Computer Color Matching and Levelness of PEG-Based Reverse Micellar Decamethyl cyclopentasiloxane (D5) Solvent-Assisted Reactive Dyeing on Cotton Fiber

The color matching and levelness of cotton fabrics dyed with reactive dye, in a non-aqueous environmentally-friendly medium of decamethylcyclopentasiloxane (D5), was investigated using the non-ionic surfactant reverse-micellar approach comprised of poly(ethylene glycol)-based surfactant. The calibration dyeing databases for both conventional water-based dyeing and D5-assisted reverse micellar dyeing were established, along with the dyeing of standard samples with predetermined concentrations. Computer color matching (CCM) was conducted by using different color difference formulae for both dyeing methods. Experimental results reveal that the measured concentrations were nearly the same as the expected concentrations for both methods. This indicates that the D5-assisted non-ionic reverse micellar dyeing approach can achieve color matching as good as the conventional dyeing system. The levelness of the dyed samples was measured according to the relative unlevelness indices (RUI), and the results reveal that the samples dyed by the D5 reverse micellar dyeing system can achieve good to excellent levelness comparable to that of the conventional dyeing system.


Introduction
Textile wet processing has long been considered to be a significant consumer of water and producer of effluents [1,2]. In the conventional water-based reactive dyeing process, large amounts of salt were required to eliminate the repulsion force between dye molecules and the fiber surface, in order to enhance dye affinity and accelerate dye absorption [3]. To reduce effluent discharges and save environmental cost, alternatives such as low-salt or salt-free dyeing [4][5][6], dyeing using natural mordants [7][8][9], reusing wastewater for dyeing [10] and supercritical carbon dioxide dyeing [11][12][13] have been tried and are being used.
For reasons related to the need for ecological friendliness, energy saving and safety of the working environment, the use of reverse micelles has also been considered as one of the mediums for non-aqueous dyeing [14,15]. The reverse micelles are nanoscale spherical aggregates, which are self-assembled by certain surfactants in non-polar media [16,17]. Their ability to solubilize a small amount of water in the interior region can provide a stable aqueous micro-environment (water-pool) in non-aqueous media. Attempts have been made to dye cotton with non-ionic reverse micelles in non-aqueous media [18][19][20]. A previous study has also investigated the feasibility of dyeing cotton samples; (d) measuring the color levelness of dyed samples; and (e) comparing the differences between conventional water-based dyeing and D5 solvent-assisted dyeing.

Materials and Reagents
Commercially ready-for-dyeing pure cotton interlock knitted fabrics, 40 wales per inch (wpi) and 40 course per inch (cpi), were first cleaned with a solution containing 2 g/L soda ash and 2 g/L soap with a thermal-control shaker at a temperature of 90 • C for 30 min, thoroughly rinsed with cold water and dried at room temperature. After that, the cleaned fabrics were conditioned at relative humidity (RH) of 65% ± 2% and 20 ± 2 • C for at least 24 h before further experiment. Nonionic surfactant poly(ethylene glycol) (12) tridecyl ether (C 13 H 27 (OCH 2 CH 2 ) n OH) (Sigma-Aldrich, St. Louis, MO, USA) (n=12) was used. Decamethylcyclopentasiloxane (D5) (Green Earth Cleaning, Kansas City, MO, USA) was used as the organic solvent, and n-octanol (Alfa Aesar, Heysham, UK) was used as the co-surfactant in the dyeing process. Sodium chloride (NaCl) (VWR Chemicals, Leuven, Belgium), i.e., salt, was used as the exhausting agent, in conventional dyeing. Soda ash (Na 2 CO 3 ) (Sigma-Aldrich, St. Louis, MO, USA) was used as the color fixing agent. Both surfactant, solvent, co-surfactant, sodium chloride and soda ash were of reagent grade. Three reactive dyes (types of Levafix Yellow CA, Levafix Red CA and Levafix Blue CA dye) (Dystar, Shanghai, China) were directly used without further purification.

