Indian Journal of Fibre & Textile Research Vol. 28. December 2003 , pp. 437-443

Combined desizing, scouring and bleaching of cotton using ozone

M Prabaharana

Department of Textile Technology, Anna University, Chennai 600025, India

and

J Venkata Rao

Northern India Textile Research Association , Ghaziabad 20 I 002, India

Received I AI/gl/st 2002; accepted 28 October 2002

An attempt has been made to desi ze, scour and bleach grey cotton fabric simultaneously using ozone. The mechanical. chemical and dy.:ing properties of the ozonized fabric have been compared with those of the fabric subjected to conventional desizing, scouring and hydrogen peroxide bleaching. To achieve a high degree of whiteness with minimum damage to cellulose, a two-stage process is suggested wherein the ozonized fabric is further treated with hydrogen peroxide. An acceptable degree of whiteness (ready for dyeing) can be obtained by using ozone in a vcry short time. Thi s process has additional advantages such as savings in thermal energy, water and chemicals.

Keywords: Bleaching. Cotton, Desizing. Dyeing. Scouring

1 Introduction The impurities present in grey cotton fabric are

sizing ingredients, fat, waxes, pectins and natural colouring matter. Efficient removal of these impurities during grey preparation is essential to guarantee proper dyeing, printing and finishing processes. Till today, the most commonly accepted sequence of operations for cotton grey preparation is acid or enzyme desizing, alkali scouring and hypochlorite or hydrogen peroxide bleaching. This sequence of operations is time consuming and needs a large quantity of water, energy and a variety of chemicals. Therefore in the past, different approaches have been made to simplify cotton grey preparation. One approach was to carry out all the three processes simultaneously (combined process). Some of the combined processes proposed are: • A combined desizing, scouring and bleaching of medium and fine varieties of cotton fabrics was sug­gested by Parikh i

, which involves the use ofDTPA as stabi lizer for hydrogen peroxide. • BTRA developed a single-stage process in kier which is claimed to produce high degree of whiteness with minimum damage to cellulose2

.

"To whom all the correspondence should be addressed. Phone: 510330; Fax: 00351- 0253-510339; E-mail: mprabaharan @yahoo.com Present address : Department of Polymer Engineering, University of Minho, P-4800-058 , Guimaracs, Portugal

• Sahakari3 worked out a process which is based on solvent-nonionic emu lsion system known as ScoUl'ex process.

• Gulrajani et al. 4 suggested a solvent-assisted aq ue­ous process in which sodium hydroxide (scouring agent) has been replaced by a solvent-nonionic sur­factant-p ine oil combination along with hydrogen peroxide.

Other combined processes reported in the literature use tetrapotassium perhydroxy phosphate (KPP). sodium dipersulfate (SPS) and various combinations of sequestering agents, wetting agents and perox ide stabilizers5

. In almost all these combined processes. the benefits of combining the processes are generally derived at the expense of some quality. Hence. they were accepted by the industry with a lot of reservations.

The increasing demand for the conservation of natural resources and environmental protection has forced the researchers to look for the processes which can be carried out at a low temperature using small amount of water in a short duration without the use of harmful chemica ls such as hypochlorite. To fulfil these objectives, attempts are now being made to bleach cotton using ozone, ozone-UV radiati on. peracetic acid and enzymes. Ozone being a powerful oxidizing agent can bleach cellulose much faster than hydrogen peroxide and hypochlorite. Today. it is

438 INDIAN J. FIBRE TEXT RES .. DECEMBER 2003

be ing used , to a large extent, as a substitute for chlorine in pulp bleaching6

. 7. However, in cotton bleaching it is still at the infant stage. In 1995, two patents have been awarded for bleac hing of cotton wi th ozone. These patents claim a continuous process using low-temperature plasma for des izing and scouring and ozone in the presence of UV for b eac hing8

. 9. Takahashi and Kai ga lO have studied the e 'fect of ozone concentration and trea tment ti me on tlJe properties of bleached fabric s. It is expec ted that in a few yea rs from now an enzyme cocktail will be developed which will perform continuous des izing, scouring and bleaching of cotton II. The ad vantage of using enzymes is that it is ecofriendl y and saves sign ificant amount of energy.

