Indian Journal of Fibre & Textile Research Vol. 26. September 200 1, pp. 296-301

Role of fibre properties in colour non-uniformity of dyed fabrics

K P Chellamania, M Arulmozhi & K Kumarasamy

The South India Textile Research Association, Coimbatore 641 014, India

Received 22 February 2000; accepted II May 2000

The vari ation in colour uniformity (1'1£) of dyed fabric s has been measured by spectrophotometer. Difference in micronaire va lue. maturity ratio and fluorescence have been identified as the major fibre properties which influence shade variation in dyed fabrics. A comprehensive study has been carried out to establi sh the limits for variation in above fibre properties of the mi xing used for spinning yarns in order to maintain high level of colour uniformity in dyed fabrics. Image process ing technique was employed in the study for assess ing cotton fibre maturity. Fibre properties of cotton mixing for maintaining acceptable 1'1£ of I in dyed fabri cs have been suggested using prediction expression connecting fi bre properti es with 1'1£. It has al so been observed that the fabric s made out of man-made fibres exhibit shade variation after dyeing. The ex tent of 1'1£ va lue in these fabrics. as measured by spectrophotometer, has been found to be around 0.5.

Keywords: Colour non-uniformity, Fluorescence, Image processing, Maturity ratio, Micronaire value

1 Introduction The export of yarns , fabrics, garments and other

textile materi als has been increasing rapidly during the last few years and as a result it now accounts for about 35% of our total export earnings. One of the major problems identified in the export market with the Indian textiles (particularly with the knitted goods) is the non-uniformity in colour.

Streakiness in fabrics arises due to the variations in the contributions that the individual yarns make to the appearance of a fabric . Each yarn's contribution can be represented by a reflectance value measured by spectrophotometer. If the reflectance value of every yarn in a fabric is identical, there will be no streakiness . Variations in reflectance value can be caused by the variations in dye on the fibre, fibre cross-sections, denier of yarn, tightness/ length of yarn in fabric , spacing between wales or knit loops in fabrics I , etc. As per the studies conducted at the International Centre for Textile Research and Development, while making fabrics using yarns from different sources, the difference in micronaire value of the cottons from which the different yarns (used for knitting) are made should be less than 0.2 units to avo id streaks in dyed fabrics2

•

Colour uniformity in piece-dyed fabrics is an important fabric property and is used as a criterion for j udgi ng the va lue of the fabric. However, the norms/

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standards for variation in fibre properties (that go into the mixing) of the yarns (used in the fabric) to maintain colour uniformity in dyed fabrics are not available at present.

Man-made fibres like polyester and viscose do not have the problem of fibre immaturity and, therefore, shade variation in fabrics made out of these fibres could be expected to be low. However, no data are available in literature on the attainable level of shade variation in man-made fibre fabrics.

Hence, the present study has been carried out with the following objectives: (i) to establish the limits for variation in fibre properties of the cottons used in a mixing (for producing yarns) to control colour non­uniformity in dyed fabrics, and (ii) to study the level of shade variation in fabrics made out of man-made fi bre yarns.

2 Materials and Methods

2.1 Cotton Fibres

Eight spinnings were carried out in this investigation for cotton fibres.Two spinnings were done using single cottons and six spinnings using mixings. In the six mixings used, the difference in micronaire value varies between 0.32 and 1.31 . The maturity ratio by image processor varies between 0.75 and 0.85. Appropriate counts (based on overall fibre quality) were spun in each spinning trial. The properties of the cotton mixings used and the counts spun are g iven in Table I.

CHELLAMANI et at.: ROLE OF FIBRE PROPERTIES IN COLOUR NON-UNIFORMITY OF DYED FABRICS 297

Table I-Fibre/mixing properties and count spun

Fibre/ Difference in Av. maturity Count spun mixing mlcron3lre ratio (Ne)

value

Single cotton 0 0.77 40s KH &40s CH

Mixing 0.32 0.85 40sCH

Mixing 0.87 0.78 40sCH Mixing 1.25 0.82 40sCH Single cotton 0 0.75 62s KH Mixing 0.39 0.80 62s KH Mixing 0.69 0.84 62s KH Mixing 1.31 0.81 62s KH