Preparation of Calibration Curves
Cotton interlock knitted fabrics were used. Calibration dyeing was conducted at a liquor-to-goods ratio of 50:1. The dyeing recipe was as shown in Table 1. The dyed sample used for preparing the calibration curves is termed as the "batch sample". The amount of NaCl added was based on the concentration of reactive dye used (percentage on weight of fabric, % owf) ( Table 1). Figure 1 depicts the dyeing profile of conventional water-based dyeing. The cotton fabric was first immersed in the dye liquor, and the prepared dye liquor was then poured into a shaking water bath. The dyeing process was conducted at 60 • C with 165 shakes/min for 40 min. After that, the corresponding amount of Na 2 CO 3 (based on Table 1) was added into the dye liquor for color fixation. The fixation process was then conducted at 60 • C with 165 shakes/min for 60 min. In order to remove the unfixed dye and residues on the fabrics, soaping was conducted after the fixation process by immersing the dyed cotton fabric in soap solution (2 g/L) and rinsing for 15 min at 60 • C with a shaking speed of 165 shakes/min. The cotton fabric was then rinsed with water, air-dried and conditioned at relative humidity of 65% ± 2% and 20 ± 2 • C for at least 24 h before further experiment.

Preparation of PEG-Based Reverse Micelle Dye Liquors
Series of PEG-based reverse micelles were prepared by a simple injection method at room temperature. The non-ionic surfactant (PEG) and co-surfactant (n-octanol) were first premixed (with agitation). The surfactant to co-surfactant ratio was 1:8, and the molar ratio was 1:1.49 (in weight ratio). The surfactant/co-surfactant mixture was then dissolved in D5 solvent to obtain the organic surfactant solution for facilitating the self-assembly of PEG-based reverse micelles. The solvent volume to cotton weight ratio was 8:1. A controlled amount of reactive dye aqueous solution was then added into the reverse micellar system dropwise. After the injection of the dye solution, the mixture was vigorously stirred for 2 min until a well-dispersed solution with reverse micelle encapsulated reactive dye was obtained.

Preparation of Calibration Curves
Cotton interlock knitted fabrics were used. Calibration dyeing (with dye concentrations of 0.5%, 1.5%, 2.5%, 3.5% and 6.0% owf) was conducted at surfactant to water ratio of 1:30, in molar ratio (0.033:1 in mole ratio or 1:0.778 in weight ratio). The dyed sample used for preparing the calibration curves is termed as the "batch sample".

Reverse Micellar Dyeing Process
The reverse micellar dye liquor was prepared by adding the corresponding amount of surfactant, co-surfactant, dye solutions, D5 solvent and deionized water without adding salt. The dyeing profile of reverse micellar D5 solvent dyeing is as shown in Figure 2. The cotton fabric was first immersed in the reverse micellar dye liquor, and the prepared dye liquor was poured into a shaking water bath. The dyeing process was conducted at 70 °C with 165 shakes/min for 40 min. After that, the corresponding amount of soda ash (Table 2) was added into the dye liquor for dye fixation. The fixation process was conducted at 70 °C with 165 shakes/min for 60 min. Soaping was conducted after the fixation process by immersing the dyed cotton fabric in soap solution (2 g/L) and rinsing for 15 min at 60 °C with the shaking speed of 165 shakes/min. The rinsed cotton fabric was then further rinsed with water, air-dried and conditioned at relative humidity of 65% ± 2% and 20 ± 2°C for at least 24 h prior to further experiment.   Series of PEG-based reverse micelles were prepared by a simple injection method at room temperature. The non-ionic surfactant (PEG) and co-surfactant (n-octanol) were first premixed (with agitation). The surfactant to co-surfactant ratio was 1:8, and the molar ratio was 1:1.49 (in weight ratio). The surfactant/co-surfactant mixture was then dissolved in D5 solvent to obtain the organic surfactant solution for facilitating the self-assembly of PEG-based reverse micelles. The solvent volume to cotton weight ratio was 8:1. A controlled amount of reactive dye aqueous solution was then added into the reverse micellar system dropwise. After the injection of the dye solution, the mixture was vigorously stirred for 2 min until a well-dispersed solution with reverse micelle encapsulated reactive dye was obtained.