In the present work, an attempt has been made to combine a ll the three grey preparatory processes, such as desiz ing, scouring and bleaching. Here, ozone is used to des ize, dewax and decolour the grey cotton fabric. To improve the fabric prope rti es, two-s tage bleac hing is sugges ted wherein the fabric is treated with ozone followed by hydroge n perox ide.

2 Materials and Methods 2.1 Materia ls

Commercial grey cotton fabric (ends/in , 134; pickslin , 78; and we ight, 139.6ghr?) sized with native starch was used for the study. The che micals and dyes (monochl orotriazinyl and dich lorotri azinyl) used were of ana lyti ca l and commercia l grades respec tive ly.

2.2 Apparatus T he equipment used fo r the bleaching with ozone

has three components: the ozone generator, the appl icator and the ozone destroyer. The ozone generator of 8g/h capacity was supplied by Ozone Tek Ltd , India. The input for the generator is oxygen from a pressurized cylinder. The generator supplies the required concentration of ozone - oxygen mixture to the applicato r. The applicator is a glass cylindrical tube wi th a diffuser at the bottom. The gas mixture is pumped continuously at the required flow rate into the app li cator through the diffuser. The wet fabric sa mples are suitably placed in the applicator for the required time. The spent mixture is passed through the ozone destruction unit which has a heating e lement, where the ozone is completely converted into oxygen be fore be ing re leased into the atmosphere. An ozone analyzer is mounted on the applicator to measure ozone concentration both at the entry and the exit of the appl icator.

2.3 Procedure

Wet grey cotton fabrics having -24% mo isture content (wet pickup) were placed in the applicati on chamber and ex posed to 100 g/m3 concentration of ozone-oxygen gas mixture at pH 5 us ing acetic acid for a specified time (1- 7 min) at room te mperature

(about 30°C) for the combined desi zing, scouring and bleaching. The gas mixture flow was maintained at a constant rate of 0 .5 litre/min for the all ex periment s.

Two-stage bleaching of grey cotton fabric s was first carried o ut with ozone and then w ith hydrogen peroxide. T he wet grey fabric s with -24% moisture content were first bleac hed with ozone (lOOg/m ' ) at

pH 5 for I , 2 and 3 min and washed. These fabric s were the n bleached with hyr.j '(ogen peroxide for 45. 30 and 15 min respect ive ly using standard procedure. A ll the bleached sampl es were then hot soap washed at 80°C, cold washed, dri ed and fina ll y condi tioned before testing for the properties.

Grey fabrics were a lso acid des ized , a lka li scoured and bleached with hydrogen perox ide and cal cium hypochlorite usi ng standard procedures.

2.4 Tests The bleached samples were tes ted for whitene s

index (W I) using the following Harri son eq uati on l 2:

WI = 100-(R67o-R.l3o)

where R represents the refl ec tance va l ues at 670 and 430 nm.

The pe r cent strength loss and pe r cent change in e longati o n-at- brea k were determined by the te ns ile tes ting method according to Indian standards procedure 13. The ex tent of che mi cal modifi ca ti o n was measured in terms of aldehyde groups (copper number), carboxyl groups, fluidit y and degree of polymerization us ing Indi an standards procedures l

.l

1( •.

The per cent we ig ht loss was measu red by weighing the conditioned samples before and afte r bleac hing l 7

The per cent dewaxing was measured by sox hlct ex traction 18 and the absorbency was tested accordi ng to AA TCC test method 39- 1980.

In o rder to assess the dyeing properti es of ozone bl eached sa mpLs, the fab ric s w ith WI rang in g fro m 80% to 90% were separately d yed to medium shade (2. 5% owm) w ith two types of reacti ve d yes. The dye ing proced ures were as spec ifi ed by the dye man ufac turers . T he co lour y ie ld of the dyed sa mples was eva lu ated using a Hitachi spectrophoto meter LJ-3210 and by app lying the fol lowing Kube ika-Muilk

• 10

eq uat Ion -:

f

PRABAHARAN & RAO: COMBINED DES IZI NG, SCOUR ING AND BLEACH ING OF COTION 439

KIS= [(l-R/ 12Rl - [(l-Ro)2/2Rol

where, R is the decimal fraction of the reflectance of dyed fabric ; Ro, the dec imal fraction of the reflectance of undyed fabric; K, the absorption coefficient; and S, the scattering coefficient.

