CHAPTER 4 EFFECT OF HUMID CONDITIONS ON THE COLOUR APPEARANCE OF DYED COTTON FABRICS

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1 59 CHAPTER 4 EFFECT OF HUMID CONDITIONS ON THE COLOUR APPEARANCE OF DYED COTTON FABRICS 4.1 INTRODUCTION Surface colour measurement and matching are of great importance in the very wide range of goods manufactured in various industries. Matching of the colour of the goods has to be carried out in such a way that it is within the discrimination limits of the human eye. The colour of an object is the response of a complex interaction of incident light on an object and the optical characteristics of the object. Such interaction depends on the amount of colouring matter present and other foreign matter such as moisture, chemical additives etc present in it. In the case of textile materials especially hygroscopic ones, their moisture content varies with respect to the humidity level of the atmosphere. This leads to variation in the interaction of light, which would inturn, affect their colour. Since the colour of textile materials is influenced by the moisture content, the atmospheric humidity should be taken in to account while measuring the colour. It is well known that when light falls on textile materials scattering takes place at the surface. The extent of such scattering depends on the surface characteristics of these materials. In addition to this, light also undergoes diffusion through the material resulting in absorption and scattering within the material. Finally, light comes out of the material as diffuse reflection, which depends on the extent of surface and internal

2 60 scattering that take place (Munsell et al 1933, Billmeyer and Smith 1967). Internal scattering of textile materials depends on number of dye molecules, number of other molecules, which may be air, water or chemical compounds present in it. When a dyed textile material undergoes transformation from the dry to wet state, it results in reduction in reflectance (Jahagirdar et al 2002). This drop in reflectance is due to reduced light scattering. Allen and Goldfinger (1971) noted that a decrease in scattering efficiency would provide more opportunity for absorption of light in the sample and thus contribute in its brightness. Dalton et al (1995) reported that the change in reflectance value of a fabric due to the influence of moisture is purely a physical phenomenon. This is in agreement with observations of Rieker and Gerlinger s (1984) that changes in reflectance properties of a fabric are physically attributable to changes in its surface properties. Kubelka-Munk theory is widely used for colour prediction of dye or pigment mixtures in dyed textile materials. As they are opaque and light absorbing/scattering materials, single-constant Kubelka-Munk theory is applied successfully to describe its complex-subtractive colour mixing. Since the theory does not deal with surface phenomena and medium in which the substrate is embedded, surface correction becomes essential to minimise the inaccuracy. It is always useful to take into account the change in the surrounding medium while assessing the dry colour of a textile material from its wet colour (Tsoutseos and Nobbs 1998 and 2001). In actual situation, the dyed textile materials are exposed to different humid conditions i.e., low level of moisture, which may have effect on their colour. The distribution of moisture could also affect the colour. In such situation, correct colour communication through the whole supply chain and colour reproduction during dyeing becomes difficult. Attempts have not been made in the direction of assessment of colour of the textile materials by taking humid conditions into account. The analysis carried out on the effect of humid

3 61 conditions expressed as relative humidity on the colour of cotton fabrics dyed with direct dyes is reported. 4.2 MATERIALS AND METHODS The particulars on fabric, chemicals, methods of preparation, determination of dye uptake and conditioning of fabric at various humid conditions to obtain various bound water levels are given in various sections of Chapter RESULTS AND DISCUSSION Effect of Humidity on the Colour Fibres carry moisture in the form of bound and unbound water content. The bound water content in the cotton fibre can be as high as 19% (Nakamura et al 1983). However, studies pertaining to the effect of bound water on colour measurement of wet substrates have not been reported. According to Goldfinger et al (1970) the change in colour arises from the change in refractive index as water (1.33) is substituted for air (1). The increase in moisture content within a substrate effectively changes the refractive index at the fibre-air interface to a higher value. The literature review reveals that the previous researchers have examined the role of wet pick up ranging between 50 to 120% in the colour measurement of wet textile fabrics (Goldfinger et al 1970, Allen and Goldfinger 1971, Allen et al 1972 and 1973, Dalton et al 1995, Manian et al 2000, Tsoutseos and Nobbs 2000, Jahagirdar et al 2002, Tiwari and Jahagirdar 2007). Smith (1979) has also reported about the effect of drying wet fabric to around 20% moisture content on its colour. But in actual situation the moisture content of a dyed fabric may fluctuate due to variations in the RH of the surrounding environment. Such variations influence the interaction of light with the fabric and its colour