Preparation of Calibration Curves
Cotton interlock knitted fabrics were used. Calibration dyeing (with dye concentrations of 0.5%, 1.5%, 2.5%, 3.5% and 6.0% owf) was conducted at surfactant to water ratio of 1:30, in molar ratio (0.033:1 in mole ratio or 1:0.778 in weight ratio). The dyed sample used for preparing the calibration curves is termed as the "batch sample".

Reverse Micellar Dyeing Process
The reverse micellar dye liquor was prepared by adding the corresponding amount of surfactant, co-surfactant, dye solutions, D5 solvent and deionized water without adding salt. The dyeing profile of reverse micellar D5 solvent dyeing is as shown in Figure 2. The cotton fabric was first immersed in the reverse micellar dye liquor, and the prepared dye liquor was poured into a shaking water bath. The dyeing process was conducted at 70 • C with 165 shakes/min for 40 min. After that, the corresponding amount of soda ash ( Table 2) was added into the dye liquor for dye fixation. The fixation process was conducted at 70 • C with 165 shakes/min for 60 min. Soaping was conducted after the fixation process by immersing the dyed cotton fabric in soap solution (2 g/L) and rinsing for 15 min at 60 • C with the shaking speed of 165 shakes/min. The rinsed cotton fabric was then further rinsed with water, air-dried and conditioned at relative humidity of 65% ± 2% and 20 ± 2 • C for at least 24 h prior to further experiment.

Preparation of PEG-Based Reverse Micelle Dye Liquors
Series of PEG-based reverse micelles were prepared by a simple injection method at room temperature. The non-ionic surfactant (PEG) and co-surfactant (n-octanol) were first premixed (with agitation). The surfactant to co-surfactant ratio was 1:8, and the molar ratio was 1:1.49 (in weight ratio). The surfactant/co-surfactant mixture was then dissolved in D5 solvent to obtain the organic surfactant solution for facilitating the self-assembly of PEG-based reverse micelles. The solvent volume to cotton weight ratio was 8:1. A controlled amount of reactive dye aqueous solution was then added into the reverse micellar system dropwise. After the injection of the dye solution, the mixture was vigorously stirred for 2 min until a well-dispersed solution with reverse micelle encapsulated reactive dye was obtained.

Preparation of Calibration Curves
Cotton interlock knitted fabrics were used. Calibration dyeing (with dye concentrations of 0.5%, 1.5%, 2.5%, 3.5% and 6.0% owf) was conducted at surfactant to water ratio of 1:30, in molar ratio (0.033:1 in mole ratio or 1:0.778 in weight ratio). The dyed sample used for preparing the calibration curves is termed as the "batch sample".

Reverse Micellar Dyeing Process
The reverse micellar dye liquor was prepared by adding the corresponding amount of surfactant, co-surfactant, dye solutions, D5 solvent and deionized water without adding salt. The dyeing profile of reverse micellar D5 solvent dyeing is as shown in Figure 2. The cotton fabric was first immersed in the reverse micellar dye liquor, and the prepared dye liquor was poured into a shaking water bath. The dyeing process was conducted at 70 °C with 165 shakes/min for 40 min. After that, the corresponding amount of soda ash ( Table 2) was added into the dye liquor for dye fixation. The fixation process was conducted at 70 °C with 165 shakes/min for 60 min. Soaping was conducted after the fixation process by immersing the dyed cotton fabric in soap solution (2 g/L) and rinsing for 15 min at 60 °C with the shaking speed of 165 shakes/min. The rinsed cotton fabric was then further rinsed with water, air-dried and conditioned at relative humidity of 65% ± 2% and 20 ± 2°C for at least 24 h prior to further experiment.   Simulated dyeing with a known concentration of dye was conducted to predict the dye concentration of the calibration dyeing of both conventional water-based and reverse micellar methods. Cotton interlock knitted fabrics were used. The dyed fabrics were regarded as the standard samples used for color matching. The concentration of dye of the standard sample for preparing the mixture of color was as shown in Table 3.