3 Results and Discussion

It is reported in the li terature that the quality of ozone bleached cotton fabric depends on ozone concentration , treatment time, pH and the amount of moisture present on the fabric lO

, 19, 20 . The degree of whiteness and the extent of mechanical and chemical damages to cellulose are fo und to increase with the increase in either ozone concentration or treatment time. However, the latter is found to have more detrimental effect than the former. The results indicate that the quality of ozone bleached fabric could be brought in line by using the hydrogen perox ide bleaching, provided that the ozone bleaching is first carried out in a very short time wi th hi gh ozone concentration . It is also reported that the presence of moisture during bleaching has a very significant effect on the degree of whiteness obtained and the rate of bleac hing. It is found that the 8% moisture content (standard moisture rega in) has very little bleaching effect, but it increases very rapidly as the per cent moisture content increases up to 24%. A further increase in moisture content (up to 80%) retards the bleaching act ion and thereafter the amount of moisture present seems to have no effec t on the time taken for bleaching. The pH also plays a significant role in decid ing the degree of whiteness obtained and the rate of bleaching. It is found that wi th the increase

in pH from 2 to 9 there is a marginal decrease in WI; a further increase in pH up to 12 results in a ve ry rapid decrease in WI. Based on these findings, the present inves tigation has been carried out where in the grey cotton fabric with 24% moisture content is bleac hed with 100 g/m3 ozone at pH 5. The properties of ozoni zed fabrics with those of fa brics processed by conventiona l methods have a lso been compared.

Table 1 shows the properties of ozone bleac hed fabrics. It is observed that with the increase in treatment time there is an increase in WI, strength loss and ce llulose modification (in terms of copper number, carboxy l group, fluidity and degree of polymerization) of the bleached fabrics. For an acceptable degree of whiteness (84%) to dye medium shades, the strength loss of ozone bleached fabr ic is same as that of hydrogen peroxide bl eached fabric . However, when WI is further increased , the strength loss is least for hydrogen peroxide followed by calci um hypochl orite and ozone. Table 1 also shows the change in e longati on-at-break of ozone and conventionally bleached fabrics . There is no change in e longation-at-break when fa bric is treated with ozone, while it is significantly high for conventionall y processed samples. The reason for this might be due to the fact that the latter have undergone a number of hot-wet treatments for a long period during desizing. scouring and bleac hing. During such treatments. the fabric is prone to shrink considerably, resulting in high crimp and thread dens ity which leads to high e longati on. On the other hand , ozone bleached grey fa brics are subj ected to onl y a hot soap wash and a cold wash for a short duration. The changes in fabric construction parameters due to the wet process ing

Table 1- Properties of fabrics bleached with ozone and conventional methods

Property Fabric bleached with

Ozone Hydrogen peroxide Calcium hypochlorite

I" 3" Sa 7" 30" 40" 50" 60" 30" 40" 50" 60"

Whiteness index 80 84 88 90 80 84 88 90 80 84 88 90

Strength loss, % 14 16 23 27 14 15 17 18 14 17 2 1 23

Elongation-at-break. % 0 0 0 40 44 49 5 1 39 42 49 5 1

Weight loss, % 5 6 6.5 7 11 II II II II 11 11 11

Dc-wax ing, % 24 31 35 38 57 57 57 57 57 57 57 57

Copper No. 0 .12 0.27 0 .36 0.49 0 .04 005 0 .06 0 .07 0 .04 0.05 0 . 17 0 .24

Carboxyl con tent 5.38 6.00 6 97 8. 15 5 .19 5.59 5 .75 6. 10 5.65 5.85 6.5 1 7.2 1

Fluidity 4. 18 5. 13 7.80 12.0 3. 10 3.25 3.65 4.25 3.95 4.8 1 (135 7.15

Degree of polymerizati on 2 150 1965 1654 1308 2345 2325 226] 21 15 2259 2085 1795 1691

"Treatment time (min)

440 INDIAN 1. FIBRE TEXT. RES., DECEMBER 2003

Table 2 - Change in fabric construction parameters due to processing

Parameter Fabric Grey -= __ -=-_B::.1:.::e=:ac:.:.:h:.::.ed=---___ _

Ozone Peroxide Hypochlorite

Endslin 134 134 135 135

Picks/in 78 78 85 85

WJrp crimp, % 17 18 21 21

Weft crimp. % 7 7 7 7

Weight. g/m2 139.6 130 141.2 141.2

operations of hydrogen peroxide, calcium hypochlorite and ozone bleached samples are given in Table 2.

