Indian Journal of Textile ResearchVol. 6, June 1981, pp. 45-52

Electron Microscopic Studies on Cotton Knits Durable Press Finished byDifferent Techniques*

J VARGHESE & D M PRASADt

The Bombay Textile Research Association, Bombay 400086

W B ACHWAL

Department of Chemical Technology, University of Bombay, Bombay 400019&

K M PARALIKAR & S M BETRABETt

Cotton Technological Research Laboratory, Bombay 400019

Received 18 March 1981; accepted 12 April 1981

Two cotton knitted fabrics, one of honeycomb and the other of sinkerbody structure, were durable press finished withdimethylol dihydroxyethylene urea (DMDHEU) using three techniques, viz. pad-cure, moist-cure and wet-fix treatment withand without the polymeric stiffening additive Daikanol AMH in the treatment bath. Scanning and transmission electronmicrographs of the finished knitted fabrics showed that the finishing conditions employed in each technique have a profoundinfluence on the extent of swelling and uniform distribution of DMDHEU crosslinks in the fimshed fabrics. The moist-curefinished samples showed more uniform swelling and distribution of crosslinks as compared to the pad-cure and wet-fix treatedsamples. The use of the polymeric stiffening additive Daikanol AMH in the wet-fix treatment bath gave a mixed cross-sectionof both the collapsed and swollen structures with predominance of the latter.

Knitted fabrics have a tremendous technologicaladvantage over the woven fabrics in their speed ofproduction, and with the advent of synthetic fibrestheir share in the world textile industry has beenincreasing steadily. However, the share of cotton knitsis limited, especially in the field of outerwear garmentsbecause of their poor dimensional stability and themussy appearance after laundering and drying.Knitted goods made from synthetic fibres are free fromthese defects. Moreover, they are able to retainpressed-in creases and appear durable-press finished.To overcome these defects of cotton knits, efforts wereinitiated in USA to impart durable press finish tocotton knits without sacrificing their superior comfortcharacteristics and stretchability. Detailed in­vestigations have been carried out on the (1)use ofdimethylol ethylene urea (DMEU) and dimethyloldihydroxyethylene urea (DMDHEU) as crosslinkingagents t , (2)addition of polyvinyl alcohol andpolyoxyethylene glycols of high molecular weight asgrafting agents in' the treatment bath2, (3) merceri­zation prior to crosslin king to minimize the burstingstrength loss3, and (4)role of compressive shrinkage or

• Presented at the 22nd technological conference of ATIRA, BTRA& SITRA (2&3 February 1981).tPresent address: Textile Science Department, Purdue University,West Lafayette, Ind. 47907, USA.t Present address: The Bombay Textile Research Association,Bombay 400 086

some other induced shrinkage along with crosslinkingtreatment to improve the dimensional stability of thefinished cotton knits4. In all these studies, the standardpad-dry-cure technique was employed for impartingdurable press finish. Recently, the use of three finishingtechniques, viz. pad-cure, moist-cure and wet-fixtreatment (with and without the polymeric stiffeningagent), for finishing of two cotton knitted fabrics hasbeen reported5,6. It was found that the moist-curetechnique could impart good dimensional stability anddurable press (DP) rating with low bursting streng.thand Accelerotor weight losses. Water imbibition wasminimum for pad-cure finished samples, somewhathigher for moist-cure samples and highest for both thewet-fix treated samples which were close to the controlvalues. Thus, the state of swelling of the fibre at thetime of crosslinking has a profound influence on theperformance of these durable press finished cottonknits. Hence, it was thought worthwhile to examinethem under the electron microscope to find out theextent to which these varying finishing treatments alterthe surface and ultrastructural characteristics of thefibres and the eventual influence of these changes onthe performance of the finished cotton knits.

Materials and Methods

Two cotton knits, one of honeycomb and the otherof sinkerbody structure made of 18s yarns were used.The fabric construction details anll the physico-

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INDIAN J TEXT RES., VOL. 6, JUNE 1981

Table I-Construction Details and Physico-chemical TestResults of the Two Knitted Cotton Fabrics

• Mention: of a product by trade name does not imply anyrecommendation of the same by authors in preference to similarother products.

chemicaljtest results of the two knitted fabrics are givenin Table 1.These fabrics had undergone pressure scourin keir and sodium hypochlorite bleaching as perdetails given elsewhere 7. DMDHEU used was acommercial product with 43% solid content. DaikanolAMH*, the polymeric stiffening agent used in the wet­fix treatrpent bath, is an acrylic based water solubleproduct; lit was used as such.

