Indian Journal of Fibre & Textile ResearchVol. 23, December 1998, pp. 257-260

Green technology in textile processing: Part II - Bleaching ofpolyester/cotton fabric

H T Deo' & A I Wasif

Department of Chemical Technology, University of Mumbai, Mumbai 400 019, India

Received 28 October 1997; accepted 19 November 1997

Two new formulations have been prepared for bleaching of polyester/cotton (48:52) dhoti fabricsand the results of bleaching with these formulations have been compared with those of the conventionalprocess. The new formulations result in significant savings in steam, electricity and water. A total savingof 10.67% in the cost of polyester/cotton dhoti preparation has been reported. These formulations arealso eco-friendly as they avoid use of toxic bleaching agents, require lesser concentration of chemicalsand generate reduced load of effluent. Biological oxygen demand and chemical oxygen demand lowerby 38% and 35% respectively have been reported.

Keywords: Bleaching, Eco-formulations, Eco-processes, Polyester/cotton dhoti

1IntroductionThe chemical processing of textile consumes

large quantities of water. Consumption ~f watervaries from fibre to fibre and the quantities of pol-lutants that form the pollution load going intowaste water depending on a particular chemicalprocess followed. Synthetic fibres contain spin-finishes, coning oils, sizes and adventitious dirt as .impurities. A mild scouring treatment removesthese impurities. Although the bleaching of poly-ester component is not required in the case of fab-ric to be dyed later, it is essential to obtain purewhite goods since the polyester fibres get discol-oured during heat treatment. This is particulariytrue in case of polyester/cotton dhotis, which havebecome very popular in rural and semi-urban areasof our country I.

Cotton dhoti, in general, and polyester/cottondhoti, in particular, are very popular as the tradi-tional dress in rural and semi-urban areas in Indiaand in some countries in Asia and Africa. Polyes-ter/cotton blend fabrics are normally bleached withsodium hypochlorite, followed by sodium chlorite

. and hydrogen peroxide. Though sodium hypo-chlorite imparts whiteness to cotton component, itdoes not produce good white polyester componentof the blend. Further, sodium hypochlorite is not

"The author has changed his name from H T Lokhande toH T Deo. All the correspondence should be addressed to him.

eco-friendly due to its chlorite content. With hy-drogen peroxide also, polyester does not getbleached satisfactorily. Sodium chlorite, however,has a bleaching action on polyester as well as cot-ton. While most other bleaching agents produceonly an ivory white on synthetic fibre fabric, so-dium chlorite renders the fabric a pure intrinsicchalk white. But sodium chlorite under acidic con-ditions is extremely corrosive to plant and equip-ment and the addition of sodium nitrate to preventcorrosion leads to pollution of water'. The chlorinedioxide fumes evolved during bleaching are verytoxic. Various chemicals and their mixtures toprotect the stainless steel from attack by sodiumchlorite or chlorous acid have been reported).Butcher" reported the use of fluorescent whiteningagents in place of hazardous sodium chlorite. Re-cent developments like use of various eco-friendlyenzymes in desizing and scouring have been re-ported'". Besides these, some noteworthy work inthe development of polymeric dispersants likesugar folymer based cleansing agent has been re-ported.

In the present work, two formulations have beenprepared which are aimed at avoiding toxic/bannedchemicals, reducing the concentration of chemicalsand thereby reducing waste concentration, reusingthe wash water containing low pollutiori loads andreducing steam and electricity consumption duringbleaching of polyester/cotton (48:52) dhoti fabric.

258 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1998

The chlorine-based bleaching agents have beenreplaced by chlorine-free bleaching system basedon acidified potassium permanganate solutions.

2 Materials and Methods

2.1 MaterialsPolyester/cotton (48:52) dhoti fabric having the

following specifications was used : warp count,60s; weft count, 80s; ends/in, 80; and picks/in, 80.

Sulphuric acid, hydrochloric acid, calcium chlo-ride, magnesium sulphate, ferric chloride, mercuricsulphate, silver sulphate, potassium dichromate,ferrous ammonium sulphate, all of AnalaR grade,were used. The. non-cresylic wetting agent MercinNC (M/s Ahura Chemicals, Mumbai) was used.

2.2 Methods

2.2.1 DesizingSeparate de sizing step has been eliminated in

conventional method and in formulations I and IIas the size consists of water soluble materials suchas polyvinyl alcohol and modified starches.