Table 2-Process parameters used for the study

Process sequence

Blow room

Blow room lap linear density, g/m

Card

40s KH

450

Card sliver hank (Ne) 0.15

Comber preparatory & comber

Ribbon lap weight, g/m Comber noil, % Comber sliver hank (Ne)

Draw frame

Draw frame sliver hank 0.15 (Ne) Delivery speed, mlmin

- First passage 500 -Second passage 500

Speed frame

Roving hank (Ne) 1.80 TM 1.25

Ring frame

Spindle speed, rpm 14400 TM 3.73

40sCH

450

0.15

48 18

0.15

0.15

62s KH

450

0.18

0.18

250 500 500

1.80 2.40 1.25 1.55

15100 16000 3.41 3.90

Major process particulars employed during spinning are given in Table 2. The yams were knitted and dyed in 3 shades (light, medium and dark) and each shade in 3 colours (blue, orange and magenta). The colour uniformity of the fabrics was measured using spectrophotometer.

2.2 Man-made Fibres Polyester and viscose are the two man-made fibres

used in this investigation. The properties of these fibres were:

Polyester - Fineness, 1.2 den and length, 40mm Viscose -Fineness, l.5 den and length, 38mm

40s Hosiery yarns were spun using these fibres.

Fig. I-Schematic layout showing data and signal flows between hardware components [PC - Personal computer, CCD - Charge coupled device, and CPU -Central processing unit]

These yarns were knitted and dyed in 3 shades(light, medium and dark). The colour uniformity of the dyed fabrics was estimated using spectrophotometer.

2.3 Image Processing - Concept and Application Generally, the cotton fibre maturity is measured by

treating the fibres with caustic soda and then carrying out microscopical examination. Since the fibres are alkali treated in this method, the measured values do not represent the real condition of the fibres during mechanical or chemical processing3

. But image processing is a fundamental, unbiased and quicker technique for measuring fibre maturity. Hence, the same was used in this study.

Image processor is an instrument with a microscope, a video camera and a sample handling equipment. It also has a PC with special hardware for digiti sing and processing of the camera images. The camera images are analysed using special software. Fig.1 shows a chart depicting a schematic layout of the experimental set up and the data and signal flows between hardware components. The hardware converts real microscopic (grey scale) images into a two-dimensional digital matrix. Every matrix element correlates to a grey scale value of a pixel. This digitising method enables computers to analyse and manipulate images.

Image processing involves three distinct steps as given below:

-Image pre-processing: It improves picture quality by way of noise reduction, contrast enhancement, etc.

-Segmentation: In this process, relevant objects are separated from the background.

-Object recognition and interpretation: In this process, the segmented objects are allocated to one of the problem- specific object classes. Identification

298 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 2001

of objects is based on certain descriptors, sets of data describing the form, size, texture, etc. The steps involved in image processing are

schematically shown in Fig. 2.

2.3.1 Measurement of Cotton Fibre Maturity using Image Processor

Using image processor, the degree of fibre thickening in cotton fibres is measured4

.5

. Degree of fibre thickening 'C' is given by

C = Cross - sectional area of the total fibre cell wall .

Area of the circle of the same perimeter

In Fig. 3, the shaded portion represents the total cross-sectional area (A) of fibre cell wall and the dotted portion , the perimeter (P). As per definition,

A A 47rA Degree of f;bre ,h;cken;ng = ,,[:n J = :~ = p '

Degree of fibre thickening is also known as fibre circularity.

For a cotton fibre sample, the maturity ratio is defined as

Feed Dac k - Co ntrol

Output orResuIts

Fig. 2-Steps involved in di gi tal image processing

Fig. 3-Cross-section of a cotton fibre

No. of fibres with circularity ratio of 0 .5 and higher

No. of fibres with circularity ratio of 0 .25 and higher

Image processor is used to assess the maturity ratio of cotton fibres and these maturity ratio values are made use of in developing expressions to predict shade variation in dyed fabrics .

2.4 Shade Variation using Spectrophotometer

The reflectance value for each fabric was measured at 30 different places using spectrophotometer. While measuring reflectance value, three parameters, namely L, a and b are obtained6

.

The range in the values of L, 0 and b for each fabric was calculated and denoted as I1L, l1a and I1b respectively . Then, colour non-uniformity (11£) is given by

11£ = (I1L2 + 1102 + I1b2) 112.