The loss in weight after bleaching is an indication of the removal of impurities, such as sizing ingredients and natural waxes. During bleaching, ozone reacts with cellulose as well as non-cellulosic constituents such as starch and waxes present in the fabric. Chemically starch is poly a-glucopyranose in which straight chain (amylose) and branched chain (amylopecti n) polymers are present. These constituents are insoluble in water but can be solubi lized by oxidation. Due to high oxidation potential of ozone, it readily oxidizes these constituents, thereby solubilising them. During oxidation, the 1-4 linked D-glucopyranose unit of starch is attacked by ozone, resulting in the formation of carbonyl and carboxyl groups as well as the cleavage of glycosidic bonds21

. Hence, one would expect depolymerization of the starch to occur through the attack of ring connecting the ether linkages. It is well known that the waxes are esters of higher fatty alcohols and fatty acids. The principal components of waxes are considered to be long chain alcohols and acids, saturated and unsaturated hydrocarbons, resin and resin acids, sterols and sterol g lucosides . Hence, the reactivity of waxes towards ozone largely depends on their chemical nature. It is generally agreed that while reacting with unsaturated hydrocarbon present in waxes, ozone gives an ozon ide which, on hydrolysis in water, breaks down into a mixture of aldehydes and acids. Ozone may also give an aldehyde and acid type products by attacking the bond between carbonyl and alkoxy group present in the saturated waxes22

. Therefore, one would expect that during grey bleaching of cotton with ozone there is a possibility that some of these impurities are also removed, resulting in weight loss after bleac hing. Table 1 also shows this phenomenon

that with the increase in treatment time there is an increase in weight loss. The extent of impuriti es removed during ozonation is less than that of conventional method of desizing, scouring and bleaching. However, a weight loss ranging between 5.0% and 7.0 % (depending upon treatment time) during ozonation clearly demonstrates that the ozone is capable of desizing and dewax ing to some ex tent. The extent of dewaxing during ozone bleaching is shown in the Table 1. It is found that with the increase in treatment time the efficiency of dewaxing increases . To achieve an acceptable degree of whiteness (84%), the degree of dewaxing is found to be 31 %. Though it is less when compared to the degree of dewaxing of fabrics scoured with alkali, the water drop absorbency is found to be satisfactory « 3 s). In this context, it is worthwhile to recall the fla sh scouring techniques such as vaporloc system, where the absorbency is satisfactory even though the extent of dewaxing is low because the wax present on the fibre surface develops cracks due to flas h scouring and so the absorbency is satisfactory. It has al so been reported in the I iterature that there is no correlation between water adsorption of cotton fibre and its natural wax content. ]n fac t, it is ma inl y the di stribution of the residual wax on the f ibre surface that determines water absorbency. Hence. it is necessary to reduce the wax content to a leve l at which it can no longer form a continuous protecti ve film around the fibre so that the subseq uent wet operations like dye ing could be carried out without any problem23

.

It is well known that a higher degree of chemical modification in cellulose during ozone bleaching than that in conventional bleaching is because of the very high oxidation potential of ozone. Whil e bleac hing cotton fabrics with ozone, it is generall y accepted that the cellulose mol ec ules a re ox idi zed into oxy-cellulose . Hence, in the present work , to study the influence of ozone on cellulose chemical modification, copper number, carboxy l groups content, fluidity and degree of polymerization are measured. The results (Table 1) indicate that with the increase in treatment time the extent of chemical modification is increased. Thi s modification is found to be higher ' than that caused during conventional hydrogen peroxide bleaching, especially at hi gher degree of WI.