2.2.2 ~ .caching

Conventional MethodThe polyester/cotton dhoti fabric was grey mer-

cerised with a solution containing 44° Tw (245 g/I)sodium hydroxide and 0.25% non-cresylic wettingagent for one min at 30°C followed by hot and coldwash with water. Scouring was then carried outusing 2%( owf) sodium carbonate and 0.25% (owf)non-ionic detergent at 70°C for 2 h. The scouredfabric after hot and cold wash was subjected tochemicking with sodium hypochlorite (3.0 g/lavailable chlorine) for 2 h followed by washing.The bleaching was carried out with 5.0 g/l sodiumchlorite, 2.0 g/l sodium nitrate and 2.0 g/l formicacid (85%) at boil for 3 h followed by two washingtreatments. The fabric was then subjected to per-ox!de bleaching with 0.28% (owf)hydrogen per-OXide (50%), 0.4% (owf) sodium silicate, 0.2%(owf) sodium hydroxide and 0.11% (owf) wettingagent at 85°C for 3 h. The fabric was finallywashed twice before drying.

Formulation I

The grey fabric was hot mercerised with a solu-tion containing 35"Tw (182 g/l) sodium hydroxideand 0.125% non-cresylic wetting agent at 70°C for45 s followed by hot and cold wash with water.

The fabric was subjected to bleaching with 0.3%(ow~ potassium permanganate, 98% suIphuric acid(1/4 part of permanganate) and glacial acetic acid(3/4Ih part of permanganate) at 60°C for 30 minfollowed by washing. The fabric was then treatedwith 4.0 g/l oxalic acid at 85°C for 30 min andwashed. Finally, peroxide bleaching was carriedout with 0.2% (owf) hydrogen peroxide (50%),0.2% (owf) stabilizer AWNI, 0.2% (owf) sodiumhydroxide and 0.1% (owf) detergent at 85°C for 3h. The fabric was finally SUbjected to hot wash andcold wash with water before drying.Formulation /I

The processing sequence was same as for for-mulation I. However, potassium permanganate andhydrogen peroxide leftover from formulation Iwere put to reuse after replenishment. The replen-ishment of permanganate bleaching bath was donewith 0.18% (owf) potassium permanganate and thatof peroxide bleaching bath with 0.15% (owf) hy-drogen peroxide (50%), 0.15% (owf) stabiliserAWNI, 0.1% (owf) sodium hydroxide and 0.05%(owf) detergent.

The wash liquor obtained in the cold washing ofhot mercerised fabric in formulation I was put toreuse in formulation II for hot wash after merceri-sation. Similarly, the wash liquor obtained inwashing of perrnanganate-treated fabric in formu-lation Iwas reused for washing of permanganate-treated fabric in formulation II. The wash waterobtained from the second wash after peroxidebleaching in formulation I was reused for first washafter peroxide bleaching in formulation II.

2.2.3 TestsSuspended solids, total dissolved solids, bio-

chemical oxygen demand (BOD), chemical oxygendemand (COD), pH, tensile strength, whiteness,yellowness, whiteness retention, copper numberand carboxyl values were determined as reportedearliers,

3 Results and DiscussionThe waste water generated in the various stages

of pretreatment of polyester/cotton (48:52) dhotifabric was analysed for pH, total solids, total dis-solved solids, suspended solids, BOD and CODand the results are given in Table 1. Bleaching ofpolyester/cotton (48: 52) dhoti fabric generates lessvolume of effluent and the pollution load is alsolow~r :s compared ~o that in bleaching of cottonfabrics . However, In the conventional method ,

I, DEO & WASIF: GREEN TECHNOLOGY IN TEXTILE PROCESSING: PART II 259

rather large quantity of water is consumed gener-ating higher pollution load. To curtail water con-sumption and reduce pollution load, formulations Iand II were prepared. The hot mercerisation treat-ment with mild alkali and detergent at 70°C servesthe purpose of desizing, scouring and mercerisingin a single operation ·in formulations I and II,thereby shortening the processing sequence. Themercerising effluent from formulations I and IIhave higher total solids, total dissolved solids, sus-pended solids, BOD and COD as compared to thatfor the effluent from the conventional mercerisingmethod. This may be due to the removal of signifi-cant quantities of both natural and added impuritiesin hot mercerisation. However, the effluent gener-ated in hot mercerisation is much smaller in vol-ume as compared to the scouring effluent of con-ventional method.

The total BOD for the formulation I is 640 mg/Icomprising 60 mg/I in bleaching and 580 mg/I inmercerising as compared to 1028 mg/I for the con-ventional method which comprises 880 mg/I in

scouring, 92 mg/I in bleaching and 56 mg/I in mer-cerising. Similarly, the COD value for the formu-lation I is 1816 mg/I as compared to 2797 mg/l forthe conventional method. Thus, reduction in theBOD and COD values is by about 38% and 35%respectively.

To take care of the absorbable organohalogens(AOX) problem, sodium hypochlorite bleachingand sodium chlorite bleaching, generally followedin the conventional method, were phased out informulations I and II and instead eco-friendly po-tassium permanganate bleaching was adopted. Thepermanganate bleaching effluent (formulations Iand II) showed lower values of total solids totaldissolved solids, suspended solids, BOD and' CODas compared to that by the effluent of the conven-tional method. It may be seen from Table I that theoverall pollution load is maximum for conventionalmethod followed by formulation II and formulation I.

