Indian Journal of Fibre & Textile ResearchVol. 23, December 1998, pp. 250-256

Optimization of non-silicate stabilizers for bleaching of cottonknitted goods

M Chakraborty & Amit Dayal

Northern India Textile Research Association, Ghaziabad 201 002, India

and

M L Gulrajani

Textile Technology Department, Indian Institute of Technology, New Delhi 110 016, India

Received 9 April 1997; revised received and accepted 14 August 1997

The study gives a comparative view on the performance of silicate and non-silicate stabilizers andoptimizes the peroxide concentration in the bleaching process. The uniformity of dyeing and the colourdifference have also been studied on fabrics bleached using silicate and non-silicate stabilizers. Resultsindicate that the optimum amount of peroxide needed for bleaching is 3% owf, the inference being madefrom the whiteness index and yam tenacity data. The bending length of the samples bleached using non-silicate stabilizers is smaller compared to that for the samples bleached by using sodium silicate, indi-cating a softer handle. The values of relative unlevelness index show that the uniformity of dyeing is ex-cellent in all the cases i.e. the use of non-silicate stabilizer during bleaching has no adverse effect ondyeing.

Keywords: Bleaching, Cotton knitted fabric, Non-silicate stabilizer, Yam tenacity, Whiteness index

1 IntroductionPreparatory process has great significance in

textile wet processing. In cotton knitted goods, thepreparatory processes involve scouring, bleachingand washing. There has been a continuousdevelopment in the area of bleaching. Sodiumhypochlorite bleaching is definitely cheaper, but itgives rise to high AOX (adsorbable organic halogencompounds) value':", Further, it leads to yellowingof the fabric on storage'. Sodium chlorite bleaching,developed subsequently, has problems due to thecorrosive nature of CI02 evolved during the process.A major disadvantage is that it cannot be combinedwith other processes. Though it has AOX valuelower than hypochlorite, it is not completely freefrom this problem.

Considering the ecological requirements,hydrogen peroxide has emerged as the most viablebleaching chemical.It is free from AOX problemsand can be combined with the scouring operation.However, sodium silicate, employed as a stabilizerin peroxide bleaching, tends to precipitate out ofsolution in hard water or upon acidification', Hence,thrust has been on substituting silicate with non-

silicate stabilizers. In fact, improvement in theproperties of jute fibres has been reported with theuse of non-silicate stabilizer during bleaching'.

In the present study, an attempt has been made tostudy silicate and non-silicate stabilized peroxidebleaching. Both single-stage and two-stagebleaching processes have been studied aiming at theoptimization of the peroxide concentration in thebleaching bath. The uniformity of dyeing and thecolour difference have also been studied on fabricsusing silicate and non-silicate stabilizers.

2 Materials and Methods2.1 Materials2.1.1 Fabric

Cotton knitted (single jersey) fabric having thefollowing specifications was used: courses/inch, 44;wales/inch, 38; weight, 128 g/nr' ; and count, 26sNe.

2.1.2 WaterThe water used during scouring and bleaching

operations had the following qualities: pH, 7.26;hardness, 296 ppm;1DS, 589 ppm; and totalalkalinity, 46 ppm. Alkalinity and hardness were

CHAKRABORTY et al.: BLEACHING OF COTION KNITIED GOODS

measured in terms of calcium carbonate. Duringdyeing, 0.857 gIl EDTA was added to water to makeit soft.

2.1.3 Non-silicate StabilizerNon-silicate stabilizer having the following

characteristics was used:

Physical formChemical natureSolubilityCompatibility

Off-white paste/liquidAnionicSoluble in waterStable to hard water andalkali; compatible withanionic and non-ionicauxiliaries.

2.2 Methods2.2.1 Whiteness

The Hunter whiteness has been reported as thewhiteness index (WI). The Hunter whiteness isexpressed as L-3b, where Land bare CIE co-ordinates. WI was measured on computer colourmatching system (Macbeth Color-Eye 3100).

2.2.2 AbsorbencyThis was determined as per IS: 2349-1963.

2.2.3 Yarn TenacityYarn tenacity was measured on Tensorapid

instrument using 20cm yarn gauge length and 50mm/min elongation speed.

