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

Transcription

1 Indian Journal of Fibre & Textile Research Vol. 26. September 200 1, pp Role of fibre properties in colour non-uniformity of dyed fabrics K P Chellamani a, M Arulmozhi & K Kumarasamy The South India Textile Research Association, Coimbatore , India Received 22 February 2000; accepted II May 2000 The vari ation in col our 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 yarn s in order to maintain high level of colour uniformity in dyed fabrics. Image process ing technique was employed in the study for assessing 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 variati on 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 th e 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 th e 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 avoid streaks in dyed fabrics 2 Colour uniformity in piece-dyed fabrics is an important fabric property and is used as a criterion for j udgi ng the value of the fabric. However, the norms/ "To whom all the correspondence shou ld be addressed. Phone: ; Fax: ; si tra@vsnl.com 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 The maturity ratio by image processor varies between 0.75 and 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 given in Table I.

2 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 s KH &40s CH Mixing sCH Mixing sCH Mixing sCH Single cotton s KH Mixing s KH Mixing s KH Mixing s KH Process sequence Blow room Table 2-Process parameters used for the study 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 TM sCH s KH 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 processing 3. 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

3 298 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 Measurement of Cotton Fibre Maturity using Image Processor Using image processor, the degree of fibre thickening in cotton fibres is measured 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 circul arity. For a cotton fibre sample, the maturity ratio is defined as Feed Dac k - Control 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 obtained 6. 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 = (I1L I1b 2 ) 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. Output orresuits Fig. 2-Steps involved in di gi tal image processing 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: Fig. 3-Cross-section of a cotton fibre 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 available, the val ues of coefficients and constant were obtained and 11 is given by

4 CHELLAMANI et al.: ROLE OF FIBRE PROPERTIES IN COLOUR NON-UNIFORMITY OF DYED FABRICS 299 _1- = x difference In mlcronalre /).E value x average maturity ratio A high correlation of 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% 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 available 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) 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 Colour Non-uniformity While Using Carded and Combed Yarns in the Fabric The average /).E values in dyed fabrics while using combed yarns (combed yarns are from the same mixing used fo r carded yarns with 18% noil extraction) 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 va lues Fibre/mixing Count spun t1e value % Error" (Ne) Actual Predicted Single cotton 40s CH & 40s KH Mixing 40s CH Mixing 40s CH Mixing 40sCH Single cotton 62s KH Mixing 62s KH Mixing 62s KH Mixing 62s KH "% Error = [(actual-predicted)/actual] x 100 Table 4-Reduction in t1e value Mean Difference in % Reduction in t1e micronaire value Light to medium Light to dark Colour Table 5-t1E values for different colours Shade Light Medium Dark Blue Orange Magenta 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 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

5 300 INDIAN J. FIBRE TEXT. RES., SEPTEMBER ' 1.0 "" 0 c::: < U Q "" I'l ::a ~ (J) "" + :3 0.7 ;; - LIGHT + MEDIUM -*- DARK "" <I o ,._ li< ~!-< ;Z i AVERAGE MATURITY RATIO Fig. 4-Ratio of DE values (combed/carded) vs average maturity ratio for different shades _1_ = X difference in micronaire value!j.e 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) 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 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 % Error a ---- spun fibre Actual Predicted (Ne) content % Single cotton 40s KH & sCH Mixing 40sCH Mixing 40s CH Mixing 40sCH Single cotton 62s KH Mixing 62s KH Mixing 62s KH Mixing 62s KH Mean a % Error = [(actual-predicted)/actualj x 100 Table 7 - Fibre properties to produce fabrics with a!:le of 1.0 Combination Fibre properties Difference Av. maturity Immature fibre in micronaire ratio by image content by value processor caustic soda method, % 2 3 Table 8 - Cotton Fluorescence values of cottons used in the study Fluorescence value S S LK 92.5 LRA 98.5 MCUS 98.8 J 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 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 fibres 8.

6 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 100% Viscose 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 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 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) 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, Thibodeaux D P, Developmellt of calibration cottons for fibre maturity, paper presented at the Cotton Conference, Bremen, March Thibodeaux D P, Determinatioll of cotto II maturity by image analysis, paper presented at the Cotton Conference, Bremen , March Billmeyer F W (Jr) & Saltzman M, Principle of Colour Techllology (Wiley Inter-science, New York), 1981, 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, March 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.