4 62 (Tsoutseos and Nobbs 2000). Inadequate assessment of the role of RH on the colour of substrates can complicate colour communication throughout the supply chain and adversely affect the production/reproduction of colour. In the study, samples dyed with different combinations of direct dyes and conditioned at various RH levels from 25% to 85% in order to obtain fabrics with different levels of moisture content were considered. The moisture contents obtained for 25%, 45%, 65% and 85% RH levels were 2.7%, 4.8%, 7.2% and 10.8% odwf respectively. Since the cotton fabric can hold up to 19% moisture content as bound moisture (Nakamura et al 1983), all the above conditioned fabrics carry only bound water in them. The effect of bound water content on the colour of these dyed fabrics was analysed and reported here. Figure 4.1 shows the K/S values of the Twill fabric, T1, dyed to different depths of shade and conditioned at various RH levels. It shows that the colour strength of the fabrics, for all dye combinations and depths of shade, increase with increase in relative humidity. It is already stated that when the relative humidity is increased to 85%, the bound water content of the fabric increases to 10.8%. This increase in bound water content in the fabric changes the refractive index of the medium in which light travel from air to fibre and fibre to air. The difference between the dry fibre (RI = 1.58) and fibre with bound water (RI = 1.33) content is that the former has only air (RI = 1) as medium inside whereas the later has a combination of air and bound water as medium inside. When light is incident on dyed fabric, the lower the difference between the refractive indices of mediums involved, the higher is the intensity of the colour perceived due to reduced light scattering effect. In the present case the fabrics which are conditioned at different relative humidity levels appear darker because of partial replacement of air by bound water which has higher refractive index. The effect becomes more pronounced as the replacement of air by bound water increases. In addition,

5 63 Y RY R YB RYB RB B Figure 4.1 Effect of humidity on the depth of colour of Twill Fabric, T1 the increase in colour strength is higher for dark shades compared to the light shades when bound moisture content increases. The K/S value and depth of colour of other twill fabric T2, plain fabrics P1 and P2 dyed with all combination of dyes are given in Figures A1.1 and A1.2 and also follow the same trend with respect to RH. The practical significance of the change in colour with respect to RH was analysed by calculating the colour difference ( E * ab) of the dyed fabrics conditioned at various RH levels compared to a standard substrate conditioned at 0% RH. A summary of results for fabric T1 are given in Table 4.1. It is shown that colour differences between the standard and fabrics conditioned at 25% RH are not appreciable for any of the dye combinations with the exception of the samples dyed with C.I. Direct Yellow 106.

6 64 Table 4.1 Effect of change in humidity on E * ab among T1 Twill fabric samples based on bone dry samples as standard Dye Direct Red 243 (R) Direct Yellow 106 (Y) Direct Blue 85 (B) RY YB RB RYB E * ab Values % Shade Change in %RH Levels 0 to 25 0 to 45 0 to 65 0 to

7 65 The Yellow dye applied has a high luminosity factor (Shah and Gandhi 1990) which causes a large change in colour for a small change in moisture. However, results also show that colour differences between the standard and samples conditioned at RH 45%, for the majority of dye combinations, are greater than 1.5. Samples dyed with C.I. Direct Blue 85, or combinations of C.I. Direct Red 243 and C.I. Direct Blue 85 at 45% RH, however, exhibit slightly lower E * ab values, although the values are still greater than one. This may be due to the relatively low reflectance of these samples and having less number of polar groups in their structure (Figure 3.3) in comparison to other dyed samples, which might affect the role of increased moisture content on colour until the change in moisture content reaches higher levels. It can also be inferred that fabrics with low reflectance values exhibit an insignificant change in colour with slight variations in moisture content. In addition, colour differences were also obtained among samples conditioned at 65% RH and those under other RH levels to assess the significance of variation in relation to standard conditions. Results are shown in Table 4.2. It can be seen that colour differences between the fabrics conditioned at 65% and 45% RH are not appreciable for any of the dye combinations with the exception of the samples dyed with C.I. Direct Yellow 106 and combination of Red and Yellow. However, results also show that colour differences between the samples conditioned at 65% and 25%, 85% RH and dried fabric, for the majority of dye combinations, are greater than 1.5. The E * ab values of other fabrics T2, P1 and P2 dyed with all combination of dyes are given in Tables A1.1 A1.6 and also follow the same trend. The colour difference value between the sample/bulk and standard play an important role in the dyeing industry. Although colour difference