2.2.4 Relative Unlevelness Index (RUI)The RUI was calculated using the following

formula:700 700

RUI= L CA,VA = L (SA IR).vAA=4oo A=4oo

where C'J,. is the coefficient of variance of reflectancevalues; V'J,., the phototopic relative luminousefficiency function; S'J,. , the standard deviation of thereflectance values; and R, -the average ofreflectance values.

2.2.5 Colour EvaluationColour evaluation was done on a computer colour

matching system (Macbeth Color-Eye 3100). Thecolour difference DE is measured in terms of DL'Da' and Db' which are the differences in CIE co-ordinates.

2.2.6 Bending LengthBending length was measured as per ASTM

standard (D-1388-64).

251

1.2.7 BleachingLaboratory hank dyeing machine (open vessel)

and laboratory high-temperature high-pressuredyeing machine (closed vessel) were used forrecipes I and II, whereas for all other recipes(III- VI),only laboratory hank dyeing machine was used. Therecipes used in the experiments were as per theguidelines of the manufacturers.

Bleaching was carried out on scoured samples(for recipes I and II) and grey samples (for recipesIII - VI). The grey fabric was scoured with thefollowing recipe:

Caustic soda : 2-3% owfLissapol D : 0.5% owfpH : 10-11Temperature : At boilTime : 5 h-The details of the various experimental bleaching

recipes are given below. In all the experiments, theM:L ratio was kept at 1:20. Hydrogen peroxide hasbeen referred as peroxide in these recipes.Recipe!

Sodium hydroxideSoda ashWetting agent(Lissapol D)Sodium silicate : 2.0% owfPeroxide (50%) : 0.5-5% owfTemperature : 95°CTime : 2 hpH : 10.5-11.0Fabric : ScouredExperiments were carried out with different

concentrations (0.5-5.0% owf) and residual peroxideestimated with O.IN sodium thiosulphate solution aslisted in Table 1.

: 0.6%owf: 1.0% owf: 0.5% owf

RecipeIJ

Peroxide (50%)Stabilizer NS'(ICI)Caustic sodaLissapol DTemperature and time

: 2.5 mlll (5% owf): 1.7 gIl: 1.5 gIl: 0.5 gIl: Temp. raised from60°C to 95°C in 30 minand then maintained at95°C for 1 h

: 10-11: Scoured

pHFabric

INDIAN J. FIBRE TEXT. RES., DECEMBER 1998

Table I-Results of open system (hank dyeing machine) bleaching

H202(50%) H202 consumed WI (Hunter Yam tenacity Yam strength%owf %owf whiteness index) gfltex loss, %

0.5 0.46 68.90 13.08 3.91.0 0.98 73.62 13.29 2.41.5 1.38 75.22 12.27 9.52.0 1.97 74.49 12.18 10.53.0 2.98 76.53 1\.33 16.84.0 3.96 77.85 8.73 35.95.0 4.89 76.62 9.74 28.4

5.0 4.4 76.29 11.56 15.15.0 5.0 76.62 9.74 28.4

59.10 13.61

252

Stabilizer

Recipe ISodium silicate

Recipe IIStabilizer NSSodium silicate'Scoured sample

The experiment was also repeated by substitutingstabilizer NS by sodium silicate (5.0 %owt).

Recipe IIICombined scouring and bleaching with non-

silicate stabilizer Superstat VP, marketed by SuperProducts (rapid bleaching process):

Caustic soda : 15 gIlSodium silicate : 10 gIlSuperstat VP : 5 gIlImerol Xlliq : 5 gIlMineral turpentine : 5 gIlPeroxide (35%) : 30 gIlFabric : GreyImerol Xl and turpentine were emulsified

together under stirring. Caustic soda and sodiumsilicate were dissolved in water. A dispersion ofSuperstat VP was made separately and all the threewere mixed together. Peroxide was added in the last.

