Indian Journal of Fibre & Textile ResearchVol. 15, June 1990, Pp 59-64

Role of dye structure in improved dyeing of cotton with direct dyes inpresence of a redox system and influence of glucose in improving

direct dye uptake on cotton

G L Bhalla, J S Rawat & Arnita Malik

Shrirarn Institute for Industrial Research, 19, University Road, Delhi 110007, India

Received 14 September 1989; accepted 26 February 1990

The dyeing of a cotton fabric with direct dye in presence and absence of a redox system (potassiumpersulphate/ammonium persulphate as oxidant and glucose as reducing agent) as well as in presence of areducing agent (glucose) alone has been investigated. The dyeing properties studied are dye exhaustion,colour strength and wash fastness. The results of the study indicate that imptovement is dye dependentand varies with the chromophore of the dye, irrespective of the auxochrome groups. When a redox systemis used, the dyeing process proceeds via a free radical mechanism and a covalent bond is formed by thereaction of dye free radical and cellulose free radical. The increased life of free radical (dye or cellulose),due to stabilization by resonance, results in increased probability of reaction and thus improvement indyeing properties. Improvement is also observed when glucose alone is used since glucose increases theaccessible regions of the fibre to dyes. The probability of dye absorption thus increases due to the inter-action of dispersive forces and, to some extent, H-bonding with the cellulose substrate. The use of glu-cose for aftertreatment to improve the dye fixation is time-saving as well as economical.

Keywords: Cotton fabric, Direct dyes, Dyeing, Glucose, Hunter coordinates, Redox system

1 IntroductionThe dyeing of cotton/viscose with direct dyes

proceeds via simple hydrogen bonding of dye anionwith cellulose or through the interaction of Van derWaaI forces. The fabrics dyed with direct dyes showpoor wash fastness and low % dye exhaustion,which leads to wastage of dye.

The use of redox systems to induce polymeriza-tion onto cellulose and modified cellulose has beenextensively studied 1.2. However, their use in improv-ing the dyeing properties of such substrates is verylimited". After the initial study on dyeing of wool,nylon and blends of wool/polyacrylic with acid dyesproved to be successful">, it was extended to cottonwhich also showed positive results", This paper re-ports the effect of the use of a redox system as wellas a reducing agent alone in improving the directdyeing of cotton. The main aim of the paper is to in-vestigate the structural aspects of the systems stud-ied.

2 Materials and Methods

Mill scoured and bleached plain-weave fabric(weight, 102 g/m") having 32 ends/em and 20

picks/ em was used. The dyes used for the study aregiven in Table 1 along with their C'L Nos and struc-tures. All other chemicals used, viz. glucose, pota-ssium persulphate, ammonium persulphate and so-dium sulphate, were of laboratory grade.

Two sets of experiments were performed with theredox system (potassium/ammonium persulphate asoxidizing agent and glucose as reducing agent, in eq-uimolar concentration of 0.015 M) and with the re-ducing agent (glucose) alone (0.030 M). In the firstset, the fabric was dyed in a dye-bath (material-to-liquor ratio, 1 : 40) containing 2% dye (owf), 10%sodium sulphate (owf) and redox system for 1 h at70°C. In the second set, the redox system or reduc-ing agent was added in the dye-bath for fixation on-ly. The study on fixation was further extended forcomparing the results with those obtained by otherconventional fixing methods, i.e. use of fixing agent(Fixogen Universal Quality Product), formaldehyde,etc. The dyed samples were treated with 2% (owf)fixing agent and formaldehyde. The fixing agent wasapplied after cooling the dye-bath and keeping thefabric dipped for 10 min, while formaldehyde treat-ment was carried out at 70-80°C for 30 min with0.5% acetic acid.

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INDIAN J. FIBRE TEXT. RES., JUNE 1990

Table 1- Particulars 01dyu usedDye Nameof dye C.I.No.of dye Structure of dyeNo.

OH CH3Atul Viscose Dir~ct @-N=NL§:©.. .. ~N=N~Orange A Orange 108

NH-C-NHII S03Na0

OH OH

2 Cibo Dir~ct @-~N~.. .. ~N=N-@Orong~ SE Orange 26SO:!N NH-C-NH

a ~ S03NaOH

3 Atul Direct @-N=N--@-N=N~ .. ./§)Brilliant Dir~ct

Viol~t Extra Violet 9 S03No NH

OH

4 Atul Direct Dir~ct @-N-~ .. .@@rN=N-@Red 12B Red31

NH S03No

•• OH OH NH25 Atul ",NOO-N=N-@-@-N=N~

Dev~lop~d Direct

Block T Blu~2S0:3NQ

.. OC~ .. S03NoNH2 N~

6 Atul Dir~ct Direct @$TN=N-@-@-N=N1$@Bord~Qux BW Red 7

S03Na S03No

2.1 Analysis of SamplesThe first set of dyed fabrics was analyzed visually

with respect to change in shade. However, the sec-ond set of dyed samples was analyzed for % dye ex-haustion, colour strength (KIS), Hunter coordinates(L, a, b) and wash fastness as per the followingmethods:

2.1.1 Dye ExhaustionAll the dyes (except dye 5) were extracted from

the samples with a mixture (4 : 1) of dimethylforma-mide (DMF) and hydrochloric acid (5%). Due to theincomplete extraction of dye 5, the samples weredestroyed with 70% sulphuric acid. The absorbancewas noted on a lJV spectrophotometer and com-pared with that of the standard dye solution for ob-taining % dye exhaustion.

2.1.2 Colour Strength (KIS)and Hunter CoordinatesThe reflectance values of the samples were noted

60

for wavelengths ranging from 400 to 700 nm withan interval of 20 nm. The KIS values were calculat-ed at the wavelength where minimum reflectancewas observed using the Kubelka-Munk equation:

KIS= (1- R)22R

where R is the fraction of reflectance.

The Hunter coordinates L, a and b were calculat-ed? from the tristimulus values x, y, z using the fol-lowing equations:L= 10 yl!2

17.5 (1.02 x- y) 7 (y- 0.84 z )a = 1/2 b = 112

Y YThe higher values of a and b indicate brightness,

which is more due to redness and yellowness re-spectively, and the negative values indicate green-

BHALLA et al.: DIRECT DYE UPTAKE ON COlTON

ness and blueness which are.more towards the dullside. The lower the value of I!, the greater is thedepth.

2.1.3 Wash Fastness and StainingThe testing was carried out as per the specific-

ations laid down in colour index", The samples werestitched in an undyed cloth and subjected to vigo-rous shaking in a soap solution containing 5 g/l soapand 2 g/l sodium carbonate at 60°C in a launderome-ter for 30 min (material-to-liquor ratio, 1: 50).These samples were then washed and dried.The wash fastness was assessed by comparing

both the washed and unwashed cloths with the greyscale for the change in colour, and the staining wasassessed by comparing the stained cloth with thegrey scale as specified by the British Standard Insti-tute and approved by the Society of Dyers and Co-lourists.

3 ResultsOn visual assessment of the first set of fabric dyed

with redox system in the dye-bath.change in colourwas observed in case of dyes 4-6. No significantchange was observed in case of dye 1.The % dye exhaustion and colour strength of the

second set of dyed samples are given in Table 2,which show slight improvement in % exhaustionand KIS in the case of samples dyed with dyes 1 and2 in the presence of redox system. The improve-ment is less in samples dyed in presence of glucosealone. In the case of samples dyed with dyes 3 and 4there is a slight decrease in % exhaustion and colour

strength with redox system while with glucose alone,not 'much change is observed. For samples dyedwith dyes 5 and 6 the KIS values are higher in boththe cases but exhaustion shows a decreasing trend.These results are further substantiated by the

Hunter coordinates, the values of which are given inTable 3. The use of glucose increases the depth ofsamples which is comparabJc or slightly less thanthat obtained with redox system in case of dyes 1and 2. The values of a and b show no definite trend,suggesting that brightness, when observed visually,shows no significant change. In case of samples dyedwith dyes 3 and 4, in presence of redox system andreducing agent alone, the values of L show no signi-ficant change in the depth of samples. The bright-ness of the samples indicates that in the case of sam-ple dyed with dye 3, the increasing value of bandthe lowering value of a compensate each other. Butin the case of sample dyed with dye 4 the results in-dicate dullness on the basis of b values, although notmuch change is observed visually. In the case ofsamples dyed with dye 5 the L values are consistentwith the colour strength. Thus, with increased co-lour strength, the L values are lower, while in thecase of dye 6, the depth is lower as indicated by theL values. In the case of dye 5 the values of a and bare comparatively more with reducing agent, sug-gesting better brightness. Visual observations wereconsistent with the a and b values.The results of wash fastness and staining test are

given in Table 4. The samples dyed with dye 3showed the poorest wash fastness. The samples dy-ed in presence of glucose showed good wash fast-

Table 2 - Dye exhaustion and colour strength of samples dyed with and without redox system

Dye No. Dye exhaustion (%) and KIS values".Without redox With redox K2S2OHI With redox (NH4)2S20HI With only

system glucose glucose glucose

1 55.4 61.8 60.8 57.6(11.52) (15.68) (14.40) (12.35)

2 88.3 97.0 92.3 88.28(14.16) (14.70) (14.52) (14.17)

3 59.0 ~9.4 57.0 63.4(10.13) (9.03) (10.35) ( 11.52)

4 71.2 71.2 71.9 75.6(5.62) (5.44) (5.21) (6.30)

5 83.0 64.2 64.2 93.6(11.80) (15.20) (13.69) (16.57)

6 90.3 76.7 74.8 96.9(20.77) (20.75) (14.70) (20.70)

"The figures within parentheses indicate KIS values.