One would expect a low colour yie ld during reactive dyeing with the increase in copper number and carboxyl group content. Therefore, another

"

PRABAHARAN & RAO: COMBINED DESIZING, SCOURING AN D BLEACHING OF COTTON 44 1

method to assess the quality of bleached fabric was used to study the amount of reactive dye uptake after bleaching. Dyeing experiments were carried out on fabr ics having WI of 80, 84, 88 and 90%. The colour yield of these dyed samples was compared with that of dyed fabr ics that are processed by conventional methods. The KIS values for different WI are shown in Figs 1 and 2. It can be seen from these figures that with the increase in whiteness index the KIS values decrease for all the fabrics bleac hed with ozone, hydrogen peroxide and calcium hypochlorite. Thi s indicates that during bleaching the hydroxyl groups are converted into carboxyl or aldehyde groups . For the acceptable degree of WI (84%), the dye uptake of the samples bleached with ozone is nearly same as that of the samples bleached with hydrogen perox ide and calcium hypochlorite. However, at higher degrees of WI, the dye uptake is more for the samples bleached with hydrogen peroxide followed by calcium hypochlorite and ozone. The wash fastness properties of dyed samples bleached with ozone were found to be same as those of peroxide bleached samples. Thi s result further confirms that for the acceptable degree of WI, the quality of ozone

VI Ql OJ ro >

I.°r -----------;=========;j --Ozone

- - - - Hv<*"ogen peroxide

0.8 ....... Calcium hypochlorite

------- - --

~ 0.6 .. .. .

VI

'" OJ iii

0.4

0.3 80

5 . .

> 4

~

84 88

Whiteness Index ,%

Fig 1- KIS values of bleached fabrics dyed with monochlorotriazinyl dye

90

I Ozone - I - - - - Hydrogen peroxide I - .. - - - :~~~~ hYPOC hl~

28~0-----~8~4------8~8-----~

Whiteness Index ,%

Fig 2- KIS values of bleached fabri cs dyed with dichlorotri aziny l dye

bleached fabric is in line with that of conventiona ll y processed fabrics.

From the above comparison, it could be concluded that up to a certain degree of whiteness (84%), which is sufficient for dye ing medium to dark shades with reactive dyes, the grey fabric processed with ozone exhibits a quality which is comparable to the qual ity of fabric processed by conventional method of desizing, scouring and hydrogen peroxide bleac hing. Beyond this degree of whiteness which is necessary for dyeing pastel shades with reactive dyes. the quality of ozonized fabric is inferior to that of fabri cs processed by conventional method . In the conventional method, two-stage bleaching is ofte n used to obtain high degree of whiteness with minimum cellulose degradation at a competitive cost to dye pastel shades and also for OBA treatment. The best example is hypochlorite bleaching fo ll owed by hydrogen peroxide bleaching. It is reported in the literature24

.25 that the ozone can be used as the sole­

bleaching agent in pulp bleaching, provided that the required brightness is about 60 - 70%. For hi gh brightness levels, it is used in a two-stage bleac hing that would include treatment with other bleac hing agents like oxygen, peroxide, chlorine dioxide and hypochlorites. This kind of two-stage bleac hing is recommended to minimize severe loss in yie ld and fibre strength properties. In light of the above practice, in the present work an attempt has been made to carry out two-stage bleaching of grey fabr ic with ozone followed by hydrogen peroxide to achieve a high degree of whiteness with minimum cellulose degradati on.

The wet grey fabr ics with 24% moi sture content at pH 5 were first bl eac hed with ozone (l00 g/m3

) for 1,2 and 3 min and then bl eac hed with hydrogen peroxide for 45 , 30 and 15 min respectivel y. The process conditions were so adjusted that the fina l fabrics have 90% WI. The quality paramete rs of these bleached fabrics are reported in Table 3. Thi s table a lso shows the fabric properti es afte r single­stage bl eaching with ozone, hydrogen pe roxide and calcium hypoc hl orite separate ly to ac hi eve 90% WI. It can be seen from thi s tab le that with the increase in ozone treatment time, the r;trength loss, copper number, carboxyl group content and fluidity increase and the degree of polymerizat ion decreases . However, the strengt h loss, fluidity and degree of polymer.iza ti on of grey fabric trea ted with ozone fo r 1 min followed by hydrogen peroxide are nearly sa me as that of the fabric bleached with hydroge ll

442 INDI AN 1. FIBRE TEXT. RES ., DECEMB ER 2003

Table 3 - Properties of two-stage and single- stage bleached fabrics to obtain 90% WI [Control grey fabri c - Whiteness index, 70%; Fluidity, 2.61; and Degree of po lymeri zation. 2571]