Fabric was impregnated in the above liquor andthen batched, well covered and kept rotating for 3h.It was then re-padded in the same liquor andsteamed at 100°-120°C for lh and rinsed well.Recipe IV (Scourex Process)

Scouring agent"Peroxide (50%)Sodium silicateDisodium hydrogen

phosphate and NaOH in1:1 molar ratioto give pH

TimeTemperatureFabric

: 2%owf: l%owf: l.1%owf

: 11: 6h: 70°C: Grey

a Scouring agent" composition: Pine oil, 50%; Emulsifier(Niogen EA635), 40%; and Perchloroethylene, 10%.'

Recipe IV(A)Experiments were done with Scourex process by

taking different concentrations of H202 (1%-6%owt) and processed at 95°C for 2h.

Recipe VCombined scouring and bleaching process used

in some modem mills. The chemicals used were ofClariant India Ltd.

Sandoclean PC liqSirrix 2UDI liqCaustic sodaSoda ashPeroxide (50%)Stabilizer AWNITemperatureTimepHFabric

: 1 gpl } at 50°C for 15 min: 2 gpl: 2-3% owf: 0.5% owf: 1-2% owf: 0.25-0.5% owf: 95°C: 1.5 h: 10-11: Grey

Recipe VI

Conventional combined scouring and bleachingprocess used in some textile mills:

Caustic sodaSodium silicatePeroxide (50%)Soda ashTemperatureTimeFabric

: 3%owf: 8%owf: 6%owf: 1% owf: 95°C: 2 h: Grey

Recipe VII

Industrial trial was taken on 100 kg of fabricusing this recipe.

CHAKRABORTY et al.: BLEACHING OF COTION KNITIED GOODS 253

SandocIean PC liqSirrix 2UDI liqCaustic sodaSoda ashPeroxide (50%)Stabilizer AWNITemperatureTimepHFabric

: 1 gpl 1at 50°C for 15 min: 2 gpl: 3% owf: 0.5% owf: 3%owf: 0.5% owf: 95°C: 1.5 h: 10-11: Grey (40s Neinterlocked): WinchMachine used

2.2.8 DyeingThe samples obtained after bleaching using

sodium silicate and non-silicate stabilizers AWNIand NS were dyed with four different reactivecolours and evaluated on a computer colourmatching system (Macbeth Color-Eye 3100). Thereactive dyes used were: Ichofix Orange HE2R(Dyechem), Serifix Red HE 8B (Serene), IchofixGolden Yellow HER (Dyechem) and Serifix NavyBlueHR (Serene).

3 Results and DiscussionThe results of the experiments with recipe I and

recipe II are given in Tables I and 2 respectively. Itis observed that for sodium silicate stabilizedbleaching, the whiteness index of the bleachedfabric increases with an increase in the peroxideconcentration. However, at concentrations of 3%owf and above the WI is almost constant. It is clearfrom Table 1 that on increasing the peroxideconcentration from 3% owf to 4% owf, the WIincreases by just about I%, whereas the loss in yamstrength of the bleached fabric almost doubles.Thus, considering the whiteness and the strengthloss, around 3% (owf) H202 is the optimumconcentration of peroxide for bleaching the scouredknitted fabric.

Using the data given in Table 1, the best fitequations relating WI and %loss in yam strength toH202 (% owf) used initially were found out usingthe SYSTAT software. From these modeledequations, the respective curves were plotted againstthe H202 concentration used. These curves areshown in Figs I and 2 alongwith their respectiveequations.

It is evident from Table I that with non-silicatestabilizer NS, the whiteness equivalent to that withsilicate is obtained but the %Ioss in yam strength isreduced to almost half.

The absorbency in all these cases was less thanone second. The peroxide consumption duringbleaching is almost equal to peroxide used initially.Fig.3 shows the curve for peroxide consumed versusperoxide used initially.