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INDIAN 1. FIBRE TEXT. RES., JUNE 1990

Table 3-Hunter coordinates of samples dyed with and without redox system I

Dye No. Hunter Without redox With redox With redox With onlycoordinate system K2S2OS/ (NH4hS20S/ glucose

glucose glucose

I L 51.6 60.0 50.1 50.7a 52.7 51.87 49.9 52.5b 26.27 26.96 26.11 26.55

2 L 51.5 49.8 50.8 56.8a 52.99 32.49 49.78 5~.26b -27.4 -26.63 -26.2 -27.5

3 L 28.3 27.9 27.2 27.0a 26.69 24.84 25.74 27.0b -24.11 -23.86 -18.01 -21.98

I·

4 L 45.6 45.7 45.6 45.0a 53.6 51.32 51.58 46.0b -6.29 -8.26 -7.77 -8.33

5 L 20.02 19.44 20.2 18.9a 2.22 .2.60 4.25 1.77b -8.52 -8.9 -14.02 - 11.61

6 L 25.8 26.8 35.3 23.8a 31.55 35.26 33.39 23.4b 3.85 0.56 0.63 2.93

Table 4- Wash fastness and staining of samples dyed with and without redox system

Dye No. Wash fastness Staining test

Without With redox With redox .• Glucose Without With redox !With redox Glucoseredox K2S2OS/ (NH4hS20S/ redox K2SZOS/ (NH~hS20K/system glucose glucose system glucose glucose

1 4 4 4 4 4 4·5 4·5 4-52 3 3 3 3-4 2-3 3 3 3-43 1-2 2 1-2 2 1-2 1-2 1-2 24 3 2-3 2-3 3-4 2-3 2-3 2-3 2-35 4 3 3-4 4-5 3 3 2-3 36 2 2-3 2-3 3 3 3 2-3 3

Table 5- Wash fastness and staining of samples dyed with andwithout aftertreatment

Aftertreatment Brill. Violet Extra Boredeaux BW

Wash Staining Wash Stainingfastness fastness

1-2 1-2 2-3 3

1-2 2

2 2 3 3

1-2 2 3 3-4

Nil

Formaldehyde +acetic acid

Glucose

Fixing agent(Fixogen)

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ness, which is comparable or slightly better than thatobtained with redox system. Similar results were ob-served in the case of staining scale reading.The comparative results of the fixation methods

for the dyes 2 and 6 are given in Table 5. The sam-ples treated with glucose showed improvement inwash fastness and minimum staining. Visually, thebrightness and depth of these samples was muchbetter.

4 DiscussionThe chromophore group of the dye is responsible

for its colour, which is further modified by the na-ture and number of auxochrome groups. As the aux-ochromic and chromophoric groups vary, the co-

BHALLA et al.: DIRECT DYE UPTAKE ON COTTON

lour of dye varies. The dyes selected for the presentstudy contained similar chromophores but differentauxochromes in different numbers. The results ob-tained are discussed on the basis of the nature of thechromophoric group.

The improvement in dyeing has been explainedon the basis of a free radical mechanism", which isdiscussed below:

2 e e.(j) S20e- -2504 • +H20 - HS04 + OH

2- e. 2-(ij) S20e +g-S04 + 504 + ge

( Glucose)(iii) C.II-H+R.- C.II. + R-H

e ••(Wh.r. R. = 504· ,g ,OH,.tc.)

(iv) O-OH + R.- 0-0. + R,-H

( Oy.)(v) 0-0.+ C"I·-C~II-0- 0

(C.llulos.-Dy. bond)

The reducing agent initiates the decomposition ofthe oxidizing agents present and generates free ra-dicals which react with the cellulose and dye to gen-erate cellulose free radicals and dye free radicals,providing additional reaction pathways whichshould lead to improvement in dyeing properties.This reaction will be further favoured by the in-creased stability of either cellulose free radical ordye free radical.

But in the case when reducing agent alone is used,improvement observed in most of the cases is dueto increase in the accessible regions of the fibre todyes as a result of the opening up of the cellulosestructure. The probability of dye adsorption thus in-creases due to dispersive force interaction and, tosome extent, H-bonding with the cellulose substrate.Improvement in properties such as wash fastnessand dye exhaustion can be attributed to the abovefactors.