Table 2 shows the results of the physical andchemical analysis of polyester/cotton (48:52) dhotifabric. The whiteness values and Yellowness index

Table I-Waste water analysis results

Process pH Total solids Total dissolved Suspended Biochemical oxy- Chemical oxy-mg/I solids solids gen demand (5 gen demand

mg/I mg/I days at 20°C) mg/I

Scouringmg/I

Conventional 9.1 9885 9125 760 880 1998Formulation IFormulation II

Bleaching

Conventional 7.4 2695 2480 215 92 549Formulation I 5.2 1864 1698 166 60 304Formulation II 5.2 2228 2020 208 71 366

Mercerising

Conventional 8.7 1865 1761 104 56 250Formulation I 8.2 6492 5872 620 580 1512(Hot mercerisation)Formulation II 8.3 6728 5072 656 605 1587(Hot mercerisation)

Table 2-Physical and chemical test data for bleached polyester/cotton (48:52) dhoti fabric

Bleaching treatment Tensile strength Loss in tensile Whiteness Wh iteness Yellowness Copper Carhoxylkgf strength % retention index number content

% % meq/100gWarp Wen Warp Weft

Control (Untreated) 21 27.5 64.2 13.9Conventional method 19.5 25.2 7.14 8.36 86.4 85.7 6.1 0.19 0.6Formulation I 19.8 26.0 5.71 5.45 86.8 85.9 6.1 0.17 0.5Formulation II 19.6 26.0 6.6 5.45 86.3 84.6 6.2 0.18 0.5

260 INDIAN J. FIBRE TEXT. RES., DECEMBER 1998

Tabld-Savings in steam, electricity and water for bleached polyester/cotton(48:52) dhoti fabric"

Savings withFormulation 1 Formulation II

._--Steam Electricity Water Steam Electricity Water

kg kWh litres kg kWh litres

2000 16.78 32,000 2000 16.78 32,00042 8000 42 8000

1000 15 1000 15

16,000

~3000 73.78 40,000 3000 73.78 56,000Rs 1800 Rs 221.34 Rs 220 Rs 1800 Rs 221.34 Rs 308

Rs 2241.34 Rs 2329.34

Savings due to

Avoidance of scouringAvoidance of chemickingAvoidance of sodium chloritebleachingReuse of water

Total savingsSavings in monetary terms/dayTotal monetary savings per dayfor bleaching 2000 kg dhotifabric

• Savings based on assumptions given in Appendix I

values obtained with formulations I and II arecomparable to those obtained by the conventionalmethod. Slightly lower values of copper number,carboxyl content and loss in tensile strength forsamples bleached with formulations I and II indi-cate that no additional damage has been done informulations I and II as compared to conventionalbleaching.

Table 3 shows the projected savings in steam,electricity and water on bleaching polyester/cotton(48:52) dhoti fabric in a process house with for-mulations I and II instead of conventional method.The savings are based on the assumptions given inAppendix I. It is observed that the formulation Iresults in savings of 3000 kg steam, 73.78 kWhelectricity and 40,000 litres of water per day,whereas the formulation II results in savings of3000 kg steam, 73.78 kWh electricity and 56,000litres of water. The cost of bleaching polyes-ter/cotton dhoti by the conventional method is Rs10.50 per kg. The data show that with formulationsI and II there is a saving of Rs 1.12 per kg in a pro-cess house. Thus, there is a total saving of 10.67%in the cost of polyester/cotton (48:52) dhoti fabricpreparation.

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3 Hetherington P W, J Soc Dyers Colour, 84(7) (i 968) 359.4 Butcher J V, Rev Prog Color, 4 (1913) 90.5 Chopra S, Book of papers. 35thjoint technological confer-

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6 Srivastava H C, Book of papers. 33rd joint technologicalconference of ATIRA. BTRA. NITRA and SITRA (BTRA.Bombay), 1992, 135.

7 Sharma M, Book of papers. national seminar on eco-friendly pathways to textile finishing and agrowaste utili-sation (Central Institute for Research on Cotton Technol-

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(1998) 177.

Appendix I-Steam, 'electricity and water required for proc-essing 4000 m (approx. 2000 kg) of polyester/cotton dhotifabric at shop floor level

Steam consumption for scouring= 1 kg/kg of fabricSteam consumption for peroxide boil=1 kg/kg offabricSteam consumption for formulation bleaching (permanganatebleaching)=0.5 kg / kg of fabricSteam consumption for sodium chlorite bleaching=O.5 kg / kgoffabricRating ofkier circulation pump=5 HP n.75 kW)Motor of washing machine=5 HP (3.75 kW)Average production of washing machine= 100 m1minWater requirement for washing after scouring=16 litres/kg offabricWater requirement for washing operation=4 IitreS/kg of fabricCost of steam=Re 0.60/kgCost ofelectricity=Rs 3 / kWhCost ofwater=Rs 55 per 10,000 litres