79r------------------------------~

l. __ --

IIIIIIIIIZIII!:::c~a:III>-Z;:):c

i)·9 I·a 2·7 3·6 4·5PEROXIDE CONCENTRATION;'" OWF

Fig. I--Hunter whiteness vs initial peroxide concentration forsodium silicate stabilized bleaching in open systemRegression equation: Y= 59.158 + 26.353 X - 15.704 Xl + 3.976X 3-0.35 X4, where Y = Hunter whiteness, and X = Concentrationof HPl (% owf).For the equation, R = 0.998, R 2= 0.996, Std. error = 0.632, andF-ratio = 166.179

Table 2-Results of close system (HTHP machine) bleaching

Stabilizer HP2 (50%) H202consumed WI (Hunter Yam tenacity Yam strength%owf %owf whiteness index) gf/tex loss, %

Recipe ISodium silicate 0.5 0.46 72.19 12.14 10.8

\.0 0.84 74.23 11.95 12.21.5 1.33 75.58 11.36 16.5

Recipe IIStabilizer NS 5.0 4.6 74.60 11.83 12.7Sodium silicate 5.0 4.6 76.39 11.15 18.1

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

Table 2 shows that the whiteness of the knittedfabric is better in closed system compared to that inopen system (Table I) at the same peroxideconcentration. This is owing to better circulation ofbleaching liquor and hence better penetration in theclosed system. This is also supported by the higher

40.-----------------------------~

30>-t-

U~z••••..z 20~>-

!IIIIII

g 10~

0·9 U 2-7 3-6 4·5PEROXIDE OONCENTRATION ,"I. OWF

Fig. 2-Loss in yam tenacity vs initial peroxide concentrationfor sodium silicate stabilized bleaching in open systemRegression equation: Y = 6.139 X - 0.779 X 4 + 0.408 X s_0.051 X6,where Y= % loss in yam tenacity, andX= Concen-tration of HPz (% owf)For the equation, R = 0.997, RZ = 0.999, Std. error = 2.316 andF-ratio = 120.43

5r---------------------------m

....u.'~o

1 2 3 ,PEROXIDE USED INITIALLY, ".CNiF

Fig. 3--Peroxide consumed vs peroxide used initially for so-dium silicate stabilized bleaching in open systemRegression equation: Y = 0.982 X, where Y = Peroxide con-sumed (% owf), andX= Peroxide used initially (% owf)For the equation. R = 1.0, RZ = 1.0, Std. error = 0.042, and F-ratio = 31730.039

loss of yarn strength in a closed system compared tothat in open system for the same peroxideconcentration.

From the results of the combined scouring andbleaching process (Table 3), it may be seen that thewhiteness is almost same for all recipes withperoxide concentration at or above 3% owf. Forscourex process employing sodium silicate asstablizer, a concentration of 3% (owf) H202 isoptimum above which the whiteness index isconstant.

In some mills which follow the conventionalbleaching, 6% (owf) HPz is used, From our study,it is indicated that in this process (recipe VI) the WIis 74.44 whereas the non-silicate stabilizer (recipeV) gives the same whiteness (WI=75.07) at 3%(owf) H20z (Table 3). Recipe III using acombination of silicate and non-silicate stabilizergives almost the same whiteness (WI=74.65) at2.1% (owf) H202• This shows that non-silicatestabilizer gives the same results at a lower peroxideconcentration and hence can be used as a substitutewithout any adverse effect on the fabric quality ..

Table 3-Results of combined scouring and bleachingprocesses

Process HPz Hunter Yam(Recipe) . (50%) whiteness tenacity

%owf index gf/tex

Rapid bleaching 2.1 74.65 13.52(recipe III)Scourex 1.0 72.67 13.68(recipe IVA) 2.0 73.98 13.77

3.0 75.10 13.874.0 73.60 13.655.0 75.38 13.616.0 75.13 12.48

Modem bleaching 3.0 75.07 12.72(recipe V)Conventional 6.0 74.44 14.98bleaching (recipe VI)Grey (standard) 48.80 8.6

Table 4--Results of industrial trial (Recipe VII)

Stabilizer %HP2 Hunter Yamused whiteness tenacity

index gf/tex

3 72.26 13.75

50.57 9.71

AWNI (Clariant)Grey (standard)

fabric

CHAKRABORTY et al.: BLEACHING OF COTION KNITIED GOODS 255

Parameter Orange HE2RStabilizer-eA WNI NS

Red HE 8B

Table 5----Colour evaluation of bleached fabrics taking sodium silicate stabilized bleaching as standard

NavyBlueHRAWNI NS

DL· 1.63 1.23 1.06Da" -1.01 -0.45 0.84Db· -2.07 -0.45 0.34DE 2.82 4.69 1.38

Based on the above studies, an industrial trial wastaken on Winch with a non-silicate stabilizer (recipeVII) and results are given in Table 4. It is observedthat the yam tenacity is almost same to that obtainedin the laboratory. This is indicative of the industrialtrial being commensurate with the laboratoryexperiments. The whiteness index data also supportthe above fact.