8 66 Table 4.2 Effect of change in humidity on E * ab among T1 Twill fabric samples based on standard samples conditioned at 65% RH Dye C.I.Direct Red 243 (R) C.I.Direct Yellow 106 (Y) C.I.Direct Blue 85 (B) RY YB RB RYB % Shade E * ab based on change in %RH Levels 65 to 0 65 to to to

9 67 tolerances are specific to each dyeing unit, but in majority of the cases where critical evaluations of colour are important, E * ab is confined to 1 or lower values. Since a small change in moisture content of the fabric affects the colour of the material, there is a possibility of misinterpretation of E * ab values. This indicates that there is a need for giving due importance to RH while measuring the colour Role of Humidity on the Colour of Fabrics Having Identical Dye Uptake In order to bring out the true effect of increase in RH on the colour of fabrics dyed with various combinations of dyes, K/S value was calculated for all these fabrics at a particular dye uptake (2g/100g of fabric) as explained below. Initially, the amount of dye present in 100g of fabric was calculated using the actual dye uptake values given in Table 4.3 and %shades for all the dyes. Then plots were drawn between the amount of dye present in 100g of fabric (instead of % shade) and corresponding K/S value at each RH level. The plot drawn for twill T1 fabric and C.I. Direct Red 243 at 85% RH is given in Figure 4.2. The plot also shows a best-fit straight line and the corresponding equation. With help of the equation, K/S was calculated for the 2g/100g of fabric for all RH levels. The same steps were followed for all fabrics, dye combinations and RH levels. In order to compare the effect of RH on colour of fabric dyed with various combinations of dyes, the %increase in K/S value was calculated with respect to the K/S value of 0% RH conditioned sample for all fabrics and dye combinations. The %increase in K/S value, when the RH is increased from 0% to different levels, for all combination of dyes for fabric

10 68 T1 is given in Figure 4.3. It shows that the %increase in K/S value increases gradually up to 65% RH and then increases drastically. Table 4.3 Calculated % dye uptake of fabrics Dyed samples R Y B RY YB RB RYB % Shade % Dye uptake Twill Plain T 1 T 2 P 1 P

11 69 Figure 4.2 Plot with best fit line and equation between dye up take per 100g of fabric and K/S value for the fabric T1, dyed with C.I. Direct Red 243 and conditioned at 85% RH Figure 4.3 Effect of increase in humidity on the increase in depth of colour of Twill fabric, T1 having same dye uptake (2g/100g of fabric)

12 70 It clearly brings out the fact that at higher RH levels there is migration of moisture towards the surface of the fibre due to capillary condensation, which in turn results in reduced scattering of light and increase in depth of the colour. The figure also reveals that the increase in colour is more prominent for the fabric dyed with C.I. Direct Yellow 106 at all RH levels than the fabrics dyed with all other combination of dyes. The fabric dyed with C.I. Direct Blue 85 shows lower value in %increase in colour compared to all other fabrics at all RH levels. The %increase in K/S value, when the RH is increased from 0% to different levels for all combination of dyes for other fabrics T2, P1 and P2 is given in Figures A1.3 and A1.4 and also found to follow the same trend Effect of Fabric Structure and Humidity on Colour The effect of fabric structure on change in colour of the fabrics dyed with various combinations of dyes and exposed to different RH levels was analysed by determining the K/S value for a dye uptake of 3 g/m 2 of fabric as explained below. Initially, the amount of dye present in 100g of fabric was calculated using the actual dye uptake values given in Table 4.3 and % shades for all dyes. From that value the amount of dye present in one m 2 of fabric was calculated using GSM (Table 3.1) value of the fabric. Then plots were drawn between the amount of dye present in one m2 of fabric and corresponding K/S value at a particular RH level. The plot drawn for twill T1 fabric and Direct Red 243 at 85% RH is given in Figure 4.4. The plot also shows a best-fit straight line and the corresponding equation. With help of the equation, K/S was calculated for the 3g/m 2 of fabric for all RH levels. The same steps were followed for all fabrics, dye combinations and RH levels.