DMDHEU finishing of the two cotton knit fabricst

was carried out using three techniques, viz. pad-cure,moist-cute and wet-fix treatment, with and withou't theadditioniofDaikanolAMH in the treatment bath. Forall the treatments, the concentration of DMDHEUused is dpressed on 100%solids basis. On the basis ofa previous study, the optimum concentration ofDMDHEU and other chemicals and the conditionsemployea for each technique were arrived at. Thedetails !are reported elsewhere7• F~r pad-curetreatment, bath containing 5% DMDHEU and20 g/Iitretmixed catalyst of MgClz.6HzO and NH4CIin the molar ratio of 1:1.3 was prepared. The knittedfabrics w.ereimmersed in the solution, padded througha mangle with an expression of 120% (taking care tohave minimum distortion of the fabric) and then put onpin frames with slight stretch to keep the fabric in tautcondition. The fabrics were then cured straight at160°C fQr 3min and soaped at 60°C with a solution ofsoap (2 g/litre) and soda ash (1 g/Iitre) for 20 minfollowed by hot and cold water rinses. Finally, thefabrics were centrifuged and line dried, For moist-curefinishing, the treatment bath containing 6%DMDHEU was adjusted to pH 1.5 with conc. HC!.The fabrjicsamples were padded with a wet pick-up of120%, fixed on pin frame with minimum stretch andthen dried at 80°C for 6.25 min in the case ofhoneycomb and for 5min in the case of sinkerbody soas to have 8-10% moisture content on both the driedfabrics. These fabrics were stored flat in sealed

Parameter

CuAm fluidity, poises-1Non-cotton content, % Owf*Absorbency, secWpit/CpitWeight/m2~ g

• Owf, on ihe weight of the fabric.tWpi, wales per inch.t Cpi, coarses per inch.

Honeycomb2.70.66

I16/11223

Sinkerbody2.70.83I

28/31161

polyethylene bags for 24hr and then rinsed well withcold water, neutralized with 2 g/Iitre sodium carbonatesolution, again washed and then soaped, centrifugedand line dried, as in pad-cure treatment. For wet-fixtreatment without the additive, the bath containing40% DMDHEU was adjusted to pH 1.0 with conc.HC!. The fabrics, after padding with the same pick-upas in moist-cure finishing, were stored flat in wetcondition in sealed polyethylene bags for 48 hI'.Rinsing, neutralization and soaping were carried outas per details given above. For wet-fix treatment withthe additive Daikanol AMH, the treatment bathcontaining 15%DMDHEU and 1.5%Daikanol AMHwas adjusted to pH 1.0 with conc. HC!. The fabricsamples were treated in the same manner as in the wet­fix treatment without the additive. All the fabric

samples were conditioned at 65% RH and roomtemperature (28°C) for 24 hI' prior to carrying out thephysical tests.

The following parameters Were measured, the testmethod used being given within parentheses:Cuprammonium (CuAm) fluidity (ASTM D-539-53),absorbancy (AATCC No. 79-1975), non-cottoncontent (AATCC No. 97-1975), bursting strength(ASTM D-231-61), DP rating (AATCC No. 124-1967)using hand-wash and line drying procedure, andAccelerotor weight loss (AATCC No. 93-1974) usingCarborundum Universal Liner No. 320 at a rotorspeed of 2000 rpm for 1.5min. The water imbibitionwas determined by centrifuging the water soakedsamples at 3000rpm for 5min on Roto-Uni centrifugeand then weighing before and after b~ne-drying. Thedimensional stability was determined by taking themeasurements before and after soaping from 25 cmmarks made on the fabrics in both the directions.

Soaping was carried out by a slightly modifiedAATCC method No. 135-1973 wherein a domestic

washing machine was used for laundering under thefollowing conditions: temperature, 60 ± 3°C;non-ionicdetergent, 1g/litre; material to liquor ratio, 1:20; totalload (with dummy), 1kg; and wash cycle, 5min. Thiswas followed by two rinses, each of 2min duration,with warm water (41 ± 3°C) and cold water. Nitrogenwas estimated by Kjeldahl's method.

