Chemical structure of CEL ENMs
Figure 2 shows the IR-spectrum of the electrospun nanofibers. For neat CA NFM, the FTIR spectrum showed absorption bands present at 1368 cm− 1 (symmetric C-CH3 bending), 1231 cm− 1 (asymmetric stretching of carboxylate group) and 1046 cm− 1 (asymmetric C-O-C stretching) from the pyranose ring. Additionally, the peak located at 1745 cm− 1 is assigned to the carbonyl band (C = O) of the acetate group in the CA ENM (Khatri et al., 2016). The results are in agreements with previous reports (Khatri et al., 2013a, Khatri et al., 2014).
After deacetylation process, the typical absorption bands of CA were disappeared, and one absorption peak appeared significantly at around 3400 cm− 1 (-OH group) (Fig. 2). This shows a complete conversion of CA ENM into CEL ENM (Khatri et al., 2013a, Khatri et al., 2013c). The IR-spectrum of dyed CEL ENM has retained its chemical structure (result is not shown here), the interaction in between the CEL ENM and the dye molecules is not evident in the IR-spectrum. This is possibly due to the small amount of dye molecules used in during the experiments.
UV-Vis spectrophotometry of dye solution
Figure 3 shows the UV-vis transmittance spectra of indigo dye solution. It is evident that the transmittance spectra of CN indigo dye solution exhibited 55% transmittance peak at 500nm wavelength, and aniline free indigo dye exhibited 62% transmittance peak at the same wavelength.
Effect of dye solution pH
The pH is the most impacting parameter in order to get maximum color yield during dyeing of CEL ENM’s with synthetic indigo. The dyeing of pre-wetted CEL ENM’s was carried out using dye concentration of 3.33 g/L, sodium hydrosulphite 5 g/L during variation in pH at (10.5, 11, 11.5, 12, and 12.5). Figure 4 exhibits the effect of pH on color yield during dyeing process. The result shows that by increasing the pH of the dye solution from 10.5 to 12.5, color yield of the dyed samples was increased and the maximum color yield obtained at pH of 12.5. Further increase in the pH, reduced the color yield of the dyed samples because at higher pH, CEL ENM when dipped in alkaline bath acquires more negative charge and more is the repulsion occurs between dye and fiber, resulting in reduced dye uptake (Chakraborty and Chavan, 2004). Therefore, we considered the 12.5 pH as optimized value for further experiments.
Effect of sodium hydrosulphite concentration
To investigate the effect of sodium hydrosulphite concentration, dyeing of pre-wetted samples of CEL ENM’s was carried out at optimized pH of 12.5 by using 3.33 g/L dye concentration during variation in sodium hydrosulphite concentration ( 3, 4, 5, 6, 7 g/L). Figure 5 represents the effect of sodium hydrosulphite concentration on color yield and it reveals that by increasing the amount of sodium hydrosulphite concentration, color yield of the dyed samples was increased gradually. The maximum color yield was obtained at 7 g/L and was considered for further experiments.
Effect of dyebath temperature
For the optimization of dye bath temperature we consider the optimized value of pH and sodium hydrosulphite concentration by using 3.33 g/L dye concentration throughout the variation in dye bath temperature (22, 32, 42, 52, 62 ᵒC). Figure 6 reflects the effect of dye bath temperature on the color yield of dyed samples and reveals that the color yield increased by increasing the dye bath temperature from 22ᵒC to 52ᵒC. Further increment in the dye bath temperature slightly decreases the color yield of the dyed samples may be due to the affinity of the solubilized dye for CEL ENM decreases with the increase in temperature. Hence we selected 52ᵒC dye bath temperature for further trials.
Effect of dye concentration
Figure 7 shows the effect of dye concentration on color build up property of the dyed CEL ENM were investigated using the previously optimized conditions. Five differing dye concentrations (1.11, 2.22, 3.33, 4.44, 5.55 g/L) were used and the color build up properties were measured in terms of K/S values. As expected, the color yield increased with increasing dye concentration. However, further increment in the dye concentration has slightly reduced the K/S of CEL ENM. Hence, 5.55 g/L may be the saturation level for dyeing of CEL ENM with synthetic indigo dye by continuous method.
Comparison between the conventional (CN) and aniline free synthetic indigo dye
Effect of dye concentration on color build up property of the CN and aniline free synthetic indigo dye were investigated using the previously optimized conditions. Five differing dye concentrations (1.11, 2.22, 3.33, 4.44, 5.55 g/L) of both dyes were used and the color build up properties were measured in terms of K/S values. The results reported in Fig. 8 demonstrate an increase in the K/S with increasing dye concentration for both dyes. The K/S values for dyed CEL ENMs by aniline free indigo dye are slightly higher in comparison to the CN indigo dye during continuous dyeing method. The profiles of K/S results for both dyes were same demonstrating the uniform behavior of both dyes at the optimized conditions.
Effect of dye concentration on colorimetric values of dyed CEL ENMs
Figure 9 shows the images of the undyed and dyed samples for shade visualization. It can be seen that the dye have been applied uniformly on the ENMs.