13 71 Figure 4.4 Plot with best fit line and equation between dye up take per square meter of fabric and K/S value for the fabric T1, dyed with C.I. Direct Red 243 and conditioned at 85% RH The %increase in K/S value with respect to RH for all types of fabrics and for the C.I. Direct Red 243 is shown in Figure 4.5. It shows that the %increase in K/S value is higher for twill fabrics than that for the plain fabrics. The increase in depth of the colour with respect to increase in RH falls between 27.7% and 76.7%. It is due to the higher surface area of the twill fabrics, which is about 4.6% to 15.6% higher than that of plain fabrics considered. The effective surface area utilized for colour measurement was calculated as per the procedure explained below. In order to analyse the effect of RH on colour of fabrics having different weave and specifications, the area utilized for color measurement was calculated using the model developed for all fabrics. The schematic diagram of the model used for this study is given in Figure 4.6.

14 72 Figure 4.5 Effect of increased humidity on the depth of colour of various fabrics containing a normalized amount (3 g/m 2 ) of C.I. Direct Red 243 The increase in dimension of the fibres present in the yarn/fabric exposed to different RH levels due to swelling is accommodated within the voids in the yarn. In this model the yarn is considered as cylindrical in shape and assumed that only front side of the fabric (50%) is exposed for colour measurement. The effective area used for colour measurement was calculated by multiplying the area of one repeat (A) as mentioned in Figure 4.6 and the total number of repeats (T) present in the area utilized for colour measurement in spectrophotometer (A*T). Area (A) of one repeat of the plain and twill fabrics indicated in figure was calculated using the formulae given below.

15 73 Weft Warp (i) Weft Warp (ii) Figure 4.6 Repeating unit for (i) plain and (ii) 1/3 twill fabrics

16 74 For plain fabric, A = 2 [( R 1 H 1 )C 1 + ( R 2 H 2 )C 2 ] (4.1) For twill fabric, A = 4[( R a H a )C a + ( R b H b )C b ] (4.2) where, R 1, R a Radius of warp yarn R 2, R b Radius of weft yarn H 1 = 2(P s -R 2 ) where, P s Distance between two adjacent picks H 2 = 2(E s -R 1 ) where, E s Distance between two adjacent ends H a = 4P s -6R a (4.3) H b = 4E s -2R b (4.4) C 1, C a Warp crimp and C 2, C b Weft crimp. The total number of repeats present in the area utilized for colour measurement in spectrophotometer is calculated using the formula given below. Total number of repeats (T) present in the area used for colour measurement Area of aperture in the spectrophotometer = l*b (4.5) where, l*b -Area of one repeat l = 2E s for plain, l = 4E s for twill b = 2P s for plain, b = 4P s for twill

17 75 The effective area used for colour measurement was calculated using the above formula and is given in Table 4.4. Table 4.4 Effective exposed area of fabrics used for colour measurement S. No. Type of Fabric Total Exposed Area (mm 2 ) 1 T T P P The Table 4.4 also clearly indicates that the surface area of both twill fabrics is higher than that of plain fabrics. Since the twill fabric has higher surface area, the incident light has better interaction with fabric surface, dye molecules and moisture present in it than plain fabric during the assessment of colour. Due to the better interaction of light with fabric and other molecules present in it, the twill fabric shows higher %increase in colour than plain fabric when the RH level increases to higher value. The %increase in K/S value with respect to RH for all types of fabrics and for other combinations of dyes is shown in Figure A1.5 and also found to follow the same trend. Since the fabric geometry and humidity has influence on the colour, care should be taken on both the parameters while measuring the colour. 4.4 CONCLUSIONS The study clearly reveals that not only wetting, but also moisture absorbed by dyed fabrics from the humid atmosphere has an appreciable effect on the measured depth of colour. The difference, expressed by both change in K/S and E * ab, between conditioned samples at 0% RH and those

18 76 at other humidity levels is higher when the RH is above 45%. Further the study reveals that the difference in colour due to moisture absorption is gradual up to 65% RH and drastic above this level. With respect to standard conditions the difference between samples at 65% RH and those at other humidity levels is also more pronounced when the RH% is increased. Although the amount of dye on fabrics was normalized for various woven structures examined, it was found that the fabric geometry plays an important role in the measurement of colour. Indeed the two twill fabrics studied exhibited higher increases in their depth of colour than plain woven fabrics at all relative humidity levels examined. Therefore, due importance should be given to the relative humidity of the atmosphere while measuring the colour of dyed cotton fabrics.