Bleaching Treatment time, min agent Ozone Peroxide Hypoch lorite

45

2 30

3 15

II 7

III 60

IV 60

I - Ozone followed by hydrogen peroxide II -Ozone III - Hydrogen peroxide

Strength loss, %

18

19

23

27

18

23

peroxide. The copper number and carboxyl group content for two-stage bleached fabrics are higher than that of the hydrogen peroxide bleac hed samples. From thi s data, it may appear that the extent of chem ica l modification due to ozone treatment fo llowed by hydrogen peroxide bl eac hing is very hi gh. However, it should be noted that two-stage bleaching is carried out on grey fabric whi le the conventional bleaching with hydrogen peroxide or ca lci um hypochlorite is carried out after des iz ing and scouring. The presence of residual size materia ls and natura l impuriti es, which carry acidic and reducing groups in the grey two-stage bleached fabric, seem to be respons ible for high coppe r numbe r and carboxy l groupS26. It is also noted that the quality parameters of two-stage bleached fabric s are found to be better than that of the fabric bleac hed with only ozone or calcium hypochlorite. A re markab le drop in degree of pol ymerization is observed when fabric is treated separa te ly with ozone and calcium hypoc hl orite to achieve 90% wh iteness .

To study the suitability of two-stage bleaching to dye pastel shades, experiments were carried out with two types of reactive dyes . The fabrics were dyed to 0 .1 % (ow m) shade and analyzed for KIS values. These va lues are compared with that of dyed fabrics that are bleached by only ozone and by conventional methods. These results are reported in Table 3. Here a lso, as expected , the KIS values are found to decrease with the inc rease in ozone treatment time for both the reactive dyes studied. However, the KIS values for the fabrics bleached with ozone for 1 min followed by hydrogen peroxide are same as that of the fabrics bleached with hydrogen peroxide and higher than that

Copper number

0.17

0.22

0 .32

0.49

0.07

0 .24

Carboxyl Fluidity Degree of content po lymeri zation

5.85 4.48 2084

6.21 5.72 1970

6 .55 7.90 1663

8. 15 12.0 1308

6. 10 4.25 2 115

7.2 1 7 .15 1691

IV - Calcium hypochlorite MCT - Monochlorotriazinyl dye OCT - Dichlorotriazinyl dye

KIS MCT OCT dye dye

0 .55 2.70

0 .53 2.42

0.46 1.40

0 .3 1 0.70

0 .54 273

0.51 2. 16

of the fa brics bleached with calcium hypoc hlorite and ozone a lone. Thi s clearly indicates that the quality of two-stage bleached fabric is same as that of fabric s bleached with hydrogen peroxide and higher than that of fabric bleached with calcium hypoc hlorite and ozone alone. Therefore, the above work illustrates that it is possi ble to achieve a high degree of whiteness by two-stage bleaching of grey fabric, the qual ity of which is comparable to the quality of fabric processed by conventional method of des izing, scouring and hydrogen perox ide bleac hing.

4 Conclusions

The above study shows that g rey preparation with ozone can be completed in one or two minute. Thi s process has additional advantages such as sav ings in the rma l energy, water and chemicals. For an acceptable degree of whiteness which is suffi c ient fo r dye ing medium and dark shades with reacti ve dyes. the quality of the fabrics processed with ozone is comparable with that of the fabrics processed by conventional method of desi z ing, scouring and hydrogen peroxide bleaching. For hi gh degree of whiteness , undoubtedly the quality of fabrics bleac hed with hydrogen pe roxide is found to be superior to that of the fabrics bleached with ozone and ca lci um hypochlorite . Howe ve r, it can be said that the re is a scope to improve the quality of ozonized fabrics. A two-stage bleaching of g rey fabri c w ith ozone followed by hydrogen peroxide is recommended for achieving a high degree of whiteness with minimu m cell ulose modification , which may be suitabl e fo r dye ing pastel shades.

PRABAHARAN & RAO: COMBINED DESIZING, SCOURING AN D BLEAC HI NG OF COTTON 443

Acknowledgement One of the authors (MP) thanks the Council of

Scientific and Industrial Research, Govemment of India, for awarding Research Associateship during his research work.

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