In addition to K/S values, the calorimetric coordinates (L*, a*, b*, C* and ho) were measured for each dye concentration and the results are compiled in Table 1. It can be observed that with the increased concentrations of dye, the lightness values (L*) were decreased which shows that the dyed webs became darker. This trend supports the results shown in Fig. 7, as negative a* designate for greenness, and negative b* designated for blueness. Hence, the obtained results indicate that the dye is bluer in tone; this can be proved by hue angle ho which shows all values within the bluer region.
Table 1
Effect of CN synthetic indigo dye concentration on calorimetric values of dyed CEL ENMs
dye conc. g/L
|
L*
|
a*
|
b*
|
C*
|
ho
|
---|
1.11
|
40.63
|
-1.36
|
-23.65
|
23.69
|
266.71
|
2.22
|
30.11
|
-2.26
|
-20.35
|
20.47
|
276.34
|
3.33
|
31.85
|
-2.49
|
-18.50
|
18.67
|
262.34
|
4.44
|
31.18
|
-1.19
|
-20.60
|
20.64
|
266.69
|
5.55
|
30.22
|
-0.16
|
-22.51
|
22.51
|
270.41
|
The calorimetric values (L*, a*, b*, C* and ho) were also measured for aniline free synthetic indigo dye concentration; and the results are compiled in Table 2. It can be observed that with the increased concentrations of dye, the lightness values (L*) were decreased which shows that the dyed webs became darker. This trend supports the results shown in Fig. 8, as positive a* designate for redness, i.e. with increasing dye concentration the dyed ENMs are moving towards redder region, and negative b* designated for blueness. Hence, the obtained results indicate that the dye is bluer in tone; this can be proved by hue angle ho which shows all values within the bluer region.
Table 2
Effect of aniline free synthetic indigo dye concentration on calorimetric values of dyed CEL ENMs
dye conc. (g/L)
|
L*
|
a*
|
b*
|
C*
|
ho
|
---|
1.11
|
36.26
|
1.58
|
-26.39
|
26.43
|
273.42
|
2.22
|
28.63
|
2.82
|
-19.78
|
19.98
|
278.11
|
3.33
|
27.20
|
4.70
|
-21.50
|
22.01
|
282.34
|
4.44
|
24.43
|
5.05
|
-14.01
|
14.89
|
289.84
|
5.55
|
24.05
|
5.20
|
-14.72
|
15.62
|
289.46
|
Dye Solution Analysis as Effluent
Table 3 shows the properties of CN and aniline free synthetic indigo dyeing effluent used in this study. The results revealed that the effluent is basic in pH with high TDS content. In Pakistan, the untreated effluent discharge from dyeing industries has TDS content up to 2500 mg/L. During the testing everything performed exactly the same as it would with conventional indigo, there was just one important difference is no aniline and that aniline impurities are toxic to humans, causing skin allergies, damage to major organs and genetic defects, as well as being linked to cancer. Aniline is also toxic to aquatic life, which is an issue as two-thirds of the 400 metric tons of aniline waste on an annual basis ends up in the environment as wastewater discharge. Therefore, we used an alternative system that is aniline free indigo dye which make our industry sustainable by removing a hazardous impurity and to protect the workers, consumers and the environment with cleaner waterways.
Table 3
Analysis of dye solution as effluent
Dye
|
pH
|
TDS (mg/L)
|
COD (mg/L)
|
---|
CN synthetic indigo dye
|
12.3
|
1641
|
1940
|
Aniline free synthetic indigo dye
|
12
|
1193
|
1635
|
Colorfastness Properties
Table 4 presents the effect of colorfastness to washing and light of CEL ENM dyed with CN and aniline free synthetic indigo at optimum conditions. The colorfastness to washing (change in color) was excellent, and the results of colorfastness to washing (staining on multifiber) ratings were also excellent probably. The results of colorfastness to light test was excellent with no color change of the dyed sample. The excellent colorimetric properties demonstrate that the novel CEL ENM (colored and breathable) can potentially be deliberated as future apparels for casual and fashion.
Table 4
Colorfastness of CEL ENM dyed with CN and aniline free synthetic indigo dye
Dye
|
Washing fastness
|
Light fastness
|
---|
Change in color
|
Staining on Multifiber a
|
Blue wool reference
|
|
CT
|
CO
|
PA
|
PES
|
PAC
|
WO
|
CN synthetic indigo dye (Liq.)
|
4
|
4.5
|
4.5
|
4
|
4.5
|
5
|
4.5
|
7
|
Aniline free synthetic indigo dye
|
4
|
4.5
|
4.5
|
4.5
|
5
|
5
|
4.5
|
7
|
a CT, cellulose triacetate; CO, cotton; PA, polyamide; PES, polyester; PAC, polyacrylonitrile; WO, wool |
Morphology of Electrospun Fibers
The surface morphology of ENMs is demonstrated in Fig. 10. CEL ENM displayed a smooth morphology without beads suggesting that the electrospinning parameters (solution and process) used were optimum (Fig. 10a). Significantly, dyed CEL ENMs displayed almost similar morphology (Fig. 10b-c) in comparison to the undyed CEL ENM revealing good resistance to indigo dyeing conditions used. Similar findings were reported in previous studies for dyeing of CEL ENMs (Khatri et al., 2013a, Khatri et al., 2013c).