Attempts were made to (i) relate the colour non­uniformity in cotton fabrics with difference in micronaire value and average maturity ratio, and (ii) assess the extent of colour non-uniformity in polyester and viscose fibre fabrics.

3 Results and Discussion

3.1 Colour Non-uniformity in Cotton Fabrics

The pattern of shade variation in cotton knits dyed using light, medium and dark shades is discussed.

3.1.1 Light Shades

It is reasonable to suppose that micronaire value difference in mixing and average maturity ratio influence colour non-uniformity. With a view to quantify the association between M and the two fibre properties under question, various statistical models were attempted. The reciprocative model of the following type was found to give the best fit:

where y=l1£

Xl = difference in micronaire value X2 = average maturity ratio (determined by image

processor) A & B = regression equation coefficients, and

C = constant. By solving the eight equations avai lable, the val ues

of coefficients and constant were obtained and 11£ is given by

CHELLAMANI et al.: ROLE OF FIBRE PROPERTIES IN COLOUR NON-UNIFORMITY OF DYED FABRICS 299

_1- =0.8558-0.9990 x difference In mlcronalre /).E

value + 1.0836 x average maturity ratio A high correlation of 0 .975 was obtained between

actual and predicted values of /)'E. Actual and predicted /).E values are given in Table 3. The mean error of estimate is around 8.2%.

3.1.2 Medium and Dark Shades Normally, when a fabric is dyed using medium!

dark shades, the extent of colour non-uniformity could be expected to be low as compared to that while using light shades. This is due to the additional amount of dye molecules avai lable for the fibres in yarn in the medium and dark shades as compared to that of light shades. The chances of leveling up of differences in fibre/yarn characteristics by the dye molecules is more in medium and dark shades.

While moving from light to medium shades, the /).E

values get reduced, on an average by 5% (reduction in individual cases varies between 3% and 7%) . Similarly, from light to dark shades the average reduction in /).E is around II % (reduction in individual cases varies between 4% and 13%). Generally, the extent of reduction in /).E while moving from li ght to medium or from light to dark is less when the difference in micronaire value in the mixing is also less (Table 4) .

3.1.3 Colour Non-uniformity WIllie Using Different Colours

The average /).E values for blue, orange and magenta colours in all three shades are given in Table 5.

Three colours (blue, orange and magenta) were selected to represent three different areas of the colour spectrum. In the case of each colour as one would move from light to dark, the colour di fference /).E decreases slightly. This is due to the availability of more dye molecules in dark shade which levels off the difference in maturity levels, if any . Each colour being of different structures, the behaviour among each other cannot be compared. But each of them behaves in a similar manner towards mature and immature fibres , only their extent of reaction differs.

3.1.4 Colour Non-uniformity While Using Carded and Combed Yarns in the Fabric

The average /).E values in dyed fabrics whi le using combed yarns (combed yarns are from the same mixing used fo r carded yarns with 18% noil ex traction) are about 20% lower as compared to that when carded yarns are used in light and medium shades, with difference in individual cases varying between 5% and 30%.

Table 3-Difference between actual and predicted t1E values

Fibre/mixing Count spun t1E value % Error" (Ne) Actual Predicted

Single cotton 40s CH & 40s KH 0.562 0.592 5.277

Mixing 40s CH 0.764 0.686 10. 176

Mixing 40s CH 1.438 1.202 16.405

Mixing 40sCH 1.825 2.018 10.562

Single cotton 62s KH 0.592 0.599 1.240

Mixing 62s KH 0.785 0.750 4.440

Mixing 62s KH 0.793 0.929 17.11 9

Mixing 62s KH 2.368 2.354 0.595

Mean 8.230

"% Error = [(actual-predicted)/actual] x 100

Table 4-Reduction in t1E value

% Reduction in t1E Difference in micronaire value Light to medium Light to dark

0.30 4.50 8.04

0.85 6.88 11.08

1.25 8.50 13.20

Table 5-t1E values for different colours

Colour Shade Light Medi um Dark

Blue 1.141 1.014 0.9 15

Orange 1.272 1.127 1.043

Magenta 1.437 1.264 1.139

It is well known that combing preferentially removes fine and immature fibres. Due to this, the difference in micronaire value is expected to come down and the average maturity ratio is expected to go up. Hence, this could be the reason for the reduction in /).E values in combed yarns. However, in dark shades, combed yarn fabrics have only about 8% lower /).E compared to that of carded yarn. This is again due to the more leveling-off effect in dark shades .

