 em - 1 for O-H,2900 em -1 for C-H, 1732 em -1 for C = 0 stretchingvibrations, and 1620-1680 em - 1for aromatic skeletalvibrations, The absorption bands below 1500 cm - 1

are highly superimposed and do not provide usefulinformation about various functional groups. Thenature and intensity of absorption bands at 3400em - rand 2900 em - 1 did not change appreciably afterscouring or bleaching treatments since cellulose wasretained as a major component in the treated fabrics.The free or esterified carboxylic acid groups give riseto absorption band at 1732 em - 1 for the grey fabric.This functional group probably originates from thepectin component or lignin-carbohydrate esterfunctions or both. The latter type of functional grouphas been shown to exist in allied vegetable fibres> - 26

and hence its presence in linen appears to be highlyprobable. The scouring treatment ruptures the esterbonds and neutralizes the free carboxylic acid groups.The liberated carboxylate anions absorbed stronglyin the region of 1550-1610 em - 1and 1300-1400 em -1

and hence could not be identified separately due tostrong interference of absorption bands alreadypresent in that region in the IR spectrum of the greyfabric. From the hemicellulose and lignin contents ofthe bleached fabrics (Table 3), it is apparent that thesecomponents are retained to various degrees in all thesamples. In spite of this, the band at 1732 cm -1 wasabsent from all the samples, indicating that thecarboxylic acids exist as carboxylate anions in thebleached fabrics (Fig.3).

4 ConclusionsFor a given set of bleaching sequence, the scouring

parameters at pre-bleaching stage playa key role indetermining the ultimate whiteness and durabilityparameters of the linen. The optimized scouringformulation 1 has been found to be better in thisrespect. Extensive removal of the NCs produced awhiteness of >- 7.5% w.r.t. the standard white tile andthis was set as a target value for industrial application.The single-stage bleaching always produced lowlevels of whiteness (-47-57%), irrespective of themethod of scouring and the nature of bleaching agent.Application of B2-B)-B3 combination bleachsequence on fabrics scoured by either recipe I or recipe2 produced a whiteness of '" 79%, which wellexceeded the target value, but the loss in tensilestrength and cellulosic degradation were less in thecase of optimally scoured fabric. Depending on the

GOSWAMI et al.:SCOURING AND BLEACHING OF UNEN 33

end use, the B2-B3 sequence may also be utilized toachieve a whiteness of -68-70%.

AcknowledgementThe authors gratefully acknowledge the financial

support received from the management executivesMr G L Moondra, Mr A N Choudhury and Mr H COaga and the technical assistance received from Mr AChakrabarty, Mr S Kar, Mr S Mukherjee and Mr BGhosh, all of Jaya Shree Textiles. They are alsothankful to Mr 0 K Sinha of Institute of JuteTechnology, Calcutta, Mr S. Varadrajan ofBTRA,Bombay, and Dr A K Mukhopadhyay of IJIRA,Calcutta, for technical assistance.

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