Dyes 1 and 2 contain the chromophoric group- NH - C - NH -. Whatever be the auxochrome••:9:group, attack by a free radical results in the forma-tion of dye free radical, which is stabilized by the re-sonating structures:

-NH-C-NH---NH-C-NH--NH-C-NH-II .". .".:q: .q. .~.

1NH-C-NH-etc. II

:0;

The lone pairs are labile and can be stabilized.This increases the half life of the free radical andthus the probability of reaction with cellulose freeradical increases. The improvement in dye exhaus-tion, and hence KIS, lowers the value of L in case ofredox system.

In dyes 3 and 4, the chromophore group p~esentis - NH - group. The dye free radical ( - NH - )does not stabilize by itself. Thus, the availability ofdye free radical decreases as compared to that in thecase of samples dyed with dyes 1 and 2. This causesdecrease in % dye exhaustion and colour strength.Further, the oxidizing agent present oxidizes the dyeto some extent, resulting in decrease in colourstrength.

The dyes 5 and 6 do not contain any chromophoreother than the azo group that can form free radical.The auxochrome group - NH2 present in thesedyes is susceptible to oxidation. Thus, the oxidizingagent oxidizes the - NH2 group, resulting in changein shade. The colour strength is higher in the sam-ples dyed with dye 5 as the oxidized dye is associat-ed with the samples while in samples dyed with dye6 the colour strength is less as the dye associated isless. When reducing agent alone is used, the surfacearea available for dye adsorption increases due tothe opening up of the cellulose structure, the ad-sorbed dye being bound to this newly available sur-face through the operation of dispersive forces and,to a lesser extent, H-bonding. Thus, the % dye ex-haustion increases with the increase in wash fastnessof the dyed fabric and this is reflected in the experi-mental data. Some evidence for an increase inH-bonding between dye and cellulose has been ob-tained from the IR studies of dyed and undyed fa-bric where slight shift of band towards the lower en-ergy is observed. In dyes 1 and 2, the probability todye adsorption due to some H-bonding is slightsince these dyes have lesser availability of the lonepair. However, in dyes 3-6, dye adsorption due toH-bonding can increase due to the presence of- NH - group in dyes 3 and 4 and - NH2 group indyes 5 and 6. The experimental data on % dye ex-haustion and wash fastness for these dyes supportthe above conclusions. This method can, therefore,be used as an aftertreatment.

A comparison of the aftertreatment methodsshows that the improvement in dyeing with fixingagent and formaldehyde is dye dependent whilewith glucose, improvement is observed in most ofthe cases. The use of fixing agent and formaldehydechanges the shade in case of dyes having - NH2group. For example, in the case of dye 6 the - NH2groups react with formaldehyde and bring aboutbleaching of colour. In case of other dyes with dif-

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INDIAN J. FIBRE TEXT. RES .. JUNE 1990

ferent auxochrome groups, slight change in shade isobserved. The reaction takes place as follows:

H+- NH2+ HCHO ----<••.- N= CH2+ H20

This double bond formation results in bleachingof colour as it does not absorb in the visible regionof the spectrum.

The use of glucose for aftertreatment to improvethe fixation of dye is advantageous as the process iscarried out at the exhaustion temperature, whichmakes the process economical due to saving of la-bour, time and energy.

5 ConclusionsThe improvement in direct dyeing with the use of

redox system suggest that an additional free radicalmechanism operates which helps in the formation ofmore dye-cellulose bonds. Improvement in dye-ing, when glucose alone is used, is due to the open-ing up the cellulose structure, which increases thesurface area available for dye adsorption; the dye isbound by the dispersive forces and, to some extent,by H-bonding with the cellulose. The results indic-ate that the improvement in dyeing with the use ofredox system is dye dependent while with glucose,improvement is observed in most of the cases. Thus,

64

the use of glucose for aftertreatment is more econ-omical and easy compared to the conventionalmethods of aftertreatment.

AcknowledgementThe authors are thankful to the Department of

Science and Technology for sponsoring the work.They are also thankful to Dr J.K. Nigam, Director,and to Dr DA. Dabholkar, Joint Director, both ofSRIFIR, for encouragement during the work.

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(1985)15;132(1985)53.5 Dyeing of wool, silk and nylon with acid dyes by low tempera-

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(1986) 14.7 Wyszecki G & Stiles W S, Colour science: concepts and meth-

ods, quantitative data and [ormulas (John Wiley and SonsInc.) 1967,460.

8 Colour index (The Society of Dyers and Colorists and Am-erican Association of Textile Chemists and Colorists) 1956,4045.

9 Robert T 0 Connor, Instrumental analysis of cotton celluloseand modified cotton cellulose (Marcel Dekker Inc., NewYork) 1972,65.