For scanning electron microscopic studies, fabricsample (1 x 1cm) was mounted on a specimen stubmade conductive by coating with AujPd alloy andthen examined in Cambridge Sterioscan S-150. TheSEM was operated at 5 kV and the specimen was tiltedthrough 30°. For transmission electron microscopicstudies, ultra-thin cross-sections of the fibres from theknitted fabric were examined by the layer expansiontechnique using Hitachi HU lIE at 75kV acceleratingpotentia!.

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VARGHESE et at.: ELECTRON MICROSCOPIC STUDIES ON DURABLE PRESS FINISHED COTTON KNITS

Table2-Data on Knitted Fabric SamplesUsed for Electron MicroscopicStudies

Sample

Treatment Area shrinkageNitrogenDP ratingBurstingAccel. weightWaterNo.

%% Owft strength, psiloss. ~~imbibition, %-------- ~---,----- -------H*stHSHSHSHSHS

IControl --0.080.042.92.483l031.91.54241

2Pad-cure 761.221.063.93.642599.711.42125

(5% DMDHEU) 3Moist-cure I10.870.874.63.950743.92.42726

(6% DMDHEU) 4Wet-fix 231.121.104.74.053662.62.63537

40% DMDHEU) 5Wet-fix 440.640.654.63.860753.02.74043

(15% DMDHEU + l.5%) Daikanol AMH)* H, honeycomb; t S, sinkerbody; and t Owf, on the weight of the fabric.

I------='"

/

Results and DiscussionData obtained from various tests carried out on the

two knitted fabrics, finished with different con­centrations ofDMDHEU for different techniques, aregiven in Table 2 and the electron micrographs areshown in Figs I-II. Admittedly, these electronmicrographs are limited in the area of the sampleexamined. However, they are the typical repre­sentatives chosen from a series of electron micrographstaken for each sample.

Fig. 1 shows the scanning electron micrograph(SEM) of bleached honeycomb knitted fabric (control)at high magnification. The wrinkle on the fibre surfaceis clearly seen with a few compression hands. Thefabric has a DP rating of 2.9, bursting strength 83 psi,Accelerotor weight loss 1.9%, and water imbibitionvalue of 42% (Table 2). In contrast, SEM of the fabricfinished with 5% DMDHEU by the pad-curetechnique shows a highly crosslinked and collapsedstructure (Fig. 2). This is evident from the rod-likeappearance of fibres with much less swelling andsmoothening of the fibre surface with occasional inter­fibre bridging. The surface of fibres finished by moist­cure technique is very smooth (Fig. 3). Further, thiscrosslinking treatment has caused uniform fibre tofibre binding in the yarn of the knitted fabric with somedeposit of resin. Also, good swelling of the fibres,which results in more uniform adhesion of fibre tofibre, is observed, while in the case of pad-cure finishedsample, only occasional adhesion with much lessswelling is observed. Fig.4 shows the electronmicrograph of wet-fix treated sample wherein a largenumber of fibres in the yarn are found to be damaged.The surface of the fibres is badly eroded, whicheventually resulted in breakage of a large number offibres in the yarn. This fibre damage can be attributedto the prolonged storage under wet acid conditions (i.e.48 hr at pH 1.0) for the completion of crosslinking

Fig. I-Scanning electron micrograph of bleached honeycombknitted fabric (control)

Fig. 2-Scanning electron micrograph of honeycomb knitted fabriccrosslinked by pad-cure technique

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INDIAN J TEXT RES., VOL. 6, JUNE 1981

reaction of DMDHEU with cotton. The addition of

the polymeric stiffening agent, Daikanol AMH, in the

treatment bath reduced this damage considerably(Fig. 5). This is reflected in its higher bursting strengthof 60 psi as against 53 psi for the sample wet-fix treatedwithout this additive (Table 2).

Scannidg electron micrographs of sinkerbodyknitted fabric finished by various techniques showed asimilar trend as above, except that the extent ofswelling and inter-fibre bridging in pad-cum finishedsamples and the cementing of fibres in moist-cure

Fig. 3-Scanning electron micrograph of honeycomb knitted fabriccrosslinked by moist-cure technique

Fig. 4-Scanning electron micrograph of honeycomb knitted fabric'crosslinked by wet-fix technique

48

finished samples were of a much lower order. The

surface damage of fibres finished by wet-fix techniquewas also negligible and, therefore, the protective actiondue to the addition of the polymeric stiffening agentDaikanol AMH was also not visible.