The relationship between the ratio of /).E value (combed/carded) and average maturity ratio for different shades is shown in Fig. 4.

3.1.5 Prediction Expression Connecting Shade Variation with Immature Fibre Content as Measured by Caustic Soda Method

Only very few mills have image processor and hence the expression connecting shade variation with maturity ratio could not be made use of by many mi lis . Hence, a separate expression 7 to predict colour non-uniformity from maturity measurements based on caustic-soda method was also developed and is given by

300 INDIAN J. FIBRE TEXT. RES ., SEPTEMBER 2001

5' 1.0

"" 0 c::: < + U 0.9

- - - .,._ - - - - - -lI<

Q "" I'l ::a 0 0.8 ~ (J)

"" + :3 0.7

;; - LIGHT + MEDIUM -*- DARK

"" <I ... 0.6 o ~ !-<

.+

;Z 0.5+-----.----.-- - -----.-----i 0.75 0.78 0.81 0.84 0.87

AVERAGE MATURITY RATIO

Fig. 4-Ratio of DE values (combed/carded) vs average maturity ratio for different shades

_1_ = 2.064 - 0.552 X difference in micronaire value !J.E - 0.025 x immature fibre content

A correlation of 0.98 was obtained between actual and predicted !J.E. The average error of estimate is around 8.6% (Table 6).

3.1.6 Fibre Properties to Produce Fabrics with Acceptable Level ofM

Five dyed fabric samples accepted in the market (passed after inspection) were procured and their !J.E values estimated. The estimation revealed that !J.E of 1.0 in combed yarn fabrics is generally accepted by the buyers. Hence, to maintain !J.E at this level, the level at which the fibre properties are to be maintained is given in Table 7.

3.1.7 Fluorescence Value and Shade Variation in Cotton Fabrics

Studies at Special Instrument Laboratory Inc., USA, showed that ultraviolet fluorescence of cotton fibres is a major contributing factor in the creation of dye streaks and barre in fabrics. In a study by Swift Spinning Mills Inc., a close relation was shown between cotton's reflectance in UV light and the shade to which it dyes. Hence, the fluorescence values of the 6 cottons used in the project were measured using UV fibre glow and these are given in Table 8.

The influence of fluorescence value of cotton on fabric shade variation was assessed for unbleached, half bleached and full bleached fabrics . Surprisingly in all the cases, cotton fluorescence was found to have no significant influence on colour non-uniformity in fabrics. This may be probably due to the very narrow range of the fluorescence value of cottons used in the

Table 6-Error of estimate between actual and predicted DE values

Fibre/mixing Count Immature !:lE value % Errora

----spun fibre Actual Predicted (Ne) content

%

Single cotton 40s KH & 14.5 0.562 0.590 4.921 40sCH

Mixing 40sCH 19.0 0.764 0.712 6.857 Mixing 40s CH 30.2 1.438 1.223 14.970 Mixing 40sCH 34.5 1.825 2.003 9.772 Single cotton 62s KH 17.2 0.592 0.614 3.794 Mixing 62s KH 17.5 0.785 0.712 9.310 Mixing 62s KH 21.5 0.793 0.879 10.837 Mixing 62s KH 37.5 2.368 2.564 8.282

Mean 8.590

a % Error = [(actual-predicted)/actualj x 100

Table 7 - Fibre properties to produce fabrics wi th a !:lE of 1.0 Combination Fibre properties

2

3

Difference Av. maturity Immature fibre in micronaire ratio by image content by

value processor caustic soda method, %

0.50

0.65

0.80

0.60

0.73

0.86

30.0

27.5

25.0

Table 8 - Fluorescence values of cottons used in the study

Cotton Fluorescence value

S4 109.1

S6 110.7

LK 92.5

LRA 98.5

MCUS 98.8

J34 87.8

study. Hence, it is planned to select some more cottons with a wide range in their colour value and then assess its impact on fabric shade variation.