Establishment of Calibration Curves
Color yield of the dyed fabrics was measured by Color Eye 7000A Spectrophotometer (X-Rite, Grand Rapids, MI, USA). The reflectance and K/S values were measured. The K/S sum value was obtained by the summation of the K/S values measured from a 400-700 nm wavelength. The face of the dyed fabrics was measured. The condition of measurement was set under specular reflection with a large aperture diameter (30 mm). The opacity of the fabric was assured by folding the fabric two times. The calibration curves were prepared in graphic form by a plot of the K/S sum value versus the concentration of dye (%). Illuminant D 65 and a standard observer of 10 • were used.
The color yield, expressed as the K/S value, was calculated by Equation (1) at wavelengths of 400-700 nm with 10-nm intervals within the visible spectrum. The higher the K/S value, the more is the dye uptake and the better is the color yield.
where K is the absorption coefficient, depending on the concentration of the colorant, S is the scattering coefficient, caused by the dyed substrate, and R is the reflectance of the colored sample [41].

Prediction of the Dye Recipe with Different Color Difference Equations
Nine color difference equations were used for predicting the dye recipe, including CIE L*a*b*, CIE L*u*v*, ANLAB, Hunter lab, FMC2, JPC 79, CMC 1.0, BFD 1.0 and CIE94 1.0. Color yields of samples dyed with different concentrations of dye were measured by the Color Eye 7000A Spectrophotometer (X-Rite, Grand Rapids, MI, USA) using illuminant D 65 under specular reflection and a standard observer of 10 • with a large aperture diameter (30 mm). The color yield, expressed as the K/S value, was calculated by Equation (2).
K/S mixture = (K/S) 1 + . . . + (K/S) n = K 1 /S sub + . . . + K n /S sub + K sub /S sub , where K 1 to K n are coefficients of the absorption of the dyes, K sub and S sub are the coefficients of absorption and scattering of the substrate, respectively [41].

Levelness Measurement
The relative unlevelness indices (RUI) are obtained by using the reflectance values of the three randomly-selected spots on standard and batch-dyed samples over the visible spectrum (λ = 400-700 nm) at intervals of 10 nm. The measurement was conducted by Color Eye 7000A Spectrophotometer (X-Rite, Grand Rapids, MI, USA) using illuminant D 65 under specular reflection and a standard observer of 10 • with a large aperture (30 mm).
The RUI mainly consisted of four equations: (a) the standard deviation (s λ ) of reflectance values measured at a specific wavelength (Equation (3)); (b) the uncorrected relative unlevelness index (RUI u ) corrected relative unlevelness index (RUI c ) obtained by the summation of the coefficients of variation of the reflectance values (Equation (5)); and (d) the relative unlevelness index (RUI) obtained by multiplying the coefficients of variation of reflectance by the photopic relative luminous efficiency function (V λ ) (Equation (6)) [34]. The suggested interpretation of the RUI values is as listed in Table 4.