Fig. 6 illustrates the typical cross-sectionalmorphology of untreated cotton fibre ext;acted from

'the knitted honeycomb fabric. It shows profuselayering and microfibrils at a higher magnification. Incontrast, the cross-section of pad-cure finished sample

(Fig. 7) shows a highly crosslinked a~d collapsedstructure. The ultrastructure of the fibres extracted

from honeycomb fabric finished with 6% DMDHEUby moist-cure technique shows a swollen and openstructure (Fig. 8). The distribution of cross links in thefibre finished by this technique seems to be moreuniform than that in the fibre finished bythe pad-curetechnique, which is also in agreement with theobservation made earlier from scanning electronmicrographs (Fig. 3). This is reflected in the higherbursting strength, better abrasion resistance andhigher water imbibition of moist-cure finished samplesas against those finished by the pad-cure technique(Table 2). Further, the fabric shows excellent DP rating(4.6) accompanied by negligible area shrinkage (1%)which shows the presence of effective DMDHEU

. crosslinks.

On finishing with 40% DMDHEU by the wet-fixtechnique, the fabric shows slight improvement inbursting strength and abrasion resistance, but markedincrease in water imbibition compared with either pad­cure or moist-cure finished samples. This indicates thatthe effective cross links are few and far "between. The

Fig. 5-Scanning electron micrograph of honeycomb knitted fabriccrosslinked by wet-fix technique with Daikanol AMH

VARGHESE et al.: ELECTRON MICROSCOPIC STUDIES ON DURABLE PRESS FINISHED COTTON KNITS

electron microscopic examination shows openstructure, especially at higher magnification (Fig. 9).The use of the polymeric stiffening agent DaikanolAMH in the wet-fix treatment bath reducesconsiderably the amount of DMDHEU required forobtaining satisfactory low shrinkage values onlaundering, as the polymer additive gets embeddedalong with DMDHEU crosslinks within the cellulosematrice and keeps the celh.ilose network in a moredistended swollen state. The electron microscopical

(b)

Fig. 6-Transmission electron micrograph by layer expansiontechnique of untreated cotton fibre extracted from honeycombknitted fabric [(a) low magnification and (b)high magnification]

examination of fibres extracted from this fabricrevealed two types of cross-sections, viz. openstructure as in Fig. 6 and highly crosslinked structureas in Fig. 7, the former being more predominant. It

may be noted that this fabric has high water imbibitionof 40%, which is very near to that of the control.However, from its higher area shrinkage value of 4%against 2% for the 40% DMDHEU wet-fix treated'~ample, it is evident that the effective cross Jinks arevery few and the yarn mobility is not restricted by the

(a)

(b)

Fig. 7-Transmission electron micrograph by layer expansiontechnique of fibre extracted from honeycomb knitted fabriccrosslinked by pad-cure technique [(a) low magnification and

(b) high magnification]

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INDIAN J TEXT RES., VOL. 6, JUNE 1981

presence of this polymeric additive, unlike in the case of

polyvinyl alcohol as additive in DMDHEU finishing ofcotton knits by the pad-dry-cure technique2.

The sinkerbody knitted fabric possesses a muchhigher bursting strength for both the control andtreated samples as compared to the respectivehoneycomb knitted fabric. Further, these sampleshave shown comparable values for area shrinkage andwater imbibition and definitely lower values for DPrating. Transmission electron microscopic observation

of fibres lextracted from sinkerbody knitted fabrics

(b)

Fig. 8-Transmission electron micrograph by layer expansiontechnique of fibre extracted from honeycomb knitted fabriccrosslinked by moist-cure technique [(a) low magnification and

(b) high magnification]

50

after various durable press finishing treatments did notshow marked differences as in the case of honeycomb·fabric, except in the case of wet-fix treatment whereinthe polymeric additive Daikanol AMH was used in thebath. This may be due to the tight knit structure of thesinkerbody fabric which makes the control of themoisture content of this fabric during the variousfinishing operations rather difficult: However, fromFig. 10, which shows a typical fibre cross-section ofsinkerbody fabric crosslinked by the semi-swollenmoist-cure process, it is evident that the fibres are neither

(b)