3.2 Colour Non-uniformity in Man-made Fibre Fabrics

3.2.1 Common Defects in Polyester and Viscose Fibres

It is a common experience that imported man-made fibres (polyester and viscose) exhibit better spinning performance and yarn quality as compared to their indigenous counterparts.

The main reason identified for this differential behaviour of indigenous fibres is the presence of defects, namely oligomers, fused fibres, differential filament diameter, undrawn filaments, longitudinal cracks & cavities, tendered fibres and peeling-off of fibrous layers in indigenous man-made fibres8

.

CHELLAMANI et al.: ROLE OF FIBRE PROPERTIES IN COLOUR NON-UNIFORMITY OF DYED FABRICS 301

Table 9-/1£ values for polyester and viscose knitted fabrics

Shade

Light

Medium

Dark

Fibre type

100% Polyester

0.55

0.51

0.44

100% Viscose

0.481

0.460

0.440

3.2.2 Shade Variation in Polyester and Viscose Fabrics Even though man-made fibres do not have the

problem of immaturity, they do have impurities as already explained in section 3.2.1. Hence, the fabrics made out of man-made fibres may also be expected to have a certain level of shade variation. To ascertain this, the knitted fabrics made out of 100% polyester and 100% viscose were dyed in Orange Bril. M~R in three shades and tested for shade variation. Yarn count of 40s was spun and knitted from these fibres. The 13.£ values are given in Table 9. It is observed that the polyester fabrics have a 13.£ of around 0 .50 and the viscose fabrics, around 0.46. As one would proceed from light to dark shade, the 13.£ values tend to reduce in both the fabrics, the per cent reduction being around 20% for polyester and 10% for viscose.

4 Conclusions 4.1 Difference in micronaire value and average

maturity ratio of cottons in the mixing (as measured by image processor) are identified as the major fibre parameters affecting shade variation in dyed fabrics . A prediction expression connecting fibre properties with colour non-uniformity (13.E) (as measured by spectrophotometer) has been developed.

4.2 In dyed fabrics, while moving from light to medium shades, the 13.£ values get reduced by 5%. Similarly, while moving from light to dark shades, the average reduction in 13.£ is around 11 %.

4.3 The 13.£ values of dyed fabrics made out of combed yarns are lower by 20% in light and medium shades and by 8% in dark shades as compared to that of carded yarn fabrics.

4.4 Fibre properties of cotton mixIng to maintain 13.£ in dyed fabrics at acceptable level are suggested.

4.5 Fabrics made out of man-made fibres also exhibit colour non-uniformity after dyeing. The extent of shade variation (13.E) in these fabrics, as measured by spectrophotometer, is found to be around 0.5 .

Acknowledgement The authors are thankful to Mr. M.P.S. Ravindran

and Mr M.K. Vittopa of Spinning Division for conducting the various trials and for statistical computation connected with this project.

References I Davis H, McGregor R, Pastore C & Timble N, Text Res J. 66

(1996) 533. 2 The effect of micronaire on fabric barre, Text Top. XIX(4)

(1990). 3 Sundaram V, Basu A K , Krishna Iyer K R , Narayanan S S &

Rajendran T P, Handbook of Cotton in India (Indian Society for Cotton Improvement, Mumbai), 1999,303.

4 Thibodeaux D P, Developmellt of calibration cottons for fibre maturity, paper presented at the Cotton Conference, Bremen, 11-14 March 1998.

5 Thibodeaux D P, Determinatioll of cotto II maturity by image analysis, paper presented at the Cotton Conference, Bremen. 11-14, March 1998.

6 Billmeyer F W (Jr) & Saltzman M, Principle of Colour Techllology (Wiley Inter-science, New York), 1981, 103.

7 Chellamani K P & Janakiraman K P, Illfluence of fibre properties on shade variation in dyed fabrics , paper presented at the 39'h Joint Technological Conference of ATIRA, BTRA, SITRA & NITRA, New Delhi, 21-22 March 1998

8 Chellamani K P, Gnanasekar K & Gunasekaran R, Studies on fibre-yarn relationships and drafting force and its variability during mechanical processing of man-made fibres, SITRA Res Rep, 36(5) (1991) 15.