Reflectance Values of the Dyed Samples
Figures 3-5 depict reflectance curves of red, yellow and blue color, respectively, of the batch-dyed samples, while Figure 6 presents reflectance curves of the color mixture of dyed standard samples. With regards to Figures 3-5, the results show that reflectance curves of red, yellow and blue color of the samples batch dyed by using the conventional water-based method and the reverse micellar D5-assisted method do not overlap at dye concentrations from 0.5% to 6.0%. Reflectance values of the sample batch dyed in water from 0.5% to 6.0% are higher than those of the sample dyed in D5. This indicates that color yield of the sample batch dyed in water was lower with lower dye absorption in the fiber, and the shade thus appeared to be lighter when compared with the batch sample dyed in D5. Figure 6 shows that reflectance values of the standard samples dyed in water with 0.3-3.0% dye concentrations are higher than standard samples dyed in D5, and no overlap of the reflectance curves was found. This indicates that the color yield of the standard samples dyed in water from 0.3 to 3.0% dye concentrations was lower with lower dye absorption in the fiber, and the shade thus appeared to be lighter when compared with the standard samples dyed in D5. Figures 3-6, although the reflectance values of batch and standard dyed samples are not similar, the shapes of the reflectance curves of the batch and standard samples dyed by conventional and reverse micellar D5-assisted methods are similar to each other, without any peak shifting of the curves found. This indicates that the use of D5 for the preparation of PEG-based reverse micelles and as the medium for solvent-assisted dyeing can be one of the alternative methods to achieve electrolyte-free and water-saving dyeing of cotton fiber and for computer color matching (CCM) without chromatic shift. The possible reason was the use of the PEG-based reverse micellar system, which could strongly reduce the ionization effect between reactive dye molecules and the cotton fiber and improve the swelling of cotton fiber [15,18]. shifting of the curves found. This indicates that the use of D5 for the preparation of PEG-based reverse micelles and as the medium for solvent-assisted dyeing can be one of the alternative methods to achieve electrolyte-free and water-saving dyeing of cotton fiber and for computer color matching (CCM) without chromatic shift. The possible reason was the use of the PEG-based reverse micellar system, which could strongly reduce the ionization effect between reactive dye molecules and the cotton fiber and improve the swelling of cotton fiber [15,18].   shifting of the curves found. This indicates that the use of D5 for the preparation of PEG-based reverse micelles and as the medium for solvent-assisted dyeing can be one of the alternative methods to achieve electrolyte-free and water-saving dyeing of cotton fiber and for computer color matching (CCM) without chromatic shift. The possible reason was the use of the PEG-based reverse micellar system, which could strongly reduce the ionization effect between reactive dye molecules and the cotton fiber and improve the swelling of cotton fiber [15,18].            Figure 7 show that the K/S sum values of red, yellow and blue colors of the samples dyed in the reverse micellar D5 system are higher than samples dyed in water, indicating that the use of the reverse micellar D5 dyeing system can achieve better color yield than the conventional dyeing system.

Linearity of the Calibration Curves
The linearity of the calibration curves was calculated in terms of R-square (R 2 ), a method widely applied for measuring the goodness-of-fit of a regression [42]. The value of R-square ranged from 0 to 1, where zero meant non-linear and one meant linear in structure. As presented by Figure 7 and Table 5, the values of R-square of water-dyed fabrics ranged from 0.9605 (water yellow) to 0.9937 (water red), whereas the R-square of D5-dyed fabrics ranged from 0.9630 (D5 blue) to 0.9933 (D5 red). This indicates that the calibration curves of samples dyed using the conventional and the D5 reverse micellar system are almost linear in structure and, thus, suitable for computer color matching. water, indicating that the use of the reverse micellar D5 dyeing system can achieve better color yield than the conventional dyeing system. The linearity of the calibration curves was calculated in terms of R-square (R 2 ), a method widely applied for measuring the goodness-of-fit of a regression [42]. The value of R-square ranged from 0 to 1, where zero meant non-linear and one meant linear in structure. As presented by Figure 7 and Table  5, the values of R-square of water-dyed fabrics ranged from 0.9605 (water yellow) to 0.9937 (water red), whereas the R-square of D5-dyed fabrics ranged from 0.9630 (D5 blue) to 0.9933 (D5 red). This indicates that the calibration curves of samples dyed using the conventional and the D5 reverse micellar system are almost linear in structure and, thus, suitable for computer color matching.   Table 6 shows the color matching recipes of standard samples dyed by the conventional water-based method with different color difference formulae. It reveals that color matching recipes generated for   Table 6 shows the color matching recipes of standard samples dyed by the conventional water-based method with different color difference formulae. It reveals that color matching recipes generated for Sample 1 are similar (yellow: 0.12; blue: 0.08; and red: 0.07) even though different color difference formulae were used. The color matching recipes generated for Sample 2 (yellow: 0.62; blue: 0.50; and red: 0.44) and Sample 3 (yellow: 1.1; blue: 0.86; and red: 0.96) are also quite constant except when using JPC79 (yellow: 0.53; blue: 0.45; and red: 0.45) and FMC2 (yellow: 0.93; blue: 0.82; and red: 0.83), respectively. This indicates that the variations of color matching recipes generated for samples dyed in 0.3%, 1.5% and 3.0% concentrations are generally small. Table 7 shows color matching recipes of standard samples dyed by the D5 solvent-assisted method with different color difference formulae. It is observed that color matching recipes generated for Sample 4 are similar (yellow: 0.56; blue: 0.53; and red: 0.54) although different color difference formulae were used. Constant results are also found for recipes generated for Sample 5 (yellow: 0.60; blue: 0.46; and red: 0.49) and Sample 6 (yellow: 1.33; blue: 1.11; and red: 1.24) except when using Hunter lab, JPC 79 and CIE94 1.0 (yellow: 0.66; blue: 0.63; and red: 0.58) and CIE L*u*v* and FMC2 (yellow: 1.15; blue: 0.86; and red: 1.07), respectively. This indicates that color matching recipes generated for samples dyed by the solvent-assisted method have slightly larger variations than fabrics dyed by the conventional method.  Tables 8 and 9 show the color matching difference and the percentage difference between standard samples dyed by the conventional method and the D5 solvent-assisted method using different color difference formulae. The results reveal that most of the measured concentrations generated by using the conventional water-based method are below the theoretical concentrations, while concentrations generated by using the D5 solvent-assisted dyeing method are mostly higher than the expected concentrations. Measured values below the expected concentrations can be explained by the fact that the dye molecules are insufficient to be evenly distributed in the fabrics while the reason for measured values being higher than the expected concentration is the formation of dye aggregates, which may influence the absorption and scattering of light.