Fig. 9-Transmission electron micrograph by layer expansiontechnique of fibre extracted from honeycomb knitted fabriccrosslinked by wet-fix technique [(a) low magnification and (b) high

magnification]

VARGHESE el at.: ELECTRON MICROSCOP1C STUDIES ON DURABLE PRESS FINISHED COTTON KNITS

(a)

(b)

Fig. 10-Transmission electron micrograph by layer expansion

technique of sinkerbody knitted fabric crosslinked by moist-cure

technique [(a) low magnification and (b) high magnification]

in the completely collapsed state as in the case of pad­cure finished samples (Fig. 7) nor as swollen and openstructured as for the moist-cure finished honeycombfabric (Fig. 8). Thus, they may be considered aspartially swollen. From the above discussion it is clearthat the morphology of fibres of both the fabricssubjected to wet-fix treatment using the additiveDaikanol AMH is similar in most respects. The typicalultrastructural morphology of such fibres extractedfrom sinkerbody fabric is shown in Fig. 11.The fibre is

(a)

(b)

Fig. 11-Transmission electron micrograph by layer expansiontechnique of sinkerbody knitted fabric crosslinked by wet-fix

technique with Daikanol AMH [(a) low magnification and (b) highmagnification]

swollen with considerable open structure, though thefusion of lamellae due to crosslinking is perceptible,especially in the outer portions of the cross-sections.The polymeric additive Daikanol AMH seems to haveacted as a barrier to the diffusion of the reagents andthereby prevented them from reaching the centralregion of the fibre. The water imbibition values ofthese finished fabrics (Sample 5, Table 2) almostapproach their control values and thus tally with theabove electron microscopic observations.

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INDIAN J TEXT RES., VOL. 6, JUNE 1981

Conclus;ons(1) Scanning electron micrographs of honeycomb

knitted fabric durable press finished by pad-curetechnique show a typical crosslinked collapsedstructure with occasional inter-fibre bridging, whilethe moist-cure finished fabrics show good swelling offibres with more uniform adhesion of fibres alld fibre

to fibre bridging with some deposit of resin on the fibresurface. Wet-fix treatment without the polymericadditive shows that the surface of the fibres was badlyeroded, which eventually resulted. in breakage of alarge number of fibres in the yarn. The use ofpolymeric additive Daikanol AMH in the wet-fixtreatment bath lessened this damage considerably.

(2) Scanning electron micrographs of sinkerbodyfabric finished by various techniques showed a similartrend as above, except that the effect of variousfinishing techniques used was discernible to a lessere~tent than those observed in the case of honeycombfabric.

(3)Transmission electron micr9graphs of both thefabrics finished by various techniques showed thetypical collapsed structure of fibres in the case of pad­cure finished samples, semi-swollen and uniformlycrosslinked cross-section for moist-cure 'finished

samples, and an open swollen structure in the case ofwet-fix treated samples.

(4)The use of the polymeric additive DaikanolAMH in the wet-fix treatment bath shows a mixture ofboth collapsed and swollen structures with the

52

predominance of the latter for both the fabrics. Theoutside edge of the fibre showed some crosslinking,while the core of the fibre remained unaffected,showing that the polymeric additive has acted as abarrier to the diffusion of the reagents into the coreregion of the fibres.

Acknowledgement

This study was undertaken as a part of PL-480Project No. ARS-28 (USDA Grant No. FG-In-542)and the authors are grateful to the USDA authoritiesand to the Directors of the Department of ChemicalTechnology, University of Bombay, Bombay, andCotton Technological Research Laboratory, Bombay,for their keen interest.

References

1 Andrews B A K, Frick J G (Jr) & Reid J D, Knitter, 33 (1969) 26.

2 Andrews B A K, Frick J G (Jr) & Reid J D, Text Chem Color, 3(1971) 39.

3 Andrews B A K, Frick J G (Jr), Reid J D & Gautreaux G A, TextChem Color, 4 (1972) 21.

4 Varburg G B, Frick J G (Jr) & Reid J D, Knitter, 35 (1971) 25.

5 Pasad D M, Varghese J & Achwal W B, Am Dyestuff Reptr, 69(1980) 24.

6 Pasad D M, Varghese J & Achwal W B, Am Dyestuff Reptr, 69(1980) 38.

7 Pasad D M, Studyies infinishirtg, M.Sc. (Tech) thesis, Universityof Bombay, 1980.