Colour evaluation data of samples dyed afterbleaching with different peroxide stabilizers aregiven in Table 5. The sample bleached using sodiumsilicate as stabilizer was taken as the standard. Thedata show that the samples bleached using non-silicate stabilizers AWNI and NS have invariablylighter shade with all the dyes. However, the degreeof lightness varies for different dyes. The Da* andDb * values are less than I, except in a few cases.The total colour difference DE in all these cases isgreater than I, indicating a considerable differencein colour of these samples compared to the standard.This difference is mostly due to the depth of theshade as shown by the data. It is only in case ofOrange HE2R and Golden Yellow HER that thetonal variations also add significantly to DE value.

Table 6 shows the KJS values for the the dyedfabrics. It is seen that the KJS values for the samplesbleached in a non-silicate stabilized system arelower compared to those for the samples bleached insodium silicate stabilized system. This indicates thatthe former are lighter in shade compared to thelatter.

Table 7 shows the relative unlevelness index(RUT) values for the dyed samples. It is seen that thelevelness is excellent for the samples bleached byemploying either sodium silicate as stabilizer ornon-silicate stabilizers. Thus, it can be said that thedyeing uniformity is excellent in all the cases.

The bending length data of samples bleachedusing silicate and non-silicate stabilizers (Table 8)show that the bending length is smaller for the.

Golden Yellow HERAWNI NS AWNI NS

2.16 0.37 -0.17 2.87 3.60-0.49 -0.35 0.69 -0.89 -0.64

0.70 0.25 1.68 0.77 0.542.32 0.57 1.82 3.11 3.70

Table 6--KlS values for dyed samples

Dye KlSvalueSodium AWNI NSsilicate

Orange HE2R 15.6210 11.3421 12.2732Red HE 8B 16.4287 14.4738 13.1246Golden Yellow HER 8.7461 8.5939 9.6350Navy BlueHR 7.7153 6.2036 5.8610

Table 7-Relative un levelness index (RUI) for dyed samples

Dye RUI for system with stabilizerSodium AWNI NSsilicate

Orange HE2R 0.119 0.153 0.084RedHE8B 0.101 0.220 0.181Golden Yellow HER 0.100 0.060 0.156Navy J3\ue HR 0.154 0.173 0.141

Table &-Bending length of fabrics

Fabric Bending length (em) for system withstabilizer

Silicate AWNI NS

GreyScouredWhite (bleached)Dyed

1.72.0

1.92.0

2.11.85

2.22.1

samples bleached by employing non-silicatestabilizer compared to the one bleached with silicatestabilizer. The same trend is observed in the case ofdyed fabric. Thus, it can be said that the systememploying non-silicate stabilizers give a softer

.handle to the fabric, which can be assigned to theelimination of precipitate deposition as in the caseof silicates.

4 ConclusionsThe optimum peroxide concentration is 3% owf

for two-stage process' as well for single-stageprocess like scourex or modem bleaching process.A comparison of these processes shows that non-

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

silicate stabilizers can be used in place of silicatewithout any adverse effect on the uniformity andlevelness of dyeing. The bending length data showthat the stiffuess of the samples bleached in non-silicate stabilized system is lower compared to thatin silicate stabilized system, thereby indicaung thatthe problem arising due to silicate deposition isavoided. Hence, non-silicate stabilizers caneffectively substitute silicate and maintain the fabricquality by avoiding the problems of precipitation asfaced by silicates. The results of industrial trialsupport the findings of the laboratory experiments.

AcknowledgementThe authors are thankful to the Ministry of

Textiles, Govt. of India, for sponsoring the projectof which this study is a part. They are also thankfulto Mr. S.D. Chaudhary of Kautilya Industries Pvt.Ltd, Delhi, for allowing them to take industrial trialin his mill.

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