Computer Color Matching
Moreover, the difference between theoretical and measured concentration can be explained by the linearity of the calibration curves. Generally speaking, the higher the linearity of the calibration curves, the higher would be the accuracy of the result, and the difference between theoretical and measured concentration would be smaller. It was also observed that at a higher concentration (3%), exemplified by Sample 3 of water-based dyeing and Sample 6 of D5 solvent-assisted dyeing, the difference and percentage difference between the theoretical and measured concentration is greater than that at a lower concentration for both methods, and this could be explained by the degree of dye aggregation and the amount of dye molecules. At a high concentration, the possibility of the formation of dye aggregates and having insufficient dye molecules for an even distribution in the fabrics would be generally higher than that at low concentration. Furthermore, from Tables 7 and 8, the results show that at a high concentration (3%), the difference in recipe prediction for Sample 3 dyed in water and Sample 6 dyed in D5 is large. This means that the recipe prediction accuracy of sample dyed in D5 at a high concentration is lower than that in water. Nevertheless, this difference is considered to be tolerable and, thus, acceptable for computer color matching.    Table 10 illustrates the relative unlevelness indices (RUI) and visual levelness assessment of water-dyed and D5-dyed fabrics. Values of RUI of water-dyed fabrics are between 0.02 and 0.34, whereas the RUIs of D5-dyed fabrics are between 0.05 and 0.40. This means that the standard and the batch dyed samples, by the conventional water-based method and the D5 solvent-assisted method, can achieve good to excellent levelness, indicating that most dye molecules were in non-aggregate form throughout the dyeing process and they were well dispersed within the fabric in the reverse micellar D5 dyeing system.

Conclusions
The computer color matching and levelness of cotton fabrics dyed in non-aqueous medium of decamethylcyclopentasiloxane (D5) with reactive dye was investigated using a non-ionic surfactant reverse-micellar approach comprised of poly(ethylene glycol)-based surfactant and was compared with the conventional water-based dyeing system. The reflectance values of the samples were measured, and no chromatic change was found when the non-ionic reverse micellar D5-assisted dyeing approach was used. The calibration dyeing databases for both dyeing methods were established. The structure of the calibration curves was found to be almost linear with R-square ranging from 0.9605 to 0.9937, and the K/S sum values of D5-dyed samples were higher than water-dyed samples at each calibrated dye concentration. Computer color matching was conducted by using several color difference formulae, and the results reveal that the measured concentrations are nearly the same as the expected concentrations for both methods. This indicates that good color matching can be achieved by using the D5-assisted reverse micellar dyeing system. The levelness of the dyed samples was measured according to the relative unlevelness indices (RUI), and the results indicate that samples dyed by the D5 reverse micellar dyeing system can achieve good to excellent levelness, comparable with